Improvement of ZnO and SnO2 Hydrogen GAs Sensors

I

Republic of Iraq

Ministry of Higher Education

& Scientific Research

University of Baghdad

College of Science

Improvement of ZnO and SnO2

Hydrogen Gas Sensors

A thesis

Submitted to the Committee of College of Science,


In Partial Fulfillment of the Requirements for the Degree of

Doctor of Philosophy in

Physics

By

Qahtan Ghatih Hial

B. Sc. 1994

M. Sc. 1997

Supervised By

Dr. Abdulla M. Suhail

Dr. Wasan R. Saleh

2011 A D

1432A H

II

Supervisor certification

We certify that this thesis was prepared by Mr. Qahtan Ghatih Hial

under our supervision at the Physics Department, College of Science,

University of Baghdad as a partial requirement for the degree of doctor of

philosophy in Physics.

Signature:

Name: Abdulla M. Suhail

Title: Assist. Professor

Address: College of Science,


Date: November , 2011

Signature:

Name: Wasan R. Saleh


Address: College of Science,



In view of the available recommendation, I forward this thesis for

debate by the Examining Committee.

Signature:

Name: Dr. Raad M. S. Al-Haddad

Title: Professor

Address: Collage of Science, University of Baghdad

Date: November 29, 2011

Suhail Wasan

Raad

III

Examination Committee Certification

We certify that we have read the thesis entitled “Improvement of ZnO

and SnO2 Hydrogen Gas Sensors” as an examining committee, examined the

Student “Qahtan Ghatih Hial” in its contents, and that in our opinion it meets

the standard of a thesis for the degree of Doctor of Philosophy in Phys-

ics/Optoelectronics.

Signature: Title: Professor

Name: Dr. Raad M. S. Al-Haddad Date: November 29, 2011 Chairman

Signature:

Title: Professor Name: Dr. Izzat M. AL-Essa

Date: November , 2011 Member

Signature:

Title: Professor

Name: Dr. Emad Kh. Al-Shakarchi Date: November 27, 2011

Member

Signature:


Name: Dr. Mayada Bedry Al-Quzweny


Member

Signature: Title: Assist. Professor

Name: Abdulla M. Suhail


Supervisor


Name: Dr. Bassam Ghalib Rasheed


Member


Name: Wasan R. Saleh


Co-Supervisor

Approved by the Dean of college of Science

Signature:

Title: Professor

Name: Dr. Saleh Mahdi Ali

The Dean of the College of Science

Date: December 5, 2011

Raad

Suhail Wasan

Bassam M. B. Q.

Emad

Salih M. Ali

IV

ABSTRACT

Spray – pyrolyzed palladium – doped metal oxides (zinc oxide

ZnO and tin oxide SnO2) nano films have been prepared on glass sub-

strates and explored as a fast response sensor to hydrogen reducing gas.

Both ZnO and SnO2 sensing films are obtained via chemical spray pyrol-

ysis deposition (SPD) technique at around 450 0C spraying temperature

with atmospheric air as the carrier gas. Zinc chloride ZnCl2 and zinc ace-

tate Zn(CH3COO)2.2H2O starting materials have been exploited in spray-

ing precursor solutions of ZnO thin film whereas, stannous chloride dihy-

drate SnCl2.2H2O is used in obtaining tin oxide SnO2. The SPD technique

has proven its simplicity and reliability in realizing polycrystalline in

nature ZnO films which crystallized along the (002) phase with preferen-

tial orientation along the c – axis of the ZnO hexagonal wurtzite structure

as verified by the XRD structural analysis. The films exhibit high trans-

mission in the visible range of the electromagnetic spectrum with an av-

erage transmittance value of up to 95 %, and present a sharp ultraviolet

cut – off at approximately 380 nm. The transmission but not the estimated

direct band gap Eg increased with decreasing film thickness. Scanning

Electron Microscope SEM and Atomic Force Microscope AFM surface

morphology studies of the ZnO films reveal a uniform distribution of

porous spherical – shaped nanostructure grains of 20 nm diameter. The

electrical characterization of the sprayed thin films shows that they are

highly resistive, but that their properties vary considerably when the

measurements are conducted in vacuum or in air.

For both ZnO and SnO2 metal oxides, the doped sensor exhibit an

increase of the conductance upon exposure to hydrogen gas of various

concentrations and at different operating temperatures, showing excellent

sensitivity.

V

It was found that the sensing mechanism of hydrogen gas in the

present metal oxide sensors is mostly related to the enhancement of ad-

sorption of atmospheric oxygen. The excellent selectivity and the high

sensitivity for hydrogen gas can be achieved by surface promotion of

ZnO/SnO2 metal oxide films. The observed conductance change in Pd –

doped ZnO sensors after exposure to H2 gas (3%) is about two times as

large as that in the undoped ZnO sensors.

The variation of the operating temperature of the film has led to a

significant change in the sensitivity of the sensor with an ideal operating

temperature of about 250 ± 25 0C after which sensor sensitivity decreas-

es. The sensitivity of the ZnO thin films changes linearly with the in-

crease of the gas concentration.

The response – recovery time of Pd:ZnO materials to hydrogen gas

is characterized to be relatively extremely short. ZnO thin films of 20 –

time dipping in palladium chloride solution have the highest sensitivity of

97% and extremely short response time of 3 s, which fit for practice since

it is crucial to get fast and sensitive gas sensor capable of detecting toxic

and flammable gases well below the lower explosion limit (4% by vol-

ume for H2 gas).

For SnO2 sensing elements, the optimum operating temperature is

around 210 0C and 95.744 % sensitivity to 4.5% H2: air mixing ratio.

VI

Dedicated to

All Those Who Care…

Including…

Her

VII

Acknowledgments

It would be impossible to express my thanks on this page to all those who

have supported me, without whose help I could never have come so far. I will attempt,

at least, to satisfy the barest demands of decency by saying a few words here.

Firstly, I would like to thank my advisor Dr. Abdulla M. Suhail for giving me

the opportunity to work on a challenging and interesting project over the past three

years and for his discussions that always challenged me to look at things from a dif-

ferent perspective. I would also like to thank Dr. Wasan R. Al-azawi for her utmost

valuable feedback collaboration on this research and always backing me up.

I am really indebted to the Ministry of Higher Education & Scientific Re-

search, and the Physics Department – College of Science of Baghdad University for

the unceasing generous patronage of the postgraduates.

My sincere appreciation goes to my colleagues at the Electro – optics & Nano-

technology Research Group: Dr. Osama, Dr. Suded, Assistant lecturer Miss Ghaida,

Assistant lecturer Mr. Omar. Also, to my wonderful group postgraduates: Miss Hind,

Miss Hanan, Mrs. Fatin and Mr. Aqil for their continuous encouragement and support.

To the Thin Film Research Group, I would like to express my extreme grati-

tude and indebtedness for collaborating on this research and for all of the material

support provided. Also, the great help of the XRD, AFM at the Material Physics &

Chemistry Research Establishment labs at the Ministry of Science and Technology are

acknowledged. This thesis would not have been possible without their willingness to

work with me.

No gratitude is sufficient to repay the endless love of my parents and family,

who have stood behind me from my first steps through all the moments of skinned

knees and shaken confidence. They read me my first book, and never failed to call

when my studies overwhelmed me. They gave me the ground I could stand on when-

ever the path ahead seemed dim. No son (or brother) could ask for better.

VIII

Curriculum Vitae

July 1, 1972 ................................................................................................... Born – Iraq.

1994 ..................................................... B.Sc., Physics/Physics – Baghdad University

1997........................................ M.Sc., Physics/Laser Technology – Baghdad University

2002 – 2007......................................................... Assist. Lecturer – Physics Department

2007 – October 31, 2011 .............................. Ph. D. Postgraduate – Physics Department

PUBLICATIONS

Journal Articles

[1] Q. G. Al-zaidi, Abdulla. M. Suhail, Wasan R. Al-azawi, Palladium – doped

ZnO thin film hydrogen gas sensor, Applied Physics Research Vol. 3, No.

1, pp. 89 – 99, (2011).

[2] H. A. Thjeel, A. M. Suhail, A. N. Naji, Q. G. Al-zaidi , G. S. Muhammed,

and F. A. Naum, Fabrication and characteristics of fast photo response high

responsivity ZnO UV detector, Sensors and Actuators A: Physicsl, revised

manuscript submitted for publication.

[3] A. M. Suhail, O. A. Ibrahim, H. I. Murad, A. M. Kadim and Q. G. Al-zaidi,

Enhancement of white light generation from CdSe/ZnS core – shell system

by adding organic pyrene molecules, Journal of Luminescence, revised

manuscript submitted for publication.

:إال قال في غده إني رأيت أنه ال يكتب أحد كتابا في يومه

م هذا " ."لكان أفضل، ولو ترك هذا لكان أجمللو غير هذا لكان أحسن، ولو زيد هذا لكان يستحسن، ولو قد

.النقص على جملة البشر وهذا من أعظم العبر، وهو دليل على استيالء

العمـــاد األصفهانـــي

Assuming that he’s not dead, Qahtan can best be reached at his “lifetime” email address of:

[email protected]

Mobile No.: +009647702981421

mailto:[email protected]

IX

Contents

Page

Abstract ........................................................................................................................ IV

Dedication .................................................................................................................... VI

Acknowledgments...................................................................................................... VII

Curriculum Vitae ...................................................................................................... VIII

List of Tables ............................................................................................................. XII

List of Figures ........................................................................................................... XIII

List of Symbols .......................................................................................................... XX

Chapter 1 Motivation and Project Objectives ................................................................ 1

1.1 Motivation ........................................................................................................... 1

1.2 Gas Sensor Applications ..................................................................................... 4

1.3 Focus of current research .................................................................................... 6

1.4 Thesis Outline ...................................................................................................... 6

Chapter 2 Working Principles of Semiconductor Metal Oxide Gas Sensors ................ 8

2.1 Adsorption Mechanisms ...................................................................................... 8

2.2 Non-Stoichiometry in Semiconductors ............................................................. 11

2.3 Gas Sensor Operation: Catalysis and Adsorption ............................................. 13

2.4 Semiconductor Metal Oxide Gas Sensors ......................................................... 21

2.5 Gas Sensor Metrics ............................................................................................ 27

2.5.1 Sensitivity ............................................................................................... 27

2.5.2 Selectivity .............................................................................................. 28

2.5.3 Stability .................................................................................................. 29

2.5.4 Response and Recovery Times ............................................................... 30

2.6 Sensing Mechanism ........................................................................................... 31

2.7 Factors Influencing the Performance ................................................................ 33

2.7.1 Long term effects / Baseline Drift .......................................................... 33

2.7.2 Sensor surface poisoning ........................................................................ 34

2.8 Optimization of Sensor Performance ................................................................ 34

2.8.1 Use of Catalyst ........................................................................................ 34

X

2.8.1.1 Spill over Mechanism ................................................................ 36

2.8.1.2 Fermi Energy Control ................................................................ 37

2.8.2 Grain size effects .................................................................................... 39

2.8.3 Thickness dependence ............................................................................ 40

2.8.4 Temperature Modulation ........................................................................ 41

2.8.5 Filters for Selectivity............................................................................... 42

2.8.6 AC and DC measurements ...................................................................... 43

2.9 Zinc Oxide ......................................................................................................... 45

2.9.1 Properties of Zinc Oxide ......................................................................... 45

2.9.2 Defects chemistry.................................................................................... 49

2.9.3 Spray pyrolysis deposition technique ..................................................... 55

2.9.3.1 The deposition process and atomization models ........................ 59

2.9.3.2 Deposition parameters ................................................................ 63

I. Substrate temperature............................................................... 63

II. Influence of Precursors ............................................................ 63

III. Spray Rate ................................................................................ 64

IV. Other Parameters ...................................................................... 65

2.9.4 Metal Oxide Gas Sensors ........................................................................ 65

Chapter 3 Experimental Procedure .............................................................................. 78

3.1 Gas Sensor Fabrication ...................................................................................... 78

3.2 Spray pyrolysis experimental set up .................................................................. 80

3.3 Precursor solution .............................................................................................. 81

3.4 The determination of film thickness .................................................................. 83

3.5 Surface modification of ZnO by palladium noble metal ................................... 84

3.6 Al Interdigitated Elecrtodes (IDE) .................................................................... 85

3.7 Gas sensor testing system .................................................................................. 87

3.8 Sensor testing protocol ...................................................................................... 89

3.9 Crystalline structure of the prepared ZnO thin films ........................................ 91

3.10 Thin film surface topography ............................................................................ 92

3.11 Optical properties ............................................................................................. 92

3.12 Tin oxide (SnO2) hydrogen gas sensor.............................................................. 94

XI

Chapter 4 Results and Discussion ................................................................................ 95

4.1 ZnO thin film deposition ................................................................................... 95

4.2 Crystalline structural properties of the ZnO thin film ....................................... 98

4.3 Surface topography and morphology studies ................................................. 100

4.4 Optical Properties ............................................................................................ 105

4.5 Electrical Properties ........................................................................................ 108

4.5.1 Resistance – Temperature Characteristic .............................................. 108

4.5.2 I – V characteristic of the zinc oxide films ........................................... 110

4.5.3 AC Impedance Spectroscopy ................................................................ 112

4.6 Gas Sensing Measurements ............................................................................. 114

4.6.1 Sensing Characteristics of Pure ZnO towards hydrogen gas ................ 114

4.6.2 Sensing characteristics of Pd – doped ZnO towards hydrogen gas ...... 118

4.7 Operation temperature of the sensor ............................................................... 119

4.8 Tin oxide (SnO2) hydrogen gas sensor ............................................................ 122

4.8.1 Crystalline structure and morphology of undoped SnO2 thin film ....... 122

4.8.2 Optical properties of the undoped tin oxide SnO2 thin films ................ 125

4.8.3 Sensing characteristics of pure SnO2 towards hydrogen gas ................ 126

4.8.4 Sensing characteristics of Pd – doped SnO2 towards hydrogen gas ..... 128

4.9 Conclusions and Future work Proposals ......................................................... 132

References ....................................................................................................... 135

XII

List of Tables

Table page

1. Table 1.1 ----------------------------------------------------------------------- 5

Examples of application for gas sensors and electronic noses

2. Table 2.1 ----------------------------------------------------------------------- 9

Comparison of physisorption and chemisorption

3. Table 2.2 --------------------------------------------------------------------- 10

Temperature ranges associated with molecular and dissociative oxy-

gen adsorption reactions.

4. Table 2.3 --------------------------------------------------------------------- 46

Zn – O crystal structure data.

5. Table 2.4 --------------------------------------------------------------------- 48

Typical properties of zinc oxide.

6. Table 2.5 --------------------------------------------------------------------- 55

Characteristics of atomizers commonly used for SPD.

7. Table 3.1 --------------------------------------------------------------------- 83

Optimum thermal spray pyrolysis deposition conditions for the prepa-

ration of ZnO thin films

8. Table 4.1 --------------------------------------------------------------------- 96

Spray pyrolysis deposition optimum parameters

9. Table 4.2 -------------------------------------------------------------------- 100

Crystalline structure, Miller indices and d spacings of the as – deposit-

ed ZnO crystal planes

10. Table 4.3 -------------------------------------------------------------------- 100

Crystalline structure, Miller indices and d spacings of the Pd – doped

ZnO crystal planes.

XIII

LIST OF FIGURES

Figure Page

1. Figure 2.1 -------------------------------------------------------------------- 15

Microstructure and energy band model of a gas sensitive SnO2 thick

film. The potential barriers form as a result of oxygen adsorption.

2. Figure 2.2 -------------------------------------------------------------------- 17

The nature of oxygen species adsorbed on ZnO as reported by several

researchers.

3. Figure 2.3 -------------------------------------------------------------------- 18

The energy barriers in the transformation from reactants (A + B) to

products (C + D). The uncatalyzed reaction is characterized by a large

activation energy (Eg), while the barrier to product formation is low-

ered (Ec) when a catalyst is used.

4. Figure 2.4 -------------------------------------------------------------------- 22

Schematic view of gas sensing reaction in (a) Compact layer and (b)

Porous layer. a: grain boundary model. b: open neck model. c: closed

neck model.

5. Figure 2.5 ------------------------------------------------------------------- 23

Schematic of a compact layer with geometry and energy band repre-

sentation; Z0 is the thickness of the depleted surface layer; Zg is the

thickness of the surface and eVS the band bending. (a) A partly deplet-

ed compact layer (“thicker”) and (b) A completely depleted layer

(“thinner”).

6. Figure 2.6 ------------------------------------------------------------------- 24

Schematic representation of a porous sensing layer with geometry and

energy band for small and large grains. λD Debye length, Xg grain

size.

7. Figure 2.7 -------------------------------------------------------------------- 25

Schematic of a porous layer with geometry and surface energy band

with necks between grains; Zn is the neck diameter; Z0 is the thickness

of the depletion layer and eVS the band bending. (a) a partly depleted

necks and (b) a completely depleted necks.

8. Figure 2.8 -------------------------------------------------------------------- 26

Influence of particle size and contacts on resistances and capacitances

in thin films are shown schematically for a current flow I from left to

right.

XIV

9. Figure 2.9 -------------------------------------------------------------------- 30

Drawing showing how response and recovery times are calculated

from a plot of sensor conductance versus time.

10. Figure 2.10 ------------------------------------------------------------------- 35

Illustration of the catalyst effect. Nano – particles, having higher sur-

face area, act as catalysts. Here, R stands for reducing gas.

11. Figure 2.11 ------------------------------------------------------------------- 36

Mechanism of sensitization by metal or metal oxide additive.

12. Figure 2.12 ------------------------------------------------------------------- 37

Illustration of Spill Over caused by catalyst particles on the surface of

the grain of the polycrystalline particle.

13. Figure 2.13 ------------------------------------------------------------------- 38

An adequate dispersion of the catalysts is required in order to effec-

tively affect the grains of the semi-conducting material to serve the

implied purpose of increase in sensitivity.

14. Figure 2.14 ------------------------------------------------------------------- 39

Schematic models for grain – size effects.

15. Figure 2.15 ------------------------------------------------------------------- 44

Equivalent circuit for the different contributions in a thin film gas sen-

sor; intergranular contact, bulk and electrode contact.

16. Figure 2.16 ----------------------------------------------------------------- 46

T – X diagram for condensed Zn- O system at 0.1 MPa.

17. Figure 2.17 ------------------------------------------------------------------- 47

Many properties of zinc oxide are dependent upon the wurtzite hexag-

onal, close-packed arrangement of the Zn and O atoms, their cohe-

siveness and void space [59].

18. Figure 2.18 ------------------------------------------------------------------- 49

Ellingham diagram of oxides.

19. Figure 2.19 ------------------------------------------------------------------- 50

Various types of point defects in crystalline materials.

20. Figure 2.20 ------------------------------------------------------------------- 60

Schematic diagram of chemical spray pyrolysis unit

21. Figure 2.21 ------------------------------------------------------------------- 62

XV

Spray processes (A, B, C, and D) occurring with increase in substrate

temperature.

22. Figure 3.1 -------------------------------------------------------------------- 78

Schematic of a typical gas sensor structure.

23. Figure 3.2 -------------------------------------------------------------------- 81

Spray pyrolysis experimental set up.

24. Photo plate 3.1 -------------------------------------------------------------- 82

A: experimental set up of the spray pyrolysis deposition SPD. B: Air

atomizer. C: Gemo DT109 temperature controller, and D: Digital bal-

ance with the magnetic stirrer.

25. Figure 3.3 -------------------------------------------------------------------- 85

Vacuum system for the vaporization from resistance – heated sources.

When replacing the transformer and heater with an electron gun, va-

porization by means of an electron beam occurs.

26. Figure 3.4 -------------------------------------------------------------------- 86

A schematic diagram of the IDE masks utilized in this work.

27. Figure 3.5 -------------------------------------------------------------------- 87

Gas sensor testing system.

28. Photo plate 3.2 -------------------------------------------------------------- 88

A photo of the sensor testing system.

29. Figure 3.6 -------------------------------------------------------------------- 91

A schematic diagram of the gas sensor basic measurement electrical

circuit.

30. Photo plate 3.3 -------------------------------------------------------------- 92

LabX XRD – 6000 Shimadzu diffractometer unit.

31. Photo plate 3.4 -------------------------------------------------------------- 93

Ultra 55 SEM unit from ZEISS.

32. Photo plate 3.5 -------------------------------------------------------------- 93

AA3000 Scanning Probe Microscope SPM, tip NSC35/AIBS, from

Angstrom Advance Inc.

33. Photo plate 3.6 -------------------------------------------------------------- 94

Optima sp-3000 plus UV-Vis-NIR spectrophotometer.

34. Figure 4.1 -------------------------------------------------------------------- 96

XVI

A photo of spray pyrolyzed ZnO thin film on glass samples.

35. Figure 4.2 -------------------------------------------------------------------- 97

Scanning Electron Micrograph photo of spray pyrolyzed ZnO thin

film on glass.

36. Figure 4.3 -------------------------------------------------------------------- 97

Enlarged photos of Al interdigitated electrodes IDE evaporated on

ZnO thin film sample. A: 1 – mm finger spacing IDE on glass, and B:

0.4 – mm finger spacing IDE on silicon.

37. Figure 4.4 -------------------------------------------------------------------- 99

XRD crystal structure of as deposited ZnO thin film prepared from 0.1

M Zinc Chloride aqueous precursor.

38. Figure 4.5 -------------------------------------------------------------------- 99

XRD crystal structure of Pd – doped ZnO thin film prepared from 0.1

M Zinc Chloride aqueous precursor.

39. Figure 4.6 ------------------------------------------------------------------- 101

Scanning Electron Micrograph of ZnO film prepared at a) 400 0C and

the inset b) 200 0C.

40. Figure 4.7 ------------------------------------------------------------------- 102

Scanning Probe Microscope images of zinc oxide thin film spray py-

rolysed on glass substrate at 450 0C spraying temperature with the

precursor of 0.2 M zinc acetate dissolved in 100 mL distilled water.

41. Figure 4.8 ------------------------------------------------------------------- 103

Scanning Probe Microscope images of zinc oxide thin film spray py-

rolysed on glass substrate at 450 0C spraying temperature with the

precursor of 0.2 M zinc acetate dissolved in 100 mL isopropyl alcohol.

42. Figure 4.9 ------------------------------------------------------------------- 104

Granularity cumulation distribution report of ZnO thin film deposited

at 450 0C on glass substrate using 0.2 M zinc acetate in distilled water

precursor solution.

43. Figure 4.10 ------------------------------------------------------------------ 105

Transmission spectra of ZnO thin films of different thicknesses

sprayed on – glass at 400 0C temperature. The precursor was 0.1 M

dissolved in distilled wa-ter except the 189.34 – nm thick sample

which was a 0.2 M zinc acetate dissolved in 3:1 volume ratio isopro-

pyl alcohol and distilled water.

XVII

44. Figure 4.11 ------------------------------------------------------------------ 106

Absorption spectra of ZnO thin films of different thicknesses sprayed

on – glass at 400 0C temperature. The precursor was 0.1 M zinc ace-

tate dissolved in distilled water.

45. Figure 4.12 ------------------------------------------------------------------ 107

Plots of (αhν)2 vs. photon energy hν for ZnO thin films of different

energy gaps and thicknesses.

46. Figure 4.13 ------------------------------------------------------------------ 108

Relationship of energy gap Eg of sprayed ZnO thin films with film

thickness.

47. Figure 4.14 ------------------------------------------------------------------ 109

The variation of resistance of the spray – pyrolyzed deposited zinc ox-

ide film of 668 nm film thickness with temperature.

48. Figure 4.15 ------------------------------------------------------------------ 110

The I – V characteristic in dark and under UV illumination.

49. Figure 4.16 ------------------------------------------------------------------ 111

The effect of vacuum on base line current of a ZnO thin film at 200 0C

and 10 v bias voltage.

50. Figure 4.17 ------------------------------------------------------------------ 112

The I – V characterization of sprayed ZnO film in the temperature

range from RT to 300 0C.

51. Figure 4.18 ------------------------------------------------------------------ 113

The Cole – Cole plot for the impedance spectrum of the films at room

temperature. The inset is the R-C equivalent circuit of the simulation

of the impedance spectrum.

52. Figure 4.19 ------------------------------------------------------------------ 114

Sensing behavior of ZnO thin film at 6 v bias voltage and 210 degrees

temperature to traces of hydrogen reducing gas mixing ratio in air of

3%, 2%, and 1% respectively.

53. Figure 4.20 ------------------------------------------------------------------ 115

The sensitivity dependence of as – deposited ZnO sensor on hydrogen

gas mixing ratio.

54. Figure 4.21 ------------------------------------------------------------------ 116

XVIII

Transient responses of ZnO thin film (245 nm thick) at 210 0C testing

temperature upon exposure to hydrogen gas of mixing ratios of 1%,

2%, and 3% respectively.

55. Figure 4.22 ------------------------------------------------------------------ 117

Response and recovery time of the sensor as a function of testing gas

mixing ratio at a testing temperature of 210 0C and bias voltage of 6 v.

56. Figure 4.23 ------------------------------------------------------------------ 117

I - V characteristics of undoped ZnO gas sensor to 5%, 3%, and 1%

Hydrogen gas mixture in air and at 200 degrees temperature.

57. Figure 4.24 ------------------------------------------------------------------ 118

The switching behavior of the Pd – sensitized ZnO thin film maximum

con-ductance to hydrogen of 3% H2:air mixing ratio at 200 0C and bias

voltage of 10 v.

58. Figure 4.25 ------------------------------------------------------------------ 119

Effect of the testing temperature on the Pd – sensitized ZnO thin film

maximum conductance to hydrogen of 3% H2:air mixing ratio and bias

voltage of 10 v.

59. Figure 4.26 ------------------------------------------------------------------ 121

The variation of sensitivity with the operating temperature of the Pd –

doped ZnO gas sensor.

60. Figure 4.27 ------------------------------------------------------------------ 122

Transient responses of Pd – sensitized ZnO thin film (668 nm thick) as

exposed to hydrogen gas of mixing ratio of 3% and at three different

testing temperatures of (1) 250, (2) 300, and (3) 350 0C successively.

61. Figure 4.28 ------------------------------------------------------------------ 123

X – ray diffraction (XRD) pattern of SnO2 thin film spray pyrolyzed

on glass substrate at temperature of 450 0C.

62. Figure 4.29 ------------------------------------------------------------------ 124

AFM image of undoped SnO2 thin film deposited at 450 0C on glass

substrate with the precursor being tin dichloride dehydrate dissolved

in isopropyl alcohol.

63. Figure 4.30 ------------------------------------------------------------------ 125

Transmission spectra of undoped SnO2 thin films of different thick-

nesses deposited at 450 0C on glass substrates.

64. Figure 4.31 ------------------------------------------------------------------ 126

XIX

Absorption coefficient versus the photon energy for energy gap esti-

mation of undoped SnO2 thin films of different thicknesses deposited

at 450 0C on glass substrates.

65. Figure 4.32 ------------------------------------------------------------------ 127

Sensitivity behavior of undoped tin oxide SnO2 thin film to different

hydrogen concentrations. The bias voltage was 5.1 v with the tempera-

ture set to 210 0C.

66. Figure 4.33 ------------------------------------------------------------------ 127

Sensitivity versus H2 gas concentration of undoped tin oxide SnO2 thin

film. The bias voltage was 5.1 v with the temperature set to 210 0C.

67. Figure 4.34 ------------------------------------------------------------------ 128

Sensing behavior of Pd – doped SnO2 gas sensor to different H2 : air

mixing ratios. The tests were performed at 210 0C temperature and 10

v bias.

68. Figure 4.35 ------------------------------------------------------------------ 129

Response transient of Pd – doped SnO2 gas sensor to different H2 : air

mixing ratios. The tests were performed at 210 0C temperature and 10

v bias.

69. Figure 4.36 ------------------------------------------------------------------ 130

Sensitivity and Response time as a function of the H2 test gas mixing

ratio. The test was performed at 210 0C and 10 v bias on SnO2 sample

sprayed over the IDE and surface coated with 20 PdCl2 layers sprayed

at 400 0C over the film.

70. Figure 4.37 ------------------------------------------------------------------ 130

Transient responses of SnO2 thin film of 248 nm thick at 150, 175, and

210 0C testing temperature upon exposure to1% H2:air gas mixing ra-

tio.

71. Figure 4.38 ------------------------------------------------------------------ 131

Variation of sensor response current with temperature of Pd - doped

SnO2 thin film exposed to 4.5% hydrogen gas mixing ratio in air and

at 10 v bias voltage.

XX

LIST OF SYMBOLS

ε Static dielectric constant

λ Wavelength nm

μ (electron) mobility cm2.s

-1.v

-1

ρ Density kg/m3

σ Conductivity Ω.cm

τrec Recovery time s

τres Response time s

Ω Ohm

qVS Surface Potential Barrier eV

AFM Atomic Force Microscope

CVD Chemical Vapor Deposition

CSP Chemical Spray Pyrolysis

DMM Digital Multi Meter

ENC Electro Neutrality Condition

G Conductance (electrical) S

IDE Interdigitated Electrode

kB Boltzmann’s constant J/K

LEL Lower Explosion Limit %

LD Debye length (LD≡λD) nm

Nd Concentration of Donors cm-3

ns Concentration of Electrons cm-3

NO Nitric oxide

N2O Nitrous Oxide

NO2 Nitrogen dioxide

NTCR Negative Temperature Coefficient of Resistance

PID Proportional–Integral–Derivative Controller

ppm Parts Per Million

PTCR Positive Temperature Coefficient of Resistance

R Resistance (electrical) Ω

S Siemens

sccm Standard Cubic Centimeter per Minute

SEM Scanning Electron Microscope

SMO Semiconductor Metal Oxide

t90 Time to accomplish 90% of sensor response change s

TEM Transmission Electron Microscope

VOC Volatile Organic Compound

XRD X-Ray Diffraction

Z0 Depletion Region nm

1

Chapter 1

Motivation and Project Objectives

Introduction

The purpose of this chapter is to provide a general framework and

introduction for the work presented in the current Ph.D. project. This

chapter is divided into four sections, addressing the research motivations,

objectives, focus of current research and thesis outline.

1.1 Motivation

Sensors are devices that produce a measurable change in output in

response to a specified input stimulus [1]. This stimulus can be a physical

stimulus like temperature and pressure or a concentration of a specific

chemical or biochemical material. The output signal is typically an elec-

trical signal proportional to the input variable, which is also called the

measurand. Sensors can be used in all three phases of matter although gas

and liquid sensors are the most common.

The presence of a reducing/oxidizing gas at the surface of certain

metal oxide semiconductors changes its electrical resistance R. It is this

phenomenon that has spurred the use of these materials in the detection of

a gaseous ambient. The theoretical basis for semiconductor gas sensors

arose in 1950, when Carl Wagner proposed a concept to explain the de-

composition of nitrous oxide (N2O) on zinc oxide (ZnO) [2]. He made the

novel assumption that an exchange of electrons was taking place between

the gaseous N2O and the solid ZnO, which possessed a layer of adsorbed

oxygen. A few years later, Brattain et al. found that ambient gas produced

changes in potential between an electrode and a germanium surface [3].

These findings were explained in a theory outlining the existence of do-

2

nor and acceptor traps that lead to the generation of a space charge layer

on the surface of the germanium. A working gas sensor was realized in

1962, when Seiyama et al. detailed the use of ZnO thin films in the detec-

tion of such gases as ethanol (C2H6O) and carbon dioxide (CO2) [4]. It

was in that same year that Naoyoshi Taguchi issued a patent for a gas

sensor based on tin oxide (SnO2) [5]. As such, gas sensors based on SnO2

are typically referred to as Taguchi sensors and are commercially availa-

ble through Figaro Engineering Inc. [6].

The Taguchi gas sensor is a partially sintered SnO2 bulk device

whose resistance in air is very high and drops when exposed to reducing

gases such as combustibles (H2, CO, CH4, C3H8) or volatile organic va-

pors and it has enjoyed a substantial popularity because of its ease of

fabrication, low cost, robustness, and their sensitivity to a large range of

reductive and oxidative gases [7]. In addition to research on understand-

ing the fundamentals of the sensing mechanism, the studies on ZnO and

SnO2 sensors have been directed on enhancing the sensor performance

through the addition of noble metals (Pt, Pd, etc.), synthesis of thick and

thin film sensors, and doping with other semiconductors [7-10]. Other

metal oxides such as Fe2O3, TiO2, WO3 and Co3O4 have also been used as

gas sensors. Despite these broad studies in the semiconductor sensor area,

problems such as insufficient gas selectivity, slow response and recovery

times, inability to detect very low gas concentrations, and degradation of

the sensor performance by surface contamination still persist. Thus, there

is a growing need for chemical sensors with novel properties.

The principle mechanism for gas detection in metal oxides in am-

bient air is the ionosorption of oxygen at its surface, which produces a

depletion layer (for n-type semiconductors), and hence reduces conduc-

tivity [11]. Here, ionosorption refers to the process where a species is

3

adsorbed and undergoes a delocalized charge transfer with the metal ox-

ide. This can then be used to measure reducing and oxidizing gases, as

they will change the amount of ionosorbed oxygen, and therefore the

conductivity of the metal oxide.

At higher temperatures the adsorption and desorption rates of oxy-

gen are faster, resulting in a greater response (sensitivity) and a lower

response time for the gas sensor. However, the physical properties of the

metal oxides place an upper limit on the temperatures that can be used. If

the temperature is too high, the stability and reliability of the sensors

diminishes because of possible coalescence and structural changes [12].

Furthermore, as temperature increases, the charge – carrier concentration

will increase and the Debye length, LD, will decrease, resulting in less

sensitivity [13]. In most cases, the optimal temperature for metal oxide

gas sensors is between 200 0C and 500

0C [17].

There are two well – known ways for improving the gas sensing

properties of these films. The first is to add noble metals for their catalyt-

ic activity and to dope the film, with many reports showing that it leads to

better sensitivity and stability, e.g. [14, 15]. The second is to reduce grain

size, which has been shown to increase sensitivity [16]. This is because

the depletion layer caused by ionosorption has a greater effect on the

conduction channel of the grain as the grain size decreases. Consequently,

there is great interest in using nanoparticles in gas sensors, since they can

be used to make films with very small grain sizes. These two approaches

have been experimented in the current research to maximize the sensitivi-

ty and enhance the response time of the metal oxide ZnO/SnO2 thin film

– based hydrogen gas sensors. Thus, the surface modification of the ZnO

sensing element with palladium has greatly enhanced the sensor sensitivi-

4

ty and response time of minute – grain size ZnO thin films made possible

through using organic solvent other than water in spraying precursor.

1.2 Gas Sensor Applications

Gases are the key measurands in many industrial or domestic activ-

ities. In the last decade the specific demand for gas detection and moni-

toring has emerged particularly as the awareness of the need to protect the

environment has grown. Gas sensors find applications in numerous fields

[17, 18]. Two important groups of applications are the detection of single

gases (as NOx, NH3, O3, CO, CH4, H2, SO2, etc.) and the discrimination

of odours or generally the monitoring of changes in the ambient. Single

gas sensors can, for examples, be used as fire detectors, leakage detectors,

controllers of ventilation in cars and planes, alarm devices warning the

overcoming of threshold concentration values of hazardous gases in the

work places. The detection of volatile organic compounds (VOCs) or

smells generated from food or household products has also become in-

creasingly important in food industry and in indoor air quality, and multi-

sensor systems (often referred to as electronic noses) are the modern gas

sensing devices designed to analyze such complex environmental mix-

tures [19]. In Table 1.1 [17] examples of application for gas sensors and

electronic noses are reported.

Industry currently employs many varieties of gas sensing systems

for monitoring and controlling emissions from their processes. Applica-

tions exist in the steel, aluminum, mineral, automotive, medical, agricul-

tural, aroma and food industries.

Analytical instruments, based on optical spectroscopy and electro-

chemistry, are widely used in the scientific community. These instru-

ments give precise analytical data, however are costly, slow, and cumber-

5

some and require highly qualified personnel to operate. Current trends are

to improve low cost, solid state gas sensor performance in order to obtain

high linearity, sensitivity, selectivity and long term stability [19].

The only practical way to monitor air quality or provide a mean to

alert a human of potential danger is by direct gas sensing. A gas sensor

can form part of an early warning system, notifying the appropriate au-

thorities or provide the feedback signals to a process control system. To

achieve this, a gas sensor system must be capable of accurate and stable

in-situ real time measurements. Environmental factors such as operating

temperature, vibration, mechanical shock, chemical poisoning, as well as

Applications

Automobiles

Car ventilation control

Filter control

Gasoline vapour detection

Alcohol breath tests

Safety

Fire detection

Leak detection

Toxic/flammable/explosive gas detectors

Boiler control

Personal gas monitor

Indoor air quality

Air purifiers

Ventilation control

Cooking control

Environmental control

Weather stations

Pollution monitoring

Food

Food quality control

Process control

Packaging quality control (off-odours)

Industrial production

Fermentation control

Process control

Medicine

Breath analysis

Disease detection

Table 1.1: Examples of applications for gas sensors and electronic noses [17].

6

various device characteristics (accuracy, resolution, physical size and

cost) must also be taken into consideration.

1.3 Focus of current research

The main focus of the present thesis is on the improvement of semi

conducting metal oxide (SMO) thin film based gas sensors (with special

emphasis on SnO2 and ZnO) and their characterization. An objective

analysis of the various substrates used by several investigators is per-

formed as a part of this work. The gas sensitive zinc oxide and tin oxide

films are deposited by chemical spray pyrolysis deposition technique with

air blast atomization at 400 – 450 0C spraying temperature. The character-

ization of the as deposited film is performed by XRD, absorption, trans-

mission, scanning electron microscope SEM and atomic force microscope

AFM. Too much effort has been spent to maximize the sensitivity S and

reduces the response τres and recovery τrec times of the sensing element

upon exposing to hydrogen reducing gas H2 of various concentrations C

and at different operating temperatures T. The catalytic effect of the pal-

ladium noble metal and grain size effect are exploited to accomplish these

vital objectives. The thesis also describes the development of the gas

sensor test setup which has been used to measure the sensing characteris-

tics of the sensor.

1.4 Thesis Outline

The current thesis is organized into four chapters. The first chapter

is a general introduction where the overview of the field of study and

scope of the work carried out is outlined. The second chapter entitled

“Working principles of semiconductor metal oxide gas sensors” briefly

describes the working principle of the kind of sensors developed and

surveys the various methods used currently to improve the sensor charac-

7

teristics. The fabrication and characterization of the sensing element as

well as testing the sensors towards hydrogen reducing gas is dealt with in

the third chapter. Moreover, the development of the gas sensor testing

chamber and the protocol to use it are also detailed in this chapter. The

subsequent chapter (chapter four), a discussion of the experiments carried

out and the results obtained in the development of gas sensors is submit-

ted. At the end of the chapter four, the conclusions and the scope for fu-

ture work are summarized.

8

Chapter 2

Working Principles of Semiconductor Metal Oxide Gas Sensors

Background

Background information relevant to gas sensor technology is intro-

duced in this chapter. Adsorption of gases on the oxide surface is dis-

cussed in the first section as it is of fundamental importance in sensors

built using metal oxide materials. Particular emphasis is placed on the

adsorption of oxygen because most sensors operate in air and oxygen is

the dominant adsorbed species in this case. The mechanism where an

oxide transforms gas – surface interactions into a measurable electrical

signal is reviewed with a focus on the effects of particle size on this phe-

nomenon. The current understanding of the gas sensing mechanism and a

brief discussion of theoretical and empirical models proposed for semi-

conductor metal oxide (SMO) gas sensors are discussed. The metrics by

which gas sensor performance is judged are defined in this chapter and an

introduction to SMO gas sensors is presented. Background information is

concluded with a discussion of reported spray pyrolysis deposition tech-

nique for oxide semiconductors.

2.1 Adsorption Mechanisms

Physical adsorption (physisorption) is defined as an adsorption

event where no geometric change occurs to the adsorbed molecule and

van der Waals forces are involved in the bonding between the surface and

adsorbate [11]. Chemical adsorption (chemisorption) is the formation of a

chemical bond between the molecule and the surface during the adsorp-

tion process and requires an activation energy (e.g. ~0.5eV for chemi-

sorption of oxygen on SnO2) [20]. Chemisorption is a much stronger

bond than physisorption and the characteristics of each are summarized in

9

Table 2.1. Two types of chemisorption occur on the surface of metal ox-

ides: (1) molecular or associative chemisorption, in which all the atomic

bonds are preserved in the adsorbed molecule; and (2) dissociative chem-

isorption, where bonding within the adsorbed molecule decomposes and

molecular fragments or ions are bound to oxide surface. Molecular chem-

isorption is the most probable type of adsorption for molecules that pos-

sess free electrons or multiple bonds. Gas molecules with single bonds

tend to react via dissociative chemisorption; however; there is an activa-

tion energy associated with dissociation. The type of chemisorbed oxygen

on the surface of a metal oxide is dependent on the temperature of the

system. Barsan and Weimar compiled results from a survey of the litera-

ture concerning oxygen adsorption on SnO2 and correlated the adsorbed

oxygen species to temperature where techniques such as infrared spec-

troscopy, temperature programmed desorption and electron paramagnetic

resonance were used [21]. Table 2.2 summarizes the temperature ranges

associated with each species of oxygen adsorption.

Physisorption Chemisorption

Intermolecular (van der Waals) Force Covalent Bonding

(adsorbate & surface)

Low Temperature High Temperature

Low Activation Energy

(<< 0.5 eV)

High Activation Energy

(> 0.5 eV)

Low Enthalpy Change

(ΔH < 20kJ/mol)

High Enthalpy Change

(50kJ/mol < ΔH < 800kJ/mol)

Reversible Reversible at High Temperature

Adsorbate energy state unaltered Electron density increases at interface

Multilayer formation possible Monolayer surface coverage

Table 2.1: Comparison of physisorption and chemisorption [20].

10

In the reaction shown in Table 2.2, (g) indicates the gaseous form,

(ads) indicates the molecule or ions that are adsorbed on a surface and e-

is an electron initially in the metal oxide. Oxygen interactions with the

surface of an oxide are of utmost importance in gas sensing. Oxygen is a

strong electron acceptor on the surface of a metal oxide. A majority of

sensors operate in an air ambient; therefore, the concentration of oxygen

on the surface is directly related to the sensor electrical properties. The

conversion to O- or O

2- at elevated temperatures are useful in gas sensing

since only a monolayer of oxygen ions are present with these strongly

chemisorbed species [20, 22].

Desorption is the opposite reaction to adsorption where the chemi-

cal bonds are broken, the adsorbed atoms are removed from the surface,

and electrons are injected back into the material. Desorption is achieved

by thermal stimulation up to a specific temperature or by reactions with

other gaseous species. A desorption process that is isothermal occurs

when, for instance, a reducing gas such as carbon monoxide (CO) is in-

troduced into the surrounding atmosphere. Oxygen is consumed in a reac-

tion with the CO to form carbon dioxide (CO2) as written in equation 2.1.

CO(g) + O−(ads) → CO2(g) + e− (2.1)

Temperature Range (°C) Adsorption Reaction(s)

Room Temp.< T < 175°C O2 (g) + e− → O2

− (ads) O2 (g) + 2e− → O2

2− (ads)

175°C < T < 500°C O2 (g) + 2e− → 2O− (ads)

T > 500°C O− (ads) + e− → O2− (ads) O2 (g) + 4e− → 2O2− (ads)

Table 2.2: Temperature ranges associated with molecular and dissociative oxygen

adsorption reactions [20].

11

where the extra electron generated is injected back into the metal oxide.

This desorption reaction results in a lower surface coverage of oxygen

adsorbates which influences the electrical properties of the oxide.

2.2 Non – Stoichiometry in Semiconductors

The relevance of non – stoichiometry to the transport properties of

metal oxide semiconductors will now be explored using ZnO as an exam-

ple. It is well – known that ZnO is stoichiometrically deficient in oxygen

traced to either zinc interstitials or oxygen vacancies. To begin with, the

notation of Kröger and Vink [23] must first be introduced.

A defect is characterized by the charge it carries relative to the sur-

rounding crystal lattice [23]. A defect’s superscript denotes this relative

charge, with a dot (˙) being a single positive charge and a prime (′) denot-

ing a negative charge. Neutrals are written with either an x (X) or no su-

perscript. A subscript is used to denote the lattice site of the defect, with i

(i) being used to signify an interstitial atom. Vacancies are represented

with the letter V, as in VO ⋅⋅ , which denotes a doubly charged vacancy

occupying an oxygen lattice site. Electrons and holes may be signified by

e and h, respectively.

The equilibrium constant, K(T), for the general chemical reaction

of reactants A, B and products C, D [19]:

aA + bB → cC + dD (2.2)

can be written as:

K(T) =[C]c[D]d

[A]a[B]b= e−

∆G

KT (2.3)

12

where ΔG represents the standard change in free energy for the reaction.

An oxygen vacancy, known as a type of Schottky defect, can be generated

in ZnO through the following reaction:

ZnZn + OO → ZnZn + VO∙∙ + 2e′ +

1

2O2(g) (2.4)

It is conventional practice to denote the left side of (2.4) as “nil”. As seen

in (2.4), a positively charged oxygen vacancy is compensated by the gen-

eration of electrons, thus leading to n-type conductivity of ZnO.

The mass action constant for (2.4) can be written as:

KR = [VO∙∙](pO2)

1

2 ⋅ n2 (2.5)

where pO2denotes the partial pressure of oxygen, [VO∙∙] and n represent the

oxygen vacancy and electron concentrations, respectively. To solve for

[VO∙∙] or n, one must evoke the electroneutrality condition (ENC), which

states that the concentration of positive defects present in the material

must equal the concentration of negative defects. The ENC for (2.4) is:

2[VO∙∙] = 𝑛 (2.6)

using (2.6) and solving for [VO∙∙] or n in (2.5) yields:

[VO∙∙] = (

KR

4)

1

3

(pO2)−1

6 (2.7)

and

n = (2)1

3(KR)1

3(pO2)−1

6 (2.8)

Thus, the logarithmic concentration of oxygen vacancies and electrons in

ZnO, plotted against log (pO2), is shown to have what is termed a -1/6

pO2 dependence. The electronic conductivity, 𝜎, is given by:

13

σ = q(2)1

3(KR)1

3(pO2)−

1

6μe (2.9)

where q is the electronic charge and 𝜇𝑒is the electronic mobility.

An oxygen deficiency in ZnO may also be realized through the

formation of a Zn interstitial, known as a type of Frenkel defect. This can

be formed through the following reaction:

nil = Zni⋅⋅ + 2e′ +

1

2O2(g) (2.10)

using the ENC for this reaction and substituting it into the proper equa-

tion for the equilibrium constant will also yield a -1/6 pO2 dependence.

2.3 Gas Sensor Operation: Catalysis and Adsorption

The electrical conductivity of a semiconductor is dictated in large

part by the concentration of electrons or holes present in the material. In

certain metal oxide semiconductors, the majority charge carrier concen-

tration changes as a result of an interaction with a gaseous species [4].

The resulting change in conductance may be quite large and provides the

basis for semiconductor gas sensor operation. This behavior is unlike

metals, where the adsorption of a gas may cause small conductance

changes due to a modification of charge carrier mobility [6]. As an exam-

ple, recall the experiments of Wagner on the decomposition of N2O on

ZnO. If the generation of two electrons proceeds as in (2.4), the decom-

position reaction was proposed as follows [2]:

2e− + N2O → N2 + O2−(ads. ) (2.11)

O2−(ads. ) + N2O → N2 + O2 + 2e− (2.12)

The adsorption of oxygen in (2.11) would result in an increase in

ZnO resistivity due to the capture of majority charge carriers. The subse-

14

quent reaction between the adsorbed oxygen and N2O in (2.12) acts to

restore the supply of conduction electrons and thus, an increase in con-

ductivity may be observed. It is this simple and reversible change in

charge concentration that drives the use of metal oxide gas sensors.

For a visual perspective, a schematic of an n – type semiconductor

tin dioxide SnO2 thick film with an accompanying band structure model

is shown in figure 2.1 [19]. For conduction to occur, an electron must

pass from one grain to the next. While there exists an ample concentra-

tion of electrons in the bulk of the material, adsorbed oxygen has cap-

tured electrons near the surface of the film. The electrons that bind to the

adsorbed oxygen leave behind positively charged donor ions. An electric

field develops, between these positive donor ions and the negatively

charged adsorbed oxygen ions, which serves to impede the flow of elec-

trons between neighboring grains. The barrier generated by the electric

field has a magnitude of eVS, where e is the electronic charge and VS is

the potential barrier. The magnitude of VS increases as more oxygen ad-

sorbs on the film surface. Utilizing the Boltzmann equation, the concen-

tration of electrons, ns, that possesses ample energy to cross the barrier

and reach a neighboring grain is given by:

ns = Ndexp (−eVs

kT) (2.13)

where Nd is the concentration of donors, k is Boltzmann’s constant, and T

is the temperature. Since conductance (or resistivity) is proportional to ns,

an increase in the adsorbed oxygen content will raise eVS and thus, fewer

electrons will cross the potential barrier. This may be empirically moni-

tored as an increase in resistivity. The introduction of a reducing gas will

reverse this effect, lowering the potential barrier and decreasing resistivi-

ty. It is this reducing gas, often termed the analyte, whose presence is of

interest.

15

SnO2

Figure 2.1: Microstructure and energy band model of a gas sensitive SnO2 thick film.

The potential barriers form as a result of oxygen adsorption [19].

EF

O-

O-

O-

O-

O-

O-

O-

O-

O-

O-

O-

O-

O-

Grain Grain

Thermionic

emission

EC

𝑒𝑉𝑆1 ;𝐺1 = 𝐺0exp(−𝑒𝑉𝑆1

𝐾𝑇)

1

2O2

− Electron – depleted

region

(a) In air

O-

O-

O

O-

O-

O-

O

O-

O-

O-

O

Grain Grain

𝑒𝑉𝑆2 ;𝐺2 = 𝐺0exp(−𝑒𝑉𝑆2

𝐾𝑇)

𝑠𝑒𝑛𝑠𝑖𝑡𝑖𝑣𝑖𝑡𝑦 ⇉ 𝑆 =𝐺2

𝐺1= exp (

𝑒 𝑉𝑆1−𝑉𝑆2

𝐾𝑇)

CO

CO2

CO CO CO2

CO2

(b) In the presence

of reducing gas

16

Using an example of ZnO in the detection of hydrogen, the following

reactions may occur [24]:

2e− + O2 → 2O−(ads. ) (2.14)

2O−(ads. ) + 2H2 → 2H2O + 2e− (2.15)

The reaction of H2 with adsorbed oxygen on the surface of ZnO

(2.15) will result in a measurable reduction in the resistivity. It should be

noted that adsorbed oxygen may exist in multiple forms. Takata et al.

proposed that oxygen adsorbed on ZnO is transformed with increasing

temperature in the following manner [25]:

O2 → O2− → 2O− → 2O2− (2.16)

of these forms, O2 is considered fairly inactive due to its non – dissocia-

tive state. With regards to O2− and O

−, electron spin resonance (ESR)

studies have shown that O− is far more reactive than O2− [26]. The nature

of adsorbed oxygen on ZnO as reported by several researchers is shown

in figure 2.2.

A catalyst acts to increase the rate at which a chemical reaction ap-

proaches equilibrium, without permanently being altered in the process

[27]. In the reactions detailed in (2.17) – (2.18) and (2.14) – (2.15), ZnO

acts as a heterogeneous catalyst, as it is a phase distinct from the reactants

and products. To illustrate the phenomenon of catalysis, consider the

following reaction:

A + B → C + D (2.17)

Two possible paths in which this reaction may proceed are shown

in figure 2.3 [27]. In the absence of a catalyst, the reaction of (2.17) is

characterized by a large activation energy, Ea. When a catalyst such as

ZnO or SnO2 is used, the gaseous products adsorb onto the metal oxide

17

surface with an exothermic heat of adsorption ΔH (State I). The reaction

to form adsorbed products then proceeds with a lower activation energy

Ec (State II). It is evident from figure 2.3 that if ΔH is too large, the gase-

ous reactants are strongly adsorbed and Ec may become too large for the

reaction to proceed. An undesirably low activation energy will cause the

reaction to be energetically easier, but will result in fewer products avail-

able for the reaction.

A gas molecule approaching the surface of a solid will be subject

to an attractive potential [28]. This potential is the origin of adsorption

and arises from the multitude of unsatisfied bonds that exist at the surface

of the solid. The adsorbed species is often called the adsorbent and the

solid surface is termed the adsorbate [11]. Physical adsorption, or phy-

sisorption, occurs as a result of electrostatic and van der Waals forces that

exist between the adsorbent and adsorbate. Heats of adsorption for phy-

sisorption tend to be low, with ΔH values typically in the range of 2 – 15

kcal/mole [29]. In the case of chemical adsorption, (termed chemisorp-

Figure 2.2: The nature of oxygen species adsorbed on ZnO as reported by several

researchers [25].

18

tion), the adsorbent forms a chemical bond with the solid surface. Values

for ΔH tend to be higher for chemisorption and are often in the range of

15 – 200 kcal/mole [29]. As chemisorption tends to provide the necessary

catalysis conditions, it is often the adsorption mode of interest when dis-

cussing semiconductor gas sensors.

If the electrical conductivity of a semiconductor is to be used for

gas detection, then changes in conductivity must be proportional to the

concentration of the gaseous analyte. To understand this relationship,

adsorption kinetics must be discussed. The residence time, of an ad-

sorbed atom is given by [28]:

= exp (∆H

T) (2.18)

C+D

A+B

Ener

gy

Ea

Ec ΔH

I

II

Reaction coordinate

Figure 2.3.The energy barriers in the transformation from reactants (A + B) to products

(C + D). The uncatalyzed reaction is characterized by a large activation energy (Ea),

while the barrier to product formation is lowered (Ec) when a catalyst is used [27].

19

where is related to surface vibration time and R is the universal gas

constant. The surface coverage, S, of a gaseous species is dependent on

both and the flux, F, of gas molecules per unit area per second through:

= (2.19)

Typical units for S are molecules per cm2. Relating the gas flux to

the pressure through the kinetic theory of gases will yield:

= (N

√2 T) exp (

∆H

T) (2.20)

where NA is Avogrado’s number, P is the partial pressure of the gas, and

M is the average molar weight of the gaseous species. Experimental

curves of S plotted as a function of P at a given temperature are known as

adsorption isotherms [28]. One particular isotherm derived by Irving

Langmuir is of key interest in the field of semiconductor gas sensors [30].

It is based on two assumptions [27]:

1) Adsorption terminates upon the completion of one monolayer.

2) There exists neither surface heterogeneity nor interaction among

adsorbed species.

While these assumptions are to some degree impractical for real

surfaces, modified isotherms have been developed [29]. Regardless, the

derivations of Langmuir provide a sound qualitative relationship between

surface coverage and gas concentration.

Using assumption 1), any gaseous molecule will reflect off a sur-

face when striking an adsorbed species. Thus, if So denotes a completely

covered surface, then a concentration of S adsorbed molecules will result

in So – S available sites [28]. The fluxes for both reflected molecules, FR,

and adsorbed molecules, FA, are given by:

R = (

) = (1 −

) (2.21)

substitution of FA into (2.19) and subsequent rearrangement will yield:

20

=

+ =

a

+ a (2.22)

The constant a is comprised of the grouping of terms, with the ex-

ception of P, from (2.20). If θ = (S/So), where θ is defined as the degree

of coverage, then (2.22) takes the following form:

=b

1 + b (2.23)

where b = (a/So).

When the degree of surface coverage is proportional to the partial

pressure, changes in the electrical conductivity may be related to gas

concentration. Inspection of (2.23) shows that if bP is small, θ is propor-

tional to P. However, if bP >> 1, then θ approaches unity and the lack of

proportionality makes the gas sensor insensitive to coverage. If a compe-

tition ensues for surface sites between two gas species, A and B, then

(2.23) becomes [27]:

=b

1 + b + b =

b 1 + b + b

(2.24)

If bBPB >> bAPA, then the equations of (2.24) become:

=b

1 + b =

b 1 + b

(2.25)

in the case that bBPB is >> 1, the equations of (2.25) reduce to:

b

1 + b 1 (2.26)

Thus, if the conductivity of the gas sensor is strongly dependent on spe-

cies A, then the concentration of either A or B may be measured. Howev-

er, if the conductivity possesses a strong dependence on species B, it will

become independent of the gas concentration as approaches 1.

The rate of a reaction between A and B may be given by [27]:

a e = k (2.27)

21

If the rate in (2.27) is much higher than the rate of adsorption of say, A,

then will fall to zero and will increase. As an example, note that the

coverage of oxygen, represented by in (2.26) on an n-type semicon-

ductor is quite low. The coverage of a reducing gas, in (2.26) is quite

high. As the reaction rate between the oxygen and the reducing species

increases, falls to zero, enabling the reducing gas to be detected with a

high degree of sensitivity.

2.4 Semiconductor metal oxide gas sensors

Metal oxide semiconductor gas sensors are, essentially, gas de-

pendent resistors [31]. A broad range of metal oxides are known for their

gas sensing properties, each with a unique sensitivity and selectivity.

Their detection principle is based on a modulation of their electrical con-

duction properties by surface adsorbed gas molecules. The sensitive layer

is deposited onto a substrate with a set of electrodes for measuring re-

sistance changes and heating the sensitive layer; normally 2 – point re-

sistance measurements are accurate enough for gas sensors. The used

metal – oxides are n – or p – type semiconductors, due to the presence of

oxygen – vacancies in the bulk. Generally the conductance or the re-

sistance of the sensor is monitored as a function of the concentration of

the target gases. Additionally the performance of the sensor depends on

the measurement parameters, such as sensitive layer polarization or tem-

perature, which are controlled by using different electronic circuits.

The elementary reaction steps of gas sensing will be transduced in-

to electrical signals measured by appropriate electrode structures. The

sensing itself can take place at different sites of the structure depending

on the morphology. They will play different roles, according to the sens-

ing layer morphology. An overview is given in figure 2.4.

22

A simple distinction can be made between:

Compact layers; the interaction with gases takes place only at the

geometric surface (Figure 2.5, such layers are obtained with most

of the techniques used for thin film deposition such as pulsed laser

deposition, sputtering etc.) and

Porous layers; the volume of the layer is also accessible to the gas-

es and in this case the active surface is much higher than the geo-

metric one (Figure 2.6, such layers are characteristic to thick film

techniques and RGTO (Rheotaxial Growth and Thermal Oxida-

tion).

For compact layers, gases only interact with metal oxides at geo-

metrical surfaces, resulting in a surface depletion layer through the film

either partly (a) or completely (b) (Figure 2.5). Whether the sensor oper-

ates under a partly or a completely depleted, the condition is determined

Figure 2.4: Schematic view of gas sensing reaction in (a) Compact layer and (b) Po-

rous layer. A: grain boundary model. B: open neck model. C: closed neck model [32]

Sensitive layer

Electrodes

Substrate

C B

A

Product Gas

Product

(a) Compact layer

Gas

(b) Porous layer

Gas

adsorp-

tion

Product desorption

23

by the ratio between layer thickness Zg and Debye length λD (or LD). For

partly depleted layers, when surface reactions do not influence the con-

duction in the entire layer (Zg >Z0 see Figure 2.5), the conduction process

takes place in the bulk region (of thickness Zg −Z0, much more conduc-

tive than the surface depleted layer).

Formally two resistances occur in parallel, one influenced by sur-

face reactions and the other not; the conduction is parallel to the surface,

and this explains the limited sensitivity. Such a case is generally treated

as a conductive layer with a reaction-dependent thickness.

For the case of completely depleted layers in the absence of reduc-

ing gases, it is possible that exposure to reducing gases acts as a switch to

the partly depleted layer case (due to the injection of additional free

charge carriers) [32].

(a)

Surface band

bending

Conducting

channel

Volume not accessible

to gases

Product Gas

Current flow

Z0

Zg

Energy

eVS Z

Z

X

Zg

Z

Energy

eVS

e∆VS

Eb

(b)

Figure 2.5: schematic of a compact layer with geom-

etry and energy band representation; Z0 is

the thickness of the depleted surface lay-

er; Zg is the thickness of the surface and

eVS the band bending. (a) A partly deplet-

ed compact layer (“thicker”) and (b) A

completely depleted layer (“thinner”)

[32].

24

q∆VS <KB T

Flat band condition

Figure 2.6: Schematic representation of a porous sensing layer with geometry and

energy band for small and large grains. λD Debye length, Xg grain size [33].

X

Energy

Eb

q∆VS

Xg< λD

Current flow

Small grains

Extended surface influence

Gas

Xg

Xg> λD

Eb

2X0

Energy

Current

flow

qVS

X

X

Z

Product Large grains

25

It is also possible that exposure to oxidizing gases acts as a switch

between partly depleted and completely depleted layer cases depending

on the initial state of the sensing film.

Figure 2.6 illustrates the conduction model of a porous sensing lay-

er with geometry and surface energy band for small and large grains. For

large grains, conduction can be hindered by the formation of depletion

layers at surface/bulk regions and grain boundaries; the presence of ener-

gy barriers blocks the motion of charge carriers. In contrast, flat band

conditions dominate in case of small grains, allowing fast conduction for

this case [34]. For porous layers the situation may be complicated further

by the presence of necks between grains (Figure 2.7). It may be possible

to have all three types of contribution presented in figure 2.8 in a porous

eVS

Zn

Z

X

eVS

(b)

(a) eVS

Zn

Zo

Z Z

X

Current

flow Conduction

Channel

Energy

Zn- 2Z0

Figure 2.7: schematic of a porous layer with geometry and surface energy band with necks

between grains; Zn is the neck diameter; Z0 is the thickness of the depletion layer and eVS

the band bending. (a) a partly depleted necks and (b) a completely depleted necks [32].

26

layer:

I. Surface/bulk (for large enough necks Zn >Z0, figure 2.5).

II. Grain boundary (for large grains not sintered together), in which

conduction can be hindered by the formation of depletion layers at

surface/bulk regions and grain boundaries; the presence of energy

barriers blocks the motion of charge carriers.

III. Flat bands conditions which dominate in case of small grains and

small necks, allowing fast conduction for this case.

Of course, what was mentioned for compact layers, i.e. the possible

switching role of reducing gases, is valid also for porous layers. For small

grains and narrow necks, when the mean free path of free charge carriers

becomes comparable with the dimension of the grains, a surface influence

Model

Geometric

Electronic

Band

Electrical equiva-

lent circuit

(Low current)

Figure 2.8 : Influence of particle size and contacts on resistances and capacitances in thin

films are shown schematically for a current flow I from left to right [35].

Grain

boundary

Surface

bulk nano crystal

𝐿𝐷>12

O2−

X and

I

Z

Schottky contact

Metal

EF

Metal

LD

X

CO effect

EV

EC

EF

X

EC

EV Surface

LD O2

−

EF

bulk

Z

LD

EF

X

Surface

Bulk

27

on mobility should be taken into consideration. This happens because the

number of collisions experienced by the free charge carriers in the bulk of

the grain becomes comparable with the number of surface collisions; the

latter may be influenced by adsorbed species acting as additional scatter-

ing centers [34].

2.5 Gas Sensor Metrics

There are several measures of the performance of a gas sensor. The

“3S” parameters are often cited in the literature as sensitivity, stability,

and selectivity [20]. Sensitivity is the most frequently studied of these

parameters in the literature. Korotcenkov recently pointed out that stabil-

ity in nanoparticle – based sensors may be equally as important as sensi-

tivity when operating at elevated temperatures [36]. Selectivity has also

been extensively examined in the literature since this is a crucial factor

when creating a commercially viable device [37, 38]. Other issues such as

response time, recovery time and re – producibility have been less inten-

sively studied. All of these factors are important for building a microsen-

sor or microsensor array. These parameters are defined here as most of

them will be applied in this research.

2.5.1 Sensitivity

The response of a sensor upon the introduction of a particular gas

species is called the sensitivity (S The most general definition of sensitiv-

ity applied to solid – state chemi – resistive gas sensors is a change in the

electrical resistance (or conductance) relative to the initial state upon

exposure to a reducing or oxidizing gas component). The sensitivity de-

pends on many factors including the background gas composition, rela-

tive humidity level, sensor temperature, oxide microstructure, film thick-

ness and gas exposure time. One of the most common methods is to re-

28

port the ratio of the electrical resistance (R) in air to the resistance meas-

ured when a gas is introduced as shown in equation 2.28 [39]:

R

RG =

G

(2.28)

where R is the electrical resistance and G is the electrical conductance

and the subscript “AIR” indicates that background is the initial dry air

state and the subscript “GAS” indicates the analyte gas has been intro-

duced.

Another common approach to report S is shown in equations 2.29

and 2.30 [40]:

=|∆R|

R 100 =

RG −R

R 100 (2.29)

|∆ |

100

100 (2.30)

The values calculated using the above equations scale from zero

while the values from equation 2.28 scale from 1. The relationship be-

tween the S values (using G as a metric) from 2.28 and equation 2.30 is

simply to add 1 to the reported value and they are equivalent. Both values

are acceptable and useful metrics for gas sensor response testing. In the

current research, the percentage conductance change has been selected as

the value of sensitivity as calculated in equation 2.30 (S = ΔG/Go) be-

cause it scales more intuitively from a value of zero.

2.5.2 Selectivity

Selectivity is defined as the ability to discriminate a particular gas

species from the background atmosphere. This is a task where metal ox-

ide based sensors face significant challenges and show poor discrimina-

tion between gas species. Selectivity has been defined as the ratio of sen-

sitivities to a particular gas as shown in equation 2.31.

⁄ =SG

SG (2.31)

29

In this work only a single gas was introduced in a background of dry air

so this ratio does not describe the ability of the sensor to pick out a par-

ticular gas species from a complex mixture of gases. Selectivity has also

been applied to describe the ability of a sensor (or array) to detect and

distinguish a particular gas species in a mixture containing multiple ana-

lyte gases [41]. As with sensitivity there are many factors (e.g. tempera-

ture gas flow rate, device electrode material, etc.) that contribute to the

selectivity of a sensor [41]. A sensor array uses the combination of sig-

nals from multiple sensors to test and improve the selectivity of a system.

This is why analytically orthogonal (opposite) signals such as those from

n – type and p – type materials in a sensor array are valuable for data

analysis algorithms to enhance selectivity. Selectivity is not a focus of

this study. It will become a more important issue once other reducing

and/or oxidizing gases become available.

2.5.3 Stability

Stability measures the capability of a sensor to maintain sensitivity

over durations of time for a particular gas species. Stability is measured

in terms of baseline “drift” which is the change in baseline conductance

over some duration of time at a particular temperature. Here we define

drift as the change in baseline conductance relative to the initial conduct-

ance as written in equation 2.32 below:

D = | 1 −

1 − 0| (2.32)

Drift is reported in units of Siemens per hour (S/hr). Elvin R.

Beach III [20] reported the drift over durations of 48 hours by individual-

ly introduceing reducing and oxidizing gases during testing to simulate

more realistic conditions the sensor would operate under.

30

2.5.4 Response and Recovery Times

The response time (τres) of a gas sensor is defined as the time it

takes the sensor to reach 90% of maximum/minimum value of conduct-

ance upon introduction of the reducing/oxidizing gas [42]. Similarly, the

recovery time (τrec) is defined as the time required to recover to within

10% of the original baseline when the flow of reducing or oxidizing gas

is removed. Figure 2.9 shows how this is measured from sensor data

plotting the conductance as a function of time.

Xu et al., [43] prepared ultrathin Pd nanocluster film capable of detecting

2% H2 with a rapid response time down to tens of milliseconds (~ 70 ms)

and is sensitive to 25 ppm hydrogen, detectable by a 2% increase in con-

ductance due to the hydrogen – induced palladium lattice expansion. Four

years earlier that time, in 2001, Favier et al., [44] obtained a comparable

ultrafast response time of less than 75 ms towards H2 : N2 gas mixture

from 2 to 10% at room temperature using palladium mesowire arrays.

Conductance

Time

90% of maximum

conductance Maximum conductance

Recovered to within 10%

of original baseline

τres τrec

Figure 2.9: drawing showing how response and recovery times are calculated

from a plot of sensor conductance versus time [20].

31

2.6 Sensing Mechanism

Both thin – film and bulk semiconducting metal oxide materials

have been widely used for the detection of a wide range of chemicals

such as H2, CO, NO2, NH3, H2S, ethanol, acetone, human breath, and

humidity. The sensing mechanism of metal oxide gas sensors mainly

relies on the change of electrical conductivity contributed by interactions

between metal oxides and surrounding environment. The exchange of

electrons between the bulk of a metal oxide nanostructure grain and the

surface states takes place within a surface layer (charge depletion layer),

thus, contributing to the decrease of the net charge carrier density in the

nanomaterial conductance channel. This will also lead to band bending

near the surface of both conduction and valence bands. The thickness of

the surface layer is of the order of the Debye length/radius λD of the sens-

ing material which can be expressed as the following formula obtained in

the Schottky approximation [32]:

ω = λD (eVS

kT)

12

(2.33)

λD = (εε0kT

e2n0)

12

(2.34)

where ω is the width of the surface charge region that is related to the

Debye length λD of the nanomaterial, ε0 is the absolute dielectric con-

stant, ε is the relative dielectric permittivity of the structure, k is the

Boltzmann’s constant, T is the temperature, e is the elementary charge, n0

is the charge carrier concentration and VS is the adsorbate – induced band

bending. The Debye length λDis a quantum value for the distribution of

the space charge region. It is defined as the distance to the surface at

which the band bending is decreased to the 1/e – th part of the surface

value [11].

32

The conductance of 1-D metal oxide nanomaterial can be ex-

pressed as [45]

=1

=

A

ρl= n0eμ

π(D − 2ω)2

4l (2.35)

where R is the electrical resistance, ρ is the resistivity, no is the ini-

tial/nominal charge carriers concentration, e is the electron charge, μ is

the mobility of electrons, l is the length of the nanomaterial, D is the di-

ameter of the nanomaterial, and w is the width of surface charge region

that is related to the Debye length of the nanomaterial. Likewise, the

electrical conductance of ZnO nanofilms can be expressed as dependent

[24] upon the charge carriers’ concentration:

=1

=

A

ρl=

no|e|μA

l (2.36)

here A, l are the area and length of the nanofilm channel, respectively.

Therefore, the change in electrical conductance of the nanofilm exposed

to gas atmosphere is determined by the change in electrical charge carri-

ers’ concentration ∆n [12]:

∆ =∆no|e|μA

l (2.37)

The gas sensitivity S is given by [24]:

𝑆 =|∆ |

100 =

∆nS

no (2.38)

According to this expression, higher gas sensitivity could be obtained by

a larger modulation in the depletion region of the ZnO metal oxide nano-

film. The width of the depletion region is inversely proportional to the

square root of the free charge carrier concentration.

When the radius of ZnO nanostructure (grain) is of the order of or

less than Debye length/radius, the conductive channel is reduced substan-

tially. The modulation of the depletion region width can also be produced

33

by the control of electron density in the metal oxide ZnO nanostructure,

i.e. by means of surface defects.

Generally speaking, the response of the chemoresistors in ambient

environment can be defined as [13]:

= g − a

g 100 =

4

D ωa − ωg =

4

D√𝜀𝜀0𝑒𝑛0

(VSa

12 − V

Sg

12 ) (2.39)

where Gg and Ga are the conductance of ZnO nanostructure in H2 gas and

in air ambient, respectively, n0 is carrier concentration in air. VSa and VSg

are the adsorbance – induced band bending in air and in H2 gas, respec-

tively. According to this equation, enhancement of H2 gas sensitivity can

be realized by controlling the geometric factor (4/D), electronic character-

istics (εε0/en0), and adsorption induced band bending (VSa

12 − V

Sg

12 ) due

to adsorption on the ZnO nanostructure surface. This can be done by

doping, or by using modulation of operation temperature which is not

desirable for H2 gas sensors on single ZnO nanowire. Another way is to

make use of geometric parameters, of which the grain size is at the fore-

front of the parameters used for sensitivity enhancement.

2.7 Factors influencing the performance

2.7.1 Long term effects / Baseline Drift:

Baseline refers to the conductance of the sensor in clean air.

Changes over long operating times of both baseline and sensitivity are all

important in utilization of the sensors. These determine the frequency at

which the calibration checks should be carried out and the frequency at

which the sensors may have to be replaced. They can only be determined

over long periods of time and no method by which the process can be

accelerated is valid [18].

34

2.7.2 Sensor surface poisoning

The surface of ZnO and other oxides may become unstable because

of “poisons”. Sulfur (as H2S) is a potential poison that can block the cata-

lytic activity of Pd on the surface. Wagner et al found instability due to

the presence of H2S in commercial SnO2 based sensors. Another domi-

nant poison is chlorine gas. Thus it is important in the development of

sensors to be aware of the other reactive gases in the measurand environ-

ment [18].

2.8 Optimization of Sensor Performance

2.8.1 Use of Catalyst

Metal oxide gas sensors need a catalyst deposited on the surface of

the film to accelerate the reaction and to increase the sensitivity. A cata-

lyst is a material that increases the rate of chemical reactions without

itself getting changed. It does not change the free energy of the reaction

but lowers the activation energy. Catalysts are supposed to, and do, im-

part speed of response and selectivity to gas sensors [45].

The catalytic surface reaction used for gas sensing makes this field

close to that of heterogeneous catalysis, with the only difference that in

catalysis one is mainly interested in the products of the reaction whereas

in gas sensing one is interested in the reactants as shown in the figure

2.10. This approach is considered relatively standard in fields such as

heterogeneous catalysis but so far it has rarely been applied to solid-state

gas sensors.

The chosen catalyst influences the selectivity of sensor. Ideally, if

one wants to detect a particular gas in a mixture of gases, one will like a

catalyst combination that catalyzes the oxidation of the gas of interest and

35

does not catalyze the oxidation of any other gas. Unfortunately, such

ideal combinations are not easily found [46].

The widespread applicability of semi-conducting oxides such as

SnO2 or ZnO, as gas sensors is related both to the range of conductance

variability and to the fact that it responds to both oxidizing and reducing

gases.

Small amounts of noble metal additives, such as Pd or Pt are com-

monly dispersed on the semiconductor as activators or sensitizers to im-

prove the gas selectivity, sensitivity and to lower the operating tempera-

ture [47]. There are two ways in which the catalysts can affect the inter-

granular contact region and hence affect the film resistance. The first one

is the spill over mechanism and the other is Fermi energy control.

Catalytic theory proposed, as spillover and Fermi energy control,

have not led to a widely accepted catalyst mechanism that predicts or

explains sensor behavior in different environments [48]. In spite of all the

work reported, a deep analysis of the material – gas interaction and its

influence on the sensor electrical response is still lacking to completely

Figure 2.10: Illustration of catalyst effect. Nano – particles, having higher

surface area, act as catalysts. Here, R stands for reducing gas [46].

O-

O-

O-

O-

O-

O- O

-

O-

O-

O-

O-

O-

O-

O-

O-

O-

O-

O-

RH2+O2 RH2+H2O

36

understand the role played by the additives on the gas sensing mecha-

nism. A model for increase in sensitivity using nanoparticles has been

explained by activated charge carrier creation and tunneling through po-

tential barrier [48].

2.8.1.1 Spill over Mechanism

Spillover mechanism is a well-known effect in heterogeneous ca-

talysis and is probably most active with metal catalysts. This interaction

is a chemical reaction by which additives assist the redox process of met-

al oxides. The term spillover refers to the process, illustrated in figure

2.11, namely the process where the metal catalysts dissociates the mole-

cule, then the atom can ‘spillover’ onto the surface of the semiconductor

support. At appropriate temperatures, reactants are first adsorbed on to

the surface of additive particles and then migrate to the oxide surface to

react there with surface oxygen species, affecting the surface conductivi-

ty. For the above processes to dominate the film resistance, the spilled-

over species must be able to migrate to the inter-granular contact as

shown in the figure 2.12.

Figure 2.11: Mechanism of sensitization by metal or metal oxide additive [48].

O-

O-

O-

O-

H

H

H

H H2O H2O

Activation of gas followed

by spilling over

Change in surface

oxygen concentration

M

Acceptor of electrons

Change in redox state of additive

O-

O-

O-

O-

e

H2O H2

M

Electronic sensitization Chemical sensitization

37

Thus, for a catalyst to be effective there must be a good dispersion

of the catalysts, as shown in the figure 2.13, so that catalyst particles are

available near all inter – granular contacts. Only then can the catalysts

affect the important inter-granular contact resistance. The inverse effect

may also occur [49] when a nascent oxygen or gas atom is newly formed

from a reaction on a metal oxide site. The nascent atom may migrate to a

metal site and desorbs into gas molecule from there. This is called reverse

spillover or the porthole effect.

2.8.1.2 Fermi Energy Control

The second interaction is the electronic sensitization (figure

2.11); in which additives interact electronically with the metal – oxide as

a sort of electron donor or acceptor. For example, changes in the work

function of the additive due to the presence of a gas will cause a change

in the Schottky barrier between the metal and the oxide and thus, a

change in conductivity. This simply means that oxygen adsorption on the

O2

O-

O-

O-

O- O-

O-

O-

O-

O-

O-

O-

H2

Figure 2.12: Illustration of Spill Over caused by catalyst particles on the surface

of the grain of the polycrystalline particle [46].

38

catalyst removes electrons from the catalyst and the catalyst, in turn,

removes electrons from the supporting semiconductor. Figure 2.11 illus-

trates the situation with Fermi energy control.

Figure 2.13 demonstrates the catalyst, by Fermi energy control, to

dominate the depletion of electrons form the semiconductor surface, but

the poor catalyst dispersion prohibits any influence on the inter-granular

contact resistance. In other words, oxygen adsorbed on the catalyst re-

moves electrons from the catalyst and the catalyst, in turn, removes elec-

trons from the nearby surface region of semiconductor. But if only a few

catalyst particles are on each semiconductor particle, only a small portion

of the semiconductor surface will have a surface barrier controlled by the

catalyst. Then, the chances of a catalyst particle being near enough to the

inter – granular contact to control its surface barrier will be small [46].

Figure 2.13 (b) shows the more desired situation where one has a

good dispersion of the catalyst particles such that the depleted regions, at

the surface of a metal-oxide, overlap and the influence of the catalyst

extends to the inter-granular contact.

(b) Need adequate catalyst dispesion

(Particle separation < 500 A)

Electron

flow

O

2

O-

(a) Poor catalyst dispersion

Catalyst

Figure 2.13: An adequate dispersion of the catalysts is required in order to effective-

ly affect the grains of the semi-conducting material to serve the implied purpose of

increase in sensitivity [46].

39

2.8.2 Grain size effects

One of the most important factors which affect the sensing proper-

ty of semiconducting gas sensors is the microstructure of polycrystalline

element [48]. Each crystallite of semiconductor oxide in the element has

an electron depleted surface to a depth of L in air, where L is determined

the by Debye length LD (or λD) and the strength of chemisorptions. The

grain size effects are pictorially depicted in figure 2.14. If the diameter D

of the crystallite is comparable to 2L, the whole crystallite will be deplet-

ed of electrons and this will cause the gas sensitivity of the element to the

reducing gas to change with D. The crystallites in the gas sensing ele-

ments are connected to the neighboring crystallites either by grain bound-

ary contacts or by necks. In the case of grain boundary contacts the elec-

trons should move across potential barrier, the height of which changes

with surrounding atmosphere .The gas sensitivity in this case is inde-

pendent of the grain size. In the case of conduction through necks, elec-

trons move through the channel penetrating through each neck. The aper-

Core region

(Low re-

sistance)

Space – charge

region

(High re-

sistance)

𝐷 ≥ 2𝐿 (𝑛𝑒𝑐𝑘 𝑐𝑜𝑛𝑡𝑟𝑜𝑙)

𝐷 ≫ 2𝐿 (𝑔𝑟𝑎𝑖𝑛 𝑏𝑜𝑢𝑛𝑑𝑎𝑟𝑦 𝑐𝑜𝑛𝑡𝑟𝑜𝑙)

𝐷 < 2𝐿 (𝑔𝑟𝑎𝑖𝑛 𝑐𝑜𝑛𝑡𝑟𝑜𝑙)

Figure 2.14: Schematic models for grain – size effects [31]

40

ture of the channel is attenuated by the surface space charge layer. This

model is related to the grain size through the neck size.

It has been found out experimentally by Yamazoe et al, in 1991

[50] that the neck size X is proportional to D with a proportionality con-

stant of 0.8 ± 0.1. For D>>2L, conduction of electrons in the sensing

element is dominated by conduction through grain boundary contacts

(grain boundary control). For D≥2L, neck control forms the primary

mechanism of conductivity modulation (neck control). For D<2L, the

electrical resistance of the grains dominates whole resistance of the sen-

sor and thus sensitivity is controlled by grains themselves (grain control).

2.8.3 Thickness dependence

Thin and thick film sensing layers differ not only in their thickness

but also in their microstructures and can thus lead to rather different

transducer functions [48, 51]. The sensitivity of the layers depends

strongly on the layer thickness. In the case, that the thickness of the elec-

tron depleted surface thickness is about the size of a film, high gas sensi-

tivity can be expected. Thus, sensitivity of the metal oxide sensor is di-

rectly influenced by the size of the oxygen induced depletion layer at the

film surface relative to the thickness of the bulk semiconductor. In gen-

eral, when the depletion width equals the film thickness, more sensitivity

is expected [51].

Adsorption of the atmospheric oxygen on the surface of sensing

film, results in an increase of the resistance of sensing thin film. Upon

exposing to reducing gas, reduction in depletion layer depth occurs thus

decreasing the resistance of the film.

When the depletion depth is more or less equal to the thickness of

sensing film, the resistance will be high and hence contributing for the

higher sensitivity. However, it has been pointed out that the columnar

41

growth of gas sensitive film leads to the thickness-independent gas sensi-

tivity of sensor.

It has also been shown that the thickness of the sensitive layer does

play a role in determining the sensitivity of the sensor for different gases

[50]. The thin SnO2 layer, (thickness 50-300 nm) mainly responds to the

oxidizing gases such as Ozone and NO2 whereas thick films (thickness

15-80 μm) respond to reducing gases like CO and CH4. However, upon

reducing the temperature of the sensor, the thick film showed a signifi-

cant response to oxidizing gases. This behavior can be explained with the

diffusion reaction model. . A model for the sensing mechanism in thick-

film has been presented in [50].

2.8.4 Temperature modulation

The temperature of the sensor surface is one of the forefront char-

acterization parameters. Firstly, adsorption and desorption are tempera-

ture activated processes, thus dynamic properties of the sensors viz. re-

sponse time, recovery etc. depends on the temperature. The surface cov-

erage, co-adsorption, chemical decomposition or other reactions are also

temperature dependent, resulting in different static characteristics at dif-

ferent temperatures. On the other hand, temperature has an effect on the

physical properties of the semiconductor sensor material such as charge

carrier concentration, Debye length, work function etc.

The optimum range of temperature for an effective sensor response

corresponds to that where the material is able to catalytically reduce or

oxidize the target gas, simultaneously changing the electrical properties

of the sensor material. The rate of reaction depends on the exact reducing

agent under study. It is found that, with a given reducing agent, there is

peak in the sensitivity: If the temperature is too low, the rate of reaction is

too slow to give a high sensitivity, whereas if the temperature is too high,

42

the overall oxidation reaction proceeds so rapidly that the concentration

of reducing agent [R] at the surface becomes diffusion limited and con-

centration seen by sensor approaches to zero [48]. At such temperatures,

the whole target gas concentration reaching the material surface could be

reduced/oxidized without producing a perceptible electrical change on the

metal-oxide material. The sensitivity again is low. However, on the one

hand, temperature should be high enough to allow gas reaction on the

material surface. The operating temperature is chosen empirically to pro-

vide the highest sensitivity to the determinate gases. So, a clear compre-

hension of the relation between the sensing material, catalytic properties

and the sensor electrical response is indispensable to understand the

whole gas sensing mechanism.

For each sensor-gas combination, an optimum temperature be-

tween these limits must be used. When higher degrees of selectivity are

needed, sensor arrays are used (sometimes termed “electronic noses”),

where the different response of different sensors is used for identifying

the gaseous species by pattern matching [52]. With such sensor array, the

lack of selectivity of the single metal-oxide gas sensor can also be over-

come by processing the signals of the same kind of sensor devices at

different operating temperatures or of the device using different materials

at the same temperature [53, 54].

2.8.5 Filters for selectivity

The use of filters forms another approach to improve the selectivity

of gas sensors. These filters either consume gases that one does not wish

to pass to the gas sensor or to permit the passage of selected gases to the

sensor. Their use is to a great extent empirical. For example, Ogawa et al

claim that ultra-fine SnO2 rejects methanol. Carbon cloth and low porosi-

ty materials are used to prevent highly reactive or large molecules from

43

reaching the sensor. Silica can be used to increase hydrogen sensitivity,

as hydrogen passes more freely through a silica surface layer. Similarly

Teflon is helpful in stopping H2O reaching the sensor and Zirconia can be

used at high temperature to pass oxygen [17].

2.8.6 AC and DC measurements

The sensors resistance change is the best-known sensor output sig-

nal and in most cases determined at constant operation temperature and

by DC measurement. The inherently noisy behavior of the resistor, 1/f

noise also known as flicker noise in the DC resistance measurements can

often approach the desired sensitivity threshold of the sensor. AC re-

sistance measurements are one way to overcome prohibitive 1/f noise, but

they incur more complex measurement electronics and calibration repro-

ducibility issues. AC measurements are more frequently used in imped-

ance spectroscopy at modeling level. Udo Weimar and Wolfgang Göpel

have reported [55] that sensitivity and selectivity of the gas sensors can

be improved by applying different conduction measurement methods viz.

DC and AC conduction measurement methods. They have shown that the

use of different contact arrangements and monitoring at different fre-

quencies make it possible to discriminate between different gases.

The equivalent circuit for the different contributions; intergranular

contact, bulk and electrode contact is illustrated in figure 2.15. Intergran-

ular contact: the ionosorption of oxygen at the grain surface results in the

creation of a potential barrier and the corresponding depletion layers at

the intergranular contacts. An intergranular contact can be represented

electronically by a resistor Rgb (due to the high resistive depletion layers)

and a capacitor Cgb (due to the sandwiching of high resistive depletion

layers between two high conductive ‘plates’ of bulk material) in parallel.

The electrode contact can also be represented by a (RC) element. The

44

values of the resistor RC and the capacitor CC are independent of the am-

bient gas atmosphere. The bulk contribution can be represented by a re-

sistor Rb, whose resistance value is hardly influenced by changes in the

ambient atmosphere.

Gas

Large grains

X

Z

Product

Xg

Xg> λD

Eb

2X0

Energy

Current

flow

X

… …

qVS ∆Φ

𝑅 𝐶 ~𝑛𝑏exp(∆Φ

𝑘𝐵𝑇)

𝐶 𝐶 ~(𝜀

∆Φ)0.5

𝑅 𝑔𝑏 ~𝑛𝑏exp(𝑞𝑉𝑆𝑘𝐵𝑇

)

𝐶 𝑔𝑏 ~(𝜀

𝑞𝑉𝑆)0.5

𝑅 𝑏 ~𝑛𝑏 𝑅 𝑏 ~𝑛𝑏

Figure 2.15: Equivalent circuit for the different contributions in a thin film gas sen-

sor; intergranular contact, bulk and electrode contact.

45

2.9 Zinc oxide

2.9.1 Properties of Zinc oxide

Zinc oxide, ZnO, is an interesting II – VI compound semiconductor

with a wide direct band gap of 3.4 eV at room temperature [56]. It is a

widely used material in various applications such as gas sensors, UV

resistive coatings, piezoelectric devices, varistors, surface acoustic wave

(SAW) devices and transparent conductive oxide electrodes [57]. In the

early 2000 ‘s, ZnO also attracted attention for its possible application in

short – wavelength light emitting diodes (LEDs) and laser diodes (LDs)

because the optical properties of ZnO are similar to those of GaN [56].

Figure 2.16 shows the phase diagram of the Zn – O binary system

[58]. The equilibrium solid phase of the condensed Zn – O system at 0.1

MPa hydrostatic pressure are the hexagonal closed packed (hcp) Zn with

a very narrow composition range, the hexagonal compound, ZnO (49.9 to

50.0 at % O), with a narrow but significant composition range, and a

cubic peroxide, ZnO2 (~66.7 a . O), with unknown composition range.

Even though the existence of ZnO2 has been reported, its nature and tem-

perature of formation are unknown. At elevated hydrostatic pressure, a

face centered cubic (fcc) modification of ZnO is stable. Also, it has been

reported that ZnO can exist metastably at room temperature in either of

two cubic modifications with structure of ZnO (sphalerite) and NaCl

(rock salt) types [58]. Table 2.3 summarizes data related to Zn – O crystal

structures.

ZnO crystals are composed of alternate layers of zinc and oxygen

atoms disposed in a wurtzite hexagonal close – packed structure with a

longitudinal axis (c – axis) as shown in figure 2.17 [59]. The oxygen

atoms (ions) are arranged in close hexagonal packing, with zinc ions

46

Stable phases at 0.1 MPa Other phases

Zn ZnO (I) ZnO2 ZnO (II)(a) ZnO (III)

Composition,

at. % O ~0 49.9 to 50.0 ~66.7 ~50 ~50

Pearson

Symbol hP2 hP4 cP12 cF8 cF8

Space group P63/mmc P63mc Pa3 Fm3(-)m F4(-)3m

Prototype Mg ZnO

(wurtzite)

FeS2

(pyrite) NaCl

ZnS

(sphalerite)

Figure 2.16: T – X diagram for condensed Zn- O system at 0.1 MPa [58].

O – Rich

Boundary

Unknown

Zn – Rich Boundary

49.999 Liq. ~0.005

Liq. ~ 7 10−7

T 0C

800

600

400

419.58 0

Liquid

0 10 20 30 40 50 60

At. %

ZnO2

ZnO

~50.00

O

70

~419.80

Z

n

(Zn)~O

(Zn)

M.P.

Table 2.3: Zn – O crystal structure data [58]

47

occupying half the tetrahedral interstitial positions with the same relative

arrangement as the oxygen ions. In the crystal structure, both zinc and

oxygen ions are coordinated with four ions of the opposite charge, and

the binding is strong ionic type. Owing to the marked difference in size,

these ions fill only 44% of the volume in a ZnO crystal leaving some

relatively large open spaces (0.095 nm). Typical properties of ZnO are

listed in Table 2.4 [58, 60] and Ellingham diagram including ZnO is

shown in figure 2.18 [61].

Pure zinc oxide, carefully prepared in a laboratory, is a good insu-

lator. However, its electrical conductivity can be increased many folds by

special heat treatments and by the introduction of specific impurities into

the crystal lattice. ZnO can even be made to exhibit metallic conductivity

as for transparent electrodes similar to ITO. In general, 0.5 – 1% addi-

Figure 2.17: Many properties of zinc oxide are dependent upon the wurtzite

hexagonal, close-packed arrangement of the Zn and O atoms, their cohesiveness

and void space [59].

O

Zn

Zn

Zn O

O

Zn

O

Zn

O

48

tions of trivalent cations (e.g. Al and Cr) decrease the resistivity of ZnO

by about 10 orders of magnitude. [58].

Property Value

Crystal structure Hexagonal, wurtzite

Molecular weight Zn:65.38, O:16 and ZnO:81.38

Lattice parameters at 300 K (nm) a0: 0.32495

c0: 0.52069

Density (g cm-3

) 5.606 or

4.21 x 1019

ZnO molecules/mm3

Stable phase at 300 K Wurtzite

Melting point (ºC) 1975

Thermal conductivity 0.6, 1-1.2

Linear thermal expansion coefficient a0: 6.5 10

-6

c0: 3.0 10-6

dielectric constant 𝜀0 = 8.75, 𝜀∞ = 3.75

Refractive index 2.008, 2.029

Energy band gap (eV) Direct, 3.37

Intrinsic carrier concentration (cm-3

)

<106

max n-type doping: n ~ 1020

max p-type doping: p ~ 1017

Debye temperature 370 K

Lattice energy 964 kcal/mole

Exciton binding energy (meV) 60

Pyroelectric constant 6.8 Amp./sec/cm2/K x 10

10

Piezoelectric coefficient D33 = 12 pC/N

Electron effective mass 𝑚𝑒∗ 0.24

Electron Hall mobility, n-type at 300 K (cm2V

-1s

-1) 200

Hole effective mass 𝑚ℎ∗ 0.59

Hole Hall mobility, p-type at 300 K (cm2V

-1s

-1) 5 – 50

Table 2.4: Typical properties of zinc oxide [58, 60].

49

2.9.2 Defects chemistry

Many properties of crystals, most particularly electrical, are deter-

mined by imperfections, e.g. defects. Point defects are defined as devia-

tions from the perfect atomic arrangement: missing ions, interstitial ions

and their associated valence electrons as shown in figure 2.19. A principal

difference between point defects in ionic solids and those in metals is that

in the former, all such defects can be electrically charged. Ionic defects

Figure 2.18: Ellingham diagram of oxides [61].

50

are point defects that occupy lattice atomic positions, including vacan-

cies, interstitial and substitutional solutes. Electronic defects are devia-

tions from the ground state electron orbital configuration of a crystal,

formed when valence electrons are excited into higher orbital energy

levels. Such an excitation may create an electron in the conduction band

and/or an electron hole in the valence band of the crystal. In terms of

spatial positioning, these defects may be localized near atom sites, in

which case they represent changes in the ionization state of an atom, or

may be delocalized and move freely through the crystal.

An equivalent way to view the formation of defects is as a chemi-

cal reaction, for which there is an equilibrium constant. Chemical reac-

tions for the formation of defects within a solid must obey mass, site and

charge balance. In this respect they differ somewhat from ordinary chem-

ical reactions, which must obey only mass and charge balance. Site bal-

ance means that the ratio of cation to anion sites of the crystal must be

preserved, although the total number of sites can be increased or de-

creased.

Vacancy

Vacancy Substitutional

impurity

Interstitial

impurity

Figure 2.19: various types of point defects in crystalline materials [62].

A A A A

B A A

A A A A

A A

A A A A

51

For example, the Schottky disorder for NaCl and Frenkel disorder

for ZnO, respectively, can be written using Kröger – Vink notation as:

null = VNa′ + VCl

⋅ (2.40)

and

ZnZnX = Zni

⋅⋅ + VZn′′ (2.41)

where null indicates the creation of defects from a perfect lattice. The

respective mass – action equilibrium constants are:

KScho ky = [VNa′ ] ⋅ [VCl

⋅ ] (2.42)

and

KF enkel = [Zni⋅⋅] ⋅ [VZn

′′ ] (2.43)

The brackets denote concentration, usually given in mole fraction. Writ-

ing the equilibrium constant as the product of concentrations implies that

the thermodynamic activity of each defect is equal to its concentration.

The free energies for these quasichemical reactions are simply the

Schottky or Frenkel formation energy, and the equilibrium constant is

given by:

KScho ky = KS∘ exp (−

∆HS

kT) (2.44)

and

KF enkel = KF∘ exp (−

∆HF

kT) (2.45)

The equilibrium constant is a function of temperature only and the prod-

uct of the cation and anion vacancy concentrations is a constant at fixed

52

temperature. Furthermore, when only intrinsic defects are present, the

concentration of anion and cation vacancies must be equal for charge

neutrality considerations,

[VNa′ ] = [VCl

⋅ ] = KS∘ 1 2⁄ exp (−

∆HS

2kT) (2.46)

and

[Zni⋅⋅] = [VZn

′′ ] = KF∘ 1 2⁄ exp (−

∆HF

2kT) (2.47)

In Kröger – Vink notation, free electrons and holes do not themselves

occupy lattice sites. The process of forming intrinsic electron – hole pairs

is excitation across the band gap, which can be written as the intrinsic

electronic reaction:

null = e′ + h⋅ (2.48)

The equilibrium constant for this reaction is:

Ke = n. p = [e′]. [h⋅] = NC. NVexp (− g

kT) (2.49)

where NC and NV are the density of state of conduction band and valence

band, respectively, and Eg is the energy band gap of the material.

When electrons and holes are tightly bound to an ion, or otherwise

localized at a lattice site, the whole is considered to be one ionic defect.

Thus, the valence state of defects such as vacancies and interstitials can

vary. For instance, an oxygen vacancy can in principle take on different

valence states (VO⋅⋅ VO

⋅ and VOX ), as can cation interstitials, e.g., Zni

⋅⋅, Zni⋅

and ZniX in the wurtzite structure compound ZnO.

53

Equilibration of ionic solids with an ambient gas plays an im-

portant role in determining defect structure.

For example, the reduction of ZnO can be written as the removal of

oxygen to the gas phase leaving behind doubly charged oxygen vacancies

or cation interstitials:

OOX =

1

2O2(g) + VO

⋅⋅ + 2e′ o OOX =

1

2O2(g) + Zni

⋅⋅ + 2e′ (2.50)

The equilibrium constant for the creation of double ionized oxygen va-

cancies is:

KR = n2. [VO⋅⋅].

O2

12 = KR

0 . exp (− R

kT) (2.51)

In ZnO, the electron is a major electronic charge carrier. Thus, the con-

ductivity of ZnO is:

σ ∝ n = 2 ⋅ [VO⋅⋅] = 2 ⋅ (2KR

0)1

3 ⋅ exp (− R

3kT) ⋅ (O2)

−16 (2.52)

With background acceptor,

[A′] = 2 ⋅ [VO⋅⋅] (2.53)

The conductivity of ZnO is:

σ ∝ n = (2KR0)

12 ⋅ exp (−

R

2kT) ⋅ (O2)

−14 ⋅ [A′]

−12 (2.54)

Also, the reduction of ZnO can be expressed by creating single charged

oxygen vacancies or cation interstitials:

OOX =

1

2O2(g) + VO

⋅ + e′ o OOX =

1

2O2(g) + Zni

⋅ + e′ (2.55)

54

The equilibrium constant for the creation of single ionized oxygen vacan-

cies is:

KR = n. [VO⋅ ].

O2

12 = KR

0 . exp (− R

kT) (2.56)

Thus, the conductivity of semiconducting ZnO in which single ionized

oxygen vacancies are dominant is:

σ ∝ n = [VO⋅ ] = (KR

0)1

2 ⋅ exp (− R

2kT) ⋅ (O2)

−14 (2.57)

With background acceptor,

[A′] = [VO⋅ ] (2.58)

The conductivity of ZnO is:

σ ∝ n = (KR0) ⋅ exp (−

R

kT) ⋅ (O2)

−12 ⋅ [A′] −1 (2.59)

Thus, investigating the conductivity of ZnO in reducing environments can

assist in determining the valence state of defects and the activation energy

for releasing electrons.

Substituted foreign atoms can also enhance the semiconducting

properties of ZnO. In the presence of selected metallic vapors at elevated

temperatures, the foreign metallic atom replaces a portion of the Zn at-

oms. The zinc atoms, on release from their lattice positions, diffuse to the

crystal surface where they are vaporized. This substitution process can

substantially alter the crystal properties, depending upon the nature, con-

centration and valence of the foreign atom. Optical and electrical proper-

ties are two of the several areas that can be readily modified.

55

2.9.3 Spray pyrolysis deposition technique

Chemical spray pyrolysis (CSP) is used for depositing a wide vari-

ety of thin films, which are used in devices like solar cells, sensors, solid

oxide fuel cells etc. It has evolved into an important thin film deposition

technique and is classified under chemical methods of deposition [63].

This method offers a number of advantages over other deposition pro-

cesses, the main ones being scalability of the process, cost – effectiveness

with regard to equipment costs and energy needs, easiness of doping,

operation at moderate temperatures (100 – 500 °C) which opens the pos-

sibility of wide variety of substrates, control of thickness, variation of

film composition along the thickness and possibility of multilayer deposi-

tion. Spray pyrolysis has been used for several decades in the glass indus-

try [64] and in solar cell production [65].

Typical spray pyrolysis equipment consists of an atomizer, precur-

sor solution, substrate heater, and temperature controller. The following

atomizers, table 2.5, are usually used in spray pyrolysis technique: air

blast (the liquid is exposed to a stream of air) [66], ultrasonic (ultrasonic

frequencies produce the short wavelengths necessary for fine atomiza-

tion) [67, 68] and electrostatic (the liquid is exposed to a high electric

field) [69, 70].

Key parameters of this process are the atomization technique, aero-

sol transport (carrier gas, pressure, distance, and reactor geometry), sub-

Atomizer Droplet size µm Atomization rate

cm3/min.

Droplet velocity

m/s

Pressure 10-100 3-no limit 5-20

Nebulizer 0.1-2 0.5-5 0.2-0.4

Ultrasonic 1-100 <2 0.2-0.4

Electrostatic 0.1-10

Table 2.5: Characteristics of atomizers commonly used for SPD [63].

56

strate temperature and material, the relative expansion coefficients of the

film and the substrate upon which it is deposited, and most importantly,

the chemical composition of the solution (both solvent and precursor salt

types). [71].

Many studies were made on CSP process since the pioneering work

by Hill and Chamberlin in 1964 on CdS films for solar cells [65]. Several

reviews on this technique have also been published.

Siu and Kwok made a detailed study of the properties of the Cu2S/CdS

thin film solar cells formed on chemically sprayed CdS films. Good and

reproducible films could be obtained using a spray-rate of 2.8 ml min-1

and a substrate temperature of around 340 0C. They demonstrated that

cells made on sprayed films could compete well with cells made on evap-

orated films, especially when cost is also considered [72].

Henry et al. reviewed CSP technique in which properties of specific films

of oxide superconductors in relation to deposition parameters and their

device applications were discussed in detail [73].

Brown and Bates discussed the preparation, properties and applications,

as solar cell, of spray – coated CulnSe2 thin films at 250 0C deposition

temperature [74].

Song et al., presented a preparation procedure of spray pyrolyzed un-

doped and aluminium doped zinc oxide thin films for solar cell. They

investigated the effects of the various deposition parameters and vacuum

– annealing of ZnO. ZnO:Al thin films with a transmittance at about 80%

and a resistivity as low as 3.5 x 10-3

Ω.cm were obtained using CSP dep-

osition route [75].

57

Next, in 1995, Roh et al., [76] did employ ultrasonic nozzle to deposit

CdS thin films on SAW devices intended for SO2 gas sensing.

Polycrystalline tin oxide SnO2 films with nano – size crystallites (8 – 20

nm) were prepared by Korotcenkov et al., [77]. The crystallites with ori-

entation (110) or (200) plane parallel to substrate, forming the surface of

the film, were predominant in the sprayed SnO2 films. The latter factor is

important in influencing the gas sensitivity characteristics similar to the

grain size effect.

Different atomization techniques and properties of metal oxide, chalco-

genide and superconducting films prepared using CSP were discussed by

Patil [78]. The results proved that film properties depended on the prepa-

ration conditions and could be easily tailored via optimizing spraying

parameters viz. substrate temperature, spray rate, precursor concentration

etc.

After that, Ebothe and El Hichou [79] examined the role of different

spraying flow rates deposition parameter f, between 1 and 8 mil min-1

, on

the surface irregularities evolution of a sprayed ZnO thin film of the same

thickness e evaluated at 1 mm by always adjusting the deposition time, t,

to the f value. This thickness has been confirmed from cross-sectional

images of the samples examined by scanning electron microscopy (SEM)

using a LEO 982 set. The substrate – nozzle distance d=44.5 cm is kept

constant and the optimal spraying temperature used is 450 0C. The XRD

results revealed that the variation of f has no effect on the material’s

structure as it remains hexagonal and has (002) preferred growth orienta-

tion which is normal to the substrate plane.

58

In 2005, Perednis and Gaukler gave an extensive review on the effect of

spray parameters on films as well as models for thin film deposition by

CSP [63].

Gümüs et al. [66] reported highly transparent ZnO thin films that had

successfully been prepared by pyrolytically spraying zinc acetate solution

on glass substrate at 400 °C using air as a carrier gas. The X-ray diffrac-

tion analysis shows that film is polycrystalline in nature and exhibits

excellent crystalline structure with (002) preferential orientation perpen-

dicular to substrate surface. The grain size is estimated to be 40 nm. Opti-

cal measurements show that the film possesses a high transmittance of

over 90 % in the visible region and a sharp absorption edge near 380 nm.

Envelope method is employed to calculate the refractive index and ex-

tinction coefficient as a function of wavelength. The film has a 3.27 eV

optical direct band gap which is close to the elsewhere – reported value

(3.25 – 3.27 eV) [69].

Recently, Sahay et al. [80] analyzed the optical and electrical properties

of a ZnO thin film obtained by spraying a 0.1 M zinc acetate precursor on

glass substrate held at 370 oC temperature. The optical energy gap for the

film of different thicknesses is estimated to be in the range 2.98 – 3.09

eV. The film exhibits thermally activated electronic conduction and the

activation energies depending on the film thickness. Moreover, the con-

ducted impedance spectra contained a single arc with a non – zero inter-

section with the real axis in the high frequency region.

Next, using simple, flexible and cost-effective ultrasonic spray pyrolysis

(USP) technique, Babu et al. prepared Al – doped ZnO (AZO) thin films

at substrate temperatures around 475 0C. Zinc acetate dehydrate (Zn

(CH3COO)2.2H2O) and Aluminum acetylacetonate (C15H21AlO6) were

59

used as precursors and the solvent was a mixture of de – ionized water,

methanol and acetic acid. The obtained films are polycrystalline with a

hexagonal wurtzite structure and are preferentially oriented in the (002)

crystallographic direction. Grain sizes varied from 21.3 to 25.3 nm based

on substrate temperature. An average transmission of 75% is observed

and the optical band gap of AZO films is varied from 3.26 to 3.29 eV

with the increase in substrate temperature [81].

In the early 2011, Gledhill et al. [82] prepared highly transparent, conduc-

tive ZnO films deposited by spray pyrolysis of zinc acetate – based solu-

tion. Quality films yielded as the spraying process is analogous to an

aerosol assisted chemical vapour deposition rather than a droplet deposi-

tion spray pyrolysis technique. Aluminum – doped zinc oxide (ZnO:Al)

films are grown with free charge carrier concentrations of more than

1020

cm−3

. The carrier density and mobility are measured by both Hall

probe and near infrared spectroscopy. Film growth and grain size, mor-

phology and orientation have been altered using an increased percentage

of ZnCl2 in the precursor, which resulted in a 10 – fold increase in charge

carrier mobility (~10 cm2V

−1 s

−1). An investigation is presented correlat-

ing the composition of the precursor solution with the chemical, structur-

al, electrical and optical properties of the grown films.

2.9.3.1 The deposition process and atomization models

CSP technique involves spraying a solution, usually aqueous, con-

taining soluble salts of the constituents of the desired compound onto a

heated substrate. Typical CSP equipment consists of an atomizer, a sub-

strate heater, temperature controller and a solution container. Additional

features like solution flow rate control, improvement of atomization by

electrostatic spray or ultrasonic nebulization can be incorporated into this

basic system to improve the quality of the films. To achieve uniform large

60

area deposition, moving arrangements are used where either nozzle or

substrate or both are moved. The schematic diagram of a typical spray

unit is depicted in figure 2.20.

Only crude models about the mechanism of spray deposition and

film formation have been developed. There are too many processes that

occur sequentially or simultaneously during the film formation by CSP.

These include atomization of precursor solution, droplet transport, evapo-

ration, spreading on the substrate, drying and decomposition. Understand-

ing these processes will help to improve film quality.

Deposition process in CSP has three main steps: atomization of

precursor solutions, transportation of the resultant aerosol and decompo-

sition of the precursor on the substrate. Atomization of liquids has been

Compressed

Air

Gas Regulator

Valve

Figure 2.20: Schematic diagram of chemical spray pyrolysis unit [83].

Spray

Solution

Power Supply

Heater

Substrate

Hot Plate

Temperature

Controller

Power Supply

Electronic

Controller

Mechanical

System

Thermocouple

Spray nozzle

61

investigated for years. It is important to know which type of atomizer is

best suited for each application and how the performance of the atomizer

is affected by variations in liquid properties and operation conditions. Air

blast, ultrasonic and electrostatic atomizers are normally used. Among

them, air blast atomization is the simplest. However this technique has

limitation in obtaining reproducible droplets of micrometer or submicron

size and in controlling their distribution [63, 83].

In ultrasonic nebulized atomization, precursor solutions are fogged

using an ultrasonic nebulizer [68]. The vapour generated is transported by

carrier gas to the heated substrate. Precursor solution is converted to

small droplets by ultrasonic waves and such droplets are very small with

narrow size distribution and have no inertia in their movement. Pyrolysis

of an aerosol produced by ultrasonic spraying is known as pyrosol pro-

cess [10, 75]. Advantage of this technique is that the gas flow rate is in-

dependent of aerosol flow rate, unlike the case of air blast spraying.

Electrostatic spray deposition technique has gained significance

only in recent years. Electrostatic atomization of liquids was first reported

by Zeleny [84]. Jaworek et al. published an article on this type of atomi-

zation [70]. A positive high voltage applied to the spray nozzle generated

a positively charged spray. Stainless steel discs acted as cathode and the

droplets under electrostatic force moved towards the hot substrate where

pyrolysis took place. In electrostatic spray, depending on the spray pa-

rameters, various spraying modes were obtained. They were classified as

cone – jet mode and multi – jet mode. Cone – jet mode split into multi –

jet mode with increase in electric field, where number of jets increased

with applied voltage.

62

The reaction process taking place in CSP is interesting. Many

models exist for the decomposition of precursor. Many simultaneous

processes occur when a droplet hits the substrate surface: evaporation of

residual solvent, spreading of droplet and salt decomposition. Vigue and

Spitz proposed that the following processes occur with increasing sub-

strate temperature [85]. Figure 2.21 given below illustrates the four pos-

sible processes that occur with increasing temperature.

In process A, droplet splashes on substrate, vaporizes and leaves a dry

precipitate in which decomposition occurs.

In process B, solvent evaporates before the droplet reaches the surface

and precipitate impinges on the surface where decomposition occurs.

In process C, solvent vaporizes as droplet approaches the substrate, then

solid melts and sublimes and vapour diffuses to substrate to undergo het-

erogeneous reaction there.

In process D, at highest temperature, the metallic compound vaporizes

before it reaches the substrate and chemical reaction takes place in vapour

phase.

Substrate

Finely divided solid

product

Vapour

Precipitate

D C B A

Figure 2.21: spray processes (A, B, C, and D) occurring with increase in substrate

temperature [85, 63].

63

Most of the spray pyrolysis deposition is of type A or B and our discus-

sion will be focused on these two.

2.9.3.2 Deposition parameters

Properties of film deposited depends on various deposition parame-

ters like substrate temperature, nature of spray and movement of spray

head, spray rate, type of carrier gas, nature of reactants and solvents used.

The effect of some important spray parameters are discussed here.

I Substrate temperature

Substrate temperature plays a major role in determining the proper-

ties of the films formed. It is generally observed that higher substrate

temperature results in the formation of better crystalline films [66, 80].

Grain size is primarily determined by initial nucleation density and re-

crystallization. Recrystallization into larger grains is enhanced at higher

temperature [86]. By increasing the substrate temperature, the film mor-

phology can be changed from cracked to dense and then to porous [87].

Variation of substrate temperature over different points results in non-

uniform films. Composition and thickness are affected by changes in

substrate temperature which consequently affect the properties of depos-

ited films. Though surface temperature is a critical factor, most investiga-

tors have not known the actual surface temperature of the substrate. Also,

maintenance of substrate temperature at the preset value and its uniformi-

ty over large area are challenging. Spraying in pulses or bursts also has

been used to assure that surface temperature is reasonably constant [86].

II Influence of precursors

The precursor used for spraying is very important and it extremely

affects the film properties. Solvent, type of salt, concentration and addi-

tives influence the physical and chemical properties of the films [71].

64

For ZnO thin films grown by spray pyrolysis, it was found that organic

salts (e.g., zinc acetate Zn(CH3COO)2.2H2O) are preferable over inorgan-

ic ones such as chlorides and nitrates. In the case of inorganic salts, un-

wanted etching processes, caused by acids formed as a result of the pre-

cursor decomposition, lead to degradation of the films performance.

Similarly, organic solvents are preferable over water due to a better drop-

let size distribution and, also, due to additional heat transfer toward the

sample surface by their burning. It was observed that transparency of as

deposited ZnO films increased when ethanol was used instead of water as

solvent for zinc acetate [71].

III Spray rate

Spray rate is yet another parameter influencing the properties of

films formed. It has been reported that properties like crystallinity, sur-

face morphology, resistivity and even thickness are affected by changes in

spray rate [88].

It is generally observed that smaller spray rate favours formation of

better crystalline films. Smaller spray rate requires higher deposition time

for obtaining films of the same thickness prepared at higher spray rate.

Also, the surface temperature of substrate may deviate to a lower value at

high spray rate. These two factors may contribute to the higher crystal-

linity at small spray rates. Decrease in crystallinity at higher spray rates is

observed in sprayed CuInS2 thin films [88]. Decrease in crystallinity

usually results in increased resistivity of the films.

Surface morphology of the films varies with spray rate. Higher

spray rate results in rough films. Also, it is reported that films deposited

at smaller spray rates are thinner due to the higher re-evaporation rate.

65

IV Other parameters

Parameters like height and angle of spray head, angle or span of

spray, type of scanning, pressure and nature of carrier gas etc., influence

the properties of deposited films. Different types of spray heads which

produce different spray patterns are commercially available. Relative

motion of the substrate holder and spray head should ensure maximum

uniformity and large area coverage.

2.9.4 Metal oxide gas sensors

The idea of using semiconductors as gas sensitive devices leads

back to 1952 when Brattain and Bardeen first reported that gas adsorption

on germanium semiconductor surface caused a variation in its electrical

conductivity [3]. The first realization of a working gas sensor was in

1962, when Seiyama et al. detailed the use of zinc oxide ZnO thin films

in the detection of gases as ethanol (C2H6O) and carbon dioxide (CO2)

[4]. Naoyoshi Taguchi, in that same year, published a patent for a tin

oxide (SnO2) – based gas sensor [5]. From then, the detection of hydro-

gen (H2), oxygen (O2) and hydrocarbon by means of surface conductivity

changes on various metal oxide crystals and thin films have been pro-

posed and demonstrated [89]. Although, many metal oxides have been

successfully demonstrated in gas sensing, SnO2 and ZnO have been in-

tensively investigated fundamentally and commercially due to the attrac-

tive structural, optical and electrical properties they possess and its easy

fabrication in thin film form with various methods [12, 13, 14, 41, 69, 92]

as well as, its improved sensor performance by addition of dopants [41,

47, 58, 93, 95].

The sensing performance (magnitude of gas response (sensitivity),

selectivity, sensing temperature, response /recovery time and so on) de-

66

pends on the electronic and structural properties of the sensor material

[36]. The sensing parameters can be promoted by the addition of metal

additives such as Al, Sn, Sb, Cu, Pt and Pd. These additives greatly en-

hance the gas sensor sensitivity, shorten the response time, and shift the

volcano – shaped correlations between gas response and temperature

toward the lower temperature side [48].

Nanto et al., [38] prepared a sensor with a high sensitivity and an excel-

lent selectivity for ammonia by using sputtered ZnO thin films. The sen-

sor exhibited a negative resistance change on exposure to oxidizing am-

monia gas (NH3) whereas the change became positive for exposure to

many other reducing inflammable and organic gases (H2, methane CH4,

butane C4H10, acetone C3H6O, ethanol C2H5OH). The resistance change

and the selectivity of the sensor were enhanced by doping group III metal

impurities such as AI, In, and Ga. On exposure to 200 ppm ammonia gas,

the resistance changed about three times as large as that in the undoped

ZnO sensors. The lower limit of the detection for ammonia gas was about

1 ppm at a working temperature of 350 0C. The sensing mechanism of

ammonia gas was related to the enhancement of adsorption of atmospher-

ic oxygen.

Five years later, in 1991, in a study to develop a cheap smell sensor capa-

ble of detecting the various gases, the freshness of sea foods and drinks,

and the fragrance from wine and coffee, Nanto et al., [90] investigated the

gas sensitivity of RF and DC magnetron – sputtered aluminum doped

zinc oxide (ZnO:Al) thin film on corning 17059 glass substrate with a

film thickness of about 300 nm. The sensitivity measurements were car-

ried out at an operating temperature of 200 – 350 °C in air. A testing gas

of 200 ppm was introduced by using an injector into the testing glass bell

– jar. The resistance of the sensor changed on exposure to odor from rot-

67

ten sea foods such as oyster, squid, sardine or fragrance from wine and

coffee. The high sensitivity of the sensor for the odor from rotten sea

foods was attributed to the high sensitivity of the sensor for trimethyla-

mine N(CH3)3 in the odor.

Hong et al., [91] successfully demonstrated the identification of methan-

ethiol CH3SH, trimethylamine (CH3)3N, ethanol C2H5OH and CO gases

in the 0.1 – 100 ppm concentration range by a gas recognition system

using a thin film oxide semiconductor micro gas sensor array and the

neural – network pattern recognition technique. The sensing materials of

1 wt. % Pd – doped SnO2, 6 wt. % A1203 – doped ZnO, WO3 and ZnO

were used for the gas sensor array whose power consumption was only 65

mW at 300 0C, and the back-propagation algorithm was applied as the

supervised learning rule. The recognition probability of the neural-

network was 100% for the discrimination of the gases and concentrations

used in the work.

Next, Mitra et al., [92] investigated the gas response of chemically depos-

ited ZnO films using a sodium zincate bath. The ZnO films prepared by

this method were highly resistive, signifying the presence of a large den-

sity of oxygen adsorbed acceptor – like trap states (O2− , O

-, etc.). Prelim-

inary studies of gas sensing characteristics, performed at 150 – 375 0C

temperature range, indicated that the Pd – sensitized ZnO films respond

strongly to 1 vol% H2, which is on the lower side of the hazardous explo-

sion range for H2 (4 –75 %). The ZnO film – based sensors exhibited

excellent sensitivity (more than 99%) at 200 0C and the response time

was reasonably rapid.

Nunes et al., [93] reported a maximum value of sensitivity for zinc oxide

(ZnO) thin films sensor of high electrical resistivity and low thickness

68

upon being exposed to methane (CH4), hydrogen (H2) and ethane (C2H6)

reductive gases. The sensitivity of the sensor increased with operating

temperature and its highest values were obtained at 200 ºC for methane

and hydrogen, while it occurred at 100 ºC for the test with ethane. More-

over, the sensitivity of the ZnO thin films changed linearly with the in-

crease of the gas concentration. The ZnO sensor demonstrated low selec-

tivity since it detects the presence of several gases. The increase of the

selectivity could be promoted by the use of an appropriated catalytic

metal such as Pd or Au.

Roy and Basu [94] explored the selectivity, towards dimethylamine

(DMA) (CH3)2NH and H2 test gases at different temperatures, of a good

quality undoped ZnO films deposited on glass and quartz by a novel CVD

technique using a 0.5 M zinc acetate as the starting solution. They ob-

tained faster response and higher sensitivity. The operating temperature

played a key role in the selectivity of such sensors, with the optimum

operating temperature being 300 0C.

B. Licznerski [19] observed that thick – film gas sensors based on semi-

conductive tin dioxide are suitable for detection of explosive and toxic

gases and vapours. Sensitivity, selectivity and stability of sensors working

in different temperature depend on the way the tin dioxide and additives

were prepared. A construction also plays an important role. He presented

an attitude towards the evaluation of transport of electrical charges in

semiconductive grain layer of SnO2, when dangerous gases appeared in

the surrounding atmosphere.

Bârlea et al., [8] characterized SnO2 metal oxide gas sensors deposited on

a ceramic cylinder heated to the functioning temperature (100 – 400 °C),

and exposed it to an atmosphere containing a reducing gas: carbon mon-

69

oxide (CO), liquid petroleum gas (LPG, mainly butane) and methane gas.

They investigated the influence of the supplied electric current to the

sensor sensitivity and the response time and recovery time. The electric

resistance of the semiconducting material dramatically modified, even at

very low gas concentrations. The response time was longer for lower

supply voltage (many seconds, even minutes) and became very short

(under 1 second) at the greatest voltages.

Gas sensing with fast response and recovery times is at the forefront of

gas sensing characteristic parameters particularly when it comes to

providing early alert of inflammable and toxic gas leaks. Several re-

searches are concerned on optimizing this parameter.

Xu et al., [43] demonstrated an ultrafast, ultrasensitive hydrogen gas

sensor based on self – assembled monolayer promoted 2 – dimensional

palladium film on glass substrate. Their enhanced sensor was sensitive

enough to detect hydrogen levels as low as 25 ppm with a fast response

time in tens of milliseconds (~70 ms) upon exposure to 2% H2, a con-

centration below the hydrogen explosion limit range of 4 – 75% for effec-

tive alarming.

Chou et al., [95] investigated the structural and sensing properties of

ZnO:Al films as an ethanol vapor gas sensor obtained by RF magnetron

sputtering on Si substrate using Pt as interdigitated electrodes. The struc-

tural characteristics revealed that flat and well – defined columnar films

with c – axis textured were formed. The film exhibited good sensitivity to

the ethanol vapors with quick response – recovery characteristics, and it

was found that the sensitivity for detecting 400 ppm ethanol vapor was

~20 at an operating temperature of 250 °C. The high sensitivity, fast re-

70

covery, and reliability implied that ZnO:Al seemed to be a promising

semiconducting material for the detection of ethanol vapor.

Patil et al., [41] explored the characterization and ethanol gas sensing

properties of pure and doped ZnO thick films prepared by the screen

printing technique. Pure zinc oxide was almost insensitive to ethanol.

Thick films of (1 wt%) Al2O3 – doped ZnO were observed to be highly

sensitive to ethanol vapours at 300 °C. Doping Al2O3 into zinc oxide

created surface misfits and since it is reported that the surface misfits,

calcination temperature and operating temperature could affect the micro-

structure and gas sensing performance of the sensor. The sensor showed

very rapid response and recovery to ethanol vapours. Moreover, it had

good selectivity to ethanol against LPG, NH3, CO2, Cl2 and H2 at 300 °C.

Mitra & Mukhopadhyay [96] studied the methane (CH4) sensitivity, re-

sponse time and recovery process of a chemically fabricated ZnO thin

film semiconducting layer and a Pd – catalyst layer coated on the surface

of the semiconducting ZnO. The sensor exhibited reasonable sensitivity

of about 86% to 1vol. % methane (CH4) in air at 200 0C optimum operat-

ing temperature.

The effect of film thickness on sensor performance was investigated by

Liewhiran and Phanichphant [42]. They fabricated ZnO thick films using

flame spray pyrolysis (FSP) on Al2O3 substrate interdigitated with Au

electrodes with various thicknesses (5, 10, 15 μm). The gas sensing char-

acteristics to ethanol (25 – 250 ppm) evaluated as a function of film

thickness at 400 °C in dry air, displayed the tendency of the sensitivity to

decrease with increasing film thickness and response time; with the thin-

nest sensing film (5 μm) showed the highest sensitivity and the fastest

response time (to 250 ppm, S=801, τres =5 s). They discussed the behavior

71

on the basis of diffusively and reactivity of the gases inside the oxide

films. The sensing characteristics were deteriorated evidently with in-

creasing film thicknesses. The recovery times were quite long within

minutes.

Aroutiounian et al., [97] reported hydrogen sensors working at and close

to room temperature and made of porous silicon covered by the TiO2-x or

ZnO<Al> thin films. The sensitivity of manufactured structures to 1000

– 5000 ppm H2 showed the possibility of realizing a durable, highly sen-

sitive and selective hydrogen sensor within its lower and upper explosion

limits of 4 – 75% by volume. The sensor had a relatively short time of

response and recovery (~20 s). Such sensors could also be a part of a

silicon integral circuit. The same research group grew Aluminum –

doped ZnO nano – size films on glass ceramic substrates by high – fre-

quency magnetron sputtering method [99]. Pt layer and gold interdigitat-

ed ohmic contacts were evaporated on the prepared films by the ion –

beam sputtering method. Sensitivity measurements in the temperature

range 40 −100 0C to different concentrations of hydrogen (1000 − 5000

ppm) in air was investigated. The glass ceramic/ZnO<Al>/Pt structure

showed sufficient sensitivity to hydrogen at the pre-heating of the work-

ing body already up to 40 0С.

The grain or crystallite size of the sensing element is one of the most

important factors affecting sensing properties, especially sensitivity. The

gas response to different gases is related to a great extent to the surface

state and morphology of the material [13]. The use of micro and

nanostructured films is advantageous due to the high surface to volume

ratio the nanostructure exhibits, offering faster response and higher gas

sensitivities to low concentration of the tested gases and eliminating the

need of operation at elevated temperatures. Some groups have explored

72

nano grain – sized ZnO structures and tested it for gas sensitivity. Mat-

thew Szeto [98] fabricated ZnO chemical gas sensors from prepared ZnO

nanoplatelets and their sensitivities to H2 gas were investigated under

conditions of varying concentration, sensor temperature, and intensity of

UV light. It was found that at room temperature and a source voltage of

5V that the ZnO sensor had the best sensitivity of greater than 85% to H2

gas at 50 ppm and was most sensitive in the absence of UV radiation. At

130 0C temperature, the ZnO sensor showed sensitivities near 100% to

~50 ppm of H2, where it both responded and recovered faster. Memory

effect previously observed was also non – existent at temperatures near

and above 100°C. The sensor was also observed to be both slower in

response and lower in sensitivity in the absence of UV light in a darkened

chamber.

Savu et al., [16] deposited a Porous nano and micro crystalline tin oxide

films by RF Magnetron Sputtering and doctor blade techniques, respec-

tively. Electrical resistance and impedance spectroscopy measurements,

as a function of temperature and atmosphere, were performed in order to

determine the influence of the microstructure and working conditions

over the electrical response of the sensors. The conductivity of all sam-

ples increases with the temperature and decreases in oxygen, as expected

for an n-type semiconducting material. The improved sensitivity and

response times at the 200 °C working temperature are due to the higher

rate of gas adsorption/desorption. The impedance plots indicate the exist-

ence of two time constants related to the grains and the grain boundaries.

The Nyquist diagrams at low frequencies reveal the changes that took

place in the grain boundary region, with the contribution of the grains

being indicated by the formation of a second semicircle at high frequen-

cies. The better sensing performance of the doctor bladed samples can be

73

explained by their lower initial resistance values, bigger grain sizes and

higher porosity.

Also, Korotcenkov et al., [36] reviewed the pioneering influence of mor-

phological and crystallographic structural parameters (i.e., film thickness,

grain size, agglomeration, porosity, faceting, grain network, surface ge-

ometry, and film texture) on the gas sensor main analytical characteristics

(absolute magnitude and selectivity of sensor response (S), response time

(τres), recovery time (τrec), and temporal stability). A comparison of stand-

ard polycrystalline sensors and sensors based on one – dimension struc-

tures was conducted. The structural parameters of metal oxides were

found to be the most important factors for controlling response parame-

ters of resistive type gas sensors. Thus, it was shown that the decreasing

of thickness, grain size and degree of texture was the best way to decrease

time constants of metal oxide sensors. However, it was concluded that

there is no universal decision for simultaneous optimization of all gas-

sensing characteristics. One have to search for a compromise between

various engineering approaches because adjusting one design feature may

improve one performance metric but considerably degrade another.

Kiriakidis et al., [100] exhibited highly porous ZnO films with character-

istic c-axis columnar growth structure deposited on glass substrates in a

home-made aerosol spray pyrolysis system at 350 0C. Their sensing re-

sponse to low ozone concentrations was evaluated at room temperature.

Films have shown to produce clear response signals to ozone concentra-

tions as low as 16 ppb with a response time of 1 min, demonstrating the

potential of applying these films as sensing elements in future metal ox-

ide gas sensing devices.

74

Al-hardan et al., [101] prepared ZnO thin films by thermal oxidation of

Zn metal at 400 0C for 30 and 60 min. The XRD results showed that the

Zn metal was completely converted to ZnO with a polycrystalline struc-

ture. The sensors had a maximum response to H2 at 400 0C and showed

stable behavior for detecting H2 gas in the range of 40 to 160 ppm. Film

oxidized for 60 min in oxygen flow exhibited higher response than that of

the 30 min oxidation which was approximately 4000 for 160 ppm H2 gas

concentration. The sensor with higher resistance yields higher response to

the gas under test. The sensing mechanism was modeled according to the

oxygen – vacancy model.

Tamaekong et al [102] investigated the gas sensing properties toward

hydrogen (H2) of ZnO nanoparticles doped with 0.2 – 2.0 at. % Pt which

were successfully produced in a single step by flame spray pyrolysis

(FSP) technique. ZnO nanoparticles paste was coated on Al2O3 substrate

interdigitated with gold electrodes to form thin films by spin coating

technique. The gas sensing properties toward hydrogen gas revealed that

the 0.2 at. % Pt/ZnO sensing film exhibited an optimum H2 sensitivity of

~164 at hydrogen concentration in air of 1 volume % at 300 0C and a low

hydrogen detection limit of 50 ppm at 300 0C operating temperature.

Al-hardan et al., [103] synthesized undoped and 1 at. % chromium (Cr) –

doped ZnO by RF reactive co-sputtering for oxygen gas sensing applica-

tions. The prepared films showed a highly c – oriented phase with a dom-

inant (002) peak at a Bragg angle of around 34.28 o. The Cr – doped ZnO

sensor has been shown to have a lower operating temperature of around

250 0C and enhanced sensitivity than previously reported. Good stability

and repeatability of the sensor were demonstrated when tested under

different concentration of oxygen atmosphere. The enhancement was

likely attributed to the higher oxidation state of the chromium.

75

Also, Al-hardan et al., [104] investigated the mechanism of hydrogen

(H2) gas sensing in the range of 200 – 1000 ppm of RF-sputtered ZnO

films. The I – V characteristics as a function of operating temperature

proved the ohmic behavior of the contacts to the sensor. The complex

impedance spectrum (IS) of the ZnO films exhibited a single semicircle

with shrinkage in the diameter as the temperature increased and as the

hydrogen concentration was increased in the range from 200 ppm to 1000

ppm.

One month later, Al-hardan et al. [12] studied the gas sensing properties

of RF reactively sputtered ZnO thin film towards volatile organic com-

pounds VOC in which the sensitivity of the sensor was the highest

( ~ 100 ) for 500 ppm acetone in comparison to that of isopropanol

and ethanol. An optimum operating temperature for maximum sensitivity

of 400 0C for the above vapors was obtained. The sensor showed a stable,

reversible and repeatable behavior in the acetone concentration of 15 up

to 1000 ppm. They explained the sensing mechanism in accordance with

the ionosoption model. The same ZnO based sensor also exhibited good

sensitivity for vinegar test in the concentration range of 4% to 9% and the

maximum sensitivity to vinegar test application was obtained at 400 0C.

The work revealed the validity of using ZnO gas sensor in estimating the

acid concentrations of the vinegars for food requirements [106].

Hussain et al., [105], grew ultra – fine thin films of pure and SnO doped

ZnO nanosensor on gold interdigitated ceramic substrate by ultrasonic

aerosol assisted chemical vapor deposition technique (UAACVD) at

around 450 °C temperature and under 5 Pa oxygen atmospheric pressure.

Both doped and undoped ZnO thin films sensing characteristic measure-

ments verified the nanosensor suitability for detecting ethanol vapor at a

temperature range of 60 – 150 °C. At room temperature (25 °C), the re-

76

sponse and recovery time of the sensor increased many orders of magni-

tude compared to 60 °C. Sensitivity of the ZnO sensor demonstrated

linear dependence with the increase of gas concentration. 1 % SnO dop-

ing of ZnO enhanced the sensitivity of the film drastically and thus im-

proved its detecting efficiency.

Later, Al-zaidi et al., [107] demonstrated spray – pyrolyzed palladium –

doped ZnO thin film deposited on glass substrate to be a fast hydrogen

gas sensor. The prepared ZnO films were doped by dipping in palladium

chloride PdCl2. Sensitivity dependence on the temperature and test gas

concentration was tested and the optimum operation temperature was

determined at around 280 oC. The response time of 2-3 s of the doped

ZnO film was so fast to detect flammable H2 leaks well below the lower

explosion limit (LEL) of 4%.

Chen et al., [118] successfully prepared tin dioxide SnO2 thin films with

interesting fractal features by pulsed laser deposition techniques under

different substrate temperatures. Tin oxide is a unique material of wide-

spread technological applications, particularly in the field of environ-

mental functional materials. New strategies of fractal assessment for tin

dioxide thin films formed at different substrate temperatures are of fun-

damental importance in the development of microdevices, such as gas

sensors for the detection of environmental pollutants. Fractal method

was applied to the evaluation of this material. The measurements of car-

bon monoxide gas sensitivity confirmed that the gas sensing behavior

was sensitively dependent on fractal dimensions, fractal densities, and

average sizes of the fractal clusters. The random tunneling junction net-

work mechanism was proposed to provide a rational explanation for this

gas sensing behavior. The formation process of tin dioxide nanocrystals

and fractal clusters could be reasonably described by a novel model.

77

In spite of the many researches on the as – deposited and doped

metal oxide ZnO and SnO2 – based gas sensors prepared via different

depositions techniques, there is still a need to obtain simple, cost – effec-

tive, sensitive and fast response gas sensor towards inflammable hydro-

gen reducing gas. Thus, in this work the fabrication and sensing charac-

teristics of undoped and Pd – doped ZnO and SnO2 semiconductor metal

oxides by chemical spray pyrolysis deposition are presented. The effect

of Pd doping on the sensitivity S, operating temperature and the response

time of both metal oxides gas sensing elements to hydrogen (H2) gas of

different concentrations is examined.

78

Chapter 3

Experimental Procedure

Introduction

The present chapter gives a detailed account of the work carried

out for the development of Zinc Oxide and tin oxide thin film based gas

sensors. The different steps followed in this regard are described. Chemi-

cal spray pyrolysis deposition on glass and silicon substrates, which is

used in the present work, is discussed in details. Following it is a discus-

sion on the characterization of the deposited gas sensitive ZnO and SnO2

materials. Details of the experimental set up made for testing and study-

ing the performance of the developed gas sensors is also presented in this

chapter. The results obtained and analyses of data on the performance of

the sensors fabricated are presented at the next chapter.

3.1 Gas Sensor Fabrication

The schematic conception of a typical simple metal oxide gas sen-

sor is illustrated in figure 3.1 below.

The different steps that have been followed for the realization of a

semiconductor metal oxide, ZnO or SnO2, gas sensor are outlined below:

Selection of substrate

Figure 3.1: Schematic of a typical gas sensor structure. Thicknesses are not to scale.

ZnO film (150 nm)

Electrode: Pt (200 nm)/ Ta (25 nm)

film

Insulation layer: SiO2 layer (1 μm)

Substrate: Si wafer

H2 H2

H2

H2

SiO2 layer

Pt electrode

Si wafer

ZnO film

Electrical Measurement

79

Substrate cleaning procedure

Deposition of gas sensitive thin film

Surface sensitization of the prepared thin film by palladium

noble metal catalyst.

Deposition of Al interdigitated electrodes IDE on the sensi-

tive film and attachment of leads for electrical measurement.

Fabrication of gas sensor testing system.

The substrate refers to the base on which the gas sensing material is de-

posited. A substrate used for gas sensing application should ideally be

[18]:

Good conductor of heat: The ability of a material to conduct heat is

quantified by either thermal conductivity or thermal diffusivity,

thereby, determining the power consumption of the sensor.

Electrically insulating: This is to ensure that the electrons generat-

ed due to gas – solid interaction are not being grounded by a con-

ducting substrate.

Rugged.

Stable and inert in measurand environment.

Inexpensive.

Capable of influencing the microstructure favorably (porous films

with granular microstructure).

Render itself suitably for cleaning.

The sensitivity of gas sensors depends mainly on the grain size of

the gas sensitive material [36]. The grain size of a given material on a

substrate is known to depend on the wettability of the substrate. Sub-

strates with lower surface tension are believed to result in smaller grain

size. Glass, having the lowest surface tension, resulted in the smallest

80

ZnO thin film grain size [18]. The large grain size of ZnO particles de-

posited on sintered alumina is probably due to of the rough topography

exhibited by the substrate itself.

3.2 Spray pyrolysis experimental set up

Chemical techniques for the preparation of thin films have been

studied extensively because such processes facilitate the designing of

materials on a molecular level. Spray pyrolysis, one of the chemical tech-

niques applied to form a variety of thin films, results in good productivity

from a simple apparatus. In the current research, zinc oxide thin films are

deposited on glass substrates employing locally – made spray pyrolysis

deposition chamber whose main components set up is illustrated in the

schematic diagram of figure 3.2. It is essentially made up of a precursor

solution, carrier gas assembly connected to a spray nozzle, and a tempera-

ture – controlled hot plate heater.

The atomizer, illustrated in the photo plate 3.1-B, has an adjustable

copper capillary tube nozzle of 0 - 0.8 mm inner diameter clamped to a

holder and supported by a metal tripod. The nozzle is driven by a com-

pressed atmospheric air. The prepared precursor solution is pumped

through the metal nozzle with a solution flow rate ranging from 1 to 2

mL/min. Due to the air pressure of the carrier gas; a vacuum is created at

the tip of the nozzle to suck the solution from the tube after which the

spray starts [63]. To regulate spraying time, a 16 – Bar Tork solenoid

valve controlled by an adjustable timer has been incorporated. The atom-

izer and the 1500 Watts hot plate heater are enclosed in a 1 1

1 𝑚3 ventilation hood, photo plate 3.1-A. A 220 V a.c. power was ap-

plied to the heater and temperature was measured using a type K (nickel-

chromium) thermocouple and precision digital temperature controller

(GEMO DT109 photo plate 3.1-C).

81

3.3 Precursor solution

A 0.2 M concentration precursor solution of zinc acetate dihydrate

Zn(CH3COO)2.2H2O (molecular weight 219.4954 g/mole) has been pre-

pared by dissolving a solute quantity of 4.389908 g of

Zn(CH3COO)2.2H2O (as weighed by a 10−4 g - precision balance) in 100

mL isopropyl alcohol C3H9O (the solvent). A magnetic stirrer is incorpo-

rated for this purpose for about 10 – 15 minutes to facilitate the complete

dissolution of the solute in the solvent. Furthermore, aqueous precursor of

Zinc chloride ZnCl2 (molecular weight 136.3146 g/mole) dissolved in

distilled water has also been employed in getting ZnO thin films. Organic

0 Substrate

Sprayer

Holder with

stand

Spray

cone

Air Nozzle

Substrate heater

Capillary Tube

Compressed

Air Tube

Thermocouple

Temperature

Controller

30 cm

Measuring

Cylinder

Ventilation Fan

Solenoid Valve

And Timer

0450°C

Air in

Precursor

Solution

Figure 3.2: Spray pyrolysis experimental set up

82

solvents are preferable over distilled water because the former enables the

attainment of homogeneous, highly – transparent, thin films of small

grain size [71].

Prior to depositing the films, the substrates, which are commercial

glass slides of 76×25×1 mm3 dimensions, are firstly cleaned by dipping

in distilled water to remove the dust and then are ultrasonically cleaned in

methanol for about 10 min. Finally they are soaked in distilled water,

dried, and polished with lens paper. The pretreatment of the substrates is

carried out to facilitate nucleation on the substrate surface. Presence of

contamination on the substrate surface is one of the reasons of the ap-

pearance of pinholes and film inhomogeneity [71].

Photo plate 3.1: A: experimental set up of the spray pyrolysis deposition SPD. B: Air atomiz-

er. C: Gemo DT109 temperature controller, and D: Digital balance with the magnetic stirrer.

B

Needle

in nozzle

Air exit

A

C D

83

The spray rate is usually in the range 2 – 3 mL min-1

. The optimum

carrier gas pressure for this rate of solution flow is around 5 kg cm-2

. At

lower pressures, the size of the solution droplets becomes large, which

results in the presence of recognized spots on the films and then reduction

of transparency. This situation increases the scattering of light from the

surface and then reduces the transmittance of the films.

The spray pyrolytic substrate temperature is maintained within 450

± 5 °C during the deposition. Film thickness is controlled by both the

precursor concentration and the number of sprays, or alternatively, spray-

ing time. Thus, a 4 – second spray time is maintained during the experi-

ment. The normalized distance between the spray nozzle and substrate

was fixed at 30 cm. Table 3.1 summarizes the optimized thermal pyroly-

sis deposition conditions for the preparation of ZnO thin films that were

employed in the current research.

3.4 The determination of film thickness

The thickness of the films is determined using a micro gravimet-

rical method. The films deposited on clean glass slides whose mass had

previously been determined. After the deposition, each substrate itself is

weighted again to determine the quantity of deposited ZnO. Measuring

Spray parameters Values

Concentration of precursor 0.2 M

Volume of precursor sprayed 100 mL

Solvent isopropyl alcohol

Substrate temperature 450 0C

Spray rate 2.3 mL/min.

Carrier gas pressure 1 bar

Nozzle-substrate distance 30 cm

Table 3.1.: Optimum thermal spray pyrolysis deposition conditions for the preparation

of ZnO thin films.

84

the surface area of the deposited film, taking account of ZnO specific

weight of the film, the thickness is determined using the relation:

=∆mZnO

A ∙ ρ (3.1)

where A is the actual area of the film in cm2, ∆mZnO

is the quantity of

deposited zinc oxide, and ρ is the specific weight of ZnO. Film thickness

was also confirmed and verified from cross sectional SEM image.

3.5 Surface modification of ZnO by palladium noble metal

Metal oxide gas sensors need a catalyst deposited on the surface of

the film to accelerate the reaction and to increase the sensitivity, impart

speed of response and selectivity [41].

Small amounts of noble metal additives, such as Pd or Pt are com-

monly dispersed on the semi conducting as activators or sensitizers to

improve the gas selectivity, sensitivity and to lower the operating temper-

ature [41, 48]. Many methods have already been tested for this purpose,

for example bulk doping during calcination, sol-gel technology, spray

pyrolysis deposition, thermal evaporation, CVD, laser ablation, magne-

tron sputtering, impregnation by salt solution. With the help of these

methods, it was possible to form on a surface of metal oxides surface

clusters of various components with sizes from 0.1 to 8 nm [16].

For the above reasons, the surface of the deposited ZnO thin films

were catalyzed using successive multiple dipping (or spraying) of the

prepared samples with a 50 ccm (0.0564 Molar) solution made up of

dissolving 1% by weight palladium chloride PdCl2 (Mwt.= 177.3256

g/mole) in ethanol alcohol C2H5OH. Each sample was successively

sprayed 10, 15, 20, 25, and 30 times of 4s spray interval and at 400 de-

grees hot plate heater temperature. Eventually, the sensitized samples

85

were heat – treated at the same temperature for a period of one hour in

atmospheric air. About 20 – time palladium spray or dipping was found to

be optimum for fast and sensitive zinc oxide H2 gas sensor.

3.6 Al Interdigitated Elecrtodes (IDE)

Figure 3.3 illustrates a schematic diagram of the thermal evapora-

tion system (Edward type E306A unit) which is used to thermally evapo-

rate the aluminum electrodes layer on the ZnO sample via the metal

Variable power

supply transformer

Boat

Valve

Pirani gauge

Valve

Rotary pump

Diffusion pump

High vacuum

Valve

Penning

gauge

Bell jar

Substrate holder

Bus bars

Cylindrical

shield

Ring shield

Shutter

Figure 3.3: Vacuum system for the vaporization from resistance – heated sources.

When replacing the transformer and heater with an electron gun, vaporization by

means of an electron beam occurs.

Glow ring Thickness

monitor

86

mask. Figure 3.4 illustrates two 8 and 10 – finger interdigitated electrode

IDE metal masks which were utilized in this work. The samples were

fixed in the evaporation system. The thickness t of the evaporated alumi-

num electrode was estimated using the following formula:

=m

2πR2ρ (3.2)

Where R is the separation distance of the tungsten boat to the substrate

holder, 𝜌 represents the density of aluminum (specific gravity of 2.7) and

22 mm

19 mm

1 mm

2 mm

3 mm

2 mm

14 mm

0.4 mm

0.4 mm

3 mm

3 mm

0.4 mm

13.6

mm

2 mm

3 mm

10 mm

15 mm

25 mm

2 mm

2 mm

Figure 3.4.: A schematic diagram of the IDE masks utilized in this work.

87

m is the mass of Al used during evaporation.

3.7 Gas sensor testing system

A schematic cross sectional view of the gas sensor testing system,

test chamber and photos of the mounted sensor and test chamber are illus-

trated schematically in figure 3.5 and in the photograph plate 3.2 respec-

tively. The unit consists of a vacuum – tight stainless steel cylindrical test

chamber of diameter 163 mm and of height 200 mm with the bottom base

made removable and of O – ring sealed. The effective volume of the

chamber is 4173.49 cc; it has an inlet for allowing the test gas to flow in

and an air admittance valve to allow atmospheric air after evacuation.

20 cm

16.3 cm

8 – pin feed through

Output to

vacuum

pump

Test gas in

Gas Manifold 2 cm

O –ring seal

V A

Ω

Needle Valve

Vacuum gage

3 mm

Auxiliary inlet

436

450

Gas

Flow meter

ZnO Sensor

PC – interfaced

DMM

Temp. Controller

Exhaust

USB

Cable

Air Flow

meter

Hydrogen Air

Relief

valve

Vacuum Pump

Figure 3.5: Gas sensor testing system

88

Another third port is provided for vacuum gauge connection.

A multi – pin feed through at the base of the chamber allows for

the electrical connections to be established to the heater assembly as well

as to the sensor electrodes via spring loaded pins [15].

The heater assembly consists of a hot plate and a k – type thermo-

couple inside the chamber in order to control the operating temperature of

the sensor. The thermocouple senses the temperature at the surface of the

film exposed to the analyte gas. The PC – interfaced multi meter, of type

UNI-T UT81B, is used to register the variation of the sensor conductance

(reciprocal of resistance) exposed to predetermined air – hydrogen gas

mixing ratio. The chamber can be evacuated using a rotary pump to a

rough vacuum of 1 10−3 ba . A gas mixing manifold is incorporated to

Photo plate 3.2.: A photo of the sensor testing system

Testing Chamber

H2 gas

Vacuum Pump

Air Supply

Temp Controller

DC Power

Supply

UNI-T81 DMM

Gas Flow

Meter

Gas Needle

Valve

Pressure Gauge

UNI-T81 DMM

89

control the mixing ratios of the test and carrier gases prior to being inject-

ed into the test chamber. The mixing gas manifold is fed by zero air and

test gas through a flow meter and needle valve arrangement. This ar-

rangement of mixing scheme is done to ensure that the gas mixture enter-

ing the test chamber is premixed thereby giving the real sensitivity.

3.8 Sensor testing protocol

The following is the protocol used in the operation of the test set-

up.

The test chamber is opened and the sensor placed on the heater.

The necessary electrical connections between the pin feed through

and the sensor spring loaded pins and the thermocouple are made.

Doing so, the test chamber is closed.

Then, the rotary pump is switched on to evacuate the test chamber

to approximately 1 10−3 ba . Setting the sensor desired operat-

ing temperature is done using the PID temperature controller.

After that, using the needle valves the flow rate of the carrier and

test gases flow meters is adjusted.

Next, the gas of known concentration in mixing chamber is al-

lowed to flow to the test chamber by opening the two-way valve.

Measurement of the current variation of the sensor for the known

concentration of test gas mixing ratio is observed by the PC – inter-

faced digital multimeter DMM.

After the measurement, the needle valve of the test gas is closed to

allow the sensor to recover to the base line current value I0.

90

The above measurements are repeated for the other required tem-

peratures and/or concentrations of the test gas.

The process of achieving a known concentration of test gas for

measurement is described below:

The flow meter connected to zero air cylinder is set to a known value (say

1000 sccm) using the needle valve. Then, the flow meter connected to the

test gas is set to the required value to achieve the desired concentration.

For example if 1000 ppm (0.1%) of test gas is required, the flow rate of

test gas is set to 1 sccm while keeping the flow rate of zero air at 1000

sccm.

A schematic diagram of the electrical circuit used for gas sensor

measurements is illustrated in figure 3.6. When the sensor is connected as

shown in the basic circuit, output across the load resistor (VRL) increas-

es/decreases as the sensor's resistance (RS) decreases/increases, depend-

ing on the analyte gas concentration and its type; i.e., whether it is a re-

ducing or oxidizing gas. A DC power supply feeds an adjustable bias

voltage Vb from 0 to 15 volts across the sensor resistance RS and the cor-

responding current of the circuit is measured via the digital multimeter

DMM whose signal is directly being interfaced to the PC for further anal-

ysis. The hot plate heater of the sensor is supplied by a 220 V A.C voltage

controlled by the GEMO DT109 PID temperature controller (not shown

in the figure) together with its k – type (nickel-chromium) thermocouple.

The same above electrical circuit was exploited to investigate the I – V

characteristic of the sensitive sensing material at various temperatures

both in pure air and in gas – containing atmosphere.

91

3.9 Crystalline structure of the prepared ZnO thin films

The crystalline structure is analyzed by a SHIMADZU 6000 X-ray

diffractometer (illustrated in photo plate 3.3) using Cu K𝛼 radiation

(1.5406 Å) in reflection geometry. A proportional counter with an operat-

ing voltage of 40 kV and a current of 30 mA is used. XRD patterns are

recorded at a scanning rate of 0.08333° s-1

in the 2𝜃 ranges from 20° to

60°.

PC – interfaced

DMM

Figure 3.6: A schematic diagram of the gas sensor basic measurement elec-

trical circuit.

RL

RS RH

A

220 V AC DC

Power

Supply

0 -15 V

Gas

Vb

92

3.10 Thin film surface topography

The surface topography is analyzed with Ultra 55 scanning electron

microscope SEM from ZEISS, with its photo plate 3.4 illustrated below.

Also, it is employed for thin film thickness measurement.

The morphological surface analysis is carried out employing an

atomic force microscope, AFM, (AA3000 Scanning Probe Microscope

SPM, tip NSC35/AIBS) shown in photo plate 3.5, from Angstrom Ad-

vance Inc.

3.11 Optical properties

The optical properties are examined via Optima sp-3000 plus UV-

Vis-NIR (Split-beam Optics, Dual detectors) spectrophotometer equipped

with a xenon lamp. Photo plate 3.6 illustrates this spectrophotometer.

Photo plate 3.3.: LabX XRD – 6000 Shimadzu diffractometer unit.

93

Photo plate 3.4.: Ultra 55 SEM unit from ZEISS.

Photo plate 3.5.: AA3000 Scanning Probe Microscope SPM, tip NSC35/AIBS), from Ang-

strom Advance Inc.

94

3.12 Tin oxide (SnO2) hydrogen gas sensors

In addition to the zinc oxide – based hydrogen gas sensor, and for

comparison purposes, undoped and palladium doped tin oxide thin films

have been prepared on glass substrates using the same deposition/doping

procedures described previously for ZnO thin films. The 0.2 – M concen-

tration spraying precursor is obtained by dissolving 4.5126 g stannous

chloride dihydrate SnCl2.2H2O (molecular weight 225.63 g/mole) solute

in 100 mL isopropyl alcohol C3H9O solvent. Likewise, the dissolution

process is facilitated by a magnetic stirrer for10 minutes. The structural,

optical and sensing properties of the prepared films are studied to reduc-

ing hydrogen gas environments at different operating temperatures and H2

gas mixing ratios.

Photo plate 3.6.: Optima sp-3000 plus UV-Vis-NIR spectrophotometer.

95

Chapter 4

Results and discussion

Introduction

In the preceding chapter, experimental setup and methods are de-

scribed. In this chapter, the detailed experimental results and sensing

performance characteristics of the ZnO and SnO2 thin films to hydrogen

gas will be presented.

Some important factors of sensor characteristics will be investi-

gated here. These include sensitivity, response and recovery time, the

optimum operating temperature, and gas concentration. Other characteris-

tics that we didn't measure such as selectivity, stability, repeatability,

resolution, and hysteresis, will not be covered.

We start with a comprehensive outline of ZnO thin film deposition,

its crystalline structure; optical and electrical properties. Surface mor-

phology characteristics will be examined as well. The sensitivity, transi-

ent response, and temperature effects will be analyzed in details for both

metal oxides sensing elements. At the end, results discussion of the sen-

sor performance will be outlined.

4.1 ZnO thin film deposition

Initially, zinc oxide thin films are obtained by spray pyrolytic de-

composition of 0.1 M zinc chloride ZnCl2 aqueous solution precursor.

Later on, 0.2 M precursor of zinc acetate dehydrate Zn(CH3COO)2.2H2O

dissolved in water, organic solvent, and mixture of both, are also used in

the realization of zinc oxide thin films. Spraying temperature, the crucial

parameter, is varied between 370 and 500 oC with the optimum spraying

temperature being around 450 oC. Table 4.1 outlines the optimum deposi-

tion conditions employed in the current research.

96

Figure 4.1: A photo of spray pyrolyzed ZnO thin film on glass samples

Zinc chloride aqueous precursor

Zinc acetate aqueous precursor

Figure 4.1 illustrates a zinc oxide thin film photos of 0.2 M zinc chloride

and zinc acetate dehydrate aqueous precursors sprayed on glass substrates

at 450 0C temperature.

The above ZnO thin films have a thickness of the order of 950 –

1200 nm as estimated by the weighing method and verified in figure 4.2

with cross sectional view of the scanning electron microscope SEM im-

age from which film thickness is estimated to be 1541 nm. Moreover,

film thickness is calculated using an interference method, a procedure

Spray parameters Values

Concentration of precursor 0.2 M

Volume of precursor sprayed ~100 mL

Solvent Isopropyl Alcohol

Substrate temperature 450 0C

Spray rate ~2.3 mL/min.

Carrier gas pressure 1 bar

Nozzle – substrate distance 30 cm

Table 4.1: spray pyrolysis deposition optimum parameters.

97

developed for calculating the thickness of thin films is given elsewhere

[108].

Two masks of different fingers spacing (1 mm and 0.4 mm) were

used to evaporate the Al interdigitated electrodes IDE and figure 4.3

Figure 4.3: Enlarged photos of Al interdigitated electrodes IDE evaporated on ZnO

thin film sample. A: 1 – mm finger spacing IDE on glass, and B: 0.4 – mm finger

spacing IDE on silicon

A B

Figure 4.2: Scanning Electron Micrograph photo of spray pyrolyzed ZnO thin

film on glass

98

shows two of these electrodes after being evaporated over the ZnO thin

film layer deposited on glass and silicon substrates.

4.2 Crystalline structural properties of the ZnO thin film

The structure and lattice parameters of undoped and Pd – doped

ZnO films are analyzed by a LabX XRD 6000 SHIMADZU XR – Dif-

fractometer with Cu Kα radiation (wavelength 1.54059 Å, voltage 30 kV,

current 15 mA, scanning speed = 4 °/min) as illustrated in figure 4.4 and

the effect of the Pd dopant on the structure of the film is displayed in

figure 4.5. Diffraction pattern spectra are obtained with 2𝜃 starting from

20 ° to 50 ° at 6 ° glancing angle. In both the as – deposited and Pd –

doped ZnO thin films, the X – ray diffraction spectra possess one sharp

and three small peaks. It means that the film is polycrystalline with crys-

tal planes (100), (002), (101) and (102). The film is crystallized in the

hexagonal wurtzite phase and presents a preferential orientation along the

c – axis indicated by the plane (002). The result is in a good agreement

with data mentioned in the literature (JCPDF card no 36-1451) [109]. The

strongest peak, observed at 2𝜃 = 34.3646 ° (d = 0.260 nm), can be at-

tributed to the (002) plane of the hexagonal ZnO. Another major orienta-

tion present is (101) observed at 2𝜃 = 36.1732 °. The other orientations

like (100) and (102) at 2𝜃 = 31.7013 ° and 47.4424 °, respectively are

also seen with comparatively lower intensities. Therefore, the crystallites

are highly oriented with their c – axes perpendicular to the plane of the

substrate. It is worth to mention here that a small intensity peak appears

in the Pd – doped film at 2𝜃 = 40.32 o which belongs to the plane (111) of

the palladium. The lattice constants: a = 3.24982 Å, c = 5.20661 Å. The c

– axis lattice constant of the ZnO thin film was calculated from XRD data

as 5.20 nm. This value is consistent with the one obtained by Gumu et al

[66]. The (002) peak full width at half maximum (FWHM) is 0.1958 0.

99

0.1958 °., while 2𝜃and d values are given in Tables 4.2 and 4.3, respec-

tively.

0

500

1000

1500

2000

2500

20 25 30 35 40 45 50

I [

CP

S]

Theta - 2Theta [Degree]

(002)

(101)

(102) (100)

XRD 6000 SHIMADZU XR-Diffractometer

Figure 4.4: XRD crystal structure of as deposited ZnO thin film (thickness =1178 nm)

prepared from 0.1 M Zinc Chloride aqueous precursor on glass substrate.

0

200

400

600

800

1000

1200

1400

1600

1800

20 25 30 35 40 45 50

I [

CP

S]

Theta - 2Theta [Degree]

(101)

(002)

(100) (102)

XRD 6000 SHIMADZU XR-Diffractometer

Pd (111)

Figure 4.5: XRD crystal structure of Pd – doped ZnO thin film (thickness =1178 nm)

prepared from 0.1 M Zinc Chloride aqueous precursor on glass substrate.

100

4.3 Surface topography and morphology studies

Figure 4.6 (a) shows the surface micrograph of zinc oxide film

prepared at 400 0C which consists of a uniform distribution of spherical –

shaped nanostructure grains with a diameter of about 20 nm. This struc-

ture repeats throughout the materials with closely packed to each other

indicating good adhesiveness of film with the substrate. The grains size

seen is comparable with the value calculated from x-ray diffraction stud-

ies. Al-Hardan et al., had a uniform distribution of RF – sputtered ZnO

nanostructure grains with a similar average grain size diameter [12]. Film

prepared at 200 0C spraying temperature, displayed in the inset (b) of

Peak No. 2Theta

deg.

dExp.

dTheo I/I1 FWHM

deg.

Intensity

counts

Integrated

Int.

counts

1 31.6946 2.82084 2.857884 8 0.179 104 854

2 34.383 2.60618 2.65 100 0.1958 1355 8020

3 36.1701 2.48141 2.515484 13 0.2329 170 1287

4 47.4654 1.91393 1.943173 6 0.2588 82 578

Peak

No.

2Theta

deg. dTheo Å dExp. Å I/I1

FWHM

Deg.

Intensity

counts

Integrated

Int.

counts

1 31.7013 2.8578838 2.82026 5 0.179 56 379

2 34.3646 2.65 2.60754 100 0.1958 1166 6624

3 36.1732 2.5154837 2.4812 11 0.2329 131 938

4 47.4424 1.9431734 1.9148 7 0.2588 82 630

Table 4.2: Crystalline structure, Miller indices and d spacings of the as – deposited

ZnO crystal planes.

Table 4.3: Crystalline structure, Miller indices and d spacings of the Pd – doped ZnO

crystal planes.

101

figure 4.6, demonstrates a discontinuous nature. E. Arca et al. believe that

at low temperature the droplet splashes onto the substrate with lesser

decomposition which leads to porous and less adhesive film which could

sometimes be observed visually [71].

The surface morphology of the undoped ZnO films as observed

from the AFM micrograph (figures 4.7 and 4.8) confirms that the grains

are uniformly distributed within the scanning area (5 μm Χ 5 μm), with

individual columnar grains extending upwards. This surface characteristic

is important for applications such as gas sensors and catalysts [101]. It

was found that using isopropyl alcohol organic solvent other than water is

preferred. This is due to a better droplet size distribution and, also, due to

additional heat transfer toward the sample surface resulted from alcohol

burning [71]. The root mean square (rms) of the film surface roughness

deposited at 450 0C using precursor of zinc acetate dissolved in distilled

Figure 4.6: Scanning Electron Micrograph of ZnO film prepared at a) 400 0C and the

inset b) 200 0C

a

b

102

CSPM Imager Surface Roughness Analysis Image size: 20000.00*20000.00 nm Amplitude parameters: Sa(Roughness Average) 31.1 [nm] Sq(Root Mean Square) 39.2 [nm] Ssk(Surface Skewness) 0.101 Sku(Surface Kurtosis) 3.12 Sy(Peak – Peak) 304 [nm] Sz(Ten Point Height) 293 [nm]

CSPM Imager Surface Roughness Analysis Image size: 10000.00*10000.00 nm Amplitude parameters: Sa(Roughness Average) 31.4 [nm] Sq(Root Mean Square) 39.9 [nm] Ssk(Surface Skewness) 0.0412 Sku(Surface Kurtosis) 3.15 Sy(Peak – Peak) 283 [nm] Sz(Ten Point Height) 267 [nm]

CSPM Imager Surface Roughness Analysis

Image size: 2000.00*2000.00 nm Amplitude parameters: Sa(Roughness Average) 13.4 [nm] Sq(Root Mean Square) 17 [nm] Ssk(Surface Skewness) -0.366 Sku(Surface Kurtosis) 3.17 Sy(Peak – Peak) 108 [nm] Sz(Ten Point Height) 105 [nm]

Figure 4.7: Scanning Probe Microscope images of zinc oxide thin film spray pyrolysed on

glass substrate at 450 oC spraying temperature with the precursor of 0.2 M zinc acetate

dissolved in 100 mL distilled water.

103


Image size: 20000.00*20000.00 nm Amplitude parameters: Sa(Roughness Average) 15 [nm] Sq(Root Mean Square) 25.8 [nm] Ssk(Surface Skewness) 0.766 Sku(Surface Kurtosis) 9.04 Sy(Peak – Peak) 223 [nm] Sz(Ten Point Height) 223 [nm]


Image size: 5000.00*5000.00 nm Amplitude parameters: Sa(Roughness Average) 1.57 [nm] Sq(Root Mean Square) 2.21 [nm] Ssk(Surface Skewness) 1.35 Sku(Surface Kurtosis) 10.3 Sy(Peak – Peak) 34.7 [nm] Sz(Ten Point Height) 33.4 [nm]


Image size: 2000.00*2000.00 nm Amplitude parameters: Sa(Roughness Average) 15 [nm] Sq(Root Mean Square) 25.8 [nm] Ssk(Surface Skewness) 0.766 Sku(Surface Kurtosis) 9.04 Sy(Peak – Peak) 223 [nm] Sz(Ten Point Height) 223 [nm]

Figure 4.8: Scanning Probe Microscope images of zinc oxide thin film spray pyrolysed on

glass substrate at 450 oC spraying temperature with the precursor of 0.2 M zinc acetate

dissolved in 100 mL isopropyl alcohol.

104

water is about 17 nm, indicating that the surface of the spray deposited

ZnO thin film is very smooth. This value increases to 25.8 nm and the

grain size decreases from 250 to 65 nm as zinc acetate is dissolved in

isopropyl alcohol organic solvent.

The higher nucleation with a lower growth rate, results in a fine

grains of the films. Further, the grain size of the film can also be deduced

from the AFM micrograph and the distribution of grain size value is ob-

served between a minimum of about 20 nm and a maximum of about 130

nm as illustrated in figure 4.9. The statistical mean grain size of the film

deposited at 450 °C is about 57.76 nm, which is a little bit higher than the

crystallite size calculated from the XRD profile (49.15211 nm).

0

20

40

60

80

100

120

0

20

40

60

80

100

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190

Cum

ula

tio

n

%

Per

centa

ge

%

Diameter nm

Granularity Cumulation Distribution Chart

Sample: ZnO_01 Code: 009 Line No.: lineno Grain No.:1072 Instrument: CSPM Date: 2011-03-29

Avg. Diameter: 57.76 nm <=10% Diameter: 20.00 nm <=50% Diameter: 50.00 nm <=90% Diameter: 100.00 nm

Figure 4.9: Granularity cumulation distribution report of ZnO thin film deposited at 450 0C on glass substrate using 0.2 M zinc acetate in distilled water precursor solution.

105

4.4 Optical properties

Figure 4.10 shows the optical transmittance spectra of the ZnO thin

films. Approximately, all the films demonstrate more than 60% transmit-

tance at wavelengths longer than 400 nm, which is comparable with the

values for the ZnO thin films deposited by Ju-Hyun Jeong [69] using

electrostatic spray deposition ESD method, P. P. Sahay [80] using SPD

method, B. J. Babu [81] by ultrasonic spraying USP scheme. Below 400

nm there is a sharp fall in the T% of the films, which is due to the strong

absorbance of the films in this region. It has been observed that the over-

all T% increases with the decrease in the film thickness. This happens

due to the overall decrease in the absorbance with the decrease in film

thickness [80]. The relationship between the morphology of the sample

and the solvent composition is straightforward. A smooth, homogeneous,

good quality layer can be obtained by just using organic solvents (isopro-

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

200 300 400 500 600 700 800 900

Tra

nsm

issi

on

Wavelength nm

613 nm

523 nm 279 nm

189 nm

Figure 4.10: Transmission spectra of ZnO thin films of different thicknesses sprayed

on – glass at 400 0C temperature. The precursor was 0.2 M zinc acetate dissolved in

distilled water except for the 189.34 – nm thick sample which was a 0.2 M dis-

solved in 3:1 volume ratio isopropyl alcohol and distilled water.

106

pyl alcohol or methanol) while the surface gets rougher with increasing

water content as it has experimentally been noticed during spray.

The morphology of the film has a direct influence on the optical

properties of the coating (figure 4.10). Increasing water content leads to a

significant decrease in the transparency of the film. It is also worth noting

that the growth rate (related to the film thickness) increases with the wa-

ter content. This has already been observed in the literature in the case of

SnO2 [110]. Nevertheless the above mentioned decrease in transmission

is not caused by the thickness of the sample, but it is a consequence of the

scattering losses at the rough surface [71].

The absorption spectra of the films are shown in figure 4.11. These

spectra reveal that films grown under the same parametric conditions

have low absorbance in the visible/ near infrared region while absorbance

is high in the ultraviolet region.

The absorption coefficient (α) was calculated using Lambert law as

0

0.5

1

1.5

2

2.5

200 300 400 500 600 700 800 900

Ab

sorb

ance

Wavelength nm

523 nm

613 nm

279 nm

189 nm

Figure 4.11: Absorption spectra of ZnO thin films of different thicknesses sprayed on –

glass at 400 0C temperature. The precursor was 0.2 M zinc acetate dissolved in distilled

water.

107

follows [111]:

log 0 = d log e → 2.30258 A = d (4.1)

where I0 and I are the intensities of the incident and transmitted light

respectively, A is the optical absorbance and d is the film thickness.

The absorption coefficient (α) was found to follow the relation

h = A (h − g )1 2⁄ (4.2)

where A is a constant and Eg is the optical energy gap. Plots of (αhυ)2

versus the photon energy (hυ) in the absorption region near the funda-

mental absorption edge indicate direct allowed transition in the film mate-

rial [66], as shown in figure 4.12. Extrapolating the straight line portion

of the plot (αhυ)2 versus (hυ) for zero absorption coefficient value gives

the optical band gap (Eg), and its dependence on film thickness t is illus-

trated in figure 4.13. The band gap of the films varied slightly between

3.21 eV to 3.224 eV as the film thickness is changed from 189 nm to 613

0

2

4

6

8

10

12

14

16

2 2.5 3 3.5 4

(αh

ν)2

cm-2

. eV

2

Χ

10

10

hν eV

Figure 4.12: Plots of (αhν)2 vs. photon energy hν for ZnO thin films of different

energy gaps Eg and thicknesses t.

Eg= 3.210 eV, t=613 nm

Eg= 3.216 eV, t=523 nm

Eg= 3.220 eV, t=279 nm

Eg= 3.224 eV, t=189 nm

108

nm. As it is obvious from the plot above, the variation of thickness t for

the sprayed ZnO thin films has no effect on the estimated values of Eg.

4.5 Electrical properties

4.5.1 Resistance – temperature characteristic

The film is initially tested to confirm its semi conducting behavior.

The sensor is placed on a heater and its resistance is measured as the

temperature is ramped up from 50 0C to 350

0C in the dry air atmosphere.

Figure 4.14 shows the variation of resistance of the spray – pyrolyzed

deposited zinc oxide films of 668 nm film thickness with temperature.

The variation of the resistance with the temperature reveals that resistance

of the film decreases as the temperature increases from room temperature

3.15

3.2

3.25

100 200 300 400 500 600 700

Ener

gy g

ap E

g

eV

Film thickness t nm

Figure 4.13: Relationship of the extrapolated energy gap Eg of sprayed ZnO thin

films at different film thicknesses.

109

to 200 0C showing a typical negative temperature coefficient of resistance

(NTCR) due to thermal excitation of the charge carriers in semiconductor

[104]. Above 240 0C, sensor film displays positive temperature coeffi-

cient of resistance (PTCR) as temperature increases further, which may

be due to the saturation of the conduction band with electrons promoted

from shallow donor levels caused by oxygen vacancies. At this point an

increase in temperature leads to a decrease in electron mobility and a

subsequent increase in resistance. Similar observations are made by other

research groups [104, 18]. According to Al-Hardan et. al., [12] below 150

0C temperature, oxygen adsorption at the surface is mainly in the form of

O2− , while above 150

0C, chemisorbed oxygen is present in the form of

O− or O

−2. Due to the conversion of O2

− into O− or O

−2, oxygen adsorbs

the additional electron from the zinc oxide, which is attributed to increase

in the resistance of the sensor film as temperature rises further. At tem-

perature higher than 275 0C up to 300

0C, the film resistance is not greatly

affected by the temperature variation, probably due to the equilibrium

0

100

200

300

400

500

600

700

800

900

1000

0 50 100 150 200 250 300 350 400

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

0.02

Res

ista

nce

kΩ

Temperature C

Co

nd

uct

ance

S

Figure 4.14: The variation of resistance of the spray – pyrolyzed deposited zinc

oxide film of 668 nm film thickness with temperature.

110

obtained between the two competing processes: thermal excitation of

electrons and the oxygen adsorption. Finally, at temperature higher than

300 0C the resistance decreases again, probably because of the dominant

excitation of electrons and desorption of electron species [12]. The tem-

perature range 200 – 240 0C is suitable for sensor operation due to the

small temperature dependence of the sensor [12].

4.5.2 I – V characteristic of the zinc oxide films

I – V characteristics of nanostructured ZnO films are shown in fig-

ure 4.15. Both dark current and current under illumination increase linear-

ly for both positive and negative applied bias voltages up to ±12 V. How-

ever, when observations are made in vacuum, the dark current increases

due to decrease in resistance [112], figure 4.16.

In air, the dark base line current decreases due to increase in re-

sistance. The resistance variation in air is attributed to the effect of oxy-

gen chemisorption.

-10

-8

-6

-4

-2

0

2

4

6

8

10

-14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12 14

Cu

rren

t μ

A

Bias Voltage V

UV - illuminated

Dark

Figure 4.15: The I–V characteristic at room temperature of undoped ZnO film of

1178 - nm thickness in dark and under UV illumination.

111

It is generally accepted that oxygen is chemiadsorbed at a surface

site such as oxygen vacancy in the form of an ionized oxygen atom or

molecule, i.e. O− or O2−, resulting in a reduced concentration of free elec-

trons at the surface and the observed reduction in the conductivity or dark

current [112, 113].

The effect of temperature on the I – V characteristics is depicted in

figure 4.17. It confirms the enhancement of the current with temperature

from room temperature up to 200 0C. As the temperature is increased,

more electrons have sufficient energy to surmount the barrier height be-

tween the grains.

It can be observed that there is a decrease in the measured current

as the temperature is further raised above 200 0C indicating an increase in

the film’s resistance. This effect is observed in the chemisorption region

at elevated temperatures (250 – 500 0C) [104] where the oxygen is ad-

sorbed at the surface of the metal oxide that enable an electron trapping.

Hence the charge carrier density is reduced which leads to an increase in

the resistance of the ZnO. This reaction can be expressed as follows:

10

15

20

25

30

35

40

0 50 100 150 200 250 300 350 400

Curr

ent

μ

A

Time s

maximum

vacuum

Atmospheric air Atmospheric

air

Vacuum

pump ON

Vacuum

pump OFF

Figure 4.16: The effect of vacuum on base line current of a ZnO thin film at 200 0C

and 10 v bias voltage.

112

1

2O2 + e− → O− (4.3)

where O2 is the adsorbed oxygen molecules, O- is the chemisorbed oxy-

gen and e- is the trapped electrons from the ZnO surface. In this region,

the film resistance shows weak dependence on the temperature, as the

equilibrium is achieved for the thermal excitation of electrons and oxygen

adsorption processes.

4.5.3 AC impedance spectroscopy

Figure 4.18 shows the Cole – Cole plot of the impedance spectrum

of ZnO thin film at room temperature. It was observed that as the temper-

ature of the films increases above room temperature (300 K), the imped-

ance spectra begin to distort and therefore, experimental observations

cannot be carried out above room temperature. The spectrum at room

temperature contains only a single arc, but the arc has a non-zero inter-

section with the real axis in the high frequency region. Also, the center of

-15

-10

-5

0

5

10

15

-12 -8 -4 0 4 8 12

Cu

rren

t

μA

Bias voltage v

36 0C

50 0C

100 0C

200 0C

300 0C300 0C

200 0C

100 0C

50 0C

36 0C

Figure 4.17: The I–V characterization of sprayed ZnO film in the temperature range

from RT to 300 0C.

113

each arc lies below the real axis at a particular angle of depression θ. This

indicates that in our films the relaxation time τ is not single – valued but

is distributed continuously or discretely around a mean τm = ωm-1

(the

ideal case). The angle θ is related to the width of the relaxation time dis-

tribution and as such is an important parameter [80].

The low frequency arc is interpreted as due to the grain boundary

effect and the high frequency arc is attributable to the grain effect, in

agreement with the conventional view. In this experiment, the arc is ob-

served in the low frequency region. This indicates that the electric

transport mechanism is associated with the grain boundaries.

A simple R – C equivalent circuit, shown in the inset of figure

4.18, is used to simulate the impedance spectrum. The values of real and

imaginary components for such a circuit are given by:

Z′ = S +

(1 + ω2C 2

2 ) (4.4)

Z′′ = −ωC

2 2

(1 + ω2C 2

2) (4.5)

0

10000

0 10000 20000 30000

Z''

Ω

Z' Ω

Figure 4.18: The Cole-Cole plot for the impedance spectrum of the films at room tempera-

ture. The inset is the R-C equivalent circuit of the simulation of the impedance spectrum.

RS

RP

CP

114

The values of RS, RP and CP of the circuit are estimated using the

experimental values. These values are fed back in equations 4.4 and 4.5

to evaluate Z’ and Z” for the spectrum. The simulated curve is shown in

the same figure 4.18 (dotted line). A close agreement between the two

shows that a simple circuit shown in the inset of figure 4.18 can be used

to analyze the Cole – Cole plot for impedance spectrum.

4.6 Gas sensing measurements

4.6.1 Sensing characteristics of pure ZnO towards hydrogen gas

The gas sensing characteristics of the as – sprayed ZnO film are

carried out for H2 reducing gas at different mixing ratios and operating

temperatures. A known amount of target gas is introduced after the ohmic

resistance of the sensor material gets stabilized. The recovery

characteristics (when the target gas is withdrawn) are also monitored as a

function of time. Figure 4.19 demonstrates sensor test at 6v bias voltage

and operating temperature of 210 0C. The hydrogen : air mixing ratio is

set at 3%, 2%, 1%, respectively. The ZnO sample is prepared from zinc

30

40

50

60

70

80

90

0 100 200 300 400 500 600

Cu

rren

t μ

A

Time s

3%

H 2%

H 1%

H

Figure 4.19: Sensing behavior of pure ZnO thin film at 6 v bias voltage and 210 0C tem-

perature to traces of H2 reducing gas mixing ratio in air of 3%, 2%, and 1% respectively.

115

acetate of 0.2 M precursor solution sprayed on the aluminum interdigitat-

ed electrodes (IDE) of 1– mm finger spacing and at 450 0C spraying tem-

perature.

The variation of sensor sensitivity S, as estimated using equation

2.30 with test gas mixing ratio C is illustrated in figure 4.20. The figure

displays that the sensitivity of the sensor is linear in the low gas concen-

tration region up to 2%, which benefits an actuator by enabling it to de-

tect different concentrations of combustible gases and organic vapors

[95], whereas, the sensitivity tends to saturate in the high gas concentra-

tion. This may be due to a saturation of adsorption of H2 atoms at the Al

electrode/ZnO nanofilm interface and lack of adsorbed oxygen ions at the

nanofilm surface to react with gas molecules [114]. [101] obtained a con-

sistent behavior on ZnO thin film prepared by thermal oxidation exposed

to H2 gas up to 120 ppm at 400 0C.

40

45

50

55

60

0 0.5 1 1.5 2 2.5 3 3.5

Sen

siti

vit

y

%

Hydrogen : air mixing ratio %

Figure 4.20: The sensitivity dependence of as – deposited ZnO sensor on hydrogen gas

mixing ratio

116

Figure 4.21 exhibits the transient response as a function of H2 gas

concentration for the as deposited, 668 – nm thick, ZnO sensing element

at 210 0C. The sensitivity of the ZnO gas sensor increases as the H2 gas

concentration is increased from 1% (10000 ppm) to 3% (30000 ppm) and

it drops relatively rapidly when the H2 gas is removed , indicating that the

gas sensor has a good response for different H2 concentrations. Besides, it

takes almost the same time for the sensor to reach the maximum sensitivi-

ty for different H2 concentrations. This result is consistent with the con-

clusion for the dominance of operation temperature for the response time

[115].

The response and recovery times of the undoped sensor as a func-

tion of testing gas mixing ratio is illustrated in figure 4.22. Both response

and recovery of the sensor have the same monotonicity behavior as the

hydrogen target gas concentration increases. They both decrease with

increasing hydrogen concentration up to 2% at which the lowest response

and recovery times of 21s and 15 s are observed. Figure 4.23 illustrates a

0

10

20

30

40

50

60

0 50 100 150 200

Sen

siti

vit

y

%

Time s

Figure 4.21: Transient responses of ZnO thin film (668 nm thick) at 210 0C testing

temperature upon exposure to hydrogen gas of mixing ratios of 1%, 2%, and 3%

respectively.

3%

2%

1%

117

comparison of the I – V characteristic curves of the undoped sensor from

0 up to 10 v of 2–v increment both in atmospheric air and in three H2 gas

containing air ambients.

A linear dependence is dominant for the measured maximum cur-

0

20

40

60

80

100

120

140

0

5

10

15

20

25

30

35

0 0.5 1 1.5 2 2.5 3 3.5

Rec

over

y t

ime

s

Res

po

nse

tim

e

s

Hydrogen : air mixing ratio %

Figure 4.22: Response and recovery time of the sensor as a function of testing gas

mixing ratio at a testing temperature of 210 0C and bias voltage of 6 v.

0

1

2

3

4

5

6

7

8

9

0 2 4 6 8 10 12

Max

imu

m c

urr

ent

Imax

m

A

Bias Voltage v

Air

1% H2 3%H

5% H2

Figure 4.23: I - V characteristics of undoped ZnO gas sensor to 5%, 3%, and 1%

Hydrogen gas mixture in air and at 200 degrees temperature

118

rent Imax with its value almost doubled once the reducing gas being inside

the testing chamber.

4.6.2 Sensing characteristics of Pd – doped ZnO towards hydrogen gas

Figure 4.24 shows the switching behavior of the ZnO gas sensor

followed film surface modification with 20 palladium chloride PdCl2

layers. A drastic enhancement in sensor sensitivity is achieved when it is

exposed to 3% hydrogen gas traces in air.

The temperature at which the test is carried out was 200 0C with a

10 – v bias voltage. As can be seen from the figure 4.24, both response

and recovery times ( 𝑒 = | 0 − 10 |) are much faster as compared

to figure 4.19. The response and recovery time values at the level of 90%

and at 3% of H2, are about 3 s and 116 s, respectively.

For the above two gas traces, the sensitivity was 91.5528% and

Figure 4.24 the switching behavior of the Pd – sensitized ZnO thin film maximum con-

ductance to hydrogen of 3% H2:air mixing ratio at 200 0C and bias voltage of 10 v.

0

200

400

600

800

1000

1200

1400

1600

1800

0 100 200 300 400 500 600 700 800

Co

nd

uct

ance

μ

S

Time sec.

Rise time = 3 sec

H2 OFF

H2 OFF

H2 ON

Recovery time = 116 s

H2 ON

119

94.3787% respectively which are comparatively twice as that for the

undoped ZnO sensor. These drastic performance enhancements in the

ZnO based H2 gas sensor are believed to be due to the role of the palladi-

um noble metal surface promotion [43, 116] which lowers the reaction

activation energy and the low grain size ZnO crystallites of the prepared

sensing layer made possible by using organic solvent precursors [71].

4.7 Operation temperature of the sensor

One of the most important disadvantages of ZnO gas sensors is the

high temperature required for the sensor operation (200 – 500 ºC). For

that reason, the effect of the operation temperature on the thin films sensi-

tivity was studied with the aim of optimizing the operation temperature to

the lowest possible value.

The gas sensitivity tests performed at room temperature show no

variation on the film conductivity, even with the increase of the gas con-

centration. The increase in the operation temperature leads to an im-

provement of the films sensitivity. Figure 4.25 illustrates the results of

how the maximum conductance Gmax depends on the temperature T for

0

500

1000

1500

2000

2500

3000

3500

0 50 100 150 200 250 300 350 400

Max

imu

m C

on

du

ctan

ce G

max

μS

Temperature T 0C

Figure 4.25: Effect of the testing temperature on the Pd – sensitized ZnO thin film

maximum conductance to hydrogen of 3% H2:air mixing ratio and bias voltage of

10 v.

120

hydrogen sensor based on surface – promoted, with Palladium, ZnO sens-

ing layer of about 1178 nm thickness. It is seen that the film maximum

conductance Gmax goes through a maximum on changing T, with the best

operating temperature at around 280 ºC. Roughly speaking, the increase

of Gmax (the left side of the maximum) results from an increase in the rate

of surface reaction of the target gas, while the decrease of Gmax (the right

side) results from a decrease in the utility of the gas sensing layer. At the

temperature of the maximum conductance (response), the target gas mol-

ecules have optimum penetration depth into the gas sensing grains (large

utility) i.e., optimum reactivity for the diffusion in the whole sensing

layer, as well as for exerting sufficiently large interaction with the surface

(large gas response coefficient). This explains qualitatively why the corre-

lations between Gmax and T take a volcano shape for semiconductor metal

oxide gas sensors [48].

It is evident from the figure that the ZnO film shows a negative

temperature coefficient of resistance (NTCR) up to ~ 270 oC, whereas

above 285 oC it shows a positive temperature coefficient (PTCR). Similar

behavior was obtained by other researchers [18, 101].

The sensitivity of the sensor was calculated after the response had

reached steady state condition as a function of the operating temperature

in the range of 150 to 350 oC with temperature increment of 50

oC. The

test was achieved by recording the change in the current I (and ultimately,

the conductance G) upon exposure to a specific concentration of the hy-

drogen target gas which was kept constant at 3% in atmospheric air. The

variation of sensitivity with the operating temperature is shown in figure

4.26. The sensitivity increased as the operating temperature increased,

reaching a maximum value (~ 97%) at 250 oC, and decreased thereafter

with further increase in the operating temperature. It is suggested that 250

121

oC is the optimum operating temperature for high sensitivity of the sen-

sor.

The sensitivity as well as response time is temperature dependent

since the chemical kinetics governing the solid-gas interface reaction is

temperature dependent [117].

Figure 4.27 illustrates the transient responses of palladium - pro-

moted ZnO thin film (245 nm thick) as exposed to 3% H2: air gas mixing

ratio and at three different testing temperatures of 250, 350, and 300 0C

successively. Here, the surface Pd promotion is achieved by spraying

(rather than dipping) of the palladium chloride PdCl2 solution. The tem-

perature at which PdCl2 solution is sprayed was 400 0C. After 20 sprays,

the Pd – promoted ZnO sample was heat treated for 1 hour in atmospheric

air. As it is obvious from the figure, maximum sensitivity S of 93.01228

% is obtained at a temperature around 300 0C with a comparatively fast

response time of ~ 4 s and a baseline recovery time of 72 s.

60

70

80

90

100

0 50 100 150 200 250 300 350 400

Sen

siti

vit

y

%

Temperature 0C

Figure 4.26: The variation of sensitivity with the operating temperature of the Pd –

doped ZnO gas sensor.

122

There seems to be no noticeable difference in sensing behavior of

the ZnO hydrogen gas sensor as its surface is promoted by either dipping

or spraying. However, it is worth to mention here that the palladium layer

over the ZnO surface, resulted by spraying, looks more homogeneous

than that obtained by dipping.

4.8 Tin oxide (SnO2) hydrogen gas sensor

4.8.1 Crystalline structure and morphology of undoped SnO2 thin film

It is known that tin dioxide SnO2 has a tetragonal rutile crystalline

structure (known in its mineral form as cassiterite) [118]. The unit cell

consists of two metal atoms and four oxygen atoms. Each metal atom is

situated amidst six oxygen atoms which approximately form the corners

of a regular octahedron. Oxygen atoms are surrounded by three tin atoms

which approximate the corners of an equilateral triangle. The lattice pa-

rameters are a= 4.7382 Å, and c= 3.1871 Å. Figure 4.28 shows the X-ray

diffraction (XRD) pattern of the SnO2 thin film prepared on glass sub-

0

20

40

60

80

100

0 50 100

Sen

siti

vit

y

%

Time s

Figure 4.27: Transient responses of Pd – sensitized ZnO thin film (245 nm thick) as

exposed to hydrogen gas of mixing ratio of 3% and at three different testing tempera-

tures of (1) 250, (2) 350, and (3) 300 0C successively.

1

2

3

123

strate at 450 °C spraying temperature. The major diffraction peaks of

some lattice planes can be indexed to the tetragonal unit cell structure of

SnO2 with lattice constants a= 4.738 Å and c= 3.187 Å, which are con-

sistent with the standard values for bulk SnO2 (JCPDS-041-1445) [119].

There are six peaks with 2θ values of 26.72418 0, 34.03094

0, 38.00002

0,

43.45998 0, 51.91576

0 and 54.85798

0 corresponding to SnO2 crystal

planes peaks of (110), (101), (200), (211), (220), and (002) respectively.

No characteristic peaks belonging to other tin oxide crystals or impurities

were detected. In our films, the XRD spectrum showed predomination of

the peaks, corresponding to reflection from the crystallographic (110),

(101) planes, parallel to the substrate. The intensity of other peaks is

small. It indicates that the current films are textured. At that, the degree of

the texturing depends on kind of sprayed solution we used, and increases

while using water solution instead of alcohol solution of SnCl2.

0

20

40

60

80

100

120

140

160

15 20 25 30 35 40 45 50 55 60 65

Inte

nsi

ty

I C

PS

Theta 2 -Theta degrees

(110)

(101)

(200)

(220)

SnO2

(211)

(002)

Figure 4.28: X-ray diffraction (XRD) pattern of SnO2 thin film spray pyrolyzed

on glass substrate at temperature of 450 oC.

124

The high intensity of these peaks suggests that these thin films

mainly consist of the crystalline phase.

The surface morphology of the undoped SnO2 thin films, as re-

vealed by the AFM image, is shown in figure 4.29 on a scanning area of

2000 nm x 2000 nm. The average roughness, Ra of the sample is of the

order of 1.26 nm, whereas the peak – to – valley roughness, RPV takes

value of up to 12.8 nm. This result indicates that the coating surface mor-

Figure 4.29: AFM image of undoped SnO2 thin film deposited at 450 oC on

glass substrate with the precursor being tin dichloride dehydrate dissolved in

isopropyl alcohol.

125

phology of SnO2 thin films is almost perfectly smooth with nanosize

grains. The estimated grain size of undoped films is in the range of 57.6 –

68.8 nm.

4.8.2 Optical properties of the undoped tin oxide SnO2 thin films

Figure 4.30 shows the transmittance spectra obtained at the wave-

length between 300 – 850 nm. The optical transmission depends on the

film thickness. The increase of the film thickness leads to higher absorp-

tion and thus reducing the transmittance. The average visible transmit-

tance calculated in the wavelength ranging 400 – 700 nm varied between

~58 and 80 %.

The calculated values of the direct optical energy gap varied be-

tween 3.49 and 3.79 eV for SnO2 thin films depending on film thickness

as obviously illustrated in figure 4.31. The variations of the optical ener-

gy gap could be attributed to changes in the film defect density.

The band gap decreases with the increase of the film thickness

from 145 nm to 466 nm. The decrease of band gap with the increase of

film thickness implies that SnO2 is an n-type semiconductor [80]. This

0.00%

20.00%

40.00%

60.00%

80.00%

100.00%

200 300 400 500 600 700 800 900

Tran

smis

sio

n

%

Wavelength nm

t=240.294 nm

t=145.633 nm

t=466.024 nm

t=466 nm

t=145 nm

t=240 nm

Figure 4.30: Transmission spectra of undoped SnO2 thin films of different

thicknesses deposited at 450 oC on glass substrates.

126

decrease of band gap may be attributed to the presence of unstructured

defects, which increase the density of localized states in the band gap and

consequently decrease the energy gap [80].

4.8.3 Sensing characteristics of pure SnO2 towards hydrogen gas

The percentage response, S, of the pure SnO2 towards hydrogen

gas of different mixing ratios has been explored. The successive tests

were performed at a bias voltage of 5.1v and a 210 0C operating tempera-

ture. The results are shown in figure 4.32.

As it is apparent from the figure, the sensor sensitivity to hydrogen

gas increases linearly with H2 test gas mixing ratio up to 3% H2 (S~83%)

after which the sensitivity tends to saturate with increasing the analyte

gas. A maximum sensitivity value of 88% has been registered when the

sensor is exposed to 4% H2 gas in air (figure 4.33). Moreover, it’s worth

to indicate here that both the response and recovery times of the undoped

SnO2 gas sensor decrease with increasing the hydrogen gas mixing ratio,

0

5

10

15

20

25

1.5 2 2.5 3 3.5 4 4.5

(αh

ν)2

eV

2 c

m-2

Χ1

01

0

hν eV

Sample 1 thickness t=240.294 nm , Eg=3.76 eV Sample 2 thickness t=145.633 nm , Eg=3.79 eV Sample 3 thickness t=466.024nm , Eg=3.49 eV

Figure 4.31: Absorption coefficient versus the photon energy for energy gap esti-

mation of undoped SnO2 thin films of different thicknesses deposited at 450 oC on

glass substrates.

127

with the shortest response and recovery times being about 29, 127 s re-

spectively at 4% H2:air gas mixing ratio. Both response and recovery

times were measured as 90% of the conductance change ∆ that the

sensor experiences upon the step introduction of the H2 reducing gas.

0

10

20

30

40

50

60

70

80

90

100

0 500 1000 1500

Sen

siti

vity

S

%

Time t s

1% H2

2% H2

3% H2

4% H2

Figure 4.32: Sensitivity behavior of undoped tin oxide SnO2 thin film to different hydro-

gen concentrations. The bias voltage was 5.1 v with the temperature set to 210 0C.

Figure 4.33: Sensitivity versus H2 gas concentration of undoped tin oxide SnO2 thin

film. The bias voltage was 5.1 v with the temperature set to 210 0C.

30

40

50

60

70

80

90

100

0% 1% 2% 3% 4%

Sen

siti

vity

S

%

H2:air mixing ratio C %

128

4.8.4 Sensing characteristics of Pd – doped SnO2 towards hydrogen gas

In a similar way to that done for the ZnO thin film sensing element,

and in order to enhance the sensing characteristics of the tin oxide SnO2 –

thin film based sensors, its surface is promoted by 20 palladium layers

applied by the same spraying method used to prepare the films. Figure

4.34 shows the representative real – time electrical responses of a 145 nm

– thick PdCl2 promoted SnO2sensing element to H2 gas concentrations up

to 4.5% in air. The test was performed at 210 0C sensing temperature and

10 v bias voltage.

A drastic enhancement in sensor sensitivity towards hydrogen gas

is achieved in which, the sensor maximum current increased upon being

exposed to successively incrementing hydrogen gas concentrations. Fig-

0

100

200

300

400

500

600

700

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200

Cu

rren

t μ

A

Time s

3.3% H2 4.5% H2

2% H2

1% H2

0.5% H2

pulse due to H2 remaining in the

tubing of H2 when the manifold is cracked

open; NF is still closed

Current increased upon switching ON of

rotary - from atmosphere to

vacuum

Figure 4.34: Sensing behavior of Pd – doped SnO2 gas sensor to different H2 : air

mixing ratios. The tests were performed at 210 0C temperature and 10 v bias.

129

ure 4.35 exhibits the transient sensitivity S variation with hydrogen gas

concentration.

The relationship of sensor response time versus hydrogen:air mix-

ing ratio is plotted in figure 4.36. The maximum sensitivity obtained is

95.744 % at 4.5% H2 gas concentration in air which is slightly less than

that reported by Mitra [117]. Also, the shortest response time of about 24

s was observed at 2% hydrogen gas concentration. This speed of response

is faster than that obtained by Sunita Mishra et al. [120]

Figure 4.37 shows the switching behavior to 4.5% H2 gas of the

SnO2 gas sensor followed film surface modification with 20 palladium

chloride PdCl2 layers. The response for three operating temperatures is

compared. It is evident from the figure that the sensor sensitivity increas-

es, and its response time decreases, with increasing the operating temper-

ature for the sensor.

Figure 4.35: Response transient of Pd – doped SnO2 gas sensor to different H2 : air

mixing ratios. The tests were performed at 210 degrees temperature and 10 v bias.

0

20

40

60

80

100

0 250 500 750 1000 1250 1500 1750 2000 2250

Sen

siti

vity

%

Time s

0.5% H2

1% H2

2% H2

3.3% H2 4.5% H2

130

Figure 4.37: Transient responses of SnO2 thin film of 248 nm thick at 150, 175, and

210 0C testing temperature upon exposure to 4.5% H2:air gas mixing ratio.

0

20

40

60

80

100

0 100 200 300 400 500

Sen

siti

vity

%

Time s

210 0C

150 0C

175 0C

Figure 4.36: Sensitivity and Response time as a function of the H2 test gas mixing

ratio. The test was performed at 210 0C and 10 v bias on SnO2 sample sprayed over

the IDE and surface coated with 20 PdCl2 layers sprayed at 400 0C over the film.

0

20

40

60

80

100

0

10

20

30

40

50

60

70

80

0 1 2 3 4 5

Sen

siti

vity

%

Res

po

nse

tim

e s

H2 mixing ratio %

131

The optimum operating temperature for the palladium – doped tin

oxide hydrogen gas sensor was found to be around 210 0C. The response

versus temperature plot, figure 4.38, demonstrated a volcano - shape

relationship.

0

100

200

300

400

500

600

700

100 125 150 175 200 225 250 275 300

Max

imu

m c

urr

ent

Im

ax.

μ

A

Temperature T 0C

Figure 4.38: variation of sensor response current with temperature of Pd - doped SnO2

thin film exposed to 4.5% hydrogen gas mixing ratio in air and at 10 v bias voltage.

132

4.7 Conclusions and future work proposals

In this study, the influence of thin film processing conditions on

the properties and gas sensing performance of spray pyrolyzed Pd –

doped ZnO and SnO2 thin films have been investigated.

The spray pyrolysis deposition has proved to be a relatively simple

and reliable deposition technique in acquiring high quality thin films of

different types and at a wide range of deposition temperatures. This

method has the advantages of low cost, easy-to-use, safe and efficient

route to coat large surface areas in mass production.

The spray pyrolyzed ZnO thin films obtained from non – organic

solvent (distilled water) are observed to be comparatively of less trans-

parency, with thickness of about 1178 nm, as measured by weighing

difference method and confirmed via the SEM unit.

The XRD diffraction pattern proves that the prepared ZnO on glass

substrate is highly c – axis oriented, giving a peak at Bragg angle equal to

34.38 o, which belong to the (0 0 2) phase of the hexagonal wurtzite struc-

ture of the ZnO.

The nearly ohmic behavior of I–V characteristics reveals that the

prepared films contain high carrier concentrations.

AFM investigations reveal a porous morphology of spherical parti-

cles. The electrical characterization of the sprayed thin films shows that

they are highly resistive, but that their properties vary considerably when

the measurements are conducted in vacuum or in air.

Both spray pyrolyzed Pd – doped ZnO and SnO2 thin – film sen-

sors demonstrated high sensitivity, relatively fast, and excellent selectivi-

ty to hydrogen reducing gas. Thus, they exhibit an increase in the con-

ductance for exposure to hydrogen gas of different concentrations and

operating temperatures, showing excellent sensitivity. It is found that the

133

sensing of hydrogen gas in our metal oxide sensors is related to the en-

hancement of adsorption of atmospheric oxygen. The excellent selectivity

and the high sensitivity for hydrogen gas can be achieved by surface

modification of ZnO films. The observed conductance change in Pd –

doped ZnO sensors after exposure to H2 gas (3%) is about two times as

large as that in the undoped ZnO sensors.

The variation of the operating temperature of the film leads to a

significant change in the sensitivity of the sensor with an ideal operating

temperature of about 250 ± 25 0C after which sensor sensitivity decreas-

es. The sensitivity of the ZnO thin films changes linearly with the in-

crease of the gas concentration. For tin oxide sensors, the optimum tem-

perature is 210 0C.

The response – recovery time of Pd:ZnO sensing element to hy-

drogen gas is very fast. ZnO thin films of 20 – time dipping (or spraying

with) in palladium chloride solution have the highest sensitivity of 97%

and extremely short response time of 3 s, which fit for practice since it is

crucial to get fast and sensitive gas sensor capable of detecting toxic and

flammable gases well below the lower explosion limit (4% by volume for

H2 gas). The current high sensitivity and the fast response time are com-

parable to that obtained by Mitra et al., [92].

There is a great effort to fabricate hand – held, low power con-

sumption gas sensing elements that possesses high sensitivity, fast re-

sponse and recovery characteristic, and can operate at room temperature.

The heating element in the current ZnO and SnO2 gas sensors is bulky

and requires a 220 v A.C current source, consuming much power. This

issue can be handled by applying a thin film heating element on the back

of the substrate. The thin film is composed of a spraying solution consist-

ed of 100 g SnCl4.5H2O, 4g SbCl3, 6 g ZnCl2, 50 cc H2O, and 10 cc HCl

equivalent to 93.2% SnO2, 5.5% Sb2O3 and 1.3% ZnO. Using this ap-

134

proach, Mochel [64] obtained such thin films of an electrical resistance of

42 ohms per square. This value increased to 56 ohms per square upon

passing an alternating current, equivalent to 300 Watts at 110 v, to the

deposited film. Doing so, the sample temperature increased to 825 0C in

just a few minutes. Seven heating and cooling cycles caused no substan-

tial change in the resistance and other properties.

135

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[120] S. Mishra, C. Ghanshyam, N. Ram, S. Singh, R. P. Bajpai And R. K. Bedi,

Alcohol sensing of tin oxide thin film prepared by sol – gel process, Bull.

Mater. Sci., Vol. 25, No. 3, pp. 231– 234, (2002).

I

راقــــــة العـــــجمهوري

يــــث العلمـــي والبحــــم العالــــوزارة التعلي

داد ــــــــــة بغـــــــــجامع

وم ـــــــــة العلــــــــــــكلي

الزنك وأوكسيد القصدير متحسس أوكسيد حسينت

لغـاز الهايدروجين

أطروحة مقدمة إلى

داد ـة بغـجامع –وم ـكلية العلمجلس

ة ــــوراه فلسفـة دكتـل درجـات نيـزء من متطلبـوهي ج

اء ـفي الفيزي

من قبل

ـالـع حيـقحطــان كـاطـ

9114بكالوريوس علوم في الفيزياء

9111ر علوم في الفيزياء ـــــماجستي

رافــــأش

صالـحوسـن رشيـــد د.

لــن سهيــدهللا محسـعبد.

ميالدي 1199 هجري9441

II

الخالصــــــه

المطعم بالباليديوم على قواعد وأوكسيد القصدير ZnOتم تحضير اغشيه نانويه ألوكسيد الزنك

الكيميائي الحراري وتم تفحصها كمتحسس سريع االستجابه لغاز لتحللزجاجيه باستخدام طريقه ا

450الكيميائي الحراري بدرجه حراره حوالي تحللالهايدروجين المختزل. أستخدمت تقنية ال

وكسيدوأ والهواء الجوي كغاز حامل لتحضير األغشيه المتحسسه ألوكسيد الزنك درجه مئويه

وأسيتات ZnCl2التي طبقت هي كل من أمالح كلوريد الزنك تذريه. كانت محاليل الالقصدير

SnCl2.2H2Oبينما أستخدمت ماده كلوريد القصدير Zn(CH3COO)22H2Oالزنك

بساطتها ومعوليتها في الكيميائي الحراري تحللأثبتت تقنيه ال. للحصول على أوكسيد القصدير

cوبأتجاه على امتداد المحور (002)ل على أغشيه رقيقه متعدده التبلور وذات طور والحص

لتركيب أوكسيد الزنك السداسي كما اظهرته تحليالت التركيب لحيود األشعه السينيه. أظهرت

األغشيه المحضره نفاذيه عاليه عند المدى المرئي للطيف الكهرومغناطيسي وبمعدل وصل الى

نانومترتقريبا. 481وعتبة قطع عند األشعه فوق البنفسجيه ذات الطول الموجي %95حوالي

فجوة الطاقه المباشره المحسوبه لألغشيه بنقصان سمك الغشاء الرقيق. ليس نفاذيه وازدادت ال

القوه الذريه ألغشيه جهرااللكتروني الماسح وم جهرأظهرت نتائج الدراسات السطحيه بالم

اوكسيد الزنك تشكل توزيع منتظم لحبيبات مساميه نانويه التركيب دائريه الشكل ذات اقطار

. بينت الخصائص الكهربائيه لألغشيه الرقيقه المحضره بهذه التقنيه مقاوميتها نانومتر 11بحدود

العاليه وان هذه الخصائص تغيرت تغيرا ملحوظا عند أجراء القياسات في الهواء اوفي الفراغ.

المطعم تحسسيه فائقه حيث أزدادت SnO2و ZnOسا اوكسيد المعدن متحسسكل من ابدى

لغاز الهايدروجين ذا تراكيز متنوعه وعند درجات حراريه مختلفه. المواصله له بتعرضه

لغاز الهيدروجين يتعلق بتحسن امتزاز االوكسجين معادنسيد الاأكمتحسس وجد أن تحسس

الجوي. االنتقائيه الممتازه والتحسسيه العاليه لغاز الهيدروجين يمكن تحقيقها بالمعامله السطحيه

. لقد كان مقدار التغير بالمواصله ألوكسيد الزنك المطعم /أوكسيد القصديرألغشيه اوكسيد الزنك

% حوالي مرتان بقدر مثيلتها للغشاء الغير 4 بالباليديوم بتعرضه لغاز الهايدروجين ذا تركيز

مطعم.ال

أدى تغيير درجة الحرارة التي يعمل عندها متحسس أوكسيد الزنك الى تغير ملحوظ في حساسيته

250يجة الحراره المثلى لألشتغال حوالكانت درحيث ± ت بعدها ضأنخف درجه مئويه 25

حساسية المتحسس. كان تغير التحسسيه لغشاء أوكسيد الزنك خطيا بزياده تركيز الغاز.

III

. نسبيارقصنه مفرط الكواألفاقه لمادة أوكسيد الزنك المطعم بالباليديوم ب –زمن األستجابه يتميز

% وزمن 11كلوريد الباليديوم اعلى تحسسيه قدرها في مره 11لقد اظهرت األغشيه المغطسه

ثانيه وهذا مالئم عمليا طالما أنه يعتبر من االمور الحاسمه الحصول 4استجابه مفرط القصر

على متحسس غازي سريع االستجابه والتحسس له القدره على كشف الغازات الملتهبه والسامه

4راكيز قليله دون الحد االدنى ألنفجار هذه الغازات )لغاز الهيدروجين يكون هذا الحد وعند ت

.)%

درجه مئويه 191ولعناصر التحسس ألوكسيد القصدير فقد كانت درجه حرارة االشتغال المثلى

%. 4.5 لغاز هيدروجين بتركيز قدره % 14.144وبنسبه مئويه للتحسس قدرها

Improvement of ZnO and SnO2 Hydrogen GAs Sensors

Education

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