Improvement of ZnO and SnO2 hydrogen gas sensors

Presenter Qahtan Al-zaidi

Nanotechnology & Optoelectronics Research Group

E-mail: qahtaniliya@yahoo.co.uk Mobile:+009647702981421

Monday, October 31, 2011 Optoelectronics Research Group 1

Baghdad University-College of Science

Department of Physics

Nanotechnology & Optoelectronics Research Group

• The word sensor traces back to the Latin “sentire“ means “to perceive”

• Technically, what is a sensor?

• Chemical gas sensor: A branch of chemical sensing

Chemical sensors mimic the tongue and nose function

SMO gas sensors advantages:

Compact small size

Low cost

Maintenance free

Long life – around 10 years compared to 1-2 years for catalytic/electrochemical types

Feasibility to combine control circuits, signal conditioning

Drawbacks :

• Lack of sensitivity

• Slow response time

• Broad selectivity

• High power consumption

• Life cycle and efficiency of the sensor Monday, October 31, 2011 Optoelectronics Research Group 4

To develop a reliable SMO thin film H2 gas sensor

To maximize the sensing selectivity and responsive by means of noble metal catalytic effect.

To explore the Structural, surface morphology, optical , and electrical properties

To investigate the sensing characterization parameters of the ZnO and SnO2 thin films.

04 sec

Substrate

Sprayer

Holder with stand

Spray cone

Air Nozzle

Substrate heater

Capillary Tube

Compressed Air Tube

Thermocouple

Temperature

Controller

Measuring Cylinder

Ventilation Fan

Solenoid Valve And Timer

Air in

Figure 3.1: Spray pyrolysis experimental set up

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

Zinc chloride aqueous precursor

Zinc acetate aqueous precursor

0.4 mm

13.6 mm

Figure 3.3.: A schematic diagram of the IDE masks utilized in this work.

16.3 cm

8 – pin feed through

Output to vacuum pump

Test gas in

Gas Manifold 2 cm

O –ring seal

Digital Multimeter

Needle Valve

Vacuum gage

Auxiliary inlet

Gas Flow meter

ZnO Sensor

PC – interfaced DMM

Temp. Controller

Exhaust

USB Cable

Air Flow meter

Hydrogen Air

Relief valve

Vacuum Pump

Figure 3.3: Gas sensor testing system

PC – interfaced DMM

220 V AC

DC Power Supply

0 -15 V Gas

ZEISS Ultra 55 SEM unit

20 25 30 35 40 45 50

Theta - 2Theta [Degree]

(102) (100)

XRD 6000 SHIMADZU XR-Diffractometer

Peak No. 2Theta

deg. dExp. Å

dTheo Å I/I1 FWHM

Intensity

counts

Integrate

d 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

20 25 30 35 40 45 50

Theta - 2Theta [Degree]

(100) (102)

XRD 6000 SHIMADZU XR-Diffractometer

Pd (111)

Figure 4.10: Transmission spectra of ZnO thin films of different thicknesses sprayed on – glass at 400 0C temperature. .

200 300 400 500 600 700 800 900

Wavelength nm

613.68 nm

523.586 nm

279.847 nm

189.34 nm

200 300 400 500 600 700 800 900

Wavelength nm

523.586 nm

613.680nm

279.847 nm

189.340 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.

2 2.5 3 3.5 4

hν eV

Figure 4.12: Plots of (αhν)2 vs. photon energy hν for ZnO thin films of different energy gaps and thicknesses.

3.21 eV, 613.68 nm

3.216 eV, 523.586 nm

3.22 eV, 279.847 nm

3.224 eV, 189.34 nm

100 200 300 400 500 600 700

Film thickness t nm

Figure 4.13: Relationship of energy gap Eg of sprayed ZnO thin films with film thickness.

Figure 4.2: Scanning Electron Micrograph photo of spray pyrolyzed ZnO thin film on glass Monday, October 31, 2011 Optoelectronics Research Group 25

Figure 4.6: Scanning Electron Micrograph of ZnO film prepared at a) 400 0C and the inset b) 200 0C

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.

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190

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

0 50 100 150 200 250 300 350 400

Temperature ͦC

Figure 4.14: The variation of resistance of the spray – pyrolyzed deposited zinc oxide film of 668 nm film thickness with temperature.

-14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12 14

Bias Voltage V

UV - illuminated

Figure 4.15: The I–V characteristic in dark and under UV illumination.

0 50 100 150 200 250 300 350 400

Time s

maximum vacuum

Atmospheric air

Atmospheric

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.

-12 -8 -4 0 4 8 12

Bias voltage v

100 0C

200 0C

300 0C300 0C

200 0C

100 0C

Figure 4.17: The I–V characterization of sprayed ZnO film in the temperature range from RT to 300 0C.

0 10000 20000 30000

Figure 4.18: 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.

0 100 200 300 400 500 600

Time s

2% H2 1% H2

Figure 4.19: Sensing behavior of pure ZnO thin film at 6 v bias voltage and 210 0C temperature to traces of H2 reducing gas

mixing ratio in air of 3%, 2%, and 1% respectively.

0 0.5 1 1.5 2 2.5 3 3.5

Hydrogen : air mixing ratio %

Figure 4.20: The sensitivity dependence of as – deposited ZnO sensor on hydrogen gas mixing ratio

0 50 100 150 200

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.

0 0.5 1 1.5 2 2.5 3 3.5

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 2 4 6 8 10 12

Bias Voltage v

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

Figure 4.24 the switching behavior of the Pd – sensitized ZnO thin film maximum conductance to hydrogen of 3% H2:air

mixing ratio at 200 0C and bias voltage of 10 v.

0 100 200 300 400 500 600 700 800

Time sec.

Rise time = 3 sec

H2 OFF

Recovery time = 116 s

trise =6 s

trecovery =3.9 min.

100 150 200 250 300 350 400

Temperature 0C

Variation of max. Conductance with sensor temperature

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. Monday, October 31, 2011 Optoelectronics Research Group 41

0 50 100 150 200 250 300 350 400

Temperature 0C

Figure 4.26: The variation of sensitivity with the operating temperature of the Pd – doped ZnO gas sensor.

0 50 100

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

Hydrogen:Air mixing ratio

Undoped ZnO

Pd - doped ZnO

15 20 25 30 35 40 45 50 55 60 65

Theta 2 -Theta degrees

Figure 4.28: X-ray diffraction (XRD) pattern of SnO2 thin film spray pyrolyzed on glass substrate at temperature of 450 oC.

20.00%

40.00%

60.00%

80.00%

100.00%

200 300 400 500 600 700 800 900

hν eV

t=240.294 nm

t=145.633 nm

t=466.024 nm

Figure 4.30: Transmission spectra of undoped SnO2 thin films of different thicknesses deposited at 450 oC on glass substrates.

1.5 2 2.5 3 3.5 4 4.5

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 estimation of undoped SnO2 thin films of different

thicknesses deposited at 450 oC on glass substrates.

0 500 1000 1500

Time t s

Figure 4.32: Sensitivity behavior of undoped tin oxide SnO2 thin film to different hydrogen concentrations. The bias

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

0% 1% 1% 2% 2% 3% 3% 4% 4% 5%

H2:air mixing ratio C %

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.

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200

Time s

3.3% H2

4.5% 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 -

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.

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.

0 250 500 750 1000 1250 1500 1750 2000 2250

Time s

0.5% H2

3.3% H2

4.5% H2

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 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

H2 mixing ratio %

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 50 100 150 200 250 300 350 400 450 500

Time s

210 0C

150 0C

175 0C

100 125 150 175 200 225 250 275 300

Temperature T oC

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.

1 2 3 4

Hydrogen:Air mixing ratio %

Undoped SnO2

Pd-Doped SnO2

0.01 0.02 0.03 0.04

Hydrogen:Air mixing ratio

Pd - doped SnO2

Pd - doped ZnO

Improvement of ZnO and SnO2 hydrogen gas sensors

Education

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