Improvement of ZnO and SnO2 hydrogen gas sensors
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Transcript of Improvement of ZnO and SnO2 hydrogen gas sensors
Presenter Qahtan Al-zaidi
Nanotechnology & Optoelectronics Research Group
E-mail: [email protected] 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
Monday, October 31, 2011 Optoelectronics Research Group 2
Chemical sensors mimic the tongue and nose function
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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.
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04 sec
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
Air in
Figure 3.1: Spray pyrolysis experimental set up
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Monday, October 31, 2011 Optoelectronics Research Group 7
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
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Zinc chloride aqueous precursor
Zinc acetate aqueous precursor
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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 3.3.: A schematic diagram of the IDE masks utilized in this work.
Monday, October 31, 2011 Optoelectronics Research Group 10
20 cm
16.3 cm
8 – pin feed through
Output to vacuum pump
Test gas in
Gas Manifold 2 cm
O –ring seal
Digital Multimeter
V A
Needle Valve
Vacuum gage
3 mm
Auxiliary inlet
43665
450
Gas Flow meter
ZnO Sensor
PC – interfaced DMM
Temp. Controller
Exhaust
USB Cable
Air Flow meter
Hydrogen Air
Relief valve
Vacuum Pump
Figure 3.3: Gas sensor testing system
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Monday, October 31, 2011 Optoelectronics Research Group 12
PC – interfaced DMM
RL
RS RH
A
220 V AC
DC Power Supply
0 -15 V Gas
Vb
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Monday, October 31, 2011 Optoelectronics Research Group 15
ZEISS Ultra 55 SEM unit
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0
500
1000
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I [
CP
S]
Theta - 2Theta [Degree]
(002)
(101)
(102) (100)
XRD 6000 SHIMADZU XR-Diffractometer
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Peak No. 2Theta
deg. dExp. Å
dTheo Å I/I1 FWHM
deg.
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
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I [
CP
S]
Theta - 2Theta [Degree]
(101)
(002)
(100) (102)
XRD 6000 SHIMADZU XR-Diffractometer
Pd (111)
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Figure 4.10: Transmission spectra of ZnO thin films of different thicknesses sprayed on – glass at 400 0C temperature. .
0
0.1
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1
200 300 400 500 600 700 800 900
Tra
nsm
issi
on
Wavelength nm
613.68 nm
523.586 nm
279.847 nm
189.34 nm
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0
0.5
1
1.5
2
2.5
200 300 400 500 600 700 800 900
Ab
sorb
ance
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.
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0
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2 2.5 3 3.5 4
(αh
ν)2
cm
-2 .
eV2
Χ1
01
0
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
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3.212
3.214
3.216
3.218
3.22
3.222
3.224
3.226
100 200 300 400 500 600 700
Ener
gy g
ap E
g
eV
Film thickness t nm
Figure 4.13: Relationship of energy gap Eg of sprayed ZnO thin films with film thickness.
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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
a
b
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Monday, October 31, 2011 Optoelectronics Research Group 28
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
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60
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100
120
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40
60
80
100
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190
Per
cen
tage
%
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
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0.002
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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.
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-8
-6
-4
-2
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4
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8
10
-14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12 14
Cu
rren
t
μA
Bias Voltage V
UV - illuminated
Dark
Figure 4.15: The I–V characteristic in dark and under UV illumination.
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10
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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.
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-25
-15
-5
5
15
25
-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.
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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 temperature. The inset
is the R-C equivalent circuit of the simulation of the impedance spectrum.
RS
RP
CP
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30
40
50
60
70
80
90
0 100 200 300 400 500 600
Cu
rren
t μ
A
Time s
3% H2
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.
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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
Monday, October 31, 2011 Optoelectronics Research Group 36
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%
Monday, October 31, 2011 Optoelectronics Research Group 37
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120
140
0
5
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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.
Monday, October 31, 2011 Optoelectronics Research Group 38
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1
2
3
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5
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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%H2
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
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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.
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800
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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
trise =6 s
trecovery =3.9 min.
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87
1687
2954
3177
1857
0
500
1000
1500
2000
2500
3000
3500
100 150 200 250 300 350 400
Max
. Co
nd
uct
ance
µS
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
60
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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.
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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 temperatures of (1) 250, (2) 350, and (3) 300 0C successively.
1
2
3
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Monday, October 31, 2011 Optoelectronics Research Group 44
0
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100
1 2 3
Sen
siti
vity
%
Hydrogen:Air mixing ratio
Undoped ZnO
Pd - doped ZnO
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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)
(211)
(002)
Figure 4.28: X-ray diffraction (XRD) pattern of SnO2 thin film spray pyrolyzed on glass substrate at temperature of 450 oC.
Monday, October 31, 2011 Optoelectronics Research Group 45
Monday, October 31, 2011 Optoelectronics Research Group 46
0.00%
20.00%
40.00%
60.00%
80.00%
100.00%
200 300 400 500 600 700 800 900
Tran
smis
sio
n
%
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.
Monday, October 31, 2011 Optoelectronics Research Group 47
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 estimation of undoped SnO2 thin films of different
thicknesses deposited at 450 oC on glass substrates.
Monday, October 31, 2011 Optoelectronics Research Group 48
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 hydrogen concentrations. The bias
voltage was 5.1 v with the temperature set to 210 0C.
Monday, October 31, 2011 Optoelectronics Research Group 49
30
40
50
60
70
80
90
100
0% 1% 1% 2% 2% 3% 3% 4% 4% 5%
Sen
siti
vity
S
%
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.
Monday, October 31, 2011 Optoelectronics Research Group 50
0
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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.
Monday, October 31, 2011 Optoelectronics Research Group 51
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
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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
Monday, October 31, 2011 Optoelectronics Research Group 52
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.
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120
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0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Sen
siti
vity
%
Res
po
nse
tim
e s
H2 mixing ratio %
Monday, October 31, 2011 Optoelectronics Research Group 53
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 50 100 150 200 250 300 350 400 450 500
Sen
siti
vity
%
Time s
210 0C
150 0C
175 0C
Monday, October 31, 2011 Optoelectronics Research Group 54
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600
700
100 125 150 175 200 225 250 275 300
Max
imum
curr
ent
Im
ax.
μ
A
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.
Monday, October 31, 2011 Optoelectronics Research Group 55
Monday, October 31, 2011 Optoelectronics Research Group 56
0
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50
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100
1 2 3 4
Sen
siti
vity
%
Hydrogen:Air mixing ratio %
Undoped SnO2
Pd-Doped SnO2
Monday, October 31, 2011 Optoelectronics Research Group 57
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0.01 0.02 0.03 0.04
Sen
siti
vity
%
Hydrogen:Air mixing ratio
Pd - doped SnO2
Pd - doped ZnO
Monday, October 31, 2011 Optoelectronics Research Group 58
Monday, October 31, 2011 Optoelectronics Research Group 59