Improvement of ZnO and SnO2 hydrogen gas sensors
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This document summarizes research presented on nanotechnology and optoelectronics by Qahtan Al-zaidi from the Baghdad University College of Science, Department of Physics, Nanotechnology & Optoelectronics Research Group. The presentation covered the development of tin oxide and zinc oxide thin film gas sensors for hydrogen detection through spray pyrolysis. Characterization of the structural, optical, and electrical properties of the films was discussed along with testing of the sensor response to varying concentrations of hydrogen gas.
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Improvement of ZnO and SnO2 hydrogen gas sensors
- 1. Baghdad University-College of Science Department of Physics Nanotechnology & Optoelectronics Research Group Presenter Qahtan Al-zaidi Nanotechnology & Optoelectronics Research Group E-mail: qahtaniliya@yahoo.co.uk Mobile:+009647702981421 Monday, October 31, 2011 Optoelectronics Research Group 1
- 2. • 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
- 3. Chemical sensors mimic the tongue and nose function Monday, October 31, 2011 Optoelectronics Research Group 3
- 4. 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
- 5. 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. Monday, October 31, 2011 Optoelectronics Research Group 5
- 6. Ventilation Fan Compressed Air Tube Measuring Air Nozzle Cylinder Capillary Tube Sprayer Spray 30 cm cone Holder with stand Solenoid Valve And Timer 04 sec Substrate Temperature Controller Substrate heater Thermocouple Air in Figure 3.1: Spray pyrolysis experimental set up Monday, October 31, 2011 Optoelectronics Research Group 6
- 7. Monday, October 31, 2011 Optoelectronics Research Group 7
- 8. 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 Monday, October 31, 2011 Optoelectronics Research Group 8
- 9. Zinc chloride aqueous precursor Zinc acetate aqueous precursor Monday, October 31, 2011 Optoelectronics Research Group 9
- 10. 15 mm 10 mm 0.4 mm 0.4 mm 2 mm 0.4 mm 13.6 mm 25 mm 2 mm 2 mm 3 mm 3 mm 3 mm 22 mm 3 mm 19 mm 1 mm 14 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
- 11. Vacuum gage Test gas in Auxiliary inlet USB 3 mm Cable 16.3 cm 20 cm PC – interfaced ZnO DMM Sensor O –ring seal Temp. 436 Controller 65 2 cm V A Gas Manifold 450 Gas Output to Flow meter Air vacuum 8 – pin feed through Flow pump meter Needle Valve Hydrogen Air Digital Multimeter Relief Exhaust valve Vacuum Pump Figure 3.3: Gas sensor testing system Monday, October 31, 2011 Optoelectronics Research Group 11
- 12. Monday, October 31, 2011 Optoelectronics Research Group 12
- 13. PC – interfaced DMM A RH RS Vb 220 V AC DC Power Supply Gas 0 -15 V RL Monday, October 31, 2011 Optoelectronics Research Group 13
- 14. Monday, October 31, 2011 Optoelectronics Research Group 14
- 15. Monday, October 31, 2011 Optoelectronics Research Group 15
- 16. ZEISS Ultra 55 SEM unit Monday, October 31, 2011 Optoelectronics Research Group 16
- 17. Monday, October 31, 2011 Optoelectronics Research Group 17
- 18. 2500 XRD 6000 SHIMADZU XR-Diffractometer (002) 2000 1500 I [CPS] 1000 500 (101) (100) (102) 0 20 25 30 35 40 45 50 Theta - 2Theta [Degree] Monday, October 31, 2011 Optoelectronics Research Group 18
- 19. Integrate 2Theta FWHM Intensity Peak No. dExp. Å dTheo Å I/I1 d Int. deg. deg. counts 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 Monday, October 31, 2011 Optoelectronics Research Group 19
- 20. 1800 (002) XRD 6000 SHIMADZU XR-Diffractometer 1600 1400 1200 1000 I [CPS] 800 600 400 (101) Pd (100) 200 (111) (102) 0 20 25 30 35 40 45 50 Theta - 2Theta [Degree] Monday, October 31, 2011 Optoelectronics Research Group 20
- 21. 1 189.34 nm 0.9 0.8 279.847 nm 523.586 nm 0.7 0.6 613.68 nm Transmission 0.5 0.4 0.3 0.2 0.1 0 200 300 400 500 600 700 800 900 Wavelength nm Figure 4.10: Transmission spectra of ZnO thin films of different thicknesses sprayed on – glass at 400 0C temperature. . Monday, October 31, 2011 Optoelectronics Research Group 21
- 22. 2.5 613.680nm 2 523.586 nm 1.5 Absorbance 279.847 nm 1 189.340 nm 0.5 0 200 300 400 500 600 700 800 900 Wavelength 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. Monday, October 31, 2011 Optoelectronics Research Group 22
- 23. 16 3.22 eV, 279.847 nm 14 12 3.216 eV, 523.586 nm 3.224 eV, 189.34 nm 10 Χ1010 (αhν)2 cm-2 . eV2 8 3.21 eV, 613.68 nm 6 4 2 0 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. Monday, October 31, 2011 Optoelectronics Research Group 23
- 24. 3.226 3.224 3.222 Energy gap Eg eV 3.22 3.218 3.216 3.214 3.212 100 200 300 400 500 600 700 Film thickness t nm Figure 4.13: Relationship of energy gap Eg of sprayed ZnO thin films with film thickness. Monday, October 31, 2011 Optoelectronics Research Group 24
- 25. Figure 4.2: Scanning Electron Micrograph photo of spray pyrolyzed ZnO thin film on glass Monday, October 31, 2011 Optoelectronics Research Group 25
- 26. a b Figure 4.6: Scanning Electron Micrograph of ZnO film prepared at a) 400 0C and the inset b) 200 0C Monday, October 31, 2011 Optoelectronics Research Group 26
- 27. Monday, October 31, 2011 Optoelectronics Research Group 27
- 28. Monday, October 31, 2011 Optoelectronics Research Group 28
- 29. 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. 100 120 Granularity Cumulation Distribution Chart 100 80 80 60 Percentage % 60 40 40 20 20 0 0 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 Diameter nm 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 Monday, October 31, 2011 Optoelectronics Research Group 29
- 30. 0.02 1000 0.018 900 0.016 800 0.014 700 0.012 600 Conductance S Resistance kΩ 0.01 500 0.008 400 0.006 300 0.004 200 0.002 100 0 0 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. Monday, October 31, 2011 Optoelectronics Research Group 30
- 31. 10 UV - illuminated 8 Current μA Dark 6 4 2 0 -14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12 14 -2 -4 -6 -8 -10 Bias Voltage V Figure 4.15: The I–V characteristic in dark and under UV illumination. Monday, October 31, 2011 Optoelectronics Research Group 31
- 32. 40 maximum vacuum Vacuum pump OFF 35 30 Vacuum pump ON Current μA 25 Atmospheric air 20 Atmospheric air 15 10 0 50 100 150 200 250 300 350 400 Time s Figure 4.16: The effect of vacuum on base line current of a ZnO thin film at 200 0C and 10 v bias voltage. Monday, October 31, 2011 Optoelectronics Research Group 32
- 33. 25 0 36 0C 36 C 0 50 0C 50 C 15 100 0C100 0C 200 0C200 0C 300 0C 300 0C 5 Current μA -5 -15 -25 -12 -8 -4 0 4 8 12 Bias voltage v Figure 4.17: The I–V characterization of sprayed ZnO film in the temperature range from RT to 300 0C. Monday, October 31, 2011 Optoelectronics Research Group 33
- 34. 10000 RP RS CP Z'' Ω 0 0 10000 20000 30000 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. Monday, October 31, 2011 Optoelectronics Research Group 34
- 35. 90 3% H2 2% H2 1% H2 80 70 Current μA 60 50 40 30 0 100 200 300 400 500 600 Time s 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. Monday, October 31, 2011 Optoelectronics Research Group 35
- 36. 60 55 Sensitivity % 50 45 40 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 Monday, October 31, 2011 Optoelectronics Research Group 36
- 37. 60 3% 2% 50 1% 40 Sensitivity % 30 20 10 0 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. Monday, October 31, 2011 Optoelectronics Research Group 37
- 38. 35 140 30 120 25 100 Response time s Recovery time s 20 80 15 60 10 40 5 20 0 0 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. Monday, October 31, 2011 Optoelectronics Research Group 38
- 39. 9 5% H2 3%H2 8 1% H2 7 6 Maximum current Imax mA Air 5 4 3 2 1 0 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 Monday, October 31, 2011 Optoelectronics Research Group 39
- 40. 1800 H2 OFF H2 OFF 1600 1400 trise =6 s 1200 Rise time = 3 sec Conductance μS 1000 Recovery time = 116 s trecovery =3.9 min. 800 600 400 H2 ON H2 ON 200 0 0 100 200 300 400 500 600 700 800 Time sec. 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. Monday, October 31, 2011 Optoelectronics Research Group 40
- 41. Variation of max. Conductance with sensor temperature 3500 3177 3000 2954 2500 Max. Conductance µS 2000 1857 1687 1500 1000 500 87 0 100 150 200 250 300 350 400 Temperature 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. Monday, October 31, 2011 Optoelectronics Research Group 41
- 42. 100 90 Sensitivity % 80 70 60 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. Monday, October 31, 2011 Optoelectronics Research Group 42
- 43. 100 3 1 80 2 60 Sensitivity % 40 20 0 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% 0 Monday, October 31, 2011 three different testing Optoelectronics (1) 250, (2) 350, and (3) 300 C successively. and at temperatures of Research Group 43
- 44. 100 90 80 70 60 Sensitivity % Undoped ZnO 50 Pd - doped ZnO 40 30 20 10 0 1 2 3 Hydrogen:Air mixing ratio Monday, October 31, 2011 Optoelectronics Research Group 44
- 45. 160 140 (101) 120 (110) 100 Intensity I CPS 80 (211) 60 (200) 40 (220) (002) 20 0 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. Monday, October 31, 2011 Optoelectronics Research Group 45
- 46. Monday, October 31, 2011 Optoelectronics Research Group 46
- 47. 100.00% t=145.633 nm 80.00% t=240.294 nm 60.00% t=466.024 nm Transmission % 40.00% 20.00% 0.00% 200 300 400 500 600 700 800 900 hν eV 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
- 48. 25 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 20 Χ1010 (αhν)2 eV2 cm-2 15 10 5 0 1.5 2 2.5 3 3.5 4 4.5 hν 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
- 49. 100 4% H2 90 3% H2 80 70 2% H2 60 Sensitivity S % 50 1% H2 40 30 20 10 0 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. Monday, October 31, 2011 Optoelectronics Research Group 49
- 50. 100 90 80 Sensitivity S % 70 60 50 40 30 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. Monday, October 31, 2011 Optoelectronics Research Group 50
- 51. 700 4.5% H2 600 pulse due to H2 3.3% H2 remaining in the tubing of H2 when the manifold is cracked open; NF is still closed 500 2% H2 400 1% H2 Current μA Current increased upon switching ON of rotary - 300 from atmosphere to vacuum 200 0.5% H2 100 0 0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 Time s 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
- 52. 100 4.5% H2 3.3% H2 80 2% H2 60 1% H2 Sensitivity % 40 0.5% H2 20 0 0 250 500 750 1000 1250 1500 1750 2000 2250 Time s 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. Monday, October 31, 2011 Optoelectronics Research Group 52
- 53. 80 120 70 100 60 80 50 Response time s Sensitivity % 40 60 30 40 20 20 10 0 0 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 H2 mixing ratio % 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. Monday, October 31, 2011 Optoelectronics Research Group 53
- 54. 100 210 0C 80 175 0C 60 Sensitivity % 40 150 0C 20 0 0 50 100 150 200 250 300 350 400 450 500 Time s 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. Monday, October 31, 2011 Optoelectronics Research Group 54
- 55. 700 600 500 Maximum current Imax. μA 400 300 200 100 0 100 125 150 175 200 225 250 275 300 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. Monday, October 31, 2011 Optoelectronics Research Group 55
- 56. 100 90 80 70 60 Sensitivity % 50 Undoped SnO2 Pd-Doped SnO2 40 30 20 10 0 1 2 3 4 Hydrogen:Air mixing ratio % Monday, October 31, 2011 Optoelectronics Research Group 56
- 57. 100 90 80 70 60 Sensitivity % 50 Pd - doped SnO2 Pd - doped ZnO 40 30 20 10 0 0.01 0.02 0.03 0.04 Hydrogen:Air mixing ratio Monday, October 31, 2011 Optoelectronics Research Group 57
- 58. Monday, October 31, 2011 Optoelectronics Research Group 58
- 59. Monday, October 31, 2011 Optoelectronics Research Group 59