Improvement of ZnO and SnO2 Hydrogen GAs Sensors

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Improvement of ZnO and SnO2 Hydrogen GAs Sensors

  1. 1. Republic of IraqMinistry of Higher Education& Scientific ResearchUniversity of BaghdadCollege of Science Improvement of ZnO and SnO2 Hydrogen Gas Sensors A thesis Submitted to the Committee of College of Science, University of Baghdad 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 ByDr. Abdulla M. Suhail Dr. Wasan R. Saleh2011 A D 1432A H I
  2. 2. Supervisor certification We certify that this thesis was prepared by Mr. Qahtan Ghatih Hialunder our supervision at the Physics Department, College of Science,University of Baghdad as a partial requirement for the degree of doctor ofphilosophy in Physics.Signature: Suhail Signature: WasanName: Abdulla M. Suhail Name: Wasan R. SalehTitle: Assist. Professor Title: Assist. ProfessorAddress: College of Science, Address: College of Science,University of Baghdad University of BaghdadDate: November , 2011 Date: November , 2011 In view of the available recommendation, I forward this thesis fordebate by the Examining Committee.Signature: RaadName: Dr. Raad M. S. Al-HaddadTitle: ProfessorAddress: Collage of Science, University of BaghdadDate: November 29, 2011 II
  3. 3. Examination Committee Certification We certify that we have read the thesis entitled “Improvement of ZnOand SnO2 Hydrogen Gas Sensors” as an examining committee, examined theStudent “Qahtan Ghatih Hial” in its contents, and that in our opinion it meetsthe standard of a thesis for the degree of Doctor of Philosophy in Phys-ics/Optoelectronics. Signature: Raad Title: Professor Name: Dr. Raad M. S. Al-Haddad Date: November 29, 2011 ChairmanSignature:Title: Professor Signature: Emad Title: ProfessorName: Dr. Izzat M. AL-Essa Name: Dr. Emad Kh. Al-ShakarchiDate: November , 2011 Date: November 27, 2011Member MemberSignature:M. B. Q. Signature:BassamTitle: Assist. Professor Title: Assist. ProfessorName: Dr. Mayada Bedry Al-Quzweny Name: Dr. Bassam Ghalib RasheedDate: November 28, 2011 Date: November 29, 2011Member Member Signature: Suhail Signature:WasanTitle: Assist. Professor Title: Assist. ProfessorName: Abdulla M. Suhail Name: Wasan R. SalehDate: November , 2011 Date: November , 2011Supervisor Co-Supervisor Digitally signedApproved by the Dean of college of Science Qahta by Qahtan IliyaSignature:Salih M. Ali DN: cn=Qahtan IliyaTitle: ProfessorName: Dr. Saleh Mahdi AliThe Dean of the College of Science n Iliya Date: 2012.05.19 15:07:50 -0700Date: December 5, 2011 III
  4. 4. ABSTRACT Spray – pyrolyzed palladium – doped metal oxides (zinc oxideZnO 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 temperaturewith 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 techniquehas proven its simplicity and reliability in realizing polycrystalline innature ZnO films which crystallized along the (002) phase with preferen-tial orientation along the c – axis of the ZnO hexagonal wurtzite structureas 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 ultravioletcut – off at approximately 380 nm. The transmission but not the estimateddirect band gap Eg increased with decreasing film thickness. ScanningElectron Microscope SEM and Atomic Force Microscope AFM surfacemorphology studies of the ZnO films reveal a uniform distribution ofporous spherical – shaped nanostructure grains of 20 nm diameter. Theelectrical characterization of the sprayed thin films shows that they arehighly resistive, but that their properties vary considerably when themeasurements are conducted in vacuum or in air. For both ZnO and SnO2 metal oxides, the doped sensor exhibit anincrease of the conductance upon exposure to hydrogen gas of variousconcentrations and at different operating temperatures, showing excellentsensitivity. IV
  5. 5. It was found that the sensing mechanism of hydrogen gas in thepresent metal oxide sensors is mostly related to the enhancement of ad-sorption of atmospheric oxygen. The excellent selectivity and the highsensitivity for hydrogen gas can be achieved by surface promotion ofZnO/SnO2 metal oxide films. The observed conductance change in Pd –doped ZnO sensors after exposure to H2 gas (3%) is about two times aslarge as that in the undoped ZnO sensors. The variation of the operating temperature of the film has led to asignificant change in the sensitivity of the sensor with an ideal operatingtemperature 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 gasis characterized to be relatively extremely short. ZnO thin films of 20 –time dipping in palladium chloride solution have the highest sensitivity of97% and extremely short response time of 3 s, which fit for practice sinceit is crucial to get fast and sensitive gas sensor capable of detecting toxicand flammable gases well below the lower explosion limit (4% by vol-ume for H2 gas). For SnO2 sensing elements, the optimum operating temperature isaround 210 0C and 95.744 % sensitivity to 4.5% H2: air mixing ratio. V
  6. 6. Dedicated toAll Those Who Care… Including… Her VI
  7. 7. Acknowledgments It would be impossible to express my thanks on this page to all those whohave 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 methe opportunity to work on a challenging and interesting project over the past threeyears 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 utmostvaluable 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 forthe 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 materialsupport provided. Also, the great help of the XRD, AFM at the Material Physics &Chemistry Research Establishment labs at the Ministry of Science and Technology areacknowledged. This thesis would not have been possible without their willingness towork 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 skinnedknees and shaken confidence. They read me my first book, and never failed to callwhen 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. VII
  8. 8. Curriculum VitaeJuly 1, 1972 ...................................................................................................Born – Iraq.1994 ..................................................... B.Sc., Physics/Physics – Baghdad University1997........................................ M.Sc., Physics/Laser Technology – Baghdad University2002 – 2007.........................................................Assist. Lecturer – Physics Department2007 – October 31, 2011 .............................. Ph. D. Postgraduate – Physics DepartmentPUBLICATIONSJournal 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. :‫إِنِّي رأَيت أَنَّهُ الَ يَكتُب أَحد كتَابا ً فِي يَومه إِالّ قَال فِي غده‬ َِِ َ ِ ِ ِ ٌ َ ْ ُ َ ".‫"لَو غيِّر هَذا لَكانَ أَحْ سن، ولَو زيد هَذا لَكانَ يُستَحسن، ولَو قُدم هَذا لَكان أَفضل، ولَو تُرك هَذا لَكانَ أَجْ مل‬ َ َ َ َ َ َ ْ َ َ َ ِّ َ َ ْ َ َ َ ِ َ َ َ َ َ ُ .‫وهَذا من أَعظَم العبَر، وهُو دلِيل علَى اِستِيالَء النُقص علَى جُملَة البَشَر‬ ِ ِ َ ِ ِ َ ٌ َ َ ِ ِ ِ ْ ِ َ َ ‫العمـــاد األصفهانـــي‬ Assuming that he’s not dead, Qahtan can best be reached at his “lifetime” email address of: qahtaniliya@yahoo.co.uk Mobile No.: +009647702981421 VIII
  9. 9. Contents PageAbstract ........................................................................................................................ IVDedication .................................................................................................................... VIAcknowledgments...................................................................................................... VIICurriculum Vitae ...................................................................................................... VIIIList of Tables ............................................................................................................. XIIList of Figures ........................................................................................................... XIIIList of Symbols .......................................................................................................... XXChapter 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...................................................................................................... 6Chapter 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 IX
  10. 10. 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 ........................................................................ 65Chapter 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 X
  11. 11. 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 XI
  12. 12. List of TablesTable page1. Table 1.1 ----------------------------------------------------------------------- 5 Examples of application for gas sensors and electronic noses2. Table 2.1 ----------------------------------------------------------------------- 9 Comparison of physisorption and chemisorption3. 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 films8. Table 4.1 --------------------------------------------------------------------- 96 Spray pyrolysis deposition optimum parameters9. Table 4.2 -------------------------------------------------------------------- 100 Crystalline structure, Miller indices and d spacings of the as – deposit- ed ZnO crystal planes10.Table 4.3 -------------------------------------------------------------------- 100 Crystalline structure, Miller indices and d spacings of the Pd – doped ZnO crystal planes. XII
  13. 13. LIST OF FIGURES Figure Page1. 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. XIII
  14. 14. 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 unit21.Figure 2.21 ------------------------------------------------------------------- 62 XIV
  15. 15. 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 XV
  16. 16. 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. XVI
  17. 17. 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 XVII
  18. 18. 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 XVIII
  19. 19. 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. XIX
  20. 20. 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 eVAFM Atomic Force MicroscopeCVD Chemical Vapor Deposition CSP Chemical Spray PyrolysisDMM Digital Multi MeterENC 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 dioxideNTCR Negative Temperature Coefficient of Resistance PID Proportional–Integral–Derivative Controller ppm Parts Per MillionPTCR Positive Temperature Coefficient of Resistance R Resistance (electrical) Ω S Siemenssccm Standard Cubic Centimeter per MinuteSEM Scanning Electron MicroscopeSMO Semiconductor Metal Oxide t90 Time to accomplish 90% of sensor response change sTEM Transmission Electron MicroscopeVOC Volatile Organic CompoundXRD X-Ray Diffraction Z0 Depletion Region nm XX
  21. 21. Chapter 1Motivation and Project ObjectivesIntroduction The purpose of this chapter is to provide a general framework andintroduction for the work presented in the current Ph.D. project. Thischapter 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 inresponse to a specified input stimulus [1]. This stimulus can be a physicalstimulus like temperature and pressure or a concentration of a specificchemical or biochemical material. The output signal is typically an elec-trical signal proportional to the input variable, which is also called themeasurand. Sensors can be used in all three phases of matter although gasand liquid sensors are the most common. The presence of a reducing/oxidizing gas at the surface of certainmetal oxide semiconductors changes its electrical resistance R. It is thisphenomenon that has spurred the use of these materials in the detection ofa gaseous ambient. The theoretical basis for semiconductor gas sensorsarose in 1950, when Carl Wagner proposed a concept to explain the de-composition of nitrous oxide (N2O) on zinc oxide (ZnO) [2]. He made thenovel assumption that an exchange of electrons was taking place betweenthe gaseous N2O and the solid ZnO, which possessed a layer of adsorbedoxygen. A few years later, Brattain et al. found that ambient gas producedchanges in potential between an electrode and a germanium surface [3].These findings were explained in a theory outlining the existence of do- 1
  22. 22. nor and acceptor traps that lead to the generation of a space charge layeron the surface of the germanium. A working gas sensor was realized in1962, 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]. Itwas in that same year that Naoyoshi Taguchi issued a patent for a gassensor based on tin oxide (SnO2) [5]. As such, gas sensors based on SnO2are 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 devicewhose resistance in air is very high and drops when exposed to reducinggases such as combustibles (H2, CO, CH4, C3H8) or volatile organic va-pors and it has enjoyed a substantial popularity because of its ease offabrication, low cost, robustness, and their sensitivity to a large range ofreductive and oxidative gases [7]. In addition to research on understand-ing the fundamentals of the sensing mechanism, the studies on ZnO andSnO2 sensors have been directed on enhancing the sensor performancethrough the addition of noble metals (Pt, Pd, etc.), synthesis of thick andthin film sensors, and doping with other semiconductors [7-10]. Othermetal oxides such as Fe2O3, TiO2, WO3 and Co3O4 have also been used asgas sensors. Despite these broad studies in the semiconductor sensor area,problems such as insufficient gas selectivity, slow response and recoverytimes, inability to detect very low gas concentrations, and degradation ofthe sensor performance by surface contamination still persist. Thus, thereis 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 adepletion layer (for n-type semiconductors), and hence reduces conduc-tivity [11]. Here, ionosorption refers to the process where a species is 2
  23. 23. adsorbed and undergoes a delocalized charge transfer with the metal ox-ide. This can then be used to measure reducing and oxidizing gases, asthey will change the amount of ionosorbed oxygen, and therefore theconductivity 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 lowerresponse time for the gas sensor. However, the physical properties of themetal oxides place an upper limit on the temperatures that can be used. Ifthe temperature is too high, the stability and reliability of the sensorsdiminishes because of possible coalescence and structural changes [12].Furthermore, as temperature increases, the charge – carrier concentrationwill increase and the Debye length, LD, will decrease, resulting in lesssensitivity [13]. In most cases, the optimal temperature for metal oxidegas sensors is between 200 0C and 500 0C [17]. There are two well – known ways for improving the gas sensingproperties 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 tobetter sensitivity and stability, e.g. [14, 15]. The second is to reduce grainsize, which has been shown to increase sensitivity [16]. This is becausethe depletion layer caused by ionosorption has a greater effect on theconduction channel of the grain as the grain size decreases. Consequently,there is great interest in using nanoparticles in gas sensors, since they canbe used to make films with very small grain sizes. These two approacheshave 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 ZnOsensing element with palladium has greatly enhanced the sensor sensitivi- 3
  24. 24. ty and response time of minute – grain size ZnO thin films made possiblethrough 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 theenvironment has grown. Gas sensors find applications in numerous fields[17, 18]. Two important groups of applications are the detection of singlegases (as NOx, NH3, O3, CO, CH4, H2, SO2, etc.) and the discriminationof odours or generally the monitoring of changes in the ambient. Singlegas sensors can, for examples, be used as fire detectors, leakage detectors,controllers of ventilation in cars and planes, alarm devices warning theovercoming of threshold concentration values of hazardous gases in thework places. The detection of volatile organic compounds (VOCs) orsmells 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 gassensing devices designed to analyze such complex environmental mix-tures [19]. In Table 1.1 [17] examples of application for gas sensors andelectronic noses are reported. Industry currently employs many varieties of gas sensing systemsfor 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- 4
  25. 25. some and require highly qualified personnel to operate. Current trends areto improve low cost, solid state gas sensor performance in order to obtainhigh linearity, sensitivity, selectivity and long term stability [19]. Table 1.1: Examples of applications for gas sensors and electronic noses [17]. 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 The only practical way to monitor air quality or provide a mean toalert a human of potential danger is by direct gas sensing. A gas sensorcan form part of an early warning system, notifying the appropriate au-thorities or provide the feedback signals to a process control system. Toachieve this, a gas sensor system must be capable of accurate and stablein-situ real time measurements. Environmental factors such as operatingtemperature, vibration, mechanical shock, chemical poisoning, as well as 5
  26. 26. various device characteristics (accuracy, resolution, physical size andcost) must also be taken into consideration.1.3 Focus of current research The main focus of the present thesis is on the improvement of semiconducting metal oxide (SMO) thin film based gas sensors (with specialemphasis on SnO2 and ZnO) and their characterization. An objectiveanalysis of the various substrates used by several investigators is per-formed as a part of this work. The gas sensitive zinc oxide and tin oxidefilms are deposited by chemical spray pyrolysis deposition technique withair 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 microscopeAFM. Too much effort has been spent to maximize the sensitivity S andreduces the response τres and recovery τrec times of the sensing elementupon exposing to hydrogen reducing gas H2 of various concentrations Cand at different operating temperatures T. The catalytic effect of the pal-ladium noble metal and grain size effect are exploited to accomplish thesevital objectives. The thesis also describes the development of the gassensor 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 chapteris a general introduction where the overview of the field of study andscope of the work carried out is outlined. The second chapter entitled“Working principles of semiconductor metal oxide gas sensors” brieflydescribes the working principle of the kind of sensors developed andsurveys the various methods used currently to improve the sensor charac- 6
  27. 27. teristics. The fabrication and characterization of the sensing element aswell as testing the sensors towards hydrogen reducing gas is dealt with inthe third chapter. Moreover, the development of the gas sensor testingchamber and the protocol to use it are also detailed in this chapter. Thesubsequent chapter (chapter four), a discussion of the experiments carriedout 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. 7
  28. 28. Chapter 2Working Principles of Semiconductor Metal Oxide Gas SensorsBackground 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 sensorsbuilt using metal oxide materials. Particular emphasis is placed on theadsorption of oxygen because most sensors operate in air and oxygen isthe dominant adsorbed species in this case. The mechanism where anoxide transforms gas – surface interactions into a measurable electricalsignal is reviewed with a focus on the effects of particle size on this phe-nomenon. The current understanding of the gas sensing mechanism and abrief discussion of theoretical and empirical models proposed for semi-conductor metal oxide (SMO) gas sensors are discussed. The metrics bywhich gas sensor performance is judged are defined in this chapter and anintroduction to SMO gas sensors is presented. Background information isconcluded with a discussion of reported spray pyrolysis deposition tech-nique for oxide semiconductors.2.1 Adsorption Mechanisms Physical adsorption (physisorption) is defined as an adsorptionevent where no geometric change occurs to the adsorbed molecule andvan der Waals forces are involved in the bonding between the surface andadsorbate [11]. Chemical adsorption (chemisorption) is the formation of achemical 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 strongerbond than physisorption and the characteristics of each are summarized in 8
  29. 29. Table 2.1. Two types of chemisorption occur on the surface of metal ox-ides: (1) molecular or associative chemisorption, in which all the atomicbonds are preserved in the adsorbed molecule; and (2) dissociative chem-isorption, where bonding within the adsorbed molecule decomposes andmolecular 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 Table 2.1: Comparison of physisorption and chemisorption [20]. Physisorption Chemisorption Covalent Bonding Intermolecular (van der Waals) Force (adsorbate & surface) Low Temperature High Temperature Low Activation Energy High Activation Energy (<< 0.5 eV) (> 0.5 eV) Low Enthalpy Change High Enthalpy Change (ΔH < 20kJ/mol) (50kJ/mol < ΔH < 800kJ/mol) Reversible Reversible at High Temperature Adsorbate energy state unaltered Electron density increases at interface Multilayer formation possible Monolayer surface coveragetend to react via dissociative chemisorption; however; there is an activa-tion energy associated with dissociation. The type of chemisorbed oxygenon the surface of a metal oxide is dependent on the temperature of thesystem. Barsan and Weimar compiled results from a survey of the litera-ture concerning oxygen adsorption on SnO2 and correlated the adsorbedoxygen species to temperature where techniques such as infrared spec-troscopy, temperature programmed desorption and electron paramagneticresonance were used [21]. Table 2.2 summarizes the temperature rangesassociated with each species of oxygen adsorption. 9
  30. 30. Table 2.2: Temperature ranges associated with molecular and dissociative oxygenadsorption reactions [20]. Temperature Range (°C) Adsorption Reaction(s) O2 (g) + e− → O− (ads) 2 Room Temp.< T < 175°C O2 (g) + 2e− → O2− (ads) 2 175°C < T < 500°C O2 (g) + 2e− → 2O− (ads) O− (ads) + e− → O2− (ads) T > 500°C O2 (g) + 4e− → 2O2− (ads) 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 thesurface of an oxide are of utmost importance in gas sensing. Oxygen is astrong electron acceptor on the surface of a metal oxide. A majority ofsensors operate in an air ambient; therefore, the concentration of oxygenon the surface is directly related to the sensor electrical properties. Theconversion to O- or O2- at elevated temperatures are useful in gas sensingsince only a monolayer of oxygen ions are present with these stronglychemisorbed 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 achievedby thermal stimulation up to a specific temperature or by reactions withother gaseous species. A desorption process that is isothermal occurswhen, 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) 10
  31. 31. where the extra electron generated is injected back into the metal oxide.This desorption reaction results in a lower surface coverage of oxygenadsorbates which influences the electrical properties of the oxide.2.2 Non – Stoichiometry in Semiconductors The relevance of non – stoichiometry to the transport properties ofmetal oxide semiconductors will now be explored using ZnO as an exam-ple. It is well – known that ZnO is stoichiometrically deficient in oxygentraced to either zinc interstitials or oxygen vacancies. To begin with, thenotation 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 relativecharge, 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 vacancyoccupying an oxygen lattice site. Electrons and holes may be signified bye and h, respectively. The equilibrium constant, K(T), for the general chemical reactionof reactants A, B and products C, D [19]: aA + bB → cC + dD (2.2)can be written as: [C]c [D]d ∆G K(T) = = e−KT (2.3) [A]a [B]b 11
  32. 32. where ΔG represents the standard change in free energy for the reaction.An oxygen vacancy, known as a type of Schottky defect, can be generatedin ZnO through the following reaction: ∙∙ 1 ZnZn + OO → ZnZn + VO + 2e′ + O2 (g) (2.4) 2It is conventional practice to denote the left side of (2.4) as “nil”. As seenin (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: 1 ∙∙ K R = [VO ](pO2 )2 ⋅ n2 (2.5) ∙∙where pO2 denotes the partial pressure of oxygen, [VO ] and n represent theoxygen vacancy and electron concentrations, respectively. To solve for ∙∙[VO ] or n, one must evoke the electroneutrality condition (ENC), whichstates that the concentration of positive defects present in the materialmust 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: 1 KR 3 1 [VO ] = ( ) (pO2 )−6 ∙∙ (2.7) 4and 1 1 1 n = (2)3 (K R )3 (pO2 )−6 (2.8)Thus, the logarithmic concentration of oxygen vacancies and electrons inZnO, plotted against log (pO2 ), is shown to have what is termed a -1/6pO2 dependence. The electronic conductivity, 𝜎, is given by: 12
  33. 33. 1 1 1 σ = q(2)3 (K R )3 (pO2 )−6 μe (2.9)where q is the electronic charge and 𝜇 𝑒 is the electronic mobility. An oxygen deficiency in ZnO may also be realized through theformation of a Zn interstitial, known as a type of Frenkel defect. This canbe formed through the following reaction: 1 nil = Zn⋅⋅ + 2e′ + O2 (g) i (2.10) 2using 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 largepart by the concentration of electrons or holes present in the material. Incertain 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 thebasis for semiconductor gas sensor operation. This behavior is unlikemetals, where the adsorption of a gas may cause small conductancechanges due to a modification of charge carrier mobility [6]. As an exam-ple, recall the experiments of Wagner on the decomposition of N 2O onZnO. If the generation of two electrons proceeds as in (2.4), the decom-position reaction was proposed as follows [2]: 2e− + N2 O → N2 + O2− (ads. ) (2.11) O2− (ads. ) + N2 O → N2 + O2 + 2e− (2.12) The adsorption of oxygen in (2.11) would result in an increase inZnO resistivity due to the capture of majority charge carriers. The subse- 13
  34. 34. quent reaction between the adsorbed oxygen and N2O in (2.12) acts torestore the supply of conduction electrons and thus, an increase in con-ductivity may be observed. It is this simple and reversible change incharge concentration that drives the use of metal oxide gas sensors. For a visual perspective, a schematic of an n – type semiconductortin dioxide SnO2 thick film with an accompanying band structure modelis shown in figure 2.1 [19]. For conduction to occur, an electron mustpass 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 theadsorbed oxygen leave behind positively charged donor ions. An electricfield develops, between these positive donor ions and the negativelycharged adsorbed oxygen ions, which serves to impede the flow of elec-trons between neighboring grains. The barrier generated by the electricfield has a magnitude of eVS, where e is the electronic charge and VS isthe 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 barrierand reach a neighboring grain is given by: eVs ns = Nd exp (− ) (2.13) kTwhere Nd is the concentration of donors, k is Boltzmann’s constant, and Tis the temperature. Since conductance (or resistivity) is proportional to n s,an increase in the adsorbed oxygen content will raise eVS and thus, fewerelectrons will cross the potential barrier. This may be empirically moni-tored as an increase in resistivity. The introduction of a reducing gas willreverse this effect, lowering the potential barrier and decreasing resistivi-ty. It is this reducing gas, often termed the analyte, whose presence is ofinterest. 14
  35. 35. 1 − Electron – depleted O region 2 2 O- O- O- O- O- O- - O O- Grain Grain (a) In air O- O- O- O- O- Thermionic emission 𝑒𝑉 𝑆1 𝑒𝑉 𝑆1 ; 𝐺1 = 𝐺0 exp(− ) 𝐾𝑇 EC EF CO2 CO - O O- O O- O- Grain Grain (b) In the presence of reducing gas O- O- O O O- O- CO CO2 CO2 CO 𝑒𝑉 𝑆2 𝑒𝑉 𝑆2 ; 𝐺2 = 𝐺0 exp(− ) 𝐾𝑇 𝐺2 𝑒 𝑉 𝑆1 − 𝑉 𝑆2 𝑠𝑒𝑛𝑠𝑖𝑡𝑖𝑣𝑖𝑡𝑦 ⇉ 𝑆 = = exp ( ) 𝐺1 𝐾𝑇Figure 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]. SnO2 15
  36. 36. Using an example of ZnO in the detection of hydrogen, the followingreactions may occur [24]: 2e− + O2 → 2O− (ads. ) (2.14) 2O− (ads. ) + 2H2 → 2H2 O + 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 benoted that adsorbed oxygen may exist in multiple forms. Takata et al.proposed that oxygen adsorbed on ZnO is transformed with increasingtemperature in the following manner [25]: O2 → O− → 2O− → 2O2− 2 (2.16)of these forms, O2 is considered fairly inactive due to its non – dissocia-tive state. With regards to O− and O−, electron spin resonance (ESR) 2studies have shown that O− is far more reactive than O− [26]. The nature 2of adsorbed oxygen on ZnO as reported by several researchers is shownin 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), ZnOacts as a heterogeneous catalyst, as it is a phase distinct from the reactantsand products. To illustrate the phenomenon of catalysis, consider thefollowing reaction: A+B→ C+D (2.17) Two possible paths in which this reaction may proceed are shownin figure 2.3 [27]. In the absence of a catalyst, the reaction of (2.17) ischaracterized by a large activation energy, Ea. When a catalyst such asZnO or SnO2 is used, the gaseous products adsorb onto the metal oxide 16
  37. 37. Figure 2.2: The nature of oxygen species adsorbed on ZnO as reported by severalresearchers [25].surface with an exothermic heat of adsorption ΔH (State I). The reactionto form adsorbed products then proceeds with a lower activation energyEc (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 thereaction to proceed. An undesirably low activation energy will cause thereaction 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 subjectto an attractive potential [28]. This potential is the origin of adsorptionand arises from the multitude of unsatisfied bonds that exist at the surfaceof the solid. The adsorbed species is often called the adsorbent and thesolid surface is termed the adsorbate [11]. Physical adsorption, or phy-sisorption, occurs as a result of electrostatic and van der Waals forces thatexist between the adsorbent and adsorbate. Heats of adsorption for phy-sisorption tend to be low, with ΔH values typically in the range of 2 – 15kcal/mole [29]. In the case of chemical adsorption, (termed chemisorp- 17
  38. 38. EaEnergy A+B ΔH Ec II I C+D Reaction coordinateFigure 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 (E a),while the barrier to product formation is lowered (Ec) when a catalyst is used [27].tion), the adsorbent forms a chemical bond with the solid surface. Valuesfor ΔH tend to be higher for chemisorption and are often in the range of15 – 200 kcal/mole [29]. As chemisorption tends to provide the necessarycatalysis 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 forgas detection, then changes in conductivity must be proportional to theconcentration 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]: ∆H = exp ( ) (2.18) T 18
  39. 39. where is related to surface vibration time and R is the universal gasconstant. The surface coverage, S, of a gaseous species is dependent onboth 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 tothe pressure through the kinetic theory of gases will yield: N ∆H =( ) exp ( ) (2.20) √2 T Twhere NA is Avogrado’s number, P is the partial pressure of the gas, andM is the average molar weight of the gaseous species. Experimentalcurves of S plotted as a function of P at a given temperature are known asadsorption isotherms [28]. One particular isotherm derived by IrvingLangmuir 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 realsurfaces, modified isotherms have been developed [29]. Regardless, thederivations of Langmuir provide a sound qualitative relationship betweensurface 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 completelycovered surface, then a concentration of S adsorbed molecules will resultin 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: 19
  40. 40. a = = (2.22) + +a 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 degreeof coverage, then (2.22) takes the following form: b = (2.23) 1+bwhere b = (a/So). When the degree of surface coverage is proportional to the partialpressure, changes in the electrical conductivity may be related to gasconcentration. 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 ofproportionality 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 b = = (2.24) 1+b +b 1+b +b If bBPB >> bAPA, then the equations of (2.24) become: b b = = (2.25) 1+b 1+bin the case that bBPB is >> 1, the equations of (2.25) reduce to: b 1 (2.26) 1+bThus, 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 willbecome independent of the gas concentration as approaches 1. The rate of a reaction between A and B may be given by [27]: ae=k (2.27) 20
  41. 41. 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 thecoverage 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 quitehigh. As the reaction rate between the oxygen and the reducing speciesincreases, falls to zero, enabling the reducing gas to be detected with ahigh 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 theirgas 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 layeris 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 usedmetal – oxides are n – or p – type semiconductors, due to the presence ofoxygen – vacancies in the bulk. Generally the conductance or the re-sistance of the sensor is monitored as a function of the concentration ofthe target gases. Additionally the performance of the sensor depends onthe 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. Thesensing itself can take place at different sites of the structure dependingon the morphology. They will play different roles, according to the sens-ing layer morphology. An overview is given in figure 2.4. 21
  42. 42. Gas Product Gas Product desorption adsorp- tion (a) Compact layerGas Product B C A Sensitive layer (b) Porous layer Electrodes Substrate 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] 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 22
  43. 43. 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- Gas Product Surface band bending Volume not accessible (a) Z eVS to gases Z0 Current flow Zg Conducting channel Z Energy X eVS (b) ZFigure 2.5: schematic of a compact layer with geom- e∆VS etry and energy band representation; Z0 is the thickness of the depleted surface lay- Zg er; Zg is the thickness of the surface and eVS the band bending. (a) A partly deplet- ed compact layer (“thicker”) and (b) A Eb Energy completely depleted layer (“thinner”) [32]. 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]. 23
  44. 44. Large grains Product Gas Z X Energy Current flow qVS Eb Xg> λD Xg X 2X0 Small grains Current flow q∆VS <KB T Energy Flat band condition q∆VS Eb Xg< λD X Extended surface influenceFigure 2.6: Schematic representation of a porous sensing layer with geometry andenergy band for small and large grains. λD Debye length, Xg grain size [33]. 24
  45. 45. 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 Z (b) eVS Zn X eVS Z Z (a) eVSCurrent Zo flow Zn Conduction Zn- 2Z0 X Channel Energy 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]. 25
  46. 46. 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 Surface Grain Model bulk nano crystal Schottky contact − boundary O2 Geometric Z X and Metal I 𝐿 𝐷>1 2 CO effect EC Electronic EC EF EF EF EF Band LD − LD LD O2 EV EV bulk Surface Metal Z X X X SurfaceElectrical equiva- lent circuit (Low current) Bulk 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]. 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 26
  47. 47. on mobility should be taken into consideration. This happens because thenumber of collisions experienced by the free charge carriers in the bulk ofthe grain becomes comparable with the number of surface collisions; thelatter 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 theseparameters 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 alsobeen extensively examined in the literature since this is a crucial factorwhen creating a commercially viable device [37, 38]. Other issues such asresponse 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 ofthem will be applied in this research.2.5.1 Sensitivity The response of a sensor upon the introduction of a particular gasspecies is called the sensitivity (S The most general definition of sensitiv-ity applied to solid – state chemi – resistive gas sensors is a change in theelectrical resistance (or conductance) relative to the initial state uponexposure 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- 27
  48. 48. 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 G = (2.28) RGwhere R is the electrical resistance and G is the electrical conductanceand the subscript “AIR” indicates that background is the initial dry airstate and the subscript “GAS” indicates the analyte gas has been intro-duced. Another common approach to report S is shown in equations 2.29and 2.30 [40]: |∆R| RG −R = 100 = 100 (2.29) R R |∆ | 100 100 (2.30) The values calculated using the above equations scale from zerowhile 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 issimply to add 1 to the reported value and they are equivalent. Both valuesare acceptable and useful metrics for gas sensor response testing. In thecurrent research, the percentage conductance change has been selected asthe 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 gasspecies 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 ⁄ = (2.31) SG 28
  49. 49. In this work only a single gas was introduced in a background of dry airso 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 alsobeen applied to describe the ability of a sensor (or array) to detect anddistinguish 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 theselectivity 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 fromn – type and p – type materials in a sensor array are valuable for dataanalysis algorithms to enhance selectivity. Selectivity is not a focus ofthis study. It will become a more important issue once other reducingand/or oxidizing gases become available.2.5.3 Stability Stability measures the capability of a sensor to maintain sensitivityover durations of time for a particular gas species. Stability is measuredin terms of baseline “drift” which is the change in baseline conductanceover some duration of time at a particular temperature. Here we definedrift as the change in baseline conductance relative to the initial conduct-ance as written in equation 2.32 below: 1 − D=| | (2.32) 1− 0 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 simulatemore realistic conditions the sensor would operate under. 29
  50. 50. 2.5.4 Response and Recovery Times The response time (τres) of a gas sensor is defined as the time ittakes the sensor to reach 90% of maximum/minimum value of conduct-ance upon introduction of the reducing/oxidizing gas [42]. Similarly, therecovery time (τrec) is defined as the time required to recover to within10% of the original baseline when the flow of reducing or oxidizing gasis removed. Figure 2.9 shows how this is measured from sensor dataplotting the conductance as a function of time. 90% of maximum Maximum conductance conductance Conductance Recovered to within 10% of original baseline τres τrec Time Figure 2.9: drawing showing how response and recovery times are calculated from a plot of sensor conductance versus time [20].Xu et al., [43] prepared ultrathin Pd nanocluster film capable of detecting2% 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. Fouryears earlier that time, in 2001, Favier et al., [44] obtained a comparableultrafast response time of less than 75 ms towards H2 : N2 gas mixturefrom 2 to 10% at room temperature using palladium mesowire arrays. 30

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