GROWTH OF INDIUM OXIDE BASED NANOSTRUCTURES BY PLASMA ASSISTED REACTIVE THERMAL EVAPORATION FOR PHOTOELECTROCHEMICAL APPLICATION

Tekspenuh

(1)of. M. al. ay. a. GROWTH OF INDIUM OXIDE BASED NANOSTRUCTURES BY PLASMA ASSISTED REACTIVE THERMAL EVAPORATION FOR PHOTOELECTROCHEMICAL APPLICATION. U. ni. ve r. si. ty. AZIANTY BINTI SARONI. FACULTY OF SCIENCE UNIVERSITY OF MALAYA KUALA LUMPUR. 2018.

(2) M. al. ay. a. GROWTH OF INDIUM OXIDE BASED NANOSTRUCTURES BY PLASMA ASSISTED REACTIVE THERMAL EVAPORATION FOR PHOTOELECTROCHEMICAL APPLICATION. si. ty. of. AZIANTY BINTI SARONI. U. ni. ve r. THESIS SUBMITTED IN FULFILMENTOF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF PHYSICS FACULTY OF SCIENCE UNIVERSITY OF MALAYA KUALA LUMPUR 2018.

(3) UNIVERSITY OF MALAYA ORIGINAL LITERARY WORK DECLARATION Name of Candidate: AZIANTY BINTI SARONI Matric No: SHC140057 Name of Degree: DOCTOR OF PHILOSOPHY Title of Project Paper/Research Report/Dissertation/Thesis (“this Work”):. a. GROWTH OF INDIUM OXIDE BASED NANOSTRUCTURES BY PLASMA ASSISTED REACTIVE THERMAL EVAPORATION FOR PHOTOELECTROCHEMICAL APPLICATION. ay. Field of Study: EXPERIMENTAL PHYSICS I do solemnly and sincerely declare that:. ni. ve r. si. ty. of. M. al. (1) I am the sole author/writer of this Work; (2) This Work is original; (3) Any use of any work in which copyright exists was done by way of fair dealing and for permitted purposes and any excerpt or extract from, or reference to or reproduction of any copyright work has been disclosed expressly and sufficiently and the title of the Work and its authorship have been acknowledged in this Work; (4) I do not have any actual knowledge nor do I ought reasonably to know that the making of this work constitutes an infringement of any copyright work; (5) I hereby assign all and every rights in the copyright to this Work to the University of Malaya (“UM”), who henceforth shall be owner of the copyright in this Work and that any reproduction or use in any form or by any means whatsoever is prohibited without the written consent of UM having been first had and obtained; (6) I am fully aware that if in the course of making this Work I have infringed any copyright whether intentionally or otherwise, I may be subject to legal action or any other action as may be determined by UM.. Date:. U. Candidate’s Signature. Subscribed and solemnly declared before, Witness’s Signature. Date:. Name: Designation:. ii.

(4) GROWTH OF INDIUM OXIDE BASED NANOSTRUCTURES BY PLASMA ASSISTED REACTIVE THERMAL EVAPORATION FOR PHOTOELECTROCHEMICAL APPLICATION ABSTRACT In this work, indium oxide based nanostructures were grown in a home-built system using plasma assisted reactive thermal evaporation (PARTE) followed by nitrogen plasma assisted in-situ thermal annealing. The main concern of this work is to grow various types. a. of indium oxide based nanostructures for photoelectrochemical (PEC) water splitting. ay. application using this technique. The first part of experiment is focused on the growth of In2O3 composite nanostructures by nitrogen plasma assisted in-situ thermal annealing at. al. filament temperatures (Tf ) varies from 1300 to 1800 °C and with the radio frequency (rf). M. power fixed at 50 and 150 W. The in-situ thermal annealing transformed the composition. of. of the composite nanostructures from In2O3/InN to In2O3/WO3 and In2O3/W2N with increase in Tf at both rf power. High Tf, has the effect of transforming the morphology of. ty. the nanostructures from spherical-shaped grains of In2O3/InN composite nanostructures. si. to rhomboidal shaped grains with significant crystal facets of In2O3/W2N. The research. ve r. revealed that there was an obvious reduction of crystallite size when the rf power was increased. Consequently, this also resulted in tailoring of the optical energy gap (𝐸𝑔 ) from. ni. the ultraviolet (UV) to visible region of the electromagnetic spectrum. In the second part. U. of this work, self-catalyzed In2O3 nanowires and composite nanowires were grown at different substrate temperatures (Ts) and growth time. The nanowires exhibited rod-like and needle-like morphologies with tetrahedron facet at the sidewalls. The diameter decreased and length increased with increase in Ts and growth time. Apart from that, the variation in these parameters resulted in the appearance of tapered and branches nanowires. Most importantly, this work has highlighted the importance of Tf during the nitrogen plasma assisted in-situ thermal annealing in modifying the surface morphology of In2O3 nanostructures and transforming In2O3 nanowires to In2O3/W2N composite iii.

(5) nanowires resulting in effective incorporation of W and N adatoms into the as-grown In2O3 nanowires. The presence of W2N nanostructures for both composite nanostructures caused the red-shifted of the 𝐸𝑔 , which promoted enhanced photon absorption thus increased photocurrent density for effective PEC water splitting application. Keywords:. Indium. oxide,. Composite. nanostructures,. Nanowires,. PARTE,. U. ni. ve r. si. ty. of. M. al. ay. a. Photoelectrochemical. iv.

(6) PENUMBUHAN STRUKTUR NANO BERASASKAN INDIUM OKSIDA OLEH PENYEJATAN TERMA REAKTIF DENGAN BANTUAN PLASMA UNTUK KEGUNAAN FOTOELEKTROKIMIA ABSTRAK Dalam kerja ini, struktur nano berasaskan indium oksida telah ditumbuhkan daripada penyejatan terma reaktif dengan bantuan plasma binaan sendiri diikuti dengan terma penyepuhlindapan bantuan nitrogen plasma in-situ. Keutamaan dalam kerja ini adalah. a. untuk menumbuhkan pelbagai jenis struktur nano berasaskan indium oksida dengan. ay. menggunakan teknik ini bagi aplikasi penguraian air fotoelektrokimia (PEC). Bahagian pertama eksperimen ini tertumpu kepada pertumbuhan In2O3 nanostruktur komposit. al. dengan terma penyepuhlindapan bantuan nitrogen plasma in-situ pada suhu filament (Tf). M. yang berbeza dari 1300 °C ke 1800 °C dan kuasa radio frekuensi (rf) ditetapkan pada 50. of. W and 150 W. Terma penyepuhlindapan in-situ telah mengubah komposisi nanostruktur komposit daripada In2O3/InN ke In2O3/WO3 dan In2O3/W2N dengan peningkatan Tf di. ty. kedua-dua kuasa radio frekuensi. Tf ,yang tinggi telah memberi kesan kepada perubahan. si. nanostruktur komposit morfologi daripada butiran berbentuk sfera In2O3/InN kepada. ve r. butiran berbentuk romb In2O3/W2N dengan kristal facet. Kajian ini telah mendedahkan pengurangan ketara saiz hablur apabila kuasa radio frekuensi meningkat. Akibatnya ia. ni. memberi keputusan kepada membolehkan pengawalan jurang tenaga optik (𝐸𝑔 ) spektrum. U. elektromagnetik daripada julat ultraviolet (UV) dan cahaya nampak. Di bahagian kedua, pemangkinan sendiri In2O3 nanowayar dan komposit nanowayar telah ditumbuhkan dengan mengubah suhu substrat (Ts) dan masa pembinaan. Nanowayar mempamerkan morfologi berbentuk rod dan jarum dengan faset tetrahedral di susur dinding. Diameter berkurang dan panjang bertambah dengan peningkatan suhu substrat dan masa pembinaan. Tambahan pula, ia memberi keputusan kepada pembentukan nanowayar tirus dan bercabang. Yang paling penting, dalam kerja ini telah menekankan kepentingan Tf semasa terma penyepuhlindapan bantuan nitrogen plasma di situ dengan mengubah v.

(7) sruktur nano In2O3 permukaan morfologi dan perubahan In2O3 nanowayar kepada In2O3/W2N komposit nanowayar memberi keputusan keberkesanan penggabungan W dan N adatoms ke dalam pembinaan asal In2O3 nanowayar. Kehadiran W2N nanostruktur pada kedua-dua komposit nanostrukutur menyebabkan fenomena anjakan merah dimana telah menambahbaik penyerapan foton yang seterusnya meningkatkan ketumpatan fotoarus untuk aplikasi penguraian air fotoelektrokimia yang berkesan. Katakunci:. Indium. oksida,. Nanostruktur. komposit,. PARTE,. U. ni. ve r. si. ty. of. M. al. ay. a. Fotoelektrokimia. Nanowayar,. vi.

(8) ACKNOWLEDGEMENTS All praise is to ‘ALLAH THE ALMIGHTY’ for giving me the opportunity, patience and guidance in completing this modest work successfully. I am indebted to many for their support over the course of research and writing of this thesis. First and absolutely foremost, a million of thanks dedicated to my superior, Dr. Goh Boon Tong for sharing his unique views of science, constant interests, advices,. a. suggestions and constructive criticism. He has introduced and taught me the plasma. ay. assisted reactive thermal evaporation technique method for nanostructures growth consequently improve my knowledge of physics particularly in solid state concepts which. al. serve for add depth and perspective of materials sciences specifically in semiconductor. M. nanomaterials. Likewise, the same expression goes to my second supervisor, Professor. of. Datin Dr. Saadah Abdul Rahman, who had been a great of a help in accomplishing this research via numerous ways with tremendous efforts.. ty. This thesis may not be successfully completed without the love and support from my. si. family members especially my husband, Abdul Aziz Bin Rozi, my mother, Saniah Binti. ve r. Sabitah, my father, Saroni Bin Martubi and my siblings who have given endless love, prayers and encouragement during my studies. I’m also grateful to the Ministry of Higher. ni. Education (MOHE) and MARA University of Technology (UiTM) for awarding the. U. SLAB/SLAI scholarship scheme to pursue this fruitfully PhD journey. I like to. acknowledge the financial given by University of Malaya which is the Postgraduate Research Fund (PPP) of PG073-2015A. Last but not least, a huge thanks to the colleagues from Low Dimensional Materials Research Centre for their supporting attitude, advise, understanding and attention towards the completion of my thesis. Once again, thank you so very much for the bountiful of blessings and endless support.. vii.

(9) TABLE OF CONTENTS ABSTRACT ....................................................................................................................iii ABSTRAK ....................................................................................................................... v ACKNOWLEDGEMENTS .......................................................................................... vii TABLE OF CONTENTS.............................................................................................viii LIST OF FIGURES ...................................................................................................... xii. a. LIST OF TABLES ....................................................................................................... xxi. ay. LIST OF SYMBOLS AND ABBREVIATIONS ...................................................... xxii. al. CHAPTER 1: INTRODUCTION .................................................................................. 1 Background on Materials Used ............................................................................... 1. 1.2. Metal Oxides and Nitrides in Photoelectrochemical Processes ............................... 5. 1.3. Growth Techniques of In2O3 Based Nanostructures ............................................... 8. 1.4. Research Problems................................................................................................. 10. 1.5. Motivations and Research Objectives ................................................................... 12. 1.6. Outline of Thesis.................................................................................................... 14. ve r. si. ty. of. M. 1.1. ni. CHAPTER 2: LITERATURE REVIEW .................................................................... 16 Introduction............................................................................................................ 16. 2.2. Common Deposition Techniques .......................................................................... 16. U. 2.1. 2.3. 2.2.1. Chemical Vapor Deposition (CVD) ......................................................... 16. 2.2.2. Hydrothermal Method .............................................................................. 20. 2.2.3. Pulsed Laser Deposition (PLD) ................................................................ 21. 2.2.4. Radio Frequency (rf) Magnetron Sputtering ............................................ 24. 2.2.5. Thermal Evaporation Technique .............................................................. 25. Structure Properties ............................................................................................... 35. viii.

(10) 2.4. Optical Properties .................................................................................................. 39. 2.5. Electrode Materials in Photoelectrochemical Water Splitting............................... 41. CHAPTER 3: SAMPLES PREPARATION AND CHARACTERIZATION TECHNIQUES .............................................................................................................. 50. 3.3. Reaction Chamber .................................................................................... 51. 3.1.2. RF Generator and Matching Impedance................................................... 53. 3.1.3. Hot-filament Power Supply ...................................................................... 55. 3.1.4. Pumping System ....................................................................................... 56. 3.1.5. Gas Supply ............................................................................................... 58. 3.1.6. Heat Supply .............................................................................................. 59. M. al. ay. a. 3.1.1. of. Deposition Procedures ........................................................................................... 60 Pre-deposition ........................................................................................... 63. 3.2.2. Deposition ................................................................................................ 65. 3.2.3. Post-deposition ......................................................................................... 68. ty. 3.2.1. si. 3.2. Plasma Assisted Reactive Thermal Evaporation ................................................... 51. Characterization Techniques ................................................................................. 68. ve r. 3.1. 3.3.1. Morphological Study ................................................................................ 69. U. ni. 3.3.1.1 Field Emission Scanning Electron Microscopy (FESEM) ........ 69. 3.3.2. Elemental Compositions Analysis ............................................................ 72 3.3.2.1 X-ray Photoelectron Spectroscopy (XPS) ................................. 72. 3.3.3. Structural Properties ................................................................................. 75 3.3.3.1 X-ray Diffraction (XRD) ........................................................... 75 3.3.3.2 Raman Scattering Spectroscopy ................................................ 78 3.3.3.3 High Resolution Transmission Electron Microscopy (HRTEM) .................................................................................. 79. 3.3.4. Optical Properties ..................................................................................... 81 ix.

(11) 3.3.4.1 UltraViolet Visible Near Infrared Spectroscopy (UV-Vis-NIR) Spectroscopy ..................................................... 81 3.3.5. Photoelectrochemical Measurement ......................................................... 85. CHAPTER 4: IN2O3 BASED COMPOSITE NANOSTRUCTURES ...................... 90 Introduction............................................................................................................ 90. 4.2. In2O3 Nanostructures: Effects of Filament-to-Substrate Distance ......................... 91. 4.3. Effect of Nitrogen Plasma Assisted In-Situ Thermal Annealing .......................... 92. ay. a. 4.1. Effect of Substrate Temperature during Growth ...................................... 93. 4.3.2. Effect of Filament Temperature at rf Power 50 W ................................... 99. 4.3.3. Effect of Filament Temperature at rf Power 150 W ............................... 113. M. al. 4.3.1. Proposed Growth Mechanism.............................................................................. 132. 4.5. Summary .............................................................................................................. 135. of. 4.4. ty. CHAPTER 5: IN2O3 BASED NANOWIRES ........................................................... 137 Introduction.......................................................................................................... 137. 5.2. Effect of Substrate Temperature and Growth Time ............................................ 138. 5.3. Effects of Nitrogen Plasma Assisted In-Situ Thermal Annealing ....................... 153. 5.4. Proposed Growth Model of a Self-catalyzed In2O3 Nanowire ............................ 160. 5.5. Summary .............................................................................................................. 163. U. ni. ve r. si. 5.1. CHAPTER 6: PHOTOELECTROCHEMICAL BEHAVIOUR OF IN2O3 BASED NANOSTRUCTURES .................................................................................. 164 6.1. Introduction.......................................................................................................... 164. 6.2. Photoelectrochemical Responses of Composite Nanostructures ......................... 164 6.2.1. Variations of Temperature at rf Power 50 W ......................................... 164. 6.2.2. Variations of Temperature at rf Power 150 W ....................................... 171 x.

(12) 6.3. Intrinsic In2O3 Nanowires and In2O3/W2N Composite Nanowires ...................... 179. 6.4. Summary .............................................................................................................. 183. CHAPTER 7: CONCLUSION AND SUGGESTIONS FOR FUTURE WORK .. 185 7.1. Overview of the Research Methodology ............................................................. 185. 7.2. Important Findings............................................................................................... 186. 7.3. Suggestions for Future Work ............................................................................... 190. ay. a. REFERENCES ............................................................................................................ 192. U. ni. ve r. si. ty. of. M. al. LIST OF PUBLICATIONS AND PAPER PRESENTED.......................................206. xi.

(13) LIST OF FIGURES The solar spectrum with respect to wavelength (Tada & Jin, 2016)….... 6. Figure 1.2:. Band energy levels of various semiconductors with different Egs (Abe, 2010)………………………………………………………………….. 7. Figure 1.3:. Absolute conduction band and valence band energy levels for (a) suitable, & (b) unsuitable semiconducting photocatalysts with respect to normal hydrogen electrode (NHE) and vacuum (Vac) (Babu et al., 2015)...................................................................................................... 7. Figure 1.4:. Photoelectrochemical water splitting systems using (a) n-type semiconductor photoanode, & (b) p-type semiconductor photocathode (Abe, 2010).…………………………………………............................ 9. Figure 2.1:. FESEM images of (a & b) Nanowires grown at 875 °C and high magnification image, respectively, & (c & d) Nanowires grown at 950 °C and high magnification image, respectively (Singh et al., 2009)........ 18. Figure 2.2:. (a) FESEM images of as-synthesized hierarchical In2O3 nanostructures, (b) High magnification of SEM images, (c) HRTEM image of the ultrathin In2O3 nanowire branches grown on the backbone nanowire, & (d) XRD pattern of In2O3 nanostructures (Shen et al., 2011)………………………………………………………………….. 19. Figure 2.3:. FESEM images of (a) In2O3 micro-flower, (b - d) In2S3/In2O3 composite with TTA of 0.02, 0.05, and 0.08 g, respectively, & (e) The elemental mapping image of In, O, and S (Zhang et al., 2016a)……..… 21. ve r. si. ty. of. M. al. ay. a. Figure 1.1:. Experimental set-up of PLD process (Popescu et al., 2016)…………… 22. ni. Figure 2.4:. U. Figure 2.5:. FESEM images of ITO nanostructure films deposited at (a) 0.1, (b) 0.5, (c) 1, & (d) 2 mbar (Savu & Joanni, 2006).……………………………. 23. Figure 2.6:. Surface morphologies of ITO nanostructure films deposited at (a) 0.001, (b) 0.02, & (c) 2.0 Pa (Yoko et al., 2017)……………………... 23. Figure 2.7:. AFM images of as-deposited undoped and Nb doped In2O3 films deposited by RF magnetron sputtering technique on quartz substrate at various wt. % of Nb (Krishnan et al., 2017)...…………………………. 25. Figure 2.8:. (a) Schematic diagram of evaporation process by HWCVD, & (b) Growth mechanism by VLS mode (Ohring, 2002; Yu & Lee, 2014).…. 26. xii.

(14) Figure 2.9:. Vapor pressures of elements employed in semiconductor materials (Honig, 1969)…………………………………………………………. 27. Figure 2.10: (a) Tappered In2O3 nanotowers, (b) Lateral In2O3 with the truncated octahedron on the tip, & (c) Growth mechanism of In2O3 nanotowers (Tuzluca et al., 2018; Yan et al., 2007)…………………………............ 28 Figure 2.11: (a & b) WO3 thin film prepared at substrate temperature of 350 and 600 °C, respectively, & (c) Cross section image of WO3 deposition region on Si substrate. The inset image shows the interplanar distance of WO3 (Corrêa et al., 2014). ………………………………….......................... 30. al. ay. a. Figure 2.12: (a - e) FESEM images of the InN thin films grown at applied rf power of 50, 100, 150, 200, and 250 W, respectively, & (f) Dependence of the elemental percentage of the InN films on the applied rf power (Ganesh et al., 2015)……………………………………………………………. 31. M. Figure 2.13: SEM images of (a & b) core-shell PbS/Sn:In2O3 prepared at 100 and 200 °C, respectively, & (c & d) branched PbIn2S4/Sn:In2O3 nanowires prepared at 300 and 400 °C, respectively (Zervos et al., 2017)……...… 33. of. Figure 2.14: (a & b) TEM and HRTEM images of PbS/Sn:In2O3 core-shell nanowires prepared at 200 °C respectively (Zervos et al., 2017)……..... 34. ve r. si. ty. Figure 2.15: (a) TEM images of branched PbIn2S4/Sn:In2O3 nanowires prepared at 400 °C, (b) EDS spectra of the PbIn2S4/Sn:In2O3 nanowires, (c) High magnification images of PbIn2S4, & (d - e) HRTEM images of the In2S3, In2O3, and PbIn2S4, respectively (Zervos et al., 2017)……..……. 34. ni. Figure 2.16: XRD pattern of (a) In2O3, & (b) InN nanostructures (Barick & Dhar, 2015; Wei et al., 2018)………………………………………………… 36. U. Figure 2.17: XRD pattern of (a) WO3, & (b) W2N nanostructures (Cao et al., 2009; Kim et al., 2016b).……………………………………..……………… 36 Figure 2.18 Raman spectrum of (a) In2O3, (b) InN, & (c) W2N nanostructures (Chakrapani et al., 2009; Jiang et al., 2013; Liu et al., 2011).………...... 38 Figure 2.19: XPS narrow scan spectra of (a) In 3d, (b) O 1s, (c) W 4f, & (d) N 1s peak (Gan et al., 2013; Nandi et al., 2015).………………………….… 39 Figure 2.20: (a) In2O3 (001) thin film grown by molecular beam epitaxy on YSZ (001) substrate, & (b) Cross section image of In2O3 thin film, demonstrating a closed film with flat surface (Bierwagen, 2015).…….. 40. xiii.

(15) Figure 2.21: Typical analysis of (a) UV-Vis absorption spectra, & (b) Tauc plot (Liu et al., 2017b)……….………..……...…………………………….. 41 Figure 2.22: SEM images of (a) Nanocubes of In2O3, (b) Ni-Fe/In2O3 composite nanostructure, & (c) Square In2O3 nanowires. The inset show high magnification of SEM image (Chaudhari & Singh, 2015; Gan et al., 2013; Meng et al., 2014)……………………………………………..... 44 Figure 2.23: Typical (a) Linear sweep voltammetry, (b) EIS-Nyquist plot, (c) Bode plot, & (d) Mott Schottky plot (Liu et al., 2016)………………….......... 48. ay. a. Figure 2.24: (a) UV-Vis absorption spectra, (b) Linear sweep voltammetry, & (c) EIS-Nyquist plot of In2O3/ZnO@Ag composite nanowires (Liu et al., 2017a)..................................................................................................... 48 Schematic diagram of PARTE system including (A) Reaction chamber, (B) Rf power supply, (C) Hot-filament power supply, (D) Vacuum evacuation system, (E) Gas supply, & (F) Substrate heating supply.…….…………………………………………………………... 52. Figure 3.2:. (a) Schematic diagram of the reaction chamber of PARTE, & (b) Photograph image of the reaction chamber.…………………..…….…. 53. Figure 3.3:. (a) Manual impedance matching network, & (b) Rf generator……..….. 54. Figure 3.4:. Photograph of (a) Plasma treatment, (b) Deposition, (c) Hot-filament supply voltage, & (d) Filament temperature detection by Reytek, Raynger 3i………………………...…………………………………... 56. ve r. si. ty. of. M. al. Figure 3.1:. Photograph of vacuum pumping system………………………………. 57. Figure 3.6:. Photograph of mass flow controller and gas lines……………………... 59. Figure 3.7:. Photograph of (a) Heating cartridge and thermocouple, & (b) Temperature controller and voltage regulator………….…………….... 61. Figure 3.8:. Diagram of research methodology involved in this work.……………... 62. Figure 3.9:. (a) Schematic diagram of the main components of a FESEM, (b) Signals generated when the electron beam strikes the specimen, & (c) Photograph of Hitachi SU 8000 SEM…………………………….…… 71. U. ni. Figure 3.5:. Figure 3.10: (a) Schematic diagram for photoelectric process, (b) Basic schematic diagram for XPS instrument, (c) Photograph of PHI Quantera II, & (d) Photograph of Thermo VG Scientific CLAM2 electron spectrometer… 73. xiv.

(16) Figure 3.11: Typical XPS deconvolution spectra of In 3d5/2…………………….….. 74 Figure 3.12: (a) Schematic diagram of the X-ray diffraction process, (b) Schematic diagram of Bragg’s law, & (c) Photograph of PANalytical Empyrean XRD diffractometer…............................................................................ 77 Figure 3.13: (a) Typical XRD pattern of In2O3, & (b) Fitting curve of preferred orientation peak……………………………………………………….. 77 (a) Schematic diagram of the Raman spectroscopy process, (b) Energy level diagram of Raman scattering phenomena, & (c) Photograph of Renishaw inVia Raman Microscope…………………………………... 79. a. Figure 3.14:. al. ay. Figure 3.15: (a) Schematic diagram of TEM, (b) Signals generated when the electron beam strikes the sample in the HRTEM, & (c) Photograph of TEM (JOEL JEM-2100F) with an accelerating voltage of 200 kV……. 81. M. Figure 3.16: (a) Comparison of the absorption electronic energy level diagram of UV-Vis and IR, (b) Schematic diagram of UV-visible spectrometer, & (c) Photograph of Lambda 750 UV-Vis-NIR spectrometer.…………... 84. of. Figure 3.17: (a) Typical Tauc plot, & (b) Fitting curve of linear slope from Tauc plot…………………………………………………………………….. 85. ve r. si. ty. Figure 3.18: (a & b) Photograph of photoelectrochemical measurement set-up, (c) Schematic diagram of a basic configuration of photoelectrochemical cell, (d) Photograph of the real-time measurement during the photoelectrochemical processes, & (e) Real image of the working electrode with the deposited sample.…………………………….……. 88. ni. Figure 3.19: Typical plot of (a) LSV graph, (b) EIS-Nyquist, (c) Bode, & (d) MottSchottky.………………………………………………………............. 89 FESEM images of In2O3 nanostructures grown by plasma assisted reactive thermal evaporation at different df-s of (a) 1, (b) 3, & (c) 5 cm... 93. Figure 4.2:. FESEM images of In2O3 composite nanostructures grown by nitrogen plasma assisted in-situ thermal annealing at different substrate temperature of (a) 150, (b) 200, (c) 250, (d) 300, (e) 350, & (f) 400 °C.. 95. Figure 4.3:. XRD pattern of In2O3 composite nanostructures grown at different substrate temperature………………………………..……….………... 97. U. Figure 4.1:. xv.

(17) (a) Raman spectra of In2O3 composite nanostructures grown at different substrate temperature, & (b) A typical deconvoluted of 𝐸 2 (high) and 𝐴1 (LO) at 300 °C…………………………………..……… 98. Figure 4.5:. FESEM images of the In2O3 composite nanostructures prepared by nitrogen plasma assisted in-situ thermal annealing at rf power 50 W with different filament temperatures of (a) 1300, (b) 1400, (c) 1500, (d) 1600, (e) 1700, & (f) 1800 °C.………………..……………………. 101. Figure 4.6:. XRD pattern of In2O3 composite nanostructures prepared by nitrogen plasma assisted in-situ thermal annealing at different filament temperature of 50 W applied rf power.…………………..……..……… 104. Figure 4.7:. Raman spectra of In2O3 composite nanostructures prepared by nitrogen plasma assisted in-situ thermal annealing at different filament temperature of 50 W applied rf power.………………………………… 106. Figure 4.8:. Wide scan XPS spectra of In2O3 composite nanostructures prepared by nitrogen plasma assisted in-situ thermal annealing at different filament temperature of 50 W applied rf power.……………………………….. 108. Figure 4.9:. XPS narrow scan and its deconvoluted spectra of (a) O 1s, (b) In 3d5/2, (c) N 1s, & (d) W 4f of composite nanostructures prepared at 1300 °C... 108. of. M. al. ay. a. Figure 4.4:. si. ty. Figure 4.10: XPS narrow scan and its deconvoluted spectra of (a) O 1s, (b) In 3d5/2, (c) N 1s, & (d) W 4f of composite nanostructures prepared at 1700 °C... 109. ve r. Figure 4.11: Relative integrated intensity plot against filament temperature of (a) In-In, In-N, In-O, & (b) WO3, WO2, W2N bonds……………………... 110. U. ni. Figure 4.12: (a) Optical transmission spectra of In2O3 composite nanostructures prepared at different filament temperature, (b) Absorption coefficient spectra against wavelength, (c) Tauc plot of the In2O3 composite nanostructures, & (d) Variation of the optical band gap energy against filament temperature……....................................................................... 112 Figure 4.13: FESEM images of the In2O3 composite nanostructures prepared by nitrogen plasma assisted in-situ thermal annealing at rf power 150 W with different Tf of (a) 1300, (b) 1400, (c) 1500, (d) 1600, (e) 1700, & (f) 1800 °C…………………………………………………………….. 114 Figure 4.14: XRD pattern of the In2O3 composite nanostructures prepared by nitrogen plasma assisted in-situ thermal annealing at different filament temperature of 150 W applied rf power……........................................... 116. xvi.

(18) Figure 4.15: Raman spectra of the In2O3 composite nanostructures prepared by nitrogen plasma assisted in-situ thermal annealing at different filament temperature of 150 W applied rf power……………………................... 118 Figure 4.16: (a & b) TEM images of In2O3/InN nanostructures prepared by nitrogen plasma assisted in-situ thermal annealing at Tf of 1300 ºC, (c) HRTEM image of In2O3/InN for the selected empty box in (a), & (d - g) FFT images of the In2O3 and InN nanostructure in (c) (HRTEM analysis at rf power 50 W)………………………………........................................ 120. ay. a. Figure 4.17: (a & b) TEM image of In2O3/WO3 nanostructures prepared by nitrogen plasma assisted in-situ thermal annealing at Tf of 1400 °C, (c) HRTEM image of In2O3/WO3 for the selected empty box in (a), & (d & e) FFT images of the In2O3 and WO3 nanostructures, respectively as shown in (c) (HRTEM analysis at rf power 150 W)……………………………... 121. M. al. Figure 4.18: (a) TEM image of In2O3/WO3 nanostructures, (b) Selected area perform the elemental mapping, & (c) EDX spectra and elemental maps of the In (red), W (green), N (blue), and O (grey)……………...… 122. ty. of. Figure 4.19: (a) TEM image of In2O3/W2N nanostructures prepared by nitrogen plasma assisted in-situ thermal annealing at Tf of 1500 °C, (b) HRTEM image of In2O3/W2N for the selected empty box in (a), & (c) FFT image of the In2O3 and W2N nanostructure in (b) (HRTEM analysis at rf power 50 W)………............................................................................... 123. ni. ve r. si. Figure 4.20: (a) TEM image of In2O3/W2N nanostructures prepared by nitrogen plasma assisted in-situ thermal annealing at Tf of 1800 °C, (b) HRTEM image of In2O3/W2N for the selected empty box in (a), & (c) FFT image of the In2O3 and W2N nanostructure in (c) (HRTEM analysis at rf power 150 W)………………………………………………………..... 124. U. Figure 4.21: (a) FESEM image of In2O3/W2N nanostructures, (b) TEM image perform the elemental mapping, & (c) EDX spectra and elemental maps of the In (red), W (green), N (blue), and O (grey)…………...….... 125 Figure 4.22: Wide scan XPS spectra of In2O3 composite nanostructures prepared by nitrogen plasma assisted in-situ thermal annealing at different filament temperature of 150 W applied rf power……………………................... 127 Figure 4.23: XPS narrow scan and its deconvoluted spectra of (a) O 1s, (b) In 3d5/2, (c) N 1s, & (d) W 4f of nanostructure composite prepared at Tf 1700 °C……………………………………………………………………… 127. xvii.

(19) Figure 4.24: Relative integrated intensity plot against filament temperature of (a) In-In, In-N, In-O, & (b) WO3, WO2, W2N bonds.…………………….... 129 Figure 4.25: (a) Optical transmission spectra of In2O3 composite nanostructures prepared at different filament temperature, (b) Absorption coefficient spectra against wavelength, (c) Tauc plot of the In2O3 composite nanostructures, & (d) Variation of the optical energy gap against filament temperature..…………………………...…………………….. 131. a. Figure 4.26: Proposed growth mechanism of (a) As-prepared In2O3, (b) In2O3/InN, (c) In2O3/WO3, & (d) In2O3/W2N nanostructures prepared by nitrogen plasma assisted in-situ thermal annealing.………………...................... 134 FESEM images of the nanowires prepared by plasma assisted reactive thermal evaporation at different substrate temperatures and growth time of (a - c) 200 °C, 5 mins, (d - f) 300 °C, 5 mins, & (g - i) 200 °C, 30 mins………………………………………………………………... 140. Figure 5.2:. Cross sectional analysis of (a & b) SE images at low and high electron accelerating voltages respectively, & (c & d) High magnification of SE and BSE images respectively, of In2O3 nanowires…….......................... 142. Figure 5.3:. (a & b) FESEM images at the base and near the tip of In2O3 nanowires, & (c & d) The respective EDX spectra of the selected area (highlighted in the image) at the base and tip of In2O3 nanowires………..………….. 143. Figure 5.4:. XRD patterns of the In2O3 nanowires prepared by plasma assisted reactive thermal evaporation at different substrate temperature and growth time…………………………………………………………… 145. ve r. si. ty. of. M. al. ay. Figure 5.1:. (a) TEM image of In2O3 nanowire prepared at 300 °C, 5 mins by plasma assisted reactive thermal evaporation, (b) TEM image of the tip of the nanowire, (c & d) HRTEM image of InN and In2O3 at the selected point 𝑐 and 𝑑, respectively as shown in (b) with its corresponding FFT (inset), (e) TEM image of the base of the nanowire, & (f) HRTEM image of the base of nanowire as shown in (e) with its corresponding FFT (inset)………………………………...................... 148. U. ni. Figure 5.5:. Figure 5.6:. XPS analysis of (a) Wide scan spectra of In2O3 nanowires, (b - d) Narrow scan and its deconvoluted spectra of In 3d5/2 peak of sample prepared at 200 °C, 5 mins; 300 °C, 5 mins, 200 °C, 30 mins respectively, & (e) Relative integrated intensity plot of In-In, In-N, InO bond of the nanowire samples……………………………………..... 150. xviii.

(20) (a) Optical transmission spectra of In2O3 nanowires, (b) Absorption coefficient spectra against wavelength, (c) Tauc plot of the In2O3 nanowires, & (d) Variation of optical energy gap at various parameter.……………………………………………………………... 152. Figure 5.8:. (a) FESEM images of In2O3 nanowires grown by PARTE, (b) FESEM images In2O3/W2N composite nanowires grown by nitrogen plasma assisted in-situ thermal annealing, & (c & d ) Piled terraces at the sidewall of the In2O3 nanowires and In2O3/W2N composite nanowires, respectively…………………………………………………………… 154. Figure 5.9:. XRD pattern of In2O3 nanowires and In2O3/W2N composite nanowires……………………………………………………………... 155. ay. a. Figure 5.7:. M. al. Figure 5.10: XPS analysis of (a) Wide scan spectra of In2O3 nanowires and In2O3/W2N composite nanowires, & (b - e) Narrow scan of In2O3/W2N composite nanowires and its deconvoluted spectra of In 3d5/2, O 1s, W 4f and N 1s peak, respectively..……………..…………………….…… 158. of. Figure 5.11: (a) Absorption coefficient spectra against photon wavelength, & (b) Tauc plot of the In2O3 nanowires and In2O3/W2N composite nanowires……………………………………………………………... 159. si. Linear sweep voltammetry of In2O3/InN, In2O3/WO3, and In2O3/W2N composite nanostructures electrode prepared by nitrogen plasma assisted in-situ thermal annealing at rf power 50 W.…………..………. 165. ve r. Figure 6.1:. ty. Figure 5.12: Proposed growth mechanism of In2O3 nanowires grown by PARTE at (a) 200 °C, 5 mins, (b) 300 °C, 5 mins, & (c). 200 °C, 30 mins.………... 162. (a) EIS-Nyquist plots of In2O3/InN, In2O3/WO3, and In2O3/W2N composite nanostructures at rf power 50 W, & (b - d) EIS-Nyquist plot at lower frequency range of In2O3/InN, In2O3/WO3, and In2O3/W2N, respectively.………………………………….………..……………… 167. U. ni. Figure 6.2:. Figure 6.3:. (a) Bode plots of In2O3/InN, In2O3/WO3 and In2O3/W2N composite nanostructures, & (b) Electron recombination lifetime plot against filament temperatures.……………………………………………….... 168. Figure 6.4:. (a - c) Mott-Schottky plot of In2O3/InN, In2O3/WO3, and In2O3/W2N composite nanostructures, respectively, & (d) Dependence of flat band potential and donor density of the composite nanostructure at different filament temperature………….…………………................................... 170. xix.

(21) Linear sweep voltammetry of In2O3/InN, In2O3/WO3, and In2O3/W2N composite nanostructures electrode prepared by nitrogen plasma assisted in-situ thermal annealing at rf power 150 W.………..………... 172. Figure 6.6:. (a) EIS-Nyquist plots of In2O3/InN, In2O3/WO3, and In2O3/W2N composite nanostructures at rf power 150 W, & (b - d) EIS-Nyquist plot at lower frequency range of In2O3/InN, In2O3/WO3, and In2O3/W2N, respectively…………..……………..……......................... 174. Figure 6.7:. (a) Bode plots of In2O3/InN, In2O3/WO3, and In2O3/W2N composite nanostructure, & (b) Electron recombination lifetime plot against filament temperatures.………………………………………………… 175. Figure 6.8:. Mott-Schottky plot of (a) In2O3/InN, (b) In2O3/WO3, & (c) In2O3/W2N composite nanostructures.……..…………………………………….... 176. Figure 6.9:. Schematic diagram of proposed charge transfer mechanisms in PEC processes of the In2O3/W2N composite nanostructures.……………...... 178. M. al. ay. a. Figure 6.5:. of. Figure 6.10: (a) Linear sweep voltammetry, & (b) EIS-Nyquist plots of In2O3 nanowires and In2O3/W2N composite nanowires.……….…………….. 181. ty. Figure 6.11: (a) Bode plots, & (b) Electron recombination lifetime plot of In2O3 nanowires and In2O3/W2N composite nanowires.………….………….. 181. ve r. si. Figure 6.12: (a) Mott-Schottky plot, & (b) Dependence of flat band potential and donor density of the In2O3 nanowire and In2O3/W2N composite nanowires.…………………………………………………………….. 182. U. ni. Figure 6.13: Transient photocurrent density responses of (a) composite nanostructures, & (b) nanowires………………………………………. 184. xx.

(22) LIST OF TABLES Summaries of the crystal structures and properties of indium and tungsten based oxides and nitrides…..………………………………… 4. Table 2.1:. Summaries of various In2O3 based nanostructures used as electrode material in photoelectrochemical water splitting.…………………….. 49. Table 3.1:. Deposition parameter detail on the growth of In2O3 based nanostructures..……………………………………….…………......... 67. Table 3.2:. Parameter use in FESEM analysis...…………………………………... 71. Table 3.3:. Integrated peak area value of In 3d5/2 spectra.………………………… 74. Table 4.1:. Crystallite size of In2O3, InN, WO3 and W2N of nanostructures prepared by nitrogen plasma assisted in-situ thermal annealing at different filament temperature.……….…………………….…………. 104. Table 4.2:. Crystallite size of In2O3, InN, WO3, and W2N of nanostructures prepared by nitrogen plasma assisted in-situ thermal annealing at different filament temperature.…...…………………………………… 117. Table 4.3:. Elemental analysis on the In2O3 nanostructures.……………………… 125. Table 5.1:. Elemental analysis of base and tip In2O3 nanowires grown at 300 °C for 5 mins.…………………………………….………………………. 143. ve r. si. ty. of. M. al. ay. a. Table 1.1:. Crystallite size of In2O3 nanowires prepared by plasma assisted reactive thermal evaporation.…………………………......................... 146. Table 6.1:. The values of Rs, Rct, Diameter of semicircle, flat band potential, and donor density of the composite nanostructures at rf power 50 W……... 171. U. ni. Table 5.2:. Table 6.2:. The values of Rs, Rct, Diameter of semicircle, flat band potential, and donor density of the composite nanostructures at rf power of 150 W….. 177. Table 6.3:. The values of Rs, Rct, Diameter of semicircle, flat band potential, and donor density of the nanowires.……………………...…...................... 182. xxi.

(23) LIST OF SYMBOLS AND ABBREVIATIONS :. Absorption coefficient. C. :. Capacitance of the space charge region. 𝑅𝑐𝑡. :. Charge transfer resistance. ECB. :. Conduction band energy. 𝑁𝑑. :. Donor density. e-. :. Electron. 𝜏𝑛. :. Electron recombination lifetime. χ. :. Electronegativity of the semiconductor. q. :. Elementary electron charge. 𝐸𝑒. :. Energy of free electrons vs. hydrogen. Tf. :. Filament temperature. df-s. :. Filament-to-Si substrate distance. 𝑉𝑓𝑏. :. Flat band potential. fmax. :. Frequency maximum. A. :. Interfacial area. :. Onset potential. :. Optical energy gap. :. Permittivity of free space. Eg. :. Phonon vibration mode. ℎ𝑣. :. Photon energy. R. :. Reflection. . :. Relative dielectric constant. Ts. :. Substrate temperature. 𝑅𝑠. :. Solution resistance. ni. 𝐸𝑔. U. o. ay. al. M. of. ty. si. ve r. Eon. a. 𝛼. xxii.

(24) :. Valence band energy. CVD. :. Chemical vapor deposition. EDX. :. Energy-dispersive X-ray spectroscopy. EIS. :. Electron impedance spectroscopy. FESEM. :. Field emission scanning electron microscopy. FFT. :. Fast Fourier transform. FWHM. :. Full width at half maximum. HRTEM. :. High resolution transmission electron microscopy. ay. a. EVB. Hot wire chemical vapor deposition. In. :. Indium. InN. :. Indium nitride. In2O3. :. Indium oxide. IPCE. :. Incident photon to current conversion efficiency. LO. :. Longitudinal optical. LSV. :. Linear sweep voltammetry. MFC. :. Mass flow controller. MS. :. Mott-Schottky. :. Nitrogen. :. Normal hydrogen electrode. Ni. :. Nickel. PARTE. :. Plasma assisted reactive thermal evaporation. PEC. :. Photoelectrochemical. PLD. :. Pulsed laser deposition. Pt. :. Platinum. sccm. :. Standard cubic centimeters per minute. SE. :. Secondary electron. U. ni. NHE. M of. ty. si. ve r. N2. al. HWCVD :. xxiii.

(25) :. Silicon. SS. :. Stainless steel. TEM. :. Transmission electron microscopy. TCOs. :. Transparent conducting oxide. TO. :. Transversal optical. T. :. Transmission. UV. :. Ultraviolet. UV-Vis. :. Ultraviolet visible spectroscopy. VB. :. Valence band. W. :. Tungsten. W2N. :. Tungsten nitride. WO3. :. Tungsten oxide. XPS. :. X-ray photoelectron spectroscopy. XRD. :. X-ray diffraction. YSZ. :. Yttria-stabilized zirconia. U. ni. ve r. si. ty. of. M. al. ay. a. Si. xxiv.

(26) CHAPTER 1: INTRODUCTION 1.1. Background on Materials Used. Wide bandgap metal oxides are particularly useful for optoelectronic and energy harvesting applications such as in LEDs, conducting electrodes, lasers, and fuel cells due to their unique physical and superior electronic properties. Research on In2O3 started in the last century and was focused mainly on investigating its potential application as a coating material for optical mirrors and lenses. In2O3 is produced by the oxidation of. ay. a. evaporated indium layers at high temperatures in ambient air resulting in the formation of polycrystalline In2O3 (Rupprecht, 1954). In2O3 has been extensively investigated of. al. late for its high electron carrier mobility of ~200 cm2/Vs, (Bourlange et al., 2009; Galazka. M. et al., 2013; Koida et al., 2006; Ohta et al., 2000; Scherer et al., 2012; Weiher, 1962), high electrical conductivity (Girtan et al., 2000), lower electrical resistivity, extremely. of. high optical transparency, and high reflectance in infrared region (Zhang et al., 2016c).. ty. This makes it extremely useful in optoelectronic devices, specifically as transparent conducting oxides (TCOs) (Bierwagen, 2015; Ho et al., 2011), field effect transistors. si. (Kim et al., 2016; Shen et al., 2011; Zou et al., 2013) and gas sensing devices (Li et al.,. ve r. 2013; Ma et al., 2016). Also, its high electron carrier mobility, electrical conductivity, high optical absorption in visible light, and high chemical stability has made In2O3 very. ni. promising material for photoelectrochemical (PEC) electrodes in water splitting. U. processes (Mu et al., 2012; Zhang et al., 2013). Besides indium oxide, InN which belongs to the group III-nitride (AlN, GaN, and InN). semiconductor, has various remarkable properties such as small direct energy gap, smallest effective mass, extremely high saturation velocity, high mobility, and large electron drift velocity at room temperature (Davydov et al., 2002; Valdueza et al., 2010; Wu et al., 2002; Yun et al., 2010). Thus, InN is considered a highly favorable material for electronic and photonic devices including high speed field-effect transistors, terahertz 1.

(27) emitters (Ascázubi et al., 2004), high efficiency solar cells, near infrared LEDs, and LDs (Davydov et al., 2002; Wu et al., 2012). On the other hand, WO3 materials have shown to have interesting contributions mainly in X-ray tubes, fireproofing fabrics, and gas sensors, and in ceramics and paint applications. Robert Oxland discovered WO3 while he was preparing tungsten trioxide and sodium tungstate. WO3 is basically a wide energy gap metal oxide material that has. a. a strong absorption coefficient in the solar spectrum, stable physiochemical properties,. ay. and strong resistance to the effects of photocorrosion. These properties have made WO3. al. a good candidate for electrodes in the PEC water splitting process (Bamwenda & Arakawa, 2001; Chong et al., 2015). In addition, the chromism effect of WO3 which is. M. related to color changes caused by external disruptions such as voltage, changing gases,. of. heating, or irradiation of light offers much potential for its application in smart windows,. al., 2010).. ty. flat panel displays, and optical memory devices (Niklasson & Granqvist, 2007; Patel et. si. W2N, belongs to a class of transition metal nitrides, a popular compound used in. ve r. conductive layers and smart windows (Toth, 1971). It has been extensively studied as a cathode catalyst for fuel cells (Lee et al., 1993), storage nodes in flash memories,. ni. hardwearing-resistant protective coatings for cutting tools (Lévy et al., 1999), gate. U. electrodes in metal oxide semiconductor field effect transistors (Masaru et al., 2000), and diffusion barriers in microelectronic devices (Wang et al., 2008). Recent reports have. highlighted its potential application as a lithium-ion battery anode (Nandi et al., 2015). W2N also shows high photoelectrochemical performance under irradiation of visible light (Chakrapani et al., 2009; Jing et al., 2014; Shi et al., 2015). These properties of indium and tungsten based oxides and nitrides have prompted research on In2O3 based composite nanostructures in various interesting applications. For 2.

(28) instance, the growth of In2O3-TiO2 nanocomposites by hydrothermal oxidation has been widely developed and researched in the last two decades (Babu & Srivastava, 1989). The nanocomposites are prepared specifically for photocatalytic activity (Babu et al., 1990; Mu et al., 2012; Wu et al., 2015). Furthermore, the various In2O3 based nanocomposites such as, In2O3/SnO2 (Li et al., 2013), In2S3/In2O3 (Zhang et al., 2016a), and CuO/In2O3 (Kumar et al., 2015) have been intensively investigated for gas sensing, photocatalytic processes and light emitting applications, respectively. Tungsten-based nanocomposites,. ay. a. tungsten nitride nanoparticles decorated on nitrogen-rich porous graphene-like carbon nanosheets (WNx-NRPGC) (Zhu et al., 2018) and iron-doped tungsten oxide. al. nanoplate/reduced graphene oxide nanocomposite (Fe-WOxP/rGO) (Wondimu et al.,. M. 2018) have recently been reported as being good electrodes for hydrogen evolution. of. reactions.. The crystal structures and properties of indium and tungsten based oxides and nitrides. ty. are of interest to researchers as they provide new explanations for the growth of novel. si. nanostructures in novel and interesting applications. The crystal structures and properties. U. ni. ve r. of these materials are tabulated in Table 1.1.. 3.

(29) al ay. a. Table 1.1: Summaries of the crystal structures and properties of indium and tungsten based oxides and nitrides.. Lattice parameter (Å) Material. Crystal structure. Bcc. 𝒃. Electron effective. Electron mobility,. 𝑬𝒈 (eV). mass, 𝒎∗𝒆.  cm2/Vs). 0.15𝑚0. 200. 0.11𝑚0. 250. NR. 80. NR. NR. 𝒄. M. 𝒂. Optical energy gap,. 10.117. ~3.6 - 3.7 (direct). In2O3. W2N. Hexagonal. 7.298. Cubic. ty. 5.690. ~2.8 (indirect). of. 14.510. rs i. 3.530. 4.126. U. NR = Not reported. Wurtzite. 5.487. ve. WO3. 5.487. ni. InN. Hexagonal. ~0.7. ~3.25-3.65 (amorphous). 3.899. ~2.6 - 2.7 (crystalline) ~2.2. 4.

(30) 1.2. Metal Oxides and Nitrides in Photoelectrochemical Processes. In semiconductor oxide and nitride materials, the optical energy gap (𝐸𝑔 ) plays an essential role in the absorption of light. By absorbing sufficient energy from incident photons, electrons from valence bands (VB) transfer to the conduction band (CB) and leave a hole which subsequently generates electron/hole pairs between the energy gaps. These electron/hole pairs play a role in chemical reaction for the redox process to occur.. a. In fact, several criteria applied show that semiconductor photocatalysts have the. ay. capability of performing water splitting processes and this can be explained based on the. al. following: 1.. The optical energy gap of semiconductors in the visible region of the. M. electromagnetic spectrum (~< 3 eV) makes them capable of absorbing 45% solar. of. illumination at optimal conditions as illustrated in Figure 1.1. 2.. Photocatalytic water splitting using a single photocatalyst requires the bandgap of. ty. the semiconductor to straddle the reduction and oxidation potentials of water,. si. which are +0 and +1.23 V vs the normal hydrogen electrode (NHE) at pH =0 as. ve r. depicted in Figure 1.2. Practically, the semiconductor must have a band gap energy of at least 1.6 eV in order to split water. For a redox process to occur the conduction band edge must be more negative. ni. 3.. U. than the water reduction potential while the valence band edge must be more positive than the water oxidation potential (Figure 1.3).. It is worth noting how semiconductor photocatalysts can be suitable or unsuitable for PEC processes. As shown in Figure 1.3(a), the materials are considered suitable for PEC processes if the VB of the semiconductor edge is deeper than the O2/H2O oxidation potential. In contrast, the material is unsuitable if the CB minima are lower than the thermodynamic requirements (Figure 1.3(b)). Suitable semiconducting photocatalysts. 5.

(31) include materials such as GaN, TiO2, In2O3, SiC, GaP, and CdS while SnO2, NiO2, BaTiO3, ZnO, WO3, Fe2O3, Cu2O, CuO, and MoS2 are considered unsuitable because. of. M. al. ay. a. their CB levels are mostly lower than the water reduction potential.. U. ni. ve r. si. ty. Figure 1.1: The solar spectrum with respect to wavelength (Tada & Jin, 2016).. 6.

(32) a ay al. U. ni. ve r. si. ty. of. M. Figure 1.2: Band energy levels of various semiconductors with different Egs (Abe, 2010).. Figure 1.3: Absolute conduction band and valence band energy levels for (a) suitable, & (b) unsuitable semiconducting photocatalysts with respect to normal hydrogen electrode (NHE) and vacuum (Vac) (Babu et al., 2015).. 7.

(33) Additionally, PEC performances can be influenced by the types of semiconductors i.e., whether they are the n-type or p-type. N-type semiconductors are dominated by electrons termed majority carriers and the holes are called minority carriers, whereas p-type semiconductors are dominated by holes as majority carriers and the electrons are minority carriers. For n-type semiconductor electrodes, when the photon energy is higher than its energy gap, the absorption of photon energy results in the photoexcited electrons to transfer to a counter electrode (e.g., Pt) through the outer circuit. Hence, the electrons. ay. a. induce a reduction process of the H2O molecules to H2molecules. The holes are then transferred to the semiconductor surface and oxidize the H2O molecules to O2 (Figure. al. 1.4(a)). In contrast, for the p-type semiconductor, the reduction of water occurs on the. M. semiconductor surface by the photoexcited electrons, while H2O molecules are oxidized resulting in the production of electrons near the counter electrode, as shown in Figure. of. 1.4(b). By coupling with narrow band gap materials, the absorption of light in the. ty. semiconductor is extended into the visible region. The shifting of the band gap is done via (i) modification of VB, (ii) adjustment of CB, and (iii) continuous modulation of VB. si. and/or CB (Tong et al., 2012). The purpose of shifting the band gap is to efficiently. ve r. improve the transportation of photoexcited charges along a preferable route for redox. ni. reactions and to effectively render the separation of photoexcited electrons/holes. Growth Techniques of In2O3 Based Nanostructures. U. 1.3. Due to the attractive properties of In2O3, several growth techniques have been. extensively investigated including chemical vapor deposition (CVD) (Kim et al., 2005; Shen et al., 2011; Singh et al., 2009), hydrothermal process (Chen et al., 2015; Yang et al., 2017; Zhang et al., 2016a), pulsed laser deposition (PLD) (Li et al., 2003; Savu & Joanni, 2006; Tarsa et al., 1993; Warmsingh et al., 2004; Yoko et al., 2017), rf-magnetron sputtering (Krishnan et al., 2017; Wei et al., 2011), and evaporation (Chu et al., 2008;. 8.

(34) Corrêa et al., 2014; Ganesh et al., 2015; Meng et al., 2014; Nandan et al., 2011; Yan et al., 2007; Zervos et al., 2017). These techniques will be described in Chapter 2. Physical growth methods have been widely used in the growth of In2O3 nanostructures; however, the chemical growth technique reported by Liu and co-workers has successfully grown the monodisperse In2O3 at high temperatures using organic solutions (Liu et al., 2005). For the PEC devices, the structure of the grown In2O3 nanostructures should be. a. stable in electrolytes and be capable of performing high efficiency photon-to-. ay. hydrogen/oxygen conversion. Therefore, it is advantageous to have simple, reproducible,. al. and controlled properties of the growth technique to obtain high quality nanostructures. The uniform distribution of In2O3 nanowires are achieved by simple evaporation and. M. mixing In2O3 and carbon powder utilizing a carbothermal reduction method at 1100 °C. of. for 20 mins. The evaporated sample is deposited on Si substrate coated with a 10 nm Au. U. ni. ve r. si. ty. layer.. Figure 1.4: Photoelectrochemical water splitting systems using (a) n-type semiconductor photoanode, & (b) p-type semiconductor photocathode (Abe, 2010).. 9.

(35) To improve absorption in the visible region, the nanowires are annealed in hydrogen ambient by hydrogenated In2O3 nanowires at 400, 500, and 600 °C (Meng et al., 2014). Recently, controllable of In2O3/In2S3 microflower heterostructures have been successfully grown by hydrothermal process. The uniform assembly of In2S3 nanoflakes on the surface of the In2O3 microflowers has been reported to show enhanced visiblelight photoactivity (Zhang et al., 2016a).. a. Research Problems. ay. 1.4. Despite In2O3 being widely used and having superior properties for PEC applications,. al. several ambiguities remain in understanding its energy band configurations and. M. fundamental optical absorptions for such purposes. Generally, the application of In2O3 in PEC processes is limited by its large energy gap (~ 3.75 eV) (Zou et al., 2013) which is. of. absorbed in the UV region (low sunlight absorption of 3 %) resulting in a low sunlight. ty. conversion quantum efficiency (Linsebigler et al., 1995). The direct and wide band gaps of In2O3 are only suitable for harvesting a small range of high energy photons. In addition,. si. In2O3 usually contains a high density of oxide states which can act as a trap within the. ve r. crystal structure resulting in a recombination of electron/hole pairs. The photogenerated majority carriers are then lost in these states before reaching the electrode/electrolyte. ni. interfaces. The wide In2O3 energy gap is incompatible with its electrochemical potential. U. which is specifically lower than the energy of the oxidation potential. In addition, the short diffusion length of the charge carriers in the In2O3 structure reduces the migration of carriers resulting in low redox capabilities (Scaife, 1980). These drawbacks suggest the importance of extending the photoresponse of In2O3 towards the visible regions, and one possible technique is by incorporating a narrow energy gap semiconductor photocatalyst. Several attempts have been reported using heterostructure composites of. 10.

(36) AgCl/H2WO4·H2O (Wang et al., 2008a), ZnFe2O4/In2O3 (Zhang et al., 2016b), TiO2/In2O3 (Wu et al., 2015), and Ni-Fe/In2O3-WO3 (Chaudhari & Singh, 2017). In recent years, maximizing the absorption of visible light has received much interest together with the growth of large surface area nanostructures such as nanorods, nanowires, and nanotubes. Various techniques employed to grow these nanostructures have been reported as being able to increase the number of photogenerated majority. a. carriers thus leading to the enhancement of redox performances in the PEC. Further, the. ay. large surface area nanostructures increase the diffusion of photogenerated carriers in the. al. electrode/electrolyte interface which facilitates charge separation and reduces the electron/hole pair recombination (Kargar et al., 2013). The enhancement of PEC water. M. splitting by In2O3 nanowire electrodes has been reported in (Meng et al., 2014; Sariket et. of. al., 2017; Zhou et al., 2011). These works show that there is much potential to investigate. ty. metal or In2O3 based composite nanomaterials specifically for PEC applications. In most cases, the growth of crystalline In2O3 nanostructures requires high reaction. si. temperatures and expensive deposition systems. Apart from that, it is rather challenging. ve r. to control the high-symmetry cubic crystals of the In2O3 nanowires during the growth process (Hadia & Mohamed, 2013). In general, important factors determining the quality. ni. of nanowires are size uniformity, dimensionality, growth direction, and dopant. U. distribution (Savu & Joanni, 2006). With regard to the metal catalyzed grown nanowires, Au, Ag, or Ni are often used to facilitate the high oriented growth of cubic crystals in In2O3 nanowires (An et al., 2013; Kim et al., 2011; Papageorgiou et al., 2011). However,. these metal catalysts produce undesirable dopants or contamination, or include the presence of additional phases, i.e., alloy particles. This could lead to the formation of stacking faults and dislocations that adversely affect the properties of the nanowire particularly the electrical and photoelectrochemical ones (Hadia & Mohamed, 2013;. 11.

(37) Mohammad, 2011). Thus, developing a technique to grow crystalline In2O3 nanostructures using low reaction temperatures and a cost-effective deposition system is one of motivations to address the research problems mentioned above. 1.5. Motivations and Research Objectives Based on the aforementioned applications of In2O3 nanostructures especially in. optoelectronics and PEC, further investigations are needed in the area of enhancing In2O3. ay. a. properties by the fabrication of composite nanostructures or their intrinsic nanowires. In addition, the low-temperature growth, low-costs, and large-scale production of these. al. transparent conducting materials are of much interest to industry.. M. The bandgap-engineering of In2O3 nanostructures by tuning their 𝐸𝑔 into visible. of. region had been reported in recent years (Gan et al., 2011; Kumar et al., 2015; Yang et al., 2013; Zhang et al., 2013; Zhang et al., 2016a). The narrow bandgap of grown In2O3. ty. nanostructures can result in high absorbance intensity in the visible region resulting in. si. significantly improving photon-energy harvesting efficiency. In addition, the narrowing. ve r. of the energy gap by the formation of core-shell structures, such as In2O3/CaIn2O4 (Chang et al., 2007), In2O3-In2S3 (Yang et al., 2013), In2O3-CdSe (Shinde et al., 2014),. ni. hydrogenated-In2O3 (Meng et al., 2014), and PbS/Sn:In2O3, (Zervos et al., 2017) have. U. also been reported recently. Although formation core-shell nanostructures has been extensively studied, the formation of heterojunctions or composites materials has achieved substantial improvements in various applications such as for sensors, optoelectronic devices, and also for energy harvesting in fuel cells (Gan et al., 2011; Ma et al., 2016; Singh et al., 2011). This has been reported in several renowned international papers.. 12.

(38) In this work, In2O3 based composite nanostructures and nanowires are grown using simple plasma assisted reactive thermal evaporation. This thermal evaporation technique has been previously reported to grow high deposition rate thin films and nanostructured materials (Alizadeh et al., 2016a; Chu et al., 2008; Corrêa et al., 2014; Ganesh et al., 2015; Singh et al., 2011; Yan et al., 2007; Zervos et al., 2017). The thermal evaporation which is operated using a hot-filament is a simple approach where the metal source is evaporated in an inert gas environment or in ambient plasma. Under the plasma. ay. a. environment, the generated energetic or excited N radicals are capable of enhancing the surface reactions and increasing surface mobility of the growth adatoms. The low-energy. al. ion bombardment effect by nitrogen or argon radicals is capable of increasing the surface. M. nucleation of the In2O3 single crystals. Furthermore, hydrogen plasma has been proven effective in removing oxide contaminants and in activating the surface nucleation of. of. metal particle for the self-catalyzing growth of the metal oxide nanowires. The thermally. ty. assisted plasma is also able to increase the incorporation of the energetic radicals into the. si. nucleation sites as dopants or composite heterostructures.. ve r. The plasma assisted in-situ thermal annealing is employed on the as-grown nanostructure and nanowire samples to form composite nanostructures at the similar low. ni. Ts as well. These composite nanostructures can be formed in the phase of metal oxides. U. and metal nitrides nanostructures and the growth process can be done without breaking vacuum conditions. The novelty of this work is the growth of In2O3 based composite. nanostructures and nanowires using a one-step simple evaporation technique. The nitrogen plasma assisted in-situ thermal annealing for these composite nanostructure materials has not been previously reported elsewhere. The key objective is to demonstrate that this technique can control the precise growth of composite nanostructures at low cost as well as address environmental concerns.. 13.

(39) As such, the objectives of this research were as follows: 1. To investigate the growth of In2O3 based composite nanostructures by plasma assisted reactive thermal evaporation and nitrogen plasma assisted in-situ thermal annealing; 2. To examine the morphology, structure, composition, and optical properties of the treated In2O3 based composite nanostructures by nitrogen plasma assisted in-situ. a. thermal annealing;. ay. 3. To investigate the effects of growth conditions on the properties of the treated. al. In2O3 based composite nanostructures;. optimizing growth conditions;. M. 4. To tune the 𝐸𝑔 of In2O3 based composite nanostructures into the visible region by. of. 5. To determine the photoelectrochemical performances of the In2O3 based composite nanostructures; and. ty. 6. To propose the growth mechanisms of In2O3 based composite nanostructures and. si. In2O3 nanowires by nitrogen plasma assisted in-situ thermal annealing and plasma. Outline of Thesis. ni. 1.6. ve r. assisted reactive thermal evaporation, respectively.. U. The thesis covers 7 chapters including literature review studies on In2O3. nanostructures (Chapter 2), preparation technique and sample characterization of In 2O3 nanostructures (Chapter 3), results and discussion of the structure, optical and PEC properties (Chapter 4 - 6), and end up with conclusion and suggestions for future works (Chapter 7). Chapter 1 highlights the background of materials and their properties, application of metal oxides and nitrides in PEC processes and growth techniques of In2O3. based nanostructures. The chapter ends with presentation of the research problems, motivations and objectives of the work. In Chapter 2, the growth of In2O3 nanostructures 14.

(40) by various techniques are described comprehensively. The structural and optical properties of the related materials as well as semiconductor photocatalyst for PEC are reviewed. From the literature works perspective, the effect of various surface morphologies and 𝐸𝑔 of In2O3 nanostructures towards PEC performances have been clarified and discussed in the last topic of Chapter 2. Chapter 3 describes the sample preparation method for both In2O3 based composite. a. nanostructures and nanowires as well as the characterization technique implement in this. ay. study. The basic principle theory of each characterization technique is briefly explained.. al. The results and discussion disclosed in this thesis is covered in Chapters 4, 5, and 6. Chapter 4 discusses the formation of the In2O3 based composite nanostructures by. M. nitrogen plasma assisted in-situ thermal annealing. Effect on the structural and optical. of. properties by varying the rf power of 50 and 150 W during in-situ thermal annealing were studied and briefly explained. The proposed growth mechanism of the In2O3 composite. ty. nanostructures are presented in the last section of Chapter 4. Chapter 5 discusses the. si. results on the formation of In2O3 nanowires by self-catalyzed growth. Effect on the. ve r. growth conditions at different Tss and deposition times will be discussed. Investigation of the treated In2O3 nanowires by nitrogen plasma assisted in-situ thermal annealing will. ni. also be presented. The chapter ends with the proposed growth mechanism of self-. U. catalyzed In2O3 nanowires. Chapter 6 focuses on the PEC properties for both In2O3 based composite nanostructures and nanowires. The summary and conclusions of this work will be presented in Chapter 7 including the significance of findings and suggestions for future works.. 15.