隨著石油能源的耗竭以及其造成全球暖化的影響，以氫能源作為一種可再生能源受到許多矚目，其中致力於光催化產氫的研究是一大潛力能源再生方法，而本研究中利用兩種效應提高光催化產氫效率：(1) 建立異質接面協助電子的傳輸以及 (2) 建立壓電電場使照光產生的電子電洞對趨向相反方向以降低再結合率。
在本論文中，透過調控二硫化鉬及氧化鋅的混合比例，並且以水熱法製程產生異質接面及適當中間相-硫化鋅，此具有壓電性質的三元催化劑-氧化鋅微米柱/硫化鋅顆粒/二硫化鉬奈米花之組合可在模擬太陽光(使用AM-1.5G之濾光片)及超音波震盪下達到最高的產氫效率表現，此時樣品的產氫效率為10.42 mmol/(g·h), 超過純氧化鋅及二硫化鉬樣品分別是7倍以及33倍以上。此明顯的增益效率可以歸因於在異質結構間適當的能帶匹配以及三元催化劑ZnO/ZnS/MoS2共存下具有多種電子傳遞路徑。
此外，三元催化劑之壓電-光催化產氫效率相較於僅以物理接觸混合的同比例下之氧化鋅-二硫化鉬組合高出了4倍，說明了中間相-硫化鋅的產生大幅提升異質接面傳遞電子的能力。接下來透過比較三元異質結構於純照光以及同時照光及超音波震盪時效率差異可相差至2倍以上確認在外力施加於材料時產生之壓電電場在助益載子分離率進而提升產氫效率是相當重要的。

In recent decades, hydrogen energy as promising renewable energy has drawn a lot of attention due to the shortage of fossil fuel and the threatening of global warming. In most researches on hydrogen production, photocatalysts play the key role to overcome the bottleneck of low efficiencies. Tremendous efforts on pursuing high H2 production has been investigated. The concept of suppressing the recombination rate of photo-induced carriers can be achieved by co-catalysts and piezo-potential to drive the e-/h+ in the opposite directions.
In this thesis, enhanced piezo-photocatalytic properties for hydrogen production with ZnO microrods (MRs) / ZnS nanoparticles (NPs) / MoS2 nanoflowers (NFs) have been demonstrated under simulated solar light (with AM 1.5 G filter) and ultrasonication. The ternary heterostructures can be obtained by varying the weight ratio of MoS2 to ZnO in the hydrothermal method. When the moderate formation of ZnO/ZnS/MoS2 in the whole system, the highest H2 production rate, exceeding that of the pure ZnO and MoS2 samples by 7 times and 33 times, respectively. This can be achieved by using 5 mM Na2S and 5 mM Na2SO3 as sacrificial reagents in water under both 300 W xenon arc lamp and 200 W ultrasonication. The high piezo-photocatalytic H2 production activity can be ascribed to the suitable band alignments among the ternary phases and the variable charge transfer pathways between multi interfaces in the heterostructures.
Additionally, the physically-mixed ZnO-MoS2 sample shows less improvement in H2 production, thus, the formation of ZnS with a hydrothermal method plays a key role in this work. Furthermore, the H¬2 production under both simulated solar light and ultrasonication can exceed that of under solely simulated solar light by 2.3 times. The efficiency was significantly improved by the modulation of piezo-potential to suppress the recombination of photo-induced carriers.

Contents
Abstract I
摘要 III
Ackmowledgements IV
致謝 V
Contents VI
Chapter 1 Introduction 1
1.1 Hydrogen energy 1
1.2 Hydrogen production by photocatalytic water splitting 3
1.3 Heterostructured materials 4
1.4 Piezo-photonic effect 5
1.4.1 Principles of piezoelectric property 5
1.4.2 Enhanced catalysis reaction by piezo-phototronic effect 6
1.5 Physical properties of zinc oxide (ZnO) 9
1.5.1 Structure and properties 9
1.5.2 ZnO as photo-, piezo-(sono-), photo-piezocatalysts 10
1.6 Physical properties of molybdenum disulfide (MoS2) 12
1.6.1 Structure and properties 12
1.6.2 MoS2 as photo-, piezo-, photo-piezocatalysts 13
1.7 Formation of heterostructures via ion-exchange method 15
1.8 Motivations 17
Chapter 2 Experimental Section 18
2.1 Experimental flowchart 18
2.1 Experimental procedures 19
2.1.1 Preparation of ZnO via hydrothermal method 19
2.1.2 Preparation of MoS2 via hydrothermal method 20
2.2.3 Preparation of MZ heterostructures via hydrothermal method 21
2.2.4 Measurements of H2 production 23
2.2.5 Quantum yield measurement 24
2.3 Experimental details 25
2.3.1 Scanning Electron Microscope (SEM) 25
2.3.2 X-Ray Diffractometer (XRD) 26
2.3.3 Transmission Electron Microscope (TEM) 27
2.3.4 X-Ray photoelectron spectroscopy (XPS) 29
2.3.5 Ultraviolet-Visible Spectroscopy (UV-Vis Spectroscopy) 30
2.3.6 Piezoelectric Force Microscopy (PFM) 31
2.3.7 Gas Chromatography (GC) 32
Chapter 3 Results and Discussion 33
3.1 Characterizations of pristine materials 33
3.1.1 Synthesis of ZnO microrods(MRs) 33
3.1.2 Synthesis of MoS2 nanoflowers (NFs) 34
3.2 H2 production efficiencies with pristine ZnO and MoS2 38
3.3 Characterizations of the heterostructures 40
3.3.1 SEM analysis 40
3.3.2 XRD analysis 42
3.3.3 TEM and EDS analysis 44
3.3.4 XPS analysis 47
3.3.5 UV-Vis spectroscopy analysis 49
3.2.6 Piezoelectric Force Microscopy (PFM) analysis 50
3.4 H2 production with MZ heterostructure 51
3.4.1 Piezo-photocatalytic production with MZ heterostructures 51
3.2.7 Influence of new phase and piezo-potential on catalytic H2 production 53
3.2.8 Comparison on H2 production with other relevant works 58
Chapter 4 Summary and Conclusions 59
Chapter 5 Future prospects 61
5.1 Immobilized piezo-photocatalysts 61
5.2 Plasmon-enhanced piezo-photocatalytic H2 production 63
References 65

References
1. Quarton CJ, Tlili O, Welder L, Mansilla C, Blanco H, Heinrichs H, Leaver J, Samsatli NJ, Lucchese P, Robinius M. The curious case of the conflicting roles of hydrogen in global energy scenarios. Sustainable Energy & Fuels 4, 80-95 (2020).

2. Turner J. A realizable renewable energy future. Science (New York, NY) 285, 687-689 (1999).

3. Zhang P, Zhang J, Gong J. Tantalum-based semiconductors for solar water splitting. Chemical Society Reviews 43, 4395-4422 (2014).

4. Zhu S, Wang D. Photocatalysis: basic principles, diverse forms of implementations and emerging scientific opportunities. Advanced Energy Materials 7, 1700841 (2017).

5. Abe R. Recent progress on photocatalytic and photoelectrochemical water splitting under visible light irradiation. Journal of Photochemistry and Photobiology C: Photochemistry Reviews 11, 179-209 (2010).

6. Liu Y, Zhang S, He J, Wang ZM, Liu Z. Recent progress in the fabrication, properties, and devices of heterostructures based on 2D materials. Nano-Micro Letters 11, 13 (2019).

7. Li X, Shen R, Ma S, Chen X, Xie J. Graphene-based heterojunction photocatalysts. Applied Surface Science 430, 53-107 (2018).

8. Curie P, Curie J. Développement par compression de l'électricité polaire dans les cristaux hémièdres à faces inclinées. Bulletin de Minéralogie 3, 90-93 (1880).

9. Liang Z, Yan C-F, Rtimi S, Bandara J. Piezoelectric materials for catalytic/photocatalytic removal of pollutants: recent advances and outlook. Applied Catalysis B: Environmental 241, 256-269 (2019).

10. Zhang Y, Huang X, Yeom J. A floatable piezo-photocatalytic platform based on semi-embedded ZnO nanowire array for high-performance water decontamination. Nano-Micro Letters 11, 11 (2019).

11. Hong D, Zang W, Guo X, Fu Y, He H, Sun J, Xing L, Liu B, Xue X. High piezo-photocatalytic efficiency of CuS/ZnO nanowires using both solar and mechanical energy for degrading organic dye. ACS Applied Materials & Interfaces 8, 21302-21314 (2016).

12. Zhao Y, Fang Z-B, Feng W, Wang K, Huang X, Liu P. Hydrogen production from pure water via piezoelectric-assisted visible-light photocatalysis of CdS nanorod arrays. ChemCatChem 10, 3397-3401 (2018).

13. Guo L, Zhong C, Cao J, Hao Y, Lei M, Bi K, Sun Q, Wang ZL. Enhanced photocatalytic H2 evolution by plasmonic and piezotronic effects based on periodic Al/BaTiO3 heterostructures. Nano Energy 62, 513-520 (2019).

14. Pan L, Sun S, Chen Y, Wang P, Wang J, Zhang X, Zou J-J, Wang ZL. Advances in piezo-phototronic effect enhanced photocatalysis and photoelectrocatalysis. Advanced Energy Materials 10, 2000214 (2020).

15. Abbood HA, Alabadi A, Al-Hawash AB, Abbood AA, Huang K. Square CdS micro/nanosheets as efficient photo/piezo-bi-catalyst for hydrogen production. Catalysis Letters 150, 3059–3070 (2020).

16. Wang Z, Hu T, He H, Fu Y, Zhang X, Sun J, Xing L, Liu B, Zhang Y, Xue X. Enhanced H2 production of TiO2/ZnO nanowires co-using solar and mechanical energy through piezo-photocatalytic effect. ACS Sustainable Chemistry & Engineering 6, 10162-10172 (2018).

17. Ong CB, Ng LY, Mohammad AW. A review of ZnO nanoparticles as solar photocatalysts: synthesis, mechanisms and applications. Renewable and Sustainable Energy Reviews 81, 536-551 (2018).

18. Wang ZL, Song J. Piezoelectric nanogenerators based on zinc oxide nanowire arrays. Science 312, 242-246 (2006).

19. Qi K, Cheng B, Yu J, Ho W. Review on the improvement of the photocatalytic and antibacterial activities of ZnO. Journal of Alloys and Compounds 727, 792-820 (2017).

20. Kandavelu V, Kastien H, Thampi KR. Photocatalytic degradation of isothiazolin-3-ones in water and emulsion paints containing nanocrystalline TiO2 and ZnO catalysts. Applied Catalysis B: Environmental 48, 101-111 (2004).

21. Ansari SA, Khan MM, Ansari MO, Lee J, Cho MH. Biogenic synthesis, photocatalytic, and photoelectrochemical performance of Ag–ZnO nanocomposite. The Journal of Physical Chemistry C 117, 27023-27030 (2013).

22. Low J, Yu J, Jaroniec M, Wageh S, Al-Ghamdi AA. Heterojunction photocatalysts. Advanced Materials 29, 1601694 (2017).

23. Chang Y-C. Complex ZnO/ZnS nanocable and nanotube arrays with high performance photocatalytic activity. Journal of Alloys and Compounds 664, 538-546 (2016).

24. Ye Y, Wang K, Huang X, Lei R, Zhao Y, Liu P. Integration of piezoelectric effect into a Au/ZnO photocatalyst for efficient charge separation. Catalysis Science & Technology 9, 3771-3778 (2019).

25. Lops C, Ancona A, Di Cesare K, Dumontel B, Garino N, Canavese G, Hérnandez S, Cauda V. Sonophotocatalytic degradation mechanisms of Rhodamine B dye via radicals generation by micro- and nano-particles of ZnO. Applied Catalysis B: Environmental 243, 629-640 (2019).

26. Tian H, Chin ML, Najmaei S, Guo Q, Xia F, Wang H, Dubey M. Optoelectronic devices based on two-dimensional transition metal dichalcogenides. Nano Research 9, 1543-1560 (2016).

27. Pumera M, Loo AH. Layered transition-metal dichalcogenides (MoS2 and WS2) for sensing and biosensing. TrAC Trends in Analytical Chemistry 61, 49-53 (2014).

28. Kuc A. Low-dimensional transition-metal dichalcogenides. Chemical Modelling: Volume 11. The Royal Society of Chemistry, 1-29 (2015).

29. Li X, Zhu H. Two-dimensional MoS2: Properties, preparation, and applications. Journal of Materiomics 1, 33-44 (2015).

30. Li S, Zhao Z, Yu D, Zhao J-Z, Su Y, Liu Y, Lin Y, Liu W, Xu H, Zhang Z. Few-layer transition metal dichalcogenides (MoS2, WS2, and WSe2) for water splitting and degradation of organic pollutants: understanding the piezocatalytic effect. Nano Energy 66, 104083 (2019).

31. Wu JM, Sun Y-G, Chang W-E, Lee J-T. Piezoelectricity induced water splitting and formation of hydroxyl radical from active edge sites of MoS2 nanoflowers. Nano Energy 46, 372-382 (2018).

32. Jia S, Su Y, Zhang B, Zhao Z, Li S, Zhang Y, Li P, Xu M, Ren R. Few-layer MoS2 nanosheet-coated KNbO3 nanowire heterostructures: piezo-photocatalytic effect enhanced hydrogen production and organic pollutant degradation. Nanoscale 11, 7690-7700 (2019).

33. Zhang R, Xie J, Wang C, Liu J, Zheng X, Li Y, Yang X, Wang H-E, Su B-L. Macroporous ZnO/ZnS/CdS composite spheres as efficient and stable photocatalysts for solar-driven hydrogen generation. Journal of Materials Science 52, 11124-11134 (2017).

34. Prabhu YT, Navakoteswara Rao V, Shankar MV, Sreedhar B, Pal U. The facile hydrothermal synthesis of CuO@ZnO heterojunction nanostructures for enhanced photocatalytic hydrogen evolution. New Journal of Chemistry 43, 6794-6805 (2019).

35. Kumar SG, Rao KSRK. Zinc oxide based photocatalysis: tailoring surface-bulk structure and related interfacial charge carrier dynamics for better environmental applications. RSC Advances 5, 3306-3351 (2015).

36. Sang HX, Wang XT, Fan CC, Wang F. Enhanced photocatalytic H2 production from glycerol solution over ZnO/ZnS core/shell nanorods prepared by a low temperature route. International Journal of Hydrogen Energy 37, 1348-1355 (2012).

37. Momirlan M, Veziroglu TN. The properties of hydrogen as fuel tomorrow in sustainable energy system for a cleaner planet. International Journal of Hydrogen Energy 30, 795-802 (2005).

38. Wu Z, Tang C, Zhou P, Liu Z, Xu Y, Wang D, Fang B. Enhanced hydrogen evolution catalysis from osmotically swollen ammoniated MoS2. Journal of Materials Chemistry A 3, 13050-13056 (2015).

39. Liu N, Guo Y, Yang X, Lin H, Yang L, Shi Z, Zhong Z, Wang S, Tang Y, Gao Q. Microwave-assisted reactant-protecting strategy toward efficient MoS2 electrocatalysts in hydrogen evolution reaction. ACS Applied Materials & Interfaces 7, 23741-23749 (2015).

40. Anto Jeffery A, Nethravathi C, Rajamathi M. Two-Dimensional Nanosheets and Layered Hybrids of MoS2 and WS2 through Exfoliation of Ammoniated MS2 (M = Mo,W). The Journal of Physical Chemistry C 118, 1386-1396 (2014).

41. Kumar V, Wang X, Lee PS. Formation of hexagonal-molybdenum trioxide (h-MoO3) nanostructures and their pseudocapacitive behavior. Nanoscale 7, 11777-11786 (2015).

42. Chithambararaj A, Bhagya Mathi D, Rajeswari Yogamalar N, Chandra Bose A. Structural evolution and phase transition of [NH4]6Mo7O24·4H2O to 2D layered MoO3−x. Materials Research Express 2, 055004 (2015).

43. Jiang J, Wang M, Ma L, Chen Q, Guo L. Synthesis of uniform ZnO/ZnS/CdS nanorod films with ion-exchange approach and photoelectrochemical performances. International Journal of Hydrogen Energy 38, 13077-13083 (2013).

44. Zhao D, Zhou Y, Deng Y, Xiang Y, Zhang Y, Zhao Z, Zeng D. A novel and reusable RGO/ZnO with nanosheets/microparticle composite photocatalysts for efficient pollutants degradation. ChemistrySelect 3, 8740-8747 (2018).

45. Choi YI, Lee S, Kim SK, Kim Y-I, Cho DW, Khan MM, Sohn Y. Fabrication of ZnO, ZnS, Ag-ZnS, and Au-ZnS microspheres for photocatalytic activities, CO oxidation and 2-hydroxyterephthalic acid synthesis. Journal of Alloys and Compounds 675, 46-56 (2016).

46. Eda G, Yamaguchi H, Voiry D, Fujita T, Chen M, Chhowalla M. Photoluminescence from chemically exfoliated MoS2. Nano Letters 11, 5111-5116 (2011).

47. Shi Y, Zhou Y, Yang D-R, Xu W-X, Wang C, Wang F-B, Xu J-J, Xia X-H, Chen H-Y. Energy level engineering of MoS2 by transition-metal doping for accelerating hydrogen evolution reaction. Journal of the American Chemical Society 139, 15479-15485 (2017).

48. Lee D, Ahn H, Shin H, Um Y. ZnS buffer layers grown by modified chemical bath deposition for CIGS solar cells. Journal of Electronic Materials 47, 3483-3489 (2018).

49. Zou L, Qu R, Gao H, Guan X, Qi X, Liu C, Zhang Z, Lei X. MoS2/RGO hybrids prepared by a hydrothermal route as a highly efficient catalytic for sonocatalytic degradation of methylene blue. Results in Physics 14, 102458 (2019).

50. Cheon SY, Yoon J-S, Oh KH, Jang KY, Seo JH, Park JY, Choi S-I, Seo WS, Lee G, Nam KM. Sonochemical synthesis of ZnO-ZnS core-shell nanorods for enhanced photoelectrochemical water oxidation. Journal of the American Ceramic Society 100, 3825-3834 (2017).

51. Zong X, Na Y, Wen F, Ma G, Yang J, Wang D, Ma Y, Wang M, Sun L, Li C. Visible light driven H2 production in molecular systems employing colloidal MoS2 nanoparticles as catalyst. Chemical Communications, 4536-4538 (2009).

52. Chen Y, Tan L, Sun M, Lu C, Kou J, Xu Z. Enhancement of photocatalytic performance of TaON by combining it with noble-metal-free MoS2 cocatalysts. Journal of Materials Science 54, 5321-5330 (2019).

53. Liu Y, Yan X, Kang Z, Li Y, Shen Y, Sun Y, Wang L, Zhang Y. Synergistic effect of surface plasmonic particles and surface passivation layer on ZnO nanorods array for improved photoelectrochemical water splitting. Scientific reports 6, 29907 (2016).

54. Lee SL, Chang CJ. Recent Progress on Metal Sulfide Composite Nanomaterials for Photocatalytic Hydrogen Production. Catalysts 9, 25 (2019).

55. Kumaravel V, Imam MD, Badreldin A, Chava RK, Do JY, Kang M, Abdel-Wahab A. Photocatalytic Hydrogen Production: Role of Sacrificial Reagents on the Activity of Oxide, Carbon, and Sulfide Catalysts. Catalysts 9, 35 (2019).

56. Majeed I, Nadeem MA, Hussain E, Badshah A, Gilani R, Nadeem MA. Effect of deposition method on metal loading and photocatalytic activity of Au/CdS for hydrogen production in water electrolyte mixture. International Journal of Hydrogen Energy 42, 3006-3018 (2017).

57. Kumar S, Kumar A, Navakoteswara Rao V, Kumar A, Shankar MV, Krishnan V. Defect-Rich MoS2 Ultrathin Nanosheets-Coated Nitrogen-Doped ZnO Nanorod Heterostructures: An Insight into in-Situ-Generated ZnS for Enhanced Photocatalytic Hydrogen Evolution. ACS Applied Energy Materials 2, 5622-5634 (2019).

58. Kumar S, Reddy NL, Kushwaha HS, Kumar A, Shankar MV, Bhattacharyya K, Halder A, Krishnan V. Efficient Electron Transfer across a ZnO–MoS2–Reduced Graphene Oxide Heterojunction for Enhanced Sunlight-Driven Photocatalytic Hydrogen Evolution. ChemSusChem 10, 3588-3603 (2017).

59. Bak D, Kim JH. Oxidation driven ZnS Core-ZnO shell photocatalysts under controlled oxygen atmosphere for improved photocatalytic solar water splitting. Journal of Power Sources 389, 70-76 (2018).

60. Madhusudan P, Wang Y, Chandrashekar BN, Wang W, Wang J, Miao J, Shi R, Liang Y, Mi G, Cheng C. Nature inspired ZnO/ZnS nanobranch-like composites, decorated with Cu(OH)2 clusters for enhanced visible-light photocatalytic hydrogen evolution. Applied Catalysis B: Environmental 253, 379-390 (2019).

61. Lv H, Liu Y, Tang H, Zhang P, Wang J. Synergetic effect of MoS2 and graphene as cocatalysts for enhanced photocatalytic activity of BiPO4 nanoparticles. Applied Surface Science 425, 100-106 (2017).

62. Lakshmana Reddy N, Navakoteswara Rao V, Mamatha Kumari M, Kakarla RR, Ravi P, Sathish M, Karthik M, Muthukonda Venkatakrishnan S, Inamuddin. Nanostructured semiconducting materials for efficient hydrogen generation. Environmental Chemistry Letters 16, 765-796 (2018).

63. Huang Y-S, Hsiao Y-C, Tzeng S-H, Wu Y-H, Perng T-P, Lu M-Y, Chueh Y-L, Chen L-J. Vastly improved solar-light induced water splitting catalyzed by few-layer MoS2 on Au nanoparticles utilizing localized surface plasmon resonance. Nano Energy 77, 105267 (2020).

64. Hong KS, Xu HF, Konishi H, Li XC. Direct Water Splitting Through Vibrating Piezoelectric Microfibers in Water. Journal of Physical Chemistry Letters 1, 997-1002 (2010).

65. Su Y, Zhang L, Wang W, Li X, Zhang Y, Shao D. Enhanced H2 evolution based on ultrasound-assisted piezo-catalysis of modified MoS2. Journal of Materials Chemistry A 6, 11909-11915 (2018).

66. Zhu B, Lin B, Zhou Y, Sun P, Yao Q, Chen Y, Gao B. Enhanced photocatalytic H2 evolution on ZnS loaded with graphene and MoS2 nanosheets as cocatalysts. Journal of Materials Chemistry A 2, 3819-3827 (2014).

67. Lin E, Qin N, Wu J, Yuan B, Kang Z, Bao D. BaTiO3 nanosheets and caps grown on TiO2 nanorod arrays as thin-film catalysts for piezocatalytic applications. ACS Applied Materials & Interfaces 12, 14005-14015 (2020).

68. Qian W, Zhao K, Zhang D, Bowen CR, Wang Y, Yang Y. Piezoelectric material-polymer composite porous foam for efficient dye degradation via the piezo-catalytic effect. ACS Applied Materials & Interfaces 11, 27862-27869 (2019).

69. Zhang Y, He S, Guo W, Hu Y, Huang J, Mulcahy JR, Wei WD. Surface-plasmon-driven hot electron photochemistry. Chemical Reviews 118, 2927-2954 (2018).

70. Xiang D, Liu Z, Wu M, Liu H, Zhang X, Wang Z, Wang ZL, Li L. Enhanced piezo-photoelectric catalysis with oriented carrier migration in asymmetric Au−ZnO nanorod array. Small 16, 1907603 (2020).