氫能擁有高能量密度且零污染之優勢，產物只有水且沒有副產物，極度適合成為永續的能量來源，作為光電元件，氧化鋅具有低成本和易於製造的優勢，但氧化鋅為寬能隙半導體，能隙約為3.37eV，外觀顏色通常為白色，對太陽光的利用範圍只在紫外光波段，為了大幅增加對光線的利用率，本研究藉由在不同溫度下退火的策略，製造出具有內孔或內外孔兼具的奈米柱，孔洞可在能隙之間添加缺陷能階，將光線trap在材料內部，能夠合成出外觀顏色為黑色的氧化鋅，光的吸收率高達95%。氧化鋅是眾所周知具有的壓電特性的材料之一，透過施加正向力或彎曲等外力，會在材料中會誘發壓電電位，壓電電位可用於分離照光後產生的電子電洞對，甚至驅動外部電路中的電流，對光電化學產氫可達到增益的效果，本研究將對不同孔隙率的奈米柱進行探討，包括其光電化學產氫效率及壓電特性。
　　由結果看來，光電化學產氫效率並未隨著退火溫度上升而持續上升，初合成的氧化鋅在進行350°C退火後具有最佳值，其光電流密度為初合成氧化鋅的5-6倍增益，若溫度繼續上升，因奈米柱表面會形成p-type的薄層，此薄層不利於光電化學產氫的反應進行，雖光吸收高達95%，光電流仍然無法如預期有好表現。退火後，氧化鋅的壓電電流輸出有增加的趨勢，預期未來能成功結合兩種機制的增益，以達到更高的產氫效率。

Hydrogen provides zero-pollution and high energy density as a sustainable energy source, and thus developing lost cost hydrogen evolution technology, such as photocatalytic (PC) or photo-electrochemical (PEC) water splitting is highly desirable to tackle energy and environmental issues. However, hydrogen production yield from any current materials is still too low to be realized. For PEC water splitting, ZnO has been regarded as a great candidate due to its superior optoelectronic properties, low cost and ease of manufacturing. However, the main drawback is only absorption of UV region in solar light limited by its wide band gap of 3.37eV. This study demonstrates “black ZnO”, capable of absorbing over 95% of light down to 800 nm, by inducing mid-gap electronic states via annealing hydrothermally grown ZnO nanorod arrays in vacuum. The underlying mechanism is associated with the formation of inner pores as porous ZnO nanorod arrays. We demonstrate how porous ZnO enhances PEC water splitting. Finally, pores are also demonstrated to enhance the output current of piezoelectric nanogenerator.

參考文獻
[1] A. Midilli, M. Ay, I. Dincer, and M. A. Rosen, "On hydrogen and hydrogen energy strategies," Renewable and Sustainable Energy Reviews, vol. 9, no. 3, pp. 255-271, 2005.
[2] A. Kudo and Y. Miseki, "Heterogeneous photocatalyst materials for water splitting," Chem Soc Rev, vol. 38, no. 1, pp. 253-78, Jan 2009.
[3] T. Hisatomi, J. Kubota, and K. Domen, "Recent advances in semiconductors for photocatalytic and photoelectrochemical water splitting," Chem Soc Rev, vol. 43, no. 22, pp. 7520-35, Nov 21 2014.
[4] X. Li, J. Yu, J. Low, Y. Fang, J. Xiao, and X. Chen, "Engineering heterogeneous semiconductors for solar water splitting," Journal of Materials Chemistry A, vol. 3, no. 6, pp. 2485-2534, 2015.
[5] Y. Liu, B. Xiao, H. Chen, Y. Li, and S. Chang, "Decreased Surface Photovoltage of ZnO Photoanode Films via Optimal Annealing Temperature for Enhanced Photoelectrochemical Performance," Journal of Nanomaterials, vol. 2019, pp. 1-8, 2019.
[6] B.-S. Wang et al., "An overlapping ZnO nanowire photoanode for photoelectrochemical water splitting," Catalysis Today, vol. 321-322, pp. 100-106, 2019.
[7] Y. Hu et al., "Large-scale patterned ZnO nanorod arrays for efficient photoelectrochemical water splitting," Applied Surface Science, vol. 339, pp. 122-127, 2015.
[8] M. Gupta et al., "Preparation and characterization of nanostructured ZnO thin films for photoelectrochemical splitting of water," Bulletin of Materials Science, vol. 32, no. 1, pp. 23-30, 2009.
[9] Z. Liu, Q. Cai, C. Ma, J. Zhang, and J. Liu, "Photoelectrochemical properties and growth mechanism of varied ZnO nanostructures," New Journal of Chemistry, vol. 41, no. 16, pp. 7947-7952, 2017.
[10] X. Li, F. Niu, J. Su, and L. Guo, "Photoelectrochemical Performance Dependence on Geometric Surface Area of Branched ZnO Nanowires," ChemElectroChem, vol. 5, no. 23, pp. 3717-3722, 2018.
[11] Q. Li, X. Sun, K. Lozano, and Y. Mao, "Facile and Scalable Synthesis of “Caterpillar-like” ZnO Nanostructures with Enhanced Photoelectrochemical Water-Splitting Effect," The Journal of Physical Chemistry C, vol. 118, no. 25, pp. 13467-13475, 2014.
[12] A. Wolcott, W. A. Smith, T. R. Kuykendall, Y. Zhao, and J. Z. Zhang, "Photoelectrochemical Study of Nanostructured ZnO Thin Films for Hydrogen Generation from Water Splitting," Advanced Functional Materials, vol. 19, no. 12, pp. 1849-1856, 2009.
[13] K.-S. Ahn et al., "Enhancement of photoelectrochemical response by aligned nanorods in ZnO thin films," Journal of Power Sources, vol. 176, no. 1, pp. 387-392, 2008.
[14] X. Sun, Q. Li, J. Jiang, and Y. Mao, "Morphology-tunable synthesis of ZnO nanoforest and its photoelectrochemical performance," Nanoscale, vol. 6, no. 15, pp. 8769-80, Aug 7 2014.
[15] Y.-K. Hsu, Y.-G. Lin, and Y.-C. Chen, "Polarity-dependent photoelectrochemical activity in ZnO nanostructures for solar water splitting," Electrochemistry Communications, vol. 13, no. 12, pp. 1383-1386, 2011.
[16] A. U. Pawar, C. W. Kim, M. J. Kang, and Y. S. Kang, "Crystal facet engineering of ZnO photoanode for the higher water splitting efficiency with proton transferable nafion film," Nano Energy, vol. 20, pp. 156-167, 2016.
[17] R. Lv, T. Wang, F. Su, P. Zhang, C. Li, and J. Gong, "Facile synthesis of ZnO nanopencil arrays for photoelectrochemical water splitting," Nano Energy, vol. 7, pp. 143-150, 2014.
[18] H. Chen et al., "Epitaxial growth of ZnO Nanodisks with large exposed polar facets on nanowire arrays for promoting photoelectrochemical water splitting," Small, vol. 10, no. 22, pp. 4760-9, Nov 2014.
[19] W. Chen, Y. Qiu, and S. Yang, "Branched ZnO nanostructures as building blocks of photoelectrodes for efficient solar energy conversion," Phys Chem Chem Phys, vol. 14, no. 31, pp. 10872-81, Aug 21 2012.
[20] M. Baek, D. Kim, and K. Yong, "Simple but Effective Way To Enhance Photoelectrochemical Solar-Water-Splitting Performance of ZnO Nanorod Arrays: Charge-Trapping Zn(OH)2 Annihilation and Oxygen Vacancy Generation by Vacuum Annealing," ACS Appl Mater Interfaces, vol. 9, no. 3, pp. 2317-2325, Jan 25 2017.
[21] B. Zhang et al., "Anisotropic Photoelectrochemical (PEC) Performances of ZnO Single-Crystalline Photoanode: Effect of Internal Electrostatic Fields on the Separation of Photogenerated Charge Carriers during PEC Water Splitting," Chemistry of Materials, vol. 28, no. 18, pp. 6613-6620, 2016.
[22] K. Govatsi, A. Seferlis, S. G. Neophytides, and S. N. Yannopoulos, "Influence of the morphology of ZnO nanowires on the photoelectrochemical water splitting efficiency," International Journal of Hydrogen Energy, vol. 43, no. 10, pp. 4866-4879, 2018.
[23] T.-F. Hou, R. Boppella, A. Shanmugasundaram, D. H. Kim, and D.-W. Lee, "Hierarchically self-assembled ZnO architectures: Establishing light trapping networks for effective photoelectrochemical water splitting," International Journal of Hydrogen Energy, vol. 42, no. 22, pp. 15126-15139, 2017.
[24] X. Ren et al., "Photoelectrochemical water splitting strongly enhanced in fast-grown ZnO nanotree and nanocluster structures," J Mater Chem A Mater, vol. 4, no. 26, pp. 10203-10211, Jul 14 2016.
[25] Y.-C. Chen, K.-H. Yang, C.-Y. Huang, Z.-J. Wu, and Y.-K. Hsu, "Overall photoelectrochemical water splitting at low applied potential over ZnO quantum dots/nanorods homojunction," Chemical Engineering Journal, vol. 368, pp. 746-753, 2019.
[26] H. M. Chen et al., "Multi-Bandgap-Sensitized ZnO Nanorod Photoelectrode Arrays for Water Splitting: An X-ray Absorption Spectroscopy Approach for the Electronic Evolution under Solar Illumination," The Journal of Physical Chemistry C, vol. 115, no. 44, pp. 21971-21980, 2011.
[27] M. Wang et al., "N Doping to ZnO Nanorods for Photoelectrochemical Water Splitting under Visible Light: Engineered Impurity Distribution and Terraced Band Structure," Sci Rep, vol. 5, p. 12925, Aug 11 2015.
[28] S. Emin et al., "Photoelectrochemical properties of cadmium chalcogenide-sensitized textured porous zinc oxide plate electrodes," ACS Appl Mater Interfaces, vol. 5, no. 3, pp. 1113-21, Feb 2013.
[29] T. A. Nirmal Peiris, J. S. Sagu, Y. Hazim Yusof, and K. G. Upul Wijayantha, "Microwave-assisted low temperature fabrication of ZnO thin film electrodes for solar energy harvesting," Thin Solid Films, vol. 590, pp. 293-298, 2015.
[30] X. Zhang, Y. Liu, and Z. Kang, "3D branched ZnO nanowire arrays decorated with plasmonic au nanoparticles for high-performance photoelectrochemical water splitting," ACS Appl Mater Interfaces, vol. 6, no. 6, pp. 4480-9, Mar 26 2014.
[31] X. Chen, L. Liu, P. Y. Yu, and S. S. Mao, "Increasing Solar Absorption for Photocatalysis with Black Hydrogenated Titanium Dioxide Nanocrystals," Science, vol. 331, no. 6018, pp. 746-750, 2011.
[32] X. Lu et al., "Efficient photocatalytic hydrogen evolution over hydrogenated ZnO nanorod arrays," Chem Commun, vol. 48, no. 62, pp. 7717-9, Aug 11 2012.
[33] J. Shi et al., "Interface engineering by piezoelectric potential in ZnO-based photoelectrochemical anode," Nano Lett, vol. 11, no. 12, pp. 5587-93, Dec 14 2011.
[34] Ü. Özgür et al., "A comprehensive review of ZnO materials and devices," Journal of Applied Physics, vol. 98, no. 4, 2005.
[35] H. W. Kim, H. G. Na, Y. J. Kwon, H. Y. Cho, and C. Lee, "Decoration of Co nanoparticles on ZnO-branched SnO2 nanowires to enhance gas sensing," Sensors and Actuators B: Chemical, vol. 219, pp. 22-29, 2015.
[36] S. Baruah and J. Dutta, "Hydrothermal growth of ZnO nanostructures," Sci Technol Adv Mater, vol. 10, no. 1, p. 013001, Feb 2009.
[37] G. Bruno, M. M. Giangregorio, G. Malandrino, P. Capezzuto, I. L. Fragalà, and M. Losurdo, "Is There a ZnO Face Stable to Atomic Hydrogen?," Advanced Materials, vol. 21, no. 17, pp. 1700-1706, 2009.
[38] K. C. Pradel, J. Uzuhashi, T. Takei, T. Ohkubo, K. Hono, and N. Fukata, "Investigation of nanoscale voids in Sb-doped p-type ZnO nanowires," Nanotechnology, vol. 29, no. 33, p. 335204, Aug 17 2018.
[39] K. Gupta, J.-T. Lin, R.-C. Wang, and C.-P. Liu, "Porosity-induced full-range visible-light photodetection via ultrahigh broadband antireflection in ZnO nanowires," NPG Asia Materials, vol. 8, no. 9, pp. e314-e314, 2016.
[40] F. E. Osterloh, "Inorganic nanostructures for photoelectrochemical and photocatalytic water splitting," Chem Soc Rev, vol. 42, no. 6, pp. 2294-320, Mar 21 2013.
[41] X. Chen, S. Shen, L. Guo, and S. S. Mao, "Semiconductor-based photocatalytic hydrogen generation," Chem Rev, vol. 110, no. 11, pp. 6503-70, Nov 10 2010.
[42] X. Shi, L. Cai, M. Ma, X. Zheng, and J. H. Park, "General Characterization Methods for Photoelectrochemical Cells for Solar Water Splitting," ChemSusChem, vol. 8, no. 19, pp. 3192-203, Oct 12 2015.
[43] F. Zhan, Y. Yang, W. Liu, K. Wang, W. Li, and J. Li, "Facile Synthesis of FeOOH Quantum Dots Modified ZnO Nanorods Films via a Metal-Solating Process," ACS Sustainable Chemistry & Engineering, vol. 6, no. 6, pp. 7789-7798, 2018.
[44] U. Ilyas et al., "High temperature ferromagnetic ordering in c-axis oriented ZnO:Mn nanoparticle thin films by tailoring substrate temperature," International Journal of Modern Physics: Conference Series, vol. 32, 2014.
[45] J.-Y. Leem et al., "Fabrication of Porous ZnO Nanorods with Nano-sized Pores and Their Properties," Journal of the Korean Physical Society, vol. 57, no. 6, pp. 1477-1481, 2010.
[46] A. Janotti and C. G. Van de Walle, "Native point defects in ZnO," Physical Review B, vol. 76, no. 16, 2007.
[47] A. Sahai and N. Goswami, "Probing the dominance of interstitial oxygen defects in ZnO nanoparticles through structural and optical characterizations," Ceramics International, vol. 40, no. 9, pp. 14569-14578, 2014.
[48] H. V. Le, T. L. Le, U. T. D. Thuy, and P. D. Tran, "Current perspectives in engineering of viable hybrid photocathodes for solar hydrogen generation," Advances in Natural Sciences: Nanoscience and Nanotechnology, vol. 9, no. 2, 2018.