藉由海水及太陽能等自然資源搭配半導體材料光電解水，能同時轉化二氧化碳產生甲酸及氫氣，為地球永續發展及潔淨能源創新的新領域。本研究探討氮化鎵系列(GaN-based)半導體作為光電解水的工作電極，雖然氮化鎵系列材料具有良好的抗酸鹼能力，其能隙橫跨於氧氣的氧化電位及二氧化碳的還原電位兩側，能同時光電解產生氧氣、氫氣及甲酸等產物，但是由於其能隙較大只能吸收太陽能光譜中低於365波長的部分，產生的光電流密度有限，此論文將會探討氮化鎵薄膜搭配不同材料薄膜形成雙面結構探討光電解反應轉化二氧化碳。
首先我們討論寬能隙氧化鎵(β-Ga2O3)共蒸鍍銦錫氧化物(ITO)薄膜特性，較高的銦錫氧化物共蒸鍍功率下，不只薄膜能隙減少，吸收波段紅移，錫原子還會取代本來與氧原子鍵結的鎵原子，提升薄膜材料的載子濃度。經由實驗發現此薄膜在蒸鍍時提升基板的溫度至攝氏600度，能使原子在高溫下堆疊較緊密，內部缺陷較少，薄膜的面電阻較低、載子濃度較高，有利於進行光電解實驗。完成薄膜材料基本分析後，在氧化鎵共蒸鍍銦錫氧化物薄膜的另一面分別以金屬有機氣相沉積磊晶法成長氮化鎵及氮化銦鎵(InGaN)薄膜，藉由雙面結構的二次吸收來增加光電子的生成，提升轉換的效率，但由於太陽能光譜中極紫外波段的部分很低，以及氧化鎵共蒸鍍銦錫氧化物薄膜材料品質不佳，導致光電子大多被內部缺陷所複合，無法傳輸至對電極產生還原反應。
為了改善氧化鎵共蒸鍍銦錫氧化物薄膜品質不佳的問題，我們改成以金屬有機氣相沉積磊晶法在藍寶石基板正面成長氮化鎵薄膜，背面成長氮化銦鎵薄膜的雙面結構，隨著銦含量的增加可以延伸氮化銦鎵的吸收至可見光波段，吸收更大比例的太陽能光譜，此雙面結構中氮化鎵薄膜吸收短波段的光源，氮化銦鎵薄膜吸收長波段的光源，經由接線的連接，對電極可同時收集雙面結構產生的光電子進行還原反應，促使陰極反應更加劇烈，轉換出更多氫氣及甲酸。
光電化學轉換二氧化碳系統利用恆電位儀或是串接砷化鎵太陽能電池(GaAs-based solar cell)照光提供偏壓，以錫(Sn)片當陰極、銀/氯化銀(Ag/AgCl)當參考電極、1M氯化鈉(NaCl)水溶液為電解液，反應前先在陰極溶液通入二氧化碳氣體至飽和。長時間反應後收集陰極水溶液中甲酸及氫氣，經由公式換算反應後氫氣及甲酸的產率以及能量轉換效率，此論文達成相對於銀/氯化銀參考電極不施加額外偏壓的情況下，直接光電解轉化二氧化碳同時產生氧氣、氫氣及甲酸。

In this study, we investigated material properties of gallium oxide (Ga2O3) co-sputter indium tin oxide (ITO) thin film. The conductivity and photocurrent density of thin film were observed to improve through heating the substrate to 600 degrees during evaporation, which resulted better quality. We fabricate double-sided structures to increase the absorption of solar spectrum, such as Ga2O3 co-sputter ITO / n-GaN double-sided structure, Ga2O3 co-sputter ITO / n-InGaN double-sided structure, n-GaN / n-InGaN double-sided structure. To improve transfer CO¬2 to HCOOH and H2 conversion efficiency by photoelectrochemical (PEC) system, GaAs solar cell was used to assist bias. The n-GaN / n-InGaN double-sided structure electrode with solar cell to bias the photoelectrochemical system, the energy conversion efficiency for HCOOH and H2 production reached 1.44% and 9.92%, respectively.

[1]S. Pacala and R. Socolow (2004), “ Stabilization wedges: solving the climate problem for the next 50 years with current technologies,” Science, vol. 305, pp. 968-972.
[2]“ BP Statistical Review of World Energy, 2014,” 2014.
[3]M. A. DeLuchi, J. M. Ogden (1993), “ Solar hydrogen fuel cell vehicles, ” Transportation Research A, vol. 27A, pp. 255-275.
[4]J. Lédé, F. Lapicque, J. Villermaux (1983), “ Production of hydrogen by direct thermal decomposition of water,” International Journal of Hydrogen Energy, vol. 8, pp. 675-679.
[5]W. Kreuter, H. Hofmann (1998), “ Electrolysis: The important energy transformer in a world of sustainable energy,” International Journal of Hydrogen Energy, vol. 23, pp. 661-666.
[6]O. Khaselev, J. A. Turner (1998), “ A monolithic photovoltaic photoelectrochemical device for hydrogen production via water splitting,” Science, vol. 280, pp. 425-427.
[7]A. Fujishima, K. Honda (1972), “ Electrochemical photolysis of water at a semiconductor electrode,” Nature, vol. 238, pp. 37-38.
[8]A. Fujishima, K. Kohayakawa, and K. Honda (1975), “ Hydrogen production under sunlight with an electrochemical photocell,” Journal of the Electrochemical Society, vol. 122, pp. 1487-1489.
[9]A. J. Nozik, R. Memming (1996), “ Physical chemistry of semiconductor-liquid interfaces,” The Journal of Physical Chemistry, vol. 100, pp. 13061-13078.
[10]I. M. Huygens, K. Strubbe, W. P. Gomes (2000), “ Electrochemistry and Photoetching of n-GaN,” Journal of The Electrochemical Society, vol. 147, pp. 1797-1802.
[11]K. Fujii, T. Karasawa, K. Ohkawa (2005), “ Hydrogen gas generation by splitting aqueous water using n-type GaN photoelectrode with anodic oxidation,” Japanese journal of applied physics, vol. 44, pp. L543-L545.
[12]M. Halmann (1978), “ Photoelectrochemical reduction of aqueous carbon dioxide on p-type gallium phosphide in liquid junction solar cells,” Nature, vol. 275, pp. 115-116.
[13]S. Yotsuhashi, M. Deguchi, Y. Zenitani, R. Hinogami, H. Hashiba, Y. Yamada, K. Ohkawa (2011), “ Photo-induced CO2 Reduction with GaN Electrode in Aqueous System,” Applied Physics Express, vol. 4, p. 117101.
[14]S. Kim, S. J. Kim, K. H. Kim, H. D. Kim, and T. G. Kim (2014), “ Improved performance of Ga2O3/ ITO-based transparent conductive oxide films using hydrogen annealing for near-ultraviolet light-emitting diodes,” Phys. Status Solidi A, vol. 211, pp. 2569–2573.
[15]A. W. Bott (1998), “ Electrochemistry of Semiconductors,” Curr. Sep., pp. 87-91.
[16]F. Mollers, H. J. Tolle, R. Memming (1974), “ Origin of the photocatalytic deposition of noble metals on TiO2,” J. Electrochem. Soc., vol. 121, pp. 1160–1167.
[17]C. A. Grimes, O. K. Varghese, S. Ranjan (2008), Light, Water, Hydrogen: The Solar Generation of Hydrogen by Water Photoelectrolysis, New York: Springer Press, pp. 148–152.
[18]R. V. D. Krol, and M. Grätzel (2012), Photoelectrochemical Hydrogen Production, New York: Springer Press, pp. 47-49.
[19]C. A. Grimes, O. K. Varghese, S. Ranjan (2008), Light, Water, Hydrogen: The Solar Generation of Hydrogen by Water Photoelectrolysis, New York: Springer Press, pp. 120-122.
[20]R. V. D. Krol, and M. Grätzel (2012), Photoelectrochemical Hydrogen Production, New York: Springer Press, pp. 13-14.
[21]C. A. Grimes, O. K. Varghese, S. Ranjan (2008), Light, Water, Hydrogen: The Solar Generation of Hydrogen by Water Photoelectrolysis, New York: Springer Press, pp. 35-52.
[22]T. Ogita, Y. Uetake, Y. Yamashita, A. Kuramata, S. Yamakoshi, and K. Ohkawa (2015), “ InGaN photocatalysts on conductive Ga2O3 substrates,” Phys. Status Solidi A, vol. 212, pp. 1029-1032.
[23]T. Sekimoto, S. Shinagawa, Y. Uetake, K. Noda, M. Deguchi, S. Yotsuhashi, and K. Ohkawa (2015), “ Tandem photo-electrode of InGaN with two Si p-n junctions for CO2 conversion to HCOOH with the efficiency greater than biological photosynthesis,” Appl. Phys. Lett., vol. 106, p. 073902.
[24]A. J. Bard, L. R. Faulkner (2001), Eelectrochemical Methods: Fundamentals and Applications, Second Edition, New York: John Wiley & Sons, pp. 2–3.
[25]R. V. D. Krol, and M. Grätzel (2012), Photoelectrochemical Hydrogen Production, New York: Springer Press, pp. 75–77.
[26]C. A. Grimes, O. K. Varghese, S. Ranjan (2008), Light, Water, Hydrogen: The Solar Generation of Hydrogen by Water Photoelectrolysis, New York: Springer Press, pp. 126–131.
[27]S. C. Roy, O. K. Varghese, M. Paulose, and C. A. Grimes (2010), “ Toward Solar Fuels: Photocatalytic Conversion of Carbon Dioxide to Hydrocarbons,”ACS Nano, vol. 4, pp. 1259-1278.
[28]M. H. Miles and M. A. Thomason (1976), “ Periodic Variation of Overvoltages for Water Electrolysis in Acid Solutions from Cyclic Voltammetric Studies,” J. electrochem. Soc., vol. 123, pp. 1459-1661.
[29]H. Noda, S. Ikeda, Y. Oda, K. Imai, M. Maeda, and K. Ito (1990), “ Electrochemical Reduction of Carbon Dioxide at Various Metal Electrodes in Aqueous Potassium Hydrogen Carbonate Solution,” Bull. Chem. Soc. Jpn., vol. 63, pp. 2459-2462.
[30]M. G. Walter, E. L. Warren, J. R. McKone, S. W. Boettcher, Q. Mi, E. A. Santori, and N. S. Lewis (2010), “ Solar Water Splitting Cells,” Chem. Rev., vol. 110, pp. 6446-6473.
[31]R. V. D. Krol, and M. Grätzel (2012), Photoelectrochemical Hydrogen Production, New York: Springer Press, pp. 51-53.
[32]Z. B. Chen, T. F. Jaramillo, T. G. Deutsch, A. Kleiman - Shwarsctein, A. J. Forman, N. Gaillard, R. Garland, K. Takanabe, C. Heske, M. Sunkara, E. W. McFarland, K. Domen, E. L. Miller, J. A. Turner, and H. N. Dinh (2010), “ Accelerating materials development for photoelectrochemical hydrogen production: Standards for methods, definitions, and reporting protocols,” J. Mater. Res., vol.25, pp. 3–16.
[33]M. Deguchi, S. Yotsuhashi, H. Hashiba, Y. Yamada, and K. Ohkawa (2013), “ Enhanced Capability of Photoelectrochemical CO2 Conversion System Using an AlGaN/ GaN Photoelectrode,” Jpn. J. Appl. Phys., vol. 52, p. 08JF07.
[34]H. Hashiba, S. Yotsuhashi, M. Deguchi, Y. Yamada, and K. Ohkawa (2013), “ Selectivity Control of CO2 Reduction in an Inorganic Artificial Photosynthesis System,” Appl. Phys. Exp., vol. 6, p. 097102.
[35]S. Yotsuhashi, M. Deguchi, H. Hashiba, Y. Zenitani, R. Hinogami, Y. Yamada, and K. Ohkawa (2012), “ Enhanced CO2 reduction capability in an AlGaN/ GaN photoelectrode,” Appl. Phys. Lett., vol. 100, p. 243904.
[36]S. Yotsuhashi, M. Deguchi, Y. Yamada, and K. Ohkawa (2014), “ Effect of Inserted Si p-n Junction on GaN-based photo-electrochemical CO2 conversion system,” AIP Advances, vol. 4, p. 067135.
[37]K. Shimamura, E. G. Villora, K. Domen, K. Yui, K. Aoki, and N. Ichinose (2005), “ Epitaxial Growth of GaN on (1 0 0) β-Ga2O3 Substrates by Metalorganic Vapor Phase Epitaxy,” Jpn. J. Appl. Phys., vol. 44, pp. L7-L8.
[38]M. Higashiwaki, K. Sasaki, A. Kuramata, T. Masui, and S. Yamakoshi (2012), “ Gallium oxide (Ga2O3) metal-semiconductor field-effect transistors on single-crystal β-Ga2O3 (010) substrates,” Appl. Phys. Lett., vol. 10, p. 013504.
[39]T. Oshima, T. Okuno, N. Suzuki, S. Ohira, and S. Fujita (2008), “ Vertical Solar-Blind Deep-Ultraviolet Schottky Photodetectors Based on β-Ga2O3 Substrates,” Appl. Phys. Express, vol. 1, p. 011202.
[40]H. H. Tippins (1965), “ Optical absorption and photoconductivity in the band edge of β-Ga2O3,” Phys. Rev., vol. 140, pp. A316-A319.
[41]J. B. Varley, J. R. Weber, A. Janotti, and C. G. Van de Walle (2010), “ Oxygen vacancies and donor impurities in β-Ga2O3,” Appl. Phys. Lett., vol. 97, p. 142106.
[42]S. Ezhilvalavan, and T. Y. Tseng (1999), “ Short-Duration Rapid-Thermal-Annealing Processing of Tantalum Oxide Thin Films,” J. Am. Ceram. Soc., vol. 82, pp. 600-606.
[43]K. Y. Woo, J. H. Lee, K. H. Kim, S. J. Kim, and T. G. Kim (2014), “ Highly transparent conductive Ag/ Ga2O3 electrode for near-ultraviolet light-emitting diodes,” Phys. Status Solidi A, vol. 211, pp. 1760-1763.
[44]J. Wu, W. Walukiewicz, K. M. Yu, W. Shan, J. W. Ager III, E. E. Haller, H. Lu, W. J. Schaff, W. K. Metzger, and S. Kurtz (2003), “ Superior radiation resistance of In1−xGaxN alloys: Full-solar-spectrum photovoltaic material system,” J. Appl. Phys., vol. 94, pp. 6477-6482.
[45]D. Doppalapudia and S. N. Basu (1998), “ Phase separation and ordering in InGaN alloys grown by molecular beam epitaxy,” J. Appl. Phys., vol. 84, pp. 1389-1395.
[46]G. Centi and S. Perathoner (2010), “ Towards Solar Fuels from Water and CO2,” ChemSusChem., vol. 3, pp.195-208.
[47]O. K. Varghese, M. Paulose, T. J. LaTempa, and C. A. Grimes (2009), “ High-Rate Solar Photocatalytic Conversion of CO2 and Water Vapor to Hydrocarbon Fuels,” Nano Lett., vol. 9, pp. 731-737.
[48]E. E. Barton, D. M. Rampulla, and A. B. Bocarsly (2008), “ Selective Solar-Driven Reduction of CO2 to Methanol Using a Catalyzed p-GaP Based Photoelectrochemical Cell,” J. Am. Chem. Soc., vol. 130, pp. 6342-6344.
[49]T. Inoue, A. Fujishima, S. Konishi, and K. Honda (1979), “ Photoelectrocatalytic reduction of carbon dioxide in aqueous suspensions of semiconductor powders,” Nature, vol. 277, pp. 637-638.
[50]K. Sayama and H. Arakawa (1993), “ Photocatalytic decomposition of water and photocatalytic reduction of carbon dioxide over zirconia catalyst,” J. Phys. Chem., vol. 97, pp. 531-533.
[51]K. Fujii, K. Kusakabe, and K. Ohkawa (2005), “ Photoelectrochemical Properties of InGaN for H2 Generation from Aqueous Water,” Jpn. J. Appl. Phys., vol. 44, pp. 7433.
[52]E. Aharon-Shalom, A. Heller (1982), “ Efficient p-InP(Rh-H alloy) and p-InP(Re-H alloy) Hydrogen Evolving Photocathodes,” J. Electrochem. Soc., vol. 129, pp. 2865-2866.
[53]O. Khaselev and J. A. Turner (1998), “ A monolithic photovoltaic- photoelectrochemical device for hydrogen production via water splitting,” Science, vol. 280, pp. 425–427.
[54]A. Currao (2007), “ Photoelectrochemical water splitting,” CHIMIA, vol. 61, pp.815–819.
[55]S. Y. Liu, J. K. Sheu, Y. C. Lin, Y. T. Chen, S. J. Tu, M. L. Lee, and W. C. Lai (2013), “ InGaN working electrodes with assisted bias generated from GaAs solar cells for efficient water splitting,” Optics Express, vol. 21, pp. A991-996.
[56]S. T. Nishanthi, S. Iyyapushpam, B. Sundarakannan, E. Subramanian, D. Pathinettam Padiyan (2015), “ Plasmonic silver nanoparticles loaded titania nanotube arrays exhibiting enhanced photoelectrochemical and photocatalytic activities,” Journal of Power Sources, vol. 274, pp. 885-893.