Over the recent years, there has been a growing interest in energy obtained from exploiting nature as an alternative fuel for sustainable development and environmental preservation. Hydrogen, which has been suggested as the main energy source in the future, could offer an answer to the threat of global climate change and help avoid the undesirable effects of the use of fossil fuels. Furthermore hydrogen from renewable energy resources is a clean, green, virtually inexhaustible, environmentally benign energy source that could be our future energy supply. Thus, to meet the growing energy demand, hydrogen produced by solar energy can be considered as one of the desirable approaches. One of the attractive and environmentally friendly methods to harvest solar energy is photocatalytic hydrogen generation from water. [1] Because of its simplicity, water splitting using a powdered photocatalyst has become a subject of much interest, with most research focusing on the visible light sensitization of catalysts in order to effectively utilize incoming solar energy. Since the first report of TiO2-based photoelectrolysis initiated by Fujishima and Honda, [2] many attempts have been made to achieve efficient energy conversion from solar energy to hydrogen energy. However, the most significant obstacle is the lack of proper semiconductor materials that could meet the demands for efficient and stable water splitting: a band gap of 1.6 to 2.0 eV straddling the redox potentials of the water, and chemical stability. [3, 4]

In this study, construction of a Z-scheme water splitting system has been implemented with the use of various modified visible-light-responsive photocatalysts. [4] The Z-scheme system, which refers to overall water splitting using a two-step photoexcitation system mimicking photosynthesis in a green plant, is made up of two different photocatalysts for H2 and O2 evolution respectively. These photocatalysts are reacted along with a proper reversible shuttle redox couple. Recent studies have also revealed that Z-scheme water splitting proceeds even in the absence of redox couples owing to interparticle electron transfer. In these two-step photoexcitation processes, a wider range of visible light can be utilized through several photocatalysts combinations, with reduction in the energy required to drive each photocatalyst. Therefore, current research work aimed to investigate and optimize the efficiency of Z-scheme water splitting and through the use of new and/or modified past developed photocatalysts that promote overall water splitting into a stoichiometric mixture of H2 and O2 (2:1 by volume) under visible light irradiation.

My research work aimed to develop new photocatalyst materials for water splitting reaction and to achieve an artificial photosynthesis. The goals of my Ph.D research were to overcome some of the challenges in application of visible-light-driven photocatalytic materials in particular, oxynitrides, in Z-scheme two-step photocatalytic water splitting process. The major goals of my research were: (1) preparation of active visible-light-responsive photocatalyst; (2) investigation of its photocatalytic activity for water oxidation or water reduction in the presence of redox mediator relays (half reaction); (3) in the application for Z-scheme water splitting system both in the presence and absence of redox relays. Through my research work on the study of photocatalysts for Z-scheme system, the optimized photocatalysts would enhance and enable the process of splitting water into hydrogen and oxygen much more efficiently and therefore viable.

A total of 7 chapters are included in this thesis, all of which has been carried out in the time frame of three years:

Chapter 1 gives a brief introduction to the background and motivation of the research. The general principle of water splitting on semiconductor, available visible light-driven photocatalysts, Z-scheme photocatalytic hydrogen and oxygen generation and factors which affect photocatalytic efficiency, are reviewed extensively. The significance, aim and objectives of the study are described. The scope of the research and the overall structure of the thesis are also outlined in this chapter.

In Chapter 2, the effects of the preparation conditions of ZrO2-modified TaON (referred to as ZrO2/TaON) on the photocatalytic activity for H2 evolution under visible light (λ > 420 nm) were investigated. ZrO2/TaON catalysts were prepared by loading particulate Ta2O5 with ZrO2 using different zirconium precursors, followed by nitridation at 1123 K for different durations (5-20 h) under NH3 flow. Nitridation of ZrO2/Ta2O5 for 15 h resulted in the production of ZrO2/TaON, regardless of the zirconium precursors used, but the physicochemical properties varied. The highest activity was obtained for ZrO2/TaON synthesized from Ta2O5 and ZrO(NO3)2・2H2O (Zr/Ta = 0.1 by mole) after nitridation for 15 h. Physicochemical analysis suggested that a lower density of anionic defects in TaON, which was realized using highly-dispersed ZrO2 nanoparticles (10-30 nm in size), contributed primarily to the enhanced activity.

Chapter 3 covers modified tungsten trioxide (WO3) powder that is employed as water oxidation photocatalyst under visible light (λ > 420 nm) irradiation. WO3 modified with nanoparticulate Pt species (more specifically, PtO) as cocatalysts is capable of photocatalyzing water oxidation under visible light in the presence of iodate (IO3-) ions as an electron accepter under near-neutral pH conditions (pH ≒ 5.9). When PtO/WO3 was further modified with a very small amount (0.001 wt%) of a metal oxide (e.g., MnOx, CoOx, RuO2 or IrO2) as a secondary cocatalyst, the water oxidation activity was improved. Among the metal oxide cocatalysts examined, RuO2 was found to give the highest performance, with an apparent quantum yield of 14.4% at 420 nm. The results of photocatalytic reactions and photoelectrochemical analyses suggest that the main roles of the loaded PtO and RuO2 on WO3 are to promote the reduction of IO3- and water oxidation, respectively.

Chapter 4 present tantalum nitride (Ta3N5) semiconductor as a photocatalyst for solar water splitting due to its suitable band edge potentials, capable of producing hydrogen and oxygen from water under visible light (λ< 590 nm). Ta3N5 modified with various O2-evolving cocatalysts such as CoOx and IrO2, was studied as a photocatalyst for water oxidation under visible light irradiation (λ < 500 nm) in the presence of electron acceptor, AgNO3. Cocatalyst loaded Ta3N5 in particular CoOx cocatalysts, displays especially good water oxidation performance with apparent quantum efficiencies (AQY) of 2.3 % at 500-600 nm, respectively.

Subsequently, Chapter 5 also reports on highly-active Ta3N5 photocatalysts that has been modified with alkaline metal. This chapter describes on the effect of facile modification of Ta2O5 with alkaline metal salts on water oxidation in an aqueous AgNO3 solution under visible light irradiation. Compared with conventional Ta3N5, the modified Ta3N5 was characterized by better crystallinity, smaller particle sizes with smoother surface and less agglomeration, and more importantly higher photocatalytic activity by six fold. Interestingly, modification with alkaline metal salts was compatible with loading of an oxygen evolution cocatalyst such as CoOx, suggesting that functions of the modifiers were distinct from catalytic effect. The present study shows critical roles of alkali metal salts loaded onto Ta2O5 upon nitridation toward the highly-active Ta3N5.

Chapter 6, various Z-scheme photocatalysis system using the optimized photocatalyst from the previous research as mentioned in earlier chapters was studied. The Z-scheme water splitting reactions were done both in the presence and absence of redox mediators respectively. Strategies to develop a highly active Z-scheme photocatalysis system driven by electron transfer between H2- and O2-photocatalyst are discussed on the basis of the structural and activity analysis results. The present results show the possibility of constructing an efficient solar H2 production system from water using this simple Z-scheme mechanism through interparticle electron transfer.

Chapter 7 brings to a close of the past three years research work with a summary of what has been accomplished. Future research areas based on the information drawn from the results obtained are suggested. Subsequently, an outline is drawn for future work that can be carried out with useful findings from this project.

All in all, the present study demonstrates that suitable cocatalyst loading and surface modifications are effective in minimizing electron-hole recombination and maximizing absorption capability while improving the selectivity for the two-step water splitting (Z-scheme). The present results suggest that an efficient and stable utilization of solar energy using two-step photoexcited Z-scheme water splitting can be achieved by employing high absorption wavelength visible-light-driven photocatalysts. This expands the possibility of using various active (oxy)nitrides in Z-scheme water-splitting systems, by employing suitable modification method to construct reaction sites and promote Z-scheme interparticle electron transfer.

本論文は、高効率な水分解光触媒システムの実現を目指して、長波長側の光まで利用できると期待されるZスキーム型水分解反応に適した光触媒を合成し、表面修飾や反応条件がその光触媒活性に与える影響を解明し、かつ光触媒活性を向上させることを目的として行われた研究結果をまとめたものである。本論文は英語で書かれており全部で7つの章から構成されている。

第1章では、太陽エネルギーを化学エネルギーに変換する手段としての光触媒による水分解の原理、特徴について述べ、既存の水分解用光触媒を引用しながら、本研究の目的と意義、本論文の構成について記述している。さらに、酸窒化物光触媒試料の合成や修飾法、活性の評価に関する実験方法の概要と原理、及び活性評価の指針についてまとめられている。

第2章では、ZrO2修飾TaON光触媒の調製方法と可視光照射下での水素生成活性が検討されている。ZrO2前駆体の種類に応じてZrO2修飾TaONの物性が大きく変化することや、オキシ硝酸ジルコニウムを前駆体に用いることでZrO2が高分散に担持され窒化中の欠陥の生成が抑制されること、その結果として水素生成活性が向上することが構造や発光特性の分析を通じて詳細に記述されている。

第3章では、WO3への異なる2種類の助触媒の共担持法と可視光照射下での酸素生成活性の相関について述べられている。PtOが担持されたWO3にRuO2などの金属酸化物を極微量担持することで酸素生成活性が向上することを報告している。担持された各助触媒の機能は光触媒反応と光電気化学反応の両面において議論され、PtOがIO3-のI-への還元を、RuO2が水の酸化を促進していると結論されている。

第4章では、Ta3N5光触媒による酸素生成活性の向上を目的に酸素生成助触媒の担持効果について述べられている。助触媒担持条件を詳細に検討することで、光触媒活性の顕著な向上が達成されている。

第5章では出発材料をアルカリ金属塩により修飾することで酸素生成反応に高活性なTa3N5光触媒を合成できることや、その理由について述べられている。窒化過程の詳細な観察をもとに結晶性や粒子の形状を議論し、アルカリ金属タンタル酸塩の核生成の結果、結晶性が高く光触媒活性の優れたTa3N5光触媒が得られていることを結論している。

第6章では、第2章から第5章で開発した光触媒を組み合わせたZスキーム型水分解光触媒反応システムの構築について検討している。水素生成光触媒としてZrO2で修飾されたTaONやRhがドープされたSrTiO3を、酸素生成光触媒としてWO3やTa3N5、Na添加Ta3N5などを使用している。特に、RhドープSrTiO3とTa3N5を組み合わせた反応系ではレドックス対非存在下での水分解反応を詳細に検討し、助触媒の担持順序や種類、溶液のpHなどの反応条件が粒子間電子移動に重要な影響を及ぼすことを見出している。

第7章では、各章に記述された成果が総括されている。さらに本論文の成果に基づいて、助触媒の担持や表面修飾、水分解用Zスキームシステム構築における今後の課題やそれを解決するための研究指針が提案されている。

以上、本論文は高効率なZスキーム型光触媒水分解反応システムの実現を目指した研究の結果が述べられている。TaONやTa3N5の合成方法と光触媒活性の改良、WO3の表面修飾法の改良を通じて、従来よりも長波長側の光まで利用できるZスキーム型水分解反応系について十分な成果を報告している。一連の研究成果は太陽エネルギー変換システムの構築という社会的要求の高い研究分野に重要な知見を与え、その進展を促すものであると認定され、触媒工学および化学システム工学の進展に大いに貢献するものであると判断される。

よって本論文は博士(工学)の学位請求論文として合格と認められる。