Water splitting by a photoelectrochemical (PEC) cell utilizing solar energy is an attractive means of hydrogen production. In the PEC cell, a semiconductor electrode is the main equipment of the device. An n-type semiconductor and a p-type semiconductor electrode function as a photoanode and a photocathode, respectively. The combination of a photoanode and a photocathode is beneficial way to drive water splitting efficiently under sunlight because of applicability of narrow gap semiconductor materials which can capture the light of wider wavelength region by combining driving force of photocathode and photoanode. From the view point of capturing more sunlight, development of photoelectrodes with visible-light response is indispensable, because almost half of energy in sunlight is composed of visible-light.

Colored metal oxynitrides are hopeful candidates of photoanode material for sunlight driven water splitting, because some oxynitrides have proper band structure for water splitting. The chemical stability is also advantageous point of oxynitrides; some photocatalysts stably generate oxygen gas form an aqueous solution under visible-light, suggesting that the oxynitride materials have durability for PEC water oxidation. In fact, some Ti(4+), Ta(5+), and Nb(5+) based oxynitiride electrode functions as photoanode for water oxidation with relatively small applied bias voltage under visible-light.

Herein, LaTiO2N which is composed of relatively abundant metals (La, Ti) and responsive to up to 600 nm were investigated in detail as photoanode material for water splitting. From the view point of practical and large scale application, low cost and simple preparation of photoelectrodes are preferable. Particularly for oxynitride materials, the powder sample can be synthesized easily. Semiconductor powder based photoelectrodes with less inter-particle boundaries and proper connection between the particle and the conductor are desirable. Therefore, development of the preparation method is important for an efficient photoelectrode consisting of semiconductor particles.

It is known that over potential of water oxidation reaction is relatively large. Thus, decreasing the over potential is indispensable for efficient water splitting. Surface modification with co-catalyst which decreases over potential for oxidation of water significantly affects the PEC properties of a photoanode. Although IrO2 is an efficient co-catalyst material composed of noble metal, from the view point of practical application, non-noble metal co-catalysts such as cobalt catalyst are promising candidates and the catalytic effects are still debatable.

In addition, characterization of flat band potential which suggests the band structure of the semiconductor material is also important in order to judge possibilities for photocatalyst and/or photoelectorde material for water splitting. The discussion would provide statistics for development in near future photoanodes for water splitting.

Accordingly, following three topics for the LaTiO2N electrode prepared from the powder sample are focuses in this study; (1) fabrication method and effects of the structure (2) effects of surface modification with co-catalyst, (3) flat band potential of (oxy)nitiride electrodes, and hence these were investigated.

Contents of this thesis were classified as follows;

Chapter 1. General Introduction; The first chapter describes general backgrounds of the research and basic principal of PEC water splitting, characteristic of (oxy)nitride materials, role of structure in photoelectrode, and effects of co-catalyst for water oxidation.

Chapter 2. TiCl4 Treatment Effect on the Photoelectrochemical Properties of LaTiO2N Electrodes; LaTiO2N electrodes were prepared by an ink method with the powder sample and a fluorine doped tin oxide (FTO) coated glass substrate, and the PEC properties were investigated. The as-prepared LaTiO2N/FTO electrode generated low photocurrent probably because of poor connection between the particles in the LaTiO2N film and/or the particles and the FTO conductor substrate. However, LaTiO2N photoelectrodes with TiCl4 treatment which introduced TiO2 like binder into the LaTiO2N film generated several times higher photocurrent than those without TiCl4 treatment, because the introduction of titanium species facilitated migration of photoexcited electrons in LaTiO2N toward the FTO substrate.

Chapter 3. Morphological effects on the Photoelectrochemical Properties of LaTiO2N Electrodes; Effects of morphology in LaTiO2N particles which were employed for photoelectrodes prepared by an electrophoresis deposition (EPD) method on the PEC properties for water splitting were investigated. Various LaTiO2N particles synthesized from crystalline La2Ti2O7 of various particle sizes with featureless shape and micro-crystals of plate like shape derived from a flux method were used. Untreated LaTiO2N electrodes consisting of the smaller particles tended to generate higher photocurrent, whereas TiCl4-treated electrodes prepared from LaTiO2N particles of micro-meter size tended showed higher PEC performance than that consisting of the several ten nano meter sized particles, probably because of the less inter particle boundaries. A photoelectrode consisting of plate like shaped LaTiO2N micro-crystals which would be advantageous for shorter necessitate diffusion length of photogenerated holes in LaTiO2N for water oxidation generated larger photocurrent than those formed from the particles of featureless shape. These results provide a strategy for structure of an efficient semiconductor-particles-based photoelectrode which consists of the semiconductor micro-crystals appropriately fixed onto a conductor substrate.

Chapter 4. Photoelectrochemical Properties of LaTiO2N Electrode Prepared by Particle-Transfer Method; LaTiO2N photoanodes consisting of 1-3 layers of LaTiO2N micro-crystals which firmly anchored by continuous metal film were prepared by a novel particle-transfer (PT) method. The PEC properties were dependent on contact layer material which directly connected to LaTiO2N particles and functions as conductor in the electrode, and contact layer of Ta metal was proper material for the LaTiO2N photoelectrode. The LaTiO2N/Ta electrode prepared by PT method generated higher photocurrent than that fabricated by EPD method with TiCl4 treatment. The IrO2 modification was remarkably more effective to improve PEC properties of LaTiO2N photoelectrode (PT) than that (EPD), indicating that surface modification with co-catalyst is more important for the electrode (PT) than that (EPD). The LaTiO2N electrode (PT) in an electrolyte containing a redox regent showed more negative potentials where large photocurrent was obtained than that in a supporting electrolyte, suggesting that less recombination rate in LaTiO2N leads to more negative potential for large photocurrent.

Chapter 5. Modification Effects of Cobalt-based Co-catalyst on Photoelectrochemical Properties of LaTiO2N Electrode Prepared by Particle-Transfer Method; Effects of surface modification with cobalt (Co) based co-catalyst for water oxidation on PEC properties of LaTiO2N electrode prepared by PT method were investigated. Initial photocurrent density and stability of photocurrent generated from the LaTiO2N photoanode conflicts each other by controlling loading amount of Co co-catalyst. However, addition of optimal amount of Fe species into Co co-catalyst on LaTiO2N particles leaded to an improvement in stability of the photocurrent without decreasing the initial photocurrent, probably because the addition of Fe species improved dispersibility of the co-catalyst on LaTiO2N and/or the catalytic properties for water oxidation. The optimized Co-Fe/LaTiO2N photoanode generated stoichiometric amount of oxygen gas, and showed solar energy conversion efficiency of ca. 0.6 %.

Chapter 6. Flat Band Potentials of Metal (Oxy)nitride Electrodes; Flat band potentials of Ti(4+), Ta(5+), or Nb(5+) based (oxy)nitiride electrodes prepared by PT method were estimated by the Mott-Schottky plots, and band structure of the materials were estimated. Band edges of these (oxy)nitride materials straddle the potentials of water reduction and oxidation, indicating a possibility for application of these materials to photocatalyst and photoanode materials for water splitting. The band structures of some narrow gap oxynitride suggest necessity of proper modification with co-catalyst and/or band engineering for oxygen evolution reaction over the material.

Chapter 7. Summary; The results of chapter 2-6 were summarized, and outlooks of this study were discussed.

本論文は、「Study on photoelectrodes prepared from oxynitride particles for water splitting with visible-light (酸窒化物粒子から調製した可視光水分解用光電極の研究)」と題し、太陽光エネルギーを利用した水からの水素製造を目的とした光電極の開発について研究の結果をまとめたものである。本論文は英文で書かれており、全七章から構成されている。

第一章では、水分解用光電気化学セルの特徴と原理、光電極材料としての酸窒化物材料の長所、電極性能に影響を与える幾つかの要素について述べ、本研究の意義と目的が示されている。

第二章では、酸窒化物LaTiO2N粒子の塗布膜電極の光電気化学特性、および粒子層中にバインダーを堆積させるポスト処理(TiCl4処理)の効果について調べた結果を述べている。TiCl4処理はLaTiO2N塗布膜電極の光電流を増大させ、可逆水素電極に対して1.2Vで最大5倍程度増加したことが記されている。TiCl4処理後の電極のSEM、XPS、ラマンスペクトルから、アナタース型の酸化チタンがLaTiO2N粒子層中に堆積し、薄膜の抵抗を減少させたことでLaTiO2N中の電子の基板までの移動を促進したことが結論づけられている。

第三章では、LaTiO2Nの粒子形態が塗布膜電極の光電気化学特性に与える影響について検討した結果が述べられている。数十nmから数μmの粒子径のLaTiO2N粉末サンプルを用いて、電気泳動電着法により作製した電極にTiCl4処理を施した試料について検討した結果が記述されている。大きな粒子からなる電極に対してTiCl4処理による光電流の増大が大きく、その結果大きな光電流を示したことが記されている。この傾向はTiCl4処理が粒子-基板間の接合改善に効果的であり、大きな粒子からなる電極では粒界の影響が小さいために大きな光電流を示したと説明されている。また、フラックス法により調製された前駆体酸化物を用いて合成された板状のLaTiO2N微結晶からなる電極はさらに大きな光電流を示したと述べられている。

第四章では、新規の電極作製方法、粒子転写法により電極を作製し、その電極の構造評価、および光電気化学特性を検討した結果が述べられている。粒子転写法は、LaTiO2N粒子層をガラス板上に堆積させ、その上からRF-マグネトロンスパッタによって膜厚200nm程度の任意の金属材料の連続膜(コンタクト層)、続いて10μm程度のTi、またはNbの金属膜(集電層)を堆積し、堆積させた金属薄膜をLaTiO2N粒子と共に別のガラス板に転写する手法である。電子顕微鏡によって、作製した電極は単粒子層のLaTiO2N粒子が、金属層を覆っている構造になっていると結論されている。この電極の光電気化学特性はコンタクト層の材料を変化させることによって変化し、金属Taを用いた際に最も優れた光電気化学特性を示したことが述べられている。粒子転写法によって作製した電極は同じ粒子を用いて電気泳動電着法によるものよりも大きな光電流を示したことが記されている。粒子転写法による電極は、助触媒による表面修飾の効果がより顕著であったことが述べられている。

第五章では、粒子転写法によって調製したLaTiO2N電極に対する非貴金属のCo系助触媒による表面修飾の効果について研究した結果が述べられている。Co系助触媒による表面修飾の効果は、担持量の増加による表面反応促進の効果と、助触媒による入射光の吸収または散乱によるLaTiO2Nに吸収される光の減少の効果のために、最適値があることが述べられている。Co系助触媒に適量Fe種を添加することで、電流値を低下させることなく耐久性が向上することが見出されている。これはLaTiO2N上の助触媒の分散性もしくは活性がFe添加によって向上するためであると説明されている。Feを最適量添加されたCo系助触媒により表面修飾が施されたLaTiO2N電極の太陽光エネルギー変換効率は約0.6%であったことが述べられている。

第六章では、様々な材料の金属酸窒化物粒子を用い粒子転写法によって電極を作製し、これらの電極のフラットバンドポテンシャルをモットショットキープロットにより求めた結果が記されている。調べられたTi、Ta、Nbによる酸窒化物のバンド端のほとんどが、水の酸化還元電位を挟んでいて、水分解用光電極材料としての可能性が高いことが示唆されている。

第七章では、各章の結論が総括されている。また、本論文で開発された電極の研究の位置付け、および研究成果から見出された今後の光電極の開発に必要な方向性について述べられている。

以上、本研究は大規模展開に有利な粉末材料からなる水分解電極について、性能向上のための開発指針の提示、および有用な新規電極作製方法を示している。加えて、高効率な水の分解に必要不可欠な助触媒について、電極の表面修飾の効果、および触媒材料開発について検討した結果をまとめたものであり、太陽光エネルギー変換による化学エネルギー媒体を製造するシステムの構築という、社会的期待の高い研究分野に重要な知見を与え、触媒工学、および化学システム工学の進展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。