Afshin Ebrahimi

称号及び氏名博士（工学）

学位授与の日付

2010 年 3 月 31 日論

Investigations on the Preparation of Visible Light-responsive TiO2 Thin
Film Photocatalysts and Their Application to the Evolution of H2 frompure H2O and Aqueous Solutions Involving Organic Compounds(可視光応答型の二酸化チタン薄膜光触媒の調製とその水や有機 化合物を溶解した水溶液からの光触媒水素発生への応用に関する研究)

文

名論文審査委員

主査

安保正一

副査

辰巳砂昌弘

副査

井上博史

論文要旨
The development of artificial photosynthesis to produce renewable energy and othergreen energy materials is a major challenge to mankind. Efficient use of the freelyavailable resource of solar energy by its conversion into electricity as well as clean
chemical energy will be significant in reducing our dependence on fossil fuels. TiO2 is
one of the most popular oxide materials for its many applications and potential use in avariety of technologies where surface chemistry is critical, including photocatalysis.
Recently, TiO2 has been widely studied as a key component of photocatalysis in water
splitting reaction. The main objective of this study was to modify and develop newTiO2-based photocatalysts that allow the efficient absorption of visible light, thus,
enabling such modified TiO2 to initiate various desired reactions effectively even undervisible or solar light irradiation.
Chapter 1 summarized the various studies detailed in this thesis. In Chapter 2, thepreparation methods used to develop the TiO2 thin films were discussed. Among these,
sputtering methods have several advantages and, in this study, TiO2 thin films weredeposited by a radio-frequency magnetron sputtering (RF-MS). The films weredeposited using a stoichiometric TiO2 target sputtered in pure Ar at different pressures.
The aim of this study was to investigate the effect of the deposition conditions on thestructural and photocatalytic properties of the thin films before and after heat treatment.
As is shown in the studies, the crystalline phase of the TiO2 catalysts is an importantfactor that determines its activity. The influence of the substrate temperatures on thephotocatalytic activity of the TiO2 thin films was also investigated. The main influenceof temperature was found to affect the crystallinity of TiO2.
Reducing the pressure was found to decrease the total sputtering time for a specified
thickness. In this Chapter, the design of optimized Vis-TiO2 thin films for the separate

production of H2 and O2 was also discussed. Optimizing of the photocatalytic activityof Vis-TiO2 thin films prepared by a RF magnetron sputtering (RF-MS) method bycontrolling the various parameters such as sputtering pressure and target-to-substratedistance DT-S was investigated. It was shown through different characterization
experiments that the optimal sputtering pressure was 2.0 Pa. At this pressure, the highest
activity for the Vis-TiO2 thin films could be attained. When the sputtering pressure wasfixed at 2.0 Pa and the target-to-substrate distance was DT-S=75 mm, the best results wereobtained.
It was also found that chemical etching of the Vis-TiO2 thin films with HF solution
remarkably enhanced their photocatalytic activity. Such optimized HF-Vis-TiO2/Ti thinfilm electrodes exhibited a significant increase in their photocurrent under UV andvisible light irradiation as compared to untreated Vis-TiO2/Ti. SEM and BET surface
measurements revealed that the surface roughness increased and the interspace between
the columnar TiO2 crystallites were extended by HF treatment, indicating thatHF-Vis-TiO2/Ti has a shorter diffusion length for the photoformed holes to reach thesolid–liquid interfaces than untreated Vis-TiO2/Ti. The donor densities of Vis-TiO2/Ti
were found to increase after HF treatment, indicating that the photogenerated electrons ofHF (60)-Vis-TiO2/Ti can reach the TiO2 substrate interface more easily than Vis-TiO2/Ti
due to its higher conductivity. Moreover, the chemical etching of Vis-TiO2 by HF
solution was found to lead a remarkable increase in the separate evolution of H2 and O2
under visible light irradiation (λ ≥ 450 nm). Thus, HF (60)-Vis-TiO2/Ti/Pt thin film
photocatalysts were found to be successful in realizing the stoichiometric separate
evolution of H2 and O2.
Chapter 3 dealt with the post heat treatment of visible light-responsive TiO2 thin film
photocatalysts (Vis-TiO2) prepared on Ti metal foil (Vis-TiO2/Ti) or ITO glass(Vis-TiO2/ITO) substrates by the RF magnetron sputtering (RF-MS). The UV–Vis
spectra as well as photoelectrochemical performance of Vis-TiO2 were affected byvarious heat treatments such as calcination in air or ammonia. Calcination treatment in
NH3 (1.0 × 104 Pa, 673 K) was found to be particularly effective in increasing the visible
light absorption of Vis-TiO2 as well as in enhancing its photoelectrochemical
performance and photocatalytic activity. The Vis-TiO2 thin film photocatalyst wasprepared by the RF-MS method on one side and nanoparticles of Pt were deposited on
the opposite side of a Ti metal foil substrate (Vis-TiO2/Ti/Pt). The separate evolution ofH2 and O2 from H2O could be successfully achieved by using an H-shape glass cellconsisting of two aqueous phases separated by Vis-TiO2/Ti/Pt photocatalytic device and a
proton-exchange membrane. It was found that the rate of the separate evolution of H2and O2 was also dramatically enhanced by the calcination treatment of Vis-TiO2
photocatalyst in ammonia.
Chapter 4 dealt with the preparation of double-layered TiO2 thin film in order toimprove the reactivity of TiO2 thin film photocatalyst. A double-layered visible
light-responsive TiO2 thin film photocatalyst was prepared on a Ti foil substrate by aRF-MS method. The produced DL-TiO2/Ti consisted of a UV light-responsive TiO2
thin film (UV-TiO2) prepared on a Ti foil substrate which a visible light-responsive TiO2
thin film (Vis-TiO2) was prepared on it.
DL-TiO2/Ti exhibited higher
photoelectrochemical and photocatalytic performance under both UV and visible lightirradiation than a single-layered Vis-TiO2 thin film photocatalyst prepared on a Ti foilsubstrate. The optimal thickness of the UV-TiO2 thin film of DL-TiO2/Ti was found tobe about 100nm. Expanding on this work, a novel double-layered TiO2 thin film device,
DL-TiO2/Ti/Pt, was also prepared by a RF-MS method where DL-TiO2 was prepared on
one side and Pt was deposited on the other side of a Ti foil substrate. The separate
evolution of H2 and O2 from H2O was successfully achieved by using an H-type glass
cell consisting of two aqueous phases separated by the DL-TiO2/Ti/Pt and aproton-exchange membrane.
In Chapter 5, two aspects, the production of visible light responsive TiO2 thin films
and their characterizations were focused. TiO2 thin films were fabricated on quartz
substrates under similar conditions. The main part of this Chapter dealt with thephotocatalytic degradation of organic contaminants which were found in landfill leachate
and methylene blue by utilizing the Vis-TiO2 thin films. In this study, the changes in
the concentrations of COD and TOC as well as UV absorbance of the water involving
contaminants at a given wavelength were investigated. The results clearly showed thatthe novel Vis-TiO2 thin film photocatalyst led to a remarkably decrease in theconcentration of contaminants under solar light irradiation.
In Chapter 6, the Vis-TiO2 thin films developed in Chapter 2 were also applied indeveloping sandwich-type dye-sensitized solar cells (DSSC).
Two kinds ofsandwich-type dye-sensitized solar cells, DSSCVis and DSSCUV, were fabricated using
Vis-TiO2 (DSSCVis) and UV-TiO2 electrodes (DSSCUV), and their photovoltaic
performances were investigated under illumination with an AM-1.5 solar simulator lamp(100 mWcm-2). DSSCVis exhibited remarkably higher incident photon-to-electron
conversion efficiency (IPCE) than DSSCUV.
Furthermore, it was found that theoptimized solar-to-electric energy conversion efficiencies (η) reached 2.6 % for DSSCVis.
UV-Vis, SEM and photoelectrochemical investigations on DSSCVis and DSSCUV revealed
that the unique film morphology as well as band structure of Vis-TiO2 thin films play animportant role in realizing the high photovoltaic performance of DSSCVis.
Chapter 7 presented the results of the evolution of H2 from an aqueous solution
involving various light hydrogen-rich compounds such as various alcohols, hydrocarbons,
hydrazine and ammonia. As NH3 contains no carbon, it is a promising source ofhydrogen. Investigations on photocatalytic H2 evolution using different concentrations ofNH3 clearly showed that the production of hydrogen from ammonia aqueous solution.
Thus the present research unveiled that the photocatalytic hydrogen production fromammonia aqueous solution is a promising process for low-cost, low-temperature,high-purity in situ hydrogen production. Using higher concentrations of ammonia inaqueous solution led to a reasonably high production of hydrogen, and the best result wasobtained at 28%W of ammonia solution.
Finally, the results and conclusions of the core topics of Chapters 2 to 7 weresummarized in the final Chapter 8. This Chapter also presented several topics forfurther studies in unraveling the mechanisms behind the photocatalytic reactions as wellas in the development of new photocatalysts with high performance and functionality.

審査結果の要旨
本論文は、RF マグネトロンスパッタ法により可視光応答型酸化チタン薄膜光触媒（Vis-TiO2 ）を構築し、各種成膜条件の最適化による光触媒活性の高効率化を図るとともに、それを光触媒とする可視光照射下での水や有機化合物水溶液からの水素生成反応の探索を目的として行った研究をまとめたものである。また、本論文では、色素増感太陽電池の電極材料に Vis-TiO2 を応用する研究も遂行しており、次のような成果を得ている。
(1)RF マグネトロンスパッタ法における基板温度(T)、スパッタガス圧(P)、ターゲット-基板間距離(Dt-s)などの各種成膜条件が Vis-TiO2 の活性に及ぼす影響について検討し、T=873 K, P=2.0 Pa, Dt-s=75 mm の場合に最も高い可視光吸収と光触媒活性が得られることを見いだしている。さらに、HF 水溶液により Vis-TiO2 を化学的にエッチングすることで、H 型セルを用いた太陽光照射下での水からの水素と酸素の分離生成効率が向上することを明らかにしている。
(2)アンモニアガス中での焼成処理により、Vis-TiO2 の光電気化学特性や光触媒活性が向上することを見いだしている。また、各種分光測定により、アンモニアガス中での焼成処理により Vis-TiO2 中の Ti3+種が減少し、電子と正孔の再結合が抑制されるため光触媒活性が向上することを明らかにしている。
(3) Ti 基板上に紫外光応答型薄膜光触媒を成膜し、その上に Vis-TiO2 を成膜した積層型薄膜光触媒(DL-TiO2/Ti)が、一層型の Vis-TiO2/Ti より高い光触媒活性を示すことを見いだすとともに、その高い活性が Ti 基板から電解質溶液への逆電子移動の抑制に起因することを明らかにしている。
(4) Vis-TiO2 を電極とする高効率な色素増感太陽電池(DSSCVis)の構築に成功するとともに、Vis-TiO2 の伝道帯の下端が通常の TiO2 より正に位置しており、色素から Vis-TiO2への効率よい電子注入が可能となるため DSSCVis の特性が向上することを各種光電気化学測定により明らかにしている。
以上の諸成果は、可視光応答型酸化チタン薄膜光触媒の創製とその高効率化、および高効率な色素増感太陽電池の構築に貢献すること大である。また、申請者が自立して研究活動を行うに必要な能力と学識を有することを証したものである。