In order to clarify the transfer of photoexcited electrons in photocatalyst and the function of different cocatalyst, in situ infrared spectroscopy (IR) using CO as probe molecules combined with electrochemical method was used as a tool to study the potential change of Pt or Rh particles deposited on the photocatalysts particles. By frequency shift of Pt-CO and Rh-CO adsorption peaks between dark and visible light irradiation conditions, potential change of cocatalyst particles were represented and analyzed.

Also, to enhance the hydrogen evolution reaction (HER), photodeposited Pt particles were introduced as reaction sites for HER. The effects of FeOx layers deposited on p-Si electrodes were assessed and detailed analysis revealed that the increase in photocurrent was due to the effects of surface orientation and tailing of the removed FeOx.

1. IR spectra of Pt/TaON, Pt/GaN:ZnO and Pt/LaTiO2N

For Pt/GaN:ZnO and Pt/TaON, the C-O stretching vibrations of adsorbed CO on Pt shifted to a lower frequency under visible light irradiation, as shown in Figure 1. In case of Pt/GaN:ZnO, the peak frequency of linearly-bonded CO (COL) to the Pt cocatalyst was at 2047 cm(-1) under dark conditions. Under the maximum irradiation of 2.5×10(17) photon cm(-1) s(-1), the COL peak shifted to 2039 cm(-1). In case of Pt/TaON, the COL frequency under dark conditions was 2053 cm(-1). Under the maximum irradiation, the absorption peak of COL shifted to 2042 cm(-1). The observed frequency shift of -8 cm(-1) and -11 cm(-1) indicates that the potential of the Pt cocatalyst particles was shifted to a more negative value by the excitation light irradiation on GaN:ZnO and TaON. On the other hand, no frequency shift was observed for Pt/LaTiO2N under irradiation.

GaN:ZnO, TaON, and LaTiO2N are n-type semiconductors, and upward band bending is expected at the photocatalyst/electrolyte interface. The Fermi levels (EF) of the photocatalyst and the Pt should be the same. When the excitation light irradiates the photocatalyst particle, electrons in the valence band are excited to the conduction band along with the formation of a hole in the valence band. The photogenerated holes migrate to the photocatalyst/electrolyte interface by band bending at the photocatalyst/electrolyte interface, initiating an oxidation reaction at the interface. The photogenerated electrons migrate to Pt cocatalyst particles and negatively shift the potential. While Pt particles on GaN:ZnO and TaON were negatively shifted in potential by the irradiation, those on LaTiO2N did not show this shift, indicating that the photoexcited carriers in Pt/LaTiO2N did not behave as described above.

Besides, the obvious differences were observed between the slopes of Pt/GaN:ZnO and Pt/TaON. From the infrared measurements, a slope of -0.09 V/decade was observed for Pt/GaN:ZnO compared to -0.21 V/decade for Pt/TaON. A similar trend with estimated potential was also observed in the rest potential. The difference of dependent slops suggests that the different recombination process is dominant for the Pt/GaN:ZnO and Pt/TaON system. It is most likely that this recombination pathway is related to the difficulty of overall water splitting on TaON. Improving the crystallinity and decreasing the number of defects in TaON should suppress the rate of recombination of photogenerated carriers in this pathway.

2. Comparison of potential shift between Pt and Rh

Some similarities and differences of IR spectra from Pt-deposited samples were observed in Rh-deposited oxynitrides. In the case of Rh/GaN:ZnO, very similar photo-response frequency shift and light intensity dependence was confirmed with Pt/GaN:ZnO. In the case of Rh/TaON, although obvious shift also showed, slope of potential change on irradiation intensity dependence decreased to almost half of the Pt-deposited sample. In case of LaTiO2N, despite frequency shift could not observed on Pt-deposited sample; obvious frequency shift and irradiation intensity dependence were observed on Rh-deposited one. The IR spectra for Rh/LaTiO2N under dark and a series of light intensity are shown in Figure 2, together with spectra of Pt/LaTiO2N as comparison. The reasons were partly ascribed to the different work function between Pt and Rh, as well as different size of cocatalyst particles between Pt/LaTiO2N and Rh/LaTiO2N.

3. Effects of surface modification on p-Si

The PEC properties of bare and Pt-loaded p-Si (100) were examined first. Cathodic photocurrents were observed for both samples under irradiation, indicating that these electrodes are p-type semiconductors. Upon modification with Pt, both the photocurrent and onset potential showed a marked increase, from 0.2 μA/cm2 to 0.28 mA/cm2 (at 0 VRHE) and from 0.15 VRHE to 0.50 VRHE, respectively. The apparent improvement in J-E properties by noble metal modification is likely due to the decrease of overpotential in hydrogen evolution.

To investigate the effects of FeOx modifications, J-E measurements were performed on FeOx/p-Si(100) and Pt/FeOx/p-Si(100) electrodes. Before the measurements, the samples were aged for 10 h at -0.28 VRHE under illumination to bypass the induction period in the J-E measurements. Both the cathodic photocurrent and the onset potential of FeOx/p-Si (100) were higher than those of p-Si, at 0.15 mA/cm2 (at 0 VRHE) and 0.52 VRHE, respectively. Pt modification of FeOx/p-Si(100) further improved the J-E properties: the onset potential shifted to 0.85 VRHE, which is 0.35 V more positive than that of Pt/p-Si(100), and the photocurrent increased to 2.45 mA/cm2 at 0 VRHE.(see Figure 3)

The present experimental results indicate that the improvement in J-E properties of aged Pt/FeOx/p-Si (100) is caused first by the formation of surface pyramids with {111} facets. The FeOx layer, which is removed during the aging treatment, promotes the formation of {111} facets and produces surface properties conducive to hydrogen generation. This can be verified by the better photoresponse of p-Si(111) compared to p-Si(100) and the similarity in PEC properties between Pt/FeOx/p-Si(111) and Pt/FeOx/p-Si(100) with surface pyramids. One other possible factor is the tailing of FeOx, which leads to the formation of a modulated carrier concentration structure in the depth direction. The depletion layer is thus thickened, which in turn enhances charge separation and PEC properties of p-Si.

4. Conclusions

This thesis describes the study on the potential change of Pt and Rh particles deposited on oxynitride photocatalysts by infrared absorption spectroscopy using adsorbed CO as a probe molecule. The different behavior of noble metal particles deposited on photocatalysts has been found. This thesis also includes the study on the surface modification of silicon by metal oxide thin layer to improve its PEC performance. These results clarify some of the reasons for the activities of various photocatalytic materials for overall water splitting under visible light irradiation, and effect of surface modification on p-Si photoelectrode.

Figure 1Infrared spectra of CO adsorbed on Pt in Pt/GaN:ZnO (left) and Pt/TaON (right) with changing excitation light intensity. Pt/GaN:ZnO or Pt/TaON was pasted onto ITO/sapphire, and the potential of the ITO was kept at the rest potential. The sample was immersed in a 0.1 M Na2SO4 aqueous solution at pH=6 and was irradiated with 385-740 nm light.

Figure 2Infrared spectra of CO adsorbed on Pt (left) and Rh (right) deposited on LaTiO2N with a changing intensity of excitation light. Pt or Rh deposited LaTiO2N was pasted on ITO/sapphire, and the potential of the ITO was kept at the rest potential. The sample was immersed in a 0.1 M Na2SO4 aqueous solution at pH=6 and was irradiated with 385-740 nm light.

Figure 3. J-E curves of p-Si (100), FeOx/p-Si (100) and Pt/FeOx/p-Si (100) electrodes in 0.1 M Na2SO4 under intermittent illumination.

本論文は、「Spectroscopic and Electrochemical Study on Hydrogen Evolution Sites of Photocatalysts for Water Splitting(赤外分光法と電気化学を利用した水分解光触媒の水素発生サイトについての研究)」と題し、水分解のための光触媒および光触媒電極における水素生成助触媒の機能について、赤外分光測定と電気化学測定を用いて研究した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、6つの章から構成されている。

第1章では、太陽光エネルギーを化学エネルギーに変換する意義について述べ、太陽光による水素製造と他の水素製造法について論じている。また、光触媒の動作原理や用いる材料の電子構造について記されていて、本論文の研究で用いた光触媒材料の特性についても紹介されている。次に、固液界面の表面赤外分光法が解説がされていて、吸着COの赤外スペクトルにおける挙動などの既報の紹介がなされている。さらに、光触媒電極による水分解の原理が述べられている。

第2章では本論文の研究で行った実験手順や、用いた実験装置について記されている。光触媒や光触媒電極の調製法が詳細に説明されている。また、用いた赤外分光セルの構造の説明や、電気化学セルの詳細が述べられている。

第3章では、GaN:ZnO、TaON、LaTiO2Nなどの可視光水分解を達成した光触媒材料や、可視光水分解が可能な電子構造をもつが水分解が達成していない光触媒材料を取り上げ、赤外分光測定と電気化学測定によって研究した結果がまとめられている。これらの光触媒の表面に水素生成用のPt助触媒を担持した時に光照射下で助触媒の電位がどのように変化するかを、吸着COをプローブとして助触媒に吸着させ、赤外分光法によって研究した結果が述べられている。光照射による電位変化の光量依存性を、吸着COの赤外吸収波数から測定し、光励起キャリアの再結合過程の速度論的パラメーターを求めた結果について述べられている。また、同時に助触媒の電位変化を電気化学的に測定した結果も述べられている。それぞれの光触媒で助触媒の電位変化の光量依存性は異なっていることについて議論されていて、その特性と光触媒活性について考察されている。

第4章では、第3章で用いられたGaN:ZnO、TaON、LaTiO2Nなどの光触媒材料を取り上げ、これらの光触媒の表面にRh助触媒を担持した時に光照射下で助触媒の電位がどのように変化するかを、吸着COをプローブとして助触媒に吸着させ、赤外分光法によって研究した結果が述べられている。Rhは水素生成過電圧がPtよりも大きいとされる金属であるが、実際の光触媒においてはPt助触媒よりRh助触媒のほうが高い水素生成活性をもつ。赤外分光法を用いて光照射によるRh助触媒の電位変化を調べ、Pt助触媒の結果と比較考察した結果が述べられている。電気化学測定からRh助触媒は、半導体光触媒と助触媒間の電子移動の障壁がPt助触媒に比べ小さいことを明らかにしている。また、LaTiO2NのようにPt助触媒では、半導体光触媒から助触媒への電子の移動が観察されなかった光触媒材料についても、Rh助触媒では電子の移動が観察され、Rh助触媒の有効性が論じられている。

第5章では、水素生成光電極としてp型Si半導体材料を取り上げ、その表面への水素生成のための構造の構築に取り組んだ結果について述べられている。p型Si(100)は、非修飾では僅かな水素生成の光電流しか観察されないが、Pt助触媒で修飾することにより約1000倍も光電流が増強することが示されている。さらに、Feで表面を修飾してからPt助触媒を担持することにより、光電流は数倍増えることが記されている。Feで修飾したときのSi(100)表面の構造的要因やバンド構造について考察されていて、Pt助触媒に電子を取り出すための要因について議論されている。

第6章では、本論文の総括が述べられていて、本研究の将来の展望が述べられている。

以上要するに、本論文は水分解のための光触媒の水素生成助触媒への光触媒からの光励起電子の移動について、赤外分光測定や電気化学測定など物理化学的測定に基づいた研究成果をまとめたものである。本論文の研究で見出された知見は、光触媒の研究開発に触媒設計の指針を与えるもので、太陽光を用いた光触媒による水からの水素生産の実現に大きく貢献するものであり、社会的な貢献も大きい。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。