Studies on the Molecular Mechanism of Electrochemical Oxygen Evolution Reaction by the Surface luminescence Spectroscopy

本研究的主要目的是阐明水在n-TiO2和n-SrTiO_3等n型金属氧化物半导体电极上光氧化反应的分子机理，主要采用我们最近开发的表面发光光谱方法。通过这样的研究，将会发现用于水电化学氧化反应的新型活性电极材料。本文主要研究了n-TiO电极上光氧化反应的机理。虽然到目前为止，人们通常认为该反应是通过光生空穴氧化吸附OH离子产生的吸附OH自由基进行的，但本研究得出的结论是，该反应是通过靠近表面的n-TiO_2体中产生的间隙OH自由基进行的。理论考虑支持了后一种新机制，表明(1)OH^-离子可以通过平行于晶体c轴的通道渗透到n-TiO_2（金红石）体中，(2)在n-TiO_2体中产生的间隙OH-离子比吸附的OH-离子更容易被能量氧化，并且可以假设被光生成的空穴氧化，与吸附的OH-离子相反。上述新机制引起了人们极大的兴趣，为理解电极反应的机理提出了一个全新的概念。从实用的角度来看，这一机制也非常有趣，因为它表明了在这一新概念的基础上，通过对电极材料的重新研究，发现新的活性电极材料的可能性。此外，这一新机制的发现具有重要的价值，因为它清楚地表明，表面发光光谱对于电极反应的分子机制的原位研究是非常有效的。

The majn purpose of this research is to elucidate the molecular mechanism of the photooxidation reaction of water at n-type metal oxide semiconductor electrodes such as n-TiO2 and n-SrTiO_3, mainly by using a method of our recently developed surface luminescence spectroscopy. Through such research, new active electrode materials for electrochemical oxidation reaction of water will be found. We have focused our attention mainly on the mechanism of the photooxidation reaction at the n-TiO, electrode. Though it has often been assumed so far that this reaction proceeds via adsorbed OH radicals produced by the oxidation of adsorbed OH ions by photogenerated holes, it has been concluded in this re- search that this reaction proceeds via interstitial OH radicals produced in the n-TiO_2 bulk close to the surface. Theoretical considerations have sup- ported the latter new mechanism, showing that (1) OH^- ions can be assumed to penetrate into the n-TiO_2 (rutile) bulk through channels present parallel to the c-axis of this crystal, and (2) the resulting interstitial OH- ions in the n-TiO_2 bulk are energetically more easily oxidized than the adsorbed OH- ions, and can be assumed to be oxidized by the photogenerated holes, contrary to the adsorbed OH- ions. The above new mechanism is of great interest, proposing quite a new concept for the understanding of the mechanism of electrode reactions. This mechanism is also very interesting from a practical view-point, in that it indicates the possibility of finding new active electrode materials through the re-investigation of electrode materials on the basis of this new concept. Furthermore, the present finging of this new mechanism is of great value in that it clearly indicates that the surface luminescence spectroscopy is very effective for the in situ investigation of the molecular mechanism of the electrode reactions.