Two dimensional thin film surface layer structure of the promising new photo-catalyst InVO_4 for water molecule decomposition with water molecules adsorbed on the surface

摘要

A photo-catalyst able to enhance the decomposition of water molecules (hydrolysis) and able to work in the frequency range of visible light (or a wider wavelength region) is highly desirable for solar energy storing. However, to date, most of the photo-catalyst can work only in the ultra violet (UV) region (< 420 nm). The electron affinity - that can be identified as the conduction band bottom (CBB) - of the most of the photo-catalysts consists of d orbitals of transition metals (e.g. Zr, Ta, Nb, Ti), while the ionization potential ― represented by the valence band top (VBT) ― is represented by the 2p states of the oxygen atoms. Given this scenario, one could intuitively infer that a way to activate the catalyst in the longer wavelength region would be to lower the bottom of the 3d conduction band in order to make it closer to the redox level of H~+/H~2. A previous work by Z. Zou et al. indicated that InTaO_4 and InNbO_4 have a catalytic activity in the visible region up to 500nm. This has encouraged us to inspect V-based catalysts in order to get a better insight in this class of systems. The results coming from experiments have shown that, indeed, InVO_4 has a photo-catalytic activity responsible for the observed hydrogen evolution in the visible range (> 600 nm)as shown in Figure. 1 and this provided a basis for a theoretical inspection of the related electronic structure. In this paper, we report the results of our first principles calculations on the structural properties of InVO_4 and surface relaxation processes induced by the absorption of H_2O. This study is a necessary step and a valid testing ground in view of the extension of the approach to the analysis of the excitation spectrum of the catalyst surface in presence of H_2O molecules.