Origin of enhanced visible light driven water splitting by (Rh, Sb)-SrTiO3

Abstract

A systematic calculation, using hybrid density functional theory, has been carried out to investigate the origin of the enhancement of photo-conversion efficiency of Rh-doped SrTiO₃ with codoping of Sb. In the case of Rh-doped SrTiO₃, partially unoccupied states are introduced above the valence band, thus lowering the hole oxidation at the valence band (VB) drastically, which explains the poor oxygen evolution activity of Rh-doped SrTiO₃. We show that the partially occupied t_2g subset of the Rh 4d orbital is completely filled in the presence of Sb due to the transfer of the extra electron to the Rh center. As a result, acceptor states are completely passivated in the case of (Rh, Sb)-codoped SrTiO₃ and a continuous band structure with reduced band gap is formed, which is responsible for the observed enhanced photocatalytic activity of (Rh, Sb)-codoped SrTiO₃. We have shown that the relative positions of the band edges of (Rh, Sb)-codoped SrTiO₃ with respect to the water redox levels are in favor of the spontaneous release of both hydrogen and oxygen during water splitting, which is consistent with the experimental observation. We have also studied the effect of codoping in different proportions (1 : 2 and 2 : 1) of Rh and Sb. Although 1 : 2 (Rh, Sb)-codoping leads to the formation of a clean band structure with the reduction of the band gap by a larger extent, it shows lower photo-conversion efficiency due to its charge non-compensated nature. In addition, the presence of acceptor states above the VB limits the oxygen evolution efficiency of 2 : 1 (Rh, Sb)-codoped SrTiO₃. Thus, the present approach successfully reproduces the experimental features of the Rh-monodoped as well as (Rh, Sb)-codoped SrTiO₃ and also explains their origin.