Elucidation of Rh-Induced In-Gap States of Rh:SrTiO₃ Visible-Light-Driven Photocatalyst by Soft X-ray Spectroscopy and First-Principles Calculations

Abstract

The occupied and unoccupied in-gap electronic states of a Rh-doped SrTiO3 photocatalyst were investigated by X-ray emission spectroscopy and X-ray absorption spectroscopy for different Rh impurity valence states and doping levels. An unoccupied midgap Rh4+ acceptor state was found 1.5 eV below the SrTiO3 conduction band minimum. Both Rh4+ and Rh3+ dopants were found to have an occupied donor level close to the valence band maximum of SrTiO3. The density of states obtained from first-principles calculations show that all observed spectral features can be assigned to electronic states of substitutional Rh at the Ti site and that Rh:SrTiO3 is an unusual titanate compound with a characteristic p-type electronic structure. The Rh doping results in a large decrease of the bandgap energy, making Rh:SrTiO3 an attractive material for use as a visible-light-driven H2-evolving photocatalyst in a solar water splitting reaction.

Efficient Photocatalytic Hydrogen Production over Rh‐Doped Inverse Spinel Zn2TiO4

“…[6,20] Nevertheless, the high oxidation state of Rh (Rh 4 + ) is often inevitable in these studies and can be easily distinguished by an absorption peak centered at 600 nm as well as a violet color of the powders. [20] Interestingly, such an absorption peak is not discernible for any of the samples studied here and all powders remain a yellow color even though high-temperature calcination is applied during the synthesis. A high oxidation state of Rh has been confirmed to be detrimental to the photocatalytic activity as it results in the competitively consumption of photogenerated electrons.…”

“…A high oxidation state of Rh has been confirmed to be detrimental to the photocatalytic activity as it results in the competitively consumption of photogenerated electrons. [6,20] Thus, the complete removal of Rh 4 + species is highly desired for promoting efficient photocatalytic reactions. The absence of Rh 4 + species in these spinels is therefore highly beneficial although the underlying causes for the good Rh 3 + stabilization in the inverse spinel-type structure is not clear.…”

“…These observations are similar to Rh-doped SrTiO 3 in the absence of Rh 4 + species. [20] We now can have a discussion about the optical and photocatalytic properties observed before and after Rh doping according to their band structures. Charge generation processes are schematically represented in Figure 9.…”

Role of F in Improving the Photocatalytic Activity of Rh-Doped SrTiO3

“…Consequently, the band gap is reduced to 2.70 eV (ignoring the unoccupied midgap states), which is in excellent agreement with the experimentally reported band gap for Rh 4+ -doped SrTiO 3 . 32 To investigate the nature of these states, we have analyzed the DOS and PDOS ( Figure 4A) for Rh-doped SrTiO 3 . As can be seen from Figure 4A, the states below the Fermi level are partially occupied.…”

Photocatalytic Water Oxidation by a Pyrochlore Oxide upon Irradiation with Visible Light: Rhodium Substitution Into Yttrium Titanate

“…[16] Calculations [17] suggest that rhodium E g states are located within the conduction band in SrTi 1Àx Rh x O 3 , allowing visible-light excitation from the rhodium-based T 2g state to generate delocalized electrons which evolve hydrogen. However, the formation of Rh 4+ inter-gap states, which act as a recombina-tion center for photoexcited carriers, can inhibit hydrogen evolution.…”