Perovskite-type titanates have been proposed as potentially efficient photocatalytic energy conversion materials that can absorb visible sunlight and directly transfer the formed photocarriers to liquid water, thereby generating hydrogen gas. Despite the potential for efficient, cheap, and sustainable energy conversion, photocatalytic materials have so far not reached practical solar light collection efficiencies. The main reason appears to be the high recombination rate of generated photocarriers, which means that the energy of the sunlight is mostly spent on generating heat, rather than splitting water. The purpose of this work was to determine the electronic structure of Rh-doped SrTiO₃, which is known to be a moderately efficient hydrogen-evolution photocatalyst.

Fig. 1. Photographs of Rh⁴⁺:SrTiO₃ and Rh³⁺:SrTiO₃ pellets showing the different colors and the Rh in-gap state locations relative to the valence and conduction band edges of SrTiO₃. The yellow Rh³⁺:SrTiO₃ is photocatalytically more efficient due to the lack of a mid-gap recombination state.

Fig. 2. (a) X-ray emission spectra of various Rh-doped SrTiO₃ samples showing the presence of an occupied Rh state close to the top of the valence band of SrTiO₃. (b) Illustration of the XES process and the location of the Rh donor levels just above the SrTiO₃ valence band edge.

For efficient transfer of photoelectrons from a bulk photocatalyst to water, it is necessary to have a semiconductor with a conduction band located well above the reduction potential of water, while the band gap should be close to 2 eV for the best energy harvesting efficiency. Pure SrTiO₃ is a wide-gap semiconductor that satisfies the conduction band alignment requirement with water, but it is transparent for visible light and only absorbs sunlight in the ultraviolet part of the spectrum, above the band gap energy of 3.2 eV. Rhodium doping of the SrTiO₃ host semiconductor appears to be quite special, in that it creates deep impurity levels either around the mid-gap region of SrTiO₃ or close to the top of the valence band without affecting the location of the conduction band edge. Electrochemical measurements under visible light show an apparent p-type photocathode behavior, which is quite unusual for typically n-type SrTiO₃. In this work, we have used a combination of x-ray photoelectron spectroscopy (XPS) at Photon Factory beamline 13A and x-ray absorption (XAS) and emission (XES) spectroscopy at the undulator beamline BL07LSU in SPring-8.

The XPS experiments were used to investigate the violet-to-yellow color change reaction that occurs when Rh:SrTiO₃ is reduced, typically during the initial induction period of an electrochemical reaction. SrTiO₃ thin films with several different Rh doping levels were grown at various temperatures and ambient oxygen pressures. The Rh 3d XPS profile analysis showed that the color shift is caused by a change in the Rh impurity valence, with Rh⁴⁺:SrTiO₃ being violet and Rh³⁺:SrTiO₃ yellow [1]. Significant variations were also observed in the Rh content at the catalyst surface due to evaporative loss of Rh at high crystal growth temperatures.

A more detailed analysis of the in-gap energy levels was undertaken by XAS and XES analysis of powder samples. The XAS analysis, which probes unoccupied states, was used to show that a mid-gap unoccupied state appears only in samples containing the Rh⁴⁺ valence state, as illustrated in Fig. 1. Reduced Rh³⁺:SrTiO₃ powders did not show a mid-gap state. Both Rh valence states lead to the appearance of an occupied impurity level close to the top of the valence band, as shown by XES spectra in Fig. 2. In addition to an increase of spectral weight with Rh doping just above the valence band top, a shift was observed in the locations of the Rh⁴⁺ and Rh³⁺ impurity levels. A conclusion of the analysis was that the Rh³⁺:SrTiO₃ material shows higher photocatalytic activity due to the lack of a mid-gap unoccupied energy level that would lead to rapid photocarrier recombination. However, the photogenerated charge collection efficiency is limited by the lack of strong hybridization between the deep Rh levels and the O2p character valence band of SrTiO₃.

References

• [1] S. Kawasaki, K. Nakatsuji, J. Yoshinobu, F. Komori, R. Takahashi, M. Lippmaa, K. Mase, and A. Kudo, Appl. Phys. Lett. 101, 033910 (2012).
• [2] S. Kawasaki, K. Akagi, K. Nakatsuji, S. Yamamoto, I. Matsuda, Y. Harada, J. Yoshinobu, F. Komori, R. Takahashi, M. Lippmaa, C. Sakai, H. Niwa, M. Oshima, K. Iwashina, and A. Kudo, J. Phys. Chem. C 116, 24445 (2012).

Authors

• S. Kawasaki, K. Nakatsuji, S. Yamamoto, I. Matsuda, Y. Harada, 
J. Yoshinobu, F. Komori, R. Takahashi, M. Lippmaa, K. Akagi^a, C. Sakai^b, H. Niwa^b, M. Oshima^b, K. Mase^c, K. Iwashina^d, and A. Kudo^d.
• ^aTohoku University
• ^bDepartment of Applied Chemistry, University of Tokyo
• ^cHigh Energy Accelerator Research Organization
• ^dTokyo University of Science