(Invited) Interfacial Charge-Transfer Excitation at an Anatase-Rutile Junction: A Hypothesis

Abstract

Titania, the most widely used metal oxide for photocatalytic applications, is categorized into semiconductor based on its electronic energy structure, and therefore its band structure, which must impact on the physicochemical properties of titania, is very important. We have developed a novel method for identification and characterization of metal-oxide powders with energy-resolved distribution of electron traps (ERDT) measured by reversed double-beam photoacoustic spectroscopy (RDB-PAS) [1,2]. In this study, ERDT patterns of anatase and rutile titania powders as well as their mixtures were measured and analyzed to elucidate the actual position of valence-band of two kinds of crystallites. Figure 1 shows ERDT patterns of ST-21 (anatase), ST-G1 (rutile) and their thoroughly brayed 50/50 mixture as well as simulated patterns by summing up to ST-21 and ST-G1 patterns with and without lower-side energy shift (ΔE = 0.19 eV) of the former. The degree of coincidence in the ERDT-pattern shape of observed and simulated mixtures, ζ, was calculated by changing ΔE to obtain the maximum ζ, 0.70, at ΔE =0.19 eV. If valence-band (VB) electrons are excited to ETs during RDB-PAS process independently in two titania samples, the mixture may give an ERDT pattern as simple summation. The results suggest that interfacial electron excitation from VB of ST-G1, which is slightly higher than that of ST-21, to ETs in ST-21. As has been already claimed that energy of ETs measured as energy from VBT in the ERDT patterns might be overestimated and electrons in a high-DOS part of VB (h-DOS(VB)), not VBT of the least DOS, are excited to ETs [1,2]. The above mentioned consideration may suggest that h-DOS(VB) of rutile is ca. 0.19 eV higher than that of anatase. Thus, RDB-PAS analysis of ERDT patterns of mixtures of different titania samples is a novel method to clarify the relative band position of semiconducting metal oxides such as titania. [1] A. Nitta, M. Takase, M. Takashima, N. Murakami, B. Ohtani, Chem. Commun., 52, 12096–12099 (2016). [2] A. Nitta, M. Takashima, N. Murakami, M. Takase, B. Ohtani, Electrochim. Acta, 264, 83–90 (2018). Figure 1