This study is led by Prof. Wenjun Luo (College of Engineering and Applied Sciences, Nanjing University). In previous studies, charge transfer mechanism at solid/liquid interface remains a very significant but controversial topic in photocatalysis and photoelectrocatalysis. Great efforts have been made to determine the structure and composition of solid surface on working conditions by different in-situ characterization methods. However, solid surface is usually amorphous and the structure and composition depend sensitively on light, electricity, temperatures and chemical surroundings, which could change in a very short time. Moreover, proton transfer is usually involved. It is thus a great challenge to elucidate the exact structure and composition of the solid surface. Therefore, it's necessary to develop a new theory to clarify the composition, the energy level positions and the characteristics of solid surface on working conditions.
In this work, by using TiO2 as a model, Luo et al. find a new characteristic of photo-induced bipolarity of the intrinsic faradaic layer on semiconductor surface. The researchers investigated the surface composition and the potential window of the intrinsic faradaic layer by in situ XPS, in situ EPR, TOF-SIMS and electrochemical methods. A reduction faradaic layer (RFL) and an oxidation faradaic layer (OFL) are obtained under illumination simultaneously (see Figure 1). The potential window of RFL is found to locate out of the forbidden band, which is different from conventional energy level positions of surface states in semiconductors. Moreover, the roles of RFL and OFL in photocatalysis and photoelectrocatalysis were investigated. In photocatalysis, the RFL and OFL serve as electron and hole transfer mediators, while the bipolarity or mono-polarity of intrinsic faradaic layer on a semiconductor depends on the applied potential in photoelectrocatalysis.
Moreover, electrochemical potential is introduced as a simplified descriptor for the structure and composition of the solid surface, in which it is not necessary to know the exact structure and composition of the surface (see Figure 2). It is just like momentum (k) space is introduced to describe electronic states in solid physics though exact positions of electrons also cannot be measured in a solid.
