Enhanced Photocatalytic Performance of Carbon-Coated TiO2_(x) with Surface-Active Carbon Species

Carbon (C) coating on the TiO2 surface has attracted extensive research interest due to the unique properties of the conjugated materials in electron transport and photoelectronic coupling ability. However, owing to the complexity of surface C species, there is no experimental study on their structure and property. Although the C-coated TiO2_(x) photocatalyst (C/TiO2_(x) and its corresponding acid-washed sample (C*/TiO2_(x)) exhibit similar visible-light absorption, their catalytic activity is quite different. According to high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, electron spin resonance, and NMR results, the only structural difference between C/TiO2_(x) and C*/TiO2_(X) lies in the surface C species. Our NMR experimental results show that several C species (including alkoxy and carboxylate, and macromolecular graphitelike C) are present in C/TiO2_(x), whereas only macromolecular graphitelike C exists in C*/TiO2_(x). Combined with the photocatalytic activity measurements, it can be deduced that the surface graphitelike C should be the active C sites, which facilitate the separation of photoinduced electron and hole and lead to the exceptionally high photocatalytic activity for C*/TiO2_(x), whereas the alkoxy and carboxylate C species that should be the recombination centers would poison seriously the surface of C/TiO2_(x). Accordingly, the hole and electron transfer mechanism in the C-coated TiO2_(x) photocatalyst is proposed.