Enhanced Photocatalytic Performance of Carbon-Coated TiO_2–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 TiO_2–x photocatalyst (C/TiO_2–x) and its corresponding acid-washed sample (C*/TiO_2–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/TiO_2–x and C*/TiO_2–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/TiO_2–x, whereas only macromolecular graphitelike C exists in C*/TiO_2–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*/TiO_2–x, whereas the alkoxy and carboxylate C species that should be the recombination centers would poison seriously the surface of C/TiO_2–x. Accordingly, the hole and electron transfer mechanism in the C-coated TiO_2–x photocatalyst is proposed.