N-doped graphene-wrapped TiO_2 nanotubes with stable surface Ti~(3+) for visible-light photocatalysis

摘要

Due to the merits of visible-light absorption and high conductivity, Ti3+ is a main focus among the modifications of TiO2. However, surface Ti3+ is unstable. Herein, surface-stable Ti3+ in an N-doped rGO-coated TiO2 nanotube sample is prepared. The N-doping introduced Ti-C bonds and an abundance of surface Ti3+, which produced several mid-gap states. These properties allow the composite to absorb nearly the full visible-light spectrum. Owing to the advantages of a broad visible-light absorption, short diffusion distance in the nanotube, and a good heterojunction, the composite exhibits a good photocatalytic activity. Under visible light and without Pt as a cocatalyst, methylene blue was degraded within 40 mins, and the H-2 production rate is as high as 720 mu mol g(-1) h(-1). Due to the synergistic effect of N doping and Ti-C bond formation, the configuration of surface Ti3+ is stabilized by a charge-transfer resonance mechanism. The tightly coated graphene also plays a vital role in protecting the surface Ti3+. Even after storage for one year, the photocatalytic activity and the EPR signal intensity from Ti3+ paramagnetic states remain unchanged within experimental error. It suggests that the surface Ti3+ concentration, and the properties of the composite are stable with time.