Thin carbon layer coated Ti3+-TiO2 nanocrystallites for visible-light driven photocatalysis

Abstract

Black TiO₂ containing Ti³⁺ attracts enormous attention due to its excellent visible-light driven photocatalytic activity. Herein, an in situ thermal decomposition approach to synthesize uniform thin carbon coated Ti³⁺-TiO₂ nanocrystals is presented. During the oleic acid-assisted solvothermal process, the crystal size and morphology of TiO₂ were controlled through oleic acid with carboxylic acid groups. Then the residual small quantities of oleic acid anchored on TiO₂ were used as a carbon source, which could be in situ pyrolyzed into a carbon layer on TiO₂ at high temperature and under an inert atmosphere. Meanwhile, Ti⁴⁺ species were partly reduced into Ti³⁺ states/oxygen vacancies on the surface of TiO₂ due to the carbothermal reduction reaction for the carbon-encapsulated Ti³⁺-TiO₂ structure. A series of characterizations indicated that the 20–25 nm TiO₂ nanocrystals obtained were wrapped evenly by 1–2 nm carbon layers, which had an important effect on the energy band structure change of TiO₂. The presence of the carbon layer also improves the Ti³⁺ stability and the conduction behavior of the composites. The Ti³⁺ states/oxygen vacancies created on the surface of TiO₂ were responsible for the remarkable photogenerated charge separation and extended visible-light absorption range. Furthermore, Ti³⁺ states/oxygen vacancies and the carbon layer together could enhance the adsorption ability of O₂ so as to promote the photogenerated electrons captured by the adsorbed O₂, leading to a great increase in the charge separation. As a result, the composites exhibit high photocatalytic performance for organic pollutants under visible light irradiation. This simple and new method may pave the way to practical applications for efficient photocatalytic degradation under visible light.