Shed light on defect induced enhanced visible-light photocatalysis activity of rutile TiO2 nanoparticles: effects of annealing on blue-gray to light–gray transition

Abstract

Preparation of the defective TiO2 nanoparticles (NPs) have been recently in particular interest for its outstanding applications in visible-light active photocatalysis. In order to make TiO2 visible-light absorber, optical band gap should be modified especially via introducing structural defects like oxygen vacancies as an effective pathway. In this research, we prepared defective rutile TiO2 NPs with highly efficient photocatalytic activity under visible light. The NPs were synthesized by electric arc discharge in water as a cost-effective and environmentally friendly method with capability of defect formation which followed by heat treatment. The photocatalytic activity improvement mechanisms based on oxygen vacancies were supported by TEM, XPS, XRD, PL and optical spectroscopy characterizations. Spherical TiO2 NPs with average size of 23, 28, 38, and 50 nm were obtained for pristine, and annealed samples at 300, 500 and 700 °C respectively. The optical band gaps of NPs decreased from 2.9 to 2.0 eV with annealing temperature, and a concurrent color change from blue-gray to light-gray. The XPS results confirm the presence of vacant oxygen and surface OH defects in all TiO2 samples. The results reveal that concentration of surface defects and mostly the oxygen vacancies play a crucial role in visible light photoactivity of gray TiO2 nanostructures. In the synthesized TiO2 NPs, the sample with the lowest concentration of non-lattice oxygen exhibit better degradation efficiency than other samples. The results provide a flexible method for synthesis of defective rutile TiO2 as a highly efficient visible active photocatalyst nanomaterial.