Synthesis and characterization of nanocrystalline metal/nonmetal doped TiO 2 -graphene composites for photocatalytic applications

Abstract

The environmental pollution on a global scale is expected to be the greatest problem that scientists will face in the 21 st century. Semiconductor photocatalysis is green technology that allows the use of sunlight for the destruction of pollutants, thus providing an attractive route to potentially solve the problem. However, the efficiency and availability of photocatalysts which can be activated by the solar spectrum and specially indoor lighting is severely limited. Therefore, the purpose of this present work was to synthesize the metal and nonmetal doped TiO 2 –graphene composite photocatalysts in order to obtain the best photocatalytic efficiency under visible light irradiation. In first step, doped TiO 2 nanoparticles were prepared by simple sol-gel method and secondly graphene-doped/TiO 2 composites were synthesized by hydrothermal process. Different characterization techniques such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Transmission electrn microscopy (TEM), X-ray photoelectron sepectroscopy (XPS), Diffuse reflectance spetroscopy (DRS) and Photoluminscence spectroscopy (PL) were used to study the structure, morphology, chemical composition and optical properties of the prepared photocatalysts. XRD results showed that all doped and composite samples showed only anatse phase structure. It was noted that doping of TiO 2 with metals, nonmetal and also graphene incorporation did not change the anatse phase structure of TiO 2 . The average crystallite size found for all doped samples were in the range of 7-10 nm. TEM images also showed TiO 2 nanoparticles with average particle size in 8-13 nm range, which is in good agreement with XRD results. Moreover, results of all composite samples demonstrated that TiO 2 nanoparticles were successfully loaded on whole graphene sheets and hydrothermal process had played a role for the reduction of xixgraphene oxide. XPS technique confirmed that metals, nometal and graphene were present in the composite samples. Diffuse reflectance spectroscopy results demonstrated that the doping and graphene incorporation have significant effect on the optical properties of TiO 2 in visible light region. Photoluminscence results verified that metals and nometal doping and graphene introduction was effective to reduce the recombiation of photogenerated electron-hole pairs. Photocatalytic degradation of methyl orange was investigated by using different photocatalysts under visible light irradiation (λ ≥ 420 nm) for 3 h. All composite samples showed enhanced photocatalytic activty under visible light illumination. The maximum photodegradation was observed for nitrogen doped TiO 2 -graphene composite photocatalyst which is thirty time higher than commercial Degussa P25. The highest photodegradation of the composite catalyst was due to the synergetic effects of enhanced visible-light absorption, efficient charge separation, enhanced adsorptivity on the composite catalyst surface due to two-dimensional planar structure of graphene.