Facile synthesis of mesoporous titanium dioxide doped by Ag-coated graphene with enhanced visible-light photocatalytic performance for methylene blue degradation

A highly efficient and elaborately structured visible-light-driven catalyst composed of mesoporous TiO2 (MT) doped with Ag+-coated graphene (MT-Ag/GR) has been successfully fabricated by a sol-gel and solvothermal method. The as-prepared catalyst has been investigated by X-ray diffraction (XRD), photoluminescence spectroscopy (PL), X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectroscopy (DRS), scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) spectroscopy, and transmission electron microscopy (TEM). Inexpensive, stable MT was coupled with hole-accepting graphene (GR) and electron-trapping silver induced higher activities than those achieved with pure MT, MT-Ag, and MT/GR in the degradation of methylene blue (MB) in solution. Although the Ag dopant and graphene support were responsible for narrowing the band gap of TiO2 and shifting its optical response, respectively, they acted synergistically in shifting absorbance from the ultraviolet (UV) to the visible-light region with a smaller band-gap energy. Meanwhile, they also served to lower the photo-induced electron and hole recombination rate, and increased the specific surface area and the concentrations of Ti3+ ions and hydroxyl groups. In degradation studies, the effects of catalyst amount, pH, and initial MB concentration have been examined as operational parameters. A photocatalytic mechanism for the action of MT-Ag/GR is proposed, and possible reasons for the enhancement in visible-light photocatalytic efficiency are discussed.