Characterization of graphene decorated with TiO2 grown by atomic layer deposition

The great potentialities of graphene can be enhanced thanks to the functionalization of its surface, which make it suitable for different application fields. In this work the changes in electronic structure of graphene after the deposition of TiO2 and the influence of graphene in TiO2 photocatalytic activity were studied. Different thicknesses of TiO2 were grown on graphene films by Atomic Layer Deposition at 200 °C using water and TiCl4 as precursors. The changes in electronic structure of graphene were investigated by means of Raman spectroscopy and the TiO2 stoichiometry was studied by X-ray photoelectron spectroscopy (XPS). The photocatalytic activity of TiO2/Graphene material was tested under a UV-Visible irradiation (generated by a solar simulator) using UV-Vis. spectrophotometry. The results indicate that inhomogeneity and intrinsic stain effects are present within the same sample. A study on undecorated graphene showed pre-existent strain and nonintentional self-doping caused by the mismatch and the presence of charged impurities on the underlying substrate respectively. The deposition of TiO2 films with thickness ≤ 10 nm led graphene to be p-doped, while strain becomes the dominant effect increasing film thickness. Oxygen vacancies, detected by XPS, decreased exponentially increasing the thickness of the TiO2 films to reach a stoichiometric O/Ti atomic ratio above 10 nm thickness where anatase phase signal appeared. The combination of TiO2 and graphene enhanced the efficiency of the electron-hole separation of TiO2 under UV-Visible light, which gave rise to a higher photocatalytic activity tested for methyl red molecule degradation.