The great potential of graphene can be enhanced thanks to the functionalization of its surface. For this aim, different thicknesses of TiO2 were grown on graphene films by atomic layer deposition (ALD) at 200 degrees C using H2O and TiCl4 as precursors. The changes in electronic structure of graphene after the deposition of TiO2 and the influence of graphene in TiO2 photocatalytic activity under UV-Visible irradiation were studied. Results indicated the presence of inhomogeneity and intrinsic strain effects within the same sample. Undecorated graphene showed pre-existent strain due to the mismatch between graphene film and the underlying substrate, while non-intentional self-doping is caused by the presence of charged impurities. The deposition of TiO2 films with thickness <= 10 nm led graphene to be p-doped, while strain became the dominant effect increasing film thickness. Oxygen vacancies in the film decreased exponentially by increasing the film thickness leading to a stoichiometric O/Ti atomic ratio of TiO2 above 10 nm thickness. The combination of TiO2 and graphene enhanced the efficiency of electron-hole separation of TiO2 under UV-Visible light, leading to a higher photocatalytic activity tested for methyl red molecule degradation, making TiO2/Graphene hybrid material a promising candidate for the photodegradation of pollutants and water purification.