The Ti 2p peaks in all samples apart from the 0W(250) film were highly symmetric, while the Ti 2p1/2 and 2p3/2 peaks of the 0W(250) sample were asymmetric and each could be fitted into a series of two peaks: the peaks at 464.1 and 458.3 eV are assigned to Ti4+ (TiO2), while the peaks at 463 eV and 456.7 eV are attributable to a reduced state owing to the presence of an oxygen vacancy next to a Ti atom [43], as a result of the low energetic barriers for the formation of such defects in TiO2[12].