Fine Control of the Chemistry of Nitrogen Doping in TiO2: A Joint Experimental and Theoretical Study

N-doped TiO2 materials have recently attracted a considerable amount of interest due to their enhanced photoelectrochemical properties compared to pristine TiO2. However, this improvement is still not clearly correlated to the N chemistry because the attribution of the observed components in the N 1s X-ray photoelectron spectra is strongly disputed. In this context, this joint experimental and theoretical study aims at clearly distinguishing the different N atomic species and positions that can be encountered upon N-doping of titania by the N2+ ion implantation method. Core-level binding energy shifts (CLS) calculated at a quantum-chemical level for N 1s orbitals with different chemical environments nicely match the components obtained by the fitting of the N 1s XPS experimental regions and allow for its detailed interpretation. We also monitored the N atom positions as a function of the dose used for implantation. Based on classical simulations of ion/surface interactions achieved with the help of the TRIDYN software, we show that oxygen vacancies play a key role in defining the nitrogen doping type (i.e., nitrogen position in the TiO2 lattice), as further supported by DFT calculations.