Charge Carrier Dynamics in Undoped and N‑Doped Titania Nanolayers upon Ultraviolet and Visible Light Irradiation

This paper reports on the successful synthesis of visible light photoactive N–TiO2 nanolayers and the investigation of charge carrier dynamics in dependence on N-doping and irradiation wavelengths. Grazing incidence X-ray diffractometry exhibited that N-doping supports the formation of an anatase phase with a higher crystallinity than observed for undoped TiO2. Photoelectrochemical measurements gave evidence that N–TiO2 is characterized by a significantly higher incident photon conversion efficiency (IPCE) upon both UV and visible light irradiation. Photoelectrochemical impedance spectroscopy revealed that the higher IPCE of N–TiO2 in UV can be explained by a lowered charge transfer resistance, probably due to its higher crystallinity. The higher photoactivity in the visible can be explained by the incorporation of intrabandgap states upon N-doping. This is supported by X-ray photoelectron spectroscopy indicating the incorporation of N atoms in the titania layer, the observed bandgap narrowing by at least 250 meV as measured by ultraviolet–visible absorption spectroscopy, and the decrease of the work function by 50 meV, as derived from scanning Kelvin probe microscopy. Intensity-modulated photocurrent/photovoltage spectroscopy proved that the generally lower quantum yield at visible light is caused not only by the generation of less photoexcited charge carriers, but also by a higher surface hole recombination rate and hence lower hole charge transport efficiency.