Shedding Light on Aging of N-Doped Titania Photocatalysts

A detailed analysis of nitrogen dopant behavior in nanostructured microspheres of the TiO2 photocatalyst obtained by the thermally assisted reactions in aqueous sprays method has been performed for the first time using electron paramagnetic resonance, X-ray photoelectron spectroscopy, and UV–vis spectroscopy and is supported by theoretical simulation of possible defect structures. The nitrogen species were found to undergo the N• to N– transformation during sample storage under different conditions, with an activation energy of about 0.45 eV. Three main possible evolution pathways for the dopant state were identified and discussed. It was established that the most probable transformation consists of migration of an oxygen vacancy site to an interstitial nitrogen atom followed by the formation of a nonparamagnetic substitution nitrogen center. Possible diffusion routes of oxygen vacancy and corresponding energy barriers were estimated and found to be in agreement with experimental observations.