Critical influence of surface nitrogen species on the activity of N-doped TiO2 thin-films during photodegradation of stearic acid under UV light irradiation

Atmospheric-pressure chemical vapour deposition (APCVD) was used to produce a series of nitrogen-doped titania (N-TiO2) thin-films using tert-butylamine as the nitrogen source. The films were deposited as the anatase phase on glass and quartz substrates and characterised using X-ray diffraction, optical and vibrational spectroscopy and electron microscopy. The nature and location of the nitrogen species present on the surface and bulk of the films was studied by X-ray photoelectron spectroscopy. Thorough comparison amongst films with similar structural and morphological features allowed the role of nitrogen species to be evaluated during photo-oxidation of a model organic pollutant (stearic acid). Sequential photocatalytic experiments revealed a drastic decrease in the UV activity of the films which were correlated with changes involving surface nitrogen groups. The existence of concomitant nitrogen species with similar binding energies (ca. 400 eV) but different chemical nature is proposed, as well as the direct participation of at least one of these species in the oxidation reaction. A similar mechanism for the visible light activity of N-TiO2 materials is also suggested. (C) 2014 The Authors. Published by Elsevier B.V.