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

Raul Quesada-Cabrera,Carlos Sotelo-Vazquez,Jawwad A Darr,Ivan P Parkin

doi:10.1016/j.apcatb.2014.06.010

Raul Quesada-Cabrera, Carlos Sotelo-Vazquez + Show 2 more

Open Access

https://doi.org/10.1016/j.apcatb.2014.06.010

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Abstract

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. 400eV) 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.

Highlights

The strategy of doping titania (TiO2) using non-metal impurities in order to extend the photocatalytic efficiency of the semiconductor to include the visible range is one of the key challenges of photocatalysis
These N-TiO2 films were pure anatase and no traces of rutile, titanium nitride or any other nitrogen-containing structures were detected by X-ray diffraction (XRD) and Raman spectroscopy (Fig. 2)
Scanning electron microscopy (SEM) images of undoped TiO2 films revealed relatively rough surfaces formed by large star-like aggregated particles (Fig. 3(a)) whereas the NTiO2 films showed slightly more compacted surface structures with flat particles apparently merged together (Fig. 3(b))

Summary

Introduction

The strategy of doping titania (TiO2) using non-metal impurities in order to extend the photocatalytic efficiency of the semiconductor to include the visible range is one of the key challenges of photocatalysis. The discussion around nitrogen-doped TiO2 materials ( N-TiO2) has been focused on their visible light activity and its origins. Many authors have attributed the visible light activity as being due to localised N-2p midgap energy states in the band structure of TiO2 upon substitution of O2− by N3− (substitutional Ns) species in the TiO2 lattice [7]. Some authors have suggested that the visible light activity is only indirectly related to the incorporation of substitutional nitrogen Ns in the structure, but rather due to an optimum number of oxygen vacancies (VoS) inherently formed in the doping process [4,7]

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