Chemical State of Nitrogen and Visible Surface and Schottky Barrier Driven Photoactivities of N-Doped TiO₂ Thin Films

Abstract

N-doped TiO 2 thin films have been prepared by plasma enhanced chemical vapor deposition and by physical vapor deposition by adding nitrogen or ammonia to the gas phase. Different sets of N-doped TiO 2 thin films have been obtained by changing the preparation conditions during the deposition. The samples have been characterized by X-ray diffraction, Raman, UV-vis spectroscopy, and X-ray photoemission spectroscopy (XPS). By changing the preparation conditions, different structures, microstructures, and degrees and types of doping have been obtained and some relationships have been established between these film properties and their visible light photoactivity. The N1s XP spectra of the samples are characterized by three main features, one tentatively attributed to Ti-N (i.e., nitride with a binding energy (BE) of 396.1 eV) and two others with BEs of 399.3 and 400.7 eV, tentatively attributed to nitrogen bonded simultaneously to titanium and oxygen atoms (i.e., Ti-N-O like species). By controlling the deposition conditions it is possible to prepare samples with only one of these species as majority component. It has been shown that only the samples with Ti-N-O like species show surface photoactivity being able to change their wetting angle when they are illuminated with visible light. The presence of these species and an additional complex structure formed by a mixture of anatase and rutile phases is an additional condition that is fulfilled by the thin films that also present photocatalytic activity with visible light (i.e., surface and Schottky barrier driven photoactivities). The relationships existing between the reduction state of the samples and the formation of Ti-N or Ti-N-O like species are also discussed.

Surface nanostructuring ofTiO2thin films by high energy ion irradiation

“…Previous studies in literature have attributed different N 1s peaks in N-doped TiO 2 to nitride and oxinitride species of the type: Ti-N ͑396.3 eV͒, Ti-ON ͑399.3 eV͒, and Ti-NO ͑400.7 eV͒. 8 According to these previous assignments, the spectra in Fig. 3͑b͒ support the formation of a high concentration of Ti-N species in the implanted samples.…”

“…5 For these applications, it was found that the anatase crystalline phase is especially adequate to optimize many of the previous properties, and that its performance clearly improves when the film surface is nanostructured 6 and/or when it is doped with nitrogen to shift its spectral response from the ultraviolet toward the visible light spectrum. 7,8 The development of reliable techniques for surface nanostructuring of thin films is nowadays a key issue that has been approached by different physical and chemical methods. 9 In particular, considerable efforts have been devoted during the last years to the preparation of mesostructured 10 and one-dimensional TiO 2 structures ͑na-norods, nanowires, and nanotubes͒ 11,12 which have depicted unprecedented results as hydrogen sensors, self-cleaning surfaces under ultraviolet light exposure, or dye sensitized heterojunction solar cells.…”

Synthesis of dual-phase titanate/anatase with controllable morphology doped with nitrogen from a bola-amphiphile amine surfactant as template

“…The assignment of N 1s peaks in N-doped TiO 2 by XPS remains a controversial issue. The N 1s peak found at a binding energy (BE) of around 397 eV has been often assigned as substitutional N-doping, while N 1s peaks at BE4398 eV have been assigned to interstitial N-doping [30][31][32]. The binding energy of N 1s is highly dependent on the methods and conditions of preparation.…”

Metal oxide thin films and nanostructures for self-cleaning applications: current status and future prospects

“…Doping of metal ions such as Cr, V, Fe, Pb, Cu has been explored [39][40][41], but it should be noted that improvement has not been observed in all cases. Non-metal doped TiO 2 (C-N-F doped TiO 2 ) thin films are also active under visible-light irradiation and can therefore act as self-cleaning surfaces [42][43][44]. It is generally observed that non-metal doping is more promising than metal doping to produce visible-light photocatalytic activity.…”