High resolution X-ray photoemission study of nitrogen doped TiO2 rutile single crystals

The electronic structure of nitrogen doped TiO2 prepared by annealing single crystal rutile (110) substrates in NH3 at elevated temperatures was investigated using high resolution X-ray photoelectron spectroscopy. NH3 treatment at 600 C introduced N into the TiO2 lattice without concomitant surface reduction of the rutile phase. This doping leads to bandgap narrowing associated with the appearance of new N 2p electronic states above the O 2p band in valence region photoemission spectra. Surface modification at the higher temperature of 700 C also produced bandgap narrowing but at the same time led to pronounced surface reduction. 2008 Elsevier B.V. All rights reserved. The two main polymorphs of TiO2 are rutile and anatase, with bandgaps of 3.06 eV and 3.20 eV, respectively. Both polymorphs can act as photocatalysts with the ability to degrade adsorbed organic pollutants under irradiation with photons whose energy is bigger than the energy of the bulk bandgap [1,2]. Anatase is the more active phase and anatase-TiO2 photocatalysts have been successfully commercialised [3]. However interest in fundamental research in this area has remained strong. Recent work has focussed on sensitizing TiO2 to radiation with wavelengths longer than 400 nm in order to increase the efficiency of the photocatalyst in sunlight. Visible light absorption in TiO2 could in principle be achieved by introducing mid band gap donor or acceptor states associated with cation dopants. However, it has been found that such localised states promote electron or hole trapping and recombination, deactivating the catalyst [4,5]. An alternative method is to narrow the band gap of TiO2 by anion doping. Interest in this idea was aroused by Asahi et al. [6] who reported that visible light photocatalysis could be induced by nitrogen doping. The empirical observation of 0009-2614/$ see front matter 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2008.02.031 * Corresponding author. E-mail address: [email protected] (R.G. Palgrave). visible light photocatalysis has been widely replicated in both anatase and rutile phases of N-doped TiO2 [7–9] but no general consensus has emerged as to the mechanism of visible region activation. Optical measurements have shown a reduction in the energy of the near UV absorption edge with N-doping [10] but this has been variously ascribed to localised N 2p states and localised Ti 3d states as well as true band gap narrowing with mixing of N 2p and O 2p states at the top of the valence band [11,12]. Detailed measurements of the electronic structure are necessary to resolve this issue. In this Letter we report X-ray photoelectron spectra of N-doped rutile TiO2 obtained with much higher resolution than in previous work. N-Doping can be achieved during sample preparation by wet chemical procedures using Ncontaining precursors or growth of TiO2 films by molecular beam epitaxy (MBE) in the presence of a nitrogen source [12–14]. Alternatively pre-existing powder or thin film samples may be nitrided by N-ion implantation or by treatment with NH3 at elevated temperatures [7,15–18]. However attempts to achieve high doping levels with these technique have in previous work led to surface reduction of the TiO2 phase and the appearance of localised Ti 3d states in valence region photoemission spectra and Ti 2p core level structure associated with reduced Ti states [19]. In the I. Takahashi et al. / Chemical Physics Letters 454 (2008) 314–317 315 present communication we explore the reaction of NH3 with TiO2(110) single crystal surfaces. Of course TiO2(110) is not an optimal photocatalyst but it is nonetheless the most widely studied oxide surface and serves as an idealised model system in which to explore N-doping. It is shown that by optimal treatment of TiO2(110) single crystal surfaces with NH3 at 600 C it is possible to prepare samples which give the XPS signature of substitutional incorporation of N without concurrent reduction of Ti. This implies that the charge difference between O and N is compensated for by the creation of oxygen vacancies. Additionally filled states with significant photoemission intensity were observed above the valence band maximum (VBM) of undoped rutile TiO2. The XPS spectra thus provide clear evidence of band gap narrowing due to N-doping. Commercial epipolished rutile single crystals (PiKem Ltd.) were heated in recrystallised alumina boats to temperatures between 600 C and 700 C under flowing NH3 gas for periods of up to three hours. X-ray photoelectron spectroscopy was carried out using a Scienta ESCA 300 spectrometer located at Daresbury Laboratory, UK. This incorporates a rotating anode Al Ka (hm = 1486.6 eV) Xray source, a seven crystal monochromator and a 300 mm mean radius spherical sector analyser with parallel electron detection system. The effective instrument resolution was set at 0.35 eV. The N 1s and O 1s core level X-ray photoelectron spectra of rutile TiO2(110) samples heated to 600 C and 700 C for 3 h under flowing NH3 are shown in Fig. 1. 520 525 530 535 540 O 1s