XANES study of vanadium and nitrogen dopants in photocatalytic TiO₂ thin films

Abstract

We report an X-ray absorption near edge structure (XANES) study of vanadium (V) and nitrogen (N) dopants in anatase TiO thin films deposited by radio-frequency magnetron sputtering. Measurements at the Ti K and V K edges were combined with soft X-ray experiments at the Ti L, O K and N K edges. Full potential ab initio spectral simulations of the V, O and N K-edges were carried out for different possible configurations of substitutional and interstitial dopant-related point defects in the anatase structure. The comparison between experiments and simulations demonstrates that V occupies substitutional cationic sites (replacing Ti) irrespective of the film structure and dopant concentration (up to 4.5 at%). On the other hand, N is found both in substitutional anionic sites (replacing O) and as N dimers within TiO interstices. The dopants' local structures are discussed with reference to the enhanced optical absorption and photocatalytic activity achieved by (co)doping.

Charge carrier dynamics and visible light photocatalysis in vanadium-doped TiO2 nanoparticles

“…A previous electrochemical study showed that -NO2 reduction takes place at about 0.5 eV below the CBM [52]. Since sub-bandgap visible light cannot promote electronic transitions from the valence to the conduction band, the initial electron state must be an intra-gap state associated with V, which is incorporated as a substitutional V 4+ cation in the lattice of both rutile and anatase [43,44]. There is a significant spread in the calculated energy level of the electronic state localized on V 4+ , which was reported between 0.7 eV [16,20] and 1.36 eV [21] below the CBM.…”

“…Studies of the local structure of V dopants in TiO2 provide the essential prerequisite for an understanding of the optical properties and of the charge carrier dynamics. We have recently published X-ray Absorption Fine Structure (XAFS) studies which demonstrate that V always occupies a substitutional site in V-TiO2 NPs [43] and in V-TiO2 thin films [44] irrespective of whether they have mainly anatase or rutile structure. By exploiting the chemical sensitivity of XAFS, we have also performed a differential illumination High Energy Resolution Fluorescence Detected (HERFD) -X-ray Absorption Near Edge Structure (XANES) experiment on V-TiO2 NPs [45].…”

Novel K2Ti8O17 Anode via Na+/Al3+ Co-Intercalation Mechanism for Rechargeable Aqueous Al-Ion Battery with Superior Rate Capability

“…According to molecular orbital theory, the O K-edge features originate from a transition of the O 1s electron to the various partially occupied and unoccupied molecular orbitals of the oxides, considering the crystal-field splitting effects [ 31 ]. As shown in the O K-edge XANES spectrum of the pristine K 2 Ti 8 O 17 sample, the major peaks located at about 532.6 eV and 534.1 eV are assigned to excitations varying from the O 1s core level to the Ti 3d-related conduction band, splitting into t 2g and e g subbands [ 32 ]. This result confirms the partial reduction of Ti 4+ during the discharge of the K 2 Ti 8 O 17 electrode.…”

X-ray absorption spectroscopy and materials science: recent advances

“…This limitation can be overcome by doping or inclusion of metallic nanoparticles. By using XAS, including ab -initio full potential simulations, we have shown that V dopants in TiO2 nanoparticles and thin films occupy substitutional sites, irrespective of whether the oxide matrix has a rutile, anatase or mixed structure; N dopants, instead, are found both in substitutional anionic sites and as N2 dimers [1,2]. These structural studies are complemented with a quantification of materials functionality, correlated to the charge carrier dynamics studied by ultra fast optical spectroscopy [3].…”