Local Structure of V Dopants in TiO₂ Nanoparticles: X-ray Absorption Spectroscopy, Including Ab-Initio and Full Potential Simulations

Abstract

Doping with transition metals is an effective method to enhance visible light absorption in TiO2 nanoparticles and to improve the efficiency of many photocatalytic processes under solar radiation. A determination of the incorporation site of the dopant and an understanding of the local bonding arrangement and electronic structure is a necessary step for knowledge -based materials design. In this paper, we report an in-depth X-ray Absorption Spectroscopy study of V dopants in TiO2 nanoparticles deposited by gas phase condensation with a local structure similar to anatase, rutile or intermediate. The combination of K and L edge spectra in the pre-edge, edge, and extended energy regions with full potential ab-initio spectral simulations shows that V ions occupy substitutional cationic sites in the TiO2 structure, irrespective of whether it is similar to rutile, anatase or mixed.

Doping anatase TiO2 with group V-b and VI-b transition metal atoms: a hybrid functional first-principles study

“…stitutional V acts as a deep donor with the ε(+/0) transition level at 1.59 eV below the HSE CBM (or 1.37 eV below the experimental CBM). This result is in line with x-ray absorption spectroscopy results 55 and EPR measurements 56 which show that vanadium is in the charge state "V 4+ " (i.e. q = 0 in our notation), which confirms its deep character as an electron donor.…”

Element-specific channels for the photoexcitation of V-doped TiO2 nanoparticles

“…HERFD-XAS can be used to study charge-transfer processes, as shown by the recent study by some of us of hot electron transfer from Au to TiO 2 NPs induced by plasmon excitation in the metal [9]. As for the specific system investigated, we have recently demonstrated that V dopants in TiO 2 NPs are found in both 4+ and 5+ oxidation states in bulk and surface sites, respectively, and occupy substitutional sites irrespective of whether the matrix has an atomic structure similar to rutile or anatase [10]. Now, we use V Kαand Ti Kβ-detected HERFD-XANES under differential illumination to demonstrate that visible light absorption predominantly leads to electron transfer from V to long-lived Ti-related traps.…”

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

“…%. Further information about the deposition procedure can be found in previous publications [43,45].…”

“…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].…”