V-doped TiO2 Research Articles

First-principles study on magnetism and electronic structure of V-doped rutile TiO 2

We have investigated the magnetism and the electronic structure of V-doped rutile TiO 2 using the first-principles full potential linearized augmented plane-wave (FP-LAPW) method. Total energy calculations reveal that V-doped rutile TiO 2 has a stable ferromagnetic ground state. Meanwhile, the electronic structure analysis indicates that V-doped rutile TiO 2 is a half-metal within the local density approximation (LDA) while a semiconductor within the LDA + U (Hubbard coefficient). The calculated magnetic moment in V-doped rutile TiO 2 mainly arises from the V atom with a little contribution from the nearest-neighboring O atoms due to the hybridization between the V 3d states and the nearest-neighboring O 2p states.

Sintering Rates for Pristine and Doped Titanium Dioxide Determined Using a Tandem Differential Mobility Analyzer System

Sintering rates of pristine and V-doped TiO 2 were obtained using a tandem DMA system. A range of experiments were conducted to first map out the variation of mobility size of a monodisperse (by mobility) agglomerate with time at three fixed temperatures. Using relationships of the surface area to the mobility size, the sintering equation was solved to determine the activation energy and pre-exponential factor. The value of the activation energy was 236 (± 46) kJ/mol for pristine TiO 2 and 363 (± 1) kJ/mol for V-doped TiO 2 . The corresponding pre-exponential factors were 7.22 × 10 19 and 2.22 × 10 12 s/m 4 K, respectively. These values were then used to predict changes in mobility diameter at different temperatures, and good agreement was obtained with measurements. Possible reasons for faster sintering rates of V-TiO 2 relative to pristine TiO 2 were conjectured.

Electronic structures of V-doped anatase TiO 2

First-principles calculations using the full-potential linearized augmented plane-wave method have been performed to investigate the electronic structure of V-doped TiO 2 in the anatase modification. In calculations with local density approximation (LDA), V 3d states are located at the bottom of the conduction band of the TiO 2 host. The V-doped TiO 2 was shown to be a half-metal. However, in calculations with LDA+ U (Hubbard coefficient) approach that incorporates strongly correlated interactions of both Ti and V 3d electrons, the band gap of TiO 2 host was obtained as 3.18 eV, very close to the experimental result of 3.2 eV. Additionally, spin-polarized V 3d states were obtained which are gap states located in the band gap of TiO 2 host. The V-doped TiO 2 was indicated to be an insulator. An analysis of the V 3d orbital splitting in a D 2d local symmetry of the TiO 2 host indicates that the LDA+ U approach presents a more accurate description on the electronic structure of V-doped TiO 2 than the standard LDA approach. In addition, the energy of V-doped TiO 2 in the ferromagnetic phase was calculated with LDA+ U to be 0.034 eV lower than that in the anti-ferromagnetic phase. This energy difference corresponds to 400 K, close to the Curie temperature, ∼405 K in V-doped TiO 2.

Magnetic structure of V:TiO2 and Cr:TiO2 thin films from magnetic force microscopy measurements

Ferromagnetic V-doped TiO2 and Cr-doped TiO2 films were fabricated by the pulsed laser deposition technique on LaAlO3 substrates. All V∕Cr:TiO2 films are single phased anatase, well epitaxial, c-axis oriented, and strongly ferromagnetic at room temperature. Besides giving evidences for a great flatness and magnetic homogeneities of those films, magnetic force microscopy measurements implied that the V∕Cr-doped TiO2 films seem to have a diluted magnetic structure with the ferrmagnetism originated from the doped matrix rather than any type of magnetic clusters. The size of the ferromagnetic domains was assumed to be 5–10 μm.

Room temperature ferromagnetism in anatase Ti 0.95V 0.05O 2 thin films

V-doped TiO 2 thin films were grown by pulsed laser deposition technique on LaAlO 3 substrates. In the chosen range of the growth conditions, all V-doped TiO 2 films have an anatase structure and exhibit ferromagnetic behaviors at room temperature. V-doped TiO 2 films have a much larger magnetic moment than Co/Fe/Ni-doped TiO 2 films. This study has shown that the substitution of V for Ti in TiO 2 could result in a potential diluted magnetic semiconductor. Magnetic force microscopy measurements implied that the room temperature ferromagnetism does not come from V clusters but from V-doped TiO 2 matrix.

Ferromagnetism at room temperature with a large magnetic moment in anatase V-doped TiO2 thin films

V-doped TiO2 thin films were grown by laser ablation on LaAlO3 substrates. In the chosen range of the growth conditions, all V:TiO2 films have an anatase structure and exhibit semiconducting and ferromagnetic behaviors at room temperature. V:TiO2 films have a giant magnetic moment and they seem to be far better ferromagnetic than Co/Fe/Ni-doped TiO2 films. This study has proved that a few percent of V substituting for Ti in TiO2 can result in a potential diluted magnetic semiconductor.

A visible-light response vanadium-doped titania nanocatalyst by sol–gel method

A series of vanadium-doped TiO 2 catalysts were synthesized by two modified sol–gel methods. V-doped TiO 2 was found to be mainly preserved its anatase phase after calcination at 400 °C. The TEM micrographs showed the sizes of primary particles were in the range of 6–20 nm. The increase of vanadium doping promoted the particle growth, and enhanced “red-shift” in the UV-Vis absorption spectra. The XPS (X-ray photoelectron spectroscopy) could not detect vanadium indicating negligible vanadium on the surface of catalysts, furthermore, there were also no peak of vanadium oxide in the XRD patterns. XAS (X-ray absorption spectroscopy) analysis indicating V 4+ instead of V 5+ implied that vanadium either substituted Ti 4+ site or embedded in the vacancy of TiO 2 structure. Therefore, vanadium was concluded to be highly dispersed inside the TiO 2 structure. The photocatalytic activity was evaluated by the degradation of crystal violet (CV) and methylene blue (MB) under visible light irradiation. The degradation rate of CV and MB on V-doped TiO 2 were higher than those of pure TiO 2. As the results, V-doped TiO 2 possessed better absorption ability of visible light.

Fundamental Photoelectrocatalytic and Electrophoretic Mobility Studies of TIO2 and V-Doped TIO2 Thin-Film Electrode Materials

Titanium dioxide and vanadium-doped titanium dioxide prepared by the sol−gel method showed a quantum-size effect in the form of a 0.19 eV blue shift in the band gap energy compared to bulk TiO2. The presence of vanadium as a dopant did not affect the band gap energy of TiO2. However, concentrations of this dopant up to 1% atom ratio increased the pH of the isoelectric point (IEP) above the value of 5.8 found for pure TiO2. When the vanadium concentration surpassed 1%, the IEP began to decrease, dropping below the IEP of pure TiO2 at dopant concentrations greater than ca. 2%. Thin-film electrodes fabricated from TiO2 and V-doped TiO2 coated on titanium foil showed different performances in degrading formic acid and oxalic acid depending on the V content in the catalyst. Using the 5% V-doped TiO2 electrode, 82% of TOC was removed from an aqueous solution containing oxalic acid (25 mg L-1 in C) against 74% removed by the pure TiO2 thin-film electrode in 1 h. In degrading formic acid, 79% of TOC was removed using the 1% V-doped photoelectrode against 77% removed using the plain TiO2 in 1 h. Additionally, the influence of the bias potential on photoelectrocatalytic oxidation using these thin-film electrodes is discussed as well as the effect of metal doping on the flat band potential of the electrodes.

Structural study of titanium doped vanadia and vanadium doped titania catalysts

The morphologies and structures of nanostructurally assembled V2O5 doped with Ti as well as of the inverse system, V-doped TiO2, have been studied using transmission electron microscopy and Raman spectroscopy. The bulk structure of the Ti-doped vanadia particles was found to be crystallized in a rod-like shape and to have the phase composition of V2O5 with titanium atoms nonuniformly distributed over the surface. A coherent interface between supported V2O5 and TiO2 particles was found to be the main structural peculiarity of the inverse system, V-doped TiO2 (anatase). The vanadium atoms are partially exchanged for titanium atoms at the interface, which leads to a change in the bond lengths of V=O and V-O-Ti in comparison with those observed in the monolayer supported vanadia catalysts. Both materials showed good catalytic behavior in the reaction of selective reduction of NO by NH3.

Application of V-doped TiO2 as a sensor for detection of SO2

Changes in the resistivity of polycrystalline V-doped TiO2 upon exposure to SO2 in air allow application of the material as a sensor for detection of this important pollutant. Optimal sensitivity to SO2 is achieved at low doping levels, where there is also good selectivity relative to CO and CH4. Concentration of SO2 as low as 10 ppm produce a 10% change in the resistance of 0.5% V-doped TiO2, allowing facile detection of the gas at this level. Analysis of the kinetics of the resistance changes for 1% V-doped TiO2 as a function of temperature shows that the rate of initial response is only weakly activated.

Visible Light Driven V-Doped TiO2 Photocatalyst and Its Photooxidation of Ethanol

A vanadium-doped, supported TiO2 photocatalyst is presented which is quite active using visible (396−450 nm) light. The oxidation of ethanol over this catalyst was studied using 13C solid-state NMR methods that demonstrated that this catalyst photooxidizes ethanol to produce mostly carbon dioxide with small amounts of acetaldehyde, formic acid, and carbon monoxide under visible irradiation. Under UV irradiation, the catalyst has comparable activity and product distribution as a similarly prepared TiO2 thin-film monolayer catalyst.

MOCVD growth and structure of Nb- and V-doped TiO 2 films on sapphire

Nb- and V-doped TiO 2 thin films at a doping level up to 20 and 40 at%, respectively, have been grown on sapphire (0 0 0 1), (1 1 2 ̄ 0) , and (0 1 1 ̄ 2) by metalorganic chemical vapor deposition. The Nb-doped TiO 2 films are epitaxial rutile films, but the V-doped TiO 2 films exhibit phase separation. The epitaxial orientation relationships for the Nb-doped TiO 2 films were determined by X-ray diffraction ( φ scans). Rutherford backscattering and X-ray θ rocking curves reveal that the atomic alignment in the growth direction is much better for the Nb-doped TiO 2 grown on sapphire (0 0 0 1) than on sapphire (1 1 2 ̄ 0) and (0 1 1 ̄ 2) . On the other hand, the in-plane alignment for the latter films is better than that for the former. Rutherford backscattering also shows that Nb atoms substitutionally incorporate at cation sites in the rutile lattice. X-ray photoelectron spectroscopy reveals that the oxidation state of both Ti and Nb is 4+. In contrast, XPS shows Ti 4+ and V 5+ for the V-doped TiO 2 films. X-ray diffraction and atomic force microscopy indicate that the V-doped TiO 2 films are comprised of epitaxial TiO 2 rutile islands in a V 2O 5 matrix.

Supercell Model of V-Doped TiO2: Unrestricted Hartree-Fock Calculations

The host crystal space group and the point defect site symmetry group being given the procedure of generation of the imperfect space group in the supercell or periodic defect model is considered. The approach developed is applied to the symmetry analysis of the possible space groups of metal-doped rutile and anatase structures in the supercell model. The results of unrestricted Hartree-Fock calculations of the V-doped rutile electronic structure (an impurity atom charge, valence states, an impurity level position in the energy gap) are given. Supercells of 36, 60, 84 atoms chosen on the base of the symmetry analysis are calculated. The convergence of the results with the supercell increasing is demonstrated.

Supercell Model of V-Doped TiO2: Unrestricted Hartree-Fock Calculations

The host crystal space group and the point defect site symmetry group being given the procedure of generation of the imperfect space group in the supercell or periodic defect model is considered. The approach developed is applied to the symmetry analysis of the possible space groups of metal-doped rutile and anatase structures in the supercell model. The results of unrestricted Hartree-Fock calculations of the V-doped rutile electronic structure (an impurity atom charge, valence states, an impurity level position in the energy gap) are given. Supercells of 36, 60, 84 atoms chosen on the base of the symmetry analysis are calculated. The convergence of the results with the supercell increasing is demonstrated.

Characterisation of bandgap states in V-doped TiO2 by high-resolution X-ray photoemission spectroscopy

Core- and valence-level photoemission spectra of V-doped TiO2 have been measured with monochromatic Al-Kα X-ray excitation. UHV annealing of as-presented samples leads to the appearance of a photoemission peak above the O 2p valence band within the bulk bandgap of TiO2. Simultaneous changes in V core-level spectra allow this peak to be identified as arising from electrons trapped on vanadium ions to generate localised VIV centres.

Nature of donor states in V-doped SnO2

The influence of vanadium doping on the electronic structure of SnO2 has been studied by a range of techniques including X-ray and ultraviolet photoemission spectroscopy, as well as IR reflectance and magnetic susceptibility measurements. In contrast to Sb, V does not act as a shallow n-type dopant in SnO2 and V substitution appears to be largely compensated by cation vacancies or oxygen interstitials. However, a state is apparent in ultraviolet photoemission toward the bottom of the bulk bandgap of SnO2. This corresponds to electrons trapped on VIV ions close to the surface of the doped material. The binding energy of the VIV state in doped SnO2 is compared with that in V-doped TiO2 and the differences between the two materials are shown to be consistent with a simple dielectric model.