Cleavage of rutile SiO2 hemi-crystals: Insights from first-principles investigations

In the current study, less-explored low-index surfaces of rutile-like SiO₂ have been examined through first-principles electronic structure calculations. This work reports on the surface energies of the (110), (100), (101) and (001) facets of high-pressure SiO₂ polymorph, also referred to stishovite. Surface-induced atomic displacements with respect to bulk were computed using a periodic slab approach. A multitude of density functional approximations, both dispersion-free and dispersion-corrected, were applied to perform the necessary calculations. Anomalously, the (100) plane was discovered to be the most energetically favorable low-index facet. It also turned out that the (001) was the least stable cut. In addition, the level of agreement between our computed surface energies and atomic shifts for rutile SiO₂(110) and the results of the investigation prior to the present work was found to be acceptable.     

Preparation and Characterisation of Nano-Structured WO<Subscript>3</Subscript>-TiO<Subscript>2</Subscript> Layers for Photoelectrochromic Devices

Nano-structured WO₃-TiO₂ layers were prepared by the sol-gel route. To obtain transparent, porous and crack free layers up to 0.8 μ m with a single dipping cycle a templating strategy was used. As a template three-dimensionally network based on organically modified silane was introduced to the WO₃ and TiO₂ sols. The WO₃ layers were dip-coated onto the conductive glass substrate (TCO) and the TiO₂ layers on the top of the WO₃ layer. The morphology and the structure of the layers were determined by Scanning Electron Microscopy (SEM), High Resolution Transmission Electron Microscopy (HR-TEM), Energy Dispersive X-Ray Spectroscopy (EDXS), Auger and Infrared spectroscopy. SEM image of the WO₃-TiO₂ layer confirmed the nano-porosity of the layers and give the size of the particles of about 10 nm for TiO₂ and 30 nm for WO₃ layer. Further analysis indicated that the titanium sol penetrates the WO₃ layer. Particles in the WO₃ layer consist of a crystalline monoclinic WO₃ core surrounded by a 5–10 nm amorphous phase consisting of WO₃, TiO₂ and SiO₂. The WO₃-TiO₂ layers were used to assemble all solid state photoelectrochromic (PE) devices. Under 1 sun irradiation (1000 W/m²) the visible transmittance of the PE device changes from 62% to 1.6%. The colouring and bleaching processes last about 10 minutes.     

Influence of NH<Subscript>3</Subscript>-treating temperature on visible light photocatalytic activity of N-doped P<Subscript>25</Subscript>-TiO<Subscript>2</Subscript>

The influence of NH₃-treating temperature on the visible light photocatalytic activity of N-doped P₂₅-TiO₂ as well as the relationship between the surface composition structure of TiO₂ and its visible light photocatalytic activity were investigated. The results showed that N-doped P₂₅-TiO₂ treated at 600°C had the highest activity. The structure of P₂₅-TiO₂ was converted from anatase to rutile at 700°C. Moreover, no N-doping was detected at the surface of P₂₅-TiO₂. There was no simply linear relationship between the visible light photocatalytic activity and the concentration of doped nitrogen, and visible light absorption. The visible light photocatalytic activity of N-doped P₂₅-TiO₂ was mainly influenced by the synergistic action of the following factors: (i) the formation of the single-electron-trapped oxygen vacancies (denoted as V_o^·); (ii) the doped nitrogen on the surface of TiO₂; (iii) the anatase TiO₂ structure.     

Quantum-chemical study of adsorption of Ag<Subscript>2</Subscript>, Ag<Subscript>4</Subscript>, and Ag<Subscript>8</Subscript> on different parts of the TiO<Subscript>2</Subscript> surface

Quantum-chemical study of the adsorption of two-, four- and eight-atomic silver clusters on stoichiometric and partially reduced rutile (110) surface, and of silver tetramer on the surface of anatase (101) was carried out in the framework of periodic DFT model. The most energetically favorable positions of clusters on the surface of TiO₂ and the mechanism of binding the clusters with the substrate were revealed. According to the calculations, the adsorption of silver clusters on the surface of stoichiometric rutile (110) is more preferable than on the partially reduced one. The mechanism of binding the clusters with the surface of anatase and rutile is shown to be similar.     

Modeling the active centers of V<Subscript>2</Subscript>O<Subscript>5</Subscript>/SiO<Subscript>2</Subscript> and V<Subscript>2</Subscript>O<Subscript>5</Subscript>/TiO<Subscript>2</Subscript> supported catalysts. DFT theoretical analysis of optical properties

Within the framework of the density functional theory (DFT), the electronic structure of monooxodioxovanadium functional groups in tetrahedral coordination, which model the active centers (ACs) of fine supported catalysts V₂O₅/SiO₂ and V₂O₅/TiO₂, has been analyzed. The optimal structures of three ACs as possible models of monomeric and polymeric oxovanadium forms on the carriers with low vanadium content were determined. The modified DFT method involving the time dependence of Kohn-Sham equation (TDDFT) was used for the adopted AC models to calculate the energies of the excited states, and optical spectra of the absorption in 25000–60000 cm^?1 region were reconstructed on their base. The spectrum in this region is due to O → V charge transfer. The features of electronic spectra with the charge transfer for V₂O₅/SiO₂ and V₂O₅/TiO₂ catalysts and the vibrational spectra of three AC models corresponding to the monomeric and dimeric oxovanadium forms of the supported catalysts V₂O₅/SiO₂ and V₂O₅/TiO₂ were defined. The detailed interpretation of normal vibration frequencies is given. The frequencies typical of the monomeric and dimeric oxovanadium forms on the carrier surface were identified.     

Dielectric properties and I-V characteristics of (Pb<Subscript>0.4</Subscript>Sr<Subscript>0.6</Subscript>)TiO<Subscript>3</Subscript> thin films improved by TiO<Subscript>2</Subscript> buffer layers

A TiO₂ thin buffer layer was introduced between the (Pb_0.4Sr_0.6)TiO₃ (PST) film and the Pt/Ti/SiO₂/Si substrate in an attempt to improve their electrical properties. Both TiO₂ and PST layers were prepared by a chemical solution deposition method. It was found that the TiO₂ buffer layer increased the (100)/(001) preferred orientation of PST and decreased the surface roughness of the films, leading to an enhancement in electrical properties including an increase in dielectric constant and in its tunability by DC voltage, as well as a decrease in dielectric loss and leakage current density. At an optimized thickness of the TiO₂ buffer layer deposited using 0.02 mol/l TiO₂ sol, the 330-nm-thick PST films had a dielectric constant, loss and tunability of 1126, 0.044 and 60.7% at 10 kHz, respectively, while the leakage current density was 1.95 × 10⁻⁶ A/cm² at 100 kV/cm.     

Degradation of organic compounds on TiO<Subscript>2</Subscript> photocatalysts prepared by a hydrothermal method in the presence of NH<Subscript>4</Subscript>F

TiO₂ photocatalysts were synthesized by a hydrothermal method from tetraisopropyl orthotitanate (TPOT) in the presence of NH₄F with different NH₄F/Ti molar ratios (0, 0.25, and 1). The formation of a well-crystallized anatase phase of TiO₂ and the suppression of phase transition to rutile were observed, even at high calcination temperature, owing to the effects of NH₄F. The TiO₂ synthesized hydrothermally with NH₄F exhibited absorption with a shift to the longer wavelengths of the visible-light region. The hydrothermally synthesized TiO₂ with a moderate amount of NH₄F exhibited high photocatalytic activity for the degradation of alcohol diluted in water under both UV-light and visible-light irradiations.     

Fabrication and characterization of TiO<Subscript>2</Subscript>–SiO<Subscript>2</Subscript> composite nanoparticles and polyurethane/(TiO<Subscript>2</Subscript>–SiO<Subscript>2</Subscript>) nanocomposite films

TiO₂–SiO₂ composite nanoparticles were prepared by a sol–gel process. To obtain the assembly of TiO₂–SiO₂ composite nanoparticles, different molar ratios of Ti/Si were investigated. Polyurethane (PU)/(TiO₂–SiO₂) hybrid films were synthesized using the “grafting from” technique by incorporation of modified TiO₂–SiO₂ composite nanoparticles building blocks into PU matrix. Firstly, 3-aminopropyltriethysilane was employed to encapsulate TiO₂–SiO₂ composite nanoparticles’ surface. Secondly, the PU shell was tethered to the TiO₂–SiO₂ core surface via surface functionalized reaction. The particle size of TiO₂–SiO₂ composite sol was performed on dynamic light scattering, and the microstructure was characterized by X-ray diffraction and Fourier transform infrared. Thermogravimetric analysis and transmission electron microscopy (TEM) employed to study the hybrid films. The average particle size of the TiO₂–SiO₂ composite particles is about 38 nm when the molar ratio of Ti/Si reaches to1:1. The TEM image indicates that TiO₂–SiO₂ composite nanoparticles are well dispersed in the PU matrix.     

Analysis of preparation of TiO<Subscript>2</Subscript> particles by diffusion flame reactor for photodegradation of phenol and toluene

TiO₂ nanoparticles were produced in the diffusion flame reactor, and the size and anatase/rutile content of TiO₂ were examined by a Particle Size Analyzer and X-ray diffraction, respectively. Increase in fuel/O₂ ratio, initial concentration of TiCl₄ or total gas flow rate causes the larger particle size and the higher rutile composition. The photocatalytic activities of TiO₂ powders were tested on the decompositions of phenol and toluene in the aqueous solution under UV irradiation. The degradation rate increases as the TiO₂ particle size decreases and as the initial concentration of phenol or toluene increases. The photodegradation rate of phenol by TiO₂ particles is higher than that of toluene at the same process conditions. The computational method was used to simulate the gas temperature, velocity and species mass fractions inside the diffusion flame reactor during synthesis of TiO₂ nanoparticles. The measured and simulated temperature results were compared on several positions above the burner and both of them show good agreements. The typical contours of TiCl₄, TiO₂ mass fractions and gas velocities in flame reactor were presented.     

Microstructure of SiO<Subscript>2</Subscript>/TiO<Subscript>2</Subscript> hybrid electrospun nanofibers and their application in dye degradation

SiO₂/TiO₂ hybrid nanofibers were prepared by electrospinning and applied for photocatalytic degradation of methylene blue (MB). The phase structure, specific surface area, and surface morphologies of the SiO₂/TiO₂ hybrid nanofibers were characterized through thermogravimetry (TG), X-ray diffraction (XRD) analysis, Brunauer–Emmett–Teller (BET) analysis, scanning electron microscopy (SEM), etc. XRD measurements indicated that doping of silica into TiO₂ nanofibers can delay the phase transition from anatase to rutile and decrease the grain size. SEM and BET characterization proved that silica doping can remarkably enhance the porosity of the SiO₂/TiO₂ hybrid nanofibers. The MB adsorption capacity and photocatalytic activity of the SiO₂/TiO₂ hybrid nanofibers were distinguished experimentally. It was found that, although increased silica doping content could enhance the MB adsorption capacity, the intrinsic photocatalytic activity gradually dropped. The SiO₂ (10 %)/TiO₂ composite nanofibers exhibited the highest MB degradation rate, being superior to SiO₂ (20 %)/TiO₂ or pure TiO₂.     

Adsorption properties of N<Subscript>2</Subscript>O on (6,0), (7,0), (8,0), and Al-doped (6,0) zigzag single-walled carbon nanotubes: a density functional study

Abstract  

The behavior of N₂O adsorbed on the external surface of H-capped (6,0), (7,0), (8,0), and Al-doped (6,0) zigzag single-walled carbon nanotubes was studied by using density functional calculations. Geometry optimizations were carried out at the B3LYP/6-31G* level of theory using the Gaussian 03 suite of programs. We present the nature of the N₂O interaction in selected sites of the nanotubes. Binding energies corresponding to adsorption of the N₂O are calculated to be in the range 4–21 kJ mol⁻¹. More efficient binding energies cannot be achieved by increasing the nanotube diameter. We also provide the effects of N₂O adsorption on the electronic properties of the nanotubes.     

水对甲醇在Rutile-TiO2(110)-(1 ×1)表面光催化解离的影响简

采用程序升温脱附方法研究了甲醇分子吸附在真空退火后的二氧化钛（110）表面的光催化过程,对比分析了单独吸附甲醇分子以及甲醇分子与水分子共吸附情况下的光催化解离过程. 结果表明,在二氧化钛（110）表面吸附的甲醇分子对共吸附水分子的光催化解离过程并没有直接的帮助作用. 共吸附状态下的水分子也同样没有影响到甲醇的光致解离过程,但是水分子的存在抑制了甲醇光解产物甲醛的光致脱附过程,同时促进了甲酸甲酯的形成.     

Computational investigation of the adsorption and reactions of SiHx (x = 0–4) on TiO2 anatase (101) and rutile (110) surfaces

The adsorption and reactions of the SiH_x (x = 0–4) on Titanium dioxide (TiO₂) anatase (101) and rutile (110) surfaces have been studied by using periodic density functional theory in conjunction with the projected augmented wave approach. It is found that SiH_x (x = 0–4) can form the monodentate, bidentate, or tridentate adsorbates, depending on the value of x. H coadsorption is found to reduce the stability of SiH_x adsorption. Hydrogen migration on the TiO₂ surfaces is also discussed for elucidation of the SiH_x decomposition mechanism. Comparing adsorption energies, energy barriers, and potential energy profiles on the two TiO₂ surfaces, the SiH_x decomposition can occur more readily on the rutile (110) surface than on the anatase (101) surface. The results may be used for kinetic simulation of Si thin‐film deposition and quantum dot preparation on titania by chemical vapor deposition (CVD), plasma enhanced CVD, or catalytically enhanced CVD. © 2013 Wiley Periodicals, Inc.     

Temperatures and kinetics of anatase to rutile transformation in doped TiO<Subscript>2</Subscript> heated in microwave field

The influence of aliovalent ions such as Mn, Cr, Fe, Mo, and V on the temperature and kinetics of anatase to rutile phase transformation in TiO₂ heated in microwave field was studied in this work. The results indicated that heat treatment method and dopants considerably affected the anatase-to-rutile phase transition temperature and kinetics of transformation. The activation energy for anatase to rutile transformation of TiO₂ derived from the isothermal data was found to be 328.4 kJ mol^–1, which was considerably reduced by the addition of dopants in TiO₂ matrix. The activation energy for Mo, Mn and V doped samples was 252.0, 101.3 and 96.4 kJ mol^–1, respectively.     

A first principles study of Pd deposition on the TiO2(1 1 0) surface

 The adsorption of isolated Pd atoms on the (1 1 0) surface of rutile TiO₂ was investigated through ab initio embedded-cluster calculations performed at the Hartree–Fock, second-order M?ller–Plesset and Becke's three parameter hybrid method with the Lee–Yang–Parr correlation functional levels. The role played by the magnitude of the surrounding charges used in the embedding procedure was carefully analyzed. The most stable site for adsorption consisted of a fourfold hollow site in which the Pd atom was coordinated to a fivefold Ti atom, two basal oxygens, and a protruding oxygen atom. However, the adsorption energies computed after basis set superposition error corrections seemed to favor a bridge site in which the Pd atom binds two protruding oxygen atoms. A periodic slab calculation using gradient-corrected functionals and plane-wave basis sets confirmed that for full coverage, the hollow site was more stable, although Pd displacement along the fivefold Ti channels was almost free. These results agree with the experimental data obtained from scanning tunneling microscopy. Finally, the adsorption energy computed from the periodic calculations was found to be 1.88 eV. Received: 14 September 1999 / Accepted: 3 February 2000 / Published online: 19 April 2000     

Preparation of nanocrystalline TiO<Subscript>2</Subscript> powders for photocatalytic oxidation of phenol

Nanocrystalline TiO₂ powders in the anatase, rutile, and mixed phases prepared by hydrolysis of TiCl₄ solution were of ultrafine size (<7.2 nm) with high specific surface areas in the range 167 to 388 m²/g. In the photocatalytic degradation of phenol as model reaction, the photocatalytic properties of TiO₂ nanoparticles were evaluated by use of UV–vis absorption spectroscopy and total organic carbon (TOC) content. The synthetic mixed-phase TiO₂ powder calcined at 400 °C had higher activity than pure anatase or rutile; it degraded more than 90% phenol to CO₂ (evaluated by TOC) after irradiation with near UV light for 90 min at a catalyst loading of 0.4 g/L. The TOC results indicated that rutile TiO₂ crystallites of particle size 7.2 nm resulted in much better photocatalytic performance than particles of larger size. This result suggested that some intermediates, not determined by UV–vis absorption spectroscopy, existed in the solution after the photocatalytic process over the rutile TiO₂ photocatalysts of larger crystallite size.     

Surface structure for Au on (0 0 1) SrTiO3

First-principle calculations are performed to study the crystal structure, formation energies, and electronic structures of (0 0 1) SrTiO₃ surfaces with/without Au covered. The initiative Au additive layer is crystallized in a fcc structure with (0 0 1) face on SrO-terminated surface. The bimodal growth trend of Au on TiO₂-terminated surface is qualitatively consistent with the experimental observations. The defect structure of Au occupying the oxygen (O) vacancies of TiO₂-terminated surface is energetically favorable under oxygen-poor conditions, and a feature corresponding to gap states appear and the occupied Ti 3d states disappear.     

Density functional theory (DFT) study of O<Subscript>3</Subscript> molecules adsorbed on nitrogen-doped TiO<Subscript>2</Subscript>/MoS<Subscript>2</Subscript> nanocomposites: applications to gas sensor devices

Density functional theory calculations were carried out to investigate the adsorption behaviors of O₃ molecules on the undoped and N-doped TiO₂/MoS₂ nanocomposites. With the inclusion of vdW interactions, which correctly account the long-range dispersion energy, the adsorption energies and final geometries of O₃ molecules on the nanocomposite surfaces were improved. For O₃ molecules on the considered nanocomposites, the binding sites were located on the fivefold coordinated titanium atoms of the TiO₂ anatase. The structural properties of the adsorption systems were examined in view of the bond lengths and bond angles. The variation of electronic structures was also discussed in view of the density of states, molecular orbitals and distribution of spin densities. The results suggest that the adsorption of the O₃ molecule on the N-doped TiO₂/MoS₂ nanocomposite is more favorable in energy than that on the pristine one, indicating that the N-doped nanocomposite has higher sensing capability than the pristine one. This implies that the N-doped TiO₂/MoS₂ nanocomposite would be an ideal O₃ gas sensor. However, our calculations thus provide a theoretical basis for the potential applications of TiO₂/MoS₂ nanocomposites as efficient O₃ sensors, leading to very interesting results in the context of air quality measurement.     

The Primary Origin of Excellent Dielectric Properties of (Co,Nb) Co-Doped TiO2 Ceramics: Electron-Pinned Defect Dipoles vs. Internal Barrier Layer Capacitor Effect

(Co, Nb) co-doped rutile TiO₂ (CoNTO) nanoparticles with low dopant concentrations were prepared using a wet chemistry method. A pure rutile TiO₂ phase with a dense microstructure and homogeneous dispersion of the dopants was obtained. By co-doping rutile TiO₂ with 0.5 at.% (Co, Nb), a very high dielectric permittivity of ε′ ≈ 36,105 and a low loss tangent of tanδ ≈ 0.04 were achieved. The sample–electrode contact and resistive outer-surface layer (surface barrier layer capacitor) have a significant impact on the dielectric response in the CoNTO ceramics. The density functional theory calculation shows that the 2Co atoms are located near the oxygen vacancy, creating a triangle-shaped 2CoV_oTi complex defect. On the other hand, the substitution of TiO₂ with Nb atoms can form a diamond-shaped 2Nb2Ti complex defect. These two types of complex defects are far away from each other. Therefore, the electron-pinned defect dipoles cannot be considered the primary origins of the dielectric response in the CoNTO ceramics. Impedance spectroscopy shows that the CoNTO ceramics are electrically heterogeneous, comprised of insulating and semiconducting regions. Thus, the dielectric properties of the CoNTO ceramics are attributed to the interfacial polarization at the internal insulating layers with very high resistivity, giving rise to a low loss tangent.     

Wannier-type atomic orbitals for periodic systems

The results of an application of Wannier-type atomic orbitals for calculations of local properties of electronic structure for periodic systems (atomic charges and covalencies, bond orders and total valencies), published earlier by the authors, are summarized. The numerical results are given for bulk crystals with the perovskite structure ATiO₃, A=Sr,Ba,Pb and LaMnO₃ (the Bloch functions are calculated in LCAO basis), for bulk MgO crystal (the Bloch functions are calculated both in LCAO and PW basis) and for the two periodic slab models of surfaces (001) of MgO and (110) of rutile TiO₂.