Surface Segregation of Niobium and Tantalum in Titanium Dioxide. Overview

Segregation in crystals results in surface layer enrichment of selected lattice elements. The present work considers the phenomenon of segregation in solid solutions of TiO₂ doped with donor‐type elements, such as niobium and tantalum. The focus is on the effect of oxygen activity on segregation at elevated temperatures. It is shown that the effect of oxygen activity on the segregation‐induced enrichment may be used for engineering of oxide materials in general and TiO₂‐based materials in particular with controlled surface composition that is required to achieve the desired reactivity and photoreactivity for energy conversion in general and solar energy conversion in particular. The development of the related surface engineering procedures requires correct determination of oxygen activity during processing and the segregation‐induced concentration gradients within the surface layer.     

Tantalum Enrichment in Tantalum‐Doped Titanium Dioxide

This investigation has assessed the behavior of Ta enrichment in Ta‐doped TiO₂ under various conditions of controlled oxygen activity and temperature. The aim has been to establish the relationships between specific processing conditions and the resulting compositional variation within the surface and near‐surface region. Under the application of oxidizing conditions [p(O₂) = 101 kPa], it has been observed that Ta will strongly enrich the surface of Ta‐doped TiO₂ irrespective of the annealing temperature (over the range of 1173–1523 K). However, under reducing conditions [p(O₂) in the vicinity of 10^?10 Pa], Ta enrichment is observed at 1173 K, but Ta depletion from the surface and near‐surface is observed at 1348 and 1523 K. This is attributed to an apparent lack of stability of the surface phase, which could possibly be TiTa₂O₇. The results for the investigation contribute to the engineering of TiO₂‐based photoelectrode materials that possess improved charge separation properties.     

Photocatalytic oxidation of cyclohexane over TiO2 nanoparticles by molecular oxygen under mild conditions

The structure, physical characteristics and photocatalytic selective oxidation properties of nanometer‐size TiO₂ particles produced by a sol–gel method were studied by X‐ray diffraction (XRD), diffuse reflectance spectroscopy (DRS), X‐ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR) and photocatalytic selective oxidation measurements. Analysis of the XRD results shows that sol–gel‐produced TiO₂ nanoparticles have the anatase structure at annealing temperatures ≤973 K, that the rutile structure begins to emerge at annealing temperatures ≥973 K and the particles have the pure rutile structure at 1023 K. DRS indicates that the obtained TiO₂ nanoparticles exhibit a blue shift with decreasing crystallite size. Analysis of the XPS results shows that the TiO₂ nanoparticles have a lot of oxygen vacancies. The EPR spectrum of TiO₂ at 77 K is composed of a strong isotropic EPR Surface‐Ti³⁺ signal(I) at g = 1.926 and a weak broad Bulk‐Ti³⁺ signal (II) at g = 1.987. Quantitative EPR indicates that both signals show a size and temperature dependence. Photocatalytic oxidation of cyclohexane into cyclohexanol with high selectivity and activity has been obtained by activation of molecular oxygen over sol–gel‐produced TiO₂ nanoparticles under mild conditions in dry solvent, which reveals that the quantum size effect and surface state effect of nanoparticles are key points for governing the selective photocatalytic reaction. The photocatalytic oxidation mechanism under dry solvent is different from that in aqueous solutions. Copyright © 2003 Society of Chemical Industry     

Diffusion Kinetics of Indium in TiO2 (Rutile)

This work determines the self‐diffusion coefficients of indium in TiO₂ single crystal (rutile). Diffusion concentration profiles were imposed by deposition of a thin surface layer of InCl₃ on the TiO₂ single crystal and subsequent annealing in the temperature range 1073–1573 K. The diffusion‐induced concentration profiles of indium as a function of depth were determined using secondary ion mass spectrometry (SIMS). These diffusion profiles were used to calculate the self‐diffusion coefficients of indium in the polycrystalline In₂TiO₅ surface layer and the TiO₂ single crystal. The temperature dependence of the respective diffusion coefficients, in the range 1073–1573 K, can be expressed by the following formulas: and The obtained activation energy for bulk diffusion of indium in rutile (316 kJ/mol) is similar to that of zirconium in rutile (325 kJ/mol). The determined diffusion data can be used in selection of optimal processing conditions for TiO₂–In₂O₃ solid solutions.     

Bi2O3 nanoparticles incorporated porous TiO2 films as an effective p‐n junction with enhanced photocatalytic activity

Porous TiO₂ films decorated with Bi₂O₃ nanoparticles are fabricated via alkali‐hydrothermal of titanium (Ti) plate by varying the reaction time. The amorphous TiO₂ is transformed into anatase after annealing the films at 500°C in air. The p‐type Bi₂O₃ nanoparticles are successfully assembled on the surface of porous n‐type TiO₂ films through the ultrasonic‐assisted successive ionic layer adsorption and reaction (SILAR) technique to form Bi₂O₃/TiO₂ nanostructure by the two cycles. The obtained Bi₂O₃/TiO₂ films are consisted of a well‐ordered and uniform porous structure with an average pore diameter of about 100‐200 nm containing homogeneously dispersed Bi₂O₃ nanoparticles of ~5 nm diameter. Moreover, the resultant composites present excellent photocatalytic performance toward methyl blue (MB) degradation under UV and visible light irradiation, which could be mainly ascribed to the enhanced light adsorption capacity of unique composite structure and the formation of p‐n heterojunctions in the porous Bi₂O₃/TiO₂ films. This research is helpful to design and construct the highly efficient heterogeneous semiconductor photocatalysts.     

Charge distribution in nano‐scale grains of magnesium aluminate spinel

Charge distribution in magnesium aluminate spinel (MAS) results in the formation of a space‐charge region that plays a critical role in assigning functional properties. Significant theoretical advances explaining this phenomenon have been accomplished, even though quantitative experimental support from nano‐scale granular MAS is only indirect. In this work, the electrostatic potential distribution in nano‐scale grains of nonstoichiometric MAS (MgO·0.95Al₂O₃ and MgO·1.07Al₂O₃) was measured by off‐axis electron holography (OAEH) and compared to the distribution of cations and defects in this material as measured by electron energy‐loss spectroscopy (EELS). In this manner, we studied the roles of composition, grain size, and applied electric field (EF) on the formation of a space‐charge region. We quantitatively demonstrated that regardless of grain size, the vicinity of MgO·0.95Al₂O₃ grain boundaries presented an excess of Mg⁺² cations, whereas the vicinity of MgO·1.07Al₂O₃ grain boundaries included an excess of Al⁺³ cations. The degree of structural disorder (ie, the inversion parameter, i) indicated that as‐synthesized MAS were significantly disordered (i between 0.37 and 0.41), with values decreasing toward equilibrium ordering values following annealing (i between 0.27 and 0.31). The application of an external ~150 V/cm EF during annealing further enhanced lattice ordering (i between 0.16 and 0.19). Such variations in the distribution of cations and defects should determine the space‐charged potential (SCP). However, using these measurements to calculate the SCP was not possible due to the wide range of values reported for formation energies of defects (0.82‐8.78 eV). Consequently, we correlated local ionic ordering with electrostatic potential in nonstoichiometric MAS. The magnitudes of the SCP in both MgO·0.95Al₂O₃ and MgO·1.07Al₂O₃ decreased following annealing from ?3.4 ± 0.3 V and 2.0 ± 0.2 V to ?2.0 ± 0.2 V and 1.6 ± 0.1 V, respectively.     

Facile fabrication of TiO2-SiO2-C composite with anatase/rutile heterostructure via sol-gel process and its enhanced photocatalytic activity in the presence of H2O2

Constructing anatase/rutile heterostructure in TiO₂ based materials is a quite powerful approach to enhance their photocatalytic activities. Herein, by simply annealing the sol-gel derived TiO₂-SiO₂ composite in N₂ atmosphere at 850 °C, TiO₂-SiO₂-C composite (CTS-850) with anatase/rutile heterostructure has been successfully prepared, while the counterpart prepared in air contains only anatase phase. It was proven that the residual organic groups in the sol-gel process were converted into carbon species upon N₂ annealing, during which TiO₂ in the composite was partially reduced, not only leaving lots of oxygen vacancies on its surface but also promoting the phase transformation. By turning the annealing temperature and atmosphere, a series of control products were further synthesized. Among these samples, the CTS-850 showed the best photocatalytic performance toward Rhodamine B degradation in the presence of H₂O₂, which was mainly due to its lowest band gap and the enhanced sensitization of H₂O₂ by oxygen vacancies. Moreover, the photocatalytic activity of CTS-850 remained unchanged after five cycles and a proper mechanism was also proposed.     

Effect of Heat-Treatment on the Mechanism and Kinetics of the Hydrogen Evolution Reaction on Ni—P + TiO2 + Ti Electrodes

Composite Ni—P + TiO₂ + Ti layers were prepared by codeposition of Ni—P alloy with TiO₂ and Ti powders from a solution containing suspension of TiO₂ and Ti particles. The electrodeposition was carried out under galvanostatic conditions at room temperature. The layers exhibited an amorphous Ni—P matrix in which crystalline TiO₂ and Ti were embedded. On the deposit surface, the nonstoichiometric Ti oxide, Ti₁₀O₁₉, and intermetallic compounds, NiTi, formed during the electrodeposition, were also present. The heat treatment of these layers in argon leads to the crystallization of Ni—P matrix and formation of nonstoichiometric Ti oxides, detected by XRD. Electrolytic activity towards the hydrogen evolution reaction (HER) was studied on these electrode materials before and after heat treatment. The mechanism of the HER was also studied, and the kinetic parameters were determined using steady-state polarization and electrochemical impedance spectroscopy (EIS). An increase in activity occurring after heating of Ni—P + TiO₂ + Ti layers is related to TiO₂ reduction and formation of nonstoichiometric Ti oxides: Ti₁₀O₁₉(400 °C), Ti₇O₁₃(500 °C) and Ti₄O₇(800 °C). It is postulated that the increase in electrochemical activity is related to the properties of these oxides and a facility for H reduction/adsorption on their surface, as well as to the presence of NiTi intermetallics as compared with the Ni—P + TiO₂ + Ti electrode.     

Tailoring the room temperature ferromagnetism in TiO2 nanoparticles by modulating the concentration of surface oxygen vacancy via La incorporating

La-incorporated TiO₂ nanoparticles (Ti_1-xLa_xO₂) were synthesized by a sol-gel method followed by vacuum annealing at 500 °C for 4 h, and were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Raman scattering spectroscopy, and vibrating sample magnetometer (VSM), respectively. The results indicated that La ions were incorporated with TiO₂ without distorting the tetragonal anatase structure, and a small amount of La ions were doped into TiO₂ lattice by substituting the surface Ti sites, whereas most of the La atoms evenly distribute into TiO₂ matrix in the form of La₂O₃. The incorporation of La ions with TiO₂ matrix modulates the concentration of surface oxygen vacancy in Ti_1-xLa_xO₂ nanoparticles. The room temperature ferromagnetism (RTFM) in Ti_1-xLa_xO₂ nanoparticles varies with the concentration of surface oxygen vacancy due to ferromagnetic coupling interactions between singly ionized oxygen vacancy, thus RTFM can be tailored by modulating the concentration of surface oxygen vacancy via La ions incorporating.     

Production of argon free oxygen by adsorptive air separation on Ag‐ETS‐10

The purification of different components of air, such as oxygen, nitrogen, and argon, is an important industrial process. Pressure swing adsorption (PSA) is surpassing the traditional cryogenic distillation for many air separation applications, because of its lower energy consumption. Unfortunately, the oxygen product purity in an industrial PSA process is typically limited to 95% due to the presence of argon which always shows the same adsorption equilibrium properties as oxygen on most molecular sieves. Recent work investigating the adsorption of nitrogen, oxygen and argon on the surface of silver‐exchanged Engelhard Titanosilicate‐10 (ETS‐10), indicates that this molecular sieve is promising as an adsorbent capable of producing high‐purity oxygen. High‐purity oxygen (99.7+%) was generated using a bed of Ag‐ETS‐10 granules to separate air (78% N₂, 21% O₂, 1% Ar) at 25°C and 100 kPa, with an O₂ recovery rate greater than 30%. © 2012 American Institute of Chemical Engineers AIChE J, 59: 982–987, 2013     

The influence of Si-modified TiO2 on the activity of Ag/TiO2 in CO oxidation

Nanocrystalline TiO₂ and Si-modified TiO₂ with Si/Ti ratios 0.01, 0.05, 0.1, and 0.3 were prepared by the solvothermal method and employed as the supports for Ag/TiO₂ catalysts for CO oxidation reaction. The incorporation of Si into the TiO₂ lattice in the form of Ti–O–Si as revealed by FT-IR results could inhibit the agglomeration of TiO₂ crystallites, resulting in an increase of both surface area and metal dispersion. However, there existed an optimum content of Si/Ti at ca. 0.05–0.1 which resulted in an improved catalytic activity of Ag/TiO₂ in CO oxidation. Based on the O₂-temperature program desorption (O₂-TPD) results, the catalysts with appropriate amounts of Si/Ti exhibited higher amount of O₂ adsorption and much lower desorption temperature. It is suggested that the presence of Ti–O–Si promoted the formation of active oxygen species and increased the mobility of lattice oxygen so that the catalytic activity was enhanced. There was no improvement in CO oxidation activity of the Ag/TiO₂ catalyst when the Si/Ti was further increased to 0.3 due probably to the formation of amorphous SiO₂ instead of the Ti–O–Si bond.     

Ionic conductivity of tetragonal ZrO2 polycrystal doped with TiO2 and GeO2

The effects of the co-doping and the resultant co-segregation of 2 mol% TiO₂ and 2 mol% GeO₂ on the ionic conductivity and on the chemical bonding state in a tetragonal ZrO₂ polycrystal were investigated. The conductivity data and grain boundary microstructure showed that the doped Ti⁴⁺ and Ge⁴⁺ cations segregate along the grain boundary, and this segregation causes a reduction in the conductivity of both the grain interior and grain boundary and an increase in the activation energy of the grain boundary conductivity. Overall, the data indicate that the segregation retards the diffusion of oxygen anions. A first-principle molecular orbital calculation explains the retarded diffusion of the oxygen anion from a change in the covalent bonds around the dopant cations; an increase in the strength of the covalent bond between the oxygen and doped cation should work to suppress the diffusion of the oxygen anion.     

Defective Y2O3−x–driven anomalous photocatalytic enhancement using Y2O3−x–TiO2−x nanorod composite composition spreads

Synergistic photocatalysis is reported, using the optimal amounts of oxygen vacancies of high‐k materials and nanoarchitecture maneuvering by employing a combinatorial sputtering approach. The highlights include (i) the successful fabrication of samples using combinatorial sputtering; (ii) a systematic investigation of the coupling effect between Y₂O_3?x and TiO_2?x; (iii) elucidating charge carrier transport through current‐voltage (I‐V) and capacitance‐voltage (C‐V) characterizations; and (iv) providing an alternative application for high‐dielectric constant (high‐k) materials in photocatalysis. The simple yet effective composition spread technique rapidly determined that Sample 6 (4 at% Y₂O_3?x‐96 at% TiO_2?x, TiO_2?x‐rich on the Y₂O_3?x–TiO_2?x nanorod composite composition spread) exhibited the highest photocatalytic efficiency (i.e., approximately 3.4 times and 1.4 times higher than that of P25 and pure TiO_2?x nanorods, respectively). The predominant factor was determined to be electron migration along defective Y₂O_3?x nanorods to the sample surface. The extracted mobility was discovered to be an order of magnitude greater than that of pure TiO_2?x. The photoelectrochemical stability and reusability were also demonstrated.     

Effect of oxygen treatment on structure and electrical properties of Mn-doped Ca0.6Sr0.4TiO3 ceramics

Different oxygen treatment methods, including O₂ and N₂ annealing, were conducted on Ca_0.6Sr_0.4TiO₃ (CST) ceramics with varying Mn content (0?mol%, 0.5?mol% and 2.0?mol%). Structure characterization, including XRD and SEM, indicated the minimal effect of annealing on the microstructure. Grain boundaries were found to be sensitive to oxygen treatments, and annealing in O₂ resulted in increased grain boundary resistance, while in N₂ led to the opposite result. The insulating properties of bulk ceramics were found to be dominated by grain boundaries. Both the concentration and mobility of oxygen vacancies were confirmed to affect the energy storage properties to some extent in this work.     

Extremely Thin Al2O3 Surface‐Passivated Nanocrystalline ZnO Thin‐Film Transistors Designed for Low Process Temperature

Nanocrystalline ZnO (nc‐ZnO) thin‐film transistors (TFTs) exhibit inherent instability under bias/photo stresses, which originates from the oxygen molecules adsorbed on the surface of the crystal grains. The space charge region at nanocrystal surfaces that is induced by adsorbed oxygen molecules produces a high electrical potential barrier and significantly interrupts charge transport between the source and drain in nc‐ZnO TFTs. In this article, we developed high‐performance TFTs via the continuous deposition of an extremely thin Al₂O₃ layer on a nc‐ZnO channel. These devices were fabricated by atomic layer deposition at an extremely low process temperature of 150°C, including both the deposition and postannealing temperatures. The nc‐ZnO TFT with an extremely thin Al₂O₃ layer (1.8 nm) showed a significantly higher mobility (25 cm²/Vs) compared to devices without an Al₂O₃ layer (3.6 cm²/Vs). This dramatic difference was ascribed to the suppression of the chemisorption of oxygen molecules at the nanocrystal surface during thermal annealing (reducing the potential barrier width/height between adjacent nanocrystals). Furthermore, ultrathin Al₂O₃‐covered nc‐ZnO TFTs exhibited considerably enhanced electrical/photo stability due to the reduction in adsorption/desorption events of oxygen molecules on the nanocrystal surfaces (with no change in the depletion width after illumination) under gate bias or illumination stress.     

Intense Red Photoluminescence Emission of Sol–Gel‐Derived Nanocrystalline Mg2TiO4 Thin Films

In this work, undoped Mg₂TiO₄ thin films were fabricated on p‐type Si(111) substrates by the sol–gel method, and the red photoluminescence (PL) of the films is introduced and discussed. According to the experimental results, the red emission appears when the films have been thermally treated at higher temperatures, which have a long range and well‐organized crystalline arrangement. Furthermore, to have better realization of the red emission mechanism of Mg₂TiO₄ films, the optical band gap of Mg₂TiO₄ (E_g^Mg2TiO4) was estimated at ~3.7 eV; furthermore, 325 nm (corresponding energy, hν = 3.82 eV > E_g^Mg2TiO4) and 633 nm (corresponding energy, hν = 1.96 eV < E_g^Mg2TiO4) excited light sources were used to clarify the position of the defect levels. In addition, the influence of annealing atmospheres (O₂, air, and vacuum) on the red emission of our samples is also discussed. A significant variety of red emissions can be found between these annealing conditions: the red emission can be effectively enhanced by O₂ annealing, but weakened by vacuum annealing. Results reveal that the red emission of Mg₂TiO₄ thin films may be highly dependent on the completeness of the O–X–O (X = Mg, Ti) bonds.     

Synthesis of Ti1−xSnxO2 nanosized photocatalysts in reverse microemulsions

The preparation of Ti_1−xSn_xO₂ nanocrystalline photocatalysts in reverse microemulsions is reported in this work. The obtained materials have been characterised by total reflection X-ray fluorescence (TXRF), X-ray diffraction (XRD) and Raman and UV–vis spectroscopies. Very good accordance between calculated and obtained compositions is observed. Undoped TiO₂ prepared in this way crystallises in the anatase phase. Tin-doped anatase is formed with x < 0.05, while both anatase and rutile phases crystallise when x ≥ 0.05. When both phases coexist, a preferential doping of rutile seems to occur. When x = 0.10, a multiphase mixture containing TiO₂(anatase), TiO₂(rutile) and SnO₂ was formed. No significant modification of the band gap is found in any case. The photocatalytic activity of the obtained catalysts is compared employing the trichloroethylene photocatalytic degradation as a test reaction. The beneficial effect of Sn⁴⁺ in the activity of TiO₂ appears to be related to the formation of anatase–rutile mixtures, leading to the highest specific photocatalytic activity in the sample of composition Ti_0.93Sn_0.07O₂, with anatase:rutile ratio close to 3.     

Microdomain Formation,Oxidation, and Cation Ordering in LaCa2Fe3O8+y

The compound LaCa₂Fe₃O_8+y, also known as the Grenier phase, is known to undergo an order–disorder transformation (ODT) at high temperatures and oxidation has been observed when the compound is cooled in air after the ODT. In this study, we have synthesized the Grenier compound in air using traditional solid‐state reactions and investigated the structure and composition before and after the ODT. Thermal analysis showed that the material undergoes an ODT in both oxygen and argon atmospheres with dynamic, temperature dependent, oxidation upon cooling. Results from scanning transmission electron microscopy (STEM) suggest that the Grenier phase has preferential segregation of Ca and La on the two crystallographic A sites before the ODT, but a random distribution above the ODT temperature. Furthermore, STEM images suggest the possibility that oxygen excess may exist in La‐rich regions within microdomains rather than at microdomain boundaries.     

The Nanocrystalline SnO2–TiO2 System—Part I: Structural Features

The phases present and their crystal structure and microstructure in the nanocrystalline SnO₂–TiO₂ system were studied in the compositional range Sn_1?xTi_xO₂ (0.0 ≤ x ≤ 0.9). There is an apparent increase in the solubility limits in the solid solution compared to bulk crystalline SnO₂–TiO₂. No two phase region was observed with increasing TiO₂ content. Electron energy loss spectroscopy, infrared spectroscopy (FTIR), and X‐ray diffraction (XRD) of the nanopowders showed that the apparent increase in solubility is related to the systematic Ti⁴⁺ segregation on the particle surface (surface excess) at the SnO₂‐rich side, avoiding the nucleation of a second phase even at high Ti⁴⁺ contents. Is this finding in accord with Raman spectra, which suggest localized Ti‐rich sites in the absence of a second crystalline phase. Ti⁴⁺ surface excess is also lead to a modification of the surface hydroxyls and a decrease in the crystallite size of the nanoparticles (with a concomitant increase in surface area), with expected implications to catalytic and sensorial properties of these nanoparticles.     

Effect of substitution of SnO2 for TiO2 on the surface and electrocatalytic properties of RuO2+TiO2 electrodes

The effect of substituting SnO₂ for TiO₂ in RuO₂+TiO₂ electrodes has been studied by varying the SnO₂ content systematically in a series of oxides of general composition 30 mol% RuO₂+x mol% SnO₂+(70-x) mol% TiO₂. The surface properties have been investigated by voltammetric curves, the electrocatalytic activity by using O₂ evolution from 1 mol dm^–3 HClO₄ solutions as a test reaction. It has been observed that only the surface area changes at intermediate compositions as a result of morphological modifications, while the electrocatalytic activity increases dramatically as the substitution of SnO₂ for TiO₂ becomes complete. Reasons for that are discussed. The present results do not support the claim that SnO₂ depresses the electrocatalytic activity of oxide electrodes for oxygen evolution.Dedicated to Professor Dr Fritz Beck on the occasion of his 60th birthday.