Electrical properties of niobium-doped titanium dioxide. 1. Defect disorder

The present work reports the electrical conductivity and thermoelectric power for Nb-doped TiO(2) at elevated temperatures (1073-1298 K) in the gas phase of controlled oxygen activity, 10(-14) Pa < p(O(2)) < 75 kPa. It is shown that in reduced conditions the Nb-doped TiO(2) exhibits metallic-type conductivity. This finding paves the way for the development of high-performance photoelectrodes with substantially reduced internal energy losses during charge transport. The present work also determined the equilibrium constant for the formation of oxygen vacancies and titanium vacancies for Nb-doped TiO(2).     

Electrical properties and defect chemistry of TiO2 single crystal. II. Thermoelectric power

The present work reports the thermoelectric power of high-purity single-crystal TiO(2) in the temperature range 1073-1323 K and in gas phases of controlled oxygen activities, p(O(2)), in the range 10(-13) to 7.5 x 10(4) Pa. The thermoelectric power versus log p(O(2)) dependence for strongly reduced TiO(2) at p(O(2)) < 10(-5) Pa may be approximated by a slope of 1/6, which is consistent with the defect disorder governed by electronic charge compensation of oxygen vacancies. The thermoelectric power data confirm that oxygen vacancies are the predominant ionic defects. These data indicate that TiO(2) at high p(O(2)) exhibits p-type properties. It is shown that the p(O(2)) related to the n-p transition increases with increase of temperature.     

Pyrogenic iron(III)-doped TiO2 nanopowders synthesized in RF thermal plasma: phase formation, defect structure, band gap, and magnetic properties

Iron(III)-doped TiO(2) nanopowders, with controlled iron to titanium atomic ratios (R(Fe/Ti)) ranging from nominal 0 to 20%, were synthesized using oxidative pyrolysis of liquid-feed metallorganic precursors in a radiation-frequency (RF) thermal plasma. The valence of iron doped in the TiO(2), phase formation, defect structures, band gaps, and magnetic properties of the resultant nanopowders were systematically investigated using M?ssbauer spectroscopy, XRD, Raman spectroscopy, TEM/HRTEM, UV-vis spectroscopy, and measurements of magnetic properties. The iron doped in TiO(2) was trivalent (3+) in a high-spin state as determined by the isomer shift and quadrupole splitting from the M?ssbauer spectra. No other phases except anatase and rutile TiO(2) were identified in the resultant nanopowders. Interestingly, thermodynamically metastable anatase predominated in the undoped TiO(2) nanopowders, which can be explained from a kinetic point of view based on classical homogeneous nucleation theory. With iron doping, the formation of rutile was strongly promoted because rutile is more tolerant than anatase to the defects such as oxygen vacancies resulting from the substitution of Fe(3+) for Ti(4+) in TiO(2). The concentration of oxygen vacancies reached a maximum at R(Fe/Ti) = 2% above which excessive oxygen vacancies tended to concentrate. As a result of this concentration, an extended defect like crystallographic shear (CS) structure was established. With iron doping, red shift of the absorption edges occurred in addition to the d-d electron transition of iron in the visible light region. The as-prepared iron-doped TiO(2) nanopowders were paramagnetic in nature at room temperature.     

Impedance spectroscopy of reduced monoclinic zirconia

Zirconia doped with low-valent cations (e.g. Y3+ or Ca2+) exhibits an exceptionally high ionic conductivity, making them ideal candidates for various electrochemical applications including solid oxide fuel cells (SOFC) and oxygen sensors. It is nevertheless important to study the undoped, monoclinic ZrO2 as a model system to construct a comprehensive picture of the electrical behaviour. In pure zirconia a residual number of anion vacancies remains because of contaminants in the material as well as the thermodynamic disorder equilibrium, but electronic conduction may also contribute to the observed conductivity. Reduction of zirconia in hydrogen leads to the adsorption of hydrogen and to the formation of oxygen vacancies, with their concentration affected by various parameters (e.g. reduction temperature and time, surface area, and water vapour pressure). However, there is still little known about the reactivities of defect species and their effect on the ionic and electronic conduction. Thus, we applied electrochemical impedance spectroscopy to investigate the electric performance of pure monoclinic zirconia with different surface areas in both oxidizing and reducing atmospheres. A novel equivalent circuit model including parallel ionic and electronic conduction has previously been developed for titania and is used herein to decouple the conduction processes. The concentration of defects and their formation energies were measured using volumetric oxygen titration and temperature programmed oxidation/desorption.     

On the convergence of isolated neutral oxygen vacancy and divacancy properties in metal oxides using supercell models

The properties of isolated neutral oxygen vacancies and divacancies of metal oxides of increasing complexity (MgO, CaO, alpha-Al2O3, and ZnO) have been studied by means of density-functional theory within a supercell periodic approach. Vacancy formation energies, vacancy-vacancy interactions, and geometry rearrangements around these point defects have been investigated in detail. The characterization of the electronic structure of these point defects has been established by analysis of the density of states and of the topology of the electron density and of electron localization function. It is found that the chemical character of the oxide determines the properties of the oxygen vacancies. For the covalent ZnO oxide, a more complex scheme arises in which the relaxation around the oxygen vacancy is much larger leading to the formation of Zn4-like almost metallic particles in the crystal. The relationship of these structures with the crystal shear planes is discussed. The present study shows that supercells containing approximately 200-300 atoms provide converged values for the geometric and electronic structure of oxygen vacancies of these metal oxides in the point defect low concentration limit.     

N doping of TiO2(110): photoemission and density-functional studies

The electronic properties of N-doped rutile TiO2(110) have been investigated using synchrotron-based photoemission and density-functional calculations. The doping via N2+ ion bombardment leads to the implantation of N atoms (approximately 5% saturation concentration) that coexist with O vacancies. Ti 2p core level spectra show the formation of Ti3+ and a second partially reduced Ti species with oxidation states between +4 and +3. The valence region of the TiO(2-x)N(y)(110) systems exhibits a broad peak for Ti3+ near the Fermi level and N-induced features above the O 2p valence band that shift the edge up by approximately 0.5 eV. The magnitude of this shift is consistent with the "redshift" observed in the ultraviolet spectrum of N-doped TiO2. The experimental and theoretical results show the existence of attractive interactions between the dopant and O vacancies. First, the presence of N embedded in the surface layer reduces the formation energy of O vacancies. Second, the existence of O vacancies stabilizes the N impurities with respect to N2(g) formation. When oxygen vacancies and N impurities are together there is an electron transfer from the higher energy 3d band of Ti3+ to the lower energy 2p band of the N(2-) impurities.     

一种将三价钛引入锐钛型二氧化钛的新方法:用于有效的光催化产氢（英文）

伴随着科技的日益进步,人们的生活水平有了极大的提升,但是随之而来的环境污染问题也成为当前世界亟待解决的重大问题之一.太阳能是地球上最大的清洁能源来源,有效地利用太阳能将为解决环境污染问题提供巨大的帮助.而光催化技术的一个关键材料就是催化剂,催化剂对太阳能利用的好坏也就决定了光催化技术的应用前景.因此,人们对催化剂的研究也在逐步深入.1972年,Fuiishima和Honda在Nature杂志上首次报道了二氧化钛电极在紫外光照射下分解水产生氢气的现象,自此掀起了半导体光催化的研究热潮.二氧化钛因具有无毒、廉价和耐腐蚀等优点而在光催化领域广受关注.传统白色二氧化钛具有两大劣势(宽的禁带宽度与高的电子/空穴复合率)大大限制了它的应用.人们对于改变二氧化钛性质进行了大量的尝试,例如金属/非金属掺杂、形貌的改变等.黑色二氧化钛(存在大量的氧空位和三价钛)的出现极大地克服了传统白色二氧化钛的缺点,其具有窄的禁带宽度和低的电子/空穴复合速度.目前,黑色二氧化钛的制备方法大致分为以下几种:(1)高压下氢化;(2)高温常压下氢化;(3)铝热还原法;(4)溶液蒸发法;(5)化学氧化法;(6)电化学还原等.由于更简单安全(不涉及氢气使用)和更好的可见光光催化活性,三价钛自掺杂的二氧化钛从这些方法中脱颖而出.迄今为止,诱导三价钛的方法通常是能量密集型的,涉及不稳定的Ti原料(如TiO和TiH_2).研究用于制备三价钛自掺杂二氧化钛的新方法是十分必要的.我们以稳定的钛粉为原料,通过简单的方法将三价钛和氧空位成功地引入锐钛型二氧化钛.通过X射线衍射、场致发射扫描电子显微镜等技术研究了制备样品的物理化学性质,发现H_2O_2和水热反应时间对纳米棒型微观形貌的形成至关重要.通过调整诸如H_2O_2量和水热反应时间的参数,根据实验结果提出了形成这种微观结构的可能机制.更重要的是,可见光区域的光吸收受到样品中氧空位量的控制,存在氧空位的最佳值使光催化产氢活性最高.     

Chain structures of surface hydroxyl groups formed via line oxygen vacancies on TiO2(110) surfaces studied using noncontact atomic force microscopy

Structures of surface hydroxyl groups arranged on a reduced TiO2(110) surface that had line oxygen vacancies were studied using noncontact atomic force microscopy (NC-AFM). NC-AFM results revealed that by increasing the density of oxygen vacancies on the TiO2(110) surface, line oxygen vacancies were formed by removal of oxygen atoms in a bridge oxygen row on the TiO2(110) surface. After the TiO2(110) surface with the line oxygen vacancies was exposed to water, the surface showed hydroxyl chain structures that were composed of hydroxyl groups linearly arranged in a form of two rows on the line oxygen vacancies and on a neighboring bridge oxygen row. In-situ NC-AFM measurements of these surfaces exposed to water at room temperature revealed that hydroxyl chain structures were formed at the line oxygen vacancy. Annealing above 500 K was sufficient to remove the hydroxyl chain structures on the TiO2(110) surface and allowed line oxygen vacancies to reappear on the surface. The line oxygen vacancies are active sites for water dissociation. In conclusion, the formation of the hydroxyl chain structure suggests that the surface hydroxyl groups on a TiO2(110) surface can be controlled by preparing oxygen vacancy structures on the surface.     

Electrical properties and defect chemistry of TiO2 single crystal. I. Electrical conductivity

The present work reports the electrical properties of high-purity single-crystal TiO(2) from measurements of the electrical conductivity in the temperature range 1073-1323 K and in gas phases of controlled oxygen activities in the range 10(-13) to 10(5) Pa. The effect of the oxygen activity on the electrical conductivity indicates that oxygen vacancies are the predominant defects in the studied ranges of temperature and oxygen activities. The electronic and ionic lattice charge compensations were revealed at low and high oxygen activities, respectively. The determined semiconducting quantities include: the activation energy of the electrical conductivity (E(sigma) = 125-205 kJ.mol(-1)), the activation energies of the electrical conductivity components associated with electrons (E(n) = 218 kJ.mol(-1)), electron holes (E(p) = 34 kJ.mol(-1)), and ions (E(i) = 227 kJ.mol(-1)), and the enthalpy of motion for electronic defects (DeltaH(m) = 4 kJ/mol). The electrical conductivity data are considered in terms of the components related to electrons, holes, and ions. The obtained data allow the determination of the n-p demarcation line in terms of temperature and oxygen activities. The band gap determined from the electronic component of the electrical conductivity is 3.1 eV.     

Characterization of paramagnetic species in N-doped TiO2 powders by EPR spectroscopy and DFT calculations

Electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations are combined for the first time in an effort to characterize the paramagnetic species present in N-doped anatase TiO2 powders obtained by sol-gel synthesis. The experimental hyperfine coupling constants are well reproduced by two structurally different nitrogen impurities: substitutional and interstitial N atoms in the TiO2 anatase matrix. DFT calculations show that the nitrogen impurities induce the formation of localized states in the band gap. Substitutional nitrogen states lie just above the valence band, while interstitial nitrogen states lie higher in the gap. Excitations from these localized states to the conduction band may account for the absorption edge shift toward lower energies (visible region) observed in the case of N-doped TiO2 with respect to pure TiO2 (UV region). Calculations also show that nitrogen doping leads to a substantial reduction of the energy cost to form oxygen vacancies in bulk TiO2. This suggests that nitrogen doping is likely to be accompanied by oxygen vacancy formation. Finally, we propose that the relative abundance of the two observed nitrogen-doping species depends on the preparation conditions, such as the oxygen concentration in the atmosphere and the annealing temperature during synthesis.     

TiO2-yNx纳米光催化剂的制备及其可见光响应机理

利用溶胶-凝胶技术,以尿素为氮源,采用原位掺杂方式制备了TiO2-yNx纳米粉体;以亚甲基蓝(MB)溶液在可见光下的光催化降解评价其可见光催化活性;考察了体系初始pH值、N的掺杂量和焙烧温度对样品可见光催化活性的影响。 结合XRD、XPS、ESR和DRS测试技术,研究了N掺杂纳米TiO2的可见光响应机理。 研究结果表明,TiO2-yNx纳米粉体的优化制备工艺条件为：体系初始pH=0.52,掺杂比n(N)∶n(Ti)=1∶6,焙烧温度为440 ℃。 此条件下制备的样品N含量为0.77%,为单一的锐钛矿相,平均粒径为19.0 nm,具有良好的可见光催化活性。 N掺杂导致TiO2纳米粉体的表面羟基含量增加,形成了大量束缚单电子的氧空位;N取代晶格O形成了N-Ti-O和O-N-Ti键合结构。 N掺杂导致TiO2纳米粒子的吸收带边红移,对可见光的吸收能力明显增强,这表明N掺杂改变TiO2电子结构,使带隙窄化,降低光响应阈值。 N掺杂TiO2纳米粒子的可见光响应归因于N取代掺杂形成的掺杂能级与氧空位形成的缺陷能级共同作用所致。     

A new nuclear magnetic resonance algorithm to determine equilibrium constants of the species in the B(III)-H2O system

Several efforts have been attempted to study species formation by Nuclear Magnetic Resonance (NMR) in systems with several chemical equilibria present. The majority of these are qualitative and only a few have tried to relate component fractions of a distribution diagram with experimental area fractions determined from NMR spectra to obtain equilibrium constants values. In this work we present a new focus that attempts to relate the species concentration fractions in the system with area fractions beneath NMR peaks to achieve this task. 11B-NMR data of B(III)-H2O systems have been processed with the aid of formation constant values (-log *beta) obtained by potentiometry which are 9.17+/-0.01 for B(OH)3, 9.79+/-0.08 for B2O(OH)5-, 19.90+/-0.09 for B3O3(OH)4- and 38.50+/-0.04 for B5O6(OH)4-, form B(III)-H2O systems with 0.075 M< or = [B(III)]total< or = 0.700 M, in agreement with previous reports and NMR behavior. The treatment of NMR data developed in this work gives a new methodology to obtain formation constants and suggests the possibility to establish a generalization of Beer's law to NMR spectroscopy.     

Nitrogen-doped TiO2 thin films: photocatalytic applications for healthcare environments

N-doped TiO(2) has for many years received interest as visible light photocatalytic materials. Here we give our perspective on the subject with special consideration towards the use of visible light photocatalysts in the field of antimicrobial materials with applications in healthcare environments. The subject is reviewed and critiqued from synthetic techniques to characterisation and assessment of functional properties. N-doped TiO(2) has huge potential to form commercially viable antimicrobial surfaces that are easily implemented within the healthcare environment. We aim to shed light on the illusive nature of the mechanism of the different types of N-doping and comment on how these affect the properties of the catalysts themselves. Small concentrations of nitrogen doped under mild conditions lead to interstitial doping, which also promotes the creation of oxygen vacancies. Many believe that it is these oxygen vacancies that actually promote the formation of visible light photocatalysis and hence there is an indirect correlation between the interstitial doping and the photocatalysis. As the concentration of interstitial nitrogen increases the oxygen vacancies increase, however the presence of oxygen vacancies in turn encourages substitutional doping which then fills the oxygen vacancies. This cyclic relationship leads to photocatalysts that are very sensitive to changing nitrogen concentration.     

O2 and vacancy diffusion on rutile(110): pathways and electronic properties.

The binding structures and diffusion pathways of molecular oxygen on a defective TiO2(110) surface are studied by means of a recently developed first-principles string molecular dynamics approach. A variety of molecular and dissociated O2 adsorption states are identified and the kinetics of their interconversion is analyzed. These results, as well as calculations of the electronic properties and of scanning tunneling microscopy (STM) images, are used to discuss recent experimental observations of the interactions between surface oxygen vacancies and the adsorbed oxygen molecule.     

Nitrogen/gold codoping of the TiO2(101) anatase surface. A theoretical study based on DFT calculations

The interaction between implanted nitrogen atoms, adsorbed gold atoms, and oxygen vacancies at the anatase TiO(2)(101) surface is investigated by means of periodic density functional theory calculations. Substitutional and interstitial configurations for the N-doping have been considered, as well as several adsorption sites for Au adatoms and different types of vacancies. Our total energy calculations suggest that a synergetic effect takes place between the nitrogen doping on one hand and the adsorption of gold and vacancy formation on the other hand. Thus, while pre-implanted nitrogen increases the adsorption energy for gold and decreases the energy required for the formation of an oxygen vacancy, pre-adsorbed gold or the presence of oxygen vacancies favors the nitrogen doping of anatase. The analysis of the electronic structure and electron densities shows that a charge transfer takes place between implanted-N, adsorbed Au and oxygen vacancies. Moreover, it is predicted that the creation of vacancies on the anatase surface modified with both implanted nitrogen and supported gold atoms produces migration of substitutional N impurities from bulk to surface sites. In any case, the most stable configurations are those where N, Au and vacancies are close to each other.     

Reactivity of sub 1 nm supported clusters: (TiO2)n clusters supported on rutile TiO2 (110)

Metal oxide clusters of sub-nm dimensions dispersed on a metal oxide support are an important class of catalytic materials for a number of key chemical reactions, showing enhanced reactivity over the corresponding bulk oxide. In this paper we present the results of a density functional theory study of small sub-nm TiO(2) clusters, Ti(2)O(4), Ti(3)O(6) and Ti(4)O(8) supported on the rutile (110) surface. We find that all three clusters adsorb strongly with adsorption energies ranging from -3 eV to -4.5 eV. The more stable adsorption structures show a larger number of new Ti-O bonds formed between the cluster and the surface. These new bonds increase the coordination of cluster Ti and O as well as surface oxygen, so that each has more neighbours. The electronic structure shows that the top of the valence band is made up of cluster derived states, while the conduction band is made up of Ti 3d states from the surface, resulting in a reduction of the effective band gap and spatial separation of electrons and holes after photon absorption, which shows their potential utility in photocatalysis. To examine reactivity, we study the formation of oxygen vacancies in the cluster-support system. The most stable oxygen vacancy sites on the cluster show formation energies that are significantly lower than in bulk TiO(2), demonstrating the usefulness of this composite system for redox catalysis.     

Enhanced visible-light activity of titania via confinement inside carbon nanotubes

Titania confined inside carbon nanotubes (CNTs) was synthesized using a restrained hydrolysis method. Raman spectra and magnetic measurements using a SQUID magnetometer suggested the formation of remarkable oxygen vacancies over the encapsulated TiO(2) in comparison with nanoparticles dispersed on the outer surface of CNTs, extending the photoresponse of TiO(2) from the UV to the visible-light region. The CNT-confined TiO(2) exhibited improved visible-light activity in the degradation of methylene blue (MB) relative to the outside titania and commercial P25, which is attributed to the modification of the electronic structure of TiO(2) induced by the unique confinement inside CNTs. These results provide further insight into the effect of confinement within CNTs, and the composites are expected to be promising for applications in visible-light photocatalysis.     

Characterizing TiO2(110) surface states by their work function

The unreconstructed TiO(2)(110) surface is prepared in well-defined states having different characteristic stoichiometries, namely reduced (r-TiO(2), 6 to 9% surface vacancies), hydroxylated (h-TiO(2), vacancies filled with OH), oxygen covered (ox-TiO(2), oxygen adatoms on a stoichiometric surface) and quasi-stoichiometric (qs-TiO(2), a stoichiometric surface with very few defects). The electronic structure and work function of these surfaces and transition states between them are investigated by ultraviolet photoelectron spectroscopy (UPS) and metastable impact electron spectroscopy (MIES). The character of the surface is associated with a specific value of the work function that varies from 4.9 eV for h-TiO(2), 5.2 eV for r-TiO(2), 5.35 eV for ox-TiO(2) to 5.5 eV for qs-TiO(2). We establish the method for an unambiguous characterization of TiO(2)(110) surface states solely based on the secondary electron emission characteristics. This is facilitated by analysing a weak electron emission below the nominal work function energy. The emission in the low energy cut-off region appears correlated with band gap emission found in UPS spectra and is attributed to localised electron emission through Ti(3+)(3d) states.     

受主掺杂BaPbO3陶瓷的缺陷结构

采用高温平衡电导法测定高温平衡电导率随氧分压(10^－12～10⁵ Pa)的变化曲线, 阐明了受主掺杂BaPbO₃的缺陷结构, 解释了材料的导电机理. 高氧分压下, Pb离子空位缺陷占主导, 电荷补偿缺陷为空穴; 随着氧分压的下降, 材料由本征缺陷占主导向杂质缺陷占主导转变, 受主杂质成为主导缺陷, 电荷补偿缺陷为空穴; 在低氧分压下, 电荷补偿缺陷由空穴转变为氧离子空位. 受主掺杂浓度的下降, 导致高温电导率下降, 并引起本征缺陷占主导向非本征缺陷占主导的转变点向低氧分压方向移动, 同时低氧分压区域的电荷补偿缺陷由空穴转变为氧离子空位的转变点也向更低的氧分压方向移动.