ZnO氧缺陷的电子结构和光学性质

采用基于密度泛函理论的平面波超软赝势方法对ZnO_0.875的电子结构和光学性质进行了计算. 用第一性原理对含氧空位的ZnO晶体进行了结构优化处理, 计算了完整的和含氧空位的ZnO晶体的电子态密度. 结合精确计算的电子态密度分析了带间跃迁占主导地位的ZnO_0.875 材料的介电函数、吸收系数、折射系数、湮灭系数和反射系数, 并对光学性质和极化之间的联系做了详细讨论. 结果表明ZnO_0.875晶体是单轴晶体, 并且在低能区域存在因氧缺陷而造成的一些特性. 我们的研究结果为ZnO的发光特性提供新的视野, 同时为ZnO的光电子材料的设计和应用提供理论基础.     

Oxygen Vacancies in ZnO Nanosheets Enhance CO2 Electrochemical Reduction to CO

As electron transfer to CO₂ is generally considered to be the critical step during the activation of CO₂, it is important to develop approaches to engineer the electronic properties of catalysts to improve their performance in CO₂ electrochemical reduction. Herein, we developed an efficient strategy to facilitate CO₂ activation by introducing oxygen vacancies into electrocatalysts with electronic‐rich surface. ZnO nanosheets rich in oxygen vacancies exhibited a current density of ?16.1 mA cm^?2 with a Faradaic efficiency of 83 % for CO production. Based on density functional theory (DFT) calculations, the introduction of oxygen vacancies increased the charge density of ZnO around the valence band maximum, resulting in the enhanced activation of CO₂. Mechanistic studies further revealed that the enhancement of CO production by introducing oxygen vacancies into ZnO nanosheets originated from the increased binding strength of CO₂ and the eased CO₂ activation.     

Kinetics of passivity of NiTi in an acidic solution and the spectroscopic characterization of passive films

Anodic polarization of nitinol in acetic acid under galvanostatic conditions produces oxide films composed mainly of TiO₂. An exponential current-field relation is valid during ionic conduction through the growing oxide, in which the field coefficient is related to the jump distance. Transport processes in anodic films have been discussed in terms of a cooperative mechanism developed for amorphous oxide films on valve metals, in which both metal and oxygen ions were involved in ionic conduction. For more crystalline oxide structure of passive films on nitinol, formed during a prolonged potentiostatic conditions, the charge transfer takes place only through the oxygen vacancies as mobile species via a high-field-assisted mechanism. Based on the results of the Mott–Schottky analysis, these films behave as n-type semiconductors indicating that oxygen vacancies formed during the film formation and growth act as electron donors. The barrier/protecting and electronic/semiconducting properties of the passive films as well as their chemical composition were studied using electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy.     

ZrO2晶相对ZnO/ZrO2+SAPO-34双功能催化剂上合成气制烯烃反应的影响

烯烃是重要的化工原料,目前主要通过石油催化裂化得到.随着石油资源的消耗以及人们对烯烃需求的日益增长,开发非石油路线制取烯烃势在必行.合成气可以从煤、天然气和生物质等获得,由合成气作为重要的C1平台分子一步制取烯烃(STO)的过程受到了广泛关注.将合成气制甲醇/二甲醚的金属催化剂与甲醇制烯烃的分子筛催化剂耦合得到的混合双...     

Opto-electronic properties of blue phosphorene oxide with and without oxygen vacancies

Blue phosphorene is an attractive nanomaterial that exhibits some remarkable optoelectronic properties. Various studies have verified its ability to adsorb gaseous compounds and, in particular, to dissociate O₂, forming covalent bonds between phosphorus and oxygen atoms. These covalent bonds could be the reason behind the oxidation reaction that affects the blue phosphorene in normal room conditions. Theoretically, it has been demonstrated that the blue phosphorene oxide (BPO) is just as stable as the blue phosphorene. Given that metallic oxides are widely used as catalyzers and gas sensors, this opens the possibility of the BPO being presented as a gas sensor as well. For all the above, in this work the optoelectronic properties of BPO were studied, along with the generation of the oxygen vacancies. The investigation was performed within the density functional theory (DFT). In the study of the oxygen vacancy, the formation energy was calculated, and the results are similar to the formation energies of oxygen vacancies in other known oxides. It was found that the BPO with a single vacancy has a favorable energetic stability. The characterization of the vacancy is achieved using the electronic structure and the optical response. Additionally, the analysis of the adsorption of a hydrogen atom on the BPO, and the subsequent formation of hydroxide is presented.     

Spatially resolved imaging of inhomogeneous charge transfer behavior in polymorphous molybdenum oxide. I. Correlation of localized structural, electronic, and chemical properties using conductive probe atomic force microscopy and Raman microprobe spectroscopy

A detailed study of electrochemically deposited molybdenum oxide thin films has been carried out after they were sintered at 250 degrees C. Conductive probe atomic force microscopy (CP-AFM), Raman microscopy, and X-ray photoelectron spectroscopy (XPS) techniques were employed to assess the complex structural, electronic, and compositional properties of these films. Spatially resolved Raman microprobe spectroscopy studies reveal that sintered molybdenum oxide is polymorphous and phase segregated with three types of domains observed comprising orthorhombic alpha-MoO3, monoclinic beta-MoO3, and intermixed alpha-/beta-MoO3. CP-AFM studies conducted in concert with Raman microprobe spectroscopy allowed for correlation between specific compositional regions and localized electronic properties. Single point tunneling spectroscopy studies of chemically distinct regions show semiconducting current-voltage (I-V) behavior with the beta-MoO3 polymorph exhibiting higher electronic conductivity than intermixed alpha-/beta-MoO3 or microcrystalline alpha-MoO3 domains. XPS valence level spectra of beta-MoO3 films display a small structured band near the Fermi level, indicative of an increased concentration of oxygen vacancies. This accounts for the greatly enhanced electronic conductivity of beta-MoO3 as these positively charged cationic defects (anion vacancies) act to trap excess electrons. Connections between structural features, electronic properties, and chemical composition are established and discussed. Importantly, this work highlights the value of using spatially resolved techniques for correlating structural and compositional features with electrochemical behaviors of disordered, mixed-phase lithium insertion oxides.     

The nature of transition-metal-oxide surfaces

The surfaces of the 3d-transition-metal oxides form a rich and important system in which to study the effects of atomic geometry, ligand coordination and d-orbital population on surface electronic structure and chemisorption. This article considers the properties of those surfaces in terms of the types of surface structures that can exist, including steps and point defects, and their relation to the experimental data that is available for well characterized, single-crystal surfaces. The electronic structure of nearly perfect surfaces is very similar to that of the bulk for many of the oxides that have been studied; atoms at step sites also appear to have properties similar to those of atoms on terraces. Point defects are often associated with surfaces 0 vacancies and attendant transfer of electrons to adjacent metal cations. Those cations are poorly screened from each other, and the excess charge is presumably shared between two or more cations having reduced ligand coordination. Point defects are generally more active for chemisorption than are perfect surfaces, however for Ti₂O₃ and V₂O₃, whose cations have 3d¹ and 3d² electronic configurations respectively, the cleaved (047) surface is more active than are surfaces having a high density of defects. The chemisorption behavior of both nearly perfect and defect surfaces of 3d-transition-metal oxides varies widely from one material to another, and it is suggestive to correlate this with cation d-orbital population. However, too few oxides have yet been studied to draw any firm conclusions. Additional theoretical work on perfect surfaces, defects and chemisorption is also necessary in order to gain a more complete understanding of transition-metal-oxide surfaces.     

Substrate-Enabled Room-Temperature Electrochemical Deposition of Crystalline ZnMnO3

Mixed transition metal oxides have emerged as promising electrode materials for electrochemical energy storage and conversion. To optimize the functional electrode properties, synthesis approaches allowing for a systematic tailoring of the materials’ composition, crystal structure and morphology are urgently needed. Here we report on the room-temperature electrodeposition of a ternary oxide based on earth-abundant metals, specifically, the defective cubic spinel ZnMnO₃. In this unprecedented approach, ZnO surfaces act as (i) electron source for the interfacial reduction of MnO₄⁻ in aqueous solution, (ii) as substrate for epitaxial growth of the deposit and (iii) as Zn precursor for the formation of ZnMnO₃. Epitaxial growth of ZnMnO₃ on the lateral facets of ZnO nanowires assures effective electronic communication between the electroactive material and the conducting scaffold and gives rise to a pronounced 2-dimensional morphology of the electrodeposit forming – after partial delamination from the substrate – twisted nanosheets. The synthesis strategy shows promise for the direct growth of different mixed transition metal oxides as electroactive phase onto conductive substrates and thus for the fabrication of binder-free nanocomposite electrodes.     

Partial P-Type Metal Ions Doping Induced Variation of Both Crystal Structure and Oxygen Vacancy Within Cu/SnO2 Metastable Solid Solution Nanofibers for Highly Sensitive C2H2 Sensor

Partial P-type metal ions doping(PPMID) is an alternative method to further enhance the gas sensing performance of N-type metal oxides(NMOs) in contrast to that of P-N metal oxides heterojunctions, but the influences of the introduction of PPMID on the grain size and oxygen vacancies of NMOs have been rarely investigated. Herein, a simple and effective route has been demonstrated to address this problem with Cu²⁺-doped SnO₂ metastable solid solution nanofibers(CSMSSNs) as model and C₂H₂ as target molecule by combining electrospinning and calcination technique. It seems that the introduction of PPMID can also affect crystal structure and oxygen vacancies of NMOs, proven by combining X-ray diffraction(XRD) and X-ray photoelectron spectra(XPS). Thus, PPD, crystal structure and oxygen vacancies have been combined to clarify the enhanced sensing performance of Cu-doped SnO₂ metastable solid solution nanofibers angainst C₂H₂.     

A DFT Study of the Reactivity of Anatase TiO2 and Tetragonal ZrO2 Stepped Surfaces Compared to the Regular (101) Terraces

It is generally assumed that low‐coordinated sites at extended defects of oxide surfaces like steps or edges are more reactive than the regular, fully coordinated sites at the flat terraces. In this work we have considered the properties of stepped surfaces of anatase TiO₂ and tetragonal ZrO₂ by means of periodic DFT+U calculations. For both oxides, the stability of oxygen vacancies located near the step edges is compared to that of the same defects at the regular terraces. The capability of the steps to induce nucleation of metal nanoparticles on the surface has been evaluated by simulating the adsorption of a single ruthenium adatom. We conclude that, for anatase, step edges have no particular role in favouring the reduction of the oxide by reducing the cost for oxygen abstraction; in the same way, there is no special role of the stepped anatase surface in stabilizing adsorbed Ru atoms. On the contrary, step edges on zirconia display some capability to stabilise oxygen vacancies and ruthenium adatoms.     

Defects Energetics and Electronic Properties of Li Doped ZnO: A Hybrid Hartree-Fock and Density Functional Study

The electronic properties and stability of Li-doped ZnO with various defects have been stud-ied by calculating the electronic structures and defect formation energies via first-principles calculations using hybrid Hartree-Fock and density functional methods. The results from formation energy calculations show that Li pair complexes have the lowest formation energy in most circumstances and they consume most of the Li content in Li doped ZnO, which make the p-type conductance hard to obtain. The formation of Li pair complexes is the main obstacle to realize p-type conductance in Li doped ZnO. However, the formation energy ofLiZn decreases as environment changes from Zn-rich to O-rich and becomes more stable than that of Li-pair complexes at highly O-rich environment. Therefore, p-type conductance can be obtained by Li doped ZnO grown or post annealed in oxygen rich atmosphere.     

Oxygen Vacancy Diffusion and Condensation in Lithium‐Ion Battery Cathode Materials

Oxygen vacancies (OV) are native defects in transition metal (TM) oxides and their presence has a critical effect on the physicochemical properties of the oxide. Metal oxides are commonly used in lithium‐ion battery (LIB) cathodes and there is still a lack of understanding of the role of OVs in LIB research field. Here, we report on the behavior of OVs in a single‐crystal LIB cathode during the non‐equilibrium states of charge and discharge. We found that microcrack evolution in a single crystal occurs due to OV condensation in specific crystallographic orientations generated by the continuous migration of OVs and TM ions. Moreover, understanding the effects of the presence and diffusion of OVs in metal oxides enables the elucidation of most of the conventional mechanisms of capacity fading in LIBs and provides new insights for new electrochemical applications.     

Synthesis, characterization and photoluminescence properties of Ce3+-doped ZnO-nanophosphors

The present study involves the synthesis of Ce³⁺ doped ZnO nanophosphors by the zinc nitrate and cerium nitrate co-precipitation method. The synthesized nanophosphors were characterized with respect to their crystal structure, crystal morphology, particle size and photoluminescence (PL) properties using X-ray diffraction (XRD), scanning electron microscopy (SEM)/energy dispersive X-ray (EDX), transmission electron microscopy (TEM)/Energy-dispersive X-ray spectroscopy (EDS) and PL-spectroscopy respectively. XRD results revealed that ZnO nanophosphors are single phase and cubic type structures. Further, PL spectra of ZnO:Ce³⁺ nanophosphors showed green emission because of the charge transfer at single occupied oxygen vacancies with ZnO holes and red emission due to the cerium ion transitions. Intensity and fine structure of the Ce³⁺ luminescence and its temperature dependence are strongly influenced by the doping conditions. The formation of ZnO:Ce³⁺ nanophosphors was confirmed by Fourier transform infrared (FTIR) and XRD spectra.     

Beyond the structure-property relationship paradigm: influence of the crystal structure and microstructure on the Li+ conductivity of La2/3Li(x)Ti(1-x)Al(x)O3 Oxides

The crystal structures of several oxides of the La(2/3)Li(x)Ti(1-x)Al(x)O(3) system have been studied by selected-area electron diffraction, high-resolution transmission electron microscopy, and powder neutron diffraction, and their lithium conductivity has been by complex impedance spectroscopy. The compounds have a perovskite-related structure with a unit cell radical2 a(p)x2 a(p)x radical2 a(p) (a(p)=perovskite lattice parameter) due to the tilting of the (Ti/Al)O(6) octahedra and the ordering of lanthanum and lithium ions and vacancies along the 2 a(p) axis. The Li(+) ions present a distorted square-planar coordination and are located in interstitial positions of the structure, which could explain the very high ionic conductivity of this type of material. The lithium conductivity depends on the oxide composition and its crystal microstructure, which varies with the thermal treatment of the sample. The microstructure of these titanates is complex due to formation of domains of ordering and other defects such as strains and compositional fluctuations.     

Theory of electronic structure of oxide surfaces

Recent theoretical studies on the electronic structures of oxide surfaces by the DV-Xα cluster method are reviewed. The aim is to elucidate the major material and structural factors governing the surface electronic structure. Special attention is paid to the properties of the electronic states which are related to the surface chemical activities. To give a comprehensive picture of the electronic structure of oxide surfaces, topics are selected from several different types of oxides including MgO, MO (where M denotes 3D transition metals), TiO₂, SrTiO₃, ReO₃, ZnO. Electronic structure of oxygen chemisorbed on metals, which is the initial step of the oxide formation, is also studied. The important role of the partially filled d band and the influence of the chemisorption geometry is clarified in detail. Oxygen vacancy and other surface imperfections introduce localized electronic states, often with the energy level deep in the gap. They form active centers in the surface chemical processes. The charge compensation mechanism and the related peculiar properties of the polar surfaces are discussed. The chemisorption of oxygen, proton and hydroxyl on the oxide surface is investigated and the mechanism of the acidic and basic sites are discussed.     

钛表面二氧化钛膜中氧空位点缺陷传输性能的电化学研究

"钛/TiO2氧化膜/溶液"界面电极体系的电化学性能主要决定于钛表面的TiO2氧化物膜.本文利用多种电化学技术,结合半导体物理的Mott-Schottky分析和Einstein方程,研究了金属钛在1.0mol·L-1HClO4溶液中表面半导体TiO2氧化膜的生长及氧化膜中氧空位点缺陷在外加电场作用下的传输性能,并根据离子性电荷传输与电子性电荷传输对电场变化响应时间之不同特点,确定氧化膜中点缺陷扩散系数.结果表明,电极电位或阳极析氧反应对稳态电流(iss)、氧化膜的阳极化常数(α)、膜中电场强度()、以及膜中氧空位点缺陷的扩散系数(D0)等重要物理化学参数,均有显著影响,并依据氧化膜中的结构变化进行分析.     

Molybdenum‐tungsten Oxide Nanowires Rich in Oxygen Vacancies as An Advanced Electrocatalyst for Hydrogen Evolution

Electrolysis of water is a promising way to produce hydrogen fuel in large scale. The commercialization of this technology requires highly efficient non‐noble metal electrocatalysts to decease the energy input for the hydrogen evolution reaction (HER). In this work, a novel nanowire structured molybdenum‐tungsten bimetallic oxide (CTAB‐D‐W₄MoO₃) is synthesized by a simple hydrothermal method followed with post annealing treatment. The obtained metal oxides feature with enhanced conductivity, rich oxygen vacancies and customized electronic structure. As such, the composite electrocatalyst exhibits excellent electrocatalytic performance for HER in an acidic environment, achieving a large current density of 100 mA cm^?2 at overpotential of only 286 mV and a small Tafel slope of 71.2 mV dec^?1. The excellent electrocatalytic HER performance of CTAB‐D‐W₄MoO₃ is attributed to the unique nanowire structure, rich catalytic active sites and promoted electron transfer rate.     

Metal-oxide surfaces

Over the past two decades the amount of effort devoted to the study of metal oxides by surface scientists has increased significantly. The general characteristics of the electronic structure of metal-oxide surfaces are now fairly well understood, although transition-metal oxides have been more thoroughly studied than have non-transition-metal oxides. The geometric arrangement of atoms on the surfaces of a variety of metal oxides has also been determined. Extensive studies have been performed of the interaction of both molecules and metal atoms with metal oxides, where point defects are found to play a dominant role. However, our understanding of the surface properties of metal oxides is still much less compete than it is for metals and semiconductors, and there are several areas where more experimental and theoretical effort needs to be concentrated.     

A systematic study of neutral and charged 3d-metal trioxides and tetraoxides

Using density functional theory with generalized gradient approximation, we have performed a systematic study of the structure and properties of neutral and charged trioxides (MO(3)) and tetraoxides (MO(4)) of the 3d-metal atoms. The results of our calculations revealed a number of interesting features when moving along the 3d-metal series. (1) Geometrical configurations of the lowest total energy states of neutral and charged trioxides and tetraoxides are composed of oxo and∕or peroxo groups, except for CuO(3)(-) and ZnO(3)(-) which possess a superoxo group, CuO(4)(+) and ZnO(4)(+) which possess two superoxo groups, and CuO(3)(+), ZnO(3)(+), and ZnO(4)(-) which possess an ozonide group. While peroxo groups are found in the early and late transition metals, all oxygen atoms bind chemically to the metal atom in the middle of the series. (2) Attachment or detachment of an electron to∕from an oxide often leads to a change in the geometry. In some cases, two dissociatively attached oxygen atoms combine and form a peroxo group or a peroxo group transforms into a superoxo group and vice versa. (3) The adiabatic electron affinity of as many as two trioxides (VO(3) and CoO(3)) and four tetraoxides (TiO(4), CrO(4), MnO(4), and FeO(4)) are larger than the electron affinity of halogen atoms. All these oxides are hence superhalogens although only VO(3) and MnO(4) satisfy the general superhalogen formula.