Donor characteristics of transition-metal-doped oxides: Cr-doped MgO versus Mo-doped CaO

The ability of Mo (Cr) impurities in a CaO (MgO) matrix to act as charge donors to adsorbed gold has been investigated by means of scanning tunneling microscopy and density functional theory. Whereas CaO(Mo) features a robust donor characteristic, as deduced from a charge-transfer-driven crossover in the Au particles' geometry in the presence of dopants, MgO(Cr) is electrically inactive. The superior performance of the CaO(Mo) system is explained by the ability of the Mo ions to evolve from a +2 oxidation state in ideal CaO to a +5 state by transferring up to three electrons to the Au adislands. Cr ions in MgO, on the other hand, are stable only in the +2 and +3 charge states and can provide a single electron at best. Since this electron is likely to be captured by cationic vacancies or morphological defects in the real oxide, no charge transfer to Au particles takes place in this case. On the basis of our findings, we have developed general rules on how to optimize the electron donor characteristics of doped oxide materials.     

碱—碱土—稀土催化剂发射光谱的研究

用Eu~(3+)的发射光谱探讨了O~-物种的形成。对M/MgO、M/CaO体系(M为碱金属)的研究发现,Li~+、Na~+可进入MgO晶格;Li~+、Na~+、K~+可进入CaO晶格。由于Li~+不能进入La_2O_3晶格,故不能形成O~-物种,表明不同的催化剂体系在甲烷氧化偶联反应中会形成不同的活性中心。     

Mechanism of Charging of Au Atoms and Nanoclusters on Li Doped SiO2/Mo(112) Films

We present the results of supercell DFT calculations on the adsorption properties of Au atoms and small clusters (Au_n, n≤5) on a SiO₂/Mo(112) thin film and on the same system modified by doping with Li atoms. The adsorbed Li atoms penetrate into the pores of the silica film and become stabilized at the interface where they donate one electron to the Mo metal. As a consequence, the work function of the Li‐doped SiO₂/Mo(112) film is reduced and results in modified adsorption properties. In fact, while on the undoped SiO₂/Mo(112) film Au interacts only very weakly, on the Li‐doped surface Au atoms and clusters bind with significant bond strengths. The calculations show that this is due to the occurrence of an electron transfer from the SiO₂/Mo(112) interface to the adsorbed gold. The occurrence of the charge transfer is related to the work function of the support but also to the possibility for the silica film to undergo a strong polaronic distortion.     

Tuning the work function of ultrathin oxide films on metals by adsorption of alkali atoms

We report a theoretical investigation of the adsorption of alkali metal atoms deposited on ultrathin oxide films. The properties of Li, Na, and K atoms adsorbed on SiO(2)/Mo(112) and of K on MgO / Ag(100) and TiO(2)/Pt(111) have been analyzed with particular attention to the induced changes in the work function of the system, Phi. On the nonreducible SiO(2) and MgO oxide films there is a net transfer of the outer ns electron of the alkali atom to the metal substrate conduction band; the resulting surface dipole substantially lowers Phi. The change in Phi depends (a) on the adsorption site (above the oxide film or at the interface) and (b) on the alkali metal coverage. Deposition of K on reducible TiO(2) oxide films results in adsorbed K(+) ions and in the formation of Ti(3+) ions. No charge transfer to the metal substrate is observed but also in this case the surface dipole resulting from the K-TiO(2) charge transfer has the effect to considerably reduce the work function of the system.     

Screening of MgO——and Ce02-Based Catalysts for Carbon Dioxide Oxidative Coupling of Methane to C2＋ Hydrocarbons

The catalyst screening tests for carbon dioxide oxidative coupling of methane (CO2-OCM)have been investigated over ternary and binary metal oxide catalysts.The catalysts are prepared by doping MgO-and CeO2-based solids with oxides from alkali(Li2O),alkaline earth (CaO),and transition metal groups (WO3 or MnO).The presence of the peroxide (O2^2-)active sites on the Li2O2,revealed by Raman spectroscopy,may be the key factor in the enhanced performance of some of the Li2O/MgO catalysts.The high reducibility of the CeO2 catalyst,an important factor in the CO2-OCM catalyst activity,may be enhanced by the presence of manganese oxide species. The manganese oxide species increases oxygen mobitity and oxygen vacancies in the CeO2 catalyst.raman and Fourier Transform Infra Red (FT-IR)spectroscopies revealed the presence of lattice vibrations of metal-oxygen bondings and active sites in which the peaks carresponding to the buld crystalline structures of Li2O,CaO,WO2 and MnO are detected.The performance of 5%MnO/15?O/CeO2 catalyst is the most potential among the CeO2-based catalysts,although lower than the 2%Li2O/MgO catalyst.The 2%Li2O/MgO catalyst showed the most promising C2 hydrocarbons selectivity and yield at 98.0%and 5.7%,respectively.     

Screening of MgO- and CeO_2-Based Catalysts for Carbon Dioxide Oxidative Coupling of Methane to C_(2+) Hydrocarbons

The catalyst screening tests for carbon dioxide oxidative coupling of methane (CO2-OCM) have been investigated over ternary and binary metal oxide catalysts. The catalysts are prepared by doping MgO- and CeO2-based solids with oxides from alkali (Li2O), alkaline earth (CaO), and transition metal groups (WO3 or MnO). The presence of the peroxide (O2-2) active sites on the Li2O2, revealed by Raman spectroscopy, may be the key factor in the enhanced performance of some of the Li2O/MgO catalysts. The high reducibility of the CeO2 catalyst, an important factor in the CO2-OCM catalyst activity, may     

From Composites to Solid Solutions: Modeling of Ionic Conductivity in the CaF2–BaF2 System

By using calcium fluorite and barium fluorite as test materials, we demonstrated that homovalent “dopants” can greatly affect ionic conductivity through locally changing the defect density. Whilst this doping is a state‐of‐the‐art effect in the case of dopants that replace native ions of different charge (heterovalent dopants), it is a rather surprising effect at a first glance for substitutional dopants of the same charge; here, the phenomenon is not electrostatic, but elastic in nature. As a consequence of size mismatch, the smaller Ca atoms in the BaF₂ lattice favored the formation of interstitial sites that were located close to the Ca atoms, whilst doping larger Ba species into the CaF₂ phase favored vacancy formation. In terms of conductivity, and in agreement with the different mobilities, the first doping effect was favorable, whilst the other decreased conductivity. The concentration effects were formalized by a heterogeneous Frenkel reaction that was distinguished from the mean Frenkel reaction by additional (elastic) trapping that became more pronounced the lower the temperature. It was very revealing to relate this phenomenon to CaF₂–BaF₂ multilayers and composites. In very general terms, these effects in the solid solutions were understood as being the atomistic limit of the interfacial charge‐transfer that occurred at the hetero‐interface of the crystallites or films, and reflected the transition from heterogeneous doping (higher‐dimensional doping) to homogeneous doping (zero‐dimensional doping).     

From composites to solid solutions: modeling of ionic conductivity in the CaF2-BaF2 system

By using calcium fluorite and barium fluorite as test materials, we demonstrated that homovalent "dopants" can greatly affect ionic conductivity through locally changing the defect density. Whilst this doping is a state-of-the-art effect in the case of dopants that replace native ions of different charge (heterovalent dopants), it is a rather surprising effect at a first glance for substitutional dopants of the same charge; here, the phenomenon is not electrostatic, but elastic in nature. As a consequence of size mismatch, the smaller Ca atoms in the BaF(2) lattice favored the formation of interstitial sites that were located close to the Ca atoms, whilst doping larger Ba species into the CaF(2) phase favored vacancy formation. In terms of conductivity, and in agreement with the different mobilities, the first doping effect was favorable, whilst the other decreased conductivity. The concentration effects were formalized by a heterogeneous Frenkel reaction that was distinguished from the mean Frenkel reaction by additional (elastic) trapping that became more pronounced the lower the temperature. It was very revealing to relate this phenomenon to CaF(2)-BaF(2) multilayers and composites. In very general terms, these effects in the solid solutions were understood as being the atomistic limit of the interfacial charge-transfer that occurred at the hetero-interface of the crystallites or films, and reflected the transition from heterogeneous doping (higher-dimensional doping) to homogeneous doping (zero-dimensional doping).     

Mo(001)表面的CaO(001)薄膜模型催化体系的STM研究

氧化物单晶化薄膜的制备与表征是研究氧化物表面性质的重要方法,也是模型催化研究的前沿领域。本文主要综述了Fritz-Haber研究所的Hajo Freund小组在过去几年间围绕着以Mo(001)为衬底制备的CaO(001)薄膜模型催化体系而进行的表面结构和化学性质的系列研究。其中既包含了氧化物薄膜研究的共同特点,如界面效应、膜厚效应等,也包含有CaO/Mo体系独特的性质,如Mo的自发掺杂对表面性质的调控作用。在该系列研究中低温扫描隧道显微镜(LT-STM)技术的应用贯穿了方方面面,从原子结构表征到电子性质研究,从杂质缺陷的鉴别到表面物种荷电性质的分析等。STM所获得的微观信息直接从原子分子水平揭示了调控薄膜表面性质的各种控因。特别的,在理论计算的辅助下,不断深化认识氧化物掺杂调控的原理和机制,为设计新型催化剂提供重要思路。     

表面电荷转移掺杂石墨烯的研究进展

表面电荷转移掺杂是调制石墨烯电学特性的重要手段。发展高效、稳定的表面电荷转移掺杂剂对于提高石墨烯的电学和光电性能、从而推动其在电子和光电领域中的应用具有重要意义。本文围绕高效与稳定两个方面综述了近年来石墨烯表面电荷转移掺杂剂的研究现状以及掺杂石墨烯在光电器件应用方面的进展。根据掺杂剂的类型，着重介绍了最新发展的高效p型和n型掺杂剂，并概述了稳定掺杂方面的重要研究工作。此外，专门介绍了基于掺杂石墨烯透明电极的高性能光电器件。最后，根据表面电荷转移掺杂研究面临的主要挑战，对其未来的发展方向进行了展望。     

Secondary doping in poly(3,4‐ethylenedioxythiophene):Poly(4‐styrenesulfonate) thin films

The electrical and structural properties of poly(3,4‐ethylenedioxythiophene):poly(4‐styrenesulfonate) (PEDOT:PSS) thin films deposited from aqueous dispersion using different concentrations of selected secondary dopants are studied in detail. An improvement of the electrical conductivity by three orders of magnitude is achieved for dimethyl sulfoxide, sorbitol, ethylene glycol, and N,N‐dimethylformamide, and the secondary dopant concentration dependence of the conductivity exhibits almost identical behavior for all investigated secondary dopants. Detailed analysis of the surface morphology and Raman spectra reveals no presence of the secondary dopant in fabricated films, and thus the dopants are truly causing the secondary doping effect. Although the ratio of benzenoid and quinoid vibrations in Raman spectra is unaffected by doping, the phase transition in PEDOT:PSS films owing to doping is confirmed. Further analysis of temperature‐dependent conductivity reveals 1D variable range hopping (VRH) charge transport for undoped PEDOT:PSS, whereas highly conductive doped PEDOT:PSS films exhibit 3D VRH charge transport. We demonstrate that the charge ‐ hopping dimensionality change should be a fundamental reason for the conductivity enhancement. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1139–1146     

Metal Oxide Nanosheets as 2D Building Blocks for the Design of Novel Materials

Research into 2-dimensional materials has soared during the last couple of years. Next to van der Waals type 2D materials such as graphene and h-BN, less well-known oxidic 2D equivalents also exist. Most 2D oxide nanosheets are derived from layered metal oxide phases, although few 2D oxide phases can be also made by bottom-up solution syntheses. Owing to the strong electrostatic interactions within layered metal oxide crystals, a chemical process is usually needed to delaminate them into their 2D constituents. This Review article provides an overview of the synthesis of oxide nanosheets, and methods to assemble them into nanocomposites, mono- or multilayer films. In particular, the use of Langmuir–Blodgett methods to form monolayer films over large surface areas, and the emerging use of ink jet printing to form patterned functional films is emphasized. The utilization of nanosheets in various areas of technology, for example, electronics, energy storage and tribology, is illustrated, with special focus on their use as seed layers for epitaxial growth of thin films, and as electrochemically active electrodes for supercapacitors and Li ion batteries.     

Construction of an Artificial Ferrimagnetic Lattice by Lithium Ion Insertion into a Neutral Donor/Acceptor Metal–Organic Framework

Construction of a molecular system in which the magnetic lattice exhibits long‐range order is one of the fundamental goals in materials science. In this study, we demonstrate the artificial construction of a ferrimagnetic lattice by doping electrons into acceptor sites of a neutral donor/acceptor metal–organic framework (D/A‐MOF). This doping was achieved by the insertion of Li‐ions into the D/A‐MOF, which was used as the cathode of a Li‐ion battery cell. The neutral D/A‐MOF is a layered system composed of a carboxylate‐bridged paddlewheel‐type diruthenium(II,II) complex as the donor and a TCNQ derivative as the acceptor. The ground state of the neutral form was a magnetically disordered paramagnetic state. Upon discharge of the cell, spontaneous magnetization was induced; the transition temperature was variable. The stability of the magnetically ordered lattice depended on the equilibrium electric potential of the D/A‐MOF cathode, which reflected the electron‐filling level.     

In2O3-based multicomponent metal oxide films and their prospects for thermoelectric applications

Thermoelectric properties of In₂O₃–SnO₂-based multi-component metal oxide films formed by spray pyrolysis method are studied. It is shown that the introduction of additional components such as gallium and zinc can control the parameters of the deposited layers. At that, the doping with gallium is more effective for optimization of the efficiency of the thermoelectric conversion. The explanation of the observed changes in the electro-physical and thermoelectric properties of the films at the composition change is given. It is found that the main changes in the properties of multicomponent metal oxide films take place at concentrations of dopants which correspond to their limit solubility in the dominant oxide.     

Gate‐Voltage Control of Borophene Structure Formation

Boron nanostructures are easily charged but how charge carriers affect their structural stability is unknown. We combined cluster expansion methods with first‐principles calculations to analyze the dependence of the preferred structure of two‐dimensional (2D) boron, or “borophene”, on charge doping controlled by a gate voltage. At a reasonable doping level of 3.12×10¹⁴ cm⁻², the hollow hexagon concentration in the ground state of 2D boron increases to 1/7 from 1/8 in its charge‐neutral state. The numerical result for the dependence of hollow hexagon concentration on the doping level is well described by an analytical method based on an electron‐counting rule. Aside from in‐plane electronic bonding, the hybridization among out‐of‐plane boron orbitals is crucial for determining the relative stability of different sheets at a given doping level. Our results offer new insight into the stability mechanism of 2D boron and open new ways for the control of the lattice structure during formation.     

Charge transfers at metal/oxide interfaces: a DFT study of formation of K delta+ and Au delta- species on MgO/Ag(100) ultra-thin films from deposition of neutral atoms

Ultra-thin oxide films grown on a metal substrate and of thickness smaller than 1 nm may exhibit unusual properties with respect to thicker films or single crystal oxide surfaces. In a previous study [G. Pacchioni, L. Giordano and M. Baistrocchi, Phys. Rev. Lett., 2005, 94, 226104] we have suggested that a Au atom adsorbed on a MgO/Mo(100) thin film becomes negatively charged by direct electron tunneling from the Mo metal and that this is related to the low MgO/Mo(100) work function. Here we show, based on periodic DFT supercell calculations, that charge transfer can occur also in the opposite direction by adsorption of electropositive K atoms on MgO/Ag(100) films. We predict the occurrence of a charge transfer also for Au on MgO/Ag(100) films despite the fact that here the work function is 1 eV larger than in MgO/Mo(100). The formation of a layer of adsorbed negative (Au delta-/MgO/Ag) or positive (K delta+/MgO/Ag) adsorbates results in an increase or decrease, respectively, of the MgO/Ag(100) work function as predicted by the classical Gurney model for ionic adsorbates on metal surfaces.     

Transition metal-doped titanium(IV) dioxide: Characterisation and influence on photodegradation of poly(vinyl chloride)

The effects of transition metal dopants (V(IV), V(V), Mn(II), Cr(III), Mo(V), and W(V)), introduced into TiO₂, upon the rate of photodegradation of poly(vinyl chloride) (PVC) films containing TiO₂ have been measured. The rates were determined mainly by monitoring carbonyl group formation. In another set of experiments, the rates of chloride ion release from irradiated PVC particles suspended in water undergoing agitation with air or O₂ in the presence of particles of doped TiO₂ were measured electrochemically. The doping of TiO₂ (rutile) with Cr(III), V(V) or Mn(II) reduces the photoactivity of the pigment, while doping by Mo(V) or W(V) enhances its photoactivity; the results obtained from carbonyl index measurements are paralleled closely by those from chloride ion release. Even the most aggressive doped pigments were less reactive than Degussa P25 pigment, while the greatest protection to PVC film was offered by TiO₂ particles coated with Al₂O₃ or SiO₂. Overall, the photoactivity of doped TiO₂ is a complex function of dopant concentration, the energy levels of the dopants in the TiO₂ lattice, their d electronic configuration and their local distribution. Photoactivity is also linked to other factors such as crystal type, particle size distribution and surface area. There is a clear relationship between the tendency of the dopant to induce the rutile-to-anatase transition and its effect in enhancing the photoactivity of the pigment. The characterisation of the doped pigments was achieved using X-ray powder diffraction, EPR spectroscopy, diffuse reflectance UV-vis spectrophotometry, scanning optical and electron microscopy and particle size analysis using LALLS.     

Au and Pd atoms adsorbed on pure and Ti-doped SiO2/Mo(112) films

The interaction of Pd and Au atoms with a silica surface and SiO2Mo(112) ultrathin films has been studied with periodic density-functional theory-generalized gradient approximation calculations. On both unsupported and supported silica, Pd and Au are weakly bound. No charge transfer occurs to the empty Pd and Au orbitals. Differently from Au, Pd can easily penetrate with virtually no barrier into the hexagonal rings of the supported silica film and binds strongly at the SiO2-Mo interface. The same process for Au implies overcoming a barrier of 0.9 eV. Completely different is the behavior of Ti-doped silica films. Au forms a direct covalent bond with substitutional Ti at the expense of the Ti...O-Mo interface bond which breaks. The global process is exothermic by 1 eV and nonactivated, showing that Ti doping results in solid anchoring points for the adsorbed Au atoms and for nucleation and growth of small gold particles. The effect of Ti doping is less pronounced for Pd but still visible with substantial enhancement of the Pd adsorption strength.     

Control of charge recombination dynamics in dye sensitized solar cells by the use of conformally deposited metal oxide blocking layers

We report herein a methodology for conformally coating nanocrystalline TiO2 films with a thin overlayer of a second metal oxide. SiO2, Al2O3, and ZrO2 overlayers were fabricated by dipping mesoporous, nanocrystalline TiO2 films in organic solutions of their respective alkoxides, followed by sintering at 435 degrees C. These three metal oxide overlayers are shown in all cases to act as barrier layers for interfacial electron transfer processes. However, experimental measurements of film electron density and interfacial charge recombination dynamics under applied negative bias were vary significantly for the overlayers. A good correlation was observed between these observations and the point of zero charge of the different metal oxides. On this basis, it is found that the most basic overlayer coating, Al2O3 (pzc = 9.2), is optimal for retarding interfacial recombination losses under negative applied bias. These observations show good correlation with current/voltage analyses of dye sensitized solar cell fabricated from these films, with the Al2O3 resulting in an increase in V(oc) of up to 50 mV and a 35% improvement in overall device efficiency. These observations are discussed and compared with an alternative TiCl4 posttreatment of nanocrystalline TiO2 films with regard to optimizing device efficiency.     

Alkali metal salts doped pluronic block polymers as electron injection/transport layers for high performance polymer light-emitting diodes

A series of alkali metal salts doped pluronic block copolymer F127 were used as electron injection/transport layers (ETLs) for polymer light-emitting diodes with poly[2-(4-(3′,7′-dimethyloctyloxy)-phenyl)-p-phenylenevinylene] (P-PPV) as the emission layer. It was found that the electron transport capability of F127 can be effectively enhanced by doping with alkali metal salts. By using Li2CO3 (15%) doped F127 as ETL, the resulting device exhibited improved performance with a maximum luminous efficiency (LE) of 13.59 cd/A and a maximum brightness of 5529 cd/m2, while the device with undoped F127 as ETL only showed a maximum LE of 8.78 cd/A and a maximum brightness of 2952 cd/m2. The effects of the doping concentration, cations and anions of the alkali metal salts on the performance of the resulting devices were investigated. It was found that most of the alkali metal salt dopants can dramatically enhance the electron transport capability of F127 ETL and the performance of the resulting devices was greatly improved.