Properties Of Transition Metal Oxides Research Articles

Tuning electronic properties of novel metal oxide nanocrystals using interface interactions: MoO 3 monolayers on Au(1 1 1)

We report experimental and theoretical investigations of monolayer MoO 3 nanocrystals grown on Au(1 1 1), in contrast to the bilayered structure of the bulk oxide crystal. The Au surface acts as the other half of the bilayer by satisfying local bonding requirements through charge redistribution at the interface. Epitaxy with the Au lattice is achieved through the ability of the Mo–O bonds to rotate about one another. The oxide layer becomes semimetallic as it strains to enhance bonding with the Au substrate. This flexibility of the oxide lattice suggests the possibility of tuning electronic properties of transition metal oxides via interface interactions. The effects of electronic structure on the surface chemistry of oxides are in turn illustrated by H adsorption energetics on MoO 3.

Electronic phase separation in transition metal oxide systems.

Electronic phase separation is increasingly getting recognized as a phenomenon of importance in understanding the magnetic and electron transport properties of transition metal oxides. The phenomenon dominates the rare-earth manganates of the formula Ln(1-x)A(x)MnO(3)(Ln = rare earth and A = alkaline earth) which exhibit ferromagnetism and metallicity as well as charge-ordering, depending on the composition, size of A-site cations and external factors such as magnetic and electric fields. We discuss typical phase separation scenarios in the manganates, with particular reference to Pr(1-x)Ca(x)MnO(3)(x= 0.3-0.4), (La(1-x)Ln(x))(0.7)Ca(0.3)MnO(3)(Ln = Pr, Nd, Gd and Y) and Nd(0.5)Sr(0.5)MnO(3). Besides discussing the magnetic and electron transport properties, we discuss electric field effects. Rare-earth cobaltates of the type Pr(0.7)Ca(0.3)CoO(3) and Gd(0.5)Ba(0.5)CoO(3) also exhibit interesting magnetic and electron transport properties which can be understood in terms of phase separation.

Acido-Basic Properties of Selective Oxidation Catalysts — An Overview

For Abstract see ChemInform Abstract in Full Text.

Role of bifunctionality of ZrO2-Based oxide systems in no reduction with lower hydrocarbons

The catalytic properties of transition metal oxides (Cr, Ce, and Co) supported on ZrO2 synthesized by various methods, as well as the effect of rhodium on the performance of the Мe х O y /ZrO2 oxide systems in NO reduction with hydrocarbons (methane, propane–butane mixture, and propene) were studied. The acidity of the surface of the catalysts was studied by IR spectroscopy and ammonia temperature-programmed desorption. The adsorption of NO, propene, and their mixture on the Rh–Сr2О3/ZrO2 and Rh–СеO2/ZrO2 catalysts at temperatures of 293–623 K was studied by IR spectroscopy. The adsorption and co-adsorption of the reactants was found to differ significantly depending on the nature of the surface. The activity of the catalysts studied in NO selective catalytic reduction with hydrocarbons is due to the presence of the acidic functional groups on their surface.

Oxygen-mediated diffusion of oxygen vacancies on the TiO2(110) surface.

Defects such as oxygen vacancies play a crucial role in the surface properties of transition metal oxides. By means of time-resolved, high-resolution scanning tunneling microscopy, we unraveled an adsorbate-mediated diffusion mechanism of oxygen vacancies on rutile TiO2(110). Adsorbed oxygen molecules mediate vacancy diffusion through the loss of an oxygen atom to a vacancy and the sequential capture of an oxygen atom from a neighboring bridging oxygen row, leading to an anisotropic oxygen vacancy diffusion pathway perpendicular to the bridging oxygen rows.

Recent advances in synthesis and applications of transition metal containing mesoporous molecular sieves.

Work in mesoporous silica-based materials began in the early 1990s with work by Mobil. These materials had pore sizes from 20-500 A and surface areas of up to 1500 m(2) g(-1) and were synthesized by a novel liquid crystal templating approach. Researchers subsequently extended this strategy to the synthesis of mesoporous transition metal oxides, a class of materials useful in catalysis, electronic, and magnetic applications because of variable oxidation states, and populated d-bands-features not found in silicates. These materials are already showing promise in electronic and optical applications hinging on the semiconducting properties of transition metal oxides and their potential to act as electron acceptors, an important feature in the design of cathodic materials. This is the first general review of non-silicate mesoporous materials and will focus on recent advances in this area, emphasizing materials possessing unique electronic, magnetic, or optical properties. Also covered are advances in the synthesis and applications of mesostructured sulfides as well as a new class of template-synthesized platinum-based materials that show promise in heterogeneous catalysis.

Taming the ground-state and optical properties of transition metal oxides

We have implemented the so-called local density approximation (LDA) plus the Hubbard parameter U method (LDA+U), in a formalism based on the projector augmented wave (PAW) method and calculated the ground-state properties of NiO and its optical properties. Good agreement with the measured ground-state antiferromagnetic magnetic moment and optical properties are obtained for U=5 eV. The enhancement of the O 2p near the top of the valence states increases with the value of U. At intermediate U, the nature of the band gap is a mixture of charge transfer and Ni d → d excitations.

Magnetism and electronic structure of LaMnO 3 and LaCoO 3

For the description of electronic and magnetic properties of transition metal oxides we have taken into account the fine electronic structure of the Mn 3+ and Co 3+ ions, that results from the crystal-field, spin–orbit coupling, local lattice distortions (Jahn–Teller effect). Our model consistently explains the formation of the non-magnetic state of LaCoO 3 as due to the non-magnetic Co 3+-ion ground state caused by the Jahn–Teller effect via the trigonal lattice distortion and the magnetic insulating state for LaMnO 3. Our studies indicate that the orbital moment has to be unquenched in this description of 3d-ion compounds.

Diffusion–evaporation method in studying self-diffusion in metal oxides and sulphides

The diffusion–evaporation method, developed several years ago by Kofstad is shortly discussed with the background of experimental results obtained on the Mn–MnS system from evaporation kinetics and radioactive tracer experiments. Excellent agreement between self-diffusion coefficients of cations in Mn 1+ y S determined experimentally and those calculated from evaporation rate measurements clearly indicates that this novel fascinating method can successfully be utilized in studying the transport properties of transition metal oxides and sulphides in equilibrium with the metallic phase. A notable feature of this method is that diffusional transport is studied under carbon- (and hydrogen-) free conditions, while in conventional, much more difficult, tracer experiments CO–CO 2 or H 2−H 2O (H 2–H 2S) gas mixtures must be used in order to obtain such low oxidant activities.

Valence state mapping of cobalt and manganese using near-edge fine structures

The properties of transition metal oxides are related to the presence of elements with mixed valences. The spectroscopy analysis of the valence states is feasible experimentally, but a spatial mapping of valence states of transition metal elements is a challenge to existing microscopy techniques. In this paper, with the use of valence state information provided by the white lines and near-edge fine structures observed using the electron energy-loss spectroscopy (EELS) in a transmission electron microscope (TEM), a novel experimental approach is demonstrated to map the valence state distributions of Mn and Co using the ratio of white lines in the energy-filtered TEM. The valence state map is almost independent of specimen thickness in the thickness range adequate for quantitative EELS microanalysis. An optimum spatial resolution of ∼2 nm has been achieved for a two-phase Co oxides.

Mapping The Valence States of Transition Metals Across Interfaces By Energy-Filtered Tem

Some properties of transition metal oxides are related to the presence of elements with mixed valences. In electron energy-loss spectroscopy (EELS), the L or M ionization edges of transition-metal, rare-earth and actinide elements usually display sharp threshold peaks known as white-lines. EELS experiments have shown that a change in cation valence state introduces a significant change in the White-line intensity ratio [1]. With the use of valence state information provided by the white lines, an experimental approach is demonstrated here to map the valence state distributions of Mn and Co using an energy-filtered transmission electron microscope (TEM). A spatial resolution of ˜ 2 nm has been achieved. This technique should be particularly useful in studying valence states of cations in magnetic oxides.To map the distribution of ionization states, an energy window of ˜ 10 eV in width is required to isolate the L3 from L2 white lines (Figure 1).

A chemical contribution to the search for novel electronic properties in transition metal oxides: LiNiO 2

ANiO 2 ( A=Li, Na; the layered α-NaFeO 2 structure) contains Ni 3+ (or Ni 2+ L ) with S=1/2 and a 2-fold orbital degeneracy in the two-dimensional (2D) triangular (/O/Ni/O/) − unit. Though the unit seems to be ferromagnetic (FM) and hence free from spin frustration, rhombohedral LiNiO 2 does not show any well-defined long-range magnetic ordering. In contrast, NaNiO 2 is known to undergo a cooperative Jahn–Teller distortion from rhombohedral to monoclinic symmetry at such a high temperature of 490 K and then allows an antiferromagnetic (AF) inter-unit ordering at T N=20 K. These structural and magnetic dissimilarities suggest a dimensional crossover controlled by the spacer, A +: It seems to be strengthened in the more compact Li-oxide enough to switch the orbital- and spin-ordered ground state realized in the Na-oxide (the isolated type) to another one (the coupled type) that remains somehow free from these instabilities.

Mapping the Valence States of Transition-Metal Elements Using Energy-Filtered Transmission Electron Microscopy

The properties of transition-metal oxides are related to the presence of elements with mixed valences, such as Mn and Co. Spatial mapping of the valence-state distribution of transition-metal elements is a challenge to existing microscopy techniques. In this letter, using the valence-state information provided by the white lines observed in electron energy-loss spectroscopy in a transmission electron microscope (TEM), an experimental approach is demonstrated to map the valence-state distributions of Mn and Co using the energyfiltered TEM. An optimum spatial resolution of 2 nm has been achieved for two-phase Co oxides with sharp boundaries. This provides a new technique for quantifying the valence states of cations in magnetic oxides. Many physical and chemical properties of inorganic materials are determined by the elements with mixed Valences in the structural unit, 1 by which we mean that an element has two or more different valences while forming a compound. Transitionand rare-earth-metal elements with mixed valences are unique for initiation of electronic, structural, and/or chemical evolutions. We have demonstrated previously the valence states of Mn and Co in their oxides by applying electron energy-loss spectroscopy (EELS) for quantitative determination. 2-5 In EELS, the L ionization edges of transition-metal, rare-earth, and actinide compounds usually display sharp peaks at the near-edge region. These threshold peaks are known as white lines. The unoccupied 3d states form a narrow energy band, the transition of a 2pstate electron to the 3d levels, leading to the formation of white lines observed experimentally. EELS experiments have shown that a change in the valence states of cations introduces significant variation in the intensity ratio of the white lines, giving the possibility of identifying the occupation number of 3d or 4d electrons (or cation valence states) using the measured white line intensities. 6,7 The information obtained using EELS is an integration over the spatial region illuminated by the incident electron beam. In this paper, we introduce a new experimental approach which can give the distribution maps of cation valences in real space, allowing a direct identification of cations with different valence states. This is useful for studying nanocomposite magnetic oxide materials with mixed valences.

Properties of transition metal oxides with layered perovskite structure

The manganates of La n− n x Sr 1+ n x Mn n O 3 n+1 ( x=0.4; n=1, 2, ∞) were prepared to study the relationship between their electronic properties and their crystal structure; two-dimensional structure with n=1, pseudo-two-dimensional with n=2 and three-dimensional with n=∞. The n=1 manganate exhibits semiconducting behavior below room temperature. When the manganate has a higher dimensional structure with an increase in the value of n, the electric conduction changes to a more itinerant one. The n=2 manganate exhibits an electric transition from semiconductor to metal at 127 K when the temperature is reduced below room temperature. The n=∞ manganate exhibits metallic behavior below room temperature. Ferromagnetism was found when the metallic conduction occurred, i.e., in the n=2 manganate below the electric transition temperature, and in the n=∞ manganate below room temperature. From photoelectron spectroscopy, the valence band structure was found to consist of two features, due to the photoemission from the Mn 3d orbital (∼3 eV) and the O 2p orbital (∼5.5 eV). The change in the dimension of the crystal structure affects the electronic structure, that is, the ratio of the intensity of the photoemission from the Mn 3d orbital to that of the O 2p orbital increases with an increase in n.

Optical and magnetic properties of transition metal oxides

The extremely strong coupling of conduction electrons with local spins in transition metal oxides leads to many intriguing electronic and optical properties, which are currently of great interest. Among these intriguing properties, the discovery of colossal magnetoresistance in manganese oxides has sparked a number of studies on the magnetic and electronic properties of these materials as well as the exploration of the relevant new materials phase.

Electronic Structure and Phsical Properties of Transition Metal Oxides

Electronic Structure and Phsical Properties of Transition Metal Oxides

Pure and mixed titanium, niobium and vanadium as sputtered thick and thin films: Crystallographic properties and phase transitions between 300 and 1800 K II: Electron microscopy investigations of thin films

Crystallographic properties of transition metal oxides (MO y and (Ti,M)O y , with 1.9 〈 y 〈 2.5 and M = Ti, Nb and V) as thin films ( e ≈ 50 nm) deposited by r.f. sputtering, were studied by conventional transmission electron microscopy. Samples were annealed by the electron beam (up to 2000 K). Generally, the thin layers were initially amorphous. TiO y samples crystallized according to the TiO 2 anatase and rutile phases. After heating (about 1000 K), NbO y and (Ti, Nb)O y films crystallized according to the NbO 2 and (Ti, Nb)O 2 structures. At higher temperatures, different Nb 2O 5 compounds and unknown phases were formed. VO y layers generally exhibit the formation of the V 6O 13 compound, whereas (Ti, V)O y samples yielded unexpected and unknown compounds (such as V n O 2 n−1 + TiO 2, Ti m V n−m O 2 n−1 ,…).

The interaction of oxygen and carbon monoxide with iron oxide, nickel oxide and zinc oxide surfaces: A molecular orbital study

An atom superposition electron delocalization (ASED) molecular orbital technique is employed in this study of catalytic properties of transition metal oxides. Cluster models of stoichiometric iron oxide, nickel oxide and zinc oxide surfaces are used in the investigation of reactions of carbon monoxide, in the absence and presence of preadsorbed oxygen atoms. We predict that oxygen preferentially adsorbs to the atop metal site, suggesting the dissociative adsorption of molecular oxygen. Carbon monoxide, oriented vertically above the surface with the carbon end down, also prefers to adsorb on the atop site. Carbon dioxide is predicted to adsorb more weakly than carbon monoxide in the presence of preadsorbed oxygen atoms, and we find evidence for reaction of carbon monoxide with lattice oxygen as well. The structures and energy levels are examined, and theoretical results compared with experiments.

IR Studies of NH3, Pyridine, CO, and NO Adsorbed on Transition Metal Oxides

Abstract Chemisorption of small molecules is often used as a probe for By probing the surface properties of transition metal oxides. the interaction of molecules with the surface, information is often obtained on the oxidation state, the coordination symmetry, the degree of coordination unsaturation of the surface cations, the acid-base properties of the surface hydroxyl groups, and the presence and the nature of surface Lewis acid and B rosnsted acid sites. This information is deduced from experimental measurements of the adsorption isotherms, the heats of adsorption, the thermal desorption spectra, and the vibrational spectra of the adsorbate. Until recently, when high resolution electron energy loss spectroscopy became available, vibrational spectra were obtained with infrared spectroscopy. Laser Raman spectroscopy has seldom been used because of the low Raman scattering cross section of most molecules.

Electrochromic materials and devices for energy efficient windows

Numerous inorganic and organic electrochromic materials are discussed in the context of developing a film-based optical shutter for a window application. It is possible electronically to alter a window's transmission and reflection properties by use of electrochromic thin films. This allows regulation of conductive and radiative heat transfer rates, with variable optical attenuation. As a result, an aperture can be optically and thermally managed, reducing space heating and cooling loads. The properties of transition metal oxides, such as WO 3, MoO 3, Ir 2O 3 and V 2O 5 are detailed. Organic systems such as heptyl viologen and polytungsten anion are reviewed. Also, intercalated structures are discussed. Various designs of working devices are outlined with emphasis on solid-state configurations. From this quantification, materials and devices with appropriate deposition techniques for window applications are detailed.