Oxygen surface exchange kinetics of SrTi(1-x)Fe(x)O(3-δ) mixed conducting oxides

The oxygen surface exchange kinetics of mixed conducting perovskite oxides SrTi(1-x)Fe(x)O(3-δ) (x = 0, 0.01, 0.05, 0.35, 0.5) has been investigated as a function of temperature and oxygen partial pressure using the pulse-response (18)O-(16)O isotope exchange (PIE) technique. Arrhenius activation energies range from 140 kJ mol(-1) for x = 0 to 86 kJ mol(-1) for x = 0.5. Extrapolating the temperature dependence to the intermediate temperature range, 500-600 °C, indicates that the rate of oxygen exchange, in air, increases with increasing iron mole fraction, but saturates at the highest iron mole fraction for the given series. The observed behavior is concomitant with corresponding increases in both electronic and ionic conductivity with increasing x in SrTi(1-x)Fe(x)O(3-δ). Including literature data of related perovskite-type oxides Ba(0.5)Sr(0.5)Co(0.8)Fe(0.2)O(3-δ), La(0.6)Sr(0.4)Co(0.2)Fe(0.8)O(3-δ), La(0.6)Sr(0.4)CoO(3-δ), and Sm(0.5)Sr(0.5)CoO(3-δ), a linear relationship is observed in the log-log plot between oxygen exchange rate and oxide ionic conductivity with a slope fairly close to unity, suggesting that it is the magnitude of the oxide ionic conductivity that governs the rate of oxygen exchange in these solids. The distribution of oxygen isotopomers ((16)O(2), (16)O(18)O, (18)O(2)) in the effluent pulse can be interpreted on the basis of a two-step exchange mechanism for the isotopic exchange reaction. Accordingly, the observed power law dependence of the overall surface exchange rate on oxygen partial pressure turns out to be an apparent one, depending on the relative rates of both steps involved in the adopted two-step scheme. Supplementary research is, however, required to elucidate which of the two possible reaction schemes better reflects the actual kinetics of oxygen surface exchange on SrTi(1-x)Fe(x)O(3-δ).     

In situ XPS studies of perovskite oxide surfaces under electrochemical polarization

An in situ XPS study of oxidation-reduction processes on three perovskite oxide electrode surfaces was carried out by incorporating the materials in an electrochemical cell mounted on a heated sample stage in an ultrahigh vacuum (UHV) chamber. Electrodes made of powdered LaCr(1-x)Ni(x)O(3-delta) (x = 0.4, 1) showed changes in the XPS features of all elements upon redox cycling between formal Ni3+ and Ni2+ oxidation stoichiometries, indicating the delocalized nature of the electronic states involved and strong mixing of O 2p to Ni 3d levels to form band states. The surface also showed changes in adsorption capacity for CO2 upon reduction as a result of increased nucleophilicity of surface oxygen. Another perovskite oxide, La(0.5)Sr(0.5)CoO(3-delta), laser deposited as highly oriented thin films on (100) oriented yttria-stabilized zirconia (YSZ), also showed evidence of both local and nonlocal effects in the XPS features upon redox cycling. In contrast to LaCr(1-x)Ni(x)O(3-delta), redox cycling mainly affected the XPS features of cobalt with little effect on oxygen. This signifies reduced participation of O 2p states in the conduction band of this material. Small changes in surface cation stoichiometry in this film were observed and attributed to mobility of the A-site Sr dopant under polarization.     

Novel cobalt-free oxygen permeable membrane

A series of cobalt-free and low cost perovskite oxygen permeable membranes based on BaCe(x)Fe(1-x)O(3-delta)(BCF) oxides was successfully synthesized and the membrane showed both high oxygen permeability and high stability under reductive atmosphere, which will be most suitable for constructing a membrane reactor for selective oxidation of light hydrocarbons to syngas or high value corresponding oxygenates.     

Epitaxial thin films of ATiO(3-x)H(x) (A = Ba, Sr, Ca) with metallic conductivity

Epitaxial thin films of titanium perovskite oxyhydride ATiO(3-x)H(x) (A = Ba, Sr, Ca) were prepared by CaH(2) reduction of epitaxial ATiO(3) thin films deposited on a (LaAlO(3))(0.3)(SrAl(0.5)Ta(0.5)O(3))(0.7) substrate. Secondary ion mass spectroscopy detected a substantial amount and uniform distribution of hydride within the film. SrTiO(3)/LSAT thin film hydridized at 530 °C for 1 day had hydride concentration of 4.0 × 10(21) atoms/cm(3) (i.e., SrTiO(2.75)H(0.25)). The electric resistivity of all the ATiO(3-x)H(x) films exhibited metallic (positive) temperature dependence, as opposed to negative as in BaTiO(3-x)H(x) powder, revealing that ATiO(3-x)H(x) are intrinsically metallic, with high conductivity of 10(2)-10(4) S/cm. Treatment with D(2) gas results in hydride/deuteride exchange of the films; these films should be valuable in further studies on hydride diffusion kinetics. Combined with the materials' inherent high electronic conductivity, new mixed electron/hydride ion conductors may also be possible.     

Design of energy band alignment at the Zn(1-x)Mg(x)O/Cu(In,Ga)Se2 interface for Cd-free Cu(In,Ga)Se2 solar cells

The electronic band structure at the Zn(1-x)Mg(x)O/Cu(In(0.7)Ga(0.3))Se(2) interface was investigated for its potential application in Cd-free Cu(In,Ga)Se(2) thin film solar cells. Zn(1-x)Mg(x)O thin films with various Mg contents were grown by atomic layer deposition on Cu(In(0.7)Ga(0.3))Se(2) absorbers, which were deposited by the co-evaporation of Cu, In, Ga, and Se elemental sources. The electron emissions from the valence band and core levels were measured by a depth profile technique using X-ray and ultraviolet photoelectron spectroscopy. The valence band maximum positions are around 3.17 eV for both Zn(0.9)Mg(0.1)O and Zn(0.8)Mg(0.2)O films, while the valence band maximum value for CIGS is 0.48 eV. As a result, the valence band offset value between the bulk Zn(1-x)Mg(x)O (x = 0.1 and x = 0.2) region and the bulk CIGS region was 2.69 eV. The valence band offset value at the Zn(1-x)Mg(x)O/CIGS interface was found to be 2.55 eV after considering a small band bending in the interface region. The bandgap energy of Zn(1-x)Mg(x)O films increased from 3.25 to 3.76 eV as the Mg content increased from 0% to 25%. The combination of the valence band offset values and the bandgap energy of Zn(1-x)Mg(x)O films results in the flat (0 eV) and cliff (-0.23 eV) conduction band alignments at the Zn(0.8)Mg(0.2)O/Cu(In(0.7)Ga(0.3))Se(2) and Zn(0.9)Mg(0.1)O/Cu(In(0.7)Ga(0.3))Se(2) interfaces, respectively. The experimental results suggest that the bandgap energy of Zn(1-x)Mg(x)O films is the main factor that determines the conduction band offset at the Zn(1-x)Mg(x)O/Cu(In(0.7)Ga(0.3))Se(2) interface. Based on these results, we conclude that a Zn(1-x)Mg(x)O film with a relatively high bandgap energy is necessary to create a suitable conduction band offset at the Zn(1-x)Mg(x)O/CIGS interface to obtain a robust heterojunction. Also, ALD Zn(1-x)Mg(x)O films can be considered as a promising alternative buffer material to replace the toxic CdS for environmental safety.     

Spectroscopic study of the electric field induced valence change of Fe-defect centers in SrTiO3

The electrochemical changes induced by an electric field in Fe-doped SrTiO(3) have been investigated by X-ray absorption spectroscopy (XANES and EXAFS), electron paramagnetic resonance (EPR) and Raman spectroscopy. A detailed study of the Fe dopant in the regions around the anode and cathode reveals new insights into the local structure and valence state of Fe in SrTiO(3) single crystals. The ab initio full multiple-scattering XANES calculations give an evidence of the oxygen vacancy presence in the first coordination shell of iron. Differences in the length and disorder of the Fe-O bonds as extracted from EXAFS are correlated to the unequivocal identification of the defect type by complementary spectroscopical techniques to identify the valence state of the Fe-dopant and the presence of the Fe - V(?) complexes. Through this combinatorial approach, novel structural information on Fe - V(?) complexes is provided by X-ray absorption spectroscopy, and the relation of Fe-O bond length, doping level and oxidation state in SrTi(1-x)Fe(x)O(3) is briefly discussed.     

Raman spectroscopic study on the structure in the surface and the bulk shell of Ce(x)Pr(1-x)O(2-delta) mixed oxides

The difference between the surface and the bulk shell of Ce(x)Pr(1-x)O(2-delta) mixed oxides was studied by Raman spectroscopy with four different excitation lasers. Two Raman peaks appear at 465 and 570 cm(-1) under all of the four lasers. The former is attributed to the Raman active F(2g) mode of CeO2, while the latter is attributed to oxygen vacancy. On the basis of the fact that the laser with shorter wavelength is closer to the electronic adsorption of samples, it is found that the Raman information detected by excitation laser with shorter wavelength is more sensitive to the surface region of samples. An inflection is observed in the relationship of the value I570/I465 to the Ce content in Ce(x)Pr(1-x)O(2-delta). With the increase in the wavelength of excitation laser, the Ce content corresponding to the inflection decreases. Combined with the surface concentration obtained by XPS, it can be deduced that the composition of Ce(x)Pr(1-x)O(2-delta) mixed oxide particles in the surface region and the bulk shell are different, the former is enrichment of Pr component and the latter is enrichment of Ce component. The thickness of the surface layer with rich Pr component decreases with the increase in the Ce content.     

Suppression of magnetic order in Sr3Fe2O4Cl2 by Fe-site substitution by cobalt

The low-temperature topotactic reduction of Sr(3)Fe(2-x)Co(x)O(5)Cl(2) oxychloride phases with LiH allows the preparation of phases of composition Sr(3)Fe(2-x)Co(x)O(4)Cl(2) (0 ≤ x ≤ 1). The reduced phases adopt body-centered tetragonal structures which are isostructural with Sr(3)Fe(2)O(4)Cl(2) and contain square-planar (Fe/Co)O(4) centers connected into apex-linked sheets, analogous to the CuO(2) sheets present in superconducting cuprate phases. As the cobalt concentration in Sr(3)Fe(2-x)Co(x)O(4)Cl(2) is increased the antiferromagnetic order of the Sr(3)Fe(2)O(4)Cl(2) host phase is suppressed, ultimately leading to spin-glass behavior, at low temperature, in Sr(3)Fe(2-x)Co(x)O(4)Cl(2) phases with x ≥ 0.8. The limited influence of cobalt substitution on the reactions which form the Sr(3)Fe(2-x)Co(x)O(4)Cl(2) phases is discussed and contrasted to that of the related SrFeO(3-δ)-SrFeO(2) system.     

Structural, optical, and magnetic properties of Mn-doped ZnO thin film

The Zn(1-x)Mn(x)O (x = 0, 0.16, and 0.25) thin films were grown on fused quartz substrates by reactive magnetron cosputtering. X-ray-diffraction measurement revealed that all the films were single phase and had wurtzite structure with c-axis orientation. As Mn concentration increased in the Zn(1-x)Mn(x)O films, the c-axis lattice constant and band-gap energy increased gradually. In Raman-scattering studies, an additional Mn-related vibration mode appeared at 520 cm(-1). E(2H) phonon line of Zn(1-x)Mn(x)O alloy was broadened asymmetrically and redshifted as a result of microscopic structural disorder induced by Mn(2+) random substitution. The Zn(0.84)Mn(0.16)O film exhibited a ferromagnetic characteristic with a Curie temperature of approximately 62 K. However, with increasing Mn concentration to 25 at. %, ferromagnetism disappeared due to the enhanced antiferromagnetic superexchange interactions between neighboring Mn(2+) ions.     

Tunable electrical and magnetic properties of half-metallic Zn(x)Fe(3-x)O4 from first principles

The electrical and magnetic properties of Zn-doped Fe(3)O(4) at different doping concentrations of Zn have been investigated using a density functional method with generalized-gradient approximation corrected for on-site Coulombic interactions. The electronic structure calculation predicts that Zn(x)Fe(3-x)O(4) (0 ≤x≤ 0.875) is half-metallic with a full spin polarization. The hopping carrier concentration of Zn(x)Fe(3-x)O(4) decreases with increasing x, which indicates a distinct increase in the resistivity. The saturation magnetization of Zn(x)Fe(3-x)O(4) increases evidently with increasing x from x = 0 to x = 0.75 (i.e. from 4.0 to 8.3 μ(B)/f.u.) and then decreases rapidly to zero at x = 1. The robust half-metallicity, large tunability of electrical and magnetic properties of a Zn doped Fe(3)O(4) system make it a promising functional material for spintronic applications.     

Synthetic control over magnetic moment and exchange bias in all-oxide materials encapsulated within a spherical protein cage

This work focuses on the synthetic control of magnetic properties of mixed oxide magnetic nanoparticles of the general formula Fe(3-x)Co(x)O(4) (x < or = 0.33) in the protein cage ferritin. In this biomimetic approach, variations in the chemical synthesis result in the formation of single-phase Fe(3-x)Co(x)O(4) alloys or intimately mixed binary phase Fe/Co oxides, modifying the chemical structure and magnetic behavior of these particles, as characterized by static and dynamic magnetization measurements and X-ray absorption spectroscopy.     

Electronic structures and excitonic transitions in nanocrystalline iron-doped tin dioxide diluted magnetic semiconductor films: an optical spectroscopic study

Nanocrystalline iron-doped tin dioxide (Sn(1-x)Fe(x)O(2)) films with x from 0 to 0.2 were prepared on c-sapphire substrates by pulsed laser deposition. X-ray diffraction and Raman scattering analysis show that the films are of the rutile structure at low compositions and an impurity phase related to Fe(2)O(3) appears until the x is up to 0.2, suggesting the general change of lattice structure due to the Fe ion substitution. The dielectric functions are successfully determined from 0.0248 to 6.5 eV using the Lorentz multi-oscillator and Tauc-Lorentz dispersion models in the low and high photon energy regions, respectively. With increasing Fe composition, the highest-frequency transverse optical phonons E(u) shifts towards a lower energy side and can be well described by (608 - 178x) cm(-1). From the transmittance spectra, the fundamental absorption edge is found to be decreased with the Fe composition due to the joint contributions from SnO(2) and Fe(2)O(3). It can be observed that the doped films exhibit evident excitonic excitation features, which are strongly related to the Fe doping. Among them, the 6A(1g)→ 4T(2g) transition contributes to the onset of optical absorption. Moreover, the remarkable intensity reduction and a red-shift trend with the doping composition, except for the pure film, can be testified by the photoluminescence spectra. It can be concluded that the replacement of Sn with the Fe ion could induce the 2p-3d hybridization and result in the electronic band structure modification of the Sn(1-x)Fe(x)O(2) films.     

Systematic study of compositional and synthetic control of vacancy and magnetic ordering in oxygen-deficient perovskites Ca2Fe(2-x)Mn(x)O(5+y)and CaSrFe(2-x)Mn(x)O(5+y) (x = 1/2, 2/3, and 1; y = 0-1/2)

Ten compounds belonging to the series of oxygen-deficient perovskite oxides Ca(2)Fe(2-x)Mn(x)O(5) and CaSrFe(2-x)Mn(x)O(5+y), where x = 1/2, 2/3, and 1 and y ≈ 0-0.5, were synthesized and investigated with respect to the ordering of oxygen vacancies on both local and long-range length scales and the effect on crystal structure and magnetic properties. For the set with y ≈ 0 the oxygen vacancies always order in the long-range sense to form the brownmillerite structure containing alternating layers of octahedrally and tetrahedrally coordinated cations. However, there is a change in symmetry from Pnma to Icmm upon substitution of Sr for one Ca for all x, indicating local T(d) chain (vacancy) disorder. In the special case of CaSrFeMnO(5) the neutron diffraction peaks broaden, indicating only short-range structural order on a length scale of ~160 ?. This reveals a systematic progression from Ca(2)FeMnO(5) (Pnma, well-ordered tetrahedral chains) to CaSrFeMnO(5) (Icmm, disordered tetrahedral chains, overall short-range order) to Sr(2)FeMnO(5) (Pm3m, destruction of tetrahedral chains in a long-range sense). Systematic changes occur in the magnetic properties as well. While long-range antiferromagnetic order is preserved, the magnetic transition temperature, T(c), decreases for the same x when Sr substitutes for one Ca. A review of the changes in T(c) for the series Ca(2)Fe(2-x)M(x)O(5), taking into account the tetrahedral/octahedral site preferences for the various M(3+) ions, leads to a partial understanding of the origin of magnetic order in these materials in terms of a layered antiferromagnetic model. While in all cases the preferred magnetic moment direction is (010) at low temperatures, there is a cross over for x = 0.5 to (100) with increasing temperature for both the Ca(2)Fe(2-x)Mn(x)O(5) and the CaSrFe(2-x)Mn(x)O(5) series. For the y > 0 phases, while a brownmillerite ordering of oxygen vacancies is preserved for the Ca(2) phases, a disordered Pm3m cubic perovskite structure is always found when Sr is substituted for one Ca. Long-range magnetic order is also lost, giving way to spin glass or cluster-glass-like behavior below ~50 K. For the x = 0.5 phase, neutron pair distribution function (NPDF) studies show a local structure related to brownmillerite ordering of oxygen vacancies. Neutron diffraction data at 3.8 K show a broad magnetic feature, incommensurate with any multiple of the chemical lattice, and with a correlation length (magnetic domain) of 6.7(4) ?.     

Sonochemical synthesis of Ce(1-x)Fe(x)O(2-δ) (0 ≤ x ≤ 0.45) and Ce(0.65)Fe(0.33)Pd(0.02)O(2-δ) nanocrystallites: oxygen storage material, CO oxidation and water gas shift catalyst

Nanocrystalline Ce(1-x)Fe(x)O(2-δ) (0 ≤ x ≤ 0.45) and Ce(0.65)Fe(0.33)Pd(0.02)O(2-δ) of ~4 nm sizes were synthesized by a sonochemical method using diethyletriamine (DETA) as a complexing agent. Compounds were characterized by powder X-ray diffraction (XRD), X-ray photo-electron spectroscopy (XPS) and transmission electron microscopy (TEM). Ce(1-x)Fe(x)O(2-δ) (0 ≤ x ≤ 0.45) and Ce(0.65)Fe(0.33)Pd(0.02)O(2-δ) crystallize in fluorite structure where Fe is in +3, Ce is in +4 and Pd is in +2 oxidation state. Due to substitution of smaller Fe(3+) ion in CeO(2), lattice oxygen is activated and 33% Fe substituted CeO(2)i.e. Ce(0.67)Fe(0.33)O(1.835) reversibly releases 0.31[O] up to 600 °C which is higher or comparable to the oxygen storage capacity of CeO(2)-ZrO(2) based solid solutions (Catal. Today 2002, 74, 225-234). Due to interaction of redox potentials of Pd(2+/0)(0.89 V) and Fe(3+/2+) (0.77 V) with Ce(4+/3+) (1.61 V), Pd ion accelerates the electron transfer from Fe(2+) to Ce(4+) in Ce(0.65)Fe(0.33)Pd(0.02)O(1.815), making it a high oxygen storage material as well as a highly active catalyst for CO oxidation and water gas shift reaction. The activation energy for CO oxidation with Ce(0.65)Fe(0.33)Pd(0.02)O(1.815) is found to be as low as 38 kJ mol(-1). Ce(0.67)Fe(0.33)O(1.835) and Ce(0.65)Fe(0.33)Pd(0.02)O(1.815) have also shown high activity for the water gas shift reaction. CO conversion to CO(2) is 100% H(2) specific with these catalysts and conversion rate was found to be as high 27.2 μmoles g(-1) s(-1) and the activation energy was found to be 46.4 kJ mol(-1) for Ce(0.65)Fe(0.33)Pd(0.02)O(1.815).     

Polarization Rotation in the Monoclinic Perovskite BiCo(1-x) Fe(x) O(3)

The monoclinic perovskite BiCo(1-x) Fe(x) O(3) (x≈0.7) undergoes a second-order structural transition from tetragonal to monoclinic, which is accompanied by a rotation of the polarization vector from the [001] to [111] directions of a pseudo cubic cell. The crystal structure, determined by electron diffraction and powder synchrotron X-ray diffraction, was the same as that of Pb(Ti(1-x) Zr(x) )O(3) at the morphotropic phase boundary.     

Magnetization, Mössbauer and isothermal dilatometric behavior of oxidized YBa(Co,Fe)4O(7+δ)

M?ssbauer spectroscopy and magnetization studies of YBaCo(4-x)Fe(x)O(7+δ) (x = 0-0.8) oxidized at 0.21 and 100 atm O(2), indicate an increasing role of penta-coordinated Co(3+) states when the oxygen content approaches 8-8.5 atoms per formula unit. Strong magnetic correlations are observed in YBaCo(4-x)Fe(x)O(8.5) from 2 K up to 55-70 K, whilst the average magnetic moment of Co(3+) is lower than that for δ ≤ 0.2, in correlation with the lower (57)Fe(3+) isomer shifts determined from M?ssbauer spectra. The hypothesis on dominant five-fold coordination of cobalt cations was validated by molecular dynamics modeling of YBaCo(4)O(8.5). The iron solubility limit in YBaCo(4-x)Fe(x)O(7+δ) corresponds to approximately x ≈ 0.7. The oxygen intercalation processes in YBaCo(4)O(7+δ) at 470-700 K, analyzed by X-ray diffraction, thermogravimetry and controlled-atmosphere dilatometry, lead to unusual volume expansion opposing to the cobalt cation radius variations. This behavior is associated with increasing cobalt coordination numbers and with rising local distortions and disorder in the crystal lattice on oxidation, predicted by the computer simulations. When the oxygen partial pressure increases from 4 × 10(-5) to 1 atm, the linear strain in YBaCo(4)O(7+δ) ceramics at 598 K is as high as 0.33%.     

微波水热法对CoxNi(1-x)Fe2O催化活性的影响

到目前，人们已利用微波水热法合成了一些物质的超细粉体，但有关微波水热法合成粉体的催化性能研究仍没有见报导，这里，我们首次报导了在微波辐射下湿法合成的CoxNi（1-x）Fe2O4（0≤x≤1）粉体对催化分解H2O2有较高活性．微波辐射下合成CoxNi（1-x）Fe2O4催化剂的实验装置如     

α-Fe2O3掺杂对In2O3电导和气敏性能的影响

用化学共沉淀法制备了α Fe2O3掺杂的In2O3纳米晶微粉,研究了α Fe2O3掺杂对In2O3电导和气敏性能的影响. 结果表明, α Fe2O3和In2O3间可形成有限固溶体In2－xFexO3(0≤x≤0.40); Fe3＋对In2O3晶格中In3＋格位的部分取代,大大增强了阴阳离子间的结合力,导致材料中氧空位VO×数骤降、 自由电子的浓度变稀和电导下降. n(Fe3＋):n(In3＋)=5 :5的共沉淀粉于800 ℃下灼烧4 h所得的α Fe2O3掺杂In2O3传感器元件,对45 μmol•L－1 C2H5OH的灵敏度达54.0,为相同浓度干扰气体汽油的8倍多.     

CaMn(1-x)Nb(x)O3 (x < or = 0.08) perovskite-type phases as promising new high-temperature n-type thermoelectric materials

Perovskite-type CaMn(1-x)Nb(x)O(3+/-delta) (x = 0.02, 0.05, and 0.08) compounds were synthesized by applying both a "chimie douce" (SC) synthesis and a classical solid state reaction (SSR) method. The crystallographic parameters of the resulting phases were determined from X-ray, electron, and neutron diffraction data. The manganese oxidations states (Mn(4+)/Mn(3+)) were investigated by X-ray photoemission spectroscopy. The orthorhombic CaMn(1-x)Nb(x)O(3+/-delta) (x = 0.02, 0.05, and 0.08) phases were studied in terms of their high-temperature thermoelectric properties (Seebeck coefficient, electrical resistivity, and thermal conductivity). Differences in electrical transport and thermal properties can be correlated with different microstructures obtained by the two synthesis methods. In the high-temperature range, the electron-doped manganate phases exhibit large absolute Seebeck coefficient and low electrical resistivity values, resulting in a high power factor, PF (e.g., for x = 0.05, S(1000K) = -180 microV K(-1), rho(1000K) = 16.8 mohms cm, and PF > 1.90 x 10(-4) W m(-1) K(-2) for 450 K < T < 1070 K). Furthermore, lower thermal conductivity values are achieved for the SC-derived phases (kappa < 1 W m(-1) K(-1)) compared to the SSR compounds. High power factors combined with low thermal conductivity (leading to ZT values > 0.3) make these phases the best perovskitic candidates as n-type polycrystalline thermoelectric materials operating in air at high temperatures.