一种乌青铜结构光催化剂：铁电-顺电相变与光催化性质

近年来,铁电材料作为一种潜在的高活性光催化剂材料越来越多地受到研究者关注.由于晶体结构的对称性破缺导致铁电晶体中出现自发极化,在这种极化内建电场的影响下,光生电子-空穴将发生空间分离,从而有效抑制载流子复合,进一步提高光催化体系的总量子效率.文献报道了很多性能优异的铁电光伏器件,在这样的思路下,越来越多的材料被直接作为光催化材料来研究,但有关顺电-铁电差异的报道很少,铁电性与光催化性能关联的直接证据尚有欠缺.研究表明, SrxBa1-xNb2O6(SBN)材料的居里温度(TC)对组分调变有很强的依赖性,当 Sr2+含量发生改变时(x =0.32-0.82),其居里温度可在10-270oC范围内变化.因此,合成一个居里温度接近室温的 SBN材料,研究其在顺/铁电相下自发极化行为、光生电荷分离行为、光催化行为及结构演变,就可能得到一个关于铁电性与光催化性能关系的直接证据,并有助于理解二者的关联性.本文以较低温度(65°C)下发生铁-顺电相变的 Sr0.7Ba0.3Nb2O6(SBN-70)材料为模型体系,使用X射线衍射、紫外可见光谱及不同温度下的铁电及介电测试、光电测试、光催化产氢和荧光激发谱等表征技术,研究了 SBN-70光电/光催化性能差异与相结构的相关性.首先使用高温固相合成法制备了纯相的四方钨青铜型 SBN-70材料,根据紫外可见漫反射光谱表征结果,推测 SBN-70材料在热力学上可发生光催化水分解反应.对 SBN-70进行的铁电及介电测试表明 SBN-70属于典型的弛豫型铁电材料：介电峰温度宽化显著,且介电峰位置随不同测试频率发生变化.在f =1 kHz测试下,其名义居里温度约为65oC.使用高温下两步烧结法制备了致密的 Ag/SBN-70/Ag陶瓷样品.受到高强度电场(E =30 kV/cm)极化的 Ag/SBN-70/Ag样品在紫外光照射下出现了显著光伏效应,铁电回线测试表明极化后的 SBN-70材料中存在强度约为0.8 kV/cm由剩余极化导致的内建电场,当经过极化的 SBN-70经过80oC退火后,光伏效应消失.结果表明, SBN-70极化后在内建极化场的作用下,在低温铁电相表现了显著的光生电荷分离能力,在进入高温顺电相后,这种分离能力随着自发极化的消失而消失.同时我们发现担载 Pt的 SBN-70粉末样品作为光催化剂在不同温度下存在很大的光催化产氢活性差异：反应在15oC时,产氢量为4.5μmol,随着反应温度升高,其产氢量升高,在60oC时为5.3μmol,继续升高温度至80oC时,发生了反常的活性变化,即产氢活性完全消失.这种反常的活性-温度关系可能与 SBN-70的铁-顺电相变有关：进入高温顺电相(80oC)后,晶体结构回到4/mmm点群,铁电畴结构不再存在,从而丧失了作为电荷分离驱动力的自发极化.不同温度下荧光激发谱结果也暗示了 SBN-70在高于65oC附近极化结构的消失.本文结果表明,铁电相中存在的铁电极化结构确实有利于光生电荷分离,进而提高了光催化反应活性.     

Adsorption kinetic, isotherm and thermodynamic studies of Sr2+ onto hexagonal tungsten oxide

Hexagonal tungsten oxide (hex-WO₃) with exchangeable sodium and ammonium cations located in hexagonal channel was synthesized by a facile hydrothermal treatment of sodium tungstate dihydrate in concentrated HCl solution in the presence of ammonium sulfate. An attempt was made to assess the potential of hex-WO₃ for the adsorption of Sr²⁺ ions from acidic radioactive waste solutions. Adsorption of Sr²⁺ reached equilibrium very quickly in 2 h in acidic aqueous solution. Maximum removal of Sr²⁺ ions occurred at pH 4. Equilibrium studies showed that the extent of Sr²⁺ ions uptake by hex-WO₃ was better described by the Freundlich isotherm in comparison with the Langmuir model. The thermodynamic parameters showed that the adsorption of Sr²⁺ ions onto hex-WO₃ was spontaneous and exothermic under the studied conditions.     

Crystal growth,structure determination and magnetism of a new hexagonal rhodate: Ba9Rh8O24

Single crystals of Ba9Rh8O24, grown from a molten potassium carbonate flux, crystallize in the spacegroup R3c with lattice parameters of a = 10.0899(4) and c = 41.462(2) A. Magnetic measurements on oriented single crystals reveal the existence of magnetic anisotropy.     

Unit-cell intergrowth of pyrochlore and hexagonal tungsten bronze structures in secondary tungsten minerals

Structural relations between secondary tungsten minerals with general composition A_x[(W,Fe)(O,OH)₃]_·yH₂O are described. Phyllotungstite (A=predominantly Ca) is hexagonal, , , space group P6₃/mmc. Pittongite, a new secondary tungsten mineral from a wolframite deposit near Pittong in Victoria, southeastern Australia (A=predominantly Na) is hexagonal, , , space group P-6m2. The structures of both minerals can be described as unit-cell scale intergrowths of (111)_py pyrochlore slabs with pairs of hexagonal tungsten bronze (HTB) layers. In phyllotungstite, the (111)_py blocks have the same thickness, 6 Å, whereas pittongite contains pyrochlore blocks of two different thicknesses, 6 and 12 Å. The structures can alternatively be described in terms of chemical twinning of the pyrochlore structure on (111)_py oxygen planes. At the chemical twin planes, pairs of HTB layers are corner connected as in hexagonal WO₃.     

Bond lengths and valences in tungsten oxides

Equations relating bond strength (valence) to bond length have been developed for tungsten-oxygen and phosphorus-oxygen bonds. Bond-valence sums have been carried out for the different tungsten atoms in phosphate tungsten bronzes and other mixed-valence tungsten oxides and for the Mo atoms in TeMo₅O₁₆. Valences intermediate between 5 and 6 are generally found, in agreement with physical measurements that indicate delocalization of d electrons in these materials. Evidence is presented that shows that the degree of distortion of WO₆ octahedra increases with apparent oxidation state of tungsten.     

Preparation, characterization and gas-sensitive properties of nano-crystalline Cr2O3Fe2O3 mixed oxides

A series of Fe₂-_xCr_xO₃(x = 0, 0.4, 0.8, 1.2, 1.6, 2.0) mixed oxides have been prepared with the chemical coprecipitation method and characterized by specific surface area, transmission electron microscopy (TEM), powder X-ray diffraction (XRD), IR and Mössbauer spectroscopy. Single-phase Fe-Cr mixed oxide nano-crystalline powders with corundum structure are obtained, and the results of the five characterization methods are well accordant with each other. Furthermore, gas-sensitive properties of the sensors made of the oxide powders have been studied.     

Crystal growth of new perovskite and perovskite related iridates: Ba3LiIr2O9, Ba3NaIr2O9, and Ba3.44K1.56Ir2O10

Single crystals of Ba₃LiIr₂O₉, Ba₃NaIr₂O₉, and Ba_3.44K_1.56Ir₂O₁₀ were grown from hydroxide fluxes. Ba₃LiIr₂O₉ and Ba₃NaIr₂O₉ form in the 6H–BaTiO₃ or triple perovskite structure, which is derived from the hexagonal and cubic stacking of [AO₃] layers. The structure contains face-sharing Ir₂O₉ octahedra pairs, which are connected via corner shared LiO₆ (NaO₆) octahedra. Both compounds crystallize in the space group P6₃/mmc, Z=2, with a=5.7804(4) and c=14.302(1) and a=5.866(4) and c=14.596(1) for the Li and Na member, respectively. The structure of Ba_3.44K_1.56Ir₂O₁₀ is derived from the stacking of [AO₃] and mixed [A₂O] layers, and is an n=3 member of the [A_nM_n−1O_3n][A₂O] family of hexagonal perovskite related oxides. The structure of Ba_3.44K_1.56Ir₂O₁₀ consists of (Ba₃Ir₂O₉) slabs separated by [(Ba,K)₂O] layers and is isostructural with Ba₅Ru₂O₁₀. The (Ba₃Ir₂O₉) slabs contain isolated, face-sharing Ir₂O₉ octahedra pairs. The compound crystallizes in the space group P6₃/mmc, Z=2, with a=5.91330(1) Å and c=18.1792(7) Å. The magnetic moments determined from the temperature dependence of the magnetic susceptibility are low for all three oxides, which is thought to be due to a combination of spin–orbit coupling and strong exchange interactions within the iridium octahedra pairs.     

Photoluminescence of perovskite nanosheets prepared by exfoliation of layered oxides, K2Ln2Ti3O10, KLnNb2O7, and RbLnTa2O7 (Ln: lanthanide ion)

Luminescent perovskite nanosheets were prepared by exfoliation of single- or double-layered perovskite oxides, K2Ln2Ti3O10, KLnNb2O7, and RbLnTa2O7 (Ln: lanthanide ion). The thickness of the individual nanosheets corresponded to those of the perovskite block in the parent layered compounds. Intense red and green emissions were observed in aqueous solutions with Gd1.4Eu0.6Ti3O10- and La0.7Tb0.3Ta2O7-nanosheets, respectively, under UV illumination with energies greater than the corresponding host oxide band gap. The coincidence of the excitation spectrum and the band gap absorbance indicates that the visible emission results from energy transfer within the nanosheet. The red emission intensity of the Gd1.4Eu0.6Ti3O10-nanosheets was much stronger than that of the La0.90Eu0.05Nb2O7-nanosheets reported previously. The strong emission intensity is a result of a two-step energy transfer cascade within the nanosheet from the Ti-O network to Gd(3+) and then to Eu(3+). The emission intensities of the Gd1.4Eu0.6Ti3O10- and La0.7Tb0.3Ta2O7-nanosheets can be modulated by applying a magnetic field (1.3-1.4 T), which brings about a change in orientation of the nanosheets in solution. The emission intensities increased when the excitation light and the magnetic field directions were perpendicular to each other, and they decreased when the excitation and magnetic field were collinear and mutually perpendicular to the direction of detection of the emitted light.     

Oxygen ionic transport in Bi2O3-based oxides: The solid solutions Bi2O3–Nb2O5

The minimum concentration of niobium to stabilize the fluorite-type f.c.c. phase in the Bi₂O₃–Nb₂O₅ oxide system at temperatures below 996 K was ascertained to be about 10 mol%. Thermal expansion, electrical conductivity and crystal lattice parameters of the Bi(Nb)O_1.5+δ solid solutions decrease with increasing niobium content. Thermal expansion coefficients were calculated from the dilatometric data to be (10.314.5)×10⁻⁶ K⁻¹ at temperatures in the range 300–700 K and (17.526.0)×10⁻⁶ K⁻¹ at 700–1100 K. The conductivity of the Bi_{1− x} Nb _x O_1.5+δ ceramics is predominantly ionic. The p-type electronic transference numbers of the Bi(Nb)O_1.5+δ solid solutions in air were determined to be less than 0.1. Annealing at temperatures below 900 K results in a sharp decrease in conductivity of the Bi_{1− x} Nb _x O_1.5+δ ceramics. Received: 18 August 1997 / Accepted: 20 October 1997     

First-principles calculations of the structural and dynamic properties, and the equation of state of crystalline iodine oxides I2O4, I2O5, and I2O6

The structural and dynamical correlations, and the equation of state of crystalline I(2)O(4), I(2)O(5), and I(2)O(6) are investigated by first-principles calculations based on the density functional theory (DFT). The lattice dynamics results reveal distinctive features in the phonon density of states among the three crystals. The frequencies of the stretch modes in I(2)O(4) and I(2)O(5) are clearly separated from those of the other (e.g., bending) modes by a gap, with all stretch modes above the gap. In contrast, the gap in I(2)O(6) separates the highest-frequency stretch modes with other stretch modes, and there is no gap between the stretch and the other modes in I(2)O(6). The motion of iodine atoms is involved in all vibrational modes in I(2)O(5), but only in low-frequency lattice modes in I(2)O(6). In I(2)O(4), iodine atoms are involved in modes with frequency below 700 cm(-1). Van der Waals correction within our DFT calculations is found to reduce the overestimation of the equilibrium volume, with its effect on structure similar to the pressure effect. Namely, both effects significantly decrease the inter-molecular distances, while slightly increasing the bond lengths within the molecules. This causes the frequencies of some vibrational modes to decrease with pressure, resulting in negative "modes Gru?neisen parameters" for those modes. Thermodynamic properties, derived from the equation of state, of crystalline I(2)O(4), I(2)O(5), and I(2)O(6) are discussed within the quasi-harmonic approximation.     

Pd2+/Pd0 redox cycling in hexagonal YMn(0.5)Fe(0.5)O3: implications for catalysis by PGM-substituted complex oxides

Complex oxides--containing at least two different cations on crystallographically distinct sites--have recently been shown to display redox cycling of platinum group metals (PGMs), such as Pd; for example, Pd-substituted complex oxides can reversibly extrude metallic Pd under reducing conditions and then reincorporate Pd(2+) ions into the lattice under oxidizing conditions. The title compounds, YMn(0.5)Fe(0.5-x)Pd(x)O(3-δ) (0 ≤ x ≤ 0.07) crystallizing in the noncentrosymmetric YMnO(3) structure, were prepared using a sol-gel process at 800 °C, and the structures were refined from high-resolution synchrotron X-ray powder diffraction data. Their redox cycling behavior was monitored using synchrotron X-ray diffraction and EXAFS studies. In contrast to the previously studied complex oxide host compounds, YMn(0.5)Fe(0.5-x)Pd(x)O(3-δ) is only modestly tolerant to cycling: repeated redox cycling leads to the formation of PdO, which, on the time-scale of the oxidation cycles, does not reincorporate in the complex oxide lattice. Both oxidized and reduced samples were tested for the oxidation of CO to CO(2) under CO-lean conditions. YMn(0.5)Fe(0.5-x)Pd(x)O(3-δ) performs essentially as well as previously studied YFe(1-x)Pd(x)O(3-δ). The CO oxidation light-off characteristics of the hexagonal hosts are very similar to finely dispersed PdO. Despite evidence that Pd is almost fully dispersed as divalent ions in the host lattice, which is presumably accompanied by the concurrent creation of oxygen vacancies (2 Pd(2+):1 V(O(2-))), the as-prepared hexagonal materials do not display any significant improvement in catalytic activity as a function of Pd substitution level. This suggests that the corner-connected trigonal bipyramids that characterize this structural family do not enable the transport of oxygen through the bulk of the lattice. The study casts light on factors in the solid-state chemistry of precious metal-substituted complex oxides that influence the efficacy of redox cycling of the precious metal, and catalytic performance.     

FT-IR and FT-FIR studies of vanadium,molybdenum and tungsten oxides supported on different carriers

In the far IR region at low molybdenum loadings, Mo-SiO₂ catalysts present a pseudomolybdate or a polymolybdate species, while bulk-like MoO₃ appears at loadings close to the geometrical monolayer coverage. W-SiO₂ and V-SiO₂ spectra show bands close to those observed on the corresponding bulk oxides.In the case of TiO₂, Al₂O₃, ZrO₂ supported catalysts, a band is observed near 1000 cm^–1 which is assigned to the Mo=O stretching vibration of coordinatively unsaturated Mo ⁿ⁺ ions showing a stronger interaction with the support than one observed on silica.     

Depletion kinetics of low-lying states of tungsten in the presence of NO,N2O,and SO2

The gas-phase reactivities of W(a⁵D_J, a⁷S₃) with N₂O, SO₂, and NO in the temperature range of 295–573 K are reported. Tungsten atoms produced by the photodissociation of W(CO)₆. The tungsten atoms were detected by a laser-induced fluorescence technique. The removal rate constants for the 6s²5d⁴ a⁵D_l states were found to be pressure dependent for all of the reactants. Removal rate constants for the 6s¹5d⁵ a⁷S₃ state were found to be fast compared to the a⁵D_J states and often approached the gas kinetic rate constant. The reaction rates for all the states were found to be pressure independent with respect to the total pressure. Results are discussed in terms of the different electronic configurations of the states of tungsten © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 367–375 1997