Showing papers on "Perovskite (structure) published in 2009"

Multiferroic Behavior Associated with an Order−Disorder Hydrogen Bonding Transition in Metal−Organic Frameworks (MOFs) with the Perovskite ABX3 Architecture

Abstract: Multiferroic behavior in perovskite-related metal−organic frameworks of general formula [(CH3)2NH2]M(HCOO)3, where M = Mn, Fe, Co, and Ni, is reported. All four compounds exhibit paraelectric−antiferroelectric phase transition behavior in the temperature range 160−185 K (Mn: 185 K, Fe: 160 K; Co: 165 K; Ni: 180 K); this is associated with an order−disorder transition involving the hydrogen bonded dimethylammonium cations. On further cooling, the compounds become canted weak ferromagnets below 40 K. This research opens up a new class of multiferroics in which the electrical ordering is achieved by means of hydrogen bonding.

Dielectric and Piezoelectric Properties in Mn‐Modified (1−x)BiFeO3–xBaTiO3 Ceramics

Abstract: In the current work, the bulk (1−x)BiFeO3–xBaTiO3 system has been studied as a potential lead-free piezoelectric material. Barium titanate (BaTiO3) in solid solution with bismuth ferrite (BiFeO3) is observed to stabilize the perovskite structure and improve switching behavior. Samples with various content of BaTiO3 were prepared via solid-state route, and pure perovskite phase was confirmed by X-ray diffraction. Modification of the BaTiO3–BiFeO3 material with Mn improved DC resistivity by one to five orders of magnitude (7.6 × 1012 vs. 2.7 × 107Ω·m for 25 mol% BaTiO3 at room temperature) and polarization hysteresis measurements indicated “hard” ferroelectric behavior with the highest strain response at 33 mol% BaTiO3. Finally, low-field piezoelectric d33 coefficient of 116 pC/N and ferroelectric transition temperature above 450°C are reported for 25 mol% BaTiO3 composition.

Temperature-induced A–B intersite charge transfer in an A-site-ordered LaCu 3 Fe 4 O 12 perovskite

Abstract: The introduction of 'foreign' elements into transition-metal oxides (called chemical doping) can change the valence state of the metal's cations and hence modify the physical properties of the material as a whole. These changes can be dramatic, for example causing high-temperature superconductivity in copper oxides and colossal magnetoresistance in manganese oxides. Youwen Long et al. have identified an oxide system, the perovskite LaCu3Fe4O12, in which changes in valence state occur when charge is shuffled between different cations (iron and copper) in the host structure, rather than via doping. As a result, the material can be reversibly transformed from one possessing iron in an unusually high Fe3.75+ state (partnered with fairly common Cu2+ ions) to one possessing rare Cu3+ ions. These changes are reflected in the magnetic and electronic properties of the material and, intriguingly, the material contracts slightly on being warmed through the transition. The temperature sensitivity of this effect makes it a strong candidate for technological applications. This paper identifies an oxide system where changes in valence state occur as a result of charge being shuffled between different cations in the host structure, rather than via doping, this charge transfer being sensitive to temperature. As a result, the material can be reversibly transformed from one possessing iron in an unusually high Fe3.75+ state to one possessing rare Cu3+ ions. These changes are reflected in the magnetic and electronic properties of the material and, intriguingly, are accompanied by negative thermal expansion. Changes of valence states in transition-metal oxides often cause significant changes in their structural and physical properties1,2. Chemical doping is the conventional way of modulating these valence states. In ABO3 perovskite and/or perovskite-like oxides, chemical doping at the A site can introduce holes or electrons at the B site, giving rise to exotic physical properties like high-transition-temperature superconductivity and colossal magnetoresistance3,4. When valence-variable transition metals at two different atomic sites are involved simultaneously, we expect to be able to induce charge transfer—and, hence, valence changes—by using a small external stimulus rather than by introducing a doping element. Materials showing this type of charge transfer are very rare, however, and such externally induced valence changes have been observed only under extreme conditions like high pressure5,6. Here we report unusual temperature-induced valence changes at the A and B sites in the A-site-ordered double perovskite LaCu3Fe4O12; the underlying intersite charge transfer is accompanied by considerable changes in the material’s structural, magnetic and transport properties. When cooled, the compound shows a first-order, reversible transition at 393 K from LaCu2+3Fe3.75+4O12 with Fe3.75+ ions at the B site to LaCu3+3Fe3+4O12 with rare Cu3+ ions at the A site. Intersite charge transfer between the A-site Cu and B-site Fe ions leads to paramagnetism-to-antiferromagnetism and metal-to-insulator isostructural phase transitions. What is more interesting in relation to technological applications is that this above-room-temperature transition is associated with a large negative thermal expansion.

Domain Engineering for Enhanced Ferroelectric Properties of Epitaxial (001) BiFeO Thin Films

Abstract: Adv. Mater. 2009, 21, 817–823 2009 WILEY-VCH Verlag Gm Multiferroic BiFeO3 has attracted great interest due to its promising application tomagnetoelectric devices. In addition, the high remanent polarization and piezoelectric response of BiFeO3 thin films, which are comparable to those of conventional Ti-rich lead zirconia titanate, suggested BiFeO3 as a strong candidate for lead-free nonvolatile memories. BiFeO3 has a rhombohedral perovskite structure with pseudocubic lattice parameters ar1⁄4 3.96 A and ar1⁄4 0.68. Due to this low symmetry, (001)-oriented epitaxial BiFeO3 films possess the rhombohedral distortion along one of the four (111) crystallographic directions of the pseudocubic perovskite unit cell. Thus, eight possible polarization (ferroelectric) variants, which correspond to four structural (ferroelastic) domains, may form in the films, leading to complex domain patterns with both {100} and {101} twin boundaries. Such a complex domain structure can deteriorate the ferroelectric response of the system by external electric field, and complicates the examination of the coupling between magnetic and ferroelectric order parameters in BiFeO3. [3]

Materials design for perovskite SOFC cathodes

Abstract: This article focuses on perovskite materials for application as cathode material in solid oxide fuel cells. In order to develop new promising materials it is helpful to classify already known perovskite materials according to their properties and to identify certain tendencies. Thereby, composition-dependent structural data and materials properties are considered. Structural data under consideration are the Goldschmidt tolerance factor, which describes the stability of perovskites with respect to other structures, and the critical radius and lattice free volume, which are used as geometrical measures of ionic conductivity. These calculations are based on the ionic radii of the constituent ions and their applicability is discussed. A potential map of perovskites as a tool to classify simple ABO3 perovskite materials according to their electrical conduction behavior is critically reviewed as a structured approach to the search for new cathode materials based on more complex perovskites with A and/or B-site substitutions. This article also covers the approaches used to influence electronic and the ionic conductivity. The advantage of mixed ionic electronic conductors in terms of the oxygen exchange reaction is addressed and their important properties, namely the oxygen-exchange coefficient and the oxygen diffusion coefficient, and their effect on the oxygen reduction reaction are presented.

Structural diversity and retro-crystal engineering analysis of iodometalate hybrids

Abstract: With guidance from retro-crystal engineering, iodometalate structures based on MI6 octahedra of group 14 (M = Sn(II), Pb(II)) and group 15 (M = Sb(III) and Bi(III)) are analysed. The criterion of I/M ratio, with the function of indicating the degree of condensation of octahedra in inorganic networks and the average charge density at the organic–inorganic interface, is introduced to classify all of the iodometalate networks, resulting in an easy and clear way to identify isomers with different dimensionalities. Of all iodometalates, the 2D M(II)I4 anion derived from the perovskite network is special since it can be easily stabilized by a range of common organic cations. We provide here the up-to-date progress in this extensively studied field, focusing on crystal engineering of hybrids in the aim of getting materials with a reduced band gap. Relationships between the molecular layouts of cationic entities and the structures of several non-perovskite anionic networks, focusing on the organic–inorganic interface, are highlighted. Distinct dependences between different types of cations and different types of anions are revealed, although it is still unfeasible to apply them in the actual control, design, or prediction of specific hybrid structures.

Water splitting into H2 and O2 over niobate and titanate photocatalysts with (111) plane-type layered perovskite structure

Abstract: Photophysical and photocatalytic properties of A5Nb4O15 (A = Sr and Ba), Ba3LaNb3O12, ALa4Ti4O15 (A = Ca, Sr, and Ba), and La4Ti3O12 with layered perovskite structures, in which a plane in parallel with (111) of a simple perovskite structure was exposed at interlayer, were investigated. These oxides were obtained by a polymerizable complex method at 973–1473 K though only A5Nb4O15 (A = Sr and Ba) were prepared by a solid state reaction even at 1673 K. The shapes of these complex metal oxides were plate-like derived from the perovskite layered structure. These band gaps were estimated to be 3.7–4.1 eV from the onsets of diffuse reflection spectra. These oxides showed photoluminescence at 77 K. These oxides loaded with NiO cocatalysts showed activities for water splitting under UV irradiation. NiOx/BaLa4Ti4O15 and NiOx/Ba5Nb4O15 showed the highest activities among the titanates and niobates tested in the present study. NiOx/BaLa4Ti4O15 and NiOx/Ba5Nb4O15 gave 15% and 17% of quantum yields at 270 nm, respectively. Photocatalytic activities of ALa4Ti4O15 (A = Ca, Sr, and Ba) strongly depended on the alkaline earth metal ion. Pt, Au, Ni, and PbO2 were selectively photodeposited on basal or edge plane of the BaLa4Ti4O15 plate-like powder while these were randomly loaded on CaLa4Ti4O15. It was suggested that this difference in the surface property was the one of the important factors affecting photocatalytic ability for ALa4Ti4O15 (A = Ca, Sr, and Ba).

Metal–Organic Perovskites: Synthesis, Structures, and Magnetic Properties of [C(NH2)3][MII(HCOO)3] (M=Mn, Fe, Co, Ni, Cu, and Zn; C(NH2)3= Guanidinium)

Abstract: We report the synthesis, crystal structures, and spectral, thermal, and magnetic properties of a family of metal-organic perovskite ABX(3), [C(NH(2))(3)][M(II)(HCOO)(3)], in which A = C(NH(2))(3) is guanidinium, B = M is a divalent metal ion (Mn, Fe, Co, Ni, Cu, or Zn), and X is the formate HCOO(-). The compounds could be synthesized by either diffusion or hydrothermal methods from water or water-rich solutions depending on the metal. The five members (Mn, Fe, Co, Ni, and Zn) are isostructural and crystallize in the orthorhombic space group Pnna, while the Cu member in Pna2(1). In the perovskite structures, the octahedrally coordinated metal ions are connected by the anti-anti formate bridges, thus forming the anionic NaCl-type [M(HCOO)(3)](-) frameworks, with the guanidinium in the nearly cubic cavities of the frameworks. The Jahn-Teller effect of Cu(2+) results in a distorted anionic Cu-formate framework that can be regarded as Cu-formate chains through short basal Cu-O bonds linked by the long axial Cu-O bonds. These materials show higher thermal stability than other metal-organic perovskite series of [AmineH][M(HCOO)(3)] templated by the organic monoammonium cations (AmineH(+)) as a result of the stronger hydrogen bonding between guanidinium and the formate of the framework. A magnetic study revealed that the five magnetic members (except Zn) display spin-canted antiferromagnetism, with a Neel temperature of 8.8 (Mn), 10.0 (Fe), 14.2 (Co), 34.2 (Ni), and 4.6 K (Cu). In addition to the general spin-canted antiferromagnetism, the Fe compound shows two isothermal transformations (a spin-flop and a spin-flip to the paramagnetic phase) within 50 kOe. The Co member possesses quite a large canting angle. The Cu member is a magnetic system with low dimensional character and shows slow magnetic relaxation that probably results from the domain dynamics.

Double-Perovskite Anode Materials Sr2MMoO6 (M = Co, Ni) for Solid Oxide Fuel Cells

Abstract: Double-perovskites Sr2MMoO6 (M = Co, Ni) have been investigated as anode materials for a solid oxide fuel cell. At room temperature, both Sr2CoMoO6 and Sr2NiMoO6 are tetragonal (I4/m). X-ray absorption spectroscopy confirmed the presence of Co2+/Mo6+ and Ni2+/Mo6+ pairs in the oxygen-stoichiometric compounds. The samples contain a limited concentration of oxygen vacancies in the reducing atmospheres at an anode. Reoxidation is facile below 600 °C; they become antiferromagnetic at low temperatures TN = 37 and 80 K for M = Co and Ni, respectively. As an anode with a 300 μm thick La0.8Sr0.2Ga0.83Mg0.17O2.815 electrolyte and SrFe0.2Co0.8O3−δ as a cathode, Sr2CoMoO6 exhibited maximum power densities of 735 mW/cm2 in H2 and 527 mW/cm2 in wet CH4 at 800 °C; Sr2NiMoO6 shows a notable power output only in dry CH4. The high performance of Sr2CoMoO6 in wet CH4 may be due to its catalytic effect on steam reforming of methane, but some degradation of the structure that occurred in CH4 obscures identification of the ca...

Superconductivity at 17 K in (Fe2P2)(Sr4Sc2O6): a new superconducting layered pnictide oxide with a thick perovskite oxide layer

Abstract: A new layered pnictide oxide (Fe2P2)(Sr4Sc2O6) has been synthesized by solid-state reaction. This material has an alternating layer stacking structure of anti-fluorite Fe2P2 and perovskite-based Sr4Sc2O6 oxide layers. The space group of the material is P4/nmm and the lattice constants a and c are 4.016 and 15.543 A, respectively. The interlayer Fe–Fe distance corresponding to the c-axis length is the longest ever reported in the iron-based pnictides. In both magnetization and resistivity measurements, the present compound exhibited superconductivity below 17 K; this temperature is much higher than that for LaFePO and the highest in arsenic-free iron-based pnictide systems under ambient pressure.

Optical spectroscopy of two-dimensional layered $(C_{6}H_{5}C_{2}H_{4}-NH_{3})_{2}-PbI_{4}$ perovskite

Abstract: We report on optical spectroscopy (photoluminescence and photoluminescence excitation) on two-dimensional self-organized layers of $(C_{6}H_{5}C_{2}H_{4}-NH_{3})_{2}PbI_{4}$ perovskite. Temperature and excitation power dependance of the optical spectra gives a new insight into the excitonic and phononic properties of this hybrid organic/inorganic semiconductor. In particular, exciton-phonon interaction is found to be more than one order of magnitude higher than in GaAs QWs. As a result, photoluminescence emission lines have to be interpreted in the framework of a polaron model.

La(1−x)SrxCo1−yFeyO3 perovskites prepared by sol–gel method: Characterization and relationships with catalytic properties for total oxidation of toluene

Abstract: La(1-x)SrxCo(1-y)FeyO3 samples have been prepared by sol-gel method using EDTA and citric acid as complexing agents. For the first time, Raman mappings were achieved on this type of samples especially to look for traces of Co3O4 that can be present as additional phase and not detect by XRD. The prepared samples were pure perovskites with good structural homogeneity. All these perovskites were very active for total oxidation of toluene above 200 degrees C. The ageing procedure used indicated good thermal stability of the samples. A strong improvement of catalytic properties was obtained substituting 30% of La3+ by Sr2+ cations and a slight additional improvement was observed substituting 20% of cobalt by iron. Hence, the optimized composition was La0.7Sr0.3Co0.8Fe0.2O3. The samples were also characterized by BET measurements, SEM and XRD techniques. Iron oxidation states were determined by Mossbauer spectroscopy. Cobalt oxidation states and the amount of O- electrophilic species were analyzed from XPS achieved after treatment without re-exposition to ambient air. Textural characterization revealed a strong increase in the specific surface area and a complete change of the shape of primary particles substituting La3+ by Sr2+. The strong lowering of the temperature at conversion 20% for the La0.7Sr0.3Co(1-y)FeyO3 samples can be explained by these changes. X photoelectron spectra obtained with our procedure evidenced very high amount of O- electrophilic species for the La0.7Sr0.3Co(1-y)FeyO3 samples. These species able to activate hydrocarbons could be the active sites. The partial substitution of cobalt by iron has only a limited effect on the textural properties and the amount of O- species. However, Raman spectroscopy revealed a strong dynamic structural distortion by Jahn-Teller effect and Mossbauer spectroscopy evidenced the presence of Fe4+ cations in the iron containing samples. These structural modifications could improve the reactivity of the active sites explaining the better specific activity rate of the La0.7Sr0.3Co0.8Fe0.2O3 sample. Finally, an additional improvement of catalytic properties was obtained by the addition of 5% of cobalt cations in the solution of preparation. As evidenced by Raman mappings and TEM images, this method of preparation allowed to well-dispersed small Co3O4 particles that are very efficient for total oxidation of toluene with good thermal stability contrary to bulk Co3O4. (C) 2008 Elsevier B.V. All rights reserved.

Perovskite-type mixed oxides as catalytic material for NO removal

Abstract: Perovskite-type mixed oxides, with ABO3 or A2BO4 structure, are promising materials as catalysts for NO removal due to their low cost, high thermal stability and of course, good catalytic performances. Their unique structural properties (e.g. A- and B-site cations could be replaced by a foreign one without destroying the matrix structure) make it possible to study the correlations between structure properties and catalytic performances. In this work the effects of solid-state and electrochemical properties of perovskite-type catalyst on NO removal activity is reviewed. Also, current experiments regarding the catalytically active site and the reaction mechanism of NO removal on perovskite-type mixed oxides are compared and discussed.

Superconductivity at 17 K in (Fe2P2)(Sr4Sc2O6): a new superconducting layered pnictide oxide with a thick perovskite oxide layer

Abstract: A new layered oxypnictide (Fe2P2)(Sr4Sc2O6) have been synthesized by solid-state reaction. This material has an alternating layer stacking structure of anti-fluorite Fe2P2 and perovskite-based Sr4Sc2O6 oxide layers. Space group of the material is P4/nmm and lattice constants a and c are 4.016 A and 15.543 A, respectively. The interlayer Fe-Fe distance corresponding to the c-axis length is the longest ever reported in the iron-based oxypnictide systems. In both magnetization and resistivity measurements, the present compound exhibited superconductivity below 17 K, which is much higher than that of LaFePO and the highest in arsenic-free iron-based oxypnictide systems under ambient pressure.

Relaxor-PT single crystals: Observations and developments

Abstract: Relaxor-PT based ferroelectric single crystals Pb(Zn1/3Nb2/3)O3-PbTiO3 (PZNT) and Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMNT) offer high performance with ultra-high electromechanical coupling factors k33s > 0.9 and piezoelectric coefficients d33s > 1500 pC/N. However, the usage temperature range of these perovskite single crystals is limited by TRT-the rhombohedral to tetragonal phase transition temperature, which occurs at significantly lower temperatures than the Curie temperature TC, a consequence of curved morphotropic phase boundaries (MPBs). Furthermore, these 〈001〉-oriented crystals exhibit low mechanical quality Q and coercive fields, restricting their usage in high-power applications. In this survey, recent developments on binary and ternary perovskite relaxor-PT crystal systems are reviewed with respect to their temperature usage range. General trends of dielectric and piezoelectric properties of relaxor-PT crystal systems are discussed in relation to their respective TC/TRT. In addition, two approaches have been implemented to improve mechanical Q, including acceptor dopants, analogous to hard polycrystalline ceramics, and anisotropic domain engineering, enabling lowloss crystals with high coupling for high-power applications.

Perovskite oxide, process for producing the perovskite oxide, and piezoelectric device

Abstract: A process for producing a piezoelectric oxide having a composition (Ba, Bi, A)(Ti, Fe, M)O 3 , where each of A and M represents one or more metal elements. The composition is determined so as to satisfy the conditions (1) and (2), 0.98≦TF(P)≦1.02, (1) TF(BiFeO 3 )

Enhanced effective mass in doped SrTiO3 and related perovskites

Abstract: The effective mass is one of the main factors determining the Seebeck coefficient and electrical conductivity of thermo-electrics. In this ab-initio LDA-GGA study the effective mass is estimated from the curvature of electronic bands by one-band-approximation and is in excellent agreement with experimental data of Nb- and La-doped SrTiO 3 . It is clarified that the deformation of SrTiO 3 crystals has a significant influence on the bandgap, effective electronic DOS-mass and band-mass, but the electronic effect due to the e g -band flattening near the Γ -point due to Nb-doping up to 0.2 at% is the main factor for the effective mass increase. Doping of La shows a linear decrease of the effective mass; this can be explained by the different surroundings of A- and B-sites in perovskite. Substitution with other elements such as Ba on the A-site and V on the B-site in SrTiO 3 increases the effective mass as well.

Elastically driven anisotropic percolation in electronic phase-separated manganites

Abstract: Complex oxide films are highly anisotropic in the way they conduct electricity, which is due to phase separation. However, the origin of this metal–insulator phase coexistence has been unclear. Transport measurements now show that strain, rather than chemical inhomogeneity, is mainly responsible. The presence of electronic phase separation in complex materials has been linked to many types of exotic behaviour, such as colossal magnetoresistance, the metal–insulator transition and high-temperature superconductivity1,2,3,4; however, the mechanisms that drive the formation of coexisting electronic phases are still debated5,6,7,8. Here we report transport measurements that show a preferential orientation of electronic phase domains driven by anisotropic long-range elastic coupling between a complex oxide film and substrate. We induce anisotropic electronic-domain formation along one axis of a pseudocubic perovskite single-crystal thin-film manganite by epitaxially locking it to an orthorhombic substrate. Simultaneous temperature-dependent resistivity measurements along the two perpendicular in-plane axes show substantial differences in the metal–insulator transition temperature and extraordinarily high anisotropic resistivities, which indicate that percolative conduction channels open more readily along one axis. These findings suggest that the origin of phase coexistence is much more strongly influenced by strain than by local chemical inhomogeneity.

Perovskite oxide for solid oxide fuel cells

Abstract: Structure and Properties of Perovskite Oxides.- Overview of Intermediate-Temperature Solid Oxide Fuel Cells.- Ionic Conduction in Perovskite-Type Compounds.- Oxide Ion Conductivity in Perovskite Oxide for SOFC Electrolyte.- Diffusivity of the Oxide Ion in Perovskite Oxides.- Structural Disorder, Diffusion Pathway of Mobile Oxide Ions, and Crystal Structure in Perovskite-Type Oxides and Related Materials.- Perovskite Oxide for Cathode of SOFCs.- Perovskite Oxide Anodes for SOFCs.- Intermediate-Temperature Solid Oxide Fuel Cells Using LaGaO3.- Quick-Start-Up Type SOFC Using LaGaO3-Based New Electrolyte.- Proton Conductivity in Perovskite Oxides.- Proton Conduction in Cerium- and Zirconium-Based Perovskite Oxides.- Mechanisms of Proton Conduction in Perovskite-Type Oxides.- Intermediate-Temperature SOFCs Using Proton-Conducting Perovskite.- LaCrO3-Based Perovskite for SOFC Interconnects.

High-Performance Piezoelectric Single Crystals of Complex Perovskite Solid Solutions

Abstract: Relaxor-based single crystals of complex perovskite solid solutions, Pb(Mg1/3Nb2/3)O3–PbTiO3 [PMN–PT] and Pb(Zn1/3Nb2/3)O3–PbTiO3 [PZN–PT], exhibit extraordinary piezoelectric performance, with extremely high piezoelectric coefficients, very large electromechanical coupling factors, and exceptionally high strain levels. These materials outperform the currently used Pb(Zr1–xTix)O3 [PZT] ceramics, making them the materials of choice for the next generation of electromechanical transducers for a broad range of advanced applications. In this article, recent major advances in the development of piezocrystals are reviewed in terms of crystal growth, piezoelectric properties, crystal chemistry, domain structure, and device applications.

Direct Decomposition of Nitrous Oxide to Nitrogen by In Situ Oxygen Removal with a Perovskite Membrane

Abstract: Direct decomposition of N(2)O to N(2) using perovskite hollow fiber membranes is achieved by combination with in situ oxygen removal (see picture). A coupled partial methane oxidation allows N(2)-free synthesis gas to be obtained. This sustainable process combines N(2)O removal with the simultaneous production of valuable chemicals.

Tuning the orthorhombic–rhombohedral phase transition temperature in sodium potassium niobate by incorporating barium zirconate

Abstract: We report that the orthorhombic–rhombohedral phase transition temperature in sodium potassium niobate could be tuned from lower to higher than room temperature by incorporating barium zirconate into it. It is revealed that when barium zirconate is between 8 mol% and 15 mol%, the solid solution exhibits rhombohedral symmetry at room temperature. Rietveld analysis shows that the space group of the rhombohedral phase is R3m. It is suggested that a relatively larger B-cation favors the low-temperature phase which is a common phenomenon for perovskite ferroelectrics. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoelectrochemical Properties of Crystalline Perovskite Lanthanum Titanium Oxynitride Films under Visible Light

Abstract: Thin films of the Lanthanum Titanium oxynitride perovskite (LaTiOxNy) synthesized by reactive radio-frequency sputtering on conductive Nb-doped Strontium Titanate (Nb:SrTiO3) substrates are evaluated as photoelectrodes for water splitting reaction under visible light. The films are characterized by x-ray diffraction analysis, energy-dispersive spectroscopy, scanning electron and atomic force microscopy, and ultraviolet-visible spectroscopy. Thin films with polycrystalline, oriented, or epitaxial structures are obtained depending on the substrate temperature and plasma composition. The band-gap energies of the films thus prepared are in the range 2.05 - 2.35 eV. Photoelectrochemical measurements reveal that the photoactivity of these films increases with the quality of film crystallization. The flat-band position of LaTiOxNy is found to change with pH of the reactant solution. Surface modification with colloidal IrO2 is also demonstrated to result in a marked increase in photoactivity, with the modified epitaxial LaTiOxNy film exhibiting a photocurrent density of ca. 70 A cm 2 at +1.0 V vs. Ag/AgCl at pH 4.5 in aqueous Na2SO4 solution under irradiation at visible wavelengths ( > 420 nm).