Thin La0 Research Articles

Collective Synergistic Catalysis of Electrochemical CO2 Reduction on Nonstoichiometric Double Perovskites

AbstractPerovskite oxides show great promise as an alternative catalyst to the conventional nickel cermets for CO2 reduction reactions (CO2RR) in solid oxide electrolysis cells (SOECs) owing to their advantages of redox stability and coking resistance. Nevertheless, practical applications of these oxides are prevented largely by their poor CO2RR activities. Herein, a novel donor and acceptor co‐doped nonstoichiometric double perovskite, La0.3Sr1.55Fe1.5Ni0.1Mo0.4O6−δ (LSFNM), is developed with in situ exsolved FeNi3 nanoparticles to efficiently catalyze CO2RR in SOECs. Pure CO2 electrolysis over the impregnated FeNi3@LSFNM catalysts is evaluated on two types of SOECs—one with thin (ZrO2)0.89(Sc2O3)0.1(CeO2)0.01 (SSZ) electrolytes supported on 430L alloys and the other with thin La0.9Sr0.1Ga0.8Mg0.2O3−δ (LSGM) electrolytes supported on impregnated SmBa0.5Sr0.5Co2O5+δ (SBSCO)@LSGM anodes, producing unprecedently high current densities of 2.84 A cm−2 for the former and 3.07 A cm−2 for the latter at 1.5 V and 800 °C. Experimental analysis and density‐functional theory (DFT) calculations reveal collective synergistic catalysis of oxygen vacancies (), the doping Ni2+ ions and FeNi3 nanoparticles via the cooperative ‐O(CO2), and Ni(II)–C(sp) and Ni(0)–O(CO2) interactions in LSFNM, not only facilitating CO2 chemisorption on oxygen vacancies but also destabilizing and dissociating surface carbonates in the vicinity of FeNi3 spontaneously into CO.

Mechanistic insights into photo-current enhancement in photo-active SrTiO3 heterojunctions under UV illumination

Photo-active heterojunctions were prepared by growing thin La0.9Sr0.1CrO3-δ films on SrTiO3 (STO) single crystals substrates via pulsed laser deposition. The photo-electrochemical characteristics of these heterojunctions under UV illumination were investigated by DC measurements and electrochemical impedance spectroscopy (EIS). Under UV illumination, the photo-current produced by these junctions increases over time by almost an order of magnitude to 1 to 2 mA cm-2 at 350 °C. Two effects governing this increase can be distinguished: A first process, independent of the surrounding atmosphere, is related to a stoichiometry polarization of the STO single crystal upon current flow. A second process is highly sensitive to contaminants in the surrounding atmosphere and is strongly accelerated in pure, synthetic atmospheres. We suggest that it is related to photo-oxidation of the STO single crystal. EIS revealed that both processes result in a decrease of both the electronic transport resistance through the STO single crystal and, to a lesser degree, the space charge resistance of the photo-active junction. Operando EIS under illumination and with DC bias was used to record the cells' photo-power characteristics and identify individual loss processes. Owing to the illumination induced strong increase of the STO single crystal conductivity, the photo-currents of such enhanced cells are limited by the space charge resistance.

Guided vortex motion in a ferromagnet-superconductor bilayer

Vortex motion in ferromagnetic-superconducting bilayers constituted by thin manganite La0.67Sr0.33MnO3 film deposited on the top of thin YBa2Cu3O7−δ film has been investigated. Both films were deposited on uniformly twinned single-crystalline substrates and patterned into strips directed at different angles to the twin lines pattern. These conditions lead to the guided vortex motion at different angles to the periodic pinning potential. It has been established that critical current and flux flow resistance in such an anisotropic pinning landscape-scale follow the elliptic dependence on the angle of the direction of the strip.

Investigation of Ni2+-doped ceria nanorods as the anode catalysts for reduced-temperature solid oxide fuel cells

Tailoring the surface chemistry of oxides has been widely used to adjust their catalytic behavior in the energy conversion and storage devices. Herein, nanorods of Ni2+-doped ceria (Ce1-xNixO2-δ, x = 0, 0.05, 0.1, 0.15) are synthesized via a modified hydrothermal method, and evaluated as the anode catalysts for reduced-temperature solid oxide fuel cells (SOFCs). X-Ray diffraction patterns of as-synthesized powders in air imply successful incorporation of Ni2+ into the fluorite lattice of ceria for x = 0.05 and 0.1, with a secondary phase of NiO observed for x = 0.15. Transmission electron microscopy (TEM) examination confirms a rod-like morphology with a diameter of 10–13 nm and a length of 55–105 nm. Exposure of these powders in H2 at 600°C results in exsolution of some spherical Ni particles of 11 nm in diameter. Electrochemical measurements on both symmetrical anode fuel cells and functioning cathode-supported fuel cells show an order of the catalytic activity toward hydrogen oxidation - CeO2-δ < Ce0·95Ni0·05O2-δ < Ce0·9Ni0·1O2-δ. The anode polarization resistances in 97% H2 – 3% H2O are 0.24, 0.31 and 0.37 Ω⋅cm2 for Ce0·9Ni0·1O2-δ, Ce0·95Ni0·05O2-δ and CeO2-δ at 600°C, respectively. Thin (La0·9Sr0.1) (Ga0.8Mg0.2)O3-δ-electrolyte fuel cells with nanostructured SmBa0.5Sr0·5Co2O5+δ cathodes and Ce0·9Ni0·1O2-δ anodes yield the highest power densities among the investigated series of anodes, e.g., 820 mW⋅cm−2 in 97% H2 – 3% H2O and 598 mW⋅cm−2 in 68% CH3OH - 32% N2. XPS analyses of reduced nanorods indicate that the highest catalytic activities of Ce0·9Ni0·1O2-δ toward fuel oxidation reactions should be correlated to the presence of highly active Ni nanoparticles and increased surface active oxygen, as confirmed by substantially facilitated extraction of the lattice oxygen on the surface by H2 in temperature-programmed reduction (TPR) measurements.

Structural and Electric Properties of Epitaxial Na0.5Bi0.5TiO3-Based Thin Films

Substantial efforts are dedicated worldwide to use lead-free materials for environmentally friendly processes in electrocaloric cooling. Whereas investigations on bulk materials showed that Na0.5Bi0.5TiO3 (NBT)-based compounds might be suitable for such applications, our aim is to clarify the feasibility of epitaxial NBT-based thin films for more detailed investigations on the correlation between the composition, microstructure, and functional properties. Therefore, NBT-based thin films were grown by pulsed laser deposition on different single crystalline substrates using a thin epitaxial La0.5Sr0.5CoO3 layer as the bottom electrode for subsequent electric measurements. Structural characterization revealed an undisturbed epitaxial growth of NBT on lattice-matching substrates with a columnar microstructure, but high roughness and increasing grain size with larger film thickness. Dielectric measurements indicate a shift of the phase transition to lower temperatures compared to bulk samples as well as a reduced permittivity and increased losses at higher temperatures. Whereas polarization loops taken at −100 °C revealed a distinct ferroelectric behavior, room temperature data showed a significant resistive contribution in these measurements. Leakage current studies confirmed a non-negligible conductivity between the electrodes, thus preventing an indirect characterization of the electrocaloric properties of these films.

Rapid Laser Processing of Thin Sr‐Doped LaCrO3–δ Interconnects for Solid Oxide Fuel Cells

Rapid laser reactive sintering (RLRS) is an additive manufacturing process which enables the quick and efficient fabrication of a wide range of ceramic‐based cells with various geometries and microstructures. Herein, the preparation of La0.8Sr0.2CrO3–δ interconnects by RLRS for solid‐oxide fuel/electrolysis cells (SOFC/ECs) is demonstrated. Uniform perovskite structure without residual intermediate phases can be achieved by CO2 laser irradiation at 103 W with a scanning speed of 0.07–0.10 mm s−1. Narrowing the width of the underlying MgO substrate and deposition of a terpineol‐based slurry with a solid loading level of less than 4 g m−2 are found to be critical parameters to avoid cracking and delamination. The optimum laser conditions balance Cr loss and densification of La0.8Sr0.2CrO3–δ resulting in an 11‐μm‐thick RLRS‐derived La0.8Sr0.2CrO3–δ film having a high relative density of 80–95% with a low area specific resistance (ASR) of 0.003 Ω cm2 at 600 °C. This ASR is more than 30 times lower than that of furnace‐sintered La0.8Sr0.2CrO3–δ in the same thickness range. The RLRS technique is promising for quick and efficient preparation of dense and thin La0.8Sr0.2CrO3–δ which are key components for highly compact SOFC/ECs.

Controlling antiferromagnetic domains in patterned La0.7Sr0.3FeO3 thin films

Transition metal oxide thin films and heterostructures are promising platforms to achieve full control of the antiferromagnetic (AFM) domain structure in patterned features as needed for AFM spintronic devices. In this work, soft x-ray photoemission electron microscopy was utilized to image AFM domains in micromagnets patterned into La0.7Sr0.3FeO3 (LSFO) thin films and La0.7Sr0.3MnO3 (LSMO)/LSFO superlattices. A delicate balance exists between magnetocrystalline anisotropy, shape anisotropy, and exchange interactions such that the AFM domain structure can be controlled using parameters such as LSFO and LSMO layer thickness, micromagnet shape, and temperature. In LSFO thin films, shape anisotropy gains importance only in micromagnets where at least one extended edge is aligned parallel to an AFM easy axis. In contrast, in the limit of ultrathin LSFO layers in the LSMO/LSFO superlattice, shape anisotropy effects dominate such that the AFM spin axes at micromagnet edges can be aligned along any in-plane crystallographic direction.

Ozone detection based on nanostructured [formula omitted] thin films

The present paper describes the synthesis of gas sensors based on thin films perovskite-type La0.8Pb0.1Ca0.1Fe0.8Co0.2O3 over a SiO2/Si substrate with interdigitated Pt-electrodes using the drop coating method. The perovskite starting nanoparticles have been prepared by the sol-gel method and they have an orthorhombic structure with Pnma space group. The prepared thin film has been tested as a sensitive film for ozone detection and shows a promising response. The detection of low concentration of ozone (equal to 50 ppb) makes the prepared thin film a potential sensor for ozone detection.

Passivation and activation of La0.6Sr0.4FeO3 thin film electrodes.

The oxygen exchange activity of thin dense La0.6Sr0.4FeO3 electrodes prepared by pulsed laser deposition was investigated by electrochemical impedance spectroscopy and electrical conductivity relaxation below 600 °C. The value of the surface exchange coefficient (kchem) measured at 491 °C decreased from an initial 4.4 × 10-6 cm s-1 to 1.7 × 10-7 cm s-1 after aging for 7 days. The rapid decrease in the oxygen exchange rate was accompanied by the increase in the surface concentration of 'non-lattice strontium' detected by X-ray photoelectron spectroscopy. Subsequent electrochemical tests over 40 days showed that the electrode performance could be recovered by rinsing the passivated electrode in deionized water. Repeated treatments eventually also led to improved stability of electrochemical performance.

Spin current and magnetic measurements in heterostructure SrIrO3/La0.7Sr0.3MnO3

The dc voltage on the film SrIrO3 (SIO) generated under ferromagnetic resonance condition has been studied in the heterostructure based on manganite thin epitaxial films La0.7Sr0.3MnO3 (LSMO). The SIO film is a paramagnetic metal with strong spin-orbit interaction in the temperature range 20-300K. The effect is shown to be caused by two different phenomena: (1) the resonance dc electromotive force related to anisotropic magnetoresistance (AMR) in the manganite film and (2) pure spin current (spin pumping) registered by means of the inverse spin Hall effect in upper layer. We also give the results of the magnetic parameters measuring for the heterostructure SIO/LSMO using ferromagnetic resonance in a wide temperature range.

Conductivity of Manganite Films under the Action of Tension Caused by the Deformation of Substrate

The electron transport properties of strained thin La0.7Ba0.3MnO3 (LBMO) epitaxial films are studied. Films 40–100 nm in thickness were prepared by laser ablation at a temperature T = 700–800°C in pure oxygen atmosphere of 0.3–1 mBar. Ferroelectric crystal substrates (011)0.79PbMg1/3Nb2/3O3–0.21-PbTiO3 (PMN–PT) with a Curie temperature of 150°C and high piezoelectric constants were used to create a mechanical stress. The ferroelectric polarization and piezoelectric effects on the electrical parameters of LBMO films are studied.

Hybrid graphene-manganite thin film structure for magnetoresistive sensor application

An increasing demand of magnetic field sensors with high sensitivity at room temperatures and spatial resolution at micro-nanoscales has resulted in numerous investigations of physical phenomena in advanced materials, and fabrication of novel magnetoresistive devices. In this study the novel magnetic field sensor based on combination of a single layer graphene (SLG) and thin nanostructured manganite La0.8Sr0.2MnO3 (LSMO) film—hybrid graphene-manganite (GM) structure, is proposed and fabricated. The hybrid GM structure employs the properties of two materials—SLG and LSMO—on the nanoscale level and results in the enhanced sensitivity to magnetic field of the hybrid sensor on the macroscopic level. Such result is achieved by designing the hybrid GM sensor in a Wheatstone half-bridge which enables to employ in the device operation two effects of nanomaterials—large Lorentz force induced positive magnetoresistance of graphene and colossal negative magnetoresistance of nanostructured manganite film, and significantly increase the sensitivity S of the hybrid GM sensor in comparison with the individual SLG and LSMO sensors: S = 5.5 mV T−1 for SLG, 14.5 mV T−1 for LSMO and 20 mV T−1 for hybrid GM at 0.5 T, when supply voltage was 1.249 V. The hybrid GM sensor operates in the range of (0.1–2.3) T and has lower sensitivity to temperature variations in comparison to the manganite sensor. Moreover, it can be applied for position sensing. The ability to control sensor’s characteristics by changing technological conditions of the fabrication of hybrid structure and tuning the nanostructure properties of manganite film is discussed.

Confinement of magnetism in atomically thin La0.7Sr0.3CrO3/La0.7Sr0.3MnO3 heterostructures

At crystalline interfaces where a valence-mismatch exists, electronic, and structural interactions may occur to relieve the polar mismatch, leading to the stabilization of non-bulk-like phases. We show that spontaneous reconstructions at polar La0.7Sr0.3MnO3 interfaces are correlated with suppressed ferromagnetism for film thicknesses on the order of a unit cell. We investigate the structural and magnetic properties of valence-matched La0.7Sr0.3CrO3/La0.7Sr0.3MnO3 interfaces using a combination of high-resolution electron microscopy, first principles theory, synchrotron X-ray scattering and magnetic spectroscopy and temperature-dependent magnetometry. A combination of an antiferromagnetic coupling between the La0.7Sr0.3CrO3 and La0.7Sr0.3MnO3 layers and a suppression of interfacial polar distortions are found to result in robust long-range ferromagnetic ordering for ultrathin La0.7Sr0.3MnO3. These results underscore the critical importance of interfacial structural and magnetic interactions in the design of devices based on two-dimensional oxide magnetic systems.

Temperature dependence of the magnetization of La0.67Sr0.33MnO3 thin films on LaAlO3

The authors report on the interplay between magnetically ordered phases with temperature and magnetic field across compressively strained interfaces of thin La0.67Sr0.33MnO3 films on LaAlO3 substrates. From the temperature dependence of the magnetization and resistivity studies, they find two distinct temperature regimes, where this interplay is clearly exhibited. They ascribe this to the strain induced Jahn–Teller-like distortion that favors the stabilization of the d3z2−r2 orbitals and enhances superexchange between adjoining Mn atoms. The temperature and field sweep of the magnetization and electronic transport lead to a hybridization between the closely spaced energy levels of d3z2−r2 and dx2−y2 orbitals leading to the coexistence of ferromagnetic and antiferromagnetic phases. Such an observation, not reported earlier, offers new routes for the design and study of magnetic textures in variously strained interfaces between perovskite oxides.

Temperature dependence of Gilbert damping in manganite/normal metal heterostructure

The temperature dependence of the spin-pumping effect on the Gilbert damping in a bilayer based on epitaxial manganite film grown on neodymium galate substrate was investigated by measuring of the linewidth of the ferromagnetic resonance spectrum (FMR). Ferromagnetic resonance in thin ferromagnetic manganite La0.7Sr0.3MnO3 (LSMO) films is used to produce a spin current at the interface between a metallic and a ferromagnetic layer. Pt/LSMO/NGO, Au/LSMO/NGO and LSMO/NGO heterostrucutres were measured. An increase of the linewidth in FMR spectrum at the temperature lower liquid nitrogen temperature (77K) is observed .

Magnetoresistance anisotropy of ultrathin epitaxial La0.83Sr0.17MnO3 films

We present the study of temperature dependence of resistivity (ρ), magnetoresistance (MR), and magnetoresistance anisotropy (AMR) of thin epitaxial La0.83Sr0.17MnO3 films. The films with thickness from 4 nm to 140 nm were grown on an NdGaO3 (001) substrate by a pulsed injection metal organic chemical vapor deposition technique. We demonstrate that the resistivity of these films significantly increases and the temperature Tm of the resistivity maximum in ρ(T) dependence decreases with the decrease of film thickness. The anisotropy of ρ(T) dependence with respect to the electrical current direction along the [100] or [010] crystallographic axis of the film is found for ultrathin films (4–8 nm) at temperatures close to Tm. Both MR and AMR, measured in magnetic fields up to 0.7 T applied in the film plane parallel and perpendicular to the current direction, have shown strong dependence on the film thickness. It was also found that the anisotropy of magnetoresistance could change its sign from positive (thicker films) to negative (ultrathin films) and obtain very small values at a certain intermediate thickness (20 nm) when the current is flowing perpendicular to the easy magnetization axis [010]. While the positive AMR effect was assigned to the conventional magnetic ordering of manganites, the AMR of ultrathin films was influenced by the pinning of magnetization to the easy axis. The temperature dependence and change of the AMR sign with film thickness is shown to be well described by the two-region model (more strained closer to the film substrate and more relaxed further from it) assuming that the relative concentration of both regions changes with the film thickness. The possibility to use the effect of the AMR compensation for the development of scalar in-plane magnetic field sensors is discussed.

Thickness dependence of microstructure in thin La0.7Sr0.3MnO3 films grown on (1 0 0) SrTiO3 substrate

This study investigates the nanostructure of perovskite thin films, and its influence on magnetic properties. Epitaxial thin films of perovskite MnO3 with thickness ranging from nm to 320 nm were deposited on SrTiO3 (1 0 0) substrates using pulsed laser deposition techniques. X-ray diffraction, along with high-resolution transmission electron microscopy (HRTEM) investigations were carried out, and showed the presence of a second, La-rich phase in thick film. Scanning electron microscopy analysis showed thinner films to have smooth surfaces, while thicker films presented large triangular shapes emerging out of the film surface. Also HRTEM images revealed epitaxial growth for thinner films. Thicker films grow with numerous misfit dislocations leading to columnar structure, where the thickest film presents branched structure with an intermediate La-rich layer. Lattice mismatch, diffusion of Mn ions into the substrate and substituting Ti ions are known as sources of strain in films near interfaces. Magnetic properties in the structure mentioned are described by double-exchange interaction, which is very sensitive to local structure. Hence, the nanostructure of films reported here has a remarkable effect on their magnetic properties.

Polycrystalline La1-xSrxMnO3 films on silicon: Influence of post-Deposition annealing on structural, (Magneto-)Optical, and (Magneto-)Electrical properties

The integration of La1-xSrxMnO3 (LSMO) thin film technology into established industrial silicon processes is regarded as challenging due to lattice mismatch, thermal expansion, and chemical reactions at the interface of LSMO and silicon. In this work, we investigated the physical properties of thin La0.73Sr0.27MnO3 films deposited by magnetron sputtering on silicon without a lattice matching buffer layer. The influence of a post-deposition annealing treatment on the structural, (magneto-)optical, and (magneto-)electrical properties was investigated by a variety of techniques. Using Rutherford backscattering spectroscopy, atomic force microscopy, Raman spectroscopy, and X-ray diffraction we could show that the thin films exhibit a polycrystalline, rhombohedral structure after a post-deposition annealing of at least 700°C. The dielectric tensor in the spectral range from 1.7eV to 5eV determined from spectroscopic ellipsometry in combination with magneto-optical Kerr effect spectroscopy was found to be comparable to that of lattice matched films on single crystal substrates reported in literature [1]. The values of the metal-isolator transition temperature and temperature-dependent resistivities also reflect a high degree of crystalline quality of the thermally treated films.

Tape Casting of High-Performance Low-Temperature Solid Oxide Cells with Thin La0.8Sr0.2Ga0.8Mg0.2O3-δ Electrolytes and Impregnated Nano Anodes.

Low-temperature solid oxide cells (LT-SOCs), operating at 400 to 650 °C, have great potential for commercialization since they can provide lower cost and improved long-term durability. Low operating temperature can also enable high round-trip efficiency of SOCs as reversible energy storage devices. This paper describes Sr0.8La0.2TiO3-α (SLT) anode supported LT-SOC with thin La0.8Sr0.2Ga0.8Mg0.2O3-δ (LSGM) electrolyte made by tape casting, with screen printed La0.6Sr0.4Fe0.8Co0.2O3-δ (LSCF) cathode and impregnated Ni anode. Optimization of the anode functional layers is described; the best anodes had 68 vol % LSGM and 12.3 vol % Ni and yielded maximum power density of 1.6 Wcm-2 with a cell area specific resistance (ASR) of 0.21 Ωcm2 at 650 °C. Most of the cell ASR was associated with the cathode. Reversible electrolysis and fuel cell operation yielded similar characteristics with both 50% H2-50% H2O and syngas fuel. Life testing over 500 h showed that the cathode impedance stabilized after an initial break-in period; the ohmic and anode resistances, though relatively small, increased slightly with time.

A new method for quantifying the nanoscale magnetic domains

Abstract In this work, a signal transformation method to quantitatively analyze fine magnetic structures in nanometer length scales measured by magnetic force microscopy has been developed. Nanosized magnetic domains, magnetic charges with reconstructed polarity as well as quantified magnetic field contours of samples (such as ordered FePt dot arrays, hard magnetic thin films and polycrystalline La0.7Sr0.3 MnO3 (LSMO) films ) were investigated based on the basic principles of deconvolution and micromagnetics. The present technique is crucial for the analysis of fine magnetic structures, and is important for the development of next generation magnetic recording industry.