Search Results (27)

Cobalt-based perovskite oxides exhibit remarkable catalytic activity owing to abundant oxygen vacancies and mixed ionic–electronic conductivity, but they suffer from structural instability. In contrast, iron-based perovskite oxides are thermochemically stable under oxidizing and reducing conditions but are catalytically limited. To combine these complementary properties, a composite perovskite oxide was designed and prepared by infiltrating Sr_0.95Ce_0.05CoO_3−δ (SCC) into Ba_0.5Sr_0.5Fe_0.8Cu_0.2O_3−δ (BSFC). The SCC precursor solution was dropwise applied to a BSFC|SDC|BSFC symmetric cell and heat treated. Surface morphology and compositional analyses confirmed the distribution of SCC nanoparticles on the BSFC surface. High-temperature X-ray diffraction and Rietveld refinement results revealed that both BSFC and SCC retained the cubic perovskite structure (space group Pm-3m) at room temperature. No phase transition or secondary phase formation was observed during heating from 200 to 800 °C, and the peak shifts are attributed to thermal expansion and possible oxygen loss at elevated temperatures. Upon cooling, the diffraction patterns returned to their initial state, confirming a high-temperature structural stability. XPS analysis showed an increase in the satellite peak intensity associated with Fe³⁺ after SCC infiltration, and the average oxidation state of Fe decreased from 3.52 (BSFC) to 3.49 (composite perovskite oxide). The O 1s spectra revealed a higher relative content of surface-adsorbed oxygen species in the composite, indicating increased oxygen vacancy formation. Full article

We present structural, morphological, optical and photocatalytic properties of multiferroic Bi_0.98Ba_0.02FeO₃ (BBFO2) perovskite thin films prepared by a combined sol–gel and spin-coating method. X-ray diffraction (XRD) analysis revealed that all the perovskite films consisted of the stable polycrystalline rhombohedral phase structure (space group R3c) with a tolerance factor of 0.892. By using Rietveld refinement of diffractogram XRD data, crystallographic parameters, such as bond angle, bond length, atom position, unit cell parameters, and electron density measurements were computed. Scanning electron microscopy (SEM) allowed us to assess the homogeneous and smooth surface morphology of the films with a small degree of porosity, while chemical surface composition characterization by X-ray photoelectron spectroscopy (XPS) showed the presence of Bi, Fe, O and the doping element Ba. Absorption measurements allowed us to determine the energy band gap of the films, while photoluminescence measurements have shown the presence of oxygen vacancies, which are responsible for the enhanced photocatalytic activity of the material. Photocatalytic degradation experiments of Methylene Blue (MB), Methyl orange (MO), orange G (OG), Violet and Rhodamine B (RhB) performed on top of BBFO2 thin films under solar light showed the degradation of all pollutants in varying discoloration efficiencies, ranging from 81% (RhB) to 54% (OG), 53% (Violet), 47% (MO) and 43% (MB). Full article

In this work, a series of Ba_xMn_0.7Cu_0.3O₃ samples (x: 1, 0.9, 0.8, and 0.7, BxMC) was synthesized, characterized, and used as catalysts for CO oxidation reaction. All formulations were active for CO oxidation in the tested conditions. A correlation between the electrical conductivity, obtained by impedance spectroscopy, and the reducibility of the samples, obtained by H₂-TPR, was observed. The Ba_0.8Mn_0.7Cu_0.3O₃ composition (B0.8MC) showed the best catalytic performance (comparable to that of the 1% Pt/Al₂O₃ reference sample) during tests conducted under conditions similar to those found in the exhaust gases of current gasoline engines. The characterization data suggest the simultaneous presence of a high Mn(IV)/Mn(III) surface ratio, oxygen vacancies, and reduced copper species, these two latter being key properties for ensuring a high CO conversion percentage as both are active sites for CO oxidation. The reaction temperature and the reactant atmosphere composition seem to be the most important factors for achieving a good catalytic performance, as they strongly determine the location and stability of the reduced copper species. Full article

Perovskite materials are promising for thermochemical energy storage due to their ability to undergo redox cycling over a wide temperature range. Although BaCoO₃ exhibits excellent air cycling properties, its heat storage capacity in air remains suboptimal. This study introduces Na into the lattice structure to enhance oxygen vacancy formation and mobility. DFT+U simulations of the surface structure of Na-doped BaCoO_3−δ indicate that incorporating Na improves surface stability and facilitates the formation of surface oxygen vacancies. Na_xBa_1−xCoO_3−δ compounds were synthesized using a modified sol–gel method, and their properties were investigated. The experimental results demonstrate that Na doping significantly enhances the redox activity of the material. The heat storage capacity increased by above 50%, with the Na_0.0625Ba_0.9375CoO_3−δ solid solution achieving a heat storage density of up to 341.7 kJ/kg. XPS analysis reveals that Na doping increases the concentration of surface defect oxygen, leading to more active oxygen release sites at high temperatures. This enhancement in redox activity aligns with DFT predictions. During high-temperature cycling, the distribution of Na within the material becomes more uniform, and no performance degradation is observed after 300 cycles. Even after 450 cycles, Na_0.0625Ba_0.9375CoO_3−δ retains over 96% of its initial redox activity, significantly outperforming fresh BaCoO_3−δ. These findings elucidate the mechanism by which Na doping enhances the thermochemical heat storage performance of BaCoO_3−δ and provide new insights for the design of perovskite-based materials. Full article

Based on the previous work conducted by Fujii et al., NdBaInO₄ compounds present modest oxide-ion conductivities. Therefore, it has been an attractive system of significant interest. In this study, we attempted to partially substitute Ca for Nd and the total electrical conductivity was successfully improved due to the generation of oxygen vacancies. The synthesis, crystal structure, density, surface topography, and electrical properties of NdBaInO₄ and Ca-doped NdBaInO₄ have been studied, respectively. NdBaInO₄ and 10% and 20% molar fractions of Ca-doped NdBaInO₄ were synthesized through solid-state reactions. The crystal structure of them was obtained from Le Bail refinement of the XRD pattern, giving the result of the monoclinic structure, which belongs to P2₁/c space group. The highest total electrical conductivity of 4.91 × 10⁻³ S cm⁻¹ was obtained in the Nd_0.9Ca_0.1BaInO_3.95 sample at a temperature of 760 °C in the dry atmosphere and the activation energy was reduced from 0.68 eV to 0.58 eV when the temperature was above 464 °C (737 K) after doping the NdBaInO₄ with a 0.1 molar fraction of Ca²⁺. Moreover, the total conductivity of Nd_0.9Ca_0.1BaInO_3.95 in the wet atmosphere at moderate temperature was relatively higher than that in the dry atmosphere, which suggests that potential proton conduction may exist in wet atmospheres. In addition, the oxygen diffusion coefficients of Nd_0.9Ca_0.1BaInO_3.95 (D* = 1.82 × 10⁻⁸ cm²/s, 850 °C) was about two times higher than that of Nd_0.8Ca_0.2BaInO_3.90 (D* = 7.95 × 10⁻⁹ cm²/s, 850 °C) and was increased significantly by two orders of magnitude when compared with the oxygen diffusion coefficient of the undoped NdBaInO₄ (D* = 8.25 × 10⁻¹¹ cm²/s, 850 °C). Full article

Barium titanate (BaTiO₃) is a promising material for silicon-integrated photonics due to its large electro-optical coefficients, low loss, high refractive index, and fast response speed. Several deposition methods have been employed to synthesize BaTiO₃ films. Magnetron sputtering is one of these methods, which offers specific advantages for growing large-scale films. However, there is a scarcity of studies investigating the effect of sputtering target density on the quality of BaTiO₃ films. Therefore, this study aims to uncover the effect of sputtering targets on the crystal and electronic structures of epitaxial BaTiO₃ thin films. Two BaTiO₃ ceramic targets were sintered at different densities by altering the sintering temperatures. The crystal structure and chemical composition of the targets were then characterized using X-ray diffraction, Raman spectroscopy, and scanning electron microscopy with energy-dispersive X-ray spectroscopy. Subsequently, BaTiO₃ epitaxial films were grown by magnetron sputtering using these two targets. The crystal and electronic structures of the BaTiO₃ films were analyzed using high-resolution X-ray diffraction, X-ray photoemission spectroscopy, atomic force microscopy, and spectroscopic ellipsometry. Notably, the BaTiO₃ films grown with high-density targets show superior quality but contain oxygen vacancies, whereas those films synthesized with low-density targets display high surface roughness. These findings provide insights into the effect of sputtering target density on the crystal and electronic structures of epitaxial BaTiO₃ thin films. Full article

The sol–gel method, adapted to aqueous media, was used for the synthesis of BaMn_0.7Cu_0.3O₃ (BMC) and Ba_0.9A_0.1Mn_0.7Cu_0.3O₃ (BMC-A, A = Ce, La or Mg) perovskite-type mixed oxides. These samples were fully characterized by ICP-OES, XRD, XPS, H₂-TPR, BET, and O₂–TPD and, subsequently, they were evaluated as catalysts for CO oxidation under different conditions simulating that found in cars exhaust. The characterization results show that after the partial replacement of Ba by A metal in BMC perovskite: (i) a fraction of the polytype structure was converted to the hexagonal BaMnO₃ perovskite structure, (ii) A metal used as dopant was incorporated into the lattice of the perovskite, (iii) oxygen vacancies existed on the surface of samples, and iv) Mn(IV) and Mn(III) coexisted on the surface and in the bulk, with Mn(IV) being the main oxidation state on the surface. In the three reactant atmospheres used, all samples catalysed the CO to CO₂ oxidation reaction, showing better performances after the addition of A metal and for reactant mixtures with low CO/O₂ ratios. BMC-Ce was the most active catalyst because it combined the highest reducibility and oxygen mobility, the presence of copper and of oxygen vacancies on the surface, the contribution of the Ce(IV)/Ce(III) redox pair, and a high proportion of surface and bulk Mn(IV). At 200 °C and in the 0.1% CO + 10% O₂ reactant gas mixture, the CO conversion using BMC-Ce was very similar to the achieved with a 1% Pt/Al₂O₃ (Pt-Al) reference catalyst. Full article

Anodic titania nanotubes (TiO₂-NT) are very promising for use in photocatalysis and photovoltaics due to their developed surface, symmetrical structure and conductive properties, which, moreover, makes them a convenient matrix for creating various nanocomposites. Herein we propose a new facile way of synthesizing symmetrical TiO₂-NT followed by a modification with barium titanate (BaTiO₃) nanoparticles, combining the advantages of electrochemical oxidation and hydrothermal synthesis. The electrophysical and optoelectronic properties of the formed nanocomposites have been studied. An asymmetry of the current–voltage characteristics was revealed. It is shown that during the barium titanate deposition, a symmetry-breaking nanoheterojunction TiO₂/BaTiO₃ is formed. Using EPR spectroscopy, paramagnetic defects (titanium, barium and oxygen vacancies) in the samples were determined. It was observed for the first time that upon illumination of titania nanotubes modified with BaTiO₃, the asymmetrical separation of photoexcited charge carriers (electrons and holes) between TiO₂-NT and BaTiO₃ occurs, followed by the capture of electrons and holes by defects. As a result, the photoinduced charge accumulates on the defects. Full article

In this study, we analyzed the role of a series of BaMn_1−xNi_xO₃ (x = 0, 0.2, and 0.4) mixed oxide catalysts, synthesized using the sol–gel method, in NO_x-assisted diesel soot oxidation. ICP-OES, XRD, XPS, and H₂-TPR techniques were used for characterization and Temperature-Programmed Reaction experiments (NO_x-TPR and Soot-NO_x-TPR), and isothermal reactions at 450 °C (for the most active sample) were carried out to determine the catalytic activity. All samples catalyzed NO and soot oxidation at temperatures below 400 °C, presenting nickel-containing catalysts with the highest soot conversion and selectivity to CO₂. However, the nickel content did not significantly modify the catalytic performance, and in order to improve it, two catalysts (5 wt % in Ni) were synthesized via the hydrothermal method (BMN2H) and the impregnation of nickel on a BaMnO₃ perovskite as support (M5). The two samples presented higher activity for NO and soot oxidation than BMN2E (obtained via the sol–gel method) as they presented more nickel on the surface (as determined via XPS). BMN2H was more active than M5 as it presented (i) more surface oxygen vacancies, which are active sites for oxidation reactions; (ii) improved redox properties; and (iii) a lower average crystal size for nickel (as NiO). As a consequence of these properties, BMN2H featured a high soot oxidation rate at 450 °C, which hindered the accumulation of soot during the reaction and, thus, the deactivation of the catalyst. Full article

Ba_0.9A_0.1MnO₃ (BM-A) and Ba_0.9A_0.1Mn_0.7Cu_0.3O₃ (BMC-A) (A = Mg, Ca, Sr, Ce, La) perovskite-type mixed oxides were synthesised, characterised, and used for soot oxidation in simulated Gasoline Direct Injection (GDI) engine exhaust conditions. The samples have been obtained by the sol-gel method in an aqueous medium and deeply characterised. The characterization results indicate that the partial substitution of Ba by A metal in BaMnO₃ (BM) and BaMn_0.7Cu_0.3O₃ (BMC) perovskites: (i) favours the hexagonal structure of perovskite; (ii) improves the reducibility and the oxygen desorption during Temperature-Programmed Desorption (O₂-TPD) tests and, consequently, the oxygen mobility; (iii) mantains the amount of oxygen vacancies and of Mn(IV) and Mn(III) oxidation states, being Mn(IV) the main one; and (iv) for Ba_0.9A_0.1Mn_0.7Cu_0.3O₃ (BMC-A) series, copper is partially incorporated into the structure. The soot conversion data reveal that Ba_0.9La_0.1Mn_0.7Cu_0.3O₃ (BMC-La) is the most active catalyst in an inert (100% He) reaction atmosphere, as it presents the highest amount of copper on the surface, and that Ba_0.9Ce_0.1MnO₃ (BM-Ce) is the best one if a low amount of O₂ (1% O₂ in He) is present, as it combines the highest emission of oxygen with the good redox properties of Ce(IV)/Ce(III) and Mn(IV)/Mn(III) pairs. Full article

Perovskite cathodes have emerged as a promising alternative to traditional cathode materials in low-temperature solid oxide fuel cells (LT-SOFCs) due to their exceptional catalytic properties and high oxygen reduction reaction (ORR) activity. Their fast catalytic activity and chemical stability have drawn significant attention to lowering the operating temperature of SOFCs. In this study, Ba²⁺ and Bi³⁺ are doped into LaFeO₃. The aim is to investigate the catalytic activity and electrochemical performance of LT-SOFCs. The presented cathode material is characterized in terms of phase structure, surface morphology, and interface studies before being applied as a cathode in SOFCs to measure electrochemical performance. The XPS study revealed that La_1−2xBa_xBi_xFeO₃ (x = 0.1) exhibits enriched surface oxygen vacancies compared to La_1−2xBa_xBi_xFeO₃ (x = 0.2). La_1−2xBa_xBi_xFeO₃ with (x = 0.1 and 0.2) delivers a peak power density of 665 and 545 mW cm⁻² at 550 °C, respectively. Moreover, impedance spectra confirmed that La_1−2xBa_xBi_xFeO₃ with x = 0.1 exhibits lower electrode polarization resistance (0.33 Ω cm²) compared to La_1−2xBa_xBi_xFeO₃ with x = 0.2 (0.57 Ω cm²) at 550 °C. Our findings thus confirm that LBBF cathode-based SOFCs can be considered a potential cathode to operate fuel cells at low temperatures, and it will open up another horizon in the subject of research. Full article

Porous lead-free piezoelectric ceramics are characterized by their environment-friendly, light weight, and large specific surface area. The optimization of porous Na_0.5Bi_0.5TiO₃-based lead-free piezoelectric ceramics can improve piezoelectric properties, enhance force–electric coupling characteristics, and effectively promote energy conversion, expanding the application in force-electric coupling devices. This study aimed to prepare [Sm_x(Bi_0.5Na_0.5)_1−3x/2]_0.94Ba_0.06TiO₃ (x = 0, 0.01, 0.02, 0.03, 0.04) lead-free ceramics with porous structures, resulting in the piezoelectric constant d₃₃ = 131 pC/N and the plane electromechanical coupling coefficient k_p = 0.213 at x = 0.01. The presence of pores in lead-free ceramics has a direct impact on the domain structure and can cause the depolarization process to relax. Then, the soft doping of Sm³⁺ makes the A-site ion in porous (Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ ceramics occupancy inhomogeneous and generates cation vacancies, which induces lattice distortion and makes the domain wall motion easier, resulting in the improvement of piezoelectric properties and electromechanical coupling parameters. Furthermore, the piezoelectric oscillator exhibits greater resistance to resonant coupling in the radial extension vibration mode. These results infer that a combination of porosity and Sm³⁺ doping renders (Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ ceramics base material for piezoelectric resonators, providing a scientific basis for their application in force–electric coupling devices, such as piezoelectric resonant gas sensors. Full article

The oxygen reduction reaction (ORR) activity of a Cu-doped Ba_0.5Sr_0.5FeO_3−δ (Ba_0.5Sr_0.5Fe_1−xCu_xO_3−δ, BSFCux, x = 0, 0.05, 0.10, 0.15) perovskite cathode was investigated in terms of oxygen vacancy formation and valence band structure. The BSFCux (x = 0, 0.05, 0.10, 0.15) crystallized in a cubic perovskite structure ($Pm\overline{3}m$). By thermogravimetric analysis and surface chemical analysis, it was confirmed that the concentration of oxygen vacancies in the lattice increased with Cu doping. The average oxidation state of B-site ions decreased from 3.583 (x = 0) to 3.210 (x = 0.15), and the valence band maximum shifted from −0.133 eV (x = 0) to −0.222 eV (x = 0.15). The electrical conductivity of BSFCux increased with temperature because of the thermally activated small polaron hopping mechanism showing a maximum value of 64.12 S cm⁻¹ (x = 0.15) at 500 °C. The ASR value as an indicator of ORR activity decreased by 72.6% from 0.135 Ω cm² (x = 0) to 0.037 Ω cm² (x = 0.15) at 700 °C. The Cu doping increased oxygen vacancy concentration and electron concentration in the valence band to promote electron exchange with adsorbed oxygen, thereby improving ORR activity. Full article

The electrochemical reduction of molecular oxygen is a fundamental process in Solid Oxide Fuel Cells and requires high efficiency cathode materials. Two La_0.25Ba_0.25Sr_0.5Co_0.8Fe_0.2O_3−δ-based perovskite compounds were prepared by solution combustion synthesis, and characterized for their structural, microstructural, surface, redox and electrochemical properties as potential cathodes in comparison with Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ and La_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ perovskites. Results highlighted that calcination at 900 °C led to a “bi-perovskite heterostructure”, where two different perovskite structures coexist, whereas at higher calcination temperatures a single-phase perovskite was formed. The results showed the effectiveness of the preparation procedures in co-doping the A-site of perovskites with barium and lanthanum as a strategy to optimize the cathode’s properties. The formation of nanometric heterostructure co-doped in the A-site evidenced an improvement in oxygen vacancies’ availability and in the redox properties, which promoted both processes: oxygen adsorption and oxygen ions drift, through the cathode material, to the electrolyte. A reduction in the total resistance was observed in the case of heterostructured material. Full article

Metal oxide perovskite materials show promise for use as hydrogen separation membranes, but metal oxides can dehydrate in the presence of hydrogen to the point of decomposition. The stability of a material in the presence of hydrogen is necessary for an effective hydrogen separation membrane. The stability of a mixed phase metal oxide perovskite (BaCe_0.85Fe_0.15O_3-δ-BaCe_0.15Fe_0.85O_3-δ) was investigated using first-principles thermodynamics calculations based on density functional theory to examine the possible reduction processes on the surface of the material. It was found that for either phase of the material, the loss of H₂ becomes thermodynamically favorable over the formation of oxygen vacancies once oxygen vacancy defects exist on the surface. Additionally, both phases of the material become more stable with respect to the dehydration or loss of oxygen with increasing concentrations of surface oxygen vacancies. Under the conditions of commercial hydrogen production (~400–1100 K), it is more thermodynamically favorable for H₂ to desorb from the BaCe_0.85Fe_0.15O_3-δ phase. Examination of the atomic-scale structure indicates that the degree of coordination of surface metal atoms in this material may control the stability of the material in reducing environments. Full article