Search Results (24)

The photocatalytic activity of Bi₂WO₆ Aurivillius phase has been widely exploited for the degradation of a wide range of gaseous and aqueous molecules, as well as microorganisms, under the influence of visible irradiation. Strategies for the development of doped and co-doped bismuth tungstate materials require the thermodynamic data on this phase. The heat capacity of bismuth tungstate, Bi₂WO₆, was investigated using a DSC microcalorimeter on polycrystalline powder samples in the temperature range from 313 to 1103 K (40–830 °C) in two separate runs. The samples were synthesized by solid-state reaction from pure binary oxides at 1033 K (760 °C) in a platinum crucible with cover. The high temperature C_p(T) data were fitted by the Maier–Kelley equation and, from this relation, the standard molar heat capacity of γ-Bi₂WO₆ polymorph was estimated to be at 298.15 K 176.8 ± 3.9 J·K⁻¹·mol⁻¹. A reversible second-order transition of Bi₂WO₆ phase was observed in the experimental temperature range, with a peak close to 940 K (667 °C). Additionally, the extrapolation of C_p(T) to 0 K was proposed using a method based on the multiple Einstein model. Thermodynamic properties (heat capacity C_p(T), entropy S°(T), enthalpy H°(T), Gibbs energy G°(T)) of crystalline γ-Bi₂WO₆ were calculated in the temperature range of 298.15–1123 K (25–850 °C). Full article

In this work, we investigate advanced photocatalyst Bi₃TiNbO₉ as promising piezophotocatalyst in terms of the effect of synthesis methods on the surface chemistry, structure, and catalytic performance in process of contaminant removal. Samples were prepared via solid-state reaction (BTNO-900) and molten salt synthesis (BTNO-800), leading to distinct morphologies and defect distributions. SEM imaging revealed that BTNO-900 consists of agglomerated, irregular particles, while BTNO-800 exhibits well-faceted, plate-like grains. Nitrogen adsorption analysis showed that the molten-synthesized sample possesses a significantly higher specific surface area (5.9 m²/g vs. 1.4 m²/g) and slightly larger average pore diameter (2.8 nm vs. 2.6 nm). High-resolution XPS revealed systematic shifts in binding energies for Bi 4f, Ti 2p, Nb 3d, and O 1s peaks in BTNO-900, accompanied by a higher content of adsorbed oxygen species (57% vs. 7.2%), indicating an increased concentration of oxygen vacancies and surface hydroxylation due to the solid-state synthesis route. Catalytic testing demonstrated that BTNO exhibits enhanced piezocatalytic efficiency of Methylene Blue degradation (~78% for both samples), whereas BTNO-800 shows significantly reduced photocatalytic activity (45.6%) compared to BTNO-900 (84.1%), suggesting recombination effects dominate in the more defective material. Synergism of light and mechanical stress results in piezophotocatalytic degradation for both samples (92.4% and 93.4%, relatively). These findings confirm that synthesis-controlled defect engineering is a key parameter for optimizing the photocatalytic behavior of Bi₃TiNbO₉-based layered oxides and crucial role of its piezocatalytic activity. Full article

Bismuth-based oxyfluorides (BiO_xF_3−2x) have recently emerged as promising photocatalysts due to their unique electronic structures and tunable physicochemical properties. This review provides a comprehensive overview of these materials, focusing on their crystal structures, band gap characteristics, and photocatalytic performance. Particular attention is given to BiOF, Bi₇O₅F₁₁, and β-BiO_xF_3−2x, highlighting the influence of fluorine’s high electronegativity and internal electric fields on charge separation and light absorption. The potential of Aurivillius-type oxyfluorides is also discussed. Structural modifications, such as the introduction of oxygen vacancies, morphology control, and metal/non-metal doping, are examined for their effects on photocatalytic efficiency. Furthermore, various synthesis techniques and heterojunction engineering strategies involving semiconductors, carbon-based materials, and metal nanoparticles are explored to improve light harvesting and reduce charge recombination. Applications in pollutant degradation and CO₂ photoconversion are reviewed, demonstrating the versatility of these materials. Despite their promise, the challenges associated with phase identification and composition control are also emphasized, underlining the need for rigorous structural characterization. Future directions for optimizing the photocatalytic activity of bismuth-based oxyfluorides are outlined, focusing on strategies to enhance their performance. Full article

Relaxor ferroelectric film capacitors exhibit high power density with ultra-fast charge and discharge rates, making them highly advantageous for consumer electronics and advanced pulse power supplies. The Aurivillius-phase bismuth layered ferroelectric films can effectively achieve a high breakdown electric field due to their unique insulating layer ((Bi₂O₂)²⁺ layer)). However, designing and fabricating Aurivillius-phase bismuth layer relaxor ferroelectric films with optimal energy storage characteristics is challenging due to their inherently stable ferroelectric properties. In this work, lead-free CaBi_4-xLa_xTi₄O₁₅ films were synthesized using the sol–gel technique and a weakly coupled relaxor design. On one hand, the introduction of La³⁺ ions weaken the dipole–dipole interactions, thereby enhancing the relaxor behavior. Alternatively, the expansion of grain size is restricted to enhance the number of grain boundaries, which possess improved insulating properties. This leads to a higher breakdown electric field. The results indicate that CaBi_4-xLa_xTi₄O₁₅ (x = 1.0) films exhibit excellent recoverable energy storage density (70 J/cm³) and high energy efficiency (73%). Moreover, the film exhibited good temperature stability and frequency stability. This study not only identifies a promising material for dielectric film capacitors but also demonstrates that the energy storage capabilities of Aurivillius-phase bismuth layer ferroelectric films can be effectively modulated through a design incorporating weakly coupled relaxor characteristics. Full article

By using the method of high-temperature solid-phase reaction, the new piezoceramic SrBi₂Nb_2-2xW_xSn_xO₉ was obtained, where partial substitution of niobium (Nb) atoms with Sn⁴⁺ and W⁶⁺ atoms in the compound SrBi₂Nb₂O₉ occurred in the octahedra of the perovskite layer (B-position). X-ray diffraction investigations showed that these compounds are single-phase SrBi₂Nb_2-2xW_xSn_xO₉ (x = 0.1, 0.2) and two-phase SrBi₂Nb_2-2xW_xSn_xO₉ (x = 0.3, 0.4), but all of them had the structure of Aurivillius-Smolensky phases (ASPs) with close parameters of orthorhombic unit cells. It corresponded to the space group A21am. The temperature dependences of the relative permittivity ε/ε₀ and the tangent of the dielectric loss angle tan d were defined at various frequencies. It was found that doping SrBi₂Nb_2-2xW_xSn_xO₉ (x = 0.1) improved the electrophysical properties of the compound: losses decreased, and the relative permittivity increased. This result was obtained for the first time. Moreover, a new result was obtained that indicated an improvement in the electrophysical properties of SrBi₂Nb₂O₉ using the chemical element Sn (tin). This refutes the previously existing opinion about the impossibility to use Sn as a doping element. Full article

Bismuth layered structure ferroelectrics (BLSFs), also known as Aurivillius phase materials, are ideal for energy-efficient applications, particularly for solar cells. This work reports the first comprehensive detailed theoretical study on the fascinating structural, electronic, and optical properties of XNb₂Bi₂O₉ (X = Ca, Ba, Be, Mg, Sr). The Perdew–Burke–Ernzerhof approach and generalized gradient approximation (GGA) are implemented to thoroughly investigate the structural, bandgap, optical, and electronic properties of the compounds. The optical conductivity, band topologies, dielectric function, bandgap values, absorption coefficient, reflectivity, extinction coefficient, refractive index, and partial and total densities of states are thoroughly investigated from a photovoltaics standpoint. The material exhibits distinct behaviors, including significant optical anisotropy and an elevated absorption coefficient > 10⁵ cm⁻¹ in the region of visible; ultraviolet energy is observed, the elevated transparency lies in the visible and infrared regions for the imaginary portion of the dielectric function, and peaks in the optical spectra caused by inter-band transitions are detected. According to the data reported, it becomes obvious that XNb₂Bi₂O₉ (X = Ca, Ba, Be, Mg, and Sr) is a suitable candidate for implementation in solar energy conversion applications. Full article

The magnetic properties of Aurivillius-phase Bi₇Fe₃Ti3O₂₁ (BFT) and Bi_7−xGd_xFe3Ti3O₂₁, where x = 0.2, 0.4, and 0.6 (BGFT), were investigated. Ceramic material undoped (BGF) and doped with Gd³⁺ ions were prepared by conventional solid-state reaction. In order to confirm that the obtained materials belong to Aurivillius structures, XRD tests were performed. The XRD results confirmed that both the undoped and the gadolinium-doped materials belong to the Aurivillius phases. The qualitative chemical composition of the obtained materials was confirmed based on EDS tests. The temperature dependences of magnetization and magnetic susceptibility were examined for the ceramic material both undoped and doped with Gd³⁺ ions. The measurements were taken in the temperature range from T = 10 K to T = 300 K. Using Curie’s law, the value of the Curie constant was determined, and on its basis, the number of iron ions that take part in magnetic processes was calculated. The value of Curie constant C = 0.266 K, while the concentration of iron ions Fe³⁺, which influence the magnetic properties of the material, is equal 3.7 mol% (for BFT). Hysteresis loop measurements were also performed at temperatures of T = 10 K, T = 77 K, and T = 300 K. The dependence of magnetization on the magnetic field was described by the Brillouin function, and on its basis, the concentration of Fe³⁺ ions, which are involved in magnetic properties, was also calculated (3.4 mol% for BFT). Tests showed that the material is characterized by magnetic properties at low temperatures. At room temperature (RT), it has paramagnetic properties. It was also found that Gd³⁺ ions improve the magnetic properties of tested material. Full article

Bi_7-xGd_xFe₃Ti₃O₂₁ (x = (0, 0.2, 0.4, 0.6)) bismuth-layered perovskite structure compounds have been successfully prepared by a solid-state reaction. The results of X-ray studies indicate that a single-phase ceramic was obtained, characterized by an orthorhombic crystal structure for all compounds within the Fm2m space group. Microstructural analysis revealed that introducing gadolinium to the material altered the grain morphology, resulting in a more rounded grain shape and a somewhat disordered arrangement. Moreover, with higher gadolinium concentrations, there is a noticeable increase in the presence of the number of large plates. Impedance spectroscopy has been used to characterize the electrical properties of Bi_7-xGd_xFe₃Ti₃O₂₁ compounds. Full article

Pure Bi₇Fe₃Ti₃O₂₁ ceramic material and gadolinium ion (Gd³⁺)-doped ones were prepared by solid-state reaction method using simple oxides. The findings of the XRD measurements confirmed the initial author’s assumption that the dopant ions substituted in perovskite blocks influenced the dimensions of the unit cell parameters. All obtained materials are single-phase and show an orthorhombic structure with the Fm2m space group. Microstructure studies show that the admixture gadolinium doping changes the microstructure of the base material, changing grain shapes from plate-like to rounded. The temperature dependences of the electric permittivity have shown the existence of a maximum, the temperature location of which depends on both the frequency and the concentration of Gd³⁺ ions. The highest values of electric permittivity were characteristic of the material with an admixture of Gd³⁺ ions in the amount of x = 0.6 (f = 1 kHz), and the lowest values were for material with x = 0.2 (f = 1 kHz). Studies of the magnetoelectric effect have shown that the strongest coupling between magnetic and electrical properties was demonstrated by a material doped with Gd³⁺ ions in the amount of x = 0.2, for which the magnetoelectric coupling coefficient is equal to α = 12.58·10⁻⁹ s/m. Full article

In the present work, we have synthesized rare-earth ion modified Bi_4−xRE_xTi₂Fe0_.7Co_0.3O_12−δ (RE = Dy, Sm, La) multiferroic compounds by the conventional solid-state route. Analysis of X-ray diffraction by Rietveld refinement confirmed the formation of a polycrystalline orthorhombic phase. The morphological features revealed a non-uniform, randomly oriented, plate-like grain structure. The peaks evident in the Raman spectra closely corresponded to those of orthorhombic Aurivillius phases. Dielectric studies and impedance measurements were carried out. Asymmetric complex impedance spectra suggested the relaxation of charge carriers belonging to the non-Debye type and controlled by a thermally activated process. Temperature-dependent AC conductivity data showed a change of slope in the vicinity of the phase transition temperature of both magnetic and electrical coupling natures. Based on the universal law and its exponent nature, one can suppose that the conduction process is governed by a small polaron hopping mechanism but significant distortion of TiO₆ octahedral. The doping of the A-sites with rare-earth element ions and changes in the concentrations of Fe and Co ions located on the B-sites manifested themselves in saturated magnetic hysteresis loops, indicating competitive interactions between ferroelectric and canted antiferromagnetic spins. The magnetic order in the samples is attributed to pair-wise interactions between adjacent Fe³⁺–O–Fe³⁺, Co^2+/3+–O–Co^3+/2+, and Co^2+/3+–O–Fe³⁺ ions or Dzyaloshinskii–Moriya interactions among magnetic ions in the adjacent sub-lattices. As a result, enhanced magnetoelectric coefficients of 42.4 mV/cm-Oe, 30.3 mV/cm-Oe, and 21.6 mV/cm-Oe for Bi_4−xDy_xTi₂Fe0_.7Co_0.3O_12−δ (DBTFC), Bi_4−xLa_xTi₂Fe0_.7Co_0.3O_12−δ (LBTFC), and Bi_4−xSm_xTi₂Fe0_.7Co_0.3O_12−δ (SBTFC), respectively, have been obtained at lower magnetic fields (<3 kOe). The strong coupling of the Aurivillius compounds observed in this study is beneficial to future multiferroic applications. Full article

The development of visible-light-responsive (VLR) semiconductor materials for effective water oxidation is significant for a sustainable and better future. Among various candidates, bismuth tungstate (Bi₂WO₆; BWO) has attracted extensive attention because of many advantages, including efficient light-absorption ability, appropriate redox properties (for O₂ generation), adjustable morphology, low cost, and profitable chemical and optical characteristics. Accordingly, a facile solvothermal method has been proposed in this study to synthesize two-dimensional (2D) BWO nanoplates after considering the optimal preparation conditions (solvothermal reaction time: 10–40 h). To find the key factors of photocatalytic performance, various methods and techniques were used for samples’ characterization, including XRD, FE-SEM, STEM, TEM, HRTEM, BET-specific surface area measurements, UV/vis DRS, and PL spectroscopy, and photocatalytic activity was examined for water oxidation under UV and/or visible-light (vis) irradiation. Famous commercial photocatalyst–P25 was used as a reference sample. It was found that BWO crystals grew anisotropically along the {001} basal plane to form nanoplates, and all properties were controlled simultaneously by tuning the synthesis time. Interestingly, the most active sample (under both UV and vis), prepared during the 30 h solvothermal reaction at 433 K (BWO–30), was characterized by the smallest specific surface area and the largest crystals. Accordingly, it is proposed that improved crystallinity (which hindered charge carriers’ recombination, as confirmed by PL), efficient photoabsorption (using the smallest bandgap), and 2D mesoporous structure are responsible for the best photocatalytic performance of the BWO–30 sample. This report shows for the first time that 2D mesoporous BWO nanoplates might be successfully prepared through a facile template-free solvothermal approach. All the above-mentioned advantages suggest that nanostructured BWO is a prospective candidate for photocatalytic applications under natural solar irradiation. Full article

In order to meet the urgent need for high temperature piezoelectric materials with a Curie temperature over 400 °C, the Mn/Nb co-doped strategy has been proposed to improve the weak piezoelectric performance of the Aurivillius-type Na_0.5Bi_4.5Ti₄O₁₅ (NBT) high temperature piezoelectric ceramics. In this paper, the crystal structure, electrical properties, and thermal stability of the B-site Mn/Nb co-doped Na_0.5Bi_4.5Ti_4-x(Mn_1/3Nb_2/3)_xO₁₅ (NBT-100x) ceramics were systematically investigated by the conventional solid-state reaction method. The crystal structural analysis results indicate that the NBT-100x ceramics have typical bismuth oxide layer type phase structure and high anisotropic plate-like morphology. The lattice parameters and the grain sizes increase with the B-site Mn/Nb co-doped content. The electrical properties were significantly improved by Mn/Nb co-doped modifications. The maximum of the piezoelectric coefficient d₃₃ was found to be 29 pC/N for the NBT-2 ceramics, nearly twice that of the unmodified NBT ceramics. The highest values of the planar electromechanical coupling factor k_p and thickness electromechanical coupling factor k_t were also obtained for the NBT-2 ceramics, at 5.4% and 31.2%, respectively. The dielectric spectroscopy showed that the Curie temperature Tc of the Mn/Nb co-doped NBT-100x ceramics is slightly higher than that of unmodified NBT ceramics (646 °C). The DC resistivity of the NBT-2 ceramics is higher than 10⁶ Ω∙cm at 500 °C. All the results together with the good thermal stability demonstrated the Mn/Nb co-doped ceramics as an effective method to improve the NBT based piezoelectric ceramics and the potential candidates of the Mn/Nb co-doped NBT-100x ceramics for high temperature piezoelectric applications. Full article

A simple solid-state reaction was applied to synthesize Fe-doped perovskite-type PBFNO catalysts, and methylene blue decomposition studies were performed in the form of visible light according to the changes in the Fe doping content (0.4 to 1.9 mol ratio compared with Bi mol) and the amount of catalyst used (0.05 to 0.2 g used). As the Fe doping content increases, the absorbance and bang gap energy of the PBFNOs sample rapidly increase and decrease, respectively, because the Fe dopant in the PBNO lattice acts as an intermediate band between the valence and conduction bands of the PBNO and reduces the band gap energy. As a result, it showed a performance degradation of approximately 42% compared to the maximum performance. In addition, the presence of Fe dopants in the PBNO lattice greatly reduces the intensity of the photoluminescent lines. This is because the Fe dopant can play an important role in light-induced electron transfer and as a hole trap, reducing the recombination rate. Additionally, when too much photocatalyst was used (>0.1 g used), the Fe dopant played an important role as a light-induced electron transfer and hole trap, reducing the recombination rate and lowering the overall photocatalytic activity by 51%. In particular, 0.1 g of PBNO-0.2-F showed continuous catalytic activity, even when the photocatalytic reaction proceeded for 180 min. Therefore, this study demonstrates that the Fe-doped aurivillius-phase PBFNO photocatalyst is very promising for the dye manufacturing industry. Full article

A new modification of PbF(IO₃) has been obtained as single crystals from hydrothermal synthesis, alongside the known centrosymmetric, Pb(IO₃)₂, as a second phase. Measured with the Kurtz-Perry SHG method, the new crystals are phase-matchable for YAG:Nd laser radiation and demonstrate strong SHG output. According to an X-ray diffraction analysis conducted on a single crystal at low temperature, the new crystals appear monoclinic, of space group Pn, as opposed to the known orthorhombic modification of the PbF(IO₃), of space group Iba2. The new crystals were also measured at room temperature, showing an orthorhombic disordered variant of the new phase (space group C2ma, standard Abm2). This variant presents an “average structure” with the mirror plane in the group. The low-temperature X-ray single-crystal experiment allowed us to find the correct structural model, where the mirror plane was found as a twin element in the real monoclinic Pn structure. A careful crystal chemical analysis led to a whole family of nonlinear optical crystals with a common formula, AX(IO₃), A = Bi, Ba, Pb, X = O, F, (OH), currently counting six representatives, including the well-known BiO(IO₃). All of them possess common central cationic layers similar to those known in Aurivillius-type phases, with anionic iodate layers attached above and below these layers instead of the perovskite-like, or halogens. The structure–property relationships are discussed with respect to the important role of the large cations: Pb²⁺, Bi³⁺ or Ba²⁺. A number of iodates with similar structures are also analyzed. Full article

Dielectric capacitors have received increasing attention due to their high power density. The Bi-based Aurivillius phase compound Bi_3.25La_0.75Ti₃O₁₂ (BLT) is considered a potential material in the field of energy storage due to its excellent ferroelectric properties and good fatigue resistance, and temperature stability. In this paper, 0.4Bi_0.5Na_0.5TiO₃-0.6Bi_3.25La_0.75Ti₃O₁₂ (0.4NBT4BNT-0.6BLT)-thin films were prepared on Pt/Ti/SiO₂/Si substrates with the sol-gel method. The addition of BNT destroys the long-range ferroelectric order of BLT and forms nanodomains. By increasing the BNT content, the BLT is transformed from a ferroelectric state to a relaxed state, and its application in the field of energy storage is realized. The recoverable energy density is 42.41 J/cm³, and the recoverable energy storage density is relatively stable in the range of 25–200 °C with good thermal stability. The energy storage efficiency is 75.32% at ~2663 kV/cm. The leakage current density at 300 kV/cm is 1.06 × 10⁻⁹ A/cm². Full article