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Local insight into the La-induced structural phase transition in multiferroic BiFeO3 ceramics by X-ray absorption fine structure spectroscopy
摘要： Substitution of bismuth by rare-earth (RE) ions is of great technological importance to develop room-temperature BiFeO3-based multiferroic materials. Despite this interest, many fundamental properties and the structure-property correlations of RE-doped BiFeO3 remain poorly understood. Here we report a systematical experimental and theoretical exploration on the structural phase transition in Bi1-xLaxFeO3 (0 ≤ x ≤ 0.2) ceramics. By using x-ray absorption fine structure spectroscopy, we for the first time show that the La3+ dopants in fact substitute the Bi site of a secondary nanosized particles with orthorhombic Pbam symmetry instead of the long-believed parental rhombohedral R3c phase at all La3+ doping concentrations (0.001 ≤ x ≤ 0.2). This finding is further supported by complementary studies of transmission electron microscopy and thermodynamic preference, and it casts serious challenges on the prevailing assumption of La substitution on the Bi3+ site in R3c structure when x ≤ 0.1 as well as the previously proposed origin of enhanced functional properties based on morphotropic phase boundary. This new insight may ignite a revival on exploring the underlying multiferroic mechanisms in BiFeO3-based materials and facilitate the bottom-up design of novel multifunctional devices.
关键词： XAFS,BiFeO3,structural phase transition,La doping,multiferroic materials
更新于2025-09-04 15:30:14
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Direct Observation of Curved Surface Enhanced Disordering in Ag <sub/>2</sub> S Nanoparticles
摘要： Surface induced order-disorder phase transition has been widely studied on flat bulk surfaces, while such a transition is poorly understood on curved surfaces in nanoscale. Here, we report a direct observation of the dynamic behaviors of surface-initiated disordering in Ag2S nanoparticles using atomic resolution in-situ transmission electron microscopy. It was found that the disordering behavior is different from the traditional model that the disordered layer follows a logarithmical thickness dependence with temperature. In particular, the disordering is largely enhanced at higher temperature when the radius of the residual order phase is getting smaller. Moreover, the correlation length of disordered phase was found to be several times larger than the typical value for bulk surfaces. This significantly enhanced disordering in nanoparticles could be attributed to the extra driving force provided by the decreasing of order-disorder interface area.
关键词： nanoscale,Ag2S nanoparticles,in-situ TEM,phase transition,surface-initiated disordering
更新于2025-09-04 15:30:14
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Temperature Driven Phase Transition of Organic-Inorganic Halide Perovskite Single Crystals
摘要： Organic-inorganic halide perovskite single crystals undergo phase transition of being cubic, tetragonal, or orthorhombic depending on the temperature. We investigated the CH3NH3PbBr3?xIx single crystals grown by the inverse temperature crystallization method with temperature-dependent UV-Vis absorption and photoluminescence. From the temperature-dependent absorption measurement, the optical band gap is extracted by derivation of absorption spectrum fitting and Tauc plot. In our results, CH3NH3PbBr3?xIx single crystals show that an abrupt change in optical band gap, PL peak position and intensity appears around 120 K - 170 K regions, indicating the phase transition temperature.
关键词： Photoluminescence,Phase transition,Perovskite single crystal,Temperature-dependent absorption,Derivation of absorption spectrum fitting (DASF)
更新于2025-09-04 15:30:14
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Visualizing the Initial Step of Self-Assembly and the Phase Transition by Stereogenic Amphiphiles with Aggregation-Induced Emission
摘要： Many highly ordered structures with smart functions are generated by self-assembly with stimuli responsiveness. Despite the electron microscopes enable us to directly observe the end products, they are hard to visualize the initial step and the kinetic stimuli-responsive behavior of self-assembly. Here, we report the design and synthesis of stereogenic amphiphiles, namely (Z)-TPE-OEG and (E)-TPE-OEG, with aggregation-induced emission (AIE) characteristics from the hydrophobic tetraphenylethene core and thermoresponsive behavior from the hydrophilic oligoethylene glycol monomethyl ether chain. The two isomers can be easily isolated by high-performance liquid chromatography and characterized by 2D NMR spectroscopy. While (Z)-TPE-OEG self-assembles into vesicles, its (E)-cousin forms micelles in water. The initial step of their self-assembly processes can be visualized based on AIE characteristics, with the sensitivity much higher than the method based on transmittance measurement. The entrapment and release capabilities of the (Z)-stereogenic amphiphile are demonstrated by employing pyrene as a guest. The thermoresponsive behavior of the (Z)-amphiphile results in its continuous phase transition from microscopic self-assembly to macroscopic aggregation, which is successfully visualized in-situ by confocal laser scanning microscopy accompanied with AIE technique. Such kinetic process shows different stages according to the microscopic visualization and these stages have never been monitored through roughly observing the appearance of precipitates. It is anticipated that this study can deepen the understanding of the self-assembly processes for better monitoring and controlling them in different systems.
关键词： phase transition,aggregation-induced emission,self-assembly,amphiphiles,microscopic visualization
更新于2025-09-04 15:30:14
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Structural and Optical Properties of Cs2AgBiBr6 Double Perovskite.
摘要： We present a comprehensive study of the relationship between the crystal structure and optoelectronic properties of the double perovskite Cs2AgBiBr6, which has emerged as a promising candidate for photovoltaic. Based on single-crystal/powder X-ray diffraction and neutron powder diffraction, we have revealed the presence of a structural phase transition at Ts ～122K between the room-temperature cubic structure (space group F mˉ3m) and a new low-temperature tetragonal structure (I4/m). From reflectivity measurements we found that the peak exciton energy Eex ≈ 2.85 eV near the direct gap shifts proportionally to the tetragonal strain, which is consistent with the Eex being primarily controlled by a rotational degree of freedom of the crystal structure, thus by the angle Bi-Br-Ag. We observed the time-resolved photoluminescence kinetics and we found that, among the relaxation channels, a fast one is mainly present in the tetragonal phase, suggesting that its origin may lie in the formation of tetragonal twin domains.
关键词： optoelectronic properties,structural phase transition,double perovskite,photovoltaic,Cs2AgBiBr6
更新于2025-09-04 15:30:14
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Cu-Doped TiO2: Visible Light Assisted Photocatalytic Antimicrobial Activity
摘要： Surface contamination by microbes is a major public health concern. A damp environment is one of potential sources for microbe proliferation. Smart photocatalytic coatings on building surfaces using semiconductors like titania (TiO2) can effectively curb this growing threat. Metal-doped titania in anatase phase has been proven as a promising candidate for energy and environmental applications. In this present work, the antimicrobial efficacy of copper (Cu)-doped TiO2 (Cu-TiO2) was evaluated against Escherichia coli (Gram-negative) and Staphylococcus aureus (Gram-positive) under visible light irradiation. Doping of a minute fraction of Cu (0.5 mol %) in TiO2 was carried out via sol-gel technique. Cu-TiO2 further calcined at various temperatures (in the range of 500–700 ?C) to evaluate the thermal stability of TiO2 anatase phase. The physico-chemical properties of the samples were characterized through X-ray diffraction (XRD), Raman spectroscopy, X-ray photo-electron spectroscopy (XPS) and UV–visible spectroscopy techniques. XRD results revealed that the anatase phase of TiO2 was maintained well, up to 650 ?C, by the Cu dopant. UV–vis results suggested that the visible light absorption property of Cu-TiO2 was enhanced and the band gap is reduced to 2.8 eV. Density functional theory (DFT) studies emphasize the introduction of Cu+ and Cu2+ ions by replacing Ti4+ ions in the TiO2 lattice, creating oxygen vacancies. These further promoted the photocatalytic efficiency. A significantly high bacterial inactivation (99.9999%) was attained in 30 min of visible light irradiation by Cu-TiO2.
关键词： Staphylococcus aureus,phase transition,antibacterial coatings,photocatalysis,doping,Cu-doped TiO2,Escherichia coli
更新于2025-09-04 15:30:14