Synchrotron X-ray Absorption Spectroscopy Study of Local Structure in Al-Doped BiFeO3 Powders

Self-Enhancement of Water Electrolysis by Electrolyte-Poled Ferroelectric Catalyst

Efficient and durable electrocatalysts with superior activity are needed for the production of green hydrogen with a high yield and low energy consumption. Electrocatalysts based on transition metal oxides hold dominance due to their abundant natural resources, regulable physical properties, and good adaptation to a solution. In numerous oxide catalyst materials, ferroelectrics, possessing semiconducting characteristics and switchable spontaneous polarization, have been considered promising photoelectrodes for solar water splitting. However, few investigations noted their potential as electrocatalysts. In this study, we report an efficient electrocatalytic electrode made of a BiFeO3/nickel foam heterostructure, which displays a smaller overpotential and higher current density than the blank nickel foam electrode. Moreover, when in contact with an alkaline solution, the bond between hydroxyls and the BiFeO3 surface induces a large area of upward self-polarization, lowering the adsorption energy of subsequent adsorbates and facilitating oxygen and hydrogen evolution reaction. Our work demonstrates an infrequent pathway of using functional semiconducting materials for exploiting highly efficient electrocatalytic electrodes.

Construction of Pt@BiFeO3 Xerogel-Supported O-g-C3N4 Heterojunction System for Enhanced Visible-Light Activity towards Photocatalytic Degradation of Rhodamine B

Synthetic organic pigments from the direct discharge of textile effluents are considered as colossal global concern and attract the attention of scholars. The efficient construction of heterojunction systems involving precious metal co-catalysis is an effective strategy for obtaining highly efficient photocatalytic materials. Herein, we report the construction of a Pt-doped BiFeO₃/O-g-C₃N₄ (Pt@BFO/O-CN) S-scheme heterojunction system for photocatalytic degradation of aqueous rhodamine B (RhB) under visible-light irradiation. The photocatalytic performances of Pt@BFO/O-CN and BFO/O-CN composites and pristine BiFeO₃ and O-g-C₃N₄ were compared, and the photocatalytic process of the Pt@BFO/O-CN system was optimized. The results exhibit that the S-scheme Pt@BFO/O-CN heterojunction has superior photocatalytic performance compared to its fellow catalysts, which is due to the asymmetric nature of the as-constructed heterojunction. The as-constructed Pt@BFO/O-CN heterojunction reveals high performance in photocatalytic degradation of RhB with a degradation efficiency of 100% achieved after 50 min of visible-light irradiation. The photodegradation fitted well with pseudo-first-order kinetics proceeding with a rate constant of 4.63 × 10^-2 min^-1. The radical trapping test reveals that h⁺ and ^•O₂^- take the leading role in the reaction, while the stability test reveals a 98% efficiency after the fourth cycle. As established from various interpretations, the considerably enhanced photocatalytic performance of the heterojunction system can be attributed to the promoted charge carrier separation and transfer of photoexcited carriers, as well as the strong photo-redox ability established. Hence, the S-scheme Pt@BFO/O-CN heterojunction is a good candidate in the treatment of industrial wastewater for the mineralization of organic micropollutants, which pose a grievous threat to the environment.

X-ray photoemission and absorption study of the pyrochlore iridates (Eu1-xBix)2Ir2O7, 0 ⩽x ⩽ 1

The pyrochlore iridates (Eu1-xBi_x)₂Ir₂O₇(0⩽x⩽1) undergo an anomalous negative lattice expansion for small Bi-doping (x⩽0.035) (region I) and a normal lattice expansion forx⩾0.1(region II); this is accompanied by a transition from an insulating (and magnetically ordered) to a metallic (and with no magnetic ordering) ground state. Here, we investigate (Eu1-xBi_x)₂Ir₂O₇(0⩽x⩽1) using hard x-ray photoemission spectroscopy and x-ray absorption fine structure (XAFS) spectroscopy. By analyzing the Eu-L₃, Ir-L₃and Bi-L₂&L₃edges x-ray absorption near edge structure spectra and Eu-3dcore-level XPS spectra, we show that the metal cations retain their nominal valence, namely, Ir⁴⁺, Bi³⁺and Eu³⁺, respectively, throughout the series. The Ir-4fand Bi-4fcore-level XPS spectra consist of screened and unscreened doublets. The unscreened component is dominant In the insulating range (x⩽0.035), and in the metallic region (x⩾0.1), the screened component dominates the spectra. The Eu-3dcore-level spectra remain invariant under Bi doping. The extended XAFS data show that the coordination around the Ir remains well preserved throughout the series. The evolution of the valence band spectra near the Fermi energy with increasing Bi doping indicates the presence of strong Ir(5d)-Bi(6p) hybridization which drives the metal-to-insulator transition.

A comparative study for phosphate adsorption on amorphous FeOOH and goethite (α-FeOOH): An investigation of relationship between the surface chemistry and structure

Eutrophication is generally caused by excess nitrogen and phosphorus being released into surface waters by runoff. Developing adsorbents for adsorbing phosphate within soil buffer zones and/or water treatment columns may be effective methods to mitigate this problem. In this study, an amorphous FeOOH (AF) and a well-crystallized α-FeOOH (CF) was formulated to compare phosphate adsorption behavior. The physicochemical properties between these species showed significant differences in morphology, crystallization, zeta potential, and specific surface area. The AF exhibited higher phosphate uptake than CF. X-ray photoelectron spectroscopy (XPS) verified that the hydroxyl groups within AF were 13.28% higher than that in CF. The triply coordinated hydroxyl groups (μ₃-OH) associated with AF and CF appeared at different positions as shown in the diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analyses, confirming that AF contains more adsorption reactive sites (μ₃-OH). Mechanisms for monodentate formations and a stable six-member ring structure were proposed. The X-ray absorption near the edge structure (XANES) and XPS results suggested that the iron valence in AF was dominated by Fe (III). XANES also demonstrated that the amorphous structure found in the AF was caused by the disordered tetrahedron and octahedron alignments, leading to a higher phosphate adsorption.

Comparative density functional studies of pristine and doped bismuth ferrite polymorphs by GGA+U and meta-GGA SCAN+U

We analyzed the effect of nonmagnetic dopants Al, Ga, Sc, and In at the Fe-site on the phase stability, structural, and electronic properties of different bismuth ferrite (BiFeO₃) polymorphs in the framework of density functional theory with the Hubbard U correction (DFT+U). We started our consideration from the determination of the magnitude of the U parameter. First, we calculated the structural, electronic, and magnetic properties of the rhombohedral R3c-G phase of BiFeO₃ within the generalized gradient approximation (GGA) and strongly constrained and appropriately normed (SCAN) meta-GGA for different values of the U. Next, we compared these results with those obtained within the parameter-free hybrid functional. After determining the optimal values of the Hubbard U parameter we analyzed the total energies between the selected BiFeO₃ polymorphs without and with dopants within both GGA+U and SCAN+U. For all dopants the concentration was 12.5% which was close to their solubility limit in BiFeO₃ under ambient conditions. We found that none of these dopants led to the structural phase transition. However, DFT+U calculations revealed that the doping of BiFeO₃ with Al and Ga reduced the energy barrier between R3c-G and Cm-C phases whereas for Sc and In the energy difference between both phases increased. For the orthorhombic phases the considered dopants do not affect the energy barrier between them and the rhombohedral phase. In addition, the ferroelectric polarization does not change after replacing the Fe atom by the dopant for the all considered BiFeO₃ polymorphs.

NiMn-Based Bimetal-Organic Framework Nanosheets Supported on Multi-Channel Carbon Fibers for Efficient Oxygen Electrocatalysis

Developing noble-metal-free bifunctional oxygen electrocatalysts is of great significance for energy conversion and storage systems. Herein, we have developed a transformation method for growing NiMn-based bimetal-organic framework (NiMn-MOF) nanosheets on multi-channel carbon fibers (MCCF) as a bifunctional oxygen electrocatalyst. Owing to the desired components and architecture, the MCCF/NiMn-MOFs manifest comparable electrocatalytic performance towards oxygen reduction reaction (ORR) with the commercial Pt/C electrocatalyst and superior performance towards oxygen evolution reaction (OER) to the benchmark RuO₂ electrocatalyst. X-ray absorption fine structure (XAFS) spectroscopy and density functional theory (DFT) calculations reveal that the strong synergetic effect of adjacent Ni and Mn nodes within MCCF/NiMn-MOFs effectively promotes the thermodynamic formation of key *O and *OOH intermediates over active NiO₆ centers towards fast ORR and OER kinetics.

Local Structure and Magnetism of Fe2O3 Maghemite Nanocrystals: The Role of Crystal Dimension

Here we report on the impact of reducing the crystalline size on the structural and magnetic properties of γ-Fe2O3 maghemite nanoparticles. A set of polycrystalline specimens with crystallite size ranging from ~2 to ~50 nm was obtained combining microwave plasma synthesis and commercial samples. Crystallite size was derived by electron microscopy and synchrotron powder diffraction, which was used also to investigate the crystallographic structure. The local atomic structure was inquired combining pair distribution function (PDF) and X-ray absorption spectroscopy (XAS). PDF revealed that reducing the crystal dimension induces the depletion of the amount of Fe tetrahedral sites. XAS confirmed significant bond distance expansion and a loose Fe-Fe connectivity between octahedral and tetrahedral sites. Molecular dynamics revealed important surface effects, whose implementation in PDF reproduces the first shells of experimental curves. The structural disorder affects the magnetic properties more and more with decreasing the nanoparticle size. In particular, the saturation magnetization reduces, revealing a spin canting effect. Moreover, a large effective magnetic anisotropy is measured at low temperature together with an exchange bias effect, a behavior that we related to the existence of a highly disordered glassy magnetic phase.