Modularization of Regional Electronic Structure over Defect-Rich CeO2 Rods for Enhancing Photogenerated Charge Transfer and CO2 Activation.

Oxygen vacancy (OV) engineering has been widely applied in different types of metal oxide-based photocatalytic reactions. Our study has shown that the redistributed OVs resulting from voids in CeO2 rods lead to significant differences in the band structure in space. The flat energy band within the highly crystallized bulk region hinders the recombination of photogenerated carrier pairs during the transfer process. The downward curved energy band in the surface region enhances the activation of the absorbents. Therefore, the localization of the band structure through crystal structure regionalization renders V-CeO2 capable of achieving efficient utilization of photogenerated carriers. Practically, the V-CeO2 rod shows a remarkable turnover number of 190.58 µmol g-1 h-1 in CO2 photoreduction, which is ∼9.4 times higher than that of D-CeO2 (20.46 µmol g-1 h-1). The designed modularization structure in our work is expected to provide important inspiration and guidance in coordinating the kinetic behavior of carriers in OV defect-rich photocatalysts.

Atomic-scale insights into quantum-order parameters in bismuth-doped iron garnet.

Bismuth and rare earth elements have been identified as effective substituent elements in the iron garnet structure, allowing an enhancement in magneto-optical response by several orders of magnitude in the visible and near-infrared region. Various mechanisms have been proposed to account for such enhancement, but testing of these ideas is hampered by a lack of suitable experimental data, where information is required not only regarding the lattice sites where substituent atoms are located but also how these atoms affect various order parameters. Here, we show for a Bi-substituted lutetium iron garnet how a suite of advanced electron microscopy techniques, combined with theoretical calculations, can be used to determine the interactions between a range of quantum-order parameters, including lattice, charge, spin, orbital, and crystal field splitting energy. In particular, we determine how the Bi distribution results in lattice distortions that are coupled with changes in electronic structure at certain lattice sites. These results reveal that these lattice distortions result in a decrease in the crystal-field splitting energies at Fe sites and in a lifted orbital degeneracy at octahedral sites, while the antiferromagnetic spin order remains preserved, thereby contributing to enhanced magneto-optical response in bismuth-substituted iron garnet. The combination of subangstrom imaging techniques and atomic-scale spectroscopy opens up possibilities for revealing insights into hidden coupling effects between multiple quantum-order parameters, thereby further guiding research and development for a wide range of complex functional materials.

Electron energy loss spectroscopy and first-principles study of GaN via Zn doping.

The electronic structure of GaN and GaN:Zn was investigated by electron energy loss spectroscopy and first-principles calculations. In the low-loss spectrum, the interband transitions are assigned to the observed energy loss peaks. After Zn doping, impurity levels are introduced to the density of states and hybrid orbitals of N 2p and Zn 3d are formed around the Fermi level. In the nitrogen K-edge, an additional peak was observed due to the formation of donor defect states. A core-hole effect is believed to be significant for simulation of the N K-edge for both GaN and GaN:Zn.

Strengthening Effect of Nb on Ferrite Grain Boundary in X70 Pipeline Steel.

Understanding the strengthening effect of niobium on ferrite grain boundaries from the perspective of valence electron structures will help to use niobium and other microalloying elements more effectively to improve the performance of steel materials. In this paper, the effect of niobium element on ferrite grain boundary strengthening is studied based on microstructure analysis at the nanometer scale. The enrichment of niobium in pipeline steel at ferrite boundary was observed by a three-dimensional atomic probe test. Segregation of Nb is observed in the ferrite grain boundaries of X70 steel, and its maximum concentration is 0.294-0.466 at.%. The charges in the occupancy of the Fe 3d state in grain and grain boundary were 7.23 and 7.37, respectively, based on quantitative analysis of electron energy loss spectra (EELS). The first-principle calculation suggests that the charges in the occupancy of 3d state for grain boundary iron are 6.57 and 6.68, respectively, before and after the Nb doping (with an increase of 1.67%), which reveals a similar trend to that of the EELS results. Through Nb alloying, the 3d valence electronic density of the state of Fe in grain boundary moves to a lower energy, which can reduce the total energy of the system and make the grain boundary more stable. Meanwhile, the charges in the occupancy of the 3d state for Fe in the grain boundary increases, providing more electrons for grain boundary bonding. These improve the strength and toughness of the material. This work provides a fundamental understanding for pipeline steel strengthening by element alloying.

Electronic structure of multiferroic BiFeO3: Electron energy-loss spectroscopy and first-principles study.

The electronic structure of BiFeO3 has been investigated by using electron energy loss spectrum and first-principle calculations. Assignments of the individual interband transitions have been accomplished by comparing the interband transition energy with the calculated PDOS. The DOS is mainly divided into two regions, the hybridized region of O 2p with Fe 3p in the valence band and that of O 2p hybridized with Bi 6p in the conduction band. From the simulation of high energy-loss near-edge structure, the core-hole effect is believed to be more significant. The feature groups for the experimental spectra of O K-edge and Fe L2,3-edge are consistent with simulation results.

Effect of asymmetric morphology on coupling surface plasmon modes and generalized plasmon ruler.

Plasmon coupling in aggregated noble metal systems can provide a path to manipulate the optical response purposefully and possesses a wide range of application. Previously, most studies focused on the coupling behavior of Ag-Ag dimers with the same shape. However, plasmon coupling between nanoparticles at different morphologies can provide a new way to modulate optical properties due to broken of symmetry. In this work, we investigate systematically the coupling modes of asymmetric Ag-Ag heterodimers consisting of different morphologies by the boundary element method (BEM). Herein nanoparticles with different surface curvatures (modified by rounding parameter e) are constructed and combined as asymmetric Ag-Ag heterodimers. Simulated electron energy loss spectroscopy (EELS) spectra and eigenmodes are combined to analyze the evolution of coupling modes. The mode energy degeneracy and degeneracy breaking phenomena are found, while the charge states are always not degenerate, for the first time by modulating symmetry of the morphology. It is also found that coupling gap mode G2 can only be excited for Ag-Ag heterodimers with quite small separation distance, and will be greatly influenced by nanogap morphology. The rounded effect can also cause distinct blue shift of bounding dipolar modes. These results provide the possibility to modulate optical response by using different asymmetric dimers effectively. In contrast, optical response of high-order coupling modes is less sensitive to topographic effect than that of low-order coupling modes. Moreover, plasmon ruler for asymmetric Ag-Ag heterodimers is investigated and we demonstrate that a generalized plasmon ruler is applicable to predict the relative shift of coupling dipolar due to change of separation distance.