Designing a Periodic Structure with a Composition Gradient Stack Unit to Realize a Good Comprehensive Dielectric Property of Yttrium Iron Garnet Multilayer Film.

Y3Fe5O12 (YIG) thin films are highly needed in microwave devices, but the low saturation magnetization and low dielectric constant greatly limit the application of YIG thin films. It was reported that the ion substitution, for example, Pr3+, could increase the dielectric constant of Y3-xPrxFe5O12 (YPrxIG). Unfortunately, the dielectric loss would also be significantly increased. In this work, [YPr0.20IG/YPr0.15IG/YPr0.10IG]N multilayer films were fabricated via the chemical solution deposition method, by designing a periodic structure with the [YPr0.20IG/YPr0.15IG/YPr0.10IG] composition gradient stack. In comparison to the average composition of YPr0.15IG, high saturation magnetization, high dielectric constant, and low loss were successfully simultaneously achieved in the multilayer structure. The N = 6 film exhibited a higher saturation magnetization of 252.8 emu/cm3 than the value (213.1) of the YPr0.15IG (average composition) film. The dielectric constant of the N = 6 film reached 25.6 in contrast to the value of 18.3 for the YPr0.15IG film at 12.4 GHz, which was the contribution of the rapid flip of the electric dipole of a single-unit dielectric material and the accumulation of interface charge. Furthermore, the dielectric loss of the film with N = 6 decreased to 0.0036 compared with the value (0.0102) of the average composition film. This work demonstrated a strategy of designing a periodic structure with a composition gradient stack unit to realize a good comprehensive dielectric property through taking advantage of the multiple effects of "coherent growth, component matching, and interface accumulation".

Architecture Design and Catalytic Activity: Non-Noble Bimetallic CoFe/fe3 O4 Core-Shell Structures for CO2 Hydrogenation.

Non-noble metal catalysts now play a key role in promoting efficiently and economically catalytic reduction of CO2 into clean energy, which is an important strategy to ameliorate global warming and resource shortage issues. Here, a non-noble bimetallic catalyst of CoFe/Fe3 O4 nanoparticles is successfully designed with a core-shell structure that is well dispersed on the defect-rich carbon substrate for the hydrogenation of CO2 under mild conditions. The catalysts exhibit a high CO2 conversion activity with the rate of 30% and CO selectivity of 99%, and extremely robust stability without performance decay over 90 h in the reverse water gas shift reaction process. Notably, it is found that the reversible exsolution/dissolution of cobalt in the Fe3 O4 shell will lead to a dynamic and reversible deactivation/regeneration of the catalysts, accompanying by shell thickness breathing during the repeated cycles, via atomic structure study of the catalysts at different reaction stages. Combined with density functional theory calculations, the catalytic activity reversible regeneration mechanism is proposed. This work reveals the structure-property relationship for rational structure design of the advanced non-noble metallic catalyst materials with much improved performance.

Structural evolution of MXenes and their composites for electromagnetic interference shielding applications.

Nowadays, the extensive utilization of electronic devices and equipment inevitably leads to severe electromagnetic interference (EMI) issues. Therefore, EMI shielding materials have drawn considerable attention, and great effort has been devoted to the exploration of high-efficiency EMI shielding materials. As a novel kind of 2D transition metal carbide material, MXenes have been widely investigated for EMI shielding in the past few years due to their extraordinary electrical conductivity, large specific surface area, light weight, and easy processability. In view of the great achievements in MXene-based materials for EMI shielding, herein, we reviewed the recent studies on the structural design and evolution of MXenes and their composites for EMI shielding. First, the methods for structural control of MXenes, including HF etching, in situ HF etching, fluorine-free etching, electrochemical etching, and molten salt etching, are systematically summarized. Then we illustrate the fundamental relationship between the microstructure of MXenes and the EMI shielding mechanism. In the following, the effects of different synthesis methods and structures of MXene-based composite materials as well as their EMI shielding performances are comprehensively discussed. Lastly, future prospects for the development of MXene-based composite materials in EMI shielding applications are commented on.

Chemical Vapor Deposition Mechanism of Graphene-Encapsulated Au Nanoparticle Heterostructures and Their Plasmonics.

Direct encapsulation of graphene shells on noble metal nanoparticles via chemical vapor deposition (CVD) has been recently reported as a unique way to design and fabricate new plasmonic heterostructures. But currently, the fundamental nature of the growth mechanism of graphene layers on metal nanostructures is still unknown. Herein, we report a systematic investigation on the CVD growth of graphene-encapsulated Au nanoparticles (Au@G) by combining an experimental parameter study and theoretical modeling. We studied the effect of growth temperature, duration, hydrocarbon precursor concentration, and extent of reducing (H2) environment on the morphology of the products. In addition, the influence of plasma oxidation conditions for the surface oxidation of gold nanoparticles on the graphene shell growth is evaluated in combination with thermodynamic calculations. We find that these parameters critically aid in the evolution of graphene shells around gold nanoparticles and allow for controlling shell thickness, graphene shell quality and morphology, and hybrid nanoparticle diameter. An optimized condition including the growth temperature of ∼675 °C, duration of 30 min, and xylene feed rate of ∼10 mL/h with 10% H2/Ar carrier gas was finally obtained for the best morphology evolution. We further performed finite-element analysis (FEA) simulations to understand the equivalent von Mises stress distribution and discrete dipolar approximation (DDA) calculation to reveal the optical properties of such new core-shell heterostructures. This study brings new insight to the nature of CVD mechanism of Au@G and might help guiding their controlled growth and future design and application in plasmonic applications.

Solution-Processed All-V2 O5 Battery.

Exploring new battery technologies will promote the advance of energy storage systems. Designing a symmetrical-structured rechargeable battery with the same electrode materials is a meaningful exploration for battery technology. Here, a solution-processed all-V2 O5 rechargeable battery with V2 O5 as both anode and cathode is presented, in which the anionic/cationic redox reactions are decoupled by precisely clamping its working potential windows. The battery shows good electrochemical performance with high capacity of 151 mAh g-1 at 0.10 C, good rate performance with 70% capacity retention when the current increases from 0.10 to 5 C, and promising cycling stability over 83% capacity retention after 900 cycles at 1 C. Moreover, the battery is highly profitable for simplified fabrication and scalable production, which benefits from its symmetrical configuration as well as the solution-processed strategy. This work offers a new paradigm to construct advanced symmetrical energy storage devices.

Origin of Fracture-Resistance to Large Volume Change in Cu-Substituted Co3 O4 Electrodes.

The electrode materials conducive to conversion reactions undergo large volume change in cycles which restrict their further development. It has been demonstrated that incorporation of a third element into metal oxides can improve the cycling stability while the mechanism remains unknown. Here, an in situ and ex situ electron microscopy investigation of structural evolutions of Cu-substituted Co3 O4 supplemented by first-principles calculations is reported to reveal the mechanism. An interconnected framework of ultrathin metallic copper formed provides a high conductivity backbone and cohesive support to accommodate the volume change and has a cube-on-cube orientation relationship with Li2 O. In charge, a portion of Cu metal is oxidized to CuO, which maintains a cube-on-cube orientation relationship with Cu. The Co metal and oxides remain as nanoclusters (less than 5 nm) thus active in subsequent cycles. This adaptive architecture accommodates the formation of Li2 O in the discharge cycle and underpins the catalytic activity of Li2 O decomposition in the charge cycle.

Electrochemistry of Selenium with Sodium and Lithium: Kinetics and Reaction Mechanism.

There are economic and environmental advantages by replacing Li with Na in energy storage. However, sluggishness in the charge/discharge reaction and low capacity are among the major obstacles to development of high-power sodium-ion batteries. Among the electrode materials recently developed for sodium-ion batteries, selenium shows considerable promise because of its high capacity and good cycling ability. Herein, we have investigated the mechanism and kinetics of both sodiation and lithiation reactions with selenium nanotubes, using in situ transmission electron microscopy. Sodiation of a selenium nanotube exhibits a three-step reaction mechanism: (1) the selenium single crystal transforms into an amorphous phase Na0.5Se; (2) the Na0.5Se amorphous phase crystallizes to form a polycrystalline Na2Se2 phase; and (3) Na2Se2 transforms into the Na2Se phase. Under similar conditions, the lithiation of Se exhibits a one-step reaction mechanism, with phase transformation from single-crystalline Se to a Li2Se. Intriguingly, sodiation kinetics is generally about 4-5 times faster than that of lithiation, and the kinetics during the different stages of sodiation is different. Na-based intermediate phases are found to have improved electronic and ionic conductivity compared to those of Li compounds by first-principles density functional theory calculations.