Substitutional Cd Dopant as Photohole Transfer Mediator Boosting Photoelectrochemical Solar Energy Conversion of 2D Cd-ZnIn2 S4 Photoanode.

Fast recombination dynamics of photocarriers competing with sluggish surface photohole oxidation kinetics severely restricts the photoelectrochemical (PEC) conversion efficiency of photoanode. Here, a defect engineering strategy is developed to regulate photohole transfer and interfacial injection dynamics of 2D ZnIn2 S4 (ZIS). Via selectively introducing substitutional Cd dopant at Zn sites of the ZIS basal plane, energy band structure and surface electrochemical activity are successfully modulated in the Cd-doped ZIS (Cd-ZIS) nanosheet array photoanode. Comprehensive characterizations manifest that a shallow acceptor level induced by Cd doping and superior electrochemical activity make surface Cd dopants simultaneously act as capture centers and active sites to mediate photohole dynamics at the reaction interface. In depth photocarrier dynamics analysis demonstrates that highly efficient photohole capture of Cd dopants brings about effective space separation of photocarriers and acceleration of surface reaction kinetics. Therefore, the optimum 2D Cd-ZIS achieves excellent PEC solar energy conversion efficiency with a photocurrent density of 5.1 mA cm-2 at 1.23 VRHE and a record of applied bias photon-to-current efficiency (ABPE) of 3.0%. This work sheds light on a microstructure design strategy to effectively regulate photohole dynamics for the next-generation semiconducting PEC photoanodes.

Spin Polarization of 2D Weyl Semimetal Fe2Sn Enabling High Hydrogen Evolution Reaction Activity.

It is well known that magnetic field is one of the effective tools to improve the activity of hydrogen evolution reaction (HER), but considering the inconvenient application of an external magnetic field, it is essential to find a ferromagnetic material with high HER activity itself. Fortunately, recent study has shown that the two-dimmention (2D) Fe2Sn monolayer is a stable ferromagnetic topological Weyl semimetal material with high Tc of 433 K. Here, we report the Fe2Sn monolayer can be used as an alternative HER catalyst compared with expensive platinum (Pt). Our first-principles results show that the Gibbs free energy (ΔGH*) value of the spin polarized Fe2Sn monolayer is -0.06 eV, much better than that without considering spin polarization (-1.23 eV). Moreover, the kinetic analysis demonstrates that the HER occurs on the Fe2Sn monolayer according to the Volmer-Tafel mechanism with low energy barriers. Hence, our findings provide obvious evidence for spin-polarization-improved HER activity, paving a new way to design high-performance HER catalysts.

Optimized electromagnetic wave absorption ofα-Fe2O3@MoS2nanocomposites with core-shell structure.

Core-shell structures and interfacial polarization are of great significance to meet the diversified requirements of microwave attenuation. Herein,α-Fe2O3@MoS2nanocomposites are fabricated via a simple two-step hydrothermal process, in which MoS2nanosheets as the shell are self-assembled andα-Fe2O3microdrums are used as the core to constitute a special flower-like morphology with core-shell structure. This structure can provide more interface contact to achieve strong interfacial polarization and possibly offer more multiple reflection and scattering of electromagnetic waves. Furthermore, the microwave dissipation performances ofα-Fe2O3@MoS2nanocomposites can be significantly improved through construction of core-shell structure and flower-like morphology, controlling the content ofα-Fe2O3microdrums and adjusting the filler loading ratios. This work proves that the as-synthesized nanocomposites achieve excellent effective absorption bandwidth and outstanding electromagnetic wave absorption capabilities due to their special interfaces, core-shell structures and good impedance matching conditions. Therefore,α-Fe2O3@MoS2nanocomposites are expected to be a novel and desirable candidate for high-performance electromagnetic wave absorbers.

Large perpendicular magnetic anisotropy of transition metal dimers driven by polarization switching of a two-dimensional ferroelectric In2Se3 substrate.

Large perpendicular magnetic anisotropy (MA) is highly desirable for realizing atomic-scale magnetic data storage which represents the ultimate limit of the density of magnetic recording. In this work, we study the MA of transition metal dimers Co-Os, Co-Co and Os-Os adsorbed on two-dimensional ferroelectric In2Se3 (In2Se3-CoOs, In2Se3-OsCo, In2Se3-CoCo and In2Se3-OsOs) using first-principles calculations. We find that the Co-Os dimer in In2Se3-CoOs has a total magnetic anisotropy energy (MAE) of ∼40 meV. The MAE arising from the Os atom in In2Se3-CoOs is up to ∼60 meV. Such large MAE is attributed to the high spin-orbit coupling constant and the onefold coordination of the Os atom. In addition, perpendicular MA can be enhanced in In2Se3-CoOs and induced in In2Se3-OsCo, In2Se3-CoCo and In2Se3-OsOs by the ferroelectric polarization reversal of In2Se3. We demonstrate that the enlargement of exchange splitting of dxy/dx2-y2 and dxz/dyz orbitals for Os atoms in In2Se3-OsOs, Co atom in In2Se3-CoOs and Os and Co atoms in In2Se3-OsCo is responsible for the increase of MAE; while, for the upper Co atom in In2Se3-CoCo and the Os atom in In2Se3-CoOs, the energy rise of the dz2 orbital owing to the change of the crystal field effect by the reversal of ferroelectric polarization results in the increase of MAE.

Orientational Alignment of Oxygen Vacancies: Electric-Field-Inducing Conductive Channels in TiO2 Film to Boost Photocatalytic Conversion of CO2 into CO.

Oxide semiconductors are widely used in the photocatalytic fields, and introducing oxygen vacancies is an effective strategy to improve their photocatalytic efficiency. However, oxygen vacancies in the bulk often act as the recombination centers of electron-hole pairs, which accelerates the recombination of electron-hole pairs. In this paper, we propose a strategy of electric field treatment and apply it to a TiO2 film with oxygen vacancies to promote the photocatalytic efficiency. After treatment by an electric field, the conductive channels consisting of oxygen vacancies are formed in the TiO2 film, which greatly decreases the resistance by almost 6 × 103 times. The yield of CO can reach up to 1.729 mmol gcat-1 h-1, which is one of the best performances among the reported TiO2-based catalysts. This work provides an effective and feasible way for enhancing photocatalytic activity through an electric field, and this method is promising for wide use in the field of catalysis.

Enhancing the Curie temperature of two-dimensional monolayer CrI3 by introducing I-vacancies and interstitial H-atoms.

The discovery of two-dimensional monolayer CrI3 provides a promising possibility for developing spintronic devices. However, the low Curie temperature is an obstacle for practical applications. Here, based on the consideration of the superexchange interaction of ferromagnetic coupling, we investigate the effect of introducing I-vacancies and interstitial H-atoms on the Curie temperature of monolayer CrI3 by using first-principles calculations and Monte Carlo simulations. Our theoretical conclusions show that the Curie temperature of Cr8I23 (CrI2.875), Cr8I22 (CrI2.75) and Cr8I24H (CrI3H0.125) significantly increases to 97.0, 82.5 and 112.4 K, respectively. Moreover, the magnetic moment of the Cr atom increases from 3.10 to 3.45 and 3.46µB in monolayers Cr8I23 and Cr8I22, respectively. We provide more alternative approaches to effectively enhance the Curie temperature of monolayer CrI3, which will help both theoretical and experimental researchers to directly predict the change in Curie temperature of CrI3 and its analogs through structural information.

Critical phenomenon in the room-temperature ferromagnet Ce0.65Mg0.35Co3 prepared by high-pressure annealing.

Numerous studies have showed evidence that high-pressure annealing (HPA) can modify the crystal and electronic structure significantly, which thus probably alters the magnetic ordering with a different universality class. In this work, we investigate the effects of HPA on the critical behaviors of magnetization in a room-temperature ferromagnet Ce0.65Mg0.35Co3. We observe the HPA compound after annealing at 2 GPa undergoing a second-order phase transition with a decreased Curie temperature. Using the DC magnetization data, the critical exponents ß, γ and δ are calculated independently by three methods including the modified Arrott plot, the Kouvel-Fisher plot, and critical isotherm analysis. The obtained critical parameters together with the magnetization data obey the scaling equation of state, indicating that they are intrinsic and unambiguous. Furthermore, we notice that HPA not only reduces the intensity of exchange coupling, but also elongates the exchange range with J(r) ∼r-4.467, which leads to a universality class different from that of the conventional compound and the existing theoretical models.

Surface-state-mediated interfacial charge dynamics between carbon dots and ZnO toward highly promoting photocatalytic activity.

Design of hybrid systems for photocatalytic application tends to be restricted by lacking interfacial coupling and fast charge recombination in the body competing with interface dynamics. In this work, the reduced carbon dots (rCDs) with numerous surface hydroxyl groups were deliberately anchored onto flower-like ZnO spheres with a highly exposed surface area to form heterointerfaces with sufficient interfacial electronic coupling. The incorporated rCDs evidently promote the light harvesting and charge separation of the binary hybrid system, resulting in highly enhanced photocatalytic Cr(VI) degradation performance. Ultrafast time-resolved spectra reveal that the surface C-OH bonds of rCDs play a crucial role at the heterointerfaces to regulate the charge dynamics. The long-lived surface C-OH states not only act as electron donors but also become electron mediators to rapidly capture the photoelectrons from the intrinsic state in the time-domain of 1 ps and induce a much longer lifetime for achieving highly efficient photoelectron injection from rCDs to ZnO. These results manifest that rCDs can be a promising photosensitizer to apply in photocatalytic pollutant treatment and energy conversion fields.

Controllable Negative Thermal Expansion by Mechanical Pulverizing in Hexagonal Mn0.965Co1.035Ge Compounds.

Negative thermal expansion (NTE) material as a compensator is very important for accurately controlling the thermal expansion of materials. Along with the magnitude of the coefficient of thermal expansion, the operating temperature window of the NTE materials is also a major concern. However, only a few of the NTE materials possess both a large operating temperature range and a large thermal expansion coefficient. To explore this type of new NTE material, the Mn0.965Co1.035Ge fine powders were prepared by mechanical ball milling (BM). These fine powders show a largely extended NTE operation temperature window simultaneously possessing a giant thermal expansion coefficient. For samples treated with different BM times, such as the BM-0.5h, BM-4h, and BM-12 h samples, the operating temperature window (Δ T) and linear thermal expansion coefficient (αL) are 167 K (222-389 K) and ∼ -63 ppm/K, 221 K (140-360 K) and ∼ -41.3 ppm/K, and 208 K (234-442 K) and ∼ -40 ppm/K, respectively, which are larger than most well-known NTE materials. More strikingly, all BM samples have a large constant linear NTE coefficient with an ultrawide temperature window covering room temperature. For these three samples, these values are ∼ -52 ppm/K (140 K), ∼ -58.3 ppm/K (110 K), and ∼ -65 ppm/K (80 K), respectively. The origin of the excellent NTE properties is discussed based on the thermomagnetic measurements and X-ray absorption spectroscopic results.

A New Design of a Single-Device 3D Hall Sensor: Cross-Shaped 3D Hall Sensor.

In this paper, a new single-device three-dimensional (3D) Hall sensor called a cross-shaped 3D Hall device is designed based on the five-contact vertical Hall device. Some of the device parameters are based on 0.18 µm BCDliteTM technology provided by GLOBALFOUNDRIES. Two-dimensional (2D) and 3D finite element models implemented in COMSOL are applied to understand the device behavior under a constant magnetic field. Besides this, the influence of the sensing contacts, active region's depth, and P-type layers are taken into account by analyzing the distribution of the voltage along the top edge and the current density inside the devices. Due to the short-circuiting effect, the sensing contacts lead to degradation in sensitivities. The P-type layers and a deeper active region in turn are responsible for the improvement of sensitivities. To distinguish the P-type layer from the active region which plays the dominant role in reducing the short-circuiting effect, the current-related sensitivity of the top edge (Stop) is defined. It is found that the short-circuiting effect fades as the depth of the active region grows. Despite the P-type layers, the behavior changes a little. When the depth of the active region is 7 µm and the thickness of the P-type layers is 3 µm, the sensitivities in the x, y, and z directions can reach 91.70 V/AT, 92.36 V/AT, and 87.10 V/AT, respectively.

Giant barocaloric effects with a wide refrigeration temperature range in ethylene vinyl acetate copolymers.

Solid-state cooling technology based on the caloric effects of phase-transition materials has been a research hotspot due to its environmental friendliness and high efficiency, but limited for practical applications due to its narrow working temperature region. Here, we report giant barocaloric effects based on pressure-driven liquid-solid phase transitions in elastic copolymers of ethylene and vinyl acetate. Giant adiabatic temperature changes of up to 29.6/-26.9 K are directly observed under rapid compressions/decompressions of 400 MPa near the liquid-solid phase transition points. Strikingly, since both the solid and the liquid sides can show giant barocaloric effects, a very broad refrigeration temperature region of more than 110 K is achieved in these copolymers. Furthermore, a cooling prototype is designed to demonstrate the potential applications of these liquid/elastic barocaloric materials. Our study sheds light on exploring liquid-solid phase transition materials for the next-generation refrigerators.

Vertical-orbital band center as an activity descriptor for hydrogen evolution reaction on single-atom-anchored 2D catalysts.

The d-band center descriptor based on the adsorption strength of adsorbate has been widely used in understanding and predicting the catalytic activity in various metal catalysts. However, its applicability is unsure for the single-atom-anchored two-dimensional (2D) catalysts. Here, taking the hydrogen (H) adsorption on the single-atom-anchored 2D basal plane as example, we examine the influence of orbitals interaction on the bond strength of hydrogen adsorption. We find that the adsorption of H is formed mainly via the hybridization between the 1s orbital of H and the vertical dz2orbital of anchored atoms. The other four projected d orbitals (dxy/dx2-y2, dxz/dyz) have no contribution to the H chemical bond. There is an explicit linear relation between the dz2-band center and the H bond strength. The dz2-band center is proposed as an activity descriptor for hydrogen evolution reaction (HER). We demonstrate that the dz2-band center is valid for the single-atom active sites on a single facet, such as the basal plane of 2D nanosheets. For the surface with multiple facets, such as the surface of three-dimensional (3D) polyhedral nanoparticles, the d-band center is more suitable.

Advancing Applications of Black Phosphorus and BP-Analog Materials in Photo/Electrocatalysis through Structure Engineering and Surface Modulation.

Black phosphorus (BP), an emerging 2D material semiconductor material, exhibits unique properties and promising application prospects for photo/electrocatalysis. However, the applications of BP in photo/electrocatalysis are hampered by the instability as well as low catalysis efficiency. Recently, tremendous efforts have been dedicated toward modulating its intrinsic structure, electronic property, and charge separation for enhanced photo/electrocatalytic performance through structure engineering. Simultaneously, the search for new substitute materials that are BP-analogous is ongoing. Herein, the latest theoretical and experimental progress made in the structural/surface engineering strategies and advanced applications of BP and BP-analog materials in relation to photo/electrocatalysis are extensively explored, and a presentation of the future opportunities and challenges of the materials is included at the end.

Two-dimensional bimetallic phosphide ultrathin nanosheets as non-noble electrocatalysts for a highly efficient oxygen evolution reaction.

Highly efficient non-noble metal oxygen evolution reaction (OER) catalysts are urgently needed for the practical application of electrochemical energy technology. Herein, we report two-dimensional (2D) bimetallic phosphide (Co1-xFexP) ultrathin nanosheets as new OER catalysts. The two-dimensional (2D) morphology of the nanosheets and the synergistic effect between different transition-metal elements made contributions to the OER catalysis. By optimizing the doping ratio of the Fe atoms, the Co0.8Fe0.2P nanosheets showed the best OER performance with a small overpotential of 270 mV versus a rotating hydrogen electrode at a current density of 10 mA cm-2 and low Tafel slope of 50 mV dec-1 in an alkaline electrolyte. Moderate iron doping improved the degree of oxidation at the surface of CoP nanosheets and preserved the conductive and chemically stabilizing host, thereby enhancing the OER activity. Our findings could aid the rational design of novel non-layered 2D nanomaterial OER catalysts.

Nonvolatile Control of Magnetocaloric Operating Temperature by Low Voltage.

The limited operating temperature is the main obstacle for the practical applications of magnetic refrigeration. In this work, the voltage control of magnetocaloric effect (MCE) is investigated in a La0.7Sr0.3MnO3 (LSMO)/CeO2/Pt device. Different from the conventional method of volatile manipulating MCE by large-voltage-induced strain, nonvolatile manipulation of magnetocaloric operating temperature with good stability is realized in the LSMO film by applying low voltages of less than 2.3 V. The experimental results demonstrate that the magnetic entropy change peak temperature for the LSMO film can be extended from 302 to 312 K by voltage. This nonvolatile effect can be well-understood with the resistive switching mechanism and has potential in promoting microscale refrigeration technology.

Double Perovskites as Model Bifunctional Catalysts toward Rational Design: The Correlation between Electrocatalytic Activity and Complex Spin Configuration.

The oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) properties of double perovskite catalysts La2CoMnO6-δ and La2NiMnO6-δ have been investigated. The experimental results demonstrate that these samples are efficient OER and ORR catalysts. For the La2CoMnO6 compound annealed at 600 °C, its half-wave potential of ORR in alkaline media is as low as 0.75 V versus reversible hydrogen electrode. As for the correlation between electrocatalytic activities and the eg orbital occupation numbers, it is found that the eg-filling rule proposed by Shao-Horn and co-workers is valid in La2CoMnO6-δ when the eg electrons of two kinds of transition-metal ions are all considered. In the case of La2NiMnO6-δ, because the samples which prepared in different conditions all have unity average eg occupation, the eg-filling rule should be amended. We suggest that the average deviation from unity eg occupation is a suitable activity descriptor, which can be used to design perovskite catalysts with unity average eg occupation.

Ultrafine bimetallic phosphide nanoparticles embedded in carbon nanosheets: two-dimensional metal-organic framework-derived non-noble electrocatalysts for the highly efficient oxygen evolution reaction.

The development of noble-metal-free highly efficient oxygen evolution reaction (OER) catalysts is crucial for electrochemical energy technology but still challenging. Herein, ultrafine cobalt-iron bimetallic phosphide nanoparticles embedded in carbon nanosheets are synthesized using two-dimensional (2D) metal-organic frameworks (MOFs) as the precursor. The 2D morphology of the carbon matrix and the ultrafine character of Co1-xFexP nanoparticles make contributions to OER catalysis. By optimizing the molar ratio of Co/Fe atoms in MOFs, a series of Co1-xFexP/C catalysts are prepared. Among them, Co0.7Fe0.3P/C shows the best OER performance with an overpotential of 270 mV at a current density of 10 mA cm-2 and an ultralow Tafel slope of 27 mV dec-1 in an alkaline electrolyte. Moderate iron doping preserves the catalytically active sites and improves the ability to be oxidized of the surface of Co1-xFexP nanoparticles, and thus enhances the OER activity. Our finding paves the way to the rational design of the morphology and chemical composition of OER catalysts.

Enhanced Photocatalytic Performance through Magnetic Field Boosting Carrier Transport.

The promotion of magnetic field on catalytic performance has attracted extensive attention for a long time, and substantial improvements have been achieved in some catalysis fields. However, because the Zeeman energy is several orders of magnitude weaker, magnetic field seems unable to alter the band structure and has a negligible effect on semiconductor photocatalytic performance, which makes this task a great challenge. On the other hand, the spin-related behavior usually plays an important role in determining catalytic performance. For example, in some molecular catalysis, such as photosystem II, ferromagnetic alignment of the active material results in spin-oriented electrons, which are selected and accumulated at the interface, leading to great promotion of the oxygen evolution reaction activity. Here, we propose a magnetoresistance-related strategy to boost the carrier transfer efficiency and apply it in α-Fe2O3/reduced graphene oxide hybrid nanostructures (α-Fe2O3/rGO) to improve the photocatalytic performance under magnetic field. We show that both the degradation rate constant and photocurrent density of α-Fe2O3/rGO can be dramatically enhanced with the application of magnetic field, indicating the promotion of the photocatalytic performance.

Electric Field Manipulated Multilevel Magnetic States Storage in FePt/(011) PMN-PT Heterostructure.

In the current information society, the realization of a magnetic storage technique with energy-efficient design and high storage density is greatly desirable. Here, we demonstrate that, without bias magnetic field, different values of remanent magnetization (Mr) can be obtained in a FePt/0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-PT) heterostructure by applying a unipolar electric field across the substrate. These multilevel magnetic signals can serve as writing data bits in a storage device, which remarkably increases the storage density. As for the data reading, these multilevel Mr values can be read nondestructively and distinguishably using a commercial giant magnetoresistance magnetic sensor by converting the magnetic signal to voltage signal. Furthermore, these multilevel voltage signals show good retention and switching property, which enables promising applications in electric-writing magnetic-reading memory devices with low power consumption and high storage density.

Design of Hybrid Nanostructural Arrays to Manipulate SERS-Active Substrates by Nanosphere Lithography.

An easy-handling and low-cost method is utilized to controllably fabricate nanopattern arrays as the surface-enhanced Raman scattering (SERS) active substrates with high density of SERS-active areas (hot spots). A hybrid silver array of nanocaps and nanotriangles are prepared by combining magnetron sputtering and plasma etching. By adjusting the etching time of polystyrene (PS) colloid spheres array in silver nanobowls, the morphology of the arrays can be easily manipulated to control the formation and distribution of hot spots. The experimental results show that the hybrid nanostructural arrays have large enhancement factor, which is estimated to be seven times larger than that in the array of nanocaps and three times larger than that in the array of nanorings and nanoparticles. According to the results of finite-difference time-domain simulation, the excellent SERS performance of this array is ascribed to the high density of hot spots and enhanced electromagnetic field.