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.

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.

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.

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.

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.

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.

Multilevel Resistance Switching Memory in La2/3Ba1/3MnO3/0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (011) Heterostructure by Combined Straintronics-Spintronics.

We demonstrate a memory device with multifield switchable multilevel states at room temperature based on the integration of straintronics and spintronics in a La2/3Ba1/3MnO3/0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-PT) (011) heterostructure. By precisely controlling the electric field applied on the PMN-PT substrate, multiple nonvolatile resistance states can be generated in La2/3Ba1/3MnO3 films, which can be ascribed to the strain-modulated metal-insulator transition and phase separation of Manganite. Furthermore, because of the strong coupling between spin and charge degrees of freedom, the resistance of the La2/3Ba1/3MnO3 film can be readily modulated by magnetic field over a broad temperature range. Therefore, by combining electroresistance and magnetoresistance effects, multilevel resistance states with excellent retention and endurance properties can be achieved at room temperature with the coactions of electric and magnetic fields. The incorporation of ferroelastic strain and magnetic and resistive properties in memory cells suggests a promising approach for multistate, high-density, and low-power consumption electronic memory devices.

Electric field control of the magnetocaloric effect.

Through strain-mediated magnetoelectric coupling, it is demonstrated that the magnetocaloric effect of a ferromagnetic shape-memory alloy can be controlled by an electric field. Large hysteresis and the limited operating temperature region are effectively overcome by applying an electric field on a laminate comprising a piezoelectric and the alloy. Accordingly, a model for an active magnetic refrigerator with high efficiency is proposed in principle.

Excellent microwave absorption property of Graphene-coated Fe nanocomposites.

Graphene has evoked extensive interests for its abundant physical properties and potential applications. It is reported that the interfacial electronic interaction between metal and graphene would give rise to charge transfer and change the electronic properties of graphene, leading to some novel electrical and magnetic properties in metal-graphene heterostructure. In addition, large specific surface area, low density and high chemical stability make graphene act as an ideal coating material. Taking full advantage of the aforementioned features of graphene, we synthesized graphene-coated Fe nanocomposites for the first time and investigated their microwave absorption properties. Due to the charge transfer at Fe-graphene interface in Fe/G, the nanocomposites show distinct dielectric properties, which result in excellent microwave absorption performance in a wide frequency range. This work provides a novel approach for exploring high-performance microwave absorption material as well as expands the application field of graphene-based materials.

Obtaining high localized spin magnetic moments by fluorination of reduced graphene oxide.

Fluorination was confirmed to be the most effective route to introduce localized spins in graphene. However, adatoms clustering in perfect graphene lead to a low efficiency. In this study, we report experimental evidence of the generation of localized spin magnetic moments on defective graphene (reduced graphene oxide) through fluorination. More interstingly, the result shows that defects help increase the efficiency of the fluorination with regard to the density of magnetic moments created. Fluorinated reduced graphene oxide can have a high magnetic moment of 3.187 × 10(-3) µB per carbon atom and a high efficiency of 8.68 × 10(-3) µB per F atom. It may be attributed to the many vacancies, which hinder the clustering of F atoms, and introduce many magnetic edge adatoms.

Electric control of magnetism at room temperature.

In the single-phase multiferroics, the coupling between electric polarization (P) and magnetization (M) would enable the magnetoelectric (ME) effect, namely M induced and modulated by E, and conversely P by H. Especially, the manipulation of magnetization by an electric field at room-temperature is of great importance in technological applications, such as new information storage technology, four-state logic device, magnetoelectric sensors, low-power magnetoelectric device and so on. Furthermore, it can reduce power consumption and realize device miniaturization, which is very useful for the practical applications. In an M-type hexaferrite SrCo(2)Ti(2)Fe(8)O(19), large magnetization and electric polarization were observed simultaneously at room-temperature. Moreover, large effect of electric field-controlled magnetization was observed even without magnetic bias field. These results illuminate a promising potential to apply in magnetoelectric devices at room temperature and imply plentiful physics behind them.

Structural, morphological, and magnetic study of nanocrystalline cobalt-nickel-copper particles.

Nanocrystalline Co(x)Ni(y)Cu(100-x-y) particles were synthesized by the reduction of metal acetates in a mixture of polyol and Tween 80. Inductively coupled plasma (ICP) analysis revealed that the actual wt% of Co, Ni, and Cu in these nanoparticles was nearly the same as in the starting solutions. The structures of the particles were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED) spectroscopy, and vibrating sample magnetometry (VSM). The results of XRD and VSM confirmed that there was no metastable alloying in the particles. The particles were composites, consisting of nanoscale crystallites of face-centered cubic (fcc) Cu, face-centered cubic (fcc) Ni, and face-centered cubic (fcc) Co. During preparation the nucleation of Cu occurred first; then small Cu nuclei acted as cores for the precipitation of Co and Ni. The particles showed an increase in saturation magnetization (M(s)) as the concentration of Co or Ni in the particles was increased. The changes of both M(s) and coercivity of the particles with increasing annealing temperatures were studied. The coercivity of the particles was very high; it could reach as high as 489 Oe for Co34.3Ni31.2Cu34.5) .