Search

Surface adsorbates and surrounding matrix species have been demonstrated to affect the properties of nanoscale ferroelectrics and nanoscale ferroelectric composites; potentially counteracting performance losses encountered that can occur in small particle sizes. In this work we investigate the effects of nonpolar oleic acid (OA) and polar tetrafluoroborate (BF 4 - ) ligand capping on the surface of various sizes of BaTiO 3 nanocubes with combined neutron diffraction and neutron pair distribution function (PDF), density functional theory (DFT), and ab initio molecular dynamics (AIMD) methods. The low real space PDF region provides an unobstructed view of rhombohedral (split short and long) Ti-O distances in BaTiO 3 nanocubes, mimicking the well-established order-disorder local structure found in bulk BaTiO 3 . Interestingly, the intermediate-range order in nanocubes is found to be orthorhombic, rather than tetragonal. We conclude that polar ligands adsorbed at BaTiO 3 surfaces stabilize the correlation length scale of local rhombohedral distortions in ferroelectric nanoparticles relative to nonpolar ligands.

Nanoscale 12(5), 3121-3128 (2020). doi:10.1039/C9NR09818F, Published by RSC Publ., Cambridge

We report on the design of abundant and highly active VB2 for hydrogen production. Density functional theory (DFT) calculations have predicted very high HER activity of the graphene-like B-layer, the V-terminated {100} layer, and the mixed V/B-terminated {101} layer of VB2. Bulk samples and nanoparticles of VB2 were synthesized and tested for their HER performance. The results indicate that both bulk and nano-VB2 are active for HER, consistent with theoretical predictions. In addition, the HER activity of VB2 is significantly increased at the nanoscale if compared to the bulk, reaching an overpotential of 192 mV at 10 mA/cm2 current density. Increased surface area and higher density of active sites are responsible for the higher nanoscale activity, making nano-VB2 the best HER boride to date in terms of abundance, stability, and activity in acidic solution.

Two different boron layers, flat (graphene-like) and puckered (phosphorene-like), found in the crystal structure of Mo2B4 show drastically different activities for hydrogen evolution, according to Gibbs free energy calculations of H-adsorption on Mo2B4. The graphene-like B layer is highly active, whereas the phosphorene-like B layer performs very poorly for hydrogen evolution. A new Sn-flux synthesis permits the rapid single-phase synthesis of Mo2B4, and electrochemical analyses show that it is one of the best hydrogen evolution reaction active bulk materials with good long-term cycle stability under acidic conditions. Mo2B4 compensates its smaller density of active sites if compared with highly active bulk MoB2 (which contains only the more active graphene-like boron layers) by a 5-times increase of its surface area.

Non-noble metal nanomaterials (molybdenum sulfides, phosphides, carbides, and nitrides) have recently emerged as highly active electrocatalysts for the hydrogen evolution reaction (HER). Molybdenum borides in contrast have not been studied for their HER activity at the nanoscale, however, they were recently shown to be already efficient HER catalysts in the bulk (microscale). In this study, we report on the first nanocrystalline molybdenum boride (MoB2) synthesized by a simple, one-step, relatively low temperature (650 °C) and environmentally benign redox-assisted solid state metathesis (SSM) reaction. The obtained MoB2 nanospheres exhibit a low onset overpotential of 154 mV at 10 mA cm−2, a Tafel slope of 49 mV dec−1 and high stability. Furthermore, density functional theory (DFT) calculations show that several surfaces are active and that the optimum evolution of H2 occurs at a hydrogen coverage between 75% and 100% on the B-terminated {001} surface. These experimental and theoretical results open new avenues to design new architectures of inexpensive and highly efficient boron-based HER catalysts, such as boride nanospheres (with maximum active sites) or materials with B-terminated surfaces (e.g. {001} nanosheets of AlB2-type borides or even the recently discovered borophene and related 2D compounds).

Among van der Waals (vdW) layered ferromagnets, Fe

Among van der Waals (vdW) layered ferromagnets, Fe3GeTe2 (FGT) is an excellent candidate material to form FGT/heavy metal heterostructures for studying the effect of spin-orbit torques (SOT). Its metallicity, strong perpendicular magnetic anisotropy built in the single atomic layers, relatively high Curie temperature (Tc ∼ 225 K), and electrostatic gate tunability offer a tantalizing possibility of achieving the ultimate high SOT limit in monolayer all-vdW nanodevices. In this study, we fabricate heterostructures of FGT/Pt with 5 nm of Pt sputtered onto the atomically flat surface of ∼15-23 nm exfoliated FGT flakes. The spin current generated in Pt exerts a damping-like SOT on FGT magnetization. At ∼2.5 × 1011 A/m2 current density, SOT causes the FGT magnetization to switch, which is detected by the anomalous Hall effect of FGT. To quantify the SOT effect, we measure the second harmonic Hall responses as the applied magnetic field rotates the FGT magnetization in the plane. Our analysis shows that the SOT efficiency is comparable with that of the best heterostructures containing three-dimensional (3D) ferromagnetic metals and much larger than that of heterostructures containing 3D ferrimagnetic insulators. Such large efficiency is attributed to the atomically flat FGT/Pt interface, which demonstrates the great potential of exploiting vdW heterostructures for highly efficient spintronic nanodevices.

Mesostructured manganese oxide (Mn3 O4 ) is prepared by a soft-templating method employing sodium dodecyl sulfate (SDS) as a structure-directing agent. By removing the template from the as-prepared mesostructured Mn3 O4 by extraction or calcination, we successfully synthesized highly crystallized mesoporous Mn3 O4 or Mn2 O3 , respectively, with different crystalline structures.

We report a simple soft-templating strategy for the synthesis of nanoporous crystalline nickel sulfide with two-dimensional (2-D) morphology. The nickel sulfide phases and morphologies are varied by changing the hydrothermal temperatures applied. Furthermore, the nanoporous nickel sulfide (PNS) flakes can be hybridized with reduced graphene oxide (rGO) sheets. As compared to bare PNS flakes, the PNS/rGO composite, containing 40% rGO, exhibits a superior electrochemical performance in terms of specific capacitance and cyclic stability. The specific capacitance of this composite is evaluated by a three-electrode system, and it shows the highest specific capacitance of 1312 F g−1 at a scan rate of 5 mV s−1. In addition, this composite is also assembled to form an asymmetric supercapacitor with zeolitic imidazolate framework (ZIF-8)-derived carbon as a negative electrode, which gives a highest specific capacitance of 47.85 F g−1 at 2 A g−1, a high energy density of 17.01 W h kg−1, and a high power density of 10 kW kg−1.

We report the synthesis of electrode materials based on one-dimensional -NiMoO4 nanorods for application as a supercapacitor. The structure and morphology of the electrodes were characterized by powder X-ray diffraction, Raman analysis, X-ray photoelectron spectroscopy, and scanning and transmission electron microscopy. The maximum specific capacitance obtained from electrochemical measurements on a standard three-electrode system was 730 Fg(-1) at a scan rate of 5 mVs(-1). Furthermore, an asymmetric supercapacitor (ASC) was fabricated in which the NiMoO4 nanorods were used as the positive electrode and reduced graphene oxide was used as the negative electrode. The maximum specific energy obtained from the ASC study was 12.31 Whkg(-1) at a specific current of 0.5 Ag-1 in an aqueous KOH electrolyte. This ASC cell showed good stability with 85% capacitance retention up to 2000 cycles. These results reveal that our ASC shows a high performance and specific energy as well as good cycling stability.

Non-precious NiMoO 4 nanorods with carbon and graphene have been designed for methanol oxidation via one pot hydrothermal method. The physicochemical and electrocatalytic features of these catalysts are characterized. Among the three catalysts, carbon modified NiMoO 4 shows an enhanced catalytic activity in terms of current density, onset potential, cyclic stability and high tolerance to intermediate towards methanol electro-oxidation. Moreover, the NiMoO 4 /C catalyst delivers a current density of 49 mA cm −2 at low onset potential of 0.45 V ( vs . Hg/HgO) in 1 M KOH and 2.0 M methanol electrolyte. This greater electrocatalytic activity is attributed to the unique 1D microstructure of NiMoO 4 nanorods with well distributed carbonaceous material, which enhances the efficient transport of electron/ion kinetics at the electrode and electrolyte interfaces. From this observation, it is concluded that the carbon modified NiMoO 4 nanorods could be a promising alternate non-noble electrocatalysts for direct methanol fuel cell (DMFC) applications.

Two different boron layers, flat (graphene-like) and puckered (phosphorene-like), found in the crystal structure of Mo

Most nanomaterials, such as transition metal carbides, phosphides, nitrides, chalcogenides, etc., have been extensively studied for their various properties in recent years. The similarly attractive transition metal borides, on the contrary, have seen little interest from the materials science community, mainly because nanomaterials are notoriously difficult to synthesize. Herein, a simple, general synthetic method toward crystalline transition metal boride nanomaterials is proposed. This new method takes advantage of the redox chemistry of Sn/SnCl2 , the volatility and recrystallization of SnCl2 at the synthesis conditions, as well as the immiscibility of tin with boron, to produce crystalline phases of 3d, 4d, and 5d transition metal nanoborides with different morphologies (nanorods, nanosheets, nanoprisms, nanoplates, nanoparticles, etc.). Importantly, this method allows flexibility in the choice of the transition metal, as well as the ability to target several compositions within the same binary phase diagram (e.g., Mo2 B, α-MoB, MoB2 , Mo2 B4 ). The simplicity and wide applicability of the method should enable the fulfillment of the great potential of this understudied class of materials, which show a variety of excellent chemical, electrochemical, and physical properties at the microscale.

A novel composite membrane, derived from a simple combination of a phosphate based alkylimidazolium ionic liquid used as an anhydrous proton conductor in sulfonated poly (ether ether ketone) polymer matrix, was first proposed and characterized. Observed thermal, mechanical and chemical stabilities of ionic liquid doped composite membranes were finest to use in fuel cell applications. The effective anhydrous proton conductivity with respect to various ionic liquid compositions was studied as a function of different temperatures. Under anhydrous environment the composite membranes were found to have a comparatively good conductivity in the range of 1.25–3.16 mS cm–1 at 145 °C as measured by impedance analyzer. Fuel cell performance with SPEEK/50% IL doped composite membrane gave a maximum power density of 203 mW/cm2 at 145 °C. This study foresees that the new composite membranes could be a promising water-free anhydrous proton transport membrane for fuel cells.