Defect-rich honeycomb-like nickel cobalt sulfides on graphene through rapid microwave-induced synthesis for ultrahigh rate supercapacitors.

Nickel cobalt sulfides (NCS) are regarded as potential energy storage materials due to the versatile valent states and rich electrochemical activity, but their sluggish synthesis process and inferior rate performance hinder them from large-scale application. Herein, microwave-induced strategy has been employed for efficient synthesis of honeycomb-like NCS/graphene composites, which are explored as ultrahigh rate battery-type electrodes for supercapacitors. Due to the internal heat mechanism, the synthesis time of NCS by microwave could be shortened from hours to minutes. Density functional theory was simulated to uncover the interfacial effect between NCS and graphene, and the resulted Schottky barrier is in favor of enhancing redox activity and capacity. Ultimately, the obtained defect-rich nickel cobalt sulfides/graphene with thermal treatment (NCS/G-H) could exhibit a high specific capacitance of 1186 F g-1 at 1 A g-1 and sustain 89.8% capacity even after the increase of current density over 20 times, which is much superior to bare NCS and NCS/graphene. Furthermore, the assembled NCS/G-H hybrid supercapacitor delivers supreme energy density of 46.4 Wh kg-1, and retains outstanding long-term stability of 89.2% after 10 k cycles. These results indicate that the synthesized NCS/G-H by time-saving microwave-induced liquid process could be served as high rate materials for supercapacitors.

Microporous 2D NiCoFe phosphate nanosheets supported on Ni foam for efficient overall water splitting in alkaline media.

The development of high-performance non-precious electrocatalysts for both H2 and O2 evolution reactions (HER and OER activities) and overall water splitting is highly desirable but remains a grand challenge. Herein, we report a facile method to synthesize ultrathin, amorphous, porous, oxygen and defect enriched NiCoFe phosphate nanosheets (NSs). Owing to their microporous confinement in a 2D orientation, which can reduce the ion transport resistance during electrochemical processes, and defect enriched structure with higher electrochemically active surface area, these NiCoFe phosphate porous nanosheets supported on nickel foam (NiCoFe phosphate NSs/NF) facilitate the diffusion of gaseous products (H2 and O2) and exhibit remarkable catalytic performance and outstanding stability for both HER, OER and overall water splitting in an alkaline electrolyte (1.0 M KOH). For the OER electrocatalyst, 2D NiCoFe phosphate NSs/NF was oxidized to NiCoFe oxides/hydroxides on the catalyst surface and exhibited remarkable OER activity with a low overpotential of only 240 mV needed to reach a current density of 10 mA cm-2. For HER, 2D NiCoFe phosphate NSs/NF afforded a current density of 10 mA cm-2 at a low overpotential of only -231 mV. Furthermore, employing 2D NiCoFe phosphate NSs/NF as the electrocatalyst for both the anode and the cathode, a water splitting electrolyzer was able to reach 10 mA cm-2 at a cell voltage of 1.52 V with robust durability. Various characterization techniques indicated that the long term stability and the activity for overall water splitting are due to the porosity, the electrochemically active constituents, and synergistic effects. This work could be inspiring in the design of Earth abundant and highly efficient electrocatalysts for overall water splitting, especially for OER.

Multimetallic nanosheets: synthesis and applications in fuel cells.

Two-dimensional nanomaterials, particularly multimetallic nanosheets with single or few atoms thickness, are attracting extensive research attention because they display remarkable advantages over their bulk counterparts, including high electron mobility, unsaturated surface coordination, a high aspect ratio, and distinctive physical, chemical, and electronic properties. In particular, their ultrathin thickness endows them with ultrahigh specific surface areas and a relatively high surface energy, making them highly favorable for surface active applications; for example, they have great potential for a broad range of fuel cell applications. First, the state-of-the-art research on the synthesis of nanosheets with a controlled size, thickness, shape, and composition is described and special emphasis is placed on the rational design of multimetallic nanosheets. Then, a correlation is performed with the performance of multimetallic nanosheets with modified and improved electrochemical properties and high stability, including for the oxygen reduction reaction (ORR), hydrogen evolution reaction (HER), formic acid oxidation (FAO), methanol oxidation reaction (MOR), ethanol oxidation reaction (EOR), and methanol tolerance are outlined. Finally, some perspectives and advantages offered by this class of materials are highlighted for the development of highly efficient fuel cell electrocatalysts, featuring low cost, enhanced performance, and high stability, which are the key factors for accelerating the commercialization of future promising fuel cells.

Trimetallic PtCoFe Alloy Monolayer Superlattices as Bifunctional Oxygen-Reduction and Ethanol-Oxidation Electrocatalysts.

A synthesis strategy for the preparation of trimetallic PtCoFe alloy nanoparticle superlattices is reported. Trimetallic PtCoFe alloy monolayer array of nanoparticle superlattices with a large density of high index facets and platinum-rich surface are successfully prepared by coreduction of metal precursors in formamide solvent. The concentration of cetyl trimethyl ammonium bromide plays a vital role for the formation of a monolayer array of nanoparticle superlattices, while the size of nanoparticles is determined by NaI. The prepared monolayer array of nanoparticle superlattices is the superior catalyst for oxygen reduction reaction as well as for ethanol oxidation owing to their specific structural and compositional characteristics.

A generic "micro-Stoney" method for the measurement of internal stress and elastic modulus of ultrathin films.

Accurate measurement of the mechanical properties of ultra-thin films with thicknesses typically below 100 nm is a challenging issue with an interest in many fields involving coating technologies, microelectronics, and MEMS. A bilayer curvature based method is developed for the simultaneous determination of the elastic mismatch strain and Young's modulus of ultra-thin films. The idea is to deposit the film or coating on very thin cantilevers in order to amplify the curvature compared to a traditional "Stoney" wafer curvature test, hence the terminology "micro-Stoney." The data reduction is based on the comparison of the curvatures obtained for different supporting layer thicknesses. The elastic mismatch strain and Young's modulus are obtained from curvature measurements of cantilevers before and after the film deposition. The data reduction scheme relies on both analytical and finite element calculations, depending on the magnitude of the curvature. The experimental validation has been performed on ultra-thin low pressure chemical vapor deposited silicon nitride films with thickness ranging between 54 and 133 nm deposited on silicon cantilevers. The technique is sensitive to the cantilever geometry, in particular, to the thickness ratio and width/thickness ratio. Therefore, the precision in the determination of the latter quantities determines the accuracy on the extracted elastic mismatch strain and elastic modulus. The method can be potentially applied to films as thin as a few nanometers.

Tuning TiO2 nanoparticle morphology in graphene-TiO2 hybrids by graphene surface modification.

We report the hydrothermal synthesis of graphene (GNP)-TiO2 nanoparticle (NP) hybrids using COOH and NH2 functionalized GNP as a shape controller. Anatase was the only TiO2 crystalline phase nucleated on the functionalized GNP, whereas traces of rutile were detected on unfunctionalized GNP. X-Ray Photoelectron spectroscopy (XPS) showed C-Ti bonds on all hybrids, thus confirming heterogeneous nucleation. GNP functionalization induced the nucleation of TiO2 NPs with specific shapes and crystalline facets exposed. COOH functionalization directed the synthesis of anatase truncated bipyramids, bonded to graphene sheets via the {101} facets, while NH2 functionalization induced the formation of belted truncated bipyramids, bonded to graphene via the {100} facets. Belted truncated bipyramids formed on unfunctionalized GNP too, however the NPs were more irregular and rounded. These effects were ascribed to pH variations in the proximity of the functionalized GNP sheets, due to the high density of COOH or NH2 groups. Because of the different reactivity of anatase {100} and {101} crystalline facets, we hypothesize that the hybrid materials will behave differently as photocatalysts, and that the COOH-GNP-TiO2 hybrids will be better photocatalysts for water splitting and H2 production.

Decoration of graphitic surfaces with Sn nanoparticles through surface functionalization using diazonium chemistry.

Composites of tin nanoparticles (Sn NP) and graphene are candidate materials for high capacity and mechanically stable negative electrodes in rechargeable Li ion batteries. A uniform dispersion of Sn NP with controlled size is necessary to obtain high electrochemical performance. We show that the nucleation of Sn particles on highly ordered pyrolitic graphite (HOPG) from solution can be controlled by functionalizing the HOPG surface by aryl groups prior to Sn deposition. On the contrary, we observe heterogeneous deposition of micrometer sized Sn islands on HOPG subjected to oxidation prior to deposition in the same conditions. We demonstrate that functional groups act as nucleation sites for Sn NP nucleation, and that homogeneous nucleation of small particles can be achieved by combining surface functionalization with diazonium chemistry and appropriate stabilizers in solution.