Xuewu Ou

Polymer-Embedded Fabrication of Co2P Nanoparticles Encapsulated in N,P-Doped Graphene for Hydrogen Generation

We developed a method to engineer well-distributed dicobalt phosphide (Co2P) nanoparticles encapsulated in N,P-doped graphene (Co2P@NPG) as electrocatalysts for hydrogen evolution reaction (HER). We fabricated such nanostructure by the absorption of initiator and functional monomers, including acrylamide and phytic acid on graphene oxides, followed by UV-initiated polymerization, then by adsorption of cobalt ions and finally calcination to form N,P-doped graphene structures. Our experimental results show significantly enhanced performance for such engineered nanostructures due to the synergistic effect from nanoparticles encapsulation and nitrogen and phosphorus doping on graphene structures. The obtained Co2P@NPG modified cathode exhibits small overpotentials of only -45 mV at 1 mA cm(-2), respectively, with a low Tafel slope of 58 mV dec(-1) and high exchange current density of 0.21 mA cm(-2) in 0.5 M H2SO4. In addition, encapsulation by N,P-doped graphene effectively prevent nanoparticle from corrosion, exhibiting nearly unfading catalytic performance after 30 h testing. This versatile method also opens a door for unprecedented design and fabrication of novel low-cost metal phosphide electrocatalysts encapsulated by graphene.

Graphene-templated growth of hollow Ni3S2 nanoparticles with enhanced pseudocapacitive performance

Droplet-shape hollow Ni3S2 nanoparticles, as well as corresponding partially nickel-filled nanoparticles, of narrow diameter distribution and uniform dispersion were successfully synthesized on two-dimensional graphene templates using a facile process with moderate reaction conditions. The nanoparticle composites were examined as electrochemical supercapacitor materials for energy storage application. We found that the shape of the nanoparticles is dominantly droplet-shape, with shape complementary to graphene support, which ensures good contact between them. The height of the nanoparticles increases linearly with the diameter with a coefficient of 0.44 from the fitting results, and the average height/diameter ratio of those nanoparticles is about 0.6, evidence that the nanoparticles have strong interaction with the graphene template, partially because of graphene–nickel ion interaction which ensures good surface wetting. Such a composite of droplet-shape hollow Ni3S2 nanoparticles grown on reduced graphene oxides (rGOs) exhibits a high specific capacitance of 1022.8 F g−1 at scanning rate of 2 mV s−1, with a value of 1015.6 F g−1 obtained at a discharge current density of 1 A g−1. Improvement of the rate capability can be further obtained by partially filling the hollow core with nickel metal, as 93.6% of the specific capacitance is retained with this structure by increasing the discharge density from 1 A g−1 to 10 A g−1. Our method provides a new approach for controlling the structure of graphene-based nanocomposites, with the potential for use in high performance supercapacitor applications.

One-step synthesis of Ni3S2 nanoplatelets on graphene for high performance supercapacitors

Here, we present a one-step hydrogen reduction synthesis of Ni3S2 nanoplatelets on graphene surface by using NiSO4·3N2H4/GO as precursor. In this process, we have demonstrated that hydrazine molecule, which can coordinate with NiSO4 in the form of pink precipitation, not only contributes to the formation of Ni3S2 nanoplatelets structure, but also enhances the efficiency of SO42− to S24− conversion compared with NiSO4/GO. Supercapacitors made from the obtained Ni3S2/rGO composite exhibits a specific capacitance of 912.2 F g−1 at 2 mV s−1 scanning rate, and 875.6 F g−1 at galvanostatic discharge current density of 1 A g−1, along with exceptional rate capability of 83.2% at discharge current density from 1 A g−1 to 10 A g−1 as well as good cycling stability. We attribute the excellent performance from the improved contact between graphene and the planar Ni3S2 structure, which strengthens the synergistic effect with graphene as conductive support and Ni3S2 nanoplatelets as the pseudocapacitive materials. This method allows the direct and efficient preparation of Ni3S2, and provides a simple route to integrate them with graphene for energy storage applications.

Recoil Effect and Photoemission Splitting of Trions in Monolayer MoS2

The 2D geometry nature and low dielectric constant in transition-metal dichalcogenides lead to easily formed strongly bound excitons and trions. Here, we studied the photoluminescence of van der Waals heterostructures of monolayer MoS2 and graphene at room temperature and observed two photoluminescence peaks that are associated with trion emission. Further study of different heterostructure configurations confirms that these two peaks are intrinsic to MoS2 and originate from a bound state and Fermi level, respectively, of which both accept recoiled electrons from trion recombination. We demonstrate that the recoil effect allows us to electrically control the photon energy of trion emission by adjusting the gate voltage. In addition, significant thermal smearing at room temperature results in capture of recoil electrons by bound states, creating photoemission peak at low doping level whose photon energy is less sensitive to gate voltage tuning. This discovery reveals an unexpected role of bound states for photoemission, where binding of recoil electrons becomes important.

Engineering sub-100 nm Mo(1−x)WxSe2 crystals for efficient hydrogen evolution catalysis

The edge site of two-dimensional (2D) transition metal dichalcogenides (TMDs) is active towards the hydrogen evolution reaction (HER). Herein, a feasible synthesis of sub-100 nm molybdenum/tungsten diselenide [Mo(1−x)WxSe2] crystals is described. The abundant edge exposure and heteroatom-doping synergistically boost the catalysis of HER by this material. In this work, sub-100 nm Mo(1−x)WxSe2 single crystals were grown on a nitrogen-doped multiwall carbon nanotube before being applied as HER electrocatalysts. At x = 0.13 ± 0.02, the Mo(1−x)WxSe2 shows optimal HER catalytic performance with low overpotentials (70 and 129 mV) required to achieve current densities of −1 and −10 mA cm−2, respectively, along with a Tafel slope of 53.6 mV dec−1 and an exchange current density of 49.5 μA cm−2. Density functional theory (DFT) calculations indicate that the Gibbs free energy of the HER process at the edge site of the crystals reaches a minimum value of 0.06 eV, which is lower than when the reaction is catalysed on Pt active sites. This study provides a general approach to increasing the edge proportion of the catalyst material and activating the terrace of the 2D materials for catalysis, which may be of benefit to the design and fabrication of other TMDs-based compounds.

Recyclable 3D Graphene Aerogel with Bimodal Pore Structure for Ultrafast and Selective Oil Sorption from Water

Development of next-generation porous sorbents to overcome the challenges, such as low uptake capacity, slow sorption rate, and non-recyclability, associated with conventional sorbents is of utmost importance. Herein, we report the synthesis of a highly porous graphene aerogel (GA) with a unique three-dimensional hierarchical bimodal porous network of macro and meso-pores via a facile hydrothermal technique; this aerogel has sorption capacity that is more than 5 times that of conventional commercial sorbents. Fluoroalkyl silane functionalization of the GA surface results in a significant reduction in its water sorption from 20 g g−1 to 5 g g−1 due to the GA surface becoming more hydrophobic, which renders it useful in practical application to selectively remove oil from seawater. Moreover, the sorption rate of the GA for oils and organic solvents has been found to be extremely fast, and saturation of the GA is completed in a few seconds. This is attributed to its unique meso–macro bimodal porous structure with large pore channels called macro-pores or voids of various sizes ranging from 300 nm to over 10 μm, which facilitate mass transport into its inner mesopores of 14–18 nm at high rate. Finally, the GA is shown to be a highly recyclable material due to its good mechanical strength, where the oil- and organic solvent-sorbed GA can be efficiently recovered using thermal or chemical methods for several sorption–desorption cycles without significant loss in its capacity, which also makes the process cost effective and environmentally friendly.