Xing-You Lang

Controllable Growth and Transfer of Monolayer MoS2 on Au Foils and Its Potential Application in Hydrogen Evolution Reaction

Controllable synthesis of monolayer MoS2 is essential for fulfilling the application potentials of MoS2 in optoelectronics and valleytronics, etc. Herein, we report the scalable growth of high quality, domain size tunable (edge length from ∼ 200 nm to 50 μm), strictly monolayer MoS2 flakes or even complete films on commercially available Au foils, via low pressure chemical vapor deposition method. The as-grown MoS2 samples can be transferred onto arbitrary substrates like SiO2/Si and quartz with a perfect preservation of the crystal quality, thus probably facilitating its versatile applications. Of particular interest, the nanosized triangular MoS2 flakes on Au foils are proven to be excellent electrocatalysts for hydrogen evolution reaction, featured by a rather low Tafel slope (61 mV/decade) and a relative high exchange current density (38.1 μA/cm(2)). The excellent electron coupling between MoS2 and Au foils is considered to account for the extraordinary hydrogen evolution reaction activity. Our work reports the synthesis of monolayer MoS2 when introducing metal foils as substrates, and presents sound proof that monolayer MoS2 assembled on a well selected electrode can manifest a hydrogen evolution reaction property comparable with that of nanoparticles or few-layer MoS2 electrocatalysts.

Nanoporous gold supported cobalt oxide microelectrodes as high-performance electrochemical biosensors

Tremendous demands for electrochemical biosensors with high sensitivity and reliability, fast response and excellent selectivity have stimulated intensive research on developing versatile materials with ultrahigh electrocatalytic activity. Here we report flexible and self-supported microelectrodes with a seamless solid/nanoporous gold/cobalt oxide hybrid structure for electrochemical nonenzymatic glucose biosensors. As a result of synergistic electrocatalytic activity of the gold skeleton and cobalt oxide nanoparticles towards glucose oxidation, amperometric glucose biosensors based on the hybrid microelectrodes exhibit multi-linear detection ranges with ultrahigh sensitivities at a low potential of 0.26 V (versus Ag/AgCl). The sensitivity up to 12.5 mA mM⁻¹ cm⁻² with a short response time of less than 1 s gives rise to ultralow detection limit of 5 nM. The outstanding performance originates from a novel nanoarchitecture in which the cobalt oxide nanoparticles are incorporated into pore channels of the seamless solid/nanoporous Au microwires, providing excellent electronic/ionic conductivity and mass transport for the enhanced electrocatalysis.

Dendritic, transferable, strictly monolayer MoS2 flakes synthesized on SrTiO3 single crystals for efficient electrocatalytic applications.

Controllable synthesis of macroscopically uniform, high-quality monolayer MoS2 is crucial for harnessing its great potential in optoelectronics, electrocatalysis, and energy storage. To date, triangular MoS2 single crystals or their polycrystalline aggregates have been synthesized on insulating substrates of SiO2/Si, mica, sapphire, etc., via portable chemical vapor deposition methods. Herein, we report a controllable synthesis of dendritic, strictly monolayer MoS2 flakes possessing tunable degrees of fractal shape on a specific insulator, SrTiO3. Interestingly, the dendritic monolayer MoS2, characterized by abundant edges, can be transferred intact onto Au foil electrodes and serve as ideal electrocatalysts for hydrogen evolution reaction, reflected by a rather low Tafel slope of ∼73 mV/decade among CVD-grown two-dimensional MoS2 flakes. In addition, we reveal that centimeter-scale uniform, strictly monolayer MoS2 films consisting of relatively compact domains can also be obtained, offering insights into promising applications such as flexible energy conversion/harvesting and optoelectronics.

Integrated Solid/Nanoporous Copper/Oxide Hybrid Bulk Electrodes for High-performance Lithium-Ion Batteries

Nanoarchitectured electroactive materials can boost rates of Li insertion/extraction, showing genuine potential to increase power output of Li-ion batteries. However, electrodes assembled with low-dimensional nanostructured transition metal oxides by conventional approach suffer from dramatic reductions in energy capacities owing to sluggish ion and electron transport kinetics. Here we report that flexible bulk electrodes, made of three-dimensional bicontinuous nanoporous Cu/MnO2 hybrid and seamlessly integrated with Cu solid current collector, substantially optimizes Li storage behavior of the constituent MnO2. As a result of the unique integration of solid/nanoporous hybrid architecture that simultaneously enhances the electron transport of MnO2, facilitates fast ion diffusion and accommodates large volume changes on Li insertion/extraction of MnO2, the supported MnO2 exhibits a stable capacity of as high as ~1100 mA h g−1 for 1000 cycles, and ultrahigh charge/discharge rates. It makes the environmentally friendly and low-cost electrode as a promising anode for high-performance Li-ion battery applications.

Al 13 @Pt 42 Core-Shell Cluster for Oxygen Reduction Reaction

To increase Pt utilization for oxygen reduction reaction (ORR) in fuel cells, reducing particle sizes of Pt is a valid way. However, poisoning or surface oxidation limits the smallest size of Pt particles at 2.6 nm with a low utility of 20%. Here, using density functional theory calculations, we develop a core-shell Al13@Pt42 cluster as a catalyst for ORR. Benefit from alloying with Al in this cluster, the covalent Pt-Al bonding effectively activates the Pt atoms at the edge sites, enabling its high utility up to 70%. Valuably, the adsorption energy of O is located at the optimal range with 0.0–0.4 eV weaker than Pt(111), while OH-poisoning does not observed. Moreover, ORR comes from O2 dissociation mechanism where the rate-limiting step is located at OH formation from O and H with a barrier of 0.59 eV, comparable with 0.50 eV of OH formation from O and H2O on Pt(111).

Chemical vapor deposition of monolayer WS2 nanosheets on Au foils toward direct application in hydrogen evolution

Monolayer tungsten disulfide (WS2), a typical member of the semiconducting transition metal dichalcogenide family, has drawn considerable interest because of its unique properties. Intriguingly, the edge of WS2 exhibits an ideal hydrogen binding energy, which makes WS2 a potential alternative to Pt-based electrocatalysts for the hydrogen evolution reaction (HER). Here, we demonstrate for the first time the successful synthesis of uniform monolayer WS2 nanosheets on centimeter-scale Au foils using a facile, low-pressure chemical vapor deposition method. The edge lengths of the universally observed triangular WS2 nanosheets are tunable from ∼100 to ∼1,000 nm. The WS2 nanosheets on Au foils featuring abundant edges were then discovered to be efficient catalysts for the HER, exhibiting a rather high exchange current density of ∼30.20 μA/cm2 and a small onset potential of ∼110 mV. The effects of coverage and domain size (which correlate closely with the active edge density of WS2) on the electrocatalytic activity were investigated. This work not only provides a novel route toward the batch-production of monolayer WS2 via the introduction of metal foil substrates but also opens up its direct application for facile HER.

Electron Scattering and Electrical Conductance in Polycrystalline Metallic Films and Wires: Impact of Grain Boundary Scattering Related to Melting Point

For electrical conductance in polycrystalline metallic films and wires, the reflection coefficient of electrons at grain boundaries is explored and found to be proportional to the square root of the melting points of metals. As validated by available experimental results, this exploration enables classical models to take an essential role in theoretically predicting the electrical conductance of low-dimensional metals. One thus sees that the mechanism dominating the suppression of electrical conductance is transformed from the surface scattering into the grain boundary scattering as the ratio of film thickness (or wire diameter) to grain size rises. Furthermore, the impact of grain boundary scattering becomes less important for metals with lower melting points.

Facile Synthesis of Non-Graphitizable Polypyrrole-Derived Carbon/Carbon Nanotubes for Lithium-ion Batteries

Graphite is usually used as an anode material in the commercial lithium ion batteries (LIBs). The relatively low lithium storage capacity of 372 mAh g–1 and the confined rate capability however limit its large-scale applications in electrical vehicles and hybrid electrical vehicles. As results, exploring novel carbon-based anode materials with improved reversible capacity for high-energy-density LIBs is urgent task. Herein we present TNGC/MWCNTs by synthesizing tubular polypyrrole (T-PPy) via a self-assembly process, then carbonizing T-PPy at 900 °C under an argon atmosphere (TNGC for short) and finally mixing TNGC with multi-walled carbon nanotubes (MWCNTs). As for TNGC/MWCNTs, the discharge capacity of 561 mAh g−1 is maintained after 100 cycles at a current density of 100 mA g−1. Electrochemical results demonstrate that TNGC/MWCNTs can be considered as promising anode materials for high-energy-density LIBs.

Dependence of the blocking temperature in exchange biased ferromagnetic/antiferromagnetic bilayers on the thickness of the antiferromagnetic layer

Based on the size-dependent exchange interaction of antiferromagnets, the blocking temperature in exchange-biased ferromagnetic/antiferromagnetic bilayers is modelled as a function of the thickness of the antiferromagnetic layer. It is found that this temperature decreases with decreasing layer thickness, which agrees with the available experimental measurements for Fe3O4/CoO, NiO/NiFe, CoNiO/NiFe, IrMn/NiFe, Py/IrMn, IrMn/CoFe, FeMn/NiFe, MnPt/CoFe and FeF2/Fe bilayers. The correspondence between the model predictions and experimental evidence results in a deeper understanding of the present models for the above phenomena.

Single-crystalline Ni(OH)2 nanosheets vertically aligned on a three-dimensional nanoporous metal for high-performance asymmetric supercapacitors

Transition-metal hydroxides (TMHOs) or oxides (TMOs) with layered crystalline structures are attractive electrode materials for high-density charge storage in electrochemical supercapacitors. However, their randomly stacked nanostructures on conductive reinforcements, typically carbon materials, exhibit only modest enhancement of rate capability because of poor electron and ion transports that are limited by highly anisotropic conductivity, excessive grain boundaries and weak TMHO or TMO/C interfaces. Here we report a hybrid electrode design to tackle all three of these problems in layered Ni(OH)2 for high-performance asymmetric supercapacitors, wherein the single-crystalline Ni(OH)2 nanosheets are vertically aligned on a three-dimensional bicontinuous nanoporous gold skeleton with epitaxial Au/Ni(OH)2 interfaces (NP Au/VA Ni(OH)2). As a result of the unique nanoarchitecture, the pseudocapacitive behavior of Ni(OH)2 is dramatically enhanced for ensuring a volumetric capacitance as high as ∼2911 F cm−3 (∼2416 F g−1 for the constituent Ni(OH)2) in the NP Au/VA Ni(OH)2 electrode with excellent rate capability. Asymmetric supercapacitors assembled with this NP Au/VA Ni(OH)2 electrode and activated carbon have a high gravimetric energy of 31.4 W h kg−1 delivered at an exceptionally high power density of 100 kW kg−1 with excellent cycling stability.