Weihua Han

Construction of hierarchical ZnCo2O4@NixCo2x(OH)6x core/shell nanowire arrays for high-performance supercapacitors

Rational design and synthesis of core/shell nanostructures as binder-free electrodes has been believed to be an effective strategy to improve the electrochemical performance of supercapacitors. In this work, hierarchical ZnCo2O4@NixCo2x(OH)6x core/shell nanowire arrays (NWAs) have been successfully constructed by electrodepositing NixCo2x(OH)6x nanosheets onto hydrothermally grown ZnCo2O4 nanowires and investigated as a battery-type electrode for hybrid supercapacitors. Taking advantage of the hierarchical core/shell structures and the synergetic effect between ZnCo2O4 nanowires and NixCo2x(OH)6x nanosheets, the optimised core/shell electrode exhibits remarkable electrochemical performance with a high areal capacity (419.1 μA h cm−2), good rate capability and cycling stability. Moreover, the assembled ZnCo2O4@NixCo2x(OH)6x//activated carbon (AC) hybrid device can be reversibly cycled in a large potential range of 0–1.7 V and deliver a maximum energy density of 26.2 W h kg−1 at 511.8 W kg−1. Our findings indicate that the hierarchical ZnCo2O4@NixCo2x(OH)6x core/shell NWAs have great potential for applications in energy storage devices.

Highly Flexible Freestanding Porous Carbon Nanofibers for Electrodes Materials of High-Performance All-Carbon Supercapacitors

Highly flexible porous carbon nanofibers (P-CNFs) were fabricated by electrospining technique combining with metal ion-assistant acid corrosion process. The resultant fibers display high conductivity and outstanding mechanical flexibility, whereas little change in their resistance can be observed under repeatedly bending, even to 180°. Further results indicate that the improved flexibility of P-CNFs can be due to the high graphitization degree caused by Co ions. In view of electrode materials for high-performance supercapacitors, this type of porous nanostructure and high graphitization degree could synergistically facilitate the electrolyte ion diffusion and electron transportation. In the three electrodes testing system, the resultant P-CNFs electrodes can exhibit a specific capacitance of 104.5 F g(-1) (0.2 A g(-1)), high rate capability (remain 56.5% at 10 A g(-1)), and capacitance retention of ∼94% after 2000 cycles. Furthermore, the assembled symmetric supercapacitors showed a high flexibility and can deliver an energy density of 3.22 Wh kg(-1) at power density of 600 W kg(-1). This work might open a way to improve the mechanical properties of carbon fibers and suggests that this type of freestanding P-CNFs be used as effective electrode materials for flexible all-carbon supercapacitors.

Cobalt sulfide nanosheets coated on NiCo2S4 nanotube arrays as electrode materials for high-performance supercapacitors

Hierarchical hybrid electrodes were successfully fabricated by electrodeposition of ultrathin cobalt sulfide (CoSx) nanosheets on NiCo2S4 nanotube arrays grown on Ni foam for high-performance supercapacitors. The hierarchical NiCo2S4@CoSx core/shell nanotube arrays exhibit a high areal capacitance (4.74 F cm−2 at a current density of 5 mA cm−2), a good rate capability (2.26 F cm−2 at 50 mA cm−2) and cycle stability (76.1% capacitance retention after 1500 cycles at a high current density of 50 mA cm−2), which are much better than those of NiCo2S4 nanotubes. Such superior electrochemical performance could be attributed to the smart configuration of the two electroactive materials, which can provide more pathways for electron transport and improve the utilization rate of the electrode materials. This effective strategy shows the feasibility of designing and fabricating metal sulfides with core/shell hybrid structures as electrode materials for high-performance supercapacitors.

Strain-Gated Piezotronic Transistors Based on Vertical Zinc Oxide Nanowires

Strain-gated piezotronic transistors have been fabricated using vertically aligned ZnO nanowires (NWs), which were grown on GaN/sapphire substrates using a vapor-liquid-solid process. The gate electrode of the transistor is replaced by the internal crystal potential generated by strain, and the control over the transported current is at the interface between the nanowire and the top or bottom electrode. The current-voltage characteristics of the devices were studied using conductive atomic force microscopy, and the results show that the current flowing through the ZnO NWs can be tuned/gated by the mechanical force applied to the NWs. This phenomenon was attributed to the piezoelectric tuning of the Schottky barrier at the Au-ZnO junction, known as the piezotronic effect. Our study demonstrates the possibility of using Au droplet capped ZnO NWs as a transistor array for mapping local strain. More importantly, our design gives the possibility of fabricating an array of transistors using individual vertical nanowires that can be controlled independently by applying mechanical force/pressure over the top. Such a structure is likely to have important applications in high-resolution mapping of strain/force/pressure.

Vertically aligned CdSe nanowire arrays for energy harvesting and piezotronic devices.

We demonstrated the energy harvesting potential and piezotronic effect in vertically aligned CdSe nanowire (NW) arrays for the first time. The CdSe NW arrays were grown on a mica substrate by the vapor-liquid-solid process using a CdSe thin film as seed layer and platinum as catalyst. High-resolution transmission electron microscopy image and selected area electron diffraction pattern indicate that the CdSe NWs have a wurtzite structure and growth direction along (0001). Using conductive atomic force microscopy (AFM), an average output voltage of 30.7 mV and maximum of 137 mV were obtained. To investigate the effect of strain on electron transport, the current-voltage characteristics of the NWs were studied by positioning an AFM tip on top of an individual NW. By applying normal force/stress on the NW, the Schottky barrier between the Pt and CdSe was found to be elevated due to the piezotronic effect. With the change of strain of 0.12%, a current decreased from 84 to 17 pA at 2 V bias. This paper shows that the vertical CdSe NW array is a potential candidate for future piezo-phototronic devices.

Nano‐Newton Transverse Force Sensor Using a Vertical GaN Nanowire based on the Piezotronic Effect

In this paper, we explore the piezotronic effect in a GaN nanowire under a transverse force. The force was applied by bending the end of a single NW using an atomic force microscope (AFM) tip. Our results show that GaN NWs can be used to transduce a shear/bending force into a dramatic current change through the NW due to the piezotronic effect. Owing to the local piezopotential generated by the applied force, the barrier height of the Schottky contact between the GaN NW and the platinum AFM tip can be modulated. Using this transduction mechanism, the transverse force can be correlated to the natural logarithm of the current. Our results indicate that the force sensitivity is about 1.24 ± 0.13 ln(A)/nN, and a force resolution better than 16 nN is demonstrated. The nN sensitivity of GaN NWs shows the potential for piezoelectric semiconductor NWs to act as the main building blocks for micro-/nanometersized force sensor arrays and high spatial resolution artifi cial skin.

Ultraflexible Transparent Film Heater Made of Ag Nanowire/PVA Composite for Rapid-Response Thermotherapy Pads

Ultraflexible transparent film heaters have been fabricated by embedding conductive silver (Ag) nanowires into a thin poly(vinyl alcohol) film (AgNW/PVA). A cold-pressing method was used to rationally adjust the sheet resistance of the composite films and thus the heating powers of the AgNW/PVA film heaters at certain biases. The film heaters have a favorable optical transmittance (93.1% at 26 Ω/sq) and an outstanding mechanical flexibility (no visible change in sheet resistance after 10 000 bending cycles and at a radius of curvature ≤1 mm). The film heaters have an environmental endurance, and there is no significant performance degradation after being kept at high temperature (80 °C) and high humidity (45 °C, 80% humidity) for half a year. The efficient Joule heating can increase the temperature of the film heaters (20 Ω/sq) to 74 °C in ∼20 s at a bias of 5 V. The fast-heating characteristics at low voltages (a few volts) associated with its transparent and flexibility properties make the poly(dimethylsiloxane)/AgNW/PVA composite film a potential candidate in medical thermotherapy pads.

Synthesis on Winged Graphene Nanofibers and Their Electrochemical Capacitive Performance

Assembly techniques of graphene have attracted intense attention since their performance strongly depends on the manners in which graphene nanosheets are arranged. In this work, we demonstrate a viable process to synthesize winged graphene nanofibers (G-NFs) which could generate optimized pore size distribution by the fiber-like feature of graphene. The G-NF frameworks were achieved by processing the precursor graphene oxide nanosheets with the following procedures: microwave (MW) irradiation, salt addition, freeze-drying, and chemical reduction. The resultant framework composed of winged G-NFs with a diameter of 200-500 nm and a length of 5-20 μm. Moreover, the crimp degree of G-NFs can be rationally controlled by MW irradiation time. A formation mechanism of such winged G-NFs based on the synergistic effects from MW irradiation and solution ionic strength change has been proposed. With a practice in flexible electrode, after decorated with amorphous MnO2, the G-NF frameworks shows an enhanced specific capacitance compared to graphene nanosheets (G-NSs). This research has developed a controllable method to synthesis G-NFs, which can offer hierarchical pore structures, this kind of graphene nanostructure might enhance their performance in supercapacitor and related fields.

Synthesis of cadmium sulfide quantum dot-decorated barium stannate nanowires for photoelectrochemical water splitting

We report the fabrication of cadmium sulfide (CdS) quantum dot-decorated barium stannate (BaSnO3) nanowires and their application as photoanodes for photoelectrochemical water splitting. First, polycrystalline BaSnO3 nanowires, which have a perovskite structure, were prepared by electrospinning their polyvinylpyrrolidone polymer precursors and calcining the resultant polymer fibres. Then, CdS quantum dots were decorated onto the BaSnO3 nanowires by a wet-chemical method. Our results show that the hybrid photoanode made of the CdS quantum dot-decorated BaSnO3 nanowires exhibits a high photocurrent density up to 4.8 mA cm−2 at 0 V (vs. saturated calomel electrode), which corresponds to a hydrogen generation rate of 71.8 μmol (h cm2)−1 with a faradaic efficiency of around 80%. Its favourable performance was attributed to the effective charge separation at the type II staggered gap heterojunction formed at the BaSnO3/CdS interface, and the low charge recombination in BaSnO3 nanowires during transport. Our findings indicate that the water splitting performance of photoelectrochemical cells can be highly improved by rationally building a type II band alignment heterojunction with sensitizing quantum dots and wide band gap semiconductor nanowires which have a low charge recombination rate during transport.

Controlled Synthesis on Ag Nanowires for Conductive Transparent Electrodes

One-dimensional silver nanomaterials, nanorods, and nanowires (NWs) were achieved by a solution-phase method. The geometry of the silver nanostructures was successfully tuned by adding polyvinylpyrrolidone (PVP) of different molecular weights. A possible mechanism, involving the suppression effect from PVP on the growth rates at different crystal facets, is used to explain the PVP-dependent morphology of the Ag nanowires achieved in this experiment. To evaluate its potential application in transparent electrodes, we also investigated the electrical conductance and optical transmittance after NWs were transferred to transparent polyethylene terephthalate substrates. Our findings indicated a possible and simple approach to control the morphology and then the functional properties of nanostructured silvers.