Shaochun Tang

Ag Dendrite-Based Au/Ag Bimetallic Nanostructures with Strongly Enhanced Catalytic Activity

Dendritic Ag/Au bimetallic nanostructures have been synthesized via a galvanic replacement reaction (GRR) of Ag dendrites in a chlorauric acid (HAuCl4) solution. After short periods of time, one obtains structures with protruding flakes; these will mature into very porous structures with little Ag left over. The morphological, compositional, and crystal structural changes involved with reaction time t were analyzed by using scanning and transmission electron microscopy (SEM and TEM, respectively), energy-dispersive X-ray spectrometry (EDX), and X-ray diffraction. High-resolution TEM combined with EDX and selected area electron diffraction confirmed the replacement of Ag with Au. A proposed formation mechanism of the original Ag dendrites developing pores while growing Au flakes cover this underlying structure at longer reaction times is confirmed by exploiting surface-enhanced Raman scattering (SERS). Catalytic reduction of 4-nitrophenol (4-NP) by sodium borohydride (NaBH4) is strongly enhanced, implying promising applications in catalysis.

A Review on Diverse Silver Nanostructures

This article reviews recent advances in the utilization of various water based synthesis routes towards the shape-controlled synthesis of silver nanoparticles and microstructures in a diverse range of shapes and sizes from several nanometers to micrometers. A variety of very simple one-pot methods, at times employing commercial microwave ovens, inexpensive low power ultrasound cleaners, or two-electrode electro-chemistry, can be surprisingly effective in the controlled synthesis of a wide range of nanostructured products, if only parameters are carefully chosen. Many approaches which are adopted include synthesis of Ag nanostructures with various shapes in solution, doping of Ag nanoparticles on unmodified silica and on/inside carbon spheres, kinetically controlled growth of Ag micro-particles with novel nanostructures on flat substrates, and galvanic replacement towards bimetallic Ag-Au dendrites and carbon composites. Characterizations of shape, composition and microstructure are carried out via scanning and transmission electron microscopy, various spectroscopy methods, N 2 absorption measurements and suchlike. The involved growth mechanisms are investigated in order to discover new means towards better control. Size, location and shape control, including micro- and nanostructure features, allows tuning the products properties towards desired applications. We focus on the optical properties and catalytic activities, but also the stability of compounds can be an issue of interest.

Highly catalytic spherical carbon nanocomposites allowing tunable activity via controllable Au–Pd doping

We report the synthesis of highly catalytic spherical carbon composite particles with Au-Pd bimetallic nanoparticle doping using a microwave-assisted technique that allows control over the location of the nanoparticles (NPs), putting them into stable interior, but still near-surface locations (within a 100 nm thick shell). First, composite particles with Pd NPs inside of nanoporous carbon spheres (CSs) were synthesized. Subsequent immersion of the composite particles in HAuCl(4) solutions containing PVP led to an addition of Au near the Pd. Au-Pd/CS composites with Au:Pd atomic ratios varying from 0.4 to 4.6 were prepared. The growth of Au and its location relative to the carbons surface and the Pd are discussed. The catalytic activity towards the reduction of 4-nitrophenol is tunable via the Au:Pd atomic ratio. Optimizing the composition increases the activity a hundredfold over that of the corresponding monometallic Pd/CS. The catalytic activity arises from the synergy between different contributing mechanisms, here especially the interaction between the carbon matrix and metals, metal-metal interfaces, and the hydrogen absorption capabilities of Pd.

A high energy density asymmetric all-solid-state supercapacitor based on cobalt carbonate hydroxide nanowire covered N-doped graphene and porous graphene electrodes

In order to achieve high energy densities, an asymmetric all-solid-state supercapacitor is developed by synthesizing a novel composite of cobalt carbonate hydroxide (CCH) nanowire covered N-doped graphene (NG) as positive and porous NG as negative electrodes. The CCH–NG composite is obtained from a one-step hydrothermal method, where optimization of the CCH content triples the specific capacitance of porous NG, reaching 1690 F g−1 at 1.0 A g−1. The optimal composite exhibits a remarkable cycling stability retaining 94.2% of the initial capacitance after 10 000 cycles, and good rate capability (still 1358 F g−1 at 10 A g−1). The assembled asymmetric supercapacitor based on the optimal composite has a high discharge areal capacitance of 153.5 mF cm−2 (at 1.0 mA cm−2), can cycle reversibly in the high-voltage region of 0–1.9 V, and thus provide superior energy and power densities (0.77 W h m−2 and 25.3 W m−2).

High-Performance Flexible Solid-State Carbon Cloth Supercapacitors Based on Highly Processible N-Graphene Doped Polyacrylic Acid/Polyaniline Composites.

Improving the solubility of conductive polymers to facilitate processing usually decreases their conductivity, and they suffer from poor cycling stability due to swelling-shrinking during charging cycles. We circumvent these problems with a novel preparation method for nitrogen-doped graphene (NG) enhanced polyacrylic acid/polyaniline (NG-PAA/PANI) composites, ensuring excellent processibility for scalable production. The content of PANI is maximized under the constraint of still allowing defect-free coatings on filaments of carbon cloth (CC). The NG content is then adjusted to optimize specific capacitance. The optimal CC electrodes have 32 wt.% PANI and 1.3 wt.% NG, thus achieving a high capacitance of 521 F/g at 0.5 F/g. A symmetric supercapacitor made from 20 wt.% PANI CC electrodes has more than four times the capacitance (68 F/g at 1 A/g) of previously reported flexible capacitors based on PANI-carbon nanotube composites, and it retains the full capacitance under large bending angles. The capacitor exhibits high energy and power densities (5.8 Wh/kg at 1.1 kW/kg), a superior rate capability (still 81% of the 1 A/g capacitance at 10 A/g), and long-term electrochemical stability (83.2% retention after 2000 cycles).

Super-hydrophobic multilayer coatings with layer number tuned swapping in surface wettability and redox catalytic anti-corrosion application

The wetting characteristic of a metal surface can be controlled by employing different coating materials and external stimuli, however, layer number (n) modulated surface swapping between hydrophobicity and hydrophilicity in a multilayer structure to achieve prolonged anti-corrosion ability was not taken into consideration. In this study, we proposed a layer-by-layer (LbL) spin assembled polyaniline-silica composite/tetramethylsilane functionalized silica nanoparticles (PSC/TMS-SiO2) coating with the combined effect of super-hydrophobicity and enhanced anti-corrosion ability. Interestingly, the hierarchical integration of two coating materials with inherently different surface roughness and energy in a multilayer structure allows the wetting feature to swap from hydrophobic to hydrophilic state by modulating n with decreasing hydrophilicity. The samples with odd n (TMS-SiO2 surface) are hydrophobic while the samples with even n (PSC surface) exhibits the hydrophilic character. The TMS-SiO2 content was optimized to achieve super-hydrophobic coating with significantly high water contact angle (CA) 153° ± 2° and small sliding angle (SA) 6° ± 2°. Beside its self-cleaning behavior, the electro-active PSC/TMS-SiO2 coating also exhibits remarkably enhanced corrosion resistance against aggressive media. The corrosion resistance of the coating was remained stable even after 240 h of exposure, this enhancement is attributed to super-hydrophobicity and anodic shift in corrosion potential.

3D nitrogen-doped graphene/Co(OH)2-nanoplate composites for high-performance electrochemical pseudocapacitors

We report a simple hydrothermal synthesis of nanocomposites, constructed by 3D nitrogen-doped graphene (NG) networks with hexagonal Co(OH)2 nanoplates, which are optimized for applications as electrochemical pseudocapacitor materials. Single-crystalline Co(OH)2 plates are distributed homogeneously inside the conductive interconnected NG networks. The 71% Co(OH)2 weight content achieves a capacitance of 952 F g−1 at 1.0 A g−1, more than triple that of the pure NG and nearly four times that of Co(OH)2 plates; moreover, this value exceeds the recently reported values for 2D graphene/Co(OH)2 composites. Capacitance retention over 2000 cycles is still high as 95%. The improvements are attributed to the regular morphology of Co(OH)2 and the 3D porosity, which prevents the stacking of the Co(OH)2 plates as well as the composite, and the continuously connected pores and highly conductive NG networks, which facilitate electron and ion transport.

A naphthyl-imide-based epoxy resin: Cure kinetics and application in carbon fiber reinforced composites

A novel heat-resistant epoxy resin based on N, N’-bis(5-hydroxy-1-naphthyl) pyromellitic diimide was prepared and its cure kinetics with 4,4-diaminodiphenysulfone (DDS) was investigated by differential scanning calorimetry (DSC) under non-isothermal and isothermal conditions. Under non-isothermal condition, the effective activation energy calculated from the advanced isoconversional method (AICM) varies with curing conversion, which is different from that obtained by Kissinger’s model. This should be attributed to the formation of highly crosslinked network later in the curing stage. The isothermal curing process at different temperatures could be well fitted by Kamal’s model in the initial curing stage but deviates subsequently. The deviation is corrected by introducing a diffusion factor in Kamal’s model. In addition, the naphthyl-imide epoxy resin is used to prepare carbon fiber reinforced composites and their applications were explored. The composites exhibit a high glass transition temperature, low moisture absorption, sufficient flame retardance and especially very low tensile strength loss at high temperature.

Flexible Asymmetric Supercapacitors Based on Nitrogen-Doped Graphene Hydrogels with Embedded Nickel Hydroxide Nanoplates.

To push the energy density limit of supercapacitors (SCs), new electrode materials with hierarchical nano-micron pore architectures are strongly desired. Graphene hydrogels that consist of 3 D porous frameworks have received particular attention but their capacitance is limited by electrical double layer capacitance. In this work, we report the rational design and fabrication of a composite hydrogel of N-doped graphene (NG) that contains embedded Ni(OH)2 nanoplates that is cut conveniently into films to serve as positive electrodes for flexible asymmetric solid-state SCs with NG hydrogel films as negative electrodes. The use of high-power ultrasound leads to hierarchically porous micron-scale sheets that consist of a highly interconnected 3 D NG network in which Ni(OH)2 nanoplates are well dispersed, which avoids the stacking of NG, Ni(OH)2 , and their composites. The optimal SC device benefits from the compositional features and 3 D electrode architecture and has a high specific areal capacitance of 255 mF cm-2 at 1.0 mA cm-2 and a very stable, high output cell voltage of 1.45 V, which leads to an energy density of 80 μW h cm-2 even at a high power of 944 μW cm-2 , considerably higher than that reported for similar devices. The devices exhibit a high rate capability and only 8 % capacitance loss over 10 000 charging cycles as well as excellent flexibility with no clear performance degradation under strong bending.

Co dendrite based bimetallic structures with nanoflake-built Pt covers and strong catalytic activity

Platinum-cobalt bimetallic structures have been synthesized in high yield via a galvanic replacement of Co dendrites in an aqueous K(2)PtCl(6) solution at room temperature. Increasing the K(2)PtCl(6) concentration results in different surface morphologies. Starting from furry coatings, a higher Pt/Co ratio leads to very rough surfaces built with upright standing nanoflakes, and finally a relatively smooth, cauliflower-like cover is obtained. The growth is discussed as the interplay of several mechanisms like the concentration driven nucleation, consumption of Co substrate, stoichiometry of the replacement reaction, and the electron transfer through the growing flakes. While the mono metallic structures are catalytically inactive towards the reduction of 4-nitrophenol by sodium borohydride, the catalytic activity of the bimetallic structures is quite high and optimized by a Pt atomic percentage of 27%. This indicates a catalytic role of the bimetallic interfaces.