Graphene-Wrapped Polyaniline Hollow Spheres As Novel Hybrid Electrode Materials for Supercapacitor Applications

Polyaniline hollow spheres (PANI-HS)@electrochemical reduced graphene oxide (ERGO) hybrids with core–shell structures have been fabricated <i>via</i> a solution-based coassembly process. The hollow nanostructured designing for the PANI-HS greatly enlarges the specific surface area, providing high electroactive regions and short diffusion lengths for both charge and ion transport. The wrapping of ERGO sheets on the PANI-HS can offer highly conductive pathways by bridging individual PANI-HS together, thus facilitating the rate and cycling performance of supercapacitors. The specific capacitance of PANI-HS36@ERGO hybrids can reach 614 F g^–1 at a current density of 1 A g^–1. Furthermore, the capacitance of the PANI-HS36@ERGO hybrids maintains 90% after 500 charging/discharging cycles at a current density of 1 A g^–1, indicating a good cycling stability. The greatly enhanced electrochemical performance can be ascribed to the synergic effects of the two components of PANI-HS and ERGO, suggesting that the PANI-HS@ERGO hybrids as novel electrode materials may have potential applications in high-performance energy storage devices

Immobilization of Co–Al Layered Double Hydroxides on Graphene Oxide Nanosheets: Growth Mechanism and Supercapacitor Studies

Layered double hydroxides (LDHs) are generally expressed as [M²⁺_1–<i>x</i>M³⁺_<i>x</i> (OH)₂] [A^<i>n</i>–_{<i>x</i>/<i>n</i>}·<i>m</i>H₂O], where M²⁺ and M³⁺ are divalent and trivalent metal cations respectively, and A is <i>n</i>-valent interlayer guest anion. Co–Al layered double hydroxides (LDHs) with different sizes have been grown on graphene oxide (GO) via in situ hydrothermal crystallization. In the synthesis procedure, the GO is partially reduced in company with the formation of Co–Al LDHs. The morphology and structure of LDHs/GO hybrids are characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The growth mechanism of LDHs on GO nanosheets is discussed. Moreover, both LDHs and LDHs/graphene nanosheets (GNS) hybrids are further used as electrochemical supercapacitor materials and their performance is evaluated by cyclic voltammetry (CV) and galvanostatic charge/discharge measurements. It is shown that the specific capacitances of LDHs are significantly enhanced by the hybridization with GNS

Bimetallic Platonic Janus Nanocrystals

We demonstrate the creation of Ag-based bimetallic platonic Janus nanostructures by confining galvanic replacement reaction at a nanoscale interface on highly symmetrical nanostructures such as Ag nanocubes and nanooctahedra using reactive microcontact printing (μCP). The extent of galvanic replacement reaction can be controlled kinetically to derive Janus nanostructures with Au nanodots deposited on either one or multiple facets of Ag nanocubes. The selective deposition of Au dots on a single facet of Ag nanocubes breaks the cubic symmetry and brings about unique and anisotropic plasmonic responses. High-resolution cathodoluminescence hyperspectral imaging of single Janus nanocube demonstrates that surface plasmon resonances corresponding to Au and Ag can be excited at different spots on one Janus nanocube. In addition, we demonstrate the fabrication of alternating Janus/non-Janus segments on 2D Ag nanowires by using a line-patterned polydimethylsiloxane (PDMS) stamp for galvanic replacement. Aside from Au, Pt and Pd can also be selectively deposited onto Ag nanocubes. These Janus nanostructures may find important applications in the field of plasmon-enhanced catalysis

Nitrogen-Doped Graphene Nanoribbons as Efficient Metal-Free Electrocatalysts for Oxygen Reduction

Nitrogen-doped graphene nanoribbon (N-GNR) nanomaterials with different nitrogen contents have been facilely prepared via high temperature pyrolysis of graphene nanoribbons (GNR)/polyaniline (PANI) composites. Here, the GNRs with excellent surface integration were prepared by longitudinally unzipping the multiwalled carbon nanotubes. With a high length-to-width ratio, the GNR sheets are prone to form a conductive network by connecting end-to-end to facilitate the transfer of electrons. Different amounts of PANI acting as a N source were deposited on the surface of GNRs via a layer-by-layer approach, resulting in the formation of N-GNR nanomaterials with different N contents after being pyrolyzed. Electrochemical characterizations reveal that the obtained N_8.3-GNR nanomaterial has excellent catalytic activity toward an oxygen reduction reaction (ORR) in an alkaline electrolyte, including large kinetic-limiting current density and long-term stability as well as a desirable four-electron pathway for the formation of water. These superior properties make the N-GNR nanomaterials a promising kind of cathode catalyst for alkaline fuel cell applications

Ni-Doped Graphene/Carbon Cryogels and Their Applications As Versatile Sorbents for Water Purification

Ni-doped graphene/carbon cryogels (NGCC) have been prepared by adding resorcinol and formaldehyde to suspension of graphene oxide (GO), using Ni²⁺ ions as catalysts for the gelation process to substitute the usually used alkaline carbonates. The metal ions of Ni²⁺ have elevated the cross-linking between GO and RF skeletons, thus strengthening the whole cryogel. The as-formed three-dimensional (3D) interconnected structures, which can be well-maintained after freeze-drying of the hydrogel precursor and subsequent carbonization under an inert atmosphere, exhibit good mechanical properties. During the carbonization process, Ni²⁺ ions are converted into Ni nanoparticles and thus embedded in the interconnected structures. The unique porosity within the interconnected structures endows the cryogels with good capability for the extraction of oils and some organic solvents while the bulk form enables its recycling use. When ground into powders, they can be used as adsorbents for dyestuffs. Therefore, the as-obtained cryogels may find potential applications as versatile candidates for the removal of pollutants from water

Nanoporous Gold Nanoframes with Minimalistic Architectures: Lower Porosity Generates Stronger Surface-Enhanced Raman Scattering Capabilities

Current synthesis of gold nanoframes has only demonstrated morphological control over wall thickness and wall length. Here, we demonstrate the ability to control the nanoscale porosity of these nanoframes, using a templated seed-mediated approach. The porosity on these nanoporous gold nanoframes (NGNs) is tuned by controlling the crystallite size of Au nanoparticles deposited on the AgCl templates. The yield of the NGNs approaches 100%. Despite its minimalist architectural construction, the NGNs are mechanically robust, retaining its morphology even after multiple centrifugation and sonication rounds. We further highlight that decreasing the porosity on the NGN leads to improved surface-enhanced Raman scattering (SERS) enhancement. Increasing the constituent Au crystallite size decreases the porosity, but increases the surface roughness of NGN, hence leading to greater SERS enhancement. The introduction of porosity in a gold nanoframe structure through our synthesis method is novel and generic, suggesting the extendibility of our method to other types of templates

Manipulating the d‑Band Electronic Structure of Platinum-Functionalized Nanoporous Gold Bowls: Synergistic Intermetallic Interactions Enhance Catalysis

Bicontinuous nanoporous gold (NPG) is a high-performance catalyst characterized by its excellent electrochemical stability and immense active surface area with high electrolyte accessibility. However, the intrinsic catalytic activity of NPG is still lower compared to other metals (such as Pt), thus impeding its applicability in a commercial catalytic system. Herein, we incorporate secondary Pt metal with inherently strong catalytic activities into a zero-dimensional (0D) nanoporous gold bowl (NPGB) to develop Pt-NPGB bimetallic catalyst. Our strategy effectively exploits the highly accessible surface area of NPGB and the manipulative d-band electronic structure brought about by the synergistic intermetallic interaction for enhanced catalytic performance and durability. Deposition of Pt on the NPGB catalyst directly modulates its d-band electronic structure, with the electronic energy of Pt-NPGBs tunable between −3.93 to −4.24, approximating that of chemically resistant gold (−4.35 eV). This is vital to weaken the binding strength between Pt active sites and intermediate poisoning species. Together with the high Pt electrochemical active surface area (ECSA) of 17.1 mA/μg_Pt facilitated by NPGB cocatalyst, such synergistic effect enables the superior performance of Pt-NPGB hybrids over commercial Pt/C in methanol oxidation reaction (MOR), where an 11-fold and 227-fold better catalytic activity and durability are demonstrated even after an extended duration of 3600 s. Our study is therefore the first demonstration of NPGB on the exploitation of precisely modulated synergistic effect at the electronic level to control and boost catalytic performance. Furthermore, the chemically inert NPGB possesses an intrinsically higher gold surface area and electrolyte accessibility unique to 0D nanoparticle, hence empowering it as an immensely attractive cocatalytic platform extendable to a wide range of secondary metals. This is important to promote the catalytic performance for diverse electrochemical applications, especially in the field of energy, synthetic chemistry, and also environmental toxin degradation

A Chemical Route To Increase Hot Spots on Silver Nanowires for Surface-Enhanced Raman Spectroscopy Application

The effective number of surface-enhanced Raman spectroscopy (SERS) active hot spots on plasmonic nanostructures is the most crucial factor in ensuring high sensitivity in SERS sensing platform. Here we demonstrate a chemical etching method to increase the surface roughness of one-dimensional Ag nanowires, targeted at creating more SERS active hot spots along Ag nanowire’s longitudinal axis for increased SERS detection sensitivity. Silver nanowires were first synthesized by the conventional polyol method and then subjected to chemical etching by NH₄OH and H₂O₂ mixture. The surfaces of silver nanowires were anisotropically etched off to create miniature “beads on a string” features with increased surface roughness while their crystallinity was preserved. Mapping of single-nanowire SERS measurements showed that the chemical etching method has overcome the limitation of conventional one-dimensional Ag nanowires with limited SERS active area at the tips to produce etched Ag nanowires with an increase in Raman hot spots and polarization-independent SERS signals across tens of micrometers length scale