Weng Weei Tjiu

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 via 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.

Hybridization of graphene sheets and carbon-coated Fe3O4 nanoparticles as a synergistic adsorbent of organic dyes

Magnetic graphene–Fe3O4@carbon (GFC) hybrids with hierarchical nanostructures have been synthesized and their application as an adsorbent for the removal of organic dyes has been investigated. Graphene–Fe3O4 hybrids were first prepared via a facile one-pot solvothermal process, then carbonaceous coatings on Fe3O4 nanoparticles of nanometer thickness were synthesized by a hydrothermal carbonization process using eco-friendly glucose as a carbon source. Graphene sheets acting as two-dimensional (2D) substrates can effectively prevent the Fe3O4 nanoparticles from aggregating and enable a good dispersion of these magnetic nanoparticles. The carbonaceous layer protects the Fe3O4 nanoparticles in acidic environments and greatly enhances the specific surface area of the hybrids which is beneficial for the removal of organic dyes, such as methylene blue (MB). The resultant GFC hybrids exhibit great adsorption properties not only in water but also in acidic environments, and about 86% and 77% of the dye removal efficiency can be retained after five adsorption–desorption cycles in water and 1 M HCl, respectively. The rapid and efficient adsorption of organic dyes from water as well as acid suggests that the GFC hybrids have potential environmental applications as alternatives to commercial materials in wastewater treatment for the removal of organic dyes.

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

Layered double hydroxides (LDHs) are generally expressed as [M(2+)(1-x)M(3+)(x) (OH)(2)] [A(n-)(x/n)·mH(2)O], where M(2+) and M(3+) are divalent and trivalent metal cations respectively, and A is n-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.

Synthesis of Fe nanoparticles@graphene composites for environmental applications

Fe nanoparticles@graphene composites (FGC) are successfully synthesized by using graphene oxide (GO) as a supporting matrix. GO is first treated with Fe(3+) to form Fe(3+)@GO complexes. Then, by adding NaBH(4) solution, Fe(3+) and GO are simultaneously reduced in situ to Fe and graphene respectively, forming FGC hybrid composites. The structures, properties and applications of the hybrids thus obtained are investigated by X-ray diffraction, Raman spectroscopy, Fourier transformed infrared spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, thermogravimetric analysis and magnetization measurements. The hybrids are also evaluated for decolorization of methyl blue solution, a model dye in wastewater of dyeing industry. Compared with bare Fe particles, the high removal capacities of FGC are due to the increased adsorption sites in the hybrids, which are achieved by inhibiting the particle aggregation and reducing the size of Fe nanoparticles.

Carbon Nanotubes Bridged with Graphene Nanoribbons and Their Use in High‐Efficiency Dye‐Sensitized Solar Cells

To improve the practical application of functional nanomaterials, it is critically important, but often challenging, to extend their excellent properties from the nanoscale to the macroscopic scale. For example, carbon nanotubes (CNTs) have been widely studied as a new family of electrode materials for various optoelectronic and electronic devices, owing to their unique structure and remarkable electronic and catalytic properties. However, CNTs were typically aggregated to form networks with many boundaries, which significantly inhibited rapid charge transport. The resulting CNT-based electrodedevices showed much lower efficiency than expected. To solve the above problem and achieve high performance at the macroscopic scale, a general strategy is to design novel structures to realize effective interactions among CNTs at the molecular scale. To this end, nature provides excellent models for efficiently performing complex functions through the creation of elaborate structures. Well-known examples are blood vessels, which are constructed with trunks interconnected by a lot of branches, a paradigmatic structure to rapidly deliver nutrients throughout our bodies. Inspired by nature, herein we discuss the development of a new structure in which CNTs are bridged by graphene nanoribbons. Briefly, multiwalled CNTs are partially unzipped to form nanoribbons with one end on the mother CNT and the other on a neighboring CNT. Due to the strong p–p interaction between nanotube and nanoribbon, and the high charge mobility in nanoribbons, produced electrons can be rapidly transported among CNTs to macroscopically achieve high performance. As a demonstration, dye-sensitized solar cells (DSCs) with graphene-nanoribbon-bridged CNTs as counter electrodes (Figure 1) showed an energy conversion efficiency of up to 8.23%, compared with 7.61% for a conventional platinum counter electrode under similar conditions. Multiwalled CNTs with a diameter of 20–40 nm and a wall number of 20–30 were primarily studied (Supporting Information, SFigure S1). The CNTs were chemically unzipped to produce graphene oxide nanoribbons (GONRs; Figure S2). The degree of unzipping could mainly be increased by increasing the amount of potassium permanganate. X-ray diffraction (XRD) analysis was undertaken to quantitatively determine the degree of unzipping. Figure S3 shows typical XRD patterns for the mixtures of GONR and CNT with different GONR weight percentages. As the amount of GONR in the mixture increased, the characteristic GONR peak (2q= 11.28) gradually increased while the CNT peak (2q= 26.18) gradually decreased. Based on the intensity of their characteristic peaks, a relationship curve between GONR weight percentage and intensity ratio was obtained. This curve could be then used to calculate the weight percentage of GONRs in the resulting hybrid (Figure S4). Figure 2 compares the XRD patterns of pristine CNTs, GONR/CNT hybrids with different GONR weight percentages, and pure GONR. The GONR weight percentages in three hybrid samples were calculated as approximately 16%, 55%, and 85%. For simplicity, they were defined as GONR16%/CNT, GONR55%/CNT and GONR85%/CNT. CNTs could be also completely unzipped to form pure nanoribbons. Raman spectroscopy was used to monitor structural changes during the unzipping process of CNTs (Figure S5). The D band at ca. 1350 cm 1 gradually broadened and increased in intensity as the reaction progressed, a result which corresponds to increasing GONR weight percentage. As a result, the ratio between the intensities of the D and G bands (ID/IG) was enhanced, which indicates a decrease in Figure 1. a) the structure of dye-sensitized solar cells based on R-GONR-bridged CNTs as counter electrode. b) the mechanism of rapid electron transport in the counter electrode.

Facile preparation of water-dispersible graphene sheets stabilized by acid-treated multi-walled carbon nanotubes and their poly(vinyl alcohol) composites

Graphene has attracted tremendous interest in reinforcing fillers due to its unique structure and excellent physical properties. However, efficient reinforcement has been largely limited because graphene tends to agglomerate within a polymer matrix. In this study, direct reduction of graphene oxide (GO) in water in the presence of acid-treated multi-walled carbon nanotubes (t-CNTs) results in a homogeneous dispersion of reduced graphene oxide (r-GO) and t-CNT hybrids. The three-dimensional r-GO/t-CNTs hybrids (abbreviated as G-CNT hybrids) possess unique properties, making them ideal reinforcing fillers for polymer nanocomposites. Poly(vinyl alcohol) (PVA) composite containing G-CNT hybrids is then prepared by a simple water casting method. Due to the synergistic interaction of the two kinds of nanofillers, the tensile strength and Youngs modulus of the resulting PVA nanocomposite filled with only 0.6 wt% G-CNT hybrids are significantly improved by about 77% and 65%, respectively. The results indicate that the nanohybrids are well dispersed throughout the PVA matrix and form strong interfacial interactions with the matrix. Besides, the presence of G-CNT hybrids slightly increases the thermal stability at 5% weight loss and remarkably improves the amount of residues of PVA nanocomposites. For comparison, we also prepare a PVA composite with the addition of only t-CNTs and find that its mechanical properties show limited enhancement owing to the decreased length-to-diameter ratio and structural defects of CNTs by acid treatment. Our work therefore provides a new way for the preparation of hybrid carbon nanomaterials with unique structure and excellent properties for fabricating high-performance polymer nanocomposites.

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 N8.3-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.

One-step synthesis of zero-dimensional hollow nanoporous gold nanoparticles with enhanced methanol electrooxidation performance

Nanoporous gold with networks of interconnected ligaments and highly porous structure holds stimulating technological implications in fuel cell catalysis. Current syntheses of nanoporous gold mainly revolve around de-alloying approaches that are generally limited by stringent and harsh multistep protocols. Here we develop a one-step solution phase synthesis of zero-dimensional hollow nanoporous gold nanoparticles with tunable particle size (150-1,000 nm) and ligament thickness (21-54 nm). With faster mass diffusivity, excellent specific electroactive surface area and large density of highly active surface sites, our zero-dimensional nanoporous gold nanoparticles exhibit ~1.4 times enhanced catalytic activity and improved tolerance towards carbonaceous species, demonstrating their superiority over conventional nanoporous gold sheets. Detailed mechanistic study also reveals the crucial heteroepitaxial growth of gold on the surface of silver chloride templates, implying that our synthetic protocol is generic and may be extended to the synthesis of other nanoporous metals via different templates.

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 Ni(2+) ions as catalysts for the gelation process to substitute the usually used alkaline carbonates. The metal ions of Ni(2+) 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, Ni(2+) 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.

Electrospun carbon nanofibers decorated with Ag-Pt bimetallic nanoparticles for selective detection of dopamine.

Electrospun nanoporous carbon nanofibers (pCNFs) decorated with Ag-Pt bimetallic nanoparticles have been successfully synthesized by combining template carbonization and seed-growth reduction approach. Porous-structured polyacrylonitrile (PAN) nanofibers (pPAN) were first prepared by electrospinning PAN/polyvinylpyrrolidone (PVP) blend solution, followed by subsequent water extraction and heat treatment to obtain pCNFs. Ag-Pt/pCNFs were then obtained by using pCNFs as support for bimetallic nanoparticle loading. Thus, the obtained Ag-Pt/pCNFs were used to modify glassy carbon electrode (GCE) for selective detection of dopamine (DA) in the presence of uric acid (UA) and ascorbic acid (AA). This novel sensor exhibits fast amperometric response and high sensitivity toward DA with a wide linear concentration range of 10-500 μM and a low detection limit of 0.11 μM (S/N = 3), wherein the interference of UA and AA can be eliminated effectively.