Shu Huang

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.

Facile Fabrication of Functionalized Graphene Sheets (FGS)/ZnO Nanocomposites with Photocatalytic Property

Functionalized graphene sheets (FGS)/ZnO nanocomposites were fabricated via thermal treatment method, using graphene oxide as a precursor of graphene, Zn(NH(3))(4)CO(3) as a precursor of zinc oxide, and poly(vinyl pyrrolidone) as an intermediate to combine zinc with carbon materials. Thermogravimetric analysis, X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM), and scanning electron microscopy (SEM) were used to characterize crystal structure and morphology of FGS/ZnO nanocomposites. It was shown that the well-dispersed ZnO nanoparticles were deposited on FGS homogeneously. The composites exhibited photocatalytic activity to decompose rhodamine 6G efficiently under low-power ultraviolet (UV) light. This facile and low-cost method makes the composite a perfect candidate in applications of catalysis and other areas.

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.

Electrospinning of Polyvinylidene Difluoride with Carbon Nanotubes: Synergistic Effects of Extensional Force and Interfacial Interaction on Crystalline Structures

Polyvinylidene difluoride (PVDF) solutions containing a very low concentration of single-walled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWCNTs) of similar surface chemistry, respectively, were electrospun, and the nanofibers formed were collected using a modified rotating disk collector. The polymorphic behavior and crystal orientation of the nanofibers were studied using wide-angle X-ray diffraction and infrared spectroscopy, while the nanotube alignment and interfacial interactions in the nanofibers were probed by transmission electron microscopy and Raman spectroscopy. It is shown that the interfacial interaction between the SWCNTs and PVDF and the extensional force experienced by the nanofibers in the electrospinning and collection processes can work synergistically to induce highly oriented beta-form crystallites extensively. In contrast, the MWCNTs could not be well aligned along the nanofiber axis, which leads to a lower degree of crystal orientation.

From Waste to Functional Additive: Toughening Epoxy Resin with Lignin

A novel approach to toughen epoxy resin with lignin, a common waste material from the pulp and paper industry, is presented in this article. First, carboxylic acid-functionalized alkali lignin (AL-COOH) was prepared and subsequently incorporated into anhydride-cured epoxy networks via a one-pot method. The results of mechanical tests show that covalent incorporation of rigid AL-COOH into epoxy networks can significantly toughen the epoxy matrix without deteriorating its tensile strength and modulus. The addition of 1.0 wt % AL-COOH gives increases of 68 and 164% in the critical stress intensity factor (K(IC)) and critical strain energy release rate (G(IC)), respectively, relative to that of neat epoxy. This article opens up the possibility of utilizing low-cost and renewable lignin feedstocks as effective toughening agents for thermoset polymers.

Assembling Exfoliated Layered Double Hydroxide (LDH) Nanosheet/Carbon Nanotube (CNT) Hybrids via Electrostatic Force and Fabricating Nylon Nanocomposites

In this paper, Co-Al layered double hydroxide (LDH) and carbon nanotubes (CNT) have been assembled to form exfoliated LDH nanosheet/carbon nanotube hybrids via electrostatic force. The assembling process and nanostructures of exfoliated LDH/CNT hybrids were investigated by zeta potential, Raman spectroscopy, transmission electron microscopy (TEM), and X-ray diffraction (XRD). The assembly mechanism of LDH with CNT was also discussed. Furthermore, the unique three-dimensional (3D) hybrids thus prepared were used as reinforcing nanofillers to enhance the performance of polyamide 6 (PA6). It was found that the synergic effect of the CNT and LDH nanoplatelets resulted in homogeneous dispersion of the hybrid nanofillers throughout the PA6 matrix and strong combination with the matrix, thus providing an efficient mechanical improvement for the PA6 nanocomposites.

Complexes of Polydopamine-Modified Clay and Ferric Ions as the Framework for Pollutant-Absorbing Supramolecular Hydrogels

Clay-based functional hydrogels were facilely prepared via a bioinspired approach. Montmorillonite (clay) was exfoliated into single layers in water and then coated with a thin layer of polydopamine (PDOPA) via in situ polymerization of dopamine under basic aqueous conditions. When a small amount of ferric salt was added into aqueous suspensions of the polydopamine-coated clay (D-clay), D-clay and Fe(3+) ions could rapidly self-assemble into three-dimensional networks through the formation of coordination bonds. Consequently, supramolecular hydrogels were formed at very low D-clay contents. Rheological measurements show that the D-clay/Fe(3+) hydrogels exhibit fairly elastic response in low stain range, and have self-healing capability upon removal of applied large stress. More importantly, the hydrogels can be used as adsorbents to effectively remove Rhodamine 6G (Rh6G), an organic pollutant, from water. UV-vis absorption spectra of the Rh6G-loaded hydrogels show bands related to π-π stacking interactions between the aromatic moieties of PDOPA and Rh6G, confirming the formation of PDOPA/Rh6G complex on the surface of D-clay.

Aqueous stabilization of graphene sheets using exfoliated montmorillonite nanoplatelets for multifunctional free-standing hybrid films via vacuum-assisted self-assembly

We report a simple and efficient method to fabricate flexible multifunctional free-standing montmorillonite-graphene (MMT-G) hybrid films viavacuum filtration. Aqueous colloidal dispersion of reduced graphene oxide (r-GO), stabilized by MMT nanoplatelets with variable content of r-GO, has been obtained by direct reduction of graphene oxide (GO) in the presence of exfoliated MMT nanoplatelets. The formation of stable dispersion of graphene sheets in water is a result from two contributing factors, i.e. the hydrogen-bonding interaction and the crosslinking effects originated from sodium ions acting as “crosslinkers” between r-GO sheets and MMT nanoplatelets. Through filtration, the two-dimensional and positively charged MMT and r-GO can self-assemble to form highly oriented hybrid films, where MMT and r-GO stack with each other in a fashion of interlocking arrangements. The MMT-G hybrid films thus obtained show excellent flexibility, electrical conductivity and fire-retardant properties. Therefore, it might be useful to further study and understand the non-covalent interactions between nanoclay and graphene in aqueous media, and open a new strategy for graphene applications in colloidal chemistry.

Highly conductive graphene by low-temperature thermal reduction and in situ preparation of conductive polymer nanocomposites

Polydopamine-coated graphene oxide (DGO) films exhibit electrical conductivities of 11,000 S m(-1) and 30,000 S m(-1) upon vacuum annealing at 130 °C and 180 °C, respectively. Conductive poly(vinyl alcohol)/graphene and epoxy/graphene nanocomposites show low percolation thresholds due to the excellent dispersibility of the DGO sheets and their effective in situ reduction.

Functionalization of graphene and grafting of temperature- responsive surfaces from graphene by ATRP ''on water''

Water-dispersible graphene with temperature-responsive surfaces has successfully been synthesized by grafting poly(N-isopropylacrylamide) (PNIPAM) from graphene via surface-initiated atom transfer radical polymerization (ATRP). First, graphene surfaces are functionalized with aminophenol groups by diazonium reaction on water. Subsequently, bromoisobutyrate groups are covalently attached to the phenol-functionalized graphene (G-OH) surface by esterification of 2-bromoisobutyrate with the hydroxyl groups, forming bromoisobutyrate-functionalized graphene (G-Br). Finally, PNIPAM is then grafted from G-Br via ATRP. Data from Raman spectroscopy, 1H NMR, and transmission electron microscopy (TEM) confirm that PNIPAM chains grow from graphene by ATRP. Thermogravimetric analysis shows that the amount of PNIPAM grown from the graphene increases with the increase of monomer ratios. TEM images also show that functionalized polymer structures (PNIPAM cluster or agglutination) on graphene sheets can be well tuned by controlled polymerization. The obtained graphene-PNIPAM (G-PNIPAM) composite has PNIPAM surface which is highly sensitive to the temperature change. This temperature-responsive and water-dispersible G-PNIPAM composite may find potential applications in environmental devices as well as controlled release drug delivery.