Si Lei Phua

A biomimetic approach to enhancing interfacial interactions: polydopamine-coated clay as reinforcement for epoxy resin.

A facile biomimetic method was developed to enhance the interfacial interaction in polymer-layered silicate nanocomposites. By mimicking mussel adhesive proteins, a monolayer of polydopamine was constructed on clay surface by a controllable coating method. The modified clay (D-clay) was incorporated into an epoxy resin, it is found that the strong interfacial interactions brought by the polydopamine benefits not only the dispersion of the D-clay in the epoxy but also the effective interfacial stress transfer, leading to greatly improved thermomechanical properties at very low inorganic loadings. Rheological and infrared spectroscopic studies show that the interfacial interactions between the D-clay and epoxy are dominated by the hydrogen bonds between the catechol-enriched polydopamine and the epoxy.

Thin MoS2 Nanoflakes Encapsulated in Carbon Nanofibers as High-Performance Anodes for Lithium-Ion Batteries

In this work, highly flexible MoS2-based lithium-ion battery anodes composed of disordered thin MoS2 nanoflakes encapsulated in amorphous carbon nanofibrous mats were fabricated for the first time through hydrothermal synthesis of graphene-like MoS2, followed by electrospinning and carbonization. X-ray diffraction as well as scanning and transmission electron microscopic studies show that the as-synthesized MoS2 nanoflakes have a thickness of about 5 nm with an expanded interlayer spacing, and their structure and morphology are well-retained after the electrospinning and carbonization. At relatively low MoS2 contents, the nanoflakes are dispersed and well-embedded in the carbon nanofibers. Consequently, excellent electrochemical performance, including good cyclability and high rate capacity, was achieved with the hybrid nanofibrous mat at the MoS2 content of 47%, which may be attributed to the fine thickness and multilayered structure of the MoS2 sheets with an expanded interlayer spacing, the good charge conduction provided by the high-aspect-ratio carbon nanofibers, and the robustness of the nanofibrous mat.

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.

Transition‐Metal‐Ion‐Mediated Polymerization of Dopamine: Mussel‐Inspired Approach for the Facile Synthesis of Robust Transition‐Metal Nanoparticle–Graphene Hybrids

Inspired by the high transition-metal-ion content in mussel glues, and the cross-linking and mechanical reinforcement effects of some transition-metal ions in mussel threads, high concentrations of nickel(II), cobalt(II), and manganese(II) ions have been purposely introduced into the reaction system for dopamine polymerization. Kinetics studies were conducted for the Ni(2+)-dopamine system to investigate the polymerization mechanism. The results show that the Ni(2+) ions could accelerate the assembly of dopamine oligomers in the polymerization process. Spectroscopic and electron microscopic studies reveal that the Ni(2+) ions are chelated with polydopamine (PDA) units, forming homogeneous Ni(2+)-PDA complexes. This facile one-pot approach is utilized to construct transition-metal-ion-PDA complex thin coatings on graphene oxide, which can be carbonized to produce robust hybrid nanosheets with well-dispersed metallic nickel/metallic cobalt/manganese(II) oxide nanoparticles embedded in PDA-derived thin graphitic carbon layers. The nickel-graphene hybrid prepared by using this approach shows good catalytic properties and recyclability for the reduction of p-nitrophenol.

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.

Simultaneous catalyzing and reinforcing effects of imidazole-functionalized graphene in anhydride-cured epoxies

In this study, an imidazole-functionalized graphene (G-IMD) was prepared from graphene oxide by a facile one-pot method. The functionalized graphene not only showed improved organic compatibility but also could simultaneously play the roles of a cure accelerator and reinforcement for anhydride-cured epoxies. Our results showed that G-IMD could successfully catalyze the curing reaction without the addition of any routine accelerator. Thermal and mechanical properties of the epoxy–G-IMD nanocomposites were systematically studied at different filler loadings. Compared with neat epoxy resin, tensile strength and Youngs modulus of the nanocomposites were enhanced by 97% and 12%, respectively, at only 0.4 wt% G-IMD loading. Dynamic mechanical analysis and electron microscopic results revealed that the drastic improvements in mechanical properties could be attributed to the homogeneous dispersion of G-IMD and covalent bonding at the interface, which effectively improved the efficiency of load transfer between the matrix and graphene.

A high throughput method for preparation of highly conductive functionalized graphene and conductive polymer nanocomposites

Highly conductive graphene sheets were prepared by coating graphene oxide with polydopamine (PDA) followed by reduction with hydrazine. Polyacrylonitrile/graphene nanocomposites prepared via solution blending exhibit high electrical conductivities at very low graphene loadings owing to the good exfoliation and relatively planar conformation of the PDA-coated graphene in the polymer matrix.

Tailoring surface hydrophilicity of porous electrospun nanofibers to enhance capillary and push-pull effects for moisture wicking.

In this article, liquid moisture transport behaviors of dual-layer electrospun nanofibrous mats are reported for the first time. The dual-layer mats consist of a thick layer of hydrophilic polyacrylonitrile (PAN) nanofibers with a thin layer of hydrophobic polystyrene (PS) nanofibers with and without interpenetrating nanopores, respectively. The mats are coated with polydopamine (PDOPA) to different extents to tailor the water wettability of the PS layer. It is found that with a large quantity of nanochannels, the porous PS nanofibers exhibit a stronger capillary effect than the solid PS nanofibers. The capillary motion in the porous PS nanofibers can be further enhanced by slight surface modification with PDOPA while retaining the large hydrophobicity difference between the two layers, inducing a strong push-pull effect to transport water from the PS to the PAN layer.

Nanocups-on-microtubes: a unique host towards high-performance lithium ion batteries

In this work, unique carbonaceous nanocups, densely attached on a free-standing hollow microfibrous mat, were prepared via a mussel-inspired biomimetic polydopamine (PDA)-coating process using electrospun porous microfibers as the templates, followed by annealing. Electron microscopic studies show that the diameters and depths of the ellipsoid-shaped nanocups are in the range of a few hundred nanometers, and they have small openings of less than 100 nm, allowing the cups to act as nano-chambers to host other functional materials as well as nano-reactors for the synthesis of embedded nanostructures. To demonstrate the functions of such a unique hollow structure, the nanocups were used to host a MoS2 precursor, and through hydrothermal treatment, MoS2 nanosheets were effectively trapped in the nanocups. The MoS2-in-nanocups were used as an anode in lithium ion batteries. Good cyclability and excellent rate capacity (around 520 mA h g−1 at 2 A g−1) were achieved owing to the efficient charge transport provided by the good contact of the MoS2 nanosheets with the conductive nanocups and surrounding electrolyte. The nanocups could also act as buffering chambers to effectively accommodate the volume expansion of MoS2 during cycling.

Nacre-like composite films based on mussel-inspired ‘glue’ and nanoclay

Inspired by the brick-and-mortar structure of nacre and catechol–ferric ion complexes in marine mussel adhesive fibers, we utilized polydopamine (PDA) as “super glue” and clay nanosheets as “bricks” to fabricate nacre-like polydopamine-coated clay (D-clay) films using a simple vacuum filtration-assisted assembly method. The D-clay films were subsequently immersed in an aqueous solution of Fe3+ ions, allowing Fe3+ ions to diffuse in and cross-link the nanosheets. The morphologies and structures of D-clay and D-clay/Fe3+ films were characterized using field emission scanning electron microscopy, energy dispersive X-ray element mapping and two-dimensional X-ray diffraction. The results show that the diffusion of Fe3+ ions into the D-clay films induces morphological rearrangement of the D-clay platelets, leading to improved alignment and denser packing of the platelets in the films. Synergic combination of the improved packing structure and strong cross-linking coordination bonds result in significantly enhanced mechanical properties for the D-clay/Fe3+ films. Moreover, the nacre-like nanocomposite films exhibit excellent fire-shielding properties upon exposure to open flame as PDA can be easily carbonized in the combustion process.