Shiren Wang

Wettability and Surface Free Energy of Graphene Films

Graphene sheets were produced through chemical exfoliation of natural graphite flake and hydrazine conversion. Subsequently, graphene sheets were assembled into a thin film, and microscale liquid droplets were placed onto the film surface for measurement of wettability and contact angle. It is found that a graphene oxide sheet is hydrophilic and a graphene sheet is hydrophobic. Isolated graphene layers seem more difficult to wet in comparison to graphite, and low adhesion work was found in the graphene-liquid interface. Approximation of solid-liquid interfacial energy with the equation of state theory was applied to determine the graphene surface energy. The results indicate that surface energy of graphene and graphene oxide is 46.7 and 62.1 mJ/m2, respectively, while natural graphite flake shows a surface free energy of 54.8 mJ/m2 at room temperature. These results will provide valuable guidance for the design and manufacturing of graphene-based biomaterials, medical instruments, structural composites, electronics, and renewable energy devices.

Enhancing thermoelectric properties of organic composites through hierarchical nanostructures

Organic thermoelectric (TE) materials are very attractive due to easy processing, material abundance, and environmentally-benign characteristics, but their potential is significantly restricted by the inferior thermoelectric properties. In this work, noncovalently functionalized graphene with fullerene by π-π stacking in a liquid-liquid interface was integrated into poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate). Graphene helps to improve electrical conductivity while fullerene enhances the Seebeck coefficient and hinders thermal conductivity, resulting in the synergistic effect on enhancing thermoelectric properties. With the integration of nanohybrids, the electrical conductivity increased from ~10000 to ~70000 S/m, the thermal conductivity changed from 0.2 to 2 W·K−1m−1 while the Seebeck coefficient was enhanced by around 4-fold. As a result, nanohybrids-based polymer composites demonstrated the figure of merit (ZT) as high as 6.7 × 10−2, indicating an enhancement of more than one order of magnitude in comparison to single-phase filler-based polymer composites with ZT at the level of 10−3.

Functionalization of graphene sheets through fullerene attachment

Fullerene-functionalized graphene shows a hierarchical structure, and is promising for many potential applications. In this paper, a new method is presented to synthesize such a novel nanostructure. Graphite flakes were intercalated to graphite oxide, and then functionalized with zero-dimensional fullerene nanocrystals. The resultant compounds were characterized by FT-IR spectroscopy, UV–Vis spectroscopy, atomic force microscopy, and transmission electron microscopy. The characterization results confirmed that fullerene crystals were successfully attached onto the single-layer graphene sheets and significantly facilitated the exfoliation of graphite to monolayer graphene. The fullerene grafted on the graphene surface serves as a space impediment to prevent the re-stacking of exfoliated graphene sheets. This attempt provides an effective route for large-scale exfoliation and functionalization of monolayer graphene, and is expected to significantly facilitate the application of graphene in the electronic devices, energy storage, and functional materials.

Thermoelectric properties of porous multi-walled carbon nanotube/polyaniline core/shell nanocomposites

Porous polyaniline (PANI)-coated multi-walled carbon nanotube (MWNT) core/shell nanohybrids were fabricated through in situ polymerization and subsequently assembled into macroscopic composites. N(2) adsorption/desorption analysis indicated that the volume of nanopores increased significantly, which could make a significant contribution to phonon scattering. Thermal annealing was also carried out to improve the Seebeck coefficient of the as-produced nanocomposites. The optimal sample showed electrical conductivity of 14.1 S cm(-1), a Seebeck coefficient of 79.8 μV K(-1) and thermal conductivity of 0.27 W mK(-1), resulting in a highest figure of merit (ZT) of 0.01 at a very low loading of MWNTs (<1 wt%). These results will provide a potential direction to enhance thermoelectric performance of organic materials and also facilitate the application of organic materials in thermal energy harvesting or cooling.

Efficient photothermal therapy of brain cancer through porphyrin functionalized graphene oxide

Current clinical treatments including surgical resection, radiation therapy and chemotherapy for brain cancer result in high mortality due to the complex structure of the brain aggressiveness of brain cancer. Recently, non-invasive photothermal therapy (PTT) using near infrared laser irradiation has been developed as an alternative emerging therapy for brain cancer. In this paper, a biocompatible porphyrin functionalized graphene oxide (PGO) with high absorbance at 808 nm is synthesized as a photothermal platform for brain cancer therapy. Graphite oxide is exfoliated and conjugated with porphyrin through π–π interactions. This PGO is two times more stable than reduced graphene oxide (rGO) in aqueous solution. Most importantly, the efficiency of the photo-thermal conversion of PGO is increased by 89% and 33% compared to graphene oxide and rGO under 808 nm laser irradiation, causing ablation of a large number of brain cancer cells in vitro. This PGO platform containing active functional groups allows specific targeting in PTT without harming healthy cells and tissues.

3D printing of an extremely tough hydrogel

Because of their low viscosity and large gelling temperature range, precise 3D printing of agar hydrogels has not been achieved even though the agar double network hydrogels are tough and self-recoverable. In this work, a super tough agar double network hydrogel was precisely printed by adding alginate. The addition of alginate not only increases the ink viscosity and printable period, but also improves its rheological characteristics towards precise processing control. Moreover, the entanglement of the alginate chains with the agar double network hydrogel restricts the agar helical chain bundles from pulling out under stress, which toughens the hydrogel.

Direct laser initiation and improved thermal stability of nitrocellulose/graphene oxide nanocomposites

We report on the enhancement and possible control of both laser ignition and burn rates of Nitrocellulose (NC) microfilms when doped with graphene oxide (GO). A Nd:YAG (1064 nm, 20 ns) laser is used to ignite GO-doped NC films at low temperatures. The effect of GO on the doping concentration of the activation energies of laser ignition and thermal stability of the NC films is studied. The activation energy of laser ignition decreases with increasing GO/NC weight ratio and attains a constant value with higher concentrations. This behavior is accompanied by an increase in the thermal stability.

Tough and fully recoverable hydrogels

In natural cartilage, collagen fibers form the extracellular matrix, while aggrecan entangles with these fibers and provides cartilage with its osmotic properties, which are critical to resist cyclic compressive loads. In this paper, a hydrogel was fabricated via the entanglement of a bio-inspired nanostructure (chondroitin sulfate-coated vinyl silica nanoparticles, CS-SNP) within an agar/poly(acrylamide) double network hydrogel. The highly charged chondroitin sulfate groups provide additional compression resistance within the macromolecular chains, while the solid silica cores anchor these entanglements. The presence of the CS-SNP not only improved the compressive modulus, compressive strength, fracture toughness, and fatigue resistance of this hydrogel, but also ensured the full recovery of all these properties after thermal heating. This tough, fully recoverable, and robust hydrogel is a promising material for applications with strong mechanical requirements.

Large-Area Preparation of High-Quality and Uniform Three-Dimensional Graphene Networks through Thermal Degradation of Graphene Oxide–Nitrocellulose Composites

We demonstrate a simple method to prepare high-quality and uniform three-dimensional (3D) graphene networks through thermal degradation of graphene oxide (GO)-nitrocellulose composites over a large area. The nitrocellulose simultaneously acts as a support and aids in the reduction of GO by exothermic decomposition. The graphene networks have tunable porous morphology where the pore size can be controlled by adjusting the concentration of GO in the composite. This new technique is a very simple method to obtain 3D graphene networks and has the potential to produce 3D graphene-modified substrates for use in energy storage and conversion applications, in supporting frameworks of catalyst, and in sensors. In this report, the prepared 3D graphene networks were directly used as the electrodes of supercapacitors without using a binding agent and/or conducting additive with a high specific capacitance of 162.5 F g(-1) at 0.5 A g(-1) current density.

Enhanced electrical conductivity in mesoporous 3D indium-tin oxide materials

Free-standing, three-dimensional (3D) indium-tin oxide monoliths with an interconnected pore structure have been prepared using a non-alkoxide preparative method. This approach aids in the formation of stable frameworks with a regular pore structure and considerable electrical conductivity.