Mingqiang Zhong

Graphene/poly(vinylidene fluoride) composites with high dielectric constant and low percolation threshold

In aiming to obtain highly flexible polymer composites with high dielectric performance, graphene/poly(vinylidene fluoride) (PVDF) composites with a multi-layered structure were proposed and prepared. Graphene sheets were prepared by reducing graphene oxide using phenylhydrazine, which could effectively alleviate aggregation of the graphene sheets. A two-step method, including solution casting and compression molding, was employed to fabricate the graphene/PVDF composites. The composites showed an alternative multi-layered structure of graphene sheets and PVDF. Due to their unique structure, the composites had an extremely low percolation threshold (0.0018 volume fraction of graphene), which was the lowest percolation threshold ever reported among PVDF-based polymer composites. A high dielectric constant of more than 340 at 100 Hz could be obtained within the vicinity of the percolation threshold when the graphene volume fraction was 0.00177. Above the percolation threshold, the dielectric constant continued to increase and a maximum value of as high as 7940 at 100 Hz was observed when the graphene volume fraction was 0.0177.

Self-assembly of NiO nanoparticles in lignin-derived mesoporous carbons for supercapacitor applications

We demonstrate the self-assembly of highly dispersed NiO nanoparticles embedded in lignin-derived mesoporous carbon (MPC) frameworks. Self-assembly is induced by evaporation of the solvent from a mixture of metal-containing liquid crystalline mesophases of lignin-derived polymers and transition metal nitrate hydrate, which yielded NiO@MPC nanostructures at 600 °C under a N2 atmosphere. The microstructure and morphology of the NiO@MPC are characterized by XRD, TEM and BET. The results show that the NiO nanoparticles are highly dispersed in a mesoporous carbon matrix. The NiO@MPC composites show metal oxide contents in the range 49–79 wt%, high surface areas (503–802 m2 g−1), uniform pore sizes (≈3.7 nm), various porous distributions and large pore volumes (0.46–0.68 cm3 g−1). Electrochemical studies were carried out by measurement of cyclic voltammetry (CV) and charge–discharge tests. The results demonstrate that the NiO@MPC composites have high specific capacitance (880.2 F g−1 at a current density of 1.0 A g−1) and display good retention. 90.9% of the specific capacitance is retained when the current density changes from 1 A g−1 to 10 A g−1 in the charge–discharge tests and 93.7% of the specific capacitance is retained after 1000 charge–discharge cycles. Thus, the NiO@MPC composites are promising supercapacitor electrode materials.

Visible light responsive sulfated rare earth doped TiO2@fumed SiO2 composites with mesoporosity: Enhanced photocatalytic activity for methyl orange degradation

Visible light (VL) responsive mesoporous sulfated rare earth ions (Nd(3+), La(3+), Y(3+)) incorporated TiO2@fumed SiO2 photocatalysts were prepared by sol-gel method with P123 (EO20PO70EO20) as a template. The resultant samples were characterized by X-ray diffraction (XRD), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), N2 adsorption-desorption measurements (BET), UV-vis diffuse reflectance spectroscopy, photoluminescence (PL) spectra, Fourier transform infrared spectroscopy (FTIR) and thermal analyses (TG-DTA). In comparison with nondoped sample, RE-doped samples showed not only an increase in the surface areas and pore volumes, but also an inhibition of titania phase transition from anatase to rutile. Photo-degradation results revealed that RE-doped samples could greatly improve the photocatalytic activity, and the experimental degradation rates of methyl orange (MO) were higher than that catalyzed by undoped samples and Degussa P-25, obeyed the order of Nd(3+)>La(3+)>Y(3+). Nd-doped sample expressed the highest photoactivity and the optimal dosage was 0.25mol%, which resulted in MO degradation rates of 99.8% and 90.05% irradiation under UV for 60min and VL (λ>400nm) for 40h, respectively. The enhanced photocatalytic activity could be attributed to the higher specific area, good crystallinity, strong VL absorption and effective separation of photogenerated electron-hole pairs in the catalyst.

Self-assembly between graphene sheets and cationic poly(methyl methacrylate) (PMMA) particles: preparation and characterization of PMMA/graphene composites

In this study, we presented a simple approach to prepare poly(methyl methacrylate) (PMMA)/graphene composites based on the self-assembly between graphene oxide (GO) sheets and cationic PMMA emulsion particles. Briefly, cationic PMMA emulsion particles were first synthesized by a soap-free emulsion polymerization process, in which methacryloyloxyethyl trimethyl ammonium chloride was used as the emulsifier, and then blended with the aqueous solution of GO. Through electrostatic attraction, the exfoliated GO sheets were tightly adhered on the PMMA particles. The GO sheets could be reduced in situ into graphene sheets by a chemical method, without the aggregation. The structure of the prepared composites and the influences of GO and graphene sheets on the properties of PMMA were investigated. Both GO and graphene sheets can increase the glass transition temperature and storage modulus of PMMA. Moreover, graphene sheets provided a more significant reinforcement effect.

Polyelectrolyte/mesoporous silica hybrid materials for the high performance multiple-detection of pH value and temperature

Mesoporous silica nanoparticles have been widely adopted in energy, biology and medicine due to their well-ordered and stable structures. Nevertheless, few attempts have been made to study these materials as a sensing tool. Herein, we report a “smart” sensor for the dual-detection of the pH value and temperature, which was implemented with environmentally responsive polyelectrolyte/mesoporous silica electrodes. Using SBA-15 silica as the framework, we functionalized the internal mesopores with DMAMEA monomer via surface-initiated RAFT polymerization (“grafting-from” method). By controlling the degree of polymerization, the pore size and the specific surface area can be precisely controlled. When the degree of polymerization was optimized to 75, the hybrid material showed significant sensitivity in response to the pH value in the range of 4–10 and optimally responded to the temperature at 39 °C, setting a pH value of 10. The ionic conductivities of the template Fe(CN)64−/3− and Ru(NH3)62+/3+ ions were switchable in different conditions. These results suggest that the polyelectrolyte/mesoporous silica hybrid materials could have potential for application in dual-functional sensors in environmental detection.

Effect of nanoporous structure and polymer brushes on the ionic conductivity of poly(methacrylic acid)/anode aluminum oxide hybrid membranes

Anode aluminum oxide (AAO) porous materials have been widely used in ionic translocation for many biological and chemical studies. However, the lack of stimuli-response of this material limits its applications for the precise control of ionic transportation by the external environment. In this study, we functionalized the internal nanopores of AAO membranes to generate polyelectrolyte-filled pH-responsive membranes whose ionic conductivity could be readily controlled by changing the pH value. AAO membranes with different pore sizes (25, 75 and 100 nm) were modified with poly(methacrylic acid) (PMAA) by a “grafting-to” approach. Thermogravimetric and SEM analysis revealed that the extent of PMAA infiltration strongly depends upon the relative sizes of the nanopores and the PMAA concentration. Increasing the size of the nanopores enables the infiltration of PAA solution with a higher concentration. Electrochemical impedance spectroscopy demonstrated that the membrane conductivity decreases from 7.87 × 10−4 S cm−1 at pH 1 to 5.72 × 10−5 S cm−1 at pH 7. The functionalized AAO nanopores showed significant sensitivity to pH value, whereas a valve effect was observed in the pH range between 4 and 5. Our fabricated PMAA-AAO membranes show promising potential to be used as pH sensors and smart valves in micro-/nano-total analysis chips for biomedical and chemical applications.

Synthesis and characterization of “comb-like” poly(ionic liquid-co-styrene): expected applications in graphene dispersion and CO2 separation

Poly(ionic liquid)s (PILs) with well-defined architectures have shown significant potential in various fields, but establishing an easy method for their synthesis remains a challenge. Herein, comb-like copolymers with polystyrene and PIL as backbones and side chains, respectively, were prepared by directly polymerizing ionic liquid monomer by atom transfer radical polymerization using poly(styrene-co-4-vinylbenzyl chloride) as macroinitiator. NMR, GPC, and FTIR were used to characterize the molecular weights and compositions of macroinitiators and resultant copolymers. Results demonstrated successful synthesis, and also indicated that graft density and side chain length of the comb-like copolymers can be readily controlled by tuning macroinitiator composition and polymerization time. DSC and EIS measurements were further performed to investigate the thermal and electrochemical properties of comb-like copolymers with various architectures. Both glass transition temperature and impedance were found to highly depend on structure, particularly on the graft density of side chains. Furthermore, these comb-like copolymers were used both as stabilizer to disperse reduced graphene oxide (RGO) nanoplatelets in organic solvent and as an additive for N2/CO2 separation membranes. As compared to neat PIL, a highly stable suspension of RGO in propylene carbonate and a composite membrane with enhanced N2/CO2 selectivity were obtained in the presence of comb-like copolymers. Given the many advantages of diverse compositions, multiple groups, and tunable structure of PILs, the comb-like copolymers show great promising in stabilizing carbon materials and membrane separation.

Graphite oxide platelets functionalized by poly(ionic liquid) brushes and their chemical reduction

In this paper, graphite oxide (GO) platelets functionalized by poly(ionic liquid) brushes were prepared by surface-initiated atom transfer radical polymerization (SI-ATRP). The chemical reduction of these functionalized platelets was also investigated. The functionalized platelets and their reduced products were characterized and confirmed by Fourier transform infrared spectroscopy, thermogravimetric analysis, ζ potential measurements, four-probe electrical measurements, and high-resolution transmission electron microscopy. Results demonstrated that the poly(ionic liquid) brushes could be grafted from the GO surface by SI-ATRP. The surface charges of the GO platelets surface transformed from negative to positive. Upon reduction by hydrazine, the functionalized platelets were partially reduced, as suggested by the observation that reduced GO exhibits electrical conductivity three magnitudes higher than that of original GO. Although partially reduced GO platelets were not as conductive as reduced GO without functionalization, they can be homogenously dispersed in water due to the presence of poly(ionic liquids) brushes.

Improvement in comprehensive properties of poly(methyl methacrylate)-based gel polymer electrolyte by a core-shell poly(methyl methacrylate)-grafted ordered mesoporous silica

A novel strategy, herein, is demonstrated for improving comprehensive properties of poly (methyl methacrylate)(PMMA)-based gel polymer electrolyte (GPE) with a core-shell PMMA-grafted ordered mesoporous silica (OMS-g-PMMA). The OMS-g-PMMA was synthesized by surface-initiated atomtransfer radical polymerization of methyl methacrylate from the exterior surface of OMS particle. A series of PMMA-based GPE membrances, filled with the OMS-g-PMMA of different contents, were further prepared by solution casting technique. The OMS-g-PMMA was confirmed to possess regular core-shell structure, in which a PMMA shell is chemically grafted to the exterior surface of silica core remaining intact mesoporous characteristics. Compared to the bare OMS, the OMS-g-PMMA is found to more effectively improve the comprehensive properties of PMMA-based GPE including ionic conductivity, thermal stability, and mechanical properties as well. For the PMMA-based GPE filled with 15 phr OMS-g-PMMA, the ionic conductivity at 25°C reaches 1.59 × 10-4 S ċ cm-1, which is higher by nearly two orders than that of the corresponding filler-free parent GPE. Meanwhile, the tensile strength and Youngs modulus increase by 2.39 and 2.41 times, respectively, with an improvement in glass-transition temperature (Tg) about 10°C. The excellent comprehensive properties make the PMMA-based GPE filled with OMS-g-PMMA as potential candidate for electrochemical devices.

Decolorization of rhodamine B using hydrogen peroxide and H3PW12O40@C photocatalyst synthesized in situ under ultraviolet irradiation

AbstractThe H3PW12O40@C photocatalysts synthesized in situ from dodetungstophosphoric acid (TPA) and soluble starch were studied and characterized by FTIR, XRD, scanning electron microscope, and energy dispersive X-ray analysis. The degradation of rhodamine B (RhB) wastewater with H3PW12O40@C photocatalysts and hydrogen peroxide was investigated under ultraviolet irradiation. The effect of different factors and synergetic effect on the degradation of RhB were studied and the mechanism of catalytic oxidation of RhB was discussed. The results showed that the RhB was degraded efficiently. The decoloration efficiency of the RhB was up to 94.6% with initial pH value of 6 and RhB concentration of 100 mg/L under better reaction conditions (i.e. hydrogen peroxide concentration of 12 mM, PW30 (represented raw materials formula according to weight ratio of TPA to soluble starch is 30%) mass of 1.0 g/L, reaction temperature of 25°C, the high pressure mercury lamp power of 500 W, and the radiation time of 70 min). De...