Viet Hung Pham

Chemical functionalization of graphene sheets by solvothermal reduction of a graphene oxide suspension in N-methyl-2-pyrrolidone

We report a simple and effective method for reducing and functionalizing graphene oxide into chemically converted graphene by solvothermal reduction of a graphene oxide suspension in N-methyl-2-pyrrolidone (NMP). Graphene oxide sheets were functionalized by free radicals generated during heating of NMP in the presence of air. The degree of functionalization was easily controlled by manipulating the reduction time. High functionalized solvothermally reduced graphene oxide (STRG) shows superior dispersibility in various organic solvents, while slightly functionalized STRG shows excellent electrical conductivity. The superior dispersibility of highly functionalized STRG in organic solvents was attributed to the steric effect of functionalized groups on the surface of STRG sheets. Free-standing STRG paper that was reduced for 1 h exhibited electrical conductivity as high as 21600 S m−1, while the dispersibility of STRG that was reduced for 5 h was as high as 1.4 mg mL−1.

Highly Conductive Poly(methyl methacrylate) (PMMA)-Reduced Graphene Oxide Composite Prepared by Self-Assembly of PMMA Latex and Graphene Oxide through Electrostatic Interaction

We report a simple, environmentally friendly approach for preparing highly conductive poly(methyl methacrylate)-reduced graphene oxide (PMMA-RGO) composites by self-assembly of positively charged PMMA latex particles and negatively charged graphene oxide sheets through electrostatic interactions, followed by hydrazine reduction. The PMMA latex was prepared by surfactant-free emulsion polymerization using a cationic free radical initiator, which created the positive charges on the surface of the PMMA particle. By mixing PMMA latex with a graphene oxide dispersion, positively charged PMMA particles easily assembled with negatively charged graphene oxide sheets through electrostatic interaction. The obtained PMMA-RGO exhibited excellent electrical properties with a percolation threshold as low as 0.16 vol % and an electrical conductivity of 64 S/m at only 2.7 vol %. Moreover, the thermomechanical properties of PMMA-RGO were also significantly improved. The storage modulus of PMMA-RGO increased by about 30% at 4.0 wt % RGO at room temperature while the glass transition temperature of PMMA-RGO increased 15 °C at only 0.5 wt % RGO.

Chemical reduction of an aqueous suspension of graphene oxide by nascent hydrogen

One of the major challenges in the chemical reduction of graphene oxide is increasing the C/O atomic ratio of the chemically converted graphene. In this paper, we report a simple and effective method to reduce aqueous suspensions of graphene oxide using nascent hydrogen generated in situ by the reaction between Al foil and HCl, Al foil and NaOH and Zn powder and NaOH. The nascent hydrogen-reduced graphene oxides (nHRGOs) were characterized by elemental analysis, UV-vis spectra, Raman spectra, X-ray photoelectron spectroscopy, thermogravimetric analysis and electrical conductivity measurements. The reduction efficiency of graphene oxide strongly depended on the reaction medium and the rate of nascent hydrogen generation. The best nHRGO achieved a C/O atomic ratio greater than 21 and a bulk electrical conductivity as high as 12 500 S m−1, corresponding to the nascent hydrogen generated from the reaction between Al foil and HCl. Since nascent hydrogen could be produced on a metal surface upon oxidation in solution, other metals with low standard reduction potentials, such as Mg, Mn, and Fe, can be applied to reduce graphene oxide.

Superhydrophobic Silanized Melamine Sponges as High Efficiency Oil Absorbent Materials

Superhydrophobic sponges and sponge-like materials have attracted great attention recently as potential sorbent materials for oil spill cleanup due to their excellent sorption capacity and high selectivity. A major challenge to their broad use is the fabrication of superhydrophobic sponges with superior recyclability, good mechanical strength, low cost, and manufacture scalability. In this study, we demonstrate a facile, cost-effective, and scalable method to fabricate robust, superhydrophobic sponges through the silanization of commercial melamine sponges via a solution-immersion process. The silanization was achieved through secondary amine groups on the surface of the sponge skeletons with alkylsilane compounds, forming self-assembled monolayers on the surface of sponge skeletons. This resulted in our ability to tune the surface properties of the sponges from being hydrophilic to superhydrophobic with a water contact angle of 151.0°. The superhydrophobic silanized melamine sponge exhibited excellent sorption capacity for a wide range of organic solvents and oils, from 82 to 163 times its own weight, depending on the polarity and density of the employed organic solvents and oils, and high selectivity and outstanding recyclability with an absorption capacity retention greater than 90% after 1000 cycles. These findings offer an effective approach for oil spill containment and environmental remediation.

Superior conductive polystyrene – chemically converted graphene nanocomposite

The polystyrene–chemically converted graphene composite (PS-CCG) prepared by solution blending followed by compression molding, exhibited a percolation threshold as low as 0.19 vol.% and an electrical conductivity as high as 72.18 S m−1 at only ∼2.45 vol.%. The superior electrical conductivity of PS-CCG is the result of the combination of high electrical conductivity of CCG and the good dispersion of the nanofiller in PS matrix. The thermal properties of polystyrene were greatly improved upon addition of a small amount of CCG. The onset decomposition temperature of the PS-CCG increased by approximately 60 °C at 0.19 vol% of CCG loading. The mechanical properties of the PS-CCG were also affected by CCG loading. The storage modulus in the glassy region was enhanced by about 28% at 1.94 vol.% of CCG loading.

Superior dispersion of highly reduced graphene oxide in N,N-dimethylformamide.

Here, we report the effect of temperature on the extent of hydrazine reduction of graphene oxide in N,N-dimethylformamide (DMF)/water (80/20 v/v) and the dispersibility of the resultant graphene in DMF. The highly reduced graphene oxide (HRG) had a high C/O ratio and good dispersibility in DMF. The good dispersibility of HRGs is due to the solvation effect of DMF on graphene sheets during the hydrazine reduction, which diminishes the formation of irreversible graphene sheet aggregates. The dispersibility of the HRGs was varied from 1.66 to 0.38 mg/mL when the reduction temperature increased from 25 °C to 80 °C. The dispersibility of the HRGs was inversely proportional to the electrical conductivity of the HRGs, which varied from 17,400 to 25,500 S/m. The relationships between the C/O ratio, electrical conductivity, and dispersibility of the HRGs were determined and these properties were found to be easily controlled by manipulating the reduction temperature.

Graphene network on indium tin oxide nanodot nodes for transparent and current spreading electrode in InGaN/GaN light emitting diode

We report a device that combines indium tin oxide (ITO) nanodot nodes with two-dimensional chemically converted graphene (CCG) films to yield a GaN-based light emitting diode (LED) with interesting characteristics for transparent and current spreading electrodes for the potential use in the ultraviolet region. The current-voltage characteristics and electroluminescence output power performance showed that CCG network on ITO nanodot nodes operated as a transparent and current spreading electrode in LED devices.

Temperature-dependent photoluminescence from chemically and thermally reduced graphene oxide

Temperature-dependent photoluminescence (PL) of graphene oxide (GO) reduced with hydrazine and heat has been measured to investigate the effect of reduction type on the bandgap of the reduced GO. Nitrogen functionalities formed in the hydrazine-treated GO were responsible for a strong localization of carriers that caused in a fluctuation in PL peak position with temperature. The intensity of C-OH peak was relatively low in the heat-treated GO, indicating that raising temperature facilitated the removal of hydroxyl groups, resulting in larger sp2 domain size and smaller bandgap energy.

Dispersibility of reduced alkylamine-functionalized graphene oxides in organic solvents

The alkylamine functionalization of graphene oxide is well known as an efficient approach to prepare reduced functionalized graphene oxide (RFGO) that is highly dispersible in organic solvents. Herein, we systematically investigated the effects of long-chain alkylamine functionalization of graphene oxide on the organic solvent dispersibility and electrical conductivity of RFGO. Three kinds of alkylamines, octylamine, dodecylamine and hexadecylamine, were chosen as functionalization agents. The alkylamine functionalization of graphene oxide was characterized by X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, thermogravimetric analysis and X-ray diffraction. RFGO using octylamine exhibited the best electrical conductivity of greater than 180 S/m. All of the RFGOs had excellent dispersibility, up to 3.0 mg/mL, in organic solvents, with Hansen solubility parameters in the range of 6.3<(δ(p)+δ(h))<13.7.

A catalytic and efficient route for reduction of graphene oxide by hydrogen spillover

In this paper, an efficient catalytic route for the reduction of graphene oxide (GO) is described using hydrogen spillovered platinum (Pt) nanoparticles at room temperature. Pt nanoparticles were decorated on GO sheets using a hydrogen reduction of chloroplatinic acid. The Pt nanoparticles served as a catalyst for the process of hydrogen disassociation to atomic hydrogen, which spillovered onto the GO sheets which acted as a strong reducing agent for the reduction of GO. Reduced graphene oxide (RGO) obtained in this manner has a C/O atomic ratio as high as 22, which is one of the highest values ever reported for chemically converted graphenes. The electrical conductivity was greater than 8000 S m−1. Electrochemical studies revealed that the RGO–Pt hybrid exhibits excellent electrocatalytic activity toward the methanol oxidation reaction, with high CO tolerance and long-term stability.