Water-tolerant graphene oxide as a high-efficiency catalyst for the synthesis of propylene carbonate from propylene oxide and carbon dioxide

Graphene oxide (GO) was found to be a metal-free, water-tolerant and high-efficiency catalyst towards the cycloaddition of carbon dioxide (CO₂) to propylene oxide (PO) for the synthesis of propylene carbonate (PC) at room temperature (RT) and atmospheric pressure without the need for a solvent. Using GO as catalyst and tetrabutylammonium bromide (Bu₄NBr) as co-catalyst, PO is rapidly converted to PC with 96% yield and 100% selectivity under relatively mild conditions (100 °C, 2.25 MPa, 1 h). The effects of catalyst amount, temperature, time and water (H₂O) addition on the reaction were investigated. It is found that the presence of a proper amount of H₂O enhances the conversion of epoxide remarkably. A comparison of the catalytic activities of a number of reduced graphene oxide (r-GO) samples under similar reaction conditions revealed that it is the hydroxyl groups (rather than the carboxyl groups) on GO that form hydrogen bonds with PO, and act synergistically with halide anions to promote the cycloaddition reaction. A possible mechanism is proposed.     

Simultaneous reduction,exfoliation, and nitrogen doping of graphene oxide via a hydrothermal reaction for energy storage electrode materials

Nitrogen (N)-doped graphene (NG) sheets were prepared using (NH₄)₂CO₃ and an aqueous dispersion of graphene oxide (GO) by an eco-friendly hydrothermal reaction. The in situ produced ammonia played an important role in the simultaneous nitrogen doping, the reduction and exfoliation of GO. The (NH₄)₂CO₃/GO mass ratio and reaction temperature were varied to investigate the effects on the N doping level. The elemental analysis determined from the X-ray photoelectron spectroscopy showed that the nitrogen content of the NG was about 10.1 at.% and the oxygen content decreased significantly due to the hydrothermal reduction of GO. The electrochemical performances of the NG sheets increased with increasing doped N content. The highest specific capacitance of 295 F g⁻¹ at a current density of 5 A g⁻¹ and the highest specific surface area of 412 m² g⁻¹ were observed with the sample processed at 130 °C. The retention of the specific capacitance was maintained at ∼89.8% after 5000 charge–discharge cycles. These results imply that NG sheets obtained by this simple eco-friendly approach are suitable for use in high performance energy storage electrode materials.     

The size and dispersion effect of modified graphene oxide sheets on the photocatalytic H2 generation activity of TiO2 nanorods

Nano graphene oxide (NGO) was produced by further refluxing graphene oxide (GO) sheets in HNO₃, and carboxylic acid functionalized graphene oxide (GO–COOH) was obtained by a simple etherification reaction between GO and chloroacetic acid. The GO, GO–COOH and NGO sheets are combined with TiO₂ nanorods by a two-phase assembling method, and confirmed by transmission electronic microscopy. The GO–TiO₂, GO–COOH–TiO₂ and NGO–TiO₂ composites are used in a comparative study of photocatalytic H₂ generation activity under UV light irradiation. The H₂ generation rate of TiO₂ nanorods was slightly increased from 15 to 30 mL h⁻¹ g⁻¹ by replacing oleic acid ligands with hydrophilic dopamine, and significantly increased to 105 mL h⁻¹ g⁻¹ after combining with GO sheets. The further comparative study shows that GO–COOH–TiO₂ composite has higher H₂ generation rate of 180 mL h⁻¹ g⁻¹ than that of GO–TiO₂ and NGO–TiO₂ composites.     

An efficient method of producing stable graphene suspensions with less toxicity using dimethyl ketoxime

A highly stable graphene suspension has been prepared using dimethyl ketoxime (DMKO) as reductant. Nitrogen was doped into the graphene plane at the same time as the graphene oxide (GO) sheets were reduced. X-ray photoelectron spectroscopy indicated that the C/O ratio of graphene was significantly increased after GO was treated with DMKO and the quantity of nitrogen incorporated into the graphene lattice was 3.67 at.%. The electrical conductivity of the graphene paper was found to be ～10² S m^?1, which was 5 orders of magnitude better than that of GO, and this demonstrated the effective chemical reduction of GO. The mechanism of the chemical reaction of GO with DMKO was also discussed. The as-produced graphene material showed good capacitive behavior and long cycle life with a specific capacitance of ～140 F g^?1.     

Highly thermal conductive composites with polyamide-6 covalently-grafted graphene by an in situ polymerization and thermal reduction process

The thermal conductive polyamide-6/graphene (PG) composite is synthesized by in situ ring-opening polymerization reaction using ε-caprolactam as the monomer, 6-aminocaproic acid as the initiator and reduced graphene oxide (RGO) as the thermal conductive filler. The generated polyamide-6 (PA6) chains are covalently grafted onto graphene oxide (GO) sheets through the “grafting to” strategy with the simultaneous thermal reduction reaction from GO to RGO. The homogeneous dispersion of RGO sheets in PG composite favors the formation of the consecutive thermal conductive paths or networks at a relatively low GO sheets loading, which improves the thermal conductivity (λ) from 0.196 W m⁻¹ K⁻¹ of neat PA6 to 0.416 W m⁻¹ K⁻¹ of PG composite with only 10 wt% GO sheets loading.     

Hollow graphene spheres self-assembled from graphene oxide sheets by a one-step hydrothermal process

We developed a one-step hydrothermal method to assemble graphene oxide (GO) sheets into hollow graphene spheres (HGSs), using only a GO/H₂SO₄ aqueous suspension as the starting material. Scanning electron microscope, focused ion beam scanning electron microscope and transmission electron microscope images show that the as-prepared HGSs vary from 1 to 3 μm in diameter and have a hollow interior structure. The as-prepared HGSs show a high capacitance of 207 F g⁻¹, as well as good rate capability and cycling stability when used as electrode materials for supercapacitors.     

Tubular graphene nanoribbons with attached manganese oxide nanoparticles for use as electrodes in high-performance supercapacitors

Graphene nanoribbons (GNRs) with tubular shaped thin graphene layers were prepared by partially longitudinal unzipping of vapor-grown carbon nanofibers (VGCFs) using a simple solution-based oxidative process. The GNR sample has a similar layered structure to graphene oxide (GO), which could be readily dispersed in isopropyl alcohol to facilitate electrophoretic deposition (EPD). GO could be converted to graphene after heat treatment at 300 °C. The multilayer GNR electrode pillared with open-ended graphene tubes showed a higher capacitance than graphene flake and pristine VGCF electrodes, primarily due to the significantly increased surface area accessible to electrolyte ions. A GNR electrode with attached MnO₂ nanoparticles was prepared by EPD method in the presence of hydrated manganese nitrate. The specific capacitance of GNR electrode with attached MnO₂ could reach 266 F g⁻¹, much higher than that of GNR electrode (88 F g⁻¹) at a discharge current of 1 A g⁻¹. The hydrophilic MnO₂ nanoparticles attached to GNRs could act as a redox center and nanospacer to allow the storage of extra capacitance.     

Isothermal exfoliation of graphene oxide by a new carbon dioxide pressure swing method

Graphene oxide (GO) was prepared by a modified Hummers’ method. GO was modified using a simple CO₂ pressure swing technique to obtain exfoliated GO. The microcrystalline structures and morphologies were characterized using the X-ray diffraction and scanning electron microscope/transmission electron microscope measurements. The textural properties were investigated by N₂ (77 K) adsorption/desorption isotherms. CO₂ (298 K, 30 bar) and H₂ (298 K, 100 bar) adsorption experiments were conducted to investigate their adsorption behaviors. The results indicated that the best sample had a specific surface area of 547 m²/g and total pore volume of 2.468 cm³/g. According to the results, CO₂ pressure swing method can be used to increase the efficiency of exfoliation and expansion of the graphitic interlayers in GO.     

Electrochemically synthesized graphene/polypyrrole composites and their use in supercapacitor

Composite films consisting of polypyrrole (PPy) and graphene oxide (GO) were electrochemically synthesized by electrooxidation of 0.1 M pyrrole in aqueous solution containing appropriate amounts of GO. Simultaneous chronoamperometric growth profiles and frequency changes on a quartz crystal microbalance showed that the anionic GO was incorporated in the growing GO/PPy composite to maintain its electrical neutrality. Subsequently, the GO was reduced electrochemically to form a reduced GO/PPy (RGO/PPy) composite by cyclic voltammetry. Specific capacitances estimated from galvanostatic discharge curves in 1 M H₂SO₄ at a current density of 1 A g^?1 indicated that values for the RGO/PPy composite were larger than those of a pristine PPy film and the GO/PPy composite. In the case of 6 mg mL^?1 GO for the preparation of GO/PPy, a high specific capacitance of 424 F g^?1 obtained at the electrochemically prepared RGO/PPy composite indicated its potential for use as an electrode material for supercapacitors.     

Functionalized three-dimensional graphene networks for high performance supercapacitors

Three-dimensional (3D) thermal reduced graphene network (TRGN) deposition on Ni foam without any conductive agents and polymer binders was successfully synthesized by dipping Ni foam into graphene oxide (GO) suspension and subsequent thermal reduction process. The direct and close contact between thermal reduced graphene and Ni foam is beneficial to the enhanced conductivity of the electrode, as well as the improvement of ion diffusion/transport into the electrode. Additionally, low-temperature reduction of GO possesses a large amount of stable oxygen-containing groups that can provide high pseudocapacitance. As a result, the TRGN electrode delivers a high specific capacitance of 442.8 F g⁻¹ at 2 mV s⁻¹ in 6 mol L⁻¹ KOH. Moreover, symmetric supercapacitor based on TRGN exhibits a maximum energy density of 30.4 Wh kg⁻¹ based on the total mass of the two electrodes in 1 mol L⁻¹ Na₂SO₄ electrolyte, as well as excellent cycling stability with 118% of its initial capacitance after 5000 cycles.     

Non-metal catalytic synthesis of graphene from a polythiophene monolayer on silicon dioxide

Direct synthesis of graphene without metal catalysts on a dielectric substrate is a major goal in graphene-based electronics and is an increasingly popular nanotechnology alternative to metal oxide semiconductor technology. However, current methods for the synthesis of these graphenes have many limitations, including the use of metal catalyst. Herein, we report a facile approach to the direct synthesis of graphene sheets based on the self-assembled monolayers (SAMs) technique. The new method for metal catalyst-free direct synthesis of a graphene sheet is through a solution-processable, inexpensive, easy, and reproducible cross-linked polythiophene self-assembled monolayer (SAM) that is formed via the [4 + 2] π cycloaddition reaction of π-electron conjugated thiophene layer self-assembled on the dielectric silicon dioxide substrate. The bifunctional molecules were carefully designed to create an SAM via silanization of alkoxy silane groups on the SiO₂ substrate, and at the other end, a thin cross-linked polythiophene layer via a [4 + 2] π-electron cycloaddition reaction of π-electron conjugated thiophene SAM. By heating the cross-linked polythiophene SAM up to 1000 °C under a high vacuum, single-layered or few-layered graphene sheets were successfully prepared on the dielectric silicon oxide substrate.     

Structural,magnetic, and textural properties of iron oxide-reduced graphene oxide hybrids and their use for the electrochemical detection of chromium

Superparamagnetic Fe₃O₄ nanoparticles were anchored on reduced graphene oxide (RGO) nanosheets by co-precipitation of iron salts in the presence of different amounts of graphene oxide (GO). A pH dependent zeta potential and good aqueous dispersions were observed for the three hybrids of Fe₃O₄ and RGO. The structure, morphology and microstructure of the hybrids were examined by X-ray diffraction, transmission electron microscopy (TEM), Fourier transform infrared spectroscopy, Raman and X-ray photoelectron spectroscopy. TEM images reveal lattice fringes (d₃₁₁ = 0.26 nm) of Fe₃O₄ nanoparticles with clear stacked layers of RGO nanosheets. The textural properties including the pore size distribution and loading of Fe₃O₄ nanoparticles to form Fe₃O₄–RGO hybrids have been controlled by changing the concentration of GO. An observed maximum (～10 nm) in pore size distribution for the sample with 0.25 mg ml^?1 of GO is different from that prepared using 1.0 mg ml^?1 GO. The superparamagnetic behavior is also lost in the latter and it exhibits a ferrimagnetic nature. The electrochemical behavior of the hybrids towards chromium ion was assessed and a novel electrode system using cyclic voltammetry for the preparation of an electrochemical sensor platform is proposed. The textural properties seem to influence the electrochemical and magnetic behavior of the hybrids.     

Sonication-free dispersion of large-area graphene oxide sheets using internal pressure from release of intercalated carbon dioxide

A new concept to disperse graphene sheets without sonication, significant size reduction, and any further removal of intercalating materials, is demonstrated using ejection pressure of CO₂ gas captured by water molecules dissolved in the interlayer of graphene and the ice structure surrounding it. After graphene oxide (GO) layers are swelled with water molecules, CO₂ gas is dissolved in the swelled interlayer of GO. The CO₂ gas within the GO layers is well maintained by the transformation of excess water molecules outside of interlayer to solid ice at low temperature (around −30 °C). The resulting CO₂-incorporating GOs are well dispersed in various solvents by the ejection pressure of stored CO₂ without sonication, and without additional treatment for removal of the intercalating material as surrounding ices spontaneously melt. We show that the lateral length of these well-dispersed GO sheets is 10 times larger than that of GO sheets obtained by the conventional sonication method.     

Microwave-assisted,rapid cycloaddition of allyl glycidyl ether and CO2 by employing pyridinium-based ionic liquid catalysts

This study investigated the use of pyridinium-based ionic liquids (ILs) as an efficient catalyst for the rapid solvent-free microwave-assisted cycloaddition of allyl glycidyl ether (AGE) and CO₂ to yield allyl glycidyl carbonate (AGC) under moderate reaction conditions. The cycloaddition reaction occurred over a short reaction time of 30 s, resulting in a high turnover frequency (TOF) ranging from 200 to 7000 h^− 1. The effects of alkyl chain length and anion of pyridinium-based catalysts on the cycloaddition reactivity were studied. The effects of reaction parameters such as the amount of catalyst, microwave power, CO₂ pressure, and reaction time were also investigated.     

Low-cost preparation of a conductive and catalytic graphene film from chemical reduction with AlI3

AlI₃ synthesized by I₂ and Al in ethanol was used as reductive agent to directly obtain flexible reductive graphene oxide (RGO) films with high conductivity of 5320 S/m from graphene oxide (GO) films at a low temperature of 80 °C. This reductive method has provided a low-cost and effective route for large-scale production of graphene with high catalytic activity. Structural evolution during the reduction of GO was studied by Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy. The RGO films served as counter electrode exhibited high electrochemical activity.     

High peroxidase catalytic activity of exfoliated few-layer graphene

Few-layer graphene prepared from graphite exfoliated by chitosan has a preserved structure without oxidation or destruction of the sp² character of the carbon plane and exhibits a higher peroxidase catalytic activity than that of graphene oxide (GO) and its reduced form. The peroxidase catalytic activity of as-obtained few-layer graphene is 45 times higher than that of GO and 4 times higher than that of reduced GO with the same concentration of 30 μg mL⁻¹ and the detection limit of hydrogen peroxide is 10 nM. The excellently catalytic performance can be attributed to the fast electron transfer on the surface of few-layer graphene, which is further confirmed by electrochemical characterization. The as-prepared few-layer graphene has been used to determine hydrogen peroxide in three real water samples with satisfactory results.     

Ag nanoparticles anchored NiO/GO composites for enhanced capacitive performance

Hierarchical nickel oxide/graphene oxide (NiO/GO) and nickel oxide/graphene oxide/silver (NiO/GO/Ag) heterostructures were sucessfully fabricated as high-performance supercapacitors electrode materials by using a hydrothermal process and a photoreduction process. The experimental results showed that the NiO/GO/Ag heterostructure electrodes showed better electrochemical performance than those of NiO/GO and bare NiO nanosheets. The NiO/GO/Ag electrode exhibited a higher specific capacitance of 229 F g⁻¹ at a current density of 1 A g⁻¹, higher than that of 161 F g⁻¹ for NiO/GO composites. Furthermore, NiO/GO/Ag electrode also showed good rate capability (still 200 F g⁻¹ at 6 A g⁻¹) and cycling stability (24% loss after 2000 repetitive cycles at a scan rate of 20 mV s⁻¹). The enhanced capacitive performance of the NiO/GO/Ag composites was mainly attributed to the introduction of Ag nanoparticles, which increased the electrical conductivities of the composites, and promoted the electron transfer between the active components. This study suggested that NiO/GO/Ag composites were a promising class of electrode materials for high performance energy storage applications.     

Synthesis of bimetallic PtPd nanocubes on graphene with N,N-dimethylformamide and their direct use for methanol electrocatalytic oxidation

Bimetallic PtPd nanocubes supported on graphene nanosheets (PtPdNCs/GNs) were prepared by a rapid, one-pot and surfactant-free method, in which N,N-dimethylformamide (DMF) was used as a bi-functional solvent for the reduction of both metal precursors and graphene oxide (GO) and for the surface confining growth of PtPdNCs. The morphology, structure and composition of the thus-prepared PtPdNCs/GNs were characterized by transmission electron microscopy (TEM), high resolution TEM, energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy. Because no surfactant or halide ions were involved in the proposed synthesis, the prepared PtPdNCs/GNs were directly modified onto a glassy carbon electrode and showed high electrocatalytic activity for methanol oxidation in cyclic voltammetry without any pretreatments. Moreover, with the synergetic effects of Pt and Pd and the enhanced electron transfer by graphene, the PtPdNCs/GNs composites exhibited higher electrocatalytic activity (j_p = 0.48 A mg⁻¹) and better tolerance to carbon monoxide poisoning (I_f/I_b = 1.27) compared with PtPd nanoparticles supported on carbon black (PtPdNPs/C) (j_p = 0.28 A mg⁻¹; I_f/I_b = 1.01) and PtNPs/GNs (j_p = 0.33 A mg⁻¹; I_f/I_b = 0.95). This approach demonstrates that the use of DMF as a solvent with heating is really useful for reducing GO and metal precursors concurrently for preparing clean metal–graphene composites.     

The production of organogels using graphene oxide as the gelator for use in high-performance quasi-solid state dye-sensitized solar cells

We report the formation of organogels of 3-methoxypropionitrile (MPN) using graphene oxide (GO) as the gelator and the use of these gels as the quasi-solid state electrolyte of dye-sensitized solar cells (DSCs). GO–MPN gels are prepared by mechanically grinding or ultrasonicating GO in MPN. The GO sheets form 3-dimensional solid networks in the gels, which hold the MPN solvent. The GO loading can be as low as 2.5 wt.% for the gel formation. Gel electrolytes were prepared by dispersing I₂, 1-methyl-3-propylimidazolium iodide, guanidine thiocynate and 4-tert butyl pyridine into GO–MPN gels, and these were used for DSCs. The GO sheets can be fragmented into small pieces by ultrasonication, and smaller GO sheets can lead to a higher diffusion constant of the triiodide and a higher photovoltaic efficiency for the DSCs. DSCs with a GO–MPN gel electrolyte exhibit a photovoltaic efficiency of 6.70% under AM 1.5 G illumination (100 mW cm⁻²), quite close to that (7.18%) of the control liquid DSCs.     

In situ synthesis of ultrathin 2-D TiO2 with high energy facets on graphene oxide for enhancing photocatalytic activity

Two kinds of TiO₂ with novel structures, interpenetrating anatase TiO₂ tablets (IP-TiO₂), and overlapping anatase TiO₂ nanosheets (OL-TiO₂) with exposed {0 0 1} facets, are synthesized. The graphene oxide (GO) supported ultrathin TiO₂ nanosheets (OL-TiO₂/GO) is also prepared by one-pot hydrothermal method. The microscopic feature, morphology, phase, and nitrogen adsorption–desorption isotherms are characterized. The performance of photocatalytic degradation of methyl blue is also measured. Compared with IP-TiO₂, the OL-TiO₂ with GO possess higher photocatalytic efficiency. The GO can improve the photocatalytic property by increasing specific surface area, accelerating the separation of electron–hole pairs, as well as extending the electron life. The growth process of TiO₂ nanosheets on graphene oxide layers probably follows a step-growth mechanism with F⁻ as morphology controlling agent. The steps on the surface can improve the photocatalytic activity further due to the increase of dangling bonds of 5-coordinated Ti (Ti_5c) which are considered to be the active sites in the photocatalytic reaction.