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.     

Green reduction of graphene oxide by aqueous phytoextracts

The reduction of graphene oxide (GO) by phytochemicals was investigated using aqueous leaf extracts of Colocasia esculenta and Mesua ferrea Linn. and an aqueous peel extract of orange (Citrus sinensis). The prepared GO and phytoextract reduced GO (RGO) were characterized by ultraviolet–visible spectroscopy, Raman spectroscopy and Fourier transform infrared analyses to provide a clear indication of the removal of oxygen-containing groups from the graphene and the formation of RGO. The extent of reduction was determined from elemental analysis. Formation of few layers of graphene was indicated by transmission electron microscopy. The obtained RGO exhibited good specific capacitance (17–21 Fg^?1), high electrical conductivity (3032.6–4006 Sm^?1) and high carbon to oxygen ratio (5.97–7.11).     

Excellent electrocatalytic performance of Pt nanoparticles on reduced graphene oxide nanosheets prepared by a direct redox reaction between Na2PtCl4 and graphene oxide

A simple and environment-friendly method was used to prepare Pt/reduced graphene oxide (Pt/RGO) hybrids. This approach used a redox reaction between Na₂PtCl₄ and graphene oxide (GO) nanosheets and a subsequent thermal reduction of the material at 200 °C for 24 h in a vacuum oven. In contrast to other methods that use an additional reductant to prepare Pt nanoparticles, the Pt²⁺ was directly reduced to Pt⁰ in the GO solution. GO was used as the reducing agent, the stabilizing agent and the carrier. The resulting Pt/RGO hybrid was characterized by X-ray diffraction, thermo-gravimetric analysis, X-ray photoelectron spectroscopy, transmission electron microscopy and energy-dispersive X-ray spectroscopy. Electrochemical measurements showed that the Pt/RGO hybrids exhibit good activity as catalysts for the electro-oxidation of methanol and ethanol in acid media. Interestingly, the Pt/RGO hybrids showed better electrocatalytic activity and stability for the oxidation of methanol than Pt/C and Pt/RGO hybrids made from other Pt precursors. This indicates that the Pt/RGO hybrids should have great potential applications in direct methanol and ethanol fuel cells.     

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.     

Dielectric and electromagnetic wave absorption properties of reduced graphene oxide/barium aluminosilicate glass–ceramic composites

BaAl₂Si₂O₈ (BAS) glass–ceramic powders were prepared by sol–gel method. Graphene oxide (GO)/BAS mixture powders were prepared by a simple mixing process of GO and BAS. Dense and uniform reduced graphene oxide (RGO)/BAS composites were fabricated by the hot-pressing of GO/BAS, which was accompanied by the in-situ thermal reduction of GO. Microstructure, phase composition, dielectric and electromagnetic wave (EM) absorption properties of RGO/BAS were investigated. The results reveal that RGO can promote the hexacelsian-to-celsian phase transformation of BAS. In the frequency range from 8 GHz to 12 GHz, the complex permittivity of RGO/BAS increases with increasing RGO content. The composite with 1.5 wt% of RGO shows good EM absorbing ability. When the sample thickness is 2.1 mm, the minimum reflection coefficient (RC) reaches −33 dB, and the effective absorption bandwidth is more than 3.1 GHz.     

Graphene oxide hole injection layer for high-efficiency polymer light-emitting diodes by using electrophoretic deposition and electrical reduction

Graphene oxide (GO) is a fascinating nanomaterial with tremendous potential for electronic devices which show high performance when the properties of GO are properly tuned like reduced graphene oxide (RGO). Here, we report a simple, cost effective and precisely controllable methods to fabricate high-quality RGO films as a hole injection layer (HIL) for high efficiency polymer light-emitting diodes (PLEDs). First, GO is electrophoretically deposited and electrically reduced to produce RGO films. The deposition and reduction of films are electrical methods; hence the thickness and the degree of reduction for GO films can be easily manipulated by controlling electrical parameters and time. The performance of PLEDs with RGO film as HIL is measured and compared with GO and (poly(3,4-ethylenedioxythiophene)polystyrene sulfonate) (AI4083), and shows better results for luminance and current density. The optimized maximum luminance and current efficiency of the PLEDs with RGO as HIL are found to be 12830 cd/m² and 3.35 cd/A, respectively.     

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.     

Large scale production of highly conductive reduced graphene oxide sheets by a solvent-free low temperature reduction

A novel one-pot process that can produce freestanding reduced graphene oxide (RGO) sheets in large scale through a mechanochemical method is presented, which is based on a 1:1 adduct of hydrazine and carbon dioxide (H₃N⁺NHCO₂⁻, solid hydrazine). We were able to synthesize RGO sheets by grinding solid hydrazine with graphene oxide (GO), followed by storing the mixed powder at 50 °C for 10 min. No solvents, nor large vessels, nor post-annealing at high temperatures are required. The resulting RGO sample was characterized by elemental analysis, X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, Brunauer–Emmett–Teller measurement, thermo gravimetric analysis, Fourier transform infrared spectroscopy, solid state nuclear magnetic resonance spectroscopy, and conductivity measurement. It exhibits excellent conductivity and possesses a high specific surface area. This reduction method was successfully applied for the fabrication of inkjet-printed RGO devices on a flexible substrate.     

Synthesis of reduced graphene oxide–Fe3O4 multifunctional freestanding membranes and their temperature dependent electronic transport properties

A simple and scalable method for the synthesis of reduced graphene oxide (RGO) based conductive and magnetic multifunctional films (membranes) is reported. A RGO–iron oxide (Fe₃O₄) freestanding film is fabricated using a versatile chemical route followed by vacuum infiltration. Temperature dependent electronic transport properties of the magnetic GO and RGO films were measured using a four probe technique from room temperature to 15 K. A conduction mechanism based on variable range hopping is suggested for explaining of the electronic conductivity variations. Possible applications of this multifunctional membrane are also discussed.     

Highly efficient synthesis of graphene/MnO2 hybrids and their application for ultrafast oxidative decomposition of methylene blue

A highly efficient method has been reported to fabricate the reduced graphene oxide/MnO₂ (RGO/MnO₂) hybrid materials, a kind of catalysts for oxidative decomposition of methylene blue (MB). The pristine suspension of graphene oxide/manganese sulfate (GO/MnSO₄) produced by the modified Hummers method is in situ transformed into GO/MnO₂ composites in combination with KMnO₄, and then further into RGO/MnO₂ composites by means of glucose-reduction. It is found that MnO₂ nanoparticles with the size of 20–30 nm are uniformly distributed in the structure of RGO. A series of composites with different mass ratios of RGO to MnO₂ has been proved superior catalytic activities, much higher than that of the bare MnO₂ for decomposition of MB dye in the presence of H₂O₂. Typically, 50 mL of MB (50 mg L⁻¹) can be completely decolorized and nearly 66% mineralized at 50 °C in 5 min with 10 mg of the RGO/MnO₂ hybrid. According to the adsorption–oxidation–desorption mechanism, the high activity of RGO/MnO₂ composites for decomposition of MB is closely related to the positive synergistic effect of RGO and MnO₂ with the assistance of H₂O₂.     

One-step electrodeposition synthesis of silver-nanoparticle-decorated graphene on indium-tin-oxide for enzymeless hydrogen peroxide detection

Silver-nanoparticles-decorated reduced graphene oxide (rGO) was electrodeposited on indium tin oxide (ITO) by a cyclic voltammetry method. The results of X-ray diffraction, Fourier-transform infrared transmission spectroscopy and Raman spectroscopy confirmed the simultaneous formation of cubic phase silver nanoparticles and reduction of GO through the electrodeposition process. Field emission scanning electron microscope images showed a uniform distribution of nanometer-sized silver nanoparticles with a narrow size distribution on the RGO sheets, which could only be achieved using silver ammonia complex instead of silver nitrate as precursor. The composite deposited on ITO exhibited notable electrocatalytic activity for the reduction of H₂O₂, leading to an enzymeless electrochemical sensor with a fast amperometric response time less than 2 s. The corresponding calibration curve of the current response showed a linear detection range of 0.1–100 mM (R² = 0.9992) while the limit of detection was estimated to be 5 μM.     

Hydrogen reduced graphene oxide/metal grid hybrid film: towards high performance transparent conductive electrode for flexible electrochromic devices

The metal grid and reduced graphene oxide (RGO) are both promising transparent conductive materials for replacing the indium tin oxide (ITO) in flexible optoelectronics. However, the large empty area that exists in the grid together with the relatively high sheet resistance of RGO hinder both the materials for practical applications. In this work, we report for the first time a novel strategy for efficient combination of the metal grid and RGO by using a newly developed room-temperature reduction technique. The obtained RGO/metal grid hybrid films not only overcome the shortcomings of individual components but exhibit enhanced optical and electrical performances (R_s = 18 Ω sq⁻¹ and T = 80%) and excellent flexural endurance. With this hybrid film as the window electrode, a highly flexible electrochromic device with excellent stability and ultra-fast response shorter than 60 ms has been successfully fabricated. Considering its high efficiency, high quality, low cost and large area, the strategy would be particularly useful for economically fabricating various metal grid/RGO films which are quite promising high performance transparent and conductive materials for next generation optoelectronic devices.     

A green and facile one-pot synthesis of Ag–ZnO/RGO nanocomposite with effective photocatalytic activity for removal of organic pollutants

In this study, Ag–ZnO/reduced graphene oxide (Ag–ZnO/RGO) composite was synthesized by a green and facile one-step hydrothermal process. Aqueous suspension containing Ag and ZnO precursors with graphene oxide (GO) sheets was heated at 140 °C for 2 h. The morphology and structure of as-synthesized particles were characterized by field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, and Photoluminescence (PL) spectroscopy which revealed the formation of composite of metal, metal oxide and RGO. It was observed that the presence of Ag precursor and GO sheets in the hydrothermal solution could sufficiently decrease the size of ZnO flowers. The hybrid nanostructure, with unique morphology, obtained from this convenient method (low temperature, less time, and less number of reagents) was found to have good photocatalytic and antibacterial activity. The perfect recovery of catalyst after reaction and its unchanged efficiency for cyclic use showed that it will be an economically and environmentally friendly photocatalyst.     

Hydrogen-assisted pulsed KrF-laser irradiation for the in situ photoreduction of graphene oxide films

We report on the use of pulsed KrF-laser irradiation for the in situ reduction of graphene oxide (GO) films under both vacuum and partial hydrogen pressure. By exposing GO films to 500 pulses of a KrF-laser, at a fluence of 10 mJ/cm², their sheet resistance (R_s) is dramatically reduced from highly insulating (∼10¹⁰ Ω/sq) to conductive values of ∼3 kΩ/sq. By increasing the laser fluence, from 10 to 75 mJ/cm², we were able to identify an optimal fluence around 35 mJ/cm² that leads to highly conductive films with R_s values as low as 250 Ω/sq and 190 Ω/sq, under vacuum (10⁻⁵ Torr) and 50 mTorr of H₂, respectively. Raman spectroscopy analyses confirmed the effective reduction of the KrF-laser irradiated GO films through the progressive recovery of the characteristic 2D band of graphene. Furthermore, systematic Fourier-transform infrared spectroscopy analysis has revealed that KrF-laser induced reduction of GO preferentially occurs through photodissociation and removal of carboxyl (COOH) and alcohol (OH) groups. A direct correlation is established between the electrical resistance of photoreduced GO films and their COOH and OH bond densities. The KrF-laser induced reduction of GO films is found to be more efficient under H₂ background than under vacuum. It is concluded that our KrF-laser reduced GO films mainly consist of turbostratic graphite built from randomly organized few-layers-graphene building blocks, which contains some residual oxygen atoms and defects. Finally, by monitoring the KrF-laser fluence, it is shown that reduced GO films combining optical transmission as high as ∼80% along with sheet resistance as low as ∼500 Ω/sq can be achieved with this room-temperature and on-substrate process. This makes the laser-based reduction process developed here particularly attractive for photovoltaic hybrid devices using silicon substrates.     

Reduction of graphene oxide at the interface between a Ni layer and a SiO2 substrate

Reduction of graphene oxide (GO) was carried out on SiO₂ using a thin Ni overlayer as a catalyst. A Ni/GO/SiO₂ structure was heated at 800 °C in high vacuum for 6 min. After removing the Ni overlayer, formation of graphene was confirmed by Raman spectroscopy. For the Ni overlayer thinner than 40 nm, GO was reduced to graphene on-site. For the thicker Ni overlayer, however, GO was completely decomposed and graphene was formed in a segregation and/or precipitation process. The use of GO with a thin Ni overlayer enabled on-site and transfer-free fabrication of graphene without use of such flammable gases as methane and hydrogen.     

Reduced graphene oxide anchored with δ-MnO2 nanoscrolls as anode materials for enhanced Li-ion storage

We developed a one-pot in situ synthesis procedure to form nanocomposite of reduced graphene oxide (RGO) sheets anchored with 1D δ-MnO₂ nanoscrolls for Li-ion batteries. The as-prepared products were characterized by X-ray diffraction (XRD), Raman spectra, X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscope (FESEM) and transmission electron microscope (TEM). The electrochemical performance of the δ-MnO₂ nanoscrolls/RGO composite was measured by galvanostatic charge/discharge cycling and electrochemical impedance spectroscopy. The results show that the δ-MnO₂ nanoscrolls/RGO composite displays superior Li-ion battery performance with large reversible capacity and high rate capability. The first discharge and charge capacities are 1520 and 810 mAh g⁻¹, respectively. After 50 cycles, the reversible discharge capacity is still maintained at 528 mAh g⁻¹ at the current density of 100 mAh g⁻¹. The excellent electrochemical performance is attributed to the unique nanostructure of the δ-MnO₂ nanoscrolls/RGO composite, the high capacity of MnO₂ and superior electrical conductivity of RGO.     

The effect of graphene dispersion on the mechanical properties of graphene/epoxy composites

The effect of dispersion state of graphene on mechanical properties of graphene/epoxy composites was investigated. The graphene sheets were exfoliated from graphite oxide (GO) via thermal reduction (thermally reduced GO, RGO). Different dispersions of RGO sheets were prepared with and without ball mill mixing. It was found that the composites with highly dispersed RGO showed higher glass transition temperature (T_g) and strength than those with poorly dispersed RGO, although no significant differences in both the tensile and flexural moduli are caused by the different dispersion levels. In particular, the T_g was increased by nearly 11 °C with the addition of 0.2 wt.% well dispersed RGO to epoxy. As expected, the highly dispersed RGO also produced one or two orders of magnitude higher electrical conductivity than the corresponding poorly dispersed RGO. Furthermore, an improved quasi-static fracture toughness (K_IC) was measured in the case of good dispersion. The poorly and highly dispersed RGO at 0.2 wt.% loading resulted in about 24% and 52% improvement in K_IC of cured epoxy thermosets, respectively. RGO sheets were observed to bridge the micro-crack and debond/delaminate during fracture process due to the poor filler/matrix and filler/filler interface, which should be the key elements of the toughening effect.     

Facile hydrothermal synthesis and optical limiting properties of TiO2-reduced graphene oxide nanocomposites

TiO₂/reduced graphene oxide (RGO) nanocomposites Gx (RGO titania nanocomposite, x grams tetrabutyl titanate per 0.03 g RGO, x = 0.25, 0.50, 1.00) were prepared by a hydrothermal method: graphene oxide was reduced to RGO in a 2:1 water:ethanol mixture in the presence of varying quantities of tetrabutyl titanate, which deposited as TiO₂ on the RGO sheets. The nanocomposites were characterized by a combination of Fourier transform infrared spectroscopy, diffuse reflectance ultraviolet–visible spectroscopy, photoluminescence spectroscopy, Raman spectroscopy, X-ray powder diffraction, X-ray photoelectron spectroscopy and transmission electron microscopy studies. The nanocomposite G0.25 exhibits enhanced nonlinear optical properties compared to its individual components, which is ascribed to a combination of mechanisms. The role of defects and electron/energy transfer in the optical limiting performance of G0.25 was clarified with the help of Raman and photoluminescence spectroscopies. Intensity-dependent switching between reverse saturable absorption and saturable absorption behavior was observed with the G0.50 nanocomposite.     

Fabrication and properties of reduced graphene oxide reinforced yttria-stabilized zirconia composite ceramics

Fully dense yttria-stabilized zirconia (YSZ) ceramics reinforced with reduced graphene oxide (RGO) were fabricated by spark plasma sintering (SPS), and their electrical, thermal, and mechanical properties were investigated. Graphene oxide (GO) was exfoliated by a short sonification in dimethylformamide (DMF)/water solution and uniformly mixed with ZrO₂ powders. The microstructure of the composites showed that undamaged RGO sheets were homogeneously distributed throughout matrix grains. The electrical conductivity of YSZ composites drastically increased with the addition of RGO, and it reached 1.2 × 10⁴ S/m at 4.1 vol.%. However, the thermal diffusivity increased only 12% with RGO addition. The hardness decreased slightly with RGO addition, whereas the fracture toughness significantly increased from 4.4 to 5.9 MPa^1/2. The RGO pull-out and crack bridging contributed to the improved fracture toughness.     

MoO3/reduced graphene oxide composites as anode material for sodium ion batteries

MoO₃/reduced graphene oxide (MoO₃/RGO) composites were successfully prepared via a facile one-step hydrothermal method, and evaluated as anode materials for sodium ion batteries (SIBs). The crystal structures, morphologies and electrochemical properties of the as-prepared samples were characterized by X-ray diffraction, field-emission scanning electron microscopy, cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge/discharge tests, respectively. The results show that the introduction of RGO can enhance the electrochemical performances of MoO₃/RGO composites. MoO₃/RGO composite with 6 wt% RGO delivers the highest reversible capacity of ~208 mA h g⁻¹ at 50 mA g⁻¹ after 50 cycles with good cycling stability and excellent rate performance for SIBs. The excellent sodium storage performance of MoO₃/RGO should be attributed to the synergistic effect between MoO₃ and RGO, which offers the increased electrical conductivity, the facilitated electron transfer ability and the buffering of volume expansion.