Facile synthesis of reduced graphene oxide/CeO2 nanocomposites and their application in supercapacitors

The combination of the attractive properties of graphene with excellent characteristics of other functional nanomaterials has become a popular pathway for achieving applications in multiple fields. Herein, reduced graphene oxide (RGO)/CeO₂ nanocomposites with enhanced capacitive performance were designed and synthesized by a facile two-step approach with a self-assembly method followed by thermal treatment. The structure, morphology and composition of the resulting RGO/CeO₂ nanocomposites were systematically investigated. The presence of RGO can prevent the aggregation and control the structures of the CeO₂ nanocrystals in the annealing process. The nanocomposites as electrode materials for supercapacitor exhibited an enhanced capacitive performance due to the synergic effect between RGO nanosheets and CeO₂ nanocrystals. The excellent capacitive performance of the RGO/CeO₂ nanocomposites offer great promise for supercapacitor applications.     

Green preparation of reduced graphene oxide using a natural reducing agent

A simple and efficient method was introduced for the high-conversion preparation of graphene oxide (GO) from large graphite flakes (average flake size=100 μm) using a simplified Hummer׳s method. Natural reducing agents such as lemon juice and vinegar were compared with hydrazine (N₂H₄) as potential reducing agents. Graphene was prepared by chemical reduction of GO because this method was low cost and could be used for large-scale graphene production. This one-pot graphene preparation was performed at room temperature. Different degrees of oxidation of graphite flakes were obtained by stirring graphite in a mixture of sulfuric acid and potassium permanganate at different oxidation times, and highly exfoliated GO sheets were produced. GO was subsequently reduced effectively by lemon juice, a new, green, and potential reducing agent with pH 2.3. This reduced GO exhibited a high electrical conductance of 24.6 μS attributed to its higher C/O ratio (≈8:2) compared with other samples.     

One-pot synthesis of CdS sensitized TiO2 decorated reduced graphene oxide nanosheets for the hydrolysis of ammonia-borane and the effective removal of organic pollutant from water

A hybrid material of reduced graphene oxide (RGO) sheets decorated with CdS-TiO₂ NPs was prepared through a facile one-pot hydrothermal method. The assembly of CdS-TiO₂ nanoparticles (NPs) on RGO sheets was in-situ produced. As-synthesized nanocomposites were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), energy disperse X-ray spectrum (EDS), fourier transform infrared spectroscopy (FTIR), and photoluminescence spectroscopy (PL). The obtained nanocomposites exhibited a good photocatalytic activity for the visible-light-induced decomposition of methylene blue (MB) dye and hydrolysis of ammonia borane. The results showed that by incorporation of CdS and TiO₂ NPs on graphene oxide sheets the photocatalytic efficiency was enhanced. The significant enhancement in the photocatalytic activity of CdS-TiO₂/RGO nanocomposites under visible light irradiation can be ascribed to the effect of CdS by acting as electron traps in TiO₂ band gap. Reduced graphene oxide worked as the adsorbent, electron acceptor and a photo-sensitizer to efficiently enhance the dye photo decomposition. Such nanocomposite photocatalyst might find potential application in a wide range of fields, including hydrogen energy generation, air purification, and wastewater treatment.     

Nano-sized graphene oxide as sole surfactant in miniemulsion polymerization for nanocomposite synthesis: Effect of pH and ionic strength

Miniemulsion polymerization of styrene using AIBN as initiator at 70 °C has been performed with nano-dimensional graphene oxide (GO) sheets as surfactant (no conventional surfactants employed) with a view to exploring the effects of pH and ionic strength (NaCl concentration). The pH value of the emulsion exerted a relatively minor influence on the polymerization, with a somewhat narrower particle size distribution being obtained at pH = 3.2 relative to pH = 2.4 and 5.2. The ionic strength had a more significant effect – the presence of a suitable concentration of NaCl resulted in increased colloidal stability and narrower particle size distribution. The results are explained in terms of the effects of pH on degree of ionization of COOH groups of GO and the influence of ionic strength on the electric double layer, and have implications with regards to synthesis of polymer/graphene nanocomposite materials for a variety of applications.     

Materials and electrode designs of high-performance NiCo2S4/Reduced graphene oxide for supercapacitors

NiCo₂S₄ is one of the most promising bimetallic sulfides for use in energy-storage systems, but more studies are needed to endow NiCo₂S₄ with a high electrochemical reaction capability and reversibility. In this work, we present rationally materials design of an optimal NiCo₂S₄ nanoparticle in a reduced graphene oxide (RGO) matrix as a NiCo₂S₄/RGO nanocomposite. Furthermore, we report the improvements in the materials technology, demonstrating the NiCo₂S₄/RGO nanocomposite electrode with an excellent specific capacitance of 963–700 F g⁻¹ at 1–15 A g⁻¹, high capacitance retention of 70%, and long cycle life of 3000 cycles. The practical application is showcased in an asymmetric supercapacitor with a high active-material loading. The NiCo₂S₄/RGO nanocomposite shows a high energy density of 31 Wh kg⁻¹ at a power density of 987 W kg⁻¹ and maintains an excellent density of 23 Wh kg⁻¹ at a high power density of 7418 W kg⁻¹. The outstanding electrochemical utilization and stability of the NiCo₂S₄/RGO nanocomposite confirm that our systematic optimization in the materials science and technology in terms of the active-material synthesis, the electrode development, and the device design/fabrication would benefit the future development of high-performance supercapacitors.     

Enhanced electrocatalytic activity of a polyoxometalates-based film decorated by gold nanoparticles

A K₆P₂MoW₁₇O₆₂-based film decorated by gold nanoparticles was prepared by layer-by-layer self-assembly method. It was fabricated on quartz, silicon and ITO substrates in a progressive and uniform manner monitored by UV–vis spectroscopy, and characterized by atomic force microscopy (AFM), cyclic voltammograms (CVs) and electrochemical impedance spectra (EIS). A set of experimental results revealed that the incorporation of Au nanoparticles into K₆P₂MoW₁₇O₆₂-based film enhanced the conductivity of the film, leading to several or more than tenfold increase in electrocatalytic efficiency of this film for reduction of H₂O₂, IO₃⁻, NO₂⁻ compared to the no use of Au nanoparticles. It also has been found that presence of Au nanoparticles endowed the film electrocatlytic activity towards oxidation of ascorbic acid and the photo-luminescence property arising from inter electron transitions between occupied 5d bands to 6sp conduction bands of AuNPs.     

Efficient synthesis of graphene sheets using pyrrole as a reducing agent

The efficient synthesis of graphene sheets using pyrrole as a reducing agent was explored. The obtained graphene sheets were dispersible in organic solvents such as ethanol, isopropanol, N,N-dimethylformamide, N-methylpyrrolidone, dimethylsulfoxide, tetrahydrofuran, and acetone. During this reduction reaction, pyrrole was oxidized, forming oxidation product of pyrrole and adsorbed on the graphene sheets surface by π–π interaction. The oxidation product of pyrrole acted as a capping agent for graphene sheets by preventing re-stacking and formed organically dispersible graphene. The formation of graphene and its crystalline nature was indicated by the transmission electron microscopy and the atomic force microscopy analysis. Raman, X-ray photoelectron spectroscopy and X-ray diffraction provided the evidence for graphene formation from graphene oxide precursor. Furthermore, the reduced oxygen content and N 1s peak observed by the X-ray photoelectron spectroscopy analysis of graphene sheets confirmed the reduction reaction and presence of adsorbed oxidation product on the surface of graphene sheets. The resulting graphene sheets were readily dispersible in solvents and easily to process.     

Effect of UV irradiation on magnetic behavior of reduced graphene oxide decorated with nickel nanostructure

Reduced graphene oxide (RGO) decorated with nickel has been synthesized via an in-situ reduction of graphene oxide (GO) and nickel nitrate using NaOH and hydrazine. The starting materials Ni (NO₃)₂ and GO were taken in two different ratios and the products formed were designated as RGNi₂ and RGNi₁. The formation of the composite was confirmed by the appearance of X-ray diffraction peaks at 44.5°, 51.9°, 76.5° corresponding to Ni and at 24.8°and 43.2° for RGO. The RGNi₂ was irradiated with UV light (λ=254 nm) for different durations (2, 6, 12, 24 and 48 h). Intensity ratio of d and g-bands (I_d/I_g) of Raman spectra increases from 1.18 to 1.47 over the duration of irradiation period (2–48 h). The magnetization measurements using the vibrating sample magnetometer (VSM) of these samples reveal their ferromagnetic behavior. The calculated saturation magnetization (M_S) value of Ni, RGNi₁and RGNi₂ is 47.86, 30.56 and 8.25 emu/g respectively and the corresponding coercivity (H_C) value is found to be 181, 227 and 296 Oe. The M_S of RGNi₂ is found to increase to 10.65 emu/g after 48 h of irradiation. This enhancement in the M_S(~23%) with irradiation may be due to defect formation by the UV light.     

Fabrication of hexagonal cerium oxide nanoparticles decorated nitrogen-doped reduced graphene oxide hybrid nanocomposite as high-performance microwave absorbers in the Ku band

With the aim to obtain microwave absorbers simultaneously possessing broad absorption bandwidth, strong absorption intensity and thin matching thickness, nitrogen-doped reduced graphene oxide decorated by cerium oxide particles (NRGO/CeO₂) hybrid nanocomposite was prepared through a hydrothermal and calcination two-step route. Results of micromorphology analysis showed that numerous hexagonal CeO₂ nanoparticles were evenly anchored on the crumpled surfaces of NRGO. Moreover, both nitrogen doping and hybridization with RGO could notably strengthen the microwave absorption capacity of CeO₂. Remarkably, the NRGO/CeO₂ hybrid nanocomposite exhibited the minimum reflection loss of ?57.2 dB at 13.4 GHz (Ku band) under a matching thickness of 1.66 mm and maximum absorption bandwidth of 4.6 GHz (from 13.2 to 17.8 GHz) at an ultrathin thickness of only 1.5 mm. Meanwhile, the hybrid nanocomposites displayed strong absorption intensity (≤-20 dB, 99% absorption) in almost the whole measured thicknesses range. Furthermore, the relationship between absorption intensity and filler loadings was uncovered. The potential microwave absorption mechanisms were further revealed. Therefore, this work opened a novel idea for designing RGO-based hybrid nanocomposites as high-performance microwave absorbers.     

Surfactant-free synthesis of homogeneous nano-grade cadmium sulfide grafted reduced graphene oxide composite as a high-activity photocatalyst in visible light

Zero-dimensional cadmium sulfide (CdS) nanoparticles with small size (∼50 nm) were grafted on the two-dimensional reduced graphene oxide (RGO) nanosheet via a facile hydrothermal method without any surfactant to synthesize CdS@RGO nanocomposites in this paper. The structural analysis confirms the strong attachment and interaction between CdS and RGO in CdS@RGO photocatalyst, which leads to a higher photocatalytic efficiency (95.3%) with superior anti-corrosion stability (almost no change of efficiency over three repeated experiments) to that of pure CdS in visible light. The unique hybrid nanostructure of CdS@RGO can effectively prevent the self-corrosion of CdS and facilitate the separation of electron-hole pairs. Consequently, these outstanding photocatalytic performances of CdS@RGO endow it with a promising prospect for the degradation of organic pollutants and this work can be extended to other graphene-based inorganic semiconductor composites.     

Supercritical alcohols as solvents and reducing agents for the synthesis of reduced graphene oxide

We report on a facile, simple, and green graphene oxide (GO) reduction method based on a supercritical alcohol approach. The influence over the chemical, thermal, morphological, and textural properties of reduced graphene oxides (RGOs) of five different alcohols in their supercritical conditions – methanol, ethanol, 1-propanol, 2-propanol, and 1-butanol – was investigated in detail. Although the thermal stabilities and Fourier-transform infrared spectra of RGOs produced using the different alcohols are very similar, a substantial difference in the carbon-to-oxygen ratios measured by X-ray photoelectron spectroscopy and Brunauer–Emmett–Teller surface areas are observed. The RGO produced using supercritical ethanol exhibited a much higher carbon-to-oxygen ratio of 14.4 and a much larger surface area of 203 m²/g compared with that produced using the other supercritical alcohols. Raman spectra showed that the RGOs produced using supercritical ethanol and supercritical 2-propanol retained more of the graphitic structure. X-ray diffraction analysis revealed that RGOs produced using supercritical 1-propanol and supercritical 1-butanol retained at least two different interlayer spacings. The deoxygenation mechanism of GO in supercritical ethanol is proposed based on gas and liquid product analysis.     

Preparation of WO3-reduced graphene oxide nanocomposites with enhanced photocatalytic property

In this work, WO₃-reduced graphene oxide (RGO) nanocomposite was synthesized via a simple one-pot hydrothermal method. The synthesized nanocomposite was characterized by SEM, XRD, EDX, UV–vis spectroscopy, N₂ adsorption/desorption, photocurrent response, electrochemical impedance spectroscopy and Raman spectroscopy. The superior contact between WO₃ and RGO sheets in the nanocomposite facilitates the photocatalytic degradation of methylene blue and evolution of oxygen. The cause of the enhanced photocatalytic performance could ascribe to the highly facilitated electron transport by the synergistic effect between WO₃ and RGO sheets, as well as suppressing the electron hole pair recombination in the nanocomposite.     

Microwave-assisted one-pot synthesis of metal/metal oxide nanoparticles on graphene and their electrochemical applications

An effective synthesis strategy of hybrid metal (PtRu)/metal oxide (SnO₂) nanoparticles on graphene nanocomposites is developed using a microwave-assisted one-pot reaction process. The mixture of ethylene glycol (EG) and water is used as both solvent and reactant. In the reaction system for the synthesis of SnO₂/graphene nanocomposite, EG not only reduces graphene oxide (GO) to graphene, but also results in the formation of SnO₂ facilitated by the presence of a small amount of water. On the other hand, in the reaction system for preparation of PtRu/graphene nanocomposites, EG acts as solvent and reducing agent for reduction of PtRu nanoparticles from their precursors and reduction of graphene from graphene oxide. Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM) characterizations confirm the feasibility of the microwave-assisted reaction system to simultaneously reduce graphene oxide and to form SnO₂ or PtRu nanoparticles. The as-synthesized SnO₂/graphene hybrid composites show a much higher supercapacitance than the pure graphene, and the as-prepared PtRu/graphene show much better electrocatalytic activity for methanol oxidation compared to the commercial E-TEK PtRu/C electrocatalysts.     

Template-free synthesis of large anisotropic gold nanostructures on reduced graphene oxide

The morphologies/dimensions of Au nanostructures can be tailored by merely controlling the reduction degree of graphene oxide surface. Au nanoparticles, long Au nanowires, and semicircular-shaped Au nanoplates are in situ synthesized on slightly, moderately, and highly reduced graphene oxide films respectively, without the need of any templating agent.     

Gas phase mineralized graphene as core/shell nanosheet supports for single-site olefin polymerization catalysts and in-situ formation of graphene/polyolefin nanocomposites

A versatile gas phase mineralization process affords nanosheets containing a functionalized graphene (FG) core and a thin silica shell. The number of cycles, exposing FG to sequenced tetrachlorosilane and water vapors, controls the silica content and the silica shell thickness. The resulting high surface area core/shell nanosheets, containing 22 to 34 wt.-% silica, are used to immobilize single-site catalysts. During polymerization, the FG/silica nanosheets are uniformly dispersed in ultrahigh molecular weight polyethylene. This catalytic polymerization filling process, exploiting the encapsulation of graphene in a silica shell, is of interest to prepare electrically insulating carbon/polyolefin composite materials with high thermal conductivity useful in lightweight engineering.     

Electrochemical reduction of graphene oxide in organic solvents

Graphene oxide (GO) cast on conductive substrates was electrochemically reduced in some organic solvents. The amount of electricity required for the almost complete reduction of GO was 2.0 C for 1 mg GO, corresponding to attaching of a one-electron reducible species to each benzene ring in graphene. The electrochemically reduced GO film gave an electrical conductivity of about 3 S cm⁻¹ and exhibited a relatively high specific capacitance of 147.2 F g⁻¹ in propylene carbonate. The electrochemical reduction of GO was feasible on Al foils as well.     

Structure and synthesis of graphene oxide

Graphene oxide (GO) is one typical two-dimension structured and oxygenated planar molecular material. Researchers across multiple disciplines have paid enormous attention to it due to the unique physiochemical properties. However, models used to describe the structure of GO are still in dispute and ongoing to update. And currently, synthesis methods for mass production are seemingly abundant but in fact, dominated by a few core methodologies. To update with the state-of-art opinions and progresses, herein we present a mini critical review regarding the synthesis of GO as well as its models and simulations of structure. Also, we discuss the perspectives.     

One-pot synthesis of Ag-TiO2/reduced graphene oxide nanocomposite for high performance of adsorption and photocatalysis

The Ag-TiO₂/r-GO nanocomposite was synthesized via a facile one-pot solvothermal method. X-ray diffraction (XRD), Transmission electron microscopy (TEM),High resolution transmission electron microscopy(HRTEM), UV–vis diffuse reflectance spectroscopy (DRS), Fourier transformed infrared spectroscopy (FT-IR), Photoluminescence (PL) and N₂ adsorption-desorption were used for the characterization of prepared samples. The adsorbent and photocatalytic performance of prepared samples were evaluated by remove of Rh B dyes and reduction of CO₂. Both the adsorbent and photocatalytic ability of all the Ag-TiO₂/r-GO samples were much higher than pure hollow TiO₂. The excellent adsorbent capacity can be attributed to the large BET surface area and the enhanced photocatalytic activity can be assigned to the predominant properties of graphene and the localized surface plasmon(LSPR) effect of Ag nanoparticles.     

Green synthesis of gold nanostructures using pear extract as effective reducing and coordinating agent

Biosynthesis of gold nanoparticles and nanoplates (GNPs) was accomplished using aqueous fractions of pear extract as a safe, reducing, particle-stabilizing, and shape-directing agent. The maximum yields of spherical gold nanoparticles having the average sizes of 40, 20, and 10 nm were achieved at 30, 60, and 90 °C, respectively, at a pear extract concentration of 45% (v/v). The maximum yield of gold nanoplates was obtained with sizes ranging from 20 to 400 nm, particularly at reaction temperatures of 30, 60, and 90 °C, at a pear extract concentration of 5% (v/v). The surface chemistry analysis of the GNPs suggests that the sugars and peptides or proteins as key biomolecules of the pear extract play a crucial role in the reduction of Au(III), subsequently resulting in healthy capping. Therefore, this environmentally friendly synthesis method of GNPs for the particular type of morphologies is expected to be a competitive alternative to existing physical and chemical methods.     

Hydrazine hydrate-induced hydrothermal synthesis of MnFe2O4 nanoparticles dispersed on graphene as high-performance anode material for lithium ion batteries

Herein, a MnFe₂O₄/graphene (MnFe₂O₄/G) nanocomposite has been synthesized via a facile N₂H₄·H₂O-induced hydrothermal method. During the synthesis, N₂H₄·H₂O is employed to not only reduce graphene oxide to graphene, but also prevent the oxidation of Mn²⁺ in alkaline aqueous solution, thus ensuring the formation of MnFe₂O₄/G. Moreover, MnFe₂O₄ nanoparticles (5–20 nm) are uniformly anchored on graphene. MnFe₂O₄/G electrode delivers a large reversible capacity of 768 mA h g⁻¹ at 1 A g⁻¹ after 200 cycles and high rate capability of 517 mA h g⁻¹ at 5 A g⁻¹. MnFe₂O₄/G holds great promise as anode material in practical applications due to the outstanding electrochemical performance combined with the facile synthesis strategy.