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.     

Reduced chemically modified graphene oxide for supercapacitor electrode

An efficient active material for supercapacitor electrodes is prepared by reacting potassium hydroxide (KOH) with graphene oxide followed by chemical reduction with hydrazine. The electrochemical performance of KOH treated graphene oxide reduced for 24 h (reduced chemically modified graphene oxide, RCMGO-24) exhibits a specific capacitance of 253 F g^-1 at 0.2 A g^-1 in 2 M H₂SO₄ compared to a value of 141 F g^-1 for graphene oxide reduced for 24 h (RGO-24), and good cyclic stability up to 3,000 cycles. Interestingly, RCMGO-24 demonstrated a higher specific capacitance and excellent cycle stability due to its residual oxygen functional groups that accelerate the faradaic reactions and aid in faster wetting. This non-annealed strategy offers the potential for simple and cost-effective preparation of an active material for a supercapacitor electrode.     

Preparation of manganese monoxide@reduced graphene oxide nanocomposites with superior electrochemical performances for lithium-ion batteries

Manganese monoxide (MnO) nanowire@reduced graphene oxide (rGO) nanocomposites are synthesized using a simple hydrothermal method combined with a calcination process. The structural and morphological characterization of the composites indicates that the MnO nanowires homogeneously anchor on both sides of the cross-linked rGO. The nanocomposites exhibit a high surface area of 126.5?m² g^?1. When employed as an anode material for lithium-ion batteries, the nanocomposites exhibit a reversible capacity of 1195 mAh g^?1 at a current density of 0.1?A?g^?1, with a high charge-discharge efficiency of 99.2% after 150 cycles. The three-dimensional architecture of the present materials exhibits high porosity and electron conductivity, significantly shortening the diffusion path of lithium ions and accelerating their reaction with the electrolyte, which greatly improves the lithium-ion storage properties. These excellent electrochemical performances make the composite a promising electrode material for lithium-ion batteries.     

Potentiostatically deposited polypyrrole/graphene decorated nano-manganese oxide ternary film for supercapacitors

A simple method based on potentiostatic polymerization was developed for the preparation of ternary manganese oxide-based nanocomposite films. The ternary nanocomposites, which were characterized using x-ray diffraction spectroscopy and x-ray photoelectron spectroscopy, showed that the manganese oxide within the film consisted of MnO₂ and Mn₂O₃. Electrochemical measurements showed that the ternary nanocomposite electrode exhibited high specific capacitance (up to 320.6 F/g), which was attributed to the morphology of a polypyrrole/graphene/manganese-oxide (PPy/GR/MnO_x) ternary nanocomposite. The experimental approach maximized the pseudocapacitive contribution from redox-active manganese oxide (MnO_x) and polypyrrole (PPy), as well as the electrochemical double layer capacitive (EDLC) characteristic from graphene (GR) sheets. Long cyclic measurements indicated that the specific capacitance of the ternary nanocomposite film could retain 93% of its initial value over 1000 charge/discharge cycles, in the potential range of −0.2 to 0.7 V versus silver/silver chloride electrode (Ag/AgCl).     

Polydopamine-assisted formation of Co3O4-nanocube-anchored reduced graphene oxide composite for high-performance supercapacitors

Tailoring transition-metal oxide nanoparticles with two-dimensional carbon has become a favorite way to improve their electrochemical performance. In this study, a composite of reduced graphene oxide was anchored by Co₃O₄ nanocubes and easily prepared with the assistance of polydopamine (PDA), using a combination of hydrothermal reaction and pyrolysis (Co₃O₄@PDA-rGO). Polydopamine, which possesses abundant catechol and amine groups, could be easily grafted onto graphene oxide to reduce the aggregation of graphene particles. Furthermore, PDA provided active sites, i.e., catechol and amine groups, which coordinated with Co²⁺, enabling enrichment of metal ions on the surface of graphene. After the pyrolysis of Co²⁺-containing PDA-grafted graphene at 400 °C, the Co²⁺ ions were converted into Co₃O₄ nanocubes, while the PDA carbonized to form N-doped porous carbon on the surface of graphene. The resulting product, Co₃O₄@PDA-rGO, demonstrated extraordinary supercapacitive behavior with good cycling stability owing to its unique porous structure as well as the intimate contact between Co₃O₄ and the carbon matrix.     

Silver nanowire–graphene hybrid nanocomposites as label for sensitive electrochemical immunoassay of alpha-fetoprotein

A facile and sensitive mediator-free electrochemical immunoassay was developed for determination of alpha-fetoprotein (AFP) in human serum with a sandwich-type mode by using silver nanowire–graphene hybrid nanocomposites (AgNW–GPs) as label. One-pot hydrothermal method was used for the synthesis of the AgNW–GPs with the aid of trisodium citrate. The as-prepared AgNW–GP was not only utilized for the label of horseradish peroxidase-conjugated anti-AFP (HRP-anti-AFP) with highly loading capacity, but also displayed good electrochemical behaviors. Compared with the conventional HRP-anti-AFP-based electrochemical immunoassay, the sensitivity of the proposed immunoassay by using the synthesized nanolabels was greatly improved. Under optimal conditions, the electrochemical immunosensor exhibited a wide linear range of 0.05–400 ng/mL with a low detection limit (LOD) of 5 pg/mL AFP (at 3s_B). Intra- and inter-assay coefficients of variation were below 10% and 8.5%, respectively. The selectivity and stability of the immunosensor were acceptable. In addition, the methodology was also validated by assaying 16 positive serum samples and 7 negative serum samples, receiving a good correlation with the results obtained from the referenced electrochemiluminescence method.     

Preparation, characterization, and supercritical carbon dioxide foaming of polystyrene/graphene oxide composites

Graphene oxide (GO) was prepared by oxidation of graphite using the Hummers method, and was modified by isocyanate to obtain dispersed GO sheets in dimethylformamide. Polystyrene (PS)/GO composites were prepared by solution blending, and their morphologies and properties were characterized. The addition of GO increased the glass transition temperature of the PS/GO composites. The storage modulus and thermal stability of the composites were also improved compared with PS. Foams of PS and PS/GO composites were prepared by supercritical carbon dioxide foaming. The composite foams exhibited slightly higher cell density and smaller cell size compared with the PS foam, indicating the GO sheets can act as heterogeneous nucleation agents.     

Synthesis and characterization of aerogel-like mesoporous nickel oxide for electrochemical supercapacitors

Highly porous NiO was prepared via a combination of sol-gel process with supercritical drying method in this paper. The as-synthesized NiO samples exhibit 80–90% porosity and high surface area, ie, 180.5–325.6 m²g⁻¹. Cyclic voltammetric and chronopotentiometric measurements indicated the aerogel-like NiO in 1 mol.L⁻¹ KOH solution to behave capacitive well due to its uniform mesoporous microstructure. It was also observed that post-heating temperature plays a critical role in the mesoporous nature of the aerogel-like materials. An optimal heating temperature of 300^∘C was found to favor the formation of mesopores, which account for the large specific capacitance of as high as 125 F.g⁻¹. The average specific capacitance of the aerogel-like NiO was observed to be about 75–125 F.g⁻¹ between a potential window of 0–0.35 V vs. SCE.     

Facile solvothermal synthesis of a graphene nanosheet-bismuth oxide composite and its electrochemical characteristics

This work demonstrates a novel and facile route for preparing graphene-based composites comprising of metal oxide nanoparticles and graphene. A graphene nanosheet-bismuth oxide composite as electrode materials of supercapacitors was firstly synthesized by thermally treating the graphene-bismuth composite, which was obtained through simultaneous solvothermal reduction of the colloidal dispersions of negatively charged graphene oxide sheets in N,N-dimethyl formamide (DMF) solution of bismuth cations at 180 °C. The morphology, composition, and microstructure of the composites together with pure graphite oxide, and graphene were characterized using powder X-ray diffraction (XRD), FT-IR, field emission scanning electron microscopy (FESEM), transmission electron microscope (TEM), thermogravimetry and differential thermogravimetry (TG-DTG). The electrochemical behaviors were measured by cyclic voltammogram (CV), galvanostatic charge-discharge and electrochemical impedance spectroscopy (EIS). The specific capacitance of 255 F g⁻¹ (based on composite) is obtained at a specific current of 1 A g⁻¹ as compared with 71 F g⁻¹ for pure graphene. The loaded-bismuth oxide achieves a specific capacitance as high as 757 F g⁻¹ even at 10 A g⁻¹. In addition, the graphene nanosheet-bismuth oxide composite electrode exhibits the excellent rate capability and well reversibility.     

Preparation of Three-Dimensional Co3O4/graphene Composite for High-Performance Supercapacitors

Here, we developed a simple and efficient route for the preparation of three-dimensional (3D) Co₃O₄-anchored graphene composites using the sacrificial template-assisted method and the subsequent deposition process of Co₃O₄ nanoparticles. As structural guiding materials, polystyrene (PS) spheres provide 3D porous architectures with a high surface area. 3D porous graphene materials serve as conductive supporters for the deposition of Co₃O₄ nanoparticles through precipitation growth. The 3D porous composite structures of Co₃O₄/graphene composites were intensively investigated using scanning electron microscope, transmission electron microscope, and X-ray diffraction. The 3D Co₃O₄/graphene composites show a high specific capacitance of 328?F?g^?1 with efficient and fast charge–discharge process in aqueous 6?M KOH electrolyte. In addition, the composites provide a good cycle lifetime, which retained 98% capacitance retention over 2000 cycles.     

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.     

Synthesis,optical and electrochemical properties of ZnO nanowires/graphene oxide heterostructures

Large-scale vertically aligned ZnO nanowires with high crystal qualities were fabricated on thin graphene oxide films via a low temperature hydrothermal method. Room temperature photoluminescence results show that the ultraviolet emission of nanowires grown on graphene oxide films was greatly enhanced and the defect-related visible emission was suppressed, which can be attributed to the improved crystal quality and possible electron transfer between ZnO and graphene oxide. Electrochemical property measurement results demonstrated that the ZnO nanowires/graphene oxide have large integral area of cyclic voltammetry loop, indicating that such heterostructure is promising for application in supercapacitors.     

Preparation and tribological properties of novel boehmite/graphene oxide nano-hybrid

Novel boehmite/graphene oxide nano-hybrid (GO–GPTS–AlOOH) was prepared through a simple covalent bond method, which was subsequently explored as lubricant additive. For this purpose, the 3-glycidoxypropyl-trimethoxysilane (GPTS) was first chemically grafted on nano-boehmite (AlOOH) to fabricate the modified boehmite (GPTS–AlOOH). Then the GPTS–AlOOH was anchored on graphene oxide (GO) nanosheets to prepare GO–GPTS–AlOOH nano-hybrid through a coupled reaction. The structure, composition and morphology of GO–GPTS–AlOOH was characterized by FT-IR, XRD, TG/DTG, SEM and TEM, revealing that nano-boehmite was uniformly coated on GO surface. More importantly, tribological properties of GO–GPTS–AlOOH as lubricating oil additive were investigated using a ball-on-disc testing machine and a four-ball machine. It was found that the friction reduction and anti-wear ability of lubricant oil containing GO–GPTS–AlOOH hybrid was highly improved compared to bare base oil (VHVI8). Specifically, friction coefficient (COF), wear scar diameter (WSD) and wear rate were reduced by 14%, 28% and 73%, respectively. The enhancement can be attributed to the synergistic effect of the nanobearing mechanism and ultimate strengthen of graphene sheets between the frictional interfaces.     

Seedless growth of zinc oxide flower-shaped structures on multilayer graphene by electrochemical deposition

A seedless growth of zinc oxide (ZnO) structures on multilayer (ML) graphene by electrochemical deposition without any pre-deposited ZnO seed layer or metal catalyst was studied. A high density of a mixture of vertically aligned/non-aligned ZnO rods and flower-shaped structures was obtained. ML graphene seems to generate the formation of flower-shaped structures due to the stacking boundaries. The nucleation of ZnO seems to be promoted at the stacking edges of ML graphene with the increase of applied current density, resulting in the formation of flower-shaped structures. The diameters of the rods/flower-shaped structures also increase with the applied current density. ZnO rods/flower-shaped structures with high aspect ratio over 5.0 and good crystallinity were obtained at the applied current densities of −0.5 and −1.0 mA/cm². The growth mechanism was proposed. The growth involves the formation of ZnO nucleation below 80°C and the enhancement of the growth of vertically non-aligned rods and flower-shaped structures at 80°C. Such ZnO/graphene hybrid structure provides several potential applications in sensing devices.     

聚丙烯/氧化石墨烯复合材料的制备，结构及性能

以石墨、浓硫酸、高锰酸钾和双氧水等为原料,通过Hummers法制备了氧化石墨烯(GO)分散液,对其冷冻干燥得到GO粉体,将GO粉体与熔融聚丙烯(PP)树脂共混制备PP/GO复合材料,采用FTIR、AFM、TEM、XRD、DSC及导热仪和氧指数测定仪等对GO及PP/GO复合材料的结构和性能进行了表征。结果表明,GO能够以双片层形式均匀地分散在PP基体中,GO/PP复合材料具有致密均匀的微观结构,其力学性能、耐热、阻燃和热传导等性能比对照样品(单纯PP树脂)有显著提高。当GO掺量为0.4%(以PP的质量为基准,下同)时,PP/GO复合材料的拉伸强度、弯曲强度和冲击强度比对照样品分别提高了29.6%、33.6%和62.7%,熔点从154.5℃提高为174.2℃,热导率提升了205.3%,极限氧指数从18.0提高到27.6。     

Exploring the methods of synthesis,functionalization, and characterization of graphene and graphene oxide for supercapacitor applications

Graphene and graphene oxide, are attracting more attention over the last decades in the area of supercapacitor research and researchers concentrate on extensive exploration, owing to their dominating electrical conductivity, combined with mechanical properties. This review is a panoramic approach, giving insights into various aspects related to graphene and graphene oxide such as their properties, production methods, functionalities, and their applications in supercapacitors. The study ought to be beneficial to novice as well as to the domain experts. Various properties of both materials are explored and both synthesis methods are elaborated. Extra emphasis is given to bring out the role of graphene and graphene oxide in promoting the performance of supercapacitors. Synthesis methods are tabulated based on the evaluation metrics like specific capacitance and capacitance retention. Finally, the application of graphene and graphene oxide in supercapacitors are highlighted. Before concluding, perspectives along with challenges for further development are proposed and are expected to facilitate researchers in shedding light on further studies in this explorative area.     

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.     

Fabrication and electrochemical capacitance of hierarchical graphene/polyaniline/carbon nanotube ternary composite film

A film composed of graphene (GN) sheets, polyaniline (PANI) and carbon nanotubes (CNTs) has been fabricated by reducing a graphite oxide (GO)/PANI/CNT precursor prepared by flow-directed assembly from a complex dispersion of GO and PANI/CNT, followed by reoxidation and redoping of the reduced PANI in the composite to restore the conducting PANI structure. Scanning electron microscope images indicate that the ternary composite film is a layered structure with coaxial PANI/CNT nanocables uniformly sandwiched between the GN sheets. Such novel hierarchical structure with high electrical conductivity perfectly facilitates contact between electrolyte ions and PANI for faradaic energy storage and efficiently utilizes the double-layer capacitance at the electrode–electrolyte interfaces. The specific capacitance of the GN/PANI/CNT estimated by galvanostatic charge/discharge measurement is 569 F g⁻¹ (or 188 F cm⁻³ for volumetric capacitance) at a current density of 0.1 A g⁻¹. In addition, the GN/PANI/CNT exhibits good rate capability (60% capacity retention at 10 A g⁻¹) and superior cycling stability (4% fade after 5000 continuous charge/discharge cycles).     

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.     

MnO anchored reduced graphene oxide nanocomposite for high energy applications of Li-ion batteries: The insight of charge-discharge process

In the present work, a new class of anode material for high energy applications of Li-ion battery is prepared by easy and large-scale producible process. Herein, the nanocomposite of MnO and reduced graphene oxide (rGO) is prepared by anchoring MnO nanoparticles into 3D matrix of rGO hydrogel followed by annealing process. The composite which has homogeneous distribution of MnO particles on conducting rGO layers demonstrated superior electrochemical performance such as high reversible capacity, stable cycle life and better rate capability. It has shown initial discharge capacity of 2358 mAh g⁻¹ and retained 570 mAh g⁻¹ after 100 cycles as compared to pristine MnO which shown initial discharge capacity of 820 mAh g⁻¹ and retained only 45 mAh g⁻¹ after 100 cycles. The retained capacity of new MnO/rGO anode is much higher than the theoretical capacity of conventional graphite anode. Moreover, the MnO/rGO nanocomposite shows six times higher Li⁺ ion diffusion of 4.18 × 10⁻¹² cm² s⁻¹ as compared to 6.84 × 10⁻¹³ cm² s⁻¹ of MnO. In addition, the study provides insight of charge-discharge process, which conducted in initial, discharge and charge states of pristine MnO and MnO/rGO composite using ex-situ X-ray diffraction and X-ray photon spectroscopy techniques.