rGO Layers Research Articles

Sulfur-Doped and Bio-Resin-Derived Hard Carbon@rGO Composites as Sustainable Anodes for Lithium-Ion Batteries.

Hard carbon derived from fossil products is widely used as anode material for lithium-ion batteries. However, there are still several main shortcomings such as high cost, and poor rate performance, which restrict its wide application. Then tremendous efforts have been devoted to developing biomaterials in the battery applications. Recently, especially agricultural and industrial by-products have attracted much attention due to the electric double-layer capacitors. Herein, we report the sulfur-doped hard carbon (SHC) materials from the tannin-furanic resins (TF-Resin) of the derived agricultural by-products, followed by enveloping rGO on its surface through the hexadecyl trimethyl ammonium bromide. SHC provides sites for the storage of lithium, while the rGO layers can offer a highly conductive matrix to achieve good contact between particles and promote the diffusion and transport of ions and electrons. As a result, the SHC@rGO shows excellent lithium storage performance with initial discharge capacity around 746 mAh g−1 at a current density of 50 mA g−1, and shows superb stability keeping capacity retention of 91.9% after 200 cycles. Moreover, even at a high current density of 2,000 mAg−1, SHC@rGO still delivers a specific capacity of 188 mAg−1. These desired promising properties are active to the implement in the possible practical application.

Cobalt Phosphide-Embedded Reduced Graphene Oxide as a Bifunctional Catalyst for Overall Water Splitting.

It is highly desirable to design high-efficiency stable and low-price catalysts in the electrocatalysis field. Herein, we reported a cobalt phosphide (Co2P)-loaded reduced graphene oxide (rGO) composite catalyst (rGO/Co2P) prepared via the convenient hydrothermal and H2 reduction methods. The rGO/Co2P catalyst reduced at 800 °C (rGO/Co2P-800) shows superior electrocatalytic activities for hydrogen evolution reaction and oxygen evolution reaction in 1.0 M KOH solution, achieving an overpotential of 134 and 378 mV, respectively, at a current density of 10 mA cm–2. Moreover, the catalyst can not only maintain stability for a long time in alkaline solution but also in acid media because of the protection of the rGO layers. The superior performance of this catalyst is attributed to the synergy between the carbon layer and transition-metal phosphides. The Co2P nanoparticles have a high degree of dispersion, which prevents agglomeration, thereby exposing more active sites. Moreover, rGO protects the exposed metal particles while providing more electroconductivity to the material. This work provides an efficient route for the development of bifunctional electrocatalysts with excellent performance and stability, which provides new ideas toward overall water splitting.

Highly Thermally Conductive Dielectric Nanocomposites with Synergistic Alignments of Graphene and Boron Nitride Nanosheets

AbstractElectrically insulating polymer dielectrics with high energy densities and excellent thermal conductivities are showing tremendous potential for dielectric energy storage. However, the practical application of polymer dielectrics often requires mutually exclusive multifunctional properties such as high dielectric constants, high breakdown strengths, and high thermal conductivities. The rational assembly of 2D nanofillers of boron nitride nanosheets (BNNS) and reduced graphene oxide (rGO) into a well‐aligned micro‐sandwich structure in polyimide (PI) composites is reported. The alternating stacking of rGO and BNNS synergistically exploits the large difference in their electrical conductivities to yield a high dielectric constant with a moderate breakdown strength. Moreover, the distinctively separated rGO and BNNS layers give rise to higher thermal conductivities of composites than those containing mixed fillers because of reduced phonon scattering at the interfaces between two identical fillers, as verified by molecular dynamics simulations. Consequently, the micro‐sandwich nanocomposite prevails over the PI film with a simultaneously high dielectric constant of ≈579, a high energy density (43‐fold higher than PI) and an excellent thermal conductivity (11‐fold higher than PI) at a low hybrid filler content of only 2.5 vol%. The multifunctional nanocomposites developed in this work are promising for flexible dielectrics with excellent heat dissipation.

ELECTROSORPTION OF SALT IONS USING TITANIUM DIOXIDE-DECORATED rGO AS A PROMISING ELECTRODE CANDIDATE

Transition metal oxide (TMO) nanoparticles are particularly important in capacitive deionization desalination due to their wide areas of application. The performance of rGO decorated with TiO2 nanoparticles has been investigated. Hammer method synthesizes the electrodes, followed by a hydrothermal treatment. The morphological properties of the rGO and rGO-15TiO2 were characterized by FE-SEM and TEM measurements. The TEM images indicated the random distribution of titanium oxides in the rGO layer, while SEM images showed that the TiO2 homogeneously covered and distributed on rGO sheets for rGO-15TiO2. The performance of the prepared nanocomposite electrode's electrochemical conduct was assessed by cyclic voltammetry (CV) ex-situ. In the three-electrode system, was observed from -0.6 to 0.6 V in 1 M aqueous NaCl. The results illustrated that rGO-15(TiO2) electrode showed the best performance in the CDI, achieving the highest specific capacitance of 473.9/g at 25mV/sec. The results indicating that rGO-15TiO2 is a novel electrode material for CDI.

Alternate Layers of Cobalt Doped Tungsten Oxide and Reduced Graphene Oxide Composite as Electrode Material for Supercapacitor

Composites consist of layers of cobalt doped tungsten oxide and reduced graphitic oxide was prepared using jet nebulized spray pyrolysis technique. A mixture of ammonium tungstate and cobalt acetate solution was coated on glass substrate at 380 °C using jet nebulized spray pyrolysis technique. Spray technique is used to form the reduced graphene oxide (rGO) layer over the first layer and again ammonium tungstate and cobalt acetate solution coated over the surface of rGO and then calcined in nitrogen atmosphere at 450 °C for 2 h. The composite were characterized using XRD, SEM, PL, CV and EIS studies. The SEM and XRD reveal the layered structure. Cyclic voltammetry reveals the high specific capacitance 500 Fg-1 at 5 mV s-1 scan rate for 2 M Na2SO4 as electrolyte. Layered structure of CoWO4/rGO/CoWO4 composite enhances the specific capacitance. EIS gives the electrical series resistance of 7.9 Ω. The layered CoWO4/rGO/CoWO4 composite can be suitable electrode materials for supercapacitor.

Development of a new synthetic strategy for highly reduced graphene oxide-CdS quantum-dot nanocomposites and their photocatalytic activity

Cadmium sulfide (CdS) is a vital wide-bandgap semiconductor that plays an important role in many fields. In this study, we grew CdS quantum dots on graphene oxide (GO) sheets using a novel chemical bath deposition approach. Highly reduced graphene oxide-CdS (rGO-CdS) nanocomposites were successfully obtained. H2S/N2 gas played a key role in the preparation of these nanocomposites, as it provided a source of S and reduced the GO. The size distribution and crystal structure of the CdS quantum dots and the reduction degree of GO in the rGO-CdS nanocomposites were investigated using various characterization techniques and the results showed that the GO was successfully reduced. Furthermore, CdS quantum dots (approximately 5–6 nm in size) with a hexagonal wurtzite structure were uniformly distributed on the rGO layers. The pollutant degradation performances of the rGO-CdS nanocomposites under visible light were studied, and the degradation mechanism was analyzed in detail.

A novel dual-tasking hollow cube NiFe2O4-NiCo-LDH@rGO hierarchical material for high preformance supercapacitor and glucose sensor

Binary transition metal oxides as electroactive materials have continuously aroused grumous attention due to their high theoretical specific capacitance, high valtage window, and multiple oxidation states. However, the tiny specific surface area, poor conductivity and unsatisfactory cycle stability limit their practical application. Hence, a synthetic strategy is designed to fabricate a dual-tasking hollow cube nickel ferrite (NiFe2O4) - based composite (NiFe2O4-NiCo-LDH@rGO) with hierarchical structure. The composite is constructed by firstly preparing hollow NiFe2O4 from cube-like Ni - Fe bimetallic organic framework (NiFe-MOF), and then integrating nickel cobalt layered double hydroxide (NiCo-LDH) nanowires, together with reduced graphene oxide (rGO) via pyrolysis in conjuction with hydrothermal method. The NiFe2O4 possessing cubic hollow structure contributes to a huge accessible surface area, meanwhile alleviates large volume expansion/contraction effect, which facilitates suffcient permeation of the electrolyte and rapid ion/charge transport, and results in high cycling stability. The introduction of layered NiCo-LDH results in hierarchical structure and thus offers maximum contact areas with electrolyte, which heightens the specific capacitance of obtained composite and enhances the electro-catlytic activity towards oxidation of glucose. Furthermore, rGO layer greatly improves the electrical conductivity and ion diffusion/transport capability of composite. Benefiting from the unique structure and individual components of NiFe2O4-NiCo-LDH@rGO composite, the electrode delivers a high specific capacitance (750 C g−1) and superb durability. Simultaneously, the asymmetrical device based on NiFe2O4-NiCo-LDH@rGO as positive electrode delivers remarkable energy density (50 Wh kg−1). Moreover, NiFe2O4-NiCo-LDH@rGO exhibits good sensing performance with a sensitivity of 111.86 µA/µM cm−2, the wide linear range of 3.500 × 10−5 − 4.525 × 10−3 M, and the detection limit of 12.94 × 10−6 M with a signal to noise ratio of 3. Consequently, the NiFe2O4-NiCo-LDH@rGO could provide a prospective notion constructing bifunctional materials with hollow-cube hierarchical structure in the field of supercapacitors and electrochemical sensors.

Facile synthesis of ultrafine rGO/WO3 nanowire nanocomposites for highly sensitive toxic NH3 gas sensors

We introduce a facile and scalable synthesis of rGO/WO3 nanocomposites by hydrothermal method for gas sensing applications. The characterization of rGO/WO3 nanocomposites by some advanced techniques such as scanning electron microscopy, high resolution transmission electron microscopy and Raman spectroscopy revealed that high quality rGO/WO3 nanocomposites included single crystal WO3 nanowires (average diameter of 10 nm) entangled by thin rGO layers. Sensing measurements demonstrated that the rGO/WO3 nanocomposite-based sensor can detect highly toxic NH3 gas at low concentrations ranging from 20 ppm to 500 ppm and fulfills practical applications. The developed gas sensors based on rGO/WO3 nanocomposites have significant application prospects in environment pollution monitoring at detection limit of about 138 ppb. We also discussed the gas sensing mechanism of the rGO/WO3 nanocomposites based on the p-n junction.

Colloidal dispersion of Nb2O5/reduced graphene oxide nanocomposites as functional coating layer for polysulfide shuttle suppression and lithium anode protection of Li-S battery

Functional separator, which bridges anode, electrolyte and cathode together, has the potential to offer a good solution for efficient polysulfide diffusion inhibition and anode protection of Li-S battery. Herein, a novel ultra-thin multifunctional separator is prepared by a facile coating of colloidal dispersion of Nb2O5/reduced graphene oxide nanocomposites (rGO) onto porous polypropylene (PP) matrix. Benefiting from the physical blocking effect of rGO layer and chemisorption of Nb2O5, the shuttle of polysulfides has been greatly suppressed. Meanwhile, the rGO layer functioning as a conductive upper current collector can improve the sulfur utilization, while the Nb2O5 with high activity promotes the transformation of sulfur-containing species. With the assistant of Nb2O5-rGO function layer, the sulfur cathode shows significantly improved electrochemical performance with a high specific capacity of 1328 mAh g−1 at 0.2C and 754 mAh g−1 retained after 200 cycles. The sulfur cathode also exhibits excellent rate capability and stable Coulombic efficiency of 91% without the addition of LiNO3 in the electrolyte. Moreover, the presence of thin Nb2O5-rGO layer also prevents the lithium surface corrosion and the dendrite growth in the lithium anode.

Synthesis and characterisation of reduced graphene oxide-boron nitride nanotubes hybrid

Boron nitride nanotubes (BNNTs) and reduced graphene oxide (rGO) have unique features but show different properties. Typically, BNNT with a bandgap of 6 eV is an electrically insulating material, while graphene can be a metallic or semiconducting material. In this study, rGO was synthesised by Hummer's method and reduction process. BNNTs were produced from co-precipitation. The production of a new hybrid structure of rGO and BNNTs was performed by using the liquid-phase exfoliation and sonication method. The morphological evaluation of the hybrid showed that layers were successfully exfoliated. The BNNTs were effectively incorporated with rGO layers. The composite layers were overlapped and transparent, where the BNNTs under the rGO layers were clearly visible. The absorption analysis showed that the hybrid absorbed more light than pure rGO and BNNTs. As a result of the analyses, it was evident that the hybrid nanocomposite containing rGO and BNNTs were successfully produced.

Ultrafine Ru nanoclusters anchored on cucurbit[6]uril/rGO for efficient hydrogen evolution in a broad pH range.

We report for the first time ultrafine Ru nanoclusters (NCs) anchored on thin reduced graphene oxide (rGO) via introduction of the macrocyclic compound cucurbit[6]uril (CB[6]). The nanocomposite exhibits a comparable or better HER performance compared to Pt/C in acidic, alkaline as well as neutral media, which greatly broadens its application in different types of electrolyzers.

In Situ Growth of CuWO4 Nanospheres over Graphene Oxide for Photoelectrochemical (PEC) Immunosensing of Clinical Biomarker.

Procalcitonin (PCT) protein has recently been identified as a clinical marker for bacterial infections based on its better sepsis sensitivity. Thus, an increased level of PCT could be linked with disease diagnosis and therapeutics. In this study, we describe the construction of the photoelectrochemical (PEC) PCT immunosensing platform based on it situ grown photo-active CuWO4 nanospheres over reduced graphene oxide layers (CuWO4@rGO). The in situ growth strategy enabled the formation of small nanospheres (diameter of 200 nm), primarily composed of tiny self-assembled CuWO4 nanoparticles (2–5 nm). The synergic coupling of CuWO4 with rGO layers constructed an excellent photo-active heterojunction for photoelectrochemical (PEC) sensing. The platform was then considered for electrocatalytic (EC) mechanism-based detection of PCT, where inhibition of the photocatalytic oxidation signal of ascorbic acid (AA), subsequent to the antibody–antigen interaction, was recorded as the primary signal response. This inhibition detection approach enabled sensitive detection of PCT in a concentration range of 10 pg·mL−1 to 50 ng.mL−1 with signal sensitivity achievable up to 0.15 pg·mL−1. The proposed PEC hybrid (CuWO4@rGO) could further be engineered to detect other clinically important species.

Engineering of a bowl-like Si@rGO architecture for an improved lithium ion battery via a synergistic effect

In this work we propose a facile template-sacrificing method to prepare bowl-like silicon@reduced graphene oxide (Si@rGO) hybrids as a high-performance anode for lithium ion batteries (LIBs). Uniform SiO2 spheres were initially synthesized and wrapped by GO, forming a three-dimensional (3D) skeleton. After reduction and etching, Si nanoparticles were obtained and evenly distributed on the flexible rGO layer, resulting in a bowl-like nanoarchitecture. A benefit of this novel structure is that the volume change of Si can be confined during the charge–discharge process. As a result, the Si@rGO anode exhibited a high first discharge capacity of ∼1890 mAh g−1 with a Coulombic efficiency of 90.79% at a current density of 0.1 A g−1. After 100 cycles, a stable specific capacity of 450 mAh g−1 was achieved, which is twice that of pure Si nanospheres (208 mAh g−1) and rGO (260 mAh g−1). Moreover, when the current density increased to 1 A g−1, the specific capacity of Si@rGO was 100 mAh g−1, whereas it was 34 mAh g−1 for Si nanospheres, demonstrating the advantage of Si@rGO. By analyzing the electrochemical behavior, it is found that the outstanding LIB performance of Si@rGO can be ascribed to the involvement of rGO which constructs the 3D nanoarchitecture that acts as a buffer layer to stabilize the Si and promotes Li+ diffusion as well as the conductivity of the electrodes. This work highlights the significance of the microstructure for lithium ion storage performance of Si-based nanocomposites.

An ultralight, supercompressible, superhydrophobic and multifunctional carbon aerogel with a specially designed structure

Compressible and ultralight carbon aerogels are attractive due to its compressibility, elasticity and conductivity. However, it is still a great challenge to enrich the properties of carbon aerogel to meet various requirements. Herein, we report an untralight, supercompressible, fatigue resistant, superhydrophobic and fire-resistant and multifunctional CNF-GO/glucose-kaolin carbon aerogel (C-NGGK) carbon aerogel. To achieve such excellent performances, calcined GO, CNFs, glucose and kaolin are used for forming low-density and continuous wave-shape rGO layers, reinforcing the mechanical strength of carbon layers, realizing superelasticity and fatigue resistance and resulting in a superhydrophobic surface for C-NGGK, respectively. The as-prepared C-NGGK demonstrates excellent superhydrophobicity with the water contact angle (WCA) of 124.9°, and the absorption efficiency of the C-NGGK samples for different oils and organic solvents are 75–255 times their own weight. These advantages show that the C-NGGK can be an ideal candidate for oil/water separation. In addition, there is also the prospect to be used for pressure sensors, while other potential applications include three-dimensional electrode materials for supercapacitors and batteries, catalyst carriers and various wearable devices.

Sparked Reduced Graphene Oxide for Low-Temperature Sodium-Beta Alumina Batteries

Wetting Na metal on the solid electrolyte of a liquid Na battery determines the operating temperature and performance of the battery. At low temperatures below 200 °C, liquid Na wets poorly on a solid electrolyte near its melting temperature (Tm = 98 °C), limiting its suitability for use in low-temperature batteries used for large-scale energy-storage systems. Herein, we propose the use of sparked reduced graphene oxide (rGO) that can improve the Na wetting in sodium-beta alumina batteries (NBBs), allowing operation at lower temperatures. Experimental and computational studies indicated rGO layers with nanogaps exhibited complete liquid Na wetting regardless of the surface energy between the liquid Na and the graphene oxide, which originated from the capillary force in the gap. Employing sparked rGO significantly enhanced the cell performance at 175 °C; the cell retained almost 100% Coulombic efficiency after the initial cycle, which is a substantial improvement over cells without sparked rGO. These results suggest that coating sparked rGO is a promising but simple strategy for the development of low-temperature NBBs.

Influence of carboxymethyl cellulose content on structures and electrochemical behaviors of reduced graphene oxide films

Film-like charge-storage devices have attracted considerable attention owing to the emergence of flexible, bendable and wearable devices. Reduced graphene oxide (rGO), which is similar to graphene, is a promising deformable electrode for charge storage. However, film-like rGO has a densely stacked structure and poor wettability to the electrolyte, which are challenging issues. In this study, carboxymethyl cellulose/graphene oxide (CG) films with different amounts of carboxymethyl cellulose (CMC) were prepared. CMC, which is a derivative of cellulose and a water-soluble polymer, functioned as an activation agent. The CG films were converted into reduced CG (rCG), which had enhanced electrochemical properties because of the increase in the accessible areas to the electrolyte ions and the pseudocapacitance due to the thermal reduction process. The CG films possessed bending property and were retained after reduction. However, the bending property decreased after reduction when the CMC was added in excess. The thickness of the rCG films increased as vacancies were formed between stacked rGO layers, and basic oxygen functional groups capable of electrochemically performing a Faradaic redox reaction were created by CMC activation. The areal specific capacitance of the rCG films increased by a factor of 30 compared with the rGO film before cycling. With increasing charging/discharging cycles, the specific capacitance of the rCG films drastically increased owing to synergistic effects of electro-activation and pseudocapacitance.

Gallium dopant-induced tunable electrical properties of reduced graphene oxide using metal organic chemical vapor deposition

In this study, we develop a gallium-doped reduced graphene oxide (Ga-rGO) material with desirable electrical properties by a metal-organic chemical vapor deposition system using the pulsed mode technique. The effect of doping level on Ga bonding in rGO layer controlled via the pulsed mode of doping conditions, such as dopant source injection and interruption durations, is investigated using the X-ray photoelectron spectroscopy (XPS). The XPS data indicated that the pulsed mode of doping conditions resulted in a higher Ga composition and a larger O–Ga bonding portion in the Ga-rGO layer. Additionally, the Dirac point of Ga-rGO layer with the pulsed mode was observed to shift toward positive voltage in the bottom-gate field effect transistor device, leading to a p-type behavior. The higher work function of Ga-rGO affected the Schottky contact between the Ga-rGO layer and n-type Si leading to lower fermi-energy level in the energy-band structure. Consequently, this work proposes a simplistic approach of Ga doping in graphene materials that can modulate the tunable electrical properties.

Enhanced nucleate boiling using a reduced graphene oxide-coated micropillar

Critical heat flux (CHF) enhancement is necessary in order to ensure a high operating limit for two-phase cooling applications. As the boiling is developed, formation of vapor film layer becomes vigorous, which causes CHF. Here, a graphene-coated micropillar structure (GMS) is proposed in order to enhance boiling heat transfer by suppressing vapor film formation on the surface. The GMS is designed to separate the bubble nucleation region from the liquid supply region in order to enhance CHF. By controlling the height of the micropillar, we obtained a structure in which the rGO layer is coated at the top of the micropillar array with high aspect ratio of the micropillar. In particular, the GMS consists of a reduced graphene oxide (rGO) porous mesh layer and a micropillar array layer. The rGO porous structure facilitated bubble nucleation by providing a suitably sized cavity. The micropillar array, which has excellent wicking performance, is located below the rGO porous layer in order to provide a capillary pumping to the vapor bubbles. Consequently, the GMS provides a significantly improved heat transfer coefficient and CHF of 288% and 152%, respectively, compared to the plain surface.

Exfoliating two-dimensional materials into few layers via optimized environmentally-friendly ternary solvents

Exfoliation of two-dimensional (2D) materials is an issue of concern among scientific researchers. This is because many solvents such as N, N-dimethylformamide and N-methyl-2-pyrrolidone that are capable of better dispersion of 2D materials are relatively toxic and nonvolatile. This work focused on the reasonable design and mixture of two or three less toxic and volatile solvents based on Hansen solubility parameters theory to demonstrate the excellent exfoliation of 2D materials particularly reduced graphene oxide (rGO) and black phosphorus (BP). Polyvinylpyrrolidone (PVP) was introduced as a surfactant to functionalize rGO to help improve its dispersion. Results showed that PVP could effectively functionalize graphene. Few layers of rGO and BP were facilely achieved with 2–3 nm thickness from the use of the designed solvent mixtures, indicating the accomplishment of solvent mixtures in exfoliation/dispersion roles instead of the use of other toxic and nonvolatile solvents.

Generation of nanocellular TPU/reduced graphene oxide nanocomposite foams with high cell density by manipulating viscoelasticity

Previous preparation of nanocellular thermoplastic polyurethane (TPU)-based foams was at low foaming temperature and so long saturation time is needed. In this work, nanocellular structure with high cell density (up to 2.52 × 1012 cells/cm3) is developed by foaming TPU nanocomposites with 0.5 and 0.75 vol% reduced graphene oxide (rGO) at higher foaming temperatures. The formation of such nanocellular structure is ascribed to appropriately higher elasticity and obviously higher melt strength of the TPU nanocomposites, which result from a percolation structure of well exfoliated and dispersed rGO layers and the higher temperature. The nanocellular TPU nanocomposite foams with high cell density exhibit obviously higher dielectric permittivity value in low frequency range, which is ascribed to enhanced interfacial polarizations because of higher rGO content and possible dielectric breakdown of gas in large numbers of the nano-sized cells.