rGO Network Research Articles

Tuning the structure and composition of graphite-phase polymeric carbon nitride/reduced graphene oxide composites towards enhanced lithium-sulfur batteries performance

Lithium-sulfur (Li-S) batteries have been attractive alternatives to lithium-ion (Li-ion) batteries due to the high theoretical capacity of sulfur cathode. However, the polysulfide shuttling effect is detrimental to the long-term cycling stability. Chemically absorptive host materials provide an effective way to mitigate the dissolution of lithium polysulfide. Carbon nitride (C3N4) is one of the effective host materials with strong interaction with polysulfide species. The low electronic conductivity, however, is unfavorable for high sulfur utilization. In this work, we report the controlled synthesis of porous, well-interconnected C3N4/reduced graphene oxide (rGO) aerogels as hybrid sulfur host using a simple hydrothermal reaction followed by freeze-drying, which combines the structural merits of both highly conductive rGO networks and chemically active C3N4. By further tuning the structure/morphology and the ratio between C3N4 and rGO, we have demonstrated the C3N4/rGO composites with optimized 1:2 ratio of C3N4:rGO (termed as CG12) exhibits not only very high sulfur utilization but also excellent rate capability compared to other C3N4/rGO composites, pure rGO, and C3N4. The compositionally and structurally tailored CG12 also shows stable cycling performance over 400 cycles with a low decay rate of 0.09% per cycle.

Colloidally synthesized MoSe2 nano-flowers anchored on three-dimensional porous reduced graphene oxide thin films as advanced counter electrode for dye-sensitized solar cells

In this letter, MoSe2 nanosheets with flower-like nanostructure was in-situ grown on pre-coated three-dimensional (3D) porous reduced graphene oxide (rGO) thin films on fluorine-doped tin oxide glass by a facile hydrothermal method. The synergistic effect between the highly catalytic MoSe2 nanostructure and the highly conductive and large surface-area 3D rGO network endows the resultant MoSe2/rGO composite excellent electrocatalytic ability. As expected, dye-sensitized solar cells (DSSCs) prepared with MoSe2/rGO thin films as counter electrode (CE) exhibited a conversion efficiency of 6.56%, which was higher than that of DSSCs with sputtered Pt CE (6.08%).

Red Phosphorus Nanodots on Reduced Graphene Oxide as a Flexible and Ultra-Fast Anode for Sodium-Ion Batteries.

Sodium-ion batteries offer an attractive option for potential low cost and large scale energy storage due to the earth abundance of sodium. Red phosphorus is considered as a high capacity anode for sodium-ion batteries with a theoretical capacity of 2596 mAh/g. However, similar to silicon in lithium-ion batteries, several limitations, such as large volume expansion upon sodiation/desodiation and low electronic conductance, have severely limited the performance of red phosphorus anodes. In order to address the above challenges, we have developed a method to deposit red phosphorus nanodots densely and uniformly onto reduced graphene oxide sheets (P@RGO) to minimize the sodium ion diffusion length and the sodiation/desodiation stresses, and the RGO network also serves as electron pathway and creates free space to accommodate the volume variation of phosphorus particles. The resulted P@RGO flexible anode achieved 1165.4, 510.6, and 135.3 mAh/g specific charge capacity at 159.4, 31878.9, and 47818.3 mA/g charge/discharge current density in rate capability test, and a 914 mAh/g capacity after 300 deep cycles in cycling stability test at 1593.9 mA/g current density, which marks a significant performance improvement for red phosphorus anodes for sodium-ion chemistry and flexible power sources for wearable electronics.

Studies on synergetic effect of rGO-NiCo2S4 nanocomposite as an effective counter electrode material for DSSC

Abstract Nanocomposites of reduced graphene oxide (rGO) and NiCo 2 S 4 with different amount of graphene oxide (GO) are synthesized through a one- step solvothermal method and their catalytic activity towards I - /I 3 - redox electrolyte for dye-sensitized solar cell (DSSC) application are reported. The growth mechanism of the pristine hierarchical marigold like microspheres of NiCo 2 S 4 that formed without rGO and the nanocomposite of rGO-NiCo 2 S 4 are also proposed. Electrochemical studies confirmed the synergetic effect of nickel and cobalt ions with the high electrical conductive rGO networks that enhance the electrocatalytic activity of NiCo 2 S 4 nanostructures. The synergistic effect between NiCo 2 S 4 and rGO may be attributed to the higher conductivity of rGO and the inverse spinel crystal structure of NiCo 2 S 4 that have more octahedral catalytic active sites of Co 3+ . The amount of graphene oxide plays the important role of controlling the DSSC performance and the power conversion efficiency. The efficiency achieved for the rGO-NiCo 2 S 4 counter electrode (CE) based DSSC is 8.15%, which is remarkably higher than that of pristine NiCo 2 S 4 (7.36%), and Pt (7.23%) under the same experimental conditions.

Ultrafine TiO2 and Nb2O5 Nanoparticles on Reduced Graphene Oxide As Anode Materials for Lithium and Sodium Ion Batteries

The potential practical application of electric vehicles and large-scale energy storage system, have prompted extensive research in the development of electrode materials with high performance and high rate capability. Ultrafine TiO2 and Nb2O5 nanocrystals/reduced graphene oxide (TiO2/rGO and Nb2O5/rGO) were prepared by controllable hydrolysis and followed by thermal treatment. First, hydrolysis of titanium or niobium precursor formed amorphous hydroxide thin layer on graphene oxide (GO). After annealing at 450 °C under forming gas, amorphous hydroxide layer converted to TiO2 or Nb2O5 nanocrystals with particle size of around 5 nm or 3 nm, respectively, and GO reduced to rGO with enhanced conductivity. TiO2/rGO and Nb2O5/rGO were demonstrated as promising anode materials for high performance lithium ion batteries and sodium ion batteries. The architecture of ultrafine TiO2 or Nb2O5 nanocrystals anchored on highly conductive rGO network and can not only enhance charge transfer and buffer the volume change during lithiation/delithiation or sodiation/desodiation processes, but also provide more active surface area for lithium or sodium ion storage, resulting in superior rate and cycle performance. Ex situ XPS analysis revealed that the sodium ion storage mechanism in Nb2O5 could be accompanied with Nb5+/Nb4+ redox reaction and the ultrafine Nb2O5 NCs provide more surface area to accomplish the redox reaction.

Exploring molecular and spin interactions of Tellurium adatom in reduced graphene oxide

The transport of spin information fundamentally requires favourable molecular architecture and tunable spin moments to make the medium pertinent for spintronic. We report on achieving coherent molecular-spin parameters for rGO due to Tellurium (Te) adatom. Initially, GO prepared using graphite, was modified into rGO by in situ incorporation of 1 (w/w)% of Te. Both the systems were subjected to ESCA, FTIR, Raman dispersion, ESR spectroscopy, and electron microscopy. Analysis revealed that, Te substantially reacted with epoxides, carbonyl, and carboxylate groups that improved C-to-O ratio by twice. However, the spin splitting character, between Te and C, seems to be quenched. Moreover, Te altered the dynamical force constant between CC and CC that generated the mechanical stress within rGO network. The layer conjugation, nature of folding, symmetry, and electronic states of the edges were also affected by precipitation and entrapment of Te. The calculated dynamic molecular Raman and ESR spin parameters indicated that, Te acted as a bridging element for long range spin transport. This is particularly due to, the p-orbital moments of Te contributing, vectorially, to spin relaxation process operative at broken inversion symmetry sites. Our study suggests that, facile addition of Te in rGO is useful to achieve favourable spintronic properties.

Conversion of p to n-type reduced graphene oxide by laser annealing at room temperature and pressure

Physical properties of reduced graphene oxide (rGO) are strongly dependent on the ratio of sp2 to sp3 hybridized carbon atoms and the presence of different functional groups in its structural framework. This research for the very first time illustrates successful wafer scale integration of graphene-related materials by a pulsed laser deposition technique, and controlled conversion of p to n-type 2D rGO by pulsed laser annealing using a nanosecond ArF excimer laser. Reduced graphene oxide is grown onto c-sapphire by employing pulsed laser deposition in a laser MBE chamber and is intrinsically p-type in nature. Subsequent laser annealing converts p into n-type rGO. The XRD, SEM, and Raman spectroscopy indicate the presence of large-area rGO onto c-sapphire having Raman-active vibrational modes: D, G, and 2D. High-resolution SEM and AFM reveal the morphology due to interfacial instability and formation of n-type rGO. Temperature-dependent resistance data of rGO thin films follow the Efros-Shklovskii variable-range-hopping model in the low-temperature region and Arrhenius conduction in the high-temperature regime. The photoluminescence spectra also reveal less intense and broader blue fluorescence spectra, indicating the presence of miniature sized sp2 domains in the vicinity of π* electronic states, which favor the VRH transport phenomena. The XPS results reveal a reduction of the rGO network after laser annealing with the C/O ratio measuring as high as 23% after laser-assisted reduction. The p to n-type conversion is due to the reduction of the rGO framework which also decreases the ratio of the intensity of the D peak to that of the G peak as it is evident from the Raman spectra. This wafer scale integration of rGO with c-sapphire and p to n-type conversion employing a laser annealing technique at room temperature and pressure will be useful for large-area electronic devices and will open a new frontier for further extensive research in graphene-based functionalized 2D materials.

2D Layered Graphitic Carbon Nitride Sandwiched with Reduced Graphene Oxide as Nanoarchitectured Anode for Highly Stable Lithium-ion Battery

Two dimensional (2D) nanomaterials with high gravimetric capacity and rate capability are a key strategy for the anode of a Li-ion battery, but they still pose a challenge for Li-ion storage due to limited conductivity and an inability to alleviate the volume change upon lithiation and delithiation. In this paper, we report the construction of a 3D architecture anode consisting of exfoliated 2D layered graphitic carbon nitride (g-C3N4) and reduced graphene oxide (rGO) nanosheets (CN-rGO) by hydrothermal synthesis. First, bulk g-C3N4 is converted to nanosheets to increase the edge density of the inert basal planes since the edges act as active Li-storage sites. This unique 3D architecture, which consists of ultrathin g-C3N4 nanosheets sandwiched between conductive rGO networks, exhibits a capacity of 970mAhg−1 after 300 cycles, which is 15 fold higher than the bulk g-C3N4. The tuning of the intrinsic structural properties of bulk g-C3N4 by this simple bottom-up synthesis has rendered a 3D architectured material (CN-rGO) as an effective negative electrode for high energy storage applications.

Ultrafine TiO2 nanoparticles on reduced graphene oxide as anode materials for lithium ion batteries

TiO2 nanoparticles/reduced graphene oxide (TiO2/rGO) was prepared by controllable hydrolysis and followed by thermal treatment. First, hydrolysis of titanium isopropoxide formed amorphous titanium hydroxide thin layer on graphene oxide (GO). After annealing at 450°C under forming gas, amorphous titanium hydroxide layer converted to TiO2 nanoparticles with particle size of around 5nm and GO reduced to rGO with enhanced conductivity. TiO2/rGO was demonstrated as promising anode materials for high performance lithium ion batteries. The high operation potential window of TiO2 in lithium ion battery can prevent the formation of solid electrolyte interface and subsequently enhance the safety and cycle performance. The high rate and high cycling performance of lithium ion batteries could be ascribed to that the ultrafine TiO2 nanoparticles provide high active surface area and the strong connection between TiO2 and high conductive rGO network can enhance the charge transfer.

Ni(OH)2 nanosheets grown on porous hybrid g-C3N4/RGO network as high performance supercapacitor electrode

A porous hybrid g-C3N4/RGO (CNRG) material has been fabricated through a facile hydrothermal process with the help of glucose molecules, and serves as an efficient immobilization substrate to support ultrathin Ni(OH)2 nanosheets under an easy precipitation process. It was found that the g-C3N4 flakes can uniformly coat on both sides of the RGO, forming sandwich-type composites with a hierarchical structure. It is worth noting that the introduction of the g-C3N4 can effectively achieve the high dispersion and avoid the agglomeration of the nickel hydroxide, and significantly enhance the synthetically capacitive performance. Owning to this unique combination and structure, the CNRG/Ni(OH)2 composite possesses large surface area with suitable pore size distribution, which can effectively accommodate the electrolyte ions migration and accelerate efficient electron transport. When used as electrode for supercapacitor, the hybrid material exhibits high supercapacitive performance, such as an admirable specific capacitance (1785 F/g at a current density of 2 A/g), desirable rate stability (retain 910 F/g at 20 A/g) and favorable cycling durability (maintaining 71.3% capacity after 5000 cycles at 3 A/g). Such desirable properties signify that the CNRG/Ni(OH)2 composites can be a promising electrode material in the application of the supercapacitor.

Controllable nanoscale engineering of vertically aligned MoS2 ultrathin nanosheets by nitrogen doping of 3D graphene hydrogel for improved electrocatalytic hydrogen evolution

Three-dimensional (3D) porous molybdenum disulfide/nitrogen-doped reduced graphene oxide (MoS2/N-rGO) hydrogels were fabricated through a facile and controllable one-pot hydrothermal method. The nanosized MoS2 ultrathin nanosheets were uniformly and vertically dispersed on the rGO framework after nitrogen incorporation. The incorporated nitrogen in rGO played a key role for nano-scaling of MoS2 due to the protonation at pyridinic N-doping sites on carbon surface. The vertically aligned edge of nanosized MoS2 sheets, nitrogen incorporation of rGO and 3D network structure made the MoS2/N-rGO highly efficient for hydrogen evolution reaction, with improved double-layer capacitance and turnover frequency, small onset overpotential of 119 mV, low Tafel slope of 36 mV·decade−1 and superior long-time catalytic stability.

Preparation and characterization of PEDOT:PSS/reduced graphene oxide–carbon nanotubes hybrid composites for transparent electrode applications

Hybrid composites have been prepared by solution processing method using various MWCNT concentrations in reduced graphene oxide/poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (rGO-PEDOT:PSS) matrix. Structural and morphological characteristics of the prepared composite films have been characterized by X-ray diffraction, scanning electron microscopy, field emission scanning electron microscopy, Raman spectroscopy, UV–visible spectroscopy and Fourier transform infrared spectroscopy. The electrical transport measurement of composites conducted by four-point contacts shows increase in conductivity with MWCNT wt% in the composite system. The enhanced electrical conductivity as a result of improved dispersion of MWCNTs and rGO has been observed barely affecting the optical properties in the visible region. The observed improvement in electrical properties barely affecting its optical properties in the visible region can be attributed to synergistic effect of MWCNTs and rGO networks on the composite. This result suggests that such hybrid composite materials could be used as transparent conductor applications in optoelectronic devices.

A gigantically increased ratio of electrical to thermal conductivity and synergistically enhanced thermoelectric properties in interface-controlled TiO2-RGO nanocomposites.

We report synergistically enhanced thermoelectric properties through the independently controlled charge and thermal transport properties in a TiO2-reduced graphene oxide (RGO) nanocomposite. By the consolidation of TiO2-RGO hybrid powder using spark plasma sintering, we prepared an interface-controlled TiO2-RGO nanocomposite where its grain boundaries are covered with the RGO network. Both the enhancement in electrical conductivity and the reduction in thermal conductivity were simultaneously achieved thanks to the beneficial effects of the RGO network, and detailed mechanisms are discussed. This led to the gigantic increase in the ratio of electrical to thermal conductivity by six orders of magnitude and also the synergistic enhancement in the thermoelectric figure of merit by two orders. Our results present a strategy for the realization of 'phonon-glass electron-crystals' through interface control using graphene in graphene hybrid thermoelectric materials.

Reduced graphene oxide supported hierarchical flower like manganese oxide as efficient electrocatalysts toward reduction and evolution of oxygen

The slow kinetics of the electrochemical oxygen reduction reaction (ORR) is a major bottleneck for the development of alkaline fuel cells. Herein, a facile and effective hydrothermal route has been developed to synthesize hierarchical with controllable structures of manganese oxides (Mn2O3) and reduced graphene oxides supported Mn2O3 composites. The hierarchal metal oxides were formed by self-assembly of very small Mn2O3 nanoparticles as evident from transmission electron microscopy. The nature of chelating agents has found to be the key factor whose subtle variation can effectively control the morphology of the Mn2O3. Electrochemical investigations indicate that the both pure and supported Mn2O3 demonstrate bifunctional catalytic activity toward the four-electron electrochemical reduction of oxygen and evolution reaction in alkaline media. Due to the 3D assembled hierarchical architecture of Mn2O3 composed of rose- and petal-like nano-particle arrays and the interconnection of reduced graphene oxides (rGO) network, as well as the synergetic effects of rGO as a support, revealing the pronounce impact on the electrocatalytic activity the Mn2O3 composites with high current density and excellent durability as cathode material which is comparable with other electrocatalysts. The synthetic approach provides a general platform for fabricating well-defined pure and supported metal oxides with prospective applications as low-cost catalysts for alkaline fuel cells.

A Safe High-Performance All-Solid-State Lithium-Vanadium Battery with a Freestanding V2O5 Nanowire Composite Paper Cathode.

The electronic conductivity and structural stability are still challenges for vanadium pentoxide (V2O5) as cathode materials in batteries. Here, we report a V2O5 nanowire-reduced graphene oxide (rGO) composite paper for direct use as a cathode without any additives for high-temperature and high-safety solid polymer electrolyte [PEO-MIL-53(Al)-LiTFSI] lithium-vanadium batteries. The batteries can show a fast and stable lithium-ion-storage performance in a wide voltage window of 1.0-4.0 V versus Li+/Li at 80 °C, in which with an average capacity of 329.2 mAh g-1 at 17 mA g-1 and a stable cycling performance over 40 cycles are achieved. The excellent electrochemical performance is mainly ascribed to integration of the electronic conductivity of rGO and interconnected networks of the V2O5 nanowires and solid electrolyte. This is a promising lithium battery for flexible and highly safe energy-storage devices.

Free-standing electrodes composed of carbon-coated Li 4 Ti 5 O 12 nanosheets and reduced graphene oxide for advanced sodium ion batteries

Abstract A free-standing electrode composed of carbon-coated Li 4 Ti 5 O 12 nanosheets and reduced graphene oxide (designated as LTO-C/RGO) is fabricated for Na storage by modified vacuum filtration and subsequent annealing. In this process, graphene oxide with negative charges and LTO-C nanosheets with abundant charged ions are self-assembled into the nanocomposite based on the screening effect of electrostatic repulsion. The unique structure of the confined LTO-C nanosheets in a highly conductive interconnected RGO network not only promotes the reaction kinetics and structural stability of the electrodes during Na + insertion/extraction, but also provides plenty of interfacial sites for Na + adsorption giving rise to additional interfacial Na storage. The free-standing LTO-C/RGO anode for sodium ion battery exhibits a high capacity of 166 mAhg −1 at 1 C, good rate capability of 98.7 mAhg −1 at 5 C, and superior cyclic performance of 114 mAhg −1 at 2 C after 600 cycles. The materials boasting superior Na storage have large potential in high-performance sodium ion batteries in portable, flexible and wearable electronics.

Controlled synthesis of three-dimensional reduced graphene oxide networks for application in electrode of supercapacitor

Three-dimensional reduced graphene oxide network deposition on Ni foam (3D rGO/NF) was successfully synthesized by a new process. Here, the structure and loading of rGO deposition on Ni foam could be easily controlled by the synthesis conditions. Furthermore, it was directly applied as the electrode of supercapacitor, in which the electrochemical behaviors of 3D rGO/NF with various structures were further compared in detail. As a result, the rGO film deposition on backbones of Ni foam electrode delivered a higher specific capacitance comparing to rGO aerogel deposition on Ni foam. The result was attributed to that the direct and close contact between rGO film and Ni foam was beneficial to the enhanced conductivity of the electrode, as well as the improvement of ion diffusion/transport into the electrode for the rGO film deposition on backbones of Ni foam. The works provided a new way to controlled synthesis of rGO network at large scale for in various applications.

Ultrafine Nb2O5 Nanocrystal Coating on Reduced Graphene Oxide As Anode Materials for Sodium Ion Battery

Sodium ion battery (SIB) is a promising candidate for large-scale and practical application because of its lower costs and widespread of sodium resource.1 However, for SIBs, the intrinsic difference between sodium and lithium (ca. 55% larger and 330% heavier) lead to slower kinetics and lower energy densities.1 Nevertheless, it is reported that the enhance SIBs performance could be obtained by introducing the proper sodium host.2 For example, recent advances indicated that the ordered mesoporous Nb2O5-C delivered a reversible capacity of ~175 mA h/g and excellent cycle stability (up to 300 cycles).3 However, the low rate performance (<100 mA h/g at rate of 1A/g) cannot meet the requirements of fast charge-discharge electrochemical devices. In this work, ultrafine Nb2O5 nanocrystal/reduced graphene oxide (Nb2O5/rGO) was demonstrated as a promising anode material for sodium ion battery with high rate performance and high cycle durability. The Nb2O5/rGO was synthesized by controllable hydrolysis of niobium ethoxide and following treated at high annealing temperature in forming gas. Transmission electron microscopy (TEM) showed that ultrafine Nb2O5 nanocrystal with average particle size of 3 nm was uniformly deposited on rGO. Highly conductive rGO network and ultrafine Nb2O5 not only can enhance charge transfer and buffer the volume change during sodiation/desodiation process, but also could provide more active surface area for sodium ion storage, resulting in superior rate performance and cycle performance. XPS and XRD analyses revealed that the sodium ion storage mechanism in Nb2O5 could be accompanied with Nb5+/Nb4+ redox reaction and the ultrafine Nb2O5 nanocrystal provide more surface area to finish the redox reaction. Reference: 1. N. Yabuuchi, K. Kubota, M. Dahbi and S. Komaba, Chemical Reviews, 2014, 114, 11636-11682. 2. Y.-U. Park, D.-H. Seo, H.-S. Kwon, B. Kim, J. Kim, H. Kim, I. Kim, H.-I. Yoo and K. Kang, Journal ofthe American Chemical Society, 2013, 135, 13870-13878. 3. H. Kim, E. Lim, C. Jo, G. Yoon, J. Hwang, S. Jeong, J. Lee and K. Kang, Nano Energy, 2015, 16, 62-70.

Ultrafine Nb2O5 Nanocrystal Coating on Reduced Graphene Oxide as Anode Material for High Performance Sodium Ion Battery.

Ultrafine niobium oxide nanocrystals/reduced graphene oxide (Nb2O5 NCs/rGO) was demonstrated as a promising anode material for sodium ion battery with high rate performance and high cycle durability. Nb2O5 NCs/rGO was synthesized by controllable hydrolysis of niobium ethoxide and followed by heat treatment at 450 °C in flowing forming gas. Transmission electron microscopy images showed that Nb2O5 NCs with average particle size of 3 nm were uniformly deposited on rGO sheets and voids among Nb2O5 NCs existed. The architecture of ultrafine Nb2O5 NCs anchored on a highly conductive rGO network can not only enhance charge transfer and buffer the volume change during sodiation/desodiation process but also provide more active surface area for sodium ion storage, resulting in superior rate and cycle performance. Ex situ XPS analysis revealed that the sodium ion storage mechanism in Nb2O5 could be accompanied by Nb(5+)/Nb(4+) redox reaction and the ultrafine Nb2O5 NCs provide more surface area to accomplish the redox reaction.

Real-time and in situ enzyme inhibition assay for the flux of hydrogen sulfide based on 3D interwoven AuPd-reduced graphene oxide network

A highly sensitive enzyme inhibition analytical platform was established firstly based on paper-supported 3D interwoven AuPd-reduced graphene oxide (rGO) network (NW) for real-time and in situ analysis of H2S released from cancer cells. The novel paper working electrode (PWE) with large electric conductivity, effective surface area and unusual biocompatibility, was fabricated via controllably assembling rGO and AuPd alloy nanoparticles onto the surface of cellulose fibers and into the macropores of paper, which was employed as affinity matrix for horseradish peroxidase (HRP) loading and cells capture. It was the superior performances of AuPd-rGO-NW-PWE that made the loaded HRP exhibit excellent electrocatalytic behavior to H2O2, bring the rapid enhancement of current response. After releasing H2S, the current response would be obviously decreased due to the efficient inhibition effect of H2S on HRP activity. The inhibition degree of HRP was directly proportional to the amount of H2S, and so, the flux of H2S released from cells could be recorded availably. Thus, this proposed enzyme inhibition cyto-sensor could be applied for efficient recording of the release of H2S, which had potential utility to cellular biology and pathophysiology.