Graphene oxide reduction by standard industrial reducing agent: thiourea dioxide

The surface chemistry of graphene oxide (GO) can be modified by the chemical reduction of oxygen-containing groups using L-ascorbic acid (L-AA). Being able to "tune" the surface hydrophobicity of GO in a controlled manner, with a well-defined level of reduction, provides a valuable tool for understanding and controlling interactions with hydrophobic surfaces. Numerous analytical and chemical methods have been used to determine the extent of reduction in chemically reduced graphene oxide (CRGO) samples. However, many of these methods are limited by their laborious nature, cost, or lack of sensitivity in resolving oxygen content in samples that have only been reduced for short periods of time, making them inappropriate for rapid use with multiple samples. Here, we have used ultraviolet (UV), Raman, and attenuated total reflection infrared (ATR-IR) spectroscopy to monitor the chemical reduction of GO. These three techniques are simple, rapid, nondestructive, accurate, and widely available. The data set from each technique has been correlated and modeled against a reference data set (carbon to oxygen ratio obtained from elemental analysis) using partial least squares regression (PSLR). Using this approach, the chemical reduction of GO was quantified from UV (r2 = 0.983, RMSECV = 0.049), Raman (r2 = 0.961, RMSECV = 0.073) and ATR-IR (r2 = 0.993, RMSECV = 0.032) data. ATR-IR enabled identification of the different oxygen-containing groups on GO, and coupled with chemometric modeling, provides an excellent approach for the routine quantitative analysis of the chemical reduction of GO.

Chemical reduction of graphene oxide (GO) in an aqueous polymer/GO dispersion invariably causes restacking and precipitation of reduced GO (rGO) sheets leading to spontaneous phase separation. This has been a significant challenge limiting the commercial production of aqueous polymer/rGO paints and coatings. In this study, we developed in situ reduction strategies to chemically reduce GO while maintaining the colloidal stability of an aqueous polymer/rGO dispersion. An aqueous polymer/GO latex was prepared using miniemulsion polymerization, which was subsequently reduced in situ using four different chemical reducing agents (hydroiodic acid, sodium borohydride, ascorbic acid, and hydrazine hydrate) to obtain colloidally stable polymer/rGO dispersions. The reduction of GO was confirmed using X-ray photoelectron spectroscopy (XPS), Raman, and X-ray diffraction (XRD). The colloidal stability of the reduced dispersions was maintained for over 6 months under ambient storage conditions. Stable polymer/rGO dispersions were drop cast to form nanocomposite films at ambient temperature. Finally, the electrical conductivity measurement of polymer/rGO films revealed the highest conductivity for hydrazine hydrate-reduced films (∼6.6 S m–1), which was marginally higher than for hydroiodic acid-reduced films (∼3.4 S m–1). The electrical conductivity of sodium borohydride- and ascorbic acid-reduced films was two orders of magnitude lower than for hydrazine hydrate- and hydroiodic acid-reduced films. Overall, the present results pave a path for the commercial production of colloidally stable aqueous polymer/rGO composite paints and coatings.

Graphene, a single layer of sp2-hybridized carbon atoms in a hexagonal (two-dimensional honey-comb) lattice, has attracted strong scientific and technological interest due to its novel and excellent optical, chemical, electrical, mechanical and thermal properties. The solution-processable chemical reduction of Graphene oxide (GO is considered as the most favorable method regarding mass production of graphene. Generally, the reduction of GO is carried out by chemical approaches using different reductants such as hydrazine and sodium borohydride. These components are corrosive, combustible and highly toxic which may be dangerous for personnel health and the environment. Hence, these reducing agents are not promising choice for reducing of graphene oxide (GO). As a consequence, it is necessary for further development and optimization of eco-friendly, natural reducing agent for clean and effective reduction of GO. Ascorbic acid, an eco-friendly and natural reducing agents, having a mild reductive ability and nontoxic property. So, the aim of this research was to green synthesis of GO with ascorbic acid. For this purpose, the required amount of NaOH and ascorbic acid were added to GO solution (0.5 mg/ml) and were heated at 95 °C for 1 hour. According to the X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and electrochemical results, GO were reduced with ascorbic acid like hydrazine with better electrochemical properties and ascorbic acid is an ideal substitute for hydrazine in the reduction of graphene oxide process.

The reduction of graphene oxide (GO) thin films deposited on substrates is crucial to achieve a technologically useful supported graphene material. However, the well-known thermal reduction process cannot be used with thermally unstable substrates (e.g., plastics and paper), in addition photo-reduction methods are expensive and only capable of reducing the external surface. Therefore, solid-state chemical reduction techniques could become a convenient approach for the full thickness reduction of the GO layers supported on thermally unstable substrates. Here, a novel experimental procedure for quantitative reduction of GO films on paper by a green and low-cost chemical reductant (L-ascorbic acid, L-aa) is proposed. The possibility to have an effective mass transport of the reductant inside the swelled GO solid (gel-phase) deposit was ensured by spraying a reductant solution on the GO film and allowing it to reflux in a closed microenvironment at 50 °C. The GO conversion degree to reduced graphene oxide (r-GO) was evaluated by Fourier transform infrared spectroscopy (FT-IR) in attenuated total reflectance (ATR) mode and X-ray Diffraction (XRD). In addition, morphology and wettability of GO deposits, before and after reduction, were confirmed by digital USB microscopy, scanning electron microscopy (SEM), and contact angle measurements. According to these structural characterizations, the proposed method allows a bulky reduction of the coating but leaves to a GO layer at the interface, that is essential for a good coating-substrate adhesion and this special characteristic is useful for industrial exploitation of the material.

A simple, effective, and environmentally benign approach for the reduction of graphene oxide (GO) using apple juice as a reducing agent has been reported in this work. The advantage of this approach is that it eliminates the need for using toxic reducing agents or external stabilizers to carry out the chemical reduction of GO. The reduction, mediated by the extracted maleic acid from apple juice, was found to take place under mild conditions in the aqueous medium. The formation of layered graphitic structure and random orientation of graphene was confirmed by X-ray diffraction study. Appearance of a broad band at 24.9°, corresponds to d-spacing of ~0.342 nm) elucidates successful reduction. Successful reduction of GO was further confirmed by UV–visible spectroscopy. The plasmon peak at 230 nm has been red-shifted to 263 nm corroborating the electronic conjugation in MRGO nanosheets was restored. Raman spectroscopy revealed the formation of graphene with defects. Since the method proposed is economical, it can be used for the large-scale production of reduced GO.

For the goal of synthesizing high performance supercapacitor electrode materials to meet the pressing requirements of energy storage, nitrogen doped activated graphene (N-AG) was successfully synthesized by employing two steps as KOH activating and following nitrogen doping using a hydrothermal method. Continuously increased ID/IG ratios were observed in Raman spectra after KOH activation and N-doping process, indicating increased defects due to the etching effect of KOH and the introduced nitrogen defects on graphene sheet. Stacked graphene with damaged sheets on the surface was clearly seen in transmission electron microscopy (TEM) image after the KOH etching. The successful dopant of N was demonstrated by X-ray photoelectron spectroscopy (XPS) spectra and energy-dispersive X-ray spectroscopy (EDS) mapping, confirming this effective hydrothermal N doping method. The N-AG exhibited largely enhanced capacitance (186.63 F/g) and cycling stability compared with that of nitrogen doped graphene (N-G, 50.88 F/g) and activated graphene (AG, 58.38 F/g) due to the combined positive effects of KOH activation and N-doping. This facile hydrothermal method is of great significance of scaling the production and the synthesized N-AG through combining the KOH activation and nitrogen doping processes represents a promising alternative candidate to commonly used electrode materials in supercapacitor. Experimental Graphite oxide (GO) was synthesized following the modified Hummers method.1 For the synthesis of nitrogen doped graphene (N-G) and KOH activated graphene (AG), 26.0 g homogeneous GO solution (0.338 g GO) was first transferred to a 40 mL teflon lined hydrothermal autoclave, 3 mL NH4OH solution or well dissolved KOH solution (4.05 g KOH, mass KOH : GO = 12:1) was added, the solution was diluted to 40 mL by DI water and sealed maintaining at 150 ºC for 3 hrs. After cooling down, the precipitates were collected by filtration and washed with water for 3 times, the products were dried naturally for 72 hours. Reduced graphene oxide (RGO) was also obtained following the same procedure using simple GO solutions as a control experiment. For the synthesis of N-AG, 0.2 g synthesized AG powders were uniformly grinded and dissolved in 30 mL water, which undergo a 30 min sonication to achieve a uniform distribution, the AG solution was then transferred to a 40 mL autoclave added with 1.80 mL NH4OH solution (same NH4OH/AG ratio as that of NH4OH/GO). Similar procedures were following proceeded to obtain the N-AG powder products. Results and Discussion The Raman spectroscopy and TEM have successfully demonstrated the increased defects and edge parts on graphene after the activation with KOH. Typical N peaks deconvoluted from XPS and uniform distribution of N observed from EDS mapping clearly indicate the successful doping of N using this facile hydrothermal method. Activation and N-doping processes can both contribute to the enhancement of energy storage through increasing active electronic sites and surface area. Increased capacitance, energy and power densities as well as cycling stability after the activation and N-doping processes are confirmed by cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy measurements,2demonstrating the effectiveness of this hydrothermal method and potential of this N-AG for practical energy storage applications. Conclusion A facile hydrothermal synthesis method combining KOH activation and N-doping was successfully employed to synthesize nitrogen doped activated graphene, which is economically and effectively at a very high yield, implying feasibility for mass production. The synthesized N-AG exhibited relatively decreased oxygen containing groups with increased defects due to the etching nature of KOH and N containing defects were also introduced with this effective N doping method. These unique properties endow them as promising electrodes for supercapacitors with high capacity, excellent rate capability, and long-term stability. Due to the possibility of scalable synthesis and prominent properties, the N-AG can offer attractive opportunities for both fundamental study and potential industrial applications in catalysis, adsorption, energy storage, and energy conversion systems. Acknowledgments The financial supports from University of Tennessee Knoxville are kindly acknowledged. Figure 1. Charge−discharge of RGO, N-G, AG and N-AG with a current density as 1 A/g. Background is a schematic representation of the synthesis process.

Chemical reduction of graphene oxide (GO) using nontoxic reductants is a promising and green approach to produce graphene with versatile functionalities in large scale. However, the relatively low reduction extent of the reduced graphene oxide (RGO) is the main hindrance for many applications. In the present work, significantly enhanced reduction of aqueous RGO suspension is achieved by the chemical reduction with combination of tea polyphenols (TPs) and aluminum flake. In comparison with the RGO reduced by TPs only, the defect of the RGO achieved by the combination of TPs and aluminum flake decrease by 6.70%. The sp 2 structure fraction in the RGO increases by 9.67% due to the synergistic effect between TPs and aluminum flake. In addition, a possible mechanism for the synergistic effect between TPs and aluminum flake on the enhanced reduction of RGO is proposed. This approach provides new significant insights on the efficient and environmentally friendly preparation of RGO with much improved quality.

Here we report a highly conductive polypropylene-graphene nonwoven composite via direct coating of melt blown polypropylene (PP) nonwoven fabrics with graphene oxide (GO) dispersions in N,N-dimethylformamide (DMF), followed by the chemical reduction of GO with hydroiodic acid (HI). GO as an amphiphilic macromolecule can be dispersed in DMF homogeneously at a concentration of 5 mg/mL, which has much lower surface tension (37.5 mN/m) than that of GO in water (72.9 mN/m, at 5 mg/mL). The hydrophobic PP nonwoven has a surface energy of 30.1 mN/m, close to the surface tension of GO in DMF. Therefore, the PP nonwoven can be easily wetted by the GO/DMF dispersion without any pretreatment. Soaking PP nonwoven in a GO/DMF dispersion leads to uniform coatings of GO on PP-fiber surfaces. After chemical reduction of GO to graphene, the resulting PP/graphene nonwoven composite offers an electrical conductivity of 35.6 S m-1 at graphene loading of 5.2 wt %, the highest among the existing conductive PP systems reported, indicating that surface tension of coating baths has significant impact on the coating uniformity and affinity. The conductivity of our PP/graphene nonwoven is also stable against stirring washing test. In addition, here we demonstrate a monolithic supercapacitor derived from the PP-GO nonwoven composite by using a direct laser-patterning process. The resulted sandwich supercapacitor shows a high areal capacitance of 4.18 mF/cm2 in PVA-H2SO4 gel electrolyte. The resulting highly conductive or capacitive PP/graphene nonwoven carries great promise to be used as electronic textiles.

Hydrothermal growth of ZnO/GO hybrid as an efficient electrode material for supercapacitor applications

Chemical reduction of graphene oxide is one of the main routes of preparation for large quantities of graphenes. A wide range of reducing agents was described for this task, such as hydroquinone, ascorbic acid, saccharides, proteins, hydrazine, or sodium borohydride. With exception of sodium borohydride and hydrazine, no “standard” organic chemistry agents have been described for reduction of graphene oxides. Lithium aluminum hydride (LAH) is a very powerful reducing agent frequently used in organic synthetic methodologies to convert several types of oxygen containing carbon moieties with a well-known reduction mechanism. Here, we describe, for the first time, the use of LAH toward the reduction of graphene oxide and compare its reduction strength to that of hydrazine and sodium borohydride, which are generally adopted in such application. We show that LAH is far more efficient in reducing oxygen functionalities present on graphene oxide. This is a step forward toward applicability of “standard” organic chemistry reducing agents for reduction of graphene oxides.

As a novel two‐dimensional carbon material, graphene has fine potential applications in the fields of electron transfer agent and supercapacitor material for its excellent electronic and optical property. However, the challenge is to synthesize graphene in a bulk quantity. In this paper, graphite oxide was prepared from natural flake graphite by Hummers’ method through liquid oxidization, and the reduced graphene oxide was obtained by chemical reduction of graphene oxide using NH3·H2O aqueous solution and hydrazine hydrate. The raw material graphite, graphite oxide, and reduced graphene oxide were characterized by X‐ray diffraction (XRD), attenuated total reflectance‐infrared spectroscopy (ATR‐IR), and field emission scanning electron microscope (SEM). The results indicated that the distance spacing of graphite oxide was longer than that of graphite and the crystal structure of graphite was changed. The flake graphite was oxidized to graphite oxide and lots of oxygen‐containing groups were found in the graphite oxide. In the morphologies of samples, fold structure was found on both the surface and the edge of reduced graphene oxide.

We present a general study on a high performance supercapacitor based on intercalated reduced graphene oxide with carbon black nanoparticles. Graphene oxide sheets were synthesized by oxidation and exfoliation of natural graphite and were reduced using hydroiodic acid in the presence of carbon black nanoparticles. Graphene paper was fabricated by one-step procedure via simultaneous reducing and drying the aqueous solution of mixed carbon black nanoparticles and graphene oxide on a conductive substrate. Transmission electron microscopy confirmed the intercalation of carbon black nanoparticles into reduced graphene oxide sheets, preventing them from restacking during the fabrication of paper. Results confirmed that the electrochemical performance of the reduced graphene oxide paper as a supercapacitor electrode was improved by intercalation of carbon black nanoparticles into reduced graphene oxide sheets and also the assembled supercapacitor becomes more efficient with graphene oxide separator. Cyclic voltammetry of the prepared electrodes in 30 wt% KOH solution at 10 mV/s scan rate showed that adding only 5 wt% of carbon black to graphene oxide increases specific capacitance from 118 F/g to 129 F/g. In addition, the maximum specific capacitance (139 F/g) was obtained by adding 15 wt% of carbon black and that increased to 142 F/g by the use of graphene oxide paper as a separator. Furthermore, electrochemical impedance spectroscopy by Nyquist plot showed that charge transfer resistance in electrodes decreases from 18 in the reduced graphene oxide to 6 in the reduced graphene oxide paper with intercalated carbon black nanoparticles and ions diffusion occurs easier than previous.

With unique structure and extraordinary electronic, thermal, and mechanical properties, graphene fascinates the scientific community. Due to its hydrophobic feature, preparation of a stable and highly concentrated graphene dispersion without the assistance of dispersing agents has generally been considered a challenge. Chemical reduction of graphene oxide (GO) is one of the most important methods for preparing a graphene dispersion. The aggregation of graphene sheets is a key reason to destabilize the resulting dispersion during conversion of aqueous GO dispersion to graphene. In this study, by replacing mechanical stirring with ultrasonic irradiation, the aggregation of various intermediates is effectively suppressed during the process of reduction of GO. Hence, a stable graphene dispersion with a high concentration of 1 mg.mL(-1) and relatively pure graphene sheets are achieved, and the as-prepared graphene paper exhibits a high electric conductivity of 712 S.m(-1). Ultraviolet-visible absorption spectroscopy and X-ray photoelectron spectroscopy show that ultrasound is the essence of enhancing chemical reaction rate. Fourier transformed infrared spectra and Raman spectra indicate that ultrasound has less damage to the chemical and crystal structures of graphene.

Preparation of Ag/reduced graphene oxide reinforced copper matrix composites through spark plasma sintering: An investigation of microstructure and mechanical properties

Ultrafast room-temperature reduction of graphene oxide by sodium borohydride, sodium molybdate and hydrochloric acid