Dual Path Mechanism in the Thermal Reduction of Graphene Oxide

Graphene is considered to be one of the most interesting materials because of its unique two-dimensional structure and properties. However, commercialization and large-scale production of graphene still face great challenges at the moment. Thermal reduction of graphene oxide (GO) can be an effective method to fabricate graphene in large scale, but the need for inert gas protection and high reaction temperature leads to high cost of production, thus limiting the production capacity of graphene. In this paper, for the first time we report a facile, safe, and scalable method to achieve simultaneous thermal reduction and nitrogen doping of GO in air at much lower reaction temperature while upholding a high-quality end product. The reduction and nitrogen doping of GO are evidenced by ultraviolet-visible absorption spectroscopy, X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The nitrogen-doped reduced GO (NrGO) fabricated via this method has a high carbon/oxygen ratio of 15 and a nitrogen content of 11.87 atom %. The NrGO is also investigated by applying it as an electrocatalyst for the oxygen reduction reaction. As a result, the catalytic activity has presented itself as much higher than that of the undoped rGO.

For the past decade, researchers have been trying to understand the mechanism of the thermal reduction of graphite oxide. Because deuterium is widely used as a marker in various organic reactions, we wondered if deuterium-labeled graphite oxide could be the key to fully understand this mechanism. Graphite oxides were prepared by the Hofmann, Hummers, Staudenmaier, and Brodie methods, and a deuterium-labeled analogue was synthesized by the Hofmann method. All graphite oxides were analyzed not only using the traditional techniques but also by gas chromatography-mass spectrometry (GC-MS) during exfoliation in hydrogen and nitrogen atmospheres. GC-MS enabled us to compare differences between the chemical compositions of the organic exfoliation products formed during the thermal reduction of these graphite oxides. Nuclear analytical methods (Rutherford backscattering spectroscopy, elastic recoil detection analysis) were used to calculate the concentrations of light elements, including the ratio of hydrogen to deuterium. Combining all of these results we were able to determine graphite oxide's thermal reduction mechanism. Carbon dioxide, carbon monoxide, and water are formed from the thermal reduction of graphite oxide. This process is also accompanied by various radical reactions that lead to the formation of a large amount of carcinogenic volatile organic compounds, and this will have major safety implications for the mass production of graphene.

In situ thermal reduction (ISTR) of graphene oxide (GO) dispersed in a polymer matrix has attracted broad interest due to its great potential as an environmentally friendly and commercially viable process to prepare polymer/graphene nanocomposites (PGNs). In this work, the ISTR of GO in two dramatically different conditions, quiescent melt and sheared melt, was comparatively studied. Comprehensive characterization of the bulk composites and the extracted graphene-based powders from composites, as well as the results of an independent parallel plate experiment, revealed that the GO in the sheared melt has a higher reduction degree than that in the quiescent melt within identical processing temperatures and times. On the basis of our results, we hypothesize that the more intense reduction of GO in the sheared melts relative to the quiescent melts is associated with the enhanced π–π stacking and the possible radical reaction between polymers and GO sheets.

Controlled oxygen functional groups on reduced graphene using rate of temperature for advanced sorption process

In a two-dimensional carbon system for graphene (GP), three carbon electrons in four hybridized bonding electrons (2s12px12py12pz1) form strong in-plane sp2 bonds consisting of a honeycomb structure, and a fourth electron spreads out over the top or bottom of the layer as a π electron. The π electrons play an important role in the coupling interactions of multilayered GPs. A two-dimensionally spread C=C resonance structure and the hybridized electrons confined in GP are directly related to graphene’s unique characteristics, such as its ballistic electron conduction, high thermal conduction, and high mechanical strength (Geim & Novoselov, 2007; Lee et al., 2008). These fundamental properties have been well studied using high-quality graphene produced by the “top-down” physical exfoliation (McAllister et al., 2007) and solvation-assisted exfoliation of graphite (Lotya et al., 2009) and GP thin films produced using chemical vapor deposition (Kim et al., 2009; Obraztsov, 2009). Meanwhile, thermally reduced GP from graphene oxide (GO) (Gao et al., 2009; Jeong et al., 2009), produced from graphite using various chemical oxidation routes (Brodie, 1859; Hummers & Offerman, 1958; Hirata et al., 2004), has attracted considerable attention as a potential material for use in various industrial applications such as photovoltaic cells, capacitors, sensors, and transparent electrodes (Geim & Novoselov, 2007; Stoller et al., 2008; Wang et al., 2008; Liu et al., 2009). This is because of not only its potential in significantly lowering the cost of mass-produced graphene but also the simple, nonchemical, thermal conversion of GO powder or film to GP powder or film, respectively (Titelman et al., 2005; (a) Jeong et al., 2009; (b)). However, the thermal reduction of GO is a very complex phenomenon because of the thermal-energy-induced multistep removal processes of intercalated H2O molecules and oxide groups of –COOH (carboxyl group), –OH (hydroxyl group), and >O (epoxy group). It should be noted that in chemical reduction, individual GO sheets in the solution phase are chemically reduced by the strong chemical base (Titelman et al., 2005; Ju et al., 2010). Therefore, the thermal reduction of GO and resultant GP needs to be studied in great detail. In this research, X-ray diffraction (XRD) was used to probe the temperature-dependent evolution of the interlayer distance (d002) of GO/GP films and powders within temperature ranges of room temperature (RT) to 1000°C and RT to 2000°C, respectively. XRD results show a detailed thermal reduction of GO with the removal of intercalated H2O molecules and oxide groups, defect formation, lattice contraction and exfoliation mechanisms of the GO/GP sheets, the folding and unfolding of the GO/GP layers, and a bottom-up layer stacking toward bulk graphite.

Sugar blowing-assisted reduction and interconnection of graphene oxide into three-dimensional porous graphene

Graphene oxide (GO) and reduced graphene oxide (rGO) synthesised from GO, has a promising future in fields ranging from electronics to energy technologies[1]. GO may be synthesized by the modified Hummer’s method[2], where a mixture of potassium permanganate and concentrated sulfuric acid forms the ground pillar for the oxidation of graphite to GO. rGO can be synthesized by a broad range of methods, with the chemical and the thermal reduction routes being very common[3]. The synthesis mechanism of GO by the modified Hummer’s method is still unknown, even though the active oxidizing species dimanganese heptoxide has been suggested as the main redox active specie[4]. The mechanism of the thermal reduction of GO to rGO is also unknown. We present results from i n situ synchrotron X-ray diffraction (XRD) experiments of syntheses and thermal reduction of GO. The in situ synthesis of GO was performed by placing a mixture of permanganate and sulphuric acid in a capillary next to graphite. The synthesis was then initiated by gently pushing the fluid mixture into the powder with N2 gas. The in situ XRD of the GO synthesis showed how the oxidation reaction proceeds in three separate stages, as seen in Figure 1. The first stage was the dissolution of potassium permanganate, followed by an intercalation stage and subsequent formation of crystalline material. The GO 001 diffraction peak was observed early during the synthesis, in the second stage, and the intensity of the 001 diffraction increased during the third stage. The in situ XRD results of the thermal reduction of GO to rGO showed a dependence on the temperature ramping and addition of diamond powder. Syntheses were measured at 1, 5, 10, 20 and 50 °/min temperature ramps. The syntheses were performed in a capillary with GO being heated by a hot air blower under constant N2 flow. Three stages were observed for the reduction process; a GO stage, an amorphous stage and a rGO stage. The change in stage was defined from the changing of the d-value of the initial 001 GO peak, see Figure 2. The initial GO diffraction pattern changed during the heating and more diffraction peaks were observed. The results showed that the nature of the rGO material depends heavily on both temperature and additives. These in situ XRD studies revealed the crystalline intermediates and final product of synthesis by a modified Hummer’s method and the diffractional change during the thermal reduction of GO. The stages observed for both syntheses illuminate how important it is to consider the experimental parameters dependent on the application; they might even have to be optimized separately. As the future use of GO and rGO is expanding and the commercialization of these products are enhanced, the syntheses mechanisms may be of increasing interest. [1] M. Segal, Nat Nano, 4 (2009) 612-614. [2] Hummer and Offeman, J. Am. Chem. Soc. (1958) 1339-1339 [3] Pei and Cheng, Carbon (2012) 3210-3228 [4] Dreyer, Park, Bielawski and Ruoff, Chem. Soc. Rev. (2010) 228-240 Figure 1

Online tracking of the thermal reduction of graphene oxide by two-dimensional correlation infrared spectroscopy

Effect of the morphology of thermally reduced graphite oxide on the mechanical and electrical properties of natural rubber nanocomposites

Divergent mechanisms for thermal reduction of graphene oxide and their highly different ion affinities

Thermal reduction of graphite oxide and its derivatives under argon atmosphere has been studied by means of thermogravimetric analysis. Carbon materials prepared via thermal reduction of graphite oxide derivatives in argon at 900°С during 3 h have been used for deposition of platinum from H2PtCl6 solutions. Pt particles supported on the support catalyze liquid-phase hydrogenation of nitrobenzene and dec-1-ene under atmospheric pressure of H2. Thermal reduction of the supports based on graphite oxide results in the formation of the structural defects significantly enhancing the catalytic activity.

Thermal reduction of graphite oxide (GO) is considered as a prospective method for the preparation of high-performance graphene-based materials. However, this method has certain limitations, and the major is that this exothermic process is difficult to control. In this research, we focused on the kinetic studies of the reduction of graphite oxide using non-isothermal differential calorimetry (DSC) method. Six GO nanocomposites with dyes were tested to study the shift in kinetic parameters. The apparent reaction order is determined to be ca 0.7 for the thermal decomposition of pure GO, while in the presence of dye molecules it increases sometimes reaching a value of 2.0 for higher dye concentrations. Decisively, the thermal decomposition of pure GO can be presented as an intermediate between a zero- and first-order reaction, while the introduction of dye molecules turns a certain part of the energy consumption via the bimolecular pathway. Our research revealed that the process of GO thermal decomposition can be operated and properties of the final product (reduced graphene oxide (rGO) and its derivatives) can be adjusted more precisely using additive molecules, which interact with GO sheets.

Adsorption of chlorinated poly(ethylene-co-vinyl acetate)-g-maleic anhydride copolymer and in situ-generated polyaniline (PANI) on thermally reduced graphene oxide (TRGO) platelets was studied in the current study. The adsorption was characterized structurally and morphologically through thermogravimetric analysis, differential scanning calorimetry (DSC), elemental analysis, infra-red and Raman spectroscopy, X-ray diffraction and microscopy. The amount of copolymer adsorption reached a plateau of 0.22 g per g of TRGO, when the initial copolymer to TRGO weight ratio of 1 was used. In the case of PANI modification, much higher extent of adsorption of 0.92 g/g of TRGO (without reaching plateau) was observed due to in situ polymer synthesis and the absence of any steric hindrance to the chains. Shift in the DSC melting transition temperatures of copolymer also indicated that some change in the polymer chain morphology took place after immobilization of polymer on TRGO. PANI modification led to significant reduction in peak melting point from 175C to 140 °C owing to the hindrance in polymer crystallization. The basal plane spacing in the TRGO platelets increased the copolymer adsorption as the 0 0 1 basal plane diffraction shifted from 27° 2Θ for pristine TRGO to 22.5° 2Θ for modified TRGO. For the PANI modified TRGO, no diffraction signal corresponding to TRGO was observed due to extensive adsorption of polymer on the surface. A much thicker polymer phase wrapping the TRGO platelets was observed for PANI modified TRGO. This was also observed through EFTEM and EDX, where the presence of Cl and N (along with other atoms) indicated layer of copolymer and PANI, respectively on the surface of the platelets. EELS analysis also confirmed the semi-crystalline nature of the modified TRGO resulting from the adsorption of semi-crystalline polymers on TRGO. The adsorption approaches used in the study demonstrate successful generation of the functional nanomaterials with tunable extent of surface coverage and potential of employing diverse surface modifications.

In this present study, a highly conductive thermally reduced graphene oxide (TRGO) was synthesized by a low temperature thermal reduction method using RF heating, under an argon-hydrogen atmosphere. The crystallinity and morphology were examined by X-ray diffraction, Raman spectroscopy and TEM analysis. The chemical structure including the functional groups present on TRGO was studied using X-ray photoelectron spectroscopy and FTIR analysis. The studies reveal that thermal reduction of graphene oxide was successful under the experimental conditions and that the TRGO had high crystallinity. Further, the performance of the as-prepared TRGO was tested as a counter electrode (CE) in a dye-sensitized solar cell (DSSC). The maximum power conversion efficiency (PCE) obtained was 4.86% for TRGO under one sun illumination, which is comparable to that of a platinum CE-based DSSC (5.24%). The electrocatalytic activity and electron transfer kinetics were examined by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and Tafel measurements. The series resistance (Rs) and charge transfer resistance (Rct) values were found to be 35.4 Ωcm-2 and 56.40 Ωcm-2 for TRGO. The results reveal that the TRGO had an electrocatalytic performance similar to that of Pt, making TRGO-CEs promising alternatives to the conventional Pt-CEs in DSSCs.

Pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate), an antioxidant widely applied in the plastic industry, was used to assist thermal reduction of graphene oxide (GO) on a cotton fabric in air. l-ascorbic acid was also applied for comparison. GO was deposited on the fabric by the padding method. Reduction of GO deposited on fiber surfaces at 180 and 220 °C in air imparted electrical conductivity. For all the materials the conductivity worsened after the reduction during cooling and during the first hours of storage at room conditions. However, the most stable effect of GO reduction was achieved using pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate), which assisted the reduction of GO at both 180 and 220 °C, and allowed to obtain the cotton fabric with a stable surface resistivity of 6.6 and 3.7 MΩ/sq, respectively. Moreover, superhydrophobicity of the conductive fabric was achieved by modification with methyltrichlorosilane in an anhydrous environment.