Graphene Oxide Synthesis, Properties and Characterization Techniques: A Comprehensive Review

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 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.

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.

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.

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

Effects of probe and bath ultrasonic treatments on graphene oxide structure

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.

Despite great recent progress, the processing of graphene oxide (GO) sheets in polymers are often one of the most challenging steps in fabricating graphene/polymer nanocomposites. The challenge is how to achieve high levels of dispersion and reduction of GO simultaneously, without any residual reducing agents in the composites. In this work, microwave irradiation is applied as a remote source to in situ reduce GO sheets embedded in the poly (vinyl alcohol) (PVA) matrix, which process maximizes the advantage of GO's solvent processability. The active response of GO to microwave irradiation allows for the uniform and selective heating of GO, leading to a minutes‐quick reduction of GO without physical damage to the polymer matrix. Significant improvements in the mechanical properties, electrical conductivities and glass transition temperatures (Tgs) of GO/PVA nanocomposites are achieved, and possible reasons for the improvements are also discussed. POLYM. COMPOS., 40:170–178, 2019. © 2017 Society of Plastics Engineers

The association of cellular toxicity with the physiochemical properties of graphene‐based materials is largely unexplored. A fundamental understanding of this relationship is essential to engineer graphene‐based nanomaterials for biomedical applications. Here, an in vitro toxicological assessment of graphene oxide (GO) and reduced graphene oxide (RGO) and in correlation with their physiochemical properties is reported. GO is found to be more toxic than RGO of same size. GO and RGO induce significant increases in both intercellular reactive oxygen species (ROS) levels and messenger RNA (mRNA) levels of heme oxygenase 1 (HO1) and thioredoxin reductase (TrxR). Moreover, a significant amount of DNA damage is observed in GO treated cells, but not in RGO treated cells. Such observations support the hypothesis that oxidative stress mediates the cellular toxicity of GO. Interestingly, oxidative stress induced cytotoxicity reduces with a decreasing extent of oxygen functional group density on the RGO surface. It is concluded that although size of the GO sheet plays a role, the functional group density on the GO sheet is one of the key components in mediating cellular cytotoxicity. By controlling the GO reduction and maintaining the solubility, it is possible to minimize the toxicity of GO and unravel its wide range of biomedical applications.

The hydrothermal process was used to prepare Mn3O4/x%GO nanocomposites (NC’s) having different ratios of the Mn3O4 nanoparticles (NP’s) on the surface of graphene oxide (GO) sheet. SEM image showed that the Mn3O4 NP’s were distributed over the surface of GO sheet. HRTEM images exhibited the lattice fringe arising from the (101) plane of the Mn3O4 NP’s having the interplanar d-spacing of 0.49 nm decorating on the surface of GO. The electronic absorption spectra of Mn3O4/x%GO NC’s also show broad bands from 250 to 550 nm. These bands arise from the d–d crystal field transitions of the tetrahedral Mn3+ species and indicate a distortion in the crystal structure. Photo-catalytic activity of spinel ferrite Mn3O4 NP’s by themselves was low but photo-catalytic activity is enhanced when the NP’s are decorating the GO sheet. Moreover, the Mn3O4/10%GO NC’s showed the best photo-catalytic activity. This result comes from the formation of Mn–O–C bond that confirm by FT-IR. This bond would facilitate the transfer of the photoelectrons from the surfaces of the NP’s to the GO sheets. PL emission which is in the violet–red luminescent region shows the creation of defects in the fabricated Mn3O4 NP’s nanostructures. These defects create the defect states to which electrons in the VB can be excited to when the CB. The best degradation efficiency was achieved by the Mn3O4 NP’s when they were used to decorate the GO sheets in the Mn3O4/10%GO NC’s solution.HighlightsLattice fringe of Mn3O4 with an interplanar d-spacing of 0.49 nm for (101) plane.Photocatalytic activity of spinel ferrite Mn3O4 nanoparticles by itself is low.Number of photoelectrons created depends on number of Mn3O4 on a given area of GOThe bonding of the Mn3O4 to the GO sheet would be though a Mn–O–C junction.The degradation processes were accelerated by Mn3O4/10%GO nanocompositesGraphic abstract

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.

Thermo-oxidative resistance of crosslinked polyethylene (XLPE) coated by hybrid coatings containing graphene oxide

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.

Graphene oxide (GO) is a promising precursor to produce graphene-family nanomaterials for various applications. Their potential health and environmental impacts need a good understanding of their cellular interactions. Many factors may influence their biological interactions with cells, and the lateral dimension of GO sheets is one of the most relevant material properties. In this study, a model bacterium, Escherichia coli ( E. coli ), was used to evaluate the antibacterial activity of well-dispersed GO sheets, whose lateral size differs by more than 100 times. Our results show that the antibacterial activity of GO sheets toward E. coli cells is lateral size dependent. Larger GO sheets show stronger antibacterial activity than do smaller ones, and they have different time- and concentration-dependent antibacterial activities. Large GO sheets lead to most cell loss after 1 h incubation, and their concentration strongly influences antibacterial activity at relative low concentration (<10 μg/mL). In contrast, when incubating with small GO sheets up to 4 h, the inactivation rate of E. coli cells continues increasing. The increase of small GO sheet concentration also results in persistent increases in their antibacterial activity. In this study, GO sheets with different lateral sizes are all well dispersed, and their oxidation capacity toward glutathione is similar, consistent with X-ray photoelectron spectroscopy and ultraviolet-visible absorption spectroscopy results. This suggests the lateral size-dependent antibacterial activity of GO sheets is caused by neither their aggregation states, nor oxidation capacity. Atomic force microscope analysis of GO sheets and cells shows that GO sheets interact strongly with cells. Large GO sheets more easily cover cells, and cells cannot proliferate once fully covered, resulting in the cell viability loss observed in the followed colony counting test. In contrast, small GO sheets adhere to the bacterial surfaces, which cannot effectively isolate cells from environment. This study highlights the importance of tailoring the lateral dimension of GO sheets to optimize the application potential with minimal risks for environmental health and safety.

Light-induced oxygen sensing using ZnO/GO based gas sensor