Large scale production of highly conductive reduced graphene oxide sheets by a solvent-free low temperature reduction

A novel one-pot process that can produce freestanding reduced graphene oxide (RGO) sheets in large scale through a mechanochemical method is presented, which is based on a 1:1 adduct of hydrazine and carbon dioxide (H₃N⁺NHCO₂⁻, solid hydrazine). We were able to synthesize RGO sheets by grinding solid hydrazine with graphene oxide (GO), followed by storing the mixed powder at 50 °C for 10 min. No solvents, nor large vessels, nor post-annealing at high temperatures are required. The resulting RGO sample was characterized by elemental analysis, X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, Brunauer–Emmett–Teller measurement, thermo gravimetric analysis, Fourier transform infrared spectroscopy, solid state nuclear magnetic resonance spectroscopy, and conductivity measurement. It exhibits excellent conductivity and possesses a high specific surface area. This reduction method was successfully applied for the fabrication of inkjet-printed RGO devices on a flexible substrate.     

柔性透明导电氧化物薄膜的制备及应用进展

柔性透明导电氧化物薄膜以其可挠曲、柔性好、质量轻等优点在柔性薄膜太阳能电池、有机发光二极管及汽车隔热膜等领域具有较好的应用前景。综述了透明导电氧化物(TCO)薄膜的种类、目前柔性透明导电氧化物薄膜的制备技术及优缺点,对柔性TCO薄膜在各个领域的应用和未来研究方向进行了展望：柔性透明导电氧化物兼具柔性、透明性和导电性,因柔性衬底大多不耐高温,应选择合适的衬底材料和制备方法,开发成本低、绿色环保、资源丰富、高性能的柔性TCO薄膜对提高光电子产业竞争力具有重要作用。     

Comparative studies on single-layer reduced graphene oxide films obtained by electrochemical reduction and hydrazine vapor reduction

The comparison between two kinds of single-layer reduced graphene oxide (rGO) sheets, obtained by reduction of graphene oxide (GO) with the electrochemical method and hydrazine vapor reduction, referred to as E-rGO and C-rGO, respectively, is systematically studied. Although there is no morphology difference between the E-rGO and C-rGO films adsorbed on solid substrates observed by AFM, the reduction process to obtain the E-rGO and C-rGO films is quite different. In the hydrazine vapor reduction, the nitrogen element is incorporated into the obtained C-rGO film, while no additional element is introduced to the E-rGO film during the electrochemical reduction. Moreover, Raman spectra show that the electrochemical method is more effective than the hydrazine vapor reduction method to reduce the GO films. In addition, E-rGO shows better electrocatalysis towards dopamine than does C-rGO. This study is helpful for researchers to understand these two different reduction methods and choose a suitable one to reduce GO based on their experimental requirements.     

Solid-phase electrochemical reduction of graphene oxide films in alkaline solution

Graphene oxide (GO) film was evaporated onto graphite and used as an electrode to produce electrochemically reduced graphene oxide (ERGO) films by electrochemical reduction in 6 M KOH solution through voltammetric cycling. Fourier transformed infrared and Raman spectroscopy confirmed the presence of ERGO. Electrochemical impedance spectroscopy characterization of ERGO and GO films in ferrocyanide/ferricyanide redox couple with 0.1 M KCl supporting electrolyte gave results that are in accordance with previous reports. Based on the EIS results, ERGO shows higher capacitance and lower charge transfer resistance compared to GO.     

Multiscale patterning of graphene oxide and reduced graphene oxide for flexible supercapacitors

A simple and facile method for multiscale, in-plane patterning of graphene oxide and reduced graphene oxide (GO–rGO) was developed by region-specific reduction of graphene oxide (GO) under a mild irradiation. The UV-induced reduction of graphene oxide was monitored by various spectroscopic techniques, including optical absorption, X-ray photoelectron spectroscopy (XPS), Raman, and X-ray diffraction (XRD), while the resultant GO–rGO patterned film morphology was studied on optical microscope, scanning electron microscope (SEM), and atomic force microscope (AFM). Flexible symmetric and in-plane supercapacitors were fabricated from the GO–rGO patterned polyethylene terephthalate (PET) electrodes to show capacitances up to 141.2 F/g.     

Flexible conductive graphene paper obtained by direct and gentle annealing of graphene oxide paper

We report the fabrication of flexible conductive graphene paper through a direct and gentle annealing process of graphene oxide paper. Thermal treatments at 700 °C under argon or hydrogen atmosphere directly applied to parent graphene oxide paper lead to a significant removal of disruptive oxygen-containing functional groups, and to a considerable recovery of the sp² network structure. Detailed comparison of chemical and combined chemical–thermal treatments by scanning electronic microscopy (SEM), Raman, X-photoelectron spectroscopy (XPS) and conductivity measurements underline the high efficiency of the direct annealing process. The resulting highly reduced graphene oxide paper exhibits electrical conductivities as high as 8100 S/m representing an increase of five orders of magnitude with respect to the parent graphene oxide paper, which significantly outperforms the results of chemical treatments. Moreover, our direct and gentle thermal reduction allows maintaining the structural integrity and mechanical flexibility of the parent graphene oxide paper thus overcoming problems of brittleness typically encountered in annealing processes. Our approach sets the base for an easy, cost-effective and environmentally friendly fabrication route for flexible conducting graphene paper of great application potential as flexible electrodes in various fields of technology.     

Highly conductive graphene by low-temperature thermal reduction and in situ preparation of conductive polymer nanocomposites

Polydopamine-coated graphene oxide (DGO) films exhibit electrical conductivities of 11?000 S m(-1) and 30?000 S m(-1) upon vacuum annealing at 130 °C and 180 °C, respectively. Conductive poly(vinyl alcohol)/graphene and epoxy/graphene nanocomposites show low percolation thresholds due to the excellent dispersibility of the DGO sheets and their effective in situ reduction.     

In situ chemical reduction and functionalization of graphene oxide for electrically conductive phenol formaldehyde composites

We report an efficient one-step approach to reduce and functionalize graphene oxide (GO) during the in situ polymerization of phenol and formaldehyde. The hydrophilic and electrically insulating GO is converted to hydrophobic and electrically conductive graphene with phenol as the main reducing agent. Simultaneously, functionalization of GO is realized by the nucleophilic substitution reaction of the epoxide groups of GO with the hydroxyl groups of phenol in an alkali condition. Different from the insulating GO and phenol formaldehyde resin (PF) components, PF composites are electrically conductive due to the incidental reduction of GO during the in situ polymerization. The electrical conductivity of PF composite with 0.85 vol.% of GO is 0.20 S/m, nearly nine orders of magnitude higher than that of neat PF. Moreover, the efficient reduction and functionalization of GO endows the PF composites with high thermal stability and flexural properties. A striking increase in decomposition temperature is achieved with 2.3 vol.% of GO. The flexural strength and modulus of the PF composite with 1.7 vol.% GO are increased by 316.8% and 56.7%, respectively.     

Simultaneous surface functionalization and reduction of graphene oxide with octadecylamine for electrically conductive polystyrene composites

Simultaneous surface functionalization and reduction of graphene oxide (GO) was realized by simple refluxing of GO with octadecylamine (ODA) without the use of any reducing agents. The presence of the long octadecyl chain made the hydrophilic GO hydrophobic, evidenced by the selective dispersion of the ODA-functionalized GO (GO–ODA) in chloroform solvent rather than in water. Interestingly, different from the insulating GO, GO–ODA became electrically conductive due to the reduction in the presence of ODA. The electrical conductivity of GO–ODA was further increased by incidental thermal reduction during the compression-molding of its polystyrene (PS) composites at 210 °C, which exhibited a sharp transition from electrically insulating to conducting with a low percolation threshold. The high conductivity of the PS/GO–ODA composites is attributed to the improved dispersion and the reduction of GO–ODA in comparison with GO.     

Reduction of graphene oxide films on Al foil for hybrid transparent conductive film applications

We report the reduction of graphene oxide (G-O) films on Al foil using hydrogen as a reducing agent generated during the etching of Al foil in an aqueous solution of hydrochloric acid (HCl). Complete etching of the Al substrate results in simultaneous reduction and a free standing film composed of stacked and overlapped reduced graphene oxide (RG-O) platelets. Generation of hydrogen at the G-O/Al interface increases the reduction efficiency of this method that is demonstrated in better electrical conductivity of the obtained films compared to the RG-O films reduced by the similar method but using remote Al foil in HCl solution and hydrazine reduced RG-O films. By transferring the free standing RG-O films onto Ag NW films, hybrid transparent conductive films (TCFs) with opto-electrical properties comparable to that of ITO films were obtained.     

Preparation and characterization of highly conductive transparent films with single-walled carbon nanotubes for flexible display applications

Dense, aligned single-walled carbon nanotubes (SWCNTs) were obtained by nitric acid treatment and the subsequent removal of metal impurities by HCl. The highly purified SWCNTs were dispersed with sodium dodecyl sulfate in order to obtain a stabilized suspension for spray coating on flexible polyethylene terephthalate (PET) substrate. The low sheet resistance of the resulting thin conductive film on the PET substrate was due to the interconnecting networks of highly purified SWCNT bundles. These bundles formed strong crisscross networks of nanotubes clustered together with well defined channels, thus improving the electrical and optical properties of the film. Its sheet resistance varied from 956 to 472 Ω/square with 85% optical transmittance at a wavelength of 550 nm. The films may be potential candidates for flexible display applications.     

Synthesis and characterization of electrically conductive polyethylene-supported graphene films

We describe a simple mechanical approach for low-density polyethylene film coating by multilayer graphene. The technique is based on the exfoliation of nanocrystalline graphite (few-layer graphene) by application of shear stress and allows to obtain thin graphene layers on the plastic substrate. We report on the temperature dependence of electrical resistance behaviors in films of different thickness. The experimental results suggest that the semiconducting behavior observed at low temperature can be described in the framework of the Efros-Shklovskii variable-range-hopping model. The obtained films exhibit good electrical conductivity and transparency in the visible spectral region.

PACS

72.80.Vp; 78.67.Wj; 78.66.Qn; 85.40.Hp     

Green reduction of graphene oxide by aqueous phytoextracts

The reduction of graphene oxide (GO) by phytochemicals was investigated using aqueous leaf extracts of Colocasia esculenta and Mesua ferrea Linn. and an aqueous peel extract of orange (Citrus sinensis). The prepared GO and phytoextract reduced GO (RGO) were characterized by ultraviolet–visible spectroscopy, Raman spectroscopy and Fourier transform infrared analyses to provide a clear indication of the removal of oxygen-containing groups from the graphene and the formation of RGO. The extent of reduction was determined from elemental analysis. Formation of few layers of graphene was indicated by transmission electron microscopy. The obtained RGO exhibited good specific capacitance (17–21 Fg^?1), high electrical conductivity (3032.6–4006 Sm^?1) and high carbon to oxygen ratio (5.97–7.11).     

The reduction of graphene oxide

Graphene has attracted great interest for its excellent mechanical, electrical, thermal and optical properties. It can be produced by micro-mechanical exfoliation of highly ordered pyrolytic graphite, epitaxial growth, chemical vapor deposition, and the reduction of graphene oxide (GO). The first three methods can produce graphene with a relatively perfect structure and excellent properties, while in comparison, GO has two important characteristics: (1) it can be produced using inexpensive graphite as raw material by cost-effective chemical methods with a high yield, and (2) it is highly hydrophilic and can form stable aqueous colloids to facilitate the assembly of macroscopic structures by simple and cheap solution processes, both of which are important to the large-scale uses of graphene. A key topic in the research and applications of GO is the reduction, which partly restores the structure and properties of graphene. Different reduction processes result in different properties of reduced GO (rGO), which in turn affect the final performance of materials or devices composed of rGO. In this contribution, we review the state-of-art status of the reduction of GO on both techniques and mechanisms. The development in this field will speed the applications of graphene.     

One-step scalable preparation of graphene nanosheets and high-thermal-conductivity flexible graphene films

It is well known that graphene nanosheets (GNSs) have many excellent properties. However, it has been a difficult thing to exfoliate graphite into GNSs in a controllable and scalable manner. In this research, a new strategy named xylitol-assisted ball milling exfoliation (XABME) was developed for the scalable preparation of GNSs. The experimental results characterized by a series of measurements showed that GNSs were successfully exfoliated by the XABME strategy. The structure of the prepared nanosheets was featured by large lateral size and ultra-small thickness. Furthermore, the prepared GNSs easily achieved high production yield (≈54%). Lastly, the as-obtained GNSs and cellulose nanofibers (CNF) were compounded to form some nanomaterial films. The prepared films exhibited excellent flexibility and higher thermal conductivity, with the in-plane thermal conductivity of 90 wt% GNS film (8.0 W/(m·K)) being 11.4 times higher than that of the film without GNSs. This shows that GNSs could effectively enhance the thermal conductivity of the CNF matrix and indicate that these prepared films have great potentials in the thermal management of portable mobile devices.     

Simultaneous sulfonation and reduction of graphene oxide as highly efficient supports for metal nanocatalysts

The sulfonated reduced graphene oxide (S-rGO) as supports and size-controlled Pt nanoparticles (NPs) for proton exchange membrane fuel cell (PEMFC) catalysts was investigated. The S-rGO was fabricated by a lyophilization-assisted method from a liquid mixture of GO and (NH₄)₂SO₄ with a subsequent thermal treatment in inert gas. Sulfonic acid groups were grafted on GO and a reduction of GO was achieved simultaneously. Transmission electron microscope (TEM) results showed a uniform deposition of Pt NPs on S-rGO (Pt/S-rGO) with a narrow particle size distribution ranging from 2 to 5 nm in diameter. A higher catalytic activity of this novel Pt/S-rGO catalyst was revealed in comparison with that of Pt/GO, Pt/rGO and conventional Pt/C catalysts by cyclic voltammetry and oxygen reduction reaction measurements due to an enhanced triphase boundary. Significantly, the Pt/S-rGO catalyst also presented an excellent electrochemical stability. This new catalyst thus holds a great potential application in PEMFCs in terms of enhanced activity and durability.     

Room temperature ammonia sensing based on graphene oxide integrated flexible polyvinylidenefluoride/cerium oxide nanocomposite films

ABSTRACT

In this work, novel room temperature (RT) ammonia (NH₃) sensors comprising graphene oxide (GO) integrated polyvinylidenefluoride (PVDF)/cerium oxide (CeO₂) nanocomposite films have been prepared via simple solution casting technique. The structural and morphological characteristics of flexible tertiary PVDF/CeO₂/GO nanocomposite films have been investigated using various analytical techniques and their NH₃ gas-sensing performance was evaluated at RT and the relevant sensing mechanism was established. The flexible PVDF/CeO₂/GO nanocomposite films responded strongly to NH₃ gas with enhanced gas sensing properties at RT as compared with various other volatile organic compounds (VOCs) such as acetone, ethanol, formaldehyde and toluene.