High quality graphene sheets from graphene oxide by hot-pressing

We report a simple and effective route to convert graphene oxide sheets to good quality graphene sheets using hot pressing. The reduced graphene oxide sheets obtained from graphene oxide by low temperature thermal exfoliation are annealed at 1500 °C and 40 MPa uniaxial pressures for 5 min in vacuum. No appreciable oxygen content was observed from X-ray photoelectron spectroscopy and no D peak was detected in the Raman spectrum. The graphene sheets produced had a much higher electron mobility (1000 cm² V⁻¹ S⁻¹) than other chemically modified graphenes.     

Hydrophobic precipitation of carbonaceous spheres from fructose by a hydrothermal process

We report a new mechanism of hydrophobic ripening for the formation of carbonaceous spheres by the dehydration of saccharides in a hydrothermal aqueous environment using fructose as a model precursor material. We investigated the formation of carbonaceous spheres from fructose in aqueous solutions under hydrothermal conditions. The spheres were found to contain 65.7 wt.% C, 4.3 wt.% H and 30.0 wt.% O, implying incomplete dehydration of the fructose. The spheres, typically ranging between 400 nm and 10 μm in diameter, are found to be constructed entirely of primary particles of ～5 nm. The chemical structure of the carbonaceous spheres and the chemical compositions of residual solutions were analysed using solid state and solution ¹³C nuclear magnetic resonance and Fourier transform infrared spectroscopy. Based on these results, a four-step mechanism for the formation and growth of carbonaceous spheres has been proposed.     

一步水热法制备还原氧化石墨烯改性涤纶无纺布及性能研究

采用无还原剂的一步水热法得到还原氧化石墨烯(rGO)改性涤纶无纺布,研究了改性涤纶无纺布的导电性、耐水洗性、疏水性、导热性和耐磨性,并与传统浸渍法处理后用水合肼还原氧化石墨烯改性的涤纶无纺布进行比较。结果表明:一步水热法在水热反应温度为180℃、反应5 h时的条件下,制备的改性涤纶无纺布具有优异的导电性能,其电阻率为52.6Ω·m;一步水热法制得的rGO改性涤纶无纺布具有良好的耐水洗性能,经过10次水洗涤处理后,其电阻率从52.6Ω·m增加到193.4Ω·m,而传统浸渍法制得的rGO改性涤纶无纺布的电阻率从213.1Ω·m增加到8.1×10⁵Ω·m;一步水热法制得的rGO改性涤纶无纺布具有优异的疏水性、导热性和耐磨性,其水接触角由改性前的47.42°增加到160.85°;经过1 000次磨损实验之后,其电阻率由磨损前的52.6Ω·m增加到1 226.8Ω·m。     

一步法制备硅烷改性氧化石墨烯

采用一步电解剥离法制备硅烷改性氧化石墨烯,通过阳极电泳沉积法得到硅烷改性氧化石墨烯-氧化铝复合薄膜。以傅里叶变换红外光谱(FTIR)、热重分析(TG)、X射线衍射(XRD)、拉曼光谱、扫描电镜(SEM)、电化学阻抗谱等分析方法对其结构进行表征。结果表明,硅烷成功接枝到了氧化石墨烯上,令其热稳定性得以改善。硅烷改性氧化石墨烯表面剥落均匀,石墨的晶格结构在电化学剥离过程中得到保留。与单一氧化铝薄膜相比,硅烷改性氧化石墨烯氧化铝复合薄膜的腐蚀电位正移,腐蚀电流密度降低,阻抗弧半径增大,耐蚀性得到改善,寿命延长。     

Ammonia gas sensors based on chemically reduced graphene oxide sheets self-assembled on Au electrodes

We present a useful ammonia gas sensor based on chemically reduced graphene oxide (rGO) sheets by self-assembly technique to create conductive networks between parallel Au electrodes. Negative graphene oxide (GO) sheets with large sizes (>10 μm) can be easily electrostatically attracted onto positive Au electrodes modified with cysteamine hydrochloride in aqueous solution. The assembled GO sheets on Au electrodes can be directly reduced into rGO sheets by hydrazine or pyrrole vapor and consequently provide the sensing devices based on self-assembled rGO sheets. Preliminary results, which have been presented on the detection of ammonia (NH₃) gas using this facile and scalable fabrication method for practical devices, suggest that pyrrole-vapor-reduced rGO exhibits much better (more than 2.7 times with the concentration of NH₃ at 50 ppm) response to NH₃ than that of rGO reduced from hydrazine vapor. Furthermore, this novel gas sensor based on rGO reduced from pyrrole shows excellent responsive repeatability to NH₃. Overall, the facile electrostatic self-assembly technique in aqueous solution facilitates device fabrication, the resultant self-assembled rGO-based sensing devices, with miniature, low-cost portable characteristics and outstanding sensing performances, which can ensure potential application in gas sensing fields.     

Electrostatic transparency of graphene oxide sheets

The interaction of graphene oxide of varying reduction degrees with dielectric and metallic surfaces is probed in this study, in order to assess the influence that the supporting substrate has on the electronic properties of as-produced graphene oxide and its reduced form. Lateral inhomogeneities in the distribution of substrate trapped charged impurities are found to affect the electronic properties of reduced graphene oxide, giving rise to significant in-plane variations of the local electrostatic potential on reduced one-layer sheets supported on dielectric substrates. On the contrary, no such surface potential fluctuations are identified on as-produced graphene oxide sheets, or on graphene oxide layers deposited on a metallic substrate. Thicker, two-layer reduced graphene oxide sheets show effective screening of the electrostatic effects caused by charge impurities trapped in the substrate. The current study provides a useful account of the limitations that device performance could face when attempting to tune the electronic structure of graphene oxide via functionalization, highlighting the role of substrate-related disorder affecting the behaviour of nanodevices. The role of the substrate is particularly important for applications where electronic properties of graphene oxide are especially targeted, such as transparent conducting films, sensors and electrochemical devices.     

Synthesis and optical properties of ZnSe micro-grasses and microspheres grown on graphene oxide sheets by the hydrothermal method

Zinc selenide (ZnSe) micro-grasses and microspheres have been successfully grown on graphene oxide sheets by the hydrothermal method. The morphologies, structures, chemical compositions and optical properties of the as-synthesized graphene oxide (GO)/ZnSe microstructures have been characterized by X-ray power diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectrometer (EDS), ultraviolet-visible (UV–vis) absorption spectroscopy and photoluminescence (PL) spectroscopy. By adjusting the concentration of NaOH and EDTA, needle-like, coral grass-like, orchid-like, and spherical ZnSe microstructures have been synthesized.     

Templated patterning of graphene oxide using self-assembled monolayers

A cost-effective and efficient route for graphene oxide (GO) patterning through site-selective deposition of GO onto a photopatterned self-assembled monolayer (SAM) of octadecylphosphonic acid on the surface of native oxide of titanium was demonstrated. In this process, SAMs on titanium oxide was first patterned via ultraviolet light exposure through a mask, subsequent drop-casting of GO aqueous dispersion on the substrate resulted in the site-selective deposition of GO on the hydrophilic areas. The formation of GO patterns is evidenced by Raman mapping. Atomic force microscopy studies indicate that the thickness of patterned GO can be controlled by adjusting the concentration of the GO dispersion. Scanning electron microscopy, and inverted fluorescence microscopy of large area GO patterns demonstrate this process is scalable and that the fluorescent property of nanoscale GO sheets is still reserved after deposition.     

Graphite mediated reduction of graphene oxide monolayer sheets

Large area graphene oxide (GO) monolayer sheets were transferred on Si and SiO₂/Si substrates by Langmuir–Blodgett technique and reduced by heat treatment in the presence of graphite powder, inside an evacuated and sealed enclosure. Reduction at 1000 °C results in a decrease of sheet thickness to ∼0.5 nm without any morphological changes, and de-oxygenation (O/C ratio 5%) accompanied by enhancement in sp²-C to 84%. Red-shift of G-peak to ∼1585 cm⁻¹, decrease in I(D)/I(G) ratio and appearance of an intense G′-peak as a single Lorentzian, are indicative of substantial reduction in defects and restoration of graphitic network. Ultraviolet photoelectron spectroscopy (UPS) studies show large increase in density of states (DOS) in the immediate vicinity of Fermi level and decrease in work function after reduction. Bottom gated field effect transistors fabricated with isolated RGO sheets display ambipolar behavior, with charge neutrality point at a positive gate voltage, indicating p-type nature, consistent with UPS results. RGO sheets exhibit conductivity of (2–3) × 10³ S/cm and field effect mobility of (20–45) cm²/Vs, which are substantially higher than the values usually reported for RGO sheets, particularly those obtained by chemical reduction followed by heat treatment.     

Carborane functionalized graphene oxide,a precursor for conductive self-assembled monolayers

A new, advanced graphene oxide based material modified with the extremely stable o-carboranoyl 1,2-C₂B₁₀H₁₁ unit was created. The carboranoyl-functionalized graphene oxide was synthesized from Li[1,2-C₂B₁₀H₁₁] and COCl-functionalized graphene-oxide, which was prepared by converting the graphene oxide –COOH groups to COCl with SOCl₂. The carboranoyl-functionalized graphene oxide was characterized using infrared and X-ray photoelectron spectroscopy. The morphology was investigated via transmission, scanning electron, and atomic force microscopies. The electrochemical properties of o-carboranoyl functionalized graphene oxide was studied using cyclic voltammetry, electrochemical impedance spectroscopy and electric force microscopy. This new graphene-based material could in future be used as a precursor for conductive self-assembled monolayers.     

水热剥离法制备氧化石墨烯及其结构性能与应用

基于水热剥离法,以膨胀石墨为原料和阳离子表面活性剂为插层剂制备出氧化石墨烯。利用X射线衍射、红外光谱、拉曼光谱、热失重法和冷场扫描电镜等表征了水热处理后的膨胀石墨的晶体结构、表面官能团和形貌等,并初步研究了处理后的膨胀石墨对聚乙烯醇(PVA)薄膜的力学增强作用。结果表明:阳离子表面活性剂十六烷基三甲基溴化铵对膨胀石墨的剥离效果最好,97.6%的膨胀石墨形成了非晶的剥离层,即无规分散的氧化石墨烯片层,1.8%的膨胀石墨经插层剂处理后层间距增大,形成插层结构;添加质量分数为1.0%的经阴离子表面活性剂处理的膨胀石墨即可明显提高PVA薄膜的拉伸断裂强度。     

Shaping graphene oxide by electrochemistry: From foams to self-assembled molecular materials

The ability to control the three-dimensional architecture of graphene-based materials following a rational design is essential for technological applications. Here we demonstrate that the electrochemical etching can be used as a surgical tool to tailor the morphology of graphene electrodes and to impart special features, like micrometric channels and controlled mesoporosity (foams). The final materials, thanks to the high surface area, can represent a promising class of carbon-based supercapacitors. Otherwise, new materials can be prepared using a bottom-up strategy that exploits the self-assembly of the graphene oxide quantum dots produced during the electrochemical erosion. The advantages of this second approach reside not only in the possibility to downscale the control over the spatial organization as compared to the use of conventional micrometric graphene sheets, but also in the introduction of the intrinsic luminescent properties of the quantum dots in the final material. As a proof of concept we report the preparation of luminescent nanospheres by exploiting the self-organization of the graphene oxide quantum dots around frozen water nuclei.     

Decoration of graphene oxide sheets with luminescent rare-earth complexes

Graphene oxide sheets (GOSs) functionalized with rare-earth (RE) complexes were prepared using a noncovalent approach. The adsorption of RE complexes onto GOSs, as well as the individual nature of the hybrids, was confirmed. The GOS–RE complex hybrids and their dispersion can emit bright red luminescence, which makes them useful in many practical fields, such as biological labeling and anti-counterfeiting.     

Restoration of graphene from graphene oxide by defect repair

A simple and efficient method to repair defects in graphene oxide (GO) is reported, accompanied by a simultaneous reduction process by a methane plasma. The graphene after repair is of high quality. For a typical monolayer after repair and reduction, the minimum sheet resistance at the Dirac point and the Raman D/G peak intensity ratio are about 9.0 kΩ/□ and ～0.53, respectively.     

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.     

Easy preparation of ultrathin reduced graphene oxide sheets at a high stirring speed

This work explored the synthesis of rGO sheets from graphene oxide (GO) using hydrazine solvent as reducing agent through chemical reduction. Meanwhile, GO films with a 2D structure were prepared from graphite flakes (starting material with an average flake size of 150 nm) by an Improved Hummer׳s method. Results showed that the chemical oxidation of graphite flakes carried out at room temperature could be used to prepare GO sheets in the initial stage. The conversion of GO into large-area rGO sheets with ~85% of carbon content could then be achieved by chemical reduction. RGO sheets with a lateral dimension of up to ~45 nm were obtained, which indicated the formation of an extremely thin layer of rGO sheets. A high degree of GO reduction was also realized using a high stirring speed (1200 rpm) for 72 h in a mixture of acids and potassium permanganate, resulting in a high carbon content of rGO with a large lateral dimension and area. Overall, our Improved Hummer׳s method with a high stirring speed (1200 rpm) for 72 h provided an easy approach to the preparation of large-area and ultrathin rGO sheets.     

Size-controlled synthesis of graphene oxide sheets on a large scale using chemical exfoliation

The synthesis of graphene oxide (GO) sheets with controlled size on a large scale was developed using chemical exfoliation by simply controlling the oxidation and exfoliation procedure. The GO samples prepared under different conditions, which all have excellent water dispersion, are characterized by thermal gravimetric analysis, Ultraviolet-visible spectroscopy, X-ray diffraction and atomic force microscopy. It is found that as longer oxidation times and more oxidants are used, the mean size of the GO sheets, which has a Gaussian distribution, decreases from ∼59,000 to ∼550 nm².     

Novel CuS hollow spheres fabricated by a novel hydrothermal method

A thioglycolic acid (TGA) assisted hydrothermal process has been developed to synthesize submicron-sized copper sulfide (CuS) hollow spheres via the reaction between copper sulfate (CuSO₄) and thioacetamide (CH₃CSNH₂). X-ray diffraction, transmission electron microscopy and scanning electron microscopy were employed to characterize the obtained product. Furthermore, the possible mechanism and the critical factors for the TGA-assisted hydrothermal synthesis of the CuS hollow spheres have been preliminarily presented.