Modelling very large magnetoresistance of graphene nanoribbon devices

We model a very large, tunable magnetoresistance (MR) in a graphene nanoribbon field-effect transistor, without artificial engineering of ferromagnetic contacts. A high MR of nearly 100% (about 50%) is obtained at low temperature (room temperature). We show that the MR ratio can be further tuned by using conventional electric field, in agreement with a recent experiment. The simulation results indicate that the large MR of GNRs stems from reduction of the bandgap of GNRs in the presence of applied magnetic field related to the 0(th) Landau level in graphene. Interestingly, in a more realistic device with edge roughness, the MR ratio is not degraded but rather enhanced, caused by the magnetic field induced spatial separation of the transport states. The large, tunable MR of GNRs resistant to edge roughness is attractive to device applications.     

聚苯胺电荷输运及磁阻的研究进展

对聚苯胺体系的电荷输运机制、聚苯胺纳米结构与纳米复合结构的磁阻研究现状进行了综述。电荷在聚苯胺体系内通过跳跃方式输运,磁阻现象主要是由量子退相干效应和波函数收缩效应造成。磁阻效应的强弱与体系结构紧密相关,掺杂剂、掺杂程度和氧化剂所引起的结晶性会影响磁阻效应,聚苯胺复合纳米结构内纳米填充物和聚苯胺之间协同作用也会影响磁阻特性。对设计和合成高效聚苯胺磁阻材料具有指导意义。     

Structural and charge transport characteristics of graphene layers obtained from CVD thin film and bulk graphite materials

We report an experimental comparative study of graphene layers produced by micromechanical cleavage of bulk graphite materials of different origins and graphite films obtained by plasma enhanced chemical vapor deposition (PECVD). Structural characteristics of these materials were evaluated using Raman spectroscopy and electron microscopy. Field effect transistors (FETs) based on the PECVD graphene were produced using electron beam lithography. Conductivity, carrier mobility and other characteristics of the PECVD graphene obtained from Raman and FET tests were similar to the properties of graphene flakes obtained from bulk graphite materials. Taking into account the scalability of the CVD fabrication, these results confirm the possible industrial use of graphene films obtained by this method.     

Observation of photocurrent generation in electrodeposited zinc oxide layers

Wurtzite zinc oxide (ZnO) films with band gap energy of about 3.3 eV were deposited onto conductive substrates by electrodeposition from a simple aqueous zinc nitrate solution at 335 K. The films showed optical transmission around 70% in the visible light region. The electrical resistance showed a strong dependence on cathodic potential. The decrease in electrical resistance could be observed with light irradiation irrespective of the cathodic potential and was attributed to the excitation of electrons from valence band to conduction band by light with wavelength below 375 nm. A photocurrent of about 0.10 A was generated for a cell composed of Ag electrode/Au/ZnO/NESA glass substrate under conditions of irradiation by sunlight.     

Observation of mechanical percolation in functionalized graphene oxide/elastomer composites

We have covalently functionalized graphene oxide (GO) with octadecylamine (ODA) to form GO–ODA. This material can be dispersed in tetrahydrofuran (THF) and subsequently formed into composites with polymers such as thermoplastic polyurethane (TPU). We have characterized the mechanical properties of composites of GO–ODA in TPU. No increase in stiffness was observed at loading levels below 2.5 vol%. Reinforcement appeared to start sharply at this volume fraction and subsequently increased as a power law with increasing volume fraction. This behavior is typical of percolation and shows that the low-strain stress is not increased until the functionalized graphene flakes form a percolating network. Slightly different behavior is observed for properties related to material failure. The ultimate tensile strength increased linearly with graphene content up to the percolation threshold before subsequently falling off. Similarly the strain at break was constant below the percolation threshold but fell off dramatically above it. This work shows the importance of network formation in the reinforcement of elastomeric materials.     

Bio-inspired functionalization and redox charge transfer of graphene oxide sponges for pseudocapacitive electrodes

Bio-inspired and environmentally friendly chemical functionalization method for reducing of graphene oxide (GO) sponge using dopamine derivatives is developed. The GO sponge is readily modified by poly(norepinephrine) (p(Nor)) through a simple wet chemistry, and mechanical properties of the GO sponge is improved with structural stability. The oxidative polymerization of norepinephrine can considerably remove oxygen-containing species from the GO sponge surface as well as provide redox active quinone moieties for pseudocapacitive electrodes. Furthermore, three-dimensional (3D) macroporous networks of the reduced GO (rGO) sponge provide a large surface area and continuous ion pathways. As a result, p(Nor)-functionalized rGO (p(Nor)@rGO) sponge exhibits enhanced capacitive performance. The p(Nor)@rGO sponge shows that a specific capacitance of 232.1 F g⁻¹ (2.5 times higher than that of chemically converted rGO sponge) and good cyclic stability over 2000 charge/discharge cycling tests.     

Theoretical study on Janus graphene oxide membrane for water transport

Graphene oxide (GO) membranes have received considerable attention owing to their outstanding water-permeation properties; however, the effect of the membrane’s microstructures (such as the distribution of oxidized and pristine regions) on the transport mechanism remains unclear. In this study, we performed molecular simulations to explore the permeation of a water–ethanol mixture using a new type of Janus GO membranes with different orientations of oxidized and pristine surfaces. The results indicate that the oxidized upper surface endows the GO membrane with considerable water-capture capability and the in-built oxidized interlayer promotes the effective vertical diffusion of water molecules. Consequently, using the optimized Janus GO membrane, infinite water selectivity and outstanding water flux (~40.9 kg⋅m⁻²⋅h⁻¹) were achieved. This study contributes to explaining the role of oxidized regions in water permeation via GO membranes and suggests that Janus GO membranes could be used as potential candidates for water–ethanol separation.     

Focusing on energy and optoelectronic applications: a journey for graphene and graphene oxide at large scale

Carbon is the only element that has stable allotropes in the 0th through the 3rd dimension, all of which have many outstanding properties. Graphene is the basic building block of other important carbon allotropes. Studies of graphene became much more active after the Geim group isolated "free" and "perfect" graphene sheets and demonstrated the unprecedented electronic properties of graphene in 2004. So far, no other individual material combines so many important properties, including high mobility, Hall effect, transparency, mechanical strength, and thermal conductivity. In this Account, we briefly review our studies of bulk scale graphene and graphene oxide (GO), including their synthesis and applications focused on energy and optoelectronics. Researchers use many methods to produce graphene materials: bottom-up and top-down methods and scalable methods such as chemical vapor deposition (CVD) and chemical exfoliation. Each fabrication method has both advantages and limitations. CVD could represent the most important production method for electronic applications. The chemical exfoliation method offers the advantages of easy scale up and easy solution processing but also produces graphene oxide (GO), which leads to defects and the introduction of heavy functional groups. However, most of these additional functional groups and defects can be removed by chemical reduction or thermal annealing. Because solution processing is required for many film and device applications, including transparent electrodes for touch screens, light-emitting devices (LED), field-effect transistors (FET), and photovoltaic devices (OPV), flexible electronics, and composite applications, the use of GO is important for the production of graphene. Because graphene has an intrinsic zero band gap, this issue needs to be tackled for its FET applications. The studies for transparent electrode related applications have made great progress, but researchers need to improve sheet resistance while maintaining reasonable transparency. Proposals for solving these issues include doping or controlling the sheet size and defects, and theory indicates that graphene can match the overall performance of indium tin oxide (ITO). We have significantly improved the specific capacitance in graphene supercapacitor devices, though our results do not yet approach theoretical values. For composite applications, the key issue is to prevent the restacking of graphene sheets, which we achieved by adding blocking molecules. The continued success of graphene studies will require further development in two areas: (1) the large scale and controlled synthesis of graphene, producing different structures and quantities that are needed for a variety of applications and (2) on table applications, such as transparent electrodes and energy storage devices. Overall, graphene has demonstrated performance that equals or surpasses that of other new carbon allotropes. These features, combined with its easier access and better processing ability, offer the potential basis for truly revolutionary applications and as a future fundamental technological material beyond the silicon age.     

Enhanced transport properties in polymer electrolyte composite membranes with graphene oxide sheets

Polymer electrolyte membranes have been widely investigated for high performance fuel cells. Here, we report the synthesis of ionic conductive Nafion/graphene oxide (GO) composite membranes for application in direct methanol fuel cells. GOs interact with both the non-polar backbone and the polar ionic clusters of Nafion because of their amphiphilic characteristics attributable to hydrophobic conjugation and hydrophilic functional groups. Accordingly, GO sheets serve to modify the microstructures of two domains of Nafion. In particular, the transport properties of Nafion are favorably manipulated by the incorporation of GO. This modulated the ionic channels of Nafion and decrease methanol crossover while preserving ionic conductivity. Furthermore, strong interfacial interactions due to the insertion of GO nanofillers into the Nafion matrix improve the thermal and mechanical properties of the material. In particular, we exploit Nafion/GO composite membrane as electrolyte material for direct methanol fuel cell (DMFC) in order to resolve current issue of methanol crossover. This composite membrane-based DMFC compared to the Nafion 112-based DMFC remarkably enhanced cell performance, especially in severe operating conditions.     

Template-free synthesis of large anisotropic gold nanostructures on reduced graphene oxide

The morphologies/dimensions of Au nanostructures can be tailored by merely controlling the reduction degree of graphene oxide surface. Au nanoparticles, long Au nanowires, and semicircular-shaped Au nanoplates are in situ synthesized on slightly, moderately, and highly reduced graphene oxide films respectively, without the need of any templating agent.     

The evolution of surface charge on graphene oxide during the reduction and its application in electroanalysis

A series of reduced graphene oxide (RGO) colloids with different amounts of surface charges was prepared. The change in surface charge at different pH values was detected by zeta potential measurement, and the evolution of oxygen-containing functional groups attached to the RGOs was analysed by Fourier-transform infrared spectroscopy and ultraviolet-visible absorption spectroscopy. Results showed that the edge phenolic hydroxyl and carboxyl groups made more contributions to the negative surface charge compared with the basal-plane hydroxyl and epoxy groups. Electrical impedance spectroscopy results proved that the surface charge of RGOs significantly affected their electrochemical properties. Furthermore, GO and RGOs (graphene oxide materials) were also used to construct electrochemical sensors for quantitive measurement of Cu²⁺ by differential pulse anodic stripping voltammetry. The results revealed that the increase in negative surface charge on RGO enhanced its electrocatalytic activity for Cu²⁺ reduction. Thus, considering that studies on the properties of graphene oxide materials can be simplified by the surface charge, its analysis is an important means of material characterisation.     

离子组成对氧化石墨烯在饱和多孔介质中迁移行为的影响

采用室内饱和石英砂柱一维渗流模拟实验,研究离子强度相同、离子组成(钠吸附比)不同的电解质溶液对氧化石墨烯(GO)在地下环境中迁移行为的影响。通过测定GO穿透过程的沉积动力学曲线,结合数学模型与界面化学理论,对其迁移沉积机理进行分析。结果显示,当钠吸附比从0增加到∞时,砂柱出流最大浓度升高,GO回收率增加,穿透实验沉积速率、去除效率和吸附效率分别从0.356 min^-1、1.04×10^-2、0.054减小到0.039 min^-1、1.1×10^-3、0.003。研究认为布朗扩散是造成GO颗粒与石英砂粒相互接触的主要机制,Ca²⁺浓度变化是造成不同钠吸附比环境下GO不同迁移行为的主要因素,其机理在于Ca²⁺与GO发生电中和与桥接作用,改变了颗粒间的静电斥力与引力平衡,进而影响了GO颗粒粒度与形态,并最终强化了砂柱对GO的截留效应。     

Impact of synthesis conditions on physicochemical and transport characteristics of graphene oxide laminates

In this study, the effect of oxidation conditions during the synthesis process of graphene oxide (GO) flakes on transport characteristics of GO laminates is investigated. Transport properties of the GO laminates synthesized by different oxidation methods are characterized by measuring their ionic conductivity (via proton conductivity) and mass diffusion of species (via methanol permeability). These properties are observed to be significantly different for each sample owing to the difference in their physicochemical properties. It is determined that the two key parameters that dictate the transport characteristics of a GO laminate are the GO oxidation level and flake size. Through a parametric study, it is determined that at a constant oxidation level methanol permeability decreases linearly with increasing the GO mean flake size while changes in proton conductivity remain insignificant. This behavior is attributed to difference in adopted conduction pathways of protons and methanol molecules. With increasing the oxidation level, proliferation of surface defects is deductively reasoned to be the dominant factor responsible for a large increase in the measured methanol permeability. The proton conductivity is also significantly increased with increasing the oxidation level because of greater number of ion exchange sites, shortened transport pathway and increased GO flakes inter-layer spacing.     

Ultrafast water-selective permeation through graphene oxide membrane with water transport promoters

Graphene oxide (GO), as a representative two-dimensional material, has shown great prospect in developing high-performance separation membranes via forming ordered and tunable nanochannels. However, for aqueous molecular separations, the implementation of an excellent separation performance remains a critical challenge due to the membrane swelling phenomenon and the trade-off effect between permeation flux and separation factor. Herein, a facile and tunable approach is presented for introducing water transport promoters into GO interlayer channels to construct water transport highways. The combination of covalently cross-linked channel structure, facilitated water-selective sorption, and expedited water-preferential diffusion overcome the trade-off effect, achieving a superior performance from an ultrathin GO membrane with a flux of 5.94 kg/m²∙h and a water/butanol separation factor of 3,965, which exceeds the performance of state-of-the-art membranes for water/butanol separation. The strategy proposed here is straightforward, holding great potential to produce high-efficiency GO and other two-dimensional (2D)-material membranes for precise aqueous molecular separations.     

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.     

不同氧化程度氧化石墨烯氨气敏感性能及机理

基于改进的Hummers法,通过改变氧化剂KMnO_4用量制备了各种含氧官能团含量差异明显的氧化石墨烯(GOs)水相分散液,采用旋涂法制备了厚度均一的GOs气敏元件。利用XRD、FTIR、XPS对样品的结构、官能团种类及含量进行了分析;利用气敏测试系统对GOs气敏元件的NH_3敏感性能进行了测试。结果表明:GOs含有羟基(—OH)、环氧基〔—CH(O)CH—〕等含氧官能团,随KMnO4用量的增加,GOs中羟基(—OH)的相对含量(XPS测得)先增加后减少,当m(KMnO_4)∶m(石墨)=3∶1时,—OH的相对含量最高。不同氧化程度的GOs气敏元件对NH_3灵敏度与其—OH的相对含量呈正相关性,GOs中—OH相对含量为43.75%时,气敏元件对体积分数为0.008%的NH_3最大灵敏度达到78%,且有较好的稳定性和重复性,重复性误差为3.1%。GOs对NH_3分子的响应存在两种机制:NH_3分子进入GOs片层间水分子层后水解形成NH_4~+的离子电导,和GOs结构层上含氧官能团对NH_3分子吸附后形成氢键的电荷转移。     

Enhanced UV- and visible-light driven photocatalytic performances and recycling properties of graphene oxide/ZnO hybrid layers

Light induced catalytic processes have attracted significant attention during the last years for wastewater treatment due to their efficiency in decomposition of organic contaminants. In this study we report the synthesis of graphene oxide (GO)/ZnO hybrid layers with high photocatalytic efficiency using laser radiation. The results show that the hybrid layers exhibit much improved photodecomposition efficiency as compared to pure GO or ZnO both under UV and visible-light irradiation. The enhanced photocatalytic efficiency of the hybrid as compared to the reference pure ZnO and GO layers was attributed to the contribution of GO to the separation and transport of the photogenerated charge carriers. Additionally, under visible light irradiation the organic molecules can act as first sensitizers in the degradation process. The recyclability of the layers was also investigated through repetitive photodegradation cycles under UV- or visible-light irradiation. After consecutive degradation runs, the hybrid photocatalyst layers were still stable and retained high degradation efficiency, ensuring reusability. The photocatalytic activity of the layers was correlated with the gradual change of their chemical structure during consecutive degradation cycles. Owing to the high photodegradation efficiency, reusability, and ease of recovery the synthesised hybrid layers consisting of easily available materials are suitable for environmental purification applications.     

Mechanical characterization of graphene oxide reinforced epoxy at different strain rates

Strain rate has significant effect on mechanical behavior of the thermoset polymers. The rate sensitivity is more complicated for thermoset nanocomposites, which compose of two quite different types of materials. Nanofiller‐reinforced epoxy resin is widely used in the industry. In the present work, epoxy resin is reinforced by 0.05 to 0.7 wt% nanographene oxide (GO). The strain rate sensitivity of the fabricated nanocomposites is investigated through compressive test carried out at the strain rates of 0.001–1,900 s^?1. The stress–strain curves of the nanocomposites indicated considerable difference between the low‐strain and high‐strain‐rate responses of the specimens. The results showed that the compressive strength of the nanocomposites was improved by more than 100% at high strain rates with respect to the low strain rates. Also, the addition of nano‐GO had influence on compressive strength enhancement but not as significant as the effect of strain rate. It was observed that the effect of GO was less important for higher strain rates. The experimental compressive strength and modulus of elasticity of the nanocomposites were casted in empirical relations for low and high strain rates for various filler weight percentages. Scanning electron microscopy was also used to examine the quality of GO dispersion. POLYM. ENG. SCI., 59:1636–1647 2019. © 2019 Society of Plastics Engineers