Synthesis and photoluminescence of F and N co-doped reduced graphene oxide

F and N co-doped reduced graphene oxide (F,N-RGO) was synthesized by annealing the mixture of N-doped reduced graphene oxide (N-RGO) and XeF₂ at 180 °C. The photoluminescence (PL) properties of F,N-RGO were studied. The results show that F,N-RGO exhibits two strong ultraviolet (UV) PL centered at 365 and 310 nm, and the PL at 310 nm can be appropriately tuned by tailoring the N content further.     

Chemical state of nitrogen in carbon aerogels issued from phenol-melamine-formaldehyde gels

Chemical states of nitrogen in the carbon aerogels synthesized from phenol-melamine-formaldehyde were investigated by elemental analysis, X-ray photoelectron spectroscopy and nitrogen adsorption. It is found that the carbon aerogels are rich in mesopores and the nitrogen content of the carbon aerogels increases from 1.6 to 3.1 wt. % with increasing melamine to phenol ratios. Over two-thirds of nitrogen are on periphery of graphene layers (pyridinic-N, pyrrolic-N and/or pyridonic-N, pyridine N-oxide) and less than one-third in central position of graphene layers (quaternary-N). Polymerisation at low pH might cause a preferred location of nitrogen on periphery of graphene layer.     

Nitrogen doped epitaxial graphene on 4H-SiC(0001) – Experimental and theoretical study

The experimental and theoretical investigations of morphological and electronic properties of nitrogen-doped epitaxial graphene grown by chemical vapor deposition on 4H-SiC(0001) are discussed. It is shown that presence of nitrogen significantly affects the graphene growth process and leads to an increase in the concentration of defects (in the form of holes). Macro- and nanoscale investigations confirm the formation of pyridinic-N, pyrrolic-N and graphitic-N configurations within graphene layers. The relative concentrations of these configurations change as a function of global nitrogen concentration. Additionally, it is reported that the incorporated nitrogen results in inhomogeneous doping and a few nanometers wide spatial modification of the local density of states. Finally, the SiC substrate is also modified during the nitrogen doping process. To quantify the impact of the substrate modification on electronic structure of graphene the non-intercalated and hydrogen-intercalated doped graphene layers are compared. The presented complementary study sheds light on properties of N-doped graphene and its dependence on nitrogen concentration.     

Polyacrylonitrile nanocomposite with carbon nanostructures: a review

Polyacrylonitrile (PAN) is one of the versatile commercially important acrylic polymers. It is a well-known polymer due to its enhanced mechanical, thermal, and chemical properties. Various nanofillers have been incorporated in PAN to significantly improve the mechanical, thermal, and electrical properties of resulting nanocomposite. This review comprehends efforts devoted to PAN-based nanocomposite reinforced with carbon nanotube, graphene, and graphene oxide. The interaction between PAN and carbon nanostructure has been concentrated to develop high-performance nanocomposite. The scientific and technological development in the field of PAN/carbon nanofiller nanocomposite particularly in membranes, biosensor, lithium–sulfur batteries, supercapacitor, and photocatalysts has also been expressed. Moreover, future prospects in scientific and technological disciplines have been addressed.     

Evaluation of Graphene/WO<Subscript>3</Subscript> and Graphene/CeO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Structures as Electrodes for Supercapacitor Applications

The combination of graphene with transition metal oxides can result in very promising hybrid materials for use in energy storage applications thanks to its intriguing properties, i.e., highly tunable surface area, outstanding electrical conductivity, good chemical stability, and excellent mechanical behavior. In the present work, we evaluate the performance of graphene/metal oxide (WO₃ and CeO _x ) layered structures as potential electrodes in supercapacitor applications. Graphene layers were grown by chemical vapor deposition (CVD) on copper substrates. Single and layer-by-layer graphene stacks were fabricated combining graphene transfer techniques and metal oxides grown by magnetron sputtering. The electrochemical properties of the samples were analyzed and the results suggest an improvement in the performance of the device with the increase in the number of graphene layers. Furthermore, deposition of transition metal oxides within the stack of graphene layers further improves the areal capacitance of the device up to 4.55 mF/cm², for the case of a three-layer stack. Such high values are interpreted as a result of the copper oxide grown between the copper substrate and the graphene layer. The electrodes present good stability for the first 850 cycles before degradation.     

Exploring the methods of synthesis,functionalization, and characterization of graphene and graphene oxide for supercapacitor applications

Graphene and graphene oxide, are attracting more attention over the last decades in the area of supercapacitor research and researchers concentrate on extensive exploration, owing to their dominating electrical conductivity, combined with mechanical properties. This review is a panoramic approach, giving insights into various aspects related to graphene and graphene oxide such as their properties, production methods, functionalities, and their applications in supercapacitors. The study ought to be beneficial to novice as well as to the domain experts. Various properties of both materials are explored and both synthesis methods are elaborated. Extra emphasis is given to bring out the role of graphene and graphene oxide in promoting the performance of supercapacitors. Synthesis methods are tabulated based on the evaluation metrics like specific capacitance and capacitance retention. Finally, the application of graphene and graphene oxide in supercapacitors are highlighted. Before concluding, perspectives along with challenges for further development are proposed and are expected to facilitate researchers in shedding light on further studies in this explorative area.     

Recent Advances in Electrochemical Performances of Graphene Composite (Graphene-Polyaniline/Polypyrrole/Activated Carbon/Carbon Nanotube) Electrode Materials for Supercapacitor: A Review

The latest trend in the direction of miniaturized portable electronic devices has brought up necessitate for rechargeable energy sources. Among the various non conventional energy devices, the supercapacitor is the promising candidate for gleaning the energy. Supercapacitor, as a new energy device that colligates the gap between conventional capacitors and batteries, it has attracted more attention due to its high power density and long cycle life. Many researchers work on, synthesizing new electrode material for the development of supercapacitor. The electrode material possesses salient structure and electrochemical properties exhibit the efficient performance of the supercapacitor. Graphene has high carrier mobility, thermal conductivity, elasticity and stiffness and also has a theoretical specific capacitance of 2630 m²g^??1 corresponds to a specific capacitance of 550 Fg^??1. This article summarizes and reviews the electrochemical performance and applications of various graphene composite materials such as graphene/polyaniline, graphene/polypyrrole, graphene/metal oxide, graphene/activated carbon, graphene/carbon nanotube as an electrode materials towards highly efficient supercapacitors and also dealt with symmetric, asymmetric and hybrid nature of the graphene based supercapacitor.     

Graphene nanoplate-MnO2 composites for supercapacitors: a controllable oxidation approach

Graphene nanoplate-MnO(2) composites have been synthesized by oxidising part of the carbon atoms in the framework of graphene nanoplates at ambient temperature. The composites were characterized by means of X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry (CV). It was found that the oxidation extent of the carbon atoms in the graphene framework in these composites was dependent on the reaction time, which also influenced their microstructure, morphology and electrochemical properties. Compared with MnO(2) nanolamellas, the nanocomposite prepared with a reaction time of 3 h reveals better electrochemical properties as a supercapacitor electrode material.     

Facile synthesis and strongly microstructure-dependent electrochemical properties of graphene/manganese dioxide composites for supercapacitors

Graphene has attracted much attention since it was firstly stripped from graphite by two physicists in 2004, and the supercapacitor based on graphene has obtained wide attention and much investment as well. For practical applications of graphene-based supercapacitors, however, there are still many challenges to solve, for instance, to simplify the technological process, to lower the fabrication cost, and to improve the electrochemical performance. In this work, graphene/MnO₂ composites are prepared by a microwave sintering method, and we report here a relatively simple method for the supercapacitor packaging, i.e., dipping Ni-foam into a graphene/MnO₂ composite solution directly for a period of time to coat the active material on a current collector. It is found that the microwave reaction time has a significant effect on the microstructure of graphene/MnO₂ composites, and consequently, the electrochemical properties of the supercapacitors based on graphene/MnO₂ composites are strongly microstructure dependent. An appropriately longer microwave reaction time, namely, 15 min, facilitates a very dense and homogeneous microstructure of the graphene/MnO₂ composites, and thus, excellent electrochemical performance is achieved in the supercapacitor device, including a high specific capacitance of 296 F/g and a high capacitance retention of 93% after 3,000 times of charging/discharging cycles.

PACS

81.05.ue; 78.67.Sc; 88.80.fh     

石墨烯/聚苯胺纳米复合材料界面相互作用的分子动力学模拟

石墨烯具有独特的二维结构和优异的力学、电学性能,将其与聚苯胺复合得到的石墨烯/聚苯胺(Gr/PANI)纳米复合材料在微波吸收、超级电容、电子器件等领域具有广泛的应用前景。为研究Gr/PANI纳米复合材料界面相互作用的微观机理,利用分子动力学方法考察了Gr/PANI体系的相互作用能、相互作用构型以及石墨烯与PANI之间的对关联函数。温度、能量演化曲线和相互作用能分析表明,Gr/PANI体系在较短的时间内达到平衡,Gr/PANI体系为热力学稳定体系。相互作用构型显示PANI分子与石墨烯之间存在较强的相互吸引作用。对关联函数分析表明,Gr/PANI纳米复合材料界面存在近程强非键相互作用,较强的界面相互作用主要源于石墨烯与PANI都具有sp²杂化的π共轭结构。     

Simultaneous reduction and N-doping of graphene oxides by low-energy N2+ ion sputtering

An easy and catalyst-free method was used to obtain N-doped reduced graphene oxides (N-RGO) through low-energy N₂⁺ ion sputtering of graphene oxides (GO). The simultaneous reduction and N-doping of GO during the sputtering were systematically investigated by X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure and Raman spectroscopy. The N-doping and reduction levels, which are determined by the N/C and O/C atomic ratios from the quantitative XPS analysis, respectively, can be easily controlled by varying the N₂⁺ ion sputtering time. In addition, three different N species, namely, nitrile-like N, graphitic N and pyridinic N, can be distinguished in N-RGO.     

A First-Principles Study of the Role of Quaternary-N Doping on the Oxygen Reduction Reaction Activity and Selectivity of Graphene Edge Sites

Density functional theory (DFT) was used to investigate O₂ chemisorption on the edge sites of graphene doped with quaternary nitrogen (N-graphene). The location of the doped quaternary N within the graphene cluster was systematically varied to determine the effect of interior versus edge doping on the reactivity of the edge graphene sites. Model 1b, where a quaternary-N atom is at the zigzag edge of the graphene cluster, is found to be the most favored structure and strongly adsorbs O₂ molecule via a “two feet” geometry. For this most stable O₂ binding configuration, the potential-dependent free energy of reaction for the subsequent oxygen reduction reaction (ORR) steps was evaluated. The favored four electron-proton transfer mechanism passes through a dissociative O*+OH* state instead of an OOH* intermediate, followed by a series of reduction steps to produce water. At the equilibrium potential for ORR of 1.23 V-NHE, the protonation of O* and OH* both show uphill steps, but the production of O* is facile with a small overpotential. An applied potential of ?0.15 V-NHE is required to facilitate the protonation of OH* to water, a larger overpotential than observed experimentally. While solvent effects may reduce this overpotential, our results suggest that the edge of the N-graphene is very active towards activation of O₂ and production of O* and OH* but because of strong binding of the oxygen atom, the subsequent steps of the ORR reaction will be hindered. Mechanisms that have OH* formed at the edge site and then move to adjacent sites for more facile protonation will have to be explored in the future.     

Scalable and high-yield production of exfoliated graphene sheets in water and its application to an all-solid-state supercapacitor

One of the most critical issues in graphite exfoliation is realizing efficient, low-cost, eco-friendly, and scalable production of graphene for energy storage applications. The most promising strategies for exfoliating graphite to single-layer graphene sheets in scalable quantities with nearly non-oxidized content is the exfoliation of graphite by using an environmentally friendly solution. Herein, we demonstrate a universal exfoliation principle that uses imidazole, which has an anionic nature and π-conjugated heterocyclic coplanar structure, as the exfoliant for the successful production of large quantities of graphene suspensions in water. The exfoliant interacts with both surfaces of the exfoliated graphene sheets and the aqueous solution, significantly improving graphene dispersion (1 mg/mL) in water. Exfoliation in aqueous solutions produced graphene in high yield (>90%, ⩽3 layers), with a large lateral size (μm) and high quality (I_D/I_G ratio ≈ 0.1). The electrical conductivity of the graphene paper is 131.7 S/cm, which is superior to the values reported from exfoliated graphene prepared using solution processes. An all-solid-state supercapacitor with a new design fabricated using the atomically thin and flat conductive exfoliated graphene sheets delivered an ultrahigh area capacitance (∼71.9 mF/cm²). Therefore, imidazole -assisted exfoliation has great potential for scalable preparation of graphene suspensions for supercapacitor applications.     

A study on 3D graphene synthesized directly on Glass/FTO substrates: Its Raman mapping and optical properties

In this study, vertically oriented (3D) graphene nanostructures (VGN) were obtained on Glass and Fluorine doped tin oxide (FTO) substrates at relatively low temperatures (approaximatelly at 400 °C) by using plasma enhanced chemical vapor deposition (PECVD) technique. VGNs were characterized using Raman, Uv–Vis and SEM spectroscopies, respectively. Raman spectroscopy showed that growth temperature significantly affected VGN formation and film quality. The layer structures of these VGNs were also confirmed by Raman mapping. The most important data obtained from this study is that although the applied radio frequency (RF) power is the same, the growth temperature significantly affects the graphene formation and morphologies. This was attributed to the fact that CH₄ decomposition was very difficult at low temperatures. The optical transmittance of VGNs ranged from 50% to 95% (at 550 nm wavelength) and their electrical conductivity was also determined to be between 5.94 kohm/sqr and 11.2 kohm/sqr. The electrical and optical properties of VGNs, which have a large surface area, indicated that they may be an alternative material for sensor, solar cell and supercapacitor applications.     

用于脑电图监测的石墨烯/聚丙烯酸共聚酯/织物复合电极的制备与性能

为制备脑电检测用聚合物基柔性复合电极片,以石墨烯、丙烯酸(AA)、丙烯酸甲酯(MA)和丙烯酸乙酯(EA)为原料,通过原位聚合的方法制得了系列聚丙烯酸共聚酯/石墨烯复合乳液,并通过与织物浸渍的方法制备了复合电极片。研究了石墨烯含量变化对复合电极片电阻的影响、各单体含量对电极片柔性的影响,以及复合电极片用于脑电监测的性能。结果表明,当石墨烯的质量分数约为46.5%、反应温度80 ℃,反应时间3 h,AA、MA与EA的体积投料比为0.2∶1∶1.8,织物采用聚丙烯无纺布并与聚丙烯酸共聚酯复合乳液浸渍后,制备的复合电极片在室温下具有良好的柔性、导电及力学强度,适合用于脑电图监测,脑电信号强度及其稳定性与商用电极（Ag/AgCl）相比性能相近,且不需要涂抹导电膏,使用方便,不会污染皮肤及对皮肤造成损伤。     

Modeling the evolution of graphene agglomeration and the electrical and mechanical properties of graphene/polypropylene nanocomposites

Graphene agglomeration tends to develop with the increase of graphene loading. In this article, we present a unified model that first considers the evolution of graphene agglomeration and then incorporates it into the calculation of electrical and mechanical properties of agglomerated graphene/polymer nanocomposites. In the evolution of graphene agglomerates, a modified nanoparticle distance in terms of yield strength is introduced, while in the calculation of composite properties, a two-scale framework that consists of the graphene-rich agglomerates and the remainder as the graphene-poor region is constructed. In electrical conduction, electron tunneling is modeled through Simmons formula and its difference with the widely used Cauchy function is compared. We highlight that both Simmons and Cauchy functions could well describe the interfacial tunneling activity, but the former is physics-based while the latter is statistics-based. In the calculation of nonlinear elastoplastic response, a field-fluctuation method is adopted. We also demonstrated how the presented model gives rise to experimentally consistent electrical and mechanical properties of graphene/polypropylene nanocomposites, and how filler agglomeration hinders the performance of the materials.     

The synthesis of shape-controlled polypyrrole/graphene and the study of its capacitance properties

Polypyrrole (PPy)/graphene nainocomposite was prepared by methyl orange (MO) reactive template. By changing the amount of MO, the morphology of PPy can be controlled to range from nanoparticle to nanowire. Transmission electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and X-ray diffraction results demonstrated that the composites were successfully synthesized. The different morphologies of PPy/graphene nanocomposites had certain effects on the supercapacitor performance. The experimental results showed that the capacitance of PPy (nanoparticle)/graphene was higher than that of PPy (nanowire)/graphene. As a model, PPy (nanoparticle)/graphene was used to construct a supercapacitor. By changing the amount of pyrrole monomer, the performance of the supercapacitor prepared from different PPy content was studied in detail.     

Tuning electronic and magnetic properties of partially hydrogenated graphene by biaxial tensile strain: a computational study

Using density functional theory calculations, we have investigated the effects of biaxial tensile strain on the electronic and magnetic properties of partially hydrogenated graphene (PHG) structures. Our study demonstrates that PHG configuration with hexagon vacancies is more energetically favorable than several other types of PHG configurations. In addition, an appropriate biaxial tensile strain can effectively tune the band gap and magnetism of the hydrogenated graphene. The band gap and magnetism of such configurations can be continuously increased when the magnitude of the biaxial tensile strain is increased. This fact that both the band gap and magnetism of partially hydrogenated graphene can be tuned by applying biaxial tensile strain provides a new pathway for the applications of graphene to electronics and photonics.     

Studies of oxygen reduction reaction active sites and stability of nitrogen-modified carbon composite catalysts for PEM fuel cells

A non-precious nitrogen-modified carbon composite (NMCC) catalyst is synthesized by the pyrolysis of cobalt, iron-ethylenediamine-chelate complexes on silica followed by chemical and pyrolysis treatments. Pyrolysis temperature and time have a remarkable impact on the content and the type of the nitrogen-containing functional groups in the NMCC catalysts, which affect their catalytic activity and stability. Based on the analysis of the nitrogen functional groups before and after the stability tests, the ORR active sites of the NMCC catalysts are proposed to be pyridinic-N and quaternary-N functional groups. However the pyridinic-N group is not stable in the acidic environment due to the protonation reaction.