Aqueous Dispersions of Graphene from Electrochemically Exfoliated Graphite

A facile and environmentally friendly synthetic strategy for the production of stable and easily processable dispersions of graphene in water is presented. This strategy represents an alternative to classical chemical exfoliation methods (for example the Hummers method) that are more complex, harmful, and dangerous. The process is based on the electrochemical exfoliation of graphite and includes three simple steps: 1) the anodic exfoliation of graphite in (NH₄)₂SO₄, 2) sonication to separate the oxidized graphene sheets, and 3) reduction of oxidized graphene to graphene. The procedure makes it possible to convert around 30 wt % of the initial graphite into graphene with short processing times and high yields. The graphene sheets are well dispersed in water, have a carbon/oxygen atomic ratio of 11.7, a lateral size of about 0.5–1 μm, and contain only a few graphene layers, most of which are bilayer sheets. The processability of this type of aqueous dispersion has been demonstrated in the fabrication of macroscopic graphene structures, such as graphene aerogels and graphene films, which have been successfully employed as absorbents or as electrodes in supercapacitors, respectively.     

Phosphorus and Halogen Co‐Doped Graphene Materials and their Electrochemistry

Doping of graphene materials with heteroatoms is important as it can change their electronic and electrochemical properties. Here, graphene is co‐doped with n‐type dopants such as phosphorus and halogen (Cl, Br, I). Phosphorus and halogen are introduced through the treatment of graphene oxide with PX₃ gas (PCl₃, PBr₃, and PI₃). Graphene oxides are prepared through chlorate and permanganate routes. Detailed chemical and structural characterization demonstrates that the graphene sheets are covered homogeneously by phosphorus and halogen atoms. It is found that the amount of phosphorus and halogen introduced depends on the graphene oxide preparation method. The electrocatalytic effect of the resulting co‐doped materials is demonstrated for industrially relevant electrochemical reactions such as the hydrogen evolution and oxygen reduction reactions.     

Inherent Electrochemistry and Activation of Chemically Modified Graphenes for Electrochemical Applications

Graphene research is currently at the frontier of electrochemistry. Many different graphene‐based materials are employed by electrochemists as electrodes in sensing and in energy‐storage devices. Because the methods for their preparation are inherently different, graphene materials are expected to exhibit different electrochemical behaviors depending on the functionalities and density of defects present. Electrochemical treatment of these “chemically modified graphenes” (CMGs) represents an easy approach to alter surface functionalities and consequently tune the electrochemical performance. Herein, we report a preliminary electrochemical characterization of four common chemically modified graphenes, namely: graphene oxide, graphite oxide, chemically reduced graphene oxide, and thermally reduced graphene oxide. These CMGs were compared with graphite as a reference material. Cyclic voltammetry was used to ascertain the chemical functionalities present and to understand the potential ranges in which the materials were electroactive. Electrochemical treatment with either an oxidative or a reductive fixed potential were then carried out to activate these chemically modified graphenes. The effects of such electrochemical treatments on their electrocatalytic properties were then investigated by cyclic voltammetry in the presence of well‐known redox probes, such as [Fe(CN)₆]^4?/3?, Fe^3+/2+, [Ru(NH₃)₆]^2+/3+, and ascorbic acid. Thermally reduced graphene oxide exhibited the best electrochemical behavior amongst all of the CMGs, with the fastest rate of heterogeneous electron transfer (HET) and the lowest overpotentials. These findings will have far‐reaching consequences for the evaluation of different CMGs as electrode materials in electrochemical devices.     

Transition Metal (Mn,Fe, Co,Ni)‐Doped Graphene Hybrids for Electrocatalysis

The development of electrocatalysts is crucial for renewable energy applications. Metal‐doped graphene hybrid materials have been explored for this purpose, however, with much focus on noble metals, which are limited by their low availability and high costs. Transition metals may serve as promising alternatives. Here, transition metal‐doped graphene hybrids were synthesized by a simple and scalable method. Metal‐doped graphite oxide precursors were thermally exfoliated in either hydrogen or nitrogen atmosphere; by changing exfoliation atmospheres from inert to reductive, we produced materials with different degrees of oxidation. Effects of the presence of metal nanoparticles and exfoliation atmosphere on the morphology and electrocatalytic activity of the hybrid materials were investigated using electron microscopy, energy‐dispersive X‐ray spectroscopy, X‐ray photoelectron spectroscopy, and cyclic voltammetry. Doping of graphene with transition metal nanoparticles of the 4th period significantly influenced the electrocatalysis of compounds important in energy production and storage applications, with hybrid materials exfoliated in nitrogen atmosphere displaying superior performance over those exfoliated in hydrogen atmosphere. Moreover, nickel‐doped graphene hybrids displayed outstanding electrocatalytic activities towards reduction of O₂ when compared to bare graphenes. These findings may be exploited in the research field of renewable energy.     

Exfoliation and Noncovalent Functionalization of Graphene Surface with Poly-N-Vinyl-2-Pyrrolidone by In Situ Polymerization

Heteroatom functionalization on a graphene surface can endow the physical and structural properties of graphene. Here, a one-step in situ polymerization method was used for the noncovalent functionalization of a graphene surface with poly-N-vinyl-2-pyrrolidone (PNVP) and the exfoliation of graphite into graphene sheets. The obtained graphene/poly-N-vinyl pyrrolidone (GPNVP) composite was thoroughly characterized. The surface morphology of GPNVP was observed using field emission scanning electron microscopy and high-resolution transmission electron microscopy. Raman spectroscopy and X-ray diffraction studies were carried out to check for the exfoliation of graphite into graphene sheets. Thermogravimetric analysis was performed to calculate the amount of PNVP on the graphene surface in the GPNVP composite. The successful formation of the GPNVP composite and functionalization of the graphene surface was confirmed by various studies. The cyclic voltammetry measurement at different scan rates (5–500 mV/s) and electrochemical impedance spectroscopy study of the GPNVP composite were performed in the typical three-electrode system. The GPNVP composite has excellent rate capability with the capacitive property. This study demonstrates the one-pot preparation of exfoliation and functionalization of a graphene surface with the heterocyclic polymer PNVP; the resulting GPNVP composite will be an ideal candidate for various electrochemical applications.     

Selective Removal of Hydroxyl Groups from Graphene Oxide

Graphene has a wide range of potential applications, thus tremendous efforts have been put into ensuring that the most direct and effective methods for its large‐scale production are developed. The formation of graphene materials from graphene oxide through a chemical reduction method is still one of the most preferred routes. Numerous methods starting from various reducing agents have been developed to obtain near‐pristine graphene sheets. However, most of the reducing agents are not mechanistically supported by classical organic chemistry knowledge and of those that are supported, they are only theoretically capable of, at most, reducing oxygen‐containing groups on graphene oxide to hydroxyl groups. Herein, we present a mechanistically proven method for the selective defunctionalisation of hydroxyl groups from graphene oxide that is based on ethanethiol–aluminium chloride complexes and provides a graphene material with improved properties. The structural, morphological and electrochemical properties of the graphene materials have been fully characterised based on high‐resolution X‐ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, scanning electron microscopy, electrochemical impedance spectroscopy and cyclic voltammetry techniques. Our analyses showed that the obtained graphene materials exhibited high heterogeneous electron‐transfer rates, low charge‐transfer resistance and high conductivity as compared to the parent graphene oxide. Moreover, the selective defunctionalisation of hydroxyl groups could potentially allow for the tailoring of graphene properties for various applications.     

Graphene Sheet Orientation of Parent Material Exhibits Dramatic Influence on Graphene Properties

The production of graphene from various sources has garnered much attention in recent years with the development of methods that range from “bottom‐up” to “top‐down” approaches. The top‐down approach often requires thermal treatment to obtain a few‐layered and lowly oxygenated graphene sheets. Herein, we demonstrate the production of graphene through oxidation and thermal‐reduction/exfoliation of two sources of differently orientated graphene sheets: multiwalled carbon nanotubes (MWCNTs) and stacked graphene nanofibers (SGNFs). These two carbon‐nanofiber‐like materials have similar axial (length: 5–9 μm) and lateral dimensions (diameter: about 100 nm). We demonstrate that, whereas SGNFs exfoliate along the lateral plane between adjacent graphene sheets, carbon nanotubes exfoliate along its longitudinal axis and leads to opening of the carbon nanotubes owing to the built‐in strain. Subsequent thermal exfoliation leads to graphene materials that have, despite the fact that their parent materials exhibited similar dimensions, dramatically different proportions and, consequently, materials properties. Graphene that was prepared from MWCNTs exhibited dimensions of about 5000×300 nm, whereas graphene that was prepared from SGNFs exhibited sheets with dimensions of about 50×50 nm. The density of defects and oxygen‐containing groups on these materials are dramatically different, as are the electrochemical properties. We performed morphological, structural, and electrochemical characterization based on TEM, SEM, high‐resolution X‐ray photoelectron spectroscopy, Raman spectroscopy, and cyclic voltammetry (CV) analysis on the stepwise conversion of the target source into the exfoliated graphene. Morphological and structural characterization indicated the successful chemical and thermal treatment of the materials. Our findings have shown that the orientation of the graphene sheets in starting materials has a dramatic influence on their chemical, material, and electrochemical properties.     

Graphene Oxide as a Platform for Copper Pentacyanonitrosylferrate Nanoparticles and their Behavior in the Electro‐oxidation of N‐Acetylcysteine

This work describes the preparation of graphene oxide by the Modified Hummers Method and the chemical modification of its surface with nanoparticles of copper pentacyanonitrosylferrate(III) (GOCuNP). The materials obtained were characterized by Raman spectroscopy, x‐ray photoelectron spectroscopy and transmission electron microscopy. The GOCuNP was characterized by cyclic voltammetry using a graphite paste electrode that presented electrocatalytic response for N‐acetylcysteine with detection limit of 2.97×10^?5 mol L^?1 at concentration range of 3.00×10^?5 to 6.00×10^?3 mol L^?1 of N‐acetylcysteine. By this way, the bimetallic complex formed is included in the list of materials obtained as potential candidates for the construction of electrochemical sensors for N‐acetylcysteine detection.     

氧化石墨烯/聚吡咯插层复合材料的制备和电化学电容性能

以FeCl₃-甲基橙(MO)为模板, 通过化学原位聚合法成功制备出氧化石墨烯/聚吡咯(GO/PPy)插层复合材料. 采用X射线衍射(XRD)、傅里叶变换红外(FTIR)光谱、扫描电镜(SEM)和透射电镜(TEM)等测试技术对复合材料进行物性表征. 此外, 利用循环伏安、恒电流充放电和交流阻抗测试方法对复合材料在两种不同水系电解液(1 mol·L^-1 Na₂SO₄和1 mol·L^-1 H₂SO₄)中的电化学性能进行了研究. 结果显示: 氧化石墨烯和聚吡咯表现出各自优势并发挥协同作用, 使得GO/PPy插层复合材料在中性和酸性电解液中都显示出可观的比电容. 电流密度为0.5 A·g^-1时, GO/PPy 插层复合材料在Na₂SO₄和H₂SO₄电解液中的比电容分别为449.1 和619.0 F·g^-1, 明显高于纯PPy的比电容. 经过800 次循环稳定性测试后, 两种不同电解液中, 复合材料初始容量的保持率分别为92%和62%. 其中酸性电解液体系中初始容量更大, 而中性溶液中具有更稳定的循环性能.     

Precise Tuning of Surface Composition and Electron‐Transfer Properties of Graphene Oxide Films through Electroreduction

Graphene materials are generally prepared from the exfoliation of graphite oxide (GO) to graphene oxide, followed by subsequent chemical or thermal reduction. These methods, although efficient in removing most of the oxygen functionalities from the GO material, lack control over the extent of the reduction process. We demonstrate here an electrochemical reduction procedure that not only allows for precise control of the reduction process to obtain a graphene material with a well‐defined C/O ratio in the range of 3 to 10, but also one that is able to tune the electrocatalytic properties of the reduced material. A method that is able to precisely control the amount and density of the oxygen functionalities on the graphene material as well as its electrochemical behaviour is very important for several applications such as electronics, bio‐composites and electrochemical devices.     

石墨烯/聚苯胺复合材料的制备及其电化学性能

以苯胺和氧化石墨烯(GO)为原料, 采用电化学方法制备了石墨烯/聚苯胺(GP)复合材料. 利用X射线衍射(XRD)、扫描电镜(SEM)、拉曼(Raman)光谱、X射线光电子能谱分析(XPS)对其结构、微观形貌进行了表征,并对复合材料电化学性能进行了测试. 结果表明, 复合材料保持了石墨烯的基本形貌, 聚苯胺颗粒均匀地分散在石墨烯表面, 复合材料在500 mA·g^-1的电流密度下比电容达到352 F·g^-1, 1000 mA·g^-1下比电容为315 F·g^-1, 经过1000 次的充放电循环后容量保持率达到90%, 远大于石墨烯和聚苯胺单体的比电容. 复合材料放电效率高, 电解质离子易于在电极中扩散和迁移.     

Electrochemistry at chemically modified graphenes

Electrochemical applications of graphene are of great interest to many researchers as they can potentially lead to crucial technological advancements in fabrication of electrochemical devices for energy production and storage, and highly sensitive sensors. There are many routes towards fabrication of bulk quantities of chemically modified graphenes (CMG) for applications such as electrode materials. Each of them yields different graphene materials with different functionalities and structural defects. Here, we compare the electrochemical properties of five different chemically modified graphenes: graphite oxide, graphene oxide, thermally reduced graphene oxide, chemically reduced graphene oxide, and electrochemically reduced graphene oxide. We characterized these materials using transmission electron microscopy, Raman spectroscopy, high-resolution X-ray photoelectron spectroscopy, electrochemical impedance spectroscopy, and cyclic voltammetry, which allowed us to correlate the electrochemical properties with the structural and chemical features of the CMGs. We found that thermally reduced graphene oxide offers the most favorable electrochemical performance among the different materials studied. Our findings have a profound impact for the applications of chemically modified graphenes in electrochemical devices.     

Modular Covalent Graphene Functionalization with C60 and the Endohedral Fullerene Sc3N@C80: A Facile Entry to Synthetic‐Carbon‐Allotrope Hybrids

Two novel graphene‐fullerene hybrid structures, containing C₆₀ and endohedral Sc₃N@C₈₀ bound to graphene, instead of the formerly used graphene oxide, were efficiently synthesized via a reductive activation/exfoliation approach starting from pristine graphite. The structures of these multifunctional hybrid systems were unambiguously characterized by statistical Raman spectroscopy, TG‐MS, TG‐GC‐MS, and LD‐TOF mass spectroscopy, confirming the covalent bonding of the respective C₆₀/Sc₃N@C₈₀ moieties to the pristine graphene. Furthermore, assisted by temperature‐dependent Raman spectroscopy studies the corresponding defunctionalization processes were also investigated. Finally, the formation of a carbon allotrope hybrid material on the basis of C₆₀/Sc₃N@C₈₀ moieties coupled to graphene could be visualized by HRTEM.     

Concurrent Phosphorus Doping and Reduction of Graphene Oxide

Doped graphene materials are of huge importance because doping with electron‐donating or electron‐withdrawing groups can significantly change the electronic structure and impact the electronic and electrochemical properties of these materials. It is highly important to be able to produce these materials in large quantities for practical applications. The only method capable of large‐scale production is the oxidative treatment of graphite to graphene oxide, followed by its consequent reduction. We describe a scalable method for a one‐step doping of graphene with phosphorus, with a simultaneous reduction of graphene oxide. Such a method is able to introduce significant amount of dopant (3.65 at. %). Phosphorus‐doped graphene is characterized in detail and shows important electronic and electrochemical properties. The electrical conductivity of phosphorus‐doped graphene is much higher than that of undoped graphene, owing to a large concentration of free carriers. Such a graphene material is expected to find useful applications in electronic, energy storage, and sensing devices.     

Layered compounds based on perforated graphene

A method to obtain previously unknown layered structure composed of stacks of perforated graphene sheets is developed. The method consists in the thermal decomposition of graphite oxide in the concentrated H₂SO₄ and H₃PO₄ medium. In order to confirm the presence of holes in graphene layers, a large set of chemical and physicochemical analysis methods are applied. Based on a new matrix, treated thermally and chemically, layered compounds are obtained: oxide, fluoride, and fluoroxide of two types. The obtained compounds are analyzed by transmission electron microscopy, infrared absorption spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, thermogravimetric analysis, and powder X-ray diffraction.     

Thermal exfoliation of electrochemically synthesized graphite intercalation compound with perrhenic acid

In present work, we describe the synthesis of graphite intercalation compounds with perrhenic acid (HReO₄-GIC) through the anodic oxidation of graphite in aqueous perrhenic acid solution and their thermal exfoliation. Due to electrochemical treatment of graphite in perrhenic acid solution, ReO₄⁻ ions are intercalated into interlayer spaces of graphite. Anodic oxidation of graphite in HReO₄ solution leads to the formation of 3-stage GIC. Simultaneously, some amount of perrhenic acid becomes deposited on the graphite surface and edges. In the next step, thermal treatment of the previously synthesized GIC was performed, causing both the exfoliation of graphitic structure and transformation of perrhenic acid into rhenium oxides on the surface of graphene layers. The yielded product was exfoliated graphite-ReO₂/ReO₃ composite. The obtained composite was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and Raman spectroscopy. Additionally, specific surface area of the exfoliated materials was measured.

     

Synthesis of Phosphorus‐Doped Graphene and its Wide Potential Window in Aqueous Supercapacitors

Phosphorus‐doped (P‐doped) graphene with the P doping level of 1.30 at % was synthesized by annealing the mixture of graphene and phosphoric acid. The presence of P was confirmed by elemental mapping and X‐ray photoelectron spectroscopy, while the morphology of P‐doped graphene was revealed by using scanning electron microscopy and transmission electron microscopy. To investigate the effect of P doping, the electrochemical properties of P‐doped graphene were tested as a supercapacitor electrode in an aqueous electrolyte of 1 M H₂SO₄. The results showed that doping of P in graphene exhibited significant improvement in terms of specific capacitance and cycling stability, compared with undoped graphene electrode. More interestingly, the P‐doped graphene electrode can survive at a wide voltage window of 1.7 V with only 3 % performance degradation after 5000 cycles at a current density of 5 A g^?1, providing a high energy density of 11.64 Wh kg^?1 and a high power density of 831 W kg^?1.     

Platelet Graphite Nanofibers for Electrochemical Sensing and Biosensing: The Influence of Graphene Sheet Orientation

Here, we demonstrate that platelet graphite nanofibers (PGNFs) exhibit fast heterogeneous electron‐transfer rates for a wide variety of compounds such as FeCl₃, ferrocyanide, dopamine, uric acid, ascorbic acid, and the reduced form of β‐nicotinamide adenine dinucleotide. The electrochemical properties of PGNFs are superior to those of multiwalled carbon nanotubes (MWCNTs) or graphite microparticles (GMPs). Transmission electron microscopy and Raman spectroscopy reveal that this arises from the unique graphene sheet orientation of such platelet nanofibers, which accounts for their unparalleled high ratio of graphene edge planes versus basal planes.     

PtPd-nanoparticles supported by new carbon materials

Nanocomposites based on PtPd nanoparticles with chemical ordering like disordered solid solution on surface of multilayer graphene have been prepared through thermal shock of mechanically obtained mixture of double complex salt [Pd(NH₃)₄][PtCl₆] and different carbon materials–exfoliated graphite, graphite oxide and graphite fluoride. An effect of original carbon precursors on formation of PtPd bimetallic nanoparticles was studied using X-ray absorption spectroscopy (XAFS), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). It was shown that the distribution of bimetallic nanoparticles over the multilayer graphene surface as well as the particles size distribution is controlled by the graphene precursors. For all nanocomposites, the surface of the nanoparticles was found to be Pd-enriched. In case when the thermal exfoliated graphite and graphite oxide were used as the graphene precursors a thin graphitized layer covered the nanoparticles surface. Such a graphitized layer was not observed in the nanocomposite, which used the fluorinated graphite as the precursor.     

The Formation of Large‐Area Conducting Graphene‐Like Platelets

The treatment of a suspension of graphite oxide (GO) with sodium azide leads to a material that, after reduction, features amino groups at the top and bottom of the sheets. These groups react through microcontact printing with an isothiocyanate monolayer on a silicon oxide substrate to form covalent bonds that strongly attach to the particles on the surface. With ultrasonication it is possible to obtain exfoliation of the sheets that are not covalently bound to the surface leaving single‐layer platelets attached to the substrate. The azido derivative can be also used to functionalize the graphene oxide with long alkylic chains through a click chemistry approach. This functionalization results in the exfoliation of this material in dimethylformamide. The novel materials were fully characterized by different techniques including IR spectroscopy, thermogravimetric analysis (TGA), scanning and transmission electron microscopy (SEM and TEM), X‐Ray photoelectron spectroscopy (XPS), and solid state NMR spectroscopy. The material with amino groups, after the reduction step, is conductive with a resistivity only approximately seven times larger than that of unprocessed graphite. This implies that after reduction of the GO, the conjugated sp² network is largely restored. We consider this to be an important step towards a chemical approach for forming conducting large‐area platelet films of single‐layer graphene.