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.     

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.     

Graphene materials preparation methods have dramatic influence upon their capacitance

Graphene related materials are widely expected to play a major role as materials for the construction of supercapacitors. We demonstrate here that graphene oxides prepared by various well-established methods exhibit dramatically different capacitances. We exfoliated graphite oxide sonographically to graphene oxide (GO) and we reduced GO by chemical or electrochemical means to chemically reduced graphene oxide (CRGO) and electrochemically reduced graphene oxide (ERGO); in addition, graphite oxide was thermally exfoliated leading to thermally reduced graphene oxide (TRGO). We found clear dependence of weight specific capacitance upon amount of oxygen containing groups presented on the surface of these graphenes. GO exhibits the lowest and TRGO the highest values of weight specific capacitance.     

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.     

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.     

Labeling Graphene Oxygen Groups with Europium

Graphene‐related materials contain chemically bonded oxygen atoms in the form of epoxy, hydroxy, carboxy, and carbonyl groups. It is important to determine the quantity of oxygen atoms and to understand their position on the graphene sheet. However, visualization of these groups by standard methods is a challenge. Here, we utilize europium(III) as a selective label for oxygen‐containing groups. We studied three different graphene‐related materials: 1) graphene oxide, 2) chemically reduced graphene oxide, and 3) thermally reduced graphene oxide (the number of oxygen containing groups decreases from material 1 to 3). We show that it is possible to efficiently use Eu as a label of oxygen‐containing groups. This Eu label could be applied to determine the precise location of oxygen‐containing groups on graphene sheets and also induce novel optical, electrochemical, and catalytic properties.     

Mesomeric Effects of Graphene Modified with Diazonium Salts: Substituent Type and Position Influence its Properties

In the last decade, graphene and graphene derivatives have become some of the most intensively studied materials. Tuning of the electronic and electrochemical properties of graphene is of paramount importance. In this study, six diazonium‐modified graphenes containing different functional groups according to the diazonium salt precursor were investigated. These diazonium moieties have a strong mesomeric (resonance) effect and act as either electron‐donating or ‐withdrawing species. Different graphene precursors, such as thermally and chemically reduced graphenes were studied. All the products were characterized in detail by elemental combustion analysis, FTIR spectroscopy, Raman spectroscopy, high‐resolution X‐ray photoelectron spectroscopy (XPS), and cyclic voltammetry. Resistivity and zeta potential measurements were consistent with theoretical (DFT) calculations. The results show that chemical modification of graphene by diazotation strongly influences its properties, creating a huge application potential in microelectronics, energy storage and conversion devices, and electrocatalysis.     

Electrochemistry of graphene: new horizons for sensing and energy storage

Graphene is a new 2D nanomaterial with outstanding material, physical, chemical, and electrochemical properties. In this review, we first discuss the methods of preparing graphene sheets and their chemistry. Following that, the fundamental reasons governing the electrochemistry of graphene are meaningfully described. Graphene is an excellent electrode material with the advantages of conductivity and electrochemistry of sp² carbon but without the disadvantages related to carbon nanotubes, such as residual metallic impurities. We highlight important applications of graphene and graphene nanoplatelets for sensing, biosensing, and energy storage. © 2009 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 9: 211–223; 2009: Published online in Wiley InterScience ( www.interscience.wiley.com ) DOI 10.1002/tcr.200900008     

Fluorographites (CFx)n Exhibit Improved Heterogeneous Electron‐Transfer Rates with Increasing Level of Fluorination: Towards the Sensing of Biomolecules

Halogenated sp² materials are of high interest owing to their important electronic and electrochemical properties. Although methods for graphite and graphene fluorination have been extensively researched, the fundamental electrochemical properties of fluorinated graphite are not well established. In this paper, the electrochemistry of three fluorographite materials of different carbon‐to‐fluorine ratio were studied: (CF_0.33)_n, (CF_0.47)_n, and (CF_0.75)_n. Our findings reveal that the carbon‐to‐fluorine ratio of fluorographite will impact the electrochemical performance. Faster heterogeneous electron‐transfer (HET) rates and lowered oxidation potentials for ascorbic acid and uric acid are progressively obtained with increasing fluorine content. The fluorographite (CF_0.75)_n was in fact found to exhibit the most improved electrochemical performances with the fastest HET rates and significantly lowered overpotentials in the oxidation of ascorbic acid. Analytical parameters such as sensitivity and linearity were subsequently investigated by applying the fluorographite (CF_0.75)_n in the analysis of ascorbic acid and uric acid, which can be simultaneously detected. We determined good linear responses towards the detection of both ascorbic and uric acid. Fluorographites outperform graphites in sensing applications, which will have a profound impact on applications of fluorographites and fluorographene in sensing and biosensing.     

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.     

Highly Hydrogenated Graphene through Microwave Exfoliation of Graphite Oxide in Hydrogen Plasma: Towards Electrochemical Applications

Hydrogenated graphenes exhibit a variety of properties with potential applications in devices, ranging from a tunable band gap to fluorescence, ferromagnetism, and the storage of hydrogen. We utilize a one‐step microwave‐irradiation process in hydrogen plasma to create highly hydrogenated graphene from graphite oxides. The procedure serves the dual purposes of deoxygenation and concurrent hydrogenation of the carbon backbone. The effectiveness of the hydrogenation process is investigated on three different graphite oxides (GOs), which are synthesized by using the Staudenmaier, Hofmann, and Hummers methods. A systematic characterization of our hydrogenated graphenes is performed using UV/Vis spectroscopy, SEM, AFM, Raman spectroscopy, FTIR spectroscopy, X‐ray photoelectron spectroscopy (XPS), combustible elemental analysis, and electrical conductivity measurements. The highest hydrogenation extent is observed in hydrogenated graphene produced from the Hummers‐method GO, with a hydrogen content of 19 atomic % in the final product. In terms of the removal of oxygen groups, microwave exfoliation yields graphenes with very similar oxygen contents despite differences in their parent GOs. In addition, we examine the prospective application of hydrogenated graphenes as electrochemical transducers through a cyclic voltammetry (CV) study. The highly hydrogenated graphenes exhibit fast heterogeneous electron‐transfer rates, suggestive of their suitability for electrochemical applications in electrodes, supercapacitors, batteries, and sensors.     

基于还原石墨烯/纳米银复合材料的电化学传感器测定双酚A

石墨烯特有的褶皱层状结构以及银纳米粒子良好的催化性能,使其在电化学方面具有良好的应用潜能.本研究以柠檬酸钠为还原剂,通过水热反应原位制备出还原石墨烯/纳米银复合材料(rGO/AgNPs),用于修饰玻碳电极,研究了双酚A的电化学行为.循环伏安法(CV)和方波伏安法(SWV)的实验结果表明,双酚A可以在rGO/AgNPs修饰电极表面发生快速的氧化还原反应,基于此实现了对双酚A的高灵敏检测.在最优条件下,双酚A的氧化峰电流与其浓度在0.1~40.0μmol/L范围内呈良好的线性关系(r2=0.996),检出限为50.7 nmol/L(S/N=3).将其用于实际环境和塑料样品中双酚A的检测,回收率为91.7%~102.9%.     

The Structural Stability of Graphene Anticorrosion Coating Materials is Compromised at Low Potentials

Corrosion of engineered structures is a major problem causing an estimated economic loss of more than 2 trillion US dollars annually worldwide. Graphene has recently emerged as highly promising, low‐cost, and transparent anticorrosion coating material. Herein, it is shown that a multilayer graphene film grown on Ni by chemical vapor deposition undergoes abrupt stability failure under galvanic‐corrosion conditions. The multilayer graphene coating was examined by optical microscopy, SEM, energy dispersive X‐ray spectroscopy, Raman spectroscopy, and cyclic voltammetry after exposure to potentials between 600 and 1300 mV in alkaline solution. A fast and simple electrochemical method is proposed to sensitively quantify the damage caused by the applied potential bias. It is based on quantification of the oxidation signals generated by the underlying Ni‐metal catalyst that is exposed by damage to the graphene film. It is shown that film damage can start at potentials as low as 900 mV and that macroscopic and extensive damage can be caused at potentials above 1000 mV. In addition, once the graphene film has been damaged, the corrosion rate of the underlying metal is significantly increased. These findings are of great importance for potential applications of multilayer graphene films in coating metal structures with huge industrial and economic implications.     

One‐Pot Synthesis,Characterization, and Enhanced Photocatalytic Activity of a BiOBr–Graphene Composite

Herein, a chemically bonded BiOBr–graphene composite (BiOBr–RG) was prepared through a facile in situ solvothermal method in the presence of graphene oxide. Graphene oxide could be easily reduced to graphene under solvothermal conditions, and simultaneously BiOBr nanoplates with pure tetragonal phase were grown uniformly on the graphene surface. The structure and photoelectrochemical properties of the resulting materials were characterized by transmission electron microscopy (TEM), X‐ray diffraction (XRD), Fourier‐transform infrared (FTIR) spectroscopy, Raman spectroscopy, X‐ray photoelectron spectroscopy (XPS), and impedance and photocurrent action measurements. The combination of BiOBr and graphene introduces some properties of graphene into the photocatalysis reaction, such as excellent conductivity, adsorptivity, and controllability. A remarkable threefold enhancement in the degradation of rhodamine B (RhB) was observed with as‐prepared BiOBr–RG as compared with pure BiOBr under visible light (λ>420 nm). The enhanced photocatalytic activity could be attributed to the great adsorptivity of dyes, the extended photoresponse range, the negative shift in the Fermi level of BiOBr–RG, and the high migration efficiency of photoinduced electrons, which may effectively suppress the charge recombination.     

n‐Type Reduced Graphene Oxide Field‐Effect Transistors (FETs) from Photoactive Metal Oxides

Graphene is of considerable interest as a next‐generation semiconductor material to serve as a possible substitute for silicon. For real device applications with complete circuits, effective n‐type graphene field effect transistors (FETs) capable of operating even under atmospheric conditions are necessary. In this study, we investigated n‐type reduced graphene oxide (rGO) FETs of photoactive metal oxides, such as TiO₂ and ZnO. These metal oxide doped FETs showed slight n‐type electric properties without irradiation. Under UV light these photoactive materials readily generated electrons and holes, and the generated electrons easily transferred to graphene channels. As a result, the graphene FET showed strong n‐type electric behavior and its drain current was increased. These n‐doping effects showed saturation curves and slowly returned back to their original state in darkness. Finally, the n‐type rGO FET was also highly stable in air due to the use of highly resistant metal oxides and robust graphene as a channel.     

Synthesis of Graphene Oxide by Oxidation of Graphite with Ferrate(VI) Compounds: Myth or Reality?

It is well established that graphene oxide can be prepared by the oxidation of graphite using permanganate or chlorate in an acidic environment. Recently, however, the synthesis of graphene oxide using potassium ferrate(VI) ions has been reported. Herein, we critically replicate and evaluate this new ferrate(VI) oxidation method. In addition, we test the use of potassium ferrate(VI) for the synthesis of graphene oxide under various experimental routes. The synthesized materials are analyzed by a number of analytical methods in order to confirm or disprove the possibility of synthesizing graphene oxide by the ferrate(VI) oxidation route. Our results confirm the unsuitability of using ferrate(VI) for the oxidation of graphite on graphene oxide because of its high instability in an acidic environment and low oxidation power in neutral and alkaline environments.     

Chemically Modified Graphene: The Influence of Structural Properties on the Assessment of Antioxidant Capacity

Graphene materials obtained by different synthetic routes possess dissimilar amount of defects and surface functionalities, which can influence their electrochemical performance towards the detection of electroactive probes. Oxygen‐containing groups can be either detrimental to the heterogeneous charge transfer or promote favorable interactions between the graphene surface and the analyte of interest, depending on the structure of the latter. Here, we compared three chemically modified graphenes, obtained by various procedures and carrying different amounts of oxygen functionalities, for the detection of standard gallic acid, a compound commonly used as an index of the antioxidant capacity of food and beverages. We found that electrochemically reduced graphene provided the best electrochemical performance in terms of calibration sensitivity, selectivity, and linearity of response. Our findings are important in order to understand the suitability of graphene platforms for the assessment of food quality.     

Geographical and Geological Origin of Natural Graphite Heavily Influence the Electrical and Electrochemical Properties of Chemically Modified Graphenes

Natural graphite is an important precursor for the production of chemically modified graphenes in bulk quantities for electrochemical applications. These natural graphites have varying fundamental properties due to the different geological processes and environments at their points of origin, which are expected to affect their chemical reactivity and hence the properties of the derived graphene materials. Four different natural graphites with known geographical and geological origins were exposed to a modified Hummers oxidation method and the resulting graphite oxides were studied. The graphite oxides were shown to have different extents of oxidation and types of oxygen groups, which directly influenced their electrochemical properties. These differences were propagated further in the subsequent chemical reduction of the graphite oxides, and the reduced graphene oxides exhibited significantly different reduction efficiencies and electrical conductivities. These findings show that the choice of natural graphite of known origin is important to synthesize chemically modified graphenes with a desired set of properties.     

石墨烯/聚吡咯纳米纤维超级电容器电极材料的制备及其电化学性能

超级电容器因其具有较高的循环稳定性和较好的能量密度而成为储能器件中的研究热点,其电极材料及制备方法是决定超级电容器电化学性能的关键因素。 本文以聚环氧乙烷-聚环氧丙烷-聚环氧乙烷三嵌段共聚物(P123)为软模板,通过一步原位聚合法成功地制备了石墨烯/聚吡咯纳米纤维(GR/PPy NF)复合超级电容器电极材料。 通过X射线衍射(XRD),X射线光电子能谱(XPS)、透射电子显微镜(TEM)和傅里叶变换红外光谱仪(FT-IR)等对复合材料的结构和形态进行了系统的表征。 利用电化学方法对GR/PPy NF复合电极材料的电化学性能进行了系统的分析。 结果表明,在电流密度0.5 A/g下,纳米复合材料的比电容量高达969.5 F/g,在充放电600圈之后,仍可保留初始比电容的88%,展示了良好的电容性能及循环稳定性。 GR/PPy NF制备简单,性能优异,是一种很有前途的能量转换/存储材料。