Hydroboration of graphene oxide: towards stoichiometric graphol and hydroxygraphane.

Covalently functionalized graphene materials with well-defined stoichiometric composition are of a very high importance in the research of 2D carbon material family due to their well-defined properties. Unfortunately, most of the contemporary graphene-functionalized materials do not have this kind of defined composition and, usually, the amount of heteroatoms bonded to graphene framework is in the range of 1-10 at. %. Herein, we show that by a well-established hydroboration reaction chain, which introduces -BH2 groups into the graphene oxide structure, followed by H2O2 or CF3COOH treatment as source of -OH or -H, we can obtain highly hydroxylated compounds of precisely defined composition with a general formula (C1O0.78H0.75)n, which we named graphol and highly hydroxylated graphane (C1(OH)0.51H0.14)n, respectively. These highly functionalized materials with an accurately defined composition are highly important for the field of graphene derivatives. The enhanced electrochemical performance towards important biomarkers as well as hydrogen evolution reaction is demonstrated.

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.

Capacitance of p- and n-doped graphenes is dominated by structural defects regardless of the dopant type.

Graphene materials possess attractive properties that can be used for the fabrication of supercapacitors with enhanced energy-storage performance. It has been shown that both boron and nitrogen doping of graphene can improve the intrinsic capacitance of the material relative to the undoped precursor. We address the question of whether p-doping (using boron as dopant) or n-doping (using nitrogen as dopant) leads to increased capacitance relative to undoped graphene materials. Using thermal exfoliation we synthesized both boron- and nitrogen-doped graphene materials and measured capacitance relative to the undoped material. After a full characterization by SEM analysis, X-ray photoelectron spectroscopy, Raman spectroscopy, gamma-ray activation analysis, Brunauer-Emmett-Teller analysis, and electrochemical techniques we demonstrate that the doping process does not lead to enhancement of capacitive behavior and that the main characteristic influencing capacitance is the presence of structural defects within the graphitic structure, independent of doping level.

Transition metal-depleted graphenes for electrochemical applications via reduction of CO2 by lithium.

Graphene has immense potential for future applications in the electrochemical field, such as in supercapacitors, fuel cells, batteries, or sensors. Graphene materials for such applications are typically fabricated through a top-down approach towards oxidation of graphite to graphite oxide, with consequent exfoliation/reduction to yield reduced graphenes. Such a method allows the manufacture of graphenes in gram/kilogram quantities. However, graphenes prepared by this method can contain residual metallic impurities from graphite which dominate the electrochemical properties of the graphene formed. This dominance hampers their electrochemical application. The fabrication of transition metal-depleted graphene is described, using ultrapure CO2 (with benefits of low cost and easy availability) and elemental lithium by means of reduction of CO2 to graphene. This preparation method produces graphene of high purity with electrochemical behavior that is not dominated by any residual transition metal impurities which would dramatically alter its electrochemical properties. Wide application of such methodology in industry and research laboratories is foreseen, especially where graphene is used for electrochemical devices.

Boron-doped graphene and boron-doped diamond electrodes: detection of biomarkers and resistance to fouling.

Doped carbon materials are of high interest as doping can change their properties. Here we wish to contrast the electrochemical behaviour of two carbon allotropes - sp(3) hybridized carbon as diamond and sp(2) hybridized carbon as graphene - doped by boron. We show that even though both materials exhibit similar heterogeneous electron transfer towards ferro/ferricyanide, there are dramatic differences towards the oxidation of biomolecules, such as ascorbic acid, uric acid, dopamine and ß-nicotinamide adenine dinucleotide (NADH). The boron-doped graphene exhibits much lower oxidation potentials than boron-doped diamond. The stability of the surfaces towards NADH oxidation product fouling has been studied and in the long term, there is no significant difference among the studied materials. The proton/electron coupled reduction of dopamine and nitroaromatic explosive (TNT) takes place on boron-doped graphene, while it is not observable at boron-doped diamond. These findings show that boron-doped sp(2) graphene and sp(3) diamond behave, in many aspects, dramatically differently and this shall have a profound influence upon their applicability as electrochemical materials.

Searching for magnetism in hydrogenated graphene: using highly hydrogenated graphene prepared via Birch reduction of graphite oxides.

Fully hydrogenated graphene (graphane) and partially hydrogenated graphene materials are expected to possess various fundamentally different properties from graphene. We have prepared highly hydrogenated graphene containing 5% wt of hydrogen via Birch reduction of graphite oxide using elemental sodium in liquid NH3 as electron donor and methanol as proton donor in the reduction. We also investigate the influence of preparation method of graphite oxide, such as the Staudenmaier, Hofmann or Hummers methods on the hydrogenation rate. A control experiment involving NaNH2 instead of elemental Na was also performed. The materials were characterized in detail by electron microscopy, infrared spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy both at room and low temperatures, X-ray fluorescence spectroscopy, inductively coupled plasma optical emission spectroscopy, combustible elemental analysis and electrical resistivity measurements. Magnetic measurements are provided of bulk quantities of highly hydrogenated graphene. In the whole temperature range up to room temperature, the hydrogenated graphene exhibits a weak ferromagnetism in addition to a contribution proportional to field that is caused not only by diamagnetism but also likely by an antiferromagnetic influence. The origin of the magnetism is also determined to arise from the hydrogenated graphene itself, and not as a result of any metallic impurities.

Sulfur-doped graphene via thermal exfoliation of graphite oxide in H2S, SO2, or CS2 gas.

Doping of graphene with heteroatoms is an effective way to tailor its properties. Here we describe a simple and scalable method of doping graphene lattice with sulfur atoms during the thermal exfoliation process of graphite oxides. The graphite oxides were first prepared by Staudenmaier, Hofmann, and Hummers methods followed by treatments in hydrogen sulfide, sulfur dioxide, or carbon disulfide. The doped materials were characterized by scanning electron microscopy, high-resolution X-ray photoelectron spectroscopy, combustible elemental analysis, and Raman spectroscopy. The ζ-potential and conductivity of sulfur-doped graphenes were also investigated in this paper. It was found that the level of doping is more dramatically influenced by the type of graphite oxide used rather than the type of sulfur-containing gas used during exfoliation. Resulting sulfur-doped graphenes act as metal-free electrocatalysts for an oxygen reduction reaction.

Purification of carbon nanotubes by high temperature chlorine gas treatment.

Carbon nanotubes (CNTs) have a tremendous amount of potential to become useful components for future practical applications that may become a part of everyday life. While the sp(2) carbon itself is a rather chemically inert material, the issue of residual metal nanoparticle catalysts remains a prominent barrier in the utilization of CNTs in many areas due to the strong influence of these metallic impurities on the redox chemistry of biomarkers. Even with a standard purification procedure, CNTs have been shown to still contain residual metal nanoparticle catalysts. As such, presented in this paper is an improved purification technique for treating the CNTs with the highly reactive Cl2 gas at an elevated temperature of 1000 °C for 10 min, which would result in the vaporization of the metallic impurities as MxCly, leading to a large decrease in the amount of metallic nanoparticle impurities within the CNTs. By means of electrochemistry and X-ray fluorescence analysis, we demonstrate that the behaviour of such Cl2 treated CNTs showed a significant shift towards that of high purity CNTs, with a dramatic decrease in the influence of the residual metallic impurities on the electrochemical behaviour of CNTs. Therefore it is suggested that the Cl2 treatment of carbon nanotubes is a highly promising route towards the production of pure CNTs.

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.

Halogenation of graphene with chlorine, bromine, or iodine by exfoliation in a halogen atmosphere.

Nanoarchitectonics on graphene implicates a specific and exact anchoring of molecules or nanoparticles onto the surface of graphene. One such example of an effective anchoring group that is highly reactive is the halogen moiety. Herein we describe a simple and scalable method for the introduction of halogen (chlorine, bromine, and iodine) moieties onto the surface of graphene by thermal exfoliation/reduction of graphite oxide in the corresponding gaseous halogen atmosphere. We characterized the halogenated graphene by using various techniques, including scanning and transmission electron microscopy, Raman spectroscopy, high-resolution X-ray photoelectron spectroscopy, and electrochemistry. The halogen atoms that have successfully been attached to the graphene surfaces will serve as basic building blocks for further graphene nanoarchitectonics.

High-pressure hydrogenation of graphene: towards graphane.

The conversion of graphene to graphane is of high importance from a technological and scientific point of view. We present here a scalable method for the hydrogenation of graphene based on thermal exfoliation of graphite oxide in a hydrogen atmosphere under high pressure (60-150 bar) and temperature (200-500 °C). This method does not require a plasma source and is able to produce gram quantities of the material. The properties of the resultant hydrogenated graphene were studied by scanning and transmission electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, infrared spectroscopy and combustible elemental analysis techniques. Sheet and specific resistance of the graphene and hydrogenated graphene were measured. This scalable synthesis method has great potential to serve as a pathway towards the mass production of graphane.

Influence of parent graphite particle size on the electrochemistry of thermally reduced graphene oxide.

Electrochemical applications of graphene are of very high importance. For electrochemistry, bulk quantities of materials are needed. The most common preparation of bulk quantities of graphene materials is based on oxidation of graphite to graphite oxide and subsequent thermal exfoliation of graphite oxide to thermally reduced graphene oxide (TR-GO). It is important to investigate to which extent a reaction condition, that is, composition of the oxidation mixture and size of graphite materials, influences the properties of the resulting materials. We characterised six graphite materials with a range of particle sizes (0.05, 11, 20, 32, 35 and 41 µm) and the TR-GO products prepared from them by use of scanning electron microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy. Cyclic voltammetric performance of the TR-GO samples was compared using ferro/ferricyanide and ascorbic acid. We observed no correlation between size of initial graphite and properties of the resultant TR-GO such as density of surface defects, amount of oxygen-containing groups, or rate of heterogeneous electron transfer (HET). A positive correspondence between HET rate and high defect density as well as low amounts of oxygen functionalities was noted. Our findings will have profound influence upon practical fabrication of graphene for applications in sensing and energy storage devices.

Noble metal (Pd, Ru, Rh, Pt, Au, Ag) doped graphene hybrids for electrocatalysis.

Metal decorated graphene materials are highly important for catalysis. In this work, noble metal doped-graphene hybrids were prepared by a simple and scalable method. The thermal reductions of metal doped-graphite oxide precursors were carried out in nitrogen and hydrogen atmospheres and the effects of these atmospheres as well as the metal components on the characteristics and catalytic capabilities of the hybrid materials were studied. The hybrids exfoliated in nitrogen atmosphere contained a higher amount of oxygen-containing groups and lower density of defects on their surfaces than hybrids exfoliated in hydrogen atmosphere. The metals significantly affected the electrochemical behavior and catalysis of compounds that are important in energy production and storage and in electrochemical sensing. Research in the field of energy storage and production, electrochemical sensing and biosensing as well as biomedical devices can take advantage of the properties and catalytic capabilities of the metal doped graphene hybrids.

Graphenes prepared by Staudenmaier, Hofmann and Hummers methods with consequent thermal exfoliation exhibit very different electrochemical properties.

Large-scale fabrication of graphene is highly important for industrial and academic applications of this material. The most common large-scale preparation method is the oxidation of graphite to graphite oxide using concentrated acids in the presence of strong oxidants and consequent thermal exfoliation and reduction by thermal shock to produce reduced graphene. These oxidation methods typically use concentrated sulfuric acid (a) in combination with fuming nitric acid and KClO(3) (Staudenmaier method), (b) in combination with concentrated nitric acid and KClO(3) (Hofmann method) or (c) in the absence of nitric acid but in the presence of NaNO(3) and KMnO(4) (Hummers method). The evaluation of quality and applicability of the graphenes produced by these various methods is of high importance and is attempted side-by-side for the first time in this paper. Full-scale characterization of thermally reduced graphenes prepared by these standard methods was performed with techniques such as transmission and scanning electron microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy. Their applicability for electrochemical devices was further evaluated by means of cyclic voltammetry techniques. We showed that while Staudenmaier and Hofmann methods (methods that do not use potassium permanganate as oxidant) generated thermally reduced graphenes with comparable electrochemical properties, the graphene prepared by the Hummers method which uses permanganate as oxidant showed higher heterogeneous electron transfer rates and lower overpotentials as compared to graphenes prepared by the Staudenmaier or Hofmann methods. This clearly shows that the methods of preparations have dramatic influences on the materials properties and, thus, such findings are of eminent importance for practical applications as well as for academic research.

Impedimetric immunoglobulin G immunosensor based on chemically modified graphenes.

Immunosensors which display high sensitivity and selectivity are of utmost importance to the biomedical field. Graphene is a material which has immense potential for the fabrication of immunosensors. For the first time, we evaluate the immunosensing capabilities of various graphene surfaces in this work. We propose a simple and label-free electrochemical impedimetric immunosensor for immunoglobulin G (IgG) based on chemically modified graphene (CMG) surfaces such as graphite oxide, graphene oxide, thermally reduced graphene oxide and electrochemically reduced graphene oxide. Disposable electrochemical printed electrodes were first modified with CMG materials before anti-immunoglobulin G (anti-IgG), which is specific to IgG, was immobilized. The principle of detection lies in the changes in impedance spectra of the redox probe after the attachment of IgG to the immobilized anti-IgG. It was found that thermally reduced graphene oxide has the best performance when compared to the other CMG materials. In addition, the optimal concentration of anti-IgG to be deposited onto the modified electrode surface is 10 µg ml(-1) and the linear range of detection of the immunosensor is from 0.3 µg ml(-1) to 7 µg ml(-1). Finally, the fabricated immunosensor also displays selectivity for IgG.

Nanoporous carbon materials for electrochemical sensing.

Nanoporous carbon materials are highly important materials for a wide array of applications. Here we show that nanoporous carbon can act as highly active materials for electrochemical sensing. We observed that nanoporous carbon material exhibits a faster heterogeneous electron transfer than graphite and pure carbon nanotubes. Nanoporous carbon exhibits a superior electrochemical performance for sensing of important biomarkers such as dopamine, ascorbic acid, uric acid, NADH, DNA bases, and forensic-related compounds such as nitroaromatic explosives.

Hydrogen-bonding interactions between water and the one- and two-electron-reduced forms of vitamin K1: applying quinone electrochemistry to determine the moisture content of non-aqueous solvents.

Vitamin K(1) (VK(1)) was shown by voltammetry and coulometry to undergo two chemically reversible one-electron reduction processes in acetonitrile (CH(3)CN) containing 0.2 M Bu(4)NPF(6) as the supporting electrolyte. The potential separation between the first and second electron-transfer steps diminished sequentially with the addition of water, so that at a H(2)O concentration of approximately 7 M (approximately 13% v/v) only one process was detected, corresponding to the reversible transfer of two electrons per molecule. The voltammetric behavior was interpreted on the basis of the degree of hydrogen bonding between the reduced forms of VK(1) with water in the solvent. It was found that the potential separation between the first and second processes was especially sensitive to water in the low molar levels (0.001-0.1 M); therefore, by measuring the peak separation as a function of controlled water concentrations (accurately determined by Karl Fischer coulometric titrations) it was possible to prepare calibration curves of peak separation versus water concentration. The calibration procedure is independent of the type of reference electrode and can be used to determine the water content of CH(3)CN between 0.01 and 5 M, by performing a single voltammetric scan in the presence of 1.0 mM VK(1). The voltammetry was also investigated in dichloromethane, dimethylformamide, and dimethyl sulfoxide. The reduction processes were monitored by in situ electrochemical UV-vis spectroscopy in CH(3)CN over a range of water concentrations (0.05-10 M) to spectroscopically identify the hydrogen-bonded species.