J.M.D. Tascón

Graphene Oxide Dispersions in Organic Solvents

The dispersion behavior of graphene oxide in different organic solvents has been investigated. As-prepared graphite oxide could be dispersed in N, N-dimethylformamide, N-methyl-2-pyrrolidone, tetrahydrofuran, and ethylene glycol. In all of these solvents, full exfoliation of the graphite oxide material into individual, single-layer graphene oxide sheets was achieved by sonication. The graphene oxide dispersions exhibited long-term stability and were made of sheets between a few hundred nanometers and a few micrometers large, similar to the case of graphene oxide dispersions in water. These results should facilitate the manipulation and processing of graphene-based materials for different applications.

Raman microprobe studies on carbon materials

Abstract The Raman spectra of a set of 29 different carbon materials, that included all types of carbonaceous solids of practical interest, were analyzed. The main objectives were to establish whether there is any general tendency where any material can be included, and whether the Raman spectrum (i.e., parameters deduced from it) is sufficient to characterize the degree of structural order in a carbon material. Changes in the spectral profile exhibited the tendencies expected from the graphitization degree of the solids. Deconvolution of Raman spectra allowed separation of the contributions of G, D, D′, and D′′ bands; in addition, a Gaussian band centered at 1330 cm−1, never described before and attributable to impurity ions, was found in the spectra of activated carbons. The width of D band correlates well with the degree of disorder over the entire order-disorder interval, being more adequate than the G-band width as a disorder parameter for disordered materials. Both D- and G-band positions are little useful as measures of disorder, since the former is practically constant whereas the latter varies in a random form. Finally, the graphitization path described by the G′/G band width ratio versus G-band width is evidence of a qualitative difference between the most ordered materials, which become divided into two groups.

Atomic Force and Scanning Tunneling Microscopy Imaging of Graphene Nanosheets Derived from Graphite Oxide

Graphene nanosheets produced in the form of stable aqueous dispersions by chemical reduction of graphene oxide and deposited onto graphite substrates have been investigated by atomic force and scanning tunneling microscopy (AFM/STM). The chemically reduced graphene oxide nanosheets were hardly distinguishable from their unreduced counterparts in the topographic AFM images. However, they could be readily discriminated through phase imaging in the attractive regime of tapping-mode AFM, probably because of differences in hydrophilicity arising from their distinct oxygen contents. The chemically reduced nanosheets displayed a smoothly undulated, globular morphology on the nanometer scale, with typical vertical variations in the subnanometer range and lateral feature sizes of approximately 5-10 nm. Such morphology was attributed to be the result of significant structural disorder in the carbon skeleton, which originates during the strong oxidation that leads to graphene oxide and remains after chemical reduction. Direct evidence of structural disorder was provided by atomic-scale STM imaging, which revealed an absence of long-range periodicity in the graphene nanosheets. Only structured domains a few nanometers large were observed instead. Likewise, the nanosheet edges appeared atomically rough and ill-defined, though smooth on the nanometer scale. The unreduced graphene oxide nanosheets could only be imaged by STM at very low tunneling currents (approximately 1 pA), being visualized in some cases with inverted contrast relative to the graphite substrate, a result that was attributed to their extremely low conductivity. Complementary characterization of the unreduced and chemically reduced nanosheets was carried out by thermogravimetric analysis as well as UV-visible absorption and X-ray photoelectron and Raman spectroscopies. In particular, the somewhat puzzling Raman results were interpreted to be the result of an amorphous character of the graphene oxide material.

Structure and Reactivity of Perovskite-Type Oxides

Publisher Summary This chapter discusses the structure and reactivity of perovskite-type oxides. Perovskite-type oxides have the general formula ABO 3 (A, cation of larger size) and are structurally similar to CaTiO 3 , the mineral that gave its name to that group of compounds. These materials are first studied because of their important physical properties such as ferro-, piezo-, and pyroelectricity, magnetism and electrooptic effects. The most numerous and most interesting compounds with the perovskite structure are oxides. Some hydrides, carbides, halides, and nitrides also crystallize with this structure. The chapter reviews only the study of oxides and their behavior in the gas solid interface and in heterogeneous catalysis. An important characteristic of perovskites, mentioned in the chapter, is their susceptibility of partial substitution in both A and B positions. This provides a wealth of isomorphic compounds that can easily be synthesized. Given the extensive range of possibilities in the tailoring of their chemical and physical properties, there is no doubt that new reactions can be studied, where these oxides can participate as catalytic agents.

Highly stable performance of supercapacitors from phosphorus-enriched carbons

Phosphorus-rich microporous carbons (P-carbons) prepared by a simple H(3)PO(4) activation of three different carbon precursors exhibit enhanced supercapacitive performance in 1 M H(2)SO(4) when highly stable performance can be achieved at potentials larger than the theoretical decomposition potential of water. This ability of P-carbons greatly enhances the energy density of supercapacitors that are capable of delivering 16 Wh/kg compared to 5 Wh/kg for the commercial carbon. An intercept-free multiple linear regression model confirms the strongest influence of phosphorus on capacitance together with micropores 0.65-0.83 nm in width that are the most effective in forming the electric double layer.

Preparation of graphene dispersions and graphene-polymer composites in organic media

Graphene nanosheets in the form of chemically reduced graphene oxide have been prepared in organic media without the need to chemically functionalise the starting graphene oxide nanosheets. The preparation procedure is simple and similar to that previously used for the production of stable aqueous dispersions of graphene nanosheets. The resulting organic dispersions are homogeneous, exhibit long-term stability and are made up of graphene sheets a few hundred nanometres large. The ability to prepare graphene dispersions in organic media facilitates their combination with polymers, such as polyacrylonitrile and poly(methyl methacrylate), to yield homogeneous composites.

Comparative performance of X-ray diffraction and Raman microprobe techniques for the study of carbon materials

This paper compares the information provided by X-ray diffraction and Raman spectrometry in terms of the structural order in a wide set of carbon solids. A special emphasis is placed in checking the validity of the commonly used formula of Tuinstra and Koenig and establishing the magnitude of errors potentially derived from its application. For this, a total number of 45 carbon materials aiming to cover the whole spectrum of properties and applications of these solids were jointly characterised by X-ray diffraction and Raman microprobe spectrometry. The comparison of d 002 interlayer spacing and the ratio of D to G Raman band intensities allows one to conclude that both techniques are complementary rather than equivalent. The different types of factors affecting the D and G band intensities and widths are discussed, it being concluded that their contributions are difficult to separate. The overall conclusion is that Tuinstra and Koenigs formula is valid only as a first approximation to L a values, and that errors as high as 100% are possible, so that, whenever feasible, direct measurements by XRD are recommended.

Environmentally friendly approaches toward the mass production of processable graphene from graphite oxide

Graphene has attracted a great deal of scientific interest in latter years owing to its unique properties, with many prospective applications being actively investigated at present. However, the actual implementation of graphene in technological uses will depend critically on the development of appropriate methodologies for its mass production. In this regard, one of the most promising approaches is based on the exfoliation and reduction of graphite oxide. Graphenes derived from graphite oxide can be prepared at low cost and high throughput, can be further processed in a number of solvents, and are chemically versatile, among other attractive features. In an environment-conscious world, the availability of green approaches toward graphene production would also constitute an added advantage. During the last year, different environmentally friendly methods for the production of graphene from graphite oxide have emerged, which we highlight here. These are based on solvothermal and electrochemical processes, as well as on the use of green reductants. Several open questions and possible future directions for this research topic are also discussed.

Oxygen plasma modification of pitch-based isotropic carbon fibres

An isotropic carbon fibre was surface-treated by microwave oxygen plasma at different conditions and characterised by scanning electron microscopy (SEM), scanning tunneling microscopy (STM), N2/CO2 adsorption, Raman spectrometry, X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD). It is shown that the structure of the fibre suffers only limited alterations upon plasma treatment in such a way that the local disorder on its surface, which was already large in the fresh material, barely increases after the plasma exposure, as detected by Raman measurements. At the nanometre scale, STM images revealed a moderate increase in surface roughness. Evidence for chemical changes undergone by the fibre following the etching was provided by XPS and TPD, showing that stable oxygen functionalities were introduced by the plasma exposure, a result of practical importance for the application of this treatment not only to this type of carbon fibre, but to carbon materials in general. It was also observed that very gentle plasma exposures were generally sufficient to provide the fibre surface with a large amount of oxygen functional groups and that more intense treatments had a negative effect in this respect (i.e. they were not able to supply oxygen to the surface in larger amounts than the softer treatments did).

Activated carbons by pyrolysis of coffee bean husks in presence of phosphoric acid

Abstract Activated carbons (Acs) were prepared by pyrolysis of coffee bean husks in presence of phosphoric acid (chemical activities). Husks from Colombian coffee beans were impregnated with aqueous solutions of H3PO4 following a variant of the incipient wetness method. Diffenent concentrations were used to produce impregnation ratios of 30, 60, 100 and 150 wt.%. Activation was carried out under argon flow by heating to 723 K with 1 h soaking time. The porous texture of the obtained ACs was characterized by physical adsorptions of N2 at 77 K and CO2 at 273 K. The impregnation ration had a strong influence on the pore structure of these Acs, which could be easily controlled by simply varying the proportion of H3PO4 used in the activation. Thus, low impregnation ratio led to essentially microporous Acs. At intermediate impregnation ratios, ACs with wider pore size distribution (from micropores to mesopores) were obtained. Finally, high impregnation ratios yielded essentially mesoporous carbons with high surface area and pore volume.