Amelia Martínez-Alonso

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.

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.

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.

Pyrolysis of apple pulp: chemical activation with phosphoric acid

Chemical activation of apple pulp with phosphoric acid has been carried out to prepare activated carbons. The materials were characterized by elemental analysis, N2 adsorption (77 K), CO2 adsorption (273 K) and X-ray diffraction. The effects of temperature, soaking time, impregnation ratio and degree of washing were studied. Maximum development of porosity was observed upon heating to 723 K plus 1 h soaking. The amount of phosphoric acid used in the impregnation step strongly influenced the porous texture, micropores being predominant at low impregnation ratios whereas larger amounts of phosphoric acid produced wide micropores and mesopores. The washing sep also had a crucial effect on the porosity accessible to different adsorbates. The results suggest the feasibility of the process from the point of view of both porous texture and carbon yield.

Towards full repair of defects in reduced graphene oxide films by two-step graphitization

The complete restoration of a perfect carbon lattice has been a central issue in the research on graphene derived from graphite oxide since this preparation route was first proposed several years ago, but such a goal has so far remained elusive. Here, we demonstrate that the highly defective structure of reduced graphene oxide sheets assembled into free-standing, paper-like films can be fully repaired by means of high temperature annealing (graphitization). Characterization of the films by X-ray photoelectron and Raman spectroscopy, X-ray diffraction and scanning tunneling microscopy indicated that the main stages in the transformation of the films were (i) complete removal of oxygen functional groups and generation of atomic vacancies (up to 1,500 °C), and (ii) vacancy annihilation and coalescence of adjacent overlapping sheets to yield continuous polycrystalline layers (1,800–2,700 °C) similar to those of highly oriented graphites. The prevailing type of defect in the polycrystalline layers were the grain boundaries separating neighboring domains, which were typically a few hundred nanometers in lateral size, exhibited long-range graphitic order and were virtually free of even atomic-sized defects. The electrical conductivity of the annealed films was as high as 577,000 S·m−1, which is by far the largest value reported to date for any material derived from graphene oxide, and strategies for further improvement without the need to resort to higher annealing temperatures are suggested. Overall, this work opens the prospect of truly achieving a complete restoration of the carbon lattice in graphene oxide materials.Graphical abstract

Effects of plasma oxidation on the surface and interfacial properties of ultra-high modulus carbon fibres

Abstract An ultra-high modulus (UHM) carbon fibre was submitted to an oxygen plasma treatment. The effects of this treatment on the physical and chemical properties of the carbon surfaces were investigated by using surface characterisation techniques. SEM and STM studies were performed in order to determine the changes in the surface morphology. Observations on the nanometre scale lead to the conclusion that the plasma oxidation “cleaned” the original surfaces of carbonaceous impurities. XPS analysis of the treated fibres revealed a very significant increase of oxygen content. Single-fibre epoxy composites were prepared from as-received and plasma-treated fibres, and fragmentation tests were performed in order to characterise fibre/matrix interfacial adhesion. Raman spectroscopy has been used to map the strain along the fibre during tensile loading of the matrix, and the distribution of interfacial shear stress has been obtained. The quality of the interface improved dramatically after the surface treatment, supporting the ability of cold plasma oxidation to enhance the adhesion of UHM carbon to epoxy matrices. It is concluded that the increase of the oxygen surface content and the removing of the outermost layers may contribute in a co-operative way to the improvement on fibre/matrix adhesion.

Effects of plasma oxidation on the surface and interfacial properties of carbon fibres/ polycarbonate composites

Two types of carbon fibres, ultra-high modulus (pitch-based) and high strength (PAN-based), were submitted to an oxygen plasma treatment. Single filament tests were performed to evaluate the effects of the treatment on the mechanical properties of the fibres. Weibull analysis of the tensile data revealed no substantial changes in the tensile strength after fibre oxidation. Fragmentation tests performed on carbon fibre/polycarbonate composites showed that the plasma treatment increases significantly the interfacial shear strength. This demonstrates the ability of the cool plasma oxidation to enhance the adhesion of carbon fibres to polycarbonate. The possible influences of the structural and chemical properties of the carbon surfaces upon the final response of the interface were investigated using inverse gas chromatography (IGC) at infinite dilution. The increase of surface functional groups induced by the plasma treatment seems to be the main reason for the improvement of the interfacial adhesion.

Tuning of texture and surface chemistry of carbon xerogels

The influence of different activation processes on the textural and surface chemical properties of carbon xerogels was studied. Carbon xerogels were prepared by the conventional sol-gel approach using resorcinol and formaldehyde; two different pHs of sol-gel processing led to carbon materials with distinct pore size distributions. The materials were subjected to controlled activation by three different methods: activation by oxygen plasma, activation by HNO(3), and activation by diluted air. Treatments with HNO(3) and diluted air created oxygen groups on the external surface as well as inside the pore channels, whereas plasma is more suitable for introducing oxygen groups selectively on the external surface. Nevertheless, it was shown that samples with wider pores can be oxidized to some extent on the pore interiors by plasma. Significant changes in total surface area by air activation were observed.