EELS studies of carbide derived carbons

Carbide derived carbons (CDCs) have been synthesized from VC, WC, TaC, NbC, HfC and ZrC at T = 1000 °C, and from TiC at T = 700-1200 °C via a chlorination reaction. The CDCs have been studied by means of high resolution transmission electron microscopy and electron energy-loss spectroscopy (EELS). These studies show that the structures of CDCs are strongly dependent upon the nature of the starting carbide and the synthesis temperature. The structures range from very disordered nanoporous carbon, consisting of randomly curved graphene layers, to compact spherical entities of compactly packed graphitic layers. EELS studies of carbon core-loss spectra show that the relative sp² content in these carbons varies between 83% and 98%. The low-loss part of the EEL spectra shows that the positions of the π-plasmons and the bulk plasmons are more dependent on the carbide precursor, than the synthesis temperature. The densities of the CDC particles have been estimated using the bulk plasmon positions as well as the sp³ content determined from the C-K edge spectra. The densities calculated from sp³ are close to the pycnometric ones, while the densities calculated from bulk plasmon positions are lower and reflects the nanostructure of CDCs.     

Ordered mesoporous carbide derived carbons for high pressure gas storage

The design of advanced porous materials is crucial for the development of new energy storage systems for mobile applications. In the following a new class of highly porous carbon structures is applied in gas storage. Ordered mesoporous carbide derived carbons (OM-CDC) were synthesized by chlorination of mesostructured silicon carbide ceramics (OM-SiC). Resulting OM-CDC structures were characterized by nitrogen physisorption methods and small angle X-ray scattering demonstrating high specific surface areas and bimodal pore size distributions by varying the synthesis and chlorination conditions. The adsorption properties could be further enhanced by reductive hydrogen treatment. Storage capacities for mobile applications dependant on the synthesis conditions were investigated in high pressure hydrogen and methane adsorption with extraordinary high uptakes compared to micro- and mesoporous reference materials. In addition, the adsorption kinetics are studied in dynamic n-butane adsorption.     

Nanostructure,porosity and electrochemical performance of chromium carbide derived carbons

This paper presents the results from the investigation of the influence of the chlorination temperature, the carbide crystal structure, the Cr/C ratio and physicochemical properties of CrCl₃ on the morphology, nanostructure, textural properties and electrochemical performance of CDCs. Electron microscopy and its analytical associated techniques reveal that these carbons, mainly composed by disordered graphene layers, evolve into graphitic nanostructures as a result of increasing the Cr/C content, the reaction temperature and the template effect of the etched CrCl₃ halide. Their textural analysis indicates the formation of micro/mesoporous carbons with a pore width below 1.5 nm, surface area as high as 835 m²/g and exhibit capacitive behavior in aqueous electrolyte.     

Classifying nanostructured carbons using graphitic indices derived from Raman spectra

Raman scattering is known to be a unique characterization tool for carbon-based materials such as graphite, diamond, graphene and nanostructured carbons. In this article, we propose a new parameter for estimating the crystallinity of nanostructured carbons. This new parameter estimates the length of the curved graphene planes. To take into account the effect of curvature, the importance of the second-order band at about 2700 cm⁻¹ is emphasized. Using this new parameter, we propose a classification of different nanostructured carbons. The relevance of this new parameter for characterizing nanostructured carbons is confirmed by transmission electron microscopy.     

Raman spectroscopy studies of meteoritic diamonds

The confocal Raman micro-spectroscopy was used for extraterrestrial carbonaceous matter characterisation. Different allotropic forms of carbon, mainly graphite and lonsdaleite, were identified in various meteorites: ordinary chondrites DaG 610 and NWA 869, in NWA 3118 carbonaceous chondrite, and in iron meteorites Morasko and in Sikhote Alin.     

In situ Raman spectroscopy studies of catalysts

In situ Raman spectroscopy is rapidly becoming a very popular catalyst characterization method because Raman cells are being designed that can combine in situ molecular characterization studies with simultaneous fundamental quantitative kinetic studies. The dynamic nature of catalyst surfaces requires that both sets of information be obtained for a complete fundamental understanding of catalytic phenomena under practical reaction conditions. Several examples are chosen to highlight the capabilities of in situ Raman spectroscopy to problems in heterogeneous catalysis: the structural determination of the number of terminal M=O bonds in surface metal oxide species that are present in supported metal oxide catalysts; structural transformations of the MoO₃/SiO₂ and MoO₃/TiO₂ supported metal oxide catalysts under various environmental conditions, which contrast the markedly different oxide–oxide interactions in these two catalytic systems; the location and relative reactivity of the different surface M–OCH₃ intermediates present during CH₃OH oxidation over V₂O₅/SiO₂ catalysts; the different types of atomic oxygen species present in metallic silver catalysts and their role during CH₃OH oxidation to H₂CO and C₂H₄ epoxidation to C₂H₄O; and information about the oxidized and reduced surface metal oxide species, isolated as well as polymerized species, present in supported metal oxide catalysts during reaction conditions. In summary, in situ Raman spectroscopy is a very powerful catalyst characterization technique because it can provide fundamental molecular‐level information about catalyst surface structure and reactive surface intermediates under practical reaction conditions. This revised version was published online in August 2006 with corrections to the Cover Date.     

Raman spectroscopy studies of the high-temperature evolution of the free carbon phase in polycarbosilane derived SiC ceramics

The Raman spectra of a number of SiC ceramics synthesized from polycarbosilane at 1200 °C and annealed at 1400, 1600, 1800 and 2000 °C have been recorded using laser excitation wavelength of 532 nm. The peak positions, their intensities (I_D/I_G) and full width at half maximum (FWHM) were used to obtain information about the degree of disorder in the free carbon phases. The increasing ordering with annealing temperature was confirmed by lower FWHM values and G-peak positions obtained from the SiC ceramics annealed at higher temperature. However, the I_D/I_G has shown to be the highest point at 1600 °C, which illustrates that the temperature is one critical point of the microstructure evolution of the free carbon phase changing amorphous to turbostratic with increasing temperatures. Obviously, the oxidation behaviors of the SiC ceramics are significantly affected by the microstructures of the free carbon phases. In the SiC ceramics with above 1600 °C annealing, the oxidation temperatures of the SiC phases are postponed more than 100 °C, because they are surrounded by the free carbon phases.     

Kinetic studies of polyurethane polymerization with Raman spectroscopy

In this study, the polymerization kinetics of an uncatalyzed polyester based thermoplastic polyurethane formulation was characterized with Raman spectroscopy. Measuring the normalized scattering intensity of a band originating from the TPU diisocyanate, conversion was calculated as a function of time. Kinetic parameters obtained from these experiments correlated well with those obtained from analogous calorimetric experiments and with literature values. It was concluded that Raman spectroscopy is a powerful tool for characterizing the polymerization kinetics of polyurethanes in situ.     

New insights on electrochemical hydrogen storage in nanoporous carbons by in situ Raman spectroscopy

In situ Raman spectroscopy was exploited to analyze the interaction between carbon and hydrogen during electrochemical hydrogen storage at cathodic conditions. Two different activated carbons were used and characterized by different electrochemical techniques in two electrolytes (6 M KOH and 0.5 M Na₂SO₄). The in situ Raman spectra collected showed that, in addition to the D and G bands associated to the graphitic carbons, two bands appear simultaneously at about 1110 and 1500 cm⁻¹ under cathodic conditions, and then they disappear when the potential increases to more positive values. This indicates that carbon–hydrogen bonds are formed reversibly in both electrolytes during cathodic conditions. Comparing the two activated carbons, it was confirmed that, in both electrolytes, the hydrogenation of carbon atoms is produced more easily for the sample with lower amount of surface oxygen groups. In KOH medium, for the two samples, the formation of carbon–hydrogen bonds proceeds at more positive potential with respect to the thermodynamic potential value for hydrogen evolution. Furthermore, changes in the shape of the D band (due to an intensity increase of the D1 band) during the formation of carbon–hydrogen bonds suggest that hydrogenation of the carbon atoms increases the number of edge planes.     

IR studies of carbons—I: IR photothermal beam deflection spectroscopy

IR spectroscopic measurements of carbons are briefly reviewed. The new technique of IR photothermal beam deflection Fourier transform spectroscopy (IR-PDS) is outlined and the applications of IR-PDS to carbon materials is described. IR-PDS can be used to examine carbons routinely under the rigorously-controlled conditions required for surface studies.     

Temperature studies of polycarbonate using Fourier transform Raman spectroscopy

Summary The temperature dependence of the Fourier transform Raman spectrum of polycarbonate has been investigated. C–O–C stretching and C–H stretching modes were found to be temperature-sensitive. The changes noted indicate the presence of a cis-trans conformation for the crystalline polymer, while the cis-cis conformation predominates in amorphous polycarbonate.     

The external surface of microporous carbons,derived from adsorption and immersion studies

Three typical samples of active carbons have been investigated by using immersion calorimetry and adsorption techniques (t-plot, $\text{t}\text{F}$ method of Dubinin and direct analysis of the adsorption isotherm before and after prefilling), in order to obtain their external surface area. The different techniques lead to consistent results.     

Raman spectroscopy studies of concentrated vanadium redox battery positive electrolytes

Spectroscopic changes in highly concentrated vanadium(V)-sulfate solutions to be used in the vanadium redox battery are consistent with the presence of more than one V(V)-sulfate species. The results of Raman spectroscopy indicate that the major species in highly acidic conditions are VO₂SO₄ ^–, VO₂(SO₄)₂ ^3–, VO₂(HSO₄)₂ ^–, VO₃ ^–, V(V) dimers with V₂O₃ ⁴⁺ and V₂O₄ ²⁺ central units. The nature and amount of these species depends upon the V(V) and total sulfate concentrations as well as on S to V and H⁺ to V ratios in the positive half-cell electrolyte. V(V) forms V₂O₃ ⁴⁺, VO₂(SO₄)₂ ^3– and their copolymer species at higher total sulfate concentrations, which tends to stabilize the vanadium (V) positive electrolyte in the vanadium redox battery. The V(V) and V(IV) species show the least interaction with each other. Ageing of concentrated V(V) solutions at elevated temperature (50 °C) produces decomposition of species causing formation of V₂O₅ precipitates with a decrease in the amount of vanadium polymer.     

Raman spectroscopy mapping of amorphized zones beneath static and dynamic Vickers indentations on boron carbide

Three-dimensional models of amorphized zones beneath quasistatic and dynamic Vickers indentations on boron carbide were constructed using micro-Raman spectroscopy. The square of amorphized zone depth varied linearly with load and the maximum amorphized area occurred beneath the indentation imprint in accord with the maximum shear stress under Hertzian contact. Reduced measurements of amorphization intensity at loads above 10 N may be due to a loss of subsurface amorphized material through lateral cracks. Utilizing an expanding cavity model with power-law (n = 0.79–0.80) and linear (E_p = 0.39–0.45) strain hardening responses, finite element simulations were conducted to determine the critical values of stress and strain required to cause amorphization. These simulations suggest that amorphization may initiate at von Mises stresses and equivalent plastic strains above 6.6 GPa and 0.026, respectively. These results may be useful for validating computational models of boron carbide under complex loading scenarios (e.g., ballistic impact).     

Variable temperature and stretching cell for Raman spectroscopy studies of polymers

A variable temperature cell for use in laser spectroscopy studies of polymers and for the drawing of compounds is described. ‘In situ’ polarized Raman spectra of polyethylene at different draw ratios and different temperatures are recorded. The depolarization factors allow assignment of the vibrational types to the Raman active modes and the chain orientation study.     

Small angle X-ray scattering studies on carbons derived from polyfurfuryl alcohol and polyfurfuryl alcohol-ferrocene copolymers

Small angle X-ray scattering techniques have been used to characterize the porous nature of glassy carbons derived from polyfurfuryl alcohol (PFA) and from PFA-ferrocene derivative copolymers. Samples were prepared under controlled heating conditions within the temperature range 970–2500°C. For carbons produced only from PFA it is shown that each contains micropores of a narrow size distribution. As heat treatment temperature increases there is a progressive development of both pore structure and of order within the carbon matrix. A comparison is made of specific surface area results for PF'A carbons as measured by small angle X-ray procedures and by CO₂ at ?78°C. The small angle X-ray results for the materials produced from the iron-containing precursors indicate the presence of two distinct regions of heterogeneity in the carbon matrix. One is microporous and the other is of larger pores of a much wider size distribution. It is also suggested that these larger pores account for these carbons having lower tensile strength and higher electrical resistivity than a corresponding PFA carbon.     

Model for Raman scattering from incompletely graphitized carbons

Laser Raman spectroscopy has been applied to the study of various carbons ranging from highly graphitized to amorphous materials. A calculation is presented for such materials based on the assumption of short correlation lengths for normal modes that break the wave-vector selection rules. This calculation leads to expressions for the first-order Raman scattering intensity in terms of the vibrational density of states of a single graphite layer weighted by the electron-phonon couping of the modes to the electromagnetic radiation.     

In situ surface Raman spectroscopy studies of oxygen adsorbed on electrolytic silver

In situ Raman spectroscopy has been employed to investigate oxygen adsorption on electrolytic silver catalyst under industrial conditions for methanol oxidation to formaldehyde. Both adsorbed atomic and molecular oxygen species are shown to exist on the silver surface in O₂ flow above 870 K. The peroxide species is determined to be a precursor to atomic adsorbed oxygen. In consideration of the industrial process, the molecular mechanism of the partial oxidation of methanol and the adsorption mechanism of oxygen on electrolytic silver surface are discussed.