Electronic and magnetic properties of acid-adsorbed nanoporous activated carbon fibers

Nanoporous activated carbon fibers (ACFs) feature 3D disordered networks of nanographite domains that consist of stacks of 3-4 nanographene sheets each. At the peripheries of the nanographene sheets, the localized spins of a nonbonding edge state are created. The magnetic properties of the edge-state spins in ACFs are investigated in relation to the interaction with acid molecules adsorbed in the nanopores. HCl molecules condensed physisorptively in the nanopores mechanically compress the nanographite domains, resulting in the reduction of the magnetic moments of the edge-state spin in the form of a magnetic switching effect. HNO₃ molecules, which have significant oxidation ability, are subjected to charge transfer from the nanographite domains. This induces a two-step reduction in the edge-state spin concentration, wherein the first and second steps are related to the charge transfer with the nanographene sheets that directly face the nanopores and that with the nanographene sheets in the interior of the nanographite domains, respectively. In diluted HNO₃, the blockade by water molecules prevents the interaction of the HNO₃ molecules with the interior nanographene sheets, allowing for charge transfer only with the graphene sheets that directly face the nanopores; this results in a single-step change in the spin concentration.     

Fabrication of porous carbon monoliths with a graphitic framework

Macro/mesoporous carbon monoliths with a graphitic framework were synthesized by carbonizing polymeric monoliths of poly(benzoxazine-co-resol). The overall synthesis process consists of the following steps: (a) the preparation of polymeric monoliths by co-polymerization of resorcinol and formaldehyde with a polyamine (tetraethylenepentamine), (b) doping the polymer with a metallic salt of Fe, Ni or Co, (c) carbonization and (d) the removal of inorganic nanoparticles. The metal nanoparticles (Fe, Ni or Co) formed during the carbonization step catalyse the conversion of a fraction of amorphous carbon into graphitic domains. The resulting carbon monoliths contain >50 wt.% of graphitic carbon, which considerably improves their electrical conductivity. The use of tetraethylenepentamine in the synthesis results in a nitrogen-containing framework. Textural characterization of these materials shows that they have a dual porosity made up of macropores and mesopores (∼2–10 nm), with a BET surface area in the 280–400 m² g⁻¹ range. We tested these materials as electrodes in organic electrolyte supercapacitors and found that no conductive additive is needed due to their high electrical conductivity. In addition, they show a specific capacitance of up to 35 F g⁻¹, excellent rate and cycling performance, delivering up to 10 kW kg⁻¹ at high current densities.     

Metal-functionalized single-walled graphitic carbon nitride nanotubes: a first-principles study on magnetic property

The magnetic properties of metal-functionalized graphitic carbon nitride nanotubes were investigated based on first-principles calculations. The graphitic carbon nitride nanotube can be either ferromagnetic or antiferromagnetic by functionalizing with different metal atoms. The W- and Ti-functionalized nanotubes are ferromagnetic, which are attributed to carrier-mediated interactions because of the coupling between the spin-polarized d and p electrons and the formation of the impurity bands close to the band edges. However, Cr-, Mn-, Co-, and Ni-functionalized nanotubes are antiferromagnetic because of the anti-alignment of the magnetic moments between neighboring metal atoms. The functionalized nanotubes may be used in spintronics and hydrogen storage.     

SAXS investigation of nanoporous structure of thermal-modified carbon materials

The article investigates the effect of thermal modification of porous carbon material (PCM), obtained from plant feedstock, on its morphology and fractal structure by small-angle X-ray scattering (SAXS) method. The analysis of the scattering intensity curves serve the basis for calculating the parameters of the PCM porous structure: the Porod constant, the Porod invariant, average pore radius, specific surface area, and mass and surface fractal dimensions. It has been found out that the PCMs obtained have fractal structure, formed from mass and surface fractals, the sizes of which increase at the growth of temperature and modification time.

PACS

81.05.Uw; 61.05.cf; 82.47.Aa     

Intrinsic wettability of graphitic carbon

The wettability of graphitic carbon can impact the behavior of many carbonaceous materials in a variety of systems involving water, for example, the carbon aerosols–cloud interaction that affects the climate and the penetration of micropores by aqueous solution in activated carbons. Previous studies on this topic have been limited to the basal surface of graphite. Apparent water-contact angle (WCA) values reported in the literature for the basal surface of graphite or graphene sheet vary from 35° to 126°, due to factors such as surface contamination and roughness. The intrinsic wettability of graphitic carbon remains uncertain. Here we report the first experimental attempt of quantifying the intrinsic wettability of the edge surface of graphite. We show that the basal surface is intrinsically hydrophilic with a WCA of about 61°; the edge surface is more hydrophilic than the basal surface and has an intrinsic WCA likely less than 40°. We also show that the apparent WCA is highly sensitive to sample preparation procedures. Annealing, a commonly used method for removing surface organic contaminants, could result in an underestimated WCA due to oxidation by the trace O₂ in the inert gases.     

Formation of graphitic rods in carbon/carbon composites reinforced with carbon nanotubes

The formation of graphitic rods with a carbon nanotube (CNT) in the center was observed in CNT-reinforced phenolic resin-based carbon/carbon composites heat treated at 2000 °C. TEM characterization indicated that the carbon surrounding the CNT has a much better degree of graphitization compared to the carbon in most of the matrix. The formation temperature (2000 °C) of the graphitic rod is lower than for stress graphitization and normal graphitization of phenolic resin.     

The efficient removal of dyes over magnetic Ni embedded on mesoporous graphitic carbon material

Magnetically separable Ni embedded on graphitic mesoporous carbon (NMC) material was fabricated through a facile “sol–gel” route using glucose, nickle nitrate, poly(ethylene glycol)–poly(propylene glycol)–poly(ethylene glycol) (P123) and tetraethyl orthosilicate as carbon source, catalyst and magnetic precursor, soft template and porogen. The obtained NMC material exhibited highly graphitic degree with high surface area of 790 m² g^?1, large pore size at 3.9 nm and pore volume of 0.69 cm³ g^?1. The saturation magnetization was enhanced to 6.82 emu g^?1 because of aggregation of magnetic Ni particles to clusters. NMC material showed excellent removal to Rhodamine B and the adsorption capacity reached to 168.1 mg g^?1 within 120 min. NMC material could be easily separated by an external magnet and reused after ethanol extraction.     

Bromination of graphitic pitch-based carbon fibers

Bromination by exposure to bromine vapor at room temperature was effective for inhibiting the oxidation of carbon-carbon composites in the form of PAN-based carbon fibers impregnated with resin, which was subsequently carbonized at 1000 or 2000°C. The oxidation rate was decreased by up to 43%. In addition, electrical resistivity was decreased by up to 39%. However, the tensile modulus was decreased by 16–17% and tensile strength was decreased by 17–20%; the ductility was not affected. The increased oxidation resistance may be due to bromine adsorption or the electron transfer from graphite to bromine. The bromine resided in the matrix of the composite, particularly in pores within the matrix. Bromination was also attempted by the electrochemical method, but it resulted in negligible weight uptake in the carbon-carbon composite. Carbon-carbon composites that had been graphitized at 2700°C were damaged by bromination.     

Properties of nanoporous carbon material with one-dimensional conductivity

The nanoporous carbon material (PCM) prepared from petroleum coke or organic compounds that simulate the structure of coke can possess both one-dimensional and three-dimensional conductivity, which is described by the Mott law. It is proposed to consider these materials as two different PCMs with different conductivity, which has an effect on the properties of catalysts, for example, in the cathodes of solid polymer fuel cells and in the production of an anode material and the nanoparticles of 3d metals (iron, cobalt, and nickel), when PCMs are used as the supports of catalysts (or substrates).     

Characterisation of steam-treated nanoporous carbide-derived carbon of TiC origin: structure and enhanced electrochemical performance

Modifications of pore size distribution and structural order of nanoporous carbide-derived carbon (CDC) materials with variety of surface areas and pore sizes were investigated using physical activation by etching with water vapour. Variable etching duration was used to explore the activation impact on the pore size distribution and the adsorption behaviour of TiC-derived carbon. A distribution of micro- and mesopores, modified during physical activation, was studied using N₂ and CO₂ adsorption. Notable impact of preceding carbon structure on the activation product was revealed by the results of scanning electron microscopy, powder X-ray diffraction and Raman spectroscopy. An infrared spectroscopy, energy dispersive spectroscopy and X-ray photoelectron spectroscopy confirmed that water-induced etching of CDC followed by high-temperature treatment in inert gas atmosphere does not change notably the total amount of surface oxygen, however, leads to the changes in a composition of oxygen containing functional groups in post-activated carbon. The electrochemical evaluation was performed in triethylmethylammonium tetrafluoroborate/acetonitrile electrolyte to elaborate the structure-electrochemical properties relationships on post-activated nanoporous CDC materials. It was observed that the degree of improvement in double-layer capacitance achievable with a steam-treatment significantly depends on the preceding properties of CDC prior treatment, whereby the highest capacitance, ~?160 Fg^?1, was reached for the steam-treated TiC-derived CDC made at 800 °C, which clearly is a very promising material for the electrical double-layer capacitor.     

Near net-shape fabrication and properties of graphitic carbon foam with a continuous network of graphite nanosheets

A near net-shape graphitic carbon foam (GCF) with a continuous network of graphite nanosheets was prepared by direct carbonization of epoxy resin filled with nano-Al₂O₃. The effects of carbonization temperature on the properties of the resulting carbon foams were investigated by SEM, TEM, XRD, Raman, thermal conductivity and compression strength test. The results show that the as-prepared GCF can maintain well dimensional stability upon carbonization. The carbothermal reaction between the nano-Al₂O₃ and carbon foam matrix greatly influences the microstructure of carbon foam and promotes its growth of the continuous network of graphite nanosheets. In addition, the GCF prepared at 1700 °C possesses a compressive strength of 2.34 MPa with a bulk density of 0.19 g cm^-3, and meanwhile presents a high graphitization degree of 65.12% and a thermal conductivity of 2.02 W/mK. The continuous network of graphite nanosheets favors the enhancement of thermal conductivity of carbon foam and simultaneously prevents the decline of compressive strength further.     

Methanol oxidation activities of Pt nanoparticles supported on microporous carbon with and without a graphitic shell

Platinum nanoparticles were prepared as catalysts supported on microporous amorphous carbon with and without a graphitic carbon shell. The electro-oxidation of methanol in acidic solutions at room temperature was used as a probe reaction to explore the effect of the carbon structure on catalysis. CO anodic stripping voltammetry and cyclic voltammetry both recorded enhanced performance for the catalyst supported on the carbon with a graphitic shell. Some rationalizations of the possible roles of the graphitic shell are provided.     

Mechanical properties and structure of a nanoporous sodium borosilicate glass

The nanohardness (H) and microhardness (H _M) of sodium borosilicate glasses with and without nanopores were studied. From nanoindentation measurements, along with the hardness H, the Young’s modulus E was derived. While both H and H _M varied between ∼10 GPa and ∼7 GPa for the bulk glass, the values for nanoporous specimens were one order of magnitude lower at about 0.5 GPa. The Young’s moduli were found to be ∼82 GPa and ∼5 GPa for bulk and porous glasses, respectively. Cracks and pileups were observed, respectively, arising from microindents and nanoindents in the bulk glass, whereas none of them could be detected in the nanoporous material. The molecular structures of both glasses studied by X-ray diffraction are similar. The text was submitted by the authors in English.     

Facile synthesis of Y-doped graphitic carbon nitride with enhanced photocatalytic performance

Yttrium-doped graphitic carbon nitride (Y/g-C₃N₄) catalysts were prepared via a facile pyrolysis method with urea used as a precursor and yttrium nitrate as the Y source. Characterization results show that an appropriate doping ratio of Y can be embedded into in-planes of g-C₃N₄. The Y/g-C₃N₄ catalysts are characterized by hierarchical porosity, large specific surface area, and large pore volume. Introduction of Y species effectively extends the spectral response of g-C₃N₄ from ultraviolet to visible region and decelerates the recombination of photogenerated electrons and holes. Because of these properties, the Y/g-C₃N₄ catalysts show an enhanced photocatalytic performance in rhodamine B degradation under visible light.