The dispersibility of platelet-type graphite nanofibers (PGNFs), an archetype of carbon material with a surface dominated by graphitic edge planes, has been measured in 28 solvents and rationalized ...on the basis of solvent surface tension and Hansen solubility parameters. Successful solvents possess surface tensions of ∼25–35mJm−2 and substantial values of the hydrogen-bonding Hansen parameter (δH∼14–16MPa1/2), and many of them are alcohols, such as 1-butanol, ethanol or cyclohexanol. Such result is mainly attributed to the fact that the PGNF edge planes are decorated with oxygen functional groups. The dispersion behavior of the nanofibers could be changed to that typically exhibited by carbon nanotubes and graphene by means of a high temperature annealing that converted their surface edge planes to curved basal planes.
Thermal decomposition of poly(p-phenylene benzobisoxazole) (PBO) has been studied between room temperature and 1123 K. Two types of material (regular and high-modulus) were studied, which yielded ...almost equivalent results. Thermogravimetry and differential thermal analysis allowed establishing the main stages of the pyrolytic degradation of the material. On the basis of the thermal analysis results, samples were decomposed at several controlled temperatures and characterized by elemental analysis, infrared spectroscopy, X-ray diffraction, atomic force microscopy, and scanning tunneling microscopy. At temperatures below 933 K the polymer retains its original conformation and becomes stabilized by enhancement of its crystallinity. The decomposition takes place in a single step and the main changes occur within a very narrow temperature interval (983−993 K). Formation of polyaramides as intermediates in the decomposition process was detected. These amide bonds subsequently degrade by homolytic breaking, yielding nitriles. The final carbonaceous residue is rich in nitrogen and retains a certain degree of anisotropy, a fact that was explained by the conservation of crystallinity at an intermediate decomposition stage.
In the present work, the thermal transformations of Nomex poly(m-phenylene isophthalamide) fibers have been investigated up to a temperature of 1173 K. The main stages of the pyrolytic degradation of ...the fibers were determined by thermal analysis, and their chemical and morphological evolution through the different steps was subsequently followed by Fourier transform infrared spectroscopy (FTIR) and atomic force microscopy (AFM) measurements, respectively. The degradation starts with the cleaving of hydrogen bonds at approximately 633 K, which leads to a disordering of the polyaramide chains on the nanometer scale. The next decomposition step takes place between 673 and 873 K with the disruption of the amide bonds, the subsequent breaking of the polyaramide chains into smaller units, and their condensation into large polyaromatic compounds. From 873 K onward, the reaction progresses by the dehydrogenation of the polyaromatic structures and their arrangement into graphite-like assemblies, resulting in the final fibrous carbon which is obtained at 1173 K.
Graphene materials are attractive for use in novel aqueous electrochemical energy storage devices, including aqueous zinc-ion batteries (AZIBs) and hybrid capacitors (AZICs). Ideally, such materials ...should be readily accessible by eco-friendly routes and possess physicochemical features beneficial for the intended application. Here, we propose an anodic exfoliation strategy, using a proper combination of common salts/bases as the aqueous electrolyte, for the preparation of highly oxidized graphenes with some control over the populations of their oxygen groups and retention of electrical conductivity (∼102–103 S m−1). As an active cathode material for AZIC cells, the hydrophilic, carboxyl-enriched anodic graphene processed into a compact film outperformed reduced graphene oxides derived from common routes (e.g., Hummers' method), in terms of capacity and rate capability. Furthermore, the performance of this new anodic graphene was enhanced by combining it with a multifunctional biomolecule (flavin mononucleotide), which promoted the cathode wettability and provided extra capacity due to its redox-active center, particularly at higher currents. The carboxyl-enriched graphene was also shown to act as an effective coating layer for the protection of the zinc metal electrode in AZICs/AZIBs, extending its cycle life for longer than is usually attained with carbon-based protective coatings.
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•Environmentally friendly approach to anodic graphene with tunable oxygen groups.•Carboxyl-enriched anodic graphene offers advantages over reduced graphene oxide.•Advantageous active cathode material for aqueous zinc-ion hybrid capacitors (AZICs).•Effective coating to protect the zinc anode in AZICs and zinc-ion batteries (AZIBs).
Poly(
p-phenylene terephthalamide) (PPTA) and poly(
p-phenylene benzobisoxazole) (PBO) fibers were exposed to an oxygen plasma under equivalent conditions. The resulting changes in the surface ...properties of PPTA and PBO were comparatively investigated using inverse gas chromatography (IGC) and atomic force microscopy (AFM). Both non-polar (
n-alkanes) and polar probes of different acid–base characteristics were used in IGC adsorption experiments. Following plasma exposure, size-exclusion phenomena, probably associated to the formation of pores (nanoroughness), were detected with the largest
n-alkanes (C
9 and C
10). From the adsorption of polar probes, an increase in the number or strength of the acidic and basic sites present at the fiber surfaces following plasma treatment was detected. The effects of the oxygen plasma treatments were similar for PPTA and PBO. In both cases, oxygen plasma introduces polar groups onto the surfaces, involving an increase in the degree of surface nanoroughness. AFM measurements evidenced substantial changes in the surface morphology at the nanometer scale, especially after plasma exposure for a long time. For the PBO fibers, the outermost layer – contaminant substances – was removed thanks to the plasma treatment, which indicates that this agent had a surface cleaning effect.
The pyrolysis behavior of Nomex poly(
m-phenylene isophtalamide) fibers under argon was studied using thermoanalytical and infrared spectroscopic methods to get direct information on the progressive ...changes undergone by the solid material and its carbon fiber residues. Simultaneous thermogravimetry (TG)–differential thermal analysis (DTA) measurements were carried out in a thermobalance in order to establish the different steps in the thermal degradation of Nomex. Samples pyrolyzed to various carefully controlled extents were prepared in the same thermobalance by heating under the same conditions to different temperatures selected as a function of the TG/DTA results. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) spectra of all samples were obtained in order to follow changes in chemical composition along the pyrolysis process. Rupture of hydrogen bonds takes place in the 300–400°C interval, being followed by a complex thermal event involving two weight loss steps and three endothermal DTA peaks. An intermediate aryl nitrile forms from heterolytic decomposition of Nomex above 433°C and decomposes totally at 698°C. From 473°C onwards, the DRIFTS spectra evidence a drop in intensity of amide bands in parallel with increases in primary amine and carbonyl groups. Total disappearance of amide bands (607°C) is followed by a DTA exotherm at 627°C attributed to condensation reactions. Accordingly, CC skeletal vibrations in polyaromatic networks develop confirming the occurrence of a progressive loss of heteroatoms and an increase in aromaticity.
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•Sub-surface mechanical damage was evidenced for the most extensive ground graphite.•Oxygen functionalization prevailed over etching of graphitic material for the shorter plasma ...exposure.•The degree of graphitic order increased with increasing oxygen plasma exposure time as a result of etching.
Two types of high-surface area graphite having different specific surface areas (HSAG100 and HSAG300) were treated in a low-pressure microwave discharge oxygen plasma for 3 and 10 min. The physico-chemical properties at surface scale of the resulting materials were investigated by N2 physisorption, Raman spectroscopy, temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). For both materials, an increase of the degree of graphitic order was observed following the longer oxygen plasma exposure. Likewise, the 10-min plasma treatment gave place to surface-functionalized materials without compromising their thermal stability and aromatic network. Particularly, for HSAG100, the longer plasma exposure contributed to the development of an edge surface-functionalized graphite. In the case of as-received HSAG300, the presence of a mechanically damaged sub-surface ascribable to the milling process could be evidenced.
Two-dimensional (2D) MoSe2 nanosheets were investigated as a catalyst for nitroarene reduction in the context of water treatment and demonstrated to exhibit a higher activity than their more studied ...MoS2 counterpart. Density functional theory (DFT) calculations disclosed a feasible nitroarene reduction pathway on 2D MoSe2, which involved passivation of the active site by oxygen from the nitro group and its reactivation by a reducing hydride. DFT analysis also provided an explanation for the isomer selectivity displayed by the 2D MoSe2 catalyst (towards, e.g., 2-, 3- and 4-nitroaniline). To facilitate their practical implementation, the bare MoSe2 nanosheets were coated with colloidal stabilizers, and were also immobilized on polymer foam, where they could be re-used for several consecutive catalytic cycles with no significant loss of activity. Overall, the present results open the prospect of 2D transition metal dichalcogenides beyond MoS2 as competitive nanocatalysts for the reduction of nitroarenes and other organic compounds.
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•2D transition metal dichalcogenides-based nanocatalysts beyond MoS2.•2D MoSe2 outperforms 2D MoS2 as catalyst for nitroarene reduction.•Immobilization of MoSe2 flakes on polymeric foam facilitate re-use in water treatment.•A feasible nitroarene reduction pathway on 2D MoSe2 is disclosed.•Different adsorption energetics at the active sites may introduce isomer selectivity.
Microporous carbon molecular sieves for separating gaseous mixtures have been prepared through chemical vapour deposition (CVD) of methane on activated carbon cloth obtained from Nomex aramid fibre. ...The activated carbon fibres were subjected to CVD of methane for different periods of time. The textural characterization of the resulting materials was assessed by physical adsorption of gases (N
2 and CO
2) and vapours (dichloromethane, benzene and cyclohexane) as well as immersion calorimetry into the same liquids. A direct visualization of the changes induced by CVD on the micropore mouth structure of the activated carbon cloth was provided by scanning tunnelling microscopy (STM). The validity of the materials for separating CO
2/CH
4 and O
2/N
2 was tested by assessing the kinetics of adsorption of the corresponding gases. Carbon molecular sieves with good selectivity for CO
2/CH
4 separation and showing acceptable CO
2 adsorption capacity were obtained.
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