► Control of the reduction parameters on chemical reductions of graphene oxide (GO) leads to different deoxygenation degrees. ► Reduction proceeds by elimination of the most labile functional groups, ...mostly located on basal plane positions. ► GO sheets are composed of intermingled oxidized and non-oxidized regions, whose relative area depends on the reduction degree.
Chemical reduction of exfoliated graphite oxide (graphene oxide) has become one of the most promising routes for the mass production of graphene sheets. Nonetheless, the material obtained by this method exhibits considerable structural disorder and residual oxygen groups, and reports on their microscopic structure are quite scarce. We have investigated the structure and chemistry of graphene oxide samples reduced to different degrees using atomic force and scanning tunneling microscopy (AFM/STM) as well as X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD), respectively. TPD and XPS results indicate that reduction proceeds mainly by eliminating the most labile oxygen groups, which are ascribed to epoxides and hydroxyls on basal positions of the graphene plane. AFM/STM shows that the sheets are composed of buckled oxidized regions intermingled with flatter, non-oxidized ones, with the relative area of the latter increasing with the reduction degree.
Gold interdigitated microelectrodes (Au-IDA) modified with either graphene flakes exfoliated using flavin mononucleotide (FMN) (Gr-FMN) or graphene flakes and platinum nanoparticles (Pt-Gr-FMN) have ...been studied in the oxidation of uric acid (UA). An electrochemical method for the detection and quantification of UA in phosphate buffer solution at physiological pH (PBS, 0.25M, pH7) in the absence and presence of ascorbic acid (AA) has been studied by cyclic voltammetry. The quantification of UA was investigated by cyclic voltammetry, presenting an oxidation peak at 0.99V with both modified electrodes. Linearity range of 60–578μM and 60–345μM has been found for Gr-FMN/Au-IDA and Pt-Gr-FMN/Au-IDA electrodes, respectively. Limits of detection of 18μM were obtained for both electrodes, and the repeatability was studied at 177μM providing 4% and 8% for Gr-FMN/Au-IDA and Pt-Gr-FMN/Au-IDA, respectively. AA interference has been studied by cyclic voltammetry, showing two clearly separated oxidation peaks, at 0.99V for UA oxidation and at 0.74V for Gr-FMN/Au-IDA and 0.70V for Pt-Gr-FMN/Au-IDA for AA oxidation. Linearity range has been studied in presence of 250μM AA obtaining a working range of 60–578μM for Gr-FMN/Au-IDA electrode and of 60–288μM with Pt-Gr-FMN/Au-IDA electrode. Limits of detection remain at 18μM for both electrodes and the repeatability was studied at 177μM providing 8% and 14% for Gr-FMN/Au-IDA and Pt-Gr-FMN/Au-IDA electrodes respectively.
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A direct route for the synthesis of activated Co3O4 nanosheets that avoids the need for reduction post-treatments is reported for the first time.
Our strategy yields materials with a high potential ...towards energy- and environment-related applications.
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Two-dimensional Co3O4 nanosheets have emerged as attractive materials for use in a number of relevant technological applications. To exhibit a competitive performance in such uses, however, their structure needs to be activated, which is frequently accomplished via post-synthesis reduction strategies that introduce oxygen vacancies and increase the number of active Co(II) sites. Here, we investigate a direct route for the synthesis of activated Co3O4 nanosheets that avoids reduction post-treatments, yielding materials with a high potential towards energy- and environment-related applications. The synthesis relied on an interim amorphous cobalt oxide material with nanosheet morphology, which upon calcination afforded Co3O4 nanosheets having Co(II) sites in quantities similar to those usually found for Co3O4 nanostructures activated by reduction post-treatments. When tested as electrodes for charge storage, the nanosheets demonstrated a competitive behavior in terms of both capacity and rate capability, e.g., a gravimetric capacity of ∼293 mAh g−1 at 1 A g−1 with 57% retention at 60 A g−1 was measured for nanosheets calcined at 350 °C. The materials were shown to be efficient catalysts for the reduction of nitroarenes (4-nitrophenol and 4-nitroaniline), outperforming other Co3O4 nanostructures, as well as effective adsorbents for the removal of organic dyes (methyl orange, methylene blue) from water.
The implementation of green approaches towards the preparation of graphene and graphene-based materials with enhanced functionality from graphite oxide has been relatively little explored. ...Particularly, the use of bioreductants and the testing of their relative efficacies is an incipient area of research. Here, a pool of 20 environmentally friendly, natural antioxidants have been tested for their ability to reduce graphene oxide. These antioxidants were mostly vitamins, amino acids and organic acids. By establishing a protocol to systematically compare and optimize their performance, several new efficient bioreductants of graphene oxide have been identified, namely, pyridoxine and pyridoxamine (vitamin B6), riboflavin (vitamin B2), as well as the amino acids arginine, histidine and tryptophan. These biomolecules were used to prepare reduced graphene oxide–silver nanoparticle hybrids that displayed colloidal stability in water in the absence of additional dispersants. Particularly, hybrids prepared with pyridoxamine exhibited a combination of long-term colloidal stability and exceptionally high catalytic activity among silver nanoparticle-based catalysts in the reduction of p-nitrophenol with NaBH4. Thus, in addition to expanding substantially the number of green reductants available for graphene oxide reduction, the present results underline the idea that proper selection of bioreductant can be relevant to achieve graphene-based materials with improved performance.
The oxidation and etching of reduced graphene oxide (RGO) by thermal oxidation in air, microwave oxygen plasma, ultraviolet-generated ozone, and scanning tunneling microscopy (STM) lithography have ...been investigated. This type of graphene exhibited a higher reactivity toward oxidation than that of pristine graphite (taken as a reference carbon material), which could be related to its intrinsically defective structure. Etching of RGO as a result of thermal oxidation in air was started at as low a temperature as 400 °C, as compared to ∼500 °C for graphite, indicating that the defects present on the former are different in nature from those found on the surface of pristine graphite. The morphological evolution of individual RGO sheets upon oxidative attack was consistent with the sheets being essentially a patchwork of minute domains (a few to several nanometers large) with two distinct reactivities, higher (lower) reactivity associated with defective (defect-free) domains. The introduction of oxygen functional groups on the basal plane of RGO was revealed directly by X-ray photoelectron spectroscopy and indirectly through STM. STM lithography enabled discrimination between oxidation proper (introduction of oxygen groups) and etching (desorption of the groups as CO or CO2) of the RGO sheets through control of the applied bias voltage. The former was visualized in the STM images as locally depressed features of electronic origin, whereas etching led to the generation of actual trenches on the sheets. Taken together, the present results provide insight into the reactivity of RGO and suggest potential practical applications involving controlled oxidation of this type of graphene.
Activated carbon fibers derived from polyaramids exhibit an outstandingly homogeneous pore size distribution. Here, we review the work carried out on the porosity characteristics of carbons prepared ...from these polymers, specifically Kevlar poly(p-phenylene terephthalamide) and Nomex poly(m-phenylene isophthalamide). First, studies on the thermal decomposition of both polyaramid fibers and the reactivity of the resulting chars are reviewed in connection with the properties of the resulting products. Then, the porous texture of activated carbon fibers (ACFs) prepared by different methods (physical and chemical activation) is examined, with particular emphasis on the use of some novel methods to investigate microporosity in solids. Finally, the relationship between porosity and behavior of polyaramid-based carbons in several different types of application is analyzed.
Metal-assisted reduction of graphene oxide (GO) has recently emerged as a fast, efficient and room-temperature method towards the preparation of chemically derived graphene, but according to the ...mechanisms of reduction that have been proposed, not all relevant metals ( e.g. , Ag) should be a priori effective for this purpose. Here, we show that aqueous GO dispersions can be very efficiently reduced at room temperature with NaBH 4 using Ag nanoparticles (Ag NPs) as catalysts, either generated in situ from appropriate precursors (AgNO 3 ) or added to the dispersions as pre-formed objects. We propose and investigate a reduction mechanism that involves the charging of the Ag NPs with excess electrons obtained from the oxidation of a product of the spontaneous hydrolysis of NaBH 4 in the aqueous medium. These excess electrons are then transferred to the GO sheets, triggering their reduction. The catalytic and electrochemical performance of the reduced GO–Ag NP hybrids that result from this process has also been examined. In particular, the hybrids are seen to exhibit very high catalytic activity in the reduction of 4-nitrophenol to 4-aminophenol as a model reaction, and are also effective towards the electrochemical reduction of H 2 O 2 .
Activated carbon fibers were prepared from Nomex® poly(
m-phenylene isophthalamide) by either H
3PO
4 activation, H
3PO
4–CO
2 activation, or simply CO
2 or steam activation. These treatments ...converted amide groups from the polymer precursor into complex and heterogeneously distributed nitrogen functionalities. TPD, XPS and XANES were used to study the effects of these treatments on the local bonding environment around nitrogen in the resulting carbons. These analytical techniques showed that nitrogen atoms are present in the 6-membered rings located at the edges of condensed polyaromatic systems as pyridine-like sp
2 nitrogen (N1 or N2) or in the interior, where nitrogen replaces one carbon atom and is bonded to three carbon neighbors (N3). The occurrence of a species (N2) hypothetically related to a pyridinic cycle bearing oxygen substituents or intracyclic oxygen atoms could be correlated with the degree of oxidation of the carbon surface. Assuming that a relative N3 increase is indicative of aromatization and that the reverse, correlated with a N2 increase, is indicative of surface oxidative denitrogenation, the ratio between these nitrogen species revealed that aromatization and oxidative denitrogenation processes occur sequentially or simultaneously to different extents according to the type of carbon activation and to the burn-off degree. Physical activation involves thermal aromatization reactions during the carbonization stage and the subsequent isothermal activation one. In this second activation stage, co-occurring thermal oxidation reactions lead to a less intense denitrogenation during CO
2 activation than during steam activation. H
3PO
4 activation induces the largest nitrogen retention in the final product in a double process of aromatization and “auto-activation” producing a moderate oxidative attack of nitrogen. However, an increase of the H
3PO
4 ratio fostered the oxidation of the carbon surface and consequently enhanced nitrogen gasification during the subsequent activation.
Electrochemical characterization has been carried out for electrodes prepared of several activated carbon fiber samples derived from poly (
m-phenylene isophthalamide) (Nomex) in an aqueous solution. ...Depending on the burn-off due to activation the BET surface area of the carbons was in the order of 1300–2800
m
2
g
−1, providing an extensive network of micropores. Their capability as active material for supercapacitors was evaluated by using cyclic voltammetry and impedance spectroscopy. Values for the capacitance of 175
F
g
−1 in sulfuric acid were obtained. Further on, it was observed that the specific capacitance and the performance of the electrode increase significantly with increasing burn-off degree. We believe that this fact can be attributed to the increase of surface area and porosity with increasing burn-off.
► We overview recent developments in the liquid-phase processing of graphite and graphite derivatives for the mass production of graphene. ► Processing of graphite oxide affords aqueous and organic ...suspensions of reduced graphene oxide sheets. ► Direct exfoliation of pristine graphite yields stable graphene suspensions in organic solvents and water-surfactant solutions. ► Structurally, graphene oxide is a patchwork of defect-free and oxidized, disordered regions, the former expanding upon reduction.
Liquid-phase processing of graphite and graphite derivatives is one of the most promising methodologies for the mass production of graphene. Here, we present a brief overview of the main developments in this research area over the last few years, together with our own contributions to the field. Particularly, we discuss the preparation of graphenes both in aqueous and organic media by reduction of exfoliated graphite oxide as well as by direct exfoliation of pristine graphite, highlighting some of the obstacles that have been encountered along the way and the approaches proposed to overcome them. Some fundamental aspects of graphenes derived from graphite oxide, specifically their structure and reactivity, are also considered.
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