A. Martínez-Alonso

Environmentally friendly approaches toward the mass production of processable graphene from graphite oxide

Graphene has attracted a great deal of scientific interest in latter years owing to its unique properties, with many prospective applications being actively investigated at present. However, the actual implementation of graphene in technological uses will depend critically on the development of appropriate methodologies for its mass production. In this regard, one of the most promising approaches is based on the exfoliation and reduction of graphite oxide. Graphenes derived from graphite oxide can be prepared at low cost and high throughput, can be further processed in a number of solvents, and are chemically versatile, among other attractive features. In an environment-conscious world, the availability of green approaches toward graphene production would also constitute an added advantage. During the last year, different environmentally friendly methods for the production of graphene from graphite oxide have emerged, which we highlight here. These are based on solvothermal and electrochemical processes, as well as on the use of green reductants. Several open questions and possible future directions for this research topic are also discussed.

Surface chemical modifications induced on high surface area graphite and carbon nanofibers using different oxidation and functionalization treatments

Two graphitic carbon materials with different edge to basal plane ratio, high surface area graphite (HSAG) and graphitized carbon nanofibers (CNFs), were oxidized by two methods, aqueous-HNO(3) wet oxidation and oxygen plasma oxidation. Characterization of the materials by temperature-programmed desorption, thermogravimetry and X-ray photoelectron and Raman spectroscopies indicated that the amount and nature of oxygen surface groups introduced depended on the oxidation method and on the structure of the original material. While surface sites within the layers were only oxidized by oxygen plasma, surface sites at the edges of graphene layers were oxidized by both treatments being the wet oxidation more effective. Modification of the oxidized materials with a diamine or a triamine molecule resulted in the formation of ammonium carboxylate salt species on the carbon surface.

Synthesis, characterization and dye removal capacities of N-doped mesoporous carbons.

Nitrogen-doped ordered mesoporous carbons were synthesized by chemical vapor deposition, using acetonitrile as carbon and nitrogen source and SBA-15 as mesoporous silica template. Their porous texture, structural order and surface chemistry were studied as a function of the experimental conditions (acetonitrile stream concentration and deposition time). A non-doped ordered mesoporous carbon was also prepared by the same procedure using propylene as carbon source. Methylene blue, methyl orange and fuchsin acid were selected as probe molecules to investigate the dye adsorption behavior on the ordered mesoporous carbons. Both N-doped and non-doped ordered mesoporous carbons adsorbed large amounts of these three dyes demonstrating the importance of mesoporosity, especially for the adsorption of larger dyes (e.g. fuchsin acid). The presence of nitrogen functional groups was detrimental for the adsorption of the basic dye (methylene blue). On the other hand, the nitrogen functionalities improved the adsorption kinetics for both acid and basic dyes, and the N-doped samples achieved 100% of their maximum adsorption capacities in less than 15 min.

A comparison between physically and chemically driven etching in the oxidation of graphite surfaces.

The etching of graphite surfaces by two different types of oxidative treatments, namely dielectric barrier discharge (DBD) air plasma and ultraviolet-generated ozone (UVO), has been investigated and compared by means of scanning tunneling microscopy (STM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Although the attack is initiated in both cases with the formation of individual, isolated atomic-scale defects (in particular, atomic vacancies), its subsequent evolution indicated that different mechanisms drive the surface modification in the two types of treatment, which greatly differ in etching selectivity. Thus, physical processes (i.e., ion bombardment) dominate the attack by DBD air plasma, which are not present in the case of UVO oxidation. The effects of the different etching mechanisms on the graphite surface structure, as visualized by STM down to the atomic scale, are discussed and found to be consistent with the Raman spectroscopy and XPS data. This type of information can be relevant when selecting the most appropriate type of surface modification of carbon materials for specific purposes.

Highly efficient silver-assisted reduction of graphene oxide dispersions at room temperature: mechanism, and catalytic and electrochemical performance of the resulting hybrids

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 NaBH4 using Ag nanoparticles (Ag NPs) as catalysts, either generated in situ from appropriate precursors (AgNO3) 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 NaBH4 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 H2O2.

Surface modification of high-performance polymeric fibers by an oxygen plasma. A comparative study of poly(p-phenylene terephthalamide) and poly(p-phenylene benzobisoxazole)

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.

Porosity Development in Carbon Nanofibers by Physical and Chemical Activation

In this Work we Have Compared the Effects of Physical Activation with CO2 and Chemical Activation with KOH on Porosity Development in Vapor Grown Carbon Nanofibers (CNFs). both Physical and Chemical Activations Result in Micro- and Mesoporosity Development in the Studied Cnfs. under this Work’s Conditions, Chemical Activation with KOH Was More Efficient than Physical Activation with CO2 in Terms of Surface Area Increase Regarding the Fresh Material (7.5-Fold versus 4-Fold, Respectively, under the Optimal Conditions Found for each Type of Activation). Atomic Force Microscopy Indicated that, although the CNF Samples Retained their Fibrous Morphology upon both Physical and Chemical Activation, the Latter Treatment Brought about Noticeable Changes in their Nanometer-Scale Structure. Likewise, an Appreciable Decrease in Nanofiber Diameter Following both Types of Activation Was Noticed. However, such Diameter Reduction Could Not Account for the Increase in Specific Surface Area of the Activated Materials, which Has to Be Attributed to Porosity Development. X-Ray Diffraction Studies Showed that both Physical as Chemical Activation Take Place Mainly on the Disordered Skin of the Cnfs but in a Different Way. Thus, Physical Activation Removes the More Amorphous Areas from the CNF Skin by Gasification (which Increases their Structural Order), while upon Chemical Activation with KOH, the Carbon Material Is Oxidized to a Carbonate, and the Alkali Hydroxide Is Reduced to Metallic Potassium, which Becomes Intercalated between the Graphene Layers of the Carbon Material, Leading to a Certain Expansion of the Structure.

Adsorption and microcalorimetric measurements on activated carbons prepared from Polyethylene Terephtalate

This paper deals with the characterization of activated carbons obtained from Polyethylene Terphtalate (PET). This has been carried out by using several techniques. Among them immersion calorimetry of several organic vapours (n-hexane, benzene, cyclohexane and 2,2-DMB) and adsorption of the same vapours. Nice agreement is found between the textural characteristics determined by both techniques.

Impact of the Carbonization Atmosphere on the Properties of Phosphoric Acid-activated Carbons from Fruit Stones

Activated carbons were obtained by phosphoric acid activation of a mixture of fruit stones (apricot and peach) in two different atmospheres (argon and air) at various temperatures in the 400–1000°C range. The evolution of several characteristic parameters of the resulting carbons (bulk density, yield, BET surface area, ultramicropore, supermicropore and mesopore volumes and cation-exchange capacity) with activation temperature was examined. The above parameters were re-calculated on a volume basis (practical effectiveness) and a volume-yield basis (economic efficiency). It was concluded that carbons obtained in an argon atmosphere exhibit some practical advantages over those obtained in air regarding cation adsorption, although those obtained in air may represent an interesting alternative regarding porous structure.

Synthetic Carbons Derived from a Styrene–Divinylbenzene Copolymer Using Phosphoric Acid Activation

Activated carbons were obtained by phosphoric acid activation of a porous chloromethylated and sulphonated styrene—divinylbenzene copolymer in two different atmospheres (argon and air) at various temperatures in the range 400–1000°C. The development with activation temperature of several characteristic parameters of the resulting carbons, i.e. bulk density, yield, surface area, meso-, micro- and ultra-micropore volumes, and cation-exchange capacity, was examined. All these parameters were recalculated relative to the volume of adsorbent (to obtain their practical effectiveness) and then related to the same quantity of precursor (to yield their economic efficiency). It is concluded that the carbons obtained in an argon atmosphere exhibit some practical advantages over those obtained in air regarding cation adsorption, although those obtained in air at low temperatures may represent an interesting alternative.