The preparation of solution-processable graphene from graphite oxide typically involves a hydrazine reduction step, but the use of such a reagent in the large-scale implementation of this approach is ...not desirable due to its high toxicity. Here, we compare the deoxygenation efficiency of graphene oxide suspensions by different reductants (sodium borohydride, pyrogallol, and vitamin C, in addition to hydrazine), as well as by heating the suspensions under alkaline conditions. In almost all cases, the degree of reduction attainable and the subsequent restoration of relevant properties (e.g., electrical conductivity) lag significantly behind those achieved with hydrazine. Only vitamin C is found to yield highly reduced suspensions in a way comparable to those provided by hydrazine. Stable suspensions of vitamin C-reduced graphene oxide can be prepared not only in water but also in common organic solvents, such as N,N-dimethylformamide (DMF) or N-methyl-2-pyrrolidone (NMP). These results open the perspective of replacing hydrazine in the reduction of graphene oxide suspensions by an innocuous and safe reductant of similar efficacy, thus facilitating the use of graphene-based materials for large-scale applications.
The preparation of aqueous graphene dispersions by exfoliation of pristine graphite in the presence of a wide range of surfactants is reported. High graphene concentrations, up to about 1
mg
mL
−1, ...were obtained with the use of some non-ionic surfactants. The dispersions consisted of single- and few-layer graphene platelets with their basal planes virtually free of even atomic-sized (point) defects. The potential utility of such highly concentrated dispersions toward the low-cost, large-scale manipulation and processing of graphene was demonstrated by processing them into electrically conductive, free-standing paper-like films.
The use of UV light to trigger different processes involving graphene oxide sheets suspended in aqueous medium at room temperature has been investigated. These processes include (1) deoxygenation of ...the sheets in the absence of photocatalysts, reducing agents and stabilizers, (2) selective nucleation and growth of metal nanoparticles on the sheets to yield graphene-based hybrids and (3) decomposition of the dye molecule rhodamine B in the presence of only graphene oxide. Photoinduced heating of the suspended graphene oxide sheets by intense UV irradiation (∼1
W
cm
−2 delivered at the surface of the dispersion) was interpreted to generate at high temperature and reactive environment strictly localized at the sheets and their immediate aqueous medium, which in turn brings about the mentioned processes. In addition to providing a simple route toward reduction of graphene oxide dispersions, the present results suggest that intense UV light can be used to promote reactions at ambient conditions with this material that would otherwise require high temperatures, chemical reactants and/or catalysts.
The stable dispersion of graphene flakes in an aqueous medium is highly desirable for the development of materials based on this two-dimensional carbon structure, but current production protocols ...that make use of a number of surfactants typically suffer from limitations regarding graphene concentration or the amount of surfactant required to colloidally stabilize the sheets. Here, we demonstrate that an innocuous and readily available derivative of vitamin B2, namely the sodium salt of flavin mononucleotide (FMNS), is a highly efficient dispersant in the preparation of aqueous dispersions of defect-free, few-layer graphene flakes. Most notably, graphene concentrations in water as high as ∼50 mg mL–1 using low amounts of FMNS (FMNS/graphene mass ratios of about 0.04) could be attained, which facilitated the formation of free-standing graphene films displaying high electrical conductivity (∼52000 S m–1) without the need of carrying out thermal annealing or other types of post-treatment. The excellent performance of FMNS as a graphene dispersant could be attributed to the combined effect of strong adsorption on the sheets through the isoalloxazine moiety of the molecule and efficient colloidal stabilization provided by its negatively charged phosphate group. The FMNS-stabilized graphene sheets could be decorated with nanoparticles of several noble metals (Ag, Pd, and Pt), and the resulting hybrids exhibited a high catalytic activity in the reduction of nitroarenes and electroreduction of oxygen. Overall, the present results should expedite the processing and implementation of graphene in, e.g., conductive inks, composites, and hybrid materials with practical utility in a wide range of applications.
The stabilization of reduced graphene oxide (RGO) sheets in aqueous dispersion using a wide range of surfactants of anionic, non-ionic and zwitterionic type has been investigated and compared under ...different conditions of pH, surfactant and RGO concentration, or sheet size. The observed differences in the performance of the surfactants were rationalized on the basis of their chemical structure (e.g., alkylic vs. aromatic hydrophobic tail or sulfonic vs. carboxylic polar head), thus providing a reference framework in the selection of appropriate surfactants for the processing of RGO suspensions towards particular purposes. RGO-surfactant composite paper-like films were also prepared through vacuum filtration of the corresponding mixed dispersions and their main characteristics were investigated. The composite paper-like films were also electrochemically characterized. Those prepared with two specific surfactants exhibited a high capacitance in relation to their surfactant-free counterpart.
The long‐term trend of increasing phytoplankton net primary production (NPP) in the Arctic correlates with increasing light penetration due to sea ice loss. However, recent studies suggest that ...enhanced stormy wind mixing may also play a significant role enhancing NPP. Here, we isolate the role of sea ice and stormy winds (hereafter high‐frequency winds) using an eddy‐permitting ice‐ocean‐biogeochemical model configured for the North Atlantic and the Arctic. In the model, the presence of high‐frequency winds stimulates nutrient upwelling by producing an earlier and longer autumn‐winter mixing period with deeper mixing layer. The early onset of autumn mixing results in nutrients being brought‐up to near‐surface waters before the light becomes the dominant limiting factor, which leads to the autumn bloom. The enhanced mixing results in higher nutrient concentrations in spring and thus a large spring bloom. The model also shows significant iron limitation in the Labrador Sea, which is intensified by high‐frequency winds. The effect of sea ice loss on NPP was found to be regionally dependent on the presence of high‐frequency winds. This numerical study suggests high‐frequency winds play significant role increasing NPP in the Arctic and sub‐Arctic by alleviating phytoplankton nutrient limitation and that the isolated effect of sea ice loss on light plays a comparatively minor role.
Plain Language Summary
The amount of phytoplankton (i.e., small floating algae) in the Arctic Ocean has been increasing in response to sea ice losses, which increases light penetration and reduces light limitation on growth. However, the parallel increase in storm activity can also enhance phytoplankton growth by reducing nutrient limitation through increasing surface ocean mixing. Here, we use a numerical ocean—sea ice model with marine biogeochemistry to separate the contribution of stormy winds and sea ice loss on the phytoplankton growth. In the model, stormy winds induce more ocean mixing. This results in a seasonally larger amount of nutrient in surface waters with the capacity of sustaining a larger phytoplankton bloom in spring and autumn if light conditions are optimal for growth. The model shows a significant iron limitation in the Labrador Sea, which is intensified by high‐frequency winds. The contribution of sea ice loss to phytoplankton growth was only significant in the presence of stormy winds. This numerical study suggests high‐frequency winds play significant role increasing phytoplankton growth in the Arctic and sub‐Arctic by alleviating phytoplankton nutrient limitation and that the isolated effect of sea ice loss on light plays a comparatively minor role.
Key Points
High‐frequency winds stimulate surface mixing and nutrient upwelling enhancing primary production and biogenic carbon export
The model shows significant iron limitation in the Labrador Sea, which is intensified by high‐frequency winds
Sea ice loss contributes to increasing primary production only in Hudson Bay and in the presence of high‐frequency winds
► 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.
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.
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.
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