Abstract
Graphene oxides (GO) of highly polydisperse size distribution were prepared by the Brodie method and their dispersion stability was characterized. Exfoliation and fractionation led to well-defined particle populations in the Nano, classical Colloidal (submicron) and Micrometer size ranges, as revealed by atomic force microscopy and light scattering measurements. Time-resolved dynamic light scattering experiments revealed that aggregation processes are fully impeded in the intermediate pH regime of 3–13 in the absence of electrolytes. While the resistance against salt-induced aggregation increases with the pH due to the progressive ionization of the surface functional groups of GO sheets, their dispersions are inherently unstable at supramillimolar concentrations of strong acids and submolar concentrations of bases, in line with the DLVO theory. However, the aggregation behavior quantified by the critical coagulation concentrations (CCCs) shows surprisingly substantial platelet size dependence. The CCC of Nano Brodie-GO reaches 360 mm at pH = 12, which is one of the highest values ever reported for GO aqueous dispersions. These results provide useful quantitative information to design processable GO dispersions of pH- and size-tunable stability for future applications.
Highlights
Much effort has been recently applied to design and fabricate multifunctional graphene-based materials both as bulk solids and as ultrathin films [1e5]
(1) the “Nano Graphene oxides (GO)” fraction was obtained by a combined application of centrifugation and filtration, (2) the “Colloid GO” fraction was collected upon a single centrifugation run and (3) the “Micron graphite oxide” fraction was isolated by differential centrifugation
Dramatic stability enhancement is quite obvious: the progressive deprotonation of the surface functional groups gives rise to higher surface potential and to stronger electrical double layer repulsion, which can be screened at higher ionic strengths
Summary
Much effort has been recently applied to design and fabricate multifunctional graphene-based materials both as bulk solids and as ultrathin films [1e5]. Visual observation on the sedimentation may give a practically useful semi-quantitative picture on the storage stability of particle dispersions, this can be very misleading concerning the true colloidal stability This is because (1) the aggregation state strongly depends on the time window investigated, (2) larger primary particles may settle even in the absence of aggregation and (3) both single and aggregated particles forming a coherent structure may exhibit hindered settling due to multiparticle hydrodynamic interactions and remain dispersed for extended time periods [38]. Significantly different CCC values have been recently published by Wang et al [43] for GO particles in the above salt solutions These strongly inconsistent data reflect that a wide range of experimental parameters such as pH [44], the degree of oxidation [45] and exfoliation along with the size of particles [46] may affect the aggregation behavior of GO in electrolyte solutions. Since the existing knowledge was accumulated solely for GO prepared by the Hummers’ method, a special aspect of the present research lies in the choice of a unique GO sample obtained by Brodie’s protocol [49]
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