## 1 Introduction
However, some types of nanoparticles have become promising, such as the family of carbon-based nanostructures. Graphite [10; 14], single and multi-walled nanotubes [12; 15; 16; 17; 18], nanohorns [6; 19; 20], and graphene [21; 22; 23], were widely considered for solar energy applications. Carbon-based nanoadditives reach higher thermal conductivity enhancements at lower concentrations than other families of nanoparticles (metals, metal oxides). Moreover, the dispersion of carbon-based nanoadditives on a base fluid usually causes its conversion into a dark fluid. Graphene stands out as one of the most interesting carbon allotropes [24], thanks to its high thermal conductivity and its outstanding mechanical and electrical properties. However, graphene is hydrophobic, thus its stable dispersion in aqueous fluids is a difficult task. On the other hand, graphene oxide entails lower dispersability problem in water, but its thermal conductivity is dramatically reduced with respect to pristine graphene. A balance between dispersability in water and promising thermal properties is shown by reduced graphene oxide and functionalized graphene [25; 26]. As for the nanoadditive morphology, it is worth noting that commercial multilayer structures of functionalized graphene, nanosheets or nanoplatelets partially retain the original properties of the single-layer form with a notably lower production cost [22]. Various studies have investigated the potential of nanofluids containing graphene derivatives as heat-transfer media. [PERSON] et al. [27] measured the main thermophysical properties affecting the heat-transfer behaviour of a water/propylene glycol nanofluid containing functionalized graphene nanoplatelets. The authors highlighted that the 0.25 wt% nanoadditive concentration ensured the best compromise between thermal conductivity enhancement and viscosity increase. Similarly, [PERSON] et al. [28] determined the thermophysical properties of sulfonic acid-functionalized graphene nanoplatelets dispersed in a mixture of ethylene glycol and water. Nevertheless, they found that, for the studied nanoparticle concentrations (up to 0.5 wt%), the heat-transfer properties of the fluid did not improve considerably. [PERSON] et al. [29] investigated the heat-transfer behaviour of a water-based nanofluid containing sulfonic acid-functionalized graphene nanoplatelets, using a tube-in-tube heat exchanger. They found that the 0.5 wt% nanoplatelet concentration represented the optimal loading for convective heat-transfer performance. Although some studies addressed the optical properties of nanofluids containing graphene oxide [30], reduced graphene oxide [31], or even conventional graphene [32], very few researchers have specifically studied the potential of graphene nanoplatelets nanofluids as sunlight absorbers, which still remains an open research line [21; 22]. As regards direct solar steam generation, various authors reported on the efficient solar steam generation by gold [33], silicon [34] and carbon nanostructures [13; 35; 36]. However, to the best of our knowledge, no research involving graphene nanoplatelets exploitation for direct solar steam generation has been addressed yet. Preliminary investigations [21; 22] have shown the promising perspectives for DASC and direct solar steam generation of low-concentration functionalized graphene nanoplatelet-based dispersions. Therefore, the present work completes the property assessment of these nanofluids, reporting on their detailed and comprehensive physical profile. Two different types of nanofluids, consisting of polycarboxylate chemically modified and sulfonic acid-functionalized graphene nanoplatelets dispersions in water at two nanoadditive concentrations, 0.005 and 0.05 wt%, have been prepared and characterized. Elemental analyses of the employed nanopowders and pH studies of the dispersions were performed. The rheological behaviour and dynamic viscosity of the different-loaded nanofluids were determined in a wide temperature range.