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Direct absorption of sunlight with carbon nanofluids is a facile way to efficiently harvest solar-thermal energy, but poor dispersion stability of nanofluids under elevated temperatures severely limits their applications. Herein, we report a general strategy to prepare stably-dispersed medium-temperature solar-thermal nanofluids by employing crumpled particles of ethanol-wetted graphene oxide sheets as the self-dispersible photothermal converters. The crumpled particles simultaneously possess intensively-deformed surface structure, a low density and a small particle size. Such features weaken the inter-particle van der Waals attraction and gravitational sedimentation of the crumpled particles, thereby enabling their long-term stable dispersion within commercial thermal storage oils. Uniform dispersion of nanofluids was maintained after continuous heating for 2 weeks under a heating temperature up to 200 °C. The homogenously dispersed nanofluids also achieved high solar absorptance with low particle loadings, and retained the large heat capacity and suitable viscosity. These combined advantageous thermophysical properties enabled consistent high-performance medium-temperature direct absorption-based nanofluidic solar-thermal energy harvesting under large-flux solar illumination. [Display omitted] [ABSTRACT FROM AUTHOR]

Direct absorption and conversion of sunlight by optical nanofluids into storable heat is an efficient and facile way to harness abundant clean solar energy, but achieving long-term stable dispersion of the nanofluids is challenging under elevated temperatures. Herein, we report the preparation of mesoporous crumpled graphene particles as self-dispersible solar absorbers that can achieve homogenous dispersion within medium-temperature silicone oil. The mesoporous crumpled graphene particles were synthesized by using an aerosol-assisted capillary compression process and a subsequent oxidative-etching reaction. By controlling the etching time, the introduced mesopores can tune the apparent density of the crumpled particles into a value comparable with the dispersing base fluid. The crumpled surface structure, small particle size and compatible density weaken the inter-particle van der Waals attraction and gravitational sedimentation tendency. The resultant silicone oil nanofluids have maintained their uniform dispersion under a heating temperature of 200 °C for 50 h. The stable uniform dispersion together with the high solar absorptance, the large specific heat capacity and the proper viscosity enabled the application of prepared nanofluids for consistent high-performance medium-temperature solar-thermal energy harvesting under concentrated solar irradiation. [Display omitted] • Mesopores are introduced to crumpled graphene particles to tailor the density. • Stable dispersion of mesoporous crumpled graphene particles in silicone oil is achieved. • The nanofluids have high solar absorptance, large heat capacity and suitable viscosity. • Medium-temperature nanofluidic solar-thermal energy harvesting is demonstrated. [ABSTRACT FROM AUTHOR]