Self-dispersible graphene quantum dots in ethylene glycol for direct absorption-based medium-temperature solar-thermal harvesting.

Poor dispersion stability of carbon nanofluids is one of the key issues limiting their solar-thermal applications especially under medium-to-high temperatures. Herein, this work reported a facile way to prepare stably dispersed graphene quantum dot-ethylene glycol (GQD-EG) medium-temperature solar-thermal nanofluids. The hydroxyl-terminated GQDs were synthesized by a scalable hydrothermal approach. The obtained GQDs have a small particle size, narrow particle size distribution and are self-dispersible within EG fluids. The GQD-EG nanofluids maintained their uniform dispersion after continuous heating at 180 °C for 7 days. The hydrogen bonding between the hydroxyl group on the surface of GQDs and the EG molecules helped achieve homogenous dispersion of GQDs in the nanofluids, and the small particle size and low density of GQDs helped mitigate the sedimentation tendency. The dispersed GQD-EG nanofluids have demonstrated broadband absorption of sunlight, high specific heat capacity and low viscosity, which are all desired for high-performance direct absorption-based solar-thermal energy. The prepared GQD-EG nanofluids have exhibited consistent volumetric harvesting of solar-thermal energy under concentrated solar illumination with a heating temperature up to 170 °C.