Molecularly engineered boron nitride nanosheet nanofluids with Rayleigh-Bénard-type convection for medium-temperature direct absorption solar collection

Medium-temperature direct absorption solar collectors (DASCs) remain limited by insufficient optical absorption, poor heat transfer, and inadequate long-term stability of the working fluid. Here, few-layer hexagonal boron nitride nanosheets were coated with polydopamine (PDA) and further grafted with tetradecanamine (TDA) to produce an oil dispersible BNNS@PDA-TDA nanofluid. The PDA layer provided broadband solar absorption, whereas TDA grafting enabled surfactant-free dispersion in heat-transfer oil. The resulting nanofluid increased thermal conductivity by up to 16.7 % relative to the base oil, with only a minor viscosity increase. Optical experiments and FDTD simulations showed that PDA coating dominated the absorption enhancement, while nanosheet thinning reduced scattering and improved the absorption ratio. Under simulated solar irradiation, reverse irradiation established a buoyancy-unstable thermal field and promoted Rayleigh-Bénard-type convection, producing a more uniform temperature field and a maximum photothermal conversion efficiency of 69.52 % at 40 ppm. The nanofluid also exhibited smaller transmittance changes and more reproducible temperature responses during cyclic heating under reverse irradiation than under forward irradiation. These results show that interfacial molecular engineering combined with irradiation-direction control can yield stable, high-efficiency nanofluids suitable for medium-temperature DASC applications.