Development of superior stable two-dimensional montmorillonite nanosheet based working nanofluids for direct solar energy harvesting and utilization

Nanofluids showing enhanced thermal diffusivity and thermal conductivity have potential applications in the fields of thermal science, engineering, and sustainable energy. It is still a challenge, however, to facilely prepare nanofluids with good dispersion stability using an environmentally friendly method. In the present study, natural layered montmorillonite was exfoliated into two-dimensional montmorillonite nanosheets (MtNS) using a physical ultrasonic method in aqueous solutions. The obtained MtNS suspensions can be directly used as nanofluids without any further operation or any addition of dispersants. Owing to electrostatic repulsion and hydration repulsion among the MtNS nanoparticles in aqueous solution, MtNS/water nanofluids show excellent dispersion stability even over a long time. In addition, owing to the good light absorption ability of MtNS and the thermal diffusion properties of nanofluids, MtNS/water nanofluids exhibit outstanding photothermal conversion efficiency and dramatic solar thermal energy collection performance. Experimental results also proved that suspensions with a high MtNS concentration and small MtNS particle size increase the efficiency of solar energy harvesting. In addition, the stable dispersion of MtNS in fluids leads to extremely good suspension sustainability and recycling performance of the nanofluids. The facile and green preparation method, low cost, excellent dispersion stability, and good solar energy collection performance of MtNS/water nanofluids enable them to be promising solar energy collection mediums. The large-scale application of MtNS/water nanofluids can take full advantage of green solar energy, which can be used in green buildings and contribute to building a sustainable society. •MtNS/water with superior stability has been developed.•MtNS/water nanofluid can be prepared with facile and green method.•The electrostatic repulsion and hydration repulsion lead to stable suspension.•Outstanding photo-thermal conversion of MtNS/water has been proved.•MtNS/water nanofluid shows huge potential in solar energy utilization.