Photothermal properties of a nanofluid containing graphene oxide-copper nanoparticle hybrid materials prepared by using <i>Paramignya trimera</i> extract

) extract as a reducing and stabilizing agent, producing uniformly distributed Cu nanoparticles (∼8.5 nm) on GO sheets. The prepared nanofluids (0.01-0.04 vol%) exhibited enhanced thermal conductivity and optical absorption compared with deionized water. The thermal conductivity increased by 4-15% in the temperature range 30-55 °C. Zeta potential analysis revealed values of 38.5, 32.6, 30.8, and 26.2 mV, indicating good colloidal stability up to 0.03 vol% and moderate stability with incipient aggregation at 0.04 vol%. Under AM 1.5 solar irradiation, the GO-CuNP nanofluids demonstrated photothermal conversion efficiencies of 35-62%, which are higher than those of nanofluids containing GO and CuNPs only. This was attributed to synergistic plasmonic-graphenic absorption and improved heat transport within the hybrid structure. These findings highlight the potential of hybrid nanofluids as efficient solar-absorbing media for solar thermal applications.