Size- and pH-dependent stability, thermal degradation kinetics, and photothermal behavior of graphene oxide-based surfactant-free nanofluids

The stability and thermal degradation kinetics of a nanofluid critically govern its use. Herein, the stability and thermal degradation kinetics of three variants of graphene oxide-based nanofluids are investigated, considering the nanoparticle size and pH of the medium. For this, graphene oxide (GO)-, amine-functionalized graphene oxide (f-GO)-, and reduced graphene oxide (rGO)-based nanofluids with different particle sizes were prepared at a concentration of 0.1 wt.% and at various pH values ranging from 3 to 9. It was found that f-GO nanofluids with a particle size of approximately 55 nm remained stable for up to 90 days at pH 7 and showed higher thermal conductivity, i.e., 0.698 W m⁻¹ K⁻¹ at 25°C, than the other variants. Additionally, at the same pH, f-GO-3 nanofluid showed a higher photothermal response than the other nanofluids. An optimum transparent insulating material thickness (TIM) of 2 mm was obtained for improved photothermal behavior. Analysis of thermal degradation kinetics revealed that rGO showed better thermal stability and higher activation energy (E_a) at higher temperatures than GO and f-GO nanoparticles. Thus, f-GO and rGO nanofluids with a particle size of around 50 nm may be chosen depending on the application requirements involving high temperatures or long-duration storage.