Stability study of graphene oxide based aqueous nanofluids for solar absorption application

In this study, the stability of graphene oxide based aqueous nanofluids are tested under boiling–condensation conditions. The graphene oxide nanofluids remain very stable, and their transmittance in solar spectral region varies less than $6\%$ after $630g$ centrifugation for $3$ h at $90^{\circ}$C without boiling. However, when undergoing boiling and condensation processes, the solar transmittance of the graphene oxide nanofluids declines quickly, from $38$ to $4\%$, during the first $24$ h of testing, before leveling out in the final $120$ h of testing. A decrease in the fluid transmittance is due to the partial reduction of graphene oxide nanosheets, as evidenced by X-ray photoelectron spectrometer and Fourier transform infrared spectroscopy technique measurements. It is surprising that thermal reduction of graphene oxide in aqueous fluids occurs at such a low temperature $(\sim100^{\circ}$C$)$, when undergoing boiling and condensation. This temperature is much lower than the previously reported thermal reduction temperature $(180^{\circ}$C and above$)$ without boiling. The low-temperature thermal reduction of graphene oxide may be attributed to the bubble cavitation associated with boiling in aqueous fluids.