Fabrication and Stabilization of Oxidized Carbon Black Nanoparticle Dispersion in Aqueous Solution for Photothermal Conversion Enhancement

[Image: see text] This study aims to explore oxidized carbon black nanoparticles (OCB-NPs) capped with an inorganic surfactant dispersed in water, as a carbon black water-based nanofluid, on photothermal conversion enhancement. We used ultraviolet–visible (UV–vis) absorption spectroscopy and zeta potential analyzers to identify the optimal concentration of sodium hexametaphosphate (SHMP) as an inorganic surfactant for OCB-NPs in order to determine the maximum value of UV–vis light absorption and absolute zeta potential. Then, the concentrations of 0.025–0.1 wt % OCB water-based nanofluid with SHMP were formulated by an ultrasonic bath for the examination of rheological behavior, thermal conductivity, and heating rate. The results indicated that the heating rate improvement of the water-based nanofluid involving 0.1 wt % OCB-capped with SHMP after irradiation by UV–vis light with wavelengths ranging from 220 to 380 nm, which is included in the solar spectrum, and an intensity of 205 W/m(2) increased by approximately 66%, compared to the base fluid in the cyclic flow system. Furthermore, after a 1 month storage period, the dispersion stabilization of water-based nanofluid including 0.1 wt % OCB-capped with SHMP reached 98%, as estimated by the UV–vis spectrophotometer.