Spray pyrolysis-based synthesis of self-floating black TiO_2-x microspheres for solar-driven interfacial evaporation

While solar-driven interfacial evaporation (SIE) using self-floating monodispersed photothermal microspheres has emerged as a promising method for eco-friendly and continuous clean water production without performance degradation by contaminant accumulation, few studies have been conducted on self-floating SIE due to the lack of appropriate synthesis methods for the self-floating, large-size photothermal particles. In this study, a facile and versatile spray pyrolysis-based process is demonstrated to synthesize self-floating black TiO_2-x microspheres. This process facilitates defect engineering within the bulk region of large-sized metal oxides to enhance photothermal properties, a capability that is challenging to achieve with conventional reduction-based methods. The physicochemical properties as a function of process conditions were systematically investigated. The results demonstrated that high concentrations of oxygen vacancies, which were introduced within the bulk region of the black TiO_2-x microspheres, extended the light absorption range, improved absorptivity across a wide spectrum, and enhanced photo-to-heat conversion by promoting non-radiative recombination. Consequently, the synthesized defective TiO_2-x microspheres exhibited an outstanding solar-driven interfacial evaporation rate of 0.532 kgm⁻² h⁻¹. While floating independently without hydrophilic support, the evaporation performance of the self-floating TiO_2-x microspheres is 1.63-fold faster than that of bulk water evaporation under light irradiation with an intensity of 1 kWm⁻².