High-performance interfacial water evaporation of black TiO_2-x with high-concentration bulk oxygen vacancies

While black TiO_2-x nanoparticles (NPs) have emerged as promising photothermal materials due to exceptional light absorption properties and intrinsic stability for reliable interfacial water evaporation (IWE) system, their lower evaporation rate (0.8–1.53 kg m^-2h⁻¹) than carbon materials remains a significant challenge. This poor evaporation rate is attributed to the low photo-to-heat conversion property of black TiO_2-x nanoparticles with a microstructure containing oxygen vacancies exclusively in the surface region—a consequence of the common reduction-based synthesis process. Herein, we achieved an outstanding evaporation rate of 2.12 kg m^-2h⁻¹, significantly higher than previous results under 1sun illumination due to the high-concentration bulk oxygen vacancies that promote the photo-to-heat conversion of black TiO_2-x nanoparticles. Moreover, it demonstrated that black TiO_2-x NPs exhibited a higher evaporation rate than carbon black, a typical carbonaceous photothermal material, under identical conditions. Furthermore, an innovative synthesis method utilizing the ultrasonic spray pyrolysis (USP) process and synthesis mechanism is proposed to introduce high-concentration bulk oxygen vacancies. This method overcomes the unavoidable drawbacks of conventional reduction-based synthesis methods, including surface-concentrated oxygen vacancies of the black TiO_2-x. This work provides valuable insights into the defect engineering of metal oxides for high-performance photothermal materials.