Display omitted •Oxygen-deficient TiO2-x NPs are prepared by one-pot ultrasonic spray pyrolysis.•The one-pot synthesis mechanism for NPs with bulk defects is specifically proposed.•High-concentration bulk defects demonstrate to promote photo-to-heat conversion.•Black TiO2-x shows good an evaporation rate of 2.12 kg m-2h−1 due to bulk defects.•The higher evaporation rate is achieved in black TiO2-x compared to carbon black. While black TiO2-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 TiO2-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 TiO2-x nanoparticles. Moreover, it demonstrated that black TiO2-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 TiO2-x. This work provides valuable insights into the defect engineering of metal oxides for high-performance photothermal materials.