Synthesis of Black TiO x Nanoparticles by Mg Reduction of TiO 2 Nanocrystals and their Application for Solar Water Evaporation

TiO x ( x < 2) nanoparticles with tunable colors from white to gray to blue–gray to black are synthesized by magnesium (Mg) reduction of white P25 TiO 2 nanocrystals followed by removal of excess Mg with aqueous HCl and distilled water. Increasing amounts of Mg smoothly decrease the oxygen content in TiO x which is responsible for the gradual increase in light absorption and concomitant darkening of its color from white to black with decreasing values of x . The as‐synthesized TiO x nanoparticles are spin‐coated onto the surface of a stainless steel mesh followed by surface superhydrophobization in order to test their performance as a solar water evaporator. Results from the tests show that the black TiO x efficiently generates water vapor with a solar thermal conversion efficiency as high as 50% under solar‐simulated light irradiance at an intensity of 1000 W m –2 (1 Sun). Moreover, TiO x nanoparticles have inherent advantages over other materials used for solar water desalination, such as their tunable light absorption, low‐cost, low‐toxicity, superhydrophobicity, and chemical stability.