Selective photochromism in a self-coated WO3/WO3−x homojunction: enhanced solar modulation efficiency, high luminous transmittance and fast self-bleaching rate

Pursuing excellent photochromic (PC) properties is still a longstanding challenge for energy-efficient smart coatings. Herein, we have prepared a novel series of self-coated crystalline WO3-amorphous WO3−x homojunctions by a one-step hydrothermal process, in which oxalic acid was used as a capping agent to prevent polycondensation and the crystallization of [WO6]. The as-prepared nanopowders with tight interfaces have a large specific surface area and rich surface electrons, which leads to a dramatic PC property reaching up to ΔTsol = 64.74% at the near-infrared (NIR) range of 1000–2600 nm with high luminous transmittance (Tlum = 94.6% in the virgin state and Tlum = 32.47% in the colored state). Meanwhile, the as-prepared crystalline WO3-amorphous WO3−x homojunction reveals fast reversible PC circulation; most particularly, the self-bleaching rate increases at least three times—the self-bleaching time is less than 8 h. Moreover, as the microstructure of the homojunction is tuned, the solar modulation range can be selective and tuned, so that the solar modulation efficiency is up to the best energy-saving state by controlling the main absorption according to the solar energy distribution.