Reversible photochromic W18O49: Mechanism revealing and performance improvement for smart windows

W 18 O 49 has widely used in photothermal application and photocatalyst due to its strong Localized Surface Plasmon Resonance (LSPR) effect and unique oxygen-deficient structure. In addition, the unstable structure and easily changed valence state of W 5+ make W 18 O 49 potentially photochromic. However, the generally synthesized blue W 18 O 49 particles was in a saturated colored state, delivering a great challenge in exploring its photochromic features, which thus have seldom been studied. Herein, we found that the bleached W 18 O 49 obtained by mild-heating the solvothermal-synthesized blue W 18 O 49 nanoparticles in air had considerable reversible photochromic feature, which can be colored in 1 min under UV irradiation. The photochromic mechanism of W 18 O 49 was revealed through the XRD, XPS and FTIR results. The chromic process was mainly attributed to the changed W 5+ content under different conditions. The photochromic performance was systematically investigated on the W 18 O 49 -based film and which can be significantly improved by Ti doping. Finally, the great potential of the films in the energy-saving smart windows application has verified, and a 10 at.% Ti-doped W 18 O 49 film delivered outstanding optical performance with luminous transmittance ( T lum ) of 87.53% and solar energy modulation efficiency (Δ T sol ) of 30.13%.