Photochromic behavior of Li-stabilized MoO3 sol–gels

Photochromic behavior of Li-doped MoO3 sol–gels prepared by the peroxo sol–gel method was studied in details using various experimental techniques, including UV–vis, XRD, Raman, EPR, and XPS. The lithium doping has drastically enhanced the stability of the MoO3 sol–gels. Upon UV-light irradiation from a high-pressure mercury lamp, the Li-doped MoO3 sol turned from yellowish into deep blue, and the sol in ethanol exhibited much more intense color change than the sol in water. The UV-light exposure has the effect of re-arranging the structural units of MoO3 and building up short-range order inside the solid. The formation of a blue colored bronze in the photo-irradiated sols because of the reduction of Mo6+ to Mo5+ is also evidenced by the experimental data. Based on these data, a mechanism for the photochromic behavior is proposed.