STUDY OF THE PHOTOCHROMIC PROPERTIES OF AMORPHOUS MOO3 FILMS USING RAMAN MICROSCOPY

Raman microprobe spectroscopy was applied to the study of the photochromic properties of thin amorphous MoO3 (a-MoO3) films. The films became blue during UV irradiation, a process that involves a partial reduction of Mo-VI, and were bleached by electrochemical oxidation. Such coloration-bleaching cycles can be repeated many times, Raman peaks assigned to the three Mo-O stretching modes (singly, doubly, and triply coordinated oxygen) were observed to shift reversibly during the coloration-bleaching cycles. Raman peak shifts occurring during the coloration process were consistent with predictions of Mo-O bond strengths based on bond lengths in crystalline MoO3 and hydrogenated MoO3 presented in the literature. Disorder increased in the a-MoO3 structure as a result of the UV-induced coloration, as evidenced by increases in the Raman peak width, and decreased in the bleaching process. These changes were also reversible. We conclude that the local structure in the colored state in a-MoO3 is similar to that in the crystalline hydrogen bronze HxMoO3, where x less than or equal to 0.36.