Effect of UV light illumination in humid air on the optical and electronic properties of the orthorhombic α-MoO3 and monoclinic β-MoO3

Abstract

We present reflectivity measurements on monoclinic [Formula: see text], orthorhombic [Formula: see text]-[Formula: see text], and monoclinic [Formula: see text]-[Formula: see text] in a wide frequency range of 190–2500 nm. The extracted optical conductivity [[Formula: see text]] showed that [Formula: see text] has a metallic character while [Formula: see text]-[Formula: see text] and [Formula: see text]-[Formula: see text] have an insulating behavior. In addition, the photochromic properties of both [Formula: see text]-[Formula: see text] and [Formula: see text]-[Formula: see text] have been studied. The [Formula: see text] spectra for both samples showed a different spectral weight of the optical transition due to the formation of color center bands, which formed as a result of UV exposure. The spectral weight of optical transition from the bulk sixfold cations [Formula: see text] to [Formula: see text] cations is higher in case of the illuminated [Formula: see text]-[Formula: see text] sample than the illuminated [Formula: see text]-[Formula: see text] sample. The XRD results showed that both [Formula: see text]-[Formula: see text] and [Formula: see text]-[Formula: see text] were transformed to monoclinic molybdenum oxide dihydrate ([Formula: see text]) after exposure to UV irradiation in humid air. The [Formula: see text] spectra revealed that photoinjection of hydrogen into the [Formula: see text]-[Formula: see text] film is higher than in the case of the [Formula: see text]-[Formula: see text] film. In addition, the time domain fluorescence lifetime imaging microscopy data showed that the lifetime due to the optical transition from surface fourfold cations [Formula: see text] to [Formula: see text] cations in the case of illuminated [Formula: see text]-[Formula: see text] is higher than that for the illuminated [Formula: see text]-[Formula: see text] for the same optical transition. Meaning that, in the case of illuminated [Formula: see text]-[Formula: see text], the surface [Formula: see text] cations disperse and penetrate into the bulk, lowering the spectral weight of the [[Formula: see text] [Formula: see text]] dimers and enhancing the spectral weight of the bulk centers.