Coloration of molybdenum oxide thin films synthesized by spray pyrolysis technique

Photochromiccoatings of molybdenum oxide films were deposited on glass substrates by spray pyrolysis technique. The samples were prepared at two substrate temperatures of 250 °C and 400 °C using spray time varying between 1 and 5 min while the other spray operating parameters are fixed at their optimum values. The crystalline nature and crystallite size of the films were investigated by grazing angle X-ray diffraction. The surface topography was characterized by atomic force microscopy roughness data and particle size histograms. The X-ray diffraction patterns prove that the films deposited at substrate temperature of 250 °C and different spray time are amorphous while the films deposited at 400 °C and different spray time are crystalline with only α-phase. The particle size distribution for the prepared samples are found to be in the range from 70 to 100 nm. The particle size distribution tends to be narrow as the spray time increases which indicates the improvement in homogeneity of the samples. The optical absorption spectra of selected colorless samples before and after UV light irradiation was recorded. Two distinct absorption bands were observed: a sharp one related to the energy gap and a broader one at lower energy of 1.03 eV to which the blue color of the sample after UV irradiation is related. This latter band is fitted with three sub-bands E1, E2 and E4 using a Lorentzian model. The intensity and position of these sub-bands depended on UV light exposure time. The sub-bands E1 and E4 are related to the bulk and surface paramagnetic color centers while the E2 band is related to pairing diamagnetic color centers.