The evolution of the Mo5+ oxidation state in the thermochromic effect of MoO3 thin films deposited by rf magnetron sputtering

Molybdenum trioxide (MoO3) thin films were deposited on glass substrates by rf reactive magnetron sputtering using a metallic molybdenum target. The thermochromic effect was induced by exposing the samples to either argon or air at specific temperatures within the range 25?300 C. Optical absorption spectra indicated that the samples annealed in argon show a higher thermochromic response than the samples annealed in air. Moreover, the average luminous transmittance T?lum was investigated as a function of the annealing temperature, Ta. The highest change of T?lum was found in samples treated in argon, where T?lum shifted from 71.69% at room temperature (RT) to 44.58% at 300 C. For samples treated in air, T?lum decreased from 71.69% at RT down to 67.47% at 200 C, but increased up to 79.08% at 300 C. In addition, the annealing in argon at 300 C revealed the formation of two coloration bands around 550 nm and 900 nm, which are associated with an oxygen vacancy defect state (Mo5+O5) and an intervalence charge transfer transition of a hexa-coordinated defect state (Mo5+O6), respectively. Raman spectra, HR-TEM images, and X-ray diffraction measurements showed that the samples treated in air displayed mainly the orthorhombic ??MoO3 phase with small indications of the monoclinic ??MoO3 phase, whereas the treatment in argon led to a mixture of ??MoO3 and the Magneli phase Mo9O26. The dependence of the Mo5+ oxidation state on Ta, was analyzed by X-ray photoemission spectroscopy (XPS) using deconvolution methods. For the annealing in air, it was found that the Mo5+ signal of the Mo 3d3/2 peak increased slightly from 25 to 200 C but exhibited a steep decrease at 300 C; on the contrary, this signal showed a slight increase in the whole range for the anoxic atmosphere. These results show that the Mo5+ oxidation state and the optical density display a similar dependence on Ta for both types of annealing atmosphere.