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, T_a. 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 α–MoO₃ phase with small indications of the monoclinic β–MoO₃ phase, whereas the treatment in argon led to a mixture of β–MoO₃ and the Magneli phase Mo₉O₂₆. The dependence of the Mo⁵⁺ oxidation state on T_a, 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 3d_3/2 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 T_a for both types of annealing atmosphere.