Effects of natural aging on the surface compositions and optical properties of thermochromic vanadium oxide films

We experimentally investigate the long-term atmospheric durability of thermochromic vanadium oxide (VO_X) films. The VO_X films were deposited on glass substrates by reactive direct current magnetron sputtering and were then in-situ annealed at the preparation temperature ≤350 °C. The X-ray photoelectron spectroscopy (XPS) was applied to investigate the surface compositions of the natural aged films, while the optical properties of the films were characterized by the spectrophotometer and spectroscopic ellipsometry. The XPS results reveal that the oxygen-deficient VO_X (X < 2.5) components possessing remarkable chemical changes is preferred to react with the O₂ and H₂O in the air and become high oxidation state compounds. However, it is worth to note that the optical properties of the films shows extra-low variation performance (e.g. linear increasing rate of transmittance ~0.00448% per day @ λ = 600 nm). The aging mechanism of the VO_X films in air was interpreted in terms of the interface aging and the diffusion-reaction aging on basis of the above experimental results. This study would provide valuable experience of the natural aging of VO_X films.