Enhancing the NIR shielding ability and controlling the surface wettability of RF magnetron sputter deposited WO3-x thin films by tuning the oxygen vacancies

Enhancement in the near-infrared (NIR) shielding property of tungsten oxide (WO3-x, 0 < x < 1) thin films by the induction of further oxygen vacancies via vacuum annealing is discussed in this work. The property is comprehensively analysed by UV–Vis–NIR spectroscopy and variable angle spectroscopic ellipsometry techniques. From transmittance spectra, an average visible transmittance of ∼48% along with an average NIR transmittance of ∼15% (with Tvis/Tsol ratio of 1.49) is obtained for WO3-x thin films vacuum annealed at 500 °C for 1.5 h. The pronounced NIR absorption is attributed to the enhanced polaron absorption and surface plasmon resonance effects produced by the increased oxygen vacancies in the film. In order to realize their potential as energy saving window coatings, thermal shielding test is performed in which vacuum annealed WO3-x thin films could restraint the indoor temperature of the heat box efficiently. X-ray diffractometry, field emission scanning electron microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy measurements were also performed on the films for elucidating structural, morphological, and compositional characteristics. In addition, contact angle measurements revealed that vacuum annealing could reduce the surface wettability of WO3-x thin films considerably. The present work is significant in the field of development of NIR shielding materials for energy conserving smart window applications, especially for buildings and automobiles.