Influence of thermal modification on the gasochromic properties of WO ₃ thin films fabricated by electron beam evaporation

This paper describes the effect of post-deposition annealing on the structural and gasochromic properties of <i>W</i><i>O</i>₃ thin films deposited by electron beam evaporation and additionally decorated with a Pd catalyst layer of varying thickness. The <i>W</i><i>O</i>₃ layers were annealed at 800°C, which led to a phase transition from an amorphous to a monoclinic crystal structure, accompanied by an increase in surface roughness from 1.3 to 66 nm and the formation of a discontinuous island-like morphology with grain sizes up to 3 µm. The structural changes had a significant effect on the optical response of the layers to hydrogen. For the annealed samples, the absolute change in light transmission (<i>Δ</i><i>T</i>) reached 20.2-20.9% for the 1.5 nm thick Pd catalyst and 7.7-9.2% for the 5 nm thick Pd catalyst at a wavelength of 850 nm. The corresponding optical response was 184-186% for the thin Pd layer and 353-396% for the thick Pd layer, depending on the hydrogen concentration (25-1000 ppm). The response time was reduced from ∼10min at 25 ppm to <4min at 1000 ppm, while the recovery time to the original state in air remained below 66 s under all conditions. XPS studies confirmed the reduction of <i>W</i>⁶⁺ to <i>W</i>⁵⁺ under the influence of hydrogen and reversible transition <i>P</i><i>d</i><i>O</i>→<i>P</i><i>d</i>, which correlates with the observed optical changes. The results show that annealing increases crystallinity and modifies porosity, which, in combination with the Pd catalyst directly affects the kinetics and magnitude of the gasochromic response.