In situRaman spectroscopy of H2 interaction with WO3 films

Abstract

It is well known that WO₃ interacts efficiently with H₂ gas in the presence of noble metals (such as Pd, Pt and Au) at elevated temperatures, changing its optical behaviors; and that its crystallinity plays an important role in these interactions. For the first time, we investigated the in situRaman spectra changes of WO₃ films of different crystal phases, while incorporating Pd catalysts, at elevated temperatures in the presence of H₂. The Pd/WO₃ films were prepared using RF sputtering and subsequently annealed at 300, 400 and 500 °C in air in order to alter the dominant crystal phase. The films were then characterized using SEM, XRD, XPS, and both UV-VIS and Raman spectroscopy. In order to fundamentally study the process, the measurements were conducted when films were interacting with 1% H₂ in synthetic air at elevated sample temperatures (20, 60, 100 and 140 °C). We suggest that the changes of Raman spectra under such conditions to be mainly a function of the crystal phase, transforming from monoclinic to a mix phase of monoclinic and orthorhombic achieved via increasing the annealing temperature. The as-deposited sample consistently shows similar Raman spectra responses at different operating conditions upon H₂ exposure. However, increasing the annealing temperature to 500 °C tunes the optimum H₂ response operating temperature to 60 °C.