Alterations in the complex refractive index of copper oxide thin films as sensing effect for hydrogen sulfide monitoring

Throughout the last three decades cuprous (Cu₂O) and cupric oxide (CuO) have been subject of extensive investigations of their material properties. This research was mainly driven by potential applicability as a photovoltaic or doping material. However, CuO/Cu₂O layers show a specific reaction towards hydrogen sulfide (H₂S), making it a good candidate as highly selective gas sensor material. On this account thin film samples of CuO and Cu₂O have been investigated with regard to their specific surface interactions with H₂S gas. Changes in morphology, chemical composition, and alterations in the complex refractive index have been thoroughly examined in order to understand possible sensing effects. Raman spectroscopy was used for verifying the films composition after heat treatment. Transmission and reflection characteristics in the extended UV/Vis regime (350–1,100 nm) of initially prepared samples and after exposure to well-defined doses of H₂S were recorded. A distinct increase in transmissivity was observed for Cu₂O films in the wavelength region λ = 550–900 nm. An initial conditioning effect was observed from consecutive measurements. Absorptivity characteristics and optical band gaps were derived, showing an absorptivity shift of CuO thin films after exposure towards H₂S. A specific optical read-out based on total internal reflection was set-up, offering a transient monitoring of the materials surface interactions with the gas phase. Changes in the response, in terms of intensity variations, were reproducibly shown for low concentrations of 5 ppm of H₂S.