Decoration of multi-walled carbon nanotubes with CuO/Cu₂O nanoparticles for selective sensing of H₂S gas

In this study, multi-walled carbon nanotubes (MWCNTs) were decorated by the Cu₂O/CuO nanoparticles for gas sensing. For this purpose, Cu layer with different thicknesses of 3, 6, and 9 nm was coated on multi-walled carbon nanotubes using sputtering technique for different times of 1, 2, and 3 min at 25°C, respectively, followed by annealing at 500°C to produce isolated Cu₂O/CuO islands. The synthesized products were fully characterized and their expected morphology, chemical composition and phases were confirmed. The gas sensors were fabricated and the optimal sensing temperatures for H₂S sensing was found to be relatively low (150 °C). With control of the size of the Cu₂O/CuO nanoparticles, very high sensor response ((R_g – R_a)/R_a x 100) of about 1244 % to 1 ppm H₂S gas was obtained, with response time and recovery time of 219 and 77 s, respectively. Since sensor response, response time, and recovery time became significantly higher, shorter, and shorter, respectively, by the Cu₂O/CuO functionalization, the associated mechanisms were explained in regard to the selective sensitivity to H₂S gas. In fact, Cu₂O/CuO decorations not only increased the effective surface area for sensing studies, but also they acted as effective catalyst for H₂S gas. By considering the energy bands, among the multi-walled carbon nanotubes-comprising interfaces, the multi-walled carbon nanotubes/CuS heterointerfaces were effective in the enhancing the sensing behavior.