Graphene Langmuir-Schaefer films Decorated by Pd Nanoparticles for NO₂ and H₂ Gas Sensors

NO₂ and H₂ gas sensing by few-layer graphene (FLG) were studied in dependence on the annealing and decoration of graphene by palladium nanoparticles (NPs). Graphene was deposited onto SiO₂ (500 nm)/Si substrates by a modified Langmuir-Schaefer technique. A solution of FLG flakes in 1-methyl-2-pyrrolidone was obtained by a mild sonication of the expanded milled graphite. FLG films were characterized by atomic force microscopy, X-ray diffraction, Raman spectroscopy, and the Brunnauer-Emmett-Teller method. Average FLG flake thickness and lateral dimension were 5 nm and 300 nm, respectively. Drop casting of Pd NP (6–7 nm) solution onto FLG film was applied to decorate graphene by Pd. The room temperature (RT) resistance of the samples was stabilized at 15 kΩ by vacuum annealing. Heating cycles of FLG film revealed its semiconducting character. The gas sensing was tested in the mixtures of dry air with H₂ gas (10 to 10 000 ppm) and NO₂ gas (2 to 200 ppm) between RT and 200 °C. The response of 26 % to H₂ was achieved by FLG with Pd decoration at 70 °C and 10 000 ppm of H₂ in the mixture. Pure FLG film did not show any response to H₂. The response of FLG with Pd to 6 ppm of NO₂ at RT was ≥ 23 %. It is 2 times larger than that of the pure FLG sample. Long term stability of sensors was studied.