Hydrogen sensing properties of protective-layer-coated single-walled carbon nanotubes with palladium nanoparticle decoration

Protective-layer-coated single-walled carbon nanotubes (SWNTs) with palladium nanoparticle decoration (Pd-SiO2-SWNTs) were fabricated and their sensing properties for hydrogen (H-2) were investigated. SWNTs were coated with a 3-4 nm thick SiO2 layer by pulsed laser deposition and subsequently decorated with Pd nanoparticles by electron beam evaporation. Even though the SWNTs were completely surrounded by a protective layer, Pd-SiO2-SWNTs responded to H-2 down to a concentration of 1 part per million. Compared with the Pd nanoparticle-decorated SWNTs without a protective layer (Pd-SWNTs), Pd-SiO2-SWNTs exhibited highly stable sensor responses with variations of less than 20%; Pd-SWNTs showed a variation of 80%. The density of the Pd-SWNTs significantly decreased after the sensing test, while that of the Pd-SiO2-SWNTs with the netlike structure remained unchanged. The hydrogen sensing mechanism of the Pd-SiO2-SWNTs was attributed to the chemical gating effect on the SWNTs due to dipole layer formation by hydrogen atoms trapped at the Pd-SiO2 interface. Moreover, the relationship between H-2 concentration and sensor response can be described by the Langmuir isotherm for dissociative adsorption.