Hydrogen sensing characteristics of Pt/TiO2/MWCNTscomposites

Platinum-doped titanium-dioxide/multi-walled carbon nanotubes (Pt/TiO 2/MWCNTs) composites were prepared by a sol-gel method and characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermo-gravimetry (TG) and micro-Raman spectroscopy (MRS). Regardless the nominal C/Ti molar ratio (0.3-17.0), only the anatase phase of TiO2 was detected. However, on the composite with the highest C/Ti molar ratio, the formation of a more structurally disordered and non-stoichiometric anatase phase seemed to be favored. Small Pt nanoparticles, whose size ranged from 1 to 10 nm, were observed dispersed on the surface of composite samples. Electrical characteristics and hydrogen sensing properties of Pt/TiO2/MWCNTs composite films deposited on inter-digitated ceramic substrates were analyzed in the temperature interval from room temperature (RT) to 100 °C. The electrical conductivity of the composite films was several orders of magnitude higher than that of pure titania, allowing electrical measurements at RT. Pt/TiO2/MWCNTs composite films showed a response to hydrogen concentration, up to 100%, in nitrogen even at RT. On the basis of the results obtained, a "spill-over" mechanism, in which hydrogen molecules are first chemisorbed and dissociated on platinum, and finally spill out of the Pt surface, diffusing into the TiO2 surface layer, with MWCNTs providing a preferential pathway to the current flow, can be proposed to explain the hydrogen sensing mechanism on these sensors.