Hydrogen sensing characteristics of Pt/TiO2/MWCNTs composites

Platinum-doped titanium-dioxide/multi-walled carbon nanotubes (Pt/TiO2/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 (0.3e17.0), 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.