On the hydrogen sensing mechanism of Pt/TiO2/CNTs based devices

Titanium-dioxide/carbon-nanotubes (TiO2/CNTs) composites, prepared by sol–gel, were loaded with 2 wt% of platinum by wet impregnation, annealed in slightly reducing atmosphere (5% H2 in Ar) and utilized to fabricate resistive hydrogen sensors, able to operate, in helium, at near room temperature in presence of high gas concentrations (up to 100%). A systematic investigation of the nano-hybrids was conducted by the aid of several techniques in order to shed light on the sensing mechanism, clarify the role played by each component of the Pt/TiO2/CNTs active material and explain the reason for the change of the sensor responsiveness with the variation of CNT load (3.4–59.7 wt%). For this purpose, morphology of the nanocomposites and phase, crystalline arrangement and stoichiometry of the oxide were investigated. Results obtained allow sketching a qualitative band diagram of the nanocomposites and attributing the sensor working mechanism to the quenching of the CNT conductance following the oxide mediated electron transfer from the metal. The differences in stoichiometry, agglomeration degree of the nano-sized TiO2 phase and surface contact with CNTs introduced by the variation of the CNT load account for the changes in responsiveness of the sensors fabricated with the Pt/TiO2/CNTs composite investigated.