There is an increasing demand for metal oxide-based gas sensors, such as TiO₂, ZnO, SnO₂, and BaTiO₃ for flammable and toxic gas detection (H₂ and CO), due to their simplicity and low production costs. During processing of n-type metal oxides in low pO₂ atmospheres, adsorption of oxygen occurs at grain boundaries, which can become charged due to fast electron transfer from the conduction band to the adsorbed oxygen, forming ionosorbed oxygen species (O₂^-, O^-, O₂^-). The H₂ sensing mechanism in metal oxides is thought to occur due to the interaction of H₂ with ionosorbed oxygen species, which causes lowering of the grain boundary Schottky barriers, resulting in increased electronic conductivity. However, this mechanism is still debated. Additionally, the incorporation of a thin surface layer of Pt can facilitate the dissociative chemisorption of H₂ and increase the sensor sensitivity towards H₂.
In our recent work, we have used AC impedance spectroscopy to monitor the conduction properties during oxidation of spark plasma sintered BaTiO₃ (BTO) at 723 K, followed by exposure of the oxidized material to dry H₂ (298 K – 448 K). Under low pH₂, our results demonstrated that the conductivity increased by several orders of magnitude, due to a decrease in the resistance of the BTO grain boundary regions. H₂/D₂ studies confirmed that electrons were the prevailing charge carrier, and that conductivity changes were reversible when switching from H₂ to argon atmospheres. Therefore, an increase in conductivity was attributed to H₂ dissociation on Pt, under mixed H₂/Ar atmospheres, which releases electrons into the conduction band. Conversely, a decrease in conductivity, under an Ar atmosphere, is due to the reassociation of H₂ on Pt, which withdraws electrons from the BTO conduction band. This reassociation process is kinetically slow, implying that protons must hop along grain boundary O^- species to the Pt surface to form H2.
Pt-catalysed H₂ dissociation and spillover onto O^- species was confirmed using ambient pressure photoelectron spectroscopy (AP-XPS). O^- species were formed after controlled oxidation of n-type BTO, and OH groups were formed during exposure to H₂. There was a proportional signal decrease for O^- and an increase in OH groups as the H₂ pressure was increased in the AP-XPS system (1 mbar argon to 4 mbar H₂), demonstrating O^- groups act as adsorption sites for H⁺. Therefore, our results indicate, that for Pt-BTO systems, the increase in conductivity is through electron release due to the dissociation of H₂, rather than direct interaction of H₂ with ionosorbed O^- species. The degree of oxidation and concentration of O^- species controls the initial resistivity under argon, sensor switching amplitude, and tentatively speaking, the rate of reassociation of H₂; hence, the kinetic switching response of the sensor. Therefore, it may be possible to control the H₂ sensing characteristics by changing the material porosity during spark plasma sintering, and degree of post sintering oxidation.