Hydrogen spillover drives room temperature sensing on spark plasma sintered BaTiO3 with Pt electrodes

Abstract

Chemiresistive gas sensing using metal oxide semiconductors has been rationalised in terms of reactions between gas phase species and the surface of the metal oxide either through oxygen vacancy creation/passivation or ionosorbed charged oxygen species; however, no convincing spectroscopic evidence has been observed for the formation of gas sensing charged oxygen surface states. We have investigated the H₂ sensing characteristics of Pt-coated BaTiO₃ prepared by spark plasma sintering using a combination of electrochemical impedance spectroscopy (EIS) and synchrotron-based near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS). We reveal that for undoped BaTiO₃, oxygen vacancies are formed at the surface of platinum-coated BaTiO₃ after reduction in hydrogen at 1000 °C and, after controlled reoxidation in air at 470 °C, are passivated by hydroxyl species detected using NAP-XPS over the temperature range 22–150 °C. EIS was used to monitor the change in electrical properties for both the reoxidation treatment at 470 °C and H₂ gas sensing over the temperature range of 22–175 °C while moving through the Curie temperature of BaTiO₃. NAP-XPS was used to determine the BaTiO₃ surface species present as a function of H₂ pressure and temperature. The results are discussed in terms of oxygen vacancy electron donor creation at the BaTiO₃–Pt interface and grain boundaries, H₂ dissociation on Pt and spillover onto BaTiO₃, driving electronic transport in the conduction band, reaction of dissociated H₂ with surface hydroxyl species, and the presence of temperature- and H₂ concentration-dependent colossal dielectric permittivity and Schottky barrier height. As the H₂ partial pressure increased, the grain boundary Arrhenius activation energies and pre-exponential factors followed the Meyer-Neldel rule, indicating a possible relationship between activation energy and frequency of collisions in charge trapping centres. These observations confirm the presence of surface reactions between dissociated hydrogen, oxygen vacancies, adsorbed H₂O, and hydroxyl species that possibly mediate the H₂ spillover process, which drives the decrease in electrical resistance at the grain boundary regions.