A mechanistic study of hydrogen gas sensing by PdO nanoflake thin films at temperatures below 250 °C

Abstract

We prepared PdO nanoflake thin films on the SiO₂ substrate by reactive sputter deposition, and studied their sensing response to H₂ at temperatures between 25 and 250 °C. In addition to the oxygen ionosorption model, which is used to describe the early H₂ sensing response over the temperature range studied, the H₂ sensing kinetics of the PdO thin films can be separated into three temperature regimes: temperatures below 100 °C, around 150 °C and above 200 °C. At temperatures below 100 °C, PdO reduction is the dominant reaction affecting the H₂ sensing behavior. At temperatures around 150 °C, Pd reoxidation kinetically competes with PdO reduction leading to a complicated sensing characteristic. Active PdO reduction by H₂ promotes the continuing growth of Pd nanoislands, facilitating dissociative oxygen adsorption and thus the subsequent Pd reoxidation in the H₂–dry air gas mixture. The kinetic competition between the PdO reduction and reoxidation at 150 °C leads to the observation of an inverse of the increase in the sensor conductivity. At temperatures above 200 °C, the PdO sensor exhibits a sensor signal monotonically increasing with the H₂ concentration, and the H₂ sensing behavior is consistent with the Mars–van-Krevelen redox mechanism.