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

We prepared PdO nanoflake thin films on the SiO2 substrate by reactive sputter deposition, and studied their sensing response to H-2 at temperatures between 25 and 250 degrees C. In addition to the oxygen ionosorption model, which is used to describe the early H-2 sensing response over the temperature range studied, the H-2 sensing kinetics of the PdO thin films can be separated into three temperature regimes: temperatures below 100 degrees C, around 150 degrees C and above 200 degrees C. At temperatures below 100 degrees C, PdO reduction is the dominant reaction affecting the H-2 sensing behavior. At temperatures around 150 degrees C, Pd reoxidation kinetically competes with PdO reduction leading to a complicated sensing characteristic. Active PdO reduction by H-2 promotes the continuing growth of Pd nanoislands, facilitating dissociative oxygen adsorption and thus the subsequent Pd reoxidation in the H-2-dry air gas mixture. The kinetic competition between the PdO reduction and reoxidation at 150 degrees C leads to the observation of an inverse of the increase in the sensor conductivity. At temperatures above 200 degrees C, the PdO sensor exhibits a sensor signal monotonically increasing with the H-2 concentration, and the H-2 sensing behavior is consistent with the Mars-van-Krevelen redox mechanism.