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CO Gas Sensing by PdO Nanoflake Thin Film

In this work, we deposited PdO nanoflake thin films on the SiO2/Si substrates by reactive sputter deposition, and studied CO gas sensing properties of the PdO thin film. The thin film has the surface morphology characterized by interconnected flake-like nanostructures, which have a flake height of 130-150 nm and a thickness of 15-20 nm (Fig. 1). Because of the nanoflake morphology, the PdO thin film has a very large surface area providing enormous adsorption sites for CO molecules. The very thin flake thickness allows the depletion region induced by the charge transfer between CO adsorbates and the PdO surface to occupy a large part of the flake volume, and the electrical properties of the thin film are therefore dramatically changed. As a result, the PdO thin film must have a high sensitivity toward CO gas sensing. The electrical measurement was conducted in a vacuum system that has the base pressure of 5x10 torr. When CO gas is introduced to the film surface at room temperatures, the electrical resistance of the PdO thin film increases rapidly. This observation indicates that negative charges transfer from CO adsorbates to the PdO nanoflake, which is a p-type semiconductor, thereby inducing the reduction in the majority carrier density. The PdO thin film has a good CO sensing performance at 200C, including fast response time (<10 sec), high sensitivity and good reproducibility (Fig. 2). The responsivity of the PdO thin film to CO increases with the CO partial pressure, and increases with the film temperature up to ~200C, from where it decreases with increasing the film temperature. According to x-ray photoelectron spectroscopy and thermal desorption mass spectroscopy analyses, the PdO surface is reduced by CO at temperatures higher than 150C under the sensing condition. This observation seems to indicate that CO adsorbates on the thin metallic Pd surface layer can still influence the electrical property of the underlying PdO nanoflakes. Study on sensing selectivity of the PdO thin film is being undertaken, and more results will be presented in the meeting.