On the influence of nano-sized palladium clusters at the surface of SnO 2 thin films on the gas response

Abstract

Dispersions of small catalyst particles on metal oxide sensitive layers are commonly used as catalytic activators in gassensing devices. Platinum and palladium are a well known catalyst, but their capability to increase the selectivity of metal oxide gas sensors is however far from being well understood and is thus still a matter of investigation. One recent trend is the size reduction of the catalyst particles. We present an approach to optimize the specific response to gases by using specially prepared nano-sized palladium clusters on highly dense sputtered polycrystalline SnO₂. For the deposition of the palladium clusters energetic cluster impact technique is used. Using this deposition technique, charged clusters are accelerated by an electric field of several keV and directed onto the substrate. The deposited clusters consist of 3000 palladium atoms in average, forming clusters of approx. 4 nm in diameter. Structural and morphological analyses were performed. Gas measurements were carried out. The Pd-nano-clusters covering the sensitive layer significantly affect the gas sensitivities and the corresponding dynamic response. For example the sensors reaction to CO exposure is significantly different in two points in comparison to a sensor without nano-sized clusters: even at room temperature CO can be detected and the sensitivity is increased at all investigated operation temperatures of the sensor. Nevertheless the CO sensitivity maximum still is around 400 °C, which corresponds with the maximum of O^- adsorption on the surface. Palladium acts as a catalyst for dissociating O²- to O^-.