HRTEM and STEM‐EELS study of thin films Pt‐CeOx nanocatalysts for on‐chip fuel cell technology

Abstract

Platinum (Pt) is a versatile element in catalysis that efficiently mediates a multitude of chemical reactions. Unfortunately, Pt is a rare noble metal and its high price exceeding that of gold, limits large‐scale applications. Therefore, not surprisingly, reducing amount of Pt is the major driving force in catalysis research. There are two strategies to tackle this challenge: to replace noble metal by others, less expensive materials; and to use platinum as efficiently as possible. In this study, we handle both of them by growing extremely porous Pt‐CeO 2 structures prepared It was shown that sputtered thin cerium oxide films containing Pt, which had been deposited on the anode side of a fuel cell, exhibited a higher specific power compared to a conventional Pt−Ru catalyst [1]. Besides the large scale fuel cells, there is also an increasing interest in miniature fuel cells fabricated on silicon, which could be used as an on‐chip power supply for portable electronic devices. In this study, nanometric Pt‐ceria thin films were characterized by TEM after elaboration by physical vapor deposition on various substrates (silicon, carbon foils, intermediate carbon films). The deposited layers exhibited different morphologies linked to the different substrates. It is shown that the roughness of the layers is dependent on the deposition conditions, the amount of deposited material and the type of the carbon substrate. The change of these parameters results in growth of flat, mushroom‐like or noodle‐like structures (Fig. 1). By the optimization and the suitable combination of materials we can tune the morphology of the catalysts. In addition to the substrate type, many effects as the formation of carbides or silicates at the interface, an interaction of ceria with platinum and the presence of the porosity influenced also the structure and the chemistry of the deposited layers [2,3]. In all samples, crystallites corresponding mainly to CeO 2 and to a less extent to CeC 2 crystallographic structures were observed (Fig. 1). STEM‐EELS measurements have been carried out on layers grown on silicon with and without intermediate carbon layer. Data analysis of the M 4,5 white lines of cerium have pointed out a variation of cerium oxidation state from Ce 4+ to a mixture of Ce 3+ and Ce 4+ depending of the localization of the measurement (Fig. 2).