Influence of gas atmospheres and ceria on the stability of nanoporous gold studied by environmental electron microscopy and in situ ptychography

Abstract

A novel complementary approach of electron microscopy/environmental TEM and in situ hard X-ray ptychography was used to study the thermally induced coarsening of nanoporous gold under different atmospheres, pressures and after ceria deposition. The temperature applied during ptychographic imaging was determined by IR thermography. While using elevated temperatures (room temperature – 400 °C) and realistic gas atmospheres (1 bar) we achieved for the first time a spatial resolution of about 20 nm during hard X-ray ptychography. The annealing of pure and ceria stabilized nanoporous gold in different atmospheres revealed that the conditions have a tremendous influence on the coarsening. The porous structure of the samples was stable up to approximately 800 °C in vacuum, whereas pronounced changes and coarsening were observed already at approximately 300 °C in oxygen containing atmospheres. A layer of ceria on the nanoporous gold led to an improvement of the stability, but did not alleviate the influence of the gas atmosphere. Different behaviors were observed, such as coarsening and even material loss or migration. The results suggest that additional mechanisms beyond surface diffusion need to be considered and that microscopic studies aimed at more realistic conditions are important to understand the behavior of such materials and catalysts.