Effect of precursor stoichiometry on the morphology of nanoporous platinum sponges

Nanoscale sponges formed by de-alloying suitable metallic alloys have a wide variety of potential applications due to their enhanced catalytic, optical, and electrochemical properties. In general, these materials have a bi-continuous, vermicular morphology of pores and ligaments with a fibrous appearance; however, other morphologies are sometimes reported. Here, we investigate how stoichiometry and process parameters control the characteristics of sponges formed from thin film precursors of AlxPt. Materials deposited at elevated temperatures and with mole fraction of Al between 0.65 and 0.90 produce the classic isotropic fibrous sponges with a morphology that varies systematically with precursor stoichiometry; however, de-alloying of material deposited at room temperature produced unusual isotropic foamy sponges. The evidence suggests that formation of a conventional fibrous sponge requires an equilibrated precursor whereas foamy morphologies will result if the precursor is metastable. Modeling was used to investigate the range of possible morphologies. As stoichiometry changed in the model system, the average mean and Gaussian curvature of the sponges systematically changed, too. The evolution of these shapes passed through certain special morphologies; for example, modelled structures with 0.80 Al had a zero average Gaussian curvature and might represent a structural optimum for some applications. These observations provide a means to control sponge morphology at the nanoscale.