New Plasma-Enabled Synthesis of Designer Nanoporous Metallic Films

Nanoporous metal films (NPMFs) are metallic layers of nanometric thickness containing voids, similar to pores in three-dimensional materials. This structure allows the presence of a high number of low coordinated atoms that determine the physicochemical properties of the films. NPMFs are used in transparent electrodes, plasmonically active materials with very high surface-to-volume ratios, and as active electrode materials for energy conversion and sensing. Generally, NPMFs are obtained by wet chemical dealloying, which involves the selective chemical etching of the less-noble metals from the alloy, which cannot be completely removed for thermodynamic reasons. The remainder of the less noble element (usually dopant levels) is difficult to precisely control and affects the characteristics of the resulting NPMFs. Additionally, the requirement of an alloy restricts the minimum thickness that can be manufactured with the desired porous structure/interconnected solid phase.<br/>Here, we report a novel dry synthesis method based on the plasma treatment of metal nanoparticles formed by physical vapor deposition that produces ultra-thin NPMFs with highly curved and adjustable pore structures, and thicknesses in the order of the diameter of the original nanoparticles. Our approach is general, requires no solution-processing or harsh chemicals, and can be applied to many metals including non-noble ones (e.g. Au, Pt, Ni, and Fe) and their combinations. Nanopore and ligament sizes can be easily tuned by control of plasma conditions (type of gas, pressure, etc.). The resulting NPMFs are impurity-free, remarkably stable, and highly active for electrocatalysis. We demonstrate various NPMFs using different metals and their combinations, including several that have thus far not been shown to form NPMFs. We also present their remarkable performance, including their optical and electrical properties, as well as their exemplary electrocatalytic performance. We expect that these novel nanomaterials, synthesized by the plasma-based dry method we developed, will allow for their incorporation into many new applications and extensively advance the NPM field.