Surface Area and Microstructure of Metallic Nanowire Foams*

The COVID-19 pandemic has shown the urgent need for the development of efficient, durable, reusable and recyclable face masks for the deep submicron size range. Nanowire-based low-density metal foams, synthesized by electrodeposition and sintering, have recently been shown to exhibit extremely large surface areas as well as outstanding filtration efficiencies (>96.6%) in the PM_0.3 regime with breathability comparable to N-95 respirators [1]. Here we report the analysis of the microstructure of the foams, detailing how the growth parameters influence the overall surface area and characteristic feature size, as well as the influence of the microstructure on filtration efficiency. The foams have an overall low density, up to 30% of the bulk density. We have found that nanogranules deposited on the nanowires during electrodeposition greatly increase the surface area until the foam reaches about 13% bulk density, at which point the nanogranules begin to coalesce and surface area begins to decrease. Surprisingly, the overall surface area gained from the nanogranules has little correlation with an improvement in capture efficiency in the lowest density foams, and that the two parameters with the highest correlation are the nanowire density and diameter. These results demonstrate promising directions to optimize the foam microstructure for highly efficient submicron particulate filtration.

[1]. J. Malloy, A. Quintana, C. J. Jensen, and Kai Liu, Nano Letters, 21, 2968-2974 (2021).