A D V A N C E D M A T E R I A L S & P R O C E S S E S | S E P T E M B E R 2 0 2 1 2 8 NANOPOROUS METAL FOAM: EFFICIENT AND REUSABLE SUBMICRON FILTRATION MEDIA A new type of material based on nanoporous copper foams using metallic nanowires as building blocks has promising applications in filtration media, smart masks, and air cleaners. James Malloy* and Kai Liu,* Department of Physics, Georgetown University, Washington, and Department of Physics, University of California, Davis Alberto Quintana and Christopher J. Jensen, Department of Physics, Georgetown University, Washington MATERIALS SCIENCE AND CORONAVIRUS SERIES T he World Health Organization (WHO) has identified air pollution as the worst global environmen- tal killer, causing millions of premature deaths annually [1] . The vast majority of pollutant particulates, including those in smog, are below 0.3 μm. These par- ticulates pose the most health risks as they can penetrate deep into the respi- ratory system. Airborne droplets and aerosols also play a key role in spread- ing COVID-19. Those at 1 μm size and below are particularly worrisome be- cause they can suspend in air for long periods of time and travel long distanc- es. There is an urgent need to develop filtration materials that provide crit- ical defense against deep submicron particulates. Airborne particulates in the deep submicron size range are the hard- est to filter, where conventional filtra- tion mechanisms are least effective. *Member of ASM International Current filters have various limitations, e.g., N95 masks rely on electrostatics, whichmakes themhard to clean and re- use. Their efficiency decreases to below 60% when the electrostatic charges are lost [2] . The sheer volume of waste that face masks generate globally amid the COVID-19 pandemic also poses signifi- cant environmental challenges, as poly- mer-based materials take hundreds of years to decompose [3] . SYNTHESIS To address this challenge, a new type of filtration material based on nanoporous metal foams using metal- lic nanowires (NWs) as building blocks has been developed [4] . Copper foams were fabricated using electrodeposit- ed NWs following a cross-linking and freeze-drying technique [5] . Anodized aluminum oxide (AAO) templates with 200 nm pore size and 60 μm thickness were used for the electrochemical deposition of NWs [6-8] . Subsequently, NWs were liberated from the AAO mem- brane into deionized water. The NW-wa- ter suspension was then freeze-cast by liquid nitrogen into the desired shape and pumped in vacuum to sublimate the ice. The resultant free-standing foam was further strengthened by sin- tering at 300°C, while simultaneously undergoing multiple oxidation/reduc- tion cycles [5] . For this study, metallic foams with a density of 1% of the Cu bulk density were achieved after this first synthesis step of electrodeposi- tion and sintering. They were further strengthened with a second electro- deposition step (referred to as 2ED-Cu hereafter) that introduced addition- al deposited materials. The final foam density was tuned between 2% - 30% of Cu bulk density. As illustrated by scan- ning electron microscopy (SEM) images Fig. 1 — SEM images of (a) 5%, (b) 15%, and (c) 30% density 2ED-Cu foams, respectively, adapted with permission fromMalloy et al. [4] .