In some examples, a method of making a filter includes heating a metal substrate to precipitate a first phase on a surface of the metal substrate from a metal alloy, the metal substrate defining a plurality of apertures configured to allow a gas to pass through the apertures. The metal substrate is the metal alloy including a first metal and a second metal. The method further includes growing a plurality of carbon nanotubes (CNTs) on the surface of the first metal of the first phase, and the CNTs are configured to capture at least one particle.