where f is the volume fraction of the Cu nanoparticles, ε Cu and ε air are the wavelength-dependent permittivity of the Cu and air, respectively. Note that it was air rather than aqueous electrolyte used for calculation because the empirical data were measured ex-situ in the dry state. Then, considering three stratified mediums (inset of the Figure ), the thickness-dependent reflectivity ℛ of the porous Cu film is obtained (eq. 2):(64, 65)