Fourier transform infrared spectroscopy (FTIR) reflectance spectra were measured in a Bruker VERTEX 70 equipment with incident s-polarized light at an angle of 60° with respect to the sample surface normal. The spectra were acquired in reflectance configuration at incidence angle of 30° with respect to the surface normal, a resolution of 2 cm -1 and s-polarization. The measurements were conducted at room temperature, after a 30 min N2 purge, with 2 cm -1 resolution and averaged over 50 scans. A thin film of gold was used as reference.