The optical properties of the investigated samples were ana- lyzed by UV–vis optical characterization. The inset of Fig. 4 reports the absorbance for ZnO nanofibers annealed at 550 °C. The ab- sorbance (A) was obtained by the transmittance (T) and reflectance (R) measured spectra, in accordance with the following equation: A% 1⁄4 100 T% R%. The nanofibers clearly show an optical ab- sorption in the UV part of the spectrum, as expected due to the energy gap of the material [1]. Optical spectra were deeply ana- lyzed by the Tauc model, which describes the light absorption process in the amorphous semiconductors [17]. For direct transi- tions (that is the case of ZnO [18]) the Tauc law can be written as follow [19]: