Electrochromic properties of Nb2O5 sol-gel coatings

Abstract

Sol-gel niobium oxide coatings are promising electrochromic materials. The sols have been prepared by a sonocatalytic mixing of NbCl5 powder, butanol and acetic acid. Thermal analysis (DTA/TG) coupled to mass spectrometry has been performed on Nb2O5 precipitates to quantitatively analyse the effluents. Transparent and defect-free single and multilayers coatings have been deposited on ITO-coated glass by a dip-coating process and then calcined between 400 degrees C and 600 degrees C. The coatings structure change from amorphous to crystalline (TT form) and the later ones are highly textured. The films present a reversible and fast insertion/extraction kinetics for Li+ ions. After insertion the amorphous coatings present a grey-brown color, while the crystalline ones are dark blue. The maximum charge density exchanged with a three-layer 200 nm thick coating sintered at 600 degrees C was 16 mC/cm(2) with a corresponding spectral transmission change practically wavelength-independent varying from 80% to 20%. The coloring efficiency determined at lambda = 600 nm was 22 cm(2)/C.