Optical and electrochemical characteristics of niobium oxide films prepared by sol-gel process and magnetron sputtering - A comparison

Electrochromic niobia (Nb2O5) coatings were prepared by the sol-gel-spin-coating and d.c. magnetron sputtering techniques. Parameters were investigated fbr the process fabrication of sol-gel spin coated Nb2O5 films exhibiting high coloration efficiency comparable with that d.c. magnetron sputtered niobia films. X-ray diffraction studies (XRD) showed that the sol-gel deposited and magnetron sputtered films heat treated at temperatures below:450 degrees C were amorphous, whereas those heat treated at higher temperatures were slightly crystalline. X-ray photoelectron spectroscopy (XPS) studies showed that the stoichiometry of the films was Nb2O5. The refractive index and electrochromic coloration were found to depend on the preparation technique. Both films showed low absorption and high transparency in the visible range. We found that the n, k values of the sol-gel deposited films to be lower than for the sputtered films. The n and k values were n=1.82 and k=3x10(-3), and n=2.28 and k=4x10(-3) at 530 nm for sol-gel deposited and sputtered films, respectively. The electrochemical behavior and structural changes were investigated in 1 M LiClO4/propylene carbonate solution. Using the electrochemical measurements and X-ray photoelectron spectroscopy, the probable electrode reaction with the lithiation and delithiation is Nb2O5+x Li++x e(-) <-> LixNb2O5. Cyclic voltametric (CV) measurements showed that both Nb2O5 films exhibits electrochemical reversibility beyond. 1200 cycles without change in performance. ''In situ'' optical measurement revealed that those films exhibit an electrochromic effect in the spectral range 300 < lambda < 2100 nm but remain unchanged in the infrared spectral range. The change in visible transmittance was 40% for 250 nm thick electrodes. Spectroelectrochemical measurements showed that spin coated films were essentially electrochromically equivalent to those prepared by d.c. magnetron sputter deposition.