Optical and Electrochemical Characteristics of Niobium Oxide Films Prepared by Sol-Gel Process and Magnetron Sputtering: A Comparison

Electrochromic niobia (Nb₂0₅) coatings were prepared by the sot-gel spin-coating and d.c. magnetron sputtering techniques. Parameters were investigated for the process fabrication of sol-gel spin coated Nb₂0₅ films exhibiting high coloration efficiency comparable with that d.c. magnetron sputtered niobia films. X-ray diffraction studies (XRD) showed that the sot-gel deposited and magnetron sputtered films heat treated at temperatures below 450°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 Nb₂0₅. 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 sot-gel deposited films to be lower than for the sputtered films. The n and k values were n = 1.82 and k = 3 × 10⁻³, and n = 2.28 and k = 4 × 10⁻³ at 530 urn for sot-gel deposited and sputtered films, respectively. The electrochemical behavior and structural changes were investigated in 1 M LiC10₄/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⁻ ↔ Li_xNb₂0₅. Cyclic voltametric (CV) measurements showed that both Nb₂0₅ 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 < λ < 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 electrochemically equivalent to those prepared by d.c. magnetron sputter deposition.