Thermally induced evolution of sol–gel grown WO3 films on ITO/glass substrates

• Published in: Applied surface science Vol. 297; pp. 195 - 204
• Main Authors: Caruso, T., Castriota, M., Policicchio, A., Fasanella, A., De Santo, M.P., Ciuchi, F., Desiderio, G., La Rosa, S., Rudolf, P., Agostino, R.G., Cazzanelli, E.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 01-04-2014; Elsevier
• Subjects: Condensed matter: electronic structure, electrical, magnetic, and optical properties; Condensed matter: structure, mechanical and thermal properties; Cross-disciplinary physics: materials science; rheology; Exact sciences and technology; Photoelectron spectroscopy; Physics; Raman spectroscopy; Sodium diffusion; Sol–gel; Thin film annealing; Tungsten trioxide; Thin film annealing; Sodium diffusion; Sol–gel; Raman spectroscopy; Photoelectron spectroscopy; Tungsten trioxide; Raman spectra; Annealing; Inorganic compounds; Sol-gel; Transition element compounds; Glass; Thin films; Tungsten oxide; Sodium; Transition elements; Tungsten; Diffusion; Sol-gel process
• ISSN: 0169-4332; 1873-5584
• DOI: 10.1016/j.apsusc.2014.01.154

•Tungsten trioxide films deposited by sol–gel route on ITO–glass substrates.•Formation of rod-like and platelet shaped nanocrystalline structures.•Mixed WO3 and Na2WO4 phases. Different evolution of bulk and surface structure.
The electronic, morphological and structural properties of WO3 thin films, synthesized via a sol–gel route and deposited on ITO/glass substrates by spin-coating, were analyzed as a function of annealing temperature (100–700°C range) by Scanning Electron Microscopy, Atomic Force Microscopy, micro-Raman spectroscopy, X-ray Diffraction and Photoelectron Spectroscopy. We have found evidence of two competing processes when the film is annealed at high temperatures (600–700°C): a structural phase transition from amorphous to crystalline WO3 and a temperature-activated diffusion of sodium ions, from the substrate into the WO3 film, which induces the formation of sodium tungstate. The surface of the films was found to be oxygen deficient after deposition but reverted to fully oxidized WO3 after high temperature annealing in air. The annealing also induced a restructuring of the films with formation of nano-crystalline aggregates. The influence of film thickness on these processes was also investigated.