PhD: Chemical deposition and characterization of inorganic electrochromic thin films

Sani Demiri
Institute of Chemistry, Faculty of Natural Sciences and Mathematics, Ss. Cyril and Methodius University, Skopje, Macedonia
June, 2012
 

Abstract

Five new chemical methods for deposition of electrochromic thin films of inorganic materials are designed. The developed methods are simple and do not require the use of special equipment and technique. The films are prepared on electroconductive FTO glass substrates and analyzed by different methods. The film composition and structure are determined by X-ray powder diffraction, infrared and Raman spectroscopy and differential thermal analysis. The surface morphology of the thin films is studied by SEM and AFM. The redox processes in different aqueous and non-aqueous electrolytes are examined by cyclic voltammetry. The electro-chromic properties are characterized by VIS spectrometry. The prepared films comprise different chemical compositions: Prussian blue, sodium vanadium oxide bronzes, birnessite-type potassium manganese oxides, manganese(IV) oxides obtained by transformation of manganese(II) carbonate and hydrated tungsten(VI) oxide.
The Prussian blue thin films are easily prepared by successive immersion of the substrates into an acidic aqueous solution of Fe2(SO4)3 and K4[Fe(CN)6]. These thin films exhibit stability, excellent reversibility and good electrochromic characteristics, which make these films favorable for electrochromic devices.
The electrochromic thin films of sodium vanadium bronzes have been prepared by mixing of aqueous solutions of sodium metavanadate and acetic acid. The thin films exhibit two-step electrochromism (color change from yellow-orange to green, and then to blue) related with the redox reactions between V5+ and V4+ ions acompanied with intercalation/deintercalation of alkaline ions from electrolyte.
The chemical deposition of potassium manganese oxide films has been performed at room temperature by a reaction of aqueous solutions of potassium permanganate and manganese(II) chloride. The manifested electrochromic behaviour of the films with color change between brown and pale-yellow is due to the electrochemical transformations between Mn4+ and Mn3+ ions. The aqueous KNO3 is found to be the best electrolyte regarding the electrochromic characteristics of the thin films.
The manganese(II) carbonate thin films have been deposited precipitation using urea from aqueous solution of Mn2+ ions. The as-deposited MnCO3 films are trans-formed into electrochromically active manganese(IV) oxide films by thermal treatment and electrochemical cycling.
The chemical bath deposition from solutions containing sodium tungstate and hydrochloric acid produces thin films of hydrated tungsten(VI) oxide. The elec-trochromic properties are significantly improved by use of sulfuric acid as electrolyte.
The achieved results for the studied inorganic thin films in respect to the difference in the transmittance between the bleached and colored states are promising for their application in different electrochromic devices.