Preparation and Properties of Electrochemically Deposited Tungsten Oxide Films

Abstract

Tungsten oxide films of thicknesses 1095 nm have been cathodically deposited onto ITO coated glass substrates kept at room temperature, at a rate of 13 nm/min. The electrolyte solution has been prepared by adding 0.025 M tungsten (dissolved in 30% H2O2) to 0.25 M H2SO4. XRD observations showed that all films are amorphous. The film resistivity at room temperature was found to be 9 × 106 Ω cm, and from the resistivity–temperature dependence, the thermal activation energy was obtained as 0.29 eV. The refractive index, n, and the extinction coefficient, k, have been computed from the corrected transmittance and reflectance over the spectral range 400 to 800 nm. Absorption versus photon energy curves give an indirect transition with optical energy gap of 3.25 and 3.36 eV for the as‐deposited and coloured states, respectively. The electrochromic properties of the prepared films have been investigated in situ and both solar transmittance and optical density have been evaluated in the coloured and bleached states. The electrochemical coloration and bleaching were performed in an electrolyte of 0.4 M H2SO4 solution. The coloration efficiency has been evaluated as 50 cm2/C at λ = 600 nm, which is comparable with the results obtained from films prepared by other methods.

Microstructural and electrochromic properties of tungsten oxide thin films produced by surfactant mediated electrodeposition

“…The poor electrochromic performance of the annealed films (as compared to the as-deposited film) is also a consequence of severe cracking and densification. Contrary to a previous report by Hutchins et al [22], wherein surface cracks on electrodeposited WO 3 films had little or no effect on the electrochromic response of the film, in the present study, cracks in the films have an adverse effect on the film properties. Perhaps the considerable weakening of interactive forces (controlled by the of water/solvent molecules and these are no longer prevalent at 500 8C) operative between the substrate and the metal oxide deposit which is responsible for its tearing off from the substrate and this is accelerated in a liquid electrolyte.…”

“…3c and d show the gradual formation of grains with distinct grain boundaries and the progressive loss of porosity. The cracks in the heat-treated films develop due to thermal stresses generated in the film caused by an uncontrolled rate of evaporation of water and solvent molecules from the film during annealing and the film shrinks to release its free energy [22].…”

Vapor growth of electrochromic thin films of transition metal oxides

“…The absorption lines around 447 cm À1 are typical for stretching modes of Mo-O of orthorhombic MoO 3 . The absorption band at 725 cm À1 , appearing in the IR spectrum of annealed film, is characteristic for the stretching mode of the monoclinic b-phase of MoO 3 [17] or can be connected with WQO bending vibrations [18]. The absorption band at 807 cm À1 observed for 200 1C film is attributed to n s (O-Mo-O) vibrations-characteristic for a-MoO 3 , and the same time this band at 807 cm À1 is assigned to W-O stretching vibration.…”

Influence of Structure on Optical Properties of WO<SUB>3</SUB> Thin Films Deposited by Sputtering Method

“…This value is used not only with amorphous films, but also with films containing grains in nano sizes [13,22]. Figure 2 depicts the energy diagrams of crystal WO 3 monoclinic for ideal case (a) and sample with oxygen vacancies (b) [23].…”

“…Therefore, their optical parameters like reflection, transmission and optical absorption can be modified by varying the external voltage or the ambient parameters. The WO 3 thin films were done by different deposition methods: spray pyrolysis [11], sol-gel [12], anodizing [13], thermal evaporation [3,14], electron beam [15] and magnetron sputtering [16][17][18][19]. Thin films, deposited by these methods, generally have a low crystallization temperature.…”