An Ellipsometric Study of Electrochromism in Tungsten Oxide

Abstract

A self‐nulling ellipsometer was used to follow the optical changes which occur when galvanostatic anodic oxidation of tungsten in 0.5M sulfuric acid is interrupted at 5V intervals by the application of cathodic transients which color the oxide film. The film undergoes two distinct optical changes during the cathodic cycle, the first due to the strain and anisotropy resulting from electrostriction, and the second due to the optical absorption associated with electrochromism. For color cycles of limited duration, application of an anodic current will bleach the film with little optical hysteresis and return it to its initial thickness and refractive index. Longer color cycles reach a point where further coloring is accompanied by film dissolution. The optical data fit a model in which coloring takes place by a progressive change in the entire film rather than by movement of an interface separating colored and bleached layers of a duplex film. The coloring process appears to have no clearly defined end point either in sulfuric acid or in acetic acid where the color cycle can be extended without film dissolution.

Anodic Oxidation of Tungsten in Nonaqueous Electrolyte: II . Electrochemical Coloring and Bleaching of the Oxide Film

“…The results in the previous section show that hydrogen does not build up uniformly in tungsten oxide, as was previously reported (2,3). It forms a distinct phase, HWO3, and reduction and reoxidation proceed in the same way as in the molybdenum (4, 5) and vanadium (6, 7) systems, by motion of a phase boundary across the film.…”

“…The first paper in this series (1) dealt with optical anisotropy in the oxide, and its findings are used in the optical analysis in the current study. Most of our earlier work (2,3) on the optical effect of electrochemical processes (electrochromism) in tungsten oxide was done in sulfuric acid electrolyte, but some work was done in acetic acid, and we found that the oxide could be made more absorbing optically in the nonaqueous electrolyte. We found that the optical data exhibited little hysteresis when the oxide was reduced (colored) and then reoxidized (bleached), and we fit the data to a model in which optical changes take place uniformly throughout the film.…”

Structural and Electrochromic Properties of Tungsten and Molybdenum Trioxide Electrodes in Acidic Media

“…Electrochromism of WO3 and MoO~ or their hydrates have been widely investigated because they have a potential as passive display electrodes (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16). Several mechanisms of coloration have been proposed up to date: intervalence transition W 5+ --~ W 6 § (2), electron transition between donor state and conduction band or intraband transition of conduction electron (7), small polaron transition (8), and formation of F or F + centers in reduced tungsten trioxide matrices (10).…”

<title>Performance problems of electrochromic coatings</title>

“…The oxidation charge density was varied in region of 0,5 to 4 C/cm2. In situ interferometric measurements shows that for oxidation charge density 1 , 1 C/cm2 the thickness of film is 240 nm, if refractive index for anodic oxide film with small content of water is n=2, 145 at X=632,8 nm [6,11]. The PAR Model 173 Potentiostat/Galvanostat was used both for oxidation of tungsten and cycling in the same > .::.…”