Coloration mechanism of electrochromic Na_xWO₃ thin films

Abstract

The coloration mechanism of tungsten trioxide (WO3) upon insertion of alkali ions is still under debate after several decades of research. This Letter provides new insights into the reversible insertion and coloration mechanisms of Na + ions in WO3 thin films sputterdeposited on ITO/glass substrates. A unique model based on a constrained spline approach was developed and applied to draw out ε1+iε2 from spectroscopic ellipsometry data from 0.6 to 4.8 eV whatever the state of the electrochromic active layer, i.e. as-deposited, colored or bleached. It is shown that electrochemically intercalated sodium-tungsten trioxide, NaxWO3 (x=0.1, 0.2, 0.35), exhibits an absorption band centered at ca. 1.14 eV in 2 governing the coloration mechanism.

Morphological and chemical dynamics upon electrochemical cyclic sodiation of electrochromic tungsten oxide coatings extracted by in situ ellipsometry

“…6a). This thickness evolution obtained here in situ is consistent with previous ex situ one [18]. The roughness is also found to evolve in such a manner, the colored state being slightly rougher than the bleached one (Fig.…”

“…small polarons) or electron delocalization (Drude behavior), respectively. We showed recently [18] that optical absorption in amorphous NaxWO3 follows the generalized site saturation model as it was shown for LixWO3 [19] or HxWO3 [20]. More particularly, we derived a unique optical model based on a constrained spline approach to draw out the pseudodielectric function spectra (0.6-4.8 eV) whatever the state of the EC layer, i.e., as-deposited, colored (x=0.1, 0.2 and 0.…”

Towards enhanced durability of electrochromic WO3 interfaced with liquid or ceramic sodium-based electrolytes

“…An important optical feature is that the observed characteristic dark blue coloring of the coatings indicates the reduction caused by the intercalation of M + cations [6,19,21]. Optical transmission monitoring was preferably performed about 600 nm which a noticeable absorption of the human eye occurs.…”

“…Therefore, the development of EC devices based upon Na-intercalated WO 3 should not be overlooked before detailed investigation is conducted. If WO 3 can readily operate a sodiation-desodiation process using typical three electrodes configuration in aqueous [19][20][21] or non-aqueous [22][23][24][25] electrochemistry, investigations on how to handle the degradation of the amorphous tungsten oxide under potentiodynamic conditions in an aqueous Na + -based medium are lacking. Actually, the degradation in the amorphous case has been suggested [26,27] as originating from very open microstructure with the formation of alkali metatungstate.…”

Layered Monophosphate Tungsten Bronzes [Ba(PO4)2]WmO3m−3: 2D Metals with Locked Charge‐Density‐Wave Instabilities

“…[1] Since this early discovery, they have turned into a compulsory family of related mixed valence tungsten and molybdenum oxides, based on the diversity of structural variants and displaying a number of fundamental properties, which most notably include charge-density-wave (CDW) orders assisted by Peierls like transitions, as well as superconductivity and some interesting similarities with the high-T c cuprates. In addition, they also find applications due to their electrochromic [2,3] and catalytic properties after heterovalent mixed-metal (mm) tandem replacing W V by (M IV 1/2 W VI 1/2 ), (M III 1/3 W VI 2/3 ), or (M II 1/4 W VI 3/4 ). [4,5] The molybdenum bronzes became very popular in the 80's as they provided the first examples of oxide compounds materializing CDW physics.…”