High contrast solid state electrochromic devices based on Ruthenium Purple nanocomposites fabricated by layer-by-layer assemblyw

In the last two decades, the only inorganic non-oxide materials that have attracted significant attention for incorporation in low power electrochromic displays are those belonging to the metal hexacyanoferrate (HCNF) family. In the HCNF family, Prussian Blue (PB) is by far the most studied electrochromic material. In contrast, Ruthenium Purple (RuP) has not been explored for electrochromic displays because the neutral form is insoluble in aqueous solution. Though nanocomposites incorporating polymer and inorganic materials have shown improved performance as compared to the separate constituents in terms of mechanical stability, electrochemical activity, and solar energy conversion, their use in electrochromic devices has not been well studied. In this paper, we describe the synthesis of stable Ruthenium Purple nanoparticles and the electrochromic and surface properties of Ruthenium Purple–polymer nanocomposites fabricated by the layer-by-layer (LbL) assembly approach. RuP belongs to the class of well-defined zeolite-like polynuclear inorganic materials with fixed metal ionic redox centers (e.g., Ru(II) and Fe(III)) wherein the electroneutrality is maintained during the redox process by the movement of freely diffusing cations in and out of the lattice structure. As discussed by several groups and similar to PB, RuP is present in two forms: ionized water-soluble KFe[Ru(CN)6] xH2O and a neutral insoluble compound, Fe4[Ru (CN)6]3. 4–7 The soluble RuP is present in a colloidal form and the inclusion of potassium makes it stable in aqueous media, bringing an overall negative charge, which allows LbL assembly using RuP colloids as the anions. As explained by Rosseinsky et al., the incorporation of K as a ‘supernumerary’ cation in Fe[Ru(CN)6] makes it water-soluble. RuP (Fe4[Ru (CN)6]3/KFe [Ru(CN)6] xH2O) in its natural form has a deep purple color and changes to colorless RutheniumWhite (RW), Fe3[Ru (CN)6]2 upon reduction. For the present studies, synthesis of 5 mM solution of RuP was carried out by mixing equal concentrations of potassium ruthenium cyanide (K4[Ru(CN)6]), ferric chloride (FeCl3) and potassium chloride (KCl). KCl is added to ensure the excess of potassium ions crucial for the solubility of RuP. This was further purified by ultrafiltration to remove insoluble RuP, following the procedure described by DeLongchamp et al. The composition of the as-synthesized product in powder form was confirmed by the X-ray diffraction (XRD) pattern (Fig. 1). The main peaks at 16.91 (200), 23.81 (220), 34.11 (400), 38.21 (420), and 44.31 (420) can be readily indexed as a pure face-centered cubic (fcc) phase of RuP [space group: Fm3m]. The broadening of the XRD is known to occur when the particle size reduces progressively below 200 nm. The values are in good agreement with values reported by other groups for RuP and its analogs. Transmission electron microscopy (TEM) (Fig. 2) and dynamic light scattering (DLS) (see ESI Fig. S2(a)w) of the sample were taken from the diluted RuP solution and both techniques show that the average RuP nanoparticle size is 20–30 nm, which is in the range of values reported for HCNF nanoparticles by other groups. RuP possesses a simple fcc lattice structure; the metal ion centers in the fcc structure are Ru(II) and Fe(III) with cyanide groups between these ions. The presence of (200) fcc structure is confirmed by a fast