Electrodeposition of Ni-Al layered double hydroxide thin films having an inversed opal structure: Application as electrochromic coatings

- Electrophoretic deposition of synthetic opals templates from suspension of polystyrene nanospheres is described.
- These templates were used for electrochemically induced precipitation of Ni-Al-LDHs having inverse opal (IO) structures.
- The IO of the films survived heating at temperature high enough to convert the LDH into the corresponding mix oxides.
- The inverse opal LDH coatings have improved electrochromic properties compared to similar films lacking the opal structure.
- The color changes were more reversible when the IO films were heated to convert the LDH to mix oxides.

Application of a positive potential to suspensions of mono-dispersed polystyrene beads results in electrophoretic deposition of synthetic opal coatings of uniform thickness, consisting of large domains of essentially complete monolayers of the beads, arranged in nearly perfect hexagonal order. These colloidal templates were used for electrochemically induced precipitation of Ni-Al-LDHs from mixed solution of nickel and aluminum ions in potassium nitrate in ethanol-water solutions. Extractions with organic solvents left behind distorted inverse opal coatings consisting of a Ni-Al-LDH framework surrounding large interconnected cavities. The films 3-dimensional ordered structure survived unchanged heating of the coatings at temperatures high enough (> 350 °C) to completely convert the hydroxide coatings into the corresponding mix oxides. The shrinkage of the hydroxide coatings upon loss of structural water had no visible effect of the films inverse opal structure.In situ measurements of the UV-visible spectra of the films during potential cycling show greater transmittance changes (ΔT > 20%) were obtained for the inverse opal N-Al-LDH coatings compared with non-opal structure Ni-Al-LDH coatings prepared in the same conditions (ΔT ≈ 5%). However, introduction of the inverse opal structure did not much affect the reversibility of the color changes. Both films darken rapidly upon continuous potential cycling. Reversibility of the color change was greatly improved when the film were heated at a temperature high enough to convert the Ni-Al LDH to the corresponding mix oxides.