“Nano to nano” electrodeposition of WO3 crystalline nanoparticles for electrochromic coatings

Abstract

A “nano to nano” electrodeposition approach for preparing nano-structured thin films from the dispersion of nano-objects is reported. A typical WO₃ system is demonstrated, where nanocrystalline films are electrodeposited onto transparent conductive electrodes such as ITO and Ag grid printed PET (Ag grid/PET) from the water dispersion of WO₃ nanoparticles without applying high potential, adding surfactants or polymers. The process is based on the reduction of WO₃, which eliminates the electrostatic repulsion between the nanoparticles causing film deposition on the cathode. The reduced WO₃ (HWO₃) is conductive, thus it allows further film growth towards higher thickness and coverage. The electrodeposited films consist of stacked crystalline nanoparticles, which provide a highly active surface area, facilitate the penetration of electrolyte and the intercalation/deintercalation of Li⁺ in the nanocrystals and therefore result in outstanding electrochromic performance and stability (92% contrast, 9 s coloring and 15 s bleaching, retaining 76% contrast after 1000 coloring–bleaching cycles). The thickness, electrochromic performance and surface coverage of the films are well tuned by potential and time. This novel “nano to nano” electrodeposition approach based on the electrochemical redox of nano-objects can be extended to various transition metal oxide nano-objects with different sizes and shapes.