Aqueous Processing and Spray Deposition of Polymer-Wrapped Tin-Doped Indium Oxide Nanocrystals as Electrochromic Thin Films

Plasmonic metal oxide nanocrystals are interesting electrochromic materials because they display high modulation of infrared light, fast switching kinetics, and durability. Nanocrystals facilitate solution-based and high-throughput deposition, but typically require handling hazardous nonaqueous solvents and further processing of the as-deposited film with energy-intensive or chemical treatments. We report on a method to produce aqueous dispersions of tin-doped indium oxide (ITO) by refunctionalizing the nanocrystal surface, previously stripped of its native hydrophobic ligands, with a hydrophilic poly(acrylic acid) polymer featuring a low density of methoxy-terminated poly(ethylene oxide) grafts (PAA-mPEO4). To determine conditions favoring the adsorption of PAA-mPEO4 on ITO, we varied the pH and chemical species present in the exchange solution. The extent of polymer wrapping on the nanocrystal surface can be tuned as a function of the pH to prevent aggregation in solution and deposit uniform, smooth, and optical quality spray coated thin films. We demonstrate the utility of polymer-wrapped ITO nanocrystal thin films as an electrochromic material and achieve fast, stable, and reversible near-infrared modulation without the need to remove the polymer after deposition provided that a wrapping density of ∼20% by mass is not exceeded.