Colloidal WO _<i>x</i> Quantum Dot Films with Tunable Electrochromic Performance via Surface Ligand Engineering: Implications for Flexible Electrochromic Devices

Low-dimensional tungsten oxide (WOx) nanomaterials, characterized by large active surface areas and electrochemical activity, have recently emerged as a hot research area in the field of electrochromism. In oil-phase synthesis, long-chain ligands are commonly employed for synthesizing and stabilizing colloidal WOx quantum dots (QDs) as well as preventing QDs aggregation, but they form a dense insulating coating that impedes charge transfer between QDs which must be removed during the fabrication of solid films, significantly affecting their electrochromic performance. In this work, we innovatively adopted a solid-state ligand exchange method to employ short-chain organic molecular ligands with thiol functional groups such as 1,2-ethanediol (EDT) and 3-mercaptopropanoic acid (MPA), along with halide ion ligands such as tetrabutylammonium iodide (TBAI) and cetyltrimethylammonium bromide (CTAB), to replace the original oleic acid ligands on the surface of colloidal WOx QDs which was synthesized by a solvothermal method, thereby preparing WOx QD solid films. Subsequently, the effects of different ligands on the electrical and electrochromic properties of the WOx QD films were systematically investigated. Results revealed that EDT-modified WOx QD films exhibited the highest optical modulation (80.3% at 750 nm) and fastest response times (tc/tb = 5.49/3.50 s) among other ligands-modified WOx QD films due to its lower defect state and higher ion diffusion rate, demonstrating outstanding electrochromic performance. Notably, the WOx QD films prepared by this method require no annealing treatment, supporting the development of highly efficient, flexible inorganic electrochromic devices.