Colloidal WO _x Quantum Dot Films with Tunable Electrochromic Performance via Surface Ligand Engineering: Implications for Flexible Electrochromic Devices

Low-dimensional tungsten oxide (WO x ) 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 WO x 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 WO x QDs which was synthesized by a solvothermal method, thereby preparing WO x QD solid films. Subsequently, the effects of different ligands on the electrical and electrochromic properties of the WO x QD films were systematically investigated. Results revealed that EDT-modified WO x QD films exhibited the highest optical modulation (80.3% at 750 nm) and fastest response times ( t c/ t b = 5.49/3.50 s) among other ligands-modified WO x QD films due to its lower defect state and higher ion diffusion rate, demonstrating outstanding electrochromic performance. Notably, the WO x QD films prepared by this method require no annealing treatment, supporting the development of highly efficient, flexible inorganic electrochromic devices.