Fig. 1: Investigation of structure evolution in electrochromic WO₃ nanosheets.

a A schematic illustration of the electrochromic process showing the electrons and alkali ions injected into the working electrode, where CE, WE, and RE refer to the counter electrode, working electrode, and reference electrode. b UV-Vis diffuse reflectance spectroscopy (DRS) of the pristine, coloured, and bleached WO₃ nanosheets with various thicknesses (15 nm, 30 nm, and 60 nm) and their corresponding photograph in the insets of b, demonstrating that thinner nanosheets exhibit darker colour. c Chronoamperometry plots of coloured WO₃ nanosheets with different thicknesses, showing that more electrons were injected into the thinner nanosheets. d The inductively coupled plasma (ICP) curves of the different WO₃ nanosheets, showcasing that the content of K⁺ ions in electrodes is negatively correlated to the thickness of the WO₃ nanosheets. The error bar represents the standard deviation, calculated from three repeated experiments. e XRD patterns of the pristine, coloured, and bleached WO₃ nanosheets with different thicknesses, displaying that the nanosheets of ~15 nm thick, which possess the highest electrochromic performance, undergo a distinct phase transition from monoclinic to cubic and then back to monoclinic during the electrochromic and fading processes. f. Raman spectra of the pristine, coloured, and bleached WO₃ nanosheets, demonstrating that the absorption band of the bending mode (200–400 cm^-1) and stretching vibration (600–740 cm^-1) for the W-O-W bonds is weakened at the coloured state, and indicating that the bond angle of the W-O-W bonds is enlarged after electrochromic. The blue and pink balls represent to W atoms and O atoms, respectively. g XPS spectra of the WO₃ nanosheets at various states, showing that W⁵⁺ species appeared at the coloured state. h Electron-energy loss spectra of the pristine, coloured, and bleached WO₃ nanosheets at O K-edges. The apparent suppression of the A peak in the coloured state suggests that electrons were injected into W 5 d orbitals.