Amorphous-crystalline dual phase WO3 synthesized by pulsed-voltage electrodeposition and its application to electrochromic devices

Abstract A facile one-step pulsed-voltage electrodeposition (PVE) is used to prepare nanoporous amorphous-crystalline dual-phase tungsten trioxide (WO3) thin films onto indium-doped tin oxide (ITO) glass substrates. The dual-phase WO3 films exhibit superior electrochemical and electrochromic abilities because the amorphous WO3 disordered structure provides many ion trapping sites, and the crystalline WO3 imbedded in the amorphous matrix improves the electron mobility. Besides, the highly porous morphology not only enhances the electrochemical active surface area, but also offers an efficient pathway for ion diffusion and charge transfer. The nanoporous amorphous-crystalline PVE-WO3 thin films possess a large optical modulation (ΔT) of 80.1% at 700 nm, fast switching speed (tc/tb) of 2.9 s/2.1 s, and high coloration efficiency (CE) of 141 cm2 C−1 at 700 nm. On the other hand, single phase amorphous or crystalline WO3 films exhibit inferior electrochromic performances with ΔT of 57.23% and 62.51%, tc/tb of 3.7 s/2.9 s and 4.1 s/3.3 s, and CE of 68 and 63 cm2 C−1 at 700 nm, respectively. In addition, the amorphous-crystalline PVE-WO3 films exhibit excellent cycling stability (sustain 94.5% of the initial optical modulation after 25 cycles) and maintain their colored state for a longer time under open circuit, when compared to the conventional amorphous WO3 film. These results suggest that the amorphous-crystalline dual phase WO3 prepared by one-step PVE can be employed as an economical, highly efficient, and stable electrode material in various applications such as electrochromic smart window, batteries, supercapacitors, and photoelectrochemical water splitting cells.