Topotactic layer-to-tunnel crystallization for h-WO3 films with enhanced electrochromic performance

Benefitting from large-sized ion tunnels and stable crystalline structures, hexagonal WO3 (h-WO3) is a promising electrochromic (EC) material. However, long reaction times and highly controlled reaction conditions for preparation hinder the practical application. Here, a novel method with a simple process and moderate condition, named topotactic layer-to-tunnel crystallization (TLTC), to prepare h-WO3 films is reported. The hexagonal tunnel structure is transformed from a layered structure by annealing, assisted by the structure director. The ACTEM with atomic resolution reveals the position of the structure director in the crystal lattice, which is left in the hexagonal tunnel. The mechanism of TLTC has been studied by the characterization of samples at different formation stages, and the key is the structure director, which maintains the hexagonal tunnel by steric effect. The prepared h-WO3 film exhibits significant optical modulation (65.8 % at 633 nm and 72.8 % at 1200 nm) and outstanding cyclic stability (98.1 % of initial optical modulation remained over 600 cycles). The TLTC method is based on the structure transition to construct large tunnels in lattice, providing a new technical route to prepare h-WO3 and showing great potential for EC materials manufacturing.