Facile synthesis of one-dimensional crystalline/amorphous tungsten oxide core/shell heterostructures with balanced electrochromic properties

Abstract

One-dimensional (1D) crystalline/amorphous WO₃ core–shell heterostructures were synthesized by a simple, two-step hydrothermal process. Single-crystalline hexagonal WO₃ (h-WO₃) nanorod cores with diameters of about 200 nm and lengths of 6–7 μm were obtained in the first step. Amorphous WO_x (a-WO_x) shells with thicknesses ranging from 8 to 40 nm were grown on the h-WO₃ cores in the second step. The electrochromic properties of the devices based on the h-WO₃/a-WO_x core–shell heterostructures were measured and compared with those based on the bare a-WO_x and h-WO₃ nanorods. The optical modulation of coloration/bleaching at λ = 550 nm was measured to be 44% after coloration at −3.0 V for 180 s and bleaching at +2.0 V for 90 s. The coloration and bleaching times were found to be 41 s and 6 s, respectively, and the coloration efficiency was 82 cm² C⁻¹. After cyclic stability tests for 1000 cycles, the transmittance changes compared with the initial stage at the colored and bleached states are only 2.6 and 3.4%, respectively. It appears that the h-WO₃/a-WO_x core/shell heterostructures combine the assets of the constituent phases and exhibit highly improved and balanced electrochromic properties.