Bifunctional aligned hexagonal/amorphous tungsten oxide core/shell nanorod arrays with enhanced electrochromic and pseudocapacitive performance†
Tungsten oxide possesses electrochromic and pseudocapacitive properties; integrating these two functions into a single device with improved comprehensive performance has become a hot research topic. In this paper, bifunctional aligned hexagonal/amorphous tungsten oxide core/shell nanorod arrays (denoted as h@a-WNRAs) were fabricated on fluorine-doped tin oxide substrates by a two-step process involving hydrothermal treatment and spin coating to provide enhanced electrochromic and capacitive properties. The first step involved synthesis of single-crystalline hexagonal tungsten trioxide (h-WO3) nanorod cores. The second step coated amorphous WOx (a-WOx) shells with thicknesses of 2–8 nm on the surface of the h-WO3 cores. The results indicated that the a-WOx shells strongly affected the electrochromic and pseudocapacitive properties of the heterostructure because of their high specific surface area and porous internal structure. Compared with that of pure WO3 nanorod arrays, the optimized nanostructured h@a-WNRAs showed marked improvement of electrochromic and pseudocapacitive performance. In terms of electrochromic properties, the optimized h@a-WNRAs realized a substantial optical modulation (67.7%), short response times (15 and 21 s), high coloration efficiency (101 cm2 C−1 at 800 nm), and good cycling stability. Meanwhile, the optimized h@a-WNRAs also displayed promising pseudocapacitive properties, including a high specific capacitance (885.8 F g−1 at 1 A g−1), enhanced rate capability, good cycling efficiency (91.8%), and good specific capacitance retention (57.8% after 2000 cycles). The h@a-WNRA heterostructure obtained by this facile approach represents a new idea for the preparation of bifunctional materials with excellent electrochromic and pseudocapacitive properties.