Effect of pH on the structure and morphology of W18O49 nanowires and their electrochromic properties

In wet-chemistry processes, pH value is an important parameter that can be used to control the microstructures of the as-grown nanostructures. In this work, W18O49 nanowires with crystalline@amorphous core-shell structures were synthesized by controlling the pH of the precursor solutions in a hydrothermal reaction. Due to surface functionalization by ethylene glycol (EG) groups, the hydrothermally grown W18O49 nanowires could be easily dispersed in aqueous environment, thus enabling the fabrication of electrochromic thin films of W18O49 nanowires by a simple spray-drying process. The optimized W18O49 nanowire films showed a large optical modulation (AT) of 81.1% at 633 nm, short response time (coloring/bleaching time rc/rb = 7.9 s/3.9 s), good cycle stability (AT retention was 84.5% after 1000 cycles), and the excellent memory effect (the color withstood for 36000s after turning off the voltage). Furthermore, the electrochromic device (ECD) (4 x 4 cm2) based on the W18O49 nanowires exhibits a large optical modulation of 56.6%, short response time (rc = 7.6 s, rb = 2.7 s), and superior coloration efficiency (CE) of 64 cm2 C-1. Finally, by simply amplifying the developed processes, a large-area electrochromic device (10 x 10 cm2) was successfully prepared, which implies the possibility of large-area spraying process for practical applications.