Solvothermal Synthesis of Oxygen Deficient Tungsten Oxide Nano-Particle for Dual Band Electrochromic Devices

Dual-band electrochromic smart windows can independently control near infrared (NIR) and visible (Vis) light, and are considered a promising technology for improving building energy efficiency. However, most existing dual band electrochromic materials consist of complex nanocomposites, and it is difficult to balance the delicate composition of each constituent. Recently, to solve these problems, research has been conducted on single component dual-band electrochromic materials, in particular, oxygen deficient tungsten oxide (WO3-x) nanostructures. In this study, WO3-x nanoparticles were synthesized using a simple solvothermal method, and highly dispersible inks were formulated which could be deposited at relatively low temperature via various solution-based coating methods. Various analysis techniques including XRD, HR-TEM and in-situ spectroelectrochemical measurements revealed that the oxygen-deficient WO3-x composition remained intact up to an annealing temperature of 350 °C. Under suitable thermal treatment conditions, the resulting WO3-x films could independently and dynamically control NIR and VIS light transmittance. When a moderate voltage of -0.4 V was applied, the NIR light was mainly blocked via a capacitive charge mechanism, with optical modulations of 23.81% and 72.48% at 550 nm and 1200 nm, respectively. A further increase in applied voltage to -1.2 V led to the blocking of both VIS and NIR light through faradaic Li+ ion insertion with optical modulations of 71.13% and 84.57% at 550 nm and 1200 nm, respectively. Moreover, the optical modulations were maintained for 1,000 consecutive cycles. This work demonstrates a single component dual-band electrochromic material that can be useful for various applications such as electrochromic smart windows.