Catalyst-free synthesis of tungsten oxide nanowires via thermal evaporation for fast-response electrochromic devices

Abstract

Tungsten oxide has drawn widespread attention due to its outstanding physical properties; however, the use of high temperature, low pressure, and expensive catalysts has been challenging in the synthesis of tungsten oxide nanowires. In this study, we developed a single step thermal evaporation process without any catalyst to synthesize WO_3−x nanowires on silicon substrates and indium tin oxide glass substrate safely and economically. We utilized the Taguchi method to design experiments for the synthesis of tungsten oxide nanowires and analyze the effect of processing parameters. The growth mechanism of nanowires is proposed to be a vapor–solid mechanism. Confirmed to be oxygen-vacancy-rich WO_3−x nanowires through high-resolution transmission electron microscopy studies, the nanowires have a great morphology with a high aspect ratio of around 700. The electrical resistivity of a single WO_3−x nanowire at 300 K was measured to be 2.32 × 10⁻⁵ Ω M, which is relatively low due to rich oxygen vacancies. We synthesized WO_3−x nanowires on an indium tin oxide glass substrate and assembled electrochromic devices with LiClO₄/PC as the electrolyte. The devices were demonstrated to have fast coloring and bleaching rates of less than one second at low voltage; at high voltage, the color of the devices could change to a deeper color but take more than 2 seconds to react. Compared with previous works, the electrochromic devices from this study exhibit superior response time and stability. These significant results make WO_3−x nanowires promising materials for applications in semiconductors, nanodevices, display and green technology.