Thermal oxidation of seeds for the hydrothermal growth of WO3 nanorods on ITO glass substrate

•A simple seed formation method (thermal oxidation) on sputtered W film is reported.•Crystalline seed with good adhesion to substrate is required for nanorod growth.•The appropriate temperature and period for seed formation were 500 °C and 15 min.•WO3 nanorods exhibited higher electrochromic current density than WO3 compact film.

This work reports a simple seed formation method for the hydrothermal growth of tungsten oxide (WO3) nanorods. A WO3 seed layer was prepared by thermal oxidation, where a W-sputtered substrate was heated and oxidized in a furnace. Oxidation temperatures and periods were varied at 400–550 °C and 5–60 min, respectively, to determine an appropriate seed layer for nanorod growth. Thermal oxidation at 500 °C for 15 min was found to produce a seed layer with sufficient crystallinity and good adhesion to the substrate. These properties prevented the seed from peeling off during the hydrothermal process, thereby allowing nanorod growth on the seed. The nanorod film showed better electrochromic behavior (higher current density of − 1.11 and + 0.65 mA cm− 2) than compact film (lower current density of − 0.54 and + 0.28 mA cm− 2).