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 aW-sputtered substrate was heated and oxidized in a furnace. Oxidation temperatures and periods were varied at 400-550 degrees C and 5-60 min, respectively, to determine an appropriate seed layer for nanorod growth. Thermal oxidation at 500 degrees 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)). (C) 2015 Elsevier B.V. All rights reserved.