Facile growth of aligned WO3 nanorods on FTO substrate for enhanced photoanodic water oxidation activity

We demonstrate a facile hydrothermal method to grow oriented WO3 nanorods on a transparent conductive substrate without the assistance of any seed layer or structure directing agent for photocatalytic applications. The effects of hydrothermal growth conditions such as reaction time, precursor solution, and post annealing temperature on the crystalline phase and morphology of the WO3 on the FTO substrate are discussed. XRD studies reveal that the as-prepared orthorhombic WO3·0.33H2O nanorods are transformed to the hexagonal phase by post annealing at 400 °C. Moreover, post annealing above 500 °C converts them to monoclinic WO3 nanorods on the FTO substrate, which is the photoactive crystal phase of WO3 for water oxidation. The synthesized WO3 nanorods were revealed to have a single crystalline structure by HRTEM analysis. The photoelectrochemical water splitting properties of the annealed WO3 nanorod arrays were investigated in 0.5 M Na2SO4 under AM 1.5G illumination. The optimized WO3 nanorod arrays exhibit a photocurrent of 2.26 mA cm−2 at 1.23 V (versus RHE), and an incident photon-to-current conversion efficiency (IPCE) of as high as 35% at 400 nm for the photoelectrochemical oxidation of water. This simple hydrothermal method can allow the use of WO3 for photoanodic applications with high efficiency.