Seed layer formation determines photocurrent response of hydrothermally-grown WO3 photoanodes†
Abstract
Due to its superior electrical properties and band gap suitable to absorb visible light, WO3 is an exceptional photoanode material for photoelectrochemical water splitting. The hydrothermal growth of WO3 photoelectrodes is well established, based on seed layers used to grow WO3 nanosheets directly on substrates. However, the influence of this seed layer on the photoelectrochemical performance is unknown so far. We therefore present a detailed investigation on the influence of seed layer preparation on the photoelectrochemical water oxidation performance of hydrothermally-grown WO3 photoanodes. Those thin films were carefully characterized by X-ray diffraction, electron microscopy, Raman spectroscopy, absorption spectroscopy, and multiple electrochemical and photoelectrochemical methods. A high spinning speed or long spinning duration for spin coating leads to a low seed layer density, which benefits the formation of m-WO3 resulting in an increased performance. The combination of high absorption, high donor density, low onset potential and high charge carrier injection efficiency leads to the highest photocurrents. For a phase pure m-WO3 photoanode with a film thickness of 2.3 μm and a flat band potential of 0.39 VRHE, a photocurrent density of 1.51 mA cm−2, an IPCE of 60% and high separation efficiency of 34% was observed without any co-catalyst.