Internal field engineering of WO by ion channel migration with enhanced photocatalytic oxygen evolution ability

Different from transition metals, Na ions cannot be incorporated in the WO 3 lattice, but exist in the open framework formed by corner-sharing WO 6 octahedra. Based on this channel-inserted Na-doping mechanism, herein, an in situ ion migration-based internal electric field (IEF) engineering strategy is developed. Firstly, Na channel-doped WO 3 (Na/WO 3 ) was prepared via a hydrothermal method. Driven by an increase in temperature, Na ions directionally migrated through channels towards the surface region of WO 3 , leading to an uneven charge distribution between the inner and outer side of the WO 3 structure, together with the formation of a core-shell m-WO 3 /h-WO 3 phase junction, and then the IEF from h-WO 3 (+) to m-WO 3 (−) was built. With a further increase in temperature, Na ions formed chemical bonds with O atoms belonging to the WO 6 octahedra, forming a series of core/shell heterojunctions with consecutive -O-W-O- at the interface, including m-WO 3 /T-Na 2 W 4 O 13 and m-WO 3 /T-Na 5 W 14 O 44 , which provide a pathway for the electron transfer. The distinct O 1s oxidation states between the two sides of the atomic junction interface were suggested to be the driving force for the formation of IEF in two heterojunctions. Electrons transfer to one side with a higher O 1s oxidation state along -O-W-O- bands at the interface to build IEF. Transient photocurrent measurement, photoluminescence spectroscopy, steady-state surface photovoltage spectroscopy (SS-SPV) and Mott-Schottky curves were used to investigate separation efficiencies of Na/WO 3 - T and the relationship with their relative IEF strength and activities. Results indicate that the synergistic effect of IEF and junctions is the main reason for the efficient carrier separation, finally resulting in improved photocatalytic oxygen evolution. This study highlights the synergistic effect of IEF and junctions for the efficient carrier separation and improved photocatalytic oxygen evolution.