Internal field engineering of WO3 by ion channel migration with enhanced photocatalytic oxygen evolution ability

Abstract

Different from transition metals, Na ions cannot be incorporated in the WO₃ lattice, but exist in the open framework formed by corner-sharing WO₆ 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 WO₃ (Na/WO₃) was prepared via a hydrothermal method. Driven by an increase in temperature, Na ions directionally migrated through channels towards the surface region of WO₃, leading to an uneven charge distribution between the inner and outer side of the WO₃ structure, together with the formation of a core–shell m-WO₃/h-WO₃ phase junction, and then the IEF from h-WO₃ (+) to m-WO₃ (−) was built. With a further increase in temperature, Na ions formed chemical bonds with O atoms belonging to the WO₆ octahedra, forming a series of core/shell heterojunctions with consecutive –O–W–O– at the interface, including m-WO₃/T-Na₂W₄O₁₃ and m-WO₃/T-Na₅W₁₄O₄₄, 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/WO₃-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.