Improving performance for efficient photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting is a promising green technique for renewable hydrogen production. To construct a practical PEC system, it is of great significance to develop efficient photoanodes. BiVO₄ has been identified as the most promising photoanode material because of its narrow band gap and favorable band positions for hydrogen and oxygen evolution. Nevertheless, BiVO₄ has limitations of low carrier mobility (4×10⁻² cm²·V⁻¹·s⁻¹) and short hole diffusion length (<100 nm) as a photoanode, resulting to an unsatisfactory photocurrent density (<1 mA·cm⁻² at 1.23V vs. RHE in neutral medium under AM 1.5G illumination). Therefore, it is necessary to explore a series of methods to improve the PEC performance of BiVO₄.

It has been proposed and investigated that inserting a new layer between BiVO₄ and fluorine-doped tin oxide (FTO) can improve the carrier separation efficiency. Among them, BiVO₄/WO₃ is a proven type II heterojunction. Surface and deposition of an oxygen evolution co-catalyst (OEC) layer can enhance the water oxidation kinetics. But most WO₃ arrays on FTO electrodes exhibit small array gaps (< 60 nm), which are not conducive to uniform loading of BiVO₄ nanoparticles with the sizes larger than 80 nm. Furthermore, the upper layer of BiVO₄ is coated on the bottom WO₃ layer to form a bilayer heterojunction, which exhibits a small contact area and inevitable charge recombination in the bulk and boundary of BiVO₄ particles.

Recently, a research team led by Prof. Junwang Tang from University College London, U.K. and Hai-Ying Jiang from Northwest University, China fabricated WO₃ nanobowl arrays based on monolayer colloidal crystals (MCCs) to construct a highly matched BiVO₄/WO₃ heterojunction with BiVO₄. In this novel design, the small-sized BiVO₄ nanoparticles (<90 nm) are perfectly deposited on the bottom layer of the WO₃ nanobowl with a large inner diameter of 920 nm. The size of BiVO₄ is smaller than its hole diffusion length (~100 nm), ensuring that holes are efficiently transferred to its surface. Meanwhile, a highly ordered monolayer WO₃NB array was chosen to minimize WO₃ defects at grain boundaries, reducing the interfacial resistance with FTO and increase the contact area with BiVO₄ nanoparticles. In addition, the highly matched BiVO₄/WO₃ interface can also enhance the charge separation of BiVO₄, which plays an important role in the PEC process. To enhance the water oxidation kinetics, the authors further loaded an OEC layer on the BiVO₄/WO₃NB heterojunction photoanode, which produced about 5 times higher photocurrent and IPCE than pristine BiVO₄ under one sunlight condition.

The results were published in the Chinese Journal of Catalysis.