Regulating coordinatively unsaturated Co active sites in TA@ZIF-L(Co) via tannic acid for efficient photoelectrochemical water oxidation

First, the electrons in the valence band (VB) of BiVO 4 are excited by light and leap to the conduction band (CB), leaving holes in the VB. Since the surface of BiVO 4 is covered by a TA@ZIF-L(Co) layer, the holes diffuse onto it to participate in water splitting. Meanwhile, due to the introduction of TA, the electron density on coordinatively unsaturated Co active sites is increased, which promotes charge transfer to improve the activity of metal atoms, thus further accelerating the surface OER kinetics. In addition, the phenolic hydroxyl group of TA protects and stabilizes the metal atoms during the reaction process, making the photoanode BiVO 4 /TA@ZIF-L(Co) efficient and stable. • Etching ZIF-L with TA creates defects to induce coordinatively unsaturated sites. • TA@ZIF-L(Co) was developed as an efficient surface modifier for BiVO 4 . • Regulating coordinatively unsaturated Co active site enhances η inj for PEC. • The photocurrent density is 4.21 mA cm −2 at 1.23 V RHE , 3 times higher than BiVO 4 . Metal-organic frameworks (MOFs) as a cocatalyst can enhance the photoelectrochemical (PEC) water oxidation performance to a certain extent, but still suffer from deficient exposure of active sites and low electron mobility. In this work, 2D Leaf-like zeolitic imidazolate frameworks (ZIF-L) were selected as a precursor for the generation of defective structures by tannic acid (TA) functionalization-assisted etching, and then coupling it with BiVO 4 to prepare a novel and highly efficient TA@ZIF-L(Co)/BiVO 4 photoanode by a simple impregnation method. The photocurrent density of TA@ZIF-L(Co)/BiVO 4 composite photoanode achieves 4.21 mA cm −2 at 1.23 V RHE under AM 1.5 G illumination, which is superior to that of ZIF-L(Co)/BiVO 4 , and three times that of the bare BiVO 4 , with a charge injection efficiency of up to 80 %, and its stability is also remarkably improved. Experimental results and DFT calculations showed that the introduction of TA not only improved the light absorption efficiency, but also exposed more active sites, increased the electron density of the coordinatively unsaturated Co active sites, and enhanced the polarized electric field on the surface, thus accelerating electron mobility. This work demonstrates that TA as a surface modifier has great potential to activate metal sites and change the electronic environment of MOFs for BiVO 4 -based photoanode in PEC water oxidation, providing valuable insights into the rational design of photoanodes.