Multilayer Core-Shell MOF-on-COF supported TiO₂ nanorod arrays for enhanced photoelectrochemical water splitting

The combination of enlarged electrochemical active areas and directed electron transport toward catalytically active sites holds great potential for enhancing the overall performance of artificial photoelectrochemical (PEC) hydrogen production. Herein, we present an alkyl-linked heterostructure constructed from TiO₂ nanorod arrays and a covalent organic framework, combined with a metal-organic-framework-on-covalent-organic-framework (MOF-on-COF) assembly strategy (TiO₂@COF-2@NH₂-MIL-125), for the PEC water splitting reaction. Upon light irradiation, the photoinduced charge carriers generated in COF can directionally migrate across the interface along the alkyl chains and amino group to the active sites of the TiO₂ and MOF, while the remaining holes are consumed by water oxidation, thereby enhancing the spatial separation and utilization efficiency of electron-hole (e-h⁺) pairs. Benefiting from the clever construction of the porous MOF-on-COF featuring a high specific surface area, the electrochemically active surface area of the photoanode was greatly increased. The differential charge density analysis offers compelling evidence for the presence of a dual type-II band alignment and extremely rapid charge transfer kinetics in the TiO₂@COF-2@NH₂-MIL-125 heterojunction. Consequently, TiO₂@COF-2@NH₂-MIL-125 provides remarkably superior photocurrent density (2.85 mA·cm^-2 at 1.23 V vs. reversible hydrogen electrode, RHE) among the TiO₂@COF-2 and pristine TiO₂. This work provides a new sight in integrating MOF and COF epitaxial growth for enhancing PEC applications of TiO₂ based hybrid materials.