Transition-Metal Coordination Enables Chemical Design of Covalent Organic Frameworks/HfX2 Heterostructures with a Proposed Direct Z-Scheme Pathway for Solar Water Splitting

Abstract Covalent organic frameworks (COFs) are attractive photocatalyst platforms because their electronic structures can be chemically tailored, yet overall water splitting requires the simultaneous optimization of framework stability, interfacial charge separation, carrier utilization, and surface redox thermodynamics. Here, we use transition-metal coordination as a chemical design handle to investigate COF/HfX2 (X = S, Se) heterostructures as candidates for a proposed direct Z-scheme configuration from TpMA-COF monolayers. Screening of a series of coordinated TpMA-COFs shows that Ti- and Zr-coordinated frameworks provide the most suitable building blocks, as they preserve the COF backbone, remain nonmagnetic, and exhibit narrowed band gaps with band-edge positions more favorable for hydrogen evolution than those of pristine TpMA-COF. When coupled with HfX2, these coordinated COFs form energetically favorable interfaces with pronounced charge redistribution, work-function-driven built-in electric fields, and complementary redox functions. State-resolved nonadiabatic molecular dynamics further differentiate the relative tendencies of selected electron-transfer, hole-transfer, and interband-recombination pathways among the four heterostructures, providing comparative insight into possible carrier-loss processes that may limit the proposed direct Z-scheme operation. Gibbs free-energy analysis indicates that the photocatalytic feasibility of the selected heterostructures is strongly pH dependent, with sulfide-based systems favored under acidic conditions and selenide-based systems becoming more viable in alkaline media. These results identify transition-metal coordination as a chemically useful strategy for linking frontier-state regulation, interfacial field formation, carrier-loss assessment, and pH-dependent reaction feasibility in COF-based heterostructure photocatalysts.