Tuning the Photoelectrochemical Properties of Ti/W-Modified PCN-222 Using Charge-Selective Interfaces

Metal-organic frameworks (MOFs) have attracted growing interest for photoelectrochemical (PEC) applications, including visible-light photocatalysis, CO₂ reduction, and hydrogen evolution, owing to their structural tunability and hybrid inorganic-organic nature. The Zr-based porphyrinic framework PCN-222 combines strong visible light absorption from its porphyrin linkers with robust Zr₆ clusters that act as structural and electronic backbones. Here, we report a modular strategy to tailor and optimize the PEC behavior of PCN-222 through postsynthetic metal-node substitution with Ti, pore encapsulation of phosphotungstic acid (PTA), and integration with charge-selective interfaces. The resulting PCN-222 materials exhibit photoelectrochemical activity across the entire visible range. Whereas pristine PCN-222(Zr) exhibits photocathodic behavior (photoelectron transfer to the solution and photohole collection at the FTO substrate), partial substitution of Zr with Ti inverts the current to photoanodic. Encapsulation of PTA further enhances the anodic photocurrent due to its electrocatalytic properties. Furthermore, charge-selective TiO₂ and NiO_x interlayers deposited between the FTO substrate and the MOF films enable selective extraction of photoelectrons or holes, respectively. This strategy results in a significant photocurrent enhancement, which can be attributed to effective competition of charge extraction and recombination. For PCN-222(Zr), the cathodic photocurrent increases by a factor of 7 using a NiO_x interlayer, while the current switches to photoanodic upon TiO₂ integration, illustrating the importance of efficient charge extraction. Similarly, the current direction is reversed to photocathodic for PCN-222(Zr/Ti) and PCN-222(Zr/Ti/W) when using NiO_x. We discuss the interfacial charge extraction, charge transfer and trapping mechanisms in detail, providing strategies for the design of multicomponent MOF-based systems for photoelectrochemical devices.