Unraveling the Optimal Cerium Content for Boosting the Photoresponse Activity of Mixed-Metal Zr/Ce-Based Metal-Organic Frameworks through a Photodynamic and Photocurrent Correlation: Implications on Water Splitting Efficiency

Mixed-metal-organic frameworks (MMOFs) have emergedas promisingphotocatalyst candidates in multiple reactions. For instance, thedoping of Zr-UiO-type MOFs with Ce atoms increases their photoactivityowing to a better overlap between the organic linker and Ce orbitals.However, it is not clear which is the ideal content of Ce to reachthe optimal photocatalytic performance. Herein, a series of MMOFsisostructural to UiO-66 and with napthalene-2,6-dicarboxylate (NDC)as a linker were synthesized and characterized. The Ce content wasvaried from 0 to 100% and their corresponding structural, chemical,photodynamic, and photoresponse properties were investigated. PowderX-ray diffraction shows that when the content of Ce is 12% onward,in addition to the UiO-type structure, a second crystalline structureis cosynthesized (NDC-Ce). Steady-state and femtosecond (fs) to millisecond(ms) spectroscopy studies reveal the existence of two competing processes:a linker excimer formation and an ultrafast ligand-to-cluster chargetransfer (LCCT) phenomenon from the organic linker to Zr/Ce metalclusters. The ultrafast (fs-regime) LCCT process leads to the formationof long-lived charge-separated states, which are more efficientlyphotoproduced when the content of Ce reaches 9%, suggesting that therelated material would show the highest photoactivity. Photoactionspectroscopic measurements corroborate that the sample with 9% ofCe exhibits the maximum photocatalytic efficiency, which is reflectedin a 20% increment in overall water splitting efficiency comparedwith the monometallic Zr-based MOF. The current study demonstratesthe relationship between the photodynamical properties of the MMOFsand their photocatalytic performance, providing new findings and openingnew ways for improving the design of new MOFs with enhanced photocatalyticactivities.