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

Abstract

Mixed-metal−organic frameworks (MMOFs) have emerged as promising photocatalyst candidates in multiple reactions. For instance, the doping of Zr-UiO-type MOFs with Ce atoms increases their photoactivity owing to a better overlap between the organic linker and Ce orbitals. However, it is not clear which is the ideal content of Ce to reach the optimal photocatalytic performance. Herein, a series of MMOFs isostructural to UiO-66 and with napthalene-2,6-dicarboxylate (NDC) as a linker were synthesized and characterized. The Ce content was varied from 0 to 100% and their corresponding structural, chemical, photodynamic, and photoresponse properties were investigated. Powder X-ray diffraction shows that when the content of Ce is 12% onward, in addition to the UiO-type structure, a second crystalline structure is 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 charge transfer (LCCT) phenomenon from the organic linker to Zr/Ce metal clusters. The ultrafast (fs-regime) LCCT process leads to the formation of long-lived charge-separated states, which are more efficiently photoproduced when the content of Ce reaches 9%, suggesting that the related material would show the highest photoactivity. Photoaction spectroscopic measurements corroborate that the sample with 9% of Ce exhibits the maximum photocatalytic efficiency, which is reflected in a 20% increment in overall water splitting efficiency compared with the monometallic Zr-based MOF. The current study demonstrates the relationship between the photodynamical properties of the MMOFs and their photocatalytic performance, providing new findings and opening new ways for improving the design of new MOFs with enhanced photocatalytic activities.

Solar Gas-Phase CO2 Hydrogenation by Multifunctional UiO-66 Photocatalysts

“…To further evaluate the photoinduced charge separation efficiency of UiO-66 solids and their relationship with their photocatalytic activities, photocatalysts were characterized by PL spectroscopy and transient photocurrent and EIS measurements. PL spectroscopy is commonly used in heterogeneous photocatalysis, including MOFs, to evaluate the photoexcited charge transfer and recombination processes. , Amino functionalized UiO-66 solids have a different degree of fluorescence, whereas negligible emission was found when using the nitro-functionalized solids. These results agree with some of our previous results showing that acetonitrile solutions of 2-aminoterephthalate emit much more on excitation at 266 nm than the analogous 2-nitroterephthalate acetonitrile solutions …”