This talk will include two projects: (1) Separation of photo-excited charges (electrons and holes) can exert important control over the lifetimes of photoexcited states. We address this issue in metal-organic frameworks (MOFs), which are potential photocatalysts due to their tunability that provides greater photoredox possibilities and their porosity that facilitates the diffusion of reactants, with the potential to overcome the low charge mobility of many nonporous semiconductors. We studied UiO-66, which is built from Zr6O4(OH)4 nodes and 1,4-benzene-dicarboxylate linkers, because it has superior tunability, such as complete or partial metal (Hf, Th, Ti, U, Ce) substitutions in the nodes and a variety of possible linker functionalizations. Ligand-to-metal charge transfer (LMCT) can lead to separation of photo-excited charges in the MOF, and with pure metals (Zr, Hf, Th, Ti, U, or Ce) in the nodes that LMCT is favorable only for Ce, where it is promoted by the low-lying empty 4f states of Ce4+. We propose that incorporating Ce4+ into the nodes is an effective way to facilitate LMCT in a MOF. We also found that linker functionalization is possible to engineer the electronic structure of the Ce-MOF for a desired reaction (e.g., water splitting) while preserving favorable LMCT. We also considered partial metal (Zr and Ti) substitution in the Ce-MOF and found that it is a good way to engineer the band gap and balance the charge carrier density and lifetime of the Ce-MOF. (2) The accuracy and cost of Kohn−Sham calculations depend on the exchange-correlation functional. Local functionals tend to underestimate band gaps and chemical reaction barrier heights and to delocalize polarons; the most successful functionals are hybrid functionals that are very expensive for plane-wave calculations on extended systems. We have recently developed local functionals called High Local Exchange (HLE) functionals that give semiconductor band gaps and chemical reaction barrier heights comparable to hybrid functionals without nonlocal exchange and without system-dependent parameters. We also find that HLE functionals can correctly localize holes as polarons, which other local functionals usually fail to do.

Cerium metal-organic frameworks and high local exchange density functionals for photocatalysis

256th National Meeting (2018)

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