Single-Atom Metal Oxide Sites as Traps for Charge Separation in the Zirconium-Based Metal–Organic Framework NDC–NU-1000

• Published in: Energy & fuels 2021-12, Vol.35 (23), p.19081-19095
• Main Authors: Kramar, Boris V, Phelan, Brian T, Sprague-Klein, Emily A, Diroll, Benjamin T, Lee, Sungsik, Otake, Ken-ichi, Palmer, Rebecca, Mara, Michael W, Farha, Omar K, Hupp, Joseph T, Chen, Lin X
• Format: Article
• Language: English
• ISSN: 0887-0624; 1520-5029
• DOI: 10.1021/acs.energyfuels.1c02623

Solvothermal deposition in metal–organic frameworks (MOFs) can be used to mount single-metal-atom catalytic species at chemically reactive sites on hexa-Zr­(IV)–oxy­(oxo,hydroxo,aqua) nodes in nanoscale crystallites of the MOF NDC–NU-1000 in a self-limiting fashion. Upon photoexcitation of the 1,3,6,8-tetrakis­(p-benzoato)­pyrene chromophores of the parent framework, charge transfer may occur between the chromophores and the installed heterometal sites. Extended X-ray absorption fine structure studies revealed the single-atom nature of the installed species. A combination of steady-state and ultrafast optical spectroscopy was used to uncover evidence of a charge-separated (CS) state arising in the metalated samples. The relevant dynamics were characterized with transient photoluminescence and femtosecond transient absorption spectroscopy. We find that a titanium–oxy single-atom site gives rise to the longest lived CS species compared to cobalt and nickel in a similar arrangement. This study provides guidance in designing MOF-based catalytic systems for photocatalysis and solar fuel production.