Abstract
Metal-organic frameworks (MOFs) containing d0 metals such as NH2-MIL-125(Ti), NH2-UiO-66(Zr) and NH2-UiO-66(Hf) are among the most studied MOFs for photocatalytic applications. Despite structural similarities, we demonstrate that the electronic properties of these MOFs are markedly different. As revealed by quantum chemistry, EPR measurements and transient absorption spectroscopy, the highest occupied and lowest unoccupied orbitals of NH2-MIL-125(Ti) promote a long lived ligand-to-metal charge transfer upon photoexcitation, making this material suitable for photocatalytic applications. In contrast, in case of UiO materials, the d-orbitals of Zr and Hf, are too low in binding energy and thus cannot overlap with the π* orbital of the ligand, making both frontier orbitals localized at the organic linker. This electronic reconfiguration results in short exciton lifetimes and diminishes photocatalytic performance. These results highlight the importance of orbital contributions at the band edges and delineate future directions in the development of photo-active hybrid solids.
Highlights
Metal-organic frameworks (MOFs) have attracted a great deal of interest during the last decades due to their unprecedented surface area, remarkable tuneability and the fascinating variety of possible combinations of constituting blocks
In this study we focus on the electronic properties of NH2-MIL-125(Ti), NH2-UiO-66(Zr) and NH2-UiO66(Hf) and their influence on the photocatalytic performance in hydrogen evolution reaction (HER)
Based on the earlier works by Farha who reported the synthetic path yielding the defective UiO-66(Zr)[28], Lillerud claiming the nearly stoichiometric composition of the UiO obtained with the high temperature synthesis[29], and our own characterization, it can be safely assumed that the NH2-UiO-66 materials used possess a different number of structural defectsSI
Summary
Metal-organic frameworks (MOFs) have attracted a great deal of interest during the last decades due to their unprecedented surface area, remarkable tuneability and the fascinating variety of possible combinations of constituting blocks. Despite the fact that the early reports of semiconducting properties of MOFs have been disputed[7,8], this initial misconception led to the application of frameworks based on Zn, Ti and other transition metals in a variety of photocatalytic reactions such as oxidation of organic compounds[9,10], reduction of metal ions[11] and synthesis of solar fuels[12]. NH2-MIL-125(Ti) and NH2-UiO-66(Zr) clearly stand out as the most researched among the different MOF structures tested in photocatalysis to date While these two MOFs are based on the same linker and crystallize in a rather similar topology, the metal ions constituting the inorganic nodes in the frameworks are different: Ti4+ forms octameric Ti8O8(OH)[4] rings[13] in NH2-MIL-125(Ti) and Zr4+ or Hf4+ form the M6O4(OH)[4] clusters in the UiOs14. The light absorption properties of MOFs are determined by the synergy between the organic ligand and the metal ion. In analogy to NH2-MIL-125(Ti) Li et al and Wang et al ascribed the lowest in energy absorption band to LMCT and reported EPR spectra for this Zr-based MOF as evidence[9,22,23]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
