Proceedings of nanoGe September Meeting 2017 (NFM17)
Publication date: 20th June 2016
Metal-organic frameworks (MOFs) have been recently used as custom designed materials for artificial photosynthesis, i.e. CO2 reduction and H2 evolution (HER). However, their performance is still limited, and fundamental understanding of the photocatalytic process by MOFs offers a great contribution to move forward this field.
Given the large photostability of Ti-and Zr based MOFs, these M4+-based (d0) solids are the most attractive for perspective application in photocatalysis. In this work we use two frameworks, namely NH2-MIL-125(Ti) and NH2-UiO-66(Zr), as examples to better understand photocatalytic processes in MOFs by using (spectro)electrochemistry and ultrafast spectroscopy and highlight the importance of the location of excited states (metal to organic linker) on photocatalytic performance.
Although the robust NH2-UiO-66(Zr) framework possesses excellent light absorption properties, its photoactivity is limited as a consequence of the poor overlap of the d-orbital of Zr and the π* orbital of the ligand. Both frontier orbitals (HOCO and LUCO) are localized at the organic linker, resulting in short exciton lifetimes and decreased photocatalytic performance. As a shortcut to this disadvantage, we demonstrate that the post-synthetic metal modification (PSM) of Ti4+ into the UiO-66(Zr) framework enhances its photocatalytic performance in visible light HER and proves the great impact of the metal precursor in PSM for photocatalysis.
In contrast, the good photocatalytic performance of NH2-MIL-125(Ti) is a consequence of an optimal overlap of ligand (HOMO) and metal (LUMO) orbitals that favours LMCT. Recent studies have shown that certain cobalt-base inorganic compounds can catalyse hydrogen evolution in electrocatalysis. Thus, the requirement of an extra component (photosensitizer) is mandatory for photocatalysis. Taking advantage of the porosity at MOFs, we combined the Ti-based NH2-MIL-125(Ti) and encapsulated a Co catalyst to give a boost to the photocatalyst design. Our recent studies show that the local structure and the whole MOF scaffold are involved in the reaction, demonstrating the importance of the molecular and spatial arrangement of the Co-species inside the NH2-MIL-125(Ti) pore space.
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