Doping Copper Ions in a Metal-Organic Framework (UiO-66-NH₂): Location Effect Examined by Ultrafast Spectroscopy

We constructed two types of copper-doped metal-organic framework (MOF), i.e., Cu@UiO-66-NH _2 and Cu-UiO-66-NH _2 . In the former, Cu ^2+ ions are impregnated in the pore space of the amine-functionalized, Zr-based UiO-66-NH _2 ; while in the latter, Cu ^2+ ions are incorporated to form a bimetal-center MOF, with Zr ^4+ being partially replaced by Cu ^2+ in the Zr - O oxo-clusters. Ultrafast spectroscopy revealed that the photoinduced relaxation kinetics associated with the ligand-to-cluster charge-transfer state is promoted for both Cu-doped MOFs relative to undoped one, but in a sequence of Cu-UiO-66-NH _2 > Cu@UiO-66-NH _2 > UiO-66-NH _2 . Such a sequence turned to be in line with the trend observed in the visible-light photocatalytic hydrogen evolution activity tests on the three MOFs. These findings highlighted the subtle effect of copper-doping location in this Zr-based MOF system, further suggesting that rational engineering of the specific metal-doping location in alike MOF systems to promote the photoinduced charge separation and hence suppress the detrimental charge recombination therein is beneficial for achieving improved performances in MOF-based photocatalysis.