Facet-dependent photocatalytic hydrogen production of metal–organic framework NH2-MIL-125(Ti)

Facet-dependent catalytic activity of hard materials such as metals and metal oxides is well recognized in previous works. However, it has rarely been established for metal-organic frameworks (MOFs), possibly because the soft crystals of MOFs are conceptually different from the hard solids. In this work, the surface structure of the MOF NH2-MIL-125(Ti) has been investigated by density functional theory (DFT) calculations for the first time. These calculations predict that the {110} facet has a surface energy of 1.18 J m-2, which is superior to those of the {001}, {100} and {111} facets. This difference can be attributed to the larger percentage of exposed metal clusters, which can act as active sites in catalysis. Thus, we have devised and successfully obtained a series of nanoscaled NH2-MIL-125(Ti) MOFs with controlled facets both experimentally and theoretically. The sample containing the {110} facet exhibits the highest photocatalytic hydrogen production activity and apparent quantum yield, which are approximately three times those of the sample with a dominant {111} facet.