Enhanced Photocatalytic CO₂ Reduction Activity over NH₂-MIL-125(Ti) by Facet Regulation

Abstract

Designation and optimization of facets of photocatalysts is an effective strategy to address the issue of facet-dependent photocatalytic reactions. However, studies regarding the facet effect of metal–organic frameworks (MOFs) on the photocatalytic process are in infancy. In this study, NH2-MIL-125(Ti) with different ratios of {001} and {111} facets was exactly controlled and synthesized, and it was found that the activity in photoreduction of CO2 is enhanced with gradually increasing exposed proportion of {111} facets. The {111} facets exhibit photocatalytic activity with the maximal CO and CH4 yields of 8.25 and 1.01 μmol g–1 h–1, which are 9 and 5 times higher than those of {001} facets, respectively. Also, the {111} facets give the highest quantum yields of 0.14 and 0.07% for CO and CH4 production, respectively. Steady-state and time-resolved fluorescence spectra reveal the importance of inhibiting the recombination of photoinduced electrons and holes for the sample with {111} facets. Besides, TiIII formed during the reaction process exhibits strong reducibility for CO2. Starting from NH2-MIL-125(Ti), the photocatalytic performance can be enhanced by regulating exposed {111} facets. This work not only provides a strategy for further enhancing photocatalytic performance by tuning the exposed active facets of MOFs, but also provides a deep understanding of the factors for improving the photocatalytic reduction of CO2.