Step-by-Step Improving Photocatalytic Hydrogen Evolution Activity of NH2–UiO-66 by Constructing Heterojunction and Encapsulating Carbon Nanodots

Carbon nanodots (CDs) have attracted enormous attention in the photocatalytic area for their high light-harvesting and outstanding electron transfer abilities. In this work, NH2–UiO-66 was first composited with g-C3N4 to construct an NH2–UiO-66/g-C3N4 heterojunction. Then, CDs were incorporated into the pores of NH2–UiO-66 by the pore space of the framework serving as confined nanoreactors to construct a CD@NH2–UiO-66/g-C3N4 ternary composite. The ultrasmall CDs transformed from incapsulated glucose in the pores of NH2–UiO-66 were uniformally distributed in MOFs and extensively improve the photocatalytic hydrogen evolution activity of the composite under visible-light irradiation. The optimum photocatalytic H2 evolution rate of the CD@NH2–UiO-66/g-C3N4 composite with a CD content of 2.77 wt % is 2.930 mmol·h–1·g–1 under visible-light irradiation, which is 32.4, 38.6, and 17.5 times as high as that of bulk g-C3N4, NH2–UiO-66, and NH2–UiO-66/g-C3N4, respectively. The remarkable enhancement of the photocatalytic activity should be that CDs as cocatalysts effectively increase the transport properties of electrons and efficient charge separation. Moreover, CD@NH–UiO-66/g-C3N4 nanocomposites showed excellent stability during the photocatalytic process as determined by XRD and TEM analyses for the sample after reaction. The results of the mechanism investigation reveal that CDs in the ternary composite serve as electron transfer mediation to facilitate charge separation, enhancing light absorption and extending the lifetime of photoinduced carriers. The present work shows that encapsulating CDs into the pores of MOFs is an efficient strategy to improve the activity of an MOF-based photocatalyst.