Core-Shell Structured NH<sub>2</sub>-UiO-66@TiO<sub>2</sub> Photocatalyst for the Degradation of Toluene under Visible Light Irradiation

Metal-organic frameworks (MOFs) are of significant interest for photocatalysis using visible light, but they are typically limited by the instability and high recombination ratio of photoexcited pairs.Integrating MOFs into an inorganic semiconductor is one of the most widespread methods to promote their activity.In this study, a core-shell structured MOF@TiO2 (NH2-UiO-66@TiO2) was synthesized as an efficient photocatalyst for the degradation of toluene.Pristine NH2-UiO-66 was synthesized by a hydrothermal method as the core, which was then coated with an amorphous TiO2 shell.Compared with pristine NH2-UiO-66 and other samples prepared by the direct mixing of NH2-UiO-66 and TiO2, NH2-UiO-66@TiO2 exhibited a higher degradation rate of toluene.Using NH2-UiO-66@TiO2 as a catalyst, the degradation efficiency of toluene reached 76.7% within 3 h, which is 1.48 times higher than that of NH2-UiO-66.The degradation performance was also stable in four repeated reuse experiments, and the slight deactivation was reactivated after washing with ethanol.A series of characterization methods were used to determine the physicochemical properties of NH2-UiO-66@TiO2, including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM).Using the measured physicochemical properties, the photocatalytic mechanism of NH2-UiO-66@TiO2 was explored.NH2-UiO-66 is an ideal photocatalyst, with visible-light response and a huge specific surface area (914.9 m 2 •g -1 ), which is favorable for the utilization of sunlight as well as the absorption of pollutants in indoor air.In addition, a new interface formed between the two components (NH2-UiO-66 and TiO2), which efficiently broaden the light absorption area and enhanced the utilization of photogenerated species.The photogenerated holes and electrons could transfer through the interlayer as soon as they were formed.It is speculated that holes would transfer to the HOMO of NH2-UiO-66, and then combine with H2O molecules to form hydroxyl radicals (•OH).At the same time, more electrons tended to combine with oxygen molecules in the conduction band of TiO2 rather than recombine with holes.Consequently, the recombination rate of electrons and holes decreased, while the quantity of oxygen radicals and hydroxyl radicals increased.Toluene was efficiently oxidized by these two types of radicals.Owing to the outstanding properties mentioned above, the strategy of constructing NH2-UiO-66@TiO2 is considered to be an effective approach.This work may provide new insights into the design of core-shell structured MOF@photocatalysts for the photocatalytic degradation of indoor air pollutants.