Strategies To Enhance The Photocatalytic Performance Of Zr-MOF Via Defect Engineering

Water contaminants,such as heavy metal ions and antibiotics pose a threat to both plants and human beings.Various methods and technologies to treat water contaminants,such as adsorption,filtration,and ion exchange,have been applied,but they have many limitations.Therefore,it is necessary to develop facile and environmental-friendly technologies for contaminants removal.Recently,photocatalysis has emerged as a promising method because of its energy-saving,high-efficient and sustainable features.For photocatalysis,the design of high-performance photocatalyst is the key to promote its wide application.In recent years,UiO-66,one subclass of metal-organic framework(MOF),has aroused much attention because of their high specific surface area,tunable pore size and porous structure.In this paper,we aim to enhance the photocatalytic properties of UiO-66 via defect engineering and compositing it with g-C3N4 layer through chemical vapor deposition method.The modulation of defects in UiO-66 are by changing zirconium sources,reaction conditions,and replacing Zr ions by Ti ions in original frameworks.The contributions of defects to photocatalytic performance(taking the photodegration of Cr(Ⅵ)as model)are studied.The detailed work in this paper is as follows:(1)Zirconium(Zr)precursors and synthetic methods(solvent-assisted or non-solvent assisted)were found to significantly affect the electric charge and the defect sites of UiO-66.UiO-66 with enhanced adsorption and photosensitized photocatalytic ability was then obtained.When ZrOCl2·8H2O was used as Zr precursor in a non-solvent method,defect rich UiO-66(UiO-OCl-N)was achieved with higher Cr(Ⅵ)adsorption and improved photosensitized photocatalytic performance.The positively charged framework with-OH/OH2 pairs deriving from defect sites facilitates the adsorption of anionic Cr(Ⅵ)species.As a result,UiO-66 with Cr(Ⅵ)adsorption become visible light responsive and Cr(Ⅵ)can be further removed by photosensitized photocatalytic reduction under visible light.(2)The effects of Zr precursors and Ti substitution on the photocatalytic performance of UiO-66-NH2 were studied from the perspective of defect engineering and surface chemistry.The findings show that when using ZrOCl2·8H2O as the Zr-precursor combined with the post-synthetic treatment of Ti substitution,sample U-OCl-Ti shows significant improvement in Cr(Ⅵ)adsorption(20.9 mg/g)and photocatalytic reduction of Cr(Ⅵ)under visible light.The improvement can be attributed to the highly defective framework and more-NH2 groups exposed on the surface;therefore the highest light absorption and lowest recombination rate of photo-generated electron-hole pairs are observed in U-OCl-Ti.The findings in this study contribute to a better understanding of how defects and surface chemistry affect the photocatalytic performance,thus providing some designing knowledge for the synthesis of UiO-66 or UiO-66-NH2 with high photocatalytic performance.(3)A chemical vapor deposition(CVD)assisted method is developed to fabricate UiO-66@g-C3N4,where the beneficial features of g-C3N4 and UiO-66 could be well combined and displayed.The resultant composite has an ultrathin g-C3N4 layer coating(-2 nm)and tight interfacial bonding,and exhibits an improved photocatalytic performance for tetracycline degradation as compared to pristine UiO-66 and the composites synthesized by other methods.The CVD synthesized sample can degrade 47.8 ppm tetracycline under 2 h illumination,which is almost twice than that of pristine UiO-66(25.8 ppm).The enhanced performance closely relates to the desirable textural and chemical properties of g-C3N4@UiO-66,all of which benefit from the CVD-assisted method.