Surface Functionalization Of UiO-66 For Catalysis And Humidity Sensitivity

UiO-66 has drawn much attention in the field of Metal-organic frameworks（MOFs）,and it has been applied in various applications due to its stability and tunability.Cycloaddition of CO₂ takes a dominant position for recycle carbon energy.Thus,the design of catalysts for cycloaddition of CO₂ has received extensive attention.However,only a few applications focus on UiO-66 to catalytic cycloaddition of CO₂.In this paper,we first tune the size of UiO-66 by adjusting the amount of acetic acid（10 eq and 160 eq）and synthesis time（7 h and 24 h）,named as UiO-66-X（X=1,2 3）.The size of UiO-66 grows with increasing acetic as well as increasing synthesis time,which were verified by SEM.When using it as catalysis cycloaddition of CO₂,UiO-66-3 presents best effect,which can reach a highest yield of 92%among them.According to the XRD and PL characterization,the content of defects increases as increasing concentration of acetic acid and synthesis time,indicating interaction between size and defects.Furthermore,TGA results shows that UiO-66-3 lost three linkers exposing uncoordinated metal sites to catalysis reaction,which is controlled by Acetic acid.Aimed at size modulation of UiO-66,bimetallic MOFs x Hf-UiO-66（x=0,0.25,0.5,0.75,1）were successfully formed by doping different ratio of Hf element during the in situ synthesis process.After Hf doping in the UiO-66.XRD and SEM shows that the size and structure remained,and EDS element analysis proved that Hf element exist in the samples.Furthermore,compared with Zr-UiO-66 and Hf-UiO-66,the binding energy of Zr and Hf changed,indicating synergistic effects exists due to charge transfer between Hf-O clusters and Zr-O clusters.0.5Hf-UiO-66 showed highest activity in the solvent-free cycloaddition of CO₂ with epoxides under mild conditions（60^oC,9 h）,in line with the result of PL spectrum.Thus,bimetallic synergy not only enhanced the contents of defect,but also accelerate charge transfer that improve the activity in catalysis cycloaddition of CO₂.Metal-organic frameworks（MOFs）have been proved to be outstanding candidates for humidity sensors,however,pure MOFs are still limited in hysteresis,integration,and intelligence.In this investigation,inspired by a single-site ion strategy for catalysis,UiO-66-NH₂ is first modified by 2,6-DPA（2,6-dipicolinic acid）molecules（named as U-DPA）and then combined by Zn（II）ions（noted as U-DPA-Zn）.Taking advantage of electrolyte and MOF,U-DPA-Zn exhibits significantly enhanced humidity sensing performance than UiO-66-NH₂ and U-DPA.U-DPA-Zn possesses the highest response（S_r=8.07×10³）,the shortest response-recovery time(τ_res=2 s,τ_rec=105 s),the smallest hysteresis（ΔH=0.59%）,as well as good repeatability in a wide relative humidity（RH）range（11-97%）.Thanks to its excellent properties,U-DPA-Zn shows great potential in the field of monitoring human respiration（breath detection）and water evaporation on human skin（finger detection）.This work provides an optional material for humidity sensors.