Confining Luminescent Nanodots In Zeolites:Preparation And Properties

Inorganic porous materials represented by zeolites usually have well-defined and varisized microporous channels,large specific surface areas,excellent thermal and chemical stability,which are widely applied in adsorption/separation,catalysis and ion exchange areas.In recent years,by adjusting the pore structure,framework composition,crystal morphology,and doping of active species,porous materials(PMs)have shown unprecedented excellent properties in many fields.In addition,confining quantum dots(QDs),organic molecules,metal clusters and other guest nanoparticles with photoelectric activity in the pores of inorganic PMs,will give new properties of composite materials,which is also a research hotspot in recent years.Carbon dots(CDs),as a new type of nanomaterial with size less than 20 nm,have been widely used in anti-counterfeiting,sensing,bioimaging,optoelectronics and energy storage fields.Among them,CDs with long afterglow and red fluorescence that have potential applications in anti-counterfeiting,display lighting and biological imaging are received particular attention.However,most of the reported CDs show excellent luminescence in solution,while exhibit fluorescence quenching under solid state.The recognized mechanism of such phenomena is similar to hydrogen-bonding induced organic fluorescence molecular aggregation.In solid state,CDs will carry out?-?stacking and the large conjugated system will consume the energy of excitation state,resulting in fluorescence quenching without long afterglow emission.To overcome this problem,polymer,inorganic salts,PMs and other matrices have been used to confine CDs and prepare CD-based composite materials.Among various matrices,PMs with the advantage of confined spaces compared to non-porous solid substrates such as polymers or inorganic salts will construct rigid protections for CDs.In addition,the host-guest synergy endows composites with the advantages of both CDs and PMs,providing possibilities for the design of ideal functional composites to meet the application needs in optics,catalysis and energy storage.Metal halide perovskite quantum dots(PQDs)possess fascinating photoelectric properties which have shown great potential in optoelectronic applications.However,their inherent poor stability against humidity,oxygen,light and heat limits their practical application.On the other hand,even though quantum yields(QYs)of all-inorganic Cs Pb Br₃ with green fluorescence and all-inorganic Cs Pb I₃ with red fluorescence have reached nearly 100%,the QYs of Cs Pb Cl₃ with blue fluorescence is less than 5%.Considering the development of three primary colors for balanced high-resolution lighting,exploring all-inorganic blue perovskite with high QYs is of great significance.To achieve this,the PQDs could be encapsulated into porous channels of PMs,which can not only limit their growth,reduce the size and increase the band gap,but also improve their stabilities,forming stable blue luminescent PQDs@PMs composite materials.In this thesis,we use the strategy of"confining quantum dots in porous materials"to confine CDs and PQDs into PMs such as microporous and mesoporous zeolites.Zeolite matrices can not only limit the growth of QDs,but also establish rigid protection to stabilize their structures,promoting their efficient photoluminescence.The host-guest synergy provides new thought for designing a variety of functional materials to be applied in optoelectronic,biological and anti-counterfeiting areas.The main research contents of this thesis are as follows:1.A solvent-free synthesis strategy has been proposed to prepare CDs@zeolite composites with highly efficient thermally activated delayed fluorescence(TADF)and room temperature phosphorescence(RTP)by loading CDs into Al PO-5 matrix.The advantages of solvent-free synthesis method lie in that CDs can be easily embedded into zeolite matrix rather than remaining in high polar solvent in the solvothermal synthesis system,which increases the loading capacity of CDs and product yield,thus improving the luminescence properties of CDs@zeolite composites.In addition,the synthesis procedure is simple,easy to operate and the raw materials are cheap,which facilitate the large-scale preparation of composite materials.CDs@zeolite composites have ultra-high photostability,mechanical stability and oxygen stability,which can be used as phosphors in the field of alternating-current light-emitting diode(AC-LED)supplementary lighting to reduce the harm caused by alternating stroboscopic to human eyes.2.CDs@mesoporous zeolite composite with high-efficient red fluorescence emission has been prepared by combining CDs in mesoporous silica.Firstly,CDs with red fluorescence emission are synthesized under solvothermal condition.The study shows that the as-synthesized CDs have solvent-dependent luminescence effect,with the increase of electron donor groups such as amino in solvent,the emissions of CDs gradually redshift.Through a series of characterization,it is found that there is an interaction between the carboxyl functional groups on the surface of CDs and the electron donor groups such as amino groups in solvent,which leads to the increase of energy density of CDs and the decrease of energy level,resulting in the redshift of luminescence.On the basis of this,mesoporous materials with about 6 nm channels have been prepared,through modifying amino groups in the channels,and confining CDs,CDs@mesoporous zeolite composites with solid red fluorescence emission have been successfully prepared.Such composite has good biocompatibility,which can be used as deep red fluorescent probe in biological imaging.3.A“break-and-repair”synthesis strategy has been proposed.The silicalite-1zeolite doped with tungsten is used as matrix to limit the growth of Cs Pb Br₃ QDs,and the PQDs@zeolite composite with high water stability and blue light emission has been successfully prepared.High temperature evaporation method is used to introduce perovskite precursors into zeolite framework.In this process,due to the small pore size of zeolite,the perovskite precursors will break part of the zeolite pores.In order to prevent the loss of PQDs in zeolite channels,amino acids and silane are employed to repair the broken zeolite channels.By using this strategy,PQDs with adjustable size of2-6 nm can be prepared in zeolite by controlling the reaction conditions.Based on the quantum confinement effect,the composites exhibit variable fluorescence emissions from blue light of 460 nm to green light of 510 nm.Furthermore,such encapsulation prevents the exposure of PQDs to water,promoting stable blue light emission after soaking in aqueous solution for 14 days.