Preparation And Properties Of Rare Earth Upconversion Luminescent Nanomaterials And Novel Semiconductor Nanocomposite Photocatalysts

Recently,lanthanide-doped upconversion nanoparticles(UCNPs)have attracted many attentions.Benefitting from the distinguished merits of UCNPs,they have shown potential applications in various fields,such as solar cells,cellular labeling,biological imaging,and photodynamic therapy.Despite these advantages,there exist a number of daunting challengers that may restraint the wide applications of the UCNPs.One major limitation is their low conversion efficiency due to the extremely weak absorption of the sensitizer Yb3+in the NIR region.Moreover,the most commonly used 980 nm NIR laser as an excitation source may lead to a limited penetration ability for deep-tissue imaging,and overhearing issues which result in cell and tissue damage.In order to overcome these problems,in this study,we designed and prepared the UCNPs with quenching shield sandwich structure.Through a method of organic dyes and Nd3+ co-sensitizing to realize a significant enhanced upconversion luminescence under 808 nm excitation.On the other hand,the continued deterioration of global environment and energy crisis have become serious problems that have attracted great attention all over the world.Semiconductor-based photocatalysis is a potential strategy to solve the problems through photocatalytic water splitting to produce clean energy hydrogen and degradation of organic pollutants using solar energy.However,the most widely used and representative semiconductor photocatalysts,such as TiO2 and ZnO,can only respond to UV light with low quantum efficiency under solar light irradiation,which greatly limits their practical applications.Therefore,developing new photocatalysts with low toxicity,good stability,and high activity has become a hot issue in the field of photocatalysis.In this study,we designed and prepared several new g-C3N4-based nanocomposite photocatalysts and polymetallic sulfide photocatalysts,and investigated their performances with various means.Their photocatalytic activities were evaluated by hydrogen evolution and degradation of organic pollutants.The possible mechanisms of the enhanced photocatalytic activity were proposed.These works provide valuable information for the development of new UCNPs nanomaterials and high efficient semiconductor photocatalysts.The main contents of this thesis are as follows:(1)First,we designed and prepared Nd3+-sensitized quenching-shield sandwich structural UCNPs of NaLuF4:Gd,Yb,Tm@NaLuF4:Gd,Yb@NaNdF4:Yb.There exists an intermediate layer between the core and outermost shell served as a shield that spatially isolated the Nd3+ from the activator in the core,and consequently prevented the cross-relaxation-induced UCL quenching effects between Nd3+ and Tm3+,while the efficient excitation energy transfer from Nd3+ to activator could still be achieved through the Yb-mediated core-shell-shell interface.The emission intensity of the quenching-shield sandwich structure upon 808 nm excitation reaches a maximum by optimizing the thickness of the interlayer(5 nm).It is even brighter than that of conventional 980 nm-excited UCNPs.(2)We designed and prepared Nd3+and IR-820 dye sensitized quenching-shield sandwich structural UCNPs.The dye molecules anchored on the surface of the UCNPs serve as "antenna molecules" which can broadly and strongly harvest the NIR light and subsequently efficiently transfer excitation energy to the Nd3+.UCNPs containing quenching shield sandwich structure can effectively eliminate the cross-relaxation between activator and sensitizer.The Nd3+ incorporated in the outermost shell of the UCNPs can efficiently sensitize UCL of the UCNPs under around 808 nm excitation.The results showed that the UCL intensity under 808 nm excitation of IR-820 dye-sensitized UCNPs was about 6 times higher than that of the UCNPs without the dye.Moreover,the intensity of UCL from the IR-820 dye-sensitized UCNPs excited at 808 nm was about 10 times higher than that of UCNPs excited at 980 nm using the same laser power.This strategy can not only improve the light absorption and consequently enhance the UCL of UCNPs,but also increase the light penetration depth for biological tissue and avoid the thermal effect via using 808 nm laser instead of 980 nm laser.(3)The UCNPs@SiO2@Ag/g-C3N4 composite photocatalysts with different Ag content were successfully designed and synthesized by subtly integrating UCNPs,Ag NPs and g-C3N4 nanosheets into a single nanoarchitecture.Due to the synergistic enhancement effect by the UCNPs and silver nanoparticles,the spectral response range is broadened and the rapid separation of photoinduced electron-hole pairs is promoted.The experimental results showed that the rates for degradation of RhB and hydrogen evolution were about 10 and 12 times higher than that of pristine g-C3N4.respectively.This study presents a facile strategy for the simultaneous modification of semiconductor photocatalysts with UCNPs and plasma.(4)The Co9S8 dodecahedron was obtained via deriving from metal-organic frameworks(MOFs)ZIF-67,and then a new hetero-structured nanocomposite Co9S8@ZnAgInS was prepared by growing ZnAgInS nanosheets on the surface of Co9S8.The Coo9S8@ZnAgInS nanocomposites with different Co9Sg contents were investigated.The Co9S8@ZnAgInS showed significantly higher photocatalytic activity than that of pristine components.The optimal Co9S8 content was determined to be 5%and the corresponding H2 production rate was 9395.3 ?mol g-1 h-1,which was nearly 4.3 times higher than that of pure ZnAgInS.The enhanced photocatalytic activity of Co9S8@ZnAgInS composite can be attributed to the heterojunction with hollow architecture Co9S8 and surface-grown nanosheet ZnAgInS,which promote the separation and transfer of electron-hole pairs,provide large specific surface area and a large number of active sites.This study presents a new strategy for the design and preparation of high-efficiency nanocomposite photocatalysts with MOFs materials.(5)New quantum dots(QDs)ZAGS with silver doping was synthesized by a facile hydrothermal method.The effects of silver content on the properties of ZAGS QDs and the photocatalytic activity of RhB degradation were studied.The results showed that the increase of silver content could improve the visible light utilization rate for ZAGS QDs.When the molar ratio of Zn:Ag:Ga was 1:0.2:2,it showed the highest photocatalytic activity,and further increasing the amount of silver resulted in decrease of photocatalytic activity.This study provides valuable data for the development of quaternary metal sulfides for highly efficient photocatalysis.