Controlled Synthesis Of Zr-MOFs And Investigation On Their Photocatalytic Properties And Mechanisms

Metal-organic frameworks(MOFs)as a relatively new class of porous crystalline materials are constructed from metal ions/clusters and multi-topic organic struts linked through coordination bonds.Given the flexible and diversified coordination modes,the combination of the same organic ligand and metal ion may afford a variety of MOFs with distinct structures under similar or even identical reaction conditions.This severely hampers the application of the obtained MOFs because even structurally similar MOFs have different properties.Although this phenomenon is common in the synthesis of MOFs,there has no a general solution to solve yet.Therefore,it is imperative to develop an effective approach to separate the mixed MOFs that are formed in a one-pot MOF synthesis.On the other hand,based on previous studies,metal clusters in MOFs can behave as inorganic semiconductor quantum entities,and the organic linkers could serve as antennas to activate these metal clusters upon photoexcitation.Compared with other photocatalytic materials,MOFs might be ideal photocatalysts as they have the advantages of ultra-high specific surface area,porosity,adjustability,and high CO2 adsorption capacity.As an important branch of MOFs,Zr-MOFs are promising for practical applications due to their high stability.This dissertation aims to investigate the controlled synthesis of Zr-MOFs and investigations on their photocatalysis and related mechanisms.In this dissertation,we reported a more efficient and rapid method for the synthesis of phase-pure Zr-MOFs,and investigated their photocatalytic application and related mechanism of Zr-MOFs and their composites.These findings provide important insights into rational synthesis of Zr-MOFs and the design of efficient photocatalysts over MOFs.This dissertation mainly includes the following aspects:1.Even structurally similar MOFs have different physical/chemical properties,so the synthesis of pure-phase MOFs is particularly important.We found that,a small amount of pure-phase target MOF as seed,being introduced into the reaction system in advance,the separation of diverse MOF mixtures and synthesis of phase-pure MOFs can be successfully achieved.This method is not only applicable to the Zr-porphyrin MOFs,but also extendable to the Cd-MOFs with different pore sizes,indicating the possible generality of this strategy.The experimental results also show that this method is also efficient and rapid in the synthesis of MOFs.This research work will help to further explore the performance of MOFs and may also inspire the synthesis of other crystalline materials.2.Given the weak CO2 sorption of the common semiconductors that is unfavorable to CO2 photoreduction,we proposed a strategy toward the conversion of photoreduction of CO2 to useful chemicals using broad-spectrum light-absorbing MOFs,which can effectively enrich CO2 molecules.We have employed a very stable mesoporous zirconium-porphyrin MOF,PCN-222(also called MOF-545)based on tetrakis(4-carboxyphenyl)porphyrin(H2TCPP),for the integration of CO2 capture and CO2 reduction under visible-light irradiation in the presence of triethanolamine(TEOA)as an sacrificial agent.Results have shown that PCN-222 exhibits much better activity than that of the H2TCPP ligand(molecular catalyst).In addition,we have investigated the ultrafast transient and steady-state/transient fluorescence spectral data and found that the emergence of an extremely long-lived electron trap state in PCN-222 brings about substantial suppression of the detrimental electron-hole recombination,thereby boosting the efficiency of the CO2 photoreduction.This reveals the relationship between the photocatalytic conversion efficiency of the Zr-MOF and the photoelectron-hole separation efficiency.This research work not only enables us deeper understanding of the mechanism of photogenerated carriers in the photocatalytic process of MOFs,but also would stimulate further studies toward developing more efficient MOFs photocatalysts.3.Cadmium sulfide(i.e.,CdS)is one of the most promising photocatalysts,and has been intensively studied due to its visible light response and appropriately positioned conduction band.Unfortunately,the fast recombination of photogenerated electron-hole(e-h)pairs and the limited active sites on surface restrict the application of bulk CdS particles.Meanwhile,most of MOFs suffer from light response in the high-energy spectral range only,such as UV and part of visible light.For these considerations,CdS-decorated Zr-MOF composites,namely CdS/UiO-66,have been synthesized with very small CdS nanoparticles and exhibit high photocatalytic H2 production activity from water splitting,far surpassing UiO-66 and CdS counterparts,under visible light irradiation.Moreover,ultrafast transient absorption results unambiguously demonstrate that,the emergence of a long-lived charge separated state in CdS/UiO-66 causes substantial suppression of the detrimental e-h recombination,thereby boosting the efficiency of photocatalytic H2 production.This work will not only enable our deeper understanding of electron transfer in semiconductor-MOF composites,but also stimulate intensive research toward the development of advanced composite materials for efficient photo catalysis.