Mechanism Insight Into Exciton Manipulations Over Zr-based Metal-Organic Framework Materials And Its Photocatalytic Performance

Photocatalysis can use the solar light as input energy to drive the photocatalyst for ROS generation.It has been considered as a green advanced oxidation-reduction technique for wastewater treatment due to its simple operation,low energy consumption,strong oxidative ability that can avoid secondary pollution.Zr based metal organic frameworks（Zr-MOFs）with the advantages of large surface area,high porosity and tunable structure have shown its promise as photocatalyst for environmental purification.Generally,the organic pollutants or bacteria inactivation are ROS mediated oxidative process,and the key to achieve high photocatalytic performance lies in the generation of large amount of ROS.However,the ROS generation activity is unsatisfactory due to the long time overlooked strong Coulomb interactions between the photo-induced electron and hole which leads to the fast electron-hole separation.Thus,the exploration of new strategy by taking the strong Coulomb interactions into consideration for further improving the catalytic performance of Zr-MOFs is urgently needed and still remains an enormous challenge.In this work,two kinds of novel exciton manipulation strategies over Zr based MOFs are developed to adjust the oxygen activation pathways for high-efficient and selective radical generation.“1)heterojunctions construction for exciton dissociation to improve the electron-hole separation efficiency.2)heavy atom modification to adjust the spin-orbital coupling of Zr based MOFs,boosting the exciton transformation from its singlet for to triplet state”.The main content and results are shown bellow:（1）PCN-224/g-C₃N₄ heterojunction was successfully prepared by solvothermal method.The electron-hole separation efficiency is greatly improved by using the heterojunction to drive the exciton interface cracking.HRTEM results showed that the interface of the heterojunction was closely connected,indicating that the PCN-224/g-C₃N₄ heterojunction was successfully constructed.Uv-vis and PL results showed that the construction of the heterojunction could enhance the photoabsorption intensity of the photocatalyst,and improve the separation efficiency of electron and hole.The PCN-224/g-C₃N₄-4 exhibited the best photoactivity,and the pseudo-first-order kinetic constant for phenol is 0.337 h^-1,which is 10.7 times higher than g-C₃N₄.In addition,PCN-224/g-C₃N₄ heterojunction also showed excellent performance in photocatalytic E-coli inactivation.The E-coli could be inactivated completely at 3 h,when the initial concentration of E-coli was 2×10⁷cfu/m L.The photocatalytic degradation efficiency of phenol did not decrease obviously after 4 runs,indicating the good stability of PCN-224/g-C₃N₄.The capture experiment of active species demonstrated that O₂·^-and h⁺played the major roles in photocatalytic degradation phenol.（2）PCN-224-Zn was prepared by solvothermal synthesis.Fluorescence phosphorescence spectrum results showed that the introduction of Zn into MOF can promote the transformation of electron-hole couple from singlet to triplet state,leading to the selective generation of ¹O₂ without the need of electron-hole separation.Since PCN-224-Zn was capable of O₂ activation without the need of charge separation,PCN-224-Zn exhibited efficient reactive oxygen radical production,making it showed much higher bacteria inactivation activity and organic pollutant degradation performance.The inactivation activity of PCN-224-Zn against E-coli was 3.5 times higher than pure PCN-224.Furthermore,the bacteria inactivation results showed that the generated ¹O₂ could fervently attack the proteins and phospholipids of the cell membrane,creating irreparable pores on the cell membrane to facilitate the leakage of cytoplasmic contents out of the cell,accelerating the bacterial cell destruction.（3）Au nanoparticles（Au NPs）were synthesized by reduction of chloroauricacid with trisodium citrate,and then loaded on PCN-224-Zn.The Au NPs/PCN-224-Zn photocatalyst with surface plasmon resonance was prepared.TEM was used to confirm the successful synthesis of Au NPs and Au NPs/PCN-224-Zn.The introduction of Au NPs not only improves the light adsorption property of PCN-224-Zn,but more importantly the Au NPs induced plasmon resonance under light irradiation could facilitate the transformation of exciton from singlet into triplet form.The ROS generation of Au NPs/PCN-224-Zn was significantly higher than PCN-224-Zn.Compared with pure PCN-224-Zn,the photocatalytic degradation of phenol by Au NPs/PCN-224-Zn was improved by 138%.In addition,the Au NPs/PCN-224-Zn was not deactivated after 5 cycles of degradation,indicating high stability of photocatalyst.Free quenching experiments showed that the main reactive oxygen species（ROS）of Au NPs/PCN-224-Zn in the photocatalytic degradation of phenol was ¹O₂.（4）A ternary synergistic photocatalyst H₃PW₁₂O₄₀/Pt/Ui O-66-NH₂was successfully fabricated for the first time through a simple solvothermal synthetic strategy.The prepared photocatalyst was characterized by XRD,TEM,et al,and used to reduce toxic chromium（Ⅵ）to low toxic chromium（Ⅲ）.The as-prepared H₃PW₁₂O₄₀/Pt/Ui O-66-NH₂ showed excellent photocatalytic reduction performance,the Cr（Ⅵ）with a concentration of 10 mg/L could be totally reduced into Cr（Ⅲ）under visible light for 30 min.The heterojunction of H₃PW₁₂O₄₀/Pt/Ui O-66-NH₂facilitated the separation of electrons and holes,H₃PW₁₂O₄₀ could offer bonded H⁺to improve the oxidation power of Cr（Ⅵ）,meanwhile the introduction of Pt would enhance the charge separation to provide abundant electrons,resulting in showing an effect to promote the fast Cr（Ⅵ）reduction to Cr（Ⅲ）under original p H conduction.