The Study Of Synergistic Effect For Photocatalytic Hydrogen Evolution From Water Splitting Based On Polyoxo-Titanium Clusters Coupling With Sulfide And Metal-Organic Framework MIL-101 Composite Materials

The technology of photocatalytic water splitting to produce hydrogen using solar energy has been considered as one of the most important approaches to solve the energy and environmental crises faced by mankind.However,the currently developed catalysts without precious metals as co-catalysts cannot meet the needs of practical applications because of their poor hydrogen production activity.On the basis of numerous experimental results,several approaches have been adopted toimprove photocatalytic properties of photocatalysts:（1）enhancing the utilization of solar energy and improving the light absorption efficiency;（2）increasing the active sites of the catalytic reaction;（3）promoting the transfer of photo-generated electrons and holes.In this paper,ZnIn₂S₄ and CdS,which have visible light absorption activity,were selected as the main catalysts.ZnIn₂S₄ were modified by La or Ni doping,CdS were modified by combing with NiS,forming Cd_xZn_1-xS or CdIn₂S₄ solid solutions for increasing the carrier transmission or light absorption;then,the modified main catalysts were loaded on the metal-organic framework MIL-101 by in-situ composite method for promoting the creation of active sites;finally,a series of composite photocatalysts were prepared by coupling polyoxo-titanium clusters（PTCs）with sulfide and MIL-101.In this paper,the structure-activity relationship of the catalyst is also studied,and the mechanism of catalytic activity improvement of the composite photocatalyst is reasonably inferred.The main research works are summarized as follows:（1）ZnIn₂S₄ combined with MIL-101 by one-step hydrothermal method to prepare ZnIn₂S₄/MIL-101,and then three different wide band gap PTCs[Ti₆O₄（O_iPr）₁₀（O₃P-Phen）₂（L）₂]（the polyoxo-titanium cluster is abbreviated as PTC-3,PTC-7 and PTC-9when L is isonicotinic acid,benzoic acid and bromoacetic acid,respectively）were compounded with ZnIn₂S₄/MIL-101 by solvothermal method to form ZnIn₂S₄/MIL-101/PTC-3,ZnIn₂S₄/MIL-101/PTC-7 and ZnIn₂S₄/MIL-101/PTC-9（collectively referred to as ZIS/MIL-101/PTCs）ternary composite photocatalysts.In this catalytic system,the presence of MIL-101 can reduce the size of the main catalyst ZnIn₂S₄ and increase the specific surface area of catalysts;PTCs have the dual function of tuning the energy gap of the catalyst for efficient visible light harvesting and enhancing the transfer of carriers.La-ZnIn₂S₄/MIL-101/PTCs were prepared by doping La into ZnIn₂S₄.In these photocatalytic systems,PTCs also exhibit dual functions,which can be confirmed by UV-Vis and electrochemical performance characterization.（2）MIL-101 was combined with Ni_1.0%-ZnIn₂S₄ by hydrothermal method to prepare Ni_1.0%-ZnIn₂S₄/MIL-101.Then,Ni_1.0%-ZnIn₂S₄/MIL-101 were compounded with two different PTCs[Ti₆O₄（O_iPr）₁₀（O₃P-Phen）₂（L）₂]（the polyoxo-titanium cluster is abbreviated as PTC-21 and PTC-22 when L is 3-pyridinesulfonic acid and 2-pyrazinecarboxylic acid,respectively）by solvothermal method to form Ni_1.0%-ZIS/MIL-101/PTCs ternary composite photocatalysts.The ternary composite photocatalysts have the following advantages:the doping of Ni in ZnIn₂S₄ can enhance the electron transfer and light absorption during the photocatalytic process;the existence of MIL-101 can reduce the size of Ni_1.0%-ZnIn₂S₄ and transforms it from flower-like multilevel structure to single layer nanosheet structure,exposing more active sites;the suitable band position of PTC promotes fast migration of the electrons from Ni_1.0%-ZnIn₂S₄ to PTC.In the absence of noble-metal co-catalyst,the photocatalytic hydrogen production rate of Ni_1.0%-ZIS/MIL-101/PTCs under visible light irradiation is significantly higher than that of Ni_1.0%-ZnIn₂S₄ and ZnIn₂S₄.（3）NiS and CdS were compounded by simple hydrothermal method and loaded onMIL-101 to prepare NiS/CdS/MIL-101 ternary catalyst,the optimal amount of NiS was investigated.When NiS in NiS/CdS/MIL-101 is about 5.75 wt%,the hydrogen production rate is the highest,up to 1.3 mmol g^-1_CdS h^-1,which is about 18 times and14 times higher than that of CdS/MIL-101 and NiS/CdS,respectively.UV-vis diffuse reflectance spectra and electrochemical tests show that the photocatalytic rate of NiS/CdS/MIL-101 is significantly higher than that of CdS/MIL-101,which is attributed to the fact that the existence of NiS enhances the light absorption of photocatalyst and prevents the rapid recombination of photogenerated electron-hole pairs in CdS.Scanning electron microscopy results show that the large specific surface area of MIL-101 can inhibit the aggregation of NiS/CdS and promote the reactive sites in the photocatalytic process.On the basis of the above studies,NiS/CdS/MIL-101 was compounded with zero-dimensional PTC[Ti₆O₄（O_iPr）₁₀（O₃P-Phen）₂（NA）₂]（abbreviated as PTC-20）and three-dimensional PTC[Ti₆O₄（O_iPr）₁₀（O₃P-Phen）₂][Cu₄^Ⅱ₄（NA）₂]（abbreviated as PTC-34）respectively,to form NiS/CdS/MIL-101/PTC-20 and NiS/CdS/MIL-101/PTC-34.The hydrogen production rate of NiS/CdS/MIL-101/PTC-20 and NiS/CdS/MIL-101/PTC-34 is 5.1 and 16.5 mmol g^-_CdS¹ h^-1,respectively.The Cu₄I₄ units bridge the{Ti₆P₂}clusters in the zero-dimensional structure of PTC-20 to form the three-dimensional structure of PTC-34.The evolution from zero-dimensional to three-dimensional structure not only increases the absorbtion of visible light,but also improves the separation of carriers.It shows that the dimensional structure of titanium oxide clusters can be tuned at the molecular level to increase and regulate the rate of hydrogen production under visible light.（4）Cd_xZn_1-xS/MIL-101 was prepared by one-step hydrothermal method.Then the narrow band gap PTC C₇₈H₈₉Ni O₃₄Ti₇（Ni-Toc）was coupled with Cd_0.6Zn_0.4S/MIL-101 by solvothermal method to prepare Cd_0.6Zn_0.4S/MIL-101/Ni-Toc.The Cd_0.6Zn_0.4S in the composite catalyst is transformed from micron-sized pagoda cauliflower-like to nano-particle.The particle size of Cd_0.6Zn_0.4S is reduced to 25 nm,this can greatly shorten the transmission distance of photogenerated carriers.The UV-Vis and Mott-Schottky tests confirm that Cd_0.6Zn_0.4S and Ni-Toc formed a type Ⅱ heterojunction.The transient photocurrent response and electrochemical impedance spectroscopy results prove that the formation of heterojunction can promote the separation of photogenerated electron and hole pairs.Without the presence of precious metals and under visible light,the hydrogen production rate of Cd_0.6Zn_0.4S/MIL-101/Ni-Toc reaches 46.9 mmol g^-1 h^-1,which is 13 times higher than that of Cd_0.6Zn_0.4S/MIL-101.Finally,through further electrochemical experiments,the possible electron transfer model and photocatalytic mechanism of Cd_0.6Zn_0.4S/MIL-101/Ni-Toc are constructed.（5）By in-situ self-assembly method,the narrow band gap polyoxo-titanium cluster-copper bromide supersalt[Ti₁₂（μ₃-O）₁₄（O_iPr）₁₈][Cu₄^IBr₆](abbreviated as Ti₁₂-Cu₄Br₆),narrow band gap CdIn₂S₄ and MIL-101 were compounded to form CdIn₂S₄/MIL-101/Ti₁₂-Cu₄Br₆ ternary catalyst.The measurements of XRD,SEM,TEM,XPS,UV-VisDRS,transient photocurrent response,electrochemical impedance spectroscopy and Mott-Schottky tests of all samples were carried out for clarifying the relationship between the structure and performance of catalysts.In and MIL-101 in CdIn₂S₄/MIL-101/Ti₁₂-Cu₄Br₆ make CdIn₂S₄ evolve into nanorod structure,which shorten the electron transmission path;under visible light irradiation,the active components CdIn₂S₄ and Ti₁₂-Cu₄Br₆ can simultaneously produce photogenerated electrons,increasing the number of charge carriers,and CdIn₂S₄ and Ti₁₂-Cu₄Br₆ with appropriate band position can form type Ⅱ heterojunction,which can enhance the separation and transfer of charge carriers.The catalyst is economical which does not use precious metals as co-catalysts in the catalytic reaction process.