Preparations And Photocatalytic Properties Of Ni-based Compound/semiconductor Composites

To solve the problems of gas fuel acquisition,environmental purification and organic synthetic transformation,the use of semiconductor photocatalysis technology is considered to be a more promising technical solution.As a renewable gas fuel with high energy density and ecological friendliness,hydrogen has attracted wide research interest.In the presence of semiconductor materials,light-driven water-splitting has become one of the crucial methods for obtaining high-purity hydrogen.Moreover,photocatalytic organic synthesis reactions hold a great promise for the production of high value-added compounds,as they can be performed by utilizing economical and clean solar energy under mild conditions.The catalytic efficiency of traditional semiconductor photocatalysts is often restricted by low carrier separation efficiency,few active sites,and weak light absorption capacity.Surface modification of photocatalysts is an important approach to solve these problems.As a groupⅧtransition metal,Ni bears unique electronic activity and multiple stable oxidation states（Ni（0）,Ni（I）,Ni（Ⅱ）,Ni（Ⅲ）,and Ni（Ⅳ））.The ligand scaffold can coordinate with the Ni center to form many complexes and clusters with different structures,which display potential as excellent catalysts and cocatalysts.When they are modified onto the surfaces of semiconductor nanomaterials,composite photocatalysts with high catalytic activity will be obtained.Therefore,in this thesis,we successfully constructed several composite photocatalysts by modifying Ni-based complexes or cluster molecules with different coordination environments onto the surfaces of traditional semiconductor photocatalytic materials such as g-C₃N₄,Cd S,and NH₂-MIL-125（Ti）.Owing to the intrinsic characteristics of Ni-based molecules,the photocatalytic activity of semiconductor catalysts in the processes of photocatalytic hydrogen evolution,nitroaromatic reduction and oxidation of aromatic alcohols was promoted.The main research contents are shown as follows:Using a facile room-temperature mechanical mixing method,atomically precise Ni₆（SC₂H₄Ph）₁₂ nanoclusters（Ni₆ NCs）supported on g-C₃N₄nanosheets were constructed to enhance the photocatalytic hydrogen evolution performances.Zeta potential experiments demonstrated that the electrostatic interaction between Ni₆ NCs and g-C₃N₄ led to the formation of Ni₆/g-C₃N₄ hybrid photocatalyst.Photocatalytic measurements showed that 5%-Ni₆/g-C₃N₄prepared from the system with the original Ni₆/g-C₃N₄ mass ratio of 1/20 exhibited high hydrogen production activity(5.87 mmol·h^-1·g^-1)with triethanolamine as the sacrifice agent under visible light irradiation（without the assistance of Pt）,which was about 290 times higher than that of pure g-C₃N₄.Density functional theory calculations indicated that with the loading of Ni₆ NCs,the N sites in the surface of g-C₃N₄ became active toward hydrogen evolution reaction,in which the photogenerated electrons were transferred from Ni₆ NCs to g-C₃N₄ under light irradiation.The present work provides robust evidences that atomically precise non-noble metal nanoclusters can significantly boost the photocatalytic hydrogen evolution performances of the semiconductor photocatalyst.A grinding-ultrasonic route was designed to construct nickel bis（chelate）complex（Ni（abt）₂,abt=2-aminobenzenethiolate）modified carbon nitride ultrathin nanosheets.During the grinding and ultrasonic process,Ni（abt）₂ was implanted into the interlamination of bulk carbon nitride.Under the assistance of the shear force,bulk carbon nitride was exfoliated,resulting in the formation of ultrathin carbon nitride（UCN）nanosheets.Simultaneously,Ni（abt）₂ molecules were anchored on the surfaces of as-formed UCN nanosheets due to theπ-πstacking interaction,and the Ni（abt）₂/UCN hybrids were formed.Compared with single Ni（abt）₂ and UCN,the as-obtained Ni（abt）₂/UCN nanosheets exhibited excellent photocatalytic hydrogen evolution capability.A molecule-semiconductor internal electron transmission mechanism was suggested for explaining the separation and transfer of electron-hole pairs.Density functional theory calculations demonstrated that the interface-induced electron redistribution tuned the electron density and hydrogen adsorption of the active centers,thus enhancing the photocatalytic performance of the hybrid catalyst.In addition,the as-obtained Ni（abt）₂/UCN nanosheets could also catalyze the reduction of nitroaromatics in the presence of Na BH₄.Under simulated sunlight irradiation,in 20 min,the conversion efficiency of 4-nitroaniline to 1,4-phenylenediamine was up to 97.3%,far higher than that under the condition without light irradiation（51.7%）.Finally,the possible mechanism of photoexcited electron participate in the reduction of nitroaromatic compounds was proposed.A conventional amidation reaction route was adopted to successfully construct a novel Ni tripyridine complex（Ni（terpy C）₂）-grafted Cd S nanorods composite photocatalyst.It was found that the as-constructed composite photocatalyst owned the outstanding acticvity for the dehydrogenation of benzyl alcohol under visible light with higher hydrogen evolution efficiency and higher conversion efficiency and selectivity of benzaldehyde.The photocatalytic dehydrogenation process of benzyl alcohol and the reasons for the preparation of benzaldehyde and hydrogen were investigated by capture experiment and EPR test.The study of carrier dynamics by femtosecond transient absorption spectroscopy demonstrated that the Ni（terpy C）₂on the surface of Cd S could quickly extract the photo-generated electrons of Cd S.The carrier recombination efficiency was significantly reduced.This work reveals the effect of surface active-site engineering on the photocatalysis and is expected to shed substantial inspiration on future surface modulation and design of Cd S-based semiconductor photocatalysts.Based on the different decomposition temperatures between Ni₆（PET）₁₂ clusters and NH₂-MIL-125（Ti）,Ni₆（PET）₁₂ clusters loaded on NH₂-MIL-125（Ti）were decomposed by heating in N₂ atmosphere to successfully prepare ultrasmall Ni S_1+x modified NH₂-MIL-125（Ti）composite photocatalyst,by an accurate temperature-controlled calcination procedure.When the as-prepared composite catalyst was applied to aerobic oxidation of aromatic alcohol,it showed excellent catalytic activity and selectivity to aromatic aldehyde than other catalysts.At the conversion rate of 100%,the selectivity of benzaldehyde reached 98.1%.Further investigations uncovered that Ni S_1+x acted as the co-catalyst,which significantly enhanced the separation and migration of photogenerated carriers and the light absorption ability of the NH₂-MIL-125（Ti）.It was found that superoxide radical and hole were the actual active species during the photocatalytic oxidation of benzyl alcohol,simultaneously,that the reasonable band gap width and band edge position of the composite materials restrained the excessive oxidation of aromatic alcohol.