Preparation Of MOF-derived Transition Metal Composites Catalysts And Their Performance In Photo/Electrocatalytic Water Splitting

The shortage of fossil fuels and the environmental problems caused by their use are related to the future development and survival of humanity.To realize the goal of"carbon neutrality",it is urgent to develop the clean and sustainable hydrogen energy.Photo/electrocatalytic water splitting has been regarded as the most promising ways to produce hydrogen,because of their green and easy operation features.Till now,a large number of photo/electro-catalysts have been reported,and the efficiency of water splitting has been significantly improved.However,the water splitting efficiencies obtained from the current photo/electro-catalysts are still unsatisfactory and do not meet the demand for the industrialization.Thus,it remains an active research area and challenge to design and construct more efficient photo/electro-catalysts to further promote the efficiency and reduce the cost of hydrogen production from water splitting.Recently,the materials derivated from metal-organic frameworks（MOFs）have received great attention,because these derivatives can well inherit the merits of MOFs,such as large surface area and high porosity.As well known,large surface area and high porosity not only favor the adsorption and diffusion of reactants,but also can exposure more active sites and shorten the transfer distance of charge carriers.It offers new opportunities to explore more efficient photo/electrocatalyst for water splitting.The specific studies in this thesis are as follows:（1）The poor electrical conductivity of transitional metal oxides limit their application in electrocatalytic system.To address this problem,we obtained Co O/Co by calcining Co（Ac）₂and Zn（Ac）₂in N₂atmosphere.Then,the prepared Co O/Co was converted to ZIF-67/Co by reacting with 2-methylimzdole under solvent-free condition.Finally,the obtained ZIF-67/Co was mixed with Fe SO₄by ball milling to prepare Fe-doped Co O/Co（Fe-Co O/Co）catalyst.As revealed by the electrocatalytic water splitting result,the oxygen evolution reaction（OER）overpotential（η）on Fe-Co O/Co was only 276 m V at current density of 10 m A cm^-2in alkaline condition,which was lower than those on the Fe directly doped ZIF-67（Fe-Co O-Z）and Co O/Co（Fe-Co O/Co-D）.The better performance of Fe-Co O/Co could be attributed to its better conductivity by the introduction of Co,larger surface area and homogeneously dispersed Fe doping.Furthermore,the Fe-Co O/Co also showed superior stability of electrocatalytic water splitting,the OER activity did not change significantly after 10 h reaction.（2）Cd S is a good visible light response semiconductor.However,pure Cd S is prone to undergo photocorrosion and fast recombination of photogenerated carriers.The combination of Cd S with other sulfides can alleviate the photocorrosion and recombination of photogenerated carriers.Accordingly,this work prepared porous Mo S₂/Cd S composites by reaction of Cd-MOF with Mo S₂using low temperature solvothermal sulfurization method.The content of Mo S₂was optimized in this preparation.The optimized Mo S₂/Cd S catalyst exhibited good photocatalytic hydrogen evolution reaction（HER）activity(3318μmol h^-1g^-1)and stability from water splitting.Moreover,it was also efficient for removing the organic pollutant in water,95%of rhodamine B（Rh B）and 90%of tetracycline（TC）was removed within40 min under illumination,and no obvious changes of morphology and crystalline structure were observed after four cycles.The photocatalytic degradation mechanism was investigated by radical trapping and electron paramagnetic resonance spectroscopy experiments.The results confirmed that·O₂^-was mainly attributed to the degradation of Rh B,while h⁺and.O^2-were to the degradation of TC.The excellent photocatalytic activity of Mo S₂/Cd S could be ascribed to its porous structure and intimate heterojunction.The structure improves the separation efficiency of photogenerated carrier and shortens the carrier migration distance.（3）The construction of effective photoanode is crucial for the photoelectrocatalytic（PEC）water splitting.In this work,we firstly prepared porous Cd O（Cd O-M）by calcinating Cd-MOF.Then,Cd S was loaded on the prepared Cd O-M via the successive ionic layer adsorption and reaction（SILAR）method.The content of Cd S on Cd O was optimized,and the obtained Cd S/Cd O-M catalysts were used for photoelectrocatalytic water splitting.The optimum Cd S/Cd O-M exhibited a photocurrent density of 4.9 m A cm^-2,which was 20,6 and 1.5 times higher than that of Cd O-M,Cd S and Cd S/Cd O-C（Cd O-C represents commercial Cd O）,respectively.Meanwhile,the optimum Cd S/Cd O was stable in the PEC water splitting,almost no attenuation of photocurrent after 1 h of continuous illumination test.Moreover,the applied bias photon-to-current conversion efficiency（ABPE）was calculated to be4.5%on the optimum Cd S/Cd O-M.The effective PEC water splitting performance and stability of the catalyst could be ascribed to its highly porous structure and intimate heterojunction.This work provides a feasible strategy for the preparation of effective PEC photoanodes from MOFs.（4）In order to further improve the PEC water splitting activity,this work prepared porous Zn Cd O_xcomposite by calcinating a bimetallic Zn Cd-MOF,and then loading the Cd Se by gas-liquid interface reaction self-assembly method.The prepared Cd Se/Zn Cd O_xshowed excellent PEC water splitting performance,the photocurrent density was as high as 7.8 m A cm^-2,which was 4,2 and 1.5 times higher than that of Cd Se/Zn O,Cd Se/Cd O and Cd Se/Zn Cd O_x-C（Zn Cd O_x-C was prepared by ball milling and calcination method）heterojunction,respectively.Meanwhile,its applied bias photon-to-current conversion efficiency was as high as 7.8%.The highly PEC water splitting efficiency of Cd Se/Zn Cd O_xcould be attributed to its porous double heterojunction structure.