Preparation And Properties Of The Visible-light-active Photocatalysts Based On Layered Double Hydroxides

As a new emerging technology in energy conversion and environmental purification,photocatalysis is cost-effective and environmental-friendly in realizing the conversion of solar energy to chemical energy based on the clean and inexhaustible sunlight.Up to now,many kinds of the semiconductor materials are developed and applied as photocatalysts,and most of them have powder-like structure for achieving large specific surface area and rich active sites,which also leads to agglomeration and deactivation in water.In addition,the used photocatalysts should be separated by mechanical processing methods such as filtration and centrifugation for further recycling.Therefore,most attentions are paid on the design and fabrication of the semiconductor-based photocatalysts with high activity,stable structure,excellent photocatalytic performance and easily recovered for reuse.Based on the tunability of layered double hydroxides(LDHs),visible-light-active LDHs can be obtained by changing the types of metal cations in it,which greatly enhances the light absorption and utilization rate of the LDHs-based photocatalysts.Meanwhile,its unique layered structure provides a large specific surface area,so that the active sites are fully exposed.Moreover,it was reported that the calcination of LDHs at an appropriate temperature leads it be converted into mixed metal oxides(MMOs),which have much stronger visible-light absorption than LDHs.In order to construct heterojunctions,LDHs and MMOs are utilized in the photocatalytic oxidation or reduction process for harmless removal of pollutants in water driven by visible light irradiation.In addition,the recovery and facile reuse of the photocatalysts could be solved by loading semiconductor heterojunction on the surface of 3D porous conductive Ni foam or utilizing the magnetism of the composite materials themselves.(1)A visible-light-active Ni foam@ZnO@ZnFe-LDH composite photocatalyst is fabricated with 2D/2D interconnected nanosheets of ZnO and ZnFe layered double hydroxide(ZnFe-LDH)growing on the Ni foam.Firstly,the ZnO nanosheets were vertically grown on the surface of Ni foam to obtain Ni foam@ZnO by electrodeposition.Thereafter,the visible-light-active ZnFe-LDH nanosheets were coated on ZnO nanosheets by hydrothermal method.The 3D porous Ni foam with good electrical conductivity is used as the substrate of visible-light active composite photocatalyst,so that Ni foam@ZnO@ZnFe-LDH can be directly used as photoanode in photoelectrocatalytic(PEC)process.Under the action of vislble light and applied voltage(0.5 V),the dyes(Acid Red 1 as model,10 mg/L)and hexavalent chromium ions(Cr(VI),10 mg/L)in water can be simultaneously removed by oxidation reaction on the anode and reduction reaction on the cathode within 210 min,respectively.In addition,the composite photocatalyst showed outstanding stability after five PEC process.(2)It was reported that the built-in electric field of the p-n heterojunction is widely used to promote the separation of photogenerated carriers.Herein,a visible-light active Ni foam@NiFe-LDH/Co3O4 composite was successfully prepared by loading 1D Co3O4 nanowires on the surface of 2D NiFe-LDH nanosheets to construct a p-n heterojunction supported on 3D porous Ni foam through a simple and low-cost two-step hydrothermal method for PEC process.Owning to the p-n heterojunction formed between NiFe-LDH nanosheets and Co3O4 nanowires,the visible-light absorption of the composite was increased greatly and the recombination of photoexcited electron-hole pairs were reduced effectively.Through an effective three-electrode PEC system,the organic pollutants(BPA as model,10 mg/L)and Cr(VI)(10 mg/L)in water can be removed simultaneously within 120 min under the action of visible light and a low voltage(0.7 V).Furthermore,the convenient recyclability and excellent stability of the as-prepared composite indicate a great potential in the field of environmental purification.(3)Hollow nanostructures can provide photocatalysts with large surface area,improve the light absorption and expose rich active sites,which are beneficial to light absorption and pollutants adsorption.Herein,the hollow In2O3@ZnFe2O4 heterostructures were obtained by growing ZnFe2O4 nanosheets on the outer surface of hollow In2O3 for efficient photodegradation of tetracycline(TC)under visible light.Firstly,the hollow In2O3 was prepared by calcination treatment of In-MIL-68 obtained through a solvothermal method.After that,2D ZnFe2O4 nanosheets are loaded on the surfaces of hollow In2O3 by electrodeposition and annealing methods.The hollow structure and 2D nanosheets can shorten the distance from inside to surface,the separation of photoinduced electrons and holes can be enhanced effectively.Moreover,the large surface area and abundant active sites are conducive to the progress of photocatalytic reaction.In addition,the heterojunction formed by loading 2D ZnFe2O4 nanosheets on the surface of 1D hollow In2O3 can curb the charges recombination,which contributes to the enhancement of photocatalytic performance.The optimized In2O3@ZnFe2O4 photocatalyst exhibits a 90%photodegradation rate of TC(50 mg/L)in 30 min.In addition,In2O3@ZnFe2O4 heterojunction shows excellent stability and unique magnetic property,which can be easily separated for recycled use in actual purification of organic pollutants.In summary,the construction of LDHs-based heterojunction composite is conducive to improving the absorption and utilization of visible light,and curbing the recombination of photogenerated electrons and holes.In addition,coordination with other catalytic methods(such as photoelectrocatalysis)and the introduction of specific morphology(with different dimensions or hollow structure)are both good strategies for improving the activities of the heterojunction-based photocatalysts.