Construction And Properties Research Of High Efficient Photocatalytic Materials Based On Hydrotalcite-like Compounds

With the rapid development of urbanization and industrialization,large quantities of household waste and industrial wastewater containing heavy metal ions were discharged into the environment,resulting in seriously environment pollution.Among them,hexavalent chromium（Cr（VI））as a typical heavy metal pollutant with strong migration,high toxicity and stable in water environment,which will cause severe damage to the stomach,small intestine and liver.Besides,as a human carcinogen,Cr（VI）had been listed in the first category of hazardous waste pollutants in China.However,as another form of chromium,trivalent chromium（Cr（III））is a trace element necessary for the human body with low mobility.Therefore,reducing the hight toxicity Cr（VI）into the low toxicity Cr（III）will be a promising method for the treatment of Cr（VI）-containing wastewater.Among the numerous Cr（VI）reduction methods,semiconductor photocatalysis has become increasingly promising technology because of its environmental protection,easy operation,low cost and no secondary pollution.In the process of photocatalytic reduction of Cr（VI）,the photogenerated electrons in the semiconductor migrate to the surface and reduce Cr（VI）to Cr（III）.As an important kind of clay mineral material,layered double hydroxide（LDH）has emerged as one of the most promising candidates to replace Ti O2 photocatalyst owing to its unique layered structure,adjustable host-guest chemical composition,low manufacturing cost and simplified preparation.However,the insufficient of visible-light absorption capacity and poor separation efficiency of photogenerated electron-hole pairs severely limited the application of the bare LDH.Based on the the structural properties of hydrotalcite and their derivatives,we constructed a series of LDH-based photocatalysts through metal cation regulation,surface functionalization and compound with semiconductor materials in order to improve the problems mentioned above.The crystal structure,morphology,photoresponse characteristics,photogenerated charge carriers separation and transfer behavior of the as-prepared photocatalysts were studied by various characterization methods.The photocatalytic activity of various composite photocatalysts was evaluated by Cr（VI）reduction under visible light irradiation.Furthermore,the relationships between the structure and catalytic performance of the hydrotalcite and its derivative-based composite photocatalytic materials were revealed,and the possible mechanisms for enhancing the catalytic performance of the photocatalysts were clarified.This study provides a theoretical basis and reference for the development of LDH-based photocatalysts in the application of Cr（VI）-containing wastewater treatment.Some valuable research results are obtained as follows:1.Taking the advantages of the metal elements in the brucite-like layer are dispersed at the atomic level,Cd²⁺was introduced into the layer of Zn Al LDH.The Cd S/Zn Al O composite photocatalyst was prepared by hydrothermal-calcination method.The SEM,TEM and EDS-mapping showed that the agglomeration of Cd S nanoparticles was improved significantly.The UV-vis DRS,PL and photoelectrochemistry found that the introduction of Cd S effectively enhanced the visible light absorption performance of Zn Al O.The formation of heterojunction between Cd S and Zn Al O avoided the aggregation of photogenerated electrons-holes in Cd S,which effectively improved the photocatalytic activity and stability of the Cd S/Zn Al O composite.Under visible light irradiation,The Cr（VI）reduction activity of the Cd S/Zn Al O was as high as 98.2%,and the Cr（VI）reduction efficiency was not significantly decreased after 5 cycles.2.Taking the advantages of compositional flexibility of LDH,a microspherical multilevel structure Ni Al LDH with visible light absorption was prepared.Based on that,Ni Al LDH@PDA/Fh composite photocatalytic system was constructed by using PDA as a binder to combine Ni Al LDH and Fh.The structure,morphology,surface chemical properties and photo-generated carrier separation behavior were characterized.The results indicated that the high Cr（VI）reduction activity of the composite photocatalyst originated from the synergistic effect between Ni Al LDH,PDA and Fh.Under visible light irradiation,Ni Al LDH can be excited to generate electrons.On the one hand,the strong visible light absorption capacity of PDA allows the composite to absorb more photons and generate more photogenerated electron-holes.On the other hand,the photogenerated electrons in Ni Al LDH can be transferred to Fh due to the excellent charge transport property of PDA and thus promoted the redox cycle of Fe³⁺/Fe²⁺on the surface of Fh,which accelerated the separation of photogenerated electron-hole pairs in Ni Al LDH and improved the utilization of photogenerated electrons.Subsequently,Cr（VI）was reduced by Fe²⁺or the electrons transferred to the PDA.The redox cycle of Fe³⁺/Fe²⁺was formed under the interaction of photogenerated electrons and Cr（VI）,leading to efficient reduction of Cr（VI）.3.The g-C₃N₄/Ni Al LDH composite photocatalyst was prepared through electrostatic self-assembly method.The characterization results and photocatalytic experiments showed that the Cr（VI）reduction activity of the composite material was closely related to the loading of g-C₃N₄.The 15%CN/LDH showed the highest photocatalytic activity（95.7%）and the Cr（VI）reduction efficiency did not significantly decrease after 5 cycles.The results indicated that g-C₃N₄/Ni Al LDH composite has excellent photocatalytic activity and recycling stability.Moreover,the band-matched g-C₃N₄ and Ni Al LDH were tightly coupled to form a heterojunction,which effectively promoted the separation and transfer of photogenerated electron-hole pairs.The abundant surface active sites of Ni Al LDH and efficient transfer of charge carriers in g-C₃N₄/Ni Al LDH are the important reasons for enhancing the Cr（VI）reduction efficiency.4.Taking the advantages of the confinement effect of LDH,firstly Ag Br was firstly deposited on the surface of Co Al LDH,and then the Z-scheme Ag Br@Ag/Co Al LDH composite was prepared with the photoassisted reduction.The photocatalytic activity of Ag Br@Ag/Co Al LDH was adjusted by controlling the content of Ag species.When the content of Ag species was 10%,the composite exhibited the highest Cr（VI）reduction activity（96%）under visible light.The k value was 0.0243 min^–1,which was about 12.2 times higher than that of Co Al LDH(0.0020min^–1).The results found that the Ag⁰ can serve as a bridge between Ag Br and Co Al LDH.The formation of Z-scheme heterojunction in the Ag Br@Ag/LDH effectively promoted the separation of photogenerated electron-hole pairs and kept the electrons with high reducing capability in the conduction band of LDH,leading to the excellent photocatalytic activity toward Cr（VI）reduction.5.Based on previous research,Co Al O/g-C₃N₄ p-n composite photocatalyst was constructed by the structural topology transformation of Co Al LDH.Under the visible light irradiation,CACN70 showed the highest photocatalytic reaction performance,and the reduction efficiency of Cr（VI）was as high as 98.5%.The k value was 0.0314min^–1,approximately 7.7 and 18.5 times of that of Co Al O and g-C₃N₄.The improved performance of the composite could be attributed to the induced interfacial electric field resulted from the formation of p-n heterojunction that facilitated separation and transportation of photogenerated charges.