Synthesis And Purification Performance Of CdS-Graphene Based Porous Composites For Organic Pollutant Wastewater

As a typical n-type semiconductor material,CdS is considered to be an ideal visible light-driven photocatalyst due to its suitable band gap?2.4 eV?and easily converting solar energy into chemical energy under irradiation conditions.Therefore,it is widely used in many fields including photocatalysis.However,the surface of CdS is prone to photocorrosion and photolysis under irradiation conditions,which causes cadmium ions to pollute water resources.In addition,the photo-generated carriers have high recombination rate and low quantum efficiency.These shortcomings limit the practical application of CdS.The previous literature reveals that some appropriate modification for CdS?such as metal/non-metal doping,precious metal deposition or forming composition with carbon-based material?may inhibit obviously the rapid recombination of the photogenerated electron pairs and photocorrosion.As the thinnest material known in the world today,graphene has many excellent physicochemical properties.It has received great attention in the fields of science and engineering since it was firstly prerpared by a mechanical exfoliation method in 2004.What's more,three-dimensional?3D?graphene aerogel?GA?and reduced graphene oxide?RGO?obtained by the ice crystal template method have good charge mobility and electrical conductivity,which are expected to be good modifiers for CdS.Other than that,they possess large specific surface area and abundant pores,which facilitates to adsorb organic pollutants when used as adsorbents.In this thesis,CdS-GA,CdS-g-C₃N₄-GA,CdS-RGO,CdS-TiO₂-RGO,CdS-g-C₃N₄-RGO and CdS-ZnO-RGO composites were sucessfully prepared and characterized by XRD,SEM,TEM,XPS,FT-IR,Raman,PL,DR UV-vis and N₂adsorption-desorption.Meanwhile,their adsorption and photocatalytic degradation properties for various organic pollutants were comparetively investigated.In addition,the corresponding possible photocatalytic mechanisms were also explored.The main work is as follows:?1?CdS-GA composites were prepared with a one-pot hydrothermal self-assembly combined with freeze-drying technique using cadmium acetate,thiourea and graphene oxide?GO?as raw materials.The results showed that CdS-GA had a large specific surface area and abundant pores(S_BET=175.0 m² g^-1,V_p=0.286 cm³ g^-1),and the wurtzite CdS spheres particle were uniformly encapsulated by GA.The composites exhibited high adsorption capacity and excellent photocatalytic activity for various organic contaminants.5.0 mg of CdS-GA was dispersed into 25 mL of contaminant solutions(20 mg L^-1),respectively.The corresponding total removal efficiencies for RhB,MB,AcbK,MO,and CIP were 99.4%,88.1%,70.4%,76.8%,and 87.1%within50 min,respectively.Additionly,the composites exhibited satisfactory reuseability,and the total removal rate for RhB was still remained as 97.5%of the initial value after 3cycles.?2?Graphitic carbon nitride?g-C₃N₄?was prepared with a thermal decomposition method using melamine as raw material and dispersed into the precursor liquid for preparing CdS-GA.The CdS-g-C₃N₄-GA composites were synthesized by the aforementioned hydrothermal self-assembly combined with freeze-drying technique.The results showed that the incorporation of g-C₃N₄ greatly enhanced the photocatalytic activity of CdS-GA.The degradation rates of RhB,CIP,MB,MO and AcbK were 91.4%,71.6%,56.8%,27.0%and 20.1%over CdS-g-C₃N₄-GA within 50min,respectively.They were significantly higher than those corresponding values over CdS-GA.Even the CdS-g-C₃N₄-GA composites were recycled 3 times,the total removal rate of RhB was still remained as 98.9%of the initial value,indicating that the composites had good cycle stabilities.?3?CdS powders were prepared with a hydrothermal method using cadmium acetate and thiourea as raw materials.Then,the resulting powders were dispersed into GO solution to prepare the CdS-RGO composites with abundant micro-and mesopores and large specific surface area(S_BET=84.8 m² g^-1,V_p=0.192 cm³ g^-1)by using an ice crystal template method combined with freeze-drying and calcination process.The results showed that the CdS particles were uniformly encapsulated by RGO.Compared with pure CdS and RGO,the as-prepared CdS-RGO composites showed excellent total removal performance for various pollutants.Under the present conditions,the corresponding total removal rates for RhB,MB and AcbK reached 96.9%,80.0%and64.9%,respectively.In addition,the total removal rate for RhB was still remained as98.1%of the initial value after 3 cycles,indicating that the composites had good cycle stabilities.?4?Mesoporous TiO₂ powders were prepared by a hydrothermal method using P123 as structure directing agent and butyl titanate as Ti source.Then,the resulting TiO₂ and the above CdS powders were dispersed into GO solution to prepare CdS-TiO₂-RGO ternary composites using an ice crystal template method combined with freeze-drying and calcination process.The results showed that the composites had a larger specific surface area and pore volume(S_BET=95.7 m² g^-1,V_p=0.265 cm³ g^-1)than the aforementioned CdS-RGO.Under the present conditions,the ternary composites exhibited better total removal performance for various pollutants than the pure TiO₂ and CdS-RGO.Within 30 min,the total removal rate for RhB was nearly100%over the CdS-TiO₂-RGO composites.Within 50 min,the total removal rates for MB and AcbK were 82.2%and 72.2%,respectively.The total removal rate for RhB was still remained as 97.3%of the initial value after 3 cycles,indicating that the composites had good cycle stabilities.?5?The aforementioned g-C₃N₄ and CdS powders were dispersed into GO solution to prepare the CdS-g-C₃N₄-RGO ternary composites with an ice crystal template method combined with freeze-drying and calcination process.The results showed that the composites still possessed a hierarchical porous structure.Their specific surface area and pore volume are 58.3 m² g^-1 and 0.149 cm³ g^-1,respectively.Under the present conditions,the corresponding total removal rates for RhB,MB and AcbK over CdS-g-C₃N₄-RGO were 99.0%,83.5%and 75.0%within 50 min,respectively.After 3 cycles,the total removal rate for RhB was still remained as 93.4%of the initial value,indicating that the composites had satisfactory cycle stabilities.?6?ZnO powders were prepared with a thermal decomposition method using zinc nitrate as raw material.Then,the resulting ZnO and the aforementioned CdS powders were dispersed into GO solution to prepare CdS-ZnO-RGO ternary composites using an ice crystal template method combined with freeze-drying and calcination process.The results showed that the composites,in which the CdS and ZnO particles were uniformly encapsulated by RGO,had a larger specific surface area and pore volume(S_BET=119.6 m² g^-1,V_p=0.267 cm³ g^-1)than CdS-RGO.Compared with pure ZnO and CdS-RGO,the ternary composites exhibited better purification performance for MB.Under the present conditions,the total removal rate was 88.2%within 50 min.In addition,after 3 cycles,the total removal rate of MB was still remained as 96.9%of the initial value.