Modification Of CdS Catalyst And The Performance Of Photocatalytic Hydrogen Production From Water Splitting

The consumption of the fossil fuels has caused energy crisis and the environmental pollution problems in the world.These two problems are urgent to be solved for the earth.Hydrogen energy is a kind of new energy which is clean,no carbon emission and high fuel value.The popularization of this kind of energy is beneficial for the environment.Photocatalytic hydrogen production from water splitting is a kind of technique using semiconductor catalyst to convert solar energy into chemical energy and produce hydrogen from water.This technique can store the most abundant renewable energy into hydrogen and is a kind of potential technique for hydrogen production.However,the photo-induced electrons and holes are tending to recombine with each other in pure semiconductor catalyst,which results in low photocatalytic activity for pure semiconductor catalyst.To enhance the photocatalytic activity and the of the separation efficiency semiconductor catalyst,different methods were developed in this work.In the first part of the research,MoS2/CdS composite catalyst with different MoS2 loading amount was synthesized using a one-pot hydrothermal method.The highest hydrogen production rate(406 μmol h-1)was achieved from 2.0 wt%MoS2 loaded MoS2/CdS composite catalyst.The hydrogen production rate was improved by 17 times compared with pure CdS.The catalyst was characterized using XRD,Raman,HRTEM,XPS,UV-vis DRS measurements.The results showed that heterojunction was formed between MoS2 and CdS using hydrothermal method,while heterojunction could not be formed through mechanical mixing method.Mo-S-Cd bond and the change of the binding energy was discovered by XPS measurement,indicated the formation of the heterojunction between MoS2 and CdS.The heterojunction promoted the electron transfer from CdS to MoS2 and suppressed the combination of the photo-induced electrons and holes,which resulted in the enhancement of the photocatalytic activity.In the second part,PSGM/rGO/CdS composite catalyst was synthesized using cadmium acetate and dimethyl sulfoxide(DMSO)as precursor through a solvothermal method.CdS was loaded on polymer supported 3D graphene structure.The affection of graphene oxide coating amount,cadmium acetate addition amount and solvothermal reaction temperature was studied and the proper value for these factors was chosen as 5 wt%,150mg(per 100 mg PSGM/GO microspheres)and 180℃,respectively.Then,a series of composite catalysts wereprepared with different reaction time(3 h,6 h,12 h and 24 h).The catalyst was characterized using SEM,XRD,ICP,PL,HRTEM,XPS,BET,UV-vis DRS measurements.The results showed that the CdS catalyst grew larger with the increasing of the reaction time firstly and became smaller in the later stage of the reaction.The same phenomenon was observed for the change of the crystallinity and the yield of the CdS catalyst.In the initial stage of the reaction,the CdS catalyst grew larger as CdS was produced gradually.When the precursor was completely reacted,CdS stopped growing.In the later stage,a dissolution and recrystallization process happened and CdS was dissolved.UV-vis DRS and PL measurement discovered that the band gap of the CdS catalyst reduced with the increasing of the reaction time.This result was caused by the quantum size effect.The quantum size effect also affected the photocatalytic activity of the composite catalyst.The average hydrogen production rate decreased with the increasing of the reaction time.When the reaction time was shorter,the size of the CdS catalyst loaded on the surface of the microsphere was smaller and the band gap of it was larger.The catalyst with larger band gap got higher conduction band potential and lower valance band potential,which resulted in higher thermodynamics driving force for hydrogen production reaction.And the charge carriers from the catalyst with smaller size could move to the surface of the catalyst faster.All the above result caused higher photocatalytic hydrogen production rate.The composite catalyst prepared with 3 h reaction time achieved the highest average hydrogen production rate of 175 μmol h-1.HRTEM measurement observed the formation of heterojunction between CdS and graphene.The heterojunction could promote the electron transfer from CdS to graphene.The graphene also served as active site of the hydrogen production reaction and the electrons were consumed.Then the photocatalytic activity of the catalyst was enhanced as the separation efficiency of the charge carriers was improved.The result showed that the hydrogen production rate of CdS was improved by 30.6 times.In the third part,PSGM/rGO/(ZnO/CdS)composite catalyst was synthesized using polymer supported 3D graphene structure as supporter,cadmium acetate,zinc acetate and DMSO as precursor.The total amount of cadmium acetate and zinc acetate added into 60ml DMSO reaction solution was 5 mmol.With different ratio of cadmium acetate additive amount,different composite catalysts were prepared and the photocatalytic activity was detected.The composite catalysts were characterized using XRD,SEM,XPS,UV-vis DRS,BET measurements.With 100%of zinc acetate added into the reaction solution,ZnO and little ZnS was formed in the product.When cadmium acetate was added into the reaction solution,ZnO and CdS was formed in the product.When 100%of cadmium acetate was added into the reaction solution,only CdS was formed in the product.When the additive proportion of cadmium acetate was 60%,the catalyst got the highest hydrogen production rate of 809 μmol h-1.The experimental result indicated that the formation of the composite catalyst changed with the additive proportion of cadmium acetate.The heterojunction formed between the materials improve the separation efficiency of the charge carriers and the hydrogen production rate of the catalyst.The different formation of the composite catalyst resulted in different photocatalytic activity.The pure ZnO/CdS composite catalyst(the additive proportion of cadmium acetate was 60%)got a hydrogen production rate of 451 μmol h-1.The 3D graphene structure improved the activity of ZnO/CdS.This result also indicated that synergistic effect of 3D graphene and ZnO could suppress the combination of the charge carriers and improve the hydrogen production rate of the photocatalyst.