Preparation And Hydrogen Production Performance Of Cobalt-promoted Cadmium Sulfide Composite Photocatalysts

Since the industrial revolution,the dual crisis of energy and environment has become one of the mian problems hindering the further development of society,because of the rapid increase of industralization and uncontrolled use of energy resources.Photocatalytic technology can collect solar energy and convert it into chemical energy.It can also be used to degrade various organic apollutants and heavy metal ions in wastewater.Thus,photocatalytic technology is widely concerned.Cobalt is an important member of transition metals,whose phosphides and sulfides can be used to replace the noble metals（Pt,Rh,Pd,etc.）to capture photo-generated electrons and provide active sites for hydrogen evolution reactions.CdS is one of the most ideal photocatalysts for hydrogen evolution due to its suitable band gap（～2.4 eV）.In this paper,cobalt sulfide and cobalt phosphide are loaded on CdS,and the photocatalytic activity is further improved by element doping and morphology control methods.The main contents and conclusions are as follows:（1）The Co_1-xS/CdS composite photocatalysts were synthesized by hydrothermal and solvothermal methods.Effects of the morphology of CdS and the synthesized sequence of Co_1-xS and CdS on their photocatalytic activity were also dicussed.When the morphology of CdS is nanorods and Co_1-xS grows in situ on CdS,the photocatalytic activity is highest and the evolution rate for H2 reaches 27.38 mmol·g^-1·h^-1 under visible light.Moreover,sacrificial agent is of great influence on the photocatalytic activity,lactic acid is the most suitable sacrificial agent for this system.（2）The effect of another metal M(M=Mo4+,Zn²⁺,Mn²⁺,Ag+)doped in Co_1-xS/CdS on its activity was discussed.The synthesized Co_1-xS@ZnCoS/CdS（ZCS/CdS）exhibited the highest hydrogen evolution activity.Subsequently,the influence of the molar ratios of Zn/Co and Co/（Co+Cd）on photocatalitic activity was discussed.When the molar ratios of both Zn/Co and Co/（Co+Cd）are fixed as 0.3,the 30%ZCS-0.3/CdS composite shows the highest photocatalytic activity for hydrogen evolution with 58.4 mmol·g^-1·h^-1,which is 31.4 times higher as that of CdS(1.86 mmol g^-1·h^-1)and even 2.13 times as that of Co_1-xS/CdS(27.38 mmol·g^-1·h^-1).The stability of the photocatalyst is good,and the photocatalytic hydrogen production rate remains above 91%after four cycles.（3）The CoP nanoflowers were synthesized by using Co（OH）2 nanoflowers as precursors,that was sythesized through Co-MOF mediated.Effects of the morphology of CoP（nanoflowers,nanoparticles and nanosheets）in CoP/CdS were compared on the photocatalytic activity.Results showed that CoP/CdS with CoP nanoflowers exhibits the highest activity.Activities of the CoP/CdS photocatalysts prepared through the hydrothermal method and the ultrasonicassisted physical mixing method are compared.The physical mixing method is more suitable to achieve CoP/CdS composite with high photocatalytic activity for hydrogen production.XRD patterns show that CoP is partially sulfided to form Co₃S₄ through hydrothermal process in the presence of thiourea.Changes in composition and crystallinity weaken its performance as co-catalyst.（4）The influence of CoP loading amount on the activity of the composite catalysts was discussed.Results showed that 10%CoP/CdS composite catalyst exhibits the highest catalytic activity of 164.4mmol·g^-1·h^-1 for hydrogen,which is 53 times as that of pure CdS(3.1mmol·g^-1·h^-1).The low loading of CoP has slight role of improve photocatalytic activity,and too much CoP will hinder the absorption and utilization of visible light by CdS.Moreover,the 10%CoP/CdS composite catalyst has good stability.The photocatalytic activity of the catalyst decreased obviously after the first cycle,and it remains constent during the subsequent three cycles.