New Cocatalysts On The Surface Of Cadmium Sulfide For Enhanced Photocatalytic Hydrogen Evolution Performance

In the 21st century,one of the great challenges which we are facing is the global shortage of energy supplies.Hydrogen is an efficient and clean new renewable energy,which can effectively relieve the human dependence on fossil fuels,thus reducing the environmental pollution.In all hydrogen production technologies,photocatalytic H₂-evolution from water splitting based on semiconductor is a low-cost and ideal means of hydrogen production.Among various semiconductor materials,CdS serves as one of the most competitive photocatalysts owing to its appropriate band gap?2.4eV?and strong reduction potential?-0.51 V vs SHE?for H₂-evolution.Unfortunately,the high recombination rate of photogenerated electron-hole pairs and photo-corrosion phenomenon severely limit its photocatalytic H₂-evolution performance.Cocatalysts surface modification is one of the effective ways to improve the photocatalytic H₂-evolution performance and the photoinduced-stability of CdS.To achieve a high H₂-evolution performance for cocatalyst-modified composite CdS photocatalysts,both of the rapid electron transfer?or electron capture?from CdS to cocatalyst and the efficient interfacial catalysis for H₂ production on the cocatalyst surface are highly required.In this study,to develop highly stable and efficient CdS photocatalytic materials for hydrogen production,the electron transfer-interfacial reaction and the photoinduced-stability of CdS are explored:?1?the synergistic effect of electron-transfer mediator Au and interfacial catalytic active-site SCN^-for the enhanced H₂-evolution performance of CdS;?2?the improved photocatalytic photoinduced-stability and H₂-evolution activity of CdS photocatalyst by molybdate ions.The results are as follows:First,Au nanoparticles were loaded on the CdS surface through a photodeposition process.Subsequently,a calculated amount of 0.1 mol L^-1 potassium thiocyanate?KSCN?solution was injected into the above CdS/Au suspension,and the resultant samples were referred to as CdS/Au-SCN photocatalysts.It was found that the CdS/Au-SCN photocatalyst exhibited excellent photocatalytic H₂-evolution activity compared with the bare CdS,the single loaded CdS/Au and CdS/SCN.In particular,the CdS/Au-SCN?0.5 mM?achieved the highest photocatalytic activity(109.60?mol h^-1)with an apparent quantum efficiency?QE?of 11.25%,which was clearly higher than that of CdS(47.29?mol h^-1)and CdS/Au(73.70?mol h^-1)by a factor of 2.3 and 1.5 times,respectively.Based on the above experimental results,the enhanced H₂-evolution performance of CdS/Au-SCN can be attributed to the excellent synergistic effect of Au and SCN^-,namely,the Au nanoparticle functions as an effective electron-transfer mediator for the steady capture and rapid transportation of photogenerated electrons from CdS surface,while the adsorbed SCN^-serves as the interfacial catalytic active-site to effectively adsorb H⁺ions from solution and promote the rapid H₂-evolution reaction.Hence,it is clear that the excellent synergistic effect between Au and SCN^-is account for the greatly improved photocatalytic H₂-evolution performance of CdS/Au-SCN photocatalyst.Second,cubic-phase CdS photocatalysts were synthesized by a hydrothermal method.In the presence of lactic acid sacrificial agent,a calculated amount of Na₂MoO₄ solution was injected into the CdS suspension and stirring 1 h at ambient temperature,and the resultant samples were referred to as CdS-M photocatalysts.The photocatalytic results indicated that molybdate ions can effectively restrain the photo-corrosion of CdS crystal and reduced the production of S?0?in the CdS lattice.Hence,the photocatalytic photoinduced-stability of CdS was improved.Meanwhile,the obtained CdS-M photocatalysts displayed obviously high H₂-evolution performance.When the concentration of Na₂MoO₄ was 22?mol L^-1,the H₂-evolution rate of the resultant CdS-M-22 reached 42.49?mol h^-1,which was remarkably higher than that of the pure CdS(16.90?mol h^-1)by a factor of 2.51.On the basis of the experimental results,a possible mechanism was proposed to account for the improved photocatalytic performance and photoinduced-stability of CdS-M photocatalyst.On the one hand,in the presence of lactic acid sacrificial agent,MoO₄^2-condenses to form(Mo₈O₂₆)^4-spontaneously and then combines with the protons in solution to form(H_xMo₈O₂₆)^4-x intermediate.The(H_xMo₈O₂₆)^4-x intermediate can effectively capture photogenerated electrons from the CdS conduction band and subsequently produce H₂ and(Mo₈O₂₆)^4-ions,and this process shows many similarities to the Calvin cycle in natural photosynthesis,the CdS-M photocatalysts exhibitd significantly enhanced photocatalytic H₂-evolution performance.On the other hand,molybdate ions can effectively acquire protons in solution,to promote the ionization of lactic acid,resulting in the increased concentration of lactate ions in solution.Lactate ions can more easily trap positively charged holes and the photoinduced-stability of CdS photocatalytic is remarkably enhanced.With the further research,the preparation method and the related mechanism of high-efficiency photocatalytic H₂-evolution materials co-modified with metal and the interfacial catalytic active-site will be developed.It is also highly required to deepen the understanding of the synergistic mechanism between metal and interfacial catalytic active-site and develop new hole cocatalysts or design highly efficient hole-transfer mechanism for various sulfide photocatalysts to improve their photoinduced-stability.These works are of great theoretical significance for the design and development of new efficient and stable semiconductor photocatalytic systems.