Study On Photocatalytic H₂Evolution Activity And Photoinduced Charge Behavior Of CdS Based Photocatalysts

Due to the increasing energy crisis and environmental pollution issues caused byfossil fuel, new energy is necessary to replace the fossil fuel. Hydrogen energy is onekind of efficient, green resources, which is the most likely energy to replace fossil fule.Among many methods for hydrogen evolution, the photocatalytic H2evolution usinginexhaustible solar energy has become urgently important topics nowadays. In theprocess of photocatalytic H2evolution, semiconductor photocatalysts as mediumtransformate solar energy into hydrogen energy. Most of photocatalysts, such as TiO2,SrTiO3, only use UV light. CdS is a typical II-VI semiconductor. Its conduction bandpotential is higher than the potential for H+reduction. So it can be used for H2evolution. It’s band gap is2.4eV. So CdS can use visible light for H2evolution. But,CdS also has some disadvantages, which limits the development of CdS inphotocatalysis. One is the rapid recombination of photogenerated charges in CdS,which result in the low rate for H2evolution. In order to improve the photocatalyticactivity, the noble metals, such as Pt, are needed. But the noble metals are expensive.The other disadvantage of CdS is corrosion. In order to prevent the holes oxidation ofS2-and decomposition of CdS, the electrons sacrificial agent is needed. The purposeof this paper is looking for non noble metal cocatalysts (such as CuS, Co(OH)2) andconstructing Z-scheme photocatalytic system to improve the photocatalytic activity ofCdS.We all know that the photocatalytic H2evolution process is that photogeneratedelectrons transferring to the surface of photocatalyst reduce protons to hydrogen. So, more photogenerated electrons transfer to the surface of photocatalyst, which isbeneficial to improving photocatalytic H2evolution, undoubtedly. So the study ofphotogenerated charges characteristics is necessary. So the other purpose of this paperis studying the effect of non noble metal cocatalysts and Z-scheme systemconstructing on the photogenerated charges characteristics of CdS. Those works maybe beneficial to developing efficient and inexpensive photocatalysts for watersplitting.1. CuS/CdS, CuS/Zn0.8Cd0.2S composites photocatalysts are successfullyprepared by simple cation exchange method. The results of FESEM, TEM andHRTEM show that CuS was successfully loaded on the surface of CdS. Non nobelmetal cocatalyst CuS can improve the photocatalytic H2activity of CdS. And theresults of SPV and SPC show that CuS loaded on the surface of CdS plays as a site fortrapping electrons. In order to further improve the H2evolution activity, CuS wasloaded on the surface of Zn0.8Cd0.2S. And the photocatalytic H2evolution activity isfurther improved. Due to CuS loaded, the TPV signal of CuS/Zn0.8Cd0.2S reverse,which also proves that CuS plays as a site for trapping electrons.2. Co(OH)2/CdS composites are prepared by using solvothermal method andcoprecipitation method. The result of XPS shows the presence of Co2+in the form ofCo(OH)2. Co(OH)2as cocatalyst improve the photocatalytic H2evolution activity ofCdS with UV-visible light and visible light. The result of SPV shows that Co(OH)2plays as a site for trapping electrons and more electrons are accumulated on thesurface. The TPV result shows that Co(OH)2cocatalyst enhance the photogeneratedcharge separation efficiency and photocatalytic H2evolution activity of CdS. Aftertesting for15h, the photocatalytic H2evolution activity of Co(OH)2/CdS does nothave obviously decrease, which indicates that Co(OH)2/CdS photocatalyst isrelatively stable.3. We construct a Z-Scheme photocatalystic system of CdS/WO3composite forH2evolution, by mimicking the natural photosynthesis. Although WO3alone cannotbe used for H2evolution from water splitting, it can significantly increase the H2evolution activity of CdS. Photogenerated charge characteristics of CdS/WO3were studied by surface photovoltage technology. The SPV results show thatphotogenerated electrons in conduction band of WO3recombinate with the holes inthe valence of CdS, which results more electrons accumulating on the surface of CdS.This is benefit to improving the H2evolution activity of CdS. On the other hand, thephotogenerated charges separation efficiencyof CdS is enhanced.