Preparation And Performance Of NiS-PdS/CdS Composite Photocatalyst For Photocatalytic H₂Evolution

The studies on photocatalytic splitting of water into hydrogen usingsolar energy have great scientific significance and potential application value.Although CdS, a kind of visible-light-driven photocatalyst, has suitable bandgap and conduction band position, the hydrogen-producing activity of CdS islow. Noble metals, such like Pt, Pd, Rh etc. are usually loaded on CdS ascocatalysts to enhance the hydrogen evolution performance. Some non-noblemetal cocatalysts such like WS₂, MoS₂etc. have been reported as wellrecently, although their efficiencies are relatively low. In the study, a novelcomposite photocatalyst NiS-PdS/CdS was synthesized via co-loading twodifferent kinds of cocatalysts. The aim is to enhance the activity of CdS andreduce the amount of noble metal used by the co-loading and interactionsbetween PdS and NiS.In this study, composite photocatalyst NiS-PdS/CdS has been developedthrough hydrothermal method by using CdS, palladium chloride, nickelacetate and thiourea as raw materials. The Characteristics of NiS-PdS/CdShave been studied and photocatalytic activities have been tested in sacrificialreagent. The results showed that the tetragonal crystalline phase of CdS wastransformed into a mix of tetragonal and hexagonal crystalline phase afterhydrothermal treatment, and this transformation was conducive to theenhancement of NiS-PdS/CdS photocatalytic activity.In the high temperature and high pressure environments of hydrothermalmethod, PdS and NiS were generated from Ni²⁺and Pd²⁺and firmly anchoredon the surface of CdS. This close contact played an important role in the transferring of photogenerated charge carrier. NiS and PdS dispersed well onthe surface of CdS. A small amount of Ni²⁺or Pd²⁺was doped into the crytallattice of CdS, resulting in a red shift of absorption edge of NiS-PdS/CdS.Moreover, the visible-light absorption of NiS-PdS/CdS was much higher thanthat of unloaded CdS in the wavelength range540⁷00nm. Co-loading couldeffectively enhance the transfer and separation efficiency of photogeneratedcharge carrier and reduce the nonradiative energy loss of excited electron.The duration of hydrothermal treatment and loading amout of cocatalystshave impact on the performance of NiS-PdS/CdS. While the loading amountof NiS and PdS were1.5wt%and0.4wt%respectively, The H₂evolution rateof NiS-PdS/CdS could reach up to its highest6550μmol/h in lactic acidsacrificial reagent which is6times higher than that of unloaded CdS andnearly2times higher than NiS/CdS and the apparent quantum yield couldreach to47.5%（λ=420nm）. The high activity was attributed to the “synergyeffect” of the two cocatalysts NiS and PdS loaded on the CdS surface.Moreover, some main factors influencing the photocatalytic activity ofthe composite photocatalyst system were discussed. The results demonstratedthat: The activity of NiS-PdS/CdS was determined by the raw materials,preparation methods and reacting environment. Aging treatment could changethe nanoparticle size and dispersion degree of CdS, and then influence theactivity of NiS-PdS/CdS. When CdS was aged for6days, NiS-PdS/CdSshowed relatively high activity; When CdS was hydrothermally treated for24hours, NiS-PdS/CdS（HT-24h） showed best activity; Compared to many otherraw materials, using palladium chloride, nickel acetate and thiourea as rawmaterials to synthesize NiS and PdS cocatalysts could greatly enhance theactivity of NiS-PdS/CdS（HT-24h）; the optimal hydrothermally loading timeand temperature was4hours and140℃；When the molar ratio of reactantmetal ions to S in thiourea was1:150, the rate of H₂evolution was highest.NiS-PdS/CdS（HT-24h） showed selectivity toward sacrificial reagents and show highest activity in lactic acid sacrificial reagent.