Synthesis,Characterization And Photocatalytic Properties Of Ni S/CdS Heterostructure Nanocomposite

Much of the world-wide attention has been payed to solve the problem of the depletion of fossil energy and the resultant environmental pollution.Among all the solutions,hydrogen evolution by water-splitting under sunlight has been given great hopes,because hydrogen energy is pollutant-free and of high energy density.Since the first work on the hydrogen evolution over TiO₂ photocatalyst by Honda,a large amount of semiconductor based photocatalysts have been reported for hydrogen production.Among them,semiconductor Cd S is a very attractive candidate for photocatalytic hydrogen evolution due to its potentially high activity under visible light and sufficiently negative flat-band potential for the reduction of H⁺to H₂.However,CdS alone always exhibits low hydrogen evolution rate due to the rapid recombination of photogenerated electrons and holes.In literature,it was reported that,like precious metal Pt,many transition metals（including Mo,W and Ni）sulfides would be also promising cocatalysts for promoting the photocatalytic performance over semiconducting CdS.For example,CdS coated with nickel sulfides could have high photocatalytic performance.Among all the nickel sulfides,β-NiS phase has the highest cocatalytic activity,although it is hard to prepare by conventional methods.Furthermore,the contact betweenβ-NiS and CdS is also a key factor to promote the photocatalytic hydrogen generation over semiconducting CdS.In a word,it is still a difficult task to synthesizeβ-NiS/CdS composites with close contact.Here a novelβ-NiS film modified CdS nanoflowers（NiS/CdS NFs）heterostructure nanocomposite（HSNC）was successfully synthesized via an electroless Ni plating method by using Cd（NO₃）₂·4H₂O as Cd-resource,CH₄N₂S as S-resource and Ni（CH₃COO）₂·4H₂O as Ni-resource with the presence of NaH₂PO₂·H₂O as the reducing agent.The synthesis conditions were investigated by controlling the Ni/Cd feed molar ratio and the amount of the addition of NaH₂PO₂·H₂O.The optimum condition includes the orderly mixing of 1 mmol Cd（NO₃）₂·4H₂O,3 mmol CH₄N₂S,0.6 mmol Ni（CH₃COO）₂·4H₂O and 0.6 mmol NaH₂PO₂·H₂O in 50 ml of deionized water,and hydrothermal treatment at 180°C for4 h.The growth mechanisms for the NiS/CdS NFs HSNC were proposed.The composition and structure of the NiS/CdS NFs HSNC prepared under optimum condition were characterized.The CdS NFs have strongly adhesive contact withβ-NiS films.The photocatalytic properties of the as-synthesized NiS/CdS NFs HSNC were measured in aqueous solution containing 20 vol.%lactic acid as the sacrificial agent under visible light.The highest rate for hydrogen production reached about 30.1 mmol?h^-1?g^-1 at 25°C,and the highest apparent quantum efficiency was approximately 43%at 420 nm.Furthermore,the photocatalysis mechanisms for the NiS/CdS NFs HSNC were proposed.