Synthesis, structural characterization and photocatalytic application of ZnO@ZnS core–shell nanoparticles

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Abstract
ZnO nanoparticles were synthesized by co-precipitation with no capping agent followed by covering with ZnS using a solution-based chemical method at low temperature. By variation of the solution concentrations it was found that the fully-covering ZnS shell forms by a reaction of Na2S with ZnO NPs followed by the formation of ZnS nano-crystals by the reaction of Na2S with ZnCl2. The mechanism that led to full coverage of the ZnO core is proposed to be the addition of ZnCl2 at a later stage of the growth which guarantees a continuous supply of Zn ions to the core surface. Moreover, the ZnS nano-crystals that uniformly cover the ZnO NPs show no epitaxial relationship between the ZnO core and ZnS shell. The slow atomic mobility at the low reaction temperature is attributed to the non-epitaxial uniform ZnS shell growth. The rough surface of the ZnO grains provides initial nucleation positions for the growth of the ZnS shell nano-crystals. The low growth temperature also inhibits the abnormal growth of ZnS grains and results in the homogeneous coverage of ZnS nano-crystals on the ZnO core surface. The as-synthesized ZnO@ZnS core–shell nanoparticles were used as a photocatalyst to decompose Rose Bengal dye at three different pH values. ZnO@ZnS core–shell nanoparticles perform as a more active photocatalyst at a pH of 4, while pure ZnO nanoparticles are more efficient at a pH of 7.

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