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 nanocrystals 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.
Highlights
Zinc oxide (ZnO) nanoparticles (NPs) as a cheap nontoxic semiconductor with a wide direct band gap (3.37 eV) are a promising material for different applications such as photocatalysts,[1] photodetectors,[2] gas sensors,[3] piezoelectric sensors[4] and ultraviolet lasers[5] and can be synthesized in different ways such as by the chemical vapor phase method,[6] thermal evaporation,[7] the vapor-solid technique,[8] the hydrothermal method[9] and the co-precipitation method.[10]
We show a simple fast reliable method to grow ZnO@Zinc sul de (ZnS) core–shell nanoparticles (CSNPs) with full coverage using a simple two steps low temperature chemical method
Comparing this with the X-ray diffraction (XRD) results of the work reported by Sharma et al.[25] it is clear that the grown ZnO@ZnS CSNPs in this work are pure core–shell NPs and no impurity phases can be observed
Summary
Zinc oxide (ZnO) nanoparticles (NPs) as a cheap nontoxic semiconductor with a wide direct band gap (3.37 eV) are a promising material for different applications such as photocatalysts,[1] photodetectors,[2] gas sensors,[3] piezoelectric sensors[4] and ultraviolet lasers[5] and can be synthesized in different ways such as by the chemical vapor phase method,[6] thermal evaporation,[7] the vapor-solid technique,[8] the hydrothermal method[9] and the co-precipitation method.[10]. TiO2 has been used as a photocatalyst to degrade different dyes and pollutants, but further investigation has shown that ZnO has similar efficiency of photocatalytic degradation and it is a better substitute for TiO2 in some applications.[30,31,32] Since covering ZnO with different semiconductors changes the electronic states and the surface of the ZnO NPs, this could affect its photocatalytic activity.[33] The other issue that needs to be considered is related to the photocatalytic activity of NPs at different pH values. As dyes pollutants can be found in different mediums, investigating the photocatalytic efficiency of products at different pH values can provide useful information
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