Controlled building of CdSe@ZnS/Au and CdSe@ZnS/Au₂S/Au nanohybrids

The addition of Au³⁺ to spherical amine-capped CdSe@ZnS nanoparticles in toluene at room temperature and under darkness can lead to ternary CdSe@ZnS/Au nanohybrids. We demonstrate that this happens only when the nanoparticles possess a relatively thin ZnS shell, thus showing that thickness plays a key role in gold deposition on the CdSe@ZnS nanoparticle surface. Our hypothesis is that the amine ligand acts as the reductant of Au³⁺ ions into Au⁺ ions, whose affinity for sulfur would keep them at the CdSe@ZnS surface. This interaction stabilizes the Au⁺ ion, making it less prone to reduction than a non-coordinated Au+ ion. In CdSe@ZnS with a thin shell, Au⁺ ions at the surface of, or most probably within, the ZnS shell cause the transfer of Cd²⁺ ions into the solution. Subsequently, the core Se^2– anion, which is a better reductant than the shell S^2–, reduces Au⁺ ions to Au(0), and large gold nanoparticles (AuNPs) are quickly deposited on the CdSe@ZnS surface in room temperature process, leading to ternary CdSe@ZnS/Au nanohybrids. In solution, these ternary nanohybrids progressively transform into quaternary CdSe@ZnS/Au2S/Au nanohybrids due to the reaction of the shell S^2– anion with the remaining Au⁺ at the CdSe@ZnS surface, thus leading to the growth of Au₂S nanoparticles on the CdSe@ZnS surface while Zn concomitantly leaches from the nanohybrid into the solution. Photoirradiation of the heterostructures with visible light enhances their emission efficiency. Comparatively, irradiation of the precursors, i.e., CdSe@ZnS nanoparticles, causes a drastic decrease in their emission accompanied by a blue shift of their emission maximum. The optical properties of these nanohybrids were analyzed by absorption and fluorescence (steady-state and time-resolved) spectroscopy, and the composition of the samples and the chemical states were determined by energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS), respectively. Finally, the structural and morphological characterizations of the nanohybrids were performed by bright-field transmission electron microscopy (TEM), dark-field TEM, high-resolution TEM (HRTEM), and selected-area electron diffraction (SAED).