Spatially Resolved In Situ X-ray Absorption Spectroscopy Studies of ZnS Nanoparticle Synthesis at the Water-Toluene Interface.

Abstract

Two-phase synthesis is a well-established approach for achieving precise control of the nanoparticle properties. However, studying and understanding chemical transformations in such a spatially heterogeneous system is challenging. In this work, we introduce a two-phase synthesis route for ZnS nanoparticles (ZnS NPs) at the water-toluene interface. By employing spatially resolved in situ high-energy resolution fluorescence-detected X-ray absorption spectroscopy (HERFD-XAS) combined with density functional theory (DFT) calculations, we track the diffusion of Zn²⁺ species at the interface, identify key reaction intermediates, and monitor the nucleation and growth of ZnS NPs within the toluene phase. We propose the formation of a [Zn(H₂O)₆]²⁺ complex upon dissolving Zn(Ac)₂ in water and the diffusion of Zn²⁺ ions from water to toluene driven by the formation of an octahedral [Zn(OA)₆]²⁺ complex (OA = oleylamine). Furthermore, by complementing HERFD-XAS with total X-ray scattering analysis, we show the formation of an intermediate tetrahedral [Zn(SR)₄]²⁺ complex at 60 °C and its successive transformation to noncrystalline ZnS nuclei at 80 °C and crystalline ZnS NPs starting at 100 °C. Thus, we demonstrate how in situ X-ray spectroscopy can elucidate the coordination and diffusion of Zn²⁺ ions, and, in combination with X-ray scattering studies, identify the emergence of atomic and electronic structures during the two-phase synthesis of ZnS nanoparticles.