Evolution of ZnS:Cu nanoparticle morphology during microwave-assisted hydrothermal synthesis

Doped zinc sulfide (ZnS) nanomaterials have unique optical and electrical properties making them an outstanding material for design and fabrication of photoluminescent (PL) and electroluminescent devices. Structure, elemental composition and morphology of nanoparticles determine properties of the produced devices and are highly sensitive to reaction conditions. Therefore, real-time monitoring with an opportunity to extract a sufficient quantity of material for the field tests should be explored. This work is devoted to the study of morphological evolution of Cu-doped ZnS particles synthesised by microwave-assisted hydrothermal method (MWHT). This study demonstrates the feasibility for continuous monitoring of synthesis product properties, offering valuable insights into particle growth and formation processes, while also confirming the reproducibility of the described sampling procedure. The results verify formation of a cubic ZnS phase with crystallite size increasing from 2 to 5 nm during the synthesis. In addition, formation of the hexagonal ZnO was confirmed by X-ray powder diffraction. Scanning transmission electron microscopy (STEM) images elucidated the formation and growth processes of ZnS particles with different morphological signs. One group of particles was represented as the spherical particles formed from nanoclusters in various sizes due to slow decomposition of thiourea. The second group of flower-like particles was generated through a process where thin-like plates commence to grow on the surface of spherical particles. PL intensity of the resulting products with a band positioned at 2.00 eV indicated ⁴T₁ → ⁶A₁ transition between Cu²⁺ ion energy levels and experienced a blueshift over time.