In-Depth Study of ZnS Nanoparticle Surface Properties with a Combined Experimental and Theoretical Approach

ZnS nanoparticles (NPs) were synthesized using a simple, green, and reproducible hydrothermal method. Transmission electron micrographs show polyhedral NPs having an average diameter of 21 nm; whereas the X-ray diffraction analysis is consistent with the exclusive presence of cubic ZnS; however no oxide could be detected. A comprehensive characterization of the NPs' surface was accomplished through X-ray photoelectron spectroscopy, diffuse reflectance infrared Fourier transform spectroscopy (DRIFT), Raman, and thermogravimetric analysis-differential scanning calorimetry, showing a fairly pure ZnS composition and a remarkable amount of adsorbed water molecules. The interaction capabilities of the surface were probed in situ by DRIFT using small molecules (CO, CO2, methanol, pyridine) as molecular probes. The same interactions were also theoretically studied with density functional calculations using a slab model based on the sphalerite ZnS (110) surface. By comparing theoretical and experimental vibrational shifts, insights on the nature of the interaction between molecular probes and surfaces were obtained. Water was found to alter both the structure as well as the reactivity of the surface, mediating the interaction of methanol with the surface, and allowing the conversion of CO2 into surface carbonates. Pyridine was instead evidenced to be able to replace water molecules because of its high adsorption energy (1 eV) which is in tune with the known pyridine-detection capabilities of ZnS. No -SH moieties or Lewis acid behavior of the exposed S atoms were observed.