Molecular Dynamics Simulations, Thermodynamic Analysis, and Experimental Study of Phase Stability of Zinc Sulfide Nanoparticles

A comprehensive study of the phase stability of ZnS nanoparticles was carried out using combined molecular dynamics simulations, thermodynamic analysis, and experimental investigations. Average surface energies of the sphalerite and wurtzite phases of zinc sulfide (ZnS) were calculated to be 0.86 and 0.57 J/m², respectively, using results from dynamics simulations of free faces of ZnS crystals at 300 K. Thermodynamic analysis, making use of the surface energy data, shows that smaller wurtzite nanoparticles are more thermodynamically stable than sphalerite. When the average particle size is ∼7 nm, the temperature for the transformation from sphalerite to wurtzite is 25 °C, dramatically lower than that observed in bulk material (∼1020 °C at 1 bar). The transformation from 3-nm sphalerite to wurtzite was simulated, and the activation energy was found to be only ∼5 kJ/mol. The very small activation energy may imply a different mechanism for the phase transformation in very small ZnS nanoparticles. Results of molecular dynamics simulations show that nanocrystalline sphalerite becomes more stable than wurtzite when sufficient water is adsorbed. Experimentally, when samples of synthetic ∼3-nm ZnS were heated in a vacuum over the range 350-750 °C, sphalerite transformed to wurtzite. However, there was no obvious conversion of sphalerite to wurtzite when samples were heated in air at 350 °C, probably due to the effect of chemisorbed water. The experimental data are consistent with the results of the thermodynamic analysis and molecular dynamic simulations, which indicate size dependence of ZnS phase stability and stabilization of sphalerite nanoparticles by water adsorption.