| Abstract
| - A comprehensive study of the phase stability of ZnS nanoparticles was carried out using combined moleculardynamics simulations, thermodynamic analysis, and experimental investigations. Average surface energiesof 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 thermodynamicallystable than sphalerite. When the average particle size is ∼7 nm, the temperature for the transformation fromsphalerite 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 tobe only ∼5 kJ/mol. The very small activation energy may imply a different mechanism for the phasetransformation in very small ZnS nanoparticles. Results of molecular dynamics simulations show thatnanocrystalline 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, sphaleritetransformed to wurtzite. However, there was no obvious conversion of sphalerite to wurtzite when sampleswere heated in air at 350 °C, probably due to the effect of chemisorbed water. The experimental data areconsistent with the results of the thermodynamic analysis and molecular dynamic simulations, which indicatesize dependence of ZnS phase stability and stabilization of sphalerite nanoparticles by water adsorption.
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