Kinetics of water adsorption-driven structural transformation of ZnS nanoparticles

Nanoparticles of certain materials can respond structurally to changes in their surface environments. We have previously shown that methanol, water adsorption, and aggregation-disaggregation can change the structure of 3 nm diameter zinc sulfide (ZnS). However, in prior observations of water-driven structure change, aggregation also may have taken place. Therefore, we investigated the structural consequences of water adsorption alone on anhydrous nanoparticles that were dried to minimize changes in aggregation. Using simultaneously collected small- and wide-angle X-ray scattering (SAXS and WAXS) data, we showed that water vapor adsorption alone drives a structural transformation in ZnS nanoparticles in the temperature range of 22-40 °C. The transition kinetics is strongly temperature dependent, with an activation energy of 55 ± 10 kJ/mol, consistent with atom displacement rather than bond breaking. At 50 °C, aggregate restructuring occurred, increasing the transition kinetics beyond the rate expected for water adsorption alone. The observation of isosbestic points in the WAXS data suggests that the particles do not transform continuously between the initial and the final structural state but rather undergo an abrupt change from a less ordered to a more ordered state.