Role of Atmospheric CO2 and H2O Adsorption on ZnO and CuO Nanoparticle Aging: Formation of New Surface Phases and the Impact on Nanoparticle Dissolution

Heterogeneous reactions of atmospheric gases with metal oxide nanoparticle surfaces have the,potential to cause changes in their physicochemical properties including their dissolution in aqueous media. In this study, gas-phase CO2 adsorption on ZnO and CuO nanoparticle surfaces was studied as a function of relative humidity to better understand the role of CO2 and H2O on nanoparticle aging and the influence of this,aging process on metal ion :dissolution from nanoparticles. Upon nanoparticle exposure to atmospherically relevant, pressures of CO2 under different relative humidity (RH) conditions, temporal variations of surface-adsorbed species were monitored using Fourier transmission infrared spectroscopy (FTIR). Under dry conditions, gas-phase. CO2 readily reacts with surface hydroxyl :groups present on the ZnO and CuO nanoparticle surface to form adsorbed bicarbonate, whereas the interaction of CO2 with, surface defect sites and lattice oxygen gives rise to surface-adsorbed moriodentate and bidentate carbonate species, as well as adsorbed carboxylate. With increasing relative humidity from 0 to 70%, surface speciation gradually changes to that of water solvated adsorbed carbonate, which was the only detectable surface species at the highest relative humidity investigated (70% RH). High:resolution TEM analysis of reacted ZnO, and CuO nanoparticles revealed considerable surface restructuring consistent with the precipitation of crystalline carbonate phases in the presence of adsorbed water. Furthermore, the restructuring of ZnO and CuO nanoparticles during CO2 exposure is,limited to the near surface region. Importantly, the reacted ZnO nanoparticles also Show an increase in the extent of their dissolution when placed in, aqueous media. Thus, this work provides valuable insights into reactions of atmospheric gases, CO2 and H2O, on ZnO and CuO nanoparticle surfaces and the irreversible changes such reactions can induce on natioparticle surface chemistry and behavior in aqueous media.