Atomic Force Microscopy and X‑ray Photoelectron Spectroscopy Study of NO₂ Reactions on CaCO₃ (101̅4) Surfaces in Humid Environments

In this study, alternating current (AC) mode atomic force microscopy (AFM) combined with phase imaging and X-ray photoelectron spectroscopy (XPS) were used to investigate the effect of nitrogen dioxide (NO₂) adsorption on calcium carbonate (CaCO₃) (101̅4) surfaces at 296 K in the presence of relative humidity (RH). At 70% RH, CaCO₃ (101̅4) surfaces undergo rapid formation of a metastable amorphous calcium carbonate layer, which in turn serves as a substrate for recrystallization of a nonhydrated calcite phase, presumably vaterite. The adsorption of nitrogen dioxide changes the surface properties of CaCO₃ (101̅4) and the mechanism for formation of new phases. In particular, the first calcite nucleation layer serves as a source of material for further island growth; when it is depleted, there is no change in total volume of nitrocalcite, Ca­(NO₃)₂, particles formed whereas the total number of particles decreases. This indicates that these particles are mobile and coalesce. Phase imaging combined with force curve measurements reveals areas of inhomogeneous energy dissipation during the process of water adsorption in relative humidity experiments, as well as during nitrocalcite particle formation. Potential origins of the different energy dissipation modes within the sample are discussed. Finally, XPS analysis confirms that NO₂ adsorbs on CaCO₃ (101̅4) in the form of nitrate (NO₃^–) regardless of environmental conditions or the pretreatment of the calcite surface at different relative humidity.