Atomic Force Microscopy
and X‑ray Photoelectron
Spectroscopy Study of NO<sub>2</sub> Reactions on CaCO<sub>3</sub> (101̅4) Surfaces in Humid Environments
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Abstract
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 (NO<sub>2</sub>) adsorption on calcium carbonate (CaCO<sub>3</sub>) (101̅4)
surfaces at 296 K in the presence of relative humidity (RH). At 70%
RH, CaCO<sub>3</sub> (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 CaCO<sub>3</sub> (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(NO<sub>3</sub>)<sub>2</sub>, 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 NO<sub>2</sub> adsorbs on CaCO<sub>3</sub> (101̅4) in the form of nitrate (NO<sub>3</sub><sup>–</sup>) regardless of environmental conditions or the pretreatment of the
calcite surface at different relative humidity