Size/Shape – Property Relationships of Non - Carbonaceous Inorganic Nanoparticles and their Environmental Implications - Environmental and Human Health Impacts of Nanotechnology - page 90

crystalline substance assume different shapes, this generally means that different
crystal faces comprise their surfaces. For example, consider nanoparticles of a
rock salt structured mineral. A cubic nanoparticle displays {100} faces, a truncated
cubo-octahedron displays {100}, {111} and {110} faces, and an octahedral nanopar-
ticle will display {111} faces. Different crystal faces will be more or less stable
(have different surface energies), depending upon their surface bonding. It is
expected that less stable faces would be etched more readily than more stable faces.
In our example, assuming that all other conditions are equal (same crystal structure,
composition, solution undersaturation, etc.), this would mean that the three differ-
ently shaped nanoparticles might dissolve at different rates. While the energetic
stability of crystal surfaces affects dissolving crystals of all sizes, it is particularly
important for nanoparticles because even minimal dissolution may result in their
annihilation.
In principle, these concepts are simple, but applying them to quantitatively
predict morphology dependent dissolution trends in nanoparticles is diffi cult. This
is because little is known regarding the relative stabilities of nanoscale surfaces.
The presence of surface defects, steps or kinks, which may be more evident on
nanoparticle surfaces, will also infl uence the energetics of dissolution. Another
complicating factor is the presence of coatings or other external substances, which
are discussed in the following section.

3.5.3 Effects of Nanoparticle Coatings and External Substances

As with the surfaces of bulk materials (Zhang and Nancollas, 1990; Casey and
Ludwig, 1995; Becker et al. , 2005), it has been shown that external substances, par-
ticularly those that can coordinate to nanoparticle surfaces, can strongly infl uence
nanoparticle growth and dissolution (Jun et al. , 2006 ; Li et al. , 2005, 2006b ; Yin and
Alivisatos, 2005). Anthropogenic nanoparticles released into the environment are
likely to encounter many substances that could interact strongly with or sorb onto
their surfaces, and many will already have coatings on their surfaces.
One way such coatings or sorbed species may affect dissolution is by stabilizing
particular crystal surfaces. Consider the partial dissolution of a truncated cubo-
octahedral nanoparticle composed of the rock salt structured material introduced
in Section 3.5.2. The reaction coordinate for this process is displayed in Figure 3.5.
Thermodynamically, the most favoured end product of dissolution for this system
is a sphere. (This is not to imply that a sphere is always the most favoured shape
for every system.) Imagine now adding a substance to the solution of nanoparticles
which binds to and stabilizes the {100} and {110} crystal faces. This will increase the
activation energy needed to obtain a spherical nanoparticle. Unless there is enough
thermal energy in the system to surmount this kinetic barrier, it is likely that the
process with the lower kinetic barrier (lower activation energy) will dominate. In
this scenario, the {111} faces are energetically unstable relative to the other faces,
so they will etch more readily. This etching results in an octahedrally shaped particle
rather than a spherical particle.
Coatings or external compounds can affect dissolution in other ways. For example,
a coating that forms a micellar structure around a nanoparticle might reduce the