Size/Shape – Property Relationships of Non - Carbonaceous Inorganic Nanoparticles and their Environmental Implications - Environmental and Human Health Impacts of Nanotechnology - page 91

∆ G † stab

{100} {110}

{111}

∆ G † 111

∆ G † nostab

Truncated
cubo-octahedron

All {111}
faces

Octahedron

sphere

Reaction coordinate

Figure 3.5 Reaction coordinate for the partial dissolution (etching) of a truncated cubo-
octahedral nanoparticle. For this system, the thermodynamically favoured product is a spheri-
cal nanoparticle. In the absence of stabilizing coatings or ligands, formation of a spherical
nanoparticle is kinetically favoured as well (the activation energy < 111 ). In the

presence of compounds that stabilize the {100} and {110} faces, the activation energy to
obtain a spherical nanoparticle increases. In this case, since this activation energy is greater
than that necessary to form an octahedron (energy

∆ G

†

∆ G

†

nostab

> 111 ), formation of a sphere
is no longer kinetically favoured. Etching the non-stabilized {111} faces is kinetically favoured.
In the absence of enough thermal energy in the system, etching will result in an octahedral
end product rather than a spherical one. (Figure adapted from Y.W. Jun, J.S. Choi and J.W.
Cheon (2006) Shape control of semiconductor and metal oxide nanocrystals through non-
hydrolytical colloidal routes. Angewandte Chemie , 45 , 3414-39. Copyright Wiley-VCH
Verlag GmbH & Co. KGaA. Reproduced with permission.)

∆ GG

stab

†

∆

†

activity of water at the surface. As for external compounds in the solution surround-
ing the nanoparticle, there are experimental examples of dissolution rates being
increased by compounds such as acetic acid (Meulenkamp, 1998) or human serum
albumin protein (Yang and Xie, 2006). External compounds might form stable ionic
complexes with the constituent metal ions in the nanoparticle, hence energetically
favouring dissolution. They also may alter pH, which again can affect nanoparticle
stability.
It is evident from such considerations that in any study concerning nanoparticle
dissolution, as much as possible should be known regarding the composition of the
coatings or the compounds in the solution surrounding the nanoparticle. These
characteristics can be as signifi cant as the composition of the inorganic part of the
nanoparticle.