Size/Shape – Property Relationships of Non - Carbonaceous Inorganic Nanoparticles and their Environmental Implications - Environmental and Human Health Impacts of Nanotechnology - page 94

also play a role in faster dissolution. While bulk natural lead sulfi de mostly displays
{100} faces, the lead sulfi de nanocrystals exhibit {111} and {110} faces. As described
above, these faces dissolved more quickly than the {100} faces.
These initial results have important implications for the dissolution behaviour of
nanoparticles in the environment. Larger micro-sized particles for further size
comparative rate studies are currently in the process of being synthesized.

3.5.5 Solid State Cation Movement in Nanoparticles

Another phenomenon that may affect nanoparticle degradation and fate is solid
state cation movement into or out of nanoparticles. One type of cation movement
is cation exchange, in which cations in solution replace cations in a lattice. Even if
cation exchange does not occur signifi cantly in the bulk form of a particular mate-
rial (excluding perhaps on its surfaces), this does not preclude this process from
happening fully in the nanoparticulate form or in thin fi lms (nanoscaled fi lms

< 100 nm in thickness). In the bulk forms of such materials, cation exchange is kineti-

cally controlled by the advancement of a reaction zone, along which cations and
vacancies travel. If nanoparticles are small enough, they may be as large as or
smaller than this minimum reaction zone, resulting in faster cation exchange (Son
et al. , 2004 ).
One fascinating example of this phenomenon is the room temperature complete
exchange of silver ions for cadmium in CdSe nanoparticles. Son and co-workers
synthesized spherical CdSe nanoparticles (4.2 nm in diameter) and rod shaped
CdSe nanoparticles (varying dimensions) and mixed them with solutions of silver
nitrate (AgNO 3 ). (While this was done in a toluene-methanol mixture, it should be
noted that cation exchange in nanostructures has also been observed for aqueous
systems (Mews et al. , 1994 ; Lokhande et al. , 1992; Dloczik and Koenenkamp, 2004).)
Within about 100 ms (Chan et al. , 2007), CdSe nanospheres are transformed into
Ag 2 Se nanospheres as shown in Figure 3.7. The exchange could be subsequently
reversed by adding an excess of cadmium ions and a compound that forms a stable
complex with silver (tributylphosphine). Notably, when the same reaction was
attempted with micrometre sized powders of CdSe, cation exchange was prohibited,
even over weeks. Cation exchange in this system has also been tested with Pb 2+ and
Cu 2+ , and has been demonstrated to occur with a variety of ions in metal sulfi des
in both nanoparticles and thin fi lms (Robinson et al. , 2007; Dloczik and Koenenkamp,
2004 ; Lokhande et al. , 1992 ; Mews et al. , 1994 ).
Another interesting aspect of cation movement in nanoparticles is that the mor-
phology of particles can be altered. As morphology can affect the chemical and
physical behaviour of nanoparticles, this has important implications. Two examples
are displayed in Figure 3.8. In the CdSe to Ag 2 Se conversion as described above,
smaller nanorods are converted to nanospheres. Another example of shape change
due to cation diffusion is in the synthesis of cobalt sulphide from metallic cobalt
nanospheres (Yin et al. , 2004, 2006). When reacting with elemental sulfur, the out-
wards diffusion of cobalt ions produces a hollow nanosphere.
It should be noted that such cation movement processes are contingent upon
favourable thermodynamic driving forces, as well as factors such as the rate of dif-