Size/Shape – Property Relationships of Non - Carbonaceous Inorganic Nanoparticles and their Environmental Implications - Environmental and Human Health Impacts of Nanotechnology - page 92

3.5.4 Case Study: The Dissolution of Lead Sulfi de Nanoparticles

From the previous discussion of nanoparticle dissolution, it is evident that not only
is size important, but morphology, coatings and molecules present in the surround-
ing solution. Currently, in our laboratory, the fi rst two factors are being studied. The
non-oxidative dissolution of

15 nm diameter lead(II) sulfi de (PbS) nanoparticles
in hydrochloric acid (pH 3) is being examined (Liu et al. , 2007a ). Bright - fi eld trans-
mission electron microscopy (TEM) is used to track changes in particle size and
high-resolution TEM is used to measure changes in morphology and structure.
The dissolution of nano-sized lead sulfi de (galena) may have implications for the
behaviour of both synthetic and natural nanomaterials in the environment. Lead
sulfi de is a low band gap semiconductor used in applications such as infrared detec-
tors. Nanoparticles of lead sulfi de are popular in nanoscience research and are
commercially available. As for natural systems, it is known that nanoparticulate
metal sulfi des are present in some environments, and that mineral nanoparticles
may be involved in the transport of heavy metals (Hochella et al. , 2005b, 2008 ;
Labrenz et al. , 2000 ).
Lead sulfi de nanoparticles are synthesized under inert atmosphere in organic
solution with surfactant via a previously published procedure at high temperature
(Joo et al. , 2003). This synthetic procedure produces monodispersed, highly crystal-
line nanoparticles, as confi rmed with TEM and X-ray diffraction (XRD). After an
initial washing procedure to remove excess free surfactant, nanoparticles are depos-
ited onto a carbon/gold TEM grid substrate. Having the particles on a substrate
helps to prevent aggregation, as this would complicate analysis. X-ray photoelec-
tron spectroscopy (XPS) confi rms that subsequent washing steps remove the
majority of the surfactant (although undetectable trace amounts may remain) and
that washing does not signifi cantly affect the presence of any oxidation species on
the nanoparticle surfaces.
Washed, dried grids are exposed to nitrogen-purged hydrochloric acid solutions
(pH 3) under constant stirring for varying periods. Images from samples exposed
to the acid for different times are compared with each other using TEM measure-
ments. Two interesting trends are summarized here.
Firstly, the morphology of the lead sulfi de nanoparticles changes after dissolution.
From high resolution TEM measurements (Figure 3.6), the {110} and {111} faces
are being etched more quickly than the {100} faces ({111} (data not shown). Such
results match what might be expected from our knowledge of bulk crystals.
Generally, on a crystal face, the rate at which an atom is removed from that face is
inversely proportional to the number of bonds it has (Lasaga and Luttge, 2004).
Atoms in the ideal bulk {110} and {111} faces have surface atomic coordination
numbers of four and three, respectively, while the {100} faces have an atomic coor-
dination number of fi ve. Therefore, it would be expected that the {100} faces would
etch more slowly than the {111} or {110} faces. At least for this system, these results
indicate that some of our current knowledge about bulk crystal surfaces can be
used to predict how nanoparticles might behave in the environment.
Secondly, lead sulfi de nanoparticles have been found to dissolve at surface area
normalized rates higher than those for bulk lead sulfi de by approximately one to
two orders of magnitude. This difference in dissolution rate may be attributed to

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