Environmental Engineering Reference
In-Depth Information
Spherical
: typical in non-crystalline materials
and very small crystalline particles
Tear Drop
: An extension of the spherical
morphology.
Geometric solid
: typical of crystalline
materials. May be a range of shapes inc.
cubic, tetrahedral, icosahedral, etc.
Dendritic
: Composed of nanoscale wires the
dendrite may be much larger than 100 nm.
Rod or wire
: May have a range of cross
sections including circular, cubic and
pentagonal
Dumbbell
: Formed by the growth of one
material only at the ends of a rod of another
material
Discotic
: thin flat plate often wider than 100
nm. May be a range of shapes including
hexagonal and irregular
Tetrapod
: Formed by the growth hexagonal
phase rods from a cubic seed crystal
Figure 2.5
Some reported morphologies for nanomaterials (see text for references).
3.5
3
r
3
SA:Volume Ratio =
2.5
2
1.5
1
0.5
0
0
5
10
15
20
25
30
Particle Diameter (nm)
Figure 2.6
The effect of radius on the surface area to volume ratio for a constant mass of
material.
2.3.1
Nanoparticle Surface
All nanoparticles have an exceptionally high surface area to volume ratio. This is
one of the key features of a nanoparticle and often has a dramatic effect on the
particle physical properties as well as being key in understanding its fate and behav-
iour in real environments. If a simple spherical particle is considered, the surface
area per unit mass scales to the inverse of the radius (Figure 2.6). The atoms at the
surface of any nanoparticle contribute to a signifi cant proportion of the whole
material. The fraction of atoms at the surface of a nanoparticle can be very high.
If a spherical gold particle (atomic radius = 0.144 nm) is considered, then a 2.5 nm
particle has 53% of the atoms at the surface, at 10 nm this falls to 16%, at 50 nm
this falls to 3% and at 100 nm this falls to 2%. Comparing this to a one micron
particle at 0.2% and a seven micron particle with only 0.02% clearly shows the
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