Title: Influence of synthesis parameters on iron nanoparticle size and zeta potential1
Authors:
Nikki Goldstein
Lauren F. Greenlee
Online Resource 1
Transmission electron microscopy (TEM) and field emission scanning electron microscopy
(FESEM) were used to obtain images of representative nanoparticle suspensions. The smaller
size group of approximately 10 nm obtained in many of the dynamic light scattering
measurements was observed with TEM (Figure S1).
Figure S1. Transmission electron microscopy (TEM) images of zero valent iron nanoparticles
made from FeSO4*7H2O in the presence of (a) 0.8 mol ATMP per mol Fe and (b) 0.0005 mol
CMC per mol Fe.
In general, the size of the particles in the electron microscopy images is typically smaller than
the size measurement given by dynamic light scattering (DLS) because DLS measures the
hydrodynamic diameter of the particles in solution. This measurement is based on the Brownian
motion of the particles in the solvent. The hydrodynamic diameter of a particle in a specific
solvent is calculated (by the DLS instrument) using the Stokes-Einstein equation and is
dependent on the solvent temperature, the solvent viscosity, and the translational diffusion
coefficient of the particles. To calculate hydrodynamic diameter, the Stokes-Einstein equation is
given as:
1
Contribution of NIST, an agency of the US government; not subject to copyright in the United States.
1
d (H ) 
kT
3D
where k is the Boltzmann’s constant, T is the absolute temperature,  is the solvent viscosity, and
D is the translational diffusion coefficient. The hydrodynamic diameter calculated is the
diameter of a sphere that has the same translational diffusion coefficient as the actual particle in
solution. The hydrodynamic diameter is therefore a measurement of not only the particle but any
molecules, such as organic stabilizers, that may be associated with the particle surface, as well as
an associated hydration layer of water molecules. The hydration layer, or electrical double layer,
is affected by the ionic strength of the solution. For non-spherical particles, the hydrodynamic
diameter is a function of the particle dimension that controls the particle diffusion.
For this work, electron microscopy images indicate that the iron nanoparticles obtained were
primarily spherical particles, which allows comparison between the DLS results obtained for
different synthesis methods tested. However, the different stabilizers tested might have an effect
on the particle size distributions obtained by DLS, due to their presence on the particle surface.
Larger polymers, such as CMC, might cause a larger particle size measurement by DLS than
smaller stabilizers, such as the phosphonates, because they take up more volume on the surface
of the nanoparticles. The three phosphonate molecules tested are similar in size, as compared to
CMC, but ATMP is half the molecular weight of HTPMP and could have less of an effect on the
calculated hydrodynamic diameter than HTPMP. Furthermore, the solvent choice can also have
an effect on calculated hydrodynamic diameter. The DLS instrument used was set up with the
viscosity of each solvent used (i.e., water, methanol, acetonitrile, or isopropanol), but the
conformation of the stabilizer on the surface of the particles, as well as the size of the electrical
double layer around the nanoparticle, will change with different solvents. All of these factors
tend to make the measured particle size from DLS appear larger than the size of the particles
imaged with electron microscopy.
In the FESEM images, zero valent iron nanoparticles made with FeCl3 (Figure S2) appear less
uniform than those made from FeSO4*7H2O (Figure S3), with a wider range of particle sizes
visible. The particles shown in Figure S2a and b can be compared to the DLS results shown in
Figure 2 of the main article. The FESEM image for particles formed in the presence of 0.05
DTPMP reveal two particle size groups, at approximately 50 nm and 200 nm. These two particle
sizes correspond to the bimodal particle size distribution obtained in Figure 2 (labeled as
DTPMP FeCl3). For the synthesis method that involved 0.05 HTPMP and FeCl3 (Figure S2b),
there appear to be some larger particles and a portion of the sample where individual particles are
difficult to resolve in the FESEM. This particle population may correspond to the smaller
particle size of 15.5 nm ± 1.8 nm obtained using DLS. While the microscopy images and the
DLS data appear to correspond for the DTPMP and HTPMP stabilizers, the results for ATMP are
not as straight forward. The microscopy images for 0.05 ATMP for the two different iron salts
show a more uniform particle size and particle morphology for FeSO4*7H2O (Figure S3a) than
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for FeCl3 (Figure S2c), whereas the DLS measurements (data not shown) resulted in virtually
identical particle size distribution curves. In this case, the microscopy images reveal differences
that are not detectable from the DLS data. While primarily larger particles are visible in the
FESEM image of Figure S3a, TEM images similar to those shown in Figure S1 were also
obtained for an ATMP:Fe ratio of 0.05. The microscopy results indicate that even though the
volume particle size distribution predicts most of the particle volume will be in particles with a
diameter of approximately 8 nm (main article, Figure 6a, control sample), there is a significant
portion of the particle population in the larger particle size range (main article, Figure 3a).
Finally, the FESEM image of nanoparticles synthesized from FeSO4*7H2O in the presence of 0.3
mol DTPMP per mol Fe appear to be aggregated together (Figure S3b), a result that supports
particle size distributions obtained (Figure 4 in the main article). These examples illustrate the
importance of using multiple different characterization techniques to evaluate nanoparticles after
synthesis.
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Figure S2. Field emission scanning electron microscopy (FESEM) images of zero valent iron
nanoparticles made from FeCl3 in the presence of (a) 0.05 mol DTPMP per mol Fe, (b) 0.05 mol
HTPMP per mol Fe and (c) 0.05 mol ATMP per mol Fe.
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Figure S3. FESEM images of zero valent iron nanoparticles made from FeSO4*7H2O with a
molar ratio of (a) 0.05 ATMP:Fe and (b) 0.3 DTPMP:Fe.
For higher molar ratios of ATMP:Fe and DTPMP:Fe, the particle size distributions indicate that
the majority of the particles are quite small. FESEM images (Figure S4) suggest that the larger
(~100 nm) particles may have formed from the aggregation of smaller particles, and there is
evidence of smaller structures that may have been dispersed particles in the dynamic light
scattering measurement. The small particles in the FESEM images of Figure S4 appear to be
approximately 10 nm, and the TEM images in Figure S1 confirm this conclusion. These
microscopy images appear to support the DLS results for the higher molar ratios of ATMP (main
article, Figure 3b) and DTPMP (main article, Figure 4a).
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Figure S4. High magnification ((a) 200,000x and (b) 500,000x) FESEM images of zero valent
iron nanoparticles made from FeSO4*7H2O with a molar ratio of 0.5 ATMP:Fe.
The molecular structures of the three phosphonate stabilizers tested in this work are shown in
Figure S5.
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Figure S5. Molecular structures of phosphonate stabilizers: (a) Aminotris(methylenephosphonic
acid) or ATMP, (b) Diethylenetriamine penta(methylene phosphonic) acid or DTPMP, and (c)
Bis(hexamethylene triamine penta(methylenephosphonic acid)) or HTPMP.
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