Environmental Engineering Reference
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growth step, where they cooled the flask to room temperature and hexadecylamine
solidified. They load in on top of the solid reaction mixture oleylamine, which will
act as a weak coordinating ligand and more iron and sulfur precursor with the ratio
of [S]/[Fe] = 2.05. The flask is then repurged with argon and then heated to
200 C for 9 h. This growth was repeated once more without the oleylamine that
helped to complete the growth of the cubes (some truncated cubes were observed
after just one growth). Note that this synthesis takes a total of 21 h in the reaction
steps alone, but the time may be worth the price to achieve these beautiful
structures. During the growth period the irregular pyrite nanocrystals were used as
''seeds'' to facilitate pyrite crystal growth that produced the final cube shape. The
use of the weak alkylamines ligands was claimed to help make sure that small
cubes were created. Hexadecylamine has strong interligand interactions that
facilitate the creation of the small particles. Oleylamine is added after the nucle-
ation step to help steer growth to cube structure, and it is found that if not added
then the final product is still irregularly shaped particles, albeit bigger in size. The
reduction of the ratio of sulfur to iron concentrations also played a role in the final
shape. If a ratio of [S]/[Fe] = 2 was used then pyrrhoite(FeS) impurities were
found, and if [S]/[Fe] = 2.1 the edges of the final cubes were more rounded. It is
also important to note, even with the long growth periods associated with this
synthesis, that Ostwald ripening did not occur. Interestingly, the absorbance of
these final cubes mocked those of irregular shape. Most cubic pyrite nanostructures
in the literature show absorbance in the NIR [
24
,
44
]. This could show that size
indeed affects the absorbance of these particles or that some other phenomenon is
occurring such as plasmonics. Nonetheless, this synthetic method shows great
control over particle growth and has produced some of the best crystalline particles
to date.
2.3.2 Hot Injection Synthesis
Hot injection synthetic methods have been extensively used in the past to create
nanoparticles of many different material systems. Peng et al. demonstrated the
versatility of this method in their groundbreaking reports of creating cadmium
selenide nanoparticles [
45
-
48
]. From there it has been adapted to be used in
creating a whole host of metal-chalcogenide nanoparticles [
49
-
52
]. The Law
group was the first to use hot injection to create spherical pyrite nanocrystals [
53
].
Two flasks were used in a typical synthesis, one loaded with Octadeclyamine and
FeCl
2
while the other was filled with diphenyl ether and sulfur. The iron-con-
taining flask was heated to 220 C for one hour to allow for decomposition of the
iron salt, while the sulfur flask was heated to 70 C for an hour to allow for
complete dissolution of the sulfur. The sulfur solution was then rapidly injected
into the iron-containing flask and allowed to react for several hours. Aliquots were
taken to examine the growth of the particles over time. Figure
5
shows the pro-
gression of synthesis. The seeds can be seen from the start as very small clusters
and as the reaction progresses, it forms spherical-like particles. After the creation
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