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showed an absorbance peak in the infrared (1420 nm), which is a redshift of the
bulk material.
In a following study, Wadia et al. showed that the size of these pyrite cubes
could be reduced in size [
3
]. By making use of a different precursor, iron (III)
diethyl dithiophosphate (Fe[(C
2
H
5
O)
2
P(S)S]
3
), hexadecyltrimethylammonium
bromide (CTAB), and a higher temperature nanocube with side lengths of
*100 nm were achieved. Purity of phase was proven by XRD and also confirmed
the 0.95 eV indirect band gap with X-ray absorption and resonant X-ray emission
spectroscopy. They note that in this synthesis, slightly acidic conditions and the
CTAB were critical for the creation of pure phase pyrite.
The most recent study utilizing hydrothermal synthesis for pyrite nanocrystals is
from Wang et al. [
25
]. In this report it is shown that it was possible to avoid single
precursors and also that it was possible to create shapes other than cubes by the
addition of polyvinylpyrrolidone (PVP) and polyvinyl alcohol (PVA) and sodium
hydroxide (NaOH). For precursors, ferrous chloride tetrahydrate (FeCl
2
• 4H
2
O)
and pure sulfur powder were used along with the PVA and PVP. By varying the
NaOH concentration and keeping the PVA and PVP polymer concentration constant,
it was shown that beautiful cubes, octahedral, and sphere-like crystallites are formed
with sizes of *140, *220, and *400 nm, respectively. Figure
3
shows scanning
electron microscopy (SEM) and transmission electron microscopoy (TEM) pictures
of the final particles. The reaction itself was dependent on the presence of NaOH, as
when a control reaction with out any NaOH was conducted only black grease was
obtained. A mechanism for formation of the FeS
2
was proposed as follows:
Fe
2
þ
þ
2OH
$
Fe
ð
OH
Þ
2
4S
þ
3H
2
O
!
2H
2
S+H
2
S
2
O
3
H
2
S
þ
2OH
!
S
2
þ
2H
2
O
3S
þ
6OH
!
2S
2
SO
3
3
þ
3H
2
O
Fe
2
þ
þ
S
2
!
FeS
Fe
2
þ
þ
H
2
S
!
FeS
þ
2H
þ
FeS
þ
S
!
FeS
2
FeS
þ
H
2
S
!
FeS
2
þ
H
2
NaOH plays a vital role in the final formation of these particles and explains the
lack of pyrite formation without it. It is also worthwhile to note that this mech-
anism goes through an FeS intermediate state. The reaction rate is dependent on
the inclusion of NaOH, and therefore changing the concentration should affect the
amount of FeS
2
seeds generated. With low concentrations of NaOH, less seeds are
formed and a large amount of the polymer adheres to the seeds that result in a
system where there is no preferred attachment to the different crystal facets by the
polymer. In this case, the final particles then form large sphere-like composites of
smaller particles most likely via a self-organization followed by intergrowing
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