The Renaissance of Iron Pyrite Photovoltaics: Progress, Challenges, and Perspectives - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 146

Fig. 6 SEM images of thin films created out of pyrite nanocrystals before and after sintering.
Reprinted with permission from [ 53 ]
taken to test this model by utilizing Law's synthetic method with few modifica-
tions. Like in previous works, upon injecting the sulfur into an FeCl 2 flask at
220 C nanosphere particles were created, but when lowering the injection tem-
perature to 120 C, cubes were obtained. An intermediate state of aggregated
cubes was obtained when the injection temperature of 170 C was used. Figure 8
shows TEM images and UV-V is absorption spectrums of each of the particles.
The exposed surface facets were proven to be {100} and {111} for the cubes and
nanospheres, respectively, by high resolution transmission electron microscopy.
We also have shown that by changing the iron precursor, the kinetics of the
reaction changes due to the different decomposition rates of the iron precursor.
When iron(0) pentacarbonyl (Fe(CO) 5 ) is used as the iron source, thick hexagonal
sheets of pyrite are formed with the {100} surface exposed. If iron (II) acetyl-
acetonate (Fe(acac) 2 ) is utilized for the iron source, thin random sheets with
greigite (Fe 3 S 4 ) impurities, also with the {100} face exposed, are formed. Final
shape dependence can be related to the decomposition rate of the iron precursor
that can be roughly estimated by hard/soft acid/base theory. This theory states that
like components, either soft or hard, will bond stronger than if components are
opposite. Fe +2 is classified as an intermediate strength acid and acetylacetonate is a
very hard base [ 55 ]. This difference makes Fe(acac) 2 the weakest bonded of the
three precursors and it will decompose readily. This means that there will be much
more iron monomers available for reaction, which then explains the higher iron
percent mineral greigite (Fe 3 S 4 ) showing up as impurity. When comparing Cl - ion
to acetylacetonate the chloride ion is also a hard base but not to the same mag-
nitude as acetylacetonate. Taking this into consideration, it does not decompose as