The Renaissance of Iron Pyrite Photovoltaics: Progress, Challenges, and Perspectives - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 148

decomposition is slow, less seeds will be created resulting in large ligand coverage
of the crystals that result in thicker/larger plates [ 56 ]. With these simple changes in
synthetic procedures, final shapes of pyrite nanocrystals can be controlled.
2.4 Surface Facet Activity
Now that the different synthetic methods for creating pyrite nanocrystals have been
discussed, a brief aside to discuss the different surface facet activities of pyrite will
be taken. Ennaoui et. al. showed by X-ray Photoelectron Spectroscopy that
adsorbates affect the materials properties and the different crystal surfaces show
different activity [ 4 ]. It was shown that exposing a freshly cleaved sample to
oxygen, the photovoltage increases. It is believed that the oxygen is passivating the
surface and removing defect states out of the gap. When the oxygen coverage
levels increase, no change or more changes occurred which suggests adsorption
occurs only at defect sites. When studying the {100} surface, they see that both
electron donors and electron accepters both adsorb to iron sites. H 2 O shows
coordination via oxygen though it was shown to de-adsorb completely at 300 K.
When the surface is exposed to Br 2 at low concentrations a Br - emission line is
seen, and when the dosage is increased Br 2 emission lines are increased showing
adsorption of molecular Br 2 . After annealing the samples, an interesting band
remains that is attributed to adsorbed bromine ions. The XPS data shows no
evidence of transformation to FeBr 3 or FeBr 2 , which leads to the conclusion that
the Br - ions adsorb rather strongly to the pyrite {100} surface.
We also looked at the different surface activity of pyrite surface facets by
testing the nanocrystals as photocatalysts to photo degrade methyl orange dye [ 24 ].
A set amount of the particles were put in an aqueous methyl orange solution and
put into a black box with a xenon light source that provided a 46 mW cm -2 of
power. Samples were taken every 10 min to access the intensity of the methyl
orange absorbance peak at *475 nm. Figure 9 shows the results of the experi-
ments. Cubes with {100} surface-exposed showed modest activity, while the
{111} terminated nanospheres showed no activity at all. When the intermediate
state was tested, it showed activity in between the cubes and the nanospheres,
which is evidence that these particles are indeed an intermediate, and contain both
surface facets. These differences in activity can be attributed to the exposed atoms
on each crystal face. It has been shown that {100} has iron atoms exposed and also
that this is the active site for adsorption, whereas the {111} only has sulfur which
does not seem photocatalytically active.
Interesting side effects of these experiments allowed us to not only examine the
photocatalytic activity, but also the photostability of the different facets. In the
same figure, the peak shifts to blue as a new peak grows in next to it. It was
determined that this intruding peak was caused by Fe +3 ions. It is known that pyrite
degrades in water naturally by equation [ 30 ]: