The Renaissance of Iron Pyrite Photovoltaics: Progress, Challenges, and Perspectives - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 142

between crystallites to minimize surface energy. When the NaOH concentration is
increased, more FeS 2 seeds will be generated. While there are still more seeds
present, the polymer will adhere to all the surfaces, though the {111} face still has
a higher surface energy than the {100} even with adsorbed polymer. This facili-
tates faster growth along the [111] direction than the [100] direction, resulting in
final formation of cubic structure. On raising the NaOH concentration, even more
FeS 2 seeds are produced and the polymer to seed ratio drops. This causes the
polymer starts selectively adhering to the {111} that causes [100] direction to
grow faster at the expense of the [111] direction, creating octahedral structures.
Dosages of PVP and PVA were also proven to be important. Holding NaOH
concentration constant (corresponding to creation of octahedral structure) and
changing the ratios, it was seen that the octahedral structure is still obtained, but
the size distribution was much wider. This report is an excellent example of shape
control utilizing ligand affinity to different surface facets and provides insight to
synthetic control of pyrite nanomaterial.
2.3 Solvothermal Synthesis
Synthesis of Pyrite nanocrystals has more recently shifted from hydrothermal
methods to solvothermal synthesis. This stems from the problems of hydrothermal
mentioned above, such as mostly using an expensive single precursor and long
reaction times. Solvothermal synthesis is much like hydrothermal, but does not
utilize water as its solvent and can be underdone in both autoclaves and in standard
glass reaction vessels. The solvent is usually a high boiling point organic along
with ligands to help stabilize the growth of the nanoparticle. Some of the more
common ligands utilized in pyrite synthesis are oleic acid and oleylamine. In this
section, two different solvothermal methods are examined. The first is precipitation
method (one pot method) where all the precursors are loaded into a flask and then
heated up and the particles are then precipitated out after reaction. The second is
the hot injection method (two pot) where the two precursors are put into separate
flasks and allowed to decompose separately. One of the precursors is then quickly
injected into the other, usually via syringe, to start the reaction. A key parameter is
having the precursor solutions at different temperatures upon injection, as this
causes a drop in temperature upon injection that causes a supersaturation of the
compound. Supersaturation is relieved by precipitating out small seeds of the
material, followed by growth.
2.3.1 Precipitation Method
With the goal of eliminating the use of a single precursor and the desire to create
final particles quickly, researchers have started using a one-pot precipitation
method to create pyrite nanocrystals. Yuan et al. showed by utilizing Hematite