In the field of energy materials, nanotechnology has already found many uses. Nanoparticles and the related nano-fabrication techniques have been used in catalysts for fuel cells [1
], in solar panels as the light collecting component [2
], for hydrogen production [3
] and storage [4
], as well as a host of other applications. More recently, nanotechnology has been used in thermoelectric materials, which have the potential to greatly enhance our current energy production efficiency. Thermoelectric materials rely on the Seebeck and Peltier effects to convert an electric current to a heat gradient, or vice versa
. By utilizing these phenomena, thermoelectric materials can be used to generate electricity from nearly any heat source, for example an automobile engine, in steam turbine electricity generation, or even direct geothermal energy. Until now though, thermoelectric materials have not found widespread use because of their inherently low energy conversion efficiency, described by the dimensionless figure of merit, ZT. In recent years, however, new techniques revolving around nanotechnology have been developed that have opened up the doors to improving the ZT value. Techniques such as nanostructuring allow suppression of the thermal conductivity, which is a useful tool to enhance ZT. These advancements in improving efficiency values have been exciting, yet from the aspect of material sustainability there are still many challenges left to address. The very best thermoelectric materials available today (i.e.
, PdTe, etc.
) contain either rare or toxic elements that limit their practical application [5
]. Tellurium is one such element that is present in nearly all of the high efficiency thermoelectric materials because of its beneficial electronic band properties [8
], but the element is extremely rare on Earth, making these materials increasingly more expensive. With this in mind, new sustainable thermoelectric materials must be sought out that do not rely on rare or toxic elements. To accomplish this, the nanotechnology techniques that have been pioneered in enhancing the thermoelectric efficiency of the traditional materials should now be applied to sustainable materials systems to elucidate and identify new techniques for optimizing thermoelectric properties through the material characteristics such as particle size, shape, composition, structure or interparticle properties [8
]. In this work, we developed a synthetic approach toward nanoparticles composed of copper, iron and sulfur, which is attractive because of the abundant nature of the constituent elements. Apart from this abundant nature, the Cu-Fe-S system is chosen for its structural properties, which can prove to be beneficial for good thermoelectric characteristics [10
]. Chalcopyrite has a tetragonal structure with lattice constants a
= 5.289 Å and c
= 10.423 Å. This class of compound shows a relatively large carrier mobility [11
], which is beneficial for thermoelectric performance. The synthetic approach used allows control of the nanoparticle composition by changing the metallic feeding ratio. Copper sulfide (and its related materials) is a widely studied semiconductor material [12
], and the particles can be created in a straightforward thermolysis reaction [16
]. The resulting particle characteristics are studied using techniques, such as transmission electron microscopy (TEM), X-ray diffraction (XRD) and inductively coupled plasma-optical emission spectroscopy (ICP-OES), then the material is processed into a solid pellet to characterize the Seebeck value of the material. The processing of the sample for Seebeck measurement does not include any ligand exchange or thermal treatment of the nanoparticles, which is highly beneficial since it preserves the true nanoparticle size. This fundamental difference for our materials provides a great contrast to the past studies on the thermoelectric materials composed of chalcopyrite. It is also important to note that the predominant Seebeck value for bulk chalcopyrite is N-type [8
], which is in contrast to our measured value showing P-type conductivity. While doping of the chalcopyrite material can be used to control the type of conduction, it is also possible that surface effects or quantum confinement contribute in this system as well [10
], providing further merits to this sustainable nanoparticle system for thermoelectrics.