Fig. 1 (Left) Low resolution TEM images of Fe2GeS4 NCs (J. Am. Chem. Soc., 2013, 135 (49), 18256-18259)
(Right)J-V plots of the photocurrent response from annealed, 150 nm thick CZTS nanocrystal thin films (ACS Appl. Mater. Interfaces, 2011 3, (1), 58-66)

Synthesis of photovoltaic absorber materials

The Prieto lab photovoltaic group is interested in synthesizing earth-abundant and low-toxic semiconductor nanocrystals (NCs) including Cu2ZnSn(SxSe1-x)4, FeS2, Fe2GeS4 and Cu-Sb-Se compounds for third generation solar cells. In addition to exploring novel synthetic strategies, we are also interested in addressing some of the fundamental issues (e.g. solubility, oxididation and charge transport) associated with these NC systems. We use a comprehensive approach which includes applying various ligand exchange procedures and organic and/or inorganic surface passivation methods, and studying intrinsic charge transport characteristics of NC thin films by Hall measurements and photocurrent measurements.

Fig. 2 Kinetic plots of fraction of hydrogen adsorbed(left) and desorbed(right) for doped(5% Ni)and undoped Mg NCs.

Magnesium Storage

Hydrogen is an attractive fuel source due to its unmatched earth-abundance, high gravimetric energy density as molecular hydrogen (142 MJ/kg), and clean exhaust when reacted with water. Due to their low volumetric density, conventional hydrogen storage techniques are inadequate for modern transportation applications. Magnesium hydride offers a solid state alternative with relatively high gravimetric capacity (7.6 wt. %). Unfortunately, MgH2 is plagued by slow hydrogen adsorption and desorption kinetics, requiring high temperatures and pressures for hydrogen cycling. In attempts to overcome these kinetic drawbacks, there has been a surge of research in nanostructuring and catalysis for the magnesium-based hydrogen storage system.

Our group has developed a low temperature solution phase synthesis of magnesium nanoparticles that allows for facile size control of particle size and dopant incorporation. Hydrogen uptake kinetic studies on our nanoparticles suggest that while not necessarily affecting the hydrogenation kinetics, nickel incorporation (5% by wt.) drastically increases the desorption kinetics of hydrogen from magnesium hydride nanoparticles. We are interested in studying the mechanism of this apparent catalysis in order to further understand how to improve various hydrogen storage systems.

Fig. 3 Scanning electron microscope(SEM) image of Cu2Sb nanowires

Battery research

Secondary batteries, such as lithium-ion batteries, are critical modern energy storage devices because they can quickly and efficiently store electrical energy using a simple electrochemical cell. Despite thirty years of research, there remain gaps in the battery performance of lithium-ion batteries that limit their capabilities and fail to meet the criteria necessary for future technologies, including low energy density, low power density, and short cycle life. Nanostructured, three dimensional battery materials such as 3D electrodes are promising because they can increase energy and power density per electrode area as well as increase the cycle life of the battery. Research goals in the Prieto group include developing nanostructured 3D anodes, such as nanofoams and nanowires, for secondary batteries and investigating new materials to improve overall battery performance. The research is focused on developing low temperature and environmentally friendly processing methods and using primarily earth-abundant materials to maximize performance and minimize cost and environmental impact of next generation batteries.

Fig. 4 SEM image of branched silicon nanowires

Branched silicon nanowires

Using chemical vapor deposition, silicon nanowires are synthesized in a single step using VLS (Vapor-Liquid-Solid) and/or VSS (Vapor-Solid-Solid) growth. Gold and copper are used as growth catalysts, with copper producing branched wires. The goal of this project is to understand the mechanism of growth as to have finer control of the synthesis. Ultimately, we would like to see these wires implemented and used in devices.