ABSTRACT

Cu2ZnSnS4 (CZTS) is a promising quaternary material for solar energy conversion and catalysis, but until recently, synthesis of phase pure, anisotropic CZTS nanocrystals remained a challenge. In this thesis, we demonstrate control over the morphology and composition of CZTS nanorods, controlled doping of CZTS nanorods with several transition metal cations, and we extend the synthetic method to ternary sulfide nanocrystals.

We begin by showing that the initial concentration of cationic precursors has a dramatic effect on the morphology and composition of hexagonal wurtzite CZTS nanorods. Our experiments strongly indicate that Cu(C5H7O2)2 is the most reactive of the metal precursors used, Zn(CH3CO2)2 is next, and Sn(CH3CO2)4 is the least reactive. Using this reactivity series, we are able to purposely fine-tune the morphology (dots vs. rods), composition, and degree of axial phase segregation of CZTS nanocrystals.

Next, we dope divalent, high spin transition metal ions (M2+ = Mn2+, Co2+, Ni2+) into the tetrahedral Zn2+ sites of wurtzite CZTS nanorods. The resulting Cu2MxZn1-xSnS4 (CMTS) nanocrystals retain the hexagonal crystalline structure, elongated morphology, and broad visible light absorption profile of the undoped CZTS nanorods. The nanorods doped with Mn2+ and Co2+ are EPR active, with spectra which are representative of the literature reports for these transition metals. EPR also shows that washing the nanocrystals with trioctylphosphine oxide (TOPO) is an efficient way to remove excess Mn2+ ions from the particle surface. XPS and FTIR show that Cu2MnxZn1-xSnS4 nanocrystals aggregate not through dichalcogenide bonds, but through excess metal ions crosslinking the sulfur-rich surfaces of neighboring particles. This was also observed in the ternary sulfide (thiospinel) nanocrystals.

In addition to CZTS and CMTS, we extend our synthetic method to synthesize ternary metal sulfides, namely thiospinel and pyrite phase nanocrystals. Due to their application in a variety of energy and catalysis fields, thiospinels and pyrites have attracted attention in recent years. Typically, these materials are synthesized in bulk, with high temperature methods and low surface areas, making them less than ideal for many applications. In this report we use low temperature, solvothermal methods to synthesize nanocrystalline, ternary thiospinels. The nanocrystals have a variety of morphologies, including spheres and nanorods, and show a broad absorbance in the visible range. This is the first time several of the materials are reported in nano sizes. We also report on the attempted synthesis of pyrite nanocrystals.

The results of this work will improve our ability to fabricate CZTS and related nanostructures for photovoltaics and photocatalysis. In addition, these results may help expand the synthetic applicability of CZTS and CMTS materials beyond photovoltaics and into the fields of spintronics and magnetic data storage. Synthesizing nano-scale thiospinels and pyrites may have a direct impact on the catalysis and energy fields as well.