Synthesis and Characterization of Transition Metal Nitride and Selenide Nanocrystals and Heterostructures PDF Download

Author: Robert William Lord
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Languages : en
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Book Description
The properties of nanomaterials are intimately dependent on their size, morphology, composition both elemental and structural, as well as their crystal structure or atomic arrangement. There exists a fundamental need to develop methods to precisely control and tune these parameters in order to target desirable materials. Colloidal chemistries utilize wet chemical precursors to synthesize inorganic nanomaterials from the bottom up and produce high quality materials. In addition to single component material synthesis, colloidal chemistries have been developed to synthesize multicomponent nanomaterials systems through a process called seeded growth. Seeded growth utilizes preformed nanoparticles as substrates to nucleate and grow new materials from their surfaces. This has led to the synthesis of highly complex heterostructured nanoparticles which allow for the incorporation of multiple material properties within single particle frameworks. While these techniques can control the structure and composition of the synthesized material, they do not allow for as much control over the resulting crystal structure. Other methods have been developed which allow for crystallographic templating and compositional modulation by post-synthetic cation exchange. Cation exchange utilizes molecular agents to solvate and exchange host cations in preformed crystals with those in solution while maintaining the anion sublattice relatively unperturbed. The crystallographic symmetry of the anion sublattice determines the symmetry of the final product phase. As such, cation exchange has allowed researchers to synthesize materials which are either metastable in bulk or not easily assessable through other methods. In this dissertation I discuss my efforts to utilize these synthetic tools to synthesize new and complex inorganic nanoparticles. First, I describe the seeded growth of Cu3N and Cu3PdN on Pt and Au nanocrystals. Utilizing Pt-Cu3PdN as the model system, it was observed that Cu3PdN nucleated and grew in a step-wise pathway with the initial deposition of Cu onto the surface. This was followed by the deposition of Pd onto the corners and edges of the Pt nanocubes which was followed by the coalescence and crystallization of Cu with the Pd to ultimately give Cu3PdN. When nucleating on more faceted or spherical seeds, whether Pt or Au, the resulting heterostructures took on more core@shell structures. In the absence of Pd, Cu3N nucleates indiscriminately on the surface of Pt without any of the regioselectivity seen with Cu3PdN. When utilizing Au seeds, AuCu alloy formation is observed without any apparent heterostructure formation. These observations helped us develop guidelines which are anticipated to be applicable to the formation of other ternary nitride heterostructures. Second, I discuss the synthesis of a new, metastable phase of copper selenide nanoparticles. This material was shown by EDS and XPS to adopt a nominally 2:1 stoichiometry and the XRD pattern did not match any known phase of Cu2-xSe. However, the nanoparticles did adopt a crystal structure similar to previously observed weissite Cu2-xTe. A structural model for our Cu2-xSe phase was developed utilizing a recently reported structural model for weissite Cu2-xTe which was computationally verified in collaboration with Professor Ismaila Dabo's group. Weissite-like Cu2-xSe has trigonal symmetry (space group P ̄3 m1) and is a layered structure with alternative Cu-rich and Cu-deficient layers sandwiched between layers of Se. UV-vis-NIR spectroscopy of weissite-like Cu2-xSe showed a broad plasmon absorbance band centered around 1550 nm. Lastly, I discuss my efforts to develop synthetic guidelines for the competitive synthesis of ternary copper selenide phases during their nucleation on Pt nanoparticle seeds. We showed experiments which allude to two potential synthetic pathways for the formation of CuFeSe2 and CuInSe2. It was observed that the CuInSe2 forms through a multistep pathway starting with the initial nucleation of Cu2-xSe followed by the incorporation of the In3+ through a high temperature cation exchange reaction. However, CuFeSe2 was shown to most likely to form by direct nucleation. The differences in these reactions were observed when their simultaneous nucleation was attempted, where only Pt--CuIn¬Se2 formed.