A study of novel eco-friendly nanomaterials for optoelectronic and electrochemical applications
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Nanotechnology has developed rapidly in recent decades, producing many new functional materials, devices, and systems by controlling matter at nanometer length scales (1-100 nm). Nanocrystals are of broad interest due to their tunable size- and shape-dependent physical and chemical properties, which give them enormous potential for application in future electronics, renewable energy technologies, health care, and medicine. This dissertation focuses on understanding bottom-up synthesis of nanomaterials from molecular precursors. Through systematic controlled synthesis of nanomaterials, their properties can be tuned to meet the needs of specific applications. These studies also improve our overall understanding of the relationships between nanomaterial synthesis processes and the resulting product properties. In chapter 1, we focus on copper-based alloy chalcogenide nanocrystals, particularly cation deficient nanocrystals that exhibit localized surface plasmon resonance (LSPR). This class of materials has attracted much interest due to their lack of toxicity, potential low cost, and tunable band gap and LSPR energies. Three specific systems are considered in this chapter. We present new methods for solution phase synthesis of Cu-In-S, Cu-Sn-S and Cu-Sn-Se and characterize the range of obtainable nanocrystal sizes, compositions, morphologies, and properties in each case. These materials have enormous potential for application in future low-cost solution-processed electronics and optoelectronics. In chapter 2, we use cation exchange to prepare a new heterogeneous nanostructure. We employ cation-deficient copper sulfide (Cu 2- x S) NCs as a template for preparing gold sulfide (Au 2 S) NCs. Such high-quality colloidal Au 2 S NCs are difficult to prepare directly. In this process, Au cations simultaneously diffuse into the NCs and fill Cu vacancies, as the original cations Cu diffuse out, initially forming Cu 2- x S–Au 2 S core/shell nanostructures and then pure Au 2 S nanoparticles. In chapter 3, we explore the use of degenerately doped copper chalcogenide NCs as electrocatalysts. Covellite phase Cu 2- x S nanoplates with three different doping levels were tested for their electrocatalytic activity for the oxygen reduction reaction (ORR) in alkaline solution. Additionally, the most heavily doped, and most active, Cu 2- x S nanoplates were deposited on carbon black and on reduced graphene oxide, producing significant improvement in electrical conductivity and electrocatalytic performance. This work showed that free hole-rich copper-deficient copper chalcogenides may provide a new opportunity to develop novel electrocatalysts and can be the starting point for a new class of nonprecious metal electrocatalysts. In chapter 4, we study highly graphitized nitrogen-doped nanocarbon materials derived from low-cost nitrogen-carbon precursors via a high-temperature approach in the presence of Fe, Co, Ni, and Mn. Synthesis in the presence of Fe yielded the largest tube size, followed by Co and Ni. In contrast to generating tubes, Mn produced a clot-like carbon morphology with relatively low surface area. We tested the ORR activity of these nanocarbons in both acidic and alkaline media, and demonstrated that the Fe-derived tubes exhibited the highest electrocatalytic activity, along with the highest BET surface area, electrochemically accessible surface area, and nitrogen content. This work provides an effective approach to further improve the performance of M-N-C precious metal-free electrocatalysts by optimizing morphology and surface area of nanocarbons. Chapter 5 provides a summary of the dissertation and chapter 6 discusses possible future research directions.