SYNTHESIS, INTERCONVERION, CATION INCORPORATION, AND APPLICATIONS OF PLASMONIC COPPER SULFIDE-BASED NANOMATERIALS
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A diverse array of colloidal semiconductor nanomaterials has been synthesized and studied over the past few decades. They are being explored or employed in many emerging applications based on their unique physical, chemical, and biological properties. Among these nanomaterials, copper chalcogenides have been the subject of much recent research in colloidal semiconductor nanocrystals (NCs) that exhibit localized surface plasmon resonance (LSPR) which can produce strong absorbance and scattering of light. This optical phenomenon is useful in several rapidly growing applications in theranostics, nanophotonics, and nanoelectronics.This dissertation focuses on synthesis, interconversion, heterocation incorporation and applications of plasmonic copper sulfide-based nanomaterials. Chapter 1 provides an introduction to copper sulfide NCs and describes methods for synthesizing them. It includes our recent progress on developing a chemical method for synthesis of high-quality plasmonic covellite CuS nanoplatelets (NPls), with useful absorbance at near-infrared wavelengths. LSPR was observed in these NCs and tuning of their LSPR was achieved by controlling their diameter while keeping their thickness constant (tuning their aspect ratio). We further generalized the method for preparation of CuS NPls to achieve rapid, room-temperature synthesis of various metal sulfides, including Ag2S, PbS, and CdS, with tunable optical properties. In Chapter 2, we describe and demonstrate reversible interconversion between copper sulfide NCs of different composition and crystal phase. The copper sulfide class of materials is known for its many possible crystal phases and morphologies. We studied the reversible interconversion by reducing disulfide bonds in covellite CuS and adding an organo-sulfur complex. We also revealed that covellite CuS NCs have a preferential plate-like morphology, due to the layered structure of covellite CuS. Chapter 3 mainly involves the outcomes, mechanisms, and related shape and composition evolution of incorporation of foreign cations into copper sulfide NCs. Ternary copper tin sulfide (CTS) NPls were prepared by using binary CuS NPls as the template. Two distinct CTS crystal phases were obtained using different combinations of Sn sources (Sn(II) or Sn(IV) precursors) and reducing agents. In addition, copper indium sulfide (CIS) NPls were prepared by a similar method. Then CIS NPls were further converted to biconcave (red blood cell-like) djurleite Cu1.94S NCs via cation exchange. The formation of biconcave structures is attributed to aggregation of defects produced by rapid In3+ out-diffusion. These defects migrate to the center of NPls to trigger the local collapse of the NPls. Finally, we further prepared a variety biconcave metal sulfide NPls (CdS, ZnS, MnS and CuInS2) using the biconcave Cu1.94S NPls as a template. We demonstrated that the compatibility of anion sublattice (hcp vs. fcc) plays a decisive role in controlling morphology preservation or evolution before and after cation exchange (CE) reactions. Again starting from the covellite CuS NPl templates, we incorporated monovalent Ag+ into them to produce heterogeneous copper sulfide-based nanostructures. The outcome of the incorporation process strongly depended on the initial amount of Ag+ provided. Plate-satellite CuS-Ag2S heterostructures were produced when using a small (0.1 mmol, insufficient to fully convert CuS to Ag2S) amount of Ag+, while biconcave-particle Ag2S-Ag heterodimers were produced when applying high (0.5 mmol, in excess of the amount required to fully convert CuS to Ag2S) amount of Ag+. Through this series of studies, we conclusively demonstrated that trivalent and tetravalent cations can be incorporated into reduced CuS NPls to produce homogeneous ternary alloy NPls, while the monovalent and divalent cations cannot coexist with Cu+ ions in the Cu2-xS phase. In turn, the incorporation of them leads to formation of heterogeneous NPls and finally produces copper-free metal sulfide NPls. The cation valence selectivity arises from conflicts between charge balance and coordination between Cu+ and divalent cations. Last but not least, we incorporated indium and tin into Cu1.81S-ZnS heterostructures. We demonstrate that the outcomes of cation incorporation are strongly influenced by heterocation identity and valence and by the presence of a Cu-extracting agent. The selectivity of cation incorporation depends upon both the cation itself and the hetero-domains in which CE reactions take place. The final nanocrystals (NCs) emerge in many forms including homogeneous NCs, heterodimers, core@shell nanoheterostructures (NHs) and NHs with three different domains. Chapter 4 mainly contains two examples of applications of copper chalcogenide nanomaterials, for electrocatalysis and photothermal therapy. The CuS NPls were tested for electrocatalytic activity for oxygen reduction reaction (ORR) in alkaline solution. ORR activity increased with increasing diameter of the CuS NPls, due to anisotropy of electron mobility and electrochemical activity of 2-dimensional NPls. Size-tunable Cu2-xSe nanoparticles (NPs) were used as photothermal agent in the second biological window of transparency, taking advantage of their tunable LSPR absorbance at near-infrared (NIR) wavelengths. Our results provide important insights into the relationship between the irradiation wavelength and photothermal conversion efficiency. A summary and outlook for future work in this field are presented in Chapter 5.