Nanomaterials for Energy Applications: Cases of Precious Metal and Metal Chalcogenides
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Nanomaterials have been intensively explored over the past two decades for their broad potential applications in fields including energy conversion, energy storage, emissive materials, sensors, biomedical devices, and medical therapies. These materials exhibit unique chemical and physical properties, distinct from the properties of the same materials in bulk form, due to their small size (less than 100 nm). Researchers have invested great effort not only on the investigation of preparation, surface functionalization, and size and shape control of the nanoscale materials, but also on both long-term and near-term applications in academia and industry. This dissertation focuses on synthesizing precious metal, semiconductor, and alloy nanomaterials, studying their physical and chemical properties, and investigating their promising roles in energy related applications, such as gas separation, solar cells, and thermoelectrics.Chapter 1 focuses on synthesizing and functionalizing Pd nanoparticles (NPs) for use in polymer-based membranes for CO2/H2 separation. Pd NPs of different shapes and sizes were produced by colloidal chemistry. In each case, Pd salts were chemically reduced to yield zerovalent Pd, and the growth of the NPs was restrained and controlled by capping ligands. Spherical Pd NPs with an average size of 4.5 nm were found to be compatible with polybenzimidazole (PBI) / dimethylacetamide (DMAc) solutions. The mixed Pd and PBI solutions were cast onto silicon wafers and dried to form Pd/PBI Mixed Matrix Membranes (MMMs). The Pd NPs were uniformly distributed in the MMMs, with Pd channels forming with increasing Pd loading in the membrane. As a result, the H2 permeability and the H2/CO2 selectivity of the MMMs were significantly improved at high Pd loading. The stability of the membrane to H2S and H2O was also confirmed by testing the membranes with mixed gases. Chapter 2 and 3 focus on tin(II, IV) chalcogenide nanocrystals (NCs) and the thermoelectric properties of thin films formed from them. Starting from a tin(IV) precursor, we were able to use different combinations of solvents and anion precursors to control the valence of Sn to produce tin chalcogenide and tin dichalcogenide NCs, while also varying the sizes and shapes of the NCs. These NCs were characterized by X-ray Photoelectron Spectroscopy (XPS), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Powder X-ray Diffraction (XRD), Energy Dispersive X-ray Spectroscopy (EDS), Atomic Force Microscopy (AFM), and Raman Spectroscopy. We also deposited these tin chalcogenide NC dispersions onto glass substrates and investigated the thermoelectric properties of the resulting nanostructured films. We found that annealing at relatively high temperature produced a transition from p-type to n-type behavior for SnS and SnSe films. Thin films of SnTe nanorods showed an exceptionally high dimensionless thermoelectric figure of merit (ZT) of 0.183 at 500K.Chapter 4 focuses on the phase and shape evolution of mixed (Cu, Sn) telluride NCs. Methods similar to those employed for the tin chalcogenide NCs described in chapters 2 and 3 were used to prepare and characterize these telluride NCs. With increasing Sn:Cu precursor ratio, the NCs evolved from pure CuTe, to Sn-doped CuTe, a CuTe/Cu2SnTe3 mixture, pure Cu2SnTe3, a SnTe/Cu2SnTe3 mixture, and finally pure SnTe. The shape and phase of CuTe also changed with increasing growth temperature. When the NCs grew at 140 ℃, the aspect ratio of the nanorods decreased with increasing Sn content. The Seebeck coefficients and electrical conductivity of thin films of these metal tellurides were measured at room temperature, providing the first report of thermoelectric properties of nanostructured Cu2SnTe3 films. In chapter 5, copper-based quinary chalcogenide semiconductor NPs were prepared and characterized, in a similar manner to the tin chalcogenide NCs described in chapters 2 and 3, and applied in multi-layered thin film photovoltaic structures. Copper zinc tin sulfide/selenide (CZTSSe) NPs were produced through anion exchange from CZTS NPs. Controlled reactivity of the Zn and the Se precursors enabled tuning of the composition of the CZTSSe NPs. Dispersions of these NPs, as well as colloidal CdS and ZnO NPs dispersions, were sequentially spin coated onto indium tin oxide (ITO) substrates to create a multi-layer structure. The multiple layer structure was confirmed by Focused Ion Beam Scanning Electron Microscopy (FIB-SEM). However, further work is still needed to improve the photovoltaic properties of these devices. A summary and suggestions for future work are presented in chapter 6.