Author: Suprita Jharimune
Publisher:
Published: 2020
Total Pages:
ISBN-13:
DOWNLOAD EBOOK →
With conventional methods of colloidal nanoparticle (NP) synthesis, it is often difficult to precisely control NPs size, shape, and chemical composition that are the most important factors in defining their properties and behavior. This has led to a growing interest in the transformation of easily synthesizable nanostructures into more complex ones by post-synthetic modifications such as cation exchange, seeded growth, galvanic exchange, etc. to facilitate independent control over these parameters. In order to develop rational design strategies for the synthesis of colloidal NPs, it is crucial to understand the mechanisms encompassing their growth and transformation. In this dissertation, I have worked on two projects focusing on unraveling transformation and growth mechanisms in nanostructured materials--cation exchange (CE) in CdSe NPs and shape-controlled growth of anisotropic Ag NPs. CE has been utilized as a promising approach to synthesize nanostructures that cannot be synthesized directly from its precursors. The thermodynamic feasibility of a CE system is often based on trial and error because of the unavailability of rigorous quantitative studies. Available quantitative methods are based on lattice energy and solvation energy of bulk crystals that do not account for nanoscale effects. In Chapter 2, I have demonstrated, the thermodynamics of the CE between CdSe NPs and Ag+ in solution can be quantified using isothermal titration calorimetry (ITC). In this chapter, the influence of CdSe NP diameter, capping ligands, and temperature are surveyed and a detailed description of overall thermodynamic parameters--Keq, [delta]H, [delta]S, and stoichiometry (n) is reported. Gibbs free energy ([delta]Grxn) of CE between CdSe NPs with Ag+ obtained from ITC shows an additional stabilization of ~ -14 kJ/mol compared to values calculated from qualitative methods. In Chapter 3, I extended the application of ITC to measure the influence of cation solvation on CE thermodynamics. In this chapter, the influence of spectator anions, solvents, and exchanging cation identity are surveyed. This work demonstrated the application of ITC to probe thermochemistry of nanoscale transformations under relevant solution conditions. Ag NPs have been synthesized in various shapes (cubes, octahedra, nanowires, etc.) to serve specific applications, however, the mechanism of shape transformation and role of each component in the synthesis is not explicitly understood. Ag nanocubes is primarily synthesized by polyol method, where the solvent is considered as the reducing agent and PVP the shape-directing agent. In Chapter 4, I built an experimental phase diagram for the formation of Ag nanocubes as a function of PVP monomer concentration (Cm) and molecular weight (Mw). Incorporation of PVP with aldehyde and hydroxyl end-groups in the synthesis leads to formation of Ag nanocubes and a mixture of nanocubes and nanowires respectively, indicating that a stronger reducing agent (hydroxyl) forms kinetically preferred nanowires. Measurement of relative rates of Ag+ reduction at different PVP Cm and Mw confirmed the reducing effect originates from the end-groups. Although, the end-group/Ag+ ratio is below stoichiometric, it is demonstrated, PVP end-groups induce the reduction of Ag+ (nucleation), followed by an autocatalytic reduction at a rate commensurate with AgCl dissolution. Combining nanostructure synthesis with polymer synthesis enabled us to quantitively decipher the role of ubiquitously used PVP polymer in nanoparticle shape-control. Ag octahedra are synthesized by seed-mediated growth of Ag NCs in presence of Cu2+ ions, yet, the role of Cu2+ in shape transformation of Ag is still unknown. In Chapter 5, I used several Mn+ (Ni2+, Co2+, Cu2+, Pd2+, and Au3+) to study their relative effect on Ag NCs to octahedra conversion. Guided by findings from these experiments, I proposed a detailed mechanism for the transformation process. Mn+ deposits on Ag (100) surface through underpotential deposition or galvanic exchange reaction, depending on their reduction potential with respect to Ag. This leads to faster growth of the Ag (100) facets, resulting in their disappearance and retention of the Ag (111) facets thus forming Ag octahedra. We envision, these Ag octahedra could be utilized as single atom alloy (SAA) catalysts owing to the very low amount of Mn+ on the Ag octahedra surface. Galvanic replacement reaction (GRR) is often used to synthesize complex porous metal nanostructures by reacting a template NP with a metal salt of higher reduction potential. However, thermodynamics of GRR systems have not been studied before. Motivated by the robustness of ITC in studying thermodynamics of nanocrystals transformation, in Chapter 6, I propose employing ITC to understand the thermodynamics and transformation mechanism of silver nanocube GRR with Au3+, Pd2+, and Pt2+. Preliminary results demonstrate that a combination of calorimetry and spectroscopy can be instrumental in understanding the thermodynamics of nanoparticle transformation via GRR. Additional experiments and characterization to explain the findings from preliminary experiments are suggested.
