In-situ Atomic Scale Studies of Nanoparticle-Solution Interface ProcessesPublic Deposited
While synthesis and transformation processes to produce monodisperse nanoparticles are empirically well-developed, the pathways for these reactions as well as the exact role of synthetic agents and binding characteristics of surface moieties remain poorly understood. This lack of understanding is primarily due to the paucity of information about nanoparticle structural evolution at the atomic scale and an inability to characterize the nanoparticle-solution interface. This thesis addresses such nanoscale processes through use of an approach which combines in-situ X-ray atomic scale characterization (XAFS and XRF) with nanoscale morphological parameters derived from electron microscopy and SAXS. These techniques and approach provide significant insight into the chemical pathways that define bimetallic nanoparticle growth and establish a methodology for characterizing nanoparticle structure (both of the inorganic core and molecular species coordinated to its surface) at the atomic scale. Chapter 1 presents an overview of nanoparticles and the importance of understanding their structure, in addition to outlining current challenges in characterizing the nanoparticle-solution interface and potential methods to address this challenge. This is followed by Chapter 2, a technical background of methods important to the rest of the work in this thesis, including colloidal nanoparticle synthesis and X-ray characterization techniques. In Chapter 3, through pairing XAFS-derived atomic scale information with electron microscopy and SAXS, a pathway for the transformation of citrate-capped Ag nanospheres into AgAu nanocages is proposed. Following a similar approach, the role of trace Ag in the synthesis of Au nanorods is determined and described in Chapter 4. Namely, it is shown that the anisotropic growth rate of the nanorods is directly proportional to the amount of surface Ag. In Chapter 5, the structure of CTAB, a common surface species in the synthesis of Au nanoparticles, is investigated through use of XAFS applied to small Au nanoparticles that are primarily composed of surface atoms. In this way, XAFS, a bulk technique, becomes sensitive to the nanoparticle molecular corona. Through combining this approach with molecular-scale techniques used to investigate ligand replacement on nanoparticle surfaces, Chapter 6 explores the exchange of CTAB with ligands believed to have a stronger affinity for Au nanoparticle surfaces, including BSPP and PEG-thiol. Chapters 3 – 6 each conclude with proposed future work to provide possible directions for project continuation and extension. In addition, a summary of and outlook for the broader impact of understanding nanoparticle-solution interface structure on improving nanoscale processing is also provided as a conclusion.