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FUNDAMENTAL UNDERSTANDING THE MECHANISM OF SHAPE CONTROLLED SYNTHESIS OF SILVER NANOPARTICLES
Restricted (Penn State Only)
Doctor of Philosophy
Date of Defense:
June 06, 2018
Robert Martin Rioux Jr., Dissertation Advisor
Robert Martin Rioux Jr., Committee Chair
Michael John Janik, Committee Member
Themis Matsoukas, Committee Member
Raymond Edward Schaak, Outside Member
Solution-based synthesis has been explored as a versatile method for the synthesis of metal nanoparticles (NPs) with size, shape and composition control. With extensive synthetic approaches reported in literature for various shapes of metal nanoparticles, the fundamental understanding of the shape control mechanism is still missing for the most part. Here we probed one of the most studied systems: polyol synthesis of Ag NPs and focuses on the fundamental understanding of the shape controlled mechanism. The dominant hypothesis in the literature is the preferential binding of PVP to Ag(100) facets compared to Ag(111) facets, which leads to the retainment of the Ag(100) facets along with the growth of Ag(111) facets through either a kinetic or thermodynamic pathways in order to control the shape; however, there is no unified agreement on the exact shape control mechanism in Ag NPs growth as both Ag cubes and octahedron are synthesized in the presence of PVP and other additives are also added during the shape controlled synthesis. We experimentally measured adsorption isotherms of PVP on different-shaped Ag NPs to determine the thermodynamics of PVP adsorption to Ag(100) and (111) surfaces and found the preferential binding of PVP is not responsible for thermodynamic shape control in Ag NPs synthesis. Next we built a phase diagram for the synthesis of Ag nanocubes by varying the PVP molecular weight and concentration to examine the exact roles of PVP in Ag nanocubes synthesis. The low boundary indicates the stabilizer role of PVP whereas the high boundary indicates the reducing agent role of PVP. Therefore we concluded PVP serves dual roles as stabilizer and reducing agent in Ag NPs synthesis instead of structure-directing agent. Instead of PVP, other additives (Cl- and Cu2+) added during the synthesis have an influence in Ag NPs shape control. For Ag nanocubes, increment of Cl- concentration alone at constant PVP and H+ concentration led to modification of the nanoparticle shape from truncated octahedron, to cuboctahedron, truncated cubes and cubes. Ab initio thermodynamics calculations further examine the impact Cl- in directing shape control. With increases in the Cl chemical potential, the calculated surface energy of Ag facets with adsorbed Cl- favored Ag(100) over Ag(111) (i.e, γ(100) < γ(111)) and predicts a Wulff shape terminated with an increasing (100) contribution, consistent with experimental observations. Ex-situ experiments probed the evolution of Cl- during the growth of Ag nanocubes, which involves the initially formation of AgCl nanocubes, their subsequent dissolution to release Cl-, which adsorbs onto the surface of Ag NPs to impact shape evolution through apparent thermodynamic control. For Ag octahedron, we examined the influence of foreign metal ions (Cu2+, Ni2+, Co2+, Pd2+ and Au3+) on the shape evolution. The foreign metal ions were found to selectively deposited on Ag(100) facets through either under-potential deposition or galvanic exchange reaction depending on their relative reduction potential to Ag+. With the foreign metal deposited on Ag(100), it induced a faster growth rate on Ag (100) than Ag(111) and controls the shape evolution kinetically. PVP serves as stabilizer and reducing agent in the shape controlled synthesis of Ag NPs whereas Cl- controls the nanocubes formation thermodynamically and foreign metal ions control the octahedron formation kinetically.
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