Engineering Crystal Structure, Elemental Composition, and Interfaces in Inorganic Nanomaterials via Ion Exchange

Restricted (Penn State Only)
Author:
Fenton, Julie Lynn
Graduate Program:
Chemistry
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 01, 2018
Committee Members:
  • Raymond Edward Schaak, Dissertation Advisor
  • Raymond Edward Schaak, Committee Chair
  • Thomas E Mallouk, Committee Member
  • Elizabeth A Elacqua, Committee Member
  • Robert Martin Rioux Jr., Outside Member
Keywords:
  • nanomaterials
  • ion exchange
  • synthesis
  • colloidal nanoparticles
  • metastable crystal structures
  • inorganic materials
Abstract:
Nanomaterials exhibit unique physical and chemical properties, defined by their morphology, dimensionality, elemental composition, and crystal structure. Wet-chemical colloidal methods can deliver large populations of high quality dispersed particles, but the complexities of solution-based chemistry render these approaches largely empirical. Technological advancements will require tightly regulated particles and increasingly complex nanoscale architectures, necessitating the development of a truly predictive framework of chemical principles to target and obtain particles with desired properties. Post-synthetic modification strategies have emerged as a promising alternative to direct synthetic methods, designed to isolate control over some of the property-defining features of nanomaterials into separate synthetic steps. Among these, solution-mediated ion exchange methods selectively replace the existing anion or cation sublattice of a pre-formed nanocrystal while preserving its overall size and shape, effectively decoupling control over particle morphology and elemental composition. Ion exchange has provided useful inroads to unusual morphologies, non-thermodynamic crystal structures, and multi-component complex heterostructures, facilitated by rapid reactions and generally mild conditions. In this dissertation, I build on this foundation, offering new insights into and a broadened scope of ion exchange reactions, critical steps towards the development and implementation of a broadly applicable synthetic strategy for simultaneously controlling all the property-defining features of nanomaterials. I begin by demonstrating the utility of cation exchange to selectively target multiple polymorphic crystal structures within the same material family, addressing a longstanding challenge in solid-state chemistry. Here, pre-synthesized cubic and hexagonal copper(I) sulfide nanospheres are used to template the formation of cubic zincblende and hexagonal wurtzite structures across four different metal sulfide materials, including MnS, CoS, CdS, and ZnS. All reported products retain both the anion and cation sublattice features programmed into the template material, which suggests a broad, generalizable method to select desired structural features in nanocrystals through cation exchange. Next, I discuss synthetic innovations for multi-component heterostructured nanoparticles, which limit reliance on challenging seeded-growth reactions. I demonstrate that the pre-formed interface on a model hybrid system can be preserved through sequential anion and cation exchange pathways. This strategy completely transforms the elemental composition of individual material domains, forming distinct derivative structures and new interfaces, using only one seeded growth step. Building on these insights, I present a modular divergent synthesis strategy that progressively transforms simple nanoparticle precursors into increasingly sophisticated products without relying on seeded growth methods. Here, a network of tunable interfaces are formed within pre-formed copper(I) sulfide nanoparticles by partial cation exchange reactions. Integrating this framework with additional known nanosynthetic chemistry, including exchanges, selective etching, and seeded-growth reactions, generated a large library of highly sophisticated nanoarchitectures. Finally, I define a new type of regioselectivity for these partial cation exchange processes based on the crystallographic relationships between parent and product phases. By maximizing the formation of low-strain interfaces, three distinct materials are rationally integrated within uniform spherical and rod-shaped colloidal nanoparticles to produce five unique, asymmetric heterostructured isomers, which are among the most complex particles ever generated on the nanoscale. These insights pave the way to a new paradigm in predictive control for target-driven nanocrystal synthesis in the future.