Multi-Scale Mechano-Chemistry – From Atomic Force Microscopy to Gram Scale Synthesis

Abstract

This dissertation systematically explores the fundamental connections between mechano-chemistry, nano-tribology and nano-mechanics. Mechanically induced chemical change is arguably the most primitive class of chemistry humans have explored, yet its physical origins largely remain a mystery. A theoretical framework based on chemical kinetics and transition state theory is provided to rationalize the influence of force on thermal and electrochemical reactions. Thermal reduction of graphene by a heated atomic force microscope tip and electrochemical oxidation of graphene through local anodic oxidation can be effectively measured by monitoring friction change. It is observed that applied normal force can accelerate the reactions and reduce the energy barrier by distorting the energy landscape of the underlying chemical processes. The activation length can be estimated from our experiments from accompanying MD simulations to assess the nature of contact between the tip and substrate. The force-modified energetics or Hammond effect can be utilized to alter the reaction conditions, forward and backward rates and even render reactions barrierless. The same notion of thermally activated process modulated by force was applied to describe folding and crease formation events in three-dimensional crumpled graphene structures. Force-indentation curves on crumpled structures show reversible and irreversible energy dissipations that bear indications to the quantity of crease formation events during deformation. The energy absorption characteristics of crumples depended strongly with chemistry of their starting flat sheets, where, graphene oxide showed greater amenability to form stable folds. The fundamental insights of mechano-chemistry were tested at the gram scale by introducing a novel mechano-chemical reactor. By adding force control and measuring impacts with microsecond time resolution, kinetics of particle size reduction was demonstrated to strongly depend on the magnitude of applied force. Incorporating in situ chemical measurement techniques can transform the reactor into an indispensable tool for probing solid state reactions in both labs and other research facilities. The theoretical framework, nanoscale tip-based experiments and gram scale synthesis discussed in this dissertation lay the foundation to conduct mechano-chemistry across spatial scales.