Mechanistic model for quantifying the effect of impact force on mechanochemical reactivity.

Conventional mechanochemical synthetic tools, such as ball mills, offer no methodology to quantitatively link macroscale reaction parameters, such as shaking frequency or milling ball radius, to fundamental drivers of reactivity, namely the force vectors applied to the reactive molecules. As a result, although mechanochemistry has proven to be a valuable method to make a wide variety of products, the results are seldom reproduceable between reactors, difficult to rationally optimize, and hard to ascribe to a specific reaction pathway. Here we have developed a controlled force reactor, which is a mechanochemical ball mill reactor with integrated force measurement and control during each impact. We relate two macroscale reactor parameters-impact force and impact time-to thermodynamic and kinetic transition state theories of mechanochemistry utilizing continuum contact mechanics principles. We demonstrate force controlled particle fracture of NaCl to characterize particle size evolution during reactions, and force controlled reaction between anhydrous copper(II) chloride and (1, 10) phenanthroline. During the fracture of NaCl, we monitor the evolution of particle size as a function of impact force and find that particles quickly reach a particle size of ∼100 µm largely independent of impact force, and reach steady state 10-100× faster than reaction kinetics of typical mechanochemical reactions. We monitor the copper(II) chloride reactivity by measuring color change during reaction. Applying our transition state theory developed here to the reaction curves of copper(II) chloride and (1, 10) phenanthroline at multiple impact forces results in an activation energy barrier of 0.61 ± 0.07 eV, distinctly higher than barriers for hydrated metal salts and organic ligands and distinctly lower than the direct cleavage of the CuCl bond, indicating that the reaction may be mediated by the higher affinity of Fe in the stainless steel vessel to Cl. We further show that the results in the controlled force reactor match rudimentary estimations of impact force within a commercial ball mill reactor Retsch MM400. These results demonstrate the ability to quantitatively link macroscale reactor parameters to reaction properties, motivating further work to make mechanochemical synthesis quantitative, predictable, and fundamentally insightful.

The role of mechanical force on the kinetics and dynamics of electrochemical redox reactions on graphene.

Electrochemical reactions are a critical class of processes strongly influenced by atomic scale effects, where the relationships between local chemical composition, stress, strain, and reactivity are not well understood. Here we investigate the relationship between applied stress and reaction rates for the oxygen evolution reaction on multi-layered graphene using conductive atomic force microscopy. During the reaction, oxygen groups accumulate on the surface and the oxygenation rate increases with applied load. The results also show that the rate is not uniform across the surface, where local edges and defects are more reactive than the basal plane. The results presented here are interpreted in the context of transition state theory, where applied load over the reaction coordinate linearly modifies the energy landscape. This work motivates the general efficacy of atomic force microscopy as a tool to study relationships between local mechanical surface effects and electrochemical reactivity.

Driving Surface Chemistry at the Nanometer Scale Using Localized Heat and Stress.

Driving and measuring chemical reactions at the nanoscale is crucial for developing safer, more efficient, and environment-friendly reactors and for surface engineering. Quantitative understanding of surface chemical reactions in real operating environments is challenging due to resolution and environmental limitations of existing techniques. Here we report an atomic force microscope technique that can measure reaction kinetics driven at the nanoscale by multiphysical stimuli in an ambient environment. We demonstrate the technique by measuring local reduction of graphene oxide as a function of both temperature and force at the sliding contact. Kinetic parameters measured with this technique reveal alternative reaction pathways of graphene oxide reduction previously unexplored with bulk processing techniques. This technique can be extended to understand and precisely tailor the nanoscale surface chemistry of any two-dimensional material in response to a wide range of external, multiphysical stimuli.