Studies of the Reactivity of Graphene Driven by Mechanical Distortions

Abstract

In mechanochemistry, the application of controlled forces is key to altering reaction rates and pathways to direct product yields and selectivity. However, a fundamental knowledge gap exists between what is occurring on the atomic scale in mechanically driven reactions and the resulting macroscale outcomes. Two-dimensional (2D) materials, such as graphene, proffer a model system to study the impact of mechanical forces, such as strain, on chemical reactivity, as force distributions may be applied across a well-organized atomic-scale structure comprising a single layer of C atoms. Here, using Raman micro-spectroscopy and first-principles calculations, we have investigated the reaction of graphene, under varying degrees of strain, with 4-nitrobenzenediazonium tetrafluoroborate (4-NBD). We find that only with increased out-of-plane distortion (shifting the C atoms of graphene from sp 2 toward sp 3 electronic states) would the reactivity be increased, with larger out-of-plane distortions yielding greater reactivity. Density functional theory (DFT) calculations reveal that increasing the curvature of graphene decreases the activation barrier of 4-NBD functionalization and enhances the thermodynamic favorability of the reaction. Furthermore, we find that curvature affects the orientation of the graphene 2p z orbitals, and we then relate the thermodynamic feasibility of 4-NBD functionalization with the orbital orientation. These studies point to how the precise application of forces can be used to direct the functionalization of graphene for C−C bond forming reactions, which has significant implications for controlling its corresponding electronic structure in a well-defined fashion.

Mechanochemical Molecular Motion Using Noncovalent Interactions on Graphene and Its Application to Tailoring the Adsorption Energetics

“…Therefore, a ( Δ E Δ E BE ) value of 0.25 in Figure d corresponds to ≈0.10 eV of thermodynamic drive when the ΔE BE is ≈−0.40 eV. Hence, we conclude that the thermodynamic drive is substantial enough to observe the π–π NCI-driven migration for experimentally realizable curvatures and length scales. − …”

Two-Legged Molecular Walker and Curvature: Mechanochemical Ring Migration on Graphene

“…This is in line with previous findings from our groups, showing that positive POAV angle regions (such as mountains) are more prone to out-of-plane functionalization and have stronger covalent binding with an adsorbate. 13,30 This is due to the fact that the functionalization of graphene C�C causes pyramidalization and out-of-plane distortion of the graphene C. 13,30 Therefore, regions that are closer to pyramidal geometry, that is, the regions with positive POAV angle, can offer a stronger binding.…”

“…Previous reports show that mechanical perturbations, such as strain and curvature of 2D materials, can be realized experimentally using state-of-the-art methods. − The use of mechanical distortion provides an alternate avenue to perform chemical reactions apart from thermal, photochemical, or electrochemical approaches. Furthermore, mechanochemistry enables one to realize novel chemical processes and perform directed chemical syntheses on 2D materials. − Recently, we have demonstrated that directional molecular motion can be attained on graphene by exploiting mechanical distortions .…”

Modeling Carbon Nanotube Entanglement Load Transfer: Implications for Lightweight Aerospace Structures

“…Covalent bonds are observed near the entanglement, both between the two nanotubes and between the walls of multiwalled nanotubes. These bonds form at the most extreme curvature areas at the edges of buckles due to the increased reactivity from the mechanical distortion. − The number of sp 3 hybridized carbons varies between models and over the duration of the loading simulation within a model but can range up to several hundred in a single entanglement. For example, the model shown in Figure a contains 48 sp 3 hybridized carbon atoms and the model shown in Figure c contains 141 sp 3 hybridized carbon atoms at the beginning of the loading simulation.…”