Nature of Interactions between Epoxides in Graphene Oxide

Abstract

Graphene oxide exhibits extensive disorder with a multitude of functional groups and holes making its structure an abstract concept. Multiple structural models make it a dark horse and hamper its utility despite its synthetic ease, maneuverability, and promising applications. Here we probe the impact of the epoxidation process, which is arguably the first kinetic step in graphene oxidation, to identify the induced vulnerabilities of its backbone and to cognize possible pathways for further oxidation. Probing the topological and geometrical variations in the distribution of epoxide on the graphene lattice, we find that the conformational entropy, driven by the combinatorial growth of isomers, aids and abets disorder. Graph theoretical enumeration gives 16 distinct epoxide environments within symmetrically equivalent epoxides. Their stability is primarily influenced by the steric repulsion between the oxygen lone pairs and overlap compatibility of the interepoxy C–C bond. Avoiding steric repulsion either by topology or by equatorial splaying of oxygens leads to stability, without which the network is weakened either by elongation of C–C bonds or by axial splaying of oxygens leading to uneven C–O bonds. The proposed unzipping of the underlying epoxy C–C bond through the cooperativity of strain from three-membered rings and conjugative stabilization from residual sp2 character are found to be incongruous. With an improved bonding model of epoxide, we account for the observed variations in C–C bond lengths and energetics of epoxides in different environments that facilitate strategic mechanistic control for further oxidation.

Topological Impact of Delocalization on the Stability and Band Gap of Partially Oxidized Graphene

“…Though the central C–C bond in the epoxy dimer (Figure a) is shortened (<1.5 Å), the C–O bond hardly shows any elongation (0.004 Å) compared to oxirane (1.429 Å), indicating negligible hyperconjugation. This rules out the suspected partial π-bonding of the central C–C bond (donation of C–O σ to C–O σ*), and its shortening is largely due to the bent nature of the exo-2c-2e bonds of the strained triangular ring . In the epoxide–double bond junction (Figure b), the proximate C–O bond elongates by 0.013 Å, implying a tangible increase in the π-delocalization, shortening the central C–C bond further.…”

“…However, there is a lack of fundamental understanding of the relative kinetic and thermodynamic stabilities of various delocalization topologies and their impact on the band gap. The steric repulsion between the lone pairs of proximate oxygen atoms is the dominant factor affecting the stability in completely oxidized graphene . Some experimental studies and density functional theory (DFT) calculations point to the tendency for segregation of epoxide groups from the delocalized π-bonding in partial oxidation processes. − …”

Topological Impact of Delocalization on the Stability and Bandgap of Partially Oxidized Graphene

“…The computed HOMO-LUMO gap is given within braces.Though the central C-C bond in the epoxy dimer (Figure1a) is shortened (<1.5Å), the C-O bond hardly shows any elongation (0.004Å) compared to oxirane (1.429Å), indicating negligible hyperconjugative effect. This rules out the suspected partial π-bonding of the central C-C bond (donation of C-O σ to C-O σ*), and its shortening is largely due to the bent nature of exo-2c-2e bonds of the strained triangular ring20 . In the epoxide-double bond junction (Figure1b), the proximate C-O bond elongates by 0.013Å, implying a tangible increase in the π-delocalization, shortening the central C-C bond further.…”

“…Delocalization effects like aromaticity, conjugation, and hyperconjugation are essential in stabilizing the network. The steric repulsion between oxygen lone pairs can be partially offset by lateral splaying that leads to non-equivalent C-O bonds within the epoxide ring or orthogonal axial splaying 20 . Assuming π-delocalization reduces the bandgap, epoxides are distributed prudently on the graphene lattice to vary the topology of delocalization.…”

“…However, there is a lack of fundamental understanding of the relative kinetic and thermodynamic stabilities of various delocalization topologies and their impact on bandgap. The steric repulsion between the lone pairs of proximate oxygen atoms is the dominant factor affecting the stability in completely oxidized graphene 20 . The bulk of the experimental studies and DFT calculations point to the tendency for segregation of epoxide groups from the delocalized π-bonding in partial oxidation processes [16][17][18][19] .…”