Graphene functionalization: Mechanism of carboxyl group formation

Carboxyl groups are ubiquitous in graphene-based materials. Decades ago, they were important in conferring cation-exchange properties to coal and coal-derived chars; today they are instrumental in converting graphite to graphene as well as in a wide variety of surface functionalization processes. And yet the essential mechanistic details of their formation have not received the attention they deserve. Here we perform quantum chemical calculations based on the density functional theory to reveal the elementary oxygen-transfer processes that are consistent with the abundant experimental literature on the oxidation of sp²-hybridized carbon materials. Prototypical graphene clusters decompose nitric acid to nitrogen oxides and the reactive hydroxyls. Carboxyl groups are thus formed by virtue of sequential hydroxyl attack at the carbon active sites (C_f) which weakens and cleaves the contiguous aromatic C-C bonds. A comparison with the other common oxidants (X, e.g., chlorate or permanganate) is carried out by distinguishing the 1O-down and 2O-down oxygen-transfer pathways. This constitutes an essential step toward the unification of a wide variety of oxidation processes involving semiquinone or dioxirane surface intermediates: C_f + XO₃^- = C(O) + NO₂⁻ (or ClO₂⁻) vs. C_f + XO₃^- = C(O₂⁻) + NO (or ClO) vs. C_f + XO₄^- = C(O₂) + XO₂^-.