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Reaction Kinetics of Reducing Graphene Oxide at Individual Sheet Level Studied by Twilight Fluorescence Microscopy

Kanazawa, Katsuki, Sato, Hikaru, Sano, Masahito
Journal of physical chemistry 2019 v.123 no.11 pp. 6881-6887
absorbance, activation energy, ascorbic acid, binding sites, fluorescence microscopy, graphene, graphene oxide, models, oxidation, reaction kinetics, sonication, temporal variation
Kinetics of the reduction reaction of graphene oxide (GO) by l-ascorbic acid in a solution was examined by direct observation of an individual GO sheet with an area over 100 μm² using Twilight fluorescence microscopy. Temporal changes of either increase in absorbance or decrease in autofluorescence from an individual GO sheet of a known layer number and an area were analyzed by the pseudo-first-order model, the pseudo-second-order model, and the Elovich model. An increase in the absorbance around 600 nm probes an extension of the π–electron conjugation length over multiple hexagonal units. All data at various conditions are best fitted with the Elovich model, implying that the activation energy increases as the reaction progresses. A decrease in the total autofluorescence intensity follows local deoxygenation of oxides. In this case, the pseudo-second-order model, which is based on a homogeneous surface with a constant activation energy, shows the best fit to all data. These results demonstrate that, for GO sheets containing many kinds of oxides at various binding sites, different kinetics appear depending on which elementary reactions we follow. For both cases, the rate constant was independent of the number of layers but became larger for smaller-area sheets. The area dependence is explained by the oxidation characteristics of the Hummers’ method and the sonication process to exfoliate the oxidized graphite.