Advances in the understanding of the electron-donating properties of carbon-based nanomaterials electrogenerated from graphite

• Electrolyte determines the preferential quinonoid form of carbon nanomaterial. • Spectroscopic and voltammetric evidence for quinone-superoxide adduct formation. • Nanomaterial shifts from electron-donor to electron-acceptor upon electrolyte removal. In a seminal paper, we demonstrated voltammetrically that carbon nanomaterials electrogenerated from graphite in specific carboxylic acid buffers show electron-donating properties, exhibiting a semiquinone-like state. Unexpectedly, after the terminus of the electrogeneration process, its concentration was found to increase when citrate was used as the synthesis buffer electrolyte. In this paper, we revisit and provide a mechanistic interpretation of this phenomenon. Beyond the formation of semiquinone-like species during the galvanostatic synthesis, we present voltammetric and spectroscopic (XPS) evidence for the formation of a non-electroactive superoxide-quinone adduct. It was found that the two entities, semiquinone and superoxide-quinone adduct, can convert into each other in a redox cycle mediated by molecular oxygen, which allowed rationalising the temporal evolution of the voltammetric response. It was concluded that the carboxylic acid buffer used in the electrochemical synthesis determines the quinonoid form of the carbon nanomaterial. In tartaric buffer solution, the semiquinone form predominates, whereas, in citrate and malic buffers, the quinone-superoxide adduct and an oxidised quinoid form are the dominant species, respectively. Upon the electrolyte removal, the electro-donating capacity of the nanomaterials is lost, and the nanomaterial is converted into a form with electron-accepting activity. Overall, this study highlights the crucial role of the electrolyte in shaping and sustaining the electro-donating properties of hydrophilic carbon nanomaterials in aqueous media.