Controlling the emission colour and chemical structure of carbon dots by catalysis-tuned conversion of ortho-aminophenol

Abstract

The synthesis of carbon dots (CDs) with tailored properties commonly requires time-consuming trial-and-error experimentation, in part because of a poorly understood and controlled chemical conversion of the precursor material. Here, we first report on the solid-state pyrolysis or solvothermal conversion of an ortho-aminophenol (oAP) precursor, comprising ortho-disposed amino and hydroxyl groups on a benzene ring. We find that both conversion reactions resulted in a two emission-colour product, which could be separated into distinct blue-emitting CDs (bCDs, λ_peak = 420 nm) and yellow-emitting CDs (yCDs, λ_peak = 565 nm) by repetitive column chromatography. Systematic characterization revealed that both CDs comprise a planar graphene-like interior, but that they are distinguished by that the bCDs comprise an intermixed significant amino-rich fluorophore while the yCDs instead comprise a pyridinic-rich fluorophore. This implies that the bCDs are formed via activation of the amino group of the oAP precursor, whereas the synthesis of the yCDs constituted a simultaneous activation of both the amino and hydroxyl groups. With this knowledge at hand, we managed to direct the chemical conversion of the oAP precursor to yield either solely bCDs or yCDs by adding a catalyst (either the Lewis acid AlCl₃·6H₂O or the Lewis base NaOH) that selectively and efficiently activated only one of the reaction pathways. This demonstration is important in that it shows that the synthesis of CDs with desired properties can be realized with efficient rational instead of trial-and-error means.