On the enhanced photocatalytic activity of N-doped carbon dots

Carbon nanostructures (dots) have emerged as a novel and sustainable alternative for the photocatalytic degradation of water pollutants. This work presents the synthesis of multidoped carbon nanomaterials (CNs) using a microwave-assisted method. Overall, four types of carbon nanostructures were obtained: (i) nitrogen-doped CNs (N-CDs), (ii) nitrogen and sulfur co-doped CNs (N,S-CNs), (iii) nitrogen and phosphorus co-doped CNs (N,P-CNs), and (iv) nitrogen, sulfur, and phosphorus multi-doped CNs (N,S,P-CNs). The characterization of these nanoparticles was performed via Fourier-transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS), enabling the identification of stretching modes corresponding to C=O, C–N, and N–H functional groups. Additionally, UV-vis and fluorescence spectroscopies allowed the detection of n–π* and π–π* absorption bands at ∼325 and 400 nm, along with light emission at 438 nm. High-resolution transmission electron microscopy (TEM) characterization confirmed structural and morphological differences between the nanomaterials, which exhibited sizes ranging from 1 to 100 nm, depending on the chemical composition of the starting precursors. Finally, the photocatalytic activity of the CNs towards the degradation of toluidine blue was assessed, considering the effects of morphology, composition, and both catalyst and dye concentration on photodegradation. Such a catalytic process followed pseudo-first-order kinetics, where N-CDs exhibited the highest potential for toluidine blue degradation. Our results highlight that the photocatalytic activity of carbon nanomaterials is a multifactorial process essentially driven by the formation of OH radicals, where doping and particle morphology also play a combined role in photocatalysis. This work opens a route for understanding the chemical composition and structure of photocatalytic nanocarbons and their application to the degradation of organic pollutants in water, thus offering a sustainable alternative for wastewater treatment.