Tuning the nitrogen content of carbon dots from N-rich up to oxidized carbon nitride nanoflakes

Abstract

Carbon nanodots (CD) can be prepared from simple organic precursors and renewable natural products through user-friendly processes. Functional groups, structure, and morphology of CD determine their fundamental properties. The understanding of the correlation of the latter properties with the nature of precursor materials and synthesis methods is still under debate. In this context, the present work explores a tunable CD synthesis method based on microwave-assisted hydrothermal carbonization of urea/glucose mixtures, allowing variations in CD N-content. Different reaction conditions were tested and products were widely characterized. In the absence of urea, oxidized graphitic carbon dots of ca. 3 nm and low photoluminescence quantum yields (ΦPL∼0.009) were obtained. On increasing the urea molar fraction (χu), N-rich carbon dots were synthesized, characterized by the presence of amide surface groups and enhanced photoluminescence yields (ΦPL∼0.02 for χu∼0.2). In the extreme cases of χu ≥ 0.9, partially oxidized graphitic carbon nitride nanoflakes were obtained with ΦPL∼0.10. Correlations between N:C precursors ratio and synthesis conditions with the structure and photophysical properties of the nanoparticles obtained is thoroughly analyzed and discussed. In summary, a tunable synthesis method for preparing different carbon nanomaterials from the same very abundant and cheap precursors, glucose and urea was developed.