Thermal carbonization in nanoscale reactors: controlled formation of carbon nanodots inside porous CaCO3 microparticles

Abstract

Synthesis of carbon nanodots (CNDs) in confined geometry via incorporation of dextran sulphate into pores of CaCO₃ microparticles is demonstrated. The preparation process included three steps: co-precipitation of solutions of inorganic salts and carbon source, thermal treatment and CaCO₃ matrix removal. We show that geometric constraints can be used to precisely control the amount of source material and to avoid formation of large carbon particles. Analysis of TEM data shows particle size of ~3.7 nm with narrow size distribution. Furthermore, we found that variation in pore morphology has a clear effect on CNDs structure and optical properties. CNDs with graphene oxide like structure were obtained in the nanoporous outer shell layer of CaCO₃ microparticles, while less ordered CNDs with the evidence of complex disordered carbons were extracted from the inner microcavity. These results suggest that confined volume synthesis route in CaCO3 nanopores can be used to precisely control the structure and optical properties of CNDs.

Figure 1

SEM images of CaCO₃ microparticles (a), and of a broken particle (b) (with the shell structure outlined by the dashed red lines); the size and the surface morphology of DS-loaded CaCO₃ microparticles is also shown (c).

Figure 2

TEM images of (a) CaCO3-DS microcapsules (DS initial concentration 2 mg/ml); (b) azimuthal average profiles of SAED patterns collected from microcapsules; TEM images obtained from (c) fraction 1, (d) fraction 2, (e) fraction 3 and (f) from samples synthesized in water solution.

Figure 3

(a) Raman data for fractions 1, 2 and 3. Two different spectra (designated Fraction 2a and Fraction 2b) were observed for the fraction 2. Features at around 1200 cm⁻¹ and 1450 cm⁻¹ are marked with arrows. (b) Normalized PL data for all three fractions are also shown (exc. 320 nm).