Precursor-free synthesis of carbon quantum dots and carbon microparticles in supercritical acetone

Abstract

Carbon quantum dots (CQDs) have recently received a lot of attention due to their unique physical properties, and their environmentally friendly features such as low toxicity and high biocompatibility. Supercritical fluids, which possess unusual properties such as high solubility, high diffusivity, low viscosity and zero surface tension, are now commonly used particularly in the fields of electronic, chemical and materials science and engineering. Here, we synthesise carbon nano/microparticles in supercritical acetone, in which neither external molecules nor starting materials are dissolved/dispersed. We find that carbon microparticles and nano structures such as graphene quantum dots (GQDs), carbon nano onions (CNOs) and elongated carbon nano onions (eCNOs) are self-assembled via thermal decomposition of acetone under its supercritical conditions. We also find that the carbon microparticles are in fact formed by GQDs, CNOs and eCNOs, the microparticles being physically resolved into GQDs, CNOs and eCNOs with sonication. The fluorescence features of the carbon nano structures are clarified, noting that no photobleaching was observed for at least one month. The present result may well lead to the development of facile bottom-up methodologies for synthesising nano materials in solvents under their supercritical conditions without using any external precursors/starting materials.

Fig. 1. Outline of the experimental setup and procedure.

a, b Supercritical fluid chamber. Acetone was confined in the chamber. The temperature was regulated by a heater surrounding the chamber and a PID controller. c Time variation of the temperature. Once the temperature had reached a target value, it was kept constant for 0, 0.5, 1, 1.5, 2, or 6 h and then lowered down to room temperature. d Replacement of acetone with ethanol in the chamber. e, f Filtration of the solution with a 100 nm mesh. Sub-micro/microscale materials (“Sub-micro/microscale materials I”) were separated from the solution (“Filtered solution I”). g Dispersion of “Sub-micro/microscale materials I” in ethanol. h Filtration of the solution with a 100 nm mesh. Sub-micro/microscale materials (“Sub-micro/microscale materials II”) were separated from the solution (“Filtered solution II”). i Ultrasonication of the solution. j Filtration of the solution with a 100 nm mesh. Sub-micro/microscale materials (“Sub-micro/microscale materials III”) were separated from the solution (“Filtered solution III”).

Fig. 2. SEM images, XRD pattern, and Raman spectrum of sub-micro/microscale particles produced in supercritical acetone.

a SEM image of particles. The synthetic temperature, synthetic time, and molar volume were 450 °C, 0.5 h, and 2.75 × 10⁻⁴ m³ mol⁻¹. The scale bar represents 10 µm. b SEM image of particles. The synthetic temperature, synthetic time, and molar volume were 450 °C, 1 h, and 2.75 × 10⁻⁴ m³ mol⁻¹. The scale bar represents 10 µm. c SEM image of particles. The synthetic temperature, synthetic time, and molar volume were 450 °C, 2 h, and 2.75 × 10⁻⁴ m³ mol⁻¹. The scale bar represents 10 µm. d XRD pattern of the particles. The particles were synthesised under the conditions of 450 °C, 2 h, and 2.75 × 10⁻⁴ m³ mol⁻¹ (Fig. 2c). The wavelength of the X-ray was 0.1392 nm. The (002) diffraction peak was located at $2\theta$ = 25.35°. e Raman spectrum of particles corresponding to Fig. 2c. The wavelength of the excitation laser was 515 nm. The D and G band peaks appeared at 1361 and 1596 cm⁻¹. The 2D and D + G bands were also detected at 2706 and 2922 cm⁻¹.

Fig. 3. TEM images of carbon nanostructures dissolved/dispersed in “Filtered solution III” and the interlayer distances.

Sub-micro/microscale particles were synthesised in supercritical acetone under the conditions of 450 °C (synthetic temperature), 2 h (synthetic time), and 2.75 × 10⁻⁴ m³ mol⁻¹ (molar volume of acetone), and the solution of the sub-micro/microscale materials dispersed in ethanol was ultrasonicated. a TEM image of graphene layers composing the sub-micro/microscale particles. The scale bar represents 5 nm. b TEM image of a carbon nano onion composing the sub-micro/microscale particles. The scale bar represents 5 nm. c TEM image of an elongated carbon nano onion composing the sub-micro/microscale particles. The scale bar represents 5 nm. d Grey level distribution along the red line across the graphene shown in Fig. 3a. e Grey level distribution along the red line across the carbon nano onion shown in Fig. 3b. f Grey level distribution along the red line across the elongated carbon nano onion shown in Fig. 3c.

Fig. 4. Fluorescence and absorption features of “Filtered solution I, II and III”.

a Colour of acetone before the experiment and “Filtered solution I, II and III”. The left and right photographs in each pair of them were, respectively, taken under irradiation of white light and UV light of 365 nm wavelength in a dark box. b Fluorescence spectra of “Filtered solution III”. The excitation wavelength was changed from 300 to 490 nm at 10 nm intervals. c Absorption spectrum of “Filtered solution III”.