A carbon nanobelt (CNB) is a loop of fused benzene rings and in a sense a cutout of a single-walled carbon nanotube. Various types of CNBs have been successfully synthesized in recent years1–7 and CNBs have been utilized for the development of practical devices8–10. A C60 molecule is a football shaped fullerene composed of 60 carbon atoms11. Patterns and structures formed by atoms, molecules and particles via bottom-up self-assembly are of great interest and importance not only from a scientific point of view but for the design and production of functional materials and devices, the size of which ranges from nano to macroscopic scales12–14. Colloidal particles are commonly used/utilized in mechanical, chemical and biophysical/biochemical/biomedical engineering, where size uniformity and mono dispersibility of the particles in the solvent, particularly in water, become crucial factors15–23. In this study, we investigate secondary structures formed by (6,6)CNBs1,2 and C60 molecules, which are dissolved in 1,2-dichlorobenzene (see Fig. 1 in the Supplementary Information for the molecular structure of a (6,6)CNB). We find that uniform spherical particles are formed by (6,6)CNBs and C60 molecules in 1,2-dichlorobenzene at room temperature via bottom-up self-assembly, setting the molar concentrations of (6,6)CNBs and C60 molecules at appropriate values, and furthermore those particles are monodisperse even in water. The present facile room temperature synthetic methodology may well be applied to the creation of nano/micro structures/materials using basic carbon nano units such as cycloparaphenylene (CPP, carbon nanorings) and fullerenes; e.g., C60, C70 and C59N.
The solutions of (6,6)CNBs, C60 molecules and a mixture of (6,6)CNBs and C60 molecules dissolved in 1,2-dichlorobenzene are shown in Fig. 2 in the Supplementary Information. The colour of the solution of (6,6)CNBs dissolved in 1,2-dichlorobenzene was yellowish, whereas that of C60 molecules dissolved in 1,2-dichlorobenzene was deep purple as well known. The colour of the solution changed to brown after the solutions of (6,6)CNBs and C60 molecules had been mixed together.
We found that particles were produced in 1,2-dichlorobenzene after the mixture of the two solutions in all of the cases of different ratios of the molar concentration of (6,6)CNBs to that of C60 molecules (see Table 1 in the Methods for the actual concentrations of (6,6)CNBs and C60 molecules dissolved in 1,2-dichlorobenzene). However, smooth spherical particles of a uniform diameter were formed when the ratio of the molar concentration of (6,6)CNBs to that of C60 was set at 1 : 2 (the concentrations of (6,6)CNBs and C60 molecules were, respectively, 0.35 and 0.70 µmol ml− 1). SEM images of particles formed by (6,6)CNBs and C60 molecules are shown in Fig. 1, where the ratio of the molar concentration of (6,6)CNBs to that of C60 was 1 : 2. The size of the particles increased with time (see also Fig. 3 in the Supplementary Information for the size distributions of the particles as a function of the time and Fig. 2 for the time variation of the diameter of a particle). Note that no particles were formed in the solution of (6,6)CNBs dissolved in 1,2-dichlorobenzene and in the solution of C60 molecules dissolved in 1,2-dichlorobenzene. The surface of the spherical particles was smooth and the diameter of a particle was uniform when the particles were synthesized setting the ratio of the molar concentration of (6,6)CNBs to that of C60 at 1 : 2 as mentioned, whereas the surface of the particles was uneven and the size of a particle varied when the ratio was different from 1 : 2 (see Fig. 4 in the supplementary Information for the size distributions and SEM images of particles produced when the ratio of the molar concentration of (6,6)CNBs to that of C60 was 1 : 1, 1 : 2 and 1 : 3).
The absorption spectra by the supernatant of the solution, in which a mixture of (6,6)CNBs and C60 molecules were dissolved in 1,2-dichlorobenzene, are shown in Fig. 5 in the Supplementary Information, where the ratio of the molar concentration of (6,6)CNBs to that of C60 molecules was changed (see Fig. 5(a) in the Supplementary Information) and the time variation of the absorption spectra when the ratio of the molar concentration of (6,6)CNBs to that of C60 molecules was 1 : 2 is shown in Fig. 5(b) in the Supplementary Information. Note that the wavelengths of photons absorbed by (6,6)CNBs and C60 dissolved in 1,2-dichlorobenzene had been measured and those by (6,6)CNBs were 296, 318, 355, 373 and 396 nm, while those by C60 were 297, 329 and 407 nm (see Fig. 6 in the Supplementary Information for the absorption spectra by the individual solution of C60 and (6,6)CNBs dissolved in 1,2-dichlorobenzene). It is supposed that m × (6,6)CNBs and n × C60 molecules ((m, n) = (2, 1) and (1, 1 ~ 3) in the present study. See Methods Table 1) were physically combined to form a compound of ((6,6)CNB)m-(C60)n in the solution since the absorption peaks corresponding to (6,6)CNBs and C60 decreased when the ratio of the molar concentration of (6,6)CNBs to that of C60 molecules was m : n, noting that smooth spherical particles of a uniform diameter were formed when the ratio of the molar concentration of (6,6)CNBs to that of C60 molecules was 1 : 2 as mentioned. The intensity of the absorption spectra corresponding to (6,6)CNBs and C60 molecules decreased with time (see Fig. 5(b) in the Supplementary Information, where the ratio of the molar concentration of (6,6)CNBs to that of C60 molecules was set at 1 : 2). In other words, the number of compounds; (6,6)CNB-(C60)2, increased with time and those compounds formed spherical particles.
The time variation of the diameter of a particle and the peaks of the absorption spectra corresponding to (6,6)CNBs and C60 molecules in 1,2-dichlorobenzene are shown in Fig. 2, where the ratio of the molar concentration of (6,6)CNBs to that of C60 molecules was set at 1 : 2. The diameter of a particle increased with time, whereas the amount of (6,6)CNBs and C60 molecules in the solution decreased with time, which means that the number of (6,6)CNB-(C60)2 produced in the solution and the diameter of a particles formed by (6,6)CNB-(C60)2 increased with time as mentioned.
The mean diameter of a particle formed 168 h after the mixture of the two solutions and the hydrodynamic diameter and zeta potential of a particle dispersed in distilled water are shown in Table 1, where the particles were synthesized setting the ratio of the molar concentration of (6,6)CNBs to that of C60 molecules at 1 : 2. The diameter and hydrodynamic diameter of a particle synthesized 168 h after the mixture of the two solutions were quite uniform. Importantly, the absolute value of the zeta potential of a particle dispersed in water was so high as 38.8 mV that the particles were monodisperse even in water (see Fig. 7 and Video-1 in the Supplementary Information for the precipitation process of the solution and Fig. 8 in the Supplementary Information for the time variation of the turbidity of the suspension). Note that the particles eventually precipitated in water due to their own weight, but once the suspension had been shaken, the particles evenly dispersed again thanks to the high absolute value of the zeta potential in water (see Video-2 in the Supplementary Information).
The mass spectra of particles formed 168 h after the mixture of the two solutions are shown in Fig. 9 in the Supplementary Information, where (6,6)CNBs are positively charged, while C60 molecules are negatively charged. It is clear that the particles were composed of compounds formed by (6,6)CNBs and C60.
We carried out some preliminary simulations concerning the structures formed by compounds [(6,6)CNB-(C60)m]n, where (m, n) = (1, 1), (2, 1), (2, 2) and (2, 3), based on a semi-empirical method; PM624, according to which a compound; (6,6)CNB-(C60)2, can be stably formed, but the configuration of triple compounds is not aligned in a regular form (see Fig. 10 in the Supplementary Information). A TEM image of a particle formed when the ratio of the molar concentration of (6,6)CNBs to that of C60 molecules was 1 : 2 is shown in Fig. 11 in the Supplementary Information. It is clearly shown that the particle is not formed by regularly oriented compounds.
In summary, particles composed of (6,6)carbon nanobelts and C60 molecules were synthesized via self-assembly at room temperature by mixing two solutions of (6,6)carbon nanobelts and C60 molecules dissolved in 1,2-dichlorobenzene. Smooth spherical particles of a uniform diameter were formed particularly when the ratio of the molar concentration of (6,6)CNBs to that of C60 molecules was set at 1 : 2 (the concentrations of (6,6)CNBs and C60 molecules were 0.35 and 0.70 µmol ml− 1). The absolute value of the zeta potential of the particles dispersed in distilled water was so high that the particles were monodisperse in water, which means that the particles may well be used as stable colloidal particles in water. The present synthetic methodology is so simple that it may also be applied to the creation of nano/micro structures/materials using basic carbon nano units such as [n]cycloparaphenylene (CPP, carbon nanorings) and fullerenes; e.g., C60, C70 and C59N.