Engineering Materials and Their Applications
One-Pot Sonochemical Synthesis of Carbon Nano-Onions from Silicon Carbide in Pure Water
2024 Volume 65 Issue 11 Pages 1431-1435
Details
2024 Volume 65 Issue 11 Pages 1431-1435
Carbon nanomaterials are a class of low-dimensional materials that have aroused a great deal of interest for decades. Carbon nano-onions (CNOs) are carbon nanomaterials with a wide range of applications. In this study, we report a novel process for synthesizing CNOs from SiC—the only inorganic carbon source—through one-pot sonication in pure water at room temperature. This synthesis process is more facile and can be performed under gentler conditions and lower temperatures than previous methods. The as-synthesized samples were characterized using transmission electron microscopy (TEM), Raman spectroscopy, Fourier-transform infrared spectroscopy (FTIR), and scanning transmission electron microscopy–electron energy loss spectroscopy (STEM-EELS). The TEM results revealed CNOs with diameters of approximately 20–30 nm, and the FTIR and STEM-EELS results indicated the presence of oxygen-containing functional groups on the CNOs and the growth of carbon from a SiC single crystal. The proposed method for obtaining CNOs from an inorganic carbon source via sonication provides novel insights into the CNO generation mechanism and its functionalization.
Ultrasonic waves have been extensively used to generate novel inorganic and organic materials, including carbon nanomaterials [1–4]. The chemical effects of ultrasound arise from acoustic cavitation: the formation, growth, and implosive collapse of bubbles in a liquid with a transient temperature of approximately 5000 K, pressure of approximately 1800 atm, and cooling rate in excess of 1010 K/s. Furthermore, pure water exposed to ultrasound can dissociate into OH· and H· radicals. Regarding carbon nanomaterials, the transformation of two-dimensional graphene oxide (GO) nanosheets into carbon nanotubes was achieved by sonicating GO in 70% nitric acid [5]. Monodispersed water-soluble fluorescent carbon nanoparticles were synthesized directly from glucose by a one-step alkali- or acid-assisted ultrasonic treatment [6]. The exfoliation of graphene layers from graphite powder using the probe sonication method has been previously reported [7]. Interestingly, Stollenwerk et al. reported the formation of CNOs via the ultrasonic agitation of MoS2 in isopropanol at room temperature [8]. The diameter of CNOs typically lies in the range of 1.4–50 nm, and large CNOs can reach 100 nm [9–12]. The size and structure of CNOs largely depend on the synthesis method and the carbon source. Due to their unique structure, CNOs exhibit excellent properties such as mechanical strength, electrical conductivity, and thermal stability. In addition, surface modification of CNOs can enhance their water solubility and the adsorption of heavy metal ions. Consequently, CNOs can be widely applied in electronics, energy storage, and sensors [10, 11, 13–18]. In this study, CNOs were synthesized by sonicating SiC single crystals in water at room temperature. Figure 1 shows an overall schematic of the CNO synthesis procedure. We then investigated the properties of the as-synthesized samples using transmission electron microscopy (TEM), Raman spectroscopy, Fourier-transform infrared (FTIR) spectroscopy, and scanning transmission electron microscopy–electron energy loss spectroscopy (STEM-EELS). The TEM observation revealed CNOs with diameters of 20–30 nm. Elemental mappings and line profile analyses of the samples by STEM-EELS revealed the existence of oxygen on the synthesized CNOs and the growth of carbon from a SiC single crystal.
Schematic of cavitation bubble evolution and CNO synthesis from SiC in pure water. During ultrasonic treatment, SiC transforms into graphene sheets and CNOs.
A SiC single crystal wafer (TanKeBlue Semiconductor Co., Ltd., China) was used for the synthesis. The SiC wafer was crushed and ground into fine powder using a mortar and pestle. Approximately 80% of the crushed particles were smaller than about 7 µm. Fifteen milligrams of SiC powder and 5 mL of pure water (Milli-Q, Merck KGaA, Darmstadt, Germany) were added to a 5 mL vial and sonicated for 4 h in an ultrasonic bath (GT SONIC-R2, GT SONIC, China; power 50 W, frequency 40 kHz) [4, 19]. Owing to sonication, the temperature of the sample and bath increased to approximately 60°C. After sonication, the resulting mixture was centrifuged at 2000 rpm for 10 min in a centrifugation machine (HSC-12000, AS ONE, Japan) to separate the soluble and insoluble portions. The supernatants were collected using a pipette and centrifuged at 6000 rpm for 10 min for purification. Some of these were dialyzed in a 3500-Da dialysis bag in water. The resulting samples were then characterized.
2.2 CharacterizationThe structures of the as-synthesized products were investigated via high-resolution TEM (HF-3300, Hitachi, Japan) at 300 keV. Samples for TEM observations were prepared by drop-casting dispersions onto carbon films on a copper microgrid and drying at room temperature.
Raman spectroscopy was used to confirm the carbon structure of the samples using a laser Raman microscope (NRS-4100, JASCO, Japan) with 532 nm laser light (7.0 mW). The total number of scans required to obtain Raman spectra ranging from 2000 to 750 cm−1 were three for an exposure time of 30 s. For Raman spectroscopy, the samples were prepared by drop-casting the dispersions onto a silicon wafer substrate and drying at 105°C for 24 h.
The chemical structures of the obtained samples were analyzed by FTIR spectroscopy (FT/IR-4100, JASCO, Japan). To prepare specimen disks for FTIR spectroscopy, the sample for measurement was sandwiched between two KBr disks. This combination was then placed in a pelletizer and subjected to pressure, resulting in the formation of specimen disks. The total number of scans used to record the FTIR spectra in the range of 4000 to 500 cm−1 was 320, with a resolution of 4 cm−1.
The elemental distributions of the prepared CNOs were investigated via EELS using aberration-corrected STEM (JEM-ARM200F, JEOL, Japan) at 200 keV.
Figure 2(a) shows an HRTEM image of the synthesized sample. The formation of onion-like structures was easily observed at the sample fragment edges. The magnified image in Fig. 2(b) shows that spheres with a hollow core polyhedral shape have an outer diameter of approximately 20–30 nm. In our sample, approximately 30 nm nano-onions had approximately 30 layers. The distance between the two planes in the shells was approximately 0.38 nm, slightly larger than the interplanar distance in the well-ordered graphite structure. This suggests that due to the bending of the graphite structure, the distance between the two planes increases. The corresponding selected area diffraction (SAED) pattern in Fig. 2(c) reveals a relatively high degree of graphitization in the nano-onions, which agrees well with the HRTEM results. We note that nano-onions could not be detected in the absence of sonication. In addition, we obtained the same results for commercially available SiC powder as the starting material. Raman spectroscopy was used to characterize the samples. Figure 3 shows the Raman spectra of the samples. The spectra display two wide bands at approximately 1296 and 1521 cm−1, corresponding to the disorder-induced D and G bands of graphitic carbon in a two-dimensional hexagonal lattice [20]. The high purity of CNOs was evidenced by a weak D band accompanied by a sharp G band in the Raman spectra [21]. The observed D and G bands in our sample can be attributed to the structure of the CNOs. The D and G peaks may also be attributed to the presence of very few other carbon nanoproducts in the sample. Our careful TEM examination revealed the existence of graphene nanosheets and carbon nanotubes in small numbers. We note that we could not separate CNOs from the other carbon nanoproducts by dialysis.
(a) TEM image of the prepared sample showing the formation of areas of an onion-like structure. (b) Magnified HRTEM image of the region with a hollow cored polyhedral shape highlighted by a yellow arrow in (a). (c) Corresponding SAED patterns of the CNOs exhibiting typical graphite structure diffraction rings.
Raman spectrum of the observed sample using an excitation light of 532 nm. Characteristic D and G bands for carbon materials are observed at 1296 and 1521 cm−1. The peaks at 1167 cm−1 correspond to the silicon–oxygen bond.
To provide additional insights into the chemical functionalization of these carbon nanoproducts, FTIR spectroscopy was used. The FTIR spectra of the SiC powders and the as-synthesized products are shown in Figs. 4(a) and 4(b), respectively. In Fig. 4(a), the vibrations at approximately 485, 856, and 1123 cm−1 are in agreement with the reported typical vibration band of SiC [22]. In Fig. 4(b), the strong vibrations at approximately 2954 and 2917 cm−1 can be attributed to the characteristic aliphatic CH2 vibration. The vibrations at approximately 1700–1200 cm−1 are attributed to the chemical bonds of COOH [23]. These observations indicate the existence of functional groups such as –OH and –COOH on the surface of carbon nanoproducts, which enhance their water solubility and the adsorption of heavy metal ions [24].
FTIR spectra obtained at room temperature in a N2 atmosphere for (a) SiC (black) and (b) the as-synthesized sample (red). The characteristic peak of SiC disappeared and the peaks of oxygen-containing functional group were observed in (b).
The compositional analysis and identification of functional groups embedded in CNOs were conducted using STEM-EELS. Figure 5(a) shows the EELS spectrum of the CNOs; only carbon (onset at 282 eV) and oxygen (onset at 535 eV) were detected. No silicon (onset at 99 eV) was detected in the CNOs. Figure 5(b)–(d) shows bright-field STEM images and the corresponding elemental mappings of the CNOs; C and O are shown in red and green, respectively. The uniform distribution of O (in green) in the CNOs was observed. This suggests that the synthesized CNOs contain abundant oxygen-containing functional groups on their surfaces [24].
(a) EELS spectrum of the CNOs showing C and O K-edges. (b) Bright-field STEM image of the CNOs. (c) and (d) Corresponding elemental mapping of the CNOs showing C and O in red and green, respectively.
To validate the carbon source, we investigated a SiC sample obtained from the supernatants after centrifugation. Figure 6(a) shows the EELS line profile through the edge of the SiC sample. The inset in Fig. 6(a) shows the SiC sample, where the yellow arrow indicates direction and position. A line profile starting from the SiC sample indicates that a sample contains Si and C at a distance of 0–2 nm, and then only the Si concentration decreases at a distance of 0–4 nm, ultimately becoming negligible at a distance of around 4 nm, while C remains. This result indicates that SiC particles can be effectively modified to be covered with carbon through sonication (Fig. 6(b)), and importantly, the carbon consisting of the CNOs in the present study comes from SiC. SiC is used as a ceramic material for sliding bearings and mechanical seals [25]. These findings may lead to a new way to reduce frictional forces generated by ultrasonic waves.
(a) STEM-EELS line profile taken from the inserted image of the EELS maps, showing the relative changes in Si and C across the SiC sample. The yellow arrow in the inset indicates the direction and position of the STEM line along which the EELS intensity data at the Si and C absorption energies were obtained. (b) Schematic representation of a SiC particle covered with C.
A comprehensive thermodynamic and experimental study indicated that carbon is formed on carbides such as SiC and TiC under hydrothermal conditions [26]. By contrast, the CNOs in the present study were synthesized exclusively from SiC without the use of acids or halogens, employing sonication in water as the synthesis method. In this case, it can be concluded that graphene nanosheets decomposed from SiC within bubbles via sonication in water are coiled and formed into graphite hollow polyhedrons. This result was attributed to the minimization of the surface energy of the graphene nanosheets in the liquid solution. Finally, the hollow graphite polyhedra were transformed into spherical CNOs, affording the lowest interfacial energy in the liquid solution. Simultaneously, oxygen-containing functional groups on CNOs could be induced by OH· radicals generated by ultrasonic cavitation.
We demonstrated a novel one-pot synthetic approach to produce CNOs from inorganic SiC via sonication at room temperature in pure water. The TEM results confirmed the formation of CNOs. Moreover, the Raman spectroscopy, FTIR, and STEM-EELS results suggested the existence of oxygen functional groups on the CNOs and the growth of carbon from SiC single crystals. Such oxygen functionalization can induce the formation of water-soluble CNOs, which are necessary for their biomedical and environmental applications. This synthesis strategy is extremely simple and environmentally friendly and requires no additional catalysts (e.g., Fe, Co, or Ni) or acids. The formation of CNOs from inorganic SiC has significant implications spanning a wide range of fields, including astrophysics and green chemistry [27]. Further research is required to investigate the mechanisms of sonication synthesis in inorganic/liquid systems and to develop a purification method to obtain high-purity CNOs from as-produced products. Notably, we also observed the formation of onion-like structures from CaCO3 through a similar one-pot sonochemical process, although this work has not yet been published.
The authors are grateful to M. Morita (ARIM) for technical assistance. A part of this work was supported by “Advanced Research Infrastructure for Materials and Nanotechnology in Japan (ARIM)” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Grant Number JPMXP1222UT0371 and JPMXP1223UT0343.
