Creation of metal-containing carbon onions via self-assembly in metallocene/benzene solution irradiated with an ultraviolet laser

Nanostructures are commonly created by so-called top-down ultra-fine fabrication techniques such as photolithography, x-ray lithography and etching [1, 2], whereas they can also be formed via bottom-up self-assembly processes learning from biological systems [2, 3]. Carbon nanostructures such as fullerenes, carbon nanotubes and graphene are self-assembled during the arc-discharge, laser ablation and chemical vapour deposition processes [4]. It has been shown that fluids such as carbon dioxide and benzene under near- and super-critical conditions can be used as solvents for the creation of nanostructures [5–13]. Gas–liquid coexistence curves terminate at the critical points, where large clusters are formed by fluids' molecules and as a result, incident light, scattered by the clusters, cannot penetrate the fluids, which is known as critical opalescence [14]. The physical properties, such as the specific heat and compressibility, diverge as the fluid systems approach the critical points due to the long-ranged coherent molecular clusters and the abnormal behaviour of fluids under near-critical conditions have been investigated from a universal point of view both theoretically and experimentally [14]. In terms of non-equilibrium transport phenomena occurring near the critical points, it is known that temperature and pressure perturbations propagate as acoustic waves due to the extremely low thermal diffusivity and high compressibility, which is known as the piston effect [15–18], and strong buoyancy convection is induced due to the low thermal diffusivity and high temperature coefficient of volume expansion [17–23]. In addition to their unusual characteristics, near- and super-critical fluids have been recognized as useful fluids from a technological point of view and therefore they are often used in chemical, electronic and environmental sciences and engineering. Reactions are encouraged [24], chemicals are extracted [25], semiconductors are cleaned and purified [26] and nanostructures are created [5–13] in super-critical fluids. In terms of the formation of nanostructures, carbon particles, onions, coils, needles and fibres were created in near- or super-critical carbon dioxide, benzene and their mixture [5–13]. In this paper we demonstrate a bottom-up method of producing metal-containing carbon onions using benzene under sub- and super-critical conditions. We mix metallocene such as ferrocene or cobaltocene with benzene and irradiate an ultraviolet laser into the metallocene/benzene solution. We find that iron- and cobalt-containing carbon onions are formed via self-assembly in benzene under sub- and near-critical conditions. We clarify the dependence of the structural and magnetic characteristics of the nanoparticles produced in benzene on the temperature of the benzene/metallocene solution. We explain the experimental system and procedure in section 2. We show and discuss the experimental results in section 3 and finally summarize the result obtained in this study in section 4.

An outline of the experimental system is shown in figure 1. Metallocene such as ferrocene or cobaltocene was dissolved in benzene and the metallocene/benzene solution was confined in a cylindrical container made of stainless steel. The mass concentration of metallocenes in benzene was set at 3.52, 11.76, 17.64 or 23.52 mg ml⁻¹ and the amount of benzene was set at the critical density. The inner and outer diameters and inner and outer heights of the container were, respectively, 13 and 60 mm, and 23 and 66 mm. A synthetic quartz was mounted at the top of the container for the introduction of the laser beam (see figure 1). The diameter and thickness of the quartz window was 20 and 10 mm. A platinum resistance thermometer (Pt100, Chino Co. Ltd) was set in the container wall and the temperature of the fluid was controlled by a heater installed around the container and a temperature controller (LT470, Chino Co. Ltd). The fluid conditions were changed from a sub-critical liquid–gas two-phase region to super-critical one by controlling the fluid temperature. Note that the critical temperature T_c, pressure P_c and density ρ_c of benzene are T_c = 289 °C, P_c = 4.92 MPa, ρ_c = 300 kg m⁻¹ [27]. In each experiment, 50000 pulses of a UV laser beam of 266 nm wavelength were irradiated from a neodymium doped yttrium/aluminium/garnet (Nd:YAG) laser (Brilliant Quantel Ltd Co.) into the metallocene/benzene solution, the temperature of which was set at 25, 150, 200, 250 or 290 °C. The diameter of the laser beam was 10 mm and the energy flux was 5.2 mW mm⁻². The duration of each laser pulse and the frequency of the pulse generation were 4.3 ns and 10 Hz. The beam was not focused on any particular point. After each experiment the temperature of the fluid was decreased gradually down to room temperature.

Zoom In Zoom Out Reset image size

We observed the residue products in benzene by a scanning electron microscope (SEM) (JSM-7400F, JEOL) and transmission electron microscope (TEM) (JEM-2200FS, JEOL). We also analysed the structures of the products by the selected area electron diffraction (SAED) method (JEM-2200FS, JEOL), the elementary components of the structures by energy-disperse x-ray spectroscopy (EDS) (JED-2300T, JEOL) and the magnetization by a vibrating sample magnetometer (VSM) (7407, Lake Shore Crytronics Inc.).

We irradiated UV laser into sub- and super-critical benzene, in which ferrocene or cobaltocene was dissolved, at different temperatures; 25, 150, 200, 250 and 290 °C. The relation between the structures created in the solution after the laser irradiation and the temperature of the solution is summarized in table 1. Amorphous carbon particles and metal-containing amorphous carbon particles were produced when the temperature was lower than or equal to 200 °C, whereas carbon onions and metal-containing carbon onions as well as amorphous carbon particles and metal-containing amorphous carbon particles were produced when the temperature was 250 and 290 °C. TEM images, SAED patterns and EDS mappings of carbon nanostructures formed in ferrocene/benzene and cobaltocene/benzene solutions are, respectively, shown in figures 2 and 3, where the temperature of the solution during laser irradiation was 290 °C and the mass concentration of ferrocene or cobaltocene mixed with benzene was 3.52 mg ml⁻¹. Iron-containing and cobalt-containing carbon onions were produced at both 250 and 290 °C irrespective of the differences in the mass concentration of ferrocene and cobaltocene mixed with benzene; 3.52, 11.76, 17.64 or 23.52 mg ml⁻¹. It is clearly shown that the core particles were formed by iron or cobalt. The number of metal-containing carbon onions formed at 290 °C was more or less the same as that formed at 250 °C, whereas no metal-containing carbon onion was formed when the temperature was lower than or equal to 200 °C as mentioned. The number of metal-containing carbon onions increased with the increase in the mass concentration of metallocenes mixed with benzene. We measured the diameters of the core metal particles from TEM images, targeting 1083 iron particles and 921 cobalt particles. The diameter of the core iron particles produced at 290 °C was 7.5 ± 5.2 nm, whereas that of the cobalt particles was 7.2 ± 3.6 nm. The average thickness of the carbon onions was 3.2 nm in both cases of iron- and cobalt-containing carbon onions. The gap between the adjacent graphitic layers in carbon onions was 0.34 nm [28] (see the supplementary information for TEM images of graphitic layers in metal-containing carbon onions, online supplementary data available from stacks.iop.org/ANSN/3/035010/mmedia). The diameters of the metal-containing carbon onions and core metal particles were the same irrespective of the differences in the temperature; i.e. 250 and 290 °C, and the mass concentration of metallocenes mixed with benzene. The sources of metals forming the core particles are metallocenes; i.e. ferrocene and cobaltocene, whereas those of carbon are benzene and metallocenes. The dissociation energy of one hydrogen atom from benzene is 4.90 eV [29]. Photons of 266 nm wavelength, which were irradiated into benzene in the present experiment, are absorbed by benzene [29]. The photon energy of 266 nm wavelength being 4.66 eV, it is supposed that at least two-photon absorption was occurring for hydrogen dissociation from benzene. However, the dissociation energy of the second hydrogen atom is lower than 4.90 eV once the first hydrogen atom has been dissociated [29] and therefore six-membered rings of carbon atoms may be quite easily produced. It is also possible for carbon atoms to be dissociated from metallocenes [30, 31]. It is known that the decomposition energy corresponding to Fe(cp)₂ → Fe + cp + cp is 6.8 eV [32], whereas that corresponding to Co(cp)₂ → Co + cp +cp is 5.64 eV [33]. It is therefore supposed that two-photon absorption was constantly occurring and ferrocene and cobaltocene were decomposed into iron, cobalt and two cp-rings during the irradiation of photons of 266 nm wavelength. It is also supposed that during the interval between two pulses, dissociated high-energy iron and cobalt atoms were cooled and coagulated each other to form the core particles, during which the excess energy was transferred to cp-rings and six-membered rings and as a result, those rings formed carbon onions [34]. We suppose that the diameter of iron particles was more or less the same as that of cobalt particles since the melting temperature and surface tension of iron and cobalt are similar [35] and the absorption characteristics of UV photons of 266 nm wavelength passing through ferrocene/benzene and cobaltocene/benzene solutions are also the same (see supplementary information for the absorption spectra of the ferrocene/benzene and cobaltocene/benzene solutions).

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

The magnetization curves of iron- and cobalt-containing carbon onions are shown in figure 4. The iron- and cobalt-containing carbon onions showed superparamagnetic characteristics. The saturation magnetization increased with the increase in the mass concentration of ferrocene, but in the case of cobaltocene, it did not change when the mass concentration of cobaltocene was over 17 mg ml⁻¹ since cobaltocene did not dissolve in benzene at room temperature once the mass concentration exceeded 17 mg ml⁻¹. Note that superparamagnetism of iron and cobalt is, respectively, caused by body-centred cubic (bcc) [36] and hexagonal close-packed (hcp) lattice structures [37] (see also figures 2(a) and 3(a)). The saturation magnetization of cobalt-containing carbon onions was higher than that of iron-containing carbon onions since the crystallinity of the cobalt particles was higher than that of the iron particles (see the supplementary information for TEM images of iron- and cobalt-containing carbon onions, online supplementary data available from stacks.iop.org/ANSN/3/035010/mmedia). We suppose that the difference in the crystallinity of the core particles might have been caused by the difference in the initial temperature of iron and cobalt atoms dissociated from metallocenes. The initial temperature of the cobalt atoms was higher than that of the iron atoms since the decomposition energy of cobaltocene is lower than that of ferrocene as mentioned.

Zoom In Zoom Out Reset image size

Since core magnetic particles are not oxidized thanks to carbon onions covering them, the magnetization will not deteriorate, which makes the present magnetic nanoparticles more attractive and practical, considering their application to the fields of nanoelectronics, nanomagnetics, biochemistry and biomedical science and engineering [38–41]. The operational temperature of the present methodology is as low as 290 °C, which is also favourable for practical use since coagulation of particles can be avoided during the synthesis process of the particles [42]. The present metal-containing carbon onions may well be utilized, particularly in biomedical fields; e.g. the imaging of biomolecules and cells [43], nanosurgery [44] and drug delivery [45], as well as in the fields of nano/micro electronics, magnetics and electromechanics [46]. We will be producing alloys such as Fe–Co and Fe–Ni by mixing two types of metallocenes such as ferrocene and cobaltocene, or ferrocene and nickelocene with benzene, in which case the magnetization may be changed by altering the elementary ratio.

We irradiated sub- and super-critical benzene/metallocene solutions with a laser beam of 266 nm wavelength and found that metal-containing carbon onions, which have superparamagnetic characteristics, are created. The number and diameter of metal-containing carbon onions were the same irrespective of the difference in the temperature; i.e. 250 and 290 °C, and the saturation magnetization of metal-containing carbon onions increased with the mass concentration of metallocenes mixed with benzene. The present metal-containing carbon onions may well be utilized, particularly in biomedical fields as well as in the fields of nano/micro electronics, magnetics and electromechanics.

Part of this study has been supported by a Grant for the Programme for the Strategic Research Foundation at Private Universities S1101017 organized by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) since April 2011.