Formation kinetics and photoelectrochemical properties of crystalline C₇₀ one-dimensional microstructures

Abstract

Assembling sub-nanometer fullerene molecules into ordered microstructures is a necessary step towards their applications, but concise morphology control of the ordered structures is very challenging and the formation mechanism is still a big mystery. We herein report the preparation and morphology control of different C₇₀ one-dimensional (1D) microstructures with regioisomers of xylene as good solvents and isopropyl alcohol (IPA) as a poor solvent using the interfacial precipitation method. Our systematic investigations show that the solvents participating in the formation process of these C₇₀ 1D microstructures play a critical role in determining the morphology, crystalline structure, formation process and intrinsic properties of these materials. Furthermore, we have also investigated the photoelectrochemical properties of these C₇₀ 1D microstructures, proving their potential applications in related fields.

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1 Introduction

Carbon nanomaterials with unique structures have received considerable attention during recent years. Typical examples are zero-dimensional (0D) fullerene, one-dimensional (1D) carbon nanotubes, and two-dimensional (2D) graphene.^1–3 Among them, fullerenes are particularly unique because of their definitive molecular structures and high solubility in organic solvents.⁴ Assembly of sub-nanometer fullerene molecules into nano/microstructures is a practical way to realize their ultimate applications in various fields.^5–8 Among the various methods for the preparation of fullerene nano/microstructures,^9–15 the interfacial precipitation method, using a mixture of solvents consisting of a good solvent and an antisolvent, shows great potential in the morphology control of fullerene crystals because of its facileness and versatility.^8,16–20 Compared to the various C₆₀ crystals obtained using the this method,^16–23 geometrically well-defined C₇₀ crystals^7,24–26 are less-studied and concise control of the morphologies of C₇₀ nano/microstructures is still a big challenge. Moreover, although it is widely acknowledged that the solvents play a crucial role in the morphology control or crystalline determination of the fullerene crystals, and thus subtle variation of the solvent types^17,21,22,27 or solvent ratios^{7,16,17,20,28} can remarkably change the morphological features of the products, it is still unclear how the solvents participate in the formation process during precipitation.

Herein, to gain an in-depth understanding of the effect of solvent on C₇₀-crystal formation during the precipitation process, we conducted a systematic study by employing three regioisomers of xylene as the good solvents and isopropyl alcohol (IPA) as the antisolvent to obtain various C₇₀ nano/microstructures with different morphological features. It is revealed for the first time that the content of solvent-doping in the formed crystals is a critical factor in controlling the morphologies and formation kinetics of these C₇₀ microstructures. A kinetically favored process is observed for the C₇₀/m-xylene/IPA system, which results in nanometer-sized cylindrical nanorods with a high content of solvent (9.45%). In contrast, the microstructures obtained from either the C₇₀/p-xylene/IPA or C₇₀/o-xylene/IPA system are large hexagonal prisms with relatively low solvent contents, confirming that the formation mechanisms are both thermodynamic processes. Finally, it is demonstrated that these well-ordered C₇₀ 1D microstructures exhibit excellent photoconductive properties, with spectral responses covering the ultraviolet to visible regions, highlighting their future applications as optical sensors. Our results have shed new light on the morphology control, formation mechanism and application of the less studied C₇₀ nano/microstructures, which will stimulate broad interest from across multidisciplinary fields.

2 Experimental section

Preparation of C₇₀ 1D microstructures

O-Xylene, m-xylene and p-xylene were employed as the respective good solvents for C₇₀ and IPA as the poor solvent in all cases. The concentration of C₇₀ in p-xylene or m-xylene was set to be 2 mg mL⁻¹, but the concentration in o-xylene was 4 mg mL⁻¹ because a lower concentration of C₇₀ (e.g. 2 mg mL⁻¹) in o-xylene did not trigger any precipitate under the same conditions. Typically, 5 mL of C₇₀ solution was rapidly injected into 10 mL IPA to instantly form a turbid solution. Then the solution was left for 12 h to precipitate. The suspension was separated by centrifugation and was washed with IPA several times. The samples freshly obtained from the mixed solvents without any heat treatment are designated as C70-O, C70-M, C70-P when o-xylene, m-xylene, p-xylene are used as solvents, respectively. The samples were further treated by vacuum drying at 60 °C overnight to remove the solvents absorbed on the surface. These samples are defined as C70-O-60 (solvent: o-xylene, 60 °C vacuum drying), C70-M-60 (solvent: m-xylene, 60 °C vacuum drying) and C70-P-60 (solvent: p-xylene, 60 °C vacuum drying). Then, the obtained solid samples were also annealed at 150 °C for 5 h under N₂ atmosphere to ensure the complete elimination of any intercalated solvents in the C₇₀ crystals. This leads to the determination of the phase-transformation process of the nanostructures obtained from the m-xylene/IPA system (vide infra). These samples are labeled as follows: C70-O-150 (solvent: o-xylene, 150 °C annealing under N₂), C70-M-150 (solvent: m-xylene, 150 °C annealing under N₂), C70-P-150 (solvent: p-xylene, 150 °C annealing under N₂).

Characterization

Scanning electron microscopy (SEM) images were collected using either a Nova NanoSEM 450 or a JEOL 6701F electron microscope operating at an accelerating voltage of 10 kV. A transmission electron microscope (FEI Tecnai G2 T20 or F30) was used to obtain transmission electron microscopy (TEM) images, high-resolution transmission electron microscopy (HRTEM) images and selected area electron diffraction (SAED) patterns. All X-ray diffraction measurements were carried out using an Empyrean diffractometer with Cu Kα radiation (λ = 0.1541 nm, operated at 40 kV and 40 mA). The composition of the samples was confirmed using FT-IR (Bruker Tensor-27 FT-IR) and Raman spectroscopy (Bruker VERTEX 70). Thermo-gravimetric analysis (TGA, Diamond TG/DTA) was performed to determine the solvent content in the samples. UV-vis spectra were obtained using a Lambda 750S spectrometer.

Photocurrent measurements

The photocurrent tests were carried out in a three-electrode system, consisting of a working electrode (C₇₀ microstructure film), an Ag/AgCl reference electrode and a platinum wire counter electrode, using a CHI 660E electrochemical workstation at room temperature and a 500 W xenon lamp (Saifan Photoelectronic Co., 7IPX5002) as a light source. The electrolyte was 0.1 M aqueous KCl solution. The working electrode was made as follows: indium tin oxide (ITO) was used as the current collector and IPA dispersions of the corresponding C₇₀ microstructures were deposited onto the ITO by drop-casting. Two filters were used to obtain the visible light source (400–800 nm) and UV light source (350 nm). All photoelectrochemical experiments were carried out under Ar atmosphere.

3 Results and discussion

Morphological characterizations

The SEM and TEM images of the as-prepared C₇₀ microstructures are shown in Fig. 1. Although all these microstructures display one-dimensional morphological appearances, their detailed structures and diameters/lengths are totally different from each other. C70-O and C70-P both exhibit hexagonal cross-sections with pyramidal ends but C70-P displays an appearance with two tube-like ends and a solid center (Fig. 1(c3)), which we assume may be a consequence of concentration depletion.²⁰ In contrast, the C70-M nanostructures display cylindrical solid shapes. As summarized in Table 1 and Fig. S1,† C70-O hexagonal microrods are 1.9–3.5 μm wide and 19.0–31.0 μm long with a statistical diameter of 2.7 μm and a mean length of 26.0 μm. Similarly, the widths of the C70-P microtubes range from 1.9 to 3.3 μm, and their lengths fall into the range of 19.0–31.0 μm. The statistical diameter and length of the C70-P microtubes are 2.5 μm and 23.9 μm, respectively. In sharp contrast, the C70-M nanorods are thinner and shorter with a narrow diameter distribution of 0.35–0.75 μm, a narrow length range of 5.0–11.0 μm, a mean diameter of 0.47 μm and a mean length of 9.8 μm. The morphological differences between these microstructures must be caused by the different solvents used in the experiments because the other experimental conditions are identical.

Fig. 1 SEM images (a1–c1, a2–c2) and TEM images (a3–c3) of C₇₀ 1D microstructures prepared using different solvents: (a) C70-O, (b) C70-M and (c) C70-P.

Table 1 Size distributions of C₇₀ 1D microstructures obtained from different solvents

Sample number	C70-O	C70-M	C70-P
Diameter (μm)	1.9–3.5	0.35–0.75	1.9–3.3
Length (μm)	19.0–31.0	5.0–11.0	19.0–31.0

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Crystalline configuration and composition

X-ray diffraction (XRD) measurements are effective for investigating the crystalline structures of organic and inorganic materials. The powder XRD patterns of the C₇₀ 1D microstructures obtained after 60 °C vacuum drying are shown in Fig. 2(a). It is clear that C70-P-60 and C70-O-60 display similar XRD patterns to that of C₇₀ powder, and are indexed as (001), (002), (101), (102), (110), (103) and (112) diffraction peaks, corresponding to an hcp lattice.²⁹ In contrast, C70-M-60 has an orthorhombic lattice, indexed as (002), (011), (102), (004), (024), (213) and (008) diffraction peaks, which is similar to that of the toluene-solvated phase reported previously.³⁰ The XRD patterns of untreated samples freshly obtained from the mixed solvents are shown in Fig. S2,† and are the same as those of the corresponding samples treated at 60 °C. However, after annealing at 150 °C, C70-M-150 shows an XRD pattern of the fcc phase (Fig. 2(b)), although C70-O-150 and C70-P-150 still retain their hcp structures. The unit cell dimensions of these samples determined by XRD measurements are listed in Table 2. The orthorhombic C70-M-60 crystals have unit cell dimensions of a = 10.49 Å, b = 11.14 Å, c = 35.05 Å.³¹ After annealing at 150 °C, the orthorhombic structure evolves into the fcc structure, with lattice constants of a = b = c = 14.87 Å.³¹ The space lattices of C70-O-60 and C70-P-60 both correspond to the hcp structure (c/a = 1.63),³¹ with unit cell dimensions of a = b = 11.15 Å, c = 17.91 Å for C70-O-60 and a = b = 11.10 Å, c = 17.70 Å for C70-P-60 (Table 2). They are slightly different from each other due to the incorporation of different solvent molecules. After annealing at 150 °C for 5 h, the crystalline types are not changed, but the unit cell dimensions slightly differ from the corresponding precursors, with a = b = 11.02 Å, c = 17.99 Å for C70-O-150 and a = b = 10.99 Å, c = 17.90 Å for C70-P-150 (Table 2), most probably due to the loss of the solvent molecules from the crystal cavities.

Fig. 2 XRD patterns of C₇₀ powder and as-prepared C₇₀ 1D microstructures (a) after vacuum drying at 60 °C and (b) after annealing at 150 °C for 5 h; in situ VT-XRD patterns of (c) C70-O-60, (d) C70-M-60, and (e) C70-P-60 in the temperature range between 25 °C and 400 °C; (f) TGA results for C70-O-60, C70-M-60 and C70-P-60.

Table 2 Crystalline types and unit cell dimensions of C₇₀ 1D microstructures under study

Sample	C70-O-60	C70-O-150	C70-M-60	C70-M-150	C70-P-60	C70-P-150
Crystalline type	hcp	hcp	Orthorhombic	fcc	hcp	hcp
a (Å)	11.15	11.02	10.49	14.87	11.10	10.99
b (Å)	11.15	11.02	11.14	14.87	11.10	10.99
c (Å)	17.91	17.99	35.05	14.87	17.70	17.90

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In situ variable temperature X-ray diffraction (VT-XRD) measurements were carried out to further examine the crystalline transformation processes of these C₇₀ 1D microstructures. As illustrated in Fig. 2(d), a conspicuous transformation of the XRD patterns between 100 and 150 °C is observed for C70-M-60. This temperature range is consistent with the boiling point of m-xylene (139 °C). Accordingly, we speculate that this crystalline phase transformation is caused by the loss of the solvent molecules trapped in the crystal lattice. Although previous studies reported that the evaporation of the trapped solvents in C₆₀ crystals is fast even at room temperature or under vacuum,^32,33 our results demonstrate that the elimination of the solvents in these C₇₀ crystals is rather difficult and requires a higher temperature. We assume here that the ellipsoidal C₇₀ molecules are more suitable for preventing the escape of solvent molecules from the closely packed crystal lattice. In comparison, the VT-XRD results of C70-O-60 and C70-P-60 (Fig. 2(c) and (e)) demonstrate that the hcp structures remain unchanged at high temperatures, because it is almost as stable as the closely packed fcc phase of C₇₀.³¹ A significant mass loss (9.45%) between 80 and 200 °C is observed for C70-M-60 in the TGA results (Fig. 2(f)) but much smaller values are obtained for C70-O-60 and C70-P-60 (2.68% and 3.64%, respectively). Consistent with this, the FTIR spectrum of C70-M-60 (Fig. S3a†) shows a clear solvent peak at 766.5 cm⁻¹, ascribed to the trapped m-xylene molecules,^19,27,34 but no obvious solvent absorption peaks are observed in the samples of C70-O-60 and C70-P-60 because of the low content of the trapped solvents in the lattice cavities. The much higher incorporation of m-xylene in the C₇₀ crystals compared to that of o-xylene or p-xylene can be explained by the geometrical differences, similar to the cases of C₆₀ crystals.²⁷ Combining the above results, we speculate that the formation of C70-M nanostructures should be a kinetic process whereas the formation processes of the C70-O and C70-P micro-structures are both thermodynamically favourable. It appears that a high degree of solvent-doping in the crystalline structures results in relatively small cylindrical nanorods and a low content of solvents tends to give large microstructures with hexagonal cross-sections (Table 1).

Morphology elucidation of C70-P hexagonal microstructures

Both C70-O and C70-P exhibit hexagonal cross-sections, which aroused our interest to perform lattice-resolved HRTEM studies of their internal structures. The TEM results of C70-P are shown in Fig. 3 as a representative example and those of C70-O are in Fig. S4.† From Fig. 3(a), the interplanar spacing values of 0.54 and 0.88 nm can be easily attributed to the lattice constants of the (110) and (002) planes, respectively, showing the preferential growth along the [001] direction (Fig. 3(b)). Considering the hexagonal prism structure, we conclude that the (100), (110) and (010) planes form the hexagonal prism (Fig. 3(c)). The angle between the longitudinal rod axis and the intersecting line of two pyramidal planes is 36.1° (Fig. S5†), which leads to the assignment of the lattice planes of the pyramidal tip to be (101̄), (111̄), (011) and (111) (Fig. 3(c)). A schematic model of a microtube of C70-P is drawn in Fig. 3(c). The formation of the prism-shaped C₇₀ crystals is attributed to the difference in the growth rate of the crystal facets.³⁵ In the present study, the disappearance of the (001) plane may result from the fairly rapid growth rate along this plane. Therefore, the {111} planes form the pyramidal tips at the end of the c-axis, while the {110} planes remain to form the hexagonal prisms. Since the case of C70-O is similar to that of C70-P, where the growth axis is also the [001] direction (Fig. S4b†), the schematic model for C70-P is also suitable for the interpretation of the internal structure of C70-O.

Fig. 3 (a) HRTEM and (b) fast Fourier transform (FFT) image of a microtube of C70-P which is shown in the inset of (a); (c) a schematic model for a hexagonal microtube of C70-P; (d) SEM image of C70-P microtubes.

Phase transformation model for C70-M

HRTEM measurements were also conducted to clarify the phase transformation process from C70-M-60 (orthorhombic) to C70-M-150 (fcc). Fig. 4 displays the corresponding HRTEM images and SAED patterns, proving that both are single crystals. Combining the XRD, TGA and TEM results, we propose an interpretation model for the phase transformation from the orthorhombic lattice to the fcc lattice for the C70-M nanorods (Scheme 1).³⁰ From the TGA results, the molar ratio of C₇₀ to m-xylene in C70-M-60 is calculated to be 5 : 4. Thus, the initial orthorhombic unit cell is formed of five C₇₀ molecules, inside which four m-xylene molecules are trapped (5C₇₀·4m-xylene) (Scheme 1(a)). Under higher temperatures, however, the solvents evaporate from the crystal lattice and the bct structure (defined in the fcc cell, Scheme 1(b)) is formed by the shrinkage of the c-axis of the orthorhombic cell along with an alternate shearing displacement along the b-axis (Scheme 1(d)).

Fig. 4 (a) HRTEM and (b) FFT images of a C70-M-60 nanorod, which is shown in the inset of (b). (c) HRTEM image and (d) SAED pattern of a C70-M-150 nanorod, which is shown in the inset of (d).

Scheme 1 Schematic illustration showing phase transformation from the orthorhombic phase of C70-M-60 to the fcc phase of C70-M-150. (a) The pristine orthorhombic unit cell of 5C₇₀·4m-xylene. (b) The fcc unit cell obtained after transformation (light lines) in which the bct cell is defined with bold bonds. (c) Projection from a-axis of the original orthorhombic cell. (d) Projection from the [11̄0] direction of the bct cell. The shrinkage in the c-axis and the alternate shearing displacement along the b-axis after solvent-loss cause a transformation in the bct unit cell. The shaded portion in (a) represents the (100) plane of the orthorhombic cell and that in (b) represents the (110) plane of the fcc cell. The grey dots represent C₇₀ molecules.

This model (Scheme 1) is also consistent with the HRTEM results (Fig. 4). For C70-M-60, the lattice spacing along the longitudinal rod axis is 0.52 nm, corresponding to the (200) plane. The parallel crystal spacing is 1.15 nm, which corresponds to the (011) plane. For C70-M-150, the discrete diffraction spots in Fig. 4(d) are indexed according to the fcc structure, in which the d(220) lattice spacing is also 0.52 nm, and the [110] direction is parallel to the longitudinal rod axis. As shown in Fig. 4(a) and (c), the (100) plane of the orthorhombic cell (d-spacing: 1.04 nm) and the (110) plane of the fcc cell (d-spacing: 1.04 nm) are both perpendicular to the longitudinal axis of the C₇₀ rods, and the d-spacing values of these two planes are identical. This confirms that the (100) plane of the orthorhombic cell is superposed with the (110) plane of the fcc cell, as clearly shown in Scheme 1.

Growth mechanism

To gain more information about the formation processes of these C₇₀ 1D microstructures, we collected the SEM images of the samples at different times after the injection of the corresponding C₇₀ solutions into IPA. The morphology evolution processes of these C₇₀ 1D microstructures are shown in Fig. 5 and the plausible growth mechanisms are illustrated in Scheme 2. It is evident that the C₇₀ clusters nucleate instantly after the injection. In the case of C₇₀/o-xylene/IPA, C₇₀ nanorod-bundles are formed initially,³⁶ and then they grow longer and fuse together to form the final hexagonal microrods. However, the case of C₇₀/m-xylene/IPA is markedly different. In the beginning, C₇₀ crystals grow in various directions to form large star-like crystals that subsequently disconnect to generate the microrods along with the depletion of C₇₀ nuclei. Finally, the microrods branch to form smaller rods (inset of Fig. 5(b2)), which eventually afford the individual nanorods after the matrix microrods are depleted. For the C₇₀/p-xylene/IPA system, preliminary hexagonal nuclei (Fig. 5(c1)) are formed instead of hierarchical structures consisting of cylindrical nanorods (Fig. 5(a1) and (b1)). Then, the hexagonal nuclei grow in two opposite directions to form the final hexagonal microtubes with two hollow ends. To conclude, C₇₀ crystals obtained from C₇₀/o-xylene/IPA or C₇₀/p-xylene/IPA are formed by unidirectional growth of the initial nuclei. In contrast, for C₇₀/m-xylene/IPA, the C₇₀ crystals tend to grow along multiple directions. This is a clear indication of the kinetically driven growth process, which results in smaller nanorods without preferentially exposed facets but with a high content of solvent molecules. In contrast, both the growth processes of C70-O and C70-P are thermodynamically favoured, which afford large equilibrium microstructures with preferred hexagonal prism appearances.

Fig. 5 Mechanistic studies of the corresponding C₇₀ microcrystals at different times and in different solvent systems using SEM. (a) C₇₀/o-xylene/IPA, (b) C₇₀/m-xylene/IPA, (c) C₇₀/p-xylene/IPA. (a1–c1) Samples obtained as soon as the corresponding C₇₀ solutions were added into IPA. (a2–c2) Samples obtained after 10 min growth. (a3–c3) Samples obtained after 1 h growth.

Scheme 2 Schematic illustration of the formation processes of C₇₀ 1D microstructures from different solvent interfaces; (a), (b) and (c) represent C₇₀/o-xylene/IPA, C₇₀/m-xylene/IPA and C₇₀/p-xylene/IPA, respectively. The dashed lines in (c) correspond to the tube-like structures.

In conclusion, we confirmed unambiguously that the intercalation of solvent molecules into the crystal lattice affects the molecular arrangement of the crystalline structures and determines the kinetic pathways toward aggregation.^37,38 In particular, the formation of the nanorods for C₇₀/m-xylene/IPA is a kinetically driven process, so that the initially formed orthorhombic structure evolves into the thermodynamically more stable fcc structure under heating. This is a clear observation of the solvent effect on the growth pattern of fullerene nano/microstructures.

Photoelectrochemical properties

Carbon-based materials with π-conjugated systems show excellent performances in various fields.^39–41 In this work, the photoelectrochemical properties of C₇₀ 1D microstructures are studied. Taking into consideration the differences in crystalline structure, we examine the electrochemical properties of C70-M-60 (orthorhombic), C70-M-150 (fcc) and C70-P-150 (hcp). As shown in Fig. 6, they all show fast and uniform cathodic photocurrents responding to each on–off event, both in the visible and ultraviolet (UV) regions, indicating p-type photoresponse processes. In contrast, C₆₀ films generally display anodic photocurrents, corresponding to an n-type photoresponse.⁴² From the UV-vis absorption spectra (Fig. S6†), it is evident that C₇₀ microstructures show much broader absorptions in the visible region than that of a C₇₀ solution (Fig. S6b†). This phenomenon can be caused by strong intermolecular π–π interactions upon the formation of these ordered microstructures.²⁸

Fig. 6 Photocurrent responses of C₇₀ microstructures under visible (400–800 nm, red) and UV light (350 nm, blue) irradiation. Conditions: 0.1 M aqueous KCl solution at 1 V bias voltage. (a) C70-M-60, (b) C70-M-150, (c) C70-P-150. (d) On/off ratios for different samples under visible light and UV light.

Furthermore, it should be noted that C70-P-150 has the largest dark current and photocurrent, under both visible and UV light illumination, while C70-M-60 possesses the smallest values (Fig. 6(a)–(c)). However, Fig. 6(d) clearly shows that C70-M-60 (orthorhombic) has the highest on/off ratio under both visible and UV light illumination. We propose that this phenomenon should be a consequence of the phase differences, associated tightly with the incorporation of solvent molecules. The results show promise for potential applications as optical sensors.

4 Conclusions

In summary, we have successfully synthesized various 1D microstructures of C₇₀ with three different xylene isomers (o-xylene, m-xylene, p-xylene) as good solvents and isopropyl alcohol as the antisolvent using the precipitation method. The morphology of the as-prepared samples can be readily controlled by changing the solvent. Morphology and mechanism studies reveal that for C₇₀/o-xylene/IPA or C₇₀/p-xylene/IPA, the nuclei grow in a unidirectional manner to form the final hexagonal microstructures with a low degree of solvent participation, governed by thermodynamic factors. However, the formation of the nanorods in the m-xylene/IPA system is a kinetically favored process which has a multidirectional growth feature resulting in cylindrical nanorods with a high content of solvent molecules. Our results confirm that the solvents play a crucial role in controlling the morphology, the crystalline configuration, the stability and somehow the intrinsic properties of the C₇₀ 1D microstructures. Finally, we reveal that these 1D C₇₀ microstructures show fast and uniform cathodic photocurrent responses under both visible and UV light irradiation. The present study reveals broad prospects for the morphology control and formation mechanism of C₇₀ nano/microstructures, which will fundamentally promote their future applications in related fields.

Acknowledgements

Financial support from The National Thousand Talents Program of China, NSFC (21171061, 51472095), Program for Changjiang Scholars and Innovative Research Team in University (IRT1014) and HUST is gratefully acknowledged. We thank the Analytical and Testing Center in Huazhong University of Science and Technology for related measurements.

Notes and references

1. W. Kratschmer, L. D. Lamb, K. Fostiropoulos and D. R. Huffman, Nature, 1990, 347, 354–358.
2. S. Iijima, Nature, 1991, 354, 56–58.
3. K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva and A. A. Firsov, Science, 2004, 306, 666–669.
4. K. N. Semenov, N. A. Charykov, V. A. Keskinov, A. K. Piartman, A. A. Blokhin and A. A. Kopyrin, J. Chem. Eng. Data, 2009, 55, 13–36.
5. L. K. Shrestha, Q. Ji, T. Mori, K. i. Miyazawa, Y. Yamauchi, J. P. Hill and K. Ariga, Chem.–Asian J., 2013, 8, 1662–1679.
6. H. Li, B. C. K. Tee, J. J. Cha, Y. Cui, J. W. Chung, S. Y. Lee and Z. Bao, J. Am. Chem. Soc., 2012, 134, 2760–2765.
7. C. Park, E. Yoon, M. Kawano, T. Joo and H. C. Choi, Angew. Chem., Int. Ed., 2010, 49, 9670–9675.
8. L. Wei, J. Yao and H. Fu, ACS Nano, 2013, 7, 7573–7582.
9. S. I. Cha, K. i. Miyazawa and J.-D. Kim, Chem. Mater., 2008, 20, 1667–1669.
10. C. Park, H. J. Song and H. C. Choi, Chem. Commun., 2009, 4803–4805.
11. K. Miyazawa, Y. Kuwasaki, A. Obayashi and M. Kuwabara, J. Mater. Res., 2002, 17, 83–88.
12. S. S. Babu, H. Mohwald and T. Nakanishi, Chem. Soc. Rev., 2010, 39, 4021–4035.
13. J. Kim, C. Park, J. E. Park, K. Chu and H. C. Choi, ACS Nano, 2013, 7, 9122–9128.
14. S. J. Ahn, J. Yang, K. W. Lim and H. S. Shin, J. Cryst. Growth, 2013, 363, 141–144.
15. B. Wang, X. Gao and G. Piao, J. Nanomater., 2013, 2013, 5.
16. M. Sathish and K. i. Miyazawa, J. Am. Chem. Soc., 2007, 129, 13816–13817.
17. J. Jeong, W.-S. Kim, S.-I. Park, T.-S. Yoon and B. H. Chung, J. Phys. Chem. C, 2010, 114, 12976–12981.
18. L. K. Shrestha, J. P. Hill, T. Tsuruoka, K. i. Miyazawa and K. Ariga, Langmuir, 2012, 29, 7195–7202.
19. H.-X. Ji, J.-S. Hu, L.-J. Wan, Q.-X. Tang and W.-P. Hu, J. Mater. Chem., 2008, 18, 328–332.
20. H.-X. Ji, J.-S. Hu, Q.-X. Tang, W.-G. Song, C.-R. Wang, W.-P. Hu, L.-J. Wan and S.-T. Lee, J. Phys. Chem. C, 2007, 111, 10498–10502.
21. L. K. Shrestha, Y. Yamauchi, J. P. Hill, K. i. Miyazawa and K. Ariga, J. Am. Chem. Soc., 2012, 135, 586–589.
22. M. Sathish, K. i. Miyazawa, J. P. Hill and K. Ariga, J. Am. Chem. Soc., 2009, 131, 6372–6373.
23. M. Sathish, K. Miyazawa and T. Sasaki, Chem. Mater., 2007, 19, 2398–2400.
24. D. Liu, M. Yao, L. Wang, Q. Li, W. Cui, B. Liu, R. Liu, B. Zou, T. Cui, B. Liu, J. Liu, B. Sundqvist and T. Wågberg, J. Phys. Chem. C, 2011, 115, 8918–8922.
25. K. i. Miyazawa, J. Am. Ceram. Soc., 2002, 85, 1297–1299.
26. D. Liu, W. Cui, N. Yu, R. Liu, D. Liu, Y. Xu, C. Quan, B. Liu, Q. Li and B. Liu, CrystEngComm, 2014, 16, 3284–3288.
27. M. Rana, R. B. Reddy, B. B. Rath and U. K. Gautam, Angew. Chem., Int. Ed., 2014, 53, 1–6.
28. Y. Xu, J. Guo, T. Wei, X. Chen, Q. Yang and S. Yang, Nanoscale, 2013, 5, 1993–2001.
29. M. C. Valsakumar, N. Subramanian, M. Yousuf, P. Sahu, Y. Hariharan, A. Bharathi, V. Sankara Sastry, J. Janaki, G. V. N. Rao, T. S. Radhakrishnan and C. S. Sundar, Pramana, 1993, 40, L137–L144.
30. Y. Takahashi, Chem. Phys. Lett., 1998, 292, 547–553.
31. A. P. Isakina, A. I. Prokhvatilov, M. A. Strzhemechny and K. A. Yagotintsev, Low Temp. Phys., 2001, 27, 1037–1047.
32. J.-i. Minato and K. i. Miyazawa, Carbon, 2005, 43, 2837–2841.
33. L. Wang, B. Liu, D. Liu, M. Yao, Y. Hou, S. Yu, T. Cui, D. Li, G. Zou, A. Iwasiewicz and B. Sundqvist, Adv. Mater., 2006, 18, 1883–1888.
34. L. Wang, B. Liu, S. Yu, M. Yao, D. Liu, Y. Hou, T. Cui, G. Zou, B. Sundqvist, H. You, D. Zhang and D. Ma, Chem. Mater., 2006, 18, 4190–4194.
35. D. Wang and C. Song, J. Phys. Chem. B, 2005, 109, 12697–12700.
36. Y. Qu, W. Yu, S. Liang, S. Li, J. Zhao and G. Piao, J. Nanomater., 2011, 2011, 5.
37. P. A. Korevaar, C. Schaefer, T. F. A. de Greef and E. W. Meijer, J. Am. Chem. Soc., 2012, 134, 13482–13491.
38. C. Park, J. E. Park and H. C. Choi, Acc. Chem. Res., 2014, 47, 2353–2364.
39. D. Chen, H. Zhang, Y. Liu and J. Li, Energy Environ. Sci., 2013, 6, 1362–1387.
40. H. Y. Mao, S. Laurent, W. Chen, O. Akhavan, M. Imani, A. A. Ashkarran and M. Mahmoudi, Chem. Rev., 2013, 113, 3407–3424.
41. D. Chen, H. Feng and J. Li, Chem. Rev., 2012, 112, 6027–6053.
42. E. S. Shibu, A. Sonoda, Z. Tao, Q. Feng, A. Furube, S. Masuo, L. Wang, N. Tamai, M. Ishikawa and V. Biju, ACS Nano, 2012, 6, 1601–1608.

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Footnote

† Electronic supplementary information (ESI) available: XRD, FTIR and Raman spectra, TEM and SEM images, SAED pattern, and UV-vis absorption spectra of C₇₀ 1D microstructures. See DOI: 10.1039/c5ra03678j

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