The effect of light irradiation on the growth of C₆₀–C₇₀ nanofibers

Abstract

C₆₀–C₇₀ nanofibers (FNFs) were prepared using a modified liquid–liquid interfacial precipitation (LLIP) method in a pyridine solution of C₆₀–C₇₀ soot and isopropyl alcohol (IPA) under illumination with white light in the region of 300–700 nm. The composition of the C₆₀–C₇₀ FNFs was analysed by high-pressure liquid chromatography. It is found that the concentration of C₇₀ in the C₆₀–C₇₀ FNFs is decreased with an increase of irradiation time in the C₆₀–C₇₀–pyridine solution. Regarding the morphology aspect, polarizing optical microscope (POM) and scanning electron microscope (SEM) were used and changing the irradiation time may well be a method to control the length or diameter of those fibers. In an analysis of the mechanism, it was speculated that the charge transfer (CT) complexes between fullerenes C₆₀, C₇₀ and pyridine that could be excited by photons was an explanation for this phenomenon.

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1. Introduction

Fullerene C₆₀ nanofibers (FNFs) are one form of fullerene crystals with a 1-dimensional (1-D) structure. This special crystal gets much attention due to their characteristics as fullerene C₆₀ monomer, but also due to their size and dimension as 1-D material.¹

Consequently, much research focused on the application designs of this material based on the property or structure of those fibers. For example, due to their larger specific surface area, FNFs have been frequently reported as catalyst carriers.^2,3 With the nature of semiconductors and higher Young’s modulus compared to C₆₀ crystals,⁴ FNFs could be used as electronic devices.⁵ The discovery of their superconductivity established on an electron accepter characteristic by Takeya et al. through intercalation of potassium extends their application to maglev, radiation detectors, microwave generators and so on.⁶ At the same time, FNFs polymer hybrid with potential application in electromagnetic shielding materials, highly hydrophobic materials, and antistatic materials emerged.^7,8 Even a new nanoporous carbon 1-D material, which is obtained from FNFs through thermal conversion, shows excellent electrochemical capacitance and superior sensing properties for aromatic compounds compared to commercial activated carbons.⁹

Due to their marvellous potential application in many areas, several methods have been developed for the construction of FNFs. They are (1) solution evaporation,^10,11 (2) template techniques,^12,13 (3) surfactant-assisted methods,¹⁴ volatile diffusion method,¹⁵ and (4) LLIP method.^16,17 With the advantages of no requirement of physical templates and catalysts, the LLIP method is generally acknowledged as a way of great promise for self-assembly of this 1D structure.^18,19

Meanwhile, great attention has been paid to the growth of these 1-D materials and the further development of this method, not only in the morphologies, such as aspect ratio, tube containing rate, formation mechanism discussion,²⁰ but also their compositions. Various FNFs are composed of not only C₆₀, but also C₇₀, C₆₀[C(COOC₂H₅)₂](η₂-C₆₀)Pt(PPh₃)₂ or even C₆₀–C₇₀ mixtures and have been successfully fabricated using LLIP method.^21–24 Indeed, construction of C₆₀–C₇₀ FNFs has been widely studied, since a C₆₀–C₇₀ mixture is much cheaper than C₆₀ or C₇₀ and low-cost fullerene fibers are mostly studied for practical applications. The Young’s moduli of C₆₀–C₇₀ FNFs were tested by a transmission electron microscope equipped with an atomic force microscope functionality and were found to increase with increasing C₇₀ content in the mother solutions.^25,26 However, quantitative analysis of the C₆₀–C₇₀ ratio in solid C₆₀–C₇₀ FNFs is still an urgent problem and quite meaningful for better understanding of the Young’s moduli, optical properties or even the formation process of C₆₀–C₇₀ FNFs.

Herein, based on our former research on the significant relationships between the formation of FNFs and the charge transfer (CT) adducts,^27,28 we used a CT complex, formed between C₆₀ or C₇₀ and pyridine, which is susceptible to light irradiation, to fabricate C₆₀–C₇₀ nanofibers with different C₆₀–C₇₀ ratios. By changing the irradiation time, we fabricated FNFs with different but linearly C₆₀–C₇₀ ratios characterised by using high-pressure liquid chromatography analysis (HPLC). In addition, changing the irradiation time may well be a method to control the length or diameter of those fibers through polarizing optical microscope (POM) and scanning electron microscope (SEM) analysis. An ultraviolet-visible (UV-Vis) spectrometer was used to study the mechanism.

2. Experimental

2.1 Synthesis of C₆₀–C₇₀ FNFs

Firstly, a pyridine-saturated colloid with C₆₀–C₇₀ (C₇₀ 22 mol%; MER Corp, Tucson, AZ, USA) was prepared by filtering 10 ml of 1 mg ml⁻¹ C₆₀–C₇₀–pyridine colloid according to literature procedures.^18,19 Then, the colloid was exposed to white light in the region of 300–700 nm (20 W, irradiation distance: 8 cm) at 8 °C for 0, 30, 60 and 90 min. A typical procedure for the preparation of the FNFs is as follows: 1 ml of C₆₀–C₇₀–pyridine colloid was poured into a 20 ml transparent glass bottle and 9 ml of isopropyl alcohol (IPA) was added. The mixture of colloid solution was kept at 8 °C and metallic luster fibers appeared as floating batting type material after 48 h.

2.2 Characterization of C₆₀–C₇₀ FNFs

The FNFs obtained were characterized by using polarizing optical microscope (POM, Leica DM-2500P), scanning electron microscope (SEM, JEOL JSM-6700F), high-pressure liquid chromatographic instrument (JAI HPLC-9104) and UV-Vis spectrometer (SHIMADZU UV-2450). For the purpose of electron microscopic measurements, the specimens were placed on aluminium foil as substrate. In HPLC analysis, Buckyprep column chromatography was used and toluene was selected as the mobile phase. The detection wavelength was 310 nm and column pressure is 48–51 Pa.

3. Results and discussion

3.1 Morphology analysis

Fig. 1 shows the optical microscope images of FNFs prepared in C₆₀–C₇₀–pyridine colloid with (a) 0 min, (b) 30 min, (c) 60 min and (d) 90 min irradiation. For their 1-D morphology, it is acknowledged that the anisotropy of nuclei and the selective growth of crystal are the causes.^17,29

Fig. 1 Optical microscope images of C60–C70 FNFs prepared in C₆₀–C₇₀–pyridine colloid with different time of light irradiation (a: 0 min; b: 30 min; c: 60 min; d: 90 min).

These C₆₀–C₇₀ fibers are quite long and some of them could reach millimetres in scale. The fibers in Fig. 1(b)–(d) are similar, which all tend to be fine and tangled. In comparison, a few crude and straight fibers emerged in Fig. 1(a). That shows that fibers without light irradiation have a larger distribution in diameter. Thus, we conclude that light irradiation may have a good effect on the fine morphology of the C₆₀–C₇₀ nanofibers.

Fig. 2 shows the SEM images of FNFs prepared in a C₆₀–C₇₀–pyridine colloid following (a) 0 min, (b) 30 min, (c) 60 min and (d) 90 min light irradiation, respectively. The surfaces of the fibers are flat. Meanwhile the diameters of those fibers tend to have a wide distribution from hundreds of nanometers to several microns. A random statistical analysis of FNFs shows that the average diameters of fibers prepared in C₆₀–C₇₀–pyridine solution following 0, 30, 60 and 90 min irradiation are 690, 680, 450 and 360 nm, respectively. The longer the irradiation time, the thinner the fibers. And from the SEM imagines we can see that some of them are hollow structures. Also, with lengthening the irradiation time, the diameter distribution of the FNFs gets smaller.

Fig. 2 SEM images of C₆₀–C₇₀ FNFs prepared in a C₆₀–C₇₀–pyridine colloid with different time of light irradiation (a: 0 min; b: 30 min; c: 60 min; d: 90 min).

In fact, the size distribution of C₆₀ FNFs aroused great attention since the discovery of those 1-D materials. Solvent ratio, growth temperature,²⁰ solution volume,³⁰ operation process,¹⁷ and even the area size of the interface.³¹ But for C₆₀–C₇₀ nanofibers, the composition becomes another important factor and this, apparently, is related to the distribution of the morphologies.

3.2 Component analysis

Fig. 3 shows the HPLC traces of C₆₀–C₇₀ soot and FNFs prepared in C₆₀–C₇₀–pyridine solution with different irradiation times of the C₆₀–C₇₀–pyridine solution. The retention times (t_r) of C₆₀ and C₇₀ are 12.3 min and 20 min, respectively.^25,26,32 The component percentages of C₆₀ and C₇₀ are given by the ratio of the areas of the two peaks in Fig. 3.³³ The C₇₀ content in FNFs that have not been irradiated by light is as high as 28 mol%, however, the C₇₀ content in the fibers that have been irradiated for 90 min is only 11 mol%. Combining the C₇₀ content in FNFs with 30 and 60 min light irradiation, which are 20 mol% and 15 mol%, respectively, a linear relationship could be found between C₇₀ content in the FNFs and the length of white light irradiation, as follows:

Fig. 3 HPLC of C₆₀–C₇₀ soot (a) and FNFs prepared in C₆₀–C₇₀–pyridine colloid with different times of light irradiation (b: 0 min, c: 30 min, d: 60 min, e: 90 min).

By increasing the length of light irradiation, the C₇₀ content in the FNFs decreased from 28 mol% to 11 mol%. The values of C₇₀ content (y_{C70 content}) are shown in Fig. 4 and fitted by the following curve as a function of length of light irradiation (t).

y_{C70 content} = −0.19t + 26.90 (1)

Fig. 4 Relationship between irradiation time and C₇₀ content of FNFs.

3.3 Mechanism analysis

The consistency of subsequent steps suggests that the differentiation occurs during light irradiation. As a matter of fact, during white light irradiation, a slight colour change from brown-red to dark brown has been observed. To analyse this process and based on our former researches,^27,28 we conducted an analysis of the UV-Vis spectra of this C₆₀–C₇₀ system, as shown in Fig. 5.

Fig. 5 UV-Vis spectra of C₆₀–C₇₀–pyridine solution with different irradiation times (a: 0 min; b: 30 min; c: 60 min; d: 90 min).

In Fig. 5, four peaks emerged in the region of 350–700 nm in the UV-Vis spectra of the C₆₀–C₇₀–pyridine solution. The absorption at 380 nm and 600 nm are characteristic of C₆₀.³⁴ And the peak at 406 nm corresponds to the C₇₀ molecule.³⁵ Additionally, in the UV-Vis spectra of the C₆₀–C₇₀–pyridine solution, a strong absorption at the region of 400–500 nm appears. It should be noted that the absorption in this region refer to CT adducts.^36,37 Similar absorption has been reported for various thin films of C₆₀ and C₇₀ and attributed to aggregate of fullerene molecule. Fullerene C₆₀ or C₇₀ has the ability to accept multiple electrons.^38,39 On the other hand, pyridine has a nitrogen atom with a lone pair, which allows the molecule to be an electron donor.^37,40 So, it is possible that CT adducts exist in the C₆₀–C₇₀–pyridine system in these conditions. Charge transfer reactions through photo excitation between C₆₀, C₇₀ and pyridine happened³⁷ as follows:

Such interaction between C₆₀, C₇₀ and pyridine in the pressure of light may have an influence on the fullerene cluster, which serves as nucleation site or building materials for larger structures.⁴¹ Thus, when different times of light irradiation are applied, different balances between C₆₀, C₇₀ and pyridine are established. The differentiation of each balance in cluster stucture or concentration leads to the distinction o size and component ratio of FNFs. From Fig. 5, we can see that the content of CT increases with increasing time of light irradiation. We supposed that CT can only serve C₆₀ molecules during crystal growth, but not during crystal nucleation.²⁸ When the content of CT is increasing, the speed of crystal growth will slow down. According to the theory of H. Ji,¹⁴ C₆₀ molecules prefer the corners of a hexagonal cross section, over the edges of the hexagonal cross section, and least the center portion of the hexagonal cross. With the existence of CT, the C₆₀ molecules can choose the growth sites more independently, resulting in better selectivity of the C₆₀ molecules attached to the crystal seeds, and a more uniform distribution of the diameter distribution. Additionally, we suppose that C₆₀ finds it easier to form a CT complex with pyridine compared to C₇₀, and this explains why there is a decreasing C₇₀ content with longer irradiation times.

Conclusions

C₆₀–C₇₀ soot was used to fabricate C₆₀–C₇₀ FNFs by LLIP method. With increasing time of light irradiation, the diameter of the FNFs tends to be smaller, meanwhile, the content of C₇₀ in the FNFs drops. We ascribed those phenomena to the formation of C₆₀ and C₇₀ pyridine charge-transfer adducts, which is sensitive to light.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra14937h

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