Selective growth of fullerene octahedra and flower-like particles by a liquid–liquid interfacial precipitation method for super-hydrophobic applications

Abstract

We demonstrate a simple, anisole/IPA interfacial precipitation of flower and octahedron-like fullerene micro-crystals using a liquid–liquid interfacial precipitation method. Both the flower and octahedron-like fullerene micro-crystals showed super-hydrophobic nature with a high water contact angle of 158.8°. While the other C₆₀ nanostructures including nanowhiskers, nanosheets and nanorods do not show significant water contact angle. To the best of our knowledge, this is higher than literature reported values for pure C₆₀-based nano/micro crystals. This clearly indicates that these crystals are promising candidates for super-hydrophobic applications.

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Fullerene (C₆₀)-based materials have acquired enduring attention in various fields due to their diverse morphology, unique size and shape dependent properties.¹ The recent developments in the selective and controlled synthesis of one-dimensional C₆₀-based materials like nanowhiskers, nanotubes, nanorods make it possible to study the size and morphology based physico-chemical properties.² There are number of methods such as liquid–liquid interfacial precipitation (LLIP), drop-drying method, template method, photo assisted method, high pressure and high temperature method that have been reported.^3,4 Overall, C₆₀ crystals with a few tens of nanometres to a few hundred microns size with unusual crystal shapes have been observed when different solvents were used.^5,6 Among the various preparation methods, LLIP is considered as simple and has more control on synthesis of C₆₀ nanostructures with desired size and shape.^3,7 The self-slow-aggregation of C₆₀ molecules at the liquid–liquid interface depends on the nature of the solvents and their volume ratios, which controls the morphology and size.^5,8 LLIP method was also successfully used to prepare other C₆₀-based nanostructured materials like two-dimensional nanosheets with different size and shape, and three-dimensional ‘bucky cubes’.^5,9 The C₆₀, C₇₀ and substituted fullerene nanostructures are well explored for their application in solar cells, sensors, separation, purification and fuel cells.^10–12 The recent reports on the super-hydrophobic nature of C₆₀, C₇₀ and their derivatives by Zheng et al. and Nakanishi et al. opens new area of research and development of fullerene based materials.^13,14 Due to the inherent hydrophobic nature of C₆₀ and the recent thrust in super-hydrophobic materials recognised C₆₀-based nanomaterials as promising candidate for super-hydrophobic applications. As the surface roughness and energy of the materials also play major role on the super-hydrophobic nature, it is important that the materials size and morphology should be easily tuneable.¹⁵ The recent studies on the hierarchal nanostructures of C₇₀ and functionalized fullerene derivatives confirmed that design and preparation of appropriate hierarchal nanostructures is essential to obtain good super-hydrophobicity.^13,14 It is possible to tune the super-hydrophobic nature of the C₆₀ nanostructures by altering their morphology and size. Thus, studies on super-hydrophobicity of C₆₀/C₇₀ nano/micro crystals with different shape and size prepared at different solvent interface is highly warranted to explore the potential of these materials.¹³ Here, we report the selective preparation of octahedron and flower-like C₆₀ crystals at anisole and IPA interface. The prepared C₆₀ crystals showed a high super-hydrophobic nature with high water contact angle of 158.8°, which is significantly higher than pure fullerene powder, and other fullerene nanostructures such as fullerene nanowhiskers, nanosheets and nanorods.

Fullerene micro-crystals were synthesized by mixing of fullerene saturated anisole solution with different volume of isopropyl alcohol (IPA) using LLIP method. In a typical precipitation, C₆₀ saturated anisole solution was prepared by adding excess amount (150 mg) of C₆₀ powder (99.5% pure, MTR Ltd) in 25 ml of anisole (AR grade, Merck). The mixture was ultrasonicated for 30 min followed by filtration to remove undissolved excess of C₆₀ powder. Then, 1 ml of C₆₀ saturated solution of anisole was taken in two 10 ml glass bottle and kept in ice water bath maintained at 5 °C. Similarly, IPA was taken in glass beaker and stored in ice water bath at 5 °C and calculated volume of IPA was added to C₆₀ saturated solution of anisole gently and slowly through the glass bottle wall. For all the preparation, the 1 ml of C₆₀ saturated solution was kept as constant and the volume of IPA is varied from 1 to 5 ml. The resulted solution was ultrasonicated for 30 s and kept in an incubator at 5 °C. The formation of fullerene micro particles was observed after 1 h at the interface.

The formation of fullerene microcrystals was initially confirmed using optical microscopy (Leica Microsystems with magnification ranges from 50× to 1000×). The phase formation and crystalline nature of the C₆₀ microcrystals were examined by powder X-ray diffraction (XRD) technique (X'Pert PRO, PANAlytical) using Cu Kα radiation (λ = 1.5418 Å) in the 2θ ranges from 10 to 80° at 0.02° step with a count time of 0.2 s at each step. The morphology and the particle size of the C₆₀ microcrystals were analyzed by field-emission scanning electron microscope (FE-SEM, Carl Zeiss Supra 55VP) and high resolution transmission electron microscope (HR-TEM, Tecnai G2 TF20) working at an accelerating voltage of 5–30 kV and 200 kV, respectively. Water contact angel measurement using goniometer (OCA 35 Data Physics).

The optical microscopic image of C₆₀ octahedron and flower-like micro-particles produced at anisole–IPA interface is shown in Fig. 1. It could be clearly seen the formation of flower-like micro-particles when the solvent ratio of anisole to IPA was 1 : 1. Whereas, when the ratio was increased to 1 : 5, formation of octahedron-like C₆₀ crystals are observed. It is interesting to note that there is no significant change in the size of the C₆₀ crystals in both the samples and they are uniform in size.

Fig. 1 Optical microscopic images of (a) flower-like and (b) octahedron-like C₆₀ crystals formed at liquid–liquid interface.

The morphology of the prepared C₆₀ crystals has been further analyzed by FE-SEM analysis. Fig. 2a and b shows the FE-SEM images of the prepared flower-like micro-particles at different magnifications. It could be clearly seen the formation of flower-like crystals by self-assembly of infant octahedron with an average size of 1–2 μm. It is presumed that the low volume of IPA at the 1 : 1 solvent ratio restricts the growth of individual octahedron. As expected, when IPA volume was increased from 1 : 1 to 1 : 5, the formation of individual octahedrons at the interface is observed as shown in Fig. 2c and d. The formation of C₆₀ octahedron may be due to (i) growth of individual C₆₀ octahedrons from the flower-like morphology or (ii) conversion of entire flower-like crystals into one C₆₀ octahedron. The yellow lines shown in Fig. 2d confirms the formation of C₆₀ octahedron with an average size of 1–2 μm. The histogram of particle size distribution obtained from FE-SEM images clearly shows that the size of the flower-like assembly and the individual octahedron are also most similar (Fig. S1†). This hints that the flower-like crystals may be converted into one single octahedron by overgrowth in high IPA volume at the interface.

Fig. 2 FE-SEM images of (a and b) flower-like and (c and d) octahedron-like C₆₀ crystals at different magnifications.

The crystalline nature has been further confirmed by X-ray diffraction pattern (XRD) analysis. The XRD pattern of room-temperature dried pristine C₆₀ powder, flower and octahedron-like C₆₀ crystals can be readily indexed for C₆₀ fcc crystal system (Fig. 3). All the three samples show three major peaks at 2θ values of 10.7, 17.6, and 20.6 corresponding to (111), (220), and (311) plane reflections, respectively. The observed small intensity peaks and the calculated lattice constant value a = 1.418 nm are in good correspondence with fcc crystal system.¹⁶ However, the flower and octahedron-like C₆₀ crystals showed an addition hump at 2θ = 10.3° corresponding to the presence of small amount of hexagonal crystal system (shown as *). But the absence of main hexagonal peak corresponding to (022) plane hints that the observed additional peak may be due to the solvent induced structural imperfection or transformation of fcc crystals.^5,16 Further, the existence of sp² and sp³ carbon along with slight amount of surface bound oxygen was observed in the XPS spectrum (Fig. S2†).

Fig. 3 XRD pattern of (a) pristine fullerene powder, (b) flower-like and (c) octahedron-like fullerene crystals.

The HR-TEM images of the flower and octahedron-like fullerene crystals are shown in Fig. 4. The fetal-like arrangement of infant octahedron crystals in the flower-like morphology are clearly visible (Fig. 4a and b). The observed octahedron crystals are not uniform in size but it is well grown. It is presumed that at anisole–IPA interface the formation of octahedron like C₆₀ crystals are provoked and the low volume of IPA at 1 : 1 ratio limits the growth of individual octahedrons. It is well proved that the solvent molecule plays vital role on the formation of fullerene nanostructure at the liquid–liquid interface.¹⁷ In some case, the solvent molecules will occupies the space inside the C₆₀ nanostructures and produces pores over the nanostructures when it get evaporated during the drying process.¹⁸ However, when the IPA volume was increased to 1 : 5, the formation of individual octahedrons with equivalent in size of flower-like crystals are produced without any significant pores. Certainly, the observed C₆₀ octahedron and flower-like crystals are similar in size (1–2 μm), which supports our earlier speculation that the flower-like crystals gets converted into octahedron or infant octahedrons in the flower-like crystal grows as individual octahedrons. However, there is no comprehensible evidence for the formation of octahedron-like fullerene crystals in either route. In order to check the time dependent growth of fullerene crystals, the samples prepared at 1 : 1 and 1 : 5 volume ratios were analysed after 24 h and 5 days, in both the cases the flower and octahedron-like morphology and size does not change significantly with time (Fig. S3†). Thus, it is worthy to note here that the anisole to IPA volume ratio alone has vital role on the morphology of the fullerene crystals and the possible conversion of flower to octahedron-like morphology is trivial. The HR-TEM image of C₆₀ octahedrons show clear lattice fringes with an lattice d spacing of 0.428 nm, which is in good correspondence with the (311) plane of C₆₀ fcc crystal structure (Fig. 4c).¹⁸ The observed selected area electron diffraction (SAED) pattern of C₆₀ octahedrons confirms the formation of fcc crystalline C₆₀ crystals at the anisole:IPA interface (Fig. 4c inset). Fig. 4d shows the top view of octahedron-like fullerene crystals and it is clear from the image that octahedron is ∼1–2 μm in size.

Fig. 4 (a) TEM images of flower-like fullerene crystal, (b) STEM image of flower-like fullerene crystal (c) lattice fringes and SAED pattern (inset) of flower-like fullerene crystal and (d) octahedron-like fullerene crystal.

Fullerene and fullerene derivatives are known for hydrophobic nature.^13,14 The recent demands in super-hydrophobic coatings in vehicle windshields, anti-corrosive/icing, surgical tools, medical equipment, and textile applications rekindle the possibilities of using fullerene based nanostructures. Attempts have been made to enhance the hydrophobicity of fullerene-based nano/micro crystals and significant success was realized.^10,13,14 Zheng et al. reported hierarchical fullerene architectures using C₆₀/C₇₀ microstructures with an water contact angle of 154.12°,¹³ Wei et al. reported C₆₀ hollow microspheres with an water contact angle of 156.3°,¹⁰ and Nakanishi et al. reported globular objects of fullerene derivatives with an water contact angle value 152.0°.¹⁴ Here, the thin film of flower-like fullerene crystals and octahedrons showed better super-hydrophobic nature with the water contact angle of 158.8°, while the thin film of pristine fullerene (C₆₀) showed much lower value of 87.1 (Fig. 5). Similarly, the other fullerene nanostructures such as fullerene nanowhiskers, nanosheets and nanorods prepared using the LLIP method does not show good water contact angle.^5,16,18 To the best of our knowledge, this value is higher than the literature reported values for pure C₆₀ nano/micro crystals. This clearly shows that the morphology of the fullerene nano/micro crystals has also plays vital role on the surface properties. Certainly, the high water contact angle obtained for the C₆₀ crystals prepared in anisole–IPA interface hints both the morphology and the nature of the solvents involved in the preparation has significant influence on hydrophobic nature of the fullerene crystals. It is worthy to mention here that further investigation on the formation of different fullerene morphologies at different solvent interfaces, and their surface properties is highly warranted to explore the functional properties of fullerene crystals.

Fig. 5 Photograph of water contact angle on the surface of (a) pristine fullerene and (b) flower-like fullerene crystals.

In summary, we have succeeded in the selective preparation of C₆₀ octahedron and flower-like crystals using liquid–liquid interfacial precipitation method. The electron diffraction and XRD pattern reveals the fcc crystalline nature of the C₆₀ micro-crystals. The electron microscopic analysis confirms the selective formation of octahedron and flower-like crystals when the ratio of anisole to IPA was altered. The super-hydrophobic nature of the octahedron and flower-like crystals reveals the high water contact angle of 158.8° which is higher than the literature reported values for pure C₆₀ nano/micro crystals. And, the other fullerene nanostructure such as nanowhiskers, nanosheets and nanorods prepared in the LLIP method does not shown any super-hydrophobicity. It is concluded that, in addition to the surface roughness and energy the morphology and nature of the solvent used in the LLIP method also plays vital role on the super-hydrophobicity of fullerene nanostructures. The observed high contact angle for octahedron and flower-like crystals promises their potential applications in super-hydrophobic surfaces, oil spill capture, anti-icing and corrosion resistance.

Acknowledgements

We thank CSIR, India for financial support through MULTIFUN project (CSC 0101). We thank Dr S. Sathiyanarayanan and Mr T. Bharathidasan, Corrosion and Material Protection Division, CSIR-CECRI, Karaikudi, for the water contact angle measurements.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra15846c

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