The Synthesis of C₇₀ Fullerene Nanowhiskers Using the Evaporating Drop Method

Abstract

Semiconductor nanowhiskers, particularly nanostructured whiskers based on zero-dimensional (0D) C₇₀ fullerene, are being actively discussed due to the great potential of their application in modern electronics. For the first time, we proposed and implemented a method for the synthesis of nanostructured C₇₀ fullerene whiskers based on the self-organization of C₇₀ molecules during the thermal evaporation of C₇₀ droplets on the substrate surface. We found that the onset of the synthesis of C₇₀ nanowhiskers upon the evaporation of drops of a C₇₀ solution in toluene on the substrate surface depends on the substrate temperature. We have provided experimental evidence that an increase in both the C₇₀ concentration in the initial drop and the substrate temperature leads to an increase in the geometric dimensions of C₇₀ nanowhiskers. The obtained results provide useful vision on the role of solute concentration and substrate temperature in the synthesis of one-dimensional materials.

1. Introduction

In nanoscience, nanowhiskers are considered to be filamentous crystals with a transverse size of up to 100 nm and a length that is an order of magnitude or greater than the transverse size. Semiconductor nanowhiskers are widely used today to create miniature elements of devices in microelectronics [1,2], optoelectronics [3,4], nanoengineering [5,6], solar energy [7,8,9], biomedicine [10], nanoelectromechanics [11,12] and gas sensing [13,14]. To date, there are various methods [15,16,17] for obtaining nanowhiskers of a wide range of semiconductor materials, such as growth using molecular beam epitaxy, vapor deposition, laser ablation, growth catalysts, magnetron deposition, chemical epitaxy in a high vacuum and others.

Carbon nanomaterials (fullerene, carbon nanotube and graphene) are becoming key components of nanotechnologies for the development of complex functional nanostructures. Light fullerenes (C₆₀/C₇₀) are a hollow sphere/ellipsoid carbon molecule less than 1 nm in diameter, with sp² carbon atoms located on a curved surface at the vertices of a truncated icosahedron. They have unique physical properties, particularly optical and electrical ones. A remarkable property of fullerene molecules is their ability to self-assemble over time in pure solvents to form clusters of various shapes and sizes [18,19], and the nature of the solvent plays an important role in this process [20]. Intermolecular self-assembly, reactivity and electron affinity properties give fullerenes incomparable advantages in applications such as electrocatalysts and supercapacitors [21]. In addition, they have a suitably high photosensitivity, electron mobility, antioxidant activity and radical scavenging [22,23,24]. The latter leads to a range of applications, including photodetectors [25], sensors [26], solar cells [27], LEDs [28], biomedicines [21] and drug delivery [29].

Since the discovery of C₆₀ fullerene nanowhiskers (C₆₀NWs) by the Miyazawa group in 2001 [12,30], applications in various fields have been found. A poor solvent is added to a saturated well-dissolved solution of C₆₀ and a liquid–liquid interface is formed in the middle. As a result, a supersaturated solution is formed, C₆₀ embryo crystals are nucleated at the liquid–liquid interface and long C₆₀NWs are synthesized. Although this method was initially “static” (without external influence), later “dynamic” (ultrasound, manual mixing, etc., effects) and other modified methods were developed [31,32]. Similarly, C₇₀ fullerene nanowhisker (C₇₀NW) structures were synthesized on the basis of C₇₀ fullerene in the same ways [33]. It is known that NWs formed on the basis of nanosized fullerenes are based on bottom-up technology. Traditional and newly developed strategies for the synthesis of one-dimensional micro/nanostructures based on fullerenes and their practical applications are considered in [34,35]. In this case, the regulation and control of the size and structure of one-dimensional structures is of great importance. Specialists have now found that the morphology and size of fullerene micro- and nanostructures can be easily controlled by adjusting experimental parameters such as good/poor solvent systems, solvent ratios, temperature, area size of liquid–liquid interface and various treatments [34,35,36,37]. The elliptical shape of C₇₀ molecules renders them anisotropically polarizable, and the spherical shape of C₆₀ renders it isotropic [38]. This diversifies the physico-chemical properties of the micro/nanostructures synthesized on their basis [39]. In particular, when NWs synthesized in a solution are transferred to the surface of a solid substrate, changes in their morphology occur. It should also be taken into account that the evaporation of droplets of fullerene solutions on the surface of a solid substrate leads to self-organization processes [40,41]. In this regard, there is a need to study the processes occurring in the volume of the evaporation of droplets of fullerene solutions.

In this paper, we consider the synthesis of nanostructured C₇₀ fullerene whiskers on the surface of a substrate by evaporating a microvolume drop of a C₇₀ solution. Experimental methods for controlling the geometric dimensions of the synthesized nanowhiskers are discussed.

2. Results

In our experiments, the shape of the initial drop of a fullerene solution with a volume of V ≈ 40–50 μL on a wetted flat substrate is approximately represented by a spherical cap (see Figure 1, left). It can be noted that the drops of the fullerene solution always retain a constant area of the base of the drop throughout the entire duration of the thermal evaporation. But the contact angle (φ) of the drop gradually decreases until it disappears. The fullerene drop is protected from convective air flows until complete evaporation; the drop thermal evaporation direction is perpendicular on the surface of the spherical cap. Due to the Marangoni effect along the “droplet–air” interface and the Rayleigh–Benard effect along the evaporating droplet volume (Figure 1, right), strong capillary flows appear and start the assembly of fullerene particles as well as the synthesis of different nanostructures based on them.

The SEM image of the structures formed during the evaporation of droplets of a C₇₀ solution in toluene on the substrate surface at room temperature (~24 ± 1 °C) is shown in Figure 2. Due to the constant base area of the microdroplet, a trace of C₇₀ nanostructures remains along the base of the drop, similar to a coffee ring, during the entire thermal evaporation of the solvent. An important role is played by the temperature gradient that occurs when the surface and near-surface layers of the droplet cool sharply as a result of intense toluene evaporation. It can be seen that after the complete evaporation of toluene from a microdroplet of the C₇₀ solution, large quasi-spherical C₇₀ aggregates are formed on the surface of the optical glass substrate. At the same time, the average geometric dimensions in the diameter of C₇₀ aggregates are ~600 nm. The resulting C₇₀ aggregates are porous and consist of discrete intermediate nanoaggregates with sizes up to ~40–45 nm in diameter. The most probable, from our point of view, is the following fundamental mechanism of the self-organization of C₇₀ fullerene molecules in the volume of the evaporating solution droplets and the formation of quasi-spherical nanoaggregates. A drop of a C₇₀ solution always tends to minimize its total surface energy. The latter can be achieved, in particular, as a result of the self-organization of C₇₀ molecules. Let us assume that two intermediate fullerene formations with diameters d₁ and d₂ (d₂ >> d₁) are localized in the volume of an evaporating drop of a C₇₀ solution. Then, each of these nanosized particles will tend to establish a thermodynamic equilibrium with the surrounding solution. In accordance with the well-known Gibbs–Thomson relation [42], the solubility of a larger spherical C₇₀ cluster with an average diameter d₂ in a solution will be noticeably lower than the similar solubility parameter for a smaller C₇₀ fullerene formation with the diameter d₁. Then, individual C₇₀ molecules, appearing as a result of the dissolution of the smaller formation d₁ into drops, will be deposited on the surface of larger particles d₂ to maintain the equilibrium in the system. At the same time, smaller fullerene formations will be forced to dissolve further to compensate for the C₇₀ molecules that have left the drop. As a result, inside the evaporating drop of the C₇₀ fullerene solution, a continuous diffusion transfer of the dissolved substance (C₇₀ molecules) occurs from smaller C₇₀ formations to larger quasi-spherical nanoaggregates.

We studied the process of the evaporation of a C₇₀ solution droplet on the substrate surface at different substrate temperatures in order to synthesize one-dimensional C₇₀ structures. When the K-8 optical glass substrate was heated to 28 °C, nanostructured filaments (nanowhiskers) of C₇₀ fullerene of an optimal shape were synthesized on the substrate surface (see Figure 3). The concentration of fullerene C₇₀ in the initial drop of the solution was ~1.1 × 10⁻³ mol·L⁻¹. In this case, the temperature gradient in the process of the intensive evaporation of the solvent from a microdroplet at a temperature of 28 °C made it possible to overcome some of the energy difficulties in the formation of C₇₀NWs. We could observe that X- and V-shaped C₇₀NWs were mainly synthesized in the volume of an evaporating drop of the C₇₀ molecular solution on a substrate (see Figure 3). The average geometric dimensions of C₇₀NWs are ~105 nm in width and ~750 nm in length. At the same time, we could observe that the maximum length and width of the resulting C₇₀NWs reached the values ~1.7 µm and ~200 nm, respectively.

A SEM image of C₇₀NWs, synthesized on a surface of a horizontally located glass substrate heated to T = 36 °C, is presented in Figure 4. In experiments with a fixed concentration of C₇₀ (~1.1 × 10⁻³ mol·L⁻¹) in a drop of the working solution, the effect of increasing the temperature of the substrate on the ongoing processes of the evaporation drop was studied. It was established that an increase in the substrate temperature not only led to a more accelerated nucleation and growth of C₇₀NWs but also to a noticeable increase in the final geometric dimensions of the synthesized C₇₀NWs. Therein, the distribution of C₇₀NWs on the substrate surface was getting denser. At the same time, the average length and width of the resulting C₇₀NWs reached the values ~1.8 µm and ~175 nm, respectively. The presented results proved that the size of nanowhiskers can be controlled by changing the substrate temperature at a fixed concentration of C₇₀ in the working drop.

Under the same conditions, we studied the effect of the initial concentration on the size of the synthesized nanoparticles. Figure 5 presents a SEM image of nanostructured whiskers of C₇₀ fullerene synthesized on the smooth surface of a substrate heated to T ≈ 36 °C. An increase in the fullerene concentration (up to ~1.5 × 10⁻³ mol·L⁻¹) in the initial droplet led to a noticeable increase in the final C₇₀NW size. It was easy to observe that the longest C₇₀NWs had a size of ~28 μm in length, ~2 μm in width, as well as the shortest length and width of ~6 μm and ~200–250 nm, respectively (Figure 5). So, it was shown that the geometric dimensions of the C₇₀NWs can be controlled by changing the initial concentration of the fullerene solution.

The experimental results reflecting the change in the geometric dimensions of the synthesized C₇₀NWs at a fixed concentration of C₇₀ fullerene with different substrate temperatures are presented in

Table 1. Evolution of changes in the average sizes of synthesized C₇₀NWs depending on the substrate temperature.

C/(mol·L−1) a	T/(°C) b	Average Length/μm	Average Width/nm
~1.1 × 10−3	28	0.75	105
	32	1.35	152
	36	1.8	175

^a The C₇₀ concentration in a solution. ^b The substrate temperature (T) remains constant until the droplet is completely evaporated.

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The thermal and temporal stability of C₇₀NWs is also very important for the investigation of their structure and further applications. In our case, a TEM observation was performed on synthesized C₇₀NWs. When the synthesized nanowhiskers were stored at room temperature for 2 months, microscopic observations revealed practically no changes in their morphology and structure (see Figure 6, left). The TEM image of stored C₇₀NWs (with a diameter of ~72 nm) after heating at ~120 °C for 15 min is shown in Figure 6 (right). This result indicates that the C₇₀ nanowhiskers got thinner at ~120 °C. In addition, while the overall integrity of the nanowhiskers was preserved, traces similar to nanosized “craters” appeared on their surface.

The results of this work can be used to predict and control the geometric dimensions of nanostructured whiskers based on different macromolecules. In addition, this method is useful in the synthesis of one-dimensional, two-dimensional and three-dimensional structures from a zero-dimensional material consisting of only one component, through various combined and/or separate processes. Note that synthesized C₇₀NWs are a separate class of materials that exhibit the combined properties of both C₇₀ fullerene and nanostructures. The next steps are to develop structural and functional materials using synthesized fullerene-based one-dimensional nanounits. Although today there are many approaches to create new functional materials based on C₇₀ fullerene, combining and reorganizing independent knowledge and facts with practical applications will lead to great progress in material science. The electrical properties, superconductivity and energy storage properties of fullerene C₇₀NWs are still unexplored and thus represent an excellent area for future research. These amazing properties allow them to be effectively used in many future concrete applications such as photocatalysis, solar cells, energy storage, photodynamic therapy, drug and gene delivery, electrocatalysis and sensors.

3. Discussion

We presented an experimental method for the synthesis of cost-effective and compatible C₇₀NWs in the volume of an evaporating droplet on a substrate. Our electron microscopic measurements confirm the formation of one-dimensional C₇₀NWs during the evaporation of a drop on the surface of a substrate heated from 28 °C. The difference in substrate temperature with respect to the temperature of the deposited droplet and the environment creates additional thermodynamic forces acting on C₇₀ particles, which ultimately ensure the synthesis of nanowhiskers based on them. It was found that changing both the concentration of fullerene in the initial drop and the substrate temperature provides an opportunity to tune the geometric dimensions of C₇₀NWs to the desired value.

At a fixed concentration of C₇₀ (~1.1 × 10⁻³ mol·L⁻¹) in an initial drop, changing the substrate temperature from T₁ = 28 °C to T₂ = 36 °C led to a noticeable increase in the final geometric dimensions of the synthesized C₇₀NWs. In this case, the ratio of the average length (~1.35 µm) to width (~152 nm) of the synthesized C₇₀NWs was about 9:1. At a fixed substrate temperature (T = 36 °C) with a relatively high concentration of fullerene (~1.5 × 10⁻³ mol·L⁻¹), C₇₀NWs with a maximum length and width of ~28 μm and ~2 μm, respectively, were synthesized. It was shown that the method used is effective for the synthesis of micro- and nanosized whiskers, which can be used for various purposes of “bottom-up” technology.

4. Materials and Methods

In our experiments, we used high-purity (~99.8%) powders of fullerene C₇₀ (Sigma-Aldrich, Saint Louis, MO, USA) as well as the organic solvent toluene (C₆H₅CH₃, Sigma-Aldrich). The mixture of “toluene + C₇₀ powders”, located in a hermetically sealed glass flask, was dissolved with continuous mechanical stirring at a frequency of ~1.5 Hz for 1.5 h using a programmable laboratory magnetic stirrer of the MS-11H brand, WIGO (Pruszkow, Poland). Thereafter, the C₇₀ solution was sonicated for 15 min using an ultrasonic bath brand, DC-120H. Further, dosed drops of the C₇₀ molecular solution were taken using a VITLAB dosing pipette (VITLAB GmbH, Grossostheim, Germany).

A standard K-8 optical glass with a surface roughness of ≤7 nm was used as a substrate. Before each experiment, the surface of the used glass substrate was plasma, cleaned at the nanolevel using a Plasma Cleaner device (Harrick Plasma, «PDC-002», Ithaca, NY, USA).

For experiments on heat treatment, a thermostatically controlled table was used with the possibility of heating up to 140 °C on a Peltier effect. The temperature was programmed with an accuracy of 1 °C. Using a thermocouple built into the MS8217 digital multimeter, the temperature on the glass substrate was controlled with an error of ±2%.

Drops of the C₇₀ solution were placed on a heated substrate using a VITLAB piston micropipette (VITLAB GmbH, Grossostheim, Germany) under laboratory conditions. In this case, the temperature of the solution and the environment was ~23 ± 1 °C, and the drops were protected from convective air currents until complete evaporation.

We used a high-resolution scanning electron microscope (hereinafter SEM) brand, JSM-IT200 (Joel, Tokyo, Japan), and a transmission electron microscope (hereinafter TEM), LEO-912 AB (Carl ZEISS, Oberkochen, Germany), to establish the morphological features and determine the exact geometrical sizes of one-dimensional C₇₀NWs.

5. Conclusions

For the first time, we proposed the evaporating drop method for producing nanostructured C₇₀NWs. In this case, the self-organization of the C₇₀ molecules occurs upon the thermal evaporation of the toluene from the C₇₀ droplets located on the surface of a flat glass substrate. The optimal substrate temperature for the start of the synthesis of C₇₀ fullerene nanowhiskers in the volume of droplet evaporation was experimentally established. It was shown that the geometric dimensions of the synthesized C₇₀NWs can be controlled both by changing the C₇₀ concentration in the initial droplet and by changing the temperature of the substrate used. A selective synthesis of fullerene nanowhiskers was carried out. The results of this work can be used to predict and control the geometric dimensions of nanostructured whiskers of various kinds, which will have great potential in applications such as nano- and microelectronics, solar cells, nonlinear optics, sensors and electromechanics.