The exact pathway of these mechanisms can depend on a variety of factors including the method of synthesis, the reaction conditions such as the temperature, pressure, presence of catalysts and the starting materials used. The formation mechanism of fullerene involves the following steps: Nucleation, Growth, Rearrangement, Fusion, Annealing and purification.
The proposed formation mechanism of fullerenes is based on a combination of experimental observations and theoretical models. In the early 1990s, researchers discovered that carbon clusters produced by methods such as laser ablation or arc discharge contained closed-cage structures consisting of pentagonal and hexagonal rings, which were later identified as fullerenes.
Contents
List of Figures
List of Tables
Introduction
Formation mechanism
Proposed Formation Mechanism
Special Pathways Based on Particular Intermediate Size Clusters Mechanism
The Pentagon Road Mechanism
The Fullerene Road Mechanism
Ring Stacking Mechanism
Experimental Evidence
Molecular Modelling Method
Justification for C2 Insertion-Based Growth
Criterion for Stability
Results of Molecular Modelling
Conclusion on the Experiment
Differences Between the Proposed Mechanisms
Outlook
Market Penetration
References
List of Figures
Figure 1: (a) Zero- dimensional Fullerene, (b) one- dimensional Carbon Nanotube, (c) two- dimensional Graphene and (d) three- dimensional Graphite. Adapted from Santhiran et al., (2021)
Figure 2: Larger La2@C2n endohedral fullerenes formed by incorporating guest molecule. Adapted from Mercado et al.,
Figure 3: An image of the growth mechanism of C12 to C60. Adapted from Xie et al.,
Figure 4: Formation of C24 (D6d) from precursor 1a and 1b. Adapted from Lin et al.,
Figure 5: Ring arrangement process of C60. Adapted from Curl et al.,
Figure 6: An image of (a) two pentagons and a central hexagon before C2 insertion, (b) C2 insertion leading to the addition of a new hexagon, (c) pyracylene/pyracene patch which can undergo SW transformation, (d) pyracylene/pyracene patch after SW transformation. Adapted from Khan et al., (2006)
Figure 7: Growth of C2 into C60 by C2 insertion, for n = 2–20 energies of C20 fullerene, graphene, corannulene, linear chain and ring growth. Adapted from Khan et al.,
Figure 8: An image of the growth of C20 corannulene, fullerene, graphene, linear chain and ring is modelled from C4 up to C20. Adapted from Khan et al.,
Figure 9: The graph of growth of C60 from C24 modelled by both pentagon and fullerene roads. Addition of the growth of C60 ring is also shown. Adapted from Khan et al., (2006)
List of Tables
Table 1: Description of similarities and differences between carbon nanomaterials. Adapted from Kroto et al., 1985; Dresselhaus et al 2010; Fang et al., 2020; Hass et al., 2008; Balandin et al., 2008 and Enoki et al.,
Table 2: Differences between the proposed mechanism Adapted from Kroto et al., 1985; Dresselhaus et al 2010; Fang et al., 2020; Hass et al.,
Table 3: Bottleneck and mitigation. Adapted from Kroto et al., 1985; Dresselhaus et al 2010; Fang et al., 2020; Hass et al., 2008; Balandin et al., 2008 and Enoki et al.,
Table 4 Market penetration factors. Adapted from Kroto et al., 1985; Dresselhaus et al 2010; Fang et al., 2020; Hass et al., 2008; Balandin et al., 2008 and Enoki et al.,
Introduction
Table 1: Description of similarities and differences between carbon nanomaterials. Adapted from Kroto et al., 1985; Dresselhaus et al 2010; Fang et al., 2020; Hass et al., 2008; Balandin et al., 2008 and Enoki et al., 2003.
Abbildung in dieser Leseprobe nicht enthalten
Figure 1: (a) Zero- dimensional Fullerene, (b) one- dimensional Carbon Nanotube, (c) two- dimensional Graphene and (d) three- dimensional Graphite. Adapted from Santhiran et al., (2021).
Formation mechanism
The exact pathway of these mechanism can depend on variety of factors including the method of synthesis, the reaction conditions such as the temperature, pressure, presence of catalysts and the starting materials used. The formation mechanism of fullerene involves the following steps: Nucleation, Growth, Rearrangement, Fusion, Annealing and purification (Curl et al., 1993).
Proposed Formation Mechanism
The proposed formation mechanism of fullerenes is based on a combination of experimental observations and theoretical models. In the early 1990s, researchers discovered that carbon clusters produced by methods such as laser ablation or arc discharge contained closed-cage structures consisting of pentagonal and hexagonal rings, which were later identified as fullerenes. The proposed formation mechanisms include (Goeres et al., 1991):
Special Pathways Based on Particular Intermediate Size Clusters Mechanism.
The formation mechanism of fullerenes can involve special pathways based on intermediate size clusters, which can lead to the formation of fullerenes with specific sizes and structures. One example of a special pathway involves the use of intermediate-sized carbon clusters called carbon onions. Carbon onions are made up of concentric shells of carbon atoms, like the layers of an onion. Researchers have found that by using carbon onions as starting materials, they can selectively form fullerenes with a specific number of carbon atoms, such as C60 or C70. This is because the onion structure provides a template for the formation of a closed-cage fullerene, with the number of concentric shells determining the size of the final structure (Raghavachari et al., 1992).
Another example of a special pathway involves the use of endohedral fullerenes, which are fullerenes that contain an atom or molecule inside the cage. Endohedral fullerenes can be formed by incorporating the desired guest molecule into the starting material, which can then undergo the normal pathways of fullerene formation. The presence of the guest molecule can influence the structure and stability of the final fullerene product, as well as its potential applications in areas such as drug delivery or energy storage (Strout et al., 1993).
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Figure 2: Larger La2@C2n endohedral fullerenes formed by incorporating guest molecule. Adapted from Mercado et al., 2009.
The Pentagon Road Mechanism
The pentagon road is an appealing mechanism which begins with the formation of a small carbon cluster, which may contain one or more pentagonal rings. As the cluster grows, new carbon atoms are added to the edges of the existing rings, eventually forming a curved structure resembling a tube or sphere. However, in order to close the structure and form a fullerene, additional pentagonal rings must be added to the growing cluster (Curl et al., 1994).
The pentagon road mechanism proposes that these pentagonal rings are added in a stepwise fashion, with each new ring being formed by the rearrangement and fusion of neighbouring hexagonal rings. Specifically, a pair of adjacent hexagonal rings are transformed into a pentagon and a heptagon through the addition of two carbon atoms, followed by the rearrangement of the neighbouring hexagons to accommodate the new rings. This process can be repeated multiple times, creating a zigzagging pathway of pentagonal rings that ultimately leads to the closure of the fullerene structure (Smalley et al., 1992).
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Figure 3: An image of the growth mechanism of C12 to C60. Adapted from Xie et al., 2000.
The Fullerene Road Mechanism
The fullerene road involves the growth and fusion of carbon clusters into closed-cage structures. This mechanism is thought to be responsible for the formation of the most common fullerene species, including C60 and C70 (Heath, 1991). The fullerene road begins with the nucleation of small carbon clusters, which can be initiated by a variety of methods such as laser ablation, arc discharge, or chemical vapor deposition. These clusters then grow through the addition of new carbon atoms, which may be incorporated one-by-one or in small groups. As the cluster grows, it undergoes a series of structural rearrangements, including the formation of pentagonal and heptagonal rings, which can introduce strain into the structure. The fullerene road proposes that the cluster continues to grow and rearrange until it reaches a critical size, at which point it undergoes a process of closure to form a closed-cage fullerene. The exact mechanism of closure is not fully understood, but it may involve the fusion of neighbouring carbon clusters, the collapse of a carbon nanotube, or other processes (Smalley et al., 1991).
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Figure 4: Formation of C24 (D6d) from precursor 1a and 1b. Adapted from Lin et al., 2003.
Ring Stacking Mechanism
This mechanism involves the stacking of smaller carbon rings to form larger ones, ultimately leading to the formation of fullerene cages. These rings can form from larger carbon clusters or from carbon precursors such as hydrocarbons. The small carbon rings then stack together to form larger rings. This can occur through several mechanisms, such as the addition of new rings to the edge of an existing ring or the fusion of two or more smaller rings together. As the rings continue to stack and fuse, they eventually form a closed cage-like structure like a fullerene. The resulting structure can have various sizes and shapes, depending on the number and arrangement of the carbon rings (Curl et al., 1993).
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Figure 5: Ring arrangement process of C60. Adapted from Curl et al., 1993.
Experimental Evidence
To comprehend how fullerenes are formed during their synthesis, researchers attempted to simulate the most efficient growth pathway using a semi-empirical quantum mechanics program. The model focused on C2 addition, which results in the formation of C60, and examined three primary pathways: cyclic ring, pentagon, and fullerene road (Hutter et al., 1994).
Molecular Modelling Method
For this computational model, a semi-empirical quantum mechanical approach was chosen due to its ability to accurately model nano clusters in less time. Three semi-empirical methods were employed: Modified Neglect of Differential Overlap (MNDO), Austin Model 1 (AMI), and Parameterized Model 3 (PM3). Since the electronic structures of fullerenes differ from other hydrocarbon structures, empirical methods were necessary. The MNDO method produced the most precise results in this experiment (Stewart et al., 1989).
Justification for C2 Insertion-Based Growth.
The growth of C60 can be simulated through the step-by-step addition of carbon molecules, such as C1, C2, C3, or larger. However, observations from regenerative sooting discharge's emission spectra suggest that C2 is a significant component of carbonaceous plasma. This presence of C2 may play a crucial role in the formation of fullerenes (Ahmad et al., 2000).
Abbildung in dieser Leseprobe nicht enthalten
Figure 6: An image of (a) two pentagons and a central hexagon before C2 insertion, (b) C2 insertion leading to the addition of a new hexagon, (c) pyracylene/pyracene patch which can undergo SW transformation, (d) pyracylene/pyracene patch after SW transformation. Adapted from Khan et al., (2006)
Criterion for Stability
Abbildung in dieser Leseprobe nicht enthalten
Figure 7: Growth of C2 into C60 by C2 insertion, for n = 2–20 energies of C20 fullerene, graphene, corannulene, linear chain and ring growth. Adapted from Khan et al., 2006.
In the case of fullerenes that have identical atoms, the ΔHf per carbon atom is almost equal to the negative value of the binding energy per atom, which is the energy required to extract an atom from the molecule. The stability of a molecule increases with the strength of atom binding, so the ΔHf per atom can be used as a metric for assessing molecular stability. The molecule is more stable if the value of ΔHf per atom is lower (Khan et al., 2006).
Results of Molecular Modelling
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Figure 8: An image of the growth of C20 corannulene, fullerene, graphene, linear chain and ring is modelled from C4 up to C20. Adapted from Khan et al., 2006.
The provided figure displays the growth sequence of four potential C20 isomers. When individual carbon dimers merge to create bigger carbon clusters, they tend to arrange themselves in linear chains, which is quite noticeable. As these linear chains continue to grow, additional ring structures become increasingly energetically favourable.
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Figure 9: The graph of growth of C60 from C24 modelled by both pentagon and fullerene roads. Addition of the growth of C60 ring is also shown. Adapted from Khan et al., (2006)
The provided figure demonstrates that, from an energetic perspective, growth through the fullerene route is preferable than growth through the pentagon route. Geometry-optimized structures of these closed-cage systems are also depicted. It's evident that the pentagon route is less energetically favourable than the fullerene route, which may be due to the high strain present in the edge atoms of open-cage fragments (the pentagon route).
Conclusion on the Experiment
The initial growth begins with linear carbon chains and, when n = 10, the chains start to form rings. These rings continue to grow by ingesting C2 molecules. Around n = 38, the rings appear to transition into fullerenes, and the growth of C60 proceeds through the fullerene route. According to symmetry considerations, the transition from ring to fullerene may occur much earlier than n = 38 in a collision-dominated environment during the synthesis of fullerenes (Khan et al., 2006).
Differences Between the Proposed Mechanisms
Table 2: Differences between the proposed mechanism Adapted from Kroto et al., 1985; Dresselhaus et al 2010; Fang et al., 2020; Hass et al., 2008.
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Outlook
Table 3: Bottleneck and mitigation. Adapted from Kroto et al., 1985; Dresselhaus et al 2010; Fang et al., 2020; Hass et al., 2008; Balandin et al., 2008 and Enoki et al., 2003.
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Market Penetration
Table 4 Market penetration factors. Adapted from Kroto et al., 1985; Dresselhaus et al 2010; Fang et al., 2020; Hass et al., 2008; Balandin et al., 2008 and Enoki et al., 2003.
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