Influence of Reaction Parameters on the Structural and Morphological Properties of Carbon Nanocoils Synthesized Using Al₃Y and Effect of Rh Addition

Abstract

In the contemporary world, carbon nanocoils (CNCs) act as mainspring in both fundamental and applied levels in nanotechnology advancements. They are attributed to their remarkable electrical, thermal, chemical, and mechanical properties inherited from the synthesis method; they are predestined for many potential applications. Catalytic chemical vapor deposition (CCVD) is the prevailing synthesis method for producing CNCs with controlled morphology and structural properties. In this study, Al₃YRh_x (x = 0, 0.2, 0.5, and 1.0) intermetallic catalyst series have been employed as a template for the catalytic conversion of an acetylene precursor into a solid material over the catalyst bed. The influence of reaction temperature, reaction duration, and Rh content in the catalyst in the structure and morphology of the CNCs prepared were analyzed through X-ray diffraction analysis, Raman spectroscopical investigation, and transmission electron microscopy. A detailed study on CNCS formed through spatial confinements by incorporating non-hexagonal rings in the graphitic skeleton leads to the coiling effect. Overall, nanotube synthesis has made significant progress by testifying the imperative reaction parameters in CCVD.

1. Introduction

Carbon nanocoils (CNC), also known as carbon nanosprings or carbon nanohelices, are unique and intriguing nanomaterial composed primarily of carbon atoms arranged in a helical or spiral structure. These structures are part of the broader field of carbon nanomaterials, which includes carbon nanotubes, graphene, and fullerenes. CNCs could be described as cylindrically rolled unified sheets of graphite whose diameter is a few nanometres that possess a high aspect ratio and thus can be considered a quasi-one-dimensional structure from an electric point of view.¹⁾ CNCs differ from CNTs in terms of their coiled or spring-like shape, which can vary in terms of the number of coils and the diameter of the coils, whereas CNTs are typically straight or slightly curved cylindrical structures. The sp² hybridized graphite backbone of the CNCs enacts quantum confinement and topological restraints in the circumferential direction of the tubule. CNCs are an archetypical instance of the potential nanomaterials with incomparable performance due to several physiochemical properties, including high tensile strength, good thermal conductivity,²⁾ ballistic transportation of electric current, excellent field emission properties, and nanoscale semiconducting properties. The distinct properties of 1D CNCs arise from interlayer interactions between multilayers within a single carbon nanotube or between two different nanotubes.^3,4) CNCs are radical in several fields of applications such as microwave and electromagnetic coatings,^5,6) electronic and ionic transport devices such as batteries,⁷⁾ super capacitors,^8,9) sensors,⁹⁾ actuators,¹⁰⁾ energy conversion and energy storage devices,^11,12) high aspect ratio nanotubes are required as field emission tips for applications such as field emission displays,¹³⁾ electron sources for lithography and microscopy, X-ray tubes,¹⁴⁾ vacuum microwave amplifiers, gas discharge tubes,¹⁵⁾ scanning probe tips,¹⁶⁾ due to their noteworthy electrical conductivity from the π delocalization of electrons in the graphitic rings.

The CNCs prepared for the abovementioned applications necessitate a decidedly reliable synthesis tactic capable of producing bulk quantities of highly pure materials. Some common methodologies employed for CNC production involve laser ablation,¹⁷⁾ arc discharge,¹⁸⁾ capillary infiltration,¹⁹⁾ ion-beam sputtering,²⁰⁾ electrochemical deposition,²¹⁾ and chemical vapor deposition (CVD).²²⁾ Among these methodologies, few methods lack controllability in length, diameters, and chiralities of produced carbon tubes. In contrast, others are limited due to the requirement of extreme reaction parameters for the evaporation of graphite rods. On the other hand, CVD is considered a more appropriate approach as it requires minimum pressure, low operating temperature and higher yield percentage. Currently, the CVD methods have advanced by plasma or catalyst mediation to produces unique physical and mechanical properties to the CNCs. In CCVD the CNC synthesis is stimulated by ceramic nanoparticle catalysts,²³⁾ noble metals,²⁴⁾ and semiconducting nanoparticles.²⁵⁾ CCVD is highly engrossed due to some important advantageous factors such as (i) hydrocarbons in any form that is solid, liquid or gas can be used, (ii) any diverse form of substrate can be employed, (iii) facile, economic and accessible procedure for mass production of CNCs, (iv) different forms of CNCs like aligned or entangled, straight or coiled nanotubes or a desired architecture of nanotubes on pre-defined cites of a patterned substrate can be adapted, and (v) control over the location and geometric parameters of CNC growth.

In CCVD synthesis, the yield and quality of the CNCs produced are highly dependent on the characteristics of the catalyst used. The catalyst acts as a carbon deposition initiator by decomposing hydrocarbon at temperatures lower than the spontaneous decomposition temperature of hydrocarbon via heat, and its new nucleation boosts the production of CNCs. Using Al₃Y alloy catalysts for CNC production could represent a novel approach in the field. Researchers often explore new catalysts to discover alternative methods or improve the efficiency of CNC synthesis. Al₃Y alloys can be tailored to exhibit specific catalytic properties under certain conditions. The ability to control and tune the catalytic activity is crucial for optimizing CNC growth parameters. Al₃Y catalysts possess desirable thermal stability properties that make them suitable for high-temperature CNT growth processes. As with other catalysts, forming an alloy between aluminum and yttrium may create active sites for carbon atom attachment and CNT nucleation.²⁶⁾

These three characteristics—(a) catalytic activity for the breakdown of volatile carbon molecules, (b) capacity for the synthesis of metastable carbides, and (c) carbon diffusion through and over the metallic particles—are significantly correlated with the particular ability of transition metals to promote CNC growth. Transition metals are renowned for their catalytic properties in various chemical processes, and this quality extends to their ability to facilitate CNC growth. Transition metal Y was specifically selected for its potential to enhance CNC synthesis. As a catalyst for chemical reactions, intermetallic compounds show unique properties, which can lead to enhanced control over CNT synthesis and the ability to produce CNTs with desired characteristics, such as single-walled or multi-walled tubes, specific chirality, or improved yield, thereby indicating that intermetallics may be effective also for CNC growth;²⁷⁾ so, Al₃Y was synthesized, which can be easily synthesized by arc-melting. The production of CNCs over Al₃Y is well analyzed. Rhodium is a stable and durable catalyst material, which means it can withstand harsh reaction conditions, high temperatures, and high pressures without undergoing significant degradation. This stability contributes to the long-term effectiveness of intermetallic catalysts. Therefore, doping Rh to the Al₃Y catalyst is being verified. In this work, Al₃YRh_x (x = 0, 0.2, 0.5, and 1.0) intermetallic catalyst is used to synthesize CNCs, and the influencing reaction parameters have been studied in detail.

2. Experimental Section

2.1 Synthesis of Al₃YRh_x (x = 0, 0.2, 0.5, and 1.0) catalyst

Al₃YRh_x (x = 0, 0.2, 0.5, and 1.0) alloy ingot was prepared by arc melting in a furnace under an argon (Ar) atmosphere, using Al (99.9% purity, Rare Metallic Co. Ltd.), Y (99.9% purity, Rare Metallic Co. Ltd.) and Rh (99.95% purity, Nilaco Co. Ltd.) as the raw materials. The as-cast alloy ingot was heat treated at 900°C for 48 h to ensure a homogeneous material. The heat-treated alloy ingot was crashed to obtain particles in the size range of 100 µm in mortar and sewed with mesh-sized 100 µm.

2.2 Catalytic chemical vapor deposition of CNC over Al₃YRh_x (x = 0, 0.2, 0.5, and 1.0) catalyst

Catalytic Chemical Vapor Deposition (CCVD) is an expedient method in which one or more volatile precursors decompose at the surface of the substrate, acting as a catalyst to form the desired product. Here, CCVD is an abetted procedure hired to synthesize CNCs at Al₃YRh_x (x = 0, 0.2, 0.5, and 1.0) catalyst series. Three different experiments were performed by loading the intermetallic catalyst in a quartz tube of length 800 mm and diameter of 10 mm placed at a vertical tubular furnace. In the experiment’s first phase, as soon as the furnace was made ready for operation, hydrogen gas was purged into the chamber at a flow rate of 100 ml/min for 10 min. At constant maintenance of the H₂ flow rate, the temperature of the furnace was raised from room temperature to 500°C, 600°C, 700°C, or 800°C for each operation. Once the furnace attains the desired temperature, the chamber is introduced with acetylene (C₂H₂) at a flow rate of 75 ml/min, and the flow of H₂ is reduced to 25 ml/min. This condition with a 3:1 ratio of purged gas is maintained and persistent for 30 min to permit the growth of CNCs. Then, as the reactor starts cooling, the flow of C₂H₂ is stopped, and the H₂ flow is increased. Secondly, the furnace was set ready for operation to study the impact of variation in reaction duration. H₂ gas was purged into the chamber at a flow rate of 100 ml/min for 10 min, and then the temperature was raised to 700°C. The reaction temperature was maintained constant, and the duration of purging C₂H₂ at a flow rate of 75 ml/min along with 25 ml/min of H₂ gas was varied from 10 min to 40 min in respective operations to obtain the CNCs. Thirdly, the furnace was maintained at 700°C, and the duration of C₂H₂/H₂ purging was fixed to be 30 min. The catalyst loaded into the quartz tube is varied as Al₃YRh₀, Al₃YRh_0.2, Al₃YRh_0.5, and Al₃YRh_1.0 for four different operations, and the obtained CNCs were analyzed.

After cooling, the CNC samples were collected from the tube and taken for further characterization. The yield of prepared CNCs at different temperatures was estimated using the formula below.

  

\begin{equation*} \textit{yield}_{\textit{as synthesised}}\ (\textit{wt}\%) = \left[\frac{m_{t} - m_{c}}{m_{c}}\right] \times 100 \end{equation*}

where mc and mt are the initial weight of the catalyst and the total weight of deposited carbon with the catalyst, respectively.

2.3 Instrumentations

The crystallographic and phase configurational insights of Al₃YRh_x (x = 0, 0.2, 0.5, and 1.0) alloy catalyst and prepared CNCs were obtained through Rigaku D/maxB, DMX-2200) X-ray diffractometer. X-ray photoelectron spectroscopy ESCA/Auger Laboratory (National Taiwan University, Taiwan) is applied to analyze the chemical composition of the catalyst particles quantitatively. The microstructure analysis of the catalyst is carried out in scanning electron microscopy (SEM, Hitachi S4700) and energy dispersive X-ray (EDX, HORIBA EMAX XACT) spectroscopy. The surface morphology and elemental composition of the as-prepared CNCs were studied by employing high-resolution (HR) transmission electron microscopy (H-7600, Hitachi-Japan) operating at 200 kV with EDAX. Raman spectrum of prepared CNCs was collected on an S3 Horiba HR 800UV confocal Raman spectrophotometer.

3. Results and Discussion

3.1 Catalyst characterization

X-ray diffraction analysis is a convenient procedure to provide a complete crystallographic characterization of microcrystallites. Figure 1(A) displays the x-ray diffractograms of the Al₃YRh_x (x = 0, 0.2, 0.5, and 1.0) catalyst series, which nicely coincides with the JCPDS card with PDF number 03-065-2133. The XRD pattern implies that prominent diffraction peaks at 2 theta values 17.06°, 18.48°, 23.65°, 26.89°, 28.89°, 32.18°, 33.66°, 34.50°, 37.61°, 38.32°, and 39.81° correspond to their respective lattice planes (101), (012), (104), (015), (110), (113), (021), (101), (012), (003), and (205). These lattice planes elucidate that Al₃Y crystallizes in the rhombohedral phase with lattice parameters a = 6.1950 Å, b = 6.1950 Å, and c = 21.1400 Å. The four-catalyst rhombohedral phase of the Al₃Y phase is visible, but the doping of Rh element at different atomic percentages could be verified through XPS analysis. Figure 1(B) exhibits the XPS survey spectrum showing the presence of elements, namely Al, Y, and Rh, with corresponding peaks. For the Al₃Y Catalyst, Al 2p, Al 2s, and Y 3d peaks were observed along with atmospheric carbon and oxygen peaks. For the other three catalysts with x = 0.2, 0.5, 1.0 atomic % of Rh, a distinct Rh 3d and Rh 3p is observed around 303.3 eV and 527.1 eV. The peak around 100 eV typically corresponds to the K-edge or L-edge of light elements like carbon (C). An intense peak around 285 eV corresponds to Cis spectra of carbon, and the one around 530 eV belongs to O1s spectra of oxygen.

Fig. 1

(A) X-ray diffractograms and (B) XPS survey spectrum of Al₃YRh_x (x = 0, 0.2, 0.5, and 1.0) catalyst series.

3.2 Reaction temperature dependency on CNC growth at Al₃Y intermetallic catalyst

The reaction temperature is a significant parameter influencing the growth of CNC on an Al₃Y catalyst bed. The impact of temperature on the physiochemical properties of the CNCs was verified by performing the CCVD procedure at different temperatures ranging from 500–800°C. The yield percentage of CNCs prepared at different temperatures is calculated using eq. (1) and plotted as depicted in Fig. 2(A). It is evident from the yield percentage versus temperature graph that the catalyst surface activation increases gradually on increasing the temperature from 500–700°C and suddenly drops down on further increase to 800°C. At mild temperatures, the decomposition and chemical reactivity rate of C₂H₂ molecules over the Al₃Y surface is catalytically deprived. At 500°C and 600°C, the concentration of carbon diffused over the surface and into the bulk of catalyst is very low, eventually lowering the CNC precipitation rate. On increasing temperature to 700°C, the carbon precursors are highly accelerated towards the catalyst surface. The temperature also assists the diffusion of carbon towards the bulk of catalyst to initiate spontaneous carbon production, thereby producing an increased amount of CNCs. Further, an increase in temperature to 800°C divests the activated catalyst surface by forming a carbon layer on the surface because the kinetically accelerated carbon moieties do not have enough time to diffuse onto the bulk of the catalyst as more carbon dissolves and precipitates.

Fig. 2

(A) Yield percentage versus temperature plot for CNCs prepared at different reaction temperatures; (B) X-ray diffraction patterns and (C) Raman spectral analysis of CNC -500°C, CNC -600°C, CNC -700°C and CNC -800°C samples.

Figure 2(B) represents the X-ray diffractograms of CNCs prepared at different temperatures increasing from 600°C to 800°C. The diffraction patterns of CNC -500°C, CNC -600°C and CNC -800°C have a prominent broad peak around 26.0° and a mild peak around 44.0° ascribed to the (002) and (100) planes of graphitic carbon as well indexed with JCPDS card number: 00-026-1080. Observing the XRD patterns of CNC -700°C, the graphitic carbon phase is dominated by the rhombohedral phase of the Al₃Y catalyst.

Further, Raman spectroscopy was employed to analyze the molecular interactions, structure-property relationship, and crystallinity of CNCs produced at different temperatures, whose spectra are demonstrated in Fig. 2(C). The d-band is observed around 1350 cm⁻¹, coordinated with A_1g breathing modes (Symmetric vibration) of sp² hybridized carbon structures in hexagonal aromatic rings. Based on the symmetry forbidden rule, this mode is inactive in a perfect graphitic structure. It becomes active only when disorder/defects arise in the resonance of the sp² carbon in the graphitic skeleton. On the other hand, g-band is found around 1580 cm⁻¹ attributed to the in-plane bond-stretching E_2g symmetry vibrations present for all sp² hybridized C–C bonds, either in the form of six-fold rings or chains. To further examine the quantity and quality of defects engendered in the CNC walls, the ratio between structural defects’ intensity and graphitization (I_D/I_G ratio) was calculated. The I_D/I_G ratio of CNC -500°C, CNC -600°C, CNC -700°C and CNC -800°C is calculated and given in Table 1. At lower temperatures, generating structural defects is more feasible due to vacancy-atom complex formation or intruding heptagon–pentagon structure in the surface and deep levels in the band structure, leading to mechanical scratching on the outer walls of CNCs. Thus, the I_D/I_G ratio of CNC -500°C and CNC -600°C is high and approaching 1. Whereas at higher temperatures (800°C) an exceedingly disordered state of amorphous carbon with minimum hexagonal ring structure is formed to give higher I_D/I_G ratio as the non-hexagonal rings are self-oriented to form small clusters (Fig. 3). A reduced I_D/I_G ratio specifies a decrease in the number of created defects, indicative of better quality CNCs. The I_D/I_G ratio of CNC -700°C is the lowest because the degree of the crystallinity increases as the temperature increases the diffusion and the reaction rate.

Table 1 Summary of Raman spectral analysis detailing the intensities of D and G bands along with I_D/I_G ratios of CNCs prepared at different reaction temperatures.

Fig. 3

Schematic representation of the growth mechanism of CNCs on Al₃Y intermetallic catalyst.

Transmission electron microscopy (TEM) is a practical technique to morphologically distinguish and explore the three-dimensional topographies of CNCs formed by the influence of different reaction parameters. The TEM micrographs and SAED patterns of CNC -500°C, CNC -600°C, CNC -700°C and CNC -800°C are given in Fig. 4(A)–(D). The carbon tubes produced were helically coiled and bear a resemblance to miniature telephone cord. The CNCs were erratically oriented nano coils that are homogeneously coiled to form an interconnected network of tubules with interminable stacking and entanglement. The growth temperature significantly contributes to the inheritance of helically coiled CNCs. At low growth temperature, the decomposition rate of carbon precursor into reactive carbon moiety is forcefully impulsive and slowly allows mobilization of carbon atoms towards CVD active interface, subsequently leading to the incorporation of non-hexagonal rings.

Fig. 4

TEM micrographs of CNCs prepared at different reaction temperatures (A) CNC -500°C, (B) CNC -600°C, (C) CNC -700°C and (D) CNC -800°C.

When a heptagonal ring intrudes the uniform hexagonal array of graphite sheets, it becomes a negatively curved surface, and intruding pentagonal rings form a positively curved surface. Both the surfaces were combined to form an energy minimum shape through plane buckling; thus, the coiling energy accounts for the surface tensions, the van der Waals attraction between graphitic compartments, and the curvature elastic energy. However, the buckling effect is strictly dependent on the number of heptagon-pentagon pairs in the skeleton, which is dependent on the temperature aiding the decomposition and precipitation of carbon on the surface of the catalyst. As the reaction temperature increases, the decomposition reaction is more favored, and the catalyst surface is crowded with more carbon atoms with sluggish movement, proportionally increasing the number of incorporated heptagon-pentagon pairs, which eventually converts the coiling effect into a curving effect to form a chain of fullerene-like spheres that are self-assembled and propagates the nanotube extension. The carbon nano coils were formed only up to 700°C beyond this optimum temperature; the coiling effect is deprived and forms fullerene chains, evident from the TEM observations. The SAED patterns (Fig. 5) of CNCs prepared at all different temperatures have three concentric circles concerning (002), (100), and (004) lattice planes of the graphitic frame, which coincided with the literature.

Fig. 5

SAED patterns of CNCs prepared at different reaction temperatures (A) CNC -500°C, (B) CNC -600°C, (C) CNC -700°C and (D) CNC -800°C.

3.3 Reaction duration dependency on CNC growth at Al₃Y intermetallic catalyst

The duration of flow of acetylene gas into the reaction chamber and hence of nanotube growth is varied between 10 min to 40 min at 700°C over Al₃Y intermetallic catalyst. Figure 6(A) demonstrates that the percentage of carbon yield is not steady for 10 to 20 min. In contrast, it considerably upsurges at 30 min and reduces gradually as the reaction time is further increased to 40 min. At 10 min of CNC growth, the carbon decomposition reaction is more spontaneous than the carbon deposition reaction. This scenario is reversed after 20 min, where the early carbon deposition provides nucleation sites for the growth of nanostructured carbon tubes. As the reaction duration is prolonged to 30 minutes, abundant nucleation sites ascend; thus, the impulse of nanotube growth is involuntary to give the maximum yield percentage. On further increase in reaction duration (40 min), the CNC growth rate diminishes, which is attributable to the catalyst deactivation process, i.e., the CNCs develop through a base growth mechanism which is limited when more amount of hydrocarbon diffuses onto the catalyst forming several thin layers of carbon thereby deactivating the catalyst.

Fig. 6

(A) Yield percentage versus time plot for CNCs prepared at different reaction durations; (B) X-ray diffraction patterns and (C) Raman spectral analysis of CNC - 10 min, CNC - 20 min, CNC - 30 min, and CNC - 40 min samples. TEM micrographs (D) CNC - 10 min, (E) CNC - 20 min, (F) CNC - 30 min and (G) CNC - 40 min.

The successful formation of CNCs over the Al₃Y intermetallic catalyst by the variation of reaction duration in the CCVD procedure is preliminarily confirmed through XRD studies. The X-ray diffractograms of CNC -10 min, CNC -20 min, CNC -30 min, and CNC -40 min are given in Fig. 6(B), which exhibits the characteristic diffraction peaks of graphitic planes with traces of catalyst residues. The XRD pattern shows that as the reaction duration increases, the trace of the catalyst gradually decreases, resulting in the dominance of the amorphous carbon phase. This proves that time critically governs the diffusion of hydrocarbons into the bulk of the catalyst to initiate nucleation, thereby propagating CNC growth. In the diffraction pattern of CNC-40 min, the graphitic phase is highly dominated by the Al₃Y phase because the hydrocarbon saturation in the catalyst hinders effective CNC production to give a lower yield.

Further, the structural quality of the CNCs prepared at different durations of the CCVD process is analyzed through Raman spectroscopy (Fig. 6(C)). As expected, the D-band is obtained at ∼1290 cm⁻¹, corresponding to the lattice defect or other carbonaceous impurities, and the tangential mode, G-band, is obtained at ∼1594 cm⁻¹, validating the existence of crystalline graphitic carbon. The degree of crystallinity in the time-dependent CNC samples is evaluated from the ratio integrated intensities of the D and G bands as calculated in the Table 2. The I_D/I_G ratio of CNCs prepared decreases by increasing the reaction time, elucidating that the crystallinity of graphitic carbon increases proportionally with the increase in reaction duration and attains the maximum at 30 min, beyond which the crystallinity drops again. The decrease in I_D/I_G ratio evinces that the deposition of amorphous carbon is more favored than the formation of carbon nanocoils as reaction time increases. One possible explanation is that as reaction time increases, it is harder for the precursor to diffuse to the catalyst–notube interface, resulting in carbon deposition on the nanotube wall as defects or amorphous carbon.

Table 2 Summary of Raman spectral analysis detailing the intensities of D and G bands along with I_D/I_G ratios of CNCs prepared at different reaction duration.

Furthermore, the TEM images in Fig. 6(D)–(G) reveal that 10 min of reaction duration is insufficient for the formation of CNC. In contrast, other different forms of carbon tubes were also observed. On the progressive increase in the reaction duration, the CNCs were very clearly formed, especially the CNCs prepared at 30 min, with well-defined helixes. However, the TEM micrographs of CNC -40 min show that large amounts of CNC were disrupted by acting as nucleation platform for further CNC production, resulting in highly accumulated carbon deposits with undefinable tubular structures.

3.4 Impact of catalyst composition Al₃YRh_x (x = 0, 0.2, 0.5, and 1.0) on CNC growth

Catalysts play a dynamic role in the CCVD synthesis of CNCs. Consequently, altering the composition of catalyst will tremendously impact the quality and product yield of the CNCs produced. Among the catalyst components, transition metals have a peculiar ability to endorse CNC growth based on three essential factors: (a) catalytic decomposition of volatile carbon source, (b) capacity to form metastable carbides, and (c) diffusion of carbon over and through the metallic particles. Here, the atomic percentage of Rhodium in the Al₃Y catalyst is varied as x = 0, 0.2, 0.5, and 1.0, which was used in CCVD for CNC production, and the yield percentage versus the catalyst is given in Fig. 7(A). The plot evinces that by increasing the atomic % of Rh, the yield % gradually decreases, which is attributed to the strength of catalyst–source interactions. Generally, a strong interaction between catalyst and source may reduce the sintering, improve dispersion, and minimize active metal species agglomeration. On the other hand, it may also hinder the reduction of the carbon source or its diffusion onto the bulk of the catalyst. As the yield declined with increasing Rh content, it could be proposed that a nonmagnetic transition metal component in catalyst composition may significantly impact the production of CNCs.

Fig. 7

(A) Yield percentage versus catalyst plot for CNCs prepared on different catalysts; (B) X-ray diffraction patterns and (C) Raman spectral analysis of CNC -Al₃Y, CNC -Al₃YRh_0.2, CNC -Al₃YRh_0.5 and CNC -Al₃YRh_1.0 samples. TEM micrographs (D) CNC - Al₃Y, (E) CNC - Al₃YRh_0.2 min, (F) CNC - Al₃YRh_0.5 and (G) CNC - Al₃YRh_1.0.

Further, the examination of characteristic features of CNC- Al₃Y, CNC- Al₃YRh_0.2, CNC- Al₃YRh_0.5, and CNC- Al₃YRh_1.0 is verified through XRD and corresponding diffractograms are given in Fig. 7(B). The XRD pattern shows distinct peaks of Al₃Y intermetallic catalyst in all CNC samples. The intensity of the Al₃Y phase increased as the quantity of CNC formed decreased with increasing Rh content in the catalyst, i.e., Rh metal significantly improves the decomposition and diffusion of hydrocarbon while hindering further nucleation to initiate tube propagation, resulting in catalyst bed saturation. The main drawback of Rh metal in the catalyst is that it has more interaction within the intermetallic melt where the carbon diffusion occurs. Once the carbon diffuses into the catalyst, higher interaction within the catalyst slows down the graphitic ring formation, usually called the nucleation process. Therefore, Rh in the catalyst may trigger surface diffusion of carbon but significantly hinders carbon accumulation to form a graphitic base for CNC production.

Further the inversely proportional relationship between the I_D/I_G ratio and the in-plane crystallite dimension is analyzed through Raman spectroscopical results depicted in Fig. 7(C) and Table 3. The lowest I_D/I_G ratio value is obtained for CNC prepared at Al₃Y intermetallic catalyst with Zero content of Rh, revealing the good crystalline nature of the graphitic skeleton with less structural defect. The I_D/I_G ratio increases and reaches the maximum of 1 for CNC- Al₃YRh_{1.0, proving} abundant defects and amorphous carbon in the sample.

Table 3 Summary of Raman spectral analysis detailing the intensities of D and G bands along with I_D/I_G ratios of CNCs prepared on different catalyst.

The hexagonal lattice in a pristine CNC forms a seamless, smooth, and regular structure along its entire length. When non-hexagonal rings are introduced into this lattice, they create localized irregularities. These irregularities lead to curvature variations, meaning that the curvature of the CNC is not constant along its length. Instead, it can vary, resulting in regions of the CNC that are more or less curved than others. The curvature variations caused by non-hexagonal rings can manifest as wrinkles or kinks in the CNC. Wrinkles are slight bends or undulations in the nanotube’s structure, while kinks are more pronounced bends, often forming sharp angles. These structural anomalies are a direct consequence of the disrupted lattice, and their presence can be readily observed in TEM images of CNCs, as shown in Fig. 7(D)–(G).

4. Conclusion

The present work accentuates more particularly the CCVD technique as it is scalable and standard in the synthesis of CNC and is known to be a hybrid of CVD and chemical conversion by Gas-Solid Heterogeneous Catalysis (GSHC). Novel features explicit to CCVD ascend from the catalytic conversion of a gaseous carbon precursor into a solid one-dimensional tube at the surface of catalyst particles by the influence of optimal reaction parameters. The study reveals that extremely low or high temperatures produce disordered states of amorphous carbon with non-hexagonal ring structures; thus, the degree of crystallinity decreases. The coiling effect and degree of crystallinity are also time-dependent, as the reaction duration governs formation nucleation sites and the process of catalyst deactivation. Further, the catalyst composition variation is testified by varying atomic percentages of Rh metal and observed that it significantly improves the decomposition and diffusion of hydrocarbon while hindering further nucleation to initiate tube propagation, resulting in catalyst bed saturation.

Acknowledgment

This work was supported by the Ministry of Science and Technology (Special Research Project-MOST-108-2221-E-027-063).

REFERENCES

• 1)   T.W.  Ebbesen: Annu. Rev. Mater. Sci. 24 (1994) 235–264. doi:10.1146/annurev.ms.24.080194.001315
• 2)   H.  Li and  L.  Zhou: J. Nanotechnol. 2016 (2016) 8073593. doi:10.1155/2016/8073593
• 3)  G. Dresselhaus, M.S. Dresselhaus and R. Saito: Physical Properties of Carbon Nanotubes, (World Scientific, Singapore, 1998).
• 4)  T.W. Ebbesen: Carbon Nanotubes: Preparation and Properties, (CRC Press, Boca Raton, 1996).
• 5)   M.J.  Green,  N.  Behabtu,  M.  Pasquali and  W.W.  Adams: Polymer 50 (2009) 4979–4997. doi:10.1016/j.polymer.2009.07.044
• 6)   E.T.  Thostenson,  Z.  Ren and  T.-W.  Chou: Compos. Sci. Technol. 61 (2001) 1899–1912. doi:10.1016/S0266-3538(01)00094-X
• 7)   A.C.  Grimsdale,  J.  Wu and  K.  Müllen: Chem. Commun. (2005) 2197–2204. doi:10.1039/b418172g
• 8)   P.  Simon and  Y.  Gogotsi: Nat. Mater. 7 (2008) 845–854. doi:10.1038/nmat2297
• 9)   K.  Mukai,  T.  Sugino,  K.  Kiyohara,  I.  Takeuchi,  N.  Terasawa,  D.N.  Futaba,  K.  Hata,  T.  Fukushima and  T.  Aida: Adv. Mater. 21 (2009) 1582–1585. doi:10.1002/adma.200802817
• 10)   H.  Chen,  A.  Roy,  J.-B.  Baek,  L.  Zhu,  J.  Qu and  L.  Dai: Mater. Sci. Eng. R 70 (2010) 63–91. doi:10.1016/j.mser.2010.06.003
• 11)   L.L.  Zhang and  X.  Zhao: Chem. Soc. Rev. 38 (2009) 2520–2531. doi:10.1039/b813846j
• 12)   J.  Liu,  G.  Cao,  Z.  Yang,  D.  Wang,  D.  Dubois,  X.  Zhou,  G.L.  Graff,  L.R.  Pederson and  J.-G.  Zhang: ChemSusChem 1 (2008) 676–697. doi:10.1002/cssc.200800087
• 13)   W.B.  Choi,  Y.H.  Lee,  N.S.  Lee,  J.H.  Kang,  S.H.  Park,  H.Y.  Kim,  D.S.  Chung,  S.M.  Lee,  S.Y.  Chung and  J.M.  Kim: Jpn. J. Appl. Phys. 39 (2000) 2560. doi:10.1143/JJAP.39.2560
• 14)   J.  Radhakrishnan,  P.S.  Pandian,  V.C.  Padaki,  H.  Bhusan,  K.U.B.  Rao,  J.  Xie,  J.K.  Abraham and  V.K.  Varadan: Appl. Surf. Sci. 255 (2009) 6325–6334. doi:10.1016/j.apsusc.2009.02.010
• 15)   R.  Rosen,  W.  Simendinger,  C.  Debbault,  H.  Shimoda,  L.  Fleming,  B.  Stoner and  O.  Zhou: Appl. Phys. Lett. 76 (2000) 1668–1670. doi:10.1063/1.126130
• 16)   H.  Dai,  J.H.  Hafner,  A.G.  Rinzler,  D.T.  Colbert and  R.E.  Smalley: Nature 384 (1996) 147–150. doi:10.1038/384147a0
• 17)   J.  Chrzanowska,  J.  Hoffman,  A.  Małolepszy,  M.  Mazurkiewicz,  T.A.  Kowalewski,  Z.  Szymanski and  L.  Stobinski: Phys. Status Solidi B 252 (2015) 1860–1867. doi:10.1002/pssb.201451614
• 18)   N.  Arora and  N.  Sharma: Diamond Related Materials 50 (2014) 135–150. doi:10.1016/j.diamond.2014.10.001
• 19)   M.  Li,  R.  Carter,  A.  Douglas,  L.  Oakes and  C.L.  Pint: ACS Nano 11 (2017) 4877–4884. doi:10.1021/acsnano.7b01437
• 20)   Y.  Hu,  I.  Jang and  S.  Sinnott: Compos. Sci. Technol. 63 (2003) 1663–1669. doi:10.1016/S0266-3538(03)00055-1
• 21)   L.  Hu,  Y.  Song,  J.  Ge,  J.  Zhu,  Z.  Hana and  S.  Jiao: J. Mater. Chem. A 5 (2017) 6219–6225. doi:10.1039/C7TA00258K
• 22)   M.L.  Terranova,  V.  Sessa and  M.  Rossi: Chem. Vap. Depos. 12 (2006) 315–325. doi:10.1002/cvde.200600030
• 23)   T.  Uchino,  K.N.  Bourdakos,  C.H.  de Groot,  P.  Ashburn,  M.E.  Kiziroglou,  G.D.  Dilliway and  D.C.  Smith: Appl. Phys. Lett. 86 (2005) 233110. doi:10.1063/1.1946191
• 24)   D.  Yuan,  L.  Ding,  H.  Chu,  Y.  Feng,  T.P.  McNicholas and  J.  Liu: Nano Lett. 8 (2008) 2576–2579. doi:10.1021/nl801007r
• 25)   D.  Takagi,  H.  Hibino,  S.  Suzuki,  Y.  Kobayashi and  Y.  Homma: Nano Lett. 7 (2007) 2272–2275. doi:10.1021/nl0708011
• 26)   K.  Rana,  G.  Kucukayan-Dogu and  E.  Bengu: Appl. Surf. Sci. 258 (2012) 7112–7117. doi:10.1016/j.apsusc.2012.04.008
• 27)   C.-M.  Chen,  Y.-M.  Dai,  J.G.  Huang and  J.-M.  Jehng: Carbon 44 (2006) 1808–1820. doi:10.1016/j.carbon.2005.12.043