1 Introduction

Carbon nanotube (CNT)-based devices have been produced consecutively, which implies a bright future for CNT electronics and arouses the desire for direct growth of identical semiconducting CNTs together with the semiconductor industry standard. For various approaches to control the chirality of CNTs, the solution is to deepen the understanding of the growth mechanism in chemical vapor deposition. A physical picture focused on carbon atoms transfer in CVD growth of CNTs has been established for two decades. However, the charge transfer in the growth process can be systematically studied [1].

In the last 15 years, chirality control is considered the biggest challenge in this area of research which is related to the initiation of graphitization of CNTs [2]. When the catalytic substrate bonding reaction is weak, hydrocarbon decomposition occurs at the top of the metal surface having a sharp contact angle with the substrate. The hydrocarbon (C2H2) spreads down through the metal and the CNTs precipitate from the bottom, pushing the metal particles from the substrate to the tip of the CNTs, which then precipitate from the bottom, while the upper part of the CNTs continues the C2H2 decomposition. This model is known as Tip Growth Model [3]. Subsequent hydrocarbon deposition takes place in the lower peripheral surface of the metal and dissolved carbon diffuses upward, while the CNTs grow with the catalyst particle in their base. Such a mechanism is named the base-growing model [3].

Recent advances in the selective synthesis of SWCNT by epitaxial growth model [4], in this same mechanism same other SWCNTs synthesis by bimetallic catalyst (7–8), and also same other techniques used to control mechanisms magnetic field [5] and electric field to control the chirality of SWCNTs. This model faced main challenges of catalyst size, chirality specific growth [4], and stability of the nanosize SWCNTs growth processes. The key to the various approaches to controlling the chiral index of CNTs is to expand our understanding of the mechanisms of catalytic growth in chemical vapor deposition. We recently reported on CNT's vertical growth by serving magnetic and electric fields [5] but hear difficulty for chirality controlling also we reported the development of SWCNTs by novel electric field controlling mechanism and also by changing the orientation of the substrate concerning the direction of the electric field or by applying an electric current during the growth process, the desired carbon nanotubes are formed.

The generation of charge during the growth formation of CNTs has already been studied and reported preciously. But the variation in applied DC bias and its effects on the resistive behavior during the growth of CNTs and their electrochemical properties have not been studied previously. Herein, we investigated the effect of DC voltage on Fe/Al-catalyzed CNT growth, their properties (including semi-conductive, or metallic), and their electrochemical properties also. The additional electrons arise from the generation and transfer of charge during the growth of CNTs, indicating that electrochemical processes take place during the surface reaction step. The field measurement equipment was then subsequently designed to confirm that CNT's CVD growth could be considered a basic battery system. Moreover, it was found that changes in Fermi levels in Fe–Al-based catalysts have a significant effect on the chirality of CNTs when different external electric fields were applied. These discoveries not only provide new insights into the growth of CNTs but also open up new possibilities to control the growth of CNTs using electrochemical methods.

2 Methodology

2.1 Catalyst preparation

Catalyst preparation was carried out by the simple sol–gel method, as reported elsewhere [6]. 0.5 (M) of Fe(NO3)3·9H2O was added into 100 ml deionized water and stirred for 30 min to get a clear homogeneous solution followed by the addition of 0.8 (M) of Al(NO3)3·9H2O. The whole solution mixture was initially stirred at room temperature for 24 h and 12 h more at 90 °C to get dried material. Finally, the dried sample was annealed at 450 °C to obtain a Fe2O3/Al2O3 catalyst.

2.2 CNTs growth mechanism

The furnace (KejiaFurnace KJ-1600G) temperature was increased to 950 °C to grow CNTs, with a ramp rate of 10 °C/min with an Ar atmosphere containing 5% H2 with a flow rate of 100 sccm, followed by 100 sccm of C2H2 gas was supplied to initiate the growth. Finally, after growing for 60 min, the sample was cooled down to room temperature gradually. During the CVD process, the growth of CNT may follow the following mechanism as reported elsewhere [4, 6, 7].

2.2.1 Phase 1

Initially, near 500–600 °C, the Fe2O3 phase gets changed into unstable α-Fe2O3 and then gets reduced to α-Fe3O4 by absorbing some moisture content. After phase change, the unstable α-Fe3O4 again transformed into α-Fe2O3 by reaching the growth temperature at 700 °C with increasing crystallinity.

$${\mathrm{Fe}}_{2}{\mathrm{O}}_{3}\stackrel{500-600\mathrm{^\circ{\rm C} }}{\to }\mathrm{\alpha }-{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}\stackrel{{\mathrm{H}}_{2}\mathrm{O}}{\to }\mathrm{\alpha }-{\mathrm{Fe}}_{3}{\mathrm{O}}_{4}\stackrel{\sim 700\mathrm{^\circ{\rm C} }}{\to }\mathrm{\alpha }-{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$$
(1)

2.2.2 Phase 2

Above 800 °C, both α-Fe2O3 and acetylene start to decompose because of the reduction of α-Fe2O3 into its unstable Fe3O4 phase by hydrogen atoms (from C2H2) (Eq. 2). The reduced unstable Fe3O4 instantly transformed into metallic α-Fe according to Eq. (3). Similarly, Al2O3 has also been converted into metallic α-Al because of the temperature increment above 900 °C, since Al2O3 gets reduced at higher temperature of 900–950 °C as shown in Eq. (4).

$${\mathrm{C}}_{2}{\mathrm{H}}_{2}\stackrel{800\mathrm{^\circ{\rm C} }}{\to }2\mathrm{C}+2\mathrm{H}$$
(2)
$$3\mathrm{\alpha }-{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}\stackrel{2\mathrm{H}, 900\mathrm{^\circ{\rm C} }}{\to }2\mathrm{\alpha }-{\mathrm{Fe}}_{3}{\mathrm{O}}_{4}+{\mathrm{H}}_{2}\mathrm{O}\to \mathrm{\alpha }-\mathrm{Fe}$$
(3)
$${\mathrm{Al}}_{2}{\mathrm{O}}_{3}\stackrel{950\mathrm{^\circ{\rm C} }}{\to }\mathrm{\alpha }-{\mathrm{Al}}_{2}{\mathrm{O}}_{3}\stackrel{\mathrm{H}, 950\mathrm{^\circ{\rm C} }}{\to }\mathrm{\alpha }-\mathrm{Al}$$
(4)

2.2.3 Phase 3

In the final phase, decomposed C atoms meltdown and nucleation were initiated after reaching super-saturation level on the surface of α-Fe and α-Al to form Fe–C and Al–C orientation followed by the transformation into their carbides (such as Fe3C and Al4C3), respectively. Consequently, CNTs growth was also confirmed via precipitation according to Eqs. (5–6).

$$\mathrm{\alpha }-\mathrm{Fe}+\mathrm{C}\stackrel{800-900\mathrm{^\circ{\rm C} }}{\to }\mathrm{Fe}-\mathrm{C}\stackrel{2\mathrm{Fe}, 800-900\mathrm{^\circ{\rm C} }}{\to }{\mathrm{Fe}}_{3}\mathrm{C}$$
(5)
$$\mathrm{\alpha }-\mathrm{Al}+\mathrm{C}\stackrel{> 900\mathrm{^\circ{\rm C} }}{\to }\mathrm{Al}-\mathrm{C}\stackrel{3\mathrm{Al}/2\mathrm{C}, > 900\mathrm{^\circ{\rm C} }}{\to }{\mathrm{Al}}_{4}{\mathrm{C}}_{3}$$
(6)

Thus, by the combined catalytic effect of Fe2O3/Al2O3 catalysts, the controlled growth and formation of CNTs can be carried out because Fe2O3 helps to catalyze the reaction up to 800 °C and Al2O3 helps to catalyze the reaction above 900 °C in a suitable way.


a. In situ power supply installation and measurement

During the growth reaction, in situ measurement of DC electrical bias influence on CNT (double-walled and multi-walled) synthesis has been carried out using a continuous DC power supply (VRR 3002). The growth of double-walled and multi-walled CNTs was examined within the bias range from 0.5 to 3.5 V having 0.5 V of increment. By minimizing environmental noise and connecting Keithley 2401 sourcemeter, charge generation on the surface of CNTs has been observed in terms of electrical resistance at different biases which was quite similar to the traditional emf measurement (Fig. 1).

Fig. 1
figure 1

Schematic of in situ DC supply-assisted chemical vapor deposition technique for the synthesis of CNTs

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3 Results and discussion

Figure 2 represents the variation of resistance on CNTs growth and temperature effect on the process. It was observed that initially, the resistance decreased with increasing temperature from room temperature to 950 °C (operating temperature) which was according to the Ohmic rule. Since the substrate surface and oxide catalysts were both semi-conductive, resistance came down with a steeper profile up to an initial 190 min. After reaching the operating temperature (950 °C), 100 sccm Ar/H2 flow was begun and it was found that the resistance became stable for 20 min. But as the 100 sccm C2H2 was added as a feed gas into the reaction chamber at 60 min, the resistance again started to come down slowly and after an hour, a 0.0002 Ω of resistance was dropped. This gradual resistance drop phenomenally attributes to the growth of semi-conductive CNTs.

Fig. 2
figure 2

In situ measurement of DC bias on CNTs growth showing the change of electrical resistance with feed gas

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The diffraction pattern of few-walled CNT is displayed in Fig. 3. The major characteristic peaks of the few-walled CNT samples were observed at 2θ (deg.) of 26.25, 53.8, and 57.2° corresponding to the lattice planes (h k l) of (0 0 2), (0 0 4), and (1 0 2), respectively, with a hexagonal arrangement [8]. Peak position at 31.87° corresponding to the Al4C3 at (2 0 1) plane [9]. The diffraction peak at 43.1° indicated the presence of α-Fe2O3 corresponding to the (0 1 1) plane [10]. The peak at 36° corresponds to the (1 1 1) plane was of γ″-FeN [11]. Diffraction peak at 37.6° was attributed to Fe3C at (2 2 1) [9]. 44.75° was attributable to the α-Fe and Fe3C overlapping at (1 1 0) and (2 2 0) planes and 63° was attributed to the appearance of α-Fe at the (2 0 0) [9]. A peak at 66° was corresponding to α-Al2O3 at the (4 4 0) plane [12]. In Fig. 4, it has been observed that the major characteristic diffraction peak of as-synthesized CNTs has been shifted slightly by lowering the applied potential which signifies the enhancement of growth of semi-conductive few-walled CNTs at lesser resistance [13].

Fig. 3
figure 3

XRD pattern of CNT, including metal carbides and nitride

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Fig. 4
figure 4

Diffraction patterns of CNTs synthesized at different applied electrical biases

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The FTIR spectral illustration is displayed in Fig. 5. It showed the presence of weak vibration of α-Fe2O3 and α-Al2O3 phases between 500 and 700 cm−1 [14] and also it revealed that the samples are not functionalized, i.e., pure CNTs. Some peaks appearance has been observed near 3700 cm−1 attributable to the presence of surface hydroxyl groups (–OH) because of the absorption of atmospheric moisture [15].

Fig. 5
figure 5

FTIR spectral profiles of CNTs synthesized at different applied electrical biases

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Figure 6 displays the recorded Raman profiles of FWCNT samples synthesized at different applied electrical biases from 0 to 3.5 V indicating the presence of characteristic peak intensities of RBM (radial breathing mode) (Fig. 6b and d) and G-band. However, RBM is a distinct quantum characteristic of SWCNT because of their radial vibration, and it cannot be overlooked for FWCNTs completely because of their diameters below 2 nm [16]. Moreover, the RBM is strongly correlated with the tube diameter distribution which can be extracted using the following relation [17]:

Fig. 6
figure 6

Raman spectra of FWCNTs a synthesized at different applied potentials, b RBM regions at different applied potentials, c synthesized in alumina crucible and copper plate, respectively, and d RBM regions for alumina and copper plate-synthesized FWCNTs

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$${{f}}_{\mathrm{RBM}}=\frac{{{c}}_{1}}{{{d}}_{\mathrm{CNT}}}+{{c}}_{2}$$
(7)

where f is the frequency of RBM (cm−1), dCNT is the diameter of CNT (nm), and c1 and c2 are constants having the value of 235 (cm−1 nm) and 9 (cm−1), respectively. From Fig. 6a and c, it can be observed that the D-band and G-band resonated near 1350 cm−1 and 1570 cm−1 in all the spectrums with the D/G ratio ranging between 0.4 and 0.5 representing the graphitization of the samples with the good agreement [18]. With increasing applied potential, the average diameter of the FWCNTs also increased gradually from 0.87 to 0.9 nm because of the redshift of the RBM region which was estimated using Eq. (7). Most importantly, a strong characteristic peak of the BWF band (Breit–Wigner–Fano) was found at a higher electrical bias (3.5 V) near 1535 cm−1 frequency which was attributed to the formation of metallic CNT [19]. Further, the inclusion of G’-bathe nd nandr 2700 cm−1 intensified the formation of fewer walls as well as the metallicity of the FWCNTs [16].

Figure 7a–f displays the SEM photographs of as-synthesized CNTs at different applied potentials; hence, densely entangled nanotubes were observed in all the images which were likely to be strongly connected bundles of ropes as shown. Aggregation and compaction of these nanotubes may be enhanced by the strong Van der Waal’s interaction and π–π stacking [20, 21].

Fig. 7
figure 7

SEM photographs of entangled CNTs bundles synthesized at a 0.5 V, b 1.0 V, c 1.5 V, d 2.0 V, e 2.5 V, and f 3.0 V

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The electron transfer efficiency, i.e., redox efficiency by exchanging cation–anion between electrolyte and electrode material is mainly analyzed by cyclic voltammetry. In the particular potential window, these redox phenomena get influenced by the electron cloud density and based on that energy storage capability of a material can be explored. CV measurements of CNTs synthesized at 0 V and 3.5 V against Ag/AgCl standard electrode and two different electrolyte solutions are (KOH and H2SO4) as shown in Fig. 8a–d within the potential window of − 0.4 to 6.0 V (for KOH electrolyte) and 0 V to 1.0 V (for H2SO4 electrolyte) at the different scan rates from 0.01 to 1.0 V s−1. In both the cases of CNT-0 V and 3.5 V, it can be observed that CV profiles for KOH electrolytes (Fig. 8a and c) are quite smooth compared to H2SO4-based electrolytes (Fig. 8b and c). This implies a very less oxidation–reduction reaction happening between electrolyte and CNTs which can be reflected from the specific capacitance value (KOH) in Table 1 also. The reason might be the generation of lesser numbers of electrons and less interaction on the surface of CNTs compare to H2SO4. Furthermore, in both the CV profiles of H2SO4-based electrolytes, small oxidation–reduction peaks were observed at 0 to 0.2 V and 0.8 to 1.0 V. Impedance profiles of the CNT-electrodes in Fig. 8e confirmed the pseudocapacitive nature of the material which supported the literature report also [22] having strong possibilities of development of the CNTs and CNT-based materials for the application of electrochemical energy storage system. Charge–discharge performances in Fig. 9a–d also supported the previous literature with the good agreement of possible development of pseudocapacitor [22].

Fig. 8
figure 8

Cyclic voltammograms of electrode material using a CNT synthesized at 0 V and 1(M) KOH electrolyte, b CNT synthesized at 0 V and 1(M) H2SO4 electrolyte, c CNT synthesized at 3.5 V and 1(M) KOH electrolyte, d CNT synthesized at 3.5 V and 1(M) H2SO4 electrolyte, and e electrochemical impedance (Nyquist) plot for CNT-based capacitive electrode at different electrolytes

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Table 1 The specific capacitance of CNT-based electrodes at different electrolyte and scan rates
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Fig. 9
figure 9

Charge–discharge profiles of a CNT synthesized at 0 V and 1(M) KOH electrolyte, b CNT synthesized at 0 V and 1(M) H2SO4 electrolyte, c CNT synthesized at 3.5 V and 1(M) KOH electrolyte, and d CNT synthesized at 3.5 V and 1(M) H2SO4 electrolyte

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4 Conclusion

In this study, we have observed the changes in the CNT growth phase and its physicochemical properties with the variation of applied DC potential from 0.5 to 3.0 V. At higher potential (> 3.0 V), it was found that the metallic CNT was formed and below 3.0 V, semi-conductive CNTs were grown. All the Raman spectral analyses have supported the formation of a FWCNT because of the presence of a very weak RBM region attributing the predictive diameter approx. below 1.5 nm. The electrochemical performance showed the possibility of application of synthesized FWCNTs in semiconductor-based energy storage systems. Thus, the improvement and growth-controlled CNTs synthesis with desired chirality can be deemed in upcoming time with specific findings and new species. The main focus of this study was to investigate the effect of applied DC bias during the growth of CNTs in its semi-conductive and metallic formation and their corresponding effect on the electrochemical energy storage system (supercapacitor), which has been successfully represented. These discoveries not only provide new insights into the growth of CNTs but also open up new possibilities to control the growth of CNTs depending on the requirement for the application in electrochemical energy storage systems.