Mechanism of carbon nanotube growth in expanded graphite via catalytic pyrolysis reaction using carbores P as a carbon source

Carbon nanotubes (CNTs) had potential applications in energy conversion and storage devices, and it could be prepared by expanded graphite loaded with catalyst at high temperature, however, the mechanism of carbon nanotube growth in expanded graphite need further confirmation. In this work, carbon nanotubes’ in situ growth in expanded graphite (EG) were prepared via catalytic pyrolysis reaction using carbores P as a carbon source and Co(NO3)3•6H2O as a catalyst. The results of X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive X-ray spectroscope (EDS) indicated the carbon nanotubes could generate in, EG with the presence of carbores P as a carbon source and cobalt nitrate as a catalyst. More interestingly, the growth mechanism of carbon nanotubes could be concluded by the results of differential thermal analysis-thermogravimetry-mass spectrometry (DTA-TG-MS) and X-ray photoelectron spectroscopy (XPS) analysis. The pyrolysis products of carbores P were mainly hydrocarbon gas such as CH4 gas, which reacts with Co(NO3)3·6H2O catalyst to reduces CoOx to Co particles, then the carbon form pyrolysis was deposited the on the surface catalyst Co particles and, after continuous solid dissolution and precipitation, carbon nanotubes were at last generated in EG at last.


Introduction
Expanded graphite (EG) was first prepared in the early 1860s by mixing natural graphite with strong oxidants (Xing et al., 2022).EG possesses a loose and porous vermicular morphology, which leads to a large surface area.Hence, the expanded graphite is also named worm graphite.Expanded graphite as a kind of remarkable and new carbon material has excellent corrosion resistance, high temperature resistance, high thermal conductivity, a light weight, good adsorption, and insulation resistance.Therefore, it has been widely used in the fields of hydrogen storage, fuel cells, sensors, catalysts, adsorbents, medicine, and refractories (Yang et al., 2011;Li et al., 2013;Li et al., 2010;Qi et al., 2010).
Taking the application of, EG in refractories as an example, Shantanu K Behera blended Al 2 O 3 -C slide gate plate refractories using expanded graphite as the C source (Behera and Mishra, 2015).Apart from positive oxidation resistance, the refractories exhibited good hot and cold strength as well.In 2015, Wang researched the effect of the reactivity and porous structure of, EG on microstructures and the properties of Al 2 O 3 -C refractories (Wang et al., 2015).The results showed that the mechanical properties and oxidation resistance of the Al 2 O 3 -C refractories had been improved via the addition of, EG.Subsequently, Wang synthesized a boron-and nitrogen-doped expanded graphite as efficient reinforcement for Al 2 O 3 -C refractories (Wang et al., 2017).Analogously, they also studied Al 2 O 3 -C refractories using silicon hybridized expanded graphite as an addition to enhance their mechanical properties (Wang et al., 2018).In addition, the MgO-C refractories containing expanded graphite were investigated for their thermo-mechanical properties and oxidation resistance (Zhu et al., 2013;Subham Mahato and Behera, 2014;Mahato and Behera, 2016).Some researches found that CNTs could be formed in refractory materials containing expanded graphite.Furthermore, the composites of CNTs and expanded graphite have the significance of synergistic toughening for refractory materials.Ming Luo et al. researched the Al 2 O 3 -C refractories containing CNTs, and found that the mechanical properties, such as the cold modulus of rupture and flexural modulus, were better than that of the materials without the CNTs (Luo et al., 2013).Similarly, Ming Luo et al. investigated carbon nanotubes in situ and ceramic whiskers in Al 2 O 3 -C refractories with the addition of Nicatalyzed phenolic resin.The results also indicated that the formation of the CNTs and the ceramic whiskers lead to the enhancement of mechanical properties (Luo et al., 2012a).Additionally, Ming Luo et al. thought that the strength and toughness of refractory materials could be enhanced via introducing the CNTs/EG (Luo et al., 2012b).
Up to now, chemical vapor deposition has been widely used in the growth of carbon nanotubes in expanded graphite.Expanded graphite loaded with catalyst is heated at a high temperature, and the catalyst on its surface would react with hydrocarbon gases such as acetylene to form carbon nanotubes in expanded graphite.For example, Jianguo Zhao et al. prepared carbon nanotubes in the pores of expanded graphite via chemical vapor deposition using acetylene as the carbon source (Zhao et al., 2009).In addition, Baoyan Xing et al. successfully prepared CNTs in, EG through the in situ synthesis method by selecting naphthalene as the carbon source (Xing et al., 2022).Although the carbon source for carbon nanotubes growth in expanded graphite has generated great interest, the growth mechanism of carbon nanotubes has rarely been proven by experimental methods.
Carbores P is a carbonaceous powder with very high coking yield and softening point (>220 °C), and it provides binder properties for resin-bonded magnesia-carbon (MgO-C) or dolomite bricks and alumina-carbon (Al 2 O 3 -C) refractories.And Carbores P was suited for use as a precursor of CNTs.
More interestingly, CNTs have a large surface area, enabling increased electrochemical accessibility and mechanical, chemical, and electrochemical stability, which creates the potential for CNTs to be used as a supplemental material for energy conversion and storage devices (Iqbal et al., 2019).
In this work, the cheap carbores P was used as a carbon source and Co(NO 3 ) 2 •6H 2 O as a catalyst to form carbon nanotubes in expanded graphite through catalytic pyrolysis reaction, and argon was used as a protective atmosphere.The feasibility of the catalytic cleavage of carbores P to form carbon nanotubes was investigated, and the growth mechanism of the carbon nanotubes is discussed.
2 Materials and methods

Synthesis of CNTs in EG
Carbores P was taken as the carbon source.Co(NO 3 ) 3 •6H 2 O was used as the catalyst that was 1.0 wt% of expanded graphite.Firstly, the  expanded graphite and the catalyst Co(NO 3 ) 3 water solution were thoroughly mixed, and were placed in a corundum crucible post drying.Then, graphite paper was placed in the corundum crucible.
After that, carbores P was put on the graphite paper and then heated to 550 °C in a furnace using argon as a protective atmosphere.Next, the flowing argon was closed, and the sample continue to be heated at 1,100 °C for 3 h at a rate of 3 °C/min and then cooled down with the furnace.The scheme of the reactor setup is shown in Figure 1.The sample was classified into three types, as listed in Table 1.

Characterization
The phase composition of all samples was analyzed by D/MAX2500PC X-ray diffractometer (XRD).The scanning angle and scanning speed were 10-80 °and 10 °/min, respectively.The radiation source target of the diffractometer was the Cu-target Kα.The S-4800 field emission scanning electron microscope was equipped with an energy dispersive spectrometer (EDS) and was used to observe the morphology of the synthetic sample and the quantity of sample.Differential thermal analysis-thermogravimetrymass spectrometry (DTA-TG-MS, NETZSCH STA 449F5, Germany) of the sample was carried out.The chemical state was tested by X-ray photoelectron spectroscopy (XPS, PHI5300C, United States).

Phase analysis
The phase composition of all samples was analyzed by XRD, and the results are presented in Figure 2. All samples showed a strong diffraction peak at 26.5 °, which is a typical feature of the (002) crystal plane diffraction of graphitic material.There was a visible CoO phase in samples 1 and 3. Sample 3 had a sharper peak corresponding to the (200) plane of the CoO phase than that of sample 1.It could be obtained from the local magnification figure that there was a trace phase of the cobalt metal according with 44.2 °in sample 3. It is reasonable that the phase differences of those samples should be related to the change of the Co element in the catalyst Co(NO 3 ) 3 •6H 2 O during the catalytic process (Ince Yardimci et al., 2020;Saconsint et al. et al., 2022).

Microstructure analysis
The SEM images of sample 1 are shown in Figure 3. The, EG maintained a loose and vermicular morphology.Obviously, the CNT was not generated between the clearance in the, EG but granular material had been there, and the Co element and C element existed in Sample 1 according to the result of energy dispersive spectroscopy (EDS) analysis.It could be concluded that CNTs could not generate under the condition of the Co(NO 3 ) 3 •6H 2 O catalyst without carbores P.
The SEM images of sample 2 are shown in Figure 4.The irregular precipitate formed between the pores of expanded graphite.Without the Co(NO 3 ) 3 •6H 2 O catalyst, the carbon nanotubes were not formed instead of the granules precipitate, which indicated the CNTs were not sufficiently grown in the sample without a Co(NO 3 ) 3 •6H 2 O catalyst but with carbores P.
The SEM and transmission electron microscopy (TEM) images of sample 3 are shown in Figure 5.There were a lot of curly fibers between the pores of expanded graphite, and the fibers were hollow and about 6 nm, which are carbon nanotubes (CNTs).In addition, the catalyst cobalt was located at the top of the carbon nanotubes, which lead to the formation of carbon nanotubes radially.As shown in Figure 5C, the nanotubes were typical of carbon nanotubes (CNTs) with a hollow structure.

Thermal decomposition of carbores P and expanded graphite
The thermal decomposition process of carbores P and expanded graphite was surveyed by DTA-TG-MS, and is shown in Figures 6-9.
The DTA-TG curve of carbores P is shown in Figure 6.Firstly, there were three endothermic peaks which were located at 168 °C, 340 °C, and 800 °C in the DTA curve.This indicated that gas production started and the carbon began to resolve, as the TG  curve showed a weight change from 97% to 57% between 340 °C and 558 °C.This phenomenon verified that the carbon source from the carbores P had decomposed to generate the gas.
The MS curve of carbores P is shown in Figure 7 (a).It could be concluded that the CH 4 , H 2 O, CO, and H 2 gas corresponding to the m/z = 16, m/z = 18, m/z = 28, and m/z = 2, respectively, were generated at 558 °C (Efimova et al., 2010;Hotová and Slovák, 2016), which was same as the TG-DTA curve.Interestingly, the local magnification of H 2 showed that the H 2 was produced at 700 °C-900 °C.Additionally, the MS curve of the CH 4 gas presented distinct fluctuations in the local magnification.Similarly, in comparison with the DTA-TG curve, the MS curve was slightly undulating at 800 °C.Through the comprehensive analysis of the MS and TG curves, it was found that carbores P began to produce CH 4 , H 2 O, CO, and H 2 gas in this process.
The DTA-TG curve of the expanded graphite is shown in Figure 8.As for the DTA curve, reaction peaks formed at 100 °C and 420 °C.This indicated that volatile matter began to escape from the expanded graphite at 100 °C.Additionally, it was remarkable that the weight of the expanded graphite changed from 100% to 88% in the TG curve due to its continuous decomposition.
The MS curve of the expanded graphite is shown in Figure 9.It could be concluded that the H 2 O and CO were generated with the continuous rise of temperature corresponding to the m/z = 18 and m/ z = 28, respectively (Chien and Liao, 2020;Guo et al., 2022).Compared with the MS curve of carbores P, the intensity of the CO and CO 2 generated in carbores P was more intense than that of the expanded graphite.Additionally, the production of the CH 4 and H 2 gases were not observed in, EG corresponding to the MS curve.The CH 4 and H 2 gases were actually involved in the reaction with the catalyst cobalt to form the growth of the CNTs, which demonstrated that the growth of the CNTs depended on the carbores P instead of the, EG.The results of the MS curve of the, EG was consistent with the SEM of the samples.

Valence analysis of Co
The XPS was used to survey the valency of Co which would be changed in catalytic process.The XPS survey results of sample 1 and sample 3 are shown in Figure 10.Firstly, for sample 1, the curve could be fitted by two spin-orbit doublet characteristics of Co 3+ and Co 2+ in the XPS narrow survey of Co2p (Yang et al., 2019;Gao et al., 2021).Thus, there were four peaks corresponding to two valence states of Co, which were located at 780.6 eV and 796.8 eV and could be  ascribed to Co 3+ , while the peaks located at 785.7 eV and 803 eV were attributed to Co 2+ .However, the Co 2p spectrum of sample 3 could be fitted in six peaks.The signals located at 779.9 eV and 794.8 eV could be attributed to 2p 3/2 and 2p 1/2 of Co 0 , respectively.The two peaks of 781.2 eV and 796.8 eV could be ascribed to the Co 3+ .The peaks at 787.9 eV and 804.7 eV could be assigned to Co 2+ .Through the contrastive XPS surveys of sample 1 and sample 3, it is worth mentioning that Co 0 existed in sample 3.In general, in the process of CNT growth, Co 3+ and Co 2+ are reduced into metallic cobalt, which is reduced by the H 2 gas.Thus, the Co 0 could be detected in the sample 3. Additionally, Co2p 1/2 was produced by CoO, which indicated the Co-O band existed (Liu et al., 2021).The results of the XPS coincided with the CNT growth mechanism.In theory, the growth process of CNTs could be revealed by Equations 1, (2).Firstly, the CoO is reduced by H 2 to generate Co particles.Then, the Co particles react with the CH 4 to generate CoC.At this time, the concentration of C atoms on the surface of the CoC is larger than the concentration of C atoms on the surface on the, EG.Thus, the C atoms should continuously spread due to the concentration difference of C atoms.The concentration difference of C atoms leads to the growth of CNTs.The schematic of the CNT growth mechanism is shown in Figure 11. (2)

Growth mechanism of CNTs
The schematic illustration of the CNT growth mechanism in, EG is shown in Figure 12.As is well known, the growth mechanism of  carbon nanotubes for the synthesis of hydrocarbons from transition metal catalyzed thermal decomposition is not very clear, and the side emphasis of their accounts is different.This can be summarized as the "dissolution diffusion precipitation" model.Firstly, the carbon after the decomposition of hydrocarbons contacts with the bare metal surface.After that, dissolved carbon diffuses, disperses, and is transported to other sites of metal particles.Finally, carbon is deposited and precipitated at these site in the form of carbon nanotubes.
In this work, carbores P was used as the source of the C element.First of all, the expanded graphite was heated from room temperature to 1,100 °C.In this process, many oxygenated functional groups were generated on the surface of the graphite crystal and, when the expanded graphite was heated to an elevated temperature, most of the oxygenated functional groups were decomposed into CO 2 or H 2 O, leaving the high energy unsaturated carbon dangling bonds on the surface of expanded graphite.These unsaturated carbon dangling bonds became the active sites.At the same time, carbores P cleaved to produce hydrocarbon gases such as methane and hydrogen gas at an elevated temperature.Subsequently, the hydrogen gas reduced CoO to Co particles.Next, the CoO reacted with the CH 4 to generate the CoC.The C atom contacted with the CoC and dissolved, adsorbed on the active sites of expanded graphite.Additionally, dissolved supersaturated carbon diffused in the CoC and was transported to other sites of the CoC.Ultimately, the concentration of C atoms on the surface of the CoC was larger than the concentration of C atoms on the surface on the, EG.The C atoms should continuously spread at these sites to generate carbon nanotubes.Then, the catalyst particles gradually became inactive and the carbon nanotubes stopped growing.CNTs successfully grew in, EG with the synergistic effect of carbores P and the Co(NO 3 ) 3 •6H 2 O catalyst.The growth mechanism of carbon nanotubes was that the heated carbores P produces hydrocarbon gas (methane and hydrogen gas), which reacted with the Co(NO 3 ) 3 •6H 2 O catalyst to reduce CoO x to Co particles, then the carbon form pyrolysis was deposited the on the surface catalyst Co particles, after continuous solid dissolution and precipitation, carbon nanotubes were at last generated in the, EG.

FIGURE 1
FIGURE 1Scheme of the reactor setup.

FIGURE 3
FIGURE 3 SEM and EDS images of sample 1.
FIGURE 4 SEM images of sample 2.

FIGURE 5
FIGURE 5 SEM (A,B and D) and TEM (C) images of sample 3.

7
MS curve of carbores P (A) and local magnification of H 2 and CH 4 (B).

FIGURE 8
FIGURE 8DTA-TG curve the expanded graphite.FIGURE9curve of the expanded graphite.

FIGURE 11
FIGURE 11Schematic of the CNT growth mechanism.