Impact of Thermal Annealing under Nitrogen Ambient on Structural , Micro-Raman , and Thermogravimetric Analyses of Camphoric-CNT

1 Centre of Nanoscience and Nanotechnology (NANO-SciTech Centre), Institute of Science, Universiti Teknologi MARA, Selangor, 40450 Shah Alam, Malaysia 2 School of Physics and Material Studies, Faculty of Applied Sciences, Universiti Teknologi MARA, Selangor, 40450 Shah Alam, Malaysia Department of Physics, Faculty of Sciences and Mathematics, Universiti Pendidikan Sultan Idris, Perak, 35900 Tanjung Malim, Malaysia NANO-ElecTronic Centre, Faculty of Electrical Engineering, Universiti Teknologi MARA, Selangor, 40450 Shah Alam, Malaysia


Introduction
Over the last two decades, the official birth date of carbon nanotubes (CNT) was attributed to Japanese microscopist named Sumio Iijima.He was considered a "father of CNT" at that time affiliated at NEC Corporation, Japan, perished in a paper which shows his effort on the �rst discovery and remarkable potential of a new class of nano-structured carbon material [1].It is important to notice that a common type of CNT is single-walled CNT (SWCNT) [2] and multiwalled CNT (MWCNT) [3] which can be visualized as a single and concentrically rolled-up sheet/s of graphene, respectively.CNT has gained great interests due to its superior intrinsic properties in terms of high thermal conductivity [4,5], excellent mechanical strength [6,7], and effective �eld emission characteristics [8,9] suggesting their applicability for energy storage and conversion [10], �ller in polymer nanocomposites [11], and �at-panel display [12].ere are several methods to eliminate the impurities in the as-synthesized CNT, including microwave digestion and radiation [13], re�ux in acidic medium [14], and ultrasonication [15].
In the present paper, among the various puri�cation techniques, we adopted thermal annealing treatment under nitrogen ambient coupled with different thermal annealing time by single-stage catalytic chemical vapor deposition (CVD) method.e effect of thermal annealing treatment time under nitrogen ambient on structural, micro-Raman, and thermogravimetric analysis was investigated.It demonstrated that nonoxidative gas-phase puri�cation involves the elimination processes of some organometallic and carbonaceous particles from nanotubes.However, few effective methods have been reported for the removal of as-synthesized impurities, such as amorphous carbon (a-C) and catalyst without damaging the camphoric-CNT structure.Finally, this �nding describes the removal-mechanism in terms of the strong relation between non-oxidative gas seniority upon exposure to form heat-treated camphoric-CNT was sought.

Sample Preparation.
In this study, camphoric-CNT was grown using two-stage catalytic CVD apparatus [16].e processing and experimental setup/apparatus of assynthesized camphoric-CNT were described elsewhere [17].e optimized condition of 10% w/v (weight-volume percentage) of ferrocene relative to camphor oil was placed in different alumina boats.e alumina boats were positioned side by side along the quartz tube in the �rst furnace of two-stage catalytic CVD apparatus.e pretreatment of the evaporated camphor oil and sublimated ferrocene occur at 180 ∘ C at �rst furnace.e growth phase lasted for one hour at 800 ∘ C [18].Aer the completion of growth phase, the �rst and second furnaces were shut down until room temperature was reached under constant cooling rate and then was ready for thermal annealing process.
e synthesized camphoric-CNT was then transferred to the thermal annealing system as can be seen in Figure 1.At �rst, each time, 0.5 g of synthesized camphoric-CNT was loaded into the system.e thermal annealing time was determined to �x the thermal annealing temperature.During the thermal annealing experiment, the system was kept at about 450 ∘ C in a lower �ow rate of nitrogen (∼10 sccm) and the thermal annealing times were changed from 2 to 10 minutes with 2-minute interval.Finally, at the same time, the mechanism of the thermal annealing process of each camphoric-CNT was discussed.In order to compensate such a drawback, as the time proceeds, it is hoped that more camphoric-CNT were exposed to the graphitic layer and have more chances to be attacked by the nitrogen molecules.

Characterization
Techniques. e surface morphology and structural behaviors of as-synthesized and annealed camphoric-CNT were characterized comprehensively using a �eld emission scanning electron microscope (FESEM, JEOL JSM-7600F) operated at 10 kV electron high tension (EHT) and 200 k� magni�cation.e degree of defect and graphitization of each sample was observed by a micro-Raman spectrometer (-Raman, HORIBA JOBIN HR800 Lab-Ram System) equipped with a 514.532 nm wavelengths Ar + laser excitation as the light source.e power of the laser was ad�usted by optical �lter with a 100� ob�ective lens.e -Raman spectra were recorded from 100 to 2000 cm −1 .e thermogravimetric analysis was performed by thermogravimetric analyzer (TG, Perkin Elmer Pyris 1 TGA), operating at temperatures range from room temperature to 1000 ∘ C at a heating rate of 20 ∘ C/min in nitrogen ambient.e yield of camphoric-CNT before and aer thermal annealing was determined.

Result and Discussion
3.1.Surface Morphology.e FESEM micrographs which analyzed the variation of as-synthesized and annealed (2, 4, 6, 8, and 10 minutes thermal annealing procedure) camphoric-CNT surface structures were shown in the Figure 2. In addition, the size of camphoric-CNT was determined and compared before and aer a heat treatment process.From Figure 2(a), it is clear that as-synthesized camphoric-CNT has large-diameter ranges from 49.1 to 57.3 nm.e CNT diameter was slightly decreased to 51.4 nm as the thermal annealing was for 2 minutes as in Figure 2(b).As the time annealing increases further (from 4 to 6 minutes) the tube's diameter was further decreased to smaller diameter range (26.7 to 22.1 nm).However, further increase in thermal annealing time (from 8 to 10 minutes) did not show any signi�cant changes in tube's diameter.
e modi�cation of the surface structure towards smoother morphology was also con�rmed.Figure 2(a) showed the originality of camphoric-CNT, where the surface structure looks rough, uneven with bulge-like structure.ese were believed due to a-C which was deposited on the tube wall.However, aer the thermal annealing procedures were done, the reductions of tube's diameter were clearly observed.Moreover, rough and uneven surfaces with a bulge still remain as shown in Figure 2(b).is shows that within 2-minute times of thermal annealing, procedures were not sufficient to eliminate the impurities that produced during the synthesis process.is suggested that all contaminant compounds and other impurities (i.e., a-C and byproduct carbon) were completely burnt off at 6-minute times without any camphoric-CNT structure collapse.We found that the smallest tube's diameter was obtained at 6 minutes of thermal annealing time.However, the smoother and impurities-free surface was spotted at annealing time as shown in Figures 2(d) and 2(f), respectively.is study emphatically proves that by reordering the carbon structure, the sample is almost free from a-C layer in which tube wall produces relatively hollow and bright inside.As con�rmed by FESEM micrograph as in Figure 2(f), the dense camphoric-CNT is completely transformed to translucent camphoric-CNT (marked with red circle).It is found that when they overlapped, we could see another transparent (see-through) CNT-2 behind CNT-1.
We are not matured enough to understand the principle of a removal mechanism of the world inside nanotubes unless by providing specialized laboratory equipment, we will be able to do insitu measurement.ere are many basic issues and highly debatable questions concerning the removal mechanism awaiting solid answers to ful�ll expectations [19].However, it is hard to visualize the crucial role of nitrogen atmosphere in removal-mechanism of camphoric-CNT.It is important to note that under these theoretical assumptions and experimental veri�cations, is discussed below� (i) To elucidate this, consider the relationship of kinetic energy and motion in gas phase.At a lower temperature (−273.15∘ C), the particles of nitrogen (constituent of molecules) have no kinetic energy, in which the molecules are as closely as possible in complete rest, and it de�nes two things� (a) nitrogen molecules have minimal motion and (b) they maintain only quantum-mechanical motion [20].ough nitrogen is considered as nearly inert gas.However, at higher temperature (∼450 ∘ C), nitrogen molecules can react with carbon material as thermal annealing treatment time increased.It seems more reasonable to associate the thermal annealing treatment to the nature of kinetic energy nitrogen gas.It is a certain kind of vibrational motion of its constituent nitrogen molecules called translational motion in gas.As expected, nitrogen molecules move in threedimensional space (-, -, and -axis).It means that the nitrogen molecules move in spatial degrees of freedom.We believe that a-C layer were placed and stuck together on the outer layer of camphoric-CNT.ere are several points of support in the abovementioned removal mechanism.It is speculated that nitrogen molecules behave as a oxidizing agent and a-C will be etched layer by layer by nitrogen molecules as thermal annealing treatment time increased.
(ii) Besides, nitrogen molecules act as a carrier agent.
Since a-C layer has low thermal stability at a higher temperature compared to perfect ideal CNT [21].is phenomenon could happen due to a-C layer sublimates at 450 ∘ C. Aer that, a-C layer can be directly purged out from the single-stage thermal annealing system by nitrogen gas.

Micro-Raman
Analysis.e degree of graphitization and crystal camphoric-CNT structure can be analyzed by micro-Raman spectroscopy at room temperature.Figures 3(a) and 3(b) show the micro-Raman spectra of as-synthesized and annealed camphoric-CNT using a 514.532 nm wavelengths Ar + laser excitation in the wavenumber range from 100 to 500 cm −1 and 1000 to 2000 cm −1 [22,23], respectively.Figure 3(a) shows that the normalized radial breath mode (RBM) at the lower frequency region of small tubes is sensitive to the tube diameter ( SWCNT ) and valid in the range of 0.5 <  SWCNT < 2.48 nm.By using Bandon's equation [24], the diameter of SWCNT can be evaluated from normalized intensity of RBM peak's position ( SWCNT ), as  SWCNT = 248 (nmcm −1 )/ SWCNT (cm −1 ).While for large tubes, the effect of tube diameter on the normalized RBM frequency is ignorable, this might be due to the existence of MWCNT.As shown in Figure 3(b), we can see the deconvolution spectrum of tangential mode (TM) located at (i) 1380 cm −1 assigned to the disorder-induced phonon mode (D-band) and (ii) 1600 cm −1 designated C-C stretching graphitic mode (G-band) [25].In particular interest, the narrowing of the bands is the evaluation of the   /  ratio.e lower   /  ratio corresponds for better crystallinity of graphitic structure [26].e improvement of the intensity ratio is proven by the appearance of defects in the as-synthesized camphoric-CNT which decreased due to defect reduction process as the thermal annealing treatment time increased.
According to this study, the effect of thermal annealing treatment time under nitrogen ambient can be interpreted by using micro-Raman analysis.At the band area of RBM, no peaks related to SWCNT were observed for as-synthesized, 2and 4-minute treated camphoric-CNT samples.e peaks of SWCNT appear at 6-, 8-, and 10-minute treated camphoric-CNT.e positions of  SWCNT were located at 214.4 and 278.4 cm −1 for 6 minutes, 234.4 and 293.0 cm −1 for 8 minutes, and 250.4 and 315.7 cm −1 for 10 minutes.From micro-Raman analysis, the calculated values of  SWCNT were summarized as in Table 1.e number of walls and diameter in MWCNT can be estimated by an empirical law and quickly determined using FESEM, if the nanotubes are individually distinguishable, which is suggested by Chiodarelli et al. [27].ere are many con�icting reports since the perfect ideal CNT is far from the practically producing CNT today.However, this theory is valid if the CNT is free from any contaminants (i.e., metal catalyst and a-C layer) and structural defect.
As shown in Figure 3(b) and Table 1, there is a gradual trend of decreas   /  ratio as thermal annealing treatment time increased.Moreover, it is con�rmed from the micro-Raman analysis that the lowest   /  ratio is 0.442 at 10 minutes ohermal annealing treatment time.us, the thermal annealing treatment time under nitrogen ambient has efficiently enhanced the removal of the structural defects.

ermogravimetric Analysis.
Normally, TG analysis is a part of quantitative determination to de�ne the purity of CNT.In this study, the TG analysis was performed in the oxygen ambient, while the temperature increases at rate of 20 ∘ C/min.For every sample, TG analysis was done up  to 1000 ∘ C to evaluate thermal stability and purity of assynthesized and annealed camphoric-CNT.A plot of the weight loss monitoring thermal stability of camphoric-CNT in weight changes of the sample (including camphoric-CNT, metal catalyst, and impurities) versus temperature has been shown in Figure 4. From this diagram, it is seen that 68.01, 94.71, 96.75, 98.98, 99.17, and 99.75% of the total weight came from C element for as-synthesized, 2, 4, 6, 8, and 10 minutes of thermal annealing treatment time samples.It is found that camphoric-CNT for all samples is completely burned around 600 ∘ C. e residual mass is the mass that was le due to orange-like powder in the sample pan at the end of the characterization process.e lowest residual iron catalyst was found to be 0.25% at 10 of minutes thermal annealing treatment time.

Conclusion
e in�uence of thermal annealing under nitrogen ambient on structural, micro-Raman, and TG analyses of assynthesized and annealed camphoric-CNT has been examined in to thermal annealing process; thermal annealing treatment time is the critical parameter that determines length and diameter distribution, disordered and graphitic feature, and percentage yield of camphoric-CNT.It is evaluated that the texture of camphoric-CNT changed according to the thermal annealing treatment time, by FESEM observation.Since the thermal annealing treatment time decreased the I  /I  ratio of the camphoric-CNT, it also has a signi�cant effect of graphitization degree.However, the defects in the nanotubes structure still remained but reduced in quantity; the enhancement of graphitization degree leads to a greater thermal stability that is shown in TG analysis.TG curves showed the camphoric-CNT had highest percentage yield ∼99.75% at 10 minutes thermal annealing treatment time.As a result, in order to produce camphoric-CNT of well-de�ned properties in mass production, it is advisable that further investigations required to understand the world inside nanotubes.

F 3 :
Micro-Raman spectra of camphoric-CNT grown by two-stage catalytic CVD at different thermal annealing time for the following: (a) RBM and (b) TM.Positions of  RBM ( * ) and  TM peaks are shown.
T 1: Tabulated data of  SWCNT and   /  ratio for as-synthesized and at different thermal annealing treatment time using micro-Raman analysis.