Effect of the AACVD based synthesis atmosphere on the structural properties of multi-walled carbon nanotubes

Accepted Manuscript Original article Effect of the AACVD based synthesis atmosphere on the structural properties of multi-walled carbon nanotubes Pawel Mierczynski, Oleksandr Shtyka, Marcin Kozanecki, Paulina Filipczak, Waldemar Maniukiewicz, Dmitry G. Gromov, Sergey V. Dubkov, Artem V. Sysa, Alexey Yu. Trifonov, Agnieszka Czylkowska, Malgorzata I. Szynkowska, Tomasz P. Maniecki PII: DOI: Reference:

S1878-5352(17)30148-X http://dx.doi.org/10.1016/j.arabjc.2017.08.001 ARABJC 2130

To appear in:

Arabian Journal of Chemistry

Received Date: Revised Date: Accepted Date:

31 May 2017 31 July 2017 2 August 2017

Please cite this article as: P. Mierczynski, O. Shtyka, M. Kozanecki, P. Filipczak, W. Maniukiewicz, D.G. Gromov, S.V. Dubkov, A.V. Sysa, A. Yu. Trifonov, A. Czylkowska, M.I. Szynkowska, T.P. Maniecki, Effect of the AACVD based synthesis atmosphere on the structural properties of multi-walled carbon nanotubes, Arabian Journal of Chemistry (2017), doi: http://dx.doi.org/10.1016/j.arabjc.2017.08.001

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Effect of the AACVD based synthesis atmosphere on the structural properties of multi-walled carbon nanotubes Pawel Mierczynski*a, Oleksandr Shtykaa, Marcin Kozaneckib, Paulina Filipczakb, Waldemar Maniukiewicza, Dmitry G. Gromovc, Sergey V. Dubkovc, Artem V. Sysac, Alexey Yu. Trifonovd, Agnieszka Czylkowskaa, Malgorzata I. Szynkowskaa, Tomasz P. Manieckia a

Institute of General and Ecological Chemistry, Lodz University of Technology, Zeromskiego

116, 90-924 Lodz, Poland b

Department of Molecular Physics, Lodz University of Technology, Zeromskiego 116, 90-924

Lodz, Poland c

National Research University of Electronic Technology (MIET), Zelenograd, Moscow,

Russian Federation d

Federal State Unitary Enterprise Lukin Research Institute of Physical Problems, Zelenograd

124460, Russia *Corresponding author: [email protected], [email protected] tel. + 48 42 631 31 24 Fax: 0048 42 631 31 28

Abstract The synthesis of multi-walled carbon nanotubes (MWCNTS) has been the focus of considerable research effort for more than twenty five years and it continues to receive increasing attention because of its importance to produce carbon nanotubes with suitable parameters for future applications. To the best of our knowledge this study presents for the first time the complex studies concerning the effect of aerosol-assisted chemical vapour deposition (AACVD) process conditions (including temperature (750-1200°C) and the composition of the carrier gas (N2, Ar, He, 5% H2-95% Ar, 3% H2O-97% Ar)) on the conversion of the carbon source and on the properties of the carbon nanotubes. In addition, it was also found that oxidative or reductive atmosphere applied during the AACVD process have a great impact on the quality and the degree of toluene conversion into the carbon solids obtained during the synthesis. X-ray Diffraction (XRD), Specific Surface Area and Porosity analysis (BET), Scanning Electron Microscopy with Energy-Dispersive X-ray Spectroscopy (SEM-EDS), Transmission Electron Microscopy (TEM), Raman spectroscopy and Thermogravimetric Analysis (TG) were used to characterize the carbon nanotubes. Keywords: multi-walled carbon nanotubes, MWCNTS, nanomaterial, carrier gas, aerosolassisted catalytic chemical vapour deposition (AACVD). 1

1. Introduction Since their discovery by Iijima in 1991 [1], carbon nanotubes (CNTs) have attracted great interest in most fields of science and engineering, due to their unique physical and chemical properties. These properties allow them to be used for a wide range of applications, such as polymer reinforcements for composites, materials for energy storage and in electronics, catalysis [2-9] and in medicine [10-12]. Most of the chemical processes carry on catalysts supported on carriers. This combination allows increasing catalyst activity. Carbon nanotubes have a high surface area and are characterized by a high thermal and mechanical stability. All above and specific electronic properties of CNTs provided, that they can become the ideal support for heterogeneous catalysts [13]. Carbon nanotubes are synthesized mainly by using three methods: arc discharge [14], laser ablation [15], and chemical vapour deposition (CVD) [16-19]. The feature of these methods is to provide energy to a carbon source to form carbon nanotubes. The energy source depends on the synthesis method. In an arc discharge method, current is an energy source, in the laser ablation - the high-intensity light from a laser and in CVD - heat from a furnace. Among all methods, chemical vapour deposition is the simplest, the most economical and reproducible method, which can provide large-scale production of CNTs [20]. CVD method has been developed in the 1960s and 1970s for carbon fibres and nanofibres production [21]. Since 1996 it is considered to be a prospective method for the production of carbon nanotubes in the future [21]. In CVD method carbon nanotubes are produced from the carbon-containing source (liquid, gaseous, and solid hydrocarbons), as it decomposes at high temperature (500 - 1200°C) and passes over metal catalyst (Fe, Co, and Ni) deposited on special powder carrier (e.g. Al2O3) or on a solid substrate, such as silicon wafers or glass in a tubular reactor [22]. Apart from the above mentioned transition metals, also other metals, such as: Cu, Au, Ag, Pt, and Pd were also found to catalyse various hydrocarbons for CNTs growth [3]. It is well known, that the structural features of carbon nanotubes synthesized by CVD strongly depend on the synthesis conditions [23-25]. Experimental factors having a significant influence on the properties of carbon nanotubes obtained by CVD method include: the synthesis and pre-treatment of the catalyst (nature, size and density of the catalyst, nature of the support and its surface features etc.) and the conditions for the synthesis (temperature, atmosphere, time, additives etc.). Today, twenty five years after discovery of carbon nanotubes, research efforts are still concerned on choosing the best catalyst material, support, and atmosphere of the process and 2

hydrocarbon source to achieve large-scale production of carbon nanotubes. However, to the best of our knowledge until now scientists have not yet explained, how an atmosphere of the synthesis and the temperature of the synthesis affect the conversion of the carbon source applied during aerosol-assisted chemical vapour deposition (AACVD) synthesis and on their properties. The proposed work was undertaken to fill this gap. For this, we synthesized multi-walled carbon nanotubes (MWCNTs) by AACVD method and determined the effect of parameters of the process, such as temperature and atmosphere on the physicochemical properties of the synthesised MWCNTs.

2.

Experimental

2.1

Synthesis of multi-walled carbon nanotubes Carbon nanotubes were synthesized using an aerosol-assisted chemical vapour

deposition method. This process involves an atomization of a liquid precursor solution into aerosol droplets that are distributed throughout a gaseous medium. The generated aerosol is subsequently transported into a heated reaction zone, where it undergoes rapid evaporation and/or decomposition (pyrolysis), causing growth of carbon nanomaterial. The scheme of experimental set-up used for AACVD process is presented in Figure 1. During the synthesis of carbon nanotubes, an inert carrier gas or gas mixture (argon, nitrogen, helium, 5% H2-95% Ar, 3% H2O-97% Ar) flows at constant rate (1.4 L min-1) to the atomizer connected with glass vessel filled with a mixture of 5 % ferrocene in toluene. Then, the atomized precursor droplets pass through the stainless tube, undergo evaporation and finally, the gas mixture reaches the reaction zone, where it decomposes at high temperature and subsequently growth of the carbon nanotubes on the walls of the reactor takes place.

-Figure 1-

Prior to each synthesis the inert gas was introduced to the apparatus (the glass vessel for aerosol liquid is empty) for 20 min to eliminate the air from the system. The synthesized MWCNTs in various atmosphere and temperature have been marked in table 1.

3

Table 1. Abbreviations of the MWCNTs synthesized by AACVD method in various atmospheres and temperatures. Sample

Atmosphere

Temperature

of the synthesis

of the synthesis

Abbreviation

Abbreviation

[°C] MWCNTs

argon

750

MWCNTs Ar-750

MWCNTs

argon

800

MWCNTs Ar-800

MWCNTs

argon

850

MWCNTs Ar-850

MWCNTs

argon

900

MWCNTs Ar-900

MWCNTs

argon

1200

MWCNTs Ar-1200

MWCNTs

nitrogen

750

MWCNTs N2-750

MWCNTs

nitrogen

800

MWCNTs N2-800

MWCNTs

nitrogen

850

MWCNTs N2-850

MWCNTs

nitrogen

900

MWCNTs N2-900

MWCNTs

nitrogen

1200

MWCNTs N2-1200

MWCNTs

helium

750

MWCNTs He-750

MWCNTs

helium

800

MWCNTs He-800

MWCNTs

helium

850

MWCNTs He-850

MWCNTs

helium

900

MWCNTs He-900

MWCNTs

helium

1200

MWCNTs He-1200

MWCNTs

5% H2-95% Ar

750

MWCNTs 5% H2-750

MWCNTs

5% H2-95% Ar

800

MWCNTs 5% H2-800

MWCNTs

5% H2-95% Ar

850

MWCNTs 5% H2-850

MWCNTs

5% H2-95% Ar

900

MWCNTs 5% H2-900

MWCNTs

5% H2-95% Ar

1200

MWCNTs 5% H2-1200

MWCNTs

3% H2O-97% Ar

750

MWCNTs 3% H2O-750

MWCNTs

3% H2O-97% Ar

800

MWCNTs 3% H2O-800

MWCNTs

3% H2O-97% Ar

850

MWCNTs 3% H2O-850

MWCNTs

3% H2O-97% Ar

900

MWCNTs 3% H2O-900

MWCNTs

3% H2O-97% Ar

1200

MWCNTs 3% H2O-1200

MWCNTs Ar-700-1200

MWCNTs N2-750-1200

MWCNTs He-750-1200

MWCNTs 5% H2-750-1200

MWCNTs 3% H2O-750-1200

Purification of the multi-walled carbon nanotubes Synthesized carbon nanotubes were purified in concentrated hydrochloric acid for 24 h. After the purification process carbon nanotubes were flushed with distilled water until the neutral pH was achieved. In the next step, the carbon nanotubes were calcined in air atmosphere for 4 h at 350°C, in order to oxidize or remove the amorphous carbon and other impurities from the sample. 2.2

Characterization methods 4

The specific surface area and porosity of supports and catalysts were determined by the BET based on low temperature (-196°C) nitrogen adsorption in a Micrometrics ASAP 2020 apparatus. Thermo-gravimetric TG method, equipped with differential thermal analysis DTA device Derivatograpf Type: 34-27T (MOM BUDAPEST) was used for temperatureprogrammed decomposition of synthesized multi-walled carbon nanotubes in air atmosphere. The TG-DTA measurements were carried out with sample weight  50 mg, linear heating rate of 10°C min−1, temperature range from 25 up to 1000°C. All samples were used without any preliminary treatment. Powder X-ray diffractograms were recorded on a PAN analytical Pro MPD using Cu Kradiation=154.05 pm) in 2range of 5-90°. The morphology and surface composition of the samples were studied by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS), respectively, on the equipment JSM-6010PLUS / LA Company JEOL or using S-4700 scanning electron microscope HITACHI (Japan), equipped with an energy dispersive spectrometer EDS (Thermo Noran, USA). The TEM measurements were performed using transmission electron microscope FEI Tecnai G2 20 S-Twin, equipped with EDAX energy dispersive X-ray spectrometer and HAADF detector at an accelerating voltage of 200 kV. Raman measurements were carried out with use of JobinYvon T64000 triple grating Raman spectrometer (Ar laser excitation lines - 514,5 and 488 nm) with a spectral resolution below 1 cm-1, and Fourier transform Raman spectrometer MultiRAM (Bruker) equipped with Nd:YAG laser (excitation wavelength 1064 nm) and with high-sensitivity Ge diode detector. 3.

Results and discussion

3.1

Specific surface area The specific surface areas (SSA) of the MWCNTs are given in Table 2. The results

clearly show, that the highest temperatures of the synthesis process (1200°C) cause decrease of the specific surface area of the obtained material up to values of 20-34 m2/g. It should be noted, that the values of the surface area for systems synthesized at 750 and 800°C are practically the same in the case of carbon containing systems, synthesized in argon atmosphere. The comparison of the specific surface area of MWCNTs obtained in various atmospheres at 850°C, showed that the value of SSA is in the range 39-77 m2/g. The highest specific surface area had MWCNTs He-850 system (77 m2/g). While, the lowest value of the

5

specific surface area (39 m2/g) was characterized by the MWCNTs 5 % H2-850 (Table 2) system. Table 2. BET surface area of MWCNTs obtained during the synthesis carried out at different temperatures and various carrier gases. Specific surface area [m2/g]

3.2

Carrier gas

750°C

800°C

850°C

900°C

1200°C

Ar

52

57

47

70

20

5% H2-95% Ar

47

57

39

55

20

He

45

65

77

45

34

N2

-

-

63

-

-

3% H2O-97% Ar

-

-

59

-

-

Thermogravimetric analysis The thermal behaviour of the MWCNTs was analysed by TG technique. The thermal

analyses of prepared MWCNTs were used in order to determine the influence of the atmosphere and temperature of the synthesis on the stability of the multi-walled carbon nanotubes. The TG results for samples obtained during the synthesis performed in the temperature range 750-1200°C in a various atmosphere (Ar, 5% H2-95% Ar, He, N2 and 3% H2O-Ar) are given in Fig. 2 and Table 3, respectively. -Figure 2The thermal decomposition curves recorded for all MWCNTs showed similar stability behaviour. TG curves recorded for all synthesized MWCNTs showed two or three decomposition stages in all cases. A slight weight lost at the beginning of the thermal decomposition process observed for MWCNTs on TG curves below 200°C are connected with the removal of moisture from the carbon material (e.g. TG curve recorded for carbon material synthesized at 850°C in a mixture of 5%H2-95%Ar). The next decomposition effects visible on TG curves are assigned to the removal of impurities present in the catalyst precursor introduced during the synthesis of multi-walled carbon nanotubes and/or by the oxidation of soot. This stage is observed on TG curves up to the temperature range of 400 580°C, depending on the carrier gas used during the AACVD synthesis (see Table 3). The next decomposition step is visible on TG curves starts from the temperature range 400 580°C, which is connected with the oxidation of the MWCNTs to carbon dioxide.

6

In addition, Figure 2F presents TG measurements obtained for systems synthesized at 850°C in different atmosphere. The thermal decomposition results of carbon containing material synthesized at 850°C in nitrogen, argon and helium did not show any low temperature decomposition stages, located below 200°C (see Table 3). Only MWCNTs synthesized in a mixture of 3% H2O -97% Ar and 5% H2-95% Ar atmosphere showed decomposition stages visible on TG curves starts from about 100°C. Table 3.

The results of the thermogravimetric studies for the synthesized multi-walled carbon nanotubes at different temperatures and various atmospheres.

Lp.

Material

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15 16. 17 18. 19. 20. 21. 22. 23. 24. 25.

MWCNTs Ar-750 MWCNTs Ar-800 MWCNTs Ar-850 MWCNTs Ar-900 MWCNTs Ar-1200 MWCNTs 5%H2-750 MWCNTs 5%H2-800 MWCNTs 5%H2-850 MWCNTs 5%H2-900 MWCNTs 5%H2-1200 MWCNTs 3%H2O-750 MWCNTs 3%H2O-800 MWCNTs 3%H2O-850 MWCNTs 3%H2O-900 MWCNTs 3%H2O-1200 MWCNTs He-750 MWCNTs He-800 MWCNTs He-850 MWCNTs He-900 MWCNTs He-1200 MWCNTs N2-750 MWCNTs N2-800 MWCNTs N2-850 MWCNTs N2-900 MWCNTs N2-1200

loss of weight associated with soot oxidation, removal of moisture and removal of impurities (the final temperature of step) [%] 0 (480°C) 2 (480°C) 0 (480°C) 12 (480°C) 2 (480°C) 0 (520°C) 0 (520°C) 5 (500°C) 14 (420°C) 0 (520°C) 2 (500°C) 0 (500°C) 5 (500°C) 5 (500°C) 12 (500°C) 1 (520°C) 0.5 (520°C) 1 (520°C) 11 (400°C) 6 (480°C) 0 (520°C) 0 (520°C) 2 (580°C) 0 (520°C) 0 (500°C)

loss of weight associated with the MWCNTs oxidation (the final temperature of step) [%]

Residue [%]

97.5 (900°C) 94.2 (980°C) 96 (950°C) 85.6 (890°C) 95.6 (960°C) 91 (970°C) 97.6 (950°C) 72.2 (890°C) 67 (850°C) 97.6 (970°C) 61.5 (800°C) 72.5 (820°C) 80.8 (850°C) 83.9 (900°C) 85.6 (950°C) 95.5 (900°C) 96.2 (900°C) 96.6 (900°C) 81.5 (820°C) 91.6 (820°C) 95 (920°C) 97.6 (980) 95.6 (980) 96.7 (900) 97.6 (980)

2.50 3.80 4.00 2.40 2.40 9.00 2.40 15.0 19.0 2.40 35.5 27.5 14.2 11.1 2.40 3.50 3.30 2.40 7.50 2.40 5.00 2.40 2.40 3.30 2.40

These stages are probably connected with the removal of moisture, from the carbon material. The next stages visible on TG curves were assigned to the oxidation of the soot or/and with the removal of impurities present in the catalyst precursor. Above this temperature decomposition step assigned to MWCNTs oxidation process to CO 2 was seen on all TG curves for all remaining systems. Additionally, TG results of all systems synthesized in different atmosphere showed that in all cases we observed the residue which did not 7

decompose in the investigated temperature range. This residue is iron (III) oxide, which is derived from the oxidation of iron particles, which are a component of the catalyst used during the synthesis. It is worth emphasizing the fact, that the remaining amount of iron (III) oxide in the carbon containing system was 2.4, 2.4, 4, 14.2 and 15 %wt. for systems prepared in the atmosphere of N2, He, Ar, 3% H2O-97% Ar and 5% H2-95% Ar, respectively. It is worth to note, that there is no significant weight loss below temperature range 400 580°C, indicating the quantity of non-crystalline structure of carbonaceous materials (i.e. soot or content of the impurities) is marginal in the case of the MWCNTs synthesized in various atmosphere at 750, 800, 850 and 1200°C. Different thermal behaviour was exhibited by MWCNTs systems obtained in various atmospheres at 900°C. In the case of these MWCNTs, a significant decrease in a mass in the temperature range 400-520°C depending on the atmosphere used during the synthesis was observed, which might indicate, that in this case the amount of non-crystalline carbon is significantly higher compared to the sample obtained at other temperatures. The final temperature of the oxidation process of the MWCNTs obtained at 850°C in argon, nitrogen, helium, 3% H2O-97% Ar and 5% H2-95% Ar was 890, 980, 820, 900 and 850°C, respectively. It is worth to note, that the final sample weight loss (not reaching 100%) for MWCNTs synthesized in various atmospheres indicates the presence of impurities in the investigated material, which originate from the catalyst used during the synthesis. Base on the final sample weight loss we can suggest that the content of iron (III) oxide [26, 27] in the MWCNTs is in the range 2.4 to even 35.5 % depending on the temperature of the synthesis of the multi-walled carbon nanotubes. In addition, analysis of the TG results of MWCNTs obtained during the AACVD synthesis performed in various atmosphere showed, that synthesis carried out in an inert atmosphere (He, Ar, N2) leads to higher toluene conversion (see Table 4) and allows to obtain qualitatively better product (higher quantity of MWCNTs in the product). A completely different thermal decomposition behaviour exhibited systems obtained during the synthesis performed in a mixture of 3% H2O-97% Ar or 5% H2-95% Ar in a wide range of a temperature 750 - 1200°C. For CVD synthesis performed in reductive or oxidative atmosphere, a lower conversion value of toluene and lower carbon nanotube yields in the final product were observed. The only exceptions were processes carried out at 1200°C in various atmospheres and at 800°C in a mixture of 5% H2-95% Ar, for which 100% conversion of toluene was obtained.

8

Pinault et al. [28] studied the thermal stability of raw and annealed at 1700 and 2000°C in an argon atmosphere MWCNTs. The authors confirmed various stability behaviours depending on the MWCNTs treatment. They have found that annealed carbon nanotubes were more stable compared to the raw material and did not contain any contamination from the catalyst used during the process. This result means that annealing leads to removing of the iron particles from the carbon nanomaterial, as a result of melting and evaporation processes through the open tips of MWCNTs. TG results obtained for all samples synthesized under various conditions were used to determine the conversion degree of toluene to the various allotropic forms of carbon synthesized by AACVD method, using ferrocene as a catalyst. 5% wt. of the ferrocene in toluene solution was applied during the synthesis. The mass of products is a quantity dependent on the conversion degree of the reaction substrate or substrates. If the reactant is volatile and a gas mixture has constant composition, as it is in our case, the degree of conversion of the reaction substrate is determined on the basis of the change in weight of the sample. The weight change of the sample is determined relative to the mass of the residual corresponding to 0 and 100% conversion of the toluene. For the synthesis with zero conversion of toluene to the reaction product, the mass of residue should be equal 45.35 % by weight of the sample mass. This result was calculated based on the assumption that the total iron comes from ferrocene molecule, which has been oxidized during thermal gravimetric analysis to a hematite phase, and the result was related to the mass of ferrocene without the inclusion of hydrogen atoms constituting a molecule, which is decomposed. In the second case, when a conversion of toluene during the synthesis reaches 100%, the weight of the residue after the synthesis is 2.4% by weight. The residue was calculated as the ratio of iron originating from a molecule of ferrocene oxidized to iron oxide (III) to the sum of the weight of ferrocene and toluene used in the AACVD synthesis. Thus, the degree of conversion of toluene to the carbon solids (eg. carbon nanotubes, non-crystalline carbon, etc.) can be calculated from the following formula (1):  = 100% - (((m- m1)/(m residue))∙100%)

(1)

m - mass of the residue in the final product for each synthesis. m1 - mass of the residue at 100% conversion of toluene m residue - mass of the residue at 0% conversion of toluene.

9

The conversion degree results calculated for all samples from thermogravimetric analysis are given in Table 4 according to the formula 1. As we can see from the data in the Table 4 the conversion degree strongly depends on the atmosphere of the synthesis. In the case of the AACVD synthesis carried out in an inert atmosphere like argon, helium and nitrogen the conversion degree was above 94%. Only in the case of the synthesis carried out in a helium atmosphere at 900 °C, the toluene conversion was 88.7%. These results suggested, that the synthesis carried out in an inert atmosphere proceeds by the same mechanism. The formation of carbon nanotubes takes place according to the base-growth mechanism [29]. In the first step during the synthesis resulting from the decomposition of ferrocene iron is deposited on the external wall of the quartz reactor and then the iron carbide (Fe 3C) phase is created, as a result of iron and carbon interaction. The creation of the iron carbide is essential to start carbon nanotubes formation. On the iron carbide surface an increase of carbon nanotubes took place. Indeed, on the surface of Fe3C CNTs formation is observed what was confirmed by TEM and XRD results (see appropriate paragraph). It was confirmed, that inside of the tubes and at the end of the carbon nanotube the iron containing particles are present (see TEM and XRD results - Figures 5, 6 and 7). During the synthesis carried out in a reducing (5% H2-95% Ar) or oxidizing (3% H2O97% Ar) atmosphere we have to deal with the additional processes. In the course of the synthesis in a mixture of 5% H2-95% Ar together with growth of the synthesis temperature we observed the decrease of the conversion of toluene. This result can be explained by the methanation process, which took place during AACVD synthesis. The occurrence of this process above 500°C during the reduction process of metallic catalyst supported on carbon nanotubes was confirmed in our previous works [3-5]. The occurrence of the methanation process during AACVD synthesis explains well the toluene conversion degree obtained at a given temperature (see results in Table 4). The methanation process explains the decrease in the degree of conversion with increasing of the synthesis temperature. This process is facilitated in higher temperature, but it did not occur at 1200°C due to thermodynamic limitations. This explanation agrees well with our conversion results. SEM and TEM images (see next paragraphs) collected for the MWCNTs containing samples synthesized in a various temperature showed also the presence of the metallic particles inside and on the tip of the carbon nanotubes. This result shows that the growth mechanism of carbon nanotubes is the same as in the case of the synthesis in an inert gas atmosphere. The only difference is the competitive methanation reaction that runs parallel during the synthesis, what caused decrease in the toluene conversion. 10

Different situation can be observed in the case of carrying out the synthesis in the oxidation mixture (3% H2O-97% Ar). During the synthesis the existing iron in a reaction zone is partially oxidized into magnetite phase [30], which is not active during this synthesis. This phenomenon can explain the conversion results obtained for the MWCNTs containing systems during synthesis process carried out under these conditions and additionally confirm the base growth mechanism proposed in the literature data regarding to AACVD synthesis. In addition, such reasoning also explains a large amount of residues in the case of the samples synthesized in the oxidative mixture.

Table.4.

The influence of the temperature and synthesis temperature on conversion degree of toluene during AACVD. Lp. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15 16. 17 18. 19. 20. 21. 22. 23. 24. 25.

3.3

sample MWCNTs Ar-750 MWCNTs Ar-800 MWCNTs Ar-850 MWCNTs Ar-900 MWCNTs Ar-1200 MWCNTs 5%H2-750 MWCNTs 5%H2-800 MWCNTs 5%H2-850 MWCNTs 5%H2-900 MWCNTs 5%H2-1200 MWCNTs 3%H2O-750 MWCNTs 3%H2O-800 MWCNTs 3%H2O-850 MWCNTs 3%H2O-900 MWCNTs 3%H2O-1200 MWCNTs He-750 MWCNTs He-800 MWCNTs He-850 MWCNTs He-900 MWCNTs He-1200 MWCNTs N2-750 MWCNTs N2-800 MWCNTs N2-850 MWCNTs N2-900 MWCNTs N2-1200

Conversion degree [%] 99.7 97 96.4 100 100 85.3 100 77.7 63.4 100 27 44.6 74 80.7 100 97.6 97.9 100 88.7 100 94.2 100 100 97.9 100

SEM-EDS measurements The morphology and size of the products synthesized by AACVD method at 850°C

with different atmospheres were analysed by scanning electron microscopy. -Figure 3Figure 3 presents the SEM images of the multi-walled carbon nanotubes synthesized at 850°C in different atmospheres such as: argon, a mixture of 3% H 2O-97% Ar, a mixture of 11

5% H2-95% Ar, helium and nitrogen are given, respectively. SEM micrographs of multiwalled carbon nanotubes obtained in the synthesis carried out in different atmosphere at 850°C indicate the diversity of the final product. The comparison of the SEM images recorded for all MWCNTs gave evidence that the most well-aligned carbon nanotubes were obtained during the synthesis carried out in an argon atmosphere. These carbon nanotubes have the outer diameter ranging between 56-140 nm and length up to several tens of micrometers. It should be noted, that the predominant carbon nanotubes have a larger outer diameter. Heresanu et al. [31] investigated also the morphology of the MWCNTs synthesized by AACVD method. They reported that during the synthesis process they obtained MWCNTs containing elongated particles encapsulated inside of the nanomaterial, which were aligned perpendicularly to the carpet basis. Multi-walled carbon nanotubes obtained during the synthesis carried out in a mixture of 3% H2O-97% Ar were highly entangled forming bundles and characterized by the length from 1 to 2 micrometer and outer diameter in the range 50-80 nm. This result indicates that addition of a small amount of water vapour into Ar atmosphere results in decrease of the outer diameter and causes the formation of disordered carbon nanotubes forming the clusters. Additionally, the MWCNTs obtained during the synthesis carried out in a mixture of 3% H2O-97% Ar contains large amounts of amorphous carbon and iron containing particles which are incorporated into their structure. The occurrence of these impurities inside and outside of the nanotubes structure are confirmed by the SEM images collected at various magnifications for these systems and by the TG measurements carried out for all MWCNTs 3% H2O-750-1200 systems. On the other hand, MWCNTs obtained during the synthesis performed in a mixture of 5% H2-95% Ar are longer, up to several micrometers, and less tangled compared to MWCNTs 3% H2O-750-1200 samples. In addition, the MWCNTs synthesized in this atmosphere are characterized by the smallest outer diameters in the range 30 to 63 nm as compared to the rest of the MWCNTs prepared in different atmospheres. Also in the case of the synthesis carried out in a mixture of 5% H2-95% Ar the formation of multiwalled carbon nanotubes as a final product and soot was observed. In addition, SEM images confirmed the occurrence of impurities inside and outside of multi-walled carbon nanotubes, which were introduced during the synthesis of MWCNTs. Wasel et al. [20] investigated the effect of hydrogen on the CVD synthesis of MWCNTs using xylene as a carbon source. They observed, that the quality and the quantity of the produced carbon nanotubes were directly connected with the content of the hydrogen used during the synthesis. The authors suggested also, that the role of hydrogen in the 12

MWCNTs production is to regulate spontaneous carbonisation and regulate the branching kinetics of the products obtained in the CVD process. While, Singh et al. [32] reported that the length of the produced carbon nanotubes decreases with increase of the hydrogen concentration in a mixture used during the carbon nanotubes synthesis. On the other hand, multi-walled carbon nanotubes synthesized in a helium atmosphere were characterized by a greater length, larger outer diameter (40-100 nm) and represented less tangled material than MWCNTs 3%H2O-850 system. The resulting product also contained soot as well as impurities coming from the catalyst precursor. SEM images of multi-walled carbon nanotubes synthesized at 850°C in a nitrogen atmosphere collected at various magnifications showed that during the synthesis tangled and disordered multi-walled carbon nanotubes with an outer diameter of 50-100 nm and lengths of several micrometers were obtained. The final product synthesized in argon atmosphere contained the least amount of impurities such us: amorphous carbon and iron containing compounds (Fe, Fe 3O4, Fe3C - see XRD results). The role of nitrogen in carbon nanotube formation was studied extensively by Lin et al. [33]. The authors reported that the presence of nitrogen enhanced the carbon diffusion in the catalyst. Their investigation indicated also that nitrogen could promote of the bamboo-like CNTs. They concluded that in general the role of nitrogen is to extend the surface passivation of the catalyst surface to increase the carbon diffusion. On the other hand Jung et al. [34] also instigated the role of nitrogen during the growth of CNTs. In their work [34] authors reported about two possible roles of nitrogen in the kinetics of the CNTs growth process. The first possible route is to enhance of the graphite layer on the catalyst surface. The second role is to improve the kinetics in the separation of the graphite layer from the catalyst. It has been also found that the role of nitrogen depends strongly on the catalyst materials. The morphology of MWCNTs obtained using catalytic pyrolysis method using ferrocene as a catalyst precursor and toluene as a source of carbon at various temperature in a mixture of 5% H2-95% Ar was observed using SEM technique (Fig. 4). Figure 4 shows the SEM images of MWCNTs 5% H2-750-1200 samples. It was observed that the MWCNTs aggregated together and there are many impurities in the obtained materials independently on the temperature of synthesis. The image of MWCNTs 5% H 2-750 shows that the carbon nanotubes did not have homogeneous outer diameters. In addition, it should be noted that the diameters are from 20 up to 78 nm, what can be seen on the SEM image. Increasing of the synthesis temperature leads to an increase in the outer diameter of the resulting multi-walled carbon nanotubes. The diameters of the resulting MWCNTs obtained at 800°C were between 30 to 95 nm, wherein nanotubes with larger diameter were mostly presented. The SEM image 13

of MWCNTs 5%H2-850 showed that the mutli-walled carbon nanotubes are characterized by the outer diameter in the range 30 to 95 nm. The increase of the temperature of the synthesis up to 900°C results in further increase of the outer diameter up to 130 nm (outer diameter 42 130 nm). In addition, the resulting MWCNTs contain clusters of aggregates composed of carbon nanotubes, amorphous carbon and iron containing particles (Fe and Fe3C - see XRD results - Figure 7 and 8). The MWCNTs 5%H2-1200 contains the highest amount of soot in the final product compared to the systems obtained in the same mixture at lower temperature. SEM image recorded for MWCNTs 5%H2-1200 showed carbon nanotubes with outer diameter in the range 27 - 60 nm. It is worth to note that during the synthesis the brown oil film deposited on the reactor wall probably due to the presence of free radical condensates or precursor soot was observed. The similar findings were observed by other groups [35, 36]. -Figure 4-

Figure 4 shows also that the impurities such as iron and iron carbide are present in the MWCNTs, independently on the synthesis conditions. The existing impurities were derived from the precursor of the catalyst. The highest content of iron was observed for synthesis performed at 800, 850 and 900°C. While, at 1200°C the carbon material was characterized by the lowest content of impurities. The obtained results are consistent with the thermal analysis performed for the studied systems (see thermogravimetric analysis - Figure 2). -Figure 5Figure 5 shows the EDS spectra of synthesized MWCNTs in a mixture 5%H2-95%Ar in the temperature range 750-1200°C. The EDS images clearly confirmed that during the synthesis the incorporation of metallic iron species in the structure of carbon nanotubes took place. It should be noted, that the amount of iron species (such as: metallic Fe and Fe3C) incorporated into the structure of the MWCNTs decreases with increasing of the process temperature from 900 to 1200°C. The confirmation of this trend is the TG measurements (see Figure 2). In addition to iron, other impurities such as silicon were observed on the EDS spectra for all samples. The system formed at 800°C contains the lowest content of iron in comparison with the rest of the system formed up to 900°C

3.4

TEM measurements Figure 6 shows HRTEM images of the MWCNTs 5%H2-750-1200 samples. The main

conclusions from the SEM and HRTEM results are that regardless of the synthesis conditions 14

the multi-walled carbon nanotubes were obtained as a main final product. The HRTEM images collected for MWCNTs 5%H2-750 system showed that at this temperature an inhomogeneous final product is produced in the form of multi-walled carbon nanotubes having different external diameters with impurities e.g. iron compounds (metallic Fe and Fe 3C [37]) occluded inside of the structure of MWCNTs and a lot of structural defects [28], which is also confirmed by the results of SEM-EDS measurements. HRTEM micrographs collected for MWCNTs 5%H2-750 showed that occluded impurities particles had a maximum size 30 nm. HRTEM images of synthesized at 750°C MWCNTs showed additionally a pyrolytic carbon layer present on the surface of multi-walled carbon nanotubes. In addition, highresolution transmission microscope image recorded for the MWCNTs 5%H 2-750 showed that the some part of the obtained MWCNTs have an internal diameter of 8 nm and are composed of 28 walls. HRTEM micrographs collected for the final product reveal that the MWCNTs roots are attached to the catalyst particles (see Figure 6A). In addition, the walls of the formed carbon nanotubes are corrugated and poorly ordered. Similar findings were observed by Pinault et al. [38]. Increasing the temperature of the synthesis up to 800 and 850°C consequently leads to a more homogeneous final product. The obtained product consists of a part of the MWCNTs with the inner diameter 8 and 16 nm for MWCNTs synthesized at 800 and 850 °C (see Figure 6), respectively. HRTEM micrographs collected for the MWCNTs at these two temperatures showed that the impurities of the iron based particles occluded inside of the CNTs structure were present. It is also worth to emphasize that HRTEM micrographs confirmed that some of the particles of the impurities are occluded between the walls of the CNTs as it is seen on Figure 6B, D and 6F. It has been also found that at each temperature some part of the MWCNTs contain different number of walls in the structure (number of walls for MWCNTs synthesized at 800 and 850°C was 42 and 18, respectively). -Figure 6HRTEM images of MWCNTs 5%H2-900 showed non-uniform product in the form of twisted carbon nanotubes also containing impurities occluded inside the pipes and inside of the walls forming the individual carbon nanotube. The MWCNTs are characterized by variable of the external and internal diameter. Additionally, the HRTEM and SEM measurements performed for this sample showed that the final product contains larger amount of soot and other contaminants coming from the catalyst what is also confirmed by the thermogravimetric analysis. The detailed analysis of the HRTEM images collected for the MWCNTS 5%H2-900°C showed that some part of the MWCNTs consist of 11 walls of graphene and the inner diameter is 10 nm what showed that the increasing of the synthesis 15

temperature from 850 to 900°C results in decrease of the inner diameter of created multiwalled carbon nanotubes. Further increase of the carbon nanotubes synthesis temperature up to 1200°C results in the appearance of even greater amount of soot in the final product. In addition, the HRTEM images confirmed that the created MWCNTs are characterized by an inner diameter of 6 nm and contain also a greater number of walls (about 76 walls). Also the formation of a layer of pyrolytic carbon on the surface of the outer wall of the carbon nanotubes was detected. 3.5

Phase composition studies The phase composition studies of purified MWCNTs in a concentrated HCl solution

and heated in an air atmosphere at 350°C for 4h obtained in different atmospheres and temperatures were carried out to detect crystal structure of carbon materials and to study the phase composition of MWCNTs containing materials. The results of phase composition studies performed for MWCNTs synthesized at 850°C in various atmospheres are given in Figure 7. The phase composition studies showed that for all systems the diffraction peaks assigned to the graphite like phase, metallic iron (-Fe) and iron carbide (Fe3C) phases were detected in the case of the system synthesised in argon, nitrogen and helium atmospheres. Only in the case of a mixture of 3% H2O-97% Ar, besides of the previously mentioned crystallographic phases, additional reflexes assigned to the magnetite phase [30] were visible on the XRD curve. This result confirmed that during the cooling process of the reactor chamber iron particles, which are present in the final product, are partially oxidized to magnetite phase. -Figure 7-

Figure 8 shows XRD patterns of MWCNTs 5% H2-750-1200 systems. The detailed analysis of the diffraction curves recorded for MWCNTs showed that independently on the atmosphere used during the synthesis the same crystallographic phase were detected for all samples. On the diffraction curves recorded for all samples the following crystallographic phases such us: graphite like structure, Fe3C, metallic iron (-Fe) and magnetite were identified on the diffraction curves [31]. The phase composition of the carbon nanotubes containing systems was also studied by Heresanu at al. [31]. They detected five crystallographic phases for the final product synthesized by AACVD method at 850°C. They observed reflexes on the XRD diffraction curve coming from MWCNTs,  and  iron [39], cementite (Fe3C) and iron oxide (hematite or magnetite Fe3O4) [38], respectively.

16

A systematic studies of carbon nanotubes growth mechanism using in situ timeresolved Wide Angle XRD equipment were performed by Landois et al. [29]. Based on the obtained results authors propose the mechanism of the CNTs. They claimed that the iron carbide is an origin phase of the CNTs formation. Then during the process of carbon nanotubes synthesis ferrocene vapour decomposes to the irons on which an increase of carbon nanotubes according to the base-growth mechanism took place [31, 40]. The authors attributed two major roles to the iron carbide during the synthesis. According to this statement Fe3C is a catalyst of this reaction or can be intermediate product. They have also found that Fe is the last crystalline phase, which is formed during AACVD synthesis. In addition, the presence of the -Fe inside of the carbon nanotubes can be explained by the transformation of the cementite phase in two separate phases such us: -Fe and C. Pichot et al. [39] carried out X-ray scattering experiments for MWCNTs synthesized by CVD method. Authors also confirmed the occurrence of the MWCNTs, crystalline fcc Fe and bcc -Fe phases for the material obtained during the synthesis. Their results also clearly indicate that the base of the carpet where the carbon nanotubes starts to grow consists of MWCNTs and partially oriented -Fe nanowires. While, the top of the produced MWCNTs layer contained -Fe and Fe3C particles. Dijon et al. [41] investigated the influence of the particle size and the oxidation state of the catalyst on the growth mechanism of carbon nanotubes produced via CVD method. They also confirmed that the growth process of the MWCNTs strongly depends on the atmosphere applied during the CVD process. They results showed, that the size and the chemical state of the catalyst particles have a tremendous impact on the start of growth mechanism of carbon nanotubes. The phase composition studies carried out for all samples confirmed the base-growth mechanism of CNTs formed during AACVD method [29]. Evidence of this statement is the presence of the Fe3C phase in the final product obtaining during the AACVD synthesis. Apart from the Fe3C phase, magnetite and metallic iron phases in all investigated samples were also detected. During the cooling process the iron oxide Fe3O4 is created on the prepared MWCNTs surface, which is formed during the oxidation process of the metallic iron particles. In addition, the occurrence of the metallic iron in the final product can be explained by the fact, that this phase is occluded inside of the MWCNTs, which is not available to an oxidizing atmosphere during cooling process. -Figure 8-

17

It is also well known, that the X-ray diffraction method is a classical technique to investigate the graphitization degree. The influence of the synthesis temperature on the graphitization degree of MWCNTs is shown in Figure 7 and 8, respectively. The graphitization degree of carbon nanotubes was estimated by the interlayer distance (d002) determined from the position of diffraction line d002. From the Bragg equation it is possible to obtained the interlayer d002 spacing, and further from the Maire and Mering model we can calculate the graphitization degree using following formula [42]. d = 3.354 + 0.086 (1 - g), where, d is interplane distance (d002) in angstroms, while g is graphitization percentage. It is worth to emphasize, that the higher the d002 value, the lower the degree of graphitization is. The phase composition data obtained for the MWCNT S containing systems synthesized in a mixture of 5% H2-95% Ar indicated, that all samples represent a more or less disordered structure what was confirmed by the SEM and TEM images collected for these samples. In addition, the degree of graphitization of MWCNTs indicates that the value decreases with increasing temperature of the synthesis reaching a value of 0 for multi-walled carbon nanotubes synthesized at 1200°C. The maximum degree of graphitisation was obtained for MWCNTs synthesized at 750°C for which g was 70.6% (see Table 5). The value of the degree of graphitization suggested the purity of the material obtained during the synthesis. Multi-walled carbon nanotubes characterized by the highest degree of graphitization showed higher purity. While, decreasing of the degree of graphitization can be explained by an increase in the existing structural defects and/or impurities, which include e.g. the resulting soot and other contaminants coming from the catalyst precursor.

Table 5. Structural characteristic of MWCNTS synthesized during AACVD method in a mixture of 5% H2-95% Ar in the temperature range 750-1200°C and for MWCNTS system synthesized at 850°C in various atmospheres. Lp.

Material

1. 2. 3. 4.

MWCNTs MWCNTs MWCNTs MWCNTs

1. 2. 3. 4. 5.

MWCNTs MWCNTs MWCNTs MWCNTs MWCNTs

FWH M (o ) MWCNTs He-850 26.053 3.4175 0.939 MWCNTs Ar-850 25.935 3.4327 0.915 MWCNTs N2-850 26.018 3.4219 0.847 MWCNTS 3%H2O-850 26.088 3.4129 1.180 MWCNTs obtained in a mixture of 5% H2-95% Ar MWCNTs 5%H2-750 26.353 3.3793 0.954 MWCNTs 5%H2-800 26.102 3.4112 0.936 MWCNTs 5%H2-850 25.928 3.4336 0.862 MWCNTs 5%H2-900 25.892 3.4384 0.888 MWCNTs 5%H2-1200 25.218 3.5287 2.246 TSyn [°C]/M

2theta (o )

d(002) (Å)

Degree of graphitization g [%] 26.2 8.5 21.0 31.5 70.6 33.5 7.4 1.9 0

18

3.6

Raman spectroscopy Raman spectroscopy is powerful technique for carbon nanomaterials characterization.

All Raman measurements were done for the purified material. From Raman spectrum one can obtain information about vibrational properties and electronic structure of carbon nanotubes. Raman spectra of carbon materials contain different features, such as the radial breathing mode (RBM) (below 200 cm-1), where all the carbon atoms are moving in phase in the radial direction, the G-band (around 1580 cm-1), where neighboring atoms are moving in the opposite directions along the surface of the tube as in 2D graphite, the dispersive disorder induced D-band (around 1350 cm-1) and its second order related harmonic G`-band (around 2700 cm-1) - not present in all carbon structures [43]. The RBM Raman feature is associated with a small diameter inner tube, therefore the RBM signal from MWCNT S is too weak to be observable.

-Figure 9-

Fig. 9 presents comparison of Raman spectra of exemplary MWCNTs 5%H2-750 sample measured with use of different excitation wavelengths. Normalization was done in relation to intensity of G-band - 1580 cm-1. Observation of D-band shift to lower wavenumbers with increasing excitation wavelength (488 nm - 1358 cm-1, 514.5 nm - 1353 cm-1, 1064 nm - 1284 cm-1) confirms that studied material is carbon nanotubes sample. Also changes in intensities ratio (resonance Raman effect), presence of G`-band and small full width at half maximum (FWHM) of bands are characteristics of carbon nanostructures [44]. Rao et al. [45] studied aligned multi-walled carbon nanotubes using Raman spectroscopy and they confirmed that intensity of G-band is much higher than D-band in their samples. Also antisymmetric shape of G-band in form of low intensity band at around 1625 cm-1 is characteristic for this form of MWCNTs. This feature which is quite visible in the spectra for MWCNTs and not present in SWCNT is associated with the maximum in the graphene 2D phonon density of states. TEM and SEM pictures confirm local alignment of MWCNTs synthesized in lower temperatures. -Figure 10-

Fig. 10 presents comparison of Raman spectra of five samples: three synthesized in the same atmosphere - 5% H2 - 95% Ar at different temperatures and three synthesized at the same temperature - 850°C also in a various atmospheres:, Ar, 5% H2 - 95% Ar and N2. Low 19

intensities of D-band peaks in comparison to G-band and asymmetric G-band uphold the presence of aligned MWCNTs in tested samples synthesized at temperatures 750 and 850°C. Raman spectrum of MWCNTs 5%H2-1200 sample is characteristic for disorder carbonaceous materials such as for example soot. There is lack of G`-band and peaks of D and G-bands have large FWHM. TEM pictures confirm big amounts of soot in this sample. Fig. 11 shows intensity ratios of characteristic bands in Raman spectra of tested MWCNTs samples.

-Figure 11-

The ratio of the intensities of D and G-bands is a good indicator of the MWCNTs quality. Similar intensities of these bands indicate a high quantity of structural defects and impurities. It is well known in the literature data, that the increase of the D/G intensity ratio is connected with the C deposits formed during the synthesis [24]. The results obtained in our case indicate that increasing of the synthesis temperature caused the increase of the D/G ratio (see SEM and Raman results obtained for material obtained in argon atmosphere). It is also worth noting, that the ratio between D and G bands for the final material obtained at a given temperature, but in various atmospheres is rather similar. The highest values of this ratio have samples synthesized at 1200°C, which contain the highest quantity of soot in the final product. Figure 11b presents the ratio between G’ and G bands for the synthesized material. Raman measurements indicate that the G′/G ratio is the highest for the material obtained at low temperature. Increase of the synthesis temperature result in decrease of the crystallinity of carbon nanotubes. While, the carbon nanotubes synthesized at 1200°C in various atmosphere did not showed G`-band, what is visible in Fig. 11.b. In addition, low values of D to G-band intensities ratio and high values of G` to G-band intensities ratio confirm that measured samples contain MWCNTs.

4.

Conclusions

To summarize, it was proven that the temperature and the atmosphere applied during the multi-walled carbon nanotubes synthesis by AACVD method have a great impact on the properties, quality and the quantity of the obtained MWCNTs. The outer diameter of the synthesized multi-walled carbon nanotubes and the number of walls strongly depends on the conditions applied during the AACVD synthesis. The important role of the Fe3C during the growth of the MWCNTs was confirmed. The value of graphitization degree obtained for all systems reflects the purity of the multi-walled carbon nanotubes. The TG results allow 20

estimating the conversion of the toluene during AACVD synthesis. It was also found that oxidative or reductive atmosphere have great impact on the quality and the degree of toluene conversion into the carbon solids obtained during the synthesis. The methanation or partial oxidation process of the iron particles run parallel during the AACVD process depending on the atmosphere of the reaction zone, what leads to the decrease of the conversion degree. The good stability of the obtained MWCNTS up to 500°C (besides the carbon nanotubes obtained in an atmosphere of Ar, 5% H2-95% Ar and He at 900°C) gave evidence that synthesized multi-walled carbon nanotubes can become components of nanocomposites, catalysts and useful carrier materials for heterogeneous catalysts. The results also confirm that the use of different temperatures and atmosphere of the carbon nanotubes synthesis allows controlling their parameters (e.g. diameter, number of walls). In addition, the multi-stage cleaning process and their appropriate modification may also provide the possibility of controlling their properties that further increases their utility value. Acknowledgements Funding: This study was funded by the National Science Centre (Grant no. DEC 2012/05/D/ST8/02856). Conflict of Interest: The authors declare that they have no conflict of interest.

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Captions: Figures: Fig.1. The schematic diagram of the apparatus employed for the synthesis of carbon nanotubes. Fig.2. TG analysis for samples obtained during the synthesis performed in the temperature range 750-1200°C in A) Ar, B) 5% H2-95% Ar mixture, C) He, D) N2, E) H2O-Ar and F) at 850°C in a variety of atmospheres such us: Ar, 5% H2-95% Ar, He, N2 and 3% H2O-97% Ar stream, respectively. Fig.3. Scanning electronic microscopy (SEM) images of the multi-walled carbon nanotubes prepared by AACVD method in different atmosphere (Ar, 3% H2O-97% Ar, 5% H295% Ar, He, N2) at 850°C. Fig.4. SEM images obtained for MWCNTs systems at various temperatures A) 750°C, B) 800°C, C) 850°C, D) 900°C, E) 1200°C during the AACVD synthesis performed in 5% H2-95% Ar atmosphere.

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Fig.5. EDS spectra obtained for MWCNTs systems at various temperatures A) 750°C, B) 800°C, C) 850°C, D) 900°C, E) 1200°C during the AACVD synthesis performed in 5% H2-95% Ar atmosphere. Fig.6. Images of HRTEM obtained for MWCNTs systems in a mixture of 5% H2-95% Ar at various temperatures A) and B) MWCNTs obtained at 750°C at two magnifications, C) and D) MWCNTs obtained at 800°C at two magnifications, E) and F) MWCNTs obtained at 850°C at two magnifications, G) and H) MWCNTs obtained at 900°C at two magnifications, I) and J) MWCNTs obtained at 1200°C at two magnifications. Fig.7. XRD diffraction curves recorded for carbon nanomaterials obtained at 850°C in various atmospheres A - He; B - N2; C - Ar; D - Ar - H2O during AACVD synthesis after purified process in HCl solution and heating process carried out at 350°C in an air atmosphere for 4h. Fig.8. XRD Diffraction curves recorded for carbon nanomaterial obtained in the temperature range 750 -1200°C (A - 1200; B - 900; C - 850; D - 800; E - 750) in a mixture of 5% H2-95% Ar during AACVD synthesis after purified process in HCl solution and heating process carried out at 350°C in an air atmosphere for 4h. Fig. 9.Raman spectra of multi-walled carbon nanotubes sample synthesized at 750°C in 5% H2 - 95% Ar atmosphere recorded with different excitation wavelengths: 488 nm (2,54 eV), 514,5 nm (2,41 eV), 1064 nm (1,17 eV). Fig. 10.Raman spectra of multi-walled carbon nanotubes samples: three synthesized in the same atmosphere - 5%H2 - 95% Ar at different temperatures and three synthesized at the same temperature - 850°C in different atmospheres: 5% H2 - 95% Ar, Ar and N2. Fig. 11.Comparison of intensities ratios for measured multi-walled carbon nanotubes samples: a - ratios of D-band to G-band intensities (I3500/I1580) and b - ratios of G`-band to Gband intensities (I2700/I1580).al

25

Figure

Figure 1. The schematic diagram of the apparatus employed for the synthesis of carbon nanotubes.

Figure

Fig.2 TG analysis for samples obtained during the synthesis performed in the temperature range 750-1200°C in A) Ar, B) 5% H2-95% Ar mixture, C) He, D) N2, E) H2O-Ar and F) at 850°C in a variety of atmospheres such us: Ar, 5% H2-95% Ar, He, N2 and 3% H2O-97% Ar stream, respectively.

Figure

Ar

HO-Ar 2

Ar/H2 850°C

He

N2

Fig.3. Scanning electronic microscopy (SEM) images of the multi-walled carbon nanotubes prepared by AACCVD method in different atmosphere (Ar, 3% H2O-97% Ar, 5% H295% Ar, He, N2) at 850°C.

Figure

Fig.4. SEM images obtained for MWCNTs systems at various temperatures A) 750°C, B) 800°C, C) 850°C, D) 900°C, E) 1200°C during the AACCVD synthesis performed in 5% H2-95% Ar atmosphere.

Figure

Fig.5. EDS spectra obtained for MWCNTs systems at various temperatures A) 750°C, B) 800°C, C) 850°C, D) 900°C, E) 1200°C during the AACCVD synthesis performed in 5% H2-95% Ar atmosphere.

Figure

Fig.6. Images of HRTEM obtained for MWCNTs systems in a mixture of 5% H2-95% Ar at various temperatures A) and B) MWCNTs obtained at 750°C at two magnifications, B) and C) MWCNTs obtained at 800°C at two magnifications, D) and E) MWCNTs obtained at 850°C at two magnifications, F) and G) MWCNTs obtained at 900°C at two magnifications, H) and I) MWCNTs obtained at 1200°C at two magnifications.

Figure

Fig.7. XRD diffraction curves recorded for carbon nanomaterials obtained at 850°C in various atmospheres A - He; B - N2; C - Ar; D - Ar - H2O during AACCVD synthesis after purified process in HCl solution and heating process carried out at 350°C in an air atmosphere for 4h.

Figure

Fig.8. XRD Diffraction curves recorded for carbon nanomaterial obtained in the temperature range 750 -1200°C (A - 1200; B - 900; C - 850; D - 800; E - 750) in a mixture of 5% H2-95% Ar during AACCVD synthesis after purified process in HCl solution and heating process carried out at 350°C in an air atmosphere for 4h.

Figure

4,0

Normalised intensity [a.u.]

3,5 3,0

488 514 1064

2,5 2,0 900

1050

1200

1350

1500

1650

1800

1,5 1,0 0,5 0,0 500

1000

1500

2000

2500

3000

-1

Wavenumber [cm ]

Fig. 9. Raman spectra of multi-walled carbon nanotubes sample synthesized at 750°C in 5% H2 - 95% Ar atmosphere recorded with different excitation wavelengths: 488 nm (2,54 eV), 514,5 nm (2,41 eV), 1064 nm (1,17 eV).

Figure

Normalised Intensity [a.u.]

750 C H2/Ar 850 C H2/Ar 850 C Ar

850 C N2 1200 C H2/Ar

500

1000

1500

2000

2500

3000

Wavenumber [cm-1]

Fig. 10. Raman spectra of multi-walled carbon nanotubes samples: three synthesized in the same atmosphere - 5%H2 - 95% Ar at different temperatures and three synthesized at the same temperature - 850°C in different atmospheres: 5% H2 - 95% Ar, Ar and N2.

Figure

1,2

a

He 1200 °C

1,0

N2 1200 °C

Ar 1200 °C

H2 Ar 1200 °C

I1350/I1580

0,8 H2 Ar 900 °C

0,6 Ar 850 °C

0,4

H2 Ar 850 °C H2 Ar 750 °C

He 750 °C

N2 750 °C N2 850 °C N2 800 °C

Ar 800 °C

0,2

0,0

b

Fig. 11.

Comparison of intensities ratios for measured multi-walled carbon nanotubes samples: a - ratios of D-band to G-band intensities (I3500/I1580) and b - ratios of G`-band to G-band intensities (I2700/I1580).