J Porous Mater (2012) 19:797–805 DOI 10.1007/s10934-011-9533-2
Synthesis of MWCNTs by the decomposition of acetylene over mesoporous Ni/Cr-MCM-41 catalyst and its functionalization R. Atchudan • A. Pandurangan • K. Subramanian
Published online: 29 November 2011 Ó Springer Science+Business Media, LLC 2011
Abstract Chromium incorporated MCM-41 molecular sieves with different Si/Cr ratios (Si/Cr: 50, 75, 100 and 125) were synthesized hydrothermally and Ni was loaded on them using wet impregnation method. The synthesized materials were characterized by various physico-chemical techniques such as XRD, N2 sorption isotherms, TGA, DRS-UV and HR-TEM. The honeycomb structure like well-ordered hexagonal pores was observed from HR-TEM images. The metal-containing mesoporous materials were used as a catalytic template for the growth of MWCNTs using acetylene as the carbon precursor by CVD method at 700–900 °C. The reaction parameters such as temperature and metal concentration were optimized for the better formation of MWCNTs. The deposited carbon materials were purified and characterized by XRD, SEM, HR-TEM and Raman spectroscopy techniques. The Ni/Cr-MCM-41 template influences the high yield of well ordered MWCNTs which was confirmed by HR-TEM and Raman spectroscopy. The inner and outer diameter of the MWCNTs measured from HR-TEM observations was in the range between 5–6 and 12–13 nm, respectively. The well ordered MWCNTs were treated with 1:3 ratios of HNO3 and H2SO4 mixture at 70 °C for functionalization. The functionalized MWCNTs were studied by FT-IR, SEM and TEM techniques.
R. Atchudan A. Pandurangan K. Subramanian Department of Chemistry, Anna University, Chennai 600025, India A. Pandurangan (&) Institute of Catalysis and Petroleum Technology, Anna University, Chennai 600025, India e-mail:
[email protected]
Keywords MCM-41 Mesoporous catalyst CVD MWCNT Acetylene Functionalization
1 Introduction Quasi one-dimensional carbon nanotubes (CNTs) are the graphite sheet rolled-over to form the cylinder tube material, have became an attractive material for its specific mechanical, physical, chemical, electronic and field-emission properties [1–4]. Most of the well known methods for synthesizing CNTs are arc-discharge [5], laser ablation [6] and chemical vapour deposition (CVD) method [7]. CNTs obtained from arc discharge method were highly impure and possess very low yield. Laser ablation process could produce high quality SWCNTs. But the laser vaporization of a graphite target doped with transition metal is a high energy process and not cost effective. The synthesis of multi-walled carbon nanotubes (MWCNTs) by CVD has attracted much attention due to many advantages such as high purity, high yield, low cost, selective growth and scalable technique for mass production [8, 9]. The catalytically produced tubes were adequate for many applications; especially they could be directly synthesized without major contamination by carbonaceous impurities. Generally, zeolites are limited micropores (diameter \ 1.3 nm) with low surface area and the channel size of zeolites cannot be varied. Thus, because of their lack of pore size flexibility, zeolites are not an ideal choice for the synthesis of CNTs. In 1992, researchers at Mobil Research and Development Corporation reported the synthesis of a new family of mesoporous (nanoporous) molecular sieves with exceptionally large uniform pore structures. The disclosure of M41S materials with uniform pore diameters between 2 and 30 nm, such as MCM-41 and MCM-48
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[10, 11] have attracted considerable attentions in the scientific community. These materials were especially promising catalysts and catalyst supports because of their large pore volume ([1.0 cc/g), high surface area ([1,000 m2/g), large pore diameter ([2 nm), and narrow pore size distribution. MCM-41 materials offer the opportunity to extend shape-selective catalysis beyond the micropore domains typical of zeolite materials, allowing larger molecules to be handled [12]. By introducing the metal ions, MCM-41 shows catalytic activity in synthesizing various nanostructures [13]. Hence, the present work aimed towards the synthesis of MWCNTs using Ni/Cr-MCM-41 as catalytic template by CVD method with acetylene as a carbon precursor. The effect of metal concentration and the reaction temperature were optimized for the better formation of MWCNTs. Furthermore, the CNTs were functionalized using acid treatment and the respective functional groups were confirmed by various techniques.
and heated in a hot air oven at 145 °C for 48 h. After crystallization, the materials were recovered by filtration, washed with distilled water and dried at 100 °C for 5 h. Finally the catalytic materials were obtained after removing the occluded surfactant by calcining the sample at 550 °C in atmospheric air for 5 h.
2 Experimental
2.4 Synthesis and purification of MWCNTs
2.1 Materials
The catalytic reactions for the synthesis of MWCNTs were carried out using calcined Cr-MCM-41 and Ni/Cr-MCM41. The experiments to produce MWCNTs were carried out in a simple CVD setup containing horizontal tubular furnace and gas flow control units. In a typical growth experiment, ca. 200 mg catalyst was placed in a quartz boat inside a quartz tube. The catalyst was purged with nitrogen gas at a flow rate of 100 mL/min for 30 min in order to remove the physically adsorbed water molecules and hydrogen gas was passed at a flow rate of 100 mL/min for 30 min to reduce the metal particles. The reaction was carried out using acetylene as carbon source at different temperatures (700, 800 and 900 °C) with a flow rate of 100 mL/min for 30 min. The furnace was allowed to cool in the nitrogen atmosphere and the final product formed was a black material after the completion of the reaction. The obtained material was weighed, purified and characterized. The percentage of carbon deposited due to the catalytic decomposition of acetylene was calculated from the following equation:
The chemicals used for the synthesis of MCM-41 molecular sieves were sodium metasilicate nonahydrate (Qualigens), chromium nitrate hexahydrate (Merck) and nickel nitrate hexahydrate as sources of silicon, chromium and nickel, respectively. Cetyltrimethylammonium bromide (CTAB) was used as the structure-directing agent. Sulphuric acid was used to adjust the pH of the medium. Hydrofluoric acid, hydrochloric acid and nitric acid were used for the purification of as-grown MWCNTs. Nitric acid and sulphuric acid were used for the functionalization of MWCNTs. All the above chemicals were purchased from Merck with AR (Analytical Reagent) grade and used without any further purification.
2.2 Synthesis of Cr-MCM-41 Cr-MCM-41 molecular sieves with Si/Cr ratio 50, 75, 100 and 125 samples were synthesized hydrothermally using a gel composition of SiO2:xCr2O3:0.2CTAB:0.89H2SO4: 120H2O were prepared (where x = different Si/Cr ratios). In a typical synthesis, an appropriate amount of sodium metasilicate in water was combined with an appropriate amount of chromium nitrate in distilled water and the pH of the solution was adjusted to 10.5 with constant stirring to form a gel. After 1 h, an aqueous solution of CTAB was added to it and the mixture was stirred for 1 h at room temperature. The suspension was then transferred into a 300 mL stainless steel autoclave which was further sealed
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2.3 Preparation of Ni/Cr-MCM-41 Different wt. % of Ni was loaded over Cr-MCM-41(100) by wet impregnation method. In a typical procedure, the appropriate amount of nickel nitrate was dissolved in distilled water and sonicated for 15 min; the sonicated solution was added drop by drop to the calcined Cr-MCM-41 (100) under constant stirring. The solution was dried under reduced pressure and calcined in atmospheric air at 550 °C for 4 h. The muffle furnace was cooled to room temperature and calcined materials were collected.
Carbon deposition yield (% ) ¼ ððmtot mcat Þ=mcat Þ 100 where mtot is the total mass of carbon product and catalyst and mcat is the mass of catalyst respectively. The removal of the silica phase was carried out using 40% hydrofluoric acid (HF) at ambient temperature as cited in the literature [14, 15]. About 200 mg of as-synthesized carbon sample was calcined at 450 °C for 2 h in muffle furnace to remove the carbonaceous impurities such as amorphous carbon and microcrystalline carbon. Then the
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2.5 Functionalization of MWCNTs About 500 mg of purified MWCNTs, 160 mL of 1:3 ratios of HNO3 and H2SO4 were mixed. The mixture was refluxed at 80 °C for 48 h under constant stirring. After cooling, the final mixture was filtered and then washed with distilled water. The precipitate was dried at 70 °C for 10 h in a hot air oven. The nature of the obtained f-MWCNTs samples in powder form were analyzed by FT-IR, SEM and TEM techniques.
2.6 Characterization methods The X-ray powder diffractograms of calcined catalysts and purified MWCNTs samples were obtained from PANalytical X’Pert diffractometer equipped with liquid nitrogen cooled germanium solid-state detector using Cu Ka radiation. The diffractogram of MCM-41 and CNTs were recorded in the 2h ranges of 1–10° and 5–80° respectively, at a scanning rate of 0.02° with the counting time of 5 s at each point. N2 adsorption–desorption isotherms were measured at -197 °C using a Micromeritics ASAP 2000. Prior to the experiments, samples were dried at 130 °C and evacuated overnight for 8 h in flowing argon at a flow rate of 60 mL/min at 200 °C. Surface area, pore size and pore volumes were obtained from these isotherms using the conventional BET and BJH equations. Thermal analysis was carried out in Mettler TA 3001 analyser. DRS-UV analysis was performed using a Shimadzu UV-2450 model and BaSO4 white plate were used as a reference. SEM was performed on a JEOL with beam energy of 4 kV by placing the MWCNTs on nonconductive carbon tape. The TEM images of typical samples of CNTs were obtained using a JEOL 3010 electron microscope operated at 300 kV. Samples for TEM were prepared by placing droplets of a suspension of the sample in acetone on a polymer micro grid supported on a Cu grid. FT-IR analysis was performed using Perkin Elmer and KBr were used as a reference. Raman spectra were recorded with a Micro-Raman system RM 1000 Renishaw using a laser excitation line at 532 nm (Nd-YAG), 0.5–1 mW, with 1 lm focus spot in order to avoid photodecomposition of the samples.
3 Results and discussion 3.1 Characterizations of mesoporous Cr-MCM-41 and Ni/Cr-MCM-41 3.1.1 Low angle XRD patterns XRD technique was useful for the characterization of M41S group materials. The structure of the Cr-MCM-41 and Ni impregnated Cr-MCM-41 materials was studied by XRD techniques and these low angle XRD patterns are shown in Fig. 1. The calcined MCM-41 material shows a strong peak in the 2h range of 1.8–2.88 due to (100) diffraction planes and weak peaks in the 2h range of 3.8–4.88 and 6.2–6.78 due to higher order (110), (200) and (210) diffractions indicating the formation of well-ordered mesoporous materials. All these patterns were assigned to the hexagonal symmetry [11]. XRD patterns of the calcined material confirmed that there has been no structure collapse or change of phase during the calcination. This suggests that MCM-41 has a good thermal stability after calcinations which are in agreement with Chenite et al. [17]. The intensity of X-ray diffraction peaks decrease faintly with increase in Cr concentration over MCM-41 was observed. But the hexagonal nature of host Si-MCM-41 was very well retained for all the catalysts, whereas, a little decrease in intensity was observed for 1 wt. % of Ni loaded Cr-MCM-41 (100), which lies in between the Cr-MCM-41 (50) and Cr-MCM-41 (75). This might be the formation of nickel oxide over the mesoporous matrix. The Ni metal particles have occupied over the hexagonal pores of Cr-MCM-41 (100), but the hexagonality was not much affected which was confirmed by XRD pattern. (100)
Intensity (a.u.)
calcined material was mixed with an appropriate amount of HF and stirred for 1 h and filtered. The obtained sample was further treated with nitric acid and hydrochloric acid to remove the metal particles and filtered, washed with distilled water. The filtered material was dried at 100 °C for 5 h in air atmosphere and then air oxidized at 450 °C for 2 h to remove the carbonaceous impurities [16].
799
(110)
(200)
(210) Cr-MCM-41 (125) Cr-MCM-41 (100) Cr-MCM-41 (75) 1wt. % Ni/Cr-MCM-41 (100) Cr-MCM-41 (50)
2
4
6
8
2 θ (degree) Fig. 1 XRD patterns of mesoporous Cr-MCM-41 and Ni/Cr-MCM41 catalysts
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3.1.2 Nitrogen sorption isotherms
Cr-MCM-41 (125)
Volume adsorbed (a.u.)
Cr-MCM-41 (100)
Cr-MCM-41(75) Cr-MCM-41 (50)
1Wt. % Ni/Cr-MCM-41 (100)
Adsorption Desorption
0.0
0.2
0.4
0.6
0.8
Relative Pressure (P/P0) Fig. 2 N2 sorption isotherms of mesoporous Cr-MCM-41 and Ni/CrMCM-41 catalysts
100 90
Weight loss (%)
Table 1 shows the BET surface area, pore size and pore volume of Cr-MCM-41 catalysts. Figure 2 shows the N2 sorption isotherms of the materials, a sharp inflection at relative pressures (P/P0) in the range of 0.21–0.33 was observed which corresponds to the capillary condensation of N2 in the mesopores with completely reversible isotherms characteristic of ordered mesoporous materials [18]. The obtained isotherms were found to be similar in shape and all are type IV curves. This indicates that the ordering of the hexagonal arrays of the mesopores in MCM-41 was not much affected on increasing the metal content. However, the position of the capillary condensation step shifted some extent towards lower partial pressure due to the formation of stronger metal oxygen bond compared to Si–O bond throughout the mesoporous matrix. Decreases in pore size with increase in metal content [19] was also observed faintly in the following order: Cr-MCM-41 (125) [ Cr-MCM-41 (100) [ Cr-MCM-41 (75) [ CrMCM-41 (50). Hence, there is a sudden decrease in the pore volume and lower shift in relative pressure P/P0 was observed for 1 wt. % of Ni loading over Cr-MCM-41 (100) which is due to the formation of nickel oxides over the mesoporous matrix. But the hexagonal nature was not much affected for 1 wt. % of Ni loading over Cr-MCM-41 (100). The d-spacing value decreases with increase in metal concentration over MCM-41 frame work which might be the shrinkage of hexagonal pores.
Cr-MCM-41 (50)
80
Cr-MCM-41 (75) 70
Cr-MCM-41 (100) Cr-MCM-41 (125)
60 50
3.1.3 Thermogravimetric analysis 40
Thermogravimetric analysis plots showed three distinct weight losses that depend on the framework composition is shown in Fig. 3. The first weight loss observed between 50 and 150 °C is due to the desorption of physically adsorbed water. The second stage of weight loss between 150 and 350 °C corresponds to the decomposition of the surfactant Table 1 Textural properties of mesoporous Cr-MCM-41 and Ni/CrMCM-41 Catalyst (Si/Cr ratio)
d-spacing (nm)a
Surface area (m2/g)b
Pore size (nm)b
Pore volume (cc/g)b
Cr-MCM-41 (125)
3.715
897
2.912
0.756
Cr-MCM-41 (100)
3.698
855
2.875
0.714
Cr-MCM-41 (75)
3.676
840
2.752
0.698
Cr-MCM-41 (50)
3.643
822
2.704
0.687
1 wt. % Ni/CrMCM-41 (100)
3.584
731
2.504
0.612
a
The values obtained from XRD analysis
b
The values obtained from N2 adsorption–desorption studies
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30 100
200
300
400
500
600
700
Temperature (°C) Fig. 3 TGA curves of as-synthesized mesoporous Cr-MCM-41 catalysts
species. Finally, the weight loss from 350 to 550 °C is assigned to the condensation of adjacent silanol (Si–OH) groups to form a siloxane bond [20]. Further, there was no weight loss observed from thermogram which suggests that mesoporous MCM-41 materials have high thermal stability. Thermal stability is also one of the factors for the synthesis of CNTs, since the CNTs are grown at high temperature (800 °C). 3.1.4 DRS-UV Visible measurements The diffuse reflectance UV–Visible spectrum was recorded for the calcined Cr-MCM-41 (100) catalyst in order to
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study the co-ordination environment of Cr and the related spectrum is shown in Fig. 4. Cr-MCM-41 catalysts were green in colour and a change in colour from green to yellow was observed after the calcination. The former (green) is due to the presence of trivalent chromium ions and the latter (yellow) is due to the presence of hexavalent chromium ions, viz., chromate and/or polychromate ions. Typical absorption bands were observed for kmax around 445 and 370 nm which correspond to the polychromate and chromate species respectively. In addition, a band around 270 nm due to trivalent chromium in octahedral co-ordination was also observed [21]. However form the DRS-UV studies, it can be concluded that the Cr in MCM-41 having different environment.
3.1.5 Morphology study The structures of Cr-MCM-41(100) and 1 wt. % Ni/CrMCM-41 (100) were investigated by TEM technique and the images are presented in Fig. 5. The images indicated that the synthesized materials exhibited a well ordered hexagonal array of regular pores as commonly known for MCM-41 material [22]. TEM shows a honeycomb structure of the MCM-41 materials. This indicates that the ordering of the hexagonal arrays of the mesopores in MCM-41 was not much affected with increase in the metal content. The Ni nanoparticles were located over the Cr-MCM-41 (100) by Ni impregnation. 3.2 Effect of reaction parameters and catalytic template for the growth of MWCNTs
0.5
270
370
Absorbance
0.4
0.3
445 0.2
0.1
0.0 300
400
500
600
Wavelength (nm) Fig. 4 DR-UV Visible spectrum of Cr-MCM-41 (100)
The CNTs formation were influenced by the nature of catalytic template (thermal stability, structure, porosity and surface area), reaction temperature, flow rate of carbon sources, metal concentration over the catalytic template and nature of metal. The MWCNTs growth was greatly influenced by the reaction temperature, metal concentration and nature of the metal. The effect of reaction was studied for various ratios of metal incorporating such as Si/Cr = 125, 100, 75 and 50 at 800 °C by CVD method. The lower activity at higher loading of Cr might be due to the irregular dispersion of active species and results in the formation of bulk and clustered metal particles. This reduces the existence of active sites and surface area, thereby hinders the growth of MWCNTs which was further confirmed by BET results. Also the carbon deposition decreased with decrease in metal contents due to insufficient metal contents for the better growth of MWCNTs. Hence, Cr-MCM-41 (100) was found to be the best catalyst for high yield of MWCNTs. To study the influence of
Fig. 5 HR-TEM images of a Cr-MCM-41 (100) and b 1 wt. % Ni/Cr-MCM-41 (100)
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bimetal catalyst, different wt. % (0.5, 1.0 and 1.5) of Ni was loaded over Cr-MCM-41 (100) for the growth of MWCNTs. As Ni particles were highly crystalline in nature and were found over the surface as well as the pores to further enhance the growth of MWCNTs. The 1 wt. % of Ni loaded Cr-MCM-41 (100) showed a higher deposition when compared with Cr-MCM-41 and other wt. % Ni/CrMCM-41. Ni nanoparticles might have stimulated the Cr nanoparticles and also have self activated for the growth of MWCNTs. The influence of temperature was optimised for various Si/Cr ratios of MCM-41 and different wt. % Ni/Cr-MCM41 (100) catalysts and the results are tabulated (Table 2). The influence of temperature on the MWCNT growth was also observed and it shows that the carbon deposition increases with the increase in temperature and found to be optimum at 800 °C which was marked in bold letters, further increase in temperature leads to decrease in yield.
Table 2 Effect of reaction parameters for formation of MWCNTs at reaction time of 30 min Catalyst (Si/Cr ratio)
Carbon deposition yield (%) 700 °C
800 °C
900 °C
Cr-MM-41 (125)
68
99
85
Cr-MCM-41 (100)
76
113
95
Cr-MCM-41 (75)
71
100
89
Cr-MCM-41 (50)
66
87
75
Ni/Cr-MCM-41 (100)
105
123
109
1.0 wt. % Ni/CrMCM-41 (100)
120
156
132
1.5 wt. % Ni/CrMCM-41 (100)
116
147
125
0.5 wt. %
Flow rate of acetylene (mL/min) 100
The deposition of MWCNT decreased at lower and higher temperature beyond 800 °C. At lower temperature, the rate of carbon source decomposition is low due to which the amorphous carbon deposition is relatively high. Also at higher temperature the formation of carbonaceous impurities is maximum due to the self-pyrolysis of MWCNTs. In the present work, Ni/Cr-MCM-41 mesoporous heterogeneous catalytic template was found to play a potential role for the growth of MWCNTs which was confirmed by the experimental results. Further, these better formed MWCNTs were purified and functionalized. 3.3 Characterizations of MWCNTs 3.3.1 SEM and TEM analysis The SEM images of as-grown and purified MWCNTs are shown in Fig. 6. The MWCNTs appearing as filaments are clearly seen with catalytic materials in Fig. 6a and this image also clearly indicates the formation of MWCNTs where the active ingredients are present. Absence of catalytic particles in MWCNTs was found after purification which is observed from SEM image as in Fig. 6b. This image also clearly indicates that the MWCNTs are distributed uniformly without major contamination. The TEM images of purified MWCNTs with different magnification were shown in Fig. 7. The MWCNTs produced by the CVD method contained impurities like metal catalysts, amorphous carbon and microcrystalline carbon, which normally make more complex for the detection of MWCNTs by TEM. The clear visible MWCNTs were observed after purification by acid treatment and air oxidation which indicates majority of the impurities were eliminated. The inner and outer diameter of the MWCNTs measured from HR-TEM observations were in the range of 5–6 and 12–13 nm respectively. In the present work, a very
Fig. 6 SEM images of a as-synthesised MWCNTs over 1 wt. % Ni/Cr-MCM-41(100) and b purified MWCNTs
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Fig. 7 HR-TEM images of purified MWCNTs with different magnification
G Band 2000
[002]
1582.6
24.95 100
80
D Band 1327.21
Intensity
Intensity
1500
1000
60
[100] 43.79
40
500 20
500
1000
1500
0
-1
Raman Shift (cm ) Fig. 8 Raman spectrum of purified MWCNTs
10
20
30
40
50
60
70
2θ (Degree) Fig. 9 XRD pattern of purified MWCNTs
less quantity of amorphous carbon was deposited on the surface of MWCNTs, because of the MWCNTs were prepared from C2H2 using active Ni/Cr-MCM-41. The MWCNTs exhibits a hollow core structured and ordered arrays after purification.
1,327 cm-1 corresponds to the D-band assigned to the A1g phonon. The absence of RBM below 500 cm-1 confirms the absence of SWCNTs in the synthesized carbon materials. The very high IG/ID values (IG/ID = ca. 2) obtained in the Raman spectrum clearly indicated that the synthesized CNTs possess high purity and well graphitized.
3.3.2 Raman spectroscopy study 3.3.3 X-ray diffraction pattern Raman spectroscopy is one of the most powerful tool for the characterization of CNTs. Figure 8 shows the two major peaks at 1,582 and 1,327 cm-1, which correspond to G-band (graphite band) and D-band (disorder band), respectively. The G-band observed at 1,582 cm-1 have been assigned to the Raman allowed phonon E2g (stretching) mode of graphite. The strongest peak of G-band in the spectrum indicates the formation of good arrangement of hexagonal lattice of graphite. The weak band at
XRD pattern of MWCNTs are shown in Fig. 9. The spectrum displayed a strong peak at 2h = 24.958 and weak peak at 2h = 43.798, which are assigned to (002) and (100) diffraction patterns of typical graphite, respectively. This result indicates that the MWCNTs were well graphitized [23]. The graphitized MWCNTs were confirmed by the absence of catalytic template and other impurity peaks in XRD pattern.
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Fig. 10 SEM images of f-MWCNTs with different magnification
(a)
Transmittance (a.u.)
(b)
(-C=O Group)
(-OH Group) 4000
3500
3000
2500
2000
1500
1000
500
-1
Wavenumber (cm ) Fig. 12 FT-IR spectra of a MWCNTs and b f-MWCNTs Fig. 11 TEM image of f-MWCNTs
3.4 Characterization of functionalized MWCNTs
the functional spectroscopy.
3.4.1 Surface and structural morphology studies
3.4.2 FT-IR spectroscopy
The SEM images of f-MWCNTs with different magnifications are shown in Fig. 10. In comparison with the nonfunctionalized MWCNTs (Fig. 6), the functionalized MWCNTs (f-MWCNTs) showed a different surface morphology. The non-functionalized MWCNTs have shining tube like appearance whereas a non-clear surface was observed for the later one, which might be due to functionalization. The insight morphology of f-MWCNTs is clearly observed by TEM image which is shown in Fig. 11. The outer graphene layer was functionalization after acid treatment was also observed from TEM image. In addition,
The FT-IR spectra of functionalized and non-functionalized MWCNTs are presented in Fig. 12. The bands at 3,420 and 1,650 cm-1 are due to the presence of hydroxyl and carbonyl groups respectively in f-MWCNTs (Fig. 12b), these bands are absent for non-functionalized MWCNTs in the spectrum (Fig. 12a). The band around 2,360 cm-1 is due to the adsorption of atmospheric CO2, which are found to be weak in the case of MWCNTs (Fig. 12a). The less intense band is due to the hydrophobic nature of non-functionalized MWCNTs which does not adsorb much of the atmospheric guest species. The
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hydrophilic nature of f-MWCNTs accounts for the greater adsorption of the guest species as the band is more strong and intense which is observed in Fig. 12a and b.
India, for providing Senior Research Fellow and financial support to carry out this work. The authors would like to thank the Department of Chemistry, Anna University, Chennai, India, for providing instrumentation facilities to carry out characterizations.
4 Conclusion
References
Tailored mesoporous MCM-41 catalytic templates were found to be highly ordered through various physicochemical techniques. The MWCNTs were catalytically synthesized by the decomposition of acetylene over 1 wt. % Ni/Cr-MCM-41 catalyst at 800 °C which showed the better formation when compared with Cr-MCM-41 and other wt. % Ni/Cr-MCM-41 by CVD method. The synthesis method is very simple and effective, since it involves the production of MWCNTs at low temperature (800 °C), low cost and can be industrially applicable. The formations of MWCNTs were observed from HR-TEM with inner and outer diameter of 5–6 and 12–13 nm, respectively and also the MWCNTs were observed from Raman spectrum. Raman IG/ID value (IG/ID = ca. 2) showed the high purity of MWCNTs. The XRD pattern also supports the formation of MWCNTs were graphitized. Catalytic particles were completely absent in MWCNTs after purification which was revealed by XRD pattern, SEM and TEM images. Further these purified MWCNTs were functionalized to form hydroxyl groups around (3,420 cm-1) and carbonyl groups around (1,650 cm-1) over them to enhance the thermo-mechanical applications of nanocomposites. This study proved that the 1 wt. % Ni/Cr-MCM-41 catalyst as a potential catalytic template with good thermal stability and high productivity for the synthesis of MWCNTs and also functionalization of MWCNTs. Thus, our investigation provides new catalytic method for the synthesis of MWCNTs followed by effective functionalization of them which can provide space for other applications.
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Acknowledgments The authors would like to thank the Department of Science and Technology (SR/S5/NM-35/2005) under Nanoscience and Technology Initiative, New Delhi, India, for providing financial support. One of the authors Mr. R. Atchudan would like to thank the Council of Scientific and Industrial Research (CSIR), New Delhi,
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