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Structural and surface modifications of carbon nanotubes when submitted to high temperature annealing treatments
Journal of Alloys and Compounds 536S (2012) S460–S463
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Structural and surface modifications of carbon nanotubes when submitted to high temperature annealing treatments E. Castillejos a , B. Bachiller-Baeza a,b , M. Pérez-Cadenas c , E. Gallegos-Suarez c , I. Rodríguez-Ramos a,b , A. Guerrero-Ruiz b,c , K. Tamargo-Martinez d,∗ , A. Martinez-Alonso d , J.M.D. Tascón d a
Instituto de Catálisis y Petroleoquímica, ICP-CSIC, Campus de Cantoblanco, 28046 Madrid, Spain Unidad Asociada UNED/ICP-CSIC Group for Molecular Design of Heterogeneous Catalysts, Madrid, Spain c Dpto. de Química Inorgánica y Técnica, UNED, 28040 Madrid, Spain d Instituto Nacional del Carbón, INCAR-CSIC, Apartado 73, 33080 Oviedo, Spain b
a r t i c l e
i n f o
Article history: Received 26 June 2011 Received in revised form 21 October 2011 Accepted 2 November 2011 Available online 9 November 2011 Keywords: Carbon nanotubes Annealing HRTEM Raman spectroscopy Immersion calorimetry
a b s t r a c t Multiwall carbon nanotubes (MWCNTs) were synthesized using a chemical vapour deposition procedure using acetylene as source of carbon, iron pentacarbonyl as catalyst and an inert carrier gas. An aliquot of these MWCNTs was heat-treated at 2873 K under inert atmosphere (Ar). The two carbon nanotube samples where characterized using high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy, nitrogen adsorption at 77 K, Raman spectroscopy, and immersion calorimetry in toluene, methanol and methylcyclohexane. HRTEM images confirmed that high-temperature treatment removed amorphous carbon, the graphene layers being better graphitized, and also some structural changes inside the cylindrical mesopores took place. Immersion enthalpies in toluene, in which molecules are present as aromatic functions, indicated the existence of specific – electronic interactions between such molecules and the surface of heat-treated MWCNTs. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Carbon nanotubes (CNTs) are an ideal nanoscale material that may be viewed as a cylindrical structure formed from graphene sheets and closed by fullerenoid end-caps. CNTs presents interesting and unusual properties [1,2]: (a) high purity, which eliminates self-poisoning; (b) impressive physical properties such as mechanical strength, high electrical conductivity and appropriate thermal stability; (c) the high accessibility of the active surface and the absence of any microporosity, thus eliminating diffusion and intraparticle mass transfer in the reactions medium; (d) the possibility for macroscopic shaping; (e) the possibility of tuning; and (f) the possibility of confinement effects in their inner cavity [3]. Furthermore, nitrogen adsorption studies performed on CNTs have highlighted the porous nature of these materials [4]. For multiwall CNTs (MWCNTs), the porosity can be mainly divided into inner hollow cavities of small diameter (mesopores) and external walls of aggregates formed by interaction of isolated MWCNTs (macropores). In general, the electron mobility in CNTs as well as the presence of structural defects, the helicity and the curvature of the surface of these materials, as well as the presence of an inner
∗ Corresponding author. E-mail address: [email protected] (K. Tamargo-Martinez). 0925-8388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2011.11.007
cavity are expected to affect the surface and adsorption properties of MWCNTs, possibly in a different manner if compared to activated carbon or graphite [5,6]. The above properties make MWCNTs promising candidates for applications based on the interaction of their surface with organic species, e.g., when used as reinforcement for polymers in composite materials [7], as adsorbents for organic molecules or as catalysts. Several studies suggest CNTs as effective adsorbents for organic chemicals in water treatment compared with other typical adsorbents such as activated carbon [8]. Moreover, the structure of CNTs is well defined and their surfaces are relatively uniform in contrast to of other carbon adsorbents such as activated carbons. Therefore, CNTs are considered to be a good alternative in environmental water treatments, and for these applications the hydrophilic–hydrophobic properties of the involved surfaces are key parameters. Thus, research in this area is necessary for a better understanding of organic chemical–CNT interactions, particularly by modifying the structures of the surfaces. Several studies were focused on theoretical aspects and only a limited number of studies have been conducted experimentally to prove the effect of CNT surface structures on the chemical interactions with organic species [9]. In this context, here we report the results of an experimental study where the magnitude of nonhydrophobic interaction between several organic molecules and CNTs has been determined using immersion calorimetry, a technique well known but which
E. Castillejos et al. / Journal of Alloys and Compounds 536S (2012) S460–S463
S461
Table 1 Porous texture characteristics of carbon nanotube samples. Samples
SBET (m2 /g)
Pore width (nm)
Dex (nm)
Din (nm)
CNT CNT-HT
57 54
9.9 9.6
24.5 17.5
8.7 6.5
has mot been previously used in this type of studies. Our results show the effects of high temperature treatments of MWCNTs on both their surface properties and their structural characteristics. An increase in the strength of the interaction between aromatic adsorbate and CNTs has been proved when high-temperature treatment removes defects and amorphous carbon from the CNT surface. 2. Experimental
Considering the fact tat the original CNT sample was prepared at 1023 K, the high temperature treatment at 2873 K, yielding sample CNT-HT, could be expected to give place to an elimination of structural defects of the carbon nanostructures and graphitization of the amorphous carbon phases. Indeed, HRTEM images, shown in Fig. 1, reveal that the high-temperature treated sample contains less amorphous carbon deposited on its surface. More significant is the fact that the graphene layers of this CNT-HT sample seem to be better graphitized, and also that some structural changes inside the cylindrical mesopores seem to take place. In short, CNT-HT appears poorer in surface impurities, more homogeneous structurally and with better aligned graphene layers. The SEM images (Fig. 1) show that the studied CNTs do not associate in bundles. Moreover, TEM images show that the tips at the end of the tubes for both samples are mostly closed. Next, the two materials have been characterized by N2 adsorption. The Brunauer, Emmett and Teller (BET) surface areas (SBET ) (Table 1) are similar to each other and evidence rather low values for carbon nanotubes. It can be observed that the adsorption isotherms of CNT and CNT-HT samples (Fig. 2) have nearly the
same shape, which is characteristic of mesoporous materials with cylindrical shapes. A slow increase in nitrogen uptake at low and intermediate relative pressures suggests a negligible presence of micropores in any of the two materials [12]. Adsorption data at intermediate relative pressures show a surface adsorption corresponding to monolayer formation, in which the nitrogen adsorption amount increases slowly. Finally, a sharp adsorption at high relative pressures, accompanied by hysteresis, is indicative of capillary
120
CNT CNT-HT
100 80
3
3. Results and discussion
Fig. 1. SEM (left) and TEM (right) micrographs of pristine CNT (a) and CNT-HT (b). High-resolution TEM micrographs of CNT (c) and CNT-HT (d).
cm /g
The MWCNTs studied here were synthesized at laboratory scale using a chemical vapour deposition (CVD) procedure consisting in co-feeding in an isothermal multitubular quartz reactor stabilized at 1023 K, acetylene, which is the reactant source of carbon, iron pentacarbonyl, which acts as catalyst to grow vertically aligned carbon nanotubes [10] and an inert carrier gas. This original material labelled as CNT was heat-treated at high temperature yielding the CNT-HT sample. This process was carried out in a graphite furnace at 2873 K, under argon flow (100 cm3 /min). The heating rates used were: 50 K/min from room temperature to 973 K; 100 K/min from 973 to 1273 K; 25 K/min between 1273 and 2273 K; and 10 K/min between 2273 and 2873 K. Once the desired temperature was reached, the samples were maintained at this temperature for 30 min and then were cooled to room temperature under the same argon flow. Transmission electron microscopy (TEM) micrographs were obtained in a JEOL JEM microscope at 200 kV, while scanning electron microscopy (SEM) images were obtained in a 650 FEG apparatus. This latter was operated at 10–20 kV, with the samples mounted by a double sided carbon tape prefixed on a SEM holder. N2 adsorption at 77 K was measured in an automatic Micromeritics ASAP 2010 volumetric system. Raman measurements were performed with a Jobin Yvon HR 800 micro-Raman spectrometer. The Raman spectra were obtained in air and at room temperature in the spectral range 900–3450 cm−1 . Acquisition time was 300 s and 8 spectra in different areas were recorded for each carbon nanostructure sample to guarantee a representative sampling. Deconvolution procedure was effectuated by LabSpec® software. Immersion enthalpies into several organic liquids were determined at 303 K with an isothermal calorimeter of the Tian-Calvet type, Setaram C-80, using homemade glass bulbs. The used chemicals were toluene and methanol (analytical reagent grade), and methylcyclohexane (puriss) from Fluka. Prior to the experiments, the samples were in situ outgassed at 383 K for 12 h. The experimental procedure is similar to that described in [11], with the corrections corresponding to the energies of bulb breaking and liquid vaporization having been considered. The immersion enthalpies were determined several times for each sample, and the dispersion of determinations (deviation errors) can be estimated as lower than 5%.
60 40 20 0.0
0.2
0.4
0.6
0.8
p/pº Fig. 2. N2 adsorption–desorption isotherms at 77 K.
1.0
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E. Castillejos et al. / Journal of Alloys and Compounds 536S (2012) S460–S463 Table 4 Enthalpies of adsorption of toluene, methanol and methylcyclohexane on the studied materials.
Table 2 Relative intensity ratios of Raman bands. Samples
ID /IG
ID /IG
ID /IG
CNT CNT-HT
0.50 0.12
0.27 0.04
0.28 <0.01
Samples
Toluene (J/g)
Methanol (J/g)
Methylcyclohexane (J/g)
CNT CNT-HT
17 24
16 14
15 13
G'
decreased significantly (by 75%) upon annealing (Table 2). From a qualitative viewpoint, this behavior matches up with a relative increase of the bands located in the second order frequency (Fig. 3). The peak centered at 2700 cm−1 is called G (or 2D), corresponds to the first overtone of the D band and is related to the threedimensional order (i.e. stacking of graphene layers). As Table 2 reveals, the ID /IG ratio in CNT-HT has dramatically decreased in regard to the starting material following the thermal treatment. The Raman bands placed at 2455, 2940 and 3245 cm−1 are usually weak for ordered carbon materials and the first two could be assigned to two-phonon double scattering Raman process resonance [15]. Table 3 reports the wavenumber (¯v) and full width at halfmaximum amplitude (W) for the G, D, D and G bands. Band positions did not significantly change following thermal treatment; the G band wavenumber was close in both cases to the ideal value for graphite. However, these bands significantly narrowed upon heat treatment of the studied MWCNTs (Table 3). According to these observations, Raman spectroscopy confirms an increase of the graphitic order, which is not unexpected since this treatment at 2873 K must have eliminated most of the structural and surface defects. Table 4 displays, for the two MWCNT samples, the immersion enthalpies into toluene, methanol and methylcyclohexane. For the starting CNT sample, the immersion enthalpy in toluene is slightly higher than that in methanol or methylcyclohexane. The immersion enthalpies in toluene increase following thermal treatment whereas those in methanol and methylcyclohexane remain approximately constant. As the difference in molecular dimensions between toluene (0.37 nm), methanol (0.41 nm) and even methylcyclohexane (0.48 nm) is small, this enhancement cannot be assigned to a higher accessibility of toluene to the porous structure; moreover, let us remind here that the studied materials have porosities restricted to the mesopore range (see Fig. 2 and Table 1). Characterization studies of CNTs have been used to interpret these immersion enthalpy data. The enthalpy of immersion in toluene is clearly higher for CNT-HT, a material that mostly consists of curved graphene layers (distorting the sp2 hybridization) without presence of amorphous carbon over those layers. It appears that clean layers could lightly modify the nature of the chemical interaction with the adsorbate. We can observe in Table 4 that immersion enthalpy values in toluene, a molecule in which a large –electron density is present, suggest the existence of specific – electronic interactions [16] with CNT-HT sample. This indicates that, when amorphous carbon is not present, direct – interactions can occur between the graphitic surface sheets and the –electron cloud of toluene molecule. Then, – interactions are more energetically favourable than those between non-aromatic molecules (methanol and methylcyclohexane) and the non-polar surface of mesoporous
G
Intensity (c_p_s)
CNT-HT
D
D'
CNT
1020
1530
2040
2550
3060
Wavenumber (cm-1)
Fig. 3. Raman spectra of CNT and CNT-HT samples.
condensation within large mesopores [13], which are constituted by aggregated pores formed by interaction of isolated CNTs [2]. Table 1 also provides the pore widths (obtained from the Barrett, Joyner and Halenda method, BJH), which are slightly smaller for CNT-HT, probably due to the removal of deposited amorphous carbon. The SEM and TEM images (Fig. 1) indicated that the internal pore is centered at the core of the CNTs, and that its diameter is large compared to the distance between graphene layers. From the TEM micrographs (Fig. 1), the average external (Dex ) and internal (Din ) diameters of both CNT and CNT-HT samples were determined (see Table 1). The latter data are in good agreement with those obtained from the adsorption isotherms (BJH method). Raman spectra (Fig. 3) show characteristic features of CNTs: a strong G band at ∼1580 cm−1 from the in-plane vibrations of the C–C bonds, and a large disorder-induced (D) band at ∼1350 cm−1 . The D-band is a typical sign for defective graphitic structures due to curvature effects in graphene layers and to the presence of pentagons or heptagons. The relative intensities of the D and G bands are well-known indicators for the extent of structural disorder [14]. The degree of graphitic disorder was estimated from the ID /IG ratio, for which the first-order Raman spectrum was fitted to a Lorentzian sum of the I, D, G, D and D bands. The contribution of D and D bands as determined from the ID /IG and ID /IG parameters has
Table 3 Wavenumber (¯v) and full width at half-maximum amplitude (W), both in cm−1 , for the G, D, D and G bands of Raman spectra. Samples
CNT CNT-HT
G band
D band
D band
G band
v¯
W
v¯
W
v¯
W
v¯
W
1584.0 1583.3
36.3 25.5
1352.3 1353.3
48.8 31.2
1617.4 1624.7
17.1 6.4
2697.0 2697.6
60.2 47.6
E. Castillejos et al. / Journal of Alloys and Compounds 536S (2012) S460–S463
CNTs. For small diameter carbon nanotubes, due to the increased curvature, a local sp2 –sp3 transition of the hybridization of the carbon atoms in contact with the aromatic adsorbates forming near covalent bonds could be proposed [17]. However in our case, where the diameter of the mesopores is one order of magnitude higher than the diameters of the used probe molecules, these effects can be excluded.
4. Conclusions Very high-temperature annealing of MWCNTs not only modifies their structural features, eliminating defects and impurities, but also changes their surface chemical properties. This latter effect is revealed by an improvement of the – interactions, as detected by immersion calorimetry when MWCNTs interact with aromatic organic molecules. MWCNTs exhibit high-density structural imperfections when prepared via chemical vapor deposition at relatively low temperatures. Thermal annealing process has shown to reduce these defects as well as amorphous carbon species on the CNTs. The annealed MWCNTs were structurally characterized by Raman spectroscopy and transmission electron microscopy. Heating rearranged the imperfect graphitic sheet structure and removed both the weakly bonded defects as well as amorphous carbon. Thus, the availability of surface sites for organic chemical adsorption on CNTs, as detected by immersion calorimetry, is highly dependent on the presence of amorphous carbon.
S463
References [1] P. Serp, in: P. Serp, J.L. Figueiredo (Eds.), Carbon Materials for Catalysis, Wiley & Sons, Hoboken, NJ, USA, 2009, pp. 309–372. [2] E. Castillejos, P. Serp, in D.M. Guldi, N. Martín (Eds.), Carbon Nanotubes and Related Structures: Synthesis, Characterization, Functionalization, and Applications, Wiley-VCH, Weinheim, Germany, Ch. 11, doi:10.1002/9783527629930. [3] E. Castillejos, P.J. Debouttière, L. Roiban, A. Solhy, V. Martinez, Y. Kihn, O. Ersen, K. Philippot, B. Chaudret, P. Serp, Angew. Chem. Int. Ed. 48 (2009) 2529–2533. [4] S. Inoue, N. Ichikuni, T. Suzuki, T. Uematsu, K. Kaneko, J. Phys. Chem. B 102 (1998) 4689–4692. [5] K. Masenelli-Varlot, E. McRae, N. Dupont-Pavlovsky, Appl. Surf. Sci. 196 (2002) 209–215. [6] J. Hilding, E.A. Grulke, S.B. Sinnott, D. Qian, R. Andrews, M. Jagtoyen, Langmuir 17 (2001) 7540–7544. [7] S.W. Kim, T. Kim, Y.S. Kim, H.S. Choi, H.J. Lim, S.J. Yang, C.R. Park, Carbon 50 (2012) 3–33. [8] R.Q. Long, R.T. Yang, J. Am. Chem. Soc. 123 (2001) 2058–2059. [9] B. Pan, B. Xing, Environ. Sci. Technol. 42 (2008) 9005–9013. ˜ [10] P. Sampedro-Tejedor, A. Maroto-Valiente, D.M. Nevskaia, V. Munoz, I. Rodríguez-Ramos, A. Guerrero-Ruiz, Diamond Relat. Mater. 16 (2007) 542–549. [11] R. Denoyel, J. Fernandez-Colinas, Y. Grillet, J. Rouquerol, Langmuir 9 (1993) 515–518. [12] M. Eswaramoorthy, R. Sen, C.N.R. Rao, Chem. Phys. Lett. 304 (1999) 207–210. [13] S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, Academic Press, New York, 1982. [14] A. Cuesta, P. Dhamelincourt, J. Laureyns, A. Martínez-Alonso, J.M.D. Tascón, Carbon 32 (1994) 1523–1532. [15] T. Shimada, T. Sugai, C. Fantini, M. Souza, L.G. Canc¸ado, A. Jorio, M.A. Pimenta, R. Saito, A. Grüneis, G. Dresselhaus, M.S. Dresselhaus, Y. Ohno, T. Mizutani, H. Shinohara, Carbon 43 (2005) 1049–1054. ˜ I. Rodríguez-Ramos, A. Guerrero[16] E. Castillejos-López, D.M. Nevskaia, V. Munoz, Ruiz, Langmuir 20 (2004) 1013–1015. [17] M. Menon, A.N. Andriotis, G.E. Froudakis, Chem. Phys. Lett. 320 (2000) 425–434.
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Structural and surface modifications of carbon nanotubes when submitted to high temperature annealing treatments E. Castillejos a , B. Bachiller-Baeza a,b , M. Pérez-Cadenas c , E. Gallegos-Suarez c , I. Rodríguez-Ramos a,b , A. Guerrero-Ruiz b,c , K. Tamargo-Martinez d,∗ , A. Martinez-Alonso d , J.M.D. Tascón d a
Instituto de Catálisis y Petroleoquímica, ICP-CSIC, Campus de Cantoblanco, 28046 Madrid, Spain Unidad Asociada UNED/ICP-CSIC Group for Molecular Design of Heterogeneous Catalysts, Madrid, Spain c Dpto. de Química Inorgánica y Técnica, UNED, 28040 Madrid, Spain d Instituto Nacional del Carbón, INCAR-CSIC, Apartado 73, 33080 Oviedo, Spain b
a r t i c l e
i n f o
Article history: Received 26 June 2011 Received in revised form 21 October 2011 Accepted 2 November 2011 Available online 9 November 2011 Keywords: Carbon nanotubes Annealing HRTEM Raman spectroscopy Immersion calorimetry
a b s t r a c t Multiwall carbon nanotubes (MWCNTs) were synthesized using a chemical vapour deposition procedure using acetylene as source of carbon, iron pentacarbonyl as catalyst and an inert carrier gas. An aliquot of these MWCNTs was heat-treated at 2873 K under inert atmosphere (Ar). The two carbon nanotube samples where characterized using high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy, nitrogen adsorption at 77 K, Raman spectroscopy, and immersion calorimetry in toluene, methanol and methylcyclohexane. HRTEM images confirmed that high-temperature treatment removed amorphous carbon, the graphene layers being better graphitized, and also some structural changes inside the cylindrical mesopores took place. Immersion enthalpies in toluene, in which molecules are present as aromatic functions, indicated the existence of specific – electronic interactions between such molecules and the surface of heat-treated MWCNTs. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Carbon nanotubes (CNTs) are an ideal nanoscale material that may be viewed as a cylindrical structure formed from graphene sheets and closed by fullerenoid end-caps. CNTs presents interesting and unusual properties [1,2]: (a) high purity, which eliminates self-poisoning; (b) impressive physical properties such as mechanical strength, high electrical conductivity and appropriate thermal stability; (c) the high accessibility of the active surface and the absence of any microporosity, thus eliminating diffusion and intraparticle mass transfer in the reactions medium; (d) the possibility for macroscopic shaping; (e) the possibility of tuning; and (f) the possibility of confinement effects in their inner cavity [3]. Furthermore, nitrogen adsorption studies performed on CNTs have highlighted the porous nature of these materials [4]. For multiwall CNTs (MWCNTs), the porosity can be mainly divided into inner hollow cavities of small diameter (mesopores) and external walls of aggregates formed by interaction of isolated MWCNTs (macropores). In general, the electron mobility in CNTs as well as the presence of structural defects, the helicity and the curvature of the surface of these materials, as well as the presence of an inner
∗ Corresponding author. E-mail address: [email protected] (K. Tamargo-Martinez). 0925-8388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2011.11.007
cavity are expected to affect the surface and adsorption properties of MWCNTs, possibly in a different manner if compared to activated carbon or graphite [5,6]. The above properties make MWCNTs promising candidates for applications based on the interaction of their surface with organic species, e.g., when used as reinforcement for polymers in composite materials [7], as adsorbents for organic molecules or as catalysts. Several studies suggest CNTs as effective adsorbents for organic chemicals in water treatment compared with other typical adsorbents such as activated carbon [8]. Moreover, the structure of CNTs is well defined and their surfaces are relatively uniform in contrast to of other carbon adsorbents such as activated carbons. Therefore, CNTs are considered to be a good alternative in environmental water treatments, and for these applications the hydrophilic–hydrophobic properties of the involved surfaces are key parameters. Thus, research in this area is necessary for a better understanding of organic chemical–CNT interactions, particularly by modifying the structures of the surfaces. Several studies were focused on theoretical aspects and only a limited number of studies have been conducted experimentally to prove the effect of CNT surface structures on the chemical interactions with organic species [9]. In this context, here we report the results of an experimental study where the magnitude of nonhydrophobic interaction between several organic molecules and CNTs has been determined using immersion calorimetry, a technique well known but which
E. Castillejos et al. / Journal of Alloys and Compounds 536S (2012) S460–S463
S461
Table 1 Porous texture characteristics of carbon nanotube samples. Samples
SBET (m2 /g)
Pore width (nm)
Dex (nm)
Din (nm)
CNT CNT-HT
57 54
9.9 9.6
24.5 17.5
8.7 6.5
has mot been previously used in this type of studies. Our results show the effects of high temperature treatments of MWCNTs on both their surface properties and their structural characteristics. An increase in the strength of the interaction between aromatic adsorbate and CNTs has been proved when high-temperature treatment removes defects and amorphous carbon from the CNT surface. 2. Experimental
Considering the fact tat the original CNT sample was prepared at 1023 K, the high temperature treatment at 2873 K, yielding sample CNT-HT, could be expected to give place to an elimination of structural defects of the carbon nanostructures and graphitization of the amorphous carbon phases. Indeed, HRTEM images, shown in Fig. 1, reveal that the high-temperature treated sample contains less amorphous carbon deposited on its surface. More significant is the fact that the graphene layers of this CNT-HT sample seem to be better graphitized, and also that some structural changes inside the cylindrical mesopores seem to take place. In short, CNT-HT appears poorer in surface impurities, more homogeneous structurally and with better aligned graphene layers. The SEM images (Fig. 1) show that the studied CNTs do not associate in bundles. Moreover, TEM images show that the tips at the end of the tubes for both samples are mostly closed. Next, the two materials have been characterized by N2 adsorption. The Brunauer, Emmett and Teller (BET) surface areas (SBET ) (Table 1) are similar to each other and evidence rather low values for carbon nanotubes. It can be observed that the adsorption isotherms of CNT and CNT-HT samples (Fig. 2) have nearly the
same shape, which is characteristic of mesoporous materials with cylindrical shapes. A slow increase in nitrogen uptake at low and intermediate relative pressures suggests a negligible presence of micropores in any of the two materials [12]. Adsorption data at intermediate relative pressures show a surface adsorption corresponding to monolayer formation, in which the nitrogen adsorption amount increases slowly. Finally, a sharp adsorption at high relative pressures, accompanied by hysteresis, is indicative of capillary
120
CNT CNT-HT
100 80
3
3. Results and discussion
Fig. 1. SEM (left) and TEM (right) micrographs of pristine CNT (a) and CNT-HT (b). High-resolution TEM micrographs of CNT (c) and CNT-HT (d).
cm /g
The MWCNTs studied here were synthesized at laboratory scale using a chemical vapour deposition (CVD) procedure consisting in co-feeding in an isothermal multitubular quartz reactor stabilized at 1023 K, acetylene, which is the reactant source of carbon, iron pentacarbonyl, which acts as catalyst to grow vertically aligned carbon nanotubes [10] and an inert carrier gas. This original material labelled as CNT was heat-treated at high temperature yielding the CNT-HT sample. This process was carried out in a graphite furnace at 2873 K, under argon flow (100 cm3 /min). The heating rates used were: 50 K/min from room temperature to 973 K; 100 K/min from 973 to 1273 K; 25 K/min between 1273 and 2273 K; and 10 K/min between 2273 and 2873 K. Once the desired temperature was reached, the samples were maintained at this temperature for 30 min and then were cooled to room temperature under the same argon flow. Transmission electron microscopy (TEM) micrographs were obtained in a JEOL JEM microscope at 200 kV, while scanning electron microscopy (SEM) images were obtained in a 650 FEG apparatus. This latter was operated at 10–20 kV, with the samples mounted by a double sided carbon tape prefixed on a SEM holder. N2 adsorption at 77 K was measured in an automatic Micromeritics ASAP 2010 volumetric system. Raman measurements were performed with a Jobin Yvon HR 800 micro-Raman spectrometer. The Raman spectra were obtained in air and at room temperature in the spectral range 900–3450 cm−1 . Acquisition time was 300 s and 8 spectra in different areas were recorded for each carbon nanostructure sample to guarantee a representative sampling. Deconvolution procedure was effectuated by LabSpec® software. Immersion enthalpies into several organic liquids were determined at 303 K with an isothermal calorimeter of the Tian-Calvet type, Setaram C-80, using homemade glass bulbs. The used chemicals were toluene and methanol (analytical reagent grade), and methylcyclohexane (puriss) from Fluka. Prior to the experiments, the samples were in situ outgassed at 383 K for 12 h. The experimental procedure is similar to that described in [11], with the corrections corresponding to the energies of bulb breaking and liquid vaporization having been considered. The immersion enthalpies were determined several times for each sample, and the dispersion of determinations (deviation errors) can be estimated as lower than 5%.
60 40 20 0.0
0.2
0.4
0.6
0.8
p/pº Fig. 2. N2 adsorption–desorption isotherms at 77 K.
1.0
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E. Castillejos et al. / Journal of Alloys and Compounds 536S (2012) S460–S463 Table 4 Enthalpies of adsorption of toluene, methanol and methylcyclohexane on the studied materials.
Table 2 Relative intensity ratios of Raman bands. Samples
ID /IG
ID /IG
ID /IG
CNT CNT-HT
0.50 0.12
0.27 0.04
0.28 <0.01
Samples
Toluene (J/g)
Methanol (J/g)
Methylcyclohexane (J/g)
CNT CNT-HT
17 24
16 14
15 13
G'
decreased significantly (by 75%) upon annealing (Table 2). From a qualitative viewpoint, this behavior matches up with a relative increase of the bands located in the second order frequency (Fig. 3). The peak centered at 2700 cm−1 is called G (or 2D), corresponds to the first overtone of the D band and is related to the threedimensional order (i.e. stacking of graphene layers). As Table 2 reveals, the ID /IG ratio in CNT-HT has dramatically decreased in regard to the starting material following the thermal treatment. The Raman bands placed at 2455, 2940 and 3245 cm−1 are usually weak for ordered carbon materials and the first two could be assigned to two-phonon double scattering Raman process resonance [15]. Table 3 reports the wavenumber (¯v) and full width at halfmaximum amplitude (W) for the G, D, D and G bands. Band positions did not significantly change following thermal treatment; the G band wavenumber was close in both cases to the ideal value for graphite. However, these bands significantly narrowed upon heat treatment of the studied MWCNTs (Table 3). According to these observations, Raman spectroscopy confirms an increase of the graphitic order, which is not unexpected since this treatment at 2873 K must have eliminated most of the structural and surface defects. Table 4 displays, for the two MWCNT samples, the immersion enthalpies into toluene, methanol and methylcyclohexane. For the starting CNT sample, the immersion enthalpy in toluene is slightly higher than that in methanol or methylcyclohexane. The immersion enthalpies in toluene increase following thermal treatment whereas those in methanol and methylcyclohexane remain approximately constant. As the difference in molecular dimensions between toluene (0.37 nm), methanol (0.41 nm) and even methylcyclohexane (0.48 nm) is small, this enhancement cannot be assigned to a higher accessibility of toluene to the porous structure; moreover, let us remind here that the studied materials have porosities restricted to the mesopore range (see Fig. 2 and Table 1). Characterization studies of CNTs have been used to interpret these immersion enthalpy data. The enthalpy of immersion in toluene is clearly higher for CNT-HT, a material that mostly consists of curved graphene layers (distorting the sp2 hybridization) without presence of amorphous carbon over those layers. It appears that clean layers could lightly modify the nature of the chemical interaction with the adsorbate. We can observe in Table 4 that immersion enthalpy values in toluene, a molecule in which a large –electron density is present, suggest the existence of specific – electronic interactions [16] with CNT-HT sample. This indicates that, when amorphous carbon is not present, direct – interactions can occur between the graphitic surface sheets and the –electron cloud of toluene molecule. Then, – interactions are more energetically favourable than those between non-aromatic molecules (methanol and methylcyclohexane) and the non-polar surface of mesoporous
G
Intensity (c_p_s)
CNT-HT
D
D'
CNT
1020
1530
2040
2550
3060
Wavenumber (cm-1)
Fig. 3. Raman spectra of CNT and CNT-HT samples.
condensation within large mesopores [13], which are constituted by aggregated pores formed by interaction of isolated CNTs [2]. Table 1 also provides the pore widths (obtained from the Barrett, Joyner and Halenda method, BJH), which are slightly smaller for CNT-HT, probably due to the removal of deposited amorphous carbon. The SEM and TEM images (Fig. 1) indicated that the internal pore is centered at the core of the CNTs, and that its diameter is large compared to the distance between graphene layers. From the TEM micrographs (Fig. 1), the average external (Dex ) and internal (Din ) diameters of both CNT and CNT-HT samples were determined (see Table 1). The latter data are in good agreement with those obtained from the adsorption isotherms (BJH method). Raman spectra (Fig. 3) show characteristic features of CNTs: a strong G band at ∼1580 cm−1 from the in-plane vibrations of the C–C bonds, and a large disorder-induced (D) band at ∼1350 cm−1 . The D-band is a typical sign for defective graphitic structures due to curvature effects in graphene layers and to the presence of pentagons or heptagons. The relative intensities of the D and G bands are well-known indicators for the extent of structural disorder [14]. The degree of graphitic disorder was estimated from the ID /IG ratio, for which the first-order Raman spectrum was fitted to a Lorentzian sum of the I, D, G, D and D bands. The contribution of D and D bands as determined from the ID /IG and ID /IG parameters has
Table 3 Wavenumber (¯v) and full width at half-maximum amplitude (W), both in cm−1 , for the G, D, D and G bands of Raman spectra. Samples
CNT CNT-HT
G band
D band
D band
G band
v¯
W
v¯
W
v¯
W
v¯
W
1584.0 1583.3
36.3 25.5
1352.3 1353.3
48.8 31.2
1617.4 1624.7
17.1 6.4
2697.0 2697.6
60.2 47.6
E. Castillejos et al. / Journal of Alloys and Compounds 536S (2012) S460–S463
CNTs. For small diameter carbon nanotubes, due to the increased curvature, a local sp2 –sp3 transition of the hybridization of the carbon atoms in contact with the aromatic adsorbates forming near covalent bonds could be proposed [17]. However in our case, where the diameter of the mesopores is one order of magnitude higher than the diameters of the used probe molecules, these effects can be excluded.
4. Conclusions Very high-temperature annealing of MWCNTs not only modifies their structural features, eliminating defects and impurities, but also changes their surface chemical properties. This latter effect is revealed by an improvement of the – interactions, as detected by immersion calorimetry when MWCNTs interact with aromatic organic molecules. MWCNTs exhibit high-density structural imperfections when prepared via chemical vapor deposition at relatively low temperatures. Thermal annealing process has shown to reduce these defects as well as amorphous carbon species on the CNTs. The annealed MWCNTs were structurally characterized by Raman spectroscopy and transmission electron microscopy. Heating rearranged the imperfect graphitic sheet structure and removed both the weakly bonded defects as well as amorphous carbon. Thus, the availability of surface sites for organic chemical adsorption on CNTs, as detected by immersion calorimetry, is highly dependent on the presence of amorphous carbon.
S463
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