- Email: [email protected]
Functionalization of multi-walled carbon nanotubes using water-assisted chemical vapor deposition
Journal of Solid State Chemistry 197 (2013) 517–522
Contents lists available at SciVerse ScienceDirect
Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc
Functionalization of multi-walled carbon nanotubes using water-assisted chemical vapor deposition Maofei Ran a,b, Wenjing Sun a, Yan Liu b,n, Wei Chu a,1, Chengfa Jiang a a b
College of Chemical Engineering, Sichuan University, Chengdu, Sichuan 610065, People’s Republic of China Institute of Chemical and Engineering Sciences (ICES), AnSTAR, 627833 Singapore, Singapore
a r t i c l e i n f o
abstract
Article history: Received 24 April 2012 Received in revised form 23 July 2012 Accepted 5 August 2012 Available online 15 August 2012
A simple and novel method, water-assisted chemical vapor deposition (CVD) was developed to functionalize multi-walled carbon nanotubes (MWCNTs) during the synthesis process. The functionalized MWCNTs were characterized using Raman spectroscopy, XPS, TGA, NH3-TPD, SEM and HR-TEM. It was found that new defects are introduced and the amount of acidic groups is increased on the MWCNT surface during the water-assisted CVD process. The amount of C–OH and C–O group on the MWCNT surface is found to be increased from 21.1% to 42% with water vapor assistance. Density functional theory (DFT) was employed to study the chemical behavior of water vapor molecule on the catalyst particle surface of Ni(1 1 1) cluster. Based on the experimental and DFT simulation results, a mechanism for functionalization of MWCNTs by water-assisted CVD is proposed. & 2012 Elsevier Inc. All rights reserved.
Keywords: Multi-walled carbon nanotubes Water vapor Interface functionalization DFT calculation
1. Introduction Carbon nanotubes (CNTs) are regarded as one of the most promising materials for specific applications [1–3] because of their extraordinary mechanical, electrical and thermal properties [4–7]. Chemical vapor deposition (CVD) is widely used to prepare CNTs, but the as-grown CNTs are often accompanied by amorphous carbon generation and incorporation of metallic particles. Moreover, due to their high hydrophobicity, the CNTs are prone to aggregation and precipitation in water. These properties largely limit the application of CNTs in catalysis areas, thus it is necessary to modify the raw CNTs before application. Generally, there are several approaches to modify CNTs, such as strong acid oxidation [8–10] and plasma treatment [11]. Defects and polar groups are introduced during the process of modification, which could improve the dispersion of metal particles and enhance the interaction between CNT surface and metal particles [12–16]. However, these methods are prone to generate liquid wastes and their procedures are complex, which greatly limit their further development for applications. Here, we developed a easy and green method, water-assisted CVD, to modify multi-walled carbon nanotubes (MWCNTs) during the preparation process. The water-assisted CVD has been applied to efficiently synthesis of impurity-free single-walled carbon nanotubes [17]. Hata and Amama reported that water vapor can enhance the activity and lifetime of the catalyst during water-assisted CVD [17,18].
n
Corresponding author. Fax: þ65 63166182. E-mail addresses: [email protected] (Y. Liu), [email protected] (W. Chu). 1 Fax: þ86 28 85461108. 0022-4596/$ - see front matter & 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jssc.2012.08.014
Wong reported that aligned carbon nanotube stacks with opened end caps can be synthesized by water-assisted CVD [19]. Hu and Yoshihara reported that water vapor affects the diameter distribution of SWNTs and the surface of Fe–Mo/MgO catalysts during the water-assisted CVD [20,21]. In this work, the systematic research of the influence of water vapor on the structure and surface chemistry of MWCNTs in the water-assisted CVD has been investigated. The interactions between water vapor and the surfaces of MWCNTs and surfaces of catalyst particles were investigated in detail. The functionalized MWCNTs were characterized by X-ray photoelectron spectroscopy (XPS), Raman spectra, NH3 temperature programmed desorption (NH3TPD), thermal gravimetry analysis and differential thermogravimetry (TGA–DTG), scanning transmission electron (SEM) and high resolution transmission electron (HR-TEM) microscopy. Density functional theory (DFT) calculation of the dissociation products of water molecule on the surface of Ni(1 1 1) was also carried out. A mechanism for the influence of water is proposed, based on the experimental and the theoretical results.
2. Experimental 2.1. Synthesis of the MWCNTs The synthesis catalyst (Ni–Mn/MgO) was prepared by citric acid combustion as reported previously [22]. The pristine MWCNTs (MWCNTs-p) is used as a reference, which was synthesized by CVD in a quartz tube reactor (i.d. 2.5 cm, length 80 cm) laid in a horizontal furnace with a thermocouple in its central zone. The
M. Ran et al. / Journal of Solid State Chemistry 197 (2013) 517–522
catalyst loaded in a ceramic boat was reduced at 550 1C in H2 gas for 1 h. Subsequently, the temperature was raised to 650 1C, and methane was fed at a total flow rate of 110 ml/min. After 1 h of reaction, gas was switched to Ar (100 ml/min) during cooling. The product was collected and labeled as ‘‘MWCNTs-p’’. Functionalization of MWCNTs by water-assisted CVD was carried out using the same hydrocarbon gas and flowrate. However, the methane gas was passed through a water bubbler to produce water vapor before entering the reactor. The bubbler was maintained at 0 1C to achieve a stable concentration of water vapor. The synthesized product was labeled as ‘‘MWCNTs-H2O’’. The yields of MWCNTs-p and MWCNTsH2O were recorded, which are 762.8% and 973.4%, respectively. A portion of MWCNTs-p was further purified in 50 ml of concentrated HNO3 (68 wt%) refluxing at 140 1C for 12 h. After cooling to room temperature, it was filtered and washed with deionized water until the pH of the filtrate was around 7. Finally, the product was dried at 60 1C for 12 h and labeled as ‘‘MWCNTs-HNO3’’. 2.2. Characterization The as-prepared samples of MWCNTs were characterized using a series of techniques. HR-TEM images were obtained from a JEOL JEM-2000 FX microscope at 200 kV. SEM images were obtained on a JEOL JSM-6700F system. Room temperature microRaman scattering analyses were carried out with a Renishaw spectrometer. NH3-TPD was performed on thermo TPD/R/O 1100 catalysts analyzer instrument. XPS results were recorded on the XSAM800 spectrometer with an Al Ka (hn ¼1486.6 eV) X-ray source. TGA was performed to characterize their decomposition temperature and rate. Air was used as the carrier gas for burning the samples with a heating rate of 10 1C/min. 2.3. DFT calculation method The DFT calculations were performed with the Dmol3 program implemented within the Materials Studio of Accelrys, Inc. [23]. Exchange-correlation effects were described by the generalized approximation (GGA) developed by eight gradient-corrected functionals (PW91) [24]. The optimized geometries were also subjected to full frequency analysis at the same GGA/PW91 level of theory to certify the nature of the stationary point. The nudged elastic band was adopted to find the minimum energy path (MEP). Transition state (TS) searches were performed at the same theoretical level with the complete linear synchronous transit and quadratic synchronous transition search methods [25], then the TS confirmation tool was applied to map the MEP between reactants, intermediate and products [26]. Calculations were performed on 3 3 surface unit cells with the last layer totally relaxed. The inner-shell electron of the Ni atom was replaced by the effective core potentials with a basis of double numerical plus polarization. The density convergence criterion for self-consistent field optimization was 1.0000e–6. The Brillouin zone was sampled with a G-centered 3 3 1 Monkhorst–Pack k-point mesh. The smearing was set with 0.005 Ha. Adsorbate binding energies were defined as EslabþEadsorbate–Eadsorbateþslab, where Eadsorbateþslab was the total energy of the adsorbed configuration, Eslab was the total energy of the clean Ni(1 1 1) slab, and Eadsorbate was the adsorbate energy in the gas phase.
G D Intensity (a.u.)
518
D′ ID /IG= 0.709
a
ID /IG= 0.848
b
ID /IG= 1.118 400
800
c 1200
1600
2000
Raman shfit (cm-1) Fig. 1. Raman spectra of the different MWCNT samples. ((a) MWCNTs-p, (b) MWCNTs-H2O, (c) MWCNTs-HNO3).
main bands at 1341 and 1570 cm 1 are called D and G bands, respectively [27–29]. The D band and D0 band in the Raman spectra of CNTs are induced by disorder in a double resonance process and the G band at related to the graphite tangential E2g Raman active mode [27]. Accordingly, the intensity ratio of ID/IG is a commonly used qualitative measurement for defect density, which provides information about the crystallinity of MWCNTs [28]. From the results presented in Fig. 1, the ID/IG ratio of the MWCNTs-p is 0.709. This indicates a low quantity of structural defects in the graphitic structure of MWCNTs, which can provide active sites for further functionalization [29]. As observed in Fig. 1, the ID/IG of the MWCNTs-H2O and MWCNTs-HNO3 are improved to 0.848 and 1.118, respectively. The changes in ID/IG ratio could be due to the introduction of new defects as well as changes in the geometry of MWCNTs. The defects density of MWCNTs-H2O was much lower than that of MWCNTs-HNO3 due to the weak oxidation of water vapor. 3.1.2. TGA and DTG analysis Fig. 2 shows the TGA and DTG curves of the MWCNTs-p, MWCNTs-H2O and MWCNTs-HNO3. From the TGA curves, the initial burning temperature of MWCNTs-p at 539 1C is observed. With water vapor assistance, the initial burning temperature of MWCNTs-H2O is decreased to 468 1C. It further lowered down to 453 1C for MWCNTs-HNO3. The initial burning temperature of the sample in TGA can be served as a measure of thermal stability of nanotubes in air [30,31]. The thermal stabilities of MWCNTs-H2O and MWCNTs-HNO3 are lower than that of MWCNTs-p due to the more defects are created during the water-assisted CVD and post oxidation process, respectively. From the DTG curves (Fig. 2b) of the MWCNTs-HNO3 and MWCNTs-H2O, we can see that multi-stepwise weight losses correspond to the oxidation temperatures of the fractions of MWCNTs with different defect degree [32,33]. Particularly in the DTG curve of MWCNTs-H2O, the main combustion regime is split according to the introduction of more amounts of defects on the surface of MWCNTs [34]. The data consistently implied that carbon nanotubes have inferior crystallinity could be due to more amount of defect sites were introduced by adding water vapor in the CVD, corresponding to the results of Raman analysis.
3. Results and discussion 3.1. Surface structure and chemical bonding of MWCNTs 3.1.1. Raman spectra The Raman spectra of MWCNTs-p, MWCNTs-H2O and MWCNTsHNO3 excited with a 514.5 nm laser line are shown in Fig. 1. The two
3.1.3. NH3-TPD analysis The structural defects in the graphitic structure of MWCNTs can provide active sites to introduce polar groups on the surface [29] which can be reflected in the acidity of MWCNTs. NH3-TPD was performed to inspect the difference in acidity of MWCNT samples. As shown in Fig. 3, the NH3 desorption peak around
M. Ran et al. / Journal of Solid State Chemistry 197 (2013) 517–522
MWCNTs-p MWCNTs-H2O MWCNTs-HNO3
100
519
MWCNTs-p
102 60
453°C 100 539°C
40
468°C
98 96
20
440
480
520
560
MWCNTs-H2O
Intensity (a.u.)
Weight (%)
80
MWCNTs-HNO3
600
0 100
200
300
400 500 600 700 Temperature / (°C)
800
900
0
100
200
500 300 400 Temperature / (°C)
600
700
800
Fig. 3. NH3-TPD profiles of the different MWCNT samples.
1.2
MWCNTs-p
Deriv. Weight (%/°C)
MWCNTs-H2O MWCNTs-HNO3 0.6
0.0
0
100
200
300
400 500 600 Temperature / (°C)
700
800
900
Fig. 2. TG (a) and DTG (b) curves of different MWCNT samples.
(285.3 eV), C–O (286.8 eV), COOH (289.6 eV), and p–pn transition (290.7 eV), respectively [30,38]. As shown in Fig. 4a and b, the peak intensities of functional groups on the surfaces of MWCNTsH2O increase significantly in C1s spectra in compared with MWCNTs-p. The fractions of combined C–OH and C–O groups in MWCNTs-H2O increase to 30.27% and 11.25% (Table 1b), indicating that the SP2 carbon hybridization transformed into the SP3 mode in the water-assisted CVD [39]. These verify that functional groups were grafted on the surfaces of MWCNTs-H2O. The increase in fraction of functional groups is also observed in MWCNTs-HNO3 (Table 1c). However, it is notable that the COOH groups instead of the featured p–pn transition band appeared in the MWCNTs-HNO3 (Fig. 4c), implying that the conductive p network along the MWCNTs was disrupted during the nitric acid treatment process [21]. However, the oxidant of water vapor would not disrupt the conductive p network along MWCNTs (Fig. 2b). 3.2. Morphology of MWCNTs
200 1C is ascribed to weak acidic sites and another broad peak at 450–700 1C is ascribed to the generation of medium-strength acidic sites on the MWCNT surface [35]. A small and broad peak of NH3 desorption is observed in MWCNTs-p, indicating a few acidic groups existed on the nanotube surface due to formation of defects in the MWCNT growth process. Compared with MWCNTs-p, the peak areas of NH3 desorption in MWCNTs-H2O increased at both low and high temperature, indicating the amount of acidic sites was increased during the water-assisted CVD, especially in the weak acidic sites. Such improvements of peak areas verify that polar groups can be introduced on the MWCNT surface during the waterassisted CVD. The main desorption peak in MWCNTs-HNO3 appears at 600–800 1C and is much larger than that of the other two samples, indicating that a large number of strong acidic groups were generated on the MWCNT surface after treatment in concentrated HNO3 [36].
3.1.4. XPS analysis The surface chemical bonding of MWCNTs was detected by XPS analysis, as shown in Fig. 4. The relative percentages of surface functionalized groups were determined through curve fitting of high-resolution C1s XPS spectra [37], as listed in Table 1. The main binding-energy peak (284.5 eV) is attributed to the C–C bond, while the four remaining peaks are assigned to C–OH
The morphological variation of the MWCNT samples was observed by using SEM. As shown in Fig. 5a, the nanotubes of MWCNTs-p are covered with a number of accumulations of amorphous carbon and catalytic particles (pointed out by red arrows). With water assistance (Fig. 5b), the nanotubes of MWCNTs-H2O appear to be thicker and no significant coverage of amorphous carbon entities compared with the MWCNTs-p. HR-TEM was used to give higher resolution observations of surface structure variation of the MWCNTs-p, MWCNTs-H2O, and MWCNTs-HNO3. In Fig. 6a, well-graphitized walls of nanotubes can be observed in MWCNTs-p. With water vapor assistance and after HNO3 treatment, the HR-TEM images in Fig. 6b and c present the damaged graphite sheets and new defects (pointed out by red arrows) on the surface of MWCNTs. Additional, the thickness and graphene layers of carbon nanotube walls are decreased significantly due to the functionalization of MWCNTs. These imply that oxidizing reaction sites of MWCNTs locate along its axis and the oxidations begin at the outer-layer and progressively extends to the inner-layer of MWCNTs [40]. 3.3. DFT calculation of water dissociation steps on Ni metal surface The dissociation of water directly on CNT surface should be negligible due to the huge energy barrier [41]. To investigate the
520
M. Ran et al. / Journal of Solid State Chemistry 197 (2013) 517–522
Table 1 Chemical composition of the functional groups on surfaces of MWCNTs.
Intensity (a.u)
C-C
C–OH% (285.3 eV)
C–O% (286.8 eV)
COOH% (289.6 eV)
p–p*%
a b c
10.42 30.27 21.33
10.59 11.25 15.91
0 0 13.08
6.38 5.50 0
68.72 49.56 39.81
(290.7 eV)
((a) MWCNTs-p, (b) MWCNTs-H2O, (c) MWCNTs-HNO3).
C-OH C=O
280
Samples C–C% (284.5 eV
282
284
286 288 290 Bind Energy (eV)
π − π∗
292
294
296
Intensity (a.u)
C-C
280
C-OH
282
284
C=O
π − π∗
286 288 290 Bind Energy (eV)
292
294
296
Intensity (a.u)
C-C
C-OH C=O
280
282
284
286 288 290 Bind Energy (eV)
Fig. 5. SEM and partial enlarge images (as inset) of MWCNTs-p (a) and MWCNTsH2O (b). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
COOH
292
294
296
Fig. 4. XPS patterns of the different MWCNT samples. ((a) MWCNTs-p, (b) MWCNTs-H2O, (c) MWCNTs-HNO3).
chemical behavior of water molecules, DFT simulation was used to analyze the dissociation process on the clean Ni(1 1 1) cluster surface. The results show that two-step dissociation of water molecules takes place on the surface of Ni(1 1 1). The two
different steps of the dissociation pathway are depicted in black and red colors in Fig. 7. Before the chemical dissociation, the water molecule is easily adsorbed on the top site of the Ni(1 1 1), as it is the most stable configuration with an adsorption energy of 0.31 eV, which is consistent with previous results [42–44]. The water molecule is then transformed into the bridge site with adsorption energy of 0.27 eV along the MEP of water molecule dissociation. To extract the first H from water molecule, a barrier of 0.92 eV has to be overcome. The most stable configuration for OH is to vertically adsorb on the Ni(1 1 1) slab and this would migrate into the parallel adsorption before the second step of the dissociation, which requires a cleavage energy of the O–H bond of 0.98 eV. The
M. Ran et al. / Journal of Solid State Chemistry 197 (2013) 517–522
521
Fig. 7. Energy diagram of the dissociation of water molecule on Ni(1 1 1), the first step (black color) was OH and H dissociated from water molecule, the second step (red color) was H and O dissociated from OH. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 8. Diagram of the influence of water vapor on the surface structure of MWCNTs during CVD.
less than that of OH presented on the Ni(1 1 1) surface. This result is consistent with the conclusion from XPS analysis: the percentage of –OH group reaches 30.27%, which is almost 3 times higher than the C–O group (11.25%). 3.4. Proposed water influence reaction mechanism
Fig. 6. HR-TEM images of MWCNTs-p (a), MWCNTs-H2O (b) and MWCNTsHNO3 (c). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
second dissociation barrier is 0.06 eV higher than the first but the distinction in energy barrier could be ignored in the high temperature reaction process [42]. However, the numerical value difference in adsorption energies of (OHþH)–Ni (0.05 eV) and (O þH)–Ni (2.88 eV) is obvious and this indicates that the oxidizability of O is much stronger than that of OH [43]. Thus, the oxidation reaction of oxygenic radicals with amorphous carbon can take place easily. As a result, the concentration of O becomes
Based on the experimental results and DFT calculation analyzed above, a mechanism for the influence of water vapor on the surface structure of MWCNTs in CVD is proposed, as illustrated schematically in Fig. 8. When a small amount of water vapor is introduced by hydrocarbon gas in the CVD, the water molecules are adsorbed and activated on the surfaces of Ni particles, then dissociated on the surface into OH and O radicals. These highly active radicals will react with the hanging bonds on the defects to release high energy. Thus, O and OH species bonded with the amorphous carbon and unsaturated carbon atoms in defective sites such as carbon caps will introduce functional groups on the surfaces of nanotubes, increase the acidic sites and improve the purity of MWCNTs.
4. Conclusion Functionalized MWCNTs (MWCNTs-H2O) were prepared by water-assisted CVD. The effects of water vapor on the surface structures of MWCNTs in CVD were systematically studied by Raman spectra, NH3-TPD, TGA, SEM and HR-TEM. Raman spectra
522
M. Ran et al. / Journal of Solid State Chemistry 197 (2013) 517–522
and HRTEM images show that new defects are introduced on the surface of MWCNTs-H2O. XPS analysis shows that functional groups such as carbonyl and hydroxyl groups are formed on the surfaces of MWCNTs-H2O. The increase of the acidity and decrease in thermal stability are observed in MWCNTs-H2O, which is reflected in the NH3-TPD and TGA results. Furthermore, DFT simulations of the chemical behavior of water vapor molecules on the surface of Ni(1 1 1) cluster were carried out. We have proposed a mechanism for the effect of water on the surface structure of MWCNTs in accordance with DFT simulation and experimental results. These results imply the potential for waterassisted CVD, which can be regarded as an easy, green and efficient method for functionalization of MWCNTs.
Acknowledgments This work was supported by the National Natural Science Foundation of China (20776089). We are grateful to the China Scholarship Council for providing the Ph.D. scholarship and ICES (Singapore) for financial support. References [1] H.J. Dai, J.H. Hafner, A.G. Rinzler, D.T. Colbert, R.E. Smalley, Nature 384 (1996) 147–150. [2] J Bae, J. Solid State Chem. 184 (2011) 1749–1755. [3] R.S. Ruoff, D.C. Lorents, Carbon 33 (1995) 925–930. [4] S. Wong, E. Joselevich, A. Woolley, J. Am. Chem. Soc. 120 (1998) 8557–8558. [5] S.J. Tans, A.M. Verschueren, C. Dekker, Nature 393 (1998) 49–52. [6] M.S. Arnold, A.A. Green, J.F. Hulvat, S.I. Stupp, M.C. Hersam, Nat. Nanotechnol. 1 (2006) 60–65. [7] W.Z. Qian, T. Liu, F. Wei, Z.W. Wang, H. Yu, Carbon 41 (2003) 846–848. [8] J. Chen, M.A. Hamon, H. Hu, Y.S. Chen, A.M. Rao, P.C. Eklund, R.C. Haddon, Science 282 (1998) 95–98. [9] N.O.V. Plank, L.D. Jiang, R. Cheung, Appl. Phys. Lett. 83 (2003) 2426. [10] C.G. Zhao, L.J. Ji, H.J. Liu, G.J. Hu, S.M. Zhang, M.S. Yang, Z.Z. Yang, J. Solid State Chem. 177 (2004) 4394–4398. [11] D.G. Tong, Y.Y. Luo, W. Chu, Y.C. Guo, W. Tian, Plasma Chem. Plasma Process. 30 (2010) 897–905. [12] H. Zhang, C. Lancelot, W. Chu, J.P. Hong, A.Y. Khodakov, J. Mater. Chem. 19 (2009) 9241–9249. [13] X.L. Pan, X.H. Bao, Chem. Commun. 47 (2008) 6271–6281.
[14] L.M. Shi, W. Chu, S.Y. Deng, J. Nat. Gas Chem. 20 (2011) 48–52. [15] X.L. Pan, Z.L. Fan, W. Chen, Y.J. Ding, H.Y. Luo, X.H. Bao, Nat. Mater. 6 (2007) 507–511. [16] H. Zhang, W. Chu, C.J. Zou, Z.Y. Huang, Z.B. Ye, L. Zhu, Catal. Lett. 141 (2011) 438–444. [17] K. Hata, D.N. Futaba, K. Mizuno, T. Namai, M. Yumura, S. Iijima, Science 306 (2005) 1362–1364. [18] P.B. Amama, C.L. Pint, L. McJilton, S.M. Kim, E.A. Stach, P.T. Murray, Nano Lett. 9 (2009) 44–49. [19] L.B. Zhu, Y.H. Xiu, D.W. Hess, C.P. Wong, Nano. Lett. 5 (2005) 2641–2645. [20] B.S. Hu, H. Ago, N. Yoshihara, M. Tsuji, J. Phys. Chem. C 114 (2010) 3850–3856. [21] N. Yoshihara, H. Ago, M. Tsuji, J. Phys. Chem. C 111 (2007) 11577–11582. [22] M.F. Ran, W. Chu, J. Wen, Y.F. Li, Chem. J. Chin. Univ. 30 (2009) 231–235. [23] J.P. Perdew, M. Levy, Phys. Rev. B 56 (1997) 16021–16028. [24] J.P. Perdew, J.A. Chevary, S.H. Vosko, K.A. Jackson, M.R. Pederson, D.J. Singh, Phys. Rev. B 46 (1992) 6671–6687. [25] B. Delley, J. Phys. Chem. 100 (1996) 6107–6110. [26] G. Henkelman, H. Jonsson, J. Chem. Phys. 113 (2000) 9978–9985. [27] J. Maultzsch, S. Reich, C. Thomsen, Appl. Phys. Lett. 81 (2002) 2647–2649. ¨ [28] M. Muller, J. Maultzsch, D. Wunderlich, A. Hirsch, C. Thomsen, Phys. Status Solidi B 244 (2007) 4056–4059. [29] V. Datsyuka, M. Kalyvaa, K. Papagelisb, J. Partheniosa, D. Tasisb, A. Siokoua, I. Kallitsisa, C. Galiotis, Carbon 46 (2008) 833–840. [30] J.M. Simmons, B.M. Nichols, S.E. Baker, M.S. Marcus, O.M. Castellini, C.S. Lee, J. Phys. Chem. B 110 (2006) 7113–7118. [31] F.L. Jin, K.Y. Rhee, S.J. Park, J. Solid State Chem. 184 (2011) 3253–3256. [32] B. Scheibe, E. Borowiak-Palen, R.J. Kalenczuk, Mater. Charact. 61 (2010) 185–191. [33] D. Bom, R. Andrews, D. Jacques, J. Anthony, B.L. Chen, M.S. Meier, Nano Lett. 2 (2002) 615–619. [34] Z.J. Shi, Y.F. Lian, F.H. Liao, X.H. Zhou, Z.N. Gua, Y. Zhang, J Phys. Chem. Solids 61 (2000) 1031–1036. [35] W.P. Deng, X.S. Tan, W.H. Fang, Q.H. Zhang, Y. Wang, Catal. Lett. 133 (2009) 167–174. [36] P. Serp, M. Corrias, P. Kalck, Appl. Catal. A 253 (2003) 337–358. [37] R. Verdejo, S. Lamoriniere, B. Cottam, A. Bismarck, M. Shaffer, Chem. Commun. 5 (2007) 513–515. [38] T.T. Okpalugo, P. Papakonstantinou, H. Murphy, J. McLaughlin, N.D. Brown, Carbon 43 (2005) 153–161. [39] Y. Peng, H.W. Liu, Ind. Eng. Chem. Res. 45 (2006) 6483–6488. [40] K. Peng, L.Q. Liu, H.C. Li, H. Meyer, Z. Zhang, Carbon 41 (2010) 70–76. [41] M.K. Kostov, E.E. Santiso, A.M. George, K.E. Gubbins, M.B. Nardelli, Phy. Rev. Lett 95 (2005) 136105. [42] L.C. Grabow, A.A. Gokhale, S.T. Evans, J.A. Dumesic, M. Mavrikakis, J. Phys. Chem. C 112 (2008) 4608–4617. [43] A.A. Phatak, W.N. Delgass, F.H. Ribeiro, W.F. Schneider, J. Phys. Chem. C 113 (2009) 7269–7276. [44] G.C. Wang, S.X. Tao, X.H. Bu, J. Catal. 244 (2006) 10–16.
Contents lists available at SciVerse ScienceDirect
Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc
Functionalization of multi-walled carbon nanotubes using water-assisted chemical vapor deposition Maofei Ran a,b, Wenjing Sun a, Yan Liu b,n, Wei Chu a,1, Chengfa Jiang a a b
College of Chemical Engineering, Sichuan University, Chengdu, Sichuan 610065, People’s Republic of China Institute of Chemical and Engineering Sciences (ICES), AnSTAR, 627833 Singapore, Singapore
a r t i c l e i n f o
abstract
Article history: Received 24 April 2012 Received in revised form 23 July 2012 Accepted 5 August 2012 Available online 15 August 2012
A simple and novel method, water-assisted chemical vapor deposition (CVD) was developed to functionalize multi-walled carbon nanotubes (MWCNTs) during the synthesis process. The functionalized MWCNTs were characterized using Raman spectroscopy, XPS, TGA, NH3-TPD, SEM and HR-TEM. It was found that new defects are introduced and the amount of acidic groups is increased on the MWCNT surface during the water-assisted CVD process. The amount of C–OH and C–O group on the MWCNT surface is found to be increased from 21.1% to 42% with water vapor assistance. Density functional theory (DFT) was employed to study the chemical behavior of water vapor molecule on the catalyst particle surface of Ni(1 1 1) cluster. Based on the experimental and DFT simulation results, a mechanism for functionalization of MWCNTs by water-assisted CVD is proposed. & 2012 Elsevier Inc. All rights reserved.
Keywords: Multi-walled carbon nanotubes Water vapor Interface functionalization DFT calculation
1. Introduction Carbon nanotubes (CNTs) are regarded as one of the most promising materials for specific applications [1–3] because of their extraordinary mechanical, electrical and thermal properties [4–7]. Chemical vapor deposition (CVD) is widely used to prepare CNTs, but the as-grown CNTs are often accompanied by amorphous carbon generation and incorporation of metallic particles. Moreover, due to their high hydrophobicity, the CNTs are prone to aggregation and precipitation in water. These properties largely limit the application of CNTs in catalysis areas, thus it is necessary to modify the raw CNTs before application. Generally, there are several approaches to modify CNTs, such as strong acid oxidation [8–10] and plasma treatment [11]. Defects and polar groups are introduced during the process of modification, which could improve the dispersion of metal particles and enhance the interaction between CNT surface and metal particles [12–16]. However, these methods are prone to generate liquid wastes and their procedures are complex, which greatly limit their further development for applications. Here, we developed a easy and green method, water-assisted CVD, to modify multi-walled carbon nanotubes (MWCNTs) during the preparation process. The water-assisted CVD has been applied to efficiently synthesis of impurity-free single-walled carbon nanotubes [17]. Hata and Amama reported that water vapor can enhance the activity and lifetime of the catalyst during water-assisted CVD [17,18].
n
Corresponding author. Fax: þ65 63166182. E-mail addresses: [email protected] (Y. Liu), [email protected] (W. Chu). 1 Fax: þ86 28 85461108. 0022-4596/$ - see front matter & 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jssc.2012.08.014
Wong reported that aligned carbon nanotube stacks with opened end caps can be synthesized by water-assisted CVD [19]. Hu and Yoshihara reported that water vapor affects the diameter distribution of SWNTs and the surface of Fe–Mo/MgO catalysts during the water-assisted CVD [20,21]. In this work, the systematic research of the influence of water vapor on the structure and surface chemistry of MWCNTs in the water-assisted CVD has been investigated. The interactions between water vapor and the surfaces of MWCNTs and surfaces of catalyst particles were investigated in detail. The functionalized MWCNTs were characterized by X-ray photoelectron spectroscopy (XPS), Raman spectra, NH3 temperature programmed desorption (NH3TPD), thermal gravimetry analysis and differential thermogravimetry (TGA–DTG), scanning transmission electron (SEM) and high resolution transmission electron (HR-TEM) microscopy. Density functional theory (DFT) calculation of the dissociation products of water molecule on the surface of Ni(1 1 1) was also carried out. A mechanism for the influence of water is proposed, based on the experimental and the theoretical results.
2. Experimental 2.1. Synthesis of the MWCNTs The synthesis catalyst (Ni–Mn/MgO) was prepared by citric acid combustion as reported previously [22]. The pristine MWCNTs (MWCNTs-p) is used as a reference, which was synthesized by CVD in a quartz tube reactor (i.d. 2.5 cm, length 80 cm) laid in a horizontal furnace with a thermocouple in its central zone. The
M. Ran et al. / Journal of Solid State Chemistry 197 (2013) 517–522
catalyst loaded in a ceramic boat was reduced at 550 1C in H2 gas for 1 h. Subsequently, the temperature was raised to 650 1C, and methane was fed at a total flow rate of 110 ml/min. After 1 h of reaction, gas was switched to Ar (100 ml/min) during cooling. The product was collected and labeled as ‘‘MWCNTs-p’’. Functionalization of MWCNTs by water-assisted CVD was carried out using the same hydrocarbon gas and flowrate. However, the methane gas was passed through a water bubbler to produce water vapor before entering the reactor. The bubbler was maintained at 0 1C to achieve a stable concentration of water vapor. The synthesized product was labeled as ‘‘MWCNTs-H2O’’. The yields of MWCNTs-p and MWCNTsH2O were recorded, which are 762.8% and 973.4%, respectively. A portion of MWCNTs-p was further purified in 50 ml of concentrated HNO3 (68 wt%) refluxing at 140 1C for 12 h. After cooling to room temperature, it was filtered and washed with deionized water until the pH of the filtrate was around 7. Finally, the product was dried at 60 1C for 12 h and labeled as ‘‘MWCNTs-HNO3’’. 2.2. Characterization The as-prepared samples of MWCNTs were characterized using a series of techniques. HR-TEM images were obtained from a JEOL JEM-2000 FX microscope at 200 kV. SEM images were obtained on a JEOL JSM-6700F system. Room temperature microRaman scattering analyses were carried out with a Renishaw spectrometer. NH3-TPD was performed on thermo TPD/R/O 1100 catalysts analyzer instrument. XPS results were recorded on the XSAM800 spectrometer with an Al Ka (hn ¼1486.6 eV) X-ray source. TGA was performed to characterize their decomposition temperature and rate. Air was used as the carrier gas for burning the samples with a heating rate of 10 1C/min. 2.3. DFT calculation method The DFT calculations were performed with the Dmol3 program implemented within the Materials Studio of Accelrys, Inc. [23]. Exchange-correlation effects were described by the generalized approximation (GGA) developed by eight gradient-corrected functionals (PW91) [24]. The optimized geometries were also subjected to full frequency analysis at the same GGA/PW91 level of theory to certify the nature of the stationary point. The nudged elastic band was adopted to find the minimum energy path (MEP). Transition state (TS) searches were performed at the same theoretical level with the complete linear synchronous transit and quadratic synchronous transition search methods [25], then the TS confirmation tool was applied to map the MEP between reactants, intermediate and products [26]. Calculations were performed on 3 3 surface unit cells with the last layer totally relaxed. The inner-shell electron of the Ni atom was replaced by the effective core potentials with a basis of double numerical plus polarization. The density convergence criterion for self-consistent field optimization was 1.0000e–6. The Brillouin zone was sampled with a G-centered 3 3 1 Monkhorst–Pack k-point mesh. The smearing was set with 0.005 Ha. Adsorbate binding energies were defined as EslabþEadsorbate–Eadsorbateþslab, where Eadsorbateþslab was the total energy of the adsorbed configuration, Eslab was the total energy of the clean Ni(1 1 1) slab, and Eadsorbate was the adsorbate energy in the gas phase.
G D Intensity (a.u.)
518
D′ ID /IG= 0.709
a
ID /IG= 0.848
b
ID /IG= 1.118 400
800
c 1200
1600
2000
Raman shfit (cm-1) Fig. 1. Raman spectra of the different MWCNT samples. ((a) MWCNTs-p, (b) MWCNTs-H2O, (c) MWCNTs-HNO3).
main bands at 1341 and 1570 cm 1 are called D and G bands, respectively [27–29]. The D band and D0 band in the Raman spectra of CNTs are induced by disorder in a double resonance process and the G band at related to the graphite tangential E2g Raman active mode [27]. Accordingly, the intensity ratio of ID/IG is a commonly used qualitative measurement for defect density, which provides information about the crystallinity of MWCNTs [28]. From the results presented in Fig. 1, the ID/IG ratio of the MWCNTs-p is 0.709. This indicates a low quantity of structural defects in the graphitic structure of MWCNTs, which can provide active sites for further functionalization [29]. As observed in Fig. 1, the ID/IG of the MWCNTs-H2O and MWCNTs-HNO3 are improved to 0.848 and 1.118, respectively. The changes in ID/IG ratio could be due to the introduction of new defects as well as changes in the geometry of MWCNTs. The defects density of MWCNTs-H2O was much lower than that of MWCNTs-HNO3 due to the weak oxidation of water vapor. 3.1.2. TGA and DTG analysis Fig. 2 shows the TGA and DTG curves of the MWCNTs-p, MWCNTs-H2O and MWCNTs-HNO3. From the TGA curves, the initial burning temperature of MWCNTs-p at 539 1C is observed. With water vapor assistance, the initial burning temperature of MWCNTs-H2O is decreased to 468 1C. It further lowered down to 453 1C for MWCNTs-HNO3. The initial burning temperature of the sample in TGA can be served as a measure of thermal stability of nanotubes in air [30,31]. The thermal stabilities of MWCNTs-H2O and MWCNTs-HNO3 are lower than that of MWCNTs-p due to the more defects are created during the water-assisted CVD and post oxidation process, respectively. From the DTG curves (Fig. 2b) of the MWCNTs-HNO3 and MWCNTs-H2O, we can see that multi-stepwise weight losses correspond to the oxidation temperatures of the fractions of MWCNTs with different defect degree [32,33]. Particularly in the DTG curve of MWCNTs-H2O, the main combustion regime is split according to the introduction of more amounts of defects on the surface of MWCNTs [34]. The data consistently implied that carbon nanotubes have inferior crystallinity could be due to more amount of defect sites were introduced by adding water vapor in the CVD, corresponding to the results of Raman analysis.
3. Results and discussion 3.1. Surface structure and chemical bonding of MWCNTs 3.1.1. Raman spectra The Raman spectra of MWCNTs-p, MWCNTs-H2O and MWCNTsHNO3 excited with a 514.5 nm laser line are shown in Fig. 1. The two
3.1.3. NH3-TPD analysis The structural defects in the graphitic structure of MWCNTs can provide active sites to introduce polar groups on the surface [29] which can be reflected in the acidity of MWCNTs. NH3-TPD was performed to inspect the difference in acidity of MWCNT samples. As shown in Fig. 3, the NH3 desorption peak around
M. Ran et al. / Journal of Solid State Chemistry 197 (2013) 517–522
MWCNTs-p MWCNTs-H2O MWCNTs-HNO3
100
519
MWCNTs-p
102 60
453°C 100 539°C
40
468°C
98 96
20
440
480
520
560
MWCNTs-H2O
Intensity (a.u.)
Weight (%)
80
MWCNTs-HNO3
600
0 100
200
300
400 500 600 700 Temperature / (°C)
800
900
0
100
200
500 300 400 Temperature / (°C)
600
700
800
Fig. 3. NH3-TPD profiles of the different MWCNT samples.
1.2
MWCNTs-p
Deriv. Weight (%/°C)
MWCNTs-H2O MWCNTs-HNO3 0.6
0.0
0
100
200
300
400 500 600 Temperature / (°C)
700
800
900
Fig. 2. TG (a) and DTG (b) curves of different MWCNT samples.
(285.3 eV), C–O (286.8 eV), COOH (289.6 eV), and p–pn transition (290.7 eV), respectively [30,38]. As shown in Fig. 4a and b, the peak intensities of functional groups on the surfaces of MWCNTsH2O increase significantly in C1s spectra in compared with MWCNTs-p. The fractions of combined C–OH and C–O groups in MWCNTs-H2O increase to 30.27% and 11.25% (Table 1b), indicating that the SP2 carbon hybridization transformed into the SP3 mode in the water-assisted CVD [39]. These verify that functional groups were grafted on the surfaces of MWCNTs-H2O. The increase in fraction of functional groups is also observed in MWCNTs-HNO3 (Table 1c). However, it is notable that the COOH groups instead of the featured p–pn transition band appeared in the MWCNTs-HNO3 (Fig. 4c), implying that the conductive p network along the MWCNTs was disrupted during the nitric acid treatment process [21]. However, the oxidant of water vapor would not disrupt the conductive p network along MWCNTs (Fig. 2b). 3.2. Morphology of MWCNTs
200 1C is ascribed to weak acidic sites and another broad peak at 450–700 1C is ascribed to the generation of medium-strength acidic sites on the MWCNT surface [35]. A small and broad peak of NH3 desorption is observed in MWCNTs-p, indicating a few acidic groups existed on the nanotube surface due to formation of defects in the MWCNT growth process. Compared with MWCNTs-p, the peak areas of NH3 desorption in MWCNTs-H2O increased at both low and high temperature, indicating the amount of acidic sites was increased during the water-assisted CVD, especially in the weak acidic sites. Such improvements of peak areas verify that polar groups can be introduced on the MWCNT surface during the waterassisted CVD. The main desorption peak in MWCNTs-HNO3 appears at 600–800 1C and is much larger than that of the other two samples, indicating that a large number of strong acidic groups were generated on the MWCNT surface after treatment in concentrated HNO3 [36].
3.1.4. XPS analysis The surface chemical bonding of MWCNTs was detected by XPS analysis, as shown in Fig. 4. The relative percentages of surface functionalized groups were determined through curve fitting of high-resolution C1s XPS spectra [37], as listed in Table 1. The main binding-energy peak (284.5 eV) is attributed to the C–C bond, while the four remaining peaks are assigned to C–OH
The morphological variation of the MWCNT samples was observed by using SEM. As shown in Fig. 5a, the nanotubes of MWCNTs-p are covered with a number of accumulations of amorphous carbon and catalytic particles (pointed out by red arrows). With water assistance (Fig. 5b), the nanotubes of MWCNTs-H2O appear to be thicker and no significant coverage of amorphous carbon entities compared with the MWCNTs-p. HR-TEM was used to give higher resolution observations of surface structure variation of the MWCNTs-p, MWCNTs-H2O, and MWCNTs-HNO3. In Fig. 6a, well-graphitized walls of nanotubes can be observed in MWCNTs-p. With water vapor assistance and after HNO3 treatment, the HR-TEM images in Fig. 6b and c present the damaged graphite sheets and new defects (pointed out by red arrows) on the surface of MWCNTs. Additional, the thickness and graphene layers of carbon nanotube walls are decreased significantly due to the functionalization of MWCNTs. These imply that oxidizing reaction sites of MWCNTs locate along its axis and the oxidations begin at the outer-layer and progressively extends to the inner-layer of MWCNTs [40]. 3.3. DFT calculation of water dissociation steps on Ni metal surface The dissociation of water directly on CNT surface should be negligible due to the huge energy barrier [41]. To investigate the
520
M. Ran et al. / Journal of Solid State Chemistry 197 (2013) 517–522
Table 1 Chemical composition of the functional groups on surfaces of MWCNTs.
Intensity (a.u)
C-C
C–OH% (285.3 eV)
C–O% (286.8 eV)
COOH% (289.6 eV)
p–p*%
a b c
10.42 30.27 21.33
10.59 11.25 15.91
0 0 13.08
6.38 5.50 0
68.72 49.56 39.81
(290.7 eV)
((a) MWCNTs-p, (b) MWCNTs-H2O, (c) MWCNTs-HNO3).
C-OH C=O
280
Samples C–C% (284.5 eV
282
284
286 288 290 Bind Energy (eV)
π − π∗
292
294
296
Intensity (a.u)
C-C
280
C-OH
282
284
C=O
π − π∗
286 288 290 Bind Energy (eV)
292
294
296
Intensity (a.u)
C-C
C-OH C=O
280
282
284
286 288 290 Bind Energy (eV)
Fig. 5. SEM and partial enlarge images (as inset) of MWCNTs-p (a) and MWCNTsH2O (b). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
COOH
292
294
296
Fig. 4. XPS patterns of the different MWCNT samples. ((a) MWCNTs-p, (b) MWCNTs-H2O, (c) MWCNTs-HNO3).
chemical behavior of water molecules, DFT simulation was used to analyze the dissociation process on the clean Ni(1 1 1) cluster surface. The results show that two-step dissociation of water molecules takes place on the surface of Ni(1 1 1). The two
different steps of the dissociation pathway are depicted in black and red colors in Fig. 7. Before the chemical dissociation, the water molecule is easily adsorbed on the top site of the Ni(1 1 1), as it is the most stable configuration with an adsorption energy of 0.31 eV, which is consistent with previous results [42–44]. The water molecule is then transformed into the bridge site with adsorption energy of 0.27 eV along the MEP of water molecule dissociation. To extract the first H from water molecule, a barrier of 0.92 eV has to be overcome. The most stable configuration for OH is to vertically adsorb on the Ni(1 1 1) slab and this would migrate into the parallel adsorption before the second step of the dissociation, which requires a cleavage energy of the O–H bond of 0.98 eV. The
M. Ran et al. / Journal of Solid State Chemistry 197 (2013) 517–522
521
Fig. 7. Energy diagram of the dissociation of water molecule on Ni(1 1 1), the first step (black color) was OH and H dissociated from water molecule, the second step (red color) was H and O dissociated from OH. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 8. Diagram of the influence of water vapor on the surface structure of MWCNTs during CVD.
less than that of OH presented on the Ni(1 1 1) surface. This result is consistent with the conclusion from XPS analysis: the percentage of –OH group reaches 30.27%, which is almost 3 times higher than the C–O group (11.25%). 3.4. Proposed water influence reaction mechanism
Fig. 6. HR-TEM images of MWCNTs-p (a), MWCNTs-H2O (b) and MWCNTsHNO3 (c). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
second dissociation barrier is 0.06 eV higher than the first but the distinction in energy barrier could be ignored in the high temperature reaction process [42]. However, the numerical value difference in adsorption energies of (OHþH)–Ni (0.05 eV) and (O þH)–Ni (2.88 eV) is obvious and this indicates that the oxidizability of O is much stronger than that of OH [43]. Thus, the oxidation reaction of oxygenic radicals with amorphous carbon can take place easily. As a result, the concentration of O becomes
Based on the experimental results and DFT calculation analyzed above, a mechanism for the influence of water vapor on the surface structure of MWCNTs in CVD is proposed, as illustrated schematically in Fig. 8. When a small amount of water vapor is introduced by hydrocarbon gas in the CVD, the water molecules are adsorbed and activated on the surfaces of Ni particles, then dissociated on the surface into OH and O radicals. These highly active radicals will react with the hanging bonds on the defects to release high energy. Thus, O and OH species bonded with the amorphous carbon and unsaturated carbon atoms in defective sites such as carbon caps will introduce functional groups on the surfaces of nanotubes, increase the acidic sites and improve the purity of MWCNTs.
4. Conclusion Functionalized MWCNTs (MWCNTs-H2O) were prepared by water-assisted CVD. The effects of water vapor on the surface structures of MWCNTs in CVD were systematically studied by Raman spectra, NH3-TPD, TGA, SEM and HR-TEM. Raman spectra
522
M. Ran et al. / Journal of Solid State Chemistry 197 (2013) 517–522
and HRTEM images show that new defects are introduced on the surface of MWCNTs-H2O. XPS analysis shows that functional groups such as carbonyl and hydroxyl groups are formed on the surfaces of MWCNTs-H2O. The increase of the acidity and decrease in thermal stability are observed in MWCNTs-H2O, which is reflected in the NH3-TPD and TGA results. Furthermore, DFT simulations of the chemical behavior of water vapor molecules on the surface of Ni(1 1 1) cluster were carried out. We have proposed a mechanism for the effect of water on the surface structure of MWCNTs in accordance with DFT simulation and experimental results. These results imply the potential for waterassisted CVD, which can be regarded as an easy, green and efficient method for functionalization of MWCNTs.
Acknowledgments This work was supported by the National Natural Science Foundation of China (20776089). We are grateful to the China Scholarship Council for providing the Ph.D. scholarship and ICES (Singapore) for financial support. References [1] H.J. Dai, J.H. Hafner, A.G. Rinzler, D.T. Colbert, R.E. Smalley, Nature 384 (1996) 147–150. [2] J Bae, J. Solid State Chem. 184 (2011) 1749–1755. [3] R.S. Ruoff, D.C. Lorents, Carbon 33 (1995) 925–930. [4] S. Wong, E. Joselevich, A. Woolley, J. Am. Chem. Soc. 120 (1998) 8557–8558. [5] S.J. Tans, A.M. Verschueren, C. Dekker, Nature 393 (1998) 49–52. [6] M.S. Arnold, A.A. Green, J.F. Hulvat, S.I. Stupp, M.C. Hersam, Nat. Nanotechnol. 1 (2006) 60–65. [7] W.Z. Qian, T. Liu, F. Wei, Z.W. Wang, H. Yu, Carbon 41 (2003) 846–848. [8] J. Chen, M.A. Hamon, H. Hu, Y.S. Chen, A.M. Rao, P.C. Eklund, R.C. Haddon, Science 282 (1998) 95–98. [9] N.O.V. Plank, L.D. Jiang, R. Cheung, Appl. Phys. Lett. 83 (2003) 2426. [10] C.G. Zhao, L.J. Ji, H.J. Liu, G.J. Hu, S.M. Zhang, M.S. Yang, Z.Z. Yang, J. Solid State Chem. 177 (2004) 4394–4398. [11] D.G. Tong, Y.Y. Luo, W. Chu, Y.C. Guo, W. Tian, Plasma Chem. Plasma Process. 30 (2010) 897–905. [12] H. Zhang, C. Lancelot, W. Chu, J.P. Hong, A.Y. Khodakov, J. Mater. Chem. 19 (2009) 9241–9249. [13] X.L. Pan, X.H. Bao, Chem. Commun. 47 (2008) 6271–6281.
[14] L.M. Shi, W. Chu, S.Y. Deng, J. Nat. Gas Chem. 20 (2011) 48–52. [15] X.L. Pan, Z.L. Fan, W. Chen, Y.J. Ding, H.Y. Luo, X.H. Bao, Nat. Mater. 6 (2007) 507–511. [16] H. Zhang, W. Chu, C.J. Zou, Z.Y. Huang, Z.B. Ye, L. Zhu, Catal. Lett. 141 (2011) 438–444. [17] K. Hata, D.N. Futaba, K. Mizuno, T. Namai, M. Yumura, S. Iijima, Science 306 (2005) 1362–1364. [18] P.B. Amama, C.L. Pint, L. McJilton, S.M. Kim, E.A. Stach, P.T. Murray, Nano Lett. 9 (2009) 44–49. [19] L.B. Zhu, Y.H. Xiu, D.W. Hess, C.P. Wong, Nano. Lett. 5 (2005) 2641–2645. [20] B.S. Hu, H. Ago, N. Yoshihara, M. Tsuji, J. Phys. Chem. C 114 (2010) 3850–3856. [21] N. Yoshihara, H. Ago, M. Tsuji, J. Phys. Chem. C 111 (2007) 11577–11582. [22] M.F. Ran, W. Chu, J. Wen, Y.F. Li, Chem. J. Chin. Univ. 30 (2009) 231–235. [23] J.P. Perdew, M. Levy, Phys. Rev. B 56 (1997) 16021–16028. [24] J.P. Perdew, J.A. Chevary, S.H. Vosko, K.A. Jackson, M.R. Pederson, D.J. Singh, Phys. Rev. B 46 (1992) 6671–6687. [25] B. Delley, J. Phys. Chem. 100 (1996) 6107–6110. [26] G. Henkelman, H. Jonsson, J. Chem. Phys. 113 (2000) 9978–9985. [27] J. Maultzsch, S. Reich, C. Thomsen, Appl. Phys. Lett. 81 (2002) 2647–2649. ¨ [28] M. Muller, J. Maultzsch, D. Wunderlich, A. Hirsch, C. Thomsen, Phys. Status Solidi B 244 (2007) 4056–4059. [29] V. Datsyuka, M. Kalyvaa, K. Papagelisb, J. Partheniosa, D. Tasisb, A. Siokoua, I. Kallitsisa, C. Galiotis, Carbon 46 (2008) 833–840. [30] J.M. Simmons, B.M. Nichols, S.E. Baker, M.S. Marcus, O.M. Castellini, C.S. Lee, J. Phys. Chem. B 110 (2006) 7113–7118. [31] F.L. Jin, K.Y. Rhee, S.J. Park, J. Solid State Chem. 184 (2011) 3253–3256. [32] B. Scheibe, E. Borowiak-Palen, R.J. Kalenczuk, Mater. Charact. 61 (2010) 185–191. [33] D. Bom, R. Andrews, D. Jacques, J. Anthony, B.L. Chen, M.S. Meier, Nano Lett. 2 (2002) 615–619. [34] Z.J. Shi, Y.F. Lian, F.H. Liao, X.H. Zhou, Z.N. Gua, Y. Zhang, J Phys. Chem. Solids 61 (2000) 1031–1036. [35] W.P. Deng, X.S. Tan, W.H. Fang, Q.H. Zhang, Y. Wang, Catal. Lett. 133 (2009) 167–174. [36] P. Serp, M. Corrias, P. Kalck, Appl. Catal. A 253 (2003) 337–358. [37] R. Verdejo, S. Lamoriniere, B. Cottam, A. Bismarck, M. Shaffer, Chem. Commun. 5 (2007) 513–515. [38] T.T. Okpalugo, P. Papakonstantinou, H. Murphy, J. McLaughlin, N.D. Brown, Carbon 43 (2005) 153–161. [39] Y. Peng, H.W. Liu, Ind. Eng. Chem. Res. 45 (2006) 6483–6488. [40] K. Peng, L.Q. Liu, H.C. Li, H. Meyer, Z. Zhang, Carbon 41 (2010) 70–76. [41] M.K. Kostov, E.E. Santiso, A.M. George, K.E. Gubbins, M.B. Nardelli, Phy. Rev. Lett 95 (2005) 136105. [42] L.C. Grabow, A.A. Gokhale, S.T. Evans, J.A. Dumesic, M. Mavrikakis, J. Phys. Chem. C 112 (2008) 4608–4617. [43] A.A. Phatak, W.N. Delgass, F.H. Ribeiro, W.F. Schneider, J. Phys. Chem. C 113 (2009) 7269–7276. [44] G.C. Wang, S.X. Tao, X.H. Bu, J. Catal. 244 (2006) 10–16.