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Surface modification of multi-wall carbon nanotubes by nitrogen attachment
Diamond & Related Materials 20 (2011) 965–968
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Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d
Surface modification of multi-wall carbon nanotubes by nitrogen attachment☆☆ Imre Bertóti a,⁎, Ilona Mohai a, Miklós Mohai a, János Szépvölgyi a, b a b
Institute of Materials and Environmental Chemistry, Chemical Research Center Hungarian Academy of Sciences, Budapest, Hungary Research Institute of Chemical and Process Engineering, University of Pannonia, Veszprém, Hungary
a r t i c l e
i n f o
Available online 15 May 2011 Keywords: Carbon nanotube Plasma modification Covalent N attachment XPS N bonding states
a b s t r a c t Commercial multiwall carbon nanotubes (MWCNT-s) were treated by RF activated N2 gas plasma at (nominally) room temperature. Treatment time of 5 to 10 min was applied at negative bias varying in the 0– 300 V range. Surface chemical alterations were followed by X-ray photoelectron spectroscopy (XPS). All the applied treatments resulted in a significant build-up of nitrogen in the surface of MWCNT-s. The amount of nitrogen varied between 19 and 25 at.% depending on the treatment time and, in a lesser extent, also on biasing conditions. Interestingly, the nitrogen attachment was also significant (20 at.%) when the treatment commenced without bias. Evaluating the high-resolution N1s XP spectral region, typically three different chemical bonding states of the nitrogen was delineated. Peak component at 398.3 ± 0.3 eV is assigned to C_N\C type, at 399.7 ± 0.3 eV to sp2 N in melamine-type ring structure and at 400.9 ± 0.3 eV to N substituting carbon in a graphite-like environment. Identical chemical bonding of the nitrogen was detected on the surface of highly oriented pyrolytic graphite (HOPG) and on microcrystalline graphite surfaces treated in the same way for comparison. Estimating the penetration depth of the nitrogen atoms by the SRIM program it was concluded that at the applied DC bias energy range the implanted nitrogen is incorporated in the top 2– 4 monoatomic layers of the samples. A model for the distribution of the chemically bonded nitrogen on the outer walls of the MWCNT-s is proposed. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Multi-wall carbon nanotubes (further denoted as CNT-s), due to their unique physical and chemical properties, are candidates for promising application in various areas ranging from novel field emission devices [1,2], sensors [3,4] and ending up as bio-inspired functions [5,6]. Many of these applications require some kind of surface modification of the tubes. When using them as reinforcing component [7,8] in polymeric matrices, their surface treatment for dispersion is a must. In incorporating MWCNT-s in advanced structural ceramics [9], appropriate coatings may be applied to provide protection during high temperature sintering process in oxidative environment [10]. A few attempts were made for covalent modification by O2 and NH3 using RF plasma and by N2 in microwave plasma [11,12]. In this paper, we describe an approach of nitrogen attachment to the surface of CNT-s considering this as the first step of further modification for different specific applications. 2. Experimental Commercial thin multi-wall carbon nanotubes, produced by Nanocyl S.A. via the catalytic carbon vapor deposition process, were ☆☆ Presented at the Diamond 2010, 21st European Conference on Diamond, Diamond- Like Materials, Carbon Nanotubes, and Nitrides, Budapest. ⁎ Corresponding author. Fax: + 36 1 438 1147. E-mail address: [email protected] (I. Bertóti). 0925-9635/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2011.05.011
selected for the present study. The NC 3100 research grade series containing N95% carbon have an average diameter of 9.5 nm, average length of 1.5 μm and may contain some pyrolytically deposited carbon on the surface, as specified by the manufacturer. Samples for nitriding treatments were prepared by drop wise deposition from thick sonicated toluene slurry onto a stainless steel holder (Ø 9 mm), suitable also for XPS measurements. The samples are left in air for 30 min ensuring evaporation of the toluene. A SEM image, representing the “hay-stack” arrangement of nanotubes is shown in Fig. 1. Pieces of polished commercial graphite and freshly cleaved HOPG samples of similar size were used as reference materials. Plasma treatments were performed in a Pyrex tubular reactor (Ø 33 mm, l70 mm length); built into an UHV chamber (base pressure b1 × 10−4 Pa), in N2 flow (b5 cm3/min STP) regulated by a bleeding valve to set the pressure to 5–8 × 10−1 Pa. The RF power (13.56 MHz, 100 W) was applied through a matching circuit by an outer coil. Negative DC bias of 50, 100, 200 or 300 V was applied through the steel sample holder, or the sample was grounded, corresponding to 0 V (no-bias). X-ray photoelectron spectra were recorded on a Kratos XSAM 800 spectrometer operated at fixed analyzer transmission mode, using Mg Kα1,2 (1253.6 eV) excitation. The pressure of the analysis chamber was lower than 1 × 10−7 Pa. Survey spectra were recorded in the 150– 1300 eV kinetic energy range with 0.5 eV steps. Photoelectron lines of the main constituent elements, i.e., the O1s, N1s and C1s were recorded by 0.1 eV steps and min. 1 s dwell time. Spectra were referenced to the energy of the C1s line of the sp2 type graphitic carbon, set at 284.1 ±
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I. Bertóti et al. / Diamond & Related Materials 20 (2011) 965–968
Fig. 1. Scanning electron micrograph, showing the “hay-stack” arrangement of nanotubes on the sample holder.
0.2 eV B.E. (binding energy). Quantitative analysis, based on peak area intensities after removal of the Shirley-type background, was performed by the Kratos Vision 2 and the XPS MultiQuant programs [13,14] using experimentally determined photo-ionization cross-section data of Evans et al. [15] and asymmetry parameters of Reilman et al. [16]. 3. Results and discussion 3.1. Composition and chemical structure 3.1.1. Composition Basic treatment conditions and the overall surface composition (calculated by the generally applied “infinitely thick homogeneous sample” model) of the treated CNT samples together with some HOPG and graphite ones modified in similar conditions are summarized in Table 1. The data show that 16–23 at.% nitrogen can be incorporated into the CNT surface at the applied plasma treatment even at relatively short reaction time. Only about half of that amount was found on the graphite and on almost atomically flat HOPG surface. The significant quantity of oxygen appeared on the treated CNTs, as compared to HOPG samples, is most probably the result of post-treatment oxidation of the surface when exposed to air, while transferring the sample into the XP spectrometer after the plasma treatment. The nitrogen attachment to the CNT samples does not show significant bias dependence. This may hold also for the graphite and HOPG, as well. The most prominent observation in this respect is the build-up of about 20 at.% N at zero bias. This means that the activation of the gas phase by RF energy input, i.e., creating predominantly N2+ ions, excited molecules and electrons, are providing sufficient energy for the covalent attachment of nitrogen to the carbon network. The high reactivity of the CNT-s, manifested in the large amount of nitrogen (and also oxygen) incorporated into these samples compared to the similarly bonded network of HOPG, may be related to the significant number of defects inherently present on the predominantly curved uneven-shaped multi-wall CNT-s [17,18].
3.1.2. Chemical structure The incorporation of nitrogen into the CNT-s will inevitably modify the chemical structure of the ordered graphene type carbon network. High-resolution XP spectra of the N1s energy region for a representative CNT and also HOPG and graphite sample are shown in Fig. 2. The broad peak envelopes clearly demonstrate that the nitrogen atoms are situated in different chemical environments. Performing peak fitting procedure with components of 1.75 eV half-widths, corresponding to single chemical states at the applied resolution, we found three discernable N1s chemical shifts (denoted further as N1, N2 and N3), present in all samples at 398.3 ± 0.3 eV, 399.7 ± 0.3 eV and 400.9 ± 0.3 eV, corresponding to, at least, three different chemical environments. According to Fig. 2b and c, the same chemical states of the nitrogen were detected on the surface of HOPG and on the microcrystalline graphite surfaces, although their relative amount was different. The assignment of this peak components to certain chemical bonding states were repeatedly discussed by a large number of authors in connection of the numerous experimental works devoted to the synthesis of various CNx type materials and nitrogen-containing DLC type coatings [19–30]. The existing significant controversy was broadly discussed in Refs. [28,31]. Taking into account the above data and our earlier findings, we assigned the N1 peak component at 398.3 eV to C_N\C type chemical environment, where the sp 2 nitrogen bonded to two sp 2 type carbon, like in pyridine [27–31]. Nitrogen bonded to three sp 3 type carbon atoms may show similar shift, but formation of this configuration is less probable in our case [22,31]. The N2 component at 399.7 eV is assigned to sp 2 N in diazine or triazine-type ring structure [26]. The
Table 1 Treatment conditions and the overall surface composition (calculated by the homogeneous element distribution model). Run name
Target
Time min
Bias V
N at%
O at%
PMNC-01 PMNC-07 PMNC-08 PMNC-09 PMNC-10 PMNC-11 PMCG-03 PMCG-05 PMCH-01 PMCH-09
Nanotubes Nanotubes Nanotubes Nanotubes Nanotubes Nanotubes Graphite Graphite HOPG HOPG
5 10 10 10 10 10 10 5 5 10
200 100 50 0 200 300 300 200 200 50
16.4 20.0 19.4 19.7 23.0 20.3 8.1 11.6 10.0 7.5
5.8 9.6 9.7 11.5 6.4 7.7 7.1 4.0 4.6 2.5
Fig. 2. XP spectra and peak components of the N1s energy region for a representative CNT and also for a HOPG and a graphite sample.
I. Bertóti et al. / Diamond & Related Materials 20 (2011) 965–968
N3 type N at 400.9 eV we attribute to N substituting carbon in graphite-like environment [28–30,32]. Having performed the above peak fitting procedure, the results are presented in Fig. 3, together with the total nitrogen content for each sample included in Table 1. In addition to the visualization of the quantitative differences among the samples, there are a few minor features to notice. The first one is the difference in the component ratios. While for the CNT-s the N2 type nitrogens are dominating, for the graphite and HOPG samples the N1 type is the most abundant chemical state. The least frequent chemical environment for all the carbonaceous samples is the N3 type one. It may be also of interest to note that this N3 type nitrogen environment has somewhat higher ratio in samples treated at relatively high (200–300 V) bias. Based on the available experimental data it is not possible to interpret these differences. One can speculate however, that incorporation of one N atom into the curved side-wall of the CNT ends up with an N2 type, i.e., strained C\N environment, while incorporation of the nitrogen into the ordered hexagonal graphene network will lead preferably to C_N\C sp 2 type nitrogen environment. 3.2. In-depth distribution of nitrogen Based on the applied treatment condition it is feasible to estimate the in-depth distribution of the nitrogen built in the CNT-s. The RF plasma treatment performed at 0–300 V bias was intentionally selected for limiting the modification to a few outer atomic layers of the CNT-s leaving the lower layers of the multi-wall tubes intact, thus preserving the original structure and the overall outstanding properties not altered. Penetration of nitrogen ions (N +) into carbon was calculated by the SRIM program [33]. Briefly, the “Stopping and Range of Ions in Matter” (SRIM) is a group of computer programs, which calculate interaction of ions with matter. SRIM is based on a Monte Carlo simulation method, namely the binary collision approximation with a random selection of the impact parameter of the next colliding ion. As the input parameters, it needs the ion type and energy (in the range 10 eV–2 GeV) and the material type and composition of the target. The actual in-depth distribution of the implanted nitrogen ions calculated for 50 eV, as an example, is shown in Fig. 4. The stopping range calculation was performed for the whole applied bias energy range and the results are depicted in Fig. 5. The projected energy range (Rp) representing the mean depth where the majority of ions of a given energy will stop (lower curve) and with the added in-depth straggling (upper curve) show that the implanted nitrogen will penetrate not deeper than 20 Å into carbon material. At the estimation of the depth values, we must take the energy of a single nitrogen ion/atom. As known that the majority of the bias-
Fig. 3. The total nitrogen content and the corresponding contribution of the N1, N2 and N3 components for the CNT, graphite and HOPG samples.
967
Fig. 4. The projected stopping range distribution of N ions of 50 eV energy in the carbon target.
accelerated ionized species are N2+ ions, when hitting the target surface dissociate to two nitrogen atoms possessing, as an average, half of the biasing energy [34,35]. Accordingly, at 50 V bias 25 eV, and at the 300 V bias 150 eV ion energy is applicable. For eye guidance, thickness of atomic layers is also indicated (taken 3.5 Å for interlayer distances). From all the above it follows that the treatments with the applied bias are limited essentially to the first two monolayers in the HOPG, graphite and, by analogy, also in the outer walls of the CNT. Based on the calculations depicted in Fig. 5, we attempted to calculate the C/N atomic ratio on the outer surface of the CNT-s. For the PMCH-09 HOPG sample, treated at 50 V bias, according to SRIM, that the built-in nitrogen is situated in the first two monolayers of about 7 Å thickness. Applying the Layers-on-Plane model of the XPS MultiQuant program [13], the composition of this layer can be determined, based on the measured C1s and N1s XPS intensity. The result shows that the carbon to nitrogen ratio in this layer is 3.3, resulting in 23 at.% nitrogen in this surface layer. (It is much higher than that given in Table 1, when the N content was calculated supposing to be homogeneously distributed in the volume of the sample.) Similar calculation for the PMNC-08 CNT sample, treated also at 50 V bias, was performed taking into account the cylindrical shape by applying the Layers-on-Cylinder model [14] of the XPS MultiQuant program, taking again the 7 Å thick penetration depth. For this case, the result shows an even higher degree of the modification: the C/N ratio in this layer is 1.8, resulting in 36 at.% nitrogen incorporated into the outermost two atomic layers in this case.
Fig. 5. The projected energy range (Rp) representing the mean depth where the majority of ions of a given energy will stop (lower curve) and with the added in-depth straggling (upper curve).
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4. Conclusions • Covalent attachment of nitrogen was demonstrated by applying RF plasma activated nitrogen. • Large amount of nitrogen, 19–23 at.% could be incorporated into the carbon matrix at short 5–10 min treatment time. • Three different chemical bonding states of nitrogen was delineated and assigned to C_N\C pyridine type, to sp 2 N in diazine/triazinetype ring structure and to N substituting carbon in a graphite-like environment. • Identical chemical bonding of the nitrogen was detected on the surface of highly oriented pyrolytic graphite (HOPG) and on microcrystalline graphite surfaces treated in the same way. • The implanted nitrogen is incorporated in the top 2–4 monoatomic layers of the samples according to the calculated penetration depth of the nitrogen atoms at the applied biasing energy. References [1] Y.M. Wong, W.P. Kang, J.L. Davidson, B.K. Choi, W. Hofmeister, J.H. Huang, Diamond Relat. Mater. 14 (2005) 2078. [2] H. Ahn, K. Lee, D. Kim, S. Han, Appl. Phys. Lett. 88 (2006) 093122. [3] A. Kukovecz, R. Smajda, M. Oze, B. Schaefer, H. Haspel, Z. Konya, I. Kiricsi, Phys. Status Solidi B: Basic Res. 245 (2008) 2331. [4] F. Villalpando-Paez, A. Romero, E. Munoz-Sandoval, L. Martinez, H. Terrones, M. Terrones, Chem. Phys. Lett. 386 (2004) 137. [5] J.C. Carrero-Sanchez, A.L. Elías, R. Mancilla, G. Arrellín, H. Terrones, J.P. Laclette, M. Terrones, Nano Lett. 6 (2006) 1609. [6] A.L. Elías, J.C. Carrero-Sanchez, H. Terrones, M. Endo, P. Laclette, M. Terrones, Small 3 (2007) 1723. [7] B. McCarthy, J.N. Coleman, S.A. Curran, A.B. Dalton, A.P. Davey, Z. Kónya, A. Fonseca, J.B. Nagy, W.J. Blau, J. Mater. Sci. Lett. 19 (2000) 2239. [8] B. Fragneaud, K. Masenelli-Varlot, A. Gonzalez-Montiel, M. Terrones, J. Cavaille, Chem. Phys. Lett. 419 (2006) 567. [9] C. Balázsi, Z. Shen, Z. Kónya, Zs. Kasztovszky, F. Wéber, Z. Vértesy, L.P. Biró, I. Kiricsi, P. Arató, Compos. Sci. Technol. 65 (2005) 727.
[10] M. Mohai, I. Mohai, Z. Sebestyén, A. Gergely, P. Németh, J. Szépvölgyi, Surf. Interface Anal. 42 (2010) 1148, doi:10.1002/sia.3307. [11] A. Felten, C. Bittencourt, J.J. Pireaux, G. van Lier, J.C. Charlier, J. Appl. Phys. 98 (2005) 074308. [12] B. Ruelle, S. Peeterbroeck, R. Gouttebaron, T. Godfroid, F. Monteverde, J.-P. Dauchot, M. Alexandre, M. Hecq, P. Dubois, J. Mater. Chem. 17 (2007) 157, doi: 10.1039/b613581c. [13] M. Mohai, Surf. Interface Anal. 36 (2004) 828–832, doi:10.1002/sia.1775. [14] M. Mohai, I. Bertóti, Surf. Interface Anal. 36 (2004) 805, doi:10.1002/sia.1769. [15] S. Evans, R.G. Pritchard, J.M. Thomas, J. Electron Spectrosc. Relat. Phenom. 14 (1978) 341. [16] R.F. Reilman, A. Msezane, S.T. Manson, J. Electron Spectrosc. Relat. Phenom. 8 (1976) 389. [17] J.-C. Charlier, Acc. Chem. Res. 35 (2002) 1063. [18] B. Bhushan (Ed.), Handbook of Nanotechnology, 2nd edition, Springer Berlin, Heidelberg, New York, 2007, p. 74, Chapter 3.4.6. [19] T. Ujvári, A. Kolitsch, A. Tóth, M. Mohai, I. Bertóti, Diamond Relat. Mater. 11 (2002) 1149. [20] D. Marton, K.J. Boyd, J.W. Rabalais, Int. J. Mod. Phys. B 9 (1995) 3527. [21] A. Baker, P. Hammer, Surf. Interface Anal. 25 (1997) 629. [22] S. Souto, M. Pickholz, M.C. dos Santos, F. Alvarez, Phys. Rev. B 57 (1998) 2536. [23] I. Bertóti, A. Tóth, M. Mohai, T. Ujvári, Surf. Interface Anal. 30 (2000) 538. [24] I. Bertóti, Surf. Coat. Technol. 151–152 (2002) 194. [25] W.T. Zheng, K.Z. Xing, N. Hellgren, M. Lödlung, Å. Johansson, U. Gelius, W.R. Salaneck, J-E. Sundgren, J. Electron Spectrosc. Relat. Phenom. 87 (1997) 45. [26] A.P. Dementjev, A. de Graaf, M.C.M. van de Sanden, K.I. Maslakov, A.V. Naumkin, A.A. Serov, Diamond Relat. Mater. 9 (2000) 1904. [27] A. Ronning, H. Feldermann, R. Merk, H. Hofsäss, P. Reinke, J.-U. Thiele, Phys. Rev. B 58 (1998) 2207. [28] I. Kusunoki, M. Sakai, Y. Ygari, S. Ishidzuka, T. Takami, T. Takaoka, M. NishitaniGamo, T. Ando, Surf. Sci. 492 (2001) 315. [29] P. Ayala, A. Grüneis, T. Gemming, B. Büchner, M.H. Rümmeli, D. Grimm, J. Schumann, R. Kaltofen, F.L. Freire Jr., H.D. Fonseca Filho, T. Pichler, Chem. Mater. 19 (2007) 6131. [30] P. Ayala, R. Arenal, M. Rümmeli, A. Rubio, T. Pichler, Carbon 48 (2010) 575. [31] S.E. Rodil, S. Muhl, Diamond Relat. Mater. 13 (2004) 1521. [32] Y.-C. Lin, Ch-Y. Lin, P.-W. Chiu, Appl. Phys. Lett. 96 (2010) 133110. [33] J.F. Ziegler, M.D. Ziegler, J.P. Biersack, Stopping and Range of Ions in Matter (SRIM 2006), 2006 htpp://www.SRIM.org. [34] N. Hellgren, M.P. Johansson, E. Broitman, P. Sandström, L. Hultman, J.-E. Sundgren, Thin Solid Films 382 (2001) 146. [35] K. Kutasi, Z. Donkó, M. Mohai, L. Nemes, G. Marosi, Vacuum 68 (2003) 311.
Contents lists available at ScienceDirect
Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d
Surface modification of multi-wall carbon nanotubes by nitrogen attachment☆☆ Imre Bertóti a,⁎, Ilona Mohai a, Miklós Mohai a, János Szépvölgyi a, b a b
Institute of Materials and Environmental Chemistry, Chemical Research Center Hungarian Academy of Sciences, Budapest, Hungary Research Institute of Chemical and Process Engineering, University of Pannonia, Veszprém, Hungary
a r t i c l e
i n f o
Available online 15 May 2011 Keywords: Carbon nanotube Plasma modification Covalent N attachment XPS N bonding states
a b s t r a c t Commercial multiwall carbon nanotubes (MWCNT-s) were treated by RF activated N2 gas plasma at (nominally) room temperature. Treatment time of 5 to 10 min was applied at negative bias varying in the 0– 300 V range. Surface chemical alterations were followed by X-ray photoelectron spectroscopy (XPS). All the applied treatments resulted in a significant build-up of nitrogen in the surface of MWCNT-s. The amount of nitrogen varied between 19 and 25 at.% depending on the treatment time and, in a lesser extent, also on biasing conditions. Interestingly, the nitrogen attachment was also significant (20 at.%) when the treatment commenced without bias. Evaluating the high-resolution N1s XP spectral region, typically three different chemical bonding states of the nitrogen was delineated. Peak component at 398.3 ± 0.3 eV is assigned to C_N\C type, at 399.7 ± 0.3 eV to sp2 N in melamine-type ring structure and at 400.9 ± 0.3 eV to N substituting carbon in a graphite-like environment. Identical chemical bonding of the nitrogen was detected on the surface of highly oriented pyrolytic graphite (HOPG) and on microcrystalline graphite surfaces treated in the same way for comparison. Estimating the penetration depth of the nitrogen atoms by the SRIM program it was concluded that at the applied DC bias energy range the implanted nitrogen is incorporated in the top 2– 4 monoatomic layers of the samples. A model for the distribution of the chemically bonded nitrogen on the outer walls of the MWCNT-s is proposed. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Multi-wall carbon nanotubes (further denoted as CNT-s), due to their unique physical and chemical properties, are candidates for promising application in various areas ranging from novel field emission devices [1,2], sensors [3,4] and ending up as bio-inspired functions [5,6]. Many of these applications require some kind of surface modification of the tubes. When using them as reinforcing component [7,8] in polymeric matrices, their surface treatment for dispersion is a must. In incorporating MWCNT-s in advanced structural ceramics [9], appropriate coatings may be applied to provide protection during high temperature sintering process in oxidative environment [10]. A few attempts were made for covalent modification by O2 and NH3 using RF plasma and by N2 in microwave plasma [11,12]. In this paper, we describe an approach of nitrogen attachment to the surface of CNT-s considering this as the first step of further modification for different specific applications. 2. Experimental Commercial thin multi-wall carbon nanotubes, produced by Nanocyl S.A. via the catalytic carbon vapor deposition process, were ☆☆ Presented at the Diamond 2010, 21st European Conference on Diamond, Diamond- Like Materials, Carbon Nanotubes, and Nitrides, Budapest. ⁎ Corresponding author. Fax: + 36 1 438 1147. E-mail address: [email protected] (I. Bertóti). 0925-9635/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2011.05.011
selected for the present study. The NC 3100 research grade series containing N95% carbon have an average diameter of 9.5 nm, average length of 1.5 μm and may contain some pyrolytically deposited carbon on the surface, as specified by the manufacturer. Samples for nitriding treatments were prepared by drop wise deposition from thick sonicated toluene slurry onto a stainless steel holder (Ø 9 mm), suitable also for XPS measurements. The samples are left in air for 30 min ensuring evaporation of the toluene. A SEM image, representing the “hay-stack” arrangement of nanotubes is shown in Fig. 1. Pieces of polished commercial graphite and freshly cleaved HOPG samples of similar size were used as reference materials. Plasma treatments were performed in a Pyrex tubular reactor (Ø 33 mm, l70 mm length); built into an UHV chamber (base pressure b1 × 10−4 Pa), in N2 flow (b5 cm3/min STP) regulated by a bleeding valve to set the pressure to 5–8 × 10−1 Pa. The RF power (13.56 MHz, 100 W) was applied through a matching circuit by an outer coil. Negative DC bias of 50, 100, 200 or 300 V was applied through the steel sample holder, or the sample was grounded, corresponding to 0 V (no-bias). X-ray photoelectron spectra were recorded on a Kratos XSAM 800 spectrometer operated at fixed analyzer transmission mode, using Mg Kα1,2 (1253.6 eV) excitation. The pressure of the analysis chamber was lower than 1 × 10−7 Pa. Survey spectra were recorded in the 150– 1300 eV kinetic energy range with 0.5 eV steps. Photoelectron lines of the main constituent elements, i.e., the O1s, N1s and C1s were recorded by 0.1 eV steps and min. 1 s dwell time. Spectra were referenced to the energy of the C1s line of the sp2 type graphitic carbon, set at 284.1 ±
966
I. Bertóti et al. / Diamond & Related Materials 20 (2011) 965–968
Fig. 1. Scanning electron micrograph, showing the “hay-stack” arrangement of nanotubes on the sample holder.
0.2 eV B.E. (binding energy). Quantitative analysis, based on peak area intensities after removal of the Shirley-type background, was performed by the Kratos Vision 2 and the XPS MultiQuant programs [13,14] using experimentally determined photo-ionization cross-section data of Evans et al. [15] and asymmetry parameters of Reilman et al. [16]. 3. Results and discussion 3.1. Composition and chemical structure 3.1.1. Composition Basic treatment conditions and the overall surface composition (calculated by the generally applied “infinitely thick homogeneous sample” model) of the treated CNT samples together with some HOPG and graphite ones modified in similar conditions are summarized in Table 1. The data show that 16–23 at.% nitrogen can be incorporated into the CNT surface at the applied plasma treatment even at relatively short reaction time. Only about half of that amount was found on the graphite and on almost atomically flat HOPG surface. The significant quantity of oxygen appeared on the treated CNTs, as compared to HOPG samples, is most probably the result of post-treatment oxidation of the surface when exposed to air, while transferring the sample into the XP spectrometer after the plasma treatment. The nitrogen attachment to the CNT samples does not show significant bias dependence. This may hold also for the graphite and HOPG, as well. The most prominent observation in this respect is the build-up of about 20 at.% N at zero bias. This means that the activation of the gas phase by RF energy input, i.e., creating predominantly N2+ ions, excited molecules and electrons, are providing sufficient energy for the covalent attachment of nitrogen to the carbon network. The high reactivity of the CNT-s, manifested in the large amount of nitrogen (and also oxygen) incorporated into these samples compared to the similarly bonded network of HOPG, may be related to the significant number of defects inherently present on the predominantly curved uneven-shaped multi-wall CNT-s [17,18].
3.1.2. Chemical structure The incorporation of nitrogen into the CNT-s will inevitably modify the chemical structure of the ordered graphene type carbon network. High-resolution XP spectra of the N1s energy region for a representative CNT and also HOPG and graphite sample are shown in Fig. 2. The broad peak envelopes clearly demonstrate that the nitrogen atoms are situated in different chemical environments. Performing peak fitting procedure with components of 1.75 eV half-widths, corresponding to single chemical states at the applied resolution, we found three discernable N1s chemical shifts (denoted further as N1, N2 and N3), present in all samples at 398.3 ± 0.3 eV, 399.7 ± 0.3 eV and 400.9 ± 0.3 eV, corresponding to, at least, three different chemical environments. According to Fig. 2b and c, the same chemical states of the nitrogen were detected on the surface of HOPG and on the microcrystalline graphite surfaces, although their relative amount was different. The assignment of this peak components to certain chemical bonding states were repeatedly discussed by a large number of authors in connection of the numerous experimental works devoted to the synthesis of various CNx type materials and nitrogen-containing DLC type coatings [19–30]. The existing significant controversy was broadly discussed in Refs. [28,31]. Taking into account the above data and our earlier findings, we assigned the N1 peak component at 398.3 eV to C_N\C type chemical environment, where the sp 2 nitrogen bonded to two sp 2 type carbon, like in pyridine [27–31]. Nitrogen bonded to three sp 3 type carbon atoms may show similar shift, but formation of this configuration is less probable in our case [22,31]. The N2 component at 399.7 eV is assigned to sp 2 N in diazine or triazine-type ring structure [26]. The
Table 1 Treatment conditions and the overall surface composition (calculated by the homogeneous element distribution model). Run name
Target
Time min
Bias V
N at%
O at%
PMNC-01 PMNC-07 PMNC-08 PMNC-09 PMNC-10 PMNC-11 PMCG-03 PMCG-05 PMCH-01 PMCH-09
Nanotubes Nanotubes Nanotubes Nanotubes Nanotubes Nanotubes Graphite Graphite HOPG HOPG
5 10 10 10 10 10 10 5 5 10
200 100 50 0 200 300 300 200 200 50
16.4 20.0 19.4 19.7 23.0 20.3 8.1 11.6 10.0 7.5
5.8 9.6 9.7 11.5 6.4 7.7 7.1 4.0 4.6 2.5
Fig. 2. XP spectra and peak components of the N1s energy region for a representative CNT and also for a HOPG and a graphite sample.
I. Bertóti et al. / Diamond & Related Materials 20 (2011) 965–968
N3 type N at 400.9 eV we attribute to N substituting carbon in graphite-like environment [28–30,32]. Having performed the above peak fitting procedure, the results are presented in Fig. 3, together with the total nitrogen content for each sample included in Table 1. In addition to the visualization of the quantitative differences among the samples, there are a few minor features to notice. The first one is the difference in the component ratios. While for the CNT-s the N2 type nitrogens are dominating, for the graphite and HOPG samples the N1 type is the most abundant chemical state. The least frequent chemical environment for all the carbonaceous samples is the N3 type one. It may be also of interest to note that this N3 type nitrogen environment has somewhat higher ratio in samples treated at relatively high (200–300 V) bias. Based on the available experimental data it is not possible to interpret these differences. One can speculate however, that incorporation of one N atom into the curved side-wall of the CNT ends up with an N2 type, i.e., strained C\N environment, while incorporation of the nitrogen into the ordered hexagonal graphene network will lead preferably to C_N\C sp 2 type nitrogen environment. 3.2. In-depth distribution of nitrogen Based on the applied treatment condition it is feasible to estimate the in-depth distribution of the nitrogen built in the CNT-s. The RF plasma treatment performed at 0–300 V bias was intentionally selected for limiting the modification to a few outer atomic layers of the CNT-s leaving the lower layers of the multi-wall tubes intact, thus preserving the original structure and the overall outstanding properties not altered. Penetration of nitrogen ions (N +) into carbon was calculated by the SRIM program [33]. Briefly, the “Stopping and Range of Ions in Matter” (SRIM) is a group of computer programs, which calculate interaction of ions with matter. SRIM is based on a Monte Carlo simulation method, namely the binary collision approximation with a random selection of the impact parameter of the next colliding ion. As the input parameters, it needs the ion type and energy (in the range 10 eV–2 GeV) and the material type and composition of the target. The actual in-depth distribution of the implanted nitrogen ions calculated for 50 eV, as an example, is shown in Fig. 4. The stopping range calculation was performed for the whole applied bias energy range and the results are depicted in Fig. 5. The projected energy range (Rp) representing the mean depth where the majority of ions of a given energy will stop (lower curve) and with the added in-depth straggling (upper curve) show that the implanted nitrogen will penetrate not deeper than 20 Å into carbon material. At the estimation of the depth values, we must take the energy of a single nitrogen ion/atom. As known that the majority of the bias-
Fig. 3. The total nitrogen content and the corresponding contribution of the N1, N2 and N3 components for the CNT, graphite and HOPG samples.
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Fig. 4. The projected stopping range distribution of N ions of 50 eV energy in the carbon target.
accelerated ionized species are N2+ ions, when hitting the target surface dissociate to two nitrogen atoms possessing, as an average, half of the biasing energy [34,35]. Accordingly, at 50 V bias 25 eV, and at the 300 V bias 150 eV ion energy is applicable. For eye guidance, thickness of atomic layers is also indicated (taken 3.5 Å for interlayer distances). From all the above it follows that the treatments with the applied bias are limited essentially to the first two monolayers in the HOPG, graphite and, by analogy, also in the outer walls of the CNT. Based on the calculations depicted in Fig. 5, we attempted to calculate the C/N atomic ratio on the outer surface of the CNT-s. For the PMCH-09 HOPG sample, treated at 50 V bias, according to SRIM, that the built-in nitrogen is situated in the first two monolayers of about 7 Å thickness. Applying the Layers-on-Plane model of the XPS MultiQuant program [13], the composition of this layer can be determined, based on the measured C1s and N1s XPS intensity. The result shows that the carbon to nitrogen ratio in this layer is 3.3, resulting in 23 at.% nitrogen in this surface layer. (It is much higher than that given in Table 1, when the N content was calculated supposing to be homogeneously distributed in the volume of the sample.) Similar calculation for the PMNC-08 CNT sample, treated also at 50 V bias, was performed taking into account the cylindrical shape by applying the Layers-on-Cylinder model [14] of the XPS MultiQuant program, taking again the 7 Å thick penetration depth. For this case, the result shows an even higher degree of the modification: the C/N ratio in this layer is 1.8, resulting in 36 at.% nitrogen incorporated into the outermost two atomic layers in this case.
Fig. 5. The projected energy range (Rp) representing the mean depth where the majority of ions of a given energy will stop (lower curve) and with the added in-depth straggling (upper curve).
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