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MWCNTs composite
Nuclear Instruments and Methods in Physics Research B 410 (2017) 222–229
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Effect of carbon nanotubes irradiation by argon ions on the formation of SnO2-x/MWCNTs composite S.N. Nesov a, P.M. Korusenko a,⇑, S.N. Povoroznyuk a,b, V.V. Bolotov a, E.V. Knyazev a, D.A. Smirnov c,d a
Omsk Scientific Center, Siberian Branch of the Russian Academy of Sciences, Karl Marx Avenue 15, 644040 Omsk, Russia Omsk State Technical University, Mira Avenue 11, 644050 Omsk, Russia c St. Petersburg State University, 198504 St. Petersburg, Russia d Institute of Solid State Physics, Dresden University of Technology, D-01062 Dresden, Germany b
a r t i c l e
i n f o
Article history: Received 20 June 2017 Received in revised form 8 August 2017 Accepted 25 August 2017
Keywords: Argon ions Irradiation Functionalization Composite Tin oxide Multi-walled carbon nanotubes XPS XANES
a b s t r a c t Methods of transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and X-ray absorption near edge structure (XANES) were used to investigate the influence of irradiation by argon ions of multi-walled carbon nanotubes (MWCNTs) on the composites SnO2-x/MWCNTs formation. It has been shown that irradiation of the MWCNTs leads to the formation of structural defects and oxygen-containing groups in the walls of carbon nanotubes. The presence of the latest in MWCNTs walls leads to chemical interaction of the tin oxide with the surface of MWCNTs through formation of chemical SnAOAC bonds at the interfaces ‘‘Tin oxide-MWCNTs” at synthesis of the composite by CVD method. It has been established that the crystalline structure of the tin oxide clusters forming on surface of MWCNTs depends essentially on the defectiveness in the structure of the outer walls of carbon nanotubes. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction The interest to nano-structured composite materials based on carbon nanotubes (CNTs), decorated with nanoparticles or layers of metal oxides, is stimulated by a variety of their potential practical use in various fields of industry and technology. The distribution of metal oxide nanoparticles on the carbon nanotubes surface which has a high surface area and electrical conductivity leads to high functional characteristics of such composites. In particular, the composites based on arrays of multi-walled CNTs (MWCNTs) and tin oxide (SnOx/MWCNTs, 1 < x < 2) have been successfully tested as an anode material for lithium-ion batteries, as well as the material for the gas sensors [1–4]. It is known that CNTs with low level of structural defects exhibit sufficiently weak reactivity, which prevents fixing and uniform distribution of the metal oxides particles on their surface during the formation of composites. Increased adhesion of metal oxides particles to CNTs surface is most often achieved by functionalization of carbon nanotubes using wet chemical methods due to the ⇑ Corresponding author at: Omsk Scientific Centre, Siberian Branch, Russian Academy of Sciences, Karl Marx Avenue, 15, Omsk 644024, Russia. E-mail address: [email protected] (P.M. Korusenko). http://dx.doi.org/10.1016/j.nimb.2017.08.040 0168-583X/Ó 2017 Elsevier B.V. All rights reserved.
formation of structural defects and linking of oxygen-containing groups [5,6]. Another way to increase the surface activity of CNTs is introduction structural defects using ion irradiation (with a various the type of ions, energy and the irradiation dose), which may lead to the formation of various structural defects (vacancies, vacancy clusters, etc.). The presence of such defects in the external graphene layers of CNTs increases the number of dangled chemical bonds and twofold coordinated carbon atoms, which, as it has been shown in [7], significantly increase the reactivity of graphene plane. Chemical interaction of the deposited metal oxide particles of with the surface of carbon nanotubes increases the mechanical properties of the composite, significantly affects the processes of charge transport across the interphase boundaries, which significantly increases the functional characteristics of composites based on CNTs and metal oxides. Previously using TEM and Raman data we have shown that irradiation of MWCNTs with argon ions leads to the dramatic changes in nanotube structure. There are numerous discontinuities of graphene planes forming scaly-like structure in the MWCNTs. Also, there seems to be a well-defined amorphization of the walls of carbon nanotubes [8]. In this paper, by Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge
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structure (XANES) using synchrotron radiation, there has been studied the effect of irradiation with argon ions on the change in the structure and chemical state of the surface walls of MWCNTs. The effect of prior exposure to argon ions on the forming of composites based on arrays of MWCNTs and tin oxide has been studied. The use of the complex of methods XPS and XANES allows to conduct the analysis of the surface layers of composites as well as the analysis of the interface of ‘‘Metal oxide – MWCNTs” without the destruction and modification of the analyzed layers.
2. Material and methods 2.1. Synthesis and irradiation conditions The layers of MWCNTs were grown by chemical vapor deposition (CVD) through the pyrolysis of a mixture of acetonitrile with ferrocene (100:1). The pyrolysis was performed at atmospheric pressure at 800 °C in argon flow (150 mL/min) on substrates of silicon wafers with 100-nm thick oxide layers. Synthesis of MWCNTs continued for 12 min. The thickness of the layers of MWCNTs was 15 ± 5 lm. The average diameter of CNTs is 40 nm. The initial MWCNTs contained about 3 at.% of nitrogen, which is embedded in the nanotubes walls as pyridine, pyrrolic and graphite-like defects, and also as molecular nitrogen (N2) between the walls and in the cavities of carbon nanotubes [9]. The irradiation of MWCNTs was undertaken by a 5 keV Ar+ beam with a dose of 1 1016 ions/cm2 at room temperature. The projected ranges of argon ions in a target MWCNTs layer with a density of 0.2 g/cm3 were estimated using SRIM [10] to be 100 nm. 2.2. Composites synthesis The composites SnO2-x/MWCNTs obtained by CVD were formed on the initial and irradiated MWCNTs by thermal decomposition of hydrous SnCl22H2O at 380 °C, followed by vapor deposition on a heated substrate (MWCNTs/SiO2/Si) at 340 °C. The time taken to form the composites was 15 min. 2.3. TEM, XPS and XANES The morphologies and structures of the composites SnO2-x/MWCNTs were studied by TEM using a JEOL JEM 2200FS microscope in the bright field mode. The study of the composites atomic and electronic structure was conducted using a XPS and XANES technique at the RussianGerman beam line of BESSY II (Berlin) and experimental station PES-RGL. The photoemission (PE) and XANES spectra were measured under vacuum at a residual pressure of 3 10 10 Torr. The survey PE spectra, O 1s core level and C 1s core level spectra were recorded using synchrotron radiation with a photon energy of 850 and 400 eV, respectively. The PE spectra were collected with the hemispherical analyzer (Phoibos 150) in the fixed transmission mode, with a pass energy of 15 eV (for C 1s and O 1s core lines) and 50 eV (for survey spectra). Step energy size was 0.05 eV. The depth of the XPS analysis was 1–2 nm. The XANES spectra were obtained in the total electron yield mode by recording a sample drain current with variations in the energy of photons incident on the sample. The energy resolutions of the monochromator in the ranges of the carbon K-edge (hm 285 eV) and tin M-edge (hm 490 eV) absorption spectra were equal to 70 and 110 meV, respectively. The XANES spectra were normalized to the primary photon current from a gold covered grid recorded contemporaneously. The depth of the XANES analysis was 10–15 nm.
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3. Results and discussion 3.1. Influence of the ion irradiation on the MWCNTs electronic structure Fig. 1 shows the PE spectra of the initial and irradiated MWCNTs, measured in a wide range of binding energies (650– 0 eV). In the spectra there are observed PE lines of oxygen O 1s (binding energy of 533 eV), carbon C 1s (binding energy of 285 eV) and nitrogen N 1s (binding energy of 401 'B), as well as the lines of auger transitions C KLL (kinetic energy of 260 eV) and O KLL (kinetic energy of 510 eV). It is seen that in the spectrum of the irradiated MWCNTs a significant increase in the intensity of the oxygen line is observed. Results of the quantitative analysis indicate that oxygen content increases significantly after the irradiation (Table 1). High oxygen content is caused by the oxidation of the MWCNTs surface due to the increase of number of defects in the MWCNTs structure after of irradiation. Fig. 2a and b shows the results of a detailed analysis of PE lines of carbon (C 1s) of the initial and the irradiated MWCNTs. In accordance with the work [11,12] the most intensive component of the carbon spectrum of the initial MWCNTs with a peak at the binding energy 284.8 eV corresponds to the sp2–hybridized carbon atoms (C@C) constituting the CNT walls (Fig. 2a). A component with the peak at 285.4 eV binding energy is connected with the presence of defects in the walls of MWCNTs (sp3-hybridized amorphous carbon). Components of the spectrum with peaks at binding energies of 287, 289, 290 eV correspond to carbon, incorporated in a variety of carbon-to-oxygen chemical bonds: consisting of epoxy and phenolic groups (CAO, CAOAC, CAOH), carbonyl groups (C@O) and carboxyl oxide groups (COOH), respectively. The most highenergy component of the spectrum with a peak at the binding energy 292 eV corresponds to ‘‘shake up” satellite, which observed in spectra of ordered graphitic carbon materials [13]. As it is seen in Fig. 2b, in the C 1s PE spectrum of irradiated MWCNTs there is observed a significant increase of intensity of the spectrum components corresponding to structural defects as well as to oxygen-carbon chemical bonds (see Table 1). In addition, in the irradiated MWCNTs ‘‘shake up” satellite is not observed. The C K-edge XANES spectrum (Fig. 3, curve 1) of initial MWCNTs are represented by two resonances: the sufficiently narrow and intense p⁄ (C@C)-resonance at an photon energy of 285.3 eV, which characterizes the transitions from the C 1s core
Fig. 1. Survey PE spectra of the initial (1) and irradiated MWCNTs (2). The spectra were collected with an excitation energy of 850 eV.
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Table 1 XPS data for the MWCNTs before and after irradiation. Sample
Initial MWCNTs Irradiated MWCNTs
Concentration of elements, at.%
Relative areas of components C1s,%
C
O
N
C@C
CAC defects
CAO
C@O/COOH
96.7 88.8
1.3 9.6
2.0 1.6
69.0 44.4
21.4 35.9
5.6 13.3
4.0 6.4
Fig. 3. C K-edge XANES spectra of carbon for the initial (1) and irradiated MWCNTs (2).
Fig. 2. C 1s PE spectra of the initial (a) and irradiated MWCNTs (b). The spectra were collected with an excitation energy of 400 eV.
level to free p⁄–2pz states of the conduction band, and the r⁄ (C@C)-resonance formed by the transitions C 1s ? r⁄–2s2px,y. The r⁄ (C@C)-resonance has two local maximum at the photon energy of 291.7 and 292.8 eV. The high-energy region of the absorption edge has fine structure: spectral features at the photon energy 303 and 307 eV is observed. Therefore, the shape of the absorption CK-edge spectrum of the initial MWCNTs indicates a high degree of graphitization of the nanotube walls [14]. In the CK-edge XANES spectrum of the irradiated MWCNTs significant changes are observed (Fig. 3, curve 2). Decrease of intensity of p⁄(C@C)-resonance and shift of its maximum to lowenergy region (photon energy of 284.9 eV) is observed. In addition, the absence of fine structure in the r⁄(C@C)-resonance as well as in high-energy region of the absorption edge
(303–307 eV) is observed. All this indicates a significant increase in defectiveness of the carbon nanotubes wall structure and a possible increase in the number of sp3-hybridized carbon atoms. The intense peaks (a1) and (a2) at photon energies of 286.4 eV and 288.5 eV, respectively, associated with presence various functional oxygen-containing groups on the surface of the irradiated MWCNTs. According to [12,15,–18], the peak (a1) correspond to p⁄-resonance of carbon, being in a chemical bond with oxygen in the composition of hydroxyl (CAOH). The peak (a2) corresponds to p⁄-resonance of carbon in the double bonds (C@O) in the composition of the carboxyl, carbonyl (C@O, COOH). At the same time, high intensity at the range of photon energy of 289–291 eV is suggests the presence of states, which associated with the r⁄(CAO) states of carbon [18]. It is known [19] that the irradiation with argon ions (energy of ions >50 eV) leads to the formation of vacancies and vacancy clusters, increasing the number of dangled bonds and twofold coordinated carbon atoms on the surface of the irradiated MWCNTs. The interaction of such defects with oxygen can lead to the formation of unsaturated (single) carbon-oxygen bonds, which can increase the reactivity of the surface of carbon nanotubes [20]. 3.2. Effect of the MWCNTs irradiation on the composites SnO2-x/MWCNTs formation Fig. 4a and b shows TEM image of the composite SnO2-x/MWCNTs formed on the initial MWCNTs. As shown in the figure, the tin oxide was fixed on the surface of the MWCNTs as separate clusters which has crystalline structure. Previously using electron diffraction data we have shown that the SnO2-x clusters correspond to the tetragonal phase SnO2 [21]. About two hundreds of clusters were measured for this composite. The average clusters size is 120 nm. Fig. 4c, d, e shows TEM image of the composite SnO2-x/MWCNTs formed on the irradiated MWCNTs. As seen
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Fig. 4. TEM images of the composites SnO2-x/N-MWCNTs formed on the initial (a,b) and irradiated MWCNTs (c,d,e). Figure (e) shows the area marked in Figure (d).
(Fig. 4c) on the surface of irradiated carbon nanotubes the tin oxide clusters are distributed more uniformly than in the case of the composite formed on initial MWCNTs. The average clusters size
is 30 nm. In this case, the clusters of tin oxide is consist of differently oriented nanocrystallites with size about 5–10 nm (Fig. 4d and e). The formation of differently oriented nanocrystallites
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in clusters of tin oxide is due to apparently that its growth occurs on the defective walls of the irradiated carbon nanotubes. Fig. 5 shows C K-edge XANES spectra of the composites SnO2-x/MWCNTs formed on the initial and irradiated MWCNTs. It can be seen that the spectrum of composite formed on the initial MWCNTs (Fig. 5, curve 1) hardly differs from the spectrum of the initial carbon nanotubes (Fig. 3, curve 1) except the presence of low-intensity peak at the energy of photons 288.5 eV, which corresponds to p⁄-resonance of carbon in the C@O, COOH bonds. Low intensity of this resonance is indicating to weak oxidation of the surface of the MWCNTs during the formation of the composite. The spectrum of the composite formed on the irradiated MWCNTs (Fig. 5, curve 2) is significantly different from the spectrum of the irradiated MWCNTs (Fig. 3, curve 2). There is observed a significant decrease in the intensity of the p⁄(C@C)-resonance, as well as features (a1). Drop of the intensity p⁄(C@C)-resonance and spectral feature (a1) indicates a decrease in the number of carbon atoms in C@C (sp2) bonds and hydroxide groups (CAOH). In addition, presence of the local spectral feature (a3) at the photon energy of 290.4 eV is observed. This may indicate an interaction of the tin oxide with defective surface of MWCNTs with the formation of SnAOAC chemically bonds. In the work [22] it was shown that a chemical interaction of the graphene surface with metal oxides with the formation of CuAOAC bonds leads to the presence of states in carbon XANES spectra at the photon energy of 290.4 eV. Sn M4,5-edge XANES spectra of the composites SnO2-x/MWCNTs formed on the initial and irradiated MWCNTs are shown in Fig. 6. These spectra are reflecting the transitions from core Sn 3d level to unoccupied p and f-states of the conduction band [23]. The spectrum of Sn M-edge of the composite SnO2-x/MWCNTs formed on the initial MWCNTs (Fig. 6, curve 1), is represented by well-resolved peaks B, C, D (M5-edge of absorption) and the E, F (Sn M4-edge of absorption of tin) [20,21]. Comparison of the absorption spectrum of tin of this composite (Fig. 6, curve 1) and the spectrum of crystalline powder SnO2 (Fig. 6, curve 3) indicates that composition and structure tin oxide in the composite are close to crystalline SnO2. However, the high intensity of the pre-edge feature A and peak D in the spectrum of this composite indicates the presence of a large number of oxygen vacancies and dangled chemical bonds in the structure of tin oxide [23–25]. The Sn M-edge spectrum of the composite SnO2-x/MWCNTs formed on the irradiated MWCNTs (Fig. 6, curve 2) indicates a deterioration of the tin oxide crystalline structure. This is evidenced by
Fig. 5. C K-edge XANES spectra of carbon for the composites formed on the initial (1) and irradiated MWCNTs (2).
Fig. 6. Sn M-edge XANES spectra of tin for the composites formed on the initial (1), irradiated MWCNTs (2) and crystalline SnO2 (3).
the loss of the fine structure of the spectrum and the merge of the main peaks B-C and E-F. This indicates that the presence of structural defects, dangled bonds and various oxygen-containing groups on the surface of the MWCNTs has a significant influence on the crystal structure of the tin oxide clusters during formation composite. Deterioration of the tin oxide crystalline structure associated with presence differently oriented nanocrystallites and grain boundaries, containing a large number of structural defects in clusters of tin oxide. This is confirmed by TEM data (see Fig. 4). In addition, in the XANES spectrum of this composite the peaks A and D have a much lower intensity than in the case of the composite formed on the initial MWCNTs. Decrease of the number of oxygen vacancies in the structure of tin oxide may be because of its interaction with oxygen-containing groups on the surface of irradiated MWCNTs and also formation of SnAOAC bonds. This interaction apparently is the cause of the deterioration of the crystalline structure in the tin oxide clusters. In the survey PE spectra of the composites formed on the initial and irradiated MWCNTs (Fig. 7) the lines of tin, carbon, nitrogen, oxygen, and chlorine are observed. The results of quantitative
Fig. 7. Survey PE spectra of the composites formed on the initial (1) and irradiated MWCNTs (2). The spectra were collected with an excitation energy of 850 eV.
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S.N. Nesov et al. / Nuclear Instruments and Methods in Physics Research B 410 (2017) 222–229 Table 2 XPS data for the composites formed on the initial and irradiated MWCNTs. Composite
formed on the initial MWCNTs formed on irradiated MWCNTs
Concentration of elements, at.% C
O
Sn
N
Cl
83.1 69.3
13.7 24.5
2.0 4.2
0.8 1.1
0.4 0.9
Fig. 8. C 1s PE spectra of the composites formed on the initial (a) and irradiated MWCNTs (b). The spectra were collected with an excitation energy of 400 eV.
XPS analysis show that the content of tin on the surface of irradiated MWCNTs is over two times more than the content of tin in the composite formed on the initial MWCNTs (Table 2). A higher tin content for the same conditions of composites synthesis to indicate the increase of the adhesion of tin oxide to the surface of irradiatred MWCNTs due to the increase in the number of places of its possible fixation (structural defects, dangled bonds and oxygencontaining functional groups). This is also agreeing with the TEM data (Fig. 4). Comparative analysis of C 1s PE spectra of the composites shows a more significant oxidation of the surface of the carbon
Fig. 9. O 1s PE spectra of the composites formed on the initial (a) and irradiated MWCNTs (b). The spectra were collected with an excitation energy of 850 eV.
nanotubes in the composite formed on irradiated MWCNTs (Fig. 8a and b). In the C 1s spectrum of the composite formed on irradiated MWCNTs high intensity components are observed, corresponding to carbon in CAO, CAOAC and C@O, COOH bonds. The O 1s (oxygen) PE spectra of the composites SnO2-x/MWCNTs formed on the initial and the irradiated MWCNTs are shown in Fig. 9. In the O 1s spectrum of the composite SnO2-x/MWCNTs formed on the initial MWCNTs, components on the binding energies of 529.8 eV, 531.6 eV and 533.3 eV (Fig. 9a) are observed. Low-energy component (529.8 eV) corresponds to oxygen in the SnAO bonds [12,22,26]. High-energy components (531.6 and
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533.3 eV) correspond to oxygen in C@O and CAOAC/CAO chemical bonds, respectively [12,22]. Peak fitting of the line O 1s of the composite SnO2-x/MWCNTs formed on the irradiated MWCNTs (Fig. 9b) shows that in the spectrum the components corresponding to oxygen in the composition of the tin oxide (529.8 eV), C@O bonds (531.6 eV), and CAOAC/CAO (533.3 eV) are also present. At the same time, the intensity of the component corresponding to the singlet carbon-oxygen bonds (533.3 eV) is greater than the intensity this component in the spectrum of the composite formed on the initial MWCNTs. Moreover, in the spectrum of the composite formed on the irradiated MWCNTs the presence of an additional component with a peak at the binding energy of 530.7 eV is observed. The last is confirmed by the broadening of the line O 1s at 0.45 eV relative to the oxygen spectrum of the composite formed on the initial carbon nanotubes (Fig. 9). In the work [27,28] it has been shown that the component of oxygen spectrum corresponding to MeAOAC bonds (Me = Sn, Fe, Ag, Cu, Zr) is located on the binding energies by 1–3 eV higher than the component corresponding to oxygen in the composition of the metal oxide (MeAO). Based on this data, we assume that the observed additional component of the oxygen spectrum may be associated with the formation of SnAOAC bonds at the interfaces ‘‘Tin oxide -MWCNTs”. High content of singlet carbon-oxygen bonds (CAO, CAOAC) may indicate to possibility of the formation of SnAOAC bonds in the composite SnO2-x/MWCNTs formed on the irradiated MWCNTs. 4. Conclusions By TEM, XPS and XANES methods with the use of synchrotron radiation there have been investigated the features of the composites SnO2-x/MWCNTs formation depending on the structure and chemical state of carbon nanotube surface. It has been shown that irradiation of the MWCNTs by argon ions with an energy of 5 keV and a dose of 1016 ions/cm2 leads to the formation of structural defects in the walls of carbon nanotubes on which the linking of oxygen-containing groups (C@O, COOH, CAO, CAOAC, CAOH) takes place. It has been shown that the structural and chemical state of the nanotubes surface significantly affects the atomic and electronic structure and composition of the composites SnO2x/MWCNTs at their formation by CVD method. The irradiation of MWCNTs by argon ions lead to increases the amount of tin oxide which is deposited onto the surface of carbon nanotubes that indicates an increase in adhesion of the tin oxide to carbon nanotubes surface. Furthermore, the presence of defects and oxygencontaining groups on the surface of MWCNTs leads to chemical interaction of the tin oxide with the surface of MWCNTs through formation of chemical SnAOAC bonds at the interfaces ‘‘Tin oxide-MWCNTs”. It has been established that the structure of the tin oxide deposited on surface of MWCNTs depends essentially on the defectiveness in the structure of the outer walls of carbon nanotubes. Acknowledgments This work was partially supported by the RFBR (Russian Foundation for Basic Research, Russia), research project nos. №15-42-04308 r_sibir’_a, №16-08-00763_a, and the study was conducted using equipment provided by the Omsk Regional Collective Usage Center of SB RAS. The authors thank Yu.A. Sten’kin, K.E. Ivlev for the synthesis of initial MWCNTs and composites SnO2-x/MWCNTs, respectively. The authors are thankful to the director and administrative staff of the Helmholtz-Zentrum Berlin, and to coordinators of the Russian-German laboratory of BESSY II synchrotron.
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Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Effect of carbon nanotubes irradiation by argon ions on the formation of SnO2-x/MWCNTs composite S.N. Nesov a, P.M. Korusenko a,⇑, S.N. Povoroznyuk a,b, V.V. Bolotov a, E.V. Knyazev a, D.A. Smirnov c,d a
Omsk Scientific Center, Siberian Branch of the Russian Academy of Sciences, Karl Marx Avenue 15, 644040 Omsk, Russia Omsk State Technical University, Mira Avenue 11, 644050 Omsk, Russia c St. Petersburg State University, 198504 St. Petersburg, Russia d Institute of Solid State Physics, Dresden University of Technology, D-01062 Dresden, Germany b
a r t i c l e
i n f o
Article history: Received 20 June 2017 Received in revised form 8 August 2017 Accepted 25 August 2017
Keywords: Argon ions Irradiation Functionalization Composite Tin oxide Multi-walled carbon nanotubes XPS XANES
a b s t r a c t Methods of transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and X-ray absorption near edge structure (XANES) were used to investigate the influence of irradiation by argon ions of multi-walled carbon nanotubes (MWCNTs) on the composites SnO2-x/MWCNTs formation. It has been shown that irradiation of the MWCNTs leads to the formation of structural defects and oxygen-containing groups in the walls of carbon nanotubes. The presence of the latest in MWCNTs walls leads to chemical interaction of the tin oxide with the surface of MWCNTs through formation of chemical SnAOAC bonds at the interfaces ‘‘Tin oxide-MWCNTs” at synthesis of the composite by CVD method. It has been established that the crystalline structure of the tin oxide clusters forming on surface of MWCNTs depends essentially on the defectiveness in the structure of the outer walls of carbon nanotubes. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction The interest to nano-structured composite materials based on carbon nanotubes (CNTs), decorated with nanoparticles or layers of metal oxides, is stimulated by a variety of their potential practical use in various fields of industry and technology. The distribution of metal oxide nanoparticles on the carbon nanotubes surface which has a high surface area and electrical conductivity leads to high functional characteristics of such composites. In particular, the composites based on arrays of multi-walled CNTs (MWCNTs) and tin oxide (SnOx/MWCNTs, 1 < x < 2) have been successfully tested as an anode material for lithium-ion batteries, as well as the material for the gas sensors [1–4]. It is known that CNTs with low level of structural defects exhibit sufficiently weak reactivity, which prevents fixing and uniform distribution of the metal oxides particles on their surface during the formation of composites. Increased adhesion of metal oxides particles to CNTs surface is most often achieved by functionalization of carbon nanotubes using wet chemical methods due to the ⇑ Corresponding author at: Omsk Scientific Centre, Siberian Branch, Russian Academy of Sciences, Karl Marx Avenue, 15, Omsk 644024, Russia. E-mail address: [email protected] (P.M. Korusenko). http://dx.doi.org/10.1016/j.nimb.2017.08.040 0168-583X/Ó 2017 Elsevier B.V. All rights reserved.
formation of structural defects and linking of oxygen-containing groups [5,6]. Another way to increase the surface activity of CNTs is introduction structural defects using ion irradiation (with a various the type of ions, energy and the irradiation dose), which may lead to the formation of various structural defects (vacancies, vacancy clusters, etc.). The presence of such defects in the external graphene layers of CNTs increases the number of dangled chemical bonds and twofold coordinated carbon atoms, which, as it has been shown in [7], significantly increase the reactivity of graphene plane. Chemical interaction of the deposited metal oxide particles of with the surface of carbon nanotubes increases the mechanical properties of the composite, significantly affects the processes of charge transport across the interphase boundaries, which significantly increases the functional characteristics of composites based on CNTs and metal oxides. Previously using TEM and Raman data we have shown that irradiation of MWCNTs with argon ions leads to the dramatic changes in nanotube structure. There are numerous discontinuities of graphene planes forming scaly-like structure in the MWCNTs. Also, there seems to be a well-defined amorphization of the walls of carbon nanotubes [8]. In this paper, by Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge
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structure (XANES) using synchrotron radiation, there has been studied the effect of irradiation with argon ions on the change in the structure and chemical state of the surface walls of MWCNTs. The effect of prior exposure to argon ions on the forming of composites based on arrays of MWCNTs and tin oxide has been studied. The use of the complex of methods XPS and XANES allows to conduct the analysis of the surface layers of composites as well as the analysis of the interface of ‘‘Metal oxide – MWCNTs” without the destruction and modification of the analyzed layers.
2. Material and methods 2.1. Synthesis and irradiation conditions The layers of MWCNTs were grown by chemical vapor deposition (CVD) through the pyrolysis of a mixture of acetonitrile with ferrocene (100:1). The pyrolysis was performed at atmospheric pressure at 800 °C in argon flow (150 mL/min) on substrates of silicon wafers with 100-nm thick oxide layers. Synthesis of MWCNTs continued for 12 min. The thickness of the layers of MWCNTs was 15 ± 5 lm. The average diameter of CNTs is 40 nm. The initial MWCNTs contained about 3 at.% of nitrogen, which is embedded in the nanotubes walls as pyridine, pyrrolic and graphite-like defects, and also as molecular nitrogen (N2) between the walls and in the cavities of carbon nanotubes [9]. The irradiation of MWCNTs was undertaken by a 5 keV Ar+ beam with a dose of 1 1016 ions/cm2 at room temperature. The projected ranges of argon ions in a target MWCNTs layer with a density of 0.2 g/cm3 were estimated using SRIM [10] to be 100 nm. 2.2. Composites synthesis The composites SnO2-x/MWCNTs obtained by CVD were formed on the initial and irradiated MWCNTs by thermal decomposition of hydrous SnCl22H2O at 380 °C, followed by vapor deposition on a heated substrate (MWCNTs/SiO2/Si) at 340 °C. The time taken to form the composites was 15 min. 2.3. TEM, XPS and XANES The morphologies and structures of the composites SnO2-x/MWCNTs were studied by TEM using a JEOL JEM 2200FS microscope in the bright field mode. The study of the composites atomic and electronic structure was conducted using a XPS and XANES technique at the RussianGerman beam line of BESSY II (Berlin) and experimental station PES-RGL. The photoemission (PE) and XANES spectra were measured under vacuum at a residual pressure of 3 10 10 Torr. The survey PE spectra, O 1s core level and C 1s core level spectra were recorded using synchrotron radiation with a photon energy of 850 and 400 eV, respectively. The PE spectra were collected with the hemispherical analyzer (Phoibos 150) in the fixed transmission mode, with a pass energy of 15 eV (for C 1s and O 1s core lines) and 50 eV (for survey spectra). Step energy size was 0.05 eV. The depth of the XPS analysis was 1–2 nm. The XANES spectra were obtained in the total electron yield mode by recording a sample drain current with variations in the energy of photons incident on the sample. The energy resolutions of the monochromator in the ranges of the carbon K-edge (hm 285 eV) and tin M-edge (hm 490 eV) absorption spectra were equal to 70 and 110 meV, respectively. The XANES spectra were normalized to the primary photon current from a gold covered grid recorded contemporaneously. The depth of the XANES analysis was 10–15 nm.
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3. Results and discussion 3.1. Influence of the ion irradiation on the MWCNTs electronic structure Fig. 1 shows the PE spectra of the initial and irradiated MWCNTs, measured in a wide range of binding energies (650– 0 eV). In the spectra there are observed PE lines of oxygen O 1s (binding energy of 533 eV), carbon C 1s (binding energy of 285 eV) and nitrogen N 1s (binding energy of 401 'B), as well as the lines of auger transitions C KLL (kinetic energy of 260 eV) and O KLL (kinetic energy of 510 eV). It is seen that in the spectrum of the irradiated MWCNTs a significant increase in the intensity of the oxygen line is observed. Results of the quantitative analysis indicate that oxygen content increases significantly after the irradiation (Table 1). High oxygen content is caused by the oxidation of the MWCNTs surface due to the increase of number of defects in the MWCNTs structure after of irradiation. Fig. 2a and b shows the results of a detailed analysis of PE lines of carbon (C 1s) of the initial and the irradiated MWCNTs. In accordance with the work [11,12] the most intensive component of the carbon spectrum of the initial MWCNTs with a peak at the binding energy 284.8 eV corresponds to the sp2–hybridized carbon atoms (C@C) constituting the CNT walls (Fig. 2a). A component with the peak at 285.4 eV binding energy is connected with the presence of defects in the walls of MWCNTs (sp3-hybridized amorphous carbon). Components of the spectrum with peaks at binding energies of 287, 289, 290 eV correspond to carbon, incorporated in a variety of carbon-to-oxygen chemical bonds: consisting of epoxy and phenolic groups (CAO, CAOAC, CAOH), carbonyl groups (C@O) and carboxyl oxide groups (COOH), respectively. The most highenergy component of the spectrum with a peak at the binding energy 292 eV corresponds to ‘‘shake up” satellite, which observed in spectra of ordered graphitic carbon materials [13]. As it is seen in Fig. 2b, in the C 1s PE spectrum of irradiated MWCNTs there is observed a significant increase of intensity of the spectrum components corresponding to structural defects as well as to oxygen-carbon chemical bonds (see Table 1). In addition, in the irradiated MWCNTs ‘‘shake up” satellite is not observed. The C K-edge XANES spectrum (Fig. 3, curve 1) of initial MWCNTs are represented by two resonances: the sufficiently narrow and intense p⁄ (C@C)-resonance at an photon energy of 285.3 eV, which characterizes the transitions from the C 1s core
Fig. 1. Survey PE spectra of the initial (1) and irradiated MWCNTs (2). The spectra were collected with an excitation energy of 850 eV.
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Table 1 XPS data for the MWCNTs before and after irradiation. Sample
Initial MWCNTs Irradiated MWCNTs
Concentration of elements, at.%
Relative areas of components C1s,%
C
O
N
C@C
CAC defects
CAO
C@O/COOH
96.7 88.8
1.3 9.6
2.0 1.6
69.0 44.4
21.4 35.9
5.6 13.3
4.0 6.4
Fig. 3. C K-edge XANES spectra of carbon for the initial (1) and irradiated MWCNTs (2).
Fig. 2. C 1s PE spectra of the initial (a) and irradiated MWCNTs (b). The spectra were collected with an excitation energy of 400 eV.
level to free p⁄–2pz states of the conduction band, and the r⁄ (C@C)-resonance formed by the transitions C 1s ? r⁄–2s2px,y. The r⁄ (C@C)-resonance has two local maximum at the photon energy of 291.7 and 292.8 eV. The high-energy region of the absorption edge has fine structure: spectral features at the photon energy 303 and 307 eV is observed. Therefore, the shape of the absorption CK-edge spectrum of the initial MWCNTs indicates a high degree of graphitization of the nanotube walls [14]. In the CK-edge XANES spectrum of the irradiated MWCNTs significant changes are observed (Fig. 3, curve 2). Decrease of intensity of p⁄(C@C)-resonance and shift of its maximum to lowenergy region (photon energy of 284.9 eV) is observed. In addition, the absence of fine structure in the r⁄(C@C)-resonance as well as in high-energy region of the absorption edge
(303–307 eV) is observed. All this indicates a significant increase in defectiveness of the carbon nanotubes wall structure and a possible increase in the number of sp3-hybridized carbon atoms. The intense peaks (a1) and (a2) at photon energies of 286.4 eV and 288.5 eV, respectively, associated with presence various functional oxygen-containing groups on the surface of the irradiated MWCNTs. According to [12,15,–18], the peak (a1) correspond to p⁄-resonance of carbon, being in a chemical bond with oxygen in the composition of hydroxyl (CAOH). The peak (a2) corresponds to p⁄-resonance of carbon in the double bonds (C@O) in the composition of the carboxyl, carbonyl (C@O, COOH). At the same time, high intensity at the range of photon energy of 289–291 eV is suggests the presence of states, which associated with the r⁄(CAO) states of carbon [18]. It is known [19] that the irradiation with argon ions (energy of ions >50 eV) leads to the formation of vacancies and vacancy clusters, increasing the number of dangled bonds and twofold coordinated carbon atoms on the surface of the irradiated MWCNTs. The interaction of such defects with oxygen can lead to the formation of unsaturated (single) carbon-oxygen bonds, which can increase the reactivity of the surface of carbon nanotubes [20]. 3.2. Effect of the MWCNTs irradiation on the composites SnO2-x/MWCNTs formation Fig. 4a and b shows TEM image of the composite SnO2-x/MWCNTs formed on the initial MWCNTs. As shown in the figure, the tin oxide was fixed on the surface of the MWCNTs as separate clusters which has crystalline structure. Previously using electron diffraction data we have shown that the SnO2-x clusters correspond to the tetragonal phase SnO2 [21]. About two hundreds of clusters were measured for this composite. The average clusters size is 120 nm. Fig. 4c, d, e shows TEM image of the composite SnO2-x/MWCNTs formed on the irradiated MWCNTs. As seen
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Fig. 4. TEM images of the composites SnO2-x/N-MWCNTs formed on the initial (a,b) and irradiated MWCNTs (c,d,e). Figure (e) shows the area marked in Figure (d).
(Fig. 4c) on the surface of irradiated carbon nanotubes the tin oxide clusters are distributed more uniformly than in the case of the composite formed on initial MWCNTs. The average clusters size
is 30 nm. In this case, the clusters of tin oxide is consist of differently oriented nanocrystallites with size about 5–10 nm (Fig. 4d and e). The formation of differently oriented nanocrystallites
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in clusters of tin oxide is due to apparently that its growth occurs on the defective walls of the irradiated carbon nanotubes. Fig. 5 shows C K-edge XANES spectra of the composites SnO2-x/MWCNTs formed on the initial and irradiated MWCNTs. It can be seen that the spectrum of composite formed on the initial MWCNTs (Fig. 5, curve 1) hardly differs from the spectrum of the initial carbon nanotubes (Fig. 3, curve 1) except the presence of low-intensity peak at the energy of photons 288.5 eV, which corresponds to p⁄-resonance of carbon in the C@O, COOH bonds. Low intensity of this resonance is indicating to weak oxidation of the surface of the MWCNTs during the formation of the composite. The spectrum of the composite formed on the irradiated MWCNTs (Fig. 5, curve 2) is significantly different from the spectrum of the irradiated MWCNTs (Fig. 3, curve 2). There is observed a significant decrease in the intensity of the p⁄(C@C)-resonance, as well as features (a1). Drop of the intensity p⁄(C@C)-resonance and spectral feature (a1) indicates a decrease in the number of carbon atoms in C@C (sp2) bonds and hydroxide groups (CAOH). In addition, presence of the local spectral feature (a3) at the photon energy of 290.4 eV is observed. This may indicate an interaction of the tin oxide with defective surface of MWCNTs with the formation of SnAOAC chemically bonds. In the work [22] it was shown that a chemical interaction of the graphene surface with metal oxides with the formation of CuAOAC bonds leads to the presence of states in carbon XANES spectra at the photon energy of 290.4 eV. Sn M4,5-edge XANES spectra of the composites SnO2-x/MWCNTs formed on the initial and irradiated MWCNTs are shown in Fig. 6. These spectra are reflecting the transitions from core Sn 3d level to unoccupied p and f-states of the conduction band [23]. The spectrum of Sn M-edge of the composite SnO2-x/MWCNTs formed on the initial MWCNTs (Fig. 6, curve 1), is represented by well-resolved peaks B, C, D (M5-edge of absorption) and the E, F (Sn M4-edge of absorption of tin) [20,21]. Comparison of the absorption spectrum of tin of this composite (Fig. 6, curve 1) and the spectrum of crystalline powder SnO2 (Fig. 6, curve 3) indicates that composition and structure tin oxide in the composite are close to crystalline SnO2. However, the high intensity of the pre-edge feature A and peak D in the spectrum of this composite indicates the presence of a large number of oxygen vacancies and dangled chemical bonds in the structure of tin oxide [23–25]. The Sn M-edge spectrum of the composite SnO2-x/MWCNTs formed on the irradiated MWCNTs (Fig. 6, curve 2) indicates a deterioration of the tin oxide crystalline structure. This is evidenced by
Fig. 5. C K-edge XANES spectra of carbon for the composites formed on the initial (1) and irradiated MWCNTs (2).
Fig. 6. Sn M-edge XANES spectra of tin for the composites formed on the initial (1), irradiated MWCNTs (2) and crystalline SnO2 (3).
the loss of the fine structure of the spectrum and the merge of the main peaks B-C and E-F. This indicates that the presence of structural defects, dangled bonds and various oxygen-containing groups on the surface of the MWCNTs has a significant influence on the crystal structure of the tin oxide clusters during formation composite. Deterioration of the tin oxide crystalline structure associated with presence differently oriented nanocrystallites and grain boundaries, containing a large number of structural defects in clusters of tin oxide. This is confirmed by TEM data (see Fig. 4). In addition, in the XANES spectrum of this composite the peaks A and D have a much lower intensity than in the case of the composite formed on the initial MWCNTs. Decrease of the number of oxygen vacancies in the structure of tin oxide may be because of its interaction with oxygen-containing groups on the surface of irradiated MWCNTs and also formation of SnAOAC bonds. This interaction apparently is the cause of the deterioration of the crystalline structure in the tin oxide clusters. In the survey PE spectra of the composites formed on the initial and irradiated MWCNTs (Fig. 7) the lines of tin, carbon, nitrogen, oxygen, and chlorine are observed. The results of quantitative
Fig. 7. Survey PE spectra of the composites formed on the initial (1) and irradiated MWCNTs (2). The spectra were collected with an excitation energy of 850 eV.
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S.N. Nesov et al. / Nuclear Instruments and Methods in Physics Research B 410 (2017) 222–229 Table 2 XPS data for the composites formed on the initial and irradiated MWCNTs. Composite
formed on the initial MWCNTs formed on irradiated MWCNTs
Concentration of elements, at.% C
O
Sn
N
Cl
83.1 69.3
13.7 24.5
2.0 4.2
0.8 1.1
0.4 0.9
Fig. 8. C 1s PE spectra of the composites formed on the initial (a) and irradiated MWCNTs (b). The spectra were collected with an excitation energy of 400 eV.
XPS analysis show that the content of tin on the surface of irradiated MWCNTs is over two times more than the content of tin in the composite formed on the initial MWCNTs (Table 2). A higher tin content for the same conditions of composites synthesis to indicate the increase of the adhesion of tin oxide to the surface of irradiatred MWCNTs due to the increase in the number of places of its possible fixation (structural defects, dangled bonds and oxygencontaining functional groups). This is also agreeing with the TEM data (Fig. 4). Comparative analysis of C 1s PE spectra of the composites shows a more significant oxidation of the surface of the carbon
Fig. 9. O 1s PE spectra of the composites formed on the initial (a) and irradiated MWCNTs (b). The spectra were collected with an excitation energy of 850 eV.
nanotubes in the composite formed on irradiated MWCNTs (Fig. 8a and b). In the C 1s spectrum of the composite formed on irradiated MWCNTs high intensity components are observed, corresponding to carbon in CAO, CAOAC and C@O, COOH bonds. The O 1s (oxygen) PE spectra of the composites SnO2-x/MWCNTs formed on the initial and the irradiated MWCNTs are shown in Fig. 9. In the O 1s spectrum of the composite SnO2-x/MWCNTs formed on the initial MWCNTs, components on the binding energies of 529.8 eV, 531.6 eV and 533.3 eV (Fig. 9a) are observed. Low-energy component (529.8 eV) corresponds to oxygen in the SnAO bonds [12,22,26]. High-energy components (531.6 and
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533.3 eV) correspond to oxygen in C@O and CAOAC/CAO chemical bonds, respectively [12,22]. Peak fitting of the line O 1s of the composite SnO2-x/MWCNTs formed on the irradiated MWCNTs (Fig. 9b) shows that in the spectrum the components corresponding to oxygen in the composition of the tin oxide (529.8 eV), C@O bonds (531.6 eV), and CAOAC/CAO (533.3 eV) are also present. At the same time, the intensity of the component corresponding to the singlet carbon-oxygen bonds (533.3 eV) is greater than the intensity this component in the spectrum of the composite formed on the initial MWCNTs. Moreover, in the spectrum of the composite formed on the irradiated MWCNTs the presence of an additional component with a peak at the binding energy of 530.7 eV is observed. The last is confirmed by the broadening of the line O 1s at 0.45 eV relative to the oxygen spectrum of the composite formed on the initial carbon nanotubes (Fig. 9). In the work [27,28] it has been shown that the component of oxygen spectrum corresponding to MeAOAC bonds (Me = Sn, Fe, Ag, Cu, Zr) is located on the binding energies by 1–3 eV higher than the component corresponding to oxygen in the composition of the metal oxide (MeAO). Based on this data, we assume that the observed additional component of the oxygen spectrum may be associated with the formation of SnAOAC bonds at the interfaces ‘‘Tin oxide -MWCNTs”. High content of singlet carbon-oxygen bonds (CAO, CAOAC) may indicate to possibility of the formation of SnAOAC bonds in the composite SnO2-x/MWCNTs formed on the irradiated MWCNTs. 4. Conclusions By TEM, XPS and XANES methods with the use of synchrotron radiation there have been investigated the features of the composites SnO2-x/MWCNTs formation depending on the structure and chemical state of carbon nanotube surface. It has been shown that irradiation of the MWCNTs by argon ions with an energy of 5 keV and a dose of 1016 ions/cm2 leads to the formation of structural defects in the walls of carbon nanotubes on which the linking of oxygen-containing groups (C@O, COOH, CAO, CAOAC, CAOH) takes place. It has been shown that the structural and chemical state of the nanotubes surface significantly affects the atomic and electronic structure and composition of the composites SnO2x/MWCNTs at their formation by CVD method. The irradiation of MWCNTs by argon ions lead to increases the amount of tin oxide which is deposited onto the surface of carbon nanotubes that indicates an increase in adhesion of the tin oxide to carbon nanotubes surface. Furthermore, the presence of defects and oxygencontaining groups on the surface of MWCNTs leads to chemical interaction of the tin oxide with the surface of MWCNTs through formation of chemical SnAOAC bonds at the interfaces ‘‘Tin oxide-MWCNTs”. It has been established that the structure of the tin oxide deposited on surface of MWCNTs depends essentially on the defectiveness in the structure of the outer walls of carbon nanotubes. Acknowledgments This work was partially supported by the RFBR (Russian Foundation for Basic Research, Russia), research project nos. №15-42-04308 r_sibir’_a, №16-08-00763_a, and the study was conducted using equipment provided by the Omsk Regional Collective Usage Center of SB RAS. The authors thank Yu.A. Sten’kin, K.E. Ivlev for the synthesis of initial MWCNTs and composites SnO2-x/MWCNTs, respectively. The authors are thankful to the director and administrative staff of the Helmholtz-Zentrum Berlin, and to coordinators of the Russian-German laboratory of BESSY II synchrotron.
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