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X-ray photoelectron investigation of carbon nanostructures in iron matrix
Journal of Electron Spectroscopy and Related Phenomena 156–158 (2007) 191–194
X-ray photoelectron investigation of carbon nanostructures in iron matrix I.N. Shabanova a,b,∗ , L.G. Makarova b , N.S. Terebova a , V.I. Ladyanov a , R.M. Nikonova a a
Physical-Technical Institute of the Ural Branch of the Russian Academy of Sciences, Izhevsk, Russia b The Udmurt State University, Izhevsk, Russia Available online 3 December 2006
Abstract It is shown that both sp2 and sp3 hybridized structures form in samples modified with fullerene and prepared by fusion, which is indicative of the carbon nanostructure formation. In the samples modified with graphite and prepared by fusion, only graphite-like structures form. © 2006 Published by Elsevier B.V. Keywords: Carbon nanostructures; sp2 ; sp3 ; XPS; Satellite structure
1. Introduction For many years, the improvement of the mechanical properties of structural materials was mainly performed by the development of new alloys with new chemical and phase compositions. Lately, new ways have appeared for improving properties of structural materials, namely by the well-directed formation of micro- and nano-crystalline structure. The set of the experimental methods that are used for studying the chemical structure of carbon cluster nanostructures is limited. Therefore, one of the main tasks is the development of diagnostic methods, which will allow controlling intermediate and final results in the creation of new materials. At present, the analysis of numeral works shows that classical methods for determining shapes, sizes and compositions of carbon nanostructures are transmission electron microscopy, methods based on electron diffraction and Raman spectroscopy. However, more and more publications appear referring to the investigation of nanostructures with the use of the X-ray photoelectron spectroscopy method. Further development of the X-ray photoelectron spectroscopy method and related methods for the surface (from 1 to 10 nm) investigation will lead to an increase in the number of methods for studying compositions, electronic properties and structures of nanostructures. The X-ray photoelectron spectroscopy has been used for the determination of the type of carbon structures. The XPS method allows investigating the electron structure, chemical bond and
nearest environment of atoms. One of the important specific features of the method is its non-destructive character of action since the X-ray radiation used for photoelectron excitation does not practically cause any damages in most materials. This cannot be said about the surface analysis methods that involve ion or electron bombardment of a surface. In most cases, a sample can further be used for some other investigations after it has been studied by the XPS method. In addition, the method provides the possibility to analyze thin layers and films, which is very important for the case of the formation of fullerenes, nanotubes and nanoparticles, and to obtain the information on the sample chemical composition based on spectra, which provides the control over chemical purity of materials. The XPS method allows investigating electron structure, chemical bond, the nearest environment of atoms with the use of an X-ray photoelectron magnetic spectrometer. Russian X-ray photoelectron magnetic spectrometers with automated control system [1] are not inferior to the best foreign spectrometers in their main parameters. The preference is given to the X-ray electron magnetic spectrometer because of a number of advantages compared with electrostatic spectrometers [2], which are the high spectrum contrast, the permanency of optical efficiency and resolution capacity that are not influenced by electron energy. Moreover, the XPS method is a non-destructive investigation method. 2. Experimental
∗
Corresponding author. Tel.: +7 3412432539; fax: +7 3412250614. E-mail address: [email protected] (I.N. Shabanova).
0368-2048/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.elspec.2006.11.059
In this work, the samples prepared from iron powder modified with fullerenes or graphite were studied with the use of the
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Table 1 The investigated samples Sample no.
Sample composition
Sample form
1 2 3 4 5 6
Fe + 0.5% C60/70 Fe + 0.5% graphite Fe + 2% C60/70 Fe + 2% graphite Fe + 2% C60/70 Fe + 2% graphite
Ingot (T = 1410 ◦ C) Ingot (T = 1410 ◦ C) Powder Powder Pellet (P = 800 MPa) Pellet (P = 800 MPa)
XPS method. The samples were prepared in two ways: fusion or pressing. Modification was carried out for obtaining nanocarbon structures in metal matrices in order to improve strength properties of a material. The samples were verified with the use of X-ray diffraction, which showed that the sample structures were mainly fcc iron. The description of the samples studied is given in Table 1. The X-ray photoelectron investigations were carried out for studying the changes in the nearest environment of the carbon atoms in the samples prepared in different ways. The investigations were carried out using the X-ray photoelectron magnetic spectrometer with double focusing and instrumental resolution of 0.1 eV at the excitation of AlK␣-lines (1486.6 eV). For the XPS investigations of carbon-metal cluster nanomaterials, the method of the C 1s spectra identification by the satellite structure was employed. To do this, reference samples were studied, the carbon components of which could give the C 1s spectrum: C–H, hydrocarbons [3]; C–C (sp2 ), graphite [4]; C–C (sp3 ), diamond [5]. The spectra parameters are presented in Table 2, where Eb is binding energy, Esat is the energy characterizing the satellite position, Isat is the satellite intensity, I0 is the intensity of the main maximum. To identify the structures studied in [6], the C 1s spectra of carbon nanostructures were studied. The nanostructures were obtained in the electric arc during graphite electrode sputtering. The carbon nanostructures obtained were fullerenes C60 , single-walled and multi-walled carbon nanotubes and amorphous carbon. It is shown that in all the C 1s spectra there is a satellite structure related to different effects (a shake-up process, characteristic losses – plasmons [7]), which allows to create a calibration technique and to determine not only the energy position of the components but their intensities as well. In the C 1s spectrum of fullerenes C60 , there is a satellite with binding energy of 313 eV [8] and the relative intensity of 15% from the main peak. This satellite is characteristic of the sp2 -hybridization of the valence electrons of the carbon atoms.
In the C 1s-spectrum of single-walled carbon nanotubes, in addition to a gradually rising spectrum in the high-energy region, two satellites are observed characteristic of the C–C bonds with the sp2 - and sp3 -hybridization of the valance electrons of the carbon atoms. Consequently, in the C 1s-spectrum of the one-layer nanotubes, there are these two components with the binding energies of 284.3 and 286.1 eV and the intensities of 1:0.1 and 1:0.15 relative to their satellites and the width of 1.8 eV. The ratio between the C–C bonds with the sp2 - and those with sp3 -hybridization of valence electrons is 2. The similar situation is observed for the C 1s-spectrum of multi-walled carbon nanotubes. In the region of 313 eV of the amorphous carbon spectrum, there is a satellite characteristic of sp3 -hybridization of the valence electrons with the relative intensity of 15%. Consequently, in the C 1s spectrum there is a component characteristic of C–C bond with sp3 hybridization of the valence electrons at the distance of 27 eV from the satellite. Thus, the amorphous carbon presents carbon inclusions that are like a globe-shaped form of graphite.
Fig. 1. The experimental C 1s spectra of samples no. 1 (a) and no. 2 (b).
Table 2 The C1s spectra parameters for reference samples
C–H [3] hydrocarbons C–C (sp2 ) [4] graphite C–C (sp3 ) [5] diamond
Eb (eV)
Esat (eV)
E = Esat − Eb (eV)
FWHM (eV)
Isat /I0 , Δ = 10%
285.0 ± 0.1 284.3 ± 0.1 286.1 ± 0.1
292.0 ± 1.0 306.0 ± 1.0 313.0 ± 1.0
∼6–7 ∼22 ∼27
2.0 ± 0.1 1.8 ± 0.1 1.8 ± 0.1
0.10 0.10 0.15
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The development of a calibration method for spectra in the X-ray photoelectron investigations of reference samples allows to realize the decomposition of the C 1s spectrum to the components that determine the chemical bond, the hybridization type of s–p valence electrons of the carbon atoms and the nearest environment of the carbon atoms. Studying the nanoparticles with a known structure gives the possibility of the identification of studied carbon structures by examining the C 1s spectra shapes. The C 1s-spectra identification method developed by us was successfully used for the investigation of nanostructures in the iron matrix. 3. Results and discussion The X-ray photoelectron spectroscopy method was used to obtain the C 1s, O 1s and Fe 2p spectra. The O 1s spectra show that large amounts of adsorbed oxygen and iron oxides are present on the sample surfaces. The Fe 2p spectra are also indicative of the presence of the oxidized iron layer on the surfaces of the samples. During the experiment, the shift of the spectra was not observed. The experimental C 1s-line spectra for samples no. 1 and no. 2 are shown in Fig. 1a and b. The mathematical treatment of the C 1s spectra was performed, i.e., the background subtraction and the procedure of the spectra smoothing and decomposition. The results are given in Fig. 2. In Fig. 2a, the X-ray photoelectron spectrum of the C 1sline is displayed, which was obtained from sample no.1 without heating in the spectrometer chamber. In the high-energy region, two satellites are observed, which are characteristic of sp2 and sp3 hybridization of the valence electrons of the carbon atoms judging by their binding energies (306.0 eV and 313.0 eV, respectively). According to the data in Table 2, in the C 1s spectrum, there are components characteristic of the C–C (sp2 ) and C–C (sp3 ) bonds at distances of ∼22 and ∼27 eV. The relation of the C–C (sp2 ) bond intensity to the C–C (sp3 ) bond intensity is ∼2, which is characteristic of carbon nanostructures. In the spectrum, there are also components of C–H and C–O bonds, which characterize surface contaminations. When the sample is heated, the breakup of the C–C bonds with sp3 -type of hybridization of the valence electrons of the carbon atoms is taking place. At further heating of the sample, in the C 1s spectrum there is a component characteristic of Fe–C bonds. In addition, at heating, the Fe 2p spectra are observed on the sample surface, which is indicative of the fact that the sample surface is being cleaned during heating. In Fig. 2b, the C 1s spectrum is displayed, which is obtained from sample no. 2 without heating in the spectrometer chamber. In the high-energy region, a satellite with the binding energy of ∼306.0 eV is observed, which is characteristic of the sp2 hybridization of the valence electrons of the carbon atoms. Consequently, in the C 1s spectrum, at the distance of ∼22 there is a component characteristic of C–C bonds with sp2 hybridization. There are also components characteristic of C–H and C–O bonds in the spectrum. The X-ray photoelectron investigations of samples 3, 4, 5 and 6 show that the C 1s spectra do not have a satellite and they
Fig. 2. The X-ray photoelectron C 1s spectra obtained from samples no. 1 (a) and no. 2 (b) after the spectra were decomposed into their components.
have low intensity. Thus, only hydrocarbon contaminations are present on the surfaces of these samples. 4. Conclusion The investigations conducted have demonstrated that when iron powder is mixed with fullerene mixture C60 /C70 or with graphite powder and then the mixtures obtained are treated differently (fusion and pressing), there are strong differences in the X-ray photoelectron spectra. On the surfaces of the samples prepared from the mixture of the iron powder and the mix of fullerenes C60 /C70 and subjected to fusion, carbon nanostructures are present. On the surface of the samples prepared from the mixture of the iron powder and graphite powder, which were also subjected to fusion, graphitelike structures, are present. Acknowledgement The work is supported by the RFFR grant—Ural No. 04-0296021 and the Integration project of SO RAS.
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References [1] V.A. Trapeznikov, A.I. Yefimenko, A.V. Yevstafyev, et al. Automated Electron Magnetic Spectrometer, no. B430326. VNTITsentr., M., 1975, p. 176 (in Russia). [2] K. Siegbahn, C. Nordling, A. Fahlman, et al., Uppsala (1967) 493. [3] D. Briggs, M.P. Sih (Eds.). The Analysis of Surface with the Use of the Auger- and X-ray Photoelectron Spectroscopy. Mir, M., 1987, p. 600. [4] I.N. Shabanova Dissert. na soisk. stepeni doktora fiz.-mat. nauk. Izhevsk, 1990, p. 502.
[5] K.M. Kolobova, I.N. Shabanova, O.A. Kulyabina, et al., FMM 54 (4) (1981) 890–893. [6] L.G. Makarova, I.N. Shabanova, N.S. Terebova, Nanotekhnika 4 (2005) 55–57. [7] V.I. Nefedov. X-ray Electron spectroscopy of Chemical Compounds. Reference Book. Khimia, M., 1984, p. 256 (in Russia). [8] M.A. Khodorkovskiy, A.L. Shakhmin, N.B. Leonov, FTT 36 (3) (1994) 626–630.
X-ray photoelectron investigation of carbon nanostructures in iron matrix I.N. Shabanova a,b,∗ , L.G. Makarova b , N.S. Terebova a , V.I. Ladyanov a , R.M. Nikonova a a
Physical-Technical Institute of the Ural Branch of the Russian Academy of Sciences, Izhevsk, Russia b The Udmurt State University, Izhevsk, Russia Available online 3 December 2006
Abstract It is shown that both sp2 and sp3 hybridized structures form in samples modified with fullerene and prepared by fusion, which is indicative of the carbon nanostructure formation. In the samples modified with graphite and prepared by fusion, only graphite-like structures form. © 2006 Published by Elsevier B.V. Keywords: Carbon nanostructures; sp2 ; sp3 ; XPS; Satellite structure
1. Introduction For many years, the improvement of the mechanical properties of structural materials was mainly performed by the development of new alloys with new chemical and phase compositions. Lately, new ways have appeared for improving properties of structural materials, namely by the well-directed formation of micro- and nano-crystalline structure. The set of the experimental methods that are used for studying the chemical structure of carbon cluster nanostructures is limited. Therefore, one of the main tasks is the development of diagnostic methods, which will allow controlling intermediate and final results in the creation of new materials. At present, the analysis of numeral works shows that classical methods for determining shapes, sizes and compositions of carbon nanostructures are transmission electron microscopy, methods based on electron diffraction and Raman spectroscopy. However, more and more publications appear referring to the investigation of nanostructures with the use of the X-ray photoelectron spectroscopy method. Further development of the X-ray photoelectron spectroscopy method and related methods for the surface (from 1 to 10 nm) investigation will lead to an increase in the number of methods for studying compositions, electronic properties and structures of nanostructures. The X-ray photoelectron spectroscopy has been used for the determination of the type of carbon structures. The XPS method allows investigating the electron structure, chemical bond and
nearest environment of atoms. One of the important specific features of the method is its non-destructive character of action since the X-ray radiation used for photoelectron excitation does not practically cause any damages in most materials. This cannot be said about the surface analysis methods that involve ion or electron bombardment of a surface. In most cases, a sample can further be used for some other investigations after it has been studied by the XPS method. In addition, the method provides the possibility to analyze thin layers and films, which is very important for the case of the formation of fullerenes, nanotubes and nanoparticles, and to obtain the information on the sample chemical composition based on spectra, which provides the control over chemical purity of materials. The XPS method allows investigating electron structure, chemical bond, the nearest environment of atoms with the use of an X-ray photoelectron magnetic spectrometer. Russian X-ray photoelectron magnetic spectrometers with automated control system [1] are not inferior to the best foreign spectrometers in their main parameters. The preference is given to the X-ray electron magnetic spectrometer because of a number of advantages compared with electrostatic spectrometers [2], which are the high spectrum contrast, the permanency of optical efficiency and resolution capacity that are not influenced by electron energy. Moreover, the XPS method is a non-destructive investigation method. 2. Experimental
∗
Corresponding author. Tel.: +7 3412432539; fax: +7 3412250614. E-mail address: [email protected] (I.N. Shabanova).
0368-2048/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.elspec.2006.11.059
In this work, the samples prepared from iron powder modified with fullerenes or graphite were studied with the use of the
192
I.N. Shabanova et al. / Journal of Electron Spectroscopy and Related Phenomena 156–158 (2007) 191–194
Table 1 The investigated samples Sample no.
Sample composition
Sample form
1 2 3 4 5 6
Fe + 0.5% C60/70 Fe + 0.5% graphite Fe + 2% C60/70 Fe + 2% graphite Fe + 2% C60/70 Fe + 2% graphite
Ingot (T = 1410 ◦ C) Ingot (T = 1410 ◦ C) Powder Powder Pellet (P = 800 MPa) Pellet (P = 800 MPa)
XPS method. The samples were prepared in two ways: fusion or pressing. Modification was carried out for obtaining nanocarbon structures in metal matrices in order to improve strength properties of a material. The samples were verified with the use of X-ray diffraction, which showed that the sample structures were mainly fcc iron. The description of the samples studied is given in Table 1. The X-ray photoelectron investigations were carried out for studying the changes in the nearest environment of the carbon atoms in the samples prepared in different ways. The investigations were carried out using the X-ray photoelectron magnetic spectrometer with double focusing and instrumental resolution of 0.1 eV at the excitation of AlK␣-lines (1486.6 eV). For the XPS investigations of carbon-metal cluster nanomaterials, the method of the C 1s spectra identification by the satellite structure was employed. To do this, reference samples were studied, the carbon components of which could give the C 1s spectrum: C–H, hydrocarbons [3]; C–C (sp2 ), graphite [4]; C–C (sp3 ), diamond [5]. The spectra parameters are presented in Table 2, where Eb is binding energy, Esat is the energy characterizing the satellite position, Isat is the satellite intensity, I0 is the intensity of the main maximum. To identify the structures studied in [6], the C 1s spectra of carbon nanostructures were studied. The nanostructures were obtained in the electric arc during graphite electrode sputtering. The carbon nanostructures obtained were fullerenes C60 , single-walled and multi-walled carbon nanotubes and amorphous carbon. It is shown that in all the C 1s spectra there is a satellite structure related to different effects (a shake-up process, characteristic losses – plasmons [7]), which allows to create a calibration technique and to determine not only the energy position of the components but their intensities as well. In the C 1s spectrum of fullerenes C60 , there is a satellite with binding energy of 313 eV [8] and the relative intensity of 15% from the main peak. This satellite is characteristic of the sp2 -hybridization of the valence electrons of the carbon atoms.
In the C 1s-spectrum of single-walled carbon nanotubes, in addition to a gradually rising spectrum in the high-energy region, two satellites are observed characteristic of the C–C bonds with the sp2 - and sp3 -hybridization of the valance electrons of the carbon atoms. Consequently, in the C 1s-spectrum of the one-layer nanotubes, there are these two components with the binding energies of 284.3 and 286.1 eV and the intensities of 1:0.1 and 1:0.15 relative to their satellites and the width of 1.8 eV. The ratio between the C–C bonds with the sp2 - and those with sp3 -hybridization of valence electrons is 2. The similar situation is observed for the C 1s-spectrum of multi-walled carbon nanotubes. In the region of 313 eV of the amorphous carbon spectrum, there is a satellite characteristic of sp3 -hybridization of the valence electrons with the relative intensity of 15%. Consequently, in the C 1s spectrum there is a component characteristic of C–C bond with sp3 hybridization of the valence electrons at the distance of 27 eV from the satellite. Thus, the amorphous carbon presents carbon inclusions that are like a globe-shaped form of graphite.
Fig. 1. The experimental C 1s spectra of samples no. 1 (a) and no. 2 (b).
Table 2 The C1s spectra parameters for reference samples
C–H [3] hydrocarbons C–C (sp2 ) [4] graphite C–C (sp3 ) [5] diamond
Eb (eV)
Esat (eV)
E = Esat − Eb (eV)
FWHM (eV)
Isat /I0 , Δ = 10%
285.0 ± 0.1 284.3 ± 0.1 286.1 ± 0.1
292.0 ± 1.0 306.0 ± 1.0 313.0 ± 1.0
∼6–7 ∼22 ∼27
2.0 ± 0.1 1.8 ± 0.1 1.8 ± 0.1
0.10 0.10 0.15
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The development of a calibration method for spectra in the X-ray photoelectron investigations of reference samples allows to realize the decomposition of the C 1s spectrum to the components that determine the chemical bond, the hybridization type of s–p valence electrons of the carbon atoms and the nearest environment of the carbon atoms. Studying the nanoparticles with a known structure gives the possibility of the identification of studied carbon structures by examining the C 1s spectra shapes. The C 1s-spectra identification method developed by us was successfully used for the investigation of nanostructures in the iron matrix. 3. Results and discussion The X-ray photoelectron spectroscopy method was used to obtain the C 1s, O 1s and Fe 2p spectra. The O 1s spectra show that large amounts of adsorbed oxygen and iron oxides are present on the sample surfaces. The Fe 2p spectra are also indicative of the presence of the oxidized iron layer on the surfaces of the samples. During the experiment, the shift of the spectra was not observed. The experimental C 1s-line spectra for samples no. 1 and no. 2 are shown in Fig. 1a and b. The mathematical treatment of the C 1s spectra was performed, i.e., the background subtraction and the procedure of the spectra smoothing and decomposition. The results are given in Fig. 2. In Fig. 2a, the X-ray photoelectron spectrum of the C 1sline is displayed, which was obtained from sample no.1 without heating in the spectrometer chamber. In the high-energy region, two satellites are observed, which are characteristic of sp2 and sp3 hybridization of the valence electrons of the carbon atoms judging by their binding energies (306.0 eV and 313.0 eV, respectively). According to the data in Table 2, in the C 1s spectrum, there are components characteristic of the C–C (sp2 ) and C–C (sp3 ) bonds at distances of ∼22 and ∼27 eV. The relation of the C–C (sp2 ) bond intensity to the C–C (sp3 ) bond intensity is ∼2, which is characteristic of carbon nanostructures. In the spectrum, there are also components of C–H and C–O bonds, which characterize surface contaminations. When the sample is heated, the breakup of the C–C bonds with sp3 -type of hybridization of the valence electrons of the carbon atoms is taking place. At further heating of the sample, in the C 1s spectrum there is a component characteristic of Fe–C bonds. In addition, at heating, the Fe 2p spectra are observed on the sample surface, which is indicative of the fact that the sample surface is being cleaned during heating. In Fig. 2b, the C 1s spectrum is displayed, which is obtained from sample no. 2 without heating in the spectrometer chamber. In the high-energy region, a satellite with the binding energy of ∼306.0 eV is observed, which is characteristic of the sp2 hybridization of the valence electrons of the carbon atoms. Consequently, in the C 1s spectrum, at the distance of ∼22 there is a component characteristic of C–C bonds with sp2 hybridization. There are also components characteristic of C–H and C–O bonds in the spectrum. The X-ray photoelectron investigations of samples 3, 4, 5 and 6 show that the C 1s spectra do not have a satellite and they
Fig. 2. The X-ray photoelectron C 1s spectra obtained from samples no. 1 (a) and no. 2 (b) after the spectra were decomposed into their components.
have low intensity. Thus, only hydrocarbon contaminations are present on the surfaces of these samples. 4. Conclusion The investigations conducted have demonstrated that when iron powder is mixed with fullerene mixture C60 /C70 or with graphite powder and then the mixtures obtained are treated differently (fusion and pressing), there are strong differences in the X-ray photoelectron spectra. On the surfaces of the samples prepared from the mixture of the iron powder and the mix of fullerenes C60 /C70 and subjected to fusion, carbon nanostructures are present. On the surface of the samples prepared from the mixture of the iron powder and graphite powder, which were also subjected to fusion, graphitelike structures, are present. Acknowledgement The work is supported by the RFFR grant—Ural No. 04-0296021 and the Integration project of SO RAS.
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References [1] V.A. Trapeznikov, A.I. Yefimenko, A.V. Yevstafyev, et al. Automated Electron Magnetic Spectrometer, no. B430326. VNTITsentr., M., 1975, p. 176 (in Russia). [2] K. Siegbahn, C. Nordling, A. Fahlman, et al., Uppsala (1967) 493. [3] D. Briggs, M.P. Sih (Eds.). The Analysis of Surface with the Use of the Auger- and X-ray Photoelectron Spectroscopy. Mir, M., 1987, p. 600. [4] I.N. Shabanova Dissert. na soisk. stepeni doktora fiz.-mat. nauk. Izhevsk, 1990, p. 502.
[5] K.M. Kolobova, I.N. Shabanova, O.A. Kulyabina, et al., FMM 54 (4) (1981) 890–893. [6] L.G. Makarova, I.N. Shabanova, N.S. Terebova, Nanotekhnika 4 (2005) 55–57. [7] V.I. Nefedov. X-ray Electron spectroscopy of Chemical Compounds. Reference Book. Khimia, M., 1984, p. 256 (in Russia). [8] M.A. Khodorkovskiy, A.L. Shakhmin, N.B. Leonov, FTT 36 (3) (1994) 626–630.