XPS, EELS, AND TEM STUDY OF FLUORINATED CARBON MULTI-WALLED NANOTUBES WITH LOW CONTENT OF FLUORINE ATOMS Текст научной статьи по специальности «Химические науки»

УГЛЕРОДНЫЕ НАНОСТРУКТУРЫ

CARBON NANOSTRUCTURES

XPS, EELS, AND TEM STUDY OF FLUORINATED CARBON § MULTI-WALLED NANOTUBES WITH LOW CONTENT J

OF FLUORINE ATOMS

.c

c

Y.M. Shulga1'2, Ta-Chang Tien2, Chi-Chen Huang2, Shen-Chuan Lo2,

V.E. Muradyan1, N.F. Polyakova3, Yong-Chien Ling4 f

'c

CO

1 Institute of Problems of Chemical Physics RAs, Chernogolovka, 142432 Russia g

2 Industrial Technology Research Institute, Hsin-Chu, Taiwan 310, R.O.C. ®

3 sRI Electrical carbon products, Electrougli, 142455 Russia 4 National Tsing-hua university, hsin-Chu, Taiwan 300, R.O.C.

Carbon multi-walled nanotubes (MWNT) obtained by arc method were fluorinated at a temperature of 420 °C in a flow of F2 diluted with N2 and contained HF as catalyst. F-MWNT's with small F content (10 and 15 wt. %) were studied by means of XRS, EELS and TEM. It was shown that: 1) fluorination begins with external layers of nanotubes; 2) for the fluorinated part of the sample concentration of fluorine noticeably exceeds C2F; 3) maximum F1s line is equal to 688.2 eV and practically does not vary in an investigated range of fluorine concentration; 4) small amount (2-9 %) of ionic C-F bonds is also presented in investigated samples; 5) fluorination of MWNT may be used for production of large diameter F-SWNT.

Introduction

The fluorination of carbon nanotubes usually view as a first stage of their chemical functionalization. In the literature there is an appreciable number of the publications devoted fluorination of single-walled nanotubes (SWNT) and research of their properties. At processing SWNT by gaseous fluorine it is possible to receive samples which structure is described by formula C2F [1]. Thus the tubular structure of a carbon skeleton is kept. Fluorinated SWNT becomes soluble in spirits that does by their more convenient objects for chemical manipulations [2]. The ionic component of the bond C-F decreases with growth of concentration of fluorine in the sample that is expressed in displacement of photoelectron peak F1s to higher binding energy with increase in ratio F/C

[3]. The XPS investigations of F-SWNHs showed that the nature of C-F bonds changed from semi-ionic to co-valent with increase of the fluorination temperature

[4]. The increase in temperature of fluorination leads to SWNT destruction. Thus can be formed amorphous and turbostratic structures if process to make at 300oC [3]. The resistivity of nanotubes increases with reaction temperatures, resulting from the bandgap enlargement at high fluorine concentration. At more higher temperature (500oC) observed formation multi-walled nanotubes (MWNT) from SWNT [2].

In the found out works on fluorination of MWNT there are data which can be considered as contradicting just to the stated data. So, in work [5] affirms, that at interaction MWNT and F2 gas at 500oC the tube structure collapses and formed graphite-like structure with composition CF. Thermal stability fluorinated MWNT (F-MWNT) in an inert atmosphere is high enough — at

heating in an atmosphere of helium their weight remains constant down to 415oC [6]. In infra red spectra bonds CF is shown in a range of wave numbers from 1000 up to 1300 cm-1 [7]). It is considered to be, that reduction of ionic part of bonding C-F is accompanied by increase in frequency of valence vibrations C-F (see, for example, [8]). Unfortunately, almost in all quoted works the oxygen contents in samples under study are not known. Our experience shows, that the high oxygen contents makes fluorinated samples a less stable at heating because flying molecules COF2 are formed [9].

In this paper we present the data obtained in the results of XPS, XPS-EELS and TEM study of fluorinated multi-walls nanotubes (F-MWNT) with low content of fluorine atoms. Reaction with fluorine begins with defective sites. Thus, reaction of fluorination stopped at an initial stage can be considered as the reaction improving mechanical (for example, reinforcing) properties of MWNT. The XPS spectra of F-MWNT with high content of fluorine atoms are also obtained and discussed.

Authors of work [6] believe that in the case of arc MWNT only several inner shells are cylindrical and other layers are scroll-shaped. Scroll-shaped layers may be easy fluorinated. For coaxial cylindrical MWNT with closed ends fluorination is possible only for upper layer.

In our opinion, however, fluorination of cylindrical MWNT with the closed ends also can take place at a corresponding choice of a temperature mode. The end faces of tube which represent as half of fullerene molecule should be fluorinated first of all. At fluorination effective diameter of upper layer tube should increase (the number of double bonds decreases), van der Waals attraction between upper layer and sub-surface layers will weaken. Further, there should be a break of the closed

Статья поступила в редакцию 02.05.2006. The article has entered in publishing office 2.05.2006.

600 400

Binding Energy, eV

Fig.1. Survey XPS spectra of pristine (1) and fluorinated (2) MWNT

50

40

aP 30 ■ffi

1 20

E

10

-.O'

10

20 30

jpjvolume at 0/o

40

Fig.2. Dependence surface concentration of fluorine verses the volume concentration

carbon skeleton. For us the greatest interest would represent a case when breaking of the top layer cylindrical MWNT begins with an end face where the maximal concentration of fluorine on one carbon atom is reached. In this case fluorination of the following layer it would be accompanied sliding the top already fluorinated layer. And as a result such process we should obtain a set fluorinated SWNT. In the present work the special attention has been given to search F-SWNT with large diameter that proves the fluorination mechanism described here.

Experimental

Carbon multi-walled nanotubes were produced by arc-discharge evaporation of spectral clean graphite rods 010x170 mm, at helium pressure of 500 Torr, current density of 175 A/cm2 and voltage gap of about 23 V. Fluorination of MWNT was performed in a nickel reactor at a temperature of about 420 °C in a flow of F2 diluted with N2 at the ratio of 1:10. Gaseous fluorine was produced by electrolysis of acidic potassium trifluoride KF-2HF and contained 3% gaseous HF as catalyst. Two samples with low fluorine content (10 and 15 wt.%) and two samples with high fluorine content (39 and 55 wt.%) were selected for study.

XPS spectra were acquired using a VG ESCALAB 250 spectrometer. Photoelectron process was excited ^ using an Al Ka source with photon ™ energy of 1486.6 eV. The vacuum IT in the analysis chamber was main- .p tained at 10-9 Torr. _

Transmission electron microscopy (TEM, JEM 200CX) was adopted to analyze the microstructure of the fluorination products.

Results and discussion

Characteristic survey XPS spectrum of F-MWNT under study is presented on Fig.1. For comparison also it is shown spectrum of initial MWNT. From presented spectra it is

visible, that in both samples very small quantity of oxygen contains. The element composition of subsurface layers analyzed by method XPS is presented in Table 1. Calculation of the composition was produced by a standard technique from measured integral intensities of analytical lines (C1s, O1s and F1s) and known photoionization cross-sections. (In the table the volume content of fluorine in atomic percent is also given.)

Table 1. Volume content fluorine and calculated from XPS data subsurface composition samples under study

sample No volume content F, wt.% Composition from XPs, at.% volume content F, at.%

C O f

1 0 99.3 0.7 0 0

2 10 73.1 1.1 25.8 6.6

3 15 70.4 0.6 29.0 10.0

4 39 60.5 1.0 38.5 28.8

5 55 48.9 0.5 50.6 43.6

295 290

Binding Fig.3. C1s spectra of

Apparently from the presented data, the content of oxygen in investigated samples does not exceed 1.1 %.

Therefore in last column of the table the volume content of fluorine in atomic percent is also presented for convenience of comparison. Calculation was made in the assumption, that oxygen in the sample in general is not present. (Actually the total maintenance of oxygen in the sample was not measured.)

On Fig.2 dependence of surface concentration of fluorine verses volume is presented. It is visible, that surface concentration always exceeds volume. And, as one would expect, then content of fluorine in the sample increase the difference between volume and surface decreases. The presented data, in our opinion, can mean 1) fluorination begins with external (superficial) layers of nanotubes and 2) the part of products also has the form of tubes, horns or spheres covered by fluorine from the external site.

285 Energy, eV

fluorinated MWNT

280 .

Углеродные наноструктуры

C1s

С-С

CD £

(П

С &

с

291 288 285 Binding Energy, eV

rc-plasmon

295 290 285

Binding Energy, eV

280

Fig.4. Imposing of C1s spectra of pristine and fluorinated

(10 % fluorine) samples. On an insert one of possible approximation of C1s spectrum with help of Gauss curves is presented

Change of the form of line C1s as a result fluorination is shown in a Fig. 3. As a first approximation observable change can be described as follows: intensity of peak with Eb = 284.6 eV (carbon sp2, characteristic for graphite-like planes) decreases and intensity of peak with Eb = 289.3 eV (carbon sp3 bonded with fluorine) increases.

Let's consider now more thin details of spectra. First, full width at half maximum (FWHM) of peak with Eb = 289.3 eV it is appreciable more FWHM peak with Eb = 284.6 eV. It is connected by that ionic character of C-F bond can vary over a wide range (see Introduction), and it should be reflected not only in position of peak F1s, but also in position of peak C1s.

Secondly, from a spectrum of the sample 5 (in this sample the content of fluorine atoms is more, than in compound C2F) follows, that in the sample not fluorinated tubes (intensity of peak with Eb = 284.6 eV makes approximately 7 % from intensity of all line) are kept still. Probably, it is a question of the rests of the thickest nanotubes, present in the initial sample. From a spectrum also follows that for the fluorinated part of the sample the general concentration of fluorine noticeably exceeds those for not destroyed SWNT — C2F. Excess basically can be reached due to formation on the ends fluorinated graphite-like structures of groups CF2. However, concentration of such groups (it there corresponds the small peak at Eb = 291.2 eV marked in figure by an asterisk) it is insignificant, less than 4 %. Hence, to reaction products, caused peak with Eb = 289.3 eV, are also developed graphite sheets, a part from which fluorinated from both sizes.

Let's consider at last features of form of C1s line with the small content of fluorine (Fig.4). In insert one of variants of the description of the form of spectrum C1s fluorinated MWNT with use of Gaussian is presented. For fitting it would be possible to use more correct function 3, but as it will be shown below, Gaussian is quite enough for our purposes. Investigating procedure of fitting, we have established that minimal number of Gaussian, necessary for satisfactory fitting, is equal five. The variation of entry conditions changes position, width and intensity fitting curves a little. Two peaks which maxima for all four samples are at Eb = 284.6 ± 0.1 and 289 ± 0.1 eV have a most stability at such variations. Well enough (the deviation from average value is equal to ± 0.2 eV) has also peak at 291.3 eV. Position and width the remained two peaks are very sensitive to change of entry conditions.

Obtained data allow us to approve, that exists a little (not less than two) electronic configurations of atoms of carbon which correspond to photoelectrons with C1s binding energy laying in a range from 285,0 up to 288,4 eV. Absence of precisely expressed peaks in this energy range means, that borders between these states are washed away (smooth). Probably it is connected with structural variety of products of fluorination, including with presence of defects. Hence, for use of products of fluorination it is necessary to learn to divide them into fractions. The further speculation on this theme has no experimental bases.

Let's note here only, that it is possible to explain absence of the expressed peak in a region nearly Eb = 285.0 eV partially via high conductivity graphite-like sites of nanotubes. Actually, from the presented spectrum it obviously that C1s line of pristine MWNT is asymmetrical. Usually shoulder in higher binding energy size explained as a result of contribution of C-O spices. But oxygen concentration in our samples is very low. It may be think that asymmetry C1s line in the case of MWNT is connected with electron of conductivity (electrons at the Fermi level) as in transition metals (see, for example, [10]).

It is possible to consider as result of procedure of fitting also that fact, that FWHM of the peak corresponded sp2 atoms of carbon, increases with 0.5 up to 1.1 eV with reduction of relative intensity of this peak or with increase in the content of fluorine in the sample. In other words, with reduction of the geometrical sizes graphite-like areas fluctuation of an effective charge on carbon atom with sp2 hybridization becomes more appreciable. Probably also, that in small and greater particles relaxation processes at photo-ionization proceed on a miscellaneous.

As it was already marked, FWHM for peak with Eb = 289.4 eV is appreciable more of that for peak with Eb = 284.6 eV in case of the samples with small fluorine content. But for the sample 5 the widths of these peaks become approximately equal. There is it not only because FWHM of peak at 284.6 eV increases but also because FWHM of peak at 289.4 eV decreases with 1.3 eV at the sample 2 up to 1.0 eV at the sample 5.

Uncertainty of position of peak at 291.3 eV is connected by that this peak is located near to the wide peak connected with excitation plasma oscillation n-electrons. A source of excitation of these losses is photoelectrons C1s formed in graphite-like part of the sample. However, existence of peak at 291.3 eV at us does not raise the doubts,

since it is present also at a spectrum of the sample 5 for which intensity of n-plasmon is minimal (Fig. 4).

Intensity of line 01s is very small (Figure 1). Nevertheless the centre of gravity of the line is defined easily. Position of the centre of gravity of line 01s is presented in Table 2. It is visible, that binding energy O1s electrons for samples 1 and 5 differ on 1.4 eV. Precise correlation between position of the centre of gravity of line O1s and the fluorine content is broken in case of the sample 2. The explanation to it is easy to find — the content of oxygen in it a little bit above, than at the neighbor samples. Occurrence of O1s peak at 534.0 eV means, that sample warming up to T = 120-300° C leads it to de-fluorination with formation of volatile compound COF2 [9].

Table 2. Binding energies (in eV) for 1s electrons of C, O and F in investigated F-MWNT. FWHM of corresponding peaks presented in parentheses.

Sample* C1s O1s F1s

No C-C C-F C-F2

1 284.6 (0.47) - - 532.6 (2.7) -

2 284.6 (0.50) 289.4 (1.3) 291.5 (1.3) 533.1 (3.2) 688.2 (1.6)

3 284.6 (0.51) 289.5 (1.1) 291.7 (1.4) 532.9 (2.9) 688.2 (1.6)

4 284.6 (0.59) 289.4 (1.1) 291.4 (1.6) 533.3 (3.0) 688.1 (1.5)

5 284.7 (1.15) 289.4 (1.0) 291.2 (1.3) 534.0 (2.7) 688.4 (1.5)

< £

от

.2?

693

684

690 687

Binding Energy, eV Fig.5. F1s spectrum of F-MWNT (10 wt.% F)

*See Table 1 for sample composition

According to the data stated in introduction, we should expect change of position of peak F1s at change of concentration of fluorine. However position of peak practically does not vary in an investigated range of concentration. It agrees to that the position of peak C1s, corresponding to the main fluorine-containing of fraction of the samples, also does not vary. If to compare received by us value Eb(F1s) to value Eb(F1s) in polytetrafluoroethyl-ene -[CF2-CF2]n— (PTFE) (689 eV [11]) we can unequivocally speak, that in samples investigated by us bond C-F more ionic, than in this polymer. And it is not surprising, as in investigated samples exist n-electrons which can be displaced aside positively charged neighbor atoms. However, completely ionic bond C-F in our samples also cannot be named. For ionic bonds values Eb(F1s) = 684-686 eV [11] are characteristic. Thus, according to XPS data main part of the C-F bonds formed at fluori-nation of MWNT under described conditions lie at in-

Loss Energy, eV Fig.6. Fine structure: near C1s of pristine MWNT (1); near F1s of F-MWNT with 10 wt. % F (2); near F1s of F-MWNT with 55 wt. % F (3)

termediate position between ionic and covalent types. Probably, that changing conditions of reaction (for example, temperature) it is possible to adjust effective ion-icity of the bonds.

In the given paragraph it was a question of the basic part of communications C-F. However, if closely to look on a fine structure of spectrum F1s it is possible to notice, that besides the basic peak on a spectrum there is a shoulder from lower binding energies (Fig. 5). Fitting of a spectrum with two Gaussian shown that observable shoulder is caused by peak with Eb(F1s) = 686.2...686.6 eV which intensity makes approximately 2-9 % from total intensity of a line. By the position this peak concerns to the atoms of fluorine connected with atom of carbon or, probably, oxygen ionic type of bonding. The tendency is observed — with increase in concentration of fluorine in the sample the relative part of ionic bonds decreases. In case of SWNT the fluorine connected atoms with particles of the catalyst which do not manage to be removed from the sample completely can give the contribution to intensity of line F1s in this place.

In figure 6, the curve 1, the fine structure accompanying photoelectron peak C1s in the case of pristine MWNT is presented. The peak, separated on 6.5 eV from the main photoelectron peak C1s in side of greater binding energies, is caused by losses on excitation plasma oscillation of n-electrons (see, for example, [12]). It is visible, that this peak has high enough intensity.

Curves 2 and 3 represent the structure accompanying photoelectron peak F1s from F-MWNT with the content of fluorine of 10 and 55 %, accordingly. We shall note here, that peaks C1s and F1s are combined in such a manner that their maxima would correspond 0 on scale of loss energy. Conditions of registration of spectra are picked up in such a manner that FWHM of peaks 1s would be equal. All peaks 1s also normalized on height. Comparison of loss spectra shows, that intensity in the region of n-plasmon excitation is very small in case of the sample with 10 % and practically is absent in case of the sample of 55 % of F atoms. It means that the place of photo-ionization and a trajectory of movement of photoelectron F1s from a place of its formation to sample surface are far enough from areas with the generated n-zone. How it is possible for itself to present such situation? The unique reasonable assumption consists that F-containing prod-

Углеродные наноструктуры

Fig.7. TEM image of products of MWNT fluorination

ucts of reaction have no electric contact with nanotubes, which else it is a lot of in case of samples with the small content of fluorine. In other words, nanotubes and products their fluorination are spatially separated.

Direct TEM investigations have confirmed the conclusions made on the basis of the XPS data. As it is possible to see from figure 7, products of reaction are really separated from non-fluorinated MWNT. Separate nano-tubes have identical diameter along all its length. Among products of reaction it is possible to see small sheets with unexpectedly equal edges and the size from 20 up to 100 nanometers. Probably, top layers MWNT will consist of such sheets, which, naturally, reduce reinforcing properties of pristine nanotubes. If these sheets are formed in result of destruction of a continuous cylindrical layer at fluorination our conclusion about an opportunity of production of F-SWNT of the big diameter is incorrect. Fortunately, among products of reaction it is possible to see also the image reminding two-walled nanotube with rough walls and diameter of 5 nanometers (fig. 8). In our opinion, it of course is image F-SWNT.

Thus, from the data submitted here it is possible to draw a conclusion, that in process of fluorination probably to produce F-SWNT with big diameter.

Summary

In summary, the main results of XPS study of fluorinated (at a temperature of about 420 °C in a flow of F2 diluted with N2 and contained HF as catalyst) multi-walls nano-tubes with low content of fluorine atoms are as follows. (1) Fluorination begins with external (superficial) layers of nanotubes. (2) The part of products also has the form of tubes, horns or spheres covered by fluorine from the external site. (3) For the fluorinated part of the sample the general concentration of fluorine noticeably exceeds C2F. (4) The atoms of oxygen connected with fluorinated parts of MWNT give in the XPS spectra peak with Eb = 534.0 eV, that on 1.4 eV more than this value for the atoms connected with clean MWNT. (5) For the sample under study Eb(F1s) is equal to 688.2 eV and practically does not vary in an investigated range of fluorine concentration. (6) Small amount (2-9 %) of ionic C-F bonds is also presented in investigated F-MWNT. (7) Nanotubes and products their fluorination

Fig.8. HRTEM image of separated structure from the products of MWNT fluorination, which may be consider as fluorinated SWNT with large diameter

are spatially separated. (8) Fluorination of MWNT may be used for production of large diameter F-SWNT.

References

1. E.T.Mickelson, C.B.Huffman, A.G.Rinzler et al. Fluorination of single-wall carbon nanotubes. chem. Phys. Lett, 296, (1998) 188.

2. E.T.Mickelson, I.W.Chiang, J.L.Zimmerman et al. Salvation of fluorinated single-wall carbon nanotubes in alcohol solvents. J. Phys. Chem. B, 103, (1999) 4318.

3. K.H.An, J.G.Heo, K.G.Jeon et al. X-ray photoemission spectroscopy study of fluorinated single-walled carbon nanotubes. Appl. Phys. Lett., 80, (2002) 4235.

4. Y.Hattori, H.Kanoh, F.Okino, H.Touhara, D.Kasuya, M.Yudasaka, S.Iijima, K.Kanko, J. Phys. Chem. B108, 9614-9618 (2004).

5. A.Hamwi, H.Alvergnat, S.Bonnamy, F.Bftguin. Fluorination of carbon nanotubes. Carbon, 35, (1997) 723.

6. N.F.Yudanov, A.V.Okotrub, Yu.V.Shubin et al. Fluorination of arc-produced carbon material containing multiwall nanotubes. Chem. Mater., 14, (2002) 1472

7. T.Nakajima, S.Kasamatsu, Y.Matsuo. Synthesis and characterization of fluorinated carbon nanotube. Eur. J. Solid State Inorg. Chem., 33, (1996) 831

8. V.Gupta, T.Nakajima, Y.Ohzawa, B.'Remva. A study on the formation mechanism of graphite fluorides by Raman spectroscopy. J. Fluorine Chem., 120, (2003)143.

9. Yu. M. Shul'ga, V. E. Muradyan, V. M. Martynen-ko, B. P. Tarasov, N. V. Polyakova "Mass-Spectrometric Investigation of Gases Evolved from Fluorinated Multi-Walled Carbon Nanotubes at Heating Fullerenes", Nanotubes, and Carbon Nanostructures, 14, (2006) in press.

10. D. Briggs and J. C. Riviere, in Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, eds. D. Briggs and M. P. Seach, John Wiley, New York, 1983.

11. Handbook of XPS (Eds by C.D.Wagner, W.M.Riggs, L.E.Davis, J.F.Moulder, G.E.Muilenberg), Eden Prairie, Minnesota, 1979.

12. Yu.M.Shul'ga, V.I.Rubtsov, A.S.Lobach, "Reflection-electron— energy-loss spectra of the fullerenes C60 and C70". Z.Phys.B: Condensed Matter., 93 (1994) 327.