Study of carbon fiber surfaces by scanning tunneling microscopy, part III. Carbon fibers after surface treatments

Carbon. Vol. 32, No. 2. pp. 323-328, 1994 Copyright 0 1994Elsevier Science Ltd

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STUDY OF CARBON FIBER SURFACES BY SCANNING TUNNELING MICROSCOPY, PART III. CARBON FIBERS AFTER SURFACE TREATMENTS REN-YAN QIN* and JEAN-BAPTISTE DONNET

Centre de Recherches sur la Physico-Chimie des Surfaces Solides, 24, Avenue du President Kennedy, 68200 Mulhouse, France, and Ecole Nationale Superieure de Chimie de Mulhouse, 3 rue

CNRS,

Alfred Werner. 68093 Mulhouse, France (Received 25 January 1993, accepted in revisedform

25 Augrtst 1993)

Abstract-The

effect of surface treatments on carbon fibers has been studied by scanning tunneling microscopy (STM). The results show that over a wide range of oxidation degrees the anodic or electrochemical oxidation does not change the surface aspects, neither at microscopic scale nor at nanometric scale, while the air plasma oxidation under the experimental conditions gives rise to surface roughness and surface degradation. The results at atomic resolution have also revealed that the electrochemical treatment yields a modest oxidation on the surface ofcarbon fiber and that the oxidation attack takes place preferentially at edge sites of graphitic crystallites. The microstructure of the fibers after anodic oxidation resembles very much that of low-temperature carbonized fibers observed by STM. The structural imperfection has been tentatively interpreted in terms of the formation of intercalated graphite compounds that render the graphite layer stacking imperfect. Key WordsCarbon

fiber, surface treatment,

surface structure, scanning tunneling microscopy.

1. INTRODUCTION

The excellent mechanical properties ofcarbon fibers used for the composite materiats necessitate a good interfacial adhesion that transfers the load stress from one fiber to the other via the matrix. The mechanical behavior of this interface depends, to a large extent, on the carbon fiber surface activity, which is ensured by surface treatment. Because of the impo~ance of the surface treatment in enhancing the surface activity of carbon fibers, a great deal of work has been done in the last few decades[ I], and the interest in this area has been growing year by year[2]. In spite of this, the mechanism concerning the surface modification, as well as that of the surface properties, is still not fully understood. Scanning tunneling microscopy provides another powerful way in the study of surface[3], because of its atomic resolution. Several excellent reviews on this subject, with regard to the experimentation, the theory, and the results obtained with this technique have been published recently[4-61. The STM has been successfully applied in the study of carbon fibers[7- 13). In the previous papers[ll-131 of this series of study, we have reported the results obtained by the STM on high modulus, high strength, and activated carbon fibers, which were made from different precursors and under various conditions. In this paper, * Present address: Dept. Chem. Eng., Ecole Polytechnique. C. P. 6079, Stn. A. Montreal. Quebec, Canada, H3C 3Al.

the surface of a mesophase pitch-based carbon fiber followed by anodic oxidation as well as air plasma oxidation has been examined by STM up to atomic resolution. The results will be analysed and compared with those obtained by other techniques.

2. EXPERIM~~AL

A petroleum pesophase pitch-based graphitized carbon fiber has been used as the starting material, i.e., Fiber E in the previous paper]1 I]. It was then followed by (a) an electrochemical oxidation in industrial scale. Two anodic oxidation levels of 0.6 and 6 Aim” in current density have been applied in this work. The samples obtained under these intensities are designated as Fiber E2 and Fiber E4, respectively; (b) a surface treatment of air plasma under the pressure of 0.5 torr and the power of 75 W for 10 minutes, the treated sample is then designated as Fiber E5. All fibers are provided unsized. A commercial STM Nanoscope II (Digital Instruments Inc., Santa Barbara, California, USA) was used. The sample preparation and the experimental procedure have been described in Parts I and 11[11,12]. In the STM experiments, the constant current mode was employed. The images are presented in top view, and the surface topography is expressed by different shades of grey. The parameters adapted are setpoint current 0.5-5.0 nA, bias voltage lo-200 mV, depending on the samples analyzed and the scan size.

R.-Y. QIN et

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(a)

al.

(a)

(b)

Fig. 1. STM top view images of the carbon fibers after anodic oxidation: (a) Fiber E2, 2000 nm x 2000 nm; (b) Fiber E4, 100 nm x 100 nm.

3. RESULTS AND DISCUSSION Figures 1 and 2 show the STM images of Fibers E2, E4, and ES on a large scale. If one compares the anodically oxidized carbon fibers (Fig. 1) with

the original fiber[ 111, it can be seen that the treatment by anodic oxidation does not change the surface aspects on a microscopic scale, nor at the nanometric scale. This is in good agreement with the adsorption measurement, which has shown that there is practically no difference in specific surface area between the anodized fibers and the untreated fibers, and with the mechanical test, which has shown that the mechanical properties such as Young’s modulus remain unchanged. Our results demonstrate again that the usual electrochemical treatment provides a modest oxidation and is thus

Fig. 2. STM top view images of the air plasma oxidized Fiber E5: (a) 3000 nm x 3000 nm, two filaments; (b) 500 nm x 500 nm; (c) 200 nm X 200 nm.

Study of carbon fibers by STM 111

325

Cd)

Fig, 3. Atomic resolution STM top view images of the carbon fibers after anodic oxidation: (a) Fiber E2. 20 nm x 20 nm: (b) Fiber E4, 20 nm x 20 nm: Cc) Fiber E4. IO nm x IO nm; (d) Fiber E4, 4 nm x 4 nm.

easily controlled. A similar result has also been observed by Hoffman et cr1.[7] on a commercially treated carbon fiber. On the other hand, Fig. 2 shows that the air plasma gives rise to a certain surface roughness. This is evidenced by the etching spots that can be seen on the microscopic image (Fig. 2(a)) and more clearly on the nanometric image (Fig. 2(b) and (c)). The air plasma seems to have “burnt” a part of the surface, the ribbons and the small stripe-form stackings become less evident, comparing the original and anodically oxidized fibers. Obviously, the treatment under these conditions results in a degradation of the mechanical properties. Figure 3 presents the STM image of anodically oxidized carbon fibers at atomic resolution. It can be found that the microtextures, such as the steplike

crystallite stackings. are almost identical to those observed on the original unoxidized fiber[l I]. The results further show that the graphene basal planes are conserved after anodic oxidation, while at the edge sites of graphite planes, the tunnel current between the tip and the surface, is always very irregular (corresponding to some fuzziness on the images). This indicates the complexity of the charge, probably caused by oxygenated functional groups in these places. ESCA analysis has confirmed that the O/C atomic ratios on the surface of Fibers E2 and E4 are about 5.5% and lo%, respectively, which are 2 and 4 times higher than that ofthe virgin fiber (2.3%). The width at half-height of the graphite component (E, = 284.2 ? 0.05 eV) of the Cls peak, which is related to the degree of crystalline structuration on the surface, increases slightly (see Fig. 4), indicating

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QIN et al.

R.-Y. I .J

Width at half-height

, eV

;L,

Hn: “C 0.7 7

1sOi -

Fiber A

-

-2000

_ .& -

C

_ _a@ic_oxijatJon, A/m ’

2500

0.6

6.0-

E

E2

E4

Fig. 4. Evolution of the width at half-height of the graphite component measured by the ESCA as a function of the heat treatment temperature (HTT) and the oxidation intensity (Fibers A, C, and E refer to the previous paper[l 11).

only a small deterioration of the surface structure. Our results suggest that the attack on fibers by the electrochemical oxidation takes place preferentially at the prismatic atoms. It should be noted that a small fluctuation of the tunnel current may also be produced when the feedback parameters of the scanner are not well adjusted. This is evidently not our case, since most of the surface examined appears with good resolution. What is interesting is that we recognize here on the surface of anodically oxidized carbon fibers corrugation patterns very similar to these observed by the STM on the carbonized fibers[ 121. Such corrugation patterns indicate an imperfect crystalline structure, i.e., imperfect graphite stacking (STM image reflects the density of electronic charge of the surface, in consequence, the perfect graphite ABAB stacking is characterized usually by symmetric triangle corrugation on the STM image[l81). The exact mechanism of these observations remains unknown, may be due to the formation of intercalated graphite compounds that we will discuss later. Furthermore, it generates another question of whether this surface imperfection plays a role in improvement of adhesion, in the present case, the interlaminar shear strength (ILSS) of oxidized fiber-epoxy composite. The active edge sites obviously play an important role, but other factors may not be excluded. It is possible that the intensification of anodic oxidation could result in the intercalation of anion oxides into subsuperfacial graphitic layers through activated edge sites. These intercalated graphite compounds could prevent the graphitic layers from having perfect stacking, thus changing the distribution of the surface charge. The ESCA analysis has shown that besides the 01s peaks at 531.5 eV and 533.1 eV (binding energy), which are related to the oxygenated functional groups, a distinct peak at 530.4 eV, which is related to the oxygenated groups with polar character (OH-, CO;, etc.), appears on the surface of Fiber E4. In fact, anodization is one of the methods used to synthesize intercalated graphite compounds on carbon fibers[l4]. Nevertheless,

more studies are needed to confirm such a hypothesis. Anyway, it seems that the model that is frequently advanced and that proposes that the anodic oxidation improves the fiber-matrix adhesion by simple removal of the imperfect surface layer[ 151 is not suitable to interpret the results we obtained. As for the treatment by air plasma under present conditions, it is foreseeable that the attack on fibers is characterized by a general degradation of the surface. In Fig. 5 are some STM images of Fiber ES at atomic resolution. The atomic oxygen excited by the plasma is so energetic that it is capable of removing the carbon atoms of less ordered zones, as well as those of basal planes[ 16,171. In principle, the plasma processing is a nonselective physical sputtering method simply because it is a purely physical impact process. However, these are material- and ion-dependent thresholds for material removal or reaction that are related to the binding energy and to the material structure. This provides, under certain conditions, some selectivity in plasma sputtering, which is our case, as one can see the presence of some pronounced etching spots in Figure 5 (b). This confirms, on the other hand, the surface structure heterogeneity of the carbon fibers. 4. CONCLUSION

The usual electrochemical treatment provides a modest oxidation without changing the surface aspects of carbon fibers, while improving greatly the surface activity. The attack of fibers by anodic oxidation takes place principally at the active prismatic sites (edge sites) of crystallites. The surface of the anodically oxidized fibers presents a microstructure similar to that of carbonized fibers at low temperatures, which may be explained by the formation of intercalated graphite compounds that prevent the graphitic layers from perfect ABAB stacking. The oxidation by air plasma is very distinct from anodic treatment. An overall degradation of the surface of carbon fibers has been observed at both the microscopic scale and atomic level under experi-

327

Study of carbon fibers by STM III

(b)

(4

Fig. 5. Atomic resolution STM top view images of the air plasma oxidized Fiber E5: (a) 20 nm x 20 nm; (b) 20 nm x 20nm; (c) 5 nm x 5 nm.

mental conditions. Moreover, the plasma bombardment does not necessarily give a homogeneous treatment, because of structural heterogeneity of the surface. Our study has demonstrated that the STM is a unique and unusual tool for the study of the surface of carbon fibers and, in combination with other techniques, provides us with information that can help us to better understand the problems that concern us.

Acknowled~rment-The authors are very grateful to the regional and local authorities, the Foundation for ENSCMu., the Ministries of Education and Research and Technology of France, plus several European industries, including Degussa A. G. and ORKEM S. A., for their support.

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