Detecting Surface Oxygen Groups on Carbon Nanofibers by Phase

Langmuir 2003, 19, 7665-7668

Detecting Surface Oxygen Groups on Carbon Nanofibers by Phase Contrast Imaging in Tapping Mode AFM J. I. Paredes, A. Martı´nez-Alonso,* and J. M. D. Tasco´n Instituto Nacional del Carbo´ n, CSIC, Apartado 73, 33080 Oviedo, Spain Received October 30, 2002. In Final Form: May 30, 2003

Introduction Since the discovery of carbon nanotubes (NTs) in 19911 and the subsequent unveiling of their unique physical properties,2 there has been an increasing interest in all types of carbon nanostructures. Although current research efforts remain largely focused on NTs, an ever-growing amount of work is being devoted to related nanomaterials, most notably carbon nanofibers (NFs)3 but also others, such as the recently developed carbon nanohorns4 and ordered nanoporous carbons.5 The emphasis in this field is being placed on basic research as well as on prospective technological applications, which, in the case of NTs and NFs, include their use as polymer reinforcements,6 catalyst supports,7 field emission displays,8 gas-storage systems (mainly hydrogen),9 and membranes.10 For the fulfilment of some of these and several other potential applications, it is essential to modify the NT/NF surface properties by chemical and/or physical means.11 In this regard, a particularly useful and widespread modification approach is that of oxidation. The introduction of oxygen-containing functionalities on the NT/NF surface is important not only due to its direct effect on some of the nanostructure’s properties11,12 but also because such functionalities can * Corresponding author. Telephone: (+34) 985 11 90 90. Fax: (+34) 985 29 76 62. E-mail: [email protected]. (1) Iijima, S. Nature 1991, 354, 56. (2) Carbon Nanotubes: Synthesis, Structure, Properties, and Applications; Dresselhaus, M. S., Dresselhaus, G., Avouris, Ph., Eds.; Springer: Berlin, 2001. (3) (a) Park, C.; Engel, E. S.; Crowe, A.; Gilbert, T. R.; Rodriguez, N. M. Langmuir 2000, 16, 8050. (b) Mestl, G.; Maksimova, N. I.; Keller, N.; Roddatis, V. V.; Schlo¨gl, R. Angew. Chem., Int. Ed. 2001, 40, 2066. (c) Park, C.; Keane, M. A. Langmuir 2001, 17, 8386. (d) Ros, T. G.; van Dillen, A. J.; Geus, J. W.; Koningsberger, D. C. ChemPhysChem 2002, 3, 209. (e) Jiang, N.; Koie, R.; Inaoka, T.; Shintani, Y.; Nishimura, K.; Hiraki, A. Appl. Phys. Lett. 2002, 81, 526. (4) (a) Bekyarova, E.; Kaneko, K.; Kasuya, D.; Murata, K.; Yudasaka, M.; Iijima, S. Langmuir 2002, 18, 4138. (b) Bekyarova, E.; Kaneko, K.; Yudasaka, M.; Murata, K.; Kasuya, D.; Iijima, S. Adv. Mater. 2002, 14, 973. (5) (a) Ryoo, R.; Joo, S. H.; Kruk, M.; Jaroniec, M. Adv. Mater. 2001, 13, 677. (b) Joo, S. H.; Choi, S. J.; Oh, I.; Kwak, J.; Liu, Z.; Terasaki, O.; Ryoo, R. Nature 2001, 412, 169. (6) (a) Ajayan, P. M.; Schadler, L. S.; Giannaris, C.; Rubio, A. Adv. Mater. 2000, 12, 750. (b) Rossi, G. B.; Beaucage, G.; Dang, T. D.; Vaia, R. A. Nano Lett. 2002, 2, 319. (7) (a) Liu, Z.; Lin, X.; Lee, J. Y.; Zhang, W.; Han, M.; Gan, L. M. Langmuir 2002, 18, 4054. (b) Vieira, R.; Pham-Huu, C.; Keller, N.; Ledoux, M. J. Chem. Commun. 2002, 954. (8) (a) Bonard, J.-M.; Weiss, N.; Kind, H.; Sto¨ckli, T.; Forro´, L.; Kern, K.; Chaˆtelain, A. Adv. Mater. 2001, 13, 184. (b) Sugino, T.; Yamamoto, T.; Kimura, C.; Murakami, H.; Hirakawa, M. Appl. Phys. Lett. 2002, 80, 3808. (9) (a) Schlapbach, L.; Zu¨ttel, A. Nature 2001, 414, 353. (b) Browning, D. J.; Gerrard, M. L.; Lakeman, J. B.; Mellor, I. M.; Mortimer, R. J.; Turpin, M. C. Nano Lett. 2002, 2, 201. (10) (a) Sun, L.; Crooks, R. M. J. Am. Chem. Soc. 2000, 122, 12340. (b) Zhang, L.; Melechko, A. V.; Merkulov, V. I.; Guillorn, M. A.; Simpson, M. L.; Lowndes, D. H.; Doktycz, M. J. Appl. Phys. Lett. 2002, 81, 135. (11) (a) Hirsch, A. Angew. Chem., Int. Ed. 2002, 41, 1853. (b) Ros, T. G.; van Dillen, A. J.; Geus, J. W.; Koningsberger, D. C. Chem.sEur. J. 2002, 8, 1151.

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be used as anchor groups for further functionalization and processing of the material.13 In this context, the detection and high-resolution mapping of the oxygen groups present on the mentioned carbon nanomaterials would be extremely desirable. However, the characterization techniques normally employed for the analysis of surface chemical composition, such as X-ray photoelectron and infrared spectroscopies, lack high lateral resolution capabilities and only provide spatially averaged information,14 making the search of alternative approaches worth exploring. One possibility to consider would be the use of scanning probe microscopies, especially scanning tunneling and atomic force microscopies (STM/AFM), which are able to attain atomic resolution in the most favorable cases.15 Nevertheless, such techniques are commonly employed for investigations on surface topography but not for chemically sensitive imaging, this being particularly true in the case of graphite16 and other carbon materials.17 An exception can be found in chemical force microscopy (CFM), in which AFM tips bearing known functional groups enable direct probing of molecular interactions and imaging with chemical sensitivity. CFM has been employed, for example, to study surface oxidation of polymers.18 Very recently, a different AFM-based approach was shown to be successful for the high-resolution mapping of hydrophilic, oxygen group distributions on carbon surfaces.19 The method relies on the detection of phase changes in noncontact tapping mode AFM, and its utility was demonstrated for perfectly flat graphite surfaces with oxygenated sites created in a greatly controlled fashion. Here, as a first step in an effort to extend such methodology to nonplanar carbon nanostructures of interest, we report a tapping mode AFM study on the detection of surface oxygen groups on carbon nanofibers, introduced by plasma oxidation. Results of STM observations are also presented to address the atomic-scale organization of the NF surface prior to and after the oxidation. Experimental Section Carbon nanofibers, with a diameter of about 100 nm and produced by pyrolysis of a hydrocarbon in the presence of Fe catalyst nanoparticles, were obtained from Applied Sciences, Inc. (Cedarville, OH). Both fresh and oxygen-plasma-treated NFs were investigated. The plasma treatments were accomplished in a Technics Plasma 200-G (Kirchheim bei Mu¨nchen, Germany) chamber, where O2 was activated by means of 2.45 GHz (12) Ago, H.; Kugler, T.; Cacialli, F.; Salaneck, W. R.; Shaffer, M. S. P.; Windle, A. H.; Friend, R. H. J. Phys. Chem. B 1999, 103, 8116. (13) (a) Ebbesen, T. W.; Hiura, H.; Bisher, M. E.; Treacy, M. M. J.; Shreeve-Keyer, J. L.; Haushalter, R. C. Adv. Mater. 1996, 8, 155. (b) Ros, T. G.; van Dillen, A. J.; Geus, J. W.; Koningsberger, D. C. Chem.s Eur. J. 2002, 8, 2868. (14) (a) Ray, K.; McCreery, R. L. Anal. Chem. 1997, 69, 4680. (b) Bradley, R. H.; Daley, R.; Le Goff, F. Carbon 2002, 40, 1173. (15) Magonov, S. N.; Whangbo, M.-H. Surface Analysis with STM and AFM; VCH: Weinheim, 1996. (16) (a) You, H.-X.; Brown, N. M. D.; Al-Assadi, K. F. Surf. Sci. 1993, 284, 263. (b) Tracz, A.; Wegner, G.; Rabe, J. P. Langmuir 1993, 9, 3033. (c) Tandon, D.; Hippo, E. J.; Marsh, H.; Sebok, E. Carbon 1997, 35, 35. (d) Habenicht, S. Phys. Rev. B 2001, 63, 125419. (17) (a) Daley, M. A.; Tandon, D.; Economy, J.; Hippo, E. J. Carbon 1996, 34, 1191. (b) Paredes, J. I.; Martı´nez-Alonso, A.; Tasco´n, J. M. D. Langmuir 2001, 17, 474. (18) (a) Ton-That, C.; Campbell, P. A.; Bradley, R. H. Langmuir 2000, 16, 5054. (b) Ton-That, C.; Teare, D. O. H.; Bradley, R. H. Chem. Mater. 2000, 12, 2106. (19) (a) Paredes, J. I.; Martı´nez-Alonso, A.; Tasco´n, J. M. D. Langmuir 2002, 18, 4314. (b) Paredes, J. I.; Martı´nez-Alonso, A.; Tasco´n, J. M. D. Chem. Commun. 2002, 1790.

10.1021/la020880q CCC: $25.00 © 2003 American Chemical Society Published on Web 07/22/2003

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Figure 1. Atomic-scale STM images of the NF surface: (a) pristine, untreated NF; (b) NF following a 5 min oxygen plasma treatment.

Results and Discussion

for carbon materials. From these images, some observations regarding the atomic structure of the NF and its modification by the plasma can be made. First, the untreated NF surface (Figure 1a) lacks long-range atomicscale order such as that observed by STM on the basal plane of pristine graphite (i.e., a perfect triangular pattern with ∼0.25 nm periodicity19). Nevertheless, on a very local scale the surface is characterized by a type of ordering clearly archetypal of basal graphite: atomic-sized spots with their first neighbors at distances around 0.25 nm and disposed more or less in a triangular arrangement are usually seen on the surface. In general, the local order extends, at most, only to the second closest neighbors of a given spot. Beyond this distance, a departure from such ideal pattern takes place and several imperfections in the packing of the spots are observed. Hence, we conclude that the NF surface structure is, in essence, that of basal graphite but with a very large number of flaws in its longrange organization. Second, following the plasma treatment (Figure 1b), the surface has lost its original shortrange basal graphitic order and is now characterized by a remarkably larger degree of disorder. Atomic-sized spots are still frequently observed, and at some specific points a few of them appear to be arranged in roughly structured ensembles, but the local graphitic order that was distinctive of the pristine NF is in general no longer present. Also, the sample surface was modified by the plasma in a very uniform way, since virtually all the images recorded (of both the fresh and plasma-oxidized NFs) displayed the features outlined in Figure 1. In consequence, from a structural point of view, the plasma treatment leads to a general and quite uniform increase in the atomic-scale disorder of the NF surface. Previous studies on model HOPG have shown that oxygen plasma etching of a graphitic material results in the introduction of atomic-scale defects on its surface.19 In this regard, the present STM observations of the fresh and plasma-treated NFs are consistent with this former finding. Furthermore, it is also known that the creation of atomic defects by the plasma entails the addition of oxygen functionalities onto such defects.19 Therefore, a large number of oxygen groups are also expected to exist on the NF surface following the present plasma treatment. In point of fact, carbon fibers of the same type as those considered in this work have been shown by X-ray photoelectron spectroscopy (XPS) to undergo a large increase in surface oxygen concentration (from about 1% to 10-20% upon similar or identical oxygen plasma exposures).20 XPS survey spectra of the NF sample studied

Figure 1 shows representative atomic resolution STM images of the NF surface before (a) and after (b) a 5 min oxygen plasma treatment, which is a typical exposure time

(20) (a) Serp, Ph.; Figueiredo, J. L.; Bertrand, P.; Issi, J. P. Carbon 1998, 36, 1791. (b) Paredes, J. I.; Martı´nez-Alonso, A.; Tasco´n, J. M. D. Carbon 2002, 40, 1101.

microwave radiation at a power of 100 W. The working pressure during the treatments was 1.0 mbar. To achieve a uniform exposure of the NFs to the plasma, a small quantity (0.5 g) of sample was carefully spread over a Petri dish with a density of about 10-2 g cm-2, which was then put into the chamber. The STM and tapping mode AFM observations were carried out under ambient conditions with a Nanoscope Multimode IIIa, from Digital Instruments (Santa Barbara, CA). STM imaging was performed in constant current mode using mechanically prepared Pt/Ir (80/20) tips. Typical tunneling parameters were 10-20 mV and 3-5 nA for the bias voltage and tunneling current, respectively. For tapping mode AFM, rectangular Si cantilevers with spring constants k ∼ 40 N m-1 and resonance frequencies around 250 kHz were employed. To obtain information about the surface chemistry of the NFs, the tapping AFM measurements were performed in the noncontact regime, recording height (topography) and phase images simultaneously. Such tip-sample interaction regime was established by recording phase-distance curves and setting the free and setpoint amplitudes of cantilever oscillation accordingly. A detailed account of this procedure has been reported elsewhere.19 Likewise, to characterize reliably the surface properties of the samples, the specimens were examined at many different locations. In the case of STM, since the observations were carried out on the atomic scale (scan size, 5 nm), a few hundred images were recorded for both the pristine and plasma-treated NFs. For tapping mode AFM, the scan size was considerably larger (∼1 µm), so a smaller number of images (i.e., a few tens) was required. A specific problem that had to be addressed in the present case concerned the comparison of the phase images of the fresh NFs with those of their plasma-treated counterparts. The contrast observed in a given phase image only provides relative differences between different regions within that image, but not their absolute phase values. Consequently, phase images obtained from two different samples are, in general, not directly comparable unless a reference common to both samples is provided. In the present case, the problem was solved using pristine highly oriented pyrolytic graphite (HOPG) as a reference substrate. A small amount of the fresh or plasma-treated NFs was ultrasonically dispersed in acetone, and a drop of the thus-prepared suspension was cast onto a freshly cleaved HOPG surface and allowed to dry. This method left the HOPG surface decorated with a large number of isolated NFs, which were subsequently investigated. The phase behavior of the fresh and plasma-treated NFs could be then compared by first contrasting it with that of the pristine HOPG surface. Also, to ensure meaningful results, direct comparisons were made using sets of images obtained with the same AFM tip, having verified that its condition did not change at any point during the measurements. For this reason, phase-distance curves were regularly acquired during the imaging process and only the data from those tips which showed a thoroughly stable behavior were considered.19

Notes

Figure 2. Noncontact tapping mode AFM height (a) and corresponding phase (b) images of a pristine, untreated NF deposited onto the HOPG surface. (c) Typical line profile of the phase image taken along a direction perpendicular to the NF axis. The regions corresponding to the NF and HOPG are indicated by arrows.

in this work showed lines of C1s and O1s. The oxygen signal was stronger for the plasma-treated samples, the surface concentration for this element amounting to 13% versus only 2% in the fresh sample. In the literature, a high concentration of oxygenated functionalities has been reported following treatments of NFs of the same type as those studied in this work.20,21 High-resolution XPS spectra of reconstruction of the C1s line showed that these functionalities were carboxyl/ester, hydroxyl/ether, and carbonyl groups.21 As a next step in the investigation, phase imaging in the noncontact regime of tapping mode AFM was performed to ascertain the presence on the NF surface of oxygen groups introduced by the plasma. Figures 2 and 3 show typical tapping AFM results obtained in the mentioned regime (free amplitude, ∼25 nm; setpoint amplitude, ∼21 nm) for the untreated and 5 min plasmatreated NFs, respectively. In each case, height (Figures 2a and 3a) and corresponding phase (Figures 2b and 3b) images are provided, along with some line profiles of the phase images (Figures 2c and 3c,d) that will assist in the comparison of the two samples. The same tip was employed for acquiring the images of both figures, which, as mentioned in the Experimental Section, allows a direct comparison of the fresh with the plasma-oxidized NFs. Similar results were obtained using all tips that showed

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a stable behavior throughout the measurements. In Figure 2 (untreated sample), a single NF can be seen lying on the flat HOPG surface and running diagonally between the bottom left and the top right corners of the height and phase images. From the height image (Figure 2a), the diameter of the NF was determined to be 100-105 nm (measured as the height of the NF over the graphite substrate). In Figure 3 (plasma-treated sample), an isolated NF (95-100 nm in diameter) on the pristine HOPG surface is observed as well, in this case with one of its ends visible at the bottom part of the images. On the nanometer scale, both the fresh and plasma-oxidized NFs present a remarkably smooth topography (Figures 2a and 3a), consistent with STM observations on the same scale (images not shown) and implying that the plasma-induced structural changes of the NF surface involve only the atomic-scale order but not its general roughness. Concerning the phase images (Figures 2b and 3b), their correct analysis requires first bearing in mind that abrupt variations in surface topography, such as those arising from the presence of a step or other vertical features, have a large impact on the measured phase, as some examples in the literature demonstrate.22 This type of topographydependent behavior is rather common in scanning probe microscopy when measuring such properties as friction and adhesion.23 In the present case, its effect is clearly observed as large phase variations on the edges of the NFs, where very large slopes are present (Figures 2b and 3b), and also as smaller variations at steps on the HOPG surface (top right corner of Figure 3b). Since such effect is only of topographical origin and has no physical meaning in relation to the sample surface properties under investigation, it must be excluded from the analysis. For this reason, phase comparisons were only made between the atomically flat and pristine HOPG surface24 and the very top part of the NFs, where the sample is locally flat and presents no slope and, therefore, topographical contributions to the measured phase are absent. In Figure 2c, a representative line profile of the phase image (Figure 2b) of the pristine NF sample, taken along a direction perpendicular to the fiber axis, is presented. This and other profiles indicated that the phase of the fresh NFs was very similar (to within less than 1°) to that of pristine basal HOPG. On the other hand, the corresponding line profiles of the plasma-treated NFs (such as that shown in Figure 3c, also taken along a direction perpendicular to the NF axis) revealed a small but reproducible phase (21) (a) Bubert, H.; Ai, X.; Haiber, S.; Bru¨ser, V.; Pasch, E.; Brandl, W.; Marginean, G. Spectrochim. Acta, Part B 2002, 57, 1601. (b) Bubert, H.; Brandl, W.; Kittel, S.; Marginean, G.; Toma, D. Anal. Bioanal. Chem. 2002, 374, 1237. (22) (a) Tamayo, J.; Garcı´a, R. Appl. Phys. Lett. 1998, 73, 2926. (b) Behrend, O. P.; Odoni, L.; Loubet, J. L.; Burnham, N. A. Appl. Phys. Lett. 1999, 75, 2551. (23) (a) Sundararajan, S.; Bhushan, B. J. Appl. Phys. 2000, 88, 4825. (b) Sirghi, L.; Nakagiri, N.; Sugisaki, K.; Sugimura, H.; Takai, O. Langmuir 2000, 16, 7796. (24) We checked before and after the deposition of the NFs that the HOPG samples were free of any contaminant layer covering its surface, the presence of which could change the phase behavior of the substrate and, therefore, invalidate the data. In other experiments unrelated to this work, we have occasionally observed the accidental deposition of molecularly thin impurity layers on pristine HOPG. Such layers are easily recognizable as islands on the HOPG surface, with a slightly rough appearance in the AFM height images (compared to the perfectly flat profile of pristine HOPG) and displaying a clear contrast in the phase images. None of this was observed in the present case. We only observed at times the presence of some nanoparticles following the NF deposition, as exemplified in Figure 2b, which were attributed as coming from the raw NF sample material. Nevertheless, since such particles were localized at very specific points, the areas between them being pristine HOPG, the phase measurements were not hampered by their presence and, as a matter of fact, agreed with those attained on nanoparticle-free areas.

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Figure 3. Noncontact tapping mode AFM height (a) and corresponding phase (b) images of a 5 min oxygen-plasma-treated NF deposited on HOPG. (c) Typical line profile of the phase image obtained along a direction perpendicular to the NF axis. (d) Typical line profile of the phase image taken along the top of the NF surface and part of the HOPG surface. For both line profiles, the HOPG and NF areas are denoted by arrows.

shift of about 3° upward on the oxidized NF relative to pristine HOPG. Furthermore, such increase was considerably uniform along directions parallel to the NF axis, as can be best appreciated in Figure 3d, which shows a line profile of Figure 3b taken along the top of the NF and part of the HOPG surface: the plasma-treated NF consistently displays a higher phase than that of HOPG, which was not the case for the fresh, untreated NFs. It has been previously shown that tapping AFM phase images represent maps of the energy dissipated by the tip-sample interaction.25,26 In the noncontact regime, this implies that more dissipative areas will display higher phase values than regions dissipating less amounts of energy.26 Moreover, the mechanism of energy dissipation in such regime has been shown to be the presence of a water layer on the sample surface,26,27 so that hydrophilic surfaces, which bear thick water layers, will dissipate a greater amount of energy in the noncontact regime than hydrophobic ones, which only carry much thinner layers. Accordingly, in the present case it can be inferred that the oxidized NFs dissipate more energy and are therefore more hydrophilic than both the fresh NFs and the pristine HOPG, the latter two being comparable in this respect. The increased hydrophilicity of the plasma-treated NFs must be due to the introduction of oxygen functional groups by the plasma. These groups are sites where water adsorption is strongly favored, especially compared to the large hydrophobicity of pristine, oxygen-free carbon surfaces.28 Thus, the upward phase shift of the plasmatreated NFs observed in the noncontact regime (Figure 3c,d) is reflecting the presence of the plasma-introduced oxygen groups and provides a map of their spatial distribution along the NF surface (Figure 3d). The following observations are consistent with this interpretation: (i) The magnitude of the phase shift (∼3°) is very similar to that measured previously on model oxygencontaining carbon surfaces (HOPG with oxygenated atomic vacancies)19 and also on self-assembled monolayers with similar surface chemistry (CH3 and COOH groups).29 (ii) Figure 3d and other line profiles indicate that the lateral distribution of surface oxygen groups on the

plasma-treated NFs should be relatively uniform in the main. This is supported by the STM images (Figure 1), which indicated a uniform increase in the atomic-scale disorder over the whole NF surface, implying a uniform introduction of oxygen on the NF. (iii) The HOPG and the untreated NF exhibit a similar phase behavior, suggesting that both surfaces possess a comparable degree of hydrophobicity. Again, the STM images of the untreated NF (Figure 1a), which revealed a surface structure essentially analogous to that of basal graphite, together with the low concentrations of oxygen detected by XPS in this type of fiber, are consistent with this finding. Conclusions In the present work, phase contrast imaging in noncontact tapping mode AFM has been employed for the detection and mapping of hydrophilic oxygen groups on the surface of carbon NFs, constituting the first extension of such approach to nonplanar carbon nanostructures. It was shown that functional groups introduced by plasma oxidation were in general evenly distributed over the NF surface, which was consistent with the plasma-induced structural modifications observed on the atomic scale by STM. Challenges ahead include the application of this method to increasingly thinner carbon filaments, such as carbon NTs. For this objective to be successful, considerably sharper AFM tips (for instance, the recently developed carbon NT probes) will most likely be required. Acknowledgment. The authors acknowledge financial support from the DGICYT (Project PB98-0492). LA020880Q (25) Cleveland, J. P.; Anczykowski, B.; Schmid, A. E.; Elings, V. B. Appl. Phys. Lett. 1998, 72, 2613. (26) James, P. J.; Antognozzi, M.; Tamayo, J.; McMaster, T. J.; Newton, J. M.; Miles, M. J. Langmuir 2001, 17, 349. (27) (a) Aime´, J. P.; Boisgard, R.; Nony, L.; Couturier, G. J. Chem. Phys. 2001, 114, 4945. (b) Nony, L.; Cohen-Bouhacina, T.; Aime´, J.-P. Surf. Sci. 2002, 499, 152. (28) (a) Bradley, R. H.; Sutherland, I.; Sheng, E. J. Colloid Interface Sci. 1996, 179, 561. (b) McCallum, C. L.; Bandosz, T. J.; McGrother, S. C.; Mu¨ller, E. A.; Gubbins, K. E. Langmuir 1999, 15, 533. (29) Brandsch, R.; Bar, G.; Whangbo, M.-H. Langmuir 1997, 13, 6349.