Scanning Tunneling Microscopy Study of Graphite Oxidation in Ozone

Pit formation does not proceed continuously during oxidation and is strongly .... was automatically directed onto the sample surface in order to stop ...
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Langmuir 2003, 19, 6807-6812

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Scanning Tunneling Microscopy Study of Graphite Oxidation in Ozone-Air Mixtures Adam Tracz,*,† Gerhard Wegner,‡ and Ju¨rgen P. Rabe*,§ Polish Academy of Science, Centre of Molecular and Macromolecular Studies, Sienkiewicza 112, 90-363 Ło´ dz´ , Poland, Max-Planck-Institut fu¨ r Polymerforschung, Postfach 3148, D-55021 Mainz, Germany, and Humboldt University Berlin, Department of Physics, Newtonstrasse 15, D-10115 12489 Berlin, Germany Received January 22, 2003. In Final Form: June 9, 2003 The early stages of thermal oxidation of the basal plane of highly oriented graphite (HOPG) have been investigated in ozonized air. Scanning tunneling microscopy (STM) was employed to determine the surface topography on the nanometer scale. At temperatures within the range 300-500 °C and at O3 concentrations below 1 ppm, the abstraction of carbon atoms from the basal plane, that is, formation of pointlike defects, occurs. These defects upon further oxidation are enlarged to monolayer deep pits with diameters in the range 1-100 nm. Pit formation does not proceed continuously during oxidation and is strongly inhibited when a critical pit density is reached. The pits are not generated very close to the pit edges with the consequence of a quite uniform not purely statistical coverage of the HOPG surface with the pits. Oxidation time, temperature, and O3 concentration can be used to control the pit density and their distribution across the surface, the pit diameter, and the diameter distribution with unprecedented reproducibility. Welldefined nanostructured HOPG surfaces with a pit density in the range 10-25.000 µm-2 can be obtained.

Introduction In recent years scanning tunneling microscopy (STM) has been employed to study the formation of monolayer deep pits within the basal plane of highly oriented pyrolytic graphite (HOPG) by thermal oxidation in different gases, including air (see, e.g., refs 1-4), NO, NO2, and N2O (see, e.g., refs 5-7) or in an O2-Cl2 (ref 8) mixture. At temperatures in the range 300-700 °C, the basal plane of graphite is resistant against oxidation, and pit etching starts exclusively from naturally occurring point defects. The pit density is therefore determined by the density of these defects, which typically is as low as on the order of a few per square micrometer.1-10 During growth of the pits within the topmost graphite monolayer, they merge, which leads to the consumption of the monolayer, and simultaneously the underlying graphite plane becomes uncovered. When defects in the underlying plane become exposed to the ambient, pits within this plane will begin to grow. Such a mechanism leads to the consumption of the graphite layer by layer and to some increase of the surface roughness, the kinetics of which has been studied.2,3 Interesting applications of etch pits have been proposed, including, for example, as markers, calibrators for the z-axis of the STM, molecular containers, and specific sites for nucleation and chemical or electrochemical reactions * To whom correspondence may be addressed. E-mail: atracz@ bilbo.cbmm.lodz.pl; [email protected]. † Polish Academy of Science, Centre of Molecular and Macromolecular Studies. ‡ Max-Planck-Institut fu ¨ r Polymerforschung. § Humboldt University Berlin. (1) Chang, H.; Bard, A. J. Am. Chem. Soc. 1990, 112, 4598; 1991, 113, 5588. (2) Tracz, A.; Wegner, G.; Rabe, J. P. Langmuir 1993, 9, 3033. (3) Pakula, T.; Tracz, A.; Wegner, G.; Rabe, J. P. J. Chem. Phys. 1993, 9, 8162. (4) Zhang, D.; Cotterill, G. F.; Li, T.; O’Connor, D. J.; Wall, T. F. Fuel 1993, 72, 1454. (5) Chu, X.; Schmidt, L. Carbon 1991, 29, 1251. (6) Chu, X.; Schmidt, L. Surf. Sci. 1992, 268, 325. (7) Chu, X.; Schmidt, L. Ind. Eng. Chem. Res. 1993, 32, 1359. (8) Morozov, V. N.; Sherman, J.; Kallenbach, N. R.; Shou Ming Du; Seeman, N. C. J. Microsc. 1993, 170, 237. (9) Yang, R. T.; Wong, C. J. Chem. Phys. 1981, 75, 4471. (10) Yang, R. T.; Wong, C. J. Chem. Phys. 1982, 78, 3325.

on the HOPG surface.1 Also studies on the local deposition of metal11-17 or silicon17,18 clusters at the pit edges were reported. The influence of the etch pits, also named molecule corrals, on the formation of ordered structures within a monolayer of molecules adsorbed at the graphiteliquid interface has been investigated.19-26 Recently the influence of the etch pits on the long range ordering of polyethylene chains at the graphite-polymer melt interface was shown.27 An attempt to use the edges of the etch pits on the chemically inert basal plane as a site for chemical attachment of functional groups on HOPG for immobilization of some biological molecules8,28 or polypyrrole29 was also reported. More developments in this area may be expected soon. For any of the applications the independent control over pit size, size distribution, and pit density is desirable. The (11) Schlo¨gl, R. Chem. Z. 1994, 28, 166. (12) Hendricks, S. A.; Kim, Y. T.; Bard, A. J. J. Electrochem. Soc. 1992, 139, 2818. (13) Zoval, J. V.; Biernacki, P. R.; Penner, R. M. Anal. Chem. 1996, 68, 1585. (14) Ho¨vel, H.; Becker, Th.; Bettac, A.; Reihl, B.; Tschudy, M.; Williams, E. J. J. Appl. Phys. 1997, 81, 154. (15) Ho¨vel, H.; Becker, Th.; Bettac, A.; Reihl, B.; Tschudy, M.; Williams, E. J. Appl. Surf. Sci 1997, 115, 124. (16) Stabel, A.; Eichhorst-Gerner, K.; Rabe, J. P.; Gonza´lez-Elipe, A. R. Langmuir 1998, 14, 7324. (17) McBridge, J. D.; Van Tassell, B.; Jachmann, R. C.; Beebe, T. P., Jr. J. Phys. Chem. B 2001, 105, 3972. (18) Scheier, P.; Marsen, B.; Lonfat, M.; Schneider, W.-D.; Sattler, K. Surf. Sci. 2000, 456, 113. (19) Tracz, A.; Kalachev, A.; Wegner, G.; Rabe, J. P. Verh. Dtsch. Phys. Ges. 1994, 29, 1629. (20) Patrick, D. L.; Cee, V. J.; Beebe, T. P., Jr. Science 1994, 265, 231. (21) Patrick, D. L.; Cee, V. J.; Beebe, T. P., Jr. Lanqmuir 1994, 10, 298. (22) Patrick, D. L.; Cee, V. J.; Purcell, T. J.; Beebe, T. P., Jr. Langmuir 1996, 12, 1830. (23) Patrick, D. L.; Cee, V. J.; Beebe, T. P., Jr. J. Phys. Chem. B 1996, 100, 8478. (24) Stevens, F.; Buehner, D.; Beebe, T. P., Jr. J. Phys. Chem. B 1997, 101, 6491. (25) Patrick, D. L.; Cee, V. J.; Mortse, M. D.; Beebe, T. P., Jr. J. Phys. Chem. B 1999, 103, 8328. (26) Tracz, A.; Stabel, A.; Rabe, J. P. Langmuir 2002, 18, 9319. (27) Tracz, A.; Jeszka, J. K.; Kucin´ska, I.; Chapel, J.-P.; Boiteux, G.; Kryszewski, M. J. Appl. Polym. Sci. 2002, 86, 1329. (28) Cullen, D. C.; Lowe, C. R. J. Colloid Interface Sci. 1994, 166, 102.

10.1021/la034103h CCC: $25.00 © 2003 American Chemical Society Published on Web 07/11/2003

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natural way to control the pit density is a controlled introduction of defects within the topmost graphite monolayer. We proposed a method for a controlled creation of defects by the pretreatment of the HOPG surface at room temperature with remote microwave argon plasma.30 Subsequent thermal oxidation in air at 670 °C led to the generation and growth of pits with uniform size. The density of the pits was controlled up to 2.000 µm-2 by the time of the plasma treatment, while their size was controlled independently by the oxidation time. Other ways to increase the defect density are the bombardment of the HOPG surface with atomic ions,12,31-34 polyatomic biopolymer ions,35 or ion clusters.36,37 Ion impact allows us also to create deeper pits. An increase of the pit density was achieved furthermore by pretreatment of HOPG in a high concentration of O3 at 100 °C for 10 min before oxidation at a higher temperature.8 However, no systematic STM studies were performed to follow the stages of surface roughening. In the present work we used ozonized air as oxidizing gas with ozone concentrations in the parts per million range, that is, orders of magnitude lower than previously used. This continues our STM studies of topographic effects of graphite oxidation in order to control the surface roughness on the nanoscale.2,3,19,26,30 Experimental Section Samples of highly oriented pyrolytic graphite (HOPG) (13 × 13 × 3 mm3, qualities ZYB and ZYH) were obtained from Advanced Ceramics, Cleveland, OH. Ozone was produced by UV irradiation of air flowing through a glass chamber at a rate of 4 l min-1. Oxidation of freshly cleaved HOPG was performed in a quartz tube reactor with an inner diameter of 30 mm. One end of the reactor was placed in a cylindric, electric oven to control the temperature of the gas flowing through the tube. Samples were placed horizontally on a thin stainless steel holder attached to a quartz rod, which could be moved inside the tube. The temperature in the hot zone of the reactor was measured 2 mm above the graphite sample using a thin Ni/CrNi thermocouple. The concentration of ozone in air flowing through the reactor at different temperatures was measured using a UV photometric O3 analyzer (model 49, Thermo Enviroment Instruments Inc., USA). Since this instrument is designed for the measurement of ozone concentrations in the atmosphere, the upper limit for precise measurements was 1 ppm. Three different protocols were used for oxidation: (A) The HOPG sample was placed on the preheated sample holder and introduced for a certain period of time (several minutes) into the hot zone of the reactor in the presence of a controlled flow of ozonized air heated to the desired temperature (200-500 °C). The ozone concentration in air in the reaction zone was usually less than 1 ppm. (B) The HOPG sample was heated in the hot zone of the reactor during 2 min to a certain temperature (400-600 °C) in residual air, and then the sample was immediately (i.e. during about 0.5 s) transferred to the other end of the tube. There, that is, ∼20 cm apart from the hot zone, a system for injection of the gas onto (29) Noll, J. D.; Nicholson, M. A.; Van Patten, P. G.; Chung, C.-W.; Myrick, M. L. J. Electrochem. Soc. 1998, 145, 3320. (30) Tracz, A.; Kalachev, A.; Wegner, G.; Rabe, J. P. Langmuir 1995, 11, 2840. (31) Hahn, J. R.; Kang, H. J. Vac. Sci. Technol., A 1999, 17, 1606. (32) Hahn, J. R. Surf. Sci. 1999, 423, L216. (33) Zhu, Y.-J.; Hansen, T. A.; Ammermann, S.; McBride, J. D.; Beebe, T. P., Jr. J. Phys. Chem. B 2001, 105, 7632. (34) Zhu, Y.-J.; McBride, J. D.; Hansen, T. A.; Beebe, T. P., Jr. J. Phys. Chem. B 2001, 105, 2010. (35) Reimann, C. T.; Sullivan, P. A.; Tu¨rpitz, A.; Altmann, S.; Quist, A. P.; Bergman, A.; Oscarsson, S. O.; Sundquist, B. U. R.; Hakansson, P. Surf. Sci. 1995, 341, L1019-L1024. (36) Reimann, C. T.; Anderson, S.; Bru¨hweiler, P.; Martenson, N.; Olsson, L.; Erlandsson, R.; Henkel; Urbassek, H. M. Nucl. Instrum. Methods Phys. Res. B 1998, 140, 159. (37) Bra¨uchle, G.; Richard-Schneider, S.; Illig, D.; Rockenberger, J.; Beck, R. D.; Kappes, M. Appl. Phys. Lett. 1995, 67, 52.

Figure 1. 750 × 750 nm2 STM images of the HOPG surfaces oxidized for different times at 450 °C in air containing 0.1 ppm of ozone (protocol A). Deviation of the pit shape from circular is due to small STM drift during imaging. the sample was attached. The ozone concentration in the injected gas was several parts per million. In the absence of the hot sample holder, the temperature in this part of the tube was about 80 °C. The system allowed us to control the injection of a few milliliters of ozonized air onto the hot graphite surface. The time of injection was usually less than 1 s. At the moment when the injection was finished, a stream of N2 was automatically directed onto the sample surface in order to stop the oxidation. (C) The HOPG sample was preheated for 2 min in air at temperatures around 600 °C and then shifted to the other end of the tube (like in B) and cooled in the flow of air (4 l min-1) with a low concentration of ozone. The topography of HOPG surfaces oxidized at different conditions was studied with a home-built scanning tunneling microscope, using the constant current mode. The images were taken at 2 nA and 0.1-0.3 V using mechanically sharpened Pt/Ir tips. Images are presented as recorded without any further image processing.

Results At temperatures below 500 °C it was found that the oxidation of HOPG in ozonized air differs from the oxidation in other gases studied so far, since in the presence of ozone the carbon atoms within the basal plane are attacked, causing the formation of monolayer deep pits. The density of the pits can be orders of magnitude higher than the typical density of pits originating exclusively from natural defects, which is on the order of a few per square micrometer. It was found that the most important parameters affecting the process of pit nucleation and growth are (1) oxidation time, (2) ozone concentration, and (3) temperature. No significant influence of the sample quality on the results was found. In Figure 1 STM images are displayed, which show the influence of the oxidation time on the topography of HOPG

Graphite Oxidation in Ozone-Air Mixtures

Figure 2. Dependence of the average pit diameter D on the oxidation time t at 450 °C in air containing 0.1 ppm ozone. The inset shows the dependence plotted as D vs t1/2.

for a temperature of 450 °C and flowing air containing 0.1 ppm of ozone (protocol A). At this temperature and under our experimental conditions the pit density was relatively small and therefore the characteristic features of the oxidation process can be well illustrated. At the beginning of the reaction the main feature is that both pit density and pit diameter increase (Figures 1a-c and 2). After some time, the pit density saturates. Under the conditions employed here (temperature 450 °C and ozone concentration 0.1 ppm), the pit density reaches a plateau value of about 80 µm-2 with an average pit diameter of 40 nm after 3 min of exposure. This density is about 1 order of magnitude bigger than the pit density generated during oxidation in air or in several other gases at temperatures below 600-700 °C.1-9 This indicates that the pits formed during oxidation in ozonized air do not originate only from natural defects but that new defects are formed due to the removal of carbon atoms from the basal plane. Initially, until the pit density saturates, the pit growth is accompanied by the generation of new pits between the already existing ones. This leads naturally to some distribution of pit sizes. Further oxidation occurs exclusively at the pit edges, which results in the increase of their diameter, but new pits are not nucleated, neither within the topmost graphite monolayer between the pits nor within the underlying layer, that is, in the bottom of the pits (Figure 1c and d). The pit growth within the topmost graphite monolayer leads to its consumption, and the underlying graphite layer becomes successively exposed. For several minutes of reaction time, only the two top graphite layers are visible, although the first monolayer becomes already etched by 50% (Figure 1d). This is

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different from the reported studies on the oxidation in air, when, after consumption of about 50% of the first layer, also two layers below were already seen (in the case of both instantaneous and spontaneous pit nucleation).2,3 The nucleation of new pits begins again later when the first graphite monolayer (white fragments in Figure 1e) is already considerably consumed. When the gasification of the first layer is nearly completed, the pit formation occurs very intensely (Figure 1f). The density of the pits formed on the second layer is comparable to the density on the first one; however, the distribution of their diameters is much broader (compare Figure 1c and f). The dependence of the average pit diameter proportional to the square root of the oxidation time up to 20 min is shown in Figure 2. As one can deduce from the STM images shown in Figure 1, the generation of new pits usually does not occur very close to the pit edges. As long as pits are not too large, they remain separated. In other words, the pits oxidized in ozonized air are distributed on the surface more uniformly than expected for a purely statistical distribution. This effect is clearly seen for samples with higher pit densities (Figures 3-5 and 7), which are obtained at lower temperature and higher ozone concentration. STM images of samples oxidized for 5 min at 360 °C in constant flow of air containing ozone (see protocol A) are shown in Figure 3. As little as 0.035 ppm of ozone in air resulted in a pit density of about 130 µm-2. At ozone concentration 0.13 ppm, the pit density is about 700 µm-2. An increase of the O3 concentration results in an increase of the pit density (Figure 3), which, for 1.0 ppm, reaches a value of about 1.400 µm-2. At lower temperatures and at higher ozone concentrations, the saturation of the pit density occurs faster. For example, at 370 °C and an ozone concentration of 0.9 ppm, the pit density is stable already after 1 min (Figure 4) and amounts to about 500 µm-2. As mentioned earlier, the generation of new pits usually does not occur very close to the pit edges. In Figure 5 the histograms of the distances between the neighboring pits in a typical area of the HOPG surface as presented in Figures 3b and 4b are shown. The maximum at about 20 nm in the histogram shown in Figure 5a corresponds to the average distance between the centers of the neighboring pits. The shortest distance between pit centers is not smaller than 14 nm. Taking into account that the average pit radius is about 3 nm, this means that, during oxidation under these conditions (temperature 360 °C, ozone concentration 0.13 ppm), new pits are not generated at

Figure 3. 750 × 750 nm2 STM images of HOPG surfaces after 5 min of oxidation at 360 °C in air containing different concentrations of ozone: (a) 0.035 ppm; (b) 0.13 ppm; (c) 1.0 ppm.

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Figure 4. STM images (750 × 750 nm2) of the HOPG surfaces oxidized for different times at 370 °C in air containing 0.9 ppm of ozone (protocol A).

Figure 5. (a) STM image of the HOPG oxidized during 5 min at 360 °C in air containing 0.13 ppm of ozone and the distribution of distances between the centers of the neighboring pits. (b) STM image of the HOPG oxidized during 2 min at 370 °C in air containing 0.9 ppm of ozone and the distribution of distances between the centers of the neighboring pits. White circles in STM images show typical areas where the smallest distances between pits were measured.

distances smaller than 11-14 nm from the nearest edge of the pit. A similar analysis of the histogram in Figure 5b leads to the conclusion that, during oxidation in air containing 0.9 ppm at 370 °C, pits are only generated at distances further apart than 6-8 nm away from the edge of the nearest pit. Experiments were also performed at higher ozone concentrations. A certain amount of air with ozone was injected onto the hot sample surface (see protocol B). At ozone concentrations higher than 1 ppm, samples with a very high density of nonoverlapping pits can be obtained (25.000 µm-2 in Figure 6). In some applications the distribution of pit diameters can be useful, for example, in studies on the effect of the pit size on the formation of monolayer domains of molecules adsorbed on the surface.19,26 Graphite surfaces with a distribution of pit diameters can be obtained, when the ozone concentration changes during the experiment. This can be realized, for example, by lowering the temperature of HOPG during the experiment (protocol C). The HOPG sample was preheated for 2 min in air at temperatures around 600 °C and then cooled in the flow of air (4 L min-1) with a low concentration of ozone. Since ozone decomposes at higher temperatures, its concentration at the graphite surface was small at the beginning of the oxidation. With a decrease of the graphite temperature, the ozone concentration in air at the graphite surface was increasing. The oxidation occurs under nonequilibrium conditions (decrease of temperature and

Figure 6. STM image of the basal plane of HOPG after 0.6 s oxidation, effected by the injection of air containing a few ppm of ozone onto the sample surface. The pit density is about 25.000 µm-2. Between the pits the image exhibits a resolution on the atomic scale.

increase of the ozone concentration), resulting in a broad distribution of pit sizes. An example for such a HOPG surface is shown in Figure 7. We have checked that by varying the rate of temperature decrease and of O3 concentration change one can influence the pit diameter distribution.

Graphite Oxidation in Ozone-Air Mixtures

Langmuir, Vol. 19, No. 17, 2003 6811 Table 1. Rate of Radius Increase of Pits Formed within the HOPG Basal Plane (dr/dt in nm/min) during Oxidation in Different Gases at Two Temperatures NO2 5% N2O 10% O2 O3 10-5% O3 10-4% in Hea in Hea 100%a in air in air dr/dt at 450 °C dr/dt at 370 °C

10 3.6

3.8 1.0

1.0 0.12

5.0 3.0

a

Data for NO2, N2O, and O2 are calculated according to the results in refs 5-7.

Figure 7. 750 × 700 nm2 STM image of an HOPG surface after oxidation performed according to protocol C (see Experimental Section), that is, during cooling the sample in the flow of ozonized air. Note the broad distribution of the pit sizes.

It was found also that if the sample was removed from the reactor after a few minutes (i.e. when the pit density was saturated), and then put back again into the reactor for a second oxidation under the same conditions, no new pits were generated. Only the increase of the diameter of the pits formed in the first experiment was observed. The formation of new pits during repeated oxidation was observed only when the second treatment was performed at a lower temperature or a higher ozone concentration than the first one. A typical effect of such a repeated oxidation is illustrated in Figure 8. First the sample was oxidized for 7 min at 480 °C. At this temperature the ozone concentration was small (about 0.02 ppm). The treatment resulted in a low density of about 15 pits/µm2 with a diameter of about 45 nm. The second oxidation was performed for 4 min at 370 °C and a higher ozone concentration of 1.00 ppm. After this treatment, a high pit density of about 1.500 µm-2 with pit diameters around 10 nm appeared on the surface. As one can see in Figure 8, the pits were formed both on the basal plane between the other pits and at the bottom of bigger pits formed during the first oxidation cycle. Discussion We have shown that the oxidation of the HOPG surface in air with a very low concentration of ozone results in a very high density of monolayer pits, already at relatively low temperatures (about 300 °C). This is different from HOPG oxidation in other gases, when the removal of the carbon atoms from the basal plane was observed only during oxidation at much higher temperatures. It was proposed that atomic oxygen released from the dissociative

adsorption of various nitrous oxides on the surface is responsible for the removal of C atoms from the basal plane.7 In the investigated temperature range, the pitting of the basal plane was increasing with an increase of temperature and of time.5-7 However, despite orders of magnitude higher concentration of gases used, the densities of the pits were orders of magnitude lower compared to those reported in this paper (cf. Table 1). Therefore, we believe that, in the case of oxidation at low concentration of ozone, the mechanism responsible for nucleation of the pits is different. However, atomic oxygen may play an important role in the growth of the pits. We assume that the removal of C atoms from the basal plane leading to nucleation of the pits does not occur directly at stochastically distributed places where ozone impinges steadily from the gas phase on the graphite surface. After adsorption, the ozone molecules diffuse on the graphite surface. If the surface is perfect that is, if there are no reactive edge carbon atoms, after an average time τ the ozone molecule is consumed by a reaction which causes the generation of the first defects, which will then grow into a pit. The diffusion of oxygen toward the rim of the growing pit (or cleavage step) results in a gradient of surface concentration of oxygen species around the pit. Any ozone molecule adsorbed on the surface at a distance much larger than a distance L ) (2Dτ)1/2, where D is the surface diffusion constant, from the edges of growing pits does not feel the surface concentration gradient and moves randomly on the surface. After an average time τ, it will generate a new defect on a basal plane. This leads to an increase of the number of the growing pits. Ozone molecules which arrive close to the pit edge at a distance around a pit which is smaller than the critical distance H < L will not move randomly. Due to the surface concentration gradient of the oxygen species, it will move toward the pit edge and would not generate a defect on the basal plane near the pit edge. It will be consumed in the reaction of removal of edge carbon atoms. During pit growth, a certain area of the graphite surface corresponding to a ring of a width H, inner radius r, and outer radius

Figure 8. STM images of an HOPG surface after two oxidation experiments, the first for 7 min at 480 °C in air with 0.02 ppm of O3 and the second for 4 min at 370 °C in air with 0.95 ppm of O3.

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r + H, where r is the pit radius, is screened from the removal of C atoms from the basal plane. New pits are thus formed preferentially at distances away from the rim of a pit larger than H. This explains the quite uniform coverage of the surface by pits. After a certain time of oxidation when the distance between edges of neighboring pits becomes smaller than 2H, new pits are not formed; only the increase of the size of already formed pits is observed. During thermal oxidation of HOPG in air containing 0.13 ppm of ozone at 360 °C or 0.9 ppm at 370 °C, the value of H is on the order of 10 nm. Detailed analysis of the dependence of H on oxidation conditions (ozone concentration and temperature) and its relation to L is out of the scope of the present paper. The pit density after isothermal oxidation increases with ozone concentration, since the probability of attacking carbon atoms within the basal plane is proportional to ozone coverage. The dependence of the diameter of the pits on experimental parameters seems to be complex. In previous works the linear dependence of the pit size on the oxidation time was claimed. In some cases when the pit size distribution was narrow (pit growth started simultaneously), the kinetic parameters of oxidation were evaluated from the growth rate of the surface of the individual pit.5-7 As is shown in Figure 2, the dependence of the average pit size on the oxidation time at 450 °C is not linear, but rather the pit diameter is proportional to the square root of time (inset in Figure 2). This can be explained taking into account that the rate limiting step is not the oxidation reaction at the edges but the diffusion of the oxygen species to the edges. Under such an assumption, the overall oxidation rate may remain constant despite the increase of the amount of edge carbon atoms. The overall oxidation rate can be defined as the rate of the carbon atoms removed from the pit edges dC(t)/ dt, where C(t) is the number of carbon atoms reacted at time t. Assuming that the number of pits simultaneously growing on a certain area is N, this rate can be expressed as proportional to N d[r(t)2]/dt, where r is the pit radius. It follows that the overall rate d[C(t)]/dt is constant when r(t) is proportional to t1/2. This reasoning allows us also to explain the dependence of the growth rate on the pit density, which was observed already in earlier works on graphite oxidation in air.2,9 The total flux of diffusing oxygen species has to be divided among the number N of the reaction centers (pits). Therefore, the bigger the density of the pits is, the smaller is the flux of oxygen species, which can react with the edges of the individual pit. Consequently, for a given overall gasification rate, when the pit density is higher, the pits grow more slowly. In the ideal case for a given overall reaction rate, the diameter of the pits after a given time should be inversely proportional to the square root of their density. While the increase of the ozone concentration results in the evident increase of the pit density, its influence on the pit growth rate cannot be easily deduced from the results obtained so far. As mentioned above, the growth rate may be dependent not only on the ozone concentration but also on the pit density,2,8 which changes during the experiment. As one can see in Figure 3, the pits formed at higher ozone concentration (1.0 ppm) are smaller than the pits formed at lower ozone concentration (0.13 ppm) for the same reaction time (see Figure 3b and c). This suggests that the overall rate of the reaction of edge carbon

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atoms was in both cases similar, despite 8 times different ozone concentration. In Table 1 the values of growth rates of the pit radius (dr/dt) during oxidation at 450 and 370 °C in different gases are shown. In the case of oxidation by N2O, NO2, and O2, the rates dr/dt are calculated from the results presented in refs 5-7. In the case of oxidation in ozone/air mixtures, the rate is calculated from the average size of the pits after 5 min, assuming that the dependence r(t) is linear. The pit growth rate in the presence of ozone is higher than that during oxidation in molecular oxygen at both temperatures. As compared to the biggest rate calculated for NO2, it is, at 370 °C, comparable, and at 450 °C, it is only two times smaller. It should be noticed that the total amount of the carbon atoms removed from the basal plane after a given time (overall rate) is the highest in the case of oxidation in an ozone/air mixture despite the low ozone concentration. The amount of carbon atoms removed at a time t is proportional to the total surface of the pits, that is, to N(r(t))2. The density of pits after oxidation in NO2, N2O, and O2 is small, that is, N ) 4 µm-2 (natural defects), while, in the case of oxidation in ozonized air, N is much higher, for example, about 80 and 1500 µm-2 at 450 and 370 °C, respectively. Therefore, the value of N(r(t))2 is, in the case of oxidation in ozone, 5 times at 450 °C and 260 times at 370 °C higher than those in the case of oxidation in NO2. The results given above show that the oxidation of HOPG in air at low ozone concentrations (less than 1 ppm) is much more effective than the oxidation in pure O2 or in other gases, even where atomic oxygen was assumed to be the oxidizing agent. Conclusions From the presented results it is clear that such parameters as oxidation time, temperature, and ozone concentration in air can be used to control the process of pit formation and growth on the basal plane of HOPG. During HOPG oxidation in ozonized air, the generation of new pits does not occur very close to the pit edges. Therefore, they are distributed on the basal plane of HOPG more uniformly than expected for a purely statistical distribution. The small pits formed by oxidation in the presence of ozone can be enlarged to the desired diameter by subsequent oxidation in air (or in other gases) at conditions under which formation of additional pits does not occur, for example, in air at 400-700 °C. In such a case, one can avoid a broad distribution of pit diameters, if the process of the oxidation by ozone is stopped after a short reaction time after production of very small pits. This means that, under certain conditions, the oxidation of graphite in ozone/ air mixtures can serve to initiate the nucleation step to control, first of all, the density of the pits, while the diameter can be controlled independently. Acknowledgment. This work was partially supported by KBN Project 4 T08E06322, DLR Project Pol. 168/96, and SFB 448 “Mesoscopically Organized Composites”.We are indebted to Dr. Dieter Scharffe from the MPI fu¨r Luftchemie in Mainz for the access to the photometric ozone analyzer. Pawel Tracz is acknowledged for the help in statistical analysis of the STM images. LA034103H