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LFM and XPS study
Applied Surface Science 173 (2001) 221±232
Electrochemical intercalation of perchlorate ions in HOPG: an SFM/LFM and XPS study B. Schnydera,*, D. Alliataa,1, R. KoÈtza, H. Siegenthalerb a
b
General Energy Research, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland Department for Chemistry and Biochemistry, University of Bern, CH-3112 Bern, Switzerland Received 10 August 2000; accepted 21 November 2000
Abstract Highly oriented pyrolitic graphite (HOPG) in perchloric acid was adopted as a model system in order to elucidate the electrochemical anion intercalation process in graphite. The effects of the intercalation process were studied in terms of the changes in surface friction and on the electronic structure of the HOPG. Lateral force microscopy (LFM) combined with cyclic voltammetry (CV) and X-ray photoelectron spectroscopy (XPS) indicated that the speci®c adsorption of perchlorate ions is responsible for changes in friction occurring in proximity of the steps on the HOPG surface. The friction changes reversibly within a narrow potential window preceding intercalation. After an intercalation and deintercalation cycle the change of the friction at a step is irreversible. No change in the friction could be observed on the basal plane. The binding energies in the C 1s, O 1s and Cl 2p XPS spectra of the intercalated compound are shifted relative to those of the nonintercalated host and adsorbed perchlorate ions, which is attributed to a shifted Fermi level. # 2001 Elsevier Science B.V. All rights reserved. PACS: 71.20.T; 81.40.P; 07.79; 07.79.S; 79.60 Keywords: Intercalation compounds; HOPG; Friction; Scanning probe microscopy; Lateral force microscopy; X-ray photoelectron spectroscopy
1. Introduction Graphite was extensively studied as an electrode material in ion transfer batteries, and the macroscopic changes of its properties during intercalation/insertion have been investigated for many years [1]. In commercially available lithium-ion batteries, cations are *
Corresponding author. Tel.: 41-56-310-41-93; fax: 41-56-310-44-15. E-mail address: [email protected] (B. Schnyder). 1 Present address: INFM Ð Dipartimento di Scienze Ambientali, Universita' della Tuscia, I-01100 Viterbo, Italy.
intercalated or deintercalated during the charging or discharging processes of the electrodes. The intercalated species occupy sites in the lattice formed by the carbon atoms. However, information on a local microscopic scale has so far rarely become available from an analysis of ion intercalation processes. With the introduction of scanning probe microscopy (SPM) and more particularly of lateral force microscopy (LFM), new tools became available to probe surfaces locally, even on nanometer scale, and extract more detailed information about the dimensional and tribological characteristics of surfaces [2].
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 0 ) 0 0 9 0 2 - 8
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Beck et al. [3] had shown that perchlorate ions are among the best intercalating species in graphite at relatively low acid concentrations (2±4 M). The contributions of side reactions could be minimized by selecting the right combination of acid concentration and electrochemical potential [4]. In our laboratory we used in situ atomic force microscopy (AFM) to investigate the quantitative increase in interlayer spacing and the local aspect of the anion intercalation process into highly oriented pyrolitic graphite (HOPG). Coexisting regions with different kinetics for the intercalation and the deintercalation process were revealed. The intercalation process appears to be faster than the reverse process [5]. We also could show that the intercalation process depends on the number of graphitic layers involved [6]. In spite of the rapid commercialization of intercalation electrodes in modern batteries, several problems remain. A knowledge of the chemical nature of the intercalated species within the host graphite lattice and of the resulting electronic structure of the graphite intercalation compound (GIC) are of central importance for an understanding of the intercalation process. In the present communication HOPG in an aqueous electrolyte (perchloric acid) was adapted as a model system in order to elucidate the mechanism of the electrochemical anion intercalation process in graphite. This anion intercalation process occurs without secondary reactions like ®lm formation that are seen in the intercalation of lithium cations in HOPG in an organic electrolyte [7]. We used scanning force microscopy/lateral force microscopy (SFM/LFM) combined with cyclic voltammetry (CV) as an in situ analytical tool during the intercalation and deintercalation process of perchlorate ions in HOPG. Further, X-ray photoelectron spectroscopy (XPS) was used as an ex situ analytical tool to obtain simultaneous electronic and chemical information. Ex situ XPS studies can provide information about the chemical state of the electrode surface exposed to an electrolyte at a de®ned potential, a possibility ®rst demonstrated by Kim et al. [8]. Later Kolb et al. [9] could show that during emersion and transfer of the electrode to the UHV system, the electrochemical double layer including the counterions was preserved.
By combining the chemical surface sensitivity of XPS with the local analysis of SFM/LFM, we obtained a second set of new information about this model system. 2. Experimental HOPG samples, supplied by Advanced Ceramics (Lakewood, OH), were used as working electrodes. According to X-ray diffraction (XRD), these ZYHtype crystals have a Mosaic spread of 3.58. The HOPG samples were freshly cleaved with adhesive tape prior to each experiment. The side faces were covered with an insulating varnish (Lacomit varnish, Agar Scienti®c, Stansted) in order to prevent exposure of the HOPG edge planes to the electrolyte. We determined the bilayer step density of a freshly cleaved surface by AFM, and found it to be at least 1 mmÿ2. The density of monolayer and multilayer steps is lower (0.05 and 0.1 mmÿ2). This rather high density of defects correlates well with the mosaic spread of 3.58. Electrochemical experiments were typically carried out in aqueous 2 M HClO4 in a 3-electrode cell with a Pt-counter electrode and a suitable reference electrode (see below). The electrolyte was prepared by diluting 60% concentrated HClO4 (Fluka) with ultrapure water. Scanning probe microscopy was performed with a Park Scienti®c Instruments Model AP-100 Autoprobe CP (Sunnyvale, CA) equipped with a self-built electrochemical cell and an EG&G Versastat 270 potentiostat/galvanostat. In this in situ electrochemical cell a platinum quasi-reference electrode (Pt-QRef) [10] was used, which showed a drift of less than 10 mV/h. In all the other experiments a saturated mercury sulfate electrode (SME) served as reference electrode. In this publication all potentials are quoted against the SME. The scanning probe microscope images were acquired with a 100-mm scanner in the contact mode at constant force. The de¯ection of the laser beam at the backside of the cantilever normal to the surface represents the topographical information. At the same time the change in the lateral de¯ection contains the friction information. The typical scan rate was 1 Hz. Ultralevers from Thermomicroscopy
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were used as tips. The microscope was calibrated according to the procedure developed by Staub et al. [11]. For some experiment the so-called xtz-mode has been used. This mode permits an observation of topographical and friction changes of the sample surface as functions of time at different locations along a single line. Along the horizontal axis (x-axis), topographical or friction change information is displayed, while the time is plotted along the vertical axis (y-axis). In this mode changes in z occurring at different points along a single line can be compared during an electrochemical experiment. The XPS spectra were recorded with an ESCALAB 220i XL (VG Scienti®c) photoelectron spectrometer using monochromatic Al Ka radiation. The X-ray power was 200 W. The spectra were recorded in the CAE-mode (CAE: constant analyzer energy) with analyzer pass energies of 50 eV for the survey spectra and 20 eV for the high-resolution detail spectra. All spectra presented are normalized to the same peak height. The composition of the samples was determined by quantitative analysis of the spectra using the cross-sections of Sco®eld [12]. The base pressure in the analysis chamber during analysis was better than 5 10ÿ9 mbar. Samples prepared for the XPS measurements were all scanned (scan rate: 5 mV/s) to the desired potential, held there for 60 s and then removed from the electrolyte under potential control. HOPG can be emersed rather dry, because it is hydrophobic. Small electrolyte droplets sometimes left on surface were blown away with argon. Finally the samples were transferred immediately to the UHV-system, minimizing the exposure to air. Measured photoelectron spectra represent a convolution of the photoemission process with contributions from the instrument such as the width of the X-ray line, the monochromator and the analyzer. These instrumental contributions were determined by measuring the Fermi edge of silver with exactly the same instrument settings subsequently used for the highresolution XPS measurements (spot size 1 mm, pass energy 20 eV). The resolution function for this spectrometer condition is the derivative of the silver Fermi edge measurement [13]. This resolution function is assumed to be a Gaussian curve with in our case a full width at half maximum of 0.50 eV.
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3. Results and discussion 3.1. Material Graphite forms two different phases (hexagonal and rhombohedral) each having stacks of two-dimensional layers. The hexagonal phase is characterized by ABABAB stacking where layers B are shifted with respect to layers A. The stacking in the rhombohedral phase is ABCABC. . ., where layers C are shifted with respect to B by the same distance as layers B are shifted with respect to A. The hexagonal form is thermodynamically more stable at normal temperature and pressure, and is that commonly found [14]. During intercalation the graphite layers tend to slide and get aligned, and change their characteristic stacking from ABABAB to AIAIAI, where I is the intercalated species [15]. HOPG has a very small dispersion of its c-axis, according to XRD experiments. It consists of large crystallites with La and Lc values of about 1±10 mm [16]. Its basal plane is rather rough and contains grain boundaries, edge planes, line and point defects [17] which are preferred sites for intercalation [18]. 3.2. Cyclic voltammetry Intercalation of perchlorate ions ClO4 ÿ into HOPG basal planes was performed in 2 M HClO4. Different stages of graphite intercalation process can easily be distinguished in a cyclic voltammogram such as that shown in Fig. 1. Over a rather narrow potential range of about 200 mV three different stages of intercalation of anions ClO4 ÿ and associated solvent or
Fig. 1. Cyclic voltammogram of HOPG in 2 M HClO4; scan rate: 25 mV/s. Stages IV, III and II are indicated in the ®gure.
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acid molecules into the graphite lattice are visible in the form of anodic peaks in the positive scan direction which reappear as cathodic peaks in the negative scan [19]. The intercalation process starts at potential above 1.0 V vs. SME, which is accompanied by an increase in current. The peak positions for stage IV, III and II in the anodic scan are 1.15, 1.22 and 1.27 V vs. SME. Unfortunately, the potential required for stage I is so high that side reactions such as the evolution of CO2 or O2 took place and result in irreversible damage of the HOPG surface. Occasionally blister formation was even observed after only 20 cycles to stage IV [5], which is regarded as a rather mild treatment. This blister formation could be explained by an intercalation of electrolyte and water followed by subsurface gas evolution [20]. The main reaction involved in the intercalation process is Cx Aÿ y solv: Cx Aÿ y solv: eÿ (1) This reaction is reversible and occurs with a current ef®ciency of nearly 100% at concentrations higher than 2 M [19]. At lower concentrations of the electrolyte or very positive potentials the ef®ciency of the reaction decreases, and other reactions such as graphite oxide formation, carbon dioxide formation or water electrolysis become possible. In an earlier study [5] we could show that the process is almost reversible up to those values of potential where only stages IV and III are formed. At higher potentials where stages II and I are formed, 20±30% of the charge cannot be recovered in the cathodic scan. The excess anodic current is probably due to irreversible oxidation of the graphite or gas evolution. These results are in agreement with earlier ®ndings of Beck et al. [3]. Aronson et al. [21] already had found a value of 2 for the ratio between the intercalated solvent species (HClO4) and the intercalated perchlorate ions in fully intercalated, annealed pyrolitic graphite Cx ClO4 ÿ yHClO4 . This implies values of x 24 and y 2 in reaction (1). In their calculations a Ê was used for the distance between two value of 7.95 A intercalated graphene layers. Thus, intercalation of perchlorate and of the accompanying solvent moleÊ between cules expands the nominal spacing of 3.35 A
Ê . The neutral acid molethe graphene layers by 4.6 A cules serve to screen the Coulomb repulsion between the ClO4 ÿ ions. These authors did not discuss the presence of water in the intercalation compounds. They only mentioned that these compounds are not stable in air or water! 3.3. Friction (SFM/LFM) SFM/LFM allows to measure normal and lateral forces at nanometer scale in various environments including electrolytes [22]. In the present study we looked for a possible correlation between changes in friction properties of HOPG and the intercalation process. The in situ SFM/LFM images recorded on a potential-controlled HOPG sample at a step of four graphene layers are shown in Fig. 2 for two potentials. At 0.0 V vs. SME no friction effect was seen at the step. This is evident when comparing the forward and backward scan of the LFM images. At 0.9 V vs. SME a strong friction signal is seen at the step, while almost no change is observed on the basal planes of the HOPG surface. At this potential intercalation should not occur, and it can in fact be seen from the topographic images that the height of the step remains unchanged (same 2-scale for both topography images). Therefore, the observed friction change cannot be the result of an intercalation process. However, a slight increase in the anodic current can be seen in cyclic voltammograms at potential above 0.85 V vs. SME (it is not visible on the scale of currents used in Fig. 1). At potentials below 0.85 V vs. SME the current is purely double-layer related, where no speci®c sites are taken up by the adsorbed ions. Higher currents could also be explained in terms of a pseudocapacity process such as the oxidation/reduction of functional surface groups (quinone/hydroquinone). The change of the friction of HOPG in different electrolytes was for the ®rst time investigated with LFM by Marti and coworkers [23,24], but the origin of the friction changes occurring at graphite steps or a possible correlation with the oxidation/reduction of functional surface groups and other effects had not been elucidated. We therefore extended the potential scan to higher potentials, in order to study possible effects of the intercalation process on the friction change. In Fig. 3 the measured friction is plotted as
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Fig. 2. Topography and friction images of a step of four graphene layers on HOPG in 2 M HClO4 measured by SFM/LFM at 0.0 and 0.9 V vs. SME.
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Fig. 3. Friction at a step on HOPG as a function of electrode potential.
a function of the applied electrode potential. Friction changes were in fact detectable at potentials where intercalation occurred and the topography images revealed an increased step height (not shown here). In agreement with the already mentioned studies of Marti and coworkers [23,24], we saw a reversible change of friction at steps of HOPG in perchloric acid but no evidence for a similar phenomenon on the basal planes. The reproducibility of the friction changes at a step is shown in Fig. 4. By using the
LFM in zxt-mode, the sample potential was cycled between 0.0 and 0.9 V vs. SME while two steps on a HOPG surface (visible in the center and to the right) were imaged in topography and friction. The changes of friction with potential are clearly visible for both steps. From Fig. 4 it can be concluded that the phenomenon is reversible between these two potentials. After intercalation the friction at steps was no longer reversible. Our results indicate that the friction at steps changes permanently as soon as intercalation starts. Whether this behavior is due to chemical reactions at the graphite or to adsorbates could not be decided by on the sole basis of LFM results. For higher steps the friction change become obscured. The reason for this behavior may be that the tip is not perfect and an interaction of the ¯ank of the triangular tip shape with the edge and/or the adsorbed species is not possible or reduced to an undetectable level. 3.4. XPS XPS studies yield quantitative information about the relative concentration of species present at the
Fig. 4. Friction vs. time image (left) and the corresponding potential transient (right); potential scan rate 100 mV/s; imaging speed 1 Hz.
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Fig. 5. (a) Atomic concentration ratio Cl/C on the HOPG surface vs. electrochemical potential (SME). (b) Atomic concentration ratio O/Cl on the HOPG surface vs. electrochemical potential (SME).
surface or in the near-surface region. In our investigations, the starting potential was the open-circuit potential of HOPG, which was close to 0.1 V vs. SME. The intensity of the Cl 2p signal increases with potential over the entire range of potentials investigated (0.1±1.4 V vs. SME). This is clearly seen in Fig. 5a, where the Cl/C ratio of the atomic concentrations is plotted. Within the double-layer region between 0.1 and 1.0 V vs. SME, the ratio increases by approximately a factor of 5. Kolb [25] found a similar increase for emersed gold electrodes in 0.01 M HClO4. The increase of the Cl/C ratio is more pronounced at potentials above 1.0 V where intercalation takes place (see Fig. 5a). A similar behavior is observed for the atomic concentration ratio of O/C (not shown here). In a plot of the ratio O/Cl, which is shown in Fig. 5b, we would expect a ratio of 4, if only perchlorate ions were involved in the adsorption and intercalation process. This is de®nitely not so anywhere within the range of potentials investigated. For potentials lower than 0.9 V vs. SME a ratio of about 8
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was determined. At potentials equal to or above 0.9 V vs. SME the ratio is smaller than 5. One oxygencontaining species that could be involved in this process is water. This would mean that about four water molecules per perchlorate ion are adsorbed in the electrochemical double layer at potentials below 0.9 V vs. SME. The number of water molecules involved in the intercalation process cannot be determined from this plot alone, because in Fig. 5b we do not distinguish between oxygen due to adsorbed species and oxygen due to intercalated species, i.e. we look at the total amount of oxygen. It is readily seen, however, that the number of water molecules involved is lower at potentials equal to or above 0.9 V vs. SME. It should be noticed in particular that between 0.9 and 1.1 V, where no intercalation takes place, the ratio is below 5. In the high-resolution XPS Cl 2p spectra (see Fig. 6a) of electrodes prepared at potentials lower than 1.1 V, only one spin-split peak can be seen at a rather high binding energy (207.6 eV). This high binding energy is typical for perchlorates [26]. The spin-splitting of the Cl 2p level is clearly visible. The peak position is stable in this potential range, and this Cl 2p peak can be attributed to the adsorbed perchlorate ions forming the solution side of the electrochemical double layer. The charging of this electrochemical double layer generates the observed increase of the Cl/C ratio (see Fig. 5a). At higher potentials where intercalation does occur, a second species gives rise to an additional spin-split peak seen in the spectra. We assign this peak to the intercalated perchlorate ions. It is seen from reaction (1) that the perchlorate undergoes no change in its valence state during the intercalation process. However, there is a distinct shift in binding energy. The Cl 2p level of the intercalated species is shifted with respect to the adsorbed perchlorate by 1.7 eV (205.9 eV). The peaks of the intercalated species overlap partly with the peaks of the adsorbed perchlorate. The Cl 2p3/2 of the adsorbed species has almost exactly the same peak position as the Cl 2p1/2 peak of the intercalated species. Intercalated neutral acid molecules (HClO4) postulated by Aronson et al. [21] should give no additional shift in binding energy relative to the intercalated perchlorate ions. The XPS spectra of the O 1s level recorded at different electrode potentials (see Fig. 6b) exhibit only
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Fig. 6. Comparison of (a) Cl 2p, (b) O 1s and (c) C 1s spectra for different electrochemical potentials.
one peak at 532.6 eV in the double-layer region (0.1± 1.0 V) but two peaks when intercalation has occurred. The binding energy shift between the two peak is the same (1.7 eV) as for the Cl 2p level. An O 1s peak between 532 and 533 eV is typical for the oxygen signal of perchlorates [26]. Adsorbed water would also generate an O 1s peak in this binding energy range (532±534.5 eV) [26±28]. The O 1s peak position of adsorbed water in the electrochemical double layer and the O 1s signal of the adsorbed perchlorate seem to overlap, because only one peak is detectable in the O 1s spectra recorded at samples where only the electrochemical double layer
is charged. However, one can conclude from the O/Cl ratio plotted in Fig. 5b, which is higher than 4, that water is present. The presence of other oxygen-containing species is much less obvious. The same argument can be used for water accompanying the intercalated perchlorate. There the O/Cl ratio is smaller, so less water would move into the graphite lattice. The presence of water on the surface after transfer of the sample to an UHV-system is not unanimously accepted. On noble metal surfaces KoÈtz et al. [29,30] found the amount of water to be rather low or even below the detection limits of XPS analysis, and reasoned that water desorbs from almost any
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noble metal surface under UHV conditions. The H2O bound to a counter-ion as part of the solvation shell may be stabilized, and survive the transfer to the UHV chamber [31]. In our case we look, not at a noble metal but at a rather hydrophobic material that can be removed almost dry from an electrolyte, as mentioned earlier. From Fig. 5b we could conclude that the H2Ocontaining solvation shell of the perchlorate ions in the electrochemical double layer is still present under UHV conditions and consists of four water molecules. At a potential of about 0.9 V vs. SME the atomic ratio O/Cl is reduced to about 5, indicating that less water is present than in the pure double-layer region. The binding energy of the Cl 2p spectra did not provide any indication that the trend of adsorption of perchlorate ions would change at the potential where the friction changes. However, the trend in the change of the atomic ratio O/Cl (in Fig. 5b) is similar to that of the friction change in Fig. 3. Both the friction and the O/Cl ratio change at potentials above 0.8 V vs. SME. As mentioned earlier, this could be a result of fewer water molecules being present in the solvation shell. Although it is not possible with XPS to discriminate between ions adsorbed on basal planes and ions adsorbed at steps, we believe that this partial loss of the water in the solvation shell of perchlorate ions (speci®c adsorption [25]) adsorbed at steps could be the reason for the friction change at the steps. Speci®c adsorption can also be inferred from the current increase seen in the cyclic voltammogram. Another explanation could be that the ions are mainly accumulated at the steps. However, for geometrical reasons, the LFM tip starts to be affected by their presence, only when enough ions are adsorbed at the step to perturb the interaction forces between the tip and the sample surface. According to Fig. 3, the friction change and likewise the change in the O/Cl ratio is rather sharp. We conclude, therefore, that a mere increase in the amount of perchlorate could not be the main reason for the sharp friction change observed, the more so since at the potential where the friction change occurs, no dramatic change in the Cl/C ratio, which could produce this friction change, is visible (see Fig. 5a). The C 1s peak of graphitic carbon appears at 284.3 eV and has a long tail toward higher binding energies, which has been attributed to interaction with the conduction band electrons [32]. Adsorbed
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perchlorate ions give no special feature in the C 1s spectrum (see Fig. 6c). When perchlorate ions are intercalated, the C 1s peak obtains a shoulder on the side of lower binding energies. At potentials above 1.4 eV vs. SME, the electrode will be destroyed by exfoliation. In addition, it becomes oxidized, which can be seen from an additional peak appearing in the C 1s spectra at about 286 eV. The corresponding O 1s peak overlaps with the oxygen signal generated by the other oxygen-containing species. For a more accurate determination of the peak positions, the maximum entropy method (MEM) [13] was applied to deconvolute these spectra. This method has already been applied to spectra of sul®de minerals [33], to Cr 2p spectra of thin chromium oxides and to Au 4f spectra of gold±aluminum alloys [34]. Rather small changes and peak shifts could be identi®ed by this method. If the contributions of the instrument to photoelectron emission spectra are known or can be measured, then the energy resolution function described by Splinter and McIntyre in [13] can be used to deconvolute the intrinsic spectrum from the measured one. By this method the energy resolution of the measured spectra can be enhanced by a factor of 2, which is nicely illustrated in Fig. 7 for the C 1s levels. The C 1s peak of the sample emersed at 0.5 V vs. SME has a full width half maximum (FWHM) of 0.60 eV. After the
Fig. 7. Maximum entropy deconvolution of the C 1s spectra (same as displayed in Fig. 6c) of HOPG samples emersed at different potentials.
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deconvolution procedure the FWHM of the C 1s peak is only 0.28 eV. The quality of the original spectra must be rather high for an application of the maximum entropy method. The quality of our O 1s and Cl 2p spectra was not suf®cient on account of the limited measuring time and of the rather small amount of species present. The algorithm did not converge, only the noise in the spectra could be reduced. In the C 1s spectra recorded at potentials where only adsorption occurs, an additional feature at a binding energy of 285 eV is visible (Fig. 7). Its origin is not completely clear. A possible explanation could be a C±O or C±OH bond of adsorbed species on the HOPG surface. In spectra recorded at samples at which intercalation takes place, this feature is not or not well detectable. After application of MEM, it is much easier to determine the XPS peak position for the intercalated graphene layers. The peak shift to lower binding energies is of the order of 0.7 eV for an intercalated sample held for 60 s at 1.3 V. The shift is even slightly larger for the samples polarized to lower potentials, but it is always smaller than that found for the Cl 2p and O 1s levels. The binding energy shifts of the Cl 2p and O 1s levels seem to be independent of the applied intercalation potential. In the spectra recorded at intercalated samples, all three peaks (C 1s, O 1s, Cl 2p) corresponding to intercalated species are shifted to smaller binding energies. Due to the intercalation process, the carbon atoms involved should undergo a change of their valence state C ! C and form the graphite
intercalation compound, as described in reaction (1). For conservation of charge neutrality, the negatively charged perchlorate ions, which undergo no change in valence state during intercalation, should induce positive charges on the carbon atoms. Consequently, the C 1s peak of carbon atoms involved in the intercalation process should shift in the positive direction, if a charge transfer took place. Just the opposite is observed. On the other hand, the Cl 2p and the O 1s peaks of the intercalated species should not shift at all (according to reaction (1)), but they are shifted to lower binding energies too. The observed shifts can be explained in terms of a shift in Fermi level of the intercalated layers. The resulting energy scheme relative to the vacuum level is illustrated in Fig. 8 for the C 1s and Cl 2p levels. The shifts of the O 1s level are identical with those of the Cl 2p level. A displacement of the Fermi level has been described by Wertheim et al. [35] for LiC6. They explain this behavior in terms of a screening of the core hole by the conduction electrons in LiC6. According to the scheme in Fig. 8, the C 1s binding energy of the intercalated graphene layers is increased, as expected for positively charged species Cx . This positive charge induces a binding energy shift of 1.0 eV in an electrode emersed at 1.3 V vs. SME. The binding energy of Cl 2p and also that of O 1s (not shown in the scheme) of the perchlorate remains constant, because the perchlorate ions undergoes no change in valence state. A shift of the Fermi level means that the work function of these intercalated
Fig. 8. Energy schemes relative to the vacuum level of the binding energies of HOPG with adsorbed and intercalated species of ClO4 ÿ corresponding to a potential of 0.4 and 1.3 V vs. SME, respectively.
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graphene layers is increased by 1.7 eV regardless of the intercalation potential of the electrode. Considering the Fermi energy shift, the observed energy shift for C6 ÿ in LiC6 with respect to the neutral valence state of graphite is ÿ0.8 eV [35,36]. For Cx we ®nd a shift of about 1.0 eV at an intercalation potential of 1.3 V vs. SME. In contrast to donor-intercalation compounds, the charged species appear to be fully ionized in any stage of most of the acceptor-intercalated compounds [37]. The overall charge transfer is not 100%, because the charged molecules are separated from each other by other neutral species (in our case HClO4 and probably some water). Under the assumption of completely intercalated graphite C24 ClO4 ÿ 2HClO4 [21], this would give a value of f 0:33 for the fractional charge transferred to or from the graphite per intercalated atom or molecule. Different binding energy shifts can be observed between the two C 1s peaks for different amounts of perchlorate ions intercalated into the graphite. The C 1s peak of the intercalated graphene layers moves closer to the graphitic carbon peak when more perchlorate ions are intercalated at higher potentials (see Fig. 7). Therefore, the binding energy of the intercalated graphene layers is higher at higher intercalation potentials. The work function and the Fermi level do not change or move when different intercalation potentials are applied. In contrast to LiC6 where only one C 1s peak is visible, for HOPG intercalated with perchlorate always two peaks are visible in the spectrum, even at potentials where intercalation should be almost complete (>1.3 V vs. SME). There could be two reasons for this difference. The step density and the step height are rather low. This means that with XPS, it is possible to see through the intercalated graphene layers or to see regions not reached by intercalation. But even if we could intercalate each graphene layer completely with perchlorate ions (see product of reaction (1)), it would still be possible to have carbon atoms with an electronic structure not in¯uenced by the presence of perchlorate, on account of screening of the charge. This screening effect could be generated by the presence of neutral acid or water molecules. When tilting the sample we did not see an increase in intensity of the perchlorate ion signal relative to the carbon signal or a change in relative intensities of the two carbon signals. We can exclude on the basis of this
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result that only the top layers are intercalated, since a variation in the intensity ratio between intercalated and non-intercalated species should become visible when changing the probing depth. For higher steps the intercalation is probably less effective. In stage I not all graphene layers are ®lled with perchlorate ions [6]. In the intercalation process of ions considerable energy is required to overcome the Van der Waals forces and increase the distance between the graphene layers. The required force increases with the number of graphene layers present on top of the intercalated gap. A second explanation is related to the fact that the probability of ®nding defects and vacancies increases with the number of graphene layers. These defects can prevent the perchlorate ions from intercalating, since the adjacent layers are pinned together and dif®cult to move with respect to each other. As a consequence, fewer ions can become intercalated. From this consideration it can be concluded that stage I is an average over the whole electrode surface, and is never fully reached. A series of samples were removed from the electrolyte after 60 s at a potential of 0.1 V vs. SME, prior to which they had been exposed to potentials in the range between 0.9 and 1.4 V vs. SME, also for 60 s. All these samples had more perchlorate on the surface than a sample never exposed to such a high potential. In the Cl 2p spectra of samples that had been exposed to 0.9 and 1.0 V vs. SME, only adsorbed perchlorate ions which stick to the surface or to steps were seen. In samples that had reached potentials where intercalation occurs, some intercalated species are still present in addition to the adsorbed ones. This can be concluded from the position of the Cl 2p peaks. Even in the C 1s spectra, one can detect the C 1s signal that corresponds to the intercalated graphene layers. It follows that the irreversibility of the friction changes is due to the intercalation or the incomplete deintercalation of the perchlorate ions. 4. Conclusions The intercalation of perchlorate ions in HOPG causes a friction change at steps at potentials more positive than 0.85 V vs. SME. No change of the friction can be observed on the basal plane. In a narrow potential window between 0.85 and 1.1 V
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vs. SME before the start of intercalation, the friction change is reversible. It is due to the enhanced adsorption of perchlorate ions at steps accompanied by a loss of water from the solvation shells. After intercalation and deintercalation, the change of the friction at a step of HOPG is irreversible. This irreversibility is due to the fact that it is not possible to completely deintercalate the graphite lattice. The XPS binding energies of the intercalated species in the C 1s, O 1s and Cl 2p spectra shift with respect to those of the adsorbed species or the host material. A shift in Fermi level of the intercalated ions and part of the intercalated graphene layers is able to explain the shift of these peaks, where a charge transfer occurs because they are not screened by the presence of other molecules. With the MEM it was possible to increase the resolution of the C 1s peaks by a factor of 2 and determine the peak position of the new peak more accurately. Acknowledgements Financial support of the Swiss National Science Foundation Grant No. 4036-044040 is gratefully acknowledged. Special thanks are due to S.J. Splinter and N.S. McIntyre for making available to us the Matlab program for the maximum entropy deconvolution. References [1] L.B. Ebert, Ann. Rev. Mater. Sci. 6 (1976) 181. [2] R. Wiesendanger, H.-J. Guentherodt (Eds.), Scanning Tunneling Microscopy, Vol. I/II, 1st Edition, Springer, Berlin, 1992/ 1993. [3] F. Beck, H. Junge, H. Krohn, Electrochim. Acta 26 (1981) 799. [4] J. Zhang, E. Wang, J. Electroanal. Chem. 399 (1995) 83. [5] D. Alliata, P. HaÈring, O. Haas, R. KoÈtz, H. Siegenthaler, Electrochem. Commun. 1 (1999) 5. [6] D. Alliata, R. KoÈtz, O. Haas, H. Siegenthaler, Langmuir 15 (1999) 8483. [7] J.O. Besenhard, M. Winter, J. Yang, W. Biberacher, J. Power Sources 54 (1995) 228. [8] K.S. Kim, N. Winograd, R.E. Davis, J. Am. Chem. Soc. 93 (1971) 6269. [9] D.M. Kolb, D.L. Rath, R. Wille, W.N. Hansen, Ber. Bunsenges. Phys. Chem. 87 (1983) 1108. [10] C.M.A. Brett, A.M.O. Brett, Electrochemistry Principles, Methods and Applications, Oxford University Press, Oxford, 1993, p. 138. [11] R. Staub, D. Alliata, C. Nicolini, Rev. Sci. Instrum. 66 (1995) 2513.
[12] J.H. Sco®eld, J. Electron Spectrosc. Relat. Phenom. 8 (1976) 129. [13] S.J. Splinter, N.S. McIntyre, Surf. Interface Anal. 26 (1998) 195. [14] R.L. McCreery, in: A.J. Bard (Ed.), Electroanalytical Chemistry, Vol. 17, Marcel Dekker, New York, 1991, p. 221. [15] W.R. McKinnon, R.R. Haering, in: B. Conway, R.E. White, J.O'M. Bockris (Eds.), Modern Aspects of Electrochemistry, Vol. 15, Plenum Press, New York, 1983, p. 235. [16] R.L. McCreery, in: A.J. Bard (Ed.), Electroanalytical Chemistry, Vol. 17, Marcel Dekker, New York, 1991, p. 228. [17] D. Bar-Tow, E. Peled, L. Burstein, J. Electrochem. Soc. 146 (1999) 824. [18] B. Rousseau, M. Vayer-BesancËon, H. Estrade-Szwarckopf, Solid State Commun. 99 (3) (1996) 143. [19] F. Beck, H. Krohn, Synth. Met. 7 (1983) 193. [20] K.W. Hathcook, J.C. Brum®eld, C.A. Goss, E.A. Irene, R.W. Murray, Anal. Chem. 67 (1995) 2201. [21] S. Aronson, S. Lemont, J. Weiner, Inorg. Chem. 10 (1971) 1296. [22] M. Binggeli, R. Christoph, H.E. Hintermann, O. Marti, Surf. Coat. Technol. 62 (1993) 523. [23] M. Binggeli, R. Christoph, H.E. Hintermann, O. Marti, Nanotechnology 4 (1993) 59. [24] E. Weilandt, A. Menck, O. Marti, Surf. Interface Anal. 23 (1995) 428. [25] D.M. Kolb, Z. Phys. Chem. Neue Folge 154 (1987) 179. [26] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, in: J. Chastain (Ed.), Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer, Eden Praire, MN, 1992. [27] R. KoÈtz, in: H. Gerischer, C.W. Tobias (Eds.), Advances in Electrochemical Science and Engineering, Vol. 1, VCH, Weinheim, 1990, p. 82. [28] J.C. Fuggle, in: D. Briggs (Ed.), Handbook of X-ray and Ultraviolet Photoelectron Spectroscopy, Heyden, London, 1977, p. 295. [29] R. KoÈtz, H. Neff, K. MuÈller, J. Electroanal. Chem. 215 (1986) 331. [30] H. Neff, R. KoÈtz, J. Electroanal. Chem. 151 (1983) 305. [31] E.M. Stuve, K. Bange, J.K. Sass, in: A.F. Silva (Ed.), Trends in Interfacial Electrochemistry, NATO ASI Series C, Vol. 179, Reidel, Dordrecht, 1986, p. 255. [32] C. Kozlowski, P.M.D. Sherwood, J. Chem. Soc., Faraday Trans. I 80 (1984) 2099. [33] A.R. Pratt, N.S. McIntyre, S.J. Splinter, Surf. Sci. 396 (1998) 266. [34] N.S. McIntyre, A.R. Pratt, H. Piao, D. Maybury, S.J. Splinter, Appl. Surf. Sci. 144/145 (1999) 156. [35] G.K. Wertheim, P.M.Th.M. Van Attekum, S. Basu, Solid State Commun. 33 (1980) 1127. [36] H. Momose, H. Honbo, S. Takeuchi, K. Nishimura, T. Horiba, Y. Muranaka, Y. Kozono, H. Miyadera, J. Power Sources 68 (1997) 208. [37] P. P¯uger, H.-J. GuÈntherodt, in: J. Treusch (Ed.), Intercalated Graphite Ð A Synthetic Metal. I. Introduction and Electronic Structure in FestkoÈrperprobleme (Advances in Solid State Physics), Vol. XXI, Vieweg, Braunschweig, 1981, p. 271.
Electrochemical intercalation of perchlorate ions in HOPG: an SFM/LFM and XPS study B. Schnydera,*, D. Alliataa,1, R. KoÈtza, H. Siegenthalerb a
b
General Energy Research, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland Department for Chemistry and Biochemistry, University of Bern, CH-3112 Bern, Switzerland Received 10 August 2000; accepted 21 November 2000
Abstract Highly oriented pyrolitic graphite (HOPG) in perchloric acid was adopted as a model system in order to elucidate the electrochemical anion intercalation process in graphite. The effects of the intercalation process were studied in terms of the changes in surface friction and on the electronic structure of the HOPG. Lateral force microscopy (LFM) combined with cyclic voltammetry (CV) and X-ray photoelectron spectroscopy (XPS) indicated that the speci®c adsorption of perchlorate ions is responsible for changes in friction occurring in proximity of the steps on the HOPG surface. The friction changes reversibly within a narrow potential window preceding intercalation. After an intercalation and deintercalation cycle the change of the friction at a step is irreversible. No change in the friction could be observed on the basal plane. The binding energies in the C 1s, O 1s and Cl 2p XPS spectra of the intercalated compound are shifted relative to those of the nonintercalated host and adsorbed perchlorate ions, which is attributed to a shifted Fermi level. # 2001 Elsevier Science B.V. All rights reserved. PACS: 71.20.T; 81.40.P; 07.79; 07.79.S; 79.60 Keywords: Intercalation compounds; HOPG; Friction; Scanning probe microscopy; Lateral force microscopy; X-ray photoelectron spectroscopy
1. Introduction Graphite was extensively studied as an electrode material in ion transfer batteries, and the macroscopic changes of its properties during intercalation/insertion have been investigated for many years [1]. In commercially available lithium-ion batteries, cations are *
Corresponding author. Tel.: 41-56-310-41-93; fax: 41-56-310-44-15. E-mail address: [email protected] (B. Schnyder). 1 Present address: INFM Ð Dipartimento di Scienze Ambientali, Universita' della Tuscia, I-01100 Viterbo, Italy.
intercalated or deintercalated during the charging or discharging processes of the electrodes. The intercalated species occupy sites in the lattice formed by the carbon atoms. However, information on a local microscopic scale has so far rarely become available from an analysis of ion intercalation processes. With the introduction of scanning probe microscopy (SPM) and more particularly of lateral force microscopy (LFM), new tools became available to probe surfaces locally, even on nanometer scale, and extract more detailed information about the dimensional and tribological characteristics of surfaces [2].
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 0 ) 0 0 9 0 2 - 8
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Beck et al. [3] had shown that perchlorate ions are among the best intercalating species in graphite at relatively low acid concentrations (2±4 M). The contributions of side reactions could be minimized by selecting the right combination of acid concentration and electrochemical potential [4]. In our laboratory we used in situ atomic force microscopy (AFM) to investigate the quantitative increase in interlayer spacing and the local aspect of the anion intercalation process into highly oriented pyrolitic graphite (HOPG). Coexisting regions with different kinetics for the intercalation and the deintercalation process were revealed. The intercalation process appears to be faster than the reverse process [5]. We also could show that the intercalation process depends on the number of graphitic layers involved [6]. In spite of the rapid commercialization of intercalation electrodes in modern batteries, several problems remain. A knowledge of the chemical nature of the intercalated species within the host graphite lattice and of the resulting electronic structure of the graphite intercalation compound (GIC) are of central importance for an understanding of the intercalation process. In the present communication HOPG in an aqueous electrolyte (perchloric acid) was adapted as a model system in order to elucidate the mechanism of the electrochemical anion intercalation process in graphite. This anion intercalation process occurs without secondary reactions like ®lm formation that are seen in the intercalation of lithium cations in HOPG in an organic electrolyte [7]. We used scanning force microscopy/lateral force microscopy (SFM/LFM) combined with cyclic voltammetry (CV) as an in situ analytical tool during the intercalation and deintercalation process of perchlorate ions in HOPG. Further, X-ray photoelectron spectroscopy (XPS) was used as an ex situ analytical tool to obtain simultaneous electronic and chemical information. Ex situ XPS studies can provide information about the chemical state of the electrode surface exposed to an electrolyte at a de®ned potential, a possibility ®rst demonstrated by Kim et al. [8]. Later Kolb et al. [9] could show that during emersion and transfer of the electrode to the UHV system, the electrochemical double layer including the counterions was preserved.
By combining the chemical surface sensitivity of XPS with the local analysis of SFM/LFM, we obtained a second set of new information about this model system. 2. Experimental HOPG samples, supplied by Advanced Ceramics (Lakewood, OH), were used as working electrodes. According to X-ray diffraction (XRD), these ZYHtype crystals have a Mosaic spread of 3.58. The HOPG samples were freshly cleaved with adhesive tape prior to each experiment. The side faces were covered with an insulating varnish (Lacomit varnish, Agar Scienti®c, Stansted) in order to prevent exposure of the HOPG edge planes to the electrolyte. We determined the bilayer step density of a freshly cleaved surface by AFM, and found it to be at least 1 mmÿ2. The density of monolayer and multilayer steps is lower (0.05 and 0.1 mmÿ2). This rather high density of defects correlates well with the mosaic spread of 3.58. Electrochemical experiments were typically carried out in aqueous 2 M HClO4 in a 3-electrode cell with a Pt-counter electrode and a suitable reference electrode (see below). The electrolyte was prepared by diluting 60% concentrated HClO4 (Fluka) with ultrapure water. Scanning probe microscopy was performed with a Park Scienti®c Instruments Model AP-100 Autoprobe CP (Sunnyvale, CA) equipped with a self-built electrochemical cell and an EG&G Versastat 270 potentiostat/galvanostat. In this in situ electrochemical cell a platinum quasi-reference electrode (Pt-QRef) [10] was used, which showed a drift of less than 10 mV/h. In all the other experiments a saturated mercury sulfate electrode (SME) served as reference electrode. In this publication all potentials are quoted against the SME. The scanning probe microscope images were acquired with a 100-mm scanner in the contact mode at constant force. The de¯ection of the laser beam at the backside of the cantilever normal to the surface represents the topographical information. At the same time the change in the lateral de¯ection contains the friction information. The typical scan rate was 1 Hz. Ultralevers from Thermomicroscopy
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were used as tips. The microscope was calibrated according to the procedure developed by Staub et al. [11]. For some experiment the so-called xtz-mode has been used. This mode permits an observation of topographical and friction changes of the sample surface as functions of time at different locations along a single line. Along the horizontal axis (x-axis), topographical or friction change information is displayed, while the time is plotted along the vertical axis (y-axis). In this mode changes in z occurring at different points along a single line can be compared during an electrochemical experiment. The XPS spectra were recorded with an ESCALAB 220i XL (VG Scienti®c) photoelectron spectrometer using monochromatic Al Ka radiation. The X-ray power was 200 W. The spectra were recorded in the CAE-mode (CAE: constant analyzer energy) with analyzer pass energies of 50 eV for the survey spectra and 20 eV for the high-resolution detail spectra. All spectra presented are normalized to the same peak height. The composition of the samples was determined by quantitative analysis of the spectra using the cross-sections of Sco®eld [12]. The base pressure in the analysis chamber during analysis was better than 5 10ÿ9 mbar. Samples prepared for the XPS measurements were all scanned (scan rate: 5 mV/s) to the desired potential, held there for 60 s and then removed from the electrolyte under potential control. HOPG can be emersed rather dry, because it is hydrophobic. Small electrolyte droplets sometimes left on surface were blown away with argon. Finally the samples were transferred immediately to the UHV-system, minimizing the exposure to air. Measured photoelectron spectra represent a convolution of the photoemission process with contributions from the instrument such as the width of the X-ray line, the monochromator and the analyzer. These instrumental contributions were determined by measuring the Fermi edge of silver with exactly the same instrument settings subsequently used for the highresolution XPS measurements (spot size 1 mm, pass energy 20 eV). The resolution function for this spectrometer condition is the derivative of the silver Fermi edge measurement [13]. This resolution function is assumed to be a Gaussian curve with in our case a full width at half maximum of 0.50 eV.
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3. Results and discussion 3.1. Material Graphite forms two different phases (hexagonal and rhombohedral) each having stacks of two-dimensional layers. The hexagonal phase is characterized by ABABAB stacking where layers B are shifted with respect to layers A. The stacking in the rhombohedral phase is ABCABC. . ., where layers C are shifted with respect to B by the same distance as layers B are shifted with respect to A. The hexagonal form is thermodynamically more stable at normal temperature and pressure, and is that commonly found [14]. During intercalation the graphite layers tend to slide and get aligned, and change their characteristic stacking from ABABAB to AIAIAI, where I is the intercalated species [15]. HOPG has a very small dispersion of its c-axis, according to XRD experiments. It consists of large crystallites with La and Lc values of about 1±10 mm [16]. Its basal plane is rather rough and contains grain boundaries, edge planes, line and point defects [17] which are preferred sites for intercalation [18]. 3.2. Cyclic voltammetry Intercalation of perchlorate ions ClO4 ÿ into HOPG basal planes was performed in 2 M HClO4. Different stages of graphite intercalation process can easily be distinguished in a cyclic voltammogram such as that shown in Fig. 1. Over a rather narrow potential range of about 200 mV three different stages of intercalation of anions ClO4 ÿ and associated solvent or
Fig. 1. Cyclic voltammogram of HOPG in 2 M HClO4; scan rate: 25 mV/s. Stages IV, III and II are indicated in the ®gure.
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acid molecules into the graphite lattice are visible in the form of anodic peaks in the positive scan direction which reappear as cathodic peaks in the negative scan [19]. The intercalation process starts at potential above 1.0 V vs. SME, which is accompanied by an increase in current. The peak positions for stage IV, III and II in the anodic scan are 1.15, 1.22 and 1.27 V vs. SME. Unfortunately, the potential required for stage I is so high that side reactions such as the evolution of CO2 or O2 took place and result in irreversible damage of the HOPG surface. Occasionally blister formation was even observed after only 20 cycles to stage IV [5], which is regarded as a rather mild treatment. This blister formation could be explained by an intercalation of electrolyte and water followed by subsurface gas evolution [20]. The main reaction involved in the intercalation process is Cx Aÿ y solv: Cx Aÿ y solv: eÿ (1) This reaction is reversible and occurs with a current ef®ciency of nearly 100% at concentrations higher than 2 M [19]. At lower concentrations of the electrolyte or very positive potentials the ef®ciency of the reaction decreases, and other reactions such as graphite oxide formation, carbon dioxide formation or water electrolysis become possible. In an earlier study [5] we could show that the process is almost reversible up to those values of potential where only stages IV and III are formed. At higher potentials where stages II and I are formed, 20±30% of the charge cannot be recovered in the cathodic scan. The excess anodic current is probably due to irreversible oxidation of the graphite or gas evolution. These results are in agreement with earlier ®ndings of Beck et al. [3]. Aronson et al. [21] already had found a value of 2 for the ratio between the intercalated solvent species (HClO4) and the intercalated perchlorate ions in fully intercalated, annealed pyrolitic graphite Cx ClO4 ÿ yHClO4 . This implies values of x 24 and y 2 in reaction (1). In their calculations a Ê was used for the distance between two value of 7.95 A intercalated graphene layers. Thus, intercalation of perchlorate and of the accompanying solvent moleÊ between cules expands the nominal spacing of 3.35 A
Ê . The neutral acid molethe graphene layers by 4.6 A cules serve to screen the Coulomb repulsion between the ClO4 ÿ ions. These authors did not discuss the presence of water in the intercalation compounds. They only mentioned that these compounds are not stable in air or water! 3.3. Friction (SFM/LFM) SFM/LFM allows to measure normal and lateral forces at nanometer scale in various environments including electrolytes [22]. In the present study we looked for a possible correlation between changes in friction properties of HOPG and the intercalation process. The in situ SFM/LFM images recorded on a potential-controlled HOPG sample at a step of four graphene layers are shown in Fig. 2 for two potentials. At 0.0 V vs. SME no friction effect was seen at the step. This is evident when comparing the forward and backward scan of the LFM images. At 0.9 V vs. SME a strong friction signal is seen at the step, while almost no change is observed on the basal planes of the HOPG surface. At this potential intercalation should not occur, and it can in fact be seen from the topographic images that the height of the step remains unchanged (same 2-scale for both topography images). Therefore, the observed friction change cannot be the result of an intercalation process. However, a slight increase in the anodic current can be seen in cyclic voltammograms at potential above 0.85 V vs. SME (it is not visible on the scale of currents used in Fig. 1). At potentials below 0.85 V vs. SME the current is purely double-layer related, where no speci®c sites are taken up by the adsorbed ions. Higher currents could also be explained in terms of a pseudocapacity process such as the oxidation/reduction of functional surface groups (quinone/hydroquinone). The change of the friction of HOPG in different electrolytes was for the ®rst time investigated with LFM by Marti and coworkers [23,24], but the origin of the friction changes occurring at graphite steps or a possible correlation with the oxidation/reduction of functional surface groups and other effects had not been elucidated. We therefore extended the potential scan to higher potentials, in order to study possible effects of the intercalation process on the friction change. In Fig. 3 the measured friction is plotted as
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Fig. 2. Topography and friction images of a step of four graphene layers on HOPG in 2 M HClO4 measured by SFM/LFM at 0.0 and 0.9 V vs. SME.
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Fig. 3. Friction at a step on HOPG as a function of electrode potential.
a function of the applied electrode potential. Friction changes were in fact detectable at potentials where intercalation occurred and the topography images revealed an increased step height (not shown here). In agreement with the already mentioned studies of Marti and coworkers [23,24], we saw a reversible change of friction at steps of HOPG in perchloric acid but no evidence for a similar phenomenon on the basal planes. The reproducibility of the friction changes at a step is shown in Fig. 4. By using the
LFM in zxt-mode, the sample potential was cycled between 0.0 and 0.9 V vs. SME while two steps on a HOPG surface (visible in the center and to the right) were imaged in topography and friction. The changes of friction with potential are clearly visible for both steps. From Fig. 4 it can be concluded that the phenomenon is reversible between these two potentials. After intercalation the friction at steps was no longer reversible. Our results indicate that the friction at steps changes permanently as soon as intercalation starts. Whether this behavior is due to chemical reactions at the graphite or to adsorbates could not be decided by on the sole basis of LFM results. For higher steps the friction change become obscured. The reason for this behavior may be that the tip is not perfect and an interaction of the ¯ank of the triangular tip shape with the edge and/or the adsorbed species is not possible or reduced to an undetectable level. 3.4. XPS XPS studies yield quantitative information about the relative concentration of species present at the
Fig. 4. Friction vs. time image (left) and the corresponding potential transient (right); potential scan rate 100 mV/s; imaging speed 1 Hz.
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Fig. 5. (a) Atomic concentration ratio Cl/C on the HOPG surface vs. electrochemical potential (SME). (b) Atomic concentration ratio O/Cl on the HOPG surface vs. electrochemical potential (SME).
surface or in the near-surface region. In our investigations, the starting potential was the open-circuit potential of HOPG, which was close to 0.1 V vs. SME. The intensity of the Cl 2p signal increases with potential over the entire range of potentials investigated (0.1±1.4 V vs. SME). This is clearly seen in Fig. 5a, where the Cl/C ratio of the atomic concentrations is plotted. Within the double-layer region between 0.1 and 1.0 V vs. SME, the ratio increases by approximately a factor of 5. Kolb [25] found a similar increase for emersed gold electrodes in 0.01 M HClO4. The increase of the Cl/C ratio is more pronounced at potentials above 1.0 V where intercalation takes place (see Fig. 5a). A similar behavior is observed for the atomic concentration ratio of O/C (not shown here). In a plot of the ratio O/Cl, which is shown in Fig. 5b, we would expect a ratio of 4, if only perchlorate ions were involved in the adsorption and intercalation process. This is de®nitely not so anywhere within the range of potentials investigated. For potentials lower than 0.9 V vs. SME a ratio of about 8
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was determined. At potentials equal to or above 0.9 V vs. SME the ratio is smaller than 5. One oxygencontaining species that could be involved in this process is water. This would mean that about four water molecules per perchlorate ion are adsorbed in the electrochemical double layer at potentials below 0.9 V vs. SME. The number of water molecules involved in the intercalation process cannot be determined from this plot alone, because in Fig. 5b we do not distinguish between oxygen due to adsorbed species and oxygen due to intercalated species, i.e. we look at the total amount of oxygen. It is readily seen, however, that the number of water molecules involved is lower at potentials equal to or above 0.9 V vs. SME. It should be noticed in particular that between 0.9 and 1.1 V, where no intercalation takes place, the ratio is below 5. In the high-resolution XPS Cl 2p spectra (see Fig. 6a) of electrodes prepared at potentials lower than 1.1 V, only one spin-split peak can be seen at a rather high binding energy (207.6 eV). This high binding energy is typical for perchlorates [26]. The spin-splitting of the Cl 2p level is clearly visible. The peak position is stable in this potential range, and this Cl 2p peak can be attributed to the adsorbed perchlorate ions forming the solution side of the electrochemical double layer. The charging of this electrochemical double layer generates the observed increase of the Cl/C ratio (see Fig. 5a). At higher potentials where intercalation does occur, a second species gives rise to an additional spin-split peak seen in the spectra. We assign this peak to the intercalated perchlorate ions. It is seen from reaction (1) that the perchlorate undergoes no change in its valence state during the intercalation process. However, there is a distinct shift in binding energy. The Cl 2p level of the intercalated species is shifted with respect to the adsorbed perchlorate by 1.7 eV (205.9 eV). The peaks of the intercalated species overlap partly with the peaks of the adsorbed perchlorate. The Cl 2p3/2 of the adsorbed species has almost exactly the same peak position as the Cl 2p1/2 peak of the intercalated species. Intercalated neutral acid molecules (HClO4) postulated by Aronson et al. [21] should give no additional shift in binding energy relative to the intercalated perchlorate ions. The XPS spectra of the O 1s level recorded at different electrode potentials (see Fig. 6b) exhibit only
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Fig. 6. Comparison of (a) Cl 2p, (b) O 1s and (c) C 1s spectra for different electrochemical potentials.
one peak at 532.6 eV in the double-layer region (0.1± 1.0 V) but two peaks when intercalation has occurred. The binding energy shift between the two peak is the same (1.7 eV) as for the Cl 2p level. An O 1s peak between 532 and 533 eV is typical for the oxygen signal of perchlorates [26]. Adsorbed water would also generate an O 1s peak in this binding energy range (532±534.5 eV) [26±28]. The O 1s peak position of adsorbed water in the electrochemical double layer and the O 1s signal of the adsorbed perchlorate seem to overlap, because only one peak is detectable in the O 1s spectra recorded at samples where only the electrochemical double layer
is charged. However, one can conclude from the O/Cl ratio plotted in Fig. 5b, which is higher than 4, that water is present. The presence of other oxygen-containing species is much less obvious. The same argument can be used for water accompanying the intercalated perchlorate. There the O/Cl ratio is smaller, so less water would move into the graphite lattice. The presence of water on the surface after transfer of the sample to an UHV-system is not unanimously accepted. On noble metal surfaces KoÈtz et al. [29,30] found the amount of water to be rather low or even below the detection limits of XPS analysis, and reasoned that water desorbs from almost any
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noble metal surface under UHV conditions. The H2O bound to a counter-ion as part of the solvation shell may be stabilized, and survive the transfer to the UHV chamber [31]. In our case we look, not at a noble metal but at a rather hydrophobic material that can be removed almost dry from an electrolyte, as mentioned earlier. From Fig. 5b we could conclude that the H2Ocontaining solvation shell of the perchlorate ions in the electrochemical double layer is still present under UHV conditions and consists of four water molecules. At a potential of about 0.9 V vs. SME the atomic ratio O/Cl is reduced to about 5, indicating that less water is present than in the pure double-layer region. The binding energy of the Cl 2p spectra did not provide any indication that the trend of adsorption of perchlorate ions would change at the potential where the friction changes. However, the trend in the change of the atomic ratio O/Cl (in Fig. 5b) is similar to that of the friction change in Fig. 3. Both the friction and the O/Cl ratio change at potentials above 0.8 V vs. SME. As mentioned earlier, this could be a result of fewer water molecules being present in the solvation shell. Although it is not possible with XPS to discriminate between ions adsorbed on basal planes and ions adsorbed at steps, we believe that this partial loss of the water in the solvation shell of perchlorate ions (speci®c adsorption [25]) adsorbed at steps could be the reason for the friction change at the steps. Speci®c adsorption can also be inferred from the current increase seen in the cyclic voltammogram. Another explanation could be that the ions are mainly accumulated at the steps. However, for geometrical reasons, the LFM tip starts to be affected by their presence, only when enough ions are adsorbed at the step to perturb the interaction forces between the tip and the sample surface. According to Fig. 3, the friction change and likewise the change in the O/Cl ratio is rather sharp. We conclude, therefore, that a mere increase in the amount of perchlorate could not be the main reason for the sharp friction change observed, the more so since at the potential where the friction change occurs, no dramatic change in the Cl/C ratio, which could produce this friction change, is visible (see Fig. 5a). The C 1s peak of graphitic carbon appears at 284.3 eV and has a long tail toward higher binding energies, which has been attributed to interaction with the conduction band electrons [32]. Adsorbed
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perchlorate ions give no special feature in the C 1s spectrum (see Fig. 6c). When perchlorate ions are intercalated, the C 1s peak obtains a shoulder on the side of lower binding energies. At potentials above 1.4 eV vs. SME, the electrode will be destroyed by exfoliation. In addition, it becomes oxidized, which can be seen from an additional peak appearing in the C 1s spectra at about 286 eV. The corresponding O 1s peak overlaps with the oxygen signal generated by the other oxygen-containing species. For a more accurate determination of the peak positions, the maximum entropy method (MEM) [13] was applied to deconvolute these spectra. This method has already been applied to spectra of sul®de minerals [33], to Cr 2p spectra of thin chromium oxides and to Au 4f spectra of gold±aluminum alloys [34]. Rather small changes and peak shifts could be identi®ed by this method. If the contributions of the instrument to photoelectron emission spectra are known or can be measured, then the energy resolution function described by Splinter and McIntyre in [13] can be used to deconvolute the intrinsic spectrum from the measured one. By this method the energy resolution of the measured spectra can be enhanced by a factor of 2, which is nicely illustrated in Fig. 7 for the C 1s levels. The C 1s peak of the sample emersed at 0.5 V vs. SME has a full width half maximum (FWHM) of 0.60 eV. After the
Fig. 7. Maximum entropy deconvolution of the C 1s spectra (same as displayed in Fig. 6c) of HOPG samples emersed at different potentials.
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deconvolution procedure the FWHM of the C 1s peak is only 0.28 eV. The quality of the original spectra must be rather high for an application of the maximum entropy method. The quality of our O 1s and Cl 2p spectra was not suf®cient on account of the limited measuring time and of the rather small amount of species present. The algorithm did not converge, only the noise in the spectra could be reduced. In the C 1s spectra recorded at potentials where only adsorption occurs, an additional feature at a binding energy of 285 eV is visible (Fig. 7). Its origin is not completely clear. A possible explanation could be a C±O or C±OH bond of adsorbed species on the HOPG surface. In spectra recorded at samples at which intercalation takes place, this feature is not or not well detectable. After application of MEM, it is much easier to determine the XPS peak position for the intercalated graphene layers. The peak shift to lower binding energies is of the order of 0.7 eV for an intercalated sample held for 60 s at 1.3 V. The shift is even slightly larger for the samples polarized to lower potentials, but it is always smaller than that found for the Cl 2p and O 1s levels. The binding energy shifts of the Cl 2p and O 1s levels seem to be independent of the applied intercalation potential. In the spectra recorded at intercalated samples, all three peaks (C 1s, O 1s, Cl 2p) corresponding to intercalated species are shifted to smaller binding energies. Due to the intercalation process, the carbon atoms involved should undergo a change of their valence state C ! C and form the graphite
intercalation compound, as described in reaction (1). For conservation of charge neutrality, the negatively charged perchlorate ions, which undergo no change in valence state during intercalation, should induce positive charges on the carbon atoms. Consequently, the C 1s peak of carbon atoms involved in the intercalation process should shift in the positive direction, if a charge transfer took place. Just the opposite is observed. On the other hand, the Cl 2p and the O 1s peaks of the intercalated species should not shift at all (according to reaction (1)), but they are shifted to lower binding energies too. The observed shifts can be explained in terms of a shift in Fermi level of the intercalated layers. The resulting energy scheme relative to the vacuum level is illustrated in Fig. 8 for the C 1s and Cl 2p levels. The shifts of the O 1s level are identical with those of the Cl 2p level. A displacement of the Fermi level has been described by Wertheim et al. [35] for LiC6. They explain this behavior in terms of a screening of the core hole by the conduction electrons in LiC6. According to the scheme in Fig. 8, the C 1s binding energy of the intercalated graphene layers is increased, as expected for positively charged species Cx . This positive charge induces a binding energy shift of 1.0 eV in an electrode emersed at 1.3 V vs. SME. The binding energy of Cl 2p and also that of O 1s (not shown in the scheme) of the perchlorate remains constant, because the perchlorate ions undergoes no change in valence state. A shift of the Fermi level means that the work function of these intercalated
Fig. 8. Energy schemes relative to the vacuum level of the binding energies of HOPG with adsorbed and intercalated species of ClO4 ÿ corresponding to a potential of 0.4 and 1.3 V vs. SME, respectively.
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graphene layers is increased by 1.7 eV regardless of the intercalation potential of the electrode. Considering the Fermi energy shift, the observed energy shift for C6 ÿ in LiC6 with respect to the neutral valence state of graphite is ÿ0.8 eV [35,36]. For Cx we ®nd a shift of about 1.0 eV at an intercalation potential of 1.3 V vs. SME. In contrast to donor-intercalation compounds, the charged species appear to be fully ionized in any stage of most of the acceptor-intercalated compounds [37]. The overall charge transfer is not 100%, because the charged molecules are separated from each other by other neutral species (in our case HClO4 and probably some water). Under the assumption of completely intercalated graphite C24 ClO4 ÿ 2HClO4 [21], this would give a value of f 0:33 for the fractional charge transferred to or from the graphite per intercalated atom or molecule. Different binding energy shifts can be observed between the two C 1s peaks for different amounts of perchlorate ions intercalated into the graphite. The C 1s peak of the intercalated graphene layers moves closer to the graphitic carbon peak when more perchlorate ions are intercalated at higher potentials (see Fig. 7). Therefore, the binding energy of the intercalated graphene layers is higher at higher intercalation potentials. The work function and the Fermi level do not change or move when different intercalation potentials are applied. In contrast to LiC6 where only one C 1s peak is visible, for HOPG intercalated with perchlorate always two peaks are visible in the spectrum, even at potentials where intercalation should be almost complete (>1.3 V vs. SME). There could be two reasons for this difference. The step density and the step height are rather low. This means that with XPS, it is possible to see through the intercalated graphene layers or to see regions not reached by intercalation. But even if we could intercalate each graphene layer completely with perchlorate ions (see product of reaction (1)), it would still be possible to have carbon atoms with an electronic structure not in¯uenced by the presence of perchlorate, on account of screening of the charge. This screening effect could be generated by the presence of neutral acid or water molecules. When tilting the sample we did not see an increase in intensity of the perchlorate ion signal relative to the carbon signal or a change in relative intensities of the two carbon signals. We can exclude on the basis of this
231
result that only the top layers are intercalated, since a variation in the intensity ratio between intercalated and non-intercalated species should become visible when changing the probing depth. For higher steps the intercalation is probably less effective. In stage I not all graphene layers are ®lled with perchlorate ions [6]. In the intercalation process of ions considerable energy is required to overcome the Van der Waals forces and increase the distance between the graphene layers. The required force increases with the number of graphene layers present on top of the intercalated gap. A second explanation is related to the fact that the probability of ®nding defects and vacancies increases with the number of graphene layers. These defects can prevent the perchlorate ions from intercalating, since the adjacent layers are pinned together and dif®cult to move with respect to each other. As a consequence, fewer ions can become intercalated. From this consideration it can be concluded that stage I is an average over the whole electrode surface, and is never fully reached. A series of samples were removed from the electrolyte after 60 s at a potential of 0.1 V vs. SME, prior to which they had been exposed to potentials in the range between 0.9 and 1.4 V vs. SME, also for 60 s. All these samples had more perchlorate on the surface than a sample never exposed to such a high potential. In the Cl 2p spectra of samples that had been exposed to 0.9 and 1.0 V vs. SME, only adsorbed perchlorate ions which stick to the surface or to steps were seen. In samples that had reached potentials where intercalation occurs, some intercalated species are still present in addition to the adsorbed ones. This can be concluded from the position of the Cl 2p peaks. Even in the C 1s spectra, one can detect the C 1s signal that corresponds to the intercalated graphene layers. It follows that the irreversibility of the friction changes is due to the intercalation or the incomplete deintercalation of the perchlorate ions. 4. Conclusions The intercalation of perchlorate ions in HOPG causes a friction change at steps at potentials more positive than 0.85 V vs. SME. No change of the friction can be observed on the basal plane. In a narrow potential window between 0.85 and 1.1 V
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vs. SME before the start of intercalation, the friction change is reversible. It is due to the enhanced adsorption of perchlorate ions at steps accompanied by a loss of water from the solvation shells. After intercalation and deintercalation, the change of the friction at a step of HOPG is irreversible. This irreversibility is due to the fact that it is not possible to completely deintercalate the graphite lattice. The XPS binding energies of the intercalated species in the C 1s, O 1s and Cl 2p spectra shift with respect to those of the adsorbed species or the host material. A shift in Fermi level of the intercalated ions and part of the intercalated graphene layers is able to explain the shift of these peaks, where a charge transfer occurs because they are not screened by the presence of other molecules. With the MEM it was possible to increase the resolution of the C 1s peaks by a factor of 2 and determine the peak position of the new peak more accurately. Acknowledgements Financial support of the Swiss National Science Foundation Grant No. 4036-044040 is gratefully acknowledged. Special thanks are due to S.J. Splinter and N.S. McIntyre for making available to us the Matlab program for the maximum entropy deconvolution. References [1] L.B. Ebert, Ann. Rev. Mater. Sci. 6 (1976) 181. [2] R. Wiesendanger, H.-J. Guentherodt (Eds.), Scanning Tunneling Microscopy, Vol. I/II, 1st Edition, Springer, Berlin, 1992/ 1993. [3] F. Beck, H. Junge, H. Krohn, Electrochim. Acta 26 (1981) 799. [4] J. Zhang, E. Wang, J. Electroanal. Chem. 399 (1995) 83. [5] D. Alliata, P. HaÈring, O. Haas, R. KoÈtz, H. Siegenthaler, Electrochem. Commun. 1 (1999) 5. [6] D. Alliata, R. KoÈtz, O. Haas, H. Siegenthaler, Langmuir 15 (1999) 8483. [7] J.O. Besenhard, M. Winter, J. Yang, W. Biberacher, J. Power Sources 54 (1995) 228. [8] K.S. Kim, N. Winograd, R.E. Davis, J. Am. Chem. Soc. 93 (1971) 6269. [9] D.M. Kolb, D.L. Rath, R. Wille, W.N. Hansen, Ber. Bunsenges. Phys. Chem. 87 (1983) 1108. [10] C.M.A. Brett, A.M.O. Brett, Electrochemistry Principles, Methods and Applications, Oxford University Press, Oxford, 1993, p. 138. [11] R. Staub, D. Alliata, C. Nicolini, Rev. Sci. Instrum. 66 (1995) 2513.
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