J. Phys. Chem. B 1998, 102, 10229-10233
10229
Electrochemical Scanning Tunneling Microscopy Investigation of HOPG and Silver Electrodeposition on HOPG from the Acid Room-Temperature Molten Salt Aluminum Chloride-1-Methyl-3-butyl-imidazolium Chloride Frank Endres and Werner Freyland* Institute for Physical Chemistry and Electrochemistry, UniVersity of Karlsruhe, D-76128 Karlsruhe, Germany ReceiVed: May 28, 1998; In Final Form: September 28, 1998
The electrodeposition of silver on highly oriented pyrolytic graphite (HOPG) from the acid room-temperature molten salt aluminum chloride-1-methyl-3-butyl-imidazolium chloride (molar ratio of 55/45 containing AgCl in a concentration of 0.05 mol/L) has been investigated with electrochemical scanning tunneling microscopy, cyclic voltammetry, and potential-step experiments. HOPG can be viewed on an atomic scale; at electrode potentials above +1000 mV vs the Ag/Ag+ reference it is oxidized, leading to both hole etching and protrusions. Silver electrodeposition requires an overvoltage of -300 to -350 mV. With rising overpotential, the mechanism changes from three-dimensional progressive nucleation at a finite number of active sites to three-dimensional instantaneous nucleation. In the far overpotential range the deposition mainly takes place at steps and defects between different basal planes of graphite and to a lower extent on the basal planes themselves.
Introduction Highly oriented pyrolytic graphite (HOPG) is widely used as a substrate to study the electrodeposition of metals with electrochemical scanning tunneling microscopy (EC-STM) or atomic force microscopy (AFM). In contrast to classical electrochemical methods such as cyclic voltammetry or potentialstep experiments, which give only an overall insight into the deposition process, EC-STM allows one to directly investigate on an atomic scale the processes during electrodeposition. As has been shown by several authors, the Ag electrodeposition on HOPG from aqueous solutions occurs mainly at steps between different basal planes of graphite because the graphite basal plane surface is electrochemically very inert.1-2 The formed nuclei grow via three-dimensional progressive nucleation, and the critical cluster consists of four to five Ag atoms. It has been found that nanoscopic silver particles, which nucleate and grow on the atomically smooth graphite basal plane surface, cannot be imaged by STM or by AFM because of the weak interaction with the graphite surface.3 It is known as an experimental limitation that deposition under a tunneling tip can be hindered because of reduced mass transport toward the substrate under limiting current conditions.4,5 Electrodeposition from aqueous solutions is restricted to potential ranges where water is not reduced; thus, light metals that are interesting in battery research or refractory metals, for example, cannot be deposited. Molten salts are ideal solvents for such investigations, since they have wide electrochemical windows to allow the study of the electrodeposition of most metals.6 Systems composed of aluminum chloride and different organic salts are liquid at ambient temperatures and show, depending on the stoichiometry, electrochemical windows of up to 4.4 V along with a good solubility for several metal halides and oxides.6 Such systems are also interesting for the construction of dual intercalating molten electrolyte secondary batteries.7 In an acid aluminum chloride-1-methyl-3-ethylimidazoliumchloride melt (excess of AlCl3), the Ag electrodeposition on
glassy carbon requires a high overpotential of about 300 mV. The growth of silver has been described to follow threedimensional progressive nucleation.8 In a rapid communication we showed recently that the combination of EC-STM and molten salts is possible. As an example, first results of the electrodeposition of Ag on HOPG and of HOPG degradation have been given.9 The aim of the present paper is to give a more detailed description of Ag electrodeposited on HOPG and of the irreversible oxidation of HOPG. Experimental Section 1-Methyl-3-butylimidazolium chloride (MBIC) was prepared by slowly adding freshly distilled 1-methylimidazole to 1-chlorobutane (both from Merck, for synthesis). This mixture was stirred under argon at 50° C for 14 days. The crystallization of the resulting oily product was initiated with a crystal of the same substance. After filtering off in an argon-filled glovebox and drying under vacuum, the product was recrystallized from dry acetonitrile to remove residual 1-methylimidazole. The resulting white crystals were dried under vacuum at a temperature of 55° C for 14 days. Anhydrous AlCl3 (Fluka p.a., >99%) was sublimed under vacuum one time, leading to both a fine powder and white crystals of several millimeters in diameter. The crystals were used for the preparation of the melt. In an argon-filled glovebox (oxygen and water below 1 ppm) the AlCl3 crystals were slowly added to MBIC in a molar ratio of 55/45. AgCl (p.a.) was purchased from Fluka and dissolved up to a concentration of 0.05 mol/L. The STM tips were freshly prepared from tungsten wires (250 µm diameter, >99.98%, Alfa) via an electrochemical etching process in NaOH (2 mol/L) aqueous solution according to a procedure described in ref 10. The sharpened tips were coated via electrophoresis with an electropaint (BASF ZQ 84-3225 0201) and heated to 200 °C for 10 min to harden the paint. This coating is sufficiently resistant against the molten salt without showing any detectable
10.1021/jp9824048 CCC: $15.00 © 1998 American Chemical Society Published on Web 11/20/1998
10230 J. Phys. Chem. B, Vol. 102, No. 50, 1998
Figure 1. A 2 µm × 2 (constant current mode) are observed. Parameters +900 mV vs RE; E(tip), rate, 2 Hz.
µm scan of HOPG under the molten salt in which steps between different planes during scan are the following: E(HOPG), +100 mV vs RE; set point, 300 pA; scan
degradation. The STM experiments were performed in ECSTM mode with a Molecular Imaging Pico SPM and a 2 µm STM scanner driven by a Digital Instruments Nanoscope E controller. A Teflon cell was clamped via a Viton sealing ring onto HOPG (ZYH grade, purchased from Advanced Ceramics), whose surface has been freshly cleaved with an adhesive stripe (effective surface: 5.03 × 10-5 m2). An Ag ring electrode and an Ag wire (both 1 mm diameter) in direct contact with the melt served as counter and reference electrodes, respectively. The distance between the tip and the HOPG was adjusted by the help of a stereomicroscope, and the STM was transferred into an argon-filled glovebox, where the cell was filled with the molten salt. The head was mounted in a vacuum-tight chamber on the bottom of which a beaker with dry P2O5 was placed; it was removed from the glovebox and inserted into a vibration-damped, argon-flushed stainless steel vessel. It was ensured that the microscope was placed in a clean and dry atmosphere to prevent reaction of water with the melt, liberating highly corrosive HCl. After 15 min the argon flow was stopped to reduce vibrations. This setup allows measurements over several days without any loss in quality of the melt or of the tip. All experiments were performed at room temperature. The electrochemical data were extracted from the nanoscope files with VOLT, written by S. Vinzelberg, DI-GmbH Germany. Results and Discussion Figure 1 shows a typical surface of HOPG (2 µm × 2 µm) under the molten salt at E ) +900 mV vs RE. Several steps between one and five monolayers of graphite can be observed from the upper left to the lower right. When the scan window is reduced to 5 nm × 5 nm on any of the basal planes, the hexagonal structure of HOPG can be clearly resolved, which is shown in Figure 2. This demonstrates that molten salts are well suited for studying the behavior of substrates during electrochemical reactions on an atomic scale. When the potential of the HOPG electrode is cycled between +600 and -1000 mV vs RE at a scan rate of 10 mV/s (Figure 3), silver electrodeposition in the first scan starts at a minimum overpotential of approximately -350 mV (determined from the intercept of the rising part of the peak and the x-axis), which has also been found in ref 9, and leads to a peak at -620 mV. At potentials of +220 mV vs RE in the anodic scan a typical
Endres and Freyland
Figure 2. Unfiltered 5 nm × 5 nm scan of HOPG under the molten salt (constant height mode) in which the atomic structure of the graphite basal plane is resolved. Parameters during scan are the following: E(HOPG), +150 mV vs RE; E(tip), +100 mV vs RE; set point, 2 nA; scan rate, 40 Hz.
Figure 3. CV of HOPG between +600 and -1000 mV vs RE with a scan rate of 10 mV/s. In the first scan silver electrodeposition requires an overpotential of 350 mV. In the second scan only 40 mV are required because of residual nuclei that are not completely oxidized in the oxidative scan.
stripping peak due to silver oxidation appears. In the second reductive scan two reduction peaks are observed; in addition to the peak of the first scan, but with a lower current, an additional reduction peak at -120 mV, probably due to kinetic limitations, with a much lower minimum nucleation overpotential of only -40 mV appears. This shows that the oxidation of silver after the first scan is not complete. A similar result has been found by Lorenz et al. in Ag electrodeposition studies from aqueous solutions.1 The oxidation peak is comparable to that of the first scan. If the potential is held at +600 mV for several minutes after the first scan, the electrodeposition of Ag requires again an overvoltage of about -350 mV. When the anodic limit in CV is set to a value of +1000 mV vs RE, an oxidation current flows that rises from cycle to cycle. An oxidation current is also observed when the potential of HOPG is set potentiostatically to values of more than +1000 mV vs RE. The surface of the HOPG electrode changes significantly as shown in Figure 4; several mainly monatomic steps and defects are etched, leading to a certain activation of HOPG. Depths and extensions depend in general on the potential and time applied. A direct comparison with Figure 1 also shows that protrusions can be found (arrow in Figure 4; this protrusion has a height of 5 nm). Additional experiments have shown that the protrusions are in the vicinity of the freshly etched holes and steps and that they
HOPG
Figure 4. A 2 µm × 2 µm scan of HOPG under the molten salt (constant current mode) after oxidation at potentials more positive than +1000 mV vs RE. Steps and defects between different planes are etched. Parameters during the scan are the following: E(HOPG), +600 mV vs RE; E(tip), +100 mV vs RE; set point, 300 pA; scan rate, 2 Hz.
can have heights of up to 10 nm. In all experiments the atomic structure of graphite could be clearly resolved on the protrusions; pictures are essentially of the same quality as in Figure 2. Thus, they cannot be explained with deposition products due to HOPG oxidation. If the experiment is performed on glassy carbon as substrate under identical conditions, the holes and the protrusions are not observed. The dynamics of the etching and protrusion formation could not be resolved on the time scale of the experiment. It was observed in some experiments that the protrusions disappeared from one scan to another when the potential was set to values where graphite is electrochemically inactive. Comparison of Figures 4 and 7 gives an example: the protrusion indicated in Figure 4 disappears when the electrode potential is set to a value where Ag electrodeposition occurs. Carlin et al.7 have recently reported that both the organic ion of the buffered neutral ambient-temperature molten salt AlCl31-ethyl-3-methylimidazolium chloride and AlCl4- intercalate into the graphite lattice at the cathodic and the anodic limits of the melt, respectively. They discussed the possible use of conventional graphite rods as electrode material in a dual intercalating molten electrolyte secondary battery. As a result, they found that the reversibility of the intercalation-deintercalation of AlCl4- is about 80%, this rate also being dependent on the substrate used. If intercalation of AlCl4- into the HOPG lattice is the reason for the protrusion, the swelling would correspond to an intercalation into about eight layers (taking into account the normal plane distance of 335 pm for HOPG and that of 952 pm for AlCl3 intercalated into graphite13). This is consistent with the observations in Figures 4 and 7. Therefore, intercalation of AlCl4- seems to be a possible explanation for the protrusions on HOPG. Furthermore, the degradation process observed here is probably one reason for the incomplete reversibility of the intercalation-deintercalation reaction studied in ref 7. Potential-step experiments are well suited for the investigation of the nucleation process of metals. For this purpose the potential of the working electrode is set from a value where no deposition occurs to a value where nucleation takes place. If the nucleation is three-dimensional followed by hemispherical diffusion, two limiting cases are instantaneous nucleation on a fixed number of active sites and progressive nucleation on an
J. Phys. Chem. B, Vol. 102, No. 50, 1998 10231
Figure 5. Current-time curves for the electrodeposition of Ag on HOPG at different overpotentials. The steps were performed from +500 mV vs RE to electrode potentials between -300 and -800 mV vs RE.
Figure 6. Dimensionless current-time plots (dashed graph is measured data; line is the fit). At η ) -350 mV the measured data are explained on the basis of progressive three-dimensional nucleation at a finite number of active sites (eq 3, tmax ) 23 ( 2 s). At η ) -800 mV the equation for instantaneous three-dimensional nucleation (eq 1) fits the measured data (tmax: 0.40 ( 0.01 s).
infinite number of active sites. For both cases, dimensionless equations exist that allow us to deduce the mechanism from the normalized current-time curves:11,12
( )
(
(
))
tmax I 2 -1.2564t ) 1.9542 1 - exp Imax t tmax
2
instantaneous (1)
( ) I
(
(
( ) ))
tmax t ) 1.2254 1 - exp -2.3367 t tmax
2
Imax
2
2
progressive (2) where I is the current density, Im is the maximum current, t is the time, and tm is the time at Im. Cases between these two limiting cases are considered to involve progressive nucleation on a finite number of active sites.14 In this case the theoretical transients are represented by
( ) I
Imax
2
)
tmax{1 - exp[-xt/tmax + a(1 - exp(-xt/tmax))]}2 t {1 - exp[-x + a(1 - exp(-x/a))]}2 (3)
a and x are adjustable parameters containing information about
10232 J. Phys. Chem. B, Vol. 102, No. 50, 1998
Endres and Freyland
TABLE 1: Nucleation Data of Ag on HOPG between η ) -350 and η ) -500 mV for Progressive Nucleation at a Finite Number of Active Sites According to Eqs 3-5 η/mV
tmax/s
x
a
A/s-1
N0/cm-2
-350 -400 -450 -500
23 ( 2 11.4 ( 0.6 6.60 ( 0.06 4.10 ( 0.04
1.80 ( 0.04 2.2 ( 0.1 2.2 ( 0.2 2.2 ( 0.3
0.30 ( 0.01 0.50 ( 0.03 0.50 ( 0.05 0.40 ( 0.06
0.25 ( 0.04 0.40 ( 0.06 0.7 ( 0.2 1.3 ( 0.4
1.5 ( 0.4 × 105 3.9 ( 0.6 × 105 6.5 ( 0.1 × 105 1.0 ( 0.3 × 106
the number density of the nucleation sites (N0) and the rate constants A per active site:
N0 )
x πDtmax(8πcM/F)0.5
(4)
x atmax
(5)
A)
where c is the bulk concentration of the ion to be deposited, D is the diffusion coefficient, M is the molar mass, and F is the density of the metal deposit. If there is a certain delay time for the nucleation, the time axis has to be corrected to t0, a value of which can be estimated graphically as described in ref 15. In Figure 5 the transients are shown when the electrode potential of HOPG was stepped for 40 s from +500 mV vs RE to different values negative of the equilibrium potential. At an overpotential of -300 mV no maximum is observed, whereas for higher overpotentials distinct maxima are found. If the diffusion coefficient of Ag+ is calculated from the decreasing part of the transient at η ) -800 mV, a value of (1.50 ( 0.05) × 10-10 m2/s is obtained from the Cottrell plot. If an average transient is calculated by lumping together all decreasing parts of the transients, the Cottrell plot gives a value of (1.4 ( 0.2) × 10-10 m2/s. Both values are in good agreement to the value of (1.2 ( 0.1) × 10-10 m2/s in ref 8 at 40° C. If the dimensionless eqs 1 and 2 are fitted to the measured data, neither of the two equations gives a satisfactory fit for overpotentials between -350 and -700 mV, the measured data lying between both limiting cases with a tendency toward eq 1. It is only for an overpotential of -800 mV that an almost ideal overlap between the measured and theoretical curves according to eq 1 is obtained, showing that the nucleation is completely instantaneous for this overpotential (Figure 6). At an overpotential of -350 mV, eq 3 can be fitted very well to the measured data. This shows that the nucleation is progressive but on a limited number of active sites at low overpotentials (Figure 6). Equation 3 fits the experimental data up to overpotentials of -500 mV with rising deviations. Between -600 and -700 mV, none of the equations fit, whereas at -800 mV, eq 1 fits as described above. The origin of these deviations is not yet clear. If alloying between Ag and Al occurs as has been described in ref 16, this may influence the nucleation mechanism and its dependence on the overpotential. We have calculated the rate constants and the number density of active sites from a and x, and the values are summarized in Table 1. Since the dimensionless curve at η ) -800 mV agrees well with the theoretical model, we have fit the equation for instantaneous nucleation
I ) zFD0.5c(πt)-0.5[1 - exp(-N0πkDt)]
(6)
(where I is the current density, z is the number of electrons, F is the Faraday constant, D is the diffusion coefficient of metal ion, c is the bulk concentration of the metal ion, t is time, N0 is the number density of nuclei, M is the atomic weight of the metal, F is the density of deposited metal, and k is (8πcM/F)0.5) to the measured data to obtain values for the Ag+ diffusion
Figure 7. A 2 µm × 2 µm scan of HOPG under the molten salt (constant current mode) after a 5 min potentiostatic step from +600 mV vs RE to -800 mV vs RE, the tip approached. Silver nuclei grow mainly at defects and steps between different basal planes of HOPG. Parameters during the scan are the following: E(HOPG), -120 mV vs RE; E(tip), +100 mV vs RE; set point, 300 pA; scan rate, 2 Hz.
coefficient and the number density of active sites. We obtain a diffusion coefficient of (1.50 ( 0.05) × 10-10 m2/s and 5.7 × 106 active sites cm-2. It is tempting to compare the value for the number density of active sites obtained from eq 6 at η ) -800 mV to the number of clusters in the STM images. In a window of 4 µm2 it would correspond to 0.2 active sites. A typical example is shown in Figure 7 where the surface of HOPG is presented after a 5 min potential step to -800 mV vs RE when the tip was not retracted from the sample, which corresponds to a situation where the clusters are growing. The scan rate in this example was reduced to the lowest scan rate possible (0.1 Hz) during potential stepping so that the deposition process is not disturbed. If one assumes that the clusters seen in Figure 7 (imaged after the potential step at a scan rate of 2 Hz) are growing from active sites, this would yield an estimated number of active sites from the STM observation that is roughly an order of magnitude higher than the above value determined from I(t) measurements. The silver clusters grow mainly at steps and defects between different basal planes of graphite and have typical heights and widths between 50 and 200 nm. This shows that the growth is mainly three-dimensional as predicted from the potential-step experiments. To a lesser extent and with reduced height, clusters are also formed on the graphite basal plane. This is not surprising because in the far overpotential range, metal deposition also occurs on the basal planes.3 But the tip has a certain influence on the electrodeposition: when the potential-step experiment is repeated under identical conditions but with retracted tip, it is difficult to bring the sample to the tip because the axial elongation of the scanner (850 nm maximum) is below the maximum height differences of the deposit. As a result, topographic data cannot be acquired. Generally, depending on the distance between the tip and the sample and obviously on
HOPG the geometry of the isolated tip, a more or less severe interaction between the deposited silver and the tip may occur. Conclusion The results presented here show that the structure of HOPG in a room-temperature molten salt can be resolved both on the micrometer and the nanometer scales with the EC-STM. At potentials above +1000 mV vs RE a degradation of HOPG starts, which leads to holes in the surface along with the formation of protrusions that may be caused by intercalation of AlCl4- at the anodic limit of the melt. At potentials negative of the RE, crystal growth requires an overpotential of about 300 mV, changing from three-dimensional progressive at a finite number of nucleation sites to three-dimensional instantaneous nucleation when the overpotential rises. The growth of silver clusters mainly takes place at steps and defects between different basal planes of graphite. Acknowledgment. Financial support of this work by DFG and by the Fonds der Chemischen Industrie is gratefully acknowledged. References and Notes (1) Po¨tzschke, R. T.; Gervasi, C. A.; Vinzelberg, S.; Staikov, G.; Lorenz, W. J. Electrochim. Acta 1995, 40, 1469.
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