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Effects of sonication on electrode surfaces and metal particles
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Effects of sonication on electrode surfaces and metal particles Nanette A. Madigan, Carolynne R.S. Hagan, Honghua Zhang, Louis A. Coury, Jr. * Department of Chemistry, Box 90346, Duke University, Durham, NC27708-0346, USA Received20 December 1995;revised26 April 1996
Abstract Ultrasonic irradiation at a frequency of 20 kHz has varying effects on electrode surfaces. Non-metals such as glassy carbon and Ebonex~ are severely pitted after only a few minutes of sonication in aqueous media. By contrast, metals such as Pt, Au, W and Pd remain largely undamaged after 120 s, as observed by scanning electron microscopy. The extent of damage does not appear to be related to the melting point of the material. By contrast, when electrodes are sonicated in suspensions of metal powders, particles are deposited onto electrode surfaces. The deposits were subsequently observed by scanning electron microscopy and by voltammetry. It is concluded that the ability to deposit particles on an electrode depends on both the melting point of the particles and the electrode, whereas surface damage is more closely related to the hardness of the material.
Keywords." Ultrasound; Electrode surfaces; Metal particles
1. Introduction The combination of electrochemistry with sonochemistry is an active area of research [1-14] and motivates investigation of the effects of ultrasound on different electrode materials [8,11,13]. A goal of the present study is to examine the effects of ultrasound on several types of electrode materials under the conditions commonly used for electrochemical studies. Since the current monitored in a voltammetric experiment increases with increasing electrode area, it is important to ascertain the extent of surface damage sustained by various electrodes during sonication. Results obtained will be considered in light of the differing melting points and hardnesses of the materials studied. Ultrasound has recently been employed to deposit metal particles onto gold electrodes [ 15]. Particles were subsequently 'stripped' from the surface by electrooxidation for the purposes of quantitation. It was found that copper powder (melting point, m.p., 1083°C) could easily be deposited onto a gold electrode, whereas tungsten particles (m.p. 3410°C) could not. To further understand this phenomenon, the ability to melt particles onto different electrode materials will also be explored in the present study. Consideration of the magnitude of the effects observed in terms of properties of the * Corresponding author. Email: [email protected];fax: + 1-919660-1605.
electrode (e.g., melting point, hardness) will again be of interest. Previous work on the general effects of cavitation on extended surfaces has been published by Lauterborn and co-workers [16,17]. These studies involved the use of a pulsed laser to generate cavitation near solid surfaces [ 16]. It was shown that bubbles in contact with a surface generate the highest pressures and thus are more likely to cause cavitational erosion [17]. Based on experimental evidence with aluminum surfaces, however, it was concluded that interfacial fluid jets may play a larger role in surface damage than single bubbles, and may involve acoustic transients generated by other nearby bubbles [17]. Some work has been published on the effects of ultrasonic irradiation on iron electrodes in aqueous systems [8]; glassy carbon in both water and dioxane [11]; and on the irradiation of Pt and A1 electrodes in acetonitrile [13]. The purpose of the present study is to explore the effects of aqueous sonication on common electrode materials, and to evaluate the suitability of each for sonochemical metal deposition experiments.
2. Experimental Details of the sonochemical procedures have been published previously, but will be summarized briefly
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N.A. Madigan et al./Ultrasonics Sonochemistry 3 (1996) $239-$247
here [11,15,18]. Sonications were performed in a waterjacketed, all-glass cell which allowed for placement of electrodes parallel to the tip of a direct immersion probe sonicator. The sonicator used was a Heat Systems XL2010, 475 W, 20kHz ultrasonic processor. This instrument was fitted with a tapped Ti tip, having a geometric area of 1 cm 2. For surface damage studies, a separation distance between the Ti tip and each electrode of 1.0+0.5 mm was used. The sonicator power setting was 100%, which corresponds to a vibrational amplitude of 125 jam (peak-to-peak). This amplitude was previously found to be optimal for maximizing deposition yields [15]. The sonication time was 120 s in each case. For metal deposition experiments, a power setting of 80% (96 ~'n acoustic amplitude) at a separation distance of 4_+ 0.5 mm was employed. For this work, the sonication time was 300 s. A PAR model 253 potentiostat, interfaced to a 66 MHz 80486 DX laboratory computer, was used for all electrochemical measurements. Scanning electron micrographs (SEMs) were acquired with either a Philips 501 instrument operated at 15 kV accelerating voltage or a JEOL 6400 field emission SEM (2.0 kV). Gold, platinum and glassy carbon electrodes were obtained from Bioanalytical Systems. SEM images were obtained for GC-30 grade, Tokai glassy carbon (Electrosynthesis). Tungsten, palladium (both from Aldrich) and Ebonex ~ (Electrosynthesis) rods were fabricated into disk electrodes and sealed in epoxy (EpoTek). Silver powder (Aldrich) was 99.9+% pure and averaged 2.4 ~tm in diameter. Copper powder (Aldrich) was 99% pure 'submicron' grade, and was previously determined to have a mean diameter of 300 nm [15]. Unless stated otherwise, all sonications were performed in water which was purified by reverse osmosis and then ultrafiltration (Barnstead NanoPure), and all metal slurries were 8.3 mg/ml [15].
[11]) is shown in Fig. 1A. Other than the horizontal polishing scratch visible in the image, the surface appears smooth and featureless, as has been repeatedly shown by others [22,23]. After sonication in water, however, the electrode surface becomes severely pitted as seen in Fig. lB. (Note the difference in scale between Figs. 1A and B.) When such a sonicated electrode is placed in a 20.0 mM Fe(CN)3-/1.0 M KC1 solution and the potential stepped from +0.5 to 0 V vs. Ag/AgC1, the current monitored is noticeably higher than that observed prior to sonication. To understand this effect, the apparent area can be calculated for each point along the current transient using the Cottrell equation [24]: A-
nFC*
(1)
where t is the time in seconds, C* is the ferricyanide concentration (2.0 × 10 -5 mol/cm3), D is the diffusion coefficient for ferricyanide in 1.0 M KC1 (7.63 x 10 6 cmZ/s) [25], and the other parameters have their usual significances [24]. As shown in Fig. 2A, the calculated electrode areas for both sonicated (O) and unsonicated (+) electrodes decrease with time. At short times, ferricyanide ions are
A
3. Results and discussion
3.1. Sonication of glassy carbon electrodes Glassy carbon (GC) is an isotropic, impermeable, vitreous form of carbon which has been widely employed as an electrode material [19-21]. The most commonly cited advantage of GC is its wide potential window in aqueous systems. Kinetic results obtained at GC electrodes, however, are only interpretable when a known and reproducible surface preparation (activation protocol) has been employed [20]. We have previously discussed the effects of 20 kHz ultrasound on glassy carbon, both in aqueous and non-aqueous solvents [ 11 ]. A typical scanning electron micrograph of a GC surface which was polished with decreasingly small alumina particles (from 5 down to 0.05 lam diameter
B
Fig. 1. SEMs of glassy carbon electrodes. (A) Polished surface; scale 2 Ixm. (B) After sonication in water at a distance of 1.0 m m from a Ti tip; scale 40 ~tm. Vibrational amplitude 125 ~tm; sonication time 120 s.
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time / s Fig. 2. Apparent electrode areas as a function of time for polished electrodes (+) and for electrodes after aqueous sonication for 120 s (©). (A) Glassy carbon. (B) Ebonex~. (Other experimentalparameters as in Fig. 1.) available to undergo electroreduction across the entire surface of the electrode, including the interior of pores and pits in the surface ('microscopic area'). Once all of the ferricyanide ions inside these surface irregularities are reduced, however, the predominant mode of mass transport to the surface becomes semi-infinite linear diffusion to the projected geometric ('macroscopic' or cross-sectional ) area of the surface. At this point in time (viz., tens of milliseconds for pits of the size shown in the figure [11]), the apparent area is no longer timedependent, but rather converges to a constant value. This effect is well known in electroanalytical chemistry [21,26] and has been discussed previously in the context of sonicated electrode surfaces [11,13]. As we have reported previously, however, the calculated convergent areas for G C electrodes sonicated in water are always larger than for unsonicated electrodes [ 11 ]. This is consistent with formation of surface functional groups during sonication which persist for long periods of time, and undergo slow electrolysis reactions during area measurement experiments [11]. This phenomenon was previously also observed for graphite electrodes in non-sonochemical studies [27]. In that work, pH studies were employed to show that local pH gradients arise inside pores during electrochemical experiments because redox reactions of surface carbon
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oxides invariably involve coupled proton transfers. For this reason, a slow decay in current is characteristically measured during potential step experiments on oxidecovered carbon electrodes, which can be misinterpreted as a surface roughness (area) effect [27]. Accurate knowledge of the electrode area over the relevant time scale is mandatory for both electrochemical mass transport and kinetic studies. For this reason, we are not optimistic about the use of GC electrodes for aqueous sonoelectrochemical studies. By contrast, little pitting or surface functionalization occurs when GC electrodes are sonicated in organic solvents such as dioxane [11]. This is because organic solvents 'wet' the hydrophobic glassy carbon surface more efficiently than water, thereby resulting in fewer pockets of trapped vapor at the interface which can serve as nucleation sites for cavitation. In most cases, ultrasonic irradiation in organic solvents merely cleans polishing debris from the electrode surface. Occasionally, however, GC electrodes may fracture during sonication, as shown in Fig. 3. The upper panel (A) shows a polished GC surface at low magnification. The lower image (B) is representative of fracturing evidently caused by ultrasound. The electrochemical response at such electrodes is characterized by abnor-
A
B
Fig. 3. SEMs of glassy carbon electrodes; scale 500 gm. (A) Polished surface. (B) After sonication in dioxane at a distance of 1.0 mm from a Ti tip. Vibrational amplitude 125 gm; sonication time 120 s.
N.A. Madigan et al./Ultrasonics Sonochemistrv 3 (1996) $239-$247
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mally large charging currents; thus, such catastrophic damage is easily detected without resorting to SEM. In fact, the damage to this electrode was apparent to the unaided eye. 3.2. Sonication of Ebonex ~ electrodes
Ebonex ~ is a Magneli phase titanium suboxide, predominantly a mixture of ceramic materials having the stoichiometries Ti407 and TisO 9 [28]. Its properties as an electrode material have previously been reported in the context ofvoltammetry [29] and fuel cell applications [30]. No reports have appeared concerning its robustness with respect to aqueous sonication, however. As seen in the SEM in Fig. 4A, the polished surface prior to sonication is quite porous. Features tens of microns in diameter are visible across the entire surface. Upon sonication, deep, hemispherical cavities become apparent, two of which are identified by the white frames in Fig. 4B. A higher magnification image of one such pit in a sonicated surface is presented in Fig. 4C. The pit shown is considerably deeper than the surface irregulari-
A
ties in the surrounding ceramic, and could conceivably have arisen from nucleated cavitation occurring inside a pre-existing surface depression. The apparent area determined by chronoamperometry for polished Ebonex ~ shows the usual time dependence (Fig. 2B, lower data set (+)) and converges to a value of ~0.34 cm 2 after a few hundred ms. This value is in good agreement with the calculated geometric area for this electrode of 0.332 cm 2. After sonication (Fig. 2B, upper data set (O)), the time constant for convergence of the area is dramatically longer, and the final area is significantly increased (to about 0.37 cm2). These results are qualitatively the same as those observed for GC electrodes (vide supra), and argue against the use of Ebonex@ for aqueous sonochemical work. 3.3. Sonication of metal electrodes
In contrast to GC and Ebonex ~, metal electrodes sustain little damage after aqueous sonication under our conditions. Micrographs of a Pt surface are shown in Fig. 5 for a polished electrode (A) and after sonication (B). A few irregularly shaped pits are visible on the sonicated electrode using higher magnification (B), but it is unclear that these arose from the actions of inter-
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Fig. 4. SEMs of Ebonex ~ electrodes. (A) Polished surface; scale 40 gin. (B) After sonication in water at a distance of 1.0 mm from a Ti tip; scale 40 gin. Vibrational amplitude 125 gm; sonication time 120 s. (C) Magnified view of a surface pit after sonication; scale 20 lam.
Fig. 5. SEMs of Pt electrodes. (A) Polished surface; scale 40 pm. (B) After sonication in water at a distance of 1.0 m m from a Ti tip; scale 20 gm. Vibrational amplitude 125 pro; sonication time 120 s.
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facial ultrasound. Such features are sometimes found widely scattered across unsonicated surfaces (see, for example, the three neighboring pits in the lower left quadrant of Fig. 5A). Whatever the origin of these minor irregularities, they have little consequence for electrochemical measurements made on the dc time scale. Fig. 6A shows the time evolution of the apparent areas for a polished Pt electrode (+) and for Pt, post-sonication (O). For all times greater than 50 ms, the calculated areas for both electrodes are identical and equal to the geometric area of 0.22 cm 2. A measurable and reproducible difference does exist between areas measured for the sonicated and unsonicated electrodes for times less than 50 ms, in agreement with data reported elsewhere [13]. To put this result in perspective, the scan rate (v) in a quiescent solution voltammetry experiment at 25°C corresponding to t = 5 0 ms is estimated by [24]: v-
RT
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Thus, no effects attributable to surface roughening should be observable using sonicated electrodes at scan rates slower than a few hundred mV/s. Similar results are observed for Au, Pd and W
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electrodes. Images of a gold electrode before (Fig. 7A) and after (Fig. 7B) sonication are virtually indistinguishable. The time courses of the apparent areas shown in Fig. 6B lead to the same conclusions reached for Pt electrodes. That is, identical convergent areas are reached rapidly and any (slight) differences attributed to surface roughness are manifest only for times shorter than 50 ms. Figs. 8 and 9 show the SEM images for Pd and W electrodes, respectively. In each case, panel A is for the polished electrode prior to sonication, and panel B is for the same electrode after sonication. Plots of apparent areas (not shown) once again converge on identical limiting values, and no appreciable surface roughening is apparent after 50 ms in any case. 3.4. Correlations based on physical properties
In order to understand the factors which determine whether a particular material is suitable for use as an electrode in sonoelectrochemistry, several factors may be considered. A comparison of physical properties for the materials used in the present study is presented in Table 1. These data suggest that hard materials with low Young's moduli (GC and Ebonex@) tend to pit and fracture when exposed to 20 kHz ultrasound in water. This observation is consistent with the work of Suslick,
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Fig. 7. SEMs of Au electrodes. (A) Polished surface; scale 10 gm. (B) After sonication in water at a distance of 1.0 m m from a Ti tip; scale 10 gm. Vibrational amplitude 125 gm; sonication time 120 s.
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Fig. 8. SEMs of Pd electrodes. (A) Polished surface; scale 20 p.m. (B) After sonication in water at a distance of 1.0 m m from a Ti tip; scale 20 p.m. Vibrational amplitude 125 p.m; sonication time 120 s.
Fig. 9. SEMs of W electrodes. (A) Polished surface; scale l0 p.m. (B) After sonication in water at a distance of 1.0 m m from a Ti tip; scale 10 p.m. Vibrational amplitude 125 p.m; sonication time 120 s.
who found that particles of brittle, layered, inorganic materials underwent fragmentation during sonication [31]. Dense materials with large moduli, such as the metals listed, seem better able to withstand sonication. There does not appear to be any obvious correlation between sonochemical robustness and the melting point (or thermal breakdown limit) of the materials, at least in terms of their suitabilities for use as electrodes.
materials with melting points below about 2000°C experience appreciable sonochemical aggregation. We now wish to address the effect of the hardness and melting point of the electrode material on heterogeneous metal deposition during sonication. The electrode materials examined were the same as those studied by SEM above. In these experiments, suspensions of silver particles in water were sonicated for 300 s in the presence of electrodes fabricated from disks of the materials of interest. The extent of particle deposition was determined by placing sonicated electrodes in an electrochemical cell containing 1 M KC1, and oxidizing the silver deposit ('stripping') by linear potential sweep voltammetry [24]. The upper panel (A) in Fig. 11 shows linear sweep stripping voltammograms for the different electrodes. No detectable deposition occurred onto the W electrode, and the response for the Ebonex ~ electrode was minimal. By contrast, well-formed Ag-stripping peaks were observed on Pt, Pd and Au electrodes. The trend in peak current densities observed is well explained by considering the melting points of the electrode materials, as shown in Fig. 1 IB. (For reasons that will be discussed below, the glassy carbon data were not included in this analysis.) In this plot, the melting point for Ebonex ~
3.5. Sonochemical metal deposition on electrode surfaces Our laboratory reported the observation that Cu particles can be fused to a Au electrode by sonication in organic liquids [15]. The example in Fig. 10A shows 300 nm Cu particles melted onto a gold electrode by sonication of a dimethylsulfoxide (DMSO)/Cu slurry (17 mg/ml). Our study demonstrated that selectivity in the deposition of metals was available based on the melting point of the particles used. For example, sonication of suspensions of Cu and W particles only resulted in fusion of Cu to Au electrodes. This work was motivated by the discovery of Suslick and co-workers that sonication of suspensions of metal particles results in particle aggregation and agglomeration [31 36]. The Suslick group was able to prove that only particles of
N.A. Madigan et aL/Ultrasonics Sonoehemistry 3 (1996) $239-$247
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Table 1 Comparison of properties a of electrode materials Material
Thermal limit (TL) or melting point (m.p.)
Vickers' hardness
Glassy carbon, GC-30 b Ebonex ~, porous c
3000°C (TL) 1400°C (TL) 1830 1850°C (m.p.) d 1769°C (m.p.) 1063°C (m.p.) 1552°C (m.p.) 3410-'C (m.p.) 1083°C (m.p.) 961°C (m.p.)
230 230
Pt Au Pd W Cu Ag
Young's modulus of elasticity, E (N/m 2)
Density, p (g/cm 3)
22 × 109
40-100 20-60 40-100 360-500 50 100 25 95
1.43-1.47 3.6-3.8
152 x 109 80 x 109 100 x 109 345 × 109 124 x 109 76 x 109
21.4 19.3 12.0 19.3 8.94 10.5
aUnless noted otherwise, data compiled from: R.B. Ross, Metallic Materials Specification Handbook, 4th edn (Chapman and Hall, London, 1992); E.A. Brandes and G.B. Brook, Smithells Metals Reference Book, 7th edn (Butterworth Heinemann, Oxford, 1992); C.A. Hampel, Rare Metals Handbook, 2nd edn (Chapman and Hall, London, 1961). by. Suzuki, Tokai Carbon Co., Ltd., Tech. Bull., Minato-ku, Tokyo, Japan (1987). CR.L. Clarke, Atraverda Ltd., Tech. Bull., Ampthill, Bedfordshire, UK (1993). dMelting point range for parent compound, TiO 2 (rutile phase).
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°%'00 15'00 0'00 25'00 30'00 ; 00 Melting Point / °C Fig. 10. SEMs of particles deposited on surfaces by sonication of DMSO slurries. In each case, sonication time was 300 s, vibrational amplitude 96 gm, and electrode-tip distance 4 mm. (A) 300 nm Cu particles fused to Au electrode from a 17 mg/ml suspension in DMSO. Bar in bottom center of image= 1 ~tm. (B) 2.4 ~tm Ag particles fused to a Cu/Zn alloy surface (1995 U.S. 1 cent coin) from an 8.3 mg/ml suspension. Scale 10 gm.
was taken to be that of pure TiO 2 (rutile phase, m.p. range 1830 1850°C). The trend seen in our data be attributed to
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Fig. 1 l. Results of linear sweep stripping voltammetry for 2.4 gm Ag particles deposited onto various electrode materials. Sonochemical conditions are as listed for Fig. 10B. (A) Stripping voltammograms for W, Ebonex@, Pt, Pd and Au electrodes in 1 M KCI. Potential sweep rate 20 mV/s; initial potential - 50 mV vs. Ag/AgC1. The y-axis is anodic current density. (B) Absolute values of peak heights from (A) plotted vs. melting point of electrode material.
sonochemical surface roughening effects, generating a larger exposed area on Au than, for example, Pt. As is shown in Fig. 11A, the Ag stripping current is 48 times higher for Au than Pt. Since stripping currents are first
N.A. Madiganet al./UltrasonicsSonochemistry3 (1996) $239-$247
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order in electrode area [24], the Au electrode would necessarily be required to have 48 times the microscopic area of the Pt electrode. As is obvious from Fig. 6, there is no experimentally accessible time window when such is the case. In fact, the observation that Ag could only be fused to electrodes with melting points below about 2000°C is in remarkable agreement with the conclusion of Suslick et al. that particles will aggregate during sonication of suspensions only when their melting points lie below about 2000°C [35,36].
3.6. Sonication of glassy carbon electrodes in the presence of solid particles Next, we turn to the case of glassy carbon electrodes, which were also sonicated in the presence of Ag particles, in accord with the studies described above. Three voltammograms acquired for GC electrodes after sonication in slurries of solid particles are shown in Fig. 12, along with a comparison voltammogram for a polished GC electrode. The i/V curve for the polished electrode is relatively featureless, except for a small oxidation process at about + 0.060 V (almost indiscernible on this scale). Similar surface oxidation processes have been widely reported for carbon electrodes of various types [11,27,37,38]. By contrast, after sonication of the GC surface in a slurry of particles, the oxidation current monitored in a linear sweep voltammogram in 1 M KCI is dramatically higher and exhibits a well defined peak shape. This increase in current occurs regardless of whether the particles are made of Ag or alumina. Surface redox processes ('pseudocapacitance' effects) have been reported by others to be larger in magnitude at electrodes polished with alumina than at heat treated GC surfaces [37]. It is not clear whether these surface oxides arise in our case from products of the sonolysis of water (e.g., 5 polished ,,~ h'
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3.7. Prospects for using metal deposition as a dosimetry technique Finally, we suggest the possibility that metal deposition may be useful for dosimetry in sonochemical studies. Our previous work has demonstrated that the amount of Cu deposited on a gold electrode is linear in sonication time [15], and thus represents some measure of the interfacial turbulence, microstreaming and jet formation integrated over the irradiation time. However, while electrochemical stripping techniques and suitable Au electrodes are readily available to electrochemists, they may not be routinely accessible to the general sonochemical community. Fig. 10B shows the SEM of Ag particles deposited through sonication on a 1995 U.S. 1 cent coin. The Ag was deposited from a slurry made in DMSO (8.3 mg/ml ) during a 5 min sonication, and the coin was mounted on the floor of an ordinary Pyrex ® beaker using an elastomeric adhesive (Richbond ® 'Qwik-Tac'). (As discussed in Section 2, the separation distance between the coin and the sonicator tip was 4 mm, and a vibrational amplitude of 96 gm was used.) After sonication, the silver deposit was clearly visible to the unaided eye, due in part to the difference in color between Ag and the Cu alloy on the surface of the coin. The mass of the coin also increased from 2.5020 _+0.0001 to 2.5029_+0.0001 g. (The experiment was repeated using water as the sonication solvent. Ag deposition was again observed, with the mass of a different coin increasing from 2.5134_+0.0001 to 2.5144_+0.0001 g.) Thus, this procedure may provide a simple alternative to electrochemical studies for quantifying fluid mechanical effects at interfaces during sonication. As a dosimetry technique, it could provide complementary information to thermal measurements and foil perforation techniques, since it presumably is a better indication of total turbulence than cavitation. Further studies are underway to assess the linearity of the sonication time/mass gain relationship in such systems.
0.00
E/V Fig. 12. Linear sweep voltammograms at glassy carbon electrodes in 1 M KCI. Scanrate 20 mV/s; initialpotential 0 mV vs. Ag/AgC1. Upper curve is for polishedelectrode.Lowerdashed curvesfor electrodeafter sonication in aqueous Ag suspension. Solid curve for electrode after sonication in alumina suspension. See Fig. 10B for sonication conditions.
References
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N.A. Madigan et al./Ultrasonics Sonochemistry 3 (1996) $239-$247 [5] A. Chyla, J.P. Lorimer, T.J. Mason, G. Smith and D.J. Walton, J. Chem. Soc., Chem. Commun. (1989) 603. [6] M. Mohammad, Bull. Electrochem. 6 (1990) 806. [7] H.D. Dewald and B.A. Peterson, Anal. Chem. 62 (1990) 779. [8] S.A. Perusich and R.C. Alkire, J. Electrochem. Soc. 138 (1991) 700. [9] F. Cataldo, J. Electroanal. Chem. 332 (1992) 325. [10] M. Xiping, F. Ruo, Z. Hui and W. Shuangwei, Acoust. Lett. 15 (1992) 257. [11] H. Zhang and L.A. Coury, Jr., Anal. Chem. 65 (1993) 1552. [12] J. Klima, C. Bernard and C. Degrand, J. Electroanal. Chem. 367 (1994) 297. [13] R.G. Compton, J.C. Eklund, S.D. Page, G.H.W. Sanders and J. Booth, J. Phys. Chem. 98 (1994) 12410. [14] A. Durant, H. Frangois, J. Reisse and A. Kirsch-DeMesmaeker, Electrochim. Acta 41 (1995) 277. [15] N.A. Madigan, T.J. Murphy, J.M. Fortune, C.R.S. Hagan and L.A. Coury, Jr., Anal. Chem. 67 (1995) 2781. [16] A. Vogel and W. Lauterborn, J. Acoust. Soc. Am. 84 (1988) 719. [17] A. Vogel, W. Lauterborn and R. Timm, J. Fluid Mech. 206 (1989) 299. [18] C.R.S. Hagan and L.A. Coury, Jr., Anal. Chem. 66 (1994) 399. [19] G. Dryhurst and D.L. McAllister, in: Laboratory Techniques in Electroanalytical Chemistry, eds. P.T. Kissinger and W.R. Heineman (Marcel Dekker, New York, 1984) p. 289. [20] R.L. McCreery, in: Electroanalytical Chemistry, ed. A.J. Bard (Marcel Dekker, New York, 1991) p. 221. [21 ] D.T. Sawyer, A. Sobkowiak and J.L. Roberts, Jr., Electrochemistry for Chemists, 2nd edn (Wiley, New York, 1995) p. 206.
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[22] N.M. Pontikos and R.L. McCreery, J. Electroanal. Chem. 324 (1992) 229. [23] D.E. Weisshaar and T. Kuwana, Anal. Chem. 57 (1985) 378. [24] A.J. Bard and L.R. Faulkner, Electrochemical Methods: Fundamentals and Applications (Wiley, New York, 1980). [25] M. von Stackelberg, M. Pilgram and V. Toome, Z. Elektrochem. 57 (1953) 342. [26] M. Petek, T.E. Neal and R.W. Murray, Anal. Chem. 43 (1971) 1069. [27] L.A. Coury, Jr. and W.R. Heineman, J. Electroanal. Chem. (1988) 327. [28] R.L. Clarke, Atraverda Ltd., Tech. Bull., Ampthill, Bedfordshire, UK (1993). [29] R.R. Miller-Folk, R.E. Noftle and D. Pletcher, J. Electroanal. Chem. 274 (1989) 257. [30]A. Hamnett, P.S. Stevens and R.D. Wingate, J. Appl. Electrochem. 21 (1991) 982. [31] K.S. Suslick, Science 247 (1990) 1439. [32] K.S. Suslick, D.J. Casadonte and S.J. Doktycz, Chem. Mater. 1 (1989) 6. [33] K.S. Suslick and S.J. Doktycz, J. Am. Chem. Soc. 111 (1989) 2342. [34] S.J. Doktycz and K.S. Suslick, Science 247 (1990) 1067. [35] K.S. Suslick, S.J. Doktycz and E.B. Flint, Ultrasonics 28 (1990) 280. [36] K.S. Suslick and S.J. Doktycz, Adv. Sonochem. 1 (1990) 197. [37] D.T. Fagan, I.-F. Hu and T. Kuwana, Anal. Chem. 57 (1985) 2759. [38] M. Poon and R.L. McCreery, Anal. Chem. 59 (1987) 1615.
ELSEVIER
Ultrasonics Sonochemistry3 (1996) $239-$247
Effects of sonication on electrode surfaces and metal particles Nanette A. Madigan, Carolynne R.S. Hagan, Honghua Zhang, Louis A. Coury, Jr. * Department of Chemistry, Box 90346, Duke University, Durham, NC27708-0346, USA Received20 December 1995;revised26 April 1996
Abstract Ultrasonic irradiation at a frequency of 20 kHz has varying effects on electrode surfaces. Non-metals such as glassy carbon and Ebonex~ are severely pitted after only a few minutes of sonication in aqueous media. By contrast, metals such as Pt, Au, W and Pd remain largely undamaged after 120 s, as observed by scanning electron microscopy. The extent of damage does not appear to be related to the melting point of the material. By contrast, when electrodes are sonicated in suspensions of metal powders, particles are deposited onto electrode surfaces. The deposits were subsequently observed by scanning electron microscopy and by voltammetry. It is concluded that the ability to deposit particles on an electrode depends on both the melting point of the particles and the electrode, whereas surface damage is more closely related to the hardness of the material.
Keywords." Ultrasound; Electrode surfaces; Metal particles
1. Introduction The combination of electrochemistry with sonochemistry is an active area of research [1-14] and motivates investigation of the effects of ultrasound on different electrode materials [8,11,13]. A goal of the present study is to examine the effects of ultrasound on several types of electrode materials under the conditions commonly used for electrochemical studies. Since the current monitored in a voltammetric experiment increases with increasing electrode area, it is important to ascertain the extent of surface damage sustained by various electrodes during sonication. Results obtained will be considered in light of the differing melting points and hardnesses of the materials studied. Ultrasound has recently been employed to deposit metal particles onto gold electrodes [ 15]. Particles were subsequently 'stripped' from the surface by electrooxidation for the purposes of quantitation. It was found that copper powder (melting point, m.p., 1083°C) could easily be deposited onto a gold electrode, whereas tungsten particles (m.p. 3410°C) could not. To further understand this phenomenon, the ability to melt particles onto different electrode materials will also be explored in the present study. Consideration of the magnitude of the effects observed in terms of properties of the * Corresponding author. Email: [email protected];fax: + 1-919660-1605.
electrode (e.g., melting point, hardness) will again be of interest. Previous work on the general effects of cavitation on extended surfaces has been published by Lauterborn and co-workers [16,17]. These studies involved the use of a pulsed laser to generate cavitation near solid surfaces [ 16]. It was shown that bubbles in contact with a surface generate the highest pressures and thus are more likely to cause cavitational erosion [17]. Based on experimental evidence with aluminum surfaces, however, it was concluded that interfacial fluid jets may play a larger role in surface damage than single bubbles, and may involve acoustic transients generated by other nearby bubbles [17]. Some work has been published on the effects of ultrasonic irradiation on iron electrodes in aqueous systems [8]; glassy carbon in both water and dioxane [11]; and on the irradiation of Pt and A1 electrodes in acetonitrile [13]. The purpose of the present study is to explore the effects of aqueous sonication on common electrode materials, and to evaluate the suitability of each for sonochemical metal deposition experiments.
2. Experimental Details of the sonochemical procedures have been published previously, but will be summarized briefly
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N.A. Madigan et al./Ultrasonics Sonochemistry 3 (1996) $239-$247
here [11,15,18]. Sonications were performed in a waterjacketed, all-glass cell which allowed for placement of electrodes parallel to the tip of a direct immersion probe sonicator. The sonicator used was a Heat Systems XL2010, 475 W, 20kHz ultrasonic processor. This instrument was fitted with a tapped Ti tip, having a geometric area of 1 cm 2. For surface damage studies, a separation distance between the Ti tip and each electrode of 1.0+0.5 mm was used. The sonicator power setting was 100%, which corresponds to a vibrational amplitude of 125 jam (peak-to-peak). This amplitude was previously found to be optimal for maximizing deposition yields [15]. The sonication time was 120 s in each case. For metal deposition experiments, a power setting of 80% (96 ~'n acoustic amplitude) at a separation distance of 4_+ 0.5 mm was employed. For this work, the sonication time was 300 s. A PAR model 253 potentiostat, interfaced to a 66 MHz 80486 DX laboratory computer, was used for all electrochemical measurements. Scanning electron micrographs (SEMs) were acquired with either a Philips 501 instrument operated at 15 kV accelerating voltage or a JEOL 6400 field emission SEM (2.0 kV). Gold, platinum and glassy carbon electrodes were obtained from Bioanalytical Systems. SEM images were obtained for GC-30 grade, Tokai glassy carbon (Electrosynthesis). Tungsten, palladium (both from Aldrich) and Ebonex ~ (Electrosynthesis) rods were fabricated into disk electrodes and sealed in epoxy (EpoTek). Silver powder (Aldrich) was 99.9+% pure and averaged 2.4 ~tm in diameter. Copper powder (Aldrich) was 99% pure 'submicron' grade, and was previously determined to have a mean diameter of 300 nm [15]. Unless stated otherwise, all sonications were performed in water which was purified by reverse osmosis and then ultrafiltration (Barnstead NanoPure), and all metal slurries were 8.3 mg/ml [15].
[11]) is shown in Fig. 1A. Other than the horizontal polishing scratch visible in the image, the surface appears smooth and featureless, as has been repeatedly shown by others [22,23]. After sonication in water, however, the electrode surface becomes severely pitted as seen in Fig. lB. (Note the difference in scale between Figs. 1A and B.) When such a sonicated electrode is placed in a 20.0 mM Fe(CN)3-/1.0 M KC1 solution and the potential stepped from +0.5 to 0 V vs. Ag/AgC1, the current monitored is noticeably higher than that observed prior to sonication. To understand this effect, the apparent area can be calculated for each point along the current transient using the Cottrell equation [24]: A-
nFC*
(1)
where t is the time in seconds, C* is the ferricyanide concentration (2.0 × 10 -5 mol/cm3), D is the diffusion coefficient for ferricyanide in 1.0 M KC1 (7.63 x 10 6 cmZ/s) [25], and the other parameters have their usual significances [24]. As shown in Fig. 2A, the calculated electrode areas for both sonicated (O) and unsonicated (+) electrodes decrease with time. At short times, ferricyanide ions are
A
3. Results and discussion
3.1. Sonication of glassy carbon electrodes Glassy carbon (GC) is an isotropic, impermeable, vitreous form of carbon which has been widely employed as an electrode material [19-21]. The most commonly cited advantage of GC is its wide potential window in aqueous systems. Kinetic results obtained at GC electrodes, however, are only interpretable when a known and reproducible surface preparation (activation protocol) has been employed [20]. We have previously discussed the effects of 20 kHz ultrasound on glassy carbon, both in aqueous and non-aqueous solvents [ 11 ]. A typical scanning electron micrograph of a GC surface which was polished with decreasingly small alumina particles (from 5 down to 0.05 lam diameter
B
Fig. 1. SEMs of glassy carbon electrodes. (A) Polished surface; scale 2 Ixm. (B) After sonication in water at a distance of 1.0 m m from a Ti tip; scale 40 ~tm. Vibrational amplitude 125 ~tm; sonication time 120 s.
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time / s Fig. 2. Apparent electrode areas as a function of time for polished electrodes (+) and for electrodes after aqueous sonication for 120 s (©). (A) Glassy carbon. (B) Ebonex~. (Other experimentalparameters as in Fig. 1.) available to undergo electroreduction across the entire surface of the electrode, including the interior of pores and pits in the surface ('microscopic area'). Once all of the ferricyanide ions inside these surface irregularities are reduced, however, the predominant mode of mass transport to the surface becomes semi-infinite linear diffusion to the projected geometric ('macroscopic' or cross-sectional ) area of the surface. At this point in time (viz., tens of milliseconds for pits of the size shown in the figure [11]), the apparent area is no longer timedependent, but rather converges to a constant value. This effect is well known in electroanalytical chemistry [21,26] and has been discussed previously in the context of sonicated electrode surfaces [11,13]. As we have reported previously, however, the calculated convergent areas for G C electrodes sonicated in water are always larger than for unsonicated electrodes [ 11 ]. This is consistent with formation of surface functional groups during sonication which persist for long periods of time, and undergo slow electrolysis reactions during area measurement experiments [11]. This phenomenon was previously also observed for graphite electrodes in non-sonochemical studies [27]. In that work, pH studies were employed to show that local pH gradients arise inside pores during electrochemical experiments because redox reactions of surface carbon
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oxides invariably involve coupled proton transfers. For this reason, a slow decay in current is characteristically measured during potential step experiments on oxidecovered carbon electrodes, which can be misinterpreted as a surface roughness (area) effect [27]. Accurate knowledge of the electrode area over the relevant time scale is mandatory for both electrochemical mass transport and kinetic studies. For this reason, we are not optimistic about the use of GC electrodes for aqueous sonoelectrochemical studies. By contrast, little pitting or surface functionalization occurs when GC electrodes are sonicated in organic solvents such as dioxane [11]. This is because organic solvents 'wet' the hydrophobic glassy carbon surface more efficiently than water, thereby resulting in fewer pockets of trapped vapor at the interface which can serve as nucleation sites for cavitation. In most cases, ultrasonic irradiation in organic solvents merely cleans polishing debris from the electrode surface. Occasionally, however, GC electrodes may fracture during sonication, as shown in Fig. 3. The upper panel (A) shows a polished GC surface at low magnification. The lower image (B) is representative of fracturing evidently caused by ultrasound. The electrochemical response at such electrodes is characterized by abnor-
A
B
Fig. 3. SEMs of glassy carbon electrodes; scale 500 gm. (A) Polished surface. (B) After sonication in dioxane at a distance of 1.0 mm from a Ti tip. Vibrational amplitude 125 gm; sonication time 120 s.
N.A. Madigan et al./Ultrasonics Sonochemistrv 3 (1996) $239-$247
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mally large charging currents; thus, such catastrophic damage is easily detected without resorting to SEM. In fact, the damage to this electrode was apparent to the unaided eye. 3.2. Sonication of Ebonex ~ electrodes
Ebonex ~ is a Magneli phase titanium suboxide, predominantly a mixture of ceramic materials having the stoichiometries Ti407 and TisO 9 [28]. Its properties as an electrode material have previously been reported in the context ofvoltammetry [29] and fuel cell applications [30]. No reports have appeared concerning its robustness with respect to aqueous sonication, however. As seen in the SEM in Fig. 4A, the polished surface prior to sonication is quite porous. Features tens of microns in diameter are visible across the entire surface. Upon sonication, deep, hemispherical cavities become apparent, two of which are identified by the white frames in Fig. 4B. A higher magnification image of one such pit in a sonicated surface is presented in Fig. 4C. The pit shown is considerably deeper than the surface irregulari-
A
ties in the surrounding ceramic, and could conceivably have arisen from nucleated cavitation occurring inside a pre-existing surface depression. The apparent area determined by chronoamperometry for polished Ebonex ~ shows the usual time dependence (Fig. 2B, lower data set (+)) and converges to a value of ~0.34 cm 2 after a few hundred ms. This value is in good agreement with the calculated geometric area for this electrode of 0.332 cm 2. After sonication (Fig. 2B, upper data set (O)), the time constant for convergence of the area is dramatically longer, and the final area is significantly increased (to about 0.37 cm2). These results are qualitatively the same as those observed for GC electrodes (vide supra), and argue against the use of Ebonex@ for aqueous sonochemical work. 3.3. Sonication of metal electrodes
In contrast to GC and Ebonex ~, metal electrodes sustain little damage after aqueous sonication under our conditions. Micrographs of a Pt surface are shown in Fig. 5 for a polished electrode (A) and after sonication (B). A few irregularly shaped pits are visible on the sonicated electrode using higher magnification (B), but it is unclear that these arose from the actions of inter-
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Fig. 4. SEMs of Ebonex ~ electrodes. (A) Polished surface; scale 40 gin. (B) After sonication in water at a distance of 1.0 mm from a Ti tip; scale 40 gin. Vibrational amplitude 125 gm; sonication time 120 s. (C) Magnified view of a surface pit after sonication; scale 20 lam.
Fig. 5. SEMs of Pt electrodes. (A) Polished surface; scale 40 pm. (B) After sonication in water at a distance of 1.0 m m from a Ti tip; scale 20 gm. Vibrational amplitude 125 pro; sonication time 120 s.
N.A. Madigan et al./Uhrasonics Sonochemistry 3 (1996) $239 $247
facial ultrasound. Such features are sometimes found widely scattered across unsonicated surfaces (see, for example, the three neighboring pits in the lower left quadrant of Fig. 5A). Whatever the origin of these minor irregularities, they have little consequence for electrochemical measurements made on the dc time scale. Fig. 6A shows the time evolution of the apparent areas for a polished Pt electrode (+) and for Pt, post-sonication (O). For all times greater than 50 ms, the calculated areas for both electrodes are identical and equal to the geometric area of 0.22 cm 2. A measurable and reproducible difference does exist between areas measured for the sonicated and unsonicated electrodes for times less than 50 ms, in agreement with data reported elsewhere [13]. To put this result in perspective, the scan rate (v) in a quiescent solution voltammetry experiment at 25°C corresponding to t = 5 0 ms is estimated by [24]: v-
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Thus, no effects attributable to surface roughening should be observable using sonicated electrodes at scan rates slower than a few hundred mV/s. Similar results are observed for Au, Pd and W
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electrodes. Images of a gold electrode before (Fig. 7A) and after (Fig. 7B) sonication are virtually indistinguishable. The time courses of the apparent areas shown in Fig. 6B lead to the same conclusions reached for Pt electrodes. That is, identical convergent areas are reached rapidly and any (slight) differences attributed to surface roughness are manifest only for times shorter than 50 ms. Figs. 8 and 9 show the SEM images for Pd and W electrodes, respectively. In each case, panel A is for the polished electrode prior to sonication, and panel B is for the same electrode after sonication. Plots of apparent areas (not shown) once again converge on identical limiting values, and no appreciable surface roughening is apparent after 50 ms in any case. 3.4. Correlations based on physical properties
In order to understand the factors which determine whether a particular material is suitable for use as an electrode in sonoelectrochemistry, several factors may be considered. A comparison of physical properties for the materials used in the present study is presented in Table 1. These data suggest that hard materials with low Young's moduli (GC and Ebonex@) tend to pit and fracture when exposed to 20 kHz ultrasound in water. This observation is consistent with the work of Suslick,
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Fig. 7. SEMs of Au electrodes. (A) Polished surface; scale 10 gm. (B) After sonication in water at a distance of 1.0 m m from a Ti tip; scale 10 gm. Vibrational amplitude 125 gm; sonication time 120 s.
N.A. Madigan et al./Ultrasonics Sonochemistry 3 (1996) $239 $247
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A
A
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B
Fig. 8. SEMs of Pd electrodes. (A) Polished surface; scale 20 p.m. (B) After sonication in water at a distance of 1.0 m m from a Ti tip; scale 20 p.m. Vibrational amplitude 125 p.m; sonication time 120 s.
Fig. 9. SEMs of W electrodes. (A) Polished surface; scale l0 p.m. (B) After sonication in water at a distance of 1.0 m m from a Ti tip; scale 10 p.m. Vibrational amplitude 125 p.m; sonication time 120 s.
who found that particles of brittle, layered, inorganic materials underwent fragmentation during sonication [31]. Dense materials with large moduli, such as the metals listed, seem better able to withstand sonication. There does not appear to be any obvious correlation between sonochemical robustness and the melting point (or thermal breakdown limit) of the materials, at least in terms of their suitabilities for use as electrodes.
materials with melting points below about 2000°C experience appreciable sonochemical aggregation. We now wish to address the effect of the hardness and melting point of the electrode material on heterogeneous metal deposition during sonication. The electrode materials examined were the same as those studied by SEM above. In these experiments, suspensions of silver particles in water were sonicated for 300 s in the presence of electrodes fabricated from disks of the materials of interest. The extent of particle deposition was determined by placing sonicated electrodes in an electrochemical cell containing 1 M KC1, and oxidizing the silver deposit ('stripping') by linear potential sweep voltammetry [24]. The upper panel (A) in Fig. 11 shows linear sweep stripping voltammograms for the different electrodes. No detectable deposition occurred onto the W electrode, and the response for the Ebonex ~ electrode was minimal. By contrast, well-formed Ag-stripping peaks were observed on Pt, Pd and Au electrodes. The trend in peak current densities observed is well explained by considering the melting points of the electrode materials, as shown in Fig. 1 IB. (For reasons that will be discussed below, the glassy carbon data were not included in this analysis.) In this plot, the melting point for Ebonex ~
3.5. Sonochemical metal deposition on electrode surfaces Our laboratory reported the observation that Cu particles can be fused to a Au electrode by sonication in organic liquids [15]. The example in Fig. 10A shows 300 nm Cu particles melted onto a gold electrode by sonication of a dimethylsulfoxide (DMSO)/Cu slurry (17 mg/ml). Our study demonstrated that selectivity in the deposition of metals was available based on the melting point of the particles used. For example, sonication of suspensions of Cu and W particles only resulted in fusion of Cu to Au electrodes. This work was motivated by the discovery of Suslick and co-workers that sonication of suspensions of metal particles results in particle aggregation and agglomeration [31 36]. The Suslick group was able to prove that only particles of
N.A. Madigan et aL/Ultrasonics Sonoehemistry 3 (1996) $239-$247
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Table 1 Comparison of properties a of electrode materials Material
Thermal limit (TL) or melting point (m.p.)
Vickers' hardness
Glassy carbon, GC-30 b Ebonex ~, porous c
3000°C (TL) 1400°C (TL) 1830 1850°C (m.p.) d 1769°C (m.p.) 1063°C (m.p.) 1552°C (m.p.) 3410-'C (m.p.) 1083°C (m.p.) 961°C (m.p.)
230 230
Pt Au Pd W Cu Ag
Young's modulus of elasticity, E (N/m 2)
Density, p (g/cm 3)
22 × 109
40-100 20-60 40-100 360-500 50 100 25 95
1.43-1.47 3.6-3.8
152 x 109 80 x 109 100 x 109 345 × 109 124 x 109 76 x 109
21.4 19.3 12.0 19.3 8.94 10.5
aUnless noted otherwise, data compiled from: R.B. Ross, Metallic Materials Specification Handbook, 4th edn (Chapman and Hall, London, 1992); E.A. Brandes and G.B. Brook, Smithells Metals Reference Book, 7th edn (Butterworth Heinemann, Oxford, 1992); C.A. Hampel, Rare Metals Handbook, 2nd edn (Chapman and Hall, London, 1961). by. Suzuki, Tokai Carbon Co., Ltd., Tech. Bull., Minato-ku, Tokyo, Japan (1987). CR.L. Clarke, Atraverda Ltd., Tech. Bull., Ampthill, Bedfordshire, UK (1993). dMelting point range for parent compound, TiO 2 (rutile phase).
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was taken to be that of pure TiO 2 (rutile phase, m.p. range 1830 1850°C). The trend seen in our data be attributed to
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Fig. 1 l. Results of linear sweep stripping voltammetry for 2.4 gm Ag particles deposited onto various electrode materials. Sonochemical conditions are as listed for Fig. 10B. (A) Stripping voltammograms for W, Ebonex@, Pt, Pd and Au electrodes in 1 M KCI. Potential sweep rate 20 mV/s; initial potential - 50 mV vs. Ag/AgC1. The y-axis is anodic current density. (B) Absolute values of peak heights from (A) plotted vs. melting point of electrode material.
sonochemical surface roughening effects, generating a larger exposed area on Au than, for example, Pt. As is shown in Fig. 11A, the Ag stripping current is 48 times higher for Au than Pt. Since stripping currents are first
N.A. Madiganet al./UltrasonicsSonochemistry3 (1996) $239-$247
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order in electrode area [24], the Au electrode would necessarily be required to have 48 times the microscopic area of the Pt electrode. As is obvious from Fig. 6, there is no experimentally accessible time window when such is the case. In fact, the observation that Ag could only be fused to electrodes with melting points below about 2000°C is in remarkable agreement with the conclusion of Suslick et al. that particles will aggregate during sonication of suspensions only when their melting points lie below about 2000°C [35,36].
3.6. Sonication of glassy carbon electrodes in the presence of solid particles Next, we turn to the case of glassy carbon electrodes, which were also sonicated in the presence of Ag particles, in accord with the studies described above. Three voltammograms acquired for GC electrodes after sonication in slurries of solid particles are shown in Fig. 12, along with a comparison voltammogram for a polished GC electrode. The i/V curve for the polished electrode is relatively featureless, except for a small oxidation process at about + 0.060 V (almost indiscernible on this scale). Similar surface oxidation processes have been widely reported for carbon electrodes of various types [11,27,37,38]. By contrast, after sonication of the GC surface in a slurry of particles, the oxidation current monitored in a linear sweep voltammogram in 1 M KCI is dramatically higher and exhibits a well defined peak shape. This increase in current occurs regardless of whether the particles are made of Ag or alumina. Surface redox processes ('pseudocapacitance' effects) have been reported by others to be larger in magnitude at electrodes polished with alumina than at heat treated GC surfaces [37]. It is not clear whether these surface oxides arise in our case from products of the sonolysis of water (e.g., 5 polished ,,~ h'
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3.7. Prospects for using metal deposition as a dosimetry technique Finally, we suggest the possibility that metal deposition may be useful for dosimetry in sonochemical studies. Our previous work has demonstrated that the amount of Cu deposited on a gold electrode is linear in sonication time [15], and thus represents some measure of the interfacial turbulence, microstreaming and jet formation integrated over the irradiation time. However, while electrochemical stripping techniques and suitable Au electrodes are readily available to electrochemists, they may not be routinely accessible to the general sonochemical community. Fig. 10B shows the SEM of Ag particles deposited through sonication on a 1995 U.S. 1 cent coin. The Ag was deposited from a slurry made in DMSO (8.3 mg/ml ) during a 5 min sonication, and the coin was mounted on the floor of an ordinary Pyrex ® beaker using an elastomeric adhesive (Richbond ® 'Qwik-Tac'). (As discussed in Section 2, the separation distance between the coin and the sonicator tip was 4 mm, and a vibrational amplitude of 96 gm was used.) After sonication, the silver deposit was clearly visible to the unaided eye, due in part to the difference in color between Ag and the Cu alloy on the surface of the coin. The mass of the coin also increased from 2.5020 _+0.0001 to 2.5029_+0.0001 g. (The experiment was repeated using water as the sonication solvent. Ag deposition was again observed, with the mass of a different coin increasing from 2.5134_+0.0001 to 2.5144_+0.0001 g.) Thus, this procedure may provide a simple alternative to electrochemical studies for quantifying fluid mechanical effects at interfaces during sonication. As a dosimetry technique, it could provide complementary information to thermal measurements and foil perforation techniques, since it presumably is a better indication of total turbulence than cavitation. Further studies are underway to assess the linearity of the sonication time/mass gain relationship in such systems.
0.00
E/V Fig. 12. Linear sweep voltammograms at glassy carbon electrodes in 1 M KCI. Scanrate 20 mV/s; initialpotential 0 mV vs. Ag/AgC1. Upper curve is for polishedelectrode.Lowerdashed curvesfor electrodeafter sonication in aqueous Ag suspension. Solid curve for electrode after sonication in alumina suspension. See Fig. 10B for sonication conditions.
References
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