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Time Resolved in Situ Liquid Atomic Force Microscopy and Simultaneous Acoustic Impedance Electrochemical Quartz Crystal Microbalance Measurements: A Study of Zn Deposition Emma L. Smith,* John C. Barron, Andrew P. Abbott, and Karl S. Ryder Chemistry Department, University of Leicester, Leicester, United Kingdom LE1 7RH We have studied the deposition of Zn metal onto a polished gold-coated quartz crystal with in situ tappingmode atomic force microscopy (AFM), while simultaneously recording chronoamperometric and acoustic impedance quartz crystal microbalance (QCM) measurements. We are able to demonstrate good correlation between the three techniques. Modeling of the chronoamperometric data recorded for the initial nucleation process of the same experiment suggests that nucleation initially occurred via a progressive mechanism. Crystallites of different sizes were clearly visible from the AFM images throughout the whole deposition time monitored, suggesting that sustained nucleation also occurs via a progressive mechanism. The deposition of Zn onto metal surfaces from aqueous solutions has been widely studied due to its practical and industrial importance in the protection of ferrous substrates against corrosion. Here, Zn is deposited from a novel ionic liquid (IL), or “deep eutectic solvent”, based on a eutectic mixture of choline chloride (ChCl) and ethylene glycol (EG). These deep eutectic solvents consist of a large, unsymmetrical choline cation and complex anions formed between Cl- and a hydrogen bond donor such as ethylene glycol1 or urea.2 These liquids have found applications as replacements for aqueous electrolytes in metal finishing technologies, e.g., electropolishing1,3 and electrodeposition.4-6 Many metals have now been deposited from various ionic liquid systems, and there have been some studies into electrochemical nucleation,7-9 despite this an overall understanding of the mechanism of nucleation and growth of metals from these liquids has still not been well developed. In this study we have sought to learn more about the electrochemical nucleation and growth of Zn, for * To whom correspondence should be addressed. E-mail:
[email protected]. (1) Abbott, A. P.; Capper, G.; McKenzie, K. J.; Ryder, K. S. Electrochim. Acta 2006, 51, 4420–4425. (2) Abbott, A. P.; Capper, G.; Davies, D. L.; Rasheed, R. K.; Tambyrajah, V. Chem. Commun. 2003, 70–71. (3) Abbott, A. P.; Capper, G.; McKenzie, K. J.; Glidle, A.; Ryder, K. S. Phys. Chem. Chem. Phys. 2006, 8, 4214–4221. (4) Abbott, A. P.; Boothby, D.; Capper, G.; Davies, D. L.; Rasheed, R. K. J. Am. Chem. Soc. 2004, 126, 9142–9147. (5) Abbott, A. P.; McKenzie, K. J. Phys. Chem. Chem. Phys. 2006, 8, 4265– 4279. (6) Abbott, A. P.; Capper, G.; Davies, D. L.; Rasheed, R. K. Chem.sEur. J. 2004, 10, 1–7.
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the deposition process from the choline chloride/ethylene glycol ionic liquid. In this article, we report the first study of the electrolytic nucleation of Zn metal on a gold substrate from a choline based ionic liquid using simultaneous in situ, liquid-phase, atomic force microscopy (AFM) and electrochemical acoustic impedance quartz crystal microbalance (EQCM) techniques. AFM is a very attractive technique for profiling detailed surface structures at high resolution because it is applicable to both conducting and nonconducting substrates and because it can be applied in gas and liquid phase environments.10 This attribute in combination with the flexibility afforded by the choice of application modes (e.g., contact or resonant) and contrast mechanisms makes it especially attractive for in situ studies. It represents an alternative to techniques such as STM, which are generally carried out at higher (atomic scale) resolution and are consequently much more technically demanding.11,12 When in situ AFM is combined with electrochemical techniques,13-15 it is a powerful probe for the investigation of metal deposition processes16,17 and surface reactions such as passivation, corrosion (pitting), and electrochemical dissolution.18,19 We have been interested in the combination of AFM profilometry with electrochemical acoustic impedance QCM. This offers simultaneous determination of surface structure and mass changes together with chronometric charge data (i.e., (7) Mann, O.; Freyland, W. J. Phys. Chem. C 2007, 111, 9832–9838. (8) Bomparola, R.; Caporali, S.; Lavacchi, A.; Bardi, U. Surface Coat. Technol. 2007, 201, 9485–9490. (9) Bhatt, A. I.; Mechler, A.; Martin, L. L.; Bond, A. M. J. Mater. Chem. 2007, 17, 2241–2250. (10) Cohen, S. H., Bray, M. T., Lightbody, M. L., Eds. Atomic Force Microscopy/ Scanning Tunneling Microscopy, 1st ed.; Springer, 1995. (11) Borisenko, N.; Ispas, A.; Zschippang, E.; Liu, Q.; Zein El Abedin, S.; Bund, A.; Endres, F. Electrochim. Acta 2009, 54, 1519–1528. (12) Endres, F.; Zein El Abdein, S.; Saad, A. Y.; Moustafa, E. M.; Borissenko, N.; Price, W. E.; Wallace, G. G.; MacFarlane, D. R.; Newman, P. J.; Bund, A. Phys. Chem. Chem. Phys. 2008, 10, 2189–2199. (13) Cruickshank, B. J.; Gewirth, A. A.; Rynders, R. M.; Alkire, R. C. J. Electrochem. Soc. 1992, 139 (10), 2829. (14) Hyde, M. E.; Jacobs, R.; Compton, R. G. J. Phys. Chem. B 2002, 106 (43), 11075–11080. (15) Josefowicz, J. Y.; Xie, L.; Farrington, G. C. J. Phys. Chem. 1993, 97, 11995– 11998. (16) Manne, S.; Hansma, P. K.; Massie, J.; Elings, V. B.; Gewirth, A. A. Science 1991, 251, 183. (17) Manne, S.; Massie, J.; Elings, V. B.; Hansma, P. K.; Gewirth, A. A. J. Vac. Sci. Technol., B 1991, 9 (2), 950–954. (18) Macpherson, J. V.; Unwin, P. R.; Hillier, A. C.; Bard, A. J. J. Am. Chem. Soc. 1996, 118, 6445–6452. (19) Bertrand, G.; Rocca, E.; Savall, C.; Rapin, C.; Labrune, J.-C.; Steinmetz, P. J. Electroanal. Chem. 2000, 489, 38–45. 10.1021/ac901329e CCC: $40.75 2009 American Chemical Society Published on Web 09/15/2009
current). The combination of AFM with (E)QCM20-22 has previously been reported in the literature, but the technique is technically demanding and quite costly. These latter aspects have limited its general scope and application. While many studies have been conducted on metal deposition processes, the topographical and surface morphological studies have generally been conducted ex situ and mainly in air. To our knowledge, this is the first example of this kind of study into electrochemical metal ion nucleation from ionic liquids. Friedt et al.20 have carried out simultaneous AFM and QCM measurements investigating the electrodeposition of both copper and silver onto gold from aqueous electrolytes. They related dampening effects seen in the resonance of the quartz crystal with differences in shape and height of the metal deposit. This group has previously shown that the electronic drive and control mechanism for the resonant circuits of both AFM (in tapping mode ∼250 kHz) and impedance QCM (∼10 MHz) function independently so as to allow their simultaneous application.21 Other groups have combined AFM with electrochemical methods in order to study metal deposition. For example Gewirth et al.23 have presented atom-resolved AFM images of underpotential deposition (UPD) monolayers of Pb onto Au(III). Another study was conducted by Compton et al.14 to determine the nucleation rates of the growth of individual Pb nuclei; however, this was not time-resolved, as the slow scan axis of the AFM was disabled in between potential pulses. Josefowicz et al.15 analyzed the oxidation and reduction reactions of Cu in situ by observing morphology changes on graphite and Cu working electrode surfaces using electrochemical atomic force microscopy. Simultaneous AFM and QCM measurements allow data of very different natures (mass change and viscosity in the case of QCM and topography in the case of AFM) to be recorded from a single sample, in one experiment. It is guaranteed that the same conditions are used by recording simultaneous measurements, which might not necessarily be the case for successive experiments on different instruments. Here, the topography of depositing Zn has been imaged in situ using resonant mode (tapping) liquid phase AFM, while simultaneously recording the acoustic impedance QCM and the chronoamperometric response of a polished gold-coated quartz crystal. EXPERIMENTAL SECTION Formation of the Ionic Liquid. Choline chloride (>98%) and ethylene glycol (>99%) were purchased from Aldrich and used as received. The eutectic solvent was formed by continuous stirring and gentle heating of ethylene glycol and choline chloride (2:1) until a homogeneous, colorless liquid was formed.24 Instrumentation. Electrochemical deposition of Zn was carried out onto a thin Au film which had been evaporated onto a 10 MHz polished quartz crystal (International Crystal Manufacturing (20) Friedt, J.-M.; Choi, K. H.; Frederix, F.; Campitelli, A. J. Electrochem. Soc. 2003, 150 (10), H229–H234. (21) Friedt, J.-M.; Choi, K. H.; Francis, L.; Campitelli, A. Jpn. J. Appl. Phys., Part 1 2002, 41, 3974. (22) Hayden, O.; Bindeus, R.; Dickert, F. L. Meas. Sci. Technol. 2003, 14, 1876– 1881. (23) Chen, C.; Washburn, N.; Gewirth, A. A. J. Phys. Chem. 1993, 97, 9754– 9760. (24) Abbott, A. P.; Barron, J. C.; Ryder, K. S. Trans. Inst. Met. Finish. 2009, 87, 201–207.
Figure 1. Schematic of the custom-built cell used in the experiment, designed to allow a resonating quartz crystal under electrochemical control to be placed under the AFM scanning head and immersed in liquid. A three-electrode cell consisted of an Au coated quartz crystal working electrode, Ag wire reference, and Zn foil counter electrode (not to scale).
Co., Oklahoma City), producing a piezoelectric active electrode area of 0.23 cm2. Crystal impedance spectra were recorded over a bandwidth of typically 5-200 kHz centered on the resonant frequency (depending on the nature of the interface) using a Hewlett-Packard E5061A network analyzer operating in reflectance mode. The data acquisition and review software were developed in-house using the Agilent Virtual Engineering Environment (VEE v7.52) software, and the acoustic impedance spectrum was acquired approximately once every second. Analysis, modeling, and fitting of the measured data were performed with Visual Basic for Applications (VBA) inside Microsoft Excel (Microsoft Office Pro. 2003) using protocols previously reported.25 The quartz crystal was mounted with Dow Corning 3145 RTV silicon rubber adhesive onto a cell built in-house so that one face of the crystal was exposed to the solution while the other was exposed to air (see Figure 1). AFM images were acquired using a Digital Instruments (DI, Veeco) Nanoscope IV, Dimension 3100 instrument in resonant (tapping) mode (software version 6.12) and the designated quartz liquid-mode cantilever mount supplied directly by Veeco. Image acquisition time for the in situ simultaneous EQCM/AFM experiment was approximately 1 min per image (at a digital resolution of 128 × 128 pixels). Experimental Conditions. The experiment was carried out at room temperature (23 °C). Electrolytic deposition of Zn onto the polished gold QCM crystal was accomplished in a solution of 0.3 M ZnCl2 in Ethaline (2 ethylene glycol/1 choline chloride) using potential step chronoamperometry, with a constant applied potential of -1.1 V (versus Ag wire pseudo reference).24 Preliminary EXAFS results show that the Zn coordination in this liquid is [ZnCl4]2-, not unsurprising when you consider the high chloride content of the solvent (∼5 M).26 The counter electrode was a Zn foil (see Figure 1). In order to perform the simultaneous in situ experiments, the cell with the mounted quartz crystal was placed under the tip of the AFM head and both were then submerged in the deposition solution. Zn metal was electrolytically deposited onto the Au coated crystal surface, and the tapping mode AFM was recorded continuously alongside the acoustic impedance spectrum. (25) Abbott, A. P.; Nandhra, S.; Postlethwaite, S.; Smith, E. L.; Ryder, K. S. Phys. Chem. Chem. Phys. 2007, 9, 3735–3743. (26) Abbott, A. P.; Barron, J. C.; Elhadi, M.; Frisch, G.; Gurman, S. J.; Hillman, A. R.; Smith, E. L.; Mohamoud, M. A.; Ryder, K. S. ECS Trans. 2009, 16, 47–63.
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RESULTS AND DISCUSSION Preliminary experiments were conducted in order to establish that the surface-tip interaction did not interfere with crystal resonance and that the rf output of the network analyzer did not disturb the scanning control electronics of the AFM head or the resonance feedback for the tip control. A gold coated crystal mounted onto the experimental cell was positioned under the AFM head and submerged in fresh ionic liquid that did not contain any Zn salt. The tapping mode AFM image was then recorded with and without the crystal resonating at approximately 10 MHz. No qualitative differences were observed in the AFM images with and without crystal resonance; also the center frequency and Q-factor of the acoustic impedance spectrum was unchanged by the scanning of the resonating AFM tip. This is consistent with the findings of Friedt et al.,21 who reported that when used in combination with an AFM, the QCM did not visibly modify the image shape because of its oscillation and that the out-of-plane oscillation of the quartz crystal did not affect the resolution of the AFM for typical measurements in the liquid. Probe Microscopy. Time-resolved acquisition of liquid phase AFM images and the simultaneously recorded current transient data, enable the collection of unique and detailed information on the deposition of Zn. The results of one such experiment are presented in this paper. Samples of the time-resolved AFM images are illustrated in Figure 2, and a movie of all the AFM images acquired during this experiment is available in Supporting Information. The image presented as Figure 2a shows the bare polished gold-coated quartz crystal taken in the deposition liquid electrolyte immediately before the initiation of Zn deposition. This has been included here to allow for comparison of surface roughness of the substrate with the surface features observed during the deposition. The polished gold-coated quartz crystal is free from the mechanical scratches and pits that are often observed on the native metal substrate and as a result any nucleation sites, growing crystallites and surface features evident during deposition were not obscured by other artifacts. In an earlier example of a similar methodology used to study deposition of silver from an aqueous electrolyte, single images were acquired over a period of 700-800 s.20,21 Our approach here has yielded an improvement in temporal resolution of at least 1 order of magnitude, as a single image is acquired over a period of 60 s. Figure 2b-f clearly demonstrates the successive formation of small Zn nucleation sites followed by the growth of these nuclei in terms of both their size and shape. A small amount of piezo electric drift exhibited by the scanning head was observed; nevertheless, the same features are clearly identifiable in sequential images. The shape of the crystallites was regular and welldefined. It is also clear that the crystallites have a distribution of sizes and growth rates indicating that they originate from distinct (temporally separated) nucleation events. These observations strongly suggest a progressive nucleation mechanism where nucleation events are continually occurring at different times.27 Images are only presented here for the first 600 s of the experiment, though the total length of the experiment was 1200 s. At a time-scale greater than 700 s, the crystallites became less (27) Gunawardena, G.; Hills, G.; Montenegro, I.; Scharifker, B. J. Electroanal. Chem. 1982, 138, 225–239.
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well-defined and the growing Zn crystallites appear to coalesce; this is consistent with our analysis of the chronoamperometric data presented in Figure 3 and discussed below. Chronoamperometry. Chronoamperometric data for a potential step experiment described above (Figure 2) are presented in Figure 3. The overall pattern of the i(t) trace is consistent with expectation for a diffusion limited electrodeposition process, and at short time scales, a nucleation peak is observed (tm ) 12 s). At longer time scales, an increase in current is observed as a consequence of increased electrode area and this levels out as individual crystal growth centers coalesce at t > 700 s. This latter observation is consistent with the data presented in Figure 2. An established theoretical model to study the mechanism of nucleation and growth during the early stages of electrochemical deposition has been developed by Scharifker and Hills.27 This model assumes that nucleation occurs at certain specific sites on the surface and describes the nucleation mechanism as being either instantaneous or progressive. If the rate of nucleation is rapid in comparison with the resultant rate of growth, subsequent nuclei are formed at all possible growth sites within very short times and nucleation is classified as instantaneous. Alternatively, if the rate of nucleation is slow, nucleation will continue to take place at the surface while other clusters are growing; in this case the nucleation mechanism is described as progressive. It is clear from our observations, as well as those of others,7,8,14 that the nucleation mechanism of this and other IL metal deposition systems is correlated with the nature of surface deposit; however, there exists no clear and rational understanding of this correlation. This study has been motivated by the desire to learn more about such correlations. In order to distinguish between an instantaneous or progressive nucleation process, we have performed a conventional analysis of the chronoamperometric, i(t), trace. The experimental chronoamperometric data are represented in a dimensionless plot of i2/imax2 versus t/tmax and compared with model data derived from the Scharifker and Hills equations below: (1) instantaneous nucleation followed by 3D diffusion-limited growth i2 imax2
( )[
) 1.9542
(
tmax t 1 - exp -1.2564 t tmax
)]
2
and (2) progressive nucleation followed by 3D diffusion-limited growth i2 imax2
( )[
) 1.2254
(
tmax t2 1 - exp -2.3367 t tmax2
)]
2
where t is time, i is current, tmax is the time at which the maximum current occurs, and imax is the current at the maximum point. The experimental and model data are presented together as Figure 4. The experimental data show a good fit to the progressive model although at values of t/tm > 3 (equivalent to an elapsed time of around 48 s) the fit is intermediate, probably representing a combination of limiting mechanisms. In view of the nature of the synchronous experiment, it is gratifying to note that the best-fit mechanism obtained from the i(t) data (Figure 4) is in agreement with the qualitative analysis of the AFM images (Figure
Figure 2. Tapping mode, liquid AFM images recorded in a 0.3 M ZnCl2 solution in ethaline of an Au coated resonating quartz crystal under electrochemical control (-1.1 V versus Ag wire). Images show Zn deposition at times (a) t ) 0 s (i.e., bare Au), (b) t ) 120 s, (c) t ) 240 s, (d) t ) 360 s, (e) t ) 480 s, and (f) t ) 600 s. Image data, z(x, y), were exported from the DI Nanoscope software (v6.13); each image comprises 128 × 128 data points.
2). Having made this point, it is noteworthy that the time scales of data acquisition for the chronoamperometric and AFM experiments are quite different. Whereas the data presented in Figure 4 were acquired over an elapsed time of ∼48 s, each single AFM image (Figure 3) was acquired over 60 s. Despite these limitations, the temporal resolution of the experiment described here is significantly better than that described in the only other comparable investigations.20,21
Acoustic Impedance QCM. The acoustic impedance QCM technique was used as a mass probe to investigate the rate of deposition of Zn during the experiment. Most commercial QCM instruments use a derivative frequency-lock technique to track the resonant frequency of the crystal as a function of time, potential, or other external perturbation. However, this technique does not work well with ionic liquid systems because one of their most common physical properties is high viscosity. This severely Analytical Chemistry, Vol. 81, No. 20, October 15, 2009
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Figure 3. Chronoamperometric data acquired simultaneously alongside the mass trace and AFM images. The current-time trace, i(t), for Zn deposition from a 0.3 M ZnCl2 solution in ethaline. WE ) Au coated quartz crystal; RE ) Ag wire; CE ) Zn foil. The inset shows the initial 30 s of the experiment, highlighting data used for modeling in Figure 4.
Figure 4. Normalized plot of chronoamperometric data at short timescale for nucleation analysis. 3D-progressive model (dotted line), 3Dinstantaneous model (solid line), and experimental data (b) from the inset of Figure 3.
attenuates and dampens the crystal resonance. With the use of acoustic impedance spectroscopy, the full resonance spectrum of the crystal can be recorded. Subsequently curve fitting facilitates the extraction of the peak frequency data. Additionally the shape of the acoustic impedance spectrum is coupled to the mechanical losses at the crystal/liquid interface. As a consequence the Q-factor of the resonance (below) provides an insight into the surface roughness of the solid/liquid interface. As well as this, changes in Q provide a valuable diagnostic measure for validating the conversion of frequency change, ∆f, to mass often absent in many analyses of QCM data.28-30 The Q-factor is defined by Q ) f0 /w where f0 is the frequency value at the center of resonance and w is the full width of the peak at half height (fwhh). (28) Hillman, A. R.; Efimov, I.; Skompska, M. Faraday Discuss. 2002, 121, 423– 439. (29) Hillman, A. R.; Jackson, A.; Martin, S. J. Anal. Chem. 2001, 73, 540. (30) Hillman, A. R.; Efimov, I.; Ryder, K. S. J. Am. Chem. Soc. 2005, 127, 16611.
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Figure 5. Deposited mass, ∆m, versus charge passed, Q, acquired simultaneously with the i(t) trace and AFM images during deposition from a 0.3 M ZnCl2 solution in ethaline.
Acoustic impedance (admittance) plots recorded for the quartz crystal before and after Zn deposition were very similar in shape and intensity. Q-factor calculations (Q(after)/Q(before) ) 0.914) confirmed that the shape of the resonance peak was relatively unchanged by the deposition of Zn metal. The lack of viscoelastic losses during the experiment suggests that a rigid Zn metal deposit was formed during the deposition, allowing for quantitative conversion of frequency change, ∆f, to deposited mass using the Sauerbrey31 equation. The plot of deposited mass versus charge is presented here as Figure 5; the total mass of Zn deposited during the experiment was determined as 42 µg. The mean slope of the linear regression for this plot was determined as 3.128 × 10-4 g C-1; this gives a mean current efficiency of 92% for the deposition process by comparison with the Faraday slope (equivalent to r.a.m./2F) of 3.389 × 10-4 g C-1. Consequently the QCM data show that for the combined experiment Zn is deposited at high current efficiency also described recently24 and that the deposit is both dense and rigid. CONCLUSIONS In this article we have presented, for the first time, synchronous AFM/QCM data for the electrodeposition of Zn metal on a gold substrate from an ethylene glycol/choline chloride based ionic liquid electrolyte. We have been able to demonstrate a dramatic improvement in temporal resolution for the real-time imaging over that previously presented for a similar study (albeit a different metal in aqueous solution)20,21 and have shown that in contrast to the conventional aqueous electrolytes the zinc metal is deposited from this liquid at a current efficiency that is close to quantitative (92%). We present the first nucleation analysis for Zn deposition in a eutectic, choline chloride-based ionic liquid electrolyte and have determined from the chronoamperometric data that during the initial phases of deposition the nucleation mechanism resembles a 3D progressive model. Qualitative analysis of the time-resolved in situ AFM images shows gradual emergence and growth of separate crystallite features; this observation is also consistent with a progressive nucleation mechanism. In a recent ex situ study, we have recently shown a (31) Sauerbrey, G. Z. Phys. 1959, 155, 206.
correlation between macroscopic surface morphology of electrolytic Cu coatings from eutectic ILs and microscopic nucleation mechanism.32 This understanding is critically important to emergent IL based electrolyte technologies (as alternatives to existing aqueous processes that are environmentally unsustainable) so as to offer control of the surface finish of electrolytic metal coatings. Here we present, for the first time, the facility to carry out simultaneous determinations of nucleation mechanism, from analysis of chronoamperometric data and real time in situ monitoring of surface mass and morphology; this offers a realistic prospect of developing a detailed and rational understanding of (32) Abbott, A. P.; El Ttaib, K.; Frisch, G.; McKenzie, K. J.; Ryder, K. S. Phys. Chem. Chem. Phys. 2009, 11, 4269–4277.
the correlations between the mechanisms of electrochemical nucleation and growth with surface finish and morphology. ACKNOWLEDGMENT The authors would like to acknowledge the EU under the FP6 program for funding this work through the IONMET project (www.ionmet.eu). SUPPORTING INFORMATION AVAILABLE Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org. Received for review June 19, 2009. Accepted August 20, 2009. AC901329E
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