In Situ Electrochemical Digital Holographic Microscopy; a Study of

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In Situ Electrochemical Digital Holographic Microscopy; a Study of Metal Electrodeposition in Deep Eutectic Solvents Andrew P. Abbott, Muhammad Azam, Karl S. Ryder,* and Saima Saleem Department of Chemistry, University of Leicester, Leicester LE1 7RH, U.K. ABSTRACT: This study has shown for the first time that digital holographic microscopy (DHM) can be used as a new analytical tool in analysis of kinetic mechanism and growth during electrolytic deposition processes. Unlike many alternative established electrochemical microscopy methods such as probe microscopy, DHM is both the noninvasive and noncontact, the unique holographic imaging allows the observations and measurement to be made remotely. DHM also provides interferometric resolution (nanometer vertical scale) with a very short acquisition time. It is a surface metrology technique that enables the retrieval of information about a 3D structure from the phase contrast of a single hologram acquired using a conventional digital camera. Here DHM has been applied to investigate directly the electro-crystallization of a metal on a substrate in real time (in situ) from two deep eutectic solvent (DES) systems based on mixture of choline chloride and either urea or ethylene glycol. We show, using electrochemical DHM that the nucleation and growth of silver deposits in these systems are quite distinct and influenced strongly by the hydrogen bond donor of the DES.

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the phase changes occurring in cells. Of note is the work by Depeursinge and Marquet who have studied numerous cellular processes including mechanisms of cell death using this technique.5 In the current study DHM has been used in reflectance mode to investigate the nucleation of a metal on an electrode surface during electrolytic deposition. Recently a number of studies have been carried out into the electrodeposition of metals from ionic liquids.6−10 These electrolytes have been found to be not only viable alternatives for aqueous solutions, but they also extend the range of metals and alloys which can be deposited as well as the morphology of the deposit.11 In general ionic liquids tend to lead to nanoscale deposits although this can be changed for some metals through choice of the cation.12 In the study reported here, DHM is used to probe nucleation of silver in two different ionic liquids known as deep eutectic solvents. These consist of stoichiometric mixtures of choline chloride (ChCl) with either ethylene glycol (Eg) or urea. Silver was chosen in this context as a model system that displays simple and consistent electrochemical behavior in the chosen liquids, as well as being of interest for a range of surface treatment applications. Although the electrodeposition of silver in aqueous media has been the subject of historic interest, we have been interested in the electrochemistry and deposition characteristics of Ag+ in DES media for application in a range of electronic finishes and composite coatings.13,14 The viscosity of DES media are typically much higher than aqueous media (40 and 300 times greater than that of water for 2Eg/ChCl and

igital holographic microscopy (DHM) is a fast and nondestructive surface metrology technique. DHM allows high frequency measurements to be made at interferometric resolution and it gives both qualitative and quantitative 3D information with nanometer vertical resolution and millisecond time resolution. It is a noninvasive and noncontact measurement and uses the principle of classical holography.1−3 Holograms are generated by combining a coherent reference wave with the wave received from a specimen. The holograms are recorded by a video camera and transmitted to a computer for real time numerical reconstruction.4 This provides image intensity which is comparable with classical optical microscopy. In reflection mode, the phase image reveals directly the surface topography with a subnanometer vertical resolution. In transmission, the phase image reveals the phase shift induced by a transparent specimen, which depends on its thickness and refractive index. The strength of DHM lies in off-axis configuration by which the whole information about the surface can be retrieved by a single-image acquisition “hologram” within a few microseconds. The extremely short acquisition time makes the system insensitive to vibrations and ambient light hence it is very stable and robust. DHM can be used for shape and surface characterization of high aspect ratio micro-optics, surface nanostructures, and surface roughness. The holograms have relatively low resolution, ∼300 nm, in the x and y coordinates (diffraction limit of the laser source) but high, nanometer, resolution in the z-axis. [Here, as in conventional visible light microscopy, the lateral resolution is determined by the wavelength of laser light, 630 nm and the numerical aperture of the objective lens, 1.2.] This has led to the technique being used extensively for imaging biological systems most notably © XXXX American Chemical Society

Received: January 29, 2013 Accepted: June 10, 2013

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dx.doi.org/10.1021/ac400262c | Anal. Chem. XXXX, XXX, XXX−XXX

Analytical Chemistry

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2Urea/ChCl at r.t., respectively) presenting additional challenges for scanning probe techniques.

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RESULTS AND DISCUSSION In Situ Silver Deposition. Imaging techniques have previously been used to study nucleation on metal surfaces. Figure 3. 3D topography of silver deposited from 1:2 ChCl/ethylene glycol after deposition for 37 s at a potential of −100 mV versus Ag wire: (a) raw image data and (b) background corrected data.

physically removes the deposited material and smearing of the image because of dragging of material across the surface. Finally, the precision of moving part control, such as piezoelectric transducers is another limitation. Highly viscous solutions often do not allow images to be obtained with acceptable levels of piezo-electric drift.17 In this study, DHM was used to investigate the initial stages of silver deposition from two deep eutectic solvents (1:2 ChCl/ ethylene glycol and 1:2 ChCl/urea).18−20 These electrolytes share the same choline chloride salt but are formulated using different hydrogen bond donors. The structure and coordination properties of the H-bond donor results in distinctly different physical properties and electrochemical responses for solute ions. These have been characterized and discussed previously in studies of the deposition of a variety of metal and alloy systems.21−25 The DES liquids have also been used for the deposition of silver composites, where it was shown that wear resistant coatings could be achieved using a variety of dispersed particulates.24 In the experiments reported here, the electrochemical deposition was carried out under potential control from a DES solution of silver nitrate. Gold-coated polished single crystal quartz wafers were used as an electrode substrate in order to maximize measurement contrast. These same Aucoated quartz wafers are also used in electrochemical quartz crystal microbalance studies (EQCM) from this group.14 Figure 1 shows intensity and 3D topographical images of the bare gold crystal substrate. The figure reveals a flat surface without any

Figure 1. Surface topography of the polished gold coated quartz crystal (a) intensity image and (b) 3D phase image.

Hyde and Compton et al.15 employed AFM to directly examine the properties of individual growing nuclei on an electrode surface and hence removed the need for the priori assumptions about the growth rate of diffusion zones, nucleation rate laws, etc., and the dependence of interpretation of potentiostatic transients. However, this technique has the problem that it is slow and requires a long time to obtain the image, while electrochemical processes are faster than the acquisition rate of images. Hyde and Compton were successful in demonstrating the electrochemical AFM methodology in aqueous media using a tapping (or resonant tip) mode to minimize contact of the AFM tip with the surface. This is essential to avoid perturbation of the growing nuclei. However, AFM becomes increasingly more difficult in nonaqueous media and more viscous solvents as the resonance of the tip become severely attenuated and damped.16 The probe tip can also “shield” the surface from unrestricted mass transport and may result in disturbing the nucleation and growth rate. The SPM techniques involve the movement of the probing tip over the surface which sometimes

Figure 2. Schematic presentation of the in situ electrochemical cell placed under the DHM and filled in liquid. The cell consists of a gold-coated quartz crystal working electrode, a Ag wire pseudoreference electrode, and a platinum counter electrode. B



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Figure 4. Successive in situ images of Ag(I) deposition from 1:2 ChCl/ethylene glycol at −100 mV (versus Ag wire) for 60 s. The 3D profiles are shown on the lef t with expansion images of individual features identified by the red arrows. The corresponding 2D projections are shown on the right with expanded regions indicated by the red circles.

significant surface defects such as pits or polishing lines and the arithmentic roughness, Ra, of crystal is determined from these images less than 5 nm. The polished gold coated crystal was chosen to minimize the effects of surface defects on the deposition and morphology of the silver deposit. Electrolytic deposition of silver metal was achieved using a standard three electrode cell configuration. Here the gold coated quartz crystal was working electrode, a platinum foil was used as the counter electrode and a silver wire immersed in the DES as a quasi reference electrode. The electrochemical cell used for in situ DHM study is shown in Figure 2 and was

constructed by attaching two O-rings (1 cm diameter and 4 cm diameter) to a flat polypropylene sheet. The smaller ring was positioned in the center of the larger ring and a polished gold coated quartz crystal was positioned on the small O-ring and fixed with insulating epoxy resin. The outer ring serves as the container for the liquid reservoir. Experimental methodology. First the electrode surface was placed under the objective of the microscope and the surface was brought into focus. The solution was then fed into the cell using a syringe slowly and carefully to ensure that no bubbles were introduced into the optical path. This was done until the C



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Figure 5. (a) Image shows 10 features (circled in red) selected for the analysis. Time dependence of (b) the volume of the island, (c) the mean average island diameter, and (d) the average island height deposited from 1:2 ChCl/ethylene glycol at −100 mV.

from this figure is the unprecedented time resolution of the images. Island Growth Rate. The data presented in Figure 5 show the time dependence of the average island diameter (width), the average island height and the volume of the nuclei (these parameters were measured from the data presented in Figure 4). The analysis of the nucleation and growth features was based on the selection of growing nuclei from the final image at the end of the experiment. These selected features were then identified and measured by tracing back through the growth of the selected nuclei until they disappeared. Figure 5b shows the volume of 8 growing clusters plotted versus the elapsed time. The volume plotted here is the sum of all the volumes of individual nuclei that were separately identified as contributing to subsequent cluster growth during the course of the experimental time scale. Figure 6 shows the formation of a typical cluster in real time. Figure 5c and d shows the average lateral and vertical dimensions of the growing islands (30 nuclei) and their variance with time. At the end of the experiment, the clusters are relatively flat and wide, for example, 700 × 40 nm. The data show in Figure 5c and d both show biphasic behavior with a transition time close to 30 s. This observation indicates the manifestation of two kinetic regimes, and this can be interpreted as being consistent with isolated island growth up to 30 s and then the coupled growth after 30 s. In an isolated island growth regime, the growth for lateral and vertical dimensions is independent and the vertical growth rate is less than the lateral growth rate. After 30 s coalescence of individual islands starts which results in slow down of lateral growth because of the impingement of the growing island, while the vertical growth speeds up because of the suppression of the lateral growth.

liquid formed a thin layer over the whole substrate surface. After feeding the solution, the surface was again focused. Silver was electrodeposited at a constant voltage and the sequential “holograms” were obtained every 100 miliseconds for experimental time scales of up to a few minutes. The recorded holograms were then subsequently reconstructed using the proprietary instrumental software (Koala) to obtain the phase and intensity images and 3D profiles. The phase data thus obtained were plotted (Sigma Plot, version 11) and analyzed using the image analysis software “Digimizer”. A temporal average over one second was obtained by taking the arithmetic mean of every ten phase images. The temporal average was used to reduce the noise and a representative image, acquired for 37 s, EApp. = −100 mV, in ChCl:2Eg, is shown in Figure 3. The raw image image, Figure 3a, is still qualitatively too noisy to extract useful information relating to the nucleation and growth kinetics. For this purpose, the data were corrected by subtracting the blank substrate surface. Figure 3b shows the image obtained from the background corrected data. Silver Deposition from 1:2 ChCl/Ethylene Glycol. Silver metal was deposited electrolytically on the gold crystal from a 0.1 mol dm−3 AgNO3 solution in 1:2 ChCl/ethylene glycol at an overpotential of −100 mV for a period of 60 s. The 3D topographic and the corresponding 2D projection images are shown in Figure 4. These images also show the growth of the selected nucleation features. These images show that silver is deposited through the formation of 3D hemispherical nuclei. The coalescence of these nuclei results in the formation of large silver clusters. Depletion zones develop around the growing large clusters, which minimize the chance of the formation of new nuclei and these growing clusters also compete for adatoms at the perimeter. The coalescence of these growing clusters gives rise to the formation of a film. Of particular note D



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Figure 6. Coalescence of a silver cluster as a function of time in 1:2 ChCl/ethylene glycol. 3D images are shown for each value of time, t, with corresponding 2D projections below.

Silver Deposition from 1:2 ChCl/Urea. Figure 7 shows topographic images obtained for the deposition of silver from 1:2 ChCl/urea at a step potential (−140 mV) for 100 s. The silver is deposited from a solution containing 0.1 mol dm−3 AgNO3. Deposition occurs through the formation of 3D hemispherical islands. In contrast to the deposition of silver from 1:2 ChCl/ethylene glycol, no large silver cluster formation is observed and the growth takes place through individual nuclei and small crystallites. This ultimately results in a nanostructured deposit that is often dull in appearance and friable. The time dependence of the growth of the silver crystallites is shown in Figure 8. The average lateral and vertical dimensions of the growing islands (30 islands) are shown in Figure 8c and d. Unlike silver growth from 1:2 ChCl/ethylene glycol, there is only one kinetic regime observed indicating the

absence of the coalescence of the growing nuclei. Moreover the vertical growth rate is slightly greater than the lateral growth rate. Data for the variation in volume, as a function of time, of the growing nuclei (10 nuclei) are presented in Figure 8b. This shows that the induction time for nuclei appearance and the corresponding growth rate is less, compared to 1:2 ChCl/ ethylene glycol, Figure 5b. This effect might be a consequence of the relatively high viscosity of 1:2 ChCl:urea hence the slow mass transport rate.

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CONCLUSIONS

This study has shown for the first time that DHM can be used as a new analytical approach that provides interferometric resolution (nanometer vertical scale) with a short acquisition time. It is a nondestructive and noncontact surface metrology E



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Figure 7. Sequential in situ images of the topography of silver deposited from the 1:2 ChCl/urea at 140 mV from solution containing 100 mM dm−3 AgNO3. The 3D profiles are shown on the lef t with expansion images of individual features identified by the red arrows. The corresponding 2D projections are shown on the right with expanded regions indicated by the red circles.

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technique, which enables the retrieval of a wealth of information about a 3D morphology from a single hologram acquired using a conventional digital camera. DHM has been applied to investigate directly the electro-crystallization of a metal on a substrate in real time (in situ). The technique, demonstrated here, will be generally applicable to the real time study of surface nucleation and growth processes. In addition the technique is much more convenient and less invasive for the study of electrochemical deposition than alterative scanning probe methods. In 1:2 ChCl/Eg, two growth rates are observed, at short times the lateral growth rate is greater than the vertical growth rate while at longer times the vertical growth speeds up and the lateral growth slows down because of the coalescence of the growing islands, which suppress the lateral growth. The microscopic study also removes the need for the analysis of the chronometric transients to extract the kinetic parameters, such as nuclear number density, nucleation rate, etc. This therefore removes the uncertainty about assumptions made for the nucleation and growth rates.

EXPERIMENTAL SECTION

The in situ study of silver deposition was conducted using the Lyncee Tec’s DHM R1000 reflection configured high precision optical profiler based on Digital Holographic Microscopy technology. The silver was deposited potentiostatically on the gold coated quartz crystal using an Autolab PGSTAT20 potentiostat (Ecochemie. Holland) controlled by GPES software. A conventional three electrode configuration consisting of the AT-cut polished (flat mirror) finish quartz crystal with a gold film thickness of 900 Ǻ , deposited in a keyhole shape on both sides with central disc active area of 0.211 cm2 (International Crystal Manufacturing Co., Oklahoma City, USA) as working electrode, platinum flag counter electrode, and silver wire as quasi reference electrode was used. Ag metal was deposited potentiostatically from a 0.1 M solution of AgNO3 in either 2Eg/ChCl or 2urea/ChCl at a typical deposition potential of −100 mV (versus Ag wire) for a typical period of up to 60 s. Representative diffusion controlled currents were measured at 100 μA on a 0.23 cm2 electrode F



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Figure 8. Growth kinetics of the silver crystallites deposited from 1:2 ChCl/urea: (a) selected nuclei for the analysis, (b) the volume dependence of the growing nuclei with time, (c) the mean lateral growth of nuclei versus time, and (d) the mean vertical growth dependence on time.

corresponding to a deposition rate of 1.1 × 10−7 g s−1 of silver.14 The electrode surface for deposition was a gold coated quartz wafer crystal. Each xperiment was duplicated to ensure consistency. Choline chloride (ChCl) (Aldrich, >98%), ethylene glycol (Eg) (Aldrich, >99%), urea (Aldrich, >99%), and silver nitrate (Aldrich, >99%) were used as received without further purification. 1:2 ChCl/EG and 1:2 ChCl/urea were prepared by mixing Choline chloride with ethylene glycol (EG) and urea, respectively, for the two liquids. The deep eutectic mixtures were formed by continuous stirring of two components at 60 °C until a homogeneous, colorless liquid were formed. The gold quartz crystal was place under 50× objective on XYZ stage, and the surface was focused. The silver nitrated solution was fed into the cell using the syringe until the liquid formed the thin layer over the gold coated quartz crystal. The surface is again focused using the control on XYZ stage. A series of “holograms” were obtained after every 100 ms.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like acknowledge the EU for funding under the IONMET programme, (Framework 6 project IONMET, http://www.ionmet.org/).

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REFERENCES

(1) Wolfke, M. Phys. Z. 1920, 21, 495. (2) Gabor, D. Nature (London) 1948, 161, 777. (3) Gabor, D. Proc. Roy. Soc. 1949, 197, 454. (4) Cuche, E.; Bevilacqua, F.; Depeursinge, C. Opt. Lett. 1999, 24, 291−293. (5) Kuhn, J.; Shaffer, E.; Mena, J.; Breton, B.; Parent, J.; Rappaz, B.; Chambon, M.; Emery, Y.; Magistretti, P.; Depeursinge, C.; Marquet, P.; Turcatti, G. Assay Drug Dev. Technol. 2013, 11 (2), 101. (6) Abbott, A. P.; McKenzie, K. J. Phys. Chem. Chem. Phys. 2006, 8, 4265−4279. (7) Endres, F. ChemPhysChem 2002, 3, 144−154. (8) Galinski, M.; Lewandowski, A.; Stepniak, I. Electrochim. Acta 2006, 51, 5567−5580. (9) Buzzeo, M. C.; Evans, R. G.; Compton, R. G. ChemPhysChem 2004, 5, 1106−1120.

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Corresponding Author

*Fax: + 44 (0)116 252 3789. E-mail: [email protected]. G



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(10) Abedin, S. Z. E.; Endres, F. Acc. Chem. Res. 2007, 40, 1106− 1113. (11) Abbott, A. P.; Dalrymple, I.; Endres, F.; Macfarlane, D. R. In Electrodeposition from Ionic Liquids; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2008. (12) Kazeminezhad, I.; Barnes, A. C.; Holbrey, J. D.; Seddon, K. R.; Schwarzacher, W. Appl. Phys. A: Mater. Sci. Process. 2007, 86, 373−375. (13) Abbott, A. P.; El Ttaib, K.; Frisch, G.; Ryder, K. S.; Weston, D. Phys. Chem. Chem. Phys. 2012, 14, 2443. (14) Abbott, A. P.; Nandhra, S.; Postlethwaite, S.; Smith, E. L.; Ryder, K. S. Phys. Chem. Chem. Phys. 2007, 9, 3735. (15) Hyde, M. E.; Jacobs, R.; Compton, R. G. J. Phys. Chem. B 2002, 106, 11075−11080. (16) Abbott, A. P.; Barron, J. C.; Ryder, K. S.; Smith, E. L. Anal. Chem. 2009, 81, 8466. (17) Barron, J. C. PhD Thesis, University of Leicester, Leicester, 2009. (18) Abbott, A. P.; Capper, G.; Davies, D. L.; Rasheed, R. K.; Tambyrajah, V. Chem. Commun. 2003, 70−71. (19) Abbott, A. P.; Boothby, D.; Capper, G.; Davies, D. L.; Rasheed, R. K. J. Am. Chem. Soc. 2004, 126, 9142−9147. (20) Abbott, A. P.; Capper, G.; Davies, D. L.; Rasheed, R. Inorg. Chem. 2004, 43, 3447−3452. (21) Abbott, A. P.; Capper, G.; McKenzie, K. J.; Glidle, A.; Ryder, K. S. Phys. Chem. Chem. Phys. 2006, 8, 4214−4221. (22) Abbott, A. P.; Barron, J. C.; Ryder, K. S. Trans. Inst. Metal Finish. 2009, 87, 201−207. (23) Abbott, A. P.; Capper, G.; Davies, D. L.; Rasheed, R. K.; Archer, J.; John, C. Trans. Inst. Metal Finish. 2004, 82, 14−17. (24) Abbott, A. P.; El Ttaib, K.; Frisch, G.; McKenzie, K. J.; Ryder, K. S. Phys. Chem. Chem. Phys. 2009, 11, 4269−4277. (25) Abbott, A. P.; Ryder, K. S.; Konig, U. Trans. Inst. Metal Finish. 2008, 86, 196−204.

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In Situ Electrochemical Digital Holographic Microscopy; a Study of

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