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An in Situ Synchrotron-Based Soft X-ray Microscopy Investigation of Ni Electrodeposition in a Thin-Layer Cell Benedetto Bozzini,*,† Lucia D’Urzo,† Alessandra Gianoncelli,‡ Burkhard Kaulich,‡ Mauro Prasciolu,§ Ivonne Sgura,| Elisabetta Tondo,† and Maya Kiskinova‡ Brindisi Fuel Cell Durability Laboratory, UniVersita` del Salento, S.S. 7 Brindisi-Taranto km 7 + 300, 72100 Brindisi, Italy, Sincrotrone Trieste S.C.p.A., ELETTRA, s.s. 14 km 163.5 in Area Science Park, 34012 BasoVizza, Trieste, Italy, CNR-INFM TASC National Laboratory, S.S.14 Km 163.5, Area Science Park, 34012 BasoVizza, Trieste, Italy, and Dipartimento di Matematica, UniVersita` del Salento, Via per Arnesano, 73100 Lecce, Italy ReceiVed: February 19, 2009; ReVised Manuscript ReceiVed: April 15, 2009
X-ray techniques allow one to carry out imaging with nanometric resolution in situ, during electrodeposition processes. In this paper, we describe the pioneering application of soft X-ray microscopy to Ni electrodeposition from ammonium and chloride solutions. Morphological features typical of the relevant electrochemical process in a thin-layer cell were successfully imaged and followed dynamically as a function of the applied electrochemical polarization. In particular, grainy films, dendrites, and blisters were detected and their locations were rationalized in terms of current density distribution. Furthermore, the electrochemical system implemented at the TwinMic beamline has been proved to support in situ spectroscopic work that will be described in a subsequent publication. 1. Introduction Metal electrodeposition in confined geometries is a crucial processing step in several technologies, including: ULSI fabrication (45 nm node and below),1 template electrodeposition,2 catalysis,3 sensors,4,5 biotechnology,6 as well as miscellaneous fabrication issues in nanotechnology.7-9 In fact, in technologies involving the growth of nanometric films or nanoparticle deposition steps, electrodeposition has been recognized as a unique approach, allowing one to handle features exhibiting extreme aspect ratios and to achieve dimensional and morphological control, though at the expense of notable technological efforts and complex chemistries. In particular, nanometric electrodeposited Ni features (among which are growth of ultrathin films,10 fabrication of nanocone arrays,11 nanocatalysts12) are currently regarded as highly interesting for several emerging technologies. A pilot in situ electrochemical X-ray microscopy examination was previously performed at the TwinMic beamline13 on Ag electrodes biased in neutral aqueous solutions of NaCl and (NH4)2SO4.14 Corrosion and electrodeposition morphologies were successfully studied by exploiting the high spatial resolution of the TwinMic end station in the water window region. In this paper, we report the first electrodeposition experiment carried out during in situ soft-X-ray microscopy in a thin-layer cell designed for X-ray transmission work. The chemistries considered heresbased on well-known ammonium and chloride complexes (e.g., ref 15)sgive rise to notable differences in the electrokinetic behavior that can be described after the theory developed in ref 16 and references therein contained, essentially stating that electroreduction of Ni(II) species goes on through * Corresponding author. Phone/Fax: +39-(0)832-297325. E-mail:
[email protected]. † Brindisi Fuel Cell Durability Laboratory, Universita` del Salento. ‡ Sincrotrone Trieste S.C.p.A. § CNR-INFM TASC National Laboratory. | Dipartimento di Matematica, Universita` del Salento.
the one-electron reductive adsorption of Ni(I) complexes, representing the rate-determining step. Differences in the nature and stability of complexes as well as their propensity to interact with OH- give rise to different reduction kinetics and resulting metallic structures. 2. Experimental Section Electrodeposition was carried out by injecting Ni2+ by corrosion of a Ni anode into deaerated ultrapure aqueous solutions of composition 20 mM (NH4)2SO4, and 20 mM NaCl, 1 mM HCl. A similar approach to electrodeposition in thinlayer configurations was proposed in ref 17. In this study, we applied anodic galvanostatic pulses between the Ni and Au film electrodes. The transient cell voltages, corresponding to the electrochemical conditions (electrolyte and current intensity) applied to the cell during Twinmic measurements, were recorded in duplicated cells that were fabricated in the same way as the ones tested at the beamline and are reported in Figure 1 [(A) Cl-, 1 mA, 0.1 s; (B) Cl-, 0.2 mA, 60 s; (C) NH4+ 0.5 mA, 60 s). The overall experimental strategy was to check the response of the cell to different current levels (A vs B) and bath chemistries (B vs C). Since the electrodes are nanometric in thickness and undergo anodic dissolution and cathodic overplating processes, preliminary experiments had to be run, in order to optimize the conditions giving rise to adequate morphological changes without excessive damage to the anode and with deposit thicknesses below the level that would make the cathodes opaque to X-rays. As a result of this work, the experimental conditions A, B, and C, implemented in our in situ experiments, were identified. Higher current pulses were applied for shorter times (A vs B and C) and higher current was applied to the less reactive, NH4+-based system (B vs C). Mass-transport dominates the cell kinetics at higher currents (Figure 1A), while nucleation phenomena control the cell voltage transient at lower currents (Figure 1B).
10.1021/jp901528g CCC: $40.75 2009 American Chemical Society Published on Web 05/06/2009
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Figure 1. Cell voltage transient: (A) 20 mM NaCl, 1 mM HCl, 1 mA, 0.1 s; (B) 20 mM NaCl, 1 mM HCl, 0.2 mA, 60 s; (C) 20 mM (NH4)2SO4, 0.5 mA, 60 s.
The cell geometry and the arrangement of the film electrodes is depicted in Figure 2. In this study, we employed a secondgeneration cell, based on the design proposed in ref 14 but improved for better definition of the electrolyte volume, current density distribution, and vacuum sealing. The electrochemical cell was fabricated at the TwinMic beamline of the ELETTRA synchrotron light source in collaboration with the CNR-INFM TASC Laboratory. Parts A (top view) and C (lower part, crosssection) of Figure 2 show the supported electrode assembly, consisting of a Si3N4 optical window onto which two square, 40 nm thick Au and 75 nm thick Ni electrodes have been evaporated. On top of the optical window-electrode assembly a 500 nm thick resist layer has been deposited, suitably patterned and developed, in order to define the volume of the electrolyte reservoir. The Si frame, highlighted in Figure 2B, provides mechanical resistance of the cell body and support to the Si3N4 membrane. In Figure 2C,D we show the cross-section of the whole cell, composed by the above-described electrode assembly and by the cell cover, consisting of the second optical window supported by the Si frame. The process-flow for the fabrication of the electrodes comprised the following steps: (i) chemical vapor deposition of a 100 nm thick Si3N4 layer on both sides
Bozzini et al. of a bare Si(111) double-polished wafer; (ii) opening of the optical window on the back side of the wafer by selective etching of the Si3N4 layer through a photoresist mask, removal of the mask by hot acetone, followed by wet etching in hot KOH of the Si down to the nitride layer on the opposite side, and thorough rinsing in deionized water; (iii) integration of two metal electrodes by properly masking the top side of the wafer and consecutive deposition of Au and Ni; and (iv) spinning, patterning, and development of the resist frame, designed to offer electrolyte containment and to provide part of the sealing action. The X-ray beam crosses the electrode/electrolyte assembly normally to the optical windows, and the transmitted X-rays are monitored by the detector placed behind the cell. The experiments were carried out at the TwinMic beamline in Elettra synchrotron facility (Trieste, Italy; www.elettra. trieste.it). TwinMic is a European twin X-ray microscopy station operating in the 280-2200 keV energy range. It is a multipurpose end station that combines the advantages of full-field imaging and those of scanning transmission X-ray microscopy (STXM), with easy switching between the two modes. By using zone plate diffractive optics, the X-ray beam can be focused down to sub-100 nm spot size. A photon energy resolution better than 0.1 eV is available over the entire photon energy range. Recently, a low-energy X-ray fluorescence system for elemental mapping has also been implemented in TwinMic18 coupled with the scanning transmission mode of the microscope. During the experiments, TwinMic was operated in STXM mode in the 712-865 eV energy range, across the Ni L-edge. In this particular work, we did not make special use of edgecontrast, apart from image optimization purposes, but in a series of papers in preparation, we shall report on pre- vs postedge imaging as a tool for the unambiguous identification of the electrochemically transformed metals. The specimen was rasterscanned across the X-ray probe, provided by an Au zone plate of 250 µm in diameter with 80 nm outer zone. The size of the X-ray probe has been adapted between 200 nm and 1 µm, according to the dimension of the features of interest. The absorption and phase contrast images were recorded on a IXON Andor-Technology EMCCD camera able to collect absorption and phase information simultaneously.19 A maximum dwell time of 100 ms was used during the experiments.
Figure 2. Schematic view of the electrochemical cell. Figure details are not to scale.
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Figure 3. Au electrode in NH4+-containing electrolyte. The original position of the Au film is marked in red. The stripe-shaped feature at the right of the Au electrode is a minor lithographic flaw. (A) At open-circuit potential, absorption mode. (B) After cathodic polarization at -0.5 mA for 60 s, in phase contrast mode. (C1 and C2) After cathodic polarization at -0.5 mA for 60 s, in phase contrast mode. (A and B) 759 eV, 100 µm × 100 µm, 100 × 100 pixel, 100 ms dwell time. (C1) 865 eV, 100 × 100 pixel, 75 ms dwell time. (C2) location close to C1, 20 µm × 20 µm.
3. Results and Discussion 3.1. Ammonium-Containing Electrolyte. Before the application of electrochemical polarization, the Au film electrode appears featureless (Figure 3A). After application of a galvanostatic pulse of 0.5 mA for 60 s, the morphology of the Au electrode exhibits major changes, as shown in Figure 3B,C. In Figure 3B (for the location of the imaged areas, see Figure 3D), the same area is imaged as in Figure 3A; the dark spots corresponding to electrodeposited Ni are found scattered over the whole imaged area, and the electrode seems to have grown from the original corner position, even though a faint trace of the initial Au electrode shape is retained. In Figure 3C, the edge is shown of the Au electrode, close to the resist frame. Ni dendrites can be noticed (see position R in Figure 3C1), projecting from the electrode side toward the Ni electrode, together with some Ni islands on top of the electrode (see position β in Figure 3C1). In order or rationalize the observed spatial distribution of electrochemically induced morphologies, the 3D primary current density distribution (cdd) of the thin-layer cell has been computed by the COMSOL multiphysics solver, based on the finite-element method. The robustness of the computational results with respect to meshing and the choice of
the linear integrator have been cross checked with state-ofthe-art numerical procedures and adaptive grid techniques.20 Since the domain of the mathematical model exhibits very different scales in the electrode plane with respect to the electrolyte thickness, in order to achieve a high computational accuracy, model adimesionalization proved necessary. Of course, the real cell dimensions are accounted for as coefficients of the differential equations considered; for ease of visualization, the normalized geometry and solution are shown in Figure 4. The dendritic features shown in Figure 3C are coherent with the computed current density concentration at the top of the electrode, showing that a concentration of current density lines develops close to the resist frame plane joining the two electrodes that does not propagate into the free electrolyte, close to the free edges of the electrodes (see Figure 4). 3.2. Chloride-Containing Electrolyte. The electrochemical conditions employed in this series of experiments were galvanostatic polarizations at 1 mA, 0.1 s, and 0.2 mA, 60 s (indicated as P1 and P2, respectively). We carried out imaging work in three locations, corresponding to the tip of Au cathode (location A; see Figure 5D) and the sides facing the
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Figure 4. (A) Numerical estimate of the primary current density 3D distribution in the thin-layer cell, showing a concentration of current density lines close to the resist layer. The integration results are shown in the normalized geometry adopted for numerical computation, as detailed in the text. (B) Detail of the 2D section of the 3D current density distribution computed at the resist plane in contact with the two electrodes.
Figure 5. Au electrode in Cl--containing electrolyte, 859 eV, 100 × 100 pixels, 100 ms dwell time. (A) 100 µm × 100 µm, in phase contrast mode: (A1) open-circuit potential, (A2) after application of the first cathodic pulse P1 ) -1 mA, 0.1 s; (A3) after application of a second cathodic pulse P2 ) -40 mA, 60 s. (B) 30 µm × 30 µm, in phase contrast mode, after P1. Image imperfections are due to temporary synchrotron beam instabilities. (C) 100 µm × 100 µm, absorption mode: (C1) after P1, (C2) after P2. (D) The locations of the Au electrode where imaging work has been reported.
Ni anode (location B) and the free electrolyte reservoir (location C). An image of location A of the Au electrode at open-circuit potential before application of polarization is reported in Figure 5A1. Some imperfections, due to minor lithography defects, can be noticed at the edge of the Au film. A micrograph at the same position after application of cathodic pulse P1 is shown in Figure 5A2. A continuous Ni layer is plated over the Au electrode and a typical exploded hydrogen blister can be noticed. Figure 5A3, measured after the successive pulse P2, exhibits overplating of the exploded blister, consistent with a lower current density and longer plating time. In Figure 5B, recorded in location B after application of cathodic pulse P1, one can notice the humpy morphology that is typical for electrodeposited Ni: multimodal dome-shaped noduli are found, together with isolated larger crystallites and
Bozzini et al. an ellipsoidal blister. The imperfections of the images are due to temporary instability of the synchrotron beam, but they do not impact the electrochemical message. This is coherent with the current density concentration at the top of the electrode, close to the resist frame, found by numerical computations (see Figure 4) also on the side of the electrode facing the free electrolyte (location R in Figure 4). In Figure 5C, corresponding to location C, a Ni island is deposited, attached to the Au electrode, and projecting into the electrolyte, close to the resist border. This island is found to grow after the application of the two successive cathodic pulses (Figure 5C1 after pulse P1 and Figure 5C2 after pulse P2). This growth form is found in location C owing to the higher local current density due to the proximity of the resist frame (see Figure 4). Conclusions In this paper, we report an investigation carried out by synchrotron-based in situ soft X-ray microscopy of the electrodeposition of Ni onto an Au film of nanometric thickness, in a thin-layer configuration, from aqueous solutions containing ammonium or chloride. Electrodeposition of Ni from the ammonium-containing solution is characterized by the formation of homogeneous films and dendrites in different locations of the cathode, as a function of cdd and local concentration of [Ni(NH3)6]2+. In the Cl--containing solution, the electrodeposition of Ni onto the Au electrode, gives rise, as a function of local cdd, to the formation of (i) humps and blisters and (ii) islands progressively growing at the Au/electrolyte interface. The possibility of carrying out microscopic work with nanometric resolution during metal electrodeposition and corrosion processes provides the electrochemical community with a tool of unprecedented power for the assessment of morphology development in situ. This paper demonstrates that soft X-ray microscopy is a method of choice for the dynamic, high-resolution, in situ imaging of electrochemical processes. Our approach exploits the unique capability exhibited by X-ray spectromicroscopy of combining morphological and chemical information, to obtain both types of information simultaneously, in liquid environment and under electrochemical control. Our group has already successfully carried out experiments in the spectroscopic mode that shall be published very soon. References and Notes (1) An, S.-H.; Lim, T.-H.; Kim, Y.-H.; Bae, S.-E.; Yoon, J.-H.; Lee, C.-W. Colloids Surf. A 2008, 313-314, 339–342. (2) Liu, L.; Zhou, W.; Xie, S.; Song, L.; Luo, Sh.; Shen, J.; Zhang, Z.; Xiang, Y.; Ma, W.; Ren, Y.; Wang, Ch.; Wang., G. J. Phys. Chem. C 2008, 112, 2256–2261. (3) Yu, P.; Yan, J.; Zhang, J.; Mao, L. Electrochem. Commun. 2007, 9, 1139–1144. (4) Du, D.; Chen, Sh.; Cai, J.; Tao, Y.; Tu, H.; Zhang, A. Electrochim. Acta 2008, 53, 6589–6595. (5) Zou, Y.; Xiang, C.; Sun, L.-X.; Xu, F. Biosens. Bioelectron. 2008, 23, 1010–1016. (6) Rath, S.; Sarangi, S. N.; Sahu, S. N. Nanotechnology 2008, 19, 115606. (7) Yang, Y. L.; Wang, Y. D.; Ren, Y.; He, C. S.; Deng, J. N.; Nan, J.; Chen, J. G.; Zuo, L. Mater. Lett. 2008, 62, 47–50. (8) Zong, Z.; Yu, H.; Nui, L.; Zhang, M.; Wang, C.; Li, W.; Men, Y.; Yao, B.; Zou, G. Nanotechnology 2008, 19, 315302. (9) Huang, X.; Zhu, Y.; Dou, X.; Li, G. Mater. Lett. 2008, 62, 249– 251. (10) Allongue, P.; Cagnon, L.; Gomes, C.; Gu¨ndel, A.; Costa, V. Surf. Sci. 2004, 557, 41–56. (11) Tao, H.; Li, M.; Fei, Q.; Mao, D. Nanotechnology 2008, 19, 135201. (12) Hung, K.-H.; Tzeng, Sh.-Sh.; Kuo, W.-Sh.; Wei, B.; Ko, Ts.-H. Nanotechnology 2008, 19, 295602.
Soft X-ray Microscopy during Ni Electrodeposition (13) Kaulich, B.; Bacescu, D.; Susini, J.; David, C.; Di Fabrizio, E.; Morrison, G. R.; Charalambous, P.; Thieme, J.; Wilhein, T.; Kovac, J.; Cocco, D.; Salome, M.; Dhez, O.; Weitkamp, T.; Cabrini, S.; Cojoc, D.; Gianoncelli, A.; Vogt, U.; Podnar, M.; Zangrando, M.; Zacchigna, M.; Kiskinova, M. Proc. 8th Int. Conf. X-ray Microsc. IPAP Conf. Ser. 2006, 7, 22–25. (14) Bozzini, B.; D’Urzo, L.; Gianoncelli, A.; Kaulich, B.; Kiskinova, M.; Prasciolu, M.; Tadjeddine, A. Electrochem. Commun. 2008, 10, 1680– 1683. (15) Cotton, F. A.; G. Wilkinson G. AdVanced Inorganic Chemistry; Interscience Publishers: New York, 1962; p 732. (16) Epelboin, I.; Jousselin, M.; Wiart, R. J. Electroanal. Chem. 1981, 199, 61.
J. Phys. Chem. C, Vol. 113, No. 22, 2009 9787 (17) Kolb, D. M. AdVances in Electrochemistry and Electrochemical Engineering; Gerischer, H., Tobias, C. W., Eds.; Wiley & Sons, New York, 1978; Vol. 11, p 125. (18) Alberti, R.; Klatka, T.; Longoni, A.; Bacescu, D.; Marcello, A.; De Marco, A.; Gianoncelli, A.; Kaulich, B. X-ray Spectrom.In press. (19) Morrison, G. R.; Gianoncelli, A.; Kaulich, B.; Bacescu, D.; Kovac, J.; Aoki, S.; Kagoshima, Y.; Suzuki, Y. Proc. 8th Int. Conf. X-ray Microsc. IPAP Conf. Ser. 2006, 7, 377–379. (20) COMSOL Multiphysics, V. 3.5, Modeling Guide; COSMOL: Burlington, MA, 2008.
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