Electrodeposition of Cobalt-Yttrium Hydroxide/Oxide Nanocomposite

Electrochimica Acta 56 (2011) 5142–5150

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Electrochimica Acta

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Electrodeposition of cobalt–yttrium hydroxide/oxide nanocomposite ﬁlms from particle-free aqueous baths containing chloride salts

K.M. Sivaraman a, O. Ergeneman a, S. Pané a,∗, E. Pellicer b,∗∗, J. Sort c, K. Shou a, S. Surinach˜ b, M.D. Baró b, B.J. Nelson a a Institute of Robotics and Intelligent Systems (IRIS), ETH Zürich, CH-8092 Zürich, Switzerland b Departament de Física, Facultat de Ciències, Universitat Autònoma de Barcelona, E-08193 Bellaterra, Spain c Institució Catalana de Recerca i Estudis Avanc¸ ats (ICREA) and Departament de Física, Universitat Autònoma de Barcelona, E-08193 Bellaterra, Spain article info abstract

Article history: The feasibility of growing nanostructured films composed of cobalt and yttrium hydroxide/oxide phases Received 11 December 2010 by electrodeposition is demonstrated. Particle-free aqueous solutions containing YCl3 and CoCl2 salts and Received in revised form 13 March 2011 glycine were used. The incorporation of yttrium compounds into the cobalt deposit was achieved using Accepted 14 March 2011 pulse deposition (t = 0.1 ms, t = 0.9 ms) and for cathodic pulses higher than −500 mA cm−2. Deposits Available online 22 March 2011 on off obtained were crack-free, typically with 1–5 wt% yttrium, and exhibited morphologies markedly differ- ent from the ones shown by pure cobalt deposits. Moreover, yttrium-rich films (up to 30 wt% Y) could be Keywords: deposited under certain conditions, though incipient cracking developed in this case. X-ray photoelec- Nanocomposite Cobalt tron spectroscopy analyses revealed that Y(OH)3/Y2O3 compounds were present in the films. From the Yttrium hydroxide/oxide structural viewpoint, the composites exhibited a partially amorphous/nanocrystalline character, with Pulse deposition the crystalline fractions originating from the hexagonal-close packed structure of ␣-Co. A refinement of X-ray photoelectron spectroscopy the ␣-Co crystallite size was observed in deposits containing higher weight percentage of yttrium compounds. Nanoindentation tests revealed that hardness increased with the yttrium content. This result can be explained by taking into account both the presence of intrinsically hard oxide phases and the effects promoted by incorporation of yttrium hydroxides/oxides on the ␣-Co matrix (namely, grain-refining and higher concentration of stacking faults). © 2011 Elsevier Ltd. All rights reserved.

1. Introduction surrounding the particles [6]. As a consequence, the anticipated advantageous chemical and/or physical properties of the com- Nanocomposite coatings consisting of ultra-fine ceramic parti- posite coatings are often not obtained. Furthermore, hydration cles (e.g., Al2O3, SiO2, SiC, TiN, AlN) embedded in a metal matrix forces can also hinder particle co-deposition [7]. Suitable surfac- have become the focus of widespread research in recent years tants can improve the stability of the suspension by increasing the due to their superior properties compared to purely metallic films. wettability of suspended particles, while enhancing the electro- Benefits include high specific heat, optical non-linearity, novel static adsorption of the dispersed particles on the cathode surface magnetic properties, enhanced mechanical behavior (large hard- by increasing their positive charge [8]. Nevertheless, there are ness and wear resistance) and good corrosion resistance, amongst also drawbacks associated with the use of surface-active agents. others [1–5]. Surfactants can get adsorbed on the cathode surface leading to Nanocomposite coatings can be produced by various meth- unfavorable changes in the mechanical properties of the electrode- ods, including electrodeposition. Electrodeposited nanocomposite posit, such as high internal residual stress or brittleness [9]. Due coatings are generally obtained by suspending charged ceramic to these reasons, the search for new approaches for synthesizing nanoparticles in the electrolyte and co-depositing them with the heterogeneous, multi-phase deposits with enhanced and tunable metal [1–5]. However, this method suffers from some drawbacks. properties is highly desirable. Nanoparticles can easily agglomerate due to the compressive effect Cathodic electrodeposition of certain ceramic materials from caused by the high ionic strength on the diffuse double layer their metal salts, including metal oxides/hydroxides or complex oxide compounds, has been demonstrated over the last few years [10–14]. In particular, deposition of yttrium hydroxide has received ∗ particular attention because it enhances the corrosion resistance of Corresponding author. Tel.: +41 44 632 33 12. ∗∗ Corresponding author. Tel.: +34 93 581 14 01; fax: +34 93 581 21 55. several metals and alloys such as carbon steel, stainless steel, zinc, E-mail addresses: [email protected] (S. Pané), [email protected] (E. Pellicer). bronze, aluminium and magnesium alloys [15]. Research on Y(OH)3

0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.03.058 K.M. Sivaraman et al. / Electrochimica Acta 56 (2011) 5142–5150 5143 deposition has been primarily motivated by the need to replace Table 1 −3 Cr(VI) based corrosion inhibitors by eco-friendly protective layers, Detailed composition of the employed baths, in mol dm . given that Cr(VI) is extremely toxic and carcinogenic [16]. In addi- Bath YCl3·6H2O CoCl2·6H2O Glycine (C2H5NO2) KCl H3BO3 tion to the corrosion inhibiting performance, electrodeposition of 1 0 0.1 Y(OH)3 has also been a topic of research because it can be con- 2 1 0.05 0.15 1 0.16 verted into Y2O3 (yttria) after an appropriate thermal treatment. 3 1 0.1 Yttria is highly resistant to most chemicals and has a melting point 4 1.5 0.1 of approximately 2400 ◦C. It also exhibits a low vapor pressure, optical transparency in the infrared region, high corrosion and thermal resistance, and high strength and fracture toughness [17–20]. This wherein the [Co(II)] was much smaller than the [Y(III)] (Table 1). KCl was used as a supporting electrolyte and boric acid as a buffering has prompted the use of yttria in a variety of industrial applica- ◦ tions, including optical sensors, cutting tools, protective coatings, agent. The working temperature was 60 C. This temperature was and solid oxide fuel cell (SOFC) interconnects [20–23]. selected on the basis that yttrium chemically behaves as a RE ele- ment. RE metal deposition is generally more favorable above room Cathodic electrodeposition of Y(OH)3 can be performed from either organic (typically, water–alcohol mixtures) or fully aqueous temperature [28]. The pH was kept at 4 for all the electrodeposition solvents. According to the early work by Switzer, the formation of experiments. × an electrogenerated base leads to the precipitation of Y(III) from The working electrode consisted of 5 mm 6 mm silicon chips the electrolyte as a hydroxide film on the surface of the cathode (crystal orientation 1 0 0), on top of which a titanium adhesion [24]. From ecological and health perspectives, fully aqueous elec- layer of 50 nm and a copper seed-layer of 500 nm had been suc- trolytes are certainly more desirable. However, there is a dearth cessively deposited through e-beam evaporation. Evaporation was performed using an E306A coating apparatus controlled by an e- of literature concerning the aqueous electrodeposition of Y(OH)3 Vap CVS-6 system. Before being used for electrodeposition, the films. In [25,26], Y(NO3)3 and YCl3 salts dissolved in water were copper surface was degreased by first dipping it in acetone followed used to cathodically deposit Y(OH)3 thin films and nanospheres. In ◦ by iso-propanol and then stripped of oxide and organic residues both works, Y(OH)3 was thermally treated at 600 C in air to obtain crystalline yttria. by dipping in 10% H2SO4 solution. The three electrodes were con- To date, attempts to incorporate yttrium oxides/hydroxides nected to a Gamry Reference 3000 potentiostat/galvanostat which into metallic electrodeposits have proven difficult. The production was controlled by the Gamry Framework software during gal- of nickel–yttria composite coatings by direct current electroplat- vanostatic and linear sweep voltammetry (LSV) experiments and ing from a Watts nickel plating bath containing variable amounts by the Gamry VFP-600 software during pulsed electrodeposition. −2 of micron-sized yttria particles suspended in the electrolyte was The charge density was kept at 20 C cm across all depositions. explored by McCormack et al. [27]. However, there was little or no Deposits were obtained by galvanostatic pulse plating with a − − −2 yttria inclusion in the deposits for the investigated range of yttria cathodic pulse (CP) (from 50 to 1500 mA cm ) with a duration loadings (1–5 g dm−3). The authors reported only on the effects of ton = 0.1 ms followed by an open circuit potential (OCP) with a caused by the presence of yttria dispersed in the electrolytic bath duration of toff = 0.9 ms (Fig. 1). The data acquisition frequency was on the Ni deposit texture and morphology. Hence, incorporation 10 kHz which provided 10 data points for each cycle. The deposition of a useful amount of yttrium-based compounds into electroplated was carried out under quiescent conditions. metallic films remains a challenge and requires investigation. The morphology of the deposits was observed using Zeiss NVi- The aim of the present work is to demonstrate the feasibility sion 40 and Zeiss Merlin scanning electron microscopes (SEM). of producing yttrium-containing cobalt films by electrodeposi- The chemical composition of the deposits was analyzed by energy tion from aqueous solutions. The novelty of the present synthetic dispersive X-ray spectroscopy (EDX). Data reported are based on approach relies on incorporating yttrium oxides/hydroxides into the averaging of four measurements on three replicas per sam- nanocrystalline Co films from a fully aqueous electrolyte contain- ple. The yttrium content is calculated by taking into account the weight percentages of cobalt and yttrium elements (without con- ing both YCl3 and CoCl2 salts. Glycine was chosen as a complexing agent since it shows high buffering properties and favors the co- sidering the oxygen content) normalized to 100%. For cross-section deposition of iron-group (Fe, Co and Ni) with rare earth (RE) metals SEM images, the films were embedded in epoxy resin and pol- [28]. In order to induce both nanocrystallinity of the Co matrix and ished to a mirror-like appearance using diamond paste. X-ray an effective inclusion of yttrium oxide/hydroxide into the deposits, the pulse plating technique was utilized.

2. Experimental

The electrodeposition was performed in a one-compartment three-electrode cell. A 100 ml of bath was used in the electrochemical cell. Distilled water from a Julabo ED-5 thermostat-circulator system was circulated in the outer jacket of the cell to maintain the required working temperature. A double junction Ag|AgCl reference electrode was used with 3 mol dm−3 KCl inner solution and an outer solution made of 1 mol dm−3 NaCl. An insoluble platinum wire mesh acted as counter electrode. An inert atmosphere was ensured inside the electrochemical cell by maintaining a blanket of nitrogen gas on top of the solution. The level of water in the cell and the pH of the bath were checked at the beginning and end of each deposition. The required acidity was maintained using sodium hydroxide and hydrochloric acid. To study the feasibility of elec- trodepositing yttrium-containing ﬁlms, a set of baths was prepared Fig. 1. Plot of the galvanostatic pulse with a CP of 0.1 ms and an OCP of 0.9 ms. 5144 K.M. Sivaraman et al. / Electrochimica Acta 56 (2011) 5142–5150

Table 2 Y content in the deposits as a function of applied CP for Baths 2–4. Note that the Y content is given taking into account the weight percentages of Co and Y (without considering O), normalized to 100%.

−j (mA cm−2) wt% Y

Bath 2 Bath 3 Bath 4

750 3 2 2 1000 2 3 2 1200 5 3 3 1500 3 2 2

positive potentials and its height decreases, suggesting changes in the diffusion/charge controlling factors. As expected, for a constant Y(III) concentration, a decrease in the concentration of Co(II) (Baths 2 and 3) causes a delay in the onset of deposition and also a decrease in the height of the reduction peak (Fig. 2). These trends suggest Fig. 2. Linear sweep voltammetries of Baths 1–4 recorded under quiescent condi- that addition of Y(III) species to cobalt(II)–glycine solutions causes tions, at a scan rate of 50 mV s−1. changes in the electrolyte solution which ultimately might affect the characteristics of the reduction process. photoelectron spectroscopy (XPS) analyses were carried out on Fig. 3 shows the galvanostatic cathodic pulses for the initial a PHI equipment 5500 Multitechnique using the Al K␣ radiation and the final cycles at each current density. Especially for the final (1486.6 eV), after sputtering the sample surface with Ar ions for cycles, the potentials corresponding to the pulse are clearly differ- 1 min. All spectral positions have been corrected taking C 1s peak entiated by the applied current density. The system reaches more at 284.5 eV. X-ray diffraction (XRD) patterns were obtained with negative potentials as the cathodic pulse is increased. For the last a Philips X’Pert diffractometer using the Cu K␣ radiation in the cycles, the potentials corresponding to the relaxation time seem 35–100◦ 2 range (0.03◦ step size, 10 s holding time). Nanoin- to converge to a similar potential value. The regular pattern of dentation tests were carried out in the load control mode using the CP–OCP cycles suggests that deposition proceeds in a well- a UMIS device from Fischer-Cripps Laboratories equipped with controlled manner. Berkovich pyramid-shaped diamond tip. The load–unload curves were taken on the films’ cross-section applying a maximum force 3.2. Morphology and composition of yttrium-containing cobalt of 20 mN. Hardness (H) values were obtained applying the method deposits of Oliver and Pharr [29] at the beginning of the unloading curve. Data reported corresponds to the averaging of 20 indentations per Table 2 shows the composition of the deposits obtained from sample. Baths 2 to 4. Yttrium content varied between 1 and 5 wt%, and Y was detected only in deposits obtained using galvanostatic pulses 3. Results and discussion more negative than −500 mA cm−2. Upon unaided visual inspec- tion, all the films appeared matte dark and well adherent to the 3.1. Linear sweep voltammetry and pulse galvanostatic transients substrate. Most importantly, yttrium incorporation was almost negligible under direct current electroplating conditions. The pulse During the design and development of electrolytic solutions for plating mode allowed the application of very high peak current den- incorporating yttrium into cobalt deposits, considering bath sta- sities, which could be the key factor in enabling Y incorporation. bility was essential. In this context, at 25 ◦C all glycine-containing The results show random variability in the yttrium content, with baths were stable from pH 3 through pH 5. However, at 60 ◦C, no clear tendency observed with respect to both the applied current the baths at pH 5 were not stable. Turbidity was observed but density and the [Y(III)]/[Co(II)] ratio in solution, though maximum was reversible on bringing the pH to 4. Hence, pH 4 was cho- Y incorporation was usually attained at CP of −1200 mA cm−2. sen to synthesize the cobalt electrodeposits containing yttrium Though levels of Y incorporation were not very high, clear oxides/hydroxides. It can be conjectured that, the formation of changes in morphology were observed with respect to pure cobalt insoluble species of Y3+ is promoted at pH 5. Tkachenko et al. [30] deposits (Fig. 4), indicating a clear influence of Y on the morphology point out that, in the presence of boric acid and ammonium hydrox- of the deposits. Yttrium-free cobalt films displayed rounded grains ide, there is a possibility of formation of yttrium orthoborate salts which looked highly facetted at higher magnification (Fig. 4(a) and around pH 5. Therefore, the observed turbidity might be due to the (b)), whereas elongated grains were observed when Y was incor- presence of both boric acid and the ammonium group in glycine. At porated in the deposits (Fig. 4(c) and (e)). The distinctive feature of room temperature, the baths exhibited a pink coloration character- these grains was the presence of ridge-like structures on their sur- istic of octahedral aqua complexes of Co2+. When the temperature face, which gave them a seashell-like appearance (Fig. 4(d) and (f)). of the solution was increased, the bath became violet-blue in color. The width of these ridge-like structures was in the range of a few This can be attributed to the fact that a fraction of the Co(II) ions nanometers (roughly around 10–20 nm). Also, the trends observed 2− form tetrahedral chloride [CoCl4] complexes. in the surface morphology of these deposits were independent of The characteristics of cobalt–yttrium electrodeposition in the the [Co(II)] in their deposition baths. Significantly, as opposed to the presence of glycine at pH 4 were investigated by LSV. On com- deposits mentioned in [31], these deposits were completely free of paring the response of Baths 1, 3 and 4, it can be seen that, for a micro-cracks. This can also be attributed to the use of the pulse given Co(II) concentration, the onset of deposition remains almost plating technique [32]. For a given bath, the grains became slightly the same when Y(III) is added to the bath (cf. Baths 1 and 3), but finer with an increase in the applied current density. On the other slightly shifts towards more positive potentials when Y(III) concen- hand, an increase of Y(III) concentration in solution (Bath 4) made tration is further increased (Bath 4) (Fig. 2). Simultaneously, the the grains evolve to more rounded shapes, but they still displayed diffusion-controlled reduction peak is also shifted towards more characteristic ridge-like features on their surface as in the deposits K.M. Sivaraman et al. / Electrochimica Acta 56 (2011) 5142–5150 5145

Fig. 3. Galvanostatic cathodic pulses for the initial and ﬁnal cycles for Bath 3 ([Co(II)] = 0.1 mol dm−3, [Y(III)] = 1 mol dm−3) and quiescent conditions. prepared from Bath 3. The pure cobalt deposits were always free of from a given bath, typically after depositing 10–15 ﬁlms, and for such ridge-like features. CPs more negative than j = −750 mA cm−2. The mechanism behind Yttrium-rich deposits (with Y percentages larger than 5 wt% on the growth of such yttrium-richer deposits is not yet fully under- average) could be obtained after a number of successive depositions stood. Since the depletion of Co(II) ions in solution could intuitively

Fig. 4. SEM images (secondary electrons) of deposits: (a and b) from Bath 1 ([Co(II)] = 0.1 mol dm−3, [Y(III)]=0moldm−3)atj = −1500 mA cm−2; (c and d) from Bath 3 ([Co(II)] = 0.1 mol dm−3, [Y(III)]=1moldm−3)atj = −750 mA cm−2; (e and f) from Bath 3 at j = −1500 mA cm−2. 5146 K.M. Sivaraman et al. / Electrochimica Acta 56 (2011) 5142–5150

Fig. 5. SEM images (secondary electrons) of yttrium-rich films: (a and b) 13 wt% Y deposit obtained from Bath 2 ([Co(II)] = 0.05 mol dm−3, [Y(III)] = 1 mol dm−3)at j = −1500 mA cm−2; (c and d) 19 wt% Y deposit obtained from Bath 3 ([Co(II)] = 0.1 mol dm−3, [Y(III)] = 1 mol dm−3)atj = −1000 mA cm−2; (e and f) 30 wt% Y deposit obtained from Bath 3 at j = −1200 mA cm−2. explain such enrichment, freshly prepared baths with a lower CoCl2 percentage (Fig. 5(a), (c) and (e)). Simultaneously, incipient micro- concentration were tested. However, no clear enrichment in Y was cracking appeared, which can be attributed to the stress caused observed. Thus, it is likely that concurrence of several factors during by high levels of yttrium inclusion. Such a change in morphology successive depositions (e.g., changes in the distribution of chloro towards hemispherical grains was also observed in nickel elec- and glycine–metal complexes, local pH variations, etc.) takes place, trodeposits obtained in the presence of yttria particles suspended which ultimately lead to the formation of Y-rich deposits. Despite in the electrolyte [27]. When the films were imaged at higher mag- the strong buffering properties of glycine and boric acid, the bulk nification, cauliflower-like features were observed for intermediate electrolyte became slightly acidic (pH around 3.5) after each depo- yttrium percentages (Fig. 5(b)) while the nodules found for larger sition. This decrease in pH can be correlated to an increase in the yttrium contents consisted of nanometric rod-like subgrains (see concentration of protons that are generated at the cathode as a Fig. 5(d) and (f)). replacement for the metal ions that get reduced. On the other hand, As expected, the measured thickness of the deposits was lesser local pH changes near the cathode surface are likely to take place than the theoretical value due to the intense evolution of hydrogen due to the electrogeneration of hydroxide ions. Such local changes gas during the deposition. Evolution of hydrogen gas bubbles was of pH could affect the morphology and chemical composition of noticed during the deposition process, but the bubbles did not stick deposits as well. Despite this, there was no turbidity or precipita- to the substrate. In all cases, the deposit thickness was rather regu- tion observed at the end of deposition, nor was there any change in lar across the substrate (Fig. 6(a)). A careful examination of their the color of the solution. Fig. 5 shows selected SEM images of these cross-sections revealed the existence of differences in chemical yttrium-rich deposits. At first glance, it is clear that the morphol- composition as detected qualitatively by back-scattered electron ogy of these deposits differs from the ones previously described. (BSE) imaging. Fig. 6(b) shows a typical secondary-electron SEM Namely, nodular grains developed with an increase in yttrium image (right) and its corresponding BSE image (left) of a zoomed K.M. Sivaraman et al. / Electrochimica Acta 56 (2011) 5142–5150 5147

Fig. 6. (a) Cross-sectional SEM image of a 5 ␮m thick deposit with 13 wt% Y (Bath 2, j = −1500 mA cm−2). (b) Backscattered-electrons (BSE) (left) and secondary-electrons SEM (right) images of a zoomed area. (c) Detail of two nanoindentations on deposit’s cross-section. The profile of one of the indentations is indicated with a dotted line. region. Notice that the BSE image is not affected by topological fea- Table 3 tures (e.g., changes in surface flatness due to polishing scratches). Yttrium and cobalt weight percentages (normalized to 100%) determined by EDX- spot analyses at the regions indicated in Fig. 7(a). Hence, the phase contrast is linked to variations in composition. In order to gain a better understanding on such differences, EDX-spot Region wt% Co wt% Y analyses were carried out (see Fig. 7(a) and Table 3). It was observed 1973 that yttrium is distributed across the entire thickness of the films. 28713 Interestingly, the regions enriched in yttrium coincide with deple- 38119 4946 tion in cobalt content. Conversely, larger cobalt contents coincide with a decrease in yttrium percentage. If only an alloy or a mixed cobalt–yttrium oxide had formed throughout the film, then homo- geneous distribution of Co, Y (and O) elements would have been

Fig. 7. (a) Cross-sectional secondary-electrons SEM image of a deposit with 13 wt% Y (Bath 2, j = −1500 mA cm−2). EDX mappings taken on this area for (b) Co K␣1, (c) Y L␣1 and (d) O K energies. Cobalt- and yttrium-rich zones are indicated by green and red arrows, respectively. Notice that yttrium-enriched zones coincide with depletion of cobalt. Conversely, yttrium-depleted regions coincide with cobalt enrichment. (For interpretation of the references to color in this ﬁgure caption, the reader is referred to the web version of the article.) 5148 K.M. Sivaraman et al. / Electrochimica Acta 56 (2011) 5142–5150

Fig. 8. XPS survey spectrum of an yttrium-containing Co deposit obtained at CP of j = −750 mA cm−2 from Bath 3 ([Co(II)] = 0.1 mol dm−3, [Y(III)] = 1 mol dm−3). observed. Complementary EDX mappings confirmed this observa- tion (see Fig. 7(b)–(d)). The oxygen content as determined by EDX was found to be significant and increased in yttrium-rich films.

3.3. XPS analyses

Fig. 8 shows a typical XPS survey spectrum of a deposit containing low amounts of yttrium, featuring the characteristic peaks of cobalt and yttrium elements. Copper and carbon peaks, belonging to the copper seed-layer and surface contaminants, respectively, were also identified. Oxygen was also detected, in agreement with EDX analyses, suggesting that either oxide or hydroxide groups were present in the deposits. The corresponding core-level Y3d, Co2p and O1s spectra are displayed in Fig. 9. The Co2p spectrum shows a complex structure broadened by multiplet splitting effects, the sharpest peaks at 778.5 and 793.8 eV being characteristic of metallic cobalt [33,34]. On the other hand, less intense and broadened peaks denoted by an asterisk are attributed to Co2+ compounds. According to the literature, the Co2+ binding energies in CoOOH and Co(OH)2 are about ∼780 and ∼781 eV, respectively [35]. Likewise, CoO displays characteristic reflections at around 781 and 797 eV [36]. Hence, it can be concluded that metallic cobalt is accompanied by small amounts of oxidized cobalt. This is indeed corroborated on analyzing the O1s signal. Namely, the dominant feature in the O1s spectrum at around 532.4 eV can be assigned to structure water, which is commonly detected in electrodeposited oxides and hydroxides [37]. Also, the shoulder located at lower binding energies can be deconvoluted, according to the literature, into the contributions of hydroxide OH− (531.5 eV) and lattice oxide O2− ions (530 eV). Concerning the Y3d spectrum, the two peaks centered at 158.6 eV and 156.7 eV match with the binding energies expected for Y–OH and Y–O bindings [38,39]. This leads to the hypothesis that yttrium incorporation into cobalt films chiefly proceeds via hydrolysis of Y(III) species in solution by an electrogenerated base such as OH− (Eq. (1)) [26]. As a result, colloidal Y(OH)3 forms (Eq. (2)), which tend to accumulate at the cathode vicinity and become entrapped into the growing cobalt deposit during electrodeposition, rendering a nanocomposite structure (Eq. (3)): Fig. 9. Co2p, O1s and Y3d spectra of same deposit in Fig. 8. Note that the symbol * 2+ − − in cobalt spectrum indicates weak peaks corresponding to Co compounds. 2H2O + 2e → 2OH (aq) + H2(g) (1)

3+ − Y (aq) + 3OH (aq) → Y(OH)3(ads) (2) K.M. Sivaraman et al. / Electrochimica Acta 56 (2011) 5142–5150 5149

Table 5 Hardness values, evaluated by nanoindentation on the cross-section of various deposits, as a function of yttrium content.

wt% Y H (GPa) (±0.2 GPa)

0 1.9 3 2.2 5 2.6 13 2.7 19 3.4 30 2.2

ages. This suggests that there is no formation of Co–Y alloys during film growth. Table 4 lists the full width at half maximum (FWHM) of (1 0 0), (1 0 1) and (1 1 0) hcp reflections for a series of deposits with progressively larger Y content. The FWHM values from films containing low amounts of yttrium do not vary significantly from those Fig. 10. XRD spectra of deposits with 0 wt% (black curve), 3 wt% (dark grey curve) of pure Co films. However, in films with a relatively high Y content, and 13 wt% (light grey curve) of yttrium. The Miller indices corresponding to the hcp the (1 0 0) and (1 0 1) peaks clearly broaden, suggesting that the planes of the ␣-Co phase are indicated. Sharp reflections belong to the Cu seed-layer. incorporation of yttrium hydroxide/oxide into the films promotes grain-refining of the ␣-Co phase. The crystallite size estimated by 2+ − applying the Scherrer’s formula on the (1 0 0) peak width gives val- Co (aq) + Y(OH)3(ads) + 2e → Co(s) + Y(OH)3(s) (3) ues of ∼13 nm for almost pure cobalt films and of ∼9 nm for films The detection of Y–O binding also suggests the deposition of with the highest yttrium percentages. Most importantly, the (1 0 1) some Y2O3. The concurrent presence of glycine and Co(II) ions in reflection becomes significantly wider in comparison with (1 0 0) solution could make the deposition of Y2O3 feasible. In fact, samar- and (1 1 0) reflections, which indicates a higher density of stacking ium oxides were also detected in cobalt–samarium electrodeposits faults [41]. This is evidenced in the ratio between the peaks widths obtained from chloride salts in glycine-containing electrolytes [40]. given in Table 4. Hence, the effect of yttrium hydroxide/oxide incor- Similar XPS results were obtained for deposits containing larger poration into cobalt deposits is twofold: it induces crystallite size amounts of yttrium. For these films, the higher oxygen signal refinement of the hcp phase, and it increases the amount of stacking detected by XPS can be explained on the basis of larger amounts faults. Overall, since the background broadens with yttrium incor- of Y(OH)3/Y2O3 being incorporated into the composite. The darker poration, it can be concluded that the resulting composites exhibit a regions in the BSE image of Fig. 6(b) probably correspond to mixed nanocrystalline/amorphous character. The growth of amor- Y(OH)3/Y2O3 compounds embedded in the metallic cobalt matrix, phous Y(OH)3 and Y2O3 films by physical/chemical methods is well which appears brighter. documented [26,42]. However, the direct growth of metal–metal oxide or metal–metal hydroxide composite films from particle-free 3.4. Structure of the deposits electrolytes has not yet been reported.

The structure of the deposits was analyzed by XRD. Peaks were 3.5. Mechanical properties rather broad, indicating the nanocrystalline character of the films (Fig. 10). The main reflections could be indexed on the basis of The hardness (H) of the nanocomposites was evaluated by a hexagonal close-packed (hcp) unit cell (␣-Co phase) indepen- nanoidentation. It is known that rough surfaces tend to increase dent of the baths used. Small amounts of CoO and Co(OH)2 were the scatter in the measured hardness, leading to underestimation also detected. These compounds would be likely formed during [43]. For this reason, given the rough finish of the surface of the the electrodeposition process. Moreover, the surface of cobalt is deposits, analyses were made on the cross-section of the deposits prone to oxidation in air, leading to the formation of a native oxide after proper mechanical polishing (Fig. 6(a)). Fig. 6(c) shows a detail layer as well. In addition, the broad background between 35◦ and of two nanoidentations. Table 5 lists the H values as a function of 55◦ 2␪ range suggests the presence of amorphous material. In fact, yttrium content in the films. Note that in Fig. 6 the lateral size of the electrodeposited Y(OH)3 from chloride baths was reported to be indent impressions is about 2.5–3 ␮m. Since the variations in com- fully amorphous [26]. The angular positions of the ␣-Co XRD peaks position within the films are much smaller in size (see Fig. 7), each did not vary as a function of the Y content, indicating that no Y indentation covers both cobalt and yttrium hydroxide/oxide phases was incorporated as a solid solution within the hcp Co structure. and are thus, representative of the mechanical strength of the entire Furthermore, no additional XRD peaks appeared for large Y percent- film. It can be observed that H increases with the incorporation of

Table 4 FWHM values of the (1 0 0), (1 0 1) and (1 1 0) hcp reﬂections and their ratios as a function of the yttrium content in the deposits.

Bath −j (mA cm−2) wt% Y FWHM (◦) Ratio of XRD peak widths

FWHM(1 0 0) FWHM(1 1 0) (100) (101) (110) FWHM(1 0 1) FWHM(1 0 1) 1 1500 0 0.48 0.97 0.85 0.495 0.876 500 1 0.63 1.03 1.28 0.612 1.243 750 2 0.64 0.98 1.27 0.703 1.296 2 1000 3 0.53 1.02 1.20 0.519 1.176 1500 13a 0.69 1.43 1.07 0.482 0.748 1000 19a 0.71 1.62 1.08 0.438 0.666 3 1200 30a 0.88 2.01 1.34 0.438 0.666

a Y-rich deposits. 5150 K.M. Sivaraman et al. / Electrochimica Acta 56 (2011) 5142–5150 yttrium oxide/hydroxide. In particular, H increases from 1.9 GPa (in ing from the Generalitat de Catalunya through the 2009-SGR-1292 films without Y incorporation) to 3.4 GPa in films containing 19 wt% project and from the Spanish Ministry of Science and Innova- Y. This could be due, at least in part, to the intrinsically large hard- tion (MICINN) through MAT2007-61629. The authors sincerely ness of yttrium oxide and the mechanical interactions occurring acknowledge the staff from the Servei de Microscòpia of the Uni- between the yttrium hydroxide/oxide and hcp-Co phase during the versitat Autònoma de Barcelona for their assistance with SEM course of nanoindentation experiments. The Vickers hardness of characterization. E.P. is indebted to the Generalitat de Catalunya pure Co is around 1 GPa, whereas that of Y2O3 exceeds 9 GPa [21]. for the Beatriu de Pinós postdoctoral fellowship. M.D.B. was par- Therefore, it is likely that the incorporation of Y(OH)3 and Y2O3 tially supported by an ICREA Academia award. S.P. acknowledges a causes an increase of mechanical strength. In addition, such an postdoctoral fellowship from MICINN. increase in hardness can be explained by taking into account both the grain refining effect and the higher density of stacking faults References caused by yttrium hydroxide/oxide incorporation into the films. In crystallites with small sizes, the motion of dislocations is hin- [1] A. Hovestad, L.J.J. Janssen, J. Appl. Electrochem. 25 (1995) 519. [2] M. Musiani, Electrochim. Acta 45 (2000) 3397. dered to a large extent by grain boundaries. This leads to piling up [3] L. Benea, P.L. Bonora, A. Borello, S. Martelli, F. Wenger, P. Ponthiaux, J. Galland, of dislocations, and, consequently to an increase in stress concen- J. Electrochem. Soc. 148 (2001) C461. tration and hardness. Apart from the presence of grain boundaries, [4] S. Arai, T. 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