Materials Transactions, Vol. 51, No. 10 (2010) pp. 1842 to 1846 #2010 The Japan Institute of Metals
Electrodeposition of Silver-Nickel Thin Films with a Galvanostatic Method Hyeonjin Eom1 , Byungjun Jeon2 , Donguk Kim1 and Bongyoung Yoo1;2; * 1 2
Department of Bio Nano Technology, Hanyang University, Ansan, Korea Department of Materials Science and Materials Engineering, Hanyang University, Ansan, Korea
NiAg thin films were deposited by galvanostatic electrodeposition in an electrolyte containing NiSO4 , AgNO3 and C6 H5 Na3 O7 . The variation of composition and crystallography of electrodeposited NiAg thin films with current density and electrolyte concentration was investigated. At a low current density, electrodeposition of silver was dominant, which could be induced by a comparably low reduction potential. However, nickel electrodeposition became the dominant component at a higher current density because of the mass transfer limitation of Ag ions. When 50 mA/cm2 was applied, the FCC (200) phase was observed, which implies significant enhancement of the nucleation rate by increasing the reduction potential at a high current density condition. [doi:10.2320/matertrans.M2010126] (Received April 12, 2010; Accepted July 7, 2010; Published September 1, 2010) Keywords: thin films, galvanostatic, electroplating, deposition, NiAg
NiAg alloy is a two-component alloy system consisting of magnetic and nonmagnetic metals. Magnetic and nonmagnetic multilayer metal systems have been investigated for giant magnetoresistance (GMR) behavior;1,2) however, there has been little research on the NiAg alloy system because of the alloying characteristics of these metals. Interestingly, the phase diagram3) confirms that the two metals are totally immiscible, and intermetallic phases were not observed at low temperature. Therefore, synthesizing a perfect solid solution of NiAg alloys is difficult. Few investigations on the formation of a Ni-Ag solid solution by laser deposition,4) electron beam co-evaporation,5) or ion mixing6) have been conducted. The electrodeposition technique can produce a NiAg alloy solid solution. Scheider et al. proposed the electrodeposition of NiAg thin film using a potentiostatic method and investigated the microstructure and chemical composition of the deposited layer.7) When the overpotential decreased, the content of nickel in the thin film increased because of the difference in standard reduction potentials. Even though the potentiostatic method is suitable for precisely determining the reduction potential of each metal, it is difficult to control the deposition thickness without coulombic measurement. Therefore, the galvanostatic technique was used to deposit NiAg alloy thin films, which is a more conventional technique to estimate deposition parameters, such as the deposition rate and thickness of thin films. In this work, the effect of processing parameters, such as bath concentration, current density and pH on the composition and crystallography of deposited thin films were investigated. 2.
Experimental
The electrolyte for deposition of NiAg thin film was 0.3 M C6 H5 Na3 O7 , 0.7 M NiSO4 , and (0:002 þ x)M of AgNO3 . The pH value was varied from 4.5 to 7.5 with NaOH and H2 SO4 to investigate the effects of pH on deposition. The variation of thin film composition with the ratio of metal ions was *Corresponding
author, E-mail:
[email protected]
studied by changing the concentration of AgNO3 from 0.002 to 0.005 M. Sodium citrate (C6 H5 Na3 O7 ) was added as a complexer, by which the difference of reduction potential of Ni and Ag was minimized. Electrodeposition was performed galvanostatically, which allowed more precise thickness control with time if the deposition rate could be acquired. A brass plate with a 1 cm2 area was selected for the cathode as a working electrode, and a platinum-coated titanium strip was used as an insoluble counter electrode. The temperature of the electrolyte was maintained at room temperature, and the electrolyte was stirred by a magnetic stirrer during deposition at a constant speed of 200 rpm. The film composition was determined by energy dispersive spectroscopy (EDS) (Horiba EX-250). The film morphologies were investigated by SEM (Hitachi S-4800, 15 kV), and the crystallographic structure was also studied by X-ray diffraction (Rigaku D/MAX2500/PC). 3.
Results and Discussion
The cyclicvoltammograms which were measured in the electrolytes which contain both metal ion without sodium citrate and with sodium citrate are represented on Fig. 1.
0.02
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Introduction
Current density, i / A·cm
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0.01 Ag oxidation(w Cit.) Ni oxidation(w/o Cit.)
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Potential, E / V Ag/AgCl
Fig. 1 Cyclicvoltammograms of NiAg electrolytes with and without sodium citrate. (scan rate = 50 mVs1 , pH ¼ 5:5)
Electrodeposition of Silver-Nickel Thin Films with a Galvanostatic Method
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Fig. 3 The composition of the electrodeposited NiAg thin film with different concentrations of Ag ions in the electrolyte (current density: 10 mA/cm2 , pH ¼ 5:5).
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(b) Fig. 2 (a) Dependence of NiAg thin film composition on applied current density, (b) Dependence of the current efficiency and deposition rate of NiAg thin film on applied current density (the concentration of metal ions: 0.003 M of AgNO3 , 0.7 M of NiSO4 , pH ¼ 5:5).
When sodium citrate did not added in the electrolyte, the reduction peak of silver was clearly detected at E ¼ 600 mV, and the reduction of nickel observed at the more negative potential (E ¼ 820 mV). The oxidations of each metal also detected at 400 mV and 620 mV, respectively. However, when sodium citrate was added as a complexer, such reduction peaks of Ni and Ag was significantly reduced and shifted. Such a variation of reduction peaks with citrate proposed that Ni and Ag ions were effectively complexed with citrate at pH 5.5 which was the pH condition for the electrodeposition of NiAg thin films. The composition variation of NiAg alloy thin films with current density is represented in Fig. 2(a). At a high current density of 50 mA/cm2 , the composition of the deposited film was 97Ni3Ag, indicating the reduced substance was primarily Ni. Such a high content of Ni was caused by the large concentration of Ni ions in the electrolyte. The concentration of Ni ions in the electrolyte was 0.7 M, which was twenty times greater than that of Ag. Even though the standard reduction potential of Ag (Eo ¼ þ0:799 V) was significantly more positive than Ni (Eo ¼ 0:250 V), the current density of 50 mA/cm2 sufficiently induced a high negative potential for the reduction of Ni, as well as Ag. Therefore, the ratio of the concentration of Ni and Ag ions in the electrolyte would be the primary factor for deciding the composition
at such a high current density. When the current density was decreased, the content of Ag was drastically increased after 5 mA/cm2 . When the current density decreased to 0.5 mA/cm2 , 60 at% of Ag was obtained. At a low current density of 0.5 mA/cm2 , the overpotential would be low enough to prevent the reduction of Ni. Therefore the amount of Ni was suppressed to 40 at%. The variation of deposition rate and the current efficiency of NiAg electrodeposition was proposed on Fig. 2(b). The deposition rate was almost increased linearly by increasing the current density. The current efficiency of NiAg electrodeposition was 20% at 1 mA/cm2 , which was because of the reduction of powdery deposits. Some of powdery deposits did not attached on the substrate, but remained in the electrolyte, which did not be considered as deposits on substrate. Therefore, the current efficiency from the current density lower than 1 mA/cm2 was significantly low. When the current density was 50 mA/cm2 , the efficiency was slightly decreased, because of hydrogen evolution at the high current density. Variation of the composition with the concentration of Ag ions in the electrolyte is represented in Fig. 3. The pH and current density were fixed as 5.5 and 10 mA/cm2 respectively. The content of Ag in the electrodeposit did not vary until the concentration of Ag increased from 0.002 to 0.004 M, and it increased to 30 at% when the concentration of Ag ions reached 0.005 M. The pH also affected the composition of the NiAg alloy. As shown at Fig. 4, the content of Ag was less than 10 at% at a pH of 4.0–5.5, whereas it increased to 35–40 at% when the pH was greater than 6.5. Such an incremental change of Ag might be related to the stronger stability of Ni complexing8) with citrate, as compared to the Ag-citrate complex. The stability of the Nicitrate complex was more significant at a higher pH than the weak acid condition. Hedwig et al. proved that a Ni-citrate complex with a larger molar absorptivity was formed at high pH.9) Therefore, the reduction overpotential of Ni would become more negative at higher pH, which leads to an incremental Ag content change in the NiAg thin film. The surface morphologies of NiAg alloys obtained at different current densities are represented in Fig. 5. At a low current density of 0.5 mA/cm2 , nearly 60 at% of Ag was
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H. Eom, B. Jeon, D. Kim and B. Yoo 100
Ni Ag
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Composition (at.%)
80 70 60 50 40 30 20 10 0 3.5
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Fig. 4 The composition of electrodeposited NiAg thin film with different pH electrolytes (current density: 10 mA/cm2 , concentrations: 0.003 M of AgNO3 , 0.7 M of NiSO4 ).
(a)
Needle like deposits
achieved, and dendritic growth was observed. Such a significant dendrite structure might be developed due to an extremely low concentration of Ag in the electrolyte. When the concentration of Ag was 0.003 M, it was low enough to enhance the depletion of Ag ions on the cathode surface and could cause the non-uniform nucleation of Ag even though the current density was comparably low at 0.5 mA/cm2 . A similar result was previously demonstrated wherein silver dendrites were synthesized with a low concentration of silver nitrate solution in the electrolyte, which was caused by the mass transfer limit of silver ions.10) We tried to confirm that the whole deposit obtained by 0.5 mA/cm2 was needle-like structure, or that was the combined structures of needle-like dendritic structure and 2 dimensional thin film structure. To verify this, a part of needle like deposits was removed by cotton swab. The insert image on Fig. 5(a) clearly shows that conformal thin film was located under needle-like structure. The cross sectional SEM image (Fig. 6(a)) also clearly
(b)
NiAg thin film
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(d)
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5um Fig. 5 SEM images of the surface of Ni-Ag thin films electrodeposited from 0.7 M NiSO4 and 0.003 M AgNO3 solutions on: (a) 0.5 (b) 1 (c) 5 (d) 10 and (e) 50 mA/cm2 (temperature: room temperature, pH ¼ 5:5). (inset image on (a) is the low magnification image of (a) after removing a part of needle like deposits)
Electrodeposition of Silver-Nickel Thin Films with a Galvanostatic Method
(a)
(b)
(c)
(d)
1845
(e)
Fig. 6 Cross sectional SEM images of Ni-Ag thin films electrodeposited from 0.7 M NiSO4 and 0.003 M AgNO3 solutions on: (a) 0.5 mA/cm2 , 9000 s (b) 1 mA/cm2 , 4500 s (c) 5 mA/cm2 , 900 s (d) 10 mA/cm2 , 450 s and (e) 50 mA/cm2 , 90 s (temperature: room temperature, pH ¼ 5:5).
shows that thin film structure was existed under needlelike deposits. When the current density was greater than 5 mA/cm2 , the microstructure became granular. Such a disappearance of the dendritic structures at a greater current density strongly indicates that the major component of the deposit changed from Ag to Ni. The Ni concentration was 0.7 M, which was more than twenty times greater than the concentration of Ag ions. As indicated in the SEM images (Figs. 5(d) and (e)), the grain size of the deposits decreased with increasing current density from 10 to 50 mA/cm2 because increasing the reduction overpotential increased the rate of nucleation.11) The rate of nucleation was exponentially proportional to the overpotential (), as given by bs"2 J ¼ k1 exp ð1Þ zekT where b is the factor relating the surface area S of the nucleus to the perimeter P (b ¼ P2 =4S), s is the area occupied by one atom on the surface of the nucleus, and z, e, and " are the number of electrons involved in the reaction, electron charge, and edge energy, respectively. According to this equation, the rate of nucleation could be exponentially increased by cathodic overpotential. Therefore when the stronger potential was applied by higher current density, the faster rate of nucleation introduced the larger number of density of nuclei
on the substrate, which crucially caused the decreasing of grain size. The cross sectional views of NiAg thin films obtained by various current density conditions were represented on Fig. 6. As shown SEM images, well developed thin film structures could be observed, which means that those all deposits had the thin film structures. To applied the same charge for reducing each deposit, the deposition time was varied from 9000 s to 90 s. The thicknesses of thin films obtained by the current density higher than 5 mA/cm2 were similar, but thin films obtained with current density lower than 1 mA/cm2 was significantly thinner. Such a thin layer at the lower current density condition was caused by dendritic growth behavior at the surface. As shown at the surface morphologies on Fig. 5(a) and (b), when current density was lower than 1 mA/cm2 , some part of charges must be consumed to reduce the needle-like structure, and that could not contribute to formation of the conformal thin film. The results of XRD analysis also clearly showed the change in the dominant microstructures with increasing current density (Fig. 7). When the current density was 0.5 mA/cm2 , peaks for Ag FCC (111) and Ni FCC (111) could be identified. As the current density increased to greater than 5 mA/cm2 , the Ag peak completely disappeared, and only Ni (111) and (200) peaks were observed, which
1846
H. Eom, B. Jeon, D. Kim and B. Yoo * : Substrate (brass)
*
*
*Ni FCC (111)
(e)
* Ni FCC (220)
Ni FCC (200)
(d)
a.u.
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(b)
At a low current density, the dendritic structure was dominant; however, the microstructure became granular with increasing current density. The pH value was one of the important parameters that determined the composition of the deposits, as it affected the complexing of the metal ions with citrate. Based on the XRD results, coexistence of Ag and Ni was observed at a low current density condition (0.5 mA/cm2 and 1 mA/cm2 ). When a high current density was applied (50 mA/cm2 ), a nearly developed FCC (220) orientation was observed, which is caused by a significantly high nucleation rate.
Ag FCC (111)
Acknowledgements (a)
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2θ ( ° )
Fig. 7 XRD spectra of Ni-Ag thin films electrodeposited at (a) 0.5 (b) 1 (c) 5 (d) 10 and (e) 50 mA/cm2 (concentrations: 0.003 M of AgNO3 , 0.7 M of NiSO4 , pH ¼ 5:5).
corroborates EDS analysis. Interestingly, a FCC (220) Ni peak emerged when the current density was 50 mA/cm2 . When the current density increased, the reduction overpotential also increased, which exponentially enhanced the nucleation rate.12) At a high current density, such as 50 mA/cm2 , the reduction potential could be strong enough to enhance the nucleation rate, which could cause nucleation at specific preferred orientations and other less favorable sites. 4.
Conclusions
NiAg electrodeposited thin film was obtained from the aqueous bath using the galvanostatic method. Ag60 Ni40 was obtained with a 0.5 mA/cm2 current density, and the Ni content drastically increased with increasing current density.
This research was supported by the research fund of Hanyang university (HY-2008-N) and the National Platform Technology Program from the Ministry of Knowledge Economy in Korea. REFERENCES 1) Y. Wang, L. Zhang, G. Meng, X. Peng, Y. Jin and J. Zhang: J. Phys. Chem. B 106 (2002) 2502–2507. 2) G. Xiao, J. Wang and P. Xiong: Appl. Phys. Lett. 62 (1993) 420–422. 3) M. Singleton and P. Nash: Bull. Alloy Phase Diagrams 8 (1987) 119– 121. 4) M. Sto¨rmer and H. Krebs: J. Appl. Phys. 78 (1995) 7080–7087. 5) H. Zolla and F. Spaepen: IEEE Trans. Magn. 31 (1995) 3814–3816. 6) Z. Li, J. Liu, Z. Li and B. Liu: J. Phys. Condens Matter 12 (2000) 9231– 9235. 7) M. Schneider, A. Krause and M. Ruhnow: J. Mater. Sci. Lett. 21 (2002) 795–797. 8) C. Li, X. Li, Z. Wang and H. Guo: Trans. Nonferrous Met. Soc. China 17 (2007) 1300–1306. 9) G. Hedwig, J. Liddle and R. Reeves: Aust. J. Chem. 33 (1980) 1685– 1693. 10) Q. Zhou, S. Wang, N. Jia, L. Liu, J. Yang and Z. Jiang: Mater. Lett. 60 (2006) 3789–3792. 11) P. Milan and S. Mordechay: Fundamentals of Electrochemical Deposition, 2nd ed., (John Wiley & Sons 2006) p. 115. 12) A. Rashidi and A. Amadeh: Surf. Coat. Technol. 202 (2008) 3772– 3776.
