Electrodeposition of Silver Nickel Thin Films with a Galvanostatic Method

Electrodeposition of Silver-Nickel Thin Films with a Galvanostatic Method

Hyeonjin Eom

, Byungjun Jeon

, Donguk Kim

and Bongyoung Yoo

1_{Department of Bio Nano Technology, Hanyang University, Ansan, Korea}

2_{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 C6H5Na3O7. 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

1. Introduction

NiAg alloy is a two-component alloy system consisting of magnetic and nonmagnetic metals. Magnetic and nonmag-netic 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 mixing}6) _{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 tech-nique was used to deposit NiAg alloy thin films, which is a more conventional technique to estimate deposition param-eters, 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

C6H5Na3O7, 0.7 M NiSO4, and (0:002þx)M of AgNO3. The

pH value was varied from 4.5 to 7.5 with NaOH and H2SO4

to investigate the effects of pH on deposition. The variation of thin film composition with the ratio of metal ions was

studied by changing the concentration of AgNO3from 0.002

to 0.005 M. Sodium citrate (C6H5Na3O7) 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 cm2area 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/MAX-2500/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.

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

-0.02 -0.01 0.00 0.01 0.02

Ni oxidation(w/o Cit.)

Ni oxidation(w Cit.)

Ag oxidation(w/o Cit.) Ag oxidation(w Cit.)

Ni deposition(w/o Cit.)

Current density,

i

/

A·cm

-2

Potential, E / V Ag/AgCl

0.003M of AgNO3, 0.7M of NiSO4

0.003M of AgNO3, 0.7M of NiSO4, 0.3M of sodium citrate

Ni deposition(w Cit.)

Ag deposition(w Cit.)

Ag deposition(w/o Cit.)

Fig. 1 Cyclicvoltammograms of NiAg electrolytes with and without sodium citrate. (scan rate = 50 mVs1_,pH¼5:5)

*_{Corresponding author, E-mail: byyoo@hanyang.ac.kr}

When sodium citrate did not added in the electrolyte, the

reduction peak of silver was clearly detected atE¼600mV,

and the reduction of nickel observed at the more negative

potential (E¼ 820mV). The oxidations of each metal also

detected at 400mV 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 primar-ily 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 (Eo¼ þ0:799V) was significantly

more positive than Ni (Eo¼ 0:250V), 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

respec-tively. 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 to30at% 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 Ni-citrate complex was more significant at a higher pH than the

weak acid condition. Hedwiget 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}

(a)

(b)

0 10 20 30 40 50 60

0 10 20 30 40 50 60 70 80 90 100 Composition (at.%)

Current densityi / mA·cm-2

Ni Ag

0 10 20 30 40 50

0 20 40 60 80 100 Current efficiency Deposition rate

Current density i / mA·cm-2

Current efficiency (%)

0.0 0.2 0.4 0.6 0.8 1.0 1.2 Deposition rate ( µ m · min -1 )

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,pH¼5:5).

0.001 0.002 0.003 0.004 0.005 0.006

0 10 20 30 40 50 60 70 80 90 100 Composition (at.%)

Concentration of Ag source in electrolyte (M) Ni Ag

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

3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 0

10 20 30 40 50 60 70 80 90 100

Composition (at.%)

pH

Ni Ag

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)

(b)

(c)

(d)

(e)

5um

5um 5um

5um 5um

NiAg thin film Needle like deposits

Fig. 5 SEM images of the surface of Ni-Ag thin films electrodeposited from 0.7 M NiSO4and 0.003 M AgNO3solutions on: (a) 0.5 (b) 1

(c) 5 (d) 10 and (e) 50 mA/cm2_{(temperature: room temperature,pH¼5:5). (inset image on (a) is the low magnification image of (a) after}

shows that thin film structure was existed under needle-like 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

J¼k1exp bs"2

zekT

ð1Þ

wherebis the factor relating the surface areaSof the nucleus

to the perimeterP(b¼P2_{=4S),sis 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

(d)

(e)

(a) (b)

(c)

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/cm}2_{, 4500 s (c) 5 mA/cm}2_{, 900 s (d) 10 mA/cm}2_{, 450 s and (e) 50 mA/cm}2_{, 90 s (temperature: room}

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 over-potential 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. Ag60Ni40 was

obtained with a 0.5 mA/cm2 _{current density, and the Ni}

content drastically increased with increasing current density.

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.

Acknowledgements

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.

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30 40 50 60 70 80 90

2θ ( °)

a.u.

* : Substrate (brass)

(e)

(d)

(c)

(b)

Ni FCC (220)

*

Ni FCC (200)

Ag FCC (111)

*

Ni FCC (111)

* *

(a)

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 AgNO}

3, 0.7 M