Electrodeposition of Mirror-Bright Silver in Cyanide ...

Journal of The Electrochemical Society, 156 共3兲 D79-D83 共2009兲

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0013-4651/2008/156共3兲/D79/5/$23.00 © The Electrochemical Society

Electrodeposition of Mirror-Bright Silver in Cyanide-Free Bath Containing Uracil as Complexing Agent Without a Separate Strike Plating Process Bu-Gao Xie, Jian-Jun Sun,*,z Zhi-Bin Lin, and Guo-Nan Chen MOE Key Laboratory of Analysis and Detection Technology for Food Safety, Department of Chemistry, Fuzhou University, Fuzhou 350002, China A cyanide-free silver plating bath containing uracil as a complexing agent was investigated. The electrochemical properties of a silver complex were studied on a disk glassy carbon electrode by means of cyclic voltammetry and a rotating disk electrode. The brightness and adhesion of silver deposits obtained from this silver plating bath onto copper substrates in the absence and presence of polyethyleneimine 共PEI兲 were evaluated. Mirror-bright and good adherent silver deposits could be obtained without a separate strike plating process when PEI as an additive was added. Differential capacitance studies indicated that competition adsorption between PEI and uracil molecules occurred on the silver surface. The electrochemical displacement rate of silver by base metal 共e.g., copper兲 was very slow compared with that in a typical cyanide strike bath from the results of electrochemical quartz crystal microbalance studies. The surface morphology of silver deposits characterized by scanning electron microscopy illustrated that, with the addition of PEI, the grain size of the deposits decreased from hundreds to tens of nanometers. © 2008 The Electrochemical Society. 关DOI: 10.1149/1.3046157兴 All rights reserved. Manuscript submitted September 23, 2008; revised manuscript received November 14, 2008. Published December 29, 2008.

The electrodeposition of silver from cyanide solutions is a longestablished industrial practice. The vast majority of commercial silver plating solutions in use today is remarkably similar to that described in the first patent over 160 years ago.1,2 The electrodeposition of silver from cyanide has many advantages such as the low cost and the most consistent quality of the deposits. However, it requires delicate manipulation during the process of production, transportation, storage, and disposal due to the high toxicity problems associated with the cyanides. For these reasons, a number of attempts have been made in past years to develop cyanide-free formulations. One of the important noncyanide silver plating baths is thiosulfate formulation. Leahy et al.3 studied a noncyanide acidic silver plating bath containing thiosulfate, bisulfite buffer, and sulfate. Mirror-bright silver deposits with a low porosity and higher antitarnish ability could be obtained at room temperature by adding a brightener system including turkey red oil, furfural, and methyl imidazol thiol. Sriveeraraghavan et al.4 reported that the bath composed of silver chloride and sodium thiosulfate was stable for several months without deterioration and could be operated at room temperature in the current density range of 0.5–1.25 A/dm2 with a cathode current efficiency of about 100%. But, no information was provided as to the morphology and surface roughening. Foster et al.5,6 noted that silver electrodeposition from the sodium thiosulfate solutions produced noticeably smoother surfaces than that from ammonium thiosulfate solution. Su et al.7 obtained a mirror-bright silver deposit from the thiosulfate system by the pulse plating method and the addition of organic sulfide and a heterocyclic compound containing nitrogen. However, the appearance properties of silver deposit are extremely sensitive to chloride and cupric ions in the plating bath. Succinimide silver bath was thought to be an alternative for cyanide formulation. The bright silver deposit was obtained from the noncyanide alkali bath with succinimide as a complexing agent when polyethyleneimine 共PEI兲 was added.8-11 Succinimide undergoes hydrolysis in alkaline solution, which in turn causes the pH of the plating solution, normally in the range of 8–9 or so, to become unstable, and the silver plating process requires frequent replenishment of succinimide. Compared with succinimide, the hydantoin and its derivatives act as more effective complexing agents. They are cyclic diimide possessing structural features similar to succinimide, but more resistant to hydrolysis than succinimide. Morrissey12 obtained brilliant depos-

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its by adding 2,2⬘-dipyridyl to hydantoins silver plating bath, and proposed that 5,5⬘-dimethylhydantoin was the most commercially available among a series of substituted hydantoins. No commercially available process using this silver plating process has been reported to date. Electrochemical Products Inc. claimed that E-Brite 50/50 is a stable cyanide-free alkaline silver plating process13 by which silver can be deposited directly onto brass or copper without a separate strike and a brightener. However, there is no mention about this bath composition, and the cost of the plating bath is very high. In addition, other cyanide-free silver plating baths, such as the ones involving ammonia,14,15 thiourea,16,17 and ethylenediaminetetraacetic acid,18,19 have also been proposed. These noncyanide baths have been proven to be unstable and sensitive to light or other shortcomings, and the silver ions are eventually reduced to the metal. The resulting deposits are of low quality in terms of adhesion, etc. Consequently, more efforts should be made toward this technology. The substrate pretreatment for silver plating, besides the standard steps of cleaning and etching, includes a separate silver strike plating because most base metals, such as copper and nickel, are less noble than silver. The strike bath contains a low concentration of silver ions but a high concentration of free complexing agent. In this way the tendency for electrochemical displacement by basic metal is greatly lowered, and suitable deposits could be obtained. The strike silver process is critical to offer satisfactory adhesion whether the silver bath contains cyanide or not. However, this process utilized a solution formulated with cyanide, such as 23 mM KAg共CN兲2 and 0.92 M KCN. Use of this solution would be counterproductive in resolving the cyanide elimination problem. Accordingly, it may be a significant improvement that a cyanide-free silver plating bath provides with properties of stability, satisfactory adherent deposit, and low electrochemical displacement rate by less noble metal. Uracil 共CAS number 66-22-8兲 is a pyrimidine base and exists in Ribonucleic acid 共RNA兲, but not in DNA. It belongs to a group of the most important pyrimidines playing a fundamental role in the structure and function of nucleic acids, enzymes, and drugs. The innocuous biomedical and biodegradable natures of uracil and its derivatives make them applicable in a wide range of biological and pharmaceutical fields such as mutagenic, anticancerogenic, and antithyroedien character.20,21 Additionally, uracil compounds had also been utilized as efficient corrosion inhibitors of metallic material due to their strong adsorption on metal surfaces.22-24 In this work, uracil was selected as a complexing agent for the silver plating bath. It is dissoluble and stable in alkaline solution at room temperature. Its nitrogen-containing heterocyclic structure facilitates it coordinating metal ions firmly in solution and adsorbing

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on metals surface as well, such as copper and silver. Consequently, the noncyanide silver plating bath is insensitive to light and temperature and can remain stable for several months. In addition, owing to its superior feature of adequately slow electrochemical displacement rate, silver deposit on copper plate was performed directly in a uracil silver plating bath without a separate strike plating process. A mirror-bright silver deposit with acceptable adhesion can be obtained when PEI is added. Experimental All solutions were prepared using analytical grade reagents and ultrapure water 共Millipore-Q system兲. The silver plating bath was obtained by adding Ag2SO4 to the mixed solution of uracil and KOH and its optimal composition was 0.075 M Ag2SO4, 0.9 M uracil, and 1.0 M KOH 共free concentration兲. The average molecular weight of the PEIs 共branched polymer, Alfar Aesar兲 used in the experiments was 600, 1800, 10,000, and 70,000, respectively, and those PEIs were correspondingly identified as PEI-600, PEI-1800, PEI-10,000, and PEI-70,000. Electrochemical measurements were performed with a CHI440 electroanalyzer 共Shanghai Chenhua Instrument Limited Co., China兲 with a conventional three-electrode system. A silver plate 共99.99%兲 was used as the counter electrode, and a Hg/HgO electrode filled with 1 M KOH 关E0 = 0.098 V vs normal hydrogen electrode 共NHE兲兴 as the reference electrode. All potentials were measured and reported with respect to this reference electrode unless noted otherwise. A disk glassy carbon electrode 共GCE兲 or a silver disk electrode with a diameter of 3 mm was used as the working electrode. Prior to experiments, the working electrodes were polished successively using 1.0, 0.3, and 0.05 ␮m alumina slurries and subsequently rinsed with ultrapure water. Then, the GCE was treated with an electrochemical method, which involves potential scanning between −1.5 and 1 V at a scan rate of 50 mV/s ca. 20 cycles until a steady-state electrochemical response was obtained, and silver was activated in 1% HNO3 5 min, respectively. Electrochemical quartz crystal microbalance 共EQCM兲 connected to the CHI440 was used to evaluate the electrochemical displacement rate. The gold plated on both sides of the crystal was a central circle of 5.1 mm diameter. On the gold face exposed to the electrolyte a copper layer was galvanostatically deposited 共2 mA/cm2兲 in a copper plating bath consisting of 0.1 M CuSO4, 0.1 M H2SO4, and 0.2 M C2H5OH25 before each experiment. A Hg/Hg2SO4 electrode filled with saturated potassium sulfate 共E0 = 0.65 V vs NHE兲 was used as the reference electrode during the process of copper deposition. The copper film thickness was approximately 200 nm determined by integrating the Faraday current and the reduction of resonance frequency. A freshly deposited copper electrode was thoroughly washed with ultrapure water. Then, the aggressive solution was added to the cell immediately. Cyclic voltammogram data using a static disk GCE were recorded between 0.5 and −1.1 V at a scan rate of 50 mV/s. The diffusion coefficient was researched with a rotating disk GCE at different rotation rates between −0.05 and −1.65 V. To evaluate the brightness and the adhesion of silver deposits on copper plate, the electrodeposition was carried out in one step without a strike plating process at −0.65 V. Adhesion of the deposit was evaluated qualitatively by bending the panel covered with silver deposit back and forth at 90° twice and examining the deformed panel areas for signs of cracking, blistering, or peeling. The differential capacitance 共DC兲 technique at silver electrode was carried out on a VMP3 multichannel potentiostat 共Princeton Applied Research, USA兲 by impedance measurements. The 10 mV potential amplitude and 20 Hz frequency were selected as the experimental parameters. Scanning electron microscopy images 共SEM, JEOL JSM-6700F field emission兲 were used to study the morphology of the silver deposits. The solutions were deaerated by bubbling nitrogen 共99.999% N2兲 for 30 min prior to use. All experiments were carried out at 25 ⫾ 0.1°C.

Figure 1. Typical cyclic voltammogram recorded from the base bath using a static disk GCE initiated at open circuit potential 共Eoc兲 at scan rate of 50 mV/s. The composition of the base bath is 0.075 M Ag2SO4 + 0.9 M uracil +1.0 M KOH 共free concentration兲.

Results and Discussion Electrochemical behavior of silver complex in the uracil silver plating bath.— To research the formation of silver ions with uracil in the uracil silver plating bath containing 0.075 M Ag2SO4, 0.9 M uracil, and 1.0 M KOH 共free concentration兲, the electromotive force method was performed at 25 ⫾ 0.1°C in the presence of 0.5 M KNO3. Inspection of the data revealed that AgL共OH兲− was the main component of silver complex 共L represents uracil兲. A typical cyclic voltammogram recorded from the uracil plating bath with a static disk GCE is shown in Fig. 1. The initiated potential was set at open-circuit potential 共Eoc兲 and then shifted toward the negative direction. The cathodic current density increases sharply at the potential ⬃−0.53 V and reaches a peak at −0.76 V, which indicates that silver deposition becomes diffusion-limited. During the reversing sweep, a crossover 共Eco兲 is observed at ⬃−0.58 V, suggesting that the deposition of silver occurs by threedimensional nucleation.26,27 On the anodic branch the curve shows a well-expressed current peak corresponding to the anodic oxidation of cathodically deposited silver. Following the fundamental of cyclic voltammetric stripping, the value of the anodic/cathodic charge ratio 共Qa /Qc兲 calculated by integrating the reduction current and the oxidation current is 0.95, illustrating a high current efficiency of silver electrodepositon in the plating bath. The results are in good agreement with the current efficiency measured by the coulomb technique. Figure 2 shows a series of voltammograms of silver deposition in the base bath at a scan rate of 50 mV/s using a rotating disk GCE at different rotating rates. The current–potential curves show that, at potentials more positive than −0.51 V, the current density is independent of the electrode rotation rate. At potentials more negative than −0.55 V, there is a large region of kinetic-mass transport control. At higher rotation rates, progressive extension of the mixed control region is evident in the voltammogram curves. Under these conditions, there is a linear correlation between the inverse of the limiting current and the inverse of the square root of the rotation rate according to the Koutecky–Levich equation illustrated in the inset of Fig. 2, and this equation is formulated as follows 1/j = 1/jk + 1/共0.62nFC0D2/3␯−1/6␻1/2兲 where jk is the kinetic current density, n the number of electrons transferred, F is the Faraday constant, C0 is the bulk concentration of the silver complex AgL共OH兲−, D is the diffusion coefficient of the silver complex AgL共OH兲−, ␯ is the kinematic viscosity of the



Figure 2. 共Color online兲 Voltammograms of the rotating disk GCE in the base bath at the various rotation rates indicated for each voltammogram. Inset shows the dependence of j−1 on the rotating rate ␻−1/2 共Koutecky– Levich plots兲 for silver deposition.

electrolyte, and ␻ is the angular velocity of the rotating disk electrode. From the slope of the Koutecky–Levich equation and various rotation rates of the rotating disk GCE, the diffusion coefficient in the base solution is calculated as 2.82 ⫻ 10−6 cm2 /s if ␯ is 0.01 cm2 /s. Electrochemical behavior of silver complex in the presence of PEIs.— Good adherent silver deposits can be obtained without a separate strike plating process in an additive-free uracil plating bath, but the deposit is gray, not mirror-bright. To obtain bright deposits, the effects of brighteners, such as Br−, KSeCN, cysteine, polyethylene glycol, and PEI, were examined. It was found that PEI can make a silver deposit mirror-bright. Figure 3 shows the effect of PEI molecular weight ranging from 600 to 70,000 on the voltammograms of silver. Whether PEI with different molecular weight is added or not, Eco is observed in the cathodic part of all the curves, exhibiting the behavior of silver deposition occurring by nucleation behaviors of silver deposition. Furthermore, cathodic peaks shift toward a more negative potential,

Figure 3. 共Color online兲 Dependence of cyclic voltammograms on the molecular weight of PEI at scan rate of 50 mV/s: 共a兲 additive-free, 共b兲 PEI-600, 共c兲 PEI-1800, 共d兲 PEI-10,000, and 共e兲 PEI-70,000. Inset shows the dependence of Qa on the molecular weight of PEI.

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Figure 4. 共Color online兲 Dependence of cyclic voltammograms on the PEI1800 concentrations at scan rate of 50 mV/s: 共a兲 0, 共b兲 300, 共c兲 600, 共d兲 1200, and 共e兲 2400 ppm. Inset shows the dependence of Qa on the PEI-1800 concentrations.

indicating that the reduction of silver is suppressed by added PEI. The reverse scans show the anodic stripping peak related to the dissolution of silver, which moves to more negative potentials. The dependence of the Qa of silver on PEI molecular weight is shown in the inset of Fig. 3. Qa共0兲 represents the anodic charge corresponding to the dissolution of silver in the base bath. It can be seen that the value of Qa decreases with the increasing molecular weight of PEI up to 1800, which is attributed to the differences in their adsorption ability. When the molecular weight is over 1800, the inhibition ability of PEI changes slightly. Additionally, all Qa /Qc values with different molecular weight of PEI are ⬃0.95. To understand the effect of PEI clearly, the adsorption ability and consumption rate during the silver electroplating were evaluated by using PEI-1800. Figure 4 exhibits the dependence of cyclic voltammograms and Qa 共inset兲 on the PEI-1800 concentrations. It is clear that the silver electrodeposition process is affected by the concentration of the polymer in the plating bath. Inspection of the data reveals that the reduction current density and Qa of silver decrease with the increasing of the polymer concentrations, suggesting that PEI inhibits the reduction of silver by adsorption at the silver/ solution interface. The values of Qa /Qc with different concentrations of PEI-1800 still remain at ⬃0.95, which illustrates that the current efficiency is independent of PEI concentration. Differential capacitance studies for adsorption–desorption behaviors of PEI and uracil.— With the aim of confirming the adsorption of PEI on a silver surface in an applied potential range, a silver disk electrode was employed as the working electrode in capacitance measurement, and the results are shown in Fig. 5. In KOH solution 共curve a兲, a minimum of the capacitances at ⬃−1.1 V may correspond to the OH− special adsorption. A comparison between curve a and b shows that the capacitance in the solution with PEI1800 is lower than that without PEI, which suggests that PEI-1800 can adsorb on the silver electrode surface at the potentials applied. When uracil is added to the alkaline solution, the change of capacitance shown by curve c is different from that of curves a and b. The values of capacitance change slowly between −0.1 and −0.52 V, and then increase sharply as the potential shifts toward the negative. In the region from −0.7 to − 1.2 V there are three adsorption– desorption peaks for the shape of the DC curve, which has been explained from the point of view of the molecular level adsorption of uracil.28-32 Difference of uracil behavior is considered owing to the concentration, pH of solution, and the potential applied, and so on. Moreover, the fact that the initial potential 共−0.52 V兲 of the


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Figure 5. 共Color online兲 DC curves and voltammograms 共inset兲 at a 3 mm diameter silver disk electrode in different solutions consisting of: 共a兲 1.0 M KOH, 共b兲 1.0 M KOH +600 ppm PEI-1800, 共c兲 1.0 M KOH 共free concentration兲 +0.6 M uracil, and 共d兲 1.0 M KOH 共free concentration兲 +0.6 M uracil +600 ppm PEI-1800, respectively.

rapid capacitance change in this solution is close to the value 共−0.53 V兲 of the sharp increase of current density indicates that silver electrodeposition mainly depends on the uracil adsorption– desorption behavior on the electrode surface. As PEI is added into the solution containing KOH and uracil, the capacitance values shown by curve d become smaller, especially for a more negative potential. The result implies that, even with a relatively low concentration, PEI can also adsorb and compete with uracil on the silver surface, and the reduction rate of silver is affected by the effect of PEI and uracil. In addition, at potentials below −1.3 V the abrupt increase of capacitances under these conditions was contributed by the hydrogen evolution, which is observed by the voltammogram curves in the inset of Fig. 5. Comparison of electrochemical displacement rate in the base bath and cyanide strike bath.— Poor adherent silver deposits on less noble metal, such as copper and brass, resulted from the rapid electrochemical displacement of the base metal with silver in the early stage of deposition. To resolve this problem, strike is often employed to cover the surface of the object plated with a thin silver deposit. However, if the displacement rate in a silver plating bath is very slow, the strike plating process could be left out. EQCM was used to evaluate the displacement rate in the uracil system compared with that in a common strike bath for silver plating 关0.023 M KAg共CN兲2 + 0.92 M KCN兴 as illustrated in Fig. 6. It shows frequency shifts recorded from copper substrates in both plating baths. To analyze the frequency shift in a clearer way, two curves have been divided into four regions: Ia, IIa, Ib, and IIb. In region Ia 共less than 100 s兲 a relatively fast frequency decrease is observed due to the electrochemical displacement of silver by copper, while in region IIa a slight frequency shift occurs in the strike solution. The decrease of the frequency is over 6000 Hz in region Ia, which is almost 85% of the total frequency change, indicating that the process of electrochemical displacement is almost complete and the thickness of the silver film is ⬃26.8 nm. A silver film can be observed when the displacement period reaches about 5 s in this bath, and the frequency change is about 900 Hz, which corresponds to 3.8 nm thickness. In contrast, the trend of the frequency change from the displacement reaction in the uracil bath 共curve b兲 is different. In region Ib 共about 120 s兲 the frequency decrease is only 50 Hz, corresponding to 0.24 nm thickness. In region IIb the displacement reaction is faster than that of region Ib. The total frequency shift in 300 s reaches 334 Hz, corresponding to 1.45 nm thickness. A silver

Figure 6. 共Color online兲 Frequency shifts to electrochemical displacement of copper in 共a兲 cyanide strike solution consisting of 0.023 M KAg共CN兲2 + 0.92 M KCN and 共b兲 the base bath consisting of 0.075 M Ag2SO4 + 0.9 M uracil +1.0 M KOH 共free concentration兲. Inset shows the data for the base bath on an expanded scale.

film can be observed when the displacement time reaches about 400 s. It is evident from the EQCM results that the electrochemical displacement rate in the uracil bath is slower than that in the cyanide strike solution. Thus, no strike process is needed to obtain a satisfactory adherent silver deposit on the copper substrate with the application of a uracil silver formulation. Adhesion test over copper.— Silver deposits were obtained on a standard pretreated copper substrate from a uracil silver plating bath without a separate strike plating process in the presence of PEI-1800 at different concentrations. The deposited potential was set at −0.65 V and the thicknesses of the films were controlled by depositing charge. The adhesion of the obtained deposits was tested by bending the copper substrate panel. It was found that blistering or peeling as well as signs of cracking are not observed when the thickness of film is less than 4 ␮m. However, with the addition of PEI, signs of cracking were observed when the thickness was over 5 ␮m, which might be attributed to the incorporation of PEI during the plating process that aggravated the adhesion. Surface morphology of silver deposits.— SEM was employed to gain insight into the surface morphology of the silver deposits. Figure 7 shows the SEM images of the top views of silver deposits obtained at different PEI-1800 concentrations. In the absence of PEI-1800 共Fig. 7a兲, the microstructure of the silver deposit is characterized by regularly shaped and crystalline grains. The grain sizes range from 100 to 300 nm, and the deposit is macroscopically rough and gray. When PEI is added to the base bath, it changes to brightness. The surface is smooth and the grain size decreases to 30–80 nm 共shown in Fig. 7b-e兲. No relevant microstructural difference is observed when varying the PEI-1800 concentration. The silver deposit is not bright when the level of PEI is lower than 300 ppm. Conclusions A cyanide-free silver plating bath with uracil as a complexing agent and PEI as an additive was found to be capable of producing satisfactory silver deposits over a copper substrate without a separate strike plating process. It was found that the quality of the silver deposit depends on the competing adsorption of PEI and uracil on the silver surface. With the addition of PEI, bright silver deposits with a small size were obtained. Although the thickness is limited in our research, it provides a possible cyanide-free formulation for silver electrodeposition without a strike plating process in industrial practice.



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Figure 7. SEM images of the top views of silver deposits obtained from 共a兲 the base bath, 共b兲共a兲 +300 ppm PEI-1800, 共c兲共a兲 +1200 ppm PEI-1800, 共d兲共a兲 +1200 ppm PEI-1800, 共e兲共a兲 +2400 ppm PEI-1800, respectively.

Acknowledgments The authors thank the National Science Foundation of China 共no. 90407019 and no. 20775015兲 and State Key Laboratory of Physical Chemistry of Solid Surfaces 共Xiamen University兲 for the financial support. Fuzhou University assisted in meeting the publication costs of this article.

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