-Sono-electroplating of Bismuth Film From Bi(III)-EDTA Bath

BiOCH 3 COO and EDTA-4Na were dissolved in 2 mol/dm 3 CH3COOH-2 mol/dm 3 CH 3 COONa buﬀer solution, and was adjusted to pH4.1 by adding 2 mol/dm 3 CH 3 COOH or 2 mol/dm 3 CH 3 COONa. 100 cm 3 of this electrolyte was used. Electroplated ﬁlm was obtained in the range of 10-100 mA/cm 2 . Sono-electroplating was carried out smoothly, because the mass transfer accelerated with ultrasonic agitation and Bi ion was supplied to electrode surface. The mass transfer and crystallization processes were most aﬀected with micro-jet and shock wave pressure. Best conditions of sono-electroplating were 0.10 mol/dm 3 BiY − , pH 4.0-5.5, 298 K and 10 mA/cm 2 . Exchange current density and reaction rate constant in the sonication increased compared with that in the stationary state. As for this, an electron reaction became fast by the micro-jet or a shock wave pressure. The plated ﬁlm was smoothness and denseness in sonication compared with that in stationary state. It was concluded that main factor that the surface became smooth was shock wave pressure. [DOI: 10.1380/ejssnt.2009.688]


I. INTRODUCTION
Bi is a scarce metal with an abundance in earth about the same as Ag, and a semi metallic element with unusual electronic properties due to its highly anisotropic fermi surface, low carrier concentrations, and small carrier effective masses. Bi has specific electrical, physical and chemical properties. Coatings of Bi may be particularly useful because of physical and chemical properties. Thin films had shown large magneto resistance [1][2][3], thermoelectric efficiency [4] and interesting quantum effects [5]. Bi is also used in electrochromic devices [6,7] and for contact formation on semiconductors. Highly textured Bi films had been electroplated onto semiconductor substrates [8,9]. Uses of thin Bi film electrodes in electrochemical stripping analysis offer a very attractive alternative to mercury electrodes due to the much more environmentally friendly Bi [10][11][12]. Sub-monolayer of Bi on some noble metal surfaces had shown enhanced catalytic activity for a variety of electrochemical processes, most notably the two-electron reduction of H 2 O 2 to H 2 O, often the limiting step in the reduction of O 2 in aqueous fuel cells [13,14], as well as the oxidation of formic acid on Pt [15][16][17][18][19]. Much of the Bi deposition literature had focused either on itfs under potential deposition onto noble metals or its nucleation and growth onto non-metallic substrates such as glassy carbon [20,21]. There was a paucity of Bi electrodeposition literature involving the growth of continuous films onto metallic substrates [22,23]. The one of sonication effects was strongly agitation. The collapse of cavitation bubbles in liquid was generated about 1 GPa of sock wave pressure, 120 m/s of micro-jet and 0.2 GPa of water hammer pressure during micro-jet [24][25][26][27][28][29][30][31][32][33].
In this study, Bi thin film was electroplated from the Bi(III)-EDTA bath of the largest stability constant of Bi(III)-EDTA complex ion. The crystalline orientation and smoothness of film were examined by micro-jet and shock wave pressure.

II. EXPERIMENTAL
The substrate was Cu sheet (99.9% and 0.3 mm thick) with an active area of 1 cm × 1 cm. The counter electrode was Pt plate with an active area of 2.5 cm × 4 cm, and placed 3 cm from the working electrode. The Cu substrate was polished with No. 2000 emery paper and immersed in 6 mol/dm 3 HNO 3 solution for several seconds, then rinsed with distilled water and air-dried before the experiments. BiOCH 3 COO) and EDTA-4Na were dissolved in 2 mol/dm 3 CH 3 COOH -2 mol/dm 3 CH 3 COONa buffer solution, and was adjusted to pH 4.1 by adding of 2 mol/dm 3 CH 3 COOH or 2 mol/dm 3 CH 3 COONa. 100 cm 3 of the electrolyte was used. The cell, 6 cm in diameter and 7 cm in height, was placed in an ultrasonic tank. The electrolyte maintained at 298 k. The sonication was prepared with using an ultrasonic cleaner (Honda Denshi, model W-113, 100 W). Electroplating carried out at a current density of 10 mA/cm 2 .

III. RESULTS AND DISCUSSION
The main reaction of Bi(III) and EDTA ion in pH 4.1 solution was Eq. (1). Bi(III)-EDTA complex (BiY − ) was stable in solution because the stability constant was 26.5. The solubility limit of BiY − was 0.15 mol/dm 3 in buffer solution (pH 4.1).
A. Cathodic polarization curve Figure 1 shows the cathodic polarization curves. Polarization in the sonication state was reduced comparing with the stationary state because sonication was strongly agitation. The limiting current density was about 55 mA/cm 2 in the sonication state, and about 15 mA/cm 2 in the stationary state. It was concluded that thickness of diffusion layer was thin with sonication. Table I shows the electrochemical parameters. The exchange current density, i 0 , was larger with sonication comparing with that without sonication. It was concluded that thickness of electric double layer reduced with shockwave pressure and micro-jet in cavitation effects. Transfer coefficient, α, reduced. These results may be related to increasing of agitation. Figure 2 shows the effects of current density and bath temperature on the current efficiency with sonication. Current efficiency was about 98% at 10 mA/cm 2 and 298 K, and below 25 mA/cm 2 at 313 and 333 K, and decreased with increasing of current density over 25 mA/cm 2 at all temperatures. Dendrite was deposited over 50 mA/cm 2 at 298 and 313 K, and over 75 mA/cm 2 at 333 K. Figure  3 shows the effect of pH on the current efficiency. The current efficiency was about 85-100% at 10 mA/cm 2 in the range of pH 4.0-5.5. The current efficiency was about 80% at 30 mA/cm 2 and about 60% at 50 mA/cm 2 in pH 4.1. The current efficiency decreased with increasing of pH over pH 4.5. Plated film was dendrite over pH 5.0 at 30 and 50 mA/cm 2 . Figure 4 shows the effect of BiY − concentration on the current efficiency. The current efficiency was about 45% in 0.01 mol/dm 3 , and increased with increasing of concentration, and was about 90% in range of 0.05-0.10 mol/dm 3 at 10 mA/cm 2 . The current efficiency was about 20% in 0.01 mol/dm 3 , and increased slowly with increasing of concentration and was about 55% in 0.10 mol/dm 3 at 50 mA/cm 2 . Figure 5 shows the effect of current density on the texture coefficient of electrodeposited film with sonication. The texture coefficient of (014) plane decreased and that of (102) plane increased with increasing of current density. The texture coefficient of (110) plane increased at 100mA/cm 2 . The texture coefficients of other planes were almost constant. Figure 6 shows the effect of pH on the texture coefficient of electroplated film with sonication.   Each planes with the exception of (014) and (102) plan were almost constants. The texture coefficient was not affected with pH. Figure 7 shows the surface morphology of plated film after electrolysis of 10 mA/cm 2 . Surface was smoothness and denseness in the sonication state comparing with that in the stationary state. Grain size was about 46.8 nm in the stationary state and 35.9 nm in the sonication state with X-ray analysis. It was concluded that particles were destroyed because the particle and particle had collision with the with micro-jet and shock wave pressure on the terrace of surface. Figure 8 shows the growth process of plated film. Surface was smoothness and denseness with increasing of charge because particle crushed down with the shockwave pressure. For one example, the contact theory of elastic body was examined. The diameter of contact circle (a) indicated Eq. (2) when solid (radius: r) press to flat plate with pressure W .

E. Growth process of plated film
where, E 1 and E 2 are Youngfs modules, σ 1 and σ 2 are Poissonfs ratio of ball and plate. When Bi particle assumed globosity of 20 nm dia. and Bi film was flat plate. Figure 9 shows the effect of pressure on the contact area. The particle shaved destruction or a part off with the shock wave pressure, and the size decreased. Thereafter, the particle moved to kink of surface by the microjet. The particles could be plug up in more small clearance  (pinhole and defect on surface, etc.). Therefore, the corrugation of surface decreased. The surface was smoothness, denseness and homogeneity. Or, when the shock wave pressure was applied on the particle that deposited on surface, the particle was crushed and was compressed with adjoining particle. Finally, it consisted like one particle. The space between particles and particles was reduced by the pressures, and particles filled in film or pinhole. Generally, when pressure increases, a particle is crushed. The sense, smooth and elaborate film formed. Furthermore, a film became thick with the concave part in accordance with an increasing of a plated amount, and a good film of few gloss of unevenness was formed. Surface of electroplated film was smoothness and minuteness. In this time, applying of shock wave pressure deformed crystal.
2. Polarization was smaller and limiting current density increased about 3.7 times compared with that in stationary state. The mass transfer and crystallization processes were most affected as diffusion layer was reduced with micro-jet and shock wave pressure with cavitation.
3. The texture coefficient of (014) plane decreased, and that of (110) and (022) planes increased with increasing of current density.