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5 X 10-6 1-ethyl-3-methylimidazolium tetrafluoroborate.
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1 X 10-7 1-n-butyl-1-methylpyrrolidinium bis (trifluoromethylsulfonyl) imide.
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0 X 10-7 AlCl3 with 1-Methly-3-Ethyl Imidazolium chloride.
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3 x 10-7 1-Butyl-1-Methylpyrrolidinium Trifluoro-Methanesulfonate This work.
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11 x 10-7 The dependency of the Diffusion coefficient to the viscosity and the radius of the diffusing species can be explained by the Stoke-Einstein equation, D= kT/ 6 ηr where k = Boltzmann constant, T = Kelvin temperature, η = viscosity of the solvent, r = dynamic radius of the diffusing species. Hussey et. al. [9] found that the Tin(II) exists as SnCl42- in AlCl3 with 1-Methly-3-Ethyl Imidazolium chloride ionic liquid and the low values of the diffusion coefficient was due to the increased viscosity of the ionic liquid. They also suggest that there is some degree of association between the Tin(II) with chloroaluminate ions such as AlCl4- and Al2Cl7-, which contribute to the low value of the diffusion coefficient [5]. W. Yang et. al. [6] used tetrafluoroborate, BF4- based ionic liquid, where the diffusion coefficient was higher than calculated from the chloroaluminate ionic liquid by Hussey. From the Stoke-Einstein equation, it can seen that the smaller Tin(II) tetrafluoroborate species will contribute to a slightly higher diffusion coefficient value for the Tin(II) species. Studies using trifluoromethylsulfonyl imide ionic liquids from Tachikawa et. al. [7] and this work using trifluoromethylsulfonate ionic liquid gave smaller diffusion coefficient for the Tin(II) species. It can be suggested that the complexation between the Tin (II) with trifluoromethylsulfonate and trifluoromethylsulfonyl imide, which is larger than the chloride ion and the tetrafluoroborate ion, has increased the radius of the Tin(II) species in solution. This contributes to the lower diffusion coefficient compared to the chloride and tetrafluoroborate based ionic liquids in the works of Hussey and Tachikawa.
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2 Electroplating Experiments Electroplating on macroelectrodes were carried out to estimate the plating current efficiency and hydrogen evolution reaction for the mixture of 1-Butyl-1-methyl-pyrrolidinium trifluoro- methane sulfonate (BMPOTF) with Methane Sulfonic Acid and tin methane sulfonate salts. Table 4 presents the results of current efficiencies from electroplating experiments using Tin (II) solution in ionic liquid at different current densities and Tin (II) concentrations. Scanning electron microscopy (SEM) and Energy Dispersive X-ray spectroscopy (EDX) were used to examine the surface morphology and analyze the elemental compositions of the electrodeposits. Table 4 shows the current efficiencies obtained from experiments using current densities from 1 A dm-2 (ASD) to 7 ASD for various concentrations of Tin (II) from 0. 1 M to 0. 5 M in ionic liquids solutions. Table 4: Current efficiency obtained at different Sn2+ concentration Current Density, (A/dm2) Stannous Ions, concentration, (mol/ dm3).
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26 From the results, increasing current densities for higher concentrations of Tin(II) such as 0. 4 and 0. 5 M gave decreasing current efficiencies for Tin deposition. At these conditions, the hydrogen evolution reaction becomes prominent and decreases the current efficiency for the Tin deposition. The deposits became dull and less reflecting in appearance, owing to the porous nature of the surface as can be seen in Figure 3. Scanning Electron Microscopy (SEM) from Figures 3 at 3500X magnification reveal that the deposits became less compact, less dense and more porous for higher current densities and higher concentrations of Tin(II). This can be also seen in the EDX results in Figures 4. The copper element was present in the EDX spectrum at the tin plated surface when analyzed under 20 keV EDX as shown in Figure 4. b. The higher current densities of 7 A dm-2 for 0. 5 M of Tin (II) shows copper peaks from the copper substrate, because of the porous nature of the deposits. Figure 3 Electrodeposits from a: 0. 4 M Tin (II) methanesulfonate and BMPOTF using 1 and 7 A dm-2 (ASD) b: 0. 5 M Tin (II) methanesulfonate and BMPOTF using 1 and 7 A dm-2 (ASD) Figure 4: EDX spectrum for Tin electrodeposit using 7 A dm-2 (ASD) at a: 0. 4 M and b: 0. 5 M Tin(II) methanesulfonate and BMPOTF 5 CONCLUSIONS The electrodeposition of tin in the mixture of water and air stable ionic liquid- 1-Butyl-1 methyl-pyrrolidinium trifluoro- methane sulfonate and Methane Sulfonic Acid (MSA) based Tin Methane Sulfonate salts shows promising results in this study with current efficiency as high as 99%. The deposit morphology of the mixture BMPOTF and MSA based tin coated substrates were observed by using EDX and SEM, where dense, fine and polygonal grain structures were obtained. In this study, we are convinced that water and air stable ionic liquids have a huge potential in the additive free electrodeposition of metals, especially there is very less hydrogen evolution even though mixed with the Methane Sulfonic Acid (MSA) based Tin salt. 6 RERERENCES.
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