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Table 2. The values of the Tafel slopes at 298 and 343 K. The first linear part on curves 1, corresponding to the potentials above —800 mV, yields the Tafel slope of 440 mVdec"' at 298 K and 326 mVdec™ at 343 K. Such high Tafel slope values are difficult to interpret in terms of ele- mentary charge transfer, but may be explained assuming a specific adsorption of charged intermediates, as has al- ready been described.*"'!

Table 2 The values of the Tafel slopes at 298 and 343 K. The first linear part on curves 1, corresponding to the potentials above —800 mV, yields the Tafel slope of 440 mVdec"' at 298 K and 326 mVdec™ at 343 K. Such high Tafel slope values are difficult to interpret in terms of ele- mentary charge transfer, but may be explained assuming a specific adsorption of charged intermediates, as has al- ready been described.*"'!

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Table 1. Composition of the electrolytes used in the experiments. All chemicals were supplied by Merck, (Germany), and Kemika (Croatia). ;
Table 1. Composition of the electrolytes used in the experiments. All chemicals were supplied by Merck, (Germany), and Kemika (Croatia). ;
Figure 2. Voltammograms of cathodic polarization without TI‘; T = 343 K, polarization rate 20 mVs"!, rotation speed 500 to 2000 min"!. The supposition is based on the results published by Bindra et al,'! that a very thin layer containing only a few monolayers of an insoluble AuCN, is formed in the first reaction step, followed by the reduction of this interme- diate salt to the metal gold as the final step. No limiting current has been observed in the investigated potential range. By increasing the electrolyte temperature only the reaction rate increased, while the hysteresis on the obtai- ned voltammograms became smaller (Figure 2). This in- dicates a lower influence of adsorption and direct reduc- tion of Au(CN), to the metallic gold. In the second case as well (Figure 2), the influence of rotation speed is less pronounced. As a matter of fact, a change of hydrodyna- mic conditions does not significantly influence the obtai-
Figure 2. Voltammograms of cathodic polarization without TI‘; T = 343 K, polarization rate 20 mVs"!, rotation speed 500 to 2000 min"!. The supposition is based on the results published by Bindra et al,'! that a very thin layer containing only a few monolayers of an insoluble AuCN, is formed in the first reaction step, followed by the reduction of this interme- diate salt to the metal gold as the final step. No limiting current has been observed in the investigated potential range. By increasing the electrolyte temperature only the reaction rate increased, while the hysteresis on the obtai- ned voltammograms became smaller (Figure 2). This in- dicates a lower influence of adsorption and direct reduc- tion of Au(CN), to the metallic gold. In the second case as well (Figure 2), the influence of rotation speed is less pronounced. As a matter of fact, a change of hydrodyna- mic conditions does not significantly influence the obtai-
two temperatures: at 298 K (Figure 1) and at 343 K (Figu- re 2). Hysteresis on the polarization curves (Figure 1), re- corded at lower potentials (below 750 mV) are attributed rather to a change in the mechanism of gold deposition than to a modification of the surface area, due to the gold deposited on the substrate. Figure 1. Voltammograms of cathodic polarization without Tl*; T = 298 K, polarization rate 20 mVs"', rotation speed 500 to 2000 min"!.
two temperatures: at 298 K (Figure 1) and at 343 K (Figu- re 2). Hysteresis on the polarization curves (Figure 1), re- corded at lower potentials (below 750 mV) are attributed rather to a change in the mechanism of gold deposition than to a modification of the surface area, due to the gold deposited on the substrate. Figure 1. Voltammograms of cathodic polarization without Tl*; T = 298 K, polarization rate 20 mVs"', rotation speed 500 to 2000 min"!.
Figure 4. Voltammograms of cathodic polarization with TI*; T = 343 K, polarization rate 20 mVs_!, rotation speed 500 to 2000 min"!. At higher temperature, these plateaus were recorded on- ly at lower electrode rotation speeds and disappeared com- pletely at the highest electrode rotation speed. Hysteresis is less pronounced at this temperature, as shown in Figure 4.
Figure 4. Voltammograms of cathodic polarization with TI*; T = 343 K, polarization rate 20 mVs_!, rotation speed 500 to 2000 min"!. At higher temperature, these plateaus were recorded on- ly at lower electrode rotation speeds and disappeared com- pletely at the highest electrode rotation speed. Hysteresis is less pronounced at this temperature, as shown in Figure 4.
Figure 3. Voltammograms of cathodic polarization with TI*; T = 298 K, polarization rate 20 mVs_!, rotation speed 500 and 2000 min=!. Adding thal ium ions to the electrolyte (solutions 2, 3 and 4) current density considerably increases at lower electrode potentials. The shape of voltammetric curves is changed too. T he hysteresis are now shifted towards more negative potentials, which corresponds to a mechanism of gold deposition via direct charge transfer of Au(CN)* — ions from the e ectrolyte. Also, in the presence of thallium ions, limiting current plateaus are recorded, as presented in Figure 3.
Figure 3. Voltammograms of cathodic polarization with TI*; T = 298 K, polarization rate 20 mVs_!, rotation speed 500 and 2000 min=!. Adding thal ium ions to the electrolyte (solutions 2, 3 and 4) current density considerably increases at lower electrode potentials. The shape of voltammetric curves is changed too. T he hysteresis are now shifted towards more negative potentials, which corresponds to a mechanism of gold deposition via direct charge transfer of Au(CN)* — ions from the e ectrolyte. Also, in the presence of thallium ions, limiting current plateaus are recorded, as presented in Figure 3.
Figure 5. Voltammograms of cathodic polarization — effect of TI*- ion concentration; T = 343 K, polarization rate 20 mVs_, rotation speed 500 min”. At the same time, the addition of thallium ions shifts the deposition potential towards more positive values. In Figure 5 the effect of thallium concentration on the elec- trode polarization curves is given and compared to the hallium-free electrolyte polarization curve. Only ascen- ing branches are presented here. The contribution of hallium ions on the deposition rate is evident. Current ensity increases approximately three to four times in the investigated range of the TI* concentration. t d t d
Figure 5. Voltammograms of cathodic polarization — effect of TI*- ion concentration; T = 343 K, polarization rate 20 mVs_, rotation speed 500 min”. At the same time, the addition of thallium ions shifts the deposition potential towards more positive values. In Figure 5 the effect of thallium concentration on the elec- trode polarization curves is given and compared to the hallium-free electrolyte polarization curve. Only ascen- ing branches are presented here. The contribution of hallium ions on the deposition rate is evident. Current ensity increases approximately three to four times in the investigated range of the TI* concentration. t d t d
Figure 6. AES profile analysis of deposit; C,,= 3.75 10“ mol dm T = 343 K,i=9.87 mA cm~, rotation speed 500 min. Addition of thallium into the electrolyte causes an in- crease of current density as well as a change of the gold coating structure as illustrated in Figures 7a and 7b.
Figure 6. AES profile analysis of deposit; C,,= 3.75 10“ mol dm T = 343 K,i=9.87 mA cm~, rotation speed 500 min. Addition of thallium into the electrolyte causes an in- crease of current density as well as a change of the gold coating structure as illustrated in Figures 7a and 7b.
Figure 7. SEM analysis of deposit: (a) without Tl*: T = 343 K, cur- rent density 1.5 mA cm”, rotation speed 500 min, (b) with TI* in electrolyte (C,,, = 3.75 10“ mol dm’): T = 343 K, current density 9.87 mA cm”, rotation speed 500 min’.
Figure 7. SEM analysis of deposit: (a) without Tl*: T = 343 K, cur- rent density 1.5 mA cm”, rotation speed 500 min, (b) with TI* in electrolyte (C,,, = 3.75 10“ mol dm’): T = 343 K, current density 9.87 mA cm”, rotation speed 500 min’.
Figure 8. Dependence of i! on @°; solution 1, T = 298 K. Since the diffusion coefficient of Au(CN)* solution is: D = 1.67 x 10° cm’ s and the corresponding kinematics viscosity’? v = 5.18 x 10° cm’ s", the value of K may be estimated from equation (6) and inserted into the equation (5). According to equation (5), a plot of 1/i vs. wis es- tablished giving straight lines for three different electrode potentials, as presented in Figure 8, where the intercepts represent 1/1, values and the slopes are values of K.
Figure 8. Dependence of i! on @°; solution 1, T = 298 K. Since the diffusion coefficient of Au(CN)* solution is: D = 1.67 x 10° cm’ s and the corresponding kinematics viscosity’? v = 5.18 x 10° cm’ s", the value of K may be estimated from equation (6) and inserted into the equation (5). According to equation (5), a plot of 1/i vs. wis es- tablished giving straight lines for three different electrode potentials, as presented in Figure 8, where the intercepts represent 1/1, values and the slopes are values of K.
For a constant temperature the equation (9) gives a straight line in a semi-logarithmic scale, plotting the term
For a constant temperature the equation (9) gives a straight line in a semi-logarithmic scale, plotting the term
Figure 9. Dependence of i! on w°> given for different temperatu- res C+ = 3.75 x 10° mol dm, E = 900 MV/cop:
Figure 9. Dependence of i! on w°> given for different temperatu- res C+ = 3.75 x 10° mol dm, E = 900 MV/cop:
gure 10, for the temperature of 298 K, and in Figure 11, for 343 K. The addition of thallium strongly affects the shape of the curves, giving higher values of the slopes at potentials higher than 800 mV. Also, the potential at which the slope change occurs (curves 2 and 3 in Fig. 11) decreases in the presence of thallium in the electrolyte, indicating a chan- ge in reaction mechanism. Obtained Tafel slopes do not depend on the electrode rotation speed, but depend on the temperature as shown in Table 2.
gure 10, for the temperature of 298 K, and in Figure 11, for 343 K. The addition of thallium strongly affects the shape of the curves, giving higher values of the slopes at potentials higher than 800 mV. Also, the potential at which the slope change occurs (curves 2 and 3 in Fig. 11) decreases in the presence of thallium in the electrolyte, indicating a chan- ge in reaction mechanism. Obtained Tafel slopes do not depend on the electrode rotation speed, but depend on the temperature as shown in Table 2.
Table 3. Potential of the slope change (evaluated from Figures 10 and 11).
Table 3. Potential of the slope change (evaluated from Figures 10 and 11).
Figure 12. Influence of Tl*— ions concentration on the current den- sity; polarization rate 20 mVs“', rotation speed 500 min”!
Figure 12. Influence of Tl*— ions concentration on the current den- sity; polarization rate 20 mVs“', rotation speed 500 min”!
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