3.5 Characterisation Techniques
4.1.3 Cyclic Voltammetry Using Different Anodic Switching Potentials
Alvarez et al 89 did not observe three anodic peaks nor multiple cathodic peaks and as the anodic limit used by Alvarez et al was 0.1 V vs. SCE the anodic limit used in a previous study 72 was 1.5 V vs. SCE and, it may be inferred that an electrochemical reaction was occurring between 0.1 V and 1.5 V vs.
SCE. Therefore, to investigate where this electrochemical oxidation reaction occurs, consecutive cyclic voltammetry was carried out using different anodic switching potentials. The results of these experiments are shown in figures 4.5 – 4.10. All the cyclic voltammograms used a cathodic switching potential of -1.5 V vs. SCE. Table 4.1 shows the positions and the intensities of the oxidation and main reduction peaks for the 1st, 3rd and 9th cycles of the cyclic voltammograms using different anodic switching potentials.
-3.E-03 -2.E-03 -1.E-03 0.E+00 1.E-03 2.E-03 3.E-03
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
Current (A)
Potential (V vs. SCE) Peak 1
Peak 2
Oxidation potentials
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Table 4.1 Peak potentials and peak currents for the 1st, 3rd and 9th cycles for each different switching potential for the cylic voltammograms obtained for pure Sn on Cu in the potassium bicarbonate-potassium carbonate solution. An * indicates that an ill-defined shoulder is present.
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Figure 4.5 Cyclic voltammograms of electroplated tin on copper with a thickness of 5 µm in a potassium bicarbonate-carbonate oxidation bath with an anodic switching potential of -0.75 V vs. SCE and at a scan rate of 10 mV s -1
The total cathodic charge passed and the total anodic charge passed for the anodic switching potentials of -0.75 V vs. SCE (figure 4.5) looks similar which may suggest that the oxide produced during the anodic sweep is completely reduced during the cathodic sweep. This similarity in the cathodic and anodic charges passed was observed for every cycle, which would suggest that when cycled up to an anodic potential of -0.75 V vs. SCE, the electrochemical reactions are reversible. It was observed that after the first cycle, the anodic peak, at -0.82 V vs. SCE, significantly increases in intensity while the intensity of the cathodic peak only increases slightly (table 4.1). The anodic peak also shifts slightly shifts in potential from -0.82 V to -0.84 V vs. SCE (table 4.1).
The total cathodic charge passed and the total anodic charge passed for the anodic switching potentials of -0.5 V vs. SCE (figure 4.6) looks similar in the first cycle. However, the anodic charge passed significantly increased after the first cycle while the cathodic charge passed stayed constant.
In addition a slight shoulder appeared between the two oxidation peaks, which would suggest that an additional oxidation reaction occured after the first cycle.
-5.E-03 -3.E-03 -1.E-03 1.E-03 3.E-03 5.E-03
-1.5 -1.3 -1.1 -0.9 -0.7 -0.5 -0.3 -0.1
Current (A)
Potential (V vs. SCE)
Cycle 1 Cycle 3 Cycle 9
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Figure 4.6 Cyclic voltammograms of electroplated tin on copper with a thickness of 5 µm in a potassium bicarbonate-carbonate oxidation bath with an anodic switching potential of -0.5 V vs. SCE and at a scan rate of 10 mV s -1
Figure 4.7 Cyclic voltammograms of electroplated tin on copper with a thickness of 5 µm in a potassium bicarbonate-carbonate oxidation bath with an anodic switching potential of 0 V vs. SCE and at a scan rate of 10 mV s -1 -5.E-03
-3.E-03 -1.E-03 1.E-03 3.E-03 5.E-03
-1.5 -1.3 -1.1 -0.9 -0.7 -0.5 -0.3 -0.1
Current (A)
Potential (V vs. SCE)
Cycle 1 Cycle 3 Cycle 9
Shoulder
-5.E-03 -3.E-03 -1.E-03 1.E-03 3.E-03 5.E-03
-1.5 -1.3 -1.1 -0.9 -0.7 -0.5 -0.3 -0.1
Current (A)
Potential (V vs. SCE)
Cycle 1 Cycle 3 Cycle 9
Shoulder
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Figure 4.8 Cyclic voltammograms of electroplated tin on copper with a thickness of 5 µm in a potassium bicarbonate-carbonate oxidation bath with an anodic switching potential of 0.6 V vs. SCE and at a scan rate of 10 mV s -1
The total cathodic charge passed and the total anodic charge passed for anodic switching potentials of 0 V vs. SCE and above (figures 4.7 – 4.10) are not equal. The cathodic charge passed is significantly less than the anodic charge passed, which would suggest that the oxide produced during the anodic sweep is not completely reduced during the cathodic sweep. A similar observation was also made by Alvarez et al 89, who suggested that a stable tin(IV) oxide was formed that couldn’t be fully reduced during the cathodic sweep. It can also be seen that the reduction peak in figures 4.9 and 4.10 shifted to more positive potentials after the initial cycle (table 4.1). This may suggest that an oxide is produced during the anodic sweep that cannot subsequently be reduced, i.e. the process is irreversible. It was also observed that when using switching potentials of 0 V and 0.6 V vs. SCE (figures 4.7 and 4.8) that a small anodic peak began to form between peaks 1 and 2.
For anodic switching potentials of 1.2 V and 1.5 V vs. SCE (figure 4.9 and 4.10) the additional oxidation peak is clearly present, there is also a reduction in the intensity of reduction peaks after the initial cycle. An anodic switching potential of 1.5 V vs. SCE also showed the presence of two additional reduction peaks, one at -0.5 V and another much broader peak at ~-0.95 V vs. SCE (figure 4.10).
-5.E-03 -3.E-03 -1.E-03 1.E-03 3.E-03 5.E-03
-1.5 -1.3 -1.1 -0.9 -0.7 -0.5 -0.3 -0.1
Current (A)
Potential (V vs. SCE)
Cycle 1 Cycle 3 Cycle 9
Formation of a 3rdoxidation peak
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If the position of the main reduction peak (~-1 V vs. SCE) is compared for figures 4.5 to 4.10, it can be seen that the peak shifts to more negative potentials as the anodic switching potential is increased (table 4.1). The additional reduction peak, at -0.5 V vs. SCE, increases in intensity as the number of cycles increases (figure 4.10); this additional reduction peak may have appeared to counteract or may be connected with the additional oxidation peak observed after the first cycle.
Figure 4.9 Cyclic voltammograms of electroplated tin on copper with a thickness of 5 µm in a potassium bicarbonate-carbonate oxidation bath with an anodic switching potential of 1.2 V vs. SCE and at a scan rate of 10 mV s -1
The shoulders present in figures 4.6 to 4.8 are not very well defined. This may, in part, be attributed to the scan rate of 10 mV s-1 used in these studies; a faster scan will increase the intensity of the peaks
87, however a slower scan rate may identify smaller peaks that a fast scan rate may miss. The shoulder in figure 4.9 is more clearly defined than the other shoulders, however, the maximum current is still not well defined in figure 4.10 since the peak overlaps the area created by the two adjacent peaks.
-5.E-03 -3.E-03 -1.E-03 1.E-03 3.E-03 5.E-03
-1.5 -1.3 -1.1 -0.9 -0.7 -0.5 -0.3 -0.1
Current (A)
Potential (V vs. SCE)
Cycle 1 Cycle 3 Cycle 9
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Figure 4.10 Cyclic voltammograms of electroplated tin on copper with a thickness of 5 µm in a potassium bicarbonate-carbonate oxidation bath with an anodic switching potential of 1.5 V vs. SCE and at a scan rate of 10 mV s -1
4.1.4 X-ray Photoelectron Spectroscopy Study of Single Cycles
To more fully understand the irreversible reaction that is occurring during cyclic voltammetry, x-ray photoelectron spectroscopy (XPS) analysis was carried out to investigate the oxide film present after single cyclic sweeps carried out using the same sweep parameters and anodic limits as those shown in figures 4.5 – 4.10. The surface oxide present on each sample was then studied using XPS high resolution scans within 24 h of the single cycle.
Figure 4.11 a and b show the raw data obtained using XPS for a native air-formed oxide and an electrochemically formed oxide produced at a potential of 2.0 V vs. Ag/AgCl, respectively. For both scans the left peak represents tin as oxide and the right peak represents tin as metal as the peak binding energies fit within published ranges 147. The raw data by itself needed to be split into its individual components to enable them to be quantified. To do this, a peak fitting function was carried out to best fit individual peaks to create an envelope that best matches the original scan. This was carried out for each high resolution scan, an example of which is shown in figure 4.11 c and d. Figure 4.11c shows a high resolution scan of the Sn3d5 peak for a native air-grown oxide, whilst figure 4.11d shows a high resolution scan for the Sn3d5 peak for an electrochemically formed oxide produced at
-5.E-03 -3.E-03 -1.E-03 1.E-03 3.E-03 5.E-03
-1.5 -1.3 -1.1 -0.9 -0.7 -0.5 -0.3 -0.1
Current (A)
Potential (V vs. SCE) Cycle 1 Cycle 3 Cycle 9
Additional reduction peak
109
2.0 V vs. Ag/AgCl. The blue lines represent the original scan obtained by XPS and the red lines are the envelope obtained after performing a peak fit. Figure 4.11a shows that two oxide peaks are needed to fit the envelope for the native air-formed oxide, whereas only one tin oxide peak is needed to fit the envelope for the electrochemically formed oxide (figure 4.11b); this suggests that the oxide film on the native air-formed oxide may contain a mixture of two tin oxide species. The narrow peak on the right in both figure 4.11a and 4.11b represents tin as metal and they both show that only one peak is needed to fit the envelope. It can be seen from figure 4.11 that there is much more tin oxide formed when it is formed electrochemically compared with a native air-formed oxide.
Figure 4.11 XPS high resolution scan of the Sn 3d5 peak, of a) a native air formed oxide and b) an electrochemically formed oxide produced at 2.0 V vs. Ag/AgCl, both on electroplated tin on copper samples that had been left for ~24 hr. With the corresponding peak fittings shown in c) for the native air-formed oxide and d) for the the electrochemically formed oxide
480
a) Native air-formed oxide scan b) Electrically formed oxide scan
c) Native air-formed oxide raw scan with peak fitting
d) Electrically formed oxide raw scan with peak fitting
110
Figure 4.12 XPS high resolution scans of the Snd3 peaks for electrochemically oxidised electroplated pure tin on copper formed after a single cycle with an anodic switching potential of a) -0.75 V vs. SCE b) -0.5 V vs. SCE c) 0 V vs. SCE d) 0.6 V vs.
SCE e) 1.2 V vs. SCE and f) a table showing the oxide to metal ratio for each high resolution scan and a native air-formed oxide
The XPS high resolution scans for each single cycle are shown in figure 4.12. It is seen from the high resolution Sn3d5 scans, that sweeping anodically to limits of -0.75 V and -0.5 V vs. SCE results in an oxide similar in thickness to a native air-formed oxide (figure 4.11), although there is a distinct
111
shoulder present in both scans shown showing that the oxide coating may be comprised of at least two tin oxides with different binding energies, which could mean that the oxide coating may contain both tin(II) and tin(IV) oxides (figures 4.12 a and b). This may suggest that the oxide produced during the anodic sweep is completely reduced during the cathodic sweep and subsequently a native air formed oxide was subsequently formed after removal from the oxidising bath. This would be consistent with the cyclic voltammograms shown in figures 4.5 and 4.6, as both show to have cathodic areas that are similar in size to the anodic area. However, anodically sweeping to an anodic limit of 0 V vs. SCE results in an oxide slightly thicker than that of a native air formed oxide, Again, this is consistent with the cyclic voltammograms shown in figures 4.7 to 4.9, as they all show that the cathodic area is much smaller than the anodic area. This suggests that the oxide produced during the anodic sweep is only partially reduced during the cathodic. Sweeping to anodic limits of 0.6 V and 1.2 V vs. SCE results in thick oxide films with similar intensity ratios of tin oxide to tin metal to those of electrochemically oxidised tin surfaces observed in previous studies 21,71,72. This suggests that the oxide formed during the anodic sweep is not reduced on the cathodic sweep.