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Study of electropolishing of a Zn anode in acidic medium
157
J. Electroanal. Chem., 266 (1989) 157-172 Elsevier Sequoia S.A., Lausanne - Printed
in The Netherlands
Study of electropolishing
of a 2% anode in acidic medium
M. Novak and A. Sziics Institute of General and Physrcal Chemrstry, Attila J&se/ P.O. Box 105 (Hungary) (Received
19 September
University, H-6701 Sreged,
1988; in revised form 20 February
1989)
ABSTRACT Electropolishing of a Zn anode was studied by voltammetric, open-circuit potential decay and photoelectrochemical measurements. The experiments give evidence for the existence of a solid layer on the surface. The solubility of this layer determines the surface concentration of the metal ions and thus the rate of diffusion. The thickness of the layer varies with the polarization potential but is independent of the hydrodynamic conditions. The layer is an n-type semiconductor and is probably an amorphous zinc oxide or hydroxide. Our results indicate a key role for water, but the rate determining step cannot be the diffusion of water to the electrode surface.
INTRODUCTION
Electropolishing is a well-known method for smoothing and brightening metal surfaces. Since the fundamental work of Jaquet [I], many experimental and theoretical contributions have been made in order to explain the process. It was concluded that the diffusion of some species [2-41 is rate determining. Demonstrating the presence of a solid layer on the electrode surface [5] demanded new investigations regarding its role in the electropolishing process [6-S]. These were carried out in situ, giving the possibility of a better understanding of the phenomena involved. Most of these studies were carried out on a copper anode where the possible presence of differently charged copper ions in the surface layer complicated the understanding of the process. In this paper we examine the features of a Zn anode during electropolishing, when such complications do not arise. EXPERIMENTAL
The electrolytic cell had a three-electrode arrangement with a rotating zinc anode (Koch-Light Laboratories, 99.999% purity, diameter 6 mm) embedded in Teflon. The Pt counter electrode was separated by a glass frit because the evolved small H, bubbles disturb the measurements involving light. As a reference, a saturated 0022-0728/89/.$03.50
0 1989 Elsevier Sequoia
SA
158
calomel electrode (SCE) was used. The solution was a 1 : 1 volume ratio mixture of 85% orthophosphoric acid and absolute ethanol 191. Before starting the measurements, the surface of the anode was polished mechanically, rinsed with distilled water and then electropolished for 15 min at 500 mV. After this treatment the surface became smooth and bright. In the potential decay measurements the circuit was opened by a Reed-relay and the potential was recorded with a digital waveform analyser (EMG 5500). The data were evaluated using a Videoton computer interfaced to the analyser. In the photocurrent me~urements the light source was a 450 W stabilized ~~-pressure Xe lamp. To obtain the spectral dependence of the photocurrent, the light beam crossed a high intensity monochromator (Bausch and Lomb). The incident light intensity on the electrode surface was measured at different wavelengths by means of a thermopile detector (Laser Instrumentation Ltd.) in the same arrangement as used for the photocurrent measurements, including the optical system and the solution. The spectral dependence of the quantum efficiency was calculated from these intensities as 77(S) = 100($l,,/e%)
(1)
at wavelength A, @)his the intensity and e where IP,,.,, is the measured photocurrent is the charge of an electron. The measurements were carried out without automatic ohmic compensation. but the potential data were corrected for the ohmic potential drop, calculated from the open-circuit potential decay measurements. RESULTS
AND DISCUSSION
Figure 1 shows the current-potential curves obtained after correction for the ohmic potential drop at a potential sweep rate of 90 mV/s and at different rotation rates of the anode. As the potential is increased, the current rises to a maximum, then it begins to decrease and attains a steady state. Above about 1200 mV the current increases very slightly again. In this region, after a longer electrolysis time (for example more than 10 min at 1500 mv) small bubbles appear on the surface. The polishing process occurs on the current plateau. In this region the current becomes higher with increasing rotation rate (w) of the anode. The plot of the limiting current vs. J/2 is linear, showing that the limiting process is the diffusion on the solution side of the surface layer. Open-circuit potential decays As in the method described in ref. 8, the measured potential-time data were transformed into the rate of potential change dE/dt which was considered a function of the potential. If an exponential discharge process is assumed to take place, the following relationship [lo] applies -CdE/dt=i=Aexp(BE)
(2)
159
Fig. 1. Polarization 1000 rpm.
curves at sweep rate of 90 mV SC’ and at various
rotation
rates: (a) 400, (b) 600, (c)
where C is the capacity, A and B are constants and the logarithm of the rate of potential change is a linear function of the potential. In Fig. 2 the potential relaxations from the potentials of the polishing region at w = 600 rpm are plotted after the transformation described above. It can be seen that an exponential discharge process takes place in the initial linear region of the curves (region I). The slope of the curves in this section and the extent of the potential interval in which it holds are similar. In region II the behaviour depends strongly on the polarization potential. Its interval increases with the starting potential.
Fig. 2. Ln( - dE/dr) 700, (f) 900 mV.
vs. E plots at various
polarization
potentials:
(a) - 300, (b) 100. (C) 30% (d) 500. (e)
160
Region III begins at the minimum of the curves. An important feature of this region is that the curves merge and then follow the same course. The end of this region, where the rate of potential change decreases sharply, is at - 890 mV. In the last region of potential relaxation (IV) the curves follow the same route, reaching a constant value; finally, the rest potential is attained, which is the same ( - 960 mV) in all cases. If the rotation rate of the anode is changed, the rate of relaxation changes too, but the character remains the same. Using the theory in ref. 8 concerning the depletion of the diffusion layer formed during electropolishing, the following relationship can be obtained for the change in potential: -d E,‘dt = (RT/zF)( rr2D/4S2) (3) where 6 is the thickness of the diffusion layer, which is proportional to o-‘/‘. Thus, during depletion of the diffusion layer a constant rate of potential change arises for constant S, as can be seen in the final stage in Fig. 2. This rate is linearly proportional to w, as could be confirmed experimentally. Consequently, the final region of the relaxation curves corresponds to the depletion of the diffusion layer and the dissolution of the solid layer occurs earlier. It can be seen clearly from Fig. 2 that region III differs from the others because the rate of potential change increases with decreasing potential. Accordingly, this cannot be attributed to a discharge process. It may therefore be assumed that region III is a result of dissolution of the solid surface layer. The common course after the curves merge shows that the structure of this layer is similar in all cases, but the different potential intervals suggest that the thickness of the solid layer increases with the polarization potential. This change in the thickness may cause the difference in region II, which can be attributed to relaxation of the charge distribution within solid layers of different capacities. Concerning region I, which is similar in all cases, it can be assumed that this potential difference is built up at the metal/solid layer interface and is responsible for the charge transfer. Since the diffusion flux is constant while the solid layer covers the surface (see ref. 8) the charge (Q,) corresponding to the solid layer at different polarizing potentials can be calculated as
Q, = ‘Otis
(4)
where I, is the diffusion limited current at the moment of opening the circuit, which is equal to the diffusion flux of the metal ions, and tdis is the time needed for dissolution of the solid surface layer, i.e. the time passed till the end of region III in Fig. 2. The calculated charge (Q,) vs. polarization potential is shown in Fig. 3 at w = 600 ‘pm. It can be seen that Q, increases linearly with the polarization potential in the region of polishing but the curve bends above 1200 mV. The slope of the linear section was 4.05 X 10m4 A s V’. Changing the rotation rate of the anode did not affect the charge calculated at any potential.
161
T-
-0.5
Fig. 3. The charge
olo
015 1.0
(Q,) corresponding
1.5
2:o
2.5
E/V-
to the solid layer vs. polarization
potential
at w = 600 rpm.
region show a Relaxations from polarization potentials outside the polishing different character above 1200 mV, as below 1200 mV each region of potential relaxation is varied, but the final rest potential remains the same and the end of region III is shifted to - 860 mV. Due to the appearance of bubbles and to the slightly increasing current in this region, we may assume that the different character of potential relaxation indicates a change in the anodic process which causes the bend of the curve in Fig. 3. Transient current caused by potential step Further investigations were carried out in order to see whether the assumptions made in connection with the calculation of Q, of the solid layer are valid. In these measurements the effect of different potential steps on the current was studied. The initial polarization potential was 500 mV in the polishing region. The potential step was applied in the steady state when the diffusion and the solid surface layer have already formed. The value of the initial potential (500 mV) does not involve the ohmic potential drop. As the current changes during the experiment, the ohmic drop changes, too. Thus, the total potential step is due partly to the increase in ohmic drop and partly to the increase in potential difference at the metal/solution interface. The relative importance of each varies with the current. Applying the potential step, the potential changed from its initial value in less than 50 ps. The change in current caused by a 700 mV potential step can be seen in Fig. 4. The current jumps at the moment the potential is stepped and then it returns rapidly to the initial value. The current-time behaviour could be reproduced very well. Therefore, it was possible to study the changes of the interface as a function of time and to correlate the results in different measurements. Thus, after a reset time passed following the potential step the circuit was opened and the occurrence of the potential relaxation was examined (Fig. 5). Curve a shows the relaxation from the
162
IO-+
20
0 Fig. 4. Change
LO
of current
> 60
80
100
with time on applying
t/ms a 700 mV potential step. For the marks,
see Fig. 5.
initial potential, i.e. from 500 mV. If the circuit was opened immediately after the potential step (curve b in Fig. 5) the relaxation varies in region I, the other parts remaining the same. As can be seen, the increase in the potential interval of region I is only part of the total (700 mV) step. The remaining potential change occurred as ohmic potential drop in the solution because of the increased current. If the circuit was opened after 60 ms following the potential step (see curve c in Fig. 5) region II shows a significant change too, but region III remains the same as in the initial state. After about 150 ms the current returns to the steady state and the relaxation
-1,o
-0.5
0.0
0.5
1.0 E/V
Fig. 5. Potential relaxations after different times passed following the potential state, (b) after 450 ps, (c) after 60 ms and (d) after 150 ms. See Fig. 4.
step: (a) at the initial
163
curve does not change further (curve d in Fig. 5). The increased polarization potential increases the interval of regions II and III and the ohmic drop reaches its initial value. In all of these measurements the last section of the potential relaxation, corresponding to the depletion of the diffusion layer, remained the same as it was in Fig. 2. It can be concluded from these potential step measurements that the potential distribution varies with time, indicating changes in the overall metal/solution interface. The fact that the relaxation in region II remains almost the same at short times after the potential step, while the current is twice as high as before the potential step, shows that a higher current can flow through the same solid layer in the steady state, i.e. the rate determining step cannot be the flux through this layer. The steady state can be perturbed in two different ways. One of them is to change the hydrodynamic conditions and the other, to change the potential (Fig. 5). It can be seen that in the former case the increase in current caused by increasing diffusion does not change the relaxation regions, while in the latter case, although the current is much higher than in the initial state, the current increase does not affect the relaxation in region III. Thus, it may be assumed that the diffusion process cannot be disturbed by means of the potential step and the diffusion flux of metal ions into the solution remains the same. Thus, the excess charge due to the current increased causes an increase in the thickness of the solid surface layer and accordingly a change in potential regions II and III. With increasing thickness of the solid layer, the potential drop across this layer increases and with decreasing current the ohmic drop in the solution decreases. On this basis the current-time transient can be described as follows. (i) The current flowing through the interface is due to the charge transfer at the metal/surface layer interface. It can be given as derived from the relaxation curves as I, = A exp BE,
(5)
where I, is the steady-state current at the initial potential E, (500 mV), and A and B are constants. If the potential difference at the interface varies by AE, then Z=AexpB(E,,+AE,)=Aexp(BE,)exp(BAE,)=Z,exp(BAE,) The excess potential A E, = (l/B)
difference
from eqn. (6) is
ln( Z/ZO)
(ii) The potential drop across the solid layer depends on the thickness, changes during the potential step with the excess charge. Thus, AE,,=kj-‘(Z-I,) 0
dt
(6)
(7) which
(8)
where AE,, is the change in potential difference across the solid layer due to the excess charge /d( Z - I,) dt, k is constant and Z is the current during the potential step.
164
(iii) To obtain the overall potential difference we have to take into account the increase in ohmic drop in the solution due to the increased current AE,=(I-Io)R,
(9)
where AE, is the change in ohmic drop due to the current change from I, to I and R, is the resistivity of the solution between the working electrode and the tip of the Luggin capillary. Since the potential step is constant, E = 700 mV, the equation AE = AE, + AE,, + AE, must be fulfilled. obtain
(10)
Inserting
the potentials
from eqns. (7)-(9)
into eqn. (lo),
AE=(l/R)ln(l/ZO)+klr(l-I,)dt+(Z-I,)R,
we
(II)
0
Solving this equation for the time I=:-%
ln(Z-I,)-
&
In?
(12)
where ln(l(t=O)-I,)-+
In 1(t=O)
(13)
0
C is a constant and I = Z( t = 0) if t = 0. From the measured current-time data, the parameters C/k, R,/k and l/kI,B were obtained by means of non-linear curve fitting; they are given in Table 1. In order to check the validity of the assumptions applied in eqns. (5)-(9), one has to make use of determinations of the values given in Table 1 independently from the current-time data. The value of B can be obtained from the slope of region I in Fig. 2. It yields B = 4.5 V-‘. The slope of the charge vs. electrode potential curve in the polishing region gives k = 2.48 x lo3 V’ A s-‘. On the other hand, 1, = 15.85 mA and I(t = 0) = 35.05 mA. Taking into account these values, the parameters given in the second column of Table 1 were obtained. The agreement of the parameters suggests that eqn. (11) describes the behaviour of the system properly and that the assumptions made are valid.
TABLE I Parameters of the potential step Parameter
Curve fitting
Independently
lo3 (C/k)/s
- 5.25 1.24 6.15
- 5.33 1.26 5.89
10’ (R,/k)/s lo3 (l/kl,B)/s
165
L
PA
t
200
-Iph PA 150
100
50
(
0
12
3
4
5
6
t/s
-1
1
0
Fig. 6. Photoresponse of the system excited with white light at a polarization a stationary electrode, (II) at an electrode rotating at 600 ‘pm. Fig. 7. Dark current and photocurrent rotation rate of 600 rpm. The chopper
vs. potential curves at a potential frequency was 22 Hz.
potential
2
t,"
of 500 mV. (I) At
sweep rate of 10 mV SC’ and a
If different potential steps are applied the integral of the excess current varies linearly with the potential extent of the step. The slope of the data agrees with the slope obtained from the potential relaxation data in Fig. 3. This supports the view that the charge calculated from the relaxation is indeed the one accumulated on the electrode as a solid surface layer which dissolves from the metal surface. Photoeffect of zinc anode The photoresponse of the system excited with a flash of white light is illustrated in Fig. 6, for stationary and for rotating electrodes. It can be seen that in both cases the current increases rapidly at the moment the light is switched on (section a in Fig. 6) but this is followed by a slower change in section b. The change in current depends strongly on the rotation rate of the anode. To separate the rapid response, we chopped the light with different frequencies, and the alternating photocurrent was measured as a function of potential. Figure 7 shows the dark current and the photocurrent at a rotation rate of 600 rpm and a chopper frequency of 22 Hz. As can be seen, the photocurrent can be detected only immediately before the current maximum. It rises sharply when the dark current becomes diffusion limited and in the polishing region its value is practically independent of the potential. The photocurrent increases after 1200 mV where the dark current increases slightly too, and the open-circuit measurements show a change in the structure of the surface layer. With decreasing chopper frequency the amplitude of the photocurrent increases but the character remains the same. Similar changes can be observed if the rotation rate of the anode is varied. The spectral dependence of the quantum
Fig. 8. Spectral dependence of the quantum efficiency for a stationary electrode at a chopper frequency of 1.5 Hz. (a) 800 mV polarization potential (- - -); (b) 2100 mV potential ().
efficiency, calculated with eqn. (l), is shown in Fig. 8 (curve a) for a stationary electrode held at a potential of 800 mV. It can be seen that the system can be excited in practically the same way in the whole range of wavelengths of light studied. These values remain the same at any potential in the polishing region but vary with chopper frequency and with rotation rate as in the case of white light. At potentials above 1200 mV the spectral character changes at the shorter wavelengths (Fig. 8). A peak appears at about 420 nm, but at larger wavelengths the shape remains the same as below 1200 mV. The beginning of the peak corresponds to an energy of 3 eV, which is the bandgap of ZnO [l]. From these observations it was concluded that in the polishing region an amorphous surface layer exists on the electrode, which can be excited in the whole wavelength range. Above 1200 mV, part of the surface layer is transformed to crystalline ZnO with a well-defined structure and bandgap. The fact that the photocurrent induced by monochromatic light depends in the same way on the rotation rate and on the chopper frequency as in the case of white light indicates that the curve shape in Fig. 6 is valid for the time dependence of monochromatic photocurrents as well, although we cannot measure this current-time behaviour. Namely, the slow change in current in section b of Fig. 6 is not a consequence of the use of white light (for example in the heating of the electrode or solution) but is caused by the photoresponse of the system. If the illumination time was increased, an interesting character of current change was obtained, as can be seen for a stationary electrode and at a rotation rate of 600 t-pm in Fig. 9. In the case of a rotating electrode the current increases continuously, but at a stationary electrode, the current first increases, attains a maximum, then decreases to a minimum and slowly increases again. A remarkable feature of this character is that the minimum is below the dark current level. Switching off the light after
b
lightOn 0
> 2
4
6
Fig. 9. Change in current electrode at 600 ‘pm.
8
10
12
1L
due to long illumination
tImIn
with white light at 500 mV; (a) stationary,
20(
(b) rotating
/
400.
/ c
/
d 0
2
Fig. 10. Current of illumination.
L
change
6
a
at a stationary
10 timin
electrode
0
1 2
at 500 mV on switching
Fig. 11. Change in current caused by long illumination. and closing it again after 130 ms.
Vertical
6
a
10
off the light after various
lines correspond
to opening
t/mNl
times
the circuit
168
different times of illumination changes the current, as can be seen in Fig. 10, where the dark current is taken as 0. The change in current for continuous illumination is shown in curve a. If the light is switched off at the current maximum (curve b) the current falls far below the steady-state value, then increases slowly to its original level. This character is similar to the case when the switching off takes place at the minimum (curve c). In the last case the current decreases to the original level without going far below it (curve d). Since the overall current is the sum of the photoeffect and the dark current, the illumination affects the steady state of the polishing process and not only increases, but also decreases its rate. These effects cannot be explained by taking into account the direct photoeffect only, but one has to consider other processes too. Studying the open-circuit potential relaxation after different illumination times, it was not possible to detect any significant changes. The shape and the rate of potential relaxation were practically the same as without light. Thus, the observed change in the current is not connected to a significant change in potential distribution or in the structure of the solid layer. Furthermore, one can determine from the relaxation the time needed for the dissolution of the solid layer (130 ms) and during that time no depletion of the diffusion layer occurs. If, after the photocurrent measurement, the circuit was opened after different illumination times for the period needed for dissolution and then was closed again, it was observed (Fig. 11) that the shape of the current-time curve remained the same as it was in Fig. lOa,
800
0
I 2
Fig. 12. Current Fig. 11.
4
change
600
6
8
under illumination
Fig. 13. Change in current vs. illumination 7.13, (b) 9.43, (c) 12.94 M H,O.
10
t$hn
>
0
when the circuit
time at various
1
was opened
2
3
4
for 1 s, instead
water concentrations.
5
timln
of 130 ms as in
Base solution
with (a)
169
Fig. 14. Change in current vs. illumination 8.58, (b) 8.98, (c) 9.60 M EtOH.
time at various
ethanol
concentrations.
Base solution
with (a)
although the surface layer dissolved and formed again. Thus, it may be assumed that the slow change in current during illumination is not the consequence of either the change in the solid surface layer or of a slow change in photoeffect. If the circuit was opened for a longer time (15 s), and the depletion of the diffusion layer had already begun, the shape of the current-time curve changed as shown in Fig. 12. Therefore, one has to conclude that the photoeffect or the consequences of the primary photoeffect reach out into the diffusion layer, probably through a change in concentration of the species which are involved in the process connected to the light effect. In order to reach a conclusion regarding these solution species, the influence of additional amounts of water or ethanol was studied. As the water concentration of the solution was increased, the dark current increased strongly and the time interval of current change during the illumination decreased accordingly (Fig. 13). With increasing ethanol concentration the dark current increased slightly and the time interval of current change during the illumination also increased (Fig. 14). If the diffusion rate of water is higher, the time interval between the maximum and minimum decreases and at large diffusion rates, for example when the electrode is rotated, it disappears (see Fig. 9). The shape of the current-time curve also varies under the same diffusion conditions with the intensity of light. In Fig. 15 the light intensity was one half and one quarter of the original value. It can be seen that the change in current is proportional to the intensity and the time interval of the effect increases. The change in current (under the same diffusion conditions) is proportional to some integral result of the primary photoeffect. From the photocurrent measurements it can be concluded that the surface layer behaves as an n-type semiconductor in the whole potential region because of the positive photocurrent. The effect of electron-hole pair generation induced by light may be the photodecomposition of the semiconductor (ZnO) and the oxidation of water or ethanol [ll]. The influence of the addition of water and ethanol shows that in this system water plays an important role. It seems probable that the primary photoeffect is the
. 0
2
4
6
a
Fig. 15. Effect of light intensity on change the light intensity applied for curve a.
10
12
in current.
t/mln (a) Full intensity,
(b) one half. (c) one quarter
of
oxidation of water by holes (h+) 2H,0+4h++4HH++0,
(14)
and this also causes a perturbation of the steady state of the polishing process. This perturbation may be the consequence of water consumption and/or the generation of hydrogen ions and oxygen on the surface. To find the connection between the polishing process and the primary photoeffect we have to take into account the following phenomena obtained from voltammetric measurements. The polishing current is diffusion limited (Fig. 1). From the potential step measurements we can conclude that the limiting process, which brings the current to the steady state, may be the flux of Zn 2+ ions through the solid layer or the flux of Zn2+ ions into the solution. The potential relaxation curve measured immediately after the current jump (Fig. 5b) does not show the influence of an increased current from region II, in contrast to the case where the change in current was caused by increased diffusion. Because we did not find any significant change in the charge corresponding to the solid layer if the rotation rate of the electrode was varied, i.e. the thickness of the surface solid layer did not decrease although the current increased, it seems probable that the limiting process is not the diffusion of metal ions through the solid layer. Thus, the diffusion of Zn ‘+ ions through the diffusion layer determines the polishing current, and this is the limiting process. To form a constant flux of Zn2’ ions when the concentration in the bulk of the solution and the thickness of the diffusion layer are constant, the surface concentration of metal ions must be constant, too. This is the case when the solubility of the solid surface layer determines the surface concentration, and the Zn2+ ions exist in hydrated form in the solution: ZnO + n H,O + Zn’+(H,O).
(15)
171
Thus, the dissolution of the ZnO layer is sensitive to the hydrogen ion concentration and it needs water molecules for hydration of the Zn2+ ions. If the H+ concentration increases on the surface, the increased solubility of the ZnO layer causes a higher Zn 2+ ion concentration. If the thickness of the diffusion layer remains the same, the diffusion flux increases. If the amount of water on the surface decreases, the hydration of metal ions cannot occur and thus the solubility decreases: ZnO+2
H++n
H,O + Zn2+(H20).
+ H,O
Taking into account eqn. (14) for the primary photoeffect, it can be seen that this causes a change both in the water concentration and in the H+ ion concentration. As mentioned above, these two changes have different effects on the solubility of the solid surface layer. If there is enough water on the surface to hydrate the metal ions, the increase in H+ ion concentration increases the surface concentration and the diffusion flux which determines the polishing current. If the water consumption due to the photoreaction is high, the polishing current decreases, because of the decreased solubility of the solid layer. The latter case may be realized when the diffusion rate of water to the surface is too low to compensate the excess consumption due to the photoreaction. It may be assumed that these two effects cause the change in current during long illumination (Fig. 10a). First the current increases due to the increased H+ ion concentration generated by the photoreaction. Then, as the photoreaction and the increased current consume the excess water and the diffusion rate of water is too low to compensate it, the current consumes the excess water and the current begins to decrease. After a long time, when the diffusion flux increases because of the lower surface concentration, a new steady state is approached and a current higher than the original one arises. If the diffusion rate of water is much higher, for example when the electrode is rotated, it can compensate the excess consumption rapidly and the decrease in current cannot be observed (Fig. 9a). On the basis of these observations we can concluded that water plays an important role in the process but its diffusion cannot be rate determining because its surface concentration is not limited in the steady state. The rate determining process of the electropolishing of a Zn anode is the diffusion of metal ions into the solution. Its value depends on the surface concentration of Zn” ions, which can be modified by changing the solubility of the surface layer. REFERENCES 1 2 3 4 5 6 7
P.A. Jaquet, Nature (London), 135 (1935) 1076. W.C. Elmore, J. Appl. Phys., 11 (1940) 797. J. Edwards, J. Electrochem. Sot., 100 (1953) 189. K. Kojima and C.W. Tobias, J. Electrochem. Sot., 120 (1973) 1026. M. Nova, A.K.N. Reddy and H. Wroblowa, J. Electrochem. Sot., 117 (1970) 753. B. Pointu, M. Bra&x, P. Poncet and J. Rousseau, J. Electroanal. Chem., 122 (1981) 111. M. No-& and A. Sziics, J. Electroanal. Chem., 210 (1986) 229.
172 8 9 10 11
M. NovAk and A. Sziics, J. Electroanal. Chem., 210 (1986) 237. R. Kircheim, K. Maier and G. Fiilg, J. Electrochem. Sot., 128 (1981) 1027. J.L. Ord and D.J. De&et, J. Electrochem. Sot., 116 (1969) 763. H. Gerischer, in F. Cardon (Ed.), Photovoltaic and Photoelectrochemical Solar Energy Plenum Press, New York, 1981, p. 239.
Conversion,
J. Electroanal. Chem., 266 (1989) 157-172 Elsevier Sequoia S.A., Lausanne - Printed
in The Netherlands
Study of electropolishing
of a 2% anode in acidic medium
M. Novak and A. Sziics Institute of General and Physrcal Chemrstry, Attila J&se/ P.O. Box 105 (Hungary) (Received
19 September
University, H-6701 Sreged,
1988; in revised form 20 February
1989)
ABSTRACT Electropolishing of a Zn anode was studied by voltammetric, open-circuit potential decay and photoelectrochemical measurements. The experiments give evidence for the existence of a solid layer on the surface. The solubility of this layer determines the surface concentration of the metal ions and thus the rate of diffusion. The thickness of the layer varies with the polarization potential but is independent of the hydrodynamic conditions. The layer is an n-type semiconductor and is probably an amorphous zinc oxide or hydroxide. Our results indicate a key role for water, but the rate determining step cannot be the diffusion of water to the electrode surface.
INTRODUCTION
Electropolishing is a well-known method for smoothing and brightening metal surfaces. Since the fundamental work of Jaquet [I], many experimental and theoretical contributions have been made in order to explain the process. It was concluded that the diffusion of some species [2-41 is rate determining. Demonstrating the presence of a solid layer on the electrode surface [5] demanded new investigations regarding its role in the electropolishing process [6-S]. These were carried out in situ, giving the possibility of a better understanding of the phenomena involved. Most of these studies were carried out on a copper anode where the possible presence of differently charged copper ions in the surface layer complicated the understanding of the process. In this paper we examine the features of a Zn anode during electropolishing, when such complications do not arise. EXPERIMENTAL
The electrolytic cell had a three-electrode arrangement with a rotating zinc anode (Koch-Light Laboratories, 99.999% purity, diameter 6 mm) embedded in Teflon. The Pt counter electrode was separated by a glass frit because the evolved small H, bubbles disturb the measurements involving light. As a reference, a saturated 0022-0728/89/.$03.50
0 1989 Elsevier Sequoia
SA
158
calomel electrode (SCE) was used. The solution was a 1 : 1 volume ratio mixture of 85% orthophosphoric acid and absolute ethanol 191. Before starting the measurements, the surface of the anode was polished mechanically, rinsed with distilled water and then electropolished for 15 min at 500 mV. After this treatment the surface became smooth and bright. In the potential decay measurements the circuit was opened by a Reed-relay and the potential was recorded with a digital waveform analyser (EMG 5500). The data were evaluated using a Videoton computer interfaced to the analyser. In the photocurrent me~urements the light source was a 450 W stabilized ~~-pressure Xe lamp. To obtain the spectral dependence of the photocurrent, the light beam crossed a high intensity monochromator (Bausch and Lomb). The incident light intensity on the electrode surface was measured at different wavelengths by means of a thermopile detector (Laser Instrumentation Ltd.) in the same arrangement as used for the photocurrent measurements, including the optical system and the solution. The spectral dependence of the quantum efficiency was calculated from these intensities as 77(S) = 100($l,,/e%)
(1)
at wavelength A, @)his the intensity and e where IP,,.,, is the measured photocurrent is the charge of an electron. The measurements were carried out without automatic ohmic compensation. but the potential data were corrected for the ohmic potential drop, calculated from the open-circuit potential decay measurements. RESULTS
AND DISCUSSION
Figure 1 shows the current-potential curves obtained after correction for the ohmic potential drop at a potential sweep rate of 90 mV/s and at different rotation rates of the anode. As the potential is increased, the current rises to a maximum, then it begins to decrease and attains a steady state. Above about 1200 mV the current increases very slightly again. In this region, after a longer electrolysis time (for example more than 10 min at 1500 mv) small bubbles appear on the surface. The polishing process occurs on the current plateau. In this region the current becomes higher with increasing rotation rate (w) of the anode. The plot of the limiting current vs. J/2 is linear, showing that the limiting process is the diffusion on the solution side of the surface layer. Open-circuit potential decays As in the method described in ref. 8, the measured potential-time data were transformed into the rate of potential change dE/dt which was considered a function of the potential. If an exponential discharge process is assumed to take place, the following relationship [lo] applies -CdE/dt=i=Aexp(BE)
(2)
159
Fig. 1. Polarization 1000 rpm.
curves at sweep rate of 90 mV SC’ and at various
rotation
rates: (a) 400, (b) 600, (c)
where C is the capacity, A and B are constants and the logarithm of the rate of potential change is a linear function of the potential. In Fig. 2 the potential relaxations from the potentials of the polishing region at w = 600 rpm are plotted after the transformation described above. It can be seen that an exponential discharge process takes place in the initial linear region of the curves (region I). The slope of the curves in this section and the extent of the potential interval in which it holds are similar. In region II the behaviour depends strongly on the polarization potential. Its interval increases with the starting potential.
Fig. 2. Ln( - dE/dr) 700, (f) 900 mV.
vs. E plots at various
polarization
potentials:
(a) - 300, (b) 100. (C) 30% (d) 500. (e)
160
Region III begins at the minimum of the curves. An important feature of this region is that the curves merge and then follow the same course. The end of this region, where the rate of potential change decreases sharply, is at - 890 mV. In the last region of potential relaxation (IV) the curves follow the same route, reaching a constant value; finally, the rest potential is attained, which is the same ( - 960 mV) in all cases. If the rotation rate of the anode is changed, the rate of relaxation changes too, but the character remains the same. Using the theory in ref. 8 concerning the depletion of the diffusion layer formed during electropolishing, the following relationship can be obtained for the change in potential: -d E,‘dt = (RT/zF)( rr2D/4S2) (3) where 6 is the thickness of the diffusion layer, which is proportional to o-‘/‘. Thus, during depletion of the diffusion layer a constant rate of potential change arises for constant S, as can be seen in the final stage in Fig. 2. This rate is linearly proportional to w, as could be confirmed experimentally. Consequently, the final region of the relaxation curves corresponds to the depletion of the diffusion layer and the dissolution of the solid layer occurs earlier. It can be seen clearly from Fig. 2 that region III differs from the others because the rate of potential change increases with decreasing potential. Accordingly, this cannot be attributed to a discharge process. It may therefore be assumed that region III is a result of dissolution of the solid surface layer. The common course after the curves merge shows that the structure of this layer is similar in all cases, but the different potential intervals suggest that the thickness of the solid layer increases with the polarization potential. This change in the thickness may cause the difference in region II, which can be attributed to relaxation of the charge distribution within solid layers of different capacities. Concerning region I, which is similar in all cases, it can be assumed that this potential difference is built up at the metal/solid layer interface and is responsible for the charge transfer. Since the diffusion flux is constant while the solid layer covers the surface (see ref. 8) the charge (Q,) corresponding to the solid layer at different polarizing potentials can be calculated as
Q, = ‘Otis
(4)
where I, is the diffusion limited current at the moment of opening the circuit, which is equal to the diffusion flux of the metal ions, and tdis is the time needed for dissolution of the solid surface layer, i.e. the time passed till the end of region III in Fig. 2. The calculated charge (Q,) vs. polarization potential is shown in Fig. 3 at w = 600 ‘pm. It can be seen that Q, increases linearly with the polarization potential in the region of polishing but the curve bends above 1200 mV. The slope of the linear section was 4.05 X 10m4 A s V’. Changing the rotation rate of the anode did not affect the charge calculated at any potential.
161
T-
-0.5
Fig. 3. The charge
olo
015 1.0
(Q,) corresponding
1.5
2:o
2.5
E/V-
to the solid layer vs. polarization
potential
at w = 600 rpm.
region show a Relaxations from polarization potentials outside the polishing different character above 1200 mV, as below 1200 mV each region of potential relaxation is varied, but the final rest potential remains the same and the end of region III is shifted to - 860 mV. Due to the appearance of bubbles and to the slightly increasing current in this region, we may assume that the different character of potential relaxation indicates a change in the anodic process which causes the bend of the curve in Fig. 3. Transient current caused by potential step Further investigations were carried out in order to see whether the assumptions made in connection with the calculation of Q, of the solid layer are valid. In these measurements the effect of different potential steps on the current was studied. The initial polarization potential was 500 mV in the polishing region. The potential step was applied in the steady state when the diffusion and the solid surface layer have already formed. The value of the initial potential (500 mV) does not involve the ohmic potential drop. As the current changes during the experiment, the ohmic drop changes, too. Thus, the total potential step is due partly to the increase in ohmic drop and partly to the increase in potential difference at the metal/solution interface. The relative importance of each varies with the current. Applying the potential step, the potential changed from its initial value in less than 50 ps. The change in current caused by a 700 mV potential step can be seen in Fig. 4. The current jumps at the moment the potential is stepped and then it returns rapidly to the initial value. The current-time behaviour could be reproduced very well. Therefore, it was possible to study the changes of the interface as a function of time and to correlate the results in different measurements. Thus, after a reset time passed following the potential step the circuit was opened and the occurrence of the potential relaxation was examined (Fig. 5). Curve a shows the relaxation from the
162
IO-+
20
0 Fig. 4. Change
LO
of current
> 60
80
100
with time on applying
t/ms a 700 mV potential step. For the marks,
see Fig. 5.
initial potential, i.e. from 500 mV. If the circuit was opened immediately after the potential step (curve b in Fig. 5) the relaxation varies in region I, the other parts remaining the same. As can be seen, the increase in the potential interval of region I is only part of the total (700 mV) step. The remaining potential change occurred as ohmic potential drop in the solution because of the increased current. If the circuit was opened after 60 ms following the potential step (see curve c in Fig. 5) region II shows a significant change too, but region III remains the same as in the initial state. After about 150 ms the current returns to the steady state and the relaxation
-1,o
-0.5
0.0
0.5
1.0 E/V
Fig. 5. Potential relaxations after different times passed following the potential state, (b) after 450 ps, (c) after 60 ms and (d) after 150 ms. See Fig. 4.
step: (a) at the initial
163
curve does not change further (curve d in Fig. 5). The increased polarization potential increases the interval of regions II and III and the ohmic drop reaches its initial value. In all of these measurements the last section of the potential relaxation, corresponding to the depletion of the diffusion layer, remained the same as it was in Fig. 2. It can be concluded from these potential step measurements that the potential distribution varies with time, indicating changes in the overall metal/solution interface. The fact that the relaxation in region II remains almost the same at short times after the potential step, while the current is twice as high as before the potential step, shows that a higher current can flow through the same solid layer in the steady state, i.e. the rate determining step cannot be the flux through this layer. The steady state can be perturbed in two different ways. One of them is to change the hydrodynamic conditions and the other, to change the potential (Fig. 5). It can be seen that in the former case the increase in current caused by increasing diffusion does not change the relaxation regions, while in the latter case, although the current is much higher than in the initial state, the current increase does not affect the relaxation in region III. Thus, it may be assumed that the diffusion process cannot be disturbed by means of the potential step and the diffusion flux of metal ions into the solution remains the same. Thus, the excess charge due to the current increased causes an increase in the thickness of the solid surface layer and accordingly a change in potential regions II and III. With increasing thickness of the solid layer, the potential drop across this layer increases and with decreasing current the ohmic drop in the solution decreases. On this basis the current-time transient can be described as follows. (i) The current flowing through the interface is due to the charge transfer at the metal/surface layer interface. It can be given as derived from the relaxation curves as I, = A exp BE,
(5)
where I, is the steady-state current at the initial potential E, (500 mV), and A and B are constants. If the potential difference at the interface varies by AE, then Z=AexpB(E,,+AE,)=Aexp(BE,)exp(BAE,)=Z,exp(BAE,) The excess potential A E, = (l/B)
difference
from eqn. (6) is
ln( Z/ZO)
(ii) The potential drop across the solid layer depends on the thickness, changes during the potential step with the excess charge. Thus, AE,,=kj-‘(Z-I,) 0
dt
(6)
(7) which
(8)
where AE,, is the change in potential difference across the solid layer due to the excess charge /d( Z - I,) dt, k is constant and Z is the current during the potential step.
164
(iii) To obtain the overall potential difference we have to take into account the increase in ohmic drop in the solution due to the increased current AE,=(I-Io)R,
(9)
where AE, is the change in ohmic drop due to the current change from I, to I and R, is the resistivity of the solution between the working electrode and the tip of the Luggin capillary. Since the potential step is constant, E = 700 mV, the equation AE = AE, + AE,, + AE, must be fulfilled. obtain
(10)
Inserting
the potentials
from eqns. (7)-(9)
into eqn. (lo),
AE=(l/R)ln(l/ZO)+klr(l-I,)dt+(Z-I,)R,
we
(II)
0
Solving this equation for the time I=:-%
ln(Z-I,)-
&
In?
(12)
where ln(l(t=O)-I,)-+
In 1(t=O)
(13)
0
C is a constant and I = Z( t = 0) if t = 0. From the measured current-time data, the parameters C/k, R,/k and l/kI,B were obtained by means of non-linear curve fitting; they are given in Table 1. In order to check the validity of the assumptions applied in eqns. (5)-(9), one has to make use of determinations of the values given in Table 1 independently from the current-time data. The value of B can be obtained from the slope of region I in Fig. 2. It yields B = 4.5 V-‘. The slope of the charge vs. electrode potential curve in the polishing region gives k = 2.48 x lo3 V’ A s-‘. On the other hand, 1, = 15.85 mA and I(t = 0) = 35.05 mA. Taking into account these values, the parameters given in the second column of Table 1 were obtained. The agreement of the parameters suggests that eqn. (11) describes the behaviour of the system properly and that the assumptions made are valid.
TABLE I Parameters of the potential step Parameter
Curve fitting
Independently
lo3 (C/k)/s
- 5.25 1.24 6.15
- 5.33 1.26 5.89
10’ (R,/k)/s lo3 (l/kl,B)/s
165
L
PA
t
200
-Iph PA 150
100
50
(
0
12
3
4
5
6
t/s
-1
1
0
Fig. 6. Photoresponse of the system excited with white light at a polarization a stationary electrode, (II) at an electrode rotating at 600 ‘pm. Fig. 7. Dark current and photocurrent rotation rate of 600 rpm. The chopper
vs. potential curves at a potential frequency was 22 Hz.
potential
2
t,"
of 500 mV. (I) At
sweep rate of 10 mV SC’ and a
If different potential steps are applied the integral of the excess current varies linearly with the potential extent of the step. The slope of the data agrees with the slope obtained from the potential relaxation data in Fig. 3. This supports the view that the charge calculated from the relaxation is indeed the one accumulated on the electrode as a solid surface layer which dissolves from the metal surface. Photoeffect of zinc anode The photoresponse of the system excited with a flash of white light is illustrated in Fig. 6, for stationary and for rotating electrodes. It can be seen that in both cases the current increases rapidly at the moment the light is switched on (section a in Fig. 6) but this is followed by a slower change in section b. The change in current depends strongly on the rotation rate of the anode. To separate the rapid response, we chopped the light with different frequencies, and the alternating photocurrent was measured as a function of potential. Figure 7 shows the dark current and the photocurrent at a rotation rate of 600 rpm and a chopper frequency of 22 Hz. As can be seen, the photocurrent can be detected only immediately before the current maximum. It rises sharply when the dark current becomes diffusion limited and in the polishing region its value is practically independent of the potential. The photocurrent increases after 1200 mV where the dark current increases slightly too, and the open-circuit measurements show a change in the structure of the surface layer. With decreasing chopper frequency the amplitude of the photocurrent increases but the character remains the same. Similar changes can be observed if the rotation rate of the anode is varied. The spectral dependence of the quantum
Fig. 8. Spectral dependence of the quantum efficiency for a stationary electrode at a chopper frequency of 1.5 Hz. (a) 800 mV polarization potential (- - -); (b) 2100 mV potential ().
efficiency, calculated with eqn. (l), is shown in Fig. 8 (curve a) for a stationary electrode held at a potential of 800 mV. It can be seen that the system can be excited in practically the same way in the whole range of wavelengths of light studied. These values remain the same at any potential in the polishing region but vary with chopper frequency and with rotation rate as in the case of white light. At potentials above 1200 mV the spectral character changes at the shorter wavelengths (Fig. 8). A peak appears at about 420 nm, but at larger wavelengths the shape remains the same as below 1200 mV. The beginning of the peak corresponds to an energy of 3 eV, which is the bandgap of ZnO [l]. From these observations it was concluded that in the polishing region an amorphous surface layer exists on the electrode, which can be excited in the whole wavelength range. Above 1200 mV, part of the surface layer is transformed to crystalline ZnO with a well-defined structure and bandgap. The fact that the photocurrent induced by monochromatic light depends in the same way on the rotation rate and on the chopper frequency as in the case of white light indicates that the curve shape in Fig. 6 is valid for the time dependence of monochromatic photocurrents as well, although we cannot measure this current-time behaviour. Namely, the slow change in current in section b of Fig. 6 is not a consequence of the use of white light (for example in the heating of the electrode or solution) but is caused by the photoresponse of the system. If the illumination time was increased, an interesting character of current change was obtained, as can be seen for a stationary electrode and at a rotation rate of 600 t-pm in Fig. 9. In the case of a rotating electrode the current increases continuously, but at a stationary electrode, the current first increases, attains a maximum, then decreases to a minimum and slowly increases again. A remarkable feature of this character is that the minimum is below the dark current level. Switching off the light after
b
lightOn 0
> 2
4
6
Fig. 9. Change in current electrode at 600 ‘pm.
8
10
12
1L
due to long illumination
tImIn
with white light at 500 mV; (a) stationary,
20(
(b) rotating
/
400.
/ c
/
d 0
2
Fig. 10. Current of illumination.
L
change
6
a
at a stationary
10 timin
electrode
0
1 2
at 500 mV on switching
Fig. 11. Change in current caused by long illumination. and closing it again after 130 ms.
Vertical
6
a
10
off the light after various
lines correspond
to opening
t/mNl
times
the circuit
168
different times of illumination changes the current, as can be seen in Fig. 10, where the dark current is taken as 0. The change in current for continuous illumination is shown in curve a. If the light is switched off at the current maximum (curve b) the current falls far below the steady-state value, then increases slowly to its original level. This character is similar to the case when the switching off takes place at the minimum (curve c). In the last case the current decreases to the original level without going far below it (curve d). Since the overall current is the sum of the photoeffect and the dark current, the illumination affects the steady state of the polishing process and not only increases, but also decreases its rate. These effects cannot be explained by taking into account the direct photoeffect only, but one has to consider other processes too. Studying the open-circuit potential relaxation after different illumination times, it was not possible to detect any significant changes. The shape and the rate of potential relaxation were practically the same as without light. Thus, the observed change in the current is not connected to a significant change in potential distribution or in the structure of the solid layer. Furthermore, one can determine from the relaxation the time needed for the dissolution of the solid layer (130 ms) and during that time no depletion of the diffusion layer occurs. If, after the photocurrent measurement, the circuit was opened after different illumination times for the period needed for dissolution and then was closed again, it was observed (Fig. 11) that the shape of the current-time curve remained the same as it was in Fig. lOa,
800
0
I 2
Fig. 12. Current Fig. 11.
4
change
600
6
8
under illumination
Fig. 13. Change in current vs. illumination 7.13, (b) 9.43, (c) 12.94 M H,O.
10
t$hn
>
0
when the circuit
time at various
1
was opened
2
3
4
for 1 s, instead
water concentrations.
5
timln
of 130 ms as in
Base solution
with (a)
169
Fig. 14. Change in current vs. illumination 8.58, (b) 8.98, (c) 9.60 M EtOH.
time at various
ethanol
concentrations.
Base solution
with (a)
although the surface layer dissolved and formed again. Thus, it may be assumed that the slow change in current during illumination is not the consequence of either the change in the solid surface layer or of a slow change in photoeffect. If the circuit was opened for a longer time (15 s), and the depletion of the diffusion layer had already begun, the shape of the current-time curve changed as shown in Fig. 12. Therefore, one has to conclude that the photoeffect or the consequences of the primary photoeffect reach out into the diffusion layer, probably through a change in concentration of the species which are involved in the process connected to the light effect. In order to reach a conclusion regarding these solution species, the influence of additional amounts of water or ethanol was studied. As the water concentration of the solution was increased, the dark current increased strongly and the time interval of current change during the illumination decreased accordingly (Fig. 13). With increasing ethanol concentration the dark current increased slightly and the time interval of current change during the illumination also increased (Fig. 14). If the diffusion rate of water is higher, the time interval between the maximum and minimum decreases and at large diffusion rates, for example when the electrode is rotated, it disappears (see Fig. 9). The shape of the current-time curve also varies under the same diffusion conditions with the intensity of light. In Fig. 15 the light intensity was one half and one quarter of the original value. It can be seen that the change in current is proportional to the intensity and the time interval of the effect increases. The change in current (under the same diffusion conditions) is proportional to some integral result of the primary photoeffect. From the photocurrent measurements it can be concluded that the surface layer behaves as an n-type semiconductor in the whole potential region because of the positive photocurrent. The effect of electron-hole pair generation induced by light may be the photodecomposition of the semiconductor (ZnO) and the oxidation of water or ethanol [ll]. The influence of the addition of water and ethanol shows that in this system water plays an important role. It seems probable that the primary photoeffect is the
. 0
2
4
6
a
Fig. 15. Effect of light intensity on change the light intensity applied for curve a.
10
12
in current.
t/mln (a) Full intensity,
(b) one half. (c) one quarter
of
oxidation of water by holes (h+) 2H,0+4h++4HH++0,
(14)
and this also causes a perturbation of the steady state of the polishing process. This perturbation may be the consequence of water consumption and/or the generation of hydrogen ions and oxygen on the surface. To find the connection between the polishing process and the primary photoeffect we have to take into account the following phenomena obtained from voltammetric measurements. The polishing current is diffusion limited (Fig. 1). From the potential step measurements we can conclude that the limiting process, which brings the current to the steady state, may be the flux of Zn 2+ ions through the solid layer or the flux of Zn2+ ions into the solution. The potential relaxation curve measured immediately after the current jump (Fig. 5b) does not show the influence of an increased current from region II, in contrast to the case where the change in current was caused by increased diffusion. Because we did not find any significant change in the charge corresponding to the solid layer if the rotation rate of the electrode was varied, i.e. the thickness of the surface solid layer did not decrease although the current increased, it seems probable that the limiting process is not the diffusion of metal ions through the solid layer. Thus, the diffusion of Zn ‘+ ions through the diffusion layer determines the polishing current, and this is the limiting process. To form a constant flux of Zn2’ ions when the concentration in the bulk of the solution and the thickness of the diffusion layer are constant, the surface concentration of metal ions must be constant, too. This is the case when the solubility of the solid surface layer determines the surface concentration, and the Zn2+ ions exist in hydrated form in the solution: ZnO + n H,O + Zn’+(H,O).
(15)
171
Thus, the dissolution of the ZnO layer is sensitive to the hydrogen ion concentration and it needs water molecules for hydration of the Zn2+ ions. If the H+ concentration increases on the surface, the increased solubility of the ZnO layer causes a higher Zn 2+ ion concentration. If the thickness of the diffusion layer remains the same, the diffusion flux increases. If the amount of water on the surface decreases, the hydration of metal ions cannot occur and thus the solubility decreases: ZnO+2
H++n
H,O + Zn2+(H20).
+ H,O
Taking into account eqn. (14) for the primary photoeffect, it can be seen that this causes a change both in the water concentration and in the H+ ion concentration. As mentioned above, these two changes have different effects on the solubility of the solid surface layer. If there is enough water on the surface to hydrate the metal ions, the increase in H+ ion concentration increases the surface concentration and the diffusion flux which determines the polishing current. If the water consumption due to the photoreaction is high, the polishing current decreases, because of the decreased solubility of the solid layer. The latter case may be realized when the diffusion rate of water to the surface is too low to compensate the excess consumption due to the photoreaction. It may be assumed that these two effects cause the change in current during long illumination (Fig. 10a). First the current increases due to the increased H+ ion concentration generated by the photoreaction. Then, as the photoreaction and the increased current consume the excess water and the diffusion rate of water is too low to compensate it, the current consumes the excess water and the current begins to decrease. After a long time, when the diffusion flux increases because of the lower surface concentration, a new steady state is approached and a current higher than the original one arises. If the diffusion rate of water is much higher, for example when the electrode is rotated, it can compensate the excess consumption rapidly and the decrease in current cannot be observed (Fig. 9a). On the basis of these observations we can concluded that water plays an important role in the process but its diffusion cannot be rate determining because its surface concentration is not limited in the steady state. The rate determining process of the electropolishing of a Zn anode is the diffusion of metal ions into the solution. Its value depends on the surface concentration of Zn” ions, which can be modified by changing the solubility of the surface layer. REFERENCES 1 2 3 4 5 6 7
P.A. Jaquet, Nature (London), 135 (1935) 1076. W.C. Elmore, J. Appl. Phys., 11 (1940) 797. J. Edwards, J. Electrochem. Sot., 100 (1953) 189. K. Kojima and C.W. Tobias, J. Electrochem. Sot., 120 (1973) 1026. M. Nova, A.K.N. Reddy and H. Wroblowa, J. Electrochem. Sot., 117 (1970) 753. B. Pointu, M. Bra&x, P. Poncet and J. Rousseau, J. Electroanal. Chem., 122 (1981) 111. M. No-& and A. Sziics, J. Electroanal. Chem., 210 (1986) 229.
172 8 9 10 11
M. NovAk and A. Sziics, J. Electroanal. Chem., 210 (1986) 237. R. Kircheim, K. Maier and G. Fiilg, J. Electrochem. Sot., 128 (1981) 1027. J.L. Ord and D.J. De&et, J. Electrochem. Sot., 116 (1969) 763. H. Gerischer, in F. Cardon (Ed.), Photovoltaic and Photoelectrochemical Solar Energy Plenum Press, New York, 1981, p. 239.
Conversion,