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Abnormal photocurrent of the anodic film on lead in sulfuric acid solution
JOURNAL
ELSEVIER
or
Joumal of Electroanalytical Chemistry 414 (I996) 159-161
Abnormal photocurrent of the anodic film on lead in sulfuric acid solution Yong-Qin Wan, Qun-Zhou Wang, Hou-Tian Liu, Ji-Hua Zhuang, Wei-Fang Zhou * Department of Chemistry, Fudan University, Shanghai 200433, People's Republic of China
Received II January 1996; revised 14 March 19%
Abstract .
Th~ abnormal photocurrent of ~he anodic film formed on lead in 4.5moldm- 3 H 2 S0 4 at IAV (vs. HgIHg 2 S0 4) for Ih was
.by several electr?chemlcal methods. The experimental results show the abnormal photocurrent is due to a two-phase system the film, t.e. a p-type semlco~ductor t-PbO and an n-type semiconductor Pb01.41' The composition of the two-phase system is about Pb01.l9 when the photocurrent IS zero. ~nvestt~ated
In
Keywords: Photocurrent; Anodic film; Non-stoichiometric lead oxides
1. Introduction When the potential decreases to a certain value, the photocurrent of the anodic film formed on lead in acidic or alkaline media will convert from anodic to cathodic. However, this cathodic photocurrent has a considerable intensity and still changes with potential by nearly the same rate as that of the anodic photocurrent [1-7]' This is quite different from the Jph -E response of typical semiconductors. Debatable explanations were put forward for the abnormal cathodic photocurrent. For example, Peter and coworkers [1-3] hold that the t-PbO in the anodic film on lead is a quasi-intrinsic semiconductor, and for the thin film internal photoemission of electrons from the metal into the overlaying oxide phase can occur when the field is reversed, which brings about the cathodic photocurrent. Fletcher and Matthews [4] explained it as the result of the photoreduction of non-stoichiometric p-type PbO, generated in the 02-evolution region. Pavlov et al. [7] did not mention that the photocurrent is abnormal. Several electrochemical measurements were used in the present work so as to clarify the mechanism of the above mentioned phenomena.
• Corresponding author.
2. Experimental Formation and subsequent studies of the anodic film were carried out in a 4.5moldm- 3 H 2S0 4 solution prepared from AR H 2S04 and distilled water. A section of lead (99.999%) rod, sealed with epoxy resin at the lower part of an L-shaped glass tube and exposing a circular area of 0.283 em? to the electrolyte, was used as a working electrode. A platinum plate served as a counter electrode. An HgIHg 2S04 containing the same solution as in the electrochemical cell was used as a reference electrode. All potentials reported here are referred to this electrode. A flat working electrode surface was obtained by mechanically polishing with emery paper of successively decreasing grain size down to about 10 u.m. After that, the electrode was rinsed thoroughly with distilled water and then placed into the cell. Before anodizing, a cathodic polarization at - 1.2 V for 20 min was performed to remove any oxidation product formed during the preliminary treatment. All experiments were carried out at 25 ± 2°C. The anodic film studied in the present work was grown on the working electrode in 4.5 mol dm - 3 H 2 SO4 solution for I h at 1.4 V. Open circuit decay measurement and linear sweep voltammetry were carried out using an EG & G PARC 273 potentiostat-galvanostat interfaced to a computer and controlled by EG&G PARC Model 270 software. The a.c.
0022-0728/96/$15.00 Copyright © 1996 Elsevier Science SA All rights reserved. PI! 50022-0728(96)04681-5
Y.-Q. Wan et al./ Journal of Electroanalytical Chemistry 414 (1996) 159-161
160
3.0r-------------------, I 0
15 00 N,
.....
n
,..----+---,-.··7·:·/~·"._I.·...-.-·J I
/ ..
~
/
0.5
0.0
'
'2
/XI/50 /
C4
C,
I
I
-080
C3
---Dark current Photo current
-0.30
0.20
. . . .'.
-
Fig. 1 shows the photocurrent-potential (Jph -E) plot recorded at a slow scan rate 1 mV s - I for the anodic film formed on lead at 1.4 V in 4.5 mol dm - 3 H 2 SO4 solution for 1h in the dark. It is obvious that the photocurrent is cathodic when the potential is lower than - 0.08 V. This behavior is abnormal. We also studied the open circuit decay of the anodic film formed under the same conditions as mentioned above, and recorded its photopotential-time (Eph-t) curve and potential-time (Eoc-t) curve (Fig. 2). From Fig. 2 it can be found that when the electrode is held at open circuit for 1500 s, E ph = 0 V and E oc = - 0.1 V. The latter nearly equals the potential -0.08 V when Jph = OAcm- 2 in Fig. 1. This might imply that at zero photopotential the photocurrent is zero as well. After that, the E ph is positive, which means that a p-type semiconductor exists in the film. The steady Eoc in Fig. 2 is close to the equilibrium potential of Pb!PbO, - 0.32 V [9]. Thus the p-type semiconductor would be PbO. Fig. 1 also presents the linear sweep voltammogram recorded at 1 mV s - I for the film formed on lead at 1.4 V
....._..,
06 ~ <,
w
......../ -, ~"- ....
_.'\
\
0.70
3. Results and discussion
~
. . . . . . . ••• . -
0.2
300
-0.2
900
2100
1500
E/V
voltammetry was carried out using a CH Instruments Model 660 electrochemical work station, at a scan rate of 1mV s - ], and the rms amplitude of the small alternating signal is 1mV with a frequency of 1000Hz. The photopotential and photocurrent were measured with an EG&G PARC 5209 lock-in analyzer, and an EG&G PARC 197 chopper. Incident photon fluxes were less than 4 X 10 13 em- 2 S-] in order to avoid changes in the growth rate of the given film. The wavelength of the incident light was 500 nm, shorter than the wavelength threshold of t-PbO, 650nm [8].
~
.'-;-"
-0.2
-2.0
Fig. I. jph and j vs. E for the anodic film formed on lead in 4.5 mol dm - 3 H 2 S0 4 at 1.4V for I h in the dark, scan rate I mV S-l. : Dark current, Xl/50 current coefficient by which the current values on the ordinate must be multiplied.... : Photocurrent, illuminated with chopped monochromatic light, A= 500nm.
_r
# •• # •••••- . \........... - '...
s w 0.2
/----..,.--- . . . C . 2
-6.0
.........
06
> <,
-IO~ -1.5
/
10
10 N
-0. 5 ~
/
~-3.0
-45
A
1.4.---------------------,1.4
tis
Fig. 2. Open circuit photopotential E ph and dark potential Eoc vs. t for the anodic film grown on Pb in 4.5 mol dm - 3 H 2 SO4 solution at 1.4V for I h in the dark. : Dark potential. ... : Photopotential, illuminated with chopped monochromatic light, A = 500nm.
in 4.5 mol dm - 3 for 1h. The cathodic peaks C I (0.98 V) and C, (-1.08 V) correspond to the reduction of Pb0 2 to PbS0 4 and PbS0 4 to Pb respectively [10]. The cathodic peak C 4 ( - 0.91 V) corresponds to the reduction of PbO to Pb [2,11,12]. The anodic peak A (0.93 V) may be due to the oxidation of PbOn oxides in the film to photoinactive a-Pb0 2 , such as t - PbO
+ H 20 =
a-Pb0 2
+ 2H+ + Ze"
(1)
The Pb0 2 in the film is reduced to PbS04 under peak C I in Fig. 1 as follows Pb0 2
+ soj + 2e- + 4H+= PbS04 + 2H 20
(2)
which increases the pH within the film [13,14]. This would favour reaction (1) and may be the reason for the oscillation of the photocurrent at E between 0.25 and 0.80 V in Fig. 1. Since the resistivity of a-Pb0 2 is quite low, about 10- 3 n em [15], the real part of the impedance (Z') of the
70 60 50 N
E
u
Cj
-' N
40 30 20 10 0
-1.0
-0.5
0.0
0.5
1.0
E/V Fig. 3. Z' vs. E plot of the anodic film formed on lead at 1.4V in 4.5 moldm - 3 H 2 SO4 solution for I h, recorded at 1mV s- I , f = 1000Hz.
Y.-Q. Wan et al.j Journal ofElectroanalytical Chemistry 414 (]996) 159-161
film under peak A (between 0.96 and 0.56 V) in Fig. 1 does not vary much (see Fig. 3). The part of the broad peaks C 2 (O.OOV) and C 3 ( - 0.43 V) at E between 0.56 and - 0.58 V may correspond to the reduction of the non-stoichiometric PbO! + x (0 ~ x < 1) to that with smaller x value, since at E between 0.56 and - 0.58 V Z' increases with decreasing potential (Fig. 3). This is consistent with the fact that the resistivity of PbO I + x is larger than that with a smaller value of x [16]. In Fig. 3 at - 0.58 V Z' begins to decrease rapidly, hence it can be inferred that in Fig. 1 at E between - 0.58 and - 1.0 V the PbO in the film is reduced to metallic lead. We can calculate x in the Pb0 1+ x ' e.g. at E when Jph = aAcm -2, i.e. - 0.08 V, as follows. The amount of electricity covered by the curve in Fig. 1 between - 0.08 and - 0.58 V may correspond to the x part of the PbO I + x' while that between - 0.58 and - 1.0 V may correspond to the unit part of the PbO! + x' Thus, at - 0.08 V the nonstoichiometric lead oxide is Pb01.l9' Anderson and Stems [17] found that when the x value of PbO l + x is between 1.08 and 1.41, a two-phase system is formed, i.e. t-PbO + Pb01.4J' The Pb01.41 is an n-type semiconductor with oxygen anion vacancies [17], while the t-PbO is a p-type semiconductor as mentioned above. It may be concluded that when E = - 0.08 V, Jph = oA em - 2, the Pb0 1 + x is PbO 1.19' in which the amount of Pb01.41 is nearly equal to that of t-PbO. It is reasonable that when Jph = 0 A cm - 2 the anodic photocurrent caused by Pb01.4l is compensated for by the cathodic photocurrent caused by t-PbO. In other cases, either anodic or cathodic photocurrent appears, depending on the type of semiconductor taking the major role in the photoeffect.
161
Acknowledgements This work was supported financially by the Chinese State Education Commission and the National Natural Science Foundation of China.
References [1] J.S. Buchanan, N.P. Freestone and L.M. Peter, J. Electroanal. Chern., 182 (1985) 383. [2] J.S. Buchanan and L.M. Peter, Electrochim. Acta, 33 (1988) 127. [3] S.A. Campell, L.M. Peter and J.S. Buchanan, J. Power Sources, 40 (1992) 137. [4] S. Fletcher and D.B. Matthews, J. Electroanal. Chern., 126 (1981) 131. [5] H.-S. Zhai, J.-K. You and Z.-G. Lin, Ext. Abstr., 7th Chinese Electrochern. Soc. Meet., Changchun, Peoples Republic of China, 1993, p. 137. [6] C. Pu, Ph.D. Thesis, Fudan University, Shanghai, Peoples Republic of China, 1992. [7] D. Pavlov, B. Monahov, G. Sundholrn and T. Laitinen, J. Electroanal. Chern., 305 (1991) 57. [8] J. van den Broek, Philips Res. Rep., 22 (1967) 36. [9] A.J. Bard, R. Parsons and J. Jordan (Bds.), Standard Potentials in Aqueous Solution, IUPAC, Marcel Dekker, New York, 1985. [10] Y. Yamamoto, K. Furnian, T. Ueda and M. Narnbu, Electrochirn. Acta, 37 (1992) 199 and references cited therein. [II] R.G. Barradas, D.S. Nadezhdin and N. Shah, J. Electroanal. Chern., 147(1983) 193. [12] K.R. Bullock, G.M. Trischan and R.G. Burrow, J. Electrochern. Soc., 130 (1983) 1283. [13] P. Ruetschi, J. Electrochern. Soc., 120 (1973) 331. [14] W.-B. Cai, Y.-Q. Wan, H.-T. Liu and W.-F. Zhou, J. Electroanal. Chern., 387 (1995) 95. [15] W. Mindt, J. Electrochern. Soc., 116 (1969) 1076. [16] D. Pavlov, in B.D. McNicol and D.A.J. Rand (Eds.), Power Sources for Electric Vehicles, Elsevier, Amsterdam, 1984, p. 146. [17] J.S. Anderson and M. Stems, J. Inorg. Nucl. Chern., 11 (1959) 272.
ELSEVIER
or
Joumal of Electroanalytical Chemistry 414 (I996) 159-161
Abnormal photocurrent of the anodic film on lead in sulfuric acid solution Yong-Qin Wan, Qun-Zhou Wang, Hou-Tian Liu, Ji-Hua Zhuang, Wei-Fang Zhou * Department of Chemistry, Fudan University, Shanghai 200433, People's Republic of China
Received II January 1996; revised 14 March 19%
Abstract .
Th~ abnormal photocurrent of ~he anodic film formed on lead in 4.5moldm- 3 H 2 S0 4 at IAV (vs. HgIHg 2 S0 4) for Ih was
.by several electr?chemlcal methods. The experimental results show the abnormal photocurrent is due to a two-phase system the film, t.e. a p-type semlco~ductor t-PbO and an n-type semiconductor Pb01.41' The composition of the two-phase system is about Pb01.l9 when the photocurrent IS zero. ~nvestt~ated
In
Keywords: Photocurrent; Anodic film; Non-stoichiometric lead oxides
1. Introduction When the potential decreases to a certain value, the photocurrent of the anodic film formed on lead in acidic or alkaline media will convert from anodic to cathodic. However, this cathodic photocurrent has a considerable intensity and still changes with potential by nearly the same rate as that of the anodic photocurrent [1-7]' This is quite different from the Jph -E response of typical semiconductors. Debatable explanations were put forward for the abnormal cathodic photocurrent. For example, Peter and coworkers [1-3] hold that the t-PbO in the anodic film on lead is a quasi-intrinsic semiconductor, and for the thin film internal photoemission of electrons from the metal into the overlaying oxide phase can occur when the field is reversed, which brings about the cathodic photocurrent. Fletcher and Matthews [4] explained it as the result of the photoreduction of non-stoichiometric p-type PbO, generated in the 02-evolution region. Pavlov et al. [7] did not mention that the photocurrent is abnormal. Several electrochemical measurements were used in the present work so as to clarify the mechanism of the above mentioned phenomena.
• Corresponding author.
2. Experimental Formation and subsequent studies of the anodic film were carried out in a 4.5moldm- 3 H 2S0 4 solution prepared from AR H 2S04 and distilled water. A section of lead (99.999%) rod, sealed with epoxy resin at the lower part of an L-shaped glass tube and exposing a circular area of 0.283 em? to the electrolyte, was used as a working electrode. A platinum plate served as a counter electrode. An HgIHg 2S04 containing the same solution as in the electrochemical cell was used as a reference electrode. All potentials reported here are referred to this electrode. A flat working electrode surface was obtained by mechanically polishing with emery paper of successively decreasing grain size down to about 10 u.m. After that, the electrode was rinsed thoroughly with distilled water and then placed into the cell. Before anodizing, a cathodic polarization at - 1.2 V for 20 min was performed to remove any oxidation product formed during the preliminary treatment. All experiments were carried out at 25 ± 2°C. The anodic film studied in the present work was grown on the working electrode in 4.5 mol dm - 3 H 2 SO4 solution for I h at 1.4 V. Open circuit decay measurement and linear sweep voltammetry were carried out using an EG & G PARC 273 potentiostat-galvanostat interfaced to a computer and controlled by EG&G PARC Model 270 software. The a.c.
0022-0728/96/$15.00 Copyright © 1996 Elsevier Science SA All rights reserved. PI! 50022-0728(96)04681-5
Y.-Q. Wan et al./ Journal of Electroanalytical Chemistry 414 (1996) 159-161
160
3.0r-------------------, I 0
15 00 N,
.....
n
,..----+---,-.··7·:·/~·"._I.·...-.-·J I
/ ..
~
/
0.5
0.0
'
'2
/XI/50 /
C4
C,
I
I
-080
C3
---Dark current Photo current
-0.30
0.20
. . . .'.
-
Fig. 1 shows the photocurrent-potential (Jph -E) plot recorded at a slow scan rate 1 mV s - I for the anodic film formed on lead at 1.4 V in 4.5 mol dm - 3 H 2 SO4 solution for 1h in the dark. It is obvious that the photocurrent is cathodic when the potential is lower than - 0.08 V. This behavior is abnormal. We also studied the open circuit decay of the anodic film formed under the same conditions as mentioned above, and recorded its photopotential-time (Eph-t) curve and potential-time (Eoc-t) curve (Fig. 2). From Fig. 2 it can be found that when the electrode is held at open circuit for 1500 s, E ph = 0 V and E oc = - 0.1 V. The latter nearly equals the potential -0.08 V when Jph = OAcm- 2 in Fig. 1. This might imply that at zero photopotential the photocurrent is zero as well. After that, the E ph is positive, which means that a p-type semiconductor exists in the film. The steady Eoc in Fig. 2 is close to the equilibrium potential of Pb!PbO, - 0.32 V [9]. Thus the p-type semiconductor would be PbO. Fig. 1 also presents the linear sweep voltammogram recorded at 1 mV s - I for the film formed on lead at 1.4 V
....._..,
06 ~ <,
w
......../ -, ~"- ....
_.'\
\
0.70
3. Results and discussion
~
. . . . . . . ••• . -
0.2
300
-0.2
900
2100
1500
E/V
voltammetry was carried out using a CH Instruments Model 660 electrochemical work station, at a scan rate of 1mV s - ], and the rms amplitude of the small alternating signal is 1mV with a frequency of 1000Hz. The photopotential and photocurrent were measured with an EG&G PARC 5209 lock-in analyzer, and an EG&G PARC 197 chopper. Incident photon fluxes were less than 4 X 10 13 em- 2 S-] in order to avoid changes in the growth rate of the given film. The wavelength of the incident light was 500 nm, shorter than the wavelength threshold of t-PbO, 650nm [8].
~
.'-;-"
-0.2
-2.0
Fig. I. jph and j vs. E for the anodic film formed on lead in 4.5 mol dm - 3 H 2 S0 4 at 1.4V for I h in the dark, scan rate I mV S-l. : Dark current, Xl/50 current coefficient by which the current values on the ordinate must be multiplied.... : Photocurrent, illuminated with chopped monochromatic light, A= 500nm.
_r
# •• # •••••- . \........... - '...
s w 0.2
/----..,.--- . . . C . 2
-6.0
.........
06
> <,
-IO~ -1.5
/
10
10 N
-0. 5 ~
/
~-3.0
-45
A
1.4.---------------------,1.4
tis
Fig. 2. Open circuit photopotential E ph and dark potential Eoc vs. t for the anodic film grown on Pb in 4.5 mol dm - 3 H 2 SO4 solution at 1.4V for I h in the dark. : Dark potential. ... : Photopotential, illuminated with chopped monochromatic light, A = 500nm.
in 4.5 mol dm - 3 for 1h. The cathodic peaks C I (0.98 V) and C, (-1.08 V) correspond to the reduction of Pb0 2 to PbS0 4 and PbS0 4 to Pb respectively [10]. The cathodic peak C 4 ( - 0.91 V) corresponds to the reduction of PbO to Pb [2,11,12]. The anodic peak A (0.93 V) may be due to the oxidation of PbOn oxides in the film to photoinactive a-Pb0 2 , such as t - PbO
+ H 20 =
a-Pb0 2
+ 2H+ + Ze"
(1)
The Pb0 2 in the film is reduced to PbS04 under peak C I in Fig. 1 as follows Pb0 2
+ soj + 2e- + 4H+= PbS04 + 2H 20
(2)
which increases the pH within the film [13,14]. This would favour reaction (1) and may be the reason for the oscillation of the photocurrent at E between 0.25 and 0.80 V in Fig. 1. Since the resistivity of a-Pb0 2 is quite low, about 10- 3 n em [15], the real part of the impedance (Z') of the
70 60 50 N
E
u
Cj
-' N
40 30 20 10 0
-1.0
-0.5
0.0
0.5
1.0
E/V Fig. 3. Z' vs. E plot of the anodic film formed on lead at 1.4V in 4.5 moldm - 3 H 2 SO4 solution for I h, recorded at 1mV s- I , f = 1000Hz.
Y.-Q. Wan et al.j Journal ofElectroanalytical Chemistry 414 (]996) 159-161
film under peak A (between 0.96 and 0.56 V) in Fig. 1 does not vary much (see Fig. 3). The part of the broad peaks C 2 (O.OOV) and C 3 ( - 0.43 V) at E between 0.56 and - 0.58 V may correspond to the reduction of the non-stoichiometric PbO! + x (0 ~ x < 1) to that with smaller x value, since at E between 0.56 and - 0.58 V Z' increases with decreasing potential (Fig. 3). This is consistent with the fact that the resistivity of PbO I + x is larger than that with a smaller value of x [16]. In Fig. 3 at - 0.58 V Z' begins to decrease rapidly, hence it can be inferred that in Fig. 1 at E between - 0.58 and - 1.0 V the PbO in the film is reduced to metallic lead. We can calculate x in the Pb0 1+ x ' e.g. at E when Jph = aAcm -2, i.e. - 0.08 V, as follows. The amount of electricity covered by the curve in Fig. 1 between - 0.08 and - 0.58 V may correspond to the x part of the PbO I + x' while that between - 0.58 and - 1.0 V may correspond to the unit part of the PbO! + x' Thus, at - 0.08 V the nonstoichiometric lead oxide is Pb01.l9' Anderson and Stems [17] found that when the x value of PbO l + x is between 1.08 and 1.41, a two-phase system is formed, i.e. t-PbO + Pb01.4J' The Pb01.41 is an n-type semiconductor with oxygen anion vacancies [17], while the t-PbO is a p-type semiconductor as mentioned above. It may be concluded that when E = - 0.08 V, Jph = oA em - 2, the Pb0 1 + x is PbO 1.19' in which the amount of Pb01.41 is nearly equal to that of t-PbO. It is reasonable that when Jph = 0 A cm - 2 the anodic photocurrent caused by Pb01.4l is compensated for by the cathodic photocurrent caused by t-PbO. In other cases, either anodic or cathodic photocurrent appears, depending on the type of semiconductor taking the major role in the photoeffect.
161
Acknowledgements This work was supported financially by the Chinese State Education Commission and the National Natural Science Foundation of China.
References [1] J.S. Buchanan, N.P. Freestone and L.M. Peter, J. Electroanal. Chern., 182 (1985) 383. [2] J.S. Buchanan and L.M. Peter, Electrochim. Acta, 33 (1988) 127. [3] S.A. Campell, L.M. Peter and J.S. Buchanan, J. Power Sources, 40 (1992) 137. [4] S. Fletcher and D.B. Matthews, J. Electroanal. Chern., 126 (1981) 131. [5] H.-S. Zhai, J.-K. You and Z.-G. Lin, Ext. Abstr., 7th Chinese Electrochern. Soc. Meet., Changchun, Peoples Republic of China, 1993, p. 137. [6] C. Pu, Ph.D. Thesis, Fudan University, Shanghai, Peoples Republic of China, 1992. [7] D. Pavlov, B. Monahov, G. Sundholrn and T. Laitinen, J. Electroanal. Chern., 305 (1991) 57. [8] J. van den Broek, Philips Res. Rep., 22 (1967) 36. [9] A.J. Bard, R. Parsons and J. Jordan (Bds.), Standard Potentials in Aqueous Solution, IUPAC, Marcel Dekker, New York, 1985. [10] Y. Yamamoto, K. Furnian, T. Ueda and M. Narnbu, Electrochirn. Acta, 37 (1992) 199 and references cited therein. [II] R.G. Barradas, D.S. Nadezhdin and N. Shah, J. Electroanal. Chern., 147(1983) 193. [12] K.R. Bullock, G.M. Trischan and R.G. Burrow, J. Electrochern. Soc., 130 (1983) 1283. [13] P. Ruetschi, J. Electrochern. Soc., 120 (1973) 331. [14] W.-B. Cai, Y.-Q. Wan, H.-T. Liu and W.-F. Zhou, J. Electroanal. Chern., 387 (1995) 95. [15] W. Mindt, J. Electrochern. Soc., 116 (1969) 1076. [16] D. Pavlov, in B.D. McNicol and D.A.J. Rand (Eds.), Power Sources for Electric Vehicles, Elsevier, Amsterdam, 1984, p. 146. [17] J.S. Anderson and M. Stems, J. Inorg. Nucl. Chern., 11 (1959) 272.