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electrolyte interface
Volume 132, number 2
5 December
CHEMICAL PHYSICS LETTERS
TRANSIENT PHOTOPOTENTIALS AT THE COBALT-DOPED ZINC OXIDE/ELECTROLYTE
1986
INTERFACE
Denis FICHOU 1 Laboratoire de Photochimie Sokaire,Equipe de Recherche 241 du CNRS, 2-8 rue Hemy Dunant, 94320 Thiais,France
and Andrte KIRSCH-DE
MESMAEKER *
Facultk des Sciences, UniversitdLibre de Bruxelles, CP 160, Chimie Organique, 50 Avenue F.D. Roosevelt, 1050 Brussels, Belgium Received 16 July 1986; in final form 27 September 1986
The time evolution of the pulsed-laser-induced photopotentials of pure and cobalt-sensitized ZnO electrodes is analyzed after 365 and 640 nm excitations. This analysis, as a function of various parameters, enables a determination of the influence of Co*+ ions and excitation wavelength on the kinetics of electron-hole recombination and electron transfer at
the interface to be made.
1. Introduction
Photosensitization of wide bandgap semiconducting oxides to visible light can be achieved by doping with 3d elements [l-3]. It has been shown recently that a cobalt-doped ZnO electrode yields an intense and wellstructured visible photocurrent up to 730 nm [4]. This has been interpreted as being due to d-d transitions inside the Co2+ 3d7 core, followed by a charge transfer into the band structure of the ZnO matrix. In order to check the effect of the tetrahedrally coordinated Co2+ ions on the kinetics of the photogenerated electron-hole pair recombination and electron transfer at the interface, we carried out a timeresolved study of the pulsed-laser-induced photopotentials at the pure and Co-doped ZnO/electrolyte interface as a function of the various parameters involved. ’ Present address: Department of Material Systems Engineering, Faculty of Technology, Tokyo University of Agriculture and Technology, 2-24-26 Nakamachi, Toganei-sbi, Tokyo 184, Japan. 2 Research Associate of the National Fund for Scientific Research (Belgium).
128
This technique has been applied by several authors [6-121 and by ourselves for the photosensitization of semiconductors by organic dyes [ 131. However, this is the first time that an attempt has been made to detect the influence of a photosensitizer such as Co2’ ions on the kinetic behaviour of the ZnO photopotentials, especially after excitation at two different wavelengths, i.e. at 365 nm, the fundamental band-to-band transition and at 640 nm, the d-d transition in Co2+.
2. Experimental
The electrodes were polycrystalline ZnO sinters, prepared according to a technique described elsewhere [4]. They were polished with diamond paste (down to 1 pm) and etched with 4 M HCl(15 s). The electrolyte was an aqueous solution of 0.1 M Na2S04 buffered at pH 7; 10v2 M hydroquinone (H2Q) may be added to test the influence of a reducing agent on the photopotential relaxation. The photoelectrochemical cell contained four electrodes: a working semiconducting electrode (x0.5 cm2), a large surface area Pt counter-
0 009-26 14/86/S 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
I,
Volume 132, number 2
5 December 1986
CHEMICAL PHYSICS LETTERS
electrode (=20 cm2) and a dual reference consisting of a saturated calomel electrode (SCE) capacitively coupled (470 nF) to a Pt quasi-reference. The excitation source was a Molectron DLII tunable dye laser with an 8 ns pulse width, pumped by a Molectron UV 24 nitrogen laser. The triggering system has been described previously [ 131. A pyroelectric joulemeter (Molectron 53-05 DW) connected to an oscilloscope (Philips PM 3234) allows measurement of the relative light intensity of each pulse. The ZnO electrodes were prepolarized at a given potential for a few seconds before irradiation. A solidstate electronic switch (Analog Devices, AD 75 12 DI) then opens the circuit and after a 20 ps delay the pulsed laser is triggered and the photopotential between the working electrode and the SCE is monitored on an oscilloscope (100 MHz, Philips PM 3266, 1 MS1input). The comparison of the photopotential (AV) relaxations as a function of time, for different electrodes, has to be done at a same prepolarization potential (PP) since the PP controls the charge carrier recombinations within the investigated time scale. The measured initial photopotential AV, should thus be as small as possible, so that the PP is essentially undisturbed by AV. Consequently neutral filter combinations were used such that the measured initial APo is always smaller than 120 mV. The AI’u values given in table 1 have been normalized to the same laser intensity (1.736 fl, i.e. 0.217 kW at 365 mn, and 22.5 E.1J,i.e. 2.812 kW at 640 nm, for an irradiated spot of ~0.5 cm2) such that the resulting calculated APu values are all on the linear part of the measured curve of AI’,-,versus the laser intensity.
3. Results and discussion The electron-hole (e--h+) pairs photogenerated in the space-charge region are separated by the electric field and charge the semiconductor capacitance C,. This gives rise to an initial photopotential jump AVu = AQ,,/C,.Two eventualities must be considered: (1) If the e--h+ pairs recombine, AF,-, will relax. (2) If the free holes oxidize the solution, Csc will discharge through RS + R,, where R, is the external load and RS the sum of the bulk resistance of the serniconductor and of the electrolyte. In this work, R, is such (1 Ma) that the time constant r = (R, + R,)C,:
is much larger than the recombination time so the measurements correspond to open-circuit conditions. The influence of Co2+ is examined on the initial photopotential AFu and on its time evolution after the pulse (AV,). 3.1. Initial photopotential AV, AVO is reached roughly 20 ns after the laser pulse, independent of the doping by Co2+ and of the excitation wavelength (365 or 640 nm) of ZnO:Co. Actually this rise-time corresponds to the time constant of the detection system [6,13], showing that even the e--h+ pairs generated by excitation in the Co2+ band are separated more rapidly than 20 ns. Although Co 2+ has no effect on the AV, rise-time of ZnO electrodes, increasing the Co2+ doping levels decreases AVO for excitation at 365 nm (table 1). On the contrary, there is no response to excitation at 640 nm for pure ZnO (Ep = 3.2 eV at 300 K), it appears only for Co-doped electrodes reaching a maximum at a 3.0 at% cobalt doping rate (table 1). We thus selected this sensitizing level for the kinetic studies. Since Co2+ produces fast (< 20 ns) e--h+ recombinations at 365 nm, we compared this effect with that of purposely introducing surface states. Thus, a mechanical polishing of pure ZnO decreases AV, at 365 nrn(from 356 to 178 mV, PP = +0.4 V/SC@ whereas at 640 nm it induces the appearance of a very weak AV, signal (from 0 to 11 mV, to.4 V/SCE). Thus the surface states introduced by polishing also produce fast (-G?Ons) recombinations; the decrease of AV, is
Table 1 Influence of the Co’* doping rate on the open-circuit initial photopotential APo at a prepolarization potential of +O.4 V/SCE. The electrodes are polished and etched; APO is calculated for the same laser intensity at 365 and 640 nm (see section 2) Electrode
A Vo (mv) at 365 run
A Vo (mV) at 640 nm
ZnO ZnO : Coo.ol
356 81
ZnO : Coo.03 ZnO : Coo.06
41 31 20
0 132 192
ZnO:Coo.ro
67 46
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CHEMICAL PHYSICS LETTERS
Volume 132, number 2
however less than with Co 2+. The states should be distributed between the upper edge of the valence band and the ZnO midgap since they induce a AF on irradiation with visible light, probably by electron injection into the conduction band. We also examined the influence on AVu of hydroquinone (H2Q) 10q2 M in solution, with three electrodes: etched ZnO (ZnO l), polished ZnO (ZnO 2) and etched ZnO : Co u.u3 (ZnO : Co 1). It turns out that H2Q does not affect AFu for the chemically etched electrodes (ZnO 1 and ZnO : Co 1) at 365 and 640 nm but enhances AP’u for the polished ZnO 2 (from 178 to 286 mV at 365 nm and from 11 to 36 mV at 640 nm, +0.4V/SCE). The reductant is thus able to be oxidized by the holes trapped by the ZnO 2 surface states before they recombine with electrons; with ZnO : Co 1 on the other hand, H2Q cannot inhibit recombination because it is mainly induced within the space-charge region by Co2+. 3.2. Time evolution of AV,
3.2. I. After an excitation at 365 nm Typical time-resolved photopotentials AVt are shown in figs. 1 and 2 for ZnO 1 and ZnO : Co 1, respectively, after excitation at 365 nm; the signals for
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Fig. 1. Open-circuit AV, photopotential of an unsensitized and etched ZnO electrode (ZnO 1) irradiated by a 365 nm laser pulse under a prepolariaation potential of 0.0 V/WE. (a) 100 ns div-’ ; (b) 0.5 PCSdiv-’ ; (c) 2.0 CISdiv-’ ; (d) 0.2 ms div-' .
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5 December 1986
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Fig. 2. Open-circuit photopotential of an etched Co-doped ZnO electrode (ZnO : Co 1) irradiated at 365 nm under a prepolarization potential of 0.0 V/SCE. (a) 100 ns dive1 ; (b) 0.5 ps div-’ ; (c) 0.2 ms div-’ ; (d) 2.0 ms div-‘.
ZnO 2 are qualitatively similar to those of ZnO : Co 1. The decay during the first 50 ns (figs. la and 2a) is probably due to fast recombinations within the depletion region since this decay is uninfluenced by H2Q. At longer times, ZnO 1 behaves quite differently from ZnO : Co 1 and ZnO 2. Indeed, AV does not relax with ZnO 1 (fig. Id), indicating that no e--h+ recombination occurs. Moreover, figs. lc and Id exhibit a slight AVt increase between 1 .O ~.tsand 0.3 ms, which could originate (i) from an electron injected from a trap into the conduction band or (ii) from a contribution of the Helmholtz capacitance as discussed by Wilson et al. [lo]. By contrast, with ZnO : Co 1 (and ZnO 2), AV, relaxes with several decay components up to 10 ms (figs. 2b-2d). Figs. 3 and 4 show the influence of H2Q and of the PP (prepolarization potential) on the relaxation rate of AVt for ZnO 2 and ZnO : Co 1 respectively. We have plotted the percentage of AV, remaining at different times after the laser pulse (7 ps, 0.1 ms, 0.9 ms and 9 ms) as a function of the PP for solutions with and without H2Q. The 100% level has been arbitrarily taken as the AV, measured 1 ~.lsafter the pulse. An increase in the PP and in the H2Q concentration decreases the rate of slow relaxation of ZnO 2 (fig. 3); at a PP of tl V/SCE, H2Q almost totally inhibits the AV decay. Ob-
Volume 132, number 2
CHEMICAL PHYSICS LETTERS
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5 December 1986
viously, since H2Q has an effect on AV, for ZnO 2, it is also able to prevent the slow surface recombinations. However, hydroquinone is less efficient on ZnO : Co 1 (fig. 4a), where even higher H2Q concentrations do not totally prevent relaxation, indicating that some space-charge recombination takes place via the Co2+.
a 50
Fig. 3. Percentage of residual open-circuit photopotential for a polished but unetched ZnO electrode (ZnO 2) at different times (7.0 ps; 0.1 ms; 0.9 ms; 9.0 ms) after a 365 nm laser pulse, as a function of the prepolariiation potential. The 100% level is arbitrarily taken after 1.0 ps. Full lines: without hydroquinone. Dashed lines: with 1 0m2M hydroquinone.
100
7ops
3.2.2. After an excitation at 640 nm ZnO 1 does not give a signal at this wavelength and the very small AV, measured with ZnO 2 is too weak to be analyzed. The AV, time evolution of ZnO : Co 1 after excitation in the visible range (fig. 4b) is rather similar to that in the W (fig. 4a). Nevertheless, the H2Q effect is slightly more pronounced at 640 nm (fig. 4b) than at 365 nm (fig. 4a), remaining, however, weaker than with ZnO 2 (fig. 3). This small difference between the two wavelengths could be explained by a faster oxidation of water or of the ZnO lattice, in the absence of H2Q, by holes from the valence band rather than from the Co2+ band, implying a higher recombination percentage at 640 nm than at 365 nm. In the presence of H2Q, holes from the Co2+ band would oxidize H2Q instead of recombining with electrons, leading to a greater H2Q effect at 640 nm.
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Fig. 4. Percentage of residual open-circuit photopotential of an etched Co-doped ZnO electrode (ZnO : Co 1) at different times (7.0 rrs; 0.1 ms; 0.9 ms; 9.0 ms) after the laser pulse, as a function of the prepolarization potential. The 100% level is arbitrarily taken after 1 .O ps. Full lines: without hydroquinone. Dashed lines: with 1K2 M hydroquinone. (a) At 365 nm; (b) at 640 nm.
This study shows clearly that Co2+ ions, in spite of their good photosensitizing properties to visible light, produce fast (< 20 ns) space-charge recombinations and slow AV relaxations, extending over a large time scale. These AV decays differ from the relaxation due to surface states observed with polished ZnO (ZnO 2) because, contrary to the ZnO 2 case, they cannot be totally inhibited by H2Q. Co2+ ions thus induce slow recombination between electrons and holes captured not only on the surface but also in the space charge of the semiconductor. Only a slight difference after excitation at 365 and 640 nm has been detected; this could be explained by a less efficient water or ZnO lattice oxidation by holes from the Co2+ rather than from the valence band. Further studies of transient photopotentials in such systems under short-circuit conditions, together with complements to the present study, will be published in the near future [ 141.
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Volume 132, number 2
CHEMICAL PHYSICS LETTERS
Acknowledgement
We are grateful to Professor J. Nasielski and Dr. J. Kossanyi for having initiated the collaboration between our two laboratories. One of us (DF) thanks the “Commissariat General aux Relations Internationales de la Communaute Francophone de Belgique” for financial support during a two months’ stay in the Laboratoire de Chimie Organique in Brussels.
References
111H.P. Mar&a and A.K. Ghosh, Solar Energy Mat. 1 (1979) 237.
121Y. Matsumoto, J. Kurirnoto, Y. Amagasaki and E. Sato, J. Electrochem.
Sot. 127 (1980) 2148.
131G. Bin-Daar, M.P. Dare-Edwards, J.B. Goodenough and A. Hammett, J. Chem. Sot. Faraday Trans. I 79 (1983) 1199. [41 D. Fichou, J. Pouliquen, J. Kossanyi, M. Jakani, G. Campet and J. Claverie, J. Electroanal. Chem. 188 (1985) 167; D. Fichou and J. Kossanyi, J. Electrochem. Sot. 133 (1986) 1607; D. Fichou, Th&e de Doctorat d’Etat, University of Paris, France (1986).
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[5] J.H. Richardson, S.B. Deutscher, S.P. Perone, J. Rosenthal and J.N. Ziemer, J. Electrochem. Sot. 127 (1980) 2580; J.H. Richardson, J.P. Perone, L.L. Steinmetz and S.B. Deutscher, Chem. Phys. Letters 77 (1981) 93. [6] S. Gottesfeld and S.W. Feldberg, J. Electroanal. Chem. 146 (1983) 47; Z. Hanion, N. Croitoru and S. Gottesfeld, J. Electrothem. Sot. 128 (1981) 551. [7] P.V. Kamat and M.A. Fox, J. Phys. Chem. 87 (1983) 59. [ 81 F. WiBig, K. Bitterling, K.P. Charle and F. Decker, Ber. Bunsenges. Physik. Chem. 88 (1984) 374. [9] S. Prybyla, W.S. Struve and B.A. Parkinson, J. Electrothem. Sot. 131(1984) 1587. [ 101 R.H. Wilson, T. Sakata, T. Kawai and K. Hashimoto, J. Electrochem. Sot. 132 (1985) 1082. [ 111 W. Jaegermann, T. Sakata, E. Janata and H. Tributsch, J. Electroanal. Chem. 189 (1985) 65. [ 121 K. Itoh, M. Nakao and K. Honda, J. Appl. Phys. 57 (1985) 5493. [ 131 A. Frippiat, A. Kirsch-de Mesmaeker and J. Nasielski, J. Electrochem. Sot. 130 (1983) 237; A. Frippiat and A. Kirsch-de Mesmaeker, J. Phys. Chem. 89 (1985) 1285; J. Electrochem. Sot., to be published; A. Kirsch-de Mesmaeker, M. Rochus-Dewitt and J. Nasielski, to be published. [ 141 D. Fichou and A. Kirsch-de Mesmaeker, to be published.
5 December
CHEMICAL PHYSICS LETTERS
TRANSIENT PHOTOPOTENTIALS AT THE COBALT-DOPED ZINC OXIDE/ELECTROLYTE
1986
INTERFACE
Denis FICHOU 1 Laboratoire de Photochimie Sokaire,Equipe de Recherche 241 du CNRS, 2-8 rue Hemy Dunant, 94320 Thiais,France
and Andrte KIRSCH-DE
MESMAEKER *
Facultk des Sciences, UniversitdLibre de Bruxelles, CP 160, Chimie Organique, 50 Avenue F.D. Roosevelt, 1050 Brussels, Belgium Received 16 July 1986; in final form 27 September 1986
The time evolution of the pulsed-laser-induced photopotentials of pure and cobalt-sensitized ZnO electrodes is analyzed after 365 and 640 nm excitations. This analysis, as a function of various parameters, enables a determination of the influence of Co*+ ions and excitation wavelength on the kinetics of electron-hole recombination and electron transfer at
the interface to be made.
1. Introduction
Photosensitization of wide bandgap semiconducting oxides to visible light can be achieved by doping with 3d elements [l-3]. It has been shown recently that a cobalt-doped ZnO electrode yields an intense and wellstructured visible photocurrent up to 730 nm [4]. This has been interpreted as being due to d-d transitions inside the Co2+ 3d7 core, followed by a charge transfer into the band structure of the ZnO matrix. In order to check the effect of the tetrahedrally coordinated Co2+ ions on the kinetics of the photogenerated electron-hole pair recombination and electron transfer at the interface, we carried out a timeresolved study of the pulsed-laser-induced photopotentials at the pure and Co-doped ZnO/electrolyte interface as a function of the various parameters involved. ’ Present address: Department of Material Systems Engineering, Faculty of Technology, Tokyo University of Agriculture and Technology, 2-24-26 Nakamachi, Toganei-sbi, Tokyo 184, Japan. 2 Research Associate of the National Fund for Scientific Research (Belgium).
128
This technique has been applied by several authors [6-121 and by ourselves for the photosensitization of semiconductors by organic dyes [ 131. However, this is the first time that an attempt has been made to detect the influence of a photosensitizer such as Co2’ ions on the kinetic behaviour of the ZnO photopotentials, especially after excitation at two different wavelengths, i.e. at 365 nm, the fundamental band-to-band transition and at 640 nm, the d-d transition in Co2+.
2. Experimental
The electrodes were polycrystalline ZnO sinters, prepared according to a technique described elsewhere [4]. They were polished with diamond paste (down to 1 pm) and etched with 4 M HCl(15 s). The electrolyte was an aqueous solution of 0.1 M Na2S04 buffered at pH 7; 10v2 M hydroquinone (H2Q) may be added to test the influence of a reducing agent on the photopotential relaxation. The photoelectrochemical cell contained four electrodes: a working semiconducting electrode (x0.5 cm2), a large surface area Pt counter-
0 009-26 14/86/S 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
I,
Volume 132, number 2
5 December 1986
CHEMICAL PHYSICS LETTERS
electrode (=20 cm2) and a dual reference consisting of a saturated calomel electrode (SCE) capacitively coupled (470 nF) to a Pt quasi-reference. The excitation source was a Molectron DLII tunable dye laser with an 8 ns pulse width, pumped by a Molectron UV 24 nitrogen laser. The triggering system has been described previously [ 131. A pyroelectric joulemeter (Molectron 53-05 DW) connected to an oscilloscope (Philips PM 3234) allows measurement of the relative light intensity of each pulse. The ZnO electrodes were prepolarized at a given potential for a few seconds before irradiation. A solidstate electronic switch (Analog Devices, AD 75 12 DI) then opens the circuit and after a 20 ps delay the pulsed laser is triggered and the photopotential between the working electrode and the SCE is monitored on an oscilloscope (100 MHz, Philips PM 3266, 1 MS1input). The comparison of the photopotential (AV) relaxations as a function of time, for different electrodes, has to be done at a same prepolarization potential (PP) since the PP controls the charge carrier recombinations within the investigated time scale. The measured initial photopotential AV, should thus be as small as possible, so that the PP is essentially undisturbed by AV. Consequently neutral filter combinations were used such that the measured initial APo is always smaller than 120 mV. The AI’u values given in table 1 have been normalized to the same laser intensity (1.736 fl, i.e. 0.217 kW at 365 mn, and 22.5 E.1J,i.e. 2.812 kW at 640 nm, for an irradiated spot of ~0.5 cm2) such that the resulting calculated APu values are all on the linear part of the measured curve of AI’,-,versus the laser intensity.
3. Results and discussion The electron-hole (e--h+) pairs photogenerated in the space-charge region are separated by the electric field and charge the semiconductor capacitance C,. This gives rise to an initial photopotential jump AVu = AQ,,/C,.Two eventualities must be considered: (1) If the e--h+ pairs recombine, AF,-, will relax. (2) If the free holes oxidize the solution, Csc will discharge through RS + R,, where R, is the external load and RS the sum of the bulk resistance of the serniconductor and of the electrolyte. In this work, R, is such (1 Ma) that the time constant r = (R, + R,)C,:
is much larger than the recombination time so the measurements correspond to open-circuit conditions. The influence of Co2+ is examined on the initial photopotential AFu and on its time evolution after the pulse (AV,). 3.1. Initial photopotential AV, AVO is reached roughly 20 ns after the laser pulse, independent of the doping by Co2+ and of the excitation wavelength (365 or 640 nm) of ZnO:Co. Actually this rise-time corresponds to the time constant of the detection system [6,13], showing that even the e--h+ pairs generated by excitation in the Co2+ band are separated more rapidly than 20 ns. Although Co 2+ has no effect on the AV, rise-time of ZnO electrodes, increasing the Co2+ doping levels decreases AVO for excitation at 365 nm (table 1). On the contrary, there is no response to excitation at 640 nm for pure ZnO (Ep = 3.2 eV at 300 K), it appears only for Co-doped electrodes reaching a maximum at a 3.0 at% cobalt doping rate (table 1). We thus selected this sensitizing level for the kinetic studies. Since Co2+ produces fast (< 20 ns) e--h+ recombinations at 365 nm, we compared this effect with that of purposely introducing surface states. Thus, a mechanical polishing of pure ZnO decreases AV, at 365 nrn(from 356 to 178 mV, PP = +0.4 V/SC@ whereas at 640 nm it induces the appearance of a very weak AV, signal (from 0 to 11 mV, to.4 V/SCE). Thus the surface states introduced by polishing also produce fast (-G?Ons) recombinations; the decrease of AV, is
Table 1 Influence of the Co’* doping rate on the open-circuit initial photopotential APo at a prepolarization potential of +O.4 V/SCE. The electrodes are polished and etched; APO is calculated for the same laser intensity at 365 and 640 nm (see section 2) Electrode
A Vo (mv) at 365 run
A Vo (mV) at 640 nm
ZnO ZnO : Coo.ol
356 81
ZnO : Coo.03 ZnO : Coo.06
41 31 20
0 132 192
ZnO:Coo.ro
67 46
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however less than with Co 2+. The states should be distributed between the upper edge of the valence band and the ZnO midgap since they induce a AF on irradiation with visible light, probably by electron injection into the conduction band. We also examined the influence on AVu of hydroquinone (H2Q) 10q2 M in solution, with three electrodes: etched ZnO (ZnO l), polished ZnO (ZnO 2) and etched ZnO : Co u.u3 (ZnO : Co 1). It turns out that H2Q does not affect AFu for the chemically etched electrodes (ZnO 1 and ZnO : Co 1) at 365 and 640 nm but enhances AP’u for the polished ZnO 2 (from 178 to 286 mV at 365 nm and from 11 to 36 mV at 640 nm, +0.4V/SCE). The reductant is thus able to be oxidized by the holes trapped by the ZnO 2 surface states before they recombine with electrons; with ZnO : Co 1 on the other hand, H2Q cannot inhibit recombination because it is mainly induced within the space-charge region by Co2+. 3.2. Time evolution of AV,
3.2. I. After an excitation at 365 nm Typical time-resolved photopotentials AVt are shown in figs. 1 and 2 for ZnO 1 and ZnO : Co 1, respectively, after excitation at 365 nm; the signals for
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Fig. 1. Open-circuit AV, photopotential of an unsensitized and etched ZnO electrode (ZnO 1) irradiated by a 365 nm laser pulse under a prepolariaation potential of 0.0 V/WE. (a) 100 ns div-’ ; (b) 0.5 PCSdiv-’ ; (c) 2.0 CISdiv-’ ; (d) 0.2 ms div-' .
130
5 December 1986
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Fig. 2. Open-circuit photopotential of an etched Co-doped ZnO electrode (ZnO : Co 1) irradiated at 365 nm under a prepolarization potential of 0.0 V/SCE. (a) 100 ns dive1 ; (b) 0.5 ps div-’ ; (c) 0.2 ms div-’ ; (d) 2.0 ms div-‘.
ZnO 2 are qualitatively similar to those of ZnO : Co 1. The decay during the first 50 ns (figs. la and 2a) is probably due to fast recombinations within the depletion region since this decay is uninfluenced by H2Q. At longer times, ZnO 1 behaves quite differently from ZnO : Co 1 and ZnO 2. Indeed, AV does not relax with ZnO 1 (fig. Id), indicating that no e--h+ recombination occurs. Moreover, figs. lc and Id exhibit a slight AVt increase between 1 .O ~.tsand 0.3 ms, which could originate (i) from an electron injected from a trap into the conduction band or (ii) from a contribution of the Helmholtz capacitance as discussed by Wilson et al. [lo]. By contrast, with ZnO : Co 1 (and ZnO 2), AV, relaxes with several decay components up to 10 ms (figs. 2b-2d). Figs. 3 and 4 show the influence of H2Q and of the PP (prepolarization potential) on the relaxation rate of AVt for ZnO 2 and ZnO : Co 1 respectively. We have plotted the percentage of AV, remaining at different times after the laser pulse (7 ps, 0.1 ms, 0.9 ms and 9 ms) as a function of the PP for solutions with and without H2Q. The 100% level has been arbitrarily taken as the AV, measured 1 ~.lsafter the pulse. An increase in the PP and in the H2Q concentration decreases the rate of slow relaxation of ZnO 2 (fig. 3); at a PP of tl V/SCE, H2Q almost totally inhibits the AV decay. Ob-
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5 December 1986
viously, since H2Q has an effect on AV, for ZnO 2, it is also able to prevent the slow surface recombinations. However, hydroquinone is less efficient on ZnO : Co 1 (fig. 4a), where even higher H2Q concentrations do not totally prevent relaxation, indicating that some space-charge recombination takes place via the Co2+.
a 50
Fig. 3. Percentage of residual open-circuit photopotential for a polished but unetched ZnO electrode (ZnO 2) at different times (7.0 ps; 0.1 ms; 0.9 ms; 9.0 ms) after a 365 nm laser pulse, as a function of the prepolariiation potential. The 100% level is arbitrarily taken after 1.0 ps. Full lines: without hydroquinone. Dashed lines: with 1 0m2M hydroquinone.
100
7ops
3.2.2. After an excitation at 640 nm ZnO 1 does not give a signal at this wavelength and the very small AV, measured with ZnO 2 is too weak to be analyzed. The AV, time evolution of ZnO : Co 1 after excitation in the visible range (fig. 4b) is rather similar to that in the W (fig. 4a). Nevertheless, the H2Q effect is slightly more pronounced at 640 nm (fig. 4b) than at 365 nm (fig. 4a), remaining, however, weaker than with ZnO 2 (fig. 3). This small difference between the two wavelengths could be explained by a faster oxidation of water or of the ZnO lattice, in the absence of H2Q, by holes from the valence band rather than from the Co2+ band, implying a higher recombination percentage at 640 nm than at 365 nm. In the presence of H2Q, holes from the Co2+ band would oxidize H2Q instead of recombining with electrons, leading to a greater H2Q effect at 640 nm.
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4. Conclusion
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Fig. 4. Percentage of residual open-circuit photopotential of an etched Co-doped ZnO electrode (ZnO : Co 1) at different times (7.0 rrs; 0.1 ms; 0.9 ms; 9.0 ms) after the laser pulse, as a function of the prepolarization potential. The 100% level is arbitrarily taken after 1 .O ps. Full lines: without hydroquinone. Dashed lines: with 1K2 M hydroquinone. (a) At 365 nm; (b) at 640 nm.
This study shows clearly that Co2+ ions, in spite of their good photosensitizing properties to visible light, produce fast (< 20 ns) space-charge recombinations and slow AV relaxations, extending over a large time scale. These AV decays differ from the relaxation due to surface states observed with polished ZnO (ZnO 2) because, contrary to the ZnO 2 case, they cannot be totally inhibited by H2Q. Co2+ ions thus induce slow recombination between electrons and holes captured not only on the surface but also in the space charge of the semiconductor. Only a slight difference after excitation at 365 and 640 nm has been detected; this could be explained by a less efficient water or ZnO lattice oxidation by holes from the Co2+ rather than from the valence band. Further studies of transient photopotentials in such systems under short-circuit conditions, together with complements to the present study, will be published in the near future [ 141.
136
Volume 132, number 2
CHEMICAL PHYSICS LETTERS
Acknowledgement
We are grateful to Professor J. Nasielski and Dr. J. Kossanyi for having initiated the collaboration between our two laboratories. One of us (DF) thanks the “Commissariat General aux Relations Internationales de la Communaute Francophone de Belgique” for financial support during a two months’ stay in the Laboratoire de Chimie Organique in Brussels.
References
111H.P. Mar&a and A.K. Ghosh, Solar Energy Mat. 1 (1979) 237.
121Y. Matsumoto, J. Kurirnoto, Y. Amagasaki and E. Sato, J. Electrochem.
Sot. 127 (1980) 2148.
131G. Bin-Daar, M.P. Dare-Edwards, J.B. Goodenough and A. Hammett, J. Chem. Sot. Faraday Trans. I 79 (1983) 1199. [41 D. Fichou, J. Pouliquen, J. Kossanyi, M. Jakani, G. Campet and J. Claverie, J. Electroanal. Chem. 188 (1985) 167; D. Fichou and J. Kossanyi, J. Electrochem. Sot. 133 (1986) 1607; D. Fichou, Th&e de Doctorat d’Etat, University of Paris, France (1986).
132
5 December 1986
[5] J.H. Richardson, S.B. Deutscher, S.P. Perone, J. Rosenthal and J.N. Ziemer, J. Electrochem. Sot. 127 (1980) 2580; J.H. Richardson, J.P. Perone, L.L. Steinmetz and S.B. Deutscher, Chem. Phys. Letters 77 (1981) 93. [6] S. Gottesfeld and S.W. Feldberg, J. Electroanal. Chem. 146 (1983) 47; Z. Hanion, N. Croitoru and S. Gottesfeld, J. Electrothem. Sot. 128 (1981) 551. [7] P.V. Kamat and M.A. Fox, J. Phys. Chem. 87 (1983) 59. [ 81 F. WiBig, K. Bitterling, K.P. Charle and F. Decker, Ber. Bunsenges. Physik. Chem. 88 (1984) 374. [9] S. Prybyla, W.S. Struve and B.A. Parkinson, J. Electrothem. Sot. 131(1984) 1587. [ 101 R.H. Wilson, T. Sakata, T. Kawai and K. Hashimoto, J. Electrochem. Sot. 132 (1985) 1082. [ 111 W. Jaegermann, T. Sakata, E. Janata and H. Tributsch, J. Electroanal. Chem. 189 (1985) 65. [ 121 K. Itoh, M. Nakao and K. Honda, J. Appl. Phys. 57 (1985) 5493. [ 131 A. Frippiat, A. Kirsch-de Mesmaeker and J. Nasielski, J. Electrochem. Sot. 130 (1983) 237; A. Frippiat and A. Kirsch-de Mesmaeker, J. Phys. Chem. 89 (1985) 1285; J. Electrochem. Sot., to be published; A. Kirsch-de Mesmaeker, M. Rochus-Dewitt and J. Nasielski, to be published. [ 141 D. Fichou and A. Kirsch-de Mesmaeker, to be published.