J. Phys. Chem. 1996, 100, 3215-3220
3215
Study of Charge Carrier Dynamics at Illuminated ZnO Photoanodes G. H. Schoenmakers,* D. Vanmaekelbergh, and J. J. Kelly Debye Institute, Utrecht UniVersity, P.O. Box 80,000, 3508 TA Utrecht, The Netherlands ReceiVed: August 16, 1995; In Final Form: NoVember 6, 1995X
Charge transfer and recombination at the single crystalline ZnO photoanode were studied by a combination of dc and ac techniques. The potential dependence of the spectrally resolved photoluminescence gives information on the competition between bulk recombination and electron-hole pair separation. The kinetics of surface recombination were studied by electrical and optoelectrical admittance spectroscopy. These results provide insight into the mechanisms of recombination and charge transfer at ZnO photoanodes.
1. Introduction Because of recent developments in optoelectronic device technology1 there is a renewed interest in the surface chemistry of wide bandgap II-VI semiconductors. In particular, etching mechanisms of semiconductors such as ZnSe are important for the new devices.2 The oxidation of selenides, and also of sulfides and tellurides, generally leads to the formation of a solid layer of the chalcogen on the surface. This greatly complicates the study of surface processes. Since one does not encounter this problem with the oxides, where molecular oxygen is formed, ZnO can be considered as a suitable “model” electrode for wide bandgap II-VI semiconductors. Studies of n-type ZnO single-crystal electrodes have been mainly devoted to the determination of the potential distribution and location of the band edges at the surface3,4 and to the mechanism of electrochemical processes.5-12 The reduction of oxidizing agents by majority carriers (electrons in the conduction band) has been investigated by a number of authors.5-8 From a study of the current-potential relationship resulting from the reduction of several one-equivalent oxidizing agents, Morrison5 concluded that electrons are transferred in a single step from the conduction band to the empty states of the redox system. However, results of electrical impedance and electrochemical noise measurements indicate that surface states can be involved in the reduction of redox systems.6,7 The importance of surface states for charge transfer reactions is also clear from studies of current-doubling at ZnO.5,13 Electron-hole pairs can be generated in ZnO by the absorption of photons of energy larger than the bandgap, i.e., 3.2 eV. The minority carriers (holes in the valence band) are consumed in recombination or anodic charge transfer.9-11 The latter involves the dissolution of the semiconductor or the oxidation of a reducing agent present in solution. There have been a number of studies of the competition between these charge transfer processes based on steady state methods, such as rotating ring-disk voltammetry.14,15 The results suggest that in both the anodic dissolution and the oxidation of a reducing agent an intermediate formed by localization of a hole in a surface site is involved. Light emission from ZnO photoanodes has been observed in the potential range in which holes recombine with majority carriers.16,17 Since it is generally agreed that surface recombination is nonradiative, photoluminescence indicates that at least a part of the photogenerated holes recombine in the bulk solid. Petermann et al.17 claimed a strict correlation between the photocurrent and the green luminescence at ZnO single-crystal X
Abstract published in AdVance ACS Abstracts, January 15, 1996.
0022-3654/96/20100-3215$12.00/0
electrodes. This result indicated that photoluminescence might be a useful tool for the study of the competition between bulk recombination and anodic charge transfer. Surface recombination at illuminated semiconductor electrodes can be studied effectively by two ac techniques.18-21 In the electrical impedance method, the electrode potential is harmonically modulated while the light intensity incident on the electrode is kept constant. The harmonic current response is measured. In the potential range in which (part of) the electron-hole pairs recombine at the surface, an extra electrical admittance is observed. This admittance may provide kinetic information on surface recombination.18,20 In the optoelectrical admittance method (also called intensity modulated photocurrent spectroscopy) the electrode potential is kept constant and the incident light intensity is harmonically modulated. The harmonic photocurrent response is measured. It has been shown that with this method the flux of photogenerated minority carriers toward the surface can be determined unambiguously.22 Furthermore, the kinetics of surface recombination can be studied.18,19,21 The aim of the present work is to consider the competition for photogenerated holes between anodic charge transfer and recombination processes at ZnO single-crystal electrodes. A combination of steady-state measurements (photocurrent quantum yield and spectrally resolved luminescence), and ac electrochemical methods (electrical and optoelectrical admittance) were used for this purpose. 2. Experimental Section ZnO wafers, 1 mm thick, were cut from single crystals. The hexagonal-shaped electrodes had the 0001 and 0001h surface orientation. Results will be presented for the 0001h (oxygen) face, the results for the opposite face being very similar. The samples were glued to a copper holder with silver epoxy resin which yielded a good ohmic contact and subsequently fitted in an epoxy holder. The surface area exposed to the electrolyte was 0.10 cm2. The electrochemical cell contained the working electrode, a Radiometer SCE reference electrode, and a platinum counter electrode. The cell was thermostated at 25 °C. The electrolyte solution (0.5 M K2SO4 or 1 M KI) contained reagent grade chemicals dissolved in distilled water. The dc measurements were carried out with a Wenking LT 87 potentiostat or a PAR 273A potentiostat. The electrode potential U will be referred to that of the SCE reference electrode. Electrical impedance measurements were carried out with a PAR 273A potentiostat together with a Solartron FRA © 1996 American Chemical Society
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Figure 1. Photocurrent j (solid line) plotted as a function of potential U for a ZnO electrode in 0.5 M K2SO4 solution; 1/Cp2 measured under illumination (+) and in the dark (×) at 22140 Hz is also shown as a function of potential U. The illumination wavelength was 365 nm.
1255 frequency response analyzer. Optoelectrical admittance spectroscopy was measured with a setup similar to that described before.23 Light with an energy of 3.5 eV from an argon laser (Coherent Innova 90-6) was used to illuminate the electrode; the intensity was modulated with an Isomet 1211 acousto-optic modulator. Other sources of illumination were mercury and xenon lamps in combination with a monochromator or filters. Photoluminescence measurements were carried out with a Perkin-Elmer MPF 44-B spectrofluorometer. The impedance of the electrode strongly depended on the pretreatment of the ZnO surface. Reproducible Mott-Schottky plots with almost no frequency dependence were obtained when the electrode surface was mechanically polished and subsequently etched for at least 2 min in a 10% Br2/methanol solution.
Figure 2. Photoluminescence emission spectrum of a ZnO electrode in 0.5 M K2SO4 solution at -1 V (SCE). The excitation wavelength was 365 nm.
3. Results
Figure 3. Photocurrent quantum yield j/eφ as a function of potential U; photoluminescence, I (2.4 eV), divided by the maximum photoluminescence, Im, as a function of potential, U.
In Figure 1, the photocurrent density j of a ZnO single-crystal electrode illuminated with light of 3.4 eV is presented as a function of the electrode potential U. In this figure, the parallel equivalent capacitance Cp per unit surface area is also plotted as Cp-2 vs U. It should be noted that Cp is almost independent of the measuring frequency between 100 Hz and 20 kHz in the potential range more positive than 0.1 V. The plot of Cp-2 vs U gives a straight line both under illumination and in the dark; the Mott-Schottky relationship is clearly obeyed in this range. From the slope of the Mott-Schottky plot the donor density is found to be 1.7 × 1016 cm-3. From the intercept with the potential axis the flat-band potential Ufb is determined to be -0.15 V in the dark (in an electrolyte of pH ) 7). This value is typical for low-doped samples. For higher doped samples a somewhat more negative value for Ufb was found (≈-0.3 V) in reasonable agreement with literature reports.3,4 From the measurements no clear difference was found in flat band values under illumination and in the dark. The apparent difference in Figure 1 indicates the uncertainty in the Mott-Schottky measurements. At potentials more than 0.3 V positive with respect to the flat-band value (denoted as range A) the photocurrent is independent of the potential and is directly proportional to the light intensity. This indicates that all photogenerated holes contribute to the photocurrent. From the value of the absorption coefficient for 3.4 eV light24 the penetration depth 1/R is estimated to be 40 nm. From the measured donor density and the dielectric constant of ZnO25 the width of the depletion layer in range A is calculated to be more than 100 nm. Hence it is reasonable to assume that in this range all electron-hole pairs are photogenerated in the depletion layer, and hence effectively separated. The photocurrent quantum yield (which is photo-
current density j divided by the product of the elementary electric charge e and the absorbed photon flux Φ) is equal to 1. In potential range C (negative of -0.5 V) there is no photocurrent, which means that all photogenerated holes recombine. In the intermediate potential range B the photocurrent strongly decreases with decreasing potential. In Figure 2 the photoluminescence spectrum of a ZnO electrode at -1 V (SCE) in 0.5 M K2SO4 solution is given. This spectrum resembles the cathodoluminescence spectrum of ZnO powder measured in air26 with a near-UV band, attributed to a band-band process, and a green band, attributed to recombination via a defect center. The intensity I of both the green and UV luminescence increased as the electrode potential was made more negative, reaching a potential independent value Im at -0.5 V. In contrast to III-V semiconductors, such as GaP27 and GaAs,28 the measured luminescence intensity was reproducible and was not affected by hydrogen evolution. In Figure 3, I/Im is plotted vs U for the green emission (2.4 ( 0.05 eV), together with the quantum yield j/eφ. In range C, I/Im is 1, independent of the potential. I/Im decreases strongly with increasing potential in range B and becomes almost zero in range A, as j/eφ tends to 1. The results of Figure 3 strongly suggest that the potential dependence of the luminescence intensity reflects the potential dependent recombination of photogenerated holes, which therefore cannot contribute to photocurrent. In order to check if the relationship between I/Im and U can be attributed to recombination of electron-hole pairs formed deeper than the region in which the charge carriers are effectively separated, i.e., deeper than the sum of the depletion and diffusion layer widths, the results were analyzed on the basis of the Ga¨rtner relationship.29 Ellis and co-workers30,31 and Gerischer and co-workers27 have
Charge Carrier Dynamics at Illuminated ZnO Photoanodes
Figure 4. Logarithmic plot of the photoluminescence intensity, I (2.4 eV), divided by the maximum intensity, Im, as a function of the square root of the band bending. The excitation wavelength was 365 nm.
shown that the photoluminescence intensity Iλ for light of the wavelength λ is given by
Iλ )
Φκ e-Rd 1 + RL
(1)
where κ is the ratio of the rates of radiative to radiative + nonradiative recombination, L is the diffusion length of the minority carrier, and the depletion layer width d is given by
d)
[
]
20(U - Ufb) eND
1/2
(2)
in which ND is the donor density of the semiconductor. The maximum intensity Im is reached when the width of the depletion layer d is much smaller than 1/R. Hence, Im(λ)/I(λ) is given by
Im(λ)/I(λ) ) exp[R{U - Ufb}1/2{20/eND}1/2]
(3)
In Figure 4, {ln(Im/I)} for green emission is plotted as a function of (U - Ufb)1/2. Since the readout of the photomultiplier tube was done with a digitizing oscilloscope and the photoluminescence signal is very weak in range A, the results show considerable scatter. However, for values of the band bending larger than 0.1 V, it can be seen that this plot yields a straight line, with an almost zero intercept on the ln(Im/I) axis. It can be concluded that this plot is in agreement with eq 3. From the slope of the plot of Figure 4 and the value of ND obtained from the Mott-Schottky plots, an absorption coefficient R of 2.3 × 105 cm-1 is calculated for 3.7 eV excitation, in good agreement with the value given in the literature.24 The deviation from the straight line in Figure 4 at values of the band bending smaller than 0.1 V can probably be attributed to a slight shift of the band edges at the surface (see Figure 1 and further discussion). Plots similar to those of Figure 4 are also obtained for the 3.25 eV luminescence when the ZnO electrode is illuminated with 4.5 eV light. The foregoing analysis shows that the increase in the intensity of the photoluminescence with decreasing potential can be attributed to a decrease in the width of the depletion layer and, hence, to a decrease in the width of the layer in which photogenerated electron-hole pairs escape recombination. It follows that the flux of photogenerated holes which escape bulk recombination (jh/e) must decrease with decreasing band bending in a way described by the Ga¨rtner relationship. However, an analysis showed that the photocurrent does not obey the Ga¨rtner relationship in range B, which strongly suggests that the photocurrent j differs from the electrical hole flux towards the surface jh.
J. Phys. Chem., Vol. 100, No. 8, 1996 3217
Figure 5. Optoelectrical admittance spectra for illuminated ZnO at various electrode potentials. The dc photocurrent at 1.0 V (SCE) was 50 µA cm-2. Besides the potential, the frequency of the maximum (where meaningful) is given for each spectrum. Each frequency decade contains 10 points (for each measured potential the frequency increases clockwise).
In order to investigate the extent to which either bulk recombination (partly radiative) or surface recombination (nonradiative) is responsible for the decrease of the photocurrent with decreasing potential, ac measurements were performed on illuminated ZnO electrodes. In optoelectrical admittance experiments, a harmonically modulated light intensity (Φ(ω)) is superimposed on a constant background light intensity (Φ), and the harmonic photocurrent density response (j(ω)) is measured.21 The optoelectrical admittance Y(ω) is defined as j(ω)/eφ(ω) and is, in fact, the ac photocurrent quantum yield. The optoelectrical admittance has been calculated for the case in which photogenerated minority carriers recombine at the surface.18 It was shown that Y(ω) plotted in the complex plane gives a semicircle. The highfrequency limit of Y(ω) is equal to jh/eφ and, hence, gives the flux of photogenerated minority carriers to the surface.18 The low-frequency limit of Y(ω) is equal to the steady state quantum yield j/eφ. The diameter of the semicircle is therefore equal to the loss in photocurrent quantum yield (jh - j)/eφ, due to recombination of minority carriers at the surface. The frequency pertaining to the maximum of the semicircle is given by
ωmax ) βnns + βpps
(4)
In eq 4, βn and βp are the rate constants for electron and hole trapping in surface states, and ns and ps the concentrations of electrons and holes at the surface, respectively. In Figure 5, Y(ω) is plotted in the complex plane for various values of the electrode potential. It can be seen that, in range A, Y(ω) is a point on the real axis close to one. In range B, Y(ω) is a (flattened) semicircle, which shows that photogenerated holes recombine at the surface. Possible reasons for the deviation from a semicircle have been discussed elsewhere.19,32 The diameter of the semicircle, corresponding to the fraction of the photogenerated holes lost in surface recombination (jh - j)/eφ increases with decreasing potential. At -0.1 V, an approximate value for (jh - j)/eΦ of 0.25 is obtained by extrapolation. It is also found that the high-frequency limit of Y(ω), jh/eφ decreases with decreasing potential. This decrease becomes significant in potential range B (between -0.5 and 0.4 V). This shows that a considerable fraction of the photogenerated holes are lost by recombination in the bulk. For instance, at -0.05 V, jh/eφ is estimated to be 0.7, while j/eφ is approximately 0.4. Hence, at this potential, 30% of the photogenerated holes are lost by recombination processes in the bulk, a similar percentage is lost in surface recombination, and about 40% of the photogenerated holes are involved in anodic current flow. This latter percentage is in agreement with the steady state quantum yield shown in
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Figure 7. Dependence of the additional capacitance ∆Cp,max of an illuminated ZnO electrode in 0.5 M K2SO4 solution on the photon flux, eφ, divided by the measuring frequency, ω: (0) 0.7 Hz, (4) 2.214 Hz.
Figure 6. Parallel capacitance, Cp, as a function of potential, U, for a ZnO electrode in 0.5 M K2SO4 solution in the dark (open symbols) and under illumination (closed symbols). The dc photocurrent at 1.0 V (SCE) was 40 µA cm-2. (9, 0) 2.214 Hz, (2, 4) 2214 Hz.
Figure 3. From Figure 5 it is also clear that the frequency of the maximum of the imaginary part of Y(ω) increases with decreasing potential. This is due to the fact that, with decreasing potential, the band bending decreases and, hence, ns increases (see eq 4). If we assume that, at -0.1 V, βnns . βpps and that ns is close to the bulk concentration (1.7 × 1016 cm-3), βn is found to be of the order of 10-11 cm3 s-1. The corresponding cross section for electron capture (which is βn divided by the thermal velocity of the electron) is very low (10-18 cm2), which strongly suggests that the electron is captured in a state with a negative charge. In Figure 6, the parallel equivalent capacitance of the ZnO electrode in the dark and under illumination is shown as function of potential. Under illumination, an additional capacitance peak, with a maximum value denoted as ∆Cp,max, is observed at low frequencies (2.2 Hz is shown) in the recombination range (between -0.2 and 0.2 V). According to the theory20 this capacitance peak is due to surface recombination. In ref 20 it was shown that the electrical admittance due to surface recombination, which is parallel to the capacitance of the depletion layer, contains a resistance in series with a parallel combination of a capacitance and another resistance. When this additional admittance is transformed to a parallel equivalent circuit, the imaginary part contains a frequency-dependent “capacitance”, which shows a maximum when plotted as a function of the band bending. The peak maximum is given by
∆Cp,max ) (e/2kBT)[(jh - j)/ω]
(5)
In eq 5, jh - j is the hole flux which is lost to surface recombination and ω the measuring frequency. In Figure 7, the additional peak maximum observed under illumination is plotted as a function of eφ/ω; a linear relationship is observed with a slope of 1 V-1. If it is assumed that the recombination flux (jh - j) is a constant fraction of eφ, it may be concluded that ∆Cp,max is indeed proportional to (jh - j)/ω. From the optoelectrical admittance results it is clear that (jh j) is approximately one-third of eφ. The slope of ∆Cp,max vs [(jh - j)/ω] is then about 3 V-1. This is a factor of 6 smaller than the value predicted from eq 5. Two effects may be responsible for this discrepancy. First, different centers may
Figure 8. Photoanodic quantum yield as a function of U - Ufb for illuminated ZnO in (a) 0.5 M K2SO4 solution and (b) 1 M KI solution.
be involved in surface recombination. A distribution of rate constants for electron capture βn or a not completely uniform band bending over the surface lead to a broadening and lowering of the additional capacitance peak.20 Second, the peak height ∆Cp,max can be lower than that predicted because a change in the applied potential alters not only the band bending but also the potential drop over the Helmholtz layer.32 Experimental indications that this is, in fact, the case can be found in Figures 1 and 3. First, at -0.1 V (Ufb according to the Mott-Schottky plots extrapolated from potential range A), j/eφ is still 0.3 which indicates that, at this potential, a depletion layer must exist in the solid. This is confirmed by the fact that I/Im is still about 0.4. Second, it is clear from Figure 3 that in the potential range negative with respect to -0.1 V, the dependence of I/Im on U deviates from the exponential relationship of eq 3. That again indicates that the band bending changes less than the change of the applied potential U. It is known that in iodide solution the photoanodic corrosion of ZnO is suppressed.15 In Figure 8 the photocurrent quantum yield as a function of U - Ufb of ZnO is shown for a 0.5 M K2SO4 solution (curve a) and for a 1 M KI solution (curve b). It can be seen that with iodide in solution the quantum yield in range B is considerably higher. The observation that at a given potential the increase in quantum yield due to iodide corresponds roughly to the diameter of the corresponding semicircle in Figure 5 (representing the loss due to surface recombination in indifferent electrolyte solution) suggests that iodide suppresses surface recombination. In order to check this, electrical and optoelectrical admittance measurements were performed. In contrast to the case of indifferent electrolyte no additional capacitance peak was found in iodide solution in potential range B. Furthermore, plots of the optoelectrical admittance in the
Charge Carrier Dynamics at Illuminated ZnO Photoanodes
Figure 9. Comparison of the quantum yield, photoluminescence, and IMPS results for illuminated ZnO. The dashed and solid curves show j/eφ and 1 - I/IM, respectively, as a function of potential. The highand low-frequency limits of the IMPS spectra are shown as open and closed triangles, respectively.
complex plane measured in the presence of iodide show flattened semicircles with diameters considerably smaller than those measured in indifferent electrolyte solution. The results of these electrical and optoelectrical admittance measurements support the idea that iodide indeed strongly suppresses surface recombination. 4. Discussion In this section we hope to show how a combination of the results of photoluminescence measurements and dc and ac electrochemical measurements give insight into the processes occurring near and at the illuminated ZnO/electrolyte interface. ZnO has a direct optical transition, which means that the absorption coefficient for light of energy exceeding the bandgap is large.24 Consequently, the penetration depth of the light is small and, for a not too highly doped sample, it may be smaller than the width of the depletion layer. This is the case in potential range A and it is, therefore, reasonable to assume that in this range the photocurrent quantum yield is one. This assumption is supported by the fact that the photoluminescence intensity is nearly zero in this range. In potential range B, the luminescence intensity increases steeply with decreasing potential. Our results show that, in the more positive part of this range, the dependence of intensity of both the green and the UV luminescence on the band bending is predicted by eq 1 based on the Ga¨rtner analysis. This result clearly relates the luminescence to radiative recombination processes occurring in the bulk, adjacent to the layer in which electron-hole pairs are effectively separated by diffusion and migration. Hence, if the ratio of the rates of radiative to radiative + nonradiative bulk recombination is potential independent, the relationship between the luminescence intensity and the potential provides a means to study the competition between charge separation and bulk recombination. Figure 9 shows that the results of the different measurements in indifferent electrolyte solution are consistent. In this figure the photocurrent quantum yield j/eφ (dashed curve), 1 - I/IM (solid curve) and both the high-frequency (open triangles) and low-frequency limits (closed triangles) of the IMPS spectrum are plotted as a function of the electrode potential. As mentioned above, the IMPS low-frequency limit should be equal to the steady state quantum yield (j/eφ). From Figure 9, it is clear that this is the case. The IMPS high-frequency limit should correspond to the hole flux jh/e divided by Φ. From the
J. Phys. Chem., Vol. 100, No. 8, 1996 3219 discussion of the photoluminescence it follows that jh/eφ should be equal to 1 - I/IM. It can be seen in Figure 9 that this is, indeed, the case in the potential range positive with respect to 0 V. In the potential range more negative than 0 V, the highfrequency limit was beyond the experimentally available frequency window. The fact that, for the results measured in indifferent electrolyte, the relationship between the photocurrent quantum yield and the potential is not in agreement with the Ga¨rtner relationship indicates that besides bulk recombination, (nonradiative) surface recombination is also responsible for the decrease of the photocurrent with decreasing potential. This latter result can also be seen in Figure 9 where it is clear that the 1- I/IM and the j/eφ curves do not overlap. In addition, the results of electrical and optoelectrical admittance spectroscopy show that surface recombination plays an important role; the difference between j/eφ and 1 - I/IM is the recombination current density jr divided by eφ. As a consequence, the claim, made in ref 17, that the luminescence vs potential and the photocurrent quantum yield vs potential measurements are fully complementary, is not correct. The cross section for capture of an electron in a recombination center at the surface is very low (2 orders of magnitude smaller than the geometrical cross section); this indicates that the centers are negatively charged. It seems reasonable to assume that an O•- surface radical, formed by the previous localization of a hole at an oxygen anion, is involved. The simplest kinetic scheme for the photoanodic dissolution of ZnO is the following:
(Zn2+O2-)surf + h+ f (Zn2+O-)surf
(6)
(Zn2+O-)surf + h+ f Zn2+ + Osurf
(7)
2Osurf w O2
(8)
In the first step, a hole is trapped at an O2- ion at the surface, leading to an O- radical anion. In the following step, a second hole is captured in the intermediate (Zn2+O-)surf, resulting in a zinc ion in solution and an adsorbed oxygen atom. Two such adsorbed atoms may give rise to O2 which escapes into the electrolyte. In this framework it seems reasonable to assume that surface recombination can be described by process 6:
(Zn2+O-)surf + e- w (Zn2+O2-)surf
(9)
The capture of an electron (process 9) competes with the second step of anodic dissolution (process 7). The fact that the photocurrent onset is negative with respect to the flat-band potential means that the second step of anodic dissolution competes very effectively with recombination. The photocurrent onset near the flat-band potential of ZnO photoanodes contrasts strongly with results found with III-V semiconductors for which surface recombination dominates over photoanodic dissolution, even up to large band-bending.33 In the III-V case, trapping of a hole in a surface bond leads to a positively charged intermediate, which makes the capture of an electron more favorable than the capture of a second hole. From the results measured in the presence of iodide, it could be concluded that surface recombination is suppressed considerably. Two possible explanations can be given for this suppression. The first is that holes are captured very effectively by (adsorbed) iodide, so that the capture of a hole in a surface bond (process 6) is prevented. A second explanation is that iodide
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reacts with the decomposition intermediate formed in process 6:
(Zn2+O-)surf + I- f (Zn2+O2-)surf + 1/2I2
(10)
Results of the stabilization of ZnO with different redox systems15 suggest that process 10 is favored with respect to direct hole transfer. We have attempted to study the mechanism of iodide oxidation at a ZnO photoanode by rotating ring-disk voltammetry. The oxidation rate of I- at the ZnO disc can in principle be measured by reduction of the photogenerated I2 at the ring electrode. A determination of the degree of “stabilization” of the ZnO electrode as a function of the photocurrent should enable one to distinguish between direct hole capture by I- and process 10.34 However, preliminary results showed that oxidation of I- gives adsorbed iodine at the ZnO surface. It appears that the slow desorption of iodine complicates the study of the I- oxidation by rotating ring-disk voltammetry. From the above discussion some general conclusions can be drawn. (i) Measurements of the luminescence intensity as a function of the electrode potential can be used to study the competition between bulk recombination and electron-hole pair separation. (ii) The location in the semiconductor at which recombination occurs and the kinetics of recombination processes cannot be studied with steady state measurements alone. Photocarrier losses due to recombination in the bulk and at the surface can be distinguished by the optoelectrical admittance method. The kinetics of surface recombination can be studied by electrical admittance spectroscopy. Acknowledgment. The ZnO single crystals were kindly provided by Dr. G. Dierssen of 3M Corporate Research, St. Paul, MN. References and Notes (1) Nurmikko, A. V.; Gunshor, R. L. Solid State Commun. 1994, 92, 113. (2) Bacher, G.; Illing, M.; Forchel, A.; Hommel, D.; Jorst, B.; Landwehr, G. Phys. Status Solidi B 1995, 187, 371. (3) Bohe, A. E.; Vilche, J. R.; Ju¨ttner, K.; Lorenz, W. J.; Paatsch, W. Electrochim. Acta 1989, 34, 1443. (4) Gerischer, H. Electrochim. Acta 1989, 34, 1005.
(5) Morrison, S. R. Electrochemistry at Semiconductor and Oxidized Metal Electrodes; Plenum Press: New York, 1980; p 193. (6) Cardon, F.; Gomes, W. P. Ber. Bunsenges. Phys. Chem. 1970, 74, 436. (7) Gomes, W. P.; Cardon, F. Ber. Bunsenges. Phys. Chem. 1970, 74, 431. (8) Morrison, S. R. Surf. Sci. 1969, 15, 363. (9) Fruhwirth, O.; Herzog, G. W.; Poulios, J. Surf. Technol. 1985, 24, 293. (10) Gomes, W. P.; Cardon, F. J. Solid State Chem. 1971, 3, 125. (11) Lohmann, F. Ber. Bunsenges. Phys. Chem. 1966, 70, 428. (12) Didziulis, S. V.; Butcher, K. D.; Cohen, S. L.; Solomon, E. I. J. Am. Chem. Soc. 1989, 111, 7110. (13) Fujishima, A.; Kato, T.; Maekawa, E.; Honda, K. Bull. Chem. Soc. Jpn. 1981, 54, 1671. (14) Inoue, T.; Fujishima, A.; Honda, K. Bull. Chem. Soc. Jpn. 1989, 62, 2789. (15) Inoue, T.; Fujishima, A.; Honda, K. Bull. Chem. Soc. Jpn. 1979, 52, 3217. (16) Hoyer, P.; Eichberger, R.; Weller, H. Ber. Bunsenges. Phys. Chem. 1993, 97, 630. (17) Petermann, G.; Tributsch, H.; Bogomolni, R. J. Chem. Phys. 1972, 57, 1026. (18) Vanmaekelbergh, D.; de Wit, A. R.; Cardon, F. J. Appl. Phys. 1993, 73, 5049. (19) Peter, L. M. Chem. ReV. 1990, 90, 753. (20) Vanmaekelbergh, D.; Cardon, F. J. Phys. D: Appl. Phys. 1986, 19, 643. (21) Li, J.; Peter, L. M. J. Electroanal. Chem. 1985, 193, 27. (22) Peat, R.; Peter, L. M. Appl. Phys. Lett. 1987, 51, 328. (23) Erne´, B. H.; Vanmaekelbergh, D.; Vermeir, I. E. Electrochim. Acta 1993, 38, 2559. (24) Mo¨llwo, E. Ann. Phys. 1948, 3, 230. (25) Pankove, J. I. In Solid State Physical Electronics Series; Holonyak, N., Ed.; Optical Processes in Semiconductors 413; Prentice-Hall, Inc.: Englewood Cliffs, NJ, 1971. (26) Heiland, G.; Mollwo, E.; Sto¨ckmann, F. In Solid State Physics AdVances in Research and Applications; Seitz, F., Turnbull, D., Eds.; Academic Press: New York, 1959; Vol. 8, p 191. (27) Chmiel, G.; Gerischer, H. J. Phys. Chem. 1990, 94, 1612. (28) Smandek, B.; Chmiel, G.; Gerischer, H. Ber. Bunsenges. Phys. Chem. 1989, 93, 1094. (29) Ga¨rtner, W. W. Phys. ReV. 1959, 116, 84. (30) Burk, A. A.; Johnson, P. B.; Hobson, W. S.; Ellis, A. B. J. Appl. Phys. 1986, 59, 1621. (31) Ellis, A. B. Chemistry and Structure at Interfaces; Hall, R. B., Ellis, A. B., Eds.; VCH: Deerfield Beach, FL, 1986; Chapter 6. (32) de Wit, A. R.; Vanmaekelbergh, D.; Kelly, J. J. J. Electrochem. Soc. 1992, 139, 2508. (33) Vanmaekelbergh, D.; ter Heide, R. P.; Kruijt, W. Ber. Bunsenges. Phys. Chem. 1989, 93, 1103. (34) Vanmaekelbergh, D.; Gomes, W. P. J. Phys. Chem. 1990, 94, 1571.
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