J . Phys. Chem. 1992, 96, 2272-2214
2212
Flat-Band Potential Shift In a Dye-Sensitized Zinc Oxide Electrode on Pulse Excitation S. Nakabayashi, T. Amemiya, and A. Kim* The Institute of Physical and Chemical Research (RIKEN), Wako-shi, Saitama 351 -01, Japan (Received: August 26, 1991; In Final Form: October 1 1 , 1991)
The electrode potential dependence of the photopotential in several dye-sensitized ZnO electrodes revealed that the flat-band potential is positively shifted just after the photoinduced electron injection from the dye molecule. The shift is caused by surface charges resulting from the photoionization of dye molecules adsorbed on the electrode surface.
Introduction
The flat-band potential of a semiconductor electrode may shift if the electric charge is localized on the surface. The shift caused by matching of the Fermi level of the charge accommodating in a surface state with the redox potential of the solute in solution is known as Fermi level pinning,’ which has been extensively studied for bare (unmodified) semiconductor Adsorption can change the surface properties so drastically that a new charge-trapping site is formed on the surface. An extreme example is the adsorption-induced shift of the work function of the metal crystals in high This paper reports the flat-band shift observed on the pulse excitation of dyesensitized ZnO (n-type semiconductor) electrodes. The shift is revealed by the straightforward measurement of transient photopotentials. On the pulse excitation under anodic conditions, electrons are injected from excited dye molecules to the conduction band and the oxidized form of the dye is left on the surface. The transient surface charge thus provided can cause the observed shift. The same shift must take place on stationary-light excitation, but it has probably been overlooked because the charge accumulation in a photostationary state is too small to cause an appreciable shift. Experimental Section
The experimental setup is essentially identical to that reported previously.I0 The semiconductor used for the electrode was a ZnO single crystal doped with indium at a donor density of 3.8 X m-3. Dye molecules adsorbed on the surface of the ZnO electrode were excited by a Lumonics Hyper-Dye 300 dye laser pumped by an HY-750 pulsed YAG laser. The width of the laser pulse was 6 ns, and the photon energy was tuned to the absorption energy of the dye molecule. The beam was guided by a quartz optical rod to the surface of the electrode. The electrode of a ca. cm2 area was mounted in a Ker-F cell with a Ag or Pt counter-reference electrode of a large area of 1 cm2. Photopotential signals between ZnO and the counter-reference electrode were fed into a Tektronix differential amplifier 7A22 (1 MQ, 47 pF) or 7A13 (100 MQ, 20 pF), and recorded on a Riken-Denshi TCH-4000 or LeCroy TR8828C digitizers. A battery combined with a potentiometer was set between the electrode and an input (1) Bard, A. J.; Bocarsly, A. B.; Fan, F. R. F.; Walton, E. G.; Wrighton, M. S. J . Am. Chem. SOC.1980, 102, 3671. (2) Prybyla, S.; Struve, W. S.; Parkinson, B. A. J . Elecfrochem.SOC.1984, 131, 1587. (3) Frese, Jr., K. W. J . Electrochem. SOC.1983, 130, 28. (4) McEvoy, A. J.; Etman, M.; Memming, R. J . Elecfroanal. Chem. 1985, 190, 225. ( 5 ) Etman, M. J . Phys. Chem. 1986, 90, 1844. (6) Allongue, P.; Blonkowski, S.; Lincot, D. J . Electroanab Chem. 1991, 300, 26 1. (7) Nakabayashi, S.; Kira, A. J . Phys. Chem., submitted. (8) Goddard, P. J.; Lambert, R . M. Surf. Sci. 1977, 67, 180. (9) Gavrilyuk, V. M.; Medvedev, V. K. Sou. Phys. Solid Srote 1966, 8, 1439. (IO) Nakabayashi, S.;Kira, A. J . Elecrroanal. Chem. 1991, 300, 249.
terminal of the preamplifier for control of the electrode potential. The cell and connecting lines were shielded carefully. The impedance measurement of the electrode was conducted on a YHP 4192A low frequency impedance analyzer. The dyes used in the experiments were rhodamine B perchlorate, rose bengal, eosin Y and tris(2,2’-bipyridine)ruthenium(II) perchlorate, which were adsorbed on the electrode from acetonitrile-TBAP (0.3 M) electrolyte solution containing dye at lo4 M. Tetra(neopent0xy)phthalocyanine (TNPP) was adsorbed as a thin layer by vaporization of a drop of its dilute solution in benzene. The NaClO, (0.5 M) aqueous solution was used for TNPP-adsorbed electrode because the dye does not dissolve into the aqueous electrolyte. RWultS
The Mott-Schottky plots were made for all the dye-adsorbed electrodes as well as the ZnO without the dye. All the plots were linear and gave the same value for the flat-band potential, -1.0 V vs Pt. The adsorption of the dye does not affect the flat-band potential in the dark. Irradiation of the dye-sensitized electrode by the laser pulse gives a transient photopotential whose yield and decay depend on the electrode potential (for instance, see ref 10). The photopotential does not occur in the absence of the dye. Consequently, the observed photopotential results from the electron injection from the excited state of a dye to the conduction band.I0 From the electric viewpoint, the system of the experiment is regarded as a charged capacitor connected to a measurement circuit of an approximately infiite impedance. The photopotential AE, is caused by the charge AQ injected into a condenser of a differential capacitance C(E): AQ = C(E)hE. By using the Mott-Schottky relation, the capacitance is written as a function of the electrode potential; therefore
AE = a ( -~E ~ ) ~ / ~ A Q
(1)
where a = 21/2(ecc&Vd)-1/z;e is the electron charge; to is the permittivity of the vacuum; t and Nd are the dielectric constant and the donor density of the electrode, respectively. Since the photopotential decays with time,” initial photopotentials immediately after the pulse, AE,, and the corresponding initial charge, AQo, should be used for analysis. The data for two sensitizing dyes are plotted in Figures 1 and 2 in terms of a squared form of eq 1: ( U o ) ’ = a2(AQol2(E- E d (2) The laser intensity was fixed through a series of measurements for the same electrode to make AQo constant. The flat-band potential Em of -1.0 V vs Pt, obtained from the Mott-Schottky plot, is used for the plot. Figures 1 and 2 exhibit the data for rose bengal and TNPP, respectively. Rose bengal molecules adsorbed on the surface are (1 I ) The decay of the photopotential is almost supressed by adding a supersensitization reagent like hydroquinone. The discussion for the time evolution of the photopotential will be published elsewhere.
0022-3654/92/2096-2272%03.00/00 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 5, 1992 2273
Flat-Band Potential Shifts TABLE I: Flat-Band Potential Shift in Dye-Sensitized Electrodes
rhodamine B (&,PP
- E m MS)b
dyea eosin Y
Rose Bengal +OS9
+0.45
Ru(bpyh2+ +0.22
+0.43
TNPP +0.72
9.0 X 10" 4.8 X 10" 4.1 X 10" 2.3 X 10" 1.4 X 1 O l o no/cni2 0.15 0.16 0.003 1 0.32 0.13 CH/pF aConcentration of dye except TNPP are lo4 M. bShift of the flat-band potential. CDensityof the injected electrons. Qo(in eq 2) = 1.60 X no/C cm-*. dHelmholtz capacitance estimated by eq 4. I
I
A
I
AH,
B
---?-'--'-'-: C.B.
i
'I N
O W
4
500
Eref
SOLID
11. DYE
DYE
Figure 3. Schematic diagram for the shift of the flat-band potential. A: In the dark where adsorbed dye molecules are in the reduced form (e). B: Just after the pulse-laser excitation where some of the dye molecules
are in the photooxidized form (+).
(EmPP - EmMS) are listed in Table I where the results for three other dyes are also included. From Nd = 3.8 X loz5m-3 and t = 7.9, a2is evaluated 4.6 X lo3C-*m4 V. The slope of the plot gives the values of the injected charge, AQo, from which the densities of injected electrons, no, are calculated as listed in Table I.
/ v
E-Efb
Figure 1. Dependence of the initial photopotential, Eo,on the electrode potential for a rose bengal sensitized ZnO electrode in 10-4 M rose bengal in acetonitrile-TBAP solution.
t o-
1.5
-
\ 1.0
-
I
N
> E
N
0
W
4
15
0
-
L, 0
I
*'
I
05 E-Efb
I
I
1.0
1.5
/ v
Figure 2. Same plot as Figure 1 for a TNPP-coated ZnO electrode in
NaC104 aqueous solution. in equilibrium with those dissolved in the solution, while TNPP forms a rather thick layer which is insoluble to the solution. The plots give straight lines for these different layers. It is remarkable that the lines for the experimental data do not pass the origin but cross the abscissa at substantially positive values in contrast to eq 2. Such plots can be described in terms of eq 2 if it is assumed that the flat-band potential shifted to the positive side by these values. According to this interpretation, the flat-band potential based on the transient photopotential, which will be referred to as EmPP,differs from that obtained from the MottSchottky plots, EmMS.The experimental values of the difference
Discussion For an n-type semiconductor electrode, the flat-band potential is given by12 Efi = (EA - A d - AHL- Eref (3) Here, EA is the electron affinity. Afc denotes the energy difference between the doped Fermi level and the bottom of the conduction band, which is about 0.1 eV for heavily doped metal oxides. Eref is a constant correlating the reference electrode potential to the vacuum level. AHL is the potential drop across the Helmholtz layer, which is usually small compared with the potential drop across the space charge layer in the semiconductor but may be large if there are surface charges. Since EA, Arc, and Erefare specific to the electrode material and the reference electrode, the observed difference between Empp and EmMsmust be attributed to AHL, the potential drop across the Helmholtz layer. Figure 3 depicts simplified schematic diagrams for the change of the flat-band potential. In the dark (A), the dye molecules are in the reduced form and the flat-band potential is EmMS.After the pulse excitation (B), some of them are positively charged as the result of electron transfer. This surface charge provided by the oxidized form of the dye lowers the band-edge potential and shifts the flat-band potential to EmPP. In general there is an alternative possibility that the surface charge may be stored in surface states of the crystal as the result of hole (positive-charge) transfer from the oxidized state of the dye. This possibility is not ruled out experimentally in the present study; however, it seems reasonable to disregard the surface state in the first step of discussion, since ionic crystals including ZnO is known for the least formation of surface states.13 The capacitance of the Helmholtz layer, CH,calculated in terms of C H = Qo/(Efipp- EmMS)
(4)
(12) Butler, M. A,; Ginley, D. S.J . Electrochem. SOC.1978, 25, 228. (13) Kurtin, S.;McGill, T. C.; Mead, C. A. Phys. Reu. Lett. 1969, 22, 1433.
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J . Phys. Chem. 1992, 96, 2274-2217
is listed in the Table I. The capacitance is exceptionally small for TNPP. The difference is that the dyes except for TNPP dissolve into the electrolyte solution but TNPP does not. The soluble dye adsorbed on the electrode surface is in equilibrium with the dye in the solution and probably forms a monolayer or a comparable thin layer; accordingly, the solvent dominates the Helmholtz layer. The insoluble TNPP layer is probably thicker than the above layers but porous or sparse so that it does not disturb the Mott-Schottky plot; thus, the Helmholtz layer can be dominated by the TNPP molecules. The large difference in the capacitance is qualitatively explained in terms of such a difference in the components of the Helmholtz layer. According to the parallel plate model CH = tto/d where d is the distance between the plates. The dielectric constant
is 36 for a~etonitrile'~ and 4 for phtha1ocyanh1e.l~ If the distance is the same, the capacitance for TNPP should be about 1 order of magnitude smaller than those for the other dyes. Although the observed difference seem to be larger than this calculation, the quantitative disagreement may be tolerable in the above crude model.
Acknowledgment. This work was supported by the Special Coordination Fund of the Science and Technology Agency of the Japanese Government. Professor K. Sigehara of Tokyo University of Agriculture & Technology is gratefully acknowledged for his donating us a sample of TNPP. (14) Abkowitz, M. A.; Lakatos, A. I. J . Chem. Phys. 1972, 57, 5033. (15) Huggins, C. M.; Sharbaugh, A. H. J . Chem. Phys. 1963, 38, 393.
Photoreduction of Methylviologen in Organized Molecular Assemblies: Role of the Surfactant Headgroup Hugh J. D. McManus, Young So0 Kang, and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77204-5641 (Received: September 25, 1991)
The photoreduction yield of methylviologen in the presence of a wide variety of counterions in frozen aqueous solutions was measured using electron spin resonance at 77 K. The signal intensity was also measured in frozen micellar and vesicular suspensions. By precipitating out the chloride counterion with silver perchlorate, it is shown that MV2+ appears to engage in photoinduced electron transfer with the surfactant headgroups.
Introduction
Dimethylviologen (MV2+)and its alkyl-substituted derivatives are widely studied as electron relays in photochemical systems within the area of solar energy conversion research.' Photoreduction of dimethylviologen dichloride proceeds through a charge-transfer process involving the chloride c o u n t e r i ~ n . In ~~ the presence of catalysts, such as colloidal platinum or hydrogenase, this photoreduced viologen cation may be used to produce hydrogen through the oxidation of waterO5 The use of organized molecular assemblies, such as micelles and vesicles, in the study of photoredox systems permits compartmentalization of the charge-transfer products in either the aqueous or the hydrocarbon phases of a microheterogeneous system.@ The photoreduction of dimethylviologen dichloride has been studied in both vesicularI0 and micellar suspensions." Through the addition of an alkyl chain to the viologen, it has been shown that Matsuo, T. Pure Appl. Chem. 1982,54, 1693. Hopkins, A. S.; Ledwith, A,; Stam, M. F. Chem. Commun. 1969,259. McKellar, J. F.; Turner, P. H. Phorochem. Photobiol. 1971,13,439. Ebbesen, T. W.; Ferraudi, G. J . Phys. Chem. 1983, 87, 3717. (5) Kalyanasundaram,K.; Porter, G. Proc. R. Soc. London, Ser A. 1978, 364, 29. (6) Robinson, J. N.; Cole-Hamilton, D. J . Chem. Soc. Rev. 1991, 20,49. (7) Gritzel, M. Microhererogenous Photochemical Electron Transfer; CRC: Boca Raton, FL, 1988. (8) Katz, J. J.; Hindman, J. C. In Photoinduced Conversion and Storage of Solar Energy; Connolly, J. S., Ed.;Academic Press: New York, 1981; p 27. (9) Calvin, M. Phorochem. Phorobiol. 1983, 37, 349. (10) Lukac, S.; Harbour, J . R. J . Am. Chem. Soc. 1983, 105,4248. (1 1) Colaneri, M. J.; Kevan, L.; Schmehl, R. J . Phys. Chem. 1989,93,397. (1) (2) (3) (4)
the differential hydrophilicity of the oxidized and reduced forms of N-tetradecyl-N'-methyl-4,4'-bipyrid~umdichloride (C14MV) results in a sequestering of the photoreduced cation within the hydrocarbon phase of cationic micelles.I2 Further, an increase in photoreduction yield of alkylmethylviologens in vesicular suspensions was observed with increasing hydrocarbon chain 1ength.l3J4 The increased yield was ascribed to the deeper solubilization of the longer chain viologens within the lipid surface of the aggregate, which reduced the back electron transfer through the anionic surface of the dihexadecyl phosphate vesicles. This segregation of the charge-transfer products inhibits back electron transfer, a key step in developing a viable photochemical cell. Since viologens are widely used as a model for photosynthe~ i s , *a ~ complete J ~ understanding of their photoreduction chemistry in microheterogeneous systems is important. Methylviologen undergoes photoinduced electron transfer with a wide variety of reducing anions, with both the radical yield and detailed mechanism of the photoreduction process depending upon the nature of the c ~ u n t e r i o n . ~ ~ JIn* this study, we show that a chargetransfer complex exists between MV2+and the anionic headgroup of sodium dodecyl sulfate. The yield of the photoreduced viologen (12) Brugger, P. A.; Gratzel, M. J . Am. Chem. Soc. 1980, 102, 2461. (13) McManus, H. J . D.; Kevan, L. J . Phys. Chem. 1991, 95, 10172. (14) Colaneri, M. J.; Kevan, L.; Thompson, D. H. P.; Hurst, J. K. J . Phys. Chem. 1987, 91, 4072. (15) Matsuo, T.; Sakamoto, T.; Takuma, K.; Sakura, K.; Ohsako, T. J . Phys. Chem. 1981,85, 1277. (16) Das, P. K. Tetrahedron Lett. 1981, 22, 1307. (17) Barnett, J. R.; Hopkins, A. S.; Ledwith, A. J . Chem. Soc., Perkin Trans. 2 1973, 80. (18) Ebbesen, T. W.; Levey, G.; Patterson, L. K. Nature 1982,298, 545.
0022-3654/92/2096-2274%03.~0~0 0 1992 American Chemical Society
