J. Phys. Chem. 1992, 96, 6834-6839
6834
process can be achieved by coating the ZnO with a layer of ZnSe. Acknowledgment. The work described herein was supported by the Office of the Basic Energy Sciences of the Department of Energy. This is Contribution No. NDRL-3448 from the Notre Dame Radiation Laboratory.
References and Notes (1) (a) DimitrijeviE, N. M.; Kamat, P. V. Sol. Energy 1990, 44, 83. (b) Kamat, P. V. Kinetics and Catalysis in Microheterogeneous Systems. Surfactant Sci. Ser. 1991. 38. 375. (2) (a) Henglein, A. Top. Curr. Chem. 1988,143, 113. (b) Henglein, A. Chem. Rev. 1989,89, 1861. (3) (a) Brus, L. J . Phys. Chem. 198690,2555. (b) Brus, L. Acc. Chem. Res. 1990, 23, 183. (4) Fox, M. A. React. Chem. Intermed. 1991, 15, 153. ( 5 ) Spanhel, L.; Weller, H.; Henglein, A. J. Am. Chem. SOC.1987, 109, 6632. (6) Gopidas, K. R.; Bohorquez, M.; Kamat, P. V. J . Phys. Chem. 1990, 94, 6436. (7) Butkhuzi, T. V.; Georgobiani, A. N.; Zada-Uly, Y.; El’tazarov, B. T.; Khulordava, T. V. In Luminescence of Wideband Semiconductors; Galanin, M. D., Ed.; Nova Science Publishers: New York, 1990; p 167. (8) Kouytate, D.; Ronfard-Haret; J.-C.; Kossanyi, J. J . Lumin. 1991, 50, 205. (9) Travnikov, V. V.; Freiberg, A.; Savikhim, S. F. J . Lumin. 1990, 47, 107. (10) Koch, U.; Fojtik, A.; Weller, H.; Henglein, A. Chem. Phys. Lett. 1985, 122, 507. (11) Haase, M.; Weller, H.; Henglein, A. J. Phys. Chem. 1988, 92, 482. (12) Bahnemann, D. W.; Kormann, C.; Hoffman, M. R. J . Phys. Chem. 1987, 91, 3789. (13) Spanhel, L.; Anderson, M. A. J. Am. Chem. SOC.1991,113, 2826. 114) Rabani. J. J. Phvs. Chem. 1989.93. 7707. (15) Hotchandani, S.i Kamat, P. V. j . Sbc. 1992, 139, 1630. (16) Hotchandani, S.; Kamat, P. V. Chem. Phys. Lett. 1992, 191, 320. (17) Federici, J.; Helman, W. P.; Hug, G. L.; Kane, C.; Patterson, L. K. Comput. Chem. 1985, 9, 171.
(18) (a) Nagarajan, V.; F e n d e n , R. W. J . Phys. Chem. 1985,89,2330. (b) Kamat, P. V.; Ebbescn, T. W.; DimitrijeviE, N. M.; Nozik, A. J. Chem. Phys. Lett. 1989, 157, 384. (19) (a) Brus, L. E. J . Chem. Phys. 1983, 79, 5566. (b) Brus, L. E. J . Chem. Phys. 1984,80,4403. (20) (a) Fojtik, A.; Weller, H.; Koch, U.; Henglein. A. Ber. Bunsen-Ges. Phys. Chem. 1984,88,969. (b) Weller, H.; Schmidt, H. M.; Koch, U.; Fojtik, A.; Baral, A.; Henglein, A.; Kunath, W.; Weiss, K.; Dieman, E. Chem. Phys. Lett. 1986, 124, 557. (21) Chestnoy, N.; Harris, T. D.; Hull, R.; Brus, L. E. J . Phys. Chem. 1986, 90,3393. (22) Kamat, P. V.; Chauvet, J.-P.; Fessenden, R. W. J . Phys. Chem. 1986, 90, 1389. (23) Dunstan, D. E.; Hagfeldt, A.; Almgren, M.; Siegbahn, H. 0. G.; Mukhtar, E. J. Phys. Chem. 1990, 94, 6797. (24) Gopidas, K. R.; Kamat, P. V. Mater. Lett. 1990, 9, 372. (25) Bahnemann, D.; Henglein, A.; Lilie, J.; Spanhel, L. J . Phys. Chem. 1984, 88, 709. (26) Rothenberger, G.; Moser, J.; Grltzel, M.; Serpone, N.; Sharma, D. K. J. Am. Chem. SOC.1985, 107, 8054. (27) Kamat, P. V.; Gopidas, K. R. Picosecond and Femtosecond Spectroscopy from Laboratory to Real World. SPIE 1990, 1209, 1 15. (28) Gratzel, M.; Frank, A. J . Phys. Chem. 1982, 86, 2964. (29) Kamat, P. V. J. Am. Chem. Soc. 1991, 113,9705. (30) (a) Devonshire, R.; W e b , J. J. J. Phys. Chem. 1968,72, 3815. (b) Hug, G. L. NBS Data Ser. 1981, 69, 5 5 . (31) Kamat, P. V.; DimitrijeviE, N. M.; Fessenden, R. W. J. Phys. Chem. 1987, 91, 396. (32) Kamat, P. V. Lungmuir 1985, 1, 608. (33) Kamat, P. V.; Patrick, B. In Proceedings of the 44th IS& T Meeting, Levy, B., Deaton, J., Leubner, I., Slifkin, L., Nuenter, A., Kamat, P. V., Tani, T., Eds.; The Society for Imaging Science and Technology: Springfield, VA, 1991; p 293. (34) Serpone, N.; Borgarello, E.; Grltzel, M. J . Chem. Soc., Chem. Commun. 1983, 342. (35) Kakuta, N.; Park, K. H.; Finlayson, M. F.; Ueno, A.; Bard, A. J.; Campion, A.; Fox, M. A.; Webber, S. E.; White, J. M. J . Phys. Chem. 1985, 89, 732. (36) Vogel, R.; Pohl, K.; Weller, H. Chem. Phys. Lett. 1990, 174, 241.
Charge-Transfer Processes in Coupled Semiconductor Systems. Photochemistry and Photoelectrochemistry of the Colloidal CdS-ZnO System Swat Hotchandani Centre de Recherche en Photobiophysique. UniversitC du Qutbec c i Trois RiviPres. Trois- RiviPres. QuCbec, Canada C9A 5H7
and hashant V. Kamat* Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 (Received: January 23, 1992; In Final Form: April 28, 1992)
Picosecond laser flash photolysis and photoelectrochemical studies have been carried out to elucidate the charge-transfer processes in CdS-ZnO coupled semiconductor systems. Charge injection from excited CdS into ZnO occurs within the laser pulse duration of 18 ps. Long-lived trapped charge carriers demonstrate the improved charge separation in CdS-ZnO coupled semiconductor systems. The feasibility of employing a colloidal CdS-ZnO system in a photoelectrochemical cell has been demonstrated by modifying the surface of an optically transparent electrode with ZnO and CdS colloids. Charge injection from excited CdS into ZnO particle on the electrode surface is confirmed by recording the photocurrent action spectra. An incident photon-to-photocurrent conversion efficiency of 15% has been observed for OTE/ZnO/CdS at 420 nm.
Introduction An interesting approach for achieving efficient charge separation in a semiconductor particulate system involves coupling two semiconductor particles with different energy levels (see, for example, refs 1 and 2). Upon optical excitation, photogenerated electrons accumulate at the lower-lying conduction band of one of the two semiconductors while holes accumulate at the valence band of the other semiconductor particle. This process of charge separation is considered to be very fast. For example, a picosecond laser flash photolysis study of the CdS-TiOz system has shown
that electron injection from excited CdS into TiOz occurs in less than 20 ps.’ Chargetransfer processes in the mixed semiconductor colloids CdSe-ZnS,2 CdS-Ti0z,3” CdS-ZnO,s CdS-AgI,3 Cd3P2-TiO2,6Cd3P2-Zn0,6 AgI-Ag,S,Z and Zn0-ZnS7+8have been studied by absorption and emission measurements. The improved charge separation achieved in these systems has been shown to increase the selectivity and efficiency of the reduction of the adsorbed substrate. The literature contains a few efforts at investigating the electrochemical and photoelectrochemicalproperties of coupled sem-
0022-3654/92/2096-6834$03.00 f 0 0 1992 American Chemical Society
Photoelectrochemistryof CdS-ZnO Coupled Colloids
The Journal of Physical Chemistry, Vol. 96, No. 16, 1992 6835
ZnO O
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i
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0.9 c 0
e
OTE
0
a
0.6
0.06
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- 0.3
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Figure 1. CdS-ZnO coupled semiconductor system: (a) interaction between two colloidal particles showing the principle of the charge injection process and (b) on an electrode surfaoe leading to the generation of photocurrent.
iconductor systems such as CdS-Ti02.+" It was demonstrated in these studies that the photoresponse of a large bandgap semiconductor, Ti02, can be extended into the visible region by means of electrochemical9 or chemicalloJ1deposition of CdS particles. A photon-to-current conversion efficiency of -70% has been obaewed with an electrode moditid with Ti02and CdS particles.'O We have recently succeeded in extending the photoresponse of a ZnO-CdS coupled semiconductor into the red region with chlorophyll a.ll The rectification properties of the coupled semiconductor system improved the sensitized photon-to-photocurrent conversion effciency by an order of magnitude. PbO-AgI systems have also been shown to improve the sensitization properties of photographic emulsions. In view of these interesting aspects of coupled semiconductor systems, we have now investigated the photochemical processes of the colloidal CdS-ZnO system. The schematic diagram describing the principle of the charge injection process in the CdS-ZnO system is shown in Figure 1. Upon optical excitation of CdS (E, = 2.2 eV), the photogenerated electrons are quickly transferred to ZnO while the holes accumulate at the CdS particle. At the interface where the ZnO and CdS particles interact, one can expect formation of a thin junction of Zn,CdI$3. If such a junction exists, it would further improve the process of charge separation by providing the necessary energy gradient for the flow of electrons to ZnO particles. Photovoltage and photocurrent measurements, which directly respond to the excitation wavelength, applied bias, and redox couple at the semiconductor/electrolyteinterface are usually camed out to investigate the photoelectrochemical performance of a semiconductor electrode. On the other hand, chargetransfer procespes in colloidal semiconductor systems are often investigated by fast kinetic spectroscopic techniques such as laser flash photolysis. Both these approaches have been applied in the present study to elucidate the kinetics and mechanistic details of the chargetransfer processes in the CdS-ZnO coupled semiconductor system. The application of colloidal CdS-ZnO films for the photoelectrochemical conversion of light energy into electrical energy is also described.
Experimental Section Preparation of Semiconductor Colloids. A colloidal CdS suspension in acetonitrile was prepared by exposing 1 mM Cd12 (J. T. Baker) in acetonitrile containing 1% water to H$." A colloidal suspension of ZnO (0.02 M) in ethanol was prepared by the method described by Spanhel and Anderson'4 with stoichiometric addition of LiOH ( H u h ) to an organometallic zinc complex solution. The diameters of both CdS and ZnO colloidal particles were in the range of 20-40 A. F'repnration of CoUoidd Semiconductor Films on Optically Transpanst Electrodes. The optically transparent electrodes (OTE, N a t r o n , PPG Industries) were modified with ZnO and ZnO/CdS by the procedure described earlier.llJ5 OTE/ZnO was prepared by coating colloidal ZnO on the conducting surface of a 0.5 X 5 cm2 OTE and then sintering in air at 673 K for 1 h.
0
300
400
500
0 600
Wavelength (nm) Figure 2. Absorption spectra of colloidal suspensions (a and a') 0.2 mM ZnO, (b) 1 mM CdS, and (c) 0.2 mM ZnO and 1 mM CdS in acetonitrile containing l% ethanol. (Spectrum a' is presented on an expanded scale.)
OTE/ZnO/CdS electrodes were prepared by chemically depositing CdS on colloidal ZnO-coated electrodes. OTE/ZnO was successively dipped for a minute in aqueous solutions of 0.1 M Cd(C104)2and 0.1 M Na$. The electrode was washed with water following each treatment. Usually 5-6 such treatments were camed out for the deposition of CdS. SEM studies have confmed the growth of CdS particles (particle diameter 50-100 nm) in the form of clusters on the ZnO surface. Optical Measuremeats. Absorption spectra were recorded with a Perkin-Elmer 3840 diode array spectrophotometer. Emission spectra were recorded with a SLM S-8000spectrofluorimeter. Picosecond laser flash photolysis experiments were performed in a flow cell using a mode-locked 355-nm laser pulse from a Quantel YG-501 DP Nd:YAG laser system (2-3 &/pulse, pulse width 18 ps) as the excitation source. The white continuum was generated by passing the residual fundamental output through a D 2 0 / H 2 0 solution. The time zero (At = 0 ps) in these experiments corresponds to the end of the excitation pulse. All the experiments were performed at room temperature (296 f 1 K). Photoelectrochemical Measurements. Photoelectrochemical measurements were carried out with a standard threeampartment cell consisting of a Pt-gauze (-2 cm2) counter electrode and a saturated calomel electrode (SCE) as reference. Photovoltage and photocurrents were measured with a Keithley Model 617 programmable electrometer. The photoaction spectrum was recorded with a 1000-W xenon lamp and a Bausch and Lomb high-intensity grating monochromator assembly. The light intensity was measured with a Scientech 365 power and energy meter.
-
Results and Discussion Absorption Characteristics. Absorption spectra of CdS and ZnO colloidal suspension are shown in Figure 2. The onset of absorption of CdS colloids which is 5480 nm indicates the bandgap of these colloids to be around 2.4 eV. Similarly, ZnO colloids exhibit onset of absorption at wavelengths of 5340 nm, corresponding to a bandgap of -3.5 eV. The red shift in the absorption onset of coupled Z n M d S colloids indicates a small decrease in the effective bandgap as a result of coupling the two colloids. Although Zn,Cd,-$, formed as a result of interaction between CdS and ZnO, could also influence optical properties, its direct contribution to the increased absorption could not be resolved. Quenching of CdS Emission. CdS colloids in acetonitrile emit in the red region as a result of sulfur vacancy. The emission of CdS colloids provides a convenient means to probe the photoinduced chargetransfer process at the semiconductor surface. For example, the red emission of CdS is quenched as a result of electron transfer to an adsorbed substrate such as methylene blue or methyl viologen or to a Ti02 colloid.' This emission is also quenched when a ZnO colloidal suspension is added to the colloidal CdS suspension. The emission spectra of CdS colloids recorded
6836 The Journal of Physical Chemistry, Vol. 96, No. 16, 1992
Hotchandani and Kamat
1.c
0.110
1
.-ca In c
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-
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0 0
e
a 4 5 - 0 0 4 0
Wavelength (nm)
Wavelength (nm) Figure 3. Quenching of CdS emission by ZnO colloids. The emission spectra of colloidal CdS (1 mM in acetonitrile) were recorded following the addition of ZnO colloidal suspension. The concentration of ZnO colloids were (a) 0, (b) 0.033, (c) 0.087, (d) 0.1, and (e) 0.133 mM. The excitation wavelength was 400 nm.
0.01
t
0 450
I -k
500
550
600
650
700
after the addition of various amounts of colloidal ZnO are shown in Figure 3. We attribute this quenching to the injection of an electron from the CdS colloid into the conduction band of the ZnO colloid
-
+
CdS (e;
+ ht+)
The transient absorption spectra recorded in Figure 4 codirm the charge injection process as described in reaction 2. The prompt appearance of the broad absorption in the red region suggests that electron trapping at the ZnO particle to be completed within the laser pulse duration of 18 ps. Hence, the rate constant for the charge injection process in the C d S Z n O coupled semiconductor system is estimated to be >5 X 1O1OPI. Similar ultrafast charge injection prooes~eshave also been observed in colloidal CdS-Ti02 and dye sensitization1’ processes. It has been shown in the pulse radiolysis18 and laser flash photolysis of CdS that the trapped holes at the surface vacancies cause chemical changes to generate Slurf with a rate can constant of 5 x 108 s-l (reaction 3). The formation of SSd (CdS), + ht+ [(CdS),ICd2+S-surf] (3) easily be monitored by the absorption in the visible region with a maximum around 480 nm. Spectra recorded 1 ns after the 355-nm laser pulse excitation (Figure 5) show absorption changea as a result of SSurf formatiou. In the absence of ZnO colloids (spectrum a), transient absorption (with a maximum around 480 nm) corresponding to SSud is seen. The small increase in the absorption at 480 nm with increasing ZnO concentration (spectra W) suggests increased amounts of hole trapping in CdS-ZnO systems. However, with increasing concentration of ZnO colloids, absorption in the red region (550-700 nm) also increases as trapped electrons at ZnO surface contribute to the absorption in the red region. The evidence for an improved charge separation in the CdSZnO coupled semiconductor systems is demonstrated by moniat 480 nm (Figure 6). In the absence toring the decay of Ssurf of ZnO colloids SSd is short-lived (T = 30 nd9) as it recombines with the trapped electrons. However, in the CdS-ZnO coupled semiconductor system, the trapping of electrons at the ZnO colloid makes such a recombination process less feasible. Indeed, the absorption spectrum c in Figure 6 shows that SWrf is long-lived and does not show any decay during the time scale (8 ns) of the present investigation. Although the possibility of formation of Zn,Cdl-$ at the junction of CdS and ZnO colloids exists in the CdS-ZnO system, it is rather difficult to determine the direct influence of such a junction in the charge injection process. The photophysical properties of ZnxCdl-$ have been described elsewhere.2O Currently, efforts are being made to elucidate the beneficial role of ZnxCdl-$ in achieving better charge separation. The picosecond laser flash photolysis experiments described h a t characterize the primary photochemical events in the CdS-ZnO coupled semiconductor system. The long-lived trapped charge carriers in this system further confirm the role of coupled semiconductor systems in retarding the recombination of trapped charge carriers. As shown earlier,3-sbetter charge separation in the coupled semiconductor system leads to an enhancement in the +
Wavelength (nm) F i i 4. Transient absorption spectra recorded immediately (At = 0 ps) after 355-nm laser pulse excitation of 1 mM CdS colloids in acetonitrile in the presence of (a) 0.025 mM and (b) 0.05 mM ZnO colloids. Spectrum c was recorded following the 355-nm laser pulse excitation of 0.05 mM ZnO colloids alone.
CdS 5 CdS (ecB--.hVB+)
Figure 5. Transient absorption spectra recorded at Ar = 1 ns following the 355-nm laser pulse excitation of 1 mM CdS colloidal suspension in acetonitrile containing various concentration of ZnO colloids: (a) 0, (b) 0.025, (c) 0.05, and (d) 0.075 mM. Spectrum e was recorded with 2 mM ZnO colloids in the absence of CdS colloids under the same experimental conditions as above.
(1)
CdS (ecB- or e;) ZnO 4 CdS + ZnO (e-) (2) where e; and h,+ refer to the trapped electrons and holes, respectively. Nearly 90% of 1 mM CdS emission was quenched by 0.13 mM ZnO colloids. However, Spanhel et ala5have observed the quenching efficiency of CdS emission by ZnO colloids to be poor. The efficient quenching of CdS emission by ZnO colloids in the present study suggests that more CdS particles than one are capable of interacting with a single particle of ZnO semiconductor and of participating in the charge injection process. Trrasient Absorption Studies To Probe the Process of Clnuge Injection. Picosecond laser flash photolysis studies have been shown to be useful in characterizing the photochemical processes that occur in the subnanosecond time domain. The transient absorption spectra recorded immediately after the 355-nm laser pulse excitation of CdS in the presence and in the absence of ZnO colloids are shown in Figure 4. The broad absorption (550-700 nm) observed in the presence of ZnO colloids (spectra a and b in Figure 4) is attributed to the electrons trapped at the ZnO surface. Similar spectral features were also ohserved when ZnO colloids were excited directly with 266-nm laser pulses.16 Blank experiments carried out with colloidal CdS or ZnO alone did not exhibit such transient absorption following 355-nm laser pulse excitation (e.&, see trace c in Figure 4).
The Journal of Physical Chemistry, Vol. 96, No. 16, 1992 6831
Photoelectrochemistryof CdS-ZnO Coupled Colloids
0
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0
2
4
6
8
Time (ns) Figure 6. Formation and decay of S-,& as monitored from the transient absorbance a t 480 nm. The 1 mM CdS colloidal suspension in CH$N containing various concentrations of ZnO colloids was excited with 3554111 laser pulses. The concentrations of ZnO colloids were (a) 0, (b) 0.05, and (c) 0.075 mM.
efficiency of interfacial charge transfer to the adsorbed substrate. Photoektrocbemicrrl Studies of the CdS-ZnO Coupled Semicodudor System. There have been few ~tudies"'J~,*'-~~ which describe the preparation of semiconductor particulate films on electrode surfaces. A recent study by Grgtzel and c o - ~ o r k e r s ~ ~ indicates that TiO, semiconductor films prepared by sintering TiO, particles on a conductive surface retain many of the photophysical proprties otiserved in colloidal semiconductors. Size quantization effects have also been observed in chemically deposited CdS particles on an electrode surface.21 Recent efforts to sensitize large-bandgap semiconductor films with CdS particles have highlighted the usefihess of coupled semiconductors in improving the photon-to-photocurrentconversion efficiency of the photoelectrochemical cell~.~OJ~ In view of the current interest in developing coupled semiconductor films for solar energy conversion, we have investigated the photoelectrochemical properties of CdS-ZnO films. Deposition of CdS Particles 011 ZnO J?ihm. In earlier workl1J5 we d i d the preparation of ZnO films deposited on optically transparent electrodes. CdS particles were chemically grown on the ZnO-modified electrodes by dipping them successively in Cd(C104), and Na2S solutions. This method of CdS deposition is similar to the one employed by Vogel et a1.I0 The absorption spectra of OTE/ZnO electrode which were recorded following each such treatment of CdS deposition are shown in Figure 7. Before the deposition of CdS, the OTE/ZnO electrode exhibits an onset of absorption below 450 nm. However, a red shift in the onset of absorption is observed as the deposition of CdS continues. The increased absorption in the visible region is Seen as a mult of formation of CdS clusters on the ZnO surface. STM studies'l have confirmed the CdS deposit to consist of small particles of diameters 50-100 nm. No significant changes in the absorption were seen when the CdS deposition was continued beyond five treatments. It should be noted that CdS deposition could not be camed out directly on the OTE surface. OTE/ZnO electrodes which were subjected to five such CdS deposition treatments were employed in all the photoelectrochemical experiments. These elcarodes will be referral to as OTE/ZnO/CdS. Photocurrent Action Spectra. The photoelectrochemical response of the CdS-ZnO coupled semiconductor system was evaluated by measuring the photocurrent of OTE/ZnO/CdS at various wavelengths. The incident photon-to-current conversion efficiency (IPCE) was then determined from the expressionlo." i, 1240 loo IPCE (%) = 7 (4) linc
A
Wavelength, nm
Figure 7. Absorption spectra of an OTE/ZnO electrode recorded during the chemical deposition of CdS particles of CdS by treating it with Cd(C10J2 and Na2S solutions. Spectra a-e were recorded successively after each such treatment.
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Wavelength (nm) Figure 8. Photocurrent action spectrum of (a) OTE/ZnO electrode (electrolyte: 0.02 M NaOH) and (b) OTE/ZnO/CdS electrode (electrolyte: 0.1 M Na2S and 1 M KCl). The ordinate scale is given in incident photon-to-current conversion efficiency (IPCE,96) as determined from eq 4.
where i, is the short-circuit photocurrent (A/cm2), Zinc is the incident light intensity (W/cm2), and X is the excitation wavelength (nm). The action spectrum of OTE/ZnO/CdS is compared to that of OTE/ZnO in Figure 8. The OTE/ZnO electrode responds to excitation wavelengths below 400 nm. Since ZnO is a largebandgap semiconductor (E f: 3.0eV), it can be excited with light at shorter wa~elengths.'~ however, coupling the ZnO film with CdS particles has a beneficial effect in extending its photoresponse into the visible region. An IPCE of 15% was observed at the excitation wavelength of 400 nm. The onset of absorption of OTE/ZnO/CdS is around 520 nm, corresponding to the CdS bandgap of 2.2 eV. The dependence of the incident photon-tocurrent conversion efficiency on the excitation wavelength closely matches the ahrption spectrum (Figure 7). This shows that the observed photoelectrochemical effect is initiated by the excitation of the CdS film. Thus, the charge injection process which was evident from the photophysical studies of colloidal CdS-ZnO suspension is now observed as a photoelectrochemicaleffect at an electrode modified with a CdS-ZnO coupled semiconductor particulate film. The generation of anodic photocurrent further confirms that the direction of flow of electrons is from excited CdS into ZnO. hpendence of Photovoltage and Photocurrent 011 the Incident fight Intedty. The photoelectrochemical behavior of the CdSZnO coupled semiconductor system was further evaluated by measuring its short-circuit photocurrent (i,) and open-circuit photovoltage (V,) at various incident light intensities (I.,). The dependence of the photocurrent on the incident light intensity is shown in Figure 9.
6838 The Journal of Physical Chemistry, Vol. 96, No. 16, 1992
Hotchandani and Kamat
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(mW/cm2) Figure 9. Dependence of short-circuit photocurrent (i,) of OTE/ ZnO/CdS electrode on incident light intensity (electrolyte: 0.5 M K4[Fe(CN),], 1@ M K3[Fe(CN),] at pH 12). Excitation was carried out with a halogen lamp (A > 420 nm). The insert shows the plot of In ,i vs In Iinc. nc
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Photovoltage (mV) Figure 11. Power characteristics of a photoelectrochemical cell recorded following the excitation of an OTE/ZnO/CdS electrode immersed in an aqueous solution of 0.5 M K,[Fe(CN),] and 10" M Kp[Fe(CN),] (pH 12). The excitation was carried out with the visible light (A > 420 nm) from a halogen lamp.
(A > 420 nm). The photocurrent and photovoltages which were recorded at various load resistances are presented in Figure 11. The presence of a redox couple such as Fe(CN),'/4- at the CdS/electrolyte interface assured the stability of the photoelecwhere P, trochemical cell. The fill factor (ff = P,J(V,i,), is the maximum electrical power output of the cell), determined from the i-Vcharacteristics in Figure 11 was 0.55. In an ideal situation one would expect the i-V characteristics to follow a rectangular pattern with a fill factor close to unity. However, the fill factor of the OTE/ZnO/CdS-based photoelectrochemical cell is comparable to the values reported for other photoelectrochemical cell^.^^.^^ When the OTE/ZnO/CdS electrode was illuminated with monochromatic light at 460 nm ( i C= 0.7 mW/cm2), the photoelectrochemical cell produced an o p e n h i t voltage of 450 mV and a short-circuit current of 0.06 m4/cm2. The net power conversion efficiency, 7, which is expressed as
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F i p e 10. Dependence of open-circuit photovoltage (V,) of OTE/ ZnO/CdS electrode on incident light intensity (Zinc) (electrolyte: 0.5 M K,[Fe(CN),), 10" M K3[Fe(CN),] a t pH 12). Excitation was carried out with a halogen lamp (A ,i vs V,.
> 420 nm).
7 (%) = ffV,ix/Zinc (6) in the present experiments corresponds to 2.1 %. This value of 7 is slightly lower than the values reported for other semiconductor colloidal-based photoelectrochemical cell^.^^.^^ Further optimization of the electrode design and the operating conditions is currently being attempted to improve the efficiency of OTE/ ZnO/CdS-based photoelectrochemical cells.
-
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The insert shows the plot of In
The logarithmic plot of i, versus Zinc was found to be linear (insert in Figure 9) with a slope of 1.3. This value which is close to unity indicates that photogeneration of charge carriers in the CdS-ZnO system is a monophotonic process. The dependence of photovoltage (V,) on the incident light intensity is shown in Figure 10. For a photoelectrochemical cell operating on the Schottky barrier prin~iple,2~ one can correlate V, and i, by the expression
(5) where k, T, and 4 are respectively the Boltzmann constant, absolute temperature, and electric charge and n and io are respectively the diode quality factor and the reverse saturation current. The dependence of V, on In is (and hence also on In Zi, see insert in Figure 10) confirms the validity of expression 5 in defining the photoelectrochemical parameters of the O"El/ZnO/CdS electrode. Similar behavior has also been observed for electrodes modified with ZnO particulate filmsIS and Schottky barrier-type photovoltaic cell~.*~ The values of n and io obtained from the data in Figure 10 for OTE/ZnO/CdS are 2.4 and 2.7 X lo+' A/cm*, respectively. The i-Y characteristics of a photoelectrochemical cell that employed OTE/ZnO/CdS as a photoanode were also evaluated in the photovoltaic mode following its excitation with visible light
Conclusion Electrode surface modification with thin layers of semiconductor colloids such as ZnO and CdS provides a convenient method to de&n photoelectrochemical cells for the direct conversion of light into electricity. Coupling of colloidal ZnO with CdS colloids not only extends its photoresponse into the visible but also facilitates a better charge separation. Controlling the preparative conditions or surface modification of semiconductor colloids, it should be possible to tailor the properties of the semiconductor electrode. Acknowledgment. We thank Mr. Michael de Lind van Wijngaarden for his assistance in the computer analysis of the data. The research desaibed herein was supported by the Omce of Basic Energy Sciences of the Department of Energy. This is Contribution No.NDRL-3447 from t h e Notre Dame Radiation Laboratory.
References and Notes (1) (a) Kamat, P. V.; Dimitrijevit, N. M. Sol. Energy 1990,44,83. (b) Kamat, P. V. Kinetics and Catalysis in Microheterogeneous Systems. Surfactant Sci. Ser. 1991, 38, 375. (2) Kortan. A. R.;Hull, R.;Opila, R. L.; Bawendi, M. G.; Steigenvald, M.L.; Carroll, P. J.; Brus, L. E.J. Am. Chem. Soc. 1990, 112, 1327. (3) Gopidas, K.R.; Bohorquez, M.; Kamat, P. V. J . Phys. Chem. 1990, 94, 6436. (4) Kamat, P. V.; Gopidas,
K. R. SPIE's Proceedings of the Technical Symposium on Laser Spectroscopy. SPIE 1990, 1209, 1 1 5. (5) Spanhel, L.; Weller, H.; Hcnglein, A. J. Am. Chem. SOC.1987, 109, 6632.
J. Phys. Chem. 1992,96, 6839-6843 (6) Spanhel, L.; Henglein, A.; Weller, H. Ber. Bunsen-Ges. Phys. Chem. 1987,109,6632. (7)Henglein, A.; Gutilrrez, M.;Weller, H.; Fojtik, A.; Jirkovsky, J. Ber. Bunsen-Ges. Phys. Chem. 1989,93,593. (8)Rabani, J. J . Phys. Chem. 1989, 93,7707. (9)Gerischer, H.; Labkc, M. J . Electroanal. Chem. 1986,204, 225. (10) Vogel, R.;Pohl, K.; Weller, H. Chem. Phys. Lett. 1990,174,241. (11) Hotchandani, S.;Kamat, P. V. Chem. Phys. Lett. 1992, 191,320. (12)Levy, B. In Photochemical Conversion and Storage oJSolar Energy; Pelizzetti, E., Schiavello, M., Eds.; Kluwer Academic Publishers: Boston, 1991;p 337. (13)Kamat, P. V.; DimitrijeviE; Fessenden, R. W. J . Phys. Chem. 1989, 92,2324. (14)Spanhel, L.; Anderson, M.A. J . Am. Chem. SOC.1991,113,2826. (15)Hotchandani, S.;Kamat, P. V. J. Electrochem. Soc. 1992,139,1630. (16) Kamat, P. V.; Patrick, B. J . Phys. Chem., preceding paper in this issue. (17)Kamat, P. V.; Das, S.;George Thomas, K.; George, M. V. Chem. Phys. Lett. 1991,178, 75.
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(18) Baral, S.;Fojtik, A.; Weller, H.; Henglein, A. J . Am. Chem. Soc. 1986,108, 375. (19) Kamat, P. V.; Ebbesen, T. W.; DimitrijeviE, N. M.; Nozik, A. J. Chem. Phys. Lett. 1989,157,384. (20) Kaschke, M.;Ernsting, N. P.; Muller, U.; Weller, H.Chem. Phys. Lett. 1990,168,543. (21) Hodes, G.;Albu-Yaraon, A.; Decker, F.; Motisuke, P. Phys. Rev. B 1987,36, 4215. (22)Vlachopoulos. N.; Liska, P.; Auustynski, J.; Gritzel, M. J . Phys. Chem. 1988, 110, 1216. (23)ORegan, B.; Moser, J.; Anderson, M.; Griitzel, M. J . Phys. Chem. 1990, 94,8720. (24)ORegan, B.; Griitzel, M.; Fitzmaurine, C. Chem. Phys. Lett. 1991, 183,89. (25)Mening, R. Top. Curr. Chem. 1988,143,81. (26) (a) Fan, F.-R.; Faulkner, L. J. Chem. Phys. 1978,69,3341. (b) Slgui, S.;Hotchandani, S.;Baddou, D.; Leblanc, R. M. J. Phys. Chem. 1991, 95,8807.
Concentration Dependence of Micellar Size and Composltion in Mixed Anionlc/Cationic Surfactant Solutions Studied by Light Scattering and Pulsed-Gradient FT-NMR Spectroscopy Tadashi Kato,* Hidetomo Takeuchi, and Tsutomu Seimiya Department of Chemistry, Faculty of Science, Tokyo Metropolitan University, Minamiohsawa, Hachioji, Tokyo 192-03, Japan (Received: March 13, 1992)
Light scattering and pulsed-gradient lT-NMR spectroscopy (PGNMR) have been measured for aqueous solutions of sodium bromide (OTAB) and sodium octanesulfonatefOTAB as a function decanesulfonate (Cl,,S03)/octyltrimethylammonium of total surfactant concentration, c, at different mixing ratios (the mole fraction of OTAB in the total mixed solute, x2, is 0.145). From the analysis of self-diffusion coefficients of surfactants obtained by PGNMR, the mole fraction of OTAB is determined. At concentrations much higher than the critical micelle concentration (cmc), xlm in the mixed micelle, xZm, is close to x2. As c is decreased toward the cmc, x2"' is increased toward equimolar composition. These results suggest that micelles grow with decreasing concentration. Light-scattering results are consistent with this prediction. It is also shown that in the CI,,S03/OTABsystem, the mixed micelles grow rapidly when x~~ exceeds about 0.4.
Introduction It is well-known that dectratatic repulsion between like charges of headgroup of ionic surfactants is a major factor which inmases the free energy of micelle formation.' If other surfactants with oppositely charged headgroups are incorporated into the micelles, the micelle aggregation number is therefore expected to increase. So the composition of mixed micelles is important for discussing micellar properties in mixed anionic/cationic surfactant solutions. In our previous s t ~ d i e s ,we ~ *have ~ measured the light-scattering intensities and self-diffusion coefficients of surfactants in dilute solutions (below 0.035 m ~ l d m - ~of) a sodium dodecyl sulfate (SDS)/octyltrimethylammoniumbromide (OTAB) system. In this system, precipitation occurs in the range c = (1-3) X or x2 = 0.35-0.95 where c is the total surfactant concentration and x2 is the mixing ratio expressed by the mole fraction of OTAB in the total mixed solute. So the measurements have been made in one phase region, Le., a mixed micelle region. From the analyses of the self-diffusion coefficients and light-scattering intensities, it has been shown that in the SDS-rich side, as the phase boundary between the mixed micelle region and the precipitation region is approached, i.e., as x2 is increased or c is decreased, the fraction of OTAB in mixed micelles, xZm, increases and micellar growth occurs. It has been also shown that the mixed micelles grow rapidly when x~~exceeds about 0.25. Although intereating phase behaviom of anionic/cationic system have been reportedIc8 there are only a few papers dealing with micellar properties,e12and none of them discusses the dependence of micellar composition on the total surfactant concentration. In the present study, therefore, we extend the above line of approach
to sodium decanesulfonate (Cl$303)/0TAB and sodium octanesulfonate (C8S03)/0TABsystems in order to examine the generality of our previous results. These systems were chosen because precipitation does not occur, so the critical micelle concentration (cmc) can be observed at room temperature. At the same time, the procedure for determining micellar composition is partly modified.
Experimental Section Materials. The sample of OTAB was purchased from Tokyo Kasei Co. Ltd. (>98%) and purified by recrystallization from acetone/diethyl ether mixed solvent. The samples of Cl&03 and C8S03were purchased from Tokyo Kasei Co. Ltd. for ion pair chromatography and were used without further purification. Water triply distilled from alkaline permanganate was used for the light-scatteringmeasurements. For PGNMR measurements, deuterium oxide purchased from Showa Denko Co. Ltd. (99.75%) was used. Light Scattering. Light-scattering intensities were measured by using a H t N e laser (NEC GLC 5601) and a photon-counting system composed of photomultiplier (Hamamatsu R649), homemade discriminator, and universal counter.13 Absolute intensities were calculated using the Rayleigh ratio of benzene at cm-l.14 632.8 nm, 1.184 X PulSed-Grpdkat ET-NMR. PGNMR measurements were made on protons at 99.6 MHz using an internal D 2 0 lock on a JEOL FX-100 Fourier transform NMR spectrometer. The details of the measurements are similar to those already r e ~ 0 r t e d . lThe ~ absolute magnitude of the field gradient was calibrated against
0022-3654/92/2096-6839$03.00/00 1992 American Chemical Society
