8720
J . Phys. Chem. 1990, 94. 8720-8726
Vectorial Electron 1n)ection into Transparent Semiconductor Membranes and Electric Field Effects on the Dynamics of Light-Induced Charge Separation Brian O’Regan,2 Jacques Moser,2 Marc Anderson,’ and Michael Gratzel*?* Institut de Chimie Physique, Ecole Polytechnique F$d?rale de Lausanne. CH- I015 Lausanne. Switzerland, and Water Cheniistrj*and Material Science lnstit Ute, Unicersity of Wisconsin, Madison. Wisconsin 53706 (Rereired: Ma), 7 . 1990)
Transparent titanium dioxide membranes (thickness 2.7 pm) were prepared by sintering of 8-nm colloidal anatase particles on a conducting glass support. The dynamics of charge recombination following electron injection from the excited state of RuL, ( L = 2,2’-bipyridine-4,4’-dicarboxylicacid) into the conduction band of the semiconductor were examined under potcntiostatic control of the electric field w i t h i n the space charge layer of the membrane. Biasing the Fermi level of the TiO, positive of the flat-band potential sharply reduced the recombination rate, a 1000-fold decrease being associated with ;I potcntiul chnngc of only 300 m V . Photoelectrochemical experiments performed with the same RuL,-loaded membrane in Kal-containing watcr show the onset of anodic photocurrcnt to occur in the same potential domain. Forward biasing 01‘ the nicmbranc potcntial impairs photosensitized charge injection turning on the photoluminescence of the adsorbed sensitizer.
Introduction Tlic dynmics of heterogeneous photochemical electron-transfer reactions are frequently controlled by local electrostatic potential gradients present at the This plays a crucial role in molecular devices for light-induced charge separation and solar energy conversion. In the case of semiconductor-liquid junctions. the dcpletion laycr ficld present within the solid impairs the recombination of charge carriers formed by light excitation6 With a convcntional scmiconductor electrodes, these kinetic effects of the space chargc arc difficult to monitor directly and so far few timc-resolvcd studies have been reported.’ In the present work. we take advantage of the transparent nature of a newly developed semiconductor membraneE to examine the influence of the depletion layer field on the rate of charge carrier recombination following photoinduced electron injection from surfxc-adsorbed dye molecules. Thin titanium dioxide membranes ( I ) Water Chemistry and Matcrial Scicncc Institute, University of Wisconsin. Madison. WI 53706. (2) lnstitut de Chimie Physique. Ecole Polytechnique Fidirale de Lausannc. I O 1 5 I.ausannc. Swit7crland. ( 3 ) (a) Gcrischcr. H. Angew. Chem. 1988, 100, 630. (b) Meier. H. J . Phvs. Chem. 1%5,69, 724. (c) Hauffe. K.; Range, J. 2.Nuturforsch. B 1968. 238. 736. (d) Watanabe. T.: Fujishima. A , : Honda. K . In Energy Resources Through Photochemistry and Catal.vsis: Academic Press: New York. 1983. (c) Tributsch. H.:Calvin. M. Photochem. Photobiol. 1971. 14. 95. (0 Karnet. P. V . : Fox. M. A . Chem. Phys. Lett. 1983. 102, 379. (8) Memming. R. Prog. S u r t Sci. 1984. 17. 7. (h) Krishnan. M.: Zhang. X.; Bard, A. J. J . Am. Chem. Sur. 1984. 106. 7371. ( i ) Gerischer. H.; Willig. F. Top. Curr. Chem. 1976. 61. 31. (k) Hashimoto. K.: Sakata. T . J . Phys. Chem. 1986. 90. 4474. (4) Spitlcr. M. J . Electroanul. Chem. 1987. 228,69. For related work, cf.: Blosscy. D. F. P h i Rei,. ~ 1974. 139. 5183. Willig. F. Chem. Phyc. Left. 1976. 40. 331 ( 5 ) ( a ) Photoinduced Electron 7run.rfer: Fox. M. A,. Chanon, M.. Eds.: Elsevier: Amsterdam, 1988; Part A-4. (b) Gratzel. M. Heterogeneous Phut~,[.hc,,,ri[,trlElet,rroti Transfer; CRC Prcss: Boca Raton. FL, 1989. ( 6 ) ( a ) Wrighton. M. S . Arc. Chem. Res. 1979. 12. 303. (b) Gerischer. H. Pirre App1. Chem. 1980. 52. 2649. (c) Heller. A. A r c . Chem. Res. 1981. 1 4 . 1.24.
(7) (a) Bitterling. K.: Willig. F. J . Electroanal. Chem. 1986. 204. 21 I . (b) Rlnn. M A , : Fit7gerald. E. C.: Spiller. M. T. J . Phys. Chem. 1989, 93. 61 50. (8) W e uish to draw aitcntion to studies by Fendler et 31. concerning the formation. chor~icteri7ntion.and photoelectrochemistry of sulfide semiconductor particles supported by bilayer lipid membranes, e.g.: Zhao. X . K.: Baral. S.; Rolandi. R.: Fendler. J . H. J . Am. Chem. Soc. 1988. 110. 1012 Bard et a l . invcstigatcd t h i n CdS semiconductor films, c.g.: Finlayson. M . F.: Wheeler. B. L.: Kakuta. N.: Park. K. H.:Bard, A. J.: Fox. M . A,: Webber. S. E.. White. J . M. J . P h j ~ Chem. . 1985. 89. 5676. Liu. C.: Bard. A . J . J . PhJ.7 Chvnr. 1989. 93. 7749. The TiO? membranes introduced here distinguiah thcinrclvcs by thcir transparcnt and microporous character. High light-harvcrting cfficicncics arc achicvcd in this fashion at monolayer d l c covcragc allowing for iipplication of timeresolved optical transmission spectroscupq
have been prepared on a conducting glass support allowing for potentiostatic control of the potential gradient within the semiconductor. RuL3 ( L = 2.2’-bipyridine-4,4’-dicarboxylicacid) adheres strongly to the surface of TiO19 and is used as a model chromophore. Time-resolved absorption and transient current nicasurcmcnts are applied for the first time in conjunction with laser photolysis to scrutinize the dynamics of charge carrier formation and recombination events in this system. Experimental Section
Prepuration of Transparent TiO, Membranes Supported on Conducting Glass Sheets. Transparent TiO, membranes were produced by deposition of colloidal particles on a conducting glass support. Thc procedure applied was similar to that used for the preparation of unsupported films.I0 TiOz colloid solutions were prepared by hydrolysis of titanium isopropoxide, Ti(OCH(CH,),),, as follows: Under a stream of dry nitrogen, 125 mL of Ti(OCH(CH3)2)4(Aldrich) was added to a 150-mL dropping funnel containing 20 mL of 2-propanol (Fisher, ACS reagent grade). The mixture was added over IO min to 750 mL of distilled deionized water, stirring vigorously. During the hydrolysis a white precipitate formed. Within 10 min of the alkoxide addition, 5.3 mL of 70% nitric acid (Fisher, ACS rcagcnt) was added to the hydrolysis mixture, still stirring vigorously. The mixture was then stirred for 8 h at -80 OC. The 2-propanol (and some water) was allowed to evaporate during this time. Approximately 700 mL of stable TiOzcolloidal sol resulted from this procedure. The size of the colloidal particles was ca. 8 nm and X-ray diffraction analysis showed them to consist of anatase. Crystallization occurred during the refluxing, the initial T O z precipitate being X-ray amorphous. A portion of the above sol was concentrated under vacuum at room temperature until it was visibly viscous. Depending on the iigc of the sol. the proper viscosity was reached between I50 and 200 g of TiOz per liter. Nonporous Sn02films ( F doped) on glass ucrc used for electrically conductive supports (provided by Glasstech Solar, Wheat Ridge CO). Membranes were formed on these supports by spin coating at 3000 rpm. Ti02layers thinner than 0.5 pm did not crack when fired directly in a 400 OC oven. Thicker layers cracked under any firing regime. Membranes up to 1 pm thick were formed by multiple application and firing of ( 9 ) ( a ) Desilvestro. J.; Gratzel. M.; Kavan. L.: Moser. J.; Augustynski, J. J . 4 m Chem. Soc. 1985, 107. 2988. (b) Furlong, D.N.; Wells, D.; Sasse, W . H. F. J . Ph),.r. Chem. 1986, 90. 1107. (c) Vlachopoulos, N.; Liska. P.; Augustynski. J.: Gratzel, M. J . Am. Chem. Soc. 1988. 110, 1216. (IO) Anderson. 21 A.: Gieselmann. M . J.; Xu, Q.J. Membr. Sci. 1988, 39, 243.
0022-3654/90~2094-8720$02.50~0 6 1990 American Chemical Society
Light-I nduccd Charge Separation
Thm e Journal of Ph?*sicalChemistry. Vol. 94. No. 24. 1990 8721
w POTENTIOSTAT
MONO. MONO. .
DIGITAL OSCILLOSCOPE
Figure 2. Experimental setup for time-resolved kinctic spectroscopy using a Ud:YAG laser pulse to excite the dye-derivatized semiconductor
mcmbr;inc.
Figure 1 . Sciinning clcctron micrographs showing crohs sections of thc TiOz nicmbrancs dcpositcd on conducting glass. Mngnificntions: ( a ) 3000 times: (b) 100000 times.
-0.4-pni la!.crs. After ;i final firing a t 400 " C for 1 h. the mciiibrancs \rere hcatcd in argon a t 550 "C in ;i Lindbcrg tube furn:icc under ;in argon flow of 500 niL/min. Thc argon w a s clcancd i r i t l i ;i \rater absorber and two oxygen traps (Alltcch). The iiiorpholog\ of iiicnibriinc clcctrodcs w a s cxaniincd by S E M ( I litachi), X - r i i j diffraction, iind BET analysis of N, ad-
sorption measured by a surface acoustic wave technique. Thickness and porosity have also been measured by profilometry and clipsometry. Figure 1 shows a cross section of the Ti02film obtained by scanning electron microscopy at two different magnifications. Low resolution applied in Figure la confirms the presence of a three-layer structure, the lowest being the glass support followed by the 0.5 pm thick fluorine-doped Sn02 and the 2.7 pni thick TiO, layer. High resolution (Figure I b) reveals thc TiO, film to be composed of a three-dimensional network of interconnected particles having an average size of approximately I6 nm. Apparently, significant particle growth occurs during sintering. Methods. The photocurrent potential characteristics were measured by using a xenon arc light source and a Wenkin potentiostat (Bank Electronic GmbH, F.R.G.). The photocurrent action spectrum was obtained with a Bausch and Lomb 500-nm blaze high-intensity monochromator. The monochromatic photon flux impinging on the cell was determined by a YSI Kettering Model 65 A radiometer. This agreed within 5% with the values measured by fcrrioxalate actinometry. The electrochemical system employed a single-compartment, three-electrode cell, with a platinum counter electrode and a Ag/AgCI or a H g / H g 2 S 0 4 reference electrode in addition to the T i 0 2 surface under investigation. All potentials are reported against SCE. Coating of the TiO, surface with dye was carried out by soaking M, pH the film for 2 h in an aqueous RuL3 solution ( 2 X 4). The R u L 3 was available from previous w ~ r k . ~All~ .other ~ chemicals were at least reagent grade and were used as received. The kinetics of photosensitized electron injection and, recombination were examined by laser photolysis using potentiostatic control of the Fermi level within the transparent TiO, film supported on the conducting glass substrate. A three-electrode cell was employed where the TiO, surface was kept at a 45" angle to the laser beam (frequency-doubled Nd:YAG; 20 mJ output at 530 nm; pulse width at half-height ca. I O ns) and to the analyzing light, Figure 2. The latter was passed through a monochromator prior to entering the cell. Another monochromator was placed in front of the photomultiplier tube used to monitor the time course of the optical absorbance change induced by laser excitation of the film. Transient photocurrents following sensitized electron injection into the TiO, membrane were measured with the same setup as in Figure 2 except that the analyzing light was blocked. The currents were recorded as a voltage drop over a IO-ohm resistor iidded in series to the working electrode circuit by using a digital oscilloscope with a 80-MHz bandwidth. Blank experiments performed with the T i 0 2 membranes in the absence of sensitizer gave no transient current signals. The quantum yield for charge injection from the excited state of R u L , into the conduction band of the T i 0 2 membrane was obtained from the number of 530-nm laser photons absorbed by Ru"L3 and the amount of Ru"'L3 generated during photosensiti7ation. The former was determined from the absorption spectrum
8722 0.6
O'Regan et al.
The Journal of Physical Chemistry, Vol. 94, No. 24, 1990
r
j
7
0.3
sensitizer loaded
0.1
1 I
I
1
I
I
I
400
500
600
WAVELENGTH / nm
Figure 3. Absorption spcctrn of the Ti02 membrane supported on conducting glass with and without monolayer coating of RuL,. At 480 nm, 3770 of the incoming light is absorbed by the RuL,. Spectra are corrected
for specular and diffuse reflectance. of the RuL,-loaded film (Figure 3) while the latter was derived from the bleaching of the RuL, absorbance at 480 nm immediately after thc laser pulsc." Additional experiments were carried out with an electrochemical cell placed in the sample compartment of a S L M 500C spectrofluorometer. The sctup allowcd the excitation beam of the spectrofluorometer to be used to irradiate the Ti02 transparent elcclrodc from the rear without passing through the solution. Thus, the clcctrodc luminesccnce could be examined as a function of potential. Potential control and current measurement were provided by a n IBM EC/225 potentiostat. The area of the electrode exposcd to thc solution was 2 cm2. With a monochromator bandwidth of 20 nm. I .3 cm2 of the sample was illuminated with 470-nm light.
Results Absorption and Emission Characteristics of Dye-Deriuatized Ti02 Films. Figure 3 shows absorption spectra of the Ti02 membrane supported on conducting glass with and without coating of RuL,. These were derived by combining transmittance and reflectance spectra in order to eliminate interference oscillations. The bare film exhibits apart from the band edge transition of Ti02 below 400 nm n feature rising slowly to the red due to free carrier absorption in the fluorine-doped tin oxide.', Soaking the electrode in thc aqucous RuL, solution produces its characteristic band with a maximum in the visible around 470 nm. At 480 nm the absorption due to surracc bound RuL, is 37%. Using for the extinction cocfficient at this wavelength 2.2 X IO4 M-l cm-',98 and for thc surfacc rcquircmcnt of one adsorbed RuL, molecule the expcrimcntally determinedgbvalue of 1 nm2, and assuming complctc monolayer coverage. a roughness factor of 50 is dervied for thc film. A hexagonal closc packing of 16-nm-sized spheres to a layer of 2.7-pm thickness is expected to give a 230-fold surface enlargcnicnt. That the area accessible to RuL, is significantly smaller is not surprising in view of the necking of the particles during the sintcring proccss resulting in films with microporous morphology. The luminescence of surface adsorbed RuL, was found to be strongly affcctcd by the bias voltage applied to the TiO, membranc. At 0.2 V practically no emission could be detected due to oxidative quenching of the excited state by charge injection in the conduction bandga Ru"L3
A
-
Ru11L3*
A,",
Ru"'L,
0
700
+ ecb- (TiOz)
(I)
Holding thc potcntial of thc film at -0.7 V induced the typical ( I 1 ) For the data in Figure 5 the laser flux was kept sufficiently high to excitc a l l thc sensitizer molecules. The recombination kinetics remained essentially t h e same when the laser power was reduced such that only 25%
of the dye molecules were excited. ( 1 2) Shanthi. E.: Banrjee. A.: Chopra. K . L . Thin Solid Films 1982. 88. 93.
I
1
400
500
I
700
600
WAVELENGTH I nm Figure 4. Photocurrent action spectrum of a Ti02 membrane coated with
a monolayer of RuL,. The incident photon to current conversion efficiency is plotted as a function of excitation wavelength. The membrane was immersed in aqueous 0.2 M Nal, pH 3, and a bias voltage of 0.2 V (SCE) was applied.
a
.
I
2
80 -
K K
60
-
8
40
-
a
20
-
I-
w
3
5
l
600
l
l
400
l
l
'
200
l
1
0
l
l
-200
1
1
'
1
'
1
3
-400 -600 -800
POTENTIAL / mV ( SCE )
Figure 5. Oscillograms showing the effect of the electrical potential on
the temporal behavior of the Ru"L, absorption at 480 nm after 530-nm laser flash excitation of R"L,-loaded Ti02 membrane electrodes. The electrode is immersed in an aqueous 0.2 M LiC104 solution, pH 3 . Electrode potentials indicated on the oscillograms are referenced against SCE. In Figure 5d the solution contained apart from the electrolyte 0.2 M N a l as electron donor. luminescence of Ru"L3 with a maximum a t 640 nm, Figure 8. The emission grows in gradually reaching a steady state within a few minutes after applying the polarization. Upon applying a reverse bias one observes within a few minutes complete quenching of the RuL, emission. Steady-State and Time-Resolved Photoelectrochemical Experiments. Electricity is generated with remarkable efficiency when the RuL3-coated transparent Ti0, electrode is immersed in an aqueous solution (pH 3) containing 0.2 M Nal and irradiated by visible light. The incident photon to current conversion efficiency (IPCE) attains 25% at 480 nm, the photocurrent action spectrum matches the light absorption of the film, Figure 4. Expressing the IPCE by the relation IPCE = LHEC#Ji,,qo
(2)
where L H E is the light-harvesting efficiency (0.37 at 480 nm), i#+ isnthe j quantum yield for electron injection, and qesfis the charge separation probability, one obtains C#Jinjqesc = 0.67. Therefore, at the applied bias voltage of 0.2 V, at least 67% of the injected electrons are drawn off as a current, the remainder recombining with parent cations. The kinetics of electron injection and charge recombination were examined by nanosecond laser pulse excitation of the dye-derivatized TiO, film by monitoring the changes of the absorbance at 480 nm in the 10-7-100-s time domain. The temporal behavior of the 480-nm absorbance is shown in Figure 5. The negative deflection of the signal within the laser pulse is due to rapid electron injectionga from the excited state of the RuL, in the conduction band of the Ti02 film, eq 1. The subsequent recovery
The Journal of Physical Chemistry. Vol. 94, No. 24, 1990 8723
Light-Induced Charge Separation
+ 0.15 V
- 0.45 V
6t I I
b)
I
I
I
I
I
c)
l
I
1
IJ" 0
1
2
3
4
0
5
1
2
3
TIME / ps
TIME / ps
+ 0.15 V
+ 0.15 V + I -
4
t
10'
lo6 lo5 lo4
IO
3
5
121
1
2
3
4
.
I-
z Lu
TIME I p TIME / s Figure 6. Effect of the membrane electric potential on (0) the rate
of the signal arises from the recapture of the injected charge by the oxidized sensitizer. Ru"IL3
+ e,
0.9
I 3
5
0.8
sz
0.7
a
0 I-
o
z
0.6 0.5
600
400
200
0
-200
-400
-600 -800
POTENTIAL / mV ( SCE )
Figure 8. Effect of the electric potential on the quantum yield of photosensitized clcctron injection from Ru"L, in the conduction band of the TiO, membrane. The inset shows the luminescence emission spetra of RuL, adsorbed on the surface of the membrane at tNo different potentials. Excitation wavelength 500 nm. The electrode was immersed in 0.2 M LiCIO, electrolyte, p H 3.
us to apply simple time-resolved transmission spectroscopy in conjunction with laser photolysis to unravel the salient kinetic fcatures of heterogeneous photochemical electron-transfer reactions at the semiconductor solution interface. The primary goal of the present study was to scrutinize the role of the electric potential applied to the membrane in controlling interfacial charge-transfer events associated with the photosensitization process. By applying a bias voltage to the conductive glass electrode one can adjust the Fermi level within the semiconductor membrane. The present results show that this has a dramatic influence on the yield and dynamics of light-induced charge separation at the TiO,/solution interface. In the following discussion these observations are interpreted in terms of the effect of the applied potential on the formation of a depletion layer field as well as on the occupation of trapping states for electrons within the membrane. Polarizing the electrode positively with respect to the flat-band potential of TiO, is expected to produce a depletion layer within the film. Under these conditions, the membrane carries excess positive space charge and the conduction and valence bands are bcnt upwards from the semiconductor interior to its surface. An ideal Schottky junction behaves such that in the depletion regime the externally applied voltage drops across the semiconductor, leaving the band-edge position at the surface unchanged. Such a behavior is observed for compact semiconductor electrodes whose thickness exceeds significantly that of the space charge layer. Howcvcr, the Ti02 membranes employed here are microporous, being composed of a densely packed array of colloidal antase particles fused together during sintering. Such a morphology imposes limits on the spatial extension of the depletion layer. For an individual semiconductor particle, the width of the space charge lajcr cannot exceed the particle radius. The maximal voltage difference between the center and the surface of a sphere of radius r is given bqIs
wherc LD= ( ~ , e k T / 2 e ~ N is~ )the ~ . Debye ~ length. With dielectric constant and the ionized donor concentration values of 130 and IO" ~ 1 1 respectively, 7 ~ ~ ~ the Debye length is 30 nm and the voltage difference between the surface and the center of the 16-nm-sized Ti02 particle is 0.3 mV under conditions of maximum depletion. I t is evident that the colloidal particles constituting our membrane arc not isolated from each other. Rather they are in clcctronic contact, forming a three-dimensional array of interconnected clusters. The clustering and interconnection of the particles is expected to affect the potential distribution, and more
Light-Induced Charge Separation significant electric fields are likely to be developed in such an array as compared to individual colloidal particles. Presently, efforts are undertaken to solve numerically the Poisson-Boltzmann equation for semiconductor structures that model the membranes cmploycd hcre. However, it is unlikely that our results can be explained on the basis of the local potential gradients alone. In particular, the small value of the collection efficiency of photoinjected electrons, which is only 4% even at a reverse bias voltage of more than 1 V. cannot be reconciled with the presence of large transmcmbranc potential gradients. I f a strong depletion layer field was developed across the membrane under such a polarization, a significant fraction of the photoinjected electrons would migratc to the back contact and could be drawn off as a current. Very recent experiments by Spitler et al.7b using single-crystal TiOz and multiple reflection evanescent wave optical spectroscopy to monitor the time course of photosensitized electron injection have illustrated this behavior. The results obtained with the present membrane are strikingly different from those obtained with such single-crystal semiconductor electrodes. Duc to the high ratio of dye molecules to the volume of TiOz thc potential scale on Figure 6 must be interpreted with care. The negative shift of the membrane potential due to the photoinjected chargc carriers needs to be considered. It was shown above that this shift increases with the bias voltage compensating the depletion layer field. Therefore. in the depletion regime the membrane potential immediately after the laser excitation is much closer to flat-band conditions than in the equilibrated dark state. From the dye optical density. the membrane thickness (2.7 pm), and porosity (30%). it is apparent that the electron concentration in thc TiOz at the end of the laser pulse is on the order of 1OI9 cnr3. As the bulk donor density appears to be on the order of IO" and LIS very little current flows out of the membrane. in the absence of other electron sinks these electrons will remain in the conduction band. I n such a situation the Fermi level immediately after the liiser pulse will be about 100 mV below the conduction band independent of the depletion bias applied. The particle would be csscntially in a n accumulation mode equivalent to biasing the clcctrode to about -0.5 V. i.e.. within about 100 mV of the conduction band edge. This condition would persist during most of thc rccombination pcriod and the recombination rate should be therefore independent of applied bias for any pre-laser-pulse Fermi lcvcl positive of -0.5 V. However, Figure 6 clearly demonstrates a strong effect on recombination kinetics in this potential domain. Thc results in Figure 6 can be rationalized in terms of a large concentration of electron traps that do not act as bulk donor sites. Interstitial Ti4+,Ti4+sites at grain boundaries, or compensated oxygen vacancies could all provide the requisite traps. The slow back reaction occurs between these trapped electrons and the Ru"'L3 adsorbed at the surface of the TiO?. The trapping sites arc all filled when the membrane is polarized to potentials negative of thc conduction band edge. Therefore, the rate constant of 1.5 X IO6 s-' obtained at -0.8 V is attributed to the recombination of frec conduction band electrons with Ru"'L3 parent ions. Polarizing the electrode positive of the conduction band edge empties traps and leads to a steep decline in the back electron transfer rate which decreases by a factor of ca. 100 upon applying a reverse bias of 0.3 V. Increasing the potential further has little effect on the charge recombination dynamics. I t is worth noting that thc argument of the previous two paragraphs is not dependent on the exact ionizable donor level. Any donor level significantly less than I O i 9 cm-3 will result in the same conclusion. A plausible explanation of this observation is that, as the reverse bi ;IS I S incrcnscd. dcepcr lying traps are emptied. The photoinjected clcctrons trapped on these sites recombine much more slowly than thc ones located in shallow traps since their reaction requires thermal activation or tunneling. The linear decay of the bleaching signal on a logarithmic time scale in Figure 5c is a further indication that clcctron tunneling is indced involved in the recombination process. Similar logarithmic time laws have been observed for many charge-transfer reactions involving tunneling of trapped clcctrons and kinctic models interpreting this behavior have been publishcd.Ih Thc finding that thcrc is no effect of thc applied , ' , '
The Journal of Physical Chemistry, Vol. 94, No. 24. 1990 8725 bias on the back electron transfer rate at potentials more positive than 0.3 V may be attributed to the fact that the density of trapping levels becomes small as the Fermi level approaches the middle of the band gap. I t is noteworthy that the value of the rate constant for the slow decay when the Fermi level is held at -0.52 V. kb = 3 X IOs s-l, is very similar to that obtained from colloidal solutions of RuL3-loadcd IO-nm-sized TiO, particlesqgakb = 4 X IO5 s-!. As mcntioncd earlier, the flat-band potential of these particles was determined to be -0.52 V. This suggests that the charge recombination dynamics observed previously with the colloidal dispersions involved also trapped electrons. I f applying a reverse bias to the TiO, electrode does not generate a significant net electric field across the porous film, electron transport from the particles to the back contact should occur by diffusion rather than migration. In crystalline rutile, the electron mobility is 0.5 cm2 V-' s-' corresponding to a diffusion coefficient of 0.02 cm2 SKI. Using the mean square displacement relation d = (2D1)0.5, the time required for electrons to diffuse from the center of the membrane to the tin dioxide contact would be 0.45 ~ s The . electron movement in our porous membrane is slower since the diffusion of the electronc requires hopping between traps and crossing of grain boundaries, which is expected to reduce greatly the mobility of the charge carriers. The steep rise in the photocurrent in Figure 6 occurs in a potential domain" where the inhibitive effect of the applied potential on the charge recombination approaches its maximum. (It should be noted that the back reaction is intercepted in these experiments by iodide reducing the Ru"'L3 to Ru"L3 and assisting in this way the escape of injected electrons from recombining with their parent ions.) The photocurrent attains a plateau at around 0.1 V corresponding to an incident 480-nm photon to current conversion efficiency of 25%. Since LHE is 0.37 at this wavelength and &j I , one derives from eq 2 a charge separation yield of 67%. This implies that 67% of the electrons injected into the membrane reach the back contact and arc drawn off as a current, the remainder recombining with the Ru"'L3 parent ions. This is in very good agreement with the electron collection efficiency of 65% obtained from the transient photocurrent measurement in Figure 7b under similar conditions. The portion of the recombination which is not intercepted by iodide in the steady-state experiments may be related to the first and fast component of the bleaching recovery in Figure 5b. Since the rapidly decaying fraction amounts to about one-third of the total signal, the agreement with the photocurrent and absorption measurements is practically quantitative. One intriguing feature of the fast recombination process is that its rate is insensitive to the applied potential. Upon changing the polarization of the membrane from -0.8 to 0.6 V, the rate constant for the initial component changes by less than a factor of 2. This contrasts sharply with the behavior of the second and major part of the recombination process whose rate constant decreases by almost a factor of 1000 within the same potential domain. A convenient explanation for this observation would be that not enough trap sites are available to accommodate all the injected electrons. Excess electrons would thus remain in the conduction band and in the absence of a strong electric field would recombine with the dye at a rate similar to that of all the recombination when the membrane is held negative of flat band. This hypothesis appears to be ruled out by the observation that the fraction of the injected charge that is involved in the fast recombination remains relatively constant when the number of injected electrons is varied by a factor of 4. The persistence of a fast rccombination process at positive bias can be rationalized in terms of shallow trapping sites present on or near the surface of the TiO, membrane. I f a number of such
=
(16) (a) Inokuti. M.: Hirayama. F. J . Chem. Phys. 1965. 43. 1978. (b) Tachya, M.; Mozumder, A. Chem. Phys. t e f f . 1974, 28, 87. (c) Milosavljevic, B. H.; Thomas, J . K. J. Phys. Chem. 1985, 89, 1830. ( 17) I t should be noted that the potential for the photocurrent onset as well as the rising edge of the i ( c ) curve may be displaced to more negative potentials at higher light intensities.
8726
The Journal of Physical Chemistry, Vol. 94, No. 24, 1990
surface states exist from which an electron recombines with the dyc with thc w m c ratc as a conduction band electron. the two processes will be indistinguishable in the present experiments. Altcrnativcly. the recombination of conduction band electrons may alwaq3 occur via trapping in such shallow states constituted. e.g.. bq surface Ti4+ ions thnt arc partially coordinatcd by watcr molecules. Such traps arc on or near the surface, possibly near a d j c iiiolcculc. Thus. thcy may be located within a region where no depletion I;iycr fields can be developed under reverse bias. This would exclude the posbibilitj or intercepting the chargc rcconibination rroiii such traps bq the local clcctrostatic potential gradients foriiicd i n the bulk of the membrane under rcvcrsc bias. Regarding the probability of populating such surface states, during the laser pulsc. thc clcctrons may initially occupy traps statistically rather than ncccssarily thc dccpcst traps first. Surface and bulk traps thus w i l l bccomc occupied independent of bias. In addition. an clcctrori i n ;I sliallo~t bulk trap ma} rcturn bricfly to the conduction band bcforc occupying a deeper trap. In the absence of a field this clcctron m a y rccombinc with the dyc. The crfcct of the nicnibranc potential on thc )icld of chargc injection and luminescence displaycd in Figurc 8 dcscrvcs some final coninicnts. The blcaching a t the cnd of the laser pulse can bc cxprwcd b)
whcrc AC is the change in concentration and c the absorption coefficient of the species in the subscript. The absorption coefficient of the dye ground state is 2.2 X IO4. That of the cation. thc dyc cxcitcd stiitc. and thc conduction band electron are a11 bclou 1000. Thus the dccrcnsc in the initial bleaching at potentials negative of -0.2 V indicates that some dye molecules have returned to the ground state via a process with a half-life less than 50 ns. This is much faster than the luminescence decay of the dye in oxygenated solution (-400 ns). The fast decay process could be cithcr ;I nonradiativc dccay of the dye or a new recombination process not present at more positive potentials. (A large increase in lumincsccncc docs not appear until the electrode is biased to -0.7 V . ) Although with the present experiments it is not possible to distinguish between these two possibilities. absorption of Ru(bpy), dyes to oxide surfaces where injection should not be possible has been observed to decrease the luminescence lifetime into the 50-ns time range.I8 In the absence of a new recombination process thc decrease in bleaching negative of -0.2 V is explained by a dccrciise i n the ratc of injection relative to nonradiativc decay. The ratc constant for charge injection is proportional to the cxtcnt of overlap between occupied states of the excited sensitizer rcdox system and empty electronic states in the conduction band +m
k,", =
J-,
I ' ~ * c c ~ u n o c dc E
(8)
whcrc is a frequency factor. The position of the maximum for thc distribution function of the density of occupied states. DKc. is obtained by subtracting half of the value of the reorganization energy A from the excited state redox potential. I n acidic solution. thc redox potcntinl of the Ru"L3/Ru"'L3 couple in the ground statc is 1.2 V (SCE).I9 From the energy difference between lowest cxcitcd and ground state. AE = 1.97 eV. and neglecting entropy cffccts t h x t of the cxcitcd coniplcx is derived as -0.8 V . Since A for tris(bipyridy1) complexes of ruthenium is typically close to 0.5 cV.,O the maximum of occupied states of the excited dye i s prcdicted to bc a t --0.ZZ V . This is some 0.10 cV below the conduction band edge which doc$ not result in optimal overlap. IJ
( 1 8 ) Kajiwra. T : Hnshimoto. K . : Kawai. T.: Sakata. T. J. Ph),c. Chem. 1982, 86, 4516. ( I 9) Dcsilvertro. J.: Duonghong. D.: Kleijn. M.: Gratzel. M. Chimia 1982. 4 , 102. (20) Sutin. N. I n T i r m d i n g in Biological Svs!cmc; Chance. P.. Devault.
D. C.. Schriffer. J . R.. Frauenfelder. H.. Sutin. N.. Eds.: Academic Press: Nc\r York. 1979.
O'Regan et al. The overlap should be sensitive to small shifts in the conduction band edge which could be brought about, for example, by changes in the pH near the Ti02surface. I f this mechanism is important, one would also expect the rate constant for charge injection, and hence to depend on the bulk solution pH. Due to problems w i t h the desorption of Ru"L, from the Ti02 surface, the range of pH accessible is rather small, i.e.. between 3-4.5. Nevertheless, a distinct reduction of ca. 30% in the injection yield is noted upon increasing the pH within these limits bearing out qualitatively the predictions of the kinetic model. Polarizing the membrane negative of the flatband potential (ca. -0.35 V ) induces a further decrease in Cbinj which drops to 0.5 at -0.8 V. This is rationalized in terms of a negative displacement of the Fermi level of Ti02 which moves into the conduction band under forward bias. This decreases both the driving force for electron injection and the density of unoccupied electronic states available for charge transfer, reducing the ratc of interfacial electron injection. The electric potential of the membrane could also affect the nature of linkage between the Ru"L, and the TiO, surface. The preferred adsorption sites for Ru"L3 are likely to bc Ti4+ions having high Lewis acidity. These centers serve as electron traps that would be filled upon polarizing the electrode negatively to produce accumulation layer conditions. This is expected to weaken the binding between the sensitizer and the semiconductor surface, reducing their electronic interaction, and hence the rate of electron transfer. As the dye interaction with the surface decreases, the effect of adsorption on the nonradiative decay rate, postulated above, should decrcase. This expectation is borne out by the appearance of luminescence when the electrode is biased to -0.7 V .
&,.
Conclusions
The development of a novel transparent Ti02 membrane supported on conducting glass has allowed application of time-resolved absorption spectroscopy and amperometry in conjunction with laser photolysis to scrutinize the effect of an applied bias on the photosensitized electron injection and subsequent charge separation process. Subtle alterations of the electrode potential lead to dramatic changes in both interfacial charge-transfer events. Even without a depletion layer field the electron injection proceeds with practically 100% quantum yield suppressing the sensitizer luminescence below the detection limit. The luminescence is turned on by negative polarization where an accumulation layer is produced in the membrane. This intriguing observation is rationalized in terms of the control by the electric potential of the energetics and kinetics of charge injection as well as the effect of the surface charge of the semiconductor on the binding of the sensitizer. The effect of the applied bias on the back electron transfer is most pronounced in the vicinity of the flat-band potential where a potential change of 0.3 V decreases its rate constant by a factor of almost 1000. The time-resolved and steady-state photoelectrochemical experiments confirm that 2/3 of the injected electrons reach the back contact and are drawn off as a current. This is contingent on the presence of iodide as an electron donor. I n the absence of iodide only 4% of the photoinjected charge is collected at the back contact of the membrane. Important insight into the nature of the charge-separation mechanism in the porous membrane consisting of a network of interconnected TiOz particles was derived from this analysis. Particularly, the importance of trapping sites on the dynamics of the charge-recombination process was established. Extension of these time-resolved studies to other electron- and hole-transfer reactions should yield a wealth of information on the role of the local electric field in these interfacial redox events. Acknowledgment. I t is a pleasure to acknowledge financial support of this work from the Swiss National Fund of Scientific Research. We thank Dr. Nazeeruddin for uroviding us with a sample of RuL,. Registry No. Ti02, 13463-67-7; Ru"'L,, 129448-52-8; N a l , 768 I 1
X2-5; Ru"L,, 78338-26-8; LiCIO,. 7791 -03-9.
