J . Phys. Chem. 1986,90, 1389-1394
1389
Photoelectrochemistry in Particulate Systems. 4. Photosensitization of a TiO, Semiconductor with a Chlorophyll Analogue Prashant V. Kamat,* Jean-Paul Chauvet? and Richard W. Fessenden Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 (Received: October 25, 1985)
Chlorophyllin, an analogue of chlorophyll a, when adsorbed on colloidal Ti02 can participate in the sensitization process by injecting electrons from its excited states into the conduction band of the semiconductor. Upon excitation in its absorption band, 90% of the fluorescence emission of chlorophyllin could be quenched by colloidal TiO?. The apparent association constant for the association between colloidal Ti02 and chlorophyllin, as measured from the fluorescence quenching data, was 2 X lo4 M-I. Picosecond lifetime measurements gave the rate constant for the electron injection process from the excited singlet state into the conduction band of the semiconductor as 4.2 X lo9 s-I. The net charge transfer across the sensitizer-semiconductor interface was investigated with the laser flash photolysis and time-resolved microwave absorption techniques. Analysis of the transient absorption spectrum confirmed the generation of the cation radical of chlorophyllin with a quantum yield of 0.015.
Introduction Several papers have appeared in the past illustrating the various aspects of the photoelectrochemical sensitization process at single crystal and polycrystalline semiconductor electrodes.',2 The main emphasis so far has been to extend the absorptive range of such electrode materials by attaching organic dyes of high extinction coefficients and to improve the performance of the photoelectrochemical device^.^,^ However the net quantum conversion efficiency of the photoelectrochemical sensitization process in most of these examples has remained below 1%. Chlorophyll pigments, when used as sensitizers, can extend the photoresponse of the large bandgap semiconductors such as T i 0 2 and n-Sn02. Strong absorption in the blue and red regions of the visible spectrum and its redox properties make chlorophyll pigment an ideal candidate for the purpose of photosensitization. The photoelectrochemical behavior of chlorophyll a in a membrane or as an aggregate on a platinum electrode has been reported earlier.5 Light energy conversion with monolayers of chlorophyll a and b deposited on S n 0 2electrodes have exhibited high quantum conversion efficiency (12-16%): We have employed chlorophyllin ( l ) ,a water soluble analogue of chlorophyll a as a sensitizer to CH,=CH \
NaOCCH,CH,
I1 0
CH,
I
C-ONa II 0
C-ONa
8
1
Copper Chlorophyllin , trisodium salt probe the photoelectrochemical sensitization process of a large bandgap semiconductor. The spectral and photochemical properties of chlorophyllin have been reported earlier by Oster et aL7 Total internal reflection8 and coulostatic flash techniquesg have been employed to investigate the sensitization processes occurring at the single crystal electrode surface. Semiconductor particulate systems have been found useful in probing the interfacial processes On leave from Laboratoire des Pigments Vegetaux et Substances Modeles, E.N.S. Saint Cloud, 9221 1 Saint-Cloud, France.
0022-3654/86/2090-1389$01.50/0
at the semiconductor-electrolyte interface. For example, the transparency of the colloidal semiconductor suspension facilitates direct detection of transients generated upon laser pulse excitation with fast kinetic spectroscopy."'-13 Recently, laser flash photol y ~ i s , l resonance ~~'~ Raman spectroscopy,I6 and microwave absorption t e c h n i q ~ e s ' ~have J ~ been demonstrated to be useful in probing electron injection into the conduction band of the semiconductor from the excited state of the sensitizer. In our continuing efforts to investigate the mechanistic aspects of the (1) See,for example: (a) Gerischer, H.; Willig, F. Top. Curr. Chem. 1976, 61, 31. (b) Meier, H. Photochem. Phofobiol. 1972, 16, 219. (c) Watanabe, T.; Fujishima, A.; Honda, K. In Energy Resources through Photochemistry and Catalysis, Gratzel, M., Ed.; Academic Press: New York, 1983. (2) See, for example: (a) Spitler, M.; Calvin, M. J . Chem. Phys. 1977, 67,5193. (b) Takizawa, T.; Watanabe, R.; Honda, K. J . Phys. Chem. 1980, 84, 51. (c) Matsumura, M.; Mitsuda, K.; Yoshizawa, N.; Tsubomura, H. Bull. Chem. Soc. Jpn. 1981, 54, 692. (d) Itoh, K.; Chiyokawa, Y.; Nakao, M.; Honda, K. J . Am. Chem. SOC.1984, 106, 1620. (3) See, for example: (a) Jaeger, C. D.; Fan, F. R. F.; Bard, A. J. J . Am. Chem. Soc. 1980,102,2592. (b) Gosh, P. K.; Spiro, T. G . J . Am. Chem. SOC. 1980, 102, 5543. (c) Morishima, Y.; Isono, M.; Itoh, Y.; Nazakura, S. Chem. Lett. 1981, 1149. (d) Bauldreay, J. M.; Archer, M. D. Electrochim. Acta 1983, 28, 1515. (4) (a) Fox, M. A.; Hohman, J. R.; Kamat, P. V. Can. J . Chem. 1983,61, 888. (b) Kamat, P. V.; Fox, M. A. J . Am. Chem. SOC.1984, 106, 1191. (c) Kamat, P. V.; Fox, M. A. J . Electrochem. Soc. 1984, 131, 1032. (d) Kamat, P. V. J . Elecfroanal.Chem. 1984, 163, 389. (e) Kamat, P. V.; Basheer, R.; Fox, M. A. Macromolecules 1985, 18, 1366. ( 5 ) (a) Tributsch, H.; Calvin, M. Photochem. Phofobiol. 1971, 14, 95. (b) Fong, F. K.; Winograd, N. J . A m . Chem. SOC.1976, 98,2287. (c) Fong, F. K.; Galloway, L. J . Am. Chem. SOC.1976, 100, 3594. (6) (a) Miyasaka, T.; Watanabe, T.; Fujishima, A,; Honda, K. J . Am. Chem. SOC.1978, 100, 6657. (b) Miyasaka, T.; Watanabe, T.; Fujishima, A,; Honda, K. Nature (London) 1979, 277, 638. (7) (a) Oster, G.;Broyde, S. B.; Bellin, J. S. J . Am. Chem. SOC.1964,86, 1309. (b) Oster, G.; Bellin, J. S.; Broyde, S. B. J . Am. Chem. SOC.1964, 86, 1313. (8) Natoli, L. M.; Ryan, M. A,; Spitler, M. T. J . Phys. Chem. 1985, 89, 1448. (9) (a) Kamat, P. V.; Fox, M. A. J . Phys. Chem. 1983, 87, 59. (b) Frippiat, A.; Krish-De Mesmaeker J . Phys. Chem. 1985, 89, 1285. (10) (a) Gratzel, M. Acc. Chem. Res. 1981, 14, 376. (b) Duonghong, D.; Borgarello, E.; Gratzel, M. J . Am. Chem. SOC.1981, 103, 4685. (c) Duonghong, D.; Ramsden, J.; Gratzel, M. J . Am. Chem. SOC.1982, 104, 2977. (1 1) (a) Bahnemann, D.; Henglein, A,; Spankel, L. Faraday Discuss. Chem. SOC.1984,78, 151. (b) Henglein, A. Pure Appl. Chem. 1984,56, 1215. (c) Bahnemann, D.; Henglein, A,; Lillie, J. J . Phys. Chem. 1984, 88, 709. (12) (a) Kamat, P. V. J . Photochem. 1985, 28, 513. (b) Kamat, P. V. J . Chem. SOC.,Faraday Trans. 1 1985, 81, 509. (c) Kamat, P. V . Langmuir 1985, 1, 608. (13) (a) Kiwi, J. Chem. Phys. Lett. 1981,80, 594. (b) Darwent, J. R. J . Chem. SOC.,Faraday Trans. 1, 1984, 80, 183. (c) Brown, G. T.; Darwent, J. R. J . Chem. SOC.,Faraday Trans. I 1984, 80, 1631. (14) (a) Moser, J.; Gratzel, M. J . A m . Chem. SOC.1984, 106, 6557. (b) Moser, J.; Gratzel, M.; Sharma, D. K.; Serpone, N. H e h . Chim. Acta 1985, 68, 1686. (15) Kamat, P. V.; Fox, M. A. Chem. Phys. Letf. 1983, 102, 37). (16) Rossetti, R.; Brus, L. E. J . A m . Chem. SOC.1984, 106, 4336. (17) Warman, J. M.; De Hass, M. P.; Gratzel, M.; Infelta, P. P. Nature (London) 1984, 310, 306. (18) Fessenden, R. W.; Kamat, P. V. Chem. Phys. Letf. 1986, 123, 233.
0 1986 American Chemical Society
1390 The Journal of Physical Chemistry, Vol. 90, No. 7, 1986
Kamat et al.
TABLE I: Excited-State Prowrties of Chlorophyllin in in colloidal T i 0 2 solution' susDensionb abs max, nm 406 408 fluorescence emission max, nm 662 665 fluorescence quantum yieldd 0.001 0.0001 fluorescence lifetime, ps 400 f 20 150 15 triplet abs max, nm 315, 650 C triplet quantum yield 0.012 0.0035 triplet lifetime, ps 8.9 2.8
'
'
5
0
~
-
*
" 5 v/v % ethanol-95 v/v % acetonitrile. b 2 X M colloidal TiOz in 5 v/v % ethanol-95 v/v % acetonitrile. CAbsorption due to cation radical of chlorophyllin interferred with the measurements. Chlorophyll a in methanol (& = 0.25) was used as a standard.32
photoelectrochemical sensitization process, we have now employed picosecond and nanosecond laser flash photolysis, time-resolved microwave absorption, and spectrofluorimetry techniques to study the events of the photosensitization process in colloidal TiOz and chlorophyllin system.
Experimental Section Copper chlorophyllin trisodium salt and acetonitrile (Gold
Label) were obtained from Aldrich. The colloidal TiOz suspension was prepared by the hydrolysis of titanium(1V) 2-propoxide (Alfa) in acetonitrile, as described earlier.'2*15Freshly prepared colloidal M) was diluted with acetonitrile TiOz stock solution (5 X to obtain the desired concentration of TiOz. No attempts were made to exclude the traces of 2-propanol (-0.4%) present in the colloidal semiconductor suspension and it was confirmed separately that the presence of 2-propanol did not affect the photophysical and photochemical measurements. Absorption spectra were recorded with a Cary 219 spectrophotometer. Emission spectra were recorded with a S L M single-photon-counting fluorescence spectrometer in a right angle viewing mode. Flash photolysis experiments were performed with a 355-nm laser pulse (80 mJ, pulse width 6 ns) from a Quanta Ray NdYAG laser system or a 420- or 625-nm laser pulse (10 mJ, pulse width 6 ns) from a Quanta-Ray Nd:YAG dye laser system. The details of the flash photolysis setup is described e1~ewhere.l~The experiments were performed in a rectangular quartz cell with 5-mm pathlength along the monitoring light. A typical experiment consisted of a series of 10-20 replicate shots per single measurement and the average signal was processed with a LSI-1 l microprocessor interfaced with a PDP-11/55 computer. Picosecond lifetimes were measured by exciting the samples with a 355-nm laser pulse (13 mJ, pulse width 30.ps), the third harmonic of a modelocked Nd:YAG laser (Quantel YG402). The emission from the sample was monitored at right angles to the incident beam with a Hamamatsu temporal disperser, a digital TV camera, and a temporal analyzer. Microwave absorption measurements were made with the apparatus described previouslyZowith modifications to improve the time response and to allow the phase of the microwave signal to be determined.21 The sample of TiO, suspension in benzene was contained in a fused silica cell of 3 X 7 mm i. d. The sample was prepared by dispersing TiOz powder (P-25, Degussa: surface area 50 m2/g, average particle size 30 nm) in acetonitrile and adding a known amount of chlorophyllin solution. The solvent was evaporated off and the sensitizer-coated TiOz was redispersed in benzene by sonication. Some settling of the suspension occurred. The sample was shaken to redisperse the particles immediately before the experiment. Relative dose measurements were made by reflecting part of the incident light from a silica plate on t o a pyroelectric sensor. Samples of a chlorophyllin coated Ti02 (19) Das, P. K.;Encinas, M. V.; Small, Jr., R. D.; Scaiano, J. C. J . Am. Chem. SOC.1979, 101, 6965. (20) Fessenden, R. W.; Carton, P. M.; Simamori, H.; Scaiano, J. C . J . Phys. Chem. 1982, 86, 3803. (21) (a) Fessenden, R. W.; Scaiano, J. C . Chem. Phys. Lett. 1985, 117, 103. (b) Fessenden, R. W., Hitachi, A. to be submitted.
Oo0'
460
450
5bO 550 WAVELENGTH (nm)
Figure 1. Absorption spectrum of 2
X M chlorophyllin in 5 v/v % ethanol-95 v/v % acetonitrile at various concentrations of colloidal Ti02: M; (c) 5 X lo4 M; and (d) 2 X M. (Cor(a) 0 M; (b) 5 X responding concentration of colloidal TiOz suspension was used as reference.)
>
k 2 W
I-
z 2
0
In
z
5
,
I
.
.
.
.
I
.
I
.
.
650 700 WAVELENGTH (nm)
0
$
1
750
Figure 2. Fluorescence emission spectrum of lo-' M chlorophyllin in 5 v/v % ethanol-95 v/v % acetonitrile at various concentrations of colloidal M; (e) 2.5 X IO4 M; and (d) Ti02: (a) 0 M; (b) 1 X M. (Excitation wavelength 410 nm.) Insert is an excitation spectrum (corrected for lamp profile) of chlorophyllin emission in 5 v/v % ethanol-95 v/v % acetonitrile representing the response of 622-nm emission to the excitation of Soret band.
particle suspension in benzene were prepared as described earlier.I5 All experiments were done at 22 OC. Results and Discussion Absorption and Emission Characteristics. The absorption and emission characteristics of chlorophyllin are given in Table I and Figures 1 and 2. Upon adsorption on the surface of Ti02 colloid, an increase in the extinction coefficient of both Soret and Q-bands of chlorophyllin was observed. The polar surface of Ti02 resulting from the protonation equilibriaz2
-TiOHz+ F= -TiOH -TiOH
*
-TiO-
+ Hf + H+
pK, = 4.95 pK, = 7.8
(1)
(2)
~
The Journal of Physical Chemistry, Vol. 90, No. 7, 1986 1391
Photoelectrochemistry in Particulate Systems facilitate adsorption of chlorophyllin on T i 0 2 colloid. Similar adsorption of charged sensitizers on colloidal TiO2 and its influence on the absorption and emission characteristics of the sensitizer molecule have been discussed The fluorescence emission spectrum of chlorophyllin exhibited a maximum around 662 nm and the observed fluorescence quantum yield was 0.001. The response of the emission at 622 nm to the excitation of chlorophyllin was confirmed by recording the excitation spectrum in the Soret band. The maximum of the excitation spectrum (-405 nm) matched well with the absorption maximum of chlorophyllin in 5 v/v % ethanol-95 v/v % acetonitrile (Figure 2 ) . The fluorescence yield decreased upon successive addition of colloidal TiO, to a solution of 2.0 X 10" M chlorophyllin. The marked decrease in the fluorescence yield was due to the quenching of the excited singlet state of chlorophyllin by Ti02 colloid. The observed quenching of the excited singlet of chlorophyllin could be attributed to the energy or electron transfer to the semiconductor Ti02 colloid. Similar qualitative aspects of fluorescence quenching have been discussed earlier for Ti02-erythrosin B system.I5 A quantitative approach to the sensitization process will be dealt here. Analysis of the Fluorescence Quenching Data. The quenching of the excited singlet of chlorophyllin is due to the strong adsorption of the sensitizer on TiO, colloidal particles. The consequence of such an association between colloidal TiO, and chlorophyllin on the quenching of the excited singlet of the sensitizer can be explained by considering the equilibrium between adsorbed and unadsorbed molecules of the sensitizer (CPLN) with an apparent association constant Kapp
/
/
/ /
/ /
/ /
9/
I 0.5
l/[Ti02]
I IO
15
,104M-'
Figure 3. Dependence of I/(q5p - r # ~ , , ~ ~on ) the reciprocal concentration of colloidal Ti02 in 5 v/v % ethanol-95 v/v % acetonitrile.
K.w
T i 0 2 + CPLN
e[Ti02-CPLN]
(3)
The observed fluorescence quantum yield (q5f(obsd)) of the sensitizer in the colloidal T i 0 2 suspension can be related to the fluorescence yields of the unadsorbed (+?) and the adsorbed (+/) molecules of the sensitizer by the equation +f(obsd) = (1 - CY)+?+ a&'
(4)
where a is the degree of association between the TiOzcolloid and the sensitizer. Equation 4 can be simplified to the form
+? - +f(ObSd) = a(+? - +f9 At relatively high concentrations of colloidal TiO, ( [TiO,] [SI) one can write
(5)
>>
Kapp[TiO,l a=
(6)
1 + Kapp[TiO,I
Upon substituting the value of a in eq 5, we obtain 1
+? - +f(obsd)
=-
l
+? - +f'
1
+ Kapp(+?
- +[) [Ti021
(7)
If the observed fluorescence quenching is entirely due to the associated complex of colloidal TiOz and the sensitizer, eq 7 demands a linear dependence of 1/(@ - &(obsd)) on the reciprocal concentration of TiO, colloid with an intercept equal to 1/(4p - &') and the slope equal to l / K a p p ( ~-? $0. Indeed, the straight line plot observed in Figure 3 shows this to be true. The values of &' and Kap determined from the double reciprocal plot of Figure 3 were 104and 2000 M-I, respectively. The large value of Kappdetermined from the fluorescence quenching data indicated strong association between chlorophyllin and TiOz colloid which is an essential requisite to observe the heterogeneous chargetransfer process at the semiconductor-sensitizer interface. The net quenching efficiency expressed as ((@ - +/)/&) was found to be 90% for the Ti0,-chlorophyllin system. This value sets the upper limit for the photosensitization of the TiOz colloid with the excited singlet of chlorophyllin. The ability of the excited sensitizer to inject electrons into the conduction band of the ~~
(22) Schindler, P. W.; Gamsjafer, H. Discuss. Faraday SOC.1971.52, 286.
V vs NHE Figure 4. Schematic diagram describing the conduction and valence bands for TiOz and the electron-donating energy levels for chlorophyllin.
semiconductor is determined by the energy difference between the conduction band of the semiconductor and the oxidation potential of the sensitizer in the excited state. According to the model proposed by G e r i ~ c h e and r ~ ~S ~ i t l e r , ~ the probability of an electron transfer (j,) from an excited donor (sensitizer) with an energy level 'ED*/D+ to a conduction band with an energy level E, is proportional to an exponential factor described by the expression
where L* represents the reorganization energy in the excited state and is in the range of 0.5-1.5 eV. The standard oxidation potential of the excited singlet chlorophyllin which is at -0.95 V vs. NHE25 and the conduction band of colloidal TiO, which is around -0.5 V vs. NHE yield a favorable energetics to observe the sensitization process (Figure 4). Picosecond Lifetime Measurements. The fluorescence quenching data clearly highlighted the role of the excited-singlet state of chlorophyllin in injecting charge into the conduction band of the semiconductor T i 0 , (reactions 8 and 9). Competing (23) (a) Gerischer, H. Adu. Electrochem. Eng. 1961, 1 , 139. (b) Gerischer, H.Photochem. Photobiol. 1972, 16, 243. (24) Sonntag, L. P.; Spitler, M. J. J . Phys. Chem. 1985, 89, 1453.
1392
The Journal of Physical Chemistry, Vol. 90, No. 7 , 1986 (Ti02-CPLN (So)) (TiO,-CPLN*
hv
(TiO2-CPLN* (S,))
(S,))% -! CPLN"
+ TiO,
(e)
Kamat et al.
(8)
0
(9)
reactions in the deactivation of the excited singlet of chlorophyllin in the absence of TiO, include radiative decay (kf) and internal conversion (k1J and the intersystem crossing (klK)processes which are described by CPLN* (S,) -%CPLN (So)
+ hv'
-kCPLN (So) CPLN* (SI)5CPLN* (TI) CPLN* (SI)
(10)
(1 1)
(12)
The observed fluorescence lifetime of chlorophyllin in neat solvent would then be given by 7
= 1/(kf + k1c + k,,,)
(13)
However, the observed fluorescence lifetime of chlorophyllin associated with TiO, would be rads = 1 / ( k j + kit' f km,' kct) (14) If we assume that the radiative ( k l ) and nonradiative (klc'and k,scr)decay of CPLN* (SI)associated with TiO, colloid occurs with the same rate as that in neat solvent, one could correlate the observed lifetimes by eq 1/7,dS
=
1/7
+ ket
(15)
or The experimentally observed fluorescence lifetimes of chlorophyllin were 400 ps in 5 v/v % ethanol-95 v/v % acetonitrile and 150 ps in colloidal Ti0, dispersed in 5 v/v % ethanol-95 v / v % acetonitrile. The observed decrease in the singlet lifetime of chlorophyllin when adsorbed on colloidal TiO, parallels the fluorescence quenching experiments described earlier and supports the involvement of the charge-transfer step (keJ in the quenching process. The value of k,, obtained upon the substitution of the values of 7 and 7,& in eq 16 was 4.2 X lo9 s-'. The value of k,, reported by Moser and Gratzel14 for eosin Y-colloidal TiOz in water was 8.5 X lo8 which was determined by making use of the quantum yield of the cation radical generation and the have observed fluorescence lifetime of the sensitizer. Liang et quenching of the excited singlet sensitizer on the surface of tin oxide and indium oxide surfaces. The rate constant for the electron injection process which they measured for rhodamine B coated on indium oxide glass in the absence of any solvent environment was 1.2 X 1Olo SKI. The variation in the environment and the energetics of the excited sensitizer systems is expected to influence the charge injection process at the semiconductor-sensitizer interface. Charge Injection As Studied by Laser Flash Photolysis. It has been shown earlier1@I4that laser flash photolysis could be a convenient technique to investigate the heterogeneous electron-transfer processes in colloidal semiconductor systems. If the observed fluorescence quenching of chlorophyllin results in an electron injection into the conduction band of the semiconductor (reaction 9) one would expect to see the production of cation radical of chlorophyllin. The results of the flash photolysis experiments carried out with the excitation of chlorophyllin are described below. The transient absorption spectrum recorded immediately after the laser pulse excitation of chlorophyllin in 5 v/v % ethanol and 95 v/v % acetonitrile is shown in Figure 5a. The observed transient which exhibited a complete decay with a lifetime of 8.9 bs (insert in Figure 5a) was attributed to the triplet chlorophyllin. (25) Kamat, P. V., to be submitted. ( 2 6 ) (a) Liang, Y . ;Gonsalves, A. M.; Negus, D. K. J . Phys. Chem. 1983, 87, 1. (b) Liang, Y . ;Moy, P. F.; Poole, J. A,; Ponte Gonsalves, A. M. J . Phys. Chem. 1984, 88, 245 1.
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