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J. Phys. Chem. 1989, 93, 6428-6433
ditions that favor dimerization do not simply involve a high surface concentration of 2AS molecules because the dimers persist well below monolayer coverage. The fact that neither 1,5AS nor 1AS dimerize under similar experimental conditions points to subtle orientational effects controlling dimer formation. Adsorption onto colloidal particles is a convenient means of preparing samples suitable for characterization of the spectroscopy and photodynamics of the dimer of an anthracene derivative, although these properties may be modified by the surface environment. Acknowledgment. The research described herein was supported
by the Office of Basic Sciences of the Department of Energy. This is Document No. NDRL-3064 from the Notre Dame Radiation Laboratory. The electron microscopic measurements were performed by W. Archer of the Biology Department at Notre Dame. We thank Dr. R. E. Sassoon for providing us with the sample of R u L ~ ~Dr. - , K. K. Rohatgi-Mukherjee for the samples of 1AS and 1,5AS, and Nalco Chemical Co. for the sample of alumina-coated silica (Nalco 1SJ-612). Registry No. 2AS, 57043-27-3; 1,5AS, 62510-1 1-6; 2AS dimer, 121705-41-7: IAS, 62510-10-5; AI2O3, 1344-28-1; S O 2 , 7631-86-9.
Photochemistry on Surfaces. 4. Influence of Support Material on the Photochemistry of an Adsorbed Dye K. R. Copidas and Prashant V. Kamat* Notre Dame Radiation Laboratory, Notre Dame, Indiana 46556 (Received: December 13, 1988; In Final Form: April 24, 1989)
The absorption and emission properties of rose bengal have been characterized on the surfaces of SO2, AI20,, and Ti02 particles. The intrinsic properties of these oxide supports play an important role in effecting and controlling the photochemistry of the adsorbed dye. On the AI2O3 surface, a biphotonic photoionization process leads to the production of semioxidized dye. However, a charge injection from the excited dye into the conduction band dominates on the Ti02 particles. Steady-state photolysis indicates a rapid photodegradation of the dye only on the surface of TiOz. The diffuse reflectance laser flash photolysis experiments which elucidate the mechanism of photochemical processes on these oxide supports are described.
1. Introduction Photochemical processes in heterogeneous media have gained wide popularity in recent years because of their applications in the conversion and storage of light energy.] The support materials employed in such studies can broadly be classified into two categories: (i) inorganic supports such as Si02 or A1203, which provide an ordered two-dimensional environment for effecting and controlling photochemical processes more efficiently than can be attained in homogeneous solutions; (ii) semiconductor supports such as TiO, or CdS, which directly participate in photochemical reactions either by absorbing the incident photon and transferring charge to an adsorbed molecule or by quenching the excited state of the adsorbed molecule. The mechanistic aspects of photocatalytic processes in the semiconductor particulate systems have been reviewed recently.2 Photophysical studies of pyrene, naphthalene, and several other fluorescent aromatic molecules adsorbed on silica, alumina, and zeolites have been carried out exten~ively.~-~ The fluorescence emission of these adsorbed molecules serves as a probe of the interaction between the support and the photoactive molecule and of the organization and distribution of molecules bound to the surface.I0 The restricted mobility of the excited molecule on an oxide support often increases the lifetimes of the excited states. The effect of pore size on the oxygen quenching of excited aromatic molecules adsorbed on silica has been reported recently." There also have been a number of studies that highlight the radiative and nonradiative processes of organic dyes1*and inorganic complexesI3 on oxide surfaces. In our earlier workI4-I7on the photochemistry of anthracene9-carboxylic acid on colloidal Ti02 and anthracenesulfonates on alumina-coated silica particles, we have demonstrated that the spectral changes give important information about the adsorbentadsorbate interaction as well as about the orientational effects that control the dimer formation. The transparency of the colloidal suspension in these studies facilitated the detection and charac*Address correspondence to this author.
0022-3654/89/2093-6428$01 SO10
terization of photogenerated transients by laser flash photolysis. Static quenching was found to dominate the excited-state processes such as intermolecular triplet-triplet energy transferI4 and electron transferI7 on oxide surfaces. Techniques such as microwave ad( 1 ) See for example: (a) Oelkrug, D.; Flemming, W.; Fiillerman, R.; Giinther, R.; Honnen, W.; Krabichler, G.;Schifer, M.; Uhl, S.Pure Appl. Chem. 1986,58, 1207. (b) Kalyansundaram, K. Photochemistry in Microheterogeneous Systems; Academic Press: New York, 1987. (c) Fox, M. A,, Ed. Organic Tramformations in Nonhomogeneous Media; American Chemical Society: Washington, DC, 1985; ACS Symp. Ser. No. 278. (d) Ramamurthy, V . Tetrahedron 1986, 42, 5753. (2) See for example: (a) Gratzel, M., Ed. Energy Resources through Photochemistry and Catalysis; Academic Press: New York, 1983. (b) Henglein, A. Top. Curr. Chem. 1988, 143, 113. (c) Kamat, P. V.; Dimitrijevic, N. M. Sol. Energy, in press. (3) (a) Hara, K.; De Mayo, P.; Ware, W. R.; Weedon, A. C.; Wong, G. S.K.; Wu, K. C. Chem. Phys. Lett. 1980, 69, 105. (b) Bauer, R. K.; De Mayo, P.; Natarajan, L. V.; Ware, W. R. Can. J . Chem. 1984, 62, 1219. (4) Kessler, R. W.; Wilkinson, F. J . Chem. Soc., Faraday Trans. I 1981, 77, 309. (5) Lcchmiiller, C. H.; Colborn, A. S.; Hunnicutt, M. L.; Harris, J. M. J . Am. Chem. SOC.1984, 106,4077. (6) Avnir, D.; Busse, R.; Ottolenghi, M.; Wellner, E.; Zachariasse, K. A. J . Phys. Chem. 1985, 89, 3521. (7) Francis, C.; Lin, J.; Singer, L. A. Chem. Phys. Lett. 1983, 94, 162. (8) Thomas, J . K.; Beck, G. Chem. Phys. Letr. 1983, 94, 553. (9) Suib, S. L.; Kostapapas, A. J . Am. Chem. SOC.1984, 106, 7705. ( I O ) Thomas, J. K. J . Phys. Chem. 1987, 91, 267, and references cited therein. (11) (a) Wellner, E.; Rojanski, D.; Ottolenghi, M.; Huppert, D.; Avnir, D. J . Am. Chem. SOC.1987, 109, 575. (b) Drake, J. M.; Levitz, P.; Turro, N. J.; Nitsche, K. S.;Cassidy, K. F. J. Phys. Chem. 1988, 92, 4680. (12) (a) Crackel, R. L.; Struve, W. S. Chem. Phys. Lett. 1985, 120, 473. (b) e n i r , D.; Levy, D.; Reisfeld, R. J . Phys. Chem. 1984, 88, 5956. (c) Hashimoto, K.; Hiramoto, M.; Sakata, T. J . Phys. Chem. 1988, 92, 4272. (13) (a) Kajiwara, T.; Hashimoto, K.; Kawai, T.;Sakata, T. J. Phys. Chem. 1982, 86, 4516. (b) Grutsch, P. A,; Kutal, C. J. Chem. SOC.,Chem. Commun. 1982, 893. (c) Furlong, D. N.; Sasse, W. H. F. Colloids Surf. 1983, 7, 29. (14) Kamat, P. V.; Ford, W. E. Chem. Phys. Lett. 1987, 135, 421. (15) Kamat, P. V. J. Phys. Chem. 1989, 93, 859. (16) Ford, W. E.; Kamat. P. V. J . Phys. Chem., preceding paper in this issue. (17) Kamat, P. V.; Ford, W. E. J . Phys. Chem. 1989, 93, 1405.
0 1989 American Chemical Society
Photochemistry on Surfaces sorption,18 photoacoustic s p e c t r o ~ c o p y and ,~~~ electron spin resonance 19b have also been employed to study the photochemistry on oxide supports. Diffuse reflectance laser flash photolysis has been found to be a convenient technique in obtaining spectroscopic and kinetic information of the transients generated upon laser pulse excitation of opaque solid samples in the form of powders and films.20g21 Characterization of benzophenone triplet in microcrystals and on silica surface has been demonstrated by this technique.22 In our preliminary we have shown that diffuse reflectance laser flash photolysis can be used for elucidating the mechanism of charge injection from the excited dye into the conduction band of anatase Ti02particles. We have now examined the excited-state behavior of rose bengal on various oxide supports in order to understand the role of the support material in effecting and controlling the heterogeneous photochemical reaction. Rase bengal is a member of the xanthene class of dyes whose fluorescence and triplet-triplet absorption characteristics in homogeneous solution are readily available.24 Its application in photosensitization reactions has been demonstrated in several heterogeneous media.25
The Journal of Physical Chemistry, Vol. 93, No. 17, 1989 6429 0.5 I
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WAVELENGTH, nm Figure 1. Diffuse reflectance ground-state absorption spectra of rose bengal on (a) Ti02, (b) S O 2 , and (c) on AI2O3. The surface coverage of rose bengal in each case was 0.4 mg/g of support.
SCHEME I: Schematic Diagram of the Diffuse Reflectance Laser Flash Photolysis Setup
2. Experimental Section 2.1. Materials. Purified rose bengal (RB) was obtained from Dye Tech, Inc. Anatase Ti02 powder (surface area 50 m2/g, particle diameter 30 nm) was obtained from Degussa Corp. (P-25). Silica powder (200-425 mesh, 60 A) was obtained from Aldrich. Alumina (neutral, 150 mesh, 58 A) was obtained from Aldrich. Rose bengal was adsorbed onto Si02by dispersing the particles in acetonitrile and adding a known amount of the dye solution. The suspension was stirred for 2 h and filtered. The amount of unadsorbed dye was determined from absorption measurements of the filtrate, which when subtracted from the initial amount of dye gave the amount adsorbed onto silica particles. In the cases of A1203and Ti02, almost all the dye molecules were adsorbed onto the oxide support upon stirring the A1203or TiOz particles in dye solutions for 2 h. Therefore, in these cases, the stirred samples were subjected to vacuum for evaporating off the solvent. The samples were dried in an oven at 65 "C for 24 h. 2.2. Optical Measurements. The diffuse reflectance absorption spectra of rose bengal adsorbed on oxide particles were recorded with a Cary 219 spectrophotometer with a diffuse reflectance attachment (Harrick Scientific). Corrected emission spectra of the solid samples were measured with an SLM photon counting spectrofluorometer in a front-face configuration. Relative emission yields were determined by integrating the emission spectra of different samples with same dye coverage. Time-resolved diffuse reflectance laser flash photolysis experiments were carried out in a vacuum-tight 10 X 10 X 40 mm3 rectangular quartz cell. The dried samples were degassed by subjecting them to vacuum for 3-5 h. The evacuated sample cell containing rose bengal on an oxide support was closed with a vacuum-tight Ace stopcock. Before triggering each laser pulse
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(18) Fessenden, R. W.; Kamat, P. V. Chem. Phys. Lett. 1986, 123, 233. (19) (a) Iwasaki, T.; Oda, S.; Kamada, H.; Honda, K. J. Am. Chem. SOC. 1980, 84, 1061. (b) Turro, N. J.; Waterman, K. C.; Paczkowski, M. A,; Simmt, M. B.; Cheng, C. C. Langmuir 1988, 4, 677. (20) Stone, F. S. In Surface Properties and Catalysis by Non Metals; Bonnelle, J. P., Ed.; Reidel: Dordrecht, 1983; pp 237-272. (21) Wilkinson, J. J. Chem. SOC.,Faraday Trans. 2 1986, 82, 272. (22) (a) Wilkinson, F.; Willsher, C. J. Chem. Phys. Lett. 1984, 104, 272. (b) Ikeda, N.; Imagi, K.; Masahara, H.; Nakashima, N.; Yoshihara, K. Chem. Phys. Letr. 1987, 140, 281. (c) Turro, N. J.; Simmt, M. B.; Gould, I. R. J. Am. Chem. SOC.1985, 107, 5826. (23) Kamat, P. V.;Gopidas, K. R.; Weir, D. Chem. Phys. Lett. 1988, 149, 491. (24) See for example: (a) Fleming, G. R.; Knight, A. W. E.; Morris, J. M.; Morrison, R. J. S.; Robinson, G. W. J. Am. Chem. SOC.1977.99, 4306. (b) Cramer, L. E.; Spears, K. G.J. Am. Chem. SOC.1978, 100, 224. (c) Rodgers, M. A . J. Chem. Phys. Lett. 1981, 78, 509. (d) Kamat, P. V.; Fox, M. A. J. Phys. Chem. 1984, 88, 2297. (25) See for example: (a) Lamberts, J. J. M.; Neckers, D. C. Tetrahedron 1985, 41, 2183. (b) Mau, A. W.-H.; Johansen, 0.;Sasse, W. H. F. Photochem. Photobiol. 1985.41.403. (c) Kamat, P.V.; Fox, M. A. J. Electrochem. SOC.1984, 131, 1032.
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the cell was shaken to expose a fresh surface for excitation. Oxygen- or air-saturated samples were prepared by maintaining an oxygen or air flow over the sample. The 532-nm laser pulse (10 mJ, pulse width 6 ns) from a Quanta Ray DCR-1 Nd:YAG laser system was used for the excitation of the sample. A 1000-W xenon lamp was used as the monitoring source. The diffusely reflected monitoring light from the sample was collected and focused onto a monochromator which was fitted to a photomultiplier tube, and the photomultiplier output was input to a Tektronix 7912A digitizer. The schematic diagram of the diffuse reflectance laser flash photolysis setup is shown in Scheme I, the details of which will be described elsewhere.26 A typical experiment consisted of 1-10 replicate shots per measurement, and the average signal was processed with an LSI-11 microprocessor interfaced to a PDP 11/55 computer.
3. Results 3.1. Absorption and Emission Characteristics. It has been shown previouslyIa that the absorption and emission studies are useful in examining the interaction between an adsorbed dye and support since such an interaction alters the energetics of the electronically excited molecule. The absorption spectra of rose bengal (RB) adsorbed on S O 2 , A1203, and TiOz are shown in Figure 1. The absorption maxima observed in the region of 530-550 nm are similar to the absorption characteristics of the dye in polar solvents. This indicates the existence of a polar surface environment on these particles. Without subjecting the sample to high temperatures (>200 "C), it is not possible to exclude the physisorbed water and the hydroxyl groups from the oxide surface.la (In order to avoid any dye degradation, all the samples in the present study were dried at 65 "C.) The absorption spectra of rose bengal on these particles showed a prominent shoulder (26) Weir, D. To be published
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The Journal of Physical Chemistry, Vol. 93, No. 17, 1989
Gopidas and Kamat A
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Figure 3. Diffuse reflectance ground-state absorption spectra of rose bengal: (A) on Ti02 (0.4 mg/g of TiOJ recorded after different irradiation periods of (a) 0, (b) 5 , (c) 15, (d) 30, and (e) 45 min; (B) on AI2O3(0.4 mg/g of A1203)recorded (a) before irradiation and (b) after 45-min irradiation. Irradiation was with a filtered light ( A > 450 nm) from a Hanovia quartz lamp (140 W).
0
600
550
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Figure 2. Emission spectra of rose bengal on (a) Ti0, (0.4 mg/g), (b) S O l (0.2 mg/g), and (c) A1203 (0.4 mg/g) particles. TABLE I: Excited-State Characteristics of Rose Bengal on Oxide Surfaces SiOl
absorption A,,
nm
emission A,, nm relative emission yield (&)O triplet absorption A,, nm triplet lifetime, ps RB'+ absorption, nm
535, 505 (sh) 562
TiO,
A1203
540, 500 (sh) 570
0.75 1.o 360, 600-750 380, 600-750 12 16 400-470
545, 510 (sh) 582 0.48
b b 400-470
Relative emission yield of RB on A1203is taken as unity. bTripletformation could not be observed on Ti02 particles. a
around 500 nm which is attributed to the ground-state dimer of rose bengal. The assignment of shoulder absorption at 500 nm to the dimer dye was confirmed by recording the absorption spectra at different surface coverage of RB. An increase in the ratio of Ashoulder/Apeak was observed with increasing surface coverage of RB. For example, on A1203particles AM,/Apealr increased from 0.75 to 0.85 when the RB coverage was increased from 0.2 to 2 mg/g of support. Similar increase in the shoulder absorption was also observed in SiO, and TiO, systems. Such a dimerization at submonolayer coverages has also been observed for 2anthracenesulfonate on alumina-coated silica particle^.'^ These absorption characteristics showed the importance of an oxide support in organization and orientation of the adsorbed molecule. The emission spectra of rose bengal adsorbed on Si02, A1203, and T i 0 2 (Figure 2) exhibited maxima at wavelengths 562, 570, and 582 nm, respectively. The relative fluorescence quantum yields on these particles show that the maximum fluorescence of the dye is seen on the A1203surface (Table I) . The fluorescence yield of RB on SiO, was less than that on A1203. This decrease in fluorescence yield can be attributed to the polar surface environment of SiOz since the fluorescence emission of RB is dependent on the polarity of the medium.24 The RB/TiO, sample was least fluorescent, and its fluorescence emission decreased when the sample was continuously excited with monochromatic light in the sample chamber of the spectrometer; but little change occurred during recording of the fluorescence spectrum. The RB/A1203and RB/Si02 samples did not exhibit losses in emission yield upon continuous excitation in the sample chamber of the spectrofluorometer. As shown earlier with phenosafranin/TiO, samples, such a decrease in emission results from the dye degradation on a reactive Ti0, surface.23 3.2. Steady-State Photolysis. An estimate of the photodegradation of rose bengal on oxide surfaces was obtained by recording the absorption spectra of the individual samples before
and after exposing them to the filtered light (A > 450 nm) of a 140-W Hanovia quartz lamp. The diffuse reflectance spectra of air-saturated RB/Ti02 and RB/A1203 samples recorded after various irradiation intervals are shown in Figure 3. Rose bengal adsorbed on T i 0 2 particles underwent a facile degradation upon excitation with visible light (A > 450 nm). Almost complete disappearance of the absorption band at 540 nm was seen when this sample was irradiated for 45 min (Figure 3A). Under similar conditions of exposure to light, RB/A1203 samples exhibited a small amount (
