4420
J. Phys. Chem. C 2007, 111, 4420-4424
Formation and Behavior of Singlet Molecular Oxygen in TiO2 Photocatalysis Studied by Detection of Near-Infrared Phosphorescence Toshihiro Daimon and Yoshio Nosaka* Department of Chemistry, Nagaoka UniVersity of Technology, Nagaoka 940-2188, Japan ReceiVed: January 2, 2007
Singlet molecular oxygen (1O2), which was produced with a significant yield, was detected from a powdertype TiO2 photocatalyst irradiated with a 355-nm laser pulse by monitoring its near-infrared phosphorescence at 1270 nm. Lifetime measurements for the 1O2 produced at TiO2 (Degussa P25) in various environments, such as in air and in suspensions of H2O, D2O, and ethanol, indicated that quenching takes place mainly at the TiO2 surface in the absence of reactants. Quantum yields for 1O2 generation were measured for ten commercial TiO2 photocatalysts in air and ranged from 0.12 to 0.38, while the lifetimes ranged from 2.0 to 2.5 µs. Since the quenching by the TiO2 surface is quite fast, the formation and decay processes of 1O2 have not been distinguished from the recombination of the photogenerated electron-hole pairs. The values of quantum yield suggest that 1O2 may contribute to the oxidation of some organic molecules at the irradiated TiO2 surface.
Introduction Photocatalysis with titanium dioxide (TiO2) is expected to be utilized as one of the most powerful and popular technologies1-3 for the passive cleaning of building materials by sunlight, based on its special capabilities, such as its high oxidation ability4 and superhydrophilic properties.5 However, in the oxidation process, the photocatalytic reaction may involve the formation of intermediates that are just as objectionable as the original substance. Thus, to understand what kinds of compounds can appear during the decomposition, it is very important to investigate the detailed mechanism of the individual reactions. When organic materials are decomposed in photocatalytic reactions with TiO2, several types of active oxygen species,4 such as superoxide radical anion (•O2-),6-9 hydroxyl radical (•OH),7,10 and hydrogen peroxide (H2O2),11 are reported to be generated in the primary stage. The photocatalytic reaction is initiated by electrons and positive holes, which are photoproduced within the TiO2 bulk, and at the surface they produce these active oxygen species from water and molecular oxygen in air. Even though the quantitative contribution of •OH to photocatalytic reactions is still being discussed,12,13 the production of •O2- has been established and high quantum yields have been reported, i.e., 0.4 in air and 0.8 in aqueous suspension.14 When the photoproduced •O2- is oxidized photocatalytically, it is likely to be converted to singlet molecular oxygen (1O2) at a certain ratio.15 Although 1O2 is one of the possible reactive oxygen species,16-18 only a few reports have shown experimental indications of the formation of 1O2 in TiO2 photocatalysis.19-21 These reports are based on the use of probe reagents, and thus the conclusions remain open to ambiguity.21b Therefore, the involvement of 1O2 in the reaction mechanisms for TiO2 photocatalysis has usually not been mentioned thus far. In several different areas of science, researchers have been intrigued by the physical and chemical properties of 1O2 for more than 70 years, because the O2 molecule possesses a very * Address correspondence to this author. Phone: 0258-47-9315. Fax: 0258-47-9315. E-mail:
[email protected].
unique electronic configuration, which gives rise to a number of important photophysical interactions.22 The reactions of 1O2 are associated with significant applications in several fields, including organic synthesis, bleaching processes, and, most importantly, the photodynamic therapy of cancer, which has now obtained regulatory approval for the treatment of several types of tumors.22 TiO2 photocatalysis has already been shown to be applied for cancer treatment.23 Therefore, when we employ TiO2 photocatalysts, the estimation for the amount of 1O2 and the investigation of its behavior are very important to evaluate further applications in cancer treatment. For other applications of TiO2 photocatalysis, the evaluation of 1O2 is similarly important to clarify environmental effects. The most reliable method for the detection of 1O2 is the direct measurement of its phosphorescence in the near-infrared region.22 In our preliminary research,24 the phosphorescence of the 1O2 generated on TiO2 photocatalysts in dilute aqueous suspension was detected for the first time by a gated photon counting method after excitation with a pulsed laser. The observed emission spectra having a peak at 1270 nm identified the presence of 1O2 by comparing the spectrum observed for a Rose Bengal aqueous solution as an authentic 1O2 generator.24 In the present study, the phosphorescence from photoirradiated TiO2 powders in air was able to be measured owing to the improvement of the experimental setup so as to greatly decrease the amount of scattered light. On the basis of the direct detection of the near-IR phosphorescence of the 1O2, its behavior and contribution to the photocatalytic reaction will be clarified. Experimental Section A widely used commercial TiO2 powder (Degussa P25, Japan Aerosil) was used in most of the present experiments as a photocatalyst in air and in suspensions in various media (H2O, D2O, and ethanol). Various other types of powder-type TiO2 photocatalysts used were a series of samples, TIO-06 to TIO13, which were supplied as reference photocatalysts from the Catalysis Society of Japan. A commercially available sol
10.1021/jp070028y CCC: $37.00 © 2007 American Chemical Society Published on Web 03/01/2007
Singlet Oxygen in TiO2 Photocatalysis
Figure 1. Luminescence spectra observed immediately after the 355 nm laser excitation of TiO2 (P25) in air ([), in suspensions of water (B) and ethanol (2), and for BaSO4 powder in air (solid line) and water (dotted line).
containing the brookite phase as independent crystallites (NTB01, Showa Titanium) was also used after freeze-drying. Suspensions of TiO2 powders (0.03 g/mL) were measured under vigorous stirring with a magnetic stirrer in a 1 × 1 cm2 quartz cell. The same cell was also employed to measure the powders in air. The excitation light source was the third harmonic (355 nm) of a pulsed Nd:YAG laser (5 ns, 10 Hz, Continuum Minilite-II). The beam was passed through a HOYAU350 filter and a set of dielectric multilayer film mirrors to eliminate stray light and was then used to irradiate the front of the cell to excite the TiO2. The beam diameter was 3 mm and irradiation energy was 0.5 mJ/pulse at the cell surface. The photoinduced luminescence emission from the front surface of the sample cell was collected with quartz lenses, passed through a 800-nm sharp-cut filter (HOYA, IR-80), separated by a monochromator, and then introduced into a photomultiplier (Hamamatsu, R5509-41), which was cooled to 200 K with liquid nitrogen. The signals from the photomultiplier were counted with a gated photon counter (Stanford Research, SR445 and SR400). To measure the luminescence spectra, the signals between 1.5 and 6.5 µs (i.e., a delay time of 1.5 µs and a gate width of 5.0 µs) after the laser pulse were accumulated (300 repetitions) by changing the wavelength. On the other hand, to measure the lifetime of 1O2, the signal obtained at a fixed wavelength (1270 nm) was accumulated (5000 repetitions) by changing the delay time from 0.5 to 4.0 µs with a gate width of 0.5 µs. The dark count at each delay time was 20. Results and Discussion Phosphorescence Spectra of 1O2. Figure 1 shows the luminescence spectra observed immediately after the pulsed laser irradiation of TiO2 powders in air, ethanol, and water. The emission observed below 1200 nm may be attributed to Raman scattering or luminescence from the optics, because the same emission was observed when TiO2 powder was replaced by BaSO4 powder. A similar broad luminescence spectrum with a peak at 1270 nm was observed for the 1O2 produced by dye sensitization with Rose Bengal in homogeneous solution.24 As mentioned in the Introduction, the most reliable method for the detection of 1O2 is the measurement of the phosphorescence at 1270 nm,22 which is assigned to the transition from the a1∆g state to the X3Σg- state of molecular oxygen. Thus, the luminescence spectrum characterized by a broad peak in the vicinity of 1270 nm is attributed to the phosphorescence of 1O2 and consequently the formation of 1O2 has been confirmed.
J. Phys. Chem. C, Vol. 111, No. 11, 2007 4421
Figure 2. Time profiles of the 1O2 phosphorescence intensity observed after the laser pulse excitation on TiO2 (P25) powders suspended in various mediums.
When the suspension was deoxygenated by bubbling nitrogen gas, the intensity of the luminescence decreased significantly. This observation evidenced that 1O2 is formed from molecular oxygen in air. The intensity is approximately proportional to the square root of the excitation power, indicating that 1O2 is formed with photocatalytic reaction but not with a two-photon process with laser light. Since the photoinduced electron in TiO2 solid dominantly produces •O2- at the surface,14 as mentioned above, 1O2 might be formed by the oxidation of •O2- with photoinduced holes. As shown in Figure 1, the phosphorescence intensity in ethanol was smaller than that in water. Since 1O2 undergoes no reaction with ethanol and the lifetime of 1O2 in ethanol is longer than that in water,22 the lower phosphorescence intensity of 1O2 indicates that ethanol suppresses the photogeneration process. Since the formation mechanism of the 1O2 is the reduction of O2 to form •O2- and successive oxidation of •O2-, the suppression of the 1O2 formation in ethanol could be explained by the following two possible cases. One is that the photogenerated holes are consumed during the oxidation of alcohol, because it is evidenced that holes oxidize ethanol by laser pulse excitation of TiO225 and that the ethanol radical is formed by the oxidation.26 Another is that O2 is consumed in the process of alcohol oxidation,27 and consequently the production of •O2is decreased. The former mechanism is likely to take place in the present case, because when increasing amounts of ethanol were added to the TiO2 aqueous suspension, a decrease in the yield of 1O2 correlates with an increase in the yield of •O2-. When the surface of the TiO2 photocatalyst was surrounded by air, a higher phosphorescence intensity was observed by comparison with that in aqueous suspensions, as shown in Figure 1. This observation seems to indicate that the amount of 1O2 generated in air is relatively large. It may be difficult, however, to compare precisely the phosphorescence intensities observed in different media, because the refractive index of the medium and the filling factor of the sample powder may be different. On the other hand, the phosphorescence lifetimes could be measured accurately under the different conditions. Therefore, these lifetimes in the different media are mainly compared hereafter. Dependence of Lifetime on Different Media. To examine the effect of the surrounding environment on the 1O2, the phosphorescence decay profiles were measured. Figure 2 shows the time dependence of the phosphorescence intensity after the pulse excitation on P25 TiO2 in air and in suspensions of heavy water (D2O), light water (H2O), and ethanol. By fitting linear lines to the data in Figure 2, the numerical values for the initial
4422 J. Phys. Chem. C, Vol. 111, No. 11, 2007
Daimon and Nosaka
TABLE 1: The Initial Intensity (I) and Lifetime (τ) of the 1O Phosphorescence and the Factor (f) for Quenching by 2 TiO2 (P25) Surface circumstances Air H2O D2O ethanol
I/counts
τ/µs
τo/µsa
fb
965 996 974 499
2.21 2.02 2.46 2.74
5.4 × 104 4.2 68 12
0.65 0.56 0.56 0.46
a
Lifetime of 1O2 in homogeneous madia.22,28,29 eq 1 with a common k of 7.0 × 105 s-1.
b
Calculated from
intensity and the lifetime were obtained and are listed in Table 1. The precision of these values was estimated to be (2%. The initial phosphorescence intensity in ethanol suspension was lower than that in H2O. This observation agrees well with the difference in the spectral intensities shown in Figure 1. In ethanol suspension, the 1O2 generation in TiO2 photocatalysis is decreased as stated above. The initial intensities in D2O and H2O were almost equal to each other, indicating that no isotope effect was observed in the generation process. On the other hand, the relaxation has the strong influence of an isotope effect as stated later. In Table 1, the reported phosphorescence lifetimes (τo) of 1O in homogeneous media28,29 are also listed. Since the lifetime 2 of 1O2 surrounded by air (54 ms)22 is extremely longer than that of 1O2 adsorbed on TiO2 in air, the phosphorescence is quenched significantly by the surface of TiO2. There are three possible pathways to shorten the lifetime of 1O2; electronic-tovibrational (e-V) deactivation, the charge transfer (CT) process, and the electronic energy transfer (EET) process.22 The e-V deactivation process dominates in the lifetime difference for various solvents. Since the •O2- remains on the TiO2 surface for several seconds after the formation,7 electron transfer from •O - to 1O deactivates 1O in the CT process. 2 2 2 The 1O2 formed in TiO2 photocatalysis is probably in the adsorbed state, because the precursor, •O2-, is reported to be adsorbed on the surface30,31 and the formation of 1O2 proceeds via oxidation at the surface. Therefore, the observed lifetime (τ) of 1O2 in photocatalytic systems is likely to be determined by both quenching processes, i.e., that caused by the interaction with the TiO2 surface32 and that caused by the interaction with the solvent. Since the observed lifetimes τ at the TiO2 surface are not in the order of the reported lifetime in homogeneous media τo (Table 1), this observation indicates that the degree of deactivation with TiO2 surface depends on the media of TiO2 particles. Then, the deactivation of 1O2 in different media could be analyzed by taking into account the contribution of the surface into the whole activation processes. On the basis of the above consideration, the observed lifetime τ in different media can be elucidated on the assumption that the 1O2 molecules are deactivated in parallel by the TiO2 surface and the medium with ratios of f and (1 - f), respectively,
1/τ ) f k + (1 - f)/τo
(1)
where f is the factor representing the fraction of the deactivation with TiO2 to the total deactivation observed, and k is an assumed rate constant for the deactivation when the 1O2 molecules are surrounded by TiO2 solid. Since 1O2 is on the TiO2 surface and one side contacts with TiO2, f is expected to be around 0.5. In other words, about half of the 1O2 molecules are deactivated by the energy interaction with the TiO2 surface and the remaining with the surrounding media. When we compare the observed quenching rates (1/τ) in H2O with that in D2O, f in eq 1 would be common, because the
strength of the interaction between 1O2 and the TiO2 surface in H2O would be the same as that in D2O. Thus, the parameters in eq 1 can be obtained from τ and τo in Table 1, so that k is 7.0 × 105 s-1 ()1/(1.43 µs)) and f is 0.56. The observed lifetime of 1O2 in air is shorter than that in water, while that in ethanol is longer. The observed difference in the lifetimes could be expressed by the difference in the factor f in eq 1 as shown in Table 1. Specifically, the contribution of the TiO2 surface to the quenching is large in air and smaller in ethanol compared to that in water. This observation indicates that 1O2 binds more tightly to the TiO2 surface when the catalyst is surrounded by air and more loosely in ethanol. Since the solubility of O2 in ethanol is higher than that in water, higher solvation in ethanol may cause a looser interaction with TiO2. Thus, the lifetime of 1O2 provides precious information of the surface adsorbed states of O2 during the TiO2 photocatalytic reaction. Quantum Yield of 1O2 Generation. To calculate the quantum yield for the 1O2 generation, we previously chose a solution of Rose Bengal as a standard system.24 However, this standard dye solution requires an excitation wavelength of 532 nm,33 while the excitation wavelength of the TiO2 sample is 355 nm. Therefore, in the present experiment, we employed 0.2 mol/L benzophenone in benzene solution, which can be excited at 355 nm to produce 1O2 upon dye sensitization.34 The phosphorescence intensity immediately after the excitation for benzophenone solution was 1158 counts, while that observed for P25 TiO2 powder was 1181 counts. By measuring the diffuse reflection spectrum of the TiO2 powder, 19.7% of light at 355 nm was found to be reflected, and thus 80.3% of the incident radiation should be absorbed. Therefore, the ratio of the absorbed light to the irradiated light was 0.8. On the other hand, the benzophenone solution absorbed all incident light at 355 nm, with negligible reflection. The intensity of the phosphorescence observed from 1O2 on the TiO2 surface contains the reflected light, while the emission from 1O2 in homogeneous solution takes place in all directions, without reflection. Therefore, since TiO2 powder reflects 20% of the emitting light, the phosphorescence intensity observed at the front of the powder sample should be divided by 1.20 to be compared with the phosphorescence intensity in the benzophenone solution. Taking into account these two factors in the calculation, the quantum yield of 1O2 for P25 TiO2 powder was calculated to be 0.31 ()0.29 × 1181/(0.80 × 1.20 × 1158)), based on the quantum yield with benzophenone sensitizer, which was 0.29.34 This value is consistent with the previous rough estimate of 0.2 for dilute suspensions,24 in which Rose Bengal was employed as a sensitizer. Figure 3 shows a schematic illustration of the 1O2 formation in TiO2 photocatalytic systems. Photogenerated electron-hole pairs are considered to recombine in the photocatalyst solid (path 2). However, the quantum yield of •O2- formation (path 3) is reported to be 0.4,14 and almost all •O2- are oxidized (path 4) because of the poor reactivity of •O2-.7 Upon oxidation of the •O -, molecular oxygen converts to one of the three states, 2 X3Σg-, a1∆g, and b1Σg+. Since the b1Σg+ state deactivates very rapidly to the a1∆g state,22 the production ratio of 1O2 (a1∆g) to 3O (X3Σ -) should be 2:3. Thus, when all of the photoproduced 2 g holes are used to oxidize •O2-, without recombination, the maximum quantum yield obtained is 0.4. When all of the 1O2 molecules undergo no reaction and revert back to 3O2 (path 5), the net process is equivalent to the recombination process of photoinduced electron-hole pairs (path 2). This consideration suggests that the formation and decay processes of 1O2 may
Singlet Oxygen in TiO2 Photocatalysis
J. Phys. Chem. C, Vol. 111, No. 11, 2007 4423
Figure 3. Photocatalytic processes of molecular oxygen on the TiO2 surface: (1) excitation accompanied by the formation of valence band holes (h+) and conduction band electrons (e-); (2) possible recombination between electrons and holes; (3) electron transfer to molecular oxygen 3O2 to form the superoxide radical •O2-; (4) oxidation of •O2to partly form singlet molecular oxygen 1O2; and (5) quenching of 1O2 at the TiO2 surface to revert back to triplet molecular oxygen (ads denotes adsorption, and des denotes desorption).
TABLE 2: Properties of TiO2 Powders, and the Generated Phosphorescence Intensity, Quantum Yield, and Lifetime (τ) in Air particle size/nm name of anatase TiO2 content/% anatase othersa P25 TiO-06 TiO-07 TiO-08 TiO-09 TiO-10 TiO-11 TiO-12 TiO-13 NTB-01 a
70.5 0 100 100 100 100 82.4 100 100 ∼25
20.7 15.5 13.2 21.2 13.2 11.4 18.5 28.4 16.9
45.8 (R) 60.5 (R)
60.6 (R) 50.4 (B)
phosphorescence intensity/ quantum lifetime counts yield τ/µs 1181 455 1233 959 1107 846 593 1450 800 639
0.31 0.12 0.32 0.25 0.29 0.22 0.16 0.38 0.21 0.17
2.21 2.46 2.06 2.10 2.27 1.99 2.35 2.00 2.08 2.20
Crystal phases other than anatase: R, rutile; B, brookite.
have been involved in the “recombination” in the conventional mechanism of TiO2 photocatalysis. 1O Generation from Various TiO Powders in Air. We 2 2 measured the phosphorescence for ten different kinds of TiO2 powders to investigate the differences in the lifetime and the quantum yields of 1O2. The intensity immediately after the laser excitation, the lifetime (τ) of the phosphorescence, and the quantum yield for 1O2 generation are listed in Table 2. The properties of the TiO2 powders such as anatase content and particle size are also shown in Table 2. Figure 4 represents the plots of the lifetime and the quantum yield measured for various TiO2 photocatalysts as a function of the mean particle size. The lifetime τ became slightly longer with increasing particle size (Figure 4). Since the τo value for air is extremely large, the τ is determined mainly by the interaction of 1O2 with the TiO2 surface. No specific dependence of the lifetime on the crystal phase was observed. The increase in the interaction that shortens the lifetime may originate from a possible increase in the surface energy, because the surface energy of the particle increases with decreasing diameter and the interaction force between particles is increased with the surface energy.35 The quantum yield of 1O2 depends on the particle size when the size is larger than 20 nm (Figure 4). Since the specific surface area decreases with increasing size, the decrease of the quantum yield may be explained as the decrease in the amount of adsorbed O2. TiO2 photocatalysts smaller than 20 nm showed lower quantum yield owing to the possible recombination. Since the samples with smaller particle size have lower crystallinity,
Figure 4. Quantum yield (]) and the lifetime (B) of the 1O2 generated on various TiO2 photocatalyst powers in air plotted as a function of the mean particle size.
they have many atomic vacancies and interstitial atoms that act as a recombination center. The formation of 1O2 for P25 was very much larger than that expected from the crystal size dependence in Figure 4. The present experimental results suggest that there are some interactions between anatase and rutile particles to prevent the recombination in this particular photocatalyst. Actually, there is a report showing that P25 TiO2 has a higher photocatalytic activity than the single phase of anatase.37 The suppression of recombination by means of efficient interparticle electron transfer38,39 and therefore a high production of •O2-, as a precursor of 1O2, could also be expected. For TIO-11 there may be no interaction between anatase and rutile crystallites. Conclusions The formation of 1O2 in TiO2 photocatalytic systems under various circumstances such as in suspension and in air was investigated by detecting the phosphorescence at 1270 nm in the near-infrared region. The 1O2 is likely to be formed by the oxidation of •O2-, which is generated by the reduction of surface adsorbed oxygen molecules. In ethanol suspension, the amount of 1O2 was lower than that in aqueous suspension, indicating that the oxidizing species of TiO2 are consumed in the oxidation of the alcohol. In D2O, the generation of 1O2 similarly takes place as in H2O, because the chemical properties of D2O are similar to those of H2O. The lifetimes of the 1O2 in various environments were very short (2-3 µs) and depended upon the strength of interaction with the TiO2 solid and upon the media. The short lifetime suggests that the generated 1O2 remains on the TiO2 surface without being released to the air or solution. Thus, the formation process of 1O has not hitherto been distinguishable from the recombination 2 of photogenerated electron hole pairs in TiO2 particles. The quantum yields for 1O2 generation measured for ten TiO2 photocatalysts in air ranged from 0.12 to 0.38 and increased with decreasing particle size down to about 20 nm except for P25 TiO2, which may have an effective charge separation owing to the mixed crystal phases. Since the quantum yield is significantly high, a contribution of 1O2 to the more conventional photocatalytic reactions may arise when the reactants are adsorbed on the TiO2 surface and the reaction rate is faster than the quenching rate. Acknowledgment. This work was supported in part by a Grant-in-Aid on Priority Areas (417) and a 21st COE Program
4424 J. Phys. Chem. C, Vol. 111, No. 11, 2007 (for H.D.) from the Ministry of Education, Culture, Science and Technology (MEXT), and also by Core Research for Evolution Science and Technology (CREST), under the auspices of the Japan Science and Technology Agency (JST). References and Notes (1) Fujishima, A.; Hashimoto, K.; Watanabe, T. TiO2 Photocatalysis: Fundamentals and Applications; BKC Publishing: Tokyo, Japan, 1999. (2) Photocatalysis; Kaneko, M., Ohkura, I., Eds.; Kodansha-Springer: Tokyo, Japan, 2002. (3) Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem. Photobiol. C 2000, 1, 1. (4) Agrios, A. G.; Pichat, P. J. Appl. Electrochem. 2005, 35, 655. (5) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Nature 1997, 388, 431. (6) Cai, R.; Baba, R.; Hashimoto, K.; Kubota, Y.; Fujishima, A. J. Electroanal. Chem. 1993, 360, 237. (7) Hirakawa, T.; Nosaka, Y. Langmuir 2002, 18, 3247. (8) Li, X.; Cubbage, J. W.; Jenks, W. J. Org. Chem. 1999, 64, 8525. (9) Cermenati, L.; Pichat, P.; Guillard, C.; Albini, A. J. Phys. Chem. B 1997, 101, 2650. (10) Fujihira, M.; Satoh, Y.; Osa, T. Nature 1981, 293, 206. (11) Ishibashi, K.; Fujishima, A.; Watanabe, T.; Hashimoto, K. Electrochemistry 2001, 69, 160. (12) Nosaka, Y.; Komori, S.; Yawata, K.; Hirakawa, T.; Nosaka, A. Y. Phys. Chem. Chem. Phys. 2003, 5, 4731. (13) Nakamura, R.; Nakato, Y. J. Am. Chem. Soc. 2004, 126, 1290. (14) Ishibashi, K.; Fujishima, A.; Watanabe, T.; Hashimoto, J. Phys. Chem. B 2000, 104, 4934. (15) Kuan, A. U. Science 1970, 168, 476. (16) Maldotti, A.; Molinari, A.; Amadelli, R. Chem. ReV. 2002, 102, 3811. (17) Kearns, D. R. Chem. ReV. 1971, 71, 395. (18) Brezova, V.; Gabcova, S.; Dvoranova, D.; Stasko, A. J. Photochem. Photobiol. B 2005, 79, 121. (19) Pappas, S. P.; Fischer, R. M. J. Paint Technol. 1974, 46, 65.
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