J . Phys. Chem. 1993, 97, 13284-13288
13284
Photoinduced Hole Transfer from Ti02 to Methanol Molecules in Aqueous Solution Studied by Electron Paramagnetic Resonancef Olga I. Micic,* Yuenian Zhang, Keith R. Cromack, Alexander D. Trifunac, and Marion C. Thurnauer. Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439 Received: June 29, 1993; In Final Form: September 30, 1993'
An electron paramagnetic resonance (EPR) study is reported on the paramagnetic species formed on irradiation of aqueous Ti02 colloids in the presence of methanol. In laser (308 nm) irradiated aqueous solution of methanol at 1.9 K, the observed EPR signals are centered around g = 2 and have a line shape characteristic of the CHIOH radical. The appearance of these radicals at the very low temperature of 1.9 K implicates charge transfer from the oxygen lattice holes to methanol molecules within a few monolayers of the surface of the Ti02 particles. Different radicals are formed when CH3OH is chemisorbed on Ti02. The EPR signals detected in Ti02 colloidal solution prepared in methanol and then evaporated and dissolved in water also show the formation of the CH20H radical due to oxidation of chemisorbed methanol on the Ti02 surface. These particles, with high laser pulse intensities, oxidize the chemisorbed methanol further to give the C H O radical. In addition, the CH3 radical is formed from reducing processes and observed at 6-50 K. In the absence of methanol, holes are trapped by surface hydroxide groups forming TIIV-O-TilV-O'. At higher temperature (1 50 K) in aqueous methanol solution, the C H 2 0 H radical transfers an electron to TiO2, doubling the yield of formation of surface Ti"' ions which are stable in air-free solution. When an electron scavenger such as Hg2+ is present in solution, the EPR signal of Til1' disappears since mercury ions are reduced.
Introduction
Titanium dioxide is of continuing interest because of its photoactivity in the oxidation of a wide variety of organic and inorganic substrate^.^-^ Band-gap excitation of Ti02 leads to the generation of electron-hole pairs. Transient optical absorption spectra of several intermediates formed by reactions of electrons and holes on the Ti02 surface have been reported.612 The EPR characteristics of the photogeneratedparamagnetic intermediates have also been reported.I3-2l UV irradiation of Ti02 colloids in aqueous solutions results in EPR signals for the trapped hole with characteristics that differ from that of OH' radicals and correspond to a surface oxygen anion radical intermediate covalently bound to titanium (TilV-O-TilV-O*) at pH 3-14.22 In this paper we present an EPR study whose purpose was to detect the intermediates in the photoinitiated oxidation of methanol in Ti02colloidal solutions. It was found that in aqueous solutions of Ti02 colloids covered with OH groups the presence of methanol initiates efficient hole transfer from the oxygen lattice to the methanol. Comparison is made with Ti02 colloids where methanol was irreversibly chemisorbed (TilV-O-TilV-OCHs). The methoxide groups bound on the surface trap photogenerated holes and electrons. Experimental Procedures
All chemicals and solvents used were reagent grade (Aldrich or Baker) and were used without further purification. The solvents methanol (Mallinckrodt, ChromAR), CD3OD (Sigma, 99.5% D), and ethanol (Aldrich) were used. Triply distilled water was used. The solution pH was adjusted with HC1, NaOH, or phosphate buffer. The pH was estimated with indicator paper since the colloids block the pores of glass electrodes. Oxygen was removed by bubbling with nitrogen or by vacuum degassing.UVvis absorption spectra were recorded on a Shimadzu MPS-2000 instrument. Concentrations of Ti02 colloids were determined by t Work at Argonne performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Science, US-DOE under Contrast W-3 1-109-ENG-38. f On leave from Institute Vinca, Belgrade, Yugoslavia. Maria GoeppertMayor Scholar. Abstract published in Aduance ACS Absfracrs, November 15, 1993.
0022-365419312097-13284SO4.OOlO
dissolving colloids in concentrated H2SO4 and diluting with H20, followed by addition of H202 to form the orange colored peroxide complex.23 Particle sizes were determined by transmission electron microscopy (Philips EM420) using 50-Acarbon-coated 400-mesh copper grids. Electron diffraction from Ti02 particles showed well-defined patterns of anatase titanium dioxide. Alkoxide Ti02 Colloids. Ti02 colloids with chemisorbed methanol were prepared by first hydrolyzing titanium isopropoxide in methanol in the presence of small amounts of OH- ions. The methanol solvent was evaporated and the residue dissolved in water. Ti(OCH(CH3)z)d(1.1 mL) was slowly (1 drop/s) stirred into 200 mL of absolute methanol or ethanol which contained LiOH (1.5 X M) and 0.5 mL of water. A transparent solid film was formed by evaporating 10 mL of this solution using nitrogen bubbling. Immediatelyafter formationof the film, water was added, yielding colloids with an average diameter of 60 A and anatase crystal structure. Freshly prepared aqueous colloidal solutions were used in each experiment. In these preparations, methanol is irreversibly bound to Ti02 surface sites. Similar behavior was found previously for other Hydrous Ti02 Colloids. Ti02 colloids in water were obtained by adding 4 mL of TiC14, cooled to -20 "C,dropwise to 200 mL of cold water at 0 "C and then immediatelydialyzingthis solution against cold water at 0 "C to the final pH 3. The final solution contained 0.2 M Ti02 with an average particle diameter of 60 A. This solution is stable for weeks in a refrigerator. For a solution at high pH, NaOH is rapidly added to reach pH 7 and the resulting gelatinousmass filtered and washed thoroughly with distilled water. Apparatus, Samples were excited at 71 K by a Questek 2400 excimer laser (308 nm; 120, 5 , and 0.2 mJ/pulse; 3000 pulses) or at 13 K with an ILC 300-W xenon illuminator with a 320-nm cutoff filter. After laser irradiation, the sampleswere transferred to a variable temperature dewar mounted in the EPR spectrometer (Varian E-9). In some experimentssampleswere excited directly in the EPR cavity at 4.2 and 1.9 K. Samples were checked for background EPR signals before irradiation. The g factors were calibrated by comparison to a powder 1,1 -diphenyl-2-picrylhydrazyl (DPPH) radical sample (g = 2.0037 f 0.0002). All magnetic fields are given in gauss (1 gauss = 10-4 tesla). 0
1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 50, 1993 13285
Hole Transfer from Ti02 to Methanol
3
0
3225
3
3276
3260
%
> k P
C.
CH~CHOH 2.002
I
0
I
I
I
3280
I
3910
3360
I
3
Magnetic Field (Gauss)
Figure 1. EPR (X-band) spectrum of degassed colloidal Ti02 (60-A diameter,3 X 1k2M) at pH 8 prepared in methanolicsolution,evaporated, and then dissolved in water, recorded immediately after laser irradiation at 4.2 K (a) laser intensity 120 mJ/pulse, recorded at 5.9 K, (b) laser intensity 120 mJ/pulse, recorded at 60 K; (c) laser intensity 2.5 mJ/ pulse, recorded at 5.8 K.
Results and Discussion Alkoxide Ti02 Colloids. The EPR spectra obtained from solutions of alkoxide (prepared by dissolving solid alkoxide film in water) Ti02 colloids at different temperatures and laser pulse intensities are presented in Figure 1. Three different signals around g = 2 were found. At high laser pulse intensities (>20 mJ/pulse), EPRsignals (Figure la) recordedat 6 Karecomposed of a quartet with a ratio of 1:3:3:1 (with an underlying signal) and a doublet with about a 130-G se aration, which are assigned to the methyl (CH3) and formyl ( HO) radicals, respectively. When the temperature was raised to 60 K, the signal due to the methyl radical disappeared, whereas the signal due to the formyl radical and the underlying signal that can be assigned to the methanol radical [ C H ~ O ( H ) ] ~remained S V ~ ~ (Figure 1b). The formyl radical is known to be produced by a two-photon process.28 It was not observed when laser intensities below 5 mJ/pulse were used. At these laser intensities (2.5 mJ/pulse) only the methanol radical was observed around g 2 (Figure IC). The following reactions depict the possible pathways for radical formation:
e
=
- + + + + hv
TiO,
e-
n(e-
+ Ti'V-O-Ti'V-OCH3
h+ + Ti'V-O-Ti'V-OCH3
h+)TiO,
(1)
+ CH, (2) Ti'V-O-Ti'V-OCH2 + H+ (3) Ti'V-O-Ti'V-O-
hv
Ti'V-O-Ti'V-OCH2
H,O
CHO
H,
Ti'V-O-Ti'V-OH
(4)
The intermediate TiIV-O-Ti~V-OCH2is formed in reaction 3 by a proton transfer to water or to a surface methoxide group.
D
,
3200
,
,
I
3Pso
I
I
I
I
3300
I
I
I
3:
Magnetic Field (Gauss)
Figure 2. EPR (X-band) spectrum of degassed colloidal Ti02 (60-A diameter, 3 X 1 t 2M) at pH 8 prepared in alcoholicsolutions,evaporated, and then dissolved in water, recorded immediately after laser irradiation (laser intensity 5 mJ/pulse) at 77 and 4.2 K (a) colloids prepared in CH3OH, (b) colloids prepared in CD30D, (c) colloids prepared in CH3CHzOH. Note: magnetic field scale on top for (a) and (b) and on bottom for (c).
We have no evidence for the formation of the CH30 radical during irradiation. However, this radical, if formed, could react with water or a surface methoxide group and form the CH20H radical. Deuterated methanol, CDjOD, was also used for the preparation of colloids. The EPR signal for this colloidal solution is presented in Figure 2b. The reduction in electron-nuclear hyperfine coupling found is consistent with the EPR signals in Figure 2a,b being assigned to a methanol radical. Ti02 colloids prepared with ethanol produced a quintet EPR signal (Figure 2c), which can be ascribed to TiIV-O-TiIVOCHCH3 formed by trapping the hole at the ethoxide group. In general, formation of RCHO(H) radicals is found upon irradiation of al~mina,2~-30 silica,25V ~ O Sand , ~M020327 ~ covered by alkoxide groups. In Figures I C and 2c a broad signal around g = 1.956 appears, which has been assigned to a Till' center on the Ti02 surface.17 Figure 3 shows the temperature dependence of the EPR line shape for the CH20(H) radical on Ti02 particles. As the temperature increases, the shape of the EPR absorption line changes. At about 80 K, the central line sharpens, suggesting that rotation of the radical around the C-0 axis becomes free at temperatures above 80 K. Hydrous Ti02 Colloids. The surface of Ti02 colloids prepared with TiC14 in water is completely covered by bound hydroxide ions and water molecules.3'-34 Hydroxide ions on the surface can be a trap for photogenerated holes. The EPR signal for trapped holes on hydrous Ti02colloids is presented in Figure 4 and shows resonances at 2.014 and 2.007. This signal is similar to that for the 0'-radical produced on different metal oxide surfaces;3~1 however, less ganisotropy and the absence of axial symmetry are observed.22 This can be accounted for by considering covalent bonding of surface oxygen atoms to titanium with nonequivalent
Micic et al.
13286 The Journal of Physical Chemistry, Vol. 97, No. 50, 1993
-
1.m
1.9K
n
4
30001WK-OK
906a
3oes
I
I
3z75
Magnetic Field (Gauss) Figure 3. EPR (X-band) spectrum of degassed colloidal Ti02 (60-A diameter,3 X 1p2 M) at pH 8 prepared in methanolicsolution,evaporated, and then dissolved in water, rccorded immediately after laser irradiation (laser intensity 5 mJ/pulse) at 77 K and at different temperatures.
J
1
1
1
1
1
1
3ma
1
1
1
3390
1
1
1
1
1
3380
0
Magnetic Field (Gauss) Figure 5. EPR (X-band) spectra of degassed aqueous Ti02 colloidal solution (60-Adiameter, 0.2 M) (prepared in water) at pH 6, which was immediately frozen after additionof 1 M methanol and irradiated at 1.9 K (recorded at 1.9 K, laser power 120 mJ/pulse).
g,, of the OH' radical is present in the EPR spectrum of the OH' radical adsorbed on the surface of inorganic salts,43but we could not detect this resonance in the Ti02 samples. For aqueous colloidal solutions there is no observable effect of laser pulse intensity and temperatureon the typeof radical formed, as was the case for methanol chemisorbed on Ti02 (Figure 1). In addition to the signals for trapped holes (g = 2.014, g = 2.007), the signal at g = 1.989 appears, indicating formation, from electron trapping, of Till1 in the bulk 1atti~e.I~ We also studied aqueous Ti02 solution in which methanol is added. The potential of holes trapped on surface hydroxide ions is more than that for oxidation of methanol. Methanol (1 M) was added to an aqueous Ti02 solution, and the resulting solution was immediately frozen to 77 K (followed by cooling to 1.9 K). The EPR signals (Figure 5 ) obtained after irradiation and recording at 1.9 K are assigned to the methanol radical (20-G splitting around g 2: 2) and Ti"' centers (g = 1.980, g = 1.960, and g = 1.942).17 In these experiments at 1.9 K,only c H 2 0 H radicals are found for the methanol oxidation product, suggesting that this is the first hole trapping site. At such low temperature, stepwise charge transfer is not likely, suggesting that no intermediate steps exist in the charge transfer from the Ti02 hole to the methanol (6).
X
a
> -X P
1
3
s1w
1
1
1
1
3ma
1
,
1
1
1
1
1
SOM)
3
Magnetic Field (Gauss)
I
Figure 4. EPR (X-band) spectra of degassed colloidal Ti02 (60-A diameter, 0.2 M) at pH 6 prepared in water irradiated at 6 K (laser intensity 5 mJ/pulse), recorded at different temperatures.
crystal field splittings from adjacent atoms with two resonant structures involving the exchange between the oxygen atoms in the product of reaction 5.
+
-
+
h+ Ti'V-O-Ti'V-OH Ti'V-O-Ti'V-O' H+ ( 5 ) The nature of intermediate Tilv-O-TiIV-O' does not change in the pH range 3-14.22 The signal in Figure 4 does not have the characteristicsof the OH' radical adsorbed on the colloid surface.22 The absence of the low-field resonance in Figure 4 attributed to
h+(Ti02) + C H 3 0 H
-
-
CH20H
+ TiO, + H+
(6)
At 1.9 K there is no diffusion,and the distanceof charge transfer cannot exceed 10 A. Thus, only methanol molecules located in the vicinity of a few monolayers of the surface of Ti02 particles participate in this reaction. Different radical products are observed when CH3OH is added to aqueous Ti02 solutions than when CH30H is chemisorbed on Ti02 (Figures 1 and 5). In aqueous Ti02 solutions with added methanol at high laser intensity, we found only c H 2 0 H and Tir1', while in chemisorbed C H 3 0 H on Ti02 CH3, CHzOH, CHO, and Ti1'' are formed. This suggests that methanol accepts the hole in the aqueous Ti02 solutions within a few monolayers of the surface of the particle. Electron Injection into Particles. Our results with alkoxide Ti02 colloidsin Figure 6a,b show, in addition to the signal around
The Journal of Physical Chemistry, Vol. 97, No. 50, 1993 13287
Hole Transfer from Ti02 to Methanol
9 > -X
\ I
v
a
c.
I
D
I
I
J
I
I
I
5ow
3160
I
I
,
,
9960
,
Magnetic Field (Gausa)
Figure 6. EPR (X-band) spectrum (400-G scan) of degassed colloidal Ti02 (60-Adiameter, 3 X 10-2 M)a t pH 8 prepared in alcoholicsolutions, evaporated,and then dissolved in water, immediately after being irradiated with laser ( 5 mJ) at 77 K: (a) recorded a t 8 K, (b) recorded after raising temperature to 15 K and recooling to 8 K.
g = 2 from the methanol radical, the broad asymmetric EPR signal at g = 1.981,indicating formation from electron trapping of Ti'IIon the surface.17 The change in the spectral features with temperature (Figure 6b) indicates that the methanol radical disappears at 150K while the intensity of the Ti111 signal at 1.981 doubles. At higher temperatures the methanol radical becomes mobile and can inject electrons into Ti02. Trapped electrons on Ti02 can be regarded as reduced metal centers (Ti1]') Ti'V-O-Ti'V-OcH2
-
+ C H 2 0 + H+
Ti'v-O-Till'
(7)
The signal pattern remained constant when the sample temperature was increased to room temperature and then recooled to
6 K. We found that the same reaction of CH20H cccurs on hydrous Ti02 colloids in aqueous solution in the presence of Hg2+ ions (see Figure 7). In this solution, the resonances at 1.980, 1.961, and 1.950 are assigned to Ti111 centers, while those with 20-G splitting around g 2 are assigned to the methanol radical. At 150 K (Figure 7b) the methanol radical disappears (reactions 8 and lo), while the Til11 center at 1.950 increases in intensity. At 200 K (Figure 7c), the T P center completely disappears due to reaction 9:
=
cH20H + Ti'V-O-TilV-OH
-
+ CH20 + H,O
Ti'V-O-Ti''l 2(Ti'V-O-Ti1'')
+ Hgz+ + 2 H 2 0
-
+ Hg2+
-
CH20
\I
I
I
I
I
,
I
I
I
,
,
I
a: Magnetic Field (Gauss) Figure 7. EPR (X-band) spectra of degassed aqueous Ti02 colloids (0.2 M)in the presence of methanol (1 M)and HgCl2 ( 5 X M)irradiated with laser at 77 K,recorded a t different temperatures: (a) 8 K, (b) after raising temperature to 150 K and recooling to 8 K, (c) after raising temperature to 220 K and recooling to 8 K. D
9910
SPW
9910
vs NHE]. According to our experimental results, this radical reacts with Hg2+ in solution to form HgO or by transferring an electron to Ti02 (Figure 7b), and then surfaceTi'I'centers reduce Hg2+ions at higher temperature (220 K)by direct reaction with Hg*+ ions. The redox potential (vs NHE) of the electron on Ti02 is -0.345 V at pH 6. Therefore, species with potentials more positive than this are reduced by electrons. This is the case for Hg2+ for which the redox potential (vs NHE) of the species is Eo(Hg2+/Hg+)= +0.8 V. Reduction of Hg2+with electrons that are produced by irradiation of Ti02 was observed previously.u In the presenceof Hg2+and methanol (Figure 7c) the EPR signal for Ti1]' at g = 1.950disappears when the sample temperature increases to 220 K and then is measured at 8 K. In the absence of Hg2+the surface Ti1'' center is stable at room temperature. Hg2+ions are reduced to HgO by CH2OH and Ti1'' ions.
Conclusion These experiments illustrate that the EPR technique can be used to study the mechanism of photodecomposition reactions of methanol on irradiated Ti02. The photooxidation of methanol on Ti02 has been studied because of the importance of understanding how photogenerated holes transfer from particles to organic compounds in solution and initiate photooxidation processes. In irradiated aqueous solutions of methanol at 1.9 K, EPR signals obtained are centered around g = 2 and have a line shape characteristic of the cH20(H) radical. This radical, observed a t 1.9 K,indicates that transfer of holes from the ox gen lattice to the methanol molecules occurs. At 1.9 K only the HzOH radical can be seen, and the location of the hole on the OH group could not be seen. In the same solution without methanol, holes are trapped by surface OH groups as the oxygen anion radical, Ti~vUTiIV-O'. In the aqueous solution, methanol is directly photooxidized by the hole, and products of this reaction (eH20H and Ti"') are different from those in which CH,OH is chemisorbed on the Ti02 surface (cH3, CH20H, CHO, and Ti1]*).
E
2(Ti'v-O-Ti'v-OH) 2cH20H
(8)
i
m-mK-aK
I
3
3,
I /
+ 2H+ + Hgo (9)
+ Hgo + 2H+
(IO)
The CH2OH radical, which is formed in the vicinity of TiO2, has a large negative potential, [Eo(CH20/CH20H) = -0.97 V
13288 The Journal of Physical Chemistry, Vol. 97, No. 50. 1993 References and Notes (1) O h , D. F.; Pelizzetti, E.; Serpone, N. Enuiron. Sci. Technol. 1991, 25, 1522. (2) Matthews, R. W. InPhotochemicalConuersionandStorageofSolar Energy; Peliuetti, E., Schiavello, M., Us.; Kluwer: Dordrecht, 1991. (3) Photocatalysis-Fundamentals and Applications; Serpone, N., Pelizzetti, E., Eds.; Wiley: New York, 1989. (4) Bahnemann, D. W. In Photochemical Conversion and Storage of Solar Energy; Pelizzetti, E., Schiavello, M., Eds.; Kluwer: Dordrecht, 1991. (5) Anpo, M. Research on Chemical Intermediates-11; Elsevier: Amsterdam, 1987; p 67. (6) Rothcnberger, G.; Moser, J.; Gratzel, M.; Serpone, N.; Sharma, D. K. J. Am. Chem. Soc. 1985.107, 8054. (7) Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1982,86, 241. (8) Dimitrijevic, N. M.; Savic, D.; Micic, 0. I.; Nozik, A. J. J. Phys. Chem. 1984,88, 4278. (9) Bahnemann, D.;Henglein, A.; Spanhell, L. Faraday Discuss. Chem. Soc. 1984, 78, 51. (10) Arbour, C.; Sharma, D. K.; Langford, C. H. J. Phys. Chem. 1990, 94, 331. (11) Lawless, D.; Serpone,N.; Meisel, D. J. Phys. Chem. 1991,94,5166. (12) Rajh, T.; Saponjic, Z. V.;Micic, 0. I. Langmuir 1992, 8, 1265. (13) Zwingwl, D. Solid Stare Commun. 1976, 20, 397. (14) Gonzales-Elipc, A. R.;Munuera, G.; Soria, J. J. J. Chem. Soc., Faraday Trans. 1 1979,83, 748. (15) Gonzales-Elipc, A. R.; Soria, J. J.; Munuera, G. Chem. Phys. Lett. 1978, 57, 265. (16) Anpo, M.; Shima, T.; Kubakawa, Y. Chem. Lett. 1985, 1799. (17) Howe, R. F.; Gratzcl, M. J. Phys. Chem. 1985, 89, 4495. (18) Howe, R. F.; GrHtzel, M. J. Phys. Chem. 1987, 91, 3606. (19) GrHtzel, M.; Howe, R. F. J. Phys. Chem. 1990, 94, 2566. (20) Anpo, M.; Shima,T.; Kodma, S.;Kubokawa, J. J. Phys. Chem. 1987, 91, 4305. (21) Aundaithai, M.; Kutty, T. R. N. Mater. Res. Bull. 1988,23, 1675. (22) Micic, 0.I.; Zhang, Y.; Cromack, K. R.; Trifunac, A. D.; Thurnauer, M. C. J. Phys. Chem. 1993,97, 7271.
Micic et al. (23) Thompson, R. C. Inorg. Chem. 1984,23, 1794. (24) Abrams, L.; Allen, A. 0. J. Phys. Chem. 1969, 73, 2741. (25) Melamud, E.; Reisner, M. G.; Garbatski, U. J. Phys. Chem. 1973, 77, 1023. (26) Fujuta, Y.; Hatano, M.; Yanagita, T.; Katsu, Sato, M.; Kwan, T. Bull. Chem. Soc. Jpn. 1971.44, 2884. (27) Katsu, T.; Yanagita, M.; Fujita, Y. J. Phys. Chem. 1971,75,4064. (28) Sullivan, P. J.; Koski, W. S . J. Am. Chem. Soc. 1%3,85, 384. (29) Livingston, R.; Zcldes, H. J. Chem. Phys. 1966, 44, 1245. (30) Ono, Y.; Keii, T. J. Phys. Chem. 1968,72, 2851. (31) Boehm, H. P. Discuss. Faraday Soc. 1971,52,264. (32) Tanaka, K,White, J. H. J. Phys. Chem. 1982, 86, 4708. (33) Primet, M.; Pichat, P.; Mathieu, M. V. J. Phys. Chem. 1971, 75, 1216. (34) Scafani, A.; Palmisano, L.; Shiavello, M. J. Phys. Chem. 1990,94, 829. (35) Kirklin, P. W.; Auzins, P.; Wertz, J. E. J. Phys. Chem. Solids 1965, 26, 1067. (36) Wong, N.-B.; Lunsford, J. H. J. Chem. Phys. 1971, 55, 3007. (37) Unruh, W. P.; Chen, Y.; Abraham, M. M. Phys. Rev. Lett. 1973,30, 446. (38) Schoenberg, A.; Suss, J. T.; Szapiro, S.Phys. Rev. h i t . 1971, 27, 1641. (39) Giamello. E.; Garrone, E.; Ugliengo, P.; Che, M.; Tench, A. J. J. Chem. Soc., Faraday Trans. I 1989.85, 3987. (40) Cornaz, P. F.; van Hooff, J. H. C.; Pluum, F. J.; Schuit, G. C. A. Discuss. Faraday Soc. 1966,41, 290. (41) Maffeo, B.; Herve, A. Phys. Rev. A 1976,13. 1940. (42) Nakato, Y.; Tsubomura, A.; Tsuboura, H. J. Phys. Chem. 1983,87, 2402. (43) Gunter, T. E. J. Chem. Phys. 1967,46, 3818. (44) Serpone, N.; Ah-You, Y. K.; Tran, T. P.; Harris, R.; Pelizzetti, Sol. Energy 1987, 6, 49 1.
