1396
Langmuir 1990,6, 1396-1402
Titanium Dioxide Photosensitized Reactions Studied by Diffuse Reflectance Flash Photolysis in Aqueous Suspensions of Ti02 Powder R. Barton Draper and Marye Anne Fox* Department of Chemistry, University of Texas at Austin, Austin, Texas 78712 Received November 15, 1989
Diffuse reflectance and transmission flash photolysis have been used to study the primary reaction intermediates in titanium dioxide (TiOz) photosensitized reactions of potassium iodide, 2,4,5trichlorophenol, methyl viologen dichloride, potassium tetrachloroplatinum(II), tris(1,lO-phenanthro1ine)iron(I1) perchlorate, N,N,N',N'-tetramethyl-p-phenylenediamine, and thianthrene. In the cases where the product of OH radical mediated oxidation is different from that of direct electron transfer oxidation, only products of direct electron transfer oxidation are observed. The products of direct electron transfer and OH radical mediated oxidation of iodide and tetrachloroplatinum are identical. The transient decay kinetics in TiOz powder systems appear to be first order with a Gaussian distribution of reaction rate constants reflecting the distribution of particle radii.
Introduction Diffuse reflectance flash photolysis can now be used to investigate reactions occurring on the surface of commercially available well-characterized semiconductor powders.'-lO In a diffuse reflectance flash photolysis experiment, diffusely reflected light is detected1l-I4 rather than the transmitted light analyzed in conventional transmission flash photolysis experiments. This technique allows for the study of highly scattering aqueous suspensionsof semiconductorpowders. These suspensions are much easier to characterize and manipulate than the colloidal solutions needed for transmission flash photolysis experiments. Although colloidal systems have been invaluable in the study of semiconductor-mediated photoreactions, they are not ideal systems for such investigations. For example, in the case of Ti02 colloids, the solution pH cannot be raised above about 3 without the addition of a polymeric coating to prevent the particles from flocculating.1k22 A (1) Draper, R. B.; Fox, M. A. J . Phys. Chem. 1990,94,4628-4634. ( 2 ) Wilkinson, F.; Willsher, C. J.; Uhl, S.; Honnen, W.; Oelkrug, D. J . Photochem. 1986,33, 273. (3) Pouliquen, J.; Fichou, D.; Valat, P.; Kossanyi, F.; Wilkinson, F.; Willsher, C. J. J . Photochem. 1986, 35, 381. (4) Wilkinson, F.; Willsher, C. J. J . Luminescence 1985,33, 187. (5) Wilkinson, F.; Willsher, C. J. J . Chem. Soc., Chem. Commun. 1985, 142. (6) Oelkrug, D.; Uhl, S.; Wilkinson, F.; Willsher, C. J. J . Phys. Chem. 1989,93,4551. (7) Oelkrug, D.; Flemming, W.; Fullemann, R.; Gunther, R.; Honne, W.; Krabichler, G.; Schager, M.; Uhl, S. Pure Appl. Chem. 1986,58,1207. (8) Oelkrug, C.; Krabichler, G.; Honnen, W.; Wilkinson, F.; Willsher, C. J. J. Phys. Chem. 1988,92,3589. (9) Kessler, R. W.; Oelkrug, D.; Wilkinson, F. Appl. Spectrosc. 1982, 36, 673. (10) Kessler, R. W.; Wilkinson, F. J. Chem. SOC.,Faraday Trans. 1 1981, 77, 309. (11) Wendlandt, W. W.; Hecht, G. G. Reflectance Spectroscopy; Interscience: New York, 1966; pp 46-90. (12) Wilkinson, F. J . Chem. SOC.,Faraday Trans. 2 1986, 82, 2073. (13) Kessler, R. W.; Krabichler, G.; Uhl, S.; Oelkrug, D.; Hagen, W. P.; Hyslop, J.; Wilkinson, F. Optica Acta 1983,30, 1099. (14) Oelkrug, C.; Honne, W.; Wilkinson, F.; Willsher, C. J. J. Chem. Soc., Faraday Trans. 2 1987,83, 2081. (15) Moser, J.; Graetzel, M. Helu. Chim. Acta 1982, 65, 1436. (16) Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1982, 86, 241. (17) Nosaka, Y.; Fox, M. A. Langmuir 1987,3, 1147. (18) Brown, G. T.; Darwent, F. R.; Fletcher, P. D. I. J. Am. Chem. Soc. 1985, 107, 6446.
further disadvantage of colloidal Ti02 systems in contamination of the solutions by chloride or 2-propanol. These species are introduced when titanium tetrachloride and titanium tetraisopropoxide are hydrolyzed to prepare the colloid. Powders are more robust and simpler than colloids, but studying powder-sensitized photoreactions in a fast timeresolved manner presents one big challenge: how to analyze the reaction kinetics. Fortunately, several models for heterogeneous kinetics are now a ~ a i l a b l e . ~Recently, ~-~~ we demonstrated t h a t t h e kinetics of (SCN)z*decomposition on Ti02 powders can be analyzed using a model developed by Albery for treating a variety of heterogeneous reactions. The Albery model accounts for heterogeneous kinetics with two adjustable parameters, an average rate constant and a parameter determining the width of the rate constant d i s t r i b ~ t i o n . ~ ~ - ~ ~ Now that it is possible to analyze the kinetics of at least some reactions occurring on the Ti02 powder surface, we seek to extend the use of diffuse reflectance flash photolysis in aqueous suspensions to the study of a variety of oxidation and reduction reactions involving both organic and inorganic molecules. Molecules with known aqueous (19) Graetzel, M.; Frank, A. J. Phys. Chem. 1982,86, 2964. (20) Brown, G. T.; Darwent, J. R. J. Chem. Soc., Chem. Commun. 1985, 98. (21) Rotherberger, G.; Moeser, J.; Graetzel, M.; Serpone, N.; Sharma, D. K. J. Am. Chem. SOC.1985,107,8054. (22) Dimitrijevic, N. M.; Savic, D.; Micic, 0. I.; Nozik, A. J. J. Phys. Chem. 1984,88,4278. (23) Albery, W. J.; Bartlett, P. N.; Wilde, C. P.; Darwent, J. R. J. Am. Chem. SOC.1985, 107, 1854. (24) Albery, W. J.; Brown, G. T.; Darwent, F. R.; Saievar-Iranizad, E. J . Chem. SOC.,Faraday Trans.I 1985,81,1999. (25) Brown. G. T.: Darwent. F. R.: Fletcher, P. D. I. J. Am. Chem. SOC. 1985, 107,6446. (26) Siebrand, W.; Wildman, T. A. Acc. Chem. Res. 1986,19, 238. (27) Plonka, A.; Kevan, F. J . Chem. Phys. 1984,80,5023. (28) Plonka, A.; Kroh, J.; Lefik, W.; Bogus, W. J. Phys. Chem. 1979, 83, 1807. (29) Albery, W. J.; Bartlett, P. N.; McMahon, A. J. J. Electroanul. Chem. 1985, 182, 7. (30)Albery, W. J.; Bartlett, P. N. J. Electroanal. Chem. 1982,131,137. ( 3 1 ) Rothenberger, G.; Infelta, P. P.; Graetzel, M. J. Phys. Chem. 1979, 8 . 7 - -, 1871 - - . -. (32) Hatlee, M. D.; Kozak, J. J.; Rothenberger, G.; Infelta, P. P.; Graetzel, M. J. Phys. Chem. 1980,84, 1508. (33) Curran, J. S.; Lamouche, D. J . Phys. Chem. 1983,87, 5405.
0 1990 American Chemical Society
Langmuir, Vol. 6, No. 8,1990 1397
TiOz-Photosensitized Reactions radiation chemistry have been chosen as electrontransfer reagents in order to facilitate the identification of photosensitized reduction and oxidation products. In some cases, the products of both direct oxidation and OH radical oxidation have been characterized in the literature. These oxidized transients may serve as probes into the mechanism of photosensitized oxidations occurring on Ti02 powders. When possible, both transmission and diffuse reflectance data are reported.
Experimental Section Chemicals. Titanium dioxide powder (Degussa P-25) was a mixture of rutile anatase with a BET surface area of 50 m2/g, average primary particle size of 30 nm, and an isoelectric point at pH 6.6. KI, ,A, < 280 nm (Fisher Scientific); K2PtC14,,A, = 220 nm (AESAR);Fe(Phen)&104)2, Amax = 510 nm (Matheson Coleman & Bell); N,N,N',N'-tetramethyl-p-phenylenediamine, Amax = 260 nm (Aldrich Chemical); methyl viologen dichloride, Amax C300 nm (Sigma); and thianthrene, ,A, = 255 nm (Aldrich Chemical) were used as received. 2,4,5-Trichlorophenol, ,A, = 290 nm, was obtained from Lancaster Synthesis and purified by recrystallization and sublimation before use. Colloidal Ti02 was prepared by hydrolyzing Ti(i-PrO)r (Aldrich Chemical) in 0.01 M HC104 (Fisher Scientific). Ti02 powder was suspended in Millipore-purified water by sonicating 1 g of Ti02 in 100 mL of solution for 3 min. The pH of solutions was adjusted with dilute NaOH. The unadjusted pH of a 1-g suspension of Ti02 in 100 mL of water was approximately 4.5. The suspension temperature was maintained at 21 f 1 "C, and a new aliquot of the suspension was used after approximately 10 laser pulses. Control experiments showed no reflectance change when the low laser intensities of these experiments are used to excite aqueous suspensions of Ti02 containing no solute. Flash Photolysis Equipment. Time-resolved transient transmission measurements were performed by using the third harmonic (355 nm, 15-11s pulse) from a Quantel YG 481 Q-switched Nd: YAG laser. Diffuse reflectance measurements were performed by using the third harmonic (355 nm, 15ns pulse) from a Quantel YG 580 Q-switched Nd:YAG laser for excitation. The laser beam energy was varied by using filters. Beam energies of 10-20 mJ/pulse were used for the transmission measurements, while energiesof less than 10 mJ/pulse were used for the diffuse reflectance measurements. Beam energies were measured by using a power meter. In the diffuse reflectance experiments, the area irradiated on the cuvette's front face was approximately 1 cm*, while the beam was usually focused to less than 0.5 cm2 in the transmission experiments. Other details of the experimental setup have been described elsewhere.' The reported signals are averages of data from between 3 and 10 laser pulses. Data Analysis In diffuse reflectance experiments, the initial reflectance of the sample JO (in mV) was measured by an electronic backoff unit. The voltage was then offset to zero before the laser was fired. Voltage versus time data are reported as percent absorption, A J = (Jo- Jt)/J0,where Jt is the reflected intensity after excitation. Unlike transmission measurements where log (Zo/Zt) is easily related to the transient concentration and extinction coefficient by Beer's law, A J obtained in diffuse reflectance experiments has no convenient connection with basic transient properties. The approach most often taken to relate diffuse reflectance measurements to transient properties involves measuring a different parameter, R , given by the Kubelka-Munk equation:11-14
where R = J/Io. l o , the incident analyzing light intensity, is an impossible parameter to measure in our experiments.
When dry powders are being investigated, a diffuse reflector with a known value of R may be used with an experimentally measured value of J to calculate ZO. No standard aqueous suspension of a diffuse reflector exists. The experimentally available measurement, AJ, and the Albery model discussed below are presented in this and a previous paper' as an alternative to the use of KubelkaMunk theory for the analysis of diffuse reflectance flash photolysis data. Albery A model for kinetics in heterogeneous systems has been developed by Albery and co-workers. The Albery model accounts for the kinetic behavior we observe by considering powder systems as a distribution of different sized particles. The derivation of the equations describing this model begins with the assumption that the reaction rate constant k is proportional to the particle surface area r2. It is further assumed that the natural logarithm of the particle radius is given by In r = In rav+ p x
(2) where ravis the average particle radius, p is a parameter determining the effective width of the particle size distribution, and x , the distribution variable, is allowed to take values between plus and minus infinity. These two assumptions lead to an expression for the reaction rate constant
k
exp(yx) (3) where y = 2p is now the parameter describing the width of the distribution about some average rate constant kav. This same expression for the reaction rate constant may be arrived at by using an Arrhenius type equation with a distribution of activation free energies.23 We consider it more likely that a Gaussian distribution of particle radii, rather than of reaction activation energies, exists. Equation 3 is then substituted for the conventional homogeneous rate constant in an integrated fist-order rate law. When A J is proportional to transient concentration and when x is integrated across a Gaussian distribution, one obtains kav
+-eXp(-x2) exp[-kavt exp(yx)] dx (4) exp(-x2) dx This expression describes the time dependence of the percent absorption (given the above assumptions that k is proportional to r2,concentration is proportional to AJ, and there is a Gaussian distribution of r values). In the limit of y = 0, this expression reduces to an ordinary firstorder rate law. Since the integral has no analytical solution, it must be approximated. Reference 23 gives a variable transformation which leads to more convenient integration limits, namely, zero to one. We have used t h e cautious adaptive Romberg extrapolation routine, DCADRE,34from the International Mathematical Subroutine Library (IMSL) to perform the integration of eq 4. Calculated values of A J at each of roughly 200 times were then compared to measured values, and the sum of the residuals squared was minimized by adjusting the average rate constant (kav) and the distribution parameter (y) by use of another IMSL routine; ZXSSQ. ZXSSQ is a finite difference LevenbergMarquardt routine for solving nonlinear least-squares problems.35-3' AJ -= AJO
Jm
(34) de Boor, C.In Mathematical Software; Rice, J. R., Ed.; Academic Press: New York, 1971; Chapter 7. (35) Brown, K. M.;Dennis, J. E. Numerische Mathematik 1972,18, 289.
(36) Levenberg, K.Quart. Appl. Math. 1944,2, 164.
1398 Langmuir, Vol. 6, No. 8, 1990
Draper and Fox 0.01 1
3
I
0
t m
W Z
0
-0 001 360
v
i
395
465
430
-0.001
500
Fi ure 1. Transient trammission absorptionspectrum attributed tof*- generated by flash photolysis of colloidal Ti02 stabilized by poly(viny1alcohol) in 0.2 M K I 0, 7.5; 0 , 22.5; A, 56.5; A, 107; 0,155.5ps after photolysis. Inset: decay at 400 nm. 7
P
380
WAVELENG'H (NU)
T
420 460 WAVELENGTH (NM)
i 500
Figure 3. Transient transmissionabsorptionspectrum attributed to 2,4,5-trichlorophenoxylradical generated by flash photolysis of colloidal Ti02 in 3.1 X 10-3 M 2,4,5-trichlorophenok 0,17; 0 , 50; A, 118; A, 217; 0, 317 ps after photolysis. Inset: decay at 430 nm.
0 0107---
#a L e 1 e r/ g:
i, ( n r-
- 0 001
)
Figure 2. Transient diffuse reflectance absorption spectrum attributed to 12.- generated by flash photolysis of Ti02 powder, 1g/100 mL, in 0.001 M KI: 0,2.5; 9.5; A 22; A, 58; 0, 109.5; m, 158 ps after photolysis. Inset: decay at 400 nm.
Results Transient Spectra. Iodide. A transient transmission absorption spectrum, observed after excitation of oxygenbubbled, poly(viny1 alcohol)-stabilized, colloidal TiO2, containing 0.2 M KI at pH 4, is presented in Figure 1.The maximum of this absorption spectrum, approximately 400 nm, corresponds to that of 12.- independently generated by pulse radiolysis.38 The decay of the optical density at 400 nm is essentially complete within 200 MS and follows a second-orderrate law, Figure 1inset. With the extinction coefficient of 12.- in homogeneous solution equal to 1.4 X lo4 M-l ~ m - l a, ~second-order rate constant of k = 3.1 X 10'0 M-I s-l is obtained. The transient diffuse reflectance absorption spectrum, observed after photoexcitation of an oxygen-bubbled suspension of Ti02 powder, 6 g in 100 mL of 1 X 10-3 M KI, is presented in Figure 2. Again the maximum percent absorption change occurs at approximately 400 nm. The signal, however, decays by more complex kinetics, Figure 2 inset. This decay may be analyzed by using the Albery model with k,, = 9.5 X 103 s-1 and y = 4.3. 2,4,5-Trichlorophenol. The transient transmission absorption spectrum, observed after photoexcitation of oxygen-bubbled, colloidal TiOz, containing 3.1 X 10-3 M 2,4,5-trichlorophenol, is presented in Figure 3. The maximum of this absorption spectrum is approximately 430 nm, corresponding to that of 2,4,5-trichlorophenoxyl (37) Marquardt, D. W. J. Siam. 1963,lI ( 2 ) . (38)Hug, G. L. Optical Spectra of Nonmetallic Inorganic Transient Species In Aqueous Solution, National Bureau of Standards, Department of Commerce, Report No. NSRDS-NBS 69,Washington, DC, 1981.
m 428 486 564 602 660
370
Wavelength (nm)
Figure 4. Transient diffuse reflectance absorption spectrum attributed to methyl viologen cation radical generated by flash photolysis of Ti02 powder, 1 g/100 mL in 0.04 M methyl viologen dichloride, pH 8: 0, 1.1;0 , 3.6; A, 10.3; A, 20.4; 0,30.2 ps after photolysis. Inset: decay at 390 nm. radical independently generated by pulse r a d i o l y s i ~The .~~ decay of the optical density at 430 nm followed secondorder kinetics, Figure 3 inset. With the extinction coefficient of 2,4,5-trichlorophenoxyl radical in homogeneous solution, 3600 M-' cm-l, a second-order rate constant of k = 2 X 109 M-' s-l can be calculated. Methyl Viologen Dichloride. The transient diffuse reflectance absorption spectrum, observed after photoexcitation of an oxygen-bubbled suspension of Ti02 powder, 1 g in 100 mL of 0.4 M methyl viologen dichloride (pH 8), is presented in Figure 4. The maximum percent absorption change occurs at 390 nm with less intense bands observed at approximately 480 and 600 nm. The maxima at 390 and 600 nm correspond to those of methyl viologen cation radical, MV'+, independently generated by pulse r a d i o l y s i ~ .The ~ ~ absorption ~~~ at 390 nm decays in about 40 ps and follows the Albery model, k,, = 2.4 X lo5 s-1, y = 1.1, Figure 4 inset. Potassium Tetrachloroplatinum(I1). The transient diffuse reflectance absorption spectrum, observed after photoexcitation of an oxygen-bubbled suspension of Ti02 powder, 1g in 100 mL of 3.7 X lo4 M KzPtCL, is presented in Figure 5. The maximum percent absorption change occurs at about 450 nm with a less intense shoulder at (39)Draper, R. B.; Fox, M. A.; P e l h t t i , E.; Serpone, N. J . Phys. Chem.
--.
1989. - - - -93. , 1938. ----
(40)(a) Solar, S.;Solar, W.; Getoff, N.; Holoman, J.; Sehested, K. J. Chem. Sac., Faraday Trans. I 1982,78, 2467. (41)Solar, S.; Solar, W.; Getoff, N.; Holoman, J.; Sehested, K. J. J. Chem. Soc., Faraday Trans. I 1985,81, 1101.
Langmuir, Vol. 6, No. 8, 1990 1399
Ti02-Photosensitized Reactions 0.024
O'Ol8T-ri
-
-0,001 380
480
430
530
0.000 410
580
480
Wavelength ( n m )
Figure 5. Transient diffuse reflectance absorption spectrum attributed to a Pt(II1) species generated by flash photolysis of Ti02 powder, 1g/100 mL, in 3.7 X lo-' M K2PtCL: 0,0.6;0 , 2.6; A, 6.2; A, 12.4; 0,20.9;B, 30.3 ps after photolysis. Inset:
decay at 440 nm.
550
Wavelength
620 (nm)
690
Figure 7. Transient diffuse reflectance absorption spectrum attributed to the cation radical of tetramethyl-p-phenylenediamine generated by flash photolysis of Ti02 powder, 1 g/100 (mL of acetonitrile, in 9.4 X le4M tetramethyl-p-phenylenediamine: 0,2.3;0,7.3; A,13.85 ps after photolysis. Inset: decay at 570 nm.
1
7
0
0
AJ 0
0
-0.025 380
0.
430
480
530
580
Wavelength ( n m )
Figure 6. Transient diffuse reflectance absorption spectrum attributed to Fe(Phen)s2+generated by flash photolysis of Ti02 powder, 1 g / l W mL, in 1.25 X 10-4M Fe(Phen)s(ClO&,pH 10: 0,0.7;0,2.4;A, 6.0;A, 12.2;0,20.6;. , 30.4 ps after photolysis. Inset: recovery at 500 nm.
about 410 nm. These two bands correspond to those of Pt(II1) complexes independently generated by pulse radiolysis.42~43The signal at 450 nm decays according to the Albery model with k,, = 6.7 X lo4and y = 3.7, Figure 5 inset. T i s (1,lO-phenanthroline)iron(II) Perchlorate. The transient diffuse reflectance absorption spectrum, observed after photoexcitation of an oxygen-bubbled suspension of Ti02 powder, 1 g in 100 mL of 1.3 X M Fe(Phen)3. (c104)2 (pH lo), is presented in Figure 6. In contrast to the previous examples, Fe(Phen)s2+absorbs light in the spectral region between 380 and 580 nm. This leads to the observation of a bleaching signal in the transient spectrum. The maximum bleaching occurs around 500 nm. This wavelength corresponds to the maximum absorption of Fe(phen)32+.& The signal at 500 nm recovers according to Albery kinetics with k,, = 2.3 X lo5 s-l and y = 1.5, Figure 6 inset. N,N,N,N-Tetramethyl-pphenylenediamine.The transient diffuse reflectance absorption spectrum observed after photoexcitation of an oxygen-bubbled suspension of Ti02 powder in acetonitrile, 1 g in 100 mL of 9.4 X M N,N,",ZV'-tetramenthyl-p-phenylenediamine (TMPD), is presented in Figure 7. In this system, the maximum percent absorption change occurs at about 550 nm. This (42) Adams, G. E.;Broszkiewicz, R. B.; Michael, B. D. Trans. Faraday SOC.1968,4, 1256. (43) Ghosh-Mazumdar,A. S.; Hart, E.J. Int. J . Radiat. Phya. Chem. 1969, 1, 165. (44) Siekierska, E.; Pagsberg,P. Int. J. Radiat. Phys. Chem. 1976,8, 425.
0.000 405
!
605 Wavelength ( n m )
455
505
555
655
Figure 8. Transient diffuse reflectance absorption spectrum attributed to the cation radical of thianthrene generated by flash photolysis of Ti02 powder, 1 g/100 mL of acetonitrile, in 9.1 x M thianthrene: 0,1.4; 0,4.4; A 11.1;A, 21;0,31.2p s after photolysis. Inset: decay at 585 nm.
maximum correspondsto that of the TMPD cation radical, TMPD'+, prepared by pulse r a d i o l y ~ i s .TMPD*+ ~~ is stable over 20 ps, Figure 7 inset. Thianthrene. The transient diffuse reflectance absorption spectrum, observed after photoexcitation of an oxygen-bubbled suspension of Ti02 powder in acetonitrile, 1g in 100 mL of 9.1 X M thianthrene solution, is presented in Figure 8. In this system, the maximum percent absorption change occurs a t about 550 nm, corresponding to that of the thianthrene cation radical prepared by thermal oxidation of thianthrene in 96% sulfuric acid.* The absorption at 550 nm decays according to the Albery model with k,, = 2.8 X lo4 s-l and y = 4.1, Figure 8 inset. Kinetics. Intensity Studies. In order to establish whether the intensity of the recorded diffuse reflectance signal was proportional to transient concentration, a study of signal height at time zero as a function of laser intensity was performed in the iodide and methyl viologen systems. For the methyl viologen system, a l-g Ti02 suspension in 100 mL of 0.04 M MV2+ was irradiated with a 15-11s pulse of 355-nm light, which was attenuated with filters. AJO39Owas taken as the maximum percent absorption value at 390 nm of the transient decay. When A J o is~plotted ~ against laser energy, Figure 9, the plot is linear for energies between 2 and 5 mJ/pulse. Similar treatment of data from the iodide system yields the same results. (45) Reo, P. S.; Hayon, E. J. Phys. Chem. 1976, 79,1063. (46) Shine, A. J. Dais, C. F.; Small, R. J. J. Org. Chem. 1964,29, 21.
1400 Langmuir, Vol. 6, No. 8, 1990
Draper and Fox 0.025 1
1
A J O m
!
@~
I
I
4' @
0.01 5 1.50
I 3 35
5 20
Energy (mJ/pulse)
Time ( s )
Figure 9. Plot of h l o at 390 nm vs incident laser energy obtained by flash photolysis of Ti02 powder, 1 g/100 mL in 0.04 M methyl viologen dichloride.
Figure 12. A J at 390 nm vs time obtained by flash photolysis, 4 mJ/pulse, of Ti02 powder, 1 g/100 mL,in 0.04 M methyl viologen dichloride, pH 11. Fit is to Albery model, K,, = 2.4 X 106 s-1, y
= 0.81.
Table I. Iz*- Yield and Kinetics as a Function of Laser Intensity for TiOI laser, mJ/pulse ar03m kav, 1 0 4 s-1 Y 0.010 0.009 0.008 0.009
5.5 5.2 4.3 3.1
00
2 OE-5
4 @E-5
3.8 3.6 4.5 3.5
3.6 2.5 2.5 3.9
Table 11. Intensity vs Yield and Kinetice of MV'+ Decay for TiOz Powder laser, mJ/pulse k,, 10-6 s-l Y art3390
Time ( s )
Figure 10. Plot of normalized A J decays at 390 nm obtained by flash photolysis of Ti02 powder, 1 g/100 mL, in 0.04 M methyl viologen dichloride. Laser energieswere 4.7,2.8, and 1.7 mJ/ pulse.
5 4.7 4 2.8 1.7
pH 11 2.2 2.2 2.4 2.4 2.4
0.97 0.87 0.81 0.070 0.63
0.028 0.027 0.025 0.020 0.016
5 4.7 4 1.7
pH 10 2.0 2.2 2.3 2.2
0.93 0.92 0.82 0.77
0.019 0.018 0.017 0.013
Table 111. Dependence of MV*+Yield and Kinetics on pII for TiOl Powder DH
-0 001 00
2 OE-5
4 @E-5
Time ( s )
Figure 11. AJ at 390 nm vs time obtained by flash photolysis, 5.2 mJ/pulse, of Ti02 powder, 1 g/100 mL, in 0.1 M KI. Fit is to Albery model, k,, = 3.6 X lo4 s-l and y = 2.5. These same data may be used to show that the transient decay kinetics of MV'+ do not depend on transient concentration. When normalized by AJo, the decay traces are superimpable. In Figure 10,the greatest AJom value was 0.028, while the least was 0.016. This superposition is possible in both systems. If the transients were decaying by higher order kinetics, it would not be possible. Representative fits of the Albery model to the transient decays from the iodide and methyl viologen systems are shown in Figures 11 and 12. Tables I and I1 summarize the results from these studies: A J o decreases with decreasing intensity, while ,.k and y remain relatively constant. Dependence of MV*+ Decay on pH. Table 111 presents data on the pH dependence of MV*+decay. The
7.6 7.9 8.2 8.4 9.0 9.4 9.6 10.2
kav,
s-l
2.2 1.9 2.0 2.1 2.0 2.0 1.8 1.7
Y
AJ0390
1.1 0.92 1.3 1.3 1.3 1.4 1.6 1.7
0.012 0.014 0.016 0.018 0.025 0.028 0.034 0.040
pH of a suspension of Ti02 powder, 1 g in 100 mL of 5.5 X M methyl viologen, was varied by addition of dilute NaOH. Transient decay signals were recorded and fit to the Albery model. Figure 13 gives a plot of AJ03w vs pH, and Figure 14 gives representative fits for pH 7.6,8.4, and 9.4. In these experiments,k, is constant over the pH range studied while AJ0390 increases with increasing pH.
Discussion Photoreactions Sensitized by TiO2. Irradiation of Ti02 particles with UV light excites electrons from the material's valence band into the conduction band, leaving a vacancy or hole in the valence band. Conduction band electrons (E& = -0.1 V vs SCE; H20, pH 1)and valence band holes (Evb = 3.1 V vs SCE; HzO, pH 1)47 which avoid (47) Jaeger, C.
D.;Bard, A. J. J.Phys. Chem. 1979,83, 3146.
Langmuir, Vol. 6, No. 8,1990 1401
Ti09-Photosensitized Reactions
ologen, 2,4,5-trichlorophenol, tris(1,lO-phenanthro1ine)iron(II), and N,N,N',N'-tetramethyl-p-phenylenediamine have well-characterized OH radical adducts which 0.035 0 may allow us to differentiate between direct electron transfer and OH radical mediated oxidation mechanisms. 0 Transient Spectra. 12*-. Considering the transient 0.025 0 absorption spectra obtained in transmission (Figure l), I I diffuse reflectance (Figure 2), and pulse radiolysis experiment^,^^ we assign the transient absorption a t 400 nm observed by diffuse reflectance flash photolysis of Ti02 powder in aqueous KI to Iz'-. In the photoexcited Ti02 0.005 suspension, Iz'- is formed by oxidation of I- to I' followed 7.5 8.5 9.5 10.5 by reaction of I' with excess I-.57@ This oxidation reaction PH can proceed either by direct electron transfer or via OH Figure 13. pH dependence of AJo at 390 nm obtained by flash radical oxidation of I-. Both mechanisms yield the same photolysis of Ti02 powder, 1g/100 mL in 5.5 X 10-3 M methyl product, Iz*-. The difference in the order of decomposition viologen dichloride. kinetics, second order for the colloidal system and first order for the powder system, may reflect a difference in I 1 A pH=7.6 I the 12'- occupancy of the different particles. The secondorder kinetics of 12'- decay on colloidal TiOz may reflect the diffusional encounter of singly occupied particles. In the powder system, the first-order kinetics may reflect the two-dimensionaldiffusion of 12'- molecules confined to the same particle.s5 0.0 4.OE-5 2,4,5-Trichlorophenoxyl. The transient transmission 0.010B pH=8.4 absorption spectrum presented in Figure 3 is due to 2,4,5trichlorophenoxyl radical formed by direct electrontransfer oxidation of 2,4,5-trichlorophenol by photoexcited colloidal TiO2. The cation radical of 2,4,5-trichlorophenol deprotonates, yielding this phenoxy1 radical. The same radical has been generated by oxidizing the trichlo-0.001 0.0 4.OE-5 rophenoxide anion with azide radi~al.3~ The OH radical ""901 C pH=9.4 adduct of 2,4,5-trichlorophenol, which has an absorption maximum a t 320 nm, was not observed in these AJ experiments possibly because the Ti02 colloid absorbs strongly at wavelengths less than 390 nm. We have been unable to observe the phenoxy radical in diffuse reflectance flash photolysis experiments. When -0.001 M 00 4.X-5 investigating aqueous suspensions of Ti02 in our Time (s) laboratory, we have been restricted to the observation of Figure 14. A J at 390 nm vs time data obtained by flash photransients with homogeneous extinction coefficients greater tolysis of Ti02 powder, 1 g/100 mL in 5.5 x 10-3 M methyl vithan about 7000 M-' cm-I. This limitation is imposed by ologen dichloride at (A) pH 7.6,(B)pH 8.4, and (C)pH 9.4. the sample preparation details, apparatus geometry, photoreaction efficiency, a n d t h e sensitivity of our rapid recombination reactions may be further separated instrumentation. The extinction coefficient of the pheby the chemical potential gradient that exists near the noxyl radical at 430 nm is 3600 M-l cm-l. particle surface. In electrolyte solutions, this gradient Methyl Viologen Cation Radical. Methyl viologen drives the strongly oxidizing holes toward the particle is reduced in basic suspensions of photoexcited TiOz. It surface where they participate in oxidation reactions. The has a pH-independent reduction potential of -440 mV vs weakly reducing electrons are forced away from the surface NHE, while the pH-dependent conduction band position into the bulk of the particle and later may be transferred of Ti02 shifts negatively by 59 mV/pH unit as the pH is to adsorbed oxygen. i n ~ r e a s e d .As ~ ~a result, a pH near 8 is required for the In this study, we have focused on reagents with known reduction of methyl viologen to occur. The instability of radiation chemistry. This allows us to use the analogy colloidal Ti02 in this pH range prevented us from recording between transient diffuse reflectance absorption spectra a transient transmission absorption spectrum. The and transient transmission absorption s p e ~ t r a ~ ~to- ~ O transient diffuse reflectance absorption spectrum that we identify the species formed in Ti02 powder sensitized have recorded, Figure 4, is similar to the spectrum of the reactions. Several of the reagents, particularly, methyl vimethyl viologen cation radical measured by pulse r a d i o l y ~ i s . ~However, ~ . ~ ~ in the spectra observed by pulse radiolysis no band at 480 nm is present. The OH radical (48)Ikeda, N.;Imagi, K.; Masuhara, H.; Nakashima, N.; Yishihara, K. Chem. Phys. Lett. 1987, 140, 281. adduct of methyl viologen does, however, have an (49)Wilkinson, F.;Willsher, C. J. Appl. Spectrosc. 1984,38, 897. absorption maximum in this region. This wavelength (50) Wilkinson, F.;Willsher, C. J. Chem. Phys. Lett. 1984, 104, 272. (51)Duonghong, D.; Ramsden, J.; Graetzel, M. J. Am. Chem. Soc. 1982, ~
n
-' I
n
~~
104.2977. (52)Bahnemann, D.;Henglein, A.; Spanhel, L. Faraday Discuss. Chem. Soc. 1984, 78, 151. (53)Bahnemann, D.; Henglein, A.; Spanhel, L. J. Phys. Chem. 1984, 88,709. (54)Bahnemann, D.W.;Fischer, C.-H.; Janata, E.; Henglein, A. J. Chem. Soc., Faraday Trans. I 1987,83, 2559.
~~
~
(55)Frank, A. J.; Graetzel, M.; Kozak, J. J. J. Am. Chem. Soc. 1976, 98, 3317. (56)Hatlee, M.D.;Kozak, J. J.; Rothenberger, G.; Infelta, P. P.; Graetzel, M.J. Phys. Chem. 1980,84, 1508. (57)Harvey, P. R.; Rudham, R. J.Chem. Soc., Faraday Trans. I 1988, 84,4181. (58)Moser, J.; Graetzel, M. Helu. Chim. Acta 1982, 65, 1436.
1402 Langmuir, Vol. 6, No. 8, 1990
maximum, 480 nm, is also in the same region as that of trapped holes,S2,53which might also be formed in these experiments, so no unambiguous assignment of this species may be made based on transient absorption spectra. It is intriguing to note that Bahnemann has isolated hydroxylated viologens from irradiated aqueous suspensions of TiOz." Table 111and Figures 13 and 14 further illustrate the utility of diffuse reflectance flash photolysis and the Albery model for investigating reactions in aqueous suspensions of TiO2. Above pH 8, the yield of MV'+ is a roughly linear function of pH, Figure 13. This finding contrasts with the pH dependence of MV*+yield observed in acidic colloidal solutions.51 In these solutions, a log linear relationship between MV*+ yield and pH was observed. We have observed no effect of pH on the average rate constant for MV*+decay. This is consistent with the idea that MVo+ is being oxidized by excess 0 2 to MV2+. Pt(I11). The transient diffuse reflectance absorption spectrum presented in Figure 5 is similar to those reported for Pt(II1) species prepared by pulse radiolysis of aqueous PtCb2-.42,43In photoexcited aqueous suspensions of TiOz, Pt(II1) may be formed when holes or OH radicals at the Ti02 surface oxidize adsorbed PtC1d2-. F e ( ~ h e n ) ~ 3 As + . in the PtCL2- experiments, Fe(phen)a2+is also oxidized at the photoexcited Ti02 surface, Figure 6. In this case, however, the oxidation only occurs at pH values greater than about 6. This requirement for oxidation to occur reflects the necessity of reagents being adsorbed on the Ti02 particle surface. Below pH 6, the surface of the Ti02particle is positively charged and repels cations such as Fe(phen)S2+. Above pH 6, the particle surface is negatively charged allowing adsorption of the dication. Once again oxidation appears to occur via direct electron transfer. In this case, leading to a bleaching in the transient spectrum. The substrate, Fe(phen)3(C104)2, has a maximum absorption at about 510 nm, and Fe(phen)s3+ is transparent at 510 nm.44 The OH radical adduct of Fe(phen)s2+which has an absorption maximum at about 440 nm is not observed. N,N,N,iV'-Tetramet hylene-p-phenylenediamine Cation Radical. The transient diffuse reflectance absorption spectrum observed after photoexcitation of Ti02 powder suspended in acetonitrile containing TMPD, Figure 7, is identical with that observed for the TMPD cation radical, TMPD'+. TMPD*+can be generated by pulse radiolysis by reaction of OH radical with TMPD followed by elimination of OH-.45 In photoexcited Ti02 powder suspensions in acetonitrile, TMPD'+ is probably formed by direct electron transfer oxidation since it is present immediately after the excitation pulse; the OH radical adduct was not observed. Thianthrene Cation Radical. Figure 8 presents the transient diffuse reflectance absorption spectrum of thianthrene cation radical. Direct electron transfer oxidation of thianthrene by photoexcited Ti02 powder leads to this cation radical. This transient diffuse reflectance absorption spectrum is similar to the transmission absorption spectrum observed when thianthrene is dissolved in 96 5% H2SO4. One-electron oxidation occurring in this solution is responsible for cation radical formation.46 Kinetics. Intensity Dependence. When transient transmission absorption measurements are recorded, the proportionality between optical density change and transient concentration can be assumed. Our observation of a linear relationship between transient diffuse reflectance absorption signals, AJO, and laser intensity, Figure 9,
Draper and Fox
suggests the signals we observe are proportional to transient concentration. In this figure, the concentration of M V + is proportional to laser energy for intensities between 2 and 5 mJ/pulse. The intensity dependence of reaction kinetics can also be used to distinguish between first- and second-order kinetics. If a transient lifetime depends on the initial amount of the transient present, then second-orderkinetics are indicated. If there is no concentration dependence of transient lifetime, then first-order kinetics are indicated. Since the normalized kinetic data, obtained when different amounts of 12'- and MV*+are generated by varying laser intensity, are superimposable, Figure 10,we conclude that the reactions of 12'- and MV'+ follow first-order kinetics. The most striking observation one makes on further analysis of the kinetic data is the nonlinearity of the data when it is plotted according to an integrated first-order rate law.' The Albery model discussed earlier accounts for this behavior using two adjustable parameters. In an earlier paper,l we have argued that the firstorder kinetics observed for reactions occurringon the Ti02 powder surface result from the reduced dimensionality encountered as transient species diffuse on the surface of a particle. This argument relied on an analogy between the dismutation of Br2.- on the surface of cationic micelles observed by Frank55and the disproportionation of (SCN)2'- on the surface of Ti02 powders. First-order kinetics for such surface reactions has also been predicted by Kozak.56 He considered the diffusion of energy donoracceptor pairs on a two-dimensional, spherical surface.
Conclusions After a survey of a variety of electron-transfer reagents reacting with photoexcited TiOz, we conclude that many oxidation reactions appear to occur by direct electron transfer. The possible exception to this is methyl viologen. The observation of an absorption band at 480 nm in the transient diffuse reflectance absorption spectrum of MV*+suggests an OH radical adduct may be formed in this system. Otherwise, no spectroscopic evidence for OH radical mediated oxidation has been observed on the time scales and within the wavelength range available to us. The reagents used in this survey were chosen because their radiation chemistry in water is characterized, making a comparison between transient diffuse reflectance absorption spectra and transient transmission absorption spectra possible. The assignments of transient identity which we have made have been based largely on these comparisons. In the powder systems, the kinetics of each transient's decay could be modeled by using the Albery model for dispersed first-order kinetics. This suggests that the combination of diffuse reflectance flash photolysis and the Albery kinetic model will be powerful tools for analyzing photoreactions occurring on inexpensive, commercially available semiconductor powders.
Acknowledgment. These experiments were performed at the Center for Fast Kinetics Research (CFKR) at the University of Texas at Austin. The CFKR is supported jointly by the National Institutes of Health (DRR00886) and the University of Texas at Austin. We are grateful for invaluable discussion with Mr. Jeff Byers, Dr. Stephen Atherton, and Dr. Elizabeth Gaillard. This work was supported by the National Science Foundation and the Robert A. Welch Foundation.
