8222
J . Phys. Chem. 1990, 94, 8222-8226 spin probes, supporting the formation of a clathrate structure around the probe itself. The analysis of the ESR parameters shows that the addition of urea to micellar solutions produces a decrease of the polarity and a strong increase of the microviscosity of the micellar interface. These effects are dependent on the urea concentration. The above results suggest that urea solubilizes at the micellar surface and penetrates below the surfactant polar headgroups by replacing some water molecules that solvate the hydrophobic chain and the polar headgroup of the amphiphile. These results provide information at a molecular level of the mechanism of action of urea and are in agreement with molecular dynamics and Monte Carlo calculations and with a recent Raman study on the water/acetone/urea ternary system in which it is postulated that the main mechanism of the urea action is a direct interaction with the organic molecule.49
the Temp-TMA+/SDS and C8-TEMPO/SDS systems, that urea solubilizes at the micellar surface. Further evidence comes from the results obtained with x-DSA spin probes. Table I shows that the ( A N )parameter of x-DSA probes is almost insensitive to urea addition while the correlation time increases from 20% to 30% depending on the surfactant and urea concentration. Doxylstearic acids spin probes are located deeper inside the micelle with respect to C8-TEMP0.35 In fact the deuterium modulation depth, as measured by using the electron spin echo modulation technique, is greater for C8-TEMP0 than for x-DSA for both SDS/D,O and DTAB/D20 micellar solutions, showing that this probe is more exposed to water.3s In particular x-DSA probes are located below the surfactant polar headgroups. The above results suggest that urea penetrates below the surfactant polar headgroups. This is in agreement with structural investigations of the micellar interface by ESEM.35
Acknowledgment. Thanks are due to L. Kevan for helpful discussions. Thanks are also due to MRST (Minister0 della Ricerca Scientifica e Tecnologica) for financial support. Registry No. SDS, 15 1-2 1-3; DTAB, I I 19-94-4; Temp-TMA',
Conclusions
The use of nitroxide spin probes that due to the different kind of interactions (electrostatic or hydrophobic) with SDS or DTAB micelles are located in different regions of the micellar interface allows one investigation of the site of the interaction of the urea molecule with the micelle. It is shown that the addition of urea to premicellar solutions of SDS and DTAB leads to a decrease of the microviscosity and of the polarity sensed by the nitroxide
64525-01-5; C,-TEMPO, 126328-27-6; 5-DSA, 29545-48-0; I2-DSA, 29545-47-9; 16-DSA. 53034-38- I ; urea, 57- 13-6. (49) Mizutani, Y.; Kamogawa, K.; Nakanishi, K. J . Phys. Chem. 1989, 93, 5650.
Charge-Carrier Dynamics in TiO, Powders Karl-Michael Schindler and Marinus Kunst* Abteilung Solare Energetik, Hahn- Meitner-Institut, Glienicker Strasse 100, 0 - 1 0 0 0 Berlin 39, FRG (Received: December 13, 1989; In Final Form: April 3, 1990)
Contactless transient photoconductivity measurementsin TiO, powder are presented. Large differences between the modifications anatase and rutile are observed with anatase exhibiting higher yields and longer lifetimes of photoinduced excess charge carriers. It is shown that surface modification drastically changes the electron decay kinetics, where adsorption of 2-propanol and of tetranitromethane leads to a lower and a higher decay rate, respectively. Platinization opens an additional fast decay channel. The results are discussed in view of their relevance for photocatalysis.
1. Introduction
Titanium dioxide powder has two important applications, as white pigment] and as photocatalyst.* The most important modifications of TiOz are rutile and anatase.' Anatase, which is metastable and transforms into rutile at higher temperatures,' is produced by precipitation of Ti4+compounds.' The structures consist of slightly distorted octahedrons of oxygen atoms around a titanium atom, where the modifications differ in the connection of the octahedrons.' The two applications as pigment and as photocatalyst are complementary, and the kinetics of the light-induced excess charge carriers is of fundamental importance for both applications. In the first case fast recombination of the excess charge carriers and inhibition of the charge transfer from the powder to possible reactants are required to prevent the photocatalytic degradation of the polymeric binder, which leads to "chalking" of the paint.3 In the second case slow recombination of the excess charge carriers and an efficient charge transfer of these carriers from the powder to the reactants are necessary for a high photocatalytic activity of the powder. In general, it is found that rutile is photocata-
lytically less active than a n a t a ~ e . ~ In this work transient photoconductivity measurements are chosen as a tool for the investigation of excess charge carrier kinetics in titanium dioxide powder. A contactless transient photoconductivity method, the time-resolved microwave conductivity (TRMC) method, is used to avoid contact problem^.^ This method was already successfully applied to a number of semiconductor powders.68 The kinetics of mobile charger carriers in powders is a relatively new field of research, and the interpretation of the decay of the photoconductivity is complicated. Therefore, it is convenient to study the influence of a small structure perturbation of the powders on the decay of the photoconductivity. This approach has the advantage that these treatments lead to changes of the decay behavior, which in general can be interpreted more easily. As photocatalysis implies, the reactions between photoinduced charge carriers and molecules adsorbed occur at the surface, and the most interesting way to modify the structure of the particles is to change their surface (4) Brown, J. D.; Williamson, d. L.; Nozik, A. J. J . Phys. Chem. 1985,89, 3076.
(1) Ullmanns Encyklopldie der technischen Chemie, 8. Auflage; Verlag
Chemie: Weinheim, 1983; Vol. 18, p 570 f. (2) Photoinduced Electron Transfer; Fox, M. A,, Chanon, M. Eds.; Elsevier: Amsterdam, 1988; Part D. (3) vo17. H. G.;Kampf. G.: Klaeren, A . Farbe Lack 1976. 82, 805.
0022-3654/90/2094-8222$02.50/0
(5) Kunst, M.; Beck,G. J . Appl. Phys. 1986,60, 3558. (6) Warman, J. M.; de Haas, M. P.; Gritzel, M.; Infelta, P. P. Nature 1984, 310, 306-308. (7) Fessenden, R. W.; Kamat, P. V. Chem. Phys. Lett. 1986,123,233-238. (8) Dobbin, C. J.; McIntosh, A. R.; Bolton, J. R.; Popovic, 2. D.; Harbour, J R J Chem Soc., Faraday Trans. I 1986, 82, 3625-3633.
0 1990 American Chemical Society
The Journal of Physical Chemistry, Vol. 94, No. 21, 1990 8223
Charge-Carrier Dynamics in Ti0, Powders structure. The surface modifications applied in the present work have been chosen in view of their relevance to photocatalytic processes of Ti02 powders, Le., platinization and adsorption of 2-propanol and tetranitromethane.
TiOZ
266 nm EXCITATION
AnataselRutile
2. Experimental Section The time-resolved microwave conductivity (TRMC) method is based on the measurement of the relative change A P ( t ) / P of the microwave power reflected from the sample. The change is caused by a change Au of the conductivity of the sample which is induced by a laser flash. For small perturbations a proportionality between A P ( t ) / P and Au was derived? A P ( t ) / P = AAu(t) = AeCAni(t)pi i
(1)
where A is a time-independent proportionality factor, An,(t) the change in carrier concentration, and pi the respective effective carrier mobility. This equation can be further reduced to mobile electrons in the conduction band and holes in the valence band, because the contribution of every species is weighted by its mobility and trapped charge carriers can be neglected due to their small mobility: A P ( t ) / P = A A u ( t ) = Ae(An(t)p,
+ Ap(t)ph)
(2)
where An(t) is the excess electron concentration, pn is the electron mobility in the conduction band, and A p ( t ) and C(h are the corresponding properties of holes. The signal A P ( t ) / P obtained by this technique will be called (microwave) photoconductivity. If An(t) IA p ( t ) , the change A u ( t ) of the conductivity can be attributed to excess electrons in the conduction band, because the electron mobility p, in TiO, is much larger than the hole mobility phr9
A u ( t ) = An(t)p,
(3)
Any process that decreases the number of excess electrons in the conduction band leads to a decay of the photoconductivity. This includes electron trapping, as contributions of trapped electrons to the photoconductivity can be neglected due to the small mobility of trapped electrons. Although trapping of electrons at Ti4+sites was measured by EPR spectroscopy’0 and time-resolved UV/VlS spectroscopy,” the relevance of this process for photocatalysis is probably small, as the energetic depth of the resulting Ti3+sites relative to the conduction band is only -70 meV. At room temperature this energy difference is small enough to establish an equilibrium distribution between the trap states and the conduction band, with the majority of excess electrons in the conduction band. The transient photoconductivity data will be displayed on double-logarithmic plots. Although this way of representation can obscure details of the decay processes, it is convenient because of the large signal and time range covered by the photoconductivity decay in TiOz powder. This way of representation is often used for transient photoconductivity measurements in amorphous semiconductors12, where the extended decay of the transient photoconductivity is related to the presence of broad exponential band tails within the semiconductor band gap (mobility gap). Excess charge carriers were produced by illumination with 20-ns (full width at half-maximum) pulses of a Nd:YAG laser (JK Lasers) at 266 nm. The laser intensity was varied by UGl1 filters (Schott). The T R M C experiments were performed on a Ka-band (28.5-40GHz) homemade equipment as described previou~ly.~ An increase in sensitivity is obtained by inserting the sample into a microwave cavity. The influence of adsorption of 2-propanol and tetranitromethane on Ti02 was studied by adding a sufficient amount of the liquid (9) Fonash, S. J. Solar Cell Deoice Physics; Academic Press: New York, London, 1981. (IO) Howe, R. F.; Grlitzel, M. J. Phys. Chem. 1987, 91, 3906-3909. ( I I ) Rothenberger, G.; Moser, J.; Gratzel, M.; Serpone, N.; Sharma, D. K.J . Am. Chem. S o t . 1985, 107, 8054-8059. ( I 2) Tiedje, T. Semicond. Semimet. 1984, 2/C, 207.
gg loo
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Figure 1. Photoconductivity of TiOz powders after a 2 0 4 s (fwhm) laser flash excitation at 266 nm: (a) anatase, 0.5 mJ/cm2; (b) rutile, 1 mJ/ cm2; (c) anatase, 0.1 mJ/cm2; (d) rutile, 0.2 mJ/cm2.
to wet the powder thoroughly. After addition the liquid was evaporated at room temperature until the reflected microwave power from the sample without illumination was stable. Platinization of Ti02 was achieved by mixing the appropriate amounts of a suspension of the powder and a colloidal solution of p1atin~m.I~ This mixture was evaporated at a pressure of about 30 hP. Most experiments were performed on the widely used variety of titanium dioxide that is commercially available, Le., P25 (Degussa).
3. Results and Discussion 3.1. Anatase/Rutile. For application of titanium dioxide in paints it is necessary to suppress the photocatalytic activity. As a consequence, the transfer of oxidizing species from the powder to the matrix has to be inhibited by isolating coatings of the grains such as SiO, or AI2O3or anatase has to be converted into rutile by a heat treatment,I4 since anatase powder was found to be photocatalytically more active than r ~ t i l e .This ~ difference between the two modifications usually is explained by the fact that the conduction band of anatase is about 0.3 eV more positiveI5 than that of rutile and as a consequence the excess electrons in the conduction band of anatase possess more “driving force” for the reduction of reactants than those of rutile.I6 The position of the valence band is comparable in both modifications. Therefore, the “driving force” for oxidations should be the same. Differences in photocatalytic activities for oxidations indicate differences in the kinetic behavior of excess charge carriers due to different charge carrier transport properties of the respective lattices. Whether the dynamics of the excess charge carriers plays a role for reductions was never considered. Figure 1 shows the transient change A P ( f ) / P of the reflected microwave power, Le., the transient microwave photoconductivity, after excitation of anatase (curves a and c) and rutile (curves b and d) powder by a 20-ns laser pulse at 266 nm. There are significant differences in the decay behavior of the two modifications. In the anatase powder the photoconductivity decays over an extended time range from nanoseconds to milliseconds. In the rutile powder the photoconductivity decays much faster. As mentioned above, the signal can be attributed to excess electrons in the conduction band, because the samples are n-doped and the electron mobility pn is much larger than the hole mobility p,, in both modifications. Therefore, the faster decay of the photoconductivity in rutile can be due to a faster deep-trapping rate ( 1 3) Bahnemann, D.; Henglein, A.; Spanhel, L. Faraday Discuss. Chem. SOC.1984, 78, I5 1. (14) Heller, A.; Degani, Y.; Johnson, D. W., Jr.; Gallagher, P. K. J. Phys. Chem. 1987, 91, 5987.
(15) Landolt-Bornstein Zahlenwerte und Funktionen aus Narurwissenschaff und Technik, Neue Serie; Madelung, O., Ed.;Springer Verlag: Berlin, Heidelber, New York, Tokyo, 1984; Gruppe Ill, Vol. 17g. (16) McLendon, G. Energy Resources through Photochemistry and Caralysis; Gratzel, M.. Ed.; Academic Press: New York, 1983; p 117.
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The Journal of Physical Chemistry, Vol. 94, No. 21, 1990
for electrons or a faster recombination rate of electrons and holes in rutile than in anatase. This difference between the two modifications was also observed with other samples from different sources, which points to specific differences between the two modifications caused by differences in their bulk or surface structure. The decay behavior observed in anatase (Figure 1 ) is complicated and cannot be modelled by a pure first- or secondorder decay process. It is described by a decay rate that decreases with time as was also observed in other semiconductor powders.!' As most of the bulk processes such as deep trapping, direct recombination, and recombination via a defect state obey first- or second-order kinetics, this kind of decay behavior indicates the importance of surface processes due to the relatively small volume-to-surface ratio in powders. The longer lifetime of excess electrons in anatase compared to that in rutile may explain, at least partially, the higher photocatalytic activity of anatase. I t was found by laser flash photolysis" that excess electrons are trapped in 5 2 0 ps in colloidal Ti02. This seems to contradict our results of long-lived conduction band electrons. As mentioned above, the energy gap between the trap states Ti3+ and the conduction band is small enough to establish an equilibrium distribution with most of the excess electrons in the conduction band. Unfortunately, neither of the two methods gives quantum yields for the respective species. Furthermore, it was found" that the lifetime of the trapped electrons is shorter in the colloidal Ti02 than the lifetime of excess electrons in our measurements. In the case of an equilibrium distribution they should be very similar. However, the applied laser intensities are considerably lower in our case. At high laser intensities trap-related decay channels get saturated and the second-order direct recombination becomes the main decay process as found with the laser flash photolysis. At the lowest laser intensities long-lived electrons were also found that decay over an extended time range comparable to our findings. These lower intensities are probably closer to the actual situation during photocatalytic reactions. An interesting fact is that the most successful commercial variety of T i 0 2 powder for photocatalytic applications, Le., P25, This implies in a simple picture consists of about 40% that the electrons created in the anatase part of the powder should be transferred to the rutile part because the conduction band level of anatase is more positive than the one of rutile, although this kind of consideration must be used with care as the details of the junction are not known. As a consequence, the photocatalytic activity of P25 for reductions (for example, hydrogen evolution) should be comparable to that of a rutile powder, but on the contrary, it is at least as active as anatase powders. The transient photoconductivity in P25 (Figure 2, curve b) is similar to the decay found by other groups6*' and shows an extended decay characterized by a power-law dependence on time (exponent -0.2), which is rather more like the decay behavior observed in anatase (Figure 1, curves a and c) than that in rutile (Figure 1, curves b and d). This confirms the previous assumption of a close contact of the rutile and anatase parts in P25. Completely separate particles would yield a linear superposition of the rutile and anatase signals. However, no fast rutile like decay of the photoconductivity was found. This can be explained by the following model: the short electron lifetime in rutile is due to a higher recombination rate whereas in anatase fast trapping of a part of the minority charge carriers (holes) may take place. This would reduce the availability of holes for recombination and decrease the recombination probability and finally lead to a long lifetime in anatase. In the mixed powder one would expect fast recombination like in rutile for the clectron-hole pairs created in the rutile part. However, the deep trapping of holes in the anatase part would prevent the ~
(17) Schindler, K.-M.; Kunst, M. Z . Naturforsch. 1988, 43a, 189-192. ( I 8 ) Munuera, G New Trends and Applications of Photocatalysis and Photoelectrochemistry f o r Environmental Problems; Schiavello, M., Ed.; NATO AS1 Series; in press. (19) Pichat, P.; Borgarello, E.; Disdier, J.; Herrmann, J.-M.; Pelizzetti, E.; Serpone. N. J Chem. Soc., Faraday Trans. I 1988, 84, 261. (20) Oosawa, Y.;Gratzel. M. J . Chem. SOC.,Faraday Trans. 1 1988.84, 197
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