The Journal of
Physical Chemistry
0 Copyright, 1985, by the American Chemical Society
VOLUME 89, NUMBER 12
JUNE 6, 1985
LETTERS Direct Observation of the Variation of Energy Levels In Powdered TiOp as a Function of Temperature. Beneficial Effects for Energy Conversion through Semiconductor Devices J. Kiwi Institut de Chimie-Physique, Ecole Polytechnique Fgdgrale, CH- 101 5 Lausanne, Switzerland (Received: November 7, 1984; In Final Form: February 4, 1985) The study reports in detail the change in the relative position of the Fermi level during photocatalysis in which Ti02 powder is used as a photon absorber and support for Pt-metal islands. This change is relatively small and of the order of 0.04 V when the temperature is varied from 21 to 75 O C . Favorable interfacial electron-transferkinetics is observed when the temperature is increased. With Pt-metal-loaded Ti02 Degussa P-25 dispersions, it was also observed that the edge of the valence band varies as a function of the temperature of irradiation. The pH above which CH4 formation is prevented relates to the critical potential of the oxidation of CH3COO- in the reaction media.
Introduction Particulate suspensions of T i 0 2 have recently been shown to be useful as photocatalysts for conversion of light into chemical energy.' It has also been reported that platinized T i 0 2 is active in the oxidation of cyanide,2 sulfite? acetate," and hydrocarbons.5 More recently, studies on P t / T i 0 2 redox catalysts6 have shown that temperature exerts a beneficial influence on the observed H2 yields of photoinduced water cleavage.' Several reasons were given for the observed effect such as: (a) the decrease in H2 (1) (a) Bard, A. J. Science 1980,139,207. (b) Kiwi, J.; Kalyanasundaram, K.; Grltzel, M. Structure Bonding (Berlin) 1982,49, 37. (c) Harriman, A., West, M., Eds. "Photogeneration of Hydrogen"; Academic Press: London, 1982. (d) Gratzel, M., Ed. "Energy Resources from Photochemistry and Catalysis"; Academic Press: New York, 1983. (2) Frank, S.;Bard, A. J. J . Am. Chem. SOC.1977, 99, 303. (3) Frank, S.; Bard, A. J. J . Phys. Chem. 1977,81, 1484. (4) Krauetler, B.; Bard, A. J. J. Am. Chem. Soc. 1978, 100, 5985. (5) (a) Sato, S.;White, J. Chem. Phys. Lett. 1980, 70, 131. (b) Kawai, T.; Sakata, T. Nature (London) 1980, 286, 474. (6) Kiwi, J.; Borgarello, E.; Pelizzetti, E.; Visca, M.; Grltzel, M. Angew. Chem., Int. Ed. Engl. 1980, 19, 646. (7) (a) Borgarello, E.; Kiwi, J.; Pelizzetti, E.; Visca, M.; Gratzel, M. J . Am. Chem. Soc. 1981,103,6324. (b) Kiwi, J. Chem. Phys. Lett. 1981,83, 594.
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concentration in solution with increasing temperature,' (b) the increased energy available to overcome the activation energy necessary for the reaction and, (c) the higher transfer coefficient for H+ reduction at these temperatures.8 This study has been carried out in order to further clarify the beneficial temperature effect observed on H2 production reported in Figure 7 (ref 7a). The effect of temperature observed for CHI evolution (via the photo-Kolbe reaction)8 provides direct information on the energy level of the valence band (vb) under i r r a d i a t i ~ n . ~The observed temperature effect can be correlated to the production rate of CH+ This important aspect has, until now, not been addressed.1°
Experimental Section Conduction band measurements were carried out as shown in Figure 1 according to an electrochemical method9 of collecting the photogenerated charge on a Pt-flag electrode immersed in the irradiated suspensions. The solution used had a composition as (8) (a) Kraeutler, B.; Bard, A. J. J . Am. Chem. SOC.1977, 99,7729. (b) Kraeutler, B.; Bard, A. J. J. Am. Chem. SOC.1978, 700, 2239. (9) Ward, M.; White, J.; Bard, A. J. J . Am. Chem. SOC.1983, 105, 27. (10) Sakata, T.; Kawai, T.; Hashimoto, K. J . Phys. Chem. 1984,88,2344.
0 1985 American Chemical Society
2438 The Journal of Physical Chemistry, Vol. 89, No. 12, 1985
t
Letters in pHo as function of temperature is then described by the following equation:
2oL
* 75-c n
l6
Figure 1. Dependence of rate of the change of photocurrent with time ( A i l a t ) on T for stirred and N,-purged Ti02-P25 Degussa suspension photocell. Cell conditions: TiO, powder (250 mg); HzO (100 mL); NaOAc = 1.0 M; K N 0 3 0.1 M ; MVz+ = 1 mM. Platinum collector electrode at -0.20 V vs. SCE. Light source, Osram X e 150-W lamp (100 mW/cm2).
t
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0
$+j. *--*-A 1I
\ !
1 2
8
1 4
,
8
6
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1
0
Figure 2. Dependence of methane production rate on pH for photocatalytic oxidation of acetic acid in a solution water-acetic acid mixture (solution contained: 3 mL of acetic acid, 20 mL of water) with 50 mg of 0.05%FY/Ti02 added. The pH of the mixture was adjusted by adding N a O H and did not change after a 2-h run. Light source as in Figure 1.
described in the legend to Figure 1. Our reaction vessel was a two-compartment cell of the same design and volume as employed in ref 9. The voltage variation induced under irradiation was followed with a recorder which was fit to register current variation by means of an appropriate resistance. Irradiations reported in Figure 2 were carried out under oxygen-free conditions using a 100 mW/cm2 Rofin Xe lamp. Irradiation flasks were 25 cm3 in volume. Methane was analyzed by a Gow Mac thermal conductivity detector provided with a Carbosieve 5-A column. Details of the photolytic experiments have been reported previously.' Catalyst loading of Ti02-P25 Degussa by Pt via ion exchange from Pt(NH3)4(0H)2was used.Id Pt/Ti02 powder catalysts (50 mg, 0.05%) were used in all runs. The present irradiations were carried out under the same setup as already described in the literature for photolytic CHI generation.I0 Results and Discussion Figure 1 shows determination of the conduction band as a function of temperature for Ti02-P25 Degussa slurries according to an electrochemical method already des~ribed.~ In this method, the photogenerated charge is transferred from the semiconductor to an acceptor (methylviologen) which is reoxidized on a Pt-flag electrode immersed in the irradiation suspensions. The signal corresponds to the voltage variation originating from changes in the levels of the induced photocurrent under irradiation when connected through an appropriate resistance. An extrapolation of the two straight lines Ai/At asf(pH) for the small and large changes in A i / A t enables one to obtain a point pHo. This intersection point pHo represents the pH value in eq 1. The change
Ef(pH=O) + 0.66 pH' = 2.303kT/e when the Fermi level Ef(V vs. SCE) equals the redox potential of the couple MV+,/MV+ (MV', methylviologen reduced form). From the pHo values obtained in eq 1 and the knowledge that MV+,/MV+ redox potential is -0.66 V vs. SCE, relatively small changes in the El(pH=O) values of -0.22, -0.24, -0.25, and -0.26 V were obtained for TiO, runs at 21, 35, 55, and 75 "C, respectively. Improved reaction kinetics at higher temperature has been reported to have favorable effects on the interfacial rates observed for electron transfer from the conduction band of TiO, to MV'.'' This electrqn transfer has been reported to be slow at room temperature and to increase with increasing temperature. The energy levels of the Ti0, have been shown then to depend on temperature.], The shifts in the values obtained for pHo would then be reflected in the potential shifts as seen in eq 1. In the present experiments a perturbation of this equilibrium arises then through a change of temperature of the system in which the fractional occupancy M E ) ) of any level of energy, E, is given by the expression:
The results cited in eq 1 would then mean that the fractional occupancy of a given energy level is changing with temperature according to eq 2. I f different temperatures are applied to the system, the equilibrium distribution function-giving the distribution of the electrons between allowed energy levels-will be disturbed from its equilibrium value.12c Since the band gap for wide gap semiconductor Ti02-P25Degussa (70%anatase and 30% rutile)I3 is 3.0 eV, the expected shift in the energy levels with temperature is small. This is shown by the results obtained in Figure 1. It is also known that for a semiconductor the number of current carriers increases with temperature, since the activation energy to move electrons in the band gap is more readily available. The product of negative and positive carriers is a function of temperature (Le., see ref 12a, p 19). On the other hand, temperature has also been known to modify the band gap of semiconductor materials.12dIn CdS it has been noted12athat the band gap varies from 2.7 to 2.4 eV when the temperature increases from 0 to 300 K. This is not a negligible change when considered on a millielectronvolt/K basis. Also, an increase in temperature has been reported to influence the band bending in Ti02.12CThis variation is difficult to quantify at the TiO,/electrolyte interface since band bending will also vary with the type of adsorbed molecule. In our system (Figure 1) we have donor and acceptor species diffusing toward particulate TiO, (and combination of these species). Therefore, the electron transfer taking place at the interface will depend upon the relative positions of the energy levels of the separate phases. The equilibrium carrier densities and the thermal velocities of the carriers have both been shown to be functions of the temperature.'J2 More recently, van Damme,14 Somorjai," LehnI6 and Satol' have reported UV-induced water photocleavage on T i 0 2 and SrTi03, when experiments are run at 100 OC using a relatively high-power UV arc.' These experimental results lend further
support to our observation of increased interfacial electron-transfer ( 1 1 ) (a) Moser, J.; Grltzel, M. J . Am. Chem. SOC.1983, 105, 6547. (12) (a) Smith, R. A. "Semiconductors";Cambridge University Press: Cambridge, 1976. (b) Gerisher, H. Discuss.Faraday SOC.1980, 70,416. (c) F y , N . 'Semiconductors"; Reinhold: New York, 1970. (d) Kittel, C. Introduction to Solid State Physics"; Wiley: New York, 1976. (13) (a) Kiwi, J.; Borgarello, E.; Pelizzetti, E.; Visca, M.; Gratzel, M. In "Photogenerationof Hydrogen"; Academic Press: London, 1982. (b) Rao, V.; Rajeshwar, K.; Vernecker, V.; Dubow, J. J . Phys. Chem. 1980.84, 1987. (14) Van Damme, H.; Hall, K. J . Am. Chem. SOC.1979, 101, 4373. (15) Lo, W.; Chung, Y.; Somorjai, G. Surf. Sci. 1978, 71, 199. (16) Lehn, J-M.; Sauvage, J.; Ziessel, R.; Hilarie, L. Isr. J . Chem. 1982, 22, 168. ( 1 7 ) Yamaguti, K.; Sato, S. Proc. Int. Congr. Catal. 8th 1984, 3, 477.
J . Phys. Chem. 1985,89, 2439-2442 kinetics at higher temperature. A favorable shift in semiconductor powders increasing their activity in water decomposition processes led to higher yields for H2 evolution processes as previously reported.' The implication for such an observation is that if semiconductor powders become available for processes under sunlight irradiation (where the accompanying heat effect is not negligible) cooling of such devices would not be necessary for efficient H2 generation. Figure 2 shows the dependence of the CH4 production rate on p H and temperature. Experimental conditions are described in the Figure 2 legend. At neutralalkaline pH, CH4evolution stops. As seen from this figure, CH4 is not produced above pH values of 7.0, 7.4, 8.0, and 8.4 when experimental runs were carried out at 21,35,55, and 75 OC, respectively. The pH value above which CH4 formation is prevented relates presumably to the critical potential of the oxidation of CH3COO- as recently reported.'O The edge of the valence band as function of temperature would be given by the expression'*-20 Uvb = 2.80 - 2.303(KT/e)pH V VS. N H E (3) From the observed pH dependence of CH4production from acedtic acid, the valence band of T i 0 2 is estimated to be located at 2.39 and 2.21 V vs. N H E at 21 (pH 7) and 75 OC (pH 8.4), respectively. Therefore, at alkaline pH, the oxidation of CH3COO.OH) becomes difficult and the oxidation of OH- (OH- ht begins to predominate. When the experimental setup reported in Figure 1 is used for the determination of the Fermi energy ( E f ) position, no CH4 evolution was observed. When the T i 0 2 vb position is sufficiently positive to oxidize CH3COO-, its potential is taken as the oxidation potential on the T i 0 2 surface in the reaction media. It was necessary to use a noble metal loaded Ti02 dispersion (Figure 2) and the photocatalyst used for these reactions was 0.05% Pt/Ti02.21 The process catalyzed by the Pt deposit CH3. C 0 2 ) with subsequent forms CH, (CH3COO- ht reaction of the CH3. radical and/or reaction with adsorbed H atoms.I0 The liquid-phase photo-Kolbe reaction taking place produced the following gases: CH,, C 0 2 , H2, and C2H6. The
+
+
(18) (19) (20) (21)
-
-
+
Harris, W.; Wilson R. Annu. Rev. Mater. Sci. 1978 8, 99. Hardee, K.; Bard, A. J. J. Electrochem. SOC.1977, 124, 215. Tomkiewicz, M. J. Electrochem. SOC.1979, 126, 1505. Kiwi, J.; GrHtzel, M. J . Phys. Chem. 1984, 88, 1302.
2439
last two products were produced in relative small amounts. Irradiation of acetic acid or acetic acid-water systems as shown in Figure 2 showed different rates of gaseous product evolution. Enhancement of the observed products took place upon addition of water. This experimental observation suggests the existence of other reactions in which water is involved. Enhanced product formation from acetic acid-water mixtures may result from increased [H+] formation.22 Non-Kolbe type reactions of acetic acid23taking place lead to barely detectable amounts of methanol and ethanol under the present experimental conditions. Complications in the cooxidation of acetate and water on T i 0 2 powders have been studied and reported e l ~ e w h e r e . ~In~the ~ , ~absence of a strong electron acceptor, as is the case for the chemical system shown in Figure 2, the main reaction pathway for CHI formation has been shown to be the methyl radical reaction with H atoms generated on PtBb The results shown in Figure 2 could also be interpreted in terms of the negatively charged CH3COO- ion adsorbed at pH C 6.6 (I.E.P. Degussa P-25 Ti02). It is interesting to note that CH4 formation begins to be observed only below pH 7.0 where electrostatic attraction between the semiconductor and acetate takes place. It therefore seems that strong acetate adsorption on Ti0224 is beneficial for CH4 production. In conclusion, the first experimental evidence for the variation of band energy levels of powdered T i 0 2 semiconductor particles with temperature has been presented. This observation accounts partially for the increased H2 evolution rate which has been reported for the semiconductor particles at high temperature.'
Acknowledgment. This work was supported by the Swiss National Science Foundation. Helpful discussions with Professor M. Gratzel during the preparation of the manuscript were apprecia ted . Registry No. T i 0 2 , 13463-67-7; Pt, 7440-06-4; CHI, 74-82-8; CH3COzH, 64-19-7. (22) Jaeger, C.; Bard, A. J. J. Phys. Chem. 1979, 83, 3146. (23) (a) Hofer, H.; Moat, M. Ann. Chem. 1902,323,284. (b) Beck, F. "ElektroorganischeChemie"; Verlag Chemie: Weinheim, 1974. (c) Kraeutler, B.; Bard, A. J. Nouv. J. Chim. 1979, 3, 31. (24) (a) Koudelka, M.; Sanchez, J.; Augustynski, J. J. Phys. Chem. 1982, 86,4277. (b) Parfitt, R.; Fraser, A.; Russell, D.;Farmer, C. J. Soil Sci. 1977, 28, 40.
Ultraviolet Multiphoton Ionlzatlon of the Pyrene-Blphenyl-Isooctane System: Direct Observation of Short-Lived Ion Palrs of Pyrene Catlon and Blphenyl Anlon Yoshinori Hirata* and Noboru Mataga* Department of Chemistry, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan (Received: February 8 , 1985)
The multiphoton ionization process of the pyrene-biphenyl-isooctane system has been investigated by using the picosecond and nanosecond laser spectroscopy techniques. Short-lived (within tens of picosecond) ion pairs which consist of the pyrene cation and the biphenyl anion radical have been observed directly. The lifetime of the ion pair is lo4 times smaller than that of the geminate pair observed by pulse radiolysis studies. In addition to the very short-lived ion pairs, long-lived free ions have been detected by transient photoconductivity measurements. These findings cannot be explained by ordinary diffusion-controlled theory which can reproduce satisfactorily the radiation chemical primary process of similar systems.
Introduction Under irradiation with an intense laser P&, nodnear processes such as two-photon absorption become important,' Sometimes ionic species are produced efficiently as a result of nonlinear absorption.24 On the other hand, by using a mode-locked laser (1) Miyasaka, H.; Masuhara, H.; Mataga, N. Laser Chem. 1983, I , 357.
(2) Hirata,
Y.;Mataga, N. J. Spectrosc. SOC.Jpn., in press.
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as an excitation source, it is possible to study the ultrashort dynamic behaviors of excited species. However, since the wavelength of such laser light is restricted to the visible and near-ultraviolet region in most cases, it is not easy to conduct vacuum-ultraviolet (3) Miyasaka, H.; Masuhara, H.; Mataga, N. Chem. Phys. Lett. 1983.98, 277. (4) Miyasaka, H.; Masuhara, H.; Mataga, N. J. Phys. Chem. in press.
0 1985 American Chemical Society
