J . Phys. Chem. 1991, 95, 8858-8861
8858
Effect of Isopropyl A k o h d on the Surface LocaNzathm and R e e " of Conduction-Band Electrons in Oegu6sa P25 no2. A pulee-aTlmbResdvod Microwave Conductivity Study John M. Warman,**+Mattbijs P. de Haas,? Pierre Picbat,*and Nick SerponeLs Radiation Chemistry Department, IRI, Deut University of Technology, Mekelweg 15, 2629 JB DELFT, The Netherlands; U.R.A ai( CNRS "Photocatalyse, Catalyse et Environnement", Ecole Cmtrale de Lyon, BP 163, 69131 Ecully Cedex, France: and Chemistry Department, Concordia University, MontrPal, Canada (Received: March 19, 1991)
Conduction-band electrons, formed by pulse radiolysis of Degussa P25 Ti02particles, have been monitored by time-resolved microwave conductivity and found to undergo equilibrium localization and eventual recombination a t the particle surface. In the presence of isopropyl alcohol recombination is retarded due to surface hole scavenging. The particle bulk can then be pumped with mobile electrons, which survive for seconds.
Introduction Titanium dioxide has been one of the most thoroughly studied semiconductor materials of the past decade.'+ Interest in this bulk chemical arose initially from the possibility of applying it as a photocatalyst for the light-induced decomposition of water. While the succesfull commercial application for solar energy conversion has remained elusive, the physicochemical knowledge gained from its study could eventually prove to be of importance for more general applications of photocatalytic oxidation and reduction pracesses at solid-fluid interfaces. In addition, because of the extremely widespread use of T i 0 2 as a white pigment, particularly in paints and polymers, the photochemistry occurring at the surface of particles of Ti02embedded in organic materials is of great importance in determining the degradation processes that limit the lifetime of decorative or protective layers exposed to sunlight. The sensible application in specific photocatalytic routes to fine chemical^,'.^ to the recovery of precious metal^,^^^ and to photodegradation of noxious pollutants7~*will also be dependent on a full understanding of the bulk and surface electronic processes that take place in suspensions of Ti02. Despite a massive research effort, many aspects of the electron-transfer processes occurring at the surface of bandgapexcited Ti02microcrystallites still remain uncertain. In the present work we illustrate how the pulse-radiolysis time-resolved microwave conductivity (TRMC)technique may be able to provide additional information. By combination of this information with the large body of experimental studies in which other physical or chemical probe techniques have been used, a better overal understanding of the influence of various parameters on the physicochemical processes occurring at the surface might be obtained. Experimental Section Degussa P25 Ti02powder, which consists of approximately 75% anatase, was used in all of the experiments reported in the present work. The individual microctystallites making up the powder have an average effective radius of 15 nm. While in the text the microcrystallites are dicussed as if they were completely separate entities, it is in fact well-known that the particles in flame reactor produced TiOzare sintered together in larger clusters. The powder was used both as received and after drying in a vacuum oven at 60 OC overnight. Approximately 25 mg of powder was compressed into a cylindrical cavity in a perspex block as has been fully described previo~sly.~ Isopropyl alcohol and paradioxane were both of UV spectroscopic solvent grade. The liquids were added to the powder sample by using a microliter syringe, and the amount was de-
'Delft University of Technology.
*iEcole Centrale de Lyon. Concordia University.
0022-3654/91/2095-8858~02.50/0
TABLE I: Effective Mobility of chrpc C "Formed in PulSc-ImdLtcd PZS Tib.km Dioxide Powder with Different Surface Treatments a d lmdiafion Conditiors pulse effective end-of-pulse pretreatment conditions# mobility,' lo-' m2/V s 0.22 air equilibrated single pulse 0.21 vacuum driede single pulse 1 Hz 0.22 5 Hz 0.24 0.24 25 wt % isopropyl alcohol single pulse 1 Hz 5 Hz
single pulse 5 Hz
30 wt % p-dioxane
0.37
0.75 0.23 0.78
"Single pulse = first pulse to virgin sample (average e--h+ pain per pulse per TiOz particle a.0.8); 1 Hz = 10th pulse at 1 Hz;5 Hz = 100th pulse at 5 Hz. bFrom end-of-pulse conductivity per unit daw using an average electron-hole pair formation energy of 9 eV. Covernightin a vacuum oven at 60 O C immediately prior to measurement. termined by weighing before and after. The block was placed in a microwave waveguide cell which could be irradiated with pulses of 3-MeV electrons from a Van de Graaff accelerator. Transient changes in the microwave power reflected by the cell resulting from changes in the conductivity of the TiOz sample on irradiation were monitored by using microwave detection circuitry in the 26-40-GHzrange."l The time resolution was approximately 1 ns. ~~
( I ) Pichat, P.; Fox, M. A. Photoinduced Electron Transfer, Part D Fox, M. A., Chanon. M.,Eds.; Elsevier: Amsterdam, 1988; p 241. (2) GrHtzel, M. Photofnduced Electron Tramfer, Part 0; Fox, M. A., Chanon, M..Eds.; Elsevier: Amsterdam, 1988; p 394. (3) Fox, M. A. Top. Curr. Chem. 1987, 142, 71. (4) Henglein, A. Top. Curr. Chem. 1988, 143, 113. (5) Pichat, P. In Photocatalysis and Environment; Schiavello, M.,Ed.; Kluwer: Dordrecht, 1988; p 399. ( 6 ) Serpone, N. In Photochemical Energy Conuersion;Norris Jr., J. R., Meisel, D., Eds.; Elsevier: Amsterdam, 1989; p 297. (7) Ollis, D. F.; Pelipetti, E.; Serpone, N. In Photocatalysis; Serpone, N., Peliuetti, E., Eds.; Wiley: New York, 1989; p 603. (8) Matthews, R. W. In Photochemical Conversion ondStorage of Solar Energy; Ptlizzetti, E., Schiavello, M.,Eds.; Kluwer: Dordrccht, in preas. (9) Warman, J. M.;de Haas, M.P.; Pichat, P.; Koster, T. P.M.;van der Zouwen-Assink E. A.; Mackor, A.; Cooper, R. Radial. Phys. Chem. 1991.
37, 433. (10) Warman, J. M. In The Study of Fast Processes and Transient Species by Electron Pulse Radiolysis; Baxendale, J. H.,Busi, F., Eds.;Reidel: Dordrecht, 1982; p 129. ( 1 1) Warman. J. M.; de Haas, M. P.; In Pulse Radiolysis; Tabata, Y., Ed.; CRC Press: h a Raton, FL. 1991; p 101.
Q 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 22, I991 8859
Conduction-Band Electrons in Degussa P25 T i 0 2 All of the data presented here were obtained by using 10-ns, 2.5-A pulses, which resulted in an average energy de ition per pulse within the Ti02 regions of the sample of 8 X 1 6 m 3 . For an average pair formation energy of 9 e V and an average particle radius of approximately 15 nm, this corresponds to the formation of on the average 0.8 electron-hole pairs per particle per pulse. The physically relevant parameter derived from the transient change in microwave power reflected by the sample is the radiation-induced conductivity (in S/m) per unit dose (in J/m3), Ao/D. If there is no carrier decay during the pulse, then the end-of-pulse value of Au/D, (Au/D), achieves its "initial" value given by9 (Ao/D)o =
M-) + p(+)I/Ep
(1)
+
with [p(-) p ( + ) ] the sum of the mobilities of the primary electron and hole charge carriers, and E the pair formation energy. The value of the "end-of-pulse mobifity", pwl derived from the end-of-pulse conductivity per unit dose according to eq 2, is the experimentally determinable parameter that will be used in the discussion of the results: Pcop
= Ep(Aa/D)eOp
(2)
+
The value of pcopwill be either equal to [p(-) p(+)] if no inpulse decay has occurred or will give a lower limit to this mobility sum. The values of pmPdetermined for the different media studied are listed in Table I. The sensitivity of the microwave detection system was calibrated by using Fluka "purum" anatase powder. This material has a first half-life for radiation-induced conductivity decay of many milliseconds, and pWphas been determined to be 4.3 X IO-" m2/V S?
Results and Discussion In the present work, the transient conductivity change that occurs on pulsed electron-hole pair formation within the microcrystallites of Degussa P25 titanium dioxide powder has been monitored by using the contactless time-resolved microwave conductivity (TRMC) pulse-radiolysis technique.+" Close to uniform electron-hole pair formation throughout the compressed powder samples was achieved by irradiation with pulses of 3-MeV electrons, which have a penetration depth of approximately 3 mm in bulk Ti02. As pointed out in the previous section, the irradiation conditions correspond to the deposition, on the average, of 7 eV per Ti02 particle per pulse. This is slightly less than the average energy, EP= 9 eV, required for electron-hole pair formation in Ti02using high-energy radiation? The conditions of the present experiments come close therefore to the limiting, low-intensity single-pairper-particle situation. The microwave conductivity transients corresponding to the first, single pulses given to a previously unirradiated sample are shown in Figure 1 for an unadulterated, dry sample and for samples with 25 wt 9% isopropyl alcohol and 30 wt 76 paradioxane added prior to irradiation. The amount of the additives corresponds to a layer approximately 8.5 nm thick if the liquids are uniformly distributed over the surface of the Ti02particles (surface area ca. 50 m2/g). Since P25 is nonporous, penetration into pores can be neglected. The after-pulse decay of the conductivity is seen in Figure 1 to be slightly slower in the presence of dioxane and markedly slower for isopropyl alcohol. The decay kinetics for all systems are very disperse, which makes it impossible to talk of a mean decay time. The first half-time for decay, rIl2,is therefore strictly a phenomenological parameter that has little meaning in the reaction mechanistic sense. For want of a better semiquantitative impression of the effects observed, however, the values of 7112 are found to be approximately 100 ns, 250 ns, and ca. 25 ps for the dry, dioxane, and i-PrOH samples, respectively. The elongation of the conductivity decay process by the addition of i-PrOH to pulse-irradiated or flash-photolyzed Ti02 powders has been reported p r e v i o ~ s l y .In ~ ~the ~ ~earlier work, however, the absolute magnitude of the conductivity was not determined SO that it could not be decided with certainty whether the initial
3 d
h
:j 4 1
2t
OR I
01
0
100
200
300
500
400
TINE lnri
Figure 1. Transient conductivity change following pulsed irradiation of Degussa P25 Ti02powder with IO-ns, 2.5-A pulses of 3-MeV electrons. The traces for the dry material and for 25% and 30% added isopropyl alcohol and paradioxane were all taken by using the first single pulse on a fresh, unirradiated sample.
conductivity had not also been affected. In the present work a quantitative analysis has been carried out and, as shown by the data in Figure 1 and in Table I, the end-of-pulse conductivity is in fact not significantly affected by the additives for single-pulse conditions on previously unirradiated samples. We conclude therefore that, in all cases, we are dealing with the same charge carrier(s) and that the effect of additives is to elongate the lifetime. The elongation of the lifetime affected by i-PrOH, a known surface hole scavenger,'-" would tend to confirm previous conclusions, based on the effects of surface Pt9*12and tetranitromethane,I2that the conductivity observed is mainly due to mobile electrons. The fact that the decay rate is influenced at all by the chemical composition of the fluid at the particle interface shows directly that electrons undergo their ultimate permanent localization or recombination reaction mainly at the surface rather than within the bulk of the particles. The longer lifetime of electrons in the presence of i-PrOH could be due to either a retardation of surface recombination resulting from hole scavenging or to displacement of a deep surface trap by the alcohol. We cannot distinguish between these possibilities. The former explanation would require that holes diffuse to the particle surface on a time scale close to or faster than the electron decay time in the unadulterated material, Le., a time on the order of 100 ns or less. The fact that pdioxane, which has an ionization potential 1 eV lower than i-PrOH, does not have as large an effect as the alcohol would indicate that the effect on the decay is probably not a result simply of hole transfer to the molecules of the fluid. The presence of the 0-H moiety in the alcohol would appear to play an important and necessary role, and it is known that isopropyl alcohol can displace other adsorbed molecules, for example acetone, from the surface. It is perhaps worth emphasizing that the effects shown in Figure 1 are for the first single pulses given to the virgin samples. The decrease in decay rate in the presence of the additive cannot therefore be ascribed to chemical change either within the bulk or at the particle surface resulting from prior irradiation. The decrease in the decay rate of electrons must be attributed simply to the presence of the liquid at the interface. The absolute magnitudes of the end-of-pulse charge-carrier mobilities for single pulse conditions, derived according to eq 2 are listed in Table I. The values are all close to the ca. 0.2 X lo4 m2/V s previously determined for samples of Degussa P25? Values of pw for other flame reactor prepared samples have been found to increase markedly with increasing particle size from 0.05 X lo-" to 1.5 X lo4 m2/V s in going from 5- to 50-nm r a d i ~ s . ~ (12) Schindler, K.-M.; Kunst. M. J. Phys. Chem. 1990, 94, 8222.
8860 The Journal of Physical Chemistry, Vol. 95, No. 22, 19191
i io
.
---- .-_ __-_
__
. - - -- .-
SIHSLE P U S E
300
400
0 0
100
200
SO0 TI=
Inal
Figure 2. Transient change in conductivity following pulsed irradiation ( IO-ns. 2.5-A, 3-MeV electrons) of Dcgussa P25 TiOl powder with 25 wt % isopropyl alcohol added. All traces are for a single pulse; the
bottom trace isolated by many seconds. the middk trace the last pulse of a train of IO given at 1 per second, the upper trace for the last puke of a train of I 0 0 given at 5 Hz. The latter value approaches those found for larger particle, pure anatase commercial samples? e.g., 4.3 X lo4, 5.1 X lo4, and 2.3 X lo4 mz/V s for Fluka, Tioxide, and Aldrich powders. On the basis of the effect of particle size, it was concluded that the radiation-induced conductivity transient in Jhgussa P25 results from the praence of mobile, conduction-hnd electrons in the bulk of the particles that are in equilibrium with a shallow, localized surface state, e.g., Ti* sites at 0.07 eV below the conduction band in anatase.I2 The equilibrium was assumed to be established on a time scale much shorter than the time scale of several nanoseconds of the measurements. Such a rapid equilibration is certainly possible for a bulk electron mobility on the order of 1 X lo4 mZ/V s. Taking eq 3 for the average bulk-to-surface TS
= R2/*KkeT
(3)
diffusion time,’ Tm, an estimate of approximately 10 p is obtained for a 15-nm particle. The end-of-pulse, “effective” mobility resulting from such rapid equilibrium surface localization will be related to the bulk electron mobility, K(-)~, and the fraction of electrons formed which remain in the bulk after equilibration, i.e. ”p
M(-)BN(-)B/ [N(-)B+ N(-)sI
(4)
It is assumed in (4) that the mobility of the localized surface state is very much lower than that of the bulk, conduction-band state, Le., 14-h
