Langmuir 1994,10, 643-652
643
Articles Spectroscopic, Photoconductivity, and Photocatalytic Studies of Ti02 Colloids: Naked and with the Lattice Doped with Cr3+,Fe3+,and V5+Cations Nick Serpone* and Darren Lawless The Laboratory of Pure and Applied Studies in Catalysis, Environment and Materials, Department of Chemistry and Biochemistry, Concordia University, Montreal, Quebec, Canada H3G lM8
Jean Disdier and Jean-Marie Herrmann Laboratoire de Photocatalyse, Catalyse et Environnement, URA au CNRS, Ecole Centrale de Lyon, 69131 Ecully Cedex, France Received June 28,1993. I n Final Form: November 11, 199P Colloidal Ti02particulates have been prepared using a low-temperature (0 O C ) method by controlled hydrolysis of Ti&; Ti02particulates doped in the lattice with C1”+(0.5,5,and 10wt ?6-relative to weight of TiO2-from aqueous CrCld, with Fe3+ (10 wt 3’ 6 from aqueous FeCl3) and with V6+ ( 10 wt% from aqueous NHaVO3) were also synthesized by an analogous procedure. These particulates were examined spectroscopically and by photoconductivity measurements to assess the photosensitization of titania (anatase) by incorporated metal dopants. Visible absorption bands in metal-doped particulates parallel those observed for aqueous metal cations at all levels of doping (up to 20 w t 96); these bands are not reproduced in the action spectra of photoconductivityvs wavelength,except for the absorption threshold of the Ti02 system in all cases. Their photocatalytic activity was determined by standard photoreduction (of water, H2 evolution) and photooxidation (of oxalic acid) reactions to assess the influence of metal dopants in heterogeneous photocatalysis.
Introduction Titania (anatase phase) has universally been recognized as one of the better photocatalysts in heterogeneous photocatalysis applications as it combines two important complementary features for a photocatalyst: good UV absorption efficiency for the light harvesting process and good adsorption capacities, due particularly to the density of OH- groups of amphoteric character. However, the bandgap energy (3.0-3.2eV) requires that near-UV light be used to photoactivate this very attractive photocatalyst. Unfortunately, in solar energy applications only 3 % of the solar light is absorbed. It would be advantageous, therefore, if this metal oxide semiconductor (SC) could be photosensitized by visible light. One approach we have taken in the past to sensitize Ti02 was to couple it with CdS which absorbs visible light below 520 nm (bandgap energy ca. 2.4-2.6 eV) and which can be employed in the photodecomposition of HzS to produce hydrogen. This coupling of CdS/TiOz led to the discovery of photoinduced vectorial electron transfer from CdS particulates to RuOzloaded particulates of TiO2.l Despite the positive attributes of TiOz, however, there are some drawbacks associated with its use: (i) charge carrier recombination occurs within nanoseconds;2 (ii) the band edge absorption threshold of Ti02 is 1400 nm. To circumvent these two particular limitations, a number of
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* Please address all correspondence to this author.
e Abstract publiehedin Advance ACS Abstracts, February 1,1994.
(1) Serpone, N.; Borgarello, E.; Gratzel, M. J . Chem. SOC.,Chem. Commun. 1984,342. (2) Rothenberger, G.;Moser, J.; GrHtzel, M.; Serpone,N.;Sharma, D. K. J. Am. Chem. SOC.1985,107,8054.
strategies have been proposed to improve the light absorption features and lengthen the carrier lifetime characteristics of TiOz. Surface derivatization of Ti02 with a number of organic dyes extends the sensitivity of Ti02 into the visible regiona by injection of electrons from an excited level of the dye into the SC conduction band. Iron(II1) doping of Ti02 had been shown earlier’ to increase the lifetime of electron/hole pairs from nanoseconds for naked Ti02 colloids (ca. 30ns2)to minutes and even hours. Vanadium doping increases carrier lifetime8and apparently also extends the absorption range of Ti02.B A number of other metal dopants have been incorporated in the lattice of TiOzlOand at the surface for solar energy applications.11-24 (3) Moser, J.; Gritzel, M. J. Am. Chem. SOC.1984,106, 6557. (4) Fessenden, R. W.; Kamat, P. V. Chem. Phys. Lett. 1984,123,233. (5) Kamat, P. V.; Ford, W. E. Chem. Phys. Lett. 1987,136,421. (6) Kamat, P. V.; Chauvet, J.-P.; Fessenden, R. W. J. Phys. Chem. 1986,90,1389. (7) Maser, J.; Gritzel, J.; Gallay, R. Helu. Chim. Acta 1987, 70,1596. (8) GrHtzel, M.; Howe, R. F. J. Phys. Chem. 1990, 94, 2566. (9) Moser, J. Ph.D. Diwertation,Thesis no. 616, B o l e Polytechnique FWrale de Lausanne, Switzerland, 1986. (10) Lawleas, D. PLD. Thesis,ConcordieUniversity,Montreal, Canada, February 1993.
(11) (a) Wong, W. K.; Malati, M. A. Sol. Energy 1986, 2, 163. (b) Malati, M. A.; Wong, W. K. Surf. Technol. 1984,22,305. (12) Goodenough, J. B. Studies in Inorganic Chemistry; Proceedimp of the Second European Conference, Velhoven, The Netherlanda (7-9 June, 1982) 1983; Vol. 3. (13) Finklea, H. 0. In Semiconductor Electrodes;Studies in physical and theoretical chemistry 55; Elaevier: Amsterdam, 1980, p 43, and references therein. (14) Borgarello, E.; Kiwi, J.; Gratzel, M.; Pelizzetti, E.; Vica, M. J. Am. Chem. SOC. 1982,104,2996. (15) Palmisano, L.; Augugliaro,V.;Sclafani,A.; Schiavello,M. J . Phys. Chem. 1988,92,6710.
0743-7463/94/2410-0643$04.50/0 0 1994 American Chemical Society
Serpone et al.
644 Langmuir, Vol. 10, No. 3, 1994 We have prepared a large number of Ti02 colloidal systems in which we have introduced a variety of transition metals into the lattice to examine the effect of metal dopants on both physical and chemical characteristics, together with their effect on the photocatalytic activity of Ti02.10 These were needed to examine the sensitization of Ti02 to visible light irradiation and to probe the photophysics of charge carrier separation and recombination events in the very short time of picoseconds. We have also carried out photoconductivity studies on the systems TiOd0.5 w t % Cr, TiOd10 wt % Cr, TiOd10 w t % V, and TiOdlOwt 9% Fe (weightpercent is given relative to weight of T i 0 2 ) . The first of these systemswas examined some time ago,26 but unlike the sample prepared here, the earlier system was prepared by a high-temperature technique injecting a vaporized solution of CrCl3 and T i c 4 into an oxhydric flame. Herein we examine the spectroscopic and photoconductive behavior of these specimens and correlate it with their photocatalytic activity vis-a-vis standard reduction (of water) and oxidation (of oxalicacid) reactions, the latter having also been used earlier to test the photoactivity of the TiOd0.5 wt % Cr system.26 Experimental Section Materials. All chemicals were of reagent grade quality and were used as received. The water used throughout was doubly distilled and deionized. Clear and optically transparent colloidal sole of Ti02 were prepared via the controlled hydrolysis of Tic& at low temperature,w giving a narrow size distribution of particulates peaking around 130 A. In a typical preparation, 5.2 mL of fresh, doubly distilled Tic& wae slowly added dropwise to vigorously stirred 200 mL of doubly distilled, deionized water rigorously maintained at -0 OC. The solution was subsequently dialyzed (Viscase membrane, presoaked 24 h in distilled water and then thoroughly rinsed prior to use) against about 4 L of distilled water for -8 h; the water was changed 2 to 3 times during thisperiod. The concentration of Ti02 was -15 g/L and the pH of the solution was in the range of 2.5 to 3. This method was also chosen to dope the lattice of Ti02 particles with a variety of transition metals, herein Fea+,C?+, and V.In all instances, the appropriate quantity of metal dopant (expressed as w t 9% of TiOg) was added to the solution prior to addition of T i c 4 from stock solutions: 7.3 g of purified anhydrous FeCb in 1 L of 1 M HC1, or 2.5 g of chromium metal powder dissolved in 1.012 L of concentrated HC1. For vanadium-doped Ti02 colloids, the 10 wt 9% V/TiO2 system was prepared from a 200-mL solution of 0.69 g of N&VOs to which the 5.2 mL of prepurified Tic4 was subsequently added dropwise. Solid samples of the undoped and doped Ti02 materials were obtained by rotoevaporatingthe solvent under vacuum at ambient temperature. Procedures. Absorption spectra of naked Ti02 and metaldoped Ti02 colloidalparticulates were recorded in 1-cm Suprasil quartz cells using a Shimadzu UV-266 double beam W / V I S (16) Soria, J.; Coneea,J. C.; Augugliaro, V.; Palmisano, L.; Schiavello, M.; Sclafani, A. J. Phys. Chem. 1991,95, 274. (17) Luo, Z.; Gao, Q.-H. J. Photochem. Photobiol., A 1992, 63, 367. (18) (a) Miauehima, K.;Tanaka, M.; Ami, A.; Iida, S.;Goodenough, J. B. J.Phys. Chem. Solids 1979,40,1129. (b)Bin-Daar,G.; DamEdwards, M. P.; Goodenough, J. B.; Hamnett, A. J. Chem. SOC.,Faraday Tram.
1 1988,79,1199. (19) Metikos-Hukovic,M.; Ceraj-Ceric, M. Mater. Res. Bull. 1988,23, 1535. (20) Martii,C.; Martin, I.; Rives, V.; Palmisano, L.; Schiavello, M. J. Catal. 1992,134,494. (21) Davidson, A.; Che, M. J. Phys. Chem. 1992,96,9909. (22) de Korte,P. H. M.; 't Lam,R. U. E.; Schoonman, J.; Blasee, G. J.Znorg. Nucl. Chem. 1981,43, 2261. (23) Blasee, G.; de Korte,P. H. M.; Mackor, A. J. Inorg. Nucl. Chem. 1981,43,1499. (24) Blasee, G.; Dirksen, G. J. Chem. Phys. Lett. 1981, 77,9. (25) Herrmann, J.-M.; Disdier, J.; Pichat, P. Chem. Phys. Lett. 1984, 108,618. (26) Moser, J.; Gratzel, M. Helu. Chim. Acta 1986,65, 1436.
recordingspectrophotometer;distilled water (pH 3, adjusted with HCl) was used as reference. Powder X-ray diffraction pattems of the Ti02 samples were obtained with a Phillips PW 1050-25 diffractometer using Ni-fiitered K copper radiation (A = 1.5417 A). The powder was contained in a flat holder made of Plexiglas. Photoconductivity experimentawere performed using a statictype cell described earlier." The colloidal powder was dried in an oven at 115 OC for -17 h; subsequently it was compreesed as a pellet in a frame containing two parallel gold electrodes and placed into the cell, perpendicular to the W-light beam. Wavelength-dependent experimenta were conducted using UV light illumination from a Philips HPK 125 Hg lamp emitting about 80 mW cm-2 (without attenuator). Monochromatic irradiation wavelengths were obtained using a Jobin-Yvon monochromator set on the more intense radiation lines of the mercury lamp (HPK 125); they are 578, 546, 491.6, 453.8, 403.8, 365.5, 334.1, and 313 nm. Where the photoconductivity was too small to measure accurately, optical cutoff fiiters were used. Other experimental details have been described elsewhere." The various T i 0 2 samples prepared were tested for their photocatalytic activity by illuminating the solid materials in water at pH 3 (reductive testa; formation of Hz) and in oxalic acid solutions (oxidativetesta, degradation of oxalic acid). In all cases, irradiation was carried out on aqueous Ti02 dispersions ([Ti021 = 2 g/L) using a 1OOO-W Hg/Xe lamp, operated at ca. 900 Wand fitted with a water jacket to filter out IR radiation. Illumination with near-W light (A = 300 to 400 nm)was achieved using a Coming 7-60 fiiter before the reaction flask. Visible light illumination in the spectral window 40Ck650 nm was obtained employingatandem combination of Coming 4-97 fiiter and Oriel 400-nm cutoff fiter. The photooxidative activity of the different Ti02 samples was determined by monitoring the oxidation of oxalic acid (5.0 X 1 W M pH 2.25) in aqueous suspensions of the catalyst (20 mg Ti02 systems in 10 mL of solution) illuminated with either W or visible light for 30 min. Prepurified oxygen was bubbled through the solution at a constant rate of 10 mL/min. After 30 min, stirring either in the dark or under illumination (Wor visible), the reaction was stopped and the slurry fiitered through a 0.22hm MSI Nylon 66 filter. Unreacted oxalic acid in the fiitratewas determined using a MnOl- titration method. In photoreductive tests, HZgas was detected with a Gow-Mac Model 550 gas chromatograph equipped with a 5-A molecular sieve column and a TCD detector; it was interfaced to a HP 3396A integrator.
Rssults
Spectral Properties of Metal-Doped Ti02 Particulates. Transition-metal dopants added to the anatase Ti02 lattice significantly affect its absorption characteristics; these spectra are presented in Figures 1 and 2 for systems doped with Cr3+,Fe3+, and V5+. Figure l a illustrates the absorption features of naked colloidal Ti02 (spectrum 1)and that of a chromium(II1)doped (5 wt % ) Ti02 system (spectrum 3); note also the very weak absorption tail of naked Ti02 that extends beyond 400 nm often observed in semiconductormaterials (see ref 28 for an explanation). Also shown are the spectral characteristics of Cr,3+ at the same concentration as in the doped system under otherwise identical solution conditions (spectrum 31, together with the addition spectrum 4 (spectra 1+ 2). The Cr,S+ spectrum reveals the usual two equally intense bands in the visible region at 407 and 577 nm; there are no other spectral features beyond 750 nm for this and the doped system. Spectral variations are evident between 3 and 4. The former shows a weaker band at -580 nm, consistent with a lower (27) Herrmann, J.-M.; Disdier, J.; Pichat, P. J. Chem. SOC., Faraday Tram. 1 1981, 77,2815. (28) Pankove, J. I. Optical Processes in Semiconductors; Dover Publishing, Inc.: New York, 1971; p 10.
Langmuir, Vol. 10, No. 3, 1994 645
Study of Ti02 Colloids 30
30 a
25 0 51
I !:,
0
T10,
in vacuo
r25
T10,
in 0,
i20
9 c
i15
41 x
2 0
10
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05;
I
.-o5
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I-
- - - - - 4--0 ,* ..., .? , 0 0 Q-
Wavelength, nm
3
300
Wavelength
C
(ii) Tt02/0.7wt.%Cr
(b) .
0.01
'
300
I
400
'
I
I
'
'
I
600 700 wavelength (nm)
500
I
'
I
800
Figure 1. (a)Absorption spectra: (1)13.5g/LTiOz particulates, pH 2.8; (2) a pH 2.8 solution containing the same concentration of C$+ as would be expected in Ti09 doped with 5 wt % CrS+; (3) 13.5 g/LTiOa/5wt % C$+, pH 2.8; (4) addition spectrum of (1)plus (2). (b)Absorptionspectraof C$+-doped titaniapowders at -0.7 wt % (lower)and -3.4 wt % (upper);adapted from refs 15 and 20. 1.0
I
'
1.
Q)
u C
m
0.5
ffl
n
Wavelength
U
nm
TiO,
0.0
I
9;
3
I
(nm)
Figure 3. Photoconductivity versus wavelength plots for naked Ti02 particulates showing the influence of the wavelength of irradiation under vacuum and in an oxygen atmosphere.
v
fl0
I
400 450 500 550 600
350
The concentration of the dopant also has a marked effect on the absorption properties of Ti02 sols. Thus, increase in chromium content leads to a linear increase in the absorption band at 580 nm (inset, Figure 2). The possibility that the metal dopant coexists in solution with naked Ti02 is unlikely. First, the addition absorption spectrum (curve 4of Figure la) does not correspond to the spectrum obtained for the doped material (curve 3 of Figure la). Second, a solution containing only the metal dopant (pH adjusted to that of the undoped material; i.e. pH 2 to 3) added to a solution containing only Ti02 causes immediate precipitation, suggesting that coexistence of the Craq3+and colloidal Ti02 is not possible. Photoconductivity Studies. Influence of Irradiation Wavelength in Vacuo. Variations of the photoconductivity, u, of naked Ti02 colloids in vacuo as a function of illumination wavelength are illustrated in Figure 3. The photoconductivity is negligibly small throughout all the visible wavelengths but increases sharply below 400 nm at energies of excitation above bandgap. This action spectrum parallels the absorption spectrum of naked Ti02 particulates and confirms formation of electrordhole pairs when hv 1 EBG(=3.2 eV) Ti02 + hv
3
Figure 2. Absorption spectra of Ti02 particulates and particulates doped with vanadium(V)and iron(II1) at 10 wt % doping leveL Insert: absorption spectraof Ti02 particulatesdoped with chromium(II1) at various doping levels; pH 2.7. oscillator strength of chromium centers in Ti0229and a slightly perceptible band at higher energy. Comparison of 3 and 4 shows that the Cr3+ cations must have been incorporated into the host Ti02 lattice (see Discussion). Figure l b reports the reflectance spectra of two Cr-doped Ti02 powders adapted from earlier r e p ~ r t s land ~ *shown ~ here for comparison. Figure 2 depicts the spectra of Fe3+-and Vv-doped (10 wt %) Ti02 particulates compared to that of the parent naked Ti02 sol. In both cases, the absorption threshold has been extended into the visible region to about 550 nm with the iron-doped sample showing a well-defined band a t -470 nm. The insert in Figure 2 illustrates the concentration dependence of the spectra of chromiumdoped systems which show a well-defined band at -580 nm and an ill-defined band on the low energy side of the Ti02 absorption threshold. (29) (a) Ghosh, A. K.; Marueka, H.P.J. Electrochem. SOC.1977,124, 1616. (b) Marueka, H. P.; Ghosh, A. K.Sol. Energy Mater. 1979,1,237.
+
e- + h+
(1)
The photoconductivity of 10 wt '% Cr-doped Ti02 particulates under vacuum is significantly smaller than that for naked pure Ti02 particulates ( a 10-13 W). Doping Ti02 with Cr3+ ions creates electron acceptor centers that trap electrons and decrease u.% A weak wavelength dependence is seen below the absorption threshold of Ti02 (400nm); no bands appear in the visible region in the action spectrum of Figure 4,unlike those seen in the absorption spectra of doped systams. The decrease in photoconductivity below 365 nm is unusual.% Addition of chromium greatly diminishes the photosensitivity of Ti02 and the apparent capacity of Ti02 to generate electron/hole pairs. The action spectrum of Ti02 particulates containing smaller levels of the dopant (0.5 wt 5%) shows that the photoconductivity is small and approaches the measurement threshold of W. A small increase in u does occur below 400 nm; no features are seen in the visible region. The small values of u and the weak wavelength dependence demonstrates that chromium doping is effective even a t levels as low as 0.5 wt %. Variations in monochromatic irradiation wavelengths cause no changes in u for the 10wt '% V-doped Ti02 system throughout the whole wavelength range examined (see Figure 4). In addition, values of u are negligibly small indicating an apparent inhibitory effect by vanadium
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Serpone et al.
646 Langmuir, Vol. 10, No. 3, 1994
Table 1. Electric Conductivity in Oxygen (1WzTorr) of TiOZ/Cr (10 wt %) as a Function of Different Light Fluxes Transmitted by Optical Cutoff Filters at Their Transmission Thresholds wavelength photoconductivity Coming threshold log u UV filter (refho.) (nm) (ct-9 -14.8 dark 3-71 >460 -13.28 3-73 >400 -13.23 3-75 >370 -13.25 0-52 >340 -13.26 0-54 >300 -13.27
, 0 , 21 -
D
I
0
-0
-l3.Oi
-
Ti02/10wtXFe
-14 0
400 500 Wavelength (nm)
300
600
Figure 4. Log u versus wavelength of irradiation plots for vanadium-, iron-,and chromium-doped Ti02 particulates under vacuum (- 1od Torr). doping in the formation of electron/hole pairs. Contrary to the other two doped systems, the iron-doped Ti02 specimen shows very low photoconductivity a t wavelength below 400 nm beyond which there is a gradual rise in u throughout the visible range examined. During irradiation, the photogenerated holes, h+, react with the ionosorbed species 02'- and 0- which leads to photodesorption of oxygen (eq 2) under the prevailing dynamic vacuum; the accumulated photoelectrons increase the electric photoconductivity. Concomitantly, the photoactivated oxygen {O,)* can react with adventitious hydrocarbon impurities (chemisorbed during the preparation and storage of the sample) to decontaminate the particulate surface with evolution of C02.30 O[(ads)
+ h+
-
(O,)*-,x/202(g)
(2)
Influence of Illumination Wavelength under Oxygen. Variations of u for naked Ti02 as a function of irradiation wavelengths under an atmosphere of oxygen (4.5 X le2 Torr) are shown in Figure 3. The photoconductivity is smaller by an order of magnitude relative to that observed in vacuo. No changes in u are observed throughout the visible range; an increase in u occurs at 380 nm, slightly below the absorption threshold seen under vacuum. This increase is also consistent with formation of electron/hole pairs (eq 11, but 02 diminishes the photoconductivity via scavenging electrons to give ionosorbed species (e.g. Of; eq 3). N
O,(g)
+ e-
-
O,'-(ads)
(3)
In the presence of oxygen, the photosensitivity of TiOd Cr (10 wt 5% ) decreases further to very low levels because irradiation wavelengths selected by the monochromator provided light fluxes too small to examine these particulates. A system of optical cutoff filters were therefore employed to transmit larger light fluxes at wavelengths equal to or greater than their transmission thresholds. The data summarized in Table 1show there is no response of u as a function of wavelength; this is in keeping with and confirms the earlier results of Herrmann and coworker~.~s Interchange of IlluminationlDarkness under Vacuum. The photoconductivity of naked pure Ti02 particulates in vacuo increases upon UV light illumination from a dark value of W4i2-l to about 10-8 i2-1 (Figure 5). Note that
-
(30)Formenti, M.;Courbon, H.; Juillet, F.;Linatchenko, A.;Martii, J. R.; Meriaudeau, P.; Teichner, S. J. J. Vacuum Sci. Technol. 1971,9,
947.
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-10-1
1
I
uv
,Off
on
0 U
0
1
2
3
4
5
6
7
Time (hr)
Figure5. Temporal variations of the photoconductivity of naked Ti02 particulates and particulates doped with vanadium(V), chromium(III),and iron(1II) cations at 10 wt % doping levels under UV illumination (full spectrum of the lamp). The effect of turning off the light source is also illustrated.
the values of u are generally higher than those seen in Figures 3 and 4 due to a much higher photonic flux covering the whole lamp spectrum. Suppression of illumination slightly diminishes the electric conductivity which results from electron/hole recombination and/or trapping of the mobile free electrons. However, u remains rather high because of the large number of free electrons remaining that accumulated during illumination, while the photoformed holes reacted with negatively charged ionosorbed species (e.g., 02'- radical anion^).^' Results from experiments carried out on C$+-doped Ti02 analogous to those for naked pure Ti02 particulates are illustrated in Figure 5. Illumination with the whole lamp spectral flux causes photoconductivity to reach a level ( Q-l) some 3 orders of magnitude lower than that for naked TiO2, demonstrating the effect induced by the chromium dopant in the Ti02 lattice. On suppressing illumination, the photoconductivity decreases slowly reaching levels close to measurement thresholds. This decrease is significantly greater than that shown by naked TiO2. With the UV light off, the photogenerated electrons which accumulated during illumination can either recombine with the photoholes present a t the instant of ending UV illumination or get trapped by Crs+-induced acceptor centers or get trapped by anionic oxygen vacancies (eq 4). N
Vo"
+ e- s Vow
(4a)
V,'
+ e- s V,
(4b)
or
The concentration of anionic vacancies can be increased under vacuum by chromium doping and/or by the very small size of colloidal particles. Irradiation of TiOdV particulates with the full lamp spectral flux leads to a spontaneous increase of u to 10-l1 i2-l levels (Figure 51, similar to levels reached by the analogous chromium-doped system. However, unlike the
-
Study of Ti02 Colloids
Langmuir, Vol. 10, No. 3, 1994 647
-10.54
I
-ll,o/- 6
Table 2. Lo60 of Oxalic Acid ([H&a04]1- 6 mM)with Variour Material6 Terted under Otherwise Identical Experimental Condition6 (pH 2.26). p m(%) material % law %law % loee leached deached tested (dark)* (Wlight) (viaible) (Wlight) (viaible)
pm(%)
-25
-15
-20
-10
-05
00
05
log P 0 2 ( t o r r N Figure 6. Plots showing the influence of the partial pressure of oxygen on the photoconductivity of vanadium(V)-doped Ti02 particulates (10 wt 9% doping level). -in n . *"
'i
-130i.. -20
TiOz/0.5wtXCr
,
.
,
.
-15
I
.
.
,
.
..
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...
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1
-10 - 0 5 00 log tPo,(torr))
9
.
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1
. *
10
Figure 7. Plots showing the influence of the partial pressure of oxygen on the photoconductivity of chromium(II1)-doped TiOz particulates (0.5 wt % doping level).
latter system and the naked pure Ti02 particulates, where increases in u occur via relatively slow rise kinetics following the initial rapid rise, the photoconductivity of V-doped Ti02 particles reaches a plateau immediately. On suppression of UV irradiation, the electric conductivity decreases to levels near the measurement thresholds and parallels observations on the Fe(II1)-doped system. Vanadium(V) doping also creates recombination centers for electrons that diminish the electric conductivity. The behavior of the iron-doped system, Ti02/10 wt % Fe, under illumination and in vacuo (Figure 5) is nearly identical with that of TiOz/Cr at equal doping levels. However, unlike Ti02/Cr which shows a slow decrease in conductivity when illumination is suppressed, the conductivity for the TiOZ/Fe particulates decreases rapidly initially and then more slowly as noted for the Ti02/V system. This slower but nearly identical rate of decrease of u is seen for pure naked TiO2, for Ti02/V, and for Fedoped Ti02 particulates. With illumination on again, the iron-doped system immediately shows a near 2 orders of magnitude incrase in conductivity reaching values prevalent before light suppression. The photoconductivity shows no further variations at longer irradiation times. Influence of Oxygen Pressure on Photoconductivity. The influence of oxygen pressures on the photoconductivity was examined at constant radiative flux for the systems TiOd10 wt % V and TiOd0.5 wt 5'% Cr. The data are illustrated in Figures 6 and 7, respectively, as log-log plots to permit determination of the exponent in the relationship: u = (const)(Po,)-". In both instances, the data points of photoconductivity and oxygen pressures were taken after several hours to allow the system to reach equilibrium. Plots of the data display good linear relationships with negative slopes (I%/ aPo, < 0);they confirm the n-type character of these metaldoped Ti02 systems (e- are the majority carriers), which
P25 Ti02 Degussa naked Ti02 CrS+/TiO, (0.5wt %) (5wt %) (10wt %) Fea+/TiO2 (IO wt %) VH/TiO, (10wt %)
10.2
50.4 (40.2)C 10.4 (0.2)~
13.4
39.1 (25.7) 17.0 (3.6)
18.3 23.3 24.3
29.4 (11.1) 15.9 (-2.4) 0.09(0.9) 22.7 (4.6) 19.7(-3.6) 2.1 (1.2) 20.6 (-3.7) 16.2 (-8.1) 3.0 (1.5)
0.08 (0.8) 1.7 (1.7) 3.5 (1.8)
20.3
48.4 (28.1) 23.6 (3.3)
3.7 (3.7)
2.8 (1.4)
17.3
16.2 (-1.1)
13.8 (-3.5) 9.0 (4.5)
11.3 (5.6)
Ohradiation for -30 min with either W or visible light; concentration of Ti02 or doped TiOz, 2 g/L; ambient temperatures; pH -3; maas of catalyot, 20 mg;volume, 10 cm*. LOSEmeaoured after -30 min of etirring in the dark. e Net percent low under illumination.
*
remain n-type under the various conditions used. Four regimes are evident in Figures 6 and 7, denoted A through D (the arrows indicate the direction in which the experiments were carried out). Increasing the pressure of oxygen from PO,(initial) Torr in the vanadium-doped system (Figure 6) causes a gradual linear decrease of u until Po, 0.5 Torr (slope of line A = -0.26 f 0.03); continued addition of oxygen to -3.2 Torr leads to a steeper decrease of u (line B, slope = -1.2 f 0.1). Subsequently, gradual removal of oxygen incrases u abruptly at first (line C, slope = -0.55 f 0.01); then at Po2 0.5 Torr a further decrease in the concentration of oxygen slows down the increase in photoconductivity (line D, slope = -0.30 i 0.05). A nearly parallel behavior is displayed by the 0.5 wt % Cr-doped Ti02 particulates (Figure 7), under otherwise identical conditions; the corresponding slopes area -0.2 f 0.1 (line A), -1.3 f 0.2 (line B), -0.57 f 0.02 (line C), and -0.23 i 0.003 (line D). Similar results have been obtained for other doped systems, in particular those containing Fe. Photocatalytic Activity. The photooxidation of oxalic acid was monitored to partial mineralization; formation of C02was detected qualitatively by purging the headspace gases into an alkaline solution of barium nitrate and observing formation of (BaCOs). The results (pH 2.25) are summarized in Table 2, which also shows the quantity of the metal dopant M (as ppm and as 9%) that is leached or dissolved into the solution. Degussa P-25Ti02 was included in the tests for comparative purposes; the net conversion of oxalic acid by this Ti02 is in good agreement with values previously reported by Herrmann and coworker~.~~ The amount of hydrogen gas produced from the photocleavege of H2O at pH 3 by the various materials examined here is shown in Table 3. In all cases where hydrogen gas was produced, the increase was linear with time and the rate of hydrogen gas produced (pL/h) was taken as the slope of a plot of volume of hydrogen gas produced against time.
-
-
-
Discussion Nature of M-Doped Ti02 Specimens. Titania has been used as a photocatalyst of choice in combinationwith solar or near-UV light to mineralize a number of organics. The efficiency of the process to solar energy applications (31)Herrmann, J.-M.; Mozzanega, M.-N.; Pichat, P.J. Photochem. 1983,22,333.
Serpone et al.
648 Langmuir, Vol. 10, No. 3, 1994 Table 3. Quantity of Hydrogen Gas Formed during the Photoreductionof Water at p H 3 and under Otherwise Identical Conditions. dopant rate of concentration (wt/wt % TiOz)
material tested
Hz gas
evolved OcL/h)
C@+ Fea+
0.5 5 10 10
V6+
10
+ 300
x
I Os
Fe"
is limited by the near-UV (A I 400 nm) absorption of TiO2. Considering the density of photogenerated charge carriers, only a small fraction (
