Photoassisted Degradation of Dye Pollutants. 3. Degradation of the

Degradation of the Cationic Dye Rhodamine B in Aqueous Anionic Surfactant/TiO2 Dispersions under Visible Light Irradiation: Evidence for the Need of ...
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Environ. Sci. Technol. 1998, 32, 2394-2400

Photoassisted Degradation of Dye Pollutants. 3. Degradation of the Cationic Dye Rhodamine B in Aqueous Anionic Surfactant/TiO2 Dispersions under Visible Light Irradiation: Evidence for the Need of Substrate Adsorption on TiO2 Particles J I N C A I Z H A O , * ,† T A I X I N G W U , † KAIQUN WU,† KYOKO OIKAWA,‡ HISAO HIDAKA,‡ AND NICK SERPONE§ Institute of Photographic Chemistry, Chinese Academy of Sciences, Beijing 100101, China, Frontier Research Center for the Earth Environment Protection, Meisei University, 2-1-1 Hodokubo, Hino, Tokyo 191, Japan, and Department of Chemistry & Biochemistry, Concordia University, Montreal, Canada H3G 1M8

The TiO2 photoassisted degradation of the cationic dye rhodamine B (RhB) has been examined in aqueous dispersions under visible light irradiation at wavelengths longer than 470 nm in the presence and absence of the anionic surfactant sodium dodecylbenzenesulfonate (DBS). RhB degrades slowly via a pH-independent process in TiO2 dispersions containing no DBS. The surfactant DBS adsorbs strongly on the TiO2 particles and significantly accelerates RhB degradation with initial rates reaching maximal values at the critical micelle concentration of DBS (cmc ) 1.2 mM). In the presence of DBS, rates decrease with increase in pH, an effect directly attributable to variations in the extent of adsorption of RhB with changes in the surface charge of TiO2 particles. The zeta (ζ)-potentials of TiO2 particles in RhB/DBS/TiO2 dispersions (pH 2.1) show that DBS significantly enhances RhB adsorption and correlates with an enhancement in the rate of photodegradation of RhB. The results confirm the heretofore presumed but valid notion that preadsorption on the surface of TiO2 particles is prerequisite for efficient photodegradation of RhB under visible light irradiation; moreover, the data infer that degradation occurs at the particle surface and not in the solution bulk. Present observations are consistent with a pathway in which excited RhB* injects an electron onto TiO2 (an electron-transfer mediator) that is subsequently scavenged by O2 to form the O2•- radical anion and ultimately the OH• radical, as evidenced by DMPO spin-trapping ESR experiments carried out under conditions otherwise similar to those in photodegradation which, we infer, participates in the RhB photodegradation. * To whom all correspondence should be addressed. Fax: (+86)10-6487-9375. † Chinese Academy of Sciences. ‡ Meisei University. § Concordia University. 2394

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Introduction Industrial development is pervasively connected with the disposal of a large number of various toxic pollutants that are harmful to the environment, hazardous to human health, and difficult to degrade by natural means. During the past two decades, photocatalytic processes involving TiO2 semiconductor particles under UV light illumination have been shown to be potentially advantageous and useful in several cases (1-4), although practical process efficiencies remain somewhat contentious. As a photocatalyst, TiO2 is a semiconductor with a band gap of 3.2 eV (anatase); it is only activated by ultraviolet light at wavelengths less than 385 nm to produce electrons and holes in the conduction and valence bands, respectively (5). After their migration to the particle surface in short time and in competition with other photophysical phenomena, they react with chemisorbed O2 and/ or OH-/H2O molecules to generate reactive oxygen species (e.g., O2•-, HOO•, and/or OH• radicals) that can attack organic substrates and lead to their degradation and ultimately to their total mineralization to carbon dioxide. Photodegradation processes have been scrutinized for several organic substrates, e.g., halogenated aliphatics and aromatics (6), phenols and substituted phenols (7), surfactants (8-10), and dyestuffs (11), among others. Several intermediate products have been identified and (in some cases) quantified, and plausible process pathways inferred. Economically, UV light is not the optimal candidate as a radiation source to bring about the photodegradation of dyes that are otherwise excellent visible light harvesters. TiO2 has also been exploited as an electron transfer carrier to effectively separate the photogenerated electrons and holes, with an even greater efficiency either when the TiO2 particles are modified (1214) or when such oxidants as O2 (15, 16), Fe3+ (17), or Cu2+ (16) are added to a heterogeneous dispersions. Earlier studies (18) reported that dyes can photodecompose to CO2 under visible light irradiation in aqueous TiO2 suspensions. The initial degradative pathway is different from the one under UV light illumination in the presence of semiconductor particles. In the former case, dyes, rather than TiO2 particles, are excited by the irradiating source to appropriate singlet or triplet states, a process then followed by electron injection into the conduction band (or a surface state) of the semiconductor, whereas the dyes are converted to cationic dye radicals, dye•+. The injected electron, TiO2(e-), can reduce surface chemisorbed oxidants, typically O2, to also yield the oxidizing species O2•-, HOO•, and/or OH• radicals that can bring about photooxidations. Thus, TiO2 can play an important role in electron-transfer mediation, even though it is not itself excited. Watanabe and co-workers (19a) reported analogous experiments but in aqueous CdS suspensions and identified some intermediates in the photoconversion of RhB; mechanistic details were not treated. Photoconversion of rhodamine 6G has been examined (12, 19b), and Matthews (19c) reported on the transformation of rhodamine B (RhB) and its partial mineralization under artificial UV light and sunlight UV mediated by TiO2 on sand as a support; no further details were given (19c). Thus, TiO2mediated and sunlight-driven photodegradations might provide a practical method to treat dye wastewaters. Prior adsorption appears indispensable for dyes to photodegrade on the TiO2 particle surface so as to facilitate electron injection (20). Greater adsorption should enhance the degradation rate; cationic dyes that adsorb poorly on the TiO2 surface should therefore be difficult to degrade. Under appropriate conditions, anionic surfactants such as sodium dodecylbenzenesulfonate (DBS) that strongly adsorb on S0013-936X(97)00792-X CCC: $15.00

 1998 American Chemical Society Published on Web 07/09/1998

titania particles might be able to assist poorly adsorbing dyes to coadsorb on the TiO2 surface and thereby facilitate their photodegradation. In the present study, we examine the characteristics of the photodegradation of a cationic dye, rhodamine B, under visible light illumination at wavelengths greater than 470 nm and in the presence and absence of DBS in aqueous TiO2 dispersions. Such factors as pH and variations in the concentration of DBS were examined and other studies done to infer further the details of adsorption and the subsequent degradation of the dye on TiO2 particles. Hydroxyl radicals, that might be implicated in the photodegradation process and formed in titania dispersions under visible light illumination by primary steps unlike those under UV light irradiation, have been confirmed by the characteristic ESR signals of DMPO-OH• spin adducts formed in laser illuminated TiO2/RhB/DMPO aqueous systems.

Experimental Section Materials. The TiO2 electron-transfer mediator (P-25; ca. 80% anatase, 20% rutile; BET area, ca. 50 m2 g-1) was kindly supplied by Degussa Co. Rhodamine B (tetraethylrhodamine, RhB) and sodium dodecylbenzenesulfonate (DBS) were laboratory reagent grade products and used without further purification. Deionized and doubly distilled water was used

throughout this study. Photoreactor and Light Source. A 500 W halogen lamp (Institute of Electric Light Source, Beijing) was positioned inside a cylindrical Pyrex vessel and surrounded by a circulating water jacket (Pyrex) to cool the lamp. A cutoff filter was placed outside the Pyrex jacket to completely remove all wavelengths less than 470 nm to secure irradiation with visible light only. The reactor used was designed to ensure a constant supply of atmospheric oxygen to the reaction volume. Procedures. Unless otherwise noted, aqueous TiO2 suspensions were prepared by adding 100 mg of TiO2 powder to a 50 mL solution containing the RhB dye and the anionic surfactant DBS at appropriate concentrations (see figure captions). Prior to irradiation, the suspensions were magnetically stirred in the dark for ca. 30 min to ensure establishment of an adsorption/desorption equilibrium between the TiO2, the RhB dye and atmospheric oxygen. Thereafter, the dispersion was kept under constant airequilibrated conditions. At given irradiation time intervals, samples (4 mL) were collected, then centrifuged, and filtered through a Millipore filter (pore size, 0.22 µm) to separate the TiO2 particles. The degraded solutions were analyzed by UV-vis spectra with a Shimadzu 1600A spectrophotometer after a 10-fold dilution of the filtrate to monitor the temporal loss of RhB and the possible formation of intermediate species; appropriately, initial rates of photodegradation of RhB were determined at a wavelength of 554 nm. For reactions in different pH media, the initial pH of the suspensions was adjusted by addition of either NaOH or HCl solutions.

FIGURE 1. (1) Absorption spectrum of an aqueous solution containing RhB (5 × 10-4 M) and the anionic surfactant DBS (1.2 × 10-3 M) recorded after a 10-fold dilution (2-7). Absorption spectra of aqueous TiO2 dispersions (loading 2 g L-1) containing RhB (initial concentration, 5 × 10-4 M) and the anionic surfactant DBS (1.2 × 10-3 M) during the photoassisted degradation of RhB under visible light illumination. Spectra of the dispersions were taken after removal of TiO2 by filtration through a 0.22 µm Millipore filter and after a 10-fold dilution of the filtrate. (2) Spectrum taken before irradiation, and after subsequent irradiation for (3) 0.5 h, (4) 1.0 h, (5) 2.0 h, (6) 3.0 h, and (7) 4.0 h. The zeta-potential (ζ-potential) and the specific inductive capacity of TiO2 dispersions were monitored with a Lazer Zee model 501 instrument (power, 100 VAC; 60 Hz; PEN KEM Inc., Bedford Hills, NY) in a dispersion containing TiO2 with DBS and/or RhB at pH 2.1. A Brucker model ESP 300E electron paramagnetic resonance spectrometer was used to measure the ESR signals of radical species formed in the degradation process. For this purpose, the irradiation source (wavelength, 532 nm) was a Quanta-Ray Nd:YAG pulsed laser system operated in the continuous mode at 10 Hz frequency. The settings for the ESR spectrometer were center field, 3486.70 G; sweep width, 100.0 G; microwave frequency, 9.82 GHz; power, 5.05 mW.

Results and Discussion The photodegradation of rhodamine B was carried out under visible light illumination at wavelengths greater than 470 nm in TiO2 aqueous dispersions containing the anionic surfactant DBS and in which the titania was not light activated. The characteristic absorption features of an aqueous RhB (0.5 mM) solution comprising DBS at its cmc concentration of 1.2 mM (critical micellar concentration) decreased substantially upon addition of TiO2 particles, indicating relatively strong adsorption of RhB to TiO2 particles under these conditions (compare spectra 1 and 2 in Figure 1). Subsequent illumination of the aqueous titania dispersion during a 4 h period led to a continued diminution of the RhB dye concentration in the solution bulk; concomitantly, no new absorption peaks appeared. This confirms the photodegradation of RhB, i.e., the breakup of the chromophore, rather than its discoloration or bleaching, as deethylation results in a blue-shift of the absorption bands seen in the 480-600 nm region (19a). There is little evidence of any RhB chromophore remaining at the end of the irradiation period, as evidenced by the near total loss of absorption in this wavelength range, although the presence of some aromatic intermediate species is not precluded (note, e.g., the spectral features at wavelengths below 320 nm). That such longer lived intermediates might remain is not surprising because once the visible light chromophore has been degraded, there is no longer light absorption and the TiO2VOL. 32, NO. 16, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Photodegradation of rhodamine B versus irradiation time for several systems illuminated by visible irradiation; systems were sampled, filtered through 0.22 µm Millipore filters and diluted 10fold prior to spectral analyses: (a) solution of RhB (5 × 10-4 M) and DBS (1.2 × 10-3 M) without TiO2; (b) dispersion containing TiO2 (loading, 2 g L-1) and RhB (5 × 10-4 M); (c) TiO2 dispersions (loading, 2 g L-1) containing DBS (at cmc, 1.2 × 10-3 M) and RhB (5 × 10-4 M).

SCHEME 1. Electron-Transfer Processes Subsequent to Excitation of RhB Dye

3.5-4 h (Figure 2). Compare with the TiO2/RhB system for which only ca. 45% of RhB is degraded after this illumination period. These results strongly point to the need for prior adsorption of RhB on the TiO2 particle surface for efficient degradation. Thus, addition of DBS enhances adsorption, an indispensable requirement to the photodegradation of RhB, a point often made but not always demonstrated. Under visible light illumination, the semiconductor is not excited as its absorption threshold is 385 nm; only the chemisorbed RhB is excited at wavelengths longer than 470 nm to produce singlet and triplet states (denoted here simply as RhB*ads). Subsequently, RhB*ads injects an electron into the conduction band (or to some surface state) of the semiconductor with RhB being converted to the radical cation RhB•+; this is pictorially illustrated in Scheme 1. In turn, the injected electron on the TiO2 particle, TiO2(e-), reacts with adsorbed oxidants, usually O2, to produce reactive oxygen radicals (eqs 1-7), as evidenced by ESR results (see Figure

Dyeads + hν f Dyeads*

(1)

Dye*ads + TiO2 f Dyeads•+ + TiO2(e-)

(2)

TiO2 (e-cb) + O2 f O2•-

(3)

O2•- + H+ f OOH•

(4)

OOH• + O2•- + H+ f O2 + H2O2

(5)

H2O2 + O2•- f OH• + OH- + O2

(6)

Dyeads•+ + (OH•, O2•-, and/or O2) f f degraded products (7)

mediated and visible-light-driven photodegradation process is terminated; however, we cannot preclude secondary reactions continuing to occur that might implicate remnants of radical species. Rhodamine B adsorbs rather poorly on TiO2 particulates (ca. 2%) and its photodegradation under such conditions is rather slow, necessitating about 11 h for complete disappearance in the absence of DBS but under otherwise identical conditions (see Figure 2). For comparison, control experiments in which TiO2 was absent in the RhB/DBS solutions were also carried out, experimental details and results of which are also given in Figure 2. Rhodamine B is a very stable dye under visible light illumination of a solution containing DBS but no TiO2 particles; absorption features showed no signs of spectral intensity loss during a 5 h irradiation period (curve a of Figure 2). As well, no degradation of RhB occurred in the dark in a TiO2/RhB/DBS suspension. By contrast, illumination of RhB/TiO2 dispersions led to RhB photodegradation (curve b) via zero-order kinetics: k ) (1.28 ( 0.07) × 10-3 M min-1; correlation coefficient ) 0.992. When the dispersions also contained the anionic surfactant DBS, adsorption of RhB on the surface of TiO2 particles increased significantly (to ca. 60-70%; see Figures 1 and 2) owing to surface charge neutralization by DBS and its rate of photodecomposition increased dramatically taking place via first-order kinetics: k ) (1.7 ( 0.2) × 10-2 min-1; t1/2 ) 40 min (curve c). Under these conditions, complete decomposition (but not mineralization) of the RhB chromophore occurred in less than ca. 2396

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7). Oxygen plays an additional important role here: in scavenging the electron, it suppresses recombination (back electron transfer) between RhB•+ and e-. The radical cation RhB•+ ultimately reacts with reactive oxygen radicals and/or molecular oxygen to yield intermediate products or other radical species, for which if secondary radical processes occurred might lead to mineralization (19c). In the present context, the semiconductor TiO2 acts as an electron-transfer mediator and the oxygen as an electron acceptor leading to efficient separation of the injected electron and the radical cation, thereby facilitating the degradation process. Photoelectrochemically, this is understandable since the process is thermodynamically favored owing to appropriately positioned redox potentials of the relevant couples (see also Scheme 1): E° (RhB*/RhB•+) ) -1.09 V (19a), E° (O2/O2•-) ) -0.33 V (22), E° (O2/HO2•) ) -0.037 V (22), Ecb (TiO2) ) -0.5 V, Evb (TiO2) ) 2.7 V (23). By contrast, when the TiO2 is also excited (12, 19b,c), the first step is generation of photoelectrons and photoholes on TiO2 (eq 8) followed by reactions 3-6 and by reaction 9 to yield the same active oxygen species, such as the OH• radical, which do lead to oxidative degradation and to the ultimate mineralization of many organic substrates. Clearly, there exists a basic difference in the primary steps of photodegradations depending on the excitation wavelengths.

TiO2 + hv f (e-cb) + (h+vb) f e- + h+

(8)

TiO2 (h+vb) + OH-ads f OHads• + TiO2

(9)

Measurement of the zeta (ζ)-potential of TiO2 particles and the specific inductive capacity of the dispersions were also carried out to further confirm some details regarding the coadsorption of RhB and DBS on the TiO2 particle surface. Figure 3a summarizes the changes in the ζ-potential of TiO2 particles as a function of changes in the surfactant DBS

a

b

FIGURE 3. (a) Effect of the DBS concentration on the zeta (ζ)-potential of TiO2 particles (40 mg L-1) at pH 2.1 in the presence and absence of 0.001 mM RhB; inset depicts the changes of the ζ-potential with changes in the RhB concentration in the TiO2/RhB system. (b) Effect of RhB concentration on the specific inductive capacity of a TiO2 suspension at pH 2.1 in the presence and absence of 0.1 mM DBS. concentration, in the presence and absence of the dye RhB. The ζ-potential changes significantly from positive to negative values with increasing DBS concentration, inferring that anionic DBS molecules are adsorbed on the surface of TiO2 particles. At higher DBS concentration (greater than 0.1 mM), the ζ-potential no longer changed. We infer that adsorption saturation of the TiO2 surface by DBS has been reached at concentrations greater than 0.1 mM. In the TiO2/RhB/DBS system, the ζ-potential is more positive than the ζ-potential of the TiO2/DBS system at concentrations of DBS greater than 0.1 mM, owing to the

coadsorption of positively charged RhB which leads to a partial offset of the negative charge on the TiO2 particle. The inset in Figure 3a gives the ζ-potential changes with increasing RhB concentration in a DBS-free TiO2 system. Almost no change of the ζ-potential was observed since the cationic RhB dye adsorbs rather poorly on the positively charged TiO2 surface in acidic media. The principal conclusion from the above results is that DBS adsorbs strongly on the TiO2 particles and improves/enhances the adsorption of the cationic rhodamine B dye. VOL. 32, NO. 16, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. Effect of the DBS concentration varied from 0 to 1 × 10-2 M on the initial rate of degradation of RhB (5 × 10-4 M) at pH 3.8 in TiO2 particulate dispersions (loading, 2 g L-1). Procedures for determining the RhB concentration are the same as those in Figures 1 and 2. The specific inductive capacity was assayed and is plotted in Figure 3b. This capacity reflects the concentration of conductive ions in the bulk of the TiO2 dispersion. For the TiO2/RhB system, this specific inductive capacity rises rapidly as the concentration of RhB increases, especially at RhB concentrations greater than 0.06 mM. This can be ascribed to the poor adsorption of RhB on the TiO2 particles and to an increase in the effective concentration of free RhB in the solution bulk. By contrast, the specific inductive capacity in the TiO2/DBS/RhB system (at constant DBS concentration of 0.1 mM) shows only a slight rise with an increase in the RhB concentration and even so at greater RhB concentrations. Evidently, coadsorption of the cationic RhB dye and the DBS surfactant on TiO2 does take place. At the higher concentrations, the added RhB molecules do not enter into the solution bulk but coadsorb on the TiO2 surface. Hence, the quantity of free RhB molecules in the solution bulk no longer increased with an increase in the RhB concentration. To elucidate further the relationship between the extent of adsorption and the photodegradation process, we examined the influence of the DBS concentration on the initial rates of photodegradation of RhB in TiO2 aqueous dispersions. The results are portrayed in Figure 4 as initial rates of RhB degradation versus various DBS concentrations. An increase in the DBS concentration had a significant effect on the initial rates of RhB degradation, reaching maximal values at the critical micellar concentration (cmc ) 1.2 mM) of the DBS surfactant. This confirms the above view that, at the cmc,

RhB and DBS coadsorb on the surface of TiO2 particles to the greatest extent possible. At concentrations greater than cmc, DBS forms both micelles in the solution bulk and hemimicelles on the TiO2 surface (Figure 5). Some of the RhB molecules are locked inside the micelles so that RhB* states can no longer approach the TiO2 particle and inject electrons. As well, the number of RhB molecules adsorbed on the DBSmodified TiO2 surface or in the DBS hemimicelles illustrated in Figure 5 is relatively smaller because of the equilibrium between the RhB molecules locked in the micelles and the RhB molecules coadsorbed on the TiO2 surface. Hence, the rate of degradation of RhB decreased. At concentrations of DBS lower than cmc, RhB is not efficiently coadsorbed on the TiO2 particles since DBS hemimicelles have not formed on the TiO2 surface. The possible degradation of DBS during the degradation of RhB was not examined in this work; however, it is known that DBS photodegradation by irradiated TiO2 particulates is rather slow (8, 9). The factual vision of coadsorption rests on the notion that the sulfonate moiety in the DBS molecule is easily chemisorbed to the Ti4+ ions on the TiO2 particle surface in an aqueous dispersion (8, 9) and that the anionic surfactant DBS forms hemimicelles around the critical micellar concentration on the surface of the particles. This should improve/enhance adsorption of RhB because of the hemimicellar hydrophobicity and the electrostatic attractive interaction between the anionic DBS and the cationic RhB dye (as sketched in Figure 5). As a result, RhB and DBS coadsorb on the TiO2 particle surface. Thus, both the degree of adsorption and the rate of degradation of the cationic RhB dye have dramatically increased in the presence of the anionic surfactant with respect to the case where no surfactant is present to neutralize the positive charge on the TiO2 surface under acidic conditions (pH ca. 2). A variation in pH from 3 to 10 also influences the photoassisted degradation of RhB (0.5 mM) in aqueous TiO2 dispersions in the presence of DBS at 1.2 mM concentration (Figure 6). In such dispersions, the initial rates of photodegradation of the RhB dye decreased linearly from 8.2 × 10-6 M min-1 at pH 2.5 to ∼0.5 × 10-6 M min-1 at pH 10. By contrast, there is little if any change (within experimental error) in the degradation of RhB in TiO2 dispersions containing no DBS surfactant in this pH range. Such variations in pH are expected to modify the coadsorption of RhB and DBS on the TiO2 surface, inasmuch as adsorption of the anionic surfactant decreases dramatically at the higher pHs, i.e., formation of hemimicelles is diminished significantly as the particle surface becomes negatively charged at pHs greater

FIGURE 5. Adsorption model illustrating our views on the coadsorption of RhB and DBS on the TiO2 particle surface: (a) RhB only; (b) coadsorption of RhB and DBS. 2398

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orders of magnitude lower than formation of the DMPOOH• spin adduct (k ) 3.5 × 109 M-1 s-1) (24). Alternatively, the transition from DMPO-OOH• and DMPO-O2•- to DMPOOH• was too fast (25). The ESR spectrum taken after a 180 s preillumination period at high laser photon fluences (wavelength ) 532 nm) shows that the spectrum has basically relaxed to the 0 s spectrum since the color of the RhB dye has been totally bleached along the laser beam after this time and the generation of OH• radicals is terminated. On the other hand, at the much lower photon fluences prevalent in conventional UV-vis lamps, no spin adduct DMPO-OH •was detected at ambient conditions. Consequently, the exact lifetime of the DMPO-OH• adducts could not be determined effectively under the experimental conditions employed.

Conclusions FIGURE 6. Effect of pH on the initial rate of degradation of TiO2 suspensions containing RhB (5 × 10-4 M) and with or without DBS (at cmc, 1.2 × 10-3 M) under visible light irradiation.

The rhodamine B chromophore can be photodegraded on the TiO2 particle surface under visible light illumination by a process involving electron injection sensitization of TiO2. Addition of the anionic surfactant sodium DBS to the aqueous TiO2 dispersions greatly enhanced adsorption and photodegradation of RhB. At the low pHs of the TiO2/DBS/RhB dispersions, adsorption and degradation rates of RhB were more significant than at the higher pHs because of unfavorable adsorption of the anionic surfactant in alkaline media. Maximal photodegradation rates for the RhB dye were achieved when the DBS surfactant concentration was near or at its critical micellar concentration of 1.2 mM. The present results confirm and support the notion that degradation takes place at the TiO2 semiconductor particle surface, rather than in the solution bulk. By analogy with other studies, we infer that the active oxygen species OH• radicals likely participate in the photodegradation process, although they may not be the sole species.

Acknowledgments ESR signals of the DMPO-OH• adduct in TiO

FIGURE 7. 2/DMPO/RhB dispersions at pH 4.0: volume, 0.3 mL in a special capillary for ESR measurements; TiO2, 30 mg; DMPO, 0.16 M; RhB, 2 × 10-4 M. The signals were recorded before illumination (0 s), and after preillumination for 30 and 180 s at 532 nm with a Nd:YAG pulsed laser operated in the continuous mode at 10 Hz frequency (note, pulsed irradiation was continued during the recording of the ESR signal; 40 s). than ca. 5-6 (8, 9). Adsorption of RhB on TiO2 particles is thereby affected. Again, photodegradation is closely related to adsorption of the RhB dye on the TiO2 particle surface so as to facilitate electron injection from the chemisorbed excited dye RhB* to TiO2. Figure 7 illustrates the electron paramagnetic resonance spectra of the DMPO-OH• spin adduct under ambient conditions in a DMPO/TiO2/RhB dispersion at different times of illumination at 532 nm with the 10 Hz operated pulsed Nd:YAG laser. The characteristic four peaks of the DMPOOH• adduct with intensity 1:2:2:1 observed after a 30 s preillumination period are consistent with similar spectra reported by others for the OH• adduct (9, 10). This confirms the formation of OH• radicals via eq 6 (or equivalent) during the TiO2-assisted photodegradation of rhodamine B under visible light irradiation. According to the processes summarized in reactions 1-7, the ESR signals for the spin adducts DMPO-OOH• and/or DMPO-O2•- should also have been detected. However, under our experimental conditions, these latter two spin adducts were not observed probably because they did not form due to unfavorable kinetics; rates of formation of DMPO-OOH• and DMPO-O2•- adducts are k ) 6.6 × 103 M-1s-1 and 10 M-1 s-1, respectively, clearly several

The authors appreciate the generous financial support of this work from the National Natural Science Foundation of China (No. 29677019 and No. 29725715), the Foundation of the Chinese Academy of Sciences and the China National Committee for Science and Technology. The work in Tokyo was sponsored by a Grant-in-Aid for Scientific Research from the Ministry of Education (No. 10640569), whereas the work in Montreal was sponsored by the Natural Sciences and Engineering Research Council of Canada (No. A5443). We are greateful to Mr. C. Guo for technical assistance and Prof. J. Chen for the measurement of the ESR spectra.

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Received for review September 5, 1997. Revised manuscript received May 11, 1998. Accepted May 25, 1998. ES9707926