Mechanism Investigation of Visible Light-Induced Degradation in a

Department of Physics, Nanjing University. , §. National Laboratory of Solid State Microstructures, Nanjing University. , ∥. Zhengzhou University...
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Environ. Sci. Technol. 2009, 43, 8361–8366

Mechanism Investigation of Visible Light-Induced Degradation in a Heterogeneous TiO2/Eosin Y/Rhodamine B System M I N G C A I Y I N , †,‡,§,| Z H A O S H E N G L I , †,§ J I A H U I K O U , †,‡,§ A N D Z H I G A N G Z O U †,‡,§,* Eco-materials and Renewable Energy Research Center (ERERC), Nanjing University, Nanjing 210093, P. R. China, Department of Physics, Nanjing University, Nanjing 210093, P. R. China, National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, P. R. China, and Department of Chemistry, Zhengzhou University, Zhengzhou 450052, P. R. China

Received July 6, 2009. Revised manuscript received August 24, 2009. Accepted September 9, 2009.

Visible light-induced degradation of rhodamine B (RhB) and eosin Y (EO) in a heterogeneous TiO2 P-25/EO/RhB system was investigated in the present work. The results showed that the photodegradation of RhB is enhanced significantly when EO is introduced into the P-25/RhB system. Under optimal conditions (50 mg P-25, 20 mg L-1 EO), RhB (4 mg L-1) almost decomposed completely after 35 min of visible light irradiation, though EO was photodegraded simultaneously. The possible photodegradation mechanism was studied by the examination of active species HO•, O2•- anions, or dye radical cations through adding their scavengers such as methanol, t-butanol, benzoquinone, EDTA, and the I- anion. In addition, the electron paramagnetic resonance (EPR) spin trapping technique was also used to monitor the active oxygen species formed in the photocatalytic process. Combined with the contrastive experiments under different atmospheres (N2-purged or air) and in different systems, it can be deduced that dissolved O2 plays a crucial role in dye photodegradation and the O2•- anion is possibly the major active oxygen species. The low degradation rate with the introduction of EDTA or I- indicated that dye radical cations also play a part in photodegradation. Furthermore, except for the dye-sensitized photodegradation on the P-25 surface, reaction in bulk solution also occurs in this system, leading to effective photodegradation of RhB.

Introduction Photocatalysis has many merits in terms of the removal of harmful organic compounds, wastewater treatment, and clean up of polluted air (1-4). Semiconductor photocatalysts, for example, TiO2 (5-9), have gained much attention because they can potentially utilize inexpensive and inexhaustible solar radiation. In addition, TiO2 is nontoxic, inexpensive, and more stable than other photocatalysts under ambient * Corresponding author phone: +86-25-83686630; fax: +86-2583686632; e-mail: [email protected]. † Eco-materials and Renewable Energy Research Center (ERERC), Nanjing University. ‡ Department of Physics, Nanjing University. § National Laboratory of Solid State Microstructures, Nanjing University. | Zhengzhou University. 10.1021/es902011h CCC: $40.75

Published on Web 09/30/2009

 2009 American Chemical Society

conditions, so it would be ideal for use in clean technology. However, slow reaction rate and poor solar efficiency has hindered its commercialization (10). To handle these problems, many attempts by researchers have been carried out to modify its surface or bulk properties such as doping with various metal ions or nonmetals (11-13), deposition of noble metals (14, 15), and mixing of two semiconductors (16-18). Dye sensitization is another method to eliminate these drawbacks, and until previously, various types of dyes have been used (19-28). For example, TiO2 particles sensitized by RuII [bpy-(COOH)2]32+ have demonstrated good performance (29). Furthermore, a lot of research on photocatalytic decomposition of colored or colorless organic pollutants has been done. The mechanism of dye-sensitized photodegradation of organic pollutants over TiO2 is generally regarded as follows: the dyes, rather than the TiO2 photocatalyst, are subject to visible light excitation. The excited dye molecules subsequently transfer electrons onto conduction bands (CBs) of TiO2, leading to formation of dye cationic radicals. The injected electrons then react with the dioxygen adsorbed on the surface of TiO2 and generate a series of active oxygen species such as O2•-, H2O2, and •OH (30), i.e., the CB of TiO2 acts only as a mediator for transferring electrons from the sensitizer to substrate electron acceptors on the TiO2 surface, and the valence band (VB) remains unaffected. Although the basic processes for dye-sensitized photodegradation of organic pollutants has been extensively reported, the synergistic mechanism of two dye pollutants applied to a heterogeneous TiO2 system remains unclear until now, even if it exists extensively in dye wastewater effluents. It is conceivable that the degradation process of two dyes system will be more complex compared with that of single dye because organic dye often serves as a sensitizer and substrate to be degraded (31, 32). In addition, in the field of photocatalysis, there have been many debates as to whether photoreactions occur on the catalyst surface involving reactions with positive holes (hvb+) or surface-bound •OH or in the solution with free radicals (33, 34). All of the above background information led us to investigate the photodegradation of two dyes in a heterogeneous TiO2 P-25 system, and two xanthene dyes, eosin Y (EO) and rhodamine B (RhB), were used as the model compounds in the present study. Their structures are presented as Figure 1. Compared with EO, RhB, which contains four N-ethyl groups at either side of the xanthene ring, is relatively stable in aqueous solutions upon visible light irradiation (35), probably due to the different substituent groups on the xanthene ring (36). The main purpose of this work is to examine the optimal experimental conditions with the highest photodegradation efficiency and investigate the degradation mechanism of this complex system. The investigation of this system would set a foundation for treating dye pollutants in wastewater effluents.

FIGURE 1. Molecular structures of eosin Y and rhodamine B. VOL. 43, NO. 21, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. (a) Time courses of dye concentration during photodegradation with different EO concentrations (Note: all C/Co values were obtained only by the maximum absorption in the whole absorption spectrum in order to evaluate the degradation efficiency of both dyes). (b) Time courses of EO and RhB concentrations during photodegradation with 20 and 25 mg L-1 of EO.

Experimental Section Materials and Measurements. TiO2 P-25 (a mixture of 80% anatase and 20% rutile with an average specific surface area of about 50 m2/g) was obtained from Degussa Co. of Germany. Mesoporous TiO2 was prepared in our lab according to the literature method (37). Commercial TiO2, EO, RhB, and methyl orange (MO) were obtained from Sinopharm Chemical Reagent Co., Ltd., and used as received. Stock solutions of EO (1 g L-1), RhB (500 mg L-1), and MO (500 mg L-1) were prepared by dissolving certain amount of dyes in deionized water. The deionized water used was ultrapure (18 MΩ cm) and prepared by a Barnstead purification system. 5,5Dimethyl-1-pyrroline N-oxide (DMPO) was purchased from Tokyo Chemical Industry Co., Ltd. All other chemicals were of reagent grade and used without further treatments. The specific surface areas were measured with Micrometrics TriStar 3000 surface area and porosity analyzer by the Brunauer-Emmett-Teller (BET) method. The detailed mechanism of the photocatalytic dye degradation in neutral solution was elucidated through the use of different hvb+ or •OH, O2•- scavengers and EPR tests. EPR spectra were detected using a Bruker EMX-10/12 electron paramagnetic resonance spectrometer. The settings for the EPR spectrometer were center field, 3480.0 G; sweep width, 200.0 G; microwave frequency, 9.751 GHz; and power, 19.873 mW. The light source for EPR determination was a 300 W xenon lamp equipped with a 420 nm cutoff filter. The threedimensional (3D) fluorescence spectra of EO/RhB aqueous solutions were recorded on a Varian Cary Eclipse fluorescence spectrophotometer. Photocatalytic Experiments. The concentration of RhB (or MO) was fixed to 4 mg L-1 for all experiments, and the concentration of EO varied from 10 to 25 mg L-1 under different conditions. A 300 W Xe lamp equipped with a 420 nm cutoff filter was used as a light source. The aqueous suspension (100 mL) of a certain amount of EO and RhB (or MO) and 50 mg of photocatalyst was placed in a vessel. Prior to irradiation, the suspension was magnetically stirred in the dark for about 30 min to establish an adsorptiondesorption equilibrium. The reactor was open to air (or purged with gases), and the suspension was stirred magnetically during the irradiation. At regular time intervals, an aliquot of 3-4 mL was withdrawn and filtered through a 0.45 µm syringe filter to remove the particles. During the experimental procedure, changes in the concentration of two dyes were monitored from its characteristic absorption band (generally 516 nm for EO and 553 nm for RhB) using a Cary 50 Probe UV-vis spectrophotometer. The amount of the dyes adsorbed on TiO2 was calculated from the absorbance difference (∆A ) Amix - Aeq). The purging N2 or O2 experiment was conducted at a flow rate of ∼0.08 L min-1 for 30 min prior to the 8362

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experiments, and the gas was purged during the whole experiment.

Results and Discussion Effect of EO and Catalyst Concentration on Dye Photodegradation. It is well-known that dye photodegradation rate is affected by many factors such as light source, catalyst amount, and dye concentration (38), so the optimal degradation conditions, EO and catalyst concentration, were investigated at first. The concentration of RhB was fixed to 4 mg L-1, and the catalyst amount was 50 mg in the initial experiment, while the concentration of EO was varied from 10, 15, 20, to 25 mg L-1. From the degradation courses illustrated in Figure 2a, it is shown that photocatalytic activity is enhanced gradually with an increasing proportion of EO, and optimal photocatalytic efficiency is obtained at a 5:1 molar ratio, in which the EO concentration is 20 mg L-1, and the two dyes are almost 100% decolorized in about 35 min. It is worthwhile to note that the C/C0 values in Figure 2a were obtained only through maximum absorption in order to evaluate the degradation efficiency of both dyes. The time courses of RhB concentration during photodegradation are supplied in Figure S1of the Supporting Information. However, the degradation rates of the two dyes decreased again with a further increase in EO concentration (Figure 2b), probably due to the competitive adsorption of EO and RhB on the P-25 surface. The adsorption determination results indicated that the adsorption amount of EO decreased, whereas that of RhB increased when the concentration of EO was enhanced from 20 to 25 mg L-1. Probably it is just this reduction in the EO amount on the P-25 surface that leads to the lower degradation rate of RhB. However, this phenomenon can also be elucidated as follows. If dye concentration increases continuously, the dissociative dye molecules, which cannot participate in electron transfer on the catalyst surface but absorb a part of incident light at the same time, will also increase and result in the loss of part of the incident light by acting as an internal filter, resulting in a decrease in the degradation rate (38). In order to investigate the influence of catalyst concentration on dye degradation, 30, 50, 60, or 80 mg P-25 was used for the P-25/EO/RhB system, and the degradation courses are shown in Figure 3a. It can be found that when 50 mg P-25 was used, optimal decolorization efficiency was obtained. The UV-vis spectra of the aqueous samples collected at various irradiation time intervals for 50 mg P-25 are shown in Figure 3b. In Figure 3a, during the first 10 min, the optimal decolorization efficiency observed with TiO2 of 80 mg is probably due to the higher dosage of TiO2 leading to more dye molecules being adsorbed on the surface of TiO2.

FIGURE 3. (a) Time courses of dye concentration during photodegradation with different amounts of P-25 in the P-25/EO/RhB system. (b) Temporal UV-visible absorption spectra observed for the P-25/EO/RhB system as a function of irradiation time (EO, 20 mg L-1; RhB, 4 mg L-1). It is well-known that photodegradation of RhB usually occurs via two competitive processes: (1) N-deethylation and destruction of the conjugated structure and (2) a hypsochromic shift of the absorption band presumed to result from the formation of a series of N-deethylated intermediates in a stepwise manner, which often exists (39, 40). However, no obvious absorption band shift was observed in the present system (P-25, 50 mg; RhB, 4 mg L-1; and EO, 20 mg L-1), suggesting the efficient decomposition of the conjugated xanthene ring in RhB (40, 41). However, when the EO concentration decreased to 10 mg L-1, a distinct hypsochromic shift emerged (Figure S2 of the Supporting Information). Form Figure 3b, it is shown that not only the main absorbance in the visible region but also the peaks in the UV region are reduced with irradiation, indicating that the dye chromophores and aromatic ring were destroyed, in other words, the dye molecules were degraded instead of being only decolorized (40).

Photodegradation Mechanism Photodegradation on a P-25 Surface and/or in Bulk Solution. From Figure 2, it is also shown that the introduction of EO has a great influence on the photocatalytic degradation of RhB, though EO decomposed simultaneously. In order to make the photodegradation process clear, a series of contrastive experiments were conducted. No degradation was detected in the dark in the system of EO/RhB or that of P-25/EO/RhB, signifying that the dye degradation in the present study is indeed through a photocatalytic process. However, in the absence of P-25, simultaneous degradation of EO and RhB was observed in a homogeneous EO/RhB system under visible light irradiation, and the degradation rate of RhB increased greatly compared with a single RhB system (Figures 4 and 5), though that of EO decreased to a certain extent. To the best of our knowledge, the probable pathway for homogeneous dye photodegradation in the presence of dissolved oxygen is homolysis of excited dyes into radicals or electron transfer from excited dye molecules to electron donor (for example, O2) to form dye radical cations, which are decomposed subsequently by a superoxide radical anion (O2-•) or singlet oxygen (1O2) (42). So, for the EO/RhB homogeneous system, self-photolysis and mutual interaction between two dyes (direct electron transfer between EO and RhB) may both occur (43, 44), verified by their redox potentials (E0 (EO*/EO+) ) -1.53 V, E0 (EO/EO+) ) 0.80 V, E0 (RhB*/ RhB+) ) -1.42 V, E0 (RhB/RhB+) ) 0.95 V vs NHE, pH 7) (45) and that of O2 (E0 (O2/O2•-) ) -0.28 V vs NHE, pH 7) (46), and from which it can also be concluded that RhB plays the role of stabilizer (47), namely, acting as an electron donor sacrificial agent of EO in the degradation course. From this

FIGURE 4. Time courses of dye concentration during photodegradation in different systems (Note: all C/Co values were obtained only by the maximum absorption in the whole absorption spectrum in order to evaluate the degradation efficiency of both dyes). point of view, the electron transfer from the excited RhB to EO should also be possible; however, the degradation rate of RhB is improved, on the contrary. This can be explained from the experimental fact that EO is more easily degraded than RhB under visible light irradiation, which implies that in an EO system, dye radical cations, 1O2 and O2•-, are more liable to form than in a RhB system. That is to say, more active species exist in an EO/RhB system than in a single RhB system, and accordingly, the photodegradation rate of RhB was increased greatly. A series of experiments were also conducted in order to clarify the case for a heterogeneous TiO2/EO/RhB system. The results showed that TiO2-mediated dye photodegradation proceeds and plays a major role because the degradation rate of the heterogeneous systems were much larger than that of the homogeneous ones, for example, P-25/RhB . RhB, P-25/EO > EO, and P-25/EO/RhB . EO/RhB as depicted in Figure 4. So, electron transfer from EO or RhB to the CB of TiO2 occurs and plays an important role in dye photodegradation, which can also be testified by the oxidation potentials of excited dyes (-1.53 and -1.42 V vs NHE, pH 7 for the excited singlet states of EO and RhB, respectively) and that of CB of TiO2 (-0.52 V vs NHE, pH 7) (46). However, it also shows that the electron transfer from EO to the CB of P-25 should be easier than that for RhB, consistent with the results that EO is more liable to be degraded than RhB. Furthermore, it should be noted that high temperature might promote thermal decomposition of dyes. So, a dark control in 40 °C proceeded, and the result showed no VOL. 43, NO. 21, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 5. The EO and RhB degradation courses for EO/RhB homogeneous system (a) and heterogeneous P-25/EO/RhB system (b) under air and N2-purged atmospheres, respectively (a: EO, 20 mg · L-1; RhB, 4 mg · L-1; b: EO, 10 mg · L-1; RhB, 4 mg · L-1). degradation took place after 120 min, indicating that thermal decomposition was negligible in a P-25/EO/RhB system. Effect of Photodegradation Atmosphere. Comparison of the photolysis in air equilibrium (no gas purged), N2purged, and O2-purged conditions was also undertaken to further elucidate the degradation mechanism, and some degradation courses are shown in Figure 5. For the dye self-photodegradation of a EO/RhB system, it is worthwhile to note that the degradation rates of EO and RhB under air equilibrium conditions are clearly higher than those under N2-purged condition (Figure 5a). In addition, under a N2 atmosphere, the RhB decolorization rate for a EO/RhB system is still higher than that for a single RhB system, indicating that except for the photolysis process, interaction between two dyes in the bulk solution also plays an important role in degradation, consistent with the results obtained above. Another separate experiment showed that additional O2 purging does not result in a further increase in the degradation rate, illustrating that air-equilibrated dissolved oxygen was sufficient for dye degradation. However, from Figure 5, it is also shown that under a N2 atmosphere, the degradation rates of EO and RhB in a heterogeneous P-25/EO/RhB system are higher than those in a homogeneous EO/RhB one, which suggests that in the anoxic condition, catalyst P-25, has an enhancing effect on the dye degradation, namely, the surface reaction is more effective than that in the bulk solution, also in accordance with the above results. Effect of Scavenger Agents and Investigation on Active Species. Another series of tests were also conducted to probe the mechanism responsible for this visible light-induced photocatalysis. At first, some sacrificial agents, such as tertbutanol (tBuOH) [V (tBuOH/H2O) ) 1:20], methanol [V (CH3OH/H2O) ) 1:4], EDTA (c ) 10 mmol L-1), KI (c ) 10 mmol L-1), and benzoquinone (BQ, c ) 1 mmol L-1) were added to the degradation system in order to ascertain the active species in degradation process. The results showed that when •OH scavengers, tBuOH (48), or methanol was put into the degradation systems, no significant changes occurred on dye degradation except for a hypsochromic shift of the absorption band (Figure S3a,b of the Supporting Information), showing that the •OH is not the major oxidation species in this process. However, when holes scavenger, EDTA (48) or KI (44), was introduced, the degradation rates of EO and RhB were depressed to some extent (Figure S3c,d of the Supporting Information). It is well-known that no hvb+ arises on P-25 under irradiation of visible light because its VB is unaffected, so the low degradation rate in the presence of EDTA or KI may be assigned to electron transfer between them and two dye molecules, namely, EDTA and I- act as electron donors, which can also be confirmed by their redox potentials and that of I- (E (I•/I-) ) 1.3 V (44) and E (I3-/I-) ) 0.535 V vs NHE). 8364

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FIGURE 6. EPR spectra of DMPO-•OH adducts after different periods of irradiation for P-25/RhB dispersion in H2O. Electron transfers from EDTA or KI to them make the dye radical cations return to the ground states, and correspondingly, result in a low degradation rate. Furthermore, the influence of EDTA or I- on the degradation manifests that the dye radical cations also play a part in the photocatalysis course. As depicted above, dissolved O2 plays a key role in EO and RhB degradation, but •OH has been proven not to be the major oxidation species in this process. So, BQ, an O2•quencher, was used to examine the role of O2•- (49). The great influence of BQ on degradation (only about 40% degraded after 60 min; see Figure S3e of the Supporting Information) showed that the dye photodegradation is caused by O2•- to a large degree. The EO concentration increases for panels d and e of Figure S3 of the Supporting Information are probably attributed to the changes in the surface chemistry of P-25 due to the introduction of I- anions or BQ, respectively, which further affect the adsorption of EO (anionic dye) and RhB (cationic dye). The partial desorption of EO from the P-25 surface possibly induced the increases in EO concentration. EPR Experiments. To further confirm the active species, we carried out EPR/DMPO spin trapping experiments to detect the active oxygen species in this system under visible light irradiation (50, 51). The results revealed that no DMPO-•OH or DMPO-O2•- spin adducts were detected before irradiation, while after irradiation, characteristic peaks of DMPO-•OH were obviously observed for the P-25/RhB/ DMPO system (Figure 6), though no peaks were observed for either the P-25/EO/DMPO or P-25/EO/RhB/DMPO one. This is possibly due to the intensive and fast photodegradation of EO, which acts as a quencher of O2•- and •OH and results in a competition reaction between EO and DMPO. By

FIGURE 7. EPR spectra of DMPO-O2•- adducts in DMSO after 5 min irradiation: (a) P-25/RhB and (b) P-25/RhB/EO dispersion. comparison, EO should be more active than DMPO for oxygen active species. Moreover, for the P-25/RhB/DMPO system, the intensity of DMPO-•OH increased with prolonged irradiation time (2-5 min). However, when the irradiation time was much longer (30 min), the EPR signals disappeared, similar to the literature reported (52). As reported in the literature, O2•- is unstable in aqueous solution and is easily decomposed into •OH, and the reaction rate constant of DMPO with •OH is much larger than that with O2•- (53). So, it is usual that O2•- is hard to detect, though it is formed in the reaction. In order to further verify whether O2•- is formed in the P-25/RhB/DMPO system, a polar aprotic solvent, DMSO, was used instead of water (53), and the results are illustrated in Figure 7. It is shown that O2•- signals were observed in the visible light illuminated DMSO solution of P-25/RhB/DMPO, showing that O2•- is also generated in the P-25/RhB system. However, when a little EO was added into the above P-25/RhB suspension, the EPR signal of the DMPOO2•- adduct disappeared, further approving the above presumption that EO is a quencher of active oxygen species and more active than DMPO for O2•-. From all of the above results, it could be deduced that these two dyes, EO and RhB, were eliminated by oxygenous radical oxidation under visible light irradiation to a large degree. The photodegradation on the TiO2 surface and in the bulk solution proceeds in a heterogeneous P-25/EO/ RhB system, and the former plays more a important role than the latter, which results in effective degradation of EO and RhB under the optimal conditions (50 mg P-25 and 20 mg L-1 EO). The acceleration of RhB degradation can be attributed to the mutual interaction between two dyes and the generation of more active species by the effective electron transfer from EO to the CB of TiO2. The lower degradation rate of EO in a P-25/EO/RhB system than that in single EO system can be ascribed to an electron transfer from RhB to EO. 3D Fluorescence Spectra of a Homogeneous EO/RhB System. In order to further affirm the interaction between an EO and RhB in the solution, 3D fluorescence spectra were determined, and the results (Figure S4 of the Supporting Information) showed that the interaction between EO and RhB in a homogeneous solution exists. It is enhanced along with an increase in EO to a RhB molar ration. Effect of Catalyst Property. Besides P-25, RhB degradation on two other kinds of TiO2, mesoporous TiO2 (denoted as m-TiO2, anatase, specific surface area of about 107.8 m2/g) and commercial TiO2 (denoted as c-TiO2, rutile, specific surface area of about 2.3 m2/g) were also studied, and the degradation courses (Figure S5 of the Supporting Information) showed that P-25 possesses the highest degradation efficiency, confirming that surface area and crystal phases

influence degradation, consistent with the literature reported (54). In addition, the dye degradation over insulator Al2O3 (data not shown) exhibited a much lower degradation rate than that without a catalyst, also indicating that the catalyst property plays a major role in the dye photodegradation. Photodegradation of MO. The photodegradation of MO in heterogeneous P-25/EO/MO was also investigated, and similar results to RhB were obtained (Figure S6 of the Supporting Information). That is to say, introducing EO into a P-25/MO suspension also leads to a significant improvement in MO degradation. Elimination of Likelihood of Stray UV Light. Degradation of MO in a P-25/EO/MO system with a cutoff filter of 480 nm was conducted in order to diminish the probability of stray UV light. The result indicated that no obvious changes take place in the dye photodegradation rate, which eliminates the likelihood of stray UV light.

Acknowledgments Financial support from the National Natural Science Foundation of China (50732004 and 20773064), Science and Technology Research Program of the Ministry of Education (MOE) of China (307012), and National Basic Research Program of China (973 Program, 2007CB613301, and 2007CB613305), and China Postdoctoral Science Foundation (20080441028) are gratefully acknowledged.

Supporting Information Available Figures S1-S6. This material is available free of charge via the Internet at http://pubs.acs.org.

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