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Evidence for the Active Species Involved in the Photodegradation Process of Methyl Orange on TiO2 Wenjuan Li, Danzhen Li,* Yangming Lin, Peixian Wang, Wei Chen, Xianzhi Fu, and Yu Shao Research Institute of Photocatalysis, State Key Laboratory Breeding Base of Photocatalysis, Fuzhou University, Fuzhou, 350002, P. R. China S Supporting Information *

ABSTRACT: Active species such as holes, electrons, hydroxyl radicals (•OH), and superoxide radicals (O2•−) involved in the photodegradation process of methyl orange (MO) over TiO2 photocatalyst were detected by several techniques. Using different types of active species scavengers, the results showed that the MO oxidation was driven mainly by the participation of O2•−, holes and •OH radicals. Characterized by the liquid chromatography/mass spectrometry, the transversion of the degradation products with the light irradiation time was first analyzed. Combined with the measurement of oxidation reduction potential, dissolved oxygen, conductivity, and pH values, the degradation process of MO on TiO2 under the effect of the active species was revealed. This was the first time that electrodes were introduced to track the degradation process in situ, and these parameters would be helpful to explain the degradation processes of other organic pollutants.

1. INTRODUCTION Because of its excellent long-term stability, perfect physicochemical properties, and the appropriate position of its conduction band (CB), TiO2 is employed widely in the destruction of organic pollutants. The mechanism of the photocatalytic process on TiO2 has been also extensively studied and reported.1−5 It is often reported that the adsorption of organic compounds on the semiconductor surface is a prerequisite for organic photodegradation. However, other studies suggest that the reactive radicals such as hydroxyl radicals (•OH) can diffuse into the solution to react with organic pollutants. Therefore, the adsorption of organic contaminants would increase the reaction rate but is not required in the case of radical formation.6−8 Because of the high reactivity of reactive radicals, they cannot diffuse far and the reaction has to take place close to the surface.9 Generally, once UV light (λ < 380 nm) irradiated on the TiO2 semiconductor, electrons from valence band (VB) would be excited to the CB with the formation of photoinduced charge carriers [e.g., electron (e−), positive hole (h+)]. Then the photoinduced charge carriers transfer to the surface of the semiconductor where they can participate in oxidation−reduction reactions. Therefore, the transfer of electrons in the photocatalytic process is very important, which would inhibit the recombination of electrons and holes, and thus increase the activity. The following formed active species such as superoxide radicals (O2•−) and •OH are also very reactive and easily attack the state with high electron density on the surface of TiO2 particles. Which is the main reaction pathway, through direct electron transfer between substrate and positive holes, or active speciesmediated? What is the relationship between these active species © 2012 American Chemical Society

and the degradation process? How are these active species generated and which factors affect their generation? Despite the contributions from a number of research groups, detailed mechanisms of the photocatalytic oxidation processes on the surface of TiO2 still remain obscure and controversial.10 The •OH is a strong oxidant to attack the state with high electron density. It can be generated in different ways. Among these, surface hydroxyl groups are the important sources for its generation. It is well-known that the hydroxyl groups of some metal oxides are at the origin of the acidic or basic properties of these solids. When the surface of TiO2 is fully hydroxylated, to keep the electroneutrality, the oxide ions in oxides and H2O absorbed on the surface would distribute electrons. So a special crystal face bears equal quantities of two types of hydroxide ions: a bridging OH group bound to a surface Ti4+ ion which is four coordinates with respect to the lattice oxide ions and a terminal OH group bound to a Ti4+ ion which is five coordinates with respect to the lattice oxide ions.11 The bridging OH group possesses acidic property, while the terminal OH group shows basic property. This follows from Pauling’s electrostatic valence rule.12,13 There is a net charge of zero on the surface at a definite pH of the surrounding solution; this is the zero point of charge (zpc). The zpc of TiO2 (P25) is pH 6.3.14 Water is considered as a neutral solution with pH = 7. According to Pauling’s electrostatic valence rule, when TiO2 (P25) was dispersed in water, in order to maintain the electroneutrality, the surface of TiO2 (P25) would release some acidic groups. Received: October 7, 2011 Revised: December 30, 2011 Published: January 11, 2012 3552

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In this paper, we assigned these groups to OH (acid). The relationship between the acidic groups and •OH was analyzed. As to the detection of the active species, some techniques have been developed. For example, the electron spin resonance (ESR) technique has been established15,16 for the qualitative analysis of active oxygen species. Chemiluminescence from luminol solutions is used for observing the dynamic processes of superoxide radicals.17,18 However, there are few detection techniques that can detect the active species in situ. As we all know, the generation of the active species is closely related to the transfer of electrons. The electrochemical behaviors of the electrons directly affect the dynamic processes of the degradation reactions. Therefore, it is hopeful to reveal the photocatalytic mechanism by detecting the electrochemical behaviors of the degradation processes. Herein, this paper first introduced electrodes to track the degradation process in situ, such as the electrodes of oxidation reduction potential (ORP), dissolved oxygen (DO), conductivity, and pH. Besides, some other types of measurements have been also provided to detect the active species. As to the organic pollutant, the photocatalytic degradation of methyl orange (MO) in the aqueous phase is always taken as a model reaction to evaluate the photocatalytic activity of catalysts. MO is stable under 365 nm light or visible light irradiation but easily degraded in the presence of catalysts. Its structure and degradation byproducts have been investigated and verified in many reports. Furthermore, photosensitized reactions always participate in the degradation of MO under light irradiation. Investigating the degradation process of MO could reveal the differences between the photosensitized and non-photosensitized reactions. Therefore, the typical and common degradation process of MO solution over TiO2 was chosen as our object. Our objective was to identify the main active species and determine their roles in the photodegradation of MO under UV light irradiation.

Scheme 1. The Reaction between NaOH and Acidic Hydroxyl Groups

were monitored by a DO Probe (ORION 081010MD), ORP electrode (ORION 9678BNWP), and conductivity cell (ORION 013605MD), respectively. They were all detected on a 5-star meter (Thermo Fisher Scientific Inc.) which was connected to a computer. The UV−vis spectra of various liquid samples were performed on Varian Cary 50 UV−vis spectrophotometer. In the total organic carbon (TOC) investigation, the light source and filters were the same as the photocatalytic reaction system. The TOC values were detected by a Shimadzu TOC-VCPH total organic carbon analyzer. The generation of hydroxyl radicals was investigated by the photoluminescence technique with terephthalic acid (PL-TA) using an Edinburgh FL/FS900 spectrophotometer.19 The product formation was measured by liquid chromatography mass/spectrometry (LC/MS) using Trap XCT with an electrospray ionization (ESI) interface. Chromatographic separation was carried out at 30 °C with C18 (5 μm × 250 mm × 2.0 mm) column. The mobile phase consisting of acetonitrile/ammonium acetate = 30:70 (v/v) was set at a flow rate of 0.6 mL min−1. The ESI source was set at the negative ionization mode. The MS operating conditions were optimized as follows: drying gas, 8 L min−1; curved desolvation line (CDL) temperature, 350 °C; capillary voltage, +3.5 kV. The generation of •OH radicals was also investigated by the ESR technique with DMPO.20 ESR spectra were obtained using a Bruker model A300 spectrometer with a 355 nm laser as the light source. The settings were center field, 3512 G; microwave frequency, 9.86 GHz; power, 20 mW. The vibrational bands for −OH groups were observed by Fourier transform infrared spectroscopy (FTIR). The samples were treated at 200 °C for 2 h in vacuum. IR spectra were recorded by using a Nicolet 670 FTIR spectrometer. Absorption bands were observed from 4000 to 1000 cm−1 with 32 sample scans and 4.0 cm−1 resolutions. The absorption cell was self-made with CaF2 windows. 1H magnetic-angle spinning (MAS) NMR spectra were obtained on an Infinity Plus 300 spectrometer at 300 MHz. The π/2 radiation in pulse was 4 μs and the pulse delay 3 s. Before the NMR experiments, the samples without any treatments were placed in special NMR tubes of 7.5 mm o.d. and 12 mm length. The spinning of the samples was performed in quartz rotors at a frequency of 1−7 kHz using a probe with minimal background signal. Chemical shifts were measured relative to tetramethylsilane (TMS) as an external reference. 2.4. Tests of Photocatalytic Activity. UV-light photocatalytic experiments were conducted using a quartz reactor surrounded by four 4-W UV lamps with a wavelength centered at 365 nm (Philips, TUV 4W/G4 T5). A 0.08 g portion of catalysts was loaded into 150 mL of MO solution (6.1 × 10−5 M). Prior to irradiation, the suspensions were magnetically stirred for 1 h to ensure the equilibrium of the working solution. After irradiation, at given time intervals, 3-mL aliquots were sampled and centrifuged to remove the catalyst. Then the degraded solutions were analyzed using a Varian Cary 50 Scan UV−vis spectrophotometer. The photocatalytic processes under light irradiation mentioned in this paper were all measured after the adsorption−desorption equilibrium.

2. EXPERIMENTAL METHODS 2.1. Materials. TiO2 (P25: 80% anatase, 20% rutile; 50 m2 g−1) was kindly supplied by Degussa Company. 5,5-Dimethyl1-pyrroline-N-oxide (DMPO) was obtained from Sigma Co., Ltd. Deionized water was used for the preparation of all solutions. tert-Butyl alcohol (TBA), ammonium oxalate (AO), and benzoquinone (BQ) were all from Sinopharm Chemical Reagent Co., Ltd. 2.2. Treatment of TiO2 Samples with NaOH. The surface acidic hydroxyl (OH (acid)) groups on TiO2 were treated by the method of surface acid−base, ion−exchange reactions for saturation with low concentrations of NaOH.14 The procedure was as follows: three different amounts (0.04, 0.06, and 0.08 g) of TiO2 powders were dispersed in 50 mL of NaOH solution (2 mM) at room temperature, respectively. After 12 h of stirring, the oxides of the three systems were separated from the suspensions and then dried in air at room temperature. The samples retrieved from the three treatment processes were chosen as the object to compare. This method was applied according to the reaction mechanism as shown in Scheme 1.14 2.3. Characterization of Photocatalysts. The behavior of pH values in TiO2 suspensions was monitored by Ross Ultra Combination pH electrode (ORION 8102BNUWP) with an automatic temperature compensation probe (ORION 927005MD Star ATC Probe). The dissolved oxygen (DO), oxidation reduction potential (ORP), and conductivity tests

3. RESULTS AND DISCUSSION 3.1. Detection of Active Species by Scavengers. First, the roles of the main active species such as holes, electrons, 3553

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•OH, and O2•− were investigated using different types of active species scavengers. Figure 1 shows the photocatalytic activities

This indicated that the photoreduction of MO was well catalyzed by TiO2 through electron transfer.24 But the hydrazine did not continue to be degraded without O2. In other words, the MO molecules were not degraded completely without O2. In the air-equilibrated condition shown in Figure S1b, both of the intensities of the two main absorption peaks are decreased. And from the spectral changes, the MO molecules were mineralized to some degree. We also detected the TOC values of the two systems. From the results, the mineralization of the substrate was 67% and 27% after 2 h of light irradiation under air-equilibrated and N2 bubbled conditions, respectively. Therefore, the complete degradation of MO was carried out mainly by oxidation reactions rather than reduction reactions. In other words, the dissolved oxygen (DO) played an important part in the degradation process. The participation of O2•− has been reported to be involved in the photodegradation process.25,26 In order to determine its participation in the process, BQ was added. The BQ has the ability to trap O2•− by a simple electron transfer mechanism (eq 1).27

Figure 1. Photocatalytic degradation of MO over TiO2 under different conditions with exposure to UV light: MO blank, adding BQ, adding AgNO3, adding AO, adding TBA, air-equilibrated and N2-saturated.

BQ + O2•− → BQ •− + O2

(1)

The addition of BQ provokes partial inhibition of the MO degradation as shown in Figure 1. The degradation rate was largely suppressed and the PCR was reduced to 13% after 120 min of irradiation. As mentioned in the reference, the O2•− radicals attacked preferentially aromatic rings with low electronic density (as deactivated aromatic rings in MO).28 Besides the scavengers mentioned above, we also chose some other scavengers in our experiments and studied their effect, such as O2 (electron scavenger), ethylenediaminetetraacetic acid (EDTA), KI (hole-capturer), methanol, and isopropanol (•OH scavenger). From the results, the effect of the scavengers was a little different in the degradation process. Herein, we can draw some conclusions according to the effect of different active species scavengers. Through the comparison, the MO oxidation was driven mainly by the participation of O2•−, holes, and •OH radicals. 3.2. The Generation of O2•− and •OH Radicals. To confirm the existence of O2•− and •OH radicals, the ESR spintrap technique (with DMPO) was employed to probe the nature of reactive oxygen species generated during the irradiation of the present system. From Figure S2, in the dark, there are no ESR signals observed in the presence of TiO2. After UV light irradiation, the characteristic sextet peaks of DMPO-O2•− adduct and the characteristic quartet peaks of DMPO−•OH adduct with a 1:2:2:1 ratio were observed. The generation of the two active species was closely related to the DO, holes, and surface hydroxyl groups. In the following part, we further investigate the relationship among these species in the degradation of MO. 3.3. Relationship between the Active Species and the Degradation Process. The LC/MS technique was used to probe the main byproducts of MO. Figure 2a reports the chromatograms monitored in full scan MS, corresponding to MO solutions irradiated on TiO2 particles. As it can be seen, there are some degradation byproducts variably presented at the various times. The corresponding mass spectra views of the main peaks appeared at different retention times and their structures are also shown. Figure 2b shows the changes of mass spectrum intensity of the main peaks (A) m/z = 304, (B) m/z = 290, and (C) m/z = 191 appearing at different retention times in the degradation process. Before the light irradiation,

of TiO2 in the degradation of MO under different conditions. Without the addition of scavengers, the photocatalytic conversion ratio (PCR) of the main absorption peak (464 nm) of MO on TiO2 was 95% after 120 min of irradiation. As mentioned in the Introduction, the transfer of electrons in the photocatalytic process is very important. The separation of electrons and holes is always recognized to be the initial step in the photodegradation mechanism. Therefore, to investigate the mechanism of the process, scavengers for electrons and holes were first employed to determine the specific reactive species. AgNO3 is an electron scavenger. When it was added, the PCR was reduced to 55% after 120 min of irradiation. The decrease of the amount of electrons reduced the generation of active species such as O2•− and then inhibited the degradation rate. When AO, which was often used as a hole-capturer,21 was added to the reaction system, the PCR was reduced to 64%. The decrease of the amount of holes also influenced the degradation rate. Therefore, it seemed that electrons and holes were all important to the process. Photogenerated holes may react with surface hydroxyl groups or adsorbed water to create •OH. The •OH radicals are very reactive and quickly oxidize organic species at the surface of TiO2 particles. Two milliliters of TBA was added to the solution in order to scavenge •OH radicals.22 From the result shown in Figure 1, the degradation rate is decreased a little and the PCR is reduced to 91% after 120 min of irradiation. O2 is an electron-capturer to produce O2•− which is also an important intermediate species. To test the role of the dissolved oxygen in the degradation process, N2 was bubbled through the suspension to ensure that the reaction was operated under anoxic conditions. As shown in Figure 1, the PCR of the main absorption peak is increased. It seemed that the degradation of MO was enhanced under the anoxic condition. To explain this result, we reviewed the absorption spectra of MO in the degradation process. In the presence of TiO2 under N2 bubbled conditions (shown in Figure S1a, Supporting Information), the absorption spectra of MO significantly changed. The visible absorption band around 464 nm was decreased, and a new peak at 247 nm grew up which originated from a reduction product, hydrazine.23 The spectral change was completed in 80 min. 3554

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Figure 2. (a) Chromatograms of MO solutions at different irradiation intervals: (a1) chromatogram of the original MO solution after adsorption− desorption equilibrium in the dark; (a2−a4) chromatograms of the MO solution after 20, 40, and 80 min of irradiation, respectively; corresponding mass spectra of the byproducts have also been provided; (b) mass spectrum intensity changes of the main peaks (A) m/z = 304, (B) m/z = 290, and (C) m/z = 191 appeared at different retention times in the degradation process.

there was one strong absorption peak in the chromatogram. It belonged to MO with mass peak at m/z = 304 appearing at 6.7 min. With the light irradiation, the intensity of the peak

quickly decreased especially in the first 20 min. And after 80 min of irradiation, its intensity reached a minimum and maintained the minimum under the following continued irradiation. At the 3555

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until the light was off. It seemed that the DO consumption was enhanced in the presence of TBA. Furthermore, the DO consumption in the absence of TiO2 under this condition was also detected (shown in Figure S3). From this figure, it indicated that TBA did not consume DO under light irradiation. As we all know, O2•− radicals can be produced by molecular oxygen trapped electrons. The DO consumption may demonstrate that the reactions were carried out by the support of O2•− radicals in the absence of •OH. It may also conclude that DO would be another source of •OH and DO itself would play a role in the degradation process. At the same time, with the contribution of O2•− radicals, the PCR of the reactions was still decreased in the presence of TBA. It indicated that the role of •OH was also important in the degradation process. The trap of holes may enhance the charge-carrier separation and then increase the degradation of dyes. And otherwise, the absence of holes may decrease the reaction rate because of the high activity of holes themselves. In Figure 3c, as AO is added to the TiO2/MO system, the change trend of DO consumption is increased compared with that without the additional condition. And from Figure S3, the addition of AO does not affect the DO consumption in the absence of TiO2 under light irradiation. This may indicate that the trap of holes enhanced the charge-carrier separation and more photoelectrons generated. Therefore, more photoelectrons would react with DO and thus induce the increase of the DO consumption. But as a hole-capturer, the addition of AO reduced the PCR to 64% in Figure 1. Therefore, holes also affected much on the degradation process. As shown in Figure 3, the DO consumption is enhanced in the absence of •OH. It may indicate that the generation of •OH was closely related to the DO concentration. Therefore, to realize the relationship between DO and •OH, the generation of •OH radicals was investigated by the PL-TA probing technique.19 Figure 4 shows the PL changes in the

same time, there were several main byproducts in the degradation process. One of the main byproducts with mass peak at m/z = 290 appeared at 4.0 min. Its intensity increased in the first 20 min and then started to decrease. After 120 min of irradiation, its intensity reached the minimum closely to zero. The other main byproduct with mass peak at m/z = 191 appeared at 2.4 min. Its intensity also increased in the first 20 min and then started to reduce. But after 80 min of irradiation, its intensity reached the equilibrium with a higher intensity about 3 × 105. Therefore, the changes of the intensity of the byproducts indicated how the degradation processes were carried out. The transversion of the degradation byproducts was quickly carried out in the first 20 min of light irradiation. Then the main byproducts were degraded at the same time between 20 and 80 min. Until light was irradiated for 80 min, the main MO molecule (m/z = 304) had been degraded into the main byproducts with mass peaks at (B) m/z = 290 and (C) m/z = 191. In the above part, the importance of DO was illustrated. To further expound its role, we had detected the DO consumption in situ during TiO2 photocatalysis by DO electrodes. As shown in Figure 3b, after the equilibrium of DO concentration in the

Figure 3. The consumption of DO during TiO2 photocatalysis under different conditions: (a) MO blank, (b) original TiO2/MO system, (c) adding AO, and (d) adding TBA.

original TiO2/MO system, the DO is starting to be consumed with light irradiation and the consumption reaches the maximum with 90% at 80 min. As discussed in the mass spectra, at 80 min, the peak intensity of the main MO molecule (m/z = 304) reached the minimum while for the main byproducts, they reached the equilibrium. Herein, the electrons and DO played the maximum role at this time in the degradation process. After this time, the DO consumption decreased a little but still with a large amount until the light was off. When the light was off, the photocatalytic reaction was over and the amount of DO in the system was increased again. For comparison, we also tested the changes of the DO consumption as TBA was added. As discussed in Figure 1, TBA is an excellent capturer of •OH, and its addition reduced the PCR to 90%. It indicated that •OH played a role in the degradation process. Here as shown in Figure 3d, the DO is also starting to be consumed while the light is irradiated. Differently, the DO consumption rate was much higher than that in the original system. After irradiation for less than 30 min, the DO consumption had reached 100% and maintained the value

Figure 4. PL changes in the presence of TiO2 in aqueous TA suspension under different conditions: TA blank, air-equilibrated, N2saturated, and O2-saturated.

presence of TiO2 in aqueous TA suspension under different conditions: TA blank, air-equilibrated, N2-saturated, and O2saturated. As we can see, the generation of •OH was largely inhibited under N2-saturated conditions. Compared with that under air-equilibrated condition, the effect was very obvious. Under pure O2 bubbled, the amount of •OH achieved was the 3556

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Figure 5. Changes of the (a) conductivity and (b) ORP during TiO2 photocatalysis in aqueous MO suspension.

the following reactions and acted on the degradation of MO. With the degradation of MO and the production of its byproducts, the ORP between these species was changing. It was difficult to ensure the clear redox couples which induced the changes of ORP. But the change trends can show us how the degradation carried out under the effect of the active species. As shown in Figure 2, during the first 20 min of light irradiation, the transversion of the degradation products is quickly carried out. Accordingly, as shown in Figure 5, during this time, the ORP has a large turn and the conductivity increases all the time. When the light was irradiated for 80 min, the generated amount of the active species reached the maximum. And the main MO molecular (m/z = 304) had been degraded into the main byproducts with mass peaks at (B) m/z = 290 and (C) m/z = 191. At this time, the ORP and DO consumption reached the minimum. After the time, the following reactions were not so fast any more. Although the byproduct with mass peak at (B) m/z = 290 was still degraded under the continued light irradiation in the case of radicals formation, the one with mass peak at (C) m/z = 191 was difficult to be degraded and its intensity reached the equilibrium. Therefore, during this time, the ORP increased again. But there were still active species and byproducts generated in the process, so the conductivity was increased all the time. As the light was off again, there were no photogenerated electrons and the conductivity was not increased. ORP went through a little decrease and then increased again. Besides, although the three parameters are closely related to the temperature and the species in the solutions, we can see that the temperature during the degradation process in the pure MO or TiO2/MO system is almost the same in Figure 5. Therefore, their changes in this degradation process largely depended on the active species and the byproducts in the aqueous solution. The transfer of electrons mostly contributed to the generation of active species. And the active species such as holes, •OH, and O2•− radicals directly induced the degradation of MO under light irradiation. Unfortunately, the detailed transfer process of electrons was not detected by techniques in this paper. And the radicals generated by DO including singlet oxygen (1O2), hydroxyl radical (•OH), superoxide radical (HO2• or O2•−), and hydrogen peroxide (H2O2) were not detected and distinguished by techniques, either. The following work would be carried out. 3.4. Effect of Hydroxyl Groups in the Degradation Process. In this part, first, the existence of the OH groups on TiO2 surface was investigated by FTIR and 1H MAS NMR

largest. Therefore, the DO concentration in the system influenced the generation of •OH. DO would be another source of •OH. The generation reactions are proposed in the following eqs 2−5.

ecb− + O2 → O2•−

(2)

O2•− + ecb− + 2H+ → H2O2

(3)

O2•− + H2O2 → •OH + OH− + O2

(4)

H2O2 + hν → 2 • OH

(5)

The degradation process was also detected in situ by the conductivity and ORP electrodes. Figure 5 shows the changes of the conductivity and ORP in the degradation process of MO. There were no reports about these parameters in the photocatalysis field as yet. The electrodes were introduced here for at least two reasons. First, the two electrodes can detect the degradation process in situ and then verify the results which were detected by LC/MS. Second, the two parameters may further prove the effect of DO and the other active species in the degradation process. From Figure 5a, the values of the conductivity of MO solution remain stable in the dark in the absence or presence of TiO2. With light irradiation, the conductivity in pure MO or TiO2/MO system was increased with the temperature. But the conductivity in the presence of TiO2 was higher all the time than that without TiO2. The conductivity is mainly associated with the concentration of the species in the solution. Therefore, the larger values of the conductivity in the presence of TiO2 may be due to the increase of the concentration of byproducts which can verify the degradation of MO. Besides, the conductivity was closely related to the generation of active species. The effect of the active species would be further illuminated combined with the changes of ORP in the following part. Changes of ORP in the presence of TiO2 were in accord with the DO consumption. As shown in Figure 5b, with light irradiation, the ORP is quickly reduced in the first 10 min and then the changes slow down. As light was irradiated for 80 min, ORP in the system reached the minimum (365 RmV). After that, it increased again. For this reason, we can explain it from the transfer of electrons and the generation of active species. Before irradiation, there were no electrons generated in the system. The values of ORP were kept stable in the dark. As light was irradiated, the photoelectrons started to be generated on the surface of TiO2. Then active species were generated in 3557

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hydroxyl groups bound to titanias.33−35 From Figure S4b, 1H NMR spectrum of the TiO2 contains only one signal at δ = 4.9 ppm. From ref 35, the line at 4.9 ppm can be attributed to “acidic” protons located on bridging oxygen atoms and formed weak hydrogen bonds with adjacent oxygen atoms; these are the bridging hydroxyl groups of TiO2 in the presence of anatase and rutile. Because the bridging oxygen atoms accounted much more than the terminal oxygen atoms and the samples did not experience some special treatments before or during the NMR test, the signal at the high field peak ascribed to “basic” H atoms bound to the terminal oxygen atoms was not found. From the above measurements, the surface of TiO2 was full of OH groups which provided the chances for the reactions with holes in the formation of •OH. To investigate the important role of OH groups, OH (acid) groups on TiO2 were treated with the method of surface acid−base, ion−exchange reactions for saturation with low concentrations of NaOH. The treatment procedure was shown in the experimental methods. After the treatment, the amount of OH (acid) groups on the retrieved TiO2 surface was reduced. The effect on the PCR and the formation of •OH was also detected. In Figure 6A,B, pH tests of different treated samples were conducted under dark and irradiation conditions. Figure 6A shows that the pH value of MO is 5.9 when it reaches equilibrium in the dark. As TiO2 was added to the MO solution, the pH value turned to 5.0. With the light on and off,

techniques. And the acidic property of TiO2 suspension was then explored by pH test. IR spectroscopy was devoted to the characterization of TiO2 surfaces. As reported, anatase samples present three main IR bands corresponding to titanol groups: a band at ca. 3715 cm−1 is attributed to terminal (isolated) OH group, being basic in character; a band at ca. 3675 cm−1 is assigned to a second type of OH group (possibly terminal), being neutral or weakly acidic in character; and a band at ca. 3640 cm−1 is assigned to a bridging OH group which possesses acidic property.29 For rutile samples, their IR spectra are less than straightforward to assign.30,31 According to the reported spectra in the references, the bands appeared in Figure S4a were classified. On the surface of TiO2 samples, the IR spectrum contained an intense band at 3622 cm−1, together with three weaker bands at 3717, 3469, and 3417 cm−1, and two ambiguous shoulders at 3646 and 3673 cm−1. In view of the fact that TiO2 (P25) contains ca. 20% m/m rutile, it is likely that the absorptions observed at 3717 and 3622 cm−1 actually represented a superposition of anatase and rutile absorptions, corresponding to terminal and bridging OH groups, respectively. The broad band observed at 3673 cm−1 corresponded to a terminal titanol group, while absorption bands at 3469 and 3417 cm−1 may represent the corresponding hydrogen-bonded OH.30,32 1 H MAS NMR measurements have been performed with the aim of measuring the characteristic proton chemical shifts of

Figure 6. The changes of pH values during TiO2 photocatalysis in aqueous MO suspension in the presence of different samples: (A) MO blank and original TiO2; (B) (a) samples after 0.08 g of TiO2 reacted with NaOH, (b) original TiO2 in the MO solution with pH = 9, and (c) samples after 0.08 g of TiO2 was washed with water; (C) comparison of the photocatalytic degradation of MO on the above samples under 365 nm light irradiation; and (D) changes in ESR spectra of the samples before and after treatment with NaOH under 355 nm laser irradiation. 3558

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the pH values in TiO2 suspension were still lower than pure MO. It indicated that acidic OH groups on TiO2 affected the pH values of MO solution. The original PCR of MO in the presence of TiO2 without treatment is 95% as shown in Figure 6C. In order to exclude the effect of OH species in the environmental systems and verify the existence of OH groups in TiO2, the TiO2 samples were treated with different methods as shown in Figure 6B. In Figure 6B, after TiO2 is treated with NaOH, the pH value in the TiO2 suspension is increased to 8.5 in the dark but declined after irradiation. After 120 min of irradiation, the pH value turned to about 6. The degradation rate of MO in the presence of the sample treated with NaOH is reduced to 58% as shown in Figure 6C. After TiO2 is treated with water, the pH value in TiO2 suspension increased to 6.0 in the dark but declined to 4.9 after 120 min of irradiation (Figure 6B). This was close to that in the original TiO2 suspension which was 4.7 after 120 min of irradiation. At this condition, the PCR of the sample after treatment (87%) was lower than that of the original sample, but higher than that of the sample treated with NaOH. For further comparison, we also adjusted the pH value of MO solution to about 9 in the presence of TiO2 without treatment and tested the changes of pH values. After irradiation, the changes of pH values were similar to that in the presence of the sample treated with NaOH (shown in Figure 6B). But in Figure 6C, the degradation rate of MO in the presence of the sample treated with NaOH is still lower than that in the adjustable pH value system. From the comparison, after TiO 2 was treated with NaOH, the degradation rate of the sample was the lowest in the different systems. Therefore, the reduced amount of OH (acid) groups on the TiO2 surface directly affected the generation of •OH radicals. Changes in ESR spectra of samples before and after treatment with NaOH are shown in Figure 6D. It can be clearly observed that under the same experimental conditions, after different weights of TiO2 were treated with NaOH, the signal intensity of the spin adduct decreased much compared with the original sample. The decrease of the amount of the surface hydroxyl groups reduced the generation of •OH and then influenced the photocatalytic activity. Therefore, it can be concluded that in the degradation of MO, the OH groups were generated and released continuously on the surface of TiO2 under light irradiation. They may play two roles in the system: one is to supply chances for the generation of •OH species and the other one is to affect the pH values of the degradation surroundings.

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*Tel/Fax: (+86)591-83779256. E-mail: [email protected].

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21173047, 21073036 and 20873023), National Basic Research Program of China (973 Program, 2007CB613306).

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4. CONCLUSIONS Through the discussion above, the MO oxidation by photocatalysis on TiO2 was driven mainly by the participation of O2•−, holes, and •OH radicals. The degradation processes were analyzed in detail by the characterizations of LC/MS, DO, conductivity, ORP, and pH electrodes. DO played an important role in the formation of active species such as O2•− and •OH radicals. Surface hydroxyl groups were important sources of •OH radicals. There is still a lot of tough work to do in the following to explain the degradation process of MO clearly.

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