Specific Analyses of the Active Species on Zn0.28Cd0.72S and TiO2

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Specific Analyses of the Active Species on Zn0.28Cd0.72S and TiO2 Photocatalysts in the Degradation of Methyl Orange Wenjuan Li, Danzhen Li,* Jiangjun Xian, Wei Chen, Yin Hu, Yu Shao, and Xianzhi Fu Research Institute of Photocatalysis, State Key Laboratory Breeding Base of Photocatalysis, Fuzhou UniVersity, Fuzhou, 350002, People’s Republic of China ReceiVed: July 17, 2010; ReVised Manuscript ReceiVed: October 15, 2010

As different light-driven photocatalysts, Zn0.28Cd0.72S and TiO2 showed different properties in the photocatalytic process. In this paper, the comparison was carried out among the properties of Zn0.28Cd0.72S-visible, Zn0.28Cd0.72S-UV, and TiO2-UV light systems in the degradation of methyl orange. After the addition of different types of active species quenchers and the results detected by electron spin resonance, a photoluminescence technique, and a photometric method, it was found that, in the Zn0.28Cd0.72S-UV system, O2, •OH, and holes played a bigger role, whereas in the TiO2-UV system, •OH and holes predominated; in the Zn0.28Cd0.72S-visible light system, O2 and holes contributed to the degradation. The differences between the two catalysts lead us to realize the active species and the degradation mechanism in photocatalytic process. Introduction

Experimental Methods

The toxic pollutant associated with the release of azo dyes in the environmental phases has attracted much attention nowadays.1 The possible application of heterogeneous photocatalysis2-4 for the treatment of environment pollutants has been investigated as an alternative to conventional methods. Recently, TiO2 is employed widely in the destruction of organic pollutants due to its excellent long-term stability, perfect physicochemical properties, and the appropriate position of its conduction band. However, it can only absorb a very small ultraviolet part (3-4%) of the solar light because of its wide band gap (3.2 eV for anatase). Therefore, the extensive applications of titanium dioxide as an efficient photocatalyst are constrained. Many modification methods, such as metal-ion doping, composite semiconductors, and metal layer modification, have been used to extend the light absorption of the catalyst to the visible light region but have a little effect.5-8 Therefore, some visible-lightdriven photocatalysts with high and effective activity under solar light have been actively sought. ZnxCd1-xS nanocrystals, which belong to the binary II-VI nanocrystals, have been widely used as optoelectronic devices and biomedical tags due to their novel size-tunable properties.9-18 In recent years, there has been growing attention on the application of these crystals in the photocatalytic oxidation and reduction of water under visible light irradiation, and a few researchers have used them in the degradation of dyes.19-22 With regards to TiO2 and ZnxCd1-xS photocatalysts, they showed different properties in the degradation of methyl orange (MO). To recognize the photocatalytic process in the two different kinds of systems would be helpful to explain the photocatalytic mechanism and to develop new types visible light photocatalysts. As of yet, the mechanism of the two systems has not been compared and realized. How many differences are there between them and what leads to the differences? In this paper, the differences between Zn0.28Cd0.72S and TiO2 were investigated. The reason that caused the differences was analyzed in detail.

Materials. TiO2 (P25: 80% anatase, 20% rutile; 50 m2 g-1) was kindly supplied by Degussa Co. Zn0.28Cd0.72S photocatalysts were prepared by the method previously reported.22 Horseradish peroxidase (POD) and N,N-diethyl-p-phenylenediamine (DPD) were purchased from J&K Chemical Ltd. 5,5-Dimethyl-1pyrroline-N-oxide (DMPO) was obtained from Sigma Co. All other chemicals were of analytical grade. Deionized water was used for the preparation of all solutions. All chemicals were used without further treatment. Characterizations. The UV-vis spectra of various liquid samples and diffuse reflectance spectra (DRS) were performed on a Varian Cary 50 UV-vis spectrophotometer and a Varian Cary 500 UV-vis spectrophotometer with an integrating sphere attachment ranging from 200 to 800 nm, respectively. The product formation was measured by liquid chromatography mass spectroscopy (LC-MS) using Trap XCT with an electrospray ionization (ESI) interface. Chromatographic separation was carried out at 30 °C with a C18 (5 µm × 250 mm ×2.0 mm) column. The mobile phase consisted of acetonitrile/ammonium acetate ) 30:70 (v/v), which 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. 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 (PLTA). The photoluminescence spectra were surveyed by an Edinburgh FL/FS900 spectrophotometer. The flat band potential (Vfb) of Zn0.28Cd0.72S or TiO2 was determined by the electrochemical method, which was carried out in conventional threeelectrode cells using a PAR VMP3Multi Potentiotat apparatus. The catalyst was deposited as a film on a 1 cm × 1 cm indium-tin oxide conducting glass, which served as the working electrode, the saturated calomel electrode (SCE) as the reference electrode, and Pt as the counter electrode. The electrolyte was

* To whom correspondence should be addressed. Tel/Fax: (+86)59183779256. E-mail: [email protected].

10.1021/jp106659g  2010 American Chemical Society Published on Web 11/17/2010

Analyses of Zn0.28Cd0.72S and TiO2 Photocatalysts

J. Phys. Chem. C, Vol. 114, No. 49, 2010 21483 4 W UV lamps with a wavelength centered at 365 nm (Philips, TUV 4W/G4 T5). The photoenergy density was 160 µW/cm2. The next procedure was similar to that under visible light. Results and Discussion

Figure 1. DRS of TiO2 and Zn0.28Cd0.72S powders.

0.3 M aqueous LiClO4 at pH 3.0. The Mott-Schottky plots to evaluate the flat-band potential of the semiconductor space charge region were obtained by measuring impedance spectra at a fixed frequency of 1 kHz. Electron spin resonance (ESR) spectra were obtained using a Bruker model A300 spectrometer with a 500 W Xe-arc lamp equipped with an IR-cutoff filter (λ < 800 nm) and a UV cutoff (λ > 420 nm) as a visible light source (photoenergy density ) 230 µW/cm2) and a 355 nm laser as a UV light source (photoenergy density ) 400 µW/cm2). The settings were as follows: center field, 3512 G; microwave frequency, 9.86 GHz; power, 20 mW. Tests of Photocatalytic Activity. The visible light source was a 500 W halogen lamp (Philips Electronics) positioned beside a cylindrical reaction vessel with a plane side. The system was cooled by wind and water to maintain the room temperature. The 420 and 800 nm cutoff filters were placed before the vessel to ensure that irradiation of the MO/Zn0.28Cd0.72S system occurred only by visible light wavelengths. The photoenergy density was 230 µW/cm2. A 0.04 g portion of catalysts was added to 80 mL of MO solution (6.1 × 10-5 mol L-1) in a 100 mL Pyrex glass vessel. Prior to irradiation, the suspensions were magnetically stirred in the dark for 1 h to ensure the equilibrium of the working solution. At given time intervals, 3 mL aliquots were sampled and centrifuged to remove the catalyst. The degraded solutions were analyzed using a UV-vis spectrophotometer, and the absorption peak at 464 nm was monitored. UVlight photocatalytic experiments were conducted in a quartz reactor. The catalyst (0.08 g) was loaded into 150 mL of MO solution (6.1 × 10-5 mol L-1) in the reactor surrounded by four

Figure 1 shows the DRS of TiO2 and Zn0.28Cd0.72S powders. It is obvious to see that Zn0.28Cd0.72S has an intense absorption band with steep edges in the visible light region, being different from TiO2, which has a UV absorption band. The band gap of the Zn0.28Cd0.72S was estimated to be 2.41 eV, whereas that of TiO2 was 3.2 eV, calculated by the method.23 In Figure 2, TiO2 and Zn0.28Cd0.72S showed different photocatalytic conversion ratios (PCRs) in the degradation of MO. The photodegradation activity of Zn0.28Cd0.72S was tested by the irradiation from 365 nm light and visible light, whereas TiO2 was irradiated by only 365 nm UV light. Evidently, the processes were divided into three systems, denoted as the following: Zn0.28Cd0.72S-visible light, Zn0.28Cd0.72S-365 nm light, and TiO2-365 nm light. After 5 h of irradiation, the PCR of the degradation MO over Zn0.28Cd0.72S was up to 83% under 365 nm light and 80% under visible light. With regards to TiO2 under 365 nm light, the PCR was up to 95% after 120 min of irradiation. We also did the linear fitting according to the equation, -ln(C/C0) ) kt + b. From the results of the experimental data, we can see that the degradation processes of the three systems were all in accordance with the first-order kinetics. The apparent rate constant k can be obtained from Figure 2 (inset), that is, 0.0244 min-1 (or 1.464 h-1) for TiO2, 0.348 h-1 for Zn0.28Cd0.72S under 365 nm light irradiation, and 0.334 h-1 for Zn0.28Cd0.72S under visible light irradiation. The PCR of TiO2 was much better than that of Zn0.28Cd0.72S under 365 nm light irradiation. With regards to Zn0.28Cd0.72S, the PCR under 365 nm light irradiation was a little better than that under visible light irradiation. Though TiO2 exhibited better activity under UV light irradiation, Zn0.28Cd0.72S behaved well in the range of sunlight. Therefore, in terms of the application, Zn0.28Cd0.72S should be applied more and its application would solve the problem of the low utilization efficiency of solar energy. At the same time, all the sulfides face the problem of toxicity and stability. We must pay attention to the problem and solve it first. The degradation byproducts were also identified and compared in the three systems. Figure 3 reports the chromatograms monitored in full scan MS, corresponding to MO solutions in the three systems. We shall make a point of the change trends of the intensity of byproducts in all the degradation processes in the following

Figure 2. Photocatalytic degradation of MO in the (a) TiO2-365 nm light system, (b) Zn0.28Cd0.72S-365 nm light system, and (c) Zn0.28Cd0.72Svisible light system, and the relationship between PCR and reaction time (insets).

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Figure 3. Chromatogram intensity changes of MO solutions over (a) TiO2 particles during 120 min of irradiation under 365 nm light, (b) Zn0.28Cd0.72S particles during 5 h of irradiation under 365 nm light, and (c) Zn0.28Cd0.72S particles during 6 h of irradiation under visible light appeared at different retention times.

discussion. As it can be seen, there were some degradation byproducts variably presented at the various times. The corresponding mass spectra views of the main peaks that appeared at different retention times are shown in Figures 4-6. The intensity changes indicated the conversion of the degradation byproducts. Before light irradiation, there was only one absorption peak in the chromatogram. It belonged to MO with a mass peak at m/z ) 304 that appeared at 5.5 min in the three systems. With the light irradiation, the intensity of the peak slowly reduced in all the three systems. At the same time, after irradiation, there were several main byproducts in the three degradation processes. The change trends of their intensities showed some differences. In the TiO2-365 nm light system (shown in Figure 4), the peaks with m/z ) 191 that appeared at 1.6 min and with m/z ) 290 that appeared at 3.2 min predominated. Their intensities first increased and then decreased with the irradiation time. In the Zn0.28Cd0.72S-365 nm light system (shown in Figure 5), the peak with m/z ) 290 that appeared at 3.4 min predominated, whereas the peaks, such as the m/z ) 277, 226, and 195, that appeared at 2.9, 2.0, and 2.0 min were fewer in the byproducts. In the Zn0.28Cd0.72S-visible light system (shown in Figure 6), the peaks with m/z ) 290 that appeared at 3.3 min and m/z ) 195 that appeared at 1.9 min predominated, but the change trends of their intensities were different from that of the UV systems. The peak with m/z ) 290 first increased and then decreased with the irradiation time, whereas the peak with m/z ) 195 continuously increased, but still with lower intensity at last. The main compounds with

corresponding retention times are shown at the bottom of Figures 4-6. The clear change trends in the three figures indicated the different degradation processes in the three systems. We also detected the TOC values. From the results, the mineralization of the substrate was much lower in the Zn0.28Cd0.72S-UV and Zn0.28Cd0.72S-visible light systems than that in the TiO2-UV light system. The TOC did not decrease after 6 h of irradiation because of the lagging effect. After 12 h of irradiation, the TOC decreased about 20% in the two Zn0.28Cd0.72S systems. As for the TiO2 system, the TOC decreased about 67% after 120 min of irradiation. Therefore, in the Zn0.28Cd0.72S-UV and Zn0.28Cd0.72Svisible light systems, the byproducts shown in the LCMS may be the main products. In the TiO2 system, the byproducts generated may be secondary products. On the basis of the above analysis, the degradation processes were different in the three systems. This would be due to different active species operated on, so in the next part, first, some additional conditions that affected the generation of active species were imposed on the degradation process. Figure 7 indicates time courses of MO photodegradation by Zn0.28Cd0.72S and TiO2 under different additional conditions. O2 is an electron-capturer to produce O2•-, which is an important intermediate species. To test the role of dissolved O2 in the degradation process, N2 was bubbled through the suspension to ensure that the reaction was operated without O2. As shown in Figure 7a, the PCR of the main absorption peak (464 nm) was increased in the TiO2-365 nm light system, and the absorption spectra (shown in Figure 7d) were significantly


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Figure 4. 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 photocatalytic degradation process over TiO2 particles during120 min of irradiation under 365 nm light.

Figure 5. Mass spectrum intensity changes of the main peaks (a) m/z ) 304, (b) m/z ) 290, (c) m/z ) 276, (d) m/z ) 226, and (e) m/z ) 195 appeared at different retention times in the photocatalytic degradation process over Zn0.28Cd0.72S particles during 5 h of irradiation under 365 nm light.

changed. The visible absorption band around 464 nm decreased, and a new peak grew up at 247 nm, which originated from a reduction product, hydrazine.24 The spectral change was completed in 80 min. This indicated that photoreduction of MO was well catalyzed by TiO2 through electron transfer,25 but the hydrazine did not continue to be degradated without O2. In other words, MO molecules were not completely degradated without O2. So the degradation process was actually a reduced reaction

in the TiO2-365 nm light system. The same spectral change of the MO solution was also observed in the Zn0.28Cd0.72S-365 nm light and Zn0.28Cd0.72S-visible light systems, but the spectral change took a little longer, as shown in Figure 7e,f. In the Zn0.28Cd0.72S-365 nm light system (Figure 7b), the PCR of the main absorption peak (464 nm) was reduced to 67%, whereas in the Zn0.28Cd0.72S-visible light system (Figure 7c), the PCR is only 27%. The degradation rate decreased a lot by N2 bubbling.

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Figure 6. Mass spectrum intensity changes of the main peaks (a) m/z ) 304, (b) m/z ) 290, and (c) m/z ) 195 appeared at different retention times in the photocatalytic degradation process over Zn0.28Cd0.72S particles during 6 h of irradiation under visible light.

When O2 was bubbled in the Zn0.28Cd0.72S-visible light system, the photocatalytic activity was close to that under air-equilibrated conditions. It may be deduced that the concentration of O2 in the air was enough in the degradation. Tert-butyl alcohol (TBA) is considered as an excellent capturer of •OH. It is more easily oxidized by •OH. To detect the action of •OH in the systems, 2 mL of TBA was added. From the results in Figure 7, we could see that the removal of •OH reduced the PCR of the TiO2-365 nm light system to 90% and the Zn0.28Cd0.72S-365 nm light system to 73%, whereas in the Zn0.28Cd0.72S-visible light system, the PCR was not decreased but was higher than that under air-equilibrated conditions. Holes as another reactive species in photocatalytic reactions could be captured by ammonium oxalate (AO), which was often used as a hole-capture.26 After 0.1 g of AO was added to the reaction systems, the PCRs of the three systems were all reduced, but with different effects. In the Zn0.28Cd0.72S-365 nm light system, the PCR was reduced to 65%, whereas in the TiO2365 nm light system, the PCR was reduced to 69%. In the Zn0.28Cd0.72S-visible light system, AO obviously affected the degradation process with the lowest PCR of 44%. Therefore, in the UV systems, additional TBA prohibited the degradation process and its effect was more obvious in the Zn0.28Cd0.72S system than that in the TiO2 system. In the Zn0.28Cd0.72S-visible light system, the additional TBA increased the process. When N2 was bubbled, the PCR of the main absorption peak (464 nm) of MO was increased in the TiO2365 nm light system, whereas in the two Zn0.28Cd0.72S systems, both of the PCRs were reduced. The effect of N2 was less obvious in the Zn0.28Cd0.72S-365 nm light system than that in the Zn0.28Cd0.72S-visible light system. The addition of AO reduced the PCRs of the three systems. Its effect was much more obvious in the Zn0.28Cd0.72S-visible system than that in the Zn0.28Cd0.72S-365 nm light and TiO2-365 nm light systems. In general, from the data of Figure 7, we can conclude that, in the two UV systems, the effect of AO was more obvious than that of TBA and N2. In the Zn0.28Cd0.72S-visible light system, AO and N2 affected more than that of TBA. Therefore, the main active species in the three systems can be deduced. Under UV light, in the Zn0.28Cd0.72S system, O2, •OH, and holes played a

bigger role, whereas in the TiO2 system, •OH and holes predominated. Under visible light, in the Zn0.28Cd0.72S system, O2 and holes contributed to the degradation. The active species generated in the Zn0.28Cd0.72S systems were different from that in the TiO2 system. From the results of Figure 7c, the addition of TBA in the Zn0.28Cd0.72S-visible light system has promoted the degradation process. It was contrary to the reports27,28 in which TBA would reduce the degradation. To recognize the effect of TBA and the formation of •OH, the PL-TA probing technique was used in the Zn0.28Cd0.72S systems. The PL-TA technique has been widely used in the detection of hydroxyl radicals.29 2-Hydroxylterephthalic acid, which is generated when terephthalic acid captures the hydroxyl radicals, exhibits a strong fluorescence characteristic, so we can detect the hydroxyl radicals indirectly by monitoring the fluorescence intensity changes of 2-hydroxylterephthalic acid. Figure 8 shows the fluorescence spectra of suspensions in the (a) Zn0.28Cd0.72S-365 nm light system and (b) Zn0.28Cd0.72S-visible light system. In the two systems, the fluorescence intensity increased steadily with the irradiation time. Hydroxyl radicals were indeed generated in the two systems. The ESR spin-trap method was also applied to study the active species. From ESR spectra, the intensity ratio of the main peaks that belonged to DMPO-OH• in the TiO2 system (shown in Figure 9a) was 1:2:2:1 under 355 nm laser irradiation, whereas in the Zn0.28Cd0.72S-UV system (shown in Figure 9b), the signal was poor with a ratio of about 1:1:1:1. In the Zn0.28Cd0.72S system under visible light irradiation, the signal ratio was also 1:1:1:1 (shown in Figure 9c) and the two peaks in the middle broke up. These different results between TiO2 and Zn0.28Cd0.72S demonstrated that the species that DMPO captured were different. There is no report about these signals of Zn0.28Cd0.72Svisible or UV light yet. As reported,30-32 in the liquid phase, the majority of radicals are •OH, O2•- radicals, and hydrated electrons (e-aq), although smaller quantities of other radicals, such as •H, are also formed. The e-aq has a strong reduction ability, whereas •OH has a strong oxidation ability. Therefore, in the degradation process, there are competitive reactions between the two radicals. The TBA was chosen to scavenge



Figure 7. Photocatalytic degradation of MO over the (a) TiO2-365 nm light system, (b) Zn0.28Cd0.72S-365 nm light system, and (c) Zn0.28Cd0.72Svisible light system under different conditions: in N2-saturated, O2-saturated, air-equilibrated solutions, adding TBA, or adding AO. (d-f) Absorption spectral changes of the MO solution in the presence of TiO2 or Zn0.28Cd0.72S under bubbling N2.

Figure 8. •OH-trapping PL spectra of suspensions in the (a) Zn0.28Cd0.72S-365 nm light system and (b) Zn0.28Cd0.72S-visible light system.

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Figure 9. DMPO spin-trapping ESR spectra in the (a) TiO2 water dispersion under 355 nm laser irradiation, (b) Zn0.28Cd0.72S water dispersion under 355 nm laser irradiation, and (c) Zn0.28Cd0.72S water dispersion under visible light irradiation.

Figure 10. Mott-Schottky plots for the (a) Zn0.28Cd0.72S film and (b) TiO2 film in 0.3 mol L-1 LiClO4 solution (pH ) 3).

the •OH because its relatively inert radical intermediate cannot interfere with the studied competition reactions. In the presence of TBA, the main radicals are O2•- radicals and e-aq. The signal shown in the ESR spectra may be assigned to e-aq, hydroperoxyl radical (HO2 · ), or · H,33,34 which should be further investigated. The signal in Figure 9b was much weaker than that in Figure 9c. If these signals were assigned to e-aq, thus this is the reason that induced the different results showed in Figure 7. In this case, in the presence of TBA under visible light irradiation, more e-aq would participate in increasing the degradation rate. Instead, under 365 nm light irradiation, the amount of e-aq was less than that of visible light. Therefore, the eliminated •OH and the smaller amount of e-aq reduced the degradation rate in the Zn0.28Cd0.72S-365 nm light system. It can be deduced that, in

the degradation process, the e-aq, the transfer of e-, and O2•radicals played a bigger role in the Zn0.28Cd0.72S-visible light system. This may be the reason that led to the different degradation results in the three systems. From the results of ESR and PLTA, it can be said that •OH was exactly formed in the three systems. How was it formed and is it the same source of •OH in the three systems? It is closely related to the position of the conduction bands of TiO2 and Zn0.28Cd0.72S. In the following experiment, the conduction and valence band potentials of Zn0.28Cd0.72S and TiO2 were measured. Mott-Schottky figures were usually used to measure the flat band potential (Vfb) of semiconductor particle films.35 Figure 10a is the Mott-Schottky plots for the Zn0.28Cd0.72S film in 0.3


Figure 11. Comparison of XPS spectra of O 1s on TiO2 and Zn0.28Cd0.72S particles.

mol L-1 LiClO4 solution (pH ) 3). According to our previous report,22 the slope of the line showed that Zn0.28Cd0.72S was an n-type semiconductor and its Vfb was -0.8 V. The conduction band potential (VCB) of Zn0.28Cd0.72S was about -0.9 V, which was more negative than the standard redox potential of O2/O2•(-0.33 V vs NHE).36 The valence band potential (VVB) of Zn0.28Cd0.72S was about 1.51 V and was also less than the standard redox potential of •OH/OH- (2.38 V vs NHE).37 As a result, the photogenerated electrons on irradiated Zn0.28Cd0.72S can reduce O2 to give O2•-, whereas the photogenerated holes cannot oxidize OH- to give •OH. From the same method, the slope of the line in Figure 10b showed that TiO2 was also an n-type semiconductor and its Vfb

J. Phys. Chem. C, Vol. 114, No. 49, 2010 21489 was -0.6 V. Thereby, it was deduced that the VCB of TiO2 was about -0.7 V, which was more negative than the standard redox potential of O2/O2•-. The band gap of TiO2 was estimated to be 3.2 eV in Figure 1. Hereby, the VVB of TiO2 was about 2.5 V and was more positive than the standard redox potential of •OH/ OH-. This result indicated that the photogenerated electrons on irradiated TiO2 can reduce O2 to give O2•-, whereas the photogenerated holes can oxidize OH- to give •OH. Therefore, the different electron structures of the materials decided the different sources of •OH formed. For example, the •OH species can be generated through the reaction between holes and surface hydroxyl groups or adsorbed H2O. However, the photogenerated holes cannot oxidize OH groups to give •OH in the Zn0.28Cd0.72S system, whereas in the PL-TA characterization, the •OH species existed. The generation of •OH in the Zn0.28Cd0.72S systems would have another way. There are many radicals generated by oxygen species, including the superoxide radical (HO2• or O2•-), hydrogen peroxide (H2O2), and the hydroxyl radical (•OH). O2•- can be easily generated on both Zn0.28Cd0.72S and TiO2, according to the Mott-Schottky measurements. Because of the importance of the oxygen species, first, the concentration of oxygen species on the surface of TiO2 and Zn0.28Cd0.72S were tested by XPS, and then the intermediate species, such as O2•- and H2O2, were detected by ESR and DPD methods, respectively. Figure 11 shows the comparison of XPS spectra of O 1s on TiO2 and Zn0.28Cd0.72S particles. For TiO2, the surfaces of the metal oxide usually terminate in oxide ions due to their large size and little polarizing power. In the oxide, the lattice oxide ions are rich. There is also some chemisorption of water molecules and oxygen on the surface of the catalyst. For

Figure 12. DMPO spin-trapping ESR spectra in the (a) TiO2 methanol dispersion for DMPO-O2•- under 355 nm laser irradiation, (b) Zn0.28Cd0.72S methanol dispersion for DMPO-O2•- under 355 nm laser irradiation, and (c) Zn0.28Cd0.72S methanol dispersion for DMPO-O2•- under visible light irradiation.

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Figure 13. Detection of H2O2 in (A) TiO2 water dispersions (TiO2, 0.08 g/150 mL) under 365 nm light, (B) Zn0.28Cd0.72S water dispersions (Zn0.28Cd0.72S, 0.08 g/150 mL) under 365 nm light, and (C) Zn0.28Cd0.72S water dispersions (Zn0.28Cd0.72S, 0.04 g/80 mL) under visible light. Curve a was obtained by water without catalysts; curve b was obtained by addition of DPD and POD to the dispersions after 1 h of irradiation under 365 nm light or visible light.

Zn0.28Cd0.72S particles, nonmetallic oxides, there is just some chemisorption of water molecules and oxygen on the surface. Therefore, as we can see from this figure, the intensity of O 1s peaks on the surface of TiO2 was obviously stronger than that on Zn0.28Cd0.72S particles. However, the existence of oxygen species on the surfaces of TiO2 and Zn0.28Cd0.72S could all induce the generation of the intermediate oxygen active species. O2•- radicals as an important oxygen active species were detected by the ESR technique in the three systems. As shown in Figure 12, under light irradiation, the strong and obvious peaks for DMPO-O2•- species can be observed in all the three methanolic dispersions. Especially, in the Zn0.28Cd0.72S systems, the signals were stronger and more obvious. This indicated that O2•- species existed and would participate in the three degradation processes. The DPD method38 employed for peroxide measurements was used for the detection of H2O2 that formed during the photodegradation of the dyes. Figure 13A shows the detection result of H2O2 in TiO2 water dispersions, and there were no obvious peaks observed. In the Zn0.28Cd0.72S systems shown in Figure 13B,C, in the absence of catalysts, no H2O2 was detected in the water (curve a). In the presence of Zn0.28Cd0.72S under 365 nm or visible light irradiation (curve b), when DPD and POD were put in one after another, the oxidized DPD showed obvious peaks with two absorption maxima, one at 510 nm and the other at 551 nm. It showed that, in the Zn0.28Cd0.72S-365 nm light and Zn0.28Cd0.72S-visible light systems, the intermediate H2O2 species were generated. Therefore, it not only verified the existence of O2•- species again but also was an inevitable source of •OH species.

O2•- species were produced by the reactions between photogenerated electrons and O2. The amount of photogenerated electrons is of importance in the generation of O2•- and •OH radicals. To further prove the way to generate •OH, the PL-TA probing technique has been used again in all the three systems. Through adding •OH quencher (TBA) and hole-capturer (AO), different results are presented in Figure 14. In all the three systems, the additional TBA prohibited the formation of •OH. It proved the existence of •OH and the effect of TBA as an •OH quencher. However, the addition of AO in the three systems induced the different effects. In the TiO2 system, the additional AO prohibited the formation of •OH. In this process, holes played an important role in the formation of •OH. This is in agreement with the results discussed in Figure 10. The suitable band gap of TiO2 favored the action of holes. In the Zn0.28Cd0.72S system, under either 365 nm or visible light irradiation, the additional AO increased the generation of •OH. As discussed in Figure 10, the holes cannot oxidize OH groups to give •OH in the Zn0.28Cd0.72S system. We have detected the existence of O2•- and H2O2, so it can be deduced that holes were captured by AO and the separation rate between holes and electrons was increased. More electrons were then captured by oxygen species to form O2•-. After that, the H2O2 and •OH radicals were generated in the Zn0.28Cd0.72S systems. Therefore, we can conclude that in the Zn0.28Cd0.72S systems, •OH was formed by O2•- or H2O2 species. From all these results, the different degradation mechanisms of the Zn0.28Cd0.72S and TiO2 systems were put forward. In the Zn0.28Cd0.72S system



Figure 14. PL changes in the (a) TiO2-365 nm light irradiation, (b) Zn0.28Cd0.72S-365 nm light irradiation, and (c) Zn0.28Cd0.72S-visible light irradiation systems under different conditions: in TA solutions, adding TBA, or adding AO. 365 nm or visible light irradiation

Zn0.28Cd0.72S + hν 98 + Zn0.28Cd0.72S (hVB + eCB ) (1) eCB + O2 f O•2

(2)

+ O•2 + H f HO2•

(3)

eCB + HO2• + H+ f H2O2

(4)

H2O2 + eCB f •OH + OH-

(5)

+ hVB , O•2 , eaq, HO2•, or •OH + Dyes f peroxy or hydroxylated...intermediates f degraded products (6)

In the TiO2 system 365 nm light irradiation

+ TiO2 + hν 98 TiO2 (hVB + eCB )

(7)

+ hVB + OH- f •OH

(8)

eCB + O2 f O•2

(9)

+ •OH, hVB , or O•2 + Dyes f peroxy or hydroxylated...intermediates f degraded products (10)

Conclusions The differences between TiO2 and Zn0.28Cd0.72S were studied in the degradation of MO under 365 nm and visible light irradiation. After 5 h of irradiation, the PCR of MO over Zn0.28Cd0.72S was up to 83% under 365 nm light and 80% under visible light. With regards to TiO2 under 365 nm light, the PCR was up to 95% after 120 min of irradiation. In the degradation process, different main byproducts (m/z ) 304, 290, 195, and 191) predominated in the three systems. The change trends of the intensity of the byproducts were different. Through imposing some additional conditions (N2, O2, TBA, AO) on the degradation process, different results were presented in the three systems because of the effect from active species. The removal of •OH reduced the PCR of the TiO2-365 nm light system to 90% and the Zn0.28Cd0.72S-365 nm light system to 73%, whereas in the Zn0.28Cd0.72S-visible light system, the PCR was not decreased but higher than that under air-equilibrated conditions. From the Mott-Schottky test, DPD method, and PL-TA technique, in the TiO2 system, holes played an important role in the formation of •OH. In the Zn0.28Cd0.72S system, under either 365 nm or visible light irradiation, •OH was formed by O2•- or the species generated in the process. At last, the different degradation mechanisms of Zn0.28Cd0.72S and TiO2 catalysts were put forward. The differences between the two catalysts lead us to realize the active species and the degradation mechanism in the photocatalytic process. It would guide us to prepare more efficient novel photocatalysts and promote the photocatalytic

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activity of the present catalysts. Therefore, it is worth exploring the photocatalytic mechanism through comparing different catalysts under different conditions. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (21073036 and 20873023), the National Basic Research Program of China (973 Program, 2007CB613306), and the Science Foundation of Fujian, China (JA07001, 0330-033070). References and Notes (1) Zollinger, H. Color Chemistry: Synthesis, Properties and Applications of Organic Dyes and Pigments; VCH Publishers: New York, 1987. (2) Ollis, D. F.; Pelizzetti, E.; Serpone, N. Photocatalysis. Fundamentals and Applications; Wiley: New York, 1989. (3) Ollis, D. F.; Al-Ekabi, H. Photocatalytic Purification and Treatment of Water and Air; Elsevier: Amsterdam, 1993. (4) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69–96. (5) (a) Linsebigler, A. L.; Lu, G.; Yates, J. T., Jr. Chem. ReV. 1995, 95, 735–758. (b) Fan, J.; Yates, J. T., Jr. J. Am. Chem. Soc. 1996, 118, 4686–4692. (6) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269–271. (7) Khan, S. U. M.; Al-Shahry, M.; Ingler, W. B., Jr. Science 2002, 297, 2243–2245. (8) Zhao, W.; Ma, W. H.; Chen, C. C.; Zhao, J. C. J. Am. Chem. Soc. 2004, 126, 4782–4783. (9) , M. B., Jr.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013–2016. (10) Chan, W. C. W.; Nie, S. Science 1998, 281, 2016–2018. (11) Plass, R.; Pelet, S.; Krueger, J.; Gra¨tzel, M. J. Phys. Chem. B 2002, 106, 7578–7580. (12) Zhong, X.; Feng, Y.; Knoll, W.; Han, M. J. Am. Chem. Soc. 2003, 125, 13559–13563. (13) Huang, J.; Lianos, P. Langmuir 1998, 14, 4342–4344. (14) Li, Y.; Ye, M.; Yang, C.; Li, X.; Li, Y. AdV. Funct. Mater. 2005, 15, 433–441. (15) Wada, Y.; Niinobe, D.; Kaneko, M.; Tsukahara, Y. Chem. Lett. 2006, 35, 62–63. (16) Liu, Y. K.; Zapien, J. A.; Shan, Y. Y.; Geng, C.; Lee, C. S.; Lee, S. AdV. Mater. 2005, 17, 1372–1377.

Li et al. (17) Zhai, T.; Gu, Z.; Yang, W.; Zhang, X.; Huang, J.; Zhao, Y.; Yu, D.; Fu, H.; Ma, Y.; Yao, J. Nanotechnology 2006, 17, 4644–4649. (18) Zhai, T. Y.; Zhang, X. Z.; Yang, W. S.; Ma, Y.; Wang, J. F.; Gu, Z. J.; Yu, D. P.; Yang, H.; Yao, J. N. Chem. Phys. Lett. 2006, 427, 371– 374. (19) Chen, J.; Lin, S.; Yan, G.; Yang, L.; Chen, X. Catal. Commun. 2008, 9, 65–69. (20) Wang, W.; Zhu, W.; Xu, H. J. Phys. Chem. C 2008, 112, 16754– 16758. (21) Li, W. J.; Li, D. Z.; Chen, Z. X.; Huang, H. J.; Sun, M.; He, Y. H.; Fu, X. Z. J. Phys. Chem. C 2008, 112, 14943–14947. (22) Li, W. J.; Li, D. Z.; Zhang, W. J.; Hu, Y.; He, Y. H.; Fu, X. Z. J. Phys. Chem. C 2010, 114, 2154–2159. (23) Butler, M. A. J. Appl. Phys. 1977, 48, 1914–1920. (24) Brown, G. T.; Darwent, J. R. J. Chem. Soc., Faraday Trans. 1 1984, 80, 1631–1643. (25) Yoon, M.; Seo, M.; Jeong, C.; Jang, J. H.; Jeon, K. S. Chem. Mater. 2005, 17, 6069–6079. (26) Kominami, H.; Furushok, A.; Murakami, S.; et al. Catal. Lett. 2001, 76, 31–34. (27) Edzwald, J. K. Water Sci. Technol. 1993, 27, 21–35. (28) Langlais, B.; Reckhow, D. A.; Brank, D. R. Ozone in Water Treatment: Application and Engineering; Lewis Publishers: Chelsea, MI, 1991. (29) Barreto, J. C.; Smith, G. S.; Strobel, N. H. P.; McQuillin, P. A.; Miller, T. A. Life Sci. 1995, 56, 89–96. (30) Ulick, S.; Kimberly, A. G.; Prashant, V. K. J. Phys. Chem. 1994, 98, 6343–6351. (31) David, B.; Joseph, R. J. Phys. Chem. B 2006, 110, 8750–8755. (32) George, A. Z.; Kimberly, A. G.; Prashant, V. K. EnViron. Sci. Technol. 2000, 34, 3401–3407. (33) Liu, G.; Zhao, J.; Hidaka, H. J. Photochem. Photobiol., A 2000, 133, 83–88. (34) Noda, H.; Oikawa, K.; Kamada, H. Bull. Chem. Soc. Jpn. 1992, 65, 2505–2509. (35) Redmond, G.; O’Keeffe, A.; Burgess, C.; et al. J. Phys. Chem. 1993, 97, 11081–11086. (36) Bard, A. J.; Faulkner, L. R. Electrochemical Methods Fundamentals and Applications; John Wiley & Sons: New York, 1980. (37) Bard, A. J.; Parsons, R.; Jordan, J. Standard Potentials in Aqueous Solution; Marcel Dekker: New York, 1985. (38) Bader, H.; Sturzenegger, V.; Hoigne, J. Water Res. 1988, 22, 1109–1115.

JP106659G

Specific Analyses of the Active Species on Zn0.28Cd0.72S and TiO2

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