Photocatalytic Degradation of Dyes by ZnIn2S4 Microspheres under

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J. Phys. Chem. C 2009, 113, 4433–4440

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Photocatalytic Degradation of Dyes by ZnIn2S4 Microspheres under Visible Light Irradiation Zhixin Chen,† Danzhen Li,*,† Wenjuan Zhang,† Yu Shao,† Tianwen Chen,‡ Meng Sun,† and Xianzhi Fu*,† Research Institute of Photocatalysis, State Key Laboratory Breeding Base of Photocatalysis, and Analytical and Testing Center, Fuzhou UniVersity, Fuzhou, Fujian, 350002, P. R. China ReceiVed: October 20, 2008; ReVised Manuscript ReceiVed: December 19, 2008

ZnIn2S4 microspheres were successfully synthesized by a hydrothermal method. A series of synthesis temperatures from 80 to 200 °C was investigated. The samples were characterized by X-ray diffraction, UV-vis spectroscopy, nitrogen sorption analysis, X-ray photoelectron spectroscopy, transmission electron microscopy, and scanning electron microscopy (SEM). The results indicated that the crystalloid structure and optical property of temperature series products were almost the same. The specific surface area (SBET) of ZnIn2S4 products declined with increasing synthesis temperature. The 80 °C sample had the largest SBET (85.53 m2 g-1). SEM images demonstrated that the morphology of ZnIn2S4 was marigold-like microspheres, and the 80 °C sample had a well-proportioned morphology. Several dyes (methyl orange, congo red, and rhodamine B) were applied in the ZnIn2S4 photocatalytic reactivity investigation. It showed efficient visible light photocatalytic degradation of dyes. A liquid chromatogram-mass spectrometer was used for identification of dyes and their degradation products. A large number of · OH radicals, investigated by the method of photoluminescence with terephthalic acid, were generated in the photocatalyst system. The results indicated that the · OH radicals played an important role in the superior visible photocatalytic activity of the ZnIn2S4 system. The mechanism related to the photocatalytic degradation was proposed and discussed. 1. Introduction Synthetic textile dyes and other industrial dyestuffs constitute the largest group of chemicals produced all over the world.1,2 Within the overall category of dyestuffs, azo dyes and fluorone dyes constitute a significant portion and probably have the least desirable consequences in terms of the surrounding ecosystem. It is well-known that some azo dyes, fluorone dyes, and their degradation products such as aromatic amines are highly carcinogenic. As international environmental standards are becoming more stringent (ISO 14001, October 1996), technological systems for the removal of organic pollutants such as dyes have been recently developed.3 Among them, heterogeneous photocatalytic oxidation (PCO) is widely used as a “green” technology for the decomposition of the soluble dyes in wastewater.4 Recently, much attention has been paid to developing new materials (nontitania series) as effective photocatalysts,5-12 which can work under visible light rather than ultraviolet light. In this way, sunlight and indoor light can be utilized more efficiently in the PCO process. The ternary chalcogenides ABmCn (A ) Cu, Ag, Zn, Cd, etc.; B ) Al, Ga, In; C ) S, Se, Te; m, n: Arabic numerals) have been extensively studied because of their unique optoelectronic and catalytic properties.10,13-17 Among these compounds, zinc indium sulfide as one of the AB2C4 families has attracted considerable attention because it is a potential material for application in photocatalysis, charge storage, electrochemical recording, and thermoelectricity.11,12,15,18,19 For example, Lei et al. reported a hydrothermal synthesis of ZnIn2S4 nanoparticles * Corresponding authors. Tel. and Fax: (+86)591-83779256. E-mail: [email protected] (D.L.); [email protected] (X.F.). † State Key Laboratory Breeding Base of Photocatalysis. ‡ Analytical and Testing Center.

and examined their potential application for photocatalytic water reduction under visible light irradiation.12 Intriguing ZnIn2S4 nanotubes, nanoribbons, nanowires, and microspheres have been prepared by Gou et al. on the basis of hydrothermal/solvothermal processes.18 Hierarchically porous ZnIn2S4 submicrospheres have been synthesized by Hu et al. through a microwave-solvothermal approach and showed enhanced visible light photocatalytic activity for methylene blue degradation.11 Shen et al. reported that ZnIn2S4 was synthesized via a CTAB-assisted hydrothermal method and investigated the photocatalytic hydrogen production from water under visible light irradiation.20 These studies are important for understanding the formation of complex ZnIn2S4 nanostructures and their initial applications in photocatalysis. Despite these advances, however, the methods mentioned above have two disadvantages as follows. (1) These methods relied on relative higher temperature and special reagents. (2) The description of photocatalytic process and mechanism was faint. Therefore, further development of the direct fabrication of ZnIn2S4 with a facile synthetic method and detailed investigation of photocatalytic mechanisms are quite necessary. Recently, our group21 reported a facile thermal solution synthesis of marigold-like porous ZnIn2S4 microspheres and proposed the morphology formation mechanism. Compared with the methods mentioned above, this synthetic procedure has the advantages of simplicity (without any organic solvents or templates) and low temperature (80 °C). Also, some preliminary investigation of the visible light photocatalytic activity for methyl orange degradation has been done. Herein, a hydrothermal synthesis of marigold-like ZnIn2S4 microspheres at 80-200 °C was reported. The application of ZnIn2S4 in photocatalytic reaction of azo dyes (methyl orange (MO) and congo red (CR)) and fluorone dyes (rhodamine B, RhB) has been investigated. As expected, the resulting ZnIn2S4 synthesized

10.1021/jp8092513 CCC: $40.75  2009 American Chemical Society Published on Web 02/24/2009

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Figure 1. XRD patterns of the as-prepared temperature series ZnIn2S4 microspheres.

Figure 2. UV-vis diffuse reflectance spectra of temperature series ZnIn2S4 products. Inset: spectrum of combined filters.

at 80 °C for 6 h showed efficient visible light photocatalytic degradation of MO, CR, and RhB. Furthermore, a liquid chromatogram-mass spectrometer (LCMS) was used for identification of dyes (MO and RhB) and their degradation products. The generation of · OH radicals was investigated by the method of photoluminescence with terephthalic acid (TA-PL). The mechanism related to the photocatalytic process was proposed and discussed.

system was cooled by a fan and circulating water to maintain the room temperature. Prior to irradiation, the suspension was magnetically stirred in the dark to ensure establishment of an adsorption-desorption equilibrium. The visible light source system consisted of a 500 W tungsten-halogen lamp (Philips Electronics) and a composited cutoff filter that restricted the illumination in a range of 420-800 nm (the spectrum is shown in the inset of Figure 2). Photocatalytic degradation was monitored by measuring the absorbance of solution using a Varian Cary 50 Scan UV-vis spectrophotometer. The CR and RhB test conditions were parallel to the MO test system, but the concentration of CR and RhB were 20 and 30 ppm, respectively. In the total organic carbon (TOC) investigation, the light source and filters were the same as with the photocatalytic reaction system, original MO and RhB solutions were 100 ppm, 250 mL, the amount of ZnIn2S4 was 0.20 g, the irradiation time was 30 h, and 25 mL solutions were sampled at different stages. LCMS Analysis. The determination of the dyes’ concentration and the identification of their respective byproducts were performed by a LCMS system. An Agilent 1100 series LC system (Agilent Technologies, Palo Alto, CA) was performed with a binary pump, 1100 UV-vis diode array detector, an autosample, and a column thermostat. The LCMS system was equipped with a Zorbax C18 column (150 mm × 4.6 mm i.d., 5 µm) and coupled online to a LC/MSD Trap XCT ion-trap mass spectrometer (Agilent Technologies, CA). The mass spectrometer was equipped with an ESI source and operated in positive polarity. The ESI conditions were as follows: capillary voltage 3.5 kV; end plate offset -500 V; capillary exit 100 V; nebulizer pressure 40 psi; drying gas flow 10 L min-1; temperature 350 °C. For the MO analysis, the solvents used as mobile phase were acetonitrile:0.01 M ammonium acetate (pH 6.8) ) 30:70 (V/V), flow rate was 0.6 mL min-1, 20 µL of standard or sample solution was injected, and the mass range was from 50 to 400 m/z. For the RhB analysis, the solvents used as mobile phase were methanol:water ) 80:20 (V/V), flow rate was 0.8 mL min-1, 20 µL of standard or sample solution was injected, and the mass range was from 50 to 600 m/z. Determination of · OH Radicals. In recent years, Ishibashi et al.22 have developed a photoluminescence technique by TAPL to detect · OH selectively. The reaction principle was that terephthalic acid (TA) readily reacted with · OH radicals to produce highly fluorescent product, 2-hydroxyterephthalic acid (TAOH), which emitted photoluminescence at around 426 nm on the excitation of its own 312 nm absorption band. The intensity of the peak attributed to TAOH was known to be proportional to the amount of · OH radicals formed. Therefore, the method of photoluminescence technique with TA-PL22,23 was used to detect the generation of · OH. The visible light source

2. Experimental Details Synthesis. All chemicals were analytical grade and used as received without further purification. In a typical reaction, ZnCl2 (1 mmol) and InCl3 · 4H2O (2 mmol) were added by stoichiometric ratio, and excessive thioacetamide (TAA, 6 mmol) was dissolved in a Teflon liner with 100 mL capacity containing 80 mL of deionized water. The pH was adjusted to 2.5 by hydrochloric acid, and then the Teflon liner was sealed in the stainless steel autoclave and maintained at 80 °C for 6 h. After the reaction was completed, the autoclave was cooled to room temperature naturally. The product was collected by centrifugation and washed with deionized water and absolute ethanol several times. The final sample was dried at 60 °C in a vacuum for characterization and photocatalytic reaction. The temperature series samples from 80 to 200 °C were synthesized. Characterization. The X-ray diffraction (XRD) patterns, obtained on a Bruker D8 Advance X-ray diffractometer using Cu KR1 irradiation (λ ) 1.5406 Å), were used to identify the phase constitutions in samples. The accelerating voltage and the applied current were 40 kV and 40 mA, respectively. UV-vis diffuse reflectance spectra (DRS) were obtained by a Varian Cary 500 UV-vis-NIR spectrometer. The specific surface area and porosities of the samples were measured by N2 adsorption at 77 K on a Micrometritics ASAP2020 analyzer and calculated by the Brunauer-Emmett-Teller (BET) method. All of the samples were degassed at 70 °C overnight prior to BET measurements. The general morphology of the products was examined by scanning electron microscopy (SEM) on a JEOL JSM 6700F instrument operated at 20 kV. The morphology and microstructure of the composite were further investigated by transmission electron microscopy (TEM) and highresolution TEM (HRTEM) using a JEOL JEM 2010F microscope working at 200 kV. X-ray photoelectron spectroscopy (XPS) analysis was conducted on a ESCALAB 250 photoelectron spectroscope (Thermo Fisher Scientific) at 3.0 × 10 -10 mbar with monochromatic Al KR radiation (E ) 1486.2 eV). Photocatalytic Activity Measurements. The photocatalytic degradation of MO was carried out in an aqueous solution at ambient temperature. Briefly, 40 mg of ZnIn2S4 was suspended in an 80 mL aqueous solution containing 10 ppm of MO. The

Synthesis of ZnIn2S4 Microspheres

J. Phys. Chem. C, Vol. 113, No. 11, 2009 4435

Figure 3. SEM images of the as-prepared temperature series marigold-like ZnIn2S4 microspheres: (a) 80 °C, (b) 120 °C, (c) 160 °C, and (d) 200 °C.

TABLE 1: Nitrogen Adsorption-Desorption Isotherm Results of Temperature Series ZnIn2S4 Products sample 2

-1

SBET/m g

80 °C

100 °C

120 °C

140 °C

160 °C

180 °C

200 °C

85.53

74.70

70.24

64.80

59.79

56.92

54.49

and the cutoff filters were parallel to the photocatalytic reactivity test system, and a 40 mg ZnIn2S4 catalyst was added to 80 mL of TA solution (5 mmol L-1) in a 100 mL Pyrex glass vessel. At 20 min intervals, 3 mL aliquots were sampled and centrifuged to remove the catalysts. The photoluminescence intensity of TAOH was surveyed by an Edinburgh FL/FS900 spectrophotometer. 3. Results and Discussion The phase and crystallographic structure of the products were determined by XRD. Figure 1 showed the XRD patterns of the as-prepared temperature series ZnIn2S4 products. Although their reaction temperature was different, from 80 to 200 °C, the XRD patterns of all the samples presented almost the same profiles and all the diffraction peaks could be indexed to a hexagonal phase of ZnIn2S4, which was in agreement with the literature11,18,21 and JCPDS No. 65-2023. No other impurities, such as binary sulfides, oxides, or organic compounds related to reactants, were detected by XRD analysis. Diffuse reflectance spectroscopy was used for characterizing the optical properties of materials. Figure 2 showed the UV-vis diffuse reflectance spectra of as-prepared temperature series ZnIn2S4 powders. It could be seen that the temperatures series ZnIn2S4 products had similar DRS spectra. They all had a steep absorption edge in the visible range, which indicated that the relevant band gap was due to the intrinsic transition of the nanomaterials rather than the transition from impurity levels. The XRD and DRS results indicated that the ZnIn2S4 products could be formed at 80 °C. With the reaction temperatures increasing, the products had the same crystal phase, and the absorption properties were almost uniform. The nitrogen adsorption-desorption isotherms of the ZnIn2S4 products were further investigated. The nitrogen adsorption and desorption isotherms were characteristic of a type IV isotherm

with a hysteresis loop, indicating the presence of porous structure in the products. The SBET values of the temperature series ZnIn2S4 products are enumerated in Table 1. The SBET of the ZnIn2S4 products prepared at 80 °C was the largest, 85.53 m2 g-1. With increasing reaction temperature, the data of SBET declined. When the synthesis temperature was 200 °C, the SBET was the least, 54.49 m2 g-1. The optical properties, porous structure, and large SBET would endow the as-prepared ZnIn2S4 products with potential applications of effective photocatalysis. The morphology of the as-synthesized temperature series ZnIn2S4 was investigated by SEM. Figure 3 showed the overall morphology of the products. Figure 3a indicated that the ZnIn2S4 synthesized at 80 °C was composed of a large quantity of microspheres with an average diameter of about 3-7 µm and had a unique marigold-like spherical superstructure which was made up of numerous nanosheets. The image of the sample synthesized at 120 °C was shown in Figure 3b. There were many marigold-like microspheres, but some erose fragments appeared. When the temperature was 160 °C (Figure 3c), there were a few marigold-like microspheres composed of numerous nanosheets which were marked by rectangular frames. Some nanosheets in the microspheres’ surface were collapsed partly which were marked by a pentagonal frame, and some nanosheets were collapsed completely as marked by circular frames. The sample of 200 °C was shown in Figure 3d. Compared with the samples of lower reaction temperature, the microspheres composed by nanosheets were the minority, and nanosheets collapsed completely were the majority. These results demonstrated that hydrothermal process in 80 °C for 6 h was the best synthesis condition for the well-proportioned marigold-like microspheres composed of nanosheets, and the nanosheets were collapsed incidentally when the reaction temperature increased. So, the product of 80 °C was further investigated by TEM and HRTEM.

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Figure 4. Low-magnification (a), high-magnification (b), and high-resolution TEM (c) images of the synthesized ZnIn2S4 at 80 °C.

Figure 5. (a) Visible light photocatalytic activities of MO for the temperature series ZnIn2S4 products and TiO2-xNx. (b) Time-dependent absorption spectral pattern of MO in the presence of ZnIn2S4 synthesized at 80 °C under the visible light irradiation.

Figure 6. UV-vis spectra of the CR (a) and RhB (b) solution in the presence of ZnIn2S4 under visible light irradiation in different photocatalytic reaction stages.

Figure 4a presented some individual microspheres with a zigzag circle, in accordance with the SEM images (Figure 3a). Figure 4b was the enlarged TEM image of Figure 4a. Some unequal nanosheets which assembled into fringes of a microsphere can be seen obviously. The HRTEM image is shown in Figure 4c. The lattice interplanar spacing was measured to be 0.324 nm, corresponding to the (102) plane of hexagonal ZnIn2S4. The visible light photocatalytic activity tests of temperature series ZnIn2S4 products were evaluated by degradation of MO aqueous solution under visible light irradiation. The photocatalytic results of MO were shown in Figure 5. The y-axis was defined as C/C0 (where C was the main absorption peak intensity of MO sampled at each irradiated time interval at a wavelength of 464 nm, and C0 was the absorption intensity of starting 10 ppm MO solution). There was almost no decolorization in the solution without any catalysts or with the TiO2-xNx (the amorphous TiO2 xeogel which was prepared by the sol-gel method was heated at 400 °C for 3 h under flowing NH3 gas, and then postannealed at 400 °C for 2 h in static air to gain the TiO2-xNx sample24) under visible light irradiation. As shown in Figure 5, x-axis, during 0-1 h, the absorption-desorption equilibrium was established after 30 min in dark absorption for temperature series ZnIn2S4 photocatalysts. However, the absorption ability of these samples was different. The 80 °C product had the maximal absorption ability which may be due to the largest SBET, and the 200 °C product had the minimal one relevant to the least SBET. Furthermore, it could be clearly seen that, as shown in Figure 5a, line 80, and Figure 5b, the MO was gradually photocatalytically degraded and the 80 °C product has the best photocatalytic activity. After 3 h irradiation, the solution was nearly colorless and the value of C/C0 was about zero. Considering the energy wastage, economic reason, and facility, the sample synthesized at 80 °C was selected for further investigation. Another azo dye CR was chosen as a simulative contaminant to evaluate the photocatalytic activity of ZnIn2S4 product. Similar

to the MO results, the CR solution showed no degradation without any catalyst under visible light irradiation, and the absorption-desorption equilibrium between catalyst and CR solution was established after 30 min in dark absorption. UV-vis spectral changes during the photocatalytic degradation of CR in the aqueous ZnIn2S4 dispersions under visible light irradiation were shown in Figure 6a. With the light irradiating, both the 500 and 340 nm absorption peaks of CR solution were depressed gradually. The 20 ppm CR solution was almost colorless after 5 h visible light irradiation. The photocatalytic degradation experiment of fluorone dye RhB solution was also carried out. The adsorption ability of catalyst to RhB was stronger than the MO and CR. When the ZnIn2S4 photocatalyst was added into 10 ppm RhB solution, the dispersions discolored completely after about 10 min in dark absorption. So, the 30 ppm RhB solution was applied in this experiment. The absorption-desorption equilibrium was established after 5 h in dark absorption of ZnIn2S4 photocatalyst. UV-vis spectral changes during the photocatalytic degradation of RhB in the aqueous ZnIn2S4 dispersions under visible light irradiation were shown in Figure 6b. During visible light irradiation, in the characteristic absorption band of the dye around 554 nm slight hypsochromic shifts were observed in the 10 min sample, and the absorption intensity was decreased rapidly. Furthermore, the hypsochromic shifts of the absorption maximum existed in the 20, 30, and 60 min samples. The absorption maximum was around 496 nm after 60 min irradiation. With the irradiation time prolonged, the intensity decreased gradually but no peak shift occurred. The dispersions discolored completely after irradiation for about 180 min. This hypsochromic shift of the absorption band was presumed to result from the formation of a series of N-de-ethylated intermediates in a stepwise manner. Similar phenomena were also observed during the photodegradation of sulforhodamine-B25 and crystal violet26 under visible light irradiation. The mineralization was an important target in the removal of organic pollutants from wastewater. Therefore, the resultant

Synthesis of ZnIn2S4 Microspheres

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TABLE 2: TOC Detection Results of MO and RhB Photocatalytic Reactions TOC (mg/L) dyes

original

light 5 h

light 10 h

light 20 h

light 30 h

MO RhB

40.41 58.44

39.29 38.00

38.58 31.52

38.39 30.13

38.64 30.26

CO2 in the photocatalytic reaction of MO and RhB was detected using the Ba(OH)2 solution, and the 20 mL/min flow O2 used as the carrier gas. In both MO and RhB reactions, little white precipitation was produced as BaCO3 reacted with Ba(OH)2 solution and the resultant CO2. It demonstrated that there was some CO2 produced in the photocatalytic reactions. However, it is difficult to quantitatively analyze BaCO3 (CO2) because the amount of precipitation was little. On the other hand, the TOC experiments were carried out, too. The results were displayed in Table 2. With the irradiation prolonged, the TOC values of MO showed tiny changes. The TOC values of RhB were decreased gradually at the stage of original to light 20 h but stayed nearly the same at the stage of light 20-30 h. From the above results, we concluded that the MO and RhB solutions could be photocatalytically decolorized and partly mineralized (MO, about 5%; RhB, about 48%) by ZnIn2S4 microspheres irradiated by visible light. It is known that the photocorrosion or photodissolution of catalysts might occur on the photocatalyst surface in the photocatalytic reaction. The photocatalysts before and after reaction of MO were compared by XRD and XPS investigation. Figure 7 showed the XRD patterns of ZnIn2S4 before and after reaction. The position, intensity, and ratio of peaks were nearly the same, and no new peak was created. Furthermore, the XPS was applied to test the photocatalysts’ surface chemical states. Figure 8a indicated that the main peaks were owned by elements Zn, In, and S. The comparison spectra for Zn2p, In3d, and S2p were shown in Figure 8 b-d. The binding energies of Zn2p, In3d, and S2p before and after reaction were nearly the same whether in the full survey spectrum or the detailed spectra. These results suggested that the photocatalyst ZnIn2S4 was stable in the photocatalytic reaction. LCMS was applied to the separation and identification of degradation products to study the photocatalytic process of MO and RhB solution. Figure 9a1-a5 and Figure 10a displayed the LCMS chromatograms and mass peak intensity changes of the MO solution of different irradiation intervals. The original solution after adsorption-desorption equilibrium in the dark with ZnIn2S4 only had a peak of MO in the chromatogram. It appeared at 7.2 min, and the intensity of the peak was 5.7 × 105 counts. After 1 h visible light irradiation, the intensity of the MO peak at 7.2 min in the chromatogram was reduced to 2.6 × 105 counts. When the irradiation time was 2 h, the peak at 7.2 min decreased to 1.2 × 105 counts, and a new peak appeared at 4.0 min. Furthermore, with the visible light irradiating, the little peak of 2.0 min appeared at the 3 h sample, and there were three peaks at 2.0, 4.0, and 7.2 min, respectively. When the irradiation time was prolonged to 6.5 h, the peak of 2.0 min increased to 4.3 × 104 counts, and both the 7.2 and 4.0 min peaks disappeared. Corresponding to the LCMS chromatograms results, the mass intensity changes (Figure 10a) indicated that some photocatalytic decomposition reaction occurred in the MO solution. The intensity of m/z 304, which belongs to the MO, decreased to about zero with the irradiation time increasing to 3.0 h. This result was in agreement with the LCMS and UV-vis investigation. The intensity of m/z 291 increased

Figure 7. Comparison of XRD patterns of ZnIn2S4 before and after reaction.

Figure 8. Comparison of XPS spectra of ZnIn2S4 before and after reaction: (a) survey XPS spectrum and (b-d) high-resolution spectra of Zn2p, In3d, and S2p.

gradually in the initial irradiation stage but decreased to about zero when the irradiation time was prolonged to 6.5 h. The intensity of m/z 195 increased continuously to 4.3 × 104 counts at 6.5 h with visible light irradiation. According to the analysis above, the photocatalytic degradation process of MO can be expressed in Scheme 1. The LCMS chromatograms and mass peak intensity changes of the RhB solution at different irradiation intervals were shown in Figure 9b1-b6 and Figure 10b. The original solution after adsorption-desorption equilibrium with ZnIn2S4 in the dark also only had a chromatogram peak of RhB appearing at 7.9 min. With the visible light irradiating, the intensity of the 7.9 min peak decreased continuously until 180 min, at which this peak remained only a small apophysis (Figure 9b6). A spot of shift of peak may come from the instrumental error. In addition, new peaks of 6.2 and 3.7 min appeared at 10, 20, 30, and 60 min samples (Figure 9b2, b3), and both of them were decreased continuously until no obvious peak appeared at 180 min (Figure 9b6). Corresponding to the LCMS chromatograms results, the mass intensity changes indicated that some photocatalytic decomposition reaction occurred in the RhB solution. The intensity of m/z 443, which belongs to the RhB, decreased to about zero with the irradiation time increased, according to the LCMS and UV-vis investigation. With the visible light irradiating, the intensity of m/z 415 and 387 increased in the

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Figure 9. LCMS chromatograms of the MO and RhB solution at different irradiation intervals: (a1) chromatogram of the origin MO solution after adsorption-desorption equilibrium without visible light irradiation; (a2)-(a5) chromatogram of the MO solution after 1, 2, 3, and 6.5 h irradiation, respectively; (b1) chromatogram of the origin RhB solution after adsorption-desorption equilibrium without visible light irradiation; (b2)-(b6) chromatogram of RhB solution after 10, 20, 30, 60, and 180 min irradiation, respectively.

Figure 11. · OH-trapping photoluminescence spectra of ZnIn2S4 in solution of terephthalic acid at room temperature (ex, 312 nm; em, 426 nm) (a) in light and (b) in dark. Inset: plot of the induced fluorescence intensity at 426 nm against illumination time.

Figure 10. Mass spectra view changes of the peak intensity appearing in the photocatalytic process of the MO (a) and RhB (b) solutions.

initial irradiation stage and decreased to about zero at 180 min. According to the analysis above, the photocatalytic degradation process of RhB can be expressed in Scheme 2.

Figure 11 showed the · OH-trapping photoluminescence spectra of ZnIn2S4 in TA solution at room temperature under visible light irradiation. It was evident that some active radicals were photogenerated on the ZnIn2S4 system under visible light irradiation. When the ZnIn2S4 system was irradiated by the visible light, the photoluminescence emission peak of TAOH (em: 426 nm) was continuously enhanced (Figure 11a). However, when ZnIn2S4 system was in dark conditions, the fluorescence emission peak intensity of TAOH was very weak (Figure 11b). The inset was the plot of the induced fluorescence intensity at 426 nm against illumination time. The line of the light system was slanted; however, the dark system was almost flat. In addition, the photoluminescence emission peak of TAOH cannot be detected in either dark or light system in the blank TA solution without ZnIn2S4 photocatalyst. From the above

Synthesis of ZnIn2S4 Microspheres SCHEME 1: Proposed Degradation Pathway for MO during Photocatalysis

discussion, we can conclude that the large amount of · OH was generated in the ZnIn2S4 photocatalytic system under visible light irradiation. The generation of · OH plays an important role in the superior visible photocatalytic activity of the ZnIn2S4 system. On the basis of the experiment results and the literature,27,28 the photocatalytic degradation mechanism of dyes by the ZnIn2S4 microspheres under visible light irradiation was pro-

J. Phys. Chem. C, Vol. 113, No. 11, 2009 4439 posed, as illustrated by Scheme 3 and eqs 1-6. When photocatalyst ZnIn2S4 was irradiated by visible light, an electron (ecb) in the valence band (VB) can be excited to the conduction + ) was generated in the VB simultaband (CB), and a hole (hvb neously (eq 1). With that, electrons at the photocatalyst surface were scavenged by the ubiquitously present molecular oxygen to yield first the superoxide radical anion, · O2- (eq 2). The superoxide radical anion · O2- further combined with H+ to generate · HO2 (eq 3). The · OH radical can be formed from the trapped electron after formation of the · HO2 radical (eqs 4 and 5). The generation of a large amount · OH radicals was proved by the TA-PL method. Lastly, the photocatalytic degradation of the dyes can take place through eq. 6. The active oxygen species ( · OH, · HO2, or · O-2 radicals) or the h+vb attacked the dyes, and the dyes were degraded gradually (Schemes 1 and 2). + ZnIn2S4 + hν f ZnIn2S4(ecb+hvb)

(1)

ZnIn2S4(ecb) + O2 f ZnIn2S4+ · O2

(2)

+ O2 +H f · HO2

(3)

ZnIn2S4(ecb)

+

+ · HO2+H f H2O2

H2O2+ZnIn2S4(ecb)

-

f · OH + OH

+ OH, · HO2, · O2 or hvb + dyes f peroxy or hydroxylated...intermediates f f degraded products

SCHEME 2: Proposed Degradation Pathway for RhB during Photocatalysis

(4) (5)

(6)

4440 J. Phys. Chem. C, Vol. 113, No. 11, 2009 SCHEME 3: Proposed Mechanism for the Visible Light Photocatalytic Reaction of Dyes on ZnIn2S4 Catalyst

4. Conclusions The marigold-like ZnIn2S4 microspheres were successfully synthesized by a hydrothermal method. The crystallographic structure and optical property of temperature series products were almost the same. The SBET of ZnIn2S4 microspheres decreased when the synthesized temperature increased, and the largest SBET was the 85.53 m2 g-1 (80 °C sample). The visible light photocatalytic degradation of dyes (MO, CR, and RhB) by ZnIn2S4 was investigated in detail. The ZnIn2S4 synthesized at 80 °C for 6 h showed good photocatalytic activity to dyes. The photocatalytic degraded intermediates have been probed by LCMS. A large number of · OH radicals, which played an important role in the superior visible photocatalytic activity of the ZnIn2S4 system, have been tested by the TA-PL method. The possible photocatalytic degradation mechanism was proposed and discussed. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (20537010, 20677010, and 20873023), an “863” project from the MOST of China (2006AA03Z340), the National Basic Research Program of China (973 Program: 2007CB613306), and the Natural Science Foundation of Fujian, China (2003F004, JA07001 and 0330-033070). We also gratefully thank Dr. Yilin Chen, Yidong Hou, and Qingping Wu for helpful discussion. References and Notes (1) Brown, M. A.; DeVito, S. Crit. ReV. EnViron. Sci. Technol. 1993, 23, 249–324.

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