Synthesis and Photocatalytic Activity of Calcium Antimony Oxide

CaSb2O5(OH)2 nanocrystals were synthesized via a facile microwave-hydrothermal method. The physicochemical properties of the as-synthesized CaSb2O5(OH...
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J. Phys. Chem. C 2009, 113, 13825–13831

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Synthesis and Photocatalytic Activity of Calcium Antimony Oxide Hydroxide for the Degradation of Dyes in Water Meng Sun, Danzhen Li,* Yibin Chen, Wei Chen, Wenjuan Li, Yunhui He, and Xianzhi Fu* Research Institute of Photocatalysis, State Key Laboratory Breeding Base of Photocatalysis, Fuzhou UniVersity, Fuzhou, Fujian, 350002 People’s Republic of China ReceiVed: April 11, 2009; ReVised Manuscript ReceiVed: June 13, 2009

CaSb2O5(OH)2 nanocrystals were synthesized via a facile microwave-hydrothermal method. The physicochemical properties of the as-synthesized CaSb2O5(OH)2 photocatalyst were characterized by X-ray diffraction, UV-vis diffuse reflectance spectroscopy, transmission electron microscopy, electron spin resonance, and X-ray photoelectron spectroscopy. The photocatalytic activity was evaluated by the decomposition of rhodamine B, methyl orange, and methylene blue in aqueous solution under UV irradiation. For comparison purposes, we have also investigated the activity of TiO2 (P25, Degussa Co.) under the same condition. The results revealed that the sample had a much higher photocatalytic activity than that of P25. The high activity and long-term stability of CaSb2O5(OH)2 were mainly attributed to its stronger oxidative capability and larger specific surface area in comparison with P25. This article presents a detailed analysis of the intermediates produced in the photocatalytic reactions using the liquid chromatography-mass spectrometry technique. The possible photocatalytic reaction pathways as to how CaSb2O5(OH)2 nanocrystals degrade organic dyes have also been proposed. 1. Introduction In recent years, wastewater discharged from factories and households have polluted the environment seriously. Many industrial plants have to reuse the water inside or eliminate pollutants before the wastewater pours into natural water bodies.1,2 For the purification of wastewater, several methods have been employed. However, there were still many problems, such as low concentration of pollutant and high toxicity, that have greatly limited the practical applications.3 Photocatalysis as a green technology has attracted great attention for the purpose of purifying wastewater, especially for the treatment of organic compounds at low levels.4 TiO2based photocatalytic oxidation (PCO), which is environmentally friendly, performs at room temperature, and treats pollutants at low concentration, has been recognized as a very promising technology for environmental remediation. However, the relatively low quantum efficiency of TiO2 has greatly limited its application in the treatment of organics in aqueous phase.5,6 To solve this problem, numerous efforts have been done to enhance the photocatalytic efficiency of TiO2, such as deposition of noble metals (Pt, Au, and Ag)7-9 and introduction of either anions (N, S, and C)10-15 or cations (V, Fe, and Cu).16,17 To develop non-TiO2 photocatalysts is one of the alternative approaches for treating organic pollutants at ambient conditions. Recently, some wide band gap metal semiconductors such as Zn2GeO4,18 InOOH,19 and Sr2Sb2O720 have been successfully applied to decompose volatile organic pollutants in the gas phase under UV irradiation. However, all those photocatalysts are either of low activity or too sophisticated to synthesize, costing a long reaction time and consuming a great deal of energy. So, a simple but efficient and energy saving method is greatly needed for the synthesis of the photocatalyst. * To whom correspondence should be addressed. Phone/Fax: (+86) 59183779256. E-mail: [email protected].

In our previous work, we have synthesized CaSb2O5(OH)2 nanocrystals (NCs) using a microwave-hydrothermal (M-H) method with high product yield. It was found that CaSb2O5(OH)2 could efficiently decompose benzene in the gas phase under UV light irradiation. However, the application of the CaSb2O5(OH)2 photocatalyst for environmental purification of wastewater has not been explored yet. Herein, we report the photocatalytic activity of CaSb2O5(OH)2 for the treatment of organic pollutants in wastewater for the first time. The efficient photocatalytic activity of CaSb2O5(OH)2 was mainly demonstrated by the degradation of rhodamine B (RhB), methyl orange (MO), and methylene blue (MB). The possible degradation pathways have also been discussed according to the liquid chromatography-mass spectrometry (LCMS) results. 2. Experimental Section 2.1. Preparation of Photocatalysts. All of the reagents are analytical grade and used without further purification. The M-H synthesis was performed in a single mode CEM discover system (Explorer48, CEM Co.) operating at 200 W and 2.45 GHz. K2H2Sb2O7 was first dissolved in hot distilled water to form 0.05 M solution. In a typical procedure, 8 mL of CaCl2 solution (0.1 M) was slowly dropped into 16 mL of K2H2Sb2O7 solution under continuous stirring; potassium hydroxide or hydrochloric acid solution was added to adjust the pH values of the mixture. The resultant mixture was then loaded into a 35 mL vessel (20-25 mL reaction volume), which was treated under a controllable temperature of 120-180 °C for 20 min using a microwave system. The reaction was rapidly cooled using high pressure air (40 psi) following termination of the reaction. The resulting product was collected, washed with distilled water several times, and finally dried in air at 80 °C overnight. 2.2. Characterization of Photocatalysts. The X-ray diffraction (XRD) patterns were collected on a Bruker D8 advance X-ray diffractometer at 40 kV and 40 mA with Ni-filtered Cu KR radiation. The transmission electron microscopy (TEM) and

10.1021/jp903355a CCC: $40.75  2009 American Chemical Society Published on Web 07/08/2009

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high-resolution transmission electron microscopy images were measured by a JEOL model JEM 2010 EX instrument at an accelerating voltage of 200 kV. A carbon-coated copper grid was used as the sample holder. UV-vis diffuse reflectance spectroscopy (DRS) was recorded on a UV-vis spectrophotometer (Cary-500, Varian Co.) equipped with an integrating sphere attachment. Nitrogen sorption experiment was carried out at 77 K by using Micromeritics ASAP2020 equipment. X-ray photoelectron spectroscopy (XPS) analysis was conducted on an ESCALAB 250 photoelectron spectroscopy (Thermo Fisher Scientific) at 3.0 × 10-10 mbar using an Al KR X-ray beam (1486.6 eV). A Bruker model A300 spectrometer (Bruker Instruments, Inc.) equipped with a xenon lamp (254 nm filter) was used for measurements of the electron paramagnetic resonance (EPR) signals of radicals spin-trapped by 5,5dimethyl-l-pyrroline N-oxide (DMPO). The settings were the following: center field 3512.48 G, microwave frequency 9.86 GHz, and power 6.35 mW. 2.3. Tests of Photocatalytic Activity. The photocatalytic degradations of dyes (RhB, MO, and MB) in liquid phase were conducted in a quartz tube with 4.6 cm inner diameter and 17 cm length. Four 4 W UV lamps with a wavelength centered at 254 nm (Philips, TUV 4W/G4 T5) were used as the illuminating source. A photocatalyst (0.06 g) was suspended in 150 mL of pollutant solution and stirred for 1 h to ensure the establishment of adsorption-desorption equilibrium. An aliquot (3 mL) was taken at a certain time interval during the experiment and centrifuged (TDL-5-A) to remove the powders. The filtrates were analyzed on a Varian UV-vis spectrophotometer (Cary50, Varian Co.). The percentage of degradation is reported as C/C0. C is the absorption of pollutants at each irradiated time interval of the main peak of the absorption spectrum. C0 is the absorption of the initial concentration when adsorption-desorption equilibrium was achieved. In the total organic carbon (TOC) investigation, the photocatalytic reaction was conducted under the same condition except that the reaction time was prolonged to 180 min. 2.4. LC-MS Analysis. The determination of the dyes’ concentration and the identification of their respective byproducts was performed using a LC-MS system. An Agilent 1100 series LC system (Agilent Technologies, Palo Alto, CA) was equipped with a binary pump, 1100 UV-vis diode array detector, an autosampler, and a column thermostat. The LC-MS 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, Palo Alto, CA). The mass spectrometer was equipped with an electrospray ionization (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 RhB analysis, the mobile phase was a mixture of methanol and water in the ratio 80:20 (v/v), the 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. For the MO analysis, the mobile phase was a mixture of acetonitrile and 0.01 M ammonium acetate (pH ) 6.8) in the ratio 30:70 (v/v), the 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. 3. Results and Discussion Figure 1a displayed the XRD patterns of the resultant products synthesized under pH ) 6 at various temperatures. All the diffraction peaks could be assigned to the pure phase of

Sun et al.

Figure 1. XRD patterns of the products prepared under different conditions: (a) various temperatures with pH ) 6; (b) different pH values holding at 180 °C.

CaSb2O5(OH)2 (JCPDS no. 32-0154). The distinctive peaks at 30.15°, 14.96°, 28.91°, and 50.29° matched well with the (222), (111), (311), and (440) crystal planes of CaSb2O5(OH)2, respectively. As the temperature increased, the diffraction peaks of the samples became much sharper, indicating that the products synthesized under higher temperature were better crystallized. The wide peaks indicated the small particle size of the sample, and the average particle size of the sample synthesized at 180 °C was about 10 nm calculated via the Scherrer equation. There was no trace of an impurity phase under the instrument’s resolution. The different pH values also had some influence on the preparation of the samples. Figure 1b showed the corresponding XRD patterns of the samples synthesized at 180 °C under different pH values. Under strong acidic conditions, the sample synthesized was well-crystallized with relatively sharper peaks, while under strong basic conditions, the diffraction peaks were relatively wider and weaker. The morphologies of the as-synthesized CaSb2O5(OH)2 were demonstrated in the TEM images (Figure 2a). The assynthesized CaSb2O5(OH)2 NCs had diameters in the range 5-10 nm. The representative high-resolution TEM image showing clear lattice fringes was shown in Figure 2b. The interlayer spacing of 0.30 nm corresponded to the (222) plane of CaSb2O5(OH)2. Diffuse reflectance spectroscopy is a useful tool for characterizing the optical properties of materials. Figure 3 showed the UV-vis diffuse reflectance spectra of CaSb2O5(OH)2 NCs and the absorption located at about 270 nm, corresponding to a band gap of about 4.6 eV. The nitrogen adsorption-desorption isotherm of the as-prepared CaSb2O5(OH)2 product had also been investigated. The Brunquer-Emmett-Teller (BET) specific surface area of CaSb2O5(OH)2 was about 101.8 m2 g-1, larger than that of P25 (50 m2 g-1).21 For the liquid-phase photocatalytic reaction, the sample synthesized at 180 °C, pH ) 6, and 20 min of holding time, was selected to characterize the photocatalytic activity. The results revealed that dyes in aqueous solution could be effectively oxidized over CaSb2O5(OH)2 under UV light irradiation. Temporal concentration changes of the pollutants were monitored by examining the variations in maximal absorption in UV-vis spectra. Figure 4 showed the temporal evolution of

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Figure 2. TEM images of nanocrystalline CaSb2O5(OH)2: (a) low-resolution TEM image; (b) high-resolution TEM image.

Figure 3. UV-vis diffuse reflectance spectrum of CaSb2O5(OH)2 NCs together with P25 as a comparison.

Figure 5. UV-vis spectral changes of MO in aqueous CaSb2O5(OH)2 dispersions as a function of irradiation time. Photocatalytic properties of the CaSb2O5(OH)2 sample compared with P25, MO photolysis, and the dark reaction (inset).

TABLE 1: TOC Detection Results of RhB and MO Photocatalytic Reactions TOC (mg/L)

Figure 4. (a) Photograph of the photodegradation of RhB over CaSb2O5(OH)2 NCs against irradiation time. (b) UV-vis spectral changes of RhB in aqueous CaSb2O5(OH)2 dispersions as a function of irradiation time. Photocatalytic properties of the CaSb2O5(OH)2 sample compared with P25, RhB photolysis, and the dark reaction (inset).

the spectral changes of RhB solution (15 ppm) mediated by CaSb2O5(OH)2. With the increase in degradation time, the absorption peak of RhB at 554 nm decreased gradually, and after only 40 min of irradiation, it almost disappeared. The concentration changes of RhB as a function of irradiation time in aqueous CaSb2O5(OH)2 suspension were shown in Figure 5a (inset), and P25 was used as a reference. It could be clearly seen that the activity of CaSb2O5(OH)2 was higher than that of P25. After 40 min of UV light irradiation, the decomposition ratio of RhB over CaSb2O5(OH)2 was about 99%, while that of P25 was 90%. It has also revealed that RhB could hardly be decomposed over CaSb2O5(OH)2 in the dark, while in the blank

dye

original

40 min

90 min

180 min

RhB MO

10.53 5.12

9.82 4.06

4.91 2.34

2.90 1.78

experiment without CaSb2O5(OH)2, only 16% of RhB was decomposed after 40 min of UV irradiation. Further more, CaSb2O5(OH)2 was also found more photoactive than P25 in both MO and MB photodegradations. The temporal evolution of the spectral changes of MO solution (10 ppm) mediated by CaSb2O5(OH)2 were shown in Figure 5. In the presence of CaSb2O5(OH)2, MO has been degraded gradually with the increase in irradiation time, with an efficient degradation rate higher than that of P25 (Figure 5 (inset)). In addition, MB aqueous solution (15 ppm) could also be decomposed efficiently by CaSb2O5(OH)2 (Figure S1 of the Supporting Information). To investigate the mineralization of the dyes in water, TOC measurement has also been performed. The results have been displayed in Table 1. With the increase in irradiation time, the TOC values of the solutions decreased gradually. After 180 min of UV light irradiation, the TOC value of the RhB solution was 2.90 mg/L, corresponding to a mineralization ratio of 72.5%, while that for MO was about 65.2%. So, RhB and MO solutions could be photocatalytic decolorized and mostly mineralized by the CaSb2O5(OH)2 photocatalyst. The stability of a photocatalyst is very important for its practical application. To investigate the stability of CaSb2O5(OH)2, further characterizations have also been done. Figure S2 of the Supporting Information shows the X-ray photoelectron survey spectra of the

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Figure 6. High-resolution XPS spectra of CaSb2O5(OH)2: (a) before photodegradation; (b) after MO photodegradation.

fresh and used samples. The detailed spectra for Ca2p and Sb3d are shown in Figure 6. The binding energies of Ca2p and Sb3d before and after degradation were nearly the same, respectively. It illustrated that there was no change in the oxidation state of Sb and Ca. The XRD patterns of fresh and used samples are shown in Figure S3 of the Supporting Information, and no obvious changes have been observed, indicating the stability of CaSb2O5(OH)2 photocatalyst. For the purpose of studying the photocatalytic process of RhB and MO, LC-MS was applied to determine the residual dyes and degradation byproducts. Figure 7 reported the chromatograms corresponding to solutions of RhB and MO degraded at 0, 8, 16, and 40 min, respectively. Without UV irradiation, there was only one peak in the chromatogram that appeared at 7.2

Sun et al. min, and it belonged to the RhB molecule. However, after 8 min of UV irradiation, several peaks belonging to degradation byproducts had also been observed in the chromatogram. With the prolonging of irradiation time, all the peaks of the residual RhB and degradation byproducts had gradually disappeared, indicating the removal of dye from the water. In the LC-MS analysis of MO degradation, a similar situation had been observed. After 8 min of UV irradiation, the MO peak reduced rapidly, and two new peaks appeared at 4.0 and 4.4 min in the chromatogram, respectively. After 40 min of degradation, all the peaks ascribed to the MO dye and the degradation byproducts in the chromatograms almost disappeared, indicating the removal of MO from the water. Figure 7 also provided the corresponding mass spectra of the degradation byproducts of RhB and MO, respectively. The significant mass peaks present were labeled with the corresponding m/z values. In the degradation of RhB, five strong mass peaks (m/z ) 415, 419, 282, 436, and 475) had been observed, while four strong mass peaks (m/z ) 290, 306, 174, and 192) appeared in the degradation of MO. The m/z values mentioned above might have some deviation from the molecular weight because of the loss of hydrogen. For both the dyes and byproducts, the corresponding mass peak intensities have varied gradually with the irradiation time. Figure 8a showed that the intensity of the peak (m/z ) 443) for RhB was initially 9.9 × 107 counts. After 40 min of irradiation, it gradually reduced to 8.9 × 104 counts. For the degradation

Figure 7. LC-MS chromatograms of RhB and MO solutions at different irradiation intervals: (a1) chromatogram of the original RhB solution after adsorption-desorption equilibrium in the dark; (a2)-(a4) chromatograms of the RhB solution after 8, 16, and 40 min of irradiation, respectively; (b1) chromatogram of the original MO solution after adsorption-desorption equilibrium in the dark; (b2)-(b4) chromatograms of MO solution after 8, 16, and 40 min of irradiation, respectively. Corresponding mass spectra of the byproducts have also been provided.

CaSb2O5(OH)2 Synthesis and Photocatalytic Activity

Figure 8. Mass spectrum view changes of the peak intensity appeared in photocatalytic process of (a) RhB and (b) MO solutions.

byproducts (m/z ) 415, 436, and 282), the peak intensities were first increased and then decreased gradually, indicating all those byproducts were unstable and finally degraded in a large degree. However, for the fragments (m/z ) 419 and 475), the intensities were increased in the initial 8 min of irradiation, but in the following 40 min, it had maintained a steady level, indicating these fragments were very stable and difficult to degrade. It was worth noticing that there was a byproduct having an m/z value of 475 higher than that of the parent molecule. We had inferred its structure, and there was a numerical agreement between the molecular weight and the hypothesis of the formation of carboxyl groups. In the degradation process, two of the ethyl groups (-C2H5) bonded to the nitrogen atom in the RhB molecule were oxidized to carboxyl groups (-COOH). Though prolonging the irradiation time, it might be decomposed to a large degree. In the case of MO degradation (Figure 8b), the intensity of the peak (m/z ) 304) for MO was initially 1.4 × 106 counts, and after 40 min of irradiation, it gradually reduced to 2.4 × 102 counts. For the unstable degradation byproducts (m/z ) 290 and 306), the peak intensities were also first increased and then decreased gradually. However, for the stable fragments (m/z ) 192 and 174), the intensities have no obvious change in the prolonged UV irradiation, though the intensities were not high. According to the LC-MS analysis above and the structure of RhB and MO, we have inferred the possible structures of degradation byproducts and pathways of photodegradation (Scheme 1). We have to point out that because the RhB molecule was very sophisticated, the inferred degradation fragments of RhB may have many isomeric compounds, which have also been provided in the Figure S4 of the Supporting Information. In order to detect the active species generated in the photocatalytic process, the ESR spin-trap with DMPO technique has been performed,22,23 and the result was shown in Figure 9. Under UV light irradiation, four characteristic peaks of

J. Phys. Chem. C, Vol. 113, No. 31, 2009 13829 DMPO- · OH have been obviously observed in the suspension of CaSb2O5(OH)2, while there is no signal for the suspension kept in the dark. On the other hand, when the CaSb2O5(OH)2 photocatalyst was suspended in methanol, the signals for DMPO-O2 · - can be observed only under UV light irradiation. This result provides a strong indication that the photogenerated charge carriers in CaSb2O5(OH)2 have not only strong redox ability but also were long-lived enough to react with the surfaceadsorbed O2 or H2O to produce · OH radicals. The generated · OH played an important role and was recognized as the main reactive species responsible for the degradation of pollutants. On the basis of the discussion above, a possible mechanism for the photodegradation of organic pollutants over CaSb2O5(OH)2 has been proposed, as illustrated by Scheme 2 and eqs 1-7. Under UV light irradiation, some electrons (e-) in the valence band (VB) can be excited to the conduction band (CB) causing the generation of holes (h+) in the VB simultaneously (eq 1). A portion of the photogenerated electrons would recombine with holes in the VB, while others transferred to the surface and reacted with the ubiquitously oxygen molecule to yield · O2- (eq 2). The resultant holes (h+) in the VB would react with OH- to produce · OH (eq 3). The generated · O2- would further combine with H+ to produce · HO2 (eq 4), which could react with the trapped electron to generate H2O2 (eq 5). The · OH radical can also be generated when a H2O2 molecule capture a trapped electron (eq 6). The reactive species, such as · OH, · HO2, and hVB+, all could oxidize organic pollutants in some degree (eq 7).

CaSb2O5(OH)2 f CaSb2O5(OH)2(eCB- + hVB+)

(1)

CaSb2O5(OH)2(eCB-) + O2 f CaSb2O5(OH)2 + · O2(2) + hVB + OH- f · OH

(3)

· O2- + H+ f HO2 ·

(4)

CaSb2O5(OH)2(eCB-) + · HO2 + H+ f H2O2

(5)

H2O2 + CaSb2O5(OH)2(eCB-) f · OH + OH-

(6)

· OH, · HO2, · O2-, or hVB+ + dyes f · · · f byproducts (7) To determinate the influences of · OH and hVB+ in the photocatalytic process, tert-butanol or ammonium oxalate (AO) was added in the degradation of dyes. tert-Butanol and oxalate were commonly used to quench hydroxyl radicals and holes, respectively.24,25 The results were demonstrated by Figure 10. Figure 10a presented the influences of tert-butanol and AO on the RhB degradation process. It was observed that a small amount of tert-butanol (2.0 mL) inhibited the photocatalytic degradation of RhB, especially in the initial 16 min. It could be deduced that hydroxyl radicals played a major role in the photocatalytic process. However, in the presence of AO (0.1 g), the degradation rate was only decreased slightly, indicating that the direct oxidation of pollutants by hVB+ only played a

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SCHEME 1: Proposed Degradation Pathways for Photocatalytic Degradation over the CaSb2O5(OH)2 Photocatalysta

a

(a) RhB; (b) MO.

SCHEME 2: Proposed Mechanism of the Photocatalytic Degradation of Organic Pollutants over the CaSb2O5(OH)2 Photocatalyst

Figure 9. DMPO spin-trapping ESR spectra for CaSb2O5(OH)2 in aqueous dispersion for DMPO- · OH and in methanol dispersion for DMPO-O2- · .

minor role in the degradation. Because the addition of tertbutanol and AO could not inhibit the degradation completely, some other reactive species were necessarily involved. Those species were most probably · HO2, · O2-, and so forth. A similar phenomenon was observed in the case of MO degradation (Figure 10b).

CaSb2O5(OH)2 Synthesis and Photocatalytic Activity

J. Phys. Chem. C, Vol. 113, No. 31, 2009 13831 todegradation and after MO photodegradation, the XRD patterns of CaSb2O5(OH)2 of a fresh sample and after photo-oxidation of MO, and the possible isomeric compounds of the degradation fragments with different m/z values. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes

Figure 10. Effects of hydroxyl radical and hole scavengers on the degradation of dyes: (a1)-(a3) RhB degradation in the absence of scavenger, in the presence of tert-butanol and AO, respectively; (b1)-(b3) MO degradation in the absence of scavenger, in the presence of tert-butanol and AO, respectively.

In conclusion, nanocrystalline CaSb2O5(OH)2 has been synthesized successfully via a facile microwave-hydrothermal method. The as-prepared sample has smaller particle size and larger surface area. The large band gap endows the strong oxidizing ability of CaSb2O5(OH)2 NCs. In the photocatalytic degradation of several dyes, it exhibited remarkable stabilities and activities that were even better than that of P25. On the basis of the LC-MS analysis, a mechanism for the photocatalytic degradation of organic pollutants over CaSb2O5(OH)2 has also been proposed. For the high photocatalytic activity and good stability of CaSb2O5(OH)2, it is expected to be applied in purifying other pollutants in aqueous solution. Acknowledgment. This work was financially supported by the NNSF of China (20537010, 20873023, and 20677010), 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, 2005HZ1007). Supporting Information Available: Time-dependent absorption spectral patterns of MB in the presence of CaSb2O5(OH)2 under UV irradiation, the photocatalytic activity of CaSb2O5(OH)2 toward MB together with P25 as comparison, the comparison of X-ray photoelectron survey spectra of CaSb2O5(OH)2 NCs before pho-

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