Structural Property and Catalytic Activity of New In2YbSbO7 and

Apr 29, 2010 - The photocatalytic degradation of rhodamine B with In2YbSbO7, Gd2YbSbO7 and Bi2InTaO7 followed the first-order reaction kinetics, and t...
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J. Phys. Chem. C 2010, 114, 9398–9407

Structural Property and Catalytic Activity of New In2YbSbO7 and Gd2YbSbO7 Nanocatalysts under Visible Light Irradiation Jingfei Luan,†,* Shu Wang,† Kun Ma,† Yongmei Li,‡ and Bingcai Pan† State Key Laboratory of Pollution Control and Resource Reuse, School of the EnVironment, Nanjing UniVersity, Nanjing 210093, People’s Republic of China, and State Key Laboratory of Pollution Control and Resource Reuse, School of the EnVironment, Tongji UniVersity, Shanghai 200092, People’s Republic of China ReceiVed: February 6, 2010; ReVised Manuscript ReceiVed: April 7, 2010

In2YbSbO7 and Gd2YbSbO7 were prepared first, while the structural and photocatalytic properties of them were investigated. In2YbSbO7 and Gd2YbSbO7 were crystallized with pyrochlore-type structure, cubic crystal system by space group Fd3m. The lattice parameters a for In2YbSbO7 and Gd2YbSbO7 were 10.340277 and 10.639527 Å. The band gap energies of In2YbSbO7 and Gd2YbSbO7 were 2.361 and 2.469 eV. In2YbSbO7 and Gd2YbSbO7 could photodegrade rhodamine B under visible light irradiation and they owned higher catalytic activity compared with Bi2InTaO7. Furthermore, In2YbSbO7 showed higher catalytic activity compared with Gd2YbSbO7. The photocatalytic degradation of rhodamine B with In2YbSbO7, Gd2YbSbO7 and Bi2InTaO7 followed the first-order reaction kinetics, and the first-order rate constant was 0.02466, 0.01430, and 0.00329 min-1. After visible light irradiation for 235 and 255 min with In2YbSbO7 and Gd2YbSbO7 as catalysts, complete removal and mineralization of rhodamine B was observed. The photocatalytic degradation pathway of rhodamine B was obtained in this experiment. Introduction In 1972, the first paper written by Honda and Fujishima reported electrochemical photolysis of water at a semiconductor electrode.1 Up to now, a lot of institutes and universities have studied photochemical properties of organic compounds, such as photodissociation and photocatalytic activities of organic compounds.2-13 Nowadays, photocatalysis is a rapidly expanding technology for wastewater treatment. The photocatalytic degradation process of organic compounds has several advantages such as complete mineralization, nonexistent waste disposal problem, low cost, mild temperature conditions, and mild pressure conditions.13 According to the published researches,14-17 many semiconductor compounds such as TiO2 can be used as photocatalysts under ultraviolet light irradiation. In recent years, the researches on the photocatalytic activities of some photocatalysts under visible light irradiation were reported, and some photocatalysts with different structures had been prepared to investigate the effective utilization of solar energy.18-27 Many scientific investigations on the photocatalytic degradation of aqueous organic contaminants had been reported,28-42 especially dyes coming from the textile and photographic industries were investigated widely. Among all of the dyestuffs, rhodamine B (RhB) which was one of the most common xanthene dyes was famous for its good stability as dye materials. RhB had become a common organic pollutant, a widely used photosensitizer and a quantum counter. Thus the photodegradation of RhB was important with regard to the purification of dye effluents.43-49 However, RhB was hard to be biodegraded and photolyzed directly. Meanwhile, as a N-containing dye, RhB would produce potentially carcinogenic aromatic amine45,50 when RhB underwent natural * To whom correspondence should be addressed. Phone: +86 (0) 13585206718. Fax: +86 (0) 25 83707304. E-mail: [email protected]. † Nanjing University. ‡ Tongji University.

reductive anaerobic degradation. Thus it was necessary to get knowledge of the photodegradation process of RhB under ultraviolet light and visible light irradiation.43-49 In line with the work from Fu et al.,45 RhB could be efficiently photodegraded under visible light irradiation with nanosized Bi2WO6 as catalyst. Moreover, by utilizing Pb3Nb4O13/fumed SiO2 composite to decompose RhB, Li et al.44 also achieved photocatalytic efficiency under visible light irradiation. The researchers usually use titanium dioxide as a photocatalyst, but titanium dioxide can not be used in the visible light region. In fact, other oxides such as A2B2O7 compounds are often considered to have photocatalytic properties under visible light irradiation. In our previous work,37 we had found that Bi2InTaO7 crystallized with the pyrochlore-type structure and acted as a photocatalyst under visible light irradiation and seemed to have potential for improvement of photocatalytic activity upon modification of its structure. Based on above analysis, we can assume that the replacement of Ta5+ by Sb5+, the replacement of In3+ by Yb3+, and the replacement of Bi3+ by In3+ or Gd3+ in Bi2InTaO7 may increase carriers concentration, as a result, the new photocatalysts In2YbSbO7 and Gd2YbSbO7 may own advanced photocatalytic properties. As semiconductor photocatalysts, In2YbSbO7 and Gd2YbSbO7 were never synthesized before and never used in the photocatalysis process. The molecular composition of In2YbSbO7 and Gd2YbSbO7 is very similar with that of other A2B2O7 compounds. Thus the resemblance suggests that In2YbSbO7 and Gd2YbSbO7 may possess photocatalytic properties under visible light irradiation. This paper reported the preparation process and property characterization of In2YbSbO7 and Gd2YbSbO7. The structural, photophysical and photocatalytic properties of In2YbSbO7 and Gd2YbSbO7 were also investigated in detail in this paper. A comparison among the photocatalytic properties of In2YbSbO7, Gd2YbSbO7, and Bi2InTaO7 was achieved in order to elucidate the structure-photocatalytic activity relationship in these newly synthesized compounds.

10.1021/jp1011846  2010 American Chemical Society Published on Web 04/29/2010

In2YbSbO7 and Gd2YbSbO7 Nanocatalysts Experimental Methods The novel photocatalysts were synthesized by the solid state reaction method. In2O3, Gd2O3, Yb2O3, Sb2O5, Bi2O3, and Ta2O5 with purity of 99.99% (Sinopharm Group Chemical Reagent Co., Ltd., Shanghai, China) were used as starting materials. All powders were dried at 200 °C for 4 h before synthesis. In order to synthesize In2YbSbO7, the precursors were stoichiometrically mixed, subsequently pressed into small columns, and put into an alumina crucible (Shenyang Crucible Co., Ltd., China). Finally, calcination was carried out at 1320 °C for 65 h in an electric furnace (KSL 1700X, Hefei Kejing Materials Technology CO., Ltd., China). Similarly, Gd2YbSbO7 was prepared by calcination at 1320 °C for 65 h, and Bi2InTaO7 was prepared by calcination at 1050 °C for 46 h. The crystal structures of In2YbSbO7 and Gd2YbSbO7 were analyzed by the powder X-ray diffraction method (D/MAX-RB, Rigaku Corporation, Japan) with Cu KR radiation (λ ) 1.54056 Å). The data were collected at 295 K with a step-scan procedure within the range of 2θ ) 10-100°. The step interval was 0.02° and the time per step was 1.2 s. The chemical composition of In2YbSbO7 and Gd2YbSbO7 was determined by scanning electron microscope-Xray energy dispersion spectrum (SEM-EDS, LEO 1530VP, LEO Corporation, Germany) and X-ray fluorescence spectrometer (XFS, ARL-9800, ARL Corporation, Switzerland). The In3+ content, Gd3+ content, Yb3+ content, Sb5+ content and O2content of In2YbSbO7 and Gd2YbSbO7 were determined by X-ray photoelectron spectroscopy (XPS, ESCALABMK-2, VG Scientific Ltd., U.K.). The chemical composition within the depth profile of In2YbSbO7 and Gd2YbSbO7 was examined by the argon ion denudation method when X-ray photoelectron spectroscopy was used. The optical absorption of In2YbSbO7 and Gd2YbSbO7 was analyzed with an UV-visible spectrophotometer (Lambda 40, Perkin-Elmer Corporation, USA). The surface areas of In2YbSbO7, Gd2YbSbO7, and Bi2InTaO7 were measured by the Brunauer-Emmett-Teller (BET) method (MS21, Quantachrome Instruments Corporation, USA) with N2 adsorption at liquid nitrogen temperature. The particle sizes of the photocatalysts were measured by Malvern’s mastersize-2000 particle size analyzer (Malvern Instruments Ltd., United Kingdom). The particle morphologies of In2YbSbO7 and Gd2YbSbO7 were measured by transmission electron microscope (Tecnal F20 S-Twin, FEI Corporation, USA). The photocatalytic degradation of rhodamine B (RhB) (Tianjin Kermel Chemical Reagent Co., Ltd.) was performed with 0.8 g In2YbSbO7, Gd2YbSbO7 or Bi2InTaO7 powder suspended in 300 mL of 0.0293 mM RhB solution in a pyrex glass cell (Jiangsu Yancheng Huaou Industry, China). Before visible light irradiation, the suspensions were magnetically stirred in the dark for 45 min to ensure establishment of an adsorption/desorption equilibrium among In2YbSbO7/Gd2YbSbO7/Bi2InTaO7, the RhB dye and atmospheric oxygen. The photocatalytic reaction system consisted of a 300 W Xe arc lamp with the main emission wavelength at 436 nm (Nanjing JYZCPST CO., Ltd.), a magnetic stirrer and a cutoff filter (λ > 400 nm, Jiangsu Nantong JSOL Corporation, China). The Xe arc lamp was surrounded by a quartz jacket and was positioned in a photoreactor quartz vessel (5.8 cm in diameter and 68 cm in length) by suspension circulation of RhB and the photocatalyst. An outer recycling water glass jacket maintained a constant reaction temperature (22 °C), and the solution was continuously stirred and aerated. Two mL aliquots were sampled at various time intervals. The incident photon flux I0 measured by a radiometer (Model FZA, Photoelectric Instrument Factory Beijing Normal University, China) was determined to be 4.76 × 10-6 Einstein L-1 s-1 under

J. Phys. Chem. C, Vol. 114, No. 20, 2010 9399 visible light irradiation (wavelength range of 400-700 nm). The incident photon flux on the photoreactor was varied by adjusting the distance between the photoreactor and the Xe arc lamp. The adjustment of pH value was not carried out and the initial pH value was 7.0. The concentration of RhB was determined based on the absorption at 553.5 nm which was measured by an UV-vis spectrophotometer (Lambda 40, Perkin-Elmer Corporation, USA). The inorganic products obtained from RhB degradation were analyzed by ion chromatograph (DX-300, Dionex Corporation, USA). The identification of RhB and the degradation intermediate products of RhB were attempted to be performed by gas chromatograph-mass spectrometer (GC-MS, HP 6890 Series Gas Chromatograph, AT column, 20.3 m × 0.32 mm, ID of 0.25 µm) operating at 320 °C which was connected to HP 5973 mass selective detector and a flame ionization detector with H2 as the carried gas. Intermediate products of RhB were mainly measured by liquid chromatograph-mass spectrometer (LC-MS, Thermo Quest LCQ Duo, USA, Beta Basic-C18 HPLC column: 150 × 2.1 mm, ID of 5 µm, Finnigan, Thermo, USA). Here, 20 µL of postphotocatalysis solution was injected automatically into the LC-MS system. The fluent contained 60% methanol and 40% water, and the flow rate was 0.2 mL min-1. MS conditions included an electrospray ionization interface, a capillary temperature of 27 °C with a voltage of 19.00 V, a spray voltage of 5000 V and a constant sheath gas flow rate. The spectrum was acquired in the negative ion scan mode, sweeping the m/z range from 50 to 600. Evolution of CO2 was analyzed with an intersmat IGC120-MB gas chromatograph equipped with a porapack Q column (3 m in length and an inner diameter of 0.25 in.), which was connected to a catharometer detector. The total organic carbon (TOC) concentration was determined with a TOC analyzer (TOC-5000, Shimadzu Corporation, Japan). The photonic efficiency was calculated according to the following equation:51,52

φ ) R/I0

(1)

where φ was the photonic efficiency (%), and R was the rate of RhB degradation (mol L-1 s-1), and I0 was the incident photon flux (Einstein L-1 s-1). Results and Discussion Characterization of the Photocatalysts. Figure 1 shows the TEM images of In2YbSbO7 and Gd2YbSbO7. The results revealed that In2YbSbO7 and Gd2YbSbO7 were nanosized particles and irregular shapes. It could be seen from Figure 1 that the average particle size of In2YbSbO7 was smaller than that of Gd2YbSbO7. The diameter of In2YbSbO7 was about 35 nm, while the diameter of Gd2YbSbO7 was about 45 nm. By large numbers of detections, SEM-EDS spectrum which was taken from the prepared In2YbSbO7 displayed the presence of indium, ytterbium, antimony and oxygen. Similarly, SEM-EDS spectrum that was taken from the prepared Gd2YbSbO7 also indicated the presence of gadolinium, ytterbium, antimony and oxygen. We could not detect any other elements in In2YbSbO7 or Gd2YbSbO7. Figure 2 represents the powder X-ray diffraction patterns of In2YbSbO7 and Gd2YbSbO7. Figure 3 shows the powder X-ray diffraction patterns of Gd2YbSbO7 with full-profile structure refinements of the collected data. The collected data were obtained by the RIETANTM53 program, which was based on Rietveld analysis. Simultaneously, the final refinement results

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Figure 2. X-ray powder diffraction patterns of (A) In2YbSbO7 and (B) Gd2YbSbO7.

Figure 1. TEM images of (A) In2YbSbO7 and (B) Gd2YbSbO7.

of Gd2YbSbO7 showed a good agreement between the observed intensities and calculated intensities for the pyrochlore-type structure and a cubic crystal system which had a space group Fd3m (O atoms were included in the model). The lattice parameters for In2YbSbO7 and Gd2YbSbO7 were 10.340277 and 10.639527 Å, respectively. At the same time, all diffraction peaks for In2YbSbO7 and Gd2YbSbO7 could be indexed successfully according to the structural parameters and above refinement results. The atomic coordinates and structural parameters of In2YbSbO7 and Gd2YbSbO7 are recorded in Tables 1 and 2. In the light of Figures 2 and 3, it could be concluded that In2YbSbO7 and Gd2YbSbO7 were both single phase. Additionally, the XRD results showed that In2YbSbO7 and Gd2YbSbO7 crystallized in the pyrochlore-type structure and a cubic crystal system with a space group Fd3m. Meanwhile, 2 theta angles of each reflection for In2YbSbO7 changed with In3+ being substituted by Gd3+. The lattice parameter a increased from a ) 10.340277 Å for In2YbSbO7 to a ) 10.639527 Å for Gd2YbSbO7, which indicated a decrease for lattice parameter of the photocatalyst with decrease of the replaced ionic radii, In3+ (0.92 Å) < Gd3+ (1.053 Å).

Figure 3. X-ray powder diffraction patterns and Rietveld refinements of Gd2YbSbO7 prepared by a solid state reaction method at 1320 °C (olive solid line represented experimental XRD data; red dotted line represented simulative XRD data; blue dashed line represented a difference between experimental XRD data and simulative XRD data; purple vertical line represented observed reflection positions).

TABLE 1: Structural Parameters of In2YbSbO7 Prepared by Solid State Reaction Method atom

X

Y

Z

occupation factor

In Yb Sb O(1) O(2)

0.00000 0.50000 0.50000 -0.14541 0.12500

0.00000 0.50000 0.50000 0.12500 0.12500

0.00000 0.50000 0.50000 0.12500 0.12500

1.0 0.5 0.5 1.0 1.0

From the X-ray diffraction results, it could be concluded that In2YbSbO7, Gd2YbSbO7, and Bi2InTaO7 crystallized with the same pyrochlore-type structure. The cubic system structure with space group Fd3m for Bi2InTaO7 kept unchanged using Ta5+ being substituted by Sb5+, In3+ being substituted by Yb3+, and Bi3+ being substituted by In3+ or Gd3+. The results of refinements for In2YbSbO7 generated the unweighted R factors and the factor of RP was 13.3%. Likewise, the outcome of refinements for Gd2YbSbO7 generated the unweighted R factors and the factor of RP was 11.49%. According to the work from Zou et al.,54 Bi2InNbO7 which was slightly modified in structure had a large R factor. On the basis of high purity of the precursors

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TABLE 2: Structural Parameters of Gd2YbSbO7 Prepared by Solid State Reaction Method atom

X

Y

Z

occupation factor

Gd Yb Sb O(1) O(2)

0.00000 0.50000 0.50000 -0.15469 0.12500

0.00000 0.50000 0.50000 0.12500 0.12500

0.00000 0.50000 0.50000 0.12500 0.12500

1.0 0.5 0.5 1.0 1.0

TABLE 3: Binding Energies (BE) for Key Elements compounds

In3d5/2 BE (eV)

Sb3d5/2 BE (eV)

Yb4p3/2 BE (eV)

Gd4d5/2 BE (eV)

O1s BE (eV)

Gd2YbSbO7 In2YbSbO7

530.98 530.81

346.65 346.51

143.76

444.69

530.85 531.10

that were used in this research and the EDS results that did not trace any other elements, it was impossible that the observed space groups were generated from impurities. Thus, it could be concluded that the slightly high R factors of In2YbSbO7 and Gd2YbSbO7 were resulted from slightly modified structure model. On the other hand, the changes of structure including different bond-distance distributions, thermal displacement parameters and/or occupation factors for some of the atoms were due to the defects or the disorder/order of a part of the atoms. The XPS spectra of In2YbSbO7 and Gd2YbSbO7 were also detected. These different elemental peaks which are corresponding to definite bind energies are given in Table 3. The results illustrated that the oxidation states of In, Yb, Sb, and O ions from In2YbSbO7 were +3, +3, +5, and -2, respectively. Besides, the average atomic ratio of In:Yb:Sb:O for In2YbSbO7 was 2.00:0.97:1.02:6.99 based on our XPS, SEM-EDS, and XFS results. Similarly, for Gd2YbSbO7, the oxidation states of Gd, Yb, Sb, and O ions were +3, +3, +5, and -2, respectively. In addition, the average atomic ratio of Gd:Yb:Sb:O for Gd2YbSbO7 was 2.00:0.97:1.01:6.99. Accordingly, it could be deduced that the resulting materials were highly pure under our preparation conditions. It was remarkable that there were not any shoulders and widening in the XPS peaks of In2YbSbO7 and Gd2YbSbO7, which suggested (albeit not proving) the absence of any other phases. Figures 4 and 5 represent the absorption spectra of In2YbSbO7 and Gd2YbSbO7, respectively. Compared with the well-known photocatalyst TiO2 whose absorption edge was no more than 380 nm, the absorption edges of In2YbSbO7 and Gd2YbSbO7 were found to be at 525 and 502 nm, respectively, which belonged to the visible region of the spectrum. Clearly, the obvious absorption (defined hereby as 1-transmission) did not result from reflection and scattering. Consequently, the apparent absorbance at sub-bandgap wavelengths (520-800 nm for In2YbSbO7 and 530-800 nm for Gd2YbSbO7) was higher than zero. The optical absorption near the band edge of the crystalline semiconductors obeyed the equation:55,56

ahν ) A(hν - Eg)n

(2)

where A, a, Eg, and ν represented proportional constant, absorption coefficient, band gap and light frequency, respectively. In this equation, n determined the character of the transition in a semiconductor. Eg and n could be calculated by the following steps: (i) plotting ln(ahν) versus ln(hν - Eg) and assuming an approximate value of Eg, (ii) deducing the value of n based on the slope in this graph, (iii) refining the value of

Figure 4. Upper trace: action spectra of rhodamine B degradation with In2YbSbO7 (black solid stars) as catalyst under visible light irradiation. Lower trace: absorption spectra of In2YbSbO7 (red solid line) and rhodamine B (blue dotted line).

Eg by plotting (ahν)1/n versus hν and extrapolating the plot to (ahν)1/n ) 0. According to above method, Figure 6 shows the plot of (ahν)1/n versus hν. As a result, it could be concluded that the value of Eg for In2YbSbO7 and Gd2YbSbO7 was calculated to be 2.361 and 2.469 eV, and the estimated value of n was 0.66 and 0.55 for In2YbSbO7 and Gd2YbSbO7, respectively. The results showed that Gd2YbSbO7 possessed wider band gap compared with In2YbSbO7, at the same time, the optical transition for In2YbSbO7 and Gd2YbSbO7 was directly allowed. Photocatalytic Activity. Generally, the direct absorption of band gap photons would result in the generation of electron-hole pairs within the semiconductor particles, in succession, the charge carriers began to diffuse to the surface of these particles. As a result, the photocatalytic activity for decomposing organic compounds with these semiconductor catalysts was enhanced. Changes in the UV-vis spectrum of rhodamine B upon exposure to visible light (λ > 400 nm) irradiation with the presence of In2YbSbO7 or Gd2YbSbO7 are presented in Figure 7, panels a and b, respectively. These experiments were performed under oxygen-saturation conditions ([O2]sat ) 1.02 × 10-3 M). Figure 7, panels a and b, indicated that both In2YbSbO7 and Gd2YbSbO7 could photodegrade RhB effectively under visible light irradiation. As presented in Figure 7, panels a and b, a reduction in typical RhB peaks at 553.5 and 524.5 nm was clearly noticed. The complete disappearance of the absorption peaks which presented the absolute color change from deep pink into colorless solution occurred with In2YbSbO7 as catalyst in 235 min and with Gd2YbSbO7 as catalyst in 255 min.

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Figure 5. Upper trace: action spectra of rhodamine B degradation with Gd2YbSbO7 (black solid stars) as catalyst under visible light irradiation. Lower trace: absorption spectrum of Gd2YbSbO7 (blue solid line).

Figure 6. Plot of (ahν)2 versus hν for (A) In2YbSbO7 and (B) Gd2YbSbO7.

Figure 8 is the chart for kinetics of RhB degradation with In2YbSbO7, Gd2YbSbO7 and Bi2InTaO7 as well as in the absence of photocatalysts under visible light irradiation. The results showed that the photodegradation rate of RhB was about 2.427 × 10-9 mol L-1 s-1 and the photonic efficiency was estimated to be 0.05099% (λ ) 420 nm) with In2YbSbO7 as catalyst. Similarly, the photodegradation rate of RhB was about 2.251 × 10-9 mol L-1 s-1 and the photonic efficiency was estimated to be 0.04729% (λ ) 420 nm) with Gd2YbSbO7 as catalyst. As a contradistinction, the photodegradation rate of RhB within

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Figure 7. Temporal spectral changes of aqueous solutions of rhodamine B due to visible light irradiation in the presence of (A) In2YbSbO7 or (B) Gd2YbSbO7.

Figure 8. Photocatalytic degradation of rhodamine B under visible light irradiation in the presence of In2YbSbO7 (violet empty circles), Gd2YbSbO7 (red solid stars), and Bi2InTaO7 (blue solid triangles) as well as in the absence of a photocatalyst (olive empty squares).

200 min of visible light irradiation was only 1.1 × 10-9 mol L-1 s-1 and the photonic efficiency was estimated to be 0.02311% (λ ) 420 nm) with Bi2InTaO7 as catalyst. The results showed that the photodegradation rate of RhB and the photonic efficiency with Bi2InTaO7 as catalyst were both lower than those with In2YbSbO7 and Gd2YbSbO7 as catalysts. The reduction of RhB concentration in our experiments was tiny with the absence of a photocatalyst and our results were similar to the results

In2YbSbO7 and Gd2YbSbO7 Nanocatalysts from Rao et al.57 Furthermore, when In2YbSbO7, Gd2YbSbO7, and Bi2InTaO7 were used as photocatalysts, the photodegradation conversion rate of RhB was 99.4%, 92.2%, and 45.1% after visible light irradiation for 200 min, respectively. After visible light irradiation for 235 and 255 min with In2YbSbO7 and Gd2YbSbO7 as catalysts, complete removal of rhodamine B was observed. Besides, in the situation of degradation without photocatalysts, RhB concentration decreased slightly under visible light irradiation because of the RhB dye photosensitization effect. Based on above results, the photocatalytic degradation activity of In2YbSbO7 and Gd2YbSbO7 was very high. Meanwhile, In2YbSbO7 and Gd2YbSbO7 were more suitable for RhB photodegradation than Bi2InTaO7. As a result, In2YbSbO7 and Gd2YbSbO7 showed higher photocatalytic degradation activity compared with Bi2InTaO7, and the photocatalytic degradation activity of In2YbSbO7 was higher than that of Gd2YbSbO7. The main reason was that the lattice parameter a ) 10.746410 Å for Bi2InTaO7 was larger than the lattice parameter a ) 10.639527 Å for Gd2YbSbO7, and the lattice parameter a ) 10.639527 Å for Gd2YbSbO7 was larger than the lattice parameter a ) 10.340277 Å for In2YbSbO7, which probably resulted in a decrease for the migration distance of photogenerated electrons and holes to reach the reaction site on the photocatalyst surface, subsequently, the creation of more active sites was realized, as a result, it would probably improve the photocatalytic activities with decreasing the lattice parameter of the photocatalyst. The photocatalytic performance of novel In2YbSbO7 and Gd2YbSbO7 under visible light irradiation was amazing compared with that of Bi2InTaO7, and the main reason was that the specific surface areas of these compounds were much smaller than that of titanium dioxide. BET isotherm measurements of In2YbSbO7, Gd2YbSbO7, and Bi2InTaO7 provided a specific surface area of 1.98, 1.32, and 1.26 m2 g-1, respectively, which was almost 35 times smaller than that of TiO2 which was measured to be 46.24 m2 g-1. The first-order nature of the photocatalytic degradation kinetics with In2YbSbO7, Gd2YbSbO7 and Bi2InTaO7 as catalysts is clearly demonstrated in Figure 9, panels a and b. The results showed a linear correlation between ln(C/C0) (or ln(TOC/ TOC0)) and the irradiation time for the photocatalytic degradation of RhB under visible light irradiation with the presence of In2YbSbO7, Gd2YbSbO7, and Bi2InTaO7. Here, C represented the RhB concentration at time t, and C0 represented the initial RhB concentration, and TOC represented the total organic carbon concentration at time t, and TOC0 represented the initial total organic carbon concentration. According to Figure 9a, the first-order rate constant kC of RhB concentration was estimated to be 0.02466 min-1 with In2YbSbO7 as catalyst, 0.01430 min-1 with Gd2YbSbO7 as catalyst, and 0.00329 min-1 with Bi2InTaO7 as catalyst. The different value of kC indicated that In2YbSbO7 and Gd2YbSbO7 were more suitable for the photocatalytic degradation of RhB under visible light irradiation than Bi2InTaO7. Meanwhile, for the photodegradation of RhB under visible light irradiation, In2YbSbO7 was more suitable than Gd2YbSbO7. Figure 9b showed that the first-order rate constant KTOC of TOC was estimated to be 0.02165 min-1 with In2YbSbO7 as catalyst, 0.01204 min-1 with Gd2YbSbO7 as catalyst, and 0.00317 min-1 with Bi2InTaO7 as catalyst, which indicated that the photodegradation intermediates of RhB probably appeared during the photocatalytic degradation of RhB under visible light irradiation because of the difference between kC and KTOC. The photodegradation intermediates of RhB in our experiment were identified as 1,2-benzenedicarboxylic acid, benzoic acid,

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Figure 9. Observed first-order kinetic plots for the photocatalytic degradation of rhodamine B with In2YbSbO7 (black empty circles), Gd2YbSbO7 (blue solid stars), and Bi2InTaO7 (red solid triangles) as catalysts under visible light irradiation (upper trace a: linear correlation between ln(C/C0) and the irradiation time; lower trace b: linear correlation between ln(TOC/TOC0) and the irradiation time).

pentanedioic acid, 2-hydroxypentanedioic acid, adipic acid, 2,5dihydroxybenzoic acid and terephthalic acid. On the basis of the intermediate products found in this work, a possible photocatalytic degradation pathway of RhB is proposed in Figure 10. This pathway was similar to the pathway from Horikoshi et al.58 for the photodegradation of RhB under ultraviolet light and visible light illumination assisted by microwave radiation and TiO2. In the light of the work from Zhang et al.,54 the photodegradation of RhB occurred via two competitive processes: one process was N-demethylation, and the other process was the destruction of the conjugated structure. Therefore, we believed that chromophore cleavage, opening-ring and mineralization would be the main photocatalytic degradation process of RhB in our experiment. The molecule of RhB was converted to small organic species and was mineralized into inorganic products (CO2 and water) ultimately. Figure 11 shows the amount of CO2 yielded during the photodegradation of RhB with In2YbSbO7, Gd2YbSbO7, or Bi2InTaO7 as catalyst under visible light irradiation. The amount of CO2 increased gradually with increasing reaction time when RhB was photodegraded by In2YbSbO7, Gd2YbSbO7, or Bi2InTaO7. At the same time, after visible light irradiation of 200 min, the CO2 production of 0.2431 mmol with In2YbSbO7 as catalyst was higher than the CO2 production of 0.21781 mmol with Gd2YbSbO7 as catalyst, and the CO2 production of 0.21781 mmol with Gd2YbSbO7 as catalyst was higher than the CO2 production of 0.10803 mmol with Bi2InTaO7 as catalyst.

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Figure 12. Disappearance of TOC during the visible light photocatalytic degradation of rhodamine B with In2YbSbO7 (violet empty circles), Gd2YbSbO7 (red solid stars), and Bi2InTaO7 (blue solid triangles) as catalysts.

Figure 10. Suggested photocatalytic degradation pathway scheme for rhodamine B under visible light irradiation in the presence of In2YbSbO7 and Gd2YbSbO7.

Figure 11. CO2 production kinetics during the photocatalytic degradation of rhodamine B with In2YbSbO7 (violet empty circles), Gd2YbSbO7 (red solid stars), and Bi2InTaO7 (blue solid triangles) as catalysts under visible light irradiation.

Figure 12 shows the change of TOC during the visible light photocatalytic degradation of rhodamine B with In2YbSbO7, Gd2YbSbO7, or Bi2InTaO7 as catalyst. The TOC measurements revealed the disappearance of organic carbon when the RhB

Figure 13. Suggested band structures of In2YbSbO7 and Gd2YbSbO7.

solution containing In2YbSbO7, Gd2YbSbO7, or Bi2InTaO7 was exposed under visible light irradiation. The results showed that 98.82%, 88.53%, or 43.98% of TOC decrease was obtained after visible light irradiation for 200 min when In2YbSbO7, Gd2YbSbO7, or Bi2InTaO7 was used as photocatalyst. Subsequently, after visible light irradiation for 235 or 255 min with In2YbSbO7 or Gd2YbSbO7 as catalyst, the entire mineralization of RhB was observed because of 100% TOC removal. The turnover numbers which represented the ratio between the total amount of evolved gas and dissipative catalyst were calculated to be more than 0.19 and 0.26 for In2YbSbO7 and Gd2YbSbO7 after 200 min of reaction time under visible light irradiation. These turnover numbers were evidently to prove that these reactions occurred catalytically. Similarly, when the light was turned off in this experiment, the stop of these reactions showed the obvious light response. Figure 13 shows the suggested band structures of In2YbSbO7 and Gd2YbSbO7. Newly, the electronic structures of InMO4 (M ) V, Nb, and Ta) and BiVO4 were described by Oshikiri et al.59 based on first-principles calculations. The conduction bands of InMO4 (M ) V, Nb, and Ta) were mainly composed of a dominant V 3d, Nb 4d, and Ta 5d orbitals. Similarly, the valence bands of BiVO4 were composed of a minor Bi6s orbital component and a dominant O 2p orbital component. The band structures of In2YbSbO7 and Gd2YbSbO7 should be similar to

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that of InMO4 (M ) V, Nb, and Ta) and BiVO4. Therefore, we deduced that the conduction band of In2YbSbO7 was composed of In 5p, Yb 4f, and Sb 5p orbital component, and the valence band of In2YbSbO7 was composed of O 2p orbital component. Similarly, the conduction band of Gd2YbSbO7 was composed of Gd 5d, Yb 4f, and Sb 5p orbital component, and the valence band of Gd2YbSbO7 was composed of O 2p orbital component. In2YbSbO7 and Gd2YbSbO7 could produce electron-hole pairs by absorption of photons directly, and it indicated that enough energy which was larger than the band gap energy was necessary for the photocatalytic degradation process of RhB. Mechanisms. As depicted in Figure 8, even in the absence of a photocatalyst, RhB concentration decreased under visible light irradiation, as a result, the rate of RhB degradation was estimated to be 0.07083 × 10-9 mol L-1 s-1 and the photonic efficiency was 0.00149% (λ ) 420 nm) after visible light irradiation of 200 min. It was illustrated that the decrease of RhB concentration without photocatalyst was due to direct dyesensitization effect, which was similar to the observation from Zhao et al.60 Figures 4 and 5 show the action spectra of RhB degradation with In2YbSbO7 and Gd2YbSbO7 as catalysts under visible light irradiation. A clear photonic efficiency (0.07948% at its maximal point) at wavelengths which corresponded to sub-Eg energies of the photocatalysts (λ from 400 to 620 nm) was observed for In2YbSbO7 and Gd2YbSbO7. The existence of photonic efficiency at this region revealed that photons were not absorbed by the photocatalysts. In particular, the correlation between the low-energy action spectrum and the absorption spectrum of RhB clearly demonstrated that all photodegradation results at wavelengths above 485 nm should be attributed to photosensitization by the dye RhB itself (Scheme 1).

Scheme 1: visible light

RhB(ads) 98 RhB*(ads)

(3)

RhB*(ads) + In2YbSbO7 (or Gd2YbSbO7) f + In2YbSbO7 (or Gd2YbSbO7)(e) + RhB(ads)

(4)

In2YbSbO7 (or Gd2YbSbO7)(e) + O2 f In2YbSbO7 (or Gd2YbSbO7) + •O2

(5)

According to this mechanism, RhB which was adsorbed on In2YbSbO7 (or Gd2YbSbO7) was excited by visible light irradiation. Subsequently, an electron was injected from the excited RhB to the conduction band of In2YbSbO7 (or Gd2YbSbO7) where the electron was scavenged by molecular oxygen. Scheme 1 explained the results obtained with In2YbSbO7 (or Gd2YbSbO7) as catalyst under visible light irradiation, where the photocatalyst In2YbSbO7 (or Gd2YbSbO7) could serve to reduce recombination of photogenerated electrons and holes by scavenging of electrons.61 Below 485 nm, the situation was different. The results of photonic efficiency correlated well with the absorption spectrum of In2YbSbO7 or Gd2YbSbO7. These results evidently showed that the mechanism which was responsible for the photodegradation of RhB went through band gap excitation of In2YbSbO7 or Gd2YbSbO7. Despite the detailed experiments about the effect of oxygen and water were not performed, it was logical to presume that the mechanism in the first step was similar to the

observed mechanism for In2YbSbO7 (or Gd2YbSbO7) under supra-bandgap irradiation, namely Scheme 2:

Scheme 2: visible light

In2YbSbO7 (or Gd2YbSbO7) 98 h+ + e-

(6)

e- + O2 f •O2

(7)

h+ + OH- f •OH

(8)

Former luminescent studies had shown that the closer the M-O-M bond angle was to 180° the more delocalized was the excited state,62 as a result, the charge carriers could move more easily in the matrix. The mobility of the photoinduced electrons and holes influenced the photocatalytic activity because high diffusivity indicated the enhancement of probability that the photogenerated electrons and holes would reach the reactive sites of the catalyst surface. In view of above results, the lattice parameter a ) 10.340277 Å for In2YbSbO7 was smaller than the lattice parameter a ) 10.639527 Å for Gd2YbSbO7, therefore, the photoinduced electrons and holes within In2YbSbO7 were easier and faster to reach the reactive sites of In2YbSbO7 surface compared with those within Gd2YbSbO7. As a result, the photocatalytic degradation activity of In2YbSbO7 was higher than that of Gd2YbSbO7. For Gd2YbSbO7, the Yb-O-Sb bond angle was 109.343°. Meanwhile, for In2YbSbO7, the Yb-O-Sb bond angle was 123.338°. Above results indicated that the Yb-O-Sb (or Sb-O-Sb) bond angle of Gd2YbSbO7 or In2YbSbO7 was close to 180°. Thus, the photocatalytic activity of In2YbSbO7 or Gd2YbSbO7 was consequently higher. In addition, the Yb-O-Sb bond angle of In2YbSbO7 was larger than that of Gd2YbSbO7, which resulted in an increase of photocatalytic activity for In2YbSbO7 compared with Gd2YbSbO7. The crystal structures of In2YbSbO7 and Gd2YbSbO7 were as the same as Bi2InTaO7, and only their electronic structures were a little different. For In2YbSbO7 or Gd2YbSbO7, In was 5p-block metal element, and Gd was 5dblock rare earth metal element, and Yb was 4f-block metal element, and Sb was 5p-block metal element. But for Bi2InTaO7, Ta was 5d-block metal element, and Bi was 6p-block metal element, indicating that the photocatalytic activity might be affected by not only the crystal structure but also the electronic structure of the photocatalysts. In the light of above analysis, the different effect of RhB photodegradation among In2YbSbO7, Gd2YbSbO7, and Bi2InTaO7 could be attributed mainly to the difference of their crystalline structure and electronic structure. The present results indicated that the In2YbSbO7 (or Gd2YbSbO7)-visible light photocatalysis system might be regarded as a practical method for treatment of diluted colored wastewater. These systems could be used for decolorization, purification and detoxification for textile, printing and dyeing industries in the long-day countries. Meanwhile, this system did not need high pressure of oxygen, heating or any chemical reagents. Much decolorized and detoxified water were flowed from our new system for treatment, and the results showed that the In2YbSbO7 (or Gd2YbSbO7)-visible light photocatalysis system might provide a valuable treatment technique for purifying and reusing colored aqueous effluents. Conclusions In2YbSbO7 and Gd2YbSbO7 were novel photocatalysts which were prepared by solid state reaction method for the first time.

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In our experiments, In2YbSbO7 and Gd2YbSbO7 were compared with Bi2InTaO7. By our XRD experiment, it could be concluded that In2YbSbO7 and Gd2YbSbO7 crystallized with the pyrochlore-type structure and cubic crystal system with space group Fd3m. Besides, the lattice parameters of In2YbSbO7 and Gd2YbSbO7 were found to be 10.340277 and 10.639527 Å. Meanwhile, the band gaps of In2YbSbO7 and Gd2YbSbO7 were estimated to be 2.361 and 2.469 eV, thus In2YbSbO7 and Gd2YbSbO7 showed strong optical absorption in the visible light region (λ > 400 nm). Obvious photodegradation of aqueous RhB were observed under visible light irradiation with In2YbSbO7 or Gd2YbSbO7 as catalyst. Compared with Bi2InTaO7, In2YbSbO7 and Gd2YbSbO7 demonstrated better photocatalytic activity for the photocatalytic degradation of RhB under visible light irradiation. Furthermore, In2YbSbO7 showed higher catalytic activity compared with Gd2YbSbO7 for photocatalytic degradation of RhB. The photodegradation of RhB with In2YbSbO7 or Gd2YbSbO7 as catalyst followed the first-order reaction kinetics. The apparent first-order rate constant kC of In2YbSbO7, Gd2YbSbO7, or Bi2InTaO7 was 0.02466, 0.01430, or 0.00329 min-1. The photodegradation intermediate products of RhB were identified and the end products were carbon dioxide and water. Complete organic carbon removal was examined by TOC measurements when In2YbSbO7 and Gd2YbSbO7 were used as photocatalysts. Finally, the possible photocatalytic degradation pathway of RhB was revealed in this paper. It could be concluded that In2YbSbO7 (or Gd2YbSbO7)-visible light photocatalysis system could be regarded as an effective way for treating of textile industry wastewater. Acknowledgment. This work was supported by the National Natural Science Foundation of China (No. 20877040). This work was supported by a grant from the Technological Supporting Foundation of Jiangsu Province (No. BE2009144). This work was supported by a grant from China-Israel Joint Research Program in Water Technology and Renewable Energy (No. 5). References and Notes (1) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37–38. (2) Jin, R. C.; Cao, Y. W.; Mirkin, C. A.; Kelly, K. L.; Schatz, G. C.; Zheng, J. G. Photoinduced Conversion of Silver Nanospheres to Nanoprisms. Science 2001, 294, 1901–1903. (3) Seger, B.; Kamat, P. V. Fuel Cell Geared in Reverse: Photocatalytic Hydrogen Production Using a TiO2/Nafion/Pt Membrane Assembly with No Applied Bias. J. Phys. Chem. C 2009, 113, 18946–18952. (4) Lahiri, D.; Subramanian, V.; Bunker, B. A.; Kamat, P. V. Probing Photochemical Transformations at TiO2/Pt and TiO2/Ir Interfaces Using X-Ray Absorption Spectroscopy. J. Chem. Phys. 2006, 124, 204720. (5) Kikuchi, H.; Kitano, M.; Takeuchi, M.; Matsuoka, M.; Anpo, M.; Kamat, P. V. Extending the Photoresponse of TiO2 to the Visible Light Region: Photoelectrochemical Behavior of TiO2 Thin Films Prepared by the Radio Frequency Magnetron Sputtering Deposition Method. J. Phys. Chem. B 2006, 110, 5537–5541. (6) Hamadanian, M.; Reisi-Vanani, A.; Majedi, A. Synthesis, Characterization and Effect of Calcination Temperature on Phase Transformation and Photocatalytic Activity of Cu, S-Codoped TiO2 Nanoparticles. Appl. Surf. Sci. 2010, 256, 1837. (7) Lahiri, D.; Subramanian, V.; Shibata, T.; Wolf, E. E.; Bunker, B. A.; Kamat, P. V. Photoinduced Transformations at Semiconductor/Metal Interfaces: X-Ray Absorption Studies of Titania/Gold Films. J. Appl. Phys. 2003, 93, 2575–2582. (8) Kamat, P. V. Photoinduced Transformations in SemiconductorMetal Nanocomposite Assemblies. Pure Appl. Chem. 2002, 74, 1693–1706. (9) Hamadanian, M.; Reisi-Vanani, A.; Majedi, A. Preparation and Characterization of S-Doped TiO2 Nanoparticles, Effect of Calcination Temperature and Evaluation of Photo-Catalytic Activity. Mater. Chem. Phys. 2009, 116, 376–382. (10) Ghosh, S.; Sahu, K.; Mondal, S. K.; Sen, P.; Bhattacharyya, K. A Femtosecond Study of Photoinduced Electron Transfer from Dimethylaniline to Coumarin Dyes in a Cetyltrimethylammonium Bromide Micelle. J. Chem. Phys. 2006, 125, 054509.

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