English

• 0.   Introduction

  Coloring matter wastewater in the textile and photographic industries had become a serious environ-mental problem due to its toxicity, unacceptable color, high chemical oxygen demand, and biodegradability^[1]. The presence of dyes in water was not only aesthetically unpleasant, but it also affected the transparency of water, decreased sun penetration, reduced gas solubility, and photosynthetic reactions^[2]. For the sake of solving the problem, many scientists hope to use photocatalytic techniques to degrade harmful coloring matter wastewater from polluted water before proper treatment, and these scientists had made different efforts for this career for more than 40 years^[3-5]. At present, photocatalytic degradation process had been widely used in the demolishment of organic pollutants in wastewater, especially the degradation of coloring matter^[6].

  The selection of the wavelength of the incident light was crucial for the photocatalytic degradation system, while light was a presence of energy. Previous studies had shown that the semiconductor compounds could break down most persistent organic pollutants such as coloring matter, pesticides, detergents and volatile organic compounds under ultraviolet light irradiation^[7-10] and had higher energy than visible light.

  According to the data, ultraviolet light accounted for only 4% of sunlight, while visible light accounted for about 43%. Thus it seemed more practical and significant to use visible light instead of UV light during the degradation process. Therefore, there was an urgent demand to develop new photocatalysts that responded to visible light and had higher photocatalytic efficiency. In general, most of the photocatalytic catalysts utilized in previous studies were mainly sorted into two types: one was called TiO₂-based catalyst whose maximum absorption wavelength had been extended to visible light by ion doping^[11-20] and cocatalyst recombination^[21-31], and the other was a complex oxide, such as La₂O₃, BiVO₄, Bi₁₂TiO₂₀, K₆Nb_10.8O₃₀^[32-38]. Recently, spinel-type oxides having the formula AB₂O₄ had been found to own excellent properties for degrading coloring matter in wastewater under visible light irradiation. For instance, MIn₂O₄ (M=Ca, Sr, Ba)^{[11, 39, 40]}, NiCo₂O₄^[11] and ZnFe₂O₄/MWCNTs^[41] were prepared under visible light irradiation. In addition, ZnFe₂O₄ owned outstanding property for degradation of methyl orange^[42].

  In this paper, a new type of semiconductor catalyst Sm₂FeSbO₇ which belonged to the A₂B₂O₇ family was prepared. Indigo carmine (IC) was utilized as a model contaminant to evaluate the degradation activity of Sm₂FeSbO₇ under visible light irradiation because of its wide use and hard biodegradation. In addition, the construction and photocatalytic characterization of Sm₂FeSbO₇ were also investigated in detail. For comparison, we chose the traditional photocatalyst N-doped TiO₂ (N-TiO₂) to degrade IC under visible light irradiation.

  1.   Experimental

  1.1   Synthesis of nanocatalyst

  The new photocatalyst, Sm₂FeSbO₇ was prepared by the solid-state reaction method. Sm₂O₃, Fe₂O₃ and Sb₂O₅ with a purity of 99.99% (Sinopharm Group Chemical Reagent Co., Ltd., Shanghai, China) were used as original materials. For the sake of synthesizing Sm₂FeSbO₇, the precursors were stoichiometrically mixed in a quartz mortar, then transfered into small columns and put into an alumina crucible (Shenyang Crucible Co., Ltd, Shenyang, China). And next, calcination was carried out at 800 ℃ for 35 h in an electric furnace (KSL 1700X, Hefei Kejing Materials Technology Co., Ltd, Hefei, China). The last step was sintering and grinding with a quartz mortar, and then, Sm₂FeSbO₇ powder was made. All powders were dried at 200 ℃ for 4 h before they were prepared. Nitrogen-doped titania (N-TiO₂) catalyst with tetrabutyl titanate as a titanium precursor was prepared by using the sol-gel method at room temperature. The next step was that 17 mL tetrabutyl titanate and 40 mL absolute ethyl alcohol were mixed as solution A; subsequently, solution A was added dropwise under vigorous stirring into the solution B which contained 40 mL absolute ethyl alcohol, 10 mL glacial acetic acid and 5 mL double distilled water to form transparent colloidal suspension C. Subsequently, aqua ammonia with N/Ti proportion of 8% (n/n) was added into the resulting transparent colloidal suspension under vigorous stirring condition and stirred for 1 h. Finally, the xerogel was formed after being aged for 2 days. The xerogel was ground into powder, which was calcinated at 500 ℃ for 2 h; and next, above powder was ground in an agate mortar and screened by a shaker to obtain N-doped TiO₂ powders.

  1.2   Characterization of Sm₂FeSbO₇

  The particle formation of Sm₂FeSbO₇ were measured by transmission electron microscope (TEM, Tecnal F20 S-Twin, FEI Corporation, Hillsboro, Oregon, USA) with 200 kV operating voltage. The chemical ingredient of the compound was determined by a scanning electron microscope with 20 kV operating voltage, which was equipped with X-ray energy dispersion spectrum (SEM-EDS, LEO 1530VP, LEO Corporation, Pegnitz, Germany) and X-ray photo-electron spectroscopy (XPS, ESCALABMK-2, VG Scientific Ltd., East Grinstead, UK). The Sm³⁺, Fe³⁺, Sb⁵⁺ and O^2- content of Sm₂FeSbO₇ and the valence state of the elements were also analyzed by X-ray photoelectron spectroscopy. The chemical ingredient within the depth profile of Sm₂FeSbO₇ was examined by the argon ion denudation method when X-ray photoelectron spectroscopy was utilized. The crystalline phase of Sm₂FeSbO₇ was analyzed by X-ray diffractometer (D/MAX-RB, Rigaku Corporation, Tokyo, Japan) with Cu Kα radiation (λ=0.154 056 nm). The patterns were collected at 295 K with a step-scan procedure in the range of 2θ=10°~100°. The step interval was 0.02°, and the time per step was 1 s. The accelerating voltage and applied current were 40 kV and 40 mA, respectively. Fourier transform infrared spectroscopy (FTIR, Nexus, Nicolet Corporation, Madison, Wisconsin, USA) was applied to examine the FTIR spectra of Sm₂FeSbO₇. Its spectral range is between 7 400~350 cm^-1 and the resolution is better than 0.09 cm^-1. The UV-visible diffuse reflectance spectrum of Sm₂FeSbO₇ was gauged with a Shimadzu UV-2550 UV-Visible spectrometer (Shimadzu, Santa Clara, California, USA), and BaSO₄ was utilized as the reference material.

  1.3   Photocatalytic activity experiments

  The photocatalytic activity of Sm₂FeSbO₇ was assessed with indigo carmine (IC) (C₁₆H₈N₂Na₂O₈S₂) (Tianjin Bodi Chemical Co., Ltd., Tianjin, China) as the model substance. The photoreaction was imple-mented in a photochemical reaction apparatus (Nanjing Xujiang Machine Plant, Nanjing, China). The inner structure of the reaction apparatus was as following: the lamp was put into a quartz hydrazine, which was a hollow structure, and lied in the middle of the reactor. The recycling water through the reactor kept at a near constant reaction temperature (20 ℃), and the solution was continuously stirred and aerated. Twelve holes were utilized to put quartz tubes evenly arranged around the lamp, and the distance between the lamp and each hole was equal. The photocatalyst within the IC solution was in a state of suspension under the condition of magnetic stirring. In this paper, the photocatalytic degradation of IC was carried out with 0.3 g Sm₂FeSbO₇ in a 300 mL, 29.3 μmol·L^-1 IC aqueous solution in quartz tubes with 500 W xenon lamp (400 nm < λ < 800 nm) as the visible light source. Prior to visible light irradiation, the suspensions which contained the catalyst and IC coloring matter, were magnetically stirred in darkness for 45 min to ensure the establishment of an adsorption/desorption equili-brium among Sm₂FeSbO₇, the IC coloring matter and atmospheric oxygen. During visible light illumination, the suspension was stirred at 500 r·s^-1, and the initial pH value of the IC solution was 7.0 without pH value adjustment in the reaction process. Above experiments were carried out under oxygen-saturation conditions (C_{O2, sat}=1.02 mmol·L^-1). One of the quartz tubes was taken out from the photochemical reaction apparatus at various time intervals. The suspension was filtered through 0.22 μm membrane filters. The filtrate was subsequently analyzed by a Shimadzu UV-2450 spectrometer (Shimadzu, Santa Clara, California, USA) with a detecting wavelength at 610 nm.

  The photonic efficiency (ξ) was calculated accor-ding to the following formula^[43-44]:

  $ \xi = R/{I_0} $

  (1)

  where ξ was the photonic efficiency (%), R was the degradation rate of IC (mol·L^-1·s^-1), which indicated the concentration decrement of indigo carmine within every second, and I₀ was the incident photon flux (Einstein·L^-1·s^-1). The incident photon flux, I₀, which was measured by a radiometer (Model FZ-A, Photo-electric Instrument Factory, Beijing Normal University, Beijing, China), was determined to be 4.76×10^-6 Einstein·L^-1·s^-1 under visible light irradiation (a wavelength range of 400~700 nm).

  2.   Results and discussion

  2.1   Crystal structure and optical properties

  The transmission electron microscopy (TEM) image of the prepared catalyst, Sm₂FeSbO₇, is shown in Fig. 1. It could be observed clearly from Fig. 1a that the particles of Sm₂FeSbO₇ had a nanostructure and irregular shapes. Additionally, we could also ackno-wledge that the particles of Sm₂FeSbO₇ crystallized well. Fig. 1b showed the selected area electron diffraction pattern of Sm₂FeSbO₇. It could be seen from Fig. 1b that Sm₂FeSbO₇ crystallized with a pyrochlore-type structure, a cubic crystal system and a space group Fd3m. The results showed that the lattice parameters for Sm₂FeSbO₇ were proven to be a=b=c=1.035 434 nm. According to the calculation results from Fig. 1b, the (hkl) value for the main peaks of Sm₂FeSbO₇ had been found and indexed.

  Figure 1

  

  Figure 1.  (a) TEM image of Sm₂FeSbO₇; (b) Selected area electron diffraction pattern of Sm₂FeSbO₇

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  Fig. 2 presents the scanning electron microscopy-energy-dispersive spectrometry (SEM-EDS) spectrum of Sm₂FeSbO₇. Fig. 2 indicated the presence of samarium, iron, antimony and oxygen element. In order to avoid the influence of inhomogeneity phenomenon on the selected surface, ten different specimen areas selection of Sm₂FeSbO₇ were conducted in an EDS test. The mean value of the results of above EDS spectra taken from prepared Sm₂FeSbO₇ indicated that the stoichiometric ratio of samarium, iron, antimony and oxygen was estimated to be 17.61:9.34:7.97:65.09, namely 2.00:1.06:0.90:7.40.

  Figure 2

  

  Figure 2.  (a) SEM image of Sm₂FeSbO₇; (b) EDS spectrum of Sm₂FeSbO₇ calcinated at 800 ℃ for 35 h

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  In order to get a better understanding of the chemical state of all elements on the catalyst surface, the X-ray photoelectron spectroscopy (XPS) full spectrum of Sm₂FeSbO₇ was measured and was displayed in Fig. 3. The experimental results showed that the XPS full spectrum of Sm₂FeSbO₇ contained only the corresponding elements and carbon element, and the carbon element was due to the addition of hydrocarbons which would facilitate the testing and calibration of the elements instead of a catalyst. Table 1 provides the experimental binding energy of the characteristic peaks of all elements which exist in Sm₂FeSbO₇ and the binding energy information after C correction. By comparing the XPS standard binding energy data shown in Table 1 and the chemical shifts of each element, the valence state of each element in Sm₂FeSbO₇ was determined, and the results showed that the valence state of Sm, Fe, Sb or O was +3, +3, +5 or -2.

  Figure 3

  

  Figure 3.  XPS spectra of Sm₂FeSbO₇ and every element which was contained in Sm₂FeSbO₇: (a) Full XPS spectrum of Sm₂FeSbO₇; XPS spectra of (b) Sm3d_5/2, (c) Sb3d, (d) Fe₂p and (e) O1s

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  Table 1

  

  Table 1.  Binding energies for key elements of Sm₂FeSbO₇

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  Element	Sm3d5/2	Fe2p1/2	Sb3d5/2	O1s	C1s
  Binding energy/eV	1 084.6	724.8	532.0	531.8	286.0
  Binding energy after C correction/eV	1 083.1	723.3	530.5	530.3	284.5

  Fig. 4 shows the powder X-ray diffraction pattern of Sm₂FeSbO₇ with the full-profile structure refine-ments of the collected data. The collected data were obtained by the RIETANTM ^[45] program based on the Rietveld analysis. It could be seen from Fig. 4 that Sm₂FeSbO₇ turned out to be a single phase. Additionally, the results of the final refinements for Sm₂FeSbO₇ indicated a good agreement between the observed intensities and the calculated intensities for a pyrochlore-type structure, a cubic crystal system and a space group Fd3m (O atoms were included in the model). The lattice parameters of Sm₂FeSbO₇ were a=b=c=1.035 434 nm. All the diffraction peaks of Sm₂FeSbO₇ could be successfully indexed according to the lattice constant and above refinement results as well as the space group Fd3m. The atomic coordinates and structural parameters of Sm₂FeSbO₇ are listed in Table 2. According to above results, the structural model of Sm₂FeSbO₇ which is simulated by Materials Studio software is demonstrated in Fig. 5.

  Figure 4

  

  Figure 4.  XRD patterns and Rietveld refinements of Sm₂FeSbO₇

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  Table 2

  

  Table 2.  Structural parameters of Sm₂FeSbO₇ calcinated at 800 ℃ for 35 h

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  Atom	x	y	z	Occupation factor
  Sm	0	0	0	1
  Fe	0.5	0.5	0.5	0.5
  Sb	0.5	0.5	0.5	0.5
  O(1)	-0.175	0.125	0.125	1
  O(2)	0.125	0.125	0.125	1

  Figure 5

  

  Figure 5.  Structural model of Sm₂FeSbO₇ simulated by Materials Studio software corresponding to the XRD pattern shown in Fig. 4

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  Fourier transform infrared (FTIR) spectrum analysis of Sm₂FeSbO₇ particles is investigated in this study, as shown in Fig. 6. According to Fig. 6, we could find that the absorption bands of Sm₂FeSbO₇ prepared by a solid-state reaction method at 800 ℃ were at 487 and 625 cm^-1. The strong absorption band near 487 cm^-1 should be attributed to the Sm-O vibration. The absorption band which situated at 625 cm^-1 was overlaid by the symmetric bending and stretching of the Fe-O which should be the Fe-O vibration.

  Figure 6

  

  Figure 6.  FTIR spectrum of Sm₂FeSbO₇ calcinated at 800 ℃ for 35 h

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  The absorption spectrum of Sm₂FeSbO₇ is presented in Fig. 7. For a crystalline semiconductor, the optical absorption near the band edge following Eq.(2)^[46]:

  $ \alpha h\nu = A{\left( {h\nu - {E_{\rm{g}}}} \right)^n} $

  (2)

  $ {E_{\rm{g}}} = 1\;240/\lambda $

  (3)

  Figure 7

  

  Figure 7.  Diffuse reflection spectra of Sm₂FeSbO₇

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  Where A, α, E_g, ν and λ were the proportional constant, absorption coefficient, band gap, light frequency and absorption edge, respectively. In this equation, n determined the character of the transition in a semiconductor. E_g and n could be calculated by the following steps^[47]: (ⅰ) plotting ln(αhν) versus ln(hν-E_g) by assuming an approximate value of E_g, which could be calculated by Eq.(3); (ⅱ) deducing the value of n; and (ⅲ) refining the value of E_g. From Fig. 7, we could find that the absorption edge of Sm₂FeSbO₇ was about 461 nm, meaning that the estimated E_g of Sm₂FeSbO₇ was 2.69 eV. Then, from the plot of ln(αhν) versus ln(hν-E_g), where we could find that the slope of the line part was about 1.27. Therefore, the n of Sm₂FeSbO₇ was 2. After plotting (αhν)^1/2 versus hν and extrapolating the plot to (αhν)^1/2=0, the accurate value of E_g of Sm₂FeSbO₇ was calculated as 2.46 eV. Applying the same calculation process to N-TiO₂, we found that n was 2 for N-TiO₂ and the band gap E_g of N-TiO₂ was 2.76 eV. Above results indicated that the optical transition for Sm₂FeSbO₇ or N-TiO₂ was indirectly allowed, and Sm₂FeSbO₇ possessed a narrow band gap compared with N-TiO₂. The valence-band XPS spectra (Fig. 8) showed that the valence band position of the sample had no obvious changes. According to the empirical equation:

  $ {E_{{\rm{VBM}}}} = {E_{{\rm{CBM}}}} + {E_{\rm{g}}} $

  (4)

  Figure 8

  

  Figure 8.  Valence band of Sm₂FeSbO₇

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  where E_VBM and E_CBM were the valence band maximum position and conduction band minimum position, respectively. Based on above information, the schematics depiction of the band structures of Sm₂FeSbO₇ sample was illustrated in Fig. 9. The position of the valence band was at 0.98 eV and the position of the conduction band was at 3.44 eV.

  Figure 9

  

  Figure 9.  Suggested band structure of Sm₂FeSbO₇

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  2.2   Photocatalytic properties

  The progress of photocatalysis using the semi-conductor compound as catalyst could be described briefly as follows^[48-49]. Firstly, the semiconductor comp-ound absorbed photons, resulting in the generation of electron-hole pairs within the semiconductor compound particles, subsequently, the diffusion of the charge carriers to the surface of the semiconductor compound particle would be followed; at the same time, the active sites of the surface of the semiconductor compound particles had been adsorbing a lot of organic pollutants particles; finally, the decomposition of the organic pollutants would be performed by charge carriers.

  Fig. 10 presents the changes in the UV-Vis spectra of IC under visible light irradiation (λ > 420 nm) with the presence of Sm₂FeSbO₇. Above measure-ments were performed under oxygen-saturation cond-itions (C_{O2, sat}=1.02 mmol·L^-1). It could be clearly noticed from Fig. 10 that the typical IC peaks were at 609.5 nm. An obvious color change from deep blue into a colorless solution could be observed within 200 minutes. For further comparison, Fig. 11 depicts the concentration changes of IC with Sm₂FeSbO₇ or nitrogen-doped TiO₂ (N-TiO₂) as photocatalyst under visible light irradiation, respectively.

  Figure 10

  

  Figure 10.  Spectral changes of aqueous solutions of indigo carmine due to visible light irradiation with the presence of Sm₂FeSbO₇ calcinated at 800 ℃ for 35 h

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  Figure 11

  

  Figure 11.  Photocatalytic degradation of indigo carmine under visible light irradiation with the presence of Sm₂FeSbO₇ or N-TiO₂ as photocatalyst, respectively

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  It could be seen from Fig. 11 that the photonic efficiency (λ=420 nm) was estimated to be 5.13×10^-4 or 2.58×10^-4 with Sm₂FeSbO₇ or N-TiO₂ as catalyst, respectively. When N-TiO₂ was utilized as a catalyst, the photodegradation conversion rate of IC was 55.39% after visible light irradiation for 220 minutes, while the indigo carmine was completely degradated by Sm₂FeSbO₇. The results showed that the photodegradation rate of IC and the photonic efficiency with Sm₂FeSbO₇ as a catalyst were both higher than those with N-TiO₂ as a catalyst. Above results showed that complete removal of indigo carmine was observed after visible light irradiation for 200 minutes with Sm₂FeSbO₇ as a catalyst. Besides, based on the absorbance changes of IC with light irradiation time, the kinetic curves of IC degradation under visible light irradiation were figured out. Above results demonstrated that the photocatalytic kinetics of IC degradation with Sm₂FeSbO₇ or N-TiO₂ as photo-catalyst followed a first order nature. The first-order rate constant for IC degradation was estimated to be 0.024 65 min^-1 or 0.003 77 min^-1 with Sm₂FeSbO₇ or N-TiO₂ as catalyst, respectively. This fact indicated that Sm₂FeSbO₇ was more efficient than N-TiO₂ for the photocatalytic degradation of IC under visible light irradiation.

  Fig. 12 shows the change of total organic carbon (TOC) during the photocatalytic degradation of IC with Sm₂FeSbO₇ or N-TiO₂ as catalyst under visible light irradiation. TOC was measured using a TOC detector (TOC, vario TOC, Elementar, German). It has a detection limit of 2 μg·L^-1 and an accuracy of RSD (C) < 1%. The TOC measurements revealed the disappear-ance of organic carbon when the IC solution which contained Sm₂FeSbO₇ or N-TiO₂ was exposed under visible light irradiation. The results showed that 53.26% of a TOC decrease was obtained after visible light irradiation for 220 minutes when N-TiO₂ was utilized as the photocatalyst, while TOC was completely removed by Sm₂FeSbO₇. The apparent first order rate constant k was estimated to be 0.021 57 or 0.003 64 min^-1 with Sm₂FeSbO₇ or N-TiO₂ as the photocatalyst, respectively.

  Figure 12

  

  Figure 12.  TOC plots during the photocatalytic degradation of indigo carmine under visible light irradiation with Sm₂FeSbO₇ or N-TiO₂ as photocatalyst, respectively

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  We used a carbon dioxide detector (B1040, WOST, Shenzhen, China) to quantitatively measure the concentration of carbon dioxide. Fig. 13 presents the CO₂ yield during the photocatalytic degradation of IC with Sm₂FeSbO₇ or N-TiO₂ as the photocatalyst under visible light irradiation. During the progress of IC degradation, IC was converted into smaller organic species and was ultimately mineralized to inorganic products, such as carbon dioxide and water. The amount of CO₂ increased gradually with increasing reaction time when IC was photodegraded with Sm₂FeSbO₇ or N-TiO₂ as the photocatalyst. The results showed that the production rate of CO₂ from the Sm₂FeSbO₇-IC system was higher than that from the N-TiO₂-IC system with increasing reaction time. For example, the production amount of CO₂ was 0.074 61 mmol with N-TiO₂ as the photocatalyst after a visible light irradiation of 220 minutes. However, the production amount of CO₂ was 0.140 33 mmol with Sm₂FeSbO₇ as the photocatalyst after a visible light irradiation of 220 minutes.

  Figure 13

  

  Figure 13.  CO₂ production during the photocatalytic degradation of indigo carmine with Sm₂FeSbO₇ or N-TiO₂ as photocatalyst respectively under visible light irradiation

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  The first order nature of the photocatalytic degradation kinetics with Sm₂FeSbO₇ or N-TiO₂ as catalyst is clearly exhibited in Fig. 14, which presents a linear correlation between ln(C/C₀) (or ln(TOC/TOC₀)) and the visible light irradiation time for the photocatalytic degradation of IC with the presence of the photocatalyst. In above equation, C represents the IC concentration at time t, and C₀ represents the initial IC concentration, and TOC represents the total organic carbon concentration at time t, and TOC₀ represents the initial total organic carbon concentration. According to the relationship between ln(C/C₀) and the light irradiation time, the apparent first order rate constant k was 0.024 65 min^-1 with Sm₂FeSbO₇ as catalyst and 0.003 77 min^-1 with N-TiO₂ as catalyst, indicating that Sm₂FeSbO₇ was more efficient than N-TiO₂ for the photocatalytic degradation of IC under visible light irradiation. According to the relationship between ln(TOC/TOC₀) and the light irradiation time, the apparent first order rate constant k_TOC was estimated to be 0.021 57 min^-1 with Sm₂FeSbO₇ as catalyst and 0.003 64 min^-1 with N-TiO₂ as catalyst, indicating that the photodegradation intermediate products of IC probably appeared during the photocatalytic degradation of IC under visible light irradiation.

  Figure 14

  

  Figure 14.  Observed first order kinetic plots for the photocatalytic degradation of IC with Sm₂FeSbO₇ or N-TiO₂ as catalyst under visible light irradiation

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  In order to explore the mechanism of the IC degradation with Sm₂FeSbO₇ or N-TiO₂ as photocatalyst under visible light irradiation, we also test the concentration of NO₃^- and SO₄^2-, which are shown in Fig. 15 and Fig. 16, which may be formed as the end products of nitrogen atoms and sulfur atoms that exist in IC. From Fig. 15 and Fig. 16 we could be sure that both NO₃^- and SO₄^2- appeared during IC degradation with Sm₂FeSbO₇ or N-TiO₂ as the photo-catalyst. NO₃^- and SO₄^2- ions were generated more quickly and effectively with Sm₂FeSbO₇ as photo-catalyst compared with NO₃^- and SO₄^2- ions which were generated with N-TiO₂ as photocatalyst, which was in accord with above analysis about the degradation progress of IC. According to the NO₃^- concentration in Fig. 15, we could calculate that 92.82% or 41.89% of nitrogen from IC was converted into nitrate ions with Sm₂FeSbO₇ or N-TiO₂ as the photocatalyst after visible light irradiation for 220 minutes. Meanwhile, it could be also concluded form Fig. 14 that 72.24% or 39.11% of sulfur from IC was converted into sulfate ions with Sm₂FeSbO₇ or N-TiO₂ as photocatalyst after visible light irradiation for 220 minutes. It was noteworthy that the amount of SO₄^2- or NO₃^- which was released into the solution was sharply lower than the stoichiometry value of 100%. One possible reason could be a loss of sulfur-containing volatile comp-ounds or SO₂ for the S element and nitrogen-containing volatile compounds or NH₃ for the N element. The second possible reason was a partially irreversible adsorption of some SO₄^2- and NO₃^- on the surface of the photocatalyst, which had been observed by Lachheb et al. with titanium dioxide^[50].

  Figure 15

  

  Figure 15.  NO₃^- production during the photocatalytic degradation of IC with Sm₂FeSbO₇ or N-TiO₂ as photocatalyst respectively under visible light irradiation

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  Figure 16

  

  Figure 16.  SO₄^2- production during the photocatalytic degradation of IC with Sm₂FeSbO₇ or N-TiO₂ as photocatalyst respectively under visible light irradiation

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  In order to investigate the effect of the photosensitivity on the degradation process, we used phenol as a contaminant for degradation of IC. Fig. 17 shows the photocatalytic degradation of phenol with Sm₂FeSbO₇ as a photocatalyst under visible light irradiation. The concentration of phenol was determined by high performance liquid chromatography (HPLC, Agilent 1200-DAD, Agilent Technologies Co. Ltd., Palo Alto, USA). It was obvious to discover that the photocatalytic activity was acquired while colorless phenol was selected as a contaminant model with Sm₂FeSbO₇ as photocatalyst. The photocatalytic degradation efficiency of phenol was estimated to be 97.22% by Sm₂FeSbO₇ under visible light irradiation after 200 minutes, demonstrating that Sm₂FeSbO₇ itself had photocatalytic activity and the photosensitive effect was not the main factor in the photodegradation process of IC by using Sm₂FeSbO₇ as a photocatalyst.

  Figure 17

  

  Figure 17.  Photocatalytic degradation of phenol under visible light irradiation with Sm₂FeSbO₇ as photocatalyst

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  In order to probe the stability of the catalyst Sm₂FeSbO₇, we carried out an experiment of repeated degradation of IC. Fig. 18 shows the experimental results of repeated degradation of IC under the same experimental conditions after four times of Sm₂FeSbO₇ recovery. The results showed that the removal rates of IC were 97.99%, 97.69%, 97.02% and 96.62% respectively after four reuses of Sm₂FeSbO₇. Though the degradation effect decreased slightly each time, it had a good performance with high degradability. It could be inferred that the catalytic performance of Sm₂FeSbO₇ is relatively stable and repeatable.

  Figure 18

  

  Figure 18.  Repeated photocatalytic degradation tests of IC by Sm₂FeSbO₇

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  2.3   Photocatalytic degradation pathway of IC with Sm₂FeSbO₇ as photocatalyst

  Fig. 19 manifests the graphical representations of the relative distributions of the intermediate product as a function during the photodegradation of IC. The intermediates which were generated during the degradation process of IC were detected and identified by comparison with commercial standard samples. The intermediates in our experiment were identified as follows: indigotin, isatin sulfonic acid, 2-nitrobenzoic acid, indole-2, 3-dione, o-nitrobenzaldehyde, nitro-acetophenone, anthranilic acid, oxalic acid, formic acid and acetic acid. The sulfur element was first hydrolytically removed and subsequently was oxidized and transformed into SO₄^2-. At the same time, nitrogen atoms in the -3 oxidation state produced NH₄⁺ cations which subsequently were oxidized into NO₃^- ions. Moreover, carbon and oxygen elements were turned into carbon dioxide. Accompanied by the rapid decomposition of IC, the concentration of other intermediate products first increased and then decreased by ulteriorly light irradiation, which uncovered the formation and the conversion of the reactive intermediate products. As the starts of the illumination reaction, the concentration of the three intermediates indigotin, 2-nitrobenzoic acid and formic acid gradually increased and subsequently decreased after reaching the highest concentration point. Indigotin was first detected in the second minute and reached the top concentration in the sixtieth minute. 2-nitrobenzoic acid was detected in the tenth minute and peaked in the eightieth minute. Formic acid was detected finally in the twenty-fifth minute and peaked in the one hundred and twentieth minute. And formic acid was the last intermediate product to disappear. Above results indicated that during the degradation process of IC, indigotin was formed firstly, subsequently 2-nitrobenzoic acid produced secondly, ultimately formic acid was detected thirdly. These intermediate products would be further degraded into small molecules. Above variations unambiguously uncovered that the decomposition of IC was a stepwise course.

  Figure 19

  

  Figure 19.  Temporal change in the distribution of the intermediates during the photodegradation of IC

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  Based on above results, combined with the analysis of the software simulations and experimental data, we can narrow down the degradation path of IC to a certain extent and arrive at the most likely degradation pathway of IC with Sm₂FeSbO₇ as catalyst, as shown in Fig. 20. This pathway was similar to the pathway proposed by Zhang et al.^[51] and Qu et al.^[52]. According to Fig. 20 the main degradation end products of IC were CO₂, H₂O, NO₃^- and SO₄^2-.

  Figure 20

  

  Figure 20.  Suggested photocatalytic degradation pathway scheme for indigo carmine under visible light irradiation with the presence of Sm₂FeSbO₇ prepared by a solid-state reaction method at 800 ℃ for 35 h

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  2.4   Photocatalytic degradation mechanism

  We could easily Figure out that the organic pollutants can be degraded by the photogenerated reactive species during the photocatalytic reaction, including holes (h⁺), electrons (e^-), hydroxyl radicals (·OH) and superoxide radicals (·O₂^-). In order to verify the major reactive species for inducing the degradation of IC with Sm₂FeSbO₇ as catalyst under visible light irradiation, dissociative scavenger experi-ments were demonstrated by adding different scaven-gers into the system. Superoxide dismutase (SOD) with the concentration of 66.7 mg·L^-1, ethylene diamine tetraacetic acid (EDTA) with the concentration of 10 mmol·L^-1, AgNO₃ with the concentration of 10 mmol·L^-1 and tert-butyl alcohol (TBA) with the concentration of 10 mmol·L^-1 were added into the system as scavengers to capture ·O₂^-, h⁺, e^- and ·OH, respec-tively. The photodegradation of indigo carmine over Sm₂FeSbO₇ in the face of various scavengers are expounded in Fig. 21. As shown in Fig. 21, after the addition of AgNO₃ as a scavenger for photogenerated electrons (e^-), the photodegradation efficiency of IC remained almost the same compared with the system of no scavenger, demonstrating the minor role of e^- in this photocatalytic reaction process. When TBA was used as the scavenger to eliminate ·OH, the photode-gradation activity declined somewhat. Moreover, an obvious reduction in the photocatalytic performance was observed in the presence of SOD or EDTA, which eliminated ·O₂^- or h⁺. Above results suggested that the photoreaction process was dominated by h⁺ and ·O₂^- in this system because of their considerable impact. We could derive that h⁺ contributed most to the high activity of Sm₂FeSbO₇ during the degradation of IC in the former most of the time. Additionally, ·O₂^- radicals had a high importance during IC degradation. As for ·OH radicals, they might participate in the photode-gradation process with slight catalytic activity. However, e^- showed almost no activity during the degradation of IC in the presence of Sm₂FeSbO₇.

  Figure 21

  

  Figure 21.  Photodegradation of indigo carmine over Sm₂FeSbO₇ without scavenger and in the presence of various scavengers of EDTA, SOD, AgNO₃ and TBA

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  Based on above results, a possible mechanism scheme of the charge separation and photocatalytic reaction for Sm₂FeSbO₇ is shown in Fig. 22. Firstly, photoinduced holes (h⁺) and photoinduced electrons (e^-) came into being in the surface of Sm₂FeSbO₇ particles (Eq.(5)). Secondly, organic pollutants (R) could be degraded into inorganic products with the effluence of h⁺ and e^-. Many published works^[53-55] had confirmed that two oxidative agents could be mainly concerned under visible light irradiation: ·OH radicals and ·O₂^- radicals. Then h⁺ reacted with R directly (Eq.(6~8)). Besides, the effect of dye sensitization should be taken into consideration (Eq.(9~10), IC^*: IC in the excited state, IC⁺: hole containing IC), because IC could be excited by visible light irradiation, subsequently, the sensitizing dye molecules injected electrons into the semiconductor nanocrystallites, which were collected at a conducting surface to generate the photocurrent (Eq.(11))^[56-57].

  $ {\rm{S}}{{\rm{m}}_2}{\rm{FeSb}}{{\rm{O}}_7} + h\nu \to {{\rm{h}}^ + } + {{\rm{e}}^ - } $

  (5)

  $ \left( {{\text{H}}_{\text{2}}}\text{O}\rightleftharpoons {{\text{H}}^{+}}+\text{O}{{\text{H}}^{-}} \right)+{{\text{h}}^{+}}\to {{\text{H}}^{+}}+\cdot \text{OH} $

  (6)

  $ {{\text{O}}_{2}}+{{\text{e}}^{-}}\to \cdot \text{O}_{2}^{-} $

  (7)

  $ \begin{align} &\text{R+}\cdot \text{OH/}\cdot {{\text{O}}_{\text{2}}}^{\text{-}}\text{/}{{\text{h}}^{\text{+}}}\to \text{intermediates}\to \\ &\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{degradation}\ \text{products} \\ \end{align} $

  (8)

  $ \text{IC}+h\nu \to \text{I}{{\text{C}}^{*}} $

  (9)

  $ \text{I}{{\text{C}}^{*}}+\text{S}{{\text{m}}_{2}}\text{FeSb}{{\text{O}}_{7}}\to \text{I}{{\text{C}}^{+}}+\text{S}{{\text{m}}_{2}}\text{FeSb}{{\text{O}}_{7}}\left( {{\text{e}}^{-}} \right) $

  (10)

  $ \text{S}{{\text{m}}_{2}}\text{FeSb}{{\text{O}}_{7}}\left( {{\text{e}}^{-}} \right)+{{\text{O}}_{2}}\to {{\text{O}}_{2}}^{-}+\text{S}{{\text{m}}_{2}}\text{FeSb}{{\text{O}}_{7}} $

  (11)

  Figure 22

  

  Figure 22.  Suggested photodegradation reaction mechanism of indigo carmine with Sm₂FeSbO₇ as the photocatalyst under visible light irradiation

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  The M-O-M bond angle was closer to 180°, and the excited state was more delocalized as shown by previous study^[58], thus the charge carriers could move easily in the matrix. High diffusivity due to the mobility of the photoinduced electrons and the photoinduced holes helped impel more electrons and holes to reach the reactive sites on the catalyst surface, therefore the photon efficiency of Sm₂FeSbO₇ was improved. The Sm-O-Sm bond angle of Sm₂FeSbO₇ was 180° and the Sb-O-Sb bond angle of Sm₂FeSbO₇ was 139.624° which was close to 180°. Thus, the photocatalytic activity of Sm₂FeSbO₇ was correspon-dingly higher. The crystal structures of Sm₂FeSbO₇ and N-doped TiO₂ were diverse, subsequently the electronic structures of Sm₂FeSbO₇ and N-doped TiO₂ were diverse, either. For Sm₂FeSbO₇, Sm was 6f-block metal element, and Fe was 4d-block metal element, and Sb was 5p-block metal element. Moreover, for N-doped TiO₂, Ti was 4d-block metal element, indicating that the photocatalytic activity might be affected by not only the crystal structure but also the electronic structure of the photocatalyst. The difference of the photocatalytic degradation activity of IC among Sm₂FeSbO₇ and N-doped TiO₂ could be attributed mainly to the difference of their crystalline and electronic structures.

  3.   Conclusions

  In summary, newly synthesized photocatalyst Sm₂FeSbO₇ was prepared for the first time. Sm₂FeSbO₇ showed higher photocatalytic activity compared with N-doped TiO₂ for the photocatalytic degradation of indigo carmine under visible light irradiation and the structural properties of Sm₂FeSbO₇ was characterized by some material characterization methods. The XRD results showed that Sm₂FeSbO₇ owned a pyrochlore-type structure, a cubic crystal system and a space group Fd3m. The lattice parameters of Sm₂FeSbO₇ were a=b=c=1.035 434 nm. XPS results of Sm₂FeSbO₇ indicated that the valence state of Sm, Fe, Sb or O was +3, +3, +5 or -2. The photocatalytic decomposition of IC aqueous solution was realized under visible light irradiation in the presence of Sm₂FeSbO₇ or N-doped TiO₂. The results could apparently state that the photodegradation rate of IC and the photonic efficiency with Sm₂FeSbO₇ as catalyst was higher than those with N-doped TiO₂ as catalyst, which illustrated that Sm₂FeSbO₇ exhibited higher photocatalytic activities for IC degradation under visible light irradiation compared with N-doped TiO₂. The photo-catalytic degradation of indigo carmine with Sm₂FeSbO₇ as a catalyst followed the first-order reaction kinetics. The obvious first-order rate constant of Sm₂FeSbO₇ or N-doped TiO₂ was 0.024 65 or 0.003 77 min^-1. During the photocatalytic process, the reduction of the total organic carbon, formation of inorganic products such as SO₄^2- and NO₃^-, and the evolution of CO₂ uncovered the continuous mineralization of IC. The experimental results of degradation of phenol by Sm₂FeSbO₇ demonstrated that the photosensitive effect was not the main factor in the photodegradation process of IC, and it possessed excellent repeatability. The possible photocatalytic degradation pathway of indigo carmine was obtained. The results which were obtained in our investigations proved that Sm₂FeSbO₇ (visible light) photocatalysis might be regarded as a method for the practical treatment of diluted colored waste water in the environment of room-temperature and ordinary pressure.

  Acknowledgments

  This work was supported by a grant from the fifth group of China-Israel Joint Research Program in Water Technology and Renewable Energy (Grant No.[2010]30) and the National Natural Science Foundation of China (Grant No.21277067).

  Author Contributions

  LUAN Jing-Fei was involved with all aspects of the work including visualizing, planning, and data explication. TAN Wen-Cheng carried out the experiments, analyzed the data and wrote the paper. All authors read and approved the manuscript.

  Conflicts of Interest

  The authors declare no conflict of interest.

• 1. [1]

     Song Y L, Li J T, Bo B. Water Air Soil Pollut., 2010, 213(1/2/3/4):311-317 doi: 10.1007/s11270-010-0386-0

  2. [2]

     Wang S B, Li H T. J. Hazard. Mater., 2005, 126:71-77 doi: 10.1016/j.jhazmat.2005.05.049

  3. [3]

     Xu L, Wei Y G, Guo W, et al. Appl. Surf. Sci., 2015, 332:682-693 doi: 10.1016/j.apsusc.2015.01.235

  4. [4]

     Zhang M Y, Shao C L, Guo Z C, et al. ACS Appl. Mater. Interfaces, 2011, 3:2573-2578 doi: 10.1021/am200412t

  5. [5]

     Wang J, Li J, Zhang L Q, et al. Catal. Lett., 2009, 130(3/4):551-557 doi: 10.1007/s10562-009-0009-9

  6. [6]

     Du H Y, Luan J F. Solid State Sci., 2012, 14:1295-1305 doi: 10.1016/j.solidstatesciences.2012.07.001

  7. [7]

     Benshahar Y, Banin U. Top. Curr. Chem., 2016, 374(4):149-174

  8. [8]

     Ahmad M, Ahmed E, Zhang Y W, et al. Curr. Appl. Phys., 2013, 13(4):697-704 doi: 10.1016/j.cap.2012.11.008

  9. [9]

     Yao W F, Ye J H. Catal. Lett., 2006, 110(1/2):139-142 doi: 10.1007/s10562-006-0101-3

  10. [10]

      Pena M E, Korfiatis G P, Patel M, et al. Water Res., 2005, 39(11):2327-2337 doi: 10.1016/j.watres.2005.04.006

  11. [11]

      Pham T D, Lee B K. Appl. Surf. Sci., 2014, 296:15-23 doi: 10.1016/j.apsusc.2014.01.006

  12. [12]

      Josephine G A S, Ramachandran S, Sivasamy A. J. Saudi Chem. Soc., 2015, 19(5):549-556 doi: 10.1016/j.jscs.2015.05.011

  13. [13]

      Li J L, Liu T, Sui G Z, et al. J. Nanosci. Nanotechnol., 2015, 15(2):1408-1415 doi: 10.1166/jnn.2015.9611

  14. [14]

      Chai Y Y, Wang L, Ren J. Appl. Surf. Sci., 2015, 324(45):212-220

  15. [15]

      Yan C Y, Yi W T, Yuan H M, et al. Environ. Prog. Sustainable Energy, 2014, 33(2):419-429 doi: 10.1002/ep.v33.2

  16. [16]

      Wen X J, Niu C G, Min R, et al. J. Colloid Interface Sci., 2017, 497:368-377 doi: 10.1016/j.jcis.2017.03.029

  17. [17]

      Liang H R, Guo L J. Int. J. Hydrogen Energy, 2010, 35(13):7104-7109 doi: 10.1016/j.ijhydene.2010.01.037

  18. [18]

      Song L M, Zhang S J, Chen B. Catal. Commun., 2009, 10(12):1565-1568 doi: 10.1016/j.catcom.2009.03.022

  19. [19]

      Luan J F, Zheng S R, Hao X P, et al. J. Braz. Chem. Soc., 2006, 17(7):1368-1376 doi: 10.1590/S0103-50532006000700026

  20. [20]

      Long M, Cai J, Cai W M, et al. Prog. Chem., 2006, 18(9):1065-1075 http://en.cnki.com.cn/Article_en/CJFDTotal-HXJZ200609000.htm

  21. [21]

      Rawal S B, Bera S, Lee W I. Catal. Lett., 2012, 142:1482-1488 doi: 10.1007/s10562-012-0924-z

  22. [22]

      Habibi-Yangjeh A, Shekofteh-Gohari M. Sep. Purif. Technol., 2017, 184:334-346 doi: 10.1016/j.seppur.2017.05.007

  23. [23]

      Kalantari K, Ahmad M B, Masoumi H R, et al. Int. J. Mol. Sci., 2014, 15(7):12913-12927 doi: 10.3390/ijms150712913

  24. [24]

      Vadivel S, Maruthamani D, Habibi-Yangjeh A, et al. J. Colloid Interface Sci., 2016, 480:126-136 doi: 10.1016/j.jcis.2016.07.012

  25. [25]

      Huang L Z, Liu B S. RSC Adv., 2016, 6(22):17873-17879 doi: 10.1039/C5RA25689E

  26. [26]

      Chakraborty A K, Kebede M A. React. Kinet., Mech Catal., 2012, 106(1):83-98 doi: 10.1007/s11144-012-0423-7

  27. [27]

      Chai B, Peng T Y, Mao J, et al. Phys. Chem. Chem. Phys., 2012, 14:16745-16752 doi: 10.1039/c2cp42484c

  28. [28]

      Bi J H, Fang W, Li L, et al. J. Alloys Compd., 2015, 649:28-34 doi: 10.1016/j.jallcom.2015.07.083

  29. [29]

      Gouvêa C A, Wypych F, Moraes S G, et al. Chemosphere, 2000, 40(4):427-432 doi: 10.1016/S0045-6535(99)00312-4

  30. [30]

      Tatsuma T, Saitoh S, Ngaotrakanwiwat P, et al. Langmuir, 2002, 18:7777-7779 doi: 10.1021/la026011i

  31. [31]

      Ullah K, Meng Z D, Ye S, et al. J. Ind. Eng. Chem., 2014, 20(3):1035-1042 doi: 10.1016/j.jiec.2013.06.040

  32. [32]

      Zhou J K, Zou Z G, Ray A K, et al. Ind. Eng. Chem. Res., 2007, 46:745-749 doi: 10.1021/ie0613220

  33. [33]

      Zhang X, Ai Z H, Jia F L, et al. Mater. Chem. Phys., 2007, 103:162-167 doi: 10.1016/j.matchemphys.2007.02.008

  34. [34]

      Zhang G K, Zou X, Gong J, et al. J. Alloys Compd., 2006, 425:76-80 doi: 10.1016/j.jallcom.2006.01.016

  35. [35]

      Farrukh M A, Imran F, Ali S, et al. Russ. J. Appl. Chem., 2015, 88(9):1523-1527 doi: 10.1134/S1070427215090220

  36. [36]

      Ghorai T K, Chakraborty M, Pramanik P. J. Alloys Compd., 2011, 509(32):8158-8164 doi: 10.1016/j.jallcom.2011.05.069

  37. [37]

      Xie L J, Ma J F, Xu G J. Mater. Chem. Phys., 2008, 110:197-200 doi: 10.1016/j.matchemphys.2008.01.035

  38. [38]

      Li X K, Kako T, Ye J H. Appl. Catal., A, 2007, 326:1-7 doi: 10.1016/j.apcata.2007.02.023

  39. [39]

      Shetty K, Prathibha B S, Rangappa D, et al. Mater. Today, 2017, 4(11):11764-11772 doi: 10.1016/j.matpr.2017.09.093

  40. [40]

      Jiang Y Y, Zhang J W, Hu Z Q, et al. Appl. Mech. Mater., 2014, 455:99-105 doi: 10.4028/www.scientific.net/AMM.455.99

  41. [41]

      Chen C H, Liang Y H, Zhang W D. J. Alloys Compd., 2010, 501:168-172 doi: 10.1016/j.jallcom.2010.04.072

  42. [42]

      Qiu J X, Wang C Y, Gu M Y. Mater. Sci. Eng. B, 2004, 112:1-4 doi: 10.1016/j.mseb.2004.04.012

  43. [43]

      Marugan J, Hufschmidt D, Sagawe G, et al. Water Res., 2006, 40:833-839 doi: 10.1016/j.watres.2005.12.019

  44. [44]

      Sakthivel S, Shankar M V, Palanichamy M, et al. Water Res., 2004, 38:3001-3008 doi: 10.1016/j.watres.2004.04.046

  45. [45]

      Izumi F. Rigaku J., 1989, 6(1):10-20

  46. [46]

      Tauc J, Grigorovici R, Vancu A. Phys. Status Solidi, 1966, 15:627-637 doi: 10.1002/(ISSN)1521-3951

  47. [47]

      Luan J F, Wang S, Ma K, et al. J. Phys. Chem. C, 2010, 114:9398-9407 doi: 10.1021/jp1011846

  48. [48]

      Yang Y J, Luan J F. Molecules, 2012, 17:2752-2772 doi: 10.3390/molecules17032752

  49. [49]

      Chang M J, Wang H, Li H L, et al. J. Mater. Sci., 2018, 53:3682-3691 doi: 10.1007/s10853-017-1813-z

  50. [50]

      Lachheb H, Puzenat E, Houas A, et al. Appl. Catal., B, 2002, 39:75-90 doi: 10.1016/S0926-3373(02)00078-4

  51. [51]

      张笑河, 陈国庆, 马超群.激光与光电子学进展, 2018, 55(1):013007(5 Pages) http://www.en.cnki.com.cn/Article_en/CJFDTotal-GUAN201802032.htmZHANG Xiao-He, CHEN Guo-Qing, MA Chao-Qun. Laser and Optoelectronics Progress, 2018, 55(1):013007(5 Pages) http://www.en.cnki.com.cn/Article_en/CJFDTotal-GUAN201802032.htm

  52. [52]

      Qu R J, Xu B Z, Meng L J, et al. Water Res., 2015, 68:316-327 doi: 10.1016/j.watres.2014.10.017

  53. [53]

      Huang S Q, Xu Y G, Xie M, et al. Colloids Surf. A, 2015, 478(ISSN):71-80 http://www.sciencedirect.com/science/article/pii/s0927775715002721

  54. [54]

      Subash B, Krishnakumar B, Swaminathan M, et al. Langmuir, 2013, 29:939-949 doi: 10.1021/la303842c

  55. [55]

      Sun H Q, Zhou G L, Liu S Z, et al. Chem. Eng. J., 2013, 231(9):18-25 http://www.sciencedirect.com/science/article/pii/S1385894713009261

  56. [56]

      Ding H M, Sun H, Shan Y K. J. Photochem. Photobiol. A, 2005, 169:101-107 doi: 10.1016/j.jphotochem.2004.04.015

  57. [57]

      Gadhi T A, Hernández-Gordillo A, Bizarro M, et al. Ceram. Int., 2016, 42(11):13065-13073 doi: 10.1016/j.ceramint.2016.05.087

  58. [58]

      Wiegel M, Middel W, Blasse G. J. Mater. Chem., 1995, 5(7):981-983 doi: 10.1039/jm9950500981

• 

  Figure 1  (a) TEM image of Sm₂FeSbO₇; (b) Selected area electron diffraction pattern of Sm₂FeSbO₇

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  Figure 2  (a) SEM image of Sm₂FeSbO₇; (b) EDS spectrum of Sm₂FeSbO₇ calcinated at 800 ℃ for 35 h

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  Figure 3  XPS spectra of Sm₂FeSbO₇ and every element which was contained in Sm₂FeSbO₇: (a) Full XPS spectrum of Sm₂FeSbO₇; XPS spectra of (b) Sm3d_5/2, (c) Sb3d, (d) Fe₂p and (e) O1s

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  Figure 4  XRD patterns and Rietveld refinements of Sm₂FeSbO₇

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  Figure 5  Structural model of Sm₂FeSbO₇ simulated by Materials Studio software corresponding to the XRD pattern shown in Fig. 4

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  Figure 6  FTIR spectrum of Sm₂FeSbO₇ calcinated at 800 ℃ for 35 h

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  Figure 7  Diffuse reflection spectra of Sm₂FeSbO₇

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  Figure 8  Valence band of Sm₂FeSbO₇

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  Figure 9  Suggested band structure of Sm₂FeSbO₇

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  Figure 10  Spectral changes of aqueous solutions of indigo carmine due to visible light irradiation with the presence of Sm₂FeSbO₇ calcinated at 800 ℃ for 35 h

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  Figure 11  Photocatalytic degradation of indigo carmine under visible light irradiation with the presence of Sm₂FeSbO₇ or N-TiO₂ as photocatalyst, respectively

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  Figure 12  TOC plots during the photocatalytic degradation of indigo carmine under visible light irradiation with Sm₂FeSbO₇ or N-TiO₂ as photocatalyst, respectively

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  Figure 13  CO₂ production during the photocatalytic degradation of indigo carmine with Sm₂FeSbO₇ or N-TiO₂ as photocatalyst respectively under visible light irradiation

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  Figure 14  Observed first order kinetic plots for the photocatalytic degradation of IC with Sm₂FeSbO₇ or N-TiO₂ as catalyst under visible light irradiation

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  Figure 15  NO₃^- production during the photocatalytic degradation of IC with Sm₂FeSbO₇ or N-TiO₂ as photocatalyst respectively under visible light irradiation

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  Figure 16  SO₄^2- production during the photocatalytic degradation of IC with Sm₂FeSbO₇ or N-TiO₂ as photocatalyst respectively under visible light irradiation

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  Figure 17  Photocatalytic degradation of phenol under visible light irradiation with Sm₂FeSbO₇ as photocatalyst

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  Figure 18  Repeated photocatalytic degradation tests of IC by Sm₂FeSbO₇

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  Figure 19  Temporal change in the distribution of the intermediates during the photodegradation of IC

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  Figure 20  Suggested photocatalytic degradation pathway scheme for indigo carmine under visible light irradiation with the presence of Sm₂FeSbO₇ prepared by a solid-state reaction method at 800 ℃ for 35 h

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  Figure 21  Photodegradation of indigo carmine over Sm₂FeSbO₇ without scavenger and in the presence of various scavengers of EDTA, SOD, AgNO₃ and TBA

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  Figure 22  Suggested photodegradation reaction mechanism of indigo carmine with Sm₂FeSbO₇ as the photocatalyst under visible light irradiation

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  Table 1.  Binding energies for key elements of Sm₂FeSbO₇

  Element	Sm3d5/2	Fe2p1/2	Sb3d5/2	O1s	C1s
  Binding energy/eV	1 084.6	724.8	532.0	531.8	286.0
  Binding energy after C correction/eV	1 083.1	723.3	530.5	530.3	284.5

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  Table 2.  Structural parameters of Sm₂FeSbO₇ calcinated at 800 ℃ for 35 h

  Atom	x	y	z	Occupation factor
  Sm	0	0	0	1
  Fe	0.5	0.5	0.5	0.5
  Sb	0.5	0.5	0.5	0.5
  O(1)	-0.175	0.125	0.125	1
  O(2)	0.125	0.125	0.125	1

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•