Synthesis , Property Characterization , and Photocatalytic Activity of Novel Visible Light-Responsive Photocatalyst Fe 2 BiSbO 7

Fe2BiSbO7 was synthesized by a solid-state reaction method for the first time. The structural and photocatalytic properties of Fe2BiSbO7 have been characterized. The results showed that Fe2BiSbO7 was crystallized with the pyrochlore-type structure, cubic crystal system, and space group Fd3m. The lattice parameter for Fe2BiSbO7 was a = 10.410297 Å. The photocatalytic degradation of methylene blue (MB) was realized under visible light irradiation with Fe2BiSbO7 as catalyst. Fe2BiSbO7 owned higher catalytic activity compared with Bi2InTaO7 or pure TiO2 or N-doped TiO2 for photocatalytic degradation of MB. The photocatalytic degradation of MB with Fe2BiSbO7, Bi2InTaO7, or N-doped TiO2 followed the first-order reaction kinetics, and the first-order rate constant was 0.01189, 0.00275, or 0.00333 min−1. After visible light irradiation for 230 min with Fe2BiSbO7, complete removal and mineralization of MB was observed. The reduction of the total organic carbon, the formation of inorganic products, SO4 2− and NO3−, and the evolution of CO2 revealed the continuous mineralization of MB during the photocatalytic process. The photocatalytic degradation pathway of MB was obtained. Fe2BiSbO7/(visible light) photocatalysis system was found to be suitable for textile industry wastewater treatment.


Introduction
Dye effluents from textile industries and photographic industries are becoming a serious environmental problem because of their toxicity, unacceptable color, high chemical oxygen demand content, and nonbiological degradation [1].Many conventional methods have been proposed to treat industrial effluents, but each method has its shortcomings [1][2][3][4][5][6][7].In recent years, the photocatalytic degradation processes have been widely applied as techniques of destruction of organic pollutants in wastewater and effluents, especially for degrading dyes [1,[7][8][9][10][11][12][13][14][15][16][17][18][19][20][21].However, among various dyes, methylene blue (MB) dye was difficult to be degraded and was often utilized as a model dye contaminant to estimate the activity of a photocatalyst under both ultraviolet light irradiation [18,19,22] and visible light irradiation [20,21,23,24].There were many reports about the photodegradation of MB.Unfortunately, most of these reports were carried out under UV light irradiation.Up to now, there were only few reports of MB dye degradation under visible light irradiation such as the research by Asahi et al. with a reduced TiO x (TiO 2−x N x ) as catalyst and the research by Tang et al. and Cui et al. with Pt-TiO 2 as photocatalyst [21,24].Zhang [25] utilized Ndoped TiO 2 as catalyst to degrade MB under visible light irradiation and found that the removal ratio of MB was only 35% after 180 min.It is known that ultraviolet light only occupies 4% of the solar energy.For this reason, many endeavors should be taken up to develop new visible lightresponsive photocatalysts which are capable of utilizing more visible light, which accounts for about 43% of the solar energy.Therefore, it is urgent to develop novel visible lightresponsive photocatalysts.
With the development of investigation of photocatalysis process, investigators also paid much attention to researching and developing novel photocatalysts [26][27][28][29][30]. Currently, TiO 2 was the most common photocatalyst, however, TiO 2 could not be utilized in the visible light region and could only degrade RhB under ultraviolet light irradiation which was a restrained factor for photocatalysis technology with TiO 2 as catalyst.Therefore, some efficient catalysts which could generate electron-hole pairs under visible light irradiation should be developed.Fortunately, A 2 B 2 O 7 compounds were International Journal of Photoenergy often considered to own photocatalytic properties under visible light irradiation.In our previous work [31], we have found that Bi 2 InTaO 7 was crystallized with the pyrochloretype 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.According to the above analysis, we could assume that substitution of Ta 5+ by Sb 5+ , substitution of Bi 3+ by Fe 3+ , and substitution of In 3+ by Bi 3+ in Bi 2 InTaO 7 might increase carriers concentration.As a result, a change and improvement of the electrical transportation and photophysical properties could be found in the novel Fe 2 BiSbO 7 compound which might own advanced photocatalytic properties.
Fe 2 BiSbO 7 has never been produced before and the data about its structural and photophysical properties such as space group and lattice constants have not been found previously.In addition, the photocatalytic properties of Fe 2 BiSbO 7 have not been studied by other investigators.The molecular composition of Fe 2 BiSbO 7 was very similar with other A 2 B 2 O 7 compounds.Thus the resemblance suggested that Fe 2 BiSbO 7 might possess photocatalytic properties under visible light irradiation, which was similar with those other members in A 2 B 2 O 7 family.Fe 2 BiSbO 7 also seemed to own potential for improvement of photocatalytic activity upon modification of its structure because it had been proved that a slight modification of a semiconductor structure will result in a remarkable change within photocatalytic properties [21].In this paper, Fe 2 BiSbO 7 was prepared for the first time by the solid-state reaction method and the structure and photocatalytic properties of Fe 2 BiSbO 7 were investigated in detail.The photocatalytic degradation of MB under visible light irradiation was also performed to evaluate the photocatalytic activity of Fe 2 BiSbO 7 .A comparison among the photocatalytic properties of Fe 2 BiSbO 7 , Bi 2 InTaO 7 , and N-doped TiO 2 was achieved in order to elucidate the relationship between the structure and photocatalytic activity of Fe 2 BiSbO 7 .C for 4 h before synthesis.In order to synthesize Fe 2 BiSbO 7 , the precursors were stoichiometrically mixed in a quartz mortar, subsequently pressed into small columns, and put into an alumina crucible (Shenyang Crucible Co., Ltd., China).Finally, calcination was carried out at 1020 • C for 25 h in an electric furnace (KSL 1700X, Hefei Kejing Materials Technology Co., Ltd., China).Similarly, Bi 2 InTaO 7 was synthesized by calcination at 1050 • C for 46 h.After sintering and grounding within a quartz mortar, ultrafine Fe 2 BiSbO 7 powder was fabricated.Nitrogen-doped titania (N-doped TiO 2 ) catalyst with tetrabutyl titanate as a titanium precursor was prepared via the sol-gel method at room temperature.The procedure was as follows: 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 that 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 mol% was added into the resulting transparent colloidal suspension under vigorous stirring condition and kept stirring for 1 h.Finally, the xerogel was formed after being aged for 2 days.The xerogel was grounded into powder which was calcined at 500 • C for 2 h.Finally, above powder was grounded in agate mortar and screened by shaker to obtain N-doped TiO 2 powders.

Characterization of Fe
The crystalline phase of Fe 2 BiSbO 7 was analyzed by X-ray diffractometer (D/MAX-RB, Rigaku Corporation, Japan) with CuKα radiation (λ = 1.54056).The patterns were collected at 295 K with a stepscan procedure in the range of 2θ = 10 − 95 • .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.The chemical composition of the compound was determined by scanning electron microscope-X-ray energy dispersion spectrum (SEM-EDS, LEO 1530VP, LEO Corporation, Germany), X-ray fluorescence spectrometer (XFS, ARL-9800, ARL Corporation, Switzerland), and X-ray photoelectron spectroscopy (XPS, ESCALABMK-2, VG Scientific Ltd., UK).The particle morphology of Fe 2 BiSbO 7 was observed by transmission electron microscope (Tecnal F20 S-Twin, FEI Corporation, USA).The Fe 3+ content, Bi 3+ content, Sb 5+ content, and O 2− content of Fe 2 BiSbO 7 and the valence state of elements were also analyzed by X-ray photoelectron spectroscopy (XPS).The chemical composition within the depth profile of Fe 2 BiSbO 7 was examined by the argon ion denudation method when X-ray photoelectron spectroscopy was used.UV-visible diffuse reflectance spectrum of Fe 2 BiSbO 7 was measured with a Shimadzu UV-2550 UV-Visible spectrometer, and BaSO 4 was used as the reference material.The surface areas of Fe 2 BiSbO 7 and Ndoped TiO 2 were determined by the Brunauer-Emmett-Teller (BET) method (MS-21, Quantachrome Instruments Corporation, USA) with N 2 adsorption at liquid nitrogen temperature.The particle sizes of the photocatalysts were measured by Malvern's mastersize-2000 particle size analyzer (Malvern Instruments Ltd., UK).

Photocatalytic Activity Tests.
The photocatalytic activity of Fe 2 BiSbO 7 was evaluated with methylene blue (C 16 H 18 ClN 3 S) (Tianjin Bodi Chemical Co., Ltd., China) as a model material.The photoreaction was carried out in a photochemical reaction apparatus (Nanjing Xujiang Machine Plant, China).The internal structure of the reaction apparatus is as follows: the lamp is put into a quartz hydrazine which is a hollow structure and located in the middle of the reactor.The recycling water through the reactor maintains a near constant reaction temperature (20 • C) and the solution was continuously stirred and aerated.Twelve holes which are used to put quartz tubes evenly distribute around the lamp and the distance between the lamp and each hole is equal.Under the condition of magnetic stirring, the photocatalyst within the MB solution is in the state of suspension.In this paper, the photocatalytic degradation of the MB solution was performed with 0.3 g Fe 2 BiSbO 7 in 300 mL 0.025 mM MB aqueous solution in quartz tubes with 500 W Xenon lamp (400 nm < λ < 800 nm) as visible-light source.Prior to visible light irradiation, the suspensions which contained the catalyst and MB dye were magnetically stirred in the dark for 45 min to ensure establishment of an adsorption/desorption equilibrium among Fe 2 BiSbO 7 , the MB dye, and atmospheric oxygen.During visible light illumination, the suspension was stirred at 500 rpm and the initial pH value of the MB solution was 7.0 without pH adjustment in the reaction process.The above experiments were performed under oxygen-saturation conditions ([O 2 ] sat = 1.02 × 10 −3 M).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 UV-Visible spectrometer with the detecting wavelength at 665 nm.The experimental error was found to be within ±2.2%.
The incident photon flux I o measured by a radiometer (Model FZ-A, Photoelectric Instrument Factory Beijing Normal University, China) was determined to be 4.76×10 −6  Einstein L −1 s −1 under 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 pH value adjustment was not carried out, and the initial pH value was 7.0.The inorganic products which were obtained from MB degradation were analyzed by ion chromatograph (DX-300, Dionex Corporation, USA).The identification of MB and the degradation intermediate products of MB were 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) which operated at 320 • C and was connected to HP 5973 mass selective detector and a flame ionization detector with H 2 as the carried gas.The intermediate products of MB were also measured by liquid chromatographmass spectrometer (LC-MS, Thermo Quest LCQ Duo, USA, Beta Basic-C 18 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 and the m z −1 range swept from 50 to 600.Evolution of CO 2 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 [32,33]: where ϕ is the photonic efficiency (%), R is the rate of MB degradation (Mol L −1 s −1 ), and I o is the incident photon flux (Einstein L −1 s −1 ).  1 that Fe 2 BiSbO 7 was crystallized with the pyrochloretype structure, cubic crystal system, and space group Fd3m.The lattice parameter for Fe 2 BiSbO 7 was proved to be a = 10.410297Å.According to the calculation results from Figure 1, the (h k l) value for the main peaks of Fe 2 BiSbO 7 could be found and indexed.Full-profile structure refinements of the collected X-ray diffraction data of Fe 2 BiSbO 7 were obtained by the RIETAN [34] program, which was based on Pawley analysis.The refinement results of Fe 2 BiSbO 7 are shown in Figure 2. The atomic coordinates and structural parameters of Fe 2 BiSbO 7 are listed in Table 1.The results of the final refinement for Fe 2 BiSbO 7 indicated a good agreement between the observed and calculated intensities in a pyrochlore-type structure and cubic crystal system with space group Fd3m.Our XRD results also showed that Fe 2 BiSbO 7 and Bi 2 InTaO 7 were crystallized in the same structure, and 2 theta angles of each reflection of Fe 2 BiSbO 7 changed with Fe 3+ being replaced by Bi 3+ , Bi 3+ being replaced by In 3+ , and Sb 5+ being replaced by Ta 5+ .Bi 2 InTaO 7 was also crystallized with a cubic structure by space group Fd3m and the lattice parameter of Bi 2 InTaO 7 was a = 10.746410Å.The lattice parameter of Fe 2 BiSbO 7 was a = 10.410297Å, which indicated that the lattice parameter of Fe 2 BiSbO 7 decreased compared with the lattice parameter of Bi 2 InTaO 7 because the In 3+ ionic radii (0.92 Å) or the Bi 3+ ionic radii (1.17 Å) was larger than the Fe 3+ ionic radii (0.78 Å).The outcome of refinement for Fe 2 BiSbO 7 generated the unweighted R factor, R P = 11.56% with space group Fd3m.Zou et al. [35] refined the crystal structure of Bi 2 InNbO 7 and obtained a large R factor for Bi 2 InNbO 7 , which was ascribed to a slightly modified structure model for Bi 2 InNbO 7 .Based on the high purity of the precursors which were used in this study and the EDS results that did not trace any other elements, it was unlikely that the observed space groups originated from the presence of impurities.Therefore, it was suggested that the slightly high R factor for Fe 2 BiSbO 7 was due to a slightly modified   In order to reveal the surface chemical compositions and the valence states of various elements of Fe 2 BiSbO 7 , the X-ray  3 shows the photocatalytic degradation of methylene blue under visible light irradiation in the presence of Fe 2 BiSbO 7 , Bi 2 InTaO 7 , pure TiO 2 , N-doped TiO 2 , as well as in the absence of a photocatalyst.The results showed that a reduction in typical MB peaks at 665 nm and 614.5 nm was clearly noticed and the photodegradation rate of MB was about 1.980 × 10 −9 mol L −1 s −1 and the photonic efficiency was estimated to be 0.0416% (λ = 420 nm) with Fe 2 BiSbO 7 as catalyst.Similarly, the photodegradation rate of MB was about 1.001 × 10 −9 mol L −1 s −1 and the photonic efficiency was estimated to be 0.0210% (λ = 420 nm) with N-doped TiO 2 as catalyst.Moreover, the photodegradation rate of MB was about 0.891 × 10 −9 mol L −1 s −1 and the photonic efficiency was estimated to be 0.0187% (λ = 420 nm) with Bi 2 InTaO 7 as catalyst.By contrast, the photodegradation rate of MB within 200 min of visible light irradiation was only 0.8338 × 10 −9 mol L −1 s −1 and the photonic efficiency was estimated to be 0.0175% (λ = 420 nm) with pure TiO 2 as catalyst.The photodegradation rate of MB was about 0.6830 × 10 −9 mol L −1 s −1 and the photonic efficiency was estimated to be 0.0143% (λ = 420 nm) in the absence of a photocatalyst.The results showed that the photodegradation rate of MB and the photonic efficiency with Fe 2 BiSbO 7 as catalyst were both higher than those with N-doped TiO   Fe 2 BiSbO 7 after 200 min of reaction time under visible light irradiation and this turnover number was evident to prove that this reaction occurred catalytically.Similarly, when the light was turned off in this experiment, the stop of this reaction showed the obvious light response.

Results and Discussion
Figure 5 shows the amount of CO 2 which was yielded during the photodegradation of MB with Fe 2 BiSbO 7 , Bi 2 InTaO 7 or N-doped TiO 2 as catalyst under visible light irradiation.The amount of CO 2 increased gradually with increasing reaction time when MB was photodegraded by Fe 2 BiSbO 7 , Bi 2 InTaO 7 or N-doped TiO 2 .At the same time, after 200 min visible light irradiation, the CO 2 production of 0.11063 mmol with Fe 2 BiSbO 7 as catalyst was higher than the CO 2 production of 0.05600 mmol with N-doped TiO 2 as catalyst.Meanwhile, after visible light irradiation for 200 min, the CO 2 production of 0.05600 mmol with Ndoped TiO 2 as catalyst was higher than the CO 2 production of 0.04934 mmol with Bi 2 InTaO 7 as catalyst.
The first-order nature of the photocatalytic degradation kinetics with Fe 2 BiSbO 7 , Bi 2 InTaO 7 , or N-doped TiO 2 as catalyst is clearly demonstrated in Figure 6.The results showed a linear correlation between ln(C/C o ) (or ln(TOC/TOC o )) and the irradiation time for the photocatalytic degradation of MB under visible light irradiation with the presence of Fe 2 BiSbO 7 , Bi 2 InTaO 7 , or N-doped TiO 2 .Here, C represented the MB concentration at time t, C o represented the initial MB concentration, TOC represented the total organic carbon concentration at time t, and TOC o represented the initial total organic carbon concentration.According to Figure 6, the first-order rate constant k C of MB concentration was estimated to be 0.01189 min −1 with Fe 2 BiSbO 7 as catalyst, 0.00275 min −1 with Bi 2 InTaO 7 as catalyst, and 0.00333 min −1 with N-doped TiO 2 as catalyst.The different value of k C indicated that Fe 2 BiSbO 7 was more suitable for the photocatalytic degradation of MB under visible light irradiation than N-doped TiO 2 or Bi 2 InTaO 7 .Meanwhile Ndoped TiO 2 was more suitable for the photocatalytic degradation of MB under visible light irradiation than Bi 2 InTaO 7 .Figure 6 also showed that the first-order rate constant K TOC of TOC was estimated to be 0.01101 min −1 with Fe 2 BiSbO 7 as catalyst, 0.00275 min −1 with N-doped TiO 2 as catalyst, and 0.00259 min −1 with Bi 2 InTaO 7 as catalyst, which indicated that the photodegradation intermediate products of MB probably appeared during the photocatalytic degradation of MB under visible light irradiation because of the different value between k C and K TOC .It could also be seen from Figure 6 that Fe 2 BiSbO 7 showed higher mineralization efficiency for MB degradation compared with N-doped TiO 2 or Bi 2 InTaO 7 .At the same time, N-doped TiO 2 showed higher mineralization efficiency for MB degradation compared with Bi 2 InTaO 7 .
Some inorganic ions such as NH 4 + , NO 3 − , and SO 4 2− were formed in parallel as the end products of nitrogen and sulfur atoms which existed in MB.Figures 7 and 8 showed the concentration variation of SO 4 2− and NO 3 − during photocatalytic degradation of MB with Fe 2 BiSbO 7 , Bi 2 InTaO 7 , or N-doped TiO 2 as catalyst under visible light irradiation.The results showed that the concentration of NO    that the amount of SO 4 2− ions which was released into the solution was lower than the amount of SO 4 2− which should come from stoichiometry.One possible reason could be a loss of sulfur-containing volatile compounds such as SO 2 .The second possible reason was a partially irreversible adsorption of some SO 4 2− ions on the surface of the photocatalyst which had been observed by Lachheb et al. by titanium dioxide [37].Regardless, whether the sulfate ions were adsorbed irreversibly on the surface or not, it was important to stress that the evidence for restrained photocatalytic activity was not noticed.
The photodegradation intermediate products of MB in our experiment were identified as azure B, azure A, azure C, thionine, phenothiazine, leucomethylene blue, N,Ndimethylp-phenylenediamine, phenol, and aniline.According to the intermediate products which were found in this work and the observed appearance time of other intermediate products, a possible photocatalytic degradation pathway for MB was proposed.Figure 9 shows the suggested photocatalytic degradation pathway scheme for methylene blue under visible light irradiation in the presence of Fe 2 BiSbO 7 .The molecule of MB was converted into small organic species, which were subsequently mineralized into inorganic products such as SO 4 2− ions, NO 3 − ions, CO 2 , and water ultimately.

Photocatalytic Degradation Mechanism.
The action spectra of MB degradation with Fe 2 BiSbO 7 as catalyst were observed under visible light irradiation.A clear photonic efficiency (0.0103% at its maximal point) at wavelengths which corresponded to sub-Eg energies of the photocatalysts (λ from 375 to 700 nm) was observed for Fe 2 BiSbO 7 .The existence of photonic efficiency at this region revealed that photons are not absorbed by the photocatalysts.In particular, the correlation between the low-energy action spectrum and the absorption spectrum of MB clearly demonstrated that any photodegradation results at wavelengths above 545 nm should be attributed to photosensitization effect by the dye MB itself (Scheme I).According to the mechanism which was shown in Scheme I, MB which was adsorbed on Fe 2 BiSbO 7 was excited by visible light irradiation.Subsequently, an electron was injected from the excited MB to the conduction band of Fe 2 BiSbO 7 where the electron was scavenged by molecular oxygen.Scheme I explained the results which were obtained with Fe 2 BiSbO 7 as catalyst under visible light irradiation, where the photocatalyst Fe 2 BiSbO 7 could serve to reduce recombination of photogenerated electrons and holes by scavenging of electrons [38].
Below the wavelength of 545 nm, the situation was different.The results of photonic efficiency correlated well with the absorption spectra of Fe 2 BiSbO 7 .These results evidently showed that the mechanism which was responsible for the photodegradation of MB went through band gap excitation of Fe 2 BiSbO 7 .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 Fe 2 BiSbO 7 under suprabandgap irradiation, namely Scheme II.

Scheme II. Consider
According to first principles calculations, we deduced that the conduction band of Fe 2 BiSbO 7 was composed of Fe 3d and Sb 5p orbital component, and the valence band of Fe 2 BiSbO 7 was composed of O 2p and Bi 6s orbital component.Fe 2 BiSbO 7 could produce electron-hole pairs by absorption of photons directly, and it indicated that enough energy which was larger than the band gap energy of Fe 2 BiSbO 7 was necessary for the photocatalytic degradation process of MB.
Former luminescent studies had shown that the closer the M-O-M bond angle was 180 • , the more delocalized was the excited state [39], 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.For Fe 2 BiSbO 7 , the bond angle of Bi-O-Sb was 119.76 • , which indicated that the bond angle of Bi-O-Sb was close to 180 • .Thus, the photocatalytic activity of Fe 2 BiSbO 7 was consequently higher.The crystal structure and the electronic structure of Fe 2 BiSbO 7 and N-doped TiO 2 were totally different.For Fe 2 BiSbO 7 , Fe was 3d-block metal element, and Bi was 6pblock metal element, and Sb was 5p-block metal element.But for N-doped TiO 2 , Ti was 3d-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, as well.In conclusion, the different photodegradation effect of MB between Fe 2 BiSbO 7 and Ndoped TiO 2 could be mainly attributed to the difference of their crystalline structures and electronic structures.
The present results indicated that the Fe 2 BiSbO 7 -visible light photocatalysis system might be regarded as a practical method for treatment of diluted colored wastewater.This system could be utilized for decolorization, purification, and detoxification of 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 Fe 2 BiSbO 7 -visible light photocatalysis system might provide a valuable treatment for purifying and reusing colored aqueous effluents.

Conclusions
Fe 2 BiSbO 7 was prepared by the solid-state reaction method for the first time.The structural and photocatalytic properties of Fe 2 BiSbO 7 were investigated.XRD results indicated that Fe 2 BiSbO 7 was crystallized with the pyrochlore-type structure, cubic crystal system, and space group Fd3m.The lattice parameter of Fe 2 BiSbO 7 was found to be a = 10.410297Å. Photocatalytic decomposition of aqueous MB was realized under visible light irradiation in the presence of Fe 2 BiSbO 7 , Bi 2 InTaO 7 , or N-doped TiO 2. The results showed that Fe 2 BiSbO 7 owned higher catalytic activity compared with pure TiO 2 , Bi 2 InTaO 7 , or N-doped TiO 2 for photocatalytic degradation of MB under visible light irradiation.The photocatalytic degradation of MB with Fe 2 BiSbO 7 , Bi 2 InTaO 7 , or N-doped TiO 2 as catalyst followed the firstorder reaction kinetics, and the first-order rate constant was 0.01189 min −1 , 0.00275 min −1 , or 0.00333 min −1 .Complete removal and mineralization of MB was observed after visible light irradiation for 230 min with Fe 2 BiSbO 7 as catalyst.The reduction of the total organic carbon, the formation of inorganic products such as SO 4 2− and NO 3 − , and the evolution of CO 2 revealed the continuous mineralization of MB during the photocatalytic process.The possible photocatalytic degradation pathway of MB was obtained under visible light irradiation.Fe 2 BiSbO 7 /(visible light) photocatalysis system was found to be suitable for textile industry wastewater treatment and could be used to solve other environmental chemical pollution problems.

Figure 2 :
Figure 2: Pawley refinements of XRD data for novel photocatalyst Fe 2 BiSbO 7 prepared by the solid state reaction method at 1020 • C. The solid line represents experimental X-ray diffraction pattern (-).The dot line represents simulation X-ray diffraction pattern (. ..).The tic marks represent reflection positions.A difference (observed-calculated) profile is shown beneath.

Figure 6 :
Figure 6: Observed first-order kinetic plots for the photocatalytic degradation of methylene blue with Fe 2 BiSbO 7 , Bi 2 InTaO 7 , or Ndoped TiO 2 as catalyst under visible light irradiation.

2
BiSbO 7 and N-Doped TiO 2 .Fe 2 BiSbO 7 powder was first synthesized by the solid-state reaction method.Fe 2 O 3 , Bi 2 O 3 , and Sb 2 O 5 with the purity of 99.99% were utilized as raw materials which were purchased from Sinopharm Group Chemical Reagent Co. (Shanghai, China) and used without further purification.All powders were dried at 200 3.1.CrystalStructure of Fe 2 BiSbO 7 .Figure1presents TEM image and the selected area electron diffraction pattern of Fe 2 BiSbO 7 .The TEM image of Fe 2 BiSbO 7 showed that the morphology of the Fe 2 BiSbO 7 particle was very similar and regular.It could be seen that the Fe 2 BiSbO 7 particles crystallized well and the mean particle diameter of Fe 2 BiSbO 7 was about 150 nm.SEM-EDS spectrum of Fe 2 BiSbO 7 revealed that Fe 2 BiSbO 7 was pure phase without any other impure elements and Fe 2 BiSbO 7 displayed the presence of iron, bismuth, antimony, and oxygen.It could be seen from Figure

Table 1 :
Atomic coordinates and structural parameters of Fe 2 BiSbO 7 prepared by the solid state reaction method.
structure model for Fe 2 BiSbO 7 .It should be emphasized that the defects or the disorder/order of a fraction of the atoms could result in the change of structures, including different bond-distance distributions, thermal displacement parameters, and/or occupation factors for some of the atoms.

Table 2 :
Binding energies (BE) for key elements from Fe 2 BiSbO 7 .BiSbO 7 for detecting Fe, Bi, Sb, and O was performed.The full XPS spectrum confirmed that the prepared Fe 2 BiSbO 7 contained elements of Fe, Bi, Sb, and O, which was consistent with the results of SEM-EDS.The different elemental peaks which are corresponding to definite binding energies are given in Table2.The results illustrated that the oxidation states of Fe, Bi, Sb, and O ions from Fe 2 BiSbO 7 were +3, +3, +5, and −2, respectively.Besides , the average atomic ratio of Fe : Bi : Sb : O for Fe 2 BiSbO 7 was 2.00 : 0.97 : 1.01 : 6.98 based on our XPS, SEM-EDS and XFS results.Accordingly, it could be deduced that the resulting material was highly pure under our preparation conditions.It was remarkable that there were not any shoulders and widening in the XPS peaks of Fe 2 BiSbO 7 , which suggested the absence of any other phases.3.2.Photocatalytic Properties.Generally, the direct absorption of band-gap photons would result in the generation of electron-hole pairs within Fe 2 BiSbO 7 , subsequently; the charge carriers began to diffuse to the surface of Fe 2 BiSbO 7 .As a result, the photocatalytic activity for decomposing organic compounds with Fe 2 BiSbO 7 might be enhanced.Changes in the UV-Vis spectrum of MB upon exposure to visible light (λ > 400 nm) irradiation with the presence of Fe 2 BiSbO 7 , Bi 2 InTaO 7 , or N-doped TiO 2 indicated that Fe 2 BiSbO 7 , Bi 2 InTaO 7 , or N-doped TiO 2 could photodegrade MB effectively under visible light irradiation.Figure 2or Bi 2 InTaO 7 , or pure TiO 2 as catalyst.The photodegradation rate of MB and the photonic efficiency with N-doped TiO 2 as catalyst were both higher than those with Bi 2 InTaO 7 or pure TiO 2 as catalyst.The photodegradation rate of MB and the photonic efficiency with Bi 2 InTaO 7 as catalyst were both higher than those with pure TiO 2 or the absence of a photocatalyst.The photodegradation rate of MB and the photonic efficiency with pure TiO 2 as catalyst were both higher than those with the absence of a photocatalyst.When Fe 2 BiSbO 7 , N-doped TiO 2 , Bi 2 InTaO 7 or pure TiO 2 was used as photocatalyst, BiSbO 7 could be greatly improved by enhancing the specific surface area of Fe 2 BiSbO 7 .Figure4shows the change of TOC during photocatalytic degradation of MB with Fe 2 BiSbO 7 , Bi 2 InTaO 7 , or Ndoped TiO 2 as catalyst under visible light irradiation.The TOC measurements revealed the disappearance of organic Figure 3: The absorbance pattern of methylene blue photocatalytically degraded by Fe 2 BiSbO 7 (a), and photocatalytic degradation of methylene blue under visible light irradiation in the presence of Fe 2 BiSbO 7 , Bi 2 InTaO 7 , pure TiO 2 , N-doped TiO 2 , as well as in the absence of a photocatalyst (b).
2 BiSbO 7 , or N-doped TiO 2 or Bi 2 InTaO 7 was used as photocatalyst.Consequently, after visible light irradiation for 230 min with Fe 2 BiSbO 7 as catalyst, the entire mineralization of MB was observed because of 100% TOC removal.The turnover number which represented the ratio between the total amount of evolved gas and dissipative catalyst was calculated to be more than 0.204 for BiSbO 7 , Bi 2 InTaO 7 , or N-doped TiO 2 .Monitoring the presence of ions in the solution revealed that the SO 4 2− ion concentration was 0.01849 mM, 0.00924 mM, or 0.00757 mM with Fe 2 BiSbO 7 , N-doped TiO 2 , or Bi 2 InTaO 7 as catalyst after visible light irradiation for 200 min, indicating that 63.22%, 36.94%, or 30.28% of sulfur from MB was converted into sulfate ions with Fe 2 BiSbO 7 , N-doped TiO 2 , or Bi 2 InTaO 7 as catalyst after