Synthesis, Property Characterization and Photocatalytic Activity of the Novel Composite Polymer Polyaniline/Bi2SnTiO7

A novel polyaniline/Bi2SnTiO7 composite polymer was synthesized by chemical oxidation in-situ polymerization method and sol-gel method for the first time. The structural properties of novel polyaniline/Bi2SnTiO7 have been characterized by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy and X-ray spectrometry. The lattice parameter of Bi2SnTiO7 was found to be a = 10.52582(8) Å. The photocatalytic degradation of methylene blue was realized under visible light irradiation with the novel polyaniline/Bi2SnTiO7 as catalyst. The results showed that novel polyaniline/Bi2SnTiO7 possessed higher catalytic activity compared with Bi2InTaO7 or pure TiO2 or N-doped TiO2 for photocatalytic degradation of methylene blue under visible light irradiation. The photocatalytic degradation of methylene blue with the novel polyaniline/Bi2SnTiO7 or N-doped TiO2 as catalyst followed first-order reaction kinetics, and the first-order rate constant was 0.01504 or 0.00333 min−1. After visible light irradiation for 220 minutes with novel polyaniline/Bi2SnTiO7 as catalyst, complete removal and mineralization of methylene blue was observed. The reduction of the total organic carbon, the formation of inorganic products, SO42− and NO3−, and the evolution of CO2 revealed the continuous mineralization of methylene blue during the photocatalytic process. The possible photocatalytic degradation pathway of methylene blue was obtained under visible light irradiation.


Introduction
Dye effluents from textile industries are becoming a serious environmental problem because of their toxicity, high chemical oxygen demand content, and biological degradation [1]. A lot of conventional methods have been proposed to treat industrial effluents, but each method has its shortcomings [1][2][3][4][5][6][7]. In the last decade, photocatalytic degradation processes have been widely applied as techniques for the destruction of organic pollutants in wastewater and effluents, especially degradation of dyes [1,[7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]. Among various dyes, methylene blue (MB) dye is difficult to degrade and is often utilized as a model dye contaminant to evaluate the activity of a photocatalyst both under ultraviolet light irradiation [18,19,22] and under visible light irradiation [20][21][22][23][24]. There are many reports on the photodegradation of MB. Unfortunately, most of these experiments were carried out under ultraviolet light irradiation. Up to now, there were only a few reports for MB dye degradation under visible light irradiation such as the research of Asahi et al. with a reduced TiO x (TiO 2−x N x ) as catalyst and the research of Li et al. with Pt-TiO 2 as photocatalyst [21,24]. Zhang [25] utilized N-doped TiO 2 as catalyst to degrade MB under visible light irradiation and found that the removal ratio of MB was only 35% after visible light irradiation for 180 minutes. It is known that ultraviolet light only occupies 4% of the solar energy spectrum. For this reason, there is great interest in developing new visible light-responsive photocatalysts capable of utilizing the more ample visible light spectrum, which occupies about 43% of the solar energy range. Therefore, there is an urgent need to develop novel visible light-responsive photocatalysts.
With the development of investigations into photocatalysis processes, investigators have also paid considerable attention to researching and developing novel photocatalysts [26][27][28][29]. Moreover, UV-diffuse reflectance spectroscopy and the bandgap of novel photocatalysts [30,31] also play an important role in any photocatalyst system. Up until recently, TiO 2 was the most common photocatalyst, but TiO 2 could not be utilized in the visible light region and could only degrade MB under ultraviolet light irradiation. 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 often considered to have photocatalytic properties under visible light irradiation. In our previous work [32], we had found that Bi 2 InTaO 7 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 by modification of its structure. According to above analysis, we could assume that substitution of Ta 5+ and In 3+ by Sn 4+ and Ti 4+ in Bi 2 InTaO 7 might increase the carrier concentration. As a result, a change and improvement of the electron transport and photophysical properties could be found in the novel Bi 2 SnTiO 7 compound which might display advanced photocatalytic properties. Moreover, owing to the excellent environmental stability of polyaniline, the polyaniline-hybridized Bi 2 SnTiO 7 sample should possess more advanced photocatalytic properties. Bi 2 SnTiO 7 had never been produced and the data about its structural and photophysical properties such as space group and lattice constants had not been previously reported. In addition, the photocatalytic properties of Bi 2 SnTiO 7 had not been investigated by other investigators. The molecular composition of Bi 2 SnTiO 7 was very similar to that of other A 2 B 2 O 7 compounds. Thus the resemblance suggested that Bi 2 SnTiO 7 and the polyaniline-hybridized Bi 2 SnTiO 7 might possess photocatalytic properties under visible light irradiation, like other members in the A 2 B 2 O 7 family. Bi 2 SnTiO 7 also seemed to have potential for improvement of photocatalytic activity by modification of its structure because it had been proved that a slight modification of a semiconductor structure would cause a remarkable change in photocatalytic properties [21]. In this paper, Bi 2 SnTiO 7 was prepared for the first time by the solid-state reaction method and the novel composite polymer polyaniline/Bi 2 SnTiO 7 was synthesized by the chemical oxidation in-situ polymerization method and sol-gel method for the first time. The structural and photocatalytic properties of the polyaniline-hybridized Bi 2 SnTiO 7 were investigated in detail. The photocatalytic degradation of MB under visible light irradiation was also performed to evaluate the photocatalytic activity of the polyaniline-hybridized Bi 2 SnTiO 7 . A comparison among the photocatalytic properties of the polyaniline-hybridized Bi 2 SnTiO 7 , Bi 2 InTaO 7 and N-doped TiO 2 was carried out in order to elucidate the structure-photocatalytic activity relationship in the polyaniline-hybridized Bi 2 SnTiO 7 . Figure 1 presents a TEM image, the selected area electron diffraction pattern and the SEM-EDS spectrum of Bi 2 SnTiO 7 . The TEM image of Bi 2 SnTiO 7 showed that the morphology of the Bi 2 SnTiO 7 particle was roundish and the Bi 2 SnTiO 7 particle size was uniform. It could be seen that the Bi 2 SnTiO 7 particles crystallized well and the average particle size of Bi 2 SnTiO 7 was about 180 nm. The SEM-EDS spectrum of Bi 2 SnTiO 7 revealed that Bi 2 SnTiO 7 was pure phase without any other impurities and Bi 2 SnTiO 7 displayed the presence of bismuth, tin, titanium and oxygen. It could be seen from Figure 1 that Bi 2 SnTiO 7 crystallized with the pyrochlore-type structure, cubic crystal system and space group Fd3m.

Crystal Structure of Bi 2 SnTiO 7
The lattice parameter for Bi 2 SnTiO 7 was proved to be a = 10.52582(8) Å. According to the calculation results from Figure 1, the (h k l) value for the main peaks of Bi 2 SnTiO 7 could be found and indexed.
Full-profile structure refinements of the collected X-ray diffraction data of Bi 2 SnTiO 7 were obtained by the RIETAN TM [33] program, which was based on Pawley analysis. The refinement results of Bi 2 SnTiO 7 are shown in Figure 2. The atomic coordinates and structural parameters of Bi 2 SnTiO 7 are listed in Table 1. The results of the final refinement for Bi 2 SnTiO 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 Bi 2 SnTiO 7 and Bi 2 InTaO 7 crystallized in the same structure, and 2 theta angles of each reflection of Bi 2 SnTiO 7 changed with Sn 4+ and Ti 4+ being replaced by In 3+ and Ta 5+ . Bi 2 InTaO 7 also crystallized with a cubic structure by space group Fd3m and the lattice parameter of Bi 2 InTaO 7 was a = 10.74641(0) Å. The lattice parameter of Bi 2 SnTiO 7 was a = 10.52582(8) Å, which indicated that the lattice parameter of Bi 2 SnTiO 7 decreased compared with the lattice parameter of Bi 2 InTaO 7 because the In 3+ ionic radii (0.92 Å) was larger than the Sn 4+ ionic radii (0.71Å) and the Ta 5+ ionic radii (0.68 Å) was equal to the Ti 4+ ionic radii (0.68Å). The outcome of refinement for Bi 2 SnTiO 7 generated the unweighted R factor, R P = 10.72% with space group Fd3m. Zou et al. [34] 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 . According to the high purity of the precursors which were utilized in this study and the fact which the EDS results 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 Bi 2 SnTiO 7 was owing to a slightly modified structure model for Bi 2 SnTiO 7 . It should be emphasized that the defects or the disorder/order of a fraction of the atoms could cause the change of structures, including different bond-distance distributions, thermal displacement parameters and/or occupation factors for some of the atoms.   In order to reveal the surface chemical compositions and the valence states of various elements of Bi 2 SnTiO 7 , the X-ray photoelectron spectrum of Bi 2 SnTiO 7 for detecting Bi, Sn, Ti and O was performed. The full XPS spectrum confirmed that the prepared Bi 2 SnTiO 7 contained the elements Bi, Sn, Ti and O, which was consistent with the SEM-EDS results. The different elemental peaks for Bi 2 SnTiO 7 which are corresponding to definite bind energies are given in Table 2. The results illustrated that the oxidation states of Bi, Sn, Ti and O ions from Bi 2 SnTiO 7 were +3, +4, +4 and −2, respectively. Moreover, the average atomic ratio of Bi:Sn:Ti:O for Bi 2 SnTiO 7 was 2.00:0.98:1.02:6.97 accoring to 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 Bi 2 SnTiO 7 , which suggested the absence of any other phases.

Photocatalytic Properties
Generally, the direct absorption of band-gap photons would result in the generation of electron-hole pairs within the polyaniline-hybridized Bi 2 SnTiO 7 , subsequently, the charge carriers began to diffuse to the surface of the polyaniline-hybridized Bi 2 SnTiO 7 . As a result, the photocatalytic activity for decomposing organic compounds with the polyaniline-hybridized Bi 2 SnTiO 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 the polyaniline-hybridized Bi 2 SnTiO 7 , Bi 2 InTaO 7 or N-doped TiO 2 indicated that the polyaniline-hybridized Bi 2 SnTiO 7 , Bi 2 InTaO 7 or N-doped TiO 2 could photodegrade MB effectively under visible light irradiation. Figure 3 shows the photocatalytic degradation of methylene blue under visible light irradiation in the presence of the polyaniline-hybridized Bi 2 SnTiO 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 the typical MB peaks at 665 nm and 614 nm was clearly noticed and the photodegradation rate of MB was about 2.046 × 10 −9 mol L −1 s −1 and the photonic efficiency was estimated to be 0.0430% (λ = 420 nm) with the polyaniline-hybridized Bi 2 SnTiO 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.
Furthermore, 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. As a contradistinction, the photodegradation rate of MB within 200 minutes 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 the polyaniline-hybridized Bi 2 SnTiO 7 as catalyst were both higher than those with N-doped TiO 2 or 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 the polyaniline-hybridized Bi 2 SnTiO 7 , N-doped TiO 2 , Bi 2 InTaO 7 or pure TiO 2 was utilized as catalyst, the photodegradation conversion rate of MB was 98.20%, 48.05%, 42.76% and 40.02% after visible light irradiation for 200 minutes, respectively. Furthermore, the photodegradation conversion rate of MB was 32.78% after visible light irradiation for 200 minutes with the absence of a photocatalyst because of the MB dye photo-sensitization effect [35]. After visible light irradiation for 220 minutes with the polyaniline-hybridized Bi 2 SnTiO 7 as catalyst, complete removal of MB was observed and the complete disappearance of the absorption peaks which presented the absolute color change from deep blue into colorless solution occurred.
Bi 2 SbVO 7 , Fe 2 BiSbO 7 and SrFeO 3 were also prepared by using the preparation methods from the literature reports [36][37][38]. Polyaniline-hybridized Bi 2 SnTiO 7 , Bi 2 SbVO 7 , Fe 2 BiSbO 7 and SrFeO 3 were utilized as catalysts to degrade MB under the same experimental conditions which were shown in our paper. The results are shown in Table 3. It could be seen from Table 3 that the removal rate of MB was 98.2%, 96.5%, 95.0% or 59.7% with polyaniline-hybridized Bi 2 SnTiO 7 , Bi 2 SbVO 7 , Fe 2 BiSbO 7 or SrFeO 3 as catalyst within 200 minutes under visible light irradiation. The results showed that the polyaniline-hybridized Bi 2 SnTiO 7 showed the best photocatalytic activity for photodegradation of MB compared with Bi 2 SbVO 7 , Fe 2 BiSbO 7 or SrFeO 3 . Furthermore, our N-doped TiO 2 showed low photocatalytic activity for photodegradation of MB compared with the N-doped TiO 2 which was produced by Yang et al. [39] under visible light irradiation. The reason was that N:TiO 2 molar ratio was different between our N-doped TiO 2 and Yang's N-doped TiO 2 [39], which indicated that our N-doped TiO 2 contained 62% anatase phase and 38% rutile phase, at the same time, Yang's N-doped TiO 2 [39] contained 100% anatase phase. We prepared N-doped TiO 2 by using the preparation method from Yang et al. and we found that N-doped TiO 2 showed similar results for degradation of MB. According to above results, the photocatalytic degradation activity of the polyaniline-hybridized Bi 2 SnTiO 7 was much higher than that of N-doped TiO 2 , Bi 2 InTaO 7 or pure TiO 2 . Meanwhile, N-doped TiO 2 showed higher photocatalytic degradation activity for MB photodegradation compared with Bi 2 InTaO 7 or pure TiO 2 . Bi 2 InTaO 7 showed higher photocatalytic degradation activity for MB photodegradation compared with pure TiO 2 . Pure TiO 2 was more suitable for MB photodegradation than the absence of a photocatalyst. The photocatalytic property of novel polyaniline-hybridized Bi 2 SnTiO 7 under visible light irradiation was amazing compared with that of N-doped TiO 2 or pure TiO 2 , and the main reason was that the specific surface area of the polyaniline-hybridized Bi 2 SnTiO 7 was much smaller than that of N-doped TiO 2 or pure TiO 2 . BET isotherm measurements of the polyaniline-hybridized Bi 2 SnTiO 7 , N-doped TiO 2 and pure TiO 2 provided a specific surface area of 4.29 m 2 g −1 , 45.53 m 2 g −1 and 46.24 m 2 g −1 respectively, which indicated that the photocatalytic degradation activity of the polyaniline-hybridized Bi 2 SnTiO 7 could be improved consumedly by enhancing the specific surface area of the polyaniline-hybridized Bi 2 SnTiO 7 . Figure 4 shows the change of TOC during photocatalytic degradation of MB with the polyanilinehybridized Bi 2 SnTiO 7 , Bi 2 InTaO 7 or N-doped TiO 2 as catalyst under visible light irradiation. The TOC measurements revealed the disappearance of organic carbon when the MB solution which contained the polyaniline-hybridized Bi 2 SnTiO 7 , Bi 2 InTaO 7 or N-doped TiO 2 was exposed under visible light irradiation. The results showed that 96.77% or 46.77% or 41.71% of TOC decrease was obtained after visible light irradiation for 200 minutes when the polyaniline-hybridized Bi 2 SnTiO 7 or N-doped TiO 2 or Bi 2 InTaO 7 was utilized as photocatalyst. Consequently, after visible light irradiation for 220 minutes with the polyaniline-hybridized Bi 2 SnTiO 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 0.258 for the polyaniline-hybridized Bi 2 SnTiO 7 after 200 minutes 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. Figure 5 shows the amount of CO 2 which was released during the photodegradation of MB with the polyaniline-hybridized Bi 2 SnTiO 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 the polyaniline-hybridized Bi 2 SnTiO 7 , Bi 2 InTaO 7 or N-doped TiO 2 . At the same time, after visible light irradiation of 200 minutes, the CO 2 production of 0.11129 mmol with the polyaniline-hybridized Bi 2 SnTiO 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 of 200 minutes, the CO 2 production of 0.05600 mmol with N-doped 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 the polyaniline-hybridized Bi 2 SnTiO 7 or 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 the polyaniline-hybridized Bi 2 SnTiO 7 or Bi 2 InTaO 7 or N-doped TiO 2 . Here, C represented the MB concentration at time t, and C o represented the initial MB concentration, and 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 0.01504 min −1 with the polyaniline-hybridized Bi 2 SnTiO 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 the polyaniline-hybridized Bi 2 SnTiO 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 N-doped 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 0.01290 min −1 with the polyaniline-hybridized Bi 2 SnTiO 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 the polyaniline-hybridized Bi 2 SnTiO 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 .   It could be seen from Figure 8  . At the same time, nitrogen atoms in the −3 oxidation state prduced NH 4 + cations that subsequently were oxidized into NO 3− ions. As expected, the formation kinetics with the polyaniline-hybridized Bi 2 SnTiO 7 was significantly faster than that of N-doped TiO 2 or Bi 2 InTaO 7 by using the same amount of photocatalyst. Moreover, the formation kinetics with N-doped TiO 2 was faster than that of Bi 2 InTaO 7 when using the same amount of photocatalyst. It was noteworthy 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 [40]. 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 molecule of MB was converted to 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 the polyaniline-hybridized Bi 2 SnTiO 7 as catalyst was observed under visible light irradiation. A clear photonic efficiency (0.0112% at its maximal point) at wavelengths which corresponded to sub-Eg energies of the photocatalysts (λ from 490 nm to 800 nm) was observed for the polyaniline-hybridized Bi 2 SnTiO 7 . 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 MB clearly demonstrated that any photodegradation results at wavelengths above 490 nm should be attributed to photosensitization effect by the dye MB itself (Scheme 2). According to the mechanism which was shown in Scheme 2, MB which was adsorbed on the polyaniline-hybridized Bi 2 SnTiO 7 was excited by visible light irradiation. Subsequently, an electron was injected from the excited MB to the conduction band of the polyaniline-hybridized Bi 2 SnTiO 7 where the electron was scavenged by molecular oxygen. Scheme I explained the results which were obtained with the polyaniline-hybridized Bi 2 SnTiO 7 as catalyst under visible light irradiation, where the polyaniline-hybridized Bi 2 SnTiO 7 could serve to reduce recombination of photogenerated electrons and holes by scavenging of electrons [41].
Below 490 nm, the situation was different. The results of photonic efficiency correlated well with the absorption spectra of the polyaniline-hybridized Bi 2 SnTiO 7 . These results evidently showed that the mechanism which was responsible for the photodegradation of MB went through band gap excitation of the polyaniline-hybridized Bi 2 SnTiO 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 Visible light the observed mechanism for the polyaniline-hybridized Bi 2 SnTiO 7 under supra-bandgap irradiation, namely Scheme 3: Scheme 3. The observed mechanism for the polyaniline-hybridized Bi 2 SnTiO 7 under supra-bandgap irradiation.
Polyaniline-hybridized Bi 2 SnTiO 7 h + + e ─ (4) According to first principles calculations, we deduced that the conduction band of the polyaniline-hybridized Bi 2 SnTiO 7 was composed of Ti 3d and Sn 5p orbital component, and the valence band of the polyaniline-hybridized Bi 2 SnTiO 7 was composed of O 2p and Bi 6s orbital component. The polyaniline-hybridized Bi 2 SnTiO 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 the polyaniline-hybridized Bi 2 SnTiO 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 to 180°, the more delocalized was the excited state [42]. 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. According to above results, the lattice parameter a = 10.52582(8) Å for Bi 2 SnTiO 7 was smaller than the lattice parameter α = 10.70352(7) Å for Bi 2 InTaO 7 , thus the photoinduced electrons and holes inside Bi 2 SnTiO 7 was easier and faster to reach the reactive sites on the catalyst surface compared with those of Bi 2 InTaO 7 , which showed the photocatalytic degradation activity of Bi 2 SnTiO 7 was higher than that of Bi 2 InTaO 7 . For the polyaniline-hybridized Bi 2 SnTiO 7 in this experiment, the Sn-O-Ti bond angle was 140.196° and the In-O-Ta bond angle was 118.764°, indicating that the Sn-O-Ti or In-O-Ta bond angle was close to 180°. Thus the photocatalytic activity of the polyaniline-hybridized Bi 2 SnTiO 7 was correspondingly higher. The crystal structures of Bi 2 SnTiO 7 and Bi 2 InTaO 7 were the same, but their electronic structures were considered to be somewhat different. For Bi 2 SnTiO 7 , Ti was 3d-block metal element, and Bi was 6p-block rare earth metal element, and Sn was 5p-block metal element, but for Bi 2 InTaO 7 , Ta was 5d-block metal element, and In was 5p-block metal element, indicating that the crystal structure and the electronic structure of the photocatalysts affect the photocatalytic activity. Based on above analysis, the difference of photocatalytic MB degradation between Bi 2 SnTiO 7 and Bi 2 InTaO 7 can be attributed mainly to the difference in their crystalline structure and electronic structure. The crystal structure and the electronic structure of the polyaniline-hybridized Bi 2 SnTiO 7 and N-doped TiO 2 were totally different. For N-doped TiO 2 , Ti was 3d-block metal element, indicating that the different photodegradation effect of MB between the polyaniline-hybridized Bi 2 SnTiO 7 and N-doped TiO 2 could be attributed mainly to the difference of their crystalline structure and electronic structure.
The present results indicated that the polyaniline-hybridized Bi 2 SnTiO 7 -visible light photocatalysis system might be regarded as a practical method for treatment of diluted colored waste water. This system could be utilized for decolorization, purification and detoxification of textile, printing and dyeing Visible light 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 polyaniline-hybridized Bi 2 SnTiO 7 -visible light photocatalysis system might provide a valuable treatment for purifying and reusing colored aqueous effluents.

Synthesis of the Polyaniline-hybridized Bi 2 SnTiO 7 and N-doped TiO 2
Bi 2 SnTiO 7 powder was first synthesized by the solid-state reaction method. TiO 2 , SnO 2 , Bi 2 O 3, In 2 O 3 and Ta 2 O 5 with 99.99% purity purchased from Sinopharm Group Chemical Reagent Co. (Shanghai, China) were used as raw materials that were used without further purification. All powders were dried at 200 °C for 4 h before synthesis. In order to synthesize Bi 2 SnTiO 7 , the precursors were stoichiometrically mixed in a quartz mortar, subsequently pressed into small columns and put into an alumina crucible. Finally, calcination was carried out at 1,100 °C for 30 h in an electric furnace (KSL 1700X, Hefei Kejing Materials Technology CO., LTD, China). Similarly, Bi 2 InTaO 7 was synthesized by calcination at 1,050 °C for 46 h. After sintering and grounding within a quartz mortar, ultrafine Bi 2 SnTiO 7 powder was fabricated.
A polyaniline-hybridized Bi 2 SnTiO 7 sample was prepared as follows: an amount of distilled aniline was added to 150 mL of 1M HCl, and subsequently stirred for 30 minutes to ensure that the aniline was totally dissolved. Subsequently, a certain percentage of Bi 2 SnTiO 7 was added into above solution, sonicated for 30 minutes to obtain a dispersed solution, and then stirred for 1 h. Thirdly, 0.5 g mL −1 ammonium thiosulphate (HCl) was added into the solution slowly, subsequently the mixture was stirred for 24 h. Finally, the suspension was filtered, and the precipitate was washed with alcohol and water for many times and dried at 60 °C to obtain polyaniline-hybridized-Bi 2 SnTiO 7 .
Nitrogen-doped titania (N-doped TiO 2 ) catalyst with tetrabutyl titanate as a titanium precursor was prepared by using the sol-gel method at room temperature. Tetrabutyl titanate (17 mL) and absolute ethyl alcohol (40 mL) were mixed as solution a; subsequently solution a was added dropwise under vigorous stirring into solution b that contained absolute ethyl alcohol (40 mL), glacial acetic acid (10 mL) and double distilled water (5 mL) to form a transparent colloidal suspension c. Subsequently aqueous ammonia with N/Ti proportion of 8 mol% was added into the resulting transparent colloidal suspension under vigorous stirring and stirred for 1 h. Finally, the xerogel was formed after being aged for 2 days. The xerogel was ground into powder which was calcined at 500 °C for 2 h, subsequently above powder was ground in agate mortar and screened by shaker to obtain N-doped TiO 2 powders.

Characterization of the Polyaniline-hybridized Bi 2 SnTiO 7
The crystalline phase of Bi 2 SnTiO 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 step-scan procedure in the range of 2θ = 10-100°. The step interval was 0.02° and the time per step was 1.2 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., U.K.). The particle morphology of Bi 2 SnTiO 7 was measured by transmission electron microscope (Tecnal F20 S-Twin, FEI Corporation, USA). The Bi 3+ content, Sn 4+ content, Ti 4+ content and O 2− content of Bi 2 SnTiO 7 and the valence state of elements were also analyzed by X-ray photoelectron spectroscopy (XPS). The chemical composition within the depth profile of Bi 2 SnTiO 7 was examined by the argon ion denudation method when X-ray photoelectron spectroscopy was utilized. The surface areas of Bi 2 SnTiO 7 and N-doped TiO 2 were measured 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, U.K.).

Photocatalytic Activity Tests
The photocatalytic activity of the polyaniline-hybridized Bi 2 SnTiO 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 following: the lamp is put into a quartz hydrazine which is a hollow structure and located in the middle of the reactor. The recirculating water through the reactor maintains a near constant reaction temperature (20 °C) and the solution was continuously stirred and aerated. Twelve holes which were utilized to put quartz tubes evenly distributed around the lamp and the distance between the lamp and each hole was equal. Under magnetic stirring, the photocatalyst within the MB solution was in the state of suspension. In this paper, the photocatalytic degradation of the MB solution was performed with 0.3 g polyaniline-hybridized Bi 2 SnTiO 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 minutes to ensure establishment of an adsorption/desorption equilibrium among the polyaniline-hybridized Bi 2 SnTiO 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. 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. pH 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 liquid chromatograph-mass 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 post-photocatalysis 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 5,000 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 TM 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 [43,44]: where  was the photonic efficiency (%), R was the rate of MB degradation (Mol L −1 s −1 ), and I o was the incident photon flux (Einstein L −1 s −1 ).

Conclusions
The polyaniline-hybridized Bi 2 SnTiO 7 was prepared for the first time by a chemical oxidation in-situ polymerization method and sol-gel method. The structural and photocatalytic properties of the polyaniline-hybridized Bi 2 SnTiO 7 were investigated. XRD results indicated that Bi 2 SnTiO 7 crystallized with the pyrochlore-type structure, cubic crystal system and space group Fd3m. The lattice parameter of Bi 2 SnTiO 7 was found to be a = 10.52582(8) Å. Photocatalytic decomposition of aqueous MB was realized under visible light irradiation in the presence of the polyaniline-hybridized Bi 2 SnTiO 7 , Bi 2 InTaO 7 or N-doped TiO 2. The results showed that the polyaniline-hybridized Bi 2 SnTiO 7 possessed 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 the polyaniline-hybridized Bi 2 SnTiO 7 , Bi 2 InTaO 7 or N-doped TiO 2 as catalyst followed first-order reaction kinetics, and the first-order rate constant was 0.01504 min −1 or 0.00275 min −1 or 0.00333 min −1 . Complete removal and mineralization of MB was observed after visible light irradiation for 220 minutes with the polyaniline-hybridized Bi 2 SnTiO 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. The polyaniline-hybridized Bi 2 SnTiO 7 /(visible light) photocatalysis system was found to be suitable for textile industry wastewater treatment and could be utilized to solve other environmental chemical pollution problems.