Photophysical and Photocatalytic Properties of Novel M 2 BiNbO 7 ( M = In and Ga )

Os óxidos M 2 BiNbO 7 (M = In e Ga) foram sintetizados através de reações no estado sólido, e suas propriedades estruturais e fotocatalíticas, investigadas. Os resultados indicaram que estes compostos cristalizam na estrutura do tipo pirocloro, no sistema cúbico, grupo espacial Fd-3m. Os valores estimados dos “band gaps” dos óxidos In 2 BiNbO 7 e Ga 2 BiNbO 7 são 2,52(5) e 2,57(8) eV, respectivamente. A reação fotocatalítica da decomposição de água pura foi estudada na presença dos fotocatalisadores M 2 BiNbO 7 (M = In e Ga) e irradiação no ultravioleta, através do monitoramento da formação de H 2 e de O 2 . A degradação fotocatalítica do corante azul de metileno em água, na presença destes óxidos, foi investigada sob irradiação no visível. Os catalisadores M 2 BiNbO 7 (M = In e Ga) mostraram-se mais ativos do que o P-25, nessas condições. Completa degradação do azul de metileno foi observada após irradiação no visível durante 160 minutos, na presença do fotocatalisador Ga 2 BiNbO 7 , e após 180 minutos na presença de In 2 BiNbO 7 . A diminuição do teor total de carbono (TOC) e a formação dos produtos SO 4 2– e NO 3 – confirmaram a mineralização do azul de metileno durante o processo fotocatalítico.


Introduction
Since Honda and Fujishima first observed the splitting of water on TiO 2 electrode in 1972, 1 the investigation of semiconductor photocatalysts has attracted much attention from both academic and industrial societies. 2,33][4] Up to now, some photocatalysts with different structures have been synthesized to investigate the effective utilization of solar energy.Among them, some Nb-containing photocatalysts with a pyrochlore-type structure were reported recently, such as Bi 2 MNbO 7 (M = Al 3+ , Ga 3+ , In 3+ ) 5 and Bi 2 RNbO 7 (R = Y, rare earth elements). 68][9] In particular, it was reported that 15% of the total world production of dyes is lost during the dyeing process and is released to the textile effluents, which eventually pollute the groundwater.The release of those colored waste waters in the ecosystem is a dramatic source of non-aesthetic pollution, eutrophication and perturbations in the aquatic life.Many reports have revealed that the organic dyes could be effectively degraded using the TiO 2 -based photocatalytic process; however, the degradation of a majority of organic dyes are only under UV irradiation except for some dyes, such as alizarin red, which can be degraded under visible light using the TiO 2 -based photocatalysts based on the dyesensitized process. 10,11Among different dyes, methylene blue dye (MB) is difficult to be decomposed under visible light irradiation and is usually regarded as a model dye contaminant to evaluate the activity of a photocatalyst. 12,13p to now, there were only few reports of MB dye degradation under visible light irradiation. 12,14Therefore, it is highly desirable to develop new visible light-driven photocatalysts with high activity.
It has been generally observed that numerous compounds with the A 2 B 2 O 7 pyrochlore structure display antiferroelectric phases or dielectric abnormality.However, only a few compounds display ferroelectric behavior. 15,16M 2 BiNbO 7 (M = In and Ga) belongs to the family of the A 2 B 2 O 7 compounds, but the data about its space group and lattice constants have not been reported previously.Moreover, no photocatalytic properties of M 2 BiNbO 7 (M = In and Ga) have been investigated so far.We considered that In 3+ or Ga 3+ occupying the A site and Bi 3+ occupying the B site in the A 2 3+ B 2 4+ O 7 compounds may lead to an increase in hole (carrier) concentration, and thus result in a change in the electrical transportation and photophysical properties.We also speculate that M 2 BiNbO 7 (M = In and Ga) might yield a slight modification of crystal structure and result in a change in photophysical properties.It is noteworthy that a slight modification in the structure of a semiconductor will lead to a marked change in photocatalytic properties. 8In this contribution, we prepared the M 2 BiNbO 7 (M = In and Ga) photocatalysts and the structural and photocatalytic properties of M 2 BiNbO 7 (M = In and Ga) were studied in detail.A comparison of the photocatalytic property of M 2 BiNbO 7 (M = In and Ga) with that of TiO 2 (P-25) is also provided.

Experimental
The polycrystalline samples of the photocatalysts were synthesized by a solid-state reaction method.Ga 2 O 3 , In 2 O 3 , Bi 2 O 3 and Nb 2 O 5 (China Medicine (Group) Shanghai Chemical Reagent Corporation ) with purity of 99.99% were used as starting materials.The powders were dried at 200 o C for 4 h.Then the stoichiometric amounts of precursors were mixed and pressed into small columns.At last the small columns were sintered at 1100 o C for 52 h in an alumina crucible (ShenYang Crucible Co., LTD, China) with an electric furnace (KSL 1700X, Hefei Kejing Materials Technology CO., LTD, China).The crystal structure of M 2 BiNbO 7 (M = In and Ga) was analyzed by the X-ray diffractometer (D/MAX-RB, Rigaku Corporation, Japan) with CuKα radiation (λ = 1.54056).The data were collected at 295 K with a step scan procedure in the range of 2θ = 5-100°.The step interval was 0.02° and the scan speed was 1° min -1 .The chemical composition of the compound was measured by scanning electron microscope-X-ray energy dispersion spectrum (SEM-EDX, (LEO 1530VP, LEO Corporation, Germany)) and X-ray Fluorescence spectrometer (ARL-9800, ARL Corporation, Switzerland).The optical absorption of M 2 BiNbO 7 (M = In and Ga) was analyzed with an UV-Visible spectrophotometer (Lambda 35, Perkin-Ebmer Corporation, USA).The surface areas were determined using the BET method (MS-21, Quantachrome Instruments Corporation, USA) with N 2 adsorption at liquid nitrogen temperature.
The photocatalytic degradation of aqueous MB was performed with 0.5 g Ga 2 BiNbO 7 or In 2 BiNbO 7 or TiO 2 powders suspended in 100 mL methylene blue solution (MB solution concentration was 0.0506 mol m -3 and the initial pH value of the solution was 7) in a pyrex glass cell (Jiangsu Yancheng Huaou Industry, China ).The photocatalytic reaction system consisted of a 300 W Xe arc lamp (Nanjing JYZCPST CO., LTD) and a cut-off filter (λ > 420 nm, Jiangsu Nantong JSOL Corporation, China).The concentration of MB was determined with a UV-Vis spectrometer (UV-2201, Shimadzu Corporation, Japan) with the detecting wavelength at 670 nm.The inorganic products of MB degradation were detected by ion chromatograph (DX-300, Dionex Corporation, USA).Total organic carbon (TOC) was determined with a TOC analyzer (TOC-5000, Shimadzu Corporation, Japan).J. Braz.Chem.Soc.
The photocatalytic water splitting with M 2 BiNbO 7 (M = In and Ga) as the photocatalysts was carried out in pure water (1.0 g powder catalyst, 300 mL H 2 O) under UV irradiation.The catalysts were suspended in pure water by a magnetic stirrer and the photocatalytic reaction was conducted in a gas closed circulation system with an inner-irradiation type quartz cell and a 400 W high-pressure Hg lamp (Beijing Dongsheng Glass Light Source Factory, China).

Structural properties
Figure 1 shows X-ray diffraction patterns of M 2 BiNbO 7 (M = In and Ga) sintered at 1100 o C in air.The powder X-ray diffraction analysis showed that M 2 BiNbO 7 (M = In and Ga) are single phase, which is consistent with the results from SEM-EDX.The chemical composition of M 2 BiNbO 7 (M = In and Ga) was measured with the ZAF (element number, absorption and fluorescence corrections) quantification method.The SEM-EDX analysis revealed that M 2 BiNbO 7 (M = In and Ga) had a homogenous atomic distribution with no other impure elements.An average atomic rate of Ga: Bi: Nb = 2.00: 0.98: 1.02 for Ga 2 BiNbO 7 and In: Bi: Nb = 2.00: 0.97: 1.03 for In 2 BiNbO 7 was obtained from measurements at different points.The results are in good agreement with the measurement from X-ray fluorescence spectrometer.Based on the above results, we can conclude that the resulting materials are of high purity under our preparation conditions.The morphology of M 2 BiNbO 7 (M = In and Ga) is described in Figure 2. It was shown that the particle distribution was homogeneous and the average particle diameters of In 2 BiNbO 7 and Ga 2 BiNbO 7 were estimated to be 1.5 and 1.7 μm.
Full-profile structure refinement of the collected powder diffraction data for M 2 BiNbO 7 (M = In and Ga) was conducted using the Rietveld program REITAN, 17 by which positional parameters and isotropic thermal parameters of M 2 BiNbO 7 (M = In and Ga) were refined.The atomic coordinates and isotropic thermal parameters of M 2 BiNbO 7 (M = In and Ga) are listed in Table 1 and Table 2.The result of the final refinement for M 2 BiNbO 7 (M = In and Ga) indicated a good agreement between the observed and calculated intensities in the pyrochlore type crystal structure of the cubic system with space group Fd-3m when the O atoms are included in the model.The lattice parameter is found to be a = 10.4685(5)Å for Ga 2 BiNbO 7 and a =  10.7146(5)Å for In 2 BiNbO 7 .All the diffraction peaks for M 2 BiNbO 7 (M = In and Ga) could be successfully indexed based on the lattice constant and the space group mentioned above.Our X-ray diffraction results shows that Ga 2 BiNbO 7 and In 2 BiNbO 7 crystallize in the same structure, and 2θ angles of each reflection of In 2 BiNbO 7 change with In 3+ being substituted by Ga 3+ .The lattice parameter decrease from α = 10.7146(5)Å for In 2 BiNbO 7 to α = 10.4685(5)Å for Ga 2 BiNbO 7 , which indicates a decrease in lattice parameter of the photocatalyst with decrease of the M ionic radii, Ga 3+ (0.62 Å) < In 3+ (0.80 Å).The outcome of refinements for In 2 BiNbO 7 and Ga 2 BiNbO 7 generated the unweighted R factors, R P = 12.93% and 12.47% in space group Fd-3m when the O atoms are included in the model.Bernard et al. 15 studied Bi 2 CrNbO 7 , Bi 2 InNbO 7 and Bi 2 FeSbO 7 and also observed the large R factors (15% to 20%).Zou et al. 4 refined the crystal structure of Bi 2 InNbO 7 and obtained a large R factor (15.5%) for Bi 2 InNbO 7 , which was ascribed to a slightly modified structure model for Bi 2 InNbO 7 .Note that the precursors with high purity were used in this study.The influence of minor impurities on the structure of M 2 BiNbO 7 (M = In and Ga) can be excluded, which was further supported by the fact that no impurities were detected by EDX analysis.Therefore, we speculate that the slight high R factors for M 2 BiNbO 7 (M = In and Ga) are resulted from a slightly modified structure model for M 2 BiNbO 7 (M = In and Ga).It should be emphasized that the defects or the disorder/order of a fraction of the atoms can lead to the change of structures, including different bond-distance distributions, thermal displacement parameters and/or occupation factors for some of the atoms. 18

Photophysical properties
Figure 3 shows the results of diffuse reflection spectra of the cubic M 2 BiNbO 7 (M = In and Ga) photocatalysts.In contrast to the well-known TiO 2 whose absorption edge is at about 400 nm, the newly synthesized In 2 BiNbO 7 and Ga 2 BiNbO 7 showed obvious absorption in the visible light region up to 491 and 481 nm (Obtained according to the band gaps of Ga 2 BiNbO 7 (E g = 2.57(8) eV) and In 2 BiNbO 7 (E g = 2.52(5) eV).Then use formula E g = hcλ −1 ) which indicates that M 2 BiNbO 7 (M = In and Ga) have the ability to respond to the wavelength of visible light region.Furthermore, the attribution of the second band for In 2 BiNbO 7 at about 550 nm is possibly owing to defect energy level within crystal lattice of In 2 BiNbO 7 such as oxygen vacancy energy level.It is noteworthy that the band gaps of Ga 2 BiNbO 7 and In 2 BiNbO 7 are estimated to be 2.57(8) and 2.52(5) eV, indicating narrower band gaps compared to that of Bi 2 InTaO 7 (2.92 eV). 19This may imply that the photoabsorption of M 2 BiNbO 7 (M = In and Ga) is stronger than that of Bi 2 InTaO 7 , which may result in a higher photocatalytic activity of M 2 BiNbO 7 (M = In and Ga) than that of Bi 2 InTaO 7 .In principle, the photoabsorption of the photocatalyst depends on the mobility of electron-hole pairs, which determines the probability of electrons and holes to reach reaction sites on the surface of the photocatalyst.

Photocatalytic degradation of methylene blue
In order to know if the photoreaction is induced by light, we studied the effect of the light wavelength on MB degradation.Figure 4 shows dependence of methylene blue degradation on the light wavelength after light irradiation for 90 min over M 2 BiNbO 7 (M = In and Ga) using different cut-off filters.The results showed that the photocatalytic activity of M 2 BiNbO 7 (M = In and Ga) decreased with increasing light wavelength, indicating that the change of the photocatalytic properties over M 2 BiNbO 7 (M = In and Ga) was closely relevant to light wavelength.As a result, the change of the light wavelength will influence directly the amount of photons which participate in the photoreaction.At the same time, photocatalytic degradation of MB could not occur under the dark condition.Thus we may deduce that MB degradation over M 2 BiNbO 7 (M = In and Ga) was induced by light.Furthermore, it can be seen from Figure 4 that Ga 2 BiNbO 7 showed higher photocatalytic activity (47.4% MB degradation, λ>390 nm; 37.2% MB degradation, λ>420 nm) compared with In 2 BiNbO 7 (41.7%MB degradation, λ>390 nm; 34.4% MB degradation, λ>420 nm) not only in UV light region, but also in visible light region.
MB degradation with M 2 BiNbO 7 (M = In and Ga) or TiO 2 (P-25) as the photocatalysts under visible light irradiation (λ > 420 nm) are shown in Figure 5.The results showed that the solution color changed from deep blue to colorless and MB concentration in the solution was not detectable after visible light irradiation for 160 min with Ga 2 BiNbO 7 as the photocatalyst.The initial rate of MB degradation was about 5.271×10 -6 mol s -1 m -3 .Simultaneously, a SO 4 2-ion concentration of 0.0351 mol m -3 was detected in the solution after the photocatalytic reaction for 180 min, indicating that 69.4% of sulphur from MB was turned into sulphate ion.It was obvious that aqueous MB was mainly mineralized rather than bleached under our experimental conditions.The results also showed that MB concentration in the solution was not detectable after visible light irradiation for 180 min with In 2 BiNbO 7 as the photocatalyst.The initial rate of MB degradation was 4.685×10 -6 mol s -1 m -3 and a SO 4 2-ion concentration of 0.0324 mol m -3 was detected in the solution after the photocatalytic reaction, indicating that 64.0% of sulphur from MB was converted into sulphate ion.
In comparison, aqueous MB concentration decreased only from 0.0506 to 0.0358 mol m -3 after visible light irradiation for 180 min with TiO 2 as the catalyst, and no SO 4 2-ion was detected in the solution after the photoreaction.Photobleaching of MB (MB photolysis) in the absence of catalyst was also carried out under visible light irradiation, as shown in Figure 5.The result indicated that the rate of MB photolysis was almost the same as that of MB degradation with TiO 2 as the catalyst, suggesting that TiO 2 was inactive to MB photocatalytic degradation under visible light irradiation. 12Liu et al. 20 and Xu and Langford 21 studied that alizarin red and X3B dyes could be decomposed over TiO 2 based on visible light driven dye-sensitized phenomena.Tang et al. 8 reported that photocatalytic degradation of MB over TiO 2 was also owing to dye-sensitized process under visible light irradiation.Based on above researches, we can draw a conclusion that the effect of dye-sensitized process on photocatalytic degradation of MB over TiO 2 is a little better than the effect of low capacity of visible light irradiation  to penetrate in a media that contains a fine suspension of TiO 2 .Thus the rate of MB photolysis was almost the same as that of MB degradation with TiO 2 as the catalyst.
The ultimate aim of the photodegradation of organic pollutants is to completely convert the toxic organic compounds into inorganics, such as CO 2 , SO 4 2-or NO 3 -.In the presence of M 2 BiNbO 7 (M = In and Ga), the dependence of MB degradation products on the irradiation time is compared in Figure 6.It can be seen that the concentration of SO 4 2-or NO 3 -ions increases with the increase of irradiation time.Note that the amount of SO 4 2ions released into the solution is lower than that expected from stoichiometry.The first possible reason is the loss of sulfur-containing volatile compounds such as SO 2 .The second probable explanation is given by the partially irreversible adsorption of some SO 4 2-ions on the surface of the photocatalyst as already observed. 22However, the partial irreversible adsorption of SO 4 2-ions does not restrain the photocatalytic degradation of pollutants. 22The higher amount of NO 3 -ions is owing to the stoichiometric ratio N/S =3 in the initial MB molecule.
In order to monitor whether MB is mineralized or not, the total organic carbon (TOC) was followed during visible light irradiation and the result is shown in Figure 7.The results showed that in the presence of Bi 2 InTaO 7 26.7% of TOC decrease was obtained after visible light irradiation for 180 min.On the contrary, in the presence of Ga 2 BiNbO 7 , a significantly enhanced decrease of the TOC (98.3%) was obtained after 180 min of visible light irradiation.Consequently, the complete mineralization of MB was achieved after 190 min of visible light irradiation in the presence of Ga 2 BiNbO 7 .Similarly, we also found a decrease of TOC by 96.8% after 180 min of visible light irradiation with In 2 BiNbO 7 as the photocatalyst.

Photocatalytic water splitting
Figure 8 shows the photocatalytic H 2 evolution from pure water under UV light irradiation over the M 2 BiNbO 7 (M = In and Ga) photocatalysts.H 2 evolution rates and some physical properties are listed in Table 3.It can be seen from Figure 8 that the activities of M 2 BiNbO 7 (M = In and Ga) are different and the results are listed in Table 3.It was found that H 2 evolution rates are estimated to be 72.6 μmol h -1 for Ga 2 BiNbO 7 and 54.3 μmol h -1 for In 2 BiNbO 7 , indicating that Ga 2 BiNbO 7 exhibits a larger activity than In 2 BiNbO 7 .The influence of the UV light irradiation was also investigated by light on/off shutter studies over M 2 BiNbO 7 (M = In and Ga).The H 2 evolution stopped by terminating the UV light irradiation, indicating that the reactions of H 2 evolution were initiated by UV light   irradiation.In the second run, almost the same H 2 evolution rate was obtained after the system was evacuated.In order to compare the catalytic activities of M 2 BiNbO 7 (M = In and Ga) with that of TiO 2 , water splitting with P25 as the catalyst was conducted.In the presence of P25, the rate of H 2 evolution from pure water was about 1.4 μmol h -1 in the first 15 h, which shows much lower activity than that of M 2 BiNbO 7 (M = In and Ga).
Based on the observed H 2 and O 2 evolution from pure water, it can be concluded that the conduction band levels of M 2 BiNbO 7 (M = In and Ga) are more negative than that of H 2 evolution and the valence band levels are more positive than that of O 2 evolution.Namely, M 2 BiNbO 7 (M = In and Ga) have proper band structures for the reduction of H + to H 2 and oxidation of H 2 O to O 2 , respectively.Figure 9 shows suggested band structures of M 2 BiNbO 7 (M = In and Ga).Recently, the electronic structures of InMO 4 (M = V, Nb and Ta) and BiVO 4 were reported by Oshikiri et al. based on the first principles calculations. 23The conduction bands of the InMO 4 (M = V, Nb and Ta) photocatalysts are composed of a small indium 5s orbital component (about 20%) and a dominant d orbital component coming from vanadium 3d, niobium 4d and tantalum 5d orbitals, respectively.The valence bands of the BiVO 4 photocatalyst are composed of a small Bi 6s orbital component and a dominant O 2p orbital component.The band structures and valence band levels of M 2 BiNbO 7 (M = In and Ga) should be similar to InMO 4 (M = V, Nb and Ta) and BiVO 4 due to their similar distorted pyrochlore-type structure.Therefore, we conclude that the conduction band of In 2 BiNbO 7 is consisted of Nb 4d and In 5s.The valence band of In 2 BiNbO 7 is consisted of a small Bi 6s orbital component and a dominant O 2p orbital component.Similarly, the conduction band of Ga 2 BiNbO 7 is consisted of Nb 4d and Ga 4s.The valence band of Ga 2 BiNbO 7 is almost the same as that of In 2 BiNbO 7 .
These photocatalysts consist of a three-dimensional network structure of corner-linked MO 6 (M = Bi, Nb) octahedra and the MO 6 octahedra are connected into chains with In 3+ ions or Ga 3+ ions.The shapes of AO 8 and BO 6 polyhedra vary with the O(48f) parameter x in the pyrochlore-type A 2 B 2 O 7 structure.The O(48f) parameter x is 0.375 when the O(48f) atoms are located at the position of the related fluorite-type structure. 24Thus, information on the lattice distortion can be obtained from the O(48f) parameter x in the pyrochlore-type A 2 B 2 O 7 structure.The lattice distortion is defined according to the distortion of BO 6 polyhedral from the regular octahedral.The O(48f) parameter x of these photocatalysts were attained from the Rietveld structure refinement and the results are described in Table 3.The lattice distortion was estimated to be 0.104(4) for Ga 2 BiNbO 7 and 0.054(3) for In 2 BiNbO 7 because the lattice distortion is equal to 0.375-the O(48f) parameter x.During the process of photocatalytic water splitting into H 2 and O 2 , charge separation is necessary to inhibit the recombination of the photoinduced electrons and holes.The lattice distortion is one important parameter for charge separation, and will result in the enhanced photocatalytic activity. 25,26In other words, for the photocatalysts with same crystal and electronic structure, the higher photocatalytic activity is mainly resulted from the larger lattice distortion.This conclusion is confirmed by the fact that Ga 2 BiNbO 7 with larger lattice distortion (0.104( 4)) shows higher photocatalytic activity compared to In 2 BiNbO 7 with the lattice distortion of 0.054 (3).
The research on the luminescent properties has given a conclusion that the closer the angle between the corner-linked octahedral is to 180°, the more the excited  state is delocalized. 27This indicates that the photoinduced electrons and holes can move easily if the angle between the corner-linked octahedral is close to 180 o .The mobility of the photoinduced electrons and holes also influences the photocatalytic activity because it influences the probability of electrons and holes to reach reaction sites on the catalyst surface.
The angles between the corner-linked MO 6 (M = Bi and Nb) octahedral, i.e. the M-O1-M bond angles were attained by the Rietveld structure refinement and the results are shown in Table 3. Comparing the M-O1-M bond angles and the photocatalytic activities of Ga 2 BiNbO 7 with those of In 2 BiNbO 7 , we can find that the closer the M-O1-M bond angle is to 180 o , the higher the photocatalytic activity is.The crystal structures of these photocatalysts are almost the same, but their electronic structures are considered to be different.For the M 2 BiNbO 7 (M = In and Ga) photocatalysts, indium and gallium are p-block metal elements, indicating that the photocatalytic activity may be affected by not only the crystal structure but also the electronic structure of the photocatalysts.Both of the lattice distortion and the angles between the corner-linked MO 6 (M = Bi and Nb) octahedral are possible to influence the photocatalytic activities of M 2 BiNbO 7 (M = In and Ga).
Although direct absorption of photons by the semiconductor oxide can produce electron-hole pairs in the catalysts, the gases evolution (H 2 or O 2 ) can not be observed from pure water under visible light irradiation in our experiments, possibly indicating that the larger energy than the band gap is necessary for splitting water into H 2 and O 2 by photocatalysis.

Conclusions
We prepared single phase of the M 2 BiNbO 7 (M = In and Ga) photocatalysts by solid state reaction method and investigated the structural, optical absorption and photocatalytic properties.XRD results indicated that these compounds crystallize in the pyrochlore-type structure, cubic system with space group Fd-3m.The lattice parameters of Ga 2 BiNbO 7 and In 2 BiNbO 7 are 10.4685(5) and 10.7146(5) Angstrom respectively.The band gaps of Ga 2 BiNbO 7 and In 2 BiNbO 7 were estimated to be about 2.57(8) and 2.52 (5) eV and the compounds show strong optical absorption in the visible region (λ > 420 nm).In addition, H 2 or O 2 evolution was observed from pure water respectively with M 2 BiNbO 7 (M = In and Ga) as the photocatalysts under ultraviolet light irradiation.In the presence of M 2 BiNbO 7 (M = In and Ga), photocatalytic decomposition of aqueous MB could be achieved under visible light irradiation.At the same time, the mineralization of aqueous MB led to the generation of SO 4 2-and NO 3 -and to the marked decrease of TOC during the reaction, which suggests that M 2 BiNbO 7 (M = In and Ga)/VIS system may be regarded as an effective method for treatment of the wastewater from the textile industry.For a crystalline semiconductor, it is commonly accepted that the optical absorption near the band edge follows the equation: 1,2 αhν = A(hν-E g ) n .A, α, E g and ν are proportional constant, absorption coefficient, band gap, and light frequency, respectively.Within this equation, n determines the character of the transition in a semiconductor.E g and n can be calculated by the following steps: plot ln(αhν) versus ln(hν-E g ) with an approximative value of E g , then decide the value of n with the slope of the straightest line near the band edge, at last, plot (αhν) 1/n versus hν and evaluate the band gap E g by extrapolating the straightest line to the hν axis intercept.Based on above method, the value of n for M 2 BiNbO 7 (M = In and Ga) was calculated to be 0.5 from Figure 3 of the paper, indicating that the optical transitions for these oxides are directly allowed.Figure S3 shows the Plot of (αhν) 2 versus hν for Ga 2 BiNbO 7 and In 2 BiNbO 7 .Figure S2 showed that M 2 BiNbO 7 (M = In and Ga) consisted of the network of MO 6 , which is built by forming infinite corner-sharing MO 6 octahedra with the zigzag chains along [110].This suggests that photogenerated electron-hole pairs in the M 2 BiNbO 7 (M = In and Ga) photocatalysts can move easily in this direction, which may result in a high photocatalytic activities of M 2 BiNbO 7 (M = In and Ga).
Figure S4 shows the effect of photocatalyst concentration on photocatalytic methylene blue degradation under visible light irradiation at room temperature in air for 90 min.It could be seen that MB concentration decreased with increasing photocatalyst concentration when the photocatalyst concentration was   Figure S6 shows the photocatalytic O 2 evolution from pure water under UV light irradiation over the M 2 BiNbO 7 (M = In and Ga) photocatalysts and the results are described in Table 3 of the paper.Similar to H 2 evolutions, the O 2 evolutions increased with illumination time and O 2 evolution rates also varied according to the following order: Ga 2 BiNbO > In BiNbO 7 .

Figure 1 .
Figure 1.X-ray powder diffraction patterns of the M 2 BiNbO 7 (M = In and Ga) photocatalysts at 1100 o C.

Figure 4 .
Figure 4. Dependence of methylene blue degradation on the light wavelength (λ) after light irradiation for 90 min over the M 2 BiNbO 7 (M = In and Ga) photocatalysts.

Figure 5 .
Figure 5. Photocatalytic methylene blue degradation under visible light irradiation (λ > 420 nm) at room temperature in air for 180 min in the presence of M 2 BiNbO 7 (M = In and Ga) and TiO 2 (P-25), as well as MB photolysis.

Figure 6 .
Figure 6.Evolution of SO 4 2-and NO 3 -ions in the solution with the M 2 BiNbO 7 (M = In and Ga) photocatalysts during the photocatalytic degradation of MB under visible light irradiation (λ > 420 nm).
of Pollution Control and Resource Reuse, School of Environment, Nanjing University, Nanjing, 210093 People's Republic of China b National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, People's Republic of China c State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, People's Republic of China d Eco-Materials and Renewable Energy Research Center, Nanjing University, Nanjing 210093, People's Republic of China The structural formula of MB was shown in Figure S1.The structure of M 2 BiNbO 7 (M = In and Ga) is shown in Figure S2.The structure of M 2 BiNbO 7 (M = In and Ga) is composed of the three-dimensional network of MO 6 (M = Bi, Nb), stacked along [110] and separated by a unit cell translation (10.7146(5) or 10.4685(5) Å).

Figure S1 .
Figure S1.Structural formula of methylene blue.Figure S2.The schematic structural diagram of the cubic M 2 BiNbO 7 (M = In and Ga) photocatalysts.Three-dimensional network of MO 6 stacked along [110] and separated by a unit cell translation.

Figure S2 .
Figure S1.Structural formula of methylene blue.Figure S2.The schematic structural diagram of the cubic M 2 BiNbO 7 (M = In and Ga) photocatalysts.Three-dimensional network of MO 6 stacked along [110] and separated by a unit cell translation.

Table 1 .
Structural parameters of In 2 BiNbO 7 prepared by solid state reaction method

Table 2 .
Structural parameters of Ga 2 BiNbO 7 prepared by solid state reac-Diffuse reflection spectrum of the cubic M 2 BiNbO 7 (M = In and Ga) photocatalysts prepared by a solid state reaction method.

Table 3 .
Physical properties and formation rates of H 2 or O 2 evolutions from pure water over M 2 BiNbO 7 (M=In and Ga)