Photophysical and photocatalytic properties of novel M2BiNbO7 (M = In and Ga)

ARTICLE

Photophysical and photocatalytic properties of novel M₂BiNbO₇ (M = In and Ga)

Jingfei Luan^{*, I}; Shourong Zheng^I; Xiping Hao^II; Guoyou Luan^III; Xiaoshan Wu^II; Zhigang Zou^IV

^IState Key Laboratory of Pollution Control and Resource Reuse, School of Environment, Nanjing University, Nanjing, 210093 People's Republic of China

^IINational Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, People's Republic of China

^IIIState Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, People's Republic of China

^IVEco-Materials and Renewable Energy Research Center, Nanjing University, Nanjing 210093, People's Republic of China

ABSTRACT

M₂BiNbO₇ (M = In and Ga) were synthesized by solid-state reaction method and their structural and photocatalytic properties were investigated. The results indicated that these compounds crystallize in the pyrochlore-type structure, cubic system with space group Fd-3m. In addition, the band gaps of In₂BiNbO₇ and Ga₂BiNbO₇ were estimated to be about 2.52(5) and 2.57(8) eV, respectively. For the photocatalytic water splitting reaction, H₂ or O₂ evolution was observed from pure water respectively with M₂BiNbO₇ (M = In and Ga) as the photocatalysts under ultraviolet light irradiation. Photocatalytic degradation of aqueous methylene blue (MB) dye over these compounds was further investigated under visible light irradiation. M₂BiNbO₇ (M = In and Ga) showed markedly higher catalytic activity compared to P-25 for MB photocatalytic degradation under visible light irradiation. Complete removal of aqueous MB was observed after visible light irradiation for 160 min with the Ga₂BiNbO₇ as the photocatalyst and for 180 min with the In₂BiNbO₇ as the photocatalyst. The decrease of the total organic carbon (TOC) and the formation of inorganic products, SO₄^2– and NO₃^–, demonstrated the continuous mineralization of aqueous MB during the photocatalytic process.

Keywords: inorganic photocatalysts, crystal structure, removal of methylene blue dye, band structure, visible light irradiation

RESUMO

Os óxidos M₂BiNbO₇ (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 In₂BiNbO₇ e Ga₂BiNbO₇ são 2,52(5) e 2,57(8) eV, respectivamente. A reação fotocatalítica da decomposição de água pura foi estudada na presença dos fotocatalisadores M₂BiNbO₇ (M = In e Ga) e irradiação no ultravioleta, através do monitoramento da formação de H₂ e de O₂. A degradação fotocatalítica do corante azul de metileno em água, na presença destes óxidos, foi investigada sob irradiação no visível. Os catalisadores M₂BiNbO₇ (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 Ga₂BiNbO₇, e após 180 minutos na presença de In₂BiNbO₇. A diminuição do teor total de carbono (TOC) e a formação dos produtos SO₄^2– e NO₃^– confirmaram a mineralização do azul de metileno durante o processo fotocatalítico.

Introduction

Since Honda and Fujishima first observed the splitting of water on TiO₂ electrode in 1972,¹ the investigation of semiconductor photocatalysts has attracted much attention from both academic and industrial societies.^2,3 The photocatalytic water splitting using solar energy to produce hydrogen gas is crucial owing to the emergent requirement of clean and renewable sources.^2-4 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 Bi₂MNbO₇ (M = Al³⁺, Ga³⁺, In³⁺)⁵ and Bi₂RNbO₇ (R = Y, rare earth elements).⁶

In addition, scientific interest in the photocatalytic degradation of aqueous organic pollutants has quickly increased recently.^7-9 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 TiO₂-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 TiO₂-based photocatalysts based on the dye-sensitized process.^10,11 Among 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.^12,13 Up to now, there were only few reports of MB dye degradation under visible light irradiation.^12,14 Therefore, it is highly desirable to develop new visible light-driven photocatalysts with high activity.

It has been generally observed that numerous compounds with the A₂B₂O₇ pyrochlore structure display antiferroelectric phases or dielectric abnormality. However, only a few compounds display ferroelectric behavior.^15,16 M₂BiNbO₇ (M = In and Ga) belongs to the family of the A₂B₂O₇ compounds, but the data about its space group and lattice constants have not been reported previously. Moreover, no photocatalytic properties of M₂BiNbO₇ (M = In and Ga) have been investigated so far. We considered that In³⁺ or Ga³⁺ occupying the A site and Bi³⁺ occupying the B site in the A₂³⁺B₂⁴⁺ O₇ 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 M₂BiNbO₇ (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.⁸ In this contribution, we prepared the M₂BiNbO₇ (M = In and Ga) photocatalysts and the structural and photocatalytic properties of M₂BiNbO₇ (M = In and Ga) were studied in detail. A comparison of the photocatalytic property of M₂BiNbO₇ (M = In and Ga) with that of TiO₂ (P-25) is also provided.

Experimental

The polycrystalline samples of the photocatalysts were synthesized by a solid-state reaction method. Ga₂O₃, In₂O₃, Bi₂O₃ and Nb₂O₅ (China Medicine (Group) Shanghai Chemical Reagent Corporation ) with purity of 99.99% were used as starting materials. The powders were dried at 200 ºC 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 ºC 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 M₂BiNbO₇ (M = In and Ga) was analyzed by the X-ray diffractometer (D/MAX-RB, Rigaku Corporation, Japan) with CuKa radiation (l = 1.54056). The data were collected at 295 K with a step scan procedure in the range of 2q = 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 M₂BiNbO₇ (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 N₂ adsorption at liquid nitrogen temperature.

The photocatalytic degradation of aqueous MB was performed with 0.5 g Ga₂BiNbO₇ or In₂BiNbO₇ or TiO₂ 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 (l > 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).

The photocatalytic water splitting with M₂BiNbO₇ (M = In and Ga) as the photocatalysts was carried out in pure water (1.0 g powder catalyst, 300 mL H₂O) 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).

Results and Discussion

Structural properties

Figure 1 shows X-ray diffraction patterns of M₂BiNbO₇ (M = In and Ga) sintered at 1100 ºC in air. The powder X-ray diffraction analysis showed that M₂BiNbO₇ (M = In and Ga) are single phase, which is consistent with the results from SEM-EDX. The chemical composition of M₂BiNbO₇ (M = In and Ga) was measured with the ZAF (element number, absorption and fluorescence corrections) quantification method. The SEM-EDX analysis revealed that M₂BiNbO₇ (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 Ga₂BiNbO₇ and In: Bi: Nb = 2.00: 0.97: 1.03 for In₂BiNbO₇ 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 M₂BiNbO₇ (M = In and Ga) is described in Figure 2. It was shown that the particle distribution was homogeneous and the average particle diameters of In₂BiNbO₇ and Ga₂BiNbO₇ were estimated to be 1.5 and 1.7 µm.

Full-profile structure refinement of the collected powder diffraction data for M₂BiNbO₇ (M = In and Ga) was conducted using the Rietveld program REITAN,¹⁷ by which positional parameters and isotropic thermal parameters of M₂BiNbO₇ (M = In and Ga) were refined. The atomic coordinates and isotropic thermal parameters of M₂BiNbO₇ (M = In and Ga) are listed in Table 1 and Table 2. The result of the final refinement for M₂BiNbO₇ (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 Ga₂BiNbO₇ and a = 10.7146(5) Å for In₂BiNbO₇. All the diffraction peaks for M₂BiNbO₇ (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 Ga₂BiNbO₇ and In₂BiNbO₇ crystallize in the same structure, and 2q angles of each reflection of In₂BiNbO₇ change with In³⁺ being substituted by Ga³⁺. The lattice parameter decrease from a = 10.7146(5) Å for In₂BiNbO₇ to a = 10.4685(5) Å for Ga₂BiNbO₇, which indicates a decrease in lattice parameter of the photocatalyst with decrease of the M ionic radii, Ga³⁺ (0.62 Å) < In³⁺ (0.80 Å). The outcome of refinements for In₂BiNbO₇ and Ga₂BiNbO₇ 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 Bi₂CrNbO₇, Bi₂InNbO₇ and Bi₂FeSbO₇ and also observed the large R factors (15% to 20%). Zou et al.⁴ refined the crystal structure of Bi₂InNbO₇ and obtained a large R factor (15.5%) for Bi₂InNbO₇, which was ascribed to a slightly modified structure model for Bi₂InNbO₇. Note that the precursors with high purity were used in this study. The influence of minor impurities on the structure of M₂BiNbO₇ (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 M₂BiNbO₇ (M = In and Ga) are resulted from a slightly modified structure model for M₂BiNbO₇ (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 M₂BiNbO₇ (M = In and Ga) photocatalysts. In contrast to the well-known TiO₂ whose absorption edge is at about 400 nm, the newly synthesized In₂BiNbO₇ and Ga₂BiNbO₇ showed obvious absorption in the visible light region up to 491 and 481 nm (Obtained according to the band gaps of Ga₂BiNbO₇ (E_g = 2.57(8) eV) and In₂BiNbO₇ (E_g = 2.52(5) eV). Then use formula E_g = hcl^-1) which indicates that M₂BiNbO₇ (M = In and Ga) have the ability to respond to the wavelength of visible light region. Furthermore, the attribution of the second band for In₂BiNbO₇ at about 550 nm is possibly owing to defect energy level within crystal lattice of In₂BiNbO₇ such as oxygen vacancy energy level. It is noteworthy that the band gaps of Ga₂BiNbO₇ and In₂BiNbO₇ are estimated to be 2.57(8) and 2.52(5) eV, indicating narrower band gaps compared to that of Bi₂InTaO₇ (2.92 eV).¹⁹ This may imply that the photoabsorption of M₂BiNbO₇ (M = In and Ga) is stronger than that of Bi₂InTaO₇, which may result in a higher photocatalytic activity of M₂BiNbO₇ (M = In and Ga) than that of Bi₂InTaO₇. 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 M₂BiNbO₇ (M = In and Ga) using different cut-off filters. The results showed that the photocatalytic activity of M₂BiNbO₇ (M = In and Ga) decreased with increasing light wavelength, indicating that the change of the photocatalytic properties over M₂BiNbO₇ (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 M₂BiNbO₇ (M = In and Ga) was induced by light. Furthermore, it can be seen from Figure 4 that Ga₂BiNbO₇ showed higher photocatalytic activity (47.4% MB degradation, l>390 nm; 37.2% MB degradation, l>420 nm) compared with In₂BiNbO₇ (41.7% MB degradation, l>390 nm; 34.4% MB degradation, l>420 nm) not only in UV light region, but also in visible light region.

MB degradation with M₂BiNbO₇ (M = In and Ga) or TiO₂ (P-25) as the photocatalysts under visible light irradiation (l > 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 Ga₂BiNbO₇ as the photocatalyst. The initial rate of MB degradation was about 5.271×10^-6 mol s^-1m^-3. Simultaneously, a SO₄^2– 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 In₂BiNbO₇ as the photocatalyst. The initial rate of MB degradation was 4.685×10^-6 mol s^-1 m^-3 and a SO₄^2– 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 TiO₂ as the catalyst, and no SO₄^2– ion was detected in the solution after the photo-reaction. 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 TiO₂ as the catalyst, suggesting that TiO₂ was inactive to MB photocatalytic degradation under visible light irradiation.¹² Liu et al.²⁰ and Xu and Langford²¹ studied that alizarin red and X3B dyes could be decomposed over TiO₂ based on visible light driven dye-sensitized phenomena. Tang et al.⁸ reported that photocatalytic degradation of MB over TiO₂ 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 TiO₂ 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 TiO₂. Thus the rate of MB photolysis was almost the same as that of MB degradation with TiO₂ as the catalyst.

The ultimate aim of the photodegradation of organic pollutants is to completely convert the toxic organic compounds into inorganics, such as CO₂, SO₄^2– or NO₃^–. In the presence of M₂BiNbO₇ (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 SO₄^2– or NO₃^– ions increases with the increase of irradiation time. Note that the amount of SO₄^2– ions 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 SO₂. The second probable explanation is given by the partially irreversible adsorption of some SO₄^2– ions on the surface of the photocatalyst as already observed.²² However, the partial irreversible adsorption of SO₄^2– ions does not restrain the photocatalytic degradation of pollutants.²² The higher amount of NO₃^– 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 Bi₂InTaO₇ 26.7% of TOC decrease was obtained after visible light irradiation for 180 min. On the contrary, in the presence of Ga₂BiNbO₇, 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 Ga₂BiNbO₇. Similarly, we also found a decrease of TOC by 96.8% after 180 min of visible light irradiation with In₂BiNbO₇ as the photocatalyst.

Photocatalytic water splitting

Figure 8 shows the photocatalytic H₂ evolution from pure water under UV light irradiation over the M₂BiNbO₇ (M = In and Ga) photocatalysts. H₂ evolution rates and some physical properties are listed in Table 3. It can be seen from Figure 8 that the activities of M₂BiNbO₇ (M = In and Ga) are different and the results are listed in Table 3. It was found that H₂ evolution rates are estimated to be 72.6 µmol h^-1 for Ga₂BiNbO₇ and 54.3 µmol h^-1 for In₂BiNbO₇, indicating that Ga₂BiNbO₇ exhibits a larger activity than In₂BiNbO₇. The influence of the UV light irradiation was also investigated by light on/off shutter studies over M₂BiNbO₇ (M = In and Ga). The H₂ evolution stopped by terminating the UV light irradiation, indicating that the reactions of H₂ evolution were initiated by UV light irradiation. In the second run, almost the same H₂ evolution rate was obtained after the system was evacuated. In order to compare the catalytic activities of M₂BiNbO₇ (M = In and Ga) with that of TiO₂, water splitting with P25 as the catalyst was conducted. In the presence of P25, the rate of H₂ evolution from pure water was about 1.4 µmol h^-1 in the first 15 h, which shows much lower activity than that of M₂BiNbO₇ (M = In and Ga).

Based on the observed H₂ and O₂ evolution from pure water, it can be concluded that the conduction band levels of M₂BiNbO₇ (M = In and Ga) are more negative than that of H₂ evolution and the valence band levels are more positive than that of O₂ evolution. Namely, M₂BiNbO₇ (M = In and Ga) have proper band structures for the reduction of H⁺ to H₂ and oxidation of H₂O to O₂, respectively. Figure 9 shows suggested band structures of M₂BiNbO₇ (M = In and Ga). Recently, the electronic structures of InMO₄ (M = V, Nb and Ta) and BiVO₄ were reported by Oshikiri et al. based on the first principles calculations.²³ The conduction bands of the InMO₄ (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 BiVO₄ 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 M₂BiNbO₇ (M = In and Ga) should be similar to InMO₄ (M = V, Nb and Ta) and BiVO₄ due to their similar distorted pyrochlore-type structure. Therefore, we conclude that the conduction band of In₂BiNbO₇ is consisted of Nb 4d and In 5s. The valence band of In₂BiNbO₇ is consisted of a small Bi 6s orbital component and a dominant O 2p orbital component. Similarly, the conduction band of Ga₂BiNbO₇ is consisted of Nb 4d and Ga 4s. The valence band of Ga₂BiNbO₇ is almost the same as that of In₂BiNbO₇.

These photocatalysts consist of a three-dimensional network structure of corner-linked MO₆ (M = Bi, Nb) octahedra and the MO₆ octahedra are connected into chains with In³⁺ ions or Ga³⁺ ions. The shapes of AO₈ and BO₆ polyhedra vary with the O(48f) parameter x in the pyrochlore-type A₂B₂O₇ 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.²⁴ Thus, information on the lattice distortion can be obtained from the O(48f) parameter x in the pyrochlore-type A₂B₂O₇ structure. The lattice distortion is defined according to the distortion of BO₆ 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 Ga₂BiNbO₇ and 0.054(3) for In₂BiNbO₇ because the lattice distortion is equal to 0.375—the O(48f) parameter x. During the process of photocatalytic water splitting into H₂ and O₂, 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.^25,26 In 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 Ga₂BiNbO₇ with larger lattice distortion (0.104(4)) shows higher photocatalytic activity compared to In₂BiNbO₇ 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.²⁷ This indicates that the photoinduced electrons and holes can move easily if the angle between the corner-linked octahedral is close to 180º. 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 MO₆ (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 Ga₂BiNbO₇ with those of In₂BiNbO₇, we can find that the closer the M–O1–M bond angle is to 180º, 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 M₂BiNbO₇ (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 MO₆ (M = Bi and Nb) octahedral are possible to influence the photocatalytic activities of M₂BiNbO₇ (M = In and Ga). Although direct absorption of photons by the semiconductor oxide can produce electron–hole pairs in the catalysts, the gases evolution (H₂ or O₂) 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 H₂ and O₂ by photocatalysis.

Conclusions

We prepared single phase of the M₂BiNbO₇ (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 Ga₂BiNbO₇ and In₂BiNbO₇ are 10.4685(5) and 10.7146(5) Angstrom respectively. The band gaps of Ga₂BiNbO₇ and In₂BiNbO₇ were estimated to be about 2.57(8) and 2.52(5) eV and the compounds show strong optical absorption in the visible region (l > 420 nm). In addition, H₂ or O₂ evolution was observed from pure water respectively with M₂BiNbO₇ (M = In and Ga) as the photocatalysts under ultraviolet light irradiation. In the presence of M₂BiNbO₇ (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 SO₄^2– and NO₃^– and to the marked decrease of TOC during the reaction, which suggests that M₂BiNbO₇ (M = In and Ga)/VIS system may be regarded as an effective method for treatment of the wastewater from the textile industry.

Acknowledgments

This work was supported by a grant from the Natural Science Foundation of Jiangsu Province (No. BK2006130).

Supplementary Information

Structural formula of methylene blue, the schematic structural diagram of the cubic M₂BiNbO₇ (M = In and Ga) photocatalysts, plot of (ahn)²versus hn for the M₂BiNbO₇ (M = In and Ga) photocatalysts, effect of photocatalyst concentration on photocatalytic methylene blue degradation under visible light irradiation (l > 420 nm) for 90 min, photocatalytic methylene blue degradation under visible light irradiation (l > 420 nm) at room temperature in the presence of Bi₂MNbO₇ (M = In, Al and Ga) and Bi₂InTaO₇, photocatalytic O₂ evolution over M₂BiNbO₇ (M = In and Ga) from pure water under ultraviolet light irradiation, physical properties of Bi₂MNbO₇ (M = In, Al and Ga) and Bi₂InTaO₇ are available free of charge at http://jbcs.sbq.org.br, as PDF file.

Received: December 7, 2005

Published on the web: September 26, 2006

Supplementary Information

The structural formula of MB was shown in Figure S1. The structure of M₂BiNbO₇ (M = In and Ga) is shown in Figure S2. The structure of M₂BiNbO₇ (M = In and Ga) is composed of the three-dimensional network of MO₆ (M = Bi, Nb), stacked along [110] and separated by a unit cell translation (10.7146(5) or 10.4685(5) Å).

For a crystalline semiconductor, it is commonly accepted that the optical absorption near the band edge follows the equation:^1,2ahn= A(hn-E_g)ⁿ. A, a, E_g and n 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(ahn) versus ln(hn-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 (ahn)^1/n versus hn and evaluate the band gap E_g by extrapolating the straightest line to the hn axis intercept. Based on above method, the value of n for M₂BiNbO₇ (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 (ahn)² versus hí for Ga₂BiNbO₇ and In₂BiNbO₇. Figure S2 showed that M₂BiNbO₇ (M = In and Ga) consisted of the network of MO₆, which is built by forming infinite corner-sharing MO₆ octahedra with the zigzag chains along [110]. This suggests that photogenerated electron-hole pairs in the M₂BiNbO₇ (M = In and Ga) photocatalysts can move easily in this direction, which may result in a high photocatalytic activities of M₂BiNbO₇ (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 lower than 5 g L^-1. While MB concentration kept unchanging with increasing photocatalyst concentration when the photocatalyst concentration was higher than 5 g L^-1. The possible reason is the low capacity of visible light irradiation to penetrate in a media that contains a fine suspension of large amount of photocatalyst. Thus we chose 5 g L^-1 as photocatalyst concentration.

Figure S5 shows photocatalytic methylene blue degradation under visible light irradiation (l> 420 nm) in the presence of Bi₂MNbO₇ (M = In, Al and Ga) and Bi₂InTaO₇. 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 280, 300 and 370 min with Bi₂GaNbO₇, Bi₂InNbO₇ and Bi₂AlNb₇ as the photocatalysts, respectively. However, MB concentration decreased only from 0.0506 to 0.0182 mol m-3 after visible light irradiation for 400 min with Bi₂InTaO₇ as the catalyst. The initial rates of MB degradation for Bi₂GaNbO₇, Bi₂InNbO₇, Bi₂AlNb₇ and Bi₂InTaO₇ were about 3.012´10-6, 2.811´10-6, 2.279´10-6 and 1.350´ 10-6 mol s-1 m-3, respectively. Table S1 shows the physical properties of Bi₂MNbO₇ (M = In, Al and Ga) and Bi₂InTaO₇. Table 3 of the paper shows the physical properties of M2BiNbO7(M=In and Ga). It could be seen that the surface areas of these compounds were nearly the same and their particle size were also very similar. Thus we can draw a conclusion that the photocatalytic activity of these compounds is as following order: Ga2BiNbO7 > In2BiNbO7 > Bi₂GaNbO₇ > Bi₂InNbO₇ > Bi₂AlNb₇ > Bi₂InTaO₇.

Figure S6 shows the photocatalytic O₂ evolution from pure water under UV light irradiation over the M₂BiNbO7 (M = In and Ga) photocatalysts and the results are described in Table 3 of the paper. Similar to H₂ evolutions, the O₂ evolutions increased with illumination time and O₂ evolution rates also varied according to the following order: Ga₂BiNbO₇ > In₂BiNbO₇.

References

1. Butler, M. A.; J. Appl. Phys. 1977, 48, 1914.

2. Tauc, J.; Grigorovici, R.; Vancu, A.; Phys. Stat. Sol. 1966, 15, 627.

3. Zou, Z.; Ye, J.; Arakawa, H.; J. Mater. Sci. Lett. 2000, 19, 1909.

4. Zou, Z.; Ye, J.; Arakawa, H.; Chem. Mater. 2001, 13, 1765.

5. Wang, J. H.; Zou, Z.; Ye, J.; Mater. Sci. Forum 2003, 423, 485.

* e-mail: [e-mail]

• 1. Honda, K.; Fujishima, A.; Nature 1972, 238, 37.
• 2. Zou, Z.; Ye, J.; Sayama, K.; Arakawa, H.; Nature 2001, 414, 625.
• 3. Zou, Z.; Ye, J.; Arakawa, H.; J. Phys. Chem. B 2002, 106, 13098.
• 4. Zou, Z.; Ye, J.; Arakawa, H.; J. Mater. Sci. Lett. 2000, 19, 1909.
• 5. Zou, Z.; Ye, J.; Arakawa, H.; Chem. Mater. 2001, 13, 1765.
• 6. Zou, Z.; Ye, J.; Arakawa, H.; J. Phys. Chem. B 2002, 106, 517.
• 7. Zou, Z.; Ye, J.; Oka, K.; Nishihara, Y.; Phys. Rev. Lett. 1998, 80, 1074.
• 8. Tang, J.; Zou, Z.; Yin, J.; Ye, J.; Chem. Phys. Lett. 2003, 382, 175.
• 9. Matos, J.; Laine, J.; Herrmann, J. M.; J. Catal. 2001, 200, 10.
• 10. Qu, P.; Zhao, J.; Shen, T.; Hidaka, H.; J. Mol. Catal. A 1998, 129, 257.
• 11. Xu, Y. M.; Langford, C. H.; Langmuir 2001, 17, 897.
• 12. Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y.; Science 2001, 293, 269.
• 13. Li, F. B.; Li, X. Z.; Appl. Catal. A 2002, 228, 15.
• 14. Li, F. B.; Li, X. Z.; Chemosphere 2002, 48, 1103.
• 15. Bernard, D.; Pannetier, J. ; Lucas, J.; Ferroelectrics 1978, 21, 429.
• 16. Golovshchikove, G. I.; Isupov, V. A. ; Tutov, A. G.; Nikove, A. G.; Myl, I. E.; Nikitina, P. A.; Tulinova, O. I.; Sov.Phys.Solid State 1973, 14, 2539.
• 17. Izumi, F.; J. Crystallogr. Assoc. Jpn. 1985, 27, 23.
• 18. Xu, J.; Emge, T.; Ramanujachary, K. V.; Hohn, P.; Greenblatt, M.; J. Solid State Chem. 1996, 125, 192.
• 19. Wang, J. H.; Zou, Z.; Ye, J.; Mater. Sci. Forum 2003, 423, 485.
• 20. Liu, G.; Wu, T.; Zhao, J.; Hidaka, H.; Serpone, N.; Environ. Sci. Technol. 1999, 33, 2081.
• 21. Xu, Y. M.; Langford, C. H.; Langmuir 2001, 17, 897.
• 22. Lachheb, H.; Puzenat, E.; Houas, A.; Ksibi, M.; Elaloui, E.; Guilard, C.; Herrmann, J. M.; Appl. Catal. B 2002, 39, 75.
• 23. Oshikiri, M.; Boero, M.; Ye, J.; Zou, Z.; Kido, G.; J. Chem. Phys. 2002, 117, 7313.
• 24. Subramanian, M. A.; Aravamudan, G.; Subba, G. V.; Prog. Solid State Chem. 1983, 15, 55.
• 25. Inoue, Y.; Kohno, M.; Ogura, S.; Sato, K.; Chem. Phys. Lett. 1997, 267, 72.
• 26. Kudo, A.; Kato, H.; Nakagawa, S.; J. Phys. Chem. B 2000, 104, 571.
• 27. Wiegel, M.; Middel, W.; Blasse, G.; J. Mater. Chem. 1995, 5, 981.

Publication Dates

• Publication in this collection
  29 Jan 2007
• Date of issue
  Dec 2006

History

• Received
  07 Dec 2005
• Accepted
  26 Sept 2006