Synthesis, Property Characterization, and Photocatalytic Activity of Novel Visible Light-Responsive Photocatalyst Fe_𝟐BiSbO_𝟕

Abstract

Fe₂BiSbO₇ was synthesized by a solid-state reaction method for the first time. The structural and photocatalytic properties of Fe₂BiSbO₇ have been characterized. The results showed that Fe₂BiSbO₇ was crystallized with the pyrochlore-type structure, cubic crystal system, and space group Fd3m. The lattice parameter for Fe₂BiSbO₇ was 𝑎=10.410297 Å. The photocatalytic degradation of methylene blue (MB) was realized under visible light irradiation with Fe₂BiSbO₇ as catalyst. Fe₂BiSbO₇ owned higher catalytic activity compared with Bi₂InTaO₇ or pure TiO₂ or N-doped TiO₂ for photocatalytic degradation of MB. The photocatalytic degradation of MB with Fe₂BiSbO₇, Bi₂InTaO₇, or N-doped TiO₂ followed the first-order reaction kinetics, and the first-order rate constant was 0.01189, 0.00275, or 0.00333 min⁻¹. After visible light irradiation for 230 min with Fe₂BiSbO₇, complete removal and mineralization of MB was observed. The reduction of the total organic carbon, the formation of inorganic products, SO₄²⁻ and NO₃⁻, and the evolution of CO₂ revealed the continuous mineralization of MB during the photocatalytic process. The photocatalytic degradation pathway of MB was obtained. Fe₂BiSbO₇/(visible light) photocatalysis system was found to be suitable for textile industry wastewater treatment.

1. Introduction

Dye effluents from textile industries and photographic industries are becoming a serious environmental problem because of their toxicity, unacceptable color, high chemical oxygen demand content, and nonbiological degradation [1]. Many conventional methods have been proposed to treat industrial effluents, but each method has its shortcomings [1–7]. In recent years, the photocatalytic degradation processes have been widely applied as techniques of destruction of organic pollutants in wastewater and effluents, especially for degrading dyes [1, 7–21]. However, among various dyes, methylene blue (MB) dye was difficult to be degraded and was often utilized as a model dye contaminant to estimate the activity of a photocatalyst under both ultraviolet light irradiation [18, 19, 22] and visible light irradiation [20, 21, 23, 24]. There were many reports about the photodegradation of MB. Unfortunately, most of these reports were carried out under UV light irradiation. Up to now, there were only few reports of MB dye degradation under visible light irradiation such as the research by Asahi et al. with a reduced TiO_𝑥(TiO_2−𝑥N_𝑥) as catalyst and the research by Tang et al. and Cui et al. with Pt-TiO₂ as photocatalyst [21, 24]. Zhang [25] utilized N-doped TiO₂ as catalyst to degrade MB under visible light irradiation and found that the removal ratio of MB was only 35% after 180 min. It is known that ultraviolet light only occupies 4% of the solar energy. For this reason, many endeavors should be taken up to develop new visible light-responsive photocatalysts which are capable of utilizing more visible light, which accounts for about 43% of the solar energy. Therefore, it is urgent to develop novel visible light-responsive photocatalysts.

With the development of investigation of photocatalysis process, investigators also paid much attention to researching and developing novel photocatalysts [26–30]. Currently, TiO₂ was the most common photocatalyst, however, TiO₂ could not be utilized in the visible light region and could only degrade RhB under ultraviolet light irradiation which was a restrained factor for photocatalysis technology with TiO₂ as catalyst. Therefore, some efficient catalysts which could generate electron-hole pairs under visible light irradiation should be developed. Fortunately, A₂B₂O₇ compounds were often considered to own photocatalytic properties under visible light irradiation. In our previous work [31], we have found that Bi₂InTaO₇ was 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 upon modification of its structure. According to the above analysis, we could assume that substitution of Ta⁵⁺ by Sb⁵⁺, substitution of Bi³⁺ by Fe³⁺, and substitution of In³⁺ by Bi³⁺ in Bi₂InTaO₇ might increase carriers concentration. As a result, a change and improvement of the electrical transportation and photophysical properties could be found in the novel Fe₂BiSbO₇ compound which might own advanced photocatalytic properties.

Fe₂BiSbO₇ has never been produced before and the data about its structural and photophysical properties such as space group and lattice constants have not been found previously. In addition, the photocatalytic properties of Fe₂BiSbO₇ have not been studied by other investigators. The molecular composition of Fe₂BiSbO₇ was very similar with other A₂B₂O₇ compounds. Thus the resemblance suggested that Fe₂BiSbO₇ might possess photocatalytic properties under visible light irradiation, which was similar with those other members in A₂B₂O₇ family. Fe₂BiSbO₇ also seemed to own potential for improvement of photocatalytic activity upon modification of its structure because it had been proved that a slight modification of a semiconductor structure will result in a remarkable change within photocatalytic properties [21]. In this paper, Fe₂BiSbO₇ was prepared for the first time by the solid-state reaction method and the structure and photocatalytic properties of Fe₂BiSbO₇ were investigated in detail. The photocatalytic degradation of MB under visible light irradiation was also performed to evaluate the photocatalytic activity of Fe₂BiSbO₇. A comparison among the photocatalytic properties of Fe₂BiSbO₇, Bi₂InTaO₇, and N-doped TiO₂ was achieved in order to elucidate the relationship between the structure and photocatalytic activity of Fe₂BiSbO₇.

2. Experimental

2.1. Synthesis of Fe₂BiSbO₇ and N-Doped TiO₂

Fe₂BiSbO₇ powder was first synthesized by the solid-state reaction method. Fe₂O₃, Bi₂O₃, and Sb₂O₅ with the purity of 99.99% were utilized as raw materials which were purchased from Sinopharm Group Chemical Reagent Co. (Shanghai, China) and used without further purification. All powders were dried at 200°C for 4 h before synthesis. In order to synthesize Fe₂BiSbO₇, the precursors were stoichiometrically mixed in a quartz mortar, subsequently pressed into small columns, and put into an alumina crucible (Shenyang Crucible Co., Ltd., China). Finally, calcination was carried out at 1020°C for 25 h in an electric furnace (KSL 1700X, Hefei Kejing Materials Technology Co., Ltd., China). Similarly, Bi₂InTaO₇ was synthesized by calcination at 1050°C for 46 h. After sintering and grounding within a quartz mortar, ultrafine Fe₂BiSbO₇ powder was fabricated. Nitrogen-doped titania (N-doped TiO₂) catalyst with tetrabutyl titanate as a titanium precursor was prepared via the sol-gel method at room temperature. The procedure was as follows: 17 mL tetrabutyl titanate and 40 mL absolute ethyl alcohol were mixed as solution a, subsequently solution a was added dropwise under vigorous stirring into the solution b that contained 40 mL absolute ethyl alcohol, 10 mL glacial acetic acid, and 5 mL double distilled water to form transparent colloidal suspension c. Subsequently aqua ammonia with N/Ti proportion of 8 mol% was added into the resulting transparent colloidal suspension under vigorous stirring condition and kept stirring for 1 h. Finally, the xerogel was formed after being aged for 2 days. The xerogel was grounded into powder which was calcined at 500°C for 2 h. Finally, above powder was grounded in agate mortar and screened by shaker to obtain N-doped TiO₂ powders.

2.2. Characterization of Fe₂BiSbO₇

The crystalline phase of Fe₂BiSbO₇ 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=10−95^∘. The step interval was 0.02° and the time per step was 1 s. The accelerating voltage and applied current were 40 kV and 40 mA, respectively. The chemical composition of the compound was determined by scanning electron microscope-X-ray energy dispersion spectrum (SEM-EDS, LEO 1530VP, LEO Corporation, Germany), X-ray fluorescence spectrometer (XFS, ARL-9800, ARL Corporation, Switzerland), and X-ray photoelectron spectroscopy (XPS, ESCALABMK-2, VG Scientific Ltd., UK). The particle morphology of Fe₂BiSbO₇ was observed by transmission electron microscope (Tecnal F20 S-Twin, FEI Corporation, USA). The Fe³⁺ content, Bi³⁺ content, Sb⁵⁺ content, and O²⁻ content of Fe₂BiSbO₇ and the valence state of elements were also analyzed by X-ray photoelectron spectroscopy (XPS). The chemical composition within the depth profile of Fe₂BiSbO₇ was examined by the argon ion denudation method when X-ray photoelectron spectroscopy was used. UV-visible diffuse reflectance spectrum of Fe₂BiSbO₇ was measured with a Shimadzu UV-2550 UV-Visible spectrometer, and BaSO₄ was used as the reference material. The surface areas of Fe₂BiSbO₇ and N-doped TiO₂ were determined by the Brunauer-Emmett-Teller (BET) method (MS-21, Quantachrome Instruments Corporation, USA) with N₂ adsorption at liquid nitrogen temperature. The particle sizes of the photocatalysts were measured by Malvern’s mastersize-2000 particle size analyzer (Malvern Instruments Ltd., UK).

2.3. Photocatalytic Activity Tests

The photocatalytic activity of Fe₂BiSbO₇ was evaluated with methylene blue (C₁₆H₁₈ClN₃S) (Tianjin Bodi Chemical Co., Ltd., China) as a model material. The photoreaction was carried out in a photochemical reaction apparatus (Nanjing Xujiang Machine Plant, China). The internal structure of the reaction apparatus is as follows: the lamp is put into a quartz hydrazine which is a hollow structure and located in the middle of the reactor. The recycling water through the reactor maintains a near constant reaction temperature (20°C) and the solution was continuously stirred and aerated. Twelve holes which are used to put quartz tubes evenly distribute around the lamp and the distance between the lamp and each hole is equal. Under the condition of magnetic stirring, the photocatalyst within the MB solution is in the state of suspension. In this paper, the photocatalytic degradation of the MB solution was performed with 0.3 g Fe₂BiSbO₇ in 300 mL 0.025 mM MB aqueous solution in quartz tubes with 500 W Xenon lamp (400nm<𝜆<800nm) as visible-light source. Prior to visible light irradiation, the suspensions which contained the catalyst and MB dye were magnetically stirred in the dark for 45 min to ensure establishment of an adsorption/desorption equilibrium among Fe₂BiSbO₇, the MB dye, and atmospheric oxygen. During visible light illumination, the suspension was stirred at 500 rpm and the initial pH value of the MB solution was 7.0 without pH adjustment in the reaction process. The above experiments were performed under oxygen-saturation conditions ([O₂]_sat=1.02×10⁻³ M). One of the quartz tubes was taken out from the photochemical reaction apparatus at various time intervals. The suspension was filtered through 0.22 μm membrane filters. The filtrate was subsequently analyzed by a Shimadzu UV-2450 UV-Visible spectrometer with the detecting wavelength at 665 nm. The experimental error was found to be within ±2.2%.

The incident photon flux 𝐼_𝑜 measured by a radiometer (Model FZ-A, Photoelectric Instrument Factory Beijing Normal University, China) was determined to be 4.76×10⁻⁶ Einstein L⁻¹ s⁻¹ under visible light irradiation (wavelength range of 400–700 nm). The incident photon flux on the photoreactor was varied by adjusting the distance between the photoreactor and the Xe arc lamp. The pH value adjustment was not carried out, and the initial pH value was 7.0. The inorganic products which were obtained from MB degradation were analyzed by ion chromatograph (DX-300, Dionex Corporation, USA). The identification of MB and the degradation intermediate products of MB were performed by gas chromatograph—mass spectrometer (GC-MS, HP 6890 Series Gas Chromatograph, AT column, 20.3 m × 0.32 mm, ID of 0.25 μm) which operated at 320°C and was connected to HP 5973 mass selective detector and a flame ionization detector with H₂ as the carried gas. The intermediate products of MB were also measured by liquid chromatograph—mass spectrometer (LC-MS, Thermo Quest LCQ Duo, USA, Beta Basic-C₁₈ HPLC column: 150 × 2.1 mm, ID of 5 μm, Finnigan, Thermo, USA). Here, 20 𝜇L of postphotocatalysis solution was injected automatically into the LC-MS system. The fluent contained 60% methanol and 40% water, and the flow rate was 0.2 mL min⁻¹. MS conditions included an electrospray ionization interface, a capillary temperature of 27°C with a voltage of 19.00 V, a spray voltage of 5000 V, and a constant sheath gas flow rate. The spectrum was acquired in the negative ion scan mode and the 𝑚𝑧⁻¹ range swept from 50 to 600. Evolution of CO₂ was analyzed with an intersmat IGC120-MB gas chromatograph equipped with a porapack Q column (3 m in length and an inner diameter of 0.25 in.), which was connected to a catharometer detector. The total organic carbon (TOC) concentration was determined with a TOC analyzer (TOC-5000, Shimadzu Corporation, Japan). The photonic efficiency was calculated according to the following equation [32, 33]: 𝑅𝜑=𝐼_𝑜, (1) where 𝜑 is the photonic efficiency (%), 𝑅 is the rate of MB degradation (Mol L⁻¹ s⁻¹), and 𝐼_𝑜 is the incident photon flux (Einstein L⁻¹s⁻¹).

3. Results and Discussion

3.1. Crystal Structure of Fe₂BiSbO₇

Figure 1 presents TEM image and the selected area electron diffraction pattern of Fe₂BiSbO₇. The TEM image of Fe₂BiSbO₇ showed that the morphology of the Fe₂BiSbO₇ particle was very similar and regular. It could be seen that the Fe₂BiSbO₇ particles crystallized well and the mean particle diameter of Fe₂BiSbO₇ was about 150 nm. SEM-EDS spectrum of Fe₂BiSbO₇ revealed that Fe₂BiSbO₇ was pure phase without any other impure elements and Fe₂BiSbO₇ displayed the presence of iron, bismuth, antimony, and oxygen. It could be seen from Figure 1 that Fe₂BiSbO₇ was crystallized with the pyrochlore-type structure, cubic crystal system, and space group Fd3m. The lattice parameter for Fe₂BiSbO₇ was proved to be 𝑎=10.410297 Å. According to the calculation results from Figure 1, the (h k l) value for the main peaks of Fe₂BiSbO₇ could be found and indexed.

(a)

(b)

Figure 1 

TEM image of Fe2BiSbO7 (a) and the selected area electron diffraction pattern of Fe2BiSbO7 (b).

Full-profile structure refinements of the collected X-ray diffraction data of Fe₂BiSbO₇ were obtained by the RIETAN [34] program, which was based on Pawley analysis. The refinement results of Fe₂BiSbO₇ are shown in Figure 2. The atomic coordinates and structural parameters of Fe₂BiSbO₇ are listed in Table 1. The results of the final refinement for Fe₂BiSbO₇ 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 Fe₂BiSbO₇ and Bi₂InTaO₇ were crystallized in the same structure, and 2 theta angles of each reflection of Fe₂BiSbO₇ changed with Fe³⁺ being replaced by Bi³⁺, Bi³⁺ being replaced by In³⁺, and Sb⁵⁺ being replaced by Ta⁵⁺. Bi₂InTaO₇ was also crystallized with a cubic structure by space group Fd3m and the lattice parameter of Bi₂InTaO₇ was 𝑎=10.746410 Å. The lattice parameter of Fe₂BiSbO₇ was 𝑎=10.410297 Å, which indicated that the lattice parameter of Fe₂BiSbO₇ decreased compared with the lattice parameter of Bi₂InTaO₇ because the In³⁺ ionic radii (0.92 Å) or the Bi³⁺ ionic radii (1.17 Å) was larger than the Fe³⁺ ionic radii (0.78 Å). The outcome of refinement for Fe₂BiSbO₇ generated the unweighted 𝑅 factor, 𝑅_𝑃=11.56% with space group Fd3m. Zou et al. [35] refined the crystal structure of Bi₂InNbO₇ and obtained a large 𝑅 factor for Bi₂InNbO₇, which was ascribed to a slightly modified structure model for Bi₂InNbO₇. Based on the high purity of the precursors which were used in this study and the EDS results that did not trace any other elements, it was unlikely that the observed space groups originated from the presence of impurities. Therefore, it was suggested that the slightly high 𝑅 factor for Fe₂BiSbO₇ was due to a slightly modified structure model for Fe₂BiSbO₇. It should be emphasized that the defects or the disorder/order of a fraction of the atoms could result in the change of structures, including different bond-distance distributions, thermal displacement parameters, and/or occupation factors for some of the atoms.

Table 1 

Atomic coordinates and structural parameters of Fe₂BiSbO₇ prepared by the solid state reaction method.

Figure 2 

Pawley refinements of XRD data for novel photocatalyst Fe2BiSbO7 prepared by the solid state reaction method at 1020°C. The solid line represents experimental X-ray diffraction pattern (—). The dot line represents simulation X-ray diffraction pattern (…). The tic marks represent reflection positions. A difference (observed-calculated) profile is shown beneath.

In order to reveal the surface chemical compositions and the valence states of various elements of Fe₂BiSbO₇, the X-ray photoelectron spectrum of Fe₂BiSbO₇ for detecting Fe, Bi, Sb, and O was performed. The full XPS spectrum confirmed that the prepared Fe₂BiSbO₇ contained elements of Fe, Bi, Sb, and O, which was consistent with the results of SEM-EDS. The different elemental peaks which are corresponding to definite binding energies are given in Table 2. The results illustrated that the oxidation states of Fe, Bi, Sb, and O ions from Fe₂BiSbO₇ were +3, +3, +5, and −2, respectively. Besides_, the average atomic ratio of Fe : Bi : Sb : O for Fe₂BiSbO₇ was 2.00 : 0.97 : 1.01 : 6.98 based on our XPS, SEM-EDS and XFS results. Accordingly, it could be deduced that the resulting material was highly pure under our preparation conditions. It was remarkable that there were not any shoulders and widening in the XPS peaks of Fe₂BiSbO₇, which suggested the absence of any other phases.

Table 2 

Binding energies (BE) for key elements from Fe₂BiSbO₇.

3.2. Photocatalytic Properties

Generally, the direct absorption of band-gap photons would result in the generation of electron-hole pairs within Fe₂BiSbO₇, subsequently; the charge carriers began to diffuse to the surface of Fe₂BiSbO₇. As a result, the photocatalytic activity for decomposing organic compounds with Fe₂BiSbO₇ might be enhanced. Changes in the UV-Vis spectrum of MB upon exposure to visible light (𝜆>400 nm) irradiation with the presence of Fe₂BiSbO₇, Bi₂InTaO₇, or N-doped TiO₂ indicated that Fe₂BiSbO₇, Bi₂InTaO₇, or N-doped TiO₂ 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 Fe₂BiSbO₇, Bi₂InTaO₇, pure TiO₂, N-doped TiO₂, as well as in the absence of a photocatalyst. The results showed that a reduction in typical MB peaks at 665 nm and 614.5 nm was clearly noticed and the photodegradation rate of MB was about 1.980 × 10⁻⁹mol L⁻¹ s⁻¹ and the photonic efficiency was estimated to be 0.0416% (𝜆=420 nm) with Fe₂BiSbO₇ as catalyst. Similarly, the photodegradation rate of MB was about 1.001 × 10⁻⁹mol L⁻¹s⁻¹and the photonic efficiency was estimated to be 0.0210% (𝜆=420 nm) with N-doped TiO₂ as catalyst. Moreover, the photodegradation rate of MB was about 0.891 × 10⁻⁹mol L⁻¹ s⁻¹ and the photonic efficiency was estimated to be 0.0187% (𝜆=420 nm) with Bi₂InTaO₇ as catalyst. By contrast, the photodegradation rate of MB within 200 min of visible light irradiation was only 0.8338 × 10⁻⁹ mol L⁻¹ s⁻¹ and the photonic efficiency was estimated to be 0.0175% (𝜆=420nm) with pure TiO₂ as catalyst. The photodegradation rate of MB was about 0.6830 × 10⁻⁹mol 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 Fe₂BiSbO₇ as catalyst were both higher than those with N-doped TiO₂ or Bi₂InTaO₇, or pure TiO₂ as catalyst. The photodegradation rate of MB and the photonic efficiency with N-doped TiO₂ as catalyst were both higher than those with Bi₂InTaO₇ or pure TiO₂ as catalyst. The photodegradation rate of MB and the photonic efficiency with Bi₂InTaO₇ as catalyst were both higher than those with pure TiO₂ or the absence of a photocatalyst. The photodegradation rate of MB and the photonic efficiency with pure TiO₂ as catalyst were both higher than those with the absence of a photocatalyst. When Fe₂BiSbO₇, N-doped TiO₂, Bi₂InTaO₇ or pure TiO₂ was used as photocatalyst, the photodegradation conversion rate of MB was 96.59%, 48.05%, 42.76%, and 40.02% after visible light irradiation for 200 min, respectively. Moreover, the photodegradation conversion rate of MB was 32.78% after visible light irradiation for 200 min with the absence of a photocatalyst because of the MB dye photosensitization effect [36]. After visible light irradiation for 230 min with Fe₂BiSbO₇ 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. Based on above results, the photocatalytic degradation activity of Fe₂BiSbO₇ was much higher than that of N-doped TiO₂, Bi₂InTaO₇, or pure TiO₂. Meanwhile, N-doped TiO₂ showed higher photocatalytic degradation activity for MB photodegradation compared with Bi₂InTaO₇ or pure TiO₂. Bi₂InTaO₇ showed higher photocatalytic degradation activity for MB photodegradation compared with pure TiO₂. Pure TiO₂ was more suitable for MB photodegradation than the absence of a photocatalyst. The photocatalytic property of novel Fe₂BiSbO₇ under visible light irradiation was amazing compared with that of N-doped TiO₂ or pure TiO₂, and the main reason was that the specific surface area of Fe₂BiSbO₇ was much smaller than that of N-doped TiO₂ or pure TiO₂. BET isotherm measurements of Fe₂BiSbO₇, N-doped TiO₂, and pure TiO₂ provided a specific surface area of 2.78 m² g⁻¹, 45.53 m² g⁻¹, and 46.24 m² g⁻¹, respectively, which indicated that the photocatalytic degradation activity of Fe₂BiSbO₇ could be greatly improved by enhancing the specific surface area of Fe₂BiSbO₇.

(a)

(b)

Figure 3 

The absorbance pattern of methylene blue photocatalytically degraded by Fe2BiSbO7 (a), and photocatalytic degradation of methylene blue under visible light irradiation in the presence of Fe2BiSbO7, Bi2InTaO7, pure TiO2, N-doped TiO2, as well as in the absence of a photocatalyst (b).

Figure 4 shows the change of TOC during photocatalytic degradation of MB with Fe₂BiSbO₇, Bi₂InTaO₇, or N-doped TiO₂ as catalyst under visible light irradiation. The TOC measurements revealed the disappearance of organic carbon when the MB solution which contained Fe₂BiSbO₇, Bi₂InTaO₇, or N-doped TiO₂ was exposed under visible light irradiation. The results showed that 89.51%, 46.77%, or 41.71% of TOC decrease was obtained after visible light irradiation for 200 min when Fe₂BiSbO₇, or N-doped TiO₂ or Bi₂InTaO₇ was used as photocatalyst. Consequently, after visible light irradiation for 230 min with Fe₂BiSbO₇ as catalyst, the entire mineralization of MB was observed because of 100% TOC removal. The turnover number which represented the ratio between the total amount of evolved gas and dissipative catalyst was calculated to be more than 0.204 for Fe₂BiSbO₇ after 200 min of reaction time under visible light irradiation and this turnover number was evident to prove that this reaction occurred catalytically. Similarly, when the light was turned off in this experiment, the stop of this reaction showed the obvious light response.

Figure 4 

Disappearance of total organic carbon (TOC) during photocatalytic degradation of methylene blue with Fe2BiSbO7, Bi2InTaO7, or N-doped TiO2 as catalyst under visible light irradiation.

Figure 5 shows the amount of CO₂ which was yielded during the photodegradation of MB with Fe₂BiSbO₇, Bi₂InTaO₇ or N-doped TiO₂ as catalyst under visible light irradiation. The amount of CO₂ increased gradually with increasing reaction time when MB was photodegraded by Fe₂BiSbO₇, Bi₂InTaO₇ or N-doped TiO₂. At the same time, after 200 min visible light irradiation, the CO₂ production of 0.11063 mmol with Fe₂BiSbO₇ as catalyst was higher than the CO₂ production of 0.05600 mmol with N-doped TiO₂ as catalyst. Meanwhile, after visible light irradiation for 200 min, the CO₂ production of 0.05600 mmol with N-doped TiO₂ as catalyst was higher than the CO₂ production of 0.04934 mmol with Bi₂InTaO₇ as catalyst.

Figure 5 

CO2 production kinetics during the photocatalytic degradation of methylene blue with Fe2BiSbO7, Bi2InTaO7, or N-doped TiO2 as catalyst under visible light irradiation.

The first-order nature of the photocatalytic degradation kinetics with Fe₂BiSbO₇, Bi₂InTaO₇, or N-doped TiO₂ 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 Fe₂BiSbO₇, Bi₂InTaO₇, or N-doped TiO₂. Here, 𝐶 represented the MB concentration at time 𝑡, 𝐶_𝑜 represented the initial MB concentration, TOC represented the total organic carbon concentration at time 𝑡, and TOC_o represented the initial total organic carbon concentration. According to Figure 6, the first-order rate constant k_C of MB concentration was estimated to be 0.01189 min⁻¹ with Fe₂BiSbO₇ as catalyst, 0.00275 min⁻¹ with Bi₂InTaO₇ as catalyst, and 0.00333 min⁻¹ with N-doped TiO₂ as catalyst. The different value of k_C indicated that Fe₂BiSbO₇ was more suitable for the photocatalytic degradation of MB under visible light irradiation than N-doped TiO₂ or Bi₂InTaO₇. Meanwhile N-doped TiO₂ was more suitable for the photocatalytic degradation of MB under visible light irradiation than Bi₂InTaO₇. Figure 6 also showed that the first-order rate constant 𝐾_TOC of TOC was estimated to be 0.01101 min⁻¹ with Fe₂BiSbO₇ as catalyst, 0.00275 min⁻¹ with N-doped TiO₂ as catalyst, and 0.00259 min⁻¹ with Bi₂InTaO₇ 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 Fe₂BiSbO₇ showed higher mineralization efficiency for MB degradation compared with N-doped TiO₂ or Bi₂InTaO₇. At the same time, N-doped TiO₂ showed higher mineralization efficiency for MB degradation compared with Bi₂InTaO₇.

(a)

(b)

Figure 6 

Observed first-order kinetic plots for the photocatalytic degradation of methylene blue with Fe2BiSbO7, Bi2InTaO7, or N-doped TiO2 as catalyst under visible light irradiation.

Some inorganic ions such as NH₄⁺, NO₃⁻, and SO₄²⁻ were formed in parallel as the end products of nitrogen and sulfur atoms which existed in MB. Figures 7 and 8 showed the concentration variation of SO₄²⁻ and NO₃⁻ during photocatalytic degradation of MB with Fe₂BiSbO₇, Bi₂InTaO₇, or N-doped TiO₂ as catalyst under visible light irradiation. The results showed that the concentration of NO₃⁻ or SO₄²⁻ increased gradually with increasing reaction time when MB was photodegraded by Fe₂BiSbO₇, Bi₂InTaO₇, or N-doped TiO₂. Monitoring the presence of ions in the solution revealed that the SO₄²⁻ ion concentration was 0.01849 mM, 0.00924 mM, or 0.00757 mM with Fe₂BiSbO₇, N-doped TiO₂, or Bi₂InTaO₇ as catalyst after visible light irradiation for 200 min, indicating that 63.22%, 36.94%, or 30.28% of sulfur from MB was converted into sulfate ions with Fe₂BiSbO₇, N-doped TiO₂, or Bi₂InTaO₇ as catalyst after visible light irradiation for 200 min. It could be seen from Figure 8 that the NO₃⁻ ion concentration was 0.05258 mM, 0.0351 mM, or 0.02232 mM with Fe₂BiSbO₇, N-doped TiO₂, or Bi₂InTaO₇ as catalyst after visible light irradiation for 200 min, which indicated that 70.11%, 46.80%, or 29.76% of nitrogen from MB was converted into nitrate ions with Fe₂BiSbO₇, N-doped TiO₂, or Bi₂InTaO₇ as catalyst after visible light irradiation for 200 min. The sulfur was first hydrolytically removed, and subsequently was oxidized and transformed into SO₄²⁻. At the same time, nitrogen atoms in the −3 oxidation state produced NH₄⁺ cations that subsequently were oxidized into NO₃⁻ ions. As expected, the formation kinetics with Fe₂BiSbO₇ was significantly faster than that of N-doped TiO₂ or Bi₂InTaO₇ by using the same amount of photocatalyst. Moreover, the formation kinetics with N-doped TiO₂ was faster than that of Bi₂InTaO₇ by using the same amount of photocatalyst. It was noteworthy that the amount of SO₄²⁻ ions which was released into the solution was lower than the amount of SO₄²⁻ which should come from stoichiometry. One possible reason could be a loss of sulfur-containing volatile compounds such as SO₂. The second possible reason was a partially irreversible adsorption of some SO₄²⁻ ions on the surface of the photocatalyst which had been observed by Lachheb et al. by titanium dioxide [37]. Regardless, whether the sulfate ions were adsorbed irreversibly on the surface or not, it was important to stress that the evidence for restrained photocatalytic activity was not noticed.

Figure 7 

The concentration variation of SO42− during photocatalytic degradation of methylene blue with Fe2BiSbO7, Bi2InTaO7, or N-doped TiO2 as catalyst under visible light irradiation.

Figure 8 

The concentration variation of NO3− during photocatalytic degradation of methylene blue with Fe2BiSbO7, Bi2InTaO7, or N-doped TiO2 as catalyst under visible light irradiation.

The photodegradation intermediate products of MB in our experiment were identified as azure B, azure A, azure C, thionine, phenothiazine, leucomethylene blue, N,N-dimethylp-phenylenediamine, phenol, and aniline. According to the intermediate products which were found in this work and the observed appearance time of other intermediate products, a possible photocatalytic degradation pathway for MB was proposed. Figure 9 shows the suggested photocatalytic degradation pathway scheme for methylene blue under visible light irradiation in the presence of Fe₂BiSbO₇. The molecule of MB was converted into small organic species, which were subsequently mineralized into inorganic products such as SO₄²⁻ ions, NO₃⁻ ions, CO₂, and water ultimately.

Figure 9 

Suggested photocatalytic degradation pathway scheme for methylene blue under visible light irradiation in the presence of Fe2BiSbO7.

3.3. Photocatalytic Degradation Mechanism

The action spectra of MB degradation with Fe₂BiSbO₇ as catalyst were observed under visible light irradiation. A clear photonic efficiency (0.0103% at its maximal point) at wavelengths which corresponded to sub-Eg energies of the photocatalysts (𝜆 from 375 to 700 nm) was observed for Fe₂BiSbO₇. The existence of photonic efficiency at this region revealed that photons are not absorbed by the photocatalysts. In particular, the correlation between the low-energy action spectrum and the absorption spectrum of MB clearly demonstrated that any photodegradation results at wavelengths above 545 nm should be attributed to photosensitization effect by the dye MB itself (Scheme I).

Scheme I
Consider MB−−−−−−−→_ads^VisiblelightMB^∗_adsMB^∗_ads+Fe₂BiSbO₇⟶Fe₂BiSbO₇(e)+MB⁺_adsFe₂BiSbO₇(e)+O₂⟶Fe₂BiSbO₇+⋅O₂⁻ (2)

According to the mechanism which was shown in Scheme I, MB which was adsorbed on Fe₂BiSbO₇ was excited by visible light irradiation. Subsequently, an electron was injected from the excited MB to the conduction band of Fe₂BiSbO₇ where the electron was scavenged by molecular oxygen. Scheme I explained the results which were obtained with Fe₂BiSbO₇ as catalyst under visible light irradiation, where the photocatalyst Fe₂BiSbO₇ could serve to reduce recombination of photogenerated electrons and holes by scavenging of electrons [38].

Below the wavelength of 545 nm, the situation was different. The results of photonic efficiency correlated well with the absorption spectra of Fe₂BiSbO₇. These results evidently showed that the mechanism which was responsible for the photodegradation of MB went through band gap excitation of Fe₂BiSbO₇. Despite the detailed experiments about the effect of oxygen and water were not performed, it was logical to presume that the mechanism in the first step was similar to the observed mechanism for Fe₂BiSbO₇ under suprabandgap irradiation, namely Scheme II.

Scheme II
Consider Fe₂BiSbO−−−−−−−→₇^Visiblelightℎ⁺+e⁻e⁻+O₂⟶⋅O₂⁻ℎ⁺+OH⁻⟶⋅OH (3)

According to first principles calculations, we deduced that the conduction band of Fe₂BiSbO₇ was composed of Fe 3d and Sb 5p orbital component, and the valence band of Fe₂BiSbO₇ was composed of O 2p and Bi 6s orbital component. Fe₂BiSbO₇ 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 Fe₂BiSbO₇ was necessary for the photocatalytic degradation process of MB.

Former luminescent studies had shown that the closer the M–O–M bond angle was 180°, the more delocalized was the excited state [39], as a result, the charge carriers could move more easily in the matrix. The mobility of the photoinduced electrons and holes influenced the photocatalytic activity because high diffusivity indicated the enhancement of probability that the photogenerated electrons and holes would reach the reactive sites of the catalyst surface. For Fe₂BiSbO₇, the bond angle of Bi–O–Sb was 119.76°, which indicated that the bond angle of Bi–O–Sb was close to 180°. Thus, the photocatalytic activity of Fe₂BiSbO₇ was consequently higher. The crystal structure and the electronic structure of Fe₂BiSbO₇ and N-doped TiO₂ were totally different. For Fe₂BiSbO₇, Fe was 3d-block metal element, and Bi was 6p-block metal element, and Sb was 5p-block metal element. But for N-doped TiO₂, Ti was 3d-block metal element, indicating that the photocatalytic activity might be affected by not only the crystal structure but also the electronic structure of the photocatalysts, as well. In conclusion, the different photodegradation effect of MB between Fe₂BiSbO₇ and N-doped TiO₂ could be mainly attributed to the difference of their crystalline structures and electronic structures.

The present results indicated that the Fe₂BiSbO₇-visible light photocatalysis system might be regarded as a practical method for treatment of diluted colored wastewater. This system could be utilized for decolorization, purification, and detoxification of textile, printing, and dyeing industries in the long-day countries. Meanwhile, this system did not need high pressure of oxygen, heating, or any chemical reagents. Much decolorized and detoxified water were flowed from our new system for treatment, and the results showed that the Fe₂BiSbO₇-visible light photocatalysis system might provide a valuable treatment for purifying and reusing colored aqueous effluents.

4. Conclusions

Fe₂BiSbO₇ was prepared by the solid-state reaction method for the first time. The structural and photocatalytic properties of Fe₂BiSbO₇ were investigated. XRD results indicated that Fe₂BiSbO₇ was crystallized with the pyrochlore-type structure, cubic crystal system, and space group Fd3m. The lattice parameter of Fe₂BiSbO₇ was found to be 𝑎=10.410297 Å. Photocatalytic decomposition of aqueous MB was realized under visible light irradiation in the presence of Fe₂BiSbO₇, Bi₂InTaO₇, or N-doped TiO_2. The results showed that Fe₂BiSbO₇ owned higher catalytic activity compared with pure TiO₂, Bi₂InTaO₇, or N-doped TiO₂ for photocatalytic degradation of MB under visible light irradiation. The photocatalytic degradation of MB with Fe₂BiSbO₇, Bi₂InTaO₇, or N-doped TiO₂ as catalyst followed the first-order reaction kinetics, and the first-order rate constant was 0.01189 min⁻¹, 0.00275 min⁻¹, or 0.00333 min⁻¹. Complete removal and mineralization of MB was observed after visible light irradiation for 230 min with Fe₂BiSbO₇ as catalyst. The reduction of the total organic carbon, the formation of inorganic products such as SO₄²⁻ and NO₃⁻, and the evolution of CO₂ revealed the continuous mineralization of MB during the photocatalytic process. The possible photocatalytic degradation pathway of MB was obtained under visible light irradiation. Fe₂BiSbO₇/(visible light) photocatalysis system was found to be suitable for textile industry wastewater treatment and could be used to solve other environmental chemical pollution problems.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (no. 20877040), by a grant from the Technological Supporting Foundation of Jiangsu Province (no. BE2009144), by a grant from China-Israel Joint Research Program in Water Technology and Renewable Energy (no. 5), by a grant from New Technology and New Methodology of Pollution Prevention Program from Enviromental Protection Department of Jiangsu Province of China during 2010 and 2012 (no. 201001), by a grant from The Fourth Technological Development Scheming (Industry) Program of Suzhou City of China from 2010 (SYG201006), and by a grant from the Fundamental Research Funds for the Central Universities.