Bi2O2CO3 photocatalyst: Synthesis, characterization and application in recyclable photodegradation of organic dyes under visible light irradiation

Materials Chemistry and Physics 142 (2013) 95e105

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Heterostructured Fe3O4/Bi2O2CO3 photocatalyst: Synthesis, characterization and application in recyclable photodegradation of organic dyes under visible light irradiation Gangqiang Zhu a, *, Mirabbos Hojamberdiev b, Ken-ichi Katsumata b, Xu Cai a, Nobuhiro Matsushita b, Kiyoshi Okada b, Peng Liu a, Jianping Zhou a a b

School of Physics and Information Technology, Shaanxi Normal University, Xi’an 710062, PR China Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori, Yokohama, Kanagawa 226-8503, Japan

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Sphere- and flower-like Fe3O4/Bi2O2CO3 was synthesized by hydrothermal method.
Fe3O4 nanoparticles with the size of ca. 10 nm were synthesized by chemical method.
Photocatalysts demonstrate superparamagnetic behavior at room temperature.
Photocatalysts exhibit highly efficient visible-light-driven photocatalytic activity.
Photocatalysts can be easily recovered by applying an external magnetic field.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 30 June 2012 Received in revised form 25 May 2013 Accepted 25 June 2013

Heterostructured Fe3O4/Bi2O2CO3 photocatalyst was synthesized by a two-step method. First, Fe3O4 nanoparticles with the size of ca. 10 nm were synthesized by chemical method at room temperature and then heterostructured Fe3O4/Bi2O2CO3 photocatalyst was synthesized by hydrothermal method at 180  C for 24 h with the addition of 10 wt% Fe3O4 nanoparticles into the precursor suspension of Bi2O2CO3. The pH value of synthesis suspension was adjusted to 4 and 6 with the addition of 2 M NaOH aqueous solution. By controlling the pH of synthesis suspension at 4 and 6, sphere- and flower-like Fe3O4/Bi2O2CO3 photocatalysts were obtained, respectively. Both photocatalysts demonstrate superparamagnetic behavior at room temperature. The UVevis diffuse reflectance spectra of the photocatalysts confirm that all the heterostructured photocatalysts are responsive to visible light. The photocatalytic activity of the heterostructured photocatalysts was evaluated for the degradation of methylene blue (MB) and methyl orange (MO) in aqueous solution over the photocatalysts under visible light irradiation. The heterostructured photocatalysts prepared in this study exhibit highly efficient visible-light-driven photocatalytic activity for the degradation of MB and MO, and they can be easily recovered by applying an external magnetic field. Ó 2013 Elsevier B.V. All rights reserved.

Keywords: A. Inorganic compounds A. Magnetic materials A. Nanostructures A. Semiconductors B. Chemical synthesis C. Heterostructures

* Corresponding author. Tel./fax: þ86 29 81530750. E-mail address: [email protected] (G. Zhu). 0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.06.046

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1. Introduction Water pollution with organic compounds has been one of the challenging environmental issues in many countries. In particular, synthetic dyes have become major water pollutants generated mainly from textile dyeing, paper printing, color photography and petroleum industries because about 10e15% of the total world production of synthetic dyes is lost during textile dyeing, more precisely, released into the textile effluents [1]. A wide variety of synthetic dyes, namely, azo, polymeric, anthraquinone, triphenylmethane and heterocyclic dyes, is used in textile dyeing processes. Therefore, continuous discharge of dye-bearing effluents from textile industries into natural stream and rivers poses severe environmental problems because synthetic dyes are considered to be one of the poisonous and harmful compounds for water, soil and human life. Moreover, synthetic dyes can also hinder sunlight’s penetration into the water, leading to the eutrophication and the die-off of plants and animals [2]. Due to the complex nature and difficulty of removal of synthetic dyes by conventional methods, textile dyeing industry is presently facing problems to safe discharge of wastewater. In recent years, semiconductor-based photocatalysis has attracted considerable scientific attention in the field of environmental remediation and solar energy utilization [3]. Till date, much work has been done to develop advanced materials with high photocatalytic performance. Over 150 photocatalysts, including oxides, sulfides, nitrides, and hydroxides, have been widely reported to degrade organic pollutants and split water for H2 production [4,5]. Currently, numerous research groups are actively working on the development of new photocatalysts, optimization of the crystal structure, and tailoring chemical and physical properties in order to enhance photocatalytic performance under visible light irradiation. Recently, the coupling of two semiconductors has received much interest because of enhanced photocatalytic activity resulted from the formed heterojunction structures. The combination of two semiconductors in contact having different redox energy levels of their corresponding conduction and valence bands can actually be considered as one of the most promising methods to improve the photo-generation of electrons (e) and holes (hþ), to increase the lifetime of charge carriers and to enhance the efficiency of the interfacial charge transfer to adsorbed substrate [6]. Moreover, the enhancement of surface acidity or alkalinity [7e9] and the surface population of OH groups [10,11] can additionally promote the adsorption of reaction substrates and can facilitate the generation of hydroxyl radicals (
OH), respectively. Therefore, the development of highly efficient, low cost and visible-light-active photocatalysts has been imperative from the solar energy utilization point of view. The separation of photocatalyst powders in aqueous media after photocatalytic reaction is another critical issue. So, the magnetic separation can provide a very convenient approach for collecting and recycling magnetic photocatalyst powders by applying an external magnetic field. As an important magnetic oxide material with strong ferromagnetic property, magnetite (Fe3O4) has been extensively studied for diverse applications, such as environmental protection, sensors, electronic devices, magnetic storage media, and clinical diagnosis and treatment [12e14]. Especially, magnetite has attracted recent research interest because of its good photocatalytic activity for the degradation of organic dye molecules and its excellent magnetic separation. Also, the incorporation of Fe3O4 particles into a second semiconductor matrix may prevent the aggregation of nanoparticles during recovery and can increase the durability of photocatalysts [15]. Such heterostructured photocatalysts may also have high specific surface area and well-defined pore size that can additionally enhance photocatalytic activity [16].

For instance, the heterostructured Fe3O4/TiO2 [17], Fe3O4/C/CdS [18], Fe3O4@Bi2O3 [19], Fe3O4/WO3 [20], Fe3O4@C@Cu2O [21], Fe3O4/ZnO [22], and Bi2WO6@carbon/Fe3O4 [23] photocatalysts have demonstrated excellent photocatalytic activity under visible light irradiation and good recovery after photodegradation reaction. Bismuth subcarbonate (Bi2O2CO3) is a typical “Sillén” phase, belonging to the Aurivillius-related oxide family, and has an 2 intergrowth of Bi2O2þ 2 layers and CO3 layers with the plane of the CO2 group orthogonal to the plane of the Bi2O2þ layer [24]. 2 3 Therefore, the internal layered structure of Aurivillius-related Bi2O2CO3 can cause lower growth rate along (001) axis compared to that along other axes, and thus 2D morphologies like nanosheets can easily be formed. Although Bi2O2CO3 was initially used for medical and health-care purposes and microelectrode [25,26], it has recently obtained interest for the photodegradation of organic pollutants under visible light irradiation due to its band gap of 3.1 eV. Zheng et al. [27] investigated photocatalytic activity of Bi2O2CO3 nanostructures with different morphologies and found that Bi2O2CO3 with the exposed {001} plane displays the best photocatalytic performance because of numerous oxygen defects supplied by the distorted BieO polyhedron. Liu et al. [28] synthesized Bi2O2CO3 nanosheets using urea as a carbon source and studied its band gap and electronic structures using first-principle calculations. Zhao et al. [29] fabricated Bi2O2CO3 microspheres with highly efficient visible-light-driven photocatalytic activity because of a relatively narrow band gap and a high specific surface area. Cheng et al. [30] synthesized Bi2O2CO3 hierarchical microflowers by a template-free process and these microstructures with excellent photocatalytic activity. As mentioned before, the Bi2O2CO3 nanoand microstructures are also difficult to remove from aqueous solution after photocatalytic reaction, which may undesirably increase the cost of industrial applications and may additionally pollute the treated water. The present work aims to synthesize and incorporate Fe3O4 nanoparticles in Bi2O2CO3 to obtain heterostructured Fe3O4/ Bi2O2CO3 photocatalysts with different morphologies by chemical and hydrothermal methods, respectively. By controlling the pH of synthesis suspension at 4 and 6, sphere- and flower-like Fe3O4/ Bi2O2CO3 photocatalysts were obtained, respectively. The photocatalytic activity of the heterostructured photocatalysts was evaluated for the degradation of methylene blue (MB) and methyl orange (MO) in aqueous solution over the photocatalysts under visible light irradiation. It is found that the Fe3O4/Bi2O2CO3 nanocomposite photocatalysts can be easily recovered and recycled after the photodegradation process because of the presence of magnetic Fe3O4. 2. Experimental 2.1. Synthesis of Fe3O4 nanoparticles All the chemical reagents purchased from Sinopharm Chemical Reagent Beijing Co., Ltd. (China) were of analytical grade and used as received without further purification. The Fe3O4 nanoparticles were synthesized by chemical method at room temperature. 200 mL of deionized water (Millipore Milli-Q Plus purification system, 18.2 M U cm) was poured into a bottom round flask, and subsequently, the water was deoxygenated by bubbling N2 gas for 30 min. Later, 50 mL of ammonium chloride (NH4Cl) was introduced and mixed for 10 min using a propeller stirrer. Afterwards, 20 mL of 0.02 M ferrous chloride (FeCl2) aqueous solution and 40 mL of 0.02 M ferric chloride (FeCl3) aqueous solution were simultaneously added. Immediately, a black-colored precipitate was formed. The precipitate was collected by centrifugation,

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The photodegradation of methyl orange (MO) and methylene blue (MB) in aqueous solution over the photocatalyst samples was performed under visible light irradiation using a 300W Xe arc lamp (PLS-SXE300, Beijing Trusttech Co., Ltd., China) with a cutoff filter (l > 420 nm). The temperature of the photocatalytic reaction was maintained at 25  C by running tap water in the cooling jacket of

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2.4. Photodegradation experiments

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The crystalline phases of the samples were identified by X-ray powder diffraction using a D/Max2550 X-ray diffractometer (Rigaku, Japan) with Cu Ka radiation (l ¼ 1.5406  A). The samples were scanned at a scanning rate of 5 min1 in the 2q range from 10 to 70 at 40 kV and 40 mA. The morphological features of the samples were observed using a Quanta 200 scanning electron microscope (FEI, The Netherlands) with an acceleration voltage of 15 kV, equipped with X-ray energy dispersive spectroscopy (EDX). The transmission electron microscopic (TEM) and high-resolution transmission electron microscopic (HRTEM) images and selected area electron diffraction (SAD) patterns of the samples were taken with a JEM-2100 transmission electron microscope (JEOL, Japan) with an acceleration voltage of 200 kV. X-ray photoelectron spectroscopic (XPS) analysis was performed on an ESCALAB MKII X-ray photoelectron spectrometer (VG Scientific, UK) using Mg Ka radiation. The UVevis diffuse reflectance spectra were measured by using a Lambda 950 UVeVISeNIR spectrophotometer (Perkine Elmer, USA) in the wavelength range of 200e750 nm. The Fourier transform infrared (FTIR) spectra of the samples were recorded on an IRAffinity-1 FTIR spectrometer (Shimadzu, Japan) with a maximum resolution of 4 cm1. The magnetic properties of the samples were measured using an MPMS XL-7 superconducting quantum interference device (SQUID) magnetometer (Quantum Design, USA) at room temperature.

Fig. 1 shows the XRD patterns of the Fe3O4 and Bi2O2CO3 powders synthesized by chemical method at room temperature and hydrothermal method at 180  C for 24 h, respectively. For the Fe3O4 powders prepared by chemical method at room temperature, all the diffraction peaks in the XRD pattern can be readily indexed to cubic Fe3O4 phase (JCPDS card no. 88-0315). No characteristic peaks belonging to impurity phases (Fe2O3 or FeOOH) were observed, indicating high purity of the Fe3O4 powders. The particle size of the Fe3O4 powders estimated using Scherrer’s formula is ca. 10 nm. It is noted that all the diffraction peaks in the XRD pattern of the Bi2O2CO3 powders synthesized by hydrothermal method at 180  C for 24 h match with that of standard XRD pattern (JCPDS card no. 41-1488) for tetragonal Bi2O2CO3 phase. Compared with other characteristic diffraction peaks of Bi2O2CO3, an intensive (110) diffraction peak can be seen in the XRD pattern, implying that there is a bias of orientations of the {110} crystallographic plane. Fig. 2 shows the SEM, TEM and HRTEM images of the Fe3O4 (a) and Bi2O2CO3 (bed) powders synthesized by chemical method at room temperature and hydrothermal method at 180  C for 24 h, respectively. As shown in the TEM image in Fig. 2a, the assynthesized Fe3O4 nanoparticles have apparently a near-spherical shape with the diameter of 6e12 nm, which is consistent with the estimated particle size of Fe3O4 using Scherrer’s formula (about 10 nm). The HRTEM image shown in the inset of Fig. 2a indicates that the Fe3O4 nanoparticles are single crystals. The interplanar spacing of 0.255 nm agrees well with the spacing of the (311) lattice plane of cubic Fe3O4 phase. Fig. 2b shows the SEM image of the assynthesized Bi2O2CO3 powders. As can be noticed, the final product is dominantly consisted of large-scale sphere-like structures with the size of 3e6 mm. The TEM image shown in Fig. 2c confirms that each sphere-like structure of Bi2O2CO3 powders is composed of

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2.3. Characterization

3. Results and discussion

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To obtain Bi2O2CO3 powders, 2.0 mmol of bismuth nitrate pentahydrate [Bi(NO3)3$5H2O] was dissolved in 20 mL of 1 M HNO3, and then 1.5 mmol of citric acid (C6H8O7) was added under magnetic stirring. The pH of the solution was adjusted to 4 with the addition of 2 M NaOH aqueous solution under magnetic stirring. Finally, the white-colored precursor suspension formed was transferred into a 40 mL Teflon-lined stainless steel autoclave. The autoclave was sealed and maintained at 180  C for 24 h. After hydrothermal treatment, the autoclave was cooled down to room temperature naturally. The resulting precipitate was collected by centrifugation, washed with deionized water several times and dried at 60  C for 6 h. To synthesize heterostructured Fe3O4/ Bi2O2CO3 photocatalyst, identical experimental procedure applied for the synthesis of the Bi2O2CO3 powders was further used with the addition of 10 wt% Fe3O4 nanoparticles into the white-colored precursor suspension of Bi2O2CO3. The pH of the synthesis suspension was adjusted to 4 and 6 with the dropwise addition of 2 M NaOH aqueous solution under propeller stirring. After being well homogenized for 30 min, the suspension was transferred into a 40 mL Teflon-lined stainless steel autoclave, sealed and maintained at 180  C for 24 h. After hydrothermal treatment, the autoclave was cooled down to room temperature naturally. The resulting precipitate was collected by centrifugation, washed with deionized water several times and dried at 60  C for 6 h.

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2.2. Syntheses of Bi2O2CO3 and Fe3O4/Bi2O2CO3

the reactor. The initial concentration of MO and MB was 10 mg L1, and the amount of photocatalyst used was 1.0 g L1. First, the suspension was sonicated for 10 min and then stirred with propeller stirrer in the dark for 30 min to ensure adsorptionedesorption equilibrium prior to visible light irradiation. During irradiation, 2 mL of suspension was taken out at a given time interval for subsequent MO and MB concentration analysis. The MO and MB concentration was analyzed by using a U-3010 UVevis spectrophotometer (Hitachi, Japan) against irradiation time.

Intensity [a.u.]

washed with deionized water several times and dried at 60  C for 6 h.

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Cu K 2 [deg.] Fig. 1. XRD patterns of Fe3O4 and Bi2O2CO3 powders synthesized by chemical method at room temperature and hydrothermal method at 180  C for 24 h, respectively.

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Fig. 2. SEM, TEM and HRTEM images of Fe3O4 (a), and Bi2O2CO3 (bed) powders synthesized by chemical method at room temperature and hydrothermal method at 180  C for 24 h, respectively.

can be attributed to the stretching and deformation vibration of the OeH groups of chemisorbed and/or physisorbed water molecules. The band at 548 cm1 is assigned to the bismuth-oxygen bonds. The bands at 556e637 cm1 are assigned to the FeeO bonds. The FTIR spectra of Bi2O2CO3 powders and sphere- and flower-like Fe3O4/Bi2O2CO3 photocatalysts show intensive peaks at 1351 cm1 and 844 cm1, assigning to the n3 mode of the CO2 3

Bi O CO Fe O

Intensity [a.u.]

nanoflakes. The HRTEM image taken on a single nanoflake is shown in Fig. 2d. The crystal lattice fringes can be clearly observed, and average distances between the neighboring lattice fringes are about 0.273 nm and 0.272 nm that correspond to the distances between the (110) and (110) lattice planes of tetragonal Bi2O2CO3. This suggests that the Bi2O2CO3 nanoflakes grow along the [001] direction. This finding is consistent with the XRD data in Fig. 1, indicating an intensive (110) diffraction peak. The HRTEM results reveal that the exposed basal plane is (001), featuring a square arrangement of atoms along with the instinct layer, as shown in the inset of Fig. 2d. Fig. 3 shows the XRD patterns of heterostructured Fe3O4/ Bi2O2CO3 photocatalysts synthesized by hydrothermal method at 180  C for 24 h with pH ¼ 4 (a) and pH ¼ 6 (b). All the diffraction peaks in both XRD patterns can be readily indexed to tetragonal Bi2O2CO3 phase with the space group of I4/mmm(139). The diffraction peaks with high intensity indicate high crystallinity of the as-synthesized Bi2O2CO3 powders. Due to low content (10 wt%) in the heterostructured photocatalyst and small particle size (about 10 nm), the intensity of the diffraction peaks belonging to the Fe3O4 phase is insignificant. Nevertheless, the diffraction peaks of Fe3O4 phase can only be observed in the XRD patterns at 35.7. To investigate the chemical compositions and chemical bonding in the samples, Fourier transform infrared (FTIR) spectroscopic analysis was performed. The FTIR spectra of Fe3O4 nanoparticles (a), sphere-like Fe3O4/Bi2O2CO3 photocatalyst (b), flower-like Fe3O4/ Bi2O2CO3 photocatalyst (c) and Bi2O2CO3 powders (d) are plotted in Fig. 4. In the spectra, two bands at about 3389 cm1 and 1730 cm1

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Cu K 2 [deg.] Fig. 3. XRD patterns of heterostructured Fe3O4/Bi2O2CO3 photocatalysts prepared by hydrothermal method at 180  C for 24 h with pH ¼ 4 (a) and pH ¼ 6 (b).

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Wavenumber [cm ] Fig. 4. FTIR spectra of Fe3O4 (a), heterostructured Fe3O4/Bi2O2CO3 photocatalysts hydrothermally prepared at pH ¼ 4 (b) and pH ¼ 6 (c), and Bi2O2CO3 (d).

group and the n2 mode of the CO2 3 group, respectively [31,32]. The peak centered at 1630 cm1 is assigned to a stretching mode and bending mode of adsorbed H2O. In addition, the peak at 2344 cm1 derived from the vibration of the CO2 3 group is also observed in the FTIR spectra of all the samples. The CO2 3 vibration band in the FTIR spectra of the Fe3O4 nanoparticles, Bi2O2CO3 powders and sphereand flower-like Fe3O4/Bi2O2CO3 photocatalysts could be attributed to the trapped or adsorbed CO2 impurity from the atmosphere. As mentioned above, by controlling the pH of synthesis suspension at 4 and 6, sphere- and flower-like heterostructured Fe3O4/ Bi2O2CO3 photocatalysts were hydrothermally obtained, respectively. To study the effect of the pH of synthesis suspension on morphology of the heterostructured Fe3O4/Bi2O2CO3 photocatalyst, the as-prepared samples were subjected to the SEM and TEM observations. Fig. 5 shows the SEM (a, b), TEM (c, d) and HRTEM (e) images and EDX spectrum (f) of heterostructured Fe3O4/Bi2O2CO3 photocatalyst synthesized by hydrothermal method at 180  C for 24 h with pH ¼ 4. It can be seen from the SEM and TEM images shown in Fig. 5aec that sphere-like particles with the size of 3e 5 mm are consisted of nanoflakes. The TEM image shown in Fig. 5d evidences the attachment of Fe3O4 nanoparticles with the size of about 10 nm to the flake-like nanostructures of Bi2O2CO3. The HRTEM image in Fig. 5e shows a clear interface of Fe3O4 nanoparticle and Bi2O2CO3 nanoflake. Moreover, it indicates the singlecrystalline nature of Bi2O2CO3 nanoflake. The lattice spacings of 0.272 nm and 0.273 nm correspond to the ¼ d100 0.273 nm and d110 ¼ 0.273 nm, respectively, of standard data reported for Bi2O2CO3 (JCPDS card no. 41-1488). The elemental composition of sphere-like Fe3O4/Bi2O2CO3 photocatalyst was analyzed by using an energy dispersive X-ray spectrometer (EDX) attached to scanning electron microscope (SEM). The EDX spectrum of final product shown in Fig. 5f reveals that bismuth, iron, carbon and oxygen are present. Fig. 6 shows the SEM (a, b), TEM (c, d) and HRTEM (e) images and EDX spectrum (f) of heterostructured Fe3O4/Bi2O2CO3 photocatalyst synthesized by hydrothermal method at 180  C for 24 h with pH ¼ 6. As shown in the SEM image in Fig. 6a, heterostructured Fe3O4/Bi2O2CO3 photocatalyst synthesized by hydrothermal method at 180  C for 24 h with pH ¼ 6 possesses flowerlike microstructures with the size of about 1e2 mm. The magnified SEM image of the sample shown in Fig. 6b indicates that each flower-like microstructure is constructed by self-assembly of plate-

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like nanostructures with the thickness of about 20 nm. Fig. 6c shows the TEM image of flower-like microstructures. According to the TEM image in Fig. 6d, Fe3O4 nanoparticles with the size of about 10 nm were attached to the plate-like nanostructures of Bi2O2CO3. The HRTEM image in Fig. 6e also confirms a clear interface between the Fe3O4 nanoparticles and Bi2O2CO3 nanoplates. The TEM results show that both Fe3O4 nanoparticles and Bi2O2CO3 nanoplates formed have single-crystal nature. The lattice spacings of 0.255 nm and 0.272 nm in the observed crystallite agree well with the (311) lattice plane of Fe3O4 and the (110) lattice plane of Bi2O2CO3, respectively. According to the EDS data shown in Fig. 6f, bismuth, iron, carbon and oxygen are predominant elements in the photocatalyst. In order to elucidate the elemental compositions and chemical states of Bi, O and Fe present in the photocatalyst samples, surface analysis was performed using X-ray photoelectron spectroscopy (XPS) for sphere- and flower-like Fe3O4/Bi2O2CO3 photocatalysts synthesized by hydrothermal method at 180  C for 24 h with pH ¼ 4 and pH ¼ 6, respectively. Fig. 7 shows the XPS wide scan spectra (A) and XPS spectra of Bi 4f (B), Fe 2p (C), and O 1s (D) of sphere- (a) and flower-like (b) Fe3O4/Bi2O2CO3 photocatalysts. The peak positions of different atoms were determined by internally referencing the adventitious carbon at a binding energy of 284.6 eV. Fig. 7A shows the XPS wide scan spectra of sphere- (a) and flower-like (b) Fe3O4/ Bi2O2CO3 photocatalysts. The XPS wide scan spectra clearly indicate the binding energies for Bi 4s, Bi 4f, Bi 4d, Bi 5d, Fe 2p, O 1s, and C 1s of sphere- and flower-like Fe3O4/Bi2O2CO3 photocatalysts. Fig. 7Be D shows XPS spectra of Bi 4f (B), Fe 2p (C), and O 1s (D) of sphere(a) and flower-like (b) Fe3O4/Bi2O2CO3 photocatalysts. As shown in Fig. 7B, the binding energies of Bi 4f7/2 and Bi 4f5/2 are 159.0 eV and 164.0 eV, respectively. Fig. 7C indicates two bands with the binding energies of 710.7 eV and 724.4 eV that are assignable to Fe 2p3/2 and Fe 2p1/2, respectively. Both bands consist of the Fe2þ (of FeO) and Fe3þ (of Fe2O3) peaks which are typical characteristics of the Fe3O4 structure [15,19]. Fig. 7D shows the binding energy of 535.0 for O 1s. All the results shown above provide the insight that the assynthesized heterostructured photocatalysts contain Fe3O4 and Bi2O2CO3 phases. The magnetic properties of the photocatalyst samples were studied by using a superconducting quantum interference device (SQUID) magnetometer at room temperature (300 K). Fig. 8 shows magnetization (M) versus applied field (H) curves of sphere- (red) and flower-like (black) Fe3O4/Bi2O2CO3 photocatalysts synthesized by hydrothermal method at 180  C for 24 h with pH ¼ 4 and pH ¼ 6, respectively. As shown in Fig. 8, for both photocatalyst samples the coercivity force is almost negligible at 300 K, indicating that these heterostructured Fe3O4/Bi2O2CO3 photocatalysts have superparamagnetic behavior at room temperature. The magnetization values were normalized by the sample weight using magnetic moment per gram (emu/g). So, the estimated magnetization values for sphere- and flower-like Fe3O4/Bi2O2CO3 photocatalysts are 11 emu/g and 12 emu/g, respectively. Strong magnetization of the heterostructured Fe3O4/Bi2O2CO3 photocatalysts allows them to be rapidly and conveniently separated from an aqueous solution by applying an external magnetic field. Fig. 9 shows the UVevis diffuse reflectance spectra of pure Bi2O2CO3 (black) and sphere- (red) and flower-like (green) Fe3O4/ Bi2O2CO3 photocatalysts synthesized by hydrothermal method at 180  C for 24 h. By judging the UVevis spectra of the photocatalyst samples, it can be concluded that sphere- and flower-like Fe3O4/ Bi2O2CO3 photocatalysts have strong absorption in the visible region compared to pure Bi2O2CO3 powders. In addition, a red-shift in the absorbance region was observed only for the heterostructured Fe3O4/Bi2O2CO3 photocatalysts. We believe that this might be caused by the presence of Fe3O4 nanoparticles incorporated to

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Fig. 5. SEM (a, b), TEM (c, d) and HRTEM (e) images and EDX spectrum (f) of heterostructured Fe3O4/Bi2O2CO3 photocatalyst synthesized by hydrothermal method at 180  C for 24 h with pH ¼ 4.

Bi2O2CO3 photocatalyst. These results suggest that the heterostructured Fe3O4/Bi2O2CO3 photocatalysts prepared are more effective in absorbing visible light that will profitably enhance photocatalytic activity for the degradation of organic dyes in aqueous solution under visible light irradiation. The photocatalytic activity of sphere-like Bi2O2CO3 powders and sphere- and flower-like Fe3O4/Bi2O2CO3 photocatalysts were evaluated by the degradation of methyl orange (MO) in aqueous solution under visible light irradiation at room temperature. Temporal changes in the concentration of MO were monitored by examining the variation in maximal absorption in the UVevis spectra at 463 nm. Fig. 10a shows the photodegradation of MO (C/C0) over

sphere-like Bi2O2CO3 powders and sphere- and flower-like Fe3O4/ Bi2O2CO3 photocatalysts under visible light irradiation. For comparison purposes, we additionally performed the experiments on direct photolysis of MO (a blank experiment) without any photocatalyst under identical experimental conditions. As a result, the blank experiment performed in the absence of photocatalyst shows no obvious change in the MO concentration within 40 min of reaction under visible light irradiation. As shown as a red line in Fig. 10a, the total photodegradation of MO over sphere-like Bi2O2CO3 powders is about 77% after 40 min under visible light irradiation. This indicates that sphere-like Bi2O2CO3 powders have good photocatalytic activity for the degradation of MO under visible light

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Fig. 6. SEM (a, b), TEM (c, d) and HRTEM (e) images and EDX spectrum (f) of heterostructured Fe3O4/Bi2O2CO3 photocatalyst synthesized by hydrothermal method at 180  C for 24 h with pH ¼ 6.

irradiation. More importantly, the photocatalytic activity of Bi2O2CO3 powders could be improved by the incorporation of Fe3O4 nanoparticles. As an example, the total photodegradation of MO over sphere- and flower-like Fe3O4/Bi2O2CO3 photocatalysts is 90% and 99%, respectively, after 40 min under visible light irradiation. The UVevis spectral changes of MO in aqueous solution over flower-like Fe3O4/Bi2O2CO3 photocatalyst under visible light are plotted in Fig. 10b as a function of irradiation time. It shows that the intensity of maximum absorption peak of MO at 463 nm decreases dramatically as visible light irradiation time increases and nearly disappears within 40 min.

Methylene blue (MB) was chosen to further investigate and compare the photocatalytic activity of sphere-like Bi2O2CO3 powders and sphere- and flower-like Fe3O4/Bi2O2CO3 photocatalysts. Temporal changes in the concentration of MB were monitored by examining the variation in maximal absorption in the UVevis spectra at 653 nm. Fig. 11a shows the photodegradation of MB (C/ C0) over sphere-like Bi2O2CO3 powders and sphere- and flower-like Fe3O4/Bi2O2CO3 photocatalysts under visible light irradiation. For comparison purposes, we additionally performed the experiments on direct photolysis of MB (a blank experiment) without any photocatalyst under identical experimental conditions. It can be seen

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Fig. 7. XPS wide scan spectra (A) and XPS spectra of Bi 4f (B), Fe 2p (C) and O 1s (D) of heterostructured Fe3O4/Bi2O2CO3 photocatalysts prepared by hydrothermal method at 180  C for 24 h with pH ¼ 4 (a) and pH ¼ 6 (b).

that the blank experiment performed in the absence of photocatalyst shows 6.2% degradation of MB after 40 min of reaction under visible light irradiation. The flower-like Fe3O4/Bi2O2CO3 photocatalyst (green line in Fig. 11a) shows good photocatalytic activity for the degradation of MB under visible light irradiation compared to sphere-like Bi2O2CO3 powders and sphere-like Fe3O4/

Bi2O2CO3 photocatalyst. The total photodegradation of MB over sphere-like Bi2O2CO3 powders and sphere- and flower-like Fe3O4/ Bi2O2CO3 photocatalysts is 90%, 98% and 99%, respectively, after 40 min under visible light irradiation. The UVevis spectral changes of MB in aqueous solution over flower-like Fe3O4/Bi2O2CO3

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M [emu/g]

10

flower-like Fe O /Bi O CO

(a)

sphere-like Fe O /Bi O CO

5 0 -5 -10

-20000

-10000

0

10000

20000

H [Oe] Fig. 8. Magnetization (M) versus applied field (H) curves of heterostructured Fe3O4/ Bi2O2CO3 photocatalysts prepared by hydrothermal method at 180  C for 24 h with pH ¼ 4 (a) and pH ¼ 6 (b).

Fig. 9. UVevis spectra of Bi2O2CO3 (black) and heterostructured Fe3O4/Bi2O2CO3 photocatalysts prepared by hydrothermal method at 180  C for 24 h with pH ¼ 4 (red) and pH ¼ 6 (green). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

G. Zhu et al. / Materials Chemistry and Physics 142 (2013) 95e105

1.0

1.0

0.8

0.8

(a)

0.4

sphere-like Bi 2O 2CO 3/Fe 3O 4 flower-like Bi2O2CO3 /Fe3O4 pure sphere-like Bi2O2CO3 blank

0.2

0.0 -30

-20

-10

0

10

(a)

0.6

C/C0

C/C0

0.6

0.4

pure sphere-like Bi O CO

0.2

flower-like Bi O CO /Fe O blank

0.0 20

30

103

40

-30

-20

-10

Time [min]

(b)

350

400

450

500

10

20

30

550

600

650

orginal adsorption 5 min 10 min 15 min 20 min 25 min 30 min 35 min 40 min

Adsorbance [a.u.]

Absorbance [a.u.]

orginal adsorption 5 min 10 min 15 min 20 min 25 min 30 min 35 min 40 min

300

0

40

Time [min]

300

400

photocatalyst under visible light are plotted in Fig. 11b as a function of irradiation time. It shows that the intensity of maximum absorption peak of MB at 653 nm decreases dramatically as visible light irradiation time increases and nearly disappears within 40 min. By considering its absorption edge in Fig. 9, pure Bi2O2CO3 can only absorb light <420 nm. Hence, visible light (l > 420 nm) could not excite Bi2O2CO3 to produce reactive radicals because of its wide band gap. Therefore, the degradation of MO and MB over pure Bi2O2CO3 through a photocatalytic pathway was negligible. Nevertheless, 77% MO and 90% MB were degraded by sphere-like Bi2O2CO3 powders under visible light, whereas 99% MO and 99% MB were degraded by flower-like Fe3O4/Bi2O2CO3 photocatalyst under visible light irradiation. The dye degradation mechanism of photosensitization pathway over wide band gap BiOCl (Eg ¼ 3.4 eV) semiconductor under visible light irradiation has been reported previously [33]. In the photosensitization pathway, dye molecules are considered as a photosensitizer, whereas the Bi2O2CO3 plays the key roles of electron carriers and electron acceptors. For the photocatalysis of pure Bi2O2CO3, the dye molecules (denoted as A) first absorbs the light energy to produce singlet and triplet states (denoted as A*) [33], and then the electrons can transfer to the conduction band of Bi2O2CO3 and react with O2 to generate O 2 and
OH. The A* is converted to the radical cation
Aþ that ultimately reacts with reactive oxygen radicals to yield the degraded products.

600

700

800

Wavelength [nm]

Wavelength [nm] Fig. 10. Photodegradation of MO (C/C0) over different photocatalysts under visible light irradiation (a). Temporal changes in UVevis spectra of MO in aqueous solution over flower-like Fe3O4/Bi2O2CO3 photocatalyst under visible light at different irradiation times (b).

500

(b)

Fig. 11. Photodegradation of MB (C/C0) over different photocatalysts under visible light irradiation (a). Temporal changes in UVevis spectra of MB in aqueous solution over flower-like Fe3O4/Bi2O2CO3 photocatalyst under visible light at different irradiation times (b).

Therefore, the dye molecules could be degraded by Bi2O2CO3 under visible light irradiation. Indeed, the photocatalytic activity of heterostructured Fe3O4/Bi2O2CO3 photocatalysts is higher than that of pure Bi2O2CO3 due to the synergetic effects of Fe3O4 and Bi2O2CO3. After the incorporation of Fe3O4 nanoparticles, the Fe3O4/Bi2O2CO3 photocatalysts become more effective in absorbing visible light than pure Bi2O2CO3 that will enhance the photocatalytic activity for the degradation of organic dyes under visible light irradiation. Furthermore, the coupling of a narrow-band-gap semiconductor with a wide-band-gap semiconductor not only enhances optical absorption ability but also facilitates the separation of the photogenerated charge carriers under internal field induced by different electronic band structures. Therefore, the heterostructured Fe3O4/ Bi2O2CO3 photocatalysts show higher photocatalytic activity than pure Bi2O2CO3 under visible light irradiation. The photodegradation mechanism can be described as follows:

A þ visible light/A* A* þ Bi2 O2 CO3 /$ A þ Bi2 O2 CO3 e

(1)


Fe3 O4 þ visible light/Fe3 O4 þ e þ hþ
$  O2 þ Bi2 O2 CO3 e CB / O2

(2)


(3) (4)

104

G. Zhu et al. / Materials Chemistry and Physics 142 (2013) 95e105 $ Aþ

þ


$ OH; $ O ; þO /CO 2 2 2

þ H2 O þ inorganic compounds (8)

hþ þ A/CO2 þ H2 O þ inorganic compounds

(9)

We believe that the magnetic property of Fe3O4/Bi2O2CO3 photocatalysts is essential for efficient recycling of the photocatalyst in liquid-phase reactions. As shown in Fig. 12a, when the flower-like Fe3O4/Bi2O2CO3 photocatalyst was dispersed in water, the suspension turned into a yellow color. By applying an external magnetic field, the black particles suspended in the aqueous solution were readily harvested within 30 s and the solution became transparent. It is known that the application of a photocatalyst strongly depends on its efficiency and stability. In order to propose the future application for the heterostructured Fe3O4/Bi2O2CO3 photocatalysts synthesized by hydrothermal method at 180  C for 24 h with pH ¼ 4 and pH ¼ 6, recyclability and stability of the photocatalyst samples were tested. For this purpose, flower-like Fe3O4/Bi2O2CO3 photocatalyst was selected and reused five times for the degradation of MB under visible light irradiation by keeping the experimental conditions (pH, initial MB concentration and ionic strength) unchanged. The results obtained are presented in Fig. 12bec. As shown in Fig. 12b, the flower-like Fe3O4/Bi2O2CO3 photocatalyst retains its high photocatalytic activity in five successive runs each of which lasted for 40 min under visible light irradiation. In each experimental run, the total photodegradation of MB was higher than >95%. The XRD pattern and SEM image of the photocatalyst used for five successive runs, shown in Fig. 12c, confirm recyclability and stability of the flower-like Fe3O4/Bi2O2CO3 photocatalyst for the photodegradation of MB under visible light irradiation. 4. Conclusions In summary, we have demonstrated a two-step method for the synthesis of visible-light-responsive heterostructured Fe3O4/Bi2O2CO3 photocatalyst. The Fe3O4 nanoparticles with the size of ca.10 nm were synthesized by chemical method at room temperature, and then heterostructured Fe3O4/Bi2O2CO3 photocatalyst was synthesized by hydrothermal method at 180  C for 24 h with the addition of 10 wt% Fe3O4 nanoparticles into the precursor suspension of Bi2O2CO3. Sphere- and flower-like Fe3O4/Bi2O2CO3 photocatalysts were obtained by adjusting the pH of synthesis suspension to 4 and 6, respectively. Both photocatalysts demonstrated superparam agnetic behavior at room temperature. The photocatalysts with two different morphologies exhibited highly efficient visible-lightdriven photocatalytic activity for the degradation of methyl orange and methylene blue, and they could be easily recovered and recycled by applying an external magnetic field. Acknowledgments Fig. 12. (a) Magnetic separation of flower-like Fe3O4/Bi2O2CO3 photocatalyst after photodegradation of MB under visible light irradiation. (b) Photocatalytic perfor mance of flower-like Fe3O4/Bi2O2CO3 photocatalyst after five cycles of photodegradation experiments of MB. (c) XRD pattern and TEM image (inset) of flower-like Fe3O4/Bi2O2CO3 photocatalyst after five cycles of photodegradation experiments.

$

þ $ O 2 þ H / OOH

(5)

$

þ $ O 2 þ H þ OOH/H2 O2 þ O2

(6)

 $ H2 O2 þ $ O 2 / OH þ OH þ O2

(7)

This work was financially supported by the Major Program of the National Natural Science Foundation of China (Grant no. 51102160) and the Fundamental Research Funds for the Central Universities (Program no. GK201102027). MH would also like to thank the Japan Society for the Promotion of Science (JSPS) for the financial support. References [1] H. Zollinger, Synthesis, Properties and Applications of Organic Dyes and Pigments. Colour Chemistry, John WileyeVCH Publishers, New York, 2002, pp. 92e100. [2] N. Buvaneswari, C. Kannan, J. Hazard. Mater. 189 (2011) 294e300. [3] A. Fujishima, K. Honda, Nature 238 (1972) 37e38.

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