Tunable Morphology of Bi2Fe4O9

Feb 18, 2009 - microscopy, transmission electron microscopy, high-resolution transmission electron microscopy, and UV-vis absorption spectra...
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J. Phys. Chem. C 2009, 113, 4168–4173

Tunable Morphology of Bi2Fe4O9 Crystals for Photocatalytic Oxidation Qian-Jing Ruan and Wei-De Zhang* Nano Science Research Center, School of Chemistry and Chemical Engineering, South China UniVersity of Technology, 381 Wushan Road, Guangzhou 510640, People’s Republic of China ReceiVed: NoVember 17, 2008; ReVised Manuscript ReceiVed: January 19, 2009

Orthorhombic Bi2Fe4O9 microplatelets and nanosheets were successfully synthesized through a rather facile hydrothermal process. The as-prepared samples were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy, and UV-vis absorption spectra. The photocatalytic activity of the as-prepared samples was evaluated by the degradation of methyl orange under UV and visible irradiation. The Bi2Fe4O9 nanosheets showed high photocatalytic activity toward methyl orange degradation under visible and UV irradiation, while the Bi2Fe4O9 microplatelets were active only under UV irradiation. TABLE 1: The Serial Numbers of the As-Prepared Samples and Their Synthesis Conditions

Introduction Since environmental pollution has exceeded the limit of natural self-purification capacity, semiconductor-based photocatalysis has attracted more and more attention as a kind of “green” technology for air and water purification with solar energy.1-5 It has been proved that TiO2 is undoubtedly an excellent photocatalyst for the oxidative decomposition of many organic compounds under UV irradiation.6-9 However, the relatively wide bandgap (3.2 eV) limits its further application in the visible region (λ > 410 nm).10-12 Therefore, various methods have been attempted to improve the photocatalytic activity of TiO2 under visible irradiation, such as nonmetal doping,13-15 metal deposition,16,17 semiconductor loading,18,19 etc. In recent years, some work on other semiconductor photocatalysts, like CaBi2O4, BiFeO3, SrTiO3-Fe2O3, and Bi6WO12 with visible-light-driven photocatalysis has also been reported.20-23 The bandgaps of these metal oxides are appropriate for the absorption of visible light, so they may potentially be efficient photocatalysts under visible light. In the present paper, we will introduce the semiconductor Bi2Fe4O9 as a novel photocatalyst with visible-light-driven photocatalytic activity. It is well-known that Bi2Fe4O9 is a semiconductive material, a potential gas sensitive material, and a good catalyst for ammonia oxidation to NO.24,25 The microstructure of a material plays an important role in determining the physical properties of crystals. The optical, catalytic, magnetic, and transport properties of semiconductive materials are closely related to their morphology, which has been extensively studied.26,27 For the sake of morphology control, various methods, including hydrothermal process, sol-gel, solidstate reaction, and molten-salt synthesis for the preparation of Bi2Fe4O9 have been employed.28-32 The hydrothermal process was selected for our experiment due to its low cost, simple process, and low reaction temperature. Here, we report a tunable hydrothermal process for the synthesis of Bi2Fe4O9 crystals with different morphologies. Their photocatalytic property was also studied. The experimental results indicated that both Bi2Fe4O9 microplatelets and nanosheets showed efficient degradation capacity to methyl orange (MO) under UV irradiation. However, * Corresponding author. E-mail: [email protected]. Fax: +86-2087112053. Phone: +86-20-87114099.

sample S1 S2 S3 S4

morphology

[Bi3+]/[Fe3+]

[NaOH] (M)

reaction time (h)

microplatelets microplatelets microplatelets nanosheets

1:2 1:2 1:2 1:1

4 8 12 1

24 24 24 12

Bi2Fe4O9 nanosheets showed photocatalysis to MO under visible irradiation while Bi2Fe4O9 microplatelets did not. The photocatalysis of Bi2Fe4O9 nanosheets under visible irradiation would be an advantage for further application. Experimental Section The Bi2Fe4O9 samples were prepared by hydrothermal synthesis. All the chemicals used are analytical grade reagents without further purification. In a typical synthesis of Bi2Fe4O9 microplatelets, 2 mmol of Fe(NO3)3 · 9H2O and 1 mmol of Bi(NO3)3 · 5H2O were dissolved in 10 mL of 1 M HNO3. The alkalinity of the solution was adjusted with NaOH. The concentration of NaOH was 4, 8, and 12 M for the preparation of samples S1, S2, and S3, respectively. After 15 min of magnetic stirring, the precursor solution was transferred into a Teflon-lined steel autoclave. The autoclave was sealed and then heated at 200 °C for 24 h and then allowed to cool to room temperature naturally. The final product was washed with deionized water and pure alcohol several times to remove possible residues and then dried at 65 °C for 24 h. Sample S4 was prepared with a molar ratio of Bi3+/Fe3+ of 1:1 and 1 M NaOH. The parameters for synthesis of the Bi2Fe4O9 samples are listed in Table 1. The phase composition of the obtained samples was characterized by means of a Shimadzu XD-3A X-ray powder diffractometer with Cu KR radiation (λ ) 0.154056 nm). The size and morphology of the samples were determined by scanning electron microscope (Philips XL 30 FEG) and transmission electron microscope (Philips CM 300 FEG). The UV-vis absorption spectra were measured on a UV-vis spectrophotometer (Hitachi U-3010). The photocatalytic activity of Bi2Fe4O9 microplatelets and nanosheets was evaluated under irradiation of a 300 W high-

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Tunable Morphology of Bi2Fe4O9 Crystals

Figure 1. XRD patterns of Bi2Fe4O9 crystals synthesized under various conditions.

pressure mercury lamp (λ ) 365 nm) and a 400 W metal-halide lamp (λ > 410 nm) at natural pH value. The initial concentration of MO was 15 mg · L-1 with a catalyst loading of 0.5 g · L-1. After the elapse of a period of time, a small quantity of the solution was taken, and the concentration of MO was determined by measuring the absorbance at 464 nm, using a UV-vis spectrophotometer. Each time before the absorption measurement, the sample solution was centrifuged at 4000 rpm · min-1 for 10 min to separate the catalyst powder from the solution. The absorbance was converted to the MO concentration referring to a standard curve showing a linear behavior between the concentration and the absorbance at 464 nm. Results and Discussion The purity and crystallinity of the as-prepared Bi2Fe4O9 samples were examined by X-ray diffraction (XRD). Figure 1 shows the XRD patterns of the Bi2Fe4O9 samples prepared under different conditions. All of the diffraction patterns of the samples can be indexed to the orthorhombic structure of Bi2Fe4O9 (space group: Pbam) with lattice constants of a ) 0.796 nm, b ) 0.844 nm, and c ) 0.599 nm, which is in good agreement with the standard data (JCPDS 250090). The XRD results reveal that the samples are highly crystallized and exhibit a single-phase structure. It is noted that the crystalinity of sample S4 is much lower than that of the other three samples. This low crystalinity can be attributed to the nanoscale thickness of the platelets of sample S4, which will be described later in details. From the point of application, stability is a vitally important index to catalysts. To prove the stability of Bi2Fe4O9 photocatalysts, the samples were collected after degradation experiments with MO. By examining the used photocatalysts with XRD, it was found that the XRD patterns were exactly the same as the former ones. The crystal structure of the photocatalysts did not change after the photocatalytic reactions. This reveals that the Bi2Fe4O9 crystals are stable. Nonetheless, those doped TiO2 photocatalysts are usually not stable although doping could possibly make TiO2 respond to visible light.33 Scanning electron microscopy (SEM) images are displayed in Figure 2A-F, which show the morphologies of the prepared Bi2Fe4O9 samples. In general, well-defined Bi2Fe4O9 crystals with different morphologies were obtained. Bi2Fe4O9 microplatelets with different thickness are obtained with Bi(NO3)3 · 5H2O and Fe(NO3)3 · 9H2O in a 1:2 molar ratio as starting materials at 4-12 M NaOH. Bi25FeO40 was found as an impurity in the final product at less than 4 M NaOH. Figure 2A shows an SEM image of the Bi2Fe4O9 microplatelets obtained at 4 M NaOH after reacting 24 h. The platelets are

J. Phys. Chem. C, Vol. 113, No. 10, 2009 4169 roughly square in shape, with an edge length ranging from 1.5 to 2 µm and thickness of 400-800 nm. Panels B and C of Figure 2 show SEM images of Bi2Fe4O9 microplatelets obtained at 8 and 12 M NaOH after reacting 24 h. The thickness of the Bi2Fe4O9 microplatelets obtained at 8 and 12 M NaOH is approximately 600 and 400 nm, respectively, while the edge length of most platelets is about 2 µm (Figure 2B,C). The result clearly indicated that uniform Bi2Fe4O9 microplatelets were obtained by a hydrothermal process with 1:2 molar ratio of Bi3+ and Fe3+. The thickness of the platelets decreased upon increasing the concentration of NaOH. With Bi(NO3)3 · 5H2O and Fe(NO3)3 · 9H2O in a 1:1 molar ratio as starting materials, Bi2Fe4O9 nanosheets were obtained at 1 M NaOH solution by a hydrothermal process at 200 °C for 12 h. In the case of Bi(NO3)3 · 5H2O and Fe(NO3)3 · 9H2O in a 1:1 molar ratio at the beginning, Bi2Fe4O9 with Bi25FeO40 as byproduct was obtained at less than 1 M NaOH; Bi2Fe4O9 with BiFeO3 as byproduct was obtained with more than 1 M NaOH in the precursor solution. The shape of the Bi2Fe4O9 sheets is irregular (Figure 2D-F). Figure 2D was recorded with the asprepared S4, while panels E and F of Figure 2 were obtained from the sample after being dispersed in alcohol solution for more detailed morphology. The thickness of the Bi2Fe4O9 nanosheets is approximately 25-35 nm estimated by the SEM image (Figure 2F). The energy dispersive X-ray spectroscopy (EDS) spectrum (Figure 2G) shows the atomic ratio of Bi/Fe approximately equals 1:2 in sample S4. The percentage concentration of Bi and Fe in the Bi2Fe4O9 nanosheets determined by EDS agrees with that obtained by stoichiometric analysis. The microstructures of sample S4 were further examined by transmission electron microscopy (TEM). Figure 3A shows a low-magnification TEM image of sample S4. The sheets are electron transparent, which is consistent with the thickness estimate of 25-35 nm obtained by SEM. It is worth noting that microscopic steps can be clearly observed in many of the sheets, which must result from crystal anisotropy. The ripplelike contrast as indicated by white arrows in Figure 3A is due to the strain stemming from the bending of the sheets. This is an electron-diffraction phenomenon and is often observed in nanobelts due to deformation and bending.34 The corresponding high-resolution TEM (HRTEM) image in Figure 3B shows a clearly resolved crystalline domain with a uniform interplanar spacing of 0.319 nm, which corresponds to the (121) plane of the orthorhombic Bi2Fe4O9 crystal (JCPDS 25-0090). It confirms that sample S4 is well crystallized with a single phase structure. With Bi(NO3)3 · 5H2O and Fe(NO3)3 · 9H2O in a 1:1 or 1:2 molar ratio as starting materials, pure Bi2Fe4O9 crystals can be obtained based on the results of XRD, EDS, and TEM. It is suggested that both the molar ratio of Bi3+/Fe3+ and the alkalinity in the precursor solution play key roles in preparing pure Bi2Fe4O9 by the hydrothermal process.28 In the hydrothermal synthesis of oxide crystals, OH- ions are one of the main anions that affect the nucleation and growth behavior of oxide crystals. It has been found that the aspect ratio of low-dimensional oxide nanocrystals can be controlled by adjusting the OH- concentration. The OH- ions can serve as capping agents and adsorb on certain faces of oxide crystals, which may create additional growth anisotropy and direct the crystal growth as well.35,36 The influence of NaOH concentration on the thickness of Bi2Fe4O9 platelets can be explained by analyzing the crystal structure of the orthorhombic Bi2Fe4O9. The unit cell of the orthorhombic Bi2Fe4O9 crystal is represented in Figure 4. It consists of Bi-O tetrahedra and Fe-O octahedra arrayed as layers parallel to the (001) face. The affinity of Bi3+

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Figure 2. SEM images of Bi2Fe4O9 microplatelets and nanosheets: (A) S1, (B) S2, (C) S3, and (D, E, F) S4. (G) EDS pattern of sample S4 (Bi2Fe4O9 nanosheets); the peak corresponding to Au comes from the coating for enhancing conductivity of the sample for SEM.

Tunable Morphology of Bi2Fe4O9 Crystals

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Figure 4. Schematic representation of the crystal structure of Bi2Fe4O9.

Figure 3. (A) TEM and (B) HRTEM image of sample S4 (Bi2Fe4O9 nanosheets).

and OH- is stronger than that of Fe3+ and OH-, which results in stronger adsorption of OH- ions onto (001) faces with more projecting Bi3+. Due to the hindrance effect of OH- ions on (001) faces, the growth of (001) faces is restrained at high OHconcentration. Thus, increasing OH- concentration results in the increase of the anisotropic growth of Bi2Fe4O9 crystals to be thinner platelets. The molar ratio of Bi3+/Fe3+ is another important factor for the preparation of pure orthorhombic Bi2Fe4O9.28,29,32 Our experimental results showed that pure Bi2Fe4O9 nanosheets instead of cubes were obtained with Bi(NO3)3 · 5H2O and Fe(NO3)3 · 9H2O in a 1:1 molar ratio as starting materials in 1 M NaOH precursor solution at 200 °C. Obviously, the growth habit of Bi2Fe4O9 crystals could be altered with a 1:1 molar ratio of Bi3+/Fe3+, which is higher than the stoichiometric of Bi2Fe4O9. As shown in Figure 4, Fe-O octahedra connect with Bi-O tetrahedra along [100] and [010] directions. This favors

the growth along [100] and [010] with extra Bi3+. However, the crystal growth along [001] is unfavorable with insufficient Fe3+ because Bi-O tetrahedra connect with Fe-O octahedra along the plane. This effect, in combination with the hindrance of OH- ions on the growth of (001) faces, induced the Bi2Fe4O9 crystal to grow to nanosheets under such conditions. Before photocatalytic activity evaluation, it is important to study the optical absorption of the as-prepared Bi2Fe4O9 samples because the UV-vis absorption edge is relevant to the energy band of semiconductor catalyst. The absorption spectra of samples S1, S2, and S3 are almost the same. Therefore, the spectra of sample S1 and sample S4 are chosen to be investigated in detail. Figure 5A shows the absorption spectra of sample S1 and sample S4 transformed from the diffuse reflection spectra according to the Kubelka-Munk (K-M) theory. The absorption cutoff wavelengths of sample S1 and sample S4 are about 650 and 620 nm, respectively, suggesting that the present materials can absorb visible light in the wavelength range of 410-650/ 620 nm. The energy bandgaps of the Bi2Fe4O9 samples can be estimated from the tangent lines in the plot of the square root of Kubelka-Munk functions F(R) against photo energy,37 as shown in Figure 5B. The tangent lines, which are extrapolated to (F(R))1/2 ) 0, indicate the bandgaps of sample S1 and sample S4 are 1.9 and 2.0 eV, respectively. The bandgap of sample S1 (Bi2Fe4O9 microplatelets) is slightly smaller than that of sample S4 (Bi2Fe4O9 nanosheets) because of the quantum size effect of the Bi2Fe4O9 nanosheets. With the smaller size, incontinuous highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) occur in the nanosheets, so the bandgap of the Bi2Fe4O9 nanosheets becomes wider than that of the Bi2Fe4O9 microplatelets. Such an energy bandgap is smaller than the bandgaps of TiO2 (3.2 eV), Bi2O3 (2.8 eV), Fe2O3 (2.2 eV),38 and BiFeO3 (2.18 eV).21 The smaller bandgap of Bi2Fe4O9 indicates a possibility of utilizing more visible light for photocatalysis. The photocatalytic activity of the well-crystallized Bi2Fe4O9 samples was evaluated by degradation of the typical organic contaminant MO under UV and visible irradiation. After 90 min UV irradiation without Bi2Fe4O9 photocatalysts, the degradation rate of MO was less than 3%. It reveals that MO is stable under short time UV irradiation if there is no photocatalyst involved. The photocatalytic activity of the Bi2Fe4O9 crystals for MO degradation is shown in Figure 6A. After 90 min UV irradiation

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Figure 5. (A) UV-vis diffuse reflectance spectra of samples S1 and S4, where the short dotted line is the division between UV and visible light. (B) The square root of Kubelka-Munk functions F(R) versus photon energy, where the short dotted lines are tangent of the linear part.

with samples S1-4, the MO degradation rate was about 86%, 87%, 90%, 93%, respectively. It is noted that the photocatalytic activity is S4 > S3 > S2 > S1. After 24 h of visible irradiation without Bi2Fe4O9 samples, the degradation rate of MO was about 2.5%. The photocatalytic activity under visible irradiation of the Bi2Fe4O9 samples is shown in Figure 6B. After 24 h of visible irradiation with sample S1, the MO degradation rate was only about 3%. Meanwhile, samples S2 and S3 exhibited similarly low degradation rate under the same condition (data not shown). This indicates that the Bi2Fe4O9 microplatelets are almost inactive to visible irradiation. However, after 24 h of visible irradiation with sample S4, the MO degradation rate was about 80%. In other words, Bi2Fe4O9 nanosheets (sample S4) are active to visible irradiation while Bi2Fe4O9 microplatelets are not. Compared with the normal photocatalyst TiO2, which only responds to UV irradiation, Bi2Fe4O9 nanosheets show their obvious advantage of detecting visible light irradiation. Although all of the synthesized Bi2Fe4O9 crystals show photocatalytic activity toward oxidation of MO, the nanosheets are more active than the microplatelets, whether they are under UV or visible irradiation. The following reasons suggest a possible explanation to these observations. First, the bandgap of Bi2Fe4O9 nanosheets (2.0 eV) is wider than that of Bi2Fe4O9 microplatelets (1.9 eV). Bi2Fe4O9 nanosheets with wider bandgap provide stronger oxidation property. Second, due to the fact that a photocatalytic reaction occurs at the interface between catalyst surfaces and reagents,5 it is believed that only the electron-hole pairs on catalyst surfaces can participate in the photocatalytic reaction. Because of the bigger particle size, it

Ruan and Zhang

Figure 6. (A) Photocatalysis of Bi2Fe4O9 microplatelets and nanosheets on degradation of MO under UV irradiation. (B) Photocatalysis of Bi2Fe4O9 microplatelets and nanosheets on degradation of MO under visible irradiation.

takes a longer time for the electron-hole pairs to diffuse to surfaces of microplatelets. Then, the electron-hole recombination is more likely to occur in Bi2Fe4O9 microplatelets than in Bi2Fe4O9 nanosheets during the electron-hole diffusion. Moreover, the electron-hole pairs are more inclined to recombine in Bi2Fe4O9 microplatelets than in Bi2Fe4O9 nanosheets because of the narrower bandgap of the microplatelets. The probable occurrence of electron-hole recombination greatly reduces the photocatalytic activity of Bi2Fe4O9 microplatelets. Finally, it is well-known that the surface area of a catalyst greatly affects its catalytic activity.39 The Brunauer-Emmett-Teller (BET) measurements show that the surface area of the Bi2Fe4O9 microplatelets (samples S1-3) ranges from 0.2 to 0.5 m2 · g-1 while that of the Bi2Fe4O9 nanosheets (sample S4) is 10.4 m2 · g-1. The significantly larger surface area of Bi2Fe4O9 nanosheets also results in the higher efficiency. In addition, it is noted that the UV-driven photocatalytic activity of the samples is much higher than the visible-lightdriven photocatalytic activity. The illuminant wavelength has a great impact on the quantum yield of reaction with significantly faster rate of reaction caused by shorter wavelength.40 The energy of UV irradiation (λ ) 365 nm) is stronger than that of visible irradiation (λ > 410 nm). The electrons which absorb more energetic photons are energized to an energy level that is higher than the conduction band minimum, that is, the LUMO. The higher energy electrons are less likely to recombine than those energized to the LUMO. On the basis of an overall consideration of various factors, it is reasonable to conclude that Bi2Fe4O9 nanosheets exhibit the best photocatalytic activity while Bi2Fe4O9 microplatelets do not respond to visible irradiation.

Tunable Morphology of Bi2Fe4O9 Crystals Conclusion In summary, we present a simple hydrothermal method for synthesis of Bi2Fe4O9 crystals with different morphologies. Single-phase Bi2Fe4O9 microplatelets and nanosheets are obtained by controlling Bi3+/Fe3+ molar ratio and NaOH concentration. The thickness of the Bi2Fe4O9 microplatelets decreased upon increase of the NaOH concentration. Moreover, increasing the Bi3+/Fe3+ molar ratio is favorable for the growth of Bi2Fe4O9 nanosheets. Although both Bi2Fe4O9 microplatelets and nanosheets demonstrate photocatalytic activity to the decomposition of MO under UV irradiation, the degradation over Bi2Fe4O9 nanosheets is more significantly efficient than that over Bi2Fe4O9 microplatelets under visible irradiation. Therefore, morphology plays an important role in the photocatalytic property of Bi2Fe4O9. Acknowledgment. The authors thank the Natural Science Foundation of China (No. 20773041) and the Research Fund for the Doctoral Program of Higher Education (No. 20070561008) for financial support. References and Notes (1) Mills, A.; Davies, R. H.; Worsley, D. Chem. Soc. ReV. 1993, 22, 417. (2) Mills, A.; Hunte, S. L. J. Photochem. Photobiol. A 1997, 108, 1. (3) Choi, W. Y.; Termin, A.; Hoffmann, M. R. J. Phys. Chem. 1994, 98, 13669. (4) Linsebigler, A. L.; Lu, G. Q.; J. T. Y. Jr. Chem. ReV 1995, 95, 735. (5) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69. (6) Chen, X. B.; Mao, S. S. Chem. ReV. 2007, 107, 2891. (7) Muggli, D. S.; McCue, J. T.; Falconer, J. L. J. Catal. 1998, 173, 470. (8) Fernandez, J.; Kiwi, J.; Lizama, C.; Freer, J.; Baeza, J.; Mansilla, H. D. J. Photochem. Photobiol. A 2002, 151, 213. (9) Tsumura, T.; Kojitan, N.; Izumi, I.; Iwashita, N.; Toyoda, M.; Inagaki, M. J. Mater. Chem. 2002, 12, 1391. (10) Wu, T.; Liu, G.; Zhao, J.; Hidaka, H.; Serpone, N. J. Phys. Chem. B 1998, 102, 5845. (11) Wu, T.; Liu, G.; Zhao, J.; Hidaka, H.; Serpone, N. J. Phys. Chem. B 1999, 103, 4862.

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