Bi5FeTi3O15 Hierarchical Microflowers - American Chemical Society

Oct 29, 2008 - ... 1295 Dingxi Road, Shanghai 200050, People's Republic of China ... the nanostructured Bi5FeTi3O15 is first realized, other peculiar ...
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J. Phys. Chem. C 2008, 112, 17835–17843

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Bi5FeTi3O15 Hierarchical Microflowers: Hydrothermal Synthesis, Growth Mechanism, and Associated Visible-Light-Driven Photocatalysis Songmei Sun, Wenzhong Wang,* Haolan Xu, Lin Zhou, Meng Shang, and Ling Zhang State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, People’s Republic of China ReceiVed: August 18, 2008; ReVised Manuscript ReceiVed: September 20, 2008

Nanostructured Bi5FeTi3O15 prepared by a facile hydrothermal method is reported as a new visible-lightdriven photocatalyst, which exhibits a nanoplates-built, flower-like hierarchical structure. Its peculiar growth process, from nanonets to nanoplates-built microflowers, was studied when the time-dependent experiments were carried out. The as-prepared nano-Bi5FeTi3O15 shows excellent visible-light-driven photocatalytic activity compared with bulk-Bi5FeTi3O15 and the widely used photocatalyst TiO2. The characteristics of the photocatalyst, such as the crystal and band structure, are discussed. The relationship between the physicochemical property and the photocatalytic performance of the photocatalyst is also investigated. Since the nanostructured Bi5FeTi3O15 is first realized, other peculiar properties, such as ferroelectric, magnetic and magnetoelectric properties besides the photocatalytic activity, may be revealed. 1. Introduction Photocatalytic degradation of organic contaminants is attracting extensive interest for their potential applications in remedying environmental pollution.1-3 To date, most researches on photocatalysts are focused on TiO2,4,5 which shows relatively high reactivity and chemical stability under ultraviolet (UV) light. However, the wide band gap (3.2 eV) of TiO2 limits its applications in utilizing visible light (λ > 400 nm), which accounts for 43% of the solar spectrum. Therefore, the development of efficient visible-light-induced photocatalysts is indispensable for making use of solar energy. Visible-light-driven photocatalysts, such as sulfides (e.g., CdS, CdSe.6) and oxides semiconductors (e.g., TaON,7 TiO2-xNx,1c,8 TiO2-xCx,3b,9 Sm2Ti2O5S2,10 etc.), have been widely studied. In general, the stabilities of these photocatalysts are poor or they exhibit low activities under visible-light irradiation.11 Furthermore, though doping could shift the absorption edge into the visible region and improve the photocatalytic activities, doped materials often suffer from thermal instability,12 increased carrier-recombination centers, or the requirement of expensive ion-implantation equipments.13 New visible-light-driven photocatalysts with high activity and dependable stability are needed to satisfy the requirements of future environmental applications. Thus oxide semiconductors with intrinsic narrow bandgap attract more and more interest in the studies of visible-light-driven photocatalysts because of their high stability under illumination and wide absorption of visible light. However, the conduction band levels of oxide semiconductors are usually low because the deep valence bands are formed by O 2p.14 This is a major problem in enhancing the photocatalytic activities of oxide semiconductors. Recently, it has been reported that bismuthbased oxide semiconductors are potential candidates for highly active photocatalysts, for the Bi 6s and O 2p levels can form a largely dispersed hybridized valence band,15,16 which favors the mobility of photogenerated holes and is beneficial to the oxidation reaction.17 Consequently, a great deal of effort has * Corresponding author. Phone: +86-21-5241-5295. Fax: +86-21-52413122. E-mail: [email protected].

been devoted to develop photocatalysts containing bismuth, such as BiVO4,18 Bi2WO6,19 CaBi2O4,20 Bi4Ti3O12,21 etc. Meanwhile, another point worth mentioning on the development of visiblelight-driven photocatalysts is that iron(III)-based semiconductors have also drawn increasing attention in recent years, such as Fe2O322 and BiFeO3,23 which have narrow band gaps and visiblelight-driven photocatalytic activities. It has also been found that iron(III)-doped TiO2 presented enhanced photocatalytic activity due to the replacement of TiIV by FeIII ions which can partially prevent the undesirable recombination of electron/hole pairs generated upon ultrabandgap irradiation. Besides this, it has been reported that iron(III)-doped TiO2 absorbs more visible light than pure TiO2.24 On the basis of the above studies, we conceive bismuth(III)and iron(III)-based semiconductors may act as excellent visiblelight-driven photocatalysts. To prove this idea, Bi5FeTi3O15 is chosen as an example. Before this, Bi5FeTi3O15 has become the focus of many researches because of its unique magnetoelectric property25 and its potential applications in both magnetic and ferroelectric devices.26 However, it still remains unknown if Bi5FeTi3O15 is a photocatalyst and what the photocatalytic activity is. Moreover, few studies have been carried out on the synthesis of Bi5FeTi3O15.27 All of the limited synthetic routes are based on solid-state reactions, which are usually characterized by high reaction temperature of about 1200 °C and low surface areas of the products. Since the microstructures, such as size, morphology, phase, crystallinity, surface area, etc., of a catalyst greatly affect its catalytic performance,28 the development of effective routes to Bi5FeTi3O15 with controlled microor nanostructure is necessary for the photocatalyst with high activity. Compared with other methods, hydrothermal synthesis is advantageous because the particle size, morphology, degree of aggregation, etc. could be easily tuned. 29 Therefore, a facile hydrothermal method carried out at a relatively low temperature was selected to demonstrate the possibility of preparing Bi5FeTi3O15 with controlled microstructure and to improve its photocatalytic activity. Herein we report a novel visible-light-driven photocatalyst Bi5FeTi3O15, which was prepared by a hydrothermal method at

10.1021/jp807379c CCC: $40.75  2008 American Chemical Society Published on Web 10/29/2008

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a much lower temperature of 180 °C than that at the traditional 1200 °C by solid-state reactions. The as-prepared Bi5FeTi3O15 samples exhibited nanoplates-built, flower-like hierarchical structure. A peculiar growth process, from nanonets to nanoplates-built microflowers, is revealed when the evolution process of this flower-like structure was traced. The as-prepared Bi5FeTi3O15 sample shows excellent photocatalytic activity in photodegradation of tetraethylated rhodamine (RhB) and acetaldehyde under visible light illumination. DFT calculation suggests that FeIII ions in perovskite layers are responsible for its visible light absorption. All of these studies on Bi5FeTi3O15 are revealed for the first time. 2. Experimental Section 2.1. Sample Preparation. All chemicals were analytical grade and used as received from Shanghai Chemical Company without further purification. Bi5FeTi3O15 microflowers were synthesized through a hydrothermal process. In a typical process, 1.02 g of Ti(OC4H9)4, 2.425 g of Bi(NO3)3 · 5H2O, and 0.404 g of Fe(NO3)3 · 9H2O were dissolved into 5 mL of HNO3 (4 M). Then a concentrated aqueous solution of NaOH was added dropwise into the above solution until a yellow suspension was formed. After being stirred for 2 h, the suspension was transferred into a 50 mL Teflon-lined stainless steel autoclave up to 80% of the total volume. The autoclave was heated at 180 °C for 0-72 h at autogenous pressure, and then cooled to room -temperature naturally. The resulting samples were separated by filtration, washed with deionized water and absolute alcohol several times, and then dried at 60 °C for 12 h. On the other hand, to compare with the flower-like Bi5FeTi3O15, bulk Bi5FeTi3O15 powder was prepared by a solid-state reaction according to the previous study.27c P25 (nanoscale TiO2 particles with a surface area of 50 m2 · g-1) was purchased from Degussa AG of Germany. 2.2. Characterization. The purity and crystallinity of the asprepared samples were characterized by powder X-ray diffraction (XRD) on a Japan Rigaku Rotaflex diffractometer equipped with a rotating anode with Cu Ka radiation in the range of 10-80° while the voltage and electric current were held at 40 kV and 100 mA, respectively. The scanning electron microscope (SEM) characterizations were performed on a JEOL JSM-6700F field emission scanning electron microscope. The transmission electron microscope (TEM) analyses were performed by JEOL JEM-2100F field emission electron microscope. UV-vis diffuse reflectance spectra (DRS) of the samples were measured by using a Hitachi U-3010 UV-vis spectrophotometer. Nitrogen adsorption-desorption measurements were conducted at 77.35 K on a Micromeritics Tristar 3000 analyzer. The BrunauerEmmett-Teller (BET) surface area was estimated by using the adsorption data. 2.3. Electronic Structure Calculation. First-principles calculations were performed using the all-electron Blo¨chl’s projector augmented wave (PAW) approach30 within the generalized gradient approximation (GGA), as implemented in the highly efficient Vienna ab initio simulation package (VASP).31 The k-point meshes for Brillouin zone sampling were constructed by using the Monkhorst-Pack scheme.32 A plane wave cutoff energy of 450 eV was used. Spin-polarized calculations were performed to account for the ferromagnetic nature of FeIII. 2.4. Photocatalytic Test. Photocatalytic activity of the Bi5FeTi3O15 microflowers was evaluated by the degradation of RhB and acetaldehyde under visible light irradiation of a 500 W Xe lamp with the 420 nm cutoff filter. For the degradation of RhB, the reaction cell was placed in a sealed black box of which the top was opened and the cutoff filter was placed to

Figure 1. XRD patterns of time series Bi5FeTi3O15 samples.

provide visible light irradiation. In each experiment, 0.05 g of photocatalyst was added into 100 mL of RhB solution (10-5 mol/L). Before illumination, the solution was stirred for 120 min in the dark in order to reach the adsorption-desorption equilibrium between the photocatalyst and RhB. At every 1 h interval, a 4 mL suspension was sampled and centrifuged to remove the photocatalyst particles. Then, the adsorption spectrum of the centrifugated solution was recorded with a Hitachi U-3010 UV-vis spectrophotometer. For the degradation of acetaldehyde, 0.5 g of the as-prepared photocatalyst was placed at the bottom of a gas-closed reactor at room temperature (capacity 1 L). This reactor is made of glass and has a quartz window. The reaction gas mixture (1 atm) consisted of 100 ppm of CH3CHO and N2 balance gas. Prior to commencing irradiation, the reaction system was equilibrated for about 120 min until no changes in the concentrations of acetaldehyde and CO2 were monitored. Gaseous samples (1 mL) were periodically extracted and analyzed by a gas chromatograph (GC) equipped with a flame ionization detector (N2 carrier) and a catalytic conversion furnace. 3. Results and Discussion 3.1. Phase Structure. Figure 1 displays the XRD patterns of the Bi5FeTi3O15 products with different reaction times. As shown in Figure 1, the starting precipitate precursor is amorphous before hydrothermal treatment. After hydrothermal treatment at 180 °C for 72 h, pure nanocrystalline Bi5FeTi3O15 powders were obtained. The reaction temperature is much lower than that of solid state reactions, which is usually 1200 °C. All the diffraction peaks from the 72-h sample in Figure 1 can be indexed as orthorhombic Bi5FeTi3O15 (space group Fmm2 (42), JCPDS 82-0063). No other possible impurities, such as Fe2O3, Bi2O3, or TiO2, were detected. After refinement, the cell constants of Bi5FeTi3O15 were calculated to be a ) 5.432 Å, b ) 41.149 Å, and c ) 5.469 Å, which is consistent with the data obtained from JCPDS 82-0063. Bi5FeTi3O15 belongs to the Aurivillius phases which have the general formula of [Bi2O2]2+[An-1MnO3n+1]2-, where A represents the 12-fold coordinated cation with low valence in the perovskite sublattice, M denotes the octahedral site occupied by ions with high valence, and n is the number of perovskite layers between the [Bi2O2]2+ layers.33 The perovskite sheets of Bi5FeTi3O15, which are composed of MO6 octahedrons and 12fold coordinated Bi3+, are 4 layers in thickness, with disordered Ti4+/Fe3+ (3:1 ratio) in the M sites, as shown in Figure 2. The disordered Ti4+/Fe3+ in the octahedral field would impose significant effects on the electronic structure and photophysical and photocatalytic properties of Bi5FeTi3O15, which will be discussed in detail later.

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Figure 2. Schematic crystal structure of Bi5FeTi3O15 in the polyhedron mode by the supercell method, where the layered structure can be clearly seen.

3.2. Morphology. The morphology and microstructure of the as-prepared Bi5FeTi3O15 products (hydrothermally treated at 180 °C for 72 h) were studied by the microscope images. A panoramic SEM image (Figure 3a) demonstrates that the sample consists of microflowers with diameters of 5-10 µm. The flower-like hierarchical structures are in fact constructed by square nanoplates revealed by highly magnified SEM images as shown in Figure 3b,c. Close examination demonstrates that the thickness of the nanoplates is about 40-80 nm (Figure 3d).

J. Phys. Chem. C, Vol. 112, No. 46, 2008 17837 Further TEM investigation shows the organization of such hierarchical structures. Figure 4a presents a TEM image of an individual flower with a zigzag contour and a diameter of about 5 µm, which is in accordance with that revealed by the SEM images (Figure 3a-c). HRTEM and selected area electron diffraction (SAED) give more details of the microstructure. A typical fragment peeled from the flower is displayed in Figure 4b, showing the side length of about 600 nm and the 90° angle between the two adjacent edges, which further indicates the good quality of the square nanoplate. The SAED pattern for the [010] zone axis of the nanoplate (inset of Figure 4b) exhibits a regular and clear square diffraction spot array revealing the single crystal nature of the nanoplates. The HRTEM image shown in Figure 4c was recorded on the marked area of this nanoplate. It confirms the single crystal nature of the nanoplates and clarifies the particular orientation of these nanoplates. On the basis of the HRTEM and SAED pattern, the d spacings are measured to be 0.272 and 0.273 nm, which agrees well with the lattice spacing of (200) and (002) planes of orthorhombic Bi5FeTi3O15 (space group Fmm2), respectively. These results suggest that the basal plane of the nanoplates is a (010) plane, featuring a square arrangement of atoms along its instinct a × c layer. Energy dispersive analysis of X-rays (EDAX) is conducted to confirm the element constituents of the nanoplate. Only Bi, Fe, Ti, and O are found in the spectrum except for the Cu from the Cu grid as shown in Figure 4d. Quantitative results give the

Figure 3. (a) Low-magnification and (b, c) high-magnification SEM images of the flower-like Bi5FeTi3O15. (d) Enlarged image of the surface of the flower.

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Figure 4. (a) TEM image of an individual flower-like Bi5FeTi3O15. (b) TEM image of the Bi5FeTi3O15 peeled fragment (inset: SAED pattern recorded at the marked area of this individual nanoplate). (c) HRTEM image recorded at the marked area of panel b. (d) EDAX spectrum of the as-prepared Bi5FeTi3O15 sample.

atomic ratio of 26:4.8:15:68 for Bi:Fe:Ti:O. It is close to the ideal value of 5:1:3:15 in Bi5FeTi3O15 considering instrumental error. 3.3. Growth Process of Bi5FeTi3O15 Hierarchical Flowers. To reveal how the Bi5FeTi3O15 flower-like hierarchical structures grew, time-dependent experiments were carried out. The crystallinity and morphology of the time-dependent samples are investigated by XRD, SEM, and TEM, respectively. Figure 5 gives the SEM images of these samples. As shown in Figure 5a, irregularly aggregated spherical particles with a size about 100 nm were attained at first. The corresponding XRD pattern (Figure 1) of this precursor shows no diffraction peaks, indicating the poor crystallinity of the sample at the early stage. When the reaction time was 5 h, the as-prepared sample remained as aggregated particles (Figure 5b), which did not show an obvious difference compared with the precursor. When the hydrothermal treatment was extended to 10 h, aggregated particles were still the dominant structure, but the degree of aggregations increased, as shown in Figure 5c. Some intersectant nanorods were also observed (the marked area of Figure 5c). This phenomenon indicates that the particles may grow oriented along a certain direction during the hydrothermal treatment. When the treatment time was prolonged to 24 h, 1D intersectant nanorods increased dramatically as expected (Figure 5d), implying remarkable crystal growth at this stage. The width of the intersectant nanorods is about 60 nm while the length is about 350-300 nm (Figure 5d). The XRD pattern of the 24-h

sample in Figure 1 reveals that orthorhombic Bi5FeTi3O15 has formed at this stage. As the hydrothermal treatment continued further to 48 h, more and more intersectant nanorods attached together and turned into 2D nanonet structure and then transformed into nanoplates (Figure 5e). When the treatment time was extended to 72 h, these 2D nanoplates finally evolved into hierarchical flowers as shown in Figure 5f. The panoramic SEM images of these growing samples are supplied in the Supporting Information (Figure S1). TEM and HRTEM studies further reveal the interesting growth process from nanoparticles to nanonets and then to the nanoplates. Figure 6a is the TEM image of the intersectional nanorods. It is interesting to find that the nanorods crossed with an angle of about 90°. The inset SAED patterns in Figure 6a are recorded on the marked areas of 1, 2, and 3. All three diffraction patterns in Figure 6a can be indexed to the [010] zone axis of the orthorhombic phase of Bi5FeTi3O15. Especially, the SAED pattern of the junction part (part 3 of Figure 6a) shows the bright diffraction short arcs corresponding to the diffractions of (200), (002), and (202) planes revealing the good orientation along the a and c axes at the connected part. The short arcs are attributed to the imperfect perpendicular connection of the two nanorods. Figure 6b records three attached nanorods. The outline of each nanorod can be clearly distinguished from the HRTEM image as shown in Figure 6c. The lattice planes of each nanorod align perfectly at the junction part, which implies that this special structure forms via the oriented attachment of the nanorods.

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Figure 5. SEM images of samples treated at 180 °C for different hydrothermal times: (a) 0, (b) 5, (c) 10, (d) 24, (e) 48, and (f) 72 h.

From the HRTEM and SAED pattern, the d spacing is measured to be 0.192 nm (Figure 6c). This agrees well with the lattice spacing of (202) planes, which indicates the perpendicular attached nanorods grow along the [101] and [10-1] directions, marked as r[101] and r[10-1] nanorods, respectively. When more Bi5FeTi3O15 nanorods attach perpendicularly with each other, the dense nanonet formed as shown in Figure 6d. The corresponding SAED pattern in Figure 6d exhibits a good orientation of the nanonet. The bright spots refer to the (200), (002), and (202) surfaces indicating the regular attachment of these nanorods. This is the first observation of largely perpendicularly oriented attachment of nanorods in the crystal growth

process, although there are some reports on the parallel attachment of nanorods.34 Scheme 1 illustrates the growth process of the Bi5FeTi3O15 hierarchical structures. First, Bi5FeTi3O15 nanoparticles grow anisotropically along the [101] and [10-1] directions to form the r[101] and r[10-1] nanorods. Then, the r[101] nanorods and r[10-1] nanorods perpendicularly attach to each other via the oriented attachment growth mechanism and thus 2D dense nanonets form. These interesting nanonets will grow into single crystalline nanoplates at the next stage through a process well-known as Ostwald ripening, where the gaps between the intersectional nanorods are filled. Regarding the formation of spherical microflowers, the geometric

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Figure 6. TEM and HRTEM images of the resulting sample with 24 h of hydrothermal treatment: (a, b) TEM images of the intersectional nanorods; insets of panel a: the SAED patterns of marked areas. (c) HRTEM image recorded at the marked area of panel b. (d) TEM image of the dense nanonet composed of perpendicular attached nanorods (inset: SAED pattern of the dense nanonet).

SCHEME 1: Schematic Illustration of the Possible Formation Process of Bi5FeTi3O15 Hierarchical Flower-Like Structures

constraints of building blocks should have played a key role.35 The flower-like Bi5FeTi3O15 hierarchical structures are built from the 2D nanoplates. A simple array of the nanoplates will easily generate a curvature, and the lateral engagement of these building units would lead to a spherical flower-like structure naturally. 3.4. Photophysical Properties. The Bi5FeTi3O15 sample absorbs light from UV light to visible light shorter than 600

nm based on the UV-vis diffuse reflectance spectrum shown in Figure 7. The color of the sample is yellow, which is consistent with its absorption spectrum. For a crystalline semiconductor, the optical absorption near the band edge follows the equation36 ahν ) A(hν - Eg)n/2, where a, ν, A, and Eg are the absorption coefficient, light frequency, proportionality constant, and band gap, respectively. n depends on whether the transition is direct (n ) 1) or indirect (n ) 4). According to the

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Figure 7. Typical diffuse reflection spectrum of the hierarchical flowerlike Bi5FeTi3O15 structures (prepared at 180 °C for 72 h). Figure 9. Total DOS and partial DOS of Bi5FeTi3O15 obtained by GGA.

Figure 8. Calculated band structures for Bi5FeTi3O15 by GGA along the high-symmetry axes of the Brillouin zone.

equation, the value of n is estimated to be 4, indicating indirect transition of the Bi5FeTi3O15 sample. The band gap of Bi5FeTi3O15 is estimated to be about 2.08 eV. To further understanding the visible-light-absorption property of Bi5FeTi3O15, for the first time the electronic structure of Bi5FeTi3O15 is calculated based on the DFT with GGA method on the basis of the crystal structure. The band structure of Bi5FeTi3O15 is shown in Figure 8. The total density of states (DOS) and the partial density of states of individual atoms of Bi5FeTi3O15 are shown in Figure 9. From these results, it is evident that the Fe 3d band in Bi5FeTi3O15 splits into mainly two parts, corresponding to the Fe t2g and the Fe eg, respectively. It also reveals that the Fe t2g orbits are hybridized with the O 2p orbits to form the O-Fe-O covalent bond. Furthermore, the Ti 3d states lie in a higher energy position than the Fe eg states. Therefore, we believe the valence bands of Bi5FeTi3O15 are mainly composed of O 2p + Fe t2g + Bi 6s hybrid orbits, while the conduction bands are mainly composed of the Ti 3d + Fe eg orbits. Concerning the photoexcitations in Bi5FeTi3O15, the electronic excitation from the hybridized O 2p + Fe t2g + Bi 6s orbits to the Fe eg orbits should be responsible for the absorption of visible light photons as shown in Figure 7. The other electronic excitation from the O 2p + Fe t2g + Bi 6s hybrid orbits to the Ti 3d orbits should correspond to the absorption of UV light photons. This result implies that FeIII ions located in the FeO6 octahedrons of Bi5FeTi3O15 perovskite structures play an important role in the absorption of visible light, suggesting a new route to design visible-light-driven photo-

Figure 10. UV-visible spectral changes of RhB (1 × 10-5 M) in aqueous nanocrystalline Bi5FeTi3O15 dispersions as a function of irradiation time under visible light illumination.

catalysts in perovskite compounds. In Figure 9, some energy levels mainly introduced by Fe 3d orbits appear in the band gap region, which is consistent with the absorption shoulder in the visible-light region in the UV-vis spectrum (Figure 7). It is clear that these energy levels below the Fermi level have a large slope between G and Q, suggesting a small effective mass for hole transport and the delocalized nature of the hole states within the band.37 As delocalized bands are favorable for the hopping of electrons or holes, excited holes in Bi5FeTi3O15 will move freely along these energy levels, which is helpful to improve the photocatalytic properties. 3.5. Photocatalytic Activity. Tetraethylated rhodamine, RhB, which shows a major absorption band at 553 nm, is chosen as a representative model pollutant to evaluate the photocatalytic performance of Bi5FeTi3O15. Figure 10 displays the temporal evolution of the spectral changes during the photodegradation of RhB over nanocrystalline Bi5FeTi3O15 under visible light illumination (λ > 420 nm). An apparent decrease of RhB absorption at 553 nm is observed. It has been reported that the RhB photodegradation occurs via two competitive processes: N-demethylation and the destruction of the conjugated structure.38 In the present case, there is no obvious absorption band shifting to shorter wavelengths in the RhB decomposition by Bi5FeTi3O15 photocatalyst indicating the destruction of the conjugated structure occurred. The photocatalytic activity of the

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Figure 11. The photodegradation efficiencies of RhB as a function of irradiation time by different photocatalysts.

Figure 12. Photocatalytic activity of Bi5FeTi3O15 microflowers in the degradation of acetaldehyde in air under visible light.

nanocrystalline Bi5FeTi3O15 is compared with that of the bulk Bi5FeTi3O15 and the photocatalyst TiO2 (Degussa P25). Figure 11 shows the photolysis of RhB is extremely slow without any photocatalyst and only 4% of RhB was degraded after 4 h under visible-light illumination. When nanocrystalline Bi5FeTi3O15 was applied as photocatalyst, however, about 96% of RhB was decolorized after 4 h, showing efficient photocatalytic activity under visible-light irradiation (λ > 420 nm). Meanwhile, the adsorption of RhB on the nanocrystalline Bi5FeTi3O15 in the dark was also checked. After 4 h, the concentration of RhB decreased 9% only, suggesting that the decolorizing of RhB by Bi5FeTi3O15 is mainly attributed to the photodegradation but not adsorption. For comparison, the photocatalytic property of bulk Bi5FeTi3O15 was also tested. After 4 h under visible-light irradiation, the degradation rate of RhB was only 6%, which is significantly less efficient than that for nanocrystalline Bi5FeTi3O15 under the same conditions. When the photocatalyst TiO2 with a high BET surface area of 50 m2 g-1 was applied, only 18% of RhB was photodegraded under the same conditions, indicating an obvious advantage of the nanocrystalline Bi5FeTi3O15 in making use of visible light. Besides the electronic structures as discussed above, there are at least another two possible reasons contributing to the excellent photocatalytic activity of the as-prepared Bi5FeTi3O15 sample. One is that the BET surface area of flower-like Bi5FeTi3O15 (5.85 m2 g-1) is higher than that of bulk Bi5FeTi3O15 (1.00 m2 g-1). It has been reported the increase of surface area could increase the number of active sites and promote the separation efficiency of the electron-hole pairs in photocatalytic reactions, resulting in a higher photocatalytic activity.20,28 The other may be ascribed to its nanocrystalline structure. Photocatalytic behavior is closely related to the particle size. For randomly generated charge carriers, the average diffusion time from the bulk to the surface is given by τ ) r2/π2D, where r is the grain radius and D is the diffusion coefficient of the carrier.39 If the grain radius decreases, the opportunities for the recombination of charge carriers will be reduced and more light generated charge carriers will transfer to the surface of the photocatalysts and act with the absorbed RhB molecules, which could also enhance the photocatalytic efficiency.15 From the viewpoint of application, the stability of a photocatalyst is also important. In the case of nanocrystalline Bi5FeTi3O15 microflowers, the crystal structure is very stable. After several consecutive runs for the photodegradation of RhB under visible-light illumination (λ > 420 nm), the sample remains almost the same. As shown in Figure S2 of the Supporting Information, the crystal structure of the photocatalyst does not change after 20 h of photocatalytic reaction.

The ability of the photocatalyst to oxidize acetaldehyde in air was also studied. The as-prepared Bi5FeTi3O15 sample showed significant production of CO2. After 5 h under visible light irradiation, 116 ppm of CO2 was produced in the reactor (Figure 12), which further demonstrates the excellent photocatalytic performance of the as-prepared Bi5FeTi3O15 microflowers. 4. Conclusion Nanostructured Bi5FeTi3O15 is found to be a new visiblelight-driven photocatalyst that presents much enhanced photocatalytic activities compared with the product prepared by solid state reaction and the widely used photocatalyst TiO2. DFT calculation indicates that the FeIII ions which are located in FeO6 octahedrons in perovskite structures are mainly responsible for its visible-light absorption. This provides a possible approach to search for new visible-light-driven photocatalysts in perovskite semiconductors. The Bi5FeTi3O15 sample was prepared at a much lower temperature of 180 °C by a facile hydrothermal method, which may open up new possibilities for the preparation of other complex metal oxides via mild solution-based routes. As one of the advantages of the aqueous synthetic method carried out at low temperature, the as-prepared Bi5FeTi3O15 exhibits an interesting nanoplates-built flower-like structure. The peculiar growth process, from nanonets to nanoplates-built microflowers, is revealed for the first time, which will be helpful for understanding the nanocrystal growth process and the oriented attachment mechanism in depth. Acknowledgment. This work is financially supported by the Innovation Research of the Shanghai Institute of Ceramics and the Key Project from the National Natural Science Foundation of China (No. 50732004). Supporting Information Available: Figures showing the panoramic SEM image for Bi5FeTi3O15 growing samples and XRD pattern of the nanocrystalline Bi5FeTi3O15 microflowers after the degradation of RhB for 20 h. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Fox, M. A.; Dulay, M. T. Chem. ReV. 1993, 93, 341. (b) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69. (c) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269. (2) (a) Tao, X.; Ma, W.; Zhang, T.; Zhao, J. Angew. Chem., Int. Ed. 2001, 40, 3014. (b) Ma, W.; Li, J.; Tao, X.; He, J.; Xu, Y.; Yu, J. C.; Zhao, J. Angew. Chem., Int. Ed. 2003, 42, 1029. (c) Zhao, W.; Ma, W.; Chen, C.; Zhao, J.; Shuai, Z. J. Am. Chem. Soc. 2004, 126, 4782.

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