Size-Dependent Magnetic, Photoabsorbing, and Photocatalytic

ARTICLE pubs.acs.org/JPCC

Size-Dependent Magnetic, Photoabsorbing, and Photocatalytic Properties of Single-Crystalline Bi2Fe4O9 Semiconductor Nanocrystals Qiang Zhang, Wenjie Gong, Jiaheng Wang, Xinkun Ning, Zhenhua Wang, Xinguo Zhao, Weijun Ren, and Zhidong Zhang* Shenyang National Laboratory for Material Science, Institute of Metal Research, and International Centre for Material Physics, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, People’s Republic of China

bS Supporting Information ABSTRACT: Single-crystalline Bi2Fe4O9 nanocrystals (NCs) of various sizes (from 78 to 14 nm), synthesized using the sol gel method, exhibit size-dependent magnetic properties, which can be attributed to the uncompensated spins on the surface of the antiferromagnetic NCs. These tunable magnetic properties have a profound significance and can be widely applied in spintronics. As the crystal size decreases, the energy gap of the Bi2Fe4O9 semiconductor NCs increases from 1.91 to 2.04 eV, which results from quantum confinement effect. The Bi2Fe4O9 NCs exhibit excellent photocatalytic oxidation of methylene blue (MB) under visible light irradiation with the assistance of a small amount of H2O2. Interestingly, smaller NCs exhibit higher photocatalytic activities. The size-dependence of photocatalytic properties of the Bi2Fe4O9 NCs was ascribed to the lower recombination rate of the photogenerated electron/hole pair within smaller NCs and higher specific surface areas, which allow for stronger photon absorption on the surface of smaller crystals.

1. INTRODUCTION Vigorous research efforts have been devoted to understanding the fundamental properties of semiconductors as a function of crystal size, including the size dependence of optical and electronic properties of semiconductor quantum dots,1 14 electromagnetic wave resonances of Fe/TiO2 nanostructures,15 photocatalytic performance of TiO2-based photocatalysts,16 magnetic17 and ferroelectric18 properties of single-crystalline BiFeO3 nanocrystals (NCs), spintronic properties of diluted magnetic semiconductors,19 and so forth. Recently, Bi2Fe4O9, an antiferromagnetic semiconductor with a mullite structure,20 has attracted extensive interest for its particular magnetic structures,21,22 potential photocatalytic application under ultraviolet (UV) and visible light,23 27 and unique role as a gas sensor.28 32 The crystallographic structure of Bi2Fe4O9 belongs to the orthorhombic space group Pbam, whose unit cell consisting of two formula units can be described as columns of edgesharing FeO6 octahedra connected by corner sharing FeO4 tetrahedra and bismuth atoms (as shown in Figure S1).22,33,34 Bi2Fe4O9 thus exhibits a Fe-based Cairo pentagonal lattice, and for these very competing spin interactions (Neel temperature TN = 240 260 K), the magnetic arrangement of Bi2Fe4O9 is geometrically frustrated.21 Although the antiferromagnetic structure of Bi2Fe4O9 has been investigated by several groups,21,22,35 39 the sizedependent magnetic properties of Bi2Fe4O9 NCs have not been reported yet, and the investigation into the size dependence of magnetic properties is of immense importance for fundamental researches and applications in spintronics. On the other hand, r 2011 American Chemical Society

there are a lot of studies on bismuth-based photocatalysts, such as first-principle calculations on pure and metal-doped Bi12GeO20, Bi12SiO20, Bi12TiO20,40 Bi2Ti2O7, Bi4Ti3O12,41 Bi2O2CO3 hierarchical microflowers,42 BiOBr hollow microspheres,43 and firsttime hydrothermal synthesis of oxide-fluoride Bi2TiO4F2 nanoflakes, which possess a high photocatalytic activity due to its unique layered structure and the existence of F.44 Photocatalytic properties under UV and visible light irradiance of Bi2Fe4O9 microplatelets and nanosheets were reported by Ruan and Zhang.23 This Article reports the synthesis, characterization, magnetic, photoabsorbing, and photocatalytic properties of single-crystalline Bi2Fe4O9 NCs. The mechanisms of sizedependent magnetic, photoabsorbing, and photocatalytic properties of Bi2Fe4O9 NCs are discussed systematically, and some interesting findings are obtained. For example, the Bi2Fe4O9 NCs exhibit excellent photocatalytic oxidation of methylene blue (MB) under visible light irradiation with the assistance of a small amount of H2O2. It is also found that a transformation from antiferromagnetism to weak ferromagnetism occurs when the average diameter of the nanoparticles is below 57 nm, and smaller Bi2Fe4O9 NCs exhibit higher photocatalytic activity under visible-light irradiation. Furthermore, at the same time, our samples show faster degradation rate under visible light irradiance with the aid of H2O2 than do microplatets and nanosheets prepared by Ruan and Zhang.23 Received: September 10, 2011 Revised: November 3, 2011 Published: November 11, 2011 25241

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Table 1. Synthetic Conditions and Average Grain Sizes of Bi2Fe4O9 NCs annealed temperature

annealed time

average grain size

sample

(°C)

(min)

(nm)

S1 S2

600 600

30 60

14 ( 2 21 ( 1

S3

650

30

32 ( 1

S4

650

60

42 ( 1

S5

700

30

57 ( 4

S6

700

60

78 ( 7

S7

900

30

bulk

2. EXPERIMENTAL SECTION Synthesis of Materials. Bi2Fe4O9 NCs were synthesized via the sol gel method using polyvinyl alcohol (PVA) as a complexing agent.17,18,45 All of the reagents are analytically pure. In brief, PVA (MW = 1750) was added to deionized water and dissolved by constant stirring at 80 °C to make a 5% solution. Next, Bi(NO3)3 3 5H2O (analytical reagent) of 5 mmol was added to the 5% PVA solution, and some nitrate acid was added to promote Bi(NO3)3 3 5H2O to dissolve and prevent Bi3+ from hydrolysis. Thereafter, Fe(NO3)3 3 9H2O (analytical reagent) of 10 mmol was added to the solution. During the process, the molar ratio of positively charged valences to hydroxyl groups (M3+/ OH) of PVA was maintained at 1:1. The mixture was stirred at 80 °C, after which a colloid formed with the volatilization of NOx gas resulting from the decomposition of the nitrate. Next, the sample was heated at 250 °C for 2 h. The resulting sample was carefully ground into a fine powder to obtain Bi2Fe4O9 precursor. The precursor was annealed at 600 900 °C in air for 30 60 min to obtain Bi2Fe4O9 NCs of various sizes. The parameters for annealing are listed in Table 1. X-ray Diffraction. The purity and structures were characterized in the range 10° 70° (2θ) by XRD on Rigaku D/Max-2400 with Cu Kα radiation (λ = 1.54 Å). The grain sizes were calculated by the Scherrer equation corrected for instrumental peak broadening. TEM and HRTEM. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were taken on FEI Tecnai G2 F20 operated at 200 kV. Samples were diluted in ethanol prior to imaging to allow for better imaging conditions. Next, the samples were deposited on carbon-coated copper grids and directly imaged after drying in air. Magnetic Characterization. Magnetic measurements were performed on a superconducting quantum interference device (SQUID) magnetometer. The as-prepared Bi2Fe4O9 NCs were sealed in a capsule with addition of resin. After the resin solidification, the field-cooling temperature-dependent magnetization (M) of one sample in a field of 200 Oe was acquired to determine the Neel temperature (∼260 K). Next, the fielddependent magnetizations of each sample were measured in the field range from 20 to 20 kOe at 200 K (well below the Neel temperature). The exchange bias plots of S1 were collected in the field range from 2 to 2 kOe at 5 and 200 K, respectively. UV Vis Spectra. The UV vis absorbance and diffuse reflectance spectrum (UV vis DRS) was measured between 200 and 900 nm using a Hitachi U-3010 UV vis spectrophotometer, respectively. The spectra for solid samples were collected using an integrating sphere attachment for obtaining absorbance and

Figure 1. XRD patterns of as-prepared Bi2Fe4O9 NCs.

diffused reflectance spectra for the samples. BaSO4 was chosen as the reference. For the liquid samples, absorption measurements were made using optically transparent quartz cuvettes. Photocatalytic Activity. Photocatalytic activity was evaluated by the degradation of MB under visible light irradiation. Briefly, 0.05 g of the photocatalyst and 1 mL of H2O2 were added successively into 50 mL of MB solution (10 mg/L). Before illumination, the suspension was stirred for 2 h in the dark to reach the adsorption desorption equilibrium between the photocatalyst and MB. Every 40 min, 5 mL of suspension was sampled and centrifuged. Next, the upper solution was put in a transparent quartz cuvette. The concentration of MB was determined by measuring the absorbance at 663.5 nm of the solution in the cuvette using a Hitachi U-3010 UV vis spectrophotometer.

3. RESULTS AND DISCUSSION 3.1. Typical Characterizations of As-Prepared Bi2Fe4O9 NCs. Figure 1 shows the X-ray diffraction (XRD) patterns of

as-prepared Bi2Fe4O9 samples. Evidently, all of the XRD patterns observed can be indexed to the orthorhombic structure of Bi2Fe4O9 (space group: Pbam) with lattice constants of a = 7.96 Å, b = 8.44 Å, and c = 5.99 Å, which are consistent with the standard data (JCPDS no. 25-0090). The XRD patterns reveal that all of the as-prepared samples are single-phase Bi2Fe4O9. The mean sizes of the phase-pure Bi2Fe4O9 NCs, determined by Scherrer equation, were listed in Table 1.17,18 A typical TEM image of the as-prepared Bi2Fe4O9 NCs, generated at 700 °C for 60 min using the sol gel method as described in the Experimental Section, is represented in Figure 2A (and Figure S2). Obviously, formation of singlecrystalline Bi2Fe4O9 NCs can be induced from the sharp diffraction spots of selected area electron diffraction (SEAD) data taken from the arrow-marked individual crystal (Figure 2B). A HRTEM image, taken from the individual crystal (the arrowmarked in Figure 2A) as presented in Figure 2C, confirms the single-crystalline nature of our as-prepared Bi2Fe4O9 NCs. In this figure, interplanar spacings of about 6.01 and 5.87 Å are compatible with reported values of 5.99 and 5.83 Å for bulk Bi2Fe4O9 (JCPDS no. 25-0090), and respectively correspond to 25242

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Figure 2. A typical TEM image (A) and SEAD pattern (B). (C) HRTEM image of arrow-marked Bi2Fe4O9 NCs in (A).

{001} and {110} planes of an orthorhombic Bi2Fe4O9 single crystal.17,18 3.2. Size-Dependent Magnetic Properties of the Bi2Fe4O9 NCs. Figure 3 shows the magnetization of the as-prepared Bi2Fe4O9 NCs as a function of the applied magnetic field at 200 K. Obviously, from this figure, magnetizations of our samples are a little bit larger, as compared to that of an antiferromagnet. The magnetization at a certain magnetic field increases monotonically with decreasing size of the NCs. It is noted that the magnetization of Bi2Fe4O9 NCs with a mean size of 78 nm approaches the bulk value. Meanwhile, with increasing size of the Bi2Fe4O9 NCs, the slopes of the hysteresis loops decrease. These results show that weak ferromagnetism occurs in the Bi2Fe4O9 NCs with sizes less than 57 nm. To determine whether any magnetic impurity exists in our samples, the hysteresis loop of sample S1 with the largest magnetization among the seven samples under the same magnetic field was measured at 295 K (the inset of Figure 3A). The sample S1 with weak ferromagnetism behaves paramagnetically above its Neel temperature. Thus, one can safely eliminate the possibility of the existence of ferrimagnetic γ-Fe2O3 or Fe3O4 in our samples, due to the fact that the Curie temperatures of these ferrimagnets are highly above 295 K (about 863 and 860 K, respectively). As shown in Figure 3A, the sample S5 with an average diameter of 57 nm displays typical antiferromagnetism, while for S4 with an average diameter of 42 nm, weak ferromagnetism is observed. For clarity, demagnetization plots were presented in Figure 3B.17 Usually, an antiferromagnetic lattice is comprised of two antiparallel spin sublattices. Spins within each sublattice are aligned parallel, while spins are aligned antiparallelly between two sublattices, and, as a result, the net magnetization is negligible.46 Yet at the surfaces of small antiferromagnetic crystals, some spins of one sublattice are not compensated by another sublattice, which results in an antiferromagnetic core/ferromagnetic shell (AC/FS) structure. The number of uncompensated spins increases with deceasing crystal size, and thus the magnetization of small crystals is enhanced. As is known, for the single-domain antiferromagnetic crystals, the magnetization is inversely proportional to the diameter of the crystal.17,47 Magnetization at 20 kOe as a function of 1/d is plotted in the inset of Figure 3C. For S1 (14 nm) S6 (78 nm), an approximately linear relationship is observed, which indicates that the AC/FS model can be applied to this case. Generally, exchange bias occurs when a ferromagnet is coupled interfacially with an antiferromagnet having a larger magnetic

Figure 3. (A) Magnetic hysteresis loops at 200 K for Bi2Fe4O9 NCs with different sizes. The inset shows the magnetic loop of S1 at 295 K. (B) Demagnetization plots of the Bi2Fe4O9 NCs. Magnetization of Bi2Fe4O9 NCs at 20 kOe as a function of diameter, d, is also presented in the inset. (C) Measurement of exchange bias of S1 at 5 and 200 K, respectively. The inset shows the magnetization of as-prepared Bi2Fe4O9 NCs as a function of 1/d.

anisotropy, and the antiferromagnet pins the orientation of the moment in the ferromagnet layer through the exchange interaction.48 The presence of exchange bias in the S1 sample 25243

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Figure 4. (A) UV vis absorbance spectra of samples S1 S7. The inset shows the absorption edges of S1 S7 as a function of crystal size. (B) The square root of Kubelka Munk (K M) functions F(R) versus photon energy. The inset shows energy gap as a function of crystal size.

justified the AC/FS model (Figure 3C), which can be attributed to the interfacial coupling of an antiferromagnetic core and a ferromagnetic shell in weak ferromagnetic samples S1 S4.49,50 From Figure 3C, the derived exchange bias fields of S1 at 5 and 200 K are 97 and 12 Oe, respectively. As compared to the exchange bias field (2.5 Oe at room temperature) of singlecrystalline BiFeO3 NCs, the derived exchange bias field (12 Oe at 200 K) is much larger, which can be attributed to the stronger AC/FS coupling at a much lower temperature. 3.3. Size-Dependent Photoabsorbing Properties of the Bi2Fe4O9 NCs. UV vis absorption spectra of the as-prepared Bi2Fe4O9 NCs were shown in Figure 4A. All of the absorption spectra show a similar shape. Typically, there are two absorption edges: one is above 600 nm; another is above 800 nm. The former can be ascribed to n p or p p electronic transitions, while the latter results from the d d electronic transitions of Fe.24 At the same time, the absorption edges, which can be determined by drawing tangent lines of the absorption spectra around 600 nm, exhibit a blue shift with decreasing crystal size, as is shown in the inset of Figure 4A. As is well-known, optical absorption properties of a photocatalyst are closely associated with its optical energy gap. For this purpose, the square root of the Kubelka Munk (K M) functions F(R)51 was plotted using UV vis diffuse reflectance spectra (Figure S3), as shown in Figure 4B. The optical energy gaps can be determined by the tangent lines of the square root of F(R) against photon energy (hν). The optical energy gaps against crystal size were plotted in the inset of Figure 4B. As the crystal size decreases from 78 to 14 nm, the energy gap of the Bi2Fe4O9 NCs increases from 1.91 to 2.04 eV. Considering the as-prepared samples share similar shape, the effect of symmetry breaking on the optical absorption can be ruled out.52 Thus, the increase of energy gaps stems from the same origin with the blue shift of absorption edges, that is, the quantum confinement effect.53 When the crystal size is comparable to the exciton Bohr radius,54 56 size-dependent photoabsorption arises in semiconductor materials. Actually, when the crystal size decreases, the levels around the Fermi level change from quasicontinuous to discrete state and the valence as well as conduction band edges shift, which thus results in an increase of energy gap.10 As compared to the energy gaps of TiO2 (3.2 eV) and BiFeO3 (2.18 eV),23 the Bi2Fe4O9 NCs have smaller ones, which means our samples are perhaps a promising visible light photocatalyst.

3.4. Size-Dependent Photocatalytic Activity of the Bi2Fe4O9 NCs. Figure 5A shows the photodegradation efficien-

cies of MB as a function of irradiation time with the aid of a small amount of H2O2 under visible-light illumination as described by Sun et al.24 Evidently, the MB solution with bulk Bi2Fe4O9 (S7) as photocatalyst shows no changes after the visible-light irradiation for 4 h. By contrast, samples S1 S6 exhibit a strong photocatalytic activity. After the visible-light irradiation for 4 h, the MB degradation rates with samples S1 S6 are about 91.1%, 90.5%, 89%, 87%, 83.2%, and 82%, respectively. Evidently, smaller NCs are more active than larger ones. It is necessary to elucidate the mechanism why photocatalytic activities of asprepared Bi2Fe4O9 samples show size dependence. Generally, when photocatalysts absorb UV or visible lights from illumination source, electrons in the valence band of the photocatalyst become excited and promoted to the conduction band, which thus creates the negative electron (e ) and positive hole (h+) pair. The e /h+ pair then migrates to the photocatalyst surface and therein reacts with absorbed organic reactants as a redox source, finally leading to the decomposition of the organic reactants.57 59 In this process, however, the degradation of the absorbed reactants is usually impeded by the high recombination rate of the photogenerated e /h+ pairs. For this reason, the photocatalytic efficiency depends largely on the transferring rate of the photoexcited e /h+ pairs from the inner body to the surface of the crystal. As was previously mentioned, to react with organic reagents in the catalytic process, the photoexcited e /h+ pairs must migrate to the photocatalyst surface, avoiding the recombination of e and h+ pairs. As we know, with increasing size of the Bi2Fe4O9 NCs, the recombination rate of e /h+ pairs will increase accordingly due to the increase of traveling path within the crystal body before reaching the photocatalyst surface.60,61 By contrast, a small crystal size can ensure a low recombination rate of photoexcited e /h+ pairs within the crystal body.62 Second, smaller NCs with larger specific surface areas possess a higher photon absorption rate on the surface of photocatalysts as compared to larger ones, due to quantum confinement. All of the above factors result in the photocatalytic activity of S1 > S2 > S3 > S4 > S5 > S6 > S7. Therefore, small crystals with high surface-volume ratios are preferred for a highly efficient photocatalytic process. In addition, the high photocatalytic activities of 25244

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Figure 5. (A) Photodegradation efficiency of MB as a function of irradiation time. (B) Kinetic linear simulation curve of MB photodegradation.

the sol gel-derived Bi2Fe4O9 NCs mean that oxygen vacancy defects, usually recombination centers of e /h+ pairs as mentioned above, are low enough,62,63 which can ensure that the magnetic behaviors of our samples stem from sheer size effect. To evaluate the reaction kinetics of the degradation of MB,62 the plot of ln(C0/C) versus time is plotted in Figure 5B, which shows an approximately linear dependence. The observed rate constants in the presence of S1 S7 are 0.01161, 0.01129, 0.01043, 0.00942, 0.00826, 0.00752, and 1.967  10 4 min 1, respectively, which also indicates smaller crystals possess higher photocatalytic activity. In this picture, the catalyst crystals should be as small as possible to attain high photocatalytic efficiency. At the same time, our samples show a faster degradation rate under visible light irradiance with the aid of H2O2 as compared to microplatets and nanosheets prepared by Ruan and Zhang.23

4. CONCLUSION Phase-pure single-crystalline Bi2Fe4O9 NCs were synthesized by the sol gel method, and the NCs sizes can be adjusted according to the annealing temperature and time. Our crystals exhibit size-dependent magnetic, photoabsorbing, and photocatalytic properties. The former behavior can be ascribed to the surface uncompensated spins and can be harnessed in spintronics. The size-dependent photoabsorbing properties may result from the quantum confinement effect, and the last one can be attributed to the lower recombination rate of photogenerated e /h+ pairs within smaller NCs and higher specific surface areas, which allows for stronger photon absorption on the surface of smaller samples. The weak ferromagnetism stems from sheer size effect but not oxygen vacancy defects, considering the high photocatalytic activity of Bi2Fe4O9 NCs. It is also found that a transformation from antiferromagnetism to ferromagnetism occurs when the average diameter of the nanoparticles is below 57 nm, and smaller Bi2Fe4O9 NCs exhibit higher photocatalytic activity under visible-light irradiation. ’ ASSOCIATED CONTENT

bS

Supporting Information. Schematic illustration of a Bi2Fe4O9 unit cell; two TEM images of Bi2Fe4O9 NCs prepared at 700 °C for 60 min and its size distribution; and the UV vis

diffuse reflectance spectra of samples S1 S7. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Phone: +86-24-2397-1859. Fax: +86-24-2389-1320. E-mail: [email protected].

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