Visible Light Responsive Perovskite BiFeO3 Pills and Rods with

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Visible Light Responsive Perovskite BiFeO3 Pills and Rods with Dominant {111}c Facets Linfeng Fei,† Jikang Yuan,† Yongming Hu,† Changzheng Wu,†,‡ Junling Wang,§ and Yu Wang*,† †

Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong SAR, China and Materials Research Center, The Hong Kong Polytechnic University, Hong Kong SAR, China ‡ Hefei National Laboratory for Physical Sciences at Microscale and Department of Nanomaterials and Nanochemistry, University of Science & Technology of China, Hefei, China § School of Materials Science and Engineering, Nanyang Technological University, Singapore ABSTRACT: Constraint of the growth directions or exposed facets of nano-/microstructures has been long regarded as a popular research topic due to the various anisotropic properties of crystals. We report here, for the first time, not only uniform and phase-pure but also tunable perovskite BiFeO3 crystallites with different predominantly exposed facets have been successfully synthesized via a facile one-pot hydrothermal approach at 200 °C under the presence of potassium hydroxide and polyethylene glycol. Pills and rods with dominant {111}c facets and cubes with {100}c exposed facets have been obtained by adjusting the alkaline conditions of the precursor solution. Furthermore, due to the distinct dominant facets of BiFeO3 crystallites, {111}c dominant pills and rods grant a significant enhanced visible light response, suggesting that the designed structures can give rise to better performance in future photovoltaic and photocatalytic applications of BiFeO3.

r 2011 American Chemical Society

temperature.11,12 At room temperature and 1 atm, BiFeO3 shows a rhombohedrally distorted perovskite structure with point group R3c.13 The perovskite unit cell has a lattice parameter, arh, of 3.965 Å and a rhombohedral angel, Rrh, of ca. 89.3-89.4° at room temperature. Alternatively, the unit cell can be described in a hexagonal frame of reference, with its c-axis parallel to the diagonal of the perovskite cube, i.e. [001]hexagonal [111]pseudocubic. The hexagonal lattice constants are ahex = 5.58 Å and chex = 13.90 Å.14 For the multiferroicity, BiFeO3 exhibits both spontaneous polarization (Ps) and antiferromagnetic order along the [001]hex/[111]c.15 However, it exhibits no detectable magnetism because of a spiral magnetic spin cycloid with a periodicity of about 62 nm.16 Nevertheless, BiFeO3 nanostructures continue to be regarded as primary building blocks for a range of future nanodevices due to their unique properties, such as magnetoelectric effect,17 gas-sensing properties,18 and the photovoltaic property.19 In photocatalysis research, the small band gap (∼2.2-2.8 eV)9 and good chemical stability of BiFeO3 make it an effective photocatalyst in the UV and visible light region during the photocatalytic process.20 Many techniques have been developed to fabricate various BiFeO3 nanostructures to meet this need, which suggests that

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1. INTRODUCTION The unique morphological characteristics of snow crystals have made them known as “the letter from the sky”. In scientific research, developing a technique of controlling the growth directions or exposed facets of functional materials on a nanometer- or micrometer-scale is of great importance because of its significance in determining the electrical, magnetic, and optical properties brought by a spatial geometry effect.1-3 Remarkable progress has been made in controlled fabrication or self-organization of various compounds. For example, anatase TiO2 has been studied extensively to gain highly exposed {100} facets because of the promising high reactivity in heterogeneous reactions;4-6 while Co3O4 nanorods have been found to possess excellent low temperature catalytic ability with predominant {110} planes.7 However, little attention has been paid to complex oxides such as perovskite materials despite their high values in both scientific and technological fields. Perovskite oxides and related compounds have been attracting increasing research interest over the past decade because of their potential applications in thin film capacitors, nonvolatile memory, nonlinear optics, and photoelectrochemical cells originated from their multiferroic, photocatalytic, or magnetic properties.8-10 Among them, perovskite-type bismuth ferrite (BiFeO3) has currently received the most attention as a rare multiferroic material that shows simultaneous ferroelectric and G-type antiferromagnetic orderings well above room

Received: August 29, 2010 Revised: December 26, 2010 Published: February 23, 2011 1049

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Figure 1. Plan-view and side-view (insets) of the (a) {100}c and (b){111}c facets of the BiFeO3 supercell structure.

BiFeO3 should make a competitive candidate in intended photocatalysts and optoelectronic devices. Yet until now it remains a challenge to produce phase-pure BiFeO321-23 and to tune the dominant facets of the crystallites to enhance its performance. Previous works contributed to crystallites with exposed {100}c crystal planes are shown in Figure 1a.24,25 However, based on the crystal structure of BiFeO3, [111]c/[001]hex does not only act as the directions of Ps and antiferromagnetic ordering but also works as the closest packing direction, thus possessing the highest atomic density as shown in Figure 1b. Therefore, it is quite likely that with highly exposed {111}c facets, it will be much easier for the atoms to interact with photons so as to exhibit quality optical properties. In this paper, we will demonstrate a simple and controllable approach to create perovskite BiFeO3 pills and rods with dominant {111}c facets which leads to an obvious enhancement in light response, especially at the visible light region when compared with that of {100}c exposed cubes. The synthetic strategy presented here may not only be suitable for potential large-scale applications of BiFeO3 but also provide an effective means to synthesize other perovskite crystals with tunable exposed facets.

Figure 2. XRD patterns evolution of the as-synthesized samples under different conditions: (a) pills (1 M KOH); (b) rods (2 M KOH); (c) cubes (15 M KOH).

2. EXPERIMENTS Synthesis. All chemicals were of analytical grade and used without further purification. All samples were prepared via the following hydrothermal process. 2 mmol of Fe(NO3)3 3 9H2O and Bi(NO3)3 3 5H2O together with 0.8 g of polyethylene glycol (PEG, Mw = 4000) were dissolved in 40 mL of distilled water. The pH value of the solution was adjusted by adding a certain amount of KOH, which served as a mineralizer. After being stirred for about 30 min, the suspension was transferred into a Teflon-lined steel autoclave. The autoclave was then sealed and heated at 200 °C for 3 days. The temperature was increased at a rate of 1 °C/min. The product was collected from the bottom of the autoclave after it was furnace-cooled to room temperature. After being washed with distilled water and absolute ethanol to remove possible residues and dried at 60 °C, powder products were obtained. Characterization. The obtained sample was characterized using X-ray diffraction (XRD) with a Bruker AXS D8 ADVANCE X-ray diffractometer with Cu KR radiation (λ = 0.154178 nm). Scanning electron microscopy (SEM) observations were made on a JEOL 6335F field emission system. Transmission electron microscopy (TEM) images and high resolution TEM (HRTEM) images were captured on

Figure 3. (a) Low magnification and (b) high magnification SEM images of pills.

JEOL 2011 & JEOL 2010F (field emission) transmission electron microscopes at an acceleration voltage of 200 kV for both instruments. The absorption properties were measured with a Shimadzu SolidSpec-3700DUV UV/vis-NIR spectrophotometer.

3. RESULTS AND DISCUSSION Through the experiment, we obtained three kinds of products in this PEG-assisted hydrothermal reaction by utilizing different alkaline conditions. Their X-ray diffraction (XRD) patterns are presented in Figure 2. As can be seen, all peaks can be perfectly indexed to a rhombohedral lattice with the space group R3c (JCPDS card No. 86-1518). No noticeable peaks from other phases were detected, demonstrating that single-phase and well1050

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Figure 4. TEM analysis of pills: (a) schematic of two project directions. Parts b-e are from the axial direction, while parts f and g are from the radial direction.

Figure 5. SEM image (a and b) and TEM analysis results (c-g) from the BiFeO3 rods.

crystallized BiFeO3 has been successfully obtained under these conditions. It is worth noting that the intensity of the (012)hex and (024)hex peaks for BiFeO3 samples c was clearly strengthened, suggesting that the {012}hex/{100}c planes should be dominant. As for the other two samples, the rise of the (202)hex/(111)c peak can be identified, which suggests that the anisotropic growth is perhaps preferential. These observed phenomena indicate that the products may have undergone a morphological evolution. Scanning electron microscopy (SEM) was performed to characterize the morphology and uniformity of all samples. Figure 3 shows the results of the product from 1 M KOH. High yield and well-defined BiFeO3 pills can be recognized with an average diameter of 0.5-1 μm and an average thickness of 100-300 nm. The surfaces of these pills are quite flat while the side faces seem slightly irregular. We further examined the crystallographic nature of the pills using high resolution transmission electron microscopy (HRTEM). The results were shown in Figure 4. As seen in Figure 4a, there are two typical incident directions for the electron beam under HRTEM. Morphologically, the plan-view in Figure 4b exhibits an uniform contrast which verifies the flat surface of the pills while the ripple-like contrast would be a result of the strain and bending of

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Figure 6. Low magnification (a) and magnified (b) SEM images of cubes.

the crystal.26 The selected area electron diffraction (SAED) in Figure 4c and g can be well indexed to perovskite BiFeO3, and both indicate that the axial direction is [111]c. In the HRTEM image in Figure 4d and the corresponding contrast profile in Figure 4e (acquired along the horizontal solid white line in Figure 4d), the lattice spacing of 0.23 nm corresponds to the distance between adjacent {111}c/{202}hex planes, which further confirms that the flat surfaces of the pills are {111}c. It would be rather easy to calculate that the BiFeO3 pills have over 60% {111}c facets. All crystallites are single crystals, as repeatedly observed on many other crystallites. The same characterizations were carried out on the product from 2 M KOH as in Figure 5. Summarized from SEM images, these rods possess an average diameter of 0.5-1 μm associated with an average length of 1-2 μm. About the crystallographic specifications under TEM, both the SAED and HRTEM images suggest a growth direction of [111]c; that is, the rods are simply “elongated” from the pills and hold only about 20% exposed {111}c facets. Another sample from relatively high alkaline concentration (15 M) is shown in Figure 6 which presents like excellent cubes under SEM observation with relatively rough surfaces and an edge length ranging from 5 -10 μm. The well-developed morphology suggests undoubted high crystallinity. According to the symmetries of perovskite BiFeO3 and other relevant works,24,25 all the surfaces of cubes can be ascribed to {100}c crystal faces. In other words, the cubes show almost 100% {100}c exposed facets. When samples from different reaction intervals were observed under SEM, an interesting formation process was detected, as presented in Figure 7. Figure 7a shows some large and almost amorphous crystallites (20-30 μm) after 6 h of hydrothermal treatment. When the treatment time was extended to 24 h (Figure 7b), the intermediate product generated shows an interesting microstructure, which resembles a Rubik’s Cube with an edge length of about 10 μm. Perfect cubes with sharp edges (length of ∼10 μm) were produced after a 72-h hydrothermal treatment (Figure 4c). Grounded on the above time-dependent morphology evolution process, a multistep formation mechanism is suggested as follows. First, BiONO3 is formed by hydrolyzing of bismuth nitrate together with Fe(OH)3 resulted from the reaction between iron nitrate and potassium hydroxide. These two compounds experience a dissolution process under alkaline conditions and autogenous pressure in our reaction system. Subsequently, they nucleate to form BiFeO3 amorphous colloids followed by a slow aggregation and crystallization of primary 1051

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Crystal Growth & Design particles.27,28 The reactions involved during this process are as follows: BiONO3 þ H2 O H Bi3þ þ 2OH- þ NO3 Fe3þ þ 3OH- f FeðOHÞ3 H FeO3 3- þ 3Hþ Bi3þ þ FeO3 3- f BiFeO3 V In the second step, since symmetry conservation would work better in this situation ([KOH] = 15 M), the colloids slowly change into cubic shaped crystallites and oriented-attaches29 into bigger cubes. As the growth proceeds, microcubes are gradually formed. The rough surface of the cubes should be associated with the erosion effect of KOH to crystallites at such concentration Scheme 1. Formation and Shape-Evolution Process during the Reaction

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levels. On the other hand, at lower KOH concentration levels (i. e. 1 and 2 M), the symmetry break will dominate the growth process to form pills or rods due to selective adsorption of PEG on certain crystal planes. The schematic illustration of this growth procedure is presented in Scheme 1. To study the change of the visible light response of our {111}c dominant pills and rods, their optical absorption performance was characterized. As shown in Figure 8a, the diffused reflection spectra have been transformed into the absorption ones according to the well-known Kubelka-Munk theory.30 Interestingly, in contrast to the {100}c dominant cubes, the photoabsorption edges of BiFeO3 pills and rods red-shift to the visible light region, while the absorption features below 400 nm remain almost unchanged. Furthermore, for pills with extremely high exposed {111}c, an extra strong absorption can be identified in the ca. 600-700 nm region. Based on the results and our sample BiFeO3 cubes, it is quite possible to calculate the band gap from the plot of the Kubelka-Munk function ((ahv)2) vs photon energy (hv) for direct band gap semiconductors, as presented in Figure 8b. The slope of the linear part suggests a band gap of ∼2.1 eV, which is smaller than that of bulks or thin films. Similarly, the band gap for BiFeO3 pills and rods is estimated to be even smaller (∼2.05 eV, not shown here). On the other hand, it was reported that the band gap energy would increase with decreasing crystalline size for BiFeO3 nanoparticles,31 but here in our situation, we actually observe a weak diminution together with the reducing particle size (cube > rod > pill), which further proved the effects of the highly exposed {111}c facets. From the viewpoint of utilizing solar energy as in the applications of photocatalyst and optoelectric devices, the results suggest that our pills and rods may have a greater advantage, especially under the visible light wave band.

Figure 7. SEM images of the BiFeO3 samples after (a) 6 h-, (b) 24 h-, and (c) 72 h-hydrothermal treatment with 15 M KOH.

Figure 8. (a) UV-visible absorption spectra of the BiFeO3 crystallites. (b) Plot of (ahν)2 vs photon energy (hν) for cubes, where the dotted line is the tangent of the linear part. 1052

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4. CONCLUSION In conclusion, we have developed a facile and controllable route for one-pot synthesis of phase-pure BiFeO3 pills and rods with highly exposed {111}c facets. Such pills and rods show an obviously enhanced visible light response when compared with {100}c dominant BiFeO3 cubes. The developed method has the merits of being simple, reproducible, and mass-producible. 5. ACKNOWLEDGMENT This work is financed by the Hong Kong Research Grants Council (5309/08E). The support from the Centers for Smart Materials of the Hong Kong Polytechnic University is also acknowledged.

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(25) Li, S.; Lin, Y. H.; Zhang, B. P.; Wang, Y.; Nan, C. W. J. Phys. Chem. C 2010, 114, 2903. (26) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. (27) Burda, C.; Chen, X. B.; Narayanan, R.; El-Sayed, M. A. Chem. Rev. 2005, 105, 1025. (28) Zhong, L. S.; Hu, J. S.; Liang, H. P.; Cao, A. M.; Song, W. G.; Wan, L. G. Adv. Mater. 2006, 18, 2426. (29) Zhang, J.; Huang, F.; Zhang, L. Nanoscale 2010, 2, 18. (30) Kubelka, P.; Munk, F. Z. Tech. Phys. 1931, 12, 593. (31) Li, S.; Lin, Y. H.; Zhang, B. P.; Nan, C. W.; Wang, Y. J. Appl. Phys. 2009, 105, 056105.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

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