Large-Scale Synthesis of Berlin Green Fe[Fe(CN)6] Microcubic Crystals Jianhui Yang, Haishui Wang, Lehui Lu, Weidong Shi, and Hongjie Zhang* Key Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Graduate School of the Chinese Academy of Sciences, Changchun 130022, P. R. China
CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 11 2438-2440
ReceiVed July 19, 2006; ReVised Manuscript ReceiVed August 31, 2006
ABSTRACT: Berlin green FeFe(CN)6 microcubic crystals have been successfully prepared by a simple hydrothermal process between K3[Fe(CN)6] with Na2S2O3 aqueous solution, free of any surfactant or template. The experimental results clearly show that the molar ratio of K3[Fe(CN)6] to Na2S2O3 and their concentrations are the dominant processing factors in controlling the size, morphology, and composition of the resulting products. The chemical and physical properties of inorganic micro-/ nanostructures are fundamentally related to their chemical composition, size, crystal structure, surface chemistry, and shape.1-4 Precise control of such factors allows one to not only observe unique properties of the materials but also to tune their chemical and physical properties as desired. In recent years, Prussian blue (PB) (KFeIIIFeII(CN)6) and its analogues Mim+[M′(CN)6]n- have raised renewed and growing interest in many fields, for instance, in molecular magnets,5-7 electrochemistry,8 and optics,9 because of their unique properties. In the past few years, various forms of PB and its micro-/nanostructure analogues have been synthesized.10 However, it is a challenge to prepare Prussian blue analogue particles with different shapes. According to previous reports,11,12 Prussian blue can be readily oxidized electrochemically to Berlin green (FeIIIFeIII(CN)6). The structure of this compound is shown in Figure 2 (inset).12,13 Here, we selectively obtain Berlin green Fe[Fe(CN)6] microcrystals with cubic morphology by a simple hydrothermal process between K3[Fe(CN)6] and a Na2S2O3 aqueous solution. To the best of our knowledge, this is the first time the production of Berlin green Fe[Fe(CN)6] microcubic crystal has been observed. The experimental results clearly show that the molar ratio of K3[Fe(CN)6] to Na2S2O3 and their concentrations play crucial roles in the size, morphology, and composition of the resulting products. It is interestingly note that R-Fe2O3 snowflakelike nanostructures are obtained at lower concentrations of the reactants. In a typical synthesis, 1 mmol of K3[Fe(CN)6] and an equal mount of Na2S2O3‚5H2O were dissolved in 10 mL of deionized water at room temperature to form a homogeneous solution, which was then transferred into a 15 mL Teflon-lined stainless steel autoclave, sealed, and maintained at 130 °C for 2 days. After the reaction completed, the green solid product was collected by centrifugation, washed several times with water, and then suspended in water. The resulting suspension was used for further characterization. The method can be scaled up to produce ∼0.25 g of Berlin green Fe[Fe(CN)6] microcrystals. In a similar procedure, for instance, a reaction yielding ∼2.5 g of Berlin green Fe[Fe(CN)6] microcrystals was performed in ∼100 mL of an aqueous solution. Scanning electron microscopy (SEM) measurements were made on an XL30 ESEM FEG scanning electron microscope equipped with an energy-dispersive X-ray spectroscopy (EDS) operating at an accelerating voltage of at 20 kV. The X-ray diffraction (XRD) analysis was carried out on a D/Max 2500 V/PC X-ray diffractometer using Cu (40 kV, 200 mA) radiation. Transmission electron microscopy (TEM) measurements were performed on a JEOL JEM-2000FX instrument operating at an accelerating voltage of 200 kV. * To whom correspondence should be addressed. E-mail: hongjie@ ns.ciac.jl.cn. Phone: (86) 431-5262127. Fax: (86) 431-5698041.
Figure 1. (a) Low- and (b) high-magnification SEM images of Berlin green FeFe(CN)6 microcubic crystals. (c) TEM image and corresponding electron diffraction pattern (inset) of a part of a single FeFe(CN)6 microcubic crystal. (d) EDS spectrum of the product. The silica signal is due to the silicon wafer substrate. The inset in (a) shows the photograph of the product suspended in water.
The morphology and structure of the as-prepared samples were investigated by SEM and TEM. The low-magnification image (Figure 1a) indicates that the sample mainly consists of a large quantity of cubic particles with a size of about 2-5 µm. The resulting sample is green in color, according to the photograph of the product suspended in water (inset in Figure 1a). The highmagnification image (Figure 1b) shows that the microcubic particles have extremely smooth surfaces. A typical TEM image of a
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Figure 2. Representative XRD patterns of (a) Berlin green FeFe(CN)6 microcubic crystals and (b) R-Fe2O3 snowflakelike nanostructures. The inset gives the unit cell of FeFe(CN)6.
part of a single microcubic crystal is presented in Figure 1c. The figure reveals that the microcubic crystal is built of numerous small nanoparticles and wormlike particles formed by small nanoparticles interconnecting with one another. The electron diffraction pattern (the inset in Figure 1c) reveals that a cubic symmetry diffraction spot pattern is generated, thus demonstrating that the microcrystal is a single crystal with a preferential growth direction along the {100} plane. The EDX was used to further characterize the composition of the as-prepared product. The EDX spectrum (Figure 1d) shows the presence of Fe, C, and N with an atomic ratio of 1:3:3, which matches well with that of Berlin green Fe[Fe(CN)6]. The crystal structure was determined by X-ray diffraction (XRD) measurements. Figure 2a presents the typical diffraction pattern of products, which can be readily indexed to a face-centered-cubic phase (space group Fm3m) with a lattice parameter of 10.28 Å. It
Crystal Growth & Design, Vol. 6, No. 11, 2006 2439 is worth noting that the {200} and {400} reflection peaks are sharp and strong. This observation conforms that Fe[Fe(CN)6] microcubic crystals are primarily dominated by {100} planes, and thus their {100} planes tend to be preferentially oriented parallel to the surface of the supporting substrate.14 It is worthwhile to mention that the quantity of Na2S2O3 is an important factor in determining the morphology of the final Berlin green Fe[Fe(CN)6] particles. Figure 3a shows the SEM image of Berlin green product obtained with an initial Na2S2O3:K3[Fe(CN)6] ratio of 1:2. We observe that, in addition to the cubic particles, irregular aggregates are also formed. The initial concentrations of K3[Fe(CN)6] and Na2S2O3 significantly affect the composition and morphology of the products. Figure 3b shows the image of products obtained at a decreased concentration of 1/10, under conditions that were otherwise identical to those used for preparing Berlin green Fe[Fe(CN)6]. The SEM image indicates that the product mainly consists of snowflakelike nanostructures with an average size of about 2 µm. Figure 3c shows a typical TEM image of a snowflakelike 6-fold-symmetric structure, and the corresponding electron diffraction pattern (inset) demonstrates that the snowflakelike structure is single crystalline and oriented along {0001}. The composition and phase purity were determined by EDS and XRD. The EDS spectrum (Figure 3d) suggests that the chemical compositions of the as-prepared product are composed of Fe and O elements, and that the atomic ratio (Fe:O) matches well with the stoichiometry of Fe2O3. Figure 2b shows the XRD pattern of Fe2O3 snowflakelike nanostructures. All the reflections of the pattern in Figure 2b can be indexed to the rhombohedral R-Fe2O3 (JCPDS card no. 33-664). In view of the above results, it is believed that the molar ratio of K3[Fe(CN)6] and Na2S2O3 and their concentrations have played important roles in control of the particle compositions and morphologies. When no Na2S2O3 was introduced, only R-Fe2O3 micropine dendrite are formed at higher concentrations of K3[Fe(CN)6].15 When the Na2S2O3:K3[Fe(CN)6] molar ratio is 1:2, the product is major in the cubic particles and irregular aggregates (Figure 3a). When the Na2S2O3:K3[Fe(CN)6] molar ratio is increased to 1:1, the product is major in microcubic crystals (Figure 1a). These results indicate that the right amount of Na2S2O3 in the solution is key to
Figure 3. (a) SEM image of Berlin green FeFe(CN)6 particles obtained with an initial Na2S2O3 to K3[Fe(CN)6] ratio of 1:2. (b) SEM image, (c) TEM image, and (d) EDS spectra of R-Fe2O3 snowflakelike nanostructures obtained with a 1/10 concentration of reactants. The inset in (b) shows the corresponding electron diffraction pattern of R-Fe2O3 snowflakelike nanostructures.
2440 Crystal Growth & Design, Vol. 6, No. 11, 2006 yielding Fe[Fe(CN)6] microcubic crystals. A typical redox reaction of Fe(CN)63- and S2O32- according to eq 1 yielded Fe(CN)64and S4O62-.
Communications 20340420326, 20490210, and 90306001) and the MOST of China (“973” Program, Grant 2006CB601103).
References
Fe(CN)63- + S2O32- f Fe(CN)64- + S4O62- f Fe(CN)63- + SO42- (1) Fe(CN)63- f Fe3+ f FeOOH/Fe(OH)3 f R-Fe2O3 (2) Subsequently, Fe(CN)64- should be oxidized to Fe(CN)63- and S4O62- should simultaneously be reduced to SO42- (eq 1). According to the previous report,15 Fe(CN)63- ions dissociate slowly into Fe3+ ions under hydrothermal conditions (eq 2). These Fe3+ cations can then react with undissociated Fe(CN)63- anions to form Prussian green Fe[Fe(CN)6]. The shape of an fcc nanocrystal is mainly determined by the ratio (R) between the growth rates along the {100} versus that of the {111} direction.16,17 In the case of fcc nanocrystals, the {100} crystallographic facets have lower surface energies than the {111} planes.16,10c Therefore, Prussian green Fe[Fe(CN)6] ultimately results in perfect microcubic crystals enclosed by {100} facets (Figure 1b), which possess the lowest surface energy when R ) 0.58. At lower concentrations, only R-Fe2O3 snowflakelike nanostructures are formed, because K3[Fe(CN)6] slowly decomposes and hydrolyzes under hydrothermal conditions (eq 2).15 The amount of S2O32- in the solution could affect the surface energy of various R-Fe2O3 crystal planes.18 However, it is not clear at present how the S2O32- molecules influence the growth of different crystal planes of Fe2O3 snowflakelike structures, and the detailed mechanism needs further investigation. In summary, Berlin green Fe[Fe(CN)6] microcrystals with cubic morphology have been successfully synthesized via a template- and surfactant-free hydrothermal reaction route involving K3[Fe(CN)6] and Na2S2O3. It suggests that the initial concentrations of the precursors and their molar ratio play crucial roles in the morphology and composition of the resulting product. It is interestingly noted that R-Fe2O3 snowflakelike nanostructures were obtained at the lower initial concentrations of reactants. It is believed that the controlled synthesis of these particles might bring wide applications in optics, gas sensors, catalysts, information storage, and other related fields.19 Our experimental method also provides a simple and facile route for the large-scale preparation of Berlin green Fe[Fe(CN)6] microcrystals, which could have important applications in molecular magnets because of the efficient ability of the CNanion to transfer magnetic coupling. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (Grants 20372060,
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