Sonochemical Synthesis of Prussian Blue Nanocubes from a Single-Source Precursor Xinglong Wu,† Minhua Cao,*,† Changwen Hu,*,†,‡ and Xiaoyan He† Department of Chemistry, Northeast Normal UniVersity, Changchun, P. R. China, 130024, and The Institute for Chemical Physics, Beijing Institute of Technology, Beijing, P. R. China, 100081, and Department of Chemistry, Beijing Institute of Technology, Beijing, P. R. China, 100081
CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 1 26-28
ReceiVed July 30, 2005; ReVised Manuscript ReceiVed September 28, 2005
ABSTRACT: Regular, single-crystalline nanocubes of Prussian blue, Fe4[Fe(CN)6]3, with different sizes were synthesized in large quantities by a direct dissociation of the single-source precursor K4Fe(CN)6 in acidic solution under ultrasonic conditions. The size and size distribution of the cubes strongly depends on the reaction temperature, the concentration of K4Fe(CN)6 aqueous solution, and the ultrasonic condition. The probable mechanism of formation of Prussian blue is discussed. In recent years, Prussian blue and related cyanometalate-based coordination polymers have raised renewed and growing interest in many fields, for instance, in molecular magnets,1-4 electrochemistry,5-7 and optics8 due to their unique properties. Very recently, Kaye et al.9 have reported hydrogen storage properties of the dehydrated Prussian blue analogues M3[Co(CN)6]2 (M ) Mn, Fe, Co, Ni, Cu, and Zn). Prussian blue, which is considered to be the first synthetic coordination compound,10 is a mixed-valence iron(III) hexacyanoferrate(II) compound of composition Fe4[Fe(CN)6]3‚ xH2O with a face-centered-cubic structure, in which Fe3+ in the N-coordinated sites is in the high-spin state and Fe2+ in the C-coordinated sites is in the low-spin state.11 Although many research efforts have focused on the relationship between the unit cell structure and various properties,12-16 little progress has been made toward understanding and controlling the growth process of Prussian blue17 and its analogues18,19 with different sizes and morphologies. However, this is an important subject of nanomaterial science because control over both shape and size was of crucial importance for fine-tuning properties of these nanomaterials.
Figure 1. XRD pattern of Prussian blue nanocubes.
The traditional synthesis methodology of Prussian blue and its nanalogues with the general composition Mm+ i [M′(CN)6] , is based on a direct precipitation reaction of the Mm+ cations and the [M′(CN)6]n- anions in a neutral aqueous solution. It is well-known that Prussian blue has a small solubility-product constant (Ksp ) 3.3 × 10-41), indicating that upon direct mixing, Fe3+ and [Fe(CN)6]4- immediately react to form Prussian blue. For this kind of rapid reaction, control over the morphology and size of products
Figure 2. FE-SEM images of the Prussian blue nanocubes prepared at 40 °C and 1 mmol/L of K4[Fe(CN)6] (a) at a lower magnification and (b) at a higher magnification. (c) HRTEM image of a single nanocube; right inset: SAED image of the same nanocube.
* Corresponding authors: (M.C.): E-mail:
[email protected]; (C.H.): E-mail:
[email protected]. † Northeast Normal University. ‡ Beijing Institute of Technology.
is difficult.13 Previously Mann’s group17 reported the synthesis of Prussian blue nanoparticles and nanocrystal superlattices in reverse microemulsion. In their experiment, Prussian blue nanoparticles were formed by slow photoreduction of [Fe(C2O4)3]3- to produce
10.1021/cg050371x CCC: $33.50 © 2006 American Chemical Society Published on Web 12/08/2005
Communications
Crystal Growth & Design, Vol. 6, No. 1, 2006 27
Figure 3. TEM images of Prussian blue nanocubes prepared at 40 °C but with a different concentration of K4Fe(CN)6 of (a) 1 mmol/L, (b) 10 mmol/L, (c) 100 mmol/L, and (d) prepared at 70 °C and 1 mmol/L.
Fe2+ in the presence of [Fe(CN)6]3- ions. Recently, we prepared perfect dendrite R-Fe2O3 fractals by slow decomposition and hydrolysis of K3Fe(CN)6 under hydrothermal conditions.20 In fact, many synthesis methods of nanomaterials are based on slow reaction processes. Therefore, it can be reasonably inferred that to control the morphology and size of materials, especially for those with small Ksp values, a slow reaction process is very useful. Very recently, a single-source precursor approach (SSPA) has been widely used for preparing chalcogenide,21-26 LaF3 triangular nanoplates,27 silicoaluminophosphates,28 zinc-iron oxide composites,29 and so on. However, no report on the synthesis of coordination polymers using SSPA has been published to date. In this communication, spurning the traditional synthesis methodology and using a slow reaction process, we have successfully synthesized single crystalline Fe4[Fe(CN)6]3 nanocubes using the single iron-source precursor K4Fe(CN)6 under ultrasonic conditions. To the best of our knowledge, it is the simplest method for synthesizing Prussian blue nanocubes. In a typical synthesis, 0.1 mmol of K4Fe(CN)6 was added to 100 mL of hydrochloric acid aqueous solution of 0.1 mol/L. The resulting K4Fe(CN)6 aqueous solution was kept at 40 °C for 5 h under ultrasonic conditions and then left to cool to room temperature. The obtained blue product was filtered and washed several times with distilled water and absolute ethanol and finally dried in a vacuum oven at 25 °C for 12 h. The composition of the as-synthesized product was examined by powder X-ray diffraction (Figure 1). All reflections can be indexed as a pure face-centered-cubic phase of Fe4[Fe(CN)6]3 [space group: Fm3m (no. 225)] with lattice constant a ) 10.2 Å, in accordance with the standard values for the bulk cubic Prussian blue (JCPDS card No 73-0687). Figure 2 shows the field-emission scanning electron microscope (FE-SEM) images of typical Prussian blue nanocubes synthesized with 1 mmol/L K4[Fe(CN)6] solution. It can be seen from Figure 2a that the sample is relatively uniform. The high-magnification FE-SEM image (panel b) reveals that the surfaces of the nanocubes are extremely smooth. Figure 2c is a high-resolution transmission electron microscopy (HRTEM) image
Scheme 1.
Formation Process of Prussian Blue Nanocubes
of a single nanocube with regular square shape. It displays clear lattice fringes of (200) and (020) crystal planes. The selected area electron diffraction (SAED) image (inset in Figure 2c) taken from the same nanocube shows regular diffraction spots and confirms the Fe4[Fe(CN)6]3 nanocube to be single crystalline. According to the HRTEM and SAED patterns, Fe4[Fe(CN)6]3 nanocubes are enclosed by {100} facets. Because of large stability constant (Ks ) 1.0 × 1035), [Fe(CN)6]4ions are very stable in neutral aqueous solution at room temperature, and almost no Fe2+ ions can be detected. However, in acidic solution [Fe(CN)6]4- slowly and partially dissociates to produce Fe2+; the Fe2+ ions are quickly oxidized to generate Fe3+ as the Fe2+ ions are unstable in atmosphere; finally, these Fe3+ cations react with undissociated [Fe(CN)6]4- anions to form Prussian blue Fe4[Fe(CN)6]3 (Scheme 1). As observed above (Figure 2a,b), the obtained Fe4[Fe(CN)6]3 product consists of relatively uniform nanocubes, which are bounded by {100} facets. Previously, Wang30 had reported that the shape of an fcc nanocrystal was mainly dependent on the ratio (R) between the growth rates along the 〈100〉 and 〈111〉 directions. And Fang and co-workers31-34 had synthesized metal chalcogenides nanocrystals to investigate the growth direction of fcc nanocrystals. It is generally accepted that the growth rates along different directions are dominated by the intrinsic properties of surface energy. In the case of fcc nanocrystals, because of lower surface energy of {100} crystallographic facets in comparison with those of {111} planes,35 the {100} facets will develop to increase the portion of low energetic surface. When a particle grows, facets tend to form on the low-index planes to minimize the surface
28 Crystal Growth & Design, Vol. 6, No. 1, 2006 energy.31 So, in our case, the Prussian blue Fe4[Fe(CN)6]3 nuclei grow up into perfect nanocubes enclosed by {100} facets, which possess the lowest surface energy and the smallest value of R of 0.58. When the reaction is carried out in hydrothermal or routine conditions, only asymmetrical cubes are produced. So, we can conclude that the uniform size distribution of synthesized Prussian blue nanocubes, most probably, is caused by the ultrosonication procedure. Large numbers of experiments indicated that the size distribution and size of the synthesized Prussian blue nanocubes depend strongly on the reaction temperature and the concentration of K4Fe(CN)6. As shown in Figures 2a,b and 3a, the mean edge length of the nanocubes, which is prepared at 40 °C and with a concentration of K4[Fe(CN)6] of 1 mmol/L, is about 250 nm. However, when the concentration of K4[Fe(CN)6] was increased to 10 and 100 mmol/ L, but kept at the same temperature, the edge length of the nanocubes increases to 300 and 500 nm (Figure 3b,c), respectively. Similarly, when reaction temperature was increased from 40 to 70 °C but kept at the concentration of K4[Fe(CN)6] 1 mmol/L, the edge length of the Prussian blue nanocubes also increased, from 250 to 450 nm (as shown in Figure 3a,d), as announced by Fang et al.32 But the product had a broader size distribution, which may result from the higher fluctuation in the reaction dynamics at the higher temperature. In previous work, it was proposed that the growth of nanostructures with different morphologies, sizes, compositions, and microstructures was mainly controlled by the temperature and time duration of the reaction process. For example, Fang et al. have synthesized Al2O3, MgO, and ZnS nanostructures with different morphologies, sizes, and microstructures in a controlled and simple way.36 In summary, we have described the synthesis of Prussian blue Fe4[Fe(CN)6]3 nanocubes with quite uniform size distribution by the dissociation reaction of K4[Fe(CN)6] in acidic solution under ultrasonic conditions. The size of the synthesized Prussian blue nanocubes can be tuned by controlling the reaction temperature and the concentration of the single precursor. We also put forward that slow reaction is very useful for controlling the morphology and size of nanomaterials, and our presented work is a good example. It is our hope that the slow-reaction idea can be used to direct synthesis of other nanomaterials. Acknowledgment. This work was supported by the Natural Science Fund Council of China (NSFC, Nos. 20331010, 20271007, 90406002, and 20401005) and Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP, No. 20030007014). This work was supported also by Jilin Distinguished Young Scholars Program and the Natural Science Young Foundation of Northeast Normal University.
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