Growth of Well-Defined Cubic Hematite Single Crystals: Oriented Aggregation and Ostwald Ripening Baoping Jia and Lian Gao* State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 4 1372–1376
ReceiVed March 28, 2007; ReVised Manuscript ReceiVed January 22, 2008
ABSTRACT: Single-crystalline R-Fe2O3 submicron cubes with a well-defined shape have been successively synthesized via a simple hydrothermal process. X-ray diffraction patterns, transmission electron microscopy, field emission scanning electron microscopy, and selected area electron diffraction patterns were applied to characterize the as-synthesized samples. The results indicate that the growth of single-crystalline R-Fe2O3 cubes can be attributed to cooperation of two principal mechanisms: oriented aggregation and Ostwald ripening. At the beginning, R-Fe2O3 nanorods were first fabricated and formed cube-like aggregation following a “onedimensional (1D) f three-dimensional (3D)” mode, in which these nanorods orientedly aggregated on the surfaces of some cores by sharing common {012} faces. Then, the resulting polycrystalline aggregates would fuse into a single crystal, in which Ostwald ripening process was expected to carry out because oriented aggregation alone cannot result in well-defined R-Fe2O3 cubes. The extended particle would further attract free-standing R-Fe2O3 nanorods to orientedly attach on their surface, and the fusion process was repeated until nanorods were completely consumed. This kind of cooperation of two basic mechanisms may give us a new insight into the growth of R-Fe2O3 crystals and opens a new way for controllable synthesis of other well-defined crystals in morphology and dimensionality. Introduction Inorganic nano- or micron-structures with a well-defined shape have attracted intensive interest from both theoretical and practical perspectives not only due to their special size- and shape-dependent properties but also their self-assembly potential application for a wide range of fields. A number of strategies have been designed to synthesize monodispersed crystals with various morphologies.1–6 Generally, the hard or soft templeassistant methods are mostly employed for the chemical synthesis of well-defined crystals. As a basic issue, the crystal growth has been intensively investigated based on two fundamental mechanisms: Ostwald ripening and aggregation. Ostwald ripening, which is generally believed to be the main one, can be briefly described as the growth of larger crystals at the expense of smaller ones.7,8 According to the Gibbs-Thompson equation and Fick’s first law, the chemical potential of a particle increases with a decrease in particle size, which means the equilibrium solute concentration near a small particle is higher than that near a larger one. The resulting concentration gradients would lead to the diffusion of molecular-scale species from smaller particles to larger particles through solution.9 However, recently, this traditional mechanism seems to be unable to adequately describe the crystal growth in many systems in which the crystal growth is through self-aggregation. The products resulting from this manner are usually polycrystalline particles composed of randomly oriented primary crystals. In some cases, secondary particles can be formed through aggregation of subunits in an irreversible and highly oriented fashion. Penn and Banfield10,11 present this new growth model as “oriented aggregation”, which involves spontaneous self-organization of adjacent particles in a common crystallographic orientation and joining of these particles at a planar interface to reduce overall energy of system. Over the past decades, oriented aggregation has been highlighted in a number of works involving the growth of nanoparticles and paved a route to control the size, shape, * To whom correspondence should be addressed. E-mail: liangaoc@ online.sh.cn; tel.: 0086-21-52412718; fax: 0086-21-52413122.
and morphology of the products. Up to now, several kinds of oriented aggregation have emerged, such as zero-dimensional (0D)-one dimensional (1D),12 1D-two-dimensional (2D),13 0D-2D-three-dimensional (3D),14 1D-2D-3D,15 and so on. However, owing to the complexity of the influencing factors, the controlled synthesis in morphology and dimensionality has been still tremendous challenges for scientists. R-Fe2O3 (hematite) is a typical environmentally friendly semiconductor (Eg ) 2.1 eV). Monodispersed hematite colloidal particles have been extensively applied as a pigment, anticorrosion paint, gas sensor, catalyst, antiferromagnetic material, and photoassisted electrolysis of water.16–19 It is noted that the morphology and size of R-Fe2O3 have a great impact on their chemical and physical properties. Much effort has been made in the design of various R-Fe2O3 materials with a desired structure and morphology in the past decades, especially the solvent-assistant methods. Different morphologies, such as pseudocubic, disk, wire, belt, tube and rod, have been reported over the past decades.20–27 In most cases, β-FeOOH precursor was usually first obtained via hydrothermal treatment of ferric solution and further dehydrated into R-Fe2O3 through calcinations or annealing. Sigumoto and his group20,21 have developed a gel-sol method to prepare monodispersed hematite nanoparticles with various shapes by forced hydrolysis. However, most of the resulting hematite particles exhibited porous and polycrystalline characteristics. Recently, Wang et al.28 reported the controlled synthesis of hematite nanorhombohedra, nanorods, and nanocubes by polymer-mediated hydrolysis of FeCl3, in which the corresponding growth mechanism is not discussed in detail. Therefore, it is still desired to further explore the growth of the R-Fe2O3 crystals, which can be used to develop a new method for the synthesis of R-Fe2O3 particles with wellconfined morphology. In this report, we demonstrate a simple hydrothermal synthesis of single-crystalline R-Fe2O3 submicron-cube with high uniformity, in which R-Fe2O3 nanorods were first fabricated and self-assembled into cube-like aggregation through a “1D f 3D” mode. Then, the polycrystalline aggregates of subordinate
10.1021/cg070300t CCC: $40.75 2008 American Chemical Society Published on Web 03/15/2008
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Figure 1. Experimental flowchart to prepare single-crystalline R-Fe2O3 submicron-cube.
nanorods fused into well-defined single-crystalline R-Fe2O3 cubes. This result demonstrates a new model of crystal growth with cooperation of two basic growth mechanisms and may open a new way for controllable synthesis of other well-defined crystals in morphology and dimensionality. Experimental Section Materials. All chemicals (Shanghai Chemical Reagents Company) were analytical grade and used as received. Synthesis. The detailed experimental procedure is as follows: analytically pure FeSO4 · 7H2O (10 mmol) and cetyltrimethyl ammonium bromide (CTAB) were dissolved into 100 mL of distilled water. 1 M NaOH was added drop by drop to adjust the pH value of the system at room temperature under constant magnetic stirring. When the pH value was increased up to 11, a bright yellow precipitate appeared after stirring for 30 min. Then the resulting slurry was transferred into a Teflonlined stainless steel autoclave and maintained at 180 °C for 16–36 h. The produced precipitates were washed with deionized water and ethanol repeatedly to remove the remaining surfactant, and centrifuged for several times. Finally the collected precipitates were dried in a vacuum at room temperature. Figure 1 illustrates the overall experimental flowchart to prepare single-crystalline R-Fe2O3 submicron-cube. Characterization. Phase identification was done by the powder X-ray diffraction (XRD) pattern, using D/max 2550V X-ray diffraction meter with Cu KR irradiation at λ ) 1.5406 Å. The step-scan covered the angular range 10–80° in step of 0.02°. Transmission electron microscopy observations (TEM) and selected area electron diffraction patterns (SAED) were performed on JEM-2100F electron microscope with an accelerating voltage of 200 kV. Three-dimensional observations of the morphology of products were conducted on a field emission scanning electron microscopy (FESEM, JEOL, JSM-6700F). The dimensions of the nanocrystals were estimated via measuring their sizes based on the TEM and FESEM images.
Results and Discussion Figure 2 presents the representative XRD patterns of the samples obtained at different stages, which can all be readily indexed to rhombohedral R-Fe2O3 phase according to the JCPDS file No. 84-0311. The result indicates the high purity of our products, and the well-resolved diffraction peaks reveal the good crystallinity of the R-Fe2O3 specimens. Figure 3 shows the typical SEM images of the sample obtained after hydrothermal treatment for 36 h. It can be clearly seen that these particles are well-defined cubes with uniform edge length in the range of 500–800 nm. Detailed investigation on the morphology and microstructure of the R-Fe2O3 cube was further conducted through TEM tilting experiment of an individual cubic R-Fe2O3 crystal. Figure 4a is that of the particles prior to tilting (φ ) 0). The uniform resolution and clear-cut edges all confirm the morphology of these cubic particles. All angles between the edges at the corners of the particles can be divided into two styles: 94° and 86°, which is consistent with the data from Park et al.29 When the copper grid was tilted with
Figure 2. The XRD patterns of the samples obtained at different stages. (a) 16 h; (b) 20 h; (c) 24 h; (d) 30 h; (e) 36 h.
Figure 3. (a,b) FESEM images of the well-defined R-Fe2O3 cube.
an angle (as shown in Figure 4b,c), it is observed that the particle presents a different outline (from quadrangle to hexagon). Transition of contrast appears and the center is apparently darker than the tilted edges. At the same time, the edges become faint as the tilting continues, because of the shortening travel distance for penetrating electrons. The schematic illustrations of the R-Fe2O3 cube from different angles are inset in the bottom, respectively. Excellent crystallinity of our products is also confirmed by corresponding SEAD patterns (insets in Figure 4) which clearly exhibit a single-crystal nature. After carefully analysis was done, the spots in Figure 4a can be easily indexed to (012) and (112) of rhombohedral R-Fe2O3, respectively, with the incident electron beam parallel to the [421] direction. These results indicate that the six surfaces of the cubic hematite particle are all bound by {012} planes, agreeing well with the result determined by the X-ray diffraction of oriented particulate monolayer (OPML-XRD) method.30 These well-defined R-Fe2O3 cubes, free-standing on their {012} faces, can stand one by one and self-assemble into interesting configurations. Selfassembly of different numbers of R-Fe2O3 cubes (from two to five) was shown in Figure 5. To investigate the growth mechanism of our product, control experiments with different durations of hydrothermal treatment were carried out. As shown in Figure 6a, R-Fe2O3 nanorods with diameters in the range of 80–120 nm and length up to 400 nm were fabricated when the treatment was reduced to 16 h. It is noteworthy that a small portion of cube-like nanoparticles were also fabricated (as the white arrow points to). Close observation (as shown in Figure 6b) indicates that some rods surround the cubic center and aggregate along their side surfaces. HRTEM image of the intermediate state (Figure 6c) clearly shows that the cubic crystal and nanorod have similar crystallographic orientation and share the (012) plane. In our previous
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Figure 4. (a-c) TEM tilting experiment of an individual R-Fe2O3 cube and their corresponding SEAD patterns. The insets in the bottom represent schematic illustrations of the R-Fe2O3 cube from different angles, respectively.
Figure 5. (a-d) Self-assembly of different numbers of R-Fe2O3 cubes.
research, freestanding rod-like R-FeOOH nanocrystals were first synthesized and then transformed into R-Fe2O3 nanorods by dehydration in the successive treatment. These rod-like structures grew preferentially along the [104] direction and surrounded by (012) planes as side faces.31 According to the report from Park et al.29 in their internal structure analysis of pseudocubic hematite particles, it is also pointed out that pseudocubic hematite particles are composed with rod-like subcrystals which are banded to each other by {012} planes. Penn and her group10 studied this kind of “oriented attachment” and attributed the main driving force to the tendency to decrease the high surface energy through self-organization of adjacent particles in a common crystallographic orientation and joining of these particles at a planar interface. In our case, the surfaces of cubic particle and the side surfaces of R-Fe2O3 nanorod both belong to the {012} planes. So it is rational that the nanorods would
preferentially attach around the cubic center to reduce the surface energy of whole system. With the treatment prolonging to 20 h, solid aggregates sharing rough pseudocubic configuration were obtained as shown in Figure 6d,e. Further extending the duration of treatment (24 h), it is discernible that the center of the aggregate has grown into well-shaped cubic particles around which R-Fe2O3 nanorods with polyhedral configuration attached along their side surface (Figure 6f,g). This kind structure is similar with the oriented attachment mentioned above but developed. It is obvious that these well-shaped cubic center particles were derived from the oriented aggregates of nanorods. According to the “oriented attachment” from Penn et al., nanoparticles would fuse together with their high-energy surfaces under crystallographic fusion and elimination of the high-energy faces under energy gain, which is well coincident with the development of our samples. When the treatment was extended to 30 h, the particles became bigger with clearer cubic configuration and those nanorods surrounding the cubic center were almost disappeared (as shown in Figure 6h,i). On the basis of the above results, the growth of well-defined cubic R-Fe2O3 single crystals can be summarized as follows, though it is yet difficult to establish a quantitative kinetic model at this stage. First, R-Fe2O3 nanorods were synthesized and aggregated around some core to form cube-like aggregation through oriented attachment. Then, the adjacent nanorods would fuse into the core section, through which bigger single-crystalline particles, as extended structures, were created. In the following steps, Ostwald ripening process should also take place because of the increasing difference between center part and the surrounding nanorods. It is comprehensible that “oriented attachment” alone cannot result in well-defined R-Fe2O3 single crystal and the traditional “solid-solution-solid” approach could fix the imperfection around the single-crystalline cubes. The cubic particle would further attract free-standing R-Fe2O3 nanorods attached on their surface and repeat the fusion process to grow bigger until nanorods were consumed. The whole formation of cubic R-Fe2O3 single-crystal can be summarized in the schematic illustration as shown in Figure 7. The line in Figure 8 shows
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Figure 8. Effect of reaction time on the average size of cubic particle.
Figure 9. (a,b) Typical SEM image of the R-Fe2O3 nanocrystals synthesized without CTAB.
Figure 6. TEM, HRTEM, and SEM images of the as prepared products after reaction at 180 °C for: (a,b,c) 16 h; (d,e) 20 h; (f,g) 24 h; (h,i) 30 h.
Figure 7. Schematic illustration of the formation mechanism of cubic R-Fe2O3 single crystal.
the effect of the reaction time on the average diameter of cubic particle, from which it can be noticed that the increase of size gradually slows down as the reaction is prolonged. This kind of transformation is well coincident with the change of growth mechanism from oriented aggregation to Ostwald ripening discussed above. Teo et al. have also observed similar combination through their research on the fabrication of hollow cubic nanoparticles of Cu2O and Cu.32 In our experiment, the surfactant CTAB plays an important role in the process. Sugimoto and his group30 have extensively investigated the influence of different anions on the morphology of R-Fe2O3. A recent report demonstrated that functional molecules on the surface of nanoparticles have an important influence on their self-assembly behavior.33 The selective adsorption of organic surfactants on particular crystallographic facets has been an effective way to control the size and shape of nanocrystals,
which would be an immense step forward to the controlled, bottom-up fabrication of well-defined nanostructures. In our case, CTAB would be selectively adsorbed on the surface of R-Fe2O3 crystals depending on their different surface energies. The organic additive on different facets of crystal would confine the growth of certain planes, which eventually would lead to oriented attachment and anisotropic growth of polyhedral symmetry structure. Control experiment indicates that only spherical R-Fe2O3 particles were synthesized when CTAB was not involved in the experiment under the same reaction conditions (as shown in Figure 9), which confirmed well its key effect. It is rational that when the particles were not restricted by the functional surfactant CTAB, they would grow randomly and aggregate in an isotropic way to form polycrystalline spheres. Conclusion In summary, we demonstrated a simple and effective hydrothermal method to synthesize well-defined single-crystalline R-Fe2O3 submicron-cube with high uniformity. R-Fe2O3 nanorods were first fabricated and self-assembled into polycrystalline aggregation through a “1D f 3D” mode, in which the rods surrounded some cubic center and oriented aggregated on its surfaces by sharing common {012} planes. Then, the adjacent nanorods would fuse into the core section and create extended cubic R-Fe2O3 single crystals with a well-defined shape, in which Ostwald ripening is considered to be the main mechanism because oriented aggregation alone cannot result in well-defined R-Fe2O3 cubes and the traditional “solid-solution-solid” approach can fix the imperfection around the single-crystalline cubes. The extended particles would further attract free-standing
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R-Fe2O3 nanorods orientedly attached on their surfaces and repeat the fusion process to grow bigger until nanorods were consumed. This kind of cooperation of two basic mechanisms may give us a new insight into the growth of R-Fe2O3 crystal and open a new way for controllable synthesis of other welldefined crystals in morphology and dimensionality. Acknowledgment. This work was supported by the National Key Project of Fundamental Research (Grant No: 2005CB623605) and the Shanghai Nanotechnology Promotion Center (Grant No: 0552nm045 and No: 0652nm022), respectively.
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