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J. Phys. Chem. B 2006, 110, 11196-11198
General Solution Route for Nanoplates of Hexagonal Oxide or Hydroxide Jianbo Wu, Hui Zhang, Ning Du, Xiangyang Ma, and Deren Yang* State Key Lab of Silicon Materials, Zhejiang UniVersity, Hangzhou 310027, People’s Republic of China ReceiVed: January 19, 2006; In Final Form: March 23, 2006
A citric acid (CA)-assisted hydrothermal process was used to prepare Fe2O3 hexagonal nanoplates with a lateral size of about 100 nm. In addition, the hexagonal nanoplates of Co(OH)2, MnCO3, and Ni(OH)2 were also synthesized by this route, indicative of the universality of the solution route presented herein. The morphologies and structures of the synthesized platelike nanostructures have been characterized by transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), and X-ray diffraction (XRD). Furthermore, the mechanism for the formation of the platelike nanostructures has been preliminarily discussed. It is believed that the capping molecule of CA, which inhibits crystal growth along the 〈001〉 direction due to its chelating effect, plays a critical role in the hydrothermal formation of the nanoplates.
Introduction Great interests in nanomaterials have been focused on controlling the shapes and sizes of materials and the related finding novel properties.1 The diverse nanostructures including nanoribbons, nanorods, nanowires, nanotubes, nanoplates, and nanoprisms2 are promising in technological application areas ranging from transistors, chemical and biological sensors, separations, catalysts, lasers, and light emission diodes.3 In particular, nanoplates have received intensive research due to their potential applications in nanoelectronics, data storage, and catalysis. Compared with particles and one-dimensional (1-D) nanostructures, the synthesis of two-dimensional (2-D) nanoplates has not been well investigated yet.4 To our knowledge, the morphology-controllable synthesis approaches for nanostructures can be largely classified into two strategies: one is based on the vapor phase routes, such as thermal evaporation and chemical vapor deposition,5 the other is soft chemical routes, such as hydrothermal and solvothermal processes.1,6 However, there is not yet a general approach to preparing 2-D nanostructures without the assistance of a substrate, particularly for the soft chemical routes. As a simple, cost-effective, and efficient synthesis route, the hydrothermal process has been demonstrated as a versatile pathway toward the size- and morphology-controllable nanostructures. However, until now, few reports have addressed the formation of the 2-D nanostructures.7 Enlightened by the basic idea behind the controlled growth of 1-D nanostructures by the capping molecule-assisted hydrothermal process, it is believed that 2-D nanostructures can be fabricated through the limitation of the oriented growth using the capping molecule. Jun Liu and coworkers used a seed growth procedure to obtain a complex hexagonal ZnO nanostructure with capping of the citric acid (CA), which adsorbs preferably on the (001) surface of ZnO and slows down the crystal growth along the c-axis.8 Herein, we report a general route to synthesize oxide or hydroxide hexagonal nanoplates by a CA-assisted hydrothermal process, and we suggest that CA can suppress the intrinsically anisotropic * To whom correspondence should be addressed. E-mail: mseyang@ zju.edu.cn.
growth of the compound with hexagonal structure along the 〈0001〉 direction due to the chelating effect. Experimental Section All chemicals used were analytic grade reagents without further purification. Experiment details were as follows: 4 mmol metal ion source (FeCl3, Mn(AC)2,Co(AC)2, Ni(AC)2) was dissolved into 60 mL of H2O. After 10 min of stirring, 0.5 g of CA and 10 mL of 2 M NaOH aqueous solution were subsequently introduced into the above aqueous solution under stirring, which were then transferred into Teflon-lined stainless steel autoclaves, sealed, and maintained at 200 °C for 20 h. After the reaction was complete, the resulting solid products were centrifugalized, washed with deionized water and ethanol to remove the ions possibly remaining in the final product, and finally dried at 60 °C in air. The obtained powders were collected for the following characterization. The obtained samples were characterized by X-ray powder diffraction (XRD) using a Japan Rigaku D/max-ga X-ray diffractometer with graphite monochromatized Cu KR radiation (λ ) 1.54178 Å). The scanning electron microscopy images were obtained on a FEI SIRION. Transmission electron microscopy (TEM) observation was performed with a JEM 200 CX microscope operated at 160 kV. Results and Discussion Figure 1 shows the morphological and structural characterizations of Fe2O3 nanostructures prepared by a CA-assisted hydrothermal process. From Figure 1a, a large amount of hexagonal platelike Fe2O3 nanostructures are observed. Moreover, hexagonal Fe2O3 nanoplates with a lateral size of about 100 nm and a thickness of about 20 nm are clearly shown in Figure 1b. The XRD pattern of the Fe2O3 nanoplates is shown in Figure 1c. All of the diffraction peaks can be indexed as hexagonal Fe2O3 with lattice constants a ) 5.035 Å and c ) 13.74 Å, in agreement with the reference values of JCPDS No. 33-0664. The selected area electron diffraction (SAED) pattern performed on an individual nanoplate, as shown in Figure 1d, indicates that the Fe2O3 nanoplate is single crystalline in nature. Moreover, the zone axis vertical to the horizontal plane of the
10.1021/jp0603886 CCC: $33.50 © 2006 American Chemical Society Published on Web 05/18/2006
Nanoplates of Hexagonal Oxide or Hydroxide
J. Phys. Chem. B, Vol. 110, No. 23, 2006 11197
Figure 1. Morphological and structural characterization of the Fe2O3 nanoplates synthesized by the CA-assisted hydrothermal method at 200 °C: (a) FESEM image, (b) TEM image, (c) XRD pattern, and (d) TEM image of an individual Fe2O3 nanoplate. The upper right inset corresponds to the SAED pattern of an individual hexagonal Fe2O3 nanoplate.
Figure 3. XRD pattern of other nanostructures prepared by the CAassisted hydrothermal process at 200 °C: (a) MnCO3, (b) Co(OH)2, and (c) Ni(OH)2.
Figure 2. Morphological characterization of other nanostructures synthesized by the CA-assisted hydrothermal method at 200 °C: (a and b) FESEM and TEM images of MnCO3 nanoplates, (c and d) FESEM and TEM images of Co(OH)2 nanoplates, and (e and f) FESEM and TEM images of Ni(OH)2 nanoplates.
nanoplate is along the 〈0001〉 direction (c-axis of Fe2O3 nanoplate), indicating that the growth of the Fe2O3 nanoplate along the c-axis is suppressed. To confirm the versatility of the above-mentioned method for synthesizing oxide or hydroxide hexagonal nanoplates, we also employed this method to prepare cobalt hydroxide, nickel hydroxide, and manganese carbonate hexagonal nanoplates.
Figure 2 shows the FESEM and TEM images of the abovementioned nanostructures prepared by the CA-assisted hydrothermal process. From these images, it is known that hexagonal platelike MnCO3 nanostructures with a lateral size of about 200 nm and a thickness of about 20 nm, Co(OH)2 nanostructures with a lateral size of about 500 nm and a thickness of about 20 nm, and Ni(OH)2 nanostructures with a lateral size of about 100 nm and a thickness of about 20 nm have been successfully synthesized, reasonably confirming the universality of the method presented herein to prepare hexagonal nanoplates. The XRD patterns of the above-mentioned three kinds of nanostructures are shown in Figure 3. All of the diffraction peaks in parts a, b, and c of Figure 3 can be indexed as the hexagonal MnCO3, Co(OH)2, and Ni(OH)2, respectively. In principle, a crystal growth process consists of nucleation and growth, which are affected by the intrinsic crystal structure and the external conditions including the kinetic energy barrier, temperature, time and capping molecules, and so forth.9 The crystal structures of the four materials involved in the work are all hexagonal in nature. In general, for the materials of hexagonal structure, the anisotropic growth along the c-axis is available to form the 1-D nanostructures. In a previous paper, we have
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Wu et al.
reported the preparation of FeOOH nanorods and fish-like nanostructures without the assistance of CA.10a However, herein the hexagonal nanoplates have been fabricated by the CAassisted hydrothermal process, indicating that CA plays the critical role in the formation of hexagonal nanoplates. It is proven that the spinal or acicular FeOOH nanostructures are easily formed in the hydrothermal process without the assistance of CA.10a The hydrolysis of Fe3+ in aqueous solution proceeds as follows10
nanoplates with the lateral size of 100-500 nm have been synthesized by the CA-assisted hydrothermal process. It is found that the capping molecule, that is, CA, is critical for the formation of hexagonal nanoplates due to the chelating effect. Moreover, it is anticipated that the CA-assisted hydrothermal process could be employed to synthesize other nanoplates of hexagonal compounds.
Fe3+ + 2H2O T FeO(OH) + 3H+
(1)
Fe3+ + 3H2O T Fe(OH)3 + 3H+
(2)
Acknowledgment. The authors acknowledge financial support from the Natural Science Foundation of China (60225010), the Key Project of Chinese Ministry of Education, and the Program for New Century Excellent Talents in Universities. Thanks go to Prof. Youwen Wang for the TEM and FESEM measurements.
FeO(OH) + H2O T Fe(OH)3
(3)
References and Notes
While in the CA-assisted hydrothermal process, Fe3+ ions exist in the form of the metal ligand [Fe(C6H4O7)2]5-, which will suppress the intrinsically anisotropic growth of FeOOH. During the hydrothermal process, the following reaction proceeds11
[Fe(C6H4O7)2]5- T Fe3+ + 2C6H4O74-
(4)
In the direction perpendicular to the basal plane (0001) of the hematite with the corundum structure, layers of closely packed negative oxygen atoms alternate with two 1/3 layers of positive iron atoms (bilayers).12 The negative metal ligand preferably adsorbs on the (0001) plane of the seed crystal due to the Coulombic force, which slows down crystal growth along the 〈0001〉 orientation.8 Accordingly, the intrinsically anisotropic growth of Fe2O3 along the 〈0001〉 direction is substantially suppressed and the platelike Fe2O3 is formed, which is similar to the formation of the ZnO disk reported in our previous report.13 As for Co(OH)2 and Ni(OH)2, the hydrogen atoms are isolated between layers of MO6 octahedra along the c-axis, where M is a divalent cation such as Co or Ni. The MO6 octahedra are compressed along the c-axis, creating a distorted hexagonal close packing of oxygens. Each oxygen is hydrogenated and in the ideal structure, and the O-H groups lie along the c-axis.14 Regarding MnCO3, which is classified into calcite (R-3c), along the c-axis, layers of closely packed negative carbonate ions alternate with layers of manganese ions.15 In all of the structures of Co(OH)2, Ni(OH)2, and MnCO3 as described above, the negative metal ligands preferably adsorb on the (0001) plane of the hexagonal nucleus, similar to the case of Fe2O3. It should be stated that the above explanation about the formation of the special platelike nanostructures is somewhat conjectural and phenomenological. Intensive study is necessary to substantially explore the formation mechanism of the nanoplates reported herein. Conclusion To conclude, a general solution route has been employed to control the growth of oxide or hydroxide. Various hexagonal
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