Template-Free Hydrothermal Synthesis of CeO2 Nano-octahedrons and Nanorods: Investigation of the Morphology Evolution Lai Yan,† Ranbo Yu,† Jun Chen,† and Xianran Xing*,†,‡ Department of Physical Chemistry, and Key State Laboratory for AdVanced Metals and Materials, UniVersity of Science and Technology Beijing, Beijing, China
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 5 1474–1477
ReceiVed January 30, 2008; ReVised Manuscript ReceiVed February 26, 2008
ABSTRACT: Uniform single-crystalline CeO2 nano-octahedrons and nanorods were synthesized by a facile hydrothermal synthesis process only using Ce(NO3)3 · 6H2O as cerium resource and Na3PO4 · 6H2O as mineralizer, into which no surfactant or template was introduced. By tuning the hydrothermal treatment time, the morphology evolution between the nano-octahedron and nanorod was observed. Furthermore, the synthesizing mechanism and the morphological evolution of different shapes were investigated. Unlike traditional hydrothermal synthesis of CeO2 nanostructures using strong base as precipitant, Na3PO4 does not leave any impurity in the hydrothermal reaction system and makes the process very simple to obtain and separate the octahedral and rodlike morphology. Controllable synthesis of inorganic nanocrystal morphology is of the most important challenges in the field of advanced materials because of the unique shape-dependent materials properties, which would result in a wide range of electrical, optical, or magnetic properties and open a new domain of theoretical and technological interest. As a well-known functional rare earth metal oxide, nanostructured ceria has been extensively studied and employed in various applications including solid-state fuel cells, catalysts, UV blockers, and polishing materials.1 Stimulated by both the promising applications and the interesting properties, much attention has been directed to the controlled synthesis of CeO2 nanostructures. Over the past few years, remarkable progresses have been made in the synthesis of nanosized ceria samples with different morphologies and in the investigation of their size/shape-dependent properties.2 CeO2 with morphologies such as nanoparticles,3 nanospheres,2a nanocubes,2b nanotrigulars,4 nanorods,5 nanowires,6 and nanotubes7 has been prepared via well-developed routes by many research groups. Of these methods, hydrothermal synthesis has been regarded as one of most effective and economical routes, as it has the merits of one-step low-temperature synthesis, powder reactivity, and shape control. However, in the previous hydrothermal synthesis of nanosized CeO2, special organic reagents, surfactants, or templates were added.2b,4,5,6b,7c So the development of facile, economical, and effective methods for creating controllable architectures remains an important challenge. On the other hand, although a number of nanomorphologies have been obtained, the investigation of the relation between different morphologies is still crucial for realizing morphology-controlled synthesis. In the present work, we report a simple hydrothermal synthesis process of uniform CeO2 nano-octahedrons and nanorods using Ce(NO3)3 · 6H2O as cerium resource and Na3PO4 · 6H2O as mineralizer, into which no surfactant or template was introduced. Just by tuning the hydrothermal treatment time, the morphology evolution between the nano-octahedron and nanorod was achieved. Furthermore, the synthesizing mechanism and the morphological evolution of different shapes were investigated. Unlike traditional hydrothermal synthesis of CeO2 nanostructures using strong base as precipitant, such as NaOH, KOH, or other organic alkaline, Na3PO4 does not leave any impurity in the hydrothermal reaction system and makes the process very simple to obtain and separate * Corresponding author. E-mail:
[email protected]. Tel. 86-10-62334200. Fax: 86-10-62332525. † Department of Physical Chemistry, University of Science and Technology Beijing. ‡ Key State Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing
the octahedral and rodlike morphology. Therefore, the Na3PO4assisted hydrothermal method might offer an excellent approach to design other similar nanomaterials and have potential for industrial-scale application. CeO2 nano-octahedrons and nanorods were prepared by a simple hydrothermal process. Typically, 1 mmol cerium(III) nitrate hexahydrate and 0.01 mmol trisodium phosphate hexahydrate were dissolved in 40 mL distilled water. After being stirred at room temperature for 1 h, the mixed solution was transferred into a 50 mL Teflon-lined stainless autoclave and heated at 170 °C for 12-144 h under autogenous pressure and static conditions in an electric oven. Upon leaving the solution cool to room temperature, the precipitates were separated by centrifuging, washed with distilled water and ethanol three times in turn, and then dried at 60 °C for 1 day. The products were characterized by X-ray diffractometer with Cu KR radiation (XRD, M21XVHF22), high-resolution transmission electron microscopy, selected-area electron diffraction (HRTEM, SAED, JEM-2010) and field-emission scanning electron microscopy (FE-SEM, LEO1530). Uniform single-crystalline CeO2 nano-octahedrons and nanorods were successfully obtained at different stages of a facile Na3PO4assisted hydrothermal system at 170 °C. The detailed formation process of CeO2 nanostructures was studied by FE-SEM (Figure 1) and a clear time-dependent morphology evolution process from octahedron to rodlike shapes could be observed. Large-scale CeO2 contained octahedron-like nanoparticles were obtained after 12 h (Figure 1a). Prolonging the reaction time to 24 h, the onedimensional nanorods began to grow up out of the octahedral surfaces and became longer and longer, but in contrast, the octahedron particles gradually diminished (images b and c in Figure 1). The products obtained after 144 h almost exhibited 1D nanostructure, and few ceria nano-octahedrons could be observed (Figure 1d). The typical XRD patterns of the products synthesized at different reaction times are shown in the Supporting Information, Figure S1. All detectable peaks in these patterns can be readily indexed to a pure fluorite cubic phase (space group: Fm3m) of CeO2 with lattice constant of a )0.5411 nm (JCPDS 34-0394). To provide further insight into the different CeO2 nanostructures, we also performed TEM investigations. Figure 2 presents the HRTEM, FE-SEM micrographs and the SAED pattern of a typical individual ceria nano-octahedron. As shown in low-magnification TEM image (Figure 2a) and FE-SEM micrograph (Figure 2d), the as-obtained CeO2 nanoparticles hydrothermal synthesized at 170 °C for 12 h possess perfect octahedral morphology with sharp corners and well-defined edges. The octahedral edge lengths range is from 100 to 200 nm. The HRTEM image in Figure 2b taken
10.1021/cg800117v CCC: $40.75 2008 American Chemical Society Published on Web 03/25/2008
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Figure 1. FE-SEM images of the CeO2 hydrothermal treated at 170 °C for (a) 12, (b) 24, (c) 48, and (d) 144 h.
Figure 2. (a) TEM image, (b) HRTEM image, (c) SAED pattern, and (d) FE-SEM image of a typical indivadual octahedral CeO2 nanooctahedron.
from the upper part of the nano-octahedron (Figure 2a) shows well defined 2D lattice planes and demonstrates the good crystallinity of the ceria octahedral nanoparticles. The lattice planes with d-spacing of 0.322 and 0.324 nm both correspond to (111) planes, indicating that the octahedron enclosed by eight {111} planes which were proven to be with highest surface density of atoms and the lowest surface energy for face-centered cubic structured CeO2.8 The corresponding SAED pattern showed in Figure 2c can be assigned to face-centered cubic CeO2, indicating that the octahedron is a single crystal with its [110] orientation parallel to the electron beam. A typical low-magnification TEM of CeO2 nanorods synthesized by hydrothermal treatment at 170 °C for a long time of 144 h was shown in Figure 3a. It could be observed that a large quantity of ceria nanorods with uniform rodlike structures is about 20 nm in width and several hundreds of nanometers in length. Figure 3b shows the high-resolution TEM (HRTEM) images of an individual ceria nanorod end, indicating that the nanorods are structurally uniform and well-crystalline. The interplanar distances of the clearly resolved 2D lattice planes of the HRTEM image are about 0.271 and 0.274 nm, close to the {200} lattice spacing of the cubic phase. Using corresponding fast Fourier transform (FFT) taken from the
nanorod pattern (insert Figure 3b) reflected the HRTEM image is recorded with the electron beam parallel to the [001] axis and the growth direction of the nanorod is along [100]. The growth habits always directly determine the final crystal shape, which in turn is greatly influenced by its growth conditions. On the basis of the above time-dependent morphology evolution evidence, we could hypothesize that formation of multinanostructures and morphology evolution from nano-octahedron to nanorod canberationallyexpressedasanucleation-dissolution-recrystallization mechanism.9 The schematical mechanism for the CeO2 multinanostructures obtained during different hydrothermal stages is illustrated in Figure 4. At the very beginning, when the redox reaction was carried out in the hydrothermal system at 170 °C and Na3PO4 started to hydrate, the CeO2 nanocrystals with irregular shapes were formed in the solution through a homogeneous nucleation process. The asobtained CeO2 nanoparticles could then agglomerate and selfassemble to give the well-defined nano-octahedrons with eight {111} planes enclosed. With the increase in hydrothermal treatment time, the hydrothermal conditions changed into acidic (pH ∼4), which might make CeO2 nano-octahedron dissolve and recrystallize and influence its growth habits as well. It was found that there were many small protuberances on the surfaces of the octahedron (Figure 2d), which provided many high-energy sites for nanocrystal growth.10 So, the dissolved CeO2 in the solution might nucleate onto the active sites of the small protuberances, grow along 1D direction and recrystallize into CeO2 nanorods until the CeO2 octahedrons almost completely dissolved. The recrystallization of ceria nanostructure as nanorod might be due to system acidity and the inter-reaction between certain crystal planes and phosphate ions. Phosphate ions have been proved to have structural-direction effect in other nanostructure formation. 11 Further research is still ongoing. As mentioned above, it could be found the presence of Na3PO4 was the crucial factor that induced the formation of the ceria nanostructures. So, to determine the role of the Na3PO4 in the hydrothermal system, we carried out additional comparative experiments under similar hydrothermal conditions. When Na3PO4 was not introduced to the preparation system, no products were obtained, and when Na3PO4 was substituted by other salts with an equal molar ratio, such as NaCl, Na2CO3, and Na2SO4, almost no regular shape can be discerned. With Na3PO4 as the mineralizer, the pH value of the mixed solution was about 7 before hydrothermal treatment; when the process terminated, the final pH value was about 3-4. On the fact of no existence of impurity in the final reactant solution, the proposed mechanism of hydrothermal synthesis of CeO2 nanooctahedron and nanorod using Na3PO4 as mineralizer may be as follows: at an early stage in the hydrothermal conditions, the Na3PO4
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Figure 3. (a) TEM image of CeO2 nanorods and (b) HRTEM image of an individual CeO2 nanorod and its FFT pattern (inset).
Figure 4. Schematic illustration for the multinanostructures evolution of CeO2.
hydrolysis gives rise to OH- ions, and hydrated Ce3+ ions were then oxidized by the O2 existing in the aqueous solution and formed the complex with H2O molecules or OH- ions; by the end, H2O as a polar molecule tends to take protons away from the coordinated hydroxide and results in the acidity of the reactive solution.12 The reaction mechanism may be expressed
Na3PO4 + 2H2O T 3Na+ + 2OH- + H2PO4-
(1)
Ce3+ + OH- + H2O + O2 f Ce(H2O)x(OH-)y(4-x)+
(2)
H2O)x(OH-)y(4-x)+ + H2O f CeO2 · 2nH2O + H3O+
(3)
In addition, the concentration of phosphate ions was a key to the formation of the CeO2 nanostructures. If the concentration of phosphate ions was less than 1 × 10-4 M, only a large-scale mixed morphology of octahedron and rodlike structures could be obtained instead of uniform nano-octahedrons or nanorods. On the other hand, a relatively large phosphate ion concentration (above 1 × 10-3 M) could lead impurity (cerium phosphate) into the final product (see the Supporting Information, Figure S2). Besides the reaction time and concentration of phosphate ions, experiments to determine the parameters that are also important for the formation ceria nanostructures, such as temperature and pH value, were carried out. For example, the hydrothermal reaction could not take place below the temperature of 100 °C, the nanoparticles with irregular morphology dominated at 120 °C and 1D nanostructures were easily formed above 220 °C (see the Supporting Information, Figure S3). The hydrothermal synthesis at different pH values were also investigated, which was tuned by dilute HCl or NH3 · H2O solution. Under basic conditions (pH value above 10) only ceria nanocubes are obtained and the effect of Na3PO4 is not obvious. On the contrary, if the pH value is less than 2, all the CeO2 morphology displayed uniformed 1D nanorod
structures, which confirmed that the acidic condition was in favor of the formation of CeO2 nanorods (see the Supporting Information, Figure S4). Although the morphology evolution process from octahedron to rodlike structures has been deduced, the intrinsic cause is not completely clear. Further research on the morphology evolution mechanism and formation of other kinds of ceria structures is still in progress.
Conclusions. In summary, we have demonstrated a facile approach to the synthesis of single-crystalline CeO2 with octahedral -and/or -rod-like nanostructures only using simple starting materials in a hydrothermal system without any surfactant and template. Various experimental conditions, including time, temperature, additive, pH value and concentration for the growth of CeO2 nanocrystals were investigated. The morphological evolution between nano-octahedron and nanorod can be achieved by adjusting the hydrothermal treatment time. Na3PO4 as a mineralizer played an important role in the formation and morphological evolution of CeO2 nanostructure. Based on the evidence of electron microscopy images, the morphological evolution mechanism suggested that the shapes variation from nano-octahedron to nanorod might be a phosphate ion-related dissolution-recrystallization process. These results provide an economical route for synthesis of nanosized ceria and related materials. Moreover, the clarification of the morphology evolution mechanism will open a new strategy for nanostructure controlled synthesis. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (20731001, 50725415 and 20571009), Funds of Ministry of Education of China for PCSIRT, Beijing Nova Program (2005B20), and NCET. Supporting Information Available: XRD patterns and FE-SEM images of CeO2 nanostructures obtained under various experimental
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conditions (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. (7)
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