DOI: 10.1021/cg1008347
Characteristics of CeO2 Nanocubes and Related Polyhedra Prepared by Using a Liquid-Liquid Interface
2010, Vol. 10 4537–4541
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Feng Dang,*,† Kazumi Kato,† Hiroaki Imai,‡ Satoshi Wada,§ Hajime Haneda,# and Makoto Kuwabara †
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National Institute of Advanced Industrial Science and Technology (AIST), 2266-98 Anagahora, Shimoshidami, Moriyama-ku, Nagoya, 463-8560 Japan, ‡Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522 Japan, §Yamanashi University, 4-3-11, Takeda, Kofu, 400-8511 Japan, # National Institute for Materials and Science (NIMS), 1-1 Namiki, Tukuba, 305-0044 Japan, and Kyushu University, 6-1 Kasuga-kouen Kasuga, Fukuoka, 816-8580 Japan Received June 22, 2010; Revised Manuscript Received August 17, 2010
ABSTRACT: Monodispersed CeO2 nanocubes were prepared by using a liquid-liquid interface. The morphology of CeO2 nanocrystals from the aqueous phase changed from truncated octahedron to nanocube in the toluene phase at the watertoluene interface. The CeO2 nanocubes oriented to form large cubic particles at a low molar ratio of oleic acid (OLA)/Ce. The red-shift of the absorption edge and the high concentration of Ce3þ in the CeO2 structure was identified for the CeO2 nanocrystals. The high concentration of Ce3þ in the CeO2 structure was identified as one of the main reasons for the red-shift of the absorption edge of CeO2 nanocrystals.
Introduction Ceria (cerium oxide, CeO2) has a cubic crystal structure and is attracting a great deal of attention because of its applications in conversion catalysts, three-way catalysts, fuel cells, solar cells, and metal-oxide semiconductor devices.1-4 Synthesis of ceria materials with large specific surface areas and high oxygen storage capacities has been the focus of much research. Recently, nanostructured ceria particles such as nanowires and mesocrystals have been widely studied. It is well understood that the performance of nanoparticles strongly depends on the morphology and size distribution.5 It is desirable to obtain nanoparticles with designed morphology and size distribution. Experimental and theoretical studies have indicated the different reactivities of the crystal face of ceria. The (100) face is more reactive for the catalytic properties than (111) and (110) faces because of its high surface energy. Yan et al.6 observed that the ceria nanocubes exposed by the (100) face obtained the highest oxygen-storage capacity. On the other hand, the (111) face of ceria is stable in the growth of the crystal structure. It is difficult to control the morphology and monodispersibility of ceria nanoparticles under formal reaction conditions. Adschiri et al.7 and Gao et al.8 investigated the preparation of monodispersed CeO2 nanocubes bounded by the (100) face by using an organic-inorganic two liquid phase process. In this work, we prepared and characterized monodispersed shape-controlled CeO2 nanocrystals by using a liquid-liquid interface. Experimental Section In a typical synthesis process, 15 mL of 16.7 mmol/L cerium(III) nitrate aqueous solution was added into a 50 mL autoclave, and then a 15 mL mixed solution of toluene, oleic acid (OLA, 0.6 mL, OLA/Ce 8:1), and tert-butylamine (0.15 mL) was added to the autoclave in open air without stirring. The sealed autoclave was heated at 180 °C *To whom correspondence should be addressed. E-mail:
[email protected]. r 2010 American Chemical Society
for 36 h, and then cooled to room temperature. The upper organic crude solution was centrifugated to separate the CeO2 nanocrystals. The separated CeO2 nanocrystals were redispersed into 3 mL of toluene. The molar ratio of OLA/Ce in the synthesis was controlled at 4:1, 8:1, and 10:1. The reaction time ranged from 24 h to 72 h. Ce3þ was oxidized by the dissolved oxygen in aqueous solution. High resolution transmission electron microscopy (HRTEM) and transmission electron microscopy (TEM) images were used to observe the shape-controlled CeO2 nanocrystals. The CeO2 nanocrystals were characterized by Fourier transform IR (FTIR), ultraviolet-visible (UV-vis), and photoemission (XPS) spectra.
Results and Discussion Figure 1 shows the TEM images of the CeO2 nanocrystals synthesized with a molar ratio of OLA/Ce = 8:1 at 180 °C for a reaction time of 24-72 h. Polyhedral particles about 5 nm were obtained for the nanocrystals prepared for 24 h. Besides the polyhedral particles, nanocubes initiated to form after 36 h as shown in Figure 1B. When the reaction time was increased to 48 h (Figure 1C), the main products were nanocubes and a few polyhedral particles were observed. The (111) ring of CeO2 was not identified by the electron diffraction (ED) pattern of Figure 1C, which indicated that the main products after 48 h were nanocubes. It is worth noting that the CeO2 nanocrystals can be well dispersed in toluene and easily aggregate into an ordered 2D self-assembly structure as shown in Figure 1A,C. The self-assembly structure was hexagonal packing. The dispersion of CeO2 nanocrystals was transparent and showed a yellow color as shown in Figure 1E. The size distribution of the CeO2 nanocrystals synthesized for 48 h obtained through counting more than 400 particles from TEM images is shown in Figure 1E. The particle size of the nanocrystals ranged from 4.4 to 6.4 nm, and it showed a narrow size distribution. It can be seen that the particle size did not increase through the reaction time up to 48 h and CeO2 nanocubes were developed through the morphology change of the polyhedral particles. Besides the nanocubes, irregular particles ranging from 10 to 30 nm were Published on Web 09/02/2010
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Figure 1. TEM images of the CeO2 nanocrystals prepared with a molar ratio of OLA/Ce = 8:1 at 180 °C for 24 h (A), 36 h (B), 48 h (C), and 72 h (D); and the direct image of the dispersion and size distribution of CeO2 nanocrystals prepared with a molar ratio of OLA/Ce = 8:1 at 180 °C for 48 h (E).
observed for the preparation with a reaction time of 72 h. Figure 1F shows the XRD pattern of the CeO2 nanocrystals deposited on silicon substrate synthesized for 24 h. The diffraction peaks of CeO2 were identified and the high intensity of (200) and (400) indicated the selective growth of the (100) face. Figure 2 shows the HRTEM images of the polyhedral particles and nanocubes prepared with a molar ratio of OLA/Ce = 8:1 at 180 °C for 24, 36, and 48 h. For the polyhedral particle, a hexagon was observed from the HRTEM image and the FFT of the HRTEM image showed one set of (200) planes and two sets of (111) planes. The morphology of the polyhedral particle could be imaged as the (200) face truncated octahedron as schematically represented in Figure 2A observed from the [100] direction. The HRTEM images of the nanocubes prepared for 36 h and 48 h showed a similar truncated cubic morphology and good crystallinity. Four (200) and (220) planes as well as two sets of (200) and (220) planes were identified from FFT. It indicated the (111) face truncated nanocube as schematically represented in Figure 2B,C. The morphology of nanocubes did not change through the reaction time. Figure 3 shows the TEM images of the nanocrystals prepared with concentrations of oleic acid (OLA/Ce = 4:1 and 10:1) at 180 °C for 36 h. Nanocrystals about 5 nm and large nanocubes over 15 nm were observed for the preparation at OLA/Ce = 4:1. It was identified that the large nanocubes were the aggregates of the small CeO2 nanocrystals as shown in Figure 4A. The attachment of the nanocrystals was also observed from the HRTEM image. As shown in Figure 4B, the several nanocrystals initiated aggregation through the attachment of the identical (200) faces during TEM observation. For the nanocrystals prepared at OLA/Ce = 10:1 as shown in Figure 3, the cubic nanocrystals about 5 nm were observed which were similar to those prepared at
Figure 2. HRTEM images of the CeO2 nanocrystals prepared with a molar ratio of OLA/Ce = 8:1 at 180 °C for 24 h (A), 36 h (B), and 48 h (C).
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OLA/Ce = 8:1. The particle size did not increase with the increase of the concentration of oleic acid. CeO2 nanocrystals were synthesized by using a liquidliquid interface. Figure 5 schematically illustrates the formation mechanism of CeO2 nanocrystals at the water-toluene interface. The pH value in aqueous solution was enhanced through the hydrolysis of tert-butylamine at the interface of water-toluene at high temperature for the nucleation of CeO2. The nuclei of CeO2 coated by OLA near the watertoluene interface are drawn into a bulk of the toluene solution and change into nanocubes. CeO2 has a face-centered cubic crystal structure. The morphology of the CeO2 nanocrystals is mainly decided by the growth ratio between [100] and [111] directions.5 The order of surface energy of CeO2 is γ(100) > γ(110) > γ(111). In the aqueous solution, the growth of the [100] direction of CeO2 is dominated because the (111) face has a lower surface energy. Therefore, octahedron or truncated octahedron will be obtained. Comparatively, in the toluene solution, the growth of the [111] direction of CeO2 is proceeded because the OLA prefers to coat the (100) face with
high surface energy and limits the growth of the [100] direction of CeO2 and then nanocubes will be obtained. In the observation of the nanocrystals synthesized at different aging times, the morphology change from truncated octahedron to nanocube was clearly observed. The nanocubes were obtained for an aging time from 36 to 48 h. It can be concluded that the relatively high concentration of OLA was better for the preparation of monodispersed nanocubes (OLA/Ce = 8:1, 10:1) and abnormal large nanocubes could be obtained at a low concentration of oleic acid (OLA/Ce = 4:1). To identify the interaction between the CeO2 nanocrystals and the organic ligand molecules, FTIR spectrum of the assynthesized CeO2 nanocrystals prepared with a molar ratio of OLA/Ce = 8:1 at 180 °C for 36 h was measured. As shown in Figure 6, the doublet at 2850 and 2920 cm-1 indicated the C-H stretching mode of methyl and methylene groups. The bands at 1433 and 1515 cm-1 represented the stretching frequency of the carboxylate group, which indicated that the carboxylate group in OLA chemically bonded with the Ce ions on the surface of CeO2 nanocrystals.6,9-11 No peaks corresponding to tert-butylamine were identified. It could be identified that the CeO2 nanocrystals were modified by OLA. Figure 7 shows the UV-vis spectra and the corresponding calculated bandgap of the OLA modified CeO2 nanocrystals. It can be seen that the bandgap of the CeO2 nanocrystals prepared with a molar ratio of OLA/Ce = 8:1 increased through the reaction time up to 48 h, and decreased after 72 h. For the CeO2 nanocrystals prepared at different concentrations of OLA, the bandgap decreased with the increase of the concentration of oleic acid. The bandgap of the bulk CeO2 is 3.15 eV. Although the nanocrystals were prepared at OLA/Ce = 4:1, the CeO2 nanocrystals had lower bandgaps
Figure 3. TEM images of the CeO2 nanocrystals prepared with a molar ratio of OLA/Ce = 4:1 and 10:1 at 180 °C.
Figure 5. Schematic illustration of the growth mechanism for ceria nanocrystals with different morphologies.
Figure 4. HRTEM images of the large CeO2 nanocubes (A) and the connection of CeO2 nanocrystals prepared with a molar ratio of OLA/Ce = 4:1 at 180 °C for 36 h (B).
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than that of the bulk CeO2 and showed a red-shift. The bandgap of the nanocrystals is generally influenced by the particle size and the defect in the crystal structure. Since CeO2 is a direct bandgap semiconductor, the decrease of the particle size will cause a blue shift of the absorption edge.12 Zhang et al.13 has studied the relation of the bandgap and particle size of the CeO2 nanoparticles, in which the bandgap of nanoparticles ranging from 4 to 7 nm was about 3.36 eV. This corresponded with our result of the CeO2 nanocrystals prepared at OLA/Ce = 4:1, in which the bandgap was 3.35 eV. On the other hand, the red-shift of the absorption edge of the CeO2 nanocrystals prepared at high concentration of OLA was observed in this work. Sun et al.14 have observed the red shift of the CeO2 nanoparticles. They attributed the red shift of CeO2 nanoparticles to the electron-phonon coupling. In theory, the electron phonon coupling coefficient increases with the decrease of the particle size of semiconductors.15 In certain systems, electron-phonon coupling can overcome the spatial confinement to determine the energy of excitons and determine the effective mass of carriers scattering by lattice. CeO2 is a strong electron-phonon coupling system. The existence of a large number of defects in the CeO2 nanocrystals causes electron-phonon relaxation and leads to a red-shift of the emission band. The existence of Ce3þ causes oxygen vacancy in the CeO2 structure and the concentration of
Figure 6. FTIR spectrum of the oleic acid stabilized CeO2 nanocrystals prepared with a molar ratio of OLA/Ce = 8:1 at 180 °C for 36 h.
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Ce3þ in CeO2 structure can estimate the defects in the CeO2 structure. In this work, the concentration of Ce3þ in CeO2 nanocrystals was estimated by XPS analysis. The XPS spectra of CeO2 nanocrystals prepared at different times and ratios of OLA/Ce are shown in Figure 8. Eight peaks were identified in the XPS spectra. The peaks at 882.9, 885.8, 889.2, and 898.8 eV corresponded to the V, V0 , V00 , V000 components of 3d5/2, respectively. The peaks at 901.5, 904.5, 908.2, and 916.7 eV
Figure 8. XPS spectra of the oleic acid stabilized CeO2 nanocrystals prepared for different reaction times at OLA/Ce = 8:1 and at different concentrations of oleic acid at 180 °C for 36 h.
Figure 7. UV-vis spectra and corresponding bandgaps of the oleic acid stabilized CeO2 nanocrystals prepared for different reaction times with a molar ratio of OLA/Ce = 8:1 and at different concentrations of oleic acid (C, D) at 180 °C.
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Figure 9. Concentrations of Ce4þ in the CeO2 nanocrystals calculated from XPS results.
corresponded to the U, U0 , U00 , U000 components of 3d3/2, respectively.16,17 The main peaks of V000 and U000 represented the 3d104f 0 initial electronic state corresponding to the Ce4þ ion, and the peak of V0 represented the 3d104f1 initial electronic state corresponding to the Ce3þ ion. The relative concentration of Ce4þ in CeO2 can be estimated from the area of U000 peak in the total Ce 3d region, assuming no preferential enrichment of Ce4þ over Ce3þ or vice versa. The results are shown in Figure 9. The area of U000 peak in the total Ce 3d region is 12.4% for 100% concentration of Ce4þ.18,19 The concentrations of Ce3þ in CeO2 nanocrystals increased with the increase of the ratio of OLA/ Ce and the CeO2 nanocrystals prepared at OLA/Ce = 4:1 for 36 h obtained the lowest concentration of Ce3þ in the crystal structure which was 14.2%. The CeO2 nanocrystals prepared at OLA/Ce = 8:1 for 48 h has a higher concentration of Ce4þ (83.1%) than others prepared for different times at OLA/Ce = 8:1. It can be seen that the concentration of Ce3þ corresponded with the results of the UV analysis, where CeO2 nanocrystals showed a red-shift of absorption edge obtained at a high concentration of Ce3þ. Conclusions In this work, monodispersed CeO2 nanocubes were prepared by using a water-toluene interface. The CeO2 nuclei generated in aqueous solution transferred into the toluene solution by the modifying oleic acid at the interface of watertoluene and evolved into nanocubes. The high concentration of oleic acid and the reaction time ranging from 36 to 48 h were
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better for the formation of monodispersed CeO2 nanocubes. The oleic acid modified CeO2 nanocrystals showed a red-shift of the absorption edge. The high concentration of Ce3þ in the CeO2 structure was identified for the CeO2 nanocrystals and was thought to be one of the reasons for the red-shift of the absorption edge. Considering the monodispersibility, morphology, red-shift of the absorption edge, and high concentration of Ce3þ, excellent performance can be expected for the as-synthesized CeO2 nanocrystals in the applications of catalysts, solar cells, semiconductor devices, and so on. Acknowledgment. This work was supported by the Collaborative Research Consortium of Nanocrystal Ceramics.
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