ARTICLE pubs.acs.org/crystal
Tuning Growth Modes of Ceria-Based Nanocubes by a Hydrothermal Method Takaaki Taniguchi,*,† Ken-ichi Katsumata,‡ Shingo Omata,‡ Kiyoshi Okada,‡ and Nobuhiro Matsushita‡ † ‡
Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Kumamoto, 860-8555, Japan Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
bS Supporting Information ABSTRACT: We control two growth modes, Ostwald ripening (OR) and oriented attachment (OA), to yield ceria nanocubes by a simple hydrothermal method, using an oleate modified precursor. Using a Ce(III) salt as an initial material, the OA mode is highly activated to form ceria nanocubes with well-developed (002) facets (ca. 8 nm), while OA growth is not allowed so that smaller nanocubes (ca. 5 nm) were formed if Ce(IV) salt is used for the synthesis. A combination analysis by X-ray photoelectron spectroscopy and Fourier transform IR indicates that the surface of ceria nanocubes synthesized with Ce(III) precursor have surface valence (Ce3+) sites coordinating with oleate through bidentate bonding. We propose that the difference in Ce3+ from the grain interior (Ce4+) provides the high surface energy needed to activate the OA growth mode. The luminescence spectra of ceria nanocubes grown by OA rather resemble those of a microcrystalline sample, suggesting the presence of a nonstoichiometric ceria phase, such as Ce2O3, at the domain boundary in an identical nanocube, while OR growth favors the production of point defects randomly distributed in the fluorite lattice. The OA mode was still sufficiently active to form well-defined nanocubes (ca. 8 nm) when Zr4+ was doped to 20 mol % in ceria, while increasing the Zr4+ content to 50 mol % drastically suppressed both OA and OR to form tiny spherical nanoparticles (ca. 2.0 nm).
’ INTRODUCTION Ceria (CeO2)-based oxides are important catalytic materials. They show redox properties owing to the reversible oxidation/ reduction of Ce4+ T Ce3+, which offers properties suitable for an efficient three-way catalyst (TWC) to treat exhaust gas from automobiles1 and a water-gas-shift (WGS) catalyst to produce H2 fuel.2 Moreover, the superior catalytic property of nanosized ceria serves as nanomedicine, with potent free-radical scavengers with neuroprotective3 and radioprotective properties.4 Because the chemical reaction kinetics strongly depend on surface/interface structures, the shape control of catalytic nanocrystals is important in producing high reactivity. Solution-based synthesis is a powerful approach for the preparation of nanostructures with controlled dimensions and sizes. In particular, aqueous solution methods have yielded a variety of ceria nanostructures involving nanocubes,5 10 nanorods,5,6,11,12 nanotubes,13 15 and nanocages16 at low temperatures, inexpensively, and in a less toxic manner. Among such unique nanostructures, nanocube enclosed {200} facets are an attractive architecture for driving catalytic reactions owing to the large surface area and high energy of the facets.5,6 A carboxyl acid-assisted hydrothermal method is effective in synthesizing ceria nanocubes.7 9 The carboxyl group selectively adsorbs on {200} facets to reduce the growth rate of the crystals in the Æ001æ direction and facilitates growth in the Æ111æ direction, which results in the formation of nanocubes with exposed {200} facets.7 In those studies, the ceria nanocubes are r 2011 American Chemical Society
rather small, about 4 5 nm, as a result of a strong organic binding on the surfaces that suppress the successive crystal growth by Ostwald ripening (OR), where large particles grow at the expense of smaller ones. Yang et al. recently reported the synthesis of larger ceria nanocubes (8 20 nm) by a hydrothermal method using a Ce(NO3)3 aqueous solution, toluene, oleic acid, and tertbutylamine.10 They explained that large nanocubes were grown by an oriented attachment (OA) mechanism—the attachment between two or more nanocrystals, followed by sharing a common crystallographic orientation and uniting at the planar interfaces.17 This novel growth process for ceria nanocubes is very interesting; the change in growth mode produces diverse defect states or concentrations either at the surface or in the grain, which can greatly impact the redox properties of ceria. However, the mechanisms inducing the OA of ceria nanocubes as well as the growth mode dependent defect states remains unclear. Moreover, their method provided ceria nanocubes with bimodal distribution, where small (4 nm) and large nanocubes (8 nm) coexist, which indicates that the OA and OR growth modes were uncontrollable.10 Since the discovery of the OA growth mode in hydrothermal aging of titania nanocrystals by Penn and Banfield,17 such a Received: November 29, 2010 Revised: July 19, 2011 Published: July 28, 2011 3754
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nonclassical process is frequently employed to describe the growth of one-dimensional nanoarchitectures such as nanorods and nanowires.18 20 In those cases, the dipole dipole interaction between the surfaces of two nanocrystals could be the driving force that induces one-dimensional OA yielding onedimensional nanostructures;18 there are fewer reports on the synthesis of nanocubes grown by a three-dimensional OA process,21,22 and the corresponding formation mechanism is less understood. Therefore, a detailed study on the formation mechanism of ceria nanocube could provide new insights into the OA growth of nanomaterials and the related chemical and physical properties. In this paper, we report a simple hydrothermal method to finely tune the OA and OR growth modes of ceria nanocubes. A set of synthetic experiments demonstrated that the surface valence of Ce cations plays a critical role in facilitating or suppressing the OA. We investigated the surface and optical properties to understand the defect structure relating the growth modes. Finally, the compositional contributions of a CeO2 ZrO2 binary system on the OA growth were investigated. On the basis of the available data, we suggest the idea of “surface valenceinduced oriented attachment”, to describe the OA process for ceria nanocubes, which can be applied to other oxide nanoparticle systems containing polyvalent metal ions.
’ EXPERIMENTAL SECTION 3.5 mmol of (NH4)2Ce(NO3)6, Ce(NO3)3 (Wako, 99.5%), or their mixture was dissolved in distilled water (15 mL) at room temperature. Then, 3.5 mol of C17H33COONa (Wako, analytical grade) dissolved in distilled water (15 mL) was added to the aqueous solution containing Ce ions at room temperature under vigorous stirring. Note that Ce(IV) and Ce(III) salt solution remained yellow and colorless before sodium oleate was added, indicating direct formation of a Ce(IV)Ce(III)-oleate complex. Then, 5 mL of an ammonia 25 wt % aqueous solution (Wako, analytical grade) was added to the solution mixture. The reaction mixture was placed in a polytetrafluoroethylene (PTFE) vessel (inner volume of 40 cm3). This vessel was sealed and placed inside a stainless steel autoclave, which was kept at 150 or 200 C for 1 30 h under autogenous pressure. Note that using Ce4+ for the initial material yielded powdery products with a dark brown color in the vessel, while starting from Ce3+ resulted in the formation of dark purple aggregate at the bottom. The products were readily dispersed in nonpolar solvents, such as benzene and cyclohexane, though a tiny amount of the precipitate remained. These precipitates were removed by centrifugation at 1000 rpm for 5 min. For the synthesis of Zr4+ doped ceria nanocrystals, mixed precursors of (NH4)2Ce(NO3)6, Ce(NO3)6, and ZrOCl2 (total 3.5 mmol) were used. For this synthesis, the hydrothermal temperature was 200 C for 30 h. The other experimental procedures remained the same. The products were characterized by powder X-ray diffraction (XRD) using a MAC Science MX 3VA diffractometer with Cu KR radiation (λ) 1.54056 Å, operating at 40 mA and 40 kV. Transmission electron microscopy (TEM) was performed using a Hitachi HF-2000 microscope operating at 200 kV. For TEM observation, one drop of the sample, dispersed in cyclohexane, was deposited on a holey carbon grid. XPS was recorded using a JEOL JPS-9010 with non-monochromatic Mg KR (hυ = 1253.6 eV). The room temperature infrared (IR) Fourier-transform spectra were recorded on a Jeol JIR-7000 spectrometer. The products (20 mg) were thoroughly ground with (400 mg) potassium bromide powder (KBr for IR, Wako) and subjected to IR analysis. Photoluminescence (PL) spectra of colloidal solutions dispersed in cyclohexane were measured using a Perkin-Elmer LS 55
Figure 1. Low-magnification TEM images of ceria nanocrystals synthesized by the hydrothermal method at 200 C for 30 h with (a) Ce(IV) and (b) Ce(III) precursors; (c, d) HRTEM images of ceria nanocubes synthesized by the hydrothermal method at 200 C for 30 h with Ce(III) precursor. spectrofluorometer with a Xe lamp. The excitation wavelength was 270 nm. For comparison, a PL spectrum of microcrystalline ceria powders received from Anan Kasei Co. Ltd. was measured.
’ RESULTS AND DISCUSSION In the previous study, we reported the synthesis of ceria and doped ceria nanoparticles by the hydrothermal (HT) method using oleate-modified precursors.7,23 This method yielded nanocrystals using cerium salts, ammonium hydroxide, and sodium oleate as initial materials, which play roles as the cerium source, mineralization agent, and surface stabilizer, respectively. 3755
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Figure 2. (a) Low-magnification TEM image of ceria nanocrystals synthesized by the hydrothermal method at 200 C for 30 h with a mixed Ce(IV) and Ce(III) precursor (molar ratio of Ce4+/Ce3+ = 1). (b, c) Low and high magnification TEM images of ceria nanocrystals synthesized by the hydrothermal method at 200 C for 6 h with a mixed Ce(III) and Ce(IV) precursor (molar ratio of Ce4+/Ce3+ = 1). (d) Lowmagnification TEM image of ceria nanocrystals synthesized by the hydrothermal method at 200 C for 30 h with a mixed Ce(IV) and Ce(III) precursor (molar ratio of Ce4+/Ce3+ = 3).
No additional organic solvents that complicate the reaction mechanism were used. Therefore, this simple method is suitable for understanding the OR and OA mechanisms of ceria nanocubes. In the present study, we studied the effects of HT temperature, duration, valence of the starting cerium salt (Ce(IV) and Ce(III)), and doping of Zr4+ on the crystal growth.
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We previously reported 5 nm ceria nanocubes enclosed with six {200} facets by HT treatment at 200 C for 6 h using an oleate-modified Ce(IV) precursor.7 In this case, ceria nanocubes were grown by the OR process. First, we extended the HT duration to 30 h to confirm if 5 nm nanocubes are stable products at the HT temperature. Figure 1a shows TEM images of the ceria nanoparticles synthesized hydrothermally at 200 C for 30 h using the Ce(IV) precursor. As seen, the products are highly dispersed without any tendency to agglomerate. The average size is 5 nm and they are mostly cubic or spherical. The XRD (Figure S1, Supporting Information) shows that the grain size was calculated to be 5 nm from the line broadening of a (200) reflection. These features are very similar to the products synthesized by the HT treatment for 6 h, demonstrating that the typical OR growth, where small nanocrystals are dissolved/ reprecipitated to grow the larger ones, almost finished within 6 h of the HT treatment. Thus, it can be concluded that starting from Ce(IV) only involves OR to form ceria nanocubes but does not favor OA. Second, we performed synthesis using the Ce(III) precursor. Figure 1b shows the TEM image of the ceria nanocubes synthesized by HT treatment at 200 C for 30 h using a Ce(NO3)3 precursor. The synthetic condition produced nanocubes with an average size of 8 nm having a narrow size distribution. The nanocube edges were highly developed, indicating that surface diffusion actively occurred on an identical nanocrystal to minimize the surface energy. The high-resolution transmission electron microscopy (HRTEM) image of 9 nm ceria nanocubes (Figure 1c) demonstrates that the crystallite is oriented along the [001] zone axis, exposing lattice fringes corresponding to the {200} facets. The surfaces were not atomically flat, and the steps can possibly provide active sites for catalytic reactions. The XRD pattern (Figure S1, Supporting Information) shows that the products contain no byproducts, such as cerium hydroxides. The reflection peaks appear sharper than those from the sample synthesized with Ce(IV) precursor, presenting a larger average crystallite size; the grain size was calculated to be 8 nm, which is quite close to the average size observed by TEM, ensuring that each nanocube is a single crystal. In addition, products likely formed by the fusion of two nanocube units were also observed (Figure 1d), indicating that OA was involved in the growth mechanism. The above results demonstrate that the valence of the Ce ion in the precursor had a large influence on the crystal growth mode of ceria nanocubes. To further understand the effects of the initial valence of Ce cation, the synthesis was performed using Ce3+/ Ce4+ mixed precursors. Figure 2a shows a low-magnification TEM image of ceria nanoparticles synthesized by HT treatment at 200 C for 30 h with an initial Ce3+/Ce4+ molar ratio of unity. The image shows that the products involve large nanocubes similar to those synthesized with Ce3+ precursor; however, smaller ones were formed as well, which are very similar to those obtained by the Yang’s method. Yang suggested that the smaller and larger nanocubes are predominantly grown by the OR and OA modes, respectively.10 Figure 2b shows low-magnification TEM images of products synthesized with HT treatment at 200 C for 6 h with an atomic Ce4+/Ce3+ ratio of unity in an initial material. The shorter HT duration yielded nanocrystals likely consisting of nanocube units. The HRTEM image (Figure 2c) clearly shows that the OA mode is responsible for the formation of such nanostructures. For example, a {200} surface of a truncated cubic nanocrystal (B) 3756
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Figure 3. (A) Ce 3d XP spectra and (B) IR spectra of ceria nanocrystals synthesized by hydrothermal method at 200 C for 30 h with (a) Ce(III) and (b) Ce(IV) precursors. Solid line and dot-line shown in the XP spectra are from Ce3+ and Ce4+, respectively.
attaches rather perfectly to the {200} surface of Z-like-shaped nanocrystals A to form an identical crystal. Furthermore, other nanocubes (C, D, and E) attached to the nanocrystals are formed by the crystal fusion of nanocrystals (A and B); however, the surfaces of the nanocrystals are not atomically flat and form nanospaces and dislocations at the interfaces due to the imperfect OA.24 In addition, relatively small spherical particles (G and H) are attached to the nanocubes F and E, respectively. The spherical particles are free to rotate into an orientation to achieve structural accord with the {200} nanocube’s surface. This is directly related to the reduction of surface energy for minimizing the area of high energy faces (the grain-rotation induced grain coalescence mechanism).25 27 The spherical grains would grow to be nanocubes by OR because of the preferential adsorption of oleate on the {200} surface. In this way, the multistep OA growth between multilevel nanoparticles finally leads to a large nanocube to minimize the surface energy. We note that small nanoparticles grown by the OR mode with Ce3+/Ce4+ mixed precursors (Figure 2a,b) were smaller than ones grown by the OR mode with a single Ce4+ precursor (Figure 1a). This can be interpreted that the Ce3+/Ce4+ solutes formed in OR process is strongly favored to diffuse onto nanocubes grown by the OA mode.
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Synthesis performed starting from a Ce4+-rich condition (an atomic ratio of Ce4+/Ce3+ = 75:25) obtained nanocubes with ca. 5 nm size and a very narrow size distribution (Figure 2d), indicating that their growth excluded OA. This suggests that there is a threshold Ce3+ concentration that triggers or inhibits the OA growth. When performing the synthesis with a Ce4+/Ce3+ ratio of unity, mostly, the nucleus might contain a Ce3+ concentration greater or smaller than the threshold, which activates or suppresses OA, yielding the observed bimodal distribution (Figure 2a). This can explain why Yang’s method always produced products with a bimodal size distribution.10 As they described it, the reaction to nucleate and grow ceria nanocrystals involved simultaneous oxidization and hydrolysis of Ce (III)-oleate complex. In that case, nuclei with significant Ce4+ concentrations at the surface should be formed, partially to deactivate the OA mode. We conclude that the high-yield synthesis of ceria nanocube via the OA mode in the present study is based on the direct precipitation of Ce(III)-oleate complex from adding base. In this case, most nuclei could have a sufficient Ce3+ concentration for subsequent OA growth under HT conditions. XPS and FTIR were used to determine the surface state of the nanocrystals. Figure 3A shows the Ce (3d) XP spectra for the products synthesized by the HT method at 200 C for 30 h with Ce(III) and Ce(IV) precursors. The spectra were quite different, indicating that the surface Ce valence strongly depends on the synthesis condition. According to the literature,28 the peaks at 884 and 903 eV are mainly attributed to Ce3+, whereas peaks at 882, 898, 900, and 916 eV are characteristic peaks of Ce4+. It is evident that the XP peaks from Ce3+ were dominant for the products synthesized with Ce3+ precursors. On the basis of the fact that the products contain no Ce(III)-based impurities, the XPS data means that most surface Ce ions on ceria nanocubes were in a trivalent state when the Ce(III) precursor was used as an initial material, while nanocubes prepared with the Ce(IV) precursor exhibit a Ce(IV) dominant state on their surfaces. FTIR was used to investigate the bonding state of surface cerium sites and oleate ligands. Figure 3B shows the characteristic IR bands from oleate spices in the ceria nanocubes synthesized with Ce(III) and Ce(IV) precursors. A series of sharp bands in the region from 2700 3100 cm 1 are attributable to the symmetric and asymmetric stretching of CH2 groups and the terminal CH3 group.29 We show the asymmetrical vibrations in the range 1650 1510 cm 1 and the symmetrical vibrations in the range 1400 1280 cm 1.29,30 The FTIR spectra of both samples contain several strong bands in the COO region— 1705, 1653, 1544, and 1456 cm 1. The band at 1705 cm 1 can be assigned to either the carboxylate groups or the free oleic acid. Weak intensities of the band for both samples indicate that these spices were nondominant, and carboxylate groups should mainly coordinate with surface sites. The position and separation of COO bands in the 1300 1700 cm 1 region can be used to deduce the carboxylate coordination mode, as follows: For Δ > 200 cm 1, an unidentate ligand is expected, while for Δ < 110 cm 1, it is a bidentate ligand. For a bridging ligand, Δ is between 140 and 200 cm 1.29,30 For our samples, the difference between the bands at 1544 and 1456 cm 1 is 88 cm 1, revealing bidentate coordination, while the peak at 1653 cm 1 indicates the presence of bridging coordination (Δ is 197 cm 1). Note that the peak intensities of the bands at 1544 and 1653 cm 1 are 3757
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Figure 4. PL spectra of colloidal solution of ceria nanocrystals synthesized by the hydrothermal method at 200 C for 30 h with (a) Ce(III) and (b) Ce(IV) precursors. Note that the asterisk shown in the PL spectra of microcrystalline powder is the second harmonic band of the excitation light (270 nm).
compatible in samples synthesized with the Ce(IV) precursor, while the band at 1653 cm 1 is much weaker than the band at 1544 cm 1 for the sample synthesized with the Ce(III) precursor. This indicates that Ce3+ surface sites strongly favor the formation of bidentate ligands, while Ce4+ sites coordinates with oleate ligand through bridging coordination in addition to the bidentate one, possibly owing to the much smaller ionic radius. Combination analysis by XPS and FTIR revealed that the surface of ceria nanocubes synthesized with Ce(III) precursor have a quite simple surface structure, where Ce3+ sites coordinate with oleate ligands through bidentate bonding. Such surface structure should play a role in preventing oxidization of surface Ce3+ cations and allow the OA process. In principle, the OA mechanism can be divided into (a) collisions and (b) coalescence.31 In the present case, collision were highly involved, regardless of the valence of the cerium ion used in starting materials, as nanoparticles coated oleate ligands commonly tend to agglomerate during HT aging in an aqueous solution because of the reduced surface charge by oleate ligands. Thus, the coalescence is a rate limited reaction. Our experimental data demonstrated that the Ce3+-rich surface facilitates coalescence, while the Ce4+-rich surface does not allow the process under the given HT conditions. The chemical potential for accelerating the OA process was possibly produced by the high energy state of the Ce3+-rich ceria surface. As a result, the OA process was greatly facilitated to reduce the overall energy by removing the surface energy. In fact, studies on the OA process in an aqueous growth solution always have reported when the ceria nucleus was formed with Ce(III) salts as precursors,11,12,32 34 while there is no report to date on the OA growth of ceria nanocrystals using a Ce(IV) precursor in an aqueous solution process, indicating that those nanocrystals have stabilized and thus provides an inactive surface for the OA process. It is also notable that the binding of an oleate ligand on Ce3+ surface sites with bidentate coordination was sufficiently weak such that the ligand could be removed in the OA process. In addition, oleate ligands remaining on the surface suppressed oxidization of surface Ce3+ cations and guided the development of {200} facets during HT aging at 200 C. It is suggested that the dipole dipole interaction between the surfaces of two nanocrystals can be the driving force for inducing
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OA or that preferable/unpreferable adsorption of surfactants on specific faces plays a vital role in inducing OA. We propose that the diverse surface valences from the grain interior drastically alter the surface energy to drive OA growth. This new concept is not limited to describing the growth of ceria nanocrystals but is applicable to understanding and controlling the OA process for a wide range of nanocrystals systems with multivalent metal ions. Figure 4 shows the photoluminescence (PL) spectra of ceria nanocubes synthesized with Ce3+ and Ce4+ precursors, along with microceria crystals as a reference. Three PL bands (ca. 400, 465, and 510 nm) were observed in common. As the microcrystalline sample showed the three bands, the PL bands were not correlated with the surface state produced by oleate adsorption on the nanocrystals but are closely related to the defect nature of the fluorite structure. The nanocubes synthesized with the Ce(III) precursor show a dominant band at 400 nm, while the nanocube synthesized with the Ce(IV) precursor shows visible bands more strongly. According to the literature,35 the origin of the ultraviolet band and visible bands is different. The visible bands can be responsible for defects, including oxygen vacancies, in a crystal with electronic energy levels below the 4f band, where these defects can act as radiative recombination centers. On the other hand, the emission at 400 nm can be owing to the presence of a Ce3+-containing phase, such as Ce2O3 and Ce6O11. These intermediate phases are generally formed at boundaries during high temperature sintering, so the microceria sample showed the ultraviolet band.35,36 PL data shows that the tuning growth mode is not only important in controlling the size and morphology but also engineering crystalline properties. First, the relatively strong bands in the visible region for the ceria nanocubes synthesized with the Ce(IV) precursor indicate that OR favors the formation of point defects, such as oxygen vacancies, rather than nonstoichiometric ceria phases. These nanocubes have Ce3+ cations and oxygen vacancies distributed randomly in the fluorite lattice, presumably owing to the atom-by-atom dissolution and reattachment. On the other hand, the strong detection of the UV band for nanocubes synthesized with the Ce(III) precursor should relate to the OA growth mode. As previously suggested, the Ce3+-rich surface induced the OA mode, so that the Ce3+ spices could remain at the interface that is formed by the OA growth. As a result, the nonstoichiometric phases might be formed partially at the interface, producing strong ultraviolet luminescence. In this case, an identical ceria nanocube involves the domain boundaries with different chemical composition and local structure from a domain interior, which, we believe, can greatly affect the catalytic properties. Further studies on the growth mode dependent luminescent and catalytic properties are ongoing in our research group. Along with controlling the surface structure, the substitution of cerium ions with heterometallic ions is also effective in tailoring the catalytic activity of ceria. For example, tetravalent metal ions such as Zr4+, Ti4+, Si4+37,38 are substituted to introduce lattice strain, owing to the large intervals of the ionic radiuses. Hence, a doped ceria nanocube could be an ideal catalytic material owing to their surface and compositional contributions. In the present study, we examined whether the OA growth can be used for the synthesis of CeO2 ZrO2 binary nanocubes. Figure 5a shows TEM images of 20 mol % Zr4+ doped ceria nanoparticles synthesized with the Ce(III) precursor. As shown in the figure, the products are mainly nanocubes, indicating that OA is highly active, even under the doping condition. 3758
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Figure 5. TEM images of 20 mol % (a) Zr4+ doped ceria nanocubes synthesized using Ce(III) precursors and 20 mol % (b) Zr4+ doped ceria nanocubes synthesized using Ce(IV) precursor.
The well-developed edges of the nanocubes indicate that OR or interparticle growth are highly activated as well. Doping this element using the Ce(IV) precursor resulted in the formation of small-size (ca. 3 nm) spherical or polyhedral nanocrystals, as shown in Figure 5b. This indicates that synthesis of doped ceria nanocubes could not be archived with the OR process alone because of the significant suppression of the growth modes by doping. XRD pattern of these nanocubes is shown in Figure 6A. The wide angle XRD pattern for the Zr4+ doped ceria sample corresponds to cubic CeO2 phase without remarkable reflections from monoclinic ZrO2 phases. The reflection peaks for the Zr4+ doped sample slightly shifted to higher angles, compared to the undoped sample. The detectable smaller d-spacing in the Zr4+ doped ceria sample, compared to the undoped ceria, are attributed to the smaller ionic radii of Zr4+ (0.84 Å) in an octahedral environment than that of Ce4+ (1.11 Å). Furthermore, X-ray fluorescence (XRF) analysis estimated that the atomic ratio of Ce/Zr was approximately 70:30, which is close to the value in the initial materials for the synthesis. These results indicate that binary nanocubes were grown by HT treatment, followed by coprecipitation of Ce3+/Zr4+-oleate complexes, which occurred by adding a base to form nuclei with a solid solution framework. The PL spectra (Figure 6B) show a dominant emission band at 400 nm, quite similar to the luminescent spectra of undoped ceria nanocubes synthesized with Ce(III) precursor (see Figure 4). This demonstrated that doping in this concentration did not significantly alter the luminescent properties or growth mode. However, further increasing of Zr4+ concentration (30% and 40%) gradually suppressed the yield of nanocubes, and tiny spherical nanoparticles were produced more dominantly (Figure S2, Supporting Information). Finally, increasing the Zr4+ concentration to 50 mol % yielded spherical nanocrystals of ca. 2.5 nm in diameter, and electron diffraction (ED) analysis using
Figure 6. XRD patterns and PL spectra of colloidal solution of 20 mol % Zr4+ doped ceria nanocrystals synthesized by the hydrothermal method at 200 C for 30 h. An XRD pattern (dot line) of undoped ceria nanocubes synthesized by the hydrothermal treatment at 200 C for 30 h using the Ce(III) precursor is shown for comparison.
Figure 7. TEM image, ED pattern, and XRD pattern of 50 mol % Zr4+ doped ceria nanocrystals synthesized by the hydrothermal method at 200 C for 30 h using the Ce(III) precursor.
TEM shows ring patterns corresponding to the fluorite phase (Figure 7). The corresponding XRD pattern (Figure 7) shows very broad peaks from the fluorite phase without an impurity phase, demonstrating that OA and OR processes were strongly suppressed. In this synthesis condition, the number of surface Ce3+ sites, which trigger OA, was decreased by the increase in Zr4+ content, which can rationally yield such small-size nanocrystals, following the proposed formation principle. Thus, the current 3759
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’ CONCLUSION In conclusion, we demonstrated the simple and high-yield HT synthesis of ceria-based nanocubes via the OA growth. The reason underlying this success is the direct precipitation of Ce(III) oleate complex to form nuclei with a Ce(III) rich surface that favors the OA process. Oleate ligands suppressed oxidization of surface Ce3+ cations and guided development of {200} facets during HT aging. PL spectra indicate that the ceria nanocubes grown by OA have Ce3+-based domain boundaries in an identical grain because of the OA between Ce3+-rich surfaces, while OR favored the random distribution of oxygen vacancies in ceria nanocubes. Furthermore, we could synthesize 20 mol % Zr4+ doped ceria nanocubes effectively using OA and OR growth. Increasing the Zr4+ content to 50 mol % drastically suppressed both the growth modes to form small-size (ca. 2 nm) spherical nanoparticles. This is because the number of surface Ce3+ sites, which trigger OA and OR, were decreased by the increase in Zr4+ content, which rationally yields such small-size nanocrystals. The present study revealed that diverse surface valences from inside nanocrystals can provide high surface energy, which highly activates OA. This mechanism can play a role in the OA process for other oxides and a wide range of inorganic nanocrystals containing polyvalent metal ions. ’ ASSOCIATED CONTENT
bS Supporting Information. XRD patterns of ceria nanocrystals synthesized by the hydrothermal method at 200 C for 30 h with (a) Ce(IV) and (b) Ce(III) precursors (Figure S1), and the TEM image and XRD pattern of (a) 30 mol % and (b) 40 mol % Zr4+ doped ceria nanocrystals synthesized by the hydrothermal method at 200 C for 30 h using the (III) precursors (Figure S2). This material is available free of charge via the Internet at http:// pubs.acs.org. ’ AUTHOR INFORMATION Corresponding Author
*Phone: +81- (96)-342-3659. E-mail:
[email protected]. ac.jp.
’ ACKNOWLEDGMENT We express our gratitude to Prof. T. Watanabe, K. Kishida, and R. Motoyoshi for XPS measurements and fruitful discussions. We also thank Prof. K. Kitamoto and H. Tokimori (Tokyo Institute of Technology) for assistance with TEM analyses. ’ REFERENCES (1) Kaspar, J.; Fornasiero, P.; Graziani, M. Catal. Today 1999, 50 (2), 285–298. (2) Fu, Q.; Saltsburg, H.; Flytzani-Stephanopoulos, M. Science 2003, 301 (5635), 935–938. (3) Chen, J. P.; Patil, S.; Seal, S.; McGinnis, J. F. Nat. Nanotechnol. 2006, 1 (2), 142–150. (4) Tarnuzzer, R. W.; Colon, J.; Patil, S.; Seal, S. Nano Lett. 2005 5 (12), 2573–2577.
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