Synthesis and Characterization of Mesoporous Ceria with Hierarchical

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Synthesis and Characterization of Mesoporous Ceria with Hierarchical Nanoarchitecture Controlled by Amino Acids Guijun Zhang, Zhurui Shen, Mi Liu, Chenghua Guo, Pingchuan Sun, Zhongyong Yuan, Baohui Li, Datong Ding, and Tiehong Chen* College of Chemistry, Department of Materials Chemistry, Key Laboratory of Functional Polymer Materials of MOE, and College of Physics, Nankai UniVersity, Tianjin 300071, China ReceiVed: July 28, 2006; In Final Form: October 14, 2006

In this work, we report the synthesis and characterization of mesoporous ceria with hierarchical nanoarchitectures controlled by amino acids. During the synthesis procedure, cerium oxalate precipitate was treated hydrothermally with different amino acids as crystallization modifiers, and hierarchically structured cerium oxalate precursors were obtained. Ceria can be produced after thermal decomposition of the cerium oxalate precursors. Structure and properties of the product were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, N2 adsorption analysis, and X-ray photoelectron spectroscopy (XPS) methods. The results indicate that the mesoporous ceria with hierarchical nanoarchitectures are composed of nanosized ceria crystallites as building units and possess high surface area and high concentration of oxygen vacancy. Depending on different amino acids as the crystallization modifiers, the ceria exhibit different morphologies, such as dendritic aggregation of rods, dumbbells of nanorod arrays, or aggregated spheres. It is proposed that both the type of functional side groups and the length of the side groups of the amino acids influence the morphologies of the ceria. Meanwhile, the solvent and hydrothermal treatment temperatures also play important roles in the morphological control. The method reported in this work would be regarded as a general way to fabricate mesoporous metal oxides with hierarchical nanoarchitectures.

Introduction Ceria, characterized by its unique properties of oxygen ion conductivity and oxygen storage capacity, has been applied in a great deal of technological fields. In industrial catalysis, cerium oxide has widely been used in three-way catalysis (TWC) and the fluid catalytic cracking (FCC) process.1 As an oxygen ion conductivity material, ceria has been applied in solid oxide fuel cells,2 oxygen pumps, and amperometric oxygen monitors. As a polishing agent, ceria can also be used for the chemical mechanical planarization (CMP) process in the microelectronic industry.3 It has been reported that the performances of ceriarelated materials strongly depend on the morphologies and crystallographic orientations of nanometer-sized ceria.4 In recent years, ceria materials with nanosized,4 textured,5 and onedimensional6 (including nanotubes, nanowires, and nanorods) structures have been prepared by different synthetic methods. With the use of anodic alumina membranes (AAM) as a “hard” template, ordered ceria nanowire arrays have been fabricated.7 Mesoporous ceria with a high surface area is expected to have important applications in the fields of catalysis, sensing, and fuel cells. Ordered meso-structured CeO2 have been fabricated through the self-assembly of 6-aminocaproic modified 5 nm ceria particles with the aid of triblock copolymer pluronics 123.8 Other mesoporous ceria synthesized with different surfactants as templates or via a simultaneous polymerization-precipitate reaction have also been reported.9 However, the use of templates in the syntheses are prone to import impurities in the posttreatment to remove the organic species. * Corresponding author. Phone: +86-22-23507975. Fax: +86-2223507975. E-mail: [email protected].

Recently, using amino acids as modifiers of crystal growth has received great attention in the field of morphogenesis of inorganic materials. In nature, some proteins with specific amino acids sequences have been proved to play important roles in the biomineralization process to control the sizes, morphologies, and hierarchical structures of biominerals.10 As the basic structural units of proteins, amino acids contain hydrophilic functional groups and have the ability of complex formation with metal ions. Kandori et al. carefully examined the effects of amino acids on the formation of hematite particles.11 Yu’s group used serine as the crystal growth additive and prepared scrolled tellurium nanotubes by a hydrothermal method.12 With the addition of glutamate ions, flowerlike aggregates of slender apatite fibers were synthesized.13 Aspartic or glutamic acid functionalized clixarenes were used to control the crystal growth of calcium carbonate, barium sulfate, and calcium oxalate.14 Just recently, Lu et al. reported PbS nanorod-based multilevel architectures controlled by an amino acid-mediated approach.15 All these reports prove that amino acids are versatile in the morphological fabrication of inorganic materials. The use of metal salts as precursors to produce metal oxides has been reported. For instance, SnO2 nanowires were fabricated through a solution-phase precursor route,16 ceria with the morphologies of nanoparticles, microsized rods, and spindles were fabricated after decomposition of the cerium formate precursors,6j nanosized NiO particles were obtained by decomposition of nickel oxalate,17 and ZnO nanoparticles were synthesized through decomposition of different zinc salts as precursors.18 In this paper, we report the synthesis and characterization of mesoporous CeO2 with hierarchical nanoarchitectures by a

10.1021/jp0648285 CCC: $33.50 © 2006 American Chemical Society Published on Web 12/07/2006

Mesoporous Ceria with Hierarchical Nanoarchitecture

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precursor route in the presence of amino acids. In the synthesis procedure, the precipitates of cerium oxalate were hydrothermally treated with amino acids as the crystal growth modifier. Subsequently the cerium oxalate precursors formed after hydrothermal recrystallization were transformed into mesoporous CeO2 by a facile calcination, while the precursor’s morphologies, which can be selectively synthesized and facilely tailored by varying amino acids and other experimental parameters, remained. This is the first time that amino acids are applied to control the hierarchical morphologies of metal oxides, and this precursor method would be expected to be a general way for the fabrications of mesoporous metal oxides with hierarchical nanoarchitectures. Experimental Section Synthesis of Ceria. All chemical regents are commercially available and were used as received. The chemical structures of the amino acids used in this work are shown in Table 2. In a typical synthesis procedure, 4.1 mmol of L-lysine was first dissolved in 10 mL of H2O. Next, drops of 3 M HNO3 solution were added to adjust the pH value of the solution to about 6, in order to avoid the formation of Ce(OH)3 deposition in the following steps. Then 2.0 mmol CeCl3‚7H2O was dissolved in the solution under stirring. As soon as 10 mL of 0.15 M sodium oxalate solution was added, white precipitate of cerium oxalate formed immediately. Then the solution with the precipitate was transferred into an autoclave, which was then tightly sealed and hydrothermally treated at 160 °C for 24 h in an oven. The assynthesized sample was collected through filtering and washing with water and ethanol 3 times, respectively, and then dried at 60 °C for 12 h. After calcination at 360 °C in air for 1 h, the yellowish ceria powder was obtained. When L-glutamic acid or L-aspartic acid were used as additives in the synthesis, all experimental procedures were similar to the process described above, except that the pH value of the amino acid solution was first adjusted to about 6. When L-glycine was used in the synthesis, the pH value of the amino acid solution was not adjusted. Characterization. The thermogravimetry and differential thermal analysis (TG-DTA) of the sample was performed on a Rigaku TG-DTA thermal analyzer at a linear heating rate of 20 °C/min, and R-Al2O3 was used as a reference. The FT-IR transmission spectrum was recorded with a Bruker Vector 22 instrument. Powder X-ray diffraction pattern was recorded using a Rigaku D/max-2500 diffractometer, with Cu KR radiation (λ ) 1.5406 Å) at a scanning rate of 0.01°/s. The average crystal size of ceria was calculated with the Scherrer equation based on the width of the (111) diffraction peak, which was calibrated by the single-crystalline silicon standard sample. Scanning electron microscopy (SEM) images were measured on Philips XL-30 and Shimadzu SS-550 scanning electron microscopes. Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and selected-area electron diffraction (SAED) characterizations were carried out on a Philips Tecnai F20 instrument working at 200 kV. The samples were dispersed in ethanol by sonication and were subsequently dropped on copper grids. Nitrogen adsorption and desorption isotherms were measured on a Quantachrome NOVA 2000 sorption analyzer at 77 K. The pore size distribution is calculated from the adsorption branch using the BJH (BarettJoyner-Halenda) method, and the total pore volume was obtained at the relative pressure of P/P0 ≈1. The Raman spectra were obtained using a Spex-1403 spectrometer with the emission line at 532 nm. X-ray photoelectron spectra of Ce3d and O1s were obtained with a Kratos Axis Ultra DLD instrument.

Figure 1. (a) TG and corresponding DTA curves of the cerium oxalate precursor; (b) FT-IR spectra for cerium oxalate (curve b2) and calcined samples (curve b1).

Results and Discussion In the synthesis, with the addition of sodium oxalate to the Ce3+-containing solution, a white precipitate of cerium oxalate formed immediately. During the following hydrothermal treatment in the autoclave, the cerium oxalate precipitate would undergo dissolution and recrystallization under the control of amino acids, and complex morphologies of the cerium oxalate precursors were fabricated. After decomposition of the precursor by calcination, ceria was obtained, and its morphology remained analogous to the oxalate precursor. As an example, mesoporous ceria with hierarchical nanoarchitecture synthesized under the control of L-lysine is subjected to detailed physicochemical characterizations. Thermogravimetry analysis (Figure 1a) was first performed to determine the complete conversion temperature from cerium oxalate precursor to CeO2. In the TG curve there are two main weight loss steps at the temperature ranges of 25-250 °C (7.9 wt %) and 250-650 °C (31 wt %), respectively. The corresponding DTA curve shows one endothermic peak at 195 °C and one exothermic peak at 349 °C. The first one is due to the loss of the crystallized water of Ce2(C2O4)3‚nH2O, and the value of n is calculated to be 2.6 by its weight loss. The exothermic peak at 349 °C is assigned to the decomposition of oxalate and formation of the ceria, and thus, the calcination temperature of the cerium oxalate in our experiments is set to be 360 °C. The FT-IR spectra of the as-synthesized oxalate precursor and the calcined sample are displayed in Figure 1b. For the as-synthesized sample, two peaks at 1642 and 1317 cm-1 are assigned to the stretching mode of CdO in the cerium oxalate precursor.19 The 1480 cm-1 is due to the residual lysine in the sample. The wide peak at 3450 cm-1 is indexed to the superficial adsorbed or crystallized water, and the 793 cm-1 vibration is

25784 J. Phys. Chem. B, Vol. 110, No. 51, 2006

Figure 2. XRD pattern of (a) the cerium oxalate precursor and (b) the calcined sample (curve b1) and bulk ceria (curve b2).

due to an out-of-plane bending mode of water.20 After calcination, all peaks corresponding to CdO vibrations disappear. The one near 3410 cm-1 is due to the physically adsorbed water. There appears a broad adsorption peak at around 400 cm-1, which can be attributed to the Ce-O stretching band, according to the literature.21 So the FT-IR results indicate that the cerium oxalate precursor has been converted to ceria after the calcination treatment. Figure 2a shows the XRD pattern of the as-prepared cerium oxalate precursor after the hydrothermal crystallization process. The presence of several narrow and intensive diffraction peaks indicates that the precursor is well crystallized. This diffraction pattern could only be indexed to a mixed phase of cerium oxalate hydrates with different crystallized water, whose average number has been estimated to be 2.6 by the TG analysis. However, after decomposition of the cerium oxalate by calcination, pure ceria was obtained as proved by its XRD pattern. Figure 2b shows the XRD patterns of the bulk ceria and the ceria after decomposition of the oxalate. All diffraction peaks can be indexed to a face-centered cubic phase ceria (JCPDS 34-0394), and no signals of impurities are detected. Calculated from the position of the (200) diffraction peak, the lattice constant of the obtained ceria is a ) b ) c ) 0.5434 nm, slightly larger than the value of 0.5411 nm for the bulk CeO2. For the nanosized ceria, size-induced lattice relaxation has been observed, and this effect was due to the formation of defects in the lattice.22 The enlarged lattice constant of the obtained ceria would indicate lattice expansion due to the smaller particle size. On the basis of the width of the (111) diffraction peak, the mean particle size of the obtained ceria was calculated by the Scherrer equation to be about 6 nm, agreeing with the size of the nanocrystals observed from the TEM images (Figure 3i). The nanocrystalline ceria was formed after the decomposition of the cerium oxalate precursor, and though in the XRD pattern no impurities of crystallized precursor were detected, the existence of traces of amorphous phase, which was invisible in the diffraction analysis, could not be totally ruled out.

Zhang et al. The morphologies and architectures of the as-synthesized cerium oxalate precursor under the control of L-lysine and ceria obtained after calcination were investigated by SEM (Figure 3). As shown in Figure 3a, the precursor consists of dumbbelllike particles with a yield of almost 100%. The morphology of the precursor remains stable after decomposition of the oxalate (Figure 3b). Intergrowth of “dumbbells” was generally observed, and the aggregated particles take the shape of cauliflowers (Figure 3, parts c and d). It is interesting to note that the dumbbells are composed of oriented arrays of nanorods (200400 nm width), whose cross sections are rectangular rather than circular (Figure 3, parts e and f). Dumbbell-shaped aggregates of fluorapatite have been reported and been explained by a fractal crystal growth induced by the intrinsic electric fields.23 In our study, as shown in the following, the addition of different amino acids induce different morphologies of the crystallized cerium oxalate precursors, such as bundles and aggregated spheres, and this morphological change would be explained by the variation of intrinsic electric fields induced by the presence of different amino acids. The TEM image in Figure 3h displays mesoporosity within the nanorods, and this is also confirmed by the N2 sorption analysis (discussed later). The high-resolution TEM (HRTEM) image (Figure 3i) clearly shows that the nanorod consists of a large quantity of ceria nanocrystals which pack together in different orientations to form the mesopores. The 0.32 nm spacing between two adjacent lattice planes corresponds to the separation of the (111) lattice planes of the ceria. The selected-area electron diffraction (SAED) pattern (Figure 3j) clearly displays diffraction rings, indicating that the nanorods, as building units of the dumbbell-like microstructures, are polycrystalline rather than single crystalline. To characterize the porosity of the obtained ceria materials composed of nanocrystals, N2 sorption analysis was carried out. Figure 4 displays the N2 adsorption-desorption isotherms and the corresponding pore size distribution curve calculated from the adsorption branch by the BJH method. The obtained ceria sample exhibits the isotherm of type IV, and the adsorption branch shows an uptake of adsorbed volume at low relative pressure (P/P0) of 0.1-0.4, which indicates the existence of mesopores. The BJH pore size distribution curve displays a wide pore diameter distribution from 1.5 to 8 nm, and the maximum is centered at about 2.5 nm, within the range of a mesopore (2-50 nm). At the relative pressure higher than 0.9, there is an increasing step of the nitrogen adsorption volume, suggesting the presence of larger secondary meso- or macropores24 formed due to the particles packing or the interspace among the nanorods assembled into the “dumbbells”. The pore volume of the obtained ceria is 0.11 cm3/g, and the specific surface area is 108 m2/g. The size of the ceria grains can also be calculated according to the following equation:

D)

6 FSF

(1)

where S is the specific surface area, F is the density of CeO2 (7.28 g/cm3), and packing factor F ) 1. Then the calculated value of D is around 8 nm, which is close to the particle sizes calculated from the XRD pattern and observed from TEM images. It may be pointed out that the addition of amino acids is to control the morphologies of the cerium oxalate precursors during the hydrothermal treatment (described later), while not to modify the surface area of the product, because the origin of mesopores is due to the packing of ceria nanoparticles formed by calcination of the oxalate precursor. For example, the surface



Figure 3. SEM images of (a) as-synthesized cerium oxalate precursor in the presence of L-lysine; (b-f) ceria obtained after calcination. (g-i) TEM images of nanorods detached from the ceria microstructure after ultrasonic preparation. (j) Selected-area electron diffraction (SAED) pattern of the nanorod.

area of the ceria obtained without addition of amino acids is 105 m2/g (Table 2). The oxidation state and fraction of the cerium ions on the surface of the as-synthesized and calcined samples are studied by X-ray photoelectron spectroscopy. The XPS spectra of O1s and Ce3d of the as-synthesized and calcined samples are shown in Figure 5. In the O1s spectra of the as-synthesized precursor, the main peak at 531.7 eV is attributed to the CdO oxygen in cerium oxalate and the peak at 533.2 eV is considered as the oxygen of the crystallized water. After calcination, the positions of the O1s peaks obviously shift and the main peak at a binding energy of 529.4 eV is related to the oxygen in the crystal lattice

of CeO2. According to the literature, the peak at 531.3 eV could be assigned to either the oxygen of surface hydroxyls25 or O2 absorbed on the surface.26 Here in the ceria obtained after calcination, the oxygen in the residual traces of the amorphous phase would also contribute to the 531.3 eV signal. In the spectrum of Ce3d, eight peaks are displayed regardless of the as-synthesized and the calcined samples. These peaks can be divided into four groups, assigned to four different spin orbit doublets, (V0, u0), (V1, u1), (V2, u2), and (V3, u3), respectively. Among them, those labeled “V” correspond to 3d5/2 and those labeled “u” are attributed to 3d3/2. The doublets (V0, u0), (V2, u2), and (V3, u3) are all assigned to the diversified states of Ce4+,


Zhang et al.

Figure 4. (a) N2 adsorption-desorption isotherm of the obtained ceria sample; (b) the corresponding BJH pore size distribution curve.

Figure 6. Raman spectra of bulk ceria (curve a) and dumbbell-like ceria (curve b).

high concentration of the oxygen vacancy would be expected. Tsunekawa et al. investigated the anomalous lattice expansion of the CeO2 nanoparticles, and they found that “the reduction of the valence induces an increase in the lattice constant due to the decrease in electrostatic forces”.27 Zhou and Huebner22 calculated the concentration of oxygen vacancy (VO) according to the variation of the lattice parameter on the assumption that Ce3+ maintains the same coordination number of 8 as the Ce4+ in the CeO2 crystal and the oxygen vacancies distribute randomly in the ceria crystals. The calculation of oxygen vacancies is shown as follows according to Zhou and Huebner:22

x3 1 (a′ - a0) ) c rCe3+ - rCe4+ + (rVO - rO2-) 4 4

[

Figure 5. XPS spectra of the cerium oxalate precursor and calcined samples: Ce3d spectra (left), O 1s spectra (right).

TABLE 1: Assignments and Intensities of Ce 3d XPS Peaks of Ceria Fabricated under the Control of L-Lysine Ce 3d5/2 peaks binding energy/eV precursor binding energy/eV ceria

V0

V1

V2

Ce 3d3/2 V3

u0

u1

u2

u3

881.9 885.4 887.1 899.0 901.0 904.3 908.0 915.6 882.5 884.4 889.1 898.5 900.9 902.4 907.9 916.9

assignments Ce4+ Ce3+ Ce4+ Ce4+ Ce4+ Ce3+ Ce4+ Ce4+ percentage of 14.3 33.7 7.6 2.8 9.6 25.4 3.9 2.6 peak area/% precursor percentage of 13.8 10.2 15.4 19.4 9.3 6.9 9.9 15 peak area/% ceria

whereas the doublet (V1, u1) is considered as the character of Ce3+.9e The deconvolution results of the XPS spectra are listed in Table 1, in which it is clearly indicated that the fraction of Ce3+ (intensities of V1 and u1) is greatly reduced due to the formation of CeO2 after the calcination. Although the traces of amorphous phase and the in situ photoreduction of some Ce4+ to Ce3+ during XPS measurement9e contribute partially to the signal of Ce3+, we pay more attention to the Ce3+ in the CeO2 crystal lattice as they could induce formation of the oxygen vacancies. It has been proposed that the concentration of oxygen vacancies in ceria is related to the particle sizes, and smaller nanoparticles possess a higher concentration of oxygen vacancy.22 As the hierarchically structured ceria fabricated in this work consists of nanosized crystals of ceria as building units, a

]

(2)

where a′ is the expanded lattice parameter, rCe3+, rCe4+, rVO, and rO2- are the radii of Ce3+, Ce4+, oxygen vacancy, and O2-, respectively. The parameter c is defined as the molar ratio of Ce3+/Ce4+, and the concentration of oxygen vacancy, [VO], can be obtained by [VO] ) [O2-]c/4.22 The values of the radii were taken to be rCe3+ ) 0.1283 nm, rCe4+ ) 0.1098 nm, rO2- ) 0.124 nm, and rVO ) 0.138 nm;22 a′ ) 0.5434 nm (the calcined sample), a0 ) 0.5411 nm (the bulk CeO2), and the density of CeO2 is 7.28 g/cm3; the calculated concentration of oxygen vacancies [VO] is 5.7 × 1020/cm3 for the 6 nm ceria grains. On the basis of formula 2, the oxygen vacancy concentration is mainly determined by the expansion of the lattice constant, which can be obtained through the XRD measurements. However, in literature the lattice constant of the CeO2 nanoparticles differed slightly,22,28 probably due to the different working conditions of the XRD instruments. Thus, the oxygen vacancy concentration in ceria nanoparticles reported in different literature would be varied and may not be accurately compared. The size-dependent properties of CeO2 have been studied by the Raman scattering method, and the shift of the Raman peak near 464 cm-1 was observed due to the increasing lattice constant as the particle size became smaller.28 Accordingly, the ceria samples were subjected to Raman spectroscopy as shown in Figure 6. For the bulk ceria, only one high and sharp peak at 464 cm-1 is observed in the spectrum, which can be assigned to F2g symmetry as a symmetric breathing mode of the oxygen atoms around the cerium ions.29,30 For the hierarchically structured dumbbell-like ceria synthesized under the control of L-lysine, the first-order peak was shifted 8 cm-1 to 456 cm-1, and its shape became broadened and asymmetrical. Generally the Raman peak’s position varies with the change of the interatomic force, which is characterized by the change of bond length, as well as the change of the lattice spacing. Therefore, in our experiment the shift of the Raman peak is related to the



SCHEME 1: Hierarchical Structure of the DumbbellLike Ceria Synthesized under the Control of L-Lysine

enlarged lattice space due to the smaller size of ceria crystals (6 nm).28,31,32 The relationship between the Raman shift and the change of the lattice parameter can be expressed by the following equation:29

∆ω ) -3γω0∆a/a0

(3)

where ∆ω is the frequency shift of the Raman mode, ω0 is the Raman frequency of bulk CeO2, ∆a is the change in lattice constant, a0 is the lattice constant of bulk CeO2 (0.5411 nm), and γ is the Gru¨neisen constant. The value of γ has been

calculated to be 1.44 or 1.24 in ref 28 or 29, respectively. Then in our experiment the calculated Raman frequency shift ∆ω is -8.52 or -7.34 cm-1, respectively, which are very close to the negative experimental frequency shift of 8 cm-1. Besides the peak at 456 cm-1, there are two weak second-order peaks at around 600 and 830 cm-1 (Figure 6, inset). The former one is assigned to the nondegenerate LO mode of ceria and has been attributed to the presence of defects.28,29 The latter, namely, the peak at 830 cm-1, is linked with the nanometer-scale size of the crystal grains.28 So the results of the Raman spectroscopy confirm again that the size of the ceria nanocrystals is so small (6 nm) that the crystal distortion (larger lattice constant) and oxygen vacancies appear. Based on the above experiments, we demonstrate that the hierarchically structured mesoporous CeO2 composed of nanocrystals as building units can be obtained by decomposition of the cerium oxalate precursor, which has been fabricated through hydrothermal crystallization under the control of amino acids. As an example, with L-lysine as the crystal growth modifier, the synthesis process and the structure of dumbbell-like ceria are illustrated in Scheme 1. The two levels of hierarchical structures are (1) accumulations of ceria nanoparticles to form polycrystalline nanorods with mesoporosity and (2) assemblage of these nanorods to form dumbbell-like microstructure. Possessing mesoporosity, large surface area, and rich oxygen

Figure 7. SEM images of ceria synthesized in a mixed solvent of H2O and ethanol: (a-c) volume ratio of H2O/ethanol ) 2:1; (d-f) volume ratio of H2O/ethanol ) 1:1.


Figure 8. SEM images of ceria synthesized at a hydrothermal temperature of 140 °C (a) and 180 °C (b-d).

vacancies, this hierarchically structured ceria material would be a promising candidate as a catalyst in redox reactions, as has been proved by the CO oxidation to CO2 process catalyzed by nanosized ceria.26 Here we pay more attention to the morphologies of the ceria as the hierarchical morphogenesis of ceria has never been reported before. Because the ceria is obtained by decomposition of the cerium oxalate precursor, the morphology formed during the hydrothermal crystallization of cerium oxalate under the control of amino acids dominates the morphologies. Dumbbellshaped fluorapatite23b and BaCO333 have been reported, and the formation mechanism has been explained by a fractal growth process starting from a rodlike crystal seed. According to Busch and co-workers, the progressive stages of the self-assembled growth of fluorapatite include hexagonal-prismatic seed, dumbbell shapes, twin-spheres, and spheres, and these morphologies may be controlled by intrinsic electric fields.23 In our experiments with the addition of L-lysine, only dumbbell-shaped hierarchical aggregates of ceria were obtained. However, when ethanol was used as cosolvent in the synthesis procedure,

Zhang et al. hierarchical elliptical spheres and twin-spheres were fabricated, with the volume ratio of H2O/ethanol of 2 or 1, respectively. As shown in Figure 7, the ceria ellipsoidal particles and twinspheres were composed of aggregates of aligned nanorods or flakes. By N2 adsorption measurements, the BET surface areas of these two ceria are 143 and 140 m2/g, respectively. According to formula 1, these relatively high surface areas imply that the ellipsoidal particles or twin-spheres are all composted of nanosized ceria crystals with the average size of 6 nm created by the decomposition of the oxalate precursors. Morphological control of monodispersed CaCO3 microspheres in a mixture of solvents has been reported,34 and here the mixed solvent of H2O and ethanol would probably change the solubility of the cerium oxalate, crystal growth kinetics, and the physicochemical properties of the reaction solution. According to Busch and coworkers, the crystal growth is influenced by the intrinsic electric fields,23 then the presence of the ethanol may also influence this parameter and give rise to morphological evolution of the crystallized products. Besides the solvent, the reaction temperatures for crystallization of cerium oxalate precursors influence the morphologies. The hydrothermal temperature is 160 °C for the ceria sample described above (Figure 3). When the hydrothermal temperature was changed from 160 to 140 °C, the morphology of the ceria is the aggregation of irregular particles rather than the bundles consisting of nanorods. When the reaction temperature was changed to 180 °C, cactuslike particles consisting of arrays of nanorods were obtained as shown in Figure 8. The temperature of the hydrothermal treatment would influence the pressure of the reaction system, the solubility of the products, and the kinetics of crystallization, through which the morphologies of the products would be changed with different hydrothermal temperatures. To realize the morphological control effect of the amino acids, syntheses with different kinds of amino acids as the crystallization modifiers were also performed. SEM images of ceria products synthesized without amino acid, with L-glycine, L-glutamic acid, or L-aspartic acid, respectively, are displayed in Figure 9, and their morphologies are described in Table 2.

TABLE 2: Morphology and Surface Area of Mesoporous CeO2 Obtained under Control of Different Amino Acids

a

G and m denote the functional group and the length of the side chain of the amino acid, respectively.



Figure 9. SEM images of CeO2 synthesized by using different amino acids as the additive: (a, b) without amino acid; (c, d) L-glycine; (e, f) L-glutamic acid; (g, h) L-aspartic acid.

For the control experiment without addition of amino acid, the morphology of the product is bundles consisting of branchgrown rods, which are very densely packed together. The contour of the bundles is irregular (Figure 9, parts a and b). When L-glycine is used as the additive, the ceria’s morphology resembles that of the product synthesized without amino acids and the contour of the bundles is still irregular (Figure 9, parts c and d). With the use of L-glutamic acid as the additive the obtained ceria consists of bundles formed through rods assembling, while the rods are well separated and are loosely packed. The contour of the bundles takes the shape of a cone or hemisphere (Figure 9, parts e and f). The morphological control effect of L-glycine is not very obvious, and this may be explained by the absence of any other functional side group except the carboxyl and amino group. If the different morphol-

ogies of ceria controlled by L-lysine (Figure 3) and L-glutamic acid are compared, it can be inferred that the functional groups besides the carboxyl and amino group, either -COOH or -NH2, of the amino acids, would have great impact on the crystallization morphologies of the cerium oxalate precursors. It is more interesting that if L-aspartic acid is used to replace L-glutamic acid, the ceria’s morphology varies greatly from bundles to aggregated spheres with loose interiors (Figure 9, parts g and h). The structural difference between L-glutamic acid and L-aspartic acid is only the number of carbons in the functional side groups, and this implies that the chain length of the functional groups of the amino acids would also play important roles in morphological control of the oxalate precursors. Although the detailed mechanism of the morphological control of ceria by the amino acids is difficult to interpret, it

25790 J. Phys. Chem. B, Vol. 110, No. 51, 2006 can be expected that the structural and physical properties of the amino acids, such as their coordinating abilities with metal ions, the type and length of the functional groups, ionized state in the solution, and their influence on the intrinsic electric fields,23 etc., would play critical roles in the morphogenesis of the cerium oxalate precursors during the hydrothermal crystallization process. Thus, it would be expected that other complex morphologies of ceria can be fabricated by adding other functional additives. The method introduced in this work would be expected to be a general way to fabricate mesoporous metal oxides with hierarchical nanoarchitectures, and actually we have obtained hierarchically structured mesoporous MnO2 with the BET surface area of 276 m2/g by this oxalate precursor method. Syntheses and characterizations of other metal oxides are in progress. Conclusion Hierarchical nanoarchitectures of mesoporous ceria materials have been fabricated via a precursor method. Amino acids have been used as crystal growth modifiers to control the morphologies of cerium oxalate in a hydrothermal method, and hierarchically structured cerium oxalate precursors can be obtained. After calcination, ceria composed of nanosized crystals can be produced after thermal decomposition of the cerium oxalate precursors, while the morphology of the oxalate precursor is maintained. By characterizations of XRD, SEM, TEM, XPS, N2 sorption analysis, and Raman spectroscopy, these ceria materials are proved to be mesoporous and are composed of nanosized ceria crystallites as building units, giving rise to high concentration of the oxygen vacancy. With the addition of different amino acids as the hydrothermal crystallization modifiers, the obtained ceria exhibit different morphologies, such as dendritic aggregation of rods, dumbbells of nanorods arrays, or aggregated spheres. Both the type and the length of the functional side groups of the amino acids can influence the morphologies of the ceria. Meanwhile, mixture of solvents and hydrothermal temperatures would also play important roles in the morphological control. The synthesis procedure reported in this work includes two steps: fabrication of the precursors with well-defined morphologies through the hydrothermal recrystallization method under the control of the amino acids and decomposition of the precursors. This method would be expected to be a general way to fabricate mesoporous metal oxides with hierarchical nanoarchitectures. Acknowledgment. This work was supported by the National Science Foundation of China (Grant Nos. 20373029 and 20233030) and the Chinese Ministry of Education (Foundation for University Key Teachers, and Joint-Research Fund of Nankai University and Tianjin University on Nano-Science). References and Notes (1) Trovarelli, A.; de Leitenburg, C.; Boaro, M.; Dolcetti, G. Catal. Today 1999, 50, 353. (2) Park, S. D.; Vohs, J. M.; Gorte, J. R. Nature 2000, 404, 265. (3) (a) Zanye, A.; Kumar, A.; Sikder, A. K. Mater. Sci. Eng., A 2004, R45, 89. (b) Feng, X.; Sayle, D. C.; Wang, Z. L.; Paras, M. S.; Santora, B.; Sutorik, A. C.; Sayle, T. X. T.; Yang, Y.; Ding, Y.; Her, Y. S. Science 2006, 312, 1504.

Zhang et al. (4) (a) Yu, T.; Joo, J.; Park, Y. I.; Hyeon, T. Angew. Chem., Int. Ed. 2005, 44, 7411. (b) Si, R.; Zhang, Y. W.; You, L. P.; Yan, C. H. Angew. Chem., Int. Ed. 2005, 44, 3256. (c) Masui, T.; Fujiwara, K.; Machida, K.; Adachi, G. Chem. Mater. 1997, 9, 2197. (d) Inoue, M.; Kimura, M.; Inui, T. Chem. Commun. 1999, 957. (5) Bouchara, A.; Mosser, G.; Soler-Illia, G. J. de A. A.; Chane-Ching, J. Y.; Sanchez, C. J. Mater. Chem. 2004, 14, 2347. (6) (a) Han, W. Q.; Wu, L.; Zhu, Y. J. Am. Chem. Soc. 2005, 127, 12814. (b) Tang, C.; Bando, Y.; Liu, B.; Golberg, D. AdV. Mater. 2005, 17, 3005. (c) Tang, B.; Zhuo, L.; Ge, J.; Wang, G.; Shi, Z.; Niu, J. Chem. Commun. 2005, 3565. (d) Vantomme, A.; Yuan, Z. Y.; Du, G.; Su, B. L. Langmuir 2005, 21, 1132. (e) Sun, C.; Li, H.; Wang, Z. X.; Chen, L.; Huang, X. Chem. Lett. 2004, 33, 662. (f) Yang, R.; Guo, L. Chin. J. Inorg. Chem. 2004, 20, 152. (g) Chen, H. I.; Chang, H. Y. Solid State Commun. 2005, 133, 593. (h) Wu, G. S.; Xie, T.; Yuan, X. Y.; Cheng, B. C.; Zhang, L. D. Mater. Res. Bull. 2004, 39, 1023. (i) Kuiry, S. C.; Patil, S. D.; Deshpande, S.; Seal, S. J. Phys. Chem. B 2005, 109, 6936. (j) Ho, C.; Yu, J. C.; Kwong, T.; Mak, A. C.; Lai, S. Chem. Mater. 2005, 17, 4514. (7) La, R. J.; Hu, Z. A.; Li, H. L.; Shang, A. L.; Yang, Y. Y. Mater. Sci. Eng., A 2004, 368, 145. (8) (a) Corma, A.; Atienzar, P.; Garcia, H.; Chane-Ching, J. Y. Nat. Mater. 2004, 3, 394. (b) Chane-Ching, J. Y.; Cobo, F.; Aubert, D.; Harvey, H. G.; Airiau, M.; Corma, A. Chem. Eur. J. 2005, 11, 979. (9) (a) Terribile, D.; Trovarelli, A.; de Leitenburg, C.; Dolcetti, G.; Llorca, J. Chem. Mater. 1997, 9, 2676. (b) Lyons, D. M.; Ryan, K. M.; Morris, M. A. J. Mater. Chem. 2002, 12, 1207. (c) Lundberg, M.; Skarman, B.; Cesar, F.; Wallenberg, L. R. Microporous Mesoporous Mater. 2002, 54, 97. (d) Khalil, K. M. S.; Elkabee, L. A.; Murphy, B. Microporous Mesoporous Mater. 2005, 78, 83. (e) Brezesinski, T.; Repen, C.; Iimura, K.; Smarsly, B. Chem. Mater. 2005, 17, 1683. (f) Sun, C.; Sun, J.; Xiao, G.; Zhang, H.; Qiu, X.; Li, H.; Chen, L. J. Phys. Chem. B 2006, 110, 13445. (10) Mann, S. Biomineralization: Principles and Concepts in Bioinorganic Materials Chemistry; Oxford University Press: New York, 2001. (11) Kandori, K.; Sakai, M.; Inoue, S.; Ishikawa, T. J. Colloid Interface Sci. 2006, 293, 108. (12) He, Z.; Yu, S. H.; Zhu, J. Chem. Mater. 2005, 17, 2785. (13) Zhang, H. G.; Zhu, Q.; Wang, Y. Chem. Mater. 2005, 17, 5824. (14) Jones, F.; Mocerino, M.; Ogden, M. I.; Oliveira, A.; Parkinson, G. M. Cryst. Growth Des. 2005, 5, 2336. (15) Lu, Q.; Gao, F.; Komarneni, S. Nanotechnology 2006, 17, 2574. (16) Wang, Y.; Jiang, X.; Xia, Y. J. Am. Chem. Soc. 2003, 125, 16176. (17) Wang, X.; Song, J. M.; Gao, L. S.; Jin, J. Y.; Zheng, H. G.; Zhang, Z. D. Nanotechnology 2005, 16, 37. (18) (a) Audebrand, N.; Auffredic, J. P.; Louer, D. Chem. Mater. 1998, 10, 2450. (b) Ahmad, T.; Vaidya, S.; Sarkar, N.; Ghosh, S.; Ganguli, A. K. Nanotechnologhy 2006, 17, 1236. (19) Ouyang, J. M.; Duan, L.; Tieke, B. Langmuir 2003, 19, 8980. (20) Ouyang, J. M.; Zheng, H.; Deng, S. P. J. Cryst. Growth 2006, 293, 118. (21) McDevitt, N. T.; Baun, W. L. Spectrochim. Acta 1964, 20, 799. (22) Zhou, X. D.; Huebner, W. Appl. Phys. Lett. 2001, 79, 3512. (23) (a) Kniep, R.; Busch, S. Angew. Chem., Int. Ed. Engl. 1996, 35, 2624. (b) Bush, S.; Dolhaine, H.; DuChesne, A.; Heinz, S.; Hochrein, O.; Laeri, F.; Podebrad, O.; Vietze, U.; Weiland, T.; Kniep, R. Eur. J. Inorg. Chem. 1999, 1643. (24) Ren, T. Z.; Yuan, Z. Y.; Su, B. L. Chem. Commun. 2004, 2730. (25) Rao, M. V. R.; Shripathi, T. J. Electron Spectrosc. Relat. Phenom. 1997, 87, 121. (26) Zhang, Y. W.; Si, R.; Liao, C. S.; Yan, C. H. J. Phys. Chem. B 2003, 107, 10159. (27) Tsunekawa, S.; Ishikawa, K.; Li, Z. Q.; Kawazoe, Y.; Kasuya, A. Phys. ReV. Lett. 2000, 85, 3440. (28) Spanier, J. E.; Robinson, R. D.; Zhang, F.; Chan, S. W.; Herman, I. P. Phys. ReV. B 2001, 64, 245407. (29) McBride, J. R.; Hass, K. C.; Poindexter, B. D.; Weber, W. H. J. Appl. Phys. 1994, 76, 2435. (30) Lin, X. M.; Li, L. P.; Li, G. S.; Su, W. H. Mater. Chem. Phys. 2001, 69, 236. (31) Weber, W. H.; Hass, K. C.; McBride, J. R. Phys. ReV. B 1993, 48, 178. (32) Graham, G. W.; Weber, W. H.; Peters, C. R.; Usmen, R. J. Catal. 1991, 130, 310. (33) Yu, S. H.; Colfen, H.; Antonietti, M. J. Phys. Chem. B 2003, 107, 7396. (34) Guo, X. H.; Yu, S. H.; Cai, G. B. Angew. Chem., Int. Ed. 2006, 45, 3977.

Synthesis and Characterization of Mesoporous Ceria with Hierarchical

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