Fabrication of Monodisperse CeO2 Hollow Spheres Assembled by

Oct 27, 2009 - A possible crystal growth and hollowing mechanism was suggested based on the detailed experiment. The CO oxidation properties were ...
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DOI: 10.1021/cg900898r

Fabrication of Monodisperse CeO2 Hollow Spheres Assembled by Nano-octahedra

2010, Vol. 10 291–295

Zhijie Yang, Dongqing Han, Donglin Ma, Hui Liang, Ling Liu, and Yanzhao Yang* School of Chemistry and Chemical Engineering, Shandong University, Jinan, Shandong 250100, People’s Republic of China Received July 31, 2009; Revised Manuscript Received October 5, 2009

ABSTRACT: A simple hydrothermal method has been employed to fabricate monodisperse CeO2 hollow spheres assembled by nano-octahedra. Results revealed that the as-obtained CeO2 hollow spheres were assembled by nano-octahedra with an average edge length of ca. 20 nm. A possible crystal growth and hollowing mechanism was suggested based on the detailed experiment. The CO oxidation properties were investigated, and the novel structured CeO2 hollow spheres exhibited a higher catalytic properties compared to the commercial CeO2 powders.

1. Introduction In recent years, inorganic hollow nanostructures with remarkable interior space and shell have attracted fascinating interest because of their higher specific surface area, lower density and better permeation and widespread potential applications in chemical reactors, sensors, drug delivery, catalysis and various new application fields.1-5 Up to now, much effort has been devoted to generate inorganic hollow structures including conventional templating methods as well as newly emerging template-free methods. Template-assisted synthesis has been demonstrated as a versatile approach to fabricate hollow structures. A wealth of templates including hard templates such as polymer latex particles,6 silica spheres,7 and carbon spheres,8 and soft templates such as emulsion droplets,9,10 micelles,11 and gas bubbles12 were extensively employed to prepare hollow structures. However, the template-assisted methods usually require tedious procedures, including surface modification, precursor attachment, and core removal. In general, it still remains a challenge to develop a facile one-pot, template-free route for the fabrication of inorganic hollow structures. Recently, Ostwald ripening,13-18 Kirkendall effect,4,19 and oriented attachment20,21 were recognized as novel template-free mechanisms participating in the creation of hollow features. CeO2 has been attracting great interest because of the effective and the potential technological applications in several fields such as catalysis,22,23 UV-shielding,24 sensor,25 chemical-mechanical polishing for microelectronic,26 and water treatment.27 Notably, ceria is a key component in three-way catalysts (TWC) owing to its oxygen storage capacity (OSC) associated with the ability to undergo a facile conversion between Ce (III) and Ce (IV). To date, considerable efforts have been focused on the preparation of nanostructured CeO2 due to the general improvement in its physical and chemical properties.28-31 Recently, some groups have synthesized CeO2 hollow structures through various methods.32-36 For example, Titirici et al. fabricated CeO2 hollow spheres using carbonaceous D-glucose as template.32 Li et al. fabricated CeO2-ZrO2 solid solution nanocages via Kirkendall effect.33

Chen et al. has synthesized CeO2 hollow nanocubes from selfassembly of CeO2 nanoparticles through combining oriented attachment and Ostwald ripening process.34 However, monodisperse CeO2 hollow spheres assembled by nano-octahedra have not been reported yet. In this paper, we present a facile one-pot template-free route to prepare monodisperse CeO2 hollow spheres assembled by nano-octahedra. The structure and morphology of the prepared products were characterized by using XRD, SEM, TEM, HRTEM and XPS. The formation of such CeO2 hollow spheres has been found to be controlled by reaction time and the amount of the reactants. A hollowing growth mechanism may involve Ostwald ripening is proposed based on the detailed experiment. The CO oxidation experiment was carried out to evaluate the catalytic properties of the asprepared product. 2. Experimental Section

*Corresponding author: E-mail: [email protected] Tel.: 86-53188365431; fax: 86-531-88564464.

2.1. Synthesis of Monodisperse CeO2 Hollow Spheres Constructed with Nano-octahedra. Cerious chloride (CeCl3 3 7H2O, g 99.0%), formamide (HCONH2, g 99.5%), and polyvinylpyrrolidone (PVP, K30) were purchased from Sinopharm Chemical Reagent Co. Ltd. Hydrogen peroxide (H2O2, 30%) was obtained from Laiyang, Kangde Chemical Reagent Factory. Deionized water was used throughout. In a typical synthesis of CeO2 hollow nanospheres, 0.099 g of CeCl3 3 7H2O and 0.178 g of PVP were dissolved in 19 mL of deionized water under vigorous magnetic stirring. One milliliter of formamide and 0.1 mL of H2O2 were then added into the solution under continuous stirring for 30 min. The as-formed yellow solution was transferred into a Teflon-lined autoclave of 25 mL capacity and heated for 24 h at 180 °C. After the autoclave was cooled to room temperature naturally, light brown products were collected and washed with deionized water 4 times. Finally, the products were washed with absolute ethanol and dried in an electric oven for 6 h at 70 °C. 2.2. Materials Characterization. The phase purity of the product was examined by using a Rigaku D/Max 2200PC diffractometer with a graphite monochromator and Cu KR radiation (λ=0.15418 nm). The morphology and microstructure of the products were characterized using a transmission electron microscope (TEM, JEM 100CX II) with an accelerating voltage of 80 kV, a high-resolution transmission electron microscope with an accelerating voltage of 200 kV (HRTEM, JEM-2100), and a field-emission scanning electron microscope (FE-SEM, Hitachi, S4800). X-ray photoelectron

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Figure 1. XRD pattern of the as-prepared CeO2 hollow spheres assembled by nano-octahedra.

Figure 2. (a) XPS survey spectrum and (b) Ce 3d spectrum of the CeO2 hollow spheres. spectra (XPS) were measured using a PHI 5300 X-ray photoelectron spectrometer with Al KR radiation. The binding energy reference was taken at 284.7 eV for the C1s peak. N2 adsorption-desorption isotherms were measured on a QuadraSorb SI apparatus at 77 K. The surface areas were calculated by the Brunauer-Emmett-Teller (BET) method, and the pore size distribution was calculated from the desorption branch using the Barett-Joyner-Halenda (BJH) theory. 2.3. Measurement of Catalytic Activity. The catalytic activity of the as-obtained sample was evaluated on a continuous flow fixedbed microreactor operating under atmospheric pressure. In a typical experiment, 50 mg catalyst particles were placed in the reactor. The reactant gases (1% CO, 10% O2, and 89% N2) passed through the reactor at a rate of 30 mL/min. The composition of the gas exiting the reactor was analyzed with an online infrared gas analyzer (Gasboard-3121, China Wuhan Cubic Co.), which simultaneously detects CO and CO2 with a resolution of 10 ppm. The results were further confirmed with a Shimadzu gas chromatogragh (GC-14C).

3. Results and Discussion Figure 1 shows a typical XRD pattern of the sample, which can be indexed to face-centered cubic phase of CeO2 (JCPDS card No. 34-0394). No other impurities were detected, which indicates the high purity of the sample. The sharp peaks confirm the highly crystalline nature of CeO2. To further characterize the product, we conducted XPS analysis to investigate the surface composition and chemical state of the as-prepared product. Peaks of C 1s, O 1s, and Ce 3d can be identified from survey spectra in Figure 2a. Figure 2b displays the Ce 3d spectrum of the products. We can find that several binding energy (BE) peaks were consistent with the previous report on Ce4þ, indicating the main valence of cerium in the hollow spheres was þ4.37 The morphology and structure of the obtained products were investigated by SEM and TEM as shown in Figure 3. Figure 3a displays a panoramic SEM image of the as-prepared CeO2, which shows the sample is large-scale uniform spheres with diameters of 120-140 nm. The hollow nature of the spheres could be observed from the surface pore of the

Figure 3. (a, b) SEM images, (c, d) TEM images, and (e) HRTEM image of the typical CeO2 hollow spheres.

spheres, which can be confirmed from the high-magnification SEM image shown in Figure 3b. Herein, of particular interest is that the hollow spheres were constructed by the uniform nano-octahedra with an edge length of ca. 20 nm. To confirm that the building blocks of the spheres were octahedral particles, the CeO2 spheres were cracked by ultrasonication treatment in an ultrasonic water bath for 30 min. The detailed SEM image can be found in Figure S1 (see the Supporting Information). The corresponding TEM images of the spheres shown in Figure 3c, d further clearly identify the hollow structure of the product. A strong contrast between the dark edges and the pale center confirms that all the spherical particles have a hollow interior. The high magnification TEM image in Figure 3d further reveals that the shell of hollow spheres were organized by the nano-octahedra, which is consistent with the observation from SEM images. The detailed microstructure of the nano-octahedra on the shell is investigated by the high-resolution transmission electron microscope (HRTEM). Figure 3e shows a HRTEM image of CeO2 hollow nanostructures obtained from the area marked with a white pane as shown in Figure 3d. The interplanar spacings of a single nano-octahedron are about 0.31 and 0.27 nm, which are consistent with the (111) and (200) lattice planes of CeO2, respectively. Furthermore, the Brunauer-Emmett-Teller (BET) surface area of the synthesized CeO2 hollow spheres is 14.7 m2/g, which is higher than the value (8.5 m2/g) of the commercial CeO2 powders. The N2 adsorption-desorption isotherms of both the hollow spheres and commercial CeO2 powders could be seen in Figure S2 (see the Supporting Information). The corresponding pore size

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Figure 4. TEM and SEM images of interim samples collected at different reaction times: TEM images: (a) 1, (b) 2, (c) 6, and (d) 18 h. SEM images: (e) 1, (f) 6, and (g) 18 h.

distribution curve of the hollow spheres calculated from the desorption branch by the BJH method displays a pore size distribution from 3 to 15 nm, centered at ca.4.5 nm (inset in Figure S2a, Supporting Information). Generally, 0D nanopariticles and 1D nanorods/nanowires are used as building blocks for assembly into 3D hierarchical structures. Very recently, nanocubes and nano-octahedra have been utilized as a new class of building blocks for selfassembly.38,39 Herein, nano-octahedra were employed as building blocks for the construction of CeO2 hollow spheres. To investigate the growth process of novel CeO2 hollow spheres constructed by nano-octahedra, experiments were designed to investigate the intermediate at different hydrothermal reaction times. Panels a-d of Figure 4 show TEM images of samples obtained at the reaction times of 1, 2, 6, and 18 h, respectively. It is indicated that there are obvious changes for the interior cavity of the spheres. From Figure 4a we can see that spherical particles with a solid core are obtained within the first time of 1 h, the spheres are then followed by a solid core evacuation, and a hollowing effect is observed for those with a longer reaction time of 6 h (Figure 4b, c). The hole in diameter was changed from ca. 20 nm (Figure 4b) to ca. 40 nm (Figure 4c). The inner space of the spheres is further increased to 60-70 nm in diameter when

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the reaction time is further prolonged, which can be observed from the clear contrast between the dark edges and the pale center in Figure 4d. The surface morphology of the above time-dependent spherical particles was further characterized by SEM images. From these images (Figure 4e,f), one can see that all the spheres are composed of numerous nano-octahedra, and the size of the nano-octahedra were ca. 10, 15, and 20 nm, respectively. Regarding both the formation process of hollow interiors and the size evolution of the octahedral building blocks, it is believed that the Ostwald ripening should be the underlying mechanism for the formation of the novel structured hollow spheres. The ripening process has been commonly observed in crystal growth and involves “the growth of larger crystals from those of smaller size which have a higher solubility than the larger ones”.40 Recently, Zeng and co-workers utilized this well-established phenomenon for the fabrication of a range of different hollow structures, such as TiO2,13 Cu2O,14 and ZnS.15 Herein, in our reaction system, Ce3þ can be oxidized by H2O2 to form Ce4þ at room temperature with vigorous stirring, which can be easily observed by the color changes (from colorless to yellow) of the solution. Then in the hydrothermal environment, formamide can hydrolyze to generate NH3 and formic acid, which can be confirmed by the odor of NH3 detected from the solution of formamide through heat treatment, followed by the hydrolysis of Ce4þ to form CeO2 in the NH3-rich surrounding. Once the newly CeO2 nanocrystals formed, they would aggregate together to form solid spheres driven by the minimization of the total energy of the system. These kinds of uniform solid spheres can be obtained in large quantities by limiting the reaction time to 1 h. At this stage, crystallites located in the exterior of the spheres with larger size are packed much looser than the interior. Previous studies have demonstrated that Ostwald ripening will happen during this stage because smaller, less crystallized, or less dense particles in an aggregate will be dissolved gradually, whereas larger, better crystallized, or denser particles in the same aggregate are growing.13-18 Therefore, the inner small crystallites of a sphere are undergoing mass relocation through dissolving and regrowing, whereas the outer larger ones serve as new starting growth sites. With the continuous mass transportation from the inner core to the outermost surface of the same sphere, the hollow interior occurred. In addition, XRD measurement was further employed to characterize the intermediates at different reaction time intervals. The XRD patterns (see Figure S3 in the Supporting Information) show that the crystallinity of the products is increased gradually with the reaction time prolonged, which also validates the above proposition that Ostwald ripening is the reasonable mechanism in the present system. In general, the reaction environment has a great effect on the crystal phase and morphology of the final products. Our experimental results indicated that H2O2 do play an important role on the formation of such novel structured hollow spheres. Several detailed experiments with different amounts of H2O2 such as 0, 0.05, 0.5, and 1.0 mL were performed while keeping other parameters constant. As shown in Figure 5a, when the reaction was carried out in the absence of H2O2, only loosely packed cubic particles with edge length of 100-150 nm were obtained as the sole product. When a small amount of H2O2 (0.05 mL) was introduced to the reaction solution, similar hollow spheres assembled by nano-octahedra with edge length of ca. 30 nm could be distinguished in the sample (Figure 5b). Compared with the hollow spheres in Figure 3, the interior

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Figure 7. Percentage conversion versus temperature plots for the oxidation of CO over (a) CeO2 hollow spheres assembled by nanooctahedra; (b) commercial CeO2 powders.

Figure 5. TEM images of CeO2 particles obtained with different H2O2 amounts: (a) 0, (b) 0.05, (c) 0.5, and (d) 1.0 mL.

Figure 6. (a) TEM image and (b) SEM image of CeO2 hollow nanospheres obtained without the assistance of PVP. The inset in panel b is the magnified image of rectangled part in panel b.

cavity of the spheres is much smaller due to the larger size of the building blocks. When the H2O2 amount was increased to 0.5 mL, the product consisted entirely of well-defined discrete hollow spheres with diameters in the range of 120-150 nm, and the hollow nature was observed by a strong contrast between the dark edge and pale center in the TEM image (Figure 5c). It is worth noting here that the shells of the above hollow spheres are relative smooth, which is different from the ones constructed with nano-octahedra, indicating the shells are consisted of numerous smaller nanocrystals. When the H2O2 amount was further increased to 1.0 mL, hollow structures were not further observed, and the spheres with a solid core appeared (Figure 5d). The above experimental results reveal that the amount of H2O2 markedly affects the morphologies. This can be explained that the nucleation rate of the primary nanocrystals is likely to be controlled by the amount of H2O2 because of its oxidative role in the formation of CeO2 discussed above. Under low concentration of H2O2, the primary nanocrystals would dissolve to form nano-octahedra dominated by eight {111} plans as soon as the nanocrystals form due to low rate of nucleation. Note that when a particle grows, facets tend to the low-energy facets plans to minimize the surface energy. Previous studies revealed that ceria 0D nanoparticles prepared by conventional methods predominantly expose their {111} planes because of its lowenergy configuration.41,42 However, the formation of primary nanocrystals become dominant due to high rate of nucleation under high concentration of H2O2, thus the secondary spherical structures assembled by smaller nanocrystals can be

obtained. Only solid spheres were obtained under high concentration of H2O2, which can be explained that Ostwald ripening in the same sphere is not allowed for the primary nanocrystals in the spheres were densely packed. The influences of PVP on the morphologies of CeO2 were further investigated. When the reaction was carried out without the aid of PVP, nanospheres with a diameter in the range of 50-70 nm were obtained. The hollow nature of such nanospheres could be observed by both TEM and SEM images in Figure 6. It is obvious that such hollow nanospheres are severely aggregated and have a smaller size compared to the nano-octahedron constructed ones. It can be explained that PVP may play an important role on the nucleation of primary nanocrystals. However, the exact role of PVP in the nucleation stage of nanocrystals needs to be further investigated. To demonstrate the potential application of the prepared novel structured CeO2 hollow spheres in catalysis, we evaluated the catalytic activity of the hollow spheres to that of commercial CeO2 by studying the CO oxidation as a test reaction. In the course of the reaction, Ce(IV) is attacked and reduced by CO, and an oxygen vacancy can be created. Then molecular oxygen reacts with the surface to regenerate a surface oxygen atom. Highly reactive atomic oxygen is formed because of the dissociation of molecular oxygen at the vacancy site. Finally, CO reacts with the highly reactive atomic oxygen to form CO2.22,43 Figure 7 shows the activity profiles of both the as-prepared sample and the commercial CeO2 sample. It is clear that the novel structured hollow spheres demonstrate much higher activity than commercial CeO2. For example, at 280 °C, the percentage CO conversion is 50% over CeO2 hollow spheres and only 4% over bulk counterpart. After the catalysis, the shapes of the catalysts nearly kept unchanged, which can be observed from both the TEM and SEM images (see Figure S4 in the Supporting Information). The excellent thermal stability of the hollow spheres endows its recycling performance, which is critical for practical applications. 4. Conclusion In summary, monodisperse CeO2 hollow spheres composed of nano-octahedra were synthesized by a one -step solution approach. Based on both the formation process of hollow interiors and the size evolution of the octahedral building blocks, Ostwald ripening is proposed as the reasonable mechanism in the present system. The catalytic properties of the CeO2 hollow spheres were investigated, which demonstrates the as-prepared products are promising catalyst in CO oxidation. This method is very simple, mild and economical, and it

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may be general to synthesize other metal oxide with a hollow interior. Acknowledgment. This work was supported by the Natural Science Foundation of China (Grant 20876089) and Key Technologies R&D Programme of China (Grant 2007BAD87B05). The authors thank Prof. Ding Yi for the catalytic measurement. Supporting Information Available: SEM image of the products obtained under 30 min ultrasonication treatment (Figure S1), N2 adsortion-desorption isotherm and the corresponding BJH pore size distribution curve of the as-prepared samples and commercial CeO2 powders (Figure S2), XRD patterns of the products obtained at different reaction times (Figure S3), TEM and SEM images of the products obtained after the catalytic measurement (Figure S4) (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

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