Template-free Synthesis of Single-Crystalline-like CeO2 Hollow

Oct 10, 2008 - Template-free Synthesis of Single-Crystalline-like CeO2 Hollow Nanocubes. Guozhu Chen ... E-mail: [email protected]. ... Novel single-crys...
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CRYSTAL GROWTH & DESIGN

Template-free Synthesis of Single-Crystalline-like CeO2 Hollow Nanocubes Guozhu Chen, Caixia Xu, Xinyu Song, Shuling Xu, Yi Ding, and Sixiu Sun* Key Laboratory of Colloid and Interface Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shandong UniVersity, Jinan 250100, China

2008 VOL. 8, NO. 12 4449–4453

ReceiVed March 18, 2008; ReVised Manuscript ReceiVed August 10, 2008

ABSTRACT: Novel single-crystalline-like CeO2 hollow nanocubes were synthesized through a solvothermal method using peroxyacetic acid (PAA) as the oxidant in the absence of any templates. The structure and morphology of CeO2 hollow nanocubes were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The hollow nanocubes have an average edge length of 120 nm and shell thickness of 30 nm. TEM analyses demonstrate the formation of CeO2 hollow nanocubes is ascribed to the combination of oriented attachment and Ostwald ripening. At the beginning, a well-defined cube-like geometrical structure is formed through oriented attachment of small nanocrystallites. Then, solid evacuation in the central part via Ostwald ripening leads to single-crystalline-like hollow nanocubes. It was found that both peroxide (CH3COOOH or H2O2) and solution acidity are critical factors in determining the final morphology of the products. Compared to bulky CeO2 powders, the prepared CeO2 hollow nanocubes exhibited a higher catalytic activity toward CO oxidation. Introduction Nanomaterials with a hollow structure have aroused intense interest in the past few years owing to their high specific surface area, low density and well permeation, and widespread applications in many fields.1-3 So far, two types of template-assisted approaches have been developed to fabricate such hollow nanostructures. Either soft templates of amphiphilic molecule assemblies or hard inorganic templates have been employed.4-7 For example, Zhao et al. prepared hollow SnO2 spheres in microreactor systems generated by self-assembly of sodium dodecylbenzenesulfonate (SDBS).8 Yan et al. reported the fabrication of hollow ZnS and ZnO architectures by employing Zn5(CO3)2(OH)6 microspheres as the sacrificial template through a spontaneous ion replacement reaction.9 Even though templateassisted approaches are efficient in fabricating hollow nanostructures, removing templates is tedious: calcination at elevated temperature or wet chemical etching for hard inorganic templates and repeated washing by solvents for soft templates are necessary. Recently, the utilization of some physical phenomenon, such as the Kirkendall effect,10,11 Ostwald ripening,12,13 and the oriented attachment process,14,15 provides options for the fabrication of hollow structures. Among them, both Ostwald ripening and oriented attachment have been extensively studied. Ostwald ripening is commonly referred to as the solution progress where larger grains grow up at the expense of smaller ones with relatively higher solubility.12 Oriented attachment proposed by Banfield involves spontaneous self-organization of adjacent particles to share a common crystallographic orientation and join together at a planar interface.14 They are two primary processes on the nanocrystal growth in solution following the fast nucleation of primary particles. In general, oriented aggregation and Ostwald ripening are regarded as two independent and competitive processes. However, they could coexist simultaneously under certain conditions. Zeng’s group has synthesized Cu2O hollow nanocubes from self-assembly of reductive CuO * To whom correspondence should be addressed. E-mail: [email protected]. Tel: 86-531-88365432. Fax: 86-531-88564464.

nanocrystals through combining Ostwald ripening and oriented attachment process.16 Gao et al. ascribed the formation of Fe3O4 hollow spheres to the cooperation of oriented aggregation and Ostwald ripening.17 Therefore, the combination of separate physical phenomenon listed above would be an interesting approach for synthesizing hollow structures. Ceria, one of the most important rare earth metal oxides, has been extensively studied and applied in the field of catalysis, UV blocking, fuel cells, and oxygen sensoring.18-20 Although much attention has been paid to the preparation of nanostructured CeO2, to the best of our knowledge, less effort has been made on the fabrication of CeO2 hollow structure.21-26 CeO2-ZrO2 solid solution nanocages with controllable structures via Kirkendall effect were synthesized by Li groups.27 Sun et al. fabricated CeO2 hollow spheres using carbonaceous polysaccharide microspheres as templates.28 Chang-Chien et al. utilized the mesoporous carbon hollow spheres as a solid template to prepare CeO2 hollow spheres.29 All of these hollow structures with polycrystalline character were prepared under the assistance of templates. In this work, single-crystalline-like CeO2 hollow nanocubes have been fabricated through a template-free approach. The structure and morphology of the prepared hollow nanocubes were characterized by using XRD, TEM, SEM, and XPS. TEM study verifies oriented attachment followed by Ostwald ripening is responsible for the formation of single-crystalline-like CeO2 hollow nanocubes. Preliminary catalysis testing demonstrates the single-crystalline-like CeO2 hollow nanocubes are promising catalyst in CO oxidation. Experimental Section Cerious chloride (CeCl3 · 7H2O, g99.0%) was purchased from Sinopharm Chemical Reagent Co. Ltd. Peroxyacetic acid (PAA) was obtained from the Tianjin, Hedong, Hongyan Chemical Reagent Factory. Ceria powder (CeO2, A.R. grade), hydrogen peroxide (H2O2, 30%) and nitric acid (HNO3, 65%), hydrochloric acid (HCl, 36%), sulfuric acid (H2SO4, 98%) were purchased from Shanghai Chemical Reagent Company and used as received. In a typical synthesis of CeO2 hollow nanocubes, CeCl3 · 7H2O (0.148 g) was dissolved in 14 mL of anhydrous ethanol under vigorous magnetic stirring. Two milliters of PAA solution was then added into

10.1021/cg800288x CCC: $40.75  2008 American Chemical Society Published on Web 10/10/2008

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Figure 1. XRD pattern of the single-crystalline-like CeO2 hollow nanocubes.

Figure 2. XPS wide spectrum (a) and Ce 3d spectrum (b) of the CeO2 hollow nanocubes.

the ethanol solution. The obtained slurry with a light grape wine color was transferred into a Teflon-lined steel autoclave and heated for 9 h at 160 °C in an electric oven. After the autoclave was cooled to room temperature, white-brown products were collected and washed with distilled water 4 times until the color of products changed into brown. Finally, the products were thoroughly washed with absolute ethanol, and dried in a vacuum for 24 h at 60 °C. The as-prepared samples were characterized by XRD on a Japan Rigaku D/Max-γA rotating anode X-ray diffractometer equipped with graphite-monochromatized Cu KR radiation (λ ) 1.54178 Å) at a scanning rate of 0.02° s-1 in the 2θ range from 10 to 80°. The morphology and structure of as-synthesized hollow CeO2 nanocubes were studied by FE-SEM (JSM-6700F), TEM (JEOL 6300, 100 kV), and high-resolution transmission electron microscopy (HRTEM, JEM2100, 200 kV). XPS spectra were recorded by a PHI 5300 X-ray photoelectron spectrometer with Al KR radiation. Catalytic activity was measured using a continuous flow fixed-bed microreactor at atmospheric pressure. In a typical experiment, the system was first purged with high purity N2 gas and then a gas mixture of CO/O2/N2 (1:10:89) was introduced into the reactor which contained 20 mg samples. Gas samples were 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 Chromatograph (GC14C).

Results and Discussion Normally, PAA is prepared from acetic acid and H2O2 in the presence of acidic catalyst, typically H2SO4 (the content of H2SO4: m(H2SO4) ) [m(CH3COOH) + m(H2O2)] × 3%). The reaction could be described by the following equation: H+

CH3COOH + H2O2 y\z CH3COOOH + H2O

Figure 3. TEM and FE-SEM images of the synthesized CeO2 nanocubes: (a) TEM image; (b) FE-SEM images of a broken nanocube; and (c) a typical nanocube with an approximately square morphology.

(1)

Therefore, certain amounts of H2O2 and acidic catalyst (H2SO4) always remained in PAA solutions as residues.30 Thus, upon adding PAA, a small quantity of white Ce2(SO4)3 · nH2O precipitated quickly, which was confirmed by XRD analysis (Figure S1, Supporting Information). After a few minutes, light grape wine slurry was observed. This is because both remaining H2O2 and produced CH3COOOH peroxides are strong oxidizing agents to oxidize Ce3+ to Ce4+, and the oxidized Ce4+ could form a complex ion with OOH- easily.31 Finally, the Ce4+ from complex ions is hydrolyzed to form cerium oxide (CeO2) by forced hydrolysis at elevated temperature. The remaining Ce2(SO4)3 · nH2O precipitate could be easily washed off using water because it is diffluent in water. The purity and crystallinity of the as-prepared singlecrystalline-like CeO2 hollow nanocubes were examined using XRD. The strong and sharp diffraction peaks indicate the good crystallization of the sample. All peaks in the XRD pattern (Figure 1) can be indexed as the face-centered cubic pure phase

[space group: Fm3jm (225)] of ceria (JCPDS no. 81-0792). No additional peaks in the XRD were observed, revealing the high purity of the prepared CeO2 hollow nanocubes. Figure 2 shows the XPS spectra of as-prepared products. Two strong peaks at around 883.1 and 899.2 eV in the XPS spectrum are assigned to Ce 3d5/2 and the peak at 917.6 eV is assigned to Ce 3d3/2 for the Ce4+ state. Thus, it can be concluded that the main valence of cerium in the hollow nanocubes is +4.23 Figure 3a shows TEM images of the CeO2 hollow structure featured by the obvious contrast between the dark edge and pale center. It is clear that most CeO2 particles have an approximative cube structure with a mean edge length of 120 nm and are composed of compactly arranged nanocrystals. The hollow structure and cube-like shape of CeO2 could be further confirmed by FE-SEM image of a broken CeO2 nanocube (Figure 3b). The shell thickness is 30 nm as indicated by the white arrows in Figure 3b. The high-resolution SEM image of a typical and approximately square CeO2 nanocube (Figure 3c) shows that its shell is comprised of nanoparticles less than 10

Single-Crystalline-like CeO2 Hollow Nanocubes

Figure 4. TEM images and SAED pattern of single CeO2 nanocube: (a) TEM image of a single CeO2 nanocube; (b) HRTEM image from the interior space corresponding to the square part in (a); (c) HRTEM image from the shell corresponding to rectangle part in (a); (d) SAED pattern of the hollow nanocubes.

nm, and the corners (angles included by two surfaces) of the cubes are not strictly right, which is in good agreement with the TEM results. Figure 4a shows the TEM image of a single CeO2 nanocube, and Figure 4b,c shows the HRTEM images of the interior space and shell corresponding to the square and rectangle parts in Figure 4a, respectively. Of particular interest is that the nanocrystals are arranged so well that they assembled into a single crystal by sharing identical lattices. As shown in Figure 4b,c, lattice plane spacing calculated from the HRTEM images is ca. 0.32 nm hkl (111) and is typical for cubic CeO2 structures. The diffraction pattern exhibited a remarkable single-crystal feature as shown in Figure 4d, in which the spots were assigned to (133j) and (33j1) of cubic CeO2, respectively. All these verified the single-crystalline-like feature of the prepared CeO2 hollow nanocubes. To study the formation mechanism of CeO2 hollow nanocubes, the evolution process was examined thoroughly by TEM (Figure 5) and XRD (Figure S2, Supporting Information). It is obvious that, at the early reaction stage, there are plenty of random aggregated nanocrystallites (Figure 5a) confirmed by the electron diffraction pattern selected from the circle section (inset in Figure 5a), which reveals its polycrystalline nature. When the reaction time was prolonged to 4 h, the CeO2 nanocrystallites self-aggregate to form a cube-like structure, although a few smaller nanoparticles still surround the cube with edge length of about 76 nm, as shown in Figure 5b. The loosely packed crystallites were verified by plenty of intercrystallite spaces observed in these premature cubic structures. After 6 h, hollowing takes place and results in the creation of central space indicated by the color contrast in TEM images (Figure 5c), and the cube size increases to about 110 nm. The perfect hollow structure with an average edge length of 120 nm and shell thickness of 30 nm can be observed at 9 h as shown in Figure 5d. On the basis of the above results, it is believed that both oriented attachment and Ostwald ripening should be the main

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Figure 5. TEM images of particles obtained at 160 °C at different reaction times: (a) 2 h; (b) 4 h; (c) 6 h; and (d) 9 h. Inset in (a) shows the ED pattern of circle section.

formation mechanisms for the hollow nanocubes. In the first stage, initial nanoparticles are assumed to act as the molecules of Brownian motion under solvothermal conditions. So it is expected that the growth of nanoparticles via oriented attachment shares some characteristics with the collision reactions of molecules, from the point of view that both processes produce a whole entity right after the reaction. With the increase of particle size, the motion rate of the particle decreases rapidly.32 As a result, the oriented attachment process finishes so fast that a loose structure is formed. In the second stage, the Ostwald ripening is dominant with “solid-solution-solid” mass transportation. Crystallites located in the outermost surface of aggregates are larger and would grow at the expense of smaller ones inside, so the solid evacuation occurred. Gradually outward migration of crystals would result in continuing expansion of interior space within the original aggregates finally. This is consistent with the result that less crystalline or less dense particles in a colloidal aggregate will be dissolved gradually, while larger, better crystallized, or denser particles in the same aggregate are growing.33 To explore the key factors for the hollow nanocubes formation, H2O2 instead of peroxyacetic acid was used because a certain amount of H2O2 is retained in the peroxyacetic acid and both of them are strong oxidants. Although investigations into the preparation of CeO2 have demonstrated that H2O2 has a significant influence on the size and morphology of precipitates formed, there is still no report on hollow structure.31,34 When 2 mL of H2O2 instead of peroxyacetic acid was used, the resulting product was mainly small crystallites and no hollow cubes observed as shown in Figure 6a. In contrast, hollow particles appeared (Figure 6b-d) when the CeCl3 ethanol solution was primarily acidified by concentrated H2SO4, HCl, or HNO3 before adding H2O2. Rapid precipitation upon addition of H2O2 was prevented when the solution was acidified by concentrated acid.35 Thus, the remained H2SO4 in PAA affords an acidic environment in the typical synthesis. It is noted that the composition of H2SO4 also influences the morphology of the products (Figure S3, Supporting Information). Although the detailed mechanism is not clear at this stage, both the peroxide

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to the adsorption and desorption of gas molecules on the surface of catalyst. For CO oxidation, CO reacts with the catalyst surface, forming an oxygen vacancy, which is then replenished by gas-phase oxygen thereby completing the cycle by the formation and desorption of CO2. The interconnected hollow structure in our catalysts enables better contact with the gas molecule owing to the existence of interior spaces and penetrable shell. Therefore, they reasonably exhibit better performance. The stability and recycling performance of CeO2 catalyst are important factors for practical applications. To study the stability of CeO2 hollow nanocubes, the used CeO2 samples were collected and analyzed by TEM. The hollow structure is not collapsed at high temperature (300 °C) and is well retained, as shown in the inset of Figure 7. The catalytic stability of CeO2 hollow nanocubes was also evaluated by adding a second cycle of catalytic reaction after the reactor cooled down to room temperature. It was found that the catalytic efficiency decreased very slightly (profile b in Figure 7), which demonstrates its excellent stability and recycling performance. Conclusions Figure 6. TEM images of particles obtained using H2O2 instead of peroxyacetic acid, with the CeCl3 ethanol solution primarily acidified by a certain amount of concentrated acid: (a) without acid; (b) with H2SO4; (c) with HCl; and (d) with HNO3. Inset in (a) shows the ED pattern.

In summary, single-crystalline-like CeO2 hollow nanocubes with excellent catalytic performance for CO oxidation have been successfully synthesized in the absence of any templates. On the basis of the morphology study on the evolution of CeO2 hollow nanocubes by TEM, the formation mechanism is proposed as oriented attachment followed by Ostwald ripening. The combination of such two processes leading to the formation of CeO2 hollow nanocubes is expected to help us understand the crystal growth, and provides an alternative way to synthesize other hollow nanomaterials with controlled morphology and dimensionality. Acknowledgment. This work was supported by the National 863 (2006AA03Z222), 973 (2005CB623601, 2007CB936602) Program Projects of China, the Natural Science Foundation of Shandong Province (2007ZRB01117, 2006BS04018). Y.D. is a Tai-Shan Scholar supported by the SEM-NCET and SRFROCS Programs.

Figure 7. CO conversion profiles in the presence of CeO2: (a) the freshly prepared CeO2 hollow, nanocubes; (b) the CeO2 hollow nanocubes after the first run catalysis evaluation; (c) the commercial CeO2 powder. Insets are the TEM images of (a) freshly prepared CeO2 hollow nanocubes and (b) CeO2 hollow nanocubes after the first run catalysis evaluation.

(peroxyacetic acid or H2O2) and the acidic condition played crucial roles in determining the final morphology of the products. The CO removal is of fundamental importance in air purification, fuel cell, closed-cycle CO2 laser, and many other areas. Among the various reducible oxides, CeO2 with high oxygen storage capacity had been widely used as a three-way catalyst in automobile exhaust systems. Here, the catalytic activity of CeO2 hollow nanocubes toward CO oxidation was studied by using a continuous flow fixed-bed microreactor. Figure 7 shows the activity profiles of as-prepared CeO2 hollow nanocubes along with that of a commercial CeO2 powders for comparison. As shown in Figure 7, the CO conversion increased with increasing reaction temperature for both samples. But, much higher conversion was measured in the presence of CeO2 hollow nanocubes compared with that of CeO2 powders. As an example, at 270 °C, the CO conversation of CeO2 hollow nanocubes is 56% and almost 3.5 times higher than that of the CeO2 powder. In general, the catalytic process is mainly related

Supporting Information Available: XRD patterns of the products obtained in the typical synthesis at 160 °C for 3 h (Figure S1) and at different reaction times (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org.

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