Synthesis of CeO2 nanoparticles by rapid thermal decomposition ...

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Synthesis of CeO2 nanoparticles by rapid thermal decomposition using microwave heating H. Miyazaki*1, J.-I. Kato2, N. Sakamoto2, N. Wakiya2, T. Ota3 and H. Suzuki4 This article describes the synthesis of a CeO2 fine powder by thermal decomposition of cerium oxalate and cerium nitrate powders using a microwave furnace. Plate-like crystalline CeO2 particles were obtained using the cerium oxalate precursor powder. Using the cerium nitrate powder as a precursor, spherical crystalline CeO2 particles were obtained with a primary particle diameter of 30 nm and secondary particle size of 550 nm. Keywords: Ceria, Microwave, Fine particle

Introduction Cerium oxide CeO2 is used in various applications, such as catalyst supports,1,2 electrolytes for solid oxide fuel cells,3 oxygen sensors4 and chemical polishing materials.5 Physical properties of ceria powders (particle size, morphology and surface nature) serve an important role in the technologies described above. Using a CeO2 powder for chemical–mechanical polishing (CMP) materials, a CeO2 powder requires a primary particle size of less than tens of nanometre order and secondary particle size of less than hundreds of nanometre order.6 To control the forms of the resulting CeO2 powder to improve performance, it is necessary to design fabrication methods, conditions, after treatments and so on. Microwave heating presents a promising technique to control shapes and forms of powder specimens because this heating method can provide a high heating rate and control the heat treatment time.7,8 Therefore, grain growth and sintering of a specimen powder should be controlled. The aim of this investigation is the fabrication of CeO2 fine particles with a primary particle size of less than tens of nanometre order and secondary particle size of less than hundreds of nanometre order and their application for CMP materials. In this investigation, CeO2 powder was synthesised using thermal decomposition cerium oxalate and cerium nitrate with a microwave irradiation. The particle size, grain growth and particle morphologies of the resultant

CeO2 are discussed in terms of the microwave irradiation condition and the precursor sources.

Experimental Cerium oxalate [Ce2(C2O4)3.9H2O] and cerium nitrate [Ce(NO3)3.6H2O) powders (Wako Pure Chemical Industries, Ltd) were used as starting materials; CuO powder (Wako) was used as a microwave absorbing

1 Schematic illustration of experimental apparatus

1

Department of Material Science, Interdisciplinary Faculty of Science and Engineering, Shimane University, 1060, Nishikawatsu, Matsue, Shimane 690 8504, Japan 2 Department of Materials Science, Faculty of Engineering, Shizuoka University, 3-5-1, Johoku, Hamamatsu, Shizuoka 432 8561, Japan 3 Ceramics Research Laboratory, Nagoya Institute of Technology, 10-6-29, Asahigaoka, Tajimi, Gifu 507 0071, Japan 4 Graduate School of Science and Technology, Shizuoka University, 3-5-1, Johoku, Hamamatsu, Shizuoka 432 8561, Japan *Corresponding author, email [email protected]

ß 2010 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 28 May 2009; accepted 28 August 2009 DOI 10.1179/174367509X12503626841631

2 Proﬁle of crucible temperature depending on time and input power

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Synthesis of CeO 2 nanoparticles by rapid thermal decomposition

4 Mean particle size of resultant specimens of heat treated cerium oxalate powder with various irradiation powers

microwave furnace was measured using a radiation thermometer. Figure 2 presents a profile of the crucible temperature depending on the time and input power. The resultant powder’s structure was characterised using X-ray diffraction (XRD, RINT 2200; Rigaku Corp.) with Cu Ka radiation. The resultant powder’s microstructure was observed using scanning electron microscopy (SEM, JSM-5600; JEOL). For the SEM analysis, the sample was put on the sample holder using a duplicated tape, and a gold thin film with the thickness of 10 nm was deposited by a vacuum evaporation. The resultant powder’s microstructure was also observed using transmission electron microscopy (TEM, JEM1010; JEOL) at room temperature, and the sample powders were held on a collodion film.

Results and discussion CeO2 particle synthesis by thermal decomposition of Ce2(C2O4)3.9H2O powder

3 X-ray diffraction patterns for resultant specimens heat treated with cerium oxalate powder with irradiation power of a 200 W, b 500 W and c 1 kW

material. Figure 1 presents a schematic illustration of the experimental apparatus: the CuO powder was placed outside of a crucible. Then, the precursor powder was heated in air with a microwave furnace (RE-WB30; Sharp Corp.), and the input powers were maintained at 200, 500 and 1000 W. The experimental conditions are shown in Table 1. The temperature of the crucible in the

Table 1 Experimental condition Input power, W Heating time, s Time interval, s Total number of samples

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First, Ce(C2O4)3.6H2O powder was heat treated using a microwave furnace with various input powers. Figure 3 portrays XRD patterns for the resultant specimens with input power of 200 W, input power of 500 W and input power of 1 kW. Here, precursor Ce(NO3)3.6H2O phases were observed respectively for specimens that were heat treated for ,360 s with input power of 200 W, ,180 s with an input power of 500 W and ,90 s with an input power of 1 kW. The XRD results showed that CeO2 powder was synthesised using microwave heat treatment under the conditions described in Fig. 3. The mean particle sizes of the resultant CeO2 powder were calculated using the Scherrer’s equation with obtained XRD peaks of (111), (200) and (220). Figure 4 presents particle sizes of the resultant specimens. The particle sizes of all resultant CeO2 specimens increased concomitantly with increased heat treatment times. The microstructure and particle size of the resultant specimens were observed using SEM measurements. For microstructure observations, the specimens were heat treated, yielding single phase CeO2 particles, with the minimum heat treatment time. Figure 5 shows SEM images of the resultant CeO2 specimens. The

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5 Images (SEM) of resultant specimens of heat treated cerium oxalate powder with irradiation powers of a precursor powder, b 200 W, c 500 W and d 1 kW

microstructure of a precursor specimen is presented in Fig. 5a. All resultant specimens formed rod- or platelike shapes; secondary particles were formed with aggregated particles. It is assumed that the shape of the resulting specimens is dependent on the precursor material shape (Fig. 5a).

CeO2 particle synthesis by thermal decomposition of Ce(NO3)3.6H2O powder The Ce(NO3)3.6H2O powder was heat treated with a microwave furnace under various input powers. Figure 6 shows XRD patterns for the resultant specimens and input power of 200 W, input power of 500 W and input power of 1 kW. Here, a precursor Ce(NO3)3.6H2O phase was observed respectively for specimens heat treated for ,360 s with input power of 200 W, ,120 s with an input power of 500 W and ,90 s with input power of 1 kW. Based on the XRD results, CeO2 powder was synthesised using microwave heat treatment under the conditions described for Fig. 6. The particle sizes of the resultant CeO2 powder were calculated using the Scherrer’s equation with the obtained XRD peaks of (111), (200) and (220). Figure 7 portrays the particle sizes of the resultant specimens. The particle size of the specimen was y30 nm, which was respectively heat treated ,600 s with an input power of 200 W, ,270 s with an input power of 500 W and ,120 s with input power of 1 kW. For input powers of 500 W and 1 kW, the particle sizes respectively increased concomitantly with increasing heat treatment times that were longer than 270 and 120 s. From Fig. 2, this inflection point was y600uC.

Therefore, it is speculated that the grain growth of a CeO2 particle occurred .600uC. For specimens using cerium oxalate as the precursor, the particle size of resulting CeO2 increased proportionally depending on the heat treatment time. The microstructure and particle size of the resultant specimens were measured using SEM and TEM. For observation of the microstructure, the specimens were heat treated, yielding a single phase CeO2 particle with the minimum heat treatment time. Figure 8 shows SEM images of the resultant specimens of CeO2, where the microstructure of a precursor specimen is presented in Fig. 8a. For all resultant specimens, secondary particles were formed by fine primary particles. The secondary particle size of all the specimens was y50 nm or larger; a few huge particles were observed. Figure 9 shows TEM images of the resultant specimens of CeO2. The CeO2 primary particle that was heat treated at 200 W for 360 s was spheroidal and 30 nm; the result agreed well with the particle size estimated using the results of XRD. The specimens used with other heat treatment conditions yielded similar results.

Conclusion Using cerium oxalate and cerium nitrate powders as precursor materials, CeO2 fine particles with a single phase were fabricated using microwave heating. The shape and size of the resultant particle depend on the power, the microwave irradiation time and precursor materials. The precursor material was the most effective factor to control the shape and size of resultant specimens.


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7 Mean particle size of resultant specimens of heat treated cerium nitrate powder with various irradiation powers

6 X-ray diffraction patterns for resultant specimens of heat treated cerium nitrate powder with irradiation powers of a 200 W, b 500 W and c 1 kW

8 Images (SEM) of resultant specimens of heat treated cerium nitrate powder with irradiation powers of a precursor powder, b 200 W, c 500 W and d 1 kW

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9 Images (TEM) images of resultant specimens heat treated cerium nitrate powder with irradiation powers of a 200 W, b 500 W and c 1 kW


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