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Synthesis and characterization of nanosized ceria powders by microwave–hydrothermal method
Materials Research Bulletin 41 (2006) 38–44 www.elsevier.com/locate/matresbu
Synthesis and characterization of nanosized ceria powders by microwave–hydrothermal method A. Bonamartini Corradi a, F. Bondioli a,*, A.M. Ferrari b, T. Manfredini a a
Dipartimento di Ingegneria dei Materiali e dell’Ambiente, Universita` di Modena e Reggio Emilia, Via Vignolese 905/a, 41100 Modena, Italy b Dipartimento di Scienza e Metodi dell’Ingegneria, Via Allegri 5, 42100 Reggio Emilia, Italy Received 29 November 2004; received in revised form 8 July 2005; accepted 29 July 2005
Available online 13 September 2005 Abstract Nanocrystalline ceria powders (CeO2) have been prepared by adding NaOH to a cerium ammonium nitrate aqueous solution under microwave–hydrothermal conditions. In particular the effect of the synthesis conditions (time, pressure and concentration of both the precursor and the precipitant agent solutions) on the physical properties of the crystals have been evaluated. Microwave– hydrothermal treatment of 5 min at 13.4 atm allows to obtain almost crystallized powders (amorphous phase 4%) as underlined by Rietveld-reference intensity ratio (RIR) results. # 2005 Elsevier Ltd. All rights reserved. Keywords: A. Nanostructures; A. Oxides; B. Chemical synthesis; C. X-ray diffraction
1. Introduction Nanoscopic materials consisting of particles uniform in size and shape have been attracting increasing interest for their unique chemical and physical properties and potential technological applications. Indeed, in order to reproducibly manufacture various materials of controlled properties (optical, magnetic, electric, structural, etc.), it is essential to use well-defined dispersed powders as precursors. Maximum control of a ceramic process begins with the starting powder and powder preparation. Consequently numerous unconventional preparation methods, liable to lead to suitable powders have been extensively investigated. Among the various methods, the hydrothermal crystallization [1–3] is an interesting process to directly prepare submicrometer- and nanometer-sized crystalline powders with reduced contamination and low synthesis temperature. A recent innovation to the hydrothermal method developed by Komarneni et al. [4–6] involves the introduction of microwave during the hydrothermal synthesis to increase the kinetics of crystallization by one to two orders of magnitude and sometime to lead to novel phases. Recently, we have used microwave–hydrothermal synthesis technique to synthesize metastable tetragonal zirconia and homogeneous praseodymia-stabilized tetragonal zirconia powders [7–9] with controlled chemical and physical characteristics. However, the microwave-assisted hydrothermal (MH) route is not yet explored for the synthesis of cerium oxide. Therefore, the purpose of the present work is to report the synthesis of ultrafine CeO2 powders under microwave-assisted hydrothermal route. * Corresponding author. Tel.: +39 059 2056242; fax: +39 059 2056243. E-mail address: [email protected] (F. Bondioli). 0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2005.07.044
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Cerium(IV) oxide (CeO2), with a cubic fluorite-type structure [10] is an important material because of its application as an oxygen ion conductor in solid oxide fuel cells [11,12] and oxygen monitors [13] but also as catalytic support [14] of automotive exhaust system because of its high oxygen ion conductivity. This oxide is difficult to sinter to high density at temperatures <1500 8C in air, without additives. In addition, pure CeO2 undergoes reduction at high temperatures. Accordingly, preparation of ultrafine CeO2 powders without agglomeration has been intensively investigated. Several techniques that include hydrothermal synthesis [15,16], urea-based homogeneous precipitation [17,18], coprecipitation [19], flux method [20,21] and mechanical mixing [22] have been developed for the production of ceria or cation-doped ceria particles. As a low-temperature and wet-chemical technique, hydrothermal methods offer the possibility for the synthesis of high-purity, homogeneous and ultrafine powders. In particular, the use of microwave as heating resource could offer many benefits in the form of cost saving through the reduction in processing time and energy input and may result in improved yields of target compounds. The effects of various synthesis parameters, such as concentration of Na and Ce(IV) ions in the starting solutions, reaction time and pressure on powder properties, such as degree of crystallization, crystallite size, particle size distribution and degree of agglomeration have been investigated. 2. Experimental procedure 2.1. Powder synthesis The microwave-assisted hydrothermal synthesis of CeO2 powders has been conducted using various concentrations (from 0.1 to 2.5 M) of cerium(IV) ammonium nitrate (Ce(NH4)2(NO3)6) aqueous solutions. Precursor powders have been prepared by coprecipitation with NaOH (from 1 to 10 M) under pH 9 in Teflon vessels. The precursors have been hydrothermally treated in a microwave digestion system (Model MDS-2000, CVEM Corp.). This system uses 2.45 GHz microwaves and is controlled by pressure. It can attain a maximum pressure of 13.4 atm, which is equivalent to 194 8C, based on steam tables. The reaction vessels have been connected to a pressure transducer that monitors and controls the pressure during synthesis. In order to optimize the synthesis conditions on the sample 1Ce5Na (obtained starting from Ce(NH4)2(NO3)6 1 M and NaOH 5 M) MH treatments have been conducted at pressure ranging from 3 to 14 atm for soaking time ranging from 5 to 240 min. The time, pressure and power have been computer controlled. The crystallized powders obtained have been repeatedly washed with deionized water, filtered and dried. After the last washing of the synthesized powders, the supernatant has been analyzed by ICP spectroscopy (Model Liberty 200, Variant) to evaluate the presence of sodium and the efficacy of the washing step. 2.2. Characterization Crystalline phase identification has been performed using a computer-assisted conventional Bragg–Brentano diffractometer using Cu Ka monochromatic radiation (X-ray diffraction, XRD, Model PW 3710 Philips). The XRD patterns have been collected in the 208–808 2u range at room temperature. The scanning rate has been 0.0048 s1, step size 0.028. The average crystallite size has been calculated using the Sherrer’s formula from the width of the X-ray diffraction lines [23]. To determine the amorphous phase content and thus to evaluate their crystallization degree, the powders have been analyzed by the combined Rietveld-reference intensity ratio (RIR) method [24]. A 10 wt% of tungsten (Fluka) has been added to all samples as internal standard. The mixtures, ground in an agate mortar have been side loaded in an aluminum flat holder in order to minimize the preferred orientation problems. Data have been recorded in the 58–1408 2u range (step size 0.028 and 6 s counting time for each step). The phase fractions extracted by the Rietveld-RIR refinements, using GSAS software [25] have been rescaled on the basis of the absolute weight of tungsten originally added to the mixtures as an internal standard, and therefore internally renormalized. The specific surface area of dried powders has been measured by the BET method (Model Gemini 2360, Micromeritics), using nitrogen as the adsorbate. The particle size has been also calculated, using the specific surface area data by the equation f¼
6 Sr
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where f is the average diameter of a spherical particle, S the surface area of a powder and r is the density. Sample morphology has been examined by transmission electron microscopy, TEM (Model EM400, Philips). Specimens have been prepared by dispersing the as-obtained powders in distilled water and then placing a drop of suspension on a copper grid with a transparent polymer, followed by drying. Thermal behavior of the dried samples has been studied by thermogravimetric (TG) and differential thermal (DTA) analysis in air at a heating and cooling rate of 20 8C/min, using a simultaneous TG/DTA apparatus (Model STA409, Netzsch). 3. Results and discussion 3.1. Synthesis optimization The synthesis condition optimization has been conducted on sample 1Ce5Na (obtained starting from Ce(NH4)2(NO3)6 1 M and NaOH 5 M). ICP spectroscopy showed the absence of alkali ions in the washing water, indicating the efficacy of the washing step and the chemical homogeneity of the powders. As regarding the synthesis pressure, the XRD analysis of the as-prepared powders showed the same crystalline structure for all the synthesis conditions used. Peaks have been attributed to crystalline cerium oxide, CeO2, with a cubic fluorite structure (ICDD file 34–394). In particular the diffractometric ceria evolution as a function of the hydrothermal process pressure has been reported in Fig. 1 (synthesis time 5 min). The analysis of XRD patterns of these powders indicates the crystallization of ceria at as low a temperature as 50 8C and 3.5 atm. As regarding the synthesis time effect, no evident differences are observable in XRD patterns. After only 5 min heating, the amorphous-precipitated product is a fine crystalline powders (5.1 nm 0.1 nm average particle size), as determined from XRD patterns using the Sherrer’s formula; further hydrothermal treatment slightly increases the grain size of powders, e.g., 5.4 nm in 30 min, 5.7 nm in 120 min and 6.4 nm in 240 min. After these results, in order to verify the effect of concentration of Na and Ce(IV) ions the synthesis condition have been fixed in 5 min and 13.4 atm. 3.2. Powders characterization The XRD analysis of the as-prepared powders showed the same crystalline fluorite structure for all the synthesis conditions used. The lattice parameters obtained after data refinement are in good agreement with data reported in literature (a = 0.5411 nm) [26]. The average dimension of particles as obtained from XRD patterns by the Sherrer’s formula is strongly influenced by the Ce(IV) solution concentration (Fig. 2). It is obvious that the higher the concentration of the solution, the larger
Fig. 1. XRD patterns of sample 1Ce5Na obtained for 5 min at 3, 5, 7 and 14 atm.
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Fig. 2. Average crystallite dimension (determined by Scherrer’s formula) as a function of Ce(IV) and Na ions concentration (5 min, 13.4 atm).
the CeO2 crystal grew. As the average diffusion distance for the diffusing solute is short and the concentration gradient is steep in concentrated solutions, much diffusing material passes per unit time through a unit area [27]. Moreover, the average dimension of crystallites depends on the concentration of the precipitant agent solution: the higher the concentration of the Na ion, the larger the ceria crystals. These values have been confirmed by data obtained by specific surface values. The experimental data (Table 1) show as the diminution of both precipitant agent and Ce(IV) concentration allows to specific surface area values higher and thus to finer grain size. Moreover, because the average dimensions obtained by BET data elaboration (powders dimension) are comparable with that obtained by XRD peaks analysis (crystallites dimension) it can be defined that the obtained powders do not show hard-type agglomerations. The obtained results match with TEM results. In Fig. 3, the TEM microphotograph of 1Ce1Na (5 min, 13.4 atm) powder, chosen as characteristic, is reported. The image clearly shows that the obtained powders are constituted by cubic crystals, well-dispersed and homogeneously distributed. The thermogravimetric and differential thermal curves registered on samples obtained by 5 min microwave– hydrothermal treatment at 13.4 atm (Fig. 4) show an endothermic peak at 120 8C, correlated to a weight loss due to the water absorbed by crystalline ceria, and an exothermic peak around 400 8C, correlated to a weight loss, that must be considered as due to the crystallization of the residual amorphous phase. Furthermore, thermogravimetrical analysis points out that the Ce(IV) salt solution concentration does not provoke any variations in the thermal curve behavior. The comparison between the experimental weight loss at temperature <200 8C (nearly 5% for all the synthesis conditions used) and the theoretical values for water elimination permits to Table 1 Specific surface area values and powder average diameter calculated by BET data (5 min, 13.4 atm) Ce concentration (M)
NaOH (M)
BET 3 (m2/g)
d 0.1 (nm)
0.1 0.1 0.1 0.5 0.5 0.5 1.0 1.0 1.0 2.5 2.5 2.5
1 5 10 1 5 10 1 5 10 1 5 10
207 198 205 167 151 149 165 145 143 164 160 154
4.0 4.2 4.1 5.0 5.5 5.6 5.0 5.7 5.8 5.1 5.2 5.4
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Fig. 3. TEM micrograph (180,000) of 1Ce1Na sample (5 min, 13.4 atm).
Fig. 4. TG and DTA curves of the 1Ce1Na sample (5 min) chosen as representative.
Fig. 5. Rietveld refinement of sample 1Ce1Na after 5 min microwave heating treatment.
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Table 2 Phase composition (%) and standard Rietveld agreement factors obtained for the 1Ce1Na sample Phase
0 min
5 min
5 min calcination (500 8C, 1 h)
CeO2 Amorphous Total Rwp (%) Rp (%) x2
86.8 (1) 13.1 (1) 99.9 2.38 1.86 1.35
95.7 (1) 4.2 (1) 99.9 2.57 2.02 1.59
99.9 (1) – 99.9 4.22 3.12 3.98
hypothesize that powders are mainly constituted by a partially ceria-hydrated form, CeO2nH2O (n = 0.5). Thus, the microwave heating does not change the reaction pathway characteristic of traditional hydrothermal process which involves, starting from a Ce(IV) salt, the solution and the precipitation mechanism. The obtained powder is, in fact, constituted by CeO20.5H2O and a small amount of hydroxides with amorphous nature. To better evaluate the amorphous quantity in the powders and thus to evaluate the crystallinity of the powders and the yield of the microwave-assisted process, a quantitative analysis by Rietveld-RIR method (Fig. 5) have been performed on 1Ce1Na samples obtained after 0 and 5 min of microwave–hydrothermal treatments. Moreover to verify the complete crystallization of sample >500 8C, as indicated by differential thermal analysis, the 1Ce1Na sample has been also calcined at 500 8C for 1 h before to proceed to the quantitative analysis. The obtained results reported in Table 2 show as only after 5 min of microwave treatment the amorphous phase is almost absent. This value remains constant with increasing the treatment time. The refinement on the calcined sample confirm that the exothermal event in DTA curve correspond to the complete crystallization of the powders. 4. Conclusions Adopting the microwave–hydrothermal process as synthesis method it is possible to obtain, by treating the solution at 13.4 atm for only 5 min, nanometric ceria with a low degree of agglomeration. Powders are extremely fine and morphologically controlled but they still present traces of amorphous phases (4%). The characteristics of the microwave hydrothermally synthesized powders can be compared with literature data referred to conventional hydrothermal synthesis. It is consequently manifest that the used processing method permits to obtain nanometric powders in considerably shorter treatment times. In fact if the conventional hydrothermal synthesis requires to treat the solutions for times ranging from 4 to 40 h, as reported in literature by several authors [1– 3,15,16] the use of microwaves allows to prepare ceria with similar properties in only 5 min, with a decrease of the reaction time of about 50 times. This technique is hence extremely interesting not only for the processing time and temperature used, but also for the possibility to achieve powders with controlled properties even starting from sufficiently concentrated solutions. Despite the literature indications [17,28] regarding nanometric powders that can be only obtained starting from highly diluted solutions of the cation (from 103 to 104 M), microwave–hydrothermal synthesis allows to use concentrated solutions so that it is possible to synthesize nearly 160 g of powders from a liter of solution. Acknowledgements Authors are grateful to Dott.ssa Erica Guigli who performed the experimental procedure and Dr. Paola Miselli for the assistance in XRD diffraction analysis. References [1] S. Somiya, T. Akiba, J. Eur. Ceram. Soc. 19 (1999) 81. [2] S. Somiya, M. Yoshimura, Z. Nakai, K. Hishinuma, T. Kumaki, in: J.A. Pask, A. Evans (Eds.), Microstructure, Plenum Press, New York, 1986, pp. 465–474. [3] T. Tsukada, S. Venigalla, A.A. Morrone, J.H. Adair, J. Am. Ceram. Soc. 82 (5) (1999) 1169.
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[4] S. Komarneni, R. Roy, Q.H. Li, Mater. Res. Bull. 27 (12) (1992) 1393. [5] S. Komarneni, in: J. Singh, S.M. Copley (Eds.), Novel Techniques in Synthesis and Processing of Advanced Materials, The Minerals, Metals and Materials Society, Warrendale, PA, 1995, pp. 103–117. [6] S. Komarneni, M.C. D’Arrigo, C. Leonelli, G.C. Pellacani, H. Katsuki, J. Am. Ceram. Soc. 88 (11) (1998) 3041. [7] F. Bondioli, C. Leonelli, C. Siligardi, G.C. Pellacani, S. Komarneni, in: M. Willert-Porada (Ed.), Advances in Microwave and Radio Frequency Processing, Springer-Verlag, Berlin, in press. [8] F. Bondioli, A.M. Ferrari, C. Leonelli, C. Siligardi, G.C. Pellacani, J. Am. Ceram. Soc. 84 (11) (2001) 2728. [9] F. Bondioli, C. Leonelli, T. Manfredini, A.M. Ferrari, M.C. Caracoche, P.C. Rivas, A.M. Rodrı´guez, J. Am. Ceram. Soc. 88 (3) (2005) 633–638. [10] N. Blumental, F.S. Brugner, J.E. Garner, J. Electrochem. Soc. 120 (1973) 1230. [11] C. Tian, S. Chan, in: R.A. Gerhardt, M.A. Alim, S.R. Taylor (Eds.), Materials Research Society Symposium Proceedings, vol. 500, Materials Research Society, Pittsburgh, PA, 1998, pp. 279–284. [12] T. Mon, H. Yamamura, J. Mater. Synth. Process. 6 (3) (1998) 175. [13] J.I. Hamagami, Y. Inda, T. Umegaki, K. Yamashita, Solid State Ionics 113–115 (1998) 235. [14] E. Bekyarova, P. Fornasiero, J. Kaspar, M. Graziani, Catal. Today 45 (1–4) (1998) 179. [15] N.C. Wu, E.W. Shi, Y.Q. Zheng, W.J. Li, J. Am. Ceram. Soc. 85 (10) (2002) 2462. [16] M. Hirano, E. Kato, J. Am. Ceram. Soc. 82 (3) (1999) 786. [17] E. Matijevic, W.P. Hsu, J. Colloid Interface Sci. 118 (1987) 506. [18] X. Chu, W. Chung, L.D. Schmidt, J. Am. Ceram. Soc. 76 (8) (1993) 2115. [19] T.J. Kirk, J. Winnick, J. Electrochem. Soc. 140 (1993) 3494. [20] F. Bondioli, A. Bonamartini Corradi, C. Leonelli, T. Manfredini, Mater. Res. Bull. 34 (14–15) (1999) 2159. [21] F. Bondioli, A. Bonamartini Corradi, T. Manfredini, C. Leonelli, R. Bertoncello, Chem. Mater. 12 (2) (2000) 324. [22] F. Bondioli, P. Veronesi, C. Leonelli, T. Manfredini, in: G. Muller (Ed.), Ceramics–Processing, Reliability, Tribology and Wear, vol. 12, Wiley-VCH, Weinheim, 1999, pp. 11–16. [23] B.D. Cullity, Elements of X-ray Diffraction, second ed., Addison-Wesley, Reading, MA, 1997. [24] R.L. Snider, Powder Diffr. 7 (1992) 186. [25] A.C. Larson, R.B. von Dreele, GSAS: General Structure Analysis System LANSCE, MS-H805, Los Alamos National Laboratory, Los Alamos, NM, 1998. [26] M. Wolcyrz, L. Kepinski, J. Solid State Chem. 99 (1992) 409. [27] M. Hirano, E. Kato, J. Am. Ceram. Soc. 79 (3) (1996) 777. [28] P.L. Chen, I.W. Chen, J. Am. Ceram. Soc. 76 (6) (1993) 1577.
Synthesis and characterization of nanosized ceria powders by microwave–hydrothermal method A. Bonamartini Corradi a, F. Bondioli a,*, A.M. Ferrari b, T. Manfredini a a
Dipartimento di Ingegneria dei Materiali e dell’Ambiente, Universita` di Modena e Reggio Emilia, Via Vignolese 905/a, 41100 Modena, Italy b Dipartimento di Scienza e Metodi dell’Ingegneria, Via Allegri 5, 42100 Reggio Emilia, Italy Received 29 November 2004; received in revised form 8 July 2005; accepted 29 July 2005
Available online 13 September 2005 Abstract Nanocrystalline ceria powders (CeO2) have been prepared by adding NaOH to a cerium ammonium nitrate aqueous solution under microwave–hydrothermal conditions. In particular the effect of the synthesis conditions (time, pressure and concentration of both the precursor and the precipitant agent solutions) on the physical properties of the crystals have been evaluated. Microwave– hydrothermal treatment of 5 min at 13.4 atm allows to obtain almost crystallized powders (amorphous phase 4%) as underlined by Rietveld-reference intensity ratio (RIR) results. # 2005 Elsevier Ltd. All rights reserved. Keywords: A. Nanostructures; A. Oxides; B. Chemical synthesis; C. X-ray diffraction
1. Introduction Nanoscopic materials consisting of particles uniform in size and shape have been attracting increasing interest for their unique chemical and physical properties and potential technological applications. Indeed, in order to reproducibly manufacture various materials of controlled properties (optical, magnetic, electric, structural, etc.), it is essential to use well-defined dispersed powders as precursors. Maximum control of a ceramic process begins with the starting powder and powder preparation. Consequently numerous unconventional preparation methods, liable to lead to suitable powders have been extensively investigated. Among the various methods, the hydrothermal crystallization [1–3] is an interesting process to directly prepare submicrometer- and nanometer-sized crystalline powders with reduced contamination and low synthesis temperature. A recent innovation to the hydrothermal method developed by Komarneni et al. [4–6] involves the introduction of microwave during the hydrothermal synthesis to increase the kinetics of crystallization by one to two orders of magnitude and sometime to lead to novel phases. Recently, we have used microwave–hydrothermal synthesis technique to synthesize metastable tetragonal zirconia and homogeneous praseodymia-stabilized tetragonal zirconia powders [7–9] with controlled chemical and physical characteristics. However, the microwave-assisted hydrothermal (MH) route is not yet explored for the synthesis of cerium oxide. Therefore, the purpose of the present work is to report the synthesis of ultrafine CeO2 powders under microwave-assisted hydrothermal route. * Corresponding author. Tel.: +39 059 2056242; fax: +39 059 2056243. E-mail address: [email protected] (F. Bondioli). 0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2005.07.044
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Cerium(IV) oxide (CeO2), with a cubic fluorite-type structure [10] is an important material because of its application as an oxygen ion conductor in solid oxide fuel cells [11,12] and oxygen monitors [13] but also as catalytic support [14] of automotive exhaust system because of its high oxygen ion conductivity. This oxide is difficult to sinter to high density at temperatures <1500 8C in air, without additives. In addition, pure CeO2 undergoes reduction at high temperatures. Accordingly, preparation of ultrafine CeO2 powders without agglomeration has been intensively investigated. Several techniques that include hydrothermal synthesis [15,16], urea-based homogeneous precipitation [17,18], coprecipitation [19], flux method [20,21] and mechanical mixing [22] have been developed for the production of ceria or cation-doped ceria particles. As a low-temperature and wet-chemical technique, hydrothermal methods offer the possibility for the synthesis of high-purity, homogeneous and ultrafine powders. In particular, the use of microwave as heating resource could offer many benefits in the form of cost saving through the reduction in processing time and energy input and may result in improved yields of target compounds. The effects of various synthesis parameters, such as concentration of Na and Ce(IV) ions in the starting solutions, reaction time and pressure on powder properties, such as degree of crystallization, crystallite size, particle size distribution and degree of agglomeration have been investigated. 2. Experimental procedure 2.1. Powder synthesis The microwave-assisted hydrothermal synthesis of CeO2 powders has been conducted using various concentrations (from 0.1 to 2.5 M) of cerium(IV) ammonium nitrate (Ce(NH4)2(NO3)6) aqueous solutions. Precursor powders have been prepared by coprecipitation with NaOH (from 1 to 10 M) under pH 9 in Teflon vessels. The precursors have been hydrothermally treated in a microwave digestion system (Model MDS-2000, CVEM Corp.). This system uses 2.45 GHz microwaves and is controlled by pressure. It can attain a maximum pressure of 13.4 atm, which is equivalent to 194 8C, based on steam tables. The reaction vessels have been connected to a pressure transducer that monitors and controls the pressure during synthesis. In order to optimize the synthesis conditions on the sample 1Ce5Na (obtained starting from Ce(NH4)2(NO3)6 1 M and NaOH 5 M) MH treatments have been conducted at pressure ranging from 3 to 14 atm for soaking time ranging from 5 to 240 min. The time, pressure and power have been computer controlled. The crystallized powders obtained have been repeatedly washed with deionized water, filtered and dried. After the last washing of the synthesized powders, the supernatant has been analyzed by ICP spectroscopy (Model Liberty 200, Variant) to evaluate the presence of sodium and the efficacy of the washing step. 2.2. Characterization Crystalline phase identification has been performed using a computer-assisted conventional Bragg–Brentano diffractometer using Cu Ka monochromatic radiation (X-ray diffraction, XRD, Model PW 3710 Philips). The XRD patterns have been collected in the 208–808 2u range at room temperature. The scanning rate has been 0.0048 s1, step size 0.028. The average crystallite size has been calculated using the Sherrer’s formula from the width of the X-ray diffraction lines [23]. To determine the amorphous phase content and thus to evaluate their crystallization degree, the powders have been analyzed by the combined Rietveld-reference intensity ratio (RIR) method [24]. A 10 wt% of tungsten (Fluka) has been added to all samples as internal standard. The mixtures, ground in an agate mortar have been side loaded in an aluminum flat holder in order to minimize the preferred orientation problems. Data have been recorded in the 58–1408 2u range (step size 0.028 and 6 s counting time for each step). The phase fractions extracted by the Rietveld-RIR refinements, using GSAS software [25] have been rescaled on the basis of the absolute weight of tungsten originally added to the mixtures as an internal standard, and therefore internally renormalized. The specific surface area of dried powders has been measured by the BET method (Model Gemini 2360, Micromeritics), using nitrogen as the adsorbate. The particle size has been also calculated, using the specific surface area data by the equation f¼
6 Sr
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where f is the average diameter of a spherical particle, S the surface area of a powder and r is the density. Sample morphology has been examined by transmission electron microscopy, TEM (Model EM400, Philips). Specimens have been prepared by dispersing the as-obtained powders in distilled water and then placing a drop of suspension on a copper grid with a transparent polymer, followed by drying. Thermal behavior of the dried samples has been studied by thermogravimetric (TG) and differential thermal (DTA) analysis in air at a heating and cooling rate of 20 8C/min, using a simultaneous TG/DTA apparatus (Model STA409, Netzsch). 3. Results and discussion 3.1. Synthesis optimization The synthesis condition optimization has been conducted on sample 1Ce5Na (obtained starting from Ce(NH4)2(NO3)6 1 M and NaOH 5 M). ICP spectroscopy showed the absence of alkali ions in the washing water, indicating the efficacy of the washing step and the chemical homogeneity of the powders. As regarding the synthesis pressure, the XRD analysis of the as-prepared powders showed the same crystalline structure for all the synthesis conditions used. Peaks have been attributed to crystalline cerium oxide, CeO2, with a cubic fluorite structure (ICDD file 34–394). In particular the diffractometric ceria evolution as a function of the hydrothermal process pressure has been reported in Fig. 1 (synthesis time 5 min). The analysis of XRD patterns of these powders indicates the crystallization of ceria at as low a temperature as 50 8C and 3.5 atm. As regarding the synthesis time effect, no evident differences are observable in XRD patterns. After only 5 min heating, the amorphous-precipitated product is a fine crystalline powders (5.1 nm 0.1 nm average particle size), as determined from XRD patterns using the Sherrer’s formula; further hydrothermal treatment slightly increases the grain size of powders, e.g., 5.4 nm in 30 min, 5.7 nm in 120 min and 6.4 nm in 240 min. After these results, in order to verify the effect of concentration of Na and Ce(IV) ions the synthesis condition have been fixed in 5 min and 13.4 atm. 3.2. Powders characterization The XRD analysis of the as-prepared powders showed the same crystalline fluorite structure for all the synthesis conditions used. The lattice parameters obtained after data refinement are in good agreement with data reported in literature (a = 0.5411 nm) [26]. The average dimension of particles as obtained from XRD patterns by the Sherrer’s formula is strongly influenced by the Ce(IV) solution concentration (Fig. 2). It is obvious that the higher the concentration of the solution, the larger
Fig. 1. XRD patterns of sample 1Ce5Na obtained for 5 min at 3, 5, 7 and 14 atm.
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Fig. 2. Average crystallite dimension (determined by Scherrer’s formula) as a function of Ce(IV) and Na ions concentration (5 min, 13.4 atm).
the CeO2 crystal grew. As the average diffusion distance for the diffusing solute is short and the concentration gradient is steep in concentrated solutions, much diffusing material passes per unit time through a unit area [27]. Moreover, the average dimension of crystallites depends on the concentration of the precipitant agent solution: the higher the concentration of the Na ion, the larger the ceria crystals. These values have been confirmed by data obtained by specific surface values. The experimental data (Table 1) show as the diminution of both precipitant agent and Ce(IV) concentration allows to specific surface area values higher and thus to finer grain size. Moreover, because the average dimensions obtained by BET data elaboration (powders dimension) are comparable with that obtained by XRD peaks analysis (crystallites dimension) it can be defined that the obtained powders do not show hard-type agglomerations. The obtained results match with TEM results. In Fig. 3, the TEM microphotograph of 1Ce1Na (5 min, 13.4 atm) powder, chosen as characteristic, is reported. The image clearly shows that the obtained powders are constituted by cubic crystals, well-dispersed and homogeneously distributed. The thermogravimetric and differential thermal curves registered on samples obtained by 5 min microwave– hydrothermal treatment at 13.4 atm (Fig. 4) show an endothermic peak at 120 8C, correlated to a weight loss due to the water absorbed by crystalline ceria, and an exothermic peak around 400 8C, correlated to a weight loss, that must be considered as due to the crystallization of the residual amorphous phase. Furthermore, thermogravimetrical analysis points out that the Ce(IV) salt solution concentration does not provoke any variations in the thermal curve behavior. The comparison between the experimental weight loss at temperature <200 8C (nearly 5% for all the synthesis conditions used) and the theoretical values for water elimination permits to Table 1 Specific surface area values and powder average diameter calculated by BET data (5 min, 13.4 atm) Ce concentration (M)
NaOH (M)
BET 3 (m2/g)
d 0.1 (nm)
0.1 0.1 0.1 0.5 0.5 0.5 1.0 1.0 1.0 2.5 2.5 2.5
1 5 10 1 5 10 1 5 10 1 5 10
207 198 205 167 151 149 165 145 143 164 160 154
4.0 4.2 4.1 5.0 5.5 5.6 5.0 5.7 5.8 5.1 5.2 5.4
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Fig. 3. TEM micrograph (180,000) of 1Ce1Na sample (5 min, 13.4 atm).
Fig. 4. TG and DTA curves of the 1Ce1Na sample (5 min) chosen as representative.
Fig. 5. Rietveld refinement of sample 1Ce1Na after 5 min microwave heating treatment.
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Table 2 Phase composition (%) and standard Rietveld agreement factors obtained for the 1Ce1Na sample Phase
0 min
5 min
5 min calcination (500 8C, 1 h)
CeO2 Amorphous Total Rwp (%) Rp (%) x2
86.8 (1) 13.1 (1) 99.9 2.38 1.86 1.35
95.7 (1) 4.2 (1) 99.9 2.57 2.02 1.59
99.9 (1) – 99.9 4.22 3.12 3.98
hypothesize that powders are mainly constituted by a partially ceria-hydrated form, CeO2nH2O (n = 0.5). Thus, the microwave heating does not change the reaction pathway characteristic of traditional hydrothermal process which involves, starting from a Ce(IV) salt, the solution and the precipitation mechanism. The obtained powder is, in fact, constituted by CeO20.5H2O and a small amount of hydroxides with amorphous nature. To better evaluate the amorphous quantity in the powders and thus to evaluate the crystallinity of the powders and the yield of the microwave-assisted process, a quantitative analysis by Rietveld-RIR method (Fig. 5) have been performed on 1Ce1Na samples obtained after 0 and 5 min of microwave–hydrothermal treatments. Moreover to verify the complete crystallization of sample >500 8C, as indicated by differential thermal analysis, the 1Ce1Na sample has been also calcined at 500 8C for 1 h before to proceed to the quantitative analysis. The obtained results reported in Table 2 show as only after 5 min of microwave treatment the amorphous phase is almost absent. This value remains constant with increasing the treatment time. The refinement on the calcined sample confirm that the exothermal event in DTA curve correspond to the complete crystallization of the powders. 4. Conclusions Adopting the microwave–hydrothermal process as synthesis method it is possible to obtain, by treating the solution at 13.4 atm for only 5 min, nanometric ceria with a low degree of agglomeration. Powders are extremely fine and morphologically controlled but they still present traces of amorphous phases (4%). The characteristics of the microwave hydrothermally synthesized powders can be compared with literature data referred to conventional hydrothermal synthesis. It is consequently manifest that the used processing method permits to obtain nanometric powders in considerably shorter treatment times. In fact if the conventional hydrothermal synthesis requires to treat the solutions for times ranging from 4 to 40 h, as reported in literature by several authors [1– 3,15,16] the use of microwaves allows to prepare ceria with similar properties in only 5 min, with a decrease of the reaction time of about 50 times. This technique is hence extremely interesting not only for the processing time and temperature used, but also for the possibility to achieve powders with controlled properties even starting from sufficiently concentrated solutions. Despite the literature indications [17,28] regarding nanometric powders that can be only obtained starting from highly diluted solutions of the cation (from 103 to 104 M), microwave–hydrothermal synthesis allows to use concentrated solutions so that it is possible to synthesize nearly 160 g of powders from a liter of solution. Acknowledgements Authors are grateful to Dott.ssa Erica Guigli who performed the experimental procedure and Dr. Paola Miselli for the assistance in XRD diffraction analysis. References [1] S. Somiya, T. Akiba, J. Eur. Ceram. Soc. 19 (1999) 81. [2] S. Somiya, M. Yoshimura, Z. Nakai, K. Hishinuma, T. Kumaki, in: J.A. Pask, A. Evans (Eds.), Microstructure, Plenum Press, New York, 1986, pp. 465–474. [3] T. Tsukada, S. Venigalla, A.A. Morrone, J.H. Adair, J. Am. Ceram. Soc. 82 (5) (1999) 1169.
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[4] S. Komarneni, R. Roy, Q.H. Li, Mater. Res. Bull. 27 (12) (1992) 1393. [5] S. Komarneni, in: J. Singh, S.M. Copley (Eds.), Novel Techniques in Synthesis and Processing of Advanced Materials, The Minerals, Metals and Materials Society, Warrendale, PA, 1995, pp. 103–117. [6] S. Komarneni, M.C. D’Arrigo, C. Leonelli, G.C. Pellacani, H. Katsuki, J. Am. Ceram. Soc. 88 (11) (1998) 3041. [7] F. Bondioli, C. Leonelli, C. Siligardi, G.C. Pellacani, S. Komarneni, in: M. Willert-Porada (Ed.), Advances in Microwave and Radio Frequency Processing, Springer-Verlag, Berlin, in press. [8] F. Bondioli, A.M. Ferrari, C. Leonelli, C. Siligardi, G.C. Pellacani, J. Am. Ceram. Soc. 84 (11) (2001) 2728. [9] F. Bondioli, C. Leonelli, T. Manfredini, A.M. Ferrari, M.C. Caracoche, P.C. Rivas, A.M. Rodrı´guez, J. Am. Ceram. Soc. 88 (3) (2005) 633–638. [10] N. Blumental, F.S. Brugner, J.E. Garner, J. Electrochem. Soc. 120 (1973) 1230. [11] C. Tian, S. Chan, in: R.A. Gerhardt, M.A. Alim, S.R. Taylor (Eds.), Materials Research Society Symposium Proceedings, vol. 500, Materials Research Society, Pittsburgh, PA, 1998, pp. 279–284. [12] T. Mon, H. Yamamura, J. Mater. Synth. Process. 6 (3) (1998) 175. [13] J.I. Hamagami, Y. Inda, T. Umegaki, K. Yamashita, Solid State Ionics 113–115 (1998) 235. [14] E. Bekyarova, P. Fornasiero, J. Kaspar, M. Graziani, Catal. Today 45 (1–4) (1998) 179. [15] N.C. Wu, E.W. Shi, Y.Q. Zheng, W.J. Li, J. Am. Ceram. Soc. 85 (10) (2002) 2462. [16] M. Hirano, E. Kato, J. Am. Ceram. Soc. 82 (3) (1999) 786. [17] E. Matijevic, W.P. Hsu, J. Colloid Interface Sci. 118 (1987) 506. [18] X. Chu, W. Chung, L.D. Schmidt, J. Am. Ceram. Soc. 76 (8) (1993) 2115. [19] T.J. Kirk, J. Winnick, J. Electrochem. Soc. 140 (1993) 3494. [20] F. Bondioli, A. Bonamartini Corradi, C. Leonelli, T. Manfredini, Mater. Res. Bull. 34 (14–15) (1999) 2159. [21] F. Bondioli, A. Bonamartini Corradi, T. Manfredini, C. Leonelli, R. Bertoncello, Chem. Mater. 12 (2) (2000) 324. [22] F. Bondioli, P. Veronesi, C. Leonelli, T. Manfredini, in: G. Muller (Ed.), Ceramics–Processing, Reliability, Tribology and Wear, vol. 12, Wiley-VCH, Weinheim, 1999, pp. 11–16. [23] B.D. Cullity, Elements of X-ray Diffraction, second ed., Addison-Wesley, Reading, MA, 1997. [24] R.L. Snider, Powder Diffr. 7 (1992) 186. [25] A.C. Larson, R.B. von Dreele, GSAS: General Structure Analysis System LANSCE, MS-H805, Los Alamos National Laboratory, Los Alamos, NM, 1998. [26] M. Wolcyrz, L. Kepinski, J. Solid State Chem. 99 (1992) 409. [27] M. Hirano, E. Kato, J. Am. Ceram. Soc. 79 (3) (1996) 777. [28] P.L. Chen, I.W. Chen, J. Am. Ceram. Soc. 76 (6) (1993) 1577.