Room temperature co-precipitation of nanocrystalline CeO2 and powder

Materials Letters 61 (2007) 1904 – 1907 www.elsevier.com/locate/matlet

Room temperature co-precipitation of nanocrystalline CeO2 and Ce0.8Gd0.2O1.9−δ powder M.J. Godinho a , R.F. Gonçalves a , L.P. S Santos a , J.A. Varela b , E. Longo b , E.R. Leite a,⁎ a

Centro Multidisciplinar de Desenvolvimento de Materiais Cerâmicos/LIEC/Department of Chemistry/UFSCar, C.P. 676-13565-905, São Carlos, S.P., Brazil b Centro Multidisciplinar de Desenvolvimento de Materiais Cerâmicos/LIEC Institute of Chemistry/UNESP, C.P. 355-14801-970, Araraquara, S.P., Brazil Received 9 February 2006; accepted 27 July 2006 Available online 17 August 2006

Abstract This paper describes a simple method to co-precipitate CeO2 and Ce0.8Gd0.2O1.9−δ with ammonium hydroxide from solvents such as: water, ethylene glycol, ethyl alcohol and isopropyl alcohol. Characterization by Raman spectroscopy and XRD evidenced the formation of a solid solution of gadolinium-doped ceria at room temperature. Nanometric particles with crystallite size of 3.1 nm were obtained during synthesis using ethyl alcohol as solvent. This is a promising result compared with those mentioned in the literature, in which the smallest crystallite size reported was 6.5 nm. © 2006 Elsevier B.V. All rights reserved.

1. Introduction CeO2-based ceramics are known for their wide range of applications. CeO2 (ceria) is used in abrasives [1], pigments [2], catalyst [3], oxygen-ion-conducting solid electrolytes [4], and oxygen sensors [5]. Ceria is used in three-way catalytic (TWC) converters because of its unique redox properties and high oxygen storage capacity (OSC). In TWC converters, this oxide plays several roles: it promotes the water gas reaction, disperses precious metal, and inhibits the sintering of alumina supports [6]. From the standpoint of catalysis, it is important to design nanoporous materials with an ultrahigh surface area, offering the largest possible number of active sites for catalytic reactions [7]. Praseodymium-doped CeO2 (PDC) solid solutions are of interest because of their applications as nontoxic red and orange ceramic pigments [8]. However, the major interest in ceria-based ceramics has focused on their application as SOFCs (Solid Oxide Fuel Cells) [9]. Ceria-based ceramics are ionic conductors and are highly oxygen-conductive when subjected to temperatures of around 600 °C. This conduction is made possible by the existence of

⁎ Corresponding author. E-mail address: [email protected] (E.R. Leite). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.07.152

oxygen vacancies, whose concentration can be increased by doping the material with metal oxide having smaller valence, according to the following reaction [10]: 2MO1:5

2CeO2 Y 2MCeV þ VP þ 3Ox0

ð1Þ

An essential aspect of the preparation of rare earth ceria-doped materials is the chemical homogeneity [11]. Past studies have focused on several aspects of ceria-based solid electrolyte prepared by conventional synthesis methods [12]. However, to obtain homogeneous solid-state solutions using a conventional oxide mixture requires that the starting material have a small particle size and that it can withstand long calcining times. Ceria-based ceramics can be subjected to several different synthesization methods, such as hydrothermal [10], precipitation (for oxalate [13], carbonate [8,14], peroxide [15], hydroxide [15]), polymeric precursor [16,17], complexion with citric acid [18], the flow method [19] and organometallic decomposition [20]. Djuricic and Pickering [15] studied the synthesis of pure ceria by precipitation of cerium (III) nitrate hexahydrated with ammonium hydroxide. The formation of ceria occurred after oxidation of Ce3+ to Ce4+ in solution at a high pH (≥10), leading to Ce3+ + H2O → Ce(OH)3+ + H+ + e−, with subsequent hydrolysis to Ce(OH)4 and precipitation. However, oxidation of Ce(OH)3 also occurs readily in air at room temperature, forming yellow Ce

M.J. Godinho et al. / Materials Letters 61 (2007) 1904–1907

1905

Fig. 1. XRD patterns of the a) CeO2 and the b) Ce0.8Gd0.2O1.9−δ phases obtained by precipitation with ammonium hydroxide in different solvents.

(OH)4. These authors proposed that the cerium hydroxide precipitate is transformed into cerium oxide dihydrate, according to the following equation [15]: CeðOHÞ4 YCeO2 d2H2 O

ð2Þ

The objective of this work was to investigate the synthesis of pure ceria and gadolinium-doped ceria without the need for any type of thermal treatment, using a simple co-precipitation method. 2. Experimental Different solvents were utilized to investigate the formation of CeO2 and Ce0.8Gd0.2O1.9−δ (CGO), i.e., water, ethyl alcohol, ethylene glycol and isopropyl alcohol (0.1 M). Solutions of 0.1 M of hexahydrated cerium (III) nitrate – Ce(NO3)3·6H2O – 99.9% (Aldrich) dissolved in different solvents were prepared. Gadolinium (III) oxide — Gd2O3 99.9% Aldrich, dissolved in a minimum amount of nitric acid, was then added to the solution. In this mixture (under magnetic stirring), ammonium hydroxide was added a drop at a time to complete the precipitation under controlled pH, leading to the formation of a white gel. This gel was then collected by vacuum filtration and washed to completely remove all the ammonium nitrate formed. The gel was dried at room temperature for 48 h. The synthesized material was characterized by X-ray diffraction (XRD) using CuKα radiation (Rigaku DMax 2500PC diffractometer). Raman spectroscopy was also performed and the spectra were collected at room temperature (Bruker RFS-100/S Raman spectrometer with Fourier transform). A 1064 nm YAG laser was used as the excitation source, and its power was kept at 90 mW. To characterize the particle size and morphology, the CeO2 and Ce0.8Gd0.2O1.9−δ powder was dispersed in an aqueous solution using an ultrasound probe. A drop of this solution was dripped onto a carbon-covered copper net and then placed in a sample holder for TEM imaging

(Phillips CM 200). The surface area was determined by the BET multi-point method and a Micromeritics ASAP 2000 Physi/ Chemisorption unit was used for the adsorption/desorption analysis. 3. Results and discussion Fig. 1 shows the diffraction patterns of a) CeO2 and b) Ce0.8Gd0.2O1.9−δ phases obtained by precipitation with ammonium hydroxide in different solvents. The XRD results indicated the formation of a single phase CeO2 with a cubic fluorite structure. The surface area analysis, in Table 1, shows that listed sample of pure ceria and doped ceria precipitates in ethyl alcohol presented a higher surface area (around 74 and 131 m2/g, respectively) than the samples precipitated in other solvents. It also presented an average crystallite size of 9 nm, which was calculated based on XRD data and Scherrer's equation. The Ce0.8Gd0.2O1.9−δ phase shown in Fig. 1b was obtained by the coprecipitation method in different solvents (water, ethanol, ethylene glycol and isopropyl alcohol) using ammonium hydroxide. The XRD patterns obtained from these samples revealed the formation of crystalline CeO2 with fluorite cubic structure in all the samples, indicating the possible formation of solid solution for the CGO. However, it was not possible to define the formation of the Ce0.8Gd0.2O1.9−δ solid solution phase by XRD because of the close similarity between the CeO2 and Gd2O3 XRD patterns.

Table 1 Crystallite size (in nanometers) and BET specific surface area (m2/g) of the pure ceria and gadolinium-doped ceria samples Water solution CeO2 Crystallite size 12.7 Specific 69 surface area Ce0.8Gd0.2O1.9−δ Crystallite size 5.5 Specific 114 surface area

Ethylene Ethyl alcohol Isopropyl glycol solution solution alcohol solution 9.3 42

9.0 74

9.8 48

4.1 115

3.1 131

4.3 78

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Fig. 2. a) Raman scattering of CeO2 (Aldrich) added to Gd2O3 (Aldrich) at room temperature and of CeO2 and Ce0.8Gd0.2O1.9−δ phases obtained in this work at room temperature and b) enlarged FT-Raman spectra in the of range 250–550 cm− 1 wavenumber.

To confirm the formation of gadolinium-doped ceria solid solution, FT-Raman spectra were obtained for pure ceria (produced by our method), gadolinium-doped ceria (also produced by our method) and another material consisting of a mixture of cerium oxide (IV) – CeO2 (Aldrich) and gadolinium oxide (III) – Gd2O3 (Aldrich) without any heat treatment, in the same proportions to form the Ce0.8Gd0.2O1.9−δ phase. Fig. 2a depicts three spectra obtained under the three above described conditions. The figure shows that Ce0.8Gd0.2O1.9−δ obtained by coprecipitation from an ethyl alcohol solution exhibited a spectrum similar to CeO2. However, it differed from the CeO2 and Gd2O3 mixture. This result can be examined in greater detail in Fig. 2b, which shows a magnified version of the region of the 250–550 cm− 1 wavelength. As can be seen, there is a clear vibrational mode at 465 cm− 1 which is typical of CeO2 and corresponds to the F2g symmetry of the cubic phase [21]. The vibrational mode at 360 cm− 1, however, is attributed to the band of the Gd2O3 cubic phase [22], and was only found for the CeO2 and Gd2O3 mixture. This result is a strong evidence of the formation of a solid solution at room temperature. Fig. 3 shows the DTA/TG curves of the co-precipitated Ce0.8Gd0.2O1.9−δ powder in water. The TG curves illustrate two distinct

Fig. 3. DTA/TG analysis of CGO powder co-precipitated with ammonium hydroxide in water.

stages of weight loss, but only one of the curves can be evidenced at the DTA curve, which corresponds to the endothermic peak at around 98 °C due to weight loss of the absorbed water. The second event refers to the water adsorbed and decomposition of the remaining nitrates. The DTA curve did not show the exothermic peak related to crystallization, which also evidences the formation of the Ce0.8Gd0.2O1.9−δ at room temperature. The average crystallite size of the pure ceria and gadolinium-doped ceria powders, calculated based on using XRD data and Scherrer's equation, ranged from 3.1 to 5.5 nm (Table 1) It is interesting to note also that the this is the highest surface area obtained for the oxide processed in ethanol, as reported before for undoped ceria. The HR-TEM image displayed in Fig. 4 confirms the presence of crystalline particles for the material processed at room temperature. The nanocrystal size estimated by HR-TEM is similar to that calculated by XRD evidencing the nanometric nature of this powder. The stoichiometry of the Ce0.8Gd0.2O1.9−δ phase was verified by EDX microanalysis, confirming that this method led to the complete co-precipitation of Ce and Gd cations and resulted in the formation of the solid solution. The Ce0.8Gd0.2O1.9−δ solid solution was formed in all the solvents used here. According to Djurici and Pickering [15], the formation of CeO2 occurs due to the formation of Ce(OH)4 which is rearranged to

Fig. 4. HR-TEM image of gadolinium-doped ceria.


form CeO2·2H2O. Therefore, the following reactions can be proposed for gadolinium-doped ceria: CeðNO3 Þ3 d6H2 O þ GdðNO3 Þ3 d 6H2 O þ NH4 OHYCe1−X GdX ðOHÞ4 þ NH4 NO3 þ H2 O

ð3Þ

Ce1−X GdX ðOHÞ4 YCe1−x Gdx O2−d d 2H2 OYCe1−x Gdx O2−d þ 2H2 O

ð4Þ

4. Conclusions Nanometric CeO2 and Ce0.8Gd0.2O1.9−δ phase particles were obtained by the co-precipitation method described here. This method is very practical and allows for the production of a solidstate solution at room temperature. The nanometric CeO2 and Ce0.8Gd0.2O1.9−δ powder can be used in solid oxide fuel cells as well as in the catalytic treatment of automobile exhaust fumes. Acknowledgements CNPq, FAPESP, CAPE, Centro Multidiciplinar de Desenvolvimento de Materiais Cerâmicos and Department of Chemistry of UFSCar. References [1] S.K. Kim, S. Lee, U. Paik, et al., Influence of the electrokinetic behaviors of abrasive ceria particles and the deposited plasma-enhanced tetraethylorthosilicate and chemically vapor deposited Si3N4 films in an aqueous medium on chemical mechanical planarization for shallow trench isolation, Journal of Materials Research 18 (9) (Sep 2003) 2163–2169. [2] S.T. Aruna, S. Ghosh, K.C. Patil, Combustion synthesis and properties of Ce1−xPrxO2-delta red ceramic pigments, International Journal of Inorganic Materials 3 (4–5) (Jul 2001) 387–392. [3] B. Feng, C.Y. Wang, B. Zhu, Catalysts and performances for direct methanol low-temperature (300 to 600 degrees C) solid oxide fuel cells, Electrochemical and Solid State Letters 9 (2) (2006) A80–A81. [4] T.S. Zhang, J. Ma, L.B. Kong, et al., Aging behavior and ionic conductivity of ceria-based ceramics: a comparative study, Solid State Ionics 170 (3–4) (May 31 2004) 209–217. [5] P. Jasinski, T. Suzuki, H.U. Anderson, Nanocrystalline undoped ceria oxygen sensor, Sensors and Actuators. B, Chemical 95 (1–3) (Oct 15 2003) 73–77. [6] J. Kaspar, P. Fornasiero, M. Graziani, Use of CeO2-based oxides in the three-way catalysis, Catalysis Today 50 (2) (Apr 29 1999) 285–298.

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