- Email: [email protected]
Synthesis and conductivity of cerium oxide nanoparticles
Materials Letters 64 (2010) 1254–1256
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Synthesis and conductivity of cerium oxide nanoparticles Hongyun Jin a,b, Ning Wang a, Liang Xu a, Shuen Hou a,b,⁎ a b
Faculty of Material Science and Chemistry Engineering, China University of Geosciences, Wuhan 430074, PR China Engineering Research Center of Nano-Geomaterials of Ministry of Education, China University of Geosciences, Wuhan 430074, PR China
a r t i c l e
i n f o
Article history: Received 27 January 2010 Accepted 28 February 2010 Available online 15 March 2010 Keywords: Cerium oxide Nanomaterials Alkaline fusion Characterization methods Conductivity
a b s t r a c t Cerium oxide (CeO2) nanoparticles have been synthesized through composite-hydroxide-mediated approach. The X-ray powder diffraction (XRD) measurement proved that the pure cubic CeO2 could be obtained at a low temperature region (170–220 °C). The particle size, micrograph morphology and microstructure were investigated by transmission electron microscope (TEM) and environmental scanning electron microscope (ESEM). The conductivity of as-synthesized CeO2 was measured by a standard fourprobe method. The conductivity of CeO2 increases slightly with the increase of temperature. And the conductivity increases rapidly to 0.02418 s cm− 1 at 830 °C. The product is a potential material for intermediate temperature solid oxide fuel cells (ITSOFC). © 2010 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
CeO2 has attracted much attention for its potential applications in electronic ceramic, ultra-precise polishing, gas sensor, catalysis, solid oxide fuel cells (SOFC) and so on [1–6]. For example, CeO2 is one key abrasive material for chemical–mechanical polishing in advanced integrated circuits [4]. Because of its excellent ionic conduction, doped-cerium oxide nanoparticles are promising electrolyte materials for SOFC [5]. The capacity of the modern automotive exhaust treatment catalysts containing CeO2 is much more effective than that of the predecessors due to its high “oxygen storage capacity” [6]. Therefore, the extensive synthesis of CeO2 becomes an urgent task for further research and applications. Many synthesis approaches [7–13], such as chemical solution precipitation, sol–gel method, microwave-assisted hydrothermal processes [1,7], mechano-chemical processing [8], polyvinyl pyrrolidone (PVP) solution route [9], electrochemical synthesis [10], combustion method [11], direct sono-chemical route [12] and gas– liquid co-precipitation [13] have been used for preparation of CeO2. Some special reacting conditions, such as high temperature, high pressure, capping agents, expensive or toxic solvent have been involved in the preparation of CeO2. In this contribution, we present a successful example of alkaline fusion to yield CeO2 nanoparticles [2,3].
2.1. Synthesis of CeO2 The synthesis of CeO2 nanoparticles follows three steps. First, the mixture of potassium hydroxide and sodium hydroxide (10.0 g, KOH/ NaOH = 48.5:51.5) were put into a 40 ml Teflon container and 2.0 g cerium nitrate was added. Second, the container was covered and put into an electric oven preheated to a certain temperature. The electric oven was kept at a certain temperature for different time periods. Then the Teflon container was taken out and cooled to room temperature. At last, the product was washed with deionized water and dried to yield the product. The as-synthesized CeO2 was characterized by XRD (Model: X'Pert MPD Pro), ESEM (Model: Quanta200) and TEM (Model: Tecnal G220). The phase and structure of CeO2 were identified by powder X-ray diffraction analysis with nickel filtered Cu Kα radiation (30 kV, 30 mA). The lattice parameters were determined by a least-squares refinement. The particle size was estimated from X-ray line broadening measurements, the calculation was done by (110) diffraction line of CeO2 crystal according to the Scherrer formula [14]: D = 0:89λ = ðB cosθÞ
ð1Þ
where D is the particle size; λ is the wavelength of the X-ray (Cu Kα, 1.4954 Å); θ is the Bragg diffraction angle and B is the corrected halfwidth given by ⁎ Corresponding author. Faculty of Material Science and Chemistry Engineering, China University of Geosciences, Wuhan 430074, PR China. Tel.: + 86 2767885168. E-mail address: [email protected] (S. Hou). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.02.062
2
2
2
B = Bm −Bs
2
H. Jin et al. / Materials Letters 64 (2010) 1254–1256
1255
where Bm is the measured half-width and Bs is the half-width of a standard CeO2 product. 2.2. Conductivity experiment The conductivity of as-synthesized CeO2 was measured by a standard four-probe method. Pt wire and Pt paste were used to make the four probes. The CeO2 were pressed and polished into cylinder bars and then sintered at 750 °C for 5 h. The conductivity was measured from 525 °C to 850 °C. 3. Results and discussion The reaction temperature and time affect the phase structure and particle size of the CeO2 product. We have obtained some CeO2 products at 220 °C, 200 °C, 190 °C, 180 °C, and 170 °C for 5 h, and some products at 200 °C for 0.5, 2, 5, 12 and 24 h. The XRD patterns of the as-synthesized CeO2 nanoparticles at different reacting temperature for 5 h were shown in Fig. 1. All peaks of the as-synthesis CeO2 at 170 °C, 180 °C, 190 °C, 200 °C, and 220 °C were accorded with the standard peaks of cubic CeO2 (Space grouping Fm-3 m (225)) with the PDF/JCPDS card number: 021306, and the correlation between net plane and d value was showed. It was concluded that CeO2 could be obtained between 170 °C and 220 °C. Based on the Scherrer formula, the particle sizes at higher temperature are larger than those at low temperature, which could be concluded by the reason that the increase reaction temperature could increase the CeO2 particle growth rate. The CeO2 particle size calculated by Scherrer formula is showed in Fig. 2. Liu et al. [2,3] have synthesized some oxides using molten composite-hydroxide as a dissolvent. Our synthesis approach is similar to their work. When Ce(NO3)3 and composite hydroxides were heated at 170 °C, they were melted and ionized. Ce(OH)4 was obtained primarily when ion exchanges were accelerated. Compositehydroxide has a strong absorbability causing Ce(OH)4 dehydration reaction and CeO2 was generated. As shown in Fig. 2, when the reaction temperature was 180 °C and the reaction time was 0.5 h, the
Fig. 2. Particle size of CeO2 at different reacting temperature and time.
particle size was about 37.3 nm. With the increase of reaction time, the particle size increases accordingly. The particle size also increases with the prolongation of reaction time. At low temperature or short reaction time, the crystal growth rate is less than nucleation formation rate, leading to the small particle size. We could increase the reaction temperature or prolong the reaction time to obtain the perfect CeO2 particles. Although the Ce4+ ions could be substituted by K+ or Na+ during the CeO2 crystal growth process, Ce4+ was replaced only on an extremely low quantity due to the obvious differences of ionic radius and charge number between K+ or Na+ and Ce4+. As a result, highpurity CeO2 nanoparticles could be obtained easily. The size and micro-morphology of the CeO2 products, synthesized at 200 °C for 5 h, were investigated by TEM and ESEM, as shown in Fig. 3. In the TEM images, CeO2 particles ranging from 30 nm to 60 nm were observed, and the results agree with the result of Fig. 2. We could also observe some smaller particles of 5–6 nm in size, which are cohered together to grow to the irregular particle with the size about
Fig. 1. XRD patterns of CeO2 at different reacting temperature.
1256
H. Jin et al. / Materials Letters 64 (2010) 1254–1256
Fig. 3. Electron microscope images of CeO2 (200 °C for 5 h).
50 nm like a single particle observed by TEM [3]. Since the CeO2 particles tend to lower the surface energy, we could observe the agglomeration of nanoparticles like spherical shape with the size between 50 and 200 nm in the ESEM image. In extensive production process, higher productive ratio (Ф) is desired. The productive ratio is defined as the ratio of the weight of CeO2 produced to the weight of the theoretical value calculated by the amount of Ce(NO3)3. If the loss during the process is neglected, the productive ratio is nearly 100% at higher than 200 °C for more than 5 h. By comprehensive consideration, the reaction temperature at 200 °C and the reaction time of 5 h are the optimal parameters. In this optimal condition, the CeO2 nanoparticles were generated. The conductivity σ was measured by standard four-probe method to evaluate the conductivity performance of as-synthesized CeO2 depending on temperature, as showed in Fig. 4. At room temperature, CeO2 is nonconductive. With the increase of temperature, it becomes conductive and the oxygen-ion conductivity value σ depends strongly on the temperature. The oxygen-ion conductivity value was 4.4181 × 10− 4 s cm− 1 at 530 °C. In the region of 530–680 °C, the conductivity increases slightly. Above 680 °C, it increases rapidly to a value of 0.02418 s cm− 1 at 830 °C. The assynthesized CeO2 products exhibit conductivity at 500–800 °C. For its high oxygen-ion conductivity and low conduction activation energy, lower work temperature and lower-cost comparison with zirconiabased solid electrolyte, CeO2 is a promising material for ITSOFC. The
Fig. 4. The conductivity of CeO2 at different temperature.
conductivity would be enhanced greatly after CeO2 was doped by gadolinium or samarium. 4. Conclusions A simple process to prepare CeO2 nanoparticles was presented in which pure cubic CeO2 could be obtained at 170–220 °C and the particle size increased with rising temperature or prolonging reaction time. Some ultra-fine CeO2 particles with the size of 5–6 nm would cohere together resulting in a size about 50 nm. The as-synthesized CeO2 exhibit conductivity at 500–800 °C, and it is a promising material for ITSOFC. Composite-hydroxide-mediated approach is a simpleequipments, low temperature, high productive ratio and massproduction route for the synthesis of CeO2 nanoparticles. Furthermore, this method can be applied to prepare many other nanostructures. References [1] Riccardi CS, Lima RC, dos Santos ML, Bueno PR, Varela JA. Longo E. Solid State Ion 2009;180:288–91. [2] Liu H, Hu CG, Wang ZL. Nano Lett 2006;6:1535–9. [3] Hu CG, Zhang ZW, Liu H, Gao PX, Wang ZL. Nanotechnology 2006;17:5983–7. [4] Feng XD, Sayle DC, Wang ZL, Paras S, Santora B, Sutorik T, et al. Science 2006;312: 1504–8. [5] Chiba R, Komatsu T, Orui H, Taguchi H, Nozawa K, Arai H. Electrochem Solid State Lett 2009;12:B69–72. [6] Campbell CT, Peden CHF. Science 2005;309:713–4. [7] Hao SY. Chin J Inorg Chem 2008;24:1012–6. [8] Kaspar J, Fornasiero M, Graziani M. Catal Today 1999;50:285–8. [9] Phoka S, Laokul P, Swatsitang E, Promarak V, Seraphin S, Maensiri S. Mater Chem Phys 2009;115:423–8. [10] Lair V, Ringuede A, Vermaut P, Griveau S. Phys Status Solidi C 2008;5:3492–5. [11] Chen M, Zhang PZ, Zheng XM. Catal Today 2004;93:671–3. [12] Gu LN, Meng GY. Mater Res Bull 2008;43:1555–61. [13] Iijima T, Kato K, Kuno T, Okuwaki A, Umetsu Y, Okabe T. Ind Eng Chem Res 1993;32:733–7. [14] Bondioli F, Corradi AB, Leonelli C. Mater Res Bull 1999;34:2159–60.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Synthesis and conductivity of cerium oxide nanoparticles Hongyun Jin a,b, Ning Wang a, Liang Xu a, Shuen Hou a,b,⁎ a b
Faculty of Material Science and Chemistry Engineering, China University of Geosciences, Wuhan 430074, PR China Engineering Research Center of Nano-Geomaterials of Ministry of Education, China University of Geosciences, Wuhan 430074, PR China
a r t i c l e
i n f o
Article history: Received 27 January 2010 Accepted 28 February 2010 Available online 15 March 2010 Keywords: Cerium oxide Nanomaterials Alkaline fusion Characterization methods Conductivity
a b s t r a c t Cerium oxide (CeO2) nanoparticles have been synthesized through composite-hydroxide-mediated approach. The X-ray powder diffraction (XRD) measurement proved that the pure cubic CeO2 could be obtained at a low temperature region (170–220 °C). The particle size, micrograph morphology and microstructure were investigated by transmission electron microscope (TEM) and environmental scanning electron microscope (ESEM). The conductivity of as-synthesized CeO2 was measured by a standard fourprobe method. The conductivity of CeO2 increases slightly with the increase of temperature. And the conductivity increases rapidly to 0.02418 s cm− 1 at 830 °C. The product is a potential material for intermediate temperature solid oxide fuel cells (ITSOFC). © 2010 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
CeO2 has attracted much attention for its potential applications in electronic ceramic, ultra-precise polishing, gas sensor, catalysis, solid oxide fuel cells (SOFC) and so on [1–6]. For example, CeO2 is one key abrasive material for chemical–mechanical polishing in advanced integrated circuits [4]. Because of its excellent ionic conduction, doped-cerium oxide nanoparticles are promising electrolyte materials for SOFC [5]. The capacity of the modern automotive exhaust treatment catalysts containing CeO2 is much more effective than that of the predecessors due to its high “oxygen storage capacity” [6]. Therefore, the extensive synthesis of CeO2 becomes an urgent task for further research and applications. Many synthesis approaches [7–13], such as chemical solution precipitation, sol–gel method, microwave-assisted hydrothermal processes [1,7], mechano-chemical processing [8], polyvinyl pyrrolidone (PVP) solution route [9], electrochemical synthesis [10], combustion method [11], direct sono-chemical route [12] and gas– liquid co-precipitation [13] have been used for preparation of CeO2. Some special reacting conditions, such as high temperature, high pressure, capping agents, expensive or toxic solvent have been involved in the preparation of CeO2. In this contribution, we present a successful example of alkaline fusion to yield CeO2 nanoparticles [2,3].
2.1. Synthesis of CeO2 The synthesis of CeO2 nanoparticles follows three steps. First, the mixture of potassium hydroxide and sodium hydroxide (10.0 g, KOH/ NaOH = 48.5:51.5) were put into a 40 ml Teflon container and 2.0 g cerium nitrate was added. Second, the container was covered and put into an electric oven preheated to a certain temperature. The electric oven was kept at a certain temperature for different time periods. Then the Teflon container was taken out and cooled to room temperature. At last, the product was washed with deionized water and dried to yield the product. The as-synthesized CeO2 was characterized by XRD (Model: X'Pert MPD Pro), ESEM (Model: Quanta200) and TEM (Model: Tecnal G220). The phase and structure of CeO2 were identified by powder X-ray diffraction analysis with nickel filtered Cu Kα radiation (30 kV, 30 mA). The lattice parameters were determined by a least-squares refinement. The particle size was estimated from X-ray line broadening measurements, the calculation was done by (110) diffraction line of CeO2 crystal according to the Scherrer formula [14]: D = 0:89λ = ðB cosθÞ
ð1Þ
where D is the particle size; λ is the wavelength of the X-ray (Cu Kα, 1.4954 Å); θ is the Bragg diffraction angle and B is the corrected halfwidth given by ⁎ Corresponding author. Faculty of Material Science and Chemistry Engineering, China University of Geosciences, Wuhan 430074, PR China. Tel.: + 86 2767885168. E-mail address: [email protected] (S. Hou). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.02.062
2
2
2
B = Bm −Bs
2
H. Jin et al. / Materials Letters 64 (2010) 1254–1256
1255
where Bm is the measured half-width and Bs is the half-width of a standard CeO2 product. 2.2. Conductivity experiment The conductivity of as-synthesized CeO2 was measured by a standard four-probe method. Pt wire and Pt paste were used to make the four probes. The CeO2 were pressed and polished into cylinder bars and then sintered at 750 °C for 5 h. The conductivity was measured from 525 °C to 850 °C. 3. Results and discussion The reaction temperature and time affect the phase structure and particle size of the CeO2 product. We have obtained some CeO2 products at 220 °C, 200 °C, 190 °C, 180 °C, and 170 °C for 5 h, and some products at 200 °C for 0.5, 2, 5, 12 and 24 h. The XRD patterns of the as-synthesized CeO2 nanoparticles at different reacting temperature for 5 h were shown in Fig. 1. All peaks of the as-synthesis CeO2 at 170 °C, 180 °C, 190 °C, 200 °C, and 220 °C were accorded with the standard peaks of cubic CeO2 (Space grouping Fm-3 m (225)) with the PDF/JCPDS card number: 021306, and the correlation between net plane and d value was showed. It was concluded that CeO2 could be obtained between 170 °C and 220 °C. Based on the Scherrer formula, the particle sizes at higher temperature are larger than those at low temperature, which could be concluded by the reason that the increase reaction temperature could increase the CeO2 particle growth rate. The CeO2 particle size calculated by Scherrer formula is showed in Fig. 2. Liu et al. [2,3] have synthesized some oxides using molten composite-hydroxide as a dissolvent. Our synthesis approach is similar to their work. When Ce(NO3)3 and composite hydroxides were heated at 170 °C, they were melted and ionized. Ce(OH)4 was obtained primarily when ion exchanges were accelerated. Compositehydroxide has a strong absorbability causing Ce(OH)4 dehydration reaction and CeO2 was generated. As shown in Fig. 2, when the reaction temperature was 180 °C and the reaction time was 0.5 h, the
Fig. 2. Particle size of CeO2 at different reacting temperature and time.
particle size was about 37.3 nm. With the increase of reaction time, the particle size increases accordingly. The particle size also increases with the prolongation of reaction time. At low temperature or short reaction time, the crystal growth rate is less than nucleation formation rate, leading to the small particle size. We could increase the reaction temperature or prolong the reaction time to obtain the perfect CeO2 particles. Although the Ce4+ ions could be substituted by K+ or Na+ during the CeO2 crystal growth process, Ce4+ was replaced only on an extremely low quantity due to the obvious differences of ionic radius and charge number between K+ or Na+ and Ce4+. As a result, highpurity CeO2 nanoparticles could be obtained easily. The size and micro-morphology of the CeO2 products, synthesized at 200 °C for 5 h, were investigated by TEM and ESEM, as shown in Fig. 3. In the TEM images, CeO2 particles ranging from 30 nm to 60 nm were observed, and the results agree with the result of Fig. 2. We could also observe some smaller particles of 5–6 nm in size, which are cohered together to grow to the irregular particle with the size about
Fig. 1. XRD patterns of CeO2 at different reacting temperature.
1256
H. Jin et al. / Materials Letters 64 (2010) 1254–1256
Fig. 3. Electron microscope images of CeO2 (200 °C for 5 h).
50 nm like a single particle observed by TEM [3]. Since the CeO2 particles tend to lower the surface energy, we could observe the agglomeration of nanoparticles like spherical shape with the size between 50 and 200 nm in the ESEM image. In extensive production process, higher productive ratio (Ф) is desired. The productive ratio is defined as the ratio of the weight of CeO2 produced to the weight of the theoretical value calculated by the amount of Ce(NO3)3. If the loss during the process is neglected, the productive ratio is nearly 100% at higher than 200 °C for more than 5 h. By comprehensive consideration, the reaction temperature at 200 °C and the reaction time of 5 h are the optimal parameters. In this optimal condition, the CeO2 nanoparticles were generated. The conductivity σ was measured by standard four-probe method to evaluate the conductivity performance of as-synthesized CeO2 depending on temperature, as showed in Fig. 4. At room temperature, CeO2 is nonconductive. With the increase of temperature, it becomes conductive and the oxygen-ion conductivity value σ depends strongly on the temperature. The oxygen-ion conductivity value was 4.4181 × 10− 4 s cm− 1 at 530 °C. In the region of 530–680 °C, the conductivity increases slightly. Above 680 °C, it increases rapidly to a value of 0.02418 s cm− 1 at 830 °C. The assynthesized CeO2 products exhibit conductivity at 500–800 °C. For its high oxygen-ion conductivity and low conduction activation energy, lower work temperature and lower-cost comparison with zirconiabased solid electrolyte, CeO2 is a promising material for ITSOFC. The
Fig. 4. The conductivity of CeO2 at different temperature.
conductivity would be enhanced greatly after CeO2 was doped by gadolinium or samarium. 4. Conclusions A simple process to prepare CeO2 nanoparticles was presented in which pure cubic CeO2 could be obtained at 170–220 °C and the particle size increased with rising temperature or prolonging reaction time. Some ultra-fine CeO2 particles with the size of 5–6 nm would cohere together resulting in a size about 50 nm. The as-synthesized CeO2 exhibit conductivity at 500–800 °C, and it is a promising material for ITSOFC. Composite-hydroxide-mediated approach is a simpleequipments, low temperature, high productive ratio and massproduction route for the synthesis of CeO2 nanoparticles. Furthermore, this method can be applied to prepare many other nanostructures. References [1] Riccardi CS, Lima RC, dos Santos ML, Bueno PR, Varela JA. Longo E. Solid State Ion 2009;180:288–91. [2] Liu H, Hu CG, Wang ZL. Nano Lett 2006;6:1535–9. [3] Hu CG, Zhang ZW, Liu H, Gao PX, Wang ZL. Nanotechnology 2006;17:5983–7. [4] Feng XD, Sayle DC, Wang ZL, Paras S, Santora B, Sutorik T, et al. Science 2006;312: 1504–8. [5] Chiba R, Komatsu T, Orui H, Taguchi H, Nozawa K, Arai H. Electrochem Solid State Lett 2009;12:B69–72. [6] Campbell CT, Peden CHF. Science 2005;309:713–4. [7] Hao SY. Chin J Inorg Chem 2008;24:1012–6. [8] Kaspar J, Fornasiero M, Graziani M. Catal Today 1999;50:285–8. [9] Phoka S, Laokul P, Swatsitang E, Promarak V, Seraphin S, Maensiri S. Mater Chem Phys 2009;115:423–8. [10] Lair V, Ringuede A, Vermaut P, Griveau S. Phys Status Solidi C 2008;5:3492–5. [11] Chen M, Zhang PZ, Zheng XM. Catal Today 2004;93:671–3. [12] Gu LN, Meng GY. Mater Res Bull 2008;43:1555–61. [13] Iijima T, Kato K, Kuno T, Okuwaki A, Umetsu Y, Okabe T. Ind Eng Chem Res 1993;32:733–7. [14] Bondioli F, Corradi AB, Leonelli C. Mater Res Bull 1999;34:2159–60.