- Email: [email protected]
Synthesis of nanosized Ce0.75Zr0.25O2 porous powders via an autoignition: glycine nitrate process
January 2003
Materials Letters 57 (2003) 1066 – 1071 www.elsevier.com/locate/matlet
Synthesis of nanosized Ce0.75Zr0.25O2 porous powders via an autoignition: glycine nitrate process H.S. Potdar a, S.B. Deshpande a, Y.B. Khollam b, A.S. Deshpande a, S.K. Date a,* a
Physical Chemistry Division, National Chemical Laboratory, Pune 411008, India b Department of Physics, University of Pune, Pune 411007, India Received 26 May 2002; accepted 30 May 2002
Abstract The GNP route has been successfully employed to generate nanosized, porous, stoichiometric, homogeneous Ce0.75Zr0.25O2 powders. In this route, a precursor solution is prepared by mixing glycine with an aqueous solution of mixed (Ce – Zr) metal – nitrates in their stoichiometric ratios. Initially, the glycine mixed precursor solution was heated in a petri dish to evaporate the excess water to form a viscous semitransparent material. Then the petri dish was kept in a 5-l capacity beaker covered with a metallic mesh and then the temperature was increased slowly to 190 – 200 jC to autoignite the material. The combustion was self-sustaining and very rapid, producing yellowish coloured powders. The as-prepared powders were nanosized (20 – 35 nm), ˚ . The powders having a spherical shape, and were crystallized in cubic fluorite structure with lattice parameter a0 = 5.38 A ˚ showed a very wide pore size distribution in the range of 20 – 2500 A as is shown by mercury porosimetry measurements with surface area c 40.0 m2/g. No change in crystalline phase was observed even after giving heat treatments to as-prepared powders up to 1000 jC for 4 h in air .The SEM/TEM studies on these powders confirmed their nanosized nature with porous structure. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Chemical synthesis; GNP, glycine nitrate process; Microstructure; Nanosized; Binary oxides
1. Introduction In recent years, CeO2-based oxide materials are extensively studied for their wide range of applications in fuel cells [1], gas sensors [2] and as a promoter in three-way catalyst for automotive exhaust control [3,4]. The properties like the promotion of
*
Corresponding author. E-mail address: [email protected] (S.K. Date).
noble metal dispersion, promotion of water gas shift (WGS) reaction, thermal stability and, most importantly, its oxygen storage capacity (OSC) make it an ideal promoter in TWC formations [4]. The key to oxygen storage capacity of CeO2-based materials is due to its ability to shift between Ce4 + and Ce3 + state, stable fluorite structure which in fact allows release and transport of O2 ions. It is reported [5] that the incorporation of zirconium in the lattice of CeO2 enhances further OSC by creating lattice defects and hence facilitating O2 ion mobility. The literature survey [6,7] indicated that many chemical routes such
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 9 3 2 - 1
H.S. Potdar et al. / Materials Letters 57 (2003) 1066–1071
as solid state reaction [8], coprecipitation [9,10], sol – gel [11], hydrothermal [12], microwave –hydrothermal [13], high-energy ball milling [14], etc., are used to synthesize Ce1 xZrxO2 powders. It has been reported [6] that the catalyst performance of Ce1 xZrxO2 solid solutions depends on phase composition, homogeneity and surface area of the materials. Although GNP route [14 – 17] is very widely applied to synthesize homogeneous single, binary and multicomponent oxide systems, it has not been employed to synthesize ceria –zirconia solid solutions. To the best of our knowledge, the GNP route is not yet applied in the preparation of oxides having controlled porosity and pore size distribution. Therefore, the goal of the present investigation is to see the utility and usefulness of GNP in producing the Ce1 xZrxO2 powders with controlled porosity and pore distribution. A typical composition Ce0.75Zr0.25O2 having cubic structure was chosen for the present investigation. The results related to synthesis and characterization of Ce0.75Zr0.25O2 powders are reported and discussed in this paper.
2. Experimental 2.1. Synthesis of Ce0.75Zr0.25O2 powders For the synthesis of ceria – zirconia powders, zirconyl nitrate [ZrO(NO3)2xH2O], cerrous nitrate [Ce (NO 3)36H2O] and glycine were used as starting reagents. The formula weight of [ZrO(NO3)2xH2O] was determined prior to use by gravimetric estimation and was found to be 273.4786 g. The 2.064 g of (0.0275 M) glycine and 4.4022 g of (0.055 M) NH4NO3 were dissolved in distilled water to make their solutions. The molar ratio of glycine/Zr was kept as 2/1 and that of glycine/Ce as 3/1 to achieve complete complexation of both the cations in aqueous system. To adjust the oxidant/fuel ratio, stoichiometric quantity of NH4NO3 was added. All the reagents were dissolved in requisite quantities in minimum amount of distilled water to obtain clear solutions. Initially, all the solutions were mixed together and aged in a petri dish at 90 jC for 48 h when a transparent moisture-sensitive glassy material was obtained. This glassy material was then further kept in a 5-l capacity beaker and heated on a hot plate to raise the temperature to around 200 jC.
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The material got ignited spontaneously resulting in a yellowish colored mass. During the ignition, the beaker was covered with a fine-mesh sieve to prevent the powder from flying out of the beaker. The as-collected powders were further heated at 500, 850 and 1000 jC for 4 h in air to see the its stability. 2.2. Physico-chemical characterization The as-dried GNP derived powders were characterized by using various techniques such as microanalysis, XRD, SEM/TEM, mercury porosimetry. The microanalysis was done by carrying out the combustion of as-dried material at 900 jC in O2 flow using Hosli’s apparatus for microdetermination of C, H and N. The crystalline phase analysis in asdried and calcined powders were carried out by using X-ray diffractometer (Philips PW-1710) with Cu-Ka radiation using Ni filter. The SEM (Leica, Stereoscan 440) technique was used to determine particle size, morphology and nature of agglomerates in the as-dried and calcined powders The mercury intrusion porosimetry (Autoscan, Quantachrome, USA) technique was used to determine the pore size and its distribution in the as-prepared GNP derived powders. The morphology and degree of agglomeration were also determined by TEM technique. The standard BET method was applied for measurements of the surface area.
3. Results and discussion It is reported in the literature [15 –17] that GNP route produces single, binary and multicomponent oxides with better homogeneity. However, it has not been used to synthesize Ce1 xZrxO2 powders. Therefore, the aim of the present investigation is to find the utility of this method in synthesizing the solid solution of Ce1 xZrxO2 powders and study the powder characteristics obtained by this route. The typical cubic composition Ce0.75Zr0.25O2 was chosen as it has many applications as described earlier. The XRD pattern of our as-prepared Ce0.75Zr0.25O2 powders is depicted in Fig. 1a. This pattern has six main reflections of a typical cubic fluorite structure of CeO2 corresponding to [111], [200], [220], [222] and [400] planes. It is evident from Fig. 1a that no line corresponding to
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H.S. Potdar et al. / Materials Letters 57 (2003) 1066–1071
Fig. 1. XRD patterns of (a) as-prepared GNP powder, (b) heated at 500 jC/4 h and (c) at 1000 jC/4 h in air.
either ZrO2 or CeO2 are observed which clearly indicates that Ce and Zr ions are homogeneously mixed in the as-prepared powders. The experimental data on microanalysis of as-synthesized powders are given in Table 1. It showed no contamination of carbon, however, hydrogen c 0.24% and nitrogen c 0.7% were detected in the material. This may be ascribed to the presence of adsorbed and/or trapped H2O and N2 formed during the combustion process [15]. Further, to remove these gases and check the stability of the GNP-derived material, these powders
were heat-treated at various temperatures in air for 4 h. The XRD patterns obtained for the same are given in Fig. 1b and c. The comparison of these patterns clearly established the presence of cubic phase and enhancement in crystallinity of Ce0.75Zr0.25O2 powders as increase in intensity and narrowing of all peaks corresponding to all planes is seen on heat treatment up to 1000 jC. To determine crystallite size (D) and lattice parameter (a0), the slow scans in 2h region of 24 –30j were taken. The slow scan for GNP-derived CeO2 is also included in Fig. 2a –c for comparison. It is seen from Fig. 2 that as Zr cations are introduced into the CeO2 lattice, the positions of (111), (200) reflections are shifted towards a higher 2h values indicating thereby the decrease in the lattice parameter of the solid solution compared to CeO2. The calculated lattice parameter for our Ce0.75Zr0.25O2 powders a0 = 5.38 Aj is in good agreement with the reported data [1] for the same composition. The lattice parameter a0 = 5.42 Aj obtained for pure CeO2 is also in good agreement with reported data [5] of CeO2. Furthermore, the crystallite size (D) determined from Scherrer equation equal to 11.5 nm for the as-synthesized material has increased to 16.4 nm after heat treatment at 850jC for 4 h in air. The crystallite size values for as-prepared GNP material are found to be smaller compared to CeO2 powder. Thus, the introduction of Zr4 + ions in CeO2 lattice helped to reduce the crystallite size of Ce0.75Zr0.25O2 powders (Table. 2). The EDAX analysis on the material calcined at 500 jC/4 h is given in Table 3 confirms the composition of material. The SEM photographs of the asprepared and heat-treated at 500 jC/4 h powders are shown in Fig. 3a and b, respectively. The as-prepared powder showed presence of pores ranging from 0.15 to 1 Am with spherical particles. However, Fig. 3b shows microstructure wherein the pore structure is found to be collapsed. On the other hand, agglomerated material having size 0.15 –5.0 Am is produced. It indicates that on heat treatment pores get eliminated
Table 1 Microanalysis data on as-dried GNP powder Element
Experimental percentage
C H N
– 0.24 0.70
H.S. Potdar et al. / Materials Letters 57 (2003) 1066–1071
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Table 3 EDAX analysis of GNP-derived powders Element
Theoretical percentage
Experimental percentage
Ce Zr O
65.72 14.26 20.01
64.57 15.10 20.30
with the reported observation of templated assisted synthesis of ceria – zirconia solid solutions wherein collapse of pore structure occurred after heat treatment at 500 jC in air [18]. To further substantiate this observation, a systematic study by using TEM technique was undertaken. The microstructural features observed in SEM are also seen in TEM photo-
Fig. 2. XRD of (a) as-prepared CeO2, (b) as-prepared Ce0.75Zr0.25O2 and (c) Ce0.75Zr0.25O2 heated at 850 jC/4 h in air.
and the material is sintered with increase in grains size affecting the original porous microstructure in as-derived material. This observation is consistent Table 2 Data on crystallite size and lattice parameter Material
Crystallite size (nm)
Lattice ˚) parameter (A
As-prepared CeO2 As-prepared Ce0.75Zr0.25O2 Calcined at 850 jC Calcined at 1000 jC
50.40 11.50 16.40 19.30
5.40 5.38 – 5.38
Fig. 3. SEM of (a) as-prepared Ce0.75Zr0.25O2 powder, (b) heated at 500 jC/4 h in air.
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Fig. 6. The pore size and distribution in the as-prepared Ce0.75 Zr0.25O2 powder (mercury porosimetry measurements).
Fig. 4. TEM of as-prepared Ce0.75Zr0.25O2 powder.
micrograph shown in Fig. 4. Nanosize spherical particles ranging from 20 to 35 nm are seen in the as-prepared Ce0.75Zr0.25O2 powders. This size is higher than that calculated from XRD line broadening (Scherrer equation). This indicates that the crystallites are bunched together. The larger pores of size 0.1 –0.5 Am are also seen. Thus, both SEM/ TEM studies on the as-prepared GNP material showed macroporous nature/microstructure of the material. To see whether some mesopores are formed in the material, low-angle diffraction scan was taken
and is shown in Fig. 5. It in fact, showed weak intense reflection at low 2h values. Only one broad peak was observed at a value of d = 22.60 Aj indicating thereby a long-range order in the structure is limited to few regions. This is expected because during the combustion, the temperature is increased above 900 jC [15 – 17] where the structure may get collapsed. The absence of long-range order is confirmed by TEM studies wherein well defined small spherical particles are seen. Furthermore, it is also supported by BET measurements wherein low surface area < 5 m2/g was obtained. The mercury porosimetry results on the as-prepared Ce0.75Zr0.25O2 powders are given in Fig. 6. The presence of pores of larger size or diameter of 0.05 – 0.5 Am are seen along with smaller pores of size 2 –50 nm and it was found
Fig. 5. Low-angle XRD for as-prepared Ce0.75Zr0.25O2 powder.
H.S. Potdar et al. / Materials Letters 57 (2003) 1066–1071
that these larger pores occupy all the volume during intrusion. The total surface area as high as 40.0 m2/g is observed in the as-prepared material by mercury porosimetry measurements. Thus, all the above characterization results pointed out that macroporous, stoichiometric, homogeneous Ce0.75Zr0.25O2 powders are prepared by GNP method. Further study to control pore structure by varying preparation parameters during GNP synthesis and to understand the role of glycine in producing porous microstructure is underway.
4. Conclusion GNP method is successfully employed to obtain porous, homogeneous, stoichiometric Ce0.75Zr0.25O2 powders free from carbon contamination. The powders are nanosized in nature and have spherical shape with cubic fluorite structure. No change in the phase is observed even after heat treatment up to 1000 jC in air. However, pore structure associated in the asprepared material got collapsed by heat treatment at T>500 jC/4 h in air.
Acknowledgements We thank Drs. N.R. Pavaskar and S.R. Sainkar for recording XRD and SEM photographs, respectively. We are also thankful to the DST, New Delhi, for financial support.
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References [1] A. Cabanas, J.W. Darr, E. Lester, M. Martyn, J. Mater. Chem. 11 (2001) 561. [2] S. Rossignol, Y. Madier, D. Duprez, Catal. Today 50 (1999) 261. [3] G. Colon, M. Pijolat, F. Valdivieso, J. Kaspar, E. Finocchio, M. Daturi, C. Binet, J.C. Lavalley, R.T. Baker, S. Bernal, J. Chem. Soc., Faraday Trans. 94 (1998) 3717. [4] A. Trovalli, C. Leitenburg, G. Dolcetti, Chemtech (June 1997) 32. [5] S. Rossignol, F. Gerard, D. Dupez, J. Mater. Chem. 9 (1999) 1615. [6] H.S. Roh, W.-S. Dong, K.-W. Jun, S.-E. Park, Chem. Lett. (2001) 88. [7] H.S. Roh, K.-W. Jun, W.-S. Dong, S.-E. Park, Y.-S. Baek, Catal. Lett. 74 (2001) 31. [8] T. Masui, Y. Peng, K. Machida, G. Adachi, Chem. Mater. 27 (1992) 43. [9] B. Djurcic, D. McGarry, S. Pickering, J. Mater. Sci. Lett. 12 (1993) 1320. [10] A. Hirao, T. Miwa, M. Inagaki, J. Solid State Chem. 158 (2001) 112. [11] A. Deptula, M. Carewska, T. Olczak, W. Lada, F. Croce, J. Eletrochem. Soc. 140 (1993) 2294. [12] M. Hirao, E. Kato, J. Ceram. Soc. Jpn. 104 (1996) 958. [13] H.S. Potdar, S.B. Deshpande, A.S. Deshpande, S.P. Gokale, Y.B. Khollam, A.J. Patil, S.K. Date, Mater. Chem. Phys. 74 (2001) 306. [14] C. de Litenberg, A. Trovalli, F. Zamar, G. Maschio, G. Doketa, L. Llorca, J. Chem. Soc. Commun. (1995) 2181. [15] L.A. Chick, L.R. Pederson, G.D. Maupin, J.L. Bates, L.E. Thomas, G.J. Exarhos, Mater. Lett. 10 (1990) 6. [16] J.J. Kingsley, L.R. Rederson, Mater. Lett. 18 (1993) 89. [17] L.R. Pederson, G.D. Maupin, W.J. Weber, D.J. McReady, R.W. Stephens, Mater. Lett. 10 (1991) 437. [18] D. Terribile, A. Trovarelli, J. Llorca, C. de Leitenburg, G. Dolcetti, J. Catal. 178 (1998) 299.
Materials Letters 57 (2003) 1066 – 1071 www.elsevier.com/locate/matlet
Synthesis of nanosized Ce0.75Zr0.25O2 porous powders via an autoignition: glycine nitrate process H.S. Potdar a, S.B. Deshpande a, Y.B. Khollam b, A.S. Deshpande a, S.K. Date a,* a
Physical Chemistry Division, National Chemical Laboratory, Pune 411008, India b Department of Physics, University of Pune, Pune 411007, India Received 26 May 2002; accepted 30 May 2002
Abstract The GNP route has been successfully employed to generate nanosized, porous, stoichiometric, homogeneous Ce0.75Zr0.25O2 powders. In this route, a precursor solution is prepared by mixing glycine with an aqueous solution of mixed (Ce – Zr) metal – nitrates in their stoichiometric ratios. Initially, the glycine mixed precursor solution was heated in a petri dish to evaporate the excess water to form a viscous semitransparent material. Then the petri dish was kept in a 5-l capacity beaker covered with a metallic mesh and then the temperature was increased slowly to 190 – 200 jC to autoignite the material. The combustion was self-sustaining and very rapid, producing yellowish coloured powders. The as-prepared powders were nanosized (20 – 35 nm), ˚ . The powders having a spherical shape, and were crystallized in cubic fluorite structure with lattice parameter a0 = 5.38 A ˚ showed a very wide pore size distribution in the range of 20 – 2500 A as is shown by mercury porosimetry measurements with surface area c 40.0 m2/g. No change in crystalline phase was observed even after giving heat treatments to as-prepared powders up to 1000 jC for 4 h in air .The SEM/TEM studies on these powders confirmed their nanosized nature with porous structure. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Chemical synthesis; GNP, glycine nitrate process; Microstructure; Nanosized; Binary oxides
1. Introduction In recent years, CeO2-based oxide materials are extensively studied for their wide range of applications in fuel cells [1], gas sensors [2] and as a promoter in three-way catalyst for automotive exhaust control [3,4]. The properties like the promotion of
*
Corresponding author. E-mail address: [email protected] (S.K. Date).
noble metal dispersion, promotion of water gas shift (WGS) reaction, thermal stability and, most importantly, its oxygen storage capacity (OSC) make it an ideal promoter in TWC formations [4]. The key to oxygen storage capacity of CeO2-based materials is due to its ability to shift between Ce4 + and Ce3 + state, stable fluorite structure which in fact allows release and transport of O2 ions. It is reported [5] that the incorporation of zirconium in the lattice of CeO2 enhances further OSC by creating lattice defects and hence facilitating O2 ion mobility. The literature survey [6,7] indicated that many chemical routes such
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 9 3 2 - 1
H.S. Potdar et al. / Materials Letters 57 (2003) 1066–1071
as solid state reaction [8], coprecipitation [9,10], sol – gel [11], hydrothermal [12], microwave –hydrothermal [13], high-energy ball milling [14], etc., are used to synthesize Ce1 xZrxO2 powders. It has been reported [6] that the catalyst performance of Ce1 xZrxO2 solid solutions depends on phase composition, homogeneity and surface area of the materials. Although GNP route [14 – 17] is very widely applied to synthesize homogeneous single, binary and multicomponent oxide systems, it has not been employed to synthesize ceria –zirconia solid solutions. To the best of our knowledge, the GNP route is not yet applied in the preparation of oxides having controlled porosity and pore size distribution. Therefore, the goal of the present investigation is to see the utility and usefulness of GNP in producing the Ce1 xZrxO2 powders with controlled porosity and pore distribution. A typical composition Ce0.75Zr0.25O2 having cubic structure was chosen for the present investigation. The results related to synthesis and characterization of Ce0.75Zr0.25O2 powders are reported and discussed in this paper.
2. Experimental 2.1. Synthesis of Ce0.75Zr0.25O2 powders For the synthesis of ceria – zirconia powders, zirconyl nitrate [ZrO(NO3)2xH2O], cerrous nitrate [Ce (NO 3)36H2O] and glycine were used as starting reagents. The formula weight of [ZrO(NO3)2xH2O] was determined prior to use by gravimetric estimation and was found to be 273.4786 g. The 2.064 g of (0.0275 M) glycine and 4.4022 g of (0.055 M) NH4NO3 were dissolved in distilled water to make their solutions. The molar ratio of glycine/Zr was kept as 2/1 and that of glycine/Ce as 3/1 to achieve complete complexation of both the cations in aqueous system. To adjust the oxidant/fuel ratio, stoichiometric quantity of NH4NO3 was added. All the reagents were dissolved in requisite quantities in minimum amount of distilled water to obtain clear solutions. Initially, all the solutions were mixed together and aged in a petri dish at 90 jC for 48 h when a transparent moisture-sensitive glassy material was obtained. This glassy material was then further kept in a 5-l capacity beaker and heated on a hot plate to raise the temperature to around 200 jC.
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The material got ignited spontaneously resulting in a yellowish colored mass. During the ignition, the beaker was covered with a fine-mesh sieve to prevent the powder from flying out of the beaker. The as-collected powders were further heated at 500, 850 and 1000 jC for 4 h in air to see the its stability. 2.2. Physico-chemical characterization The as-dried GNP derived powders were characterized by using various techniques such as microanalysis, XRD, SEM/TEM, mercury porosimetry. The microanalysis was done by carrying out the combustion of as-dried material at 900 jC in O2 flow using Hosli’s apparatus for microdetermination of C, H and N. The crystalline phase analysis in asdried and calcined powders were carried out by using X-ray diffractometer (Philips PW-1710) with Cu-Ka radiation using Ni filter. The SEM (Leica, Stereoscan 440) technique was used to determine particle size, morphology and nature of agglomerates in the as-dried and calcined powders The mercury intrusion porosimetry (Autoscan, Quantachrome, USA) technique was used to determine the pore size and its distribution in the as-prepared GNP derived powders. The morphology and degree of agglomeration were also determined by TEM technique. The standard BET method was applied for measurements of the surface area.
3. Results and discussion It is reported in the literature [15 –17] that GNP route produces single, binary and multicomponent oxides with better homogeneity. However, it has not been used to synthesize Ce1 xZrxO2 powders. Therefore, the aim of the present investigation is to find the utility of this method in synthesizing the solid solution of Ce1 xZrxO2 powders and study the powder characteristics obtained by this route. The typical cubic composition Ce0.75Zr0.25O2 was chosen as it has many applications as described earlier. The XRD pattern of our as-prepared Ce0.75Zr0.25O2 powders is depicted in Fig. 1a. This pattern has six main reflections of a typical cubic fluorite structure of CeO2 corresponding to [111], [200], [220], [222] and [400] planes. It is evident from Fig. 1a that no line corresponding to
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Fig. 1. XRD patterns of (a) as-prepared GNP powder, (b) heated at 500 jC/4 h and (c) at 1000 jC/4 h in air.
either ZrO2 or CeO2 are observed which clearly indicates that Ce and Zr ions are homogeneously mixed in the as-prepared powders. The experimental data on microanalysis of as-synthesized powders are given in Table 1. It showed no contamination of carbon, however, hydrogen c 0.24% and nitrogen c 0.7% were detected in the material. This may be ascribed to the presence of adsorbed and/or trapped H2O and N2 formed during the combustion process [15]. Further, to remove these gases and check the stability of the GNP-derived material, these powders
were heat-treated at various temperatures in air for 4 h. The XRD patterns obtained for the same are given in Fig. 1b and c. The comparison of these patterns clearly established the presence of cubic phase and enhancement in crystallinity of Ce0.75Zr0.25O2 powders as increase in intensity and narrowing of all peaks corresponding to all planes is seen on heat treatment up to 1000 jC. To determine crystallite size (D) and lattice parameter (a0), the slow scans in 2h region of 24 –30j were taken. The slow scan for GNP-derived CeO2 is also included in Fig. 2a –c for comparison. It is seen from Fig. 2 that as Zr cations are introduced into the CeO2 lattice, the positions of (111), (200) reflections are shifted towards a higher 2h values indicating thereby the decrease in the lattice parameter of the solid solution compared to CeO2. The calculated lattice parameter for our Ce0.75Zr0.25O2 powders a0 = 5.38 Aj is in good agreement with the reported data [1] for the same composition. The lattice parameter a0 = 5.42 Aj obtained for pure CeO2 is also in good agreement with reported data [5] of CeO2. Furthermore, the crystallite size (D) determined from Scherrer equation equal to 11.5 nm for the as-synthesized material has increased to 16.4 nm after heat treatment at 850jC for 4 h in air. The crystallite size values for as-prepared GNP material are found to be smaller compared to CeO2 powder. Thus, the introduction of Zr4 + ions in CeO2 lattice helped to reduce the crystallite size of Ce0.75Zr0.25O2 powders (Table. 2). The EDAX analysis on the material calcined at 500 jC/4 h is given in Table 3 confirms the composition of material. The SEM photographs of the asprepared and heat-treated at 500 jC/4 h powders are shown in Fig. 3a and b, respectively. The as-prepared powder showed presence of pores ranging from 0.15 to 1 Am with spherical particles. However, Fig. 3b shows microstructure wherein the pore structure is found to be collapsed. On the other hand, agglomerated material having size 0.15 –5.0 Am is produced. It indicates that on heat treatment pores get eliminated
Table 1 Microanalysis data on as-dried GNP powder Element
Experimental percentage
C H N
– 0.24 0.70
H.S. Potdar et al. / Materials Letters 57 (2003) 1066–1071
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Table 3 EDAX analysis of GNP-derived powders Element
Theoretical percentage
Experimental percentage
Ce Zr O
65.72 14.26 20.01
64.57 15.10 20.30
with the reported observation of templated assisted synthesis of ceria – zirconia solid solutions wherein collapse of pore structure occurred after heat treatment at 500 jC in air [18]. To further substantiate this observation, a systematic study by using TEM technique was undertaken. The microstructural features observed in SEM are also seen in TEM photo-
Fig. 2. XRD of (a) as-prepared CeO2, (b) as-prepared Ce0.75Zr0.25O2 and (c) Ce0.75Zr0.25O2 heated at 850 jC/4 h in air.
and the material is sintered with increase in grains size affecting the original porous microstructure in as-derived material. This observation is consistent Table 2 Data on crystallite size and lattice parameter Material
Crystallite size (nm)
Lattice ˚) parameter (A
As-prepared CeO2 As-prepared Ce0.75Zr0.25O2 Calcined at 850 jC Calcined at 1000 jC
50.40 11.50 16.40 19.30
5.40 5.38 – 5.38
Fig. 3. SEM of (a) as-prepared Ce0.75Zr0.25O2 powder, (b) heated at 500 jC/4 h in air.
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Fig. 6. The pore size and distribution in the as-prepared Ce0.75 Zr0.25O2 powder (mercury porosimetry measurements).
Fig. 4. TEM of as-prepared Ce0.75Zr0.25O2 powder.
micrograph shown in Fig. 4. Nanosize spherical particles ranging from 20 to 35 nm are seen in the as-prepared Ce0.75Zr0.25O2 powders. This size is higher than that calculated from XRD line broadening (Scherrer equation). This indicates that the crystallites are bunched together. The larger pores of size 0.1 –0.5 Am are also seen. Thus, both SEM/ TEM studies on the as-prepared GNP material showed macroporous nature/microstructure of the material. To see whether some mesopores are formed in the material, low-angle diffraction scan was taken
and is shown in Fig. 5. It in fact, showed weak intense reflection at low 2h values. Only one broad peak was observed at a value of d = 22.60 Aj indicating thereby a long-range order in the structure is limited to few regions. This is expected because during the combustion, the temperature is increased above 900 jC [15 – 17] where the structure may get collapsed. The absence of long-range order is confirmed by TEM studies wherein well defined small spherical particles are seen. Furthermore, it is also supported by BET measurements wherein low surface area < 5 m2/g was obtained. The mercury porosimetry results on the as-prepared Ce0.75Zr0.25O2 powders are given in Fig. 6. The presence of pores of larger size or diameter of 0.05 – 0.5 Am are seen along with smaller pores of size 2 –50 nm and it was found
Fig. 5. Low-angle XRD for as-prepared Ce0.75Zr0.25O2 powder.
H.S. Potdar et al. / Materials Letters 57 (2003) 1066–1071
that these larger pores occupy all the volume during intrusion. The total surface area as high as 40.0 m2/g is observed in the as-prepared material by mercury porosimetry measurements. Thus, all the above characterization results pointed out that macroporous, stoichiometric, homogeneous Ce0.75Zr0.25O2 powders are prepared by GNP method. Further study to control pore structure by varying preparation parameters during GNP synthesis and to understand the role of glycine in producing porous microstructure is underway.
4. Conclusion GNP method is successfully employed to obtain porous, homogeneous, stoichiometric Ce0.75Zr0.25O2 powders free from carbon contamination. The powders are nanosized in nature and have spherical shape with cubic fluorite structure. No change in the phase is observed even after heat treatment up to 1000 jC in air. However, pore structure associated in the asprepared material got collapsed by heat treatment at T>500 jC/4 h in air.
Acknowledgements We thank Drs. N.R. Pavaskar and S.R. Sainkar for recording XRD and SEM photographs, respectively. We are also thankful to the DST, New Delhi, for financial support.
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