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A sol–gel route using propylene oxide as a gelation agent to synthesize spherical NiAl2O4 nanoparticles
Journal of Non-Crystalline Solids 351 (2005) 2102–2106 www.elsevier.com/locate/jnoncrysol
A sol–gel route using propylene oxide as a gelation agent to synthesize spherical NiAl2O4 nanoparticles Hongtao Cui, Marcos Zayat, David Levy
*
Instituto de Ciencia de Materiales de Madrid—ICMM, CSIC, 28049 Cantoblanco, Madrid, Spain Received 14 October 2004; received in revised form 30 March 2005 Available online 13 June 2005
Abstract Spherical NiAl2O4 nanoparticles with large surface area were successfully synthesized by a sol–gel method using propylene oxide as a gelation agent. The formation process of the resulting nanoparticles is described and characterized by TG-DTA, XRD, BET and TEM. A pseudo-spinel structure was formed at 700 °C and a spinel NiAl2O4 structure was formed at 800 °C by the reaction of NiO at the surface of amorphous alumina particles. It was found that the presence of NiO or NiAl2O4 on the surface of the alumina retards the crystallization of amorphous alumina to c-Al2O3. The obtained nanoparticles experienced the decrease of particle size from 700 to 800 °C accompanying the structure transformation from pseudo-spinel to NiAl2O4 spinel. Ó 2005 Elsevier B.V. All rights reserved.
1. Introduction The sol–gel process allows the low temperature synthesis of inorganic networks by a chemical reaction in solution, generally the hydrolysis and subsequent condensation of metallic alkoxides [1,2]. The sol–gel process is a promising chemical synthetic route that offers several advantages such as high purity, high chemical homogeneity, lower calcination temperatures and particle size control. Nowadays, the sol–gel chemistry provides a means for preparing mixed oxides in which the mixing of two or more metal oxide phases can be controlled on both the molecular and the nanoscale. The different reactivity of individual components has been the major problem of the synthesis of mixed oxides, especially in the alkoxides based sol–gel process. The problem can be minimized by controlled prehydrolysis of the less reactive precursor [3], by chemical modifica-
*
Corresponding author. Tel.: +34 91 334 90 76; fax: +34 91 372 06
23. E-mail address: [email protected] (D. Levy). 0022-3093/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.04.060
tion of the precursors [4], by using single-source heterobimetallic alkoxide precursors [2] or by a non-hydrolytic sol–gel route [5]. However, the research of new chemical routes to synthesize homogeneous mixed oxides, avoiding sophisticated procedures, is still under way. NiAl2O4 is a transition metal spinel that has a usual spinel structure with aluminum in the octahedral sites and nickel in the tetrahedral sites. NiAl2O4 has been a subject of extensive research from preparation to properties. The conventional method of preparation (solid state reaction between metal oxides) needs high temperatures of calcination and long reaction periods [6], resulting in low surface area NiAl2O4. Co-precipitation [7], homogenous precipitation [8] and sol–gel [9] methods have been used to synthesize NiAl2O4 with small particle size and high surface area. The major problem of these three methods is the phase segregation due to the different reactivity of the individual precursors. Usually, it is more difficult to overcome the problem of phase segregation in the precipitation method than in the sol–gel method. A new non-alkoxide sol–gel method using epoxide as gelation agent has been developed, and some metal
H. Cui et al. / Journal of Non-Crystalline Solids 351 (2005) 2102–2106
oxide aerogels [10–13] and nanoparticles [14–16] have been prepared by this sol–gel route. In the case of mixed oxides, some metal oxide/silica composites [17,18] have been synthesized by this route. It was reported by the Lawrence Livermore National Laboratory [10,11] that metal oxide gels cannot be prepared from aqueous metal ions that have a formal charge of less than +3 by the epoxide method, because the acidities of [M(H2O)x]2+ complexes are much lower than acidities of aqueous complexes of Mn+ ions with n = 3, 4, 5, 6. This lower acidity would slow down the protonation of the added epoxide and the subsequent rise in the pH, which allows alternative side reactions to occur to a significant degree and cause precipitation to take place. In this work, we will prove the capability of the epoxide sol–gel method to synthesize mixed oxide nanoparticles with spinel structure such as NiAl2O4 having a large surface area and spherical morphology, and which contains metal ions having a formal charge of less than +3.
2. Experimental 2.1. Materials Nickel nitrate hexahydrate (Ni(NO3)2 Æ 6H2O) and propylene oxide (PPO) were obtained from Aldrich, aluminium nonahydrate (Al(NO3)3 Æ 9H2O) was from Fluka, and Ethanol was from merck. All these materials were used as received. 2.2. Preparation of NiAl2O4 spinel nanoparticles All syntheses were performed under ambient atmosphere. Clear green ethanol solutions with different Ni/ Al molar ratio were formed by dissolving Ni(NO3)2 Æ 6H2O and Al(NO3)3 Æ 9H2O in ethanol. After the addition of propylene oxide (PPO:metal ions molar ratio is 10) to the ethanol solution, an exothermic reaction occurred within a few minutes, followed by a gradual formation of a light green rigid gel from the top to the bottom of the solution. The gel was aged in a closed vessel at 50 °C for 3 h, dried in an open vessel at 50 °C for 24 h and then treated at 100 °C for additional 12 h. The resulting xerogel was ground to powder and calcined at the given temperature for 1 h to obtain NiAl2O4 nanoparticles. 2.3. Characterization The thermal behaviour (TG/DTA) of the samples was studied by a Seiko SSC/5200 (TG/DTA 320U) in static air atmosphere from ambient temperature to 1000 °C at a heating rate of 10 °C/min. The XRD patterns of the samples were measured in a Philips PW 1710 diffrac-
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tometer using CuKa radiation. Nitrogen adsorption– desorption isotherms of the samples were measured at 77 K with a Micromeritics ASAP 2010 surface area and porosity analyser. And the surface areas of the samples were calculated from the adsorption isotherms using the BET (Brunauer–Emmett–Teller) method in the P/P0 range of 0.1–0.2 and a molecular cross sectional area of nitrogen of 0.162 nm2. The morphology of the particles was observed by a JEOL 2000 transmission electron microscope working at 200 kV.
3. Result and discussion 3.1. Gel formation The formation of the gel involves the hydrolysis and condensation of hydrated metal ions. In the bimetallic system, the problem of phase segregation arises due to the different hydrolysis rates of the individual hydrated metal ions components. In the experiment, the hydrolysis and condensation rate of Al3+ alone in ethanol solution was rather fast, forming gels within a few minutes; on the contrary, that of Ni2+ alone in solution was much slower, giving a precipitate about 10 min after the propylene oxide was added. However, after the addition of propylene oxide to an ethanol mixture solution of Al3+ and Ni2+, a gel formed without any precipitation. After aging the gels for several hours at ambient temperature, a layer of green solution appeared on the upper part of the gel, showing that the hydrolysis of the Ni2+ was not complete. After two days of aging at ambient temperature or 3 h of aging at 50 °C, the light green colour of the solution disappeared, accounting for the complete hydrolysis of the Ni2+. All these facts suggest that an aluminium gel is formed first, while the Ni2+ ions remain in solution, then the hydrated nickel ions are hydrolysed and condensed at the surface of the aluminium gel surface. 3.2. Thermogravimetric analysis TG and DTA curves of the thermal degradation of the precursor gel with a Ni/Al molar ratio of 0.11 are shown in Fig. 1. There are two sharp exothermic peaks in the DTA curve around 191 °C and 337 °C accompanied by two clear weight losses in the TG curve that result from the decomposition of organic matter. Above 337 °C, only a small weight loss is observed, which is attributed to the release of water arising from condensation reaction. 3.3. Crystallographic analysis The XRD patterns of the samples prepared with different Ni/Al molar ratios from 0.034 to 0.67 and cal-
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100
337oC
90
30
DTAµV
191oC
70
10
60
0
50
TG (%)
80
20
40
-10 30
200
400
600
800
1000
Temperature (oC) Fig. 1. TG/DTA analysis of a precursor gel with a Ni/Al molar ratio of 0.11 carried out in static air.
cined at 700 °C are shown in Fig. 2. Except for the two amorphous samples with Ni/Al = 0.034 and 0.071, all samples clearly show a spinel structure. With the increase of Ni/Al molar ratio from 0.11 to 0.67, their diffraction lines show a gradual shift to lower 2h. The green coloration of the samples with Ni/Al molar ratios between 0.11 and 0.67 suggests the existence of nickel oxide, however no traces of nickel oxide were found in their XRD patterns (Fig. 2). This can be attributed to the formation of a pseudo-spinel structure as observed when an excess amount of Al2O3 is added to NiO and calcined at high temperature [19]. The increase of Ni2+ ions concentration with the increase of Ni/Al molar ratios from 0.11 to 0.67 suggests that Al3+ sites in the lattice of the pseudo-spinel are progressively substituted by Ni2+ ions. According to the standard reported data for ionic radii [20], the effective ionic radius of Ni2+ ion is
Fig. 2. XRD patterns of the samples prepared with different Ni/Al molar ratios and calcined at 700 °C (}, pseudo-spinel).
larger than Al3+ (69 pm vs 54 pm). The substitution of smaller ions (Al3+) by larger ions (Ni2+) results in the lattice expansion of the pseudo-spinel, and then leads to a peak shift toward lower 2h as observed in the XRD patterns. For the aluminium-rich sample with Ni/Al = 0.11, the position of the diffraction lines is very near to those of spinel type c-Al2O3 (JCPDS 10-0425); while samples with Ni/Al molar ratios of less than 0.11 show amorphous XRD patterns, which suggests that alumina does not crystallize at 700 °C and the concentration of NiO in the samples is not enough for the formation of pseudo-spinel structure. The XRD patterns of the samples with Ni/Al = 0.11 calcined at different temperatures provide us information on the formation temperature of the spinel structure (Fig. 3). The sample shows an amorphous phase at 500 °C, a pseudo-spinel phase and light green coloration at 700 °C, and a spinel phase and light blue coloration of NiAl2O4 (JCPDS 10-0339) at 800 and 900 °C. The latter shows a little shift in the diffraction lines towards higher 2h as compared with the pseudo-spinel phase obtained at 700 °C. The samples calcined at 800 °C show a gradual coloration change from light blue to light green with the increase of Ni/Al molar ratios from 0.034 to 0.67. In Al-rich samples with Ni/Al molar ratios between 0.034 and 0.25, a light blue coloration is observed. A light green coloration can be seen in samples with Ni/Al molar ratio between 0.30 and 0.67. The coloration change from blue to green accounts for the phase change from spinel to pseudo-spinel as observed in the XRD patterns (Fig. 4). The diffraction lines of the samples with Ni/Al between 0.034 and 0.071 are attributed to the spinel-type c-Al2O3 phase (JCPDS 10-0425), and those of the light blue samples with Ni/Al between
Fig. 3. XRD patterns of samples with Ni/Al = 0.11 calcined at different temperature ( , spinel; }, pseudo-spinel).
H. Cui et al. / Journal of Non-Crystalline Solids 351 (2005) 2102–2106 S γ-Al2O3
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S
P+S
P
Blue
Green
Green
Temperature (°C)
800 Blue
P
A
700 Green
Green
A
500
Green
0.034 0.071 0.11
0.15 0.20 0.25
0.30 0.36
0.43 0.50 0.67
Ni/Al mol ratio Fig. 5. Phase and coloration evolution with the temperature and different Ni/Al molar ratios: A, amorphous; P, pseudo-spinel; S, spinel.
Fig. 4. XRD patterns of the samples prepared with different Ni/Al molar ratios and calcined at 800 °C ( , spinel; r, c-Al2O3; }, pseudospinel).
0.11 and 0.25 are attributed the spinel phase of NiAl2O4. When the Ni/Al molar ratios increase to between 0.30 and 0.50, it is assumed that the diffraction lines of these light green samples are attributed to the mixture of the pseudo-spinel phase of NiO–Al2O3 and the spinel phase of NiAl2O4. When Ni/Al molar ratios increase to 0.67, three of the diffraction lines at 2h = 19.1, 31.6 and 60 disappeared and the left lines of the light green sample are attributed to the pseudospinel phase of NiO–Al2O3. Generally, the phase transformation temperature from amorphous alumina to cAl2O3 is between 500 and 800 °C. While, c-Al2O3 phase was only found in the samples calcined at 700 °C with Ni/Al molar ratios between 0.071 and 0.034. In the samples with Ni/Al molar ratios between 0.25 and 0.11, this is probably due to dissolution of spinel type c-Al2O3 (in all proportions) in spinel type NiAl2O4 [8,21,22]. In the samples with Ni/Al molar ratios between 0.30 and 0.67, this is probably due to the retardation of phase transformation from amorphous alumina to c-Al2O3 by NiO [23]. A graph summarizing the coloration and crystallographic phases of the different samples and different temperatures is given in Fig. 5. In view of the results obtained in this work, the particle formation process can be described as follows: the hydrated nickel ions are hydrolysed and condensed at the surface of the aluminium gel and form a NiO coating after calcination, then reacts with amorphous alumina particle surface to form a pseudo-spinel structure at 700 °C and spinel NiAl2O4 at 800 °C. During the calcination, the NiO or NiAl2O4 on the surface of amorphous alumina retards the formation of c-Al2O3.
3.4. BET surface area and transmission electron microscope analysis The BET surface area and particle size estimated from XRD diffraction lines are shown in Table 1. It could be noticed that the crystallite size estimated from the XRD patterns of the samples calcined at these three temperatures remains almost unaffected. At the same time, the BET surface area of the samples with Ni/Al = 0.11 increases from 192 m2/g at 700 °C to 263 m2/g at 800 °C, then decreases to 102 m2/g at 900 °C. This effect was also observed in the transmission electron micrographs (Fig. 6), where the samples calcined at 700 and 900 °C show large agglomerates of nanoparticles smaller than 10 nm, and the sample calcined at 800 °C shows well dispersed particles of the same crystallite size as the other samples. The increase of the BET surface from 700 to 800 °C can be attributed to the structure transformation from pseudo-spinel to NiAl2O4 spinel as observed in the XRD patterns, while the decrease in the BET between 800 and 900 °C is due to the aggregation of particles caused by the higher calcinations temperature.
4. Conclusions In this work, spherical spinel-type NiAl2O4 nanoparticles with one of the metal ions that has a formal charge of less than +3 were successfully synthesized by a sol–gel method using propylene oxide as a gelation agent. The main step in the process is that hydrated nickel ions are hydrolysed and condensed at the surface of the previously formed aluminium oxide gel. During the thermal treatment, the NiO reacts with amorphous alumina particle surface to form a pseudo-spinel structure at 700 °C, and a spinel NiAl2O4 at 800 °C. The XRD patterns indicate that the NiO or NiAl2O4 on the surface of alumina retards the crystallization of amorphous alumina to cAl2O3 at 800 °C. It has been found that the mixed oxide
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Table 1 BET surface area of the sample with Ni/Al = 0.11 calcined at different temperature Calcination temperature (°C)
Surface area 2
700 800 900 *
*
Crystal size estimated from XRD (nm)
BET (m /g)
C value
192 263 102
118 67 95
8.2 7.7 7.2
Error < 0.5% in all samples.
Fig. 6. Transmission electron micrographs of the sample with Ni/Al = 0.11 calcined at 700 (a), 800 (b) and 900 °C (c).
experienced a decrease in particle size from 700 to 800 °C accompanied by a structure transformation from pseudo-spinel to NiAl2O4 spinel. This method was found to be an effective route to synthesize mixed oxide nanoparticles with narrow size distribution and large surface area. The route described here can also applied for the synthesis of mixed oxides with other structures and more than two metal components. References ¨ zdemir, Bu¨lent Alici, J. Mater. [1] B. C ¸ etinkaya, T. Sec¸kin, I. O Chem. 8 (1998) 1835. [2] F. Meyer, R. Hempelmann, S. Mathur, M. Veith, J. Mater. Chem. 9 (1999) 1755. [3] M.B.D. Mitchell, D. Jackson, P.F. James, J. Sol–Gel Sci. Techn. 13 (1998) 359. [4] N. Yamada, I. Yoshinaga, S. Katayama, J. Sol–Gel Sci. Techn. 17 (2000) 123. [5] M. Andrianainarivelo, R.J.P. Corriu, D. Leclercq, P.H. Mutin, A. Vioux, Chem. Mater. 9 (1997) 1098. [6] J.M. Ferna´ndez Colinas, C. Otero Area´n, J. Solid State Chem. 109 (1994) 43. [7] Y. Cesteros, P. Salagre, F. Medina, J.E. Sueiras, Chem. Mater. 12 (2000) 331.
[8] P. Jeevanandam, Y. Koltypin, A. Gedanken, Mater. Sci. Eng. B 90 (2002) 125. [9] C.O. Area´n, P.M. Mentruit, A.J. Lo´pez, J.B. Parra, Coll. Surf. A 180 (2001) 253. [10] A.E. Gash, T.M. Tillotson, J.H. Satcher Jr., L.W. Hrubesh, R.L. Simpson, J. Non-Cryst. Solids 285 (2001) 22. [11] A.E. Gash, T.M. Tillotson, J.H. Satcher Jr., J.F. Poco, L.W. Hrubesh, R.L. Simpson, Chem. Mater. 13 (2001) 999. [12] D. Suh, T. Park, W. Kim, I.J. Hong, Power Sources 117 (2003) 1. [13] R.A. Reibold, J.F. Poco, T.F. Baumann, R.L. Simpson, J.H. Satcher Jr., J. Non-Cryst. Solids 319 (2003) 241. [14] Y. Xie, J. Am. Ceram. Soc. 82 (1999) 768. [15] W. Dong, C. Zhu, J. Mater. Chem. 12 (2002) 1676. [16] B.J. Clapsaddle, A.E. Gash, J.H. Satcher Jr., R.L. Simpson, J. Non-Cryst. Solids 331 (2003) 190. [17] T.M. Tillotson, W.E. Sunderland, I.M. Thomas, L.W. Hrubesh, Sol–Gel Sci. Technol. 1 (1994) 241. [18] H. Itoh, T. Tabata, M. Mkokitsu, N. Okazaki, Y. Imizu, A. Tada, J. Ceram. Soc. Jpn. 101 (1993) 1081. [19] M.V. Twigg, J.T. Richardson, Appl. Catal. A 190 (2000) 61. [20] C.E. Housecroft, A.G. Sharpe, Inorganic Chemistry, first ed., Pearson Education Limited, London, 2001. Appendices 6. [21] A. Navrotsky, B.A. Wechsler, K. Geisinger, F. Seifert, J. Am. Ceram. Soc. 69 (1986) 418. [22] K. Okada, A. Hattori, J. Am. Ceram. Soc. 83 (2000) 928. [23] G. Li, X. Huang, M. Ruan, J. Guo, Ceram. Int. 28 (2002) 165.
A sol–gel route using propylene oxide as a gelation agent to synthesize spherical NiAl2O4 nanoparticles Hongtao Cui, Marcos Zayat, David Levy
*
Instituto de Ciencia de Materiales de Madrid—ICMM, CSIC, 28049 Cantoblanco, Madrid, Spain Received 14 October 2004; received in revised form 30 March 2005 Available online 13 June 2005
Abstract Spherical NiAl2O4 nanoparticles with large surface area were successfully synthesized by a sol–gel method using propylene oxide as a gelation agent. The formation process of the resulting nanoparticles is described and characterized by TG-DTA, XRD, BET and TEM. A pseudo-spinel structure was formed at 700 °C and a spinel NiAl2O4 structure was formed at 800 °C by the reaction of NiO at the surface of amorphous alumina particles. It was found that the presence of NiO or NiAl2O4 on the surface of the alumina retards the crystallization of amorphous alumina to c-Al2O3. The obtained nanoparticles experienced the decrease of particle size from 700 to 800 °C accompanying the structure transformation from pseudo-spinel to NiAl2O4 spinel. Ó 2005 Elsevier B.V. All rights reserved.
1. Introduction The sol–gel process allows the low temperature synthesis of inorganic networks by a chemical reaction in solution, generally the hydrolysis and subsequent condensation of metallic alkoxides [1,2]. The sol–gel process is a promising chemical synthetic route that offers several advantages such as high purity, high chemical homogeneity, lower calcination temperatures and particle size control. Nowadays, the sol–gel chemistry provides a means for preparing mixed oxides in which the mixing of two or more metal oxide phases can be controlled on both the molecular and the nanoscale. The different reactivity of individual components has been the major problem of the synthesis of mixed oxides, especially in the alkoxides based sol–gel process. The problem can be minimized by controlled prehydrolysis of the less reactive precursor [3], by chemical modifica-
*
Corresponding author. Tel.: +34 91 334 90 76; fax: +34 91 372 06
23. E-mail address: [email protected] (D. Levy). 0022-3093/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.04.060
tion of the precursors [4], by using single-source heterobimetallic alkoxide precursors [2] or by a non-hydrolytic sol–gel route [5]. However, the research of new chemical routes to synthesize homogeneous mixed oxides, avoiding sophisticated procedures, is still under way. NiAl2O4 is a transition metal spinel that has a usual spinel structure with aluminum in the octahedral sites and nickel in the tetrahedral sites. NiAl2O4 has been a subject of extensive research from preparation to properties. The conventional method of preparation (solid state reaction between metal oxides) needs high temperatures of calcination and long reaction periods [6], resulting in low surface area NiAl2O4. Co-precipitation [7], homogenous precipitation [8] and sol–gel [9] methods have been used to synthesize NiAl2O4 with small particle size and high surface area. The major problem of these three methods is the phase segregation due to the different reactivity of the individual precursors. Usually, it is more difficult to overcome the problem of phase segregation in the precipitation method than in the sol–gel method. A new non-alkoxide sol–gel method using epoxide as gelation agent has been developed, and some metal
H. Cui et al. / Journal of Non-Crystalline Solids 351 (2005) 2102–2106
oxide aerogels [10–13] and nanoparticles [14–16] have been prepared by this sol–gel route. In the case of mixed oxides, some metal oxide/silica composites [17,18] have been synthesized by this route. It was reported by the Lawrence Livermore National Laboratory [10,11] that metal oxide gels cannot be prepared from aqueous metal ions that have a formal charge of less than +3 by the epoxide method, because the acidities of [M(H2O)x]2+ complexes are much lower than acidities of aqueous complexes of Mn+ ions with n = 3, 4, 5, 6. This lower acidity would slow down the protonation of the added epoxide and the subsequent rise in the pH, which allows alternative side reactions to occur to a significant degree and cause precipitation to take place. In this work, we will prove the capability of the epoxide sol–gel method to synthesize mixed oxide nanoparticles with spinel structure such as NiAl2O4 having a large surface area and spherical morphology, and which contains metal ions having a formal charge of less than +3.
2. Experimental 2.1. Materials Nickel nitrate hexahydrate (Ni(NO3)2 Æ 6H2O) and propylene oxide (PPO) were obtained from Aldrich, aluminium nonahydrate (Al(NO3)3 Æ 9H2O) was from Fluka, and Ethanol was from merck. All these materials were used as received. 2.2. Preparation of NiAl2O4 spinel nanoparticles All syntheses were performed under ambient atmosphere. Clear green ethanol solutions with different Ni/ Al molar ratio were formed by dissolving Ni(NO3)2 Æ 6H2O and Al(NO3)3 Æ 9H2O in ethanol. After the addition of propylene oxide (PPO:metal ions molar ratio is 10) to the ethanol solution, an exothermic reaction occurred within a few minutes, followed by a gradual formation of a light green rigid gel from the top to the bottom of the solution. The gel was aged in a closed vessel at 50 °C for 3 h, dried in an open vessel at 50 °C for 24 h and then treated at 100 °C for additional 12 h. The resulting xerogel was ground to powder and calcined at the given temperature for 1 h to obtain NiAl2O4 nanoparticles. 2.3. Characterization The thermal behaviour (TG/DTA) of the samples was studied by a Seiko SSC/5200 (TG/DTA 320U) in static air atmosphere from ambient temperature to 1000 °C at a heating rate of 10 °C/min. The XRD patterns of the samples were measured in a Philips PW 1710 diffrac-
2103
tometer using CuKa radiation. Nitrogen adsorption– desorption isotherms of the samples were measured at 77 K with a Micromeritics ASAP 2010 surface area and porosity analyser. And the surface areas of the samples were calculated from the adsorption isotherms using the BET (Brunauer–Emmett–Teller) method in the P/P0 range of 0.1–0.2 and a molecular cross sectional area of nitrogen of 0.162 nm2. The morphology of the particles was observed by a JEOL 2000 transmission electron microscope working at 200 kV.
3. Result and discussion 3.1. Gel formation The formation of the gel involves the hydrolysis and condensation of hydrated metal ions. In the bimetallic system, the problem of phase segregation arises due to the different hydrolysis rates of the individual hydrated metal ions components. In the experiment, the hydrolysis and condensation rate of Al3+ alone in ethanol solution was rather fast, forming gels within a few minutes; on the contrary, that of Ni2+ alone in solution was much slower, giving a precipitate about 10 min after the propylene oxide was added. However, after the addition of propylene oxide to an ethanol mixture solution of Al3+ and Ni2+, a gel formed without any precipitation. After aging the gels for several hours at ambient temperature, a layer of green solution appeared on the upper part of the gel, showing that the hydrolysis of the Ni2+ was not complete. After two days of aging at ambient temperature or 3 h of aging at 50 °C, the light green colour of the solution disappeared, accounting for the complete hydrolysis of the Ni2+. All these facts suggest that an aluminium gel is formed first, while the Ni2+ ions remain in solution, then the hydrated nickel ions are hydrolysed and condensed at the surface of the aluminium gel surface. 3.2. Thermogravimetric analysis TG and DTA curves of the thermal degradation of the precursor gel with a Ni/Al molar ratio of 0.11 are shown in Fig. 1. There are two sharp exothermic peaks in the DTA curve around 191 °C and 337 °C accompanied by two clear weight losses in the TG curve that result from the decomposition of organic matter. Above 337 °C, only a small weight loss is observed, which is attributed to the release of water arising from condensation reaction. 3.3. Crystallographic analysis The XRD patterns of the samples prepared with different Ni/Al molar ratios from 0.034 to 0.67 and cal-
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100
337oC
90
30
DTAµV
191oC
70
10
60
0
50
TG (%)
80
20
40
-10 30
200
400
600
800
1000
Temperature (oC) Fig. 1. TG/DTA analysis of a precursor gel with a Ni/Al molar ratio of 0.11 carried out in static air.
cined at 700 °C are shown in Fig. 2. Except for the two amorphous samples with Ni/Al = 0.034 and 0.071, all samples clearly show a spinel structure. With the increase of Ni/Al molar ratio from 0.11 to 0.67, their diffraction lines show a gradual shift to lower 2h. The green coloration of the samples with Ni/Al molar ratios between 0.11 and 0.67 suggests the existence of nickel oxide, however no traces of nickel oxide were found in their XRD patterns (Fig. 2). This can be attributed to the formation of a pseudo-spinel structure as observed when an excess amount of Al2O3 is added to NiO and calcined at high temperature [19]. The increase of Ni2+ ions concentration with the increase of Ni/Al molar ratios from 0.11 to 0.67 suggests that Al3+ sites in the lattice of the pseudo-spinel are progressively substituted by Ni2+ ions. According to the standard reported data for ionic radii [20], the effective ionic radius of Ni2+ ion is
Fig. 2. XRD patterns of the samples prepared with different Ni/Al molar ratios and calcined at 700 °C (}, pseudo-spinel).
larger than Al3+ (69 pm vs 54 pm). The substitution of smaller ions (Al3+) by larger ions (Ni2+) results in the lattice expansion of the pseudo-spinel, and then leads to a peak shift toward lower 2h as observed in the XRD patterns. For the aluminium-rich sample with Ni/Al = 0.11, the position of the diffraction lines is very near to those of spinel type c-Al2O3 (JCPDS 10-0425); while samples with Ni/Al molar ratios of less than 0.11 show amorphous XRD patterns, which suggests that alumina does not crystallize at 700 °C and the concentration of NiO in the samples is not enough for the formation of pseudo-spinel structure. The XRD patterns of the samples with Ni/Al = 0.11 calcined at different temperatures provide us information on the formation temperature of the spinel structure (Fig. 3). The sample shows an amorphous phase at 500 °C, a pseudo-spinel phase and light green coloration at 700 °C, and a spinel phase and light blue coloration of NiAl2O4 (JCPDS 10-0339) at 800 and 900 °C. The latter shows a little shift in the diffraction lines towards higher 2h as compared with the pseudo-spinel phase obtained at 700 °C. The samples calcined at 800 °C show a gradual coloration change from light blue to light green with the increase of Ni/Al molar ratios from 0.034 to 0.67. In Al-rich samples with Ni/Al molar ratios between 0.034 and 0.25, a light blue coloration is observed. A light green coloration can be seen in samples with Ni/Al molar ratio between 0.30 and 0.67. The coloration change from blue to green accounts for the phase change from spinel to pseudo-spinel as observed in the XRD patterns (Fig. 4). The diffraction lines of the samples with Ni/Al between 0.034 and 0.071 are attributed to the spinel-type c-Al2O3 phase (JCPDS 10-0425), and those of the light blue samples with Ni/Al between
Fig. 3. XRD patterns of samples with Ni/Al = 0.11 calcined at different temperature ( , spinel; }, pseudo-spinel).
H. Cui et al. / Journal of Non-Crystalline Solids 351 (2005) 2102–2106 S γ-Al2O3
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S
P+S
P
Blue
Green
Green
Temperature (°C)
800 Blue
P
A
700 Green
Green
A
500
Green
0.034 0.071 0.11
0.15 0.20 0.25
0.30 0.36
0.43 0.50 0.67
Ni/Al mol ratio Fig. 5. Phase and coloration evolution with the temperature and different Ni/Al molar ratios: A, amorphous; P, pseudo-spinel; S, spinel.
Fig. 4. XRD patterns of the samples prepared with different Ni/Al molar ratios and calcined at 800 °C ( , spinel; r, c-Al2O3; }, pseudospinel).
0.11 and 0.25 are attributed the spinel phase of NiAl2O4. When the Ni/Al molar ratios increase to between 0.30 and 0.50, it is assumed that the diffraction lines of these light green samples are attributed to the mixture of the pseudo-spinel phase of NiO–Al2O3 and the spinel phase of NiAl2O4. When Ni/Al molar ratios increase to 0.67, three of the diffraction lines at 2h = 19.1, 31.6 and 60 disappeared and the left lines of the light green sample are attributed to the pseudospinel phase of NiO–Al2O3. Generally, the phase transformation temperature from amorphous alumina to cAl2O3 is between 500 and 800 °C. While, c-Al2O3 phase was only found in the samples calcined at 700 °C with Ni/Al molar ratios between 0.071 and 0.034. In the samples with Ni/Al molar ratios between 0.25 and 0.11, this is probably due to dissolution of spinel type c-Al2O3 (in all proportions) in spinel type NiAl2O4 [8,21,22]. In the samples with Ni/Al molar ratios between 0.30 and 0.67, this is probably due to the retardation of phase transformation from amorphous alumina to c-Al2O3 by NiO [23]. A graph summarizing the coloration and crystallographic phases of the different samples and different temperatures is given in Fig. 5. In view of the results obtained in this work, the particle formation process can be described as follows: the hydrated nickel ions are hydrolysed and condensed at the surface of the aluminium gel and form a NiO coating after calcination, then reacts with amorphous alumina particle surface to form a pseudo-spinel structure at 700 °C and spinel NiAl2O4 at 800 °C. During the calcination, the NiO or NiAl2O4 on the surface of amorphous alumina retards the formation of c-Al2O3.
3.4. BET surface area and transmission electron microscope analysis The BET surface area and particle size estimated from XRD diffraction lines are shown in Table 1. It could be noticed that the crystallite size estimated from the XRD patterns of the samples calcined at these three temperatures remains almost unaffected. At the same time, the BET surface area of the samples with Ni/Al = 0.11 increases from 192 m2/g at 700 °C to 263 m2/g at 800 °C, then decreases to 102 m2/g at 900 °C. This effect was also observed in the transmission electron micrographs (Fig. 6), where the samples calcined at 700 and 900 °C show large agglomerates of nanoparticles smaller than 10 nm, and the sample calcined at 800 °C shows well dispersed particles of the same crystallite size as the other samples. The increase of the BET surface from 700 to 800 °C can be attributed to the structure transformation from pseudo-spinel to NiAl2O4 spinel as observed in the XRD patterns, while the decrease in the BET between 800 and 900 °C is due to the aggregation of particles caused by the higher calcinations temperature.
4. Conclusions In this work, spherical spinel-type NiAl2O4 nanoparticles with one of the metal ions that has a formal charge of less than +3 were successfully synthesized by a sol–gel method using propylene oxide as a gelation agent. The main step in the process is that hydrated nickel ions are hydrolysed and condensed at the surface of the previously formed aluminium oxide gel. During the thermal treatment, the NiO reacts with amorphous alumina particle surface to form a pseudo-spinel structure at 700 °C, and a spinel NiAl2O4 at 800 °C. The XRD patterns indicate that the NiO or NiAl2O4 on the surface of alumina retards the crystallization of amorphous alumina to cAl2O3 at 800 °C. It has been found that the mixed oxide
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Table 1 BET surface area of the sample with Ni/Al = 0.11 calcined at different temperature Calcination temperature (°C)
Surface area 2
700 800 900 *
*
Crystal size estimated from XRD (nm)
BET (m /g)
C value
192 263 102
118 67 95
8.2 7.7 7.2
Error < 0.5% in all samples.
Fig. 6. Transmission electron micrographs of the sample with Ni/Al = 0.11 calcined at 700 (a), 800 (b) and 900 °C (c).
experienced a decrease in particle size from 700 to 800 °C accompanied by a structure transformation from pseudo-spinel to NiAl2O4 spinel. This method was found to be an effective route to synthesize mixed oxide nanoparticles with narrow size distribution and large surface area. The route described here can also applied for the synthesis of mixed oxides with other structures and more than two metal components. References ¨ zdemir, Bu¨lent Alici, J. Mater. [1] B. C ¸ etinkaya, T. Sec¸kin, I. O Chem. 8 (1998) 1835. [2] F. Meyer, R. Hempelmann, S. Mathur, M. Veith, J. Mater. Chem. 9 (1999) 1755. [3] M.B.D. Mitchell, D. Jackson, P.F. James, J. Sol–Gel Sci. Techn. 13 (1998) 359. [4] N. Yamada, I. Yoshinaga, S. Katayama, J. Sol–Gel Sci. Techn. 17 (2000) 123. [5] M. Andrianainarivelo, R.J.P. Corriu, D. Leclercq, P.H. Mutin, A. Vioux, Chem. Mater. 9 (1997) 1098. [6] J.M. Ferna´ndez Colinas, C. Otero Area´n, J. Solid State Chem. 109 (1994) 43. [7] Y. Cesteros, P. Salagre, F. Medina, J.E. Sueiras, Chem. Mater. 12 (2000) 331.
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