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Improving ZnAl2O4 structure by using chelating agents
Materials Chemistry and Physics 135 (2012) 855e862
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Improving ZnAl2O4 structure by using chelating agents A.G. Khaledi a, *, S. Afshar a, H.S. Jahromi b a b
College of Chemistry, Iran University of Science and Technology, P.O. Box 1684613114, Narmak, Tehran, Iran Research Institute of Petroleum Industry (RIPI), Environment Division, Tehran, Iran
h i g h l i g h t s < Effect of sucrose, citric acid & TEA on the structure of ZnAl2O4 was investigated. < Different ratio of M:chelating agent (1:1; 1:2; 1:4) were used. < Increasing ratio of chelating agent to metal causes decreases size of particles. < ZAO prepared by Sucrose has lower size and many pores.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 February 2012 Received in revised form 20 May 2012 Accepted 25 May 2012
In this paper, ZnAl2O4 spinel powders were prepared by a solegel route using zinc nitrate and aluminium nitrate in polar solvent (e.g. water) as the starting materials from the viewpoint that they are low-cost. A study of the effect of chelating agents such as sucrose (S), citric acid (CA) and triethanolamine (TEA) with a different weight ratio on the structure of ZnAl2O4 is reported. The characterization of the synthesized ZnAl2O4 powder was performed by XRD, FT-IR, Diffuse reflectance spectroscopy (DRS) and scanning electron microscopy (SEM). The results show that the powders made by the weight ratio 1:4 (metal:chelating agent) had a better morphological structure than the other weight ratios (1:1 and 1:2). Crystallite sizes vary from 16 nm for ZAO(S) up to 26 nm for ZAO(TEA). It was found that crystallite size, specific surface area and morphology are strongly dependent on the chelating agent/metal ratio and kinds of chelating agents. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: ZnAl2O4 Nanocrystal Solegel Chelating agent
1. Introduction In recent years there has been increasing interest in the synthesis of nanocrystalline metal oxides [1]. Oxide spinels comprise a very large group of structurally related compounds many of which are of considerable technological or geological significance. Spinels exhibit a wide range of electronic and magnetic properties, including superconductivity in LiTi2O4. The iron containing and zinc containing spinels are of technological importance primarily due to their magnetic and insulating properties, respectively. The normal spinel ZnAl2O4 (gahnite) is a typical example having the general formula (X)[Y]O4, where X and Y are divalent and trivalent ions, and the ( ) and [ ] refer to the eight tetrahedrally coordinated A sites and 16 octahedrally coordinated B sites, respectively, within the cubic unit cell. In the normal spinel structure, all the divalent cations are at the A sites and all the
* Corresponding author. Tel.: þ98 912 545 58 74. E-mail address: [email protected] (A.G. Khaledi). 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2012.05.070
trivalent cations at the B sites [2]. ZnAl2O4 is also transparent to light with wavelengths above 320 nm and is hence suitable for UV optoelectronic application and for use in thermal control coatings for spacecraft [3e5]. In recent years, several types of wet-chemical techniques have been successfully used for the preparation of pure spinel powders at relatively low temperatures, such as precipitation [6,7] solegel of metal alkoxides, spray-drying, freeze-drying, modified Pechini process, and combustion synthesis conventionally, zinc aluminate powder was prepared at high temperatures by many techniques such as sintering, solid-state reaction [8], solegel [3], hydrothermal [6], and microemulsions techniques [7e9]. The main advantages of the solegel method, as compared with traditional methods, are controls over purity and composition, easy introduction of doping elements and low temperature processing. To the best of our knowledge, studies of bulk ZnAl2O4 or MgAl2O4 nanocrystals embedded in silica glass prepared at low temperatures are rarely reported [10]. Zinc aluminate (ZnAl2O4), naturally occurring as the mineral gahnite, is a member of the spinel family. At present, zinc aluminate is used as a catalyst for the dehydration of saturated alcohols to olefins, methanol and higher alcohol
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synthesis, preparation of polymethylbenzenes, synthesis of styrenes from acetophenones, and double bond isomerisation of alkenes. Furthermore, zinc aluminate can also be used as a catalyst support, since it has a high thermal stability, low acidity and a hydrophobic behaviour. Moreover, it has a strong metalesupport interaction preventing e.g. platinum and platinum/tin to sinter. Finally, zinc aluminate can be used as a second phase in glaze layers of white ceramic tiles to improve wear resistance and mechanical properties and to preserve whiteness [11]. The metal complex precursor was prepared via solegel method using citric acid as a chelating agent, which has been reported in previous works [12e14]. The citrate solegel technique offers an excellent control over the stoichiometry and allows lower processing temperature and easier introduction of dopants. This method involves the formation of a mixed-ion citrate that due to the three-ligand nature of the citric acid, resulting in a transparent three-dimensional network upon drying. This process has the advantage that it can rapidly yield fine and homogeneous powder after the pyrolysis of the gel [15]. Sucrose (C12H22O11) is a type of water-soluble and inexpensive agricultural product. In the acidic condition, eCOOH and eOH groups can be generated, which hence can form stable binding with metal ions in homogeneous solution. Therefore, sucrose can also act as an effective chelating agent like citric acid to produce fine particles. In recent years, attempts have been made by several researchers to prepare nanopowders using sucrose as a chelating agent and fuel [16e19]. Triethanolamine (TEA, N(C2H4OH)3) is an alkanolamine. It can be complex with transition metal to form stable complex [20e22] to avoid any rapid hydrolysis and condensation in existence of water [23]. The route involves dehydration of the solution of metaleTEA complex, followed by decomposition of the TEA complex. Complete dehydration of the resulting solution to dryness results in a voluminous, organic-based, black, fluffy mass. The dried carbonaceous mass is ground to fine powders to produce the precursor material. Heat treatment of the precursor results in nanosized powders [24].Furthermore, TEA can also be used as surfactants [25] in many reactions to prepare powders with good dispersivity [23]. In this paper, ZnAl2O4 spinel powders were prepared by a solegel route using zinc nitrate and aluminium nitrate in polar solvent (e.g. water) as the starting materials from the viewpoint that they are low-cost. The advantage of this method is better control that can be exercised over the precipitate with respect to crystallite size, surface area, and composition of final oxides. Then effect of chelating agents (type and amount) on the crystallite size of ZnAl2O4 spinel has been examined. Physicochemical properties of obtained materials were investigated by different analytical methods [26]. 2. Experimental details In order to prepare different catalysts synthesized in this work, the following materials were used: Aluminium nitrate [Al(NO3)3$9H2O, M ¼ 375.13 gmol1]; Zinc nitrate [Zn(NO3)2$6H2O, M ¼ 297.47 gmol1]; Citric acid [C6H8O7$H2O, M ¼ 210.14 gmol1]; Sucrose [C12H22O11, M ¼ 342.3 gmol1]; Triethanolamine (TEA) [C6H15NO3, M ¼ 149.10 gmol1]; Nitric acid [HNO3, M ¼ 63.01 gmol1]. 2.1. Synthesis of nanocrystalline ZnAl2O4 catalyst by solegel method In order to prepare Zinc Aluminate nanoparticles by solegel method, stoichiometry amounts of the metal nitrates (Zn:Al ¼ 1:2) were dissolved in distilled water to obtain a homogeneous solution.
The obtained transparent sol was poured into glass container and allowed to gel at room temperature. The gel was then dried at 110 C for 2 h. After grinding, the dried gel was calcined at 800 C for 3 h (1.6 C min1) to obtain the ZnAl2O4 powder. The resulting powders are denoted as ZAO. 2.2. Synthesis of nanocrystalline ZnAl2O4 catalyst by citrate precursor In the first step, stoichiometry amounts of the metal nitrates (Zn:Al ¼ 1:2) were dissolved in distilled water to obtain metal nitrate solution. The citric acid solutions were prepared separately by adding different amounts of citric acid into distilled water, the molar ratio of citric acid and the metal ions were 1:1, 2:1 and 4:1 in three samples. Three solutions were subsequently dropped into the nitrate solutions under continuous stirring to ensure the homogeneous solutions. After complete mixing, the solutions were heated to evaporate the water for several hours in water-bath at 70 C. The resultant complex solutions turned to light yellow transparent viscous sols. Yellow gels were obtained by evaporating of water for several more hours. The gels were then dried at 110 C in an oven until fluffy powder obtained, named the citrate precursor of ZnAl2O4. After grinding, the precursors were calcined at 800 C for 3 h to obtain the ZnAl2O4 powder samples (Fig. 1). The powders as synthesized are denoted as ZAO(CA). 2.3. Synthesis of nanocrystalline ZnAl2O4 catalyst by sucrose precursor Similar to citrate method, stoichiometry amounts of metal nitrate (Zn:Al ¼ 1:2) and the molar ratios of sucrose to the total metal ions (1:1, 2:1, 4:1) were used. In the first step, pH of solutions was adjusted to 1.0 by addition of nitric acid dropwise. Subsequent heating with continuous stirring let the ions react with sucrose completely resulting in a transparent colourless solution and continued until the solution changed into a viscous light brownish gel. During evaporation, the aqueous sucrose solution in presence of nitric acid is oxidized to saccharic acid. The gels were then dried at 110 C in an oven which resulted in a dark brown foamy mass. All precursors were ground into powders and calcined in a furnace at 800 C for 3 h to obtain the ZnAl2O4 powder (Fig. 1). The resulting powders are denoted as ZAO(S). 2.4. Synthesis of nanocrystalline ZnAl2O4 catalyst by triethanolamine (TEA) precursor First, metal nitrates were dissolved in distilled water (Zn:Al ¼ 1:2). Then, the required amount of TEA was added dropwise into the mixed solution. For TEA variation studies, a series of solutions was prepared in such a way that the TEA to a total metal ion ratio in the starting solutions was maintained at 1:1, 2:1 and 4:1. At the beginning, a gelatinous precipitate was formed with the addition of TEA; however, it dissolved when concentrated HNO3 was added to give a final pH of about 3. The clear solutions of TEAcomplexed metal nitrate were heated to evaporate the water for several hours in water-bath at 70 C with constant stirring. Continuous heating of the solutions at 110 C in an oven causes a fluffy mass. During evaporation, the nitrate ions provide an in situ oxidizing environment for TEA, which partially converts the hydroxyl groups of TEA into carboxylic acids. When complete dehydration occurred, the nitrates themselves decomposed, with the evolution of brown fumes of nitrogen dioxide, leaving behind a voluminous, organic-based, black, fluffy powder, i.e., the precursor powder. After grinding, the precursor powders were
A.G. Khaledi et al. / Materials Chemistry and Physics 135 (2012) 855e862
Zn(NO3)2.6H2O
H 2O
857
Al(NO3)3.9H2O
Solution 1
Sucrose R.T.
Colorless gel
Stirring
Citric acid
Triethanolamine Stirring
Stirring
Solution 2
Solution 2
White Precipitate
HNO3 Dried at 110◦C
Dried gel Calcined at 800◦C
ZnAl2O4
Heated at 70◦C
Light brown gel
HNO3 Heated at 70◦C
Yellow gel
Dried at 110◦C
Dried at 110◦C
Brown Sucrose Precursor
Yellow Citrate Precursor
Calcined at 800◦C
ZnAl2O4
Heated at 70◦C
Calcined at 800◦C
Brown gel Dried at 110◦C
Brown TEA Precursor Calcined at 800◦C
ZnAl2O4
ZnAl2O4
Fig. 1. Experimental procedure for preparation of ZnAl2O4 powder.
calcined at 800 C for 3 h to obtain the ZnAl2O4 powder (Fig.1). The powders as synthesized are denoted as ZAO (TEA). 2.5. Characterization of ZnAl2O4 nanopowders The crystalline structures of the resulting samples were assessed and characterized by X-ray diffractometer (D4-BRUKER and Cu-Ka radiation at 30kv and 20 mA). The chemical structure of the prepared particles was examined using Fourier Transform Infrared Spectrophotometer (FT-IR, Shimadzu-840S) in the region of 4000e400 cm1. The Diffuse Reflectance Spectra (DRS) of catalysts were recorded by UVeVis spectrophotometer (Shimadzu2550). Morphological studies were carried out using a scanning electron microscope (SEM) PhilipseXLF30 model and surface area of the nanoparticles was calculated using nitrogen absorption data at 298 K (BET analysis, MICRO MERITICS-GEMINI).
materials are fully crystalline. The average crystallite size from XRD was calculated from X-ray line broadening of the (3 1 1) diffraction line using the Scherrer’s equation ðd ¼ ð0:9lÞ=ðbcosqÞÞ, where d is the grain size, l is the wavelength of the X-ray (Cu Ka, 0.15418 nm), b is the full-width at the half-height of the peak, and q is the diffraction angle of the peak and the results are shown in Table 1. Fig. 2 shows the XRD pattern of ZAO sample (synthesized in Section 2.1), which has been used as a reference in order to compare with other synthesized samples. As can be seen, crystallite size decreases by using chelating agents. These results indicate that the prepared sample by Sucrose has smaller size than samples by CA and TEA at the same ratio. It can be noticed in Figs. 3e5, patterns aec, that the peak width increases slightly with increasing the ratio of chelating agent that means the particle size decreases at the same formation temperature. Furthermore, the surface area of the synthesized ZAO, ZAO(S), ZAO(CA) and ZAO(TEA) nanopowders are 13.65, 40.12, 38.46 and 32.17 m2 g1, respectively.
3. Result and discussion 3.1. X-ray diffraction To study the crystallization process and phase identification, the powder X-ray diffraction (XRD) analyses were performed on various ZnAl2O4 samples, which were prepared without/with chelating agents (sucrose, citric acid and TEA). The XRD patterns of samples are shown in Figs. 2e5. In the X-ray diffraction pattern of ZnAl2O4 nanopowder, peaks were observed at 2q ( ) ¼ 18.9, 31.2, 36.8, 44.8, 49.1, 55.6, 59.3 and 65.2 which can be indexed as (1 1 1), (2 2 0), (3 1 1), (4 0 0), (3 3 1), (4 2 2), (5 1 1) and (4 4 0) diffraction planes, respectively. The observed diffraction peaks in recorded XRD patterns correspond to those of the standard patterns of cubic ZnAl2O4 spinel (JCPDS, No. 05-0669). The ratios of peak intensities of prepared powder and that of standard were the same. No other impurity phases were detected in the samples. The prepared
Fig. 2. The XRD pattern of ZAO nanopowders.
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Fig. 3. The XRD patterns of ZAO(S) nanopowders; (a) M:S (1:1), (b) M:S (1:2), (c) M:S (1:4); Inset: Effect of weight ratio on the crystallite size is evident on comparing the (311) diffraction line.
3.2. FT-IR The IR spectra of the ZnAl2O4 nanoparticles prepared by Sucrose, Citric acid and TEA, after calcination (a), before calcination (b), and chelating agents (c) are shown in Figs. 6e8, respectively. All three (a) spectra of Figs. 6e8, show metaleO stretching frequencies in the range 500e900 cm1, associated with the vibrations of ZneO, AleO, and ZneOeAl bonds, indicating the formation of metal oxide, confirmed by XRD as the single-phase ZnAl2O4 spinel. Figs. 6b, 7b and 8b show broad and intense hydroxyl stretching frequencies [y(OH)], which can be attributed to the overlapping of the bands because of surface-adsorbed water (ca. 3400 cm1). The small peaks at around 1384 and 2857e2920 cm1 correspond to the presence of a trace amount of CH2 and CeH band.
Fig. 4. The XRD patterns of ZAO(CA) nanopowders; (a) M:CA (1:1), (b) M:CA (1:2), (c) M:CA (1:4); Inset: Effect of weight ratio on the crystallite size is evident on comparing the (311) diffraction line.
Fig. 6c indicates that in IR spectra of sucrose, no significant peak is present in the frequency region 1760 and 1700 cm1, which confirms the absence of vibrational modes of the carboxylate ions, while in Fig. 6b vibrational modes of frequency of carboxylate ions are present because of the use of HNO3. Observed peaks at 1070e1100 cm1 indicate stretching vibration of CeO; also in sucrose the asymmetric stretching for CeOeC was observed at 1130 cm1, which cannot be seen in Fig. 6b. This indicates that CeOeC bond was decomposed. In citric acid spectrum (Fig. 7c), the stretching vibrations for free carboxyl groups were observed between 1760 and 1700 cm1. After complexing with metal ions (Fig. 7b), two new bands occur near 1620 and 1400 cm1, representing the asymmetric stretching and symmetric stretching vibrations for carboxyl ions (COO). The
A.G. Khaledi et al. / Materials Chemistry and Physics 135 (2012) 855e862
Fig. 5. The XRD patterns of ZAO(TEA) nanopowders; (a) M:TEA (1:1), (b) M:TEA (1:2), (c) M:TEA (1:4); Inset: Effect of weight ratio on the crystallite size is evident on comparing the (311) diffraction line.
Table 1 The effect of chelating agent on the average crystallite size of ZnAl2O4 and the surface area. Product name
Ratio M:Ca
Crystallite size (nm)
SBET (m2 g1)
ZnAl2O4 (without chelating agent) ¼ ZAO
e 1:1 1:2 1:4 1:1 1:2 1:4 1:1 1:2 1:4
36.8 19.5 18.0 16.2 24.0 22.5 21.0 26.4 24.5 22.0
13.65
ZnAl2O4 (prepared by S) ¼ ZAO(S)
ZnAl2O4 (prepared by CA) ¼ ZAO(CA)
ZnAl2O4 (prepared by TEA) ¼ ZAO(TEA) a
M:C ¼ Ratio of (Zn2þ& Al3þ): Chelating Agent.
40.12
38.46
859
Fig. 6. The FT-IR spectra of: (a) ZAO(S) after calcination, (b) ZAO(S) before calcination, (c) Sucrose.
difference between yas and ys is 220 cm1, which indicates that the carboxylate groups are coordinated to the metals in a monodentate style. Fig. 8b shows the strong absorption bands at 1600 cm1, which is attributed to the various vibrational modes of the carboxylate ions, forming because of reaction of decomposed TEA with metal ions. Apart from these bands, two or three-weak bands appear in the region 700e400 cm1, which are the results of some trace amounts of metal oxides forming during an evaporation process [27]. 3.3. UVeVis diffuses reflection spectra (DRS)
32.17
Fig. 9(aed) shows a diffuse reflection spectra of synthesized ZAO (Section 2.1) and ZnAl2O4 prepared by chelating agents. As these
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Fig. 7. The FT-IR spectra of: (a) ZAO(CA) after calcination, (b) ZAO(CA) before calcination, (c) Citric acid.
Fig. 8. The FT-IR spectra of: (a) ZAO(TEA) after calcination, (b) ZAO(TEA) before calcination, (c) Triethanolamine.
can be observed, the absorption bands of ZnAl2O4 prepared by chelating agents exhibit partial hypsochromic shifts, which indicates the decrease of particle size of ZnAl2O4 nanopowders in UV light. Negative solvatochromism corresponds to hypsochromic shift (blue shift) with increasing solvent polarity. Polarity of chelating agents is related to their chemical structure. Products resulting from interaction of sucrose with nitric acid in comparison to CA and TEA exhibit higher polarity effect. The polarity of products is due to the number of carboxyl and hydroxyl groups. Moreover, addition of chelating agents decreases crystal size and crystallinity in ZnAl2O4 particles by inhibiting the growth of particles in the crystalline
network. Furthermore, partial hypsochromic shifts indicate the formation of bands between metals and chelating agents and increasing chelating agent polarity. 3.4. Investigation of chelating agents’ effect on surface morphology The surface morphology of the ZnAl2O4 without/with chelating agents is shown in Figs. 10e13. The ZAO without chelating agents Fig. 10 consists of agglomerated ZAO particles, which may cause the surface and dispersion of particles to be nonuniform whereas in the ZnAl2O4 with chelating agents formed interparticle pores are numerous and regular. Furthermore, this porous structure is
A.G. Khaledi et al. / Materials Chemistry and Physics 135 (2012) 855e862
861
Fig. 11. SEM image of ZAO(S) nanoparticles.
distributed uniformly in the matrix of ZnAl2O4. It is observed that the surface morphology of the ZnAl2O4 is greatly dependent on the type of the chelating agent. Fig. 11 shows the SEM images of ZAO(S). In the case of sucrose, addition of nitric acid helps to break sucrose into Glycolic acid, Hydroxybutyric acid and Saccharic acid, which then helps to prevent recrystallization of sugar. Presence of more carboxylic groups causes increasing of metalesucrose bands. This sample consists of many pores and particles that are distributed uniformly on the surface of ZnAl2O4 in comparison with two other chelating agents. Decomposition of sucrose generates excess heat and a huge amount of gases that help to produce the porosity in the final Zinc Aluminat. During heating, the metal ion-chelated complex is decomposed into carbon dioxide and water, and a large amount of heat is generated. These produced gases prevent agglomeration,
and help to form pores and fine particles with high surface area in the final product. The SEM of ZAO (CA) is shown in Fig. 12. In this sample, the precursor of the metal oxide is directly calcinated in air and decomposed as carbon dioxide, carbon monoxide, water and metal oxide. The existing carbon prevents completion of the reaction to ZnAl2O4 by increasing the diffusion distance of the mixed cations. This retards the crystallization of ZnAl2O4, especially for a precursor with a high ratio of citric acid to metal. In the following heattreatment, the carbon is oxidized softly in air and the crystallization of ZnAl2O4 is completed. At the same time, the pores are formed by combustion of the carbon surrounding the amorphous ZnAl2O4. Fig. 13 shows the SEM images of ZAO (TEA). It can be seen that all the micrographs show uniform distribution of the particles without serious agglomeration. The chemical process starts from homogeneous distribution of metal ions in solution. The solution after complete evaporation decomposes into a fluffy black mass, which is effectively a carbonaceous material having a mesoporous structure which is evident from its surface area. During decomposition of metal ion complexes in air, it produces nescent metal oxides, which are basically small atomic clusters with proper chemical homogeneity, embedded in this mesoporous carbonaceous material. The nescent metal oxides on rearrangement produce the desired singlephase ZnAl2O4. The decomposition of carbonaceous material
Fig. 10. SEM image of ZAO nanoparticles.
Fig. 12. SEM image of ZAO(CA) nanoparticles.
Fig. 9. The diffuse reflection spectra of: (a) ZAO, (b) ZAO(TEA), (c) ZAO(CA) and (d) ZAO(S).
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decomposed into carbon dioxide and water. The large amount of produced gases prevents agglomeration, and helps to form pores and fine particles with high surface area in the final product in comparison with two other chelating agents. References
Fig. 13. SEM image of ZAO(TEA) nanoparticles.
produces gases (such as CO, CO2, NH3, NO2, and water vapour) that help to disintegrate the agglomerated particles and help to inhibit the sintering of nanosized particles to grow bigger particles. 4. Conclusions In this investigation, it was found that the weights ratio of chelating agents to metal and kind of the chelating agents were important factors which affected surface morphology, crystal size and specific surface area of ZnAl2O4 powders. Chemical and physical properties of nanoparticles were different. In the case of sucrose, addition of nitric acid helps to break it into new carboxylic acids, which then prevents recrystallization of sugar. Presence of more carboxylic groups causes increasing of metalesucrose bands and keeps metal ions uniformly distributed throughout the viscous gel during evaporation. The metal ionesucrose complex is
[1] X. Duan, D. Yuan, Z. Sun, C. Luan, D. Pan, D. Xu, M. Lv, J. Alloys Compd 386 (2005) 311. [2] A.R. Phani, M. Passacantando, S. Santucci, J. Mater. Chem. Phys. 68 (2001) 66. [3] S. Mathur, M. Veith, M. Haas, Haoshen, N. Lecerj, V. Huch, J. Am. Ceram. Soc. 84 (2001) 1921. [4] J.F. Cordoro, US Patent No. 6,099,637, 8 August 2000. [5] J.F. Cordoro, et al., US Patent No. 5,807,909, 15 September 1998. [6] Z. Chen, E. Shi, Y. Zheng, W. Li, N. Wu, W. Zhong, J. Mater. Lett. 56 (2002) 601. [7] A.E. Giannakas, T.C. Vaimakis, A.K. Ladavos, P.N. Trikalitis, P.J. Pomonis, J. Colloid Interface Sci. 259 (2003) 244. [8] C.O. Arean, B.S. Sintes, G.T. Palomino, C.M. Carbonell, E. ScalonaPlatero, J.B. Parra Soto, J. Microporous Mater. 8 (1997) 187. [9] K. Kumara, K. Ramamoorthya, P.M. Koinkarb, R. Chandramohanc, K. Sankaranarayanana, J. Cryst. Growth 289 (2006) 405. [10] X. Duan, D. Yuan, X. Cheng, C. Luan, Z. Sun, X. Wei, S. Guo, D. Xu, M. Lv, J. Inorg. Chem. Commun. 7 (2004) 62. [11] N.J. van der Laaga, M.D. Snela, P.C.M.M. Magusinb, G. de, J. Eur. Ceram. Soc. 24 (2004) 2417. [12] h.S. Liu, J. Li, Z.R. Zhang, Z.L. Gong, Y. Yang, J. Solid State Electrochem. 7 (2003) 456. [13] H. Liu, Z. Zhang, Z. Gong, Y. Yang, J. Solid State Ionics 166 (2004) 317. [14] C. Polo Fonsecaa, M.C.A. Fantinib, S. Nevesa, J. Thin Solid Films 488 (2005) 68. [15] X.L. Duana, D.R. Yuana, L.H. Wanga, F.P. Yua, X.F. Chenga, Z.Q. Liua, S.S. Yanb, J. Cryst. Growth 296 (2006) 234. [16] M.G. Lazarraga, L. Pascual, H. Gadjov, D. Kovacheva, K. Petrov, J.M. Amarilla, R.M. Rojas, M.A. Martin-Luengo, J.M. Rojo, J. Mater. Chem. 14 (2004) 1640. [17] R.N. Das, A. Bandyopadhyay, S. Bose, J. Am. Ceram. Soc. 84 (2001) 2421. [18] Y.J. Wu, A. Bandyopadhyay, S. Bose, J. Mater. Sci. Eng. A380 (2004) 349. [19] L. Xu, B. Wei, Z. Zhang, Z. Lu, H. Gao, Y. Zhang, J. Nanotechnol. 17 (2006) 4327. [20] A.B. Panda, A. Pathak, P. Pramanik, J. Mater. Lett. 52 (2002) 180. [21] A. Sen, P. Pramanik, J. Eur. Ceram. Soc. 21 (2001) 745. [22] T. Kocareva, I. Grozdanov, B. Pejova, J. Mater. Lett. 47 (2001) 319. [23] D.Q. Liu, Z.Z. He, X.Q. Liu, J. Alloys Compd 440 (2007) 69. [24] S. Ran, L. Gao, J. Ceram. Int. 34 (2008) 443. [25] S.B. Rane, T. Seth, G.J. Phatak, J. Mater. Lett. 57 (2003) 3096. [26] H. Zhang, X. Jia, Z. Liu, Z. Li, J. Mater. Lett. 58 (2004) 1625. [27] J.-D. Tsay, T.-T. Fang, J. Am. Ceram. Soc. 82 (1999) 1409.
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Improving ZnAl2O4 structure by using chelating agents A.G. Khaledi a, *, S. Afshar a, H.S. Jahromi b a b
College of Chemistry, Iran University of Science and Technology, P.O. Box 1684613114, Narmak, Tehran, Iran Research Institute of Petroleum Industry (RIPI), Environment Division, Tehran, Iran
h i g h l i g h t s < Effect of sucrose, citric acid & TEA on the structure of ZnAl2O4 was investigated. < Different ratio of M:chelating agent (1:1; 1:2; 1:4) were used. < Increasing ratio of chelating agent to metal causes decreases size of particles. < ZAO prepared by Sucrose has lower size and many pores.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 February 2012 Received in revised form 20 May 2012 Accepted 25 May 2012
In this paper, ZnAl2O4 spinel powders were prepared by a solegel route using zinc nitrate and aluminium nitrate in polar solvent (e.g. water) as the starting materials from the viewpoint that they are low-cost. A study of the effect of chelating agents such as sucrose (S), citric acid (CA) and triethanolamine (TEA) with a different weight ratio on the structure of ZnAl2O4 is reported. The characterization of the synthesized ZnAl2O4 powder was performed by XRD, FT-IR, Diffuse reflectance spectroscopy (DRS) and scanning electron microscopy (SEM). The results show that the powders made by the weight ratio 1:4 (metal:chelating agent) had a better morphological structure than the other weight ratios (1:1 and 1:2). Crystallite sizes vary from 16 nm for ZAO(S) up to 26 nm for ZAO(TEA). It was found that crystallite size, specific surface area and morphology are strongly dependent on the chelating agent/metal ratio and kinds of chelating agents. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: ZnAl2O4 Nanocrystal Solegel Chelating agent
1. Introduction In recent years there has been increasing interest in the synthesis of nanocrystalline metal oxides [1]. Oxide spinels comprise a very large group of structurally related compounds many of which are of considerable technological or geological significance. Spinels exhibit a wide range of electronic and magnetic properties, including superconductivity in LiTi2O4. The iron containing and zinc containing spinels are of technological importance primarily due to their magnetic and insulating properties, respectively. The normal spinel ZnAl2O4 (gahnite) is a typical example having the general formula (X)[Y]O4, where X and Y are divalent and trivalent ions, and the ( ) and [ ] refer to the eight tetrahedrally coordinated A sites and 16 octahedrally coordinated B sites, respectively, within the cubic unit cell. In the normal spinel structure, all the divalent cations are at the A sites and all the
* Corresponding author. Tel.: þ98 912 545 58 74. E-mail address: [email protected] (A.G. Khaledi). 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2012.05.070
trivalent cations at the B sites [2]. ZnAl2O4 is also transparent to light with wavelengths above 320 nm and is hence suitable for UV optoelectronic application and for use in thermal control coatings for spacecraft [3e5]. In recent years, several types of wet-chemical techniques have been successfully used for the preparation of pure spinel powders at relatively low temperatures, such as precipitation [6,7] solegel of metal alkoxides, spray-drying, freeze-drying, modified Pechini process, and combustion synthesis conventionally, zinc aluminate powder was prepared at high temperatures by many techniques such as sintering, solid-state reaction [8], solegel [3], hydrothermal [6], and microemulsions techniques [7e9]. The main advantages of the solegel method, as compared with traditional methods, are controls over purity and composition, easy introduction of doping elements and low temperature processing. To the best of our knowledge, studies of bulk ZnAl2O4 or MgAl2O4 nanocrystals embedded in silica glass prepared at low temperatures are rarely reported [10]. Zinc aluminate (ZnAl2O4), naturally occurring as the mineral gahnite, is a member of the spinel family. At present, zinc aluminate is used as a catalyst for the dehydration of saturated alcohols to olefins, methanol and higher alcohol
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synthesis, preparation of polymethylbenzenes, synthesis of styrenes from acetophenones, and double bond isomerisation of alkenes. Furthermore, zinc aluminate can also be used as a catalyst support, since it has a high thermal stability, low acidity and a hydrophobic behaviour. Moreover, it has a strong metalesupport interaction preventing e.g. platinum and platinum/tin to sinter. Finally, zinc aluminate can be used as a second phase in glaze layers of white ceramic tiles to improve wear resistance and mechanical properties and to preserve whiteness [11]. The metal complex precursor was prepared via solegel method using citric acid as a chelating agent, which has been reported in previous works [12e14]. The citrate solegel technique offers an excellent control over the stoichiometry and allows lower processing temperature and easier introduction of dopants. This method involves the formation of a mixed-ion citrate that due to the three-ligand nature of the citric acid, resulting in a transparent three-dimensional network upon drying. This process has the advantage that it can rapidly yield fine and homogeneous powder after the pyrolysis of the gel [15]. Sucrose (C12H22O11) is a type of water-soluble and inexpensive agricultural product. In the acidic condition, eCOOH and eOH groups can be generated, which hence can form stable binding with metal ions in homogeneous solution. Therefore, sucrose can also act as an effective chelating agent like citric acid to produce fine particles. In recent years, attempts have been made by several researchers to prepare nanopowders using sucrose as a chelating agent and fuel [16e19]. Triethanolamine (TEA, N(C2H4OH)3) is an alkanolamine. It can be complex with transition metal to form stable complex [20e22] to avoid any rapid hydrolysis and condensation in existence of water [23]. The route involves dehydration of the solution of metaleTEA complex, followed by decomposition of the TEA complex. Complete dehydration of the resulting solution to dryness results in a voluminous, organic-based, black, fluffy mass. The dried carbonaceous mass is ground to fine powders to produce the precursor material. Heat treatment of the precursor results in nanosized powders [24].Furthermore, TEA can also be used as surfactants [25] in many reactions to prepare powders with good dispersivity [23]. In this paper, ZnAl2O4 spinel powders were prepared by a solegel route using zinc nitrate and aluminium nitrate in polar solvent (e.g. water) as the starting materials from the viewpoint that they are low-cost. The advantage of this method is better control that can be exercised over the precipitate with respect to crystallite size, surface area, and composition of final oxides. Then effect of chelating agents (type and amount) on the crystallite size of ZnAl2O4 spinel has been examined. Physicochemical properties of obtained materials were investigated by different analytical methods [26]. 2. Experimental details In order to prepare different catalysts synthesized in this work, the following materials were used: Aluminium nitrate [Al(NO3)3$9H2O, M ¼ 375.13 gmol1]; Zinc nitrate [Zn(NO3)2$6H2O, M ¼ 297.47 gmol1]; Citric acid [C6H8O7$H2O, M ¼ 210.14 gmol1]; Sucrose [C12H22O11, M ¼ 342.3 gmol1]; Triethanolamine (TEA) [C6H15NO3, M ¼ 149.10 gmol1]; Nitric acid [HNO3, M ¼ 63.01 gmol1]. 2.1. Synthesis of nanocrystalline ZnAl2O4 catalyst by solegel method In order to prepare Zinc Aluminate nanoparticles by solegel method, stoichiometry amounts of the metal nitrates (Zn:Al ¼ 1:2) were dissolved in distilled water to obtain a homogeneous solution.
The obtained transparent sol was poured into glass container and allowed to gel at room temperature. The gel was then dried at 110 C for 2 h. After grinding, the dried gel was calcined at 800 C for 3 h (1.6 C min1) to obtain the ZnAl2O4 powder. The resulting powders are denoted as ZAO. 2.2. Synthesis of nanocrystalline ZnAl2O4 catalyst by citrate precursor In the first step, stoichiometry amounts of the metal nitrates (Zn:Al ¼ 1:2) were dissolved in distilled water to obtain metal nitrate solution. The citric acid solutions were prepared separately by adding different amounts of citric acid into distilled water, the molar ratio of citric acid and the metal ions were 1:1, 2:1 and 4:1 in three samples. Three solutions were subsequently dropped into the nitrate solutions under continuous stirring to ensure the homogeneous solutions. After complete mixing, the solutions were heated to evaporate the water for several hours in water-bath at 70 C. The resultant complex solutions turned to light yellow transparent viscous sols. Yellow gels were obtained by evaporating of water for several more hours. The gels were then dried at 110 C in an oven until fluffy powder obtained, named the citrate precursor of ZnAl2O4. After grinding, the precursors were calcined at 800 C for 3 h to obtain the ZnAl2O4 powder samples (Fig. 1). The powders as synthesized are denoted as ZAO(CA). 2.3. Synthesis of nanocrystalline ZnAl2O4 catalyst by sucrose precursor Similar to citrate method, stoichiometry amounts of metal nitrate (Zn:Al ¼ 1:2) and the molar ratios of sucrose to the total metal ions (1:1, 2:1, 4:1) were used. In the first step, pH of solutions was adjusted to 1.0 by addition of nitric acid dropwise. Subsequent heating with continuous stirring let the ions react with sucrose completely resulting in a transparent colourless solution and continued until the solution changed into a viscous light brownish gel. During evaporation, the aqueous sucrose solution in presence of nitric acid is oxidized to saccharic acid. The gels were then dried at 110 C in an oven which resulted in a dark brown foamy mass. All precursors were ground into powders and calcined in a furnace at 800 C for 3 h to obtain the ZnAl2O4 powder (Fig. 1). The resulting powders are denoted as ZAO(S). 2.4. Synthesis of nanocrystalline ZnAl2O4 catalyst by triethanolamine (TEA) precursor First, metal nitrates were dissolved in distilled water (Zn:Al ¼ 1:2). Then, the required amount of TEA was added dropwise into the mixed solution. For TEA variation studies, a series of solutions was prepared in such a way that the TEA to a total metal ion ratio in the starting solutions was maintained at 1:1, 2:1 and 4:1. At the beginning, a gelatinous precipitate was formed with the addition of TEA; however, it dissolved when concentrated HNO3 was added to give a final pH of about 3. The clear solutions of TEAcomplexed metal nitrate were heated to evaporate the water for several hours in water-bath at 70 C with constant stirring. Continuous heating of the solutions at 110 C in an oven causes a fluffy mass. During evaporation, the nitrate ions provide an in situ oxidizing environment for TEA, which partially converts the hydroxyl groups of TEA into carboxylic acids. When complete dehydration occurred, the nitrates themselves decomposed, with the evolution of brown fumes of nitrogen dioxide, leaving behind a voluminous, organic-based, black, fluffy powder, i.e., the precursor powder. After grinding, the precursor powders were
A.G. Khaledi et al. / Materials Chemistry and Physics 135 (2012) 855e862
Zn(NO3)2.6H2O
H 2O
857
Al(NO3)3.9H2O
Solution 1
Sucrose R.T.
Colorless gel
Stirring
Citric acid
Triethanolamine Stirring
Stirring
Solution 2
Solution 2
White Precipitate
HNO3 Dried at 110◦C
Dried gel Calcined at 800◦C
ZnAl2O4
Heated at 70◦C
Light brown gel
HNO3 Heated at 70◦C
Yellow gel
Dried at 110◦C
Dried at 110◦C
Brown Sucrose Precursor
Yellow Citrate Precursor
Calcined at 800◦C
ZnAl2O4
Heated at 70◦C
Calcined at 800◦C
Brown gel Dried at 110◦C
Brown TEA Precursor Calcined at 800◦C
ZnAl2O4
ZnAl2O4
Fig. 1. Experimental procedure for preparation of ZnAl2O4 powder.
calcined at 800 C for 3 h to obtain the ZnAl2O4 powder (Fig.1). The powders as synthesized are denoted as ZAO (TEA). 2.5. Characterization of ZnAl2O4 nanopowders The crystalline structures of the resulting samples were assessed and characterized by X-ray diffractometer (D4-BRUKER and Cu-Ka radiation at 30kv and 20 mA). The chemical structure of the prepared particles was examined using Fourier Transform Infrared Spectrophotometer (FT-IR, Shimadzu-840S) in the region of 4000e400 cm1. The Diffuse Reflectance Spectra (DRS) of catalysts were recorded by UVeVis spectrophotometer (Shimadzu2550). Morphological studies were carried out using a scanning electron microscope (SEM) PhilipseXLF30 model and surface area of the nanoparticles was calculated using nitrogen absorption data at 298 K (BET analysis, MICRO MERITICS-GEMINI).
materials are fully crystalline. The average crystallite size from XRD was calculated from X-ray line broadening of the (3 1 1) diffraction line using the Scherrer’s equation ðd ¼ ð0:9lÞ=ðbcosqÞÞ, where d is the grain size, l is the wavelength of the X-ray (Cu Ka, 0.15418 nm), b is the full-width at the half-height of the peak, and q is the diffraction angle of the peak and the results are shown in Table 1. Fig. 2 shows the XRD pattern of ZAO sample (synthesized in Section 2.1), which has been used as a reference in order to compare with other synthesized samples. As can be seen, crystallite size decreases by using chelating agents. These results indicate that the prepared sample by Sucrose has smaller size than samples by CA and TEA at the same ratio. It can be noticed in Figs. 3e5, patterns aec, that the peak width increases slightly with increasing the ratio of chelating agent that means the particle size decreases at the same formation temperature. Furthermore, the surface area of the synthesized ZAO, ZAO(S), ZAO(CA) and ZAO(TEA) nanopowders are 13.65, 40.12, 38.46 and 32.17 m2 g1, respectively.
3. Result and discussion 3.1. X-ray diffraction To study the crystallization process and phase identification, the powder X-ray diffraction (XRD) analyses were performed on various ZnAl2O4 samples, which were prepared without/with chelating agents (sucrose, citric acid and TEA). The XRD patterns of samples are shown in Figs. 2e5. In the X-ray diffraction pattern of ZnAl2O4 nanopowder, peaks were observed at 2q ( ) ¼ 18.9, 31.2, 36.8, 44.8, 49.1, 55.6, 59.3 and 65.2 which can be indexed as (1 1 1), (2 2 0), (3 1 1), (4 0 0), (3 3 1), (4 2 2), (5 1 1) and (4 4 0) diffraction planes, respectively. The observed diffraction peaks in recorded XRD patterns correspond to those of the standard patterns of cubic ZnAl2O4 spinel (JCPDS, No. 05-0669). The ratios of peak intensities of prepared powder and that of standard were the same. No other impurity phases were detected in the samples. The prepared
Fig. 2. The XRD pattern of ZAO nanopowders.
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Fig. 3. The XRD patterns of ZAO(S) nanopowders; (a) M:S (1:1), (b) M:S (1:2), (c) M:S (1:4); Inset: Effect of weight ratio on the crystallite size is evident on comparing the (311) diffraction line.
3.2. FT-IR The IR spectra of the ZnAl2O4 nanoparticles prepared by Sucrose, Citric acid and TEA, after calcination (a), before calcination (b), and chelating agents (c) are shown in Figs. 6e8, respectively. All three (a) spectra of Figs. 6e8, show metaleO stretching frequencies in the range 500e900 cm1, associated with the vibrations of ZneO, AleO, and ZneOeAl bonds, indicating the formation of metal oxide, confirmed by XRD as the single-phase ZnAl2O4 spinel. Figs. 6b, 7b and 8b show broad and intense hydroxyl stretching frequencies [y(OH)], which can be attributed to the overlapping of the bands because of surface-adsorbed water (ca. 3400 cm1). The small peaks at around 1384 and 2857e2920 cm1 correspond to the presence of a trace amount of CH2 and CeH band.
Fig. 4. The XRD patterns of ZAO(CA) nanopowders; (a) M:CA (1:1), (b) M:CA (1:2), (c) M:CA (1:4); Inset: Effect of weight ratio on the crystallite size is evident on comparing the (311) diffraction line.
Fig. 6c indicates that in IR spectra of sucrose, no significant peak is present in the frequency region 1760 and 1700 cm1, which confirms the absence of vibrational modes of the carboxylate ions, while in Fig. 6b vibrational modes of frequency of carboxylate ions are present because of the use of HNO3. Observed peaks at 1070e1100 cm1 indicate stretching vibration of CeO; also in sucrose the asymmetric stretching for CeOeC was observed at 1130 cm1, which cannot be seen in Fig. 6b. This indicates that CeOeC bond was decomposed. In citric acid spectrum (Fig. 7c), the stretching vibrations for free carboxyl groups were observed between 1760 and 1700 cm1. After complexing with metal ions (Fig. 7b), two new bands occur near 1620 and 1400 cm1, representing the asymmetric stretching and symmetric stretching vibrations for carboxyl ions (COO). The
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Fig. 5. The XRD patterns of ZAO(TEA) nanopowders; (a) M:TEA (1:1), (b) M:TEA (1:2), (c) M:TEA (1:4); Inset: Effect of weight ratio on the crystallite size is evident on comparing the (311) diffraction line.
Table 1 The effect of chelating agent on the average crystallite size of ZnAl2O4 and the surface area. Product name
Ratio M:Ca
Crystallite size (nm)
SBET (m2 g1)
ZnAl2O4 (without chelating agent) ¼ ZAO
e 1:1 1:2 1:4 1:1 1:2 1:4 1:1 1:2 1:4
36.8 19.5 18.0 16.2 24.0 22.5 21.0 26.4 24.5 22.0
13.65
ZnAl2O4 (prepared by S) ¼ ZAO(S)
ZnAl2O4 (prepared by CA) ¼ ZAO(CA)
ZnAl2O4 (prepared by TEA) ¼ ZAO(TEA) a
M:C ¼ Ratio of (Zn2þ& Al3þ): Chelating Agent.
40.12
38.46
859
Fig. 6. The FT-IR spectra of: (a) ZAO(S) after calcination, (b) ZAO(S) before calcination, (c) Sucrose.
difference between yas and ys is 220 cm1, which indicates that the carboxylate groups are coordinated to the metals in a monodentate style. Fig. 8b shows the strong absorption bands at 1600 cm1, which is attributed to the various vibrational modes of the carboxylate ions, forming because of reaction of decomposed TEA with metal ions. Apart from these bands, two or three-weak bands appear in the region 700e400 cm1, which are the results of some trace amounts of metal oxides forming during an evaporation process [27]. 3.3. UVeVis diffuses reflection spectra (DRS)
32.17
Fig. 9(aed) shows a diffuse reflection spectra of synthesized ZAO (Section 2.1) and ZnAl2O4 prepared by chelating agents. As these
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Fig. 7. The FT-IR spectra of: (a) ZAO(CA) after calcination, (b) ZAO(CA) before calcination, (c) Citric acid.
Fig. 8. The FT-IR spectra of: (a) ZAO(TEA) after calcination, (b) ZAO(TEA) before calcination, (c) Triethanolamine.
can be observed, the absorption bands of ZnAl2O4 prepared by chelating agents exhibit partial hypsochromic shifts, which indicates the decrease of particle size of ZnAl2O4 nanopowders in UV light. Negative solvatochromism corresponds to hypsochromic shift (blue shift) with increasing solvent polarity. Polarity of chelating agents is related to their chemical structure. Products resulting from interaction of sucrose with nitric acid in comparison to CA and TEA exhibit higher polarity effect. The polarity of products is due to the number of carboxyl and hydroxyl groups. Moreover, addition of chelating agents decreases crystal size and crystallinity in ZnAl2O4 particles by inhibiting the growth of particles in the crystalline
network. Furthermore, partial hypsochromic shifts indicate the formation of bands between metals and chelating agents and increasing chelating agent polarity. 3.4. Investigation of chelating agents’ effect on surface morphology The surface morphology of the ZnAl2O4 without/with chelating agents is shown in Figs. 10e13. The ZAO without chelating agents Fig. 10 consists of agglomerated ZAO particles, which may cause the surface and dispersion of particles to be nonuniform whereas in the ZnAl2O4 with chelating agents formed interparticle pores are numerous and regular. Furthermore, this porous structure is
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861
Fig. 11. SEM image of ZAO(S) nanoparticles.
distributed uniformly in the matrix of ZnAl2O4. It is observed that the surface morphology of the ZnAl2O4 is greatly dependent on the type of the chelating agent. Fig. 11 shows the SEM images of ZAO(S). In the case of sucrose, addition of nitric acid helps to break sucrose into Glycolic acid, Hydroxybutyric acid and Saccharic acid, which then helps to prevent recrystallization of sugar. Presence of more carboxylic groups causes increasing of metalesucrose bands. This sample consists of many pores and particles that are distributed uniformly on the surface of ZnAl2O4 in comparison with two other chelating agents. Decomposition of sucrose generates excess heat and a huge amount of gases that help to produce the porosity in the final Zinc Aluminat. During heating, the metal ion-chelated complex is decomposed into carbon dioxide and water, and a large amount of heat is generated. These produced gases prevent agglomeration,
and help to form pores and fine particles with high surface area in the final product. The SEM of ZAO (CA) is shown in Fig. 12. In this sample, the precursor of the metal oxide is directly calcinated in air and decomposed as carbon dioxide, carbon monoxide, water and metal oxide. The existing carbon prevents completion of the reaction to ZnAl2O4 by increasing the diffusion distance of the mixed cations. This retards the crystallization of ZnAl2O4, especially for a precursor with a high ratio of citric acid to metal. In the following heattreatment, the carbon is oxidized softly in air and the crystallization of ZnAl2O4 is completed. At the same time, the pores are formed by combustion of the carbon surrounding the amorphous ZnAl2O4. Fig. 13 shows the SEM images of ZAO (TEA). It can be seen that all the micrographs show uniform distribution of the particles without serious agglomeration. The chemical process starts from homogeneous distribution of metal ions in solution. The solution after complete evaporation decomposes into a fluffy black mass, which is effectively a carbonaceous material having a mesoporous structure which is evident from its surface area. During decomposition of metal ion complexes in air, it produces nescent metal oxides, which are basically small atomic clusters with proper chemical homogeneity, embedded in this mesoporous carbonaceous material. The nescent metal oxides on rearrangement produce the desired singlephase ZnAl2O4. The decomposition of carbonaceous material
Fig. 10. SEM image of ZAO nanoparticles.
Fig. 12. SEM image of ZAO(CA) nanoparticles.
Fig. 9. The diffuse reflection spectra of: (a) ZAO, (b) ZAO(TEA), (c) ZAO(CA) and (d) ZAO(S).
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decomposed into carbon dioxide and water. The large amount of produced gases prevents agglomeration, and helps to form pores and fine particles with high surface area in the final product in comparison with two other chelating agents. References
Fig. 13. SEM image of ZAO(TEA) nanoparticles.
produces gases (such as CO, CO2, NH3, NO2, and water vapour) that help to disintegrate the agglomerated particles and help to inhibit the sintering of nanosized particles to grow bigger particles. 4. Conclusions In this investigation, it was found that the weights ratio of chelating agents to metal and kind of the chelating agents were important factors which affected surface morphology, crystal size and specific surface area of ZnAl2O4 powders. Chemical and physical properties of nanoparticles were different. In the case of sucrose, addition of nitric acid helps to break it into new carboxylic acids, which then prevents recrystallization of sugar. Presence of more carboxylic groups causes increasing of metalesucrose bands and keeps metal ions uniformly distributed throughout the viscous gel during evaporation. The metal ionesucrose complex is
[1] X. Duan, D. Yuan, Z. Sun, C. Luan, D. Pan, D. Xu, M. Lv, J. Alloys Compd 386 (2005) 311. [2] A.R. Phani, M. Passacantando, S. Santucci, J. Mater. Chem. Phys. 68 (2001) 66. [3] S. Mathur, M. Veith, M. Haas, Haoshen, N. Lecerj, V. Huch, J. Am. Ceram. Soc. 84 (2001) 1921. [4] J.F. Cordoro, US Patent No. 6,099,637, 8 August 2000. [5] J.F. Cordoro, et al., US Patent No. 5,807,909, 15 September 1998. [6] Z. Chen, E. Shi, Y. Zheng, W. Li, N. Wu, W. Zhong, J. Mater. Lett. 56 (2002) 601. [7] A.E. Giannakas, T.C. Vaimakis, A.K. Ladavos, P.N. Trikalitis, P.J. Pomonis, J. Colloid Interface Sci. 259 (2003) 244. [8] C.O. Arean, B.S. Sintes, G.T. Palomino, C.M. Carbonell, E. ScalonaPlatero, J.B. Parra Soto, J. Microporous Mater. 8 (1997) 187. [9] K. Kumara, K. Ramamoorthya, P.M. Koinkarb, R. Chandramohanc, K. Sankaranarayanana, J. Cryst. Growth 289 (2006) 405. [10] X. Duan, D. Yuan, X. Cheng, C. Luan, Z. Sun, X. Wei, S. Guo, D. Xu, M. Lv, J. Inorg. Chem. Commun. 7 (2004) 62. [11] N.J. van der Laaga, M.D. Snela, P.C.M.M. Magusinb, G. de, J. Eur. Ceram. Soc. 24 (2004) 2417. [12] h.S. Liu, J. Li, Z.R. Zhang, Z.L. Gong, Y. Yang, J. Solid State Electrochem. 7 (2003) 456. [13] H. Liu, Z. Zhang, Z. Gong, Y. Yang, J. Solid State Ionics 166 (2004) 317. [14] C. Polo Fonsecaa, M.C.A. Fantinib, S. Nevesa, J. Thin Solid Films 488 (2005) 68. [15] X.L. Duana, D.R. Yuana, L.H. Wanga, F.P. Yua, X.F. Chenga, Z.Q. Liua, S.S. Yanb, J. Cryst. Growth 296 (2006) 234. [16] M.G. Lazarraga, L. Pascual, H. Gadjov, D. Kovacheva, K. Petrov, J.M. Amarilla, R.M. Rojas, M.A. Martin-Luengo, J.M. Rojo, J. Mater. Chem. 14 (2004) 1640. [17] R.N. Das, A. Bandyopadhyay, S. Bose, J. Am. Ceram. Soc. 84 (2001) 2421. [18] Y.J. Wu, A. Bandyopadhyay, S. Bose, J. Mater. Sci. Eng. A380 (2004) 349. [19] L. Xu, B. Wei, Z. Zhang, Z. Lu, H. Gao, Y. Zhang, J. Nanotechnol. 17 (2006) 4327. [20] A.B. Panda, A. Pathak, P. Pramanik, J. Mater. Lett. 52 (2002) 180. [21] A. Sen, P. Pramanik, J. Eur. Ceram. Soc. 21 (2001) 745. [22] T. Kocareva, I. Grozdanov, B. Pejova, J. Mater. Lett. 47 (2001) 319. [23] D.Q. Liu, Z.Z. He, X.Q. Liu, J. Alloys Compd 440 (2007) 69. [24] S. Ran, L. Gao, J. Ceram. Int. 34 (2008) 443. [25] S.B. Rane, T. Seth, G.J. Phatak, J. Mater. Lett. 57 (2003) 3096. [26] H. Zhang, X. Jia, Z. Liu, Z. Li, J. Mater. Lett. 58 (2004) 1625. [27] J.-D. Tsay, T.-T. Fang, J. Am. Ceram. Soc. 82 (1999) 1409.