Received: 3 October 2016
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Revised: 3 October 2016
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Accepted: 24 October 2016
DOI: 10.1111/ijac.12624
ORIGINAL ARTICLE
Development of ZrO2-MgO nanocomposite powders by the modified sol-gel method Fatemeh Davar1 | Nasrin Shayan1 | Akbar Hojjati-Najafabadi2 | Vahid Sabaghi1 | Saeed Hasani3 1 Department of Chemistry, Isfahan University of Technology, Isfahan, Iran 2
Department of Materials Engineering, Malek Ashtar University of Technology, Shahin Shahr, Isfahan, Iran 3 Department of Mining and Metallurgical Engineering, Yazd University, Yazd, Iran
Correspondence F. Davar Email:
[email protected]
Abstract The ZrO2-MgO nanocomposites were synthesized using a new sol-gel method with sucrose and tartaric acid as a gel agent. The samples were characterized by thermal analysis (TG/DTA), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), energydispersive X-ray mapping (EDX mapping), and Ultraviolet-visible spectroscopy (UV-vis). The results showed that the cubic phase of ZrO2-MgO was formed in the presence of both gel agents. The average particle size of the samples synthesized with sucrose was lower (30 nm) than that of tartaric acid (50 nm). Finally, the formation mechanism and the optical properties of zirconia-magnesia have been discussed. KEYWORDS nanostructured ceramics, sol-gel, zirconia-magnesia
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| INTRODUCTION
Nowadays, nano-scaled zirconia-magnesia ceramics are among the most important categories of solid catalysts used as an active phase; they have attracted a great deal of attention due to such properties as excellent chemical and thermal stability, high porosity and large surface area.1-4 These materials play many roles in chemistry, physics, materials science, and geochemistry4; they have been used for a wide range of industrial applications such as sensors, microelectronic devices, refractory materials, biomaterials, and fuel cells.5,6 Therefore, there many challenges in the optimization of the synthesis methods used for these oxides. The most commonly used processes for fabricating nanostructured ZrO2-MgO ceramics are sol-gel,4,7,8 mechanical alloying,9,10 co-precipitation,11-13 and plasmaspraying14,15 methods. The reliability of the sol-gel method for many practical applications is largely influenced by the intrinsic properties controlled by the sol-gel processing parameters.16,17 The early stages of eutectoid decomposition in ZrO28 mol% and 11 mol% MgO were studied by Czeppe Int J Appl Ceram Technol 2016; 1–9
et al.18 They observed that the reaction products at the grain boundaries were monoclinic or tetragonal ZrO2 phases and MgO precipitates in two different cellular morphologies; also, the eutectoid decomposition of ZrO2-MgO was controlled by the interface of each phase. Tian et al.19 showed that crystalline mesoporous ZrO2-MgO solid solutions could be fabricated by a simple evaporationinduced self-assembly procedure, showing that the Mg2+ doped into the zirconia lattice could stabilize the tetragonal phase and prevent the excessive growth of zirconia crystals. Das et al.13 prepared the ZrO2-MgO composite hydrogel by co-precipitation technique in three different mole ratios by the electrolytic interaction of inorganic salts in the aqueous phase; they showed that the MgO particles occupies the intergranular positions between ZrO2 polycrystalline. This work presents the synthesis of ZrO2-MgO nanostructured composite by using the new sol-gel method in the presence of sucrose and tartaric acid as both chelating and gel agents. The mechanisms for the synthesis of ZrO2MgO nanostructured ceramics via these gel agents have been suggested too.
wileyonlinelibrary.com/journal/ijac
© 2016 The American Ceramic Society
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| MATERIALS AND METHODS
2.1 | Preparation of ZrO2-MgO nanocomposites The raw materials used in the present work were purchased from Merck company and were used as received. The ZrO2-MgO nanocomposites were prepared by the sol-gel method in various molar ratios of sucrose and tartaric acid as chelating agents. The selected chelating agents are reported in Table 1. The stoichiometric proportions of Mg (NO3)26H2O and ZrOCl28H2O were dissolved in 150 mL of deionized water. Then, according to Table 1, various ratios of sucrose and tartaric acid were separately added into the solution, and the mixture was vigorously stirred in a hotplate in 80°C to clear sols were resulted. These solutions were slowly evaporated in a hotplate at 120°C until a highly viscous amorphous gels were formed and then the gels were heated at 150°C for 1 hour. Finally, the resulting precursor powders were calcined at 700°C for 2 hours. The procedure for the synthesis of ZrO2-MgO nanocomposites has been illustrated in Figure 1.
2.2
F I G U R E 1 The procedure for the synthesis of ZrO2-MgO nanocomposite powders. [Color figure can be viewed at wileyonlinelibrary.com]
| Materials characterization
To study the thermal characteristics of the precursor decomposition, thermogravimetry (TG) and differential thermal analysis (DTA) were carried out under an air atmosphere by using Seiko instruments Model-326. The thermal analyses were performed in flowing air by heating the sample from 25 to 800°C at a rate of 20°C/min; and for this run, about 7.51 mg of dried precursor gel was placed in an alumina crucible. For structural analysis, X-ray diffraction measurements were performed. A Phillip X’pert diffractometer (Model MPD-XPERT) was used to carry out the experiment. A CuKa beam with a wavelength of (k)=1.54 A was used as the radiation source. The diffractometer was operated at 40 kV and 30 mA using a scanning step of 0.030 in 2h, and dwell time of 1 second was used. The 2h angle was varied from 20° to 80°. The FTIR
T A B L E 1 The selected chelating agents Sucrose molar ratio
Tartaric acid molar ratio
Code
1
–
S1
2
–
S2
3
–
S3
4
–
S4
–
1
T1
–
2
T2
–
3
T3
–
4
T4
F I G U R E 2 The typical Thermogravimetric analysis (TGA) and DTA curves of S4 sample. [Color figure can be viewed at wileyonlinelibrary.com]
spectra for all samples were characterized by using a Bruker TENSOR 27 infrared spectrophotometer. The IR absorbance was measured in a range between 4000 cm1 and 400 cm1. The morphology, microstructure, and chemical composition of the ZrO2-MgO nanoparticles were examined by using a field emission scanning electron microscope (FESEM; Model Mira3-XMU, TESCAN, Prague, The Czech Republic) and energy-dispersive spectroscopy with 15 kV voltages, respectively. Before taking FESEM images, the samples were Au-coated with desk sputter coater (DST3 model, nanostructured coating Co., made in Iran). The transmittance and reflectance spectra of the
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FIGURE 3
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The typical XRD analysis results of S4 and T4 samples
T A B L E 2 Microstructure characteristics of the as-synthesized MgO and ZrO2 nanoparticles Mean crystallite size (nm)
Code
MgO
20.6
S1
ZrO2
36.2
S1
MgO
41
S2
ZrO2
32.7
S2
MgO
29.2
S3
ZrO2
35.9
S3
MgO
34.2
S4
ZrO2
27.2
S4
MgO
47.7
T1
ZrO2
42.9
T1
MgO
44.0
T2
ZrO2
39.1
T2
MgO
40.2
T3
ZrO2
36.7
T3
MgO
37.8
T4
ZrO2
34.3
T4
nanocomposites were obtained by using a UV-Vis spectrophotometer (Shimadzu UV-3100; Shimadzu Company, Kyoto, Japan).
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| RESULTS AND DISCUSSION
The thermal observation of the amorphous precursor was investigated by thermogravimetry (TG) and differential thermal analysis (DTA), as shown in Figure 2. Based on the TG/DTA curve, the sample started to decompose in the temperature range of 25-220°C, and the 13% weight loss happened probably due to the dehydration of the precursor
F I G U R E 4 FTIR spectra of the precursor powder gel and ZrO2-MgO nanocomposite in various chelating agents (A) sucrose and (B) tartaric acid. [Color figure can be viewed at wileyonlinelibrary.com]
during an endothermic process. With increasing the temperature to 600°C, the weight loss was about 40% and the black colored powder was obtained, showing that some trace of carbon remained on the sample. Due to different carboxylic acid molecules (gluconic acid and trihydroxybutyric acid) resulting from the hydrolysis of sucrose, the removal of these organic compounds occurred in temperatures ranging from ~300 to 600°C. According to the DTA curve, the weight loss could be attributed to the exhaust carbon, volatile gases and the organic precursor in the gel during an exothermic process. When the calcination temperature was increased to above 600°C, the black-colored powder became white; consequently, the calcination temperature was selected to be 700°C. To determine the structural properties of nanocomposites, X-ray diffraction (XRD) analysis was performed on MgO-ZrO2 nanocomposites calcined at 700°C for 2 hours. The typical X-ray diffraction patterns shown in Figure 3
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provided structural information on the processed materials for these nanocomposites. As could be observed in Figure 3, the crystallization of both MgO and ZrO2 phases was evidenced, and no impurity peaks appeared. The MgO was crystallized in the Periclase form. It could be seen that the positions and the relative intensities of these nanocomposite peaks could be matched well with the (111), (200), (220), (311), and (222) planes, as revealed in the JCPDS No. 01-075-0447 (MgO) and JCPDS No. 00-027-0997 (ZrO2). The lattice parameter of these particles (a) was calculated using the following equation. 1 h2 þ k2 þ l 2 ¼ 2 a2 dhkl
(1)
where d(hkl) is the d-spacing value and (hkl) refers to the miller indices. For most samples, the value of lattice parameter (a) was about 4.22 A (MgO) and 5.09 A (ZrO2), showing a good agreement with the previous literature in this field.9 The Scherrer’s formula (Equation 2) was used to obtain the crystallite size listed in Table 2. D¼
0:89k b cos h
(2)
where k is the X-ray wavelength, b is the full-width at half maximum and h is the Bragg angle.7 For the calculation of correct line broadening, the XRD analysis of single crystalline Si was used to measure the instrumental line broadening; it was found out to be 0.14° at the diffraction peak angle of 28.45°. Corrected line broadening, bcorr. was calculated using the following formula
FIGURE 5
bcorr: ¼ b20 b2i
1=2
ET AL.
(3)
where b0 and bi are the values of the observed and instrumental line broadening, respectively. As shown in Table 2, by increasing the ratio of sucrose and tartaric acid from 1 to 4, the main crystallite size of MgO and ZrO2 nanoparticles was increased.6,20 The FTIR spectroscopy is a useful tool to investigate chemical structure changes and the functional group of any organic molecule. Figure 4 shows the FTIR spectra of a precursor powder and ZrO2-MgO nanocomposites at the range of the wave number 400-4000 cm1. The major features of the spectra included bands corresponding to Metal-Oxygen (M=Mg and Zr) and -OH group. The FTIR spectra of the asprepared precursor gel showed the characteristic bands of metal-oxygen stretching vibrations of MgO and ZrO2 at 427 cm1, 585 cm1, and 638 cm1; the N-O stretching and bending vibrations of NO3 groups were near 858 cm1, 1030 cm1, and 1402 cm1, respectively.21,22 The peaks around 3400-3500 cm1 corresponded to the O-H stretching mode of hydroxyl groups, as represented by a broad band, where freely vibrating -OH groups and hydrogen bonded OH groups appeared.21,23 The spectra exhibited an O-H bending band of the H2O molecules chemically adsorbed to the nanoparticle surface around 1630 cm1.23,24 The absorption bands between 1100 and 1400 cm1 and 2920 cm1 corresponded to -C-O stretching band and -C-H asymmetrical stretching, respectively.24 The metal-oxygen bonds stretching vibration mode (M-O, M=Mg, Zr) existed in the wave number range of 400-600 cm1, which could be attributed to Mg-O and Zr-O bands.21 It confirmed the formation
Field-emission scanning electron micrographs of the nanoparticles with different chelating agents (A) sucrose and (B) tartaric acid
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FIGURE 6
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Particle size distribution of the ZrO2-MgO nanoparticles prepared with different chelating agents (A) sucrose and (B) tartaric acid
of the cubic ZrO2-MgO nanocomposites structure. It could be seen that the intensity of peaks in the wave number range of 400-600 cm1 was increased by increasing the ratio of sucrose, in comparison to tartaric acid as chelating agents, confirming that the formation reaction of MgO and ZrO2
phases was complete. This could be explained by investigating the reaction of chelating agents in the aqueous acidic medium. The sucrose was converted to glucose and fructose in the acidic media. In the presence of nitrate ions, the glucose was oxidized into gluconic acid. Furthermore, the
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FIGURE 7
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The typical EDS spectra of the optimum ZrO2-MgO nanocomposite (S4)
fructose was converted to trihydroxybutyric acid.25,26 These acids formed a complex with Mg2+ and Zr4+ ions, and a clear sol was obtained. Thus, the sucrose had a higher tendency to coordinate Mg2+ and Zr4+ ions, as compared to tartaric acid molecules. The FESEM micrographs of the MgO-ZrO2 nanocomposites calcined at 700°C for 2 hours are shown in Figure 5.Furthermore, the particle size distributions of MgO-ZrO2 nanoparticles are shown in Figure 6. As could be observed, in all samples, the porous nanoparticles had been distributed homogeneously and almost uniformly. It could be attributed to the conditions of synthesis. Due to the ignition of the dried gels in the early stages of the synthesis process, a large amount of gases and volatile organic compounds was exhausted, leading to the porous microstructure. The FESEM micrographs and the mean particle size of nanoparticles in Figures 5 and 6 showed that by increasing the sucrose and tartaric acid molar ratios (from 1 to 4), the agglomeration degree and the mean nanoparticle size were decreased. In fact, by increasing the chelating agents, the complex agents content (gluconic acid and trihydroxybutyric acid) in the acidic media were increased, which could help narrow particle size distribution due to further steric hindrance of these acids-metal complexes. Furthermore, FTIR results confirmed that some carboxylic acid (the peak at 1400-1600 cm1) existed in the sample after the calcination treatment. This small amount of -COOH group could stabilize zirconia-magnesia particles and prevent them from being agglomerated.26,27 The average particle size of both samples prepared with sucrose and tartaric acid was less than 50 nm. However, the mean particle size of sucrose mediated sample was smaller than that of tartaric acid as a chelating agent; so; the mean particle
size for sucrose was reduced to 31 nm (S4). As mentioned above, the sucrose was converted to glucose and fructose in the acidic media. Due to the hydrolysis and decomposition of sucrose, a mixture of carboxylic acids was produced. They included monodentate and bi-dentate ligands that coordinated Mg2+ and Zr4+ ions better than tartaric acid (with two functional-COOH groups). Therefore, it could be concluded that using sucrose caused a narrower particle size distribution of samples, as compared to tartaric acid. Thus, the S4 sample was selected as an optimum one (the molar ratio of sucrose is 4). The first step in the formation of zirconia-magnesia nanocomposites took place by a nucleophilic attack of the OH on the central Zr4+ and Mg2+ atoms. The nature of chelating ligands noticeably changed the rate of hydrolysispolycondensation steps. The polycondensation resulted in the formation of the nanoscaled clusters of metal oxides or hydroxide molecules. According to the literature, sucrose and sucrose degradation products act as chelating agents for metal cations and templating agents, and as fuel for combustion.28,29 In this process, sucrose and tartaric acid were used as a chelating agent to keep Zr(OH)4 and Mg (OH)2 in a homogeneous solution with hydrogen bonding interaction. With raising temperatures to 110°C, polycondensation took place; in an alkaline medium at about 100°C, sucrose or tartaric acid could be slightly hydrolyzed to water soluble products, that is, gluconic acid and trihydroxybutyric acid. Moreover, the presence of sucrose and tartaric acid along with these products could also increase the viscosity of sol and reduce the rate of the crystallization of ZrO2-MgO nanocomposites, thereby favoring the formation of less agglomerated zirconia-magnesia nanocomposites.26,30-32 Sucrose and tartaric acid were decomposed
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exothermally at a low ignition temperature (480°C), leaving behind only a small amount of carbon residue. The elemental analysis was carried out by the energydispersive spectroscopy (EDS) to confirm the chemical composition of ZrO2-MgO nanoparticles. The typical EDS spectrum of the optimum nanoparticle (S4) and the corresponding elemental analysis results are presented in Figure 7 and Table 3, respectively. It was evident from the elemental analysis that the weight percentage of Mg and Zr in the ZrO2-MgO nanoparticles agreed with the designed composition and stoichiometric ratios. The elemental mapping micrographs of as-synthesized ZrO2-MgO nanocomposite are presented in Figure 8. The micrographs were labeled as Mg Ka, Zr La, and O Ka. These micrographs confirmed the homogenous distribution of the elements in the ZrO2-MgO nanocomposite; and there was no
T A B L E 3 Energy dispersive X-ray analysis results of optimum MgOZrO2 nanocomposite (S4) Element
Energy levels
Weight percentage
Atomic percentage
Mg
K
29.17
26.81
Zr
L
22.34
5.47
48.49
67.71
Remains (O, C, and etc.)
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inhomogeneous distribution due to the presence of sucrose as a chelating agent. The UV-visible absorption spectra of the whole series of the ZrO2-MgO nanocomposite samples are shown in Figure 9. As can be seen, the maximum absorption band of S1-S4 (Figure 9A) was at 282-284 nm; according to Eg=hc/k (where Eg is the energy gap, hc is Planck constant, and k is the wavelength), energy gap was equal to 4.36 eV. The samples prepared with tartaric acid (Figure 9B) had a little red shift, as compared to Figure 9A, due to bigger particle size distributions of these samples. The electron transition occurred from the valence band to the conduction band. Since the Zr4+ ion had a d0 configuration, no electronic transition was seen in this orbital for zirconia. Hence, ZrO2 did not show any absorption band in the visible region (400-800 nm). The energy gap of c-ZrO2 (3.8-6.1 eV) was higher than that of both m-ZrO2 (3.1-3.7 eV) and t-ZrO2 (2.2-2.4 eV).27 The absorption band occurred in 282-284 nm, which was equal to 4.36-4.39 eV; as a result, this energy band was attributed to c-ZrO2. On the other hand, studies have shown that the MgO nanoparticle depends on the coordination number of O2, offering different absorption bands. For example, at the coordination number of three or four of oxygen ions, MgO shows absorption bands at 230 nm (5.4 eV) and 260 nm (4.7 eV).28 Thus, it can be concluded that the
F I G U R E 8 The elemental mapping micrographs of typical optimum ZrO2-MgO nanocomposite. [Color figure can be viewed at wileyonlinelibrary.com]
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the mean crystallite size of the nanocomposites was less than 30 nm. The FTIR spectra confirmed the formation of MgO-ZrO2 nanocomposite, and the intensity of metal-Oxide (M-O) peaks was increased by using sucrose, in comparison to tartaric acid. The FESEM micrographs of the nanoparticles revealed that the size distribution of nanoparticles was narrow and the mean particle size of nanoparticles was less than 50 nm. Furthermore, by increasing the sucrose and tartaric acid molar ratios, the agglomeration degree and the nanoparticle size were decreased. The elemental mapping micrographs of typical as-synthesized ZrO2-MgO nanocomposites confirmed the homogenous distribution of the elements in the ZrO2-MgO nanocomposite. Moreover, by using sucrose as a gel agent, the as-prepared sample had a lower particle size than the sample prepared with tartaric acid molecules. The molar ratio of 4:1 for sucrose to transition metal was selected as an optimum chelating agent. The maximum UV-visible absorption spectra of the whole series of the ZrO2-MgO nanocomposites were recorded at ~284 nm, showing that the electronic transition of MgO and ZrO2 had been overlapped together. REFERENCES
F I G U R E 9 UV-vis absorption of ZrO2-MgO nanocomposites (A) S1-S4 (B) T1-T4. [Color figure can be viewed at wileyonlinelibrary.com]
MgO absorption bands (with the coordination number 3) and c-ZrO2 phase occur at the same region, and these bands (MgO and ZrO2) may have overlappings. Furthermore, according to Figure 9, there was a difference between the absorption intensity of both curves “a” and “b.” In this research, difference in particles size and the presence of the asymmetrical and symmetrical stretching vibrations of the carboxylate groups (O–C=O formed during the combustion process) led to the difference in the absorption intensity. The carboxylate groups existing in calcined sample were confirmed by FTIR results in Figure 4(A,B). Furthermore, these organic groups were not removed after the calcination process and prevented nanoparticles from being agglomerated.33-36
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| CONCLUSIONS
The ZrO2-MgO nanocomposites were prepared by the solgel method, using sucrose and tartaric acid in various molar ratios as the chelating agent. Thermal analysis showed that the ZrO2-MgO nanocomposite was formed at 700°C and the cubic phases of ZrO2-MgO were confirmed by XRD;
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How to cite this article: Davar, F., Shayan, N., Hojjati-Najafabadi, A., Sabaghi, V. and Hasani, S. (2016), Development of ZrO2-MgO nanocomposite powders by the modified sol-gel method. International Journal of Applied Ceramic Technology, 00: 000–000. doi: 10.1111/ijac.12624
