Stabilization of the Metastable Tetragonal Phase in Zirconia

Article pubs.acs.org/IECR

Stabilization of the Metastable Tetragonal Phase in Zirconia Nanopowders Synthesized via Polyacrylamide Gel Method Peyman Khajavi,†,‡ Ali Akbar Babaluo,*,†,‡ Akram Tavakoli,†,‡ and Amir Mirzaei† †

Chemical Engineering Department and ‡Nanostructure Materials Research Center (NMRC), Sahand University of Technology, P.O. Box 51335-1996, Tabriz, Islamic Republic of Iran ABSTRACT: In this work, the possibility of synthesizing stable tetragonal zirconia nanopowders via polyacrylamide gel method was investigated. In addition, the effects of the sulfation process on the stabilization of metastable tetragonal structure and tetragonal to monoclinic phase transformation were studied. Zirconium oxychloride and zirconium oxynitrate salts were used as the initial salt precursor and the calcination procedure was performed at different temperatures (650−850 °C) for 2 h with heating and cooling rates of 5 and 2 °C/min. The results showed that in the synthesized powders from zirconium oxynitrate, metastable tetragonal phase was stable up to 850 °C. Moreover, heating rate of 5 °C/min was concluded as the optimum rate to stabilize the metastable tetragonal phase up to higher temperatures. The addition of sulfate ions into the structure of sulfated zirconia powders synthesized by zirconium oxychloride retarded the tetragonal to monoclinic phase transformation, while in the case of zirconium oxynitrate, this phase transformation was accelerated. synthesize this polymorph is great.28 Using of stabilizers for stabilization of the tetragonal phase is costly. In addition, in some cases the presence of stabilizers may have unfavorable effects on the properties of the final material. Therefore, using the metastable tetragonal phase can be a proper alternative and whatever this phase remains stable up to higher temperatures it is possible to use zirconia and sulfated zirconia for more applications. For instance in reactions that are catalyzed by the sulfated zirconia catalysts, avoiding the tetragonal to monoclinic phase transformation that may occur due to the heat of reaction, can significantly prevent decreasing of the catalytic activity and performance of catalyst.7 Also in the case of zirconia membranes preparation, it will be possible to synthesize a crack free membrane by preventing the tetragonal to monoclinic phase transformation upon sintering condition, even without addition of stabilizers. Among the traditional chemical and hydrothermal synthesis routes of oxide powders, the sol−gel and precipitation techniques have been considerably used in order to synthesize zirconia and sulfated zirconia powders.29−31 Ease of controlling the homogeneity and physical characteristics during synthesis steps as well as ability to form large surface area materials at low temperatures are the advantages of the sol−gel method. However, in this method various zirconium alkoxides have been used as the precursors which are very expensive. On the other hand, in the precipitation method, various zirconium salts usually used as the precursors which are much cheaper than zirconium alkoxides. But in this technique appropriate control of powder morphology is more difficult. Furthermore, the long required time of synthesis in both methods is an important disadvantage of them.11,32−34

1. INTRODUCTION Zirconium oxide (ZrO2) is one of the most important ceramic materials and has a wide range of applications because of the unique features of its various phases. The three low-pressure crystalline phases of zirconia are the monoclinic (below 1170 °C), tetragonal (1175−2370 °C), and cubic (2370−2680 °C).1−4 The martensitic transformation from tetragonal to monoclinic phase is accompanied by a 3−5% volume expansion which may lead to considerable material fracture, and for this reason, the room temperature monoclinic phase has no practical applications. On the other hand, the tetragonal and cubic phases are appropriate for various industrial applications. Hence, due to low stability of these phases at low temperatures, generally, zirconia is doped with oxides such as CaO, MgO, Y2O3, and CeO2. Moreover, it is observed that the high temperature tetragonal phase can also exist in low temperatures even without doping any stabilizers, so-called “metastable tetragonal zirconia”.5−9 Several models and reasons have been proposed in the literature for the formation of this high temperature polymorph at room temperature such as the influence of anionic impurities, crystallite size, lattice strains, lattice defects (oxygen vacancies), and structural similarities between precursor materials and tetragonal zirconia.9,10 One of the most important applications of zirconia is used as catalysts or catalytic supports.11,12 Although pure zirconia has catalytic activity, addition of sulfate ions can significantly enhance its catalytic activity. Because of the strong acid properties of sulfated zirconia that are related to sulfate ions, it shows remarkable catalytic activity for reactions which are not catalyzed by conventional solid catalysts.13−15 The catalytic activity of sulfated zirconia catalysts depends on several parameters such as sulfation procedure and sulfate content of the catalysts.16,17 Furthermore, the tetragonal crystalline phase is a required feature of high activity sulfated zirconia catalysts.18−27 Due to special characteristics of tetragonal zirconia such as higher surface hardness and smoothness than the monoclinic phase and also catalytic activity, the interest to © 2013 American Chemical Society

Received: Revised: Accepted: Published: 164

July 12, 2013 November 29, 2013 November 29, 2013 November 29, 2013 dx.doi.org/10.1021/ie402224m | Ind. Eng. Chem. Res. 2014, 53, 164−172

Industrial & Engineering Chemistry Research

Article

Table 1. Characteristics of Used Materials materials

function

molecular formula

supplier

acrylamide (AM) N,N′-methylenebisacrylamide (MBAM) ammonium persulfate (APS) N,N,N′,N′-tetramethylethylenediamide (TEMED) zirconium oxychloride zirconium oxynitrate sulfuric acid water

monofunctional monomer difunctional monomer (cross-linker) initiator accelerator initial salt initial salt sulfating agent solvent

C2H3CONH2 (C2H3CONH2)2CH2 (NH4)2S2O8 C6H16N2 ZrOCl2·xH2O ZrO(NO3)2·xH2O H2SO4 H2O

Merck Merck Merck Merck Merck Aldrich Merck

copolymerize of monomers and obtain a transparent gel. In the case of synthesis of sulfated zirconia powder, initial salt precursor was dissolved in a sulfuric acid solution of a given molarity followed by the same procedure as the pure zirconia powder synthesis. The calcination step was performed at different temperatures (650−850 °C) for 2 h at heating and cooling rates of 5 and 2 °C/min. The final calcined pure and sulfated zirconia powders were denoted as Z x-T and SZ x-T, respectively. Where x and T are the used initial salt precursor (zirconium oxychloride (Cl) and zirconium oxynitrate (N)) and calcination temperature, respectively. 2.2. Characterization. The X-ray diffraction patterns of the synthesized powders were recorded by a Siemens D-500 diffractometer using Cu Kα radiation source in the range of 2θ = 20−80°. The presence of crystalline phase(s) in the calcined powders was identified using JCPDS data files. The average tetragonal crystallite size was calculated from (101)t diffraction peak using the standard Scherrer formula:

Synthesized powders by the above-mentioned methods mainly have amorphous structure and the tetragonal structure will appear upon increasing calcination temperature. This metastable tetragonal phase transforms to the thermodynamically stable monoclinic phase by further heating. The temperature range of these phase transformations is different and depends on the various parameters such as used method and precursor material. However, the maximum reported temperatures at which the metastable tetragonal phase can entirely exist are in the range of 600−650 °C.6,8,11,12,20,23,34−37 The polyacrylamide gel method is one of the modified sol− gel methods that can be efficiently applied for synthesis of ultrafine and highly dispersed powders. This method has many advantages compared with the other prevalent synthesis routes of zirconia and sulfated zirconia powders. It is a simple, fast, reproducible, and easily scaled up wet chemical process which provides ultrafine powders with controlled morphology at relatively low temperatures. Furthermore, in this method, low cost inorganic salts have been used as the precursors.35,38−40 In our previous work,35 we successfully developed this method for synthesis of zirconia nanopowders. Despite the significant advantages of this technique, to the best of our knowledge, there are no available reports related to the phase transformation and also stabilizing the metastable tetragonal phase in zirconia and sulfated zirconia nanopowders synthesized via this method. Considering the significant advantages of stabilizing the metastable tetragonal phase up to high temperatures, the main objectives of this work were to evaluate the performance of polyacrylamide gel method in synthesizing pure zirconia and sulfated zirconia nanopowders with metastable tetragonal phase and also to investigate the tetragonal to monoclinic phase transformation. For this purpose, the effects of initial salt precursors and calcination conditions on the stabilization of metastable tetragonal phase and tetragonal to monoclinic phase transformation were investigated.

Dt = 0.9λ /β cos θ

(1)

where Dt is the average tetragonal nanocrystallite size in nm, λ is the radiation wavelength (0.15406 nm), β is the corrected half-width at half-maximum intensity (fwhm), and θ is the diffraction peak angle. The volume fractions of tetragonal and monoclinic phases were calculated using following equations presented by Toraya et al.:41 Xm =

νm =

Im(111) + Im( 1̅ 11) Im(111) + Im( 1̅ 11) + It(101)

1.311X m 1 + 0.311X m

νt = 1 − νm

(2)

(3) (4)

where Xm is the integrated intensity ratio and νt and νm are the volume fractions of tetragonal and monoclinic phases, respectively. In comparison with the various presented methods, the proposed model by Toraya is a simple and almost accurate model in order to calculate the volume fractions of tetragonal−monoclinic binary systems.42 FT-IR spectra of the synthesized powders were studied in the range of 400−4000 cm−1 as KBr pellets on a Unicam-Mattson 1000 spectrophotometer. The particle size distribution of the synthesized powders was obtained using a laser particle size analyzer (PSA, FRITSCH, D-55743). Microstructural analyses of the synthesized powders were performed using a highresolution scanning electron microscope (SEM, VEGATESCAN). The Brunauer−Emmett−Teller (BET) surface area of the powders was measured by N2 adsorption using a Quantachrome autosorb automated gas sorption system.

2. EXPERIMENTAL SECTION 2.1. Materials Synthesis. Zirconia and sulfated zirconia nanopowders were prepared via polyacrylamide gel method35 with modification in the calcination step. The characteristics of the used materials are presented in Table 1. For synthesis of zirconia powder, first, a solution of initial salt precursor in deionized water was prepared and then acrylamide (AM) and N,N′-methylenebisacrylamide (MBAM) monomers were added into the premixed solution. The molar ratio of monomers, total concentration of the prepared solution, and the weight ratio of monomers to salt were kept constant at 22, 50% (w/v), and 2/ 1, respectively. When the mixing was completed, ammonium persulfate (APS) and N,N,N′,N′-tetramethylethylenediamide (TEMED) were added to the system which caused to 165

dx.doi.org/10.1021/ie402224m | Ind. Eng. Chem. Res. 2014, 53, 164−172


Article

Figure 1. FT-IR spectra of the zirconia and sulfated zirconia samples synthesized by different initial salts and calcined at different temperatures.

Figure 2. XRD patterns of the synthesized zirconia and sulfated zirconia samples.

samples, there are two bands at 1190 and 1350 cm−1 in the sulfated zirconia samples spectra that describe SO and SO group stretching vibrations, respectively, confirming the presence of sulfate groups in these samples. The XRD patterns of the synthesized zirconia and sulfated zirconia powders are shown in Figure 2. Figure 2a and b shows the XRD patterns of the pure zirconia powders synthesized by zirconium oxychloride and zirconium oxynitrate, respectively. The volume fraction of tetragonal

Furthermore, in order to more accurate examination of the synthesized powders, transmission electron microscopy (TEM), and selected area electron diffraction pattern (SAED) images were also obtained by a JEOL, JEM-2100 transmission electron microscope working at 200 kV.

3. RESULTS AND DISCUSSION The FT-IR spectra of the zirconia and sulfated zirconia samples are shown in Figure 1. In comparison with the pure zirconia 166



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polymeric network is high enough that it causes acceleration of the tetragonal to monoclinic phase transformation. The XRD patterns of sulfated zirconia powders synthesized by zirconium oxychloride and zirconium oxynitrate are shown in Figure 2c and d, respectively. Comparing the patterns of pure and sulfated zirconia samples calcined at different temperatures, it can be concluded that addition of sulfate ions into the structure of prepared powders by zirconium oxychloride retarded the tetragonal to monoclinic phase transformation. However, surprisingly in the case of prepared powders by zirconium oxynitrate, the presence of sulfate ions accelerated this phase transformation. From the literature, it is concluded that the addition of sulfate ions into the structure of ZrO2 delays the tetragonal to monoclinic phase transformation and leads to stabilize tetragonal phase up to higher temperatures. However, addition of further amount of sulfate ions during the synthesis of sulfated zirconia may cause to accelerate the tetragonal to monoclinic phase transformation.19,43 Since the added sulfate ions amount per mole of zirconium in the synthesized powders by both salts was identical, so the observed results in the synthesized samples by zirconium oxynitrate cannot be attributed to the addition of large amount of sulfate ions. In order to interpret these results, it is necessary to consider the effects of anionic impurities. The effect of the sulfation process on the crystalline phase and phase transformation of zirconia is quite unexpected.16 Various theories have been presented about the effect of sulfate ions on the stabilization of tetragonal phase.44 Srinivasan et al.10,45 suggested that adsorption of SO42− ions on the oxygen ion vacancy sites prevents the entrance of oxygen ions onto these sites during the cooling of calcined powders and therefore avoids the tetragonal to monoclinic phase transformation. Furthermore, the presence of sulfate ions may also increase the crystallization temperature by retarding the water loss from the amorphous zirconia during the calcination step.46 On the other hand, Wu and Yu showed that addition of a large amount of H2SO4 during the synthesis causes to the formation of αZr(SO4)2, which due to the structural dissimilarities between the α-Zr(SO4)2 and tetragonal phase, a large amount of monoclinic phase will be produced after decomposition at high temperatures (>740 °C).43 As mentioned earlier, nitrate ions have similar effects on the retarding the crystallization and also stabilization of tetragonal structure. As we know, in the zirconium oxynitrate aqueous solution, nitrate ions are present in the molecular structure and there is Zr−NO3 bond in the system. However, in the case of zirconium oxychloride precursor, chloride ions are apparently not present as a bulk impurity and there is no Zr−Cl bond in this system.47,48 Consequently, from the observed results of this work it can be concluded that similar to the sulfate ions, nitrate ions are also adsorbed on the oxygen ion vacancy sites. The simultaneous presence of sulfate and nitrate ions may act the same as a condition that a large amount of sulfate ions is present, resulting in the formation of α-Zr(SO4)2 structure that causes a considerable amount of monoclinic phase to be produced after calcination at 750 and 850 °C. In order to confirm this theory, a sulfated zirconia powder was synthesized using half value of sulfuric acid and then calcined at 850 °C with heating rate of 5 °C/min. The XRD pattern of this sample is also presented in Figure 2d (SZ N-850 (2)). As expected, the value of monoclinic phase in this sample is considerably lower than that in the sample SZ N-850 (1) synthesized with higher content of sulfate

phase of the synthesized powders calculated based on their XRD patterns is presented in Table 2. As observed in our Table 2. Volume Fraction of Tetragonal Phase and Particles Size of the Synthesized Powders sample abbr. Z N-650 Z N-750 Z N-750 (2 °C/min) Z N-850 Z N-850 (2 °C/min) Z Cl-650 Z Cl-750 Z Cl-850 SZ N-650 SZ N-750 SZ N-850 (1) SZ N-850 (2) SZ Cl-650 SZ Cl-750 SZ Cl-850

volume fraction of tetragonal phase (%)

size of particles (nm)

100 100 96.61

21.52 40.14

95.36 4.76 90.54 31.54 2.74 100 79 8.51 60.33 100 34.87 3.61

34.75 31.98 34.13 41.43

35.55

previous work,35 the synthesized zirconia powders via polyacrylamide gel method calcined at temperatures near 300 °C have amorphous structure that by increasing calcination temperature the tetragonal and then monoclinic structures are formed. As can be seen in Figure 2a, calcined powder at 650 °C mainly has tetragonal structure; however, a small amount of monoclinic phase has been formed in the sample. The amount of monoclinic phase increases with increasing calcination temperature; at 850 °C, the tetragonal to monoclinic phase transformation is approximately completed. On the other hand, as shown in Figure 2b, in the synthesized powders by zirconium oxynitrate, the metastable tetragonal phase is approximately fully stable up to 850 °C at which the tetragonal to monoclinic phase transformation initiates. The observed trend for the oxynitrate salt can be due to the presence of nitrate ions that retard the crystallization and delay the tetragonal to monoclinic phase transformation, as observed in our previous work.35 As studied in our recent work,39 reducing the heating rate in the calcination step from 20 to 5 °C/min caused a delay in the tetragonal to monoclinic phase transformation and also in obtaining finer nanoparticles. Hence, in order to evaluate the possibility of stabilizing tetragonal phase up to higher temperatures, the effect of the heating rate of 2 °C/min on the crystalline phase of calcined zirconia powder at 850 °C was investigated. The result showed that reducing the heating rate leads to accelerate phase transformation. The same result was also observed for the calcined powder at 750 °C. These results suggest that 5 °C/min can be an optimum heating rate in order to stabilize metastable tetragonal phase up to high temperatures. In fact, in higher heating rates, the polymer network has lower time to reach a given temperature and its thermal decomposition will be lower and more uniform. For this reason it is possible to effectively prevent the aggregation of powders at high temperatures. On the other hand, lower heating rates may cause delay in the phase transformation. From the observed results of this work, it can be concluded that for the heating rates lower than 5 °C/min, thermal decomposition of 167



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Figure 3. Average tetragonal nanocrystallite size and crystalline phases of the synthesized samples as a function of calcination temperature.

Figure 4. Particle size distribution of (a) Z N-650 and (b) SZ N-650 samples.

ions. This result approves that sulfate ions affect the tetragonal to monoclinic phase transformation according to the above proposed mechanism. The critical nanocrystallite size for the metastable tetragonal phase stabilization depends differently on the processing conditions and various values have been reported in the literature.8,49 Figure 3 illustrates the average tetragonal nanocrystallite size and crystalline phases of the synthesized zirconia and sulfated zirconia powders as a function of calcination temperature. As can be seen, the maximum calculated value of average tetragonal nanocrystallite size before

completion of the tetragonal to monoclinic phase transformation is 42 nm. As mentioned in the literature,8 when zirconia nanocrystals form aggregates and in the presence of hydrostatic stresses, the critical nanocrystallite size of 41 nm is calculated based on the thermodynamic consideration for stabilization of the metastable tetragonal phase. The observed size of 42 nm in this work is consistent with the calculated critical nanocrystallite size of 41 nm. Thus, it can be concluded that similar the reported conclusion in ref 8, the surface energy, the interfacial energy, and the strain energy influence on the critical nanocrystallite 168



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Figure 5. Particle size distribution of (a) Z N-650 and (b) SZ N-650 samples in nanometeric range.

Figure 6. High resolution SEM images of SZ Cl-650 (left) and SZ N-650 (right) samples.

size for the metastable tetragonal phase stabilization in our given synthesis conditions. The average particle size of the synthesized powders in this work measured by the laser particle size analyzer is presented in Table 2. Among the various analyzed samples, the particle size distribution results of Z N-650 and SZ N-650 powders are shown in Figure 4. As can be seen, the powders have bimodal

particle size distribution; the presence of particles outside the nanometeric range is due to the incomplete homogenization of samples and existence of agglomerates. For this reason, the nanometric particle size distribution of these samples was calculated as presented in Figure 5. The results confirm that zirconia nanopowders were successfully synthesized in nanomertic scale. 169



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Figure 6 illustrates morphology of samples SZ N-650 and SZ Cl-650. As can be seen, the particles have spherical shape with the average size of about 40−45 nm which are in good agreement with the particle size distribution results. Furthermore, it can be seen that both powders forms agglomerates, and the level of agglomeration in the synthesized powder from zirconium oxynitrate is much more than synthesized one from oxychloride salt. Due to the higher stability of the tetragonal phase in the synthesized powders from oxynitrate salt, it can be concluded that the presence of higher level of agglomeration in this powders is effective in retaining the tetragonal structure and inhibiting tetragonal to monoclinic phase transformation. The same result also can be found in the literature which reported that agglomeration and especially strongly bonded and hard agglomerates can efficiently prevent tetragonal to monoclinic phase transformation.50 The BET surface area of the SZ N-650 and SZ Cl-650 nanopowders was measured to be 73.16 and 86.87 m2/g, respectively. The synthesis procedure, especially the amount of added sulfate ions as well as calcination temperature, considerably affects the surface area of sulfated zirconia catalysts. Usually there are optimum values of loaded sulfate for obtaining high surface area catalysts. Furthermore, by increasing the calcination temperature the surface area significantly decreased, mainly due to the loss of sulfur at high temperatures.13,31 The synthesized nanopowders in present work have relatively high surface area after calcination at 650 °C and can be considered as active catalysts.13,15,51,52 The BET results also indicate that the SZ Cl-650 sample has a larger BET surface area that of SZ N-650. This could be due to the higher level of agglomeration in the SZ N-650 sample, which was also concluded from the SEM results. Figure 7 shows the TEM images of the Z N-650 powder. It indicates that the particles are nanometeric in size in the range

Figure 8. High resolution TEM image of the Z N-650 sample.

tetragonal structure of the powder. These obtained results, appropriately confirm the particle size analyzer, SEM, and XRD results.

4. CONCLUSIONS In the present study, the performance of the polyacrylamide gel method in synthesizing pure and sulfated zirconia nanopowders with stabilized metastable tetragonal phase was investigated. The results showed that in the synthesized powders by zirconium oxychloride, the tetragonal to monoclinic phase transformation is initiated at lower temperatures and at 850 °C is approximately completed. However, in the case of synthesized powders by zirconium oxynitrate, the metastable tetragonal phase was approximately fully stable up to 850 °C. The investigation of the heating rate in calcination step showed that heating rate of lower than 5 °C/min leads to accelerate tetragonal to monoclinic phase transformation which can be due to the high thermal decomposition of polymeric network. Thus, from the observed results of this work and our previous works, 5 °C/min was concluded as optimum heating rate of calcination in order to stabilize tetragonal phase up to high temperatures. By comparing the XRD patterns of pure and sulfated zirconia powders calcined at different temperatures, it was concluded that the addition of sulfate ions into the structure of sulfated zirconia powders synthesized by zirconium oxychloride retards the tetragonal to monoclinic phase transformation. While, in the prepared powders by zirconium oxynitrate, the presence of sulfate ions accelerates this phase transformation. Unlike the chloride ions, nitrate ions are adsorbed on the oxygen ion vacancy sites and the simultaneous presence of sulfate and nitrate ions leads to formation of αZr(SO4)2 structure; due to the structural dissimilarities between it and the tetragonal phase, a large amount of the monoclinic phase is produced after calcination at high temperatures. The particle size analyzer, SEM, and TEM results showed that the powders were successfully synthesized on the nanometric scale. In addition, the XRD results were satisfactorily confirmed by the HRTEM and SAED results.

Figure 7. TEM images and SAED pattern (inset) of the Z N-650 sample.

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of 20−25 nm. The inset shows the selected area electron diffraction (SAED) pattern which reveals that the powder is well crystallized and has tetragonal structure. The high resolution TEM (HRTEM) image of a nanoparticle is presented in Figure 8. As can be seen, the lattice spacing in the crystals is 0.296 nm which agrees well with the interplanar spacing of the (101) plane of tetragonal zirconia indicating the

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Corresponding Author

*Tel.: +98-411-3459081. Fax: +98-411-3444355. E-mail address: [email protected]. Notes

The authors declare no competing financial interest. 170



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ACKNOWLEDGMENTS We gratefully acknowledge Sahand University of Technology (SUT) for the financial support of this work. We also would like to thank the members of Nanostructure Materials Research Center (NMRC) for their help and assistance during various stages of this research.

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Stabilization of the Metastable Tetragonal Phase in Zirconia

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