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Synthesis and characterization of lead zirconate titanate powders obtained by the oxidant peroxo method
Journal of Alloys and Compounds 469 (2009) 523–528
Synthesis and characterization of lead zirconate titanate powders obtained by the oxidant peroxo method Emerson R. Camargo a,∗ , Edson R. Leite a , Elson Longo b a
LIEC-Laborat´orio Interdisciplinar de Eletroqu´ımica e Cerˆamica, Department of Chemistry, UFSCar-Federal University of S˜ao Carlos, Rod. Washington Luis km 235, CP 676, S˜ao Carlos SP, 13565-905, Brazil b Department of Biochemistry, Chemistry Institute of Araraquara, UNESP, S˜ ao Paulo State University Rua Francisco Degni, CP 355 Araraquara SP,14801-907 Brazil Received 16 October 2007; received in revised form 25 January 2008; accepted 2 February 2008 Available online 9 April 2008
Abstract Lead zirconate titanate (PbZr1−x Tix O3 ) was synthesized by the “oxidant peroxo method (OPM)” with “x” between 0.25 and 0.50. Titanium metal was dissolved into a hydrogen peroxide/ammonia aqueous solution, followed by the addition of lead and zirconium nitrate solution. The amorphous precipitated precursor obtained was crystallized by heat treatment between 400 and 1000 ◦ C. Images of transmission microscopy showed spherical particles with average diameter between 20 and 60 nm, and the presence of necks between particles treated at 700 ◦ C. All of the unpressed powders were characterized by X-ray diffractometry and FT-Raman spectroscopy. Powder samples with “x” up to 0.35 showed rhombohedral structure when treated at temperatures higher than 500 ◦ C, and tetragonal structure when “x” was higher than 0.40. Analysis of XRD and Raman spectroscopy of the precursor powders showed amorphous-like structures, however powders treated at 400 ◦ C showed a structure identified as an intermediate pyrochlore phase, independently of the Zr and Ti mole ratio. © 2008 Elsevier B.V. All rights reserved. Keywords: Oxide materials; Chemical synthesis; X-ray diffraction; Crystal structure; Inelastic light scattering
1. Introduction Lead zirconate titanate (PZT) oxide and its related solid solutions are one of the most important classes of ferroelectric material for technological applications due to properties such as piezoelectricity and non-linear electro-optic behaviour [1–4]. Several wet-chemical routes have been developed to obtain high quality nanosized powders of PZT aiming to enhance the electronic and mechanical properties of the final ceramic bodies [5–13]. In this way, one of us developed a new synthetic route called the “oxidant peroxo method”, sometimes referred simply by the acronym OPM [14–16]. This wet-chemical method of synthesis is characterized by the fundamental oxy-reduction reaction between Pb(II) ions and some water soluble peroxo complexes that leads to the formation of an amorphous and highly reactive precipitate power. This precipitate is free of those
common contaminants usually found in materials synthesized by others chemical routes, such as halides or graphitic carbon that results from the decomposition of organic material present [17]. Since in the OPM route the precipitate is formed by a molecularlevel mechanism, its composition can be efficiently controlled. It was demonstrated that the crystallization temperature is below than that reported for this and similar systems synthesized by solid-state reaction or traditional sol–gel routes even. Moreover, the OPM technique uses water as solvent and a relatively simple experimental apparatus, without the necessity of dry atmosphere or toxic compounds. In this paper, we are reporting the synthesis of several PZT nanosized powders with compositions richer in zirconium through the heat treatment of the OPM-amorphous precipitates. 2. Experimental procedure 2.1. Synthesis
∗
Corresponding author. Tel.: +55 16 3351 8090; fax: +55 16 3351 8350. E-mail addresses: [email protected] (E.R. Camargo), [email protected] (E.R. Leite), [email protected] (E. Longo). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.02.008
All of the chemical reagents used in this work were of analytical grade and they were used as received without any further purification (Table 1). Six
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Table 1 Chemicals used to prepare samples by the OPM route Chemical
Purity (%)
Origin
Titanium metal, powder 325 mesh Hydrogen peroxide, 30% aqueous solution Ammonia, 28% aqueous solution Zirconyum oxynitrate ZrO(NO3 )2 ·xH2 O Lead nitrate, Pb(NO3 )2 Nitric acid, 15% aqueous solution Acetone
99.99
Aldrich, USA Synth, Brazil Synth, Brazil Merck, Germany Merck, Germany Synth, Brazil Synth, Brazil
99.5 99.5
nominal compositions of PbZr1−x Tix O3 were prepared, with “x” in the range between 0.25 and 0.50, thereafter referred to as PZTX. For instance, the composition PbZr0.60 Ti0.40 O3 with “x = 0.40” is referred to as PZT40, what means a sample with 40% mole ratio of titanium. The samples were prepared by the OPM route through the addition of 1 g (0.02 mol) of titanium metal powder to an aqueous solution of 80 mL of hydrogen peroxide and 20 mL of ammonia aqueous solution. This solution was kept in rest in an ice-water bath for approximately 10 h, resulting in a yellow transparent aqueous solution of the soluble peroxytitanate complex ion [Ti(OH)3 O2 ]− with concentration of 0.2 mol L−1 . Stoichiometric amount of zirconium oxynitrate, which was previously analysed by gravimetric method to determine the correct mole content of zirconium, was weighted and added into diluted acid aqueous solution of nitric acid. After its dissolution, the total volume was corrected to 50 mL with distilled water and lead nitrate was added to complete the stoichiometric ratio. This general procedure was done for each composition. This solution of zirconium and lead nitrate was slowly dropped into the peroxytitanate solution under ice-water bath with stirring, resulting in a vigorous evolution of gas. An orange precipitate was formed immediately and the solution lost its yellow colour. The precipitate was filtered and washed with acetone to eliminate the adsorbed water and the nitrate ions. The washed amorphous precipitates were dried at 50 ◦ C for 5 h, ground and sieved through a 325-mesh sieve. Amounts of 0.2 g of each powder, thereafter referred to as “precursor”, were calcined between 400 and 1000 ◦ C for 1 and 8 h using closed alumina boats with a heating rate of 10 ◦ C min−1 .
2.2. Characterization Stokes and anti-Stokes Raman spectra of the unpressed powders were collected at room temperature between 100 and 2000 cm−1 and between −100 and −1200 cm−1 , respectively, using a triple monochromator (grating of 1800 grooves per mm) Raman spectrometer (Model T-64000, Jobin Yvon/Atago Bussan, France/Japan) with a CCD detector cooled with liquid nitrogen. The 514 nm line of Ar+ laser (visible region) was used as excitation source and the output power of the laser was kept between 100 and 20 mW, avoiding any modification in the physical state of the intermediate amorphous material, such as crystallization, melting or disintegration. All of the measurements were carried out in a macrochamber (laser spot diameter of tens of micrometers) using a backscattering geometry. The unpressed powder samples were characterized by X-ray diffractometry (XRD) using Cu K␣ radiation (MXP3va , MAC-Science, Japan), in the 2θ range from 5◦ to 75◦ at room temperature, 0.02◦ of step scan and rate of 2◦ /min.
3. Results and discussion 3.1. Chemistry of the precipitate It is known that aqueous solutions of Ti(IV) and hydrogen peroxide give an intense orange colour when concentrated, characteristic of peroxo complexes, such as peroxytitanate [Ti(OH)3 O2 ]− ion, often called as peroxytitanic acid. At high pH, the reaction can be described by Eq. (1): Ti0 + 3H2 O2 → [Ti(OH)3 O2 ]− + H2 O + H+
(1)
The mechanism of the peroxytitanic acid formation from the titanium metal is very complex and not completely understood [18]. Excess of hydrogen peroxide is necessary to stabilize the solution of peroxytitanic acid, which decomposes slowly with evolution of oxygen gas. When most of the H2 O2 is consumed, a yellow gel is usually formed spontaneously [19]. On the other hand, lead nitrate can be easily dissolved in neutral or acid aqueous solution, however if the pH of the Pb(II) solution is increased, lead tetrahydroxide [Pb(OH)4 ]2− is formed. But in the presence of excess of OH− , the soluble ion [Pb(OH)4 ]2− is formed and reacts immediately with hydrogen peroxide, raising the oxidation state of the lead from Pb(II) to Pb(IV), resulting in an amorphous precipitate of lead oxide, as described by Eq. (2): Pb(OH)4 2− + H2 O2 → PbO2 + 2H2 O + 2OH−
(2)
Zirconium oxynitrate (ZrO(NO3 )2 ·xH2 O) can also be easily dissolved in a lightly acid aqueous solution, for instance using diluted nitric acid, which can be referred as dissolved zirconium nitrate. Therefore, it is possible to prepare a stable solution of zirconium and lead nitrate to be dropped into the peroxytitanic solution at high pH, and in this way to obtain a precise and stoichiometric precipitate with the Pb:Zr:Ti mole ratio desired. This precipitate can be described, at least, as a complex mixture of amorphous and hydrated oxides PbO2 , TiO2 and ZrO2 that after a firing step will result in the crystallized PZT. 3.2. Formation of the PZT The isomorphic substitution of titanium by zirconium atoms induces some interesting changes in the physical properties of PZT, mainly those regarding piezoelectric properties [1]. For instance, compositions that are richer in titanium show tetragonal phase (T-PZT) while those that are richer in zirconium are rhombohedral (R-PZT). As expected, these two different crystalline structures affect the properties and the performance of the final ceramics in different ways, since the nuclei positions, together with the electron density, determine the electric polarization vector, defining the use and applications of the PZT fundamentally in terms of Ti and Zr mole composition [20]. Therefore, a rigorous control on the composition of the material is necessary for a wide technological application of the PZT oxide. Regarding the synthesis of reactive PZT powders by wetchemical routes such as the OPM method, intermediate steps are so critical to obtain good sample that it is always important to check the structural characteristics of the precursor before to continue the work. It is well known that the presence of an amorphous precursor is not a guarantee of high quality samples, but the presence of secondary crystalline phases at this step is the certain that poor samples will be obtained. The most usual way to verify the quality is collecting powder X-ray patterns (XRD) of the precursors (Fig. 1). In spite of the fact that all of these six XRD look quite similar, each of one was obtained from PZT amorphous precursors with different contents of zirconium and titanium, with the mole ratio of titanium varying from 50 to 25%. This result means that the long-range ordering of the
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Fig. 1. X-ray patterns collected at room temperature of the precursors samples before any heat treatment. Sample referred as PZT25 means that there is 25% of titanium in the PZT composition.
precursors are similar independently of the composition, or that all of the precursors seems to be amorphous independently of the composition. To confirm this proposition, Raman spectra of the precursors were also collected at room temperature and are shown in Fig. 2. It is possible to observe only the well-defined peak at 1044 cm−1 from the presence of residual nitrate ion. Usually, this residual compound is removed by a simple washing using acetone, ethanol or pure water even, but its presence can also be eliminated by heat treatment. It is important to mention that the presence of residual nitrate ion does not affect the structural properties of PZT or any compound synthesized by OPM route. But what is really important and represent structural characteristics of the amorphous precipitate, is the presence of two broad bands centred at 200 and 510 cm−1 named as A1 and A2, respectively, and indicated by arrows. These two bands were already identified in others amorphous precursors powders syn-
Fig. 2. Raman spectra collected at room temperature of the precursors samples before any heat treatment. Sample referred as PZT25 means that there is 25% of titanium and 75% of zirconium in the composition. The peak at 1044 cm−1 results from the presence of residual nitrate ion. A1 and A2 indicate the centre of broad bands that are usually identified in amorphous OPM precursors.
Fig. 3. X-ray patterns collected at room temperature of the PZT25 sample, meaning that there is 25% of titanium and 75% of zirconium in the composition, heat treated at different temperatures and times. Miller indexes on the pattern of the powder treated at 900 ◦ C for 8 h indicates pure rhombohedral phase (R-PZT). Arrows indicate the reminiscent pyrochlore-like PZT phase (py-PZT) in the pattern of the powder treated at 500 ◦ C 8 h.
thesized by the OPM route and they are a general consequence of the breakdown of the selection rules that occurs in amorphous powders [21]. Considering that the formation of the amorphous precursor is the most critical step in the OPM route, it is possible to assert that PZT, even those that are rich in zirconium, can be prepared by the OPM method through an amorphous and uniform precipitate. This characteristic seems to be general to all of the lead-based OPM synthesized compounds. It is usually found in the literature that apparently amorphous precursors sometimes result in compounds with secondary or intermediate phases after the heat treatment. For this reason, a complete series of XRD patterns of the PZT25 treated at different temperatures and time were collected and are shown in Fig. 3. It is possible to observe that the material still amorphous even when heat treated at 400 ◦ C, but well defined diffraction peaks could be observed when the powder was calcined at 500 ◦ C for 1 h and higher temperatures. However, when the XRD of the amorphous powders are compared with the patterns of crystalline PZT, it is observed that the centre of the broad peak is shifted from approximately 29◦ to 31◦ . Although it is common to find broad bands in amorphous compounds, it is easily observed that this broad band is asymmetric in the XRD of the powder treated at 400 ◦ C, probably because of some ordering in the material as consequence of the heat treatment. This initial ordering is better visualized by the arrows in Fig. 3 that indicate three broad and low intense diffraction peaks that could be assigned to the pyrochlore-like PZT phase (py-PZT), with characteristics peaks at 29◦ , 34◦ and 48◦ [22], characterizing an mixture of py-PZT and PZT. We have already identified a similar cubic pyrochlore intermediate phase
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by a combined use of thermal analysis, XRD, Raman and XAS spectroscopy at the Ti K-edge when lead titanate was synthesized through the OPM route [16]. The presence of this py-PZT in both pure lead titanate and PZT, suggests that this intermediate pyrochlore structure is a usual intermediate metastable structure formed during the crystallization of the OPM amorphous precursor. Here, we observed that when the precursor was heat treated at 500 ◦ C and higher temperatures, only R-PZT was identified and its Miller indexes could be assigned. Of course, the peaks became sharper with the increase of the treatment temperature, but what is unusual is that the 110 peak of the XRD of the powders treated between 500 and 700 ◦ C show a shoulder at the high angle side of this peak, which can be understood as the presence of the T-PZT together to the R-PZT. Basically three different XRD patterns could be identified in Fig. 3, (i) the amorphous precursor, (ii) an apparent mixture of PZT and py-PZT when the powder is heat treated at moderate temperatures and (iii) pure R-PZT of the powder treated at high temperature. Considering that Raman spectroscopy is a more sensitive tool to analyse structures than DRX [23], Raman spectra of these three representative samples were collected at room temperature and are shown in Fig. 4. The spectra of powders treated at high temperatures are quite different from that of the powder calcined at 400 ◦ C, which show a general shape quite similar to that observed in the spectrum of the amorphous precursor, even with the presence of the nitrate peak at 1044 cm−1 . As discussed before, (Fig. 3) py-PZT is also present when the precursor is heat treated at low temperature, therefore the broadness of the peaks can be also an expression of the presence of this secondary phase, since the spectrum of the nanosized pyrochlore
Fig. 4. Raman spectra collected at room temperature of the PZT25 sample, with 25% of titanium and 75% of zirconium in the composition, heat treated at different temperatures and times.
Fig. 5. X-ray patterns collected at room temperature of the PZT50, with 50% of titanium and zirconium in the composition, heat treated at different temperatures and times. Tetragonal phase was indexed as [24].
is composed of broad bands [16]. On the other hand, it is also possible to say that the broadness of the peaks is also because of the smaller particle size and lower degree of crystallization. At this point, the best interpretation is that both effects contribute to the broadness of the Raman peaks of the powders treated at low temperature. Samples treated at 800 ◦ C and higher temperature show the characteristic profile of the R-PZT spectrum. What is very important in this spectrum is the absence of a shoulder (at approximately 330 cm−1 ) at the right side of the 245 cm−1 peak.
Fig. 6. Raman spectra collected at room temperature of samples with different amounts of titanium and zirconium heat-treated at 900 ◦ C for 1 h at different temperatures and times. The right side corresponds to the Stokes and the left side to the anti-Stokes scattering.
E.R. Camargo et al. / Journal of Alloys and Compounds 469 (2009) 523–528
This absence is strong evidence that pure rhombohedral phase (R-PZT) was obtained. Powders with composition rich in titanium, such as PbZr0.50 Ti0.50 O3 where “x = 0.50”, were also prepared and three XRD patterns of samples treated at different temperatures are shown in Fig. 5. It is visible the characteristic splitting of the diffraction peak of the tetragonal structure in the XRD of the powder calcined at 1000 ◦ C, which cannot be observed in the patterns of the powder treated at lower temperatures. What is clear again is the shifting of the centre of the broad peak from approximately 29◦ in powder treated at 400 ◦ C to 32◦ in the XRD patterns of the T-PZT samples. It is another good evidence of the formation of the py-PZT structure before to the crystallization of the T-PZT or R-PZT. To observe the phase transition from tetragonal to rhombohedral structure, which is rich in zirconium, Raman spectra of several compositions were collected and are shown in Fig. 6 heat treated at 900 ◦ C for 1 h. The Raman modes are assigned on the top of the figure [25]. It is observed a progressive modification on the profile of the spectrum when the composition is changed from PbZr0.65 Ti0.35 O3 to PbZr0.55 Ti0.45 O3 (or when “x” change from 0.35 to x = 0.45). The most visible modification is the splitting of the peak at 250 cm−1 that is broad but nearly unique when x = 0.25 (rhombohedral) but clearly can be identified two peaks when x = 0.45, located at 200 and 272 cm−1 . Another interesting difference between the spectra of rhombohedral and tetragonal structures can be observed in the anti-Stokes region where the
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most intense peak located at −205 cm−1 is identifiable only for rhombohedral structure, whereas for tetragonal structure only a broad band is observed in this region. The morphological properties of the material were observed by transmission electronic microscopy (TEM). It is possible to observe in the upper image of Fig. 7a group of several nanoparticles of PZT50 already sintered, with presence of necks between them after a heat treatment at 700 ◦ C for 1 h, what cannot be considered a high temperature. Similar behaviour was found for all of the particles treated at temperatures higher than 500 ◦ C, demonstrating the high reactivity of these ultra-fine powders. The second image shows the detail of a region of contact between two PZT particles, again confirming the sinterability of these powders. 4. Conclusions Powders of spherical shape and average particle size between 20 and 60 nm of PbZr1−x Tix O3 with of “x” in the range between 0.25 and 0.50 were synthesized by the OPM method. Rhombohedral structure was observed for that samples with x up to 0.35 and tetragonal structure for samples with “x” higher than 0.40 when the powders were heat treated at temperatures higher than 500 ◦ C. Analysis of XRD and Raman spectroscopy of the precursors showed an amorphous-like structure, however powders treated at 400 ◦ C showed an structure that could be identified as an intermediate pyrochlore phase, independently of the Zr and Ti mole ratio. All the powders showed high sinterability as observed by the presence of necks between the particles treated at 700 ◦ C for 1 h. Acknowledgements This work was supported by the Brazilian agencies FAPESP through of the CMDMC/Cepid, and to CNPq, 555644/20065. X-ray patterns and Raman spectra were collected at Tokyo Institute of Technology, Japan. Thanks also to Dr. Caue Ribeiro, from Embrapa, by the TEM images. References
Fig. 7. Transmission electronic images from the PZT 50 sample heat treated at 700 ◦ C for 1 h. The upper image shows group of several nanoparticles already sintered, with presence of necks between them after a heat treatment. The second image shows the detail of a region of contact between two PZT particles.
[1] B. Jaffe, W.R. Cook, H. Jaffe, Piezoelectric Ceramics, Academic, London, 1971. [2] A.M. Glazer, P.A. Thomas, K.Z. Baba-Kishi, G.K.H. Pang, C.W. Tai, Phys. Rev. B: Condens. Matter 70 (2004) 184123. [3] G.H. Haertling, J. Am. Ceram. Soc. 82 (1999) 797–818. [4] A. Banerjee, S. Bose, Chem. Mater. 16 (2004) 5610–5615. [5] Z. De-Qing, W. Shao-Jun, S. Hong-Shan, W. Xiu-Li, C. Mao-Sheng, J. Sol–Gel Sci. Technol. 41 (2007) 157–161. [6] G. Mu, S. Yang, J. Li, M. Gu, J. Mater. Process Technol. 182 (2007) 382–386. [7] G. Xu, G. Zhao, Z. Ren, G. Shen, G. Han, Mater. Lett. 60 (2006) 685–688. [8] G. Garnweitner, J. Hentschel, M. Antonietti, M. Niederberger, Chem. Mater. 17 (2005) 4594–4599. [9] A. Abreu Jr., S.M. Zanetti, M.A.S. Oliveira, G.P. Thim, J. Eur. Ceram. Soc. 25 (2005) 743–748. [10] S. Ram, T.K. Mandal, J. Am. Ceram. Soc. 88 (2005) 3444–3448. [11] S. Linardos, Q. Zhang, J.R. Alcock, J. Eur. Ceram. Soc. 27 (2007) 231–235. [12] F. Bezzia, A.L. Costa, D. Piazzaa, A. Ruffinia, S. Albonettia, C. Galassia, J. Eur. Ceram. Soc. 25 (2005) 3323–3334.
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[13] C. Tien-I, H. Jow-Lay, L. Hong-Ping, W. Sheng-Chang, L. Horng-Hwa, L. Wu, L. Jen-Fu, J. Alloys Compd. 414 (2006) 224–229. [14] E.R. Camargo, M. Kakihana, Chem. Mater. 13 (2001) 1181–1184. [15] E.R. Camargo, J. Frantti, M. Kakihana, J. Mater. Chem. 11 (2001) 1875–1879. [16] E.R. Camargo, E. Longo, E.R. Leite, V.R. Mastelaro, J, Solid State Chem. 177 (2004) 1994–2001. [17] E.R. Camargo, E. Longo, E.R. Leite, M. Kakihana, Ceram. Int. 30 (2004) 2235–2239. [18] P. Tengval, H. Elwing, I. Lundstrom, J. Colloid Interf. Sci. 130 (1989) 405–413. [19] P. Tengvall, T.P. Vikinge, I. Lundstrom, A.B.J. Liedberg, J. Colloid Interf. Sci. 160 (1993) 10–15.
[20] J. Frantti, S. Ivanov, S. Eriksson, H. Rundl¨of, V. Lantto, J. Lappalainen, M. Kakihana, Phys. Rev. B: Condens. Matter 66 (2002) 64108. [21] M.H. Brodsky, in: M. Cardona (Ed.), Raman Scattering in Solids I, Light Scattering in Solids I, Topics in Applied Physics, vol.8, Springer, Berlin, Heidelberg, New York, 1975. [22] D. Liu, H. Zhang, W. Cai, X. Wu, L. Zhao, Mater. Chem. Phys. 51 (1997) 186–189. [23] M. Kakihana, M. Yashima, M. Yoshimura, L. Borjesson, M. Kall, Trends Appl. Spectrosc. 1 (1993) 261–311. [24] R. Tipakontitikul, S. Ananta, Mater. Lett. 58 (2004) 449–454. [25] J. Frantti, V. Lantto, Phys. Rev. B 56 (1997) 221.
Synthesis and characterization of lead zirconate titanate powders obtained by the oxidant peroxo method Emerson R. Camargo a,∗ , Edson R. Leite a , Elson Longo b a
LIEC-Laborat´orio Interdisciplinar de Eletroqu´ımica e Cerˆamica, Department of Chemistry, UFSCar-Federal University of S˜ao Carlos, Rod. Washington Luis km 235, CP 676, S˜ao Carlos SP, 13565-905, Brazil b Department of Biochemistry, Chemistry Institute of Araraquara, UNESP, S˜ ao Paulo State University Rua Francisco Degni, CP 355 Araraquara SP,14801-907 Brazil Received 16 October 2007; received in revised form 25 January 2008; accepted 2 February 2008 Available online 9 April 2008
Abstract Lead zirconate titanate (PbZr1−x Tix O3 ) was synthesized by the “oxidant peroxo method (OPM)” with “x” between 0.25 and 0.50. Titanium metal was dissolved into a hydrogen peroxide/ammonia aqueous solution, followed by the addition of lead and zirconium nitrate solution. The amorphous precipitated precursor obtained was crystallized by heat treatment between 400 and 1000 ◦ C. Images of transmission microscopy showed spherical particles with average diameter between 20 and 60 nm, and the presence of necks between particles treated at 700 ◦ C. All of the unpressed powders were characterized by X-ray diffractometry and FT-Raman spectroscopy. Powder samples with “x” up to 0.35 showed rhombohedral structure when treated at temperatures higher than 500 ◦ C, and tetragonal structure when “x” was higher than 0.40. Analysis of XRD and Raman spectroscopy of the precursor powders showed amorphous-like structures, however powders treated at 400 ◦ C showed a structure identified as an intermediate pyrochlore phase, independently of the Zr and Ti mole ratio. © 2008 Elsevier B.V. All rights reserved. Keywords: Oxide materials; Chemical synthesis; X-ray diffraction; Crystal structure; Inelastic light scattering
1. Introduction Lead zirconate titanate (PZT) oxide and its related solid solutions are one of the most important classes of ferroelectric material for technological applications due to properties such as piezoelectricity and non-linear electro-optic behaviour [1–4]. Several wet-chemical routes have been developed to obtain high quality nanosized powders of PZT aiming to enhance the electronic and mechanical properties of the final ceramic bodies [5–13]. In this way, one of us developed a new synthetic route called the “oxidant peroxo method”, sometimes referred simply by the acronym OPM [14–16]. This wet-chemical method of synthesis is characterized by the fundamental oxy-reduction reaction between Pb(II) ions and some water soluble peroxo complexes that leads to the formation of an amorphous and highly reactive precipitate power. This precipitate is free of those
common contaminants usually found in materials synthesized by others chemical routes, such as halides or graphitic carbon that results from the decomposition of organic material present [17]. Since in the OPM route the precipitate is formed by a molecularlevel mechanism, its composition can be efficiently controlled. It was demonstrated that the crystallization temperature is below than that reported for this and similar systems synthesized by solid-state reaction or traditional sol–gel routes even. Moreover, the OPM technique uses water as solvent and a relatively simple experimental apparatus, without the necessity of dry atmosphere or toxic compounds. In this paper, we are reporting the synthesis of several PZT nanosized powders with compositions richer in zirconium through the heat treatment of the OPM-amorphous precipitates. 2. Experimental procedure 2.1. Synthesis
∗
Corresponding author. Tel.: +55 16 3351 8090; fax: +55 16 3351 8350. E-mail addresses: [email protected] (E.R. Camargo), [email protected] (E.R. Leite), [email protected] (E. Longo). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.02.008
All of the chemical reagents used in this work were of analytical grade and they were used as received without any further purification (Table 1). Six
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E.R. Camargo et al. / Journal of Alloys and Compounds 469 (2009) 523–528
Table 1 Chemicals used to prepare samples by the OPM route Chemical
Purity (%)
Origin
Titanium metal, powder 325 mesh Hydrogen peroxide, 30% aqueous solution Ammonia, 28% aqueous solution Zirconyum oxynitrate ZrO(NO3 )2 ·xH2 O Lead nitrate, Pb(NO3 )2 Nitric acid, 15% aqueous solution Acetone
99.99
Aldrich, USA Synth, Brazil Synth, Brazil Merck, Germany Merck, Germany Synth, Brazil Synth, Brazil
99.5 99.5
nominal compositions of PbZr1−x Tix O3 were prepared, with “x” in the range between 0.25 and 0.50, thereafter referred to as PZTX. For instance, the composition PbZr0.60 Ti0.40 O3 with “x = 0.40” is referred to as PZT40, what means a sample with 40% mole ratio of titanium. The samples were prepared by the OPM route through the addition of 1 g (0.02 mol) of titanium metal powder to an aqueous solution of 80 mL of hydrogen peroxide and 20 mL of ammonia aqueous solution. This solution was kept in rest in an ice-water bath for approximately 10 h, resulting in a yellow transparent aqueous solution of the soluble peroxytitanate complex ion [Ti(OH)3 O2 ]− with concentration of 0.2 mol L−1 . Stoichiometric amount of zirconium oxynitrate, which was previously analysed by gravimetric method to determine the correct mole content of zirconium, was weighted and added into diluted acid aqueous solution of nitric acid. After its dissolution, the total volume was corrected to 50 mL with distilled water and lead nitrate was added to complete the stoichiometric ratio. This general procedure was done for each composition. This solution of zirconium and lead nitrate was slowly dropped into the peroxytitanate solution under ice-water bath with stirring, resulting in a vigorous evolution of gas. An orange precipitate was formed immediately and the solution lost its yellow colour. The precipitate was filtered and washed with acetone to eliminate the adsorbed water and the nitrate ions. The washed amorphous precipitates were dried at 50 ◦ C for 5 h, ground and sieved through a 325-mesh sieve. Amounts of 0.2 g of each powder, thereafter referred to as “precursor”, were calcined between 400 and 1000 ◦ C for 1 and 8 h using closed alumina boats with a heating rate of 10 ◦ C min−1 .
2.2. Characterization Stokes and anti-Stokes Raman spectra of the unpressed powders were collected at room temperature between 100 and 2000 cm−1 and between −100 and −1200 cm−1 , respectively, using a triple monochromator (grating of 1800 grooves per mm) Raman spectrometer (Model T-64000, Jobin Yvon/Atago Bussan, France/Japan) with a CCD detector cooled with liquid nitrogen. The 514 nm line of Ar+ laser (visible region) was used as excitation source and the output power of the laser was kept between 100 and 20 mW, avoiding any modification in the physical state of the intermediate amorphous material, such as crystallization, melting or disintegration. All of the measurements were carried out in a macrochamber (laser spot diameter of tens of micrometers) using a backscattering geometry. The unpressed powder samples were characterized by X-ray diffractometry (XRD) using Cu K␣ radiation (MXP3va , MAC-Science, Japan), in the 2θ range from 5◦ to 75◦ at room temperature, 0.02◦ of step scan and rate of 2◦ /min.
3. Results and discussion 3.1. Chemistry of the precipitate It is known that aqueous solutions of Ti(IV) and hydrogen peroxide give an intense orange colour when concentrated, characteristic of peroxo complexes, such as peroxytitanate [Ti(OH)3 O2 ]− ion, often called as peroxytitanic acid. At high pH, the reaction can be described by Eq. (1): Ti0 + 3H2 O2 → [Ti(OH)3 O2 ]− + H2 O + H+
(1)
The mechanism of the peroxytitanic acid formation from the titanium metal is very complex and not completely understood [18]. Excess of hydrogen peroxide is necessary to stabilize the solution of peroxytitanic acid, which decomposes slowly with evolution of oxygen gas. When most of the H2 O2 is consumed, a yellow gel is usually formed spontaneously [19]. On the other hand, lead nitrate can be easily dissolved in neutral or acid aqueous solution, however if the pH of the Pb(II) solution is increased, lead tetrahydroxide [Pb(OH)4 ]2− is formed. But in the presence of excess of OH− , the soluble ion [Pb(OH)4 ]2− is formed and reacts immediately with hydrogen peroxide, raising the oxidation state of the lead from Pb(II) to Pb(IV), resulting in an amorphous precipitate of lead oxide, as described by Eq. (2): Pb(OH)4 2− + H2 O2 → PbO2 + 2H2 O + 2OH−
(2)
Zirconium oxynitrate (ZrO(NO3 )2 ·xH2 O) can also be easily dissolved in a lightly acid aqueous solution, for instance using diluted nitric acid, which can be referred as dissolved zirconium nitrate. Therefore, it is possible to prepare a stable solution of zirconium and lead nitrate to be dropped into the peroxytitanic solution at high pH, and in this way to obtain a precise and stoichiometric precipitate with the Pb:Zr:Ti mole ratio desired. This precipitate can be described, at least, as a complex mixture of amorphous and hydrated oxides PbO2 , TiO2 and ZrO2 that after a firing step will result in the crystallized PZT. 3.2. Formation of the PZT The isomorphic substitution of titanium by zirconium atoms induces some interesting changes in the physical properties of PZT, mainly those regarding piezoelectric properties [1]. For instance, compositions that are richer in titanium show tetragonal phase (T-PZT) while those that are richer in zirconium are rhombohedral (R-PZT). As expected, these two different crystalline structures affect the properties and the performance of the final ceramics in different ways, since the nuclei positions, together with the electron density, determine the electric polarization vector, defining the use and applications of the PZT fundamentally in terms of Ti and Zr mole composition [20]. Therefore, a rigorous control on the composition of the material is necessary for a wide technological application of the PZT oxide. Regarding the synthesis of reactive PZT powders by wetchemical routes such as the OPM method, intermediate steps are so critical to obtain good sample that it is always important to check the structural characteristics of the precursor before to continue the work. It is well known that the presence of an amorphous precursor is not a guarantee of high quality samples, but the presence of secondary crystalline phases at this step is the certain that poor samples will be obtained. The most usual way to verify the quality is collecting powder X-ray patterns (XRD) of the precursors (Fig. 1). In spite of the fact that all of these six XRD look quite similar, each of one was obtained from PZT amorphous precursors with different contents of zirconium and titanium, with the mole ratio of titanium varying from 50 to 25%. This result means that the long-range ordering of the
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Fig. 1. X-ray patterns collected at room temperature of the precursors samples before any heat treatment. Sample referred as PZT25 means that there is 25% of titanium in the PZT composition.
precursors are similar independently of the composition, or that all of the precursors seems to be amorphous independently of the composition. To confirm this proposition, Raman spectra of the precursors were also collected at room temperature and are shown in Fig. 2. It is possible to observe only the well-defined peak at 1044 cm−1 from the presence of residual nitrate ion. Usually, this residual compound is removed by a simple washing using acetone, ethanol or pure water even, but its presence can also be eliminated by heat treatment. It is important to mention that the presence of residual nitrate ion does not affect the structural properties of PZT or any compound synthesized by OPM route. But what is really important and represent structural characteristics of the amorphous precipitate, is the presence of two broad bands centred at 200 and 510 cm−1 named as A1 and A2, respectively, and indicated by arrows. These two bands were already identified in others amorphous precursors powders syn-
Fig. 2. Raman spectra collected at room temperature of the precursors samples before any heat treatment. Sample referred as PZT25 means that there is 25% of titanium and 75% of zirconium in the composition. The peak at 1044 cm−1 results from the presence of residual nitrate ion. A1 and A2 indicate the centre of broad bands that are usually identified in amorphous OPM precursors.
Fig. 3. X-ray patterns collected at room temperature of the PZT25 sample, meaning that there is 25% of titanium and 75% of zirconium in the composition, heat treated at different temperatures and times. Miller indexes on the pattern of the powder treated at 900 ◦ C for 8 h indicates pure rhombohedral phase (R-PZT). Arrows indicate the reminiscent pyrochlore-like PZT phase (py-PZT) in the pattern of the powder treated at 500 ◦ C 8 h.
thesized by the OPM route and they are a general consequence of the breakdown of the selection rules that occurs in amorphous powders [21]. Considering that the formation of the amorphous precursor is the most critical step in the OPM route, it is possible to assert that PZT, even those that are rich in zirconium, can be prepared by the OPM method through an amorphous and uniform precipitate. This characteristic seems to be general to all of the lead-based OPM synthesized compounds. It is usually found in the literature that apparently amorphous precursors sometimes result in compounds with secondary or intermediate phases after the heat treatment. For this reason, a complete series of XRD patterns of the PZT25 treated at different temperatures and time were collected and are shown in Fig. 3. It is possible to observe that the material still amorphous even when heat treated at 400 ◦ C, but well defined diffraction peaks could be observed when the powder was calcined at 500 ◦ C for 1 h and higher temperatures. However, when the XRD of the amorphous powders are compared with the patterns of crystalline PZT, it is observed that the centre of the broad peak is shifted from approximately 29◦ to 31◦ . Although it is common to find broad bands in amorphous compounds, it is easily observed that this broad band is asymmetric in the XRD of the powder treated at 400 ◦ C, probably because of some ordering in the material as consequence of the heat treatment. This initial ordering is better visualized by the arrows in Fig. 3 that indicate three broad and low intense diffraction peaks that could be assigned to the pyrochlore-like PZT phase (py-PZT), with characteristics peaks at 29◦ , 34◦ and 48◦ [22], characterizing an mixture of py-PZT and PZT. We have already identified a similar cubic pyrochlore intermediate phase
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by a combined use of thermal analysis, XRD, Raman and XAS spectroscopy at the Ti K-edge when lead titanate was synthesized through the OPM route [16]. The presence of this py-PZT in both pure lead titanate and PZT, suggests that this intermediate pyrochlore structure is a usual intermediate metastable structure formed during the crystallization of the OPM amorphous precursor. Here, we observed that when the precursor was heat treated at 500 ◦ C and higher temperatures, only R-PZT was identified and its Miller indexes could be assigned. Of course, the peaks became sharper with the increase of the treatment temperature, but what is unusual is that the 110 peak of the XRD of the powders treated between 500 and 700 ◦ C show a shoulder at the high angle side of this peak, which can be understood as the presence of the T-PZT together to the R-PZT. Basically three different XRD patterns could be identified in Fig. 3, (i) the amorphous precursor, (ii) an apparent mixture of PZT and py-PZT when the powder is heat treated at moderate temperatures and (iii) pure R-PZT of the powder treated at high temperature. Considering that Raman spectroscopy is a more sensitive tool to analyse structures than DRX [23], Raman spectra of these three representative samples were collected at room temperature and are shown in Fig. 4. The spectra of powders treated at high temperatures are quite different from that of the powder calcined at 400 ◦ C, which show a general shape quite similar to that observed in the spectrum of the amorphous precursor, even with the presence of the nitrate peak at 1044 cm−1 . As discussed before, (Fig. 3) py-PZT is also present when the precursor is heat treated at low temperature, therefore the broadness of the peaks can be also an expression of the presence of this secondary phase, since the spectrum of the nanosized pyrochlore
Fig. 4. Raman spectra collected at room temperature of the PZT25 sample, with 25% of titanium and 75% of zirconium in the composition, heat treated at different temperatures and times.
Fig. 5. X-ray patterns collected at room temperature of the PZT50, with 50% of titanium and zirconium in the composition, heat treated at different temperatures and times. Tetragonal phase was indexed as [24].
is composed of broad bands [16]. On the other hand, it is also possible to say that the broadness of the peaks is also because of the smaller particle size and lower degree of crystallization. At this point, the best interpretation is that both effects contribute to the broadness of the Raman peaks of the powders treated at low temperature. Samples treated at 800 ◦ C and higher temperature show the characteristic profile of the R-PZT spectrum. What is very important in this spectrum is the absence of a shoulder (at approximately 330 cm−1 ) at the right side of the 245 cm−1 peak.
Fig. 6. Raman spectra collected at room temperature of samples with different amounts of titanium and zirconium heat-treated at 900 ◦ C for 1 h at different temperatures and times. The right side corresponds to the Stokes and the left side to the anti-Stokes scattering.
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This absence is strong evidence that pure rhombohedral phase (R-PZT) was obtained. Powders with composition rich in titanium, such as PbZr0.50 Ti0.50 O3 where “x = 0.50”, were also prepared and three XRD patterns of samples treated at different temperatures are shown in Fig. 5. It is visible the characteristic splitting of the diffraction peak of the tetragonal structure in the XRD of the powder calcined at 1000 ◦ C, which cannot be observed in the patterns of the powder treated at lower temperatures. What is clear again is the shifting of the centre of the broad peak from approximately 29◦ in powder treated at 400 ◦ C to 32◦ in the XRD patterns of the T-PZT samples. It is another good evidence of the formation of the py-PZT structure before to the crystallization of the T-PZT or R-PZT. To observe the phase transition from tetragonal to rhombohedral structure, which is rich in zirconium, Raman spectra of several compositions were collected and are shown in Fig. 6 heat treated at 900 ◦ C for 1 h. The Raman modes are assigned on the top of the figure [25]. It is observed a progressive modification on the profile of the spectrum when the composition is changed from PbZr0.65 Ti0.35 O3 to PbZr0.55 Ti0.45 O3 (or when “x” change from 0.35 to x = 0.45). The most visible modification is the splitting of the peak at 250 cm−1 that is broad but nearly unique when x = 0.25 (rhombohedral) but clearly can be identified two peaks when x = 0.45, located at 200 and 272 cm−1 . Another interesting difference between the spectra of rhombohedral and tetragonal structures can be observed in the anti-Stokes region where the
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most intense peak located at −205 cm−1 is identifiable only for rhombohedral structure, whereas for tetragonal structure only a broad band is observed in this region. The morphological properties of the material were observed by transmission electronic microscopy (TEM). It is possible to observe in the upper image of Fig. 7a group of several nanoparticles of PZT50 already sintered, with presence of necks between them after a heat treatment at 700 ◦ C for 1 h, what cannot be considered a high temperature. Similar behaviour was found for all of the particles treated at temperatures higher than 500 ◦ C, demonstrating the high reactivity of these ultra-fine powders. The second image shows the detail of a region of contact between two PZT particles, again confirming the sinterability of these powders. 4. Conclusions Powders of spherical shape and average particle size between 20 and 60 nm of PbZr1−x Tix O3 with of “x” in the range between 0.25 and 0.50 were synthesized by the OPM method. Rhombohedral structure was observed for that samples with x up to 0.35 and tetragonal structure for samples with “x” higher than 0.40 when the powders were heat treated at temperatures higher than 500 ◦ C. Analysis of XRD and Raman spectroscopy of the precursors showed an amorphous-like structure, however powders treated at 400 ◦ C showed an structure that could be identified as an intermediate pyrochlore phase, independently of the Zr and Ti mole ratio. All the powders showed high sinterability as observed by the presence of necks between the particles treated at 700 ◦ C for 1 h. Acknowledgements This work was supported by the Brazilian agencies FAPESP through of the CMDMC/Cepid, and to CNPq, 555644/20065. X-ray patterns and Raman spectra were collected at Tokyo Institute of Technology, Japan. Thanks also to Dr. Caue Ribeiro, from Embrapa, by the TEM images. References
Fig. 7. Transmission electronic images from the PZT 50 sample heat treated at 700 ◦ C for 1 h. The upper image shows group of several nanoparticles already sintered, with presence of necks between them after a heat treatment. The second image shows the detail of a region of contact between two PZT particles.
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