Hydrothermal Synthesis of Lead Zirconate Titanate Nearly Free

and washed several times first with distilled water and then with absolute ethanol, and finally oven-dried in air at 60 °C for 24 h, obtaining a ...
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Hydrothermal Synthesis of Lead Zirconate Titanate Nearly Free-Standing Nanoparticles in the Size Regime of about 4 nm Gang Xu,* Wei Jiang, Min Qian, Xinxin Chen, Zhuobin Li, and Gaorong Han Department of Materials Science and Engineering, State Key Laboratory of Silicon Materials, Zhejiang UniVersity, Hangzhou 310027, China

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 1 13–16

ReceiVed March 18, 2008; ReVised Manuscript ReceiVed October 23, 2008

ABSTRACT: A novel, facile modified hydrothermal reaction route to nearly free-standing lead zirconate titanate (PZT) particles is presented. Simple mixing of fresh coprecipitates and mineralizer KOH followed by a hydrothermal treatment at 200 °C, produced PZT nanoparticles. Mineralizer KOH plays an important role in the size control and phase formation of PZT nanoparticles. According to TEM, HRTEM, SEAD, and XRD investigations, a KOH concentration of 4 mol L-1 results in highly crystalline and nearly free-standing PZT nanopartilces in the size regime of about 4 nm. Lower concentration of KOH leads to cubic PZT particle with several micrometers. However, higher concentration should results in amorphous PZT powders and of crystal nuclei with nice thermal stability in the temperature range of room temperature to 1000 °C. This simple low-cost approach should promise us a future large-scale synthesis of free-standing PZT nanoparticles for many important applications in nanotechnology and for the fundamental studies of nanoscale ferroelectricity and the experimental demonstration of the existence of the vortex structure phase in a controlled manner. Lead zirconate titanante (PbZrxTi1-xO3, PZT) is a prototypical ferroelectric material and has attracted broad interest due to its piezoelectric, ferroelectric and electro-optic properties. The best piezoelectric and ferroelectric properties of PZT can be obtained near the morphotropic phase boundary (MPB), where the Zr:Ti ratio is around 52:48, at room temperature.1,2 PZT has a distorted perovskite structure below about 350 °C with a ferroelectric tetragonal or rhombohedral phase and consequently displays a spontaneous polarization. The polarization direction of the PZT crystal switches between two stable polarization state corresponding to the positive and negative electric bias. This particular feature makes PZT a candidate for nonvolatile ferroelectric random-access memories (NFERAM).3 Compared with bulk ferroelectrics, low-dimensional finite ferroelectric structures promise to increase the storage density of NFERAM 10 000-fold.3 But this anticipated benefit hinges on whether phase transitions and multistable states still exist in lowdimensional structures. Unfortunately, previous studies have argued that phase transitions are impossible in one-dimensional systems and become increasingly less likely as dimensionality further decreases.4-6 This inhibits the increase of the storage density of NFERAM by unlimitedly decreasing the dimension of the ferroelectric structures. Recently, the ab initio studies of ferroelectric nanoscale disks and rods of PZT solid solutions performed by Fu and co-workers demonstrated the existence of the vortex structure phase in zero-dimensional ferroelectric nanoparticles.7 The vortex structure phase in the zero-dimensional PZT ferroelectric nanoparticles is bistable (that is, the toroid moment can be equivalently parallel or antiparallel to the Z-axis). Unlike the situation in bulk ferroelectrics where states with differently oriented polarization can be accessed via a static external electric field, the vortex structure phase can be switched from one minimum state to the other by applying a time-dependent magnetic field. Storing data using switchable macroscopic toroid moment is superior to using spontaneous polarization. The minimum diameter of the PZT disks that display low-temperature structure bistability is determined to be 3.2nm, enabling an ultimate NFERAM density of 60 × 1012 bits per square inch, that is, 5 orders of magnitude large than those currently available. It is certainly desirable for use as memory bits in two- and three-dimensional information storage devices that PZT is of free-standing nanoparticulate form and very close to 3.2 nm in particle size. As we known, however, the minimum size of the * Corresponding author. Tel.: 86-571-87952341. Fax: 86-571-87952341. E-mail: [email protected].

synthesized free-standing PZT particles reported up to date is about 10-30 nm,8 far more large than 3.2nm. The hydrothermal reaction method has been widely used in preparing oxide-materials as nanoparticles.9,10 It is arguable the most common synthesis technique for the preparation of ferroelectric materials.11-13 Here, we report a modified thermal reaction approach employing a moderate concentration of KOH as mineralizer and metal hydroxide coprecipitates as reactants to synthesize nearly free-standing nanocrystalline PZT particles in the size regime of about 4nm. The synthesis of the nearly free-standing ∼4 nm PZT nanoparticles not only facilitates the high density NFERAM achieved but also the toroid moment verified in the experiment. In our synthesis process, the hydrothermal reaction was carried out in a homemade stainless-steel autoclave with a Teflon (poly[tetrafluoroethylene]) lining, chemical-grade lead nitrate (Pb(NO3)2), zirconium oxychloride (ZrOCl2 · 8H2O), and tetrabutyl titanate ((C4H9O)4Ti) were used as starting materials, potassium hydroxide (KOH) was used as a mineralizer, distilled water was used in the preparation of all aqueous solutions. Lead, zirconium, and titanium were added in the form of the coprecipitated hydroxide PbZr0.52Ti0.48O(OH)4 (PZTOH). When preparing the coprecipitated PZTOH, ammonia was used as the precipitant. In a typical synthesis procedure, based on the nominal composition of PbZr0.52Ti0.48O3, a stoichiometric amount of (Pb(NO3)2 and ZrOCl2 · 8H2O were dissolved in distilled water to form a 0.08 mol L-1 Pb2+ and Zr4+ solution, and (C4H9O)4Ti dissolved in distilled ethanol to form a 0.1 mol L-1 Ti4+ solution, respectively. Under strong stirring, the 0.1 mol L-1 Ti4+ solution was poured into the 0.08 mol L-1 Pb2+ and Zr4+ solution, forming a transparent mixed solution. Subsequently, the coprecipitated PZTOH was prepared by introducing the mixed solution into a 0.15 mol L-1 ammonia solution under stirring. To eliminate chlorine and ammonium ions, the PZTOH precipitate was filtered and washed with distilled water for six times. The fresh PZTOH precipitate was then redispersed in distilled water with vigorous stirring, followed by the addition of KOH pellets, forming a suspension. Meanwhile, the suspension was continuously stirred for 6 h. Because of existence of KOH, lead hydroxide is dissolved and the suspension becomes some transparent.14 In the final suspension, a PZTOH concentration of 0.1molL-1 and a KOH concentration of 4 mol L-1 were created. The feedstock prepared above was introduced into a 50 mL stainless-steel Teflon-lined autoclave. Hydrothermal treatment was performed by placing the autoclave in an oven and keeping it at 200 °C for 16 h, and then cooling it to room temperature in air. The products were filtered

10.1021/cg800287e CCC: $40.75  2009 American Chemical Society Published on Web 12/03/2008

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Figure 2. X-ray diffraction pattern of the as-synthesized PZT nanoparticles. All reflections can be assigned to the tetragonal perovskite phase without any obvious rhombohedral perovskite phase.

Figure 1. Representative TEM micrographs of the as-synthesized PZT nanoparticles: (a) an overview image proves the exclusive presence of PZT nanoparticles without the presence of larger particles or strong agglomerates; (b) selected area electron diffraction (SAED) pattern of (a); (c) high-resolution TEM image of a single PZT nanoparticle.

and washed several times first with distilled water and then with absolute ethanol, and finally oven-dried in air at 60 °C for 24 h, obtaining a fine white powder. The chemical composition of the nanoparticles has been determined by chemical analysis by using inductively coupled plasmaatomic emission spectroscopy (ICP-AES). The composition of the nanoparticles varies slightly from batch to batch. The ratio of Zr to Ti is exactly 0.52:0.48. However, Pb to (Zr + Ti) can vary from 0.98:1 to 1:1. X-ray diffraction (XRD) was performed on a Rigaku X-ray diffractometer with high-intensity CuKa radiation and 2θ step interval of 0.02 as well as scanning speed of 1/min. Transmission electron microscopy (TEM) images were taken with a Hitachi H-800 TEM using an acceleration voltage of 200KV. High-resolution TEM (HRTEM) images were obtained on a JEOL-2010 HRTEM using an acceleration voltage of 200 KV. Field emission scanning electron microscopy (FESEM) images were taken with a JEOL 100CX scanning electron microscope. Thermogravimetric (TG) analysis was performed with a UL-VAC-7000 thermal analyzer from roomtemperature to 1000 °C under heating rate of 10 °C/min in air. Representative transmission electron microscopy (TEM) micrographs of the PZT nanoparticles are shown in Figure 1. An overview image (Figure 1a) illustrates that the sample entirely consists of nanosized nearly individual PZT particles without the presence of large particles or strong agglomerates. Crystallinity and phase are confirmed by electron diffraction analysis, revealing diffraction rings typical for the tetragonal perovskite PZT phase (Figure 1b). Image of a single PZT nanoparticle at higher magnification (Figure 1c) shows sets of lattice fringes, giving additional evidence that the particles are highly crystalline. The particles are quite uniform in size and shape and mostly spherical. Typically, the particle diameter is in the regime of 4 nm. Further studies on the improvement of the monodispersion for obtaining a true and complete free-standing PZT nanoparticle are currently in progress.

Figure 3. (a) XRD pattern and (b) SEM image of the as-synthesized PZT powder under hydrothermal treatment with a concentration of KOH of 1 mol L-1 at 200 °C for 16 h. The XRD data were collected at a scanning speed of 4°/min.

It is well-known that ferroelectric PZT materials, whose compositions are near the MPB, can have two phases related to the perovskite structure.15 One has a tetragonal unit cell and the other has a rhombohedral unit cell In general, the coexistence of these two phases in PZT materials has been attributed to the equal free energies for tetragonal and rhombohedral phases at MPB,16,17 also to the compositional inhomogeneity.18 Figure 2 is an X-ray diffraction pattern of the synthesized PZT nanoparticles. All diffraction peaks in Figure 2 can be assigned to tetragonal PZT phase (JCPDS No. 33-0784), which is consistent with the SAED analysis (Figure 1b). However, by observation in detail the diffraction intensity of XRD between the peaks of (002) and (200) is still high. This may be attributed to the peak of (202) of rhombohedral PZT phase (JCPDS No. 73-2022), which is the third strongest diffraction peak, or the broadening of the (002) and (200) peaks of tetragonal PZT phase brought about from the reduced size of the nanoparticulate. Although the existence of rhombohedral PZT phase can not be excluded, it can be concluded that the synthesized PZT nanoparticles are mainly of tetragonal perovskite structure.

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Figure 4. (a) TEM image, (b) SAED pattern, (c) XRD pattern, and (d) TG curve of as-synthesized PZT powder under hydrothermal treatment with a concentration of KOH of 6 mol L-1 at 200 °C for 16 h. The broaden SEAD pattern demonstrates the amorphous powder is of PZT crystal nuclei. Because of the formation of crystal nuclei, the amorphous PZT powder exhibits nice thermal stability before 900 °C as pure perovskite PZT powders.

This demonstrates that the PZT nanoparticles are of highly compositional homogeneity. The crystallite size of the synthesized nanoparticle calculated from the fwhm of (110) and (101) XRD diffraction peak using Scherrer equation is about 4.4 nm, which is consistent with the observed results by TEM (Figure 1). Regarding the hydrothermal synthesis of PZT nanoparticles, it is believed that the strong basic solution brought from high concentration of KOH plays an important role. Pb(OH)2 is a amphoteric substance, showing a minimum solubility at a pH of 9.34.19 The solubility of Pb(OH)2 increases with increased pH and temperature by forming soluble species, Pb(OH)3-. 14Therefore, many Pb2+ ions are in solution under high pH and temperature. It is evident that more amount of KOH introduced into the hydrothermal reaction solution creates more Pb2+ ions, increasing the concentration of Pb(OH)3-. According to the mechanism of forming oxide from hydroxide by dehydration under hydrothermal condition,20 it is reasonable to assume that the formation of PZT is due to the dehydrating condensation between Pb(OH)3- and hydroxides of zirconium and titanium under the hydrothermal conditions of high pH, temperature, and pressure. Thus, higher concentration of KOH makes higher concentrations of monomers for the synthesis of PZT under the hydrothermal conditions. Moreover, just as we known, the formation of nanocrystals involves two steps: nucleation and growth. In the nucleation step, high concentrations of monomers facilitate a fast nucleation and the formation of a large number of crystal nuclei.21 These nuclei then grow by incorporating additional monomers still present in the reaction medium. The formation of the large number of crystal nuclei make for the synthesis of nanoparticles since the size and the morphology of the synthesized crystal strongly depend on the competition between crystal nucleation and crystal growth.22,23 In our case, the concentration of the introduced mineralizer KOH is moderated for the hydrothermal synthesis of PZT nanoparticles in the size regime of about 4nm. If the concentration of KOH is lower than 1 mol L-1, a pure tetragonal perovskite PZT powder of cube-shaped particles with several micrometers are obtained because of a few of nuclei formed. (see Figure 3) However, if the concentration of KOH is higher than 6

mol L-1, amorphous powder of PZT crystal nuclei, which possesses nice thermal stability before 900 °C as pure perovskite PZT powders, is obtained because of a burst of PZT nuclei. (see Figure 4) To investigate the effect of KOH on the composition of the synthesized PZT powder, ICP-AES was used to check the chemical compositions of the PZT powders synthesized at lower or higher concentration of KOH, respectively. The results reveal that whether the PZT powder was synthesized at lower concentration or at higher concentration of KOH, the ratio of Zr to Ti is exactly of 0.52:0.48 and the ratio of Pb to (Zr + Ti) slightly varies from 0.98:1 to 1:1, implying the compositions of the synthesized PZT powders are consistent with the designed composition. This demonstrates that the concentration of KOH only affects the PZT crystal size but not the composition. In summary, nearly free-standing tetragonal PZT nanoparticles in the size regime of about 4 nm have been successfully synthesized by a modified hydrothermal method. Mineralizer KOH plays an important role in the size control and phase formation of PZT nanoparticles. Higher concentration of KOH makes higher concentrations of monomers for the synthesis of PZT under the hydrothermal conditions, facilitating a fast nucleation and the formation of a large number of crystal nuclei. Thus, a moderate KOH concentration of 4 mol L-1 results in highly crystalline and nearly free-standing PZT nanoparticles in the size regime of about 4 nm; a lower concentration of KOH leads to cubic PZT particle several micrometers in size; and higher concentration results in amorphous PZT powders. However, the variation in concentration of KOH does not bring about departure of the chemical composition of the synthesized PZT powder from the designed composition. This simple approach should promise us a future large-scale synthesis of lead zirconate titanate and other lead-based perovskite ferroelectric nanoparticles. Further more, these PZT nanoparticles in a free-standing form should provide an ideal candidate for fundamental studies of nanoscale ferroelectricity and experimental demonstration of the existence of the vortex structure phase in PZT zero-dimensional ferroelectric nanoparticles.

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Acknowledgment. This work is supported by the National Natural Science Foundation of China under Grant 50452003.

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