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Chem. Mater. 2005, 17, 4594-4599

Articles Nonaqueous Synthesis of Amorphous Powder Precursors for Nanocrystalline PbTiO3, Pb(Zr,Ti)O3, and PbZrO3 Georg Garnweitner, Jens Hentschel, Markus Antonietti, and Markus Niederberger* Max Planck Institute of Colloids and Interfaces, Research Campus Golm, D-14424 Potsdam, Germany ReceiVed February 15, 2005. ReVised Manuscript ReceiVed July 7, 2005

A novel, facile nonaqueous synthesis route to nanocrystalline lead zirconate titanate (PZT) powders is presented. Simple mixing of lead acetylacetonate with titanium and/or zirconium alkoxides in 2-butanone, followed by a solvothermal treatment at 200 °C, produced amorphous, nanosized precursor powders. According to X-ray diffraction and transmission electron microscopy investigations, calcination at comparably low temperatures yielded highly crystalline, phase-pure powders consisting of monocrystalline particles in the size range of about 10-30 nm. Thermogravimetric and infrared analyses of the asprepared samples showed a content of organic species of about 10 wt %, which are decomposed between 200 and 330 °C. The lack of further weight loss above 400 °C proves the absence of PbO evaporation, a major drawback of previous PZT preparations. Crack-free thin films were obtained by simply casting a dispersion of precursor powders on a silicon substrate followed by thermal treatment. Scanning electron microscopy shows homogeneous films to some extent populated with regular, micrometer-sized spheres. However, thorough washing removed most of these larger objects to leave a plain film. According to atomic force microscopy measurements, the roughness of this film is in the nanometer-size regime, confirming that this route is a potential way to fabricate thin homogeneous piezoelectric films.

Introduction Research and technology of functional ceramics is predominantly focused on metal oxides with the perovskite structure, mainly compositions based on BaTiO3, SrTiO3, PbTiO3, and PbZrO3. These materials readily form solid solutions with each other and with a large number of other oxides and, thus, provide an immense variety of ferroelectric properties.1 Ferroelectric materials are characterized by two features, namely, the existence of spontaneous polarization and the possibility to reorient the polarization by an external electric field.2 This switchable electric polarization is ideal for the use of ferroelectric materials in devices for memory storage and integrated microelectronics.3-5 Perovskites also constitute the most prominent class of piezoceramics with a piezoelectric coefficient one order of magnitude larger than that of quartz.6 Lead zirconate titanate (PZT) ceramics show the broadest range of technical applications as piezoelectric * To whom correspondence [email protected].

should

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E-mail:

(1) Lines, M. E.; Glass, A. M. Principles and Applications of Ferroelectrics and Related Materials; Oxford University Press: Oxford, U.K., 2001. (2) Haertling, G. H. J. Am. Ceram. Soc. 1999, 82, 797-818. (3) Kotecki, D. E.; Baniecki, J. D.; Shen, H.; Laibowitz, R. B.; Saenger, K. L.; Lian, J. J.; Athavale, S. D.; Cabral, C., Jr.; Duncombe, P. R.; Gutsche, M.; Kunkel, G.; Park, Y.-J.; Wang, Y.-Y.; Wise, R. IBM J. Res. DeV. 1999, 43, 367-382. (4) Waser, R. Nanoelectronics and Information Technology; WileyVCH: Weinheim, Germany, 2003. (5) Bhalla, A. S.; Guo, R.; Roy, R. Mater. Res. InnoV. 2000, 4, 3-26. (6) Noheda, B. Curr. Opin. Solid State Mater. Sci. 2002, 6, 27-34.

sensors or actuators, mainly due to their optimum electromechanical coupling properties.7,8 Moreover, PZT materials have high Curie temperatures, can easily be poled, possess a wide range of dielectric constants, are easy to sinter and form solid-solution compositions with many different constituents.2 The traditional route to perovskite materials involves solidstate reactions between the individual metal oxide or carbonate powders at temperatures between 600 and 1100 °C. However, this processing technique suffers from a number of uncertainties and drawbacks. In the present context, one major flaw is that the volatility of certain compounds such as PbO during the preparation of PZT materials leads to deviation of stoichiometry. Impurities in the raw materials, lack of homogeneity due to poor mixing, and large particle sizes, which require intensive milling, are some other major problems that resulted in the development of a large number of alternative synthesis techniques. Wet-chemical procedures offer good control from the molecular precursor to the final material, providing high purity, small crystallite sizes, welldefined particle morphologies, and small particle size distributions. These soft-chemistry routes generally involve the transformation of soluble precursors, either homometallic or heterometallic (“single source”) compounds, into the metal oxide framework.9-11 (7) Kamlah, M. Continuum Mech. Thermodyn. 2001, 13, 219-268. (8) Polla, D. L.; Lorraine, F. F. Annu. ReV. Mater. Sci. 1998, 28, 563597.

10.1021/cm0503376 CCC: $30.25 © 2005 American Chemical Society Published on Web 08/11/2005

Synthesis Route to Lead Zirconate Titanate Powders

One of the key parameters for further miniaturization of electroceramic devices is the availability of ferro- and piezoelectric materials on the nanoscale. Furthermore, ceramics produced from nanophase powders show unique consolidation and compaction properties; i.e., they exhibit enhanced sintering rates, lower sintering temperature, and the small grain sizes in the final ceramic result in increased flexibility, less brittleness, and greater strength.4,12 Accordingly, the synthesis of lead-based titanate nanoparticles is of high technological and scientific interest. Nevertheless, reports about particles with sizes smaller than 100 nm are rather scarce. By far the most synthesis procedures are either based on aqueous sol-gel chemistry, mainly involving hydrothermal processes13-16 and coprecipitation,17,18 or on thermal decomposition of polymer precursors.19-22 Complementing these approaches, nonaqueous synthesis routes to metal oxide nanoparticles have become a valuable alternative to aqueous reaction approaches, allowing the preparation of a large variety of nanocrystalline particles such as TiO2,23-27 iron oxides,28-32 manganese oxides,33 CuO,34 ZrO2,35 HfO2,36,37 HfxZr1-xO2,37 In2O3,32,38,39 Nb2O5,40 SnO2,39 (9) Chandler, C. D.; Roger, C.; Hampden-Smith, M. J. Chem. ReV. 1993, 93, 1205-1241. (10) Schwartz, R. W.; Schneller, T.; Waser, R. C. R. Chim. 2004, 7, 433461. (11) Hubert-Pfalzgraf, L. G. Inorg. Chem. Commun. 2003, 6, 102-120. (12) Arora, A. AdV. Eng. Mater. 2004, 6, 244-247. (13) Moon, J.; Kerchner, J. A.; Krarup, H.; Adair, J. H. J. Mater. Res. 1999, 14, 425-435. (14) Das, R. N.; Pati, R. K.; Pramanik, P. Mater. Lett. 2000, 45, 350355. (15) Deng, Y.; Liu, L.; Cheng, Y.; Nan, C. W.; Zhao, S. J. Mater. Lett. 2003, 57, 1675-1678. (16) Cho, S. B.; Noh, J. S.; Lencka, M. M.; Riman, R. E. J. Eur. Ceram. Soc. 2003, 23, 2323-2335. (17) Liu, C.; Zou, B.; Rondinone, A. J.; Zhang, Z. J. J. Am. Chem. Soc. 2001, 123, 4344-4345. (18) Camargo, E. R.; Popa, M.; Frantti, J.; Kakihana, M. Chem. Mater. 2001, 13, 3943-3948. (19) Mandal, T. K.; Ram, S. Mater. Lett. 2003, 57, 2432-2442. (20) Arya, P. R.; Jha, P.; Subbanna, G. N.; Ganguli, A. K. Mater. Res. Bull. 2003, 38, 617-628. (21) Bose, S.; Banerjee, A. J. Am. Ceram. Soc. 2004, 87, 487-489. (22) Banerjee, A.; Bose, S. Chem. Mater. 2004, 16, 5610-5615. (23) Trentler, T. J.; Denler, T. E.; Bertone, J. F.; Agrawal, A.; Colvin, V. L. J. Am. Chem. Soc. 1999, 121, 1613-1614. (24) Kim, C. S.; Moon, B. K.; Park, J. H.; Choi, B. C.; Seo, H. J. J. Cryst. Growth 2003, 257, 309-315. (25) Niederberger, M.; Bartl, M. H.; Stucky, G. D. J. Am. Chem. Soc. 2002, 124, 13642-13643. (26) Niederberger, M.; Bartl, M. H.; Stucky, G. D. Chem. Mater. 2002, 14, 4364-4370. (27) Niederberger, M.; Garnweitner, G.; Krumeich, F.; Nesper, R.; Co¨lfen, H.; Antonietti, M. Chem. Mater. 2004, 16, 1202. (28) Redl, F. X.; Black. C. T.; Papaefthymiou, G. C.; Sandstrom, R. L.; Yin, M.; Zeng, H.; Murray, C. B.; O’Brien, S. P. J. Am. Chem. Soc. 2004, 126, 14583-14599. (29) Li, Z.; Chen, H.; Bao, H. B.; Gao, M. Y. Chem. Mater. 2004, 16, 1391-1393. (30) Hyeon, T.; Lee, S. S.; Park, J.; Chung, Y.; Na, H. B. J. Am. Chem. Soc. 2001, 123, 12798-12801. (31) Pinna, N.; Grancharov, S.; Beato, P.; Bonville, P.; Antonietti, M.; Niederberger, M. Chem. Mater. 2005, 17, 3044-3049. (32) Pinna, N.; Garnweitner, G.; Antonietti, M.; Niederberger, M. J. Am. Chem. Soc. 2005, 127, 5608-5612. (33) Seo, W. S.; Jo, H. H.; Lee, K.; Kim, B.; Oh, S. J.; Park, J. T. Angew. Chem., Int. Ed. 2004, 43, 1115-1117. (34) Hong, Z. S.; Cao, Y.; Deng, J. F. Mater. Lett. 2002, 52, 34-38. (35) Joo, J.; Yu, T.; Kim, Y. W.; Park, H. M.; Wu, F. X.; Zhang, J. Z.; Hyeon, T. J. Am. Chem. Soc. 2003, 125, 6553-6557. (36) Pinna, N.; Garnweitner, G.; Antonietti, M.; Niederberger, M. AdV. Mater. 2004, 16, 2196-2200. (37) Tang, J.; Fabbri, J.; Robinson, R. D.; Zhu, Y. M.; Herman, I. P.; Steigerwald, M. L.; Brus, L. E. Chem. Mater. 2004, 16, 1336-1342.

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and ZnO.32,41 Even more complex metal oxides such as ferrites42,43 and perovskites44-46 were obtained on the basis of nonaqueous sol-gel chemistry. In the case of BaTiO3 nanoparticles, the process involves the dissolution of metallic barium in benzyl alcohol, followed by addition of titanium isopropoxide and solvothermal treatment in an autoclave. Since this methodology is applicable to other perovskites such as SrTiO3, (Ba,Sr)TiO3, BaZrO3, and LiNbO3,44 it was promising to explore if it also works for PZT materials. However, we found that in the standard preparations lead cations were always reduced to metallic lead by benzyl alcohol. This can only be prevented by using nonreductive organic solvents. On the basis of an approach published by Goel et al.,47 we recently reported the synthesis of crystalline titania nanoparticles using ketones and aldehydes as oxygensupplying agents.48 In this work we present a nonaqueous synthesis of amorphous powder precursors that can easily be transformed into PbTiO3 (PT), Pb(Ti,Zr)O3 (PZT), and PbZrO3 (PZ) by low-temperature processing. The reaction between lead(II) acetylacetonate and titanium and/or zirconium isopropoxide in 2-butanone results in the formation of an amorphous precipitate after solvothermal treatment. This synthesis protocol enables by concept high-purity powder precursors in good yields without contamination with other inorganics such as halides or alkali metals. Calcination in the temperature range of 400-450 °C yields nanocrystalline and phasepure PbTiO3 and Pb(Ti,Zr)O3. The processing temperature is a crucial factor, because ferroelectric thin films are usually deposited on silicon substrates. Above 500 °C, the silicon substrate is seriously damaged.49 Crystalline nanoparticles of the antiferroelectric PbZrO3 could be obtained after a calcination treatment at 600 °C. The use of amorphous powders as precursors for PT, PZT, and PZ materials is particularly attractive, because powders are easy to handle and store, provide low crystallization temperature, and are, in contrast to CVD techniques, more flexible with respect to substrate structure, substrate geometry, and cost-effectiveness. As a matter of fact, industrial fabrication of microelectronic devices is mainly based on the use of tape casting techniques with particulate slurries of metal oxide particles.2,50 (38) Seo, W. S.; Jo, H. H.; Lee, K.; Park, J. T. Angew. Chem., Int. Ed. 2003, 15, 795-797. (39) Pinna, N.; Neri, G.; Antonietti, M.; Niederberger, M. Angew. Chem., Int. Ed. 2004, 43, 4345-4349. (40) Pinna, N.; Antonietti, M.; Niederberger, M. Colloids Surf., A 2004, 250, 211-213. (41) Cozzoli, P. D.; Curri, M. L.; Agostiano, A.; Leo, G.; Lomascolo, M. J. Phys. Chem. B 2003, 107, 4756-4762. (42) Sun, S. H.; Zeng, H.; Robinson, D. B.; Raoux, S.; Rice, P. M.; Wang, S. X.; Li, G. X. J. Am. Chem. Soc. 2004, 126, 273-279. (43) Zeng, H.; Rice, P. M.; Wang, S. X.; Sun, S. H. J. Am. Chem. Soc. 2004, 126, 11458-11459. (44) Niederberger, M.; Pinna, N.; Polleux, J.; Antonietti, M. Angew. Chem., Int. Ed. 2004, 43, 2270. (45) Niederberger, M.; Garnweitner, G.; Pinna, N.; Antonietti, M. J. Am. Chem. Soc. 2004, 126, 9120-9126. (46) Niederberger, M.; Garnweitner G. Mater. Res. Soc. Symp. Proc. 2005, 879E, Z9.8.1-Z9.8.5. (47) Goel, S. C.; Chiang, M. Y.; Gibbons, P. C.; Buhro, W. E. Mater. Res. Soc. Symp. Proc. 1992, 271, 3-13. (48) Garnweitner, G.; Antonietti, M.; Niederberger, M. Chem. Commun. 2005, 397-399.

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Experimental Section Synthesis. Lead(II) acetylacetonate [CH3COCHdC(O)CH3]2Pb (technical grade), titanium isopropoxide Ti[OCH(CH3)2]4 (99.999%), zirconium isopropoxide Zr[OCH(CH3)2]4‚HOCH(CH3)2 (99.99%), and 2-butanone (HPLC grade, 99.5+%) were used as-received from Aldrich. In a typical synthesis, 1.23 mmol of titanium tetraisopropoxide (350 mg) or zirconium tetraisopropoxide isopropanol complex (477 mg) were mixed with 2-butanone (5.5 mL). For the mixed oxide, 0.58 mmol of Ti(OiPr)4 and 0.65 mmol of Zr(OiPr)4‚ iPrOH were used. After stirring for 10 min, 1.23 mmol (500 mg) of lead(II) acetylacetonate was added and further stirred for 1 h. An orange to yellow, slightly turbid liquid was obtained, which was transferred into an autoclave equipped with a Teflon liner (Parr acid digestion bomb, 23 mL). The autoclave was sealed and heated in an oven to 200 °C for 24 h. In all cases, white to yellow suspensions were obtained, which were centrifuged and the precipitates washed with ethanol and dried in vacuo. From the content in inorganic material as determined by thermogravimetric analysis (TGA), the yields were calculated to about 85-90%. Calcination was carried out in air by applying a heating rate of 100 °C/h and holding the calcination temperature of 400-600 °C for 4-6 h. Films were prepared by the following procedure: First, a suspension of the as-prepared nanopowders in EtOH (about 2 wt %) was prepared by performing an extended ultrasonication treatment, using a W-450 D Digital Sonifier, Banson Ultrasonics Corp. Several drops of this turbid suspension were cast on a silicon wafer previously treated with an SPM (“piranha”) solution (1:2 H2O2: H2SO4) at 120 °C for 30 min to remove organic contaminants. After preliminary drying, first at room temperature for 30 min and then at 100 °C for 2 h, the films were rapidly heated to 500 °C, which was held for 2 h. After cooling, the samples were rinsed thoroughly with ethanol or put in an ethanol-filled beaker and subjected to ultrasonication for 30 min. Characterization. Crystallinity of the samples was checked on an Enraf-Nonius PDS-120 powder diffractometer in reflection mode, using an FR-590 generator as the source of Cu KR radiation. A Nonius CPS-120 curved position sensitive detector, showing a resolution of 2θ ) 0.018°, was employed to record the scattered radiation. Transmission electron microscopy (TEM) was performed on a Zeiss EM 912Ω instrument at an acceleration voltage of 120 kV. TGA was measured of a Netzsch thermoanalyzer TG 209, measuring under nitrogen from room temperature to 900 °C at a heating rate of 10 °C/min. Scanning-electron microscopy (SEM) was performed on a LEO 1550-Gemini instrument on the calcined films coated with gold. The films were further characterized by atomic force microscopy (AFM), which was performed on a NanoScope IIIa device (Veeco Instruments, Santa Barbara, CA) in tapping mode. Commercial silicon tips (Type NCR-W) were used with a tip radius < 20 nm, employing a force constant of 42 N m-1 and a resonance frequency of 285 kHz. The image was recorded on a 10 × 10 µm e-scanner. Inductively coupled plasma optical emission spectrometry (ICP-OES) was performed on a Perkin-Elmer Optima 3000. Elemental analyses were carried out on a Vario EL Elementar (Elementar Analysensysteme, Hanau, Germany). The FTIR-ATR spectrum was recorded on an FTS 6000 spectrometer (Bio-Rad Laboratories, Inc.) equipped with a horizontal ATR accessory (Zn-Se crystal) from Pike Technologies. (49) Calzada, M. L.; Bretos, I.; Jimenez, R.; Guillon, H.; Pardo, L. AdV. Mater. 2004, 16, 1620-1624. (50) Chan, H. M. Annu. ReV. Mater. Sci. 1997, 27, 249-282.

Figure 1. X-ray diffraction patterns of the as-prepared powders of PbTiO3 (a), Pb(Zr0.53Ti0.47)O3 (b), and PbZrO3 (c).

Results and Discussion The as-prepared, washed precipitates were analyzed by X-ray diffraction (XRD) as shown in Figure 1. All diffractograms feature a broad peak centered around 2θ ) 30°. A second, broad peak is visible at about 48° in the PT system (Figure 1a) and also for the mixed oxide (Figure 1b), whereas, in the PZ system (Figure 1c), another signal is found centered at about 58°. The broadness of these peaks indicates low crystallinity with very short ranged ordering. Such patterns have been observed for sol-gel derived lead titanate and were suggested to correspond to amorphous materials locally ordered in the cubic pyrochlore Pb2Ti2O6 structure. Although this structure is kinetically favored, it transforms irreversibly to tetragonal PT upon heat treatment.51 TEM images of the as-prepared powders (data not shown) indicate the presence of large aggregates several hundred nanometers in diameter. On their edges, however, individual particles are visible and are estimated as approximately 5-10 nm in size. It is not possible to unambiguously determine the particle morphology from these images. TGA was performed to investigate the thermal behavior of the as-prepared powders (Figure 2a). In all cases, only a slight, steady decrease in weight by about 1.8-3.1 wt % is observed up to 200 °C, which is attributed to desorption of water and ethanol from the washing process. Between 200 and 330 °C, a strong weight loss is visible, additionally featuring a small shoulder at about 320-360 °C. The height of this step ranges from 9.7 wt % in the case of PT to 11.8 wt % for PZ. The weight loss is clearly attributed to the removal of organic species adsorbed to the as-prepared powder particles. At higher temperatures, no significant weight loss was observed for any sample up to 900 °C. This result gives evidence for the absence of carbon and proves the stability of the obtained particles at high temperatures, especially regarding the commonly observed PbO loss through evaporation at temperatures above 700 °C. Due to the fact that crystallization as well as densification of the films is achieved well below 900 °C, additional compensation for any lead loss52 is not necessary in this process. (51) Camargo, E. R.; Longo, E.; Leite, E. R.; Mastelaro, V. R. J. Solid State Chem. 2004, 177, 1994-2001.

Synthesis Route to Lead Zirconate Titanate Powders

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Figure 3. XRD patterns of the calcined powders: PbTiO3 after calcination at 400 °C (a), Pb(Zr0.53Ti0.47)O3 at 450 °C (b), and PbZrO3 calcined at 600 °C (c).

Figure 2. (a) Thermogravimetric analysis of the as-prepared samples: PbTiO3 (1), Pb(Zr0.53Ti0.47)O3 (2), and PbZrO3 (3). (b) FT-IR spectrum of as-prepared PZT powders.

The adsorbed organic species can be easily characterized by IR spectroscopy. Figure 2b shows the FT-IR spectrum observed for an as-prepared PZT sample. Strong stretching vibration bands in the range ν ) 1280-1520 cm-1 are characteristic for metal-bound β-diketonates. These bands were not found in the IR spectra of calcined samples. Therefore, we infer that the as-prepared powders are covered with acetylacetonate stemming from the lead precursor, which during the synthesis process is bound to the particle surface and thus acts as a stabilizer. Due to the strong binding, desorption/decomposition does not occur at temperatures below 200 °C. The mechanism leading to the formation of lead zirconate/ titanate powders in 2-butanone can be elucidated by studying the composition of the final reaction solution after removal of the inorganic material by filtration. NMR spectroscopy data (not shown) indicate the presence of several organic species in addition to the solvent. 2-Butanol and acetone are found in quite high amounts and prove the occurrence of Meerwein-Ponndorf-Verley-like oxidation-reduction reactions, as we have reported before.45,48 In smaller quantities, 5-methyl-4-heptene-3-one and 5-methyl-4-hexene-3-one are present, the respective aldol coupling products of 2-butanone with itself and with acetone. Thus, the formation of the (52) Kakegawa, K.; Matsunaga, O.; Kato, T.; Sasaki, Y. J. Am. Ceram. Soc. 1995, 78, 1071-1075.

inorganic powders is expected to occur via aldol condensation reactions of the solvent, involving an elimination step, where formally water is released.46,47 The resulting M-OH groups can undergo inorganic condensation reactions to form M-O-M bonds, the building unit of the oxides. This is analogous to the mechanisms leading to TiO2 formation by reacting titanium tetraisopropoxide in common ketones, which we have described recently.48 The system discussed here is more complicated due to the additional presence of the lead(II) acetylacetonate species. However it is interesting to note that no condensation products of the acetylacetonate were detected in the reaction mixture other than the surfaceadsorbed ligand. To investigate the elemental composition of the amorphous powder precursors, elemental analysis was performed by ICP-OES analysis with the samples previously digested in acids. The molar Pb:Ti:Zr ratio found was 1:0.97:0 for PbTiO3, 1:0.45:0.55 for Pb(Ti0.47Zr0.53)O3, and 1:0:1.05 in the case of PbZrO3, which within the experimental error is in good agreement with the target composition. Combustion analysis was performed in order to measure the carbon and hydrogen contents. For all systems investigated, the prepared precursor materials contained about 6-7 wt % carbon and about 0.6 wt % hydrogen. Assuming that all ligands are acetylacetonate, this would result in an organic content of 9.9-11.6 wt %, which agrees well with the data obtained by TGA. Calcination treatments were performed to induce crystallization and the resulting XRD plots are shown in Figure 3. A calcination temperature of 400 °C was sufficient to obtain highly crystalline, phase-pure PT (Figure 3a). All reflections can be assigned to tetragonal PbTiO3 (JCPDS [6-452]). For the PZT system, the temperature had to be raised to 450 °C (Figure 3b). The peaks are significantly broader, indicating smaller crystallite size. They clearly correspond to Pb(Zr0.53Ti0.47)O3 (JCPDS [33-784]), except for the small signals marked with asterisks, which are attributed to remaining amorphous material. PZ had to be treated at higher temperatures to induce crystallization. After treatment at 600 °C, a pattern with much sharper reflections is obtained (Figure 3c),

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Figure 4. TEM images of the samples after calcination. PbTiO3 after heat treatment at 400 °C (a), PZT particles treated at 450 °C (b), and PbZrO3 calcined at 600 °C (c).

pointing to larger crystallites. The reflections match the orthorhombic PbZrO3 structure (JCPDS [35-739]). From the obtained XRD patterns, the crystallite sizes were calculated using the Scherrer equation. For PT the crystallite size was determined to 19.5 nm, using the (111) reflection, whereas for the PZT system, a significantly smaller size of about 10 nm was obtained (calculated from the same reflection). The (240) reflection was employed for calculating the crystal size of PZ, which amounted to 26 nm. Consequently, calcination at relatively moderate temperatures resulted in highly crystalline materials for all three systems. The crystallite size for PZT was much smaller compared to the pure titanate and zirconate systems. With increasing Zr content, the samples need to be treated at higher temperatures to induce crystallization, which reflects the general shift to higher temperature necessary for formation of crystalline PZT materials with increasing Zr/Ti ratios.53 As the temperature necessary for crystallization of pure PZ was substantially higher than those for PT and PZT, also the growth kinetics is enhanced, which leads to the larger crystallite sizes obtained for this system. The obtained nanopowders were further investigated by TEM. Figure 4 displays the micrographs of samples after calcination, PbTiO3 after heat treatment for 4 h at 400 °C (Figure 4a), Pb(Ti,Zr)O3 after 6 h at 450 °C (Figure 4b), and PbZrO3 after 6 h at 600 °C (Figure 4c). All micrographs show almost spherical or slightly ellipsoidal nanoparticles with a narrow size distribution within the same system. Although the particles are somewhat aggregated, it is clearly visible that the lead titanate and lead zirconate particles are significantly larger compared to the PZT sample. The particle sizes lie in the range of 25-35 nm for PT and 20-30 nm for PZ, whereas the PZT material consists of particles with an average diameter of about 10 nm. These results coincide well with the crystallite sizes calculated from XRD data. Films were prepared by simply casting a dispersion of the as-prepared powders in ethanol on Si wafers. After annealing at 500 °C, a smooth film was obtained, however on its surface featuring uniform, spherical particles about 2-3 µm in size, as seen in an overview SEM image of a sample prepared using a PZT powder (Figure 5a,b). A higher magnification image shows that these large balls consist of smaller particles (Figure 5b, inset). However, one can clearly distinguish between spheres featuring a defined edge and spheres with an apparently blurred edge, pointing to the fact that some beads are just attached loosely to the surface of the underlying smooth PZT film, whereas the other ones are partially incorporated in the film. From SEM images of a (53) Kutty, T. R. N.; Balachandran, R. Mater. Res. Bull. 1984, 19, 14791488.

Figure 5. SEM images of films prepared from a dispersion of PZT particles after calcination: overview picture (a) and detailed image of spheres before washing (b); overview (c), fractured spheres (d), and homogeneous film after washing (e). Electron diffraction pattern of the scratched-off film (f).

Figure 6. AFM height image of a PZT film after annealing at 500 °C (a), with section analysis (b), and in side view (c).

scratched sample (Supporting Information), the film thickness was estimated to several micrometers. On the basis of the fact that the particles are not stabilized by surface complexants other than the acetylacetonate, it is reasonable to assume that the spheres are already formed in solution, before the film coating process. Hence, there must be both large aggregates in the form of the spheres and smaller, isolated particles present in solution. The smaller particles get into contact with the substrate, and on the basis of their higher activity, they form a homogeneous film. Such a dense, homogeneous film can only be formed via sintering processes, and in fact this proves the high sintering activity of

Synthesis Route to Lead Zirconate Titanate Powders

the precursor powders already at moderate temperatures. The large spherelike aggregates retain their shape and are found sitting on or partially inside the film. It is fascinating, though, that the large aggregates show such a uniform shape and size. To remove the spheres not connected to the film, thorough washing was performed by rinsing the films with distilled water and ethanol. Most of the micrometer-sized balls can be removed by this treatment, as is evident on the overview SEM image (Figure 5c). Even more interesting, spheres previously embedded in the underlying film are fractured upon extensive washing (Figure 5d). A region only featuring the plain film is shown at higher magnification (Figure 5e). It can be clearly seen that the film is highly homogeneous and has a smooth surface. Parts of the film were scratched off and measured in a TEM instrument. The electron diffraction image of such a section is presented and proves that the material is crystalline, with all visible reflections corresponding to the tetragonal lead zirconate titanate phase (Figure 5f). An analogously treated sample was analyzed by atomic force microscopy, again after thorough washing with water and ethanol. An area without any spheres is presented in Figure 6a. Section analysis gave evidence that the film is crack-free and rather smooth with a low mean surface roughness of 0.13 nm (Figure 6b). Also the side view (Figure 6c) proves that the film is rough only on a small nanometer scale. This is remarkable considering the particle size of about 10 nm observed for the calcined powder, and this indicates that the thermal treatment is very efficient for thin film formation. Conclusions In this work, we have established a simple and facile route to crystalline PT, PZT, and PZ nanopowders. Starting from

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the commercially available metal compounds lead(II) acetylacetonate and titanium/zirconium alkoxides, the synthesis of amorphous precursor particles is achieved under nonaqueous and nonalkaline conditions using a simple ketone as solvent. The as-synthesized powders are perfect precursors for PZT materials, because they contain the metals in the correct and targeted ratio. Subsequent calcination leads to highly crystalline, phase-pure nanoparticles with crystallite sizes in the range of about 20 nm for PbTiO3 and PbZrO3 particles and about 10 nm for the PZT system. The moderate calcination temperatures circumvent the problem of PbO evaporation and the consequent change in stoichiometry. Another big advantage of powder precursors lies in the possibility of film formation by simply casting suspensions of the as-prepared particles on silicon wafers. Using this approach, we present the fabrication of homogeneous and crack-free PZT films over an area of several micrometers. With application of extensive washing treatments to remove larger aggregates sitting on top of the film, the homogeneity of the films could be extended into areas dozens of micrometers in length. Acknowledgment. The authors thank Julien Polleux for performing TEM measurements. Anne Heilig is gratefully acknowledged for performing the AFM measurements. We further thank Dr. Nadine Nassif, Rona Pitschke, and Dr. Ju¨rgen Hartmann for acquiring the SEM images and Sylvia Pirok for combustion analyses. Dr. Bernd Smarsly is acknowledged for helpful discussions regarding the film formation procedure. Supporting Information Available: SEM images of a scratched film of PZT after annealing at 500 °C. This material is available free of charge via the Internet at http://pubs.acs.org. CM0503376