Synthesis of PbTiO3 nanoplates by two-step hydrothermal method with pH-adjusting agent of ammonia solution

ABSTRACT PbTiO3 nanoplates were synthesized by a two-step hydrothermal method, and ammonia solution was chosen as a pH-adjusting agent. The effect of ammonia concentration in the second-step precursors, Pb-Ti feedstock concentration, reaction temperature, and time on crystallization and morphologies of PbTiO3 nanocrystals was investigated. The typical single-crystal PbTiO3 nanoplates with stair-like edge were formed, as the nominal ammonia concentration in the first-step precursors was 8.8 mol/L, the nominal ammonia concentration in the second-step precursors was 4.4 mol/L, the nominal Pb-Ti feedstock concentration was 0.05 mol/L, and they were synthesized at 200°C for 20 h. The thickness of the PbTiO3 nanoplates was about 45 nm, and the lateral size was about 400 nm. The ammonia solution played an important role in the formation of PbTiO3 nanoplates, and the growth mechanism of PbTiO3 nanoplates synthesized by the two-step hydrothermal method in ammonia solution was discussed.


Introduction
Lead titanate (PbTiO 3 ) is one of the typical pervoskite oxides in ferroelectric families with a high Curie temperature of 490°C, which has been widely applied in electronic devices [1][2][3]. With the increasing demand for higher integration and miniaturization of electronic devices, PbTiO 3 nanocrystals with different morphologies have been synthesized by various techniques, such as solid-state reaction, co-precipitation, sol-gel and hydrothermal methods, etc [4][5][6][7]. Among these techniques, the hydrothermal method has been widely applied in synthesis of PbTiO 3 nanocrystals due to its low cost and low-temperature characteristics [7][8][9][10][11][12][13][14][15]. In the traditional hydrothermal process, NaOH or KOH was usually used as a pH-adjusting agent, which inevitably introduced the undesirable alkali impurities to PbTiO 3 nanocrystals, and the alkali ions caused the hardening of Pb-based perovskite oxides. As pointed by Cho et al [12], the choice of pH-adjusting agent was the most important in the hydrothermal process, and the choices of pH-adjusting agent should be made on the basis that undesirable impurities could not be incorporated in the crystal structure and they could have a favorable effect on the crystal growth. They synthesized PbTiO 3 particles by the hydrothermal method with an alkali-free pH-adjusting agent of tetramethylammonium hydroxide (TMAH) to avoid the contamination of alkalis, and the pH-adjusting agent of TMAH was a critical factor in forming PbTiO 3 particles.
Takada et al. [13] also reported that Pb(Zr,Ti)O 3 cubes were synthesized by the hydrothermal method with TMAH. The Pb(Zr,Ti)O 3 cubes with single perovskite phase were synthesized at 180°C, and the size of Pb(Zr,Ti)O 3 cubes was estimated to about 1 µm. The piezoelectric properties were confirmed for single Pb(Zr,Ti)O 3 cubes using the piezoresponse force microscopy mode of a scanning probe microscope. However, TMAH is quite expensive with strong corrosive and toxic properties. It is necessary to find the ecofriendly alkali-free pH adjusting agent with low cost. In our previous study, the ammonia solution was used as the alkali-free pH-adjusting agent to take the place of TMAH, and the dendritic PbTiO 3 nanorods were formed by the hydrothermal method [14,15]. Due to the weak alkalinity of ammonia solution, only irregular dendritic PbTiO 3 nanorods were synthesized. To synthesize the PbTiO 3 nanocrystals with well crystallization, a two-step hydrothermal method was employed to synthesize PbTiO 3 nanoplates [16].
In this study, the PbTiO 3 nanocrystals were synthesized by the two-step hydrothermal method using the ammonia solution as the pH-adjusting agent, and the effect of ammonia concentration, Pb-Ti feedstock concentration, synthesis temperature, and time on crystallization and morphologies of PbTiO 3 nanocrystals was systematically investigated. The growth mechanism of PbTiO 3 nanocrystals in the ammonia solution was discussed.

Experimental details
The reagents were of analytical-grade purity and were used without further purification in this study. PbTiO 3 nanocrystals were synthesized from Pb(CH 3 COO) 2 · 3H 2 O (Macklin 99%), bis(ammonium lactate) titanium dihydroxide (C 6 H 18 N 2 O 8 Ti, TALH, Macklin 50 wt%) by the two-step hydrothermal method, and ammonia solution was used as the pH-adjusting agent. The desired amounts of Pb(CH 3 COO) 2 · 3H 2 O and TALH were dissolved in deionized water with stirring to form transparent Pb-Ti solutions. Then, ammonia solution was slowly added in the transparent Pb-Ti solutions to form the first-step precursors (30 ml) with continuous stirring. In the first-step precursors, the nominal ammonia concentration was 8.8 mol/L, and the nominal concentration of Pb-Ti feedstock varied from 0.025 to 0.1 mol/L. When the suspended gels were formed, they were centrifuged and washed with deionized water. Then, the precipitates were dispersed in the deionized water to form the suspended secondstep precursors (30 ml) with different nominal ammonia concentrations (0-13.2 mol/L). The 30 ml secondstep precursors were added to Teflon-lined autoclaves of 50 ml capacity, and they were sealed tightly. The autoclaves were heated at different synthesis temperatures (140-260°C) for different time (1-72 h), and then naturally cooled to room temperature with continuous stirring. The precipitates were centrifuged and washed with deionized water and ethanol in sequence.
These samples were analyzed by an X-ray diffractometer (XRD, D/MAX-RB) with CuKα radiation (40 kV, 30 mA). The scanning rate was 2 °/min with a scanning step of 0.02°. The morphologies of these samples were characterized by a field emission scanning electron microscope (FESEM, JSM-7500 F) and a highresolution transmission electron microscopy (HR-TEM, JEM-2100 F).

Effect of ammonia concentration in the second-step precursors
In this hydrothermal process, ammonia solution is used as the pH-adjusting agent, which has an influence on crystallization and morphologies of PbTiO 3 nanocrystals. The nominal Pb-Ti feedstock concentration in the first-step precursors was 0.05 mol/L. In the second-step precursors, the nominal ammonia concentration varied from 0, 4.4, 8.8 to 13.2 mol/L, and the corresponding pH values were 10.98, 12.06, 12.62, and 13.16. The hydrothermal synthesis was reacted at 200°C for 20 h. Figure 1 shows the XRD results of the samples synthesized with different nominal ammonia concentrations in the second-step precursors. These XRD patterns were indexed according to JCPDS No. 78-0298 (tetragonal PbTiO 3 phase with space group P4mm). All samples showed the clear and sharp diffraction peaks, which were well consistent with the diffraction peaks of tetragonal PbTiO 3 phase. These results indicated that the single-phase PbTiO 3 samples were obtained. As the nominal ammonia concentration increased from 0 to 4.4 mol/L, the intensity of diffraction peaks obviously increased. With the continuous increase of nominal ammonia concentration from 4.4 to 13.2 mol/ L, the diffraction peak intensity of PbTiO 3 samples almost did not change. Figure 2 displays the morphologies of PbTiO 3 samples synthesized with different nominal ammonia concentrations in the second-step precursors. When the PbTiO 3 sample was synthesized without ammonia, the irregular nanoparticles were observed. As the PbTiO 3 sample was synthesized with nominal ammonia concentration of 4.4 mol/L, it mainly consisted of nanoplates with about 45 nm thickness. One side of the PbTiO 3 nanoplates was relatively smooth, and the other side of the nanoplates was rough. From the rough surface, it was observed that it was composed of nanoparticles. With increasing the nominal ammonia concentration to 8.8 and 13.2 mol/L, both surfaces of the PbTiO 3 nanoplates became rough, and the nanoparticles on the surface of nanoplates obviously grew up. According to the SEM images ( Figure 2), only irregular PbTiO 3 nanoparticles were obtained without ammonia solution in the second-step precursor, and the diffraction peak intensity was weak. When there was ammonia solution in the second-step precursors, the large-size PbTiO 3 nanoplates were synthesized, and the diffraction peak intensity was strong. These results were consistent with the XRD results ( Figure 1). It indicated that the ammonia solution in the secondstep precursors played an important role to form the

Effect of Pb-Ti feedstock concentration
In this hydrothermal process, the Pb-Ti feedstock concentration in the first-step precursors varied from 0.025, 0.05, 0.075 to 0.1 mol/L. The nominal ammonia concentration in the second-step precursors was 4.4 mol/L, the hydrothermal synthesis was reacted at 200°C for 20 h.
In Figure 3, the XRD results of the samples synthesized with different Pb-Ti feedstock concentrations in the first-step precursors are shown. The single-phase PbTiO 3 samples with tetragonal structure were obtained. With the increase of Pb-Ti feedstock concentration from 0.025 to 0.1 mol/L, there was no obvious change of diffraction peak intensity of PbTiO 3 samples.
The morphologies of PbTiO 3 samples synthesized with different Pb-Ti feedstock concentrations in the first-step precursors can be observed in Figure 4. When the PbTiO 3 sample was synthesized with low Pb-Ti feedstock concentration (0.025 mol/L), it mainly consisted of nanoplates with about 30 nm thickness, and some nanoplates had holes. With increasing the Pb-Ti feedstock concentration to 0.05 mol/L, the PbTiO 3 nanoplates obviously became thicker, and the complete nanoplates were obtained. With the continuous increase of Pb-Ti feedstock concentration to 0.075 and 0.1 mol/L, the surfaces of PbTiO 3 nanoplates became rough, and the nanoparticles on the surface of nanoplates obviously grew up. Based on these results, the Pb-Ti feedstock concentration was kept at 0.05 mol/L for the next research.

Effect of synthesis temperature
In the hydrothermal process, the synthesis temperature plays a major role in formation of tetragonal perovskite PbTiO 3 structure. In this study, the Pb-Ti feedstock concentration was 0.05 mol/L in the firststep precursors, the nominal ammonia concentration in the second-step precursors was 4.4 mol/L, and the hydrothermal synthesis was reacted at different temperatures (140, 160, 180, 200, 220, 240 and 260°C) for 20 h. Figure 5 shows the typical XRD results of the samples synthesized at different synthesis temperatures. As the sample was synthesized at 140°C, the intensity of the  diffraction peaks was weak, and there were two kinds of PbTiO 3 phases. One phase was indexed to tetragonal PbTiO 3 phase with space group P4mm (JCPDS No. 78-0298), and another phase was indexed to tetragonal PbTiO 3 phase with space group PI4 (JCPDS No. 48-0105). At higher synthesis temperature (160-260°C), the single-phase PbTiO 3 (JCPDS No. 78-0298) was obtained. With increasing the synthesis temperature, the diffraction peak intensity of PbTiO 3 phase obviously increased. Figure 6 displays the typical SEM images of PbTiO 3 nanocrystals synthesized at different temperatures. When the sample was synthesized at 140°C, the sample consisted of nanoparticles, fibers and nanoplates. At synthesis temperature of 160°C, the PbTiO 3 sample mainly consisted of nanoplates. With increasing the synthesis temperature from 160 to 260°C, the size of PbTiO 3 nanoplates gradually increased, and the grains on the surface of PbTiO 3 nanoplates also grew up.

Effect of synthesis time
In the hydrothermal process, the synthesis time is also important in the phase transformations from amorphous precipitate gels to perovskite PbTiO 3 phase. In this study, the Pb-Ti feedstock concentration was 0.05 mol/L in the first-step precursors, the nominal ammonia concentration in the second-step precursors was 4.4 mol/L, and the hydrothermal synthesis was reacted at 200°C for different time (1,2,4,8,14,20,48 and 72 h).
The typical XRD results of the samples synthesized at 200°C for different time are shown in Figure 7. As the sample was synthesized at 200°C for 1 h, only several peaks with very weak intensity were observed. They were identified as TiO 2 , Pb 3 O 2 (OH) 2 , and Pb 2 Ti 2 O 6 phases, and no PbTiO 3 phase was detectable. The similar results were also reported by Bao et al [17]. When the sample was synthesized at 200°C for 2 h, the intensity of these peaks slightly increased, and the PbTiO 3 phase with weak diffraction peaks was detected. The phase transformation from amorphous precipitate gels to perovskite PbTiO 3 phase was com-   nanoplate surface gradually grew up. At synthesis time of 72 h, it was found that these nanoparticles on the nanoplate surface grew up to form nanoplates.
To identify the crystallization of PbTiO 3 nanoplates, they were characterized by the TEM technique. Figure 9 presents the typical TEM images and selected area electron diffraction (SAED) patterns of the PbTiO 3 nanoplates synthesized at 200°C for different time. When the PbTiO 3 sample was synthesized at 200°C for 4 h, the nanoplates with large-scale area were obtained as shown in Figure 9(a). The nanoplates had stair-like edges, which was reported by Takada et al [18]. The black stripe-like nanocrystals corresponded to the side of nanoplates, which indicated that the thickness of the nanoplates was about 40 nm. According to the high-resolution TEM image (the inset of Figure 9 (a)), the edge of nanoplates was rough. With increasing the synthesis time, the nanoplate thickness slightly increased, and the edges of nanoplates became smooth. The high-resolution TEM images (the insets of Figures 9(a, c, e, g) and SAED patterns (Figures 9(b, d,  f, h) confirmed that the single-crystal PbTiO 3 nanoplates were obtained at 200°C for 4-72 h, and their normal axis corresponded to the c-axis.

Growth mechanism of PbTiO 3 nanoplates synthesized by two-step hydrothermal method
In this study, there are two steps to synthesize the PbTiO 3 nanoplates. In the first step, the suspended gels were formed in the first-step precursors as chemical reaction formulas (1) and (2). After the precipitates were dispersed in the ammonia aqueous solution to form the second-step precursors, nucleation and growth were two important aspects of PbTiO 3 nanocrytal growth in the hydrothermal precursors. At the early stage of hydrothermal reaction, the intermediate nuclei of Pb 2 Ti 2 O 6 pyrochlore phase (chemical reaction formula (3)) were formed as shown in Figure 7(a). Then, the phase transformation of intermediate pyrochlore nuclei to perovskite nuclei immediately occurred as chemical reaction formula (4) according to Figure 7 (b). Finally, the intermediate phase was completely transformed into perovskite PbTiO 3 phase as shown in Figure 7(c).
As the PbTiO 3 nanocrystals were synthesized without ammonia solution in the second-step precursor, only irregular PbTiO 3 nanoparticles were observed as shown in Figure 2(   Based the above results, a mechanism for formation of PbTiO 3 nanocrystals was believed to involve dissolution and recrystallization of intermediate phase into nanoplates. However, in this study, according to the rough surface of PbTiO 3 nanoplates shown in the SEM images, the single-crystal nanoplate was composed of nanoparticles. How did these nanoparticles combine with each other to form the single-crystal PbTiO 3 nanoplate? Banfield et al. [19,20] demonstrated an important growth mechanism of oriented attachment. Oriented attachment involved spontaneous selforganization of adjacent nanoparticles so that they shared a common crystallographic orientation, followed by joining of these nanoparticles at a planar interface to reduce overall energy by removing surface energy. This growth mechanism is relevant in cases where nanoparticles are free to move in solution. During the growth of PbTiO 3 nanoplates in the hydrothermal process, the evidences of oriented attachment growth were also found. The single-crystal PbTiO 3 nanoplate sample was synthesized at 200°C for 8 h, as the nominal ammonia concentration in the secondstep precursor was 4.4 mol/L, and the Pb-Ti feedstock concentration was 0.05 mol/L. Figure 10 displays the TEM images of the PbTiO 3 nanoplate sample. In Figure 10(a), the typical stair-like PbTiO 3 nanoplate was observed. A high-resolution TEM image of two attached nanoplates with very similar orientations is shown in Figure 10(b) corresponding to area b in Figure 10(a). Lattice fringe details indicated that the dislocation at the imperfect oriented attachment interface was formed. Figures 10(c) and (d) show the highresolution TEM images of nanoparticles attached on PbTiO 3 nanoplate corresponding to areas c and d in Figure 10(a), respectively. Lattice fringe details indicated that the perfect oriented attachment was formed between the nanoparticles and nanoplate.
In the hydrothermal process, when the nominal ammonia concentration, Pb-Ti feedstock concentration, synthesis temperature and time increased, the supersaturation of PbTiO 3 nuclei increased. As the supersaturation of PbTiO 3 nuclei in the second-step precursors was low, the PbTiO 3 nuclei grew up due to the oriented attachment along the [100] and [010] directions. At the same time, the smooth surface was formed due to the Ostwald ripening. When the supersaturation of PbTiO 3 nuclei in the second-step precursors increased, the PbTiO 3 nuclei tended to be adsorbed and grow up on the surface of PbTiO 3 nanoplates due to oriented attachment.
Based on the above analysis, a schematic illustration of growth mechanism and formation process of singlecrystal perovskite PbTiO 3 nanoplates under hydrothermal conditions using ammonia solution as the pHadjusting agent is proposed as Figure 11.

Conclusions
The PbTiO 3 nanoplates were synthesized by the twostep hydrothermal method, and the ammonia solution was very important for formation of PbTiO 3 nanoplates. The typical single-crystal PbTiO 3 nanoplates with stairlike edges were formed, as the nominal ammonia concentration in the first-step precursors was 8.8 mol/L, the nominal ammonia concentration in the second-step precursors was 4.4 mol/L, the nominal Pb-Ti feedstock concentration was 0.05 mol/L, and they were reacted at 200°C for 20 h. In the two-step hydrothermal process, the intermediate nuclei of Pb 2 Ti 2 O 6 pyrochlore phase were formed at the early stage of hydrothermal reaction, then the phase transformation of intermediate pyrochlore nuclei to perovskite nuclei immediately occurred, finally, the intermediate phase was completely transformed into perovskite PbTiO 3 phase. The large-scale stair-like PbTiO 3 nanoplates were formed mainly due to oriented attachment. Figure 11. Schematic illustration of single-crystal perovskite PbTiO 3 nanoplate formation mechanism under hydrothermal conditions using ammonia as the pH-adjusting agent.