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J. Phys. Chem. C 2008, 112, 16818–16823
Hydrothermal Synthesis of Ferroelectric PbHPO4 Nanowires from a Single-Source Precursor Guang-Yi Chen,†,‡ Wen-Gang Qu,† Feng Ye,† Wan-Xi Zhang,‡ and An-Wu Xu*,† DiVision of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, UniVersity of Science and Technology of China, Hefei 230026, China, and School of AutomotiVe Engineering, Dalian UniVersity of Technology, Dalian 116024, China ReceiVed: June 29, 2008; ReVised Manuscript ReceiVed: August 31, 2008
Ferroelectric PbHPO4 nanowires have been successfully prepared from a Pb(II)-IP6 (IP6, inositol hexakisphosphate acid) complex, a novel single-source precursor, by a hydrothermal method at 180 °C for 24 h. The structure and morphology of the as-prepared PbHPO4 nanowires and the Pb(II)-IP6 complex were systematically studied by means of X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and Fourier transform infrared spectroscopy. The possible formation mechanism for PbHPO4 nanowires is also proposed. The photoluminescence (PL) and ferroelectric properties of the obtained products were investigated in detail. PL measurements indicate that under the excitation of 375 nm the products have a broad emission peak at 460 nm. The ferroelectric hysteresis loop shows that the remnant polarization (PR) is equal to its maximum polarization (PS). The one-dimensional conducting protons chains in the PbHPO4 crystal structure may be responsible for this unique ferroelectric hysteresis loop. Introduction Because of the unique size- and morphology-dependent properties, one-dimensional (1D) inorganic nanostructures such as nanowires, nanobelts, nanotubes, and nanocables have become the focus of intensive investigation in the past decade.1 These 1D nanomaterials have novel electronic, optical, and magnetic properties that are different from their bulk counterparts and can find potential applications in nanoscale devices and sensors.2,3 Although many synthetic routes such as solution-liquid-solid (SLS) growth process,4 vapor-liquid-solid (VLS) reaction growth,5 physical evaporating,6 CVD methods,7 and template-inspired approach8 have been developed to fabricate 1D nanowires; these strategies often require high temperatures, tedious procedures, special conditions, and templates or catalysts, and most of them are complicated and uncontrollable. In particular, the introduction of templates or capping reagents to the reaction system may lead to an increase of impurity in the final products. Therefore, it is necessary to develop a simple and effective route to prepare 1D inorganic nanowires. Recently, low-temperature solution-phase synthesis has become an attractive method for the synthesis of 1D nanostructured materials with high crystallinity.9 Ferroelectrics, which are a subset of piezoelectrics, are materials having a spontaneous polarization that can be reversed with the application of an appropriate electric field. This reversible polarization makes ferroelectric materials useful in memory devices, capacitors, transducers, nonlinear optics, etc.10 PbHPO4 (lead hydrogen phosphate; LHP) was first introduced as ferroelectric by Negran et al. in 1974.11 As a typical representative of the family of ferroelectric schultenite, LHP and its isomorphs become another valuable group of hydrogen-bonding ferroelectrics besides the wellstudied KDP (KH2PO4) family.12 The LHP crystal can transfer from * To whom correspondence should be addressed. Fax: 86-551-3600724. E-mail:
[email protected]. † University of Science and Technology of China. ‡ Dalian University of Technology.
Figure 1. The XRD pattern of the obtained PbHPO4 nanowires under hydrothermal treatment at 180 °C for 24 h.
the ferroelectric phase to paraelectric phase at transition temperature TC ) 310 K. The spontaneous polarization PS rises slowly with temperature below TC to saturation at about 180 K.11 The 1D ordering of protons between the two stable configurations of O-H-O bonds was thought to be the main reason for this phase transition.13 Because of the relatively simple crystal structure and its second-order phase transition saturates over an unusual wide range in the temperature of about 100 K, which make it a reference material for studying a phase transition at different ordering stages, LHP is a classical system for investigating phase transitions involving ordering of protons in hydrogen-bonded ferroelectrics.14 The nature of the precursor used for the synthesis often plays an important role in determining the shape and size of the nanomaterials. Herein, we demonstrate that well-defined PbHPO4 nanowires can be synthesized on a large scale by using Pb(II)-IP6 (IP6, inositol hexakisphosphate acid) complex as the single-source precursor by a comparatively simple and facile hydrothermal pathway. The Pb(II)-IP6 complex used in the present work is easily synthesizable and air stable. In addition, no capping agents were
10.1021/jp805881e CCC: $40.75 2008 American Chemical Society Published on Web 10/08/2008
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Figure 2. SEM images of the PbHPO4 by hydrothermal treatment at 180 °C for 24 h (a) at low magnification and (b) at high magnification.
Figure 3. TEM images (a-c), HRTEM image (d), and SAED pattern (e) of the PbHPO4 nanowires prepared by hydrothermal treatment at 180 °C for 24 h.
used in the experiments. To the best of our knowledge, this is the first experimental example that PbHPO4 nanowires have been prepared by this simple hydrothermal approach. Experimental Section Materials. Lead nitrate (Pb(NO3)2) and inositol hexakisphosphate acid (IP6, Aldrich, 50 wt% solution in water) were used
as the main reagents. Ethanol and phosphoric acid were also used for the preparation of the PbHPO4 products; all reagents were used without any further purification. Synthesis. The Pb(II)-IP6 compound was prepared as follows, 4 mmol of IP6 and 10 mmol of Pb(NO3)2 were separately dissolved in 30 mL of water, and then the metal salt solution was added drop by drop to the IP6 solution under
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Figure 4. EDS spectrum of the as-prepared PbHPO4 nanowires.
of ferroelectric hysteresis loops was carried out using a RT 66A ferroelectric tester. Results and Discussion
Figure 5. FTIR spectra of the Pb(II)-IP6 compound (a) and pure IP6 (b).
stirring for 24 h, resulting in complete precipitation of the white Pb(II)-IP6 compound. The precipitate was filtered and washed with distilled water and ethanol for three times and then finally dried at 60 °C in air. The hydrothermal synthesis of PbHPO4 nanowires using the Pb(II)-IP6 compound as the single source precursor was carried out by adding 0.3 g of this compound in a mixed solution of 15 mL of ethanol and 0.3 mL of phosphoric acid under stirring. The resulting solution was transferred to a Teflon-lined stainless steel autoclave of 20 mL capacity and sealed. The autoclave was heated to 180 °C and maintained for 24 h and then was allowed to cool to room temperature naturally. The resulting product was filtered, washed with distilled water and absolute alcohol for several times, and dried at 60 °C in air. Characterization. The X-ray diffraction (XRD) patterns of the sample were recorded on a Rigaku/Max-3A X-ray diffractometer with Cu KR radiation (λ ) 1.54056 Å). Transmission electron microscopy (TEM) images, high-resolution transmission electron microscopy (HRTEM) images, and electron diffraction (ED) patterns were taken with a JEOL-2010 microscope operating at 200 kV. Energy-dispersive X-ray spectroscopy (EDS) is attached to the JEOL-2010. Field emission scanning electron microscopy (FE-SEM) images were obtained on a JEOL JSM-6330F apparatus. The photoluminescence (PL) measurement was performed on a FLUOROLOG-3-TAU spectrophotometer. Fourier transform infrared (FTIR) spectra were recorded on a MAGNA-IR 750 FTIR spectrometer. The measurement
The XRD pattern of the obtained PbHPO4 nanowires synthesized by hydrothermal treatment at 180 °C for 24 h is shown in Figure 1. All the diffraction peaks can be readily indexed to the standard diffraction data of the monoclinic phase PbHPO4 with lattice parameters of a ) 4.683 Å, b ) 6.645 Å, and c ) 5.781 Å (JCPDS Card File No. 75-0757). No other characteristic peaks were observed, indicating that the products were pure monoclinic PbHPO4. The diffraction peaks are much broader than those of the standard pattern, indicating the formation of a small size of the crystallites. The morphology and size of the obtained product were examined by scanning electron microscopy (SEM). SEM images shown in Figure 2 indicate that the as-prepared products are mainly make up of lead hydrogen phosphate nanowires with diameters around 30-60 nm and lengths of tens of micrometers. A small portion of nanoflakes can also be observed in the sample. The obtained products were further measured with TEM, HRTEM, and ED. TEM images (parts a-c of Figure 3) further demonstrate that the obtained products have a uniform wirelike morphology. Figure 3d shows a typical HRTEM image taken from an individual PbHPO4 nanowire. Two groups of resolved lattice fringes with d-spacing of 0.465 and 0.664 nm are clearly observed, which match those of the (100) and (010) planes of the monoclinic PbHPO4. The corresponding selected area electron diffraction (SAED) pattern is shown in the Figure 3e. The bright and sharp diffraction spots in the SAED pattern confirm the single-crystal feature of each PbHPO4 nanowire. According to the SAED pattern and HRTEM image, the growth orientation of the PbHPO4 nanowire is determined to be the (010) direction. The EDS analysis of the obtained nanowires shows that the sample is composed of lead, phosphor, and oxygen elements with an atomic ratio of ∼1:1:4 (Figure 4). IP6 contains a 6-C ring with 1 H and 1 phosphate attached to each C. Because of the presence of the six phosphate groups in very close proximity, they can strongly bind various metal ions in the environments to form compounds (IP6 salts).15 After being introduced to the lead nitrate solution, the Pb2+ ions can be attracted by IP6 to form the Pb(II)-IP6 complex. Figure 5 shows the FTIR spectra of the Pb(II)-IP6 complex (Figure 5a) obtained at room temperature and the pure IP6 (Figure 5b), respectively.
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Figure 6. Two possible molecular structures of Pb(II)-IP6 compound.
Figure 7. SEM image of Pb(II)-IP6 compound.
The characteristic infrared absorption bands of Pb(II)-IP6 compound appear in the range from 1200 to 700 cm-1. In comparison to pure IP6, the absorption bands of Pb(II)-IP6 shift slightly to high frequencies. FTIR spectrum of Pb(II)-IP6 also shows two weak absorption bands around 1633 and 3400 cm-1 (data not shown). These two bands did not disappear even after the sample was fully dried. This observation indicates the existence of O-H bonds in the complex, which could be due to the crystalline H2O or unbounded P-O-H bonds.16 Although additional data are needed to illustrate the specific structures of the Pb(II)--IP6 complex, the FTIR results show the increased interactions between Pb2+ ions and P-O bonds in IP6 molecules. Martin et al.17 proposed two Ca(II)-IP6 structures in which a Ca ion coordinates with two O atoms in two separate COPO2 groups, rather than with two O atoms in one single COPO2 group. Similarly, it can be speculated that the possible structures of Pb(II)-IP6 are constructed by the same way, as shown in Figure 6. The morphology and size of the obtained Pb(II)-IP6 compound were also measured by SEM. The SEM image in Figure 7 shows that the Pb(II)-IP6 compound is mainly composed of irregular nanoparticles with the size around 300-500 nm. The Pb(II)-IP6 compound used as the single precursor is necessary for the formation of PbHPO4 nanowires in our experiments. When lead nitrate was used to replace the Pb(II)-IP6 compound, there were almost no nanowires observed in the products (Figure 8). Therefore, it is suggested that the IP6 molecules of the Pb(II)-IP6 compound play a crucial role in the formation of PbHPO4 nanowires, which act as both the Pb source and a structure-directing agent to promote the preferential
one-dimensional growth of PbHPO4 nanowires. Under hydrothermal and acidic conditions, the C-O bonds of the complex could be broken to form PbHPO4, and phosphoric acid and the decomposed products of the used precursor might attach onto the surfaces of small crystallites, thus driving the anisotropic growth of nanowires.18 The detailed formation mechanism of PbHPO4 nanowires needs to be further explored. The electric field-induced polarization of PbHPO4 nanowires was measured at room temperature with a RT 66A ferroelectric tester. As shown in Figure 9, the sample exhibits polarization hysteresis loop, which is a well-known characteristic of ferroelectric materials. It has to be noted that the sample shows a peculiar ferroelectric hysteresis loop compared to other ferroelectric materials. The remnant polarization (PR ) 4.8 µC/cm2) is equal to its maximum polarization (PS), and with voltage increasing, the polarization gradually decreases and eventually becomes zero with the coercive voltage (Vc) of 305 V. It is proposed that the conducting character of the obtained products may be responsible for this different ferroelectric behavior.19 Deep insight into this behavior needs to be further studied. The ferroelectric phase of LHP is monoclinic (space group Pc) and the original cell contains two formula units. The paraelectric phase is monoclinic P2/c, with the Pb and P atoms locating at 2-fold axes and the H atoms locating over the two sites disorderly linked by a center of inversion at the midpoint of the O-H-O bonds (Figure 10). In contrast to KDP-type ferroelectrics, where hydrogen bonds connect each PO43- group with four neighboring groups forming a three-dimensional (3D) structure, the PO43- tetrahedra in LHP are linked into linear by short hydrogen and there are no such bonds between the chains, which indicates that the proton arrangement is 1D rather than 3D in character,19 as clearly seen in Figure 10, this determines the observed ferroelectric property of LHP materials. The excitation spectrum obtained by monitoring the emission of as-prepared PbHPO4 nanowires is shown in Figure 11a. Two energy transition bands appeared at 375 and 270 nm are observed (corresponding energy transition of 3.307 and 4.592 eV, respectively). Upon photo excitation of PbHPO4 nanowires at λex ) 375 nm, the emission spectrum of the sample is presented in Figure 11b, clearly showing a broad emission peak at 460 nm, which is assigned as radiative annihilation of selftrapped excitons.20 This value is a little larger than that reported for bulk PbHPO4 (442 nm),20 implying the dimensionality and size of our sample may be responsible for this difference. Conclusions In summary, using an easily synthesizable and environmentally friendly complex (Pb(II)-IP6) as the single-source precur-
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Figure 8. SEM image and XRD pattern of products by hydrothermal treatment at 180 °C for 24 h, using Pb(NO3)2 to replace Pb(II)-IP6.
Figure 9. Ferroelectric hysteresis loops of as-obtained PbHPO4 nanowires. Figure 11. Excitation (a) and emission (b) spectra of as-prepared PbHPO4 nanowires.
special ferroelectric hysteresis loop. These nanowires should provide an ideal candidate for fundamental studies of nanoscale ferroelectricity, piezoelectricity, and paraelectricity. Acknowledgment. Support from the National Natural Science Foundation of China (20671096) and the special funding support from the Centurial Program of CAS is gratefully acknowledged. We thank Prof. L. Chen for ferroelectric measurements. References and Notes
Figure 10. Structure of the monoclinic PbHPO4 projected onto the ac plane (x and z denote axis of the index ellipsoid).
sor, we have successfully fabricated ferroelectric PbHPO4 nanowires by a simple hydrothermal method. The structure and morphology of the as-obtained PbHPO4 nanowires and the precursor complex were systematically studied by means of XRD, SEM, TEM, FTIR, and so on. The optical and ferroelectric properties of the products were also investigated in detail. PL spectra indicate that under the excitation of 375 nm; the products show a broad emission peak at 460 nm. The ferroelectric hysteresis loop shows that the remnant polarization (PR) is equal to its maximum polarization (PS). Conducting protons in the PbHPO4 crystal structure may be responsible for this
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