Shape-Selective Fabrication of Zinc Phosphate Hexagonal Bipyramids via a Disodium Phosphate-Assisted Sonochemical Route Seung-Ho Jung,† Eugene Oh,† Hanna Lim,‡ Dae-Seob Shim,† Seungho Cho,† Kun-Hong Lee,*,† and Soo-Hwan Jeong*,‡
CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 8 3544–3547
Department of Chemical Engineering, Pohang UniVersity of Science and Technology, Pohang 790-784, Korea, and Department of Chemical Engineering, Kyungpook National UniVersity, Daegu 707-701, Korea ReceiVed March 11, 2009; ReVised Manuscript ReceiVed April 15, 2009
ABSTRACT: Via a simple and facile disodium phosphate-assisted sonochemical route, highly uniform zinc phosphate hexagonal bipyramid crystals were selectively synthesized under ambient conditions. All the facets of hexagonal bipyramids are smooth. The average size (from pole to pole) and the standard deviation were 1.437 µm and 562 nm, respectively. The concentration of HPO42in the aqueous precursor solution is thought to be a key factor in getting hexagonal bipyramidal morphology. This method is simple, fast, economical, and environmentally benign. It is expected that this sonochemical technique can be readily adopted in realizing other forms of various micro/nanostructured materials. Introduction The crystallographic design of various inorganic materials has given an impetus to the development of recent materials science owing to their facet-dependent physical and chemical properties.1,2 Thus, the shape control of inorganic micro/ nanocrystals has attracted considerable attention in recent materials chemistry. As one of the most important multifunctional inorganic materials, zinc phosphate (Zn3(PO4)2) plays an important role in various applications including nontoxic anticorrosive pigments and dental cements due to its low solubility in water/biological environment and biocompatibility.3,4 Various techniques, including a hydrothermal crystallization, a polyol-mediated synthesis, and a solid-state reaction, have been investigated for the synthesis of platelike and spherical zinc phosphate micro/nanocrystals.5-7 A polyol-mediated approach is a widely used technique for producing various nanomaterials, such as metals and oxides.8 While a polyol-mediated method can produce monodispersed and nonagglomerated spherical zinc phosphate nanoparticles, it requires both a heating system due to the relatively high temperature process (up to 300 °C) and an additional cooling system for the refluxing process. Unlike the polyol-mediated approach, a solid-state reaction and a hydrothermal crystallization can produce zinc phosphate micro/ nanocrystals at low temperature (below 100 °C), however, the reaction time required for the growth of zinc phosphate crystals is too long (usually from several hours to several days). Therefore, the development of a simple and fast route to the growth of zinc phosphate micro/nanocrystals with controllable shapes under ambient conditions is very important and still challenging. Recently, a sonochemical approach has been investigated as a promising alternative technique for the fabrication of shapecontrolled inorganic nanocrystals.9,10 The reason is that this sonochemical technique is a simple, fast, and environmentally benign process.11,12 Herein, we report on the shape-selective * To whom correspondence should be addressed. E-mail:
[email protected] (K.-H.L.) and
[email protected] (S.-H.J.). Tel: +82-54-279-2271 (K.-H.L.) and +82-53-950-7597 (S.-H.J.). Fax: +82-54-279-8298 (K.-H.L.) and +82-53-9506615 (S.-H.J.). † Pohang University of Science and Technology. ‡ Kyungpook National University.
fabrication of crystalline zinc phosphate hexagonal bipyramids via a facile disodium phosphate-assisted sonochemical route. Experimental Section Zinc nitrate hexahydrate (Zn(NO3)2 · 6H2O, 98%, Aldrich) and sodium phosphate dibasic anhydrous (also known as disodium phosphate, Na2HPO4, 99%, Junsei) were used as zinc cation and phosphate anion precursors, respectively. For the synthesis of zinc phosphate hexagonal bipyramid crystals, a mixture of 0.08 M sodium phosphate dibasic anhydrous aqueous solution (50 mL) and 0.02 M zinc nitrate hexahydrate aqueous solution (50 mL) was prepared at room temperature. Total concentrations of sodium phosphate dibasic anhydrous and zinc nitrate hexahydrate in the precursor solution were 0.04 and 0.01 M, respectively. Due to the low solubility of zinc phosphate in above aqueous solution (pH ) 7.167), white powders were formed immediately. After sufficient mixing, crystalline zinc phosphate hexagonal bipyramids were obtained via a sonochemical route using above powder-containing aqueous solution. An ultrasonic wave was introduced at an intensity of 39.5 W/cm2 for 20 min by a sonochemical apparatus under ambient conditions. The frequency of the ultrasonic wave was 20 kHz. When the ultrasonic wave was introduced for 20 min, the solution temperature gradually increased and maintained at 80 °C. The zinc phosphate hexagonal bipyramid-containing solution was filtered with a polycarbonate membrane that had pores of 100 nm in diameter. Zinc phosphate hexagonal bipyramids were washed with deionized (DI) water after filtration, and then dried in an oven. The morphology of as-prepared zinc phosphate hexagonal bipyramids was observed by field emission scanning electron microscope (FESEM, Hitachi S-4300SE). The crystallinity, crystal structure, and chemical composition were characterized by X-ray diffraction (XRD, Max Science, M18XHF), Fourier transform infrared (FT-IR) spectroscopy, and transmission electron microscope (TEM, JEOL JEM-2010) with energy-dispersive X-ray (EDX) spectroscopy.
Results and Discussion Figures 1a and 1b show the FESEM images of as-prepared zinc phosphate crystals produced via a disodium phosphateassisted sonochemical route. Zinc phosphate hexagonal bipyramids with different sizes could be clearly observed in Figure 1. All the facets of hexagonal bipyramids are smooth. Due to the large size distribution of as-prepared hexagonal bipyramid particles, we calculated the average size (from pole to pole) by measuring the size of 300 particles. The average size and the standard deviation were 1.437 µm and 562 nm, respectively.
10.1021/cg900287h CCC: $40.75 2009 American Chemical Society Published on Web 06/25/2009
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Figure 1. (a) A SEM image of as-prepared zinc phosphate hexagonal bipyramids. (b) A high-magnification SEM image of an individual hexagonal bipyramid. (c) A low-magnification TEM image of an individual hexagonal bipyramid. (d-f) EDX elemental mapping images for Zn, O, and P of the individual hexagonal bipyramid shown in (c), respectively.
Ostwald ripening is known as an important factor which causes the size distribution of precipitates by the solubility difference due to size-induced energetic stability.13,14 Based on Ostwald ripening, the larger particles continue to grow, while smaller particles continue to shrink. It is thought that the size distribution of zinc phosphate hexagonal bipyramids can be explained by the Ostwald ripening phenomenon. Figure 1c shows a lowmagnification TEM image of an individual zinc phosphate hexagonal bipyramid crystal. The hexagon configuration is the two-dimensional projection image of a single hexagonal bipyramid from pole to pole. With the elemental mapping analysis shown in Figure 1d-f, the EDX spectroscopy confirmed that as-prepared hexagonal bipyramids were composed of Zn, O, and P components only (see Figure S1 of the Supporting Information). The atomic ratio of Zn/P was about 3/2, which corresponds with the Zn3(PO4)2. Figures 2a and 2b show the XRD pattern and the FT-IR spectrum of as-prepared zinc phosphate hexagonal bipyramid crystals, respectively. In the XRD pattern, the seven strong peaks at 2θ values of 23.82°, 27.62°, 29.56°, 30.14°, 32.54°, 32.84°, and 35.20° were thought to be originated from the (112), (141), (250), (051), (015), (213), and (420) crystal faces of zinc phosphate hydrate (Zn3(PO4)2 · xH2O), respectively (JCPDF 331474, 37-0316, and 37-0465). The FT-IR spectrum shown in Figure 2b demonstrated that as-prepared hexagonal bipyramid crystals were zinc phosphate from the reference FT-IR spectrum (see Figure S2 of the Supporting Information) and the previous report.6 The FT-IR spectrum shows characteristic bands related to H2O and PO43-. The broad band centered at 3391 cm-1 is attributed to the O-H stretching vibration. In addition, the strong vibrational band at 1646 cm-1 was attributed to H2O bonding. The broad bands shown at 900-1300 cm-1 (peak at 1044 cm-1) and 400-700 cm-1 were attributed to the complex stretching and bending vibrations of the PO43- group, respectively.15 From the SEM, TEM-EDX, XRD, and FT-IR analyses, it is noted that pure crystalline zinc phosphate hexagonal bipyramids were synthesized under the current disodium phosphate-assisted sonochemical approach. Control experiments were carried out to determine if ultrasonication is essential for producing zinc phosphate hexagonal
Figure 2. (a) A XRD pattern and (b) a FT-IR spectrum of zinc phosphate hexagonal bipyramids.
Figure 3. SEM images of as-prepared zinc phosphate particles without ultrasonication. (a, b) Mixed powders of platelike particles, atypical particles, and a small amount of imperfect hexagonal bipyramid particles produced by vigorous stirring for 20 min without heating. (c, d) Mixed powders of atypical particles and hexagonal bipyramid particles produced by vigorous stirring at 80 °C for 3 h.
bipyramids. For the series of experiments, a mixture of 0.08 M sodium phosphate dibasic anhydrous aqueous solution (50 mL) and 0.02 M zinc nitrate hexahydrate aqueous solution (50 mL) was prepared at room temperature. As shown in Figures 3a and 3b, when the precursor solution was only vigorously stirred for 20 min without ultrasonication and additional heating, mixed powders of platelike particles, atypical particles, and a small amount of imperfect hexagonal bipyramid particles were produced. In Figures 3c and 3d, when the precursor solution was vigorously stirred for 3 h at 80 °C, which is the steady temperature in our ultrasonic reaction, mixed powders of atypical particles and hexagonal bipyramid particles were obtained. We expect that a long reaction time of more than 3 h can be required for the selective production of hexagonal bipyramids via vigorous stirring at 80 °C. The XRD analyses confirmed that
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3Zn2+ + 3HPO42- f 3ZnHPO4 98 Zn3(PO4)2 + H3PO4 (2)
Figure 4. (a) A schematic drawing of the crystal system. (b) Platelike particles synthesized at 0.01 M Na2HPO4. (c) Hexagonal bipyramids synthesized at 0.04 M Na2HPO4. (d) partially dissolved Hexagonal bipyramids in {011} surfaces synthesized at 0.05 M Na2HPO4.
as-prepared zinc phosphate crystals synthesized with or without ultrasonication belonged to the same phase (see Figure S3 of the Supporting Information). A series of experiments showed that ultrasonication played a key role in the shape-selective fabrication of zinc phosphate hexagonal bipyramids. Ultrasound-induced cavitation is known to enhance mass transport at a microscopic level. In addition, sonochemistry involves the radicals generated during the cavitation process or other secondary chemical reactions generated by either fluid motion or the high temperature conditions generated by the cavitation process. It is well-known that cavitation bubbles produced in liquid solution during ultrasonication can instantaneously generate local spots of temperatures of 5000 K and pressures of up to 1800 atm.16-18 Therefore we believe that ultrasound-induced cavitation accelerates the formation reaction of zinc phosphate hexagonal bipyramids. Our disodium phosphate-assisted sonochemical approach provides a facile route to shape-selective fabrication of zinc phosphate hexagonal bipyramid crystals under ambient conditions. Figure 4a shows a schematic drawing of the crystal system. In Figure 4b-d, as the concentration of Na2HPO4 increased from 0.01 to 0.05 M, the shape of the zinc phosphate crystals evolved from platelike crystals to hexagonal bipyramids which show partial dissolution in {011} facets. Except for the difference in molar concentrations of Na2HPO4, three samples were prepared under the same ultrasonication intensity (39.5 W/cm2), reaction time (20 min), and molar concentration of Zn(NO3)2 · 6H2O (0.01 M). In an aqueous solution, Na2HPO4 is known to be ionized to give Na+ and HPO42- ions as follows:
Na2HPO4 f 2Na+ + HPO42-
(1)
In general, HPO42- is the stable form in a neutral pH (pH ) 7.0) solution. Considering the pH of the precursor solution (pH ) 7.2) used in our research, it is thought that HPO42- plays a key role in the formation of hexagonal bipyramids. The formation of zinc hydrogen phosphate (ZnHPO4) at the initial stage and the conversion into zinc phosphate during ultrasonication were considered as follows:5
where the symbol ))) denotes ultrasonic irradiation. Based on the capping effect of phosphate anion (PO43-),19,20 it is thought that HPO42- is bound to the specific facet of the precipitated zinc phosphate particles. Among various facets shown in Figure 4a, the {011} surfaces are more reactive than the {010} surfaces.21 When 0.01 M Na2HPO4 was used, HPO42is thought to be selectively adsorbed on {011} surfaces. As the growth along the 〈011〉 direction is suppressed by HPO42-, zinc phosphate crystal growth mainly proceeds along the 〈010〉 directions to form platelike crystals. Meanwhile, when 0.04 M Na2HPO4 was used, large amount of HPO42- can be generated. In this case, it is thought that all the surfaces are adsorbed by HPO42-. Thus, a hexagonal bipyramid shape could be formed rather than platelike crystals due to higher growth rate along the 〈011〉 directions. As the concentration of Na2HPO4 increases, the amount of HPO42- produced from the hydration of Na2HPO4 also increases. Therefore, OH- is thought to be formed as the HPO42- bond with the H2O, and this increases the overall pH of aqueous solution. When 0.05 M or a higher concentration of Na2HPO4 was used, partially dissolved hexahonal bipyramids in {011} surfaces could be formed rather than perfect hexagonal bipyramids due to the localized dissolution process of metastable {011} surfaces. The local dissolution was considered as follows:22
HPO42- + H2O f H2PO4- + OH-
(3)
)))
Zn3(PO4)2 + 4OH- 98 2Zn2+ + 2PO43- + Zn(OH)42(4) A series of experiments was conducted to investigate the effect of reaction time on the formation of zinc phosphate hexagonal bipyramids (see Figure S4 of the Supporting Information). We expect that the growth of hexagonal bipyramids is governed by a dissolution-recrystallization growth mechanism.23 Along with the dissolution-recrystallization mechanism, it is thought that the initial precipitated particles started to dissolve into the solution and grow onto large particles of zinc phosphate, and hexagonal bipyramids were produced. We found that this growth mechanism is strongly affected by concentrations of Na2HPO4 as shown in Figure 4. Control experiments were also carried out to investigate the anion effect on the formation of zinc phosphate hexagonal bipyramids. Monobasic sodium phosphate (NaH2PO4) was used instead of Na2HPO4. NaH2PO4 is known to be ionized to give Na+ and H2PO4- ions. As shown in Figure 5, bar-type zinc phosphate crystals were synthesized in the presence of H2PO4ions. From these control experiments, we believe that HPO42anions play a key role in the formation of hexagonal bipyramidal morphology. Conclusions The disodium phosphate-assisted sonochemical synthesis of crystalline zinc phosphate hexagonal bipyramids under ambient conditions has been demonstrated. It is thought that ultrasonication both provides the required energy to synthesize zinc phosphate hexagonal bipyramids and accelerates the formation reaction. The average size from pole to pole and the standard
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References
Figure 5. SEM images of bar-type zinc phosphate crystals synthesized by using (a) 0.01 M NaH2PO4 and (b) 0.04 M NaH2PO4.
deviation were 1.437 µm and 562 nm, respectively. This method is simple, fast, economical, and environmentally benign. The concentration of HPO42- in the aqueous precursor solution is thought to be a key factor in getting hexagonal bipyramidal morphology. It is expected that this sonochemical technique can be readily adopted in realizing other forms of various micro/ nanostructured materials. Acknowledgment. This work was supported by the both National Center for Nanomaterials Technology (NCNT) of the Ministry of Commerce, Industry, and Energy (MOCIE) and second phase BK21 program of the Korean Ministry of Education, Science and Technology. Supporting Information Available: TEM-EDX characterization of zinc phosphate hexagonal bipyramids, reference FT-IR spectrum of zinc phosphate, and XRD and SEM analyses of zinc phosphate crystals. This material is available free of charge via the Internet at http:// pubs.acs.org.
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CG900287H
