CRYSTAL GROWTH & DESIGN
Synthesis of PbCrO4 and Pb2CrO5 Rods via a Microwave-Assisted Ionic Liquid Method Wei-Wei Wang and Ying-Jie Zhu* State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Graduate School of the Chinese Academy of Sciences, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China Received July 19, 2004;
2005 VOL. 5, NO. 2 505-507
Revised Manuscript Received November 22, 2004
ABSTRACT: Lead chromate (PbCrO4) rods and dilead pentaoxochromate (Pb2CrO5) with bundle-like and rod-like morphologies have been successfully synthesized by a simple microwave-assisted ionic liquid method. In the presence of NaOH, single-crystalline Pb2CrO5 bundles and rods could be synthesized by microwave heating a solution containing lead acetate and potassium dichromate at 90 °C for 10 min. However, single-crystalline PbCrO4 rods could be prepared by microwave heating at 50 °C for 10 min without using NaOH. The single-crystalline Pb2CrO5 and PbCrO4 samples prepared were unstable and tended to become polycrystalline under electron beam irradiation. Introduction Room-temperature ionic liquids (RTILs) have aroused increasing interest in recent years because of their unique properties and the potential applications in various fields.1-4 For example, due to their thermal stability and no measurable vapor pressure, RTILs are actively being explored as possible “green” solvents to substitute conventional volatile organic solvents in a variety of processes, including industrially important chemical processes.5-7 There has been a recent report that the use of RTILs in industrial production of alkoxyphenylphosphine leads to an increase in productivity by a factor of 80000 compared with the conventional process.8 Recently, the advantages of RTILs in the synthesis of nanoparticles9-12 and TiO2 hollow microspheres13 have been demonstrated. However, these nanoparticles prepared in ionic liquids were spherical instead of rod-like in shape. Lead chromate (PbCrO4) is an important solid material that is widely used as a photosensitizer and yellow pigment. Dilead pentaoxochromate (Pb2CrO5) with a monoclinic c2/m structure has a large absorption coefficient and a high-speed photoresponse in the visible region of the spectrum and therefore can be used as a potential candidate for applications in photoconductors, optoelectronic devices, and reversible thermochromism materials.14 Due to their potential applications, the synthesis of PbCrO4 and Pb2CrO5 crystals with wellcontrolled size and shape is of great significance. There have been few reports on the preparation of lead chromate. Spherical PbCrO4 nanoparticles were prepared by the microemulsion method.15 PbCrO4 rods and Pb2CrO5 microparticles were synthesized by hydrothermally treating a mixture of Pb(Ac)2, K2Cr2O7, and poly(vinyl pyrrolidone) (PVP) aqueous solution at 140 °C for 20 h.16 PbCrO4 nanowires were synthesized at room temperature by the reaction of Pb(NO3)2 and K2CrO4 in aqueous solution in the presence of PVP (Mw ≈ 55000), and PbCrO4 nanowires together with the mother solution could be hydrothermally transformed * Corresponding author. E-mail:
[email protected]. Phone: +86-21-52412616. Fax: +86-21-52413122.
into amorphous PbCr3O10 nanotubes.17 Recently, by combining the advantages of both RTILs and microwave heating we have developed a new microwave-assisted ionic liquid (MAIL) method for the preparation of tellurium nanorods and nanowires.18 RTILs are very good media for absorbing microwaves due to the presence of larger positive ions with high polarizabilities. Here we demonstrate the successful synthesis of PbCrO4 rods and Pb2CrO5 bundles and rods by the simple, fast, and surfactant-free MAIL method. Experimental Procedures Chemicals. Lead acetate (Pb(CH3COO)2‚3H2O), 1-n-butyl3-methyl imidazolium tetrafluoroborate ([BMIM][BF4]), sodium hydroxide (NaOH), and potassium dichromate (K2Cr2O7) were all of analytical grade and were purchased and used without further purification. Synthesis. In a typical synthesis of Pb2CrO5, Pb(CH3COO)2‚ 3H2O was dissolved into 0.5 mL of [BMIM][BF4] to form a 0.03 M solution. Then 0.8 mL of 0.8 M aqueous NaOH and 0.2 mL of 0.12 M K2Cr2O7 aqueous solution were added, respectively, at room temperature with magnetic stirring. The mixture was microwave-heated at 90 °C for 10 min and cooled to room temperature. The red product (sample 1) was separated by centrifugation, washed with absolute ethanol two times, and dried in air at room temperature. Without using NaOH solution, orange yellow precipitate (sample 2) was obtained after microwave heating at 50 °C for 10 min. Instruments and Characterization. The microwave oven (2.45 GHz, maximum power of 300 W) used for sample preparation was a focused single-mode microwave synthesis system (Discover, CEM, USA). X-ray powder diffraction (XRD) measurements were performed on a Huber G 670 (German) X-ray diffractometer using graphite-monochromatized highintensity Cu KR1 radiation (λ ) 1.5405981 Å). The transmission electron microscopy (TEM) images and the selected-area electron diffraction (SAED) patterns were taken with a Hitachi H-800 electron microscope with an accelerating voltage of 175 kV. The optical microscopy (OM) images were taken with a BM-12 optical microscope (Shanghai Optical Instruments Factory).
Results and Discussion The XRD patterns of two typical as-synthesized samples are shown in Figure 1. As shown in Figure 1a (sample 1), the XRD pattern can be indexed to the single
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Figure 1. XRD patterns of (a) Pb2CrO5 (sample 1) and (b) PbCrO4 (sample 2).
phase of Pb2CrO5 with the monoclinic structure (space group, C2/m (no. 12); JCPDS No. 84-0678). However, without using NaOH, a different phase was obtained as shown in Figure 1b (sample 2), which can be indexed to the single phase of well-crystallized PbCrO 4 with the monoclinic structure (space group, P21/n (no. 14); JCPDS No. 73-2059). These results show that OH- in the solution has an influence on the phase formation of Pb2CrO5 and PbCrO4. The morphologies of the samples were investigated by TEM and OM. From the OM image of sample 1 (Figure 2a), one can see the bundles of Pb2CrO5 rods, looking like sheaves of straw tied in the middle (as indicated by arrows in Figure 2a). Pb2CrO5 bundles and
Wang and Zhu
rods were sensitive to electron beam irradiation. Pb2CrO5 bundles changed rapidly under electron beam irradiation. Therefore, we failed to obtain TEM micrographs of the bundles. In addition to the bundle-like morphology, half-bundle-like and rod-like morphologies were also observed in sample 1. The majority of Pb2CrO5 structures were bundles, and the minority were rods. Figure 2b shows a single Pb2CrO5 rod with a diameter of ∼500 nm and a length of ∼2.3 µm, and its corresponding SAED pattern (the inset of Figure 2b) indicates the rod is single-crystalline. Figure 2c shows a half-bundle of Pb2CrO5 rods with diameters up to ∼500 nm and lengths up to ∼12.5 µm. The SAED pattern of an individual rod in the bundle (the inset of Figure 2c) shows the single-crystalline structure of the rod. Similar bundle-like morphologies of Sb2S3 have also been reported. For example, Sb2S3 bundles were synthesized via thermal decomposition of an antimony(III) diethyldithiocarbamate complex in ethylene glycol at 290 °C for 90 min19 or in N,N-dimethylformamide (DMF) under microwave heating with a reflux system for 20 min,20 or from the precursor Sb(S2CNEt2)3 via hydrothermal treatment in deionized water at 115 °C for 10 h.21 However, to the best of our knowledge, the bundle-like morphologies of Pb2CrO5 have not been reported yet. The detailed growth mechanism of Pb2CrO5 bundles under microwave heating needs to be further investigated. In contrast, PbCrO4 rods were obtained without using NaOH (sample 2). The lengths and diameters of PbCrO4 rods range from ∼1.8 to ∼4 µm and from ∼80 to ∼300 nm, respectively (Figure 3a-c). Some rods grow parallel to each other (Figure 3a,b). Each rod is straight and has a uniform diameter along its entire length. The two ends of PbCrO4 rods are inclined planes and parallel to each other, which is different from rods prepared by other
Figure 2. (a) OM image and (b-e) TEM micrographs of sample 1. The corresponding SAED patterns are shown in the insets of images b and c; (d,e) TEM micrographs of the rod and the bundle taken after exposure to electron beam irradiation, respectively.
Synthesis of PbCrO4 and Pb2CrO5 Rods
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by the MAIL method. This method is a fast and surfactant-free route. In the presence of NaOH, Pb2CrO5 bundles and rods could be synthesized. PbCrO4 rods could be synthesized in the absence of NaOH. The single-crystalline Pb2CrO5 and PbCrO4 rods prepared were unstable and tended to become polycrystalline under electron beam irradiation. We expect that this new method may also be employed to prepare many other kinds of materials. Related research is in progress. Acknowledgment. Financial support from the Chinese Academy of Sciences under the Program for Recruiting Outstanding Overseas Chinese (Hundred Talents Program) and from the National Natural Science Foundation of China is gratefully acknowledged. We thank the Fund for Innovation Research from the Shanghai Institute of Ceramics, Chinese Academy of Sciences, and the fund from the Shanghai Natural Science Foundation. We also thank Professor Jingtai Zhao and Haohong Chen and Professor Junlin Yin for assistance in the TEM experiments. References
Figure 3. (a-d) TEM micrographs of sample 2. (d) TEM micrograph of the PbCrO4 rod taken after exposure to electron beam irradiation. The corresponding SAED patterns are shown in the insets of images c and d.
methods.16,17 Figure 3c shows a typical single rod and its corresponding SAED pattern (inset of Figure 3c). The SAED patterns taken from different positions along a randomly selected single rod were the same, indicating that these rods were single-crystalline. We also found that PbCrO4 rods and Pb2CrO5 bundles and rods are not stable under electron beam irradiation. Both Pb2CrO5 bundles and rods changed from singlecrystalline rods to many small particles after exposure to an electron beam (Figure 2d,e). Figure 3d shows that after exposure to electron beam irradiation, the singlecrystalline PbCrO4 rod changed to become many tiny particles, and the SAED pattern (the inset of Figure 3d) shows the polycrystalline nature of these particles. A similar phenomenon induced by electron beam irradiation was not reported for PbCrO4 rods prepared in aqueous solution by other methods.16,17 Other researchers have also reported that some 1-D structures changed under electron beam irradiation. For example, Ag6Mo10O33 rods have been found to be unstable under electron beam irradiation.22 Many tiny nanoparticles appeared on the Ag6Mo10O33 rod surface, and these particles tended to become amorphous after exposure to the electron beam. Bi nanotubes could transform into polycrystalline nanowires under electron beam irradiation and then into small liquid droplets with further intense beam irradiation.23 Conclusion In summary, we have demonstrated successful synthesis of PbCrO4 rods and Pb2CrO5 bundles and rods
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