Surfactant-Assisted Hydrothermal Synthesis and Magnetic Properties

Publication Date (Web): August 13, 2008 ... Abstract. Three-dimensional (3D) urchin-like MnWO4 microspheres with a diameter of ca. 1−1.2 μm assembl...
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J. Phys. Chem. C 2008, 112, 13383–13389

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Surfactant-Assisted Hydrothermal Synthesis and Magnetic Properties of Urchin-like MnWO4 Microspheres Yu-Xue Zhou, Qiao Zhang, Jun-Yan Gong, and Shu-Hong Yu* DiVision of Nanomaterials & Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, Department of Chemistry, UniVersity of Science and Technology of China, Hefei 230026, People’s Republic of China ReceiVed: May 13, 2008; ReVised Manuscript ReceiVed: June 22, 2008

Three-dimensional (3D) urchin-like MnWO4 microspheres with a diameter of ca. 1-1.2 µm assembled by nanorods with a length of 240 nm and an aspect ratio of ca. 9 have been fabricated by a cationic surfactant cetyltrimethyl ammonium bromide (CTAB) assisted hydrothermal method. The result demonstrated that CTAB played an important role as a soft template in directing growth and self-assembly of urchin-like MnWO4 microspheres, and suitable pH values and reaction temperature are also essential for the formation of urchinlike microspheres. Magnetic measurement indicates that urchin-like MnWO4 microspheres show a weak ferromagnetic ordering at low temperature due to spin-canting and surface spins of microspheres, while much shorter MnWO4 nanorods show antiferromagnetism at low temperature. 1. Introduction Design of highly ordered superstructures with specific morphologies and structures assembled by low-dimensional inorganic nanostructures is of great interest to chemists and materials scientists due to their novel properties and fundamental significance in various application fields.1 Recently, extensive work has been devoted to the investigation of effective and efficient methodologies to synthesize functional materials with complex three-dimensional (3D) nanostructures,2 such as Fe2O3 flowerlike spheres with better adsorption performance on water treatment,3 cactus-like β-Ga2O3 nanostructures with good field emission activity,4 hollow Fe3O4 spheres with excellent superparamagnetic character,5 and hollow V2O5 spheres with high capacity and remarkable reversibility in lithium batteries.6 It was well-known that MnWO4 is a very promising material in humidity sensors7 and shows potential application in the field of multiferroic materials8 due to its high sensitivity to humidity changes and its unique magnetic properties. It has been demonstrated that MnWO4 crystals can be generally synthesized by recrystallization or chemical reaction in a molten salt solution,9 grinding in a vibrating mill,10 and high-temperature thermal treatment processes11 in the past. Recently, MnWO4 nanorods, nanofibers, nanoplates, and nanocrystals have been prepared by hydrothermal or solvothermal process in solution.12 However, novel three-dimensional urchin-like MnWO4 microspheres assembled by nanorods and their magnetic properties have not been reported so far. Herein, we report an effective cationic surfactant cetyltrimethylammonium bromide (CTAB) assisted hydrothermal method to synthesize three-dimensional MnWO4 urchin-like microspheres self-assembled by nanorods using MnCl2 · 6H2O and Na2WO4 · 2H2O as raw materials. The cationic surfactant CTAB played a key role as a structure-directing agent in driving selfassembled MnWO4 urchin-like microspheres composed of nanorods. The pH value and reaction temperature also have significant influence on the morphology of particles. The * Corresponding author. Fax: + 86 551 3603040. E-mail: shyu@ ustc.edu.cn.

magnetic properties of MnWO4 microspheres and nanorods have been investigated. 2. Experimental Section All chemicals were analytical grade and were commercially available from Shanghai Chemical Reagent Co. Ltd. and used without further purification. 2.1. Synthesis of Self-Assembled Urchin-like MnWO4 Microspheres. The 3D urchin-like MnWO4 microspheres were synthesized by a cationic surfactant cetyltrimethylammonium bromide (CTAB)-assisted hydrothermal method. In a typical experiment, MnCl2 · 6H2O (0.8 mmol), Na2WO4 · 2H2O (0.8 mmol), and cetyltrimethylammonium bromide (CTAB, 0.2 mmol) were respectively dissolved in 8 mL of water. Then, the MnCl2 · 6H2O solution and the Na2WO4 · 2H2O solution were dropwise added to the CTAB solution. The pH value of the reaction solution was adjusted to a certain amount with dilute NaOH (1 M) solution and then stirred for 5 min. Finally, the mixture was transferred into a Teflon-lined autoclave with a volume of 32 mL. The autoclave was sealed and maintained at 180 °C, which was done in a digital-type temperature-controlled oven, then cooled to room temperature naturally. The obtained brown product was filtered out and washed with absolute ethanol and distilled water several times to remove impurities and dried at 60 °C for 4 h. 2.2. Characterization. The obtained samples were characterized on a (Philips X’Pert Pro Super) X-ray powder diffractometer with Cu KR radiation (λ ) 1.541874 Å). The morphology was examined with a JEOL JSM-6700F scanning electron microscope (SEM), and a transmission electron microscope (TEM) performed on a Hitachi (Tokyo, Japan) H-800 transmission electron microscope (TEM) at an accelerating voltage of 200 kV, and a high-resolution transmission electron microscope (HRTEM) (JEOL-2010) operated at an acceleration voltage of 200 kV. Energy-dispersive X-ray (EDX) analysis was obtained with an EDAX detector installed on the same HRTEM. The X-ray photoelectron spectra (XPS) were collected on an ESCALab MKII X-ray photoelectron spectrometer, using nonmonochromatized Mg KR X-ray as the excitation source. FTIR

10.1021/jp804211w CCC: $40.75  2008 American Chemical Society Published on Web 08/13/2008

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Figure 1. XRD pattern of 3D urchin-typed MnWO4 microspheres prepared with CTAB at 180 °C for 3 h, pH 10. [CTAB] ) 0.0067 mol L-1, [Na2WO4] ) [MnCl2] ) 0.0333 mol L-1.

spectra were measured on a Bruker Vector-22 FT-IR spectrometer from 4000 to 400 cm-1 at room temperature. Thermogravimetric analysis (TGA) was carried out on a TGA-50 thermal analyzer (Shimadzu Corporation) at a heating rate of 10 deg min-1 in flowing air. The magnetic measurements on powdered samples enclosed in a medical cap were carried out with a commercial superconducting quantum interference device (SQUID) magnetometer (MPMS-XL, Quantum Design Corp). The magnetization was measured under both zero field cooling (ZFC) and field cooling (FC) processes from 4 to 50 K under an applied field of 100 Oe. Magnetic-Hysteresis loops were measured at 10 and 50 K separately under magnetic field up to 2 T. 3. Results and Discussion 3.1. Self-Assembled Urchin-like MnWO4 Microspheres. Uniform 3D urchin-like MnWO4 microspheres constructed by nanorods were synthesized directly by hydrothermal treatment at 180 °C at pH 10 for 3 h. The XRD pattern of urchin-typed MnWO4 microspheres is shown in Figure 1. All the reflection peaks can be indexed to the pure, well crystalline, monoclinic structure of MnWO4 with lattice parameters of a ) 4.80 Å, b ) 5.71 Å, c ) 4.97 Å, and β ) 91.22°, which are consistent with reported values (Joint Committee on Powder Diffraction Standards (JCPDS), Powder Diffraction File No: 80-0134). Figure 2a shows a general SEM image of a typical sample composed of abundant uniform 3D urchin-like microspheres with diameters of ca. 1-1.2 µm. The as-obtained urchin-like microsphere assemblies could not be destroyed and broken into discrete individual nanorods even by subjecting their aqueous suspension to ultrasonication for much longer time, indicating that the microspheres were not a random aggregate of nanorods but the ordered self-assemble of themselves. The magnified SEM images of the urchin-like microstructures are shown in Figure 2b,c, which demonstrated that each microsphere was composed of abundant nanorods with a length of 240 nm and an aspect ratio of ca. 9. The nanorods were self-assembled in a radial way to form the 3D urchin-like microspheres. Panels a and b of Figure 3 show the bright-field TEM images of the microspheres shown in Figure 2. It was found that almost entire nanorods are assembled in a radial form from the center to the surface of spheres which looks like a sea urchin as a whole. Figure 3c shows a typical nanorod with a smooth surface and a uniform diameter along its entire backbone. The high-

Zhou et al. resolution TEM image recorded from the middle part of an individual nanorod marked by a white rectangle (Figure 3c) clearly shows that the spacing between any two adjacent lattice fringes is 4.9 Å, corresponding to that of the (100) planes of the monoclinic MnWO4 phase, and the growth direction is revealed to be the [100] direction. The inset in Figure 3c is the corresponding SAED pattern of the nanorod taken along the [010] zone axis. Both the HRTEM image and the SAED pattern show the single-crystalline nature of the MnWO4 nanorods. The XPS spectrum in Figure 4a indicated that the main peak values at 34.55, 284.61, 532.97, and 641.45 eV can be assigned readily to the binding energies of W4f, C1s, O1s, and Mn2p, respectively. All other peaks were identified separately, corresponding to Mn, W, and O elements of MnWO4 and the N element from a small amount of unwashed CTAB with very weak signal. Therefore, the XPS results confirmed that the selfassembled urchin-like microspheres were composed of pure phase MnWO4, which is in agreement with the results of EDS analysis as shown in Figure 4b. 3.2. Formation Mechanism. To explore the formation mechanism of 3D self-assembled urchin-like MnWO4 microspheres, a series of time-dependent experiments were performed. Only aggregates of nanoparticles could be observed before heat treatment (Figure 5a); however, flower-like incipient nanostructures come into being after hydrothermal treatment for 30 min (Figure 5b). While the reaction time was prolonged to 1 h, inhomogeneous urchin-like microspheres with different sizes assembled by shorter nanorods with length of ca. 180 nm became the predominant shaped particles (Figure 5c). After the reaction was prolonged to 3 h, urchin-like 3D microspheres formed (Figure 2a). It is well-known that CTAB is a cationic surfactant and CTAB molecules in solution have been proved to be a versatile “soft template” by self-assembling to form different conformations and lead to the formation of different nanostructures of functional materials.13 Recently, ZnO flower-like and tubular nanostructures, porous magnetic hollow silica nanospheres, SrCO3 whiskers, and WS2 nanotubes14 have been fabricated by using CTAB as a shape controller and stabilizer. In fact, CTAB plays the same role as a “soft template” in the present solution reaction system. The above time-dependent experiment suggests that the growth of 3D urchin-like MnWO4 microspheres goes through the Ostwald ripening process in aqueous solution. A similar formation mechanism has been reported in the synthesis of 3D nanorods assembled ZnO urchin-type and nanobelts assembled Cu2O flower-like architectures.14a,15 As a cationic surfactant, CTAB is an ionic compound that ionized completely in water: quantities of anions OH-, WO42- and cations CTA+, Mn2+ existed in strongly alkaline reaction solution. Therefore, cooperative self-assembly between ionic CTAB molecules and charged species is built via electrostatic interaction in reaction solution.14b,16 The formation of nearly spherical aggregates of nanoparticles after hydrothermal treatment for 30 min may be brought from the strong electrostatic attraction between positive CTA+ cations and negative OH- anions on the surface of nanoparticles as well as the hydrophobic interactions and van der Waals attraction caused by adjacent CTAB adsorbing on MnWO4 colloid nanoparticles. Furthermore, although there was an intrinsic tendency for nucleation growth along the 1D direction because of the anisotropic crystal structure of MnWO4 nanoparticles in strong alkaline solution,12a nanorods selfassembled into radial architectures to minimize surface energy due to CTAB adsorption on certain facets of crystals by reaction for 1 h. Finally, uniform urchin-like MnWO4 microspheres self-

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Figure 2. FESEM images of urchin-like MnWO4 microspheres prepared by hydrothermal process with CTAB as surfactant at 180 °C for 3 h, at pH 10: (a) a general review and (b, c) higher magnification images. [CTAB] ) 0.0067 mol L-1, [Na2WO4] ) [MnCl2] ) 0.0333 mol L-1.

Figure 3. (a, b) TEM images of urchin-like microspheres. (c) A typical nanorod. The insert shows the selected area electron diffraction pattern (SAED). (d) HRTEM image taken on the typical nanorod. The sample was synthesized at 180 °C for 3 h, pH 10. [CTAB] ) 0.0067 mol L-1, [Na2WO4] ) [MnCl2] ) 0.0333 mol L-1.

Figure 4. (a) XPS spectrum and (b) EDS spectrum of the obtained self-assembled urchin-like microspheres. The peaks of Cu originate from the Cu grid.

assembled by radial nanorods from the center were precipitated after the reaction time was prolonged to 3 h. There were still some CTAB molecules remaining in the final urchin-like microspheres prepared after reaction for 3 h although

no typical peaks of CTAB in its corresponding XRD pattern were observed (Figure 1), which can be inferred from the FTIR spectra and TG curve of the sample shown in Figure 6. The IR-absorption bands at 2920 and 2850 cm-1 in the MnWO4

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Figure 5. TEM images of the products prepared at 180 °C after reaction for different reaction times, pH 10: (a) 0 min, (b) 30 min, and (c) 1 h. [CTAB] ) 0.0067 mol L-1, [Na2WO4] ) [MnCl2] ) 0.0333 mol L-1.

Figure 6. (a) FT-IR spectra and (b) TG curve of urchin-like MnWO4 microspheres after reaction for 3 h at 180 °C, pH 10. [CTAB] ) 0.0067 mol L-1, [Na2WO4] ) [MnCl2] ) 0.0333 mol L-1.

Figure 7. TEM image of samples aged at 180 °C for 3 h, pH 10: (a) without adding CTAB and (b) [CTAB] ) 0.0017 mol L-1. [Na2WO4] ) [MnCl2] ) 0.0333 mol L-1.

urchin-like microspheres can be attributed to the characteristic frequencies of residual CTAB (Figure 6a),17 which is further proved by TG measurement (Figure 6b). The TG analysis results showed that the urchin-like microspheres lose H2O at 80 °C; the mass loss between 188 and 700 °C was ascribed to the decomposition and desorption of CTAB molecules. The amount of CTAB in the sample is only about 1.5 wt %. And the characteristic peaks of CTAB disappeared in FTIR spectra of sample after TGA measurement, in accord with TGA analysis results (see Figure S1 in the Supporting Information). In the absence of CTAB, disperse MnWO4 nanorods were obtained (Figure 7a), which is consistent with that reported previously.12a However, inhomogenous urchin-like nanostructures with small diameters assembled by disordered nanorods were obtained while the concentration of CTAB is 0.0017 mol L-1 (Figure 7b). The XRD patterns of disperse MnWO4 nanorods and inhomogenous urchin-like nanostructures can be indexed as the pure, well crystalline, and monoclinic

structure of MnWO4 (Powder Diffraction File No: 80-0134, see Figure S2 in the Supporting Information), which is the same as the pattern of urchin-like MnWO4 microspheres shown in Figure 1. In addition, a suitable pH value is another key factor in modifying the morphology of MnWO4 particles. The morphologies of MnWO4 nanostructures were changed at different pH values of reaction solution with dropping of NaOH dilute solution (1 M) while other reaction conditions were kept constant. Short MnWO4 nanorods with a uniform length of ca. 40 nm and an aspect ratio of ca. 1.6 at pH 7 come forth (Figure 8a) and longer MnWO4 nanorods with a length of ca. 130 nm and an aspect ratio of ca. 5 happened at pH 8 (Figure 8b). However, no urchin-like MnWO4 microspheres were obtained when the pH values were lower than 9. Urchin-like MnWO4 microspheres with a diameter of ca. 1 µm assembled by much shorter MnWO4 nanorods with a length of ca. 140 nm and an aspect ratio of ca. 5.5 were prepared at pH 9 (Figure 8c). While

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Figure 8. TEM images of the products obtained after being aged at 180 °C, for 3 h at pH (a) 7, (b) 8, (c) 9, and (d) 12. [CTAB] ) 0.0067 mol L-1, [MnCl2] ) [Na2WO4] ) 0.0333 mol L-1.

Figure 9. FESEM images of the sample prepared after aging at 160 °C for 3 h, at pH 10: (a) lower magnification and (b) higher magnification. [CTAB] ) 0.0067 mol L-1, [Na2WO4] ) [MnCl2] ) 0.0333 mol L-1.

the pH value increased to 12, relatively larger urchin-like microspheres than the particles prepared at pH 10 with diameters of 1.2 µm were obtained, which are assembled by much longer MnWO4 nanorods with a length of ca. 260 nm (Figure 8d). The XRD patterns of products obtained at different pH values can be identified as the pure and monoclinic structure of MnWO4 (Powder Diffraction File No: 80-0134, see Figure S3 of the Supporting Information). Therefore, higher pH values than 9 are essential for the formation of self-assembled urchin-like MnWO4 microspheres. It can be explained that when there were a small quantity of OH- anions at pH 7 and not enough OHanions at pH 8 on the surface of colloid nanoparticles in aqueous solution, the electrostatic attraction between the CTA+ cations and OH- anions is thus too weak to drive the self-assembly of nanoparticles and therefore random nanorods were obtained even though the amount of CTAB is enough in aqueous solution. While the pH value was higher than 9, stronger electrostatic attraction between CTA+ cations and more OH- anions outside the colloid surface directed the fabrication of MnWO4 microspheres with the increase of length of nanorods and increasing diameters of urchin-like microspheres. While the reaction temperature was lowered to 160 °C, MnWO4 microspheres with different diameters, which are constructed by much shorter nanorods, can be obtained and the nanorods aggregated without radial arrays as shown in Figure 9, the structure and morphology of the sample were a little different from that of the sample precipitated at 180 °C under the same condition. Therefore, the appropriate reaction temperature is necessary for formation of uniform radial 3D urchinlike MnWO4 microspheres. The XRD pattern of the product obtained at 160 °C after 3 h can also be indexed as pure phased MnWO4 (Powder Diffraction File No: 80-0134, see Figure S4 in the Supporting Information).

SCHEME 1: Schematic illustration of the growth process of urchin-like MnWO4 microspheres

On the basis of the above experimental observation, a schematic illustration of the mechanism is speculated as shown in Scheme 1. At the early stage of reaction, many nanoparticles with different sizes appear in the solution as shown in Scheme 1a; due to the electrostatic interaction between positive CTA+ cations and negative colloid nanoparticles coated by abundant anions, especially OH- anions, as well as the hydrophobic interactions and van der Waals attraction of CTAB molecules, the nanoparticles will be assembled together and formed a flower-like structure with CTAB (Scheme 1b); and with the reaction proceeding continuously, assembled nanoparticles will grow into nanorods according to the Ostwald ripening mechanism and urchin-like microspheres were obtained (Scheme 1c). 3.3. Magnetic Properties of Self-Assembled Urchin-like MnWO4 Microspheres. The magnetic properties of urchinlike MnWO4 microspheres obtained at pH 10 self-assembled by anisotropic nanorods, scattered MnWO4 nanorods generated by sonication of the microspheres prepared at pH 10, and shorter MnWO4 nanorods prepared at pH 7 were investigated by using a commercial superconducting quantum interference device (SQUID) magnetometer (Quantum Design, MPMS5XL). The temperature-dependent magnetization of the three samples between 4 and 50 K was measured under zero field cooling

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Figure 10. Temperature dependence of magnetization measured with an applied field of 100 Oe for samples under FC and ZFC process and magnetization-hysteresis (M-H) loops of MnWO4 microspheres. (a and b) M-T curves and M-H loops of urchin-typed MnWO4 microspheres. The inset in panel b shows an enlarged area of the center of the M-H loops. (c) M-T curves of scatterd MnWO4 nanorods produced by sonication of the corresponding microspheres. (d) M-T curves of shorter nanorods obtained at pH 7. The insert is the χmT-T curves of shorter nanorods.

(ZFC) and field cooling (FC) processes in the applied field of 100 Oe, and the field dependence of magnetization curves of urchin-like microspheres were measured from 0-2 T and -2 T-0 applied field. The magnetization curves as a function of temperature of urchin-like microspheres are shown in Figure 10a, and it can be seen that the magnetization initially shows a much slower increase with decreasing temperature from the beginning at 50 K and then shows a notable increase subsequently, which can be inferred from the FC magnetization curve, while the magnetization in the ZFC process increased monotonously to the maximum at 19.5 K with increasing temperature at low temperature and then decreased with increasing temperature; there was an irreversible divergence between FC and ZFC curves at low temperatures, and therefore it may be speculated that a weak ferromagnetism exists in MnWO4 microspheres at low temperatures. The field dependence of the magnetization curve of MnWO4 microspheres measured at 10 K lower than 19.5 K in Figure 10b displaying a weak hysteresis with coercive force of 403 Oe and remnant magnetization of 0.093 emu/g further reveals a weak ferromagnetism of sample in accordance with the results of FC and ZFC curves shown in Figure 10a, while the M-H curve obtained at 50 K shows no hysteresis, which demonstrated the paramagnetic property of the MnWO4 microspheres at much higher temperature. It was reported that the MnWO4 crystal with magnetic cation Mn2+ belongs to antiferromagnetic materials,8a,18 weak ferromagnetism of MnWO4

microspheres at low temperature probably being due to the spincanting behavior of antiferromagnetic materials19 and uncompensated number of surface spins of MnWO4 nanorods which orderly aggregated into microspheres.20 To prove the crucial role of MnWO4 nanorods in the contribution of magnetic property of MnWO4 microspheres, the M-T curves of scattered MnWO4 nanorods generated by sonication from the microspheres were also measured as shown in Figure 10c, which indicates nearly the same trends as the ZFC and FC curves of their corresponding microspheres except for a little higher tuning point temperature at 22.0 K, and may be assigned to the disordered MnWO4 nanorods compared to relatively ordered aggregated nanorods. However, the ZFC and FC curves of shorter MnWO4 nanorods with lengths of 40 nm prepared at pH 7 (Figure 10d) show the antiferromagnetic property of the sample at temperatures lower than 12.5 K and the paramagnetic property of the sample at temperatures higher than 12.5 K, therefore the TN (Neel temperature) of shorter nanorods is 12.5 K, which is nearly the same as the reported second antiferromagnetic phase transfer temperature of MnWO4 obtained by another synthesis method.19 The χmT-T curve shown in the insert of Figure 10d indicated that the value of χmT decreased in a nearly straight line with decreasing temperature, which further proves the antiferromagnetic properties of shorter nanorods at low temperature. Although the nanorods obtained at pH 7 are smaller than scattered MnWO4 nanorods at pH 10, different magnetic properties were observed and the reason may

Synthesis of Urchin-like MnWO4 Microspheres be due to the different synthesis conditions and the size effects of nanostructures. 4. Conclusion In summary, 3D urchin-like MnWO4 microspheres with a diameter of ca. 1-1.2 µm, which were assembled by MnWO4 nanorods with a length of ca. 240 nm and an aspect ratio of ca. 9, can be prepared by the surfactant CTAB assisted hydrothermal method. CTAB played a crucial role as a directing agent in the formation of 3D urchin-like microspheres in the present reaction system. The optimal pH value of the reaction solution and suitable reaction temperature are also essential for the formation of such urchin-like microspheres composed of nanorods. The MnWO4 microspheres made of nanorods obtained at pH 10 show a weak ferromagnetism at low temperature due to spin canting of antiferromagnetic materials and surface spins of the microspheres, which is different from the antiferromagnetism observed at low temperature for much shorter MnWO4 nanorods prepared at pH 7. The results indicate that the magnetic properties of MnWO4 nanocrystals are related to detailed preparation conditions, such as the pH value, as well as the size effects of nanostructures. Acknowledgment. This work was supported by the National Science Foundation of China (Nos. 50732006, 20325104, 20621061, 20671085), the Centurial Program of the Chinese Academy of Sciences, the 973 project (2005CB623601), Anhui Development Fund for Talent Personnel and Anhui Education Committee (2006Z027, ZD2007004-1), the Specialized Research Fund for the Doctoral Program (SRFDP) of Higher Education State Education Ministry, and the Partner-Group of the Chinese Academy of Sciences-the Max Planck Society. Supporting Information Available: Figures giving FTIR spectra of MnWO4 microspheres before and after TGA analysis, XRD patterns of disperse MnWO4 nanorads and inhomogeneous MnWO4 microspheres, XRD patterns of products obtained at pH values of 7, 8, 9, and 12, and an XRD pattern of MnWO4 obtained at 160 °C. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Burda, C.; Chen, X.B.; Narayanan, R.; El-Sayed, M. A. Chem. ReV 2005, 105, 1025. (b) Yang, P. D.; Kim, K. ChemPhysChem 2002, 3, 503. (c) Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Science 2001, 291, 2115. (d) Bu, W. B.; Xu, Y. P.; Zhang, N.; Chen, H. R.; Hua, Z. L.; Shi, J. L. Langmuir 2007, 23, 9002. (e) Shi, H. T.; Wang, X. H.; Zhao,

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