Fe3O4 Nanocomposites and Their

Jan 29, 2009 - electron microscopy, transmission electron microscopy, and energy dispersive spectroscopy. Tin oxide/iron oxide nanocomposites were mai...
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J. Phys. Chem. C 2009, 113, 3070–3075

Hydrothermal Synthesis of SnO2/Fe3O4 Nanocomposites and Their Magnetic Property Wei-Wei Wang* and Jia-Liang Yao School of Materials Science and Engineering, Shangdong UniVersity of Technology, Shandong 255049, People’s Republic of China ReceiVed: October 22, 2008; ReVised Manuscript ReceiVed: December 27, 2008

Combining the optical and magnetic properties of tin oxide/iron oxide binary nanostructure would greatly broaden their application. Tin oxide/iron oxide nanocomposites were synthesized by a simple hydrothermal method. The changes of their structure and morphology were investigated by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, and energy dispersive spectroscopy. Tin oxide/iron oxide nanocomposites were mainly in sheetlike morphology and highly encapsulated by the SnO2 nanoparticles or nanorods. Increasing the reaction time would lead to the transition from nanosheets to nanorods. The zero-field-cooled and field-cooled measurements were carried out to examine the magnetic property of tin oxide/iron oxide nanocomposites. All of them revealed a paramagnetic behavior. Introduction Nanocomposites containing two or more different functionalities can exhibit novel physical and chemical properties that will be essential for future technological applications. Magnetic Fe3O4 nanoparticles have been widely used not only as information storage ferrofluids in sealing and position sensing but also as promising candidates for biomolecule imaging, sensing, and separation.1-3 Based on the magnetic property of Fe3O4 nanoparticles, many efforts have been made in the controlled synthesis of Fe3O4 composites, such as SiO2/Fe3O4 core/shell nanostructures,4-6 Fe3O4/ZrO2 microspheres,7 CoFe/Fe3O4 core/ shell nanoparticles,8 and attachments of noble metals on the surface of Fe3O4.9-11 The composites have shown different optical, magnetic, or catalytic properties compared with their individual single-component materials. Among them, tin oxide (SnO2), an inexpensive large band gap (3.6 eV) semiconductor, has been used for a wide range of applications, most notably gas sensing, catalysis, and photoelectrochemical-based energy conversions.12,13 As a form of bifunctional nanomaterials, tin oxide/iron oxide nanocomposites combining their special optical property and paramagnetic property would greatly enhance their potential and broaden their application. As for tin oxide/iron oxides composites, SnO2/Fe2O3 composites have been prepared by mechanical alloying of R-Fe2O3 and SnO2 powder,14 assembling SnO2 onto the surface of R-Fe2O3 by a solution method,15,16 or a coprecipitation method.17 However, to the best of our knowledge, there have been a few reports on the synthesis of SnO2/Fe3O4 nanocomposites which may possess the magnetic properties of Fe3O4 and the optical properties of SnO2. In this paper, magnetite nanoparticles were used as the precursor to prepare the SnO2/Fe3O4 nanocomposites by a simple hydrothermal method. The nanocomposites were in sheetlike shape with nanoparticles or nanorods assembled on the surface. The nanocomposites retained the magnetic property of the precursor. Experimental Section Iron(Π) sulfate (FeSO4 · 7H2O), ethylene glycol (EG), tin(IV) chloride (SnCl4 · 5H2O), and sodium hydroxide (NaOH) were * Corresponding author. Tel.: +86-533-2782198. Fax: +86-533-2781660. E-mail: [email protected].

purchased from Sinopharm Chemical Reagent Co. Ltd. and used as received without further purification. Preparation of the precursor: 0.3 g FeSO4 · 7H2O was dissolved in a mixture of EG (7.5 mL) and deionized water (30 mL) to form a uniform solution. Then 1.25 mL of NaOH aqueous solution (5 M) was added to the above solution. Afterward, the suspension was transferred into a 50-mL Teflonlined stainless steel autoclave. The autoclave was maintained at 180 °C for 6 h without stirring and shaking. After synthesis, the products were washed three times with deionized water and pure ethanol. The collected sample was then vacuum-dried at 50 °C. Preparation of SnO2/Fe3O4: SnCl4 · 5H2O (0.88 g) and NaOH (1 g) were dissolved in a mixture of EG (6 mL) and deionized water (34 mL) to form a uniform solution. The precursor (0.1 g) was added to the above solution and underwent ultrasonic treatment to form a suspension. The starting molar ratio of Sn4+: Fe3+ was about 2. The suspension was transferred into a 50mL Teflon-lined stainless steel autoclave. The autoclave was maintained at 180 °C for different times without stirring and shaking. The following process was the same as that of the preparation of the precursor. Please refer to Table 1 for the detailed preparation conditions. X-ray powder diffraction (XRD) patterns were recorded using a D8 ADVANCE X-ray diffractometer with high-intensity Cu Ka radiation (λ ) 1.5406 Å) and a graphite monochromator. The scanning electron microscopy (SEM) micrographs were recorded on a FEI-Sirion200 field emission scanning electron microscope. The transmission electron microscopy (TEM) micrographs, selected-area electron diffraction (SAED) patterns, high-resolution transmission electron microscopy (HRTEM) micrographs, and energy dispersive spectroscopy (EDS) spectra were taken with a JEOL JEM-2100F field emission transmission electron microscope with an accelerating voltage of 200 kV. The magnetic properties of the samples were measured using a Quantum Design Physical Property Measurement System (PPMS). The zero-field-cooled (ZFC) and field-cooled (FC) measurements were performed by cooling the sample to 10 K at zero fields or in the presence of an external field of 100 Oe, respectively. All the magnetic measurements during the warming runs were carried out in a field of 100 Oe.

10.1021/jp809349d CCC: $40.75  2009 American Chemical Society Published on Web 01/29/2009

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TABLE 1: Experimental Conditions for Samples Prepared sample no.

solution

reaction temp and time

1

0.3 g FeSO4 · 7H2O + 30 mL H2O + 7.5 mL EG + 1.25 mL NaOH (5 M) same as sample 1 0.3 g FeSO4 · 7H2O + 37.5 mL H2O + 1.25 mL NaOH (5 M) 0.1 g sample 2 + 34 mL H2O +6 mL EG + 1 g NaOH same as sample 4 0.1 g sample 2 + 34 mL H2O + 6 mL EG + 0.88 g SnCl4 · 5H2O + 1 g NaOH same as sample 6 same as sample 6 same as sample 6 0.1 g sample 2 + 40 mL H2O + 0.88 SnCl4 · 5H2O + 1 g NaOH same as sample 10

180 °C for 6 h

2 3 4 5 6 7 8 9 10 11

Results and Discussion The structure and phase purity of the samples 1-3 were investigated by XRD (Figure 1). After hydrothermal treating at 180 °C for 6 h (sample 1), a black product was synthesized. Cubic Fe3O4 was the main phase of sample 1 (JCPDS file No. 65-3107, Figure 1a). It also showed weak diffraction peaks from FeOOH (JCPDS File No. 29-0713). Upon prolonging the reaction time to 12 h, sample 2 showed a similar diffraction pattern to that of sample 1, except that the diffraction peak from FeOOH became indistinct (Figure 1b). Under the same experimental conditions as that of sample 1 but without using EG, the hexagonal R-Fe2O3 phase (JCPDS file No. 33-0664) was obtained (sample 3, Figure 1c). EG is a reductant and has been used as reducing agent for the preparation of metals such as Ag and Au.18,19 In our experiment, the reducing ability of EG could protect Fe3O4 from further oxidation. Therefore, the presence of EG played an important role in the formation of Fe3O4. The morphologies of samples 1-3 were investigated with SEM. Figure 2a shows the SEM micrograph for sample 1. One can see most of the Fe3O4 particles with sizes more than 100 nm. These Fe3O4 particles have irregular shapes, and a few particles have an octagonal shape. Upon increasing the reaction time to 12 h, sample 2 showed the same morphology, except that the amount of octagonal particles with large size was less compared with sample 1 (Figure 2b). Without using EG, R-Fe2O3 particles with wide size distribution were obtained (sample 3, Figure S1). Sample 2 was used as the precursor to prepare the tin oxide/ iron oxide nanocomposites. In the mixture solvent of EG and deionized water, sample 7 mainly consisted of tetragonal SnO2

Figure 1. XRD patterns of (a) sample 1, (b) sample 2, and (c) sample 3. O stands for FeOOH.

180 °C for 12 h 180 °C for 6 h 180 °C for 1 h 180 °C for 12 h 180 °C for 1 h 180 °C for 12 h 180 °C for 24 h 180 °C for 48 h 180 °C for 12 h 180 °C for 24 h

(space group: P42/mnm, JCPDS File No. 41-1445, Figure 3a). Weak diffraction peaks from the cubic Fe3O4 (JCPDS file No. 65-3107) and FeOOH (JCPDS file No. 29-0713) were observed. The contents of Fe3O4 and SnO2 were about 6 and 94 wt % calculated by using the internal calibration procedure (the reference intensity was from the corresponding JCPDS files). It differed from the reports that SnFe2O3 nanoparticles20 or SnO2/ Fe2O3 composites15 were prepared. Upon prolonging the reaction time to 24 h (sample 8), tetragonal SnO2 was the main phase (Figure 3b). The diffraction peaks from Fe3O4 became weaker. The contents of Fe3O4 and SnO2 were about 4 and 96 wt %. The diffraction peaks from FeOOH became indistinct. Upon further increasing the reaction time to 48 h (sample 9), the diffraction peaks from Fe3O4 became indistinct and a weak diffraction peak from FeOOH was detected (Figure 3c). Therefore, the main phases of the nanocomposites were SnO2 and Fe3O4. No diffraction peaks of R-Fe2O3 were observed. It seemed that Fe3O4 could dissolve in alkaline solution, and the content of Fe3O4 decreased with longer reaction time. The SEM and TEM micrographs in Figures 4-6 illustrate the interesting morphological evolution of the composites during

Figure 2. SEM micrographs of (a) sample 1 and (b) sample 2.

Figure 3. XRD patterns of (a) sample 7, (b) sample 8, and (c) sample 9. F stands for Fe3O4; S stands for SnO2, and O stands for FeOOH.

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Figure 6. (a) and (b) SEM micrographs of sample 9; (c) and (d) TEM micrographs of sample 9. The inset is the corresponding SAED pattern.

Figure 4. (a) SEM micrograph of sample 6; (b) and (c) SEM micrographs of sample 7; (d) EDS spectrum of the composites as shown in (f); (e) EDS spectrum of the single nanosheet as shown in (g); (f) and (g) TEM micrographs of sample 7; (h) HRTEM micrograph of the nanosheet as shown in (g), The inset is the corresponding fast Fourier transform (FFT).

Figure 5. (a)-(c) SEM micrographs of sample 8; (d) EDS spectrum of nanosheets (as indicated by a circle in (f)); (e) EDS spectrum of nanorods (as indicated by a square in (f)); (f) and (g) TEM micrographs of sample 8; (h) HRTEM micrograph of the nanorod (as indicated by a square in (f)).

different reaction times (from 1 to 48 h). When hydrothermally treating Fe3O4 nanoparticles (sample 2) and Sn4+ in basic aqueous solution for 1 h (sample 6), a sheetlike morphology

was observed (Figure 4a). The surface of the sheets was smooth. Upon prolonging the reaction time to 12 h (sample 7), it showed the same sheetlike morphology (Figure 4b). The thickness of the nanosheets was about 60 nm, estimated from the standing sheet in the SEM micrograph (Figure 4c, as indicated by an arrow). After careful investigation, one can see that the surfaces of these nanosheets were not smooth and were comprised of nanoparticles with a size of about 20 nm. The side surfaces of the nanosheets were also coated with nanoparticles (Figure 4c). TEM micrographs (Figure 4f and g) confirmed that the shape of sample 7 was nanosheets with the thickness of about 60 nm (estimated from the standing sheet in Figure 4f). Energy dispersive spectroscopy (EDS, Figure 4d and e) confirmed that the composition of both nanosheets and nanoparticles consisted of Fe, Sn, and O elements (Cu and C came from the TEM copper grid of the sample holder). The EDS spectrum of a single nanosheet with relatively smooth surface (Figure 4e, the corresponding TEM micrograph as shown in Figure 4g) showed much stronger Fe and O bands than that of the composites (Figure 4d, the corresponding TEM micrograph as shown in Figure 4f). The structure of the nanosheet was further characterized by HRTEM (Figure 4h). The spacing between lattice fringes was 0.243 nm, which is consistent with the (222) plane of Fe3O4 (0.242 nm). The fast Fourier transform (FFT) was characterized by hexagonal symmetry (the inset in Figure 4h). Based on the above analysis, Fe3O4 was mainly in sheetlike morphology and SnO2 was nanoparticles. Upon increasing the hydrothermal treatment time to 24 h (sample 8), the sample still remained in sheetlike shape and the thickness of the nanosheets was less than 100 nm (Figure 5a, b and Figure S2, as indicated by an arrow). Besides nanoparticles (Figure 5a, as indicated by a circle), some nanorods grew on the surface of the nanosheets (Figure 5c and f). The size of the nanosheets did not change distinctly. It seemed that the nanorods grew at the expense of the small particles. Both nanosheets and nanorods contained Fe, Sn, and O elements (Figure 5d and e). The EDS spectrum of the nanosheet (Figure 5d, as indicated by the circle in Figure 5f) showed much stronger Fe and O bands than that of nanorods (Figure 5e, as indicated by the square in Figure 5f). The Fe element in Figure 5e may come from nanosheets due to nanorods attached to the surface of the nanosheets. We deduced SnO2 was in rod shape. HRTEM

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Figure 7. Schematic illustration of a possible formation process for SnO2/Fe3O4 nanocomposites. (1) Formation of nanosheets with nanoparticles on the surface. (2) Nanorods formed on the surface of nanosheets. (3) Formation of nanorods.

analysis also confirmed this. Figure 5h shows the HRTEM micrograph of the nanorod (as indicated by the circle in Figure 5g). The interplanar spacing of ∼0.263 nm agrees well with the spacing between the (101) plane of SnO2 (0.264 nm). Upon further increasing the hydrothermal treating time to 48 h (sample 9), the amount of nanorods was increased (Figures 6a-c). The diameters of nanorods with tetragonal cross section were about 100 nm (Figure 6b). Many nanorods grew together, and the central portion became hollow (as indicated by an arrow in Figure 6c). Compared with sample 8 (Figure 5f), it was likely that the nanosheets in the central portion dissolved with longer reaction time. As Fe3O4 was mainly in sheetlike morphology, the percentage of Fe3O4 in the composites was decreased, consistent with the XRD results (Figure 3). Figure 6d shows the magnified micrograph of the nanorods and their SAED pattern. The SAED pattern can be indexed as the [010] zone axis of SnO2. The EDS spectrum also confirmed that sample 9 consisted of Fe, Sn, and O elements (Figure S3). Based on the XRD/SEM (TEM) studies of crystals after different growth times, we proposed the following growth mechanism for the formation of SnO2/Fe3O4 composites (Figure 7). The growth process was similar to the solid-liquid (SL) process, in which the ions in the solution directly deposit on a substrate and grow into one-dimensional nanostructures due to the confined effect of the first deposited particles.21 In our experiments, tin oxide particles were first formed on the surface of the nanosheets and grew into nanorods along the surface of the nanosheets. As the starting material used in our synthesis was Fe3O4 powder, part of the Fe3O4 particles was dissolved in alkaline solution to form a sheetlike morphology, a metastable condition (sample 4, Figure 8a). Without adding Sn4+, it likely changed to stable particles with longer reaction time (sample 5, Figure 8b). Cubic Fe3O4 was the main phase (Figure 8c). The diffraction peaks of FeOOH became stronger than that of sample 2 (Figure 1b). This is because parts of Fe3O4 dissolve to form FeOOH in alkaline solution. It also showed weak diffraction peaks from the hexagonal R-Fe2O3 (JCPDS File No. 33-0664). However, sheetlike morphology was obtained, and no diffraction peaks from R-Fe2O3 were observed in the presence of Sn4+ (Figures 3-6). Sn(OH)62- anions were formed in the strong basic solution. Under hydrothermal conditions, SnO2 nanoparticles formed quickly, followed by the growth of the nuclei into rodlike-shaped crystals (Figures 4-6). In the preparation of the precursor, the presence of EG affects the final phase of the product. Here, we study the effect of EG on the preparation of the composites. Sample 10 was obtained without using EG. The main phase was tetragonal SnO2 (JCPDS File No. 41-1445, Figure 9a). A weak diffraction peak from FeOOH (JCPDS file No.29-0713) was observed. Sample 10 showed sheetlike morphology, and nanorods grew along the surface of sheets (Figure 9b and c). Upon increasing the reaction time to 24 h (sample 11), a sheetlike morphology was observed

Figure 8. SEM micrographs of (a) sample 4 and (b) sample 5; (c) XRD pattern of sample 5. F stands for Fe3O4, O stands for FeOOH, and X stands for R-Fe2O3.

Figure 9. (a) XRD pattern of sample 10, O stands for FeOOH; (b) and (c) SEM micrographs of sample 10; (d) and (e) SEM micrographs of sample 11.

(Figure 9d). However, irregular particles and some single rods were also observed (Figure 9e). Compared with the samples

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Figure 10. ZFC and FC curves measured with a field of 100 Oe: (a) sample 2 and (b) sample 8.

prepared in the presence of EG (samples 7-9), it looked like the presence of EG could hinder the transition from sheets to rods. It could be confirmed by the fact that the growth of nanorods along the surface of nanosheets needed shorter time without EG (samples 10 and 11, Figure 9). It has been reported that EG could affect the final morphology.22 In our experiment, the nanorods showed regular morphology with EG (Figures 4-6). Upon changing the molar ratio of Sn4+ to Fe3+, a sheetlike morphology also obtained. However, the surface of the nanosheets was smoother than that of sample 7 (Figure S4). In addition, nanoparticles with size about 80 nm were observed (Figure S4). The magnetization was measured as a function of temperature in the applied field of 100 Oe between 10 and 200 K using field-cooled (FC) and zero-field-cooled procedures (ZFC). When the sample was cooled at zero magnetic field, the total magnetization decreased, due to the random orientation of the magnetic moment of individual particles (Figure 10, ZFC curves). With an external magnetic field, the moments of individual particles tended to orient along the field at low temperature. With increased temperature, more and more particles reoriented their magnetic moment along the applied field and the total magnetization increased and reached the maximum at the blocking temperature (TB). For field cooling, the total magnetization increased as the temperature decreased (Figure 10). The nature of the FC and ZFC curves remained the same with the change of blocking temperature for both samples. The TB values for sample 2 and sample 8 were 134 and 119 K, respectively (Figure 10). Hence TB decreased with the addition of Sn. It may be due to the tin oxide layer, thus reducing the magnetic dipole-dipole interaction. This led to a shift in TB to lower temperature for sample 8. The divergence of magnetization below TB in the ZFC-FC curves was attributed to the existence of magnetic anisotropy barriers.23 Figure 11a and b showed the hysteresis loops of samples 2 and 8 measured at both 10 and 300 K. At 300 K, the saturation magnetization of sample 2 was 21.8 emu/g, which was smaller than that of bulk Fe3O4.24 The magnetic behavior of iron oxide nanoparticles is very sensitive to the crystallinity and particle sizes.25 The reduction in saturation magnetization was most likely attributed to the decrease in particle size. With lower temperature, the saturation magnetization increased to 24.9 emu/ g. The coercive force was negligible for both cases (Figure 11), which was the character of paramagnetic materials. Using sample 2 as the precursor, sample 8 was the SnO2/ Fe3O4. They have shown some changes in field-dependent magnetization after the incorporation of SnO2. Sample 8 showed a similar loop to that of sample 2 but a decrease of the saturation magnetization. The saturation magnetization decreased from

Figure 11. Magnetization measured at 10 and 300 K for (a) sample 2 and (b) sample 8; (c) magnetization of sample 7 measured at 300 K.

21.8 to 5.5 emu/g at 300 K and from 24.9 to 5.9 emu/g at 10 K. A reduction of saturation magnetization has been observed upon coating Fe3O4 with the nonmagnetic material SnO2. According to the XRD analysis (Figure 3), the amount of Fe3O4 in the nanocomposite was about 4%. It reflected a smaller percentage of net magnetic materials per gram of overall nanocomposite, since the magnetization was reported per gram of material. It can be confirmed by the magnetic property of sample 7 (Figure 11c). The amount of Fe3O4 in sample 7 was about 6%. Compared with sample 8, the saturation magnetization of sample 7 was about 6.8 emu/g at 300 K. At 10 K, the magnetization loop of sample 8 did not saturate, even at 40 kOe (Figure 11b). This may be caused by both thermal agitation and the surface spin canting of the small particles. 26 Conclusions SnO2/Fe3O4 nanocomposites hierarchical nanosheets with paramagnetic property have been successfully synthesized by a simple hydrothermal method. Further reaction leads to the morphology of SnO2/Fe3O4 nanocomposites changing from sheets to rods. For the preparation of the precursor, EG acts as a reducing reagent and protects Fe3O4 from being oxidized. For the SnO2/Fe3O4 nanocomposites, EG could hinder the transition

SnO2/Fe3O4 Nanocomposites process from sheets to rods. Fe3O4 could dissolve in alkaline solution and result in the content of Fe3O4 in the nanocomposites decreasing with longer reaction time. Magnetic studies reveal that both Fe3O4 and SnO2/Fe3O4 composites exhibit a small hysteresis loop at 10 and 300 K due to their small sizes. The blocking temperature (TB) and the saturation magnetization decreased as the corporation of SnO2. The decrease in saturation magnetization of SnO2/Fe3O4 nanocomposites was likely a result of the small contribution of the nonmagnetic SnO2 to the overall mass. Acknowledgment. Financial support from the National Natural Science Foundation of China (no. 50702032) is acknowledged. We also thank the Fund for Scientific Research Foundation from Shangdong University of Technology (no. 406020). Supporting Information Available: TEM micrographs of sample 3 and sample 8, EDS spectrum of sample 9 (SnO2/ Fe3O4), SEM micrographs of SnO2/Fe3O4 prepared at different molar ratios of Sn4+/Fe3+, and the UV-vis diffuse reflectance spectroscopy (DRS) of samples 2, 7-9, and SnO2. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Cao, S. W.; Zhu, Y. J.; Ma, M. Y.; Li, L.; Zhang, L. J. Phys. Chem. C 2008, 112, 1851. (2) Zhu, Y. F.; Zhao, W. R.; Chen, H. R.; Shi, J. L. J. Phys. Chem. C 2007, 111, 5281. (3) Josephson, L.; Tsung, C. H.; Moore, A.; Weissleder, R. Bioconjugate Chem. 1999, 10, 186. (4) Fang, H.; Ma, C. Y.; Wan, T. L.; Zhang, M.; Shi, W. H. J. Phys. Chem. C 2007, 111, 1065. (5) Stjerndahl, M.; Andersson, M.; Hall, H. E.; Pajerowski, D. M.; Meisel, M. W.; Duran, R. S. Langmuir 2008, 4, 3532.

J. Phys. Chem. C, Vol. 113, No. 8, 2009 3075 (6) Deng, Y. H.; Qi, D. W.; Deng, C. H.; Zhang, X. M.; Zhao, D. Y. J. Am. Chem. Soc. 2008, 130, 28. (7) Li, Y.; Leng, T. H.; Lin, H. Q.; Deng, C. H.; Xu, X. Q.; Yao, N.; Yang, P. Y.; Zhang, X. G. J. Proteome Res. 2007, 6, 4498. (8) Li, J.; Zeng, H.; Sun, S. H.; Liu, J. P.; Wang, Z. L. J. Phys. Chem. B 2004, 108, 14005. (9) Perkas, N.; Amirian, G.; Rottman, C.; Vega, F.; Gedanken, A. Ultrasonic Sonochem. 2009, 16, 132. (10) Caruntu, D.; Cushing, B. L.; Caruntu, G.; O’Connor, C. J. Chem. Mater. 2005, 17, 3398. (11) Yu, H.; Chen, M.; Rice, P. M.; Wang, S. X.; White, R. L.; Sun, S. H. Nano Lett. 2005, 5, 379. (12) Sysoev, V. V.; Button, B. K.; Wepsiec, K.; Dmitriev, S.; Kolmakov, A. Nano Lett. 2006, 6, 1584. (13) Bennici, S.; Auroux, A.; Guimon, C.; Gervasini, A. Chem. Mater. 2006, 18, 3641. (14) Jiang, J. Z.; Lin, R.; Morup, S.; Nielsen, K.; Poulsen, F. W.; Berry, F. J.; Clasen, R. Phys. ReV. B 1997, 55, 11. (15) Wang, W. W. Mater. Res. Bull. 2008, 43, 2055. (16) Zhang, D. F.; Sun, L. D.; Jia, C. J.; Yan, Z. G.; You, L. P.; Yan, C. H. J. Am. Chem. Soc. 2005, 127, 13492. (17) Xia, H. L.; Zhuang, H. S.; Zhang, T.; Xiao, D. C. Mater. Lett. 2008, 62, 1126. (18) Sun, Y. G.; Yin, Y. D.; Mayers, B. T.; Herricks, T.; Xia, Y. N. Chem. Mater. 2002, 14, 4736. (19) Tsuji, M.; Miyamae, N.; Lim, S.; Kimura, K.; Zhang, X.; Hikino, S.; Nishio, M. Cryst. Growth Des. 2006, 6, 1801. (20) Liu, F. X.; Li, T. Z.; Zheng, H. G. Phys. Lett. A 2004, 323, 305. (21) Zhu, Y. J.; Hu, X. L. Mater. Lett. 2004, 58, 1517. (22) Ding, T.; Zhang, J. R.; Hong, J. M.; Zhu, J. J.; Chen, H. Y. J. Cryst. Growth 2004, 260, 527. (23) Rondinone, A. J.; Samia, A. C. S.; Zhang, Z. J. J. Phys. Chem. B 1999, 103, 6876. (24) Shafi, K. V. P.; Ulman, A.; Dyal, A.; Yan, X. Z.; Yang, N. L.; Estourne´s, C.; Fourne´s, L.; Wattiaux, A.; White, H.; Rafailovich, M. Chem. Mater. 2002, 14, 1778. (25) Lopez-Quintela, M. A.; Rivas, J. J. Colloid Interface Sci. 1993, 158, 446. (26) Loweth, C. J.; Caldwell, W. B.; Peng, X. G.; Alivisatos, A. P.; Schultz, P. G. Angew. Chem., Int. Ed. 1999, 38, 1808.

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