J. Phys. Chem. C 2010, 114, 10101–10107
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Strategy for Ultrafine Ni Fibers and Investigation of the Electromagnetic Characteristics Chunhong Gong,†,‡ Jingwei Zhang,*,† Xuefeng Zhang,§ Laigui Yu,† Pingyu Zhang,† Zhishen Wu,† and Zhijun Zhang*,† Key Laboratory for Special Functional Materials of Ministry of Education, Henan UniVersity, Kaifeng 475004, China, College of Chemistry and Chemical Engineering, Henan UniVersity, Kaifeng 475004, China, and School of Materials Science and Engineering, Dalian UniVersity of Technology, Dalian, 116024, China ReceiVed: January 25, 2010; ReVised Manuscript ReceiVed: March 11, 2010
A simple and practical approach for synthesizing ultrafine Ni fibers under normal pressure via the reduction of Ni2+ ions by hydrazine hydrate in the absence of any templates or external magnetic field is reported. The mechanisms and a simplified model of formation for the Ni fibers are proposed, and the correlated magnetic properties and electromagnetic characteristics of the Ni products are studied systematically. The results of a series of comparative studies indicate that the feeding sequence of the reactants and the concentration of NaOH solution are critical to controlling the shape of the target products from fiber to sphere. The Ni fibers have an enhanced magnetic coercivity compared with that of the Ni spheres. The composites filled with Ni fibers have an electromagnetic wave absorbance within the frequency range 2.0-18.0 GHz stronger than that for the Ni sphere-filled composites. Specifically, by properly adjusting the matching thicknesses, a minimum reflection loss (RL) of -39.5 dB at 4.8 GHz and an absorbance band of less than -20 dB within 3-16 GHz are obtained for the Ni fiber-filled composites, showing that the Ni fibers may have promising application for electromagnetic wave absorbance. Introduction In the field of material science and technology, one of the main challenges is how to precisely control the shape and crystal structure of nanostructured magnetic products, which is critical to the practicable and efficient tailoring of their physical/ chemical properties.1-7 Usually, anisotropic magnetic nanomaterials are expected to exhibit unique magnetic properties. This is why one-dimensional (1D) magnetite nanostructures have been the focus of considerable interests in relation to their potential applications in magnetic fluids, magnetic recording devices, drug delivery, etc.8-10 The structure-directing templateassisted method and magnetic-field-induced growth route are most commonly used to prepare 1D magnetite nanostructures.11-19 These approaches have been successful in some sense toward the synthesis of 1D magnetite nanostructures. For example, the template synthesis method, which involves the electrochemical deposition or metal-organic chemical vapor deposition (CVD) of metals into the nanopores of template materials, such as carbon nanotubes, polyaniline nanotubes, and anodic alumina oxide (AAO) films,12-14 provides a versatile approach and has had considerable success in the preparation of arrays of some magnetic nanomaterials such as Fe, Co, Ni and alloy nanowires.14 As a novel driving power, magnetic field can significantly influence the movement and self-assembly behavior of magnetic nanocrystallites.15-19 In recent years, the magnetic field-directed assembly of magnetic nanocrystallites has been proven to be an efficient method for obtaining 1D magnetic materials. For example, Chen et al. reported that Co polycrystalline wires with an average length of 2 mm and diameter of 13 µm were formed by the self-assembly of Co nanocrystallites (15 nm in average †
Key Laboratory for Special Functional Materials of Ministry of Education, Henan University. ‡ College of Chemistry and Chemical Engineering, Henan University. § Dalian University of Technology.
size) under the induction of a 0.25 T external magnetic field.17 Similar to that mentioned above, polycrystalline nickel nanowires with an average length of 10 µm and diameter of about 200 nm were formed through a low-temperature hydrothermal process by magnetic induction.18 However, they still have some limitations. For example, template-based synthetic routes have drawbacks in that the introduction of the templates or other additives inevitably adds to the complexity in the synthetic procedures and the purification of final products as well. And the magnetic-field-induced growth route is inappropriate for industries due to the reliability on external magnetic fields.20,21 Thus it is imperative to develop economical and practical routes for large scale manufacturing of 1D magnetite nanostructures so as to fully exploit their peculiar properties and unique applications. In the present study, therefore, we demonstrate a new approach to the direct preparation of Ni fibers via the reduction of Ni2+ ions by hydrazine hydrate under normal pressure and in the absence of any morphology-controlling media such as external magnetic forces, template materials, surfactants or passivating materials, etc. To add clues to the controllable and tunable growth of the Ni fiber nanostructures, the dependence of the morphologies of the Ni products on the concentration of nickel chloride (NiCl2 · 6H2O), sodium hydroxide (NaOH) and the feeding sequence of the reactants have been investigated. The mechanisms and a simplified model of formation for the Ni fibers have been proposed, and the correlated magnetic properties and electromagnetic characteristics of the Ni fibers have been studied systematically. Experimental Section Synthesis. All chemicals of analytical grade were purchased from Kermel Chemical Co., Ltd. (Tianjin, China) and used without further purification. Typically, an appropriate amount
10.1021/jp100697x 2010 American Chemical Society Published on Web 05/17/2010
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Figure 1. SEM images of Ni fibers obtained at 70 °C from the reaction systems with different NaOH/Ni molar ratios: (A) 15, (B) 7.5, (C) 2.0, and (D) 0.4.
of NaOH granule and hydrazine monohydrate solution (N2H4 · H2O, concentration 80 wt %) was added orderly to an appropriate amount of nickel chloride (NiCl2 · 6H2O) solution dissolved in 50 mL of ethylene glycol (EG) under continuous stirring for about 30 min at room temperature. The resultant transparent blue solution was then, in the absence of stirring, allowed to react at 70 °C. Several minutes later, black, fluffy, solid product emerged in the reaction solution, indicating the formation of metallic Ni. The process of the reaction can be described as below:
2Ni2+ + N2H4 + 4OH- ) 2Ni + N2v + 4H2O
(1)
To ensure that the reactions were accomplished completely, each reaction process was conducted for up to 1 h. Characterization Techniques. The as-synthesized solid products were precipitated, separated, washed with ethanol several times, and dried in a vacuum oven at 40 °C for about 24 h. The structure and shape of the resultant dried samples were characterized by means of X-ray diffraction (XRD, Philips X′ Pert Pro X-ray diffractometer; Cu KR radiation, λ ) 0.154 18 nm), scanning electron microscopy (SEM, JEOL JSM-5600LV; acceleration voltage 20 kV), and transmission electron microscopy (TEM, JEOL JEM-2100 transmission electron microscope, accelerating voltage 200 kV). The magnetic hysteresis loops of the products at room temperature were measured using a vibrating sample magnetometer (VSM, Lake Shore 7300). The Ni powders were uniformly mixed with paraffin (which is transparent to electromagnetic wave), and the mixture was then pressed into cylindrical Ni/paraffin composites. The cylindrical samples were cut into toroidally shaped samples with an outer diameter of 7.00 mm and an inner diameter of 3.00 mm. The relative complex electromagnetic parameters (complex permeability εr ) εr′ + iεr′′ and complex permittivity µr ) µr′ + iµr′′)
of the toroidally shaped Ni/paraffin composite with a height of 2.00 mm and Ni content of 50% (mass fraction) were measured using an Agilent N5230A network analyzer in a frequency range of 2.0-18.0 GHz. Results and Discussion The oriented growth of 1D nanostructures is closely dependent on the experimental conditions such as reactant concentration, temperature, time, pressure, etc. The reduction performed with Ni2+ ions at a concentration of 0.05 M in 50 mL of ethylene glycol was taken as a sample to investigate the effect of alkaline concentration on the morphology of the Ni fiber products at a reaction temperature of 70 °C, when the molar ratio of NaOH/ Ni varied from 0 to 15 and that of N2H4/Ni was fixed at 19. To our surprise, an appropriate amount of NaOH is imperative for the formation of Ni fibers. Namely, at a NaOH/Ni molar ratio of 0.4, no black products were formed unless the temperature of the reaction system was increased to 90 °C. It is thus supposed that in the reaction systems containing hydrazine hydrate as a kind of alkali, a trace amount of NaOH not only acts as an alkaline agent but also may catalyze the formation of Ni nanocrystallite. Figure 1 gives the typical SEM images of the Ni fibers obtained at 70 °C from the reaction systems with a NaOH/Ni2+ molar ratio of 15, 7.5, and 2.0, where the image of the product prepared from the reaction system with a NaOH/Ni2+ molar ratio of 0.4 at 90 °C is also provided for a comparison (reaction duration 2 h). It is seen that a higher molar ratio of NaOH/Ni, e.g., 15 and 7.5, is beneficial to synthesizing uniform Ni fibers with an average diameter of about 500 nm and a length of 50 µm (Figure 1A,B). In contrast, at a lower NaOH/Ni2+ ratio, short squiggly Ni fibers were harvested (Figure 1C), and spherical/ chain-like Ni powders were obtained at a too low molar ratio of NaOH/Ni2+ (Figure 1D). This means that during the chemical reaction, the concentration of NaOH has an evident influence
Ultrafine Ni Fibers
Figure 2. XRD patterns of typical Ni fibers obtained at 70 °C from the reaction systems with a fixed NaOH/Ni molar ratio of 7.5.
on the nucleation and growth of nickel nanocrystallites. Therefore, the molar ratio of NaOH/Ni is selected as 7.5 finally. Figure 2 depicts the XRD pattern of the Ni fibers obtained from the reaction system with a NaOH/Ni molar ratio of 7.5. Three characteristic peaks for Ni (2θ ) 44.5°, 51.8°, and 76.4°), corresponding to Miller indices (111), (200), and (220), are observed, indicating that they are composed of pure facecentered cubic (fcc) Ni (PDF standard cards, JCPDS 01-1260, space group Fm3m) and do not contain impurities.22 In a similar manner, the reductions were performed at 70 °C for the reaction systems with a fixed N2H4/Ni2+ molar ratio of 19 and NaOH/Ni molar ratio of 7.5 but varied concentration of Ni2+ ions from 0.01 to 0.10 M, so as to investigate the effect of the concentration of Ni2+ ions on the morphology of the resulting Ni products. As shown in Figure 3, it seems that a higher concentration of Ni2+ ions is beneficial for producing more homogeneous and longer fibers. However, when the concentration of Ni2+ ions in the reaction system is as high as 0.10 M, many bubbles are produced in the reaction system and the corresponding product has irregular microstructure and appears as loosely connected sphere-like particulates mixed with a few irregular chains (Figure 3D). We suppose that the reason may lie in that, at a fixed N2H4/Ni2+ molar ratio of 19 but too high concentration of Ni2+ ions in the reaction system, too much N2 is produced and released to overflow the container (eq 1). As a result, the stillness of the reaction system may be destroyed and the self-assembly of Ni retarded, generating Ni particulates. Thus it is suggested to keep the concentration of Ni2+ ions in the reaction system at 0.05 M so as to retain the desired morphology and microstructure of the fiber-like target product. So far, no well accepted theory is available for illustrating the long-range self-assembly of magnetic nanocrystallites. To acquire some insight into the growth mechanism of the 1D magnetic Ni nanocrystallites, the feeding sequence of NaOH and N2H4 · H2O was reversed while the other parameters were kept the same as that for preparing the sample in Figure 1B, where the reaction systems were labeled as system I and system II, respectively. In system I, a transparent grass green solution was first obtained by mixing the EG solutions of NiCl2 and NaOH at room temperature. Then, the addition of N2H4 into the green solution turned the color to transparent blue within a few minutes (Supporting Information Figure S1-I). When the temperature was increased to 70 °C, some black fluffy solid products emerged in a few minutes, indicating the formation of metallic Ni. As the reaction proceeded, more and more products were produced. When the reaction duration was extended to about 30 min, the solution became clear and colorless, while the desired black fluffy solid product was floating atop the reaction
J. Phys. Chem. C, Vol. 114, No. 22, 2010 10103 solution, indicating the completion of the reaction and formation of homogeneous Ni fibers as shown in Figure 1B. In system II, on the contrary, N2H4 · H2O solution was first added into the green EG solution of Ni2+, generating a purple solution, followed by the addition of a NaOH granule under strong stirring, generating a cloudy indigo solution within a few minutes (Supporting Information Figure S1-II). The generation of the cloudy indigo solution indicates the formation of some solid intermediate products. Thanks to the convenience of the open system reaction, a series of precipitates could be collected at different reaction stages with the assistance of centrifuging separation and analyzed by means of SEM, TEM, and XRD. To ensure the timely interruption of the reaction, the sampling process has to be timely and expeditious. Namely, once the target samples were taken out from the reaction solution, they should be immediately put into liquid nitrogen. Therefore, the indigo precipitate was initially collected and labeled as A. Then the reaction system was heated to 70 °C and held there for a few minutes, allowing the generation of black floccules that were step by step collected as samples B, C, and D depending on reaction time. SEM observation showed that multilayer flake-like indigo precipitates with an average size of 1-2 µm in thickness and 2-3 µm in diameter were initially generated in the reactant system II (see Figure 4A). As the reation proceeded, the flakes were redissolved and some particles emerged quickly (see Figure 4B). With a further increase in the reaction duration, more and more particles were produced, and the proportion of the flakes decreased while the flakes became thinner and thinner (see Figure 4C). At the end of the reaction, the flakes disappeared and only particles with an average size of 200 nm were harvested (see Figure 4D), implying that the flakes might have acted as precursors. The XRD patterns of various samples obtained at different reaction times in system II are shown in Figure 5. It is seen that, at the initial stage of reaction, some XRD peaks with a weak intensity emerge, and they cannot be identified as any existing nickel compounds (Supporting Information Figure SII). This indicates that the resulting products may contain some unknown crystalline of nickel compound (Figure 5A). As the reaction proceeded, the characteristic peaks for the unknown crystalline of nickel compound became more obvious and three characteristic peaks for Ni (2θ ) 44.5°, 51.8°, and 76.4°) simultaneously emerged (Figure 5B). At the end of the reaction, the XRD peaks of the unknown crystalline of nickel compound became more and more weak and finally disappeared, and the final products, without any impurities, were composed of pure face-centered cubic (fcc) Ni (Figure 5C and D).23 Apparently, the feeding sequence of the reactants is critical to controlling the shape of the target products from fiber to sphere, which implies that the types of the precursors may have significant effects on the morphology of the nickel products. When the above-mentioned SEM, TEM, and XRD results are combined, the reaction processes and mechanisms of formation for Ni nanocrystallite in the two systems are primarily inferred as follows. In system I, a blue transparent solution was first obtained after NaOH and hydrazine were added orderly to the EG solution of NiCl2 at room temperature, possibly referring to the formation of some soluble nickel compound (Figure 6A). Then, Ni2+ ions were reduced equably at an appropriate temperature (e.g., 70 °C), generating small nickel nanocrystals (Figure 6B). The small Ni primary nuclei may act as seeds, allowing more metal atoms to be reduced and absorbed thereon. And at the same time, the
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Figure 3. SEM images of Ni fibers obtained at 70 °C from the reaction systems with different concentrations of Ni2+ ions: (A) 0.01 M, (B) 0.02 M, (C) 0.05 M, and (D) 0.10 M.
Figure 4. SEM micrographs of the as-prepared samples collected from system II at different reaction durations: (A) blue precipitate, (B) and (C) the mixture of blue precipitate and black floccules, and (D) the final black products. The insets are TEM images of corresponding samples.
reduced Ni atoms are liable to aggregate, forming larger particles for the sake of decreasing the surface energy (Figure 6C). As a kind of permanent magnetic material, each nickel particle with a big enough size has an inherent self-generated magnetic field. Driven by the self-generated magnetic field, it is likely that Ni particles will preferentially aggregate along the magnetic force
lines. Consequently, desired fiber-like ultrafine Ni crystallites will be harvested at properly extended reaction duration as a result of homogeneous reaction (Figure 6C,D). In system II, an unknown indigo flake-like precipitate was first formed and acted as a starting material for preparing fine nickel powders via the chemical reduction route, where Ni(II)
Ultrafine Ni Fibers
Figure 5. XRD patterns for the samples obtained at different reaction times in system II: (A) blue precipitate, (B) and (C) the mixture of blue precipitate and black floccules, and (D) the final black products.
was gradually reduced to zero-valenced Ni by hydrazine at 70 °C (Figure 7A,B). The solid phase intermediate was reduced in situ gradually with increasing reaction time, generating small nickel nanocrystals. Similarly, in the absence of any soluble polymers or surfactants for controlling the growth of the Ni crystals, small nickel nanocrystals are liable to aggregate, forming larger particles for the sake of decreasing surface energy (Figure 7C). However, in this case the nanocrystals that grow into larger particales were restricted in the solid phase and, finally, uniform spherical nickel particles were harvested as a result of the confined diffusion reaction (Figure 7D). Naturally, due to the complex conditions involved in the growth of the Ni nanocrystallites and lack of in situ observation, the detailed formation processes of ferromagnetic Ni fibers should be much more complicated than that qualitatively discussed above. Many other factors such as experimental methods, reaction conditions, and reactant types may also affect the aspect ratio of the fibers in the magnetically directed agglomeration process. For example, we have found in our previous work that the microstructure of the Ni nanocrystallites is closely dependent on the type of solvents. And only alcoholtype solvents with double hydroxyl groups seem to be helpful to the formation of Ni nanocrystallites with shape anisotropy.24
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Figure 8. Room temperature hysteresis loops of the final samples obtained from system I (A) and system II (B).
Thus, it still remains a puzzle to precisely describe the growth process of the Ni fibers. Figure 8 shows the room temperature hysteresis loops of the two types of final samples obtained from system I and system II. Both the Ni fibers and Ni particles show obvious ferromagnetic characteristics. The specific saturation magnetization (Ms) and coercivity (Hc) values for Ni fibers were calculated to be 21.5 emu/g and 203.4 Oe, respectively (Figure 8A). The specific Ms and Hc values for Ni particles were calculated to be 39.5 emu/g and 160.7 Oe, respectively (Figure 8B). At the same time, the two types of Ni samples have a lower saturation magnetization and higher coercivity than bulk Ni sample (Ms ) 54-55 emu/g, Hc ) 100 Oe) at room temperature,25 which should be closely dependent on their crystallite size, structure, and shape. Generally speaking, nanoscale magnetic materials have a lower Ms than the bulk counterparts, because the spin disorder on the surface significantly reduces the total magnetic moment, and the shape of the hysteresis loop is strongly affected not only by the specific surface area of the particles but also by the magnetic anisotropy including magnetocrystalline anisotropy, shape anisotropy, and stress anisotropy.26,27 This can largely account for the increased magnetic coercivity of Ni fiber as compared with Ni particles and bulk Ni. Thus it can be concluded that the morphology of the Ni nanocrystallite has an obvious effect on the magnetic properties, which may be well utilized to improve the coercivity of materials to one’s expectation.
Figure 6. Schematic diagram showing the formation of Ni fiber-like assemblies in system I: (A) formation of some soluble nickel compound, (B) formation of nickel primary nuclei, (C) aggregation of nickel primary nuclei and generation of short fiber-like Ni assembles, and (D) formation of longer Ni fibers.
Figure 7. Schematic diagram showing the formation of spherical Ni particles in system II: (A) formation of unknown Ni precursor, (B) redissolving of intermediate phase and in situ nucleation, (C) formation and aggregation of small nickel nanocrystals generating bigger spherical nickel particles, and (D) formation of uniform spherical nickel particles.
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Figure 9. Frequency dependence of dielectric loss (tan δE) and magnetic loss (tan δM) of Ni fiber-filled (A) and Ni particle-filled (B) paraffin composites.
Figure 10. Calculated reflection loss (RL) of Ni fiber-filled (A) and Ni particle-filled (B) paraffin-based composites with different thicknesses.
Among all potential candidates for electromagnetic wave absorbents, ultrafine magnetic materials have been attracting considerable research interest due to their novel physicochemical properties and Snoek’s limit at a high frequency level.28 It is well-known that good absorbance is mainly determined by a cooperation of dielectric losses and magnetic losses, the socalled electromagnetic matching. The electromagnetic matching is highly related to the nature of fillers and can be effectively controlled by tailoring components, microstructures, and loading content.29-32 In the present study, we have observed an abnormal electromagnetic behavior of Ni fibers in relation to Ni particles by measuring the relative permittivity (εr ) εr′ + iεr′′) and permeability (µr ) µr′ + iµr′′) at 2-18 GHz. As illustrated in Figure 9, composite A (paraffin composite filled with Ni fibers) has a higher dielectric loss factor (tan δE ) εr′′/εr′) than composite B (paraffin composite filled with Ni particles) in the whole frequency range. At the same time, composite A has a maximum dielectric loss factor of about 0.25 at 8.0 GHz, while its magnetic loss factor (tan δM ) µr′′/µr′) is analogous to that of composite B. The significantly enhanced dielectric loss of composite A should be critical to improving the electromagnetic interference absorbance by transferring electromagnetic wave energy into heat energy. In other words, the microwave absorbance of composite A is a cooperation of both dielectric and magnetic losses. Besides, the microwave absorbance of composite B is dominated by magnetic loss rather than dielectric loss. Since the two types of composites have the same mass loading of Ni fillers, it should be reasonable to attribute the difference of their microwave absorbance to the difference of their microstructure. On one hand, due to the high aspect ratio of over 100 (Figure 1B), Ni fibers in composite A more easily form networks, generating space-charge polarizations at the joints under the action of the electromagnetic field and hence increased dielectric loss of composite A, but Ni particles are more efficiently isolated by paraffin and have weaker interactions resulting in a lower dielectric loss of
composite B. On the other hand, under the action of the electromagnetic field, the electron-vibration along the axle directions of the Ni fibers could also enhance the transfer of electromagnetic wave energy into heat energy, resulting in increased electromagnetic wave absorbance of composite A. However, the electrons in the Ni particles are localized within the granules and cannot move anisotropically, resulting in a lower dielectric loss of composite B due to the lack of capacitive electron-vibration. The reflection loss, RL (dB), as an effective evaluation standard of the microwave absorbance capacity of materials, was determined from the measured values of εr′, εr′′, µr′, and µr′′. Based on transmit-line theory,33,34 the calculated RL curves of composites A and B in relation to the frequency of electromagnetic wave and thickness (1-5 mm) of absorber layer are shown in Figure 10. It can be seen that the morphology of the Ni fillers has an obvious effect on the microwave absorbing behavior. Namely, composite A with an absorber layer thickness of 2.0 mm exhibits a reflection loss of less than -10 dB at 6.6-8.8 GHz, and a minimum reflection loss of -39.5 dB is obtained at 4.8 GHz with an absorber layer thickness of 3.0 mm (Figure 10A). As expected, composite B has a weak electromagnetic wave absorbance over the whole frequency range of 2.0-18.0 GHz, and a minimum reflection loss of -10.1 dB is recorded at 16.0 GHz with an absorber layer thickness of 5.0 mm (Figure 10B). Zhang et al. reported that carbon-coated nickel nanocapsules exhibited a good microwave absorbance behavior due to a cooperation of dielectric carbon shells and ferromagnetic cores.28 In the present research, we can validate that the more excellent microwave absorbance of composite A is the result of proper electromagnetic matching, yet an unbalance between magnetic and dielectric losses exists for composite B.
Ultrafine Ni Fibers Conclusion To sum up, we demonstrate a novel synthetic approach to preparing ultrafine Ni fibers under normal pressure and in the absence of any morphology-controlling media. It has been found that the shape and microstructure are the key factors for affecting the magnetic and electromagnetic characteristics of Ni nanostructure. An enhanced dielectric loss in association with spacecharge polarization of Ni fibers contributed to improve the electromagnetic matching and hence increased electromagnetic wave absorbance of the paraffin-based composites filled with Ni fibers. The established approach may be extremely valuable in terms of the expected advantages, such as relatively lower cost, higher product purity, and better feasibility for large-scale production. We believe the present research will help to acquire new insights into the growth and self-assembly mechanisms of 1D nickel nanocrystallites and design of a new general method for preparing other magnetic metal nanostructures with a broad range of well-defined and controllable morphologies. Also it will help to fully exploit the peculiar properties and unique application of Ni fiber nanostructures. Acknowledgment. We acknowledge the financial support from the Ministry of Science and Technology of China (project of “973” plan, grant No. 2007CB607606) and National Science Foundation of China (grant Nos. 50902045/E0213 and 20971037/ B0111). Supporting Information Available: Figure showing solutions obtained from the reaction systems I and II and the XRD pattern for the intermediate sample obtained in system II. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Gong, X. Q.; Peng, S. L.; Wen, W. J.; Sheng, P.; Li, W. H. AdV. Funct. Mater. 2009, 19, 292. (2) Zhao, N.; Gao, M. Y. AdV. Mater. 2009, 21, 184. (3) Wei, X. W.; Zhu, G. X.; Liu, Y. J.; Ni, Y. H.; Song, Y.; Xu, Z. Chem. Mater. 2008, 20, 6248. (4) Dyab, A. K. F.; Ozmen, M.; Ersoz, M.; Paunov, V. N. J. Mater. Chem. 2009, 21, 3475. (5) Chen, M.; Pica, T.; Jiang, Y. B.; Li, P.; Yano, K.; Liu, J. P.; Datye, A. K.; Fan, H. Y. J. Am. Chem. Soc. 2007, 129, 6348. (6) Tu, C. F.; Du, J. J.; Yao., L.; Yang, C. H.; Ge, M. F.; Xu, C. L.; Gao, M. Y. J. Mater. Chem. 2009, 19, 1245.
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