Synthesis of Superparamagnetic Colloidal Nanochains as Magnetic

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Synthesis of Superparamagnetic Colloidal Nanochains as Magnetic-Responsive Bragg Reflectors Hui Wang,† Qian-Wang Chen,*,† Yu-Bing Sun,† and Meng-Yuan He‡ Hefei National Laboratory for Physical Sciences at Microscale and Department of Materials Science & Engineering, UniVersity of Science and Technology of China, Hefei, 230026, China, and Department of Physics, Tongji UniVersity, Shanghai, 200092, China ReceiVed: August 27, 2010; ReVised Manuscript ReceiVed: October 08, 2010

Superparamagnetic colloidal nanochains (CNCs) coated and linked by carbon were prepared by solvothermal decomposition of ferrocene at a temperature of 180-200 °C with a 0.20 T magnetic-field applied. It is demonstrated that CNCs with different intraparticle spaces (216, 186, and 173 nm) can be obtained by changing the reaction temperature. Under the induction of an external magnetic field (e.g., 0.10 T), the ethanol suspension of the three samples can diffract different wavelengths of visible light, displaying red, green, and blue color, respectively. The three samples did not show the dependence of the diffraction wavelength on the strength of induction magnetic field, which indicates that the superparamagnetic CNCs can be used as magnetic-responsive Bragg reflectors. 1. Introduction Photonic crystals (PCs) have attracted great attention because of their broadly promising applications in many areas such as sensors, optical devices, and color displays.1-5 Most recently, great progress has been made in the investigation of magneticresponsive liquid PCs, a novel type of photonic crystal.6-12 Asher and co-workers first reported the fabrication of superparamagnetic PCs from highly charged, monodisperse superparamagnetic ∼134 nm polystyrene-iron oxide composite colloidal spheres.13 The magnetic response of the colloid spheres under magnetic fields is weak relative to interparticle electrostatic forces because of their low magnetization, leading to a limited tuning range and long response time of optical diffraction. Then, Yin et al. prepared superparamagnetic magnetite colloidal nanocrystal clusters with high magnetization and high water dispersibility, which demonstrates excellent magneticresponsive optical properties, including highly tunable stop bands and rapid and full reversibility.14,15 Soon after, our group also synthesized carbon-encapsulated magnetic-responsive PCs with highly colloidal stability.16 After being stored for 8 months in an ethanol solution, the PCs can still diffract obviously visible light when a magnetic field is applied. All the PCs mentioned above are composed of colloidal magnetic nanoparticles, which can form a one-dimensional (1D) chain structure because of the balance between the electrostatic force and magnetic force when an external magnetic field is applied. The feature of these 1D chain structures is that the separation of magnetic particles varied with the strength of an externally applied magnetic field, resulting in the variation of diffraction wavelength. A Bragg mirror or Bragg reflector (BR) consists of an alternating sequence of layers of two different optical materials and exhibits extremely high reflectivity to a certain range of light wavelength. Because of their special applications in integral optics devices, many kinds of solid BR have been synthesized.17-22 * Corresponding author. Fax and Tel: +81 551 3607292. E-mail: [email protected]. † University of Science and Technology of China. ‡ Tongji University.

However, so far, a liquid BR has hardly been realized yet due to the difficulty in arranging two different optical units distributed in alternating sequence in a liquid. In principle, magnetic-responsive PCs can be used for liquid BR by fixing their interparticle spacing. However, the applied magnetic field can easily influence the stability of interparticle spacing.15 As the strength of a magnetic field increases, the diffraction wavelength becomes shorter because of the decreasing of interparticle spacing. This result means that the diffraction wavelength is susceptible to the distribution of the magnetic field around the magnet. If a sample was induced by nonuniform magnetic fields, wide-band reflection will be observed because the particle spacing is not uniform.12,15,16 In practical application of liquid BRs, you must ensure a uniform magnetic field for the sample; in that case, the manufacturing cost for such a magnetic field will be greatly increased. Therefore, it is not suitable for the realization of a magnetic-responsive liquid BR to apply a magnetic field to normal magnetic-responsive PCs. One possible design is to prepare 1D chain structures with orderly imbedded magnetic nanoparticles; under the induction of a magnetic field, the chains in a liquid medium can be arranged along the magnetic line of force with their interparticle spacing within chains remaining unchanged, forming a liquid BR. Therefore, the synthesis of such a 1D chain is a key for producing magnetic-responsive BR. It is reported that a magnetic field can induce the formation of nanowires and nanochains.23-26 Under the induction of a magnetic field, the nanoparticles produced in a system can self-assemble into nanochains that could be stabilized by coating a surface layer such as carbon to form an outer shell structure. The interparticle spacing and carbon shell graphitization are reaction-temperature-dependent. Therefore, we could prepare superparamagnetic colloidal nanochains (CNCs) with different interparticle spacing, which can be used as a magnetic-responsive BR, by controlling the reaction temperature. In this paper, we describe how we have successfully prepared superparamagnetic carbon-coated CNCs with nearly identical gaps between every two adjacent particles using a modified reported procedure.25,26 These CNCs can diffract a single

10.1021/jp1081752  2010 American Chemical Society Published on Web 10/29/2010

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wavelength (red, green, and blue) under an external magnetic field, demonstrating magnetic-responsive Bragg reflector properties. 2. Experimental Section 2.1. Materials. Ferrocene [Fe(C5H5)2, g98%], hydrogen peroxide (H2O2, 30%), and acetone (C3H6O) (g99%) were of analytic grade (AR) from the Shanghai Chemical Factory. All chemicals were used as received without further purification. 2.2. Synthesis of Superparamagnetic Colloidal Nanochains. In a typical synthesis, 0.30 g of ferrocene was dissolved in 30 mL of acetone. After 30 min of intense sonication, 1.50 mL of hydrogen peroxide was slowly added into the above mixture solution, which was then vigorously stirred for 30 min with a magnetic stirring apparatus. After that, the precursor solution was transferred to three different Teflon-lined stainless autoclaves (40 mL) with a 0.20 T external magnetic field applied and then heated to 180, 190, and 200 °C, respectively. The autoclave was cooled naturally to room temperature after 72 h. After 15 min of mild sonication, the product from the Teflonlined stainless autoclave was separated for 5 min by a 0.20 T magnet, and the supernatant was discarded under the magnetic field. The precipitates were then washed with acetone three times. Finally, the black product was dried at room temperature in a vacuum oven and defined as sample 1 (S1, 180 °C), sample 2 (S2, 190 °C), and sample 3 (S3, 200 °C), which differed by reaction temperature. 2.3. Measurement of Reflectance Spectra. The CNCs solution (1 g/L) was obtained by dissolving solid sample into alcohol solvent. Then the alcohol suspension was transferred into a cuvette. By varying the external magnetic field strength, the reflectance spectra of CNCs can be obtained using a DUV3700 spectrometer. 2.4. Sample Characterization. The X-ray powder diffraction (XRD) patterns were collected on a Japan Rigaku D/MAX-γA X-ray diffractometer equipped with Cu KR radiation (λ ) 1.541 78 Å) over the 2θ range of 10°-70°. Transmission electron microscopy (TEM) images were obtained on a Hitachi H-800 transmission electron microscope, using an accelerating voltage of 200 kV. High-resolution transmission electron microscopy (HRTEM) images were taken on a JEOL-2010 transmission electron microscope operating at 200 kV. Field emission scanning electron microscopy (FE-SEM) images were performed on a JEOL JSM-6700 M scanning electron microscope. The Raman spectrum was taken on a LABRAM-HR confocal laser micro-Raman spectrometer using an Ar+ laser with the 514.5-nm line at room temperature. The FT-IR spectrum was obtained using a Magna-IR 750 spectrometer in the range of 400-4000 cm-1 with a resolution of 4 cm-1. A superconducting quantum interference device (SQUID, Quantum Design MPMS) magnetometer was used to measure the magnetic properties of the products. The reflective spectra were measured using a DUV-3700 spectrometer in the range of 300-800 cm-1 with a resolution of 0.10 nm. 3. Results and Discussion Typical X-ray diffraction (XRD) patterns of the products (S1, S2, and S3) are shown in Figure 1a. All of the peaks can be indexed with iron oxides (JCPDS file 19-0629, magnetite, or JCPDS file 39-1346, maghemite). Meanwhile, the five peaks at 656, 486, 404, 287, and 220 cm-1 in the Raman spectrum (Figure SI, Supporting Information) can be indexed to magnetite, maghemite, and hematite, which further reveals that iron oxides are constitutive of magnetite, maghemite, and hematite.27,28 The

Figure 1. (a) X-ray diffraction patterns and (b) FT-IR spectra of the as-obtained products (S1, S2, and S3).

strong peaks at 1592 and 1357 cm-1 in the Raman spectrum (Figure SI, Supporting Information) are assigned to the G and D mode of carbon, which indicates the existence of carbon in the sample.29,30 Figure 1b shows the FT-IR spectra of the asobtained products (S1, S2, and S3). The strong peaks at 583 cm-1 for S1, 587 cm-1 for S2, and 586 cm-1 for S3 reveal the existence of magnetite in the products.31 Meanwhile, The three peaks of 1636 cm-1 (S1), 1670 cm-1 (S2), and 1618 cm-1 (S3) can be attributed to the CdC vibration, indicating the existence of carbon-bearing molecules on the surface. Figure 2 shows representative SEM images of the as-obtained products (S1, S2, and S3), which reveals that these samples contain linear chains of nanoparticles. A typical single nanochain is enlargely shown in Figure 2b,d,f, which demonstrates that these nanoparticles are coated and linked by carbon materials derived from the decomposition of ferrocene, leading to the formation of stabilized self-assembled nanochains in the reaction system. Figure 3 shows that each nanochain is, with an average length of about 2 µm, composed of 150 nm nanoparticles in varying numbers. TEM images of single nanochains from Figure 3 are shown in Figure SII (Supporting Information), which clearly demonstrates the core-shell morphology of nanochains. The intraparticle spacing of a single nanochain, measured from Figure SII (Supporting Information), is about 216 nm for S1, 186 nm for S2, and 173 nm for S3, respectively. The structure of a single nanoparticle forming CNCs has been investigated by high-resolution transmission electron microscopy (HRTEM) (Figure 4). The colloidal nanoparticle is composed of dozens of iron oxide nanocrystals with a size of about 9.8 nm (calculated by Debye-Scherrer formula) and demonstrates a shell structure with a thickness of 23 nm. The clear 2D lattice fringes are shown in Figure 4c, and the interplanar distance is about 0.288 nm, which corresponds to the (220) lattice planes of magnetite. The clear diffraction rings from the selected-area electron diffraction (SAED) pattern (Figure 4d) indicate the polycrystalline nature of the as-obtained products, whose arcs

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Figure 2. SEM images of the as-obtained products: (a, b) S1, (c, d) S2, and (e, f) S3.

Figure 3. TEM images of the as-obtained products (S1, S2, and S3).

and rings could be indexed as (111), (220), and (311) reflections for magnetite, respectively. Combined with the results of FTIR and Raman, it is suggested that the colloidal nanoparticles were coated by carbon layers. Figure 4b further confirms the amorphous nature of the carbon shell. According to previous reports, this unique nanostructure should have superparamagnetic nature,14 which was confirmed by magnetic measurements shown in Figure SIII (Supporting Information). Figure 5 displays a schematic illustration of the formation of carbon-coated CNCs. At the beginning of the reaction (stage I), the Fe-(C5H5)2 decomposes into iron particles and C5H5 fragments. Under the assistance of hydrogen peroxide, iron particles are oxidized into iron oxide nanocrystals. According to the two-stage growth model,32 iron oxide nanocrystals are likely to nucleate and grow first in a supersaturated solution and then aggregate into larger secondary colloid particles, which has been confirmed in Figure 4b. According to previous reports,25 organic materials with conjugated double bonds can be adsorbed on the surface of iron oxide particles by chemical bonding, forming core-shell structures. As the temperature increases, the organic materials further decompose into carbon to generate iron oxide-carbon core-shell nanostructures.

Because of the existence of an external magnetic field (0.20 T) in the reaction system (stage III), the secondary magnetic colloid particles will align to form linear chains in a head-to-tail configuration. The carbon shell will be graphitized in the hightemperature and high-pressure environment (stage IV), leading to the formation of carbon-coated nanochains with a high suspension stability. It is found that the intraparticle separation is temperature-dependent: a higher reaction temperature tends to yield a smaller intraparticle space. As shown in Figure 2, nanochains with three kinds of intraparticle space can be obtained as the reaction was carried out at 180, 190, and 200 °C, respectively. Figure 6 shows the photos due to strong optical diffraction of the ethanol suspensions (1 g/L) of these CNCs under the induction of an external magnetic field. Three visible colors can be obviously observed: red for S1, green for S2, blue for S3, respectively. The optical diffraction of these CNCs under an external magnetic field has angular dependence. We can only see reflected light along the direction of magnetic field, and all the photos were taken in front of the glass bottles containing CNCs. Generally, diffraction occurs when the periodicity of the order structure and the wavelength of the incident light satisfies

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Figure 4. (a, b) HRTEM image of single magnetite nanoparticle of S2 and (c, d) lattice fringe and electron diffraction (ED) pattern of nanoparticle.

Figure 5. Schematic illustration of the formation of carbon-coated CNCs: stage I, decomposition of ferrocene; stage II, aggregation of nanocrystals; stage III, alignment of nanoparticles by magnetic-fieldinduced assembly; and stage IV, graphitization of the carbon shell outside the nanoparticles.

the Bragg condition.33,34 Therefore, the diffraction phenomenon of these CNCs under magnetic fields indicates the formation of spatially periodic structures along the direction of the magnetic field. Figure 7 is the SEM images of these CNCs under magnetic fields. It shows that these CNCs can orderly align along the magnetic line of force. This type of PC exhibits optical diffraction of a single wavelength, nearly independent of the applied magnetic field, which is different from magnetically tunable PCs,13,15,16 showing potential applications as magneticresponsive liquid BRs. Figure 8 shows the reflective spectra of the ethanol suspension (1 g/L) of these CNCs (S1, S2, and S3) in response to a varying magnetic field. It is very clear that the diffraction peaks of these CNCs (S1, S2, and S3) can be obviously observed when a magnetic field was applied by comparing the reflectance spectra

Figure 6. Photograph of the as-obtained products (S1, S2, and S3) in response to an external magnetic field (at the back): left, 0 T; right, 0.20 T.

at zero magnetic field (Figure SIV, Supporting Information) and the diffraction wavelength of the three samples under a 0.10 T magnetic field centered at 613 nm (red) for S1, 526 nm (green) for S2, 480 nm (blue) for S3, respectively. As the strength of the applied magnetic field increase to 0.20 T, the diffraction wavelength of S2 and S3 stays at a fixed value of 526 and 480

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Figure 7. SEM images of the as-obtained products (S1, S2, and S3) obtained under external magnetic field (0.20 T). The arrows indicate the direction of the external magnetic field.

Figure 8. Reflectance spectra of the as-obtained products (S1, S2, and S3) under different magnetic field strength.

nm, respectively. The diffraction peak of S1 shifts only 8 nm from 613 to 605 nm as the magnetic field increases from 0.10 to 0.20 T. This is because S1 was prepared at 180 °C, lower than the temperature of the other two samples, so the carbon layer could be much softer, as the magnetic-field-induced dipolar interaction of those colloidal nanoparticles embedded in the carbon shell is strong enough; thus, the deformation of the carbon layer would occur, which will lead to colloidal nano-

particles packing more closely and the correspondent diffraction wavelength variation. It is known that a diffraction of light in the visible region might occur if the composite structure has periodic variations of the dielectric constant on the order of an optical wavelength. According to Bragg’s law,15 the lattice plane spacing of these magnetic-responsive BR (S1, S2, and S3) should be 222, 193, and 176 nm, respectively. These values are basically consistent with the actual separations measured from TEM images. Because these stable nanochains have a fixed intraparticle space, as long as a magnetic field can induce these nanochains to align along the magnetic line of force in a solution, strong optical diffraction can be observed. And the diffraction wavelength will remain unchanged under varied magnetic field strengths, which indicates the formation of magnetic-responsive BRs.35 When an external magnetic field is applied, the nanochains respond by aligning along the direction of the magnetic field. Moreover, it is confirmed that carbon-coated mgnetite nanoparticles produced by decomposition of ferrocene molecules exhibit negative ζ-potential due to the presence of negatively charged groups (carboxyl) on the carbon layer;16 the existence of electrostatic repulsion among nanochains leads to them being loosely arranged along the direction perpendicular to the magnetic field, just as shown in Figure 9. Hence, this assembly structure can be modeled as a 1D PC that consisted of two alternative layers in each period. One layer is the singlenanoparticle layer whose thickness dA can be regarded as the average diameter of the nanoparticle, and the other is the ethanol whose thickness dB is determined by the period “a” (the intraparticle distance) and dA by the relation a ) dA + dB. The total number of periods is determined by the length of the chains, which are about 10 on average. The band structure of S2 is calculated as an example to illustrate the validity of this simplified model. The corresponding parameters for S2 we measured directly from TEM image to be dA ) 150.3 nm, dB ) 35.7 nm, and a ) 186 nm. The dielectric constant and the permeability are two key parameters in the 1D PC model, since their periodicity and contrast in two adjacent layers determine the absolute photonic band gap (PBG) width. According to the literature,36 the dielectric constant of the nanoparticle layer εA ) 2.2,2 the permeability is 1 in the visible frequency range, and εB ) 24.3. The graphite carbon shell of the nanoparticle has similar dielectric constant to the magnetite inside it and the influence can be neglected without influencing the whole particles’ dielectric constant which then is taken to be that of the magnetite. With these two parameters and the thicknesses of the corresponding layers available, we can calculated the band structure of our samples when the magnetic field is applied perpendicular to the stack using the transfer-matrix method.37,38 The band structure of S2 is illustrated in Figure 9 as an example.

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Figure 9. (Left) A typical schematic of one-dimensional PC. (Right) The band structure of S2.

In the band diagram (for the convenience of comparison, we use wavelength instead of frequency here), band gap appears where no propagation of light is allowed and corresponds to relatively high reflection regions, which is the key feature of the BR. In addition, the band gap center wavelength is pointed out, which basically fits the reflection peak in our experiments (526 nm) in the allowed error range. The deviation was observed, which may have resulted from the inaccuracy of dielectric constant and length of the nanochains. Magnetic-responsive liquid PCs have been prepared by several groups,13-16 and it is suggested that colloidal magnetic nanoparticles will form nanochains when an external magnetic field is applied. However, no direct evidence exists from a solid sample to show that such nanochains did form; one might argue that another kind of orderly structure might be responsible for the optical diffraction phenomenon observed. First of all, we prepared carbon-coated magnetite nanochains, put them in ethanol to form a suspension, and found they can diffract visible lights under a magnetic field, which provide solid evidence that formation of magnetic nanochains is essential to produce magnetically responsive liquid PCs. It has been observed that magnetite nanochains with nearly identical gaps between every two adjacent particles display ferromagnetic properties because the orderly arranged magnetite particles have strong interparticle dipolar interactions,39 which indicate that our nanochains function as magnetic dipoles that could have a high potential for biotechnological and biomedical,39 micromechanical sensors, and magnetic switching applications.40,41 4. Conclusions In summary, we have successfully realized the synthesis of magnetically responsive BR composed of superparamagnetic CNCs with nearly identical gaps between every two adjacent particles. By controlling the reaction temperature, nanochains with different intraparticle space (216, 186, and 173 nm) can be prepared. Under the induction of an external magnetic field, the ethanol suspension of the three samples can diffract different wavelengths of visible light; red, green and blue colors were observed for the nanochains with intraparticle space of 216, 186, and 173 nm, respectively. This result confirms that 1D wire with orderly imbedded magnetic nanoparticles in carbon shells can be taken as a BR with two different optical units distributed in alternating sequence, which can be used in magnetically responsive liquid BRs. Acknowledgment. We acknowledge the financial support by the project of National 863 Hi-Tech Plan (2008AA06Z337) and

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