Magnetic Rattle-Type Core–Shell Particles Containing Iron

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Magnetic Rattle-Type Core−Shell Particles Containing Iron Compounds with Acid Tolerance by Dense Silica Tomohiko Okada,*,† Shoya Ozono,† Masami Okamoto,† Yohei Takeda,† Hikari M. Minamisawa,§ Tetsuji Haeiwa,∥ Toshio Sakai,† and Shozi Mishima† †

Department of Chemistry and Material Engineering, Faculty of Engineering, Shinshu University, Wakasato 4-17-1, Nagano 380-8553, Japan § Technology Division, Faculty of Engineering, Shinshu University, Wakasato 4-17-1, Nagano 380-8553, Japan ∥ Department of Computer Science and Engineering, Faculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan S Supporting Information *

ABSTRACT: Magnetic rattle-type particles, comprising magnetite or metallic iron in nonporous dense hollow silica microspheres, were fabricated by using sol−gel reactions of alkylsilyl trichlorides around droplets of aqueous iron nitrate solution in a water-in-oil emulsion. After evaporation of water within the silica capsules to leave iron salts, calcination of the dried sample was conducted to transform into a hematite (α-Fe2O3) core and porous hollow silica shell by losing alkyl groups of polyalkylsiloxane. Hydrogen gas penetrated through the silica shell and reduced hematite to magnetite (Fe3O4) at 310 °C and metallic iron (α-Fe) at 450 and 500 °C. The reduction at 310 °C resulted in largest magnetization at 12 kOe among the present magnetic particles. The core magnetic compounds were enclosed by a dense silica shell, which was transformed from porous silica by annealing in nitrogen at 700 °C. Because the magnetic particles were encapsulated by the dense silica shell, the magnetism was shown even after immersion in 1 M HCl for a longer period. Acidity was successfully imparted on this magnetic capsule by anchoring sulfonic groups covalently for its use as magnetically collectable solid acid.

1. INTRODUCTION Encapsulation of magnetic nanoparticles in confined spaces has received much attention in many areas of interest, including separation, catalysis, transportation, and biomedical science.1−6 It is necessary to prevent the nanoparticles agglomerating in order to maintain their nanomagnetic functionality. Thus, core−shell, or A@B particles (A, core; B, shell) have been prepared using various synthetic strategies, including selfassembled monolayers, layer-by-layer deposition, and sol−gel reactions.6−9 The rattle-type hollow structure (i.e., nanoparticles surrounded by interstitial hollow space) has also received increasing interest as useful nanoreactors (i.e., for drug delivery,10−12 catalysis with molecular sieving,13 and separation by magnetism14). Chemically stable magnetism, where the magnetic particles are protected from oxidation and dissolution, is a prerequisite for reusable adsorbents, catalysts, and biomedical agents. Therefore, the stability of the nanoparticles has been investigated in various atmospheres and liquids.15−24 Silica is a useful shell substance owing to its acid-tolerance23 as well as its structural and morphological forms. A dense shell, which prevents the passage water, is necessary to protect core magnetic compound. The thickness and density of silica shell can be controlled by varying the amount of silica precursors and by degree of the polycondensation, respectively, to provide chemically stable magnetic nanoparticles. We have reported a rattle-type architecture by which a polyorganosiloxane shell is deposited at the interface of a waterin-oil (W/O) emulsion to encapsulate metallic cobalt through © 2014 American Chemical Society

the sol−gel reactions of octyltrichlorosilane (OTCS) and methyltrichlorosilane (MTCS) around the droplets25,26 of Co(NO3)2 aqueous solution.27−29 In studies by which the interface of an emulsion (or a reverse micelle) has been used to obtain metal nanoparticles coated by shells,30−34 deposition of a shell at the liquid−liquid interface has been recognized as a synthetic route that is free from the need to use a solid template (i.e., organic polymer spheres). In addition, shell deposition is a useful way to occlude magnets with varied compositions of the liquid droplets for versatile magnetic properties. Here, we report chemically stable rattle-type magnetite or metallic iron nanoparticles encapsulated by dense silica hollow microcapsules. After the water droplets containing iron(III) nitrate have been stabilized by hydrolyzed OTCS in isooctane, MTCS has been added to polymerize to a polyalkylsiloxane shell, which is the precursor of the silica shell. The cooperative sol−gel reactions are followed by vaporization of water from the resulting capsules to leave the Fe salt. Calcination, which loses the alkyl groups in the shell, reduction of α-Fe2O3 by hydrogen gas, and final annealing in nitrogen resulted in magnetic particles enclosed by a dense silica (Scheme 1). Because of the dense silica shell, the magnetism persisted even after treatment with 1 M HCl without substantial erosion of the magnetic particles. Because iron is an abundant and less Received: Revised: Accepted: Published: 8759

February 10, 2014 May 5, 2014 May 6, 2014 May 6, 2014 dx.doi.org/10.1021/ie500588j | Ind. Eng. Chem. Res. 2014, 53, 8759−8765

Industrial & Engineering Chemistry Research

Article

Scheme 1. Schematic Drawing of the Procedure on Encapsulating Magnetic Particles by a Dense Silica

The acid-treated samples were recovered using neodymium magnets, whose surface magnetic field is 3.8 kOe, located at both sides of a glass bottle (outer diameter of 21 mm). The magnetic field gradient from the center of the glass bottle to the magnet was 1.9 × 102 kOe/m. The recovered samples were thoroughly washed with water before further magnetic recovery to obtain FexOy@SiO2 (z) particles, where z is the temperature for the reduction by H2 flow (z = 310, 450, and 500 °C). The amount of Fe in the FexOy@SiO2 (310) particles was estimated by ICP analysis after it was converted to aqueous form by alkali fusion. Stability tests of FexOy@SiO2 (310) for acidic solution and for oxygen gas were conducted by immersing the sample (0.05 g) in an aqueous solution of 1 M HCl (6 mL) for 5 months, and by treating in air at 600 °C for 3 h, respectively, followed by a vibrating sample magnetometer analysis. 2.3. Scale-up for the Preparation of the Core−Shell Particles. Amount of all the chemicals and reagents used for production of the W/O emulsion was increased to 10-fold; water (2.5 mL), an aqueous solution of Fe(NO3)3 (7.5 mL, 3.0 mol/kg), and OTCS (29.7 g) in isooctane (500 mL) were mixed by ultrasonic agitation. An ultrasonicator with high power (28 kHz, 300 W) was used for homogeneous mixing, because the dispersion stability of the emulsion was poor for mixing such large volume by using an ultrasonicator with low power (45 kHz, 100 W). After the ultrasonic irradiation for 10 min, the mixture was allowed to stand for 5 min at room temperature. The ultrasonic agitation (5 min) was then once performed. MTCS (13.4 g) in isooctane (100 mL) was poured into the W/O emulsion under magnetic stirring. The stirring was continued at room temperature for more than 3 h with supplying air with saturated water vapor (ca. 0.1 L/min). Filtration, washing, drying, and calcination were conduced as same as the above-described procedure. 2.4. Immobilization of Sulfonic Groups on FexOy@ SiO2. Reactions of MPS with FexOy@SiO2 (310) particles were

harmful natural resource, the resulting core−shell particles are practically advantageous in industrial chemistry and biomedical engineering, for separation, transportation, and imaging by external magnetic fields.

2. EXPERIMENTAL SECTION 2.1. Chemicals and Reagents. Octyltrichlorosilane (OTCS) and methyltrichlorosilane (MTCS) were purchased from Aldrich Chemical Co., Ltd., and Shin-Etsu Chemical Co., Ltd., respectively. Iron(III) nitrate nonahydrates, and isooctane (2,2,4-trimethylpentane), hexadecyltrimethylammonium bromide, 3-mercaptopropyl(trimethoxysilane) (MPS) were purchased from Wako Chemical Co., Ltd.. These materials were used as received. 2.2. Fabrication of Rattle-Type Magnetic Particles Encapsulated by a Dense Silica Shell (Scheme 1). A W/O emulsion was prepared based on a previous report28 by mixing water (0.22 mL), an aqueous solution of Fe(NO3)3 (0.75 mL, 3.0 mol/kg), and OTCS (2.97 g) in isooctane (50 mL) by ultrasonic agitation. After 5 min ultrasonic irradiation (45 kHz, 100 W), the mixture was allowed to stand for 5 min at room temperature. The ultrasonic agitation was repeated three times. MTCS (1.34 g) in isooctane (10 mL) was poured into the W/ O emulsion under magnetic stirring. The mixture was stirred at room temperature for more than 3 h to form a polymethylsiloxane shell around aqueous droplets. During the reaction, air with saturated water vapor (ca. 0.1 L/min) was continuously supplied. After filtration, the product was washed with isooctane and dried at 50 °C for 1 day. The dried product was heated at 120 °C in air for 1 day and then calcined at 600 °C in an electronic furnace for 3 h. The calcined solid (0.2 g) was treated under a flow of H2 (5 mL/min) at the temperatures of 310, 450, and 500 °C for 3 h, and subsequently heated at 700 °C for 3 h in a flow of N2 (10 mL/min). After the solution was cooled in the N2 flow to 50 °C, the resulting products were washed with 1 M HCl for 1 day. 8760

dx.doi.org/10.1021/ie500588j | Ind. Eng. Chem. Res. 2014, 53, 8759−8765

Industrial & Engineering Chemistry Research

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carried out using a procedure reported previously.29 Before the silylation, FexOy@SiO2 (0.52 g) was immersed in an aqueous 1 M NaOH solution (200 mL) for 12 h at room temperature. The alkali-treated particles were dispersed in a solution of hexadecyltrimethylammonium bromide (1.3 g) in water/ ethanol (1:1 v/v). The resulting solid was transferred into a solution of MPS (0.4 mL) in dry toluene (80 mL). The mixture was heated for 3 h at 60 °C and then concentrated under reduced pressure at 60 °C for 2 h to evaporate the toluene. The product was washed with a mixture of 0.1 M HCl aqueous solution and ethanol (1:1 v/v). The washed solid was allowed to react with aqueous 7 M HNO3 (50 mL) for 6 h. After the solid product was washed with water, it was magnetically collected using the neodymium magnets, and the supernatant was decanted. The product was dried at 50 °C in air. The cation exchange capacity was determined by the reaction of NaCl with the protons of the trisilylpropylsulfonic groups. The sample was allowed to react with an aqueous 4 mass % NaCl solution, and then the resulting solid was recovered with the neodymium magnet. These steps were repeated to complete the exchange reactions. The supernatant was titrated with NaOH aqueous solution. 2.5. Instruments. X-ray powder diffraction (XRD) patterns were obtained by using Rigaku RINT 2200V/PC diffractometer (monochromatic Cu Kα radiation), operated at 20 mA and 40 kV. Fourier transform-infrared (FT-IR) spectra were recorded on a JASCO FT/IR-4200 spectrophotometer using the KBr pellet method. Scanning electron micrographs were obtained on a Hitachi S-4100 field-emission scanning electron microscope (SEM) with an accelerating voltage of 15 kV. Scanning transmission electron micrographic (STEM) observations were performed on a Hitachi High-Tech HD-2300A spherical aberration corrected scanning transmission electron microscope. The transmission electron micrographs were obtained partly using a JEOL JEM-2010 transmission electron microscope. Nitrogen adsorption−desorption isotherms at −196 °C were obtained by using a BEL Japan BELSORP-mini instrument. Before the adsorption experiments, the samples were heat-treated at 473 K under reduced pressure. ICP-AES was performed on a Shimadzu ICPS-7500 spectrometer. Magnetization curves were obtained from a Toei Kogyo VSM-5S vibrating sample magnetometer (VSM).

Figure 1. XRD patterns of (a) precipitate calcined at 600 °C, (b) FexOy@SiO2 (310) (c) FexOy@SiO2 (450) (d) FexOy@SiO2 (500).

Figure 2. FT-IR spectra of (a) the dried precipitate and (b) the calcination product. Spectrum c was recorded after annealing at 700 °C in nitrogen.

using Co(NO3)2 as the starting material (0.43 ± 0.42 μm, Figure S2b),29 relatively larger spherical particles (0.92 ± 0.57 μm) were obtained, reflecting difference in the droplets size in the emulsion from the nature of the metal salts. Large-scale production of the hematite-microporous spherical silica hybrid particles (10-fold amount: yield of 9 g per one batch) was attained by scaled-up producing emulsion using a high-powered ultrasonicator (28 kHz, 300W); grain diameter of the microspheres fabricated in the large scale (0.85 ± 0.75 μm, Figure S3a in the Supporting Information) was close to that in the original scale (0.92 ± 0.57 μm, Figure S3b in the Supporting Information). The reduction of α-Fe2O3 encapsulated sample in H2 gas at 310 °C was followed by heating in N2 at 700 °C, in order to avoid oxidation of the magnetic phase. After the sample was washed with 1 M HCl, the acid-treated sample could be collected thoroughly by a magnet (FexOy@SiO2 (310)). Under our experimental conditions, it took no more than 1 min to gather the particles from the acidic solution. In the IR spectrum of the FexOy@SiO2 (310) (Figure 2c), the absorption bands attributed to the organic moieties were not observed, showing that the shell was transformed into silica. A decrease in the specific surface area by annealing from 27 to 4 m2/g suggests

3. RESULTS AND DISCUSSION 3.1. Fabrication of FexOy@SiO2 Core−Shell Particles. Ultrasonic agitation of a mixture of OTCS dissolved in 2,2,4trimethylpentane with Fe(NO3)3 aqueous solution formed a yellow W/O emulsion. A light brown precipitate was produced by addition of MTCS to the emulsion. When the dried precipitate was calcined at 600 °C, a reddish powder was obtained that contains α-Fe2O3 (hematite),35 as evidenced by the XRD pattern (Figure 1a). Because α-Fe2O3 is paramagnetic, the calcined sample did not gather near a magnet (the photograph is shown in the Supporting Information, Figure S1). The FT-IR absorption bands of the polyalkylsiloxane alkyl groups (νC−H at around 2900 cm−1) and the Si−C bond (at around 1300 cm−1) are weak, indicating that calcination turned most of the polyalkylsiloxane to silica (Figure 2). The specific surface area derived from Brunauer−Emmett−Teller (BET) equation of the calcined sample N2 adsorption isotherm was 27 m2/g. A SEM image of the calcined sample shows that the morphology is basically spherical (Supporting Information, Figure S2a). If compared with the size distribution in case of 8761

dx.doi.org/10.1021/ie500588j | Ind. Eng. Chem. Res. 2014, 53, 8759−8765

Industrial & Engineering Chemistry Research

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Figure 3. SEM image of FexOy@SiO2 (310) particles and their grain size distribution.

that the porous silica shell turned to a dense nonporous phase. It has been reported that γ-Fe2O3 (maghemite) is metastable substance; γ-Fe2O3 transforms into α-Fe2O3 (hematite) at temperatures of 200−700 °C.36,37 Because the present sample was heat-treated at 700 °C after the reduction, the observed diffraction peaks in the XRD pattern of FexOy@SiO2 (310) (Figure 1b) are ascribable to Fe3O4 (magnetite, JCPDS 880315), whose peaks are close to those of γ-Fe2O3.38 The diffraction peaks due to FeO (wüstite, JCPDS 86-2316) were also observed. Thus, it is obvious that hydrogen gas was penetrated into the porous silica shell during the reduction at 310 °C. In addition, the porous silica shell was transformed into dense nonporous silica after heat treatment at 700 °C in N2, although the magnetite remained in the silica unchanged. Judging from the SEM observation (Figure 3), the spherical shape was maintained after agitating in 1 M HCl. Almost all the grains were less than 3 μm. Although a huge grain (>10 μm) was occasionally observed, the distribution was quite small (less than 1%). The average grain size of FexOy@SiO2 (310) was 0.63 μm (see inset of Figure 3). Figure 4a−c shows SEM, TEM, and STEM dark-field images of FexOy@SiO2 (310). A rattle-type core−shell structure was observed in the TEM image for most of the particles. The STEM dark-field image confirmed that the core and shell were composed of Fe3O4 and silica, respectively, because contrast of the core is brighter than that of the shell. The majority of the Fe3O4 agglomerated inside the hollow silica spheres. Deformed silica microcapsules enclosing magnetic iron compounds were observed to some extent (Supporting Information, Figure S4). When the reduction temperature was increased to 450 °C, the diffraction peak ascribed to α-Fe39 appeared, showing the reduction of α-Fe2O3 by H2 gas (Figure 1c). Reduction of the ferrous ions of FeO proceeded as shown by the fact that the H2-treatment at 500 °C led to a decrease in the diffraction peaks due to FeO (Figure 1d). TEM images of the three samples are shown in Figure 5. Although it was difficult from the TEM to make a differentiation among these iron compounds formed depending on the reduction temperature, the iron compounds are shown to be located inside the hollow silica. 3.2. Magnetic Properties of FexOy@SiO2 Samples. Figure 6 shows the magnetization curves of the FexOy@SiO2 (310), (450), and (500) samples at room temperature. These curves exhibit hysteresis loops typical of a soft ferromagnetic behavior: an elevated magnetic susceptibility with a small coercive field. Values of magnetization at 12 kOe (Ms), remanence (Mr) and coercive force (Hc) are presented in Table

Figure 4. (a) SEM image, (b) bright-field TEM image, and (c) STEM dark-field image of FexOy@SiO2 (310).

1. The Ms value of FexOy@SiO2 (310) was 4.0 emu/g. It has been reported that bare Fe3O4 nanoparticles are nearly in their superparamagnetic regime at 27 °C with its Ms value of ∼80 emu/g (bulk magnetite).40 Considering the amount of Fe enclosed in FexOy@SiO2 (310) (6.3 mass %) and the Ms value of 4.0 emu/g, the majority of the included Fe compounds in the silica was assumed to be Fe3O4. Relatively sharp loops have been observed for the bare Fe3O4 (Mr: