Highly Ordered Ferromagnetic Mesoporous Materials: Thermal Phase

Jul 7, 2010 - (26, 27) However, a systematic investigation of the thermal phase transformation of ferrous ions to iron oxide within ordered mesoporous...
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J. Phys. Chem. C 2010, 114, 12440–12445

Highly Ordered Ferromagnetic Mesoporous Materials: Thermal Phase Transformation Pathway and Quantitative Determination Sang Hwa Kang, Jiho Lee, and Jeong Ho Chang* Korea Institute of Ceramic Engineering and Technology, Seoul 153-801, Korea ReceiVed: March 30, 2010; ReVised Manuscript ReceiVed: May 9, 2010

Highly ordered ferromagnetic mesoporous materials with a high abundance and homogeneous dispersion of iron oxide within the mesopores were prepared through thermal oxidation at various temperatures after impregnation of ferrous ions into ordered mesoporous channels. The pathway for the phase transformation to iron oxide in ordered mesoporous channels is found to be different from the conventional transformation pathway. Rokuhnite, feroxychloride, and hematite are formed from Fe2+ ions at oxidation temperatures of 80, 300, and 450 °C, respectively. The transformation pathways are identified from the phase morphology and crystallinity of the obtained products by wide- and small-angle X-ray diffraction and by analyzing the electron diffraction patterns and images. Moreover, the high abundance and homogeneous dispersion of FeO within the mesopores were quantitatively determined up to 1150 ppm/g at various temperatures. Introduction Highly ordered mesoporous materials have attracted much attention due to their outstanding structural characteristics, such as high surface area, narrow pore size distribution, and wellordered arrangement of mesopores, making them suitable for a wide variety of applications.1-5 Significant advances in synthetic approaches have made it possible to synthesize various kinds of mesoporous materials with various components. In particular, the synthesis of magnetic mesoporous materials has seen extensive research, largely owing to their ease of handling by use of a magnet.6-12 Traditionally, magnetic mesoporous materials have been obtained by embedment or incorporation of magnetic nanoparticles, such as metal or iron oxide, within mesopores.13-17 However, with these methods, it is difficult to achieve a homogeneous dispersion of the resulting magnetic iron oxides in the ordered mesoporous channels, and this may lead to clogging of the matrix pores and a decrease in the surface area and magnetic susceptibility. Several strategies to overcome this problem have been reported. A simple block-copolymerbased self-assembly approach was developed to incorporate magnetic nanoparticles in the mesoporous walls.14 However, the amount of nanoparticles that can be embedded in the framework is limited, posing a serious problem for higher magnetizations. Another route is hard templating, which involves the deposition of metal oxides within the templates and subsequent removal of the templates.18-23 This nanocasting approach is widely applicable to the preparation of metal oxides that are difficult to synthesize by conventional pathways. However, this approach suffers from a lower saturation magnetization relative to methods utilizing pure metals. The saturation magnetization of mesoporous materials depends on the compositions and amounts of magnetic phases. Recently, it was reported that an approach using a multicomponent mesostructured alloy with controllable compositions is more convenient than the hard-templating method, as the surfactant can be removed by simple extraction.24,25 Although a number of approaches for preparing magnetic mesoporous materials have been reported, a notable obstacle * To whom correspondence should be addressed. Tel: 82-2-3282-2459. Fax: 82-2-3282-7811. E-mail: [email protected].

must still be overcome: the magnetic nanoparticles within the mesopores, especially iron oxides, are readily affected by thermal variation due to their poor crystallinity and low composition ratio. In particular, calcination of the resulting particles at elevated temperatures leads to a partial phase transformation to iron oxide and rapid in situ formation of nonmagnetic hematite. It is well known that free Fe ions (ferrous ions) can be transformed to hematite via three different pathways.26,27 However, a systematic investigation of the thermal phase transformation of ferrous ions to iron oxide within ordered mesoporous silica channels during calcination has yet to be reported. To determine the pathway by which intermediates are converted to hematite during heat treatment, it is necessary to investigate the thermal phase transformation of composite particles in ordered mesoporous channels at different calcination temperatures. In this work, we report on the preparation of highly ordered ferromagnetic mesoporous materials with a high abundance and a homogeneous dispersion of iron oxide within the mesopores. We also propose the thermal phase transformation pathway with oxidation at various temperatures. Furthermore, we have carried out detailed investigations of various pathways for the phase transformation of ferrous ions to iron oxide within the ordered mesoporous channels at various thermal oxidation temperatures. We expect that the present findings will be helpful for determining the applicability of iron oxide magnetic particles in mesoporous channels in various fields. Experimental Section Materials Synthesis. In a typical preparation of the highly ordered mesoporous silica, 4.0 g of Pluronic P123 was dissolved in 30 g of water and 120 g of 2 M HCl, and then 8.5 g of tetraethyl ortho silicate (TEOS) was added into the solution at 40 °C. The mixture was aged in a stainless steel bomb at 120 °C overnight without stirring. The solid product was filtered, washed with excess water, and air-dried at room temperature. Calcination was carried out at 550 °C for 6 h. To demonstrate the thermal phase transformation to iron oxides within ordered mesoporous channels, the ferrous ion from iron chloride tetrahydroxide (FeCl2 · 4H2O) was incorporated in mesoporous

10.1021/jp1028307  2010 American Chemical Society Published on Web 07/07/2010

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Figure 1. Pathway of thermal phase transformation of ferrous ions to iron oxides within the ordered mesopores at different oxidation temperatures and initial molar concentrations of ferrous ions. The photographs represent the color change of the obtained materials at each oxidation temperature.

silica as a function of molar concentration to 3 M from 0.5 M. The thermal oxidation was achieved at room temperature to 500 °C in air. Instrumental Analyses. The structural characterization of the formed iron oxides within the ordered mesoporous channels at various oxidation temperatures was achieved by the instrumental analyses, such as wide- and small-angle XRD, DSC, TGA, and SQUID. The nitrogen adsorption and desorption isotherms were measured using a Quantachrome Autosorb-6 system. Surface areas and pore size distributions were calculated from the analysis of the adsorption branch of the isotherm using the Barrett-Joyner-Halenda (BJH) method. The pore volume was taken at the five points of P/P0. The SAXD patterns were TABLE 1: Nitrogen Adsorption and Desorption BET Parameters of Ordered Ferromagnetic Mesoporous Materials as a Function of Molar Concentration of Initial Ferrous Ions concn of initial Fe2+@MS (M)

oxidation temp (°C)

SBET (m2/g)

avg pore size (nm)

pore vol (mL/g)

0 0.5

25 80 300 450 80 300 450 80 300 450 80 300 450 80 300 450 80 300 450

722 475 443 558 396 376 277 420 208 265 238 207 288 172 234 274 95 115 226

8.8 8.8 8.7 8.7 8.7 3.6 3.6 8.7 3.6 3.6 8.7 3.6 3.6 8.7 3.6 3.6 8.7 3.6 3.6

1.03 0.86 0.65 0.65 0.72 0.39 0.39 0.71 0.18 0.18 0.42 0.30 0.30 0.30 0.38 0.38 0.18 0.49 0.49

1.0 1.5 2.0 2.5 3.0

taken with 40 kV, 160 mA Cu KR radiation using a Rigaku Denki instrument. The scattered intensity was measured over the scattering vectors, q ) (4π/λ)sin θ, where 2θ is the total scattering angle and λ is the wavelength (Cu KR radiation, λ ) 1.54 Å) generated from a rotating anode source that was monochromatized by a crystal monochromator. The scattering curves were measured by using a point focusing scintillation detector. The electron microscopic measurements, such as TEM and SEM, were achieved by a JEOL JEM-4010 TEM (400 kV) and a JEOL JSM 6700F, respectively. Infrared (IR) and ultraviolet-visible (UV-vis) spectra were obtained from a JASCO V-460 and JASCO V-550, including an attenuated total reflectance Fourier transform IR (ATR-FTIR) technique and diffused reflectance UV-vis spectroscopy, respectively. To determine Fe ions quantitatively, the ICP-OES titration was used as follows: A 0.1 g portion of sample was put into the mixture solution of 40 mL of 85% (w/w) phosphoric acid and 5 mL of 1% (w/w) ammonium metavanadate solution in 0.9 M sulfuric acid, and then 5 mL of the 48% (w/w) hydrofluoric acid was added. The solution was kept in a water bath at 60 °C to dissolve the samples perfectly. The back-titration was achieved by 0.05 N potassium dichromate after addition of (NH4)2SO4 · FeSO4 · 6H2O to reduce the vanadate completely. The equation that gives the total amounts of FeO is as follows17

%FeO ) [NK2Cr2O7 × (Vsample - Vblank) × 7.185]/msample where NK2Cr2O7 is the normality of potassium dichromate, Vsample and Vblank are the titrant volume of the sample and the blank, respectively, and msample is the initial sample weight in grams. Results and Discussion The thermal phase transformation in the mesopores was investigated as a function of oxidation temperature and initial molar concentration of ferrous ions (Figure 1). The results show that the overall transformation from ferrous ions to hematite,

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Figure 2. Wide-angle X-ray diffraction (WAXD) patterns of iron oxide containing ordered magnetic mesoporous materials at each oxidation temperature.

which is the final phase transformation product, proceeds systematically at low temperatures, that is, below 450 °C. Generally, thermal oxidation to hematite occurs at 650 °C,26,27 but in our approach, this conversion can be achieved at relatively lower temperatures within ordered mesoporous channels. The intermediates formed in the overall phase transformation of iron chloride tetrahydrate (FeCl2 · 4H2O) to hematite in the mesoporous channels are rokuhnite (FeCl2 · 2H2O) and feroxychloride (FeOCl). On the basis of the results, we conclude that the initial ferrous ions are thermally transformed to rokuhnite and feroxychloride at 80 and 300 °C, respectively. These intermediates are then transformed to hematite (R-Fe2O3) at 450 °C. The rate of transformation to hematite differs with the molar concentration of ferrous ions. The results also indicate that the phase transformation rates are strongly dependent on the molar concentration of ferrous ions and the abundant oxygen supply from the silanol groups of highly ordered mesopores. The transformation of Fe2+ to feroxychloride and then to hematite proceeds more rapidly at low ferrous ionic concentrations (0.5 M) than at high ferrous ionic concentrations (3.0 M). During calcination, the color of the magnetic particles changes from yellow to red and then to dark brown due to the formation of feroxychloride, rokuhnite, and hematite. The BET surface areas of the synthetic materials are determined by the N2 adsorption method to confirm the change in the porosity of iron oxide containing mesoporous silica as a function of calcination temperature. The results show that the ordered mesopore sizes were changed from 8.8 to 3.6 nm, as is evident from the change in the shape of the BET isotherm. This causes an increase in the crystallinity of the magnetic particles (FeO) in high concentrated incorporation, leading to a decreased surface area of the FeO particles and pore size and increased pore volume, but this is not suitable for low concentrated incorporation of FeO (Table 1).

WAXD measurements were carried out to determine the structure of each phase of the materials as a function of oxidation temperature (Figure 2). From the results, important characteristics of the iron oxides within the mesopores at a wide range of temperatures could be obtained. The XRD patterns obtained for the samples at 25, 80, 300, and 450 °C could be indexed to iron chloride tetrahydrate, rokuhnite, feroxychloride, and hematite, respectively. The reflection peaks in the diffractogram of each of the aforementioned materials corresponded with those of the references prescribed by the JCPDS cards (PDF nos. 710917, 25-1040, 72-0619, and 87-1166 for iron chloride tetrahydrate, rokuhnite, feroxychloride, and hematite, respectively). From the wide-angle XRD data recorded for various Fe2+ molar concentrations at 25, 80, 300, and 450 °C (Figure S1, Supporting Information), it was apparent that, at a given temperature, the refection peaks did not change with the molar concentration of Fe2+. Moreover, the SAXD data were used to obtain Guinier plots of the low-angle region of the XRD spectrum in order to determine the structural changes in the mesopores.29 The main scattering reflections obtained for the samples at each oxidation temperature were attributed to the (100), (110), and (200) peaks of hexagonally ordered mesoporous structures corresponding to q ) 0.615, 1.055, and 1.220 nm-1, respectively. However, the increased formation of iron oxides in the mesopores at high oxidation temperatures resulted in weak scattering, and consequently, the scattering intensity of the samples decreased (Figure S2, Supporting Information). The structure and dimension of the formed iron oxides within the mesopores were characterized by high-resolution TEM (HRTEM). The obtained TEM images and electron diffraction patterns are shown (Figure 3). At all the oxidation temperatures investigated in this study, the obtained materials retained highly ordered hexagonal arrays of mesopores (average pore size ) 10 nm) and channels. Moreover, the iron oxide species formed

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Figure 3. HR-TEM images of iron oxide containing ordered ferromagnetic mesoporous materials at different oxidation temperatures: (a) 80, (b) 300, and (c) 450 °C. (d) Low-magnification image obtained at 450 °C. (Scale bars are 20 nm for panels a-c. Insets show the electron diffraction patterns in each case.)

in the mesopores, that is, feroxychloride and hematite, could be clearly identified as black dots at 300 and 450 °C, respectively. The formation of rokuhnite at 80 °C could not be clearly observed due to the poor crystallinity of the resulting particles, but feroxychloride and hematite were clearly visible in the mesoporous channels. Hematite, the final phase transformation product formed at 450 °C, is homogeneously distributed in the mesoporous structure. Electron diffraction patterns (shown as insets in Figure 3) were used to characterize the three iron oxide speices formed within the mesopores. The obtained diffraction patterns could be indexed to rokuhnite, feroxychloride, and hematite, formed at different temperatures. These results were consistent with the XRD data obtained at the corresponding temperatures. For a quantitative analysis of the iron oxide species formed in mesopores at various oxidation temperatures, inductively coupled plasma optical emission spectrometry (ICP-OES) measurements were carried out through titration (Figure 4). Because the iron oxides were formed from Fe2+, the total Fe content in the mesopores was determined as a function of the oxidation temperature by ICP-OES. The titration method employed was a modified version of a method reported in the

Figure 4. Quantitative analysis of the iron oxide species formed within ordered mesopores as a function of oxidation temperature by ICP-OES titration.

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Figure 5. (a) Magnetization of iron oxide containing ordered mesoporous materials as a function of applied magnetic field at 300 K (9, 80 °C; ∆, 300 °C; b, 450 °C). (b) Illustration of pathway for the thermal phase transformation of Fe2+ to iron oxides within the ordered mesopores at different oxidation temperatures.

literature.28 The sample was dissolved in a HF/H3PO4 mixture by heating at 60 °C, and the resulting solution was back-titrated using potassium dichromate, ferrous ions, and ammonium sulfate. The total amount of FeO was expressed in parts per million (ppm) using the formula found in the Experimental Section. The liberated ferrous ions were immediately oxidized to ferric ions (Fe3+), and our results indicated that, as the oxidation temperature increased, the ferrous ionic concentration in the mesopores decreased due to the formation of Fe3+ ions. Furthermore, when the oxidation temperature exceeded 300 °C, the ferrous ions were completely oxidized to ferric ions. This indicated that, up to 300 °C, the Fe2+ ions coexisted with the Fe3+ ions. They were then completely converted to Fe3+ at 300 °C because of the formation of crystalline FeOCl. Hence, the initial ionic bonds were converted to covalent bonds at 300 °C. With an increase in the oxidation temperature, the total FeO concentration in the mesopores increased from 420 to 1150 ppm/g. SQUID measurements were carried out at 300 K by sweeping the magnetic field from -10 to 10 kOe in order to determine the magnetic properties of the various iron oxides formed in the mesopores at different oxidation temperatures (Figure 5a). At different oxidation temperatures, different hysteresis loop patterns were obtained for the three iron oxide phases formed within the mesoporous channels. Ordered mesoporous samples containing ferrous ions, ferricoxychlride, and hematite at 80, 300, and 450 °C were typically paramagnetic, superparamagnetic, and ferromagnetic, respectively, and showed hysteretic behaviors. The remanent magnetization and coercivity of FeOClcontaining ordered nanoporous silica were 0.073 emu/g and 805 Oe, respectively. On the other hand, the remanent magnetization and coercivity of hematite-containing ordered nanoporous silica were found to be 0.294 emu/g and 4,660 Oe, respectively. The high remanent magnetization and coercivity of these materials

were attributed to the phase transformation of Fe2+ in the mesoporous channels by thermal oxidation, and the anisotropy in the morphology of these iron particles had a marked effect on their magnetic properties. It is well known that the magnetization of ferromagnetic materials is very sensitive to the microstructure of the materials, and no shape anisotropy is observed in the case of spherical particles. However, shape anisotropy is observed when the magnetic particles are crystalline. Consequently, the increase in the coercivity of hematite at 450 °C could be attributed to the conversion of the noncrystalline ferroxychloride at 300 °C to the crystalline hematite. With an increase in the number of subparticles, the multidomain particles became single-domain particles, and as a result, the coercivity increased. Conclusion We have proposed a new method for the formation of iron oxides from free ferrous ions within ordered mesoporous channels at different oxidation temperatures. The pathway for the phase transformation from ferrous ions to iron oxides within the mesoporous channels is different from other conventional transformation pathways. We found that ferrous ions are transformed to rokuhnite, feroxychloride, and hematite at 80, 300, and 450 °C, respectively (Figure 5b). The phase transformation to various iron oxide species at each oxidation temperature was successfully confirmed by XRD, HR-TEM, and SQUID measurements, as well as BET surface area measurements. The results of this preliminary study can be used to design and synthesize magnetic nanoparticles and incorporate them into mesoporous materials. Resulting magnetic mesoporous particles having a high surface area and pore volume can be used as carriers in magnetic separation techniques.

Highly Ordered Ferromagnetic Mesoporous Materials Acknowledgment. This work was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea. Supporting Information Available: Results of nitrogen adsorption BET parameters and isotherms, wide-angle XRD data, Guinier plot from small-angle XRD, differential scanning calorimeter (DSC) data, solid-state 29Si NMR spectra, and FTIR spectra by the attenuated total reflection (ATR) method. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Bronstein, L. M. Top. Curr. Chem. 2003, 226, 55–89. (2) Inagaki, S.; Guan, S.; Ohsuna, T.; Terasaki, O. Nature 2002, 416, 304–307. (3) Trewyn, B. G.; Slowing, I. I.; Giri, S.; Chen, H. T.; Lin, V. S. Y. Acc. Chem. Res. 2007, 40, 846–853. (4) Lee, J.; Orilall, M. C.; Warren, S. C.; Kamperman, M.; Disalvo, F. J.; Wisner, U. Nat. Mater. 2008, 7, 222–228. (5) Alam, S.; Anand, C.; Ariga, K.; Mori, T.; Vinu, A. Angew. Chem., Int. Ed. 2009, 48, 7358–7361. (6) Sakurai, S.; Namai, A.; Hashimoto, K.; Ohkoshi, S. J. Am. Chem. Soc. 2009, 131, 18299–18303. (7) Dapurkar, S. E.; Selvam, P. Mater. Phys. Mech. 2001, 4, 13–16. (8) Huang, S.; Yang, P.; Cheng, Z.; Li, C.; Fan, Y.; Kong, D.; Lin, J. J. Phys. Chem. C 2008, 112, 7130–7137. (9) Kang, K.; Choi, J.; Nam, J. H.; Lee, S. C.; Kim, K. J.; Lee, S.; Chang, J. H. J. Phys. Chem. B 2009, 113, 536–543. (10) Yang, Y.; Jiang, J. S. J. Mater. Sci. 2008, 43, 4340–4343. (11) Warner, M. G.; Hutchison, J. E. Nat. Mater. 2003, 2, 272–277. (12) Lu, Y.; Yin, Y.; Mayers, T.; Xia, Y. Nano Lett. 2002, 2, 183–186. (13) Tuysuz, H.; Liu, Y.; Weidenthaler, C.; Schuth, F. J. Am. Chem. Soc. 2008, 130, 14108–14110.

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