Maghemite Nanocrystal Impregnation by Hydrophobic Surface

Jul 11, 2007 - The hydrophobic, oleic acid capped superparamagnetic maghemite nanocrystals were incorporated into the porous network via wet impregnat...
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Langmuir 2007, 23, 8838-8844

Maghemite Nanocrystal Impregnation by Hydrophobic Surface Modification of Mesoporous Silica Robert P. Hodgkins,† Anwar Ahniyaz,† Kinnari Parekh,‡,§ Lyubov M. Belova,‡ and Lennart Bergstro¨m*,† Materials Chemistry Research Group, Department of Physical, Inorganic and Structural Chemistry, Arrhenius Laboratory, Stockholm UniVersity, SE-106 91 Stockholm, Sweden, Department of Materials Science and Engineering, Royal Institute of Technology, SE-100 44 Stockholm, Sweden, and Department of Physics, Faculty of Sciences, The M S UniVersity of Baroda, Vadodara, India ReceiVed NoVember 22, 2006. In Final Form: March 13, 2007 Here, we report the design of a hybrid inorganic/organic mesoporous material through simultaneous pore engineering and hydrophobic surface modification of the intramesochannels to improve the uptake of superparamagnetic maghemite nanocrystals via impregnation techniques. The mesoporous material of the SBA-15 type was functionalized in situ with thiol organo-siloxane groups. Restricting the addition of the thiol organo-siloxane to 2 mol % yielded an inorganic/ organic hybrid material characterized by large pores and a well-ordered hexagonal p6mm mesophase. The hydrophobic surface modification promoted the incorporation of 7.5 nm maghemite (γ-Fe2O3) nanocrystals, prepared through temperature-controlled decomposition of iron pentacarbonyl in organic solvents. The hydrophobic, oleic acid capped superparamagnetic maghemite nanocrystals were incorporated into the porous network via wet impregnation from organic suspensions. Combining diffraction, microscopy, and adsorption data confirmed the uptake of the nanocrystals within the intramesochannels of the silica host. Magnetization dependencies on magnetic field at different temperatures show a constriction in the loop around the origin, which indicates immobilization of maghemite nanocrystals inside the thiol-functionalized silica host.

Introduction

* Corresponding author. Tel. +46 8 16 2368; fax +46 8 152 187; E-mail address: [email protected]. † Stockholm University. ‡ Royal Institute of Technology. § The M S University of Baroda.

host relatively small molecules. The M41S family of mesoporous silica materials11,12 provided access to larger-pore systems. Subsequent studies have extended this field to include non-ionic templated mesoporous silicas giving rise to materials with pore sizes between 3 and 27 nm.13-16 The properties of large-pore mesoporous silica inorganic supports, large surface area, a robust framework with easily accessible pore architectures with narrow and tuneable entrances giving free ingress and egress to the guest species, allows for host-guest relationships on a magnitude of order greater than microporous solids and thus has potential to include large biological molecules and nanocrystals. In addition, the ease of uniform surface modification17-20 either through an in situ approach or by postsynthetic grafting is important for many applications, e.g., catalysis,21 adsorbents,22 and optics,23 and as templates for carbon nanomaterials24 and porous single crystals.25-27 Furthermore, the incorporation of specific functionality within the inner surface of the channels can act as anchoring moieties for the guest species to prevent leaching.28 Recent work has shown how composites of nanoparticles and porous materials can act as a catalytic support system29 and as

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The design and assembly of inorganic nanocrystals with welldefined size and shape is of significant interest for both fundamental studies and various applications. The shape- and assembly-dependent1 electronic, optical, mechanical, and magnetic properties have potential applications in fields of catalysis, biotechnology, sensors, and diagnostics.2-6 Of particular interest are monodisperse magnetic nanocrystals such as magnetite (Fe3O4) and maghemite (γ-Fe2O3), which occupy themselves in applications such as high-density magnetic storage7 and biomedical sciences.8 Such particles can also improve detection and diagnosis in systems requiring magnetic resonance signal enhancing probes.9,10 Well-defined porous solids are ideal materials for selective host-guest applications due to their well-defined pore channels and uniform cavities. Microporous zeotype materials can only

10.1021/la063395u CCC: $37.00 © 2007 American Chemical Society Published on Web 07/11/2007

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Scheme 1. The Yellow Thiolate Anion (Right) Absorbs at 412 nm, Formed Through the Framework Thiol Groups Reacting with the Disulfide Group within Ellman’s Reagent (Left)

drug delivery systems.30 Mesoporous carbon with embedded magnetic nanoparticles has been synthesized by Hyeon and coworkers31 by infiltration of carbon and Fe2+ sources as electrode materials for bioelectrocatalysis. However, it has proved difficult to control the size and shape of iron oxide nanocrystals in situ where the immobilized organic capping agent can only cap the growth on one side of the nanoparticle.32 Similarly, the reduction of Pd salts to their metallic state can lead to varying colloid sizes with standard deviations of 32-59% depending on the immobilized capping agent used.29 Here, we report how monodisperse, oleic acid capped maghemite nanocrystals can be incorporated into a thiolfunctionalized mesoporous silica material of the SBA-15 type. To functionalize the silica host, we adopted an in situ approach over that of postsynthetic grafting, as this not only saves time but also delivers an enhanced optimum linkage of the functionality to the silica matrix. In addition, postsynthetic grafting decreases both the mode pore diameter and the pore volume, which is undesirable. It will be demonstrated how a functionalized material can be surface-modified in situ while the mesostructural integrity and large-pore dimension are retained. Careful loading of the desired co-condensing functional siloxane is crucial, as micellar perturbation with increasing organo-siloxane concentration can affect the long-range order of the mesostructure.33,34 (21) (a) Corma, A. Chem. ReV. 1997, 97, 2373. (b) Taguchi, A.; Schu¨th, F. Microporous Mesoporous Mater. 2005, 77, 1. (c) Maschmeyer, T.; Rey, F.; Sankar, G.; Thomas, J. M. Nature (London) 1995, 378, 159. (d) Tanev, P. T.; Chibwe, M.; Pinnavaia, T. J. Nature (London) 1994, 368, 321. (e) Thomas, J. M.; Maschmeyer, T.; Johnson, B. F. G.; Shepard, D. S. J. Mol. Catal. A: Chem. 1999, 141, 139. (f) Chao, M. C.; Lin, H. P.; Mou, C. Y.; Cheng, B. W.; Feng, C. F. Catal. Today 2004, 97, 81. (g) Thomas, J. M. Nature (London) 1994, 368, 289. (h) Ravasio, N.; Zaccheria, F.; Guidotti, M.; Psaro, P. Top. Catal. 2004, 27, 157. (i) Thomas, J. M. Angew. Chem., Int. Ed. 1999, 38, 3588. (j) Jones, M. D.; Raja, R.; Thomas, J. M.; Johnson, B. F. G.; Lewis, D. W.; Rouzard, J.; Harris, K. D. M. Angew. Chem., Int. Ed. 2003, 42, 4326. (22) Feng, X.; Fryxell, G. E.; Wang, L. Q.; Kim, A. Y.; Lui, J.; Kemmer, K. M. Science 1997, 276, 923. (23) (a) Huo, Q.; Zhao, D.; Feng, J.; Weston, K.; Buratto, S. K.; Stucky, G. D.; Schacht, S.; Schu¨th, F. AdV. Mater. 1997, 9, 974. (b) Marlow, F.; McGehee, M. D.; Zhao, D.; Chmelka, B. F.; Stucky, G. D. AdV. Mater. 1999, 11, 632. (c) Loerke, J.; Marlow, F. AdV. Mater. 2002, 14, 1745. (24) (a) Ryoo, R.; Joo, S. H.; Jun, S. J. Phys. Chem. B 1999, 103, 7743. (b) Joo, S. H.; Choi, S. J.; Oh, I.; Kwak, J.; Liu, Z.; Terasaki, O.; Ryoo, R. Nature (London) 2001, 412, 169. (c) Che, S.; Garcia-Bennett, A. E.; Liu, X.; Hodgkins, R. P.; Wright, P. A.; Zhao, D.; Terasaki, O.; Tatsumi, T. Angew. Chem., Int. Ed. 2003, 42, 3930. (25) Zhu, K. K.; Yue, B.; Zhou, W. Z.; He, H. Y. Chem. Commun. 2003, 98. (26) Dickinson, C.; Zhou, W. Z.; Hodgkins, R. P.; Shi, Y. F.; Zhao, D. Y.; He, H. Y. Chem. Mater. 2006, 18, 3088. (27) Jiao, F.; Jumas, J.-C.; Womes, M.; Chadwick, A. V.; Harrison, A.; Bruce, P. G. J. Am. Chem. Soc. 2006, 128, 12905. (28) Yiu, H. H. P.; Wright, P. A.; Botting, N. P. J. Mol. Catal. B: Enzym. 2001, 15, 81. (29) Yi, D. K.; Lee, S. S.; Ying, J. Y. Chem. Mater. 2006, 18, 2459. (30) Kim, J.; Lee, J. E.; Lee, J.; Yu, J. H.; Kim, B. C.; An, K.; Hwang, Y.; Shin, C.-H.; Park, J.-G.; Kim, J.; Hyeon, T. J. Am. Chem. Soc. 2006, 128, 688. (31) Lee, J.; Jin, S.; Hwang, Y.; Park, J.-G.; Park, H. M.; Hyeon, T. Carbon 2005, 43, 2536. (32) Zhu, J.; Ko´nya, Z.; Puntes, V. F.; Kiricsi, I.; Miao, C. X.; Ager, J. W.; Alivisatos, A. P.; Somorjai, G. A. Langmuir 2003, 19, 4396. (33) Luan, Z.; Fournier, J. A.; Wooten, J. B.; Miser, D. E. Microporous Mesoporous Mater. 2005, 83, 150. (34) Hodgkins, R. P.; Garcia-Bennett, A. E.; Wright, P. A. Microporous Mesoporous Mater. 2005, 79, 241.

Thermal decomposition of an iron-containing precursor35 was used to synthesize the maghemite nanocrystals. By controlling the ratio of surfactant to iron precursor, spherical monodisperse maghemite nanocrystals can be obtained with a diameter of 7.5 nm that potentially allow unrestricted diffusion into the host material. The nanoparticles could only be incorporated through impregnation into the intramesochannels when the mesoporous silica was hydrophobically surface modified. The uptake within the intramesochannels is confirmed through TEM, XRD, and porosimetry measurements. Magnetic data show that the composite system is superparamagnetic at room temperature and develops coercivity at low temperatures. Experimental Section Synthesis. Details of the synthesis of thiol-functionalized SBA15 and monodisperse maghemite nanocrystals can be seen in the Supporting Information. Quantification of Accessible Thiol Groups. Quantifying the accessible thiol groups within the mesoporous silica was in accordance to our previous published work34 using Ellman’s reagent (5,5′dithiobis(2-nitrobenzoic acid) (99% Aldrich). The disulfide group in the reagent reacts with the thiol groups in the silica framework to produce a thiolate anion which has a maximum absorbance value at 412 nm. Impregnation of Maghemite Nanocrystals into Thiol-Functionalized SBA-15. Centrifugation (60 000 rpm, 30 min) was applied to the preformed maghemite particles and the mother liquid decanted. The maghemite nanocrystals were redispersed in toluene. A desired amount of extracted thiol-functionalized SBA-15 was placed under vacuum at room temperature overnight (for first injection) before a desired amount of the maghemite nanocrystal suspension was injected, followed by stirring for 30 min and ultrasonication for 15 min. The solvent was removed under reduced pressure at 323 K and the composite dried under vacuum at 423 K for an average of 48 h to allow the complete evaporation of residual organic solvent. The process was repeated again with a repeat injection of the maghemite nanocrystal suspension and subsequent evaporation and drying to obtain the final thiol-functionalized mesoporous silica/maghemite nanocomposite. Instrumentation. XRD. Powder X-ray diffractograms were obtained using a Philips X’Pro PANalytical diffractometer applying Cu KR radiation (λ ) 1.5418 Å) operating in reflectance mode. The samples were loaded onto designed XRD holders. N2 Adsorption. Nitrogen adsorption/desorption isotherms were acquired using a Micromeritics ASAP 2020 instrument. The program consisting of an adsorption branch and a desorption branch was typically run at -196 °C after samples were degassed at 120 °C for 2 h once the final temperature had been maintained. Specific surface areas were calculated via the BET model at relative pressures of P/Po ) 0.06-0.3. The total pore volume was estimated from the uptake of nitrogen at a relative pressure of P/Po ) 0.99. Pore size distribution curves were obtained via the NLDFT model assuming cylindrical pore geometry. Transmission Electron Microscopy. For thiol-functionalized mesoporous silica of the SBA-15 type, a JEOL JEM-3010 microscope operating at 300 kV (Cs ) 0.6 mm, resolution 1.7 Å) was used for investigation. Micrographs were recorded using a Gatan digital (35) Park, J.; An, K.; Hwang, Y.; Park, J.-G.; Noh, H.-J.; Kim, J.-Y.; Park, J.-H.; Hwang, N.-M.; Hyeon, T. Nat. Mater. 2004, 3, 891.

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camera (model 794, Gatan 1024 × 1024 pixels, pixel size 24 × 24 µm) at 40 000-80 000× magnification. Samples were crushed and dispersed in ethanol before being deposited onto a holey carbon grid. For monodisperse maghemite nanocrystals and the mesoporous silica/maghemite nanocomposite, a JEOL JEM-2000 FX microscope operating at 200 kV was used. Micrographs were obtained using a CCD camera (model keen view, SIS analysis) at 60 000-300 000× magnifications. Calibration of the camera length for electron diffraction patterns was obtained using gold as a standard. The γ-Fe2O3 nanocrystal suspension was added dropwise onto a holey carbon grid and dried under vacuum at 150 °C. Photon Correlation Spectroscopy. Light scattering photon correlation spectroscopy for particle size distribution analysis was obtained using a Zetasizer (Nano ZS, Malvern Instruments, UK). Refractive indices for toluene and maghemite were set at 1.491 and 2.69, respectively. Magnetic Characterization. A Quantum design MPMS2 SQUID magnetometer was used to study the magnetic properties of the thiol-functionalized mesoporous silica/maghemite nanocomposite. The magnetic characterization was carried out in the temperature range 5-300 K. For the zero field cooled (ZFC) measurements, each sample was cooled down from room temperature to 5 K in the absence of external magnetic field, and the magnetic data were acquired during the warming run in a constant external field. In the field cooled (FC) measurements, each sample was initially cooled down to 5 K in the presence of magnetic field, and the FC data were recorded during the warmup cycle in the same magnetic field.

Results and Discussion Thiol-Functionalized SBA-15. Nanoparticle uptake into a nanoporous material requires that the surface and solvent interactions are tailored to facilitate diffusion and minimize aggregation and adhesion to the pore walls. Generally, nanoparticle uptake via impregnation is unlikely to solely incorporate metal nanoparticles within the pores.36 Hence, design of both the host material through simultaneous pore engineering and in situ surface modification and the guest species by controlling the size is essential for improved uptake. We have used organosiloxanes to hydrophobize the walls of the mesoporous silica. The hexagonal p6mm SBA-15 form of mesoporous silica with long-range mesoscopic ordering and 12 nm pores only forms at 2 mol % in situ loadings (based on total silica) of MPTES. It is known that increased amounts of organo-siloxanes added directly to the synthesis can perturb the micelle formation of the nonionic block copolymer, P123, and lead not only to mesostructural integrity loss37 but also to changes in the mesophase.34 Increased loadings of 5 and 10 mol % of MPTES result in a poorly defined material. Increasing the loading to 7 mol % results in a mesophase change from hexagonal p6mm to the bicontinuous cubic Ia3d structure but with a smaller mode pore diameter of 8 nm. The addition of the hydrophobic functionality via an in situ approach offers a strong linkage to the inorganic framework throughout the mesoporous silica material. With the organo-siloxane linking into the framework as the framework condenses and grows, a random orientation of the organic moiety prevails with a fraction directed within the silica walls and not protruding out to the pore channels. We determine the fraction of the thiol groups present within the intramesochannels using the disulfide in Ellman’s reagent (5,5′-dithiobis-(2-nitrobenzoic acid)) to react with the framework thiols (Scheme 1). Elemental analysis reveals that the amount of incorporated thiol-siloxane groups into the final product (0.74%) is near the amount in the sol-gel synthesis mixture (0.72%) (calculated through TGA analysis). The fraction (36) Ko´nya, Z.; Puntes, V. F.; Kiricsi, I.; Zhu, J.; Ager, J. W.; Ko, M. K.; Frei, H.; Alivisatos, P.; Somorjai, G. A. Chem. Mater. 2003, 15, 1242. (37) Yang, C.-M.; Wang, Y.; Zibrowius, B.; Schu¨th, F. Phys. Chem. Chem. Phys. 2004, 6, 2461.

Figure 1. Analysis of the extent of thiol-modification and the fraction of accessible thiol groups. Top: Temperature-dependent weight loss of the extracted thiol-modified mesoporous silica. Below: UV spectra of the extracted thiol-modified silica material treated with Ellman’s reagent (0). The absorbance of a known concentration of Cystein (O) is used as standard. Calcined siliceous silica (×) display no UV-absorbance peak at 412 nm.

of accessible framework thiol groups to Ellman’s reagent is 56% (calculated through TGA analysis; quantification of accessible thiol groups to Ellmans’s reagent) (Figure 1). To determine the fraction of accessible framework SH groups, a known amount of extracted thiol-functionalized mesoporous silica, normalized to take into account adsorbed moisture and residual surfactant, is treated according to the expected maximum content of thiols and compared to a standard (cysteine) where a known amount of the thiol groups are measured. The addition of 2 mol % MPTES to the sol-gel retains the hexagonal p6mm symmetry mesostructure and has no effect on the degree of long-range ordering. TEM micrographs (Figure 2) confirm the integrity of the well-defined, extracted thiolfunctionalized mesoporous silica with both the [110] (Figure 2a) and [001] (Figure 2b) zone axes displaying ordered onedimensional pores in a two-dimensional honeycomb arrangement. The X-ray diffractogram (Figure 3A) also confirms the presence of the long-range ordering of the thiol-functionalized material with tertiary peaks displaying a hexagonal p6mm symmetry with an enhanced degree of ordering. The structural parameters are summarized in Table 1. The nitrogen adsorption/desorption isotherm measured at 77 K on the extracted thiol-modified mesoporous silica material exhibits a type IV isotherm with H1 hysteresis expected for cylindrical pore geometry (Figure 4a). The high affinity for N2 uptake (815 cm3/g (STP)) together with a rapid rise over a narrow

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Langmuir, Vol. 23, No. 17, 2007 8841 Table 1. Unit Cell and Porosity Measurements of Extracted Thiol-Functionalized Mesoporous Silica before and after Maghemite Incorporation

material

unit cella (Å)

pore sizeb (Å

BETc (m2 g-1)

N2 uptake (cc/g (STP))

pore volumed (cc/g)

before loading after loading

123 123

120 101

875 ( 4 114 ( 0.2

815 180

1.26 0.28

a Hexagonal lattice parameter; a ) x(4/3) d102. b Pore size distribution calculated on the adsorption branch of the isotherm via the DFT model assuming cylindrical pore geometry. c BET specific surface area calculated within relative pressures of P/Po ) 0.06 - 0.3. d Total pore volume measured at a relative pressure of P/Po ) 0.99.

Figure 4. N2 adsorption/desorption isotherms of thiol-functionalized mesoporous silica (a) before and (b) after maghemite nanocrystal incorporation into the intramesochannels with corresponding pore size distribution curves (inset).

Figure 2. HRTEM micrographs of extracted thiol-modified mesoporous silica before iron oxide incorporation. TEM confirms the hexagonal mesophase viewing (a) pores perpendicular to the incident electron beam and (b) pores parallel to the beam displaying the expected two-dimensional array of mesopores.

Figure 5. Dynamic light scattering histogram of maghemite nanocrystals.

Figure 3. Low-angle X-ray diffraction patterns for: (A) extracted thiol-functionalized mesoporous silica and (B) same material after maghemite nanocrystal impregnation.

relative pressure range (P/Po ) 0.79) is also indicative of a welldefined large pore silica. A summary of the porosimetry analyses are given in Table 1. High surface area, pore volume, and a narrow pore size distribution-mode diameter of 120 Å (Figure 4a inset) together with a well-defined functionalized parent mesostructure is ideal for selective uptake of guest species.

Synthesis of Maghemite Nanocrystals. In order to successfully incorporate nanocrystals within the intramesochannels, the nanocrystals are required to be highly monodisperse within a specific size range, i.e., below the pore diameter of the silica host. Thermal decomposition of iron (III) pentacarbonyl in highboiling-point organic solvents produces metallic Fe nanoparticles. The metallic nanoparticles are oxidized to maghemite. The TEM micrograph (Supporting Information Figure S1a) shows that the synthesized nanocrystals are monodisperse with a size around 7.5 nm. The fringes reflect the crystallinity of the oxide material, and analysis of the electron diffraction ring of the bulk material (Figure S1b) is consistent with γ-Fe2O3 structure. Dynamic light scattering measurements (Figure 5) also yield an average size of 7.5 nm, consistent with TEM measurements. Hence, both the TEM and light scattering data suggest that the maghemite nanocrystals should be sufficiently small to allow access to the intramesochannels of the host silica material.

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Figure 6. Optical picture of thiol-functionalized mesoporous silica/ maghemite nanocomposite showing magnetic properties. The color change in silica confirms the uptake of maghemite and ease of separation through a magnetic field. Figure 8. High-angle X-ray diffractogram of thiol-functionalized mesoporous silica/maghemite nanocomposite. The peaks are indexed according to the maghemite cubic structure.

Figure 7. HRTEM micrographs of (A) pure siliceous SBA-15 and unsuccessful maghemite uptake into the intramesochannels. Boxed region shows maghemite nanocrystal aggregation externally at pore entrance; (B-D) monodisperse maghemite nanocrystal incorporation into SBA-15-SH 2%. The maghemite nanocrystals are of a size small enough to be incorporated within the pore and also run parallel to the direction of the pore suggesting their position within and not on the external surface.

Maghemite Nanocrystal Impregnation into Thiol-Functionalized SBA-15. The maghemite nanocrystals were incorporated within the intramesochannels by impregnation from an organic suspension. The change in color of the thiol-functionalized silica (Figure 6) indicates maghemite uptake and also the possibility to magnetically separate the nanocomposite. The hydrophilic pure siliceous solid does not incorporate maghemite nanocrystals successfully into the intramesochannels, and it was found by HRTEM (Figure 7A) that the maghemite aggregates and resides at the pore entrance rather than penetrate into the pores. Hence, it is clear that hydrophobic surface modification of the internal pore surface is required to incorporate the hydrophobic maghemite nanocrystals. Functionalization with a low loading level of MPTES (2 mol %) allows diffusion of the maghemite nanocrystals within the internal pore structure. HRTEM micrographs (Figure 7B-D) show consecutive rows of maghemite nanocrystals running parallel to the pores of the hydrophobised mesoporous material. Electron tomography would be required to show the exact location of the nanocrystals if they are inside the pores or on the surface of the material. However, we find strong indirect evidence through the XRD and N2 adsorption data that pore modification has resulted upon maghemite uptake, which strongly suggests that the nanocrystals are within the intramesochannels. Analysis of the low-angle XRD provides additional support to pore modification with maghemite incorporation (Figure 3B).

The decrease in the intensity of the secondary 1 1 and 2 0 reflections by around 60% normalized to the major 1 0 reflection in comparison with the hybrid material before maghemite uptake can be attributed to the loss of diffraction contrast between the pore and the silica wall.38 The high-angle diffractogram of the nanocomposite material is shown in Figure 8. The broad peak at around 22 2θ/deg is assigned to amorphous silica, while the higher-angle reflections are attributed to bulk γ-Fe2O3.39 Further evidence to suggest pore modification can be found in the adsorption data (Figure 4B). The decrease in N2 uptake after maghemite nanocrystal incorporation can mainly be attributed to a reduction in pore volume through pore blocking by the maghemite nanocrystals. If the maghemite nanocrystals were only at the pore entrance or at the external surface, the isotherm would be a scaled-down version of the original, i.e., a reduction in N2 uptake due to an increased density of the nanocomposite with no difference in the shape of the isotherm especially in the capillary condensation region. The capillary condensation step for the thiol-functionalized silica before maghemite nanocrystal uptake is at a relative pressure of P/Po ) 0.79, while after uptake of maghemite nanocrystals, the capillary condensation importantly broadens and stretches to lower relative pressures (P/Po ) 0.73) indicating pore modification with smaller pore diameters. Fe elemental analysis reveals a 7.6 wt % maghemite loading. Assuming a dense amorphous silica density of 2.20 g/cm3, the density of the silica wall of the mesoporous material, taking into account the known specific microporosity volume, is calculated to be 2.17 g/cm3. Upon maghemite loading, the density of the nanocomposite increases to 2.27 g/cm3, an increase of 4.3%. On the basis of density increase alone, the capacity for the adsorbate per gram of material will reduce by 4.3% for the nanocomposite material in comparison to the thiol-functionalized SBA-15 before maghemite uptake. This corresponds to a reduction in the volume of adsorbate capacity from 815 to 780 cm3/g. Hence, as the adsorbate capacity for the maghemite incorporated nanocomposite is in the region of 180 cm3/g, this strongly suggests that the density increase is of minor importance. Figure 9 shows the room-temperature magnetic response of the thiol-functionalized mesoporous silica/maghemite nanocomposite. The experimental points (solid dots) were fitted with a modified Langevin’s theory (solid line) for superparamagnetic particles, which includes a log-normal particle size distribution. (38) Bandyopadhyay, M.; Birkner, A.; van der Berg, M. W. E.; Klementiev, K. V.; Schmidt, W.; Gru¨nert, W.; Gies, H. Chem. Mater. 2005, 17, 3820. (39) Dutta, P.; Manivannan, A.; Seehra, M. S. Phys. ReV. B 2004, 70, 174428.

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Figure 9. Room-temperature magnetization curve showing Langevin’s plot for the thiol-functionalized mesoporous silica/maghemite nanocomposite.

Figure 10. Hysteresis loop of thiol-functionalized mesoporous silica/ maghemite nanocomposite measured at 300 K and 5 K; inset shows the zoom images at lower field strength at 5 K.

According to this model, the magnetization M is a function of applied field H and is given by ∞

M ) φMd

∑0 L(R)P(D) dD

(1)

where φ is particle volume fraction, Md is domain magnetization of the particle, and R ) µH/kT, where µ is the magnetic moment of the particle, k is the Boltzmann constant, T is the absolute temperature, and P(D) dD is the log-normal distribution function for spherical particles given as

[

]

below which the spins are successively locked into their sites and do not contribute to the overall magnetization. Above TB, the magnetization decreases with increasing temperature as expected. The FC magnetization curve (MFC) does follow the ZFC curve above TB and is reversible. For ideal noninteracting SP, above TB, the magnetization for both ZFC and FC states should be completely reversible, and all the hysteresis loops at these temperatures should scale into universal Langevin’s-type functional dependence. This is found to be true in the present case. The bifurcation temperature between the FC and ZFC states is observed at 125 K, which indicates the onset of coercivity of the system at that temperature. The sharp peak in the ZFC curve that appears at a relatively low temperature also indicates that the particles are monodisperse and noninteracting. For a monodisperse and noninteracting system, the maximum of the ZFC curve (blocking temperature TB) can be used to calculate the anisotropy constant following the relationship

KV ) kTB ln(τm/τ0)

(3)

with the measuring times τm ) 100 s and τ0 ) 10-10 s. The value of the anisotropy constant thus obtained for the present system with a particle diameter of 63 Å and TB ) 10 K at 50 Oe field is 1.7 × 104 erg/cc. The value of the anisotropy constant thus obtained is rather low and may be one of the reasons for a low peak value in the ZFC curve.

Conclusions

2

ln (D/D0) 1 exp P(D) dD ) dD 1/2 Dσ(2π ) 2σ2

Figure 11. Temperature dependence of the magnetization M for thiol-functionalized mesoporous silica/maghemite nanocomposite in applied fields of 50 Oe under FC (b) and ZFC (O) conditions.

(2)

D is the particle diameter, σ is the standard deviation of ln D, and ln D0 is the mean of ln D. The particle size obtained from the best fit to eq 1 is 63 Å with the domain magnetization of 150 emu/cc and a size distribution of 0.2. The system is superparamagnetic at room temperature, while it develops a coercivity of 72 Oe at 5 K, which is shown as a hysteresis loop in Figure 10. This behavior is a typical characteristic of nanosized particles, which are superparamagnetic (SP) at room temperature and develop a finite magnetic coercivity below the spin freezing (or blocking) temperature. Further, it is also observed that the loop is constricted near the origin (inset of Figure 10), which in principle indicates two ferromagnetic phases in the system. There is also a diamagnetic contribution from the mesoporous silica. The low field temperature dependence values of the magnetization M(T) in the zero field cooled (ZFC) and field cooled (FC) states were measured in a 50 Oe magnetic field (shown in Figure 11). The peak temperature in the ZFC state is observed at 10 K, which is the typical magnetic behavior of a SP material with a characteristic spin freezing (or blocking) temperature TB,

It was demonstrated that simultaneous pore engineering and in situ hydrophobic surface modification of mesoporous silica facilitate the uptake of nanocrystalline maghemite nanoparticles. The controlled design of the pore size and surface functionality is essential for successful incorporation of nanocrystals inside the intramesochannels of the silica host via impregnation. Limiting the co-condensing thiol-siloxane to 2 mol % allows for a welldefined large-pore mesostructure to be retained and results in a hydrophobic protruding functionalitysquantified through Ellman’s reagent. It was shown that the in situ surface modification allow for successful uptake of 7.5 nm oleic acid capped nanocrystals within the intramesochannels. TEM, gas adsorption, and XRD analysis supports that the nanocrystals have successful ingress to the inner pores of the surface-modified silica material. TEM clearly shows that the maghemite nanocrystals aggregate on the surface of the nonfunctionalized mesoporous silica crystals. Magnetic data shows that the nanocomposite is superparamagnetic. The nanocrystals are noninteracting, suggesting that there is no aggregation of the nanocrystals. Acknowledgment. The authors would like to thank the Swedish Research Council (VR) and the Swedish Foundation

8844 Langmuir, Vol. 23, No. 17, 2007

Hodgkins et al.

for Strategic Research for funding. Keiichi Miyasaka is thanked for visualizations within the TOC. BASF are thanked for their kind supply of the block copolymer surfactant, P123.

synthesis of monodisperse oleic acid capped iron oxide nanocrystals. Also, electron micrographs/diffraction of the nanocrystals. This material is available free of charge via the Internet at http://pubs.acs.org.

Supporting Information Available: Details of the synthesis of in situ functionalized mesoporous silica of the SBA-15 type and

LA063395U