Synthesis and Properties of γ-Fe2O3 Nanoclusters ... - ACS Publications

Lei Zhang, Georgia C. Papaefthymiou, and Jackie Y. Ying* ... A blue shift in the absorption edge relative to bulk iron oxide was noted in the UV−Vis...
1 downloads 0 Views 517KB Size
Download PDF
7414

J. Phys. Chem. B 2001, 105, 7414-7423

Synthesis and Properties of γ-Fe2O3 Nanoclusters within Mesoporous Aluminosilicate Matrices Lei Zhang,† Georgia C. Papaefthymiou,‡,§ and Jackie Y. Ying*,† Department of Chemical Engineering and Francis Bitter Magnet Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 ReceiVed: January 17, 2001

Iron oxide nanoclusters were synthesized within mesoporous MCM-41 aluminosilicate matrices via evaporationcondensation of volatile Fe(CO)5. The well-defined hexagonally packed cylindrical pore structure of MCM41 led to the derivation of γ-Fe2O3 particles with spherical and elongated morphologies. Magnetization studies and Mo¨ssbauer spectroscopy indicated that the γ-Fe2O3/MCM-41 nanocomposites exhibited interesting superparamagnetic behavior. A blue shift in the absorption edge relative to bulk iron oxide was noted in the UV-Vis spectra. Strain at the particle-support interface and quantum confinement effects played a critical role in determining the overall magnetic and optical behavior of the γ-Fe2O3/MCM-41 nanocomposites. The iron oxide nanoclusters within the MCM-41 matrix showed high thermal stability and increased magnetization when calcined at high temperatures.

Introduction Nanoclusters and nanoarrays have attracted a great deal of research attention due to their potential use as structural, catalytic, magnetic, electronic, and optical materials.1,2,3,4,5 Ferrimagnetic γ-Fe2O3 is of particular interest due to its dominant role in magnetic storage media.6 Highly dispersed γ-Fe2O3 nanoclusters possess unique magnetic and electronic characteristics, such as superparamagnetism and macroscopic quantum tunneling associated with size quantization and electronic quantum confinement effects.7 By packing magnetic nanoclusters into a host matrix, tailored interfacial confinement environment, intercluster interaction, and effective charge carrier transport may be obtained. Such composite structures have been achieved by using cross-linked ion-exchange polymers, zeolitic molecular sieves, and sol-gel derived materials.8,9,10 Crystalline zeolitic matrices can host highly ordered superstructures of monodispersed magnetic particles but are limited by their microporous dimensions (typically 0.5 to 1.3 nm). Solgel derived silica materials can support particles of larger sizes within their interconnected porous framework but suffer from a low degree of particle packing order within the amorphous matrices of broad pore size distributions. We have recently reported on two novel approaches for the synthesis of γ-Fe2O3 nanoclusters via (i) controlled chemical precipitation of iron oxide followed by silica coating11,12 and (ii) ion-exchange and oxidation of ferrous cations within a porous sulfonated silica matrix.11 Here, we describe the use of MCM-41 with hexagonally-packed mesopores of uniform diameter13 as a matrix for hosting guest nanoclusters for fundamental studies and potential nanostructured device applications. The one-dimensional cylindrical pore channels of MCM-41 can be ideally suited for simultaneous control of cluster size and morphology. Much effort has been devoted * To whom correspondence should be addressed. †Department of Chemical Engineering, M. I. T. ‡ Francis Bitter Magnet Laboratory, M. I. T. § Present address: Department of Physics, Villanova University, Villanova, PA 19085.

toward filling the mesopores with ultrafine clusters and nanowires. Nanoclusters of Fe2O3, polyaniline, Ge, GaAs, InP, Pt, Pd, Ru have been deposited within the MCM-41 inorganic matrix.14-21 In this paper, mesoporous MCM-41 matrices were used to host the synthesis of magnetic γ-Fe2O3 nanoclusters. The resulting nanocomposite materials demonstrated unique magnetic and optical properties associated with quantum confinement effects and particle-support interactions. Various experimental techniques were employed to characterize the MCM-41 matrix structure, the iron oxide phase, and the nanocluster dispersion within the porous support. Size quantization effects on the magnetic and optical properties of the γ-Fe2O3/MCM41 nanocomposites were investigated. Finally, these new findings were compared to our previously reported studies on γ-Fe2O3 nanoparticles embedded within amorphous silica matrices.11,12 Synthesis of Fe2O3 Clusters within Hexagonally-Packed Mesoporous Hosts Synthesis and Characterization of Mesoporous Aluminosilicate. Al-doped mesoporous silica was synthesized from an inorganic siliceous precursor using organic cationic trimethylammonium surfactants (C16H33(CH3)3NBr or CTAB) as a supramolecular templating agent. A 3.65 g portion of CTAB surfactants was completely dissolved in H2O. A 22.2 g portion of 27% sodium silicate solution dispersed in 50 g of water was slowly added to the surfactant solution at room temperature with vigorous stirring. After being stirred for 5 min, the pH of the mixture was adjusted to 11.5 with diluted H2SO4. A desired amount of aqueous aluminum sulfate solution was then gradually introduced to the loosely bonded silica gel and stirred for 3 h at room temperature before the sample was hydrothermally treated at 100-150 °C for a desired period. The molar composition of the wet gel could be represented as 1 SiO2: 0.1 CTAB: 120 H2O: x Al2O3 (where x ) 0 to 0.1). The solid powder obtained was washed with ethanol and water, filtered, and then calcined at 540 °C in air for 6 h to remove the organics.

10.1021/jp010174i CCC: $20.00 © 2001 American Chemical Society Published on Web 07/18/2001

Synthesis and Properties of γ-Fe2O3 Nanoclusters

Figure 1. XRD patterns of calcined mesoporous AlSi-R samples of different Al dopant concentrations aged at 100 °C: (a) R ) 5, (b) R ) 25, and (c) R ) 50.

J. Phys. Chem. B, Vol. 105, No. 31, 2001 7415

Figure 3. BJH desorption pore size distribution of calcined AlSi-25 calculated from the N2 desorption isotherm.

Figure 2. N2 adsorption-desorption isotherm of calcined AlSi-25.

The samples were designated as AlSi-R, where R was given by the precursor molar ratio of Si and Al. The long-range pore packing order of the mesoporous Aldoped silica was characterized by X-ray diffractometry (XRD) (Siemens D5000 θ-θ diffractometer operated at 45 kV and 40 mA using Ni-filtered Cu KR, radiation) and transmission electron microscopy (TEM) (JEOL 200CX, 200 kV). The XRD patterns of calcined AlSi-R shown in Figure 1 corresponded to a typical hexagonal P6m space group with the intense (100) peak as well as minor peaks corresponding to (110), (200) and (210) diffractions. N2 adsorption analysis was performed on a Micromeritics ASAP 2010 gas sorption instrument. The N2 adsorptiondesorption isotherm of the calcined AlSi-25 sample showed negligible hysteresis (Figure 2). The sample has a narrow Barrett-Joyner-Halenda (BJH) pore size distribution (Figure 3), with a Brunauer-Emmett-Teller (BET) surface area of 1217 m2/g. Introduction of Fe2O3 Nanoclusters by EvaporationCondensation. The microstructural characteristics of porous materials, such as surface area, pore diameter, and pore size distribution, would affect the local environment for hosting quantum-sized metal oxide clusters. MCM-41 materials with hexagonally packed mesopores of 2-10 nm diameter may promote the formation of ordered superstructures of the magnetic phase within their well-defined 1-D cylindrical pore channels. Both the pore morphology and the dielectric properties of the mesoporous aluminosilicate materials are attractive matrix characteristics. An evaporation-condensation apparatus was used in our synthesis (Figure 4). Fe(CO)5, a highly volatile precursor, was carefully stored under argon and kept in the dark, while

Figure 4. A schematic of the evaporation-condensation apparatus.

mesoporous aluminosilicate (AlSi-25) was degassed at 350 °C overnight. Typically, 3 mL of Fe(CO)5 and 0.2 g of mesoporous AlSi-25 were introduced to an evaporation flask and a receiving tube, respectively. After assembling the two parts, the mesoporous aluminosilicate was further evacuated for 3 h at 250 °C, which was maintained by a heating tape around the receiving tube. The receiving tube was then cooled to the desired temperature. Upon opening valves 1 and 2, Fe(CO)5 was evaporated at room temperature and condensed within the mesopores of the aluminosilicate support. Excess Fe(CO)5 was then pumped out of the receiving tube to the cold trap by opening valve 3. The mesoporous silica containing Fe(CO)5 was next subjected to a H2 stream at 100-200 °C for several hours to decompose and reduce the iron complex. The as-prepared Fe2O3/AlSi-25 nanocomposite was further calcined in a tube furnace under flowing O2 for 3 h at the desired temperature. The iron loading depended on the synthesis parameters, such as the amount of Fe(CO)5, evaporation rate, saturation time, reaction temperature, and decomposition conditions. The evaporation-condensation procedure could be repeated several times to achieve a high iron loading. Results and Discussions Microstructure of Fe2O3/Mesoporous Aluminosilicate Nanocomposites. Three Fe2O3/AlSi-25 nanocomposite samples were synthesized with different Fe loadings. Elemental analysis results from direct-current plasma emission spectroscopy (Luvak Inc.) are summarized in Table 1. BET surface area and pore volume

7416 J. Phys. Chem. B, Vol. 105, No. 31, 2001

Zhang et al.

Figure 5. XRD patterns (i: 2θ ) 1.5-20° and ii: 2θ ) 15-75°) of Fe2O3/AlSi-25 nanocomposite #2: (a) as-prepared, (b) calcined at 300 °C, (c) calcined at 500 °C, and (d) calcined at 800 °C.

TABLE 1: Characterization of Calcined Mesoporous AlSi-25 and As-Prepared Fe2O3/A1Si-25 Nanocomposites sample

Fe2O3/ AlSi-25 #1

AlSi-25

Fe loading 0 (wt %) receiving tube temperature (°C) reduction temperature and time surface area 1217 (m2/g) pore volume (cm2/g) 1.10

Fe2O3/ Fe2O3/ AlSi-25 #2 A1Si-25 #3

1.27

6.82

57.5

25

50

80

100 °C, 1 h; 200 °C, 5 h 125 °C, 5hr then 150 °C, 5 h 929

763

362

0.93

0.66

0.29

of the Fe2O3/AlSi-25 samples as determined by N2 adsorption analysis decreased with increasing Fe loading, as shown in Table 1. At a high Fe loading, substantial pore plugging by large Fe2O3 nanoclusters might have occurred. The particle agglomeration at the higher loadings led to a significant reduction of the magnetization due to strong antiferromagnetic coupling among the closely interacting particles. The XRD pattern of the as-prepared Fe2O3/AlSi-25 nanocomposite #2 showed that the hexagonally packed mesostructure of AlSi-25 was preserved (Figure 5i(a)) and no additional peaks associated with Fe2O3 crystalline phase was found (Figure 5ii (a)). As the calcination temperature increased, the d(100) spacing of the hexagonal phase decreased slightly due to pore shrinkage from condensation. A broad band at 2θ ∼ 22° associated with amorphous SiO2 was noted in all XRD patterns in Figure 5ii. No crystalline peaks were observed at high 2θ angles (15°75°), indicating the preservation of ultrafine grain sizes for iron oxide clusters upon calcination. The morphology of the Fe2O3 nanoclusters within the MCM41 matrix was studied using both TEM (Akashi 002B, 200 kV), and scanning transmission electron microscopy (STEM) (Vacuum Generators HB603) with elemental mapping. For the as-prepared and 500 °C-calcined Fe2O3/AlSi-25 nanocomposite #1, TEM bright field images showed a hexagonally packed array with dark phase contrast inside the pores when viewing normal to the axis of the hexagonally packed pores (Figure 6). The STEM dark field image showed highly dispersed bright spherical spots of ∼2-3 nm in diameter (Figure 7). The Fe elemental mapping illustrated that Fe was highly dispersed throughout the image area, with higher concentrations indicated by the bright spots. Because of image drifting during scanning, it was very difficult to precisely match the Fe locations on the elemental map with

those on the dark field image. Elemental mapping with the point probe (2 nm probe size) was thus employed to verify the Fe locations. When the X-ray probe was pointed at the bright spherical spots shown in the dark field image, strong Fe absorption was detected, indicating the presence of Fe. When the probe was moved to the adjacent dark regions, no Fe was detected. In certain areas, a lamellar-like structure was noted in the bright field image, corresponding to hexagonally packed pore arrays oriented perpendicular to the electron beam. This was viewed as elongated bright lines on the dark field image. The point mapping along the bright lines corresponded to a continuous detection of Fe absorption, indicating the presence of elongated iron oxide particles. For the sample calcined at 500 °C, a homogeneous Fe mapping with dark field image was obtained, reflecting both spherical and elongated morphologies for the iron oxide particles (Figure 8). No particle agglomeration or sintering was observed at 500 °C, suggesting a high thermal stability for Fe2O3/AlSi-25 nanocomposite #1. Magnetic and Optical Properties of γ-Fe2O3/Mesoporous Aluminosilicate Nanocomposites. For nanometer-sized single domain magnetic clusters, magnetic behavior is governed by the magnetic energy barrier defined as the product of the cluster volume and its magnetic anisotropy density.22 At a temperature T g TB (TB ) critical magnetic blocking temperature), the thermal energy is comparable to the magnetic energy barrier. The cluster magnetization vector is thermally excited to fluctuate between the easy directions of magnetization in a manner analogous to that of paramagnetic atoms in an applied field, except with a much larger magnetic moment. This is the wellknown phenomenon of superparamagnetism. For superparamagnetic particles, the total magnetic moment displays a paramagnetic behavior obeying a classical Langevin function above the TB. The macroscopic magnetic properties of our nanocomposites were investigated by superconducting quantum interference device (SQUID). magnetometry (Quantum Design, MPMS, characteristic measurement time (τm) ) 102 s) over a wide range of temperatures (5 K < T < 300 K) and applied magnetic fields (0 T < H < 3 T). Zero field cooling (ZFC) and field cooling (FC) magnetization curves were recorded, and magnetic hysteresis loops at low temperatures were studied. The internal magnetization and dynamic spin relaxation processes of the nanocomposites were also examined with Mo¨ssbauer spectroscopy. The fast 10-8 sec characteristic

Synthesis and Properties of γ-Fe2O3 Nanoclusters

Figure 6. TEM micrographs of Fe2O3/AlSi-25 nanocomposite #1: (a) as-prepared and (b) calcined at 500 °C.

measurement time for Mo¨ssbauer spectroscopy yielded information on blocking temperature, below which spin relaxation was slowed. This enabled the investigation of dynamic and static magnetic properties of the nanocomposites over a convenient temperature range and provided the capability of zero external field microscopic magnetic study. Mo¨ssbauer spectra for the nanocomposites were recorded as a function of temperature for 4.2 K < T < 300 K using a liquid helium cryostat. They were computer-fitted to theoretical curves using a nonlinear leastsquares curve fitting procedure. For nanometer-sized particles, the crystallites could be too small or poorly developed to be amenable to XRD structural determination, rendering phase identification of the iron oxide difficult. No crystalline peaks associated with the iron oxide phase were observed in the XRD studies of the nanocomposites in Figure 5, as discussed earlier. We have made use of the Mo¨ssbauer electronic parameters, the isomer shift (δ), the quadrupole splitting (∆Eo), and the internal hyperfine field (Hhf)values in conjunction with the magnetization strength of the

J. Phys. Chem. B, Vol. 105, No. 31, 2001 7417 nanocomposites to determine the iron oxide phase as γ-Fe2O3 and to establish its thermal stability against the γ f R phase transformation. Mo¨ ssbauer Studies. In the cluster size range of 2 to 4 nm, a large fraction of the iron atoms lies on the cluster surface (assuming a surface layer comprised of two atomic layers), and the Mo¨ssbauer spectra would reflect the electronic and magnetic properties of surface iron ions to a large extent. Thus, variations in surface and strain anisotropy would be reflected in (i) the quadrupole splitting, (ii) the magnetic hyperfine field, (iii) the magnetic anisotropy energy, blocking temperature, and magnetic coercivity, and (iv) the collective magnetic excitations detected below the blocking temperature. A simple idealized core-shell model is proposed to analyze the electronic and magnetic contributions from surface and interior iron cations. We assume that the highly crystalline core is uniformly magnetized and could undergo a coherent rotation when an external field (H) is applied. However, the surface spins are canted relative to the interior lattice magnetization direction due to strong pinning forces at the surface, which would induce noncoherent spin reversal modes.22,23 The Mo¨ssbauer spectra of the as-prepared Fe2O3/AlSi-25 nanocomposite #1 at various temperatures are shown in Figure 9. The overall temperature profile is consistent with the observation of magnetically ordered particles of Fe2O3 exhibiting dynamic superparamagnetic relaxation, due to thermally activated reversals of the particle magnetization.24 The dependence of the superparamagnetic relaxation time on temperature (T), magnetic anisotropy energy density (K), and particle volume (V), is given by τ ) τo exp (KV/kT), where k is Boltzmann’s constant and τo is a constant characteristic of the material.24 At high temperatures, a superparamagnetic relaxation time shorter than the Mo¨ssbauer measuring time (τ < 10-8 s) would obscure observation of the internal magnetic order of the particles, giving rise to a quadrupolar spectrum. At lower temperatures, superparamagnetic relaxation would be slowed, revealing the internal magnetic order of the particles in a magnetically split Mo¨ssbauer spectrum. At T ) 300 K, the broad absorption lines (Figure 9a) called for spectral fits with two superimposed quadrupole doublets of similar isomer shift but different quadrupole splitting values, as tabulated in Table 2. These two subcomponents were associated with interior and surface iron sites according to our proposed core-shell model for the analysis of electronic and magnetic contributions from interior and surface atoms. The larger quadrupole splitting (1.25 mm/s) was associated with surface iron atoms due to ligand coordination distortion at the surface. A ratio of Asurface:Acore ) 39:61 was obtained, as measured by the absorption area under each subspectrum. The spectra at lower temperatures were similarly fitted using the core-shell model assuming a distribution of hyperfine fields. At 4.2 K, where only magnetically split spectra were observed (Figure 9f), two magnetic subsites were poorly resolved. The smaller hyperfine field (see Table 2) compared to the corresponding values of bulk γ-Fe2O3 (496 kOe at room temperature25 and 527 kOe saturation field at 0 K26) was associated with the surface atoms. It has been known that the sharp discontinuity of the Fe-O-Fe superexchange bonds and canted spin structure on the surface would result in diminished magnetic exchange. The contribution from surface spins was greatly suppressed at 4.2 K as Asurface:Acore ) 21:79. We attribute this to the frozen core spin structure and persistent surface spin fluctuations. At intermediate temperatures, the spectra exhibit greater complexity due to the superposition of quadrupolar and

7418 J. Phys. Chem. B, Vol. 105, No. 31, 2001

Zhang et al.

Figure 7. Elemental mappings and STEM images of as-prepared Fe2O3/AlSi-25 nanocomposite #1.

Figure 8. Elemental mappings and STEM images of Fe2O3/AlSi-25 nanocomposite #1 calcined at 500 °C.

magnetic subspectra arising from the presence of a distribution of particle sizes. The observed temperature dependence of the hyperfine fields (Hhfl and Hhf2) in Table 2 indicated that below the blocking temperature, where spin reversals were suppressed, the magnetization vectors of the particles might still undergo collective magnetic excitations around the easy axis of magnetization.27 These low-energy excitations could also prevent the observed hyperfine fields from reaching their saturation values above 0 K. In addition, the surface fields (Hhf2) exhibited stronger temperature dependence compared to the core fields (Hhfl) presumably due to surface-spin fluctuation modes.28

The cluster size distribution can be obtained from the fraction f(T) of the clusters with volume greater than the critical volume (Vc) defined by superparamagnetism.29 The plot of the fraction of particles contributing to the magnetic subspectrum as a function of temperature exhibited two inflection points (see Figure 10). This suggested that there were two distinct blocking temperatures as determined by the inflection points, TB1 ) 60 K and TB2 ) 110 K, corresponding to two particle size ranges in the as-prepared Fe2O3/AlSi-25 nanocomposite #1. This was in agreement with our STEM and point X-ray probe mapping results, which indicated the presence of spherical and elongated particles. We could associate the lower blocking temperature

Synthesis and Properties of γ-Fe2O3 Nanoclusters

J. Phys. Chem. B, Vol. 105, No. 31, 2001 7419

Figure 9. Mo¨ssbauer spectra of as-prepared Fe2O3/AlSi-25 nanocomposite #1 collected at (a) 300 K, (b) 75 K, (c) 60 K, (d) 50 K, (e) 25 K, and (f) 4.2 K. The solid line is a least-squares fit to the experimental data, assuming a superposition of sites as shown above the experimental points (see text). The magnetic subcomponent was fitted to a distribution of magnetic hyperfine fields in order to reproduce the observed spectral broadening.

TABLE 2: Mo1 ssbauer Parameters for Fe2O3/AlSi-25 Nanocomposite #1 Fe2O3/AlSi-25 nanocomposite #1

as-prepared

calcination at 300 °C calcination at 500 °C bulk γ-Fe2O3 (A+B sites averaged)

T (K) 300 75 60 50 25 4.2 300 4.2 300 4.2 298

isomer shifta,b δ (mm/sec) surface core

quadrupole splittinga ∆Eo (mm/sec) surface core

0.33 0.41 0.54 0.35 0.46

0.33 0.45 0.42 0.39 0.27

1.25 1.30 1.50 1.50 1.51

0.36

0.34

1.24

0.39

0.35

1.10

0.35c

0.73 0.77 0.77 0.78 0.94 -0.026 0.74 -0.056 0.81 -0.132 0

hyperfine field Hhf (kOe) Hhf1 Hhf2 428.8 458.3 466.0 466.9 487.9

296.4 375.2 402.6 411.3 458.2

477.4

447.9

487.5

442.2 496d

a Uncertainties in δ and ∆E are ( 0.03 mm/s and ( 0.05 mm/s, respectively. b Isomer shift is relative to metallic iron at room temperature; 4.2 o K values were fixed at those obtained at room temperature + 0.12 mm/s second-order Doppler shift. cReference (27). dReference (25).

Figure 10. The fraction of particles in Fe2O3/AlSi-25 nanocomposite #1 contributing to the Mo¨ssbauer magnetic subspectrum as a function of temperature. The arrows show the two distinct transitional regions.

with a distribution of spherical particles, and the higher blocking temperature with a distribution of larger, elongated particles. The Mo¨ssbauer parameters of the as-prepared Fe2O3/AlSi25 nanocomposite #1 were consistent with the iron oxide phase

identification as γ-Fe2O3. In support of such a conclusion, we observed (i) the absence or negligible quadrupolar perturbation on the magnetic spectra and (ii) the reduced values of the hyperfine fields. Hematite (R-Fe2O3) exhibits a large quadrupolar perturbation of 0.12 mm/s at 300 K and -0.22 mm/s below the Morin transition (T ) 260 K for bulk R-Fe2O3), and a saturation hyperfine field (Hsat) of 544 kOe;30 as opposed to zero quadrupolar perturbation and a saturation field of 527 kOe for γ-Fe2O3.26 In addition, hematite has a canted anti-ferromagnetic structure with very weak magnetization. The SQUID measurements discussed below indicated that these nanocomposites possessed considerable magnetization, consistent with the ferrimagnetic spinnel structure of γ-Fe2O3. The geometrical confinement of the Fe2O3 clusters by the regular pore structure of the inorganic AlSi-25 matrix could potentially stabilize the nanoparticles at high temperatures. The thermal stability of Fe2O3/AlSi-25 nanocomposite #1 samples calcined at 300 and 500 °C was examined by Mo¨ssbauer spectroscopy. The values for the isomer shift and quadrupole splitting of both the surface and core components in the Mo¨ssbauer spectrum at 300 K remained essentially unchanged

7420 J. Phys. Chem. B, Vol. 105, No. 31, 2001

Zhang et al.

Figure 12. Plots of magnetization vs applied field for as-prepared Fe2O3/AlSi-25 nanocomposite #1 obtained at (a) 300 K and (b) 5 K.

Figure 13. Temperature dependence of the magnetization curves under (a) field cooling and (b) zero field cooling for as-prepared Fe2O3/A1Si25 nanocomposite #1. Figure 11. Mo¨ssbauer spectra collected at 4.2 or 300 K for Fe2O3/ AlSi-25 nanocomposite #1 calcined at (a) 300 °C and (b) 500 °C under O2. The solid line is a least-squares fit to the experimental data, assuming a superposition of sites as shown above the experimental points (see text). The magnetic subcomponent was fitted to a distribution of magnetic hyperfine fields in order to reproduce the observed spectral broadening.

for the nanocomposite calcined at 300 °C (Figure 11a, Table 2), indicating that no phase transition took place at this calcination temperature. Different quadrupole splitting values were obtained for both components for the sample calcined at 500 °C, suggesting an improved crystallinity due to atomic restructuring at this calcination temperature (Figure 11b, Table 2). The Mo¨ssbauer spectrum at 4.2 K gave a small quadrupole splitting perturbation value (-0.056 mm/s) for the sample calcined at 300 °C, supporting the retention of the γ-Fe2O3 phase. The 4.2 K spectrum of the sample calcined at 500 °C, however, showed a large quadrupole splitting perturbation value of -0.132 mm/s, which may be interpreted as a partial phase transformation from γ-Fe2O3 to R-Fe2O3 at this calcination temperature (∆Eo of pure R-Fe2O3 ) -0.22 mm/s at T ) 80 K).30 SQUID Studies. The isothermal magnetization of as-prepared Fe2O3/AlSi-25 nanocomposite #1 was obtained at two different temperatures (Figure 12). The hysteresis loop at 300 K gave a zero remanence (Mr) and coercivity (Hc), indicating superparamagnetic behavior. With increasing external magnetic field, the magnetization increased rapidly without saturation up to 30 kG

due to both spin canting in the ultrafine particle assembly31 and distribution in the particle sizes. The lack of magnetic saturation became more significant at low temperatures. The onset of hysteresis occurred at ∼50 K. At 5 K, a symmetric hysteresis loop was observed with an unsaturated magnetization of 57 emu/g at 30 kG applied field, a remanence of 1.5 emu/g and a coercivity of 680 G, demonstrating the ferrimagnetic nature of the nanocomposite at a low temperature. The spin canting resulting from the discontinuity of the superexchange bonds between Fe cations was responsible for the lower magnetization (compared to bulk γ-Fe2O3: Ms ) 76 emu/g) and the lack of saturation for the nanocomposite.32 Compared to bulk γ-Fe2O3 that has a remanence of 63 emu/g33 and a coercivity of 250400 G,34 the smaller remanence and larger coercivity of the nanocomposite could be attributed to the single magnetic domain clusters. For single-domain particles, coercivities are associated with the rotation of the magnetization away from the easy axis; whereas in the bulk system, coercivities reflect magnetic domain wall motion in the presence of a magnetic field.22 The temperature-dependent magnetization in an applied field of 200 G was investigated under field cooling (FC) and zero field cooling (ZFC) (Figure 13). The ZFC curve (Figure 13b) exhibited a maximum at 20 K and a superimposed contribution to the magnetization with a maximum at 50 K. The sharp Tmax at 20 K could be related to small spherical single-domain magnetic particles. Because the Tmax is closely related to the blocking temperature and increases with increasing cluster size

Synthesis and Properties of γ-Fe2O3 Nanoclusters

J. Phys. Chem. B, Vol. 105, No. 31, 2001 7421 TABLE 3: Summary of Magnetic Properties of the Fe2O3/ SiO2-based Nanocomposites as-prepared sample

Figure 14. Temperature dependence of the magnetization curves under (a) field cooling and (b) zero field cooling for Fe2O3/AlSi-25 nanocomposite #1: as-prepared, or calcined to 300 °C and 500 °C.

Figure 15. UV-vis diffuse reflectance spectra of Fe2O3/AlSi-25 nanocomposite #1: (a) as-prepared, (b) calcined at 300 °C, and (c) calcined at 500 °C.

and magnetic anisotropy constant, the broad Tmax at 50 K could be attributed to elongated particles with larger average particle volume and shape anisotropy. These blocking temperatures are lower than those observed with Mo¨ssbauer spectroscopy due to the longer measuring time associated with SQUID measurements. SQUID magnetometry was also applied to study the magnetic properties of the calcined samples. The magnetization vs temperature curves measured under FC and ZFC at 200 G showed that the total magnetization increased with calcination temperature (Figure 14). The splitting between FC and ZFC curves occurred at a lower temperature for samples calcined at a higher temperature. The improved crystallinity of the Fe2O3 core and the reduced contribution from disordered surface spins gave rise to the increase in magnetization. The partial transformation to R-Fe2O3, observed in the Mo¨ssbauer spectrum of the 500 °C-calcined sample, was not significant enough to lead to a reduction in the overall magnetization. This study indicated that Fe2O3/AlSi-25 nanocomposite #1 would be an excellent candidate for applications requiring high-temperature stability. Optical Properties: The optical property of the Fe2O3/A1Si25 nanocomposites was studied using a Cary II UV-vis spectrometer. The spectrum of the as-prepared Fe2O3/AlSi-25 nanocomposite #1 exhibited a blue shift of 70 nm relative to bulk γ-Fe2O3 (Figure 15a). The widening of the band gap was consistent with quantum confinement. Upon calcination, the absorption edge shifted to slightly higher wavelengths (Figures 15b and c), corresponding to changes in (i) the core structure of the Fe2O3 nanoclusters, and (ii) the interfacial effects from the particle-support interaction. Comparison of Fe2O3/SiO2-Based Nanocomposites Synthesized via Different Routes. The electronic, magnetic, and

Fe2O3/ Sulfonated-SiO2 nanocompositea Fe2O3/ SiO2-Coated nanocompositeb Fe2O3/AlSi-25 nanocomposite #1 a

M Hc TB TB (emu/g Fe2O3) (G) (K) (K) (at 3 T, 5 K) (at 5 K) (SQUID) (Mo¨ssbauer) 30

270

10

25

20

635

15

23

57

680

20, 50

60, 110

Reference (11). b Reference (12).

optical properties of the Fe2O3/AlSi-R nanocomposites reflected the nanometer-scaled dimensions of the magnetic constituents, the abundance of interfaces, and particle-support interactions. These effects may be further brought to focus by a comparison of the properties exhibited herewith and those for Fe2O3/ sulfonated-SiO2 nanocomposite and Fe2O3/SiO2-coated nanoparticles reported in our previous studies.11,12 The magnetic properties of the three differently prepared Fe2O3/SiO2-based nanocomposites in our current and previous studies are summarized in Table 3. A complex magnetic system was obtained within the cylindrical mesopores of MCM-41 matrix using the evaporation-condensation approach described in this study. Large magnetization and coercivity values were obtained using the AlSi-25 supports (bulk γ-Fe2O3: 76 emu/g, polymer supported γ-Fe2O3 nanoclusters: 46 emu/g,35 block copolymer supported γ-Fe2O3 nanoclusters: 39 emu/g36). This could be explained by the combination of magnetic contributions from particles of different morphologies. For single magnetic domain particles, the magnetization is significantly reduced due to the quantum size effect. The magnetic dipole moment is proportional to the particle volume. The combined effective volume of the spherical and elongated Fe2O3 nanoclusters in the cylindrical pores of AlSi-25 could be larger than that of the spherical SiO2-coated Fe2O3 particles with diameters of ∼4 nm. Therefore, a larger magnetization was obtained with the Fe2O3/ AlSi-25 nanocomposite. The coercivity of spinel-structured nanocrystals greatly depends on morphology and surface anisotropy. Compared to spherical particles, shape anisotropy in acicular particles can produce a higher coercive field, assuming an ellipsoid model with uniform magnetization. The higher coercivity of Fe2O3/AlSi-25 nanocomposite #1 compared with that of Fe2O3/SiO2-coated nanocomposite could be attributed to the presence of elongated particles in the former. On the other hand, the low coercivity in the Fe2O3/sulfonatedSiO2 nanocomposite implied that (i) the internal spin structure of the particles in this system significantly deviated from that of uniform magnetization assumed in the Stoner and Wohlfarth model37 and/or (ii) the anisotropy was dominated by strain exerted by the sulfonated SiO2 matrix on the nanoclusters. The optical properties of the three nanocomposites are compared in Figure 16. The matrix-mediated synthesis of Fe2O3 nanoclusters within sulfonated SiO2 gel involved an interconnected porous host with a relatively broad pore size distribution. This might have resulted in relatively less stress on the magnetic particles, and thus, the largest blue shift (110 nm) was obtained with the Fe2O3/sulfonated-SiO2 nanocomposite (Figure 16a). In the case of the Fe2O3/SiO2-coated sample, the presence of high stress exerted by the SiO2 coating might have substantially counteracted the quantum confinement effects, resulting in a relatively small blue shift of 20 nm (Figure 16b). The strain induced by particle-support interactions could significantly

7422 J. Phys. Chem. B, Vol. 105, No. 31, 2001

Zhang et al. References and Notes

Figure 16. UV-vis diffuse reflectance spectra of (a) Fe2O3/sulfonatedSiO2 nanocomposite, (b) Fe2O3/SiO2-coated nanocomposite, and (c) Fe2O3/AlSi-25 nanocomposite #1.

distort the particle’s crystal lattice as to produce red-shifted optical absorption as reported for a γ-Fe2O3/polymer nanocomposite system.35 The hexagonally packed array of cylindrical mesopores (2.5 nm pore diameter) of the AlSi-25 support could induce more stress on the surface of Fe2O3 clusters than the irregular pore structure of the sulfonated SiO2 gel (4 nm average pore size) but less stress than that imposed by the silica coating. The combined quantum confinement and stress effects led to the intermediate level of optical blue shift (70 nm) in Fe2O3/ AlSi-25 nanocomposite #1 (Figure 16c). Conclusions Evaporation-condensation of volatile Fe(CO)5 was used to produce Fe2O3 nanoclusters within a hexagonally packed mesoporous aluminosilicate matrix. Various Fe loadings could be obtained through control of cluster synthesis parameters. Superparamagnetism was demonstrated for the Fe2O3/AlSi-25 nanocomposites at low Fe loadings. TEM and STEM with Fe elemental mapping indicated the coexistence of spherical and elongated iron oxide particles within the cylindrical pores of AlSi-25 supports. The SQUID and Mo¨ssbauer properties were consistent with γ-Fe2O3. Two distinct relaxation phenomena were observed in both SQUID and Mo¨ssbauer measurements, associated with the presence of two sets of particle size distributions. The large magnetization and coercivity of Fe2O3/ AlSi-25 nanocomposite can be attributed to the presence of spherical and elongated magnetic particles. The Fe2O3 nanoclusters in this nanocomposite exhibited high stability against γ f R phase transition. Only a partial phase transition to R-Fe2O3 was detected by Mo¨ssbauer spectroscopy for the sample calcined at 500 °C. The high thermal stability and increased magnetization from calcination make this material an excellent candidate for high-temperature applications. A blue shift in the absorption edge was noted in the optical spectrum of this nanocomposite system. Acknowledgment. This work was supported by the Camille and Henry Dreyfus Foundation. The authors thank M. Frongillo and A. G. Garratt-Reed (MIT CMSE) for their assistance in the TEM and STEM studies. G.C.P. thanks the Centro Brasileirode Pesquisas Fı´sicas for support during her Visiting Scientist appointment in Brazil. The authors would also like to acknowledge Drs. R. Scorzelli and A. Bustamante Dominguez of the Centro Brasileiro de Pesquisas Fı´sicas for the low-temperature Mo¨ssbauer measurements.

(1) (a) Gletier, H. Prog. Mater. Sci. 1989, 33, 233. (b) Tronc, E.; Jolivet, J. P. In Nanophase Materials: Synthesis-Properties-Applications; Hadjipanayis, G. C., Siegel, R. W., Eds.; Kluwer: Boston, 1994; p 21. (c) Gleiter, H. Mater. Sci. Forum 1995, 189-190, 67. (d) Shi, J.; Gider, S.; Bakcock, K.; Awschalom, D. D. Science 1996, 271, 937. (e) Greer, A. L. Mater. Sci. Forum 1998, 269-272, 3. (2) (a) Thomas, J. M. Pure Appl. Chem. 1988, 60, 1517. (b) Ying, J. Y. AIChE J. 2000, 46, 902. (c) Gates, B. C. Chem. ReV. 1995, 95, 511. (3) (a) Simon, A. Angew. Chem., Int. Ed. Engl. 1988, 27, 159. (b) Chien, C. L. Annu. ReV. Mater. Sci. 1995, 25, 129. (c) Leslie-Pelecky, D. L.; Rieke, R. D. Chem. Mater. 1996, 8, 1770. (d) Schmid, G.; Hornyak, G. L. Curr. Opin. Solid State Mater. Sci. 1997, 2, 204. (4) (a) Valokitin, Y.; Sinzig, J.; DeJongh, L. J.; Schmid, G.; Moiseev, I. I. Nature 1997, 384, 621. (b) Luth, H. Appl. Surf. Sci. 1998, 130-132, 855. (c) Chen, W.; Ahmed, H. AdV. Imaging Electron Phys. 1998, 102, 87. (5) (a) Simon, U.; Scho¨n, G. Angew. Chem., Int. Ed. Engl. 1993, 32, 250. (b) Gehr, R. J.; Boyd, R. W. Chem. Mater. 1996, 8, 1807. (6) Berkowitz, A. E. In Nanomaterials: Synthesis, Properties and Applications; Edelstein, A. S., Cammarata, R. C., Eds.; Institute of Physics Publishing: Bristol, 1996; p 569. (7) (a) Morrish, A. H. The Principles of Magnetism; Wiley: New York, 1996; Chapter 7. (b) Stucky, G. D.; MacDougall, J. Science 1990, 247, 669. (c) Gunther, L. In Magnetic Properties of Fine Particles; Dormann, J. L.; Fiorani, D., Eds.; Elsevier: Amsterdam, 1992; p 213. (d) Dormann, J. L.; Fiorani, D.; Tronc, E. NATO ASI Series E 1994, 260, 635. (8) (a) Golden, J. H.; Deng, H.; DiSalvo, F. J.; Fre´chet, J. M. J.; Thompson, P. M. Science 1995, 268, 1463. (b) Ziolo, R. F.; Giannelis, E. P.; Shull, R. D. Nanostr. Mater. 1993, 3, 85. (c) Nguyen, M. T.; Diaz, A. F. AdV. Mater. 1994, 6, 858. (9) (a) Shull, R. D.; Ritter, J. J.; Shapiro, A. J.; Swartzendruber, L. J.; Bennett, L. H. J. Appl. Phys. 1990, 67, 4490. (b) Shull, R. D.; Kerch, H. M.; Ritter, J. J. J. Appl. Phys. 1994, 75, 6840. (c) Wang, J. P.; Luo, H. L. J. Appl. Phys. 1994, 75, 7425. (10) (a) Bein, T.; Schmiester, G.; Jacobs, P. A. J. Phys. Chem. 1986, 90, 4851. (b) Herron, N.; Wang, Y.; Eddy, M.; Stucky, G. D.; Cox, D. E.; Moller, K.; Bein, T. J. Am. Chem. Soc. 1989, 111, 530. (c) Lazaro, F. J.; Lopez, A.; Larrea, A.; Pankhurst, Q. A.; Nieto, J. M.; Corma, A. IEEE Trans. Magn. 1998, 34, 1030. (11) Zhang, L.; Papaefthymiou, G. C.; Ziolo, R. F.; Ying, J. Y. Nanostr. Mater. 1997, 9, 185. (12) Zhang, L.; Papaefthymiou, G. C.; Ying, J. Y. J. Appl. Phys. 1997, 81, 6892. (13) (a) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (b) Huo, Q.; Margolese, D. I.; Ciesla, U.; Feng, P.; Gier, T. E.; Sieger, P.; Leon, R.; Petroff, P. M.; Schu¨th, F.; Stucky, G. D. Nature 1994, 368, 317. (c) Tanev, P. T.; Chibwe, M.; Pinnavaia, T. J. Nature 1994, 368, 321. (d) Yang, H.; Kuperman, A.; Coombs, N.; Mamiche-Afara, S.; Ozin, G. A. Science 1996, 379, 703. (e) Schulz-Ekloff, G.; Rthaisky, J.; Zukol, A. J. Inorg. Mater. 1999, 1, 97. (f) Ying, J. Y.; Mehnert, C. P.; Wong, M. S. Angew. Chem., Int. Ed. 1999, 38, 56. (14) MacLachlan, M. J.; Aroca, P.; Coombs, N.; Manners, I.; Ozin. G. A. AdV. Mater. 1998, 10, 144. (15) Wu, C.-G.; Bein, T. Chem. Mater. 1994, 6, 1109. (16) Leon, R.; Margolese, D.; Stucky, G. D.; Petroff, P. M. Phys. ReV. B 1995, 52, 2285. (17) Srdanov, V. I.; Alxneit, I.; Stucky, G. D.; Reaves, C. M.; DenBaars, S. P. J. Phys. Chem. B 1998, 102, 3341. (18) Agger, J. R.; Anderson, M. W.; Pemble, M. E.; Terasaki, O.; Nozue, Y. J. Phys. Chem. B 1998, 102, 3345. (19) (a) Junges, U.; Jacobs, W.; Voiget-Martin, I.; Krutzsch, B.; Schu¨th, F. J. Chem. Soc., Chem. Commun. 1995, 2283. (b) Corma, A.; Martı´nez, A.; Martı´nez-Soria, V. J. Catal. 1997, 169, 480. (20) Mehnert, C. P.; Ying, J. Y. Chem. Commun. 1997, 2215. (b) Mehnert, C. P.; Weaver, D. W.; Ying, J. Y. J. Am. Chem. Soc. 1998, 120, 12 289. (c) Koh, C. A.; Nooney, R.; Tahir, S. Catal. Lett. 1996, 163, 148. (21) Mumukutla, R. S.; Asakura, K.; Namba, S.; Iwasawa, Y. Chem. Commun. 1998, 1425. (22) Cullity, B. D. Introduction to Magnetic Materials; AddisonWesley: Reading, 1972; Chapters 9 and 11. (23) Pankhurst, Q. A.; Pollard, R. J. Phys. ReV. Lett. 1991, 67, 248. (24) Aharoni, A. In Magnetic Properties of Fine Particles; Dormann, J. L., Fiorani, D., Eds.; Elsevier: North-Holland, 1992; p 3. (25) Kelly, H. V.; Folen, V. J.; Hass, M.; Schreiner, W. N.; Beard, G. B. Phys. ReV. 1961, 122, 1447. (26) Murad, E. In Magnetic Properties of Fine Particles; Dormann, J. L.; Fiorani, D., Eds.; Elsevier: North-Holland, 1992; p 339. (27) Mørup, S.; Topsøe, H. J. Appl. Phys. 1976, 11, 63. (28) de Jongh, L. J.; Miedema, A. R. AdV. Phys. 1974, 23, 1. (29) Kaufman, K. S.; Papaefthymiou, G. C.; Frankel, R. B.; Rosenthal, A. Biochim. Biophys. Acta 1980, 522.

Synthesis and Properties of γ-Fe2O3 Nanoclusters (30) Greenwood, N. N.; Gibb, T. C. Mo¨ssbauer Spectroscopy; Chapman Hall: London, 1971; p 241. (31) Coey, J. M. D. Phys. ReV. Lett. 1971, 27, 1140. (32) Teillet, J.; Bouree, F.; Krishnan, R. J. Magn. Magn. Mater. 1993, 123, 93. (33) Imaoka, Y. J. Electrochem. Jpn. 1968, 36, 15. (34) Morrish, H. In Crystals: Growth, Properties, and Applications; Freyhardt, H. C., Mgr Ed.; Springer-Verlag: New York, Vol. 2, 1980; p 171.

J. Phys. Chem. B, Vol. 105, No. 31, 2001 7423 (35) (a) Ziolo, R. F.; Giannelis, E. P.; Weinstein, B. A.; O’Horo, M. P.; Ganguly, B. N.; Mehrotra, V.; Russell, M. W.; Huffman, D. R. Science 1992, 257, 219. (b) Vassiliou, J. K.; Mehrotra, V.; Russell, M. W.; Giannelis, E. P.; McMichael, R. D.; Shull, R. D.; Ziolo, R. F. J. Appl. Phys. 1993, 73, 5109. (36) Sohn, B. H.; Cohen, R. E.; Papaefthymiou, G. C. J. Magn. Magn. Mater. 1998, 182, 216; (37) Stoner, E. C.; Wohlfarth, E. P. Philos. Trans. R. Soc. A 1948, 240, 599.