Journal of Non-Crystalline Solids 354 (2008) 4271–4274
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Magnetite nanowires in MCM-41 type mesoporous silica templates Zbigniew Surowiec a,*, Marek Wiertel a, Mieczysław Budzyn´ski a, Jan Sarzyn´ski a, Jacek Goworek b a b
Institute of Physics, M. Curie-Sklodowska University, 1 M. Curie-Sklodowska Sq, 20-031 Lublin, Poland Faculty of Chemistry, M. Curie-Sklodowska University, 2 M. Curie-Sklodowska Sq, 20-031 Lublin, Poland
a r t i c l e
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Article history: Available online 14 August 2008 PACS: 76.80.+y Keywords: Magnetic properties Mössbauer effect
a b s t r a c t MCM-41 mesoporous material was chosen as a template of very small Fe3O4 particles. The results of structural and magnetic studies of magnetite nanowires are reported. The average length of these nanowires is about 70 nm and their diameter is 3 nm. Magnetite polycrystalline nanowires were characterized by means of X-ray diffraction and 57Fe Mössbauer spectroscopy (MS). Almost 80% of the particles exist in a superparamagnetic state at room temperature. Mössbauer measurements also provided evidence that the composites displayed a distribution of magnetic particles by size. As a result, strong changes of superparamagnetic and magnetic relative contributions along with temperature were observed. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Low dimensionality structures such as magnetic nanowires have attracted much attention recently due to their importance for fundamental studies and a wide range of potential applications in nanotechnology. Numerous methods have been invented to fabricate nanostructures. For example, it is possible can produce characteristic features of approx. 25 nm by means of electron beam lithography (EBL) [1] or nano-imprint lithography. Preparation and magnetic properties of nanowire arrays in anodic aluminum oxide (AAO) templates have been intensively investigated recently [2–4]. Other alternative methods include the use of mesoporous ordered silica materials [5]. One of such materials which is characterized by a uniform pore diameter, a large pore volume and a large surface area is MCM-41. A hexagonal arrangement of cylindrical pores is obtained in MCM-41 with the use of the templating technique. Cylindrical micelles created by surfactant in an alkaline medium are used as condensation centers for silica from tetraethylorthosilicate (TEOS) or alkalimetal silicate. Accumulation of micellar rods leads to the creation of honeycomb-shaped micelletemplated silica. When using the templates with an alkyl chain of a varying length it is possible to control pore diameters in the range from 2 to 10 nm. Pyrolysis at 820 K is one of the standard procedures for template removal. In the analyzed case, 3.0 nm diameter magnetite nanowires arrays placed in MCM-41 mesoporous silica templates were prepared in two ways. Magnetic properties at different temperatures were investigated taking specifically the finite-size effects into account.
MCM-41 materials were prepared [6] using octadecyltrimethyl ammonium bromide (C18TAB) as a template and TEOS as a silica source. The mixture was annealed at 323 K for 10 h. The synthesis process lasted an hour while the solution was being stirred. Solids prepared in such a way were calcinated from the room temperature up to 300 °C for 2 h under an air flow and afterwards at 550 °C for 4 h in order to remove the surfactant. Nitrogen adsorption measurements at 77 K were carried out using an ASAP2405 analyzer. Nitrogen adsorption was used to measure the Brunauer–Emmett–Teller surface area and the pore size. The pore size distribution was calculated from the desorption branches of isotherms using the standard Barrett–Joyner–Halenda procedure [7]. In the second step the calcinated samples were impregnated with a 5% aqueous solution of Fe(NO3)3 9H2O containing Fe3+ ions for 0.5 h, dried (1 h) and annealed in the air atmosphere at 110 °C for 1 h. The last stage of the preparation procedure was reduction in a hydrogen atmosphere at 300 °C for 1 h (sample 1) and for 4 h (sample 2). A phase analysis of impregnated samples was carried out by means of X-ray powder diffraction (XRD) using Cu Ka radiation. Scans were obtained in the range from 10° to 90° (2h). Mössbauer spectra of powder samples were recorded using a constant acceleration spectrometer in the temperature range 300–900 K with a 57 Co(Rh) source. The center shift (CS) values were calibrated against metallic a-Fe foil at room temperature.
* Corresponding author. Tel.: +48 81 537 62 20; fax: +48 81 537 61 91. E-mail address:
[email protected] (Z. Surowiec). 0022-3093/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2008.06.032
3. Results and discussion A specific surface area (S), an average pore diameter (R) and a mean pore volume (V) were determined from the nitrogen
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Fig. 1. XRD diffraction pattern of Fe3O4 nanowires in a MCM-41 silicate template. The full curve represents the calculated pattern, the points – the observed one. Short vertical lines below the diffraction pattern indicate the calculated Bragg positions for magnetite (upper row) and for hematite (lower row).
adsorption measurements for the MCM-41 template. The values of the above physical quantities are: S = 1214 m2/g, R = 1.5 nm and V = 0.93 cm3/g [6]. The X-ray diffraction pattern of sample 1 is shown in Fig. 1. The smoothly varying peak intensity in the 10°–30° range results from the amorphous silica template. The sharp peaks correspond to the diffraction on the crystal structure of magnetite and a small part of hematite. The lattice parameter a of Fe3O4 crystalline nanowires structure is 8.34(1) Å. This parameter is slightly smaller than in case the bulk material (a = 8.39(1) Å). The average magnetite crystallite length 12.0(5) nm was deduced by the broadening of XRD diffraction peaks using the Sherrer’s formula for sample 1 and 15.2(5) nm for sample 2. The Mössbauer spectra of magnetite nanowires in the MCM-41 type silicate templates at selected temperatures for different ways of sample preparation are shown in Fig. 2. The left panel shows the spectra for sample 1. The pattern at room temperature consists of two sextets and a superimposed superparamagnetic doublet. The
Fig. 2.
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sextets originate from two non-equivalent tetrahedral and octahedral positions of Fe atoms in a ferromagnetic magnetite. The intensity ratio of the sextet peaks is about 3:2:1, indicating that the nanowires in the MCM-41 template are randomly oriented in space. The doublet component is related to the superparamagnetic phase of this iron oxide. The existence of superparamagnetic nanoparticles results from the phenomenon of relaxation due to the intrinsic finite-size effect. The line broadening of the sextets is typical of superparamagnetic nanoparticles at a temperature that is close to the blocking which is defined as a transition temperature from fast to slow relaxation [8,9]. The relative contribution of the relaxing component to the total spectrum at room temperature is about 80%. The significant quadrupole splitting value equal to 0.78(3) mm/s obviously originates from 57Fe nuclear probes in the non-spherical local surroundings coming from particle surfaces. With increasing temperature the ferrimagnetic component decreases for sample 1. Additionally, a monoline appears in the Mössbauer spectrum above 500 K. This component results from nanoparticles in the superparamagnetic state, similarly to the doublet. However, in this case 57Fe probes located in the core of nanoparticles are a signal source. They have spherical local surroundings in which there is no electric field gradient. The appearance of the monoline at temperatures considerably below the Curie temperature for bulk magnetite (858 K) indicates that it corresponds to a superparamagnetic phase and not to a regular paramagnetic phase. At the highest measurement temperature above TC (900 K) the doublet and monoline are observed contrary to the bulk Fe3O4 where only a monoline occurs. The occurrence of this doublet is a consequence of a large value of the ratio of 57 Fe probes located on the surface to those located in the volume characteristic of nanomaterials. In the right panel of Fig. 2 the Mössbauer spectra for sample 2 are depicted. The occurrence of a monoline in sample 2 spectra at room temperature is a major difference in comparison to the spectrum of sample 1. Additionally, the relative contribution of magnetic sextets is clearly greater. These facts suggest that the contribution of nanoparticles of bigger sizes is larger in sample 2.
Fe Mössbauer spectra for Fe3O4 nanowires in the MCM-41 silicate template at some selected temperatures.
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Fig. 3. Diagrams representing relative contributions for different Mössbauer spectra components at various temperatures: superparamagnetic monoline (black filled bar), superparamagnetic doublet (grey filled bar) and ferrimagnetic sextets (empty bar). Left panel – sample 1 and right panel – sample 2.
In Fig. 3 relative contributions of individual components versus temperature are presented. Gray bars relate to the superparamagnetic doublet. Black bars relate to the superparamagnetic monoline. An increase in the superparamagnetic phase contribution relative to the magnetic phases (empty bar) with an increase of temperature from 80(3)% at RT to 100% at 900 K for sample 1 and from 64(2)% at RT to 100% at 850 K for sample 2 was observed (see Fig. 4). The values of hyperfine parameters are determined from the fitting procedure. The hyperfine magnetic fields are the same for both samples in error limits and they are equal to about 49.3 and 46.0 T for tetrahedral and octahedral positions, respectively. These values are very close to those for the bulk magnetite [10]. Quadrupole splitting (QS) temperature dependences for all components are presented in Fig. 5. For both samples the QS of
the doublet in the magnetic components is close to zero and it is independent of temperature. On the other hand, the values of QS for the doublet component representing the superparamagnetic phase decrease with the increasing temperature. This dependence can be explained by the partial phase transition from magnetite to maghemite in which the QS values are equal to 0.78 and 0.55 mm/s [8], respectively. Thus the weighted mean value of QS decreases when the maghemite contribution increases. The transition of this type was observed in nanocrystalline powders [11]. Figs. 5 and 6 show temperature dependences of center shift for samples 1 and 2, respectively. In both samples the linear decrease of CS values for the magnetic sextets and the quadrupole doublets with the increasing temperature can be fully explained by the second order Doppler shift. However, the same dependence for the superparamagnetic monoline is more complicated. At room tem-
Fig. 4. Temperature dependence of quadrupole splitting values for Fe3O4 nanowires.
Fig. 5. Center shift values referred to metallic iron derived from spectra versus temperature obtained for sample 1.
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Fe Mössbauer
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fabricated in MCM-41-type mesoporous silica templates. The use of mesoporous silicate as a template makes it possible to obtain a smaller diameter of nanowires in comparison with AAO templates. The radius of channels can be controlled through the alkyl chains length. The Mössbauer spectroscopy is a very useful tool to study magnetic properties, local electric field gradients and a phase composition in nanosized crystallites. At room temperature a major part of Fe3O4 exists in the superparamagnetic state. The contribution of this phase increases with the increasing sample temperature. A different way of preparation of samples gives a different distribution of nanoparticles sizes. Longer time of reduction in a hydrogen atmosphere leads to the creation of somewhat larger particles. The significant values of quadrupole splittings in nanowires come from 57Fe probes located on the surface. The local surroundings on the surface have a reduced symmetry. The temperature dependence of quadrupole splitting in the superparamagnetic state suggests partial phase transition from magnetite to maghemite with heating of the samples.
References Fig. 6. Center shift values referred to metallic iron derived from spectra versus temperature obtained for sample 2.
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Fe Mössbauer
perature the value of CS for sample 2 (1.5 mm/s) corresponds approximately to ferrous high spin states. It seems that at higher temperatures these states are converted into low spin ones. 4. Conclusions The results presented in this paper show that iron oxide nanowires, and magnetite nanowires in particular, could be successfully
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