ARTICLE pubs.acs.org/JPCC
Fabrication and Magnetic Properties of fcc-Co Nanorods Embedded in Epitaxial Thin Films of Anatase TiO2 As a Transparent Matrix Katsura Ikemiya,*,† Yasushi Hirose,†,‡ and Tetsuya Hasegawa†,‡ † ‡
Department of Chemistry, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan Kanagawa Academy of Science and Technology, Takatsu-ku, Kawasaki 213-0012, Japan ABSTRACT: Nanocomposite thin films consisting of Co nanorods and an anatase TiO2 matrix were successfully prepared by pulsed laser deposition (PLD). Co nanorods with diameters of 9-6 nm and heights of 10-22 nm were homogeneously dispersed in the matrix as a result of phase separation during PLD growth. Magnetization measurements confirmed ferromagnetic signals comparable to those expected for Co metal, suggesting that because of the high oxygen affinity of Ti, oxidation of Co nanorods did not occur. The obtained nanocomposite films were highly transparent in the visible-light region and had a ferromagnetic Faraday ellipticity of 1.9 104 deg cm-1 at 3.5 eV.
’ INTRODUCTION Ferromagnetic metal nanoparticles, such as nanospheres and nanorods, have received much attention because of their shape- or size-dependent magnetism.1-6 In general, however, they are subject to oxidation in ambient atmospheres, resulting in deterioration of their ferromagnetic properties. Various methods have been used to protect metal nanoparticles from oxidation. These methods include coating the nanoparticles with shells such as SiO2,7-11 embedding the nanoparticles in microfibers which have an oxygen-diffusion barrier,12 and packing the nanoparticles in carbon nanotubes.13 Another effective way of preventing oxidation is to disperse the nanoparticles in polymer films, such as films of poly(methyl methacrylate). In these nanocomposites, the polymers can be regarded as matrices in which the nanoparticles are embedded. If a transparent polymer is used as the matrix, the obtained nanocomposite film shows magneto-optical effects such as Faraday rotation.6,14,15 However, the polymers are subject to photodegradation on UV irradiation, limiting the magneto-optical applications of polymerbased nanocomposites.16 In this work, we propose a new technique for the preparation, using pulsed laser deposition (PLD), of nanocomposite thin films consisting of Co nanorods dispersed in an anatase TiO2 matrix. Fcc-Co nanorods with diameters of 9-16 nm and heights of 10-22 nm were spontaneously formed and dispersed in the anatase matrix as a result of phase separation during PLD growth. TiO2 matrices tend to hold oxygen more strongly than Co oxides do, and, thus, they are expected to play a role in protecting the Co nanorods from oxidation. The obtained nanocomposite films exhibited a ferromagnetic Faraday effect as large as 1.9 104 deg cm-1 at 3.5 eV, together with sufficient chemical stability of the Co nanorods in ambient atmospheres. r 2011 American Chemical Society
’ EXPERIMENTAL SECTION Epitaxial thin films of (001)-oriented anatase TiO2 containing Co nanorods were prepared on LaSrAlO4 (LSAO) (001) singlecrystalline substrates by pulsed laser deposition (PLD). A Kr-F excimer laser (wavelength = 248 nm) operated with a laser fluence of 2 J cm-2 pulse-1 and a repetition rate of 2 Hz was used for ablation. To obtain the crystallized anatase TiO2 phase over a wider temperature range, we used a seed layer of pure TiO2, as described below. Sintered pellets of pure TiO2 and a mixture of TiO2 and CoO (molar ratio TiO2:CoO = 95:5) were used as target materials for the anatase TiO2 seed layers and Co:TiO2 films, respectively. An anatase TiO2 seed layer was first deposited on an LSAO substrate at Ts = 650 °C and PO2 = 5 10-3 Torr, and then a Co: TiO2 thin film was grown at different substrate temperatures in the range Ts = 200-400 °C and at an oxygen pressure of PO2 = 1.0 10-6 Torr. The thicknesses of the TiO2 seed layer and the Co:TiO2 film were set at 5 and 28 nm, respectively. The crystallinity and crystallographic orientation of the obtained films were evaluated by X-ray diffraction (XRD). The surface topography was characterized by atomic force microscopy (AFM). The sizes and distribution of the Co nanorods were examined by transmission electron microscopy (TEM) with the aid of energy dispersive X-ray analysis (TEM-EDX), scanning TEM (STEM), and high-angle annular-dark-field STEM (HAADFSTEM). The crystal structure of the Co nanorods was determined from nanobeam electron diffraction patterns. Magnetic properties were measured by a superconducting quantum interference device (SQUID) magnetometer (MPMS XL; Quantum Received: October 27, 2010 Revised: December 16, 2010 Published: January 11, 2011 1776
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Figure 1. XRD patterns of Co:TiO2/TiO2 films prepared at various Ts. “A” and “L” denote anatase TiO2 and LaSrAlO4, respectively.
Design, San Diego, CA, USA) and a UV-vis magneto-optical spectrometer (BH-M800UV-KC-KF; Neoark Corp., Tokyo, Japan). All measurements were performed at room temperature.
’ RESULTS AND DISCUSSION Figure 1 shows XRD patterns of the Co:TiO2 films grown on TiO2 seed layers; the films are hereafter referred to as Co:TiO2/ TiO2. As can be seen, anatase TiO2 films with (001) orientations were obtained in all of the Ts ranges examined, that is, the TiO2 crystallization temperature is decreased from 500 to 200 °C or less by the anatase TiO2 seed layer. No diffraction peaks originating from Co metal or Co compounds were detected in the XRD data. Figure 2 compares AFM images of the Co:TiO2/TiO2 films grown at Ts = 200, 250, 300, 350, and 400 °C. The film grown at Ts = 200 °C has a relatively smooth surface, with a surface roughness of ∼1 nm. In contrast, protrusions with heights of ∼4 nm are seen on the film grown at Ts = 250 °C. The shapes and densities of the protrusions are essentially unchanged up to Ts = 300 °C. However, the protrusions are significantly bigger at Ts = 350 °C, and become irregular in both size and shape at Ts = 400 °C. TEM results for the Co:TiO2/TiO2 films grown at Ts = 250, 300, and 350 °C are summarized in Figures 3 and 4. The planeview and cross-sectional TEM images in Figures 3(a)-(c) and Figures 4(a)-(c) exhibit well-defined cylindrical nanostructures with long-axes nearly perpendicular to the film planes. The nanostructures are imaged with a brighter contrast, suggesting that they are rich in a heavier element, i.e., Co. Nanobeam electron diffraction measurements at the Co-rich regions revealed that they are Co single-crystals with an fcc structure. These results clearly indicate that fcc-Co nanorods are embedded in the TiO2 matrix. The average diameters and heights of the Co nanorods estimated from the TEM images are 9 and 10 nm, 10 and 22 nm, and 16 and 17 nm for the films grown at Ts = 250, 300, and 350 °C, respectively, yielding average volumes for individual Co nanorods of 640, 1700, and 3400 nm3, respectively. The systematic increase in the average volume with increasing Ts is consistent with a kinetic picture in which Co atoms become more mobile at higher temperatures.17
Figure 2. AFM images of Co:TiO2/TiO2 films prepared at Ts = (a) 200, (b) 250, (c) 300, (d) 350, and (e) 400 °C.
Figure 3. Planar HAADF-STEM images (left) and nanobeam electron diffraction patterns (right) of Co:TiO2/TiO2 films prepared at Ts = (a) 250, (b) 300, and (c) 350 °C.
TEM-EDX point measurements of the film grown at Ts = 300 °C confirmed that the Co content in the matrix region is below the EDX detection limit (∼1%). This strongly suggests that the Co nanorods are almost completely segregated from the TiO2 matrix. Figure 5(a)-(e) shows plots of magnetization vs magnetic field curves for the Co:TiO2/TiO2 films prepared at various Ts, for application of external magnetic fields perpendicular to and parallel to the film surfaces. As can be seen, the saturated magnetization (Ms) values are almost constant in the Ts range 250-350 °C. Assuming that the magnetization in Figure 5(a)-(e) 1777
dx.doi.org/10.1021/jp110294d |J. Phys. Chem. C 2011, 115, 1776–1779
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Figure 5. (a) Perpendicular and parallel magnetization vs field curves for Co:TiO2/TiO2 films prepared at Ts = (a) 200, (b) 250, (c) 300, (d) 350, and (e) 400 °C.
Figure 6. Cobalt concentrations evaluated from the magnetization curves as a function of Ts. The dashed line denotes Co concentration in the Co:TiO2 target (5%).
Figure 4. Cross-sectional TEM results of Co:TiO2/TiO2 films prepared at Ts = (a) 250, (b) 300, (c) 350, and (d) 200 °C. (a) and (c) are HAADF-STEM images, and (b) and (d) are TEM-EDX images.
originates from fcc-Co, we estimated the Co concentrations (cCo) in Co:TiO2/TiO2 using the following relation: cCo = 100 (Ms/MCo)/(Ms/MCo þ S t danatase NA/manatase), where Ms is the saturation magnetization of each film, MCo is the magnetic moment per Co atom (1.75 μB: μB is the Bohr magneton), S is the area of each film (18-20 cm2), t is the thickness of each film (28 nm), danatase is the density of anatase (3.859 g cm-3), NA is Avogadro's number, and manatase is the molar mass of anatase (79.9 g mol-1). The obtained cCo values are 4.3, 3.7, and 3.3% for Ts = 250, 300, and 350 °C, respectively; these values are close to the Co content of the Co:TiO2 target (5%). This implies that in these films, Co exists mostly as nanorods of fcc-Co metal, that is, the Co nanorods do not undergo oxidation, even though they are embedded in the oxide matrix. This is possibly because TiO2 does not lose oxygen easily. Indeed, the Gibbs free energy (ΔG°) for the reaction 2Co þ O2 f 2CoO (ΔG°27 °C = -427 kJ/mol O2, ΔG°300 °C = -388 kJ/ mol O2) is larger than that for Ti þ O2 f TiO2 (ΔG°27 °C =
-874 kJ/mol O2, ΔG°300 °C = -826 kJ/mol O2) or that for 2TiO þ O2 f 2TiO2 (ΔG°27 °C = -780 kJ/mol O2, ΔG°300 °C = -721 kJ/mol O2).18 In the films grown at Ts = 200 and 400 °C, the saturation values are considerably lower than that expected for 5% fcc Co. At Ts = 400 °C, it is likely that Co reacts with O or Ti to form nonmagnetic oxides such as CoO and CoTiO3, resulting in a decrease in magnetization. To investigate the chemical state of Co in the film grown at Ts = 200 °C, cross-sectional TEM-EDX measurements were conducted, as illustrated in Figure 4(d). As can be seen from Figure 4(d), Co is distributed homogeneously over the entire Co:TiO2/TiO2 film region. Furthermore, small Co-rich nanoparticles are visible at the interface between the TiO2 seed layer and the Co:TiO2 film. The presence of these nanoparticles implies that at Ts = 200 °C, phase separation into Co and TiO2 is not complete. The nanoparticles at the interface are probably composed mainly of Co metal, and this is responsible for the ferromagnetic signals shown in Figure 5(a) and Figure 6. It is speculated that the remaining Co exists inside the TiO2 film as ultrasmall particles of Co metal or nonmagnetic Co oxides such as CoO.19,20 In the Co:TiO2/TiO2 films grown at Ts = 250 and 300 °C, the perpendicular magnetization shows a sharper hysteresis loop than the parallel one. This implies that the magnetic easy-axes are along the length of the rods, due to the shape anisotropy of the Co metal cylinders.21 Finally, we describe the magneto-optical properties of the Co: TiO2/TiO2 films. As shown in Figure 7(a), all of the films obtained are transparent below a photon energy of ∼3.5 eV, 1778
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’ AUTHOR INFORMATION Corresponding Author
*Phone: þ81-3-5841-4603; Fax: þ81-3-5841-4603; E-mail:
[email protected] .
’ ACKNOWLEDGMENT We wish to thank S. Konuma at Kanagawa Academy of Science and Technology (KAST) for TEM and TEM-EDX measurements, and the Japan Science and Technology Agency (JST) for financial support. ’ REFERENCES
Figure 7. (a) Transmittance and (b) Faraday ellipticity spectra of Co: TiO2/TiO2 films prepared at various Ts. External field was applied perpendicular to the film planes.
which reflects the bandgap energy of anatase TiO2 (3.2 eV). Figure 7(b) shows the Faraday ellipticity spectra of the films grown at various Ts. Only the films grown at Ts = 250-350 °C, in which fcc-Co nanorods were successfully embedded, feature Faraday effects. Notably, the Faraday ellipticity is maximized at around 3.5 eV and reaches 1.9 104 deg cm-1, which is about 1 order of magnitude larger than that of poly(methyl methacrylate) with Co nanorods.14 The characteristic peak at around 3.5 eV in Figure 7(b) is thought to be a consequence of multiple reflections inside the film; this increases the net thickness of the film and thus enhances the Faraday signal. In the present Co:TiO2 nanocomposites, the multiple reflection effect is significantly enhanced as a result of the relatively large refractive index of anatase: 2.4 at around 400 nm. A rapid decrease in the Faraday signal at
