J. Phys. Chem. C 2008, 112, 13911–13916
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Synthesis of Vertically Aligned Manganese-Doped Fe3O4 Nanowire Arrays and Their Excellent Room-Temperature Gas Sensing Ability Seon Oh Hwang,† Chang Hyun Kim,† Yoon Myung,† Seong-Hun Park,† Jeunghee Park,*,† Joondong Kim,‡ Chang-Soo Han,‡ and Jae-Young Kim§ Department of Chemistry, Korea UniVersity, Jochiwon 339-700, Korea, Nano-Mechanical Systems Research Center, Korea Institute of Machinery and Materials, Daejeon 305-343, Korea, and Pohang Accelerator Laboratory, POSTECH, Pohang 790-784, Korea ReceiVed: April 3, 2008; ReVised Manuscript ReceiVed: June 26, 2008
Vertically aligned Mn (10%)-doped Fe3O4 (Fe2.7Mn0.3O4) nanowire arrays were produced by the reduction/ substitution of pregrown Fe2O3 nanowires. These nanowires were ferromagnetic with a Verwey temperature of 129 K. X-ray magnetic circular dichroism measurements revealed that the Mn2+ ions preferentially occupy the tetrahedral sites, substituting for the Fe3+ ions. We observed that the Mn substitution decreases the magnetization, but increases the electrical conductivity. We developed highly sensitive gas sensors using these nanowire arrays, operating at room temperature, whose sensitivity showed a correlation with their bond strength of diatomic/triatomic molecules. 1. Introduction One-dimensional (1D) nanostructures have attracted considerable attention, due to their potential as building blocks for assembling active and integrated nanosystems.1 Advances in growth and characterization have led to a greater assortment of well-characterized structures. Recently, interest in room-temperature magnetic 1D nanostructures (e.g., nanorods, nanowires, and nanotubes) has been steadily increasing, because of their potential application in perpendicular data recording and spintronic devices.2 Iron oxides possess many advantages in technological applications, due to their exclusive combination of magnetic and electrical properties. One of the most important iron oxides, magnetite (Fe3O4), is known to undergo a ferrimagnetic transition at a Curie temperature (TC) of ∼850 K. It has an inverse spinel structure, symbolized as [Fe3+]A[Fe2+Fe3+]BO4, in which the A sites (tetrahedral sites) are occupied by the Fe3+ ions, and the B sites (octahedral sites) by equal numbers of Fe2+ and Fe3+ ions. Furthermore Fe3O4 undergoes a first-order metal-insulator phase transition at TV ) 120-130 K (Verwey temperature), which is accompanied by the long-range charge ordering of the Fe2+ and Fe3+ ions in the B-site sublattices. In the past few years, much effort has been devoted to the preparation of Fe3O4 low-dimensional nanostructures.3-23The current biological applications of quantum dots (QDs) include targeted drug delivery, ultrasensitive bioagent detection, gene therapy, hydrothermic cancer treatment, and magnetic resonance imaging (MRI) contrast enhancement.3-12 Moreover, their nearly full spin polarization at room temperature makes 1D nanostructures appealing for giant magnetoresistance and spin valve devices.14,18,19 It has also been demonstrated that these 1D nanostructures are potentially applicable to lithium ion battery electrodes and field emission displays.13,15 * Corresponding author, e-mail:
[email protected]. † Department of Chemistry, Korea University. ‡ Nano-Mechanical Systems Research Center, Korea Institute of Machinery and Materials. § Pohang Accelerator Laboratory, POSTECH.
The substitution of one type of iron by another transition metal ion provides a means of controlling their magnetic/ electrical properties, which extend their application range. Ishikawa et al. reported that Mn doping (up to 17%) of Fe3O4 films decreases the carrier concentration (without affecting the electron mobility), but increases their spin polarization, suggesting the substitution of the Mn ions at the tetrahedral (Td) sites.24 On the other hand, the Codoped Fe3O4 film, in which the Co2+ ions occupy the octahedral (Oh) sites, showed increased magnetic anisotropy with increasing Co concentration.25 As one of important applications, it is expected that they can be used to build gas sensors, owing to their enhanced ability to serve as active sites for gas molecules.26,27 Thus, a study of the effect of transition metal doping on the properties of the Fe3O4 nanostructures is of interest from the viewpoint of both fundamental scientific interest and its potential application to various nanodevices. In this study, we first synthesized vertically aligned Mn (10%)-doped Fe3O4 nanowire (NW) arrays on a large area of the substrates, using the reduction/substitution reaction of pregrown vertically aligned R-Fe2O3 (hematite) NW arrays. The present work is designed to provide valuable information on how the Mn doping influences the magnetic/electrical properties of the Fe3O4 NWs. Detailed analytical investigations of the electronic structures were performed using X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD). Based on the full characterization of electronic structures, we fabricated gas sensors using the Mn-doped Fe3O4 NW arrays and demonstrated their excellent sensing ability toward various gases at room temperature, due to the effective Mn doping. 2. Experimental Section The vertically aligned R-Fe2O3 NWs were prepared using the method as described elsewhere.28 The synthesis procedure can be summarized as follows. A piece of Fe foil (0.25 mm thickness) used as both the reagent and substrates were ultrasonically cleaned with ethanol, placed inside a tube reactor, and heated to 800 °C under Ar gas flow (500 sccm). The flow
10.1021/jp802943z CCC: $40.75 2008 American Chemical Society Published on Web 08/16/2008
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Figure 1. (a) SEM micrographs showing the high-density Mn-doped Fe3O4 nanowire array grown on the substrates. (b) Magnified view revealing the vertical alignment of the nanowires on the substrates and the round-shape tips. (c) TEM images showing the general morphology of the nanowires. Their average diameter is 200 nm. (d) Lattice-resolved image of a typical nanowire, whose TEM image is shown in the top inset, revealing the single-crystalline Fe3O4 crystals. The bottom inset shows the corresponding SAED pattern, confirming the [110] growth direction. The distance between the (111) planes (d111) is about 0.49 nm. (e) EDX mapping and (f) line scanning of Fe, Mn, and O elements, showing homogeneous 10% Mn doping. The corresponding STEMimage is shown in the left/top image of part e.
of oxygen (O2) with a rate of 20-50 sccm was introduced for 30 min. While keeping the Ar gas flowing, the reactor was allowed to cool to room temperature. The Fe foils were covered homogeneously with a red-colored Fe2O3 NWs. To produce the Mn-doped Fe3O4 NWs, these as-grown R-Fe2O3 NWs were placed inside reactor tubes and MnCl2 beads (Aldrich, 99.999%) were evaporated at 800 °C using Ar flow for 30 min. The red color of nanowires turned into black color after the reaction. For the Fe3O4 NWs, the Fe foil was heated to 800 °C under O2 gas flow (to produce R-Fe2O3 NWs), and then CH4 was allowed to flow for 10 min. The Fe foils were covered homogeneously with a black-colored Fe3O4 NWs. The morphology and structures of the nanowire products were analyzed by scanning electron microscopy (SEM, Hitachi S-4300), transmission electron microscopy (TEM, Jeol JEM2010, JEM 2100F), high-voltage TEM (HVEM, Jeol JEM ARM 1300S, 1.25 MV), and energy-dispersive X-ray fluorescence spectroscopy (EDX). X-ray diffraction (XRD, Philips X’Pert PRO MRD) patterns were obtained using Cu KR radiation (λ
) 1.5406 Å). The magnetic properties were measured by means of a superconducting quantum interference device (SQUID, Quantum Design) magnetometer. The XAS and XMCD measurements were carried out at the Pohang Light Source (PLS) elliptically polarized undulator beam line, 2A. The samples were introduced into an experimental chamber with a base pressure of 5 × 10-10 Torr. The spectra were collected in the total electron yield (TEY) mode. A 0.1 T electromagnet was used to switch the magnetization direction. The magnetization direction was flipped between the parallel (F+) and antiparallel (F-) directions with respect to the photon helicity vector for each data point. The degree of circular polarization (95%) of the incident light was taken into account in the spectra. We fabricated room-temperature gas sensors using the nanowire arrays. The distance between the two aluminum electrodes was 1 mm, and the lateral resistance was measured using a twoprobe configuration. For the I-V measurements of the nanowire electrodes, a parametric test equipment (Agilent E5270A) was used.
Manganese-Doped Fe3O4 Nanowire Arrays
Figure 2. (a) XRD patterns of Fe3O4 powders, Fe3O4 NWs, and Mndoped Fe3O4 NWs. (b) Magnified (311) peaks. (c) Fine-scanned XPS Fe 2p3/2 peak of Fe3O4 and Mn-doped Fe3O4 NWs. (d) Fine-scanned Mn 2p3/2 peak of MnO powders and Mn-doped Fe3O4 NWs.
3. Results and Discussion 3.1. Morphology and Composition. The vertically aligned R-Fe2O3 NWs were prepared using the oxidation reaction of Fe foils.28 The R-Fe2O3 NWs consist of single-crystalline rhombohedral R-Fe2O3 crystals grown with the [112j0] direction. Then the Mn-doped Fe3O4 NW arrays were synthesized by the reduction/substitution reaction of these pregrown Fe2O3 NWs with MnCl2 at 800 °C. Figure 1a shows a SEM micrograph of the vertically aligned Mn-doped Fe3O4 NW array with a length of ∼5 µm on the substrates. The magnified image shows that the nanowires have round-shape tips and the roots are attached directly to the substrates (Figure 1b). The TEM image reveals the straight morphology of the nanowires (Figure 1c). Their average diameter was found to be 200 nm. The lattice-resolved image of a typical nanowire, whose TEM image is shown in the top inset, reveals that it consists of single-crystalline crystals (Figure 1d). The bottom inset shows the corresponding selectedarea ED (SAED) pattern, measured at [111] zone axis, confirming the growth of single-crystalline cubic Fe3O4 crystals with the [011] direction. The growth direction remains the same as that of R-Fe2O3 NWs after the conversion. The distance between the (111) planes (d111) is about 0.49 nm, which is consistent with that of Fe3O4 (JCPDS No. 72-2303; a ) 8.4 Å). All of the nanowires we observed had the same growth direction. Parts e and f of Figure 1 correspond to the EDX mapping and line scanning of Fe, Mn, and O elements, respectively. The corresponding scanning TEM (STEM) image is shown in the left/ top image of Figure 1e. It showed that the Mn dopes homogeneously over the entire nanowire with a content ([Mn]/ ([Mn]+[Fe])) of 0.1. The EDX data is shown in the Supporting Information, Figure S1. The XPS data also show that the Mn content is 0.1 ( 0.03, as shown in the Supporting Information, Figure S2. The average Mn content was determined to be 0.1, so that the composition of the nanowires can be represented as
J. Phys. Chem. C, Vol. 112, No. 36, 2008 13913 Fe2.7Mn0.3O4. We also successfully synthesized vertically aligned single-crystalline Fe3O4 NW array. Their SEM/TEM images and EDX data are shown in the Supporting Information, Figure S3. 3.2. XRD and XPS. Figure 2a displays the XRD pattern of the Fe3O4 powder (Aldrich, 98%), Fe3O4 NWs, and Mn-doped Fe3O4 NWs. The nanowire samples show only peaks corresponding to Fe3O4 (JCPDS No: 72-2303). The Mn doping shifts the peak positions of Fe3O4 to a lower angle; e.g., ∆(2θ) ) 0.1° for the (311) peak (Figure 2b). This shift corresponds to the 0.3% increase of the lattice constants, suggesting that the Fe3+ ions are effectively substituted with the larger radius Mn2+ ions (i.e., rFe3+ ) 63 pm and rMn2+ ) 80 pm, where all have high spins at the Td sites), as we discussed below. The Supporting Information, Figure S2, includes the XRD pattern of R-Fe2O3 powder, γ-Fe2O3 nanosize powder (Aldrich), and pregrown R-Fe2O3 NWs, to show the exact assignment of Fe3O4 phase. Figure 2c displays the fine-scanned XPS Fe 2p3/2 peaks of Fe3O4 NWs and Mn-doped Fe3O4 NWs. The Fe 2p3/2 peak of Mn-doped Fe3O4 NWs shifts to the higher energy region by 0.2 eV, indicating the higher oxidation state compared to that of the Fe3O4 NWs. Figure 2d shows the fine-scanned XPS Mn 2p3/2 peaks of MnO powder and Mn-doped Fe3O4 NWs. The position of Mn 2p3/2 peak is nearly same as that of MnO, suggesting the existence of the Mn2+ ions. 3.3. Magnetization: SQUID Measurement. Figure 3a displays the magnetization (M) versus magnetic field (H) curves measured for the Mn-doped Fe3O4 NW array (at 5 and 300 K). The nanowire was separated from the substrates. Hysteresis occurs with a saturation field of 0.2 T and a saturation magnetization (Ms) of 1.0 µB. The inset displays the curve in the vicinity of H ) 0, indicating that a coercive field (HC) of 230 Oe and a remanence (Mr) of 0.2 µB, at 5 K. Figure 3b displays the field-cooled (FC) and zero-field-cooled (ZFC) magnetization versus temperature (MFC and MZFC vs T) curves, with H ) 500 Oe. An abrupt change of the magnetization occurs at TV (Verwey temperature) ) 129 K, which arises from an order-disorder transition of the Fe2+ and Fe3+ ions within the Oh sites. The value of MFC minus MZFC (MFC - MZFC) is plotted to show precisely their TV value (inset). The M-H and M-T (also MFC-MZFC vs T) curves of the Fe3O4 NWs (separated from the substrates) also show ferromagnetic behaviors and TV ) 133 K, as shown in Figure 3, parts c and d, respectively. The M-H data shows a hysteresis loop with Ms ) 4 µB, HC ) 350 Oe, and Mr ) 1.3 µB (Figure 3c and its inset), which are larger than those of the Mn-doped Fe3O4 NWs. The TV value of the present Fe3O4 NWs (133 K) is slightly higher than that of the nanowires (TV ) 110-125 K) reported by other groups, and that of the bulk (TV ) 120-130 K).14,15,17,18,22 Since the Ms is the same as the theoretical value of spin magnetic moments, the nearly defect-free nature of the present Fe3O4 NWs may be responsible for the origin of their higher TV as compared to that of the bulk. The reduced TV, Ms, HC, and Mr values of the Mn-doped Fe3O4 NWs indicate that the 10% Mn doping decreases the ferromagnetic properties of the Fe3O4 NWs. For Fe3O4 and MnFe2O4, it is predicted that the substitution of the Fe2+ (S ) 2) ions with Mn2+ ions (S ) 5/2) increases the net spin magnetic moment.29 Ishikawa et al. showed the larger spin polarization due to the substitution of the Mn ions at the Td sites.24 On the other hand, the present 10% Mn-doped Fe3O4 NWs shows the reduced magnetization. In order to derive its origin, we performed XMCD measurements at the Fe and Mn L2,3 edges, which enables us to identify the substitution sites of Mn.
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Figure 3. (a) M-H curves of the Mn-doped Fe3O4 NWs measured at 5 and 300 K. The inset represents the M-H data in the vicinity of H ) 0. (b) Temperature-dependent MFC, MZFC, and MFC - MZFC (inset) values measured with H ) 500 Oe. (c) M-H and (d) M-T curves of the Fe3O4 NWs. The insets represents the M-H data in the vicinity of H ) 0, and MFC - MZFC values.
Figure 4. (a) XAS and XMCD spectra of Fe L2,3 edge, and (b) XAS and XMCD of Mn L2,3 edge of the Mn-doped Fe3O4 NWs. Inset of part a shows the XMCD of Fe L2,3 edge of the Fe3O4 NWs.
3.4. XAS and XMCD. Figure 4a shows the Fe L2,3 edge XAS and XMCD spectra of the two different magnetization directions (∆F ) F+ - F-), for the Mn-doped Fe3O4 NWs, measured under H ) 0.1 T at 150 K. The Fe L2,3 edge bands, which result from the 2p f 3d dipole transition, are divided roughly into the L3 (2p3/2) and L2 (2p1/2) regions. The spectral line shape of the Fe L2,3 edge shows a mixture of Fe2+ and Fe3+ ions (at the Oh and Td sites). The two negative peaks of L3 correspond to the Fe2+ and Fe3+ ions occupying at the Oh sites, and their intensity ratio of Fe2+/Fe3+ peaks is 0.8, indicating the lower population of the Fe2+ ions. For comparison, the XMCD spectra of the Fe3O4 NWs were also measured, as shown in the inset. Its ratio of Fe2+/Fe3+ is about 1.5, indicating the higher population of the Fe2+ ions. The XAS and XMCD spectra of the Fe3O4 NWs and Fe3O4 powders were shown in the Supporting Information, Figure S4. Figure 4b shows the Mn L2,3 edge (2p f 3d transition) XAS and XMCD spectra. These spectra are divided into the L3 (2p3/2) and L2 (2p1/2) regions. The dominant positive peak of L3 corresponds to the Mn2+ ions occupying at the Td sites.
The opposite signs of the Mn and Fe L2,3 edge XMCD reflect the antiferromagnetic interaction between two magnetic sublattices. The Mn L2,3 edge confirms the predominant contribution of Mn2+ at the Td sites. This result is consistent with that of MnFe2O4 films, in which the Mn2+ ions (>80%) preferentially occupy the Td sites.29 A XMCD study of MnxFe3-xO4 (x ) 0, 0.18, 1.56, 1.9) nanocrystalline thin film shows consistently that the Mn2+ substitutes for the Fe3+ ions at the Td sites.30 As the Mn dopes, the population ratio of Fe3+/Fe2+ at the Oh sites increases, indicating that the Fe2+ ions are replaced by the Fe3+ ions. The increased concentration of the Fe3+ ions at the Oh sites compensates the substitution (Fe3+f Mn2+) induced charge unbalance. This result is supported by the XPS data showing that the Mn substitution induces the higher oxidation state of Fe (section 3.2). According to the spin sum rule, the spin magnetic moment (MS) and the orbital magnetic moment (MO) of Fe and Mn ions can be estimated by the integration of the dichroism (∆F) over the entire L2,3 absorption region.31 The MS value of the Fe ions is 1.24 ( 0.12 µB/Fe, and their MO value is 0.06 ( 0.01 µB/Fe, respectively. The MS and MO values of the Mn ions are estimated to be 1.45 ( 0.15 and 0.14 ( 0.01 µB/Mn, respectively. Interestingly, the MO value of the Mn ions is quite considerable, giving MO/MS ) 0.1. The larger magnetic moments of the Mn ions than those of the Fe ions and their considerable orbital moment would be ascribed to their effective substitution for Fe at the Td sites. The magnetic moment of the 10% Mn-doped Fe3O4 NWs can be roughly estimated to be 1.3 µB. This value is consistent with that measured by MPMS, which is lower than the XMCD value of Fe3O4 single crystal, 3.68 µB (at 145 K).32 However, it is still not clear why the Mn doping reduces uniquely the magnetic moments in the case of the present nanowires. We may suggest a possibility that as the Mn dopes, the grain boundaries separating adjacent grains (although not detected from the HRTEM images due to resolution limit) forms at the nanowire surface and reduces consequently the magnetic moments. 3.5. Gas Sensors. The measurements of the electrical resistance of the Mn-doped Fe3O4 and Fe3O4 NW arrays
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Figure 5. (a) Resistance changes of the Mn-doped Fe3O4 NW array, upon its exposure to EtOH, NO2, H2, and CO gas at various pressures (20, 40, 200, 400, 800, and 2000 mTorr). (b) Sensitivity (∆R/R0 in units of %) for various gases, as a function of the gas pressure. The data in the lower pressure range show a linear fit. (c) The (∆R/R0)/p values for various gases and (d) their correlation with the bond strength (D° in kJ/mol) of each gas molecule.
revealed that the substitution of Mn2+ ions increases the conductivity by a factor of ∼50 (at room temperature), as shown in Supporting Information, Figure S5. The higher conductivity of Mn-doped Fe3O4 NWs could be realized by the accommodating the conduction path of the Fe ion network at the Oh sites. However, it is expected that the higher concentration of the Fe3+ ions (at the Oh sites) would decrease the carrier (electrons) concentration, which is inconsistent with the present result.24 Therefore, we suggest that the concentration of holes becomes much larger than the concentration of electrons, which results in a surface p-type conductivity.33 Making use of the enhanced electrical conductivity of the Mn-doped Fe3O4 NWs, we fabricated room-temperature gas sensors using these nanowire arrays that form a rigid and network film on the substrates. Gas sensors were also fabricated using the F3O4 NWs, and showed negligible gas sensing ability at room temperature, but only when the temperature was increased to at least 200 °C. As the Mn-doped Fe3O4 NW array, evacuated at the pressure
