J. Phys. Chem. C 2008, 112, 10659–10662
10659
Synthesis and Room Temperature Ferromagnetism in Fe Doped NiO Nanorods S. Manna, A. K. Deb, J. Jagannath,† and S. K. De* Department of Materials Science, Indian Association for the CultiVation of Science, JadaVpur, Kolkata 700 032, India ReceiVed: December 20, 2007; ReVised Manuscript ReceiVed: May 5, 2008
NiO nanorods of diameter 50-100 nm and length 400-500 nm doped with Fe have been synthesized in the presence of ethylenediamine by the hydrothermal method. The lattice constant does not change appreciably with Fe doping. Nanorods are highly oriented along the (111) direction having hexagonal facet ends. The average aspect ratio of synthesized nanorods varies between 4 and 8. Doping of Fe into NiO introduces an optical absorption band at 220 nm. Pure NiO nanorods exhibit weak ferromagnetic and antiferromagnetic like behaviors. The introduction of Fe within NiO lattice produces a significant ferromagnetic property with a coercive field of 614 Oe. I. Introduction
II. Experimental Section
Nickel oxide (NiO) is a very promising material for its wide application in electrochromic display devices, transparent conducting electrodes, and gas sensors. NiO is a strongly correlated electron system. On site Coulomb interaction plays an important role in determining the electronic properties of NiO. Magnetic properties of nanostructured transition metal oxides are very attractive compared to those of bulk solids. Bulk NiO reveals antiferromagnetic (AFM) behavior below the Neel temperature of 523 K. The magnetic properties of NiO are affected with reduction in size to the nanometer scale. The particular magnetic phase such as superparamagnetic, superantiferromagnetic, and ferromagnetic order appears depending on the size, shape and synthesis route of nanostructured NiO.1–5 Different geometrical shapes and patterns such as nanoparticles, nanorods, and core-shell structure of NiO are prepared by sol-gel,6 reverse-micell,7 and microwave irradiation2 techniques. Among the various wet chemical methods, the hydrothermal route is widely used to synthesize naonsized transition metal oxides.8,9
Pure NiO and Fe doped NiO nanorods were synthesized by a hydrothermal method. In a typical preparation, 14.54 g nickel nitrate Ni(NO3)2, 6H2O, 56.11 g KOH, and the required amount of ferric nitrate [Fe(NO3)3, 9H2O)] were dissolved in deionized water to form a 100 mL alkali solution. In this process, we found that the solution color turned from initial green to greenish yellow with addition of KOH. Then, 6 mL of above solution was mixed with 10 mL deionized water and 50 mL pure alcohol C2H5OH, followed by adding 10 mL of ethylenediamine C2H4(NH2)2. Before being transfer to a Teflon lined autoclave with an internal volume of 110 mL, the solution mixture was pretreated under an ultrasonic water bath for 45-50 min. The hydrothermal synthesis was conducted in an electric oven at 175 °C for 24 h. After the reaction was over, greenish white crystalline products were harvested by centrifugation and thoroughly washed with deionized water several times. Finally, the samples were dried at 80 °C for 24 h in atmospheric oxygen. X-ray diffraction (XRD) patterns of the samples were recorded by high-resolution X’Pert PRO Panalytical X-ray diffractometer in the range 35-100° using CuKR radiation. The morphology of nanosized NiO was characterized by scanning electron microscope (SEM), JEOL JSM6700F. Transmission electron microscope (TEM) images were taken by high resolution TEM model no. HRTEM, JEOL 2010. Room-temperature optical absorption spectra were obtained with a UV-visible spectrophotometer (Shimadzu). Doped samples were characterized by X-ray photoelectron spectroscopy (XPS). Magnetization measurements were carried out by SQUID Magnetometer, Quantum Design, MPMS XL (evercool).
Ferromagnetic semiconductors with high spin polarization and high Curie temperature are attracting intense interest for next generation spintronic devices.10 In the past few years, most of the researchers attempted to introduce ferromagnetic order into wide band gap semiconductors like TiO2, ZnO, SnO2, In2O3, and so forth by substituting magnetic transition metal (3d) ions.11–13 Most of the semiconductors exhibit a low magnetic moment at high temperature. Transition metal monoxides such as NiO, CoO, and MnO are antiferromagnetic insulators. The replacement of the cation by other 3d elements in monoxides also generates the ferromagnetic phase at high temperature. Wang et al.14 studied ferromagnetic properties of Fe doped NiO nanoparticles of size 30-60 nm. In the present work, microstructure, optical absorption spectra, and detailed temperature and field-dependent magnetization of Fe doped nanorods NiO have been investigated. * Corresponding author. Tel. 91 33 2473 3073. Fax: 91 33 2473 2805. E-mail address:
[email protected] (S. K. De). † Technical Physics and Prototype Engineering Division, Bhabha Atomic Research Centre, Mumbai-400 085 India.
III. Results and Discussion X-ray diffraction (XRD) patterns, shown in Figure 1, exhibit six (111), (200), (220), (311), (222), and (400) characteristic peaks of cubic crystalline NiO. All these peaks appear in Fe doped samples at the same angular positions. This indicates that no impurity phase exists in the substitution of Fe at Ni site. The XRD pattern is analyzed by Rietveld method. The estimated lattice constant of NiO is 4.17846 ( 2.0 × 10-5 Å, while for the highest doped, it is 4.17750 ( 1.0 × 10-5 Å. The ionic radii of Ni2+, Fe2+, and Fe3+ are 0.69 Å, 0.74 Å, and 0.64 Å,
10.1021/jp711943m CCC: $40.75 2008 American Chemical Society Published on Web 06/25/2008
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Figure 1. XRD patterns of the Ni1-xFexO [x ) 0, 0.01, 0.02] at room temperature. The calculated and observed diffraction profiles are shown at the top with the solid line and cross markers, respectively. The vertical marks in the middle show positions calculated for Bragg reflections. The lower trace is a plot of the difference between calculated and observed intensities.
Manna et al.
Figure 3. Transmission electron micrograph of pure NiO (a) and 2% Fe doped NiO (b). Energy dispersive spectrum (c) and high resolution TEM image (d) of 2% doped NiO.
Figure 2. Scanning electron micrograph of pure NiO (a) and 2% Fe doped NiO(b).
respectively. The lattice parameters do not change with the increase of Fe content due to very close ionic radii of Ni and Fe ions. The scanning electron micrographs are displayed in Figure 2. The nanorods are aligned in a particular direction with hexagonal facet ends. The mean diameters are ranging from 50 to 100 nm. The nanorods have their uniform diameters along their lengths. The average diameter and rod-like morphology as shown in Figure 2b indicate that Fe does not influence the morphologies of doped samples. TEM image of an isolated pure NiO nanorod is shown in Figure 3a. The bunch of nanorods of the Fe doped sample are presented in Figure 3b. The average diameter determined from TEM images is about 50-100 nm. Lengths of nanorods are in the range 400-500 nm. The aspect ratios, i.e., length/diameter, lie between 4 and 8. Energy dispersive spectroscopy (EDS) for the 2% Fe doped sample is exhibited in Figure 3c. The atomic composition of constituent elements in Ni1-xFexO are determined from EDS spectra. Quantitative amounts of Fe are 0.64% and 1.42%, which are slightly lower than the amounts taken in synthesis. The high resolution TEM image is shown in Figure 3d which exhibits clear lattice fringes. Homogeneous and parallel lattice fringes imply the single crystalline behavior of prepared samples. The interval between two adjacent lattice fringes is distinctly shown by arrows in Figure 3d is 2.47 Å which corresponds to the (111) plane of the NaCl structure of NiO.
Figure 4. Optical absorption spectra of pure NiO and Fe doped NiO.
The optical absorption spectra of NiO and Fe doped NiO are shown in Figure 4. Pure NiO reveals a broad absorption band centered around 334 nm (3.7 eV). The observed energy band gap is very close to the bulk value.15 First principle electronic structure calculations including on site Coulomb interaction predicts that the top of the valence band consists of the oxygen 2p band and the bottom of the conduction band is mainly derived from Ni 3d states.16 The energy band structure suggests that the abosorbtion peak arises from 2p states of oxygen to 3d states of Ni. The present experimental value of the energy band gap is comparable to the calculated band gap of 3.38 eV. The small difference in band gap may be due to quantum size effect arising from nanostructures of NiO. A new absorption peak appears at about 220 nm for Fe doped samples in addition to the absorption band of pure NiO. The structure at 220 nm becomes more prominent with the increase of Fe concentration. The substituted Fe in NiO lattice gives rise to Fe2+ and Fe3+ states of Fe ions as found by the present XPS spectrum and earlier Mossbauer experiment.17 The electronic configurations of Fe2+ and Fe3+ are d6 and d5, respectively. The ground state of d6 including spin-orbit interaction is D5 and S6 for d5. In the presence of an octahedral crystal field, D5 splits into Eg and T2g levels. In
Fe Doped NiO Nanorods
Figure 5. Fe 2p core level X-ray photoelectron spectrum of 2% Fe doped NiO.
the octahedron symmetry, the ground state of S6 is A1g. There are several excited states whose relative positions mainly depend on the strength of the crystal field as calculated by Tanabe and Sugano.18,19 The absorption peak at lower wavelength originates from Laporte and spin-allowed transitions between ground states and excited states resulting from Fe ion. The appearance of a new peak confirms that Fe is incorporated into the NiO lattice. The incorporation and the valence state of Fe in NiO are determined from X-ray photoelectron spectroscopy (XPS). The Fe 2p core level spectrum is shown in Figure 5. Spin-orbit splitted doublet 2p3/2 and 2p1/2 peaks and satellite structures appear in XPS spectrum. The peak corresponding to metallic Fe0 is not found at binding energy 706-707 eV. The binding energy of 2p3/2 is 709-710 eV and 710-711 eV for Fe2+ and Fe3+, respectively.20,21 A broad peak around 710 eV and satellite structures indicate the existence of Fe2+ and Fe3+ in Fe doped NiO. The solvent ethylenediamine (EDA) molecule, C2H4(NH2)2, reacts with metal Ni2+ ions and a complex is formed through a bidentate ligand. In this complex, each Ni2+ is surrounded by four NH2 groups. The coordinated geometrical structure of Ni2+ is in general tetrahedral, square planar, and octahedral. The planar structure has only one easy direction to coordinate with OH- ion, which yields a one-dimensional structure. This complex decomposes at high temperature (175 °C) to NiO nanorods. An intermediate ethylenediamine complex coordinated with metal Ni ions serves as a molecular template in nucleation and one-dimensional growth.22,23 The important difference with previously reported Fe doped NiO nanopartcles14 is that the present synthesis procedure provides nanostructures at low temperature. The magnetization (M) versus magnetic field (H) for pure NiO nanorods at room temperature is shown in Figure 6. At lower field, a small hysteresis loop indicates the existence of a weak ferromagnetic component which tends to saturate at H ) 700 Oe. The M-H curve at higher field shows almost linear dependence of magnetization with applied magnetic field. The doping of Fe affects M-H behavior significantly as presented in Figure 7. A distinct ferromagnetic behavior is reflected from well-defined M-H hysteresis loop for 2% doping concentration. The loop is symmetrical. Magnetization does not saturate up to 50 000 Oe, which indicates the presence of an antiferromagnetic phase. The maximum value of magnetization is 1.10 emu/g at 50 000 Oe. Coercive field for 2% iron is about 614 Oe with
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Figure 6. Magnetization (M) versus magnetic field (H) of pure NiO at room temperature.
Figure 7. Magnetization (M) versus magnetic field (H) of Fe doped NiO at room temperature.
remanence of 0.20 emu/g. The M-H curve at room temperature indicates that the Curie temperature is above room temperature. The most attractive observation is that Fe doped nanorods reveal higher magnetization in comparison with nanopartcles.14 100 Oe field cool (FC) and zero field cooled (ZFC) magnetization as a function of temperature for NiO are shown in Figure 8. NiO nanorods show a splitting between ZFC and FC data from room temperature. In the case of NiO, both FC and ZFC magnetization have the same behavior, i.e., decrease with decrease of temperature and exhibit minima at about 30 K. The magnetic moment starts to increase with lowering of temperature. Such behavior at low temperature is also found in antiferromagnetic CuO nanoparticles.24 This anomaly is generally attributed to oxygen vacancies which induce different magnetic moments other than spin sublattice of the antiferromagnetic phase.25 FC magnetization is substantially higher than ZFC value at lower temperature. The temperature dependences of FC and ZFC for Ni0.98Fe0.02O are displayed in Figure 9. The temperature variation of FC magnetization for 2% Fe doping has a quite different behavior compared to undoped NiO as shown in Figure 8. FC magnetization increases continuously with decrease of temperature. Such characteristic behavior of FC magnetization data is attributed to ferromagnetism in Fe doped material.
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Manna et al. Transition metal Fe ion has two stable valence states, Fe2+ and Fe3+. Ferromagnetism may arise from the mixed valence state of Fe due to the double exchange interaction.29 A very small amount of Fe creates both ferromagnetic and antiferromagnetic regions. The increase of Fe ion concentration enhances the double exchange interaction, which leads to higher magnetic moment. Exchange bias originating from the coexistence of ferromagnetic and antiferromagnetic regions induces anisotropy,30 which may play an important role in ferromagnetic order. Different possible reasons for the origin of ferromangnetism make it very difficult to realize the basic mechanism. IV. Conclusion
Figure 8. Field cooled (FC) and zero field cooled (ZFC) magnetization of pure NiO.
In the hydrothermal route, ethylenediamine plays an important role in purity and morphology of synthesized nanorods. The growth of NiO nanorods in the same direction of ferromagnetic order of the sublattice favors the ferromagnetic component. The incorporation of Fe into NiO induces ferromagnetism at room temperature with high coercivity. Ferromagnetic properties are significantly improved with increase of Fe concentration. The surface spins and shape anisotropy play important roles in the magnetic properties of NiO nanorods. References and Notes
Figure 9. Field cooled (FC) and zero field cooled (ZFC) magnetization of Fe doped NiO.
Magnetic properties of NiO are modified by reduction of size. It was found that the antiferromagnetic (AFM) phase of NiO becomes superparamagnetic as the particle size reaches about 100 nm.26,27 The AFM structure of NiO consists of two spin sublattices. Each sublattice exhibits ferromagnetic order along the (111) direction. The moments of adjacent sublattice are aligned in opposite direction to generate AFM structure. The uncompensated sublattice of nanorods may contribute to ferromagnetic order. The reduction of coordination number of surface spins of nanorods may modify the magnetic properties.1 Atomic-scale magnetic model calculations of NiO nanoparticles predict that the surface sites favor multisublattice spin configurations instead of two sublattices.28 Various research groups reported that NiO nanopartcles reveal a broad maximum in the ZFC curve below room temperature, which corresponds to blocking temperature.6,27 The shape anisotropy of rodlike nanostructure can affect the magnetic property. The magnetic spins prefer to align along the long axis of rods. Moreover, nanorods are oriented along the (111) direction as evidenced from TEM micrograph, i.e., in the direction of ferromagnetic order of spins. The absence of peak in ZFC data may be due to shape anisotropy.
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