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Novel Ferrimagnetic Iron Oxide Nanopowders Kishori Deshpande,† Mikael Nersesyan,† Alexander Mukasyan,† and Arvind Varma*,§ Department of Chemical and Biomolecular Engineering, Center for Molecularly Engineered Materials, University of Notre Dame, Notre Dame, Indiana 46556, and School of Chemical Engineering, Purdue University, West Lafayette, Indiana 47907-2100
Because of their scientific and technological utility, magnetic properties of fine-particle systems have assumed great importance in recent years. Maghemite (γ-Fe2O3) and magnetite (Fe3O4) are commonly used in magnetic inks, as catalysts, and as ferrofluids for biomedical uses. In the current work, we report a novel one-step process for the synthesis of different iron oxide phases, including maghemite and magnetite, using the aqueous combustion synthesis technique. The method involves a self-sustained reaction between an oxidizer (e.g., metal nitrate) and a fuel (e.g., glycine or hydrazine). Using this approach, for the first time, spherical, nanoscale (6-10 nm) iron oxide particles with excellent ferrimagnetic properties were synthesized. While the samples have particle sizes of 1 (φ < 1) implies fuelrich (fuel-lean) conditions. Additional experiments were also conducted using complex fuels, i.e., mixtures of glycine and hydrazine, and different iron-containing precursors such as ferrous oxalate (C2H2O4‚Fe) with and without inert oxidizer, i.e., ammonium nitrate [NH4(NO3)]. Finally, separate experiments were conducted in an inert (argon) atmosphere to establish reaction mechanisms. The synthesis was carried out in a chemical reactor made of quartz (see Figure 1a), which permits experiments in different ambient atmospheres (i.e., air, oxy-
10.1021/ie049056i CCC: $30.25 © 2005 American Chemical Society Published on Web 04/14/2005
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Figure 2. Typical XRD patterns of synthesized γ-Fe2O3 and Fe3O4 powders.
Figure 1. Details of aqueous CS: (a) schematic diagram of the chemical reactor;8 (b) typical temperature-time profile of solution combustion.18
gen, argon), measurements of the reaction temperaturetime history, and process monitoring by using a digital camera (Panasonic digital camcorder model PV-DV103). The temperature was measured by type K thermocouples (127 µm; Omega Engineering Inc.) attached to a multichannel acquisition system (INET-200 controller card, Omega Engineering Inc.), with rates from 5 to 60 samplings/s. The reactants were dissolved in sufficient water and mixed thoroughly. The reaction mixture was then placed on a hot plate (Cole Parmer model 4803-00) and preheated to the water boiling point at ∼5 °C/min. The reaction can be divided into different stages depending on the characteristic temperature gradient (dT/dt), as seen in Figure 1b. For example, stage II was a constanttemperature and relatively long (∼5 min) step, during which all free and partially bound water evaporated. The next preheating stage (stage III) was characterized by a higher rate (∼12 °C/min) than stage I and ended with either a sudden (at some ignition temperature, Tig) uniform temperature rise to a maximum value, Tm (volume combustion synthesis or VCS mode) or reaction initiated in a specific hot spot followed by steady wave propagation along the mixture (self-propagating high-
Figure 3. FTIR patterns of synthesized iron oxide powders.
temperature synthesis or SHS mode). In both cases, i.e., VCS and SHS modes, the rate of medium-temperature change, dT/dt, is high and in the range 10-104 °C/s (stage IV, Figure 1b) and the duration of the hightemperature region varies from ∼10 (for the SHS mode) to 100 s (VCS mode). After cooling (stage V), the synthesized products are typically fine solid powders. Further details about the synthesis procedure and the reaction mechanisms can be found elsewhere.8 The obtained products were analyzed for phase composition and crystallinity using a X1 Advanced diffraction system (Scintag Inc.) and Fourier transform infrared (FTIR) spectroscopy (Thermo Mattson, Satellite series, model 960M0027). The powder microstructure was studied by field emission scanning electron microscopy (SEM; Hitachi model S-4500), and the specific surface area was measured from Brunauer-EmmettTeller analysis (BET; Autosorb 1C, Quantachrome Instruments). Finally, the magnetic properties of the samples at room temperature were examined using a magnetometer (LDJ model 7000A BH meter). Using this approach, different phases of iron oxide such as γ-Fe2O3 and Fe3O4 with specific surface areas up to 200 m2/g were obtained in a single step.
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Table 1. Magnetic Properties for γ-Fe2O3 Measured at Room Temperature magnetic field H (Oe)
maximum magnetic intensity Bm (G)
residual magnetization Br (G)
coercive field Hc
specific magnetization (emu/g)
remenance (emu/g)
500 1000 3000
217 297 366
87 117 128
156 184 191
34 46.5 57.3
13.6 18.3 20
Table 2. Magnetic Properties for Fe3O4 Measured at Room Temperature magnetic field H (Oe)
maximum magnetic intensity Bm (G)
residual magnetization Br (G)
coercive field Hc
specific magnetization (emu/g)
remenance (emu/g)
500 1000 3000
68 102 137
17 29 32
161 203 213
14.9 22.4 30.1
6.4 7
Results and Discussion As noted earlier, different phases of iron oxide including γ-Fe2O3 and Fe3O4, were synthesized. The crystallinity of samples was confirmed using X-ray diffraction (XRD; see Figure 2). The nanosize of the synthesized powders was evident from the observed peak broadening for the spectra. However, because the patterns for γ-Fe2O3 and Fe3O4 are similar, the phase composition was confirmed by FTIR spectra (see Figure 3). The SEM pictures of these powders reveal that the particles have similar morphology and size in the range 6-10 nm, as seen from Figure 4. These values are on the same order of magnitude as those calculated using XRD peak broadening and the Scherrer formula (∼20 nm). The various magnetic properties of γ-Fe2O3 and Fe3O4, at room temperature, are shown in Tables 1 and 2. The following unique features may be outlined. While the samples have particle sizes of 1000 °C) in the combustion wave. Further, as described elsewhere,7,8 other unique features of the synthesis (i.e., intensive gas evolution and short process duration) permit one to simultaneously obtain nanoscale powders despite the high temperature. The same synthesis factors also lead to the formation of nanosized Fe3O4 particles, with relatively high specific magnetization (see Table 2). The latter property is due to the intrinsic structure of Fe3O4, which includes both Fe2O3 and FeO lattices.1 The simultaneous high coercivity is likely related to the nanoscale powder size.12 To explain the phenomenon accurately, however, more work needs to be done and is currently under progress. Some data related to the ferrimagnetic properties of fine (
