Nearly Monodispersed Multifunctional NiCo2O4 Spinel Nanoparticles

Importantly, these nanoparticles show a high (∼83%) infrared transparency that is useful for specific solar and fuel cell electrode applications as ...
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15106

J. Phys. Chem. C 2008, 112, 15106–15112

ARTICLES Nearly Monodispersed Multifunctional NiCo2O4 Spinel Nanoparticles: Magnetism, Infrared Transparency, and Radiofrequency Absorption Seema Verma,† Hrushikesh M. Joshi,† Tushar Jagadale,† Amit Chawla,‡ Ramesh Chandra,‡ and Satishchandra Ogale*,† Physical and Materials Chemistry DiVision, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411 008, India and Institute Instrumentation Center and Center for Nanotechnology, Indian Institute of Technology, Roorkee 247 667, India ReceiVed: February 1, 2008; ReVised Manuscript ReceiVed: July 28, 2008

We report low temperature synthesis of nearly monodispersed NiCo2O4 nanoparticles by a combustion method utilizing glycine as a fuel and nitrate as oxidizer. An appropriate glycine-to-metal nitrate molar ratio favors the formation of nearly monodispersed NiCo2O4 nanoparticles. We discuss the relevant synthesis chemistry and their detailed characterization using different techniques such as X-ray diffraction, high resolution transmission electron microscopy, superconducting quantum interference device magnetometry, and Fourier transform infrared spectroscopy. We also show the interesting evolution of the phase and magnetic properties of such nanoparticles upon annealing treatment. Importantly, these nanoparticles show a high (∼83%) infrared transparency that is useful for specific solar and fuel cell electrode applications as well as significant radiofrequency (RF) absorption causing substantial heating of their aqueous dispersion that should have potential applications for magnetic hyperthermia. 1. Introduction Transparent conducting oxides have attracted tremendous interest recently because of their importance in many potential applications such as electrodes in solar cells,1 in molten carbonate fuel cells,2 or as heterogeneous optical recording media.3 Most of the transparent conducting oxides under development are n-type semiconducting materials. Unfortunately, high conductivity limits their transmissivity in the infrared (IR) region. Therefore, there is an increased demand for conducting oxide materials exhibiting optical transparency at the longer wavelengths. Toward this end, a preferred transition metal oxide spinel compound is NiCo2O4, which can provide adequate transmissivity from visible wavelength out to the 12 µm IR region. Other research interests in this binary oxide system have been pursued in the past to develop specific applications such as electrocatalyst for anodic oxygen evolution,4 inorganic and organic electrosynthesis,5 development of a supercapacitor,6 or infrared transparent conducting electrodes for flat panel displays, sensors, or optical limiters and switches.7-9 Interestingly, this material also possess interesting magnetic properties10 that can be exploited in applications such as ferrofluid technology, magnetic carriers for site-specific drug delivery and local hyperthermia, colloidal mediators for heat generator, contrast enhancement agents for magnetic resonance imaging, etc.11-13 In such contexts it is clearly important to study the nanoparticles of this system for their superparamagnetic behavior.14,15 * Corresponding author. E-mail: [email protected]; phone: 91-20-2590 2260; fax: 91-20-2590 2636. † National Chemical Laboratory. ‡ Indian Institute of Technology.

NiCo2O4 adopts a pure spinel structure in which all the Ni ions occupy the octahedral sites and the Co ions are distributed among the tetrahedral and octahedral sites.16,17 Despite numerous studies on structural, electrical, and magnetic properties of NiCo2O4;17-22 the detailed description of the distribution of the cations remains a matter of controversy. This is primarily due to the instability of NiCo2O4 in air at a temperature exceeding 673 K, which imposes the need for low temperature synthesis. Therefore, a number of attempts have been made to lower the formation temperature for this functional compound by utilizing different low-temperature synthetic routes. Some of the wet chemical synthetic routes such as coprecipitation,23,24 sol-gel,25 and thermal decomposition of the precursors such as hydroxide nitrates25,26 and hydrazine carboxylate hydrates27 have been reported for the synthesis of nanoparticles of NiCo2O4. Earlier, an attempt was also made to synthesize highly porous nanostructured aggregates of the compound by employing heterometallic alkoxide precursor in the presence of a supramolecular liquid.28 However, in spite of being interesting, all of these synthesis approaches have not been able to render the desired level of control over the phase purity and monodispersity of the formed nanoparticles. Combustion synthesis is an important processing technique that involves the exothermic reaction of an oxidizer such as metal nitrate, ammonium nitrate, or ammonium perchlorate and an organic fuel such as urea, glycine, or carbohydrazide.29-34 Parameters such as the type of fuel, fuel-to-oxidizer ratio, ignition temperature, and the water content of the precursor greatly influence the mechanism of the combustion reaction.29,35,36 Chemical energy released from the exothermic reaction can be varied with the change in the reaction conditions, thereby changing the local heat of the system associated with the

10.1021/jp804923t CCC: $40.75  2008 American Chemical Society Published on Web 09/10/2008

Monodispersed Multifunctional NiCo2O4 Spinel NPs corresponding combustion reactions. It is known that for the complete combustion reaction to take place a stoichiometric molar ratio of the metal nitrate and the fuel is required. The stoichiometry of a combustion mixture is expressed in terms of the elemental stoichiometric coefficient, Φe.37 The mixture is stoichiometric when Φe ) 1, fuel-lean when Φe > 1 and fuelrich when Φe < 1. It is known that the glycine nitrate process (GNP) of combustion presents some advantages. It is “rapid and self-sustaining” and occurs at a low temperature of 473 K. In GNP, the amino acid, glycine, serves both as a fuel for combustion, being oxidized by the nitrate ions, and as a complexant to prevent inhomogeneous precipitation of the individual component prior to combustion. However, for stoichiometric (Φe ) 1) and fuel-rich (Φe < 1) mixtures, the GNP method is well-known to give rise to submicron size particles. As reported earlier, it is expected that for fuel-lean (Φe > 1) mixtures the heat evolved is not enough, and it therefore leads to near-smoldering combustion behavior.33,38-40 This can allow a systematic variation of the particle size based on control of the metal nitrate to glycine molar ratio.41 In this work we adopt this strategy of controlled combustion behavior to synthesize NiCo2O4 nanoparticles. It is expected that an appropriate metal nitrate to glycine molar ratio with an optimum complexant and controlled combustion nature of glycine may offer a suitable flame temperature to obtain stable NiCo2O4 nanoparticles. Moreover, the controlled combustion reaction aids the multipoint rapid decomposition of the complex with the simultaneous evolution of a large amount of gases. Also, due to desegregation and heat dissipation, growth of the particles is not significant. Therefore, low decomposition temperature and simultaneous evolution of a large amount of gases results in a high degree of nucleation and slow growth rate of the particles favoring the formation of nearly monodispersed NiCo2O4 nanoparticles. We demonstrate this in our work reported here. In the present investigation we report the synthesis of nearly monodispersed NiCo2O4 nanoparticles by a combustion method utilizing glycine as a fuel. For an appropriate molar ratio of metal ions (Ni + Co) and glycine, homogeneous nanoparticles of NiCo2O4 are formed at low temperature. We report the characteristics of such multifunctional NiCo2O4 nanoparticles, including their application-worthy properties of high IR transparency and significant radiofrequency (RF) absorption, the latter having potential for application to magnetic hyperthermia. 2. Experimental Section NiCo2O4 was synthesized from AR grade chemicals by the following steps. Cobalt nitrate, nickel nitrate, and glycine were used as raw materials. Our previous studies have shown that by varying the metal-to-glycine molar ratio during the synthesis controls the particle size.41 Therefore, aqueous solutions of metal nitrates and glycine of appropriate molarity (1:0.5) were mixed. The resultant homogeneous solutions were slowly evaporated on a water bath to form a viscous gel. The gels were allowed to undergo rapid combustion reaction in a preheated furnace at 473 K. The sample was kept at the same temperature for 4 h. The as-prepared sample was named NC-473. Some samples were further annealed at 573 and 773 K and were named as per the heat treatment temperatures as NC-573 and NC-773, respectively. The samples were characterized for their phase purity and crystallinity by powder X-ray diffraction measurements (Panalytical Xpert Pro) with Cu KR radiation using Ni filter. Infrared spectra of as-prepared and annealed powders were recorded in the 400-4000 cm-1 range (Perkin-Elmer system, spectrum One B) by preparing KBr (Merck, spectroscopy grade)

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Figure 1. XRD patterns of NiCo2O4 powders (a) NC-473, (b) NC573, and (c) NC-773 and (s) simulated pattern with a ) 8.11 Å; an asterisk indicates the NiO phase.

pellets (0.1 and 0.25 wt % sample). Particle sizes were investigated by transmission electron microscope (TEM), model JEOL 1200 EX, on a carbon coated TEM copper grid after dispersing the powder in amyl acetate. HRTEM analyses were done on model FEI Technai 30 system operated at 300 kV. The fringes were generated by subtracting the noise of the original selected area with a Digital Micrograph (TM) 3.71 for GMS 1.2 Build 22. HRTEM samples were prepared by drop coating of NiCo2O4 nanoparticles dispersed in amyl acetate. Magnetic measurements were carried out using a Quantum Design MPMS system. The measurements were made between 5 and 300 K using zero-field-cooling (ZFC) and field-cooling (FC) protocols at 50 Oe, and the hysteresis loops were obtained in a magnetic field varied from +4 to -4 T. A RF absorption study was carried out with 20 MHz oscillator at a fixed nominal power of 100 W. 3. Results and Discussions Figure 1 shows the powder X-ray diffraction patterns of NiCo2O4 particles obtained at different temperatures. The XRD patterns of the NC-473 (synthesized at 473 K) and NC-573 (annealed at 573 K) samples show the formation of a homogeneous spinel phase, which corroborates well with the simulated pattern for NiCo2O4 (a ) 8.11 Å, PCPDF No. 20-0781). For the NC-773 sample (annealed at 773 K), additional reflections are observed in the XRD pattern, and these are due to the formation of NiO nanoparticles, due to the partial decomposition of spinel NiCo2O4. The sample begins to decompose above 673 K to form NiO as an impurity phase (∼15%, nominally based upon the relative intensities). The lattice parameter values for the NC-473 and NC-573 samples are calculated as 8.13 and 8.12 Å, respectively, and are slightly larger than the value reported earlier (8.11 Å).16,17 The difference is found to be on the order of 0.25 and 0.12%, respectively, and the effect can be attributed to the nanocrystalline nature of the particles formed, as reported earlier for several cases.42-44 The calculated lattice parameter value for the NC-773 sample is 8.10 Å, which shows a small shift of 0.12% from that of stoichiometric NiCo2O4 particles (8.11 Å). This implies that formation of the NiO phase may be accompanied by the formation of cobalt-rich off-stoichiometric NiCo2O4 nanoparticles, resulting into a lattice distortion from the ideal spinel structure. Other measurements are performed to further elucidate these aspects. The average crystallite size was calculated using the Scherrer equation,45 d ) (0.9λ/β cosθ), where d is the diameter in Ångstroms, β is the half-maximum line width, and λ is the

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Figure 2. (A) TEM image of NC-473 NiCo2O4 nanoparticles; inset: corresponding SAED pattern. (B) HRTEM image; inset: the lattice fringes of the square-marked nanoparticle.

wavelength of X-rays. The average crystallite size for NC-473 was found to be 7 ((1) nm. Powder XRD studies indicate that the average crystallite size of the as-prepared sample is increased after heat treatment at 573 and 773 K. Upon annealing the asprepared sample, the crystallite size grows to 8 ((1) nm and 19 ((1) nm for the NC-573 and NC-773 samples, respectively, following the diffusion mechanism.46 Figure 2A shows the TEM micrograph of the as-prepared NC-473 sample. The formation of nearly monodispersed faceted crystalline nanoparticles is clearly seen. The mean particle size is 12 ((1) nm, which is somewhat larger than that of the crystallite size of 7 ((1) nm, as estimated from XRD line width analysis. The selected area diffraction pattern (SAED), as shown in Figure 2A (inset) corresponds to that of the spinel phase, agreeing well with the results obtained from powder XRD analysis. The diffused spotty SAED pattern for the NC-473 sample indicates the nanocrystalline nature of these particles. Figure 2B shows the HRTEM micrograph with the inset showing lattice fringes of the selected area of nanoparticles marked in the same figure. The inset of Figure 2B clearly shows nanoparticles with lattice fringes with interfringe distance measured to be around 0.244 nm, which matches well with the {311} d-spacing of NiCo2O4. To see the changes in morphology and nanoparticles size upon annealing, further TEM and HRTEM studies were performed on the NC-573 and NC-773 samples. Figure 3, panels A and B, shows TEM micrographs of these particles. These clearly show the formation of nearly monodispersed particles with mean particle sizes of 13 ((1) nm and 28 ((1) nm, respectively, for the NC-573 and NC-773 samples, which are larger than that of

Verma et al. the crystallite sizes estimated from XRD data. This may be attributed to the presence of a structurally disordered thin shell on the surface of nanocrystalline particles that does not contribute to XRD intensity. The SAED pattern as shown in the insets to Figure 3, panels A and B, bring out the formation of the spinel phase. The diffused SAED pattern of NC-573 indicates the nanocrystalline nature of the particles. On the other hand, the more spotty SAED pattern of NC-773 clearly indicates particle size growth when each particle or single crystallite exists as a monolith, which diffract coherently. Panels C and D of Figure 3 show the HRTEM micrographs for the annealed NC573 and NC-773 NiCo2O4 samples, respectively. Insets to Figure 3, panels C and D, show the corresponding lattice fringes with the interfringe distances measured to be 0.467 and 0.242 nm for the annealed NC-573 and NC-773 samples, respectively. These are close to the lattice spacing of the {111} planes at 0.468 nm and {311} planes at 0.244 nm. However, an observed deviation of 0.2 and 0.8%, respectively, can be associated with lattice disorder in the off-stoichiometric Co-rich NiCo2O4 nanoparticles. Figure 4 represent the field-dependent magnetic behavior of the as-prepared and annealed samples, measured at room temperature. For the case of as-prepared NiCo2O4 nanoparticles (NC-473), the magnetization does not saturate, even for the applied magnetic field of 20 kOe, and no hysteresis (or finite coercive field) is observed (see top left inset marked A). The M-H characteristics of the as prepared sample (NC-473) are typical of superparamagnetic behavior associated with small magnetic particles,15 as expected for NC-473 with a very small particle size (7 ( 1 nm). For the annealed samples NC-573 and NC-773, although no magnetic saturation is observed, thin hysteresis loops with coercive fields of 17 and 31 Oe, respectively, (see inset marked B) are observed, indicating wide particle size distribution. It is possible that in the annealed NC573 sample a larger fraction of small particles and a smaller fraction of large particles that are not superparamagnetic are present. On the other hand, in the annealed NC-773 sample with slightly higher coercive field, the presence of a larger fraction of big particles can be inferred, as expected. The estimates of the room-temperature saturation magnetization (Ms) value for these samples are obtained by the extrapolation of M versus 1/H curves to the limit 1/H f 0. The room temperature saturation magnetization value obtained for the as prepared NiCo2O4 particles (NC-473) is ∼2.24 emu/g. An increase in the saturation magnetization value to ∼3.79 emu/g is obtained for the annealed NC-573 sample, implying growth of the particles. Interestingly, for the annealed NC-773 sample with much bigger particles and a high coercive field, a decrease in the magnetization value to ∼2.85 emu/g is obtained. This can be attributed to the presence of an antiferromagnetic NiO phase in the NC-773 sample. Lowering of magnetic order parameters (MOP) in these nanoparticles may originate from the incomplete compensation of spins between antiferromagnetic sublattices in fine NiO nanoparticles, forming magnetic dead layer on the surface of each of the particles. The sizes of the magnetic particles are determined using the Langevin function. In the superparamagnetic region, the magnetization of the nanoparticles can be described by the Langevin function,

M ) Ms[coth(mH ⁄ kT) - kT ⁄ mH] where M is the specific magnetization at field H, Ms is the saturation magnetization, m is the magnetic moment per particle at temperature T, H is the field applied, and k is the Boltzmann

Monodispersed Multifunctional NiCo2O4 Spinel NPs

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Figure 3. Panels A and B: TEM images of NC-573 and NC-773 NiCo2O4 nanoparticles, respectively; insets, corresponding SAED patterns. Panels C and D: HRTEM images of NC-573 and NC-773 NiCo2O4 nanoparticles; insets, the lattice fringes of the square-marked nanoparticle.

Figure 4. Field-dependent magnetic behavior of (a) NC-473, (b) NC573, and (c) NC-773 at 300 K. Insets: corresponding magnetization data at an enhanced scale.

constant. For H f 0, the initial magnetization is mainly determined by the larger particles, whereas for H f ∞, the magnetization mainly corresponds to the smaller particles. The magnetic particle size of the superparamagnetic particles can be calculated from the initial slope and the slope at high magnetic fields. The detailed calculation for the other superparamagnetic magnetic oxides is discussed elsewhere.41 From the fit of the Langevin function for an assembly of superparamagnetic NC-473 particles, the magnetic particle size distribution is in the range 7 ( 2 nm. This corroborates well with that of the average XRD crystallite size (7 ( 1 nm) but not with the TEM-determined size. This implies that the structural disorder in the surface layer also causes spin disorder, leading to a magnetically dead layer on the nanoparticles surface. This is likely to be associated with the surface effects arising from the broken exchange interactions and reduced coordination in tiny particles. These types of the particles are reported to be associated with core-shell morphology where magnetic order

Figure 5. FC and ZFC magnetization of the NiCo2O4 samples (a) NC473, (b) NC-573, and (c) NC-773, measured at 50 Oe.

parameter (MOP) originates primarily from the core of the particles and the non-collinear spin structure is predominantly present on the surface of each particles.47 ZFC and FC magnetization data for the as-prepared and annealed NiCo2O4 nanoparticles, measured at 50 Oe, are shown in Figure 5. The sample was cooled from room temperature to 5 K, without any external magnetic field (ZFC), and the magnetization was recorded while warming the sample in an applied field of 50 Oe. As expected, the magnetization is seen to initially increase upon warming and then to decrease after reaching a maximum value. The temperature at which the maximum is obtained is called the blocking temperature (TB).

15110 J. Phys. Chem. C, Vol. 112, No. 39, 2008 When the same sample was cooled under a magnetic field (FC) of 50 Oe, the magnetization behavior differs significantly from that of ZFC measurements, and the two together reveal information about the magnetic state of the sample. The maximum in the ZFC magnetization for the as-prepared NiCo2O4 sample (NC-473), measured at 50 Oe, is at 132 K. The bifurcation of FC and ZFC magnetization starts above TB and grows below it. It is due to the existence and distribution of the energy barriers of the magnetic anisotropy and the slow relaxation of particles below the blocking temperature.48 Interestingly, the annealed NC-773 sample having a bigger particle size of 19 ( 1 nm and a room temperature coercive field of 31 Oe shows a lower blocking temperature of ∼92 K, which is ∼40 K lower than that of the as-prepared (NC-473) sample having a particle size of 7 ( 1 nm. The cause for this needs to be sought in the presence of precipitated antiferromagnetic NiO (which is not a major phase in the sample) on one hand and the concurrent formation of off-stoichiometric cobalt-rich NiCo2O4 nanoparticles on the other. In the latter phase, excess cobalt lattice location disorder and related lattice distortion can result in the lowering of effective magnetocrystalline anisotropy, which in turn can cause lowering of TB. It may be noted that the bifurcation between ZFC and FC curves occurs much above TB due to the existence of the energy barriers of the magnetic anisotropy and the slow relaxation of particles below the blocking temperature. These data together suggest the absence of spin-glass type behavior in this system. On the other hand, the NC-573 sample of particle size 8 ( 1 nm shows interesting intermediate magnetic behavior. In this case, in the ZFC measurement done upon warming, magnetization first increases continuously above 5K, reaches a maximum at TB1 ≈ 77 K, and then decreases slowly to form a broad maxima at TB2 ≈ 160 K, implying that the annealed sample (NC-573) is comprised of mixed magnetic phases of different anisotropy energy barriers. Below TB1, the nature of the initial ZFC and FC curves indicates the presence of interparticle interactions, which can manifest through exchange or dipole-dipole interactions. This interaction can be either ferromagnetic or antiferromagnetic depending upon the orientation of each dipole leading to the frustration. This frustrated dipolar system that is predominantly present on the surface of each of the particles may behave as a spin glass. The spin glass behavior may be induced by a developing NiO type coordination on the surface that has not undergone full precipitation, as in the case of NC773. This may lead to the presence of a much larger proportion of uncompensated surface spins on the ferrimagnetic core of slightly cobalt-rich NiCo2O4 lattice. The frustration in a dipolar system tends to increase the orientation disorder of the magnetic moment, therefore thermal agitation would be enhanced with further increase in the temperature. However, this behavior is a consequence of the competition between thermal agitation of disordered magnetic moment present on the surface of the particles and the ordinary blocking behavior induced by anisotropy of the core of the particles. Therefore, above TB1 with further increase in the temperature, the magnetization decreases very slowly to form a plateau untill the temperature TB2 is reached, above which it gradually decreases due to enhanced thermal agitation. Opening up of the magnetic hysteresis loop is also expected below the blocking temperature or the temperature below which the irreversibility between the FC and ZFC magnetization is observed. This behavior is clearly seen from the field-dependent magnetization behavior shown in Figure 6. The as-prepared NiCo2O4 sample (NC-473) clearly shows the opening of the

Verma et al.

Figure 6. Field-dependent magnetic behavior of (a) NC- 473, (b) NC573, and (c) NC-773 at 5 K. Insets: corresponding magnetization data on an enhanced scale.

loop having a high coercive field of 1128 Oe. Contrary to this, very thin hysteresis loops with a coercive field of 23 and 62 Oe are obtained for the annealed samples NC-573 and NC-773 respectively. However, asymmetric hysteresis loop with a kink at low field (see insets A and B of Figure 6) may also represent the presence of two magnetic phases (NiO and Co-rich NiCo2O4). The substantial decrease in Hc values for the annealed samples is consistent with the loss of magnetocrystalline anisotropy, as discussed earlier based on lowering of TB. Following the relationship, 15 KeffV ) 25kBTB, the observed value of TB ) 132 K, and assuming that the particles are nearly spherical, the effective magnetocrystalline anisotropy constant (Keff) for the as-prepared NiCo2O4 particle is obtained as 17.0 × 105 erg/cm3. Interestingly, this is even higher than that reported for CoFe2O4 nanoparticles (∼12.0 × 105 erg/cm3).49 The larger anisotropy of nanoparticles than the corresponding bulk values is explained in terms of the additional anisotropy contributions from dipolar interactions, surface anisotropy, etc.50,51 Although increased magnetic dipolar interaction has been invoked as a source of additional anisotropy,52,41 the nature of the FC magnetization below TB, which gradually increases in our sample, indicates the presence of non-interacting particles. Therefore, the observed increment in the anisotropy should be mainly due to the surface effects. The NiCo2O4 nanoparticles synthesized in this work were further studied for their application-worthy properties of high infrared transparency and significant radiofrequency absorption, which should be explored for biomedical applications. Nickel cobalt oxide is a recently studied polaron conducting material that shows promise as an infrared transparent conducting oxide. Figure 7 shows the FTIR spectra of the NC-473 sample taken in different amounts, 0.25 and 0.1 wt % of the sample in KBr (curves a and c) and are compared with sample NC-773 taken as 0.1 wt % of sample in KBr (curve b). The background corrected infrared transparency values for the NC-473 sample are 83% and 59% at 732 cm-1 (13.7 µm) when taken in 0.1 and 0.25 wt % of KBr. This result is quite good, and the obtained percent transmittance is comparable to those reported earlier for NiCo2O4 films on Si substrate.7 Sharp bands at 562 and 645 cm-1 indicate the presence of corresponding metal-oxygen vibrations as reported earlier.53 Other features in the IR spectra of NC-473 are a very strong band at 1380 cm-1, strong shoulders at 1625 and 1700 cm-1, and a very broad band in the 3000-3700 cm-1 region. This broad band and that around 1625 cm-1 could be attributed to the O-H stretching and bending modes of physisorbed H2O molecules. Intensity

Monodispersed Multifunctional NiCo2O4 Spinel NPs

Figure 7. FTIR spectrum of NiCo2O4 samples taken in different amount in KBr pellet for (a) NC-473 (0.25 wt %), (b) NC-773 (0.1 wt %), and (c) NC- 473 (0.1 wt %).

of these absorptions decreases for the annealed sample (NC773), indicating that the surface of the smaller particles (NC473) is much more sensitive toward the adsorption of water molecules. A shoulder at 1700 cm-1 could be attributed to the presence of free carboxylate ions, indicating that the complexation of the cations may be through zwitterions leaving a -COOH group free for the autocatalytic combustion reaction to occur with nitrates. A sharp, strong band at 1383 cm-1 and a weak band at 838 cm-1 may be attributed to the presence of ionic nitrates as ν3 NO3- and ν2 NO3-, although it was earlier reported to be associated with the presence of physically surfaceadsorbed CO2.33 As expected, the residual NO3- disappears after annealing the as-prepared sample at 773 K, as seen from the FTIR spectrum of the sample NC-773 (curve b). We now show that these NiCo2O4 nanoparticles can also absorb radiofrequency (RF) radiation causing heat generation, a property of great interest to hyperthermia applications. Solutions of dispersed NiCo2O4 particles of different quantities in deionized water (1-3 mg/mL) were subjected to RF radiation (generator: 20 MHz, 100 W, capacitatively coupled into a small 5 mL glass bottle) for different concentrations and times, and the temperature rise was monitored. The temperature increase was compared to that for pure deionized distilled water. It may be noted that, generally speaking, RF radiation in the frequency range of 100-500 kHz is used for hyperthermia. Here we used 20 MHz because the corresponding generator was available to us, just to demonstrate the electromagnetic (EM) radiation absorption property of our nanoparticles. The data are presented in Figure 8. A significant temperature rise can be seen for the samples loaded with NiCo2O4 nanoparticles as a function of exposure time as compared to pure deionized water. A systematic increase in the heat generation with increase in the quantity of the dispersed nanoparticles for a fixed duration of 20 s (inset of Figure 8) further confirms efficient EM absorption by the nanoparticles. Interestingly, the temperature rise is not as significant for the annealed NC-573 sample (see inset of Figure 8). Given the significant differences between the magnetic characteristics of NC-473 and NC-573, it is tempting to suggest a magnetic origin for the EM absorption effects. However, much further work is clearly needed to identify and elucidate the heating mechanisms, which is beyond the scope of this paper. Also, more detailed studies at lower frequencies, as well as studies on toxicity, will be needed to establish the applicability of this material to magnetic hyperthermia.54

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Figure 8. Temperature rise due to RF absorption of NiCo2O4 nanoparticles (NC-473) measured for different time durations of exposure to RF irradiation. Inset: temperature rise due to RF absorption for NC-473 and NC-573 at different concentrations of NiCo2O4 nanoparticles after 20 s of RF irradiation.

4. Conclusions Nearly monodispersed NiCo2O4 nanoparticles are synthesized at low temperature by a combustion method utilizing glycine as a fuel and nitrate as oxidizer under near-smoldering combustion behavior. These nanoparticles are subjected to detailed characterizations using different techniques that reveal their multifunctionality. We show the interesting evolution of the phase and magnetic properties of such nanoparticles upon annealing treatment. Specifically, the samples annealed at 573 K show an increase in the magnetization, whereas those annealed at 773 K exhibit a decrease in magnetization due to the precipitation of fine NiO nanoparticles resulting in incomplete compensation of spins predominantly on the surface of the nanoparticles. Also, the sample annealed at 773 K shows a lower blocking temperature than that of the as-prepared sample, which can be attributed to the formation of an off-stoichiometric cobalt-rich NiCo2O4 phase causing lattice distortion and substantial loss of magnetocrystalline anisotropy. The as-synthesized nanoparticles also show high infrared transparency and significant heating by RF absorption. Acknowledgment. S.V. is grateful to Department of Science and Technology (DST), India, for a research grant. S.B.O. would like to thank BRNS (DAE, Government of India) for a research grant under CRP in spintronics and the Department of Science and Technology (DST) for the award of Ramanujan Fellowship and research grant. Thanks are also due to Dr. S. D. Dhole for RF measurements. References and Notes (1) Park, S.; Keszler, D. A.; Valencia, M. M.; Hoffman, R. L.; Bender, J. P.; Wager, J. F. Appl. Phys. Lett. 2002, 80, 4393. (2) Kuk, S. T.; Song, Y. S.; Suh, S.; Kim, J. Y.; Kim, K. J. Mater. Chem. 2001, 11, 630. (3) Iida, A.; Nishikawa, R. Jpn. J. Appl. Phys. Part 1, 1994, 33, 3952. (4) Haenen, J.; Visscher, W.; Barendrecht, E. J. Electroanal. Chem. 1986, 208, 297. (5) Roginskaya, Y. E.; Morozova, O. V.; Lubnin, E. N.; Ulitina, Y. E.; Lopukhova, G. V.; Trasatti, S. Langmuir 1997, 13, 4621. (6) Hu, C.-C.; Cheng, C.-Y. Electrochem. Solid-State Lett. 2002, 5, 43. (7) Windisch, C. F., Jr.; Exarhos, G. J.; Ferris, K. F.; Engelhard, M. H.; Stewart, D. C. Thin Solid Films 2001, 398, 45. (8) Windisch, C. F., Jr.; Exarhos, G. J.; Sharma, S. K. J. Appl. Phys. 2002, 92, 5572. (9) Windisch, C. F., Jr.; Ferris, K. F.; Exarches; G. J., J. Vac. Sci. Technol. A 2001, 19, 1647.

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