CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 4 1666–1670
Articles Synthesis of Homogeneous NiO@SiO2 Core-shell Nanostructures and the Effect of Shell Thickness on the Magnetic Properties Sonalika Vaidya,† K. V. Ramanujachary,‡ S. E. Lofland,§ and Ashok K. Ganguli*,† Department of Chemistry, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India, and Department of Chemistry and Biochemistry and Department of Physics and Astronomy, Rowan UniVersity, 201 Mullica Hill Road, Glassboro, New Jersey 08028 ReceiVed August 11, 2008; ReVised Manuscript ReceiVed January 19, 2009
ABSTRACT: We have developed a methodology to obtain NiO@SiO2 core-shell nanostructures for the first time. These nanostructures have a NiO core of 25-20 nm diameter and silica shell of 5-10 nm. The shell thickness could be increased to 25 nm with appropriate loading of tetraethylorthosilicate. The shell structure was confirmed by transmission electron microscopy, energydispersive X-ray analysis and zeta potential studies. Electron paramagnetic resonance studies indicate the presence of free radicals on the surface of the SiO2 shell. The increase in the magnetic susceptibility with shell thickness is explained by the enhancement of the free radicals associated with the silica shell. These magnetic core-shell nanostructures have potential as catalysts and in magnetic devices. Introduction Core-shell nanostructures allow several additional possibilities compared to pure nanoparticles. These are materials that have an inner core (of nanosize) surrounded by a shell, whose thickness is also of a few nanometers. The shell can alter the reactivity of the surface and impart stability and dispersibility to the core. These materials can find a wide range of applications in medicines, pharmaceuticals, and in the protection of light and moisture sensitive materials, etc.1,2 Among various oxide shells, the synthesis of silica shell on nanoparticles has its own advantages such as easy control of the deposition, controllable porosity, and optical transparency. It is a low-cost material to tailor the surface properties while maintaining the physical integrity of the core.3 Silica-coated nanostructures are very useful for biological applications because they allow surface conjugation with amines, thiols, and carboxyl groups, which in turn can be linked to biomolecules.4,5 There have been several studies on the improvement of properties of the core nanoparticles by encapsulating with a silica shell. Shih et al. have demonstrated that stability of BaTiO3 particles can be improved,6 whereas Wang et al. were able to enhance the stability and luminescence properties of Bi2S3 nanorods by coating them with silica.7 Similar improvement in stability and water dispersibility has been shown for FePt nanoparticles
coated with silica.8 In this paper, we discuss our synthetic methodology to form silica shells on NiO nanoparticles and to study the variation of the properties of these NiO@SiO2 nanostructures with shell thickness. NiO is an antiferromagnetic material with Neel temperature (TN) of 523 K. However, there are several reports which suggest that fine particles of NiO exhibit weak ferromagnetism or superparamagnetism.9 This superparamagnetism has been attributed to incomplete compensation of spins between the antiferromagnetically coupled sublattices. Earlier, we have synthesized spherical nanoparticles of NiO (25 nm) by the decomposition of nanorods of nickel oxalate.10 These nanorods were synthesized by the reverse micellar route with CTAB as the surfactant.11 A temperature-independent spontaneous magnetization and a susceptibility large compared to that of bulk NiO, was observed in the range of 25 to 300 K, suggesting uncompensated spins and/or reordering of magnetic sublattices through finite size effects. Various reports on NiO/SiO2 nanocomposites have been reported earlier. For example, NiO/SiO2 was synthesized by Zheng et al.12 using the sol-gel method, and a bimodal pore structure was observed. A dip-coating technique was used by Torres et al.13 to form nanocomposite thin films with a molar composition of 40NiO-60SiO2. Similarly, Casula et al. reported Table 1
* Corresponding author. E-mail:
[email protected]. Tel: 91-1126591511. Fax: 91-11-26854715. † Indian Institute of Technology. ‡ Department of Chemistry and Biochemistry, Rowan University. § Department of Physics and Astronomy, Rowan University.
sample A sample B sample C
NiO: SiO2
core diameter (nm)
shell thickness (nm)
1:0.37 1:1.62 1:2.33
25-100 25-30 25
5-10 5-10 25
10.1021/cg800881p CCC: $40.75 2009 American Chemical Society Published on Web 01/29/2009
Homogeneous NiO@SiO2 Core-Shell Nanostructures
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Figure 1. PXRD pattern of (a) pure NiO synthesized by the thermal decomposition of nickel oxalate nanorods. PXRD pattern of NiO@SiO2 synthesized with (b) 1:0.37, (c) 1:1.62, and (d) 1:2.33 molar ratios. Inset shows the enlarged view of the PXRD pattern indicating the amorphous region.
Figure 2. TEM micrograph of NiO@SiO2 synthesized with (a) 1:0.37 and (b) 1:1.62 molar ratios. (c) STEM micrograph of NiO@SiO2 synthesized with a 1:2.33 molar ratio.
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Figure 3. EDX spectrum of NiO@SiO2 synthesized with a 1:0.37 molar ratio by passing the beam through (a) core and shell and (b) only shell.
the synthesis of aerogel and xerogel nanocomposite material of NiO/SiO2 using a sol-gel method.14 Several NiO/SiO2 nanocomposites have been used in reactions as catalysts.15,16 However, there appears to be no literature on NiO@SiO2 core-shell nanostructures. Here, we report the synthesis and properties of NiO@SiO2 core-shell nanostructures with controlled shell thickness. Experimental Section Synthesis of NiO was carried out by the thermal decomposition of nickel oxalate nanorods at 450 °C, which were synthesized by the reverse micellar route.10 Fifty milligrams of NiO was dispersed in a 3:2 ethanol:water mixture. To this was added 3.5 mL of ammonia solution (25% w/w). Different volumes of tetraethylorthosilicate (TEOS) were added to the above solution to vary the shell thickness. After being stirred, the product was separated by centrifugation, washed with ethanol, and then dried at room temperature. The Ni:Si ratio was estimated by atomic absorption spectroscopy. Powder X-ray diffraction (PXRD) studies were carried out on a Bruker D8 Advance diffractometer with Ni-filtered Cu KR radiation. Raw data were subjected to background correction and the KR2 lines were stripped off. Zeta potential studies of NiO nanoparticles (with
and without SiO2 shell) were performed with Zetasizer Nano ZS90 (Malvern Instruments, UK). Infrared (IR) spectroscopy studies were carried out on a Nicolet Protege 460 Fourier transform infrared (FTIR) spectrometer. The data were recorded with a KBr disk in the range of 400-4000 cm-1. Transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), and energy- dispersive X-ray analysis (EDX) studies were carried out on a FEI Technai G2 20 electron microscope operated at 200 kV. Electron paramagnetic resonance (EPR) studies were carried out on a Varian X-band spectrometer at room temperature. Magnetic studies were carried out at temperatures ranging from 5 to 300 K, in applied fields of up to 10000 Oe with a Quantum Design Physical Properties Measurement System.
Results and Discussion NiO nanoparticles were obtained from the decomposition of nickel oxalate nanorods (Figure 1a). The core-shell nanostructures obtained after adding TEOS and centrifugation also showed the presence of NiO and a broad feature for 2 theta ranging from 20-30° (Figure 1b-d). This probably indicates the presence of amorphous silica-coated on NiO particles, particularly because the relative area of this broad feature (inset, Figure 1) increased with the increase in the amount of SiO2.
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Figure 6. EPR plot of NiO@SiO2 synthesized with a 1:0.37 molar ratio.
Figure 4. IR pattern of (a) pure NiO synthesized by the thermal decomposition of nickel oxalate nanorods and NiO@SiO2 synthesized with (b) 1:0.37, (c) 1:1.62, and (d) 1:2.33 molar ratios.
Figure 7. Temperature variation studies of magnetic susceptibility of NiO and NiO@SiO2 synthesized with different molar ratios of NiO: SiO2.
Figure 5. Plot of zeta potential of NiO and NiO@SiO2 synthesized with different molar ratios of NiO:SiO2. The dotted lines are only a guide to the eye.
Figure 2a shows the TEM image of sample A with the ratio of NiO: SiO2 as 1: 0.37. Cores with size ranging from 25-100 nm with a shell thickness of 5-10 nm were observed from the TEM micrograph. Figure 2b shows the TEM micrograph of sample B having a NiO:SiO2 ratio of 1:1.62. The cores have a diameter of 25-30 nm, whereas the shells have an average thickness of 5-10 nm. Cores with 25 nm diameter and a shell thickness of 25 nm were observed in the STEM image (Figure 2c) of sample C with a 1:2.33 NiO:SiO2 ratio. Some silica particles were observed in the STEM images in addition to the core-shell nanostructures in silica-rich composites. Table 1 summarizes the result for NiO@SiO2 core-shell nanostructures. NiO@SiO2 core-shell nanostructures are present in an aggregated form as observed from TEM and STEM images in Figure
2. The presence of aggregates could be attributed to the formation of H-bond between the silica shell due to the presence of Si-OH bond over the shell surface. These Si-OH bonds were formed by the hydrolysis of TEOS in presence of ammonia and water at room temperature. The presence of Si-OH bonds was also confirmed from IR spectroscopy as shown in Figure 4 (discussed later). EDX studies carried out by passing the electron-beam through the core and the shell simultaneously show the presence of Ni, O and Si from the coated particles (Figure 3a). Only peaks corresponding to Si and O (not Ni) were observed when the EDX spectrum was obtained by passing the electron beam through the shell region (Figure 3b). This strongly suggests that the core is NiO, whereas the shell is pure SiO2. From the TEM data, one can determine the ratio of the volume of the shell to that of the core. One finds volume ratios of 1 ( 0.5, 2.5 ( 1, and 25 ( 5 for samples A, B, and C, respectively. Assuming that the NiO is fully dense (density ≈ 6700 kg/m3), it suggests that the SiO2 densities are 2000 ( 1000, 3500 ( 1500, and 500 ( 100 kg/m3 for samples A, B, and C, respectively. These values are to be compared with fully dense value of about 2200 kg/m3 for silica. Although the density of the thinner shells is consistent with dense silica, the thicker shell
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(sample C) is rather porous. In fact, the actual density of the silica of sample C is probably on the lower end when one considers that the actual volume of SiO2 is larger since silica nanoparticles, identified by TEM, were not accounted for in the calculation. IR spectra of uncoated NiO (Figure 4a) showed the presence of bands at 3430, 1626, and 413 cm-1 corresponding to O-H stretching, O-H bending, and Ni-O stretching modes, respectively. IR spectra of coated particles (Figure 4b-d) showed the presence of additional strong bands at 1089-1096, 954-960, and 799-801 cm-1 corresponding to Si-O-Si and Si-O-H stretching frequencies and Si-O-Si bending frequency, respectively. Thus IR studies corroborate well with the TEM studies and indicate that the shell formation has taken place on the NiO nanoparticles. To further confirm the presence of a silica shell on NiO nanoparticles, we carried out zeta potential studies of the uncoated and coated NiO nanoparticles in water at a pH of 7 (Figure 5). Uncoated particles show a positive zeta potential, indicating that NiO bears a positive surface charge. Earlier report17 of NiO particles also showed a positive value for the zeta potential below pH of 10.8. In contrast, the coated particles show a negative value of the zeta potential. This suggests that the NiO nanoparticles were coated with silica which is likely to have negative surface charge due to presence of hydroxyl groups on the surface of silica. To further confirm that the measured zeta potential corresponds to that of NiO@SiO2 core-shell nanostructures and not just the mixture of two components, zeta potential studies were also carried out on a mixture of NiO and silica nanoparticles (synthesized by reverse micellar route). The mixture was found to have a negative potential that was near the mean of the potentials of the pure NiO nanoparticles and the NiO@SiO2 core-shell structures. In ZnO nanoparticles coated with TiO2, it has been reported that the value of the zeta potential shifted toward that of TiO2.18 EPR studies were carried out at 9.3 GHz on sample A. No signal corresponding to Ni2+ was observed in the spectrum (Figure 6), presumably because the resonant frequency of the uncompensated spins is too high because of their connected antiferromagnetic sublattices. However, a somewhat broad line with a spectroscopic splitting factor (g) ∼2 was observed and can be attributed to free radicals due to dangling bonds, expected in the amorphous SiO2 shell. Figure 6 shows the temperature dependence of magnetic susceptibility of NiO nanoparticles and core-shell structures. The observed large susceptibility of NiO can be attributed to the presence of uncompensated spins which increase as the particle size decreases. It was earlier reported19 that the magnetic susceptibility of NiO with particle size less than 100 nm show anomalous effects and the appearance of weak ferromagnetism in nanoparticles.10,20 Here we observed that the low-temperature magnetic susceptibility per mole of NiO increased with the relative amount of silica. The free radicals identified by EPR
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should increase with amount of SiO2 and give rise to the Curielike tail at low temperatures that is indicative of noninteracting spins. Conclusions A suitable methodology for the synthesis of core-shell nanostructures of NiO@SiO2 has been developed. The core-shell structures were confirmed by TEM, IR, and zeta potential studies. The size of the core varied from 25-100 nm, and the shell thickness could be increased to 25 nm although the thicker shells were somewhat porous. Increase in the magnetic susceptibility was observed with the amount of SiO2, which was attributed to the presence of dangling bonds on the silica shell leading to free radicals as indicated by EPR studies. These core-shell nanostructures may hold potential for catalysis applications. Acknowledgment. A.K.G. thanks the NSTI, Department of Science & Technology, and CSIR, Government of India, for financial support. S.V. thanks CSIR, Government of India for a fellowship. K.V.R. appreciates the support of DST, Government of India, for the award of a CP-STIO fellowship. S.E.L. acknowledges support by the University of Maryland NSF MRSEC DMR 0520471.
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