Low temperature synthesis of nickel silica nanocomposites through Sol-gel route

Preparation of nickel (Ni) nanoparticles in the silica gel matrix for various concentrations has been achieved at low temperature. The presence of metallic Ni has been confirmed by x-ray diffraction and further supported by the presence of bright filed transmission electron microscope image, which shows presence of 5 to 8 nm diameter nanoparticles. The dielectric property was studied using complex impedance analysis technique. We have observed a significant enhancement of three orders in magnitude compare to bulk silica in dielectric permittivity of silica gel matrix due to incorporation of metallic Ni. Effect of annealing is also studied.


INTRODUCTION
Though Fermi demonstrated scientifically the first nanoparticle in 1857, it was only the last two decade that the uses of nanotechnology started coming up.Traditionally particles which have diameter less than 100 nm are called nanoparticle.Metallic nanoparticles and among them transition metal nanoparticles in homogeneous matrix have attracted much interest in the recent years due to the potential application as catalyst (Chao et al., 2009;Tae-Jong et al., 2003), magneto electronics (Aktsipetrov, 2002;Rosa et al., 2005), bio imaging (Günter and Andreas, 2005;Hyeon-Min et al., 2006;Hilger et al, 2005) etc. apart from memory applications (Osaci et al., 2007;Mohr et al., 2006;Simkevitz and Naguib, 2010) which are determined by the nanoparticle size, shape, size-distribution and internal structure.Therefore, synthesis methodologies that provide routes to obtain nanostructures in a predictable, controllable and reproducible way became very important aspect.There are various report of synthesis of magnetic nanoparticle in porous medium like silica (Balakrishnan et al., 2005;Nam and Christopher, 2006;Patel et al., 2010), but most of those methods are high temperature methods resulting instability of the smallest nanoparticles.One of the solutions is to prepare the nanocomposites at low temperature region.In this paper we report successful low temperature synthesis of nickel nanoparticle embedded on silica matrix by sol-gel method.A detailed study of the dielectric properties of the metal oxide composite using complex impedance analysis is included.We also discuss the structural, morphological properties and its applications.

EXPERIMENTAL DETAILS
Appropriate amount of TEOS (tetra ethyl ortho silicate) and ethyl alcohol were mixed homogeneously to form a solution and designated as solution 1.A second solution of NiCl2 was *Corresponding author.E-mail: sarmila.dutta09@gmail.com.Tel: +91-9433933081.prepared in deionized water and ethyl alcohol marked as solution 2. Further, solution 2 was added drop wise into solution 1 under constant stirring for about 30 min and kept at room temperature for gelling.In this process the volume ratio of TEOS: water: alcohol was 1: 1: 2. Appropriate amount of NiCl2 was used to prepare 10, 15 and 20 wt % Ni within the silica matrix.The dried silica gel samples were dipped into a solution of sodium hydroxide and hydrazine for a period of 24 h at 4°C and pH was maintained at 11.The diffusion of NaOH and hydrazine into the pores of gel forms complex with Ni 2+ ions.Subsequent heating for 1 h at 200°C (determined from the DSC curve, not shown here) decompose the nickel complex entrapped in the pores of silica gel to obtain the metallic Ni 0 .The samples were cooled down to room temperature followed by washing with DI water.
For the determination of the presence of Ni, X-ray diffraction data were collected using Cu Kα radiation (λ=1.54Å, in Rigaku Ultima III).Transmission electron microscopy (TEM) images along with SAED pattern were collected using ZEOL-JEM2100 operating at 200 kV.Fourier transform infrared spectra were collected using FTIR spectroscope 200 VCE).The Complex impedance of the samples was measured over the frequency range of 1 Hz and 10 5 Hz by using an LCR meter (HIOKI 3522-50 LCR Hi-Tester) at room temperature (~300 K) in vacuum (~10 −3 mbar).FTIR spectroscopy is a great tool for knowing kinds of bonds present in the sample.Figure 2  To find out the effect of annealing the prepared samples were heated at 950°C.And Figure 4 shows the XRD plot of 20% Ni-SiO 2 nanocomposite heated at 950°C, where we have found a strong peak of well crystalline Ni at 2θ = 44.5 0 .Particle size is found to be 37 nm, computed by using Scherrer relation (REF). Frther peaks at 2θ = 20.62 0 and 23.30 0 matches reasonably well with the standard value of 2θ = 20.820 and 23.33 0 for d 100 = 4.27Å and d 022 = 3.81Å planes for crystalline silica (JCPDS card no # 86-1630).Thus we may conclude that after heating amorphous SiO 2 gets crystallized and particle size of Ni is increased.

RESULTS
For determining the applications of the prepared sample and to understand the underneath physics we carry out complex impedance analysis.For this, first the resistive (real part of impedance, Z') and reactive (imaginary part of impedance, Z'') part were measured using LCR meter.Further, we have used the following relations for our calculations (Nandya et al., 2008).Where, and are the real and imaginary part of dielectric constant, (A = sample area, l = sample thickness, = permittivity of vacuum) and is the loss tangent (Banarji et al., 2007).
Figure 5 shows the variation of Z'' with Z' (Cole-Cole plot) for different Ni concentration in SiO 2 matrix.Curve shifts to the high frequency region with increase in concentration of Ni.Inset of Figure 5 shows the variation of Z´´ versus Z´ for annealed and un-annealed sample of 20% nickel where shifting of the maximum loss shifts towards the higher frequency can be observed for the annealed sample.Figure 6(a) shows the plot of as a function of    A.c. conductivity of the sample was calculated using relation and the variation of a.c.conductivity with frequency plot is shown in Figure 7.
From figure, it can be observed that the a.c.conductivity increases both with the increase of frequency and of Ni content.Inset of Figure 7 shows the variation of ac conductivity with frequency for the annealed and unannealed sample which shows increase in ac conductivity for the higher temperature.

DISCUSSION
In Figure 1(a), Ni peak is found to be very weak and diffused.This is due the very small size Ni nanoparticle, as confirmed by the HRTEM image.The increase of Ni peak intensity for higher Ni concentration is due to increase in number of Ni nanoparticles.Based on the above investigation by XRD, HRTEM, SAED and FTIR, we proposed the following mechanism of reduction of nickel salt to metallic nickel.Around 4 0 C, in presence of sodium hydroxide, hydrazine forms a complex with Ni salt as per the following reaction We observe presence and absence of 980 cm -1 peak in the sample respectively before and after heating at 200°C, which supports the formation of Ni-Hydrazine complex and subsequent thermal decomposition of the complex during the course of reduction to metallic Ni in SiO 2 matrix.This may be according to the following two reactions (Yu et al., 2003;Cushing, 2004).Enhanced crystallinity of both metallic phase and the matrix phase, observed in the annealed sample (Figure 4), decreases the scattering and thus increases the mean free path.In turn this increases relaxation time which ultimately shifts the curve for the annealed sample to lower frequency (inset of Figure 5).We observed decreases with increasing frequency (Figure 6 (a)) for all concentrations of nickel.This happens because the dipoles, formed at the metallic Ni/SiO 2 interfaces, fail to follow at the higher frequency.And the observed increase in with Ni concentration is due to increase of surface polarization with higher concentration.The behavior of can be explained in the same way.While with increase in nickel concentration electron get better conducting path resulting to increase in conductivity (Figure 7).The increased crystalinity and particle size for the annealed sample plays an important role in increasing dielectric constant (inset of 6(a), 6(b)) and conductivity (inset of 7).At low temperature crystallinity of the matrix is very poor, so the effective electric field at the Ni nanoparticle is less compare to the effective electric filed when the matrix is well crystalline (heated at 950°C).Thus the polarization at Ni-SiO 2 interface increases with increasing crystallinity of SiO 2 matrix.Hence dielectric constant increases with temperature.While better conducting path for electron due to increased crystalinity and particle size at higher temperature results increase in ac conductivity.

Conclusion
We have successfully synthesized Ni-SiO 2 nanocomposite at low temperature with crystalline nickel nanoparticles of 5 to 8 nm in diameter.We have found that due to large number of Ni-SiO 2 interfacial polarization, dielectric constant of the SiO 2 increases three order of magnitude which form the bulk dielectric constant of silica.We also observed that this dielectric constant increases with Ni content in the SiO 2 matrix.In the annealed sample nickel nanoparticle size was increased and silica turned crystalline which results in considerable increase in a.c.conductivity but the change of dielectric constant was minimal.

Figure 1 .
Figure 1.(a) XRD of washed and unwashed sample of 10 wt % Ni content SiO2 matrix.(b) XRD pattern of the prepared samples with different concentration of Ni in SiO2 gel matrix.

Figure 1
Figure 1(a) shows the XRD pattern of the washed and unwashed sample with 10 wt% Ni.While both patterns show presence of amorphous hump of silica and (111) plane of Ni at 2θ = 44.5°(JCPDS card no # 04-0850), NaCl peaks are absent in the washed sample.Absence of sodium chloride in the washed sample argued for the importance of proper washing of the samples.Figure 1(b) shows the XRD pattern with higher intensity Ni peak for washed sample containing 15 and 20% nickel in SiO 2 matrix.FTIR spectroscopy is a great tool for knowing kinds of bonds present in the sample.Figure2(a) and 2(b) represent the FTIR spectra for different concentrations of Ni before and after heating respectively.All patterns contain peaks at 456 and 1060 cm -1 (asymmetric stretching vibration of Si-O), 790 cm -1 (N-H wagging), 1630 cm -1 (stretching H-O-H bonds) and 3500 cm

Figure 2 .
Figure 1(a) shows the XRD pattern of the washed and unwashed sample with 10 wt% Ni.While both patterns show presence of amorphous hump of silica and (111) plane of Ni at 2θ = 44.5°(JCPDS card no # 04-0850), NaCl peaks are absent in the washed sample.Absence of sodium chloride in the washed sample argued for the importance of proper washing of the samples.Figure 1(b) shows the XRD pattern with higher intensity Ni peak for washed sample containing 15 and 20% nickel in SiO 2 matrix.FTIR spectroscopy is a great tool for knowing kinds of bonds present in the sample.Figure2(a) and 2(b) represent the FTIR spectra for different concentrations of Ni before and after heating respectively.All patterns contain peaks at 456 and 1060 cm -1 (asymmetric stretching vibration of Si-O), 790 cm -1 (N-H wagging), 1630 cm -1 (stretching H-O-H bonds) and 3500 cm -1 Figure 3(a) shows HRTEM image of 15% Ni content SiO 2 sample.And the selected area electron diffraction pattern (SAED) acquired from the encircled region shows that prepared Ni particles are crystalline in nature.

Figure 3 .
Figure 3. (a) HRTEM image of the fabricated nanocomposite and (b) the corresponding SAED pattern.

Figure 5 .
Figure 5. Variation of with for different nickel percentages.Inset shows variation due to annealed temperature with 20% Ni sample.

Figure 6
Figure 6(b) shows the variation of as a function of frequency with different concentration of nickel in SiO 2 matrix while the frequency dependent variation of annealed and un-annealed sample are compared in the inset of the figure.

Figure 6 .
Figure 6.(a) Variation of with frequency for different nickel percentage.Inset shows variation due to annealed temperature with 20% Ni sample.(b) Variation of with frequency with nickel concentration.Inset shows variation due to annealed temperature with 20% Ni sample.

Figure 7 .
Figure 7. Variation of conductivity with frequency for different nickel percentage.Inset shows variation due to annealed temperature with 20% Ni sample.
The shifts of Cole-Cole plot to the high frequency region with increase in concentration of Ni can be described as follows.With increasing nickel content in SiO 2 matrix, the numbers of Ni/SiO 2 interface increases and hence scattering of electrons from Ni/SiO 2 interface increases.Therefore, relaxation time (τ) gets decreased.And as the maximum loss occurs at ω 0 where ω 0 τ = 1, since τ decreases, ω 0 increases with increasing Ni content SiO 2 matrix.Thus the Z´´ verses Z´ curves shift to the high frequency region with increase in concentration of Ni.