Journal of Nanoscience

Volume 2013 (2013), Article ID 929321, 8 pages

http://dx.doi.org/10.1155/2013/929321

Research Article

Sol-Gel Synthesis and Characterization of Nanoparticles

S. Ramesh

Department of Chemistry, Saveetha School of Engineering, Saveetha University, Chennai 605105, India

Received 31 March 2013; Revised 18 July 2013; Accepted 18 July 2013

Academic Editor: Liqiang Jing

Copyright © 2013 S. Ramesh. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

The nanoparticles have been synthesized by sol-gel technique. X-ray diffraction, scanning electron microscopy, optical absorption spectroscopy, and electron paramagnetic resonance spectroscopy were used to characterize the sample. The X-ray diffraction results indicate the formation of nanocrystalline materials in tetragonal lattice with P4₂/nnm space group. The identical distribution of elements were confirmed by scanning electron microscopy with energy dispersive X-ray spectrometry and X-ray mapping. Electron paramagnetic resonance lineshapes of the samples are obtained at various (13 K, 77 K, and 300 K) temperatures. The isotropic lineshapes of the sample B1 are attributed to dipole-dipole interaction of Ti³⁺ ions. The incorporation of Al³⁺ ions into the sample B2–B5 the isotropic nature of the lineshapes are collapsed due to the distraction in crystal field. Optical absorption spectra results reveal the presence of Ag-TiO₂ nanoparticles.

1. Introduction

Titanium-based nanoparticles have been studied intensively owing to its widespread industrial applications such as cosmetics [1], ceramics [2], superconductivity [3], solar cells [4], magnetic [5], shape memory alloys [6], and photocatalysis [7]. However, the basic research and industrial development of TiO₂ containing samples are hot topics because of its extensive potential importance, and existing difficulties are still tough to solve [8, 9]. The key problems of TiO₂ containing samples show short wavelength excitation, it may crossover by the addition of noble metals such as Ag, Au, Pt. Among them, silver and gold nanoparticles are promising due to the remarkable chemical stability and a characteristic absorption peak in visible wavelength range [10–12]. The enriched photoactivity in visible light is attributed to the silver nanoparticles act an electron traps aiding electron-hole separation [13].

In favor of some specific advanced application (optics, sensor, photocatalysis, etc.), materials are appropriate to shape as films, tubes, and fibers but in most cases TiO₂ samples are required as a powder. Various wet chemical methods such as coprecipitation [14], hydrothermal [15, 16], sol-gel technology, and combustion method [17, 18], have been developed for materials synthesis. Since the sol-gel technology is widely used and it is an effective process to produce Ag-TiO₂ nanoparticles is due to its advantages of low processing cost, energy efficiency, high production rate, and rapid productivity of fine homogeneous powder [19–23]. Characterization of materials are the root of modern technology for further progress and future requirements of advance materials. The structural and chemical properties of the materials have been progressively investigated by several advanced techniques. Electron paramagnetic resonance (EPR) spectroscopy is a very powerful and sensitive technique to ascertain the electronic structures of materials associated with unpaired electrons.

The present work focus on the synthesis of (; B1–B5) nanoparticle by sol-gel process at room temperature. The aim is to determine the structure and property of the samples by X-ray diffraction, scanning electron microscopy (SEM), differential scanning colorimetry (DSC), optical absorbance spectroscopy, EPR spectroscopy, and vibrating sample magnetometer (VSM).

2. Experimental

2.1. Synthesis of () Nanoparticles

The nanoparticles were prepared by the sol-gel technology. Reagent grade chemicals were used without further purification. The calculated amounts of Al(NO₃)₃, Ag(NO₃)₂ and TiO₂ were mixed in 2 mol·L⁻¹ nitric acid by stirring for 1 h at pH~4-5, followed by addition of 30 mL of 1.5 mol L⁻¹ citric acid solution. The resulting solution turned to yellowish sol then the sol was stirred continuously using a magnetic stirrer at 60°C until it became a transparent sticky gel. At that point, the gel was dried in an air oven at 200°C for 1 h which leads the formation of light weight porous materials due to the enormous gas evolution, and it was sintered at 850°C for 4 h to get the fine homogeneous dense powder. A pictorial representation of the synthesis method is given in Figure 1.

Figure 1: Flow chart of the sample preparation of (; B1–B5) nanoparticles.

2.2. Characterization

The room temperature powder X-ray diffraction patterns were collected using a PANalytical X’Pert Pro diffractometer in Bragg-Brentano configuration. Monochromatic Cu Kα 1, 2 (~1.54060, 1.54443 Å) radiations were used as a source of 40 kV/30 mA power. Diffractograms were collected over -range 5–80 with 0.02 steps. Unit cell dimensions and initial structural refinements were carried out by Rietveld package FullProf 2003 [24]. SEM images were recorded using Hitchi-S3400 instrument operated at 20 kV, and EDX was done by a SUPER DRYER II instrument. The X-band EPR spectral measurement at 300, 77, and 13 K were done with a JEOL JES-TE 100 ESR spectrometer system with 100 kHz field modulation and a phase sensitive detector to obtain the first derivative signal. Quartz sample tubes were used for recording the EPR spectra of powder specimens. Samples were run at low temperatures using Air Products Helitran cryostat. The magnetic field was calibrated using a Varian NMR Gauss meter, and the microwave frequency was determined using an EIP frequency meter. Since a Gauss meter and frequency meter were used to record the lineshapes in these cases, a -standard is not necessary. A Varian spectrophotometer (Model Carry 5000) at the range 200–800 nm recorded the Optical Absorption spectra in the solid phase.

3. Results and Discussion

The X-ray diffraction patterns of (; B1–B5) nanoparticles are shown in Figure 2. In the XRD patterns, peaks were indexed using least square method to avoid the difference between experimental and theoretical patterns. The results of the indexed patterns are reported in Table 1. In the XRD patterns, a new weak peak is observed at 25.7° in B2–B5, and a peak at 55° is being reduced with increasing order of value. The indexed patterns were undergone for Rietveld refinement analysis to estimate the lattice parameters. The profile was fitted with a pseudo-Voigt function, and a polynomial background subtraction was chosen for the refinement. The Rietveld refinement fit is shown in Figure 3. The lattice parameters of the samples are given in Table 2. The refinement results indicate the formation of nanocrystalline materials in tetragonal lattice with P4₂/nnm space group, and is 1 (-number of atoms in the unit cell) for all the samples. The Lattice type, space group, and values are found to be constant in the samples, possibly due to very low concentration of Al³⁺ ion present in the host lattice.

Table 1: Index lattice parameters of (; B1–B5) nanoparticles.

Sample code

2TH-Obs
2TH-Calc
D-Obs

B1
2
1
1
27.477
27.480
3.2435

2
0
2
30.186
30.170
2.9583

3
0
1
36.135
36.078
2.4837

2
2
2
38.169
38.226
2.3559

2
2
2
44.376
44.360
2.0398

5
1
2
64.511
64.516
1.4433

B2
1
1
2
25.301
25.262
3.5173

2
1
1
27.428
27.445
3.2491

3
0
1
36.070
36.035
2.4880

2
2
2
38.112
38.170
2.3593

2
2
3
44.293
44.284
2.0434

5
1
2
64.432
64.430
1.4449

B3
0
0
3
25.270
25.269
3.5215

1
0
3
28.269
28.259
3.1544

2
1
3
38.077
38.110
2.3614

2
2
3
44.258
44.247
2.0449

1
1
7
64.403
64.402
1.4455

5
1
4
77.353
77.352
1.2326

B4
1
1
2
25.288
25.252
3.5191

2
1
1
27.417
27.440
3.2505

3
0
1
36.063
36.029
2.4885

2
2
2
38.096
38.160
2.3603

2
2
3
44.281
44.269
2.0439

5
1
2
64.421
64.417
1.4451

B5
2
0
2
25.341
25.355
3.5119

3
0
0
27.474
27.530
3.2438

4
0
0
36.993
36.994
2.4281

4
1
0
38.144
38.176
2.3574

2
2
4
44.331
44.329
2.0417

6
1
3
64.471
64.470
1.4441

2
0
2
25.341
25.355
3.5119

Table 2: Lattice parameters of (; B1–B5) nanoparticles.

Sample code
B1
B2
B3
B4
B5

Lattice type
Tetragonal
Tetragonal
Tetragonal
Tetragonal
Tetragonal

a (Å
7.7535
7.7618
7.1067
7.7629
9.7120

b (Å)
7.7535
7.7618
7.1067
7.7629
9.7120

c (Å)
9.1661
9.1875
10.5651
9.1925
10.1582

Unitcell volume
551.04
553.508
533.588
553.959
958.153

Space group
P42/nnm
P42/nnm
P42/nnm
P42/nnm
P42/nnm

Figure 2: The room temperature XRD pattern of (; B1–B5) nanoparticles.

Figure 3: Rietveld profile fit of (; B1–B5) nanoparticles.

The average crystallite sizes of synthesized and sintered samples were calculated from the full width at half maximum (FWHM) of the peaks (112), (211), (213) (222), (223), (413), (512), and (514) using Debye-Scherrer formula [25]:

Williamson-Hall equation is also used to calculate average crystallite size of the sample. The Williamson-Hall equation is given bellow:

where -crystallite size, -Scherrer constant (), -wavelength of Cu Kα radiation, -corrected full width half maximum (FWHM) of the diffraction peak, -Bragg angle of the X-ray diffraction peak and -lattice strain [26]. The average crystallite sizes of the samples in the range from 48 nm to 17 nm (B1–B5), and the values are reported in Table 3. The decreasing trend was observed in the crystallite size with increasing Al³⁺ ionic concentration; it may be attributed to dominating behavior of lower ionic radius of Al³⁺ (1.82 × 10⁻¹⁰ Å) ions that were replaced with higher Ti³⁺ (2.0 × 10⁻¹⁰ Å) ions.

Table 3: Density, crystallite sizes, and porosity of (; B1–B5).

Sample code
B1
B2
B3
B4
B5

-value
0
0.25
0.50
0.75
1.0

Density (g/cm³)
Experimental
2.74
4.87
5.15
2.39
4.45

Calculated
3.06
3.30
3.63
4.06
4.57

Crystallite sizes (nm)
Scherrer
44.0
18.8
18.8
18.1
18.0

W-H plot

Density of the samples were determined using liquid displacement method (Experimental) and lattice parameters (Calculated). The experimental density values are closer to the theoretical density. However, small differences in the densities are due to porous nature of the sample. The pictorial representation of the porous nature was identified by surface morphology. The crystallite sizes and density of the samples are reported in Table 3.

Surface morphology of the samples B1, B3, and B5 along with EDX profile and X-ray mappings of randomly selected particle in B5 is shown in Figures 4(a), 4(b), and 4(c), respectively. The morphology of the samples shows globular in shape. The purity was confirmed by EDX profile and quantitative results. The EDX results show the presence of Ag, Al, Ti, and O elements in the samples with expected ratio, and no other extra elements were observed; this conforms the formations of pure (; B1–B5) nanoparticles. The X-ray mapping has become an extremely powerful technique to understand the distribution of elements in the sample. The X-ray mapping (Figure 4(c)) of the sample B5 conforms the presence of all the constituent elements distributed homogenously with resemble of their relative quantity. The homogenous distribution of the elements is due to the existence of uniform particle size.

(a)

(b)

(c)

Figure 4: (a) The SEM morphology, (b) EDX- profile, and (c) X-ray mapping of () nanoparticles.

Room temperature optical absorbance spectra of the samples are shown in Figure 5. The absorption bands appeared at 320, 424, and 583 nm are assigned with the reported data. It reveals that the band at 320 nm is attributed to the Ti³⁺ ions, and 424 nm corresponds to the presence of Ag-TiO₂ nanoparticles. The same results lead to the samples which may be used for photocatalysis [27]. The band at 424 and 583 nm is also corresponding to Ti³⁺ ions. The 3d¹ electronic configuration of Ti³⁺ ionic ground state term is ²D. In the octahedral crystal field, the degeneracy is lifted; hence, the samples show broad absorption band at about 424 and 586 nm. The corresponding bands are attributed to ²B2g−²B1g (424 nm) and ²B2g−²A1g (583 nm) transitions of Ti³⁺ ions in distracted octahedral sites [28, 29].

Figure 5: The room temperature optical absorption spectra of (; B1–B5) nanoparticles.

Due to the paramagnetic nature of Ti³⁺ ions in the samples, the EPR technique has been widely employed in characterization of the samples. It is a single progress to understand the electronic and geometrical features of elements in different chemical environments. The EPR lineshapes of the (; B1–B5) nanoparticles are shown in Figures 6(a)–6(c).

(a)

(b)

(c)

Figure 6: (a) The EPR lineshapes of (; B1–B5) nanoparticles at 300 K. (b) The EPR lineshapes of (; B1–B5) nanoparticles at 77 K. (c) The EPR lineshapes of (; B1–B5) nanoparticles at 13 K.

The spin Hamiltonian parameters of the samples were calculated using the following relation [30]:

where -Planck’s constant, -resonance frequency, -Bohr magnetron and -field strength at which the resonance occurs. Ti³⁺ ion has five isotopes: ⁴⁶Ti, ⁴⁷Ti, ⁴⁸Ti, ⁴⁹Ti, and ⁵⁰Ti within this three isotopes, three are EPR inactive (⁴⁶Ti, ⁴⁸Ti, and ⁵⁰Ti) to zero nuclear spin . The isotopes ⁴⁷Ti and ⁴⁹Ti are EPR active due to nonzero nuclear spin. The EPR lineshapes of the sample B1 shows an isotropic nature with ~1.991 is due to dipole- dipole interaction of Ti ions [31]. However, the isotropic exchange interaction usually dominates the effects of spin-orbit coupling, and magnetic dipole interaction. The electron spin resonance of Ti³⁺ (d¹) ions has been detected in only a few cases.

Generally, Ti³⁺ (d¹) ions excited states are closer to the ground state in octahedral and tetrahedral symmetry in this case EPR lineshapes can only be detected at liquid-helium temperatures due to a very short spin-lattice relaxation time [24, 32]. Consequently, the sample B1 shows broad unresolved hyperfine line at most temperatures. Once Al³⁺ ions are incorporated into the sample B2–B5, the crystal field is distracted, as the effect or the spin-lattice relaxation time becomes longer. It results the possibility of hyperfine splitting at higher temperature [33, 34] as in the case of B2-B3. The hyperfine splitting of the sample B2–B5 shows six parallel and perpendicular lines with ~2.0233 and ~2.0233, which are not well-resolved lineshapes due to the small distraction. The obtained spin Hamiltonian parameters of the samples at 13 K, 77 K, and 300 K were reported in Table 4. The EPR spectra of the samples are reasonable for ⁴⁷Ti isotopes. A single-broad line centered at is observed, and the additional lines were observed on both sides of the central line, which are not six as expected. The collapse of the extra lines may be caused by dipolar interaction between Ti³⁺ (d¹) ions.

Table 4: The spin Hamiltonian parameters of (; B1–B5) nanoparticles.

Sample code

-matrixes and -matrixes

300 K
77 K
13 K

(G)
(G)

(G)
(G)

B1
− 2.002
− 1.992
—
—
− 1.994
—
—

B2
2.046
2.022
—
—
—
—
—
—
—
—

B3
2.277

2.087
2.030
168
20
2.075
2.019
169
21

B4
2.048
2.024
—
—
—
—
—

—
—

B5
2.047
2.018
2.077
2.016
167
20
2.086
2.019
162
21

4. Conclusion

The nanoparticles of () were successfully prepared by sol-gel method. The XRD results of the samples were conformed the formation of the tetragonal lattice with P4₂/nnm space group. The morphology of the samples shows globular in shape, and the constituent elements were distributed homogenously. The optical absorbance spectra conforms the formation of Ag-TiO₂ nanoparticles and the samples. The Ag-TiO₂ nanoparticles can be used for photocatalytic activity. The EPR result shows that the hyperfine splitting is due to the ⁴⁷Ti isotopes of Ti³⁺ ions. The samples show paramagnetic behavior in room temperature.

Acknowledgments

The author would like to thank the Central Instrumentation Facility of Pondicherry University, Pondicherry, India. The author is also very grateful to Professor Mark J. Nilges, Director and Lab Manager, Illinois EPR Research Centre, Urbana, for his help to get the low temperature EPR spectra.

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