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Study on morphological, structural and dielectric properties of sol-gel derived TiO2 nanocrystals annealed at different temperatures
Journal Pre-proof Study on morphological, structural and dielectric properties of sol-gel derived TiO2 nanocrystals annealed at different temperatures
Neetu Rathore, Asita Kulshreshtha, Rajesh Kumar Shukla, Darshan Sharma PII:
S0921-4526(19)30847-6
DOI:
https://doi.org/10.1016/j.physb.2019.411969
Reference:
PHYSB 411969
To appear in:
Physica B: Physics of Condensed Matter
Received Date:
17 July 2019
Accepted Date:
25 December 2019
Please cite this article as: Neetu Rathore, Asita Kulshreshtha, Rajesh Kumar Shukla, Darshan Sharma, Study on morphological, structural and dielectric properties of sol-gel derived TiO2 nanocrystals annealed at different temperatures, Physica B: Physics of Condensed Matter (2019), https://doi.org/10.1016/j.physb.2019.411969
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Study on morphological, structural and dielectric properties of sol-gel derived TiO2 nanocrystals annealed at different temperatures Neetu Rathore1, 2, Asita Kulshreshtha1, Rajesh Kumar Shukla2, and Darshan Sharma1, 3 1Amity
School of Applied Sciences, Amity University U P, Lucknow Campus, Lucknow (India)
2Department 3Department
of Physics, University of Lucknow, Lucknow 226025 Uttar Pradesh (India) of Physics, Harish Chandra Post Graduate College, Maidagin, Varanasi 221001 Uttar
Pradesh (India) Email: [email protected]
Abstract: In the present work, TiO2 nanoparticles have been synthesized using the sol-gel method and further being annealed at different temperatures to study the influence of temperature on structural, morphological and dielectric properties of TiO2. Owing to its peculiar physical properties, it has found various applications and been an interactive material for research in the field of semiconductor physics. To study crystallographic nature of prepared TiO2 nano-particles x-ray diffraction study was done. The result shows that on increasing the annealing temperature, the structure of TiO2 nanoparticles changes from anatase phase to the mixed anatase-rutile phase. The spectroscopic study done using FTIR reveals tetragonal arrangement of Ti and O ions in the TiO2 nanoparticles. The morphological study of samples was done by SEM technique. The SEM images show that the particle size of TiO2 nanocrystals increased as a result of increasing annealing temperature. In addition, EDS study confirmed that no impurities were present in the samples. Dielectrics properties were measured at different frequencies ranging from 20Hz to 5MHz at room temperature. It is found that the TiO2 nanoparticles possess a higher value of dielectric constant and lower dielectric loss at 1 kHz. The novelty in this work is that the modified preparative conditions of TiO2 nanoparticles yield better and optimized properties related to the crystal structure, morphology, and dielectric behavior for application in optoelectronic devices, super-capacitors, spintronics, and other dielectric applications. Keywords: Nanostructured materials; sol-gel methods; sintering; crystal growth; crystal structure; dielectric studies.
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1.
INTRODUCTION Currently, among all the inorganic oxides, TiO2 is a well-known semiconductor due to its distinguished optoelectronic and electrical properties for application in sensors, photocatalysts, solar cells, and memory devices. TiO2 has a high dielectric constant, high refractive index, the good optical transparency of visible light and high chemical stability [1–5]. Because of its high dielectric constant and high breakdown strength, TiO2 serves as a good insulator and had been found suitable for wide application in the field of electronic [3–5]. It is highly useful for the fabrication of capacitors used in microelectronics devices, electronic memories and in optical filters [6–9]. The low loss factors and high dielectric permittivity of TiO2 over an ample frequency range are always of immense attention [7,10–12]. TiO2 is a member of the transition metal oxides family[2,13]. It exists in three different phases namely rutile, brookite and anatase. The TiO2 in anatase phase is kinetically more stable at lower temperatures whereas rutile phase is stable at higher temperatures. Both anatase and rutile phase possess have tetragonal crystal structure [4,13–15]. Anatase phase has a high refractive index of 2.54 and rutile is 2.75 respectively at 550 nm wavelength [16]. It is found that anatase has a lower dielectric constant than the rutile phase of TiO2 [10]. Rare earth ions (Dy3+ and Sm3+) doped TiO2 nanoparticles have been studied and found superior catalyst, dielectric properties than pristine TiO2 [17,18]. Annealing enhances physical attachment and electronic contact between the particles. The annealing parameter depends on temperature, annealing time and reaction atmosphere[1,13]. According to reported articles, Ostwald ripening is at high temperature leading to more nucleation of the nano-clusters and more growth. Larger particles grow at the expense of smaller particles [4]. Heat treatment in the synthesis process affects morphology, porosity and crystalline nature of the particles, resulting in a decrease in surface area. Annealing of the samples reduces the surface hydroxyl groups. The annealing procedure affects the oxygen vacancies and induces phase change and escalates crystalline and structural order which normally enhances the dielectric constant [2]. Many researchers investigated the phase change of TiO2 nano-particles acquired by the sol-gel method with respect to various annealing temperatures [1,4,15,19]. The electrical properties of TiO2 nanoparticles are governed by few factors such as small size, surface-to-volume ratio being high, defects in grains, quantum confinement of charge carriers. The sol-gel method is very simple to handle. It is low cost giving good homogeneity and effective compositional control. The low-temperature method controls the product’s chemical composition. TiO2 is prepared in the form of single crystal, powder, glass, film, pellet, tube, and nanowires, etc. In this work, we present the synthesis of TiO2 nanoparticles from the hydrolysis of organometallic-precursor titanium isopropoxide via condensation. Here we report the synthesis process of TiO2 nanoparticles and study of their morphological, structural, and dielectric response at different annealing temperatures. The novelty in 2
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the present work is that the modified preparative conditions of TiO2 nanoparticles yield better and tailored results that are useful for its applications in the field of photovoltaic, photocatalyst and dielectrics.
2. EXPERIMENTAL METHODS 2.1 Synthesis of TiO2 nanoparticles The present investigation is to study the impact of annealing temperature on the morphological, structural, and dielectric properties of Titanium dioxide nanoparticles. Nanoparticles of TiO2 were prepared in the laboratory using analytical grade chemicals. Sol-gel technique is used to synthesize TiO2 nanoparticles. The organic chemicals such as Titanium isopropoxide C12H28O4Ti (TTIP) were used as the main precursor, iso-propanol as a solvent and nitric acid (HNO3). In a typical synthesis process, a solution of isopropanol was prepared in distilled water and then TTIP was dissolved drop by drop in this solution. Meanwhile, the solution was constantly stirred by a magnetic stirrer at 80°C for 1 hr. After 1 hr, a homogenized solution of distilled water and concentrated nitric acid was mixed into the previously prepared solution (Iso-propanol, deionized water, and TTIP) stirred constantly heated at 60°C for 5-6 hrs. The final solution was transformed into a viscous gel. This gel was then dried by heating at 300°C for 2 hrs to allow the water and subsequently organic constituents to evaporate. After drying at 300°C white color dried product was obtained which was then grounded using agate mortar for 2-3 hrs to get nanocrystalline powder. To prepare samples for different annealing temperature the powder was annealed at 300°C, 400°, 500°C, 600°C for 2hr at a heating rate of 5°C/min in a muffle furnace to get crystallization process of TiO2 nanoparticles. All the samples annealed at different temperatures were allowed for slow cooling at room temperature. Prepared TiO2 nanoparticles sample by the procedure described above were termed as Sample A, Sample B, Sample C, Sample D for temperature 300° C, 400°C, 500°C, 600°C respectively.
2.2 Characterization X-ray diffraction profiles analysis pattern for all the samples was recorded using X-ray Diffractometer Model- Ultima IV model ( Rigaku, Japan) with CuKα radiation for λ=0.15406 nm. The diffraction data obtained for 2θ angle in the scanning range of 20 to 70°, step width-0.02° and x-ray operation 40 KV, 40 mA. Fourier transmission infrared spectra (FTIR) of TiO2 nanoparticles were recorded in the region 4000 to 500 cm-1 with the help of Model-Nicolet TM 6700, Thermo scientific USA. For the morphological studies samples were scanned using scanning electron microscope (Model- JSM 6490LV, Joel, Japan). To find out material composition EDS Model-133 from OXFORD instruments (UK)-EV Dry Detector, 3
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INCAx - act, was used. Dielectric measurements of pellets at room temperature in a frequency range 20Hz to 5 MHz and at a voltage of 1V at room temperature were performed using an LCR meter (ModelWayne Kerr 6500 P high-speed LCR meter). TiO2 powder heated at different annealing temperatures (300°C-600°C) and it was compressed by hydraulic pressure machine at a pressure of 3 tons to make pellets for dielectric measurement. Each sample measured 13mm in diameter having thickness of 1-2 mm. BAKER-Type Jo2, 7301 screw gauge was used to measure physical dimension with least count of 0.01mm. To ensure good electrical contact these pellets were kept between two circular brass discs and the whole assembly was placed for measurement between the electrodes of the LCR meter at a signal voltage of 1.0 volt. No coating is done on any of the pellets.
3. RESULT AND DISCUSSION 3.1 XRD X-ray diffraction pattern of TiO2 peaks shows related to two phases which are anatase and rutile mixed phase. X-ray diffraction pattern of prepared samples from 300°C to 600°C temperatures are shown in figure 1(a). The Bragg reflections observed at 25.25°(101), 37.74°(200), 48.04°(200), 54.3°(022), 62.7°(250) belong to anatase phase of
TiO2 and those at 27.39°(110), 36.08°(111), 41.20°(220),
44.0°(210), 56.58°(231), 64.05°(330),68.9°(301) belong to rutile phase[12]. An additional peak at 30° (121) related to brookite phase is observed in samples A, B, and C. All peaks observed in this spectra were matched well with existing data [JCPDS data no. 00-049-1433, 00-053-0619 and 00-048-128] and literature[20]. In figure 1(b) the diffraction pattern show peaks at (101) and (110) related to anatase and rutile phase for all annealing temperature. During phase transformation at annealed temperature 500°C and 600°C in the rutile phase, Ti-O bond breaking and lattice distortion at higher temperatures take place. This brings in the replacement of oxygen and production of defects together with the elaboration of innovative Ti-O bonds [21,22]. With increase of temperature the intensity of the reflection (110) corresponds to the rutile phase is increased [23]. Sharma et.al. observed the same peaks [20]. The experimental data show no additional peaks other than that for the TiO2 material. This indicates that there are no impurities present in the prepared nano-material within the sensitivity limit of XRD.
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` (a)
(b)
Figure 1. (a) X-Ray Diffraction patterns images of TiO2 nanoparticles annealed from 300°C to 600°C temperature, (b) Variation of anatase phase (101) peak and rutile phase (110) peak intensities and Bragg angle shifting with the annealed temperature at 300°C to 600°C. 5
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(a)
(b)
. (c)
(d)
Figure 2. Variation of (a) Lattice parameter (b) size (c) d (101)-value (d) c/a ratio and unit cell volume for TiO2 nanoparticles prepared at 300°C to 600°C annealing temperature. Figure 1 (a) shows the annealing temperature effect on most intense peaks of anatase (101) and rutile (110). The temperature increases at 300°C to 600°C. In figure 1 (b) Full-width half maxima (FWHM) and intensities of peaks related to both phases are calculated from the XRD data. FWHM of anatase (101) was found to decrease from 1.26˚ to 0.320˚ and for rutile (110) from 0.323˚ to 0.201˚ respectively with the increase in annealing temperatures for Sample A, B, C, and D. The increase in peak intensity at higher temperature is attributed to the increase in crystallite size. As shown in figure 2 (a) there is variation in crystallite size as the temperature changes from 300°C to 600°C. As is seen in figure 2(a), crystallite size first increases gradually for 400°C then increases sharply beyond 400°C. The increase in
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crystallite size with increasing annealing temperature may be attributed due to thermally promoted crystal growth. The annealing of TiO2 at different temperatures affects the lattice structure of TiO2 which is the main cause of transformation from amorphous to crystalline nature of prepared nanomaterial. As per the Bragg’s law, the value of Bragg angle 2θ should increase or decrease if a change in the d-spacing of the crystallographic planes. The peak shifting towards lower 2θ is associated with the tensile stress and is responsible for increase in d spacing values and higher 2θ values are attributed to d spacing due to compressive stress[24]. Figure 2(b) shows the variation of d spacing values. The lattice strain is an estimation of the circulation of lattice constants rising from crystal imperfections, like lattice dislocation. The grain growth correlated with the more and more convex nature of grain boundary and the removal of oxygen ions. The interplanar spacing (d) and lattice parameter (a, c) for the prepared samples were calculated using following Bragg’s equation for the tetragonal system [25,26], 1 𝑑2ℎ𝑘𝑙
=
(
)
ℎ2 + 𝑘2 𝑎2
𝑙2
+ 𝑐2
h, k, l stand for Miller’s indices for the crystallographic plane of TiO2 nanocrystals. The lattice constants were measured as of the lattice spacing of (101) and (200) anatase peaks. Variation of a and c is shown in fig. 2(c). The unit cell volume for annealed samples was calculated using the following equation [26]. 𝑉 = 𝑎2𝑐 Figure 2(d) shows that the ratio of c/a parameter and unit cell volume of nanocrystals of anatase (101) and (200) peaks. The volume of the unit cell and c/a ratio parameter both increase with the increase in annealing temperature. The crystallite size (Ds) of the prepared samples was calculated using the Debye-Scherrer formula [26]. 0.9 𝜆
𝐷𝑠 = 𝛽 𝑐𝑜𝑠𝜃 Where 0.9 is constant for spherical symmetry, λ is the wavelength (1.5406 Å), β is the full width at half maximum (FWHM) of the peaks, θ is the Bragg’s angle. Average crystallite size D of prepared TiO2 powder and lattice strain ε is calculated by the WilliamsonHall uniform deformation model (WH-UDM) plot, by the following equation [25]; 𝛽ℎ𝑘𝑙𝑐𝑜𝑠𝜃 = 4𝜀 𝑠𝑖𝑛𝜃 +
𝐾𝜆 𝐷𝑠
A graph is plotted between βhklcosθ on the y-axis and 4sinθ on the x-axis. The slope of the straight line obtained from linear fit data gives the value of lattice strain ε from slope and crystallite size Ds was measured from y-intercept. 7
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The parameters Ds and ε have also been calculated from the “size strain plot” by the following equation[25].
(
𝑑ℎ𝑘𝑙 𝛽ℎ𝑘𝑙 𝑐𝑜𝑠𝜃 2
)
𝜆
2 𝐾𝜆 𝑑ℎ𝑘𝑙 𝛽ℎ𝑘𝑙 𝑐𝑜𝑠𝜃
= 𝐷𝑠
(
2
𝜆
)+( )
𝜀 2 2
Lattice strain ε was estimated from intercept and crystallite size Ds from the slope of the straight-line plot of the above equation. The crystallite size and value of strain of samples for different annealing temperatures are provided in Table 1. The positive value of strain from W-H plot and size strain plot is attributed to lattice expansion for the sample[6]. For size strain plot, the sample-B was measured with negative strain value which is attributed to compressive strain and this strain may be the cause of peak broadening[6,27,28]. Table 1: Crystallite size and average lattice strain for TiO2 nanoparticles annealed at various annealing temperatures Sample A, B, C, and D.
Crystallite size (nm) of TiO2 Sample
Strain of TiO2
Scherrer
W-H
size-strain
formula
plot
plot
Sample A-300°C
6.67
6.69
5.64
60×10-4
2.879×10-4
Sample B -400°C
9.18
10.36
5.21
85×10-4
-6.955×10-6
Sample C -500°C
13.1
8.44
12.95
18×10-4
4.888×10-4
Sample D -600°C
26.6
27.78
20.75
6.288×10-4
2.2504×10-6
W-H plot
Size-strain plot
The Bragg peak is influenced by lattice strain and crystallite size in different ways and increases the intensity, shift and peak width of the 2θ peak position consequently. For the period of thermal energy relaxations, confident deformations are created depending on the temperature gradient in various directions which is the main reason for either decrease and increase in strain value. On another hand, the crystallite size is inversely correlated with strain. Hence deviation in crystalline size will change the suitable deformations. The value of tensile strain decreases for the samples annealed temperature of heat treatment from 300 to 600°C. Afterward, the reduction in the tensile strain was not much significant (Table.1). This is possibly caused by the structural relaxation of the anatase form of crystallite to the equilibrium value successively at raised temperatures.
3.2 SEM and EDS
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Surface morphology explored through SEM reveals that the size of the particle in addition to the porosity of the crystalline particle also increases from 300°C to 600°C. All samples have spherically shaped particles which are uniformly distributed. Ostwald ripening at high temperatures leads to more nucleation of the nano-clusters and more growth. In these larger particles grows at the expense of smaller particles. Figure 3(a), (b), (c), (d) represents SEM images of TiO2 nanoparticles. Its magnified images in figure 4(a),(b),(c),(d) describe crystallinity of TiO2.
Figure 3. SEM images of prepared TiO2 nanoparticles at different annealed temperature (300°C to 600°C) (a) Sample A (b) Sample B (c) Sample C and (d) Sample D.
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Figure 4. Magnified SEM images of prepared TiO2 nanoparticles at different annealing temperatures (a) Sample A (b) Sample B (c) Sample C, (d) Sample D.
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Figure 5. EDS pattern of TiO2 nanoparticles for (a) Sample A, (b) Sample B, (c) Sample C, (d) Sample D.
EDS technique is useful for investigation of the composition of the material. EDS spectra of TiO2 nanoparticles at temperatures between 300°C to 600°C are recorded and shown in figure 5. The results obtained indicate the high purity of synthesized material where Ti and O are present only. No impurities were present.
3.3 FTIR Spectra The FTIR spectrum of TiO2 samples was recorded using KBr pellets of samples in the wavelength range from 4000 to 500 cm-1 at room temperature. No absorption peak of KBr becomes visible in this region because the absorption coefficient of KBr is far less than 400 cm-1 only a complete spectrum of the sample is observed.
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Figure 6: FTIR spectra image of TiO2 samples annealed at temperatures (300°C, 400°C, 500°C,600°C) for Samples A, B, C, and D.
Figure 6 shows sharpness of the absorption peak decreases for samples at higher annealing temperatures. The shifting of peaks related to OH stretching towards a higher wavelength side for the increasing annealing temperature is observed in the spectra. The sharpness of the peak decreases may be attributed to C=N stretching. Table2. FTIR spectrum of TiO2 nanoparticles at 300°C, 400°C, 500°, 600°C. Absorption peaks (cm-1)
Functional groups
Sample A
Sample B
Sample C
Sample D
3379.3
3410
3421.2
3462.2
Alcohol (H bonded) - OH Stretch
1619.4
1618.1
1621.9
1623.8
C=N Stretch
1525
1526.6
1523.9
1515.4
C=C Stretch
1384.1
---
----
---
Symm. CO2, NO3 and Ring Stretching
621
764
764
764
Ti-O-Ti Stretching
The data related to FTIR spectra tabulated in Table 2 show the value of peaks at different temperatures and assigned to various modes. The Transmittance band around 3400 cm -1 caused by adsorption of water molecules on TiO2 surface[13,29,30]. The peak at 1623.8 cm-1 related to Ti-OH bond stretching and its 12
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deformation vibration [21,30]. The peak at 300°C in the wavelength range of 1384.1 cm -1 corresponds to the bending mode of nitrate because nitric acid was used in the synthesis method as a catalyst [31]. According to the previous study, this peak also shows the presence of isopropoxide in the structure related to CH3 symmetric deformation[21,32]. CO2 symmetric vibration band arises due to C and O bonding for molecules present in air [25,33,34]. The peak at 2919.6 cm -1 belongs to N-Alkanes-Asym CH2 indicating the presence of a residual organic compound at 400°C[7,35]. The peaks around 621cm 764cm
-1
-1
at 300°C and
at 400°C, 500°C,600°C are assigned to the characteristic vibrations of Ti-O-Ti stretching of
titanium dioxide[31].
3.4 Dielectric properties Dielectric properties of TiO2 nanoparticles were measured for Sample B, C and D. Dielectric constant is known as the capability or efficiency of a material to accumulate electrical energy. Dielectric loss is known as dissipation of energy in the appearance of heat transfer inside the dielectric material and this type of lose arises when internal friction spread over the dipoles. We have measured the deviation of dielectric loss and dielectric constant in the frequency range from 20Hz to 5MHz at room temperature. Figure 7 represents the dielectric constant and dielectric loss of different samples. In the low-frequency region, the values of dielectric constants, as well as losses, are high but it decreases rapidly as the frequency increases and it becomes approximately constant in the high-frequency region with increasing temperature [36].
(a)
(b)
Figure 7 (a) Represents Dielectric constant and (b) dielectric losses of TiO2 nanoparticles for Sample B, Sample C, Sample D at room temperature. Figure 7 (a) indicates the higher value of the dielectric constant of samples B, C, D at low-frequency region and decreasing very speedily at low-frequency region and becoming almost constant by a further increase in frequency as reported earlier [7,22]. As stated by Koop's theory the standard dielectric response shows the high value of dielectric constant, at the low-frequency region, is showing that 13
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dielectric constant decreasing with increasing frequency and almost constant(relaxation behavior) at highfrequency region [7,22,36]. The total polarization appears in the nanocrystals namely ionic, electronic, orientation and space charge polarization [7,11,37–40]. The deduction of dielectric constant values at a low-frequency region is associated with space charge polarization [2,12]. The space charge polarization and dipolar polarization are strongly depended on temperature and affect the high value of dielectric constant [39]. As the annealing temperature increases, grain size becomes larger but due to this density of grain boundaries decreases and relaxation of grain boundary appears to be suppressed or merges with a grain [4]. The grain boundary contributions are reduced and the relaxation behavior in the low frequency is also suppressed. Due to temperature increment, the interfacial polarization at lower frequency also increases which is the region of more charge accumulation and charge transfer on the grain boundaries [7,39]. Some dominant factors to influence dielectric properties are similar to the composition of the material, frequency of the applied field, temperature and density. The dielectric losses of samples B, C, D are also shown in figure 7(b). The mechanism of dielectric loss is the main reason of total defects in grain, impurities, space charge fabrication in edge layers jointly generates absorption current. Li et.al. also reported that increment in dielectric loss value is the cause of particle size reduction [7]. It can be seen for TiO2 nanoparticles from a very low-frequency region, the reduction in dielectric loss and the dielectric constant is possibly a part of the nanomaterial purity cause. Dielectric behavior and conductivity associated with each other [8]. During this hopping of electron is the main reason for conductivity. The larger value of dielectric loss happens when applied electric field frequency is equivalent to hopping frequency. The dielectric constants at 1 kHz frequency at room temperature have been measured as 584.3, 176.9 and 218.9 for Sample B, C, D respectively. The value of the lower dielectric loss at 1 kHz frequency is obtained are 0.10, 0.01 and 0.024 at room temperature intended for Sample B, C, and D respectively. The dielectric properties of annealed TiO2 nanoparticles are appreciably affected by different temperatures [11]. Sample D exhibits a low dielectric loss and high dielectric constant value which is optimized dielectric properties for colossal permittivity. A high dielectric constant 218.9 and low dielectric loss 0.024 together at 1 kHz is correlated to colossal permittivity applications for the sample annealed at high temperature [7,11].
4. CONCLUSION The nanoparticles of Titanium dioxide were successfully synthesized via sol-gel method at different annealing temperatures (300°C-600°C). XRD analysis confirmed that no impurity was found in TiO2 14
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nanoparticles. SEM study reveals an increase in crystallite size with increasing annealing temperature. EDS confirmed that no impurities were present. At the low frequency, value of dielectric loss and dielectric constant decreases rapidly and becomes nearly constant at high frequency. Dielectric properties of TiO2 results revealed that sample D has a quality dielectric implementation having higher dielectric constant value 218.95 and lower dielectric loss of 0.024. Because high dielectric constant and low loss value it may find its use as in the area of high energy storage applications and miniaturization electronic devices.
ACKNOWLEDGMENT The author is thankful to supervisors for their encouragement and guidance given during this study. Special thanks to condensed matter and Material research lab, Department of physics, Lucknow University, Lucknow for allowing to use their laboratory facilities to accomplish this experimental work and Central Instrument Facility for X.R.D analysis. All authors listed have equally contributed to this research work and preparation of the manuscript.
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Journal Pre-proof Author Contribution Statement All authors listed have equally contributed to this research work and preparation of the manuscript.
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We confirm that this work is original and has not been published elsewhere. It is also not under consideration for publication elsewhere. All authors listed have contributed sufficiently to the project to be included as authors. To the best of our knowledge, no conflict of interest, financial or other, exists. All authors have seen and approved the manuscript for submission.
Neetu Rathore, Asita Kulshreshtha, Rajesh Kumar Shukla, Darshan Sharma PII:
S0921-4526(19)30847-6
DOI:
https://doi.org/10.1016/j.physb.2019.411969
Reference:
PHYSB 411969
To appear in:
Physica B: Physics of Condensed Matter
Received Date:
17 July 2019
Accepted Date:
25 December 2019
Please cite this article as: Neetu Rathore, Asita Kulshreshtha, Rajesh Kumar Shukla, Darshan Sharma, Study on morphological, structural and dielectric properties of sol-gel derived TiO2 nanocrystals annealed at different temperatures, Physica B: Physics of Condensed Matter (2019), https://doi.org/10.1016/j.physb.2019.411969
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Study on morphological, structural and dielectric properties of sol-gel derived TiO2 nanocrystals annealed at different temperatures Neetu Rathore1, 2, Asita Kulshreshtha1, Rajesh Kumar Shukla2, and Darshan Sharma1, 3 1Amity
School of Applied Sciences, Amity University U P, Lucknow Campus, Lucknow (India)
2Department 3Department
of Physics, University of Lucknow, Lucknow 226025 Uttar Pradesh (India) of Physics, Harish Chandra Post Graduate College, Maidagin, Varanasi 221001 Uttar
Pradesh (India) Email: [email protected]
Abstract: In the present work, TiO2 nanoparticles have been synthesized using the sol-gel method and further being annealed at different temperatures to study the influence of temperature on structural, morphological and dielectric properties of TiO2. Owing to its peculiar physical properties, it has found various applications and been an interactive material for research in the field of semiconductor physics. To study crystallographic nature of prepared TiO2 nano-particles x-ray diffraction study was done. The result shows that on increasing the annealing temperature, the structure of TiO2 nanoparticles changes from anatase phase to the mixed anatase-rutile phase. The spectroscopic study done using FTIR reveals tetragonal arrangement of Ti and O ions in the TiO2 nanoparticles. The morphological study of samples was done by SEM technique. The SEM images show that the particle size of TiO2 nanocrystals increased as a result of increasing annealing temperature. In addition, EDS study confirmed that no impurities were present in the samples. Dielectrics properties were measured at different frequencies ranging from 20Hz to 5MHz at room temperature. It is found that the TiO2 nanoparticles possess a higher value of dielectric constant and lower dielectric loss at 1 kHz. The novelty in this work is that the modified preparative conditions of TiO2 nanoparticles yield better and optimized properties related to the crystal structure, morphology, and dielectric behavior for application in optoelectronic devices, super-capacitors, spintronics, and other dielectric applications. Keywords: Nanostructured materials; sol-gel methods; sintering; crystal growth; crystal structure; dielectric studies.
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1.
INTRODUCTION Currently, among all the inorganic oxides, TiO2 is a well-known semiconductor due to its distinguished optoelectronic and electrical properties for application in sensors, photocatalysts, solar cells, and memory devices. TiO2 has a high dielectric constant, high refractive index, the good optical transparency of visible light and high chemical stability [1–5]. Because of its high dielectric constant and high breakdown strength, TiO2 serves as a good insulator and had been found suitable for wide application in the field of electronic [3–5]. It is highly useful for the fabrication of capacitors used in microelectronics devices, electronic memories and in optical filters [6–9]. The low loss factors and high dielectric permittivity of TiO2 over an ample frequency range are always of immense attention [7,10–12]. TiO2 is a member of the transition metal oxides family[2,13]. It exists in three different phases namely rutile, brookite and anatase. The TiO2 in anatase phase is kinetically more stable at lower temperatures whereas rutile phase is stable at higher temperatures. Both anatase and rutile phase possess have tetragonal crystal structure [4,13–15]. Anatase phase has a high refractive index of 2.54 and rutile is 2.75 respectively at 550 nm wavelength [16]. It is found that anatase has a lower dielectric constant than the rutile phase of TiO2 [10]. Rare earth ions (Dy3+ and Sm3+) doped TiO2 nanoparticles have been studied and found superior catalyst, dielectric properties than pristine TiO2 [17,18]. Annealing enhances physical attachment and electronic contact between the particles. The annealing parameter depends on temperature, annealing time and reaction atmosphere[1,13]. According to reported articles, Ostwald ripening is at high temperature leading to more nucleation of the nano-clusters and more growth. Larger particles grow at the expense of smaller particles [4]. Heat treatment in the synthesis process affects morphology, porosity and crystalline nature of the particles, resulting in a decrease in surface area. Annealing of the samples reduces the surface hydroxyl groups. The annealing procedure affects the oxygen vacancies and induces phase change and escalates crystalline and structural order which normally enhances the dielectric constant [2]. Many researchers investigated the phase change of TiO2 nano-particles acquired by the sol-gel method with respect to various annealing temperatures [1,4,15,19]. The electrical properties of TiO2 nanoparticles are governed by few factors such as small size, surface-to-volume ratio being high, defects in grains, quantum confinement of charge carriers. The sol-gel method is very simple to handle. It is low cost giving good homogeneity and effective compositional control. The low-temperature method controls the product’s chemical composition. TiO2 is prepared in the form of single crystal, powder, glass, film, pellet, tube, and nanowires, etc. In this work, we present the synthesis of TiO2 nanoparticles from the hydrolysis of organometallic-precursor titanium isopropoxide via condensation. Here we report the synthesis process of TiO2 nanoparticles and study of their morphological, structural, and dielectric response at different annealing temperatures. The novelty in 2
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the present work is that the modified preparative conditions of TiO2 nanoparticles yield better and tailored results that are useful for its applications in the field of photovoltaic, photocatalyst and dielectrics.
2. EXPERIMENTAL METHODS 2.1 Synthesis of TiO2 nanoparticles The present investigation is to study the impact of annealing temperature on the morphological, structural, and dielectric properties of Titanium dioxide nanoparticles. Nanoparticles of TiO2 were prepared in the laboratory using analytical grade chemicals. Sol-gel technique is used to synthesize TiO2 nanoparticles. The organic chemicals such as Titanium isopropoxide C12H28O4Ti (TTIP) were used as the main precursor, iso-propanol as a solvent and nitric acid (HNO3). In a typical synthesis process, a solution of isopropanol was prepared in distilled water and then TTIP was dissolved drop by drop in this solution. Meanwhile, the solution was constantly stirred by a magnetic stirrer at 80°C for 1 hr. After 1 hr, a homogenized solution of distilled water and concentrated nitric acid was mixed into the previously prepared solution (Iso-propanol, deionized water, and TTIP) stirred constantly heated at 60°C for 5-6 hrs. The final solution was transformed into a viscous gel. This gel was then dried by heating at 300°C for 2 hrs to allow the water and subsequently organic constituents to evaporate. After drying at 300°C white color dried product was obtained which was then grounded using agate mortar for 2-3 hrs to get nanocrystalline powder. To prepare samples for different annealing temperature the powder was annealed at 300°C, 400°, 500°C, 600°C for 2hr at a heating rate of 5°C/min in a muffle furnace to get crystallization process of TiO2 nanoparticles. All the samples annealed at different temperatures were allowed for slow cooling at room temperature. Prepared TiO2 nanoparticles sample by the procedure described above were termed as Sample A, Sample B, Sample C, Sample D for temperature 300° C, 400°C, 500°C, 600°C respectively.
2.2 Characterization X-ray diffraction profiles analysis pattern for all the samples was recorded using X-ray Diffractometer Model- Ultima IV model ( Rigaku, Japan) with CuKα radiation for λ=0.15406 nm. The diffraction data obtained for 2θ angle in the scanning range of 20 to 70°, step width-0.02° and x-ray operation 40 KV, 40 mA. Fourier transmission infrared spectra (FTIR) of TiO2 nanoparticles were recorded in the region 4000 to 500 cm-1 with the help of Model-Nicolet TM 6700, Thermo scientific USA. For the morphological studies samples were scanned using scanning electron microscope (Model- JSM 6490LV, Joel, Japan). To find out material composition EDS Model-133 from OXFORD instruments (UK)-EV Dry Detector, 3
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INCAx - act, was used. Dielectric measurements of pellets at room temperature in a frequency range 20Hz to 5 MHz and at a voltage of 1V at room temperature were performed using an LCR meter (ModelWayne Kerr 6500 P high-speed LCR meter). TiO2 powder heated at different annealing temperatures (300°C-600°C) and it was compressed by hydraulic pressure machine at a pressure of 3 tons to make pellets for dielectric measurement. Each sample measured 13mm in diameter having thickness of 1-2 mm. BAKER-Type Jo2, 7301 screw gauge was used to measure physical dimension with least count of 0.01mm. To ensure good electrical contact these pellets were kept between two circular brass discs and the whole assembly was placed for measurement between the electrodes of the LCR meter at a signal voltage of 1.0 volt. No coating is done on any of the pellets.
3. RESULT AND DISCUSSION 3.1 XRD X-ray diffraction pattern of TiO2 peaks shows related to two phases which are anatase and rutile mixed phase. X-ray diffraction pattern of prepared samples from 300°C to 600°C temperatures are shown in figure 1(a). The Bragg reflections observed at 25.25°(101), 37.74°(200), 48.04°(200), 54.3°(022), 62.7°(250) belong to anatase phase of
TiO2 and those at 27.39°(110), 36.08°(111), 41.20°(220),
44.0°(210), 56.58°(231), 64.05°(330),68.9°(301) belong to rutile phase[12]. An additional peak at 30° (121) related to brookite phase is observed in samples A, B, and C. All peaks observed in this spectra were matched well with existing data [JCPDS data no. 00-049-1433, 00-053-0619 and 00-048-128] and literature[20]. In figure 1(b) the diffraction pattern show peaks at (101) and (110) related to anatase and rutile phase for all annealing temperature. During phase transformation at annealed temperature 500°C and 600°C in the rutile phase, Ti-O bond breaking and lattice distortion at higher temperatures take place. This brings in the replacement of oxygen and production of defects together with the elaboration of innovative Ti-O bonds [21,22]. With increase of temperature the intensity of the reflection (110) corresponds to the rutile phase is increased [23]. Sharma et.al. observed the same peaks [20]. The experimental data show no additional peaks other than that for the TiO2 material. This indicates that there are no impurities present in the prepared nano-material within the sensitivity limit of XRD.
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` (a)
(b)
Figure 1. (a) X-Ray Diffraction patterns images of TiO2 nanoparticles annealed from 300°C to 600°C temperature, (b) Variation of anatase phase (101) peak and rutile phase (110) peak intensities and Bragg angle shifting with the annealed temperature at 300°C to 600°C. 5
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(a)
(b)
. (c)
(d)
Figure 2. Variation of (a) Lattice parameter (b) size (c) d (101)-value (d) c/a ratio and unit cell volume for TiO2 nanoparticles prepared at 300°C to 600°C annealing temperature. Figure 1 (a) shows the annealing temperature effect on most intense peaks of anatase (101) and rutile (110). The temperature increases at 300°C to 600°C. In figure 1 (b) Full-width half maxima (FWHM) and intensities of peaks related to both phases are calculated from the XRD data. FWHM of anatase (101) was found to decrease from 1.26˚ to 0.320˚ and for rutile (110) from 0.323˚ to 0.201˚ respectively with the increase in annealing temperatures for Sample A, B, C, and D. The increase in peak intensity at higher temperature is attributed to the increase in crystallite size. As shown in figure 2 (a) there is variation in crystallite size as the temperature changes from 300°C to 600°C. As is seen in figure 2(a), crystallite size first increases gradually for 400°C then increases sharply beyond 400°C. The increase in
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crystallite size with increasing annealing temperature may be attributed due to thermally promoted crystal growth. The annealing of TiO2 at different temperatures affects the lattice structure of TiO2 which is the main cause of transformation from amorphous to crystalline nature of prepared nanomaterial. As per the Bragg’s law, the value of Bragg angle 2θ should increase or decrease if a change in the d-spacing of the crystallographic planes. The peak shifting towards lower 2θ is associated with the tensile stress and is responsible for increase in d spacing values and higher 2θ values are attributed to d spacing due to compressive stress[24]. Figure 2(b) shows the variation of d spacing values. The lattice strain is an estimation of the circulation of lattice constants rising from crystal imperfections, like lattice dislocation. The grain growth correlated with the more and more convex nature of grain boundary and the removal of oxygen ions. The interplanar spacing (d) and lattice parameter (a, c) for the prepared samples were calculated using following Bragg’s equation for the tetragonal system [25,26], 1 𝑑2ℎ𝑘𝑙
=
(
)
ℎ2 + 𝑘2 𝑎2
𝑙2
+ 𝑐2
h, k, l stand for Miller’s indices for the crystallographic plane of TiO2 nanocrystals. The lattice constants were measured as of the lattice spacing of (101) and (200) anatase peaks. Variation of a and c is shown in fig. 2(c). The unit cell volume for annealed samples was calculated using the following equation [26]. 𝑉 = 𝑎2𝑐 Figure 2(d) shows that the ratio of c/a parameter and unit cell volume of nanocrystals of anatase (101) and (200) peaks. The volume of the unit cell and c/a ratio parameter both increase with the increase in annealing temperature. The crystallite size (Ds) of the prepared samples was calculated using the Debye-Scherrer formula [26]. 0.9 𝜆
𝐷𝑠 = 𝛽 𝑐𝑜𝑠𝜃 Where 0.9 is constant for spherical symmetry, λ is the wavelength (1.5406 Å), β is the full width at half maximum (FWHM) of the peaks, θ is the Bragg’s angle. Average crystallite size D of prepared TiO2 powder and lattice strain ε is calculated by the WilliamsonHall uniform deformation model (WH-UDM) plot, by the following equation [25]; 𝛽ℎ𝑘𝑙𝑐𝑜𝑠𝜃 = 4𝜀 𝑠𝑖𝑛𝜃 +
𝐾𝜆 𝐷𝑠
A graph is plotted between βhklcosθ on the y-axis and 4sinθ on the x-axis. The slope of the straight line obtained from linear fit data gives the value of lattice strain ε from slope and crystallite size Ds was measured from y-intercept. 7
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The parameters Ds and ε have also been calculated from the “size strain plot” by the following equation[25].
(
𝑑ℎ𝑘𝑙 𝛽ℎ𝑘𝑙 𝑐𝑜𝑠𝜃 2
)
𝜆
2 𝐾𝜆 𝑑ℎ𝑘𝑙 𝛽ℎ𝑘𝑙 𝑐𝑜𝑠𝜃
= 𝐷𝑠
(
2
𝜆
)+( )
𝜀 2 2
Lattice strain ε was estimated from intercept and crystallite size Ds from the slope of the straight-line plot of the above equation. The crystallite size and value of strain of samples for different annealing temperatures are provided in Table 1. The positive value of strain from W-H plot and size strain plot is attributed to lattice expansion for the sample[6]. For size strain plot, the sample-B was measured with negative strain value which is attributed to compressive strain and this strain may be the cause of peak broadening[6,27,28]. Table 1: Crystallite size and average lattice strain for TiO2 nanoparticles annealed at various annealing temperatures Sample A, B, C, and D.
Crystallite size (nm) of TiO2 Sample
Strain of TiO2
Scherrer
W-H
size-strain
formula
plot
plot
Sample A-300°C
6.67
6.69
5.64
60×10-4
2.879×10-4
Sample B -400°C
9.18
10.36
5.21
85×10-4
-6.955×10-6
Sample C -500°C
13.1
8.44
12.95
18×10-4
4.888×10-4
Sample D -600°C
26.6
27.78
20.75
6.288×10-4
2.2504×10-6
W-H plot
Size-strain plot
The Bragg peak is influenced by lattice strain and crystallite size in different ways and increases the intensity, shift and peak width of the 2θ peak position consequently. For the period of thermal energy relaxations, confident deformations are created depending on the temperature gradient in various directions which is the main reason for either decrease and increase in strain value. On another hand, the crystallite size is inversely correlated with strain. Hence deviation in crystalline size will change the suitable deformations. The value of tensile strain decreases for the samples annealed temperature of heat treatment from 300 to 600°C. Afterward, the reduction in the tensile strain was not much significant (Table.1). This is possibly caused by the structural relaxation of the anatase form of crystallite to the equilibrium value successively at raised temperatures.
3.2 SEM and EDS
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Surface morphology explored through SEM reveals that the size of the particle in addition to the porosity of the crystalline particle also increases from 300°C to 600°C. All samples have spherically shaped particles which are uniformly distributed. Ostwald ripening at high temperatures leads to more nucleation of the nano-clusters and more growth. In these larger particles grows at the expense of smaller particles. Figure 3(a), (b), (c), (d) represents SEM images of TiO2 nanoparticles. Its magnified images in figure 4(a),(b),(c),(d) describe crystallinity of TiO2.
Figure 3. SEM images of prepared TiO2 nanoparticles at different annealed temperature (300°C to 600°C) (a) Sample A (b) Sample B (c) Sample C and (d) Sample D.
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Figure 4. Magnified SEM images of prepared TiO2 nanoparticles at different annealing temperatures (a) Sample A (b) Sample B (c) Sample C, (d) Sample D.
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Figure 5. EDS pattern of TiO2 nanoparticles for (a) Sample A, (b) Sample B, (c) Sample C, (d) Sample D.
EDS technique is useful for investigation of the composition of the material. EDS spectra of TiO2 nanoparticles at temperatures between 300°C to 600°C are recorded and shown in figure 5. The results obtained indicate the high purity of synthesized material where Ti and O are present only. No impurities were present.
3.3 FTIR Spectra The FTIR spectrum of TiO2 samples was recorded using KBr pellets of samples in the wavelength range from 4000 to 500 cm-1 at room temperature. No absorption peak of KBr becomes visible in this region because the absorption coefficient of KBr is far less than 400 cm-1 only a complete spectrum of the sample is observed.
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Figure 6: FTIR spectra image of TiO2 samples annealed at temperatures (300°C, 400°C, 500°C,600°C) for Samples A, B, C, and D.
Figure 6 shows sharpness of the absorption peak decreases for samples at higher annealing temperatures. The shifting of peaks related to OH stretching towards a higher wavelength side for the increasing annealing temperature is observed in the spectra. The sharpness of the peak decreases may be attributed to C=N stretching. Table2. FTIR spectrum of TiO2 nanoparticles at 300°C, 400°C, 500°, 600°C. Absorption peaks (cm-1)
Functional groups
Sample A
Sample B
Sample C
Sample D
3379.3
3410
3421.2
3462.2
Alcohol (H bonded) - OH Stretch
1619.4
1618.1
1621.9
1623.8
C=N Stretch
1525
1526.6
1523.9
1515.4
C=C Stretch
1384.1
---
----
---
Symm. CO2, NO3 and Ring Stretching
621
764
764
764
Ti-O-Ti Stretching
The data related to FTIR spectra tabulated in Table 2 show the value of peaks at different temperatures and assigned to various modes. The Transmittance band around 3400 cm -1 caused by adsorption of water molecules on TiO2 surface[13,29,30]. The peak at 1623.8 cm-1 related to Ti-OH bond stretching and its 12
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deformation vibration [21,30]. The peak at 300°C in the wavelength range of 1384.1 cm -1 corresponds to the bending mode of nitrate because nitric acid was used in the synthesis method as a catalyst [31]. According to the previous study, this peak also shows the presence of isopropoxide in the structure related to CH3 symmetric deformation[21,32]. CO2 symmetric vibration band arises due to C and O bonding for molecules present in air [25,33,34]. The peak at 2919.6 cm -1 belongs to N-Alkanes-Asym CH2 indicating the presence of a residual organic compound at 400°C[7,35]. The peaks around 621cm 764cm
-1
-1
at 300°C and
at 400°C, 500°C,600°C are assigned to the characteristic vibrations of Ti-O-Ti stretching of
titanium dioxide[31].
3.4 Dielectric properties Dielectric properties of TiO2 nanoparticles were measured for Sample B, C and D. Dielectric constant is known as the capability or efficiency of a material to accumulate electrical energy. Dielectric loss is known as dissipation of energy in the appearance of heat transfer inside the dielectric material and this type of lose arises when internal friction spread over the dipoles. We have measured the deviation of dielectric loss and dielectric constant in the frequency range from 20Hz to 5MHz at room temperature. Figure 7 represents the dielectric constant and dielectric loss of different samples. In the low-frequency region, the values of dielectric constants, as well as losses, are high but it decreases rapidly as the frequency increases and it becomes approximately constant in the high-frequency region with increasing temperature [36].
(a)
(b)
Figure 7 (a) Represents Dielectric constant and (b) dielectric losses of TiO2 nanoparticles for Sample B, Sample C, Sample D at room temperature. Figure 7 (a) indicates the higher value of the dielectric constant of samples B, C, D at low-frequency region and decreasing very speedily at low-frequency region and becoming almost constant by a further increase in frequency as reported earlier [7,22]. As stated by Koop's theory the standard dielectric response shows the high value of dielectric constant, at the low-frequency region, is showing that 13
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dielectric constant decreasing with increasing frequency and almost constant(relaxation behavior) at highfrequency region [7,22,36]. The total polarization appears in the nanocrystals namely ionic, electronic, orientation and space charge polarization [7,11,37–40]. The deduction of dielectric constant values at a low-frequency region is associated with space charge polarization [2,12]. The space charge polarization and dipolar polarization are strongly depended on temperature and affect the high value of dielectric constant [39]. As the annealing temperature increases, grain size becomes larger but due to this density of grain boundaries decreases and relaxation of grain boundary appears to be suppressed or merges with a grain [4]. The grain boundary contributions are reduced and the relaxation behavior in the low frequency is also suppressed. Due to temperature increment, the interfacial polarization at lower frequency also increases which is the region of more charge accumulation and charge transfer on the grain boundaries [7,39]. Some dominant factors to influence dielectric properties are similar to the composition of the material, frequency of the applied field, temperature and density. The dielectric losses of samples B, C, D are also shown in figure 7(b). The mechanism of dielectric loss is the main reason of total defects in grain, impurities, space charge fabrication in edge layers jointly generates absorption current. Li et.al. also reported that increment in dielectric loss value is the cause of particle size reduction [7]. It can be seen for TiO2 nanoparticles from a very low-frequency region, the reduction in dielectric loss and the dielectric constant is possibly a part of the nanomaterial purity cause. Dielectric behavior and conductivity associated with each other [8]. During this hopping of electron is the main reason for conductivity. The larger value of dielectric loss happens when applied electric field frequency is equivalent to hopping frequency. The dielectric constants at 1 kHz frequency at room temperature have been measured as 584.3, 176.9 and 218.9 for Sample B, C, D respectively. The value of the lower dielectric loss at 1 kHz frequency is obtained are 0.10, 0.01 and 0.024 at room temperature intended for Sample B, C, and D respectively. The dielectric properties of annealed TiO2 nanoparticles are appreciably affected by different temperatures [11]. Sample D exhibits a low dielectric loss and high dielectric constant value which is optimized dielectric properties for colossal permittivity. A high dielectric constant 218.9 and low dielectric loss 0.024 together at 1 kHz is correlated to colossal permittivity applications for the sample annealed at high temperature [7,11].
4. CONCLUSION The nanoparticles of Titanium dioxide were successfully synthesized via sol-gel method at different annealing temperatures (300°C-600°C). XRD analysis confirmed that no impurity was found in TiO2 14
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nanoparticles. SEM study reveals an increase in crystallite size with increasing annealing temperature. EDS confirmed that no impurities were present. At the low frequency, value of dielectric loss and dielectric constant decreases rapidly and becomes nearly constant at high frequency. Dielectric properties of TiO2 results revealed that sample D has a quality dielectric implementation having higher dielectric constant value 218.95 and lower dielectric loss of 0.024. Because high dielectric constant and low loss value it may find its use as in the area of high energy storage applications and miniaturization electronic devices.
ACKNOWLEDGMENT The author is thankful to supervisors for their encouragement and guidance given during this study. Special thanks to condensed matter and Material research lab, Department of physics, Lucknow University, Lucknow for allowing to use their laboratory facilities to accomplish this experimental work and Central Instrument Facility for X.R.D analysis. All authors listed have equally contributed to this research work and preparation of the manuscript.
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Journal Pre-proof Author Contribution Statement All authors listed have equally contributed to this research work and preparation of the manuscript.
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We confirm that this work is original and has not been published elsewhere. It is also not under consideration for publication elsewhere. All authors listed have contributed sufficiently to the project to be included as authors. To the best of our knowledge, no conflict of interest, financial or other, exists. All authors have seen and approved the manuscript for submission.