INVESTIGATIONS ON STRUCTURAL, OPTICAL, AND IMPEDANCE SPECTROSCOPY STUDIES OF TITANIUM DIOXIDE NANOPARTICLES

This article explains a novel synthesis for producing titanium dioxide (TiO2) nanoparticles by a sol-gel technique using titanium tetraisopropoxide as a titanium source. The synthesized nanoparticles were analyzed using many measurements like X-ray diffraction (XRD), HRTEM, absorption UV spectroscopy, FTIR, and ac impedance spectroscopy. X-ray peaks were used to calculate the crystallite size and lattice strain by Williamson–Hall method. Crystallite size calculated from x-ray diffraction using the Scherrer equation gives an approximate size and cannot be used for measurements. TiO2 nanoparticles are found to possess a tetragonal structure with a crystalline size around 12 nm. Particle size was confirmed by HRTEM images. The optical studies response for the nanoparticles showed the possible visible absorption peaks for TiO2 nanoparticles are 323 nm. Bandgap energy (Eg) of the TiO2 nanoparticle calculated from UV visible absorption spectra is discussed, and the bandgap is 3.14 eV. FTIR spectra showed vibration bands of the Ti-O network. AC Conductivity properties of TiO2 nanoparticles are studied in the frequency range 1 to 8 MHz at varying temperatures. The conductivity of the TiO2 nanoparticle is found to be constant in the low angular frequency region. Dielectric parameters were analyzed at different temperatures and frequencies.


INTRODUCTION
Today researchers and scientists are very interested in preparing and using very clean, developed energy and living. For the above-mentioned purpose, it is necessary to synthesize metal oxide with new and good properties [1]. Nanomaterials have different structures and shapes, varied with optical and electrical studies. The size of the nanoparticle decreases as the specific surface area increases. Shape control has a very important role in the nanoscience field. Many excellent research articles on the synthesis of metal oxide nanoparticles have been published in recent years [2]. To inorganic nanoparticles with a good surface action from agglomeration, many researchers have worked various techniques to improve nanoparticles; thin films and surface action related have been focused. Among the metal oxide nanoparticles, titanium dioxide (TiO 2 ) has a very important metal oxide for application in ceramic materials, gas sensors, photocatalysis, coating, and solar cell, it has been attracting the development of practical applications. TiO 2 metal oxide has two different crystal structures like tetragonal (rutile and anatase) and orthorhombic (brookite) and each has its own high potential applications [3]. TiO 2 is a good functional material due to its having good properties such as superconductivity, low cost, high refractive index, and corrosion resistance. Nowadays, TiO 2 is used as a white pigment in cosmetic and paint applications, photonic crystals, coating for bone implants, and components in electroceramics [4]. Many research articles explaining TiO 2 have different shapes like nanopowder [5], nanoparticles [6], nanorods [7], nanotubes, and nanowires [8]. TiO 2 has nice photo catalytic behavior and is mostly used as catalysts for the decay of a difference of organic and inorganic pollutants. In most cases, nanoparticles are prepared through liquid phase reaction, acid phase reaction, and gas-phase reaction. Most of the methods have been established for TiO 2 nanoparticle syntheses such as hydrothermal method [9], sol-gel technique [10], chemical vapor deposition [11], direct oxidation, and others. Among them, the sol-gel method is one of the best most controlled methods due to its possibility of deriving a unique tetragonal structure at low reaction room temperatures and excellent chemical route [12]. Sol-gel is not only a rapid method and also cleaner, faster and economical compared to other conventional methods, and this method is chosen for producing high purity TiO 2 nanoparticles for the present work. TiO 2 nanoparticles were characterized by powder X-ray diffraction method, highresolution transmission electron microscope, UV-Vis spectra, FTIR, conductivity, and dielectric studies.

Synthesis of titanium dioxide nanoparticles
In this synthesis process [13], the two different solutions 5.8 mL of titanium tetraisopropoxide (TTIP) was mixed with 20 mL deionized water and 25 mL deionized mixed with 2.5 mL of acetic acid. The acetic acid solution is mixed within drop-wise TTIP solution during stirring, stirred in continuously for 3 hours to obtain a clear white colour solution. After one hour, the solution was kept in a micro oven at 40 o C for 10 min to obtain a colloidal solution. The obtained solution was dried at 60 o C to get white colour TiO 2 nanoparticles. Prepared nanoparticles were crushed into fine powders with a mortar. As-prepared samples were characterized.

Equipments
The powder XRD method of the titanium dioxide nanoparticle was analyzed using powder XRD (PANalytical X-Pert Pro) using CuKα1, λ = 0.15 nm radiations at 30 mA, 40 kV with Cu K radiation ( = 1.54060 Å), with an angle between 10-80 o . The particle size of the TiO 2 nanoparticle was carried out from HRTEM (JOEL JEM 2000). The FT-IR spectra of the TiO 2 were recorded using Fourier transform infra-red spectroscopy (JASCO FTIR-4100), a range of wavelengths as 400 to 4000 cm -1 . The optical spectra of the TiO 2 were recorded using UV Visible spectroscopy (UV-2600) in the wavelength range 200-1200 cm -1 . AC impedanc e spectroscopic studies were analyzed (computer-controlled Zahnner zennium IM6 meter) within the frequency range of 1 to 8 MHz at different temperatures.

RESULTS AND DISCUSSION
Powder X-ray diffraction analysis Figure 1 shows the XRD patterns of the TiO 2 nanoparticles prepared by chemical sol-gel methods. The powder XRD study indicates that the nanomaterial was synthesized from pure TiO 2 phase and crystalline structure. The line broadening of the diffraction peak indicates that the prepared materials in the nanoscale range. The XRD technique was carried out over the diffraction angle in between 2 (10-80 o ). The peaks of Figure 1 can be indexed as a tetragonal phase of TiO 2 nanoparticles with a lattice constant of a = 3.7852 Å and c = 9.5139 Å, in good agreement with JCPDS card no. 21-1272 [14]. In prepared TiO 2 nanoparticles, the peak at 25.  116) and (215) with the TiO 2 anatase phase [15]. Some other phases could not be detected in the pattern. The Scherrer formula is used to calculate the crystalline size of the nanoparticles: where d is the diameter of the crystalline particle, λ is the wavelength of X-ray radiation, B is the full width at half-maximum (FWHM) of the peak and θ is the angle of diffraction. The crystalline size of the nanoparticle obtained is 12 nm, which is coincident with the HRTEM investigation shown in Figure 3.

Williamson -Hall analysis
The X-ray peak evidence crystalline size along with the large strain related to the nanoparticles. From the XRD data, the broadening (β T ) of the peaks is due to the relation of crystalline size (β d ) and lattice microstrain (β  ). The β T was corrected, corresponding to all x-ray diffraction peaks of TiO 2 nanomaterials used by the relation is The crystalline sizes were found using the Scherrer equation is The average crystallite size is around 12 nm. The microstrain devolved in nanoparticles due to crystal defect and distortion was found using the equation: From the above equations, it was confirmed that the X-ray diffraction peaks width from crystalline size changes as 1/cos strain changes as tan. Therefore, simply sum of the above equations are e = 4e tan q + l q By rearranging the above equation is [16] β cos q = 4e sin q + l The above equations are called the W-H equation. The plot is drawn with β T cos θ along the yaxis and 4sin along the x-axis ( Figure 2). The linear fit was drawn and the strain was calculated using the slope of the fit and crystallite size was found by the y-intercept. The crystalline size of TiO 2 nanoparticles is around 12 nm and micros strain is -0.009.

High-resolution transmission electron microscope (HRTEM)
High-resolution transmission electron microscope (HRTEM) was used and further explains the crystallite nature, particle size, and morphology. The HRTEM image ( Figure. 3) shows that the crystalline, spherical shape and morphology of prepared TiO 2 nanoparticles were strongly confirmed [17]. HRTEM images (Figure 3 (a), (c) and (d)) indicate that the TiO 2 nanoparticle was small, at about around 12 nm. Figure 3

Fourier transform infra-red spectroscopy (FTIR) analysis
FTIR spectra were used to analyze the various functional groups in TiO 2 nanoparticles. Figure 4 shows the FT-IR spectra of sol-gel prepared TiO 2 in the range of 500-4000 cm −1 . The peak observed at 3208.0046 cm −1 is due to the stretching vibration of the O-H group [18]. The peak obtained at 1618.948 cm -1 was attributed to the Ti-OH bending mode of water [19]. In the spectra of pure TiO 2 , the peak observed at 578.5481 cm -1 is due to the stretching vibration of Ti-O, and 1482.0269 cm -1 shows stretching vibrations of Ti-O-Ti [20]. Only the more strong peaks lie between 500 to 4000 cm -1 , and which have been attributed to formed TiO 2 nanoparticles. It measured various well-defined peaks are 648.275, 1145.57994, 1296.894,  1618.948, 3326.64, and 3496.67 cm -1 . The TiO 2 nanoparticle prepared by this method is of good quality and can be used for further applications.

UV-Visible spectroscopy analysis
The optical studies of the prepared nanoparticles were analyzed using UV-Visible spectra. Figure 5 shows the absorbance of the prepared nanomaterials in the nanoscale range at room temperature. The UV spectra between 200-1250 nm were analyzed. It shows a peak at 323 nm with absorbance, which means that it explains good absorbance in the UV region.  To find the value of energy bandgap (E g ), it is measured from the fundamental absorption. The relation in between the  and h can be written as: (E = h,  = c/) where A is a constant, E is incident photon energy,  is an absorption coefficient and E g is bandgap energy of the materials. Bandgap energy can be found accurately from the plots of hν against (αhν) 2 . The straight-line portion of the plot (hν vs (αhν) 2 to α = 0) may be obtained by bandgap energy [21]. E g value of TiO 2 nanoparticles is 3.14 eV as calculated using Figure 6. The TiO 2 nanoparticles have large bandgap energy and can be used for further more applications like photocatalytic [22].

AC conductivity analysis
As prepared TiO 2 nanoparticles were analyzed in terms of AC conductivity. In order to raise the activity of electron movement in nanostructures, the AC conductivity studying is a very important role to find whether the nanoparticle has high conductivity or not. From the band theory, the conductivity is more means that the electrons are more in the conduction band [23]. Figure 7 shows that the frequency-dependent conductivity of TiO 2 nanoparticles of different temperatures in the angular frequency range of 0.5 to 6.5 rad/s. The conductivity (σ ac ) was obtained by the relation of s = e e tan q (8) ω (ω = 2πf) is an angular frequency and ε r is the relative permittivity. Generally, conductivity can be written as Here, the first term is dependent on temperature, and the power of angular frequency will give information about hopping mechanism correlation [24]. The conductivity of TiO 2 nanoparticles was calculated in the temperature between 303 K to 383 K. At low frequencies, conductivity has constant at all temperatures, and it has been changed at the high temperatures range. This conductivity variation has occurred in the prepared samples due to the polarization effect. The conductivity of TiO 2 nanoparticles is 989 S/cm at 383 K in the angular frequency of 6.5 rad/s and 323 S/cm at 303 K. The highest temperature 383K has shown a high conductivity as compared to the other temperature.

Dielectric analysis
The frequency-dependent analysis is carried out to study the electrical homogeneity of the nanomaterial confirming the relaxation mechanism as well as the behaviour of electrical conduction [25]. This analysis helps to carry out the dielectric constant of the nanomaterial. The microelectronics field industry needs very few dielectric constant materials. The dielectric constant was observed in the present study, it is shown in Figure 8. The obtained dielectric constant for the prepared TiO 2 is very less when compared to the bulk materials. The dielectric constant value obtained is very less when it has low angular frequency compared to all different temperatures. At high angular frequencies, the dielectric constant has a constant at all temperatures, and it has been varied at the low angular frequency range.  The ε' is high at a low angular frequency area for all temperatures. It is the reason due to the space charge region at the electrode interface, which is non-Debye nature, where the space charge area with respect to angular frequency is expelled in terms of ion diffusion. At high frequencies due to the high reversal of the field at the electrode interface, the distribution of charge carriers, ions, towards the ε' decreases with an increase in temperature [26]. The dielectric loss was observed as the energy dissipated and generally consists of the distribution ion as well as from the polarization of a charge. The log ω vs ε″ plots for TiO 2 nanoparticles with different temperatures is given in Figure 9. The dielectric loss becomes very high at higher angular frequencies due to free charge motion. At low angular frequencies, the dielectric loss has varied at all temperatures, and it has been constant at the high angular frequency range. The variation in the slope is distributed to space charge effects at low frequencies. The highest temperature 383 K has shown a good result as compared to the other temperature.

CONCLUSION
The metal oxide TiO 2 nanoparticles have been prepared by sol-gel technique and characterized by XRD, HRTEM, FTIR, UV, and AC impedance techniques. The XRD study reveals the crystalline nature of the prepared nanoparticles. The microstrain was calculated by the W-H plot method and is -0.09. The particle size was calculated from the HRTEM image. The FTIR study confirms the functional group of nanoparticles. The energy band gap was calculated by the UV absorption method and is 3.14 eV. The temperature dependence of the conductivity of nanoparticles was analyzed with the different temperature ranges. The highest conductivity has been measured at 989 S/cm for 383 K in the angular frequency of 6.5 rad/s. The dielectric parameter increased with the increase in all temperatures.