Gadolinium doped Zinc oxide (Zn 1–xGdxO) nanocrystals with different percentage of Gd content ( x = 0, 0.2, 0.4, 0.6, 0.8) have been prepared by the solid state reaction method. The structural, mor-phological and chemical studies of the samples were performed by X-ray diffraction (XRD), Scanning electron microscope (SEM) and Energy dispersive X-ray (EDX) analysis. The XRD spectra confirm that all the samples have hexagonal wurtzite structure. Decrease in average crystallite size with an increase in Gd concentration is observed in XRD. SEM images show that the grain size of undoped ZnO is larger than the Gd doped ZnO, specifying the hindrance of grain growth upon Gd doping. The chemical composition of the samples was confirmed using Energy dispersive X-ray (EDX) analysis. The variation of dielectric constant ( ε r), dielectric loss (tan δ) and AC conductivity as a function of frequency is studied at room temperature in a frequency which ranges from 100 Hz - 4.5 MHz by using LCR Hi TESTER. All the samples exhibit the normal dielectric behavior, i.e. decreases with increase in frequency which has been explained in the light of Maxwell-Wagner model. The dielectric constant and dielectric loss can be varied intensely by tuning Gd concentration in Zn 1–xGd xO compounds.
ZnO is a versatile semiconductor having a wide band gap of 3.37 eV and large exciton energy of 60 meV which crystallizes in hexagonal wurtzite structure. Due to its unique physical and chemical properties, it has a wide spread application in solar cells, gas sensors, UV light emitters and surface acoustic wave (SAW) devices [
Doping ZnO with rare earth ions is of great interest for optoelectronics and spintronic applications [
Gd doped ZnO were synthesized by solid state reaction route to study their structural and dielectric properties. The chemicals used in the experiment are ZnO (99.99% pure), Gd2O3 (99.99% pure) and LiOH∙H2O (99.99% pure). These chemicals were weighed using an electronic balance in accordance with the required stoichiometry. These materials were homogeneously mixed using an agate mortar for sufficient time to get fine powders. LiOH is an inorganic and water soluble compound used as a heat transfer medium for the synthesis of Gd doped ZnO. The prepared samples were mixed with ethanol and made into slurry. It is then dried in an oven for 1 hour at 100˚C. After drying, the mixture was ground for 1 hour and made into pellets using hydraulic pelletizer. These pellets were sintered at 900˚C for 4 hours in a high temperature furnace. The pellets were again ground and used as samples for the studies.
The XRD patterns of powder samples were attained by Rigaku Miniflex 600 X-ray diffractometer. Surface morphology and chemical composition of the samples were respectively examined by scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). For dielectric and AC conductivity measurements, the powder samples were made into pellets using hydraulic pelletizer of thickness 1 - 2 mm and of diameter 10 mm by applying pressure of 130 Kg-cm2 for 1 minute. The dielectric constant, dielectric loss and AC conductivity were measured using HIOKI 3532-50 LCR Hi TESTER for a frequency range from 100 Hz to 4.5 MHz.
X-ray diffraction pattern of undoped and gadolinium doped ZnO, Zn1−xGdxO whereas (x = 0, x = 0.2, x = 0.4, x = 0.6, x = 0.8) are shown in
It is noticeable that the XRD peaks of doped samples were found shifted towards the higher 2θ values. The peak shift observed in the XRD pattern for the peak corresponding to the plane (101) depicted in
Variation of lattice parameters with Gd concentration is shown in
where t is the crystallite size, K is the shape factor, λ is the wavelength of the incident X-ray radiation, θ is the bragg angle and β is the full width at half maximum (FWHM) in radian of the peak with given (hkl) value.
It was found that crystallite size decreases with increasing Gd concentration (up to 0.6 wt.% of Gd) as shown in
It could be noted that the crystallite size of 0.8 wt.% Gd doped ZnO is larger when compared to other concentrations and it tending to the size of pure ZnO crystallites on higher doping percentage. This shows that the solu-
Concentration of gadolinium (wt.%) | a (Å) | c (Å) | c/a |
---|---|---|---|
0 | 3.25331 | 5.21137 | 1.6018 |
0.2 | 3.25118 | 5.20893 | 1.6021 |
0.4 | 3.25012 | 5.20787 | 1.6023 |
0.6 | 3.24732 | 5.20111 | 1.6016 |
0.8 | 3.25422 | 5.21253 | 1.6017 |
Concentration of gadolinium (wt.%) | Average crystalline size (nm) | Cell volume (Å3) | Specific surface area (m2/g) × 103 |
---|---|---|---|
0 | 64.5 | 47.79 | 16.435 |
0.2 | 52.1 | 47.71 | 20.346 |
0.4 | 51.2 | 47.67 | 20.704 |
0.6 | 47.1 | 47.52 | 22.506 |
0.8 | 56.0 | 47.83 | 18.929 |
bility limit of Gd ion in the ZnO crystal lattice is close to 0.6 wt.% and excess Gd ions may precipitate out on the particle surface.
The volume of the unit cell for hexagonal system has been calculated using the formula, V = 0.866a2c. The unit cell volume is completely dependent on lattice constants. From the
The specific surface area of the crystallites of the samples was also determined using XRD. The specific surface area is a material property of solids which measures the total surface area of the crystallites present in per unit of mass. It is an important parameter that can be used to determine the type and properties of a material. It is particularly significant for adsorption, heterogeneous catalysis, and reactions on surfaces. The specific surface area can be calculated by Sauter formula,
where S is the specific surface area, Dp is the size of the particle and ρ is the density of ZnO which equals to 5.606 g/cm3.
The SEM technique was employed to find the size and distribution of particles in the materials.
Microstructural variation of Gd doped ZnO compared to pure ZnO is due to the significant difference in the ionic radius of Gd3+ related to the Zn2+ in ZnO. Ionic radius of Gd3+ is 0.938 Å, which is higher to Zn2+ (0.74 Å). Therefore higher radius Gd3+ may suppress the formation of larger nuclei during the crystallization process in ZnO. As a result of this, there is a reduction in the grain size happens. Thus Gd incorporation leads to a reduction
in grain or crystallite size.
Another reason for the reduction of grain size is due to physiochemical effect. During the compound formation and sintering time, the larger radius Gd ion will enter into the lattice partially and the remaining will diffuse to the grain boundaries. This will lead to an isolated thin layer around the crystallites. There is a stress in the crystal when Gd ions get incorporated into the lattice. Those Gd ions which accumulated at the grain boundary act as a kinetic barrier for further grain displacement and thus hinder the grain growth [
The Energy Dispersive X-ray analysis show peaks correspond to the element present in the sample. The higher a peak in a spectrum, the more concentrated the element is in the spectrum. The EDX image shown in
The variation of the dielectric constant with log frequency at room temperatures is shown in
electric constant of all the samples found decreases with increasing frequency. This can be explained on the basis of Maxwell-Wagner model which is a result of the inhomogeneous medium of two-layer dielectric structure. In this model, dielectric structure is composed of well conducting grains, which are separated by the poorly conducting grain boundaries [
The observed higher value of dielectric constant at lower frequency is due to space charge polarization. While at higher frequency, polarization will lags behind the applied and hence decreases the value of dielectric constant.
Similar to dielectric constant, dielectric loss also decreases with increase in frequency and becomes constant at higher frequencies. Dielectric loss arises when the polarization lags behind the applied field and is caused by grain boundaries, impurities and imperfection in the crystal lattice [
To find the effect of Gd substitution on the dielectric constant of the present samples, re-plot dielectric constant of Zn1−xGdxO as a function of Gd concentration is shown in
The effect of Gd substitution on the dielectric loss angle is shown in
mum dielectric loss is obtained at Gd content x = 0.2. It is also observed that all the samples have a less dielectric loss at higher frequency.
The conduction mechanism in the present samples was determined from the AC conductivity measurement. The variation of AC electrical conductivity (σac) with frequency at room temperature is shown in
Gadolinium (Gd) doped Zinc oxide (ZnO) nanocrystals were synthesized by the solid state reaction route by varying Gd concentration from 0 wt.% to 0.8 wt.%. Gd incorporation in the host lattice makes a structural distortion in ZnO due to the larger ionic radius of Gd compared to that of Zn and is evident from the structural studies. Increase the concentration of Gd hinders the growth of Zn1−xGdxO nanocrystals and multiphase growth is observed at higher concentrations. The frequency dependence of dielectric studies revealed that for all the samples studied, the dielectric constant and dielectric loss was found decreased with increase of frequency (between 100 Hz and 4.5 MHz), whereas AC conductivity was found increased. The gadolinium doping has an important effect on the dielectric properties of ZnO. At low Gd3+ concentrations, slightly higher value of dielectric constant is observed. The low dielectric loss at higher frequency makes this Gd doped ZnO nanocrystal as a candidate for high frequency applications.
Author (P. P. Pradyumnan) is thankful to SERB Govt. of India major research funding, DST-FIST Govt. of India, for projects sanctioned to Dept. of Physics, University of Calicut for the equipment facilities. One of the authors (Divya) acknowledges UGC-SAP for financial support.
P. U.Aparna,N. K.Divya,P. P.Pradyumnan, (2016) Structural and Dielectric Studies of Gd Doped ZnO Nanocrystals at Room Temperature. Journal of Materials Science and Chemical Engineering,04,79-88. doi: 10.4236/msce.2016.42009