Characterization and Elastic properties of wurtzite ZnO: Ce Nanocrystallites

 

George Varughese¹*, Crysty V.G¹, Praveen G¹, K.T. Usha², A.S. Kumar³

¹Department of Physics, Catholicate College, Pathanamthitta, Kerala-689 645

²Department of Chemistry, St. Cyril’s College Adoor, Kerala, India-691 529

³SPAP, M.G. University, Kottayam, Kerala, India-686 560

*Corresponding Author E-mail: gvushakoppara@yahoo.co.in

Zinc Oxide is an extensively studied group II-VI semiconductor with optical properties that permits stable emission at room temperature having immense application in sensors, field emission and photonic devices. It exhibits a wide variety of morphologies in the nano regime that can be grown by tuning the growth habit of the ZnO crystal. ZnO nano materials doped with Cerium ions XRD, SEM, FTIR, TEM and EDS characterized the samples. The percentage of doping material is confirmed from the EDS spectra. The average crystal size of the prepared ZnO nanopowder is determined by XRD. The size of the particles increased as the annealing temperature was increased. The crystallite size varied from 13nm to 16 nm as the calcination temperature increased, are synthesized by chemical route technique. Ultrasonic velocity through doped and undoped sample is measured and compressibility is computed. The compressibility is found to be increased. Also variation of compressibility of ZnO: Ce nanofluid with various grain size have been carried out and found that compressibility increases with decrease of particle size. To study the elastic properties of the nano fluid extensively,  certain important physical parameters such as adiabatic compressibility, specific acoustic impedance, relative association, intermolecular free length, Rao’s constant, Wada’s constant etc. are evaluated using ultrasonic velocity and density.

 

 

1. INTRODUCTION:

Zinc oxide doped with Cerium  is  a transparent electro conductive and piezoelectric material. Zinc Oxide is an excellent Ultraviolet absorber and antibacterial agent. ZnO has been used in solar cells, transparent electrodes, and blue/ UV light emitting devices. This Nanomaterial is a promising candidates for nano-electronic and photonics [1,2]. It may also be possible to observe coherent emission without population inversion induced by Bose- Einstein condensation at the lower polariton band [3]. ZnO has higher exciton binding energy (60 meV), is more resistant to radiation and is multifunctional with uses in the areas as piezoelectric, ferroelectric and ferromagnetic [4].

 

ZnO: Ce is hexagonal in symmetry with Space group P6_3mc and Lattice parameters [2] a = 3.253 Å, b = 3.253 Å, c = 5.209 Å . The stable structure of ZnO is wurtzite [1]. A substantial effort is therefore placed on utilizing such properties within a new generation of short wavelength photonic devices [5]^. Hence, it is interesting to synthesize nanoparticles of ZnO doped with Ce and characterize them and subject them to elastic studies. Nanofluids have been of great interest due to their broad applications in different fields [10]. The anomalous behavior of ultrasonic velocity in sintered ZnO provides information about the pore size, pore shape and their distribution [11]. Very few nanofluids and magnetic fluids are found to be characterized using ultrasonic technique [10,11] and no work has been reported for determining the ultrasonic properties in Ce doped ZnO nanofluid. The ultrasonic velocity measurements are performed for ultrasonic characterization of the prepared fluid.

 

Aim of this investigation is to measure Ultrasonic velocity through doped and undoped sample and certain important physical parameters such as adiabatic compressibility, specific acoustic impedance, relative association, intermolecular free length, Rao’s constant, Wada’s constant etc. and study their variations with temperature and grain size.

 

2 EXPERIMENTAL:

Synthesis and characterization

ZnO nanoparticles are prepared by chemical precipitation route using 1 molar solution of Zinc acetate solution and Sodium hydroxide solution at 60^oC. The Ceric oxide solution is also mixed with the above solutions using magnetic stirrer. The precipitate obtained is washed with double distilled water many times and acetone and filtered well.  The precipitate is placed in the furnace at 150 and 180˚C. The samples are powdered well in the crucible. The orientation and crystallinity of the powder were studied using Rigaku DMAX diffractometer using Cu-Kα radiation monochromatised with a graphite crystal and high temperature attachment in θ-2θ geometry. The high temperature stage allows samples to be measured at tightly controlled temperatures from room temperature to 200^oC in open air, under vacuum or in a purge gas. The surface topography and microstructure were studied using Field Emission Scanning Electron Microscopy (FESEM). FTIR spectroscopy uses Michelson interferometer to produce an interferogram. Energy Dispersive X-ray Spectrum Analysis (EDX) was used to determine percentage composition of Ce in ZnO.

 

In this investigation, Ultra sound technique was used to measure velocity of the sound through the ZnO: Ce nanofluid. Speed of sound was measured by using a variable path, single crystal interferometer. (Mittal Enterprises,  New Delhi). The interferometer cell was filled with the test liquid, and water was circulated around the measuring cell from a thermostat. Ultrasonic velocity was measured with an accuracy ± 0.1% and error of measurement ±0.5◦C at 303K. The various acoustic parameters are evaluated using ultrasonic velocity, compressibility and density.

 

3  RESULTS AND DISCUSSIONS:

A. Determination of particle Size from XRD Pattern.

The XRD pattern, Fig. 1, consists of sharp intense peaks of ZnO which confirms the good crystalline nature of ZnO and peaks originated from (100), (002), (101), (102), (110), (103), (112), (201), (200) and (202) reflections of hexagonal ZnO [4]. The XRD techniques are widely used for the particle size determination and structure determination of nanoparticles. The patterns are compared with JCPDS file No: 80-0075 comparing the observed data with the JCPDS file. The degree of crystallinity of nanoparticles increases with annealing temperature. The percentage of lattice contraction with annealing temperature can also be studied using X-ray diffraction pattern. Particle Size, can be calculated by the formula [4, 7], Debye- Scherrer’s formula

                                                                                                                                 (1)

K=0.89, λ the X-ray wavelength = 0.154095 nm, β the full width at half maximum and θ the half diffraction angle. The crystal sizes of ZnO:Ce nano particles synthesized at 150 and 180^oC were calculated from FWHM and tabulated in Table 1.

 

TABLE 1.Variation of Grain size of ZnO:Ce  with temperature

Temperature oC	FWHM	β×103	2θ	θ	Particle size(D) nm
150	0.653	11.39	56.64	28.32	13.83
180	0.560	9.768	56.515	28.257	16.13

 

 

(a

Fig.1 (a-b) XRD patterns of ZnO:Ce nanoparticles at 150 and 180^oC.

 

 

 

From the Fig.1 (a-b) it was found that the crystallite size of ZnO nanopowder was 13.83 nm at 150˚C which increased to 16.13 nm at  temperatures 180˚C. From the above study, it is observed that there is a continuous increase  in the particle size with temperature. According to Ostwald ripening the increase in the particle size is due to the merging of the smaller particles into larger ones as suggested by Nanda et al. [8] and is a result of potential energy difference between small and large particles and can occur through solid state diffusion.

 

B. Scanning Electron Microscopy (SEM)

The Scanning Electron Micrographs of ZnO:Ce  nanomaterials synthesized under aqueous medium was carried out . The orientation growth of ZnO crystal in water is higher [6]. Spherical shaped morphology is observed in the micrograph of ZnO:Ce. The SEM pictures, Fig.3, showed distinguished spherical morphology with self aligned prismatic nanoparticles. The morphology of ZnO nanopowder as revealed by FESEM showed nanoparticle of size 13-100 nm

C.  Energy Dispersive X ray spectrum Analysis (EDAX)

This technique is used for identifying the elemental composition of the specimen. The higher a peak in a spectrum, the more concentrated the element is in the spectrum. An EDAX spectrum plot not only identifies the element corresponding to each of its peaks, but the type of X-ray to which it corresponds as well. For example, a peak corresponding to the amount of energy possessed by X-rays emitted by an electron in the L-shell going down to the K-shell is identified as a K-Alpha peak. The peak corresponding to X-rays emitted by M-shell electrons going to the K-shell is identified as a K-Beta peak. Electron Dispersive Spectrum, see Fig.3, of ZnO: Ce nanoparticle is plotted using the recorder and the EDAX data is found as Zinc oxide with 99.4% and Cerium 0.6%.

 

 

Fig.2 SEM image of ZnO:Ce Nanoparticle under high magnification

 

 

Fig 3 EDAX of ZnO: Ce nanoparticle

 

 

D. Fourier Transform Infra-red Spectrum Analysis

FTIR spectroscopy uses Michelson interferometer to produce an interferogram and the spectrum of ZnO:Ce  is shown in the [ Fig.4]. In the case of nanoparticles, due to imperfection in the structural arrangement of atoms on the surface layer, the bond length or inter atomic distance may change producing a change in atomic vibration and hence a change in infrared absorption frequency.

.  

Fig.4. Fourier Transform Infrared Spectrum of ZnO:Ce nanoparticle

It has been shown that as particle size decreases, increase in frequency for the bond (blue shift) is observed in nanoparticles when compared with undoped ZnO [15]. Bands at 416.35 cm^-1 is assigned to the stretching vibrations of Zn-O. The stretching frequency of bulk ZnO is 424 cm^-1.  Here a blue shift is observed in that frequency i.e., that frequency due to quantum confinement. Three intense bands were centered at 1384.34 cm^-1, 1041.54 cm^-1  and 1556.58  cm^-1  and are attributed to the stretching vibrations of C = O, C = C and C-H groups in acetate species, which suggests its presents as absorbed species in the surface of nanoparticles. The broad absorption peak centered at 3423.61 cm^-1 and 1626.40 cm^-1 corresponds to O-H stretching and bending frequencies of H₂O, indicating the existence of water in the surface of nanoparticles. The peak observed at 825.80 cm^-1 may be the presence of some impurities in the Zinc Acetate or Zinc Oxide nanoparticle

 

E. Transmission Electron Microcopy of ZnO: Ce nanoparticles

The morphology and particle size of ZnO: Ce sample synthesized at 60^oC were examined by using TEM as shown in fig.5.As can be seen, the particle display rod like form basically. The particle size determined from TEM varies between 13 and 50nm. The TEM showed that the particles have nanometric prism like and rod like morphologies. The cross-sections of one dimensional (1D) nanostructure include triangle, hexagonal and rectangle shape.

 

 

Fig.5 Transmission Electron Microscopic images of ZnO: Ce nanoparticle

F. Measurement of ultrasonic velocity

The ultrasonic velocity is measured using ultrasonic interferometer (Mittel enterprises, F-80 model) at a standard frequency of 2 MHz with an accuracy of 1m/s. The principle used in the measurement of velocity [10] is based on the accurate determination of the wave length in the medium. Ultrasonic waves of known frequency are produced by a quartz plate fixed at the bottom of the cell. The waves are replaced by a movable metallic plate kept parallel to the quartz plate. If the separation between these plates is exactly a whole multiple of the sound wavelength, standing waves are formed in the medium. The acoustic resonance gives rise to an electrical reaction on the generator driving the quartz plate and the anode current of the generator becomes maximum.. The anode current again becomes maximum when the distance is increased or decreased and the variation exactly one half wavelengths or multiple of it, from the knowledge of wavelength the velocity of ultrasonic wave can be obtained by the relation

 

Hence, the enhancement in velocity by the dispersion of nanoparticles is due to change in density, compressibility, bulk modulus of the nanoparticle suspension with volume fraction. The velocities of the present nanofluids are found greater than the velocity in pure matrix state, which indicates that by the dispersion of solid particles, there is a change in density and bulk modulus.   2gm of doped and undoped ZnO is dissolved in 5ml of Ethylene Glycol and taken in the cell. The velocity of ultrasonic waves of frequency 2MHz is found out. The Compressibility and Wada’s constant of ZnO increases on doping with Ce. The result indicate that ZnO:Ce material are easy to deform but difficult to compress. The elasticity of ZnO increases with increase in mole percentage of CeO₂. In order to explain the high value of elastic constant in ZnO doped with Ce³⁺, the chain entanglement mechanism in ZnO with the structural modification by alkaline earth ions was used[9].

 

An increase in bulk modulus, Table 3, or decrease in compressibility is attributed to the fact that strong cohesive interaction forces act among the molecules and atoms after the dispersion of ZnO nanoparticles in the ethylene glycol. The variation of Rao’s constant with temperature is as shown in Table 4.The rise of temperature and particle size increases Rao’s constant. The molecules of liquid are not closely packed and as such there is always some free space between them. The Wada’s constant changes with temperature are shown in Table 3. The constant decreases with rise of temperature and particle size. This shows that solute solvent molecules are coming close to each other and the space between them is decreasing with rise in temperature. This supports to the strong solute–solvent interaction in liquid solution [11]. The Wada’s constant increases with Ce³⁺ impurity doping as depicted in Table 2. It is a measure of change in internal energy of liquid solution as it undergoes a very small isothermal change. It is a measure of cohesive or binding forces between the solute and solvent interaction [12].

 

 

 

TABLE 2. Compressibility and Wada’s constant, Relative Association   of Cerium doped and undoped ZnO Nanofluids

Fluid	Reading	λ /2	Mean λ	Velocity v=f λ	Compressibility (Å)	Wada’s const. (m3/mole)(10-4)	Relative Association
Ethylene Glycol	12.208 12.116 12.028 11.936	0.092 0.088 0.092	0.181	362	67.3	8.19
ZnO:Ce nanofluid	12.5 12.412 12.328 12.238 12.15 12.066 11.978 11.891	0.088 0.084 0.09 0.088 0.084 0.088 0.087	0.174	348	14.72	2.653	5.103
Undoped ZnO nanofluid	16.915 16.841 16.76 16.67	0.084 0.090 0.081	0.17	340	15.43	2.617	5.17

 

TABLE 3.Variation of Compressibility, Wada’s constant, Bulk modulus of ZnO:Ce nanofluid with particle size and temperature

Temperature (oC)	Particle size(D) (Nm)	Ultrasonic  velocity (m/s)	Compressibility (Ao)	Wada’s constant *10-4 m3/mole	Bulk Modulus (1010)m
150	13.53	340	15.43	2.617	0.0648
180	16.13	350.9	14.38	2.643	0.0695

 

TABLE 4. Variation of Rao’s constant, Acoustic impedance, free length with particle size

Temp (oC)	Particle size (nm)	Rao’s constant   *(10-4) (m3/mole)(m/s)1/3	Acoustic impedance  Z = V × ρ 104(kg/m2/sec	Free length (Ao)
150	13.53	1.006	191.9	0.808
180	16.13	1.01	198.0	0.780

 

 

The variations of acoustic impedance with temperature and particle size are depicted in the Table 4. Acoustic specific impedance Z increases with increase in temperature and particle size which  indicates that there is strong interaction between solute and solvent[10].Intermolecular free length L_f decreases with increasing particle size, Table 4,   shows that there is enhanced molecular association which is confirmed by values of viscosity which increases with concentration [11]. Relative Association R_a of the samples, Table 2, increases on doping the sample with Ce³⁺. Relative association is the measure of extent of association of the component in the mixture. The value of relative association increases with doping indicating strong interionic interaction [12].

 

4. CONCLUSIONS:

The structure and composition of Ce doped ZnO nanoparticles were determined by XRD, FTIR, SEM, TEM and EDAX spectra analyses. Ce doped ZnO nanoparticles have been prepared by chemical co-precipitation method. The XRD results indicated that the particle size of nano ZnO: Ce is much small as compared to that of pure  nano ZnO and decreases with the Cerium loading. From the XRD results, it is clear that as temperature increases, particle size also increases. Absorption peaks in the FTIR spectrum of ZnO:Ce with different particle size were explained. TEM images confirmed the nano size and morphology of the particle. EDAX analysis has given percentage of ZnO and Ce³⁺ in the sample. Ultrasonic velocity through doped and undoped sample is measured and compressibility is computed. The compressibility is found to be increased. Also variation of compressibility of ZnO: Ce nanofluid with various grain sizes have been carried out and found that compressibility increases with decrease of particle size. To study the elastic properties of the nano fluid certain important physical parameters such as adiabatic compressibility, specific acoustic impedance, relative association, intermolecular free length, Rao’s constant, Wada’s constant etc. are evaluated using ultrasonic velocity and density. The variations in these Acoustic parameters with grain size and temperature are reported.

 

5. ACKNOWLEDGEMENT:

We express our sincere gratitude to the SPAP, Mahatma Gandhi University, Kottayam, Kerala for technical support.

 

 

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Received on 16.09.2014         Modified on 21.10.2014

Accepted on 27.12.2014         © AJRC All right reserved