Studies on Pure and Divalent Metal Doped Copper Oxide Nanoparticles

Pure and Ni 2+ doped Copper oxide (CuO) nanoparticles were synthesized by simple solvothermal method. Effect of concentration of the dopant (2.5, 5 7.5 and 10 mole%) on the properties of CuO was analyzed from X-ray diffraction pattern (XRD), SEM, EDAX, UV-VIS, electrical measurements and Photoluminescence spectroscopy. The XRD results clearly indicates that the samples are polycrystalline in nature belongs to hexagonal crystal sturcture and also due to the addition of dopants the average grain size various from 25.14 – 17.5nm. The PL results showed that Ni 2+ dopant did not give rise to a new PL signal, but it could improve the intensity of PL spectra with a appropriate Ni 2+ content, which was possibly attributed to the increase in the content of surface oxygen vacancies and defects after doping Ni 2+ .


Introduction
The oxides of transition metals are an important class of semiconductors, which have applications in magnetic storage media, solar energy transformation, electronics and catal-

V. Ponnarasan and A. Krishnan
ysis [1][2][3][4][5][6][7][8][9].Among the oxides of transition metals, copper oxide nano particles are of special interest because of their efficiency as nanofluids in heat transfer application [10].CuO is a semiconducting compound with a narrow band gap and used for photoconductive and photothermal applications [11].However, the reports on the preparation and characterization of nanocrystalline CuO are relatively few to some other transition metal oxides such as zinc oxide, titanium dioxide, tin dioxide and iron oxide.Also when copper was added as the dopant in other transistion oxide nano particles, it enhances the optical and magnetic properties of the host material [ 12].Keeping this in view, in our present study , we made an attempt to prepare copper oxide nanopartilce by simple, cost effective , solvothermal method using microwave irradiation.Ni 2+ was added as dopant in 3 different mole % viz 2.5 , 5 and 7.5 .The influence of concentration of Ni 2+ on the structural, optical, and electrical properties of CuO nano crystals was reported herein.

Materials and Methods
Analytical reagent (AR) grade copper chloride (CuCl 2 ), urea (NH 2 CONH 2 ) and Nickel chloride (NiCl 2 ) were purchased from Merck Chemicals.These compounds were used without further purification for the preparation of CuO nanoparticles.Copper chloride is mixed with urea in 1:1 molecular ratio.The fine mixture was then dissolved in 100 ml ethylene glycol with vigorous stirring for 60 minute at room temperature and kept in a domestic microwave oven (operated with frequency 2.45 GHz and power 800 W).Microwave irradiation was carried out till the solvent evaporates completely.The colloidal precipitate obtained at the end was cooled to room temperature naturally and washed several times with doubly distilled water and then with acetone to remove the impurities.The sample was then filtered and dried in atmospheric air and collected as the yield.Similarly 2.5 , 5 , 7.5 and 10 mole % NiCl 2 was added to above mixture separately for the preparation of Ni 2+ doped CuO nanoparticles.
Pure and doped CuO nanoparticles were subjected to PXRD analysis.X-ray powder diffraction patterns were obtained using an automated PANalytical X-ray powder diffractometer with monochromated CuK α radiation (λ=1.540598AU).The particle size analysis of the as-synthesized materials has been carried out using Scherrer formula.Energy dispersive X-ray analysis (EDAX) was used to estimate the composition of the materials using a Jeol make JSM 5600 LV Model attached with Energy Dispersive Spectrometer of EDAX inc.USA.Optical absorption measurements were done at room temperature using a SHIMADZU UV-2400 PC spectrometer with a medium scan speed sampling interval 0.5 in the wavelength range 200-700 nm.Photoluminescence spectrophotometry was measured on a Perkin-Elmer LS 55 spectrophotometer.The capacitance (C) and dielectric loss factor (tanδ) measurements were carried out to an accuracy of ± 1 % with Agilant 4284A LCR meter in the temperature range of 40 -150 ºC.

Powder XRD Measurement
In order to confirm the material of the grown crystals and to determine the particle size, powder X-ray diffraction (PXRD) data were collected for all the four nanocrystals prepared using an automatedX-ray powder diffractometer ( PANalytical ) in the 2θ range of 10-70° with CuK α radiation (λ = 1.54056Å).Using the observed 2θ (Bragg angle) and d (interplanar spacing), all the reflections were indexed.The PXRD patterns of pure and doped samples were shown in figure 2. The data were indexed following the procedures of Lipson and Steeple .Pure and Ni 2+ doped CuO nanocrystals belongs to the hexagonal lattice system.No peaks of impurities are found in XRD pattern.The observed PXRD data were indexed by matching with the data available for CuO in the literature (JCPDS file).The XRD data conforms that addition of dopant in CuO lattice doesn't alter its lattice, but it produces slight shift in diffraction peaks, Also the intensity of peaks get suppressed due to dopant addition, which conforms the incorporation of dopant in host lattice.Calculated crystallite size of the prepared samples were given in Table 1, which revealed that, as the concentration of dopant increases, particles size decreases, this may be due to replacement of Cu 2+ ions by Ni 2+ ions.

Sample name
Crystallite size (nm)

SEM and EDAX Analysis
Figure 2 shows the SEM image of as prepared pure and doped CuO nanoparticles.It shows that the CuO nanoparticles are cubic in shape.As the concentration of Ni 2+ decreases the size of the cubes, and at the maximum concentration of Ni 2+ , morphology of the synthesized particle changes drastically into flower like arrangement , this may due to variation in surface atom density of the dopant.The presence of dopant was confirmed by EDAX spectra shown in Figure 3.

Electrical Properties
When the nanostructure material provides a different band structure for the same concentration, one may expect pronounced electric property variation due to changes in the forbidden energy gap [12].Keeping this in view and also the fact that we have synthesized our materials with very small crystallite sizes (<1.5 nm) using microwave heating technique, we have subjected all the 5 systems considered to DC electrical measurements by using the two probe technique.The observed DC electrical conductivities of nanostructured pure and doped CuO was shown in Figure 6.The DC conductivity over a temperature range 40 to 150ºC is minimum for pure CuO.When Ni replaces Cu sites the conducting ability of the materials seems to have raised, which is maximum for 10 mole % Ni doped CuO.It is generally accepted that smaller the particle size higher the lattice defects.There are reports suggesting that lattice defects form acceptor-or donor-like levels in the forbidden energy gap and act as trapping centers for charge carriers which affect the electrical behaviour [14].

Conclusion
Pure and Ni 2+ doped ( with different concentration viz.,2.5, 5, 7.5 and 10 mole %) CuO nanocrystals were synthesized by simple solvothermal method.The as synthesis materials were conformed by EDAX and PXRD analysis.Nano regimes of the samples were analyzed by SEM and scherrer formula.The results of structural studies revealed that, Ni 2+ addition decreases the crystallite size with it's increasing concentration.This may be due to replacement of Cu 2+ ion by Ni 2+ ions, which has lower ionic radius.Reduction in particle size leads to strong quantum confinement.Also PL results showed that Ni 2+ additions

Figure 1 :
Figure 1: PXRD patterns of pure and doped CuO

Figure 5 :
Figure 5: Dc conductivity of pure and Ni doped CuO nano particles

Figure 6 :
Figure 6: Variation of Dielectric constant with log frequency for various temperature for pure and doped CuO