Structural and dielectric properties of CuO nanoparticles
Introduction
Due to their potential possibilities in various fields, the production and the characterization of p-type nanocrystalline semiconductors have attracted considerable attention. Among transition metal oxide semiconductors, cupric oxide (CuO) as narrow band gap (1.2–1.5 eV) [1] semiconductor has attracted particular attention [2], [3]. CuO has unique features and finds applications in the fields such as photoelectrochemical cells, gas sensors, solar cells, biosensors and nano devices for catalysis [4], [5], [6], [7], [8], [9], [10]. Additionally, it has been reported in recent studies that CuO has also promising possibilities in microelectronics because of its giant dielectric constant [11], [12]. It is well documented in the literature that the formation, the particle size and the dielectric behaviour of CuO strongly depend on the preparation conditions [13]. Therefore, various methods have been employed to synthesize nanostructured CuO, such as sol–gel and sonochemical method [14], thermal decomposition method [3], [15], precipitation method [16] and so on.
Structural characterization of CuO nanoparticles prepared by microwave irradiation of copper (II) acetate and sodium hydroxide as starting materials was investigated by Zhu et al., [17]. It was reported that the CuO nanoparticles have regular shape, narrow size distribution and high purity. Structural, optical and electronic properties of CuO nanoparticles with a monoclinic structure were studied by Kim et al. [18]. The O 1s and the Cu 2p peaks corresponding to the CuO nanoparticles were observed from X-ray photoelectron spectroscopy profiles. The room temperature optical band gap of CuO nanoparticles was found to be 3.63 eV. The dielectric response of high purity CuO with two different grain sizes has been investigated by Maensiri et al. [19]. These authors reported that the grain size has strong influence on the dielectric behaviour of the CuO. Though studies on structural and optical properties in CuO nanoparticles have received considerable attention of researchers, limited work has yet been performed on the electrical characterization, most of this being confined to CuO thin films [20], [21]. Recently, the electrical behaviour of copper oxide CuO ceramics sintered at 920 °C has been characterized by a combination of fixed, radio frequency capacitance measurements and impedance spectroscopy technique [22]. These authors concluded that the temperature and frequency dependent high permittivities in CuO ceramics are dominated by an electrode artifact. Structural and multiferroic properties of poly crystalline CuO samples with Co doping have been investigated by Venimadhav et al. [23]. Multiferroic nature is confirmed in the pristine poly crystalline sample with frequency independent peaks at magnetic ordering temperatures and very low dielectric permittivity at low temperatures. Magnetic and dielectric properties of CuO nanocrystalline powders have been investigated and the observed ferromagnetic and colossal dielectric constant in these nanoparticles attributed to the presence of Cu3+ in the CuO core and oxygen vacancies in the surface of nanoparticles [24]. One of the well-known powerful methods to characterize the materials and their interfaces is impedance spectroscopy technique. Impedance spectroscopy technique permits a reasonable interpretation of the observed spectra and identification of the contributions from bulk conduction, transport across electrode-sample interface and grain boundary conduction [25].
In the present work, we focused on the temperature dependence of the DC and AC conduction behaviour and dielectric relaxation phenomena in nanocrystalline CuO pellet. The origins of the observed frequency dependence of the conductivity and the giant dielectric constant have also been discussed in terms of existing theory.
Section snippets
Experimental
Analytical grade chemicals were used without employing purification process. A simple hydrolysis method was used to produce the CuO nanoparticles. The Route for the synthesis of the CuO nanoparticles was described below: At first 3.41g copper (II) chloride dehydrate (CuCl2·2H2O) was dissolved in doubly distilled water to obtain 0.2 M copper chloride solution. Then, 10 ml of 0.2 M thiourea solution was prepared separately in deionized water and was stirred for 4 h at room temperature. After 4 h, 5 ml
Structural analysis
The formation of the single-phase of monoclinic CuO under reported conditions were confirmed by XRD studies (Fig. 1). As can be seen from Fig. 1, the obtained XRD pattern consists of eleven diffraction peaks at 2θ=32.5°, 35.5°, 36.8°, 48.6°, 53.4°, 58.3°, 61.8°, 66.4°, 68.1°, 72.3°, 75.2°. All peaks in the XRD patterns of the CuO are consistent with the JCPDS (00–048–1548) data of the copper oxide with a monoclinic phase (tenorite), verifying a pure phase of CuO products.
The mean grain size was
Conclusions
CuO particles have been prepared successfully by a simple hydrolysis method. XRD analysis showed that a single-phase compound was formed exhibiting monoclinic crystalline phase. The DC and AC electrical transport properties of the CuO in the pellet form have been investigated. From the analysis of the DC results, two different activation energies were found above and below 390 K, which confirm the presence of two different conduction mechanisms. The predominant conduction mechanism in the CuO
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