Structural, optical and room-temperature ferromagnetic properties of Fe-doped CuO nanostructures

https://doi.org/10.1016/j.physe.2013.05.009Get rights and content

Highlights

  • Fe-doped CuO nanostructures were synthesized by a simple and green method.

  • The Fe-doping has obvious effect on the morphology of the pure CuO.

  • The optical property of the various nanostructures was studied.

  • Saturation magnetization increased with the increase of the Fe content.

Abstract

Pure CuO and Fe-doped CuO nanostructures with different weight ratios (0.5, 1.0, 1.5, and 2.0 at wt% of Fe) were synthesized via the microwave combustion method. The synthesized samples were characterized by X-ray diffraction (XRD), high resolution scanning electron microscopy (HR-SEM), diffuse reflectance spectroscopy (DRS), photoluminescence (PL) spectroscopy and vibrating sample magnetometry (VSM). XRD patterns refined by the Rietveld method indicated the formation of single-phase monoclinic structure and also confirmed that Fe ions successfully incorporated into CuO crystal lattice by occupying Cu ionic sites. Interestingly, the morphology was found to change considerably from nanoflowers to nano-rod and disk-shaped then to nanoparticles with the variation of Fe content. The optical band gap calculated using DRS was found to be 2.8 eV for pure CuO and increases up to 3.4 eV with increasing ‘Fe’ content. Photoluminescence measurements also confirm these results. The magnetic measurements indicated that the obtained nanostructures are found to be room temperature ferromagnetism (RTF) with an optimum value of saturation magnetization at 2.0 wt% of Fe-doped CuO, i.e. 1.2960×10−3 emu/g.

Graphical abstract

A simple and rapid microwave-assisted combustion method was developed to synthesize Fe-doped CuO nanostructures. Various morphologies were obtained with increase in Fe content.

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Introduction

The role of high frequency electromagnetic energy (microwave) to produce rapid heating along the chemical reactions has fascinated a significant attention, as a result of prominent applications in organic synthesis, materials science, biomaterial, biomedical, polymer processing, glass and mineral processing, and environmental applications [1], [2], [3]. A number of methods have been developed to synthesize CuO nanostructures, such as chemical method [4], hydrothermal process [5], thermal decomposition [6], reflux condensation [7], sonochemical synthesis [8] and so on. The above methods are known for high-energy consumption and require long reaction time. On the other hand, microwave-assisted combustion method is more favorable offering several advantages, primarily shorter processing time, energy efficiency, and the reagents are mixed at the molecular level during the combustion in microwave-oven, which provides better control of stoichiometry, various morphology, enhanced purity and homogeneity [1], [9].

Oxide-based diluted magnetic semiconductors (DMS) are gaining more attention currently, because of their potential for spintronics application [10]. Among the metal oxides, CuO is considered to be one of the most important p-type semiconductors with a narrow band gap of about 1.2–1.4 eV at room temperature. Moreover, Cu2+ ions are considered to play a significant role in the Cu-based high temperature superconductors [11], [12]. Recent investigation shows that the photon property of CuO plays an important role in room-temperature ferromagnetism (RTF). Also, ‘Cu’ has three oxidation states, Cu+, Cu2+, and Cu3+, because of which both hole and electron doping are possible [13], [14].

Generally, the metal doping makes radical changes in optical, electrical, and magnetic properties of CuO by altering its electronic structure. Transition metal doped CuO has recently attracted considerable attention for the potential application in spintronic devices because its Curie temperature is theoretically predicted to be well above the room temperature [15]. Many authors have reported the changes associated with doping of transition metal ions into CuO lattice, which have been observed experimentally in Mn [16], [17], Ni-doped CuO [18] Fe and Ni co-doped [19] and Ti, Cd and Zn [20]. Very few reports are available on ‘Fe’ doped CuO by different preparation methods like combustion synthesis [21], spray-deposited [14], co-precipitation [22], and hydrothermal [23]. Also, these methods are high energy consuming and longer reaction time compared to microwave combustion synthesis. Moreover, the magnetic properties of CuO are highly sensitive to the method of preparation and processing conditions.

Thus, the aim of the present study is to analyze the structure, microstructure, optical and magnetic properties of Fe-doped CuO nanostructures. Fe-doped CuO nanostructures were prepared by a solution phase method employing microwave combustion synthesis, with varying Fe-content of 0.5, 1.0, 1.5, and 2.0 wt%. Structure (XRD and Rietvled analysis) and microstructure (SEM) of the as-prepared Fe-doped CuO powders were investigated and correlated to the optical and magnetic properties.

Section snippets

Preparation of pure and Fe-doped CuO nanostructures

Pure and Fe-doped CuO nanostructures with 0, 0.5, 1.0, 1.5, and 2.0 wt% of Fe were synthesized by microwave combustion method using the precursors, copper nitrate, iron nitrate and urea as a fuel. All the materials used were of analar grade obtained from Merck, India and used without further purification. The constituents in the desired proportion were dissolved in deionized water and stirred for 15 min to obtain clear transparent solution. It is then poured into a silica crucible, and then

Structural investigation

The characteristic X-ray diffraction patterns of pure and Fe-doped CuO with varying Fe content were recorded in the range of 2θ between 30° and 80°. Fig. 1(a–e) show the typical diffraction peaks located at 2θ=32.52°, 35.55°, 38.73°, 48.76°, 53.41°, 58.31°, 61.57°, 66.22°, 68.14°, 72.41°, and 75.02° which correspond to [1 1 0], [−1 1 1], [1 1 1], [−2 0 2], [0 2 0], [2 0 2], [−1 1 3], [−3 1 1], [2 2 0], [3 1 1], and [0 0 4] planes respectively. The specific crystallographic planes confirmed that the formation of the

Conclusions

The present study demonstrates the successful synthesis of pure and Fe-doped CuO nanostructures via microwave combustion method. Rietveld refinements of X-ray diffraction patterns confirmed the formation of pure CuO monoclinic phase within Fe content up to 2.0 wt%, where Fe3+ replaces Cu2+ ions. The crystallite size varies in the range 19–28 nm. SEM analysis shows the formation of various morphologies flower, rod-disc-like, and particles-like nanostructures depending on Fe content. A clear blue

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