Nanocrystalline CuO films prepared by pyrolysis of Cu-arachidate LB multilayers

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Abstract

Nanocrystalline CuO films have been formed by the pyrolysis of copper arachidate LB multilayers. FTIR and UV–vis spectroscopic investigations showed that the pyrolysis was initiated at ∼100 °C and was complete at ∼300 °C. XPS and electron diffraction studies confirm the films to be single-phase copper (II) oxide. Analysis of the bright-field images showed that the film consists of uniformly distributed particles of size 2.5 ± 1.5 nm. The optical absorption edge of the CuO film did not show a measurable blue shift attributable to the nanocrystalline nature of the films. The absorption features are analysed in the background of the uncertainty prevailing in the nature and value of the absorption edge of CuO.

Introduction

Cupric Oxide (CuO) is a semiconductor that has been studied for photoconductive, photothermal and photoelectrochemical applications [1], [2], [3], [4]. It has also been known to be antiferromagnetic [5]. In recent years, it has attracted wide interest due to its monoclinic unit cell and square-planar coordination [6], which is similar to that in high Tc cuprate superconductors. Owing to the current interest in nanomaterials and their unique properties, the formation of nanocrystalline CuO has been achieved by rapid precipitation [7], spin coating [8], solid-state reaction [9], sonochemical reaction [10], sol–gel techniques [11], solvothermal route [12], electrochemical route [13] and controlled oxidation of copper nanoparticles within silica gels [14]. These studies reveal interesting size-dependent effects on the structure, optical, electrical and magnetic behaviour of nanocrystalline CuO.

Recently, very thin and homogenous metal oxide films have been obtained by the oxidation of a precursor fatty acid salt Langmuir–Blodgett (LB) multilayers [15], [16], [17], [18]. In most cases, the precursor LB multilayer is exposed to heat or UV radiation in air, resulting in oxide formation and removal of organic components. Using this method, very thin films of CuO could be obtained by UV exposure followed by heat treatment in air at 365 °C of a precursor copper arachidate LB multilayer [19]. It was also reported [19] that a direct heat treatment of the copper arachidate (CuA) multilayer resulted in a discontinuous film. Elastic recoil detection analysis was used to calculate the Cu to O ratio.

In view of the increasing interest in transition metal oxide films, in particular copper oxide, a controlled heat treatment of CuA LB multilayers in oxygen ambient has been carried out in the present work. The process of conversion of the arachidate multilayer into CuO was studied by X-ray diffraction, infrared spectroscopy and UV–vis spectroscopy. The results show that the oxidation process and the removal of organic components are completed at about 300 °C. The formation of a single-phase, nanocrystalline CuO film was confirmed by transmission electron microscopy and X-ray photoelectron spectroscopy studies.

Section snippets

Experimental

LB multilayers of copper arachidate were prepared by the conventional LB deposition technique using a KSV 3000 instrument in a clean room. Arachidic (Aldrich, 99%) with HPLC grade chloroform as solvent (1 mg/ml) was spread on an aqueous subphase containing CuCl2 (10−4 M). Deionized and ultra-filtered water (Millipore) having a resistivity of 18.2  cm was used to prepare the subphase. The subphase temperature was kept constant at 20 °C. The subphase pH of 10−4 M CuCl2 solution was found to be 6.1.

Results and discussion

The XR patterns in the 2θ range of 4–16° for the as-deposited and heat-treated CuA multilayers on quartz substrate are shown in Fig. 1. The XR pattern (a) for the as-deposited multilayer exhibits well-defined Bragg peaks corresponding to (0 0 l) reflections. The average bilayer period calculated from the peak positions was found to be 55 Å. Considering the total length of arachidate molecule to be ∼27.5 Å, the bilayer period of 55 Å indicates molecular packing with alkyl chains of copper arachidate

Conclusions

Single-phase copper (II) oxide thin films have been formed by the pyrolysis of copper arachidate LB multilayers. FTIR and UV–vis spectroscopic investigations showed that the oxidation and the removal of organic moieties were complete at ∼300 °C. TEM images showed the film to consist of a uniformly distributed particles of size 2.5 ± 1.5 nm. The absorption edge of the CuO films was found to be 1.6 eV and did not show any noticeable blue shift due to the nanocrystalline size of the films. This absence

Acknowledgments

The financial support for this work from the Department of Science and Technology, Government of India, is gratefully acknowledged. One of the authors Sukhvinder Singh is thankful to CSIR, New Delhi, for junior research fellowship.

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    Present address: Department of Physics, Tabriz University, Tabriz 51664, Iran.

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