Synthesis, characterization, magnetic properties and gas sensing applications of ZnxCu1−xFe2O4 (0.0≤x≤0.8) nanocomposites

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Abstract

In the present work ZnxCu1−xFe2O4 nanocomposites were synthesized with a wide composition range (0.0≤x≤0.8 in the steps of x=0.2) using a sol–gel method and thin films were fabricated using a spin-coating process. The synthesized materials were analyzed using X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, atomic force microscopy, differential scanning calorimetry, UV–visible absorption and infrared spectroscopic techniques for their appropriate characterization. The XRD revealed the cubic spinel structure for each composition and the minimum crystallite size was found to be 10.4 nm. In order to study the thermodynamics during the synthesis, pre- and post-annealed materials were analyzed. An influence of the zinc on the magnetic properties was also investigated using a vibrating sample magnetometer. Further, the fabricated thin films were employed as liquefied petroleum gas (LPG) and CO2 gas sensors. Films show better sensing behavior towards LPG in comparison to CO2. Thus this study not only explored the influence of different compositions of ZnxCu1−xFe2O4 nanocomposites on their surface morphologies, crystallite sizes, porosities, specific surface areas, UV–visible absorptions and magnetic properties, but also the influence of different in situ compositions on their sensing behaviors.

Introduction

Nanostructured spinel ferrites (MFe2O4, M=Co, Ni, Mn, Mg, Zn, Cu, etc.) are interesting materials due to their potential applications in modern technology because of their electronic, magnetic, electrical and catalytic properties; all of these properties are completely different from those of their bulk materials [1], [2], [3], [4]. In addition spinel-type oxides are an alternative for inexpensive and robust detection systems because of their good chemical and thermal stability under operating conditions [5], [6]. The structure of the spinel ferrites can be described as a cubic, closely packed arrangement of oxygen atoms, and M2+ and Fe3+ ions can occupy either tetrahedral (A) or octahedral (B) sites. The sensing, magnetic and electrical properties of spinel ferrites are of academic and technical interest and are dependent on their chemical composition, cation distribution, grain size and the method of preparation [7], [8], [9], [10], [11], [12].

The origin of magnetic properties of the spinel oxides is spin magnetic moment of the unpaired 3d electrons of transition element occupied by the super exchange interaction via the oxygen ions separation [13], [14], [15]. The magnetic properties such as saturation magnetization and Curie temperature are strongly dependent on the distribution of cations and type of the doping atoms [16]. The Curie temperature, magnetic moment, electrical resistivity, and lattice constant are found to be affected by substitution in the spinel lattice and are due to the formation of secondary phase on the grain boundaries [17], [18]. The basis for wide range of applications of spinels is related to the variety of transition metal cations, which can be incorporated into the lattice of the parent magnetic structure [19]. Doping in ferrite nano-crystals with various metals, such as chromium, copper, manganese and zinc is usually used to improve their sensing, electrical or magnetic properties [20], [21], [22].

Various chemical precipitation techniques have been popularly adapted to synthesize the nanoparticles [23], [24], [25]. Among these synthesis methods, it is well conceived that chemical routes are more convenient and less expensive, they have general advantages such as superior uniformity and high yielding of nanoparticles [26], [27], [28]. In the present work, mixed ferrites have been synthesized by a sol–gel method to achieve chemically homogeneous and fine particles. This method of synthesis is economical for producing large quantity of small particles. Further it offers good chemical homogeneity, high purity and low sintering temperature and time.

A gas sensor is a device which receives a stimulus and converts it into a measurable electrical/optical signal. In many industrial applications, gas sensors are used to determine gas leaks in order to prevent any harm to human health as well as to protect from any explosions that such leaks may cause. The emphasis is currently being placed on the development of sensor materials for the detection of LPG that offers high sensitivity, short response and recovery times, superior reproducibility and stability. However, most of the sensors developed so far were kinetically slow with a limited sensitivity for the detection below the permissible level of LPG and operate above room temperature, although the detection of LPG at room temperature will be very helpful for the chemical industries and research laboratories. Thus, the fabrication of the gas-sensing materials with high performances remains a challenge. Gas sensors with high performances are considered those with high sensitivity and stability, fast response, low working temperature, and tunable parameters, as per requirements. Such performances should be associated not only with the surface properties of the sensing materials but also with their microstructures (such as specific surface area, crystallite size, pores size, porosity, film thickness, etc.). The sensing properties of the spinel ferrites vary greatly with change in their chemical compositions [29], [30], [31], [32]. Therefore, in the present investigation a series of ZnxCu1−xFe2O4 was synthesized with a wide compositional range of ‘x’ (0.0≤x≤0.8). Further the influence of different compositions on its structural, optical, magnetic and sensing properties has been investigated.

The objective of our work is to fabricate a LPG sensor having high sensitivity, small response and recovery times and good reproducibility and stability. The LPG sensing characteristics of ZnxCu1−xFe2O4 thin films were investigated at room temperature which are more reliable, stable and robust in comparison to earlier investigated LPG sensors [33], [34], [35], [36], [37], [38], [39]. Surface morphological, compositional and structural investigations were performed in order to understand the advancement in the gas sensing properties.

Section snippets

Synthesis and fabrication of films

ZnxCu1−xFe2O4 nanocomposites were synthesized via the sol–gel method using stoichiometric amounts of the starting materials such as cupric nitrate [Cu(NO3)2·3H2O], zinc sulfate [ZnSO4·7H2O] and ferric nitrate [Fe(NO3)3·9H2O] taken in (1−x):x:1 M ratios, respectively. All reagents were purchased from Qualigens, India, were of analytical grades and used without further purification. We have prepared five samples of ZnxCu1−xFe2O4 corresponding to x=0.0, 0.2, 0.4, 0.6, and 0.8, and these were

Structural analysis

X-ray diffraction is used to reveal the structure of the synthesized materials because of its qualitative and nondestructive analysis. The structural analysis is essential for optimizing the properties needed for various applications. Fig. 1(a) shows the XRD patterns of ZnxCu1−xFe2O4 (x=0.0, 0.2, 0.4, 0.6 and 0.8). The data were analyzed using JCPDS standards and confirmed the formation of cubic spinel structure. The obtained XRD patterns show the extent of crystallization. As the zinc content

Conclusion

Nanostructured ZnxCu1−xFe2O4 (0.0≤x≤0.8) was successfully synthesized by a sol–gel method. The process used here is convenient, environment friendly, inexpensive and efficient for the preparation. It is evident from XRD that the diffraction peaks become broader gradually with the increase of the Zn concentration in ferrite matrix. The crystallites were found in the range of 21.5–10.4 nm size. The increment in the band gap (blue-shifting) for different compositions in the present study is due to

Acknowledgment

Satyendra Singh acknowledges the financial support provided by the University Grants Commission-India under the U.G.C.- Dr. D.S. Kothari Postdoctoral Fellowship [F.4-2/2006 (BSR)/13-1037/2013 (BSR)]. B.C. Yadav is grateful to DST-SERC, Govt. of India, Delhi for financial support in the form of Fast Track Project (SR/FTP/PS-21/2009).

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