Synthesis, characterization, magnetic measurements and liquefied petroleum gas sensing properties of nanostructured cobalt ferrite and ferric oxide

https://doi.org/10.1016/j.mssp.2014.02.048Get rights and content

Highlights

  • Synthesis of a series of nanostructured cobalt ferrite system.

  • Minimum crystallite size of cobalt ferrite nanoparticles was 7 nm.

  • Surface morphologies and LPG sensing properties depend on ferrite composition.

  • The magnetic behavior was characterized by magnetic measurements.

  • The maximum sensitivity of cobalt ferrite film sensor was 2.0 MΩ/s.

Abstract

In the present paper, a series of nanostructured cobalt ferrite systems was synthesized in different compositions via chemical co-precipitation method. The X-ray diffraction analysis of cobalt ferrite systems confirmed the formation of its nanoparticles having minimum crystallite size 7 nm. The surface morphologies of the cobalt ferrite illustrate the distribution of partially agglomerated spherical nanoparticles having particle size ~12 nm. The magnetic behaviors of the synthesized materials were characterized by magnetic measurements. Liquefied petroleum gas sensing investigations of the fabricated pellets illustrate that the cobalt ferrite synthesized in 1:1 M ratio possesses an improved response in comparison to other compositions. The maximum sensitivity of cobalt ferrite film sensor was 2.0 MΩ/s. The response and recovery times were ~30 and 60 s, respectively. The sensor was 95% reproducible after three months of fabrication of the film, showing the stability of the fabricated sensor.

Introduction

Liquefied petroleum gas is an inflammable gas, which presents many hazards to human being as well as environment. Therefore, LPG sensor has become the recent topic of research in view of industrial applications [1], [2], [3], [4], [5]. We have been interested in carrying out our investigations with a new material that possess good sensitivity for the LPG, with properties that are stable over time and thermal cycling after exposure to the various species likely to be present in the ambient. Metal-oxide nanoparticles have been the subject of much interest because of their unusual optical, electronic and magnetic properties, which often differ from the bulk due to higher surface to volume ratio [6], [7], [8], [9], [10], [11], [12]. In particular, ferric oxide is considered to be the most promising sensing materials of sensors due to the temperature dependent surface morphology [13]. Fe2O3 does not require costly noble metal catalyst to perform as a good sensor. To achieve some specificity, the sensors can be impregnated with dopants or the working temperature can be changed so that the sensor׳s resistance changes when specific gases react with the adsorbed oxygen molecules [14], [15], [16], [17], [18], [19], [20], [21]. Nowadays, the most popular strategies employed to enhance the gas sensor response are: (a) control of surface morphology in order to increase the active surface area for the adsorption of gas and (b) use of additives which act as catalyst of the solid-gas reaction, by a chemical or electronic mechanism, thus, promoting the improvement of the sensor properties [22], [23], [24], [25].

In order to enhance the sensitivity, ferric oxide was chemically modified and mixed oxides were formed. Inclusion of Co2+ (which is a transition metal ion) in ferric oxide change the properties of the base material. The additives formed the binary compounds which will influence the size and surface morphology of the ferric oxide and hence modify the properties of that. Therefore, they are able to change the sensitivity, selectivity and response of ferric oxide based gas sensor. CoO can modify the intrinsic physical properties of Fe2O3, such as (a) the electrical transport properties by introduction of new states in the band structure of Fe2O3; (b) the surface morphology, which has a vital role in the chemical reactions between oxide and gas; and (c) the grain size distribution which contributes in determining the electrical resistance of the material.

Cobalt ferrite has an inverse spinel structure in which, in the ideal state, all Co2+ ions are in octahedral sites, and Fe3+ ions are equally distributed between tetrahedral and octahedral sites. Ferrites show very good surface reactivity and they have temperature dependent surface morphology. They are an alternative for inexpensive and robust detection systems because of good chemical and thermal stability under operating conditions [22], [23], [24]. They also show remarkable catalytic properties in oxidation reactions due to the high oxygen ion mobility at the film surface and thus are highly interesting for the development of sensors [22], [23], [24]. Therefore, in the present investigation a special attention is focusing on CoFe2O4 system in order to obtain reliable gas sensor operable at room temperature.

The gas sensing properties of the ferrites are dependent on its chemical composition and nanostructural characteristics, which can be controlled in the synthesis and fabrication processes [26], [27], [28], [29], [30], [31]. In order to acquire materials with the desired physical and chemical properties, the preparation of cobalt ferrite nanoparticles through different routes has become an important area of research. For the gas sensing studies, there is a need for developing synthesis and fabrication processes that are relatively simple and yield controlled particle sizes. Cobalt ferrite nanoparticles have been synthesized using various methods, such as ball milling, co-precipitation, reverse micelles, hydrothermal methods, sol–gel, micro emulsions, laser ablation, sonochemical approaches and aerosol method. Most of these method yielded nanoparticles of the required sizes with relevant surface structures but they are difficult to apply on larger scales. As these procedures of synthesis require a lot of money, high reaction temperature, lengthy reaction period and their potential harm to the environment; we require to developing an easy and facile synthesis procedure. Therefore, in the present investigation, cobalt ferrite system was synthesized using an aqueous solution containing ferric chloride, cobaltous acetate, poly-ethylene glycol and deionized water using a chemical co-precipitation method. This method does not require the addition of any other chemicals to the solution, and it has the advantages of simplicity, a low cost, a lack of by-product effluents, and an environment friendly operation. This method produces nanoparticles that are spherical with narrow size distribution which is an important parameter for their applications as sensors. In order to investigate the effect of microstructure of cobalt ferrite system on its LPG sensing properties, a series of cobalt ferrite materials was synthesized by varying the ratio between the cobalt and ferric chlorides precursors.

Section snippets

Synthesis of materials

All reagents such as ferric chloride, cobaltous acetate and ammonium hydroxide were of analytical grade and used without further purification. The stoichiometric amount of starting materials, such as cobaltous acetate and ferric chloride were taken in 1:1, 1:2, 1:3 and 1:4 M ratios, respectively, and dissolved into required amount of distilled water to form 1 M solution. First of all, we have taken cobaltous acetate and ferric chloride in 1:1 M ratios and dissolved into respective required amount

Surface morphological and energy-dispersive X-ray analysis

Fig. 1(a)–(d) shows the surface morphologies of cobalt ferrite system prepared in 1:4, 1:3, 1:2 and 1:1 M ratios, respectively. It is clearly seen that grains of the cobalt ferrite are at nanoscale and has a number of pores. Maximum number of pores (active sites) are observed in Fig. 1(d). The pores serve as gas adsorption sites wherein the reaction of LPG with adsorbed oxygen takes place. The surface of a solid pellet can be considered as the outermost layer of atoms plus the region between 0.5

Conclusion

In this paper, we have described the synthesis of cobalt ferrite system and characterized them for structural, optical, magnetic and surface morphological properties. The XRD of the material demonstrate spinel ferrite having cubic symmetry. The effect of the composition of CoFe2O4 on the surface morphologies and LPG sensing properties were investigated. CoFe2O4 in 1:1 M ratios (P-4) showed improved sensing response in comparison to other compositions. The maximum sensitivity of cobalt ferrite

Prime novelty statement

A series of cobalt ferrite materials was synthesized by varying the ratio of the cobalt and ferric chloride precursors which exhibits significant variations in LPG sensing properties. These cobalt ferrite systems show advancement in structural, optical and surface morphologies. The effect of these properties on sensor response was studied and the possible sensing mechanism has been discussed.

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. Corresponding author is grateful to DST-BRNS for providing the grant vide sanction number 2013/34/27/BRNS/2693.

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