Elsevier

Applied Surface Science

Volume 258, Issue 22, 1 September 2012, Pages 8780-8789
Applied Surface Science

A comparative study on surface morphological investigations of ferric oxide for LPG and opto-electronic humidity sensors

https://doi.org/10.1016/j.apsusc.2012.05.091Get rights and content

Abstract

In the present work nanostructured ferric oxides were synthesized via hydroxide precipitation method without using any surfactant and size selection medium. The surface morphologies and structure of samples were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). The structural analysis confirmed the formation of Fe2O3 with α-phase and rhombohedral structure. Optical and thermal properties were investigated by using UV–visible absorption spectroscopy and differential scanning calorimetry (DSC) techniques. Pelletizations of materials were done using hydraulic press and these pellets were investigated with the exposition of liquefied petroleum gas. Variations in resistance of the pellet with time for different concentrations of LPG were recorded at room temperature (27 °C). The maximum value of average sensitivity was found ∼5 for 5 vol.% of LPG. Our results show that the LPG sensing behavior was inspired by the different kinds of surface morphologies of Fe2O3 and inferred that the spherical porous nanoparticles synthesized via hydroxide precipitation process (S-3) had best response to LPG.

Highlights

► Flower-like, elliptical and spherical shaped surface morphologies of Fe2O3. ► The structural and surface morphological investigations. ► The relationship between the surface morphology and sensing property. ► Advancement in sensitivity of LPG sensor in comparison to prior work.

Introduction

Over the past few years, the synthesis and functionalism of nanostructured ferric oxide have fascinated the researchers due to their significant potential applications [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. Ferric oxide has a wide range of applications as magnetic material [11], [12], [13], [14], [15], [16], but in recent years it has produced a large interest as a gas sensitive material because Fe2O3 does not require costly noble metal catalyst to perform as a good sensor [17], [18], [19], [20], [21], [22], [23], [24]. Ferric oxide is considered to be the most promising highly sensing materials of sensors due to the temperature dependent surface morphology and photo catalytic activity [25]. Developing new methods for the preparation of nanomaterials as well as the control of their structures, size and surface morphology have been intensively pursued not only for their fundamental scientific interest but also for many technological applications. Various shapes of iron oxide or hydroxide with controlled grain size are available and have been of great research interest. Consequently, ferric oxide nanostructures, such as nanowires, nanobelts, nanoflowers and other special nanostructures have been successfully synthesized using various approaches [26], [27], [28], [29], [30]. In applications for gas sensor devices, lower density and higher surface area are required. With this intention, several methods of preparation of ferric oxides are known. Co-precipitation, micro emulsion, pulsed wire discharge and hydrothermal processes are employed in order to obtain nanometer-sized powders [31], [32], [33], [34], [35]. Gas-phase processes such as thermal decomposition, chemical vapor deposition and thermal evaporation are favored, although these gas-phase methods generally require high temperature and expensive equipments, which may limit many potential applications whereas they have the advantage of producing high purity crystalline products in a single step [21], [22], [23], [24]. While among solution based methods, hydroxide precipitation approach is the most attractive, due to its perfect control of surface morphology, purity, crystallinity and has the advantage of soft condition and simple equipments, which is more suitable and economic for large-scale production of nanoparticles with controlled size and surface morphology. In order to investigate the influence of different surface morphologies of α-Fe2O3 nanoparticles on the gas sensing properties, we have synthesized α-Fe2O3 via hydroxide precipitation method using FeCl3·6H2O and FeSO4 as precursor materials.

The sensing mechanism of the reducing gases is based on the change in electrical resistance resulting from interaction between the gas molecule and adsorbed oxygen species on the surface of metal oxide [36], [37]. The state and amount of the oxygen adsorbed on the surface of materials are strongly dependent on the microstructure of the materials, namely, specific surface area, particle size as well as surface morphological structure. As the sensing phenomenon mainly takes place on the surface of sensing element, the surface morphology has an essential role on the sensitivity of the sensor. The gases are always adsorbed and even react at the surfaces. In addition to this, the sensitivity of the sensor depends on the method of synthesis of nanoparticles, with the efficiency of the chemical sensor increases as particle size decreases [38].

In the present investigation, we have reported flower-like, elliptical and spherical shaped surface morphologies of Fe2O3. Moreover, the as-prepared samples were fabricated into sensing pellets to measure their sensing performance toward liquefied petroleum gas at room temperature. The relationship between the surface morphology and sensing property was also investigated. The structural and surface morphological investigations have been carried out due to their significant role in electrical and gas sensing properties. Therefore, the objective of the present work is to study the evolution of surface morphologies of the pellets in order to understand the response of these to the LPG and to make a comparative assessment of these sensing elements. In addition, we have also investigated the opto-electronic humidity sensing properties of ferric oxide film fabricated on the base of an equilateral borosilicate glass prism.

Section snippets

Synthesis of material

All chemicals used in this work, such as ferric chloride hexa hydrate, ferrous sulfate and bases such as sodium hydroxide and ammonium hydroxide were of analytical reagent grade procured from Merck, India and used without any further purification. We have synthesized three samples of ferric oxide and these were labeled as S-1, S-2 and S-3.

For synthesis of S-1, ferric sulfate and ammonium hydroxide were used. Ferric sulfate solution was made by dissolving it into required amount of deionized

Surface morphological investigations

The surface morphologies of the synthesized materials S-1, S-2 and S-3 in form of pellets were analyzed using a scanning electron microscope (SEM, LEO Cambridge) and are visualized in Fig. 1(a–c) respectively. Fig. 1(a) shows that lump of Fe2O3 combining with each other form clusters leaving some spaces as pores. These pores serve as gas adsorption sites and gas sensitivity depends on the size of these pores. Usually the porous surfaces are preferred for gas-sensing because of a number of

Conclusion

In the present investigation we have successfully synthesized the ferric oxides of different surface morphologies. The employed hydroxide precipitation method of synthesis was very simple and high yielding without incorporation of any organic material. This method can be used for scale-up industrial production of ferric oxide. We have investigated the LPG sensing properties of the ferric oxides of different surface morphologies as a function of LPG concentration. A comparative exploration of

Acknowledgments

Corresponding author is grateful to Department of Science and Technology, Government of India for providing financial assistance in the form of SERC-FAST TRACK, Project SR/FTP/PS-21/2009. Mr. Satyendra Singh is thankful to Council of Scientific and Industrial Research, India for senior research fellowship vides award no. 09/107 (0331) 2008-EMR.

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