Highly sensitive and selective LPG sensor based on α-Fe2O3 nanorods

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Abstract

The α-Fe2O3 nanorods were successfully synthesized without any templates by calcining the α-FeOOH precursor in air at 300 °C for 2 h and their LPG sensing characteristics were investigated. The α-FeOOH precursor was prepared through a simple and low cost wet chemical route at low temperature (40 °C) using FeSO4·7H2O and CH3COONa as starting materials. The formation of α-FeOOH precursor and its topotactic transformation to α-Fe2O3 upon calcination was confirmed by X-ray diffraction measurement (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) analysis. The α-Fe2O3 nanorods exhibited outstanding gas sensing characteristics such as, higher gas response (∼1746–50 ppm LPG at 300 °C), extremely rapid response (∼3–4 s), relatively slow recovery (∼8–9 min), excellent repeatability, good selectivity and lower operating temperature (∼300 °C). Furthermore, the α-Fe2O3 nanorods are able to detect up to 5 ppm for LPG with reasonable response (∼15) at the operating temperature of 300 °C and they can be reliably used to monitor the concentration of LPG over the range (5–60 ppm). The experimental results clearly demonstrate the potential of using the α-Fe2O3 nanorods as sensing material in the fabrication of LPG sensors. Plausible LP G sensing mechanism of the α-Fe2O3 nanorods is also discussed.

Introduction

Liquefied petroleum gas (LPG) is widely used as fuel for domestic heating and industrially to provide a clean source of energy for burning. It is a combustible gas which mainly consists of butane (70–80%), propane (5–10%) and propylene, butylene, ethylene and methane (1–5%) [1]. It is potentially hazardous due to the high possibility of explosion accidents caused by leakage or by human error. Hence, it is crucial to detect it in its early stages to give alarm and perform effective suppression [2]. This has stimulated considerable interest for scientific research to develop reliable, efficient, simple and cost-effective chemical sensors to monitor LPG having good sensitivity and selectivity in recent years and many efforts, in this field, are today devoted to the synthesis of novel sensing materials with enhanced performance. The metal oxide semiconductors such as undoped SnO2 [3], SnO2 doped with nobel metals such as Ru [4], Au [5], Pt [5] and Pd [5], CdO [6], WO3 [7], Al doped ZnO [8] and BaTiO3 [9] have been investigated in the past decades as LPG sensing materials because of their low cost and power consumption, simplicity of fabrication and use, versatility in detecting a wide range of toxic/flammable gases and stability in harsh environments.

Hematite (α-Fe2O3) has received considerable attention in the past few years due to its application potential in many technological areas such catalysis, lithium rechargeable batteries, gas sensors, pigments and magnetic materials [10], [11], [12], [13], [14]. It is the most stable iron oxide under ambient conditions, which exhibits n-type semiconducting properties with an indirect band gap of 2.2 eV. The α-Fe2O3 has long been used as sensing material in the fabrication of gas sensors for the detection of H2 [15], LPG [16], CO [17], NO2 [11], H2S [18], O2 [19] and ethanol [20].

Recent studies reveal that the nanostructured metal oxides with reduced dimensionality (i.e. in the form of nanoparticles, nanorods, nanotubes, nanowires and nanoribbons) are promising sensing materials for highly sensitive chemical sensors due to their small grain size and large surface-to-volume ratio [6], [21], [22], [23], [24], [25]. Consequently, different methods for synthesizing the α-Fe2O3 with different morphologies and sizes such as nanoparticles [26], nanobelts [27], nanotubes [15], nanorods [28], nanowires [29] and urchin-like superstructures [30] have been reported. Nevertheless, it still remains a challenge to develop simple, low cost and versatile approaches to synthesize 1-D nanostructures of α-Fe2O3.

The gas sensors based on α-Fe2O3 nanoparticles have been widely investigated by many researchers in the past decades [31], [32]. However, so far, there are only a few reports on the gas-sensing properties of 1-D nanostructures α-Fe2O3. Zheng et al. [33] prepared the α-Fe2O3 ceramic nanofibers by the electrospining route followed by calcinations at 800 °C for 6 h in air and studied their ethanol sensing properties. These nanofibers exhibited good sensitivity (∼4.25 for 1000 ppm ethanol at 300 °C), rapid response (∼3 s) and fast recovery (∼5 s to 500 ppm). ZhangYang et al. reported on controlled synthesis of hollow sea urchin-like α-Fe2O3 nanostructures via the hydrothermal approach followed by annealing in air at 600 °C for 2 h. The sensors based on the α-Fe2O3 nanostructures showed high gas sensing responses, short response and recovery time and long term stability in detecting ammonia, formaldehyde, triethylamine, acetone and ethanol.

Within the present investigation, experiments have been carried out for the fabrication of sensitive and selective LPG sensor based on α-Fe2O3 nanorods. There is hardly any report on LPG sensor based on α-Fe2O3 nanorods. The α-Fe2O3 nanorods were synthesized without any templates by calcining the α-FeOOH precursor in air at 300 °C for 2 h. This procedure is similar to that reported by Wang et al. [14], who have synthesized α-Fe2O3 nanorods without the use of any template or organic surfactant. Thus, in this work, the α-FeOOH precursor was prepared through a simple and low cost wet chemical route at low temperature (40 °C) using FeSO4·7H2O was used as the source of Fe2+ and the CH3COONa was used as the precipitating agent to release hydroxyl ions slowly during the reaction. Sensing characteristics of the α-Fe2O3 nanorods to LPG were systematically investigated.

Section snippets

Materials

All chemicals were of analytical grade. The ferrous sulphate (FeSO4·7H2O) and sodium acetate (CH3COONa) were purchased from E-Merck (India) and were used without further purification.

Synthesis of the α-Fe2O3 nanorods

In this work, the α-Fe2O3 nanorods were synthesized without any templates by calcining the α-FeOOH precursor in air at 300 °C for 2 h. The α-FeOOH precursor was prepared through a simple and low cost wet chemical route. The FeSO4·7H2O was used as the source of Fe2+ and the CH3COONa was used as the precipitating agent

XRD results

The XRD pattern of the as-prepared precursor is shown in Fig. 1. The diffraction peak positions match well with the XRD pattern of orthorhombic α-FeOOH phase (JCPDS # 29-0713). This α-FeOOH precursor was calcined at different temperatures in the range between 300 and 600 °C in air for 2 h. The XRD patterns of the α-FeOOH precursor calcined at 300, 400, 500 and 600 °C in air for 2 h are shown in Fig. 2. The diffraction peaks in all the plots are in agreement with the standard XRD peaks, which

Conclusions

The α-Fe2O3 nanorods were successfully synthesized without any templates by calcining the α-FeOOH precursor in air at 300 °C for 2 h and their LPG sensing characteristics were investigated. The α-FeOOH precursor was prepared through a simple and low cost wet chemical route at low temperature (40 °C) using FeSO4·7H2O was used as the source of Fe2+ and the CH3COONa was used as the precipitating agent to release hydroxyl ions slowly during the reaction. The formation of α-FeOOH precursor and its

Acknowledgements

The financial support from University Grants Commission (UGC), New Delhi, India through the major research project no. F 34-3/2008 (SR) is gratefully acknowledged. We also thank Mr. T. Sai Kamaraju and Mr. B. V. Cholkar, Forevision Instruments (India) Pvt. Ltd. for FESEM analysis of the samples.

Dewyani Patil is presently working as post-doctoral fellow in Department of Materials Science and Engineering, Chungnam National University, Daejeon, South Korea. She received her master's degree (M.Sc.) in Electronics in 2004 and Ph.D. in Physics in 2010 from North Maharashtra University, Jalgaon, India. Her research interests include conducting polymers and conducting polymer composites for chemical and biological sensors.

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    Dewyani Patil is presently working as post-doctoral fellow in Department of Materials Science and Engineering, Chungnam National University, Daejeon, South Korea. She received her master's degree (M.Sc.) in Electronics in 2004 and Ph.D. in Physics in 2010 from North Maharashtra University, Jalgaon, India. Her research interests include conducting polymers and conducting polymer composites for chemical and biological sensors.

    Virendra Patil is working as project assistant in National Chemical Laboratory, Pune, India. He received his master's degree (M.Sc.) in Physical Chemistry in 2009 from University of Pune, Pune, India. His research interests include preparation of 1-D nanostructured semiconducting metal oxides for the fabrication of gas sensors.

    Pradip Patil is working as professor in Physics at Department of Physics, North Maharashtra University, Jalgaon, India. He received his master's degree (M.Sc.) in Physics in 1983 and Ph.D. degree in 1988 from University of Pune, Pune, India. He joined North Maharashtra University, Jalgaon, India as the Head, Department of Physics since its inception. His main interests include development of conducting polymers and conducting polymer nanocomposites for their applications as corrosive protective coatings, chemical and biological sensors.

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