Sol–gel route to pseudocubic shaped α-Fe2O3 alcohol sensor: preparation and characterization

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Abstract

α-Fe2O3 hydrosol was synthesized by sol–gel method with FeCl3·6H2O as the raw material and its thin films were prepared by dip-coating technique. The structure, morphology and spectroscopy of the thin films were characterized by XRD, AFM, UV–vis and FT-IR analysis methods. The results showed that uniform α-Fe2O3 monolayer and multilayers could be obtained at certain dip-coating speed. The particles were pseudocubic shaped, with the mean particle size of about 58 nm. The formation mechanism of α-Fe2O3 particles was explored by IR spectra and the change of the system acidity and conductivity. The results indicated that α-Fe2O3 particles formed from the intermediate complex β-FeOOH. The gas-sensing properties of the multilayers to alcohols with different chain length were measured at room temperature. The thin films exhibited high sensitivity and rapid response–recovery characteristics to these gases. The film can detect C3H7OH vapor concentration as low as 2 ppm. The response and recovery time of the film to CH3OH and C2H5OH vapor is within 20 and 50 s, respectively.

Introduction

The functional film of ferric oxide has been used in many fields such as semiconducting material, dielectric material, magnetic material, sensitive material and catalyst, etc. [1], [2], [3], [4]. Most of the traditional gas sensors are thick film sensors or sintered type sensors made by metal oxides, e.g. tin oxide, zinc oxide and ferric oxide. In comparison with these sensors, thin film sensors have attracted much attention due to their high sensitivity, low working temperature and good selectivity [5], [6]. In recent years, ferric oxide thin films have been prepared by liquid-phase deposition method (LPD), plasma enhanced chemical vapor deposition (PECVD), ion-sputtering and ultrasonic spray pyrolysis [7], [8], [9], [10], [11]. Nevertheless, the working temperature of these films is still higher than 100 °C. The order of the film is hard to be controlled, in which the particles are usually spherical shape. The sol–gel method is an effective way to prepare nanoparticles and thin film of inorganic oxides, by which the transparent nanoparticulate films with uniform particles size could be obtained. There are seldom reports on the preparation of α-Fe2O3 nano-films with good gas-sensing properties at room temperature by dip-coating technique.

It is well known that the properties of the materials, e.g. magnetic, optic, electric, adsorption and catalytic properties, are dependent not only on the composition, but also on the particle size and morphology. Three-dimensional ordered spherical α-Fe2O3 nanoparticulate films were prepared by Langmuir–Blodgett (LB) technique and colloid chemical method recently in our group [12]. The thin film has good gas-sensing properties to alcohols at room temperature. In order to investigate the influence of different shape of α-Fe2O3 nanoparticle on the gas-sensing properties of the material, the hydrosol of pseudocubic shaped ferric oxide was synthesized by sol–gel method with FeCl3·6H2O as the raw material. Ordered thin films of α-Fe2O3 were prepared by dip-coating technique. In the hydrolytic process of ferric chloride, the formation mechanism of α-Fe2O3 particles was explored by IR spectra and the change of the system acidity and conductivity. The structure and the gas-sensing properties of the thin film to alcohols were investigated at room temperature.

Section snippets

Experimental details

All the chemicals are A.R. The α-Fe2O3 hydrosol was synthesized by using hydrolysis method [13] with a little improvement: predetermined amounts of stock solutions of FeCl3 (3 mol dm−3) and HCl (0.2 mol dm−3) were mixed in a flask at the ratio of 1:3 (v/v), and the deionized water was added till the final concentration of Fe3+ is 0.01 mol dm−3. This mixture was preserved in water bath at 96 °C for 24 h, and then quenched to room temperature. The resulting orange-red solution was the α-Fe2O3

Structure characterization of the film

Fig. 1 shows the XRD pattern of 30-layer nanoparticle film deposited on the glass. It is observed that the interplanar spacing of d and the relative intensity of the diffraction peaks are in good agreement with those of the standard data in JCPDS 33-0664. It is said that the obtained product is the pure hexagonal α-Fe2O3 phase.

IR spectrum of the gel (powder) in the range of 1000–350 cm−1 is shown in Fig. 2. There are three absorbance bands located at 568, 480 and 386 cm−1, which are close to 567,

Conclusions

The pseudocubic shaped α-Fe2O3 sol was synthesized by sol–gel method and its formation process was investigated by IR spectra, pH and conductivity measurements. The results indicate that pseudocubic shaped α-Fe2O3 particles formed from the intermediate complex β-FeOOH. During the new phase formation and phase transfer, the pH value and conductivity of the reaction system changed accordingly. The monolayer and multilayer of the nanoparticulate film was prepared by dip-coating technique and was

Acknowledgements

We would like to thank the support of the National Natural Science Foundation of China (20101003), Research Fund for Oversea Scholars of Heilongjiang Province (1053HZ002), Youth Foundation of Heilongjiang Province and Natural Science Foundation of Heilongjiang Province and Financial Support from Laboratory of Rare Earth Chemistry and Physics, CAS.

Lihua Huo was born in 1968. She received her PhD in physical chemistry from Fuzhou University, China, in 1997. She is the professor of College of Chemistry and Chemical Technology, Heilongjiang University. Her research interests are mainly directed towards the development of nanoparticle and organic–inorganic composite materials and their application as gas sensors.

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Lihua Huo was born in 1968. She received her PhD in physical chemistry from Fuzhou University, China, in 1997. She is the professor of College of Chemistry and Chemical Technology, Heilongjiang University. Her research interests are mainly directed towards the development of nanoparticle and organic–inorganic composite materials and their application as gas sensors.

Qiang Li was born in 1978. He graduated from College of Chemistry and Chemical Technology, Heilongjiang University and received his MS degree in 2004. His studying interests are inorganic functional materials.

Hui Zhao was born in 1969. He received his PhD from Department of Chemistry, Jilin University, China, in 1999. He is professor of chemistry at Heilongjiang University. His working interests are the study of oxide electrolyte and electrode materials for SOFC and organic–inorganic catalytic materials.

Lijun Yu was born in 1981. She graduated from College of Chemistry and Chemical Technology, Heilongjiang University, in 2003. Now she is studying her first year MSc in the same college. Her studying interests are solid oxide electrolyte materials.

Shan Gao was born in 1967. He finished his postdoctoral work from Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, China, in 2000. He is the professor of College of Chemistry and Chemical Technology, Heilongjiang University. His research interests are mainly directed towards the aspect of structural chemistry and nanomaterial.

Jinggui Zhao was born in 1948. He graduated from Department of Chemistry, Fudan University in 1974. He has been professor of inorganic chemistry at Heilongjiang University since 1996. He has been working on the field of rare earth elemental chemistry and resource chemistry. His current research interests are inorganic functional materials.

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