Low-temperature H2S sensors based on Ag-doped α-Fe2O3 nanoparticles

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Abstract

Ag-doped α-Fe2O3 nanoparticles were synthesized by a chemical coprecipitation method and characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM), thermogravimetry-differential thermal analysis (TG-DTA), X-ray photoelectron spectroscopy (XPS) and Brunauer–Emmett–Teller specific surface area analysis (BET) techniques. Obtained results indicated that spherical Ag grains with size of about 5 nm are highly dispersed on the surface of α-Fe2O3 nanoparticles. The surface area of the Ag/Fe2O3 nanoparticles is several times as large as that of pure α-Fe2O3. The H2S sensing properties of these Ag/Fe2O3 sensors were systematically investigated. In comparison with pure α-Fe2O3, all of the Ag-doped sensors showed better sensing performance in respect of response, selectivity and optimum operating temperature. The effects of Ag content, calcination and operation temperature on the sensing characteristics of the Ag/α-Fe2O3 sensors were also investigated. The sensor containing 2 wt% Ag and calcined at 400 °C exhibited the maximum response to H2S at 160 °C. A possible mechanism for the influence of Ag on the H2S-sensing properties of Ag/α-Fe2O3 sensors was proposed.

Introduction

In recent years, the concern over environmental protection and increasing demands to monitor hazardous gases in industry and home has attracted extensive interests in developing gas sensors for various polluting and toxic gases. Due to the advantages of small size, low cost, simple operation and good reversibility, the semiconductor sensors have become the most promising devices among the solid-state chemical sensors. Hence, the metal oxide gas-sensing materials are widely investigated. Many semiconductor oxides such as SnO2, ZnO, Fe2O3, In2O3, WO3, and CuO, have been explored to detect the polluting, toxic and inflammable gases, such as CO, CO2, NOX, H2S, and ethanol [1], [2], [3], [4], [5], [6], [7].

Hematite (α-Fe2O3), the most stable iron oxide, is of scientific and technological importance as catalysts, pigments, ion exchangers, magnetic materials and lithium-ion batteries [8], [9], [10], [11]. Recently, α-Fe2O3 has been proved to be an important solid-state gas sensor. However, the low sensitivity, poor selectivity and high operating temperature discourage its extensive application. In order to fit the increasing demands of sensors in more complicated systems and strict conditions, many attempts have been made to improve the sensing properties of α-Fe2O3. Based on the sensitization effects of noble metals on metal oxides through chemical and/or electronic interactions, modifying the base materials with noble metals is an efficient way to the base materials to promote their response towards various gases [12], [13], [14], [15], [16].

Up to now, α-Fe2O3-based sensors have been investigated for the detection of some organic gases such as ethanol, acetone, methanol, and LPG [17], [18], [19], [20], [21]. However, there are few reports on the study of the gas sensing properties of α-Fe2O3 to H2S. In our previous works, the effects of Au, Pd and Pt on the sensing properties of α-Fe2O3 sensors to H2S have been investigated [22], [23], [24]. The gas sensing properties of α-Fe2O3 were markedly promoted by doping with such noble metals. However, the high cost discourages their extensive application. Hence, it is important to search an alternative component to replace Au, Pd and Pt. Silver is probably a suitable one.

In this paper, we reported for the first time the preparation of Ag/α-Fe2O3 sensors by a coprecipitation method. The structural properties of the prepared Ag/α-Fe2O3 nanoparticles were characterized by means of XRD, TG-DTA, TEM, HRTEM, BET and XPS. The effects of Ag content, calcination and operating temperature on the gas sensing properties to H2S were investigated in detail. The sensing mechanism of the Ag/α-Fe2O3 sensors to H2S was also discussed.

Section snippets

Preparation of Ag/α-Fe2O3

All the reagents are of analytical grade and used as purchased.

Ag/α-Fe2O3 nanoparticles were prepared by a coprecipitation method. A small quantity of polyglycol was added to an aqueous solution of AgNO3 (0.25, 0.5, 1.0, 1.5, 2.0, 3.0 and 5.0 wt%) and Fe(NO3)3·9H2O. The aqueous mixture was then added dropwise to an aqueous solution of Na2CO3 under vigorous stirring at 80 °C. The pH of the solution was adjusted by a diluted Na2CO3 aqueous solution in the reaction process. After stirring for 1 h, a

Results of characterization

Fig. 1 shows the TG-DTA curves of as-prepared Ag/FeOOH powders. A dramatic weight loss occurs around 100 °C, accompanied with an endothermic peak on the DTA curve, which may be due to the dehydration process of physically adsorbed water and the release of attached nitrates in the products. In the range of 200–400 °C, another weight loss is observed, corresponding to an obvious exothermic peak around 300 °C on the DTA curve. It can be attributed to the reaction 2FeOOH  Fe2O3 + H2O. There is no obvious

Conclusions

In summary, the Ag/α-Fe2O3 nanoparticles were successfully synthesized by a convenient chemical coprecipitation method, and Ag/Fe2O3 sensors were made and tested. The XPS analysis indicated that Ag existed in metallic form and highly dispersed on the surface of sensors. The specific surface area (SBET) of α-Fe2O3 nanopowders was remarkably promoted by the Ag-doping. Compared with pure α-Fe2O3, the Ag-doped sensors showed much higher response, better selectivity and rather lower optimum

Acknowledgement

We gratefully appreciate the financial support of the 973 program of China (No. 2005Cb623607).

Yan Wang received her M.S. degree in chemistry from Nankai University in 2006. Now she is a Ph.D. candidate in the College of Chemistry, Nankai University in China. Her research focuses on the synthesis, characterization of metal oxide nanomaterials and their gas-sensing properties.

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Yan Wang received her M.S. degree in chemistry from Nankai University in 2006. Now she is a Ph.D. candidate in the College of Chemistry, Nankai University in China. Her research focuses on the synthesis, characterization of metal oxide nanomaterials and their gas-sensing properties.

Yanmei Wang received her B.S. degree in chemistry from Shenyang Institute of Gold Technology in 1995. Now she works at Nankai University and is also a M.S. candidate. Her research is focused on the development and application of catalysis and gas-sensitive materials.

Jianliang Cao received his M.S. degree in chemistry from Nankai University in 2006. Now he is a Ph.D. candidate in the College of Chemistry, Nankai University. His research interests include the synthesis of nanomaterials and their application in catalysis.

Fanhong Kong received her B.S. degree in chemistry from Qufu Normal University in 2005. Currently, she is a master student in the College of Chemistry in Nankai University. Her research is focused on the development and application of gas-sensitive materials.

Huijuan Xia received her B.S. degree in chemistry from Liaocheng Normal University in 2006. Now she is a master student in the College of Chemistry, Nankai University. Her interest is devoted to the preparation and application of gas-sensitive materials.

Jun Zhang received his B.S. degree in chemistry from Qufu Normal University in 2006. Now he is a master student in the College of Chemistry in Nankai University. His research interest focuses on nanomaterials gas sensors.

Baolin Zhu received her Ph.D. degree in chemistry from Nankai University in 2006. Now she is a faculty in the College of Chemistry, Nankai University. Her research is focused on the preparation of nanomaterials.

Shurong Wang received her Ph.D. degree in chemistry from Nankai University in 2007. Now she is a faculty in the College of Chemistry, Nankai University. Her research covers nanomaterials, catalysis and gas sensors.

Shihua Wu received his degree in chemistry from Nankai University in 1970. At present, he is a professor in the College of Chemistry, Nankai University, where he has been working for many years in the field of preparation, characterization and catalytic and gas-sensing properties of metal oxides nanomaterials.

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