Resistive CO gas sensors based on In2O3 and InSnOx nanopowders synthesized via starch-aided sol–gel process for automotive applications

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Abstract

Pure and Sn-doped In2O3 nanopowders have been synthesized by a starch-aided sol–gel process. A detailed characterization by means of TEM and HRTEM, TG–MS, XRD and 119Sn solid-state NMR analysis has been carried out. It has shown that the grains of the samples as-synthesized and dried at 120 °C are in the nanometer range. Moreover, on the Sn-doped In2O3 sample, the homogeneous distribution of the dopant with no segregation phase effects has been demonstrated. The thermal treatment at 550 °C induced an increase of grain size up to about 30 nm and of crystallinity.

The behaviour of the resistive gas sensors based on the synthesized nanopowders in the monitoring of carbon monoxide for automotive applications has been evaluated. Electrical and sensing tests have been performed on the sensor devices in a thick film configuration depositing pure and Sn-doped In2O3 nanopowders by screen-printing over a ceramic substrate. The results have been discussed in relation to the chemical and microstructural properties of the synthesized nanopowders. The good sensing behaviour of these samples has been associated with their special features such as very small grains and high oxygen vacancies due to the peculiar reductive character of starch pyrolysis.

Introduction

Sol–gel routes allow obtaining metal oxides starting from molecular species (generally inorganic salts or metal organic compounds) via inorganic polymerization reactions involving hydrolysis followed by olation (i.e., polycondensation with preferential elimination of water) and oxolation (i.e., polycondensation with preferential elimination of alcohol) reactions. Besides the advantages of starting from atomic scale reagents, sol–gel and the chemical approaches in general are simple, versatile and inexpensive compared to physical methods [1].

Among all the possible oxides, indium-based metal oxides have been result very effective as material for gas-sensor making [2], [3], [4], [5], [6], [7], [8], [9]. This peculiar property can be further improved by reducing the indium oxide grain size to nanoscale [10]. This effect is primarily linked to the fact that, as a consequence of grain size reduction, the surface available for the gas target-sensing layer interaction is maximized. Moreover, when the particle size is very small the gas interacting zone extends over the whole grain, i.e., the particle is fully depleted [8]. The superior sensing performances of pure In2O3 and SnO2 nanopowders, with respect to corresponding bulk materials, have been exhaustively reported [2], [3], [4], [5], [6], [10], [11], [12], [13], [14].

The use of size stabilizers, such as capping agents or templates is commonly associated to the classic sol–gel process [15]. Among them, we recently reported the synthesis of various metal oxides in the presence of starch sol using ordinary salts as precursors [16], [17], [18]. Starch is an organic macromolecule essentially composed of amylose, an unbranched single chain polymer of 500–2000 units linked through α-1,4 glucosidic bonds; it behaves in water solutions as a hydrophilic non-ionic stabilizer of metal oxide and hydroxide nanoparticles prepared by the hydrolytic route. It was proved that the polysaccharide presence offers many advantages, such as high availability at low cost, clean easy biodegradation to soluble glucosidic units and original particle shape maintenance during heat treatment, owing to the possible coordination of glucosidic moieties with particle surface. Moreover, the organic hydrophilic envelope of the particles facilitates the formation of colloidal suspensions with convenient binders, which are usually used to obtain thick films through various methods, such as screen-printing.

Here, therefore we focused our attention on the sensing applications of the pure and Sn-doped In2O3 nanopowders prepared with the above-mentioned methodology.

The sensing behaviour regarding the materials under study was investigated in the monitoring of carbon monoxide (CO). Carbon monoxide is an odorless, colorless and toxic gas due to the formation of carboxyhemoglobin in the blood, which inhibits oxygen intake. At lower levels of exposure, CO causes mild effects including headaches, dizziness, disorientation and fatigue. At higher concentrations, it causes impaired vision and coordination, confusion leading to death.

For these reasons, CO monitoring at ppm levels is of outmost importance in domestic, environmental and industrial ambient. For example, CO produced from the incomplete combustion of fossil fuels from car engines causes high concentrations of CO in urban area and particularly in closed ambience such as in the cabin vehicles, garages, tunnels and underwear parking [19]. Then, as drivers are subjected to high carbon monoxide concentration when moving in these areas, CO is monitored in these ambience with the scope to control the ventilation system [20], [21], [22]. Nowadays, vehicles with devices for automatically selecting the air inlet mode adjusted according to the information provided by HC, CO and NOx sensors have been increasing, to accomplish more severe legislation about car passenger health [20]. Moreover, CO monitoring allows the ventilation systems to operate in the pulse mode, varying automatically the ventilation rate of individual fans to ensure that the CO concentration in the ambience of vehicles and indoor car parking is maintained within safe levels, further saving energy.

For these automotive air quality monitors, the CO detection limit should be about 10 ppm. Moreover, for such applications it is important that the sensor responds very fast. The air inlet valve should be closed before low-quality air is allowed into the car. A response time in the order of a few seconds is therefore required.

Section snippets

Materials

All starting chemicals were commercially available products, used without further purification. InCl3 and SnCl4 (Aldrich, Steinheim, Germany), starch from rice and H2O2 30% were BioChemika-Fluka products. α-Amylase from barley malt, type VIII-A with 2000 U, was purchased from Sigma.

Nanopowder synthesis

Two samples were prepared (pure and Sn-doped In2O3) with the starch-aided sol–gel method as follows. The precursors, pure SnCl4 and water-diluted InCl3, were added by doping funnels to a starch water suspension (50 

Microstructural characterization

Table 1 reports the main characteristics of the prepared In2O3 and In2O3–SnO2 samples. Codes A120 and B120 refer to samples dried at the temperature of 120 °C, whereas A550 and B550 are samples heat treated at 550 °C. Textural data acquired on samples calcined at higher temperature showed that pure In2O3 sample has a high surface area and porosity. On the doped sample, the presence of tin contributes to a further increase of the surface area and porosity. Textural properties are more important

Conclusions

The gas-sensing properties towards carbon monoxide of pure and Sn-doped In2O3 nanopowders synthesized by a starch-aided sol–gel technique have been investigated. The Sn-doped In2O3-based sensor showed higher conductance than In2O3, due to n-doping of Sn cations in the In2O3 lattice, and higher defectiveness than pure oxide. Both the sensors showed high responses to CO. However, the undoped In2O3 sensor showed the best performances in terms of sensitivity, response/recovery time and lower

Acknowledgements

We thank MURST-PNR 2001–2003 (FIRB art. 8) for funding. We also thank Dr. A. Sanson and Dr. E. Roncari for screen-printing of nanopowders.

Giovanni Neri received his degree in Chemistry from the University of Messina in 1980. He is full professor of chemistry and director of the Department of Industrial Chemistry and Materials Engineering of the University of Messina. His research activity, covers many aspects of the synthesis, characterization and chemical-physics of solids with particular emphasis to catalytic and sensing properties. In the latter research area his work has been focused on the preparation of metal oxide thick

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    Giovanni Neri received his degree in Chemistry from the University of Messina in 1980. He is full professor of chemistry and director of the Department of Industrial Chemistry and Materials Engineering of the University of Messina. His research activity, covers many aspects of the synthesis, characterization and chemical-physics of solids with particular emphasis to catalytic and sensing properties. In the latter research area his work has been focused on the preparation of metal oxide thick and thin films and their application in gas sensors.

    Anna Bonavita received her degree in materials engineering from the University of Messina in 1997. At present time she is at the Department of Industrial Chemistry and Materials Engineering of the University of Messina. Her research activity concerns with the preparation, characterization and development of semiconductor films for gas-sensing applications.

    Giuseppe Micali received his degree in electronic engineering from the University of Messina in 2003. At present time he is at the Department of Industrial Chemistry and Materials Engineering of the University of Messina. His research activity concerns with the implementation of software procedures for automated instrumentation control and with the electrical characterization of gas-sensing devices.

    Giuseppe Rizzo received his degree in chemistry from the University of Messina in 1999. Actually he works at the Department of Industrial Chemistry and Materials Engineering of the University of Messina. His research activity is focused on the synthesis and characterization of materials by sol–gel method both for catalytic and optical applications.

    Emanuela Callone received her degree in physics from the University of Trento in 2001 and the PhD award from the University of Pisa in the 2005. Actually she works as research scientist at the Department of Material Engineering and Industrial Technology of the University of Trento. Her research interests include both solid-state chemistry and biomaterial evolution, mainly through solid-and liquid-state NMR characterization.

    Giovanni Carturan received his degree in Chemistry from the University of Padova in 1967. Actually he is full professor at Faculty of Engineering at the University of Trento. His research interests include solid-state chemistry and material chemistry. The present research activity relates to the chemistry of nanomaterials and to the sol–gel silica entrapment of cells for drug production and bioartificial organs.

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