Ethanol sensors based on Pt-doped tin oxide nanopowders synthesised by gel-combustion

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Abstract

The ethanol sensing properties of SnO2 powders prepared by a gel-combustion method have been investigated. SnO2 powders with different average crystallite size, 〈D〉, ranging from 6 up to 100 nm, were synthesized by this process starting from metallic tin as raw material and citric acid and/or urea as fuel substances. The average crystallite size of the synthesized powders was found to depend on the nature and loading of the fuel substance used in the combustion process.

Thick film sensors based on the gel-combustion SnO2 powders, annealed at 600 °C, have shown good sensitivity to low concentrations of ethanol (50–200 ppm). The smaller particles have shown a higher sensitivity than the bigger ones. At the addition of 1 wt% Pt, a remarkable enhancement of the sensitivity to C2H5OH and response and recovery time was observed for the sample with smaller SnO2 particles. Pt was instead found to suppress CO sensitivity, thus increasing the selectivity towards ethanol.

Introduction

Solid-state gas sensors based on semiconducting materials are attractive because of their low cost, small size, and compatibility with electronic systems [1]. They offer wide opportunities to improve process control, productivity and product quality in various industrial sectors, such as in air quality monitoring and control, energy efficiency, transportation, health and safety (e.g., to detect toxic, flammable and explosive gases in mines and industrial/residential environments), medical diagnostics, material recycling and automotive industry.

Recently there is an ongoing need for the development of new chemical sensors for the automotive industry other than the well known λ-sensor. Air Quality Systems (AQS) sensors for detect high pollution levels in traffic and Ethanol Breath sensors, are some examples [2], [3], [4]. The ethanol breath analyzer is designed to detect ethanol in the breath of drivers in order to reduce the number of road accidents caused by excessive alcohol consumption. It can be also integrated in the ignition system of the car (also named ‘alcohol ignition interlock’), thus the vehicle cannot start if the driver's breath indicates an over-the limit blood–alcohol content [5]. The requirements for a such sensor are: (i) detect low concentrations of ethanol (<200 ppm); (ii) show low cross-sensitivity to pollulant present in the vehicle cabin, such as CO.

Owing to the influence of the adsorption of various gases in the environment on the electrical properties of semiconducting oxide materials, such as ZnO, TiO2, Fe2O3, WO3, and SnO2, they are commonly investigated for gas sensing applications. Among these, tin dioxide is the most widely used in practical devices. The major drawback with tin dioxide-based semiconducting gas sensors is their nonselectivity, i.e., the inability to provide clearly distinguishable response signals when exposed to a mixture of reducing gases.

This problems can be resolved considering that the gas sensing properties of tin dioxide are known to be strongly influenced by the precursor raw materials, as well as various fabrication and processing conditions that affect the microstructure of the obtained powders or by addition of dopants or other metal oxides to the tin oxide [6], [7], [8], [9], [10].

Several physical methods are used to produce tin dioxide powders, involving spray pyrolysis [11], pulsed laser ablation [12], chemical vapor deposition [13] or sputtering [14]. However, these methods require specific apparatus, vacuum conditions and high costs. Sol–gel chemistry processing is an alternative low cost method of manufacturing a wide range of nanopowders [15].

This paper reports data on the tin oxide powders for gas sensor applications prepared from a new process named gel-combustion [16], [17], [18], [19], [20]. The technique is based on the combination of chemical gelation and combustion processes. The process involves an exothermic decomposition reaction of an aqueous gel and a thermally induced anionic redox reaction. The reaction occurs at rather low temperatures, and produces very porous and slightly soft agglomerated nanostructured powders, due to homogeneous dispersion into the liquid medium and the high amount of gases developed. To our knowledge, no systematic study on the sensing properties of tin oxide nanopowders prepared by this process has been previously reported. Here, the sensing properties of SnO2 nanopowders were evaluated in the monitoring of low concentrations of reducing gas (C2H5OH and CO) with aim to develop breath ethanol sensor with good sensitivity and selectivity. The influence of the addition of Pt as a catalytic additive, on the electrical conductance, sensitivity, selectivity and response/recovery times of gel-combustion SnO2 gas sensors is also investigated.

Section snippets

Preparation of SnO2 nanopowders

The dissolution of metallic tin in nitric acid was the first step in the preparation of gel-combustion SnO2 nanopowders. Then a fuel substance [citric acid (C6H8O7, 99% Aldrich), urea ((NH2)2CO, 99.5% Fluka] or mixture of them) was added and the pH adjusted near 7 by ammonium hydroxide. Subsequently, the solvent was evaporated by continuous stirring at 80 °C, until a gel was formed. Due to the contemporary presence of nitrates and a fuel substance, the mixture ignited leading to a rapid

SnO2 nanoparticles synthesis and characterization

The procedure followed to prepare tin dioxide powders by gel-combustion is schematized in the flow-chart in Fig. 1. Metallic tin was used as material raw in order to avoids the presence of a high chloride ion concentration (which is generally considered as poison for the sensing properties) in the final nanopowders [21]. The formation of gel was favored by adding aqueous ammonium hydroxide, until a pH 7 was reached, and heating at 80 °C. Citric acid, urea and/or mixture of them were used as fuel

Conclusions

In summary, we demonstrated that the gel-combustion method can be successfully used for the preparation of nanoparticles of SnO2 for gas sensing applications. Investigating the effects of some parameters of the process (nature of the fuel substance, metal/fuel ratio) it was found that they have a strong influence on the grain size of the SnO2 powders obtained. Powders obtained from citric acid have smaller grain size than that prepared from urea. On the other hand, these latter present a lower

Acknowledgment

The authors gratefully acknowledge the financial support for this work by MIUR under the framework of FIRB-SqUARE project (contract number RBNE01Y8C3).

Giovanni Neri was born in 1956 and 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, documented by more than 105 papers on international journals and books, cover many aspects of the synthesis, characterization and chemical-physics of solids with particular emphasis to catalytic and sensing properties. In

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Giovanni Neri was born in 1956 and 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, documented by more than 105 papers on international journals and books, cover 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 was born in 1972. She received her degree in Materials Engineering from the University of Messina in 1997 and the PhD degree from University of Reggio Calabria in 2001. 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 was born in 1975. He 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.

Nicola Donato was born in Messina, Italy, in 1971. He received the laurea degree in electronic engineering from the University of Messina in 1997 and the PhD degree from University of Palermo in 2002. His current research interests temperature-dependent linear/noise characterization techniques for solid-state devices, implementation of software procedures for automated instrumentation control, characterization and modeling of thin-film sensors.

Fabio Alessandro Deorsola is graduated in materials engineering and he is at present PhD student in materials science and technology at Materials Science and Chemical Engineering Department of Politecnico di Torino. His PhD thesis concerns the synthesis of nanostructured ceramic oxides through gel combustion and reactive microemulsion processes for gas sensing applications.

Piercarla Mossino is graduated in chemistry and PhD in metallurgical engineering. At moment she is grant researcher at Materials Science and Chemical Engineering Department of Politecnico di Torino. She is involved in the synthesis of nanostructured ceramic oxides through gel combustion and wear resistant ceramic materials through self-propagating high temperature synthesis (SHS).

Ignazio Amato is professor of Ceramic Materials Science and Technology and Advanced Ceramics at Materials Science and Chemical Engineering Department of Politecnico di Torino. He has worked in the field of development and characterisation of advanced materials (ceramics, cermets, composites). He has been involved in many national and international projects. He is author of 150 papers and three patents. The actual research is focused on the structural, microstructural and mechanical characterisation of innovative nanostructured ceramic materials.

Bruno De Benedetti is full professor of metallurgy in the Department of Materials Science and Chemical Engineering of Politecnico di Torino. During his carrier he published more than 100 papers. His interest is focused on innovative materials selection criteria, in accordance with use conditions. In this field he evaluated the alternative to metallic materials constituted by innovative composite materials and ceramics. Furthermore, acting as responsible of Materials Testing Lab., he managed the testing and characterization activities of all the different kinds of materials.

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