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Highly sensitive mixed oxide sensors for the detection of ethanol

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Abstract

Sensors consisting of mixtures of tin dioxide and zinc oxide powders in a range of proportions were constructed. Each mixture was applied to an electrode-bearing alumina substrate either as a paste, or by screen printing. The responses of these sensors, and of three commercially-available Figaro sensors, to ethanol vapour in the 1–1000 parts-per-billion (ppb) range were measured. At 100 ppb of ethanol vapour, the most sensitive paste sensor (25% SnO2/75% ZnO) exhibited a response that was more than twice that of the screen-printed sensors, and almost 60 times greater than that of the most sensitive Figaro sensor (TGS822).

Introduction

The use of tin oxide as the basis of gas sensitive films has been extensively reported. Both thin [1] and thick [2] film sensors made using a variety of techniques have been studied for uses in the detection of a large range of organic and inorganic vapours. Sensors based on these materials usually possess little selectivity to any one vapour but exhibit good sensitivity to a broad range of vapours. Therefore, the sensors are commonly doped with catalytic metals such as platinum [3] or metal oxides such as copper oxide [4] to impart selectivity, increase sensitivity and enhance stability.

Sensors based on zinc oxide are not as commonly reported in the literature, but thin films have been utilised for the detection of carbon monoxide, hydrogen and methane [5]. Thin film zinc oxide sensors doped with aluminium have also been utilised for the detection of trimethylamine [6].

Sensors based on composites of zinc oxide and tin oxide have been reported [7], [8], [9]. In early reports the sintering process was described as leading to the formation of ZnSnO3 [7] and Zn2SnO4 [8] intermediates and these were found to have some selectivity to ethanol vapour and NO2 gas respectively. However, the control of sintering temperatures has led to composites without intermediate compounds [9]. The enhanced sensing characteristics of these composites were postulated to be due to hetero-contacts formed on sintering the materials and also on changes in the microstructure making the materials more porous. A range of composite materials (2–90 mol.% ZnO) have been shown to have different sensing characteristics to CO gas when compared to the pure materials. The temperature at which maximum sensitivity to CO was observed was dependent upon the composition.

The maximum sensitivities of these sensors were commonly in the tens of ppm (v/v) range. Here, we report the development of sensors that are sensitive to ethanol in the 1–1000 parts-per-billion (ppb) (v/v) range. Both screen-printed and thick-film paste sensors are described.

Section snippets

Screen-printed sensors

Zinc oxide powder (AnalaR grade, 99.5%, BDH/Merck Ltd., Lutterworth, Leicestershire, UK) and tin dioxide powder (99.999%, Janssen Chimica, Hyde, Cheshire, UK) were used in the production of the sensors. A mixture of 0.5 g of each of the oxides was ground using an agate pestle and mortar. Cellulose acetate was added to act as a binder, then the grinding of the ingredients in the pestle and mortar was continued to produce a thick, coherent paste.

Alumina substrates were prepared, each consisting of

Results and discussion

The screen-printed sensors were produced and tested initially. The choice for these sensors of a 50/50 mix by mass of tin dioxide and zinc oxide was based on earlier results that indicated that these proportions were optimum. The results for two of these sensors when exposed to ethanol vapour at concentrations of 10, 50, 100, 200, 300, 400, 500 and 1000 ppb are shown in Fig. 1. It can be seen that the sensors display an almost linear response over this range, and that the responses of different

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