Pd- and Ca-doped iron oxide for ethanol vapor sensing

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Abstract

Iron oxide thin films doped with Ca and Pd, prepared by a liquid-phase deposition method (LPD) from aqueous solution, have been investigated as potential ethanol gas sensors.

SEM and XRD analyses were used to characterize Fe2O3 LPD films. Hematite (α-Fe2O3), having an average crystallite size in the range between 20 and 30 nm, was the only crystalline phase detected on all undoped and doped films.

The electrical response towards ethanol (100–500 ppm) has been studied in the temperature range of 300–500 °C. Both Ca and Pd promoters have shown a positive effect on the sensitivity of Fe2O3 films at the lower temperature investigated, whereas at higher temperature the undoped Fe2O3 film has shown better performances. The sensing properties of undoped and doped Fe2O3 thin films towards different interfering gases like NO2, CO and NH3 have been also investigated, showing that the selectivity to ethanol benefits of the Ca addition.

Introduction

Ethanol detection is of outmost importance for a variety of applications such as in monitoring for food-quality and in the fermentation industry [1]. Another important application is in the prevention of street accident caused by guide in state of drunkness [2]. Recently it was proposed the use of integrated ethanol monitoring systems in the cellular telephones or in air quality system (AQS) of the cars for finding ethanol in human breath [3].

These kind of devices should present specific and stringent technical characteristics: sensitivity, selectivity, miniaturizability, mechanical stability and a long lifetime. Moreover, in order to ensure a wide applicability, they should have a low cost. Metal oxide semiconductor (MOS) devices respond to many of these requirements even if they present in most cases poor selectivity [4], [5]. To enhance the selectivity to target gas, an opportune tailoring of the sensing layer with the use of catalysts and/or promoters is somewhat necessary [2], [6]. However, the right choices cannot be made without a deeper knowledge of the specific effect of these chemical additives on the interaction between the sensing material and the target gas.

Recently we developed a simple technique, namely liquid-phase deposition (LPD), for the deposition of thin films of sensing materials on ceramic substrates [7]. The LPD technology has several advantages in comparison with conventional method of deposition (simple and inexpensive deposition apparatus, no requirements of toxic organic solvents or precursors, high porosity of the deposited layer, etc.). In previous papers LPD has been used for the deposition of undoped and doped Fe2O3-based films [7]. In some of these works we investigated the effects of the addition of CeO2 to thin film of Fe2O3 for the monitoring of different alcohols [8], [9]. It has been found that CeO2 has a beneficial influence on the sensing characteristics of Fe2O3 thin films towards ethanol. The effect of cerium oxide was mainly related to two factors: (i) modification of the microstructure of the sensing layer with formation of a Fe–Ce mixed oxide phase; (ii) an increase of the surface basicity. This latter factor modifies the interaction of the target gas with the sensing layer surface addressing the reaction pathway towards dehydrogenation rather than dehydration, thus increasing the sensitivity [10]. Indeed, it is well established that, among the intermediate species involved in the processes occurring on the semiconductor surface, acetaldehyde (CH3CHO) coming from dehydrogenation of ethanol has a higher sensitivity compared to dehydration product, ethylene (C2H4) [11].

In this work we investigate the addition to iron oxide films prepared by LPD of Ca with aim to enhance the ethanol sensing properties. The addition of Ca to different metal oxides has been widely investigated [12], [13]. The effects registered on the sensing properties have been mainly related to its strong basic character, which also prevents coking from organic molecules adsorption, thereby increasing the sensor lifetime.

Moreover, we have also tested here undoped and Ca-doped Fe2O3 sensing layers promoted with Pd as a catalytic modifier. The sensing properties towards different interfering gases like NO2, CO and NH3 have been also investigated.

Section snippets

Characterization measurements

Characterization analyses were performed on powders deposited by LPD on bare glass and alumina substrates in order to have a large amount of material deposited in the same deposition conditions as in the sensor fabrication.

XRD analysis was carried out on a Ital-Structures mod. APD 2000 diffractometer using a Ni-filtered Cu Kα radiation source operating at 40 kV and 30 mA. The average crystallite size, 〈D〉, was estimated from line broadening analysis of the diffraction peaks by using the Scherrer

Sensing layer characterization

Fig. 1 reports XRD spectra of the undoped and some representative Ca- and Pd-doped LPD films after the thermal treatment at 400 °C in air. The only phase detected on all undoped and doped films was well-crystallized hematite (α-Fe2O3). By line broadening analysis of the diffraction peaks, a mean diameter in the range between 20 and 30 nm has been calculated for hematite particles on all samples.

No other phases due to Ca and Pd have been detected, suggesting that the promoters are present in an

Conclusions

Data reported in this work have shown that, Ca and Pd have a positive effect on the sensitivity towards ethanol of Fe2O3 thin films operating at the temperature of 300 °C. It is hypothesized that calcium promotes the dehydrogenation of ethanol through CH3CHO as intermediate instead of C2H4, thus increasing the sensitivity. The selectivity to ethanol also benefits of the Ca addition.

Summing up, the interesting characteristics showed by the doped Fe2O3 films prepared by LPD could be of interest to

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