WO3 Nanoparticles and Nanoflakes Based Sensors for Selective Detection of Alcohols

Abstract

Ethanol is an important chemical raw material widely used in chemical industry, medicine, cosmetics, paints, and biofuels [1]. It is volatile and flammable and a human exposure to its vapours could result in headache, drowsiness, irritation of eyes and difficulty in breathing. It derives that the precise quantitative detection of ethanol vapours is of interest for many applications. Different studies were focused on semiconducting metal oxides as sensing materials for the selective ethanol detection. It is well known that, despite their high stability over time and the low cost of production, these semiconductors suffer of low selectivity, which affects their effective use for real applications.Among the wide palette of the metal oxides, tungsten oxide (WO3) is an important intrinsic n-type wide gap semiconductor, which has attracted considerable attention to detect various hazardous gases. However, it is still challenging to significantly enhance the sensibility and selectivity of WO3 based gas sensors by tuning its crystal structures, morphology and adding dopants [2]. WO3 can be a promising candidate for alcohols detection for the presence of acidic sites that can promote the alcohol decomposition through dehydration [3].In this work, the synthesis of WO3 nanoflakes (WO3-NF) was achieved via solvothermal route [4]. The peculiar morphology was characterized by SEM microscopy. XRD analysis reveals that the investigated sample is biphasic, composed by triclinic (s.g. P-1) and hexagonal (s.g. form with P63/mcm) WO3 with a phase fraction of about 97 and 3 wt.%, respectively.Sensing performance of the WO3-NF were studied and compared to those of WO3 nanoparticles synthetized by precipitation method. Tested sensors were realized by screen-printing of the functional material on alumina substrates with interdigitated electrodes. After a systematic investigation on the optimal working temperature, WO3 based sensors were tested at 250°C and 300°C for WO3-NF and WO3-NP, respectively. Gas measurements were performed testing the WO3-based sensors towards various gaseous molecules: H2S (10 ppm), H2 (50 ppm), CO (35 ppm), NO2 (1 ppm), NH3 (2 ppm) and different concentrations of methanol, ethanol and butanol (5-10, 25, 50 ppm). The sensing analysis indicates that the WO3-NF is sensitive towards alcohols, remarkably to ethanol, with negligible responses to other analytes.Fig. 1a, where the dynamic response to different concentration of ethanol for WO3-based sensors is shown, clearly highlights the enhanced sensing performance of the WO3-NF based sensor when compared to that of the WO3-NP. The influence of humidity on WO3-based sensors exposed to ethanol was also investigated (Fig. 1b). Even at low ethanol concentration (5 ppm), the influence of water vapour is limited, and after a partial drop up to 30 RH%, the WO3-based sensors performance seems to reach a stabilization, and no variations are observed up to 60 RH%. Noteworthy, the WO3-NF response to ethanol is always more than two times higher when compared to that of WO3-NP. The sensing response towards 50 ppm of methanol, ethanol and butanol are shown in Fig. 1c. While WO3-NP sensibility increases with the length of the alcohol chain, this trend is not observed for WO3-NF, which has higher response for ethanol. This peculiar behaviour will be the object of further investigation.