Exploiting Water-Mediated Ethanol Sensing by Polycrystalline ZnO at Room Temperature

Abstract

Ambient moisture can dramatically promote the response of ZnO to ethanol vapor, a hydrophilic gas. By comparing sensor responses in a broad range of humidities, we show that there is a consistent enhancement in ethanol adsorption on ZnO when physisorbed water, detected by capacitance measurements, is present. The time constants related to the capacitive signal recovery during desorption are consistent with the formation of C 2 H 5 OH-(H 2 O) n clusters that have a different desorption rate than water alone. These room temperature results indicate that surface water mediates the dynamic adsorption/re-evaporation equilibrium of solvated ethanol molecules. Thus, attention to interactions between the target gas molecules and their environment is important for understanding the mechanisms behind selective gas sensing. Moisture is always present in ambient environments, so an under-standing of water-solid interactions is important in many applied fields such as corrosion, catalysis, and sensor development. 1 The surface structure and reactivity of semiconducting metal oxides (MOX) such as ZnO, SnO 2 and WO 3 have been studied extensively for electrical gas sensing applications. These materials have highly sensitive elec-trical conductivity and capacitance responses to many gaseous species including CO, NH 3 , and volatile organic compounds (VOCs) while having low production costs and high thermal durability. 2 However, the presence of environmental H 2 O can effectively alter the reactivi-ties of MOX surfaces, which leads to difficulties in obtaining reliable and selective sensing signals for different target gases in real-world environments. 3–6