A Proton Hopping-Guided 3-D Space Charge Model for Quantitative Understanding of Humidity-Dependent Gas Sensing by TiO2 Nanoflower-Based Devices

Low temperature (90 °C) gas sensing performance, with ethanol as a test case, of TiO2nanoflowers in the presence of humidity, was investigated. It was revealed that the sensor resistance in air and in ethanol was reduced in the presence of humidity. However, such variations in sensor resistance in the humid and in the nonhumid environment are more pronounced for baseline case (in air) than that in target species exposure (in ethanol). Interaction probability of the target species, in the presence of humidity, with the TiO2nanoflower surface was governed by the proton hopping model. For quantitative understanding of the subsequent electron generation and transport phenomenon, a space charge model, corelating the influence of mutually competing and interdependent factors like oxygen vacancies, adsorbed O-species, dissociation of water and ethanol molecules on TiO2nanoflower surface, has been demonstrated. It was found that decrease in base line resistance (Δ R) gradually becomes saturated with increase in relative humidity level, possibly owing to proton hopping in the physisorbed water layers which hinders the chemisorption of water molecules at the nanoflower surfaces leading toward relatively insignificant change in space charge region.