Study on the adsorption of H2O molecules on Al3+ modified SnO2 (221) crystal plane and the application of humidity sensor

Abstract

The molecular adsorption behavior of O₂, N₂, and H₂O on the Al³⁺ modified SnO₂(221) crystal plane and its effect on conductivity were simulated using density functional theory (DFT). The results show that the H₂O molecule is chemically adsorbed on the crystal plane as characteristic adsorption species (CAS), forming a surface mode named H₂O–SnO₂–Al (221). The conductivity of H₂O–SnO₂–Al (221) is reduced by adsorbing O₂ and N₂ molecules. The simulation results were confirmed by electrochemical impedance spectroscopy (EIS) under various atmospheric conditions. Using the crystal plane as a humidity sensor to test the different humidity of the air. The results show that the conductivity increases with the rising humidity, which contradicts the results of the H₂O single molecule adsorption model. Further FTIR data shows that high humidity on the crystal plane led to the formation of aggregated water, which in turn enhanced both conductivity and capacitance.