Observation of Concentration-Dependent Inversion of Response to Oxidizing Gases in Chemoresistive Metal Oxide Gas Sensors Resulting from Humidity Interference

Chemoresistive metal oxide gas sensors typically exhibit predictable resistance changes upon exposure to oxidizing or reducing gases, depending on their semiconductor type. However, anomalous concentration-dependent inversion of response has been reported, often attributed to intrinsic material properties. In this study, we investigate such inversion phenomena in Cr₂O₃-based sensors doped with Ti(IV), W(VI), and Ni(II), alongside Ag-decorated WO₃ sensors. Initial observations suggested inversion of response to low concentrations of O₂, consistent with prior hypotheses involving carrier-type transitions. However, systematic experiments revealed that this inversion was not intrinsic but rather caused by trace water contamination in the analyte gas stream. Only through cryogenic drying of the analyte gases was the expected sensor behavior restored. Mass spectrometry confirmed water as the sole contaminant responsible. Further tests demonstrated that water induces a rapid resistance increase in p-type Cr₂O₃ sensors, mimicking an n-type response. These findings highlight the critical influence of humidity on sensor behavior and suggest that some previously reported inversion phenomena may stem from undetected water interference. This work underscores the necessity of rigorous gas stream purification in low-concentration gas sensing studies.