TiO₂ Nanowires for Humidity-Stable Gas Sensors for Toluene and Xylene

The dual-functionality sensor derived from semiconductor metal oxides operating at low temperature for low power consumption and robust stability toward humidity is a striking platform for economic and indoor air-quality monitoring. Therefore, in this work, temperature-dependent selectivity and robust stability toward carbon monoxide (CO), toluene (C7H8), and p-xylene (C8H10) are displayed by various TiO2 nanostructures synthesized following a facile hydrothermal method. The X-ray diffraction patterns confirmed the tetragonal structure of anatase TiO2. Surface studies confirmed the different morphologies, such as nanoparticles (TiO2 nanoparticles (TNPs)), nanowires (TiO2 nanowires (TNWs)), and sea-urchin-like hierarchically (HHC) arranged TiO2 nanostructures. Relatively high surface area and interconnected pore distribution were witnessed for TNWs and HHC nanostructures as compared to TNPs. In situ photoluminescence and X-ray photoelectron spectroscopy analyses confirmed the defect states of the nanostructures, and the TNWs possessed the highest concentration of oxygen vacancies and Ti3+, which influenced the dual-selectivity functionality of TNW toward C7H8 and C8H10 at 25 and 125 °C, respectively. Additionally, at an optimum working temperature of 25 °C, a response of 2.46 toward 20 ppm CO was witnessed for the HHC-based sensor and was attributed to the available surface area and active sites presented by the hierarchically arranged nanostructures. Cross-sensitivity measurements were conducted in the presence of interfering gases, which showed negligible cross-responses. The long-term stability in the presence of relative humidity and the sensing mechanism underlying the fascinating dual functionality for C7H8 and C8H10 vapor detection were discussed in detail. These findings showed that the current sensors can be employed for detection of C7H8 and C8H10 in a vastly robust and selective way with insignificant interference from ambient humidity.