An Ultrasensitive Room-Temperature H₂ Sensor Based on a TiO₂ Rutile-Anatase Homojunction

Metal oxide semiconductor hetero- and homojunctions are commonly constructed to improve the performance of hydrogen sensors at room temperature. In this study, a simple two-step hydrothermal method was employed to prepare TiO₂ films with homojunctions of rutile and anatase phases (denoted as TiO₂-R/A). Then, the microstructure of anatase-phase TiO₂ was altered by controlling the amount of hydrochloric acid to realize a more favorable porous structure for charge transport and a larger surface area for contact with H₂. The sensor used a Pt interdigital electrode. At an optimal HCl dosage (25 mL), anatase-phase TiO₂ uniformly covered rutile-phase TiO₂ nanorods, resulting in a greater response to H₂ at 2500 ppm compared with that of a rutile TiO₂ nanorod sensor by a factor of 1153. The response time was 21 s, mainly because the homojunction formed by the TiO₂ rutile and anatase phases increased the synergistic effect of the charge transfer and potential barrier between the two phases, resulting in the formation of more superoxide (O₂^-) free radicals on the surface. Furthermore, the porous structure increased the surface area for H₂ adsorption. The TiO₂-R/A-based sensor exhibited high selectivity, long-term stability, and a fast response. This study provides new insights into the design of commercially competitive hydrogen sensors.