Room-temperature NH 3 gas sensors based on Ag-doped γ-Fe 2 O 3 /SiO 2 composite films with sub-ppm detection ability

•The Ag doped γ-Fe2O3/SiO2 composite films are prepared by a sol–gel process.•The films inherit the porous structure of SiO2 film.•The sensor is able to detect 100ppb NH3 at room temperature.•The existence of SiO2 enhances the sensor sensitivity to ammonia significantly.•The Ag dopant, acting as a catalyst, enhances the sensing properties significantly.