A wide-range, efficient and selective ammonia sensor based on nanostructured ZnO film: Ag-doping effect

This study compares the structural, optical, and gas-sensing properties of spray-deposited ZnO nanostructured films with 1–3% Ag-doped ZnO (Ag-ZnO) films at room temperature (RT) for ammonia exposure ranging from 1 to 700 ppm. Structural analysis confirmed the polycrystalline nature of both ZnO and Ag-ZnO films, with enhanced orientation along the (002) plane as Ag doping concentration increases. Ag doping reduces the optical band gap from 3.276 ± 0.003 eV for ZnO to 3.194 ± 0.003 eV for 3% Ag-ZnO. Scanning electron microscopy reveals grainy surfaces for all films, with grain sizes increasing with higher Ag doping. The room temperature gas-sensing measurements showed that the ZnO, 1% Ag-ZnO, and 3% Ag-ZnO films were selective for ammonia among the tested gases and exhibited responses of 586.69 ± 6.87, 533.81 ± 3.08, and 234.62 ± 5.09, respectively, at 700 ppm ammonia exposure. Ag-doping into ZnO degraded the gas response, likely due to larger grain sizes reducing the specific surface area. All films exhibited rapid response and recovery with response/recovery times of 12/14 s for ZnO, 24/19 s for 1% Ag-ZnO, and 28/22 s for 3% Ag-ZnO. Gas response measurements over 28 days demonstrated good stability. Both ZnO and 1% Ag-ZnO films detected ammonia concentrations as low as 1 ppm, with responses of 2.29 ± 0.37 and 2.23 ± 0.36, respectively. The findings of this work suggest that spray-deposited ZnO and lower percent Ag-doped ZnO films are promising materials for constructing a cost-effective, efficient RT ammonia sensor capable of detecting a wide range of ammonia concentrations.