Influence of annealing on microstructure and NO2-sensing properties of sputtered WO3 thin films

The influences of thermal annealing on the microstructure and NO2-sensing properties of sputtered WO3 thin films were investigated. The WO3 films as-deposited at room temperature were amorphous and would crystallize to monoclinic structure when annealed at 350 °C or above. The grains and pores in the WO3 films grew larger with an increase in annealing temperature. The effective surface area and pore volume changed non-monotonously with increasing the annealing temperature. The film annealed at 350 °C showed the largest effective surface area. The film annealed at 500 °C had the largest pore volume. Among the films investigated in this study, the WO3 thin film annealed at 500 °C showed the quickest response/recovery and the highest response to 10 ppm NO2. These results indicate the importance of achieving porous structure on improving the gas sensing performance.