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 degrees 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 degrees C showed the largest effective surface area. The film annealed at 500 degrees C had the largest pore volume. Among the films investigated in this study, the WO3 thin film annealed at 500 degrees 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. (c) 2007 Elsevier B.V. All rights reserved.