Cross sensitivity and stability of NO 2 sensors from WO 3 thin film

Abstract

The H 2O, C 2H 5OH, CO, CH 4, NO and SO 2 cross sensitivity to NO 2 gas, as well as the long term stability of the electrical response of WO 3 thin films have been evaluated and discussed in the light of different preparation conditions and working temperatures. Thin films have been obtained by evaporating high purity WO 3 powder at 5 × 10 −4 Pa on sapphire substrates provided with Pt interdigital sputtered electrodes and annealed at 500°C for 6, 12 and 24 h. The film morphology, crystalline phase and chemical composition have been characterized through AFM, glancing angle XRD and XPS. The as-deposited film is amorphous with WO 3 stoichiometry on the surface, after annealing at 500°C the films are well crystallised but with preferential orientation of WO 3 along the (200) plane. The increasing of the annealing time shows a positive effect on the crystallite and grain size of the film, while the mean roughness and surface area difference slightly decrease. The binding energies of the annealed films are close to that of WO 3 and small downshifts from the characteristic binding energy of W 4 f 7 2 reflects the formation of oxygen vacancies on the longer time annealed films. All the films show the highest sensitivity to NO 2 gas (0.7–5 ppm concentration range), at 250°C working temperature. At this temperature and 1.7 ppm NO 2 the calculated sensitivities yield S = 12, S = 43 and S = 45 for 6, 12 and 24 h annealed films, respectively. No cross sensitivity has been found by exposing the WO 3 films to CO and CH 4. Negligible H 2O cross to NO 2 has resulted for the 24 h annealed film in the 40–80% relative humidity range, as well as to 300 ppm SO 2 and 10 ppm NO. Only 1000 ppm C 2H 5OH has resulted in a significant cross to the NO 2 measure. The increase in the annealing time had positive effects on the sensitivity, cross sensitivity and long term stability properties. The 45-fold increase in the resistance of the 24 h annealed on exposure to 1.7 ppm of NO 2, as well as the good long term stability properties of its electrical response, suggest the possibility of utilising the sensor for air-quality monitoring.