Xylene sensor using double-layered thin film and Ni-deposited porous alumina

A xylene sensor using a double-layered metal-oxide thin film and Ni-catalyst deposited on a porous alumina is presented. The sensing film has a double-layered structure; the first layer is Fe2O3 + TiO2 (5 mol%) + MgO (4 mol%), and the second layer is WO3 + TiO2 (10 mol%). The thicknesses of the first and second layers are 200 nm and 70 nm, respectively. Both response and stability are improved by adopting a double-layered structure. The Ni-deposited porous alumina is placed over the sensing film. The pores in the porous alumina are cylindrical. The inner diameter and the height of the pores are about 200 nm and 60 μm, respectively. Xylene gas goes through the Ni-deposited pores in the porous alumina to reach the sensing film. The catalytic action of the deposited Ni enhances the chemical activity of xylene gas and the amount of its absorption on the sensing film. Thus the sensor is highly sensitive to xylene gas. The sensor is capable of detecting xylene at a concentration as low as 0.07 ppm. Moreover, the selectivity to xylene gas is satisfactory. The response and recovery times are approximately 20 s and 300 s, respectively, at an operating temperature of 340 °C. All films and Ni are deposited by rf sputtering while the porous alumina is formed by anodic oxidation of aluminum.