Application of sol-gel TiO_2 film for an extended-gate H~+ ion-sensitive field-effect transistor

摘要

In this study, a sol—gel TiO_2 thin film has been spin-coated on a commercial ITO glass substrate as the extended-gate field effect transistor (EGFET) for hydrogen ion sensing. The as-deposited films are further annealed at various temperatures (T_a) under ambient atmosphere. It is found that the bi-layer structure of TiO_2/ITO EGFET exhibits good linear sensitivity from pH 1 to 11. Anatase TiO_2 appeared as early as T_a = 200 °C and a brookite (121) diffraction evolved at T_a = 500 °C. No prominent influence on the surface fine structures could be found at higher T_a. Due to the reduction or disappearance of the surface hydroxyl groups on TiO_2, the sensitivities of the TiO_2/ITO pH-EGFET device are rapidly reduced. However, the influence of the conductivity decay for ITO substrates annealed at high T_a could not be excluded. A maximum sensitivity 61.44 mV/pH is achieved as T_a = 300 °C. The bi-layer structure of TiO_2/ITO exhibits better long-term stability than the traditional ITO sensing membranes. In addition, the asymmetric hysteresis is more significant in alkaline buffer solutions, which could be explained by a site-binding model because the diffusion of H~+ ions into the buried sites of the sensing film is more rapid than that of OUT ions.