Application of sol–gel TiO2 film for an extended-gate H+ ion-sensitive field-effect transistor

Abstract 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 OH − ions.

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