Development of a Nb-Doped TiO_2 Semiconductor pH Sensor for the Use in High-Temperature Aqueous Solutions

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A semiconductor pH sensor using a Bh-doped TiO_2 single crystal has been developed for pH measurement in high temperature aqueous solutions. The pH response of this sensor is based on the pH-dependent change in the flatband potential, E_<fb>, of semiconductor electrode. The value of E_<fb> can be obtained by measuring the Mott-Schottky plot on the semiconductor electrode in a solution. The measurement of the plots has been done on the Nb_2O_5-doped TiO_2 single crystal electrodes with different doping levels in solutions having various composition and pH's at temperatures in the range 298-523K. It was found that the plots measured using the 0.01mass% Nb_2O_5-doped electrode at a frequency of 10Hz had good linearity under all experimental conditions used and the relationships between the values of E_<fb> and the solution pH's showed straight lines with the Nernstian slopes (-2.303RT/F). The pH of pure water at temperatures up to 523K could be measured by this electrode. The pH response of the sensor was fast, stable and not affected by redox systems in the solutions. Interfering effects were observed, however, at higher K^+ and Na^+ concentrations.