Anodic-cathodic formation of pH-sensitive TiO2-MoOx films on titanium

A new type of solid-state metal-oxide pH sensor based on TiO2-MoOx films on titanium has been fabricated by plasma electrolytic oxidation in the anodic-cathodicmode. The structural features of the films have been investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). According to XPS, the surface layers of the resulting films contain up to 5 at.% Mo in the form of oxides (MoO3, Mo2O5 or Mo4O11) and the near-surface layers additionally contain MoO2. The potentiometric pH response of the TiO2-MoOx electrode reveals near-Nernstian behavior, i.e. a linear working range from pH 2 to 12 with a slope of about 64 +/- 2 mV/pH. The mixed films are characterized by very fast response time in alkali and acids solutions shorter than 5 s. The response is not affected by the direction of the pH change and the presence of K+ and Na+ ions. The resulting electrodes have been tested as pH sensors in acid-base titration. The relative error in the determination of NaOH using Ti/TiO2-MoOx electrode does not exceed 2.5%. The possibility of using electrodes based on TiO2-MoOx films to accurately determine the alkalinity of industrial waters is shown. In addition, this sensor offers some analytical characteristics, such as high sensitivity, good reproducibility and simple preparation procedure. Compared to a glass electrode, the Ti/TiO2-MoOx electrode has several advantages, including low cost and high stability.