Effect of citrate anion incorporation on dielectric properties of anodic oxide grown on Ti-Si alloy

Anodic oxides were galvanostatically grown on sputtered Ti-12at%Si alloy at 5 mA cm-2 up to 50 V in phosphoric acid and sodium citrate aqueous electrolytes in order to investigate the effect of anions incorporation (if any) on dielectric properties of the grown oxides. Photoelectrochemical measurements supported the occurrence of incorporation of citrate anions when anodizing is carried out in a citrate-containing solution. Indeed, two different absorption thresholds were estimated due to energy transitions involving localized states in the mobility gap. Differential capacitance measurements showed higher capacitance values for anodic oxide grown in the citrate-containing solution, with a beneficial effect on the dielectric constant. The increased capacitance can be explained considering the synergistic effect of a more favourable anodizing ratio provided by the presence of Si cations from the bare alloy as well as the incorporation of foreign species due to the inward migration of citrate anions from the electrolyte during the anodizing process.