Influence of molybdenum species on growth of anodic titania

TiMo, bcc, solid-solution alloys, containing 11.5–37.0 at.% molybdenum, have been anodised galvanostatically in 0.1 mol dm −3 ammonium pentaborate and 1.0 mol dm −3 phosphoric acid electrolytes, with resultant anodic films characterised by scanning electron microscopy, transmission electron microscopy, Rutherford backscattering spectroscopy and glow discharge optical emission spectroscopy. Uniform amorphous films are formed at high current efficiency to >100 V, with formation ratios of 2.3 and 2.2 nm V −1 in the respective electrolytes, contrasting with the amorphous-to-crystalline transition of anodic titania on titanium that occurs at ∼20–50 V. Apart from minor incorporation of electrolyte species, the films comprise an outer layer of TiO 2 and an inner oxide layer containing Ti 4+ and Mo 6+ ions. The films grow by migration of both cations and anions, with Ti 4+ ions migrating faster than Mo 6+ ions that is related to the energies of Ti 4+O and Mo 6+O bonds.