Characterization of Sol‐Gel‐Derived TiO2 Coatings and Their Photoeffects on Copper Substrates

In accordance with the corrosion protection of Cu by n ‐ TiO 2 coating under illumination, the relation between the structural characterization and the photoelectrochemical properties of sol‐gel‐derived TiO 2 coating has been investigated utilizing glow discharge spectroscopy, x‐ray diffraction, and x‐ray photoelectron spectroscopy analysis techniques. As a result of the well‐known shrinkage of the gel under heat‐treatment, the thickness of the TiO 2 coating decreased with increasing heating temperature, about a 50% decrease from 200 to 800°C. Amorphous TiO 2 gel was found to be crystallized above 400°C, which gave rise to a great enhancement of the photocurrent of the TiO 2 coating. In relating these results to the photoelectrochemical behavior of TiO 2 ‐ coated Cu, it was revealed that both the significant change in the photocurrent and the existence of Cu oxides in the coating were not the direct reasons which accounted for the critical temperature for TiO 2 to impose its photoeffect on the Cu substrate. The dramatic change in the photopotential of TiO 2 ‐ coated Cu would be explained by the change of Schottky barrier at the TiO 2 / Cu interface in terms of the Fermi level pinning at the Ti 3+ defect level. Nevertheless, the increase in the photocurrent of TiO 2 coating was beneficial to move the photopotential of TiO 2 − coated Cu to a much less noble level. The degradation of the photoeffect of TiO 2 coating heated above 800°C was due to the significant diffusion of Cu into the coating.