Silicon protected with atomic layer deposited TiO2: Conducting versus tunnelling through TiO₂

The present work demonstrates that tuning the donor density of protective TiO₂ layers on a photocathode has dramatic consequences for electronic conduction through TiO₂ with implications for the stabilization of oxidation-sensitive catalysts on the surface. Vacuum annealing at 400 °C for 1 hour of atomic layer deposited TiO2 increased the donor density from an as-deposited value of 1.3 × 1019 cm -3 to 2.2 × 1020 cm-3 following the annealing step. Using an Fe(ii)/Fe(iii) redox couple it was shown that the lower dopant density only allows electron transfer through TiO₂ under conditions of weak band bending. However it was shown that increasing the dopant density to 2.2 × 1020 cm-3 allows tunneling through the surface region of TiO2 to occur at significant band bending. An important implication of this result is that the less doped material is unsuitable for electron transfer across the TiO2/electrolyte interface if the potential is significantly more anodic than the TiO₂ conduction band due to moderate to large band bending. This means that the lesser doped TiO₂ can be used to prevent the inadvertent oxidation of sensitive species on the surface (e.g. H2 evolution catalysts) as long as the redox potential of the material is significantly more anodic than the TiO₂ conduction band. Conversely, for situations where an oxidative process on the surface is desired, highly doped TiO₂ may be used to enable current flow via tunneling.