An electrochemical, microtopographical and ambient pressure x-ray photoelectron spectroscopic investigation of Si/TiO₂/Ni/electrolyte interfaces

The electrical and spectroscopic properties of the TiO₂/Ni protection layer system, which enables stabilization of otherwise corroding photoanodes, have been investigated in contact with electrolyte solutions by scanning-probe microscopy, electrochemistry and in-situ ambient pressure X-ray photoelectron spectroscopy (AP-XPS). Specifically, the energy-band relations of the p⁺-Si/ALD-TiO₂/Ni interface have been determined for a selected range of Ni thicknesses. AP-XPS measurements using tender X-rays were performed in a three-electrode electrochemical arrangement under potentiostatic controlto obtain information from the semiconductor near-surface region, the electrochemical double layer (ECDL) and the electrolyte beyond the ECDL. The degree of conductivity depended on the chemical state of the Ni on the TiO₂ surface. At low loadings of Ni, the Ni was present primarily as an oxide layer and the samples were not conductive, although the TiO₂ XPS core levels nonetheless displayed behavior indicative of a metal-electrolyte junction. In contrast, as the Ni thickness increased, the Ni phase was primarily metallic and the electrochemical behavior became highly conductive, with the AP-XPS data indicative of a metal-electrolyte junction. Electrochemical and microtopographical methods have been employed to better define the nature of the TiO₂/Ni electrodes and to contextualize the AP-XPS results.