Fermi level pinning in metal oxides: influence on photocatalysis and photoelectrochemistry

Photocatalysis and photoelectrochemical (PEC) reactions are complex processes involving both the physical properties and surface chemistry of the semiconductor photocatalyst. Their interplay applies specific limitations on the performance of different materials in light-driven reactions, often despite their optimal band structure and optical absorption. One of the ways to properly characterize the photocatalytic and PEC properties of semiconductors remains the measurement of the photopotential, which characterizes a driving force of photoinduced processes in the material. In this work, we give a general scope on the photopotential in PEC reactions that finds its origin in semiconductor physics. It is shown that the photopotential does not always play an interchangeable role with the photocurrent in comparative analysis of the photocatalytic performance of different materials. Furthermore, a correlation between the photopotential and the kinetics of methylene blue dye photocatalysis is shown for anatase-TiO₂, CeO₂ and WO₃ as photocatalysts. Fermi level pinning (FLP) in the bandgap of CeO₂ is observed limiting the photoactivity of the compound, which is attributed to the high defectivity of CeO₂. A short review is given on the possible origins of FLP in metal oxides and ways to overcome it. It is pointed out that the shift of the Fermi level after illumination of CeO₂ can trigger the chemical instability of the material accompanied by the FLP process.