The CuFeO2-Water Interface under Illumination: Structural, Electronic, and Catalytic Implications for the Hydrogen Evolution Reaction

CuFeO2 is a p-type semiconductor that has been recently identified as a promising photocathode material for photoelectrochemical water splitting. CuFeO2 can absorb solar light and promote the hydrogen evolution reaction (HER), even though the photocurrents achieved so far are still well below the theoretical upper limit. While several experimental and theoretical works have provided a detailed characterization of the bulk properties of this material, surfaces have been largely unexplored.
In this work, we perform first-principles simulations based on DFT to investigate the structure, electronic properties, and thermodynamic stability of CuFeO2 surfaces both in vacuum and in an electrochemical environment. To estimate the alignment of the band edges on the electrochemical scale, we perform ab initio molecular dynamics in explicit water, unraveling the structure of the solid/liquid interface for various surface terminations.
We then introduce a novel scheme to determine, in a self-consistent manner, the most stable termination under illumination. In this way we can link the electronic structure of the photocatalyst, the structure of the interface and the thermodynamic stability of the surface terminations to predict the behavior of the material under illumination.
In particular, we show that light absorption can induce partial reduction of the surface, giving rise to states in the gap that can pin the Fermi level, in agreement with recent measurements.
Using the free energy of adsorption of atomic hydrogen as a descriptor of the catalytic activity for the HER, we show that hydride species formed at oxygen vacancies can be highly active and could therefore be an intermediate of reaction.

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Dated: February 5, 2021