Pinning of the Fermi Level in CuFeO2 by Polaron Formation Limiting the Photovoltage for Photochemical Water Splitting

Abstract CuFeO 2 is recognized as a potential photocathode for photo(electro)chemical water splitting. However, photocurrents with CuFeO 2 ‐based systems are rather low so far. In order to optimize charge carrier separation and water reduction kinetics, defined CuFeO 2 /Pt, CuFeO 2 /Ag, and CuFeO 2 /NiO x (OH) y heterostructures are made in this work through a photodeposition procedure based on a 2H CuFeO 2 hexagonal nanoplatelet shaped powder. However, water splitting performance tests in a closed batch photoreactor show that these heterostructured powders exhibit limited water reduction efficiencies. To test whether Fermi level pinning intrinsically limits the water reduction capacity of CuFeO 2 , the Fermi level tunability in CuFeO 2 is evaluated by creating CuFeO 2 /ITO and CuFeO 2 /H 2 O interfaces and analyzing the electronic and chemical properties of the interfaces through photoelectron spectroscopy. The results indicate that Fermi level pinning at the Fe 3+ /Fe 2+ electron polaron formation level may intrinsically prohibit CuFeO 2 from acquiring enough photovoltage to reach the water reduction potential. This result is complemented with density functional theory calculations as well.