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

• Published in: Advanced functional materials Vol. 30; no. 10
• Main Authors: Hermans, Yannick, Klein, Andreas, Sarker, Hori Pada, Huda, Mohammad N., Junge, Henrik, Toupance, Thierry, Jaegermann, Wolfram
• Format: Journal Article
• Language: English
• Published: Hoboken Wiley Subscription Services, Inc 01.03.2020
• Subjects: Chemical properties; CuFeO2; Current carriers; Density functional theory; Fermi level; Fermi level pinning; Heterostructures; Materials science; Organic chemistry; Performance tests; photocatalysis; Photocathodes; Photoelectric effect; Photoelectric emission; photoelectron spectroscopy; Photoelectrons; Pinning; Platinum; Polarons; Reaction kinetics; Reduction; Silver; Water splitting
• ISSN: 1616-301X; 1616-3028
• DOI: 10.1002/adfm.201910432

CuFeO2 is recognized as a potential photocathode for photo(electro)chemical water splitting. However, photocurrents with CuFeO2‐based systems are rather low so far. In order to optimize charge carrier separation and water reduction kinetics, defined CuFeO2/Pt, CuFeO2/Ag, and CuFeO2/NiOx(OH)y heterostructures are made in this work through a photodeposition procedure based on a 2H CuFeO2 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 CuFeO2, the Fermi level tunability in CuFeO2 is evaluated by creating CuFeO2/ITO and CuFeO2/H2O interfaces and analyzing the electronic and chemical properties of the interfaces through photoelectron spectroscopy. The results indicate that Fermi level pinning at the Fe3+/Fe2+ electron polaron formation level may intrinsically prohibit CuFeO2 from acquiring enough photovoltage to reach the water reduction potential. This result is complemented with density functional theory calculations as well. Heterostructured Pt/CuFeO2, Ag/CuFeO2, and NiOxOHy/CuFeO2 hexagonal nanoplatelets made through photodeposition are tested for photochemical water reduction. However, these heterostructures demonstrate limited water reduction efficiencies. Interface experiments show that Fermi level pinning at the Fe3+/Fe2+ charge transition level intrinsically limits the CuFeO2 Fermi level from rising above the H+/H2 redox level, thus inhibiting water reduction.