Thin Films of Sodium Birnessite-Type MnO2: Optical Properties, Electronic Band Structure, and Solar Photoelectrochemistry

Inexpensive and earth-abundant, manganese oxides (MnxOy) have attracted considerable attention for catalysis, but fewer efforts have focused on their semiconducting properties. In this study, crystalline Na birnessite-type MnO2 thin films were investigated for their surface and bulk chemistry in the context of hydrogen production by photoelectrochemical water splitting. Thin films were synthesized by electrodeposition onto fluorine-doped tin oxide (FTO) substrates, imaged by scanning electron microscopy (SEM), and characterized by X-ray diffraction (XRD) as well as X-ray photoelectron spectroscopy (XPS) and UV-vis spectroscopy. Three different electrochemical methods (illuminated open circuit potential, potential of photocurrent onset, and Mott-Schottky plots) were used to probe the flat-band potential required to construct a band diagram for the material. Photostability, conductivity, and band structure are discussed as potential causes of the low external quantum efficiency (<1%) for the birnessite-type MnO2 photoanode. The position of the conduction band well below the hydrogen evolution potential likely mitigates this material's potential use in a single absorber configuration, but its chemical, optical, and electronic characteristics as shown in this work may be well-suited for a photoanode in a tandem device.