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

• Published in: Journal of physical chemistry. C Vol. 115; no. 23; pp. 11830 - 11838
• Main Authors: Pinaud, Blaise A, Chen, Zhebo, Abram, David N, Jaramillo, Thomas F
• Format: Journal Article
• Language: English
• Published: American Chemical Society 16.06.2011
• Subjects: C: Energy Conversion and Storage
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/jp200015p

Inexpensive and earth-abundant, manganese oxides (Mn x O y ) 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.