Birnessite: A Layered Manganese Oxide To Capture Sunlight for Water-Splitting Catalysis

We show a comprehensive study on the structure and electronic properties of a layered manganese oxide commonly known as birnessite. We present the effects of substituting different intercalated cations (Li⁺, Na⁺, K⁺, Be²⁺, Mg²⁺, Ca²⁺, Sr²⁺, Zn²⁺, B³⁺, Al³⁺, Ga³⁺, Sc³⁺, and Y³⁺) and the role of waters in the intercalated layer. The importance of the Jahn–Teller effect and ordering of the Mn³⁺ centers due to cation intercalation are addressed to explain the ability to tune the indirect band gap (<i>E</i>_gⁱ) from 2.63 to ∼2.20 eV and the direct band gap (<i>E</i>_g^d) from 3.09 to ∼2.50 eV. By aligning the structures’ bands, we noted that structures with Sr, Ca, B, and Al have potential for usage in water splitting, and anhydrous B-birnessite is predicted to have a suitable direct band gap for light capturing. Furthermore, we also demonstrate how the effects of cations in the bulk differ from the behavior on single layer surfaces. More specifically, we show that an indirect to direct band transition is observed when we separate the bulk into a single layer oxide. This study shows a new strategy for tuning the band gap of layered materials to capture light which may couple to its intrinsic water-splitting catalytic properties, thus resembling photosynthesis