C6h12o6

d‐Glucose (630 mg, C₆H₁₂O₆, Sigma–Aldrich, 99.5 %) was dissolved in DI water (40 mL). The above harvested FeS nanosheets (75 mg) were dispersed into the d‐glucose solution. Subsequently, the solution was transferred into a 50 mL Teflon‐lined autoclave and heated at 180 °C for 12 h. After cooling down to room temperature, the product was poured out and washed by using rinse–precipitation cycles with DI water. The product was dried at 80 °C. Finally, the harvested black powder was kept in Ar/H₂ (Ar/H₂=95:5) at 600 °C for 5 h with a heating rate of 10 °C min⁻¹.

Scheme 1 shows the synthesis strategy of Mn₂O₃ from the starting materials. To prepare MnCO₃, 0.3 mmol D-(+)-glucose (C₆H₁₂O₆, Merck Millipore) and 0.3 mmol KMnO₄ (Sigma-Aldrich, 97%) were dissolved in 60 ml deionised water at room temperature and stirred to form a homogeneous solution. Then, the homogeneous mixture was transferred into a 125 ml stainless steel autoclave, sealed and heated at 150 °C for 10 h. During the hydrothermal reaction, MnO₄⁻ was reduced by glucose and Mn²⁺ ions generated, leading to the formation of MnCO₃ according to the Eq. (5) below^{49 ,50} . $$24{\mathrm{KMnO}}_4+5{\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6=24{\mathrm{MnCO}}_3+6{\mathrm{K}}_2{\mathrm{CO}}_3+12\mathrm{KOH}+24{\mathrm{H}}_2\mathrm{O}$$ Scheme 1

A schematic presentation for the formation of Mn₂O₃.

The precipitates were collected, washed several times with absolute ethanol and deionised water and dried overnight under vacuum. The dried sample is marked as (MnCO₃ (S)). To obtain Mn₂O₃, MnCO₃ (S) was calcined at 500, 600 and 700 °C in air for 2 h and denoted as Mn₂O₃ (S500), Mn₂O₃ (S600) and Mn₂O₃ (S700), respectively. For comparison purpose, as-received MnCO₃ (Sigma-Aldrich, 98%) marked as MnCO₃ (C) was also used in this study. MnCO₃ (C) was calcined at 600 °C in air for 2 h and later denoted as Mn₂O₃ (C600).