Hierarchical 2D Sheets and 3D Flowers Composed of α-Mn₂O₃ Nanoparticles for Photoelectrochemical Water Oxidation

Hierarchical 2D sheets and 3D flower-shaped α-Mn 2 O 3 materials were successfully synthesized using a hydrothermal reaction, followed by a subsequent heat treatment. The pH of the solvent plays a crucial role in the synthesis and the hierarchical self-assembly process. At pH 8, sheet-shaped materials were produced. However, by increasing the pH of the medium, the sheets self-assembled into flower-shaped materials, where the concentration of the bridging agent (−OH) guided the self-assembly pattern. Upon heat treatment, the BET surface area significantly increased for flower-shaped materials (from 8.46 to 122.3 m 2 g –1 ) compared to sheets (from 9.35 to 26.6 m 2 g –1 ). The flower-shaped materials exhibited three-dimensional porous frameworks, which not only contributed to a high surface area but also improved electron conductivity and provided a large surface area for electrolyte contact. The band structure of α-Mn 2 O 3 suggests that the photooxidation of water to produce oxygen is favorable, as the conduction band bottom of α-Mn 2 O 3 is more positive relative to the H + /H 2 potential. These flower-shaped materials demonstrated excellent photoelectrocatalytic performance for the OER, achieving a Tafel slope of 163 mV/dec at a 341 mV overpotential value. In contrast, sheet-shaped materials showed an overpotential value of 516 mV with a Tafel slope of 213 mV/dec.