MnO 2 nanorods were synthesized by mechanochemical processing with subsequent heat treatment and their photocatalytic activity was studied on the decolourization of aqueous solution of Rhodamine B at different pH levels. A solid state redox reaction 2KMnO 4 + MnCl 2 → 3MnO 2 + 2KCl + O 2 was activated during mechanical milling. Excess KCl salt was added in the starting powder mixture to prevent agglomeration of MnO 2 nanoparticles. The milling resulted in the production of amorphous MnO 2 nanoparticles with a high surface area of 204 m 2 g -1 . Crystalline MnO 2 nanorods of diameters about 15 nm -20 nm were produced by heating the as-milled powder at 350°C for 1 hour in air. Amorphous MnO 2 nanoparticles showed higher degradation rate of Rhodamine B than crystalline MnO 2 nanorods under simulated sunlight. The degradation rate was higher under acidic conditions. This work demonstrates the potential for cost effective, green and scalable synthesis of MnO 2 nanocatalysts for environmental applications.

Calcium manganese oxide catalysts are a new class of redox catalysts with significant importance because of their structural similarity to natural oxygen-evolving complex in plant cells and the earth-abundant elemental constituents. In the present study, the photo-electrocatalytic properties of CaMn2O4 in water-splitting were investigated. CaMn2O4 powders with irregular shapes and nanowire shapes were synthesised using mechanochemical processing and a hydrothermal method, respectively. The anode in a photo-electrochemical cell was fabricated by embedding CaMn2O4 powders within polypyrrole. The results showed that CaMn2O4 induced a higher dark and light current in comparison to the control sample (polypyrrole alone). CaMn2O4 nanowires exhibited higher dark and light current in comparison to irregular-shaped CaMn2O4 powders. The difference was attributable to the higher surface area of nanowires compared to the irregular-shaped particles, rather than the difference in exposed crystal facets.

Bio-inspired calcium manganate ceramics induce higher photocurrents than MnO2 in photo-electrochemical water splitting.

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