Hierarchically Porous Calcium-containing Manganese Dioxide Nanorod Bundles with Superior Photoelectrochemical Activity

Bioinspired by the composition of the oxygen evolving complex and the fundamental role of calcium for catalysis, we have synthesized calcium-manganese oxides as promising photo-electrodes. We report the first demonstration of hierarchically porous Ca-containing MnO2 nanorod (NR) bundles as visible-light-sensitive photofunctional nanoelectrodes to fundamentally improve the performance of MnO2 for photoelectrochemical hydrogen generation. A substantial amount of Ca (up to 7.8 atom%) can be in situ incorporated into the MnO2 lattice by a simple electroplating technique because of the exceptionally small feature sizes of several nanorods. The maximum photocurrent could be successfully achieved as high as 0.42 mAcm (2), which is the best value for a MnO2 photoanode to date. Significantly, Ca-containing MnO2 photoanodes illustrated striking photoelectrochemical activity in response to visible light with a high incident photon-to-current conversion efficiency of 7% at a monochromatic wavelength of 450 nm. The improvement in photoactivity of photoelectrochemical response may be attributed to the enhanced visible-light absorption, increased charge-carrier densities, and large contact area with electrolyte owing to the synergistic effects of Ca incorporation and specific mesopore networks, thus contributing to photocatalysis. The new design of constructing highly photoactive Ca-containing MnO2 nanorod bundles sheds light on developing high-efficiency photoelectrodes for solar hydrogen generation.