Photocurrent Generation from Semiconducting Manganese Oxide Nanosheets in Response to Visible Light

Abstract

Unilamellar nanosheet crystallites of manganese oxide generated the anodic photocurrent under visible light irradiation (λ < 500 nm), while the nanosheets themselves were stable as revealed by in-plane XRD and UV−visible absorption spectra. The band gap energy was estimated to be 2.23 eV on the basis of the photocurrent action spectrum. The molecular thickness of ∼0.5 nm may facilitate the charge separation of excited electrons and holes, which is generally very difficult for strongly localized d−d transitions. The monolayer film of MnO₂ nanosheets exhibited the incident photon-to-electron conversion efficiency of 0.16% in response to the monochromatic light irradiation (λ = 400 nm), which is comparable to those for sensitization of monolayer dyes adsorbed on a flat single-crystal surface. The efficiency declined with increasing the layer number of MnO₂ nanosheets, although the optical absorption was enhanced. The recombination of the excited electron−hole pairs may become dominant when the carriers need to migrate a longer distance than 1 layer through multilayered nanosheets.