Birnessite-type manganese oxides nanosheets with hole acceptor assisted photoelectrochemical activity in response to visible light

Abstract

A cost-effective and simple electroplating technique has been developed to prepare layered manganese oxide (MnO₂) arrays as a promising material for solar hydrogen production and waste-water cleaning through photoelectrochemical (PEC) process. The microstructure of these MnO₂ nanosheets can be referenced to Birnessite-type, as characterized by Raman spectra and transmission electron microscopy. The bandgap energy of the as-grown nanosheets determined from UV-vis spectroscopy is about 2.1 eV. Mott–Schottky plots show the flat band potential of the MnO₂ nanosheets to be −0.01 V and a donor concentration of 4.68 × 10²⁰ cm⁻³. Remarkable photocurrent in response to visible light is observed in the presence of hole acceptors, such as sodium formate and methanol, which efficiently suppress the recombination loss of electron–hole pairs from localized d–d transitions within manganese ion. Meanwhile, the transient photocurrent–time responses and the effect of different hole acceptors on PEC activity are studied with an increase of respective hole acceptor concentration, and the results reveal the critical role in the process of absorption and decomposition of the hole acceptor. Significantly, the MnO₂ nanosheets exhibit an incident photon-to-electron conversion efficiency of 7% in response to the monochromatic wavelength of 400 nm, which is comparable to that from hematite (α-Fe₂O₃). These results demonstrate the nanoporous MnO₂ nanosheets have great potential in solar hydrogen applications and organic pollutant cleaning.