Exploring visible-light-responsive photocatalysts for water splitting based on novel band-gap engineering strategies.

摘要

The increasing demand for renewable energy is driving innovations in both science and technology. Hydrogen has been broadly identified as a potential clean energy carrier due to its high energy capacity (enthalpy of combustion is as high as 286 kJ/mol) and environmental friendliness (the only product after burning is water). Meanwhile, solar energy is renewable, abundant and easily available. Solar-driven H2 production from water has therefore attracted global attention in the past decades since the first report of photoelectrochemical (PEC) water splitting based on an n-type TiO2 photoanode. Many semiconductor-based materials have been synthesized and studied for their photocatalytic or PEC performance. Among them, visible-light-active photocatalytsts are more promising since the energy of the visible-light region takes up a large proportion of the whole solar spectrum. Herein, we found two novel groups of compounds, e.g. copper borates and boron carbides. We investigated the origins and performances of their photocatalytic water splitting under visible light irradiation. It is found that the visible light activities of the two groups are resulted from different band-gap engineering mechanisms. For the two copper borates compounds, e.g. CuB2O4 and Cu3B2O6, we found that the visible light activity is from the intrinsic midgap states in the two compounds. Both midgap states serve as an electron acceptor level, but they function very differently in the two copper borates. For CuB2O4, the midgap states facilitate the visible light absorption for photocatalytic water splitting, while for Cu3B2O6, the midgap states trap electrons and reduce the photocatalytic activity. For the two boron carbides, e.g. B4.3C and B13C2, they exhibit efficient photocatalytic H2 evolution and PEC H2 evolution as a stable photocathode under visible light irradiation. Interestingly, it is found that the inherent defects and structural distortions in B4.3C cause a continuum downshift of its conduction band (CB) edge that facilitates visible-light absorption and water splitting based on density functional theory (DFT) calculations. In B13C2, however, the more complicated structural defects and distortions result in a large number of midgap states between the CB and the valence band (VB), which reduce its overall photocatalytic and PEC water splitting efficiency by promoting charge recombination.