Visible-light-responsive copper(II) borate photocatalysts with intrinsic midgap states for water splitting

Two copper borates, CuB2O4 and Cu3B2O6, produce either H2 or O2 under visible light in the presence of sacrificial agents. Both copper borates exhibit intrinsic midgap states between the valence bands and conduction bands. However, structural differences in the two compounds lead to midgap states with remarkably different photocatalytic behaviors. In CuB2O4, the midgap states facilitate visible light absorption and enhance photocatalytic activity, while in Cu3B2O6 they act as trap states that reduce photocatalytic activity.