Narrowing the band gap and suppressing electron–hole recombination in β-Fe₂O₃ by chlorine doping

The effects of halogen (F, Cl, Br, I, and At) doping in the direct-band-gap β-Fe₂O₃ semiconductor on its band structures and electron–hole recombination have been investigated by density functional theory. Doping Br, I, and At in β-Fe₂O₃ leads to transformation from a direct-band-gap semiconductor to an indirect-band-gap semiconductor because their atomic radii are too large; however, F- and Cl-doped β-Fe₂O₃ remain as direct-band-gap semiconductors. Due to the deep impurity states of the F dopant, this study focuses on the effects of the Cl dopant on the band structures of β-Fe₂O₃. Two impurity levels are introduced when Cl is doped into β-Fe₂O₃, which narrows the band gap by approximately 0.3 eV. After doping Cl, the light-absorption edge of β-Fe₂O₃ redshifts from 650 to 776 nm, indicating that its theoretical solar to hydrogen efficiency for solar water splitting increases from 20.6% to 31.4%. In addition, the effective mass of the holes in halogen-doped β-Fe₂O₃ becomes significantly larger than that in undoped β-Fe₂O₃, which may suppress electron–hole recombination.