Identification of a Fe-Dependent Optical Mode in CuAl₁–ₓFeₓO₂

Delafossites are promising candidates for photocatalysis applications because of their chemical stability and absorption in the solar region of the electromagnetic spectrum. For example, CuAlO₂ has good chemical stability but has a large indirect band gap, so that efforts to improve its absorption in the solar region through alloying are investigated. The effect of dilute alloying on the optical absorption of powdered CuAl₁–ₓFeₓO₂ (x = 0.0–1.0) is measured and compared to electronic band structures calculations using a generalized gradient approximation with Hubbard parameter and spin. A new absorption feature is observed at 1.8 eV for x = 0.01, which red-shifts to 1.4 eV for x = 0.10. This feature is associated with transitions from the L-point valence band maximum to the Fe-3d state that appears below the conduction band of the spin-down band structure. The feature increases the optical absorption below the band gap of pure CuAlO₂, making dilute CuAl₁–ₓFeₓO₂ alloys better suited for solar photocatalysis.