Synthesis, Characterization, Electronic Structure, and Photocatalytic Behavior of CuGaO₂ and CuGa_1–<i>x</i>Fe_<i>x</i>O₂ (<i>x</i> = 0.05, 0.10, 0.15, 0.20) Delafossites

Abstract

The photochemical reduction of CO₂ to chemicals, such as CO and CH₄, is a promising carbon management approach that can generate revenue from chemical sales to help offset the costs associated with the use of carbon-management technologies. Delafossite materials of the general stoichiometry ABO₂ are a new class of photocatalysts being considered for this application. Symmetry breaking in these materials, by chemical substitution, modifies the band structure of the solid, which enhances optical transitions at the fundamental gap and can therefore be used to engineer the photocatalytic performance of delafossites by adjusting the alignment of band edges with chemical redox potentials and enhancing the optical activity associated with the production of photoexcited charge carriers. The photochemical activity of CuGaO₂ and CuGa_1–<i>x</i>Fe_<i>x</i>O₂ (<i>x</i> = 0.05, 0.10, 0.15, 0.20) for the reduction of CO₂ has been studied. Our results show that the CuGaO₂ materials investigated have an optical gap at ∼3.7 eV in agreement with previous literature reports. An optical feature is also observed at ∼2.6 eV, which is not as commonly reported due to a weak absorption cross section. Alloying at the B-site with Fe to form CuGa_1–<i>x</i>Fe_<i>x</i>O₂ (<i>x</i> = 0.05, 0.10, 0.15, 0.20) creates new features in the visible and near-infrared region of the optical spectra for the substituted materials. Electronic density of states calculations indicate that B-site alloying with Fe creates new midgap states caused by O atoms associated with Fe substitution sites; increased Fe concentration contributes to broadening of these midgap states. The strain caused by Fe incorporation breaks the symmetry of the crystal structure giving rise to the new optical transitions noted experimentally. The photoreduction of CO₂ in the presence of H₂O vapor using CuGaO₂ and CuGa_1–<i>x</i>Fe_<i>x</i>O₂ produces CO with little evidence for other products such as H₂ or hydrocarbons. The impact of Fe alloying with Ga on the band structure and photochemical activity of this delafossite system is discussed