Synthesis, Characterization, Electronic Structure, and Photocatalytic Behavior of CuGaO₂ and CuGa_1–xFe_xO₂ (x = 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 CuGa1–x Fe x O2 (x = 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 CuGa1–x Fe x O2 (x = 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 CuGa1–x Fe x 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.

Comparative study of the structural and optical properties of epitaxial CuFeO2 and CuFe1−xGaxO2 delafossite thin films grown by pulsed laser deposition methods

“…The fundamental direct Tauc-gap measured at 1.5 eV from transmittance is comparable to the absorption edge reported by Lekse et al [20] for CuFe 1-x Ga x O 2 with x = 0.85 who describe the position of this absorption edge as being independent of fractional Fe content. As with the studies of Lekse et al [20], an expected outcome of B-site alloying of Ga and Fe is the breaking of delafossite crystal symmetry, which is expected to modify selection rules leading to the increased availability of transition states.…”

“…As with the studies of Lekse et al [20], an expected outcome of B-site alloying of Ga and Fe is the breaking of delafossite crystal symmetry, which is expected to modify selection rules leading to the increased availability of transition states. Moreover, Huda et al [10] also have described changes from forbidden to allowed dipole transitions, in relation to changes in localized strain imposed by the substitution of smaller Y for Ga atoms into a CuYO 2 host lattice.…”

“…More specific to this work, the chemical synthesis and characterization of CuGa 1-x Fe x O 2 delafossite has been reported previously [20], where it was shown that the substitution of Ga for Fe within CuGaO 2 compounds led to a strong absorption edge measured at 1.5eV which was not observed in the host lattice material CuGaO 2 , This strong absorption was attributed to the activation of forbidden optical channels by crystal symmetry breaking within the host material rather than solely due to the hybridization of band-structures described by the empirical Vegard's law.…”

“…However, Lee et al [21] also describe reduced photocatalytic activity below UV wavelengths (hv < 3.2eV), despite CuGaO 2 having also shown weaker absorption edges around 2.7 eV in separate studies [21]. In the case of pure CuFeO 2 , despite its lower band gap and higher absorbance through the visible spectrum, it is not thought to be suitable in terms of its band edge potentials for full reduction-oxidation of H 2 O [20]. From this standpoint, and with respect to the development of both CuFeO 2 and CuGaO 2 delafossite materials for the purpose of photo-catalysis, B-site alloying presents a possible means of extending the absorption properties of a candidate delafossite electrode material into the visible spectrum, and provides a potential to enhance photo-catalytic efficiency of the material when operating within the visible spectrum close to peak solar irradiance at sea-level.…”

“…In this work, a similar approach to [20] is undertaken but with substitution of Ga for Fe within the CuFeO 2 . We have previously published an X-ray photoelectron spectroscopy study of a CuFe 1-x Ga x O 2 composite thin film and reported its chemisorption properties related to CO 2 gas exposure [22].…”