Published: December 08, 2011 r2011 American Chemical Society 1865 dx.doi.org/10.1021/jp2087225 | J. Phys. Chem. C 2012, 116, 1865–1872 ARTICLE pubs.acs.org/JPCC Synthesis, Characterization, Electronic Structure, and Photocatalytic Behavior of CuGaO 2 and CuGa 1Àx Fe x O 2 (x = 0.05, 0.10, 0.15, 0.20) Delafossites Jonathan W. Lekse,* ,† M. Kylee Underwood, ‡ James P. Lewis, ‡ and Christopher Matranga † † The National Energy Technology Laboratory, 626 Cochrans Mill Road, Pittsburgh, Pennsylvania 15236, United States ‡ Department of Physics, West Virginia University, Morgantown, West Virginia 26506-6315, United States b S Supporting Information ’ INTRODUCTION Despite the attention that renewable sources and natural gas are receiving in the energy community, coal continues to be the dominant source of power for electricity production in the United States. 1 This is due to existing infrastructure and capacity, as well as the vast natural reserves of coal found domestically. One of the largest technological challenges associated with the use of coal is managing CO 2 emissions from coal-fired power plants. Measures that have been proven to be cost-effective for industrial processes such as limestone calcination and ammonia production are not cost-effective for use in power plants. Instead, new alternatives for managing the CO 2 emissions associated with power generation need to be investigated. The photocatalytic reduction of CO 2 is a promising approach for managing CO 2 , since the energy required for conversion can potentially be supplied by naturally available sunlight. The con- version of CO 2 to C1 products, such as CO, CH 3 OH, and CH 4 , creates a product stream with industrial demand that can be sold to help offset the costs associated with the use of CO 2 management technologies. 2 Current challenges associated with the development of photocatalytic materials include low optical activity in the visible and near-infrared regions of the solar spectrum, poor product selectivity resulting from band alignment mismatches with chemi- cal redox potentials, and slow reaction kinetics from low photo- efficiencies and charge carrier recombination. Delafossite materials offer some unique material properties that can address many of these issues, making them interesting candidates for CO 2 photo- reduction applications. Delafossite itself is an iron and copper containing mineral first reported by Friedel in 1873 and named after the French mineralogist and crystallographer Gabriel Delafosse. 3 Delafossite was first noted to be the same material as synthetically produced CuFeO 2 by Pabst. 4 Delafossites have the general formula ABO 2 , where A is a 1 + metal such as Cu, Ag, Pt, or Pd and B is a 3 + metal Received: September 9, 2011 Revised: December 1, 2011 ABSTRACT: The photochemical reduction of CO 2 to chemicals, such as CO and CH 4 , is a promising carbon management approach that can generate revenue from chemical sales to help offset the costs associated with the use of carbon-manage- ment technologies. Delafossite materials of the general stoichiometry ABO 2 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 CuGaO 2 and CuGa 1Àx Fe x O 2 (x = 0.05, 0.10, 0.15, 0.20) for the reduction of CO 2 has been studied. Our results show that the CuGaO 2 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 1Àx Fe x O 2 (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 CO 2 in the presence of H 2 O vapor using CuGaO 2 and CuGa 1Àx Fe x O 2 produces CO with little evidence for other products such as H 2 or hydrocarbons. The impact of Fe alloying with Ga on the band structure and photochemical activity of this delafossite system is discussed.