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Synthesis, Characterization, Electronic Structure, and Photocatalytic Behavior of CuGaO2 and CuGa1 xFexO2 (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
bS Supporting Information 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 xFexO2 (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 xFexO2 (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 xFexO2 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.
’ 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 CO2 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 CO2 emissions associated with power generation need to be investigated. The photocatalytic reduction of CO2 is a promising approach for managing CO2, since the energy required for conversion can potentially be supplied by naturally available sunlight. The conversion of CO2 to C1 products, such as CO, CH3OH, and CH4, creates a product stream with industrial demand that can be sold to help offset the costs associated with the use of CO2 management r 2011 American Chemical Society
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 chemical redox potentials, and slow reaction kinetics from low photoefficiencies and charge carrier recombination. Delafossite materials offer some unique material properties that can address many of these issues, making them interesting candidates for CO2 photoreduction 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 CuFeO2 by Pabst.4 Delafossites have the general formula ABO2, 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 Published: December 08, 2011 1865
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Figure 1. Crystal structures of (a) hexagonal, (b) rhombohedral, and (c) randomly substituted rhombohedral delafossite.
such as Al, Ga, Y, or Fe.5 The structure of delafossite contains alternating layers of octahedrally coordinated B-site metals spaced by two coordinate, dumbbell shaped A-site metals (Figure 1). Delafossite materials have desirable properties such as stability in aqueous solutions and excellent hole mobility.6 The primary issue facing these materials is the large disparity between the fundamental band gap and the apparent optical band gap in purephase materials, which is caused by the inversion symmetry of the crystal structure.6a The transition probability for optical excitation at the fundamental band gap is weak or symmetry forbidden preventing the absorption of lower energy photons, hence these materials are often referred to as transparent conducting oxides.6a,7 As a result, most delafossites have large band gaps in the UV region of the electromagnetic spectrum, which creates challenges for using them in many photocatalytic and photovoltaic type applications.8 Recently, computational modeling has shown that B-site alloying in copper-containing, delafossite materials breaks the inversion symmetry of the crystal, modifies the band structure of the solid, and increases the absorption probability of photons at the fundamental band gap energy.7 Furthermore B-site alloying should maintain structural integrity and not lead to the formation of recombination centers. This computational prediction is exciting because it suggests that B-site alloying in delafossites can be used to engineer the band structure of these materials to improve optical absorption across the solar spectrum and to better align band edges with specific chemical redox potentials. The improvement of optical activity in the visible and nearinfrared regions of the solar spectrum has important implications for applications such as the photochemical decomposition of water or photoreduction of CO2 where catalytic efficiencies can be improved by using broader regions of the solar spectrum. Improved band alignments with chemical redox potentials will also allow one to improve the selectivity of these photochemical reactions. In this work, we investigate the viability of delafossite materials for the photochemical reduction of CO2. Specifically, CuGaO2 and CuGa1 xFexO2 (x = 0.05, 0.10, 0.15, 0.20) have been synthesized, characterized, and used for determining the applicability of this material to the photocatalytic reduction of CO2. In addition, computational modeling of these compounds has been
used to explain the changes in electronic structure seen in these materials when the B-site of CuGaO2 is alloyed with Fe. In particular, we find that UV light is needed to photochemically reduce CO2 with CuGaO2 catalysts with the primary product being CO, indicating that the conduction band alignment in this system is favorable for this reaction. A reduction in band gap energy to near-infrared frequencies is seen after Fe substitution to form CuGa1 xFexO2; however, our results show that UV light is still needed to photoreduce CO2 with CO being produced with similar yields to the unsubstituted system. This finding indicates that the Fe substitution drops the conduction band level significantly with respect to the CO reduction potential. Our results are significant as they represent the first evaluation of the engineered band structures of delafossites for photocatalytic applications.
’ EXPERIMENTAL SECTION Reagents and Synthesis of CuGaO2, CuGa1 xFexO2, and CuFeO2. The chemicals in this work were used as obtained:
(i) copper(I) oxide, 99.9%, Strem; (ii) gallium oxide, 99.99%, Alfa Aesar; and (iii) iron(III) oxide, 99+%, Aldrich. CuGaO2, CuGa1 xFexO2, and CuFeO2 were synthesized from stoichiometric mixtures of Cu2O, Ga2O3, and Fe2O3. The mixtures were ground under acetone for 15 min, and the acetone then evaporated. The mixtures were collected and pressed into pellets, which were heated in a tube furnace under flowing argon gas from room temperature to 1100 °C in 24 h, held at 1100 °C for 48 h, and then cooled to room temperature in 3 h. Correct stoichiometry of the CuGaO2, CuGa1 xFexO2, and CuFeO2 final products was assured based of the ratio of the starting materials coupled with the lack of impurities observed in the powder diffraction patterns. Physical Property Measurements. A Panalytical X’Pert Pro diffractometer was used to collect powder diffraction patterns for each sample. Scans were performed from 5 90° 2θ with a step size of 0.17° and a scan speed of 200 s/°. HighScore Plus was used to perform Rietveld refinement on the collected patterns, in order to obtain unit cell dimensions of the substituted and unsubstituted compounds. A Perkin Elmer Lambda 1050 dual-beam spectrometer was used to collect data for each compound from 200 to 2500 nm. The Kubelka Munk equation9 was then used to convert reflectance data into absorbance data for analysis. 1866
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Figure 2. Powder X-ray diffraction data showing (a) a comparison of the diffraction patterns of CuGaO2 and CuGa1 xFexO2 (x = 0.05, 0.10, 0.15, 0.20) to a reference pattern for CuGaO2. A close-up view of the diffraction peaks (b) shows a shift in peak location as a function of iron incorporation. Rietveld refinement was used to determine the unit cell volumes as a function of iron incorporation (c) and show that substituted samples followed Vegard’s Law.
The value of the band gap was then determined using a standard method in which the absorption edge is extrapolated to zero. Photocatalytic Testing. Photocatalytic experiments were performed in a gas-tight photocatalysis cell constructed using stainless steel conflat flange components fitted with two inlet/ outlet valves for gas purging, one gas chromatography (GC) sampling port (HP 5890) connected via Swagelok fittings, and one UV quartz viewport. The general procedure was to place between 0.1 and 0.2 g of sample into a pyrex beaker that had been cut down to resemble a Petri dish. The sample was spread out, and then, the beaker was placed into the cell. The cell was purged for 10 min with CO2 bubbled through 18 MΩ water. The cell was then sealed with a slightly positive CO2 pressure and illuminated using a Newport 300 W Xe arc lamp fitted with a manual shutter and an optical filter holder. Where noted, various long-pass filters were used to select wavelengths of illumination by blocking out higher energy (lower wavelength) light (Edmunds Optics UV vis filter kit no. 47 398). After illumination, 5 mL samples of headspace gas were sampled with a gastight GC syringe to quantify the reaction products. GC was performed by manually injecting the sample into a Perkin Elmer Clarus 600 gas chromatograph equipped with both TCD and FID detectors using the following temperature program: the column temperature was held at 36 °C for 0.5 min, then increased to 225 °C at a rate of 30°/min, and then held at 225 °C for 10.2 min. The column used was a Supelco Carboxen 1000 column, 15 feet long by 1/8 in. diameter, 60/80 mesh particle size. Carboxen is a
carbonaceous molecular sieve used for separating permanent gases and C1 C3 hydrocarbons. Computational Methods. FIREBALL is a self-consistent density functional theory code that makes use of an ab initio tight-binding molecular dynamics (TBMD) simulation technique (see ref 10 for a recent review about the method).10 Atomic-like confined orbitals are used as basis set to find occupied solutions to the one-electron Hamiltonian. These orbitals, known as fireballs, were introduced by Sankey and Niklewski11 and are obtained by imposing cutoffs of a predetermined radius (rc) on the atomic orbitals that serves as a boundary condition for the atomic problem (either hard or soft potentials at the cutoff radius).12 Beyond these cutoffs, calculated numerical wave functions are zero, which is similar to an “atom-in-a-box” type confinement; this has the effect of increasing the electronic energy levels. In this methodology, the Hamiltonian and the overlap matrix elements are sparse for large systems, resulting in reduced computation time. The slight excitation caused by this numerical fireball basis generally yields a better representation of charge densities for solid-state systems as the confinement accounts for Fermi compression in solids.13 For this work on delafossites, we used a single-numerical basis set for O (rc = 3.4, 3.8) and Ga (rc = 4.8, 5.7) with a single-numerical basis set with ppolarization for Fe (rc = 5.3, 5.8, 4.7) and Cu (rc = 5.1, 5.6, 4.6), and rc values are the wavefuction cutoff lengths, in atomic units, for the s, p, and d orbitals, respectively. The McWEDA functional for evaluating multicenter exchange-correlation interactions was 1867
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The Journal of Physical Chemistry C utilized throughout this work and is proven to be effective for other oxide materials.14 In the calculated supercells, a CuGaO2 primitive cell was used as the starting structure. CuGaO2 delafossite energetically favors the rhombohedral structure (space group no. 166, R3m), which contains four atoms in the unit cell. All atoms in the unit cell lie along the diagonal of the primitive cell starting with Cu at the origin, Ga in the center of the cell, and O at 1/9 and 1/9 of the way along the diagonal of the cell. The lattice vectors, which are constructed using Curtarolo’s standardized equations for RHM1 structures, are then used to repeatedly stack the primitive cells to make a 4 4 4 supercell (288 atoms).15 As experimental results have shown the volume of the synthesized materials grow linearly with the amount of substituted Fe according to Vegard’s Law, the lattice vectors and primitive cells are changed for each substitution amount accordingly. To do this, we scale the lattice constants a and α linearly as according to the following: aCuGa1 x FexO2 = (1 x) aCuGa2 + (x)aCuFeFe2 (and similarly for α), where aCuGaO2 5.976 Å (α = 28.84°), and aCuFeO2 5.989 Å (α = 29.64°). We then select x*NGa Ga atoms to be replaced by Fe using pseudorandom methods.
’ RESULTS AND DISCUSSION Synthesis and Structure. Previous computational modeling has postulated that a solid solution of two delafossites, one with a 3A cation and one with a 3B cation, would result in a material that lacks the inversion symmetry preventing the absorption of photons at the fundamental band gap energy.7 From a synthetic perspective, forming a solid solution with cations from the 3A and 3B groups is complicated by the fact that delafossite materials crystallize in one of two crystal systems, either hexagonal or rhombohedral (Figure 1). Primarily, 3A cations tend to crystallize in the rhombohedral crystal system, while cations in the 3B group tend to crystallize in the hexagonal crystal system. Due to this fact, attempts to employ traditional high temperature solidstate methods to the synthesis of a 3A/3B solid solution will likely lead to phase segregation instead of a single crystalline product. The better approach is to make a solid solution using a 3+ transition metal whose delafossite is the same crystal structure as the 3A or 3B cation with which it is being alloyed. Both CuGaO2 and CuFeO2 crystallize in the rhombohedral structure type in the ground state.16 This coupled with the similarity of their atomic radii suggest that a solid solution of the two is possible.17 In this work, CuGaO2 and four members of the solid solution, CuGa1 xFexO2 (x = 0.05, 0.10, 0.15, 0.20), were synthesized based on a previous report.18 Structural characterization of the samples was carried out via X-ray powder diffraction, which confirmed a rhombohedral crystal structure (Figure 2). The diffraction patterns obtained match the reference pattern for CuGaO216a although there is a shift to lower 2θ with increasing iron content indicating an increase in unit cell volume (Figure 2b). This is consistent with a comparison of volumes for the CuGaO2 and CuFeO2 end members of the series at 131.6 and 136.9 Å3, respectively. In order to quantify this change in unit cell volume, Rietveld refinement was performed on the diffraction patterns of each sample. The unit cell volumes were found to increase linearly with increasing iron substitution in the samples (Figure 2c) indicating that the substitution of Fe for Ga occurs randomly, according to Vegard’s Law. Optical Properties and Electronic Structure Calculations. Visually, the CuGaO2 samples were off-white with a green tint,
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Figure 3. Diffuse reflectance spectra for CuGaO2, CuGa0.85Fe0.15O2, and CuFeO2. CuGa0.85Fe0.15O2 was chosen as representative of substituted delafossites with the general formula CuGa1-xFexO2 (x = 0.05 to 0.20). Absorption features can be observed at 3.75 and 2.55 eV for CuGaO2. The spectrum of CuGa0.85Fe0.15O2 also has an absorption feature at 3.75 eV, but the band gap energy has been shifted to 1.5 eV. The spectrum for CuFeO2 has absorption features at 0.8, 1.15, and 3.8 eV. Bandgap values were determined by extrapolation of the absorption edge to zero.
while the Fe-substituted samples were black, indicating an obvious change in the electronic structure of the substituted samples. Diffuse reflectance spectroscopy of CuGaO2 (Figure 3) showed absorption features at 2.55 and 3.75 eV. The feature at 3.75 eV compares well to previous reports of 3.6 eV20 for thin films of CuGaO2.19 Calculations based on the local density approximation (LDA) have assigned this feature to a direct band gap at the L-point of the Brillouin zone.6a The optical feature at 2.55 eV in Figure 3 is not reported in the thin film delafossite literature but has been noted in particles of CuGaO2 with dimensions exceeding ∼300 20 nm.6a,19 The optical transition has been associated with an indirect fundamental band gap consistent with the band structure predicted in LDA calculations. Because this optical transition is phonon-assisted, it has a low absorption cross section and is difficult to observe in thin films where there is not much material in the path length of the spectrometer. The samples prepared in this work are bulk powders with mean particle sizes between ∼40 and 50 μm making it possible to observe both the indirect and direct transitions at 2.55 and 3.75 eV, respectively, noted in Figure 3. Pure CuFeO2 has been observed experimentally to have an indirect band gap at 1.15 eV and a direct band transition at 2.03 eV.20 Computational studies predict direct gap transitions at 1.30, 2.06, and 3.20 eV.21 Our experimental spectra for CuFeO2 show evidence for optical transitions at ∼1.10 eV and ∼3.2 eV in agreement with experiments and theory. Electronic structure calculations and experimental valence band spectroscopy indicate that Cu 3d and O 2p orbitals hybridize to form the valence band in CuFeO2.21 Experimentally, the valence band spectra show a strong hybridization of Fe 3d and O 2p states.21a The conduction band is predicted by theory to be composed primarily of Fe 3d and O 2p states. Fe-substitution in CuGaO2 resulted in new optical features in the absorption spectrum (Figure 3). In particular, the band edge noted at 2.55 eV in the pure CuGaO2 shifts to ∼1.5 eV. 1868
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Figure 4. Density of states plots for CuGaO2, CuGa0.95Fe0.05O2, and CuGa0.85Fe0.15O2. Note that, in this figure, the Fermi level for CuGaO2, marked with a solid vertical line, are not aligned to zero energy as this would not give us an accurate comparison of the three DOS plots. The Fermi levels for the Fe-alloyed materials do not vary much from that of the pure CuGaO2 structure.
The intensity and location of the feature at ∼1.5 eV appears to be independent of the Fe concentrations evaluated here. The optical feature noted at ∼3.75 eV in pure CuGaO2 is still present suggesting that the direct band gap associated with this absorption is not significantly modified by Fe substitution. We point out that the appearance of the 1.5 eV feature and retention of the 3.75 eV feature do not indicate that we have phase segregation of CuGaO2 and CuFeO2 in our CuGa1 xFexO2 product. The optical gaps shown for CuFeO2 in Figure 3 and those reported previously are significantly different than what is observed for CuGa1 xFexO2. Likewise, our XRD data in Figure 2e clearly shows evidence for a random substitution of Fe at Ga sites. The retention of the optical feature at 3.75 eV after alloying can be expected based on similar findings in the predicted band structure for CuGaO2 and Cu(Y/Ga)O2. These calculations show the absorption probabilities and band structure near the L-point in the Brillouin zone remain relatively unchanged when Ga is alloyed with CuYO2.7a This computational result illustrates that, while the band structure of delafossites will be modified by B-site alloying, some optical features should be expected to remain unchanged. Our optical and structural data for CuGa1 xFexO2 illustrate that Fe alloying is occurring at the B-site of CuGaO2. Specifically, we believe the changes in the optical features caused by Fesubstitution in CuGaO2 to form CuGa1 xFexO2 causing strain in the crystal structure helps to break the symmetry of the system and modify the electronic structure. Previous computational modeling of the electronic structure of the related compound CuY1 xGaxO2 has indicated that the narrowing of the band gap in these materials results from the lower energy Ga s-band contributions to the conduction band minimum (CBM). The alloying of Ga at the B-site also helps to break the symmetry of the crystal and enhance symmetry forbidden optical transitions near the Γ-point by altering the parity of the states involved in the transition. Our computational results suggest that a similar mechanism involving crystal strain caused by Fe incorporation and the resulting electronic structure modifications are involved
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Figure 5. DOS plots of optimized CuGaO2 and unoptimized CuGa0.85Fe0.15O2 have been plotted over the partial DOS plots of constituent Fe and O from the unoptimized CuGa0.85Fe0.15O2 to show that the states that contribute to the broadening of the energies in the conduction band come mostly from oxygen states.
for CuGa1 xFexO2. The Fe O bond distance in CuFeO2 is 2.026 Å, while the corresponding Ga O bond distance in CuGaO2 is 1.996 Å. Though subtle, when Fe is incorporated into the CuGaO2 structure, the longer Fe O bonds will generate strain on the lattice, which will affect the band structure. The electronic structure of CuGaO2 and CuGa1 xFexO2 were evaluated using the FIREBALL self-consistent density functional theory code. The random nature of the Fe substitution prevents the calculation of a true band structure, so density of states (DOS) plots were used to evaluate electronic structure (Figures 4 and 5). Figure 4 shows a clear gap in the CuGaO2 DOS that becomes occupied with increased substitution of Fe; the solid line indicates the Fermi energy, and the allowable conduction band states start at around 0.0 eV; midgap states are unallowable transitions due to crystal inversion symmetry. The broadening of states in the midgap region points to increased activity in the conduction band and a lowered band gap energy (inversion symmetry is broken and states are allowable transitions), which helps to explain the experimentally observed band gap reduction seen with iron substitution. Note that the Fermi level in this figure is not aligned to zero energy as this would not give us an accurate comparison of the three DOS plots. The Fermi energies of the Fe-alloyed materials are similar to that of the pure CuGaO2 material. Partial DOS plots were studied in additional detail in order to understand the mechanism responsible for the broadening of energies in the conduction band. In Figure 5, the DOS plots of optimized CuGaO2 and unoptimized CuGa0.85Fe0.15O2 have been plotted over the partial DOS plots of constituent Fe and O from the unoptimized CuGa0.85Fe0.15O2 to show that the states that contribute to the broadening of the energies in the conduction band come mostly from iron and oxygen states. This suggests that the Fe itself is not affecting the conduction band, but strain on the system from Fe substitution is causing a break in the inversion symmetry of the crystalline lattice. Because Fe atoms are smaller than the replaced Ga molecules, the lattice will shrink in size where Fe is introduced causing a strain on the overall system. This breaking of inversion symmetry is now 1869
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Figure 6. Amount of CO produced, normalized for sample mass and illumination time, for substituted and unsubstituted plotted against the amount of iron incorporated into the sample. All of the samples were illuminated with broadband light for 24 h. The 95% confidence intervals are shown for all samples.
allowing what were previously forbidden electronic transitions, which appear in the DOS plot as a broadening of electronic states in the band gap. The physical manifestation of this change in the electronic structure is the shift in the band gap energy. Photocatalytic Properties. Catalyst samples were placed into a custom stainless steel cell with a UV transparent window, and the cell was purged with CO2 bubbled through water. After illuminating samples of CuGaO2 and CuGa1 xFexO2 (x = 0.05, 0.10, 0.15, 0.20) with broadband light for 24 h, we observed CO (∼9 ppm g 1 h 1) and CH4 (trace) as reaction products. In order to test that the products were being generated photocatalytically, several blank runs were performed with and without catalyst in the dark at room temperature and at 50 °C. No product was observed for any of the dark experiments. Much of the previous literature concerning the photoreduction of CO2 has employed TiO2, both pure and with a transition metal cocatalyst, as a photocatalyst. Most of the reduction products, observed when using TiO2 to photoreduce CO2 in the presence of H2O, consist of hydrogenated species such as hydrogen, methane, methanol, formic acid, and formaldehyde; occasionally, CO is mentioned as a minority product.22 Previous experiments in our lab with CdSe/Pt/TiO2 heterostructures show these photocatalysts produce 48 ppm g 1 h 1 methane, 3.3 ppm g 1 h 1 methanol, and trace amounts of H2 and CO during visible light excitation (λ > 420 nm).14g Experiments performed in our lab with Cu-loaded Evonik Aeroxide TiO2 P25 using the same lamp and reaction conditions reported for our delafossites produces 78.5 ppm g 1 h 1 of hydrocarbons consisting of CH4 and C2H6 at a ratio of ∼2:1, respectively.23 The yields for the delafossite materials tested in these experiments are smaller than those observed in TiO2 systems tested under similar conditions but are more selective for the production of CO. The amount of substituted iron in the samples did not appear to influence the nature or the amount of CO produced (Figure 6). The average amount of CO varied between samples;
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Figure 7. Amount of CO produced, normalized for sample mass and illumination time, as a function of time. 15% iron substitution was chosen as a representative sample for all substituted compounds.
Figure 8. Amount of CO produced, normalized for sample mass and illumination time, as a function of the wavelength of the cutoff filter used, shows a dramatic decrease for both CuGaO2 and CuGa0.85Fe0.15O2.
however, the 95% confidence limits overlap for unsubstituted and Fe-substituted delafossite materials, indicating there is no statistically significant difference between the samples evaluated in Figure 6. Based on this result, CuGa0.85Fe0.15O2 was selected as representative of substituted samples for subsequent measurements and comparison to CuGaO2. A series of experiments was performed to determine the effect of illumination time on the performance of the delafossite catalysts. Samples of CuGaO2 and CuGa0.85Fe0.15O2 were illuminated for 1, 3, 6, 12, and 24 h (Figure 7). The greatest amount of CO production occurred in the first hour of the reaction; however, Figure 7 shows that the adjusted amount decreased as illumination time increased indicating that the 1870
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The Journal of Physical Chemistry C reactivity is decreasing with time. Additional experiments varying H2O concentration in the headspace gas mixture were conducted and may partially explain the observed decrease in reactivity. For this series of reactions, the cell was first purged as usual with CO2 bubbled through H2O. For the second reaction, the cell was purged with CO2 directly from the cylinder. For the final reaction, the cell was evacuated for 48 h using a turbo-pump system and then charged to ambient pressure with CO2 directly from the cylinder. The cells were then illuminated and analyzed as described previously. All three reactions yielded CO; however, the amount of product was found to decrease with decreasing H2O concentration in the headspace. To this end, it seems likely that the photocatalytic reaction rate in our system depends on H2O concentration in the headspace. In addition to investigating the effect of illumination time, experiments were performed using different cutoff filters in order to determine at which point in the electromagnetic spectrum the photoactivity ceases. An ideal catalyst would be able to perform photoreduction well into the visible region of the electromagnetic spectrum, and our optical spectra suggest that the substituted catalyst may have photoactivity in the visible and nearinfrared. Experiments with cutoff filters again showed that both substituted and unsubstituted samples have similar photoactivity (Figure 8). The similarity in photocatalytic behavior is interesting because of the difference in the band gaps of the substituted and unsubstituted samples. Based upon band gap values, it was expected that the substituted sample would be able to perform the photoreduction using light of wavelengths well into the visible region (see discussion below). Fe(III) substitution into metal oxides has previously been evaluated for TiO2.24 For systems where Fe(III) is incorporated as an interstitial defect, substitution results in increased photoabsorption at energies lower than the bandgap of pure TiO2, but photocatalytic activity decreases, since these Fe(III) centers act as defects and promote carrier recombination.24i,q,r For cases where Fe(III) is incorporated directly into the TiO2 lattice, authors have noted an increase in photoactivity and a reduced recombination rate for charge carriers as Fe(III) is believed to act as a trap for electrons or holes.24b,d,e,h,n p Neither mechanism is likely an accurate representation of what is occurring for the CuGa1 xFexO2 delafossites reported in this study. Our structural data (Figure 2) indicates that substitutional Fe(III) incorporation is occurring at the B-site ruling out Fe(III) centers creating interstitial or surface defects that can serve as recombination centers. Furthermore, our computational results indicate that the change in optical properties during B-site alloying of Fe(III) result from the growth of midgap states caused by lattice strain resulting from Fe(III) incorporation. The computational results indicate that the change in optical properties for the CuGa1 xFexO2 delafossites occurs from the breaking of symmetry in the crystal structure making optical transition to these new states allowed by parity. We believe the increase in photoabsorption of CuGa1 xFexO2 at visible and near-infrared energies without any resulting change in the photocatalytic activity toward CO2 reduction at these energies results from a mismatch in conduction band alignment with CO2 redox potentials. The standard reduction potential of CO2 to CO occurs at 0.53 V with respect to the NHE at pH 7.25 Our data shows that, under broadband UV illumination, photoexcited electrons in CuGaO2 are capable of accessing this redox potential indicating the conduction band edge is reasonably well aligned with the CO2 to CO potential. The FIREBALL calculations
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show that Fe substitution reduces the band gap energy primarily by lowering the conduction band edge. The increased photoabsorption at visible and near-infrared energies for CuGa1 xFexO2 means the conduction band edge of this material is dropping approximately 1 2 eV with respect to both the CO2 to CO redox potential and the conduction band edge of pure CuGaO2. The lack of photocatalytic activity at visible and nearinfrared energies CuGa1 xFexO2 seems to result from a simple mismatch of band alignment with chemical redox energies, not from carrier recombination at Fe(III) defect centers. The band structure of these delafossites should be highly tunable by using different elements used for B-site alloying. As such, future work will investigate how to accomplish B-site alloying in these systesms with better control of both optical activity and band edge alignment.
’ CONCLUSIONS Substitution of Fe for Ga in the delafossite compound CuGaO2 was demonstrated to alter the electronic structure and reduce the band gap energy from 3.75 or 2.55 eV to 1.5 eV. The resulting substituted and unsubstituted compounds were capable of reducing CO2 to CO photocatalytically. Neither the substituted nor the unsubstituted compounds demonstrated significant production of CO upon the introduction of filters likely due to the promoted electrons lacking sufficient energy to perform the reduction. Future work will concern increasing the production of CO electrochemically, gaining a better fundamental understanding of the system, and attempting the synthesis of delafossite materials with better band alignment for CO2 photoreduction. ’ ASSOCIATED CONTENT
bS
Supporting Information. Additional details of the Rietveld refinement performed in this study, diffuse reflectance plots of all samples studied in this work, and density of states plots for all of the samples studied in this work. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We thank Congjun Wang and Robert L. Thompson for technical assistance and for collecting and sharing the TiO2 data included in this work for comparison purposes. Lekse acknowledges an appointment at the National Energy Technology Laboratory administered by the Oak Ridge Institute for Science and Education. For this work, Lewis is currently funded by the National Science Foundation through NSF DMR 09-03225 and a subcontract from NETL (URS RES) for Work Activity 0004000.6.600.007.002.420.000.005 ARRA ICMI Project, Element 420, Photo Active Materials. Lewis and Underwood would like to thank D. A. Drabold for many useful discussions regarding optical and transport properties related to the disorder found in these systems; discussions of the impact of disorder on the charge transport and optical band gaps is currently under preparation with Drabold. 1871
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