Article pubs.acs.org/JPCC
Calculated Descriptors of Catalytic Activity for Water Electrolysis Anode: Application to Delafossite Oxides Kenji Toyoda,* Reiko Hinogami, Nobuhiro Miyata, and Masato Aizawa Advanced Technology Research Laboratories, Panasonic Corporation, 3-4 Hikaridai, Seika-cho, Soraku-gun, Kyoto 619-0237, Japan S Supporting Information *
ABSTRACT: Understanding the oxygen evolution reaction (OER) is crucial for improving the performance of water electrolysis. Copper delafossite oxides (CuBO2, B = transition metal) were investigated for their potential as OER catalysts using density functional theory (DFT) calculations. To identify an appropriate descriptor for OER activity, we examined the relationships between the calculated eg or t2g occupancy of the B site and the experimentally determined OER activity. The calculated t2g occupancy was found to be approximately linearly related to OER activity. We therefore propose that t2g occupancy can be employed as an appropriate descriptor of the OER activity of delafossite oxide catalysts. The delectron occupancy of active sites, estimated using theoretical calculations, can be used to search efficiently for transition metal oxide catalysts with high OER activity.
■
INTRODUCTION The electrolysis of water using renewable energy is an increasing focus of interest because it can produce large amounts of “green hydrogen”, or CO2-free hydrogen, from water.1,2 The efficiency of hydrogen production is limited by the overpotential of the
oxygen evolution reaction (OER). Highly active catalysts for OER consist chiefly of rare noble metals, such as iridium oxide (IrO2), a commercially available catalyst. There is a need, however, to design catalysts with high OER activity that use nonprecious metals rather than rare and expensive noble metals. Non-precious transition metal oxides show promise as catalysts for water electrolysis.3−10 Figure 1 shows the crystal structures of typical transition metal oxides and d-orbital splitting of an active site. The efficient design of transition metal oxide catalysts is the key to improving their performance. Suntivich et al.8 reported that a perovskite oxide (ABO3) catalyst (Figure 1a), Ba0.5Sr0.5Co0.8Fe0.2O3−δ, is predicted to possess high OER activity by estimating the electron occupancy of the eg orbital (Figure 1c) of surface transition metal cations using X-ray adsorption spectroscopy. The eg occupancy is more straightforward for predicting OER activity than the descriptors proposed by Bockris and Otagawa3 or Rossmeisl et al.6,7 In a previous paper,11 we reported on copper delafossite oxides (ABO2, A = Cu) (Figure 1b). CuRhO2 showed the best OER performance. It should be noted that the eg occupancy of the B site in CuBO2, estimated using density functional theory (DFT) calculations, was correlated with the onset potential of OER. However, the cyclic voltammetry (CV) characteristic of CuFeO2 was much lower than that of CuRhO2, although their eg occupancies were similar. The eg occupancy might, therefore, not be an appropriate descriptor of the OER activity of delafossite oxides, as the B−O axes are inclined toward the c-axis (Figure 1b). The electron occupancy of the other symmetry,
Figure 1. Typical crystal structures of (a) perovskite oxides (ABO3) and (b) delafossite oxides (ABO2). In both structures, x, y, and z coordinates are placed at the B site along the B−O axis of the BO6 octahedral. (c) The d-orbitals in the octahedral environment are split into the eg and t2g orbitals. © XXXX American Chemical Society
Received: September 12, 2014 Revised: December 25, 2014
A
DOI: 10.1021/jp5092398 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
To distinguish the eg and t2g components, we rotated the unit cells such that the B−O axes of BO6 octahedral coordination pointed
t2g orbitals (Figure 1c), is a potential descriptor as indicated by Carcia et al.12 and Vojvodic et al.10,13 To the best of our knowledge, the relationships between the calculated eg or t2g occupancies and experimentally determined OER activity have not as yet been investigated quantitatively. In this paper, we investigate copper delafossite oxides (CuBO2) with the aim of identifying an appropriate descriptor for the design of OER catalysts using DFT calculations. The eg and t2g occupancies of the B site were estimated using DFT calculations. Meanwhile, several types of CuBO 2 were synthesized to evaluate their OER activity electrochemically. The relationships between the calculated eg or t2g occupancy and the experimentally determined OER activity were examined, with the intention of shedding light on the role of covalency between d-electrons of the B site and O 2p electrons in the OER process. We discuss a more appropriate descriptor of the OER activity of CuBO2, and the application of this descriptor, estimated using theoretical calculations, to other transition metal oxides.
■
CALCULATION METHOD Crystal Structures of Copper Delafossites. The electronic states of CuBO2 were investigated using bulk DFT calculations. Several transition metals (Cr, Mn, Fe, Co, and Rh) were adopted as the element at the B site. Figure 2 depicts three types of crystal structure seen in delafossite oxides: rhombohedral (3R), hexagonal (2H), and crednerite.14−20 Here, CuMnO2 forms a crednerite structure,18 and the other CuBO2 can form both 3R- and 2H-type structures.14−16 Table 1 shows the structural data used in our calculations. DFT Calculation Conditions. Our calculations were performed using “STATE”, a first-principles molecular dynamics program.22 The Perdew−Burke−Ernzerhof (PBE) generalized gradient approximation (GGA) was used for the exchangecorrelation functional.23 Electron−ion interactions were described by pseudopotentials.24,25 Wave functions and augmented charge density were expanded using a plane-wave basis set with the cutoff energies of 25 and 225 Ry, respectively. 6 × 6 × 6, 12 × 12 × 3, and 3 × 6 × 3 k-point meshes were used to sample the Brillouin zones of the 3R-type, 2H-type, and crednerite structures, respectively. Spin polarization was included in the calculations for Cr, Mn, Fe, and Co. Estimation of eg and t2g Occupancies. The eg and t2g occupancies of the B site in CuBO2 were estimated by integrating the eg and t2g components of the partial density of states (PDOS) projected onto the B site from negative infinity (−∞) to the Fermi energy (EF). Cut radii of the B site in PDOS were set to 1.27 Å for Cr, Mn, and Rh and to 1.11 Å for Fe and Co.
Figure 2. Crystal structures of copper delafossite oxides (CuBO2). (a) Rhombohedral (3R)-type, (b) hexagonal (2H)-type, and (c) crednerite structures. a, b, and c denote the length of unit cell. β denotes the angle of the unit cell. u denotes the ratio of the O-Cu distance to c. The crystal structures are visualized using VESTA.21
Table 1. Structural Data of CuBO2 with 3R-Type, 2H-Type, and Crednerite Structures composition
structure
a (Å)
CuCrO2
3R 2H crednerite 3R 2H 3R 2H 3R 2H
2.97 2.97 5.58 3.01 3.04 2.82 3.01 3.08 3.08
CuMnO2 CuFeO2 CuCoO2 CuRhO2
b (Å)
c (Å) 17.06 11.37 5.89 17.1 11.45 17.17 11.44 17.15 11.43
2.88
β (deg)
104
u
note
0.11 0.089 (0.093, 0.0, 0.32) 0.11 0.089 0.11 0.089 0.11 0.089
a b c a d a b e b
a
Reference 17. bThe values of a and c were assumed to be the same as a and two-thirds of c in a 3R-type structure, respectively. The value of u in 2HCuFeO2 was used. cReference 18, and u denotes the position of O relative to Cu in the fractional coordinate. dReference 19. eReference 20. B
DOI: 10.1021/jp5092398 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C Table 2. Reaction Conditions for Synthesis of CuBO2 object
method
reactant
temp (°C)
time (h)
atmosphere
CuCrO2 CuMnO2 CuFeO2 CuCoO2 CuRhO2
solid-state reaction hydrothermal reaction solid-state reaction hydrothermal reaction solid-state reaction
Cu2O + Cr2O3 Cu2O + Mn2O3 Cu2O + Fe2O3 Cu2O + CoOOH Cu2O + Rh2O3
1100 210 1000 210 1050
30 60 10 60 12
air
NaOH (M) 1.5
N2 2.0 air
Table 3. Calculated eg and t2g Occupancies of the B Site in CuBO2 with 3R-Type, 2H-Type, and Crednerite Structures composition
structure
eg
t2g
CuCrO2
3R 2H crednerite 3R 2H 3R 2H 3R 2H
1.22 1.13 2.05 3.39 3.22 1.89 1.84 1.75 1.73
3.38 2.91 3.68 4.29a 3.91 7.78 6.68 5.69 5.00
CuMnO2 CuFeO2 CuCoO2 CuRhO2 a
The results were obtained using GGA+U (see Figures S2 and S3).
Figure 4. SEM images of CuBO2 (B = (a) Cr, (b) Mn, (c) Fe, (d) Co, and (e) Rh).
Laboratory), and CoOOH were used as precursors of copper delafossite oxides. Here, the precursor of CoOOH was obtained by heating Co(OH)2 (Wako Pure Chemical Industries) at a temperature of 120 °C in an oxygen atmosphere for 24 h.15 Synthesis of Copper Delafossite Oxides. Table 2 summarizes the synthesis methods of the copper delafossite oxides.14−16 Two methods were used: solid-state reaction and hydrothermal reaction. CuCrO2, CuFeO2, and CuRhO2 were produced using solidstate reactions.14,16 Equimolar mixtures of Cu2O and M2O3 (M = Cr, Fe, Rh) were ground in an agate mortar. The mixtures were pressed into pellets that were then calcined in an electric furnace at 1000 °C and above for more than 10 h. Copper delafossite powders were obtained by grinding the calcinated pellets. CuMnO2 and CuCoO2 were produced using hydrothermal reactions.15 Equimolar reactants, Cu2O, and Mn2O3 or CoOOH, in aqueous NaOH solution were sealed in a polytetrafluoroethylene (PTFE: Teflon) container. The container was then sealed in a stainless steel reactor and heated at 210 °C for 60 h. Copper delafossite powders were obtained by filtering the products. Electrochemical Measurements. The copper delafossite powders were fixed on a conductive carbon substrate made of high-density percolate graphite, HPG (Toyo Tanso), using 1.25% Nafion (Sigma-Aldrich). The loading of the catalysts was adjusted to 20 μmol/cm2. The HPG substrate with catalytic
Figure 3. Dependencies of (a) eg and (b) t2g occupancies on the B element or crystal structure in copper delafossite oxides (CuBO2). (c) Formal d-electron configuration of trivalent B (B3+) ion in CuBO2.
in the direction of the Cartesian coordinate axes,26 since the B−O axes are inclined with respect to the c-axis (Figure 2). Here, the d orbitals along the B−O axes and the other d orbitals are designated as eg and t2g orbitals, respectively (Figure 1).
■
EXPERIMENTAL METHOD Materials. Cu2 O and Rh 2O 3 (Wako Pure Chemical Industries), Cr2O3, Mn2O3, and Fe2O3 (Kojundo Chemical C
DOI: 10.1021/jp5092398 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 5. XRD patterns of CuBO2 (B = (a) Cr, (b) Mn, (c) Fe, (d) Co, and (e) Rh). Open squares indicate the peaks calculated using RIETAN-FP.28 Asterisks denote unknown peaks.
CuFeO2 with DFT+U to reproduce a band gap (Figures S2 and S3). The use of DFT+U including spin-polarization makes the eg occupancy in CuFeO2 different from that in CuRhO2, unlike in our previous calculations.11 The eg and t2g occupancies are insensitive to the difference in the two types of crystal structure: the 3R- and 2H-type structures. Furthermore, only a 3R-type structure was experimentally observed for CuBO2 (B = Cr, Fe, Co, Rh), as can be seen in Figure 5. Hence, the results for the 2Htype structure will not be shown hereinafter. In contrast to the case of crystal structure, these occupancies are strongly dependent on the B element. These results indicate that the eg and t2g occupancies are almost entirely determined by the local environment near the B site. As seen in Table 3, the t2g occupancies for CuCoO2 exceeded 6 because the electrons that had originated in the covalency between the B−O atoms had been added to them. These results indicate that not only eg but also t2g electrons contribute to B−O covalency. The calculated eg and t2g occupancies, which include covalency, can be compared. The order of the eg occupancy is Cr < Co ≈ Rh ≈ Mn < Fe, whereas that of the t2g occupancy is
powders was mounted into a rotating disk electrode (RDE) attachment and used as a working electrode (WE) for electrochemical measurements. Preparation of the WE is described in more detail elsewhere.11 The OER activity of copper delafossite oxides was evaluated in O2-saturated 1.0 M KOH aqueous solution (Wako Pure Chemical Industries) using the RDE method. The rotation speed of the WE was set at 2000 rpm using a rotator (Nikko Keisoku). The three-electrode system was controlled using an ALS-760C potentiostat, with a platinum plate and reversible hydrogen electrode (RHE) used as counter and reference electrode, respectively. The scan rate was set to 50 mV/s for CV.
■
RESULTS AND DISCUSSION Calculated eg and t2g Occupancies. Table 3 lists the calculated eg and t2g occupancies of the B site in CuBO2, and Figure 3 displays the dependencies of both occupancies on the B element or on the crystal structure. The calculated eg and t2g occupancies were based on the PDOS of B element in CuBO2 (Figure S1). Here, we calculated the electronic structures of D
DOI: 10.1021/jp5092398 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C Cr ≈ Mn ≈ Fe < Rh ≈ Co. From the dependences of the calculated eg and t2g occupancies in the B element, we deduced the formal d-electron configuration of trivalent B ions (B3+) to be that shown in Figure 3c. Here, we assumed that in the d-states of Cr3+ an up-spin electron occupies each t2g orbit and no electrons occupy the eg orbits, since Cr3+ has only three d-electrons. Mn3+ and Fe3+ were in a high spin state, as in the case of perovskite oxides. On the other hand, Co3+ was in a low spin state (Figure S1f,g), which is consistent with other experimental results12 and a previous calculation.27 Interestingly, the spin state of Co3+ in delafossite oxides is different from that in perovskite oxides.8,9 The difference between the calculated occupancies and the formal configuration of the B site should thus be correlated with the B−O covalency. The results of the calculated occupancies suggest that CuCoO2 and CuMnO2 should have a similarly active OER to CuRhO2, if the eg occupancy is a more appropriate descriptor. On the other hand, only CuCoO2 should have a highly active OER compatible with CuRhO2, if the t2g occupancy is a more appropriate one. Characterization of Copper Delafossite Powders. Figure 4 shows scanning electron microscope (SEM) images of CuBO2 (B = Cr, Mn, Fe, Co, Rh) obtained using a Hitachi S-4500. The particles of CuCrO2, CuFeO2, and CuRhO2 look smaller than those of CuMnO2 and CuCoO2. This can probably be attributed to the difference in synthesis method. To evaluate the crystal structure of delafossite oxide powder, powder X-ray diffraction (XRD) was carried out using a PANalytical X’Pert PRO-MPD diffractometer with Cu Kα radiation. Figure 5 shows XRD patterns of CuBO2 (B = Cr, Mn, Fe, Co, Rh). Here, open squares indicate peaks calculated using RIETAN-FP,28 a 3R-type delafossite structure for CuBO2 (B = Cr, Fe, Co, Rh), assuming a crednerite structure for CuMnO2. We confirmed that CuCrO2, CuFeO2, CuCoO2, and CuRhO2 form a 3R-type delafossite structure and that CuMnO2 forms a crednerite structure. The crystalline phase for CuCrO 2 , CuFeO 2 , CuCoO2, and CuRhO2 is single, whereas that for CuCoO2 is a mixture of 3R-type delafossite structure and traces of unknown compounds. Electrochemical Characteristics. To verify the effectiveness of our calculations, we evaluated the OER activity of CuBO2 electrochemically. Figure 6 shows the CV characteristics of
CuBO2. The current densities were obtained by dividing the current by the geometric surface area of the WE (0.283 cm2). Figure 6 shows only the values in the direction of increased potential. The current was dependent on the B element; the order of current density is Cr ≈ Fe ≈ Mn < Co ≈ Rh. We found that the OER activity of CuCoO2, which is composed of nonprecious metals, almost matches that of CuRhO2 reported in our previous paper. Relationships between eg or t2g Occupancy and OER Activity. To examine quantitatively the calculated eg and t2g occupancies as descriptors of OER activity, we plotted the relationships between the eg or t2g occupancy and the onset potential of the OER, as shown in Figure 7. Here, the onset
Figure 7. Relationships between (a) eg or (b) t2g occupancy and onset potential.
potential denotes the potential of the current density at 5 mA/ cm2 (expressed as a horizontal dotted line in Figure 6). The data for the delafossite oxides, including noble metals (ABO2, A = Pd, Pt),12 were added to the results for CuBO2 (see Table S1 and Figure S4). As seen in Figure 7a,b, both eg and t2g occupancies are almost entirely independent of element A. Figure 7a shows the relationship between the eg occupancy and the onset potential of the OER. The relationship shows a “volcano” relationship. The points of ACoO2 are at the top of the volcano curve, where the eg occupancies are close to 2. Our results are consistent with previous results for perovskite oxides,8 except for the point of CuMnO2. On the other hand, Figure 7b shows the relationship between the t2g occupancy and the onset potential of the OER. The relationship shows close to a linear relationship, although the point for CuRhO2 deviates slightly from it. The difference between ARhO2 and ACoO2 can be
Figure 6. Cyclic voltammetry (CV) characteristics of CuBO2 (B = Cr, Mn, Fe, Co, Rh). The horizontal dotted line denotes a current density of 5 mA/cm2, determined as the onset potential of CuBO2. E
DOI: 10.1021/jp5092398 J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
■
explained by the B−O covalency, which suggests that the B−O covalency can contribute to enhancement of the OER process. The greater the t2g occupancy of the B site, the lower the onset potential of OER or the higher the OER activity. We propose, for the following reasons, that t2g occupancy is more appropriate as an indicator of the OER activity of delafossite oxide catalysts than the eg occupancy. First, the calculated t2g occupancy shows a close to linear relationship with OER activity. Second, CuMnO2 has a lower activity than CuCoO2. Furthermore, the t2g electrons at the B sites on the surface can be reactive (Figures S6 and S7), suggesting that the covalency between the t2g electrons and O 2p electrons play a key role in the OER process. In our calculations, the t2g occupancies of the B site in bulk, not at the surface, were estimated. The results indicate that the dependence of the t2g occupancy in the bulk material on the B element is equivalent to that at the surface, although the electronic states of the B site are different from those at the surface (Figures S6 and S7). We found that the calculated eg occupancies in perovskite oxides are linearly related to the experimentally determined eg occupancies (Figure S8), which supports the notion that the eg or t2g occupancy estimated using bulk DFT calculations can be a descriptor of the OER of transition metal oxides other than delafossite oxides. Our theoretical calculations of catalyst descriptors can thus be applied to other transition metal oxides, making it possible to more easily identify non-precious metal catalysts with higher OER activity. We believe that our findings will contribute to the further understanding of the OER mechanism on the surfaces of transition metal oxides.
AUTHOR INFORMATION
Corresponding Author
*Phone +81-774-98-2517; e-mail
[email protected] (K.T.). Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The authors thank Prof. Morikawa and Dr. Hamada for their valuable suggestions and for providing the latest STATE code. The authors also thank Prof. Uchimoto and Prof. Orikasa for fruitful discussions. Numerical calculations were carried out using the computational resources of the HPCI system provided by the Information Technology Center of Nagoya University through the HPCI System Research Project (Project ID: hp130140).
■
REFERENCES
(1) Carmo, M.; Fritz, D. L.; Mergel, J.; Stolten, D. A Comprehensive Review on PER Water Electrolysis. Int. J. Hydrogen Energy 2013, 38, 4091−4934. (2) Ursúa, A.; Gandía, L. M.; Sanchis, P. Hydrogen Production from Water Electrolysis: Current Status and Future Trends. Proc. IEEE 2012, 100, 410−426. (3) Bockris, J. O’M.; Otagawa, T. The Electrocatalysis of Oxygen Evolution on Perovskites. J. Electrochem. Soc. 1984, 131, 290−302. (4) Matsumoto, Y.; Sato, E. Electrocatalytic Property of Transition Metal Oxides for Oxygen Evolution Reaction. Mater. Chem. Phys. 1986, 14, 397−426. (5) Trasatti, S. Electrocatalysis in the Anodic Evolution of Oxygen and Chlorine. Electrochim. Acta 1984, 29, 1503−1512. (6) Rossmeisl, J.; Logadottir, A.; Nørskov, J. K. Electrolysis of Water on (Oxidized) Metal Surfaces. Chem. Phys. 2005, 319, 178−184. (7) Man, I. C.; Su, H.-Y.; Calle-Vallejo, F.; Hansen, H. A.; Martínez, J. I.; Inoglu, N. G.; Kitchin, J.; Jaramillo, T. F.; Nørskov, J. K. Universality in Oxygen Evolution Electrocatalysis on Oxide Surfaces. ChemCatChem 2011, 3, 1159−1165. (8) Suntivich, J.; May, K. J.; Gasteiger, H. A.; Goodenough, J. B.; ShaoHorn, Y.; Perovskite Oxide, A. Optimized for Oxygen Evolution Catalysis from Molecular Orbital Principles. Science 2011, 334, 1383− 1385. (9) Suntivich, J.; Gasteiger, H. A.; Yabuuchi, N.; Nakanishi, H.; Goodenough, J. B.; Shao-Horn, Y. Design Principles for OxygenReduction Activity on Perovskite Oxide Catalysts for Fuel Cells and Metal-Air Batteries. Nat. Chem. 2011, 3, 546−550. (10) Vojvodic, A.; Nørskov, J. K. Optimizing Perovskites for the Water Splitting Reaction. Science 2011, 334, 1355−1356. (11) Hinogami, R.; Toyoda, K.; Aizawa, M.; Yoshii, S.; Kawasaki, T.; Gyoten, H. Active Copper Delafossite Anode for Oxygen Evolution Reaction. Electrochem. Commun. 2013, 35, 142−145. (12) Carcia, P. F.; Shannon, R. D.; Bierstedt, P. E.; Flippen, R. B. O2 Electrocatalysis on Thin Film Metallic Oxide Electrodes with the Delafossite Structure. J. Electrochem. Soc. 1980, 127, 1974−1978. (13) Vojvodic, A.; Hellman, A.; Ruberto, C.; Lundqvist, B. I. From Electronic Structure to Catalytic Activity: A Single Descriptor for Adsorption and Reactivity on Transition-Metal Carbides. Phys. Rev. Lett. 2009, 103, 146103. (14) Marquardt, M. A.; Ashmore, N. A.; Cann, D. P. Crystal Chemistry and Electrical Properties of the Delafossite Structure. Thin Solid Films 2006, 496, 146−156. (15) Sheets, W. C.; Mugnier, E.; Barnabé, A.; Marks, T. J.; Poeppelmeier, K. R. Hydrothermal Synthesis of Delafossite-Type Oxides. Chem. Mater. 2006, 18, 7−20. (16) Amrute, A. P.; Łodziana, Z.; Modelli, C.; Krumeich, F.; PérezRamírez, J. Solid-State Chemistry of Cuprous Delafossites: Synthesis and Stability Aspects. Chem. Mater. 2013, 25, 4423−4435.
■
CONCLUSIONS Using density functional theory (DFT) calculations, we investigated copper delafossite oxide (CuBO2) catalysts for use as anodes for water electrolysis. The eg and t2g occupancies of the B site were simply estimated using bulk DFT calculations that included the electrons originating from hybridization between B−O atoms. The calculated eg and t2g occupancies are highly dependent on the B element. The activity of the oxygen evolution reaction (OER) is also mostly determined by the B element. To identify an appropriate descriptor of the OER activity, we examined the relationships between the calculated eg or t2g occupancy and the experimentally determined OER activity. The calculated t2g occupancy showed a close to linear relationship with OER activity. Our examination shows that the covalency between the t2g electrons of the B site and O 2p electrons can contribute to enhancing the OER process. We therefore propose the t2g occupancy, estimated using bulk DFT calculations, to be a more appropriate descriptor of the OER activity of delafossite oxide catalysts. It should be possible to efficiently explore highly active transition metal oxide catalysts based on the theoretically calculated eg or t2g occupancy of active sites.
■
Article
ASSOCIATED CONTENT
S Supporting Information *
PDOSs of the B site in delafossite oxides (CuBO2), the effect of DFT+U on the electronic states of CuFeO2, the calculated results of PdBO2 and PtCoO2, the slab calculations for CuCrO2 and CuCoO2, and the calculated eg occupancies of the B site in perovskite oxides. This material is available free of charge via the Internet at http://pubs.acs.org. F
DOI: 10.1021/jp5092398 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C (17) Delorme, P. C. Sur quelques composés du type M2O3-Cu2O. Acta Crystallogr. 1956, 9, 200. (18) Töpfer, J.; Trari, M.; Gravereau, P.; Chaminade, J. P.; Doumerc, J. P. Crystal Growth and Reinvestigation of the Crystal Structure of Crednerite, CuMnO2. Z. Kristallogr. 1995, 210, 184−187. (19) Effenberger, H. Structure of Hexagonal Copper(I) Ferrite. Acta Crystallogr. 1991, C47, 2644−2646. (20) Shaplygin, I. S.; Prosychev, I. I.; Lazarev, V. B. The Chemistry and Properties of Complex Oxides of Rhodium. Russ. J. Inorg. Chem. 1986, 31, 1649−1652. (21) Momma, K.; Izumi, F. VESTA: A Three-dimensional Visualization System for Electronic and Structural Analysis. J. Appl. Crystallogr. 2011, 44, 1272−1276. (22) Morikawa, Y.; Ishii, H.; Seki, K. Theoretical Study of n-Alkane Adsorption on Metal Surfaces. Phys. Rev. B 2004, 69, 041403(R). (23) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (24) Troullier, N.; Martins, J. L. Efficient Pseudopotentials for PlaneWave Calculations. Phys. Rev. B 1991, 43, 1993−2006. (25) Vanderbilt, D. Soft Self-Consistent Pseudopotentials in a Generalized Eigenvalue Formalism. Phys. Rev. B 1990, 41, 7892−7895. (26) Maignan, A.; Eyert, V.; Martin, C.; Kremer, S.; Frésard, R.; Pelloquin, D. Electronic Structures and Thermoelectric Properties of CuRh1‑xMgxO2. Phys. Rev. B 2009, 80, 115103. (27) Singh, D. J. Electronic and Thermoelectric Properties of CuCoO2: Density Functional Calculations. Phys. Rev. B 2007, 76, 085110. (28) Izumi, F.; Momma, K. Three-Dimensional Visualization in Powder Diffraction. Proc. XX Conf. Appl. Crystallogr., Solid State Phenom. 2007, 130, 15−20.
G
DOI: 10.1021/jp5092398 J. Phys. Chem. C XXXX, XXX, XXX−XXX
