Solid-State Chemistry of Cuprous Delafossites: Synthesis and Stability

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Solid-state chemistry of cuprous delafossites: synthesis and stability aspects Amol P Amrute, Zbigniew #odziana, Cecilia Mondelli, Frank Krumeich, and Javier Perez-Ramirez Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 18 Oct 2013 Downloaded from http://pubs.acs.org on October 23, 2013

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Chemistry of Materials

Solid-state chemistry of cuprous delafossites: synthesis and stability aspects Amol P. Amrute,‡ Zbigniew Łodziana,§ Cecilia Mondelli,‡ Frank Krumeich,‡ and Javier PérezRamírez*,‡ ‡

Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, WolfgangPauli-Strasse 10, CH-8093 Zurich, Switzerland. § Polish Academy of Sciences, ul. Radzikowskiego 152, PL- 31-342 Kraków, Poland. KEYWORDS: Delafossites, solid-state synthesis, in situ X-ray diffraction, stability, density functional theory ABSTRACT: Cuprous delafossites exhibit exceptional electrical, magnetic, optical, and catalytic properties. Through the application of a battery of in situ and ex situ characterization methods complemented by density functional theory (DFT) calculations, we gathered an in-depth understanding of the synthesis of CuMO2 (M = Al, Cr, Fe, Ga, Mn) by the solid-state reaction of Cu2O and M2O3 and of their stability against oxidative disproportionation to CuM2O4 and CuO. TGA-DTA and XRD studies of the synthesis revealed that the nature of the M3+ cation strongly impacts: (i) the formation temperature of the delafossite phase, which occured at much lower temperature for CuCrO2 than for the other metals (1073 versus 1273-1423 K), (ii) the mechanism of formation of the CuMO2 in different atmospheres, which was found to comprise up to 4 steps in air and a single step in N2, and (iii) the kinetics of the process, which could be significantly accelerated upon mechanochemical activation of the precursors by ball milling. The identification of unstable intermediate phases and, thus, a proper description of the synthesis mechanism was only possible by the application of in situ XRD. Electron microscopy, nitrogen sorption, and mercury porosimetry analyses of the precursor oxide mixtures at different stages of the synthesis in air revealed that particle agglomeration took place prior to the solid-state reactions forming the intermediate spinel phase and the delafossite, respectively, and that these led to a substantial drop in porosity and specific surface area. Based on XRD and He pycnometry, the resulting CuMO2 were pure phase delafossite with rhombohedral structure (R-3m), except for CuMnO2 which features a monoclinic structure (C2/m). Upon heating in air, CuCrO2 retained its structure up to 1373 K, while all other delafossites decomposed, CuAlO2 at 1073 K, CuGaO2 at 873 K, CuFeO2 at 773 K, and CuMnO2 at 673 K. The DFTcalculated surface phase diagram of CuCrO2 and CuAlO2 indicated that, at elevated oxygen pressures, the terminations with 1/2 and 0 ML of Cu are the most stable for the (0001) facet. The formation enthalpy for interstitial oxygen species in the bulk is endothermic for both delafossites, while that for oxygen insertion in subsurface layers of these terminations is still endothermic for CuCrO2 but weakly exothermic for CuAlO2. These results provide an improved understanding of the chemistry of these mixed oxides, enabling their optimization for specific applications.

1. INTRODUCTION Mixed metal oxides are multifunctional materials, widely employed as ceramics, in (opto)electronic technologies, and in catalysis. They can be classified based on their chemical structure, e.g. aluminates, titanates, silicates, ferrites, chromites, zirconates, molybdates, and vanadates, or based on their crystal structure, e.g. spinels, perovskites, fluorites, delafossites, etc.1 The exceptional properties of these solids originate from the close interaction of the different metal cations in the mixed phase, which enables synergistic effects, not manifested in single metal oxides.2 In order to attain optimized materials for a specific application or to uncover new possible uses, the preparation routes should be precisely designed by controlled tuning of their main parameters. This requires a sound knowledge of synthetic aspects as well as an in-depth understanding of the characteristics of the solids at the bulk and surface levels and their impact on the deriving functions.3,4 For spinel-, perovskite- and fluorite-type materials synthesisproperty correlations have been widely established, leading to materials with optimally tailored features.1,5-7 Representative examples are the improved electrical conductivity of CaMnO3− perovskites achieved upon synthesis via the citrate rather

than the ceramic route,5 the enhanced electrochemical properties of the LiMn2O4 spinel obtained by adequate selection of the annealing temperature and the cooling rate during preparation,6 as well as the tuning of phase composition, reducibility, and catalytic behavior in methanol decomposition of Co-Fe spinels through application of mechanochemical and thermal synthesis routes.7 With regards to delafossites, the level of understanding of synthesis and property aspects could be significantly enhanced. These solids, particularly cuprous delafossites (having the general formula A+B3+O2), have attracted substantial interest due to their exceptional (thermo)electric, magnetic, and optical properties, finding diverse technological applications, namely, in the fields of optoelectronic devices, field electron emitters, light emitting diodes, laser diodes, solar cells, functional windows, and thermoelectric materials.8-10 Besides, cuprous delafossites have also been reported as catalysts for the conversion of synthesis gas to linear alcohols and aldehydes,11,12 water splitting,13 the decomposition of toxic gases originating from internal combustion engines,14 the oxidation of unburnt combustion gases and CO, selective CO oxida-

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tion,15,16 HCl oxidation,17 and as photocatalysts for reduction of CO2.18 Concerning the synthesis of delafossites, some aspects have been uncovered only in relation to materials used in optoelectronics. In particular, various delafossite compositions (i.e. partial or total replacement of Cu or M with other metals) have been demonstrated,10,19-22 a sound understanding of the hydrothermal synthesis route has been gathered, and some scattered insights, obtained through ex situ studies, have been reported for the most commonly applied ceramic route.23,24 Thus, a comprehensive study of the solid-state synthesis, considering different trivalent cations, and comprising the use of available in situ techniques is lacking. Such investigations would lead to relevant information considering the strict requirement of a precise stoichiometry to attain well-defined and reproducible behaviors. With regards to the properties, conductivity and thermoelectric features have been widely investigated both experimentally and theoretically. DFT studies thoroughly described band gap characteristics, defect energetics, and electronic structures.25-28 In contrast, the structural and electronic origin of the catalytic functions of delafossites have been rarely elucidated. Some indications have been given about the stability of cuprous delafossites in oxidative atmosphere. Indeed, copper is present in these materials as Cu(I) species, which are prone to oxidize in the presence of oxygen ultimately leading to the disproportionation into CuO and CuM2O4. Experimental studies have shown that CuFeO2 easily undergoes oxidation, but iron doping by chromium produces a more resistant material,29 that CuCrO2 does not decompose up to 1373 K,17 and that CuAlO2 is thermodynamically and kinetically more stable than CuGaO2 in air.30 Some CuMO2 have shown to possess oxygen storage capacity at lower temperatures than those causing their decomposition.31 The stability of cuprous delafossites as a function of the nature of M3+ under oxidizing conditions deserves further elucidation, as this aspect has strong implications both on conductivity and catalytic behavior. In addition to the application of adequate experimental methods, theoretical descriptions of the surface of these materials and atomistic insights into the oxidation process would be highly valuable. Herein, we gather a comprehensive understanding of the solid-state chemistry of CuMO2 (M = Al, Cr, Mn, Fe, Ga) delafossites in terms of synthesis, properties, and stability against oxidative disproportionation, through extended ex situ and in situ characterizations. Major aspects through the paper are substantiated and further elucidated by state-of-the-art DFT calculations. Our study provides insights into the reaction steps involved in the synthesis of CuMO2 in air and inert atmospheres, their formation temperature and kinetics, and the textural and morphological modifications experienced by the precursors’ mixture throughout the process. From these data, optimized synthesis conditions are derived for all of the materials. Furthermore, a molecular-level explanation of the exceptional stability of CuCrO2 against oxidation compared to the other delafossite compositions is provided. 2. EXPERIMENTAL Materials. Cu2O (Strem, 99.9%), γ-Al2O3 (Alfa Aesar, 99.997%), Cr2O3 (Strem, 99.995%), Ga2O3 (Strem, 99.998%), Mn2O3 (Aldrich, 99.999%), and Fe2O3 (Strem, 99.999%) were used as delafossites precursors. CuO was obtained by heating of Cu(NO3)2·3H2O (Alfa Aesar, 98%) in static air at 873 K

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(5 K min−1) for 15 h. Equimolar mixtures of Cu2O + M2O3 (M = Al, Cr, Ga, Mn, Fe) were homogenized for 30 min-24 h using a Retsch Planetary Ball Mills PM200 equipped with yttria-stabilized zirconia balls (10 mm o.d.) and grinding jars (50 cm3). Unless stated otherwise, a ball milling time of 30 min was applied to the mixtures employed for various treatments and for the preparation of cuprous delafossites by solid-state reaction under optimized synthesis conditions (see Section 4.2, Table 2). Characterization methods. Powder X-ray diffraction (XRD) was measured using a PANalytical X’Pert PRO-MPD diffractometer. For in situ analyses, samples were placed in a dedicated cell, exposed to air or N2 (50 cm3 STP min−1), and heated to 1373 K (10 K min−1), dwelling the temperature for 1 h every 100 K starting from 773 K. Data was recorded in the 1070° 2θ range with an angular step size of 0.017° and a counting time of 0.26 s per step. N2 sorption at 77 K was carried out using a Quantachrome Quadrasorb-SI analyzer. The samples were degassed in vacuum at 473 K for 12 h prior to the measurement. Mercury porosimetry was performed using a Micromeritics Autopore IV 9510 instrument. Samples were degassed in vacuum, followed by intrusion of mercury in the pressure range from vacuum to 418 MPa. The Washburn equation was applied to calculate the pore size distribution. Thermogravimetric analysis-differential thermal analysis (TGADTA) was carried out using a Mettler Toledo TGA/DSC 1 Star system analyzer. The measurements were conducted in air (50 cm3 STP min−1) for the homogenized oxide mixtures and in air and N2 (100 cm3 STP min−1) for CuMO2 materials, ramping the temperature from 298 to 1373 K at 10 K min−1. The density of the solids was measured by He pycnometry using a Micromeritics Accupyc II 1340 instrument. Prior to the analysis, the samples (ca. 1 g) were dried in vacuum at 423 K for 4 h. The density data were obtained averaging 50 measurements collected after allowing the system to equilibrate during 150 measurements. Scanning electron microscopy (SEM), combined with energy-dispersive X-ray (EDX) spectroscopy, was performed with field-emission microscopes, Gemini 1530 (Zeiss) or Quanta 200 (FEI), operated under high vacuum. High resolution transmission electron microscopy (HRTEM) measurements were performed using a FEI Tecnai F30 ST microscope (field emission gun, operated at 300 kV). 3. THEORETICAL METHODS Atomic level calculations were performed within the density functional theory (DFT) approach in the plane wave formulation implemented in the VASP code.32-34 The atomic cores were represented by the projector augmented-wave (PAW) method potentials35,36 with the following valence states: 3d10 and 4s1 for Cu; 3s2 and 3p1 for Al; 3d5, 4s1 for Cr; 3d5, 4s2 for Mn; 3d6, 4s2 for Fe; 4s2, 4p1 for Ga; and 2s2, 2p4 for O. The Perdew-Burke-Ernzerhof (PBE) approximation was used for the exchange-correlation function.37 The plane wave cutoff was 500 eV. A k-point density of minimum 40 Å−1 was applied for the bulk calculations, while it was reduced to ~20 Å−1 for the surface calculations. All calculations were spin polarized. The conjugate gradient method was used for atomic relaxation. The convergence criterion for internal atomic coordinates was 0.005 eV Å−1 and the total energy was converged to 2 meV per formula unit. Due to the complex electronic structure of delafossites, the standard DFT approach with local exchange-correlation func-


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tionals (such as local density approximation, LDA, or generalized gradient approximation, GGA) fails to provide an appropriate description of the band gap38 or to properly locate the position of the metal d-states and oxygen p-states in the valence band. Thus, the rotationally invariant approach of Lichtenstein,39 the so-called PBE+U method, was applied to account for the strong electron correlations. This method explicitly considers the Coulomb interactions between the electrons localized on the atomic orbitals. For copper, U = 8 eV and J = 0.9 eV were used. These values were indeed reported to enable a down shift of the Cu 3d states to 2.8 eV, that is, below the valence band maximum, in agreement with experimental XPS data.40 In the case of the other transition metals, U = 5 eV and J = 0.9 eV were applied for Cr, Fe, and Mn, and U = J = 0 eV for Al and Ga. The surface calculations were performed in the slab geometry, where a minimum of 12 Å of vacuum in the c0 direction was introduced. The (0001) facet of the rhombohedral delafossite structures was chosen and a (2 × 2R45) surface cell was used. This surface plane is parallel to the MO6 layers and exposes sheets of MO6 octahedra with copper atoms on top (vide infra, Fig. 13). For each system, 3 to 7 MO6 layers were considered. The central layer of the slab was frozen in the bulk geometry, whereas all other atoms were relaxed in order to calculate the surface energy. The adsorption or defect formation energies were calculated on one surface of the slab, the other surface being frozen. Calculations for the reference energy of gas-phase O2 were performed in the cubic cell with an edge of 12 Å and a single k-point. The reference energies for copper and copper oxides were taken from ground state calculations for Cu, Cu2O, and CuO within the PBE+U approach. The structures of these oxides were optimized with respect to the lattice parameters and the internal atomic positions. In Cu2O, copper is coordinated to two oxygen anions, formally existing as Cu+, and has no magnetic moment due to its close shell configuration. In contrast, it is coordinated to four oxygen anions in CuO, being formally Cu2+, and carries a magnetic moment of ~0.68 µB.41 Copper in the bulk structure of the delafossites is linearly coordinated to 2 oxygen anions, thus resembling that of Cu2O. Further details of the theoretical methods can be found in the Supporting Information (SI). 4. RESULTS AND DISCUSSION 4.1. Synthesis of cuprous delafossites The influence of various parameters, such as temperature, atmosphere, annealing time, and ball milling of precursors, on the solid-state synthesis of cuprous delafossites with different trivalent cations was investigated. This enabled to gain insights into the temperature, mechanism, and kinetics of their formation and to derive a set of optimized conditions to achieve single-phase materials. Furthermore, textural, compositional, and morphological changes were explored throughout the thermal treatment from the starting mixture of oxides to the resulting delafossite structure. 4.1.1. Mechanism and temperature of formation. TGA-DTA and in situ and ex situ XRD were applied to study the synthesis of CuMO2 to gather information about the evolution of crystalline phases during the process and the formation temperature (Tformation) of the delafossite phase. The TGA-DTA profiles (Fig. 1) of ball-milled equimolar mixtures of Cu2O and M2O3 in air show a slow weight gain

Figure 1. TGA-DTA profiles in air of equimolar Cu2O + M2O3 mixtures.

Figure 2. Formation temperature of the delafossites according to TGA-DTA (Fig. 1), XRD (Figs. 3, S2, and S3), and the Ellingham diagrams (Fig. 5).



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Figure 3. In situ XRD patterns at different temperatures during the synthesis of CuAlO2 and CuCrO2 in air (a) and N2 (b) from equimolar Cu2O + M2O3 mixtures. Evolution of the crystalline phases during the synthesis of CuAlO2 and CuCrO2 in air (c,e) and N2 (d,f) as determined from the in situ XRD analysis. The corresponding XRD patterns with detailed phase assignment are available in Figure S2.

followed by a rather fast weight loss (see a and b in Fig. 1), which is mostly associated with a sharp negative peak in the corresponding DTA curves (see c in Fig. 1). The weight gain in the 600-950 K range is attributed to the oxidation of Cu2O to CuO.42 This is very similar for all of the samples. The drop in weight starts at variable temperatures between 1073 and 1300 K and takes place in a narrow or more extended temperature window depending on the mixture of oxides. In line with literature data43 and as confirmed by XRD analysis of the CuAl sample collected after the TGA analysis (not shown), this signal is related to the formation of the delafossite phase. For the Cu2O + Al2O3, Cu2O + Ga2O3, Cu2O + Fe2O3, and

Cu2O + Mn2O3 mixtures, a sharp weight loss and clear negative peaks in the DTA profiles were observed, indicating formation temperatures for the corresponding delafossites comprised between 1270 and 1350 K. It is worth noting that in the case of M = Al, Ga, and Mn, the sample weight after the drop was slightly higher than that of the starting mixture, suggesting an incomplete conversion of the single oxides into delafossite. In this respect, both the formation kinetics and stability of the delafossite phase in air and N2 will be addressed later on. Furthermore, the TGA profile of the Cu2O + Mn2O3 mixture exhibits an additional weight loss at ca. 1173 K (see d in Fig. 1), which is due to the partial transformation of Mn2O3 to


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Figure 4. Steps involved in the formation of cuprous delafossites by the solid-state reaction of the single oxides. Four-step mechanism for the delafossites forming in air above 1273 K (a) and three-step route for CuCrO2, which forms below 1273 K (b).

Mn3O4.44 For the Cu2O + Cr2O3 mixture, owing to a much slower weight loss, a local minimum in the TGA curve is not clearly visible. The Tformation for CuCrO2 was thus estimated at ca. 1073 K. Based on the Tformation values derived from the TGA-DTA results (Fig. 2), the synthesis of cuprous delafossites, in general, require high temperatures (above 1273 K), with the exception of CuCrO2. Overall, these differences hint a different reactivity of M2O3 toward copper oxide and/or stability of the obtained CuMO2 (vide infra). Based on the observed Cu2O conversion into CuO during the thermal treatment, it was checked whether the use of the latter as the copper precursor would lead to a different Tformation. As exemplified by the case of the Cu-Al system, the TGADTA analysis did not evidence significant differences (Fig. S1 in SI). Thus, Cu2O was exclusively employed as the copper source in the rest of the study. In situ XRD analyses (Figs. 3 and S2) were conducted in air and N2 atmospheres for two representative systems, Cu-Al (high Tformation) and Cu-Cr (low Tformation). A qualitative inspection of the XRD patterns collected at different temperatures suggests that the formation of the delafossites occurred from the same chemical species as the starting materials in N2, while via various intermediate phases in air (Figs. 3a,b). The latter are identified as CuO, CuAl2O4, and Cu2O for the Cu-Al system, and as CuO and CuCr2O4 for the Cu-Cr system. The relative amounts of the crystalline phases identified during the synthesis were estimated by integrating the areas of the reflections specific to each phase and dividing the obtained values by the total area of all of the reflections in each pattern (Figs. 3c-f). Considering the Cu2O + Al2O3 mixture in air (Fig. 3c), the evolution of phases is as follows: from room temperature to 773 K, Cu2O was depleted and CuO was formed; from 973 to 1273 K, CuO disappeared and the mixed CuAl2O4 phase developed to a significant extent; a moderate amount of Cu2O reformed at 1273 K; from 1273 to 1373 K, the spinel and Cu2O were partially and fully depleted, respectively, and CuAlO2 was produced. Accordingly, the formation of the Cu-Al delafossite involves: (i) the oxidation of Cu2O to CuO, in line with the TGA data (Fig. 1), (ii) the reaction of the latter with Al2O3 to form CuAl2O4, (iii) the reduction of the unconverted CuO back into Cu2O, and, finally, (iv) the reac-

tion of Cu2O and the spinel to form CuAlO2. Thus, our in situ XRD study sheds new light onto the formation mechanism of CuAlO2, which was reported, based on ex situ XRD analysis, to comprise the same intermediate formation of the spinel but to subsequently involve its reaction with CuO (rather than Cu2O) to produce delafossite.43 When monitoring the same mixture in N2, owing to the inert atmosphere, the Cu2O phase was unaltered up to 1273 K and the alumina, not being consumed in the reaction with CuO, partially underwent the expected phase transition from γ to α at 1173-1273 K (Fig. 3d).45 Formation of CuAlO2 took place at 1373 K by the solid-state reaction of Cu2O and α-Al2O3. Considering the Cu2O + Cr2O3 mixture in air, major similarities as well as interesting differences were found with respect to the synthesis steps highlighted for CuAlO2. Cu2O underwent oxidation to CuO up to 673 K, as for the Cu-Al system (Fig. 3e). Formation of the CuCr2O4 spinel by reaction of CuO and Cr2O3 occurred at 773-973 K, i.e. at ca. 300 K lower temperature than for the Cu-Al system. CuO and CuCr2O4 started forming CuCrO2 at 1073 K. The delafossite synthesis was complete already at 1173 K, as only this phase was detected at and above this temperature. In contrast to the case of CuAlO2, Cu2O was not detected and, thus, did not take part in the generation of the delafossite. This is due to the fact that both the intermediates, CuO and CuCr2O4, were completely converted into CuCrO2 at the temperature (1273 K) at which Cu2O forms. In N2, CuCrO2 originated by the direct reaction of Cu2O with Cr2O3 already from 973 K (Fig. 3f). It should be noted that the degree of formation of the Cu-Al and Cu-Cr delafossites found by in situ XRD in air is in line with the TGA-DTA results, and that the same trend is observed in the case of inert atmosphere. In order to confirm whether the synthesis of the other delafossites of the high Tformation group (i.e. CuGaO2, CuFeO2, CuMnO2; Fig. 2) would occur according to the same sequence of steps found for CuAlO2, Cu2O + M2O3 (M = Ga, Fe, Mn) mixtures were treated at selected temperatures (773 K, 1173 K, and 1273-1423 K) in air or N2 for 3 h and subsequently characterized by ex situ XRD (Fig. S3). Obviously, the specific high-temperature generation of Cu2O will be indirectly considered or excluded based on the Tformation of the



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Figure 5. Ellingham diagrams showing the thermodynamic stability of the delafossite and spinel phases at different temperatures and partial O2 pressures. Data for the thermodynamic calculations was retrieved from refs. 46-50.

delafossite. In air, the detection of CuO at 773 K and of CuM2O4 and CuO at 1173 K (Fig. S3a) for the three systems agrees with the data obtained from the Cu2O + Al2O3 mixture. Surprisingly, no delafossite phase was observed in all of the diffractograms collected after calcination at 1273-1423 K. The samples in fact showed the presence of CuM2O4, CuO, and Cu4O3. As the CuMO2 formation was proven by TGA-DTA, the use of N2 for cooling of these samples after 3 h dwelling in air at their formation temperatures was envisaged to avoid the presumed oxygen-aided transformation of the obtained delafossites. Indeed, significant amounts of CuGaO2 at 1423 K, CuFeO2 at 1373 K, and CuMnO2 at 1273 K were thus attained (Fig. S3a). In N2, no CuO was detected after calcination of the mixtures at 773 K. CuGaO2 and CuFeO2 were generated as the predominant phases, with minor contributions from M2O3 and Cu2O, after thermal treatment at 1423 and 1373 K, respectively, therefore supporting the same direct synthesis route as for CuAlO2 (Fig. S3b). After 3 h dwelling at 1273 K, only two single oxides were still detected for the CuMn system, i.e. Cu2O and Mn3O4. The latter can form from Mn2O3 via oxygen loss in inert atmosphere above 1073 K.44 The absence of the CuMnO2 phase could be due to a kinetically and/or thermodynamically unfavorable reaction of this Mn precursor with Cu2O. Anyway, their full transformation into a delafossite phase would not be possible as this mixture of oxides contains less oxygen than the stoichiometrically required. A summary of the steps involved in the synthesis of cuprous delafossites by solid-state reaction in air or N2 is schematically shown in Fig. 4. Accordingly, the formation of all of the CuMO2 in air proceeds via the generation of intermediates (CuO and CuM2O4, steps C, D, H, and I) and CuMO2 results from the reaction of Cu2O and CuM2O4, if it forms above 1273 K (step F, Fig. 4a), or of CuO and CuM2O4, if it forms below 1273 K (step J, Fig. 4b). In inert atmosphere, CuAlO2, CuCrO2, CuGaO2, and CuFeO2 are produced from the direct reaction of Cu2O and the respective M2O3 (steps B, G), while CuMnO2 does not form (step A).

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Overall, the Tformation values derived from the XRD analysis are in good agreement with those obtained from TGA-DTA (Fig. 2). These experimental values were compared to thermodynamic data calculated for the Cu-Cr, Cu-Al, Cu-Ga, and CuFe systems. The Ellingham diagrams (Fig. 5) show that the phase boundary between spinel and delafossite phases under ambient air atmosphere (log p(O2) ~ − 0.7) is at ca. 1300 K for Al and Fe, 1390 K for Ga, and 1063 K for Cr. This implies that the delafossite phases will be stable only above this temperature in air. Based on the clear correspondence of these values with the TGA-TDA and XRD data (Fig. 2), thermodynamics seems to govern the formation of the delafossite phases in air. 4.1.2. Kinetics of formation. As the synthesis of the cuprous delafossites is thermodynamically controlled, the kinetics of formation of CuMO2 was studied at their Tformation as a function of the annealing time and of the duration of the activation of the starting single oxides by ball milling. In order to assess the dependence on the annealing time, samples were produced for XRD analysis dwelling the equimolar Cu2O + M2O3 mixtures at their Tformation in air or N2 for 1-30 h. The relative amounts of CuMO2 produced were estimated according to the procedure described above. In air, ca. 65% of CuAlO2 formed already in 1 h (Fig. 6a). The rate of CuAlO2 formation significantly slowed down at longer tdwell and a pure CuAlO2 phase was attained after 30 h. In air, the rate-limiting step shall be F (Fig. 4a), as the previous steps (CE) were completed up to 1273 K and the reaction mixture above this temperature exclusively consisted of Cu2O and CuAl2O4 (Fig. 3c). In N2, the formation of CuAlO2 was much slower and, after 30 h, only 70% of CuAlO2 was produced. Thus, the reaction of Cu2O with the spinel appears to be much

Figure 6. Amount of CuAlO2 (a) and CuCrO2 (b) formed in air or N2 at increasing dwelling times at their synthesis temperatures.


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Table 1. Amount of CuMO2 formed from ball-milled equimolar Cu2O + M2O3 mixtures after treatment at their respective formation temperatures for 3 h in air or N2. Starting oxide mixture Cu2O + Al2O3 Cu2O + Cr2O3

Cu2O + Ga2O3

Heating atmosphere

CuMO2 (%)

1373

static air

69

1073

static air

13

Synthesis temperature (K)

1373 1423

Cu2O + Fe2O3

1373

Cu2O + Mn2O3

1273

N2 flow

6

N2 flow

57

static air

100

static air

a

static air

a

static air

a

N2 flow

95

N2 flow N2 flow

80 50 95 90 0

a N2 was flown during cooling to avoid oxidative decomposition of the formed delafossite into CuO and CuM2O4.

faster than that with α-Al2O3 (Fig. 4a, step B). This result reveals that CuAl2O4 is more reactive than α-Al2O3. For the Cu-Cr system, the formation kinetics of the delafossite phase was faster in N2 rather than in air (Fig. 6b). Therefore, it is deduced that the reaction of Cu2O with Cr2O3 (Fig. 4b, step G) is more favored than that of CuO with CuCr2O4 (Fig. 4b, step J). Interestingly, even after 30 h annealing at 1073 K, none of the reaction yielded a pure delafossite phase, suggesting that the formation of CuCrO2 is slower than that of CuAlO2 at their Tformation. Still, annealing of the Cu2O + Cr2O3 mixture at 1373 K in air enabled the production of pure CuCrO2 in only 1 h. In the case of the Cu-Ga and Cu-Fe systems, more delafossite was attained in N2 (ca. 95%, Table 1) than in air (ca. 80 and 50%, respectively) for 3 h dwelling time (tdwell). For the Cu-Mn system, ca. 90% CuMnO2 was obtained after a 3 h treatment in air, while its formation was not observed in N2 even after 30 h annealing, due to the in situ reduction of Mn2O3 (vide supra). Pure CuGaO2, CuFeO2, and CuMnO2 were produced after 30 h calcination in air at their Tformation and cooling in N2. Taking into account the formation temperature and the annealing time, an overall trend for the kinetics of CuMO2 formation (according to the trivalent cation) in air is: Cr > Mn > Al > Fe > Ga, and in N2 is: Cr > Fe > Ga > Al. The effect of the duration of the pretreatment by ball milling was investigated on the Cu-Al and Cu-Cr systems. TGA analysis of the Cu2O + M2O3 mixtures ball-milled for 5 and 24 h indicated a lower weight gain related to the oxidation of Cu2O compared to the sample ball-milled for 30 min (Figs. S4a,b). This possibly indicates a partial oxidation of Cu2O due to the longer treatments. However, amorphization during the extended ball milling rendered the detection of the oxidized copper phase equivocal (Figs. S5a,b). The Tformation derived from the TGA profiles (see vertical dashed lines in Figs. S4a,b) was independent of the duration of ball milling, in line with the highlighted thermodynamic control of the synthesis. However, for Cu2O + Cr2O3, a much sharper weight loss was observed after ball milling for 5 or 24 h than for 30 min. This suggests that ball milling mechanochemically activated the precursors, which led to a faster kinetics of delafossite production. In

Figure 7. Amount of CuAlO2 and CuCrO2 formed in air after ball milling of their precursors for increasing times.

particular, the extent of formation of CuCrO2 as well as CuAlO2, after calcination at their Tformation for 10 h (i.e. at incomplete conversion), linearly scaled with the duration of ball milling (Figs. 7 and S5). Accordingly, longer ball milling times could be applied to reduce the required dwelling time at high temperatures to achieve a pure-phase delafossite. Combining our findings on the formation temperature – from TGA-DTA, in situ XRD, and thermodynamic data – and the formation kinetics, a set of optimal conditions for the synthesis of pure phase delafossite was derived (Table 2, vide infra) 4.1.3. Changes in morphological and textural properties during synthesis. The textural modifications of the Cu2O + Al2O3 mixture during delafossite synthesis were investigated by SEM coupled with EDXS (presented as color maps with red for Al and blue for Cu) and HRTEM. SEM-EDXS of the ball-milled equimolar mixture indicates that Cu2O and Al2O3 are present as aggregates of 1-10 µm size in close vicinity to each other (Figs. 8a,d). TEM of this sample visualizes Al2O3 as an amorphous phase, intimately mixed with crystalline Cu2O (Fig. 8g). After treatment at 873 K in air for 3 h, some agglomeration took place (Fig. 8b). The EDXS map evidences the presence of Cu and Al all over the sample, which is further corroborated by the particles distribution observed by TEM (Figs. 8e,h). It is interesting to note that, in spite of the agglomeration, the porous structure seems to be preserved. Based on the XRD analysis of the samples (Fig. S6), only the oxidation of Cu2O to CuO occurred up to this stage. No further agglomeration was observed for the mixture treated at 1073 K (not shown), but the mixed oxide CuAl2O4 phase formed to some extent, as expected (Fig. S6). For the material treated at 1373 K, SEM revealed the presence of significantly more agglomerated particles. The corresponding XRD pattern indicates CuAlO2 as the main phase in addition to CuAl2O4 and CuO (Figs. 8c,f and S6). According to TEM, quite big crystals partially surrounded by small particles are present (Fig. 8i). The HRTEM image (Fig. 8j) recorded on a thin edge of a crystal reveals lattice fringes with a distance of ca. 5.6 Å, which corresponds to the d003 value of the CuAlO2 delafossite (a = b = 2.858 Å, c = 16.958 Å).51 This finding was confirmed by the results of electron diffraction and EDXS measurements on the same crystal. Additional fringes with a larger distance (ca. 55 Å) observed in the same image point at some structural disorder. Based on the XRD pattern of this sample, the small decorating



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Table 2. Synthesis conditions, characterization data, and lattice parameters cuprous delafossites. Synthesis conditionsa

Compound

CuAlO2

Heating rate (K min−1)

Temperature (K)

Dwell time (h)

Heating atmosphere

Characterization data Cooling atmosphereb

Crystallite sizec (nm)

Powder density (g cm−3) 4.921

a = 2.858

a = 2.883

a = 2.975

a = 3.050

10

1373

30

static air

static air

38

CuCrO2

10

11731373

1-30

static air

static air

55

5.465

CuGaO2

10

1423

30

static air

N2 flow

45

5.956

CuFeO2

10

1373

30

static air

N2 flow

42

5.244

CuMnO2

10

1273

30

static air

N2 flow

41

4.919

Experimental lattice parameters (Å)

c = 16.96 c = 17.10

a = 2.976 c = 17.16

a = 3.035 c = 17.16

a = 5.596 b = 2.880 c = 5.899

β = 104.02

Calculated lattice parameters (Å)

c = 16.96 c = 17.09

a = 3.026 c = 17.23

a = 3.060 c = 17.21

a = 5.746 b = 2.937 c = 5.882

β = 103.81

a Cu2O + M2O3 mixtures homogenized by ball milling for 30 min were employed for the solid-state synthesis of delafossites. b Cooling of CuFeO2, CuMnO2, and CuGaO2 in air leads to the oxidative decomposition of the delafossite phase into CuO and the CuM2O4 spinels. c Determined by application of the Scherrer equation to the experimental XRD data in Fig. 10.

Figure 8. SEM (a-c), EDX mapping (d-f), and TEM (g-j) of the equimolar Cu2O + Al2O3 mixture as ball-milled (a, d, g) and after subsequent treatment at 873 K (b, e, h) or 1373 K(c, f, i, j) for 3 h in air. The XRD patterns of these samples are shown in Fig. S6.


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Figure 9. Changes in specific surface area at different temperatures during the synthesis of CuAlO2 and CuCrO2 in air and N2.

particles might comprise CuAl2O4 and/or CuO. Overall, these results suggest that agglomeration may represent a crucial driving force for the solid-state reactions involved in the synthesis process, as it enhances the interaction, of the mixture constituents. The porosity of the Cu2O + Al2O3 mixture as ball-milled and after treatments at different temperatures in air and N2 was studied by N2 sorption (Fig. 9). The specific surface area (SBET) of the starting sample is 24 m2 g−1 and is mostly due to the textural properties of Al2O3 (200 m2 g−1), as Cu2O possesses a SBET of only 2 m2 g−1. The SBET dropped only moderately with thermal treatments in air up to 973 K, in line with the preservation of the porous structure highlighted by TEM up to this temperature (Fig. 8h). Calcination of the mixture at 10731173 K led to a sharp drop in SBET, which is attributed to the consumption of the amorphous Al2O3 in the formation of crystalline CuAl2O4. A further slight decrease in SBET down to 3 m2 g−1 was associated with the formation of CuAlO2 at 12731373 K. The low surface area of the resulting material is in line with the complete loss of the mesoporosity characterizing the starting mixture, as derived by Hg porosimetry (Fig. S7). When treating the Cu2O + Al2O3 mixture in N2, the SBET was slightly better preserved up to 1173 K. Then, the conversion of the amorphous γ-Al2O3 into crystalline α-Al2O3 and the formation of the delafossite produced a similar drastic drop. For the Cu2O + Cr2O3 mixture, significant reduction of the original SBET was also observed in the temperature ranges corresponding to the formation of the mixed phases for this system, 673873 K (spinel) and 1073 K (delafossite) in air and 873-1073 K (delafossite) in inert atmosphere. 4.2. Structure and properties of cuprous delafossites In the delafossite structure, the Cu+ cation is linearly coordinated to two oxygen ions, while the M3+ cation (Al, Cr, Fe, Ga, Mn) forms distorted edge-shared MO6 octahedra with M in the central position. The oxygen ion remains in pseudotetrahedral coordination with one A and three M cations.52,53 The structure can be viewed as the stacking of planar layers of Cu cations in a hexagonal pattern and layers of edge-sharing MO6 octahedra (Fig. 10, top). XRD analysis of the cuprous delafossite powders synthesized under individually optimized conditions (Table 2) confirmed the formation of pure delafossites (Fig. 10): CuAlO2 (JCPDS 73-9485), CuCrO2 (JCPDS 74-0983), CuGaO2

Figure 10. Experimental and calculated XRD patterns of CuMO2. The rhombohedral structure of CuAlO2, CuCrO2, CuGaO2, and CuFeO2, and the monoclinic structure of CuMnO2 are shown on the top.

(JCPDS 41-0255), CuFeO2 (JCPDS 39-0246), and CuMnO2 (JCPDS 50-0860). The first four XRD patterns correspond to a rhombohedral structure with R-3m space group symmetry, while the last pattern indicates a monoclinic structure with C2/m space group symmetry, referred to as credenerite or distorted delafossite.53-57 The anomalous structure of CuMnO2 with respect to the other delafossites is due to a Jahn-Teller distortion, which comprises a displacement of the Mn atoms from the center of the octahedra (Fig. 10, top).58 In order to corroborate the assignments with respect to the space group symmetry, extended calculations on the bulk properties of all five compounds were performed. The ground structures for all compounds were optimized in the possible symmetries depending on the stacking of layers, i.e. P63/mmc, R-3m, and C2/m, with further relaxation of the unit cell shape and atomic positions without symmetry constraints. The calculated structural parameters are presented in Table 2. Taking into account the standard overestimation of the lattice constants known for the GGA approximation, they are in good agreement with the experimental data. The XRD patterns were calculated based on the experimental parameters (Section 2) and are a good match for the measured diffractograms. The slight shifts in the 2θ values are within the error of the calculations of the volume



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Figure 11. TGA profiles of the cuprous delafossites in N2 (a) and air (b).

of the unit cell. These results confirm a rhombohedral structure for CuAlO2, CuCrO2, CuGaO2, and CuFeO2, and a monoclinic structure for CuMnO2. The crystallite size of the synthesized delafossites, estimated by application of the Scherrer equation to the XRD data in Fig. 10, was in the range of 37-55 nm. The density of the materials, measured by He pycnometry, is in good agreement with the values reported in a mineralogy database,59 supporting the high purity of all of the CuMO2 (Table 2). As expected from the porosity study along the synthesis (Fig. 7), the SBET of the delafossite samples was ≤ 1 m2 g−1. 4.3. Stability of cuprous delafossites In order to assess the stability of the delafossite phase, the CuMO2 were alternatively exposed to N2 and air at temperatures up to 1373 K. TGA analysis of the materials in N2 indicated no appreciable weight change, suggesting that all of the studied CuMO2 were stable in this atmosphere (Fig. 11a). Contrarily, in air, a weight gain, associated to oxygen uptake, was observed for all of the samples except for CuCrO2, for which the weight change was negligible (Fig. 11b). The broad contribution in the low-temperature region (i.e. up to 600 K) is attributed to the accommodation of oxygen in the structure interstices, as the delafossite phase was unaltered up to this point (as confirmed by XRD of the spent samples), while the sharp and more consistent weight gain at high temperatures is related to the oxidative decomposition of CuMO2 into CuO and CuM2O4 (also confirmed by XRD of the spent samples). These results suggest that CuCrO2 and CuAlO2 remained intact in the whole temperature range studied, while CuMnO2, CuFeO2, and CuGaO2 were unstable. From the extent of weight gain and the stoichiometry of the decomposition reaction, it was estimated that ca. 98, 84, and 90% of these latter solids was converted, respectively. Based on their

Figure 12. XRD patterns of CuMO2 treated at different temperatures for 5 h in air. The crystalline phases identified are listed on the side of the corresponding pattern. A detailed assignment of the reflections can be found in Fig. S8.

decomposition temperatures (defined by the temperatures at which the sharp weight gain above 600 K is observed to start in the TGA profile, Fig. 11b) the ease of decomposition follows the order: CuMnO2 > CuFeO2 > CuGaO2. Nonetheless, it should be noted that, owing to the fast temperature ramping applied in the TGA measurements, the materials were exposed to different temperatures for very short times. This implies that, in case of a slow kinetics of decomposition, the latter might have remained undetected. To substantiate the TGA results, the delafossites were thus calcined in air for 5 h at different temperatures and the resulting samples were characterized ex situ by XRD (Fig. 12). In line with the thermogravimetric analysis (Fig. 11b), CuCrO2 proved stable in the whole temperature range investigated (673-1373 K) and, for CuMnO2, CuFeO2, and CuGaO2, decomposition took place at 673 K, 773 K, and 873 K, respectively. In contrast, the formation of CuO and CuAl2O4 was detected also for CuAlO2 at 1073-1173 K. Accordingly, this material is also prone to decomposition in air at high temperature, but the kinetics of the process is slower compared to CuMnO2, CuFeO2, and CuGaO2. Based on these findings, the decomposition behavior of the delafossites in air can be summarized as follows:


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CuMO2 + N2 → no decomposition, M = Cr, Al, Ga, Fe, Mn 2CuMO2 + ½O2 → CuM2O4 + CuO, M = Mn, Fe, Ga, Al 2CuCrO2 + ½O2 → no decomposition 4.4. DFT simulations In order to gain atomic-scale insights into the differences in stability of the delafossites under oxidative conditions, extensive DFT calculations were performed. First, the electronic structures of the CuMO2 were derived. As this bulk-level description did not correlate to the experimental evidences, the analysis was focused on the surface level.60 In view of the complexity of this study, surface energetics as well as the exposure and coordination of copper were elucidated only for Cu-Al, Cu-Cr, and Cu-Ga delafossites. Thereafter, the initial step of the oxidation process, i.e. the interaction of O2 with the materials, was explored. 4.4.1. Bulk and surface properties of cuprous delafossites. An in-depth analysis of the electronic structure of the bulk of all CuMO2 delafossites was conducted (see Section S1 for details). The position of the Cu d-band appeared very similar for all of the materials (Fig. S9). It was thus concluded that the differences in stability of the delafossites in contact with oxygen shall not be related to the electronic structure or to the direct oxidation of the bulk but rather to the surface properties, which were investigated for CuAlO2, CuCrO2, and CuGaO2. According to the best of our knowledge, no such theoretical calculations have been reported in the literature. Therefore, the surface phase diagram of these three compounds was calculated for the first time. For the demonstrative surface calculations, the (0001) facet of the rhombohedral delafossite polymorph was chosen. For this surface plane, terminations with 0, 1/2, and 1 monolayer (ML) of Cu were considered, where only the reconstructed surface with 1/2 ML of Cu is non-polar.61,62 Aiming at highlighting differences between materials, to avoid any ambiguity related to calculations of reference chemical potentials for multiphase transformations, a simplified approach was applied that refers the thermodynamic conditions to pure elements. This approach enables to compare CuAlO2, CuCrO2, and CuGaO2 under the same thermodynamic external conditions (see Section S1 for details). The surface energies for the (0001) facet of CuAlO2, CuCrO2, and CuGaO2 are presented in Fig. 13. Accordingly, the non-polar termination of CuCrO2 is more stable than that of CuAlO2 and CuGaO2 by 0.5 J m−2. For copper-poor (0 ML) terminations, a similar difference in stability was derived for CuCrO2 and CuGaO2 while it was doubled in the case of CuCrO2 and CuAlO2. For all three compounds, the termination with 1/2 ML of Cu is the most stable at low O2 pressures. At higher partial O2 pressures, this termination competes with the termination where copper is absent on the surface (0 ML). The latter shall be the most stable in air at elevated temperatures. The termination with 1 ML of Cu is energetically unfavorable for any of the three delafossites. Overall, the surface structure of these materials is similar and the difference in energies is related to electronic compensation effects at the surface. For CuAlO2 and CuGaO2, the analogous valence structure of Al and Ga is expected to lead to comparable surface energies. This was found in the case of the non-polar terminations (1/2 ML) of these materials, while a moderate discrepancy was observed for their polar electron-deficient terminations (0 ML). The latter likely originates from the interaction of d-

Figure 13. Surface energy of the (0001) facet of CuAlO2 (black), CuCrO2 (red), and CuGaO2 (green). Surface terminations with 1/2 ML (solid line), 0 ML (dashed line), and 1 ML (dotted line) of Cu were considered. The inset shows the surface structure (top view) of CuAlO2 for the terminations with 0 ML (a) and 1/2 ML (b) of Cu.

states of Ga and p-states of O at the valence band maximum. It is worth noting that the local environment of copper atoms at the surface differs from the bulk. Cu is linearly coordinated to two oxygen ions in the bulk, similarly to the case of Cu2O. In contrast, Cu is effectively coordinated to three oxygen ions at a relaxed surface with 1/2 ML (Fig. 13). Due to the threefold symmetry of the surface copper species, ordering of electrons on eg orbitals is not present, as it requires the fourfold symmetry of the atomic dx2−dy2 orbital. For 1 ML, each surface Cu atom has one oxygen neighbor. This surface stoichiometry is that expected based on thermodynamic stability. Deviations from this situation could exist based on kinetic effects related to the mass transport at the surface. Since the surface stabilities of CuGaO2 and CuAlO2 are similar, we focus further analysis on CuAlO2 and CuCrO2. 4.4.2. Formation of interstitial oxygen species. To gain understanding on the oxidative decomposition of delafossites, the enthalpy of formation of interstitial oxygen species in the bulk and in the vicinity of the two most stable surface terminations was calculated (Fig. 14). Based on the comparable surface stability of CuGaO2 and CuAlO2, this analysis was conducted limitedly to CuCrO2 and CuAlO2, in view of their higher practical relevance. This provides insight into the initial stage of oxygen penetration into the structures, i.e. the initial stage of CuO formation. The bulk phase was used as the reference, where the most stable configuration for interstitial oxygen species involves two oxygen atoms (thus interstitial oxygen in the bulk effectively possesses pairwise attractive interaction) located around one Cu atom and having similar local geometry to the case of CuO. In fact, for both delafossites, Cu coordinated to four oxygen anions gains the magnetic moment of ~0.7 µB, with somehow smaller moments (~0.4 µB) coupled antiferromagnetically to four nearest Cu neighbors. For CuCrO2, these moments are further coupled to the sublattice Cr. Such an atomic arrangement of copper can be considered as a precursor of CuO: Cu has four coplanar oxygen neighbors, possesses a magnetic moment, and its formal oxidation state is two. The insertion of oxygen into the bulk of the chromium delafossite was found to require less energy than for the aluminum counterpart. However, the



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ences that are induced by interstitial oxygen species in the vicinity of the oxygen-terminated surface. The change of the electron density on copper atoms is related to the occupancy of the dx2−dy2 orbital, as expected for CuO formation and indicated by the blue region around Cu in Fig. 14. While for CuCrO2 these changes are local, i.e. confined to the first Cu coordination sphere around the interstitial oxygen species, for the subsurface interstitial oxygen species in CuAlO2, the charge redistribution extends to the AlO6 layers, thus involving the charge compensation of the surface.

Figure 14. Formation enthalpy of interstitial oxygen species in the bulk and the subsurface for Al- and Cr-delafossites. The solid line refers to isolated oxygen atoms and the dashed line to pairs of oxygen atoms. The charge density difference induced by the insertion of oxygen in shown by the isosurface of the electron density 0.05e/Å3: blue indicates charge accumulation and green charge depletion.

formation enthalpy of interstitial oxygen species in the bulk is in both cases strongly endothermic. In the vicinity of the surface terminated with 1/2 ML of Cu, i.e. at low partial O2 pressures, this enthalpy moderately increases for CuCrO2 and decreases for CuAlO2. This implies that this surface is even more stable against oxygen penetration than the bulk for the Cr-delafossite, while is more prone to interaction with oxygen than the bulk for the Al-delafossite. In the case of the surface with 0 ML of Cu, i.e. at high partial O2 pressures, the differences between CuAlO2 and CuCrO2 are even more pronounced: formation of interstitial oxygen species in the subsurface layer becomes exothermic for CuAlO2, while it remains endothermic for CuCrO2. For both compounds, subsurface isolated interstitial oxygen atoms under the termination with 0 ML of Cu are more stable than pairs of interstitial oxygen atoms, which indicate lack of attractive interaction. The reduced/negative formation enthalpy of interstitial oxygen species in the vicinity of the surface represents the thermodynamic driving force for oxygen penetration into the bulk, which shall be stronger for CuAlO2, as experimentally observed. The origin of this effect is related to (i) changes of copper oxidation state in the vicinity of interstitial oxygen species, and (ii) magnetic and structural and/or electronic compensation effects in the vicinity of the surfaces. At the surface of metal oxides, the lower atomic coordination is compensated by structural relaxation, extensive charge transfer between oxygen and metal cations, or a combination of both the mechanisms. On polar surfaces, structural reconstruction can also be observed.61 In CuAlO2 and CuCrO2, for the compensated non-polar termination (1/2 ML of Cu), neither prominent interlayer relaxation nor charge transfer are observed. For the oxygen terminated (0 ML of Cu) polar surface, the polarity is compensated via extensive atomic relaxation within CuAlO2: the Cu–O distance is shortened by ~4.5% for the first oxygencopper layer, and the AlO6 layers are distorted. Within CuCrO2, the structural relaxation is much less pronounced (0.5%) for this surface, and charge redistribution occurs between Cr and O accompanied by distortion of the CrO6 octahedron. The insets in Fig. 14 show the charge density differ-

5. CONCLUSIONS A battery of complementary in situ and ex situ characterization methods coupled to extensive DFT calculations was successfully applied to gain in-depth understanding on synthesis and stability aspects of cuprous delafossites CuMO2 (M = Al, Cr, Fe, Ga, Mn). The mechanism and kinetics of formation of these materials by solid-state reaction was found to strongly depend on the nature of the M3+ cation, the annealing and cooling atmosphere, and the extent of pretreatment of the precursors mixture by ball milling. CuCrO2 was shown to form at lower temperature (1073 K), compared to the rest of the delafossites (1273-1423 K). This difference was explained based on the distinct thermodynamic stabilities of the CuMO2 phase at different temperatures and oxygen pressures. In air, the synthesis of all the cuprous delafossites was demonstrated as a multi-step process, where CuMO2 generates from the reaction of the in situ formed spinel with in situ formed CuO or Cu2O, if the delafossite formation temperature is below or above 1273 K, respectively. In inert atmosphere, CuAlO2, CuCrO2, CuGaO2, and CuFeO2 were obtained by direct reaction of Cu2O and M2O3, while CuMnO2 was not produced due to the instability of Mn2O3. Longer mechanochemical activation times correlated to larger delafossite yields and might be thus applied to reduce the annealing time at the formation temperature. The high purity delafossites obtained under the individually optimized conditions were identified of R-3m type, except for CuMnO2 which is of C2/m type. The nature of the M3+ ion was found to also affect the stability of the CuMO2 materials against oxidative disproportionation to CuM2O4 and CuO. While CuCrO2 was stable in air up to 1373 K, all other cuprous delafossites underwent oxidation. This difference in stability is rationalized thanks to insights into the Cu-Al and Cu-Cr systems by DFT analysis. The formation enthalpy of interstitial oxygen species in the bulk as well as in the subsurface layers was found to be endothermic for CuCrO2, whereas that of subsurface interstitial oxygen species resulted exothermic for CuAlO2. Our findings uncover the main synthesis parameters for cuprous delafossites via the ceramic route and their stability in oxidative environments. This information is expected to serve as a valuable guideline for optimizing the preparation of existing materials or designing new systems as well as for selecting appropriate CuMO2 for prospective catalytic applications depending on their operational demands.

A S S O C I AT E D C O N T E N T Supporting Information. Additional experimental results from thermal analysis, X-ray diffraction, and mercury porosimetry. Theoretical details of the electronic density of states of the cuprous delafossites. This information is available free of charge via the Internet at http://pubs.acs.org/.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

ACKNOWLEDGMENT The Electron Microscopy Centre of ETH Zurich is acknowledged for the use of their facilities. PL-GRID infrastructure is thanked for providing computational resources. We are indebted to Dr. D. Koziej for granting access and providing assistance for the in situ XRD measurements.

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(60) Gokhale, A. A.; Kandoi, S.; Greeley, J. P.; Mavrikakis, M.; Dumesic, J. A. Chem. Eng. Sci. 2004, 59, 4679. (61) Goniakowski, J.; Finocchi, F.; Noguera, C. Rep. Prog. Phys. 2008, 71, 016501. (62) Noguera, C. J. Phys.: Condens. Matter 2000, 12, R367.


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Solid-State Chemistry of Cuprous Delafossites: Synthesis and Stability

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