Stability of Solid-Solution Phase and the Nature of Phase Separation

Article pubs.acs.org/JPCC

Stability of Solid-Solution Phase and the Nature of Phase Separation in Ru−Zr−O Ternary Oxide Junqiu Zhu,†,∥ Xin Wang,‡,∥ Zhonghua Yi,‡ Zhongzhi Tang,† Bo Wu,‡ Dian Tang,*,†,‡ and Wei Lin*,†,§,⊥ †

Institute of Materials Research and ‡College of Material Science and Engineering, Fuzhou University, Fuzhou, Fujian 350108, China § School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States S Supporting Information *

ABSTRACT: Equilibrium phase diagram of ruthenium− zirconium oxide (Ru−Zr−O) was calculated by a combination of ab initio density functional theory and thermodynamic calculations. The phase diagrams suggest that the solubility between ruthenium oxide and zirconium oxide is very low under normal experimental conditions, in good agreement with the prior reports in literature. Also provided is the theoretical support for the reported phenomenon that doping or amorphization of Ru−Zr−O might suppress phase separation of the oxide. To study the spinodal decomposition of Ru−Zr−O, we successfully prepared an amorphous Ru0.48Zr0.52O2 film of a solid solution phase by a thermal decomposition at a low temperature (563 K). In situ transmission electron microscopy was utilized to witness the spinodal decomposition of the amorphous Ru0.48Zr0.52O2 solid solution by electron−beam annealing. The present fundamental study of the phase behavior of Ru−Zr−O provides a guideline for the phase and microstructure design of Ru-based mixed oxides for various important applications.

1. INTRODUCTION Ruthenium−zirconium oxide (Ru−Zr−O, or simply RuO2− ZrO2) is a Ru-based ternary metal oxide of great scientific importance and engineering value. First of all, RuO2−ZrO2 is an excellent electrocatalyst.1−6 RuO2 and its mixtures with other species of metal oxides have been known for their high electrocatalytic activities in broad industrial applications.7−9 Combining ZrO2 with RuO2 properly can improve the utilization, stability, or selectivity of RuO2 while effectively reducing the consumption of Ru.3,5−12 Second, RuO2−ZrO2 is a promising solid-state electrolyte.13,14 As we know, ZrO2 is of primary importance as an ionic conductor.15 The addition of RuO2 in ZrO2 was reported to improve the proton conductivity of the solid electrolyte and therefore may find this mixed oxide system broad applications as advanced energy materials.16 Other important applications of Ru−Zr−O include photoluminescence, supercapacitors, and so on.17 Albeit the widespread research interest in Ru−Zr−O for various applications, the phase behavior of this ternary metal oxide is still not clear. The early qualitative phase study of Ru−Zr−O by Hrovat et al. proposed an “empty” phase structure of Ru−Zr−O.18 By “empty”, it means that no binary compound or eutectic of Ru− Zr−O is stable at temperatures below 1405 °C, the decomposition temperature of RuO2 in air.14 Such a qualitative analysis provides an important reference for the phase behavior of regular Ru−Zr−O-based oxides; however, it does not elucidate any possible phase separation/segregation/transformation of Ru−Zr−O or any relevant mechanism; a detailed © 2012 American Chemical Society

quantitative phase analysis is needed to provide a fundamental guideline for the equilibrium and nonequilibrium phase/ microstructure design of the ternary oxide and other systems alike. In the present study, we use a combined ab initio density function theory (DFT) and thermodynamic modeling to construct a RuO2−ZrO2 phase diagram, in which the stable, unstable, and quasi-stable regimes of the solid solution phase with fluorite or rutile structure are clearly defined. In situ transmission electron microscopy (TEM) observation provides the first proof of the spinodal decomposition of (Ru1−x,Zrx)O2 solid solution.

2. CALCULATION AND EXPERIMENTAL 2.1. Thermodynamic Method. A single-phase ternary RuO2−ZrO2 oxide can be treated as a quasi-binary oxide composed of RuO2 and ZrO2.19 Thus, the formation of rutile (R-) and fluorite (F-) (Ru1−x,Zrx)O2 solid solution from its compositional oxides of the same crystal structure can be expressed by eqs 1 and 2.20,21 (1 − x)R‐RuO2 + x R‐ZrO2 = R‐(Ru1 − x , Zrx)O2

(1)

(1 − x)F‐RuO2 + x F‐ZrO2 = F‐(Ru1 − x , Zrx)O2

(2)

Received: August 23, 2012 Revised: October 24, 2012 Published: November 16, 2012 25832

dx.doi.org/10.1021/jp308310y | J. Phys. Chem. C 2012, 116, 25832−25839

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valence configurations of O(2s22p4), Ru(4s24p64d75s1), and Zr(4d25s2). The exchange-correlation terms were described by the local density approximation (LDA). Generalized gradient approximation (GGA) was used for comparison, for which the Perdew−Burke−Ernzerhof parametrization (PBE) was used.29 The eigenstates were expanded in plane-wave basis functions, and the ion cores were represented using the projectoraugmented wave (PAW) method.30 Space groups of P42/mnm and Fm3m were used for rutile and fluorite structures, respectively. A supercell containing 32 atoms was used for each case. For solid solutions, Ru and Zr atoms were treated as random distribution over the metal sublattice. The lattice geometry was optimized using Hellman−Feynman forces via a conjugate gradient method. The total energy (E) was minimized with respect to the volume (volume relaxation), the shape of the unit cell (cell external relaxation) and the position of the atoms within the cell (cell internal relaxation) fully. The optimization process was completed for an atomic force smaller than 0.05 eV Å−1. A 5 × 5 × 5 Monkhorst-Pack net was used to sample the Brillouin zone of all compounds under study. The kinetic energy cutoff was set at 520 eV. Convergence tests concerning k-points and kinetic energy cutoff show that the total energy convergence was 0). The point of δ3Gm/δx3 = 0 dictates the temperature above which complete miscibility between the compositional phases is possible, resulting in a single homogeneous phase. Figure 3 shows the phase stability diagrams of the RuO2−ZrO2 quasibinary system, calculated with the temperature-independent and the exponentially temperature-dependent Ω. As predicted, the assumption of the temperature independency of Ω leads to an unreasonably high critical point for R-Ru0.48Zr0.52O2 (12 504 K in Figure 3A) and F-Ru0.48Zr0.52O2 (5478 K in Figure 3B). In comparison, a much lower critical point is obtained for R-

G−x relations of (Ru1−x,Zrx)O2 at varied temperatures, are obtained and plotted in Figure 2. Compared with the G−x curves for the simple (mechanical) mixtures (dotted curves in Figure 2), a solid solution phase possesses a higher Gibbs free energy than the corresponding simple mixture with the same overall chemical composition at the same temperature. Therefore, at the regular experiment temperatures ( 0, the solid solution phase is thermodynamically metastable, and its decomposition follows the nucleation-andgrowth mechanism. Similar phase behaviors are obtained using the linear temperature dependency (Figure 3E,F). Thermodynamically, Figure 3C (Figure 3D) suggests that RRu1−xZrxO2 (F-Ru1−xZrxO2) is composed of the pure compositional oxides at temperatures below 800 K (1500 K). Given that the conventional preparation conditions for Ru−Zr−O typically fall within the unstable regime (the regime below the dashed curve) below 1100 K,12,14,18,49 the final products are likely mixtures of RuO2 and ZrO2 if the nucleation−growth process following the spinodal decomposition is kinetically allowed. This prediction is consistent with the experiment results by Comninellis.2 Comninellis prepared the RuO2−ZrO2 mixture by a thermal decomposition at ∼673 K; no solidsolution phase was identified. However, it should also be noted that doping and amorphization may pose a kinetic barrier against the nucleation-and-growth phase segregation and therefore may result in a relatively high solubility between the compositional oxides.50 As such, the final products are most likely composed of a Ru-rich phase (“RSS” from here after) and a Zr-rich phase (“ZSS”). Indeed, Djurado et al. found the solubility of RuO2 in Y-doped ZrO2 to be 10−12.5 mol % at a higher temperature (1173 K) [(ZrO 2 ) 0.91 (Y 2 O 3 ) 0.09 ] 1−x(RuO2)x, where 0 ≤ x ≤ 0.2.51 Such a high solubility is around the predicted quasi-stable regime in Figure 3C,D.

Figure 4. Equilibrium phase diagram of Ru−Zr−O (A) and selective enlargements (B,C). R−SS and F−SS represent rutile-phase and fluorite-phase solid solutions, respectively. The dotted lines in panel B results from the intersection of the hypothetical Tp−x curve with the left boundary of the miscibility gap of F-(Ru1−x,Zrx)O2 in Figure 3D. The dotted lines in panel C result from the intersection of the hypothetical Tp−x curve with the right boundary of the miscibility gap of R-(Ru1−x,Zrx)O2 in Figure 3C. The exponential temperature dependency of the interaction parameter as for Figure 3C,D is used.

Ru0.48Zr0.52O2 and F-Ru0.48Zr0.52O2 (3967 and 2652 K in Figure 3C,D, respectively) using the exponential temperature dependency. In the regime below the dotted curve in Figure 3C,D, where δ2Gm/δx2 < 0, spinodal decomposition of 25837

dx.doi.org/10.1021/jp308310y | J. Phys. Chem. C 2012, 116, 25832−25839

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less stable toward the right of the phase diagram. Thus, points (0. 031, Tp) on the left and (0.99, Tp) on the right in Figure 4 are the intersection points with the miscibility gaps in Figure 3C (rutile) and Figure 3D (fluorite), respectively. Enclosed in the miscibility gap and the peritectic curve is the region where mixtures of RSS and ZSS are thermodynamically stable. We note that the conventional definition of “spinodal” decomposition states that the generated phases possess the same crystal structure with the solid solution. According to Figure 4A, however, thermodynamics suggests that the solid solution decomposes into RSS and ZSS of different crystal structures. Therefore, the “spinodal” decomposition discussed above is not strictly spinodal decomposition but quasi-spinodal decomposition. 3.4. In Situ TEM Observation of Spinodal Decomposition of (Ru1−x,Zrx)O2. To provide a direct proof of the spinodal decomposition, the first task is to prepare a solid solution oxide (Ru1−x,Zrx)O2 that has a reasonably large driving force for spinodal decomposition. This is very challenging in practice because the large driving force (Figure S2 of the Supporting Information) for spinodal decomposition tends to decompose (Ru1−x,Zrx)O2 fast into RSS and ZSS phases. In fact, the conventional approach of thermal treatment and phase analysis turned out to be not applicable to this oxide system.33 To address this challenge, we prepared an amorphous solid solution of Ru0.48Zr0.52O2 by a thermal decomposition at a relatively low temperature (563 K) to freeze the random atomic mixing of the Ru and Zr. Then, we used in situ TEM to observe the (quasi-)spinodal decomposition of the oxide under electron-beam exposure. Note that this composition of Ru0.48Zr0.52O2 corresponds to that of the crossover point between the G−x curves of R- and F-(Ru1−x,Zrx)O2 (0.467, −234.1 kJ/mol atom in Figure S3 of the Supporting Information). The high-resolution TEM (HRTEM) image in Figure 5A indicates that the prepared Ru0.48Zr0.52O2 is amorphous. The single diffuse ring in the electron diffraction pattern (Figure 5B) confirms that the Ru0.48Zr0.52O2 is a singlephase amorphous oxide. As such, the driving force for the spinodal decomposition is retained in the oxide. After annealing under the electron beam for 15 min, the driving force is released and the diffuse ring splits to two new diffraction rings (Figure 6B) with a characteristic distance larger and smaller, respectively, than that of the original diffuse ring. From the calculation results in Table 2, we know that the relatively large/ small characteristic distance is attributed to an increased/ decreased molar fraction of Zr in (Ru1−x,Zrx)O2 relative to the starting composition of Ru0.48Zr0.52O2, corresponding to ZSS/ RSS, respectively. The composition change accompanying the phase separation/segregation under the in situ TEM provides a direct proof of the (quasi-)spinodal decomposition of Ru0.48Zr0.52O2. Rigorous microstructural characterizations of RSS and ZSS are undergoing.

Figure 6. HRTEM images of the electron-beam annealed Ru0.48Zr0.52O2. (A) Bright-field image and (B) selected area electron diffraction pattern.

It has been known that RuO2 decomposes at temperatures above 1685 K (in air), and Ru-containing complex metal oxides tend to decompose at high temperatures via peritectic reactions.52,53 For pure RuO2, the equilibrium decomposition temperature (Td) can be calculated from eq 28. ⎛a ⎞ g R 0 R 0 hcp ⎜ RuO2 ⎟ ( ) ln Δ0f G RuO T = G − G − G = RT O d d RuO Ru 2 2 2 ⎜p a ⎟ ⎝ O2 Ru ⎠ (28)

where aRuO2 and aRu, the activities of pure RuO2 and Ru, respectively, are equal to 1. The partial pressure of oxygen (pO2) is 0.21 atm in air. Δ0f GRRuO2 (in the unit of kJ mol−1) as a function of T can be obtained from refs 22−27 R Δ0f G RuO (Td) = −324.72 + 0.35421T − 0.00235T ln(T ) 2

(29)

4. CONCLUSIONS The lattice parameters, total energy, and bulk modulus of RRuO2, F-ZrO2, and their solid solution (Ru1−x,Zrx)O2 were calculated by ab initio DFT at 0 K. The reliability of the calculation was confirmed by comparing the calculated cell volumes of R-RuO2, F-RuO2, and F-ZrO2 with the archived data in literature. A new method was used to calculate the temperature dependence of the mixing enthalpy Hm of R- and F-(Ru1−x,Zrx)O2. The Gibbs free energy of mixing Gm and its derivatives were determined based on the thermodynamics

As a result, Td = 1685 K is calculated for R-RuO2, in agreement with the results in literature.52,53 Similarly, Td = 4112 K is calculated for F-ZrO2. Assuming Tp (∼1700 K according to refs 18, 52, and 53) as the peritectic temperature of (Ru1−x,Zrx)O2 in air, a phase diagram is plotted in Figure 4. The hypothetical Tp−x curve intersects the miscibility gaps predicted in Figure 3C,D. From the G−x relations of (Ru1−x,Zrx)O2 in Figure 2 and Figure S4 of the Supporting Information, we know that R(Ru1−x,Zrx)O2 is thermodynamically more stable than F(Ru1−x,Zrx)O2 toward the left side of the phase diagram but 25838

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calculations, the ab initio DFT results at 0 K, and the temperature-dependent Hm. Phase diagrams were further constructed and indicate a very low equilibrium solid solubility of RuO2−ZrO2 prepared under the conventional experiment conditions. In situ TEM experiment on the amorphous solid solution of Ru0.48Zr0.52O2 confirms the spinodal nature of the phase separation/segregation. Finally, it is worth pointing out that the amorphization strategy addresses the great challenge in the preparation of Ru−Zr−O with a high solid solubility. The amorphous solid solution and its enabled kinetic control of the spinodal decomposition for the first time make it possible to perform the fundamental study of the relationship between the microstructure and electrocatalytic activity of the Ru−Zr−O electrocatalyst. (Further results will be published separately.)

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ASSOCIATED CONTENT

S Supporting Information *

Comparison of the calculated structural parameters using GGA and LDA, Gm−x relationships, calculated miscibility gaps with different interaction parameters, the experimental crossover point in the G−x curve, and the lattice structures of R-RuO2 and F-ZrO2. This material 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] and [email protected]. Present Address ⊥

International Business Machines (IBM), Albany Nanotechnology Center, Albany, New York 12203, United States. Author Contributions ∥

Authors with equal contribution.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the Natural Science Foundation of China (11104031, 50971043, and 51171046), the National High Technology Research and Development Projects (863) of China (2007AA03Z325), and financial support of the International Cooperation Projects of Fujian Province (2007I004). W.L. acknowledges Dr. Yong Ding at Georgia Institute of Technology for helpful discussion of the TEM data.

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REFERENCES

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