Epitaxial Film Growth of LixCoO2 (0.6 e x e 0.9) via Topotactic Ion Exchange of Na0.8CoO2† Atsushi Mizutani,‡ Kenji Sugiura,‡ Hiromichi Ohta,‡,§ and Kunihito Koumoto*,‡ Nagoya UniVersity, Graduate School of Engineering, Furo-cho, Chikusa, Nagoya 464-8603, Japan, and CREST, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi 332-0012, Japan
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 3 755–758
ReceiVed September 22, 2006; ReVised Manuscript ReceiVed September 5, 2007
ABSTRACT: High-quality epitaxial films of Li0.93CoO2 were successfully grown on the (0001)-face of R-Al2O3 substrates via the topotactic ion exchange of Na0.8CoO2 epitaxial films. The average x-value of the LixCoO2 films was controlled from 0.60 to 0.93 by immersing in a K2S2O8 aqueous solution. The metal (xmet. e 0.72)/insulator (xins. g 0.83) phase separation was clearly observed in the range of 0.72 < x < 0.87. Seebeck coefficient (S) for the Li0.6CoO2 film was +38 µV · K-1, and it increased gradually with the x-value (x e 0.72), while no significant x-value dependence of S was observed at x > 0.72, indicating that the metal phase dominates the S-value of the metal/insulator mixture. Thermoelectric materials, which convert heat (temperature difference) to electric energy by means of the Seebeck effect (electric energy to heat conversion; Peltier effect), are applied for thermoelectric generators and Peltier coolers.1,2 Performance of thermoelectric materials is generally characterized using the dimensionless figure of merit, ZT () S2 · σ · T · κ-1, where Z, T, S, σ, and κ are figure of merit, absolute temperature, Seebeck coefficient, electrical conductivity, and thermal conductivity, respectively). Recently, alkaline- and/or alkaline-earth metal containing layered cobalt oxides, including NaxCoO2,3 SrxCoO2,4 and Ca3Co4O9,5 have attracted growing attention for the realization of thermoelectric devices because they exhibit rather large S-values, although their carrier hole concentration is very high (nh g 1021 cm-3). Positive hole carriers are generated in an AxCoO2 crystal when A-ions are subtracted from the lattice because oxidation of cobalt from Co3+ to Co4+ occurs simultaneously. Thus, clarification of the relationship between the A-ion composition parameter x and S of such layered cobalt oxides is essential for exploration of better thermoelectric layered cobalt oxides. Very recently, Lee et al.6 reported the relationship between x and S in NaxCoO2 and found that extremely large enhancement of S in NaxCoO2 (x >> 0.75) occurs at low temperatures (T ∼100 K). Although the highly Na+-incorporated NaxCoO2 is a good example to clarify the origin of rather large S-values of layered cobalt oxides, it is very difficult to handle such highly Na+incorporated NaxCoO2 because it may be unstable even at room temperature.7 In the present study, we chose LixCoO2 to clarify the relationship between x and S because the Li+ concentration x can be easily controlled by chemical treatment or electrochemical deintercalation. Further, the crystal structure (space group: R3jm, a ) 0.282 nm, c ) 1.405 nm for x ) 1)8–11 is basically similar to that of NaxCoO2.12–14 Furthermore, high-quality epitaxial films of LiCoO2 may be fabricated. High-quality epitaxial films are appropriate to clarify the intrinsic properties. Recently, we reported high-quality epitaxial film growth of Na0.8CoO2,15 which exhibits similar thermoelectric properties to those of bulk single-crystals, by reactive solid phase epitaxy (R-SPE) using a CoO epitaxial layer and NaHCO3 powder. We have also reported fabrication of high-quality epitaxial films of γ-Sr0.32Na0.21CoO27 and Ca3Co4O916 by a topotactic ion exchange of R-SPE grown Na0.8CoO2 epitaxial films. † PACS: 72.15.Jf Thermoelectric and thermomagnetic effects, 73.50.Lw Thermoelectric effects, 73.61.Le Other inorganic semiconductors * To correspondence should be addressed. E-mail: koumoto@ apchem.nagoya-u.ac.jp. ‡ Nagoya University. § CREST, Japan Science and Technology Agency.
Although we found several reports on epitaxial film growth of LiCoO2 by conventional pulsed laser deposition (PLD) techniques,17,18 we fabricated LiCoO2 epitaxial films by a topotactic ion exchange of R-SPE grown Na0.8CoO2 film at 260 °C in a eutectic salt of a LiNO3/LiCl mixture because this ion exchange method is really appropriate for high-quality epitaxial film growth of layered complex oxides with a high vapor pressure element such as Li. Then, high-quality epitaxial films of Li0.93CoO2 were successfully grown on the (0001)-face of R-Al2O3 substrates via the topotactic ion exchange of Na0.8CoO2 epitaxial films. Figure 1 schematically illustrates the fabrication procedure for LixCoO2 epitaxial films. First, a highly (111)-oriented CoO epitaxial film (thickness: 30-50 nm) was deposited on the (0001)-face of an R-Al2O3 substrate (10 mm × 10 mm × 0.5 mmt) at 600 °C by PLD using a Co3O4 sintered disk as a target (step 1). Then the surface of the CoO film was fully capped by a yttria-stabilized zirconia (YSZ) single-crystalline plate (10 mm × 10 mm × 0.5 mmt). Next, the sandwiched specimen was heated with NaHCO3 powder at 750 °C for 2 h in air to fabricate a Na0.8CoO2 epitaxial film15 (step 2). Details of the R-SPE technique are reported elsewhere.7,15,16,19–21 In the same manner as step 2, the surface of the Na0.8CoO2 film was fully capped by a YSZ plate and was subsequently treated in a eutectic LiNO3/LiCl22 mixture at 260 °C for 4 h in Ar. Finally, the specimen was cooled to room temperature and washed with distilled water to remove the LiNO3/LiCl melting mixture. High-resolution X-ray diffraction (HR-XRD, ATX-G, Cu KR1, Rigaku Co.) measurements were used to analyze the crystalline quality and orientation of the film. Figure 2a shows the out-ofplane XRD pattern of a LixCoO2 film (after step 3). Intense Bragg diffraction peaks of 000l LixCoO2 are seen along with 000l R-Al2O3. A signal due to Na+ was not detected by energy disperse X-ray spectroscopy (EDS) analysis of the film, indicating that the Na0.8CoO2 epitaxial film was fully converted into a LixCoO2 epitaxial film. Only the intense diffraction peaks of 11–20 LixCoO2 and 3–300 R-Al2O3 are observed in the in-plane X-ray Bragg diffraction pattern (Figure 2b). The 6-fold symmetry of {11–20} LixCoO2 is clearly seen in the in-plane rocking curve (Figure 2b inset), indicating that LixCoO2 was heteroepitaxially grown on the R-Al2O3 substrate with an epitaxial relationship of (0001) [11–20] LixCoO2 || (0001) [1–100] R-Al2O3. The calculated lattice parameters, a and c, of the LixCoO2 film are 0.281 and 1.408 nm, respectively, suggesting that the chemical formula of the film is Li0.93CoO2.11,23 Figure 2c shows a topographic atomic force microscope (AFM) image of the Li0.93CoO2 film. Several hexagonally shaped domains with steplike structures are seen, indicating that the Li0.93CoO2 film has a high crystal quality.
10.1021/cg060637f CCC: $40.75 2008 American Chemical Society Published on Web 02/08/2008
756 Crystal Growth & Design, Vol. 8, No. 3, 2008
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Figure 1. (Top) Schematic fabrication procedure of LixCoO2 (0.60 e x e 0.93) epitaxial films. Step 1: Pulsed laser deposition of a highly (111)oriented CoO epitaxial film on the (0001)-face of an R-Al2O3 substrate. Step 2: R-SPE growth of a Na0.8CoO2 epitaxial film. Step 3: Li+ ion exchange treatment with a LiNO3/LiCl mixture. Step 4: Li-deintercalation treatment in a K2S2O8 aqueous solution. (Bottom) Schematic crystal structures of CoO, NaxCoO2, and LixCoO2.
Figure 2. (a) Out-of-plane X-ray Bragg diffraction pattern of the Li0.93CoO2 epitaxial film grown on the (0001)-face of an R-Al2O3 substrate by a Li+ ion exchange treatment of R-SPE grown Na0.8CoO2 epitaxial film. (b) In-plane X-ray Bragg diffraction pattern of the Li0.93CoO2 epitaxial film. Inset shows the in-plane rocking curve φ-scan of the {11–20} Li0.93CoO2. (c) Topographic AFM image of the Li0.93CoO2 epitaxial film. Crosssectional profile from a to b is also shown. Several hexagonal domains are seen.
Then, the Li0.93CoO2 epitaxial film was immersed in a K2S2O824 aqueous solution (0.03 mol · L-1) at room temperature to gently subtract the Li-ion content (x-value) from the film. Peak shift of 0003 LixCoO2 to the smaller qz side was observed in the out-ofplane XRD patterns (Figure 3a) with the immersed time. It should be noted that peak broadening of 0003 LixCoO2 is observed when the film was immersed for 6-32 min (gray lines), indicating that two-phase separation occurs. These broad peaks were successfully separated into two peaks (blue and red dotted lines), which were fitted by using Pearson VII function. Since similar phase separation was also observed from 0.75 < x < 0.95 in electrochemically Li+subtracted LixCoO2 powder11 due to a first-order metal–insulator transition,25 observed phase separation in the films is considered to be an intrinsic behavior of LixCoO2. After the immersion for 32 min, the 0003 peak became a singlet.
Figure 3b shows change in the lattice parameter, c, of the LixCoO2 epitaxial films as a function of the immersed time in the K2S2O8 aqueous solution. The c-value of the insulating phase (red) increases gradually with the immersed time. On the other hand, the c-value of the metallic phase (blue) is constant (1.423 nm) in the metal–insulator mixture region, although it increases linearly after the immersion for 32 min. The ratio of the 0003 Bragg peak intensity for the metallic phase (Imet./Itot.) gradually increased with the immersed time. To estimate the x-value of the LixCoO2 films, we used a linear relationship between the c-value and x, which was reported by Menetrier et al. 11 (Figure 3b right axis). Average c-value of the mixture phase was calculated by the following equation. c(ave) ) c(metal)(Imet/Itot) + c(insulator)(Iins/Itot)
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Figure 3. Determination of Li-content (x) in LixCoO2 epitaxial films. The Li0.93CoO2 film was immersed in a K2S2O8 aqueous solution at room temperature to reduce the Li-ion content (x-value) in the film. (a) Out-of-plane XRD patterns around 0003 LixCoO2. Peaks separation was performed by using the Pearson VII function. (b) Change in lattice constant, c of the LixCoO2 epitaxial films [metal phase and insulator phase] as a function of the immersed time.
Figure 4. Optical absorption spectra of the LixCoO2 epitaxial films (x ) 0.93, 0.82, 0.76, 0.70, and 0.60). (Inset) Integrated absorption intensity of the LixCoO2 epitaxial films (0.3-0.8 eV). Photographs of the LixCoO2 (x ) 0.93 and 0.60) epitaxial films are also shown.
Figure 5. Room temperature Seebeck coefficient (S300K) and average carrier concentration (nave.) of the LixCoO2 films as a function of the Li content (x). The lines are a guide for the eye.
From these results, we judged that the x-value in Li xCoO2 was successfully controlled from 0.93 to 0.60. We then performed optical absorption measurements of the LixCoO2 films (x ) 0.93, 0.82, 0.76, 0.70, and 0.60) as shown in Figure 4 (solid lines). The optical absorption spectrum of a
Crystal Growth & Design, Vol. 8, No. 3, 2008 757 Na0.8CoO2 epitaxial film fabricated by R-SPE is also shown for comparison (dotted line). Before being immersed in the K2S2O8 aqueous solution, the film is dark brown as shown in the inset (x ) 0.93). However, after immersing in the K2S2O8 aqueous solution, the film became black due to increases in two absorption bands, A (∼0.5 eV) and B (∼1.5 eV). The shapes of absorption spectra of Li0.60CoO2 and Li0.70CoO2 epitaxial films are very similar to that of a Na0.8CoO2 epitaxial film, suggesting that the electronic structure of LixCoO2 and NaxCoO2 are similar. The linear relationship between the integrated absorption intensity (0.3-0.8 eV) and the x-value (Figure 4 inset) is clear evidence that deintercalation of the Li ions and oxidation of Co ions from Co3+ to Co4+ in the Li0.93CoO2 occur simultaneously when the film is immersed in the K2S2O8 aqueous solution. Room temperature Seebeck coefficient (S300K) of the Li xCoO2 epitaxial film, which was measured in the in-plane direction, increased gradually with the x-value (x e 0.7) and saturated when the x-value exceeds 0.7 (S300K ∼ 85 µV · K-1), while the average carrier concentration (nh 300K) monotonically decreased (Figure 4). It should be noted that the LixCoO2 films (x > 0.7) were a metal/ insulator mixture as described above. Since the observed S-value is given by Σ Si · σxxi/Σ σxxi, where Si and σxxi are the Seebeck coefficient and sheet conductivity of the i layer, respectively, the metallic phase of the LixCoO2 film dominates the observed S-value. In summary, high-quality epitaxial films of Li0.93CoO2 were successfully grown on the (0001)-face of R-Al2O3 substrates via the topotactic ion exchange of Na0.8CoO2 epitaxial films. The average x-value of the LixCoO2 films was controlled from 0.60 to 0.93 by immersing in a K2S2O8 aqueous solution. The Seebeck coefficient (S) for the Li0.60CoO2 film was +38 µV · K-1, and it increased gradually with the x-value (x e 0.72), while any significant x-value dependence of S were observed at x > 0.72. Since the metal (xmet. e 0.72)/insulator (xins. g 0.83) phase separation was clearly observed in the range of 0.72 < x < 0.87, we judged that only the metal phase contributes to S generation. The observed x-dependence of S and phase separation behavior of LixCoO2 were very similar to those of NaxCoO2. Therefore, we concluded that the LixCoO2 epitaxial film may be suitable for modeling thermoelectric properties of layered alkaline cobalt oxides.
References (1) Tritt, T. M.; Subramanian, M. A.; Bottner, H.; Caillat, T.; Chen, G.; Funahashi, R.; Ji, X.; Kanatzidis, M.; Koumoto, K.; Nolas, G. S.; Poon, J.; Rao, A. M.; Terasaki, I.; Venkatasubramanian, R.; Yang, J. MRS Bull. 2006, 31, 188 (Special Issue on Harvesting Energy through Thermoelectrics: Power Generation and Cooling). (2) DiSalvo, F. J. Science 1999, 285, 703. (3) Terasaki, I.; Sasago, Y.; Uchinokura, K. Phys. ReV. B 1997, 56, 12685. (4) Ishikawa, R.; Ono, Y.; Miyazaki, Y.; Kajitani, T. Jpn. J. Appl. Phys. Part 2 2002, 41, L337. (5) Masset, A. C.; Michel, C.; Maignan, A.; Hervieu, M.; Toulemonde, O.; Studer, F.; Raveau, B. Phys. ReV. B 2000, 62, 166. (6) Lee, M.; Viciu, L.; Li, Y.; Wang, M. L.; Foo, S.; Watauchi, R. A. P., Jr.; Cava, R. J.; Ong, N. P. Nat. Mater. 2006, 5, 537. (7) Sugiura, K.; Ohta, H.; Nomura, K.; Hirano, M.; Hosono, H.; Koumoto, K. Appl. Phys. Lett. 2006, 88, 082109. (8) Johnston, W. D.; Heikes, R. R.; Sestrich, D. J. Phys. Chem. Solids 1958, 7, 1. (9) Mendiboure, A.; Delmas, C.; Hagenmuller, P. Mater. Res. Bull. 1984, 19, 1383. (10) Molenda, J.; Stoklosa, A.; Bak, T. Solid State Ionics 1989, 36, 53. (11) Menetrier, M.; Saadoune, I.; Levasseur, S.; Delmas, C. J. Mat. Chem. 1999, 9, 1135. (12) Fouassier, C.; Matejka, G.; Reau, J. M.; Hagenmuller, P. J.; Solid State Chem. 1973, 6, 532. (13) Jansen, V. M.; Hoppe, R. Z. Anorg. Allg. Chem. 1974, 408, 104. (14) Delmas, C.; Branconnier, J. J.; Fouassier, C.; Hagenmuller, P. Solid State Ionics 1981, 3, 165. (15) Ohta, H.; Kim, S-W.; Ohta, S.; Koumoto, K.; Hirano, M.; Hosono, H. Cryst. Growth Des. 2005, 5, 25. (16) Sugiura, K.; Ohta, H.; Nomura, K.; Hirano, M.; Hosono, H.; Koumoto, K. Appl. Phys. Lett. 2006, 89, 032111.
758 Crystal Growth & Design, Vol. 8, No. 3, 2008 (17) Perkins, J. D.; Bahn, C. S.; McGraw, J. M.; Parilla, P. A.; Ginley, D. S. J. Electrochem. Soc. 2001, 148, A1302. (18) Striebel, K. A.; Deng, C. Z.; Wen, S. J.; Cairns, E. J. J. Electrochem. Soc. 1996, 143, 1821. (19) Ohta, H.; Nomura, K.; Orita, M.; Hirano, M.; Ueda, K.; Suzuki, T.; Ikuhara, Y.; Hosono, H. AdV. Funct. Mater. 2003, 13, 139. (20) Ohta, H.; Mizutani, A.; Sugiura, K.; Hirano, M.; Hosono, H.; Koumoto, K. AdV. Mater. 2006, 18, 1649. (21) Sugiura, K.; Ohta, H.; Nomura, K.; Yanagi, H.; Hirano, M.; Hosono, H.; Koumoto, K. Inorg. Chem. 2006, 45, 1894. (22) Tan, K. S.; Reddy, M. V.; Subba Rao, G. V.; Chowdari, B. V. R. J. Power Sources 2005, 147, 241. (23) Generally, the relationship between chemical composition and lattice parameters for epitaxial film is different from that for bulk materials, if epitaxial films were “directly” grown on the substrate, due to the existence of lattice mismatch. On the other hand, lattice
Communications parameters of polycrystalline films on amorphous substrate generally can be regarded as the same as those of bulks. In the present case, amorphous Na-Al-O phase can be formed at the Na0.8CoO2/ R-Al2O3 interface unintentionally during the film growth of Na0.8CoO2. The thickness of a Na-Al-O layer was typically 10–20 nm, which depends on the treatment time. Since the amorphous layer was also observed after the ion exchange treatment from Na+ to Li+, we concluded that the LixCoO2 film was not directly grown on the R-Al2O3 substrate but on the amorphous layer. Therefore, we judged the lattice parameters of LixCoO2 epitaxial films are the same as those of LixCoO2 bulks. (24) Graetz, J.; Hightower, A.; Ahn, C. C.; Yazami, R.; Rez, P.; Fultz, B. J. Phys. Chem. B 2002, 106, 1286. (25) Marianetti, C. A.; Kotliar, G.; Ceder, G. Nat. Mater. 2004, 3, 627.
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