Ca₃Co₄O₉/polycrystalline Al₂O₃: an effective template for c-axis oriented layered cobaltate thin films by chemical solution deposition

Abstract

In this paper, Bi₂Sr₂Co₂O_y and Bi₂Ba₂Co₂O_y thin films were prepared on Ca₃Co₄O₉/polycrystalline Al₂O₃ thin films by chemical solution deposition. The results show that both of the bismuth-based cobaltate thin films are self-assembled c-axis oriented, which confirms that Ca₃Co₄O₉/polycrystalline Al₂O₃ thin film can be used as an effective template to induce c-axis oriented grain growth in layered cobaltate thin films. The annealing temperature effects on Bi₂Sr₂Co₂O_y thin films were investigated, and the resistivity and Seebeck coefficient decrease with the increasing annealing temperature. The results will provide an effective template to realize self-assembled c-axis oriented layered cobaltate thin films on polycrystalline substrates by chemical solution deposition.

---

1 Introduction

As a type of high-temperature thermoelectric (TE) materials, layered cobaltates such as Bi₂Sr₂Co₂O_y (BSC222) and Ca₃Co₄O₉ (CC349) have been widely investigated in the past years since the discovery of large room-temperature Seebeck coefficient (S) and low resistivity (ρ) in NaCo₂O₄ single crystals.^1,2 The figure of merit ZT (ZT = S²T/ρκ, where κ is the thermal conductivity), which is commonly used to evaluate the performance for TE materials, can be achieved over 1 in the BSC222 and CC349 whiskers,^3,4 suggesting their potential applications in TE energy conversion fields. Moreover, these oxide materials overcome the problem of oxidation for traditional intermetallic TE alloys, especially in air under high temperatures. From a technical perspective, however, it is difficult to fabricate large-sized single crystals to satisfy the applications. Moreover, from the viewpoint of downsized and highly integrated TE modules for power generation, TE modules usage of thin films rather than bulk modules (single crystals or polycrystalline ceramics) are desirable.⁵ Therefore, it is needed to fabricate high-performance layered cobaltate-based TE thin films to realize applications. It is known that the BSC222 and CC349 have a similar layered crystal structure consisting of CdI₂-type CoO₂ layers serving as the electronic transport to achieve large S and low ρ, whereas the Bi₂Sr₂O₄/Ca₂CoO₃ blocking layers serve as phonon scattering to achieve low κ.^6–8 Such an anisotropic structure characteristic makes the BSC222 and CC349 exhibiting anisotropic properties including electrical and thermal transport properties. The previous reports have been verified that the TE properties are much better in ab-planes than that of the c-planes for layered cobaltates,^6,8 which suggest that it is desirable to obtain c-axis oriented BSC222 and CC349 thin films.

It is reported that c-axis oriented BSC222 and CC349 thin films can be obtained by rf-magnetron sputtering, pulsed laser deposition (PLD) and chemical solution deposition (CSD) methods, which are commonly prepared on single crystal substrates such as LaAlO₃, SrTiO₃, MgO and Al₂O₃.^9–14 Among all these methods, CSD, as an easy set-up and efficient route to prepare large-area thin films, has been used in fabrication of BSC222 and CC349 thin films.^10,12 On the other hand, considering the cost factor, it is desirable to prepare BSC222 and CC349 thin films on inexpensive polycrystalline substrates by CSD method. Actually, CC349 thin films on polycrystalline substrates have been reported in a few reports both by rf-magnetron sputtering and CSD methods, showing c-axis oriented characteristics.^15,16 Nevertheless, there have no reports about BSC222 thin films grown on polycrystalline substrates, although BSC222 has a better TE performance than that of CC349.¹⁷

In this work, we found that bismuth-based layered cobaltate thin films cannot be directly deposited on polycrystalline Al₂O₃ substrates by CSD method. Fortunately, however, an effective polycrystalline template CC349/polycrystalline Al₂O₃ has been successfully used to fabricate c-axis oriented bismuth-based layered cobaltate thin films. Then, the annealing temperature effects on the properties for the BSC222 thin films deposited on CC349/polycrystalline Al₂O₃ templates have been investigated. The results show that CC349/polycrystalline Al₂O₃ can be used as an effective template for the fabrication of c-axis oriented layered bismuth-based cobaltate thin films, and the derived thin films also exhibit good performance.

2 Experimental details

Ca₃Co₄O₉/polycrystalline Al₂O₃ (CP) templates were prepared using the CSD method similar to our previous report.¹⁶ In detail, calcium acetate and cobaltous acetate were used as reagents, dissolving in propionic acid (solution concentration is 0.1 M). Then, the precursor solution was deposited on polycrystalline Al₂O₃ (PAO) substrate by spin-coating with a rotation speed of 4000 rpm for 60 s, baked at 400 °C for 10 min in air, then annealed at 850 °C for an hour under oxygen by rapid thermal annealing (RTA), and repeated for six times. Bi₂Ba₂Co₂O_y (BBC222) and BSC222 thin films were prepared using bismuth, strontium, barium and cobaltous acetate as raw reagents, dissolving in propionic acid, and the solution concentrations were controlled to 0.1 M. Two precursor solutions were deposited on the CP templates with a rotation speed of 4000 rpm at a time of 60 s, baked at 400 °C for 10 min and repeated for six times. Finally, the as-baked thin films were annealed under oxygen atmosphere at 650 °C for the BBC222 and 600, 700, 800 °C for the BSC222 (abbreviated as 600BCP, 700BCP and 800BCP, respectively) by RTA for two hours.

Detailed X-ray diffraction (XRD, Philips X'pert Pro) with Cu K_α radiation was used to check up the crystal phase for all derived thin films. Field emission scanning electron microscopy (FE-SEM, FEI-designed Sirion 200) was performed to determine the surface morphologies and thickness. Temperature dependence of resistivity and Seebeck coefficient were measured using the standard four-point probe technique on a physical properties measurement system (PPMS, Quantum-designed).

3 Results and discussion

Fig. 1 presents the XRD patterns of CP template, BBC222 and BSC222 thin films, and the inset of Fig. 1 gives the sketch map of bismuth-based layered cobaltate thin films on CP templates. It can be seen that the CC349 on PAO substrate shows pure c-axis orientation, which is consistent with our previous report.¹⁶ Such a self-assembled c-axis oriented characteristic can be explained by the localized epitaxial c-axis growth on the (00l) planes of PAO substrate.¹⁵ The calculated lattice constant for the CC349 thin film by Bragg formula (2d sin θ = nλ, where d, θ, and λ is the lattice constant, diffraction angle and wavelength, respectively) is 10.72 Å, which is similar to previous reports.^16,18 It is interesting to note here that both BBC222 and BSC222 thin films can be successfully deposited on CC349 thin films, suggesting that CC349 thin film on PAO substrate can be used as an effective growth template to fabricate other layered cobaltate thin films. Moreover, there have no any detectable impurity phase when other cobaltates are deposited on the CP templates. According to the XRD patterns, both BBC222 and BSC222 thin films show perfect c-axis oriented characteristics, even though the templates are polycrystalline. The self-assembled c-axis orientation makes the bismuth-based layered cobaltates showing better TE performances than those of the randomly oriented polycrystalline bulks due to the anisotropy of these materials. The calculated c-axis lattice constant for the BBC222 thin film is about 15.38 Å, similar to that found in BBC222 bulks as reported earlier.¹⁹ Such value is larger than that of 14.76–14.88 Å for the BSC222 thin films prepared on CP templates. It is known that the 6-fold coordination Ba ionic radius (1.35 Å) is larger than that of the Sr ionic radius (1.18 Å), resulting in the lattice expansion in the blocking layers for the layered bismuth-based cobaltates and thus the enhancement of c-axis lattice constant from BSC222 to BBC222. Besides, the obvious peak shift to higher angle for the BSC222 thin film as compared with the BBC222 thin film is shown in Fig. 1, confirming the discrepancy of lattice constant for the above two thin films.

Fig. 1 XRD patterns for PAO substrate, CP template, BBC222 and BSC222 thin films. The inset shows the sketch map for the layered bismuth-based cobaltate thin films on CP templates.

In order to clarify the effectiveness of CP template on layered cobaltate thin films, BSC222 thin films were annealed under different temperatures on CP templates, and the XRD patterns are shown in Fig. 2. As can be seen, all the derived BSC222 thin films show self-assembled c-axis orientation and the crystallization quality is improved with increasing annealing temperature. In addition, based on the positions of all diffraction peaks for BSC222 in Fig. 2, the c-axis lattice constant is calculated by Bragg formula, and the variation tendency is shown in the left inset of Fig. 2. The lattice constant values (14.76–14.88 Å) are smaller than that of the BSC222 single crystals (∼14.96 Å).²⁰ Nonetheless, these values agree well with our previous report using the same solution deposition method.²¹ Also, it is found that c-axis lattice constant for BSC222 thin film monotonously decreases with increasing annealing temperature. This tendency is also confirmed by the peak shift as shown in the right inset of Fig. 2, which presents the magnified XRD patterns of (001̅0̅) peaks for the BSC222 thin films. It is known that the BSC222 can be written as Bi₂Sr₂Co₂O_9−δ due to the existence of oxygen deficiency, which will result in the coexistence of ions Co³⁺ and Co⁴⁺ in BSC222 in order to balance the charge valence. The decreased oxygen content will lead to the increase of Co³⁺ concentration while the Co⁴⁺ concentration will be decreased. Since the ionic radius is larger for the Co³⁺ (0.69 Å) than that of the Co⁴⁺ (0.67 Å), the decreased oxygen content will lead to the increased lattice constant. In our experiment, oxygen content in the derived BSC222 thin films increases with increasing annealing temperature under ambient oxygen atmosphere, which is similar to previously report.²² Thus, the 800BCP has the smallest lattice constant, and then the 700BCP, and the lattice constant is the largest for the 600BCP. Such a decreased lattice constant induced by the increasing of oxygen content and annealing temperature is accordant with that of the results for layered cobaltate single crystals and polycrystalline bulks as reported earlier.^23,24 Due to the existence of defects such as vacancies and lattice disorders in the derived BSC222 thin films, we have calculated the microstrains by Williamson–Hall plot according to the XRD patterns, β = (kλ/D) + 4ε sin θ, where β, ε and D are the peak width, stress and grain size, respectively.²⁵ It is obtained that the microstrain is decreased from −0.70% for the 600 °C-annealed thin film to −0.12% for the 800 °C-annealed thin film, which suggests that the microstrain is decreased with increasing annealing temperature due to the enhanced crystalline quality. Based on experimental results, it is reasonable to deduce that the defects will play a subtle role in determination the lattice constant. However, it will affect the transport properties.

Fig. 2 XRD patterns for PAO substrate, CP template and different temperatures annealed BSC222 thin films on CP templates. 600BCP, 700BCP and 800BCP denote the BSC222 thin films annealed under 600, 700 and 800 °C, respectively. The left inset gives the variation of lattice constant for different BSC222 thin films, and the right inset presents the magnified XRD results between 27° to 33° to give the shift of (001̅0̅) diffraction peaks for BSC222 thin films.

Fig. 3 presents the surface and cross-sectional FE-SEM images of all derived thin films. As shown in Fig. 3(a), CC349 thin film with plate-like grains can be observed. Such a polycrystalline nanoscaled CC349 thin film with a thickness of 150 nm on PAO substrate can be used as an effective growth template for the fabrication of other layered cobaltate thin films, which is verified by the former XRD results. Moreover, BSC222 thin films with relatively dense surface morphology can be well grown on the CP templates as presented in Fig. 3(b–d), and the grain size increases with increasing annealing temperature. The variation of grain size will have much effect on the thin film electrical and thermal properties, which will be discussed in the following. Besides, as observed in the insets of Fig. 3(b–d), the thin film thickness decreases with increasing annealing temperature, which can be explained by the enhanced density for the BSC222 thin films annealed under higher temperatures.

Fig. 3 Surface FE-SEM images for CP template (a), 600BCP (b), 700BCP (c) and 800BCP (d). The insets of (a–d) give the thickness of all derived thin films.

Temperature dependence of resistivity for all derived BSC222 thin films on CP templates is shown in Fig. 4. As reported earlier, for the BSC222 single crystals and the ceramic bulks as well as the thin films prepared onto single crystal substrates, resistivity versus temperature behavior shows a metal to semiconductor transition at the low temperature regime (50–150 K) due to the formation of short-range incommensurate spin density wave (IC-SDW).^20,21,26 In this study, however, the solution derived BSC222 thin films deposited on polycrystalline templates exhibit a semiconductor-like behavior within the whole measured temperature range, which can be elucidated by the grain boundary scattering induced by the formation of a large amount of defects in the thin films. Moreover, it is clearly seen that the resistivity decreases with increasing annealing temperature, which is attributed to the enhanced crystallization quality. It is known that the grain size and grain boundaries in granular thin films will influence the electrical transport properties due to the carriers scattering at the grain boundaries, namely the grain boundary barrier effect. According to the theory of Slater,²⁷ the grain size related barrier height Φ can be described by the formula Φ ∝ (X − fL)², where X is the barrier width, L is the dimension of the grain, and f is a fraction. In addition, taking the grain boundary effect into account, Seto derived the relation of carrier concentration n and Φ, n = eQ²/8εΦ, where e is the electronic charge, Q is the density of surface and ε is the dielectric permittivity.²⁸ Therefore, the BSC222 thin films with a smaller grain size L will lead to a higher barrier height Φ and a smaller carrier concentration. Consequently, the 600BCP has the highest resistivity, and then the 700BCP, and the resistivity is the lowest for the 800BCP.

Fig. 4 Temperature dependence of resistivity for derived BSC222 thin films annealed under different temperatures on the CP templates.

Fig. 5 shows the temperature dependence of Seebeck coefficient (S) for all derived BSC222 thin films on the CP templates. The positive S value suggests that the major charge carriers are holes (p-type). As can be seen, the value of S increases with decreasing annealing temperature, which can be well explained by the theoretical model of energy filtering of charge carriers.²⁹ Thin film with a smaller grain size has a higher grain boundary barrier which will eliminate more carriers with low energy, and therefore has a larger S. In this work, BSC222 thin films grown on the CP templates under different annealing temperatures show different grain sizes as confirmed by the FE-SEM results. The 600BCP sample has the smallest grain size, and then the 700BCP, and the grain size is largest for the 800BCP thin film. Therefore, the 600BCP has the largest S amongst all the BSC222 thin films on the CP templates.

Fig. 5 Temperature dependence of Seebeck coefficient for all derived BSC222 thin films annealed under different temperatures on the CP templates.

4 Conclusions

In summary, two types of bismuth-based layered cobaltate thin films Bi₂Ba₂Co₂O_y and Bi₂Sr₂Co₂O_y were prepared on Ca₃Co₄O₉/polycrystalline Al₂O₃ thin films by chemical solution deposition. The results show that the derived thin films are self-assembled c-axis oriented although the substrates are polycrystalline. The annealing temperature effects on the properties for the self-assembled c-axis oriented Bi₂Sr₂Co₂O_y thin films are investigated, showing decreases in resistivity and Seebeck coefficient with increasing annealing temperature. The successful growth of c-axis oriented layered cobaltate thin films on Ca₃Co₄O₉/polycrystalline Al₂O₃ thin films suggests that the Ca₃Co₄O₉/polycrystalline Al₂O₃ thin films can be used as effective growth templates for realization of c-axis oriented layered cobaltate thin films by chemical solution deposition.

Acknowledgements

This work was supported by the National Basic Research Program of China (2014CB931704) and by the National Nature Science Foundation of China under Contract nos 51171177 and 11174288.

Notes and references

1. I. Terasaki, Y. Sasago and K. Uchinokura, Phys. Rev. B: Condens. Matter Mater. Phys., 1997, 56, R12685.
2. J. He, Y. F. Liu and R. Funahashi, J. Mater. Res., 2011, 26, 1762–1772.
3. R. Funahashi and M. Shikano, Appl. Phys. Lett., 2002, 81, 1459.
4. R. Funahashi, I. Matsubara, H. Ikuta, T. Takeuchi, U. Mizutani and S. Sodeoka, Jpn. J. Appl. Phys., 2000, 39, L1127.
5. S. Horii, M. Sakurai, T. Okamoto, J. Shimoyama, K. Kishio, T. Uchikoshi, T. Suzuki, Y. Sakka, R. Funahashi and T. Mihara, 25th International Conference on Thermoelectrics, Vienna, 2006.
6. A. Masset, C. Michel, A. Maignan, M. Hervieu, O. Toulemonde, F. Studer, B. Raveau and J. Hejtmanek, Phys. Rev. B: Condens. Matter Mater. Phys., 2000, 62, 166.
7. A. Maignan, W. Kobayashi, S. Hébert, G. Martinet, D. Pelloquin, N. Bellido and C. Simon, Inorg. Chem., 2008, 47, 8553–8561.
8. R. Funahashi, I. Matsubara and S. Sodeoka, Appl. Phys. Lett., 2000, 76, 2385.
9. X. B. Zhu, Y. P. Sun, H. C. Lei, X. H. Li, R. Ang, B. C. Zhao, W. H. Song, D. Q. Shi and S. X. Dou, J. Appl. Phys., 2007, 102, 103519.
10. X. B. Zhu, D. Q. Shi, S. X. Dou, Y. P. Sun, Q. Li, L. Wang, W. X. Li, W. K. Yeoh, R. K. Zheng, Z. X. Chen and C. X. Kong, Acta Mater., 2010, 58, 4281–4291.
11. S. F. Wang, A. Venimadhav, S. M. Guo, K. Chen, Q. Li, A. Soukiassian, D. G. Schlom, M. B. Katz, X. Q. Pan, W. Wong-Ng, M. D. Vaudin and X. X. Xi, Appl. Phys. Lett., 2009, 94, 022110.
12. S. F. Wang, Z. C. Zhang, L. P. He, M. J. Chen, W. Yu and G. S. Fu, Appl. Phys. Lett., 2009, 94, 162108.
13. Y. Q. Zhou, I. Matsubara, W. S. Shin, N. Izu and N. Murayama, J. Appl. Phys., 2004, 95, 625.
14. R. Moubah, S. Colis, C. Ulhaq-Bouillet and A. Dinia, Appl. Phys. Lett., 2010, 96, 041902.
15. M.-G. Kang, K.-H. Cho, S.-M. Oh, J.-S. Kim, C.-Y. Kang, S. Nahm and S.-J. Yoon, Appl. Phys. Lett., 2011, 98, 142102.
16. Y. K. Fu, X. W. Tang, J. Yang, H. B. Jian, X. B. Zhu and Y. P. Sun, J. Mater. Sci. Technol., 2013, 29, 13–16.
17. K. Koumoto and T. Mori, in Thermoelectric Nanomaterials, Springer, 2013, ch. 3, p. 54.
18. R. Wei, H. Jian, X. Tang, J. Yang, L. Hu, L. Chen, J. Dai, X. Zhu and Y. Sun, J. Am. Ceram. Soc., 2013, 96, 2396–2401.
19. M. Hervieu, A. Maignan, C. Michel, V. Hardy, N. Créon and B. Raveau, Phys. Rev. B: Condens. Matter Mater. Phys., 2003, 67, 045112.
20. T. Yamamoto, K. Uchinokura and I. Tsukada, Phys. Rev. B: Condens. Matter Mater. Phys., 2002, 65, 184434.
21. R. Wei, X. Tang, Z. Hui, J. Yang, L. Hu, L. Chen, J. Dai, X. Zhu and Y. Sun, J. Am. Ceram. Soc., 2014, 97, 1841–1845.
22. N. Bouhssira, S. Abed, E. Tomasella, J. Cellier, A. Mosbah, M. S. Aida and M. Jacquet, Appl. Surf. Sci., 2006, 252, 5594–5597.
23. M. Karppinen, H. Fjellvåg, T. Konno, Y. Morita, T. Motohashi and H. Yamauchi, Chem. Mater., 2004, 16, 2790–2793.
24. X. G. Luo, X. H. Chen, G. Y. Wang, C. H. Wang, Y. M. Xiong, H. B. Song and X. X. Lu, Europhys. Lett., 2006, 74, 526.
25. A. K. Zak, W. H. A. Majid, M. E. Abrishami and R. Yousefi, Solid State Sci., 2011, 13, 251–256.
26. T. Yamamoto, I. Tsukada, K. Uchinokura, M. Takagi, T. Tsubone, M. Ichihara and K. Kobayashi, Jpn. J. Appl. Phys., 2000, 39, L747.
27. J. Slater, Phys. Rev., 1956, 103, 1631.
28. J. Y. W. Seto, J. Appl. Phys., 1975, 46, 5247.
29. B. Y. Moizhes and V. A. Nemchinsky, 11th International Conference on Thermoelectrics, Texas, 1992.

---