Layered cobalt oxide epitaxial films exhibiting thermoelectric ZT = 0.11 at room temperature

Abstract

Among many thermoelectric materials, oxide-based materials draw significant interest due to their environmental compatibility. In particular, layered cobaltite, Na_0.75CoO₂, shows a large thermoelectric power factor parallel to the layers. However, the thermal conductivity (κ) is rather high (5–7 W m⁻¹ K⁻¹), and therefore, its thermoelectric figure of merit ZT is small (∼0.03) at room temperature. Here we show that substituting the Na⁺ ion with the Ba²⁺ ion in Na_xCoO₂ drastically reduces the κ while keeping the large power factor, resulting in a large enhancement in ZT. We fabricated epitaxial films of Na_0.75CoO₂ by the reactive solid-phase epitaxy method and performed ion-exchange treatment from Na⁺ to Ba²⁺ to synthesize Ba_0.27CoO₂ films. The room temperature electrical conductivity (σ), thermopower (S), and power factor of the c-axis oriented Ba_0.27CoO₂ films along the in-plane direction were 2310 S cm⁻¹, +72 μV K⁻¹, and 1.2 mW m⁻¹ K⁻², respectively, while the κ along the in-plane, which was clarified by measuring the κ of the c-axis inclined (55°) film, was 3.3 W m⁻¹ K⁻¹. This yields a ZT (= S²σT/κ) value along the in-plane as high as 0.11 at room temperature, which is the highest among those reported for oxide thermoelectric materials except oxychalcogenide (BiO)(CuSe).

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Introduction

Thermoelectric devices convert a temperature difference into electric current and vice versa, from the Seebeck effect/Peltier effect.^1,2 Since thermoelectric devices can convert wasted heat exhausted in our daily life into electricity, they have attracted increasing attention as a potential solution for the realization of affordable and clean energy in Sustainable Development Goals (SDGs).^3,4 Previously, wasted heat from large-scale high-temperature environments, like factories, thermal power plants, and automobiles, has been targeted for thermoelectric conversion. Therefore, thermoelectric materials exhibiting a large figure of merit (ZT) at high-temperature (800–1000 K) have been developed proactively.^5–8 Nevertheless, thermoelectric conversion of small-scale exhausted heat from computers, stoves, sunlight, and our body is also important for the realization of SDGs. Thus, thermoelectric materials exhibiting a large ZT around room temperature are also required.

Further, thermoelectric materials must avoid the use of rare and toxic elements since economic and environmental viabilities are of great concern in future. Therefore, although layered bismuth telluride based thermoelectric materials exhibit large ZT (= S²σT/κ = ∼0.8,¹ where S, σ, T, and κ are the thermopower, electrical conductivity, absolute temperature, and thermal conductivity) values at room temperature, they lack the sustainability due to the existence of Te, which is rare and toxic. In this regard, oxide-based thermoelectric materials offer an excellent alternative due to their environmental compatibility.⁹ Although it is known that oxychalcogenide (BiO)(CuSe)^10,11 exhibits a large ZT ∼ 0.2 at room temperature and ∼1.4 at 650 °C, chemical instability under an oxygen atmosphere at high temperature can potentially cause problems in real application. Two of the most commonly used thermoelectric oxides, electron-doped SrTiO₃ and Ca₃Co₄O₉, exhibit large ZT values at elevated temperatures (ZT ∼ 0.3 at 1000 K as the reliable value).^12,13 However, the ZT values of these oxides are extremely small at room temperature (ZT ∼ 0.05 for electron doped SrTiO₃ and ZT ∼ 0.04 for Ca₃Co₄O₉).¹² The origin of such low ZT observed from oxides at room temperature is their high κ. For example, Na_0.75CoO₂ shows a large thermoelectric power factor (PF = S²σ, 2.38 mW m⁻¹ K⁻²) along the layers.¹⁴ However, the κ is rather high (∼7 W m⁻¹ K⁻¹), and therefore, the thermoelectric figure of merit ZT is small (∼0.03 (ref. 15 and 16)) at room temperature.

In order to overcome this difficulty, we recently investigated the effect of ion exchange from the Na⁺ ion to alkali earth ions on the κ of Na_0.75CoO₂.¹⁷ Firstly, we fabricated Na_0.75CoO₂ epitaxial films by the reactive solid-phase (R-SPE) epitaxy method.^18–20 Then, the Na⁺ ions were topotactically substituted with Ca²⁺ and Sr²⁺ ions by the conventional ion exchange treatment.^21–23 We measured the κ of the resultant epitaxial films of Na_0.75CoO₂, Ca_1/3CoO₂, and Sr_1/3CoO₂ using the time-domain thermoreflectance (TDTR) method, and the result showed that the κ parallel to this layered structure can be reduced by substituting heavier elements between the CoO₂ planes, as schematically hypothesized in Fig. 1, whereas the κ normal to the layered structure was ∼1.5 W m⁻¹ K⁻¹, independent of the element mass.¹⁷

Fig. 1 Hypothesis toward the ZT improvement of layered cobaltite, A_xCoO₂. We expected that the lattice vibration, indicated by red arrows of the rigid CoO₂ layer would be significantly suppressed by the substitution of the A ion from (a) light one to (b) heavy one. If the substitution does not affect the power factor (S²σ), the ZT would be improved significantly by the substitution.

In this study, we focused on the substitution of Ba²⁺ because the atomic mass of Ba is the heaviest among alkali earth elements. The thermoelectric properties of Ba_1/3CoO₂ were addressed in a few previous studies. Recently, Liu et al. fabricated Ba_xCoO₂ (x = 0.19, 0.28, 0.30, and 0.33) ceramic samples through the ion-exchange treatment of Na_xCoO₂ ceramics and measured the thermoelectric properties.^24,25 They showed that the resistivity (ρ), S, and κ of the Ba_xCoO₂ ceramics at room temperature were 4–30 mΩ cm (σ = 33–250 S cm⁻¹), ∼+70 μV K⁻¹, and 3–5 W m⁻¹ K⁻¹, respectively. Thus, the ZT of the Ba_xCoO₂ ceramics at room temperature was 0.012 at the largest. The PF was 0.12 mW m⁻¹ K⁻², which is an order of magnitude lower than that of the epitaxial films of Na_0.75CoO₂ (1.23 mW m⁻¹ K⁻²), Ca_1/3CoO₂ (1.08 mW m⁻¹ K⁻²), and Sr_1/3CoO₂ (1.18 mW m⁻¹ K⁻²), measured parallel to the layered structure,¹⁷ which is probably due to carrier scattering at grain or domain boundaries. On the other hand, the lowest κ was 3 W m⁻¹ K⁻¹, which is lower than that of the epitaxial films of Na_0.75CoO₂ (5.46 W m⁻¹ K⁻¹), Ca_1/3CoO₂ (6.79 W m⁻¹ K⁻¹), and Sr_1/3CoO₂ (4.51 W m⁻¹ K⁻¹) (parallel to the layered structure).¹⁷ From these points of view, we expected that Ba_1/3CoO₂ epitaxial films show an enhanced ZT parallel to the layered structure as their power factor is not significantly damaged by grain or domain boundaries.

Here we report that the Ba_1/3CoO₂ epitaxial films, which were fabricated by the ion exchange treatment of Na_0.75CoO₂ epitaxial films, exhibit significantly low κ while maintaining the large power factor and show rather large ZT = 0.11 at room temperature. This value is the highest among those reported for oxide thermoelectric materials except oxychalcogenide (BiO)(CuSe).

Experimental

Preparation of Na_0.75CoO₂ epitaxial films

We prepared Ba_1/3CoO₂ epitaxial films as schematically summarized in Fig. 2, which also shows the crystal structures of CoO, Na_0.75CoO₂, and Ba_1/3CoO₂. Na_0.75CoO₂ epitaxial films were prepared on α-Al₂O₃ substrates by the reactive solid-phase epitaxy (R-SPE) method.^18–20 First, CoO films were heteroepitaxially grown on (0001) or (11̄00) α-Al₂O₃ single-crystal substrates (10 mm × 10 mm × 0.5 mm) at 700 °C in an oxygen atmosphere (10⁻³ Pa) by the pulsed laser deposition technique (KrF excimer laser, ∼2 J cm⁻² per pulse, 10 Hz). Then, the surface of the CoO film was heated at 750 °C in air with NaHCO₃ powder. In order to keep the film surface clean, the yttria stabilized zirconia (YSZ) single crystal plate (10 mm × 10 mm × 0.5 mm) was put on the film surface during the heating. The detailed preparation process of Na_0.75CoO₂ epitaxial films is described elsewhere.^18–20

Fig. 2 Fabrication procedure of Ba_1/3CoO₂ epitaxial films. Step 1: CoO epitaxial film growth by pulsed laser deposition on α-Al₂O₃ substrates. Step 2: Na_0.75CoO₂ film growth by the reactive solid-phase epitaxy method. Step 3: Ion exchange treatment from Na to Ba. After the ion exchange treatment, the resultant film was washed with distilled water several times.

Ion exchange from Na⁺ to Ba²⁺

The ion exchange from Na⁺ to Ba²⁺ was performed by heating the Na_0.75CoO₂ epitaxial films with molten Ba(NO₃)₂ and KNO₃ at 500 °C in an alumina crucible for 1 h. After the ion exchange treatment, the resultant films were washed with distilled water. These processes were similar to the process used to prepare Sr_1/3CoO₂ and Ca_1/3CoO₂ epitaxial films by the ion exchange of Na_0.75CoO₂ epitaxial films.^22,23

Crystallographic characterization of the films

Crystallographic characteristics including the thickness, orientation, and lattice parameters of the resultant films were analyzed using high-resolution X-ray diffraction (HR-XRD, Cu Kα₁ radiation, ATX-G, Rigaku Co.). Out-of-plane Bragg diffraction patterns, in-plane Bragg diffraction patterns, rocking curves, and X-ray reflection patterns were acquired. The surface morphologies of the resultant films were observed with a scanning probe microscope (SPM, Nanocute, Hitach High-Tech Sci. Co.). Topographic atomic force microscopy (AFM) images (2 μm × 2 μm) were acquired. The atomic arrangement of the resultant films was visualized using a high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM).

Measurements of the electrical conductivity and thermopower

The electrical conductivity of the resultant films was measured by the dc four-probe method in the van der Pauw electrode configuration. The thermopower of the resultant films was measured by the conventional steady-state method. The film sample was placed on the gap (∼5 mm) between two Peltier devices. By applying the forward/reverse current to each Peltier device, temperature differences were generated in the sample. We then measured the temperature difference (ΔT) and the thermo-electromotive force (ΔV) simultaneously at room temperature, and the thermopower was calculated as the linear slope of the ΔT − ΔV plot (ESI Fig. S1†). Details of our thermopower measurement system are described elsewhere.^26,27

Measurements of the thermal conductivity

Cross-plane thermal conductivity (κ) of the resultant films was measured by time domain thermo-reflectance (TDTR, PicoTR, PicoTherm Co.). 116 nm-thick Mo was deposited on the resultant films as a transducer by dc sputtering. The decay curves of the TDTR signals were simulated using the packaged software developed by PicoTherm Co. In our simulation, to minimize the number of free parameters, we used constant values for Mo transducers, including bulk density = 10 000 kg m⁻³, heat capacity (C_p) = 250 J kg⁻¹ K⁻¹ and κ = 45.4 W m⁻¹ K⁻¹. Details of our thermal conductivity measurement system are described elsewhere.^17,28,29

Uncertainty in the ZT value

We fabricated more than 5 samples and we measured all samples. The uncertainty in the S and the σ was less than 5%, while that in the κ was ∼15%. Therefore, the uncertainty in the ZT is approximated to be ∼20%.

Results and discussion

Fig. 3 summarizes the out-of-plane XRD patterns and the topographic atomic force microscopy (AFM) images of the resultant CoO film (Fig. 3a and d), R-SPE grown Na_0.75CoO₂ film (Fig. 3b and e), and Ba²⁺ ion-exchanged film (Fig. 3c and f) grown on (0001) α-Al₂O₃ substrates. Only an intense Bragg peak of 111 CoO is seen together with the 0006 α-Al₂O₃ substrate (Fig. 3a), indicating strong (111) orientation in the CoO film. The full width at half maximum (FWHM) of the out-of-plane rocking curve (OXRC) of 111 CoO was ∼0.02° (ESI Fig. S2a†), which is almost as small as the resolution of our optical setup of the XRD. The thickness of the CoO film was 93 nm, which was evaluated using the Pendellösung fringes around the Bragg peak of 111 CoO. The heteroepitaxial growth with the relationship of (111)[11̄0] CoO‖(0001)[11̄00] α-Al₂O₃ was confirmed by the in-plane XRD patterns (ESI Fig. S3a†). Triangle shaped facets are clearly seen in the topographic AFM image of the film (Fig. 3d), confirming the heteroepitaxial growth of CoO.

Fig. 3 Crystal structural analyses of the resultant films. (a)–(c) Change in the out-of-plane XRD pattern of the resultant films grown on the (0001) α-Al₂O₃ substrate (a) after step 1, (b) after step 2, and (c) after step 3. The crystalline phase of (a) is CoO (93 nm), (b) is Na_0.75CoO₂ (182 nm), and (c) is Ba_1/3CoO₂ (200 nm). (d)–(f) Topographic AFM images of the resultant films grown on the (0001) α-Al₂O₃ substrate (a) after step 1 (CoO), (b) after step 2 (Na_0.75CoO₂), and (c) after step 3 (Ba_1/3CoO₂). (g)–(i) Topographic AFM images of the resultant films grown on the (11̄00) α-Al₂O₃ substrate (a) after step 1, (b) after step 2, and (c) after step 3. All the scale bars are 1 μm. Plate-like grains are seen in (d)–(f), whereas stripe patterns are seen in (g)–(i).

After the R-SPE treatment, only intense Bragg peaks of 000l Na_0.75CoO₂ are seen (Fig. 3b). The thickness of the Na_0.75CoO₂ film was 182 nm, roughly twice that of the CoO film. Although the FWHM of the OXRC of 0002 Na_0.75CoO₂ was ∼0.6° (ESI Fig. S2b†), hexagonal-shaped plate-like crystals with large terraces were observed in the topographic AFM image (Fig. 3e). Thus, the broadening of the FWHM of the OXRC indicates the bending of the Na_0.75CoO₂ crystal. The in-plane XRD pattern (ESI Fig. S3b†) confirmed the heteroepitaxial growth of the Na_0.75CoO₂ film on the (0001) α-Al₂O₃ substrate with the epitaxial relationship of (0001)[112̄0] Na_0.75CoO₂‖(0001)[11̄00] α-Al₂O₃. After the Ba²⁺ ion exchange treatment, the Bragg peak position of 000l is shifted to the left, indicating a lattice expansion in the out-of-plane direction (Fig. 3c). The c-axis lattice parameter is 12.225 Å, which corresponds well to the reported value (12.2 Å (ref. 25)) of Ba_1/3CoO₂. The in-plane XRD pattern (ESI Fig. S3c†) indicated that there are two ordered structures of Ba ions. The a-axis lattice parameter was calculated to be 2.829 Å. The topographic AFM image of the resultant film (Fig. 3f) is similar to that of the Na_0.75CoO₂ film.

Since the orientation of the films was inclined on the (11̄00) α-Al₂O₃ substrate, Bragg diffraction peaks were not observed in the out-of-plane Bragg patterns of the resultant CoO film, R-SPE grown Na_0.75CoO₂ film, and Ba²⁺ ion-exchanged film (data not shown). Fig. 3g–i show the topographic AFM images of the CoO (Fig. 3g), Na_0.75CoO₂ (Fig. 3h), and Ba²⁺ ion-exchanged (Fig. 3i) films grown on the (11̄00) α-Al₂O₃ substrate in the same way. Stripe patterns are seen in Fig. 3g–i, which are similar to the surface of inclined films. The heteroepitaxial growth of CoO, Na_0.75CoO₂, and Ba_1/3CoO₂ films on the (11̄00) α-Al₂O₃ substrate was confirmed by the in-plane XRD patterns (ESI Fig. S4†): [11̄0] CoO‖[0001] α-Al₂O₃, [112̄0] Na_0.75CoO₂‖[0001] α-Al₂O₃, and [112̄0] Ba_1/3CoO₂‖[0001] α-Al₂O₃.

To further clarify the orientation of the film and the atomic arrangement, we performed high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) observation. Fig. 4a and b show the cross-sectional HAADF-STEM images of the Ba²⁺ ion-exchanged films on the (0001) α-Al₂O₃ substrate. The e-beam incident direction is along the [11̄00] α-Al₂O₃. The layered atomic arrangement parallel to the substrate surface is clearly visualized (Fig. 4b and ESI Fig. S5a†). An approximately 10 nm-thick amorphous layer is also detected between the layered crystal and the substrate. Energy dispersive X-ray spectroscopy (EDS) results of the film (data not shown) revealed that the upper part of the film is composed of Ba, Co, and O. The signal from Na was negligible. The bottom amorphous part is composed of Ba, Al, and O. The concentration ratio of Ba/Co inside the film was about 0.27. These results show that the resultant film is Ba_0.27CoO₂.

Fig. 4 Cross-sectional atomic arrangement of the Ba_1/3CoO₂ films. (a) Cross-sectional HAADF-STEM image of the Ba_1/3CoO₂ epitaxial film grown on the (0001) α-Al₂O₃ substrate. The selected area electron diffraction pattern and (b) the magnified HAADF-STEM image reveals that Ba and CoO₂ layers are stacked parallel to the substrate surface. (c) Cross-sectional HAADF-STEM image of the Ba_1/3CoO₂ epitaxial film grown on the (11̄00) α-Al₂O₃ substrate. The selected area electron diffraction pattern and (d) the magnified HAADF-STEM image reveal that Ba and CoO₂ layers are inclined 55° to the substrate surface.

On the other hand, an inclined layered structure was revealed in the HAADF-STEM images of the film on the (11̄00) α-Al₂O₃ substrate (Fig. 4c and d). The e-beam incident direction is along the [0001] α-Al₂O₃. Similar to the Ba_0.27CoO₂ film on the (0001) α-Al₂O₃ substrate, the layered atomic arrangement is clearly visualized. It should be noted that the layer is inclined 55° to the substrate surface. Although the interface amorphous layer is thick (15–30 nm), the crystal structure and the chemical composition of the layered crystal layer are similar to those of the Ba_1/3CoO₂ film on the (0001) α-Al₂O₃ substrate. The inclined structure is consistent with the topographic AFM image (Fig. 3i).

Then, we measured the electrical conductivity (σ) and thermopower (S) parallel to the film surface of the (0001)-oriented Ba_0.27CoO₂ film grown on the (0001) α-Al₂O₃ substrate at room temperature. The σ was 2310 S cm⁻¹, the S was +72 μV K⁻¹, and the power factor (PF = S²σ) was 1.2 mW m⁻¹ K⁻², respectively. The PF is the same (1.2 mW m⁻¹ K⁻²) as that of the (0001)-oriented Na_0.75CoO₂ film grown on the (0001) α-Al₂O₃ substrate at room temperature. Thus, Ba ion substitution does not affect the PF.

Next, we measured the thermal conductivity (κ) normal to the film surface of the Ba_0.27CoO₂ films grown on (0001) and (11̄00) α-Al₂O₃ substrates by the TDTR method at room temperature. The κ of the Ba_0.27CoO₂ film on the (0001) α-Al₂O₃ substrate was 1.3 W m⁻¹ K⁻¹ and that on the (11̄00) α-Al₂O₃ substrate was 2.8 W m⁻¹ K⁻¹. In order to extract κ parallel to the layered structure in the Ba_0.27CoO₂ film, we used the following equation as reported previously.¹⁷

where φ, κ_obsd, κ_∥ and κ_⊥ are the inclined angle of the layers perpendicular to the film surface, the observed thermal conductivity, and thermal conductivity parallel and perpendicular to the CoO₂ layers, respectively. Note that isotropy is recovered if κ_∥ = κ_⊥, and this equation can be applied for both isotropic and anisotropic materials.²⁶ In the case of this study, φ = 90–55 = 35°, κ_⊥ = 1.3 W m⁻¹ K⁻¹ and κ_obsd = 2.8 W m⁻¹ K⁻¹, and the κ_∥ was calculated to be 3.29 W m⁻¹ K⁻¹ (ESI Fig. S6†), which is ∼60% of that of the Na_0.75CoO₂ epitaxial film as hypothesized.

Here we summarize and compare the thermoelectric properties of the Ba_0.27CoO₂ and Na_0.75CoO₂ films on the (0001) α-Al₂O₃ substrate along with reported values for Ca_1/3CoO₂ and Sr_1/3CoO₂ (ref. 22 and 23) (Fig. 5 and ESI Table S1†). Although there are some differences in the in-plane electrical conductivity (σ_ip) (Fig. 5a) and thermopower (S_ip) (Fig. 5b) of Ca_1/3CoO₂, Na_0.75CoO₂, Sr_1/3CoO₂ and Ba_0.27CoO₂, the power factor PF_ip (Fig. 5c) is almost constant (1.08–1.2 mW m⁻¹ K⁻²) and does not show clear dependence on the atomic mass of A_x. The main difference was detected in the thermal conductivity (κ) (Fig. 5d). Although the out-of-plane thermal conductivity (κ_op) was almost independent of the atomic mass of A_x, the κ_ip drastically decreased from 6.79 W m⁻¹ K⁻¹ to 3.29 W m⁻¹ K⁻¹ with increasing the atomic mass of A_x from 13.2 (Ca_1/3) to 45.3 (Ba_1/3), resulting in a significant enhancement in ZT along the layered structure. The resulting ZT_in was 0.11 at room temperature, which is the highest among those reported for oxide thermoelectric materials except oxychalcogenide (BiO)(CuSe).

Fig. 5 Thermoelectric properties of A_xCoO₂ epitaxial films at room temperature. (a) In-plane electrical conductivity (σ_ip), (b) in-plane thermopower (S_ip), (c) in-plane power factor (PF_ip), (d) in-plane and out-of-plane thermal conductivity (κ_ip, κ_op), and (e) in-plane figure of merit (ZT_ip) as a function of the atomic mass of A_x in A_xCoO₂. The κ_ip shows a clear decreasing tendency whereas the PF shows a slightly increasing tendency. These resulted in an increasing tendency of ZT. The ZT reached 0.11 when A_x = Ba_1/3.

Conclusions

In summary, we have shown that substitution of the Na⁺ ion with the Ba²⁺ ion in Na_0.75CoO₂ significantly reduces the in-plane κ while keeping the large power factor, resulting in a large enhancement of ZT. We fabricated the epitaxial films of Ba_1/3CoO₂ from Na_0.75CoO₂ epitaxial films by ion-exchange between Na⁺ and Ba²⁺. The σ, S, and power factor of the c-axis oriented Ba_1/3CoO₂ films along the in-plane were 2310 S cm⁻¹, +72 μV K⁻¹, and 1.2 mW m⁻¹ K⁻², respectively. The κ along the in-plane, which was clarified by measuring the κ of the c-axis inclined (55°) film, was 3.29 W m⁻¹ K⁻¹. The ZT along the in-plane was as large as 0.11 at room temperature, which is the highest among those reported for oxide thermoelectric materials except oxychalcogenide (BiO)(CuSe).

Author contributions

Y. T. and Y. Z. performed the sample preparation, measurements, and analyses. J. W., B. F., and Y. I. performed the STEM analyses. H. J. C. analyzed and interpreted the data. H. O. planned and supervised the project. All authors discussed the results and commented on the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was supported by Grants-in-Aid for Innovative Areas (19H05791 and 19H05788) and Grants-in-Aid for Scientific Research A (17H01314) from the JSPS. A part of this work was supported by Dynamic Alliance for Open Innovation Bridging Human, Environment, and Materials, and by the Network Joint Research Center for Materials and Devices. A part of this work was conducted at the Advanced Characterization Nanotechnology Platform of the University of Tokyo, supported by the “Nanotechnology Platform” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, Grant Number JPMXP09A20UT0090. H. J. C. acknowledges the support from the Nippon Sheet Glass Foundation for Materials Science and Engineering. Y. Z. acknowledges the support from the International Research Fellowship from the JSPS (19F19049).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ta07565e

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