Article pubs.acs.org/JPCC
Enhanced Thermoelectric Performance Induced by Misplaced Substitution in Layered Ca3Co4O9 Yanan Huang,† Bangchuan Zhao,*,† Shuai Lin,† and Yuping Sun*,‡,†,§ †
Key Laboratory of Materials Physics, Institute of Solid State Physics, and ‡High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China § Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, People’s Republic of China ABSTRACT: Thermoelectric properties of the Ca3Co4O9 system have been enhanced through an exotic route: Na doping at Co site, namely misplaced substitution. To compare, we have also performed the research of Na doping at Ca site and Co vacancy. In view of the analysis of XRD, XPS, and Raman data, Na+ ions could be suggested to enter into Co sites of [CoO2] layers in Ca3Co4−xNaxO9 (x = 0, 0.05, 0.10) and Ca sites of [Ca2CoO3] layers in Ca2.90Na0.10Co4O9, respectively. And Co ions are omitted from [CoO2] layers in Ca3Co3.90O9. Among all samples, Ca3Co3.90Na0.10O9 shows the maximum ZT value, which at room temperature reaches to ∼0.0117. Such a value is about 150% larger than that of Ca3Co4O9 and 63% larger than that of Ca2.90Na0.10Co4O9. The results indicate that the misplaced substitution is more beneficial to enhance the thermoelectric performance of Ca3Co4O9 system, compared with the traditional idea with Na doping at Ca site. Such an enhancement is mainly attributed to the combined action of electronic correlation, locally modified band structure near Fermi level, and carrier concentration.
I. INTRODUCTION Recently, a series of layered cobalt-based oxides, NaxCo2O4,1 Ca3Co4O9,2 and Bi2Sr2Co2Oy,3 have regained attention due to their good thermoelectric (TE) properties. A key to unveil their mysterious physical properties lies in a common unit: the conducting [CoO2] layers.4 Considering the coexistence of metal-like electrical transport and high thermopower characteristics, these layered cobalt-based oxides may be suitable to become the promising TE materials. In this family, it is worth noting that the Ca3Co4O9 occupies a special place,5 because it overcomes the application limitation of the volatility of Na and Bi ions at high temperatures.6 To improve the TE performance of Ca3Co4O9 system, the element doping is usually used. In the conventional route for similar system A−B-O (A, alkali/alkaline-earth/rare-earth metal; B, transition metal), the main doping elements are the A ones at A sites or/and the B ones at B sites. However, the substitutions of the B elements at A sites or the A ones at B sites have not been researched systematically in Ca3Co4O9 system, because of the large difference between their ionic radii. Actually, such a misplaced substitution idea has been explored. For example, the substitutions of Cr for Li in LiFePO4 structure,7 Cr for Ca and Ag for Co in Ca3Co4O9 one, respectively.8,9 In the former, the electric conductivity was enhanced by 8 orders of magnitude via Cr doping. Accordingly, such an enhancement would be greatly conductive to the improvement of TE performance. The latter indicated that it is feasible to execute the misplaced substitution in Ca3Co4O9 system. It is well-known that, Na doping at Ca site has improved the TE performance of Ca3Co4O9 system.10,11 However, in this system, the study of Na doping at Co site © 2015 American Chemical Society
has not been reported up to now. Considering the misfitlayered structure of Ca3Co4O9 system, it is possible to perform the misplaced substitution idea (Na doping at Co site) in theory. In Ca3Co4O9 lattice, which is an approximate expression and should be formulated as [Ca2CoO3+α][CoO2]δ (δ = b1/ b2),12 a lattice mismatch along b-axis between [Ca2CoO3+α] and [CoO2] subsystems is found in the inset of Figure 1a. The baxis lattice parameter b2 (∼2.8173(1) Å) in [CoO2] layers is much smaller than b1 (∼4.5615(2) Å) in [Ca2CoO3+α] layers.13 Moreover, in [CoO2] layers, the network of edge-shared CoO6 octahedrons constructs the two-dimensional triangular lattice of Co ions. The larger geometric frustrations both along interlayer and within [CoO2] layers make it possible to receive the larger ions in [CoO2] layers, such as, In3+ (0.8 Å),14 Na+ (1.02 Å), Ag+ (1.15 Å),9 and so on. In addition, the [CoO2] layers dominate the electronic structure and the carrier transport of this system.2,15 Therefore, the doping in [CoO2] layers will influence the physical properties of Ca3Co4O9 system more significantly. On the basis of the experimental and theoretical enlightenments as aforementioned, in the present paper, a systematic investigation of the structure, electrical/thermal transport, and TE properties of Ca3Co4−xNaxO9 (x = 0, 0.05, 0.10) was preformed. For comparing, the effect of a proper Na doping at Ca site and Co vacancy on TE properties has also been investigated. It is surprising that Na doping at Co site Received: September 9, 2014 Revised: February 15, 2015 Published: February 25, 2015 7979
DOI: 10.1021/jp512012d J. Phys. Chem. C 2015, 119, 7979−7986
The Journal of Physical Chemistry C
Article
Figure 1. (a) XRD patterns of all samples, the inset is the sketch map of Ca3Co4O9 crystal structure; (b) the cell volume V1 (for [Ca2CoO3+α]) and V2 (for [CoO2]) for all samples; (c) Rietveld refinement of XRD data on Ca3Co3.90Na0.10O9 with the experimental (black five-pointed star), calculated (red solid line), and difference (bottom blue solid line) profiles, the bars mark the peak positions of reflections corresponding to two subsystems (1) and satellites (2).
Table 1. Lattice Parameters a, b1, b2, c, β, and δ (= b1/b2) for All Samplesa
a
sample
a (Å)
b1 (Å)
b2 (Å)
c (Å)
β (deg)
δ (=b1/b2)
Ca3Co4O9 Ca3Co3.95Na0.05O9 Ca3Co3.90Na0.10O9 Ca3Co3.90O9 Ca2.90Na0.10Co4O9
4.8309(5) 4.8310(1) 4.8310(1) 4.8311(6) 4.8310(1)
4.5671(7) 4.5669(4) 4.5669(5) 4.5687(3) 4.5742(6)
2.8311(2) 2.8337(3) 2.8399(6) 2.8219(1) 2.8312(1)
10.8359(2) 10.8355(6) 10.8358(7) 10.8359(2) 10.8354(1)
98.1743(4) 98.2089(3) 98.2178(5) 98.3599(1) 98.1651(2)
1.6132(2) 1.6116(4) 1.6081(2) 1.6190(1) 1.6156(4)
b1 and b2 are the b-axis lattice parameter for [Ca2CoO3+α] and [CoO2] subsystems, respectively.
continuously triggers a zooming ZT value compared to both Na doping at Ca site and Co vacancy.
about 2 × 2 × 0.5, 3 × 1.8 × 1.2, and 5 × 2 × 1.2 mm3, respectively.
II. EXPERIMENTAL DETAILS Po ly cry stallin e samples of nom in al co mp osit io n Ca3Co4−xNaxO9 (x = 0, 0.05, 0.10), Ca2.90Na0.10Co4O9, and Ca3Co3.90O9 were synthesized by solid-state reaction method with the same processing parameters.15 CaCO3 (99.95%, AlfaAesar), Co3O4 (99.9985%, Alfa-Aesar), and Na2CO3 (99.9%, Alfa-Aesar) powders were used as raw materials, and the detailed preparation process was reported in ref8. For convenience and clearness, the samples of Ca 3Co4O 9, Ca3Co3.95Na0.05O9, Ca3Co3.90Na0.10O9, Ca2.90Na0.10Co4O9, and Ca3Co3.90O9 in the figures are abbreviated as UN, CO-0.05, CO, CA, and VA, respectively. The structure and the phase purity of all samples were examined by the powder X-ray diffraction (XRD) using a Philips X′ pert PRO X-ray diffractometer with Cu Kα radiation at 300 K. We used JANA 2006 program to carry out the Rietveld refinement of XRD data.13,16 The valence state of relevant ions was determined by the X-ray photoelectron spectroscopy (XPS) technique with Al Kα radiation at 300 K. Raman spectra were measured with the 514.5 nm line of an argon ion laser (Spectra-Physics 2017) at 300 K. The electrical/ thermal transport properties were measured by a four- or five(Hall measurement) probe method on physical property measurement system (PPMS-9T). The sample dimensions for the Hall and electrical/thermal transport measurements are
III. RESULTS AND DISCUSSION A. Structures. First of all, three types of experimental methods were performed to test the real Na-doping site, Ca or Co one. The room-temperature XRD measurement was performed and the results are displayed in Figure 1(a) for all samples. All of the diffraction peaks could be indexed to the Ca3Co4O9 structure.2,14 In order to confirm the Na-doping site, the Rietveld refinement of XRD data was carried out. In such a refinement, the super space group X2/m(0b0)s0 was applied.17,18 The fitting results are shown in Figure 1c for Ca3Co3.90Na0.10O9 as an example and the refined structural parameters are listed in Table 1. The related fitting parameters (e.g., Rp = 4.40%, Rwp = 3.34%, and χ2 = 1.34) are very low, suggesting that our samples are fairly good in quality. Because of the low Na-doping content (∼2.5%), the lattice parameters a, b1, b2, c, β, and δ (=b1/b2) are changed mildly. b2 and β exhibit a slightly increased tendency with increasing Na-doping content x at Co sites, whereas a, b1, and c are nearly unchanged. For Ca3Co3.90O9, b2 decreases slightly, β increases, and b1 almost keeps constant compared to that of Ca3Co4O9. However, b2 of Ca2.90Na0.10Co4O9 is unchanged, β decreases slightly, and b1 increases. In fact, the ionic radius of Na+ is larger than that of Co2+ (0.73 Å), Co3+ (0.545 Å), Co4+ (0.53 Å), and Ca2+ (1 Å). Na+ ions indeed enter into Co sites of [CoO2] 7980
DOI: 10.1021/jp512012d J. Phys. Chem. C 2015, 119, 7979−7986
The Journal of Physical Chemistry C
Article
Table 2. Approximate Form and Layered Expression of the Structure Formulas for All Samples Arising from the Results of Rietveld Refinement
Figure 2. (a) XPS results of Na ions for Ca3Co3.95Na0.05O9, Ca3Co3.90Na0.10O9, and Ca2.90Na0.10Co4O9; (b) enlarged Co2p3/2 peaks for Ca3Co4O9, Ca3Co3.90Na0.10O9, Ca3Co3.90O9, and Ca2.90Na0.10Co4O9; (c) XPS results of Co ions for Ca3Co4O9; the fitted results of Co2p3/2 peaks for (d) Ca3Co4O9, (e) Ca3Co3.90Na0.10O9, (f) Ca3Co3.90O9, and (g) Ca2.90Na0.10Co4O9.
only reach 0.10, because the difference of ionic radii between Na and Co ions is much large. The XPS results are shown in Figure 2. From Figure 2a, it can be seen that the peak area of Na+ ions increases obviously for Ca3Co3.90Na0.10O9 and Ca2.90Na0.10Co4O9, compared to that for Ca3Co3.95Na0.05O9. Taking Ca3Co4O9 as an example in Figure 2c, two peaks are located near ∼781 and 796 eV, corresponding to Co2p3/2 and Co2p1/2 respectively. To exhibit the variation of peak area of Co ions clearly, the enlarged Co 2p 3/2 peaks are shown in Figure 2b for Ca 3 Co 4 O 9 , Ca3Co3.90Na0.10O9, Ca3Co3.90O9, and Ca2.90Na0.10Co4O9. We can see that the Co2p3/2 peak area for Ca3Co3.90Na0.10O9 and Ca3Co3.90O9 reduces distinctly and that for Ca2.90Na0.10Co4O9 nearly remains the same, compared with that for Ca3Co4O9. However, the peak area of Ca ions declines a little (not shown here) for Ca2.90Na0.10Co4O9. It is worth mentioning, the peak area is on behalf of the element content, suggesting the Na+ ions substitute the corresponding Ca or Co ions in the present samples.
layers in Ca3Co4−xNaxO9 and Ca sites of [Ca2CoO3+α] layers in Ca2.90Na0.10Co4O9, respectively. And Co ions are omitted from [CoO2] layers of Ca3Co3.90O9. Correspondingly, the cell volumes V2 (for [CoO2]) and V1 (for [Ca2CoO3+α]) are expanded in the samples with Na doping at Co and Ca site, respectively. And V2 of Ca3Co3.90O9 is shrunk obviously as shown in Figure 1b. On the basis of the above-mentioned analysis, the structure formulas of Ca3Co4−xNaxO9 (x = 0, 0.05, 0.10), Ca2.90Na0.10Co4O9, and Ca3Co3.90O9 would be expressed as two kinds of versions as listed in Table 2, which include the real doping content and site of Na ions. In this paper, our concern is the effect of the misplaced substitution on TE performance of Ca3Co4O9 system. To express the samples with the misplaced substitution (Na doping at Co site) and the conventional substitution (Na doping at Ca site) more intuitively, we express the studied samples as the approximate form {Ca3Co4O9} shown in the second column of Table 2. It should be also noted that the Na-doping level x at Co sites can 7981
DOI: 10.1021/jp512012d J. Phys. Chem. C 2015, 119, 7979−7986
The Journal of Physical Chemistry C
Article
Table 3. Real Stoichiometry of Ca3Co4O9, Ca3Co3.90Na0.10O9, Ca3Co3.90O9, and Ca2.90Na0.10Co4O9 Arising from the Results of XPS Analysis sample
Na+/fu(x)
Ca/fu
Co/fu
Co2+/fu
Co3+/fu
Co4+/fu
Ca3Co4O9 Ca3Co3.90Na0.10O9 Ca3Co3.90O9 Ca2.90Na0.10Co4O9
0.00 0.13(1) 0.00 0.14(2)
3.00 3.00 3.00 2.90
3.93(5) 3.85(5) 3.84(1) 3.94(2)
1.43(2) 1.46(1) 1.40(1) 1.42(4)
1.04(3) 0.78(2) 1.03(2) 1.08(1)
1.46(1) 1.61(1) 1.43(1) 1.44(4)
Moreover, in Ca3Co4O9 system, holes are the major carriers, Ca ions is in the form of Ca2+, and the valence states of Co ions have three kinds +2, +3, and +4. In [CoO2] layers, the valence states of Co ions are principally +3 and +4, and their average valence is between +3 and +4 on the basis of oxygen content.19 Because the carriers can be changed over a wide range via doping, the effective valence states of Co ions can vary from Co2+, Co3+, to Co4+ to maintain charge balance.19 Here, the change of oxygen content is low. The substitutions of Na+ for Co and Ca ions are hole-type doping. Among them the Co-site doping will bring in more extra holes. But the Co vacancy is “electron-doping” like. Thus, the substitution of Na+ ions for Co or Ca and the Co vacancy will lead to the different variations of the Co valence states. In order to confirm this viewpoint, the Co 2p3/2 peak is fitted displayed in Figures 2d− g.15,20 Here, the Co 2p3/2 peak can be fitted into three peaks. The peaks with the higher, middle, and lower banding energy are attributed to Co2+, Co3+, and Co4+, respectively. On the basis of the fitted peak area ratio among them, the molar content for three kinds of Co ions is listed in Table 3. Compared to Ca3Co4O9, three prominent phenomena can be observed from Table 3. (i) For Ca3Co3.90Na0.10O9, the content of Co3+ decreases while Co4+ augments, implying the substitution of Na+ for Co ions can introduce a large number of extra holes. Accordingly, the partial Co ions are changed from Co3+ to Co4+ to keep charge balance. (ii) For Ca2.90Na0.10Co4O9, the substitution of Na+ for Ca2+ ions brings in a small number of extra holes. Therefore, only Co3+ ion increase slightly. (iii) For Ca3Co3.90O9, the contents of three kinds of Co ions all decreases and the decrease of Co4+ is more significant, indicating the introduction of additional electrons. The above experimental data indicate that the Co vacancy and the substitution of Na+ ions for Co or Ca introduce the carriers with the different kinds and numbers into the respective samples, which changes the contents of Co ions with different valence states. On the basis of the change of ionic valence state, the above results confirm that the doped Na ions are indeed introduced into the expected Co or Ca site in the respective samples. In order to further verify the Na-doping site from the point of lattice vibration, Raman spectra are also used. As we known, Raman scattering is an important technology to research lattice vibration.21 According to the information on lattice vibration, we can easily understand the structure of our samples.22 In this paper, Raman measurement was performed from 150 to 670 cm−1 on all samples and the results are shown in Figure 3a. There are nine phonon peaks locating at about P1 (171 cm−1), P2 (220 cm−1), P3 (285 cm−1), P4 (356 cm−1), P5 (410 cm−1), P6 (451 cm−1), P7 (533 cm−1), P8 (615 cm−1), and P9 (654 cm−1), respectively.22 The assignment of particular Raman lines to define atomic motions could be based on the comparison with the results of lattice dynamical calculations (LDC).21 Since the structure of Ca3Co4O9 system is complicated and mismatched, it is difficult to perform the exact LDC. Further
Figure 3. (a) Raman spectra at 300 K for all samples collected with a 514.5 nm argon ion laser, where there are nine peaks labeled by arrows. (b) Raman phonon wave numbers P1, P2, P3, P6, and P8 for all samples as a function of Na-doping site.
detailed research on this system, such as first-principles calculations and relative experimental data, should be helpful.20,21 However, in the approximation of an harmonic oscillator ω = (k/μ)1/2, where k and μ are respectively force constant and reduced mass, the heavier atoms are expected to vibrate in the low wavenumber region.23 In Figure 3a, it is reasonable to assign the phonon mode at lower frequency to heavier Ca or Co atomic vibrations, whereas those at higher frequencies are related to the motions of lighter O atoms.22 The reported Raman scatterings in RCrO3 perovskites (R = Y, La, Pr, Sm, Gd, Dy, Ho, Yb, Lu),23 NaxCoO2·yH2O,24 and (Bi, Pb)−Sr− Co−O21 have showed some evidence for such an opinion. Especially, in the latter with the same triangular [CoO2] layers as Ca3Co4O9, the vibration relevant to the heavier atoms Co was indicated to be lower than 150 cm−1.21,24 So, in view of above-mentioned theoretical and experimental evidence, P1 and P2−P9 could be assigned to the vibrations of Ca and O atoms, respectively. Because of the similarity of Co−O triangular lattice in [CoO2] layers between Ca3Co4O9 and NaxCoO2, it is reasonable to assign P6 (451 cm−1) and P8 (615 cm−1) to E1g phonon and A1g phonon respectively, compared with E1g phonon at 472 cm−1 and A1g phonon at 593 cm−1 for NaxCoO2.21,24 The E1g and A1g modes in NaxCoO2 represent in-plane and out-of-plane vibrations of O atoms in Co−O planes of [CoO2] layers, respectively.21 For the same reason between Ca3Co4O9 and Bi2−xPbxSr2Co2Oy, P2(220 cm−1) and P3(285 cm−1) may be related to the vibrations of O atoms in Co−O and Ca−O planes of [Ca2CoO3] layers, respectively.21 Figure 3b presents the evolution of band position for such five modes P1, P2, P3, P6, and P8 as a function of the doping site. The sharp low wavenumber band P1 is almost unchanged for Ca 3 Co 4 O 9 , Ca 3 Co 3.95 Na 0.05 O 9 , Ca 3 Co 3.90 Na 0.10 O 9 , and 7982
DOI: 10.1021/jp512012d J. Phys. Chem. C 2015, 119, 7979−7986
The Journal of Physical Chemistry C
Article
Figure 4. Temperature-dependent resistivity ρ(T) for (a) Ca3Co4−xNaxO9 (x = 0, 0.05, 0.10); (b) Ca3Co3.90O9, Ca3Co4O9, Ca2.90Na0.10Co4O9, and Ca3Co3.90Na0.10O9. The arrows in the figures denote the MIT temperature Tmin. The insets show the Tmin as a function of Na-doping level x and site.
the sample changes from Ca3Co3.90O9, Ca3Co4O9, and Ca2.90Na0.10Co4O9 to Ca3Co3.90Na0.10O9, indicating that the effect of Na doping at Co site on SDW state is direct and more conspicuous. To explore reasons during the evolution of conduction mechanism, the low-temperature ρ(T) data below Tmin was fitted and the thermal activated conduction model:28 ρ−1(T) = μ(T) exp(−E0/kBT) is considered to match the ρ(T) data best for all samples as shown in Figure 5, parts a and b. Here, E0 is
Ca 3 Co 3.90 O 9 , while it shows an obvious increase for Ca2.90Na0.10Co4O9, corresponding well to the mass-induced (mNa < mCa) shift expected from a harmonic oscillator.23 However, the high wavenumber bands P2, P3, P6, and P8 are considered to be affected by the lattice distortion. Reminiscent of the reports by Linet al.25 and Anget al.,26 in present system, as Ca-site ionic radius rCa (i.e., with increasing the Na content at Ca sites) increases, the tolerance factor t [=C(rCa + rO)/(rCo + rO), where C is a constant, rCo and rO are Co- and O-site ionic radius, respectively] increases, while t decreases with increasing rCo (i.e., with increasing the Na content at Co sites).26 Hence, the variations of ionic radii rCa and rCo will induce the lattice distortion, and then influence the vibrations of O atoms in the same planes with Ca or Co.23 From Figure 3b, one can see that P2 mainly keeps unchanged for all samples without any doping or vacancy in Co−O planes of [Ca2CoO3] layers. The sharp P3 is also unchanged for Ca 3 Co 4 O 9 , Ca 3 Co 3.95 Na 0.05 O 9 , Ca3Co3.90Na0.10O9, and Ca3Co3.90O9, while increases obviously for Ca2.90Na0.10Co4O9 due to the variational rCa induced by Na doping in Ca−O planes of [Ca2CoO3] layers. At the same time, both P6 and P8 decrease monotonically from Ca3Co4O9, Ca3Co3.95Na0.05O9, Ca3Co3.90Na0.10O9 to Ca3Co3.90O9 owing to the change of rCo related to Na doping or Co vacancy in Co−O planes of [CoO2] layers. To sum up, compared to Ca3Co4O9, (i) the variations of P1 and P3 verify Na ions to be doped into the Ca sites for Ca2.90Na0.10Co4O9 really; (ii) the changes of P2, P6 and P8 confirm that Na ions indeed substitute Co ones for Ca3Co3.90Na0.10O9 and Co ions are omitted for Ca3Co3.90O9 in Co−O planes of [CoO2] layers. B. Electrical Transport Properties. The temperaturedependent resistivity ρ(T) for all samples is displayed in Figure 4, parts a and b. Because of the emergence of incommensurate spin-density-wave (SDW) ordering around Tmin in Ca3Co4O9 system, partial charge carriers are located, leading to the metal− insulator transition (MIT).19 Such a transition happens in all our samples. Thus, all present a similar electrical transport behavior: metallic behavior above Tmin and semiconducting behavior below Tmin.27 For Ca3Co4−xNaxO9, the MIT temperature Tmin increases with increasing x as shown in the inset of Figure 4a, indicating that the SDW state becomes more stable. This phenomenon may stem from the enhanced random Coulomb potential caused by the disorder effect of Na doping. As shown in the inset of Figure 4b, Tmin increases gradually as
Figure 5. (a, b) Plots of lnρ against T −1; (c, d) Plots of ρ against T 2. Here the arrows denote the end temperature T*of the linear relationship for ρ ∼ T 2.
the energy gap resulting from the SDW at the Fermi surface and kB is the Boltzmann’s constant. The values of the fitting parameter R2 (denoting the fitting quality) are all more than 0.999. For Ca3Co4−xNaxO9, the obtained thermal activation energy E0 increases monotonically with increasing x as revealed in Figure 6a, suggesting that more energy is need to excite carriers. As we known, in Ca3Co4O9 system, the [Ca2CoO3] (CaO−CoO−CaO) layer consists of alternate stacks of two Ca−O planes and one Co−O plane (see the inset of Figure 1a). That is, the Co−O plane is sandwiched by the two Ca−O planes. The [Ca2CoO3] layer only play a carrier reservoir to supply carriers to the [CoO2] layer and the SDW propagates 7983
DOI: 10.1021/jp512012d J. Phys. Chem. C 2015, 119, 7979−7986
The Journal of Physical Chemistry C
Article
Figure 6. (a, b) Fitting parameters E0 and A; (c, d) room-temperature resistivity ρ300 and carrier concentration n300 K as functions of Nadoping content x and site.
within the [CoO2] layers. If the element substitution occurs at Co sites in [Ca2CoO3] layers, there would be little influence on electrical transport mechanism and then E0 would stay the same.19 The increased E0 indicates that Na-substitution for Co ions may mainly occur at Co sites in [CoO2] layers. Moreover, as displayed in Figure 6b, E0 increases gradually as the samples change from Ca3Co3.90O9, Ca3Co4O9, and Ca2.90Na0.10Co4O9 to Ca3Co3.90Na0.10O9. These results are in coherence with the variation of Tmin as revealed in the insets of Figure 4, parts a and b. As shown in Figure 4a, the resistivity ρ decreases monotonically with increasing x for Ca3Co4−xNaxO9 in the full investigated-temperature range. The resistivity also decreases as the sample changes from Ca 3 Co 3.90 O 9 , Ca3Co4O9, and Ca2.90Na0.10Co4O9 to Ca3Co3.90Na0.10O9 displayed in Figure 4b. The resistivity change can be seen clearly in Figure 6, parts c and d, where the Na-doping level and site dependences of the room-temperature resistivity ρ300 K are plotted. As aforementioned, the substitution of Na+ ions for Co or Ca is hole-type doping and the Co-site doping will introduce more extra holes, while the Co vacancy is “electron-doping” like. Thus, for Ca3Co4−xNaxO9, n300 K increases monotonically with increasing x in Figure 6c, which will result in the gradual decrease of ρ300 K. For Ca2.90Na0.10Co4O9, as shown in Figure 6d, its n300 K also increase compared to that of Ca3Co4O9 while is still smaller than that of Ca3Co3.90Na0.10O9, which leads to the smaller decrease of ρ300 K. However, for Ca3Co3.90O9, its n300 K decreases due to the vacancy of Co ions, resulting in the increase of ρ300 K compared to that of Ca3Co4O9. These results suggest that Na doping at Co site in [CoO2] layers could reduce the resistivity more effectively than Na doping at Ca site and Co vacancy, which is more beneficial to the TE performance of Ca3Co4O9 system. C. Thermopower. Parts a and b of Figure 7 show the temperature-dependent thermopower S(T) for all our samples. The S(T) curves of all samples show a strongly T-dependent behavior below 150 K, while exhibit a nearly T-independent one above 200 K. The room-temperature thermopower S300 K almost keeps constant for Ca3Co4−xNaxO9, while Na doping at Ca site and Co vacancy change the magnitude of S300 K considerably as shown in the insets of Figure 7, parts a and
Figure 7. Temperature dependence of (a, b) thermopower S(T), (c, d) thermal conductivity κ(T), and (e, f) ZT value ZT(T) for all samples. The insets of parts a−d reveal the room-temperature thermopower S300 K and thermal conductivity κ300 K as functions of Na-doping content x and site, respectively.
b. In general, in the Co-based oxide materials with spin−orbit degeneracy, the large S with a nearly T-independent behavior at higher temperatures should be made up of two parts: the thermoelectromotive force and the thermopower induced by the additional spin-entropy. The former is based on the chemical composition, the lattice structure, and the electronic structure of materials, which could be expressed by the Mott formula:19,29,30 S(T) = (π2kBT)/(3e)[d[ln σ(ε)]/dε]ε=εF. Here σ(ε) are electric conductivity with σ(ε) = n(ε)eν(ε) = D(ε)f(ε) eν(ε), where n(ε), ν(ε), D(ε), f(ε), and εF are is carrier density, mobility, density of states (DOS), Fermi function, and Fermi level. Yin etc. suggested that the local modification of DOS and band structure near εF played a crucial role in the layered Cobased oxides, which was related to electronic correlation and lattice distortion.4 The latter follows the so-called Heikes formula:31 S = kB/e{ln[(g3/g4)y/(1 − y)]}, where y is the concentration of Co4+ and g3 and g4 are the spin orbital degeneracy for Co3+ and Co4+ ions, respectively. The y value at room temperature could be deduced from the calculated n300 K. Moreover, there does not exist the evidence of spin-state transition for Co3+ and Co4+ ions in the ρ(T) curves for all Nadoping and Co-vacancy samples. Hence, for the present samples, the thermopower near room temperature is mainly dominated by three factors: electronic correlation, lattice distortion, and n300 K. To check whether the change of S is relative to the electronic correlation, we first attempt to fit the resistivity data in the Fermi liquid regime for all samples using the equation:19 ρ(T) = ρ0 + AT2, where ρ0 is the residual resistivity and A is the Fermi liquid transport coefficient. As shown in Figures 5c and 6a, for Ca3Co4−xNaxO9, T* increases while A declines monotonically with increasing x. Here, T* is the temperature where the curve deviates the linear relationship for ρ ∼ T2. In the model of dynamical mean field theory,19,32 the effective 7984
DOI: 10.1021/jp512012d J. Phys. Chem. C 2015, 119, 7979−7986
The Journal of Physical Chemistry C
Article
mass m* plays a key role for a Fermi liquid, which is predicted as 1/T* ∼ m* and A ∼ (m*)2. The changes of T* and A result in the obviously decreased m*, implying that the electronic correlation weakens in Ca3Co4−xNaxO9. Such a result is harmful to the enhancement of S300 K value. From Figures 5(d) and 6(b), we could deduce that the m* of carriers for Ca2.90Na0.10Co4O9 also decreases and the electronic correlation weakens compared to that of Ca3Co4O9. However, for Ca3Co3.90O9, its carrier m* increases and the electronic correlation enhances. Moreover, for Ca3Co4−xNaxO9, the increase of n300 K as presented in Figure 6c also goes against the improvement of S300 K. However, t decreases when Na ions are doped into Co sites as aforementioned,26 and the distortion of the CoO6 octahedron becomes stronger in [CoO2] layer.4 As a result, the bandwidth gets narrower and the mobility of eg electrons reduces, reminiscent of layered SrLnCoO4 (Ln = La, Ce, Pr, Nd, Eu, Gd, and Tb)26 and Bi2Sr2−xCaxCo2Oy.4 Finally, the spin entropy will enhance in this system because of the weak magnetic interaction induced by the narrowing of the bandwidth, which will lead to an increasing tendency of S300 K. We suggest that the role of the lattice distortion is comparable to that of the weakened electronic correlation and the increased n300 K, leading to the almost unchanged S300 K for Ca3Co4−xNaxO9. It is well-known that the on-resonant spectra of X-ray photoemission spectroscopy for Ca3Co4O9 reveal that the εF lies in the crystal-field gap of the d states in [CoO2] layers.33 In contrast, the Co 3d partial DOS in [Ca2CoO3] layers can hardly reach the εF,29 indicating that Na doping at Ca site in [Ca2CoO3] layers has little effect on the DOS of system. Thus, for Ca2.90Na0.10Co4O9, the contribution of the weakened electronic correlation and the increased n300 K (shown in Figure 6d) to S is dominated, which leads to the decrease of S300 K compared to Ca3Co4O9. However, for Ca3Co3.90O9, all three factors, the enhanced electronic correlation, the decreased n300 K and the lattice distortion, play a positive role on the thermopower, leading to the much larger S300 K than that of both Ca3Co4O9 and Ca3Co3.90Na0.10O9. D. Thermoelectric Properties. Parts c and d of Figure 7 present the temperature-dependent thermal conductivity κ for all samples. A similar κ(T) behavior was observed for all samples, while the magnitude of κ decreases obviously via both Co vacancy and Na doping at different sites. Generally, κ consists of the phonon thermal conductivity κph and the carrier thermal conductivity κch.34 κch can be calculated from Wiedemann−Franz formula, κch=LT/ρ, where L is the Lorentz number (2.45 × 10−8 V2K−2 for free electrons). The calculated κch values at 300 K are 0.048, 0.061, 0.080, 0.041, and 0.060 W/ m−1K−1 for Ca3Co4O9, Ca3Co3.95Na0.05O9, Ca3Co3.90Na0.10O9, Ca3Co3.90O9, and Ca2.90Na0.10Co4O9, respectively, which are all less than 2% of κ.10 Hence, the κph is the main source of the κ in our present samples.19 Both the Co vacancy and the Na doping with a larger ionic radius at different sites can lead to lattice disorder and structural distortion as a point-defect. As a result, the induced lattice disharmony will scatter more phonons, and then the phonon transport will be suppressed. Finally, κph and κ decrease obviously for all Co-vacancy and Na-doped samples as exhibited in the insets of Figure 7, parts c and d. On the basis of the ρ, S, and κ, we can calculate the dimensionless figure-of-merit ZT (=S2T/ρκ). The ZT as a function of temperature for all samples is shown in Figure 7, parts e and f, and ZT increases monotonically with increasing x at Co sites from 0 to 0.10. Although the ZT values of both
Ca3Co3.90O9 and Ca2.90Na0.10Co4O9 also increase compared to that of Ca3Co4O9, their values are obviously smaller than that of Ca3Co3.90Na0.10O9. That is to say, the maximum value of ZT is reached for Ca3Co3.90Na0.10O9 in the series samples. Its value reaches 0.0117 at 300 K, which is about 150% larger than that of Ca 3 Co 4 O 9 (0.0047) and 63% larger than that of Ca 2.90Na0.10Co4O9 (0.0073). The results show that an appropriate Na doping at Co site is more beneficial to enhance the TE performance of Ca3Co4O9 system than the traditional idea (Na doping at Ca site). Considering the above analysis of ZT values, we can find that the TE performance of Ca3Co4O9 could be enhanced obviously by the misplaced substitution. We hope that such a misplaced substitution idea could provide a reference or guidance for future studies in similar materials.
IV. CONCLUSIONS In conclusion, the effect of Na doping at Co site on the structure, electrical and thermal transport properties of Ca3Co4−xNaxO9 (0 ≤ x ≤ 0.10) samples have been studied. For comparing, the effect of Co vacancy and Na doping at Ca site on TE performance has also been investigated. According to the experimental results of XRD, XPS, and Raman, we conclude that Na+ ions should indeed be doped into Co sites of [CoO2] layers in Ca3Co4−xNaxO9 lattice and Ca sites of [Ca2CoO3] layers in Ca2.90Na0.10Co4O9 lattice, respectively. And Co ions are omitted from [CoO2] layers of Ca3Co3.90O9 lattice. As Na ions are introduced into the Ca3Co4−xNaxO9 lattice, the ZT value increases obviously due to the considerable decrease of resistivity and thermal conductivity. Among all samples, Ca3Co3.90Na0.10O9 shows the maximum ZT value. Its value reaches to 0.0117 at 300 K, which is about 150% larger than that of Ca3Co4O9 and 63% larger than that of Ca2.90Cr0.10Co4O9. These results indicate that, compared to Na doping at Ca site, the misplaced substitution (Na doping at Co site) provide a more effective way to enhance the TE performance of Ca3Co4O9 system.
■
AUTHOR INFORMATION
Corresponding Authors
*(B.Z.) E-mail:
[email protected]. *(Y.S.) E-mail:
[email protected]. Telephone: +86-551-65592757. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work is supported by the National Key Basic Research under Contract No. 2011CBA00111, and the National Nature Science Foundation of China under Contract Nos. 11174293 and U1232140.
■
REFERENCES
(1) Terasaki, I.; Sasago, Y.; Uchinokura, K. Large Thermoelectric Power in NaCo2O4 Single Crystals. Phys. Rev. B 1997, 56, R12685− R12687. (2) Masset, A. C.; Michel, C.; Maignan, A.; Hervieu, M.; Toulemonde, O.; Studer, F.; Raveau, B.; Hejtmanek. Misfit-Layerd Cobaltite with an Anisotropic Gaint Magnetoresistance: Ca3Co4O9. Phys. Rev. B 2000, 62, 166−175. (3) Funahashi, R.; Shikano, M. Bi2Sr2Co2Oy Whiskers with High Thermoelectric Figure of Merit. Appl. Phys. Lett. 2002, 81, 1459−1461. (4) Yin, L. H.; Ang, R.; Huang, Y. N.; Jiang, H. B.; Zhao, B. C.; Zhu, X. B. The Contribution of Narrow Band and Modulation of 7985
DOI: 10.1021/jp512012d J. Phys. Chem. C 2015, 119, 7979−7986
The Journal of Physical Chemistry C
Article
(23) Weber, M. C.; Kreisel, J.; Thomas, P. A.; Newton, M.; Sardar, K.; Walton, R. I. Phonon Raman Scattering of RCrO3 Perovskites (R = Y, La, Pr, Sm, Gd, Dy, Ho, Yb, Lu). Phys. Rev. B 2012, 85 (054303), 1−9. (24) Lemmens, P.; Choi, K. Y.; Gnezdilov, V.; Sherman, E. Y.; Chen, D. P.; Lin, C. T.; Chou, F. C.; Keimer, B. Anomalous Electronic Raman Scattering in NaxCoO2 Center Dot yH2O. Phys. Rev. Lett. 2006, 96 (167204), 1−4. (25) Yin, L. H.; Ang, R.; Huang, Z. H.; Liu, Y.; Tan, S. G.; Huang, Y. N.; Zhao, B. C.; Song, W. H.; Sun, Y. P. Exotic Reinforcement of Thermoelectric Power Driven by Ca Doping in Layered Bi2Sr2‑xCaxCo2Oy. Appl. Phys. Lett. 2013, 102 (141907), 1−5. (26) Ang, R.; Sun, Y. P.; Luo, X.; Hao, C. Y.; Song, W. H. Studies of Structural, Magnetic, Electrical and Thermal Properties in Layered Perovskite Cobaltite SrLnCoO4 (Ln = La, Ce, Pr, Nd, Eu, Gd and Tb). J. Phys. D: Appl. Phys. 2008, 41 (045404), 1−6. (27) Sugiyama, J.; Itahara, H.; Tani, T.; Brewer, J. H.; Ansaldo, E. J. Magnetism of Layered Cobalt Oxides Investigated by Muon Spin Rotation an Relaxation. Phys. Rev. B 2002, 66 (134413), 1−9. (28) Zhao, B. C.; Sun, Y. P.; Lu, W. J.; Zhu, X. B.; Song, W. H. Enhanced Spin Fluctuations in Ca3Co4‑xTixO9 Single Crystals. Phys. Rev. B 2006, 74 (144417), 1−8. (29) Takeuchi, T.; Kondo, T.; Takami, T.; Takahashi, H.; Ikuta, H.; Mizutani, U.; Soda, K.; Funahashi, R.; Shikano, M.; Mikami, M.; et al. Contribution of Electronic Structure to the Large Thermoelectric Power in Layered Cobalt Oxides. Phys. Rev. B 2004, 69 (125410), 1−9. (30) Fisher, B.; Patlagan, L.; Reisner, G. M.; Knizhnik, A. Systematics in the Thermopower of Electron-Doped Layered Manganites. Phys. Rev. B 2000, 61, 470−475. (31) Koshibae, W.; Tsutsui, K.; Maekawa, S. Thermopower in Cobalt Oxides. Phys. Rev. B 2000, 62, 6869−6872. (32) Georges, A.; Kotliar, G.; Krauth, W.; Rozenberg, M. J. Dynamical Mean-Field Theory of Strongly Correlated Fermion Systems and the Limit of Infinite Dimensions. Rev. Mod. Phys. 1996, 68, 13−125. (33) Asahi, R.; Sugiyama, J.; Tani, T. Electronic Structure of MisfitLayered Calcium Cobaltite. Phys. Rev. B 2002, 66 (155103), 1−7. (34) Wang, Y.; Sui, Y.; Wang, X. J.; Su, W. H.; Liu, X. Y. Enhanced High Temperature Thermoelectric Characteristics of Transition Metals Doped Ca3Co4O9+δ by Cold High-Pressure Fabrication. J. Appl. Phys. 2010, 107 (033708), 1−9.
Thermoelectric Performance in Doped Layered Cobaltites Bi2Sr2Co2Oy. Appl. Phys. Lett. 2012, 100 (173503), 1−4. (5) Soret, J.; Lepetit, M. B. Electronic Structure of the Ca3Co4O9 Compound from ab initio Local Interactions. Phys. Rev. B 2012, 85 (165145), 1−9. (6) Delorme, F.; Martin, C. F.; Marudhachalam, P.; Ovono, D. O.; Guzman, G. Effect of Ca Substitution by Sr on the Thermoelectric Properties of Ca3Co4O9 Ceramics. J. Alloys Compd. 2011, 509, 2311− 2315. (7) Shi, S. Q.; Liu, L. J.; Ouyang, C. Y.; Wang, D. S.; Wang, Z. X.; Chen, L. Q.; Huang, X. J. Enhancement of Electronic Conductivity of LiFePO4 by Cr Doping and its Identification by First-Principles Calculations. Phys. Rev. B 2003, 68 (195108), 1−5. (8) Huang, Y. N.; Zhao, B. C.; Lin, S.; Ang, R.; Sun, Y. P. Enhanced Thermoelectric Performance Induce by Cr Doping at Ca-Sites in Ca3Co4O9 System. J. Am. Ceram. Soc. 2014, 97, 3589−3596. (9) Han, L.; Jiang, Y.; Li, S. Y.; Su, H. M.; Lan, X. Z.; Qin, K. X.; Han, T. T.; Zhong, H. H.; Chen, L.; Yu, D. B. High Temperature Thermoelectric Properties and Energy Transfer Devices of Ca3Co4‑xAgxO9 and Ca1‑ySmyMnO3. J. Alloys Compd. 2011, 509, 8970−8977. (10) Xu, G. J.; Funahashi, R.; Shikano, M.; Matsubara, I.; Zhou, Y. Q. Thermoelectric Properties of the Bi- and Na-Substituted Ca3Co4O9 System. Appl. Phys. Lett. 2002, 80, 3760−3762. (11) Xu, G. J.; Funahashi, R.; Shikano, M.; Pu, Q. R.; Liu, B. High Temperature Transport Properties of Ca3‑xNaxCo4O9 System. Solid State Commun. 2002, 124, 73−76. (12) Miyazaki, Y.; Onoda, M.; Oku, T.; Kikuchi, M.; Ishii, Y.; Ono, Y.; Morii, Y.; Kajitani, T. Modulated Structure of the Thermoelctric Compound [Ca2CoO3]0.62CoO2. J. Phys. Soc. Jpn. 2002, 71, 491−497. (13) Wu, T.; Tyson, T. A.; Chen, H. Y.; Bai, J. M.; Wang, H.; Jaye, C. A Structural Change in Ca3Co4O9 Associated with Enhanced Thermoelectric Properties. J. Phys.:Condens. Matter 2012, 24 (455602), 1−7. (14) Huang, Y. N.; Zhao, B. C.; Ang, R.; Lin, S.; Huang, Z. H.; Yin, L. H.; Tan, S. G.; Liu, Y.; Song, W. H.; Sun, Y. P. Enhanced Electron Correlation in the In-doped Misfit-Layered Cobaltite Ca3Co4O9 Ceramics. J. Am. Ceram. Soc. 2013, 96, 791−797. (15) Huang, Y. N.; Zhao, B. C.; Ang, R.; Lin, S.; Huang, Z. H.; Tan, S. G.; Liu, Y.; Song, W. H.; Sun, Y. P. Enhanced Thermoelectric Performance and Room-Temperature Spin-State Transition of Co4+ Ions in the Ca3Co4‑xRhxO9 System. J. Phys. Chem. C 2013, 117, 11459−11470. (16) Petříček, V.; Dušek, M.; L. Palatinus Jana2006 the Crystallographic Computing System. Institute of Physics, Academy of Sciences of the Czech Republic: Prague, 2006. (17) Ling, C. D.; Aivazian, K.; Schmid, S.; Jensen, P. Structural Investigation of Oxygen Non-Stoichiometry and Cation Doping in Misfit-Layered Thermoelectric (Ca2CoO3‑x)(CoO2)δ, δ ≈ 1.61. J. Solid State Chem. 2007, 180, 1446−1455. (18) Wu, T.; Tyson, T.; Bai, J. M.; Pandya, K.; Jaye, C.; Fischer, D. A. On the Origin of Enhanced Thermoelectricity in Fe Doped Ca3Co4O9. J. Mater. Chem. C 2013, 1, 4114−4121. (19) Wang, Y.; Sui, Y.; Ren, P.; Wang, L.; Wang, X. J.; Su, W. H.; Fan, H. J. Strongly Correlated Properties and Enhanced Thermoelectric Response in Ca3Co4‑xMxO9 (M = Fe, Mn, and Cu). Chem. Mater. 2010, 22, 1155−1163. (20) Zhu, X. B.; Tang, X. W.; Shi, D. Q.; Jian, H. B.; Lei, H. C.; Yeoh, W. K.; Zhao, B. C.; Yang, J.; Li, Q.; Zheng, R. K.; Dou, S. X.; Sun, Y. P. Synthesis and Characterization of Self-Assembled c-Axis Oriented Bi2Sr3Co2Oy Thin Films by the Sol-Gel Method. Dalton Trans. 2011, 40, 9544−9550. (21) Yuan, S. K.; An, M.; Wu, Y.; Zhang, Q. M.; Luo, X. G.; Chen, X. H. Raman-Scattering Study of Misfit-Layered (Bi,Pb)-Sr-Co-O Single Crystal. J. Appl. Phys. 2007, 101 (113527), 1−4. (22) An, M.; Yuan, S. K.; Wu, Y.; Zhang, Q. M.; Luo, X. G.; Chen, X. H. Raman Spectra of a Misfit Layered Ca3Co4O9 Single Crystal. Phys. Rev. B 2007, 76 (024305), 1−5. 7986
DOI: 10.1021/jp512012d J. Phys. Chem. C 2015, 119, 7979−7986
