Improving the zT value of thermoelectrics by nanostructuring: tuning the nanoparticle morphology of Sb₂Te₃ by using ionic liquids

Abstract

A systematic study on the microwave-assisted thermolysis of the single source precursor (Et₂Sb)₂Te (1) in different asymmetric 1-alkyl-3-methylimidazolium- and symmetric 1,3-dialkylimidazolium-based ionic liquids (ILs) reveals the distinctive role of both the anion and the cation in tuning the morphology and microstructure of the resulting Sb₂Te₃ nanoparticles as evidenced by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), and X-ray photoelectron spectroscopy (XPS). A comparison of the electrical and thermal conductivities as well as the Seebeck coefficient of the Sb₂Te₃ nanoparticles obtained from different ILs reveals the strong influence of the specific IL, from which C₄mimI was identified as the best solvent, on the thermoelectric properties of as-prepared nanosized Sb₂Te₃. This work provides design guidelines for ILs, which allow the synthesis of nanostructured thermoelectrics with improved performances.

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Introduction

Thermoelectric generators (TEG) directly convert heat fluxes into useable electrical energy and therefore provide a wear- and noiseless power source.¹ The efficiency of a thermoelectric material is defined by the dimensionless figure of merit zT ((α²σ/κ)T), where α is the Seebeck coefficient, σ the specific electrical conductivity, κ the thermal conductivity as the sum of the electronic κ_el and the lattice κ_L contribution and T the absolute temperature in Kelvin. It is assumed that at least a zT ≅ 1.5 is necessary for most technical applications to become efficient and commercially viable.² Unfortunately, the electrical and thermal transport coefficients are interrelated and cannot easily be optimized independently from each other. Metals naturally show high electrical and thermal conductivities, whereas both of these are small for insulators such as ceramics. The best choices of materials for technical applications in thermoelectric devices are semiconducting materials which contain heavy elements. This inherently minimizes the thermal conductivity due to a low speed of sound of such materials, while still a sufficiently high electronic conductivity is obtained. For technical applications near room temperature, Sb₂Te₃ and Bi₂Te₃ as well as their solid ternary solutions (Sb_xBi_1−x)₂Te₃ are currently the most efficient materials due to their high electrical conductivities and high Seebeck coefficients combined with low thermal conductivities.³

Sb₂Te₃ is a tetradymite-type layered material, which has been investigated for decades since it is a narrow band-gap (E_gap 0.26 eV) semiconductor with good thermoelectric characteristics near room temperature.⁴ More recently, interest in Sb₂Te₃ increased due to its capability to serve as a topological insulator.⁵ Nanostructuring has been demonstrated theoretically and experimentally to greatly improve the figure of merit by effectively reducing the lattice contribution to the thermal conductivity⁶ while the electrical conductivity of the material is mostly unaffected. Different types of scattering centres for the heat carrying phonons such as nanoscale precipitates or grain boundaries and other interfaces have been employed for optimizing thermoelectric materials this way.⁷ Even a hierarchical design of the nano- and microstructure was developed to effectively scatter the broad spectrum of phonon wavelengths, which led to record-high zT values.⁸

Our general interest in thermoelectric materials prompted us to investigate the synthesis of binary (Sb₂Te₃, Bi₂Te₃) and ternary ([Sb_xBi_1−x]₂Te₃) materials both in solution⁹ and via gas phase based processes such as atomic layer deposition (ALD)¹⁰ and metal organic chemical vapour deposition (MOCVD)¹¹ using single-source and dual-source precursor approaches. The microwave-assisted decomposition of the single source precursor (Et₂Sb)₂Te 1 in an ionic liquid (IL) had been shown to produce highly stoichiometric Sb₂Te₃ nanoparticles,^12a while Bi₂Se₃, Bi₂Te₃ and (Sb_xBi_1−x)₂Te₃ nanoparticles were synthesized by using specific reactive ILs.^12b,c The Sb₂Te₃ nanoparticles showed exceptionally high figures of merit of up to 1.5 at 300 °C, without the need of alloying or electronic doping. This new synthetic strategy allowed an effective decoupling of electronic and phononic transport properties.^12a In our studies we made the observation that the Sb₂Te₃ particle morphology changed depending on the chemical identity of the ionic liquids, which prompted us to study their influence on the microwave-assisted decomposition of 1 in more detail and look for correlations with the thermal and electronic transport properties in the obtained material.

We herein report on our systematic study on the decomposition of 1 in different ILs, in which both the anion and the cation were systematically varied, using microwave-assisted techniques. In addition, the results from detailed transport measurement of the resulting Sb₂Te₃ nanoparticles are reported that allow for a structure–property analysis.

Results and discussion

We have recently developed a synthetic protocol that enabled us to access Sb₂Te₃ nanomaterials with a record-figure of merit by the decomposition of 1 in the ionic liquid C₄mimBr (C₄mim = 1-butyl-3-methylimidazolium) under microwave (MW) irradiation.^12a As the IL acted in this reaction not only as the solvent but also as the heat transfer medium, we herein study the specific role of the IL as the nanotemplating agent by investigating a set of ILs based on 1,3-dialkylimidazolium cations. Starting from the most prominent IL cation, 1-butyl-3-methylimidazolium (C₄mim⁺), first the counter anion was varied from Cl⁻, Br⁻, I⁻ to NTf₂⁻ (NTf₂⁻ = bis(trifluoromethanesulfonyl)amide). Variation of the IL anion not only leads to a change in fundamental physical properties of the IL such as the melting point or viscosity but also its solvation properties such as polarity. Moreover, the chosen anions range from relatively strongly coordinating (Cl⁻) to quite weakly coordinating (NTf₂⁻) anions. In the context of nanomaterial synthesis the capabilities of the IL ions to interact with the as-formed nuclei and crystal seeds is especially important as this allows for the morphology¹³ and even the phase control¹⁴ of nanomaterials. The Lewis basicity of these ILs decreased in the order of Cl⁻, Br⁻, I⁻ to NTf₂⁻.¹⁵ Similarly, variations of the cation influence the overall IL properties. Generally an increase of the melting point with increasing chain length of the alkyl group is observed for imidazolium based ILs. Symmetrically substituted imidazolium ILs typically exhibit higher melting points than asymmetrical ILs.¹⁶ Again, in the context of tuning the nanostructure of a material through the templating effect of the IL, the interaction of the IL cation with the nanomaterial needs to be considered. Imidazolium cations can interact not only electrostatically, but, as they bear acidic protons (the 2H proton of the imidazolium ring is especially acidic) and an aromatic π-system, can also undergo secondary bonding interactions such as hydrogen bonding and π-bonding. This has been found especially important in the synthesis of nanosized oxide materials.¹⁷ However, the cation size can critically influence these bonding capabilities.¹⁸ For this reason, the alkyl-chain of the C₄ × C₁mim⁺ imidazolium cation was varied from three to eight carbon atoms.

In addition to the set of 1-methyl-n-alkylimidazolium bromides, the corresponding set of symmetrically substituted cations (C_nC_nmim⁺) with n = 4, 6 and 8 were explored. Ionic liquids are known to be highly structured solvents,¹⁹ which can impact nanoparticle formation critically.²⁰ In particular, for imidazolium cations with longer alkyl chains a highly ordered structure of the IL can be expected,²¹i.e. imidazolium-based ILs with more than eleven carbon atoms in the side chain tend to form thermotropic liquid crystalline phases. The use of ordered phases as the template in nanoparticle synthesis has already been reported.²²

To obtain Sb₂Te₃ nanoparticles from various ionic liquids, in a typical reaction, 1 was added to the respective IL at 90 °C and stirred for 5 min until a homogeneous dispersion or solution was formed, which was then heated in a laboratory microwave oven first for 30 s at 100 °C, then for 5 s at 150 °C and finally for 5 min at 170 °C. The resulting colloidal solution was centrifuged (2000 rpm), washed with 10 mL of acetonitrile (7×) to completely remove the by-product SbEt₃ (Scheme 1) and dried at ambient temperature under reduced pressure. Black precipitates were obtained, which were characterized by powder X-ray diffraction (PXRD), energy dispersive X-ray analysis (EDX), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS).

Scheme 1 Synthesis of Sb₂Te₃ nanoparticles by microwave-assisted decomposition of the single source precursor 1.

General product characterization

PXRD measurements confirmed the formation of phase-pure Sb₂Te₃ in all ILs (see Fig. 1 for a representative PXRD pattern). All observed diffraction peaks can be indexed to the database pattern of Sb₂Te₃ (JCPS file 015874) and the lattice parameters were refined to a = 4.266(9) Å and c = 30.456(6) Å.

Fig. 1 Representative powder X-ray diffraction pattern of Sb₂Te₃ nanoparticles (with Cu Kα radiation) including Rietveld refinements.

A small texture effect was observed since the intensity of the 1010 reflex was somewhat smaller compared to the reference. Our samples show an intensity ratio of the 015 (28.3°) : 1010 (38.5°) : 110 (42.5°) reflex of 1 : 0.26 : 0.35, whereas this ratio in the reference was 1 : 0.35 : 0.33. A size determination of the nanoparticles typically yielded sizes of >300 nm, but these values should be taken with care due to their plate-like structure (see Fig. 4 and 5).

EDX analysis confirmed within standard deviations the stoichiometric composition of the products. In addition, no signals originating from the IL or contaminations, i.e. oxidation or hydrolysis products, were detected. These results were confirmed by infrared (IR) spectroscopy, showing no absorption band of the respective ILs on the particle surface. In contrast, the nanoparticles were shown to be partially oxidized at the surface by XPS, which is a much more surface sensitive analytical method compared to EDX and IR. Fig. 2 exemplarily shows the XPS spectra of Sb₂Te₃ nanoparticles prepared in C4mimI, while Fig. 3 displays XPS spectra of a sample obtained in C₄mimNTf₂.

Fig. 2 Representative XPS spectra of Sb₂Te₃ nanoparticles synthesized in C₄mimI.

Fig. 3 Representative XPS spectra of Sb₂Te₃ nanoparticles synthesized in C₄mimNTf₂.

The XPS spectra of the sample prepared in C₄mimI (Fig. 2) and C₄mimNTf₂ (Te and Sb spectra at the top right and bottom right in Fig. 3) clearly showed that both Sb and Te are partially oxidized, as is clearly visible from the metal oxide peaks at 530.1 eV binding energy for the Sb 3d_5/2 and at 575.9 eV for the Te 3d_5/2 peaks. These findings are in good agreement with the literature values.^12b,c,23,24 However, while only around 3% of the Te are present as an oxide in the case of Sb roughly 40% (prepared in C₄mimI) to 60% (prepared in C₄mimNTf₂) of the Sb is oxidized. The ratio of elemental Sb to elemental Te gives exactly the expected ratio of 2 : 3. This means that there is an excess of Sb at the surface and that this Sb is present as an oxide. Comparable surface oxidation reactions have been very recently observed for binary and ternary bismuth chalcogenide nanoparticles, in which Bi₂Te₃ and Bi₂Te₂Se were found to easily oxidize upon exposure to air while Bi₂Se₃ was significantly more stable toward oxidation.^12b,c,31 In addition, Sb₂Te₃ thin films were found to be easily oxidized after exposure to atmosphere and a post-deposition treatment was therefore suggested by the authors as an effective method to promote the formation of the Sb–Te bond and prevent oxidation of the thin film surface. As a consequence, the nanoparticles have to be stored and handled under inert gas conditions to avoid surface oxidation reactions. In addition, N, S, F, C and O (Fig. 3) are also found on the surface, which can be attributed to the residues of the ionic liquid (C₄mimNTf₂) and the washing solvent (CH₃CN), which can also coordinate to the nanoparticle surface.

Morphology of Sb₂Te₃nanoparticles synthesised in different ILs.
Role of the anion (An) of 1-alkyl-3-methyl-imidazolium based ionic liquids C₄mimAn. The role of the anion (Cl⁻, Br⁻, I⁻, NTf₂⁻) of the 1-butyl-3-methyl-imidazolium based ionic liquid in tuning the composition and the morphology of the resulting nanoparticles was investigated by SEM. All four samples show the formation of hexagonally shaped Sb₂Te₃ nanoplates with diameters ranging between 300–2000 nm and varying in thickness between 65–120 nm (Fig. 4).

Fig. 4 SEM images of Sb₂Te₃ nanoparticles synthesized in various ionic liquids. The nanoparticles in (A) were synthesized in C₄mimCl, (B) in C₄mimBr, (C) in C₄mimI and (D) in C₄mimNTf₂.

These platelets form larger agglomerates. Both the dimensions of the nanoplates and the type of agglomeration are strongly influenced by the IL anion. The thickness of the individual hexagonal platelets increased while changing the IL anion from Cl⁻, Br⁻, I⁻ to NTf₂⁻. Also, the association of these platelets changed from individual sandrose-type spherical aggregates over more aggregated spheres of platelets to less spherical, less extended aggregates. This observation could be correlated with the coordination ability of the IL anion. Chloride is a strongly Lewis basic, coordinating anion whereas the NTf₂ anion has a weak coordination ability. Thus, ionic liquids with rather strongly coordinating anions force the formation of thinner platelets as the vertical particle growth is hindered through the interaction of the IL anion with the particle surface. An IL with a less coordinating anion not only hinders the particle growth less, resulting in the formation of thicker platelets, but also stabilizes the particles less against further agglomeration and, in consequence, larger agglomerates are found in C₄mimNTf₂.

Influence of the alkyl chain length of asymmetrical 1-n-alkyl-3-methylimidazolium bromide ionic liquids C_nmimBr (n = 3–8). To investigate the influence of the IL cation on the morphology of the Sb₂Te₃ nanoparticles, a set of 1-n-alkyl-3-methylimidazolium bromides was synthesized and explored as the reaction medium in the synthesis of Sb₂Te₃ nanoparticles through a microwave reaction.

The chain length of the 1-n-alkyl-3-methylimidazolium cation was systematically varied from three to eight carbon atoms. Bromide was chosen in these experiments as the anion in order to be comparable with the results of previous studies.^12a In C₃mimBr exclusively isolated spherical aggregates of small platelets with diameters of 2–5 μm were formed. By increasing the side chain length of the alkyl group of the 1-n-alkyl-3-methylimidazolium cation, the size and number of these aggregates shrink. At the same time individual larger hexagonal plates of Sb₂Te₃ are formed, which have a smaller tendency to aggregate. When C₈mimBr is used in the synthesis, almost exclusively hexagonal plates are observed (Fig. 5).

Fig. 5 SEM images of Sb₂Te₃ nanoparticles synthesized in 1-n-alkyl-3-methylimidazolium bromide C_nmimBr with (a) n = 3, (b) n = 4, (c) n = 5, (d) n = 6, (e) n = 7, and (f) n = 8.

It is obvious that the IL cation has a strong influence on the nanostructure of the obtained material and two factors appear to be important: solubility of the precursor in the IL and structural order of the IL. The solubility of the precursor increases with increasing alkyl-chain length of the cation, which can be correlated to the decreasing polarity of the IL. Whilst in ILs with short alkyl chains such as C₃mimBr and C₄mimBr only dispersions of (Et₂Sb)₂Te in the IL were obtained, a full solubility of the precursor was observed for C₈mimBr. As a consequence, the tendency of the formation of inhomogeneously distributed micro-drops of 1 in the IL increases with decreasing alkyl chain length of the IL, which obviously facilitates the formation of ball-like agglomerates upon thermolysis. In contrast, thermolysis of a homogeneous solution of 1 in the IL containing long alkyl chains leads to a steady growth of the Sb₂Te₃ nanoparticles, which consequently form large sheets. In addition, it is known for 1-alkyl-3-methylimidazolium bromides that an increasing alkyl chain length of the cation leads to an increasing structural order, which may lead to the formation of lamellar, smectic liquid crystalline structures which could act as a template.^21b,c Therefore, a set of symmetrically substituted 1-n-alkyl-3-n-alkylimidazolim bromides was tested as the reaction medium.

Influence of the alkyl chain length of symmetrical 1,3-n-alkylimidazolium bromide ionic liquids C_nC_nimBr. The synthesis of Sb₂Te₃ from 1 in C_nC_nimBr with n = 4, 6 and 8 yielded in all cases a phase pure material. However, while carrying out the synthesis in C₄C₄imBr, only a dispersion of the single source precursor was obtained, whilst in C₆C₆imBr and C₈C₈imBr homogeneous solutions were formed (Fig. 6). The nanostructures of the material obtained from the different ILs show distinct differences. The trend in the change of the morphology, however, is similar to the observations made for asymmetrical imidazolium bromides.

Fig. 6 Pictures of neat C₄C₄imBr (A), C₄C₄imBr + 1 (B), neat C₈C₈imBr (C) and C₈C₈imBr + 1 (D).

The nanoparticles synthesized in C₄C₄imBr (Fig. 7A) consist of strongly agglomerated Sb₂Te₃ nanoplates. Predominately sandrose-like structures with sizes between 1 and 4 μm are formed by the aggregation of individual Sb₂Te₃ particles, whose diameters range from 300 to 1200 nm. The diameter of the individual Sb₂Te₃ platelets was found to increase with increasing alkyl-chain lengths of the IL cation. Individual nanoplates with diameters between 300 and 1500 nm were found in C₆C₆imBr (Fig. 7B), while those obtained from C₈C₈imBr (Fig. 7C) range from 300 to 2500 nm. In addition, the SEM images of the resulting nanoparticles clearly prove a decreasing agglomeration tendency of the hexagonal Sb₂Te₃ nanoplates with increasing chain length and hence increasing steric demand and coordination strength of the IL as were observed for the Sb₂Te₃ nanoparticles obtained from unsymmetrical ILs (see Fig. 5). While compact ball-like agglomerates were formed with C₄C₄imBr, the nanoparticles obtained in C₆C₆imBr show loosely agglomerated card structures, and nanoparticles synthesized in C₈C₈imBr consist of single Sb₂Te₃ sheets and to some extent slightly crooked card structures (Fig. 7). With increasing alkyl chain length of the cation, the tendency of the formation of sandrose-like structures decreases. Instead, 3D agglomeration increases until finally in C₈C₈imBr predominately large extended plates are formed. This confirms that an interplay of the precursor solubility and microstructure and the coordination ability of the IL strongly influence the microstructure formation.

Fig. 7 SEM images of Sb₂Te₃ nanoparticles synthesized in C₄C₄imBr (A), C₆C₆imBr (B) and C₈C₈imBr (C).

Whenever the single source precursor 1 has poor solubility in the IL, sandrose-like aggregates are formed. This potentially occurs due to the formation of micro-droplets, which can act as individual micro-reaction compartments. In contrast, thermolysis of homogeneously dissolved solutions of 1 in ILs of higher hydrophobicity, which increases with increasing alkyl chain length, leads to a steady growth of the Sb₂Te₃ nanoparticles. Finally the microstructure of the IL can help to guide the particle growth. C₈C₈imBr prefers the formation of a lamellar structure and thus favours the sheet-like growth of Sb₂Te₃ nanoplates.

Thermoelectric transport properties. To investigate how the nanostructure of the obtained Sb₂Te₃ material is correlated to the thermoelectric transport properties these samples were investigated in detail. For the characterization of thermoelectric transport properties, the Sb₂Te₃ nanoparticles were cold pressed to macroscopic pellets and subsequently annealed at 300 °C. After the determination of the thermoelectric transport properties, we re-investigated the material composition by EDX and XRD. According to these results we can exclude any change of the material composition as well as the formation of any additional crystalline phase during the processing process. Fig. 8 exemplarily shows two powder X-ray diffractograms of a Sb₂Te₃ sample before and after processing.

Fig. 8 PXRDs of a Sb₂Te₃ sample before and after processing (with Mo Kα radiation).

Variation of the different alkyl-chain lengths of symmetric imidazolium-based ILs C_nC_nimBr. Since the influence of the alkyl chain lengths was observed for both the unsymmetrically and symmetrically substituted imidazolium derivatives, detailed transport characterization was performed with the nanoparticles obtained from the symmetrically substituted ILs.

Fig. 8 shows the cross-section SEM images of the three pellets as-obtained from samples synthesized in C₄C₄imBr (C₄) (Fig. 8A), C₆C₆imBr (C₆) (Fig. 8B) and C₈C₈imBr (C₈) (Fig. 8C), respectively. Distinct differences between the characteristic microstructure of the three samples after the cold pressing compaction can be seen, which can be directly correlated to the morphology of the Sb₂Te₃ nanoparticles from the IL.

In C₄C₄imBr the formation of sandroses (Fig. 7A) prevailed and this microstructure is maintained in the cold pressed samples where individual spheres can be made out (Fig. 9A). In C₆C₆imBr random three dimensional aggregations of these particles occurred (Fig. 7B) and this also shows in the compacted sample (Fig. 9B). In C₈C₈imBr the formation of large, extended nanosheets took place (Fig. 7C) and the SEM image of the cross section pellets shows still individual sheets that are stacked parallel (Fig. 9C). The microstructure evoked by the individual particle morphology and aggregation impacts directly the densities of the compacted samples. The density of the samples is 5.3 g cm⁻³ (82%) for C₄C₄imBr, 5.7 g cm⁻³ (86%) for C₆C₆imBr, and 4.9 g cm⁻³ (75%) for C₈C₈imBr, respectively.

Fig. 9 Cross section SEM images of the three cold pressed Sb₂Te₃ bulk samples synthesized in the ionic liquids C₄C₄imBr (C₄) (A), C₆C₆imBr (C₆) (B) and C₈C₈imBr (C₈) (C); insets: SEM micrographs for the respective non-pressed samples.

Fig. 10 shows the thermoelectric transport properties of the three samples between room temperature and 573 K. Table 1 summarizes the thermoelectric transport data of these three pellets at room temperature. The Seebeck coefficients range from 140 μV K⁻¹ to 180 μV K⁻¹. The decomposition of 1 was shown to produce Sb₂Te₃ nanoparticles with a highly stoichiometric composition and low anti-site defect concentration, resulting in high values of the Seebeck coefficient as observed in our previous study.^12a This is observed here, too.

Fig. 10 Thermoelectric transport properties of three Sb₂Te₃ bulk samples synthesized in C₄C₄imBr, C₆C₆imBr and C₈C₈imBr.

Table 1 Density, rel. density, Hall carrier concentration n_H, Hall mobility μ_H overall and in the electrically active volume, electrical conductivity σ, thermal conductivity κ, lattice thermal conductivity κ_L, and Seebeck coefficient α at 300 K

Sample	C4	C6	C8
Ionic liquid	C4C4imBr	C6C6imBr	C8C8imBr
Density [g cm^−3]	5.3	5.7	4.9
Relative density [%]	82	86	75
n H [×10^19 cm^−3] – overall	5.5	4.9	3.1
μ H [cm^2 V^−1 s^−1] – overall	53	35	29
– In electrically active volume	64	41	39
σ [S cm ^−1]	463	272	145
κ [W m^−1 K^−1]	0.72	0.71	0.49
κ L [W m^−1 K^−1]	0.44	0.59	0.39
α [μV K^−1]	145	131	147

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The Hall carrier concentrations n_H are 5.5 × 10¹⁹ cm⁻³ (C₄C₄imBr), 4.9 × 10¹⁹ cm⁻³ (C₆C₆imBr) and 3.1 × 10¹⁹ cm⁻³ (C₈C₈imBr). The samples show a high variation in the electrical conductivity σ. The highest σ of 463 S cm⁻¹ at room temperature was observed for the sample synthesized in C₄C₄imBr and is smaller for the samples prepared in C₆C₆imBr (272 S cm⁻¹) and C₈C₈imBr (145 S cm⁻¹).

From the electrical conductivity and the Hall carrier concentration, we obtained the Hall mobility of the charge carriers, μ_H, which was corrected for the electrically active volume of the material (Fig. 11).

Fig. 11 Mobility in the electrically active volume plotted as a function of temperature for samples obtained from the respective ionic liquids.

For this, the value was normalized to the relative density of the samples.²⁵ With this correction for the density, the Hall mobilities μ_H of 64 cm² V⁻¹ s⁻¹ (C₄C₄imBr), 41 cm² V⁻¹ s⁻¹ (C₆C₆imBr) and 39 cm² V⁻¹ s⁻¹ (C₈C₈imBr) were found. There is no evident trend of the Hall mobility and the electrical conductivity with respect to the varying densities of the three samples, instead the mobility decreases with increasing chain length. Due to the nanostructure of the samples, the thermal conductivity could be reduced from 5.6 W m⁻¹ K⁻¹ parallel ∥ and 1.6 W m⁻¹ K⁻¹ ⊥ perpendicular to the c-direction²⁶ for a single crystalline Sb₂Te₃ in the range of 0.49–0.72 W m⁻¹ K⁻¹, comparable with values previously reported by Mehta et al. for Sb₂Te₃ nanoparticles.²⁷ At 490 K the thermal conductivity exhibits a minimum in all samples and increases at higher temperatures. To calculate the lattice thermal conductivity, we subtracted the electronic contribution κ_e from the total thermal conductivity, estimated from the Wiedemann–Franz dependence, κ_e = σLT. For this, the literature value of the Lorenz number (L = 1.72 × 10⁻⁸ W Ω K⁻² Single Parabolic Band Model)²⁸ was used considering a temperature independent L.

Fig. 12 clearly shows that the lattice thermal conductivity still increases. This is most likely caused by the bipolar effect known to appear in this temperature range for semiconductors with a small band gap (Sb₂Te₃: band gap E_g = 0.28 eV²⁹): at a certain temperature electron–hole-pairs are generated and an additional contribution for the thermal conductivity κ from the bipolar thermal conductivity κ_b is given. While the thermal conductivity data points towards a contribution of the bipolar effect, in principal this effect should also influence the other transport coefficients, i.e. decrease the Seebeck coefficient and increase the electrical conductivity due to minority carriers, which is not seen here.

Fig. 12 Lattice thermal conductivity in dependence of the temperature.

The most promising property combination of the transport properties is found for samples synthesized in C₄C₄imBr, which exhibited the highest charge carrier concentration, the highest charge carrier mobility and the lowest lattice thermal conductivity. The figure of merit zT reaches a maximum value of 0.72 at 550 K for the C₄C₄imBr sample (Fig. 10).

From this we conclude that the formation of individual sandrose nanostructures of Sb₂Te₃ that can be maintained in the compacted samples, gives the best combination of properties leading to high zT values. Thus, short chain length IL cations are beneficial for this. To check this hypothesis, the thermoelectric transport properties of samples obtained from ILs with short chain imidazolium cations (C₄mim) in combination with various anions that gave sandrose-like nanostructures are investigated.

Role of the anion of 1-butyl-3-methyl-imidazolium based ILs in thermoelectric properties. In order to investigate the role of the anion in the thermoelectric properties of the resulting Sb₂Te₃ nanoparticles in more detail, four Sb₂Te₃ samples were prepared under analogous reaction conditions in C₄mimCl (A), C₄mimBr (B), C₄mimI (C) and C₄mimNTf₂ (D), respectively, and then compacted to Sb₂Te₃ pellets using the same protocol. Cross section pictures (Fig. 13) of the resulting cold pressed pellets clearly demonstrate that the agglomerate structure as observed in the SEM pictures of the nanoparticles is preserved within the microstructure of the pellets (Fig. 13A–D). Sb₂Te₃ synthesized in C₄mimNTf₂ shows only a few agglomerates in the microstructure (Fig. 13D), whilst for those observed from C₄mimCl shows that those rose-like structures are still preserved.

Fig. 13 Cross section SEM images of four cold pressed Sb₂Te₃ bulk samples synthesized in C₄mimCl (A), C₄mimBr (B), C₄mimI (C) and C₄mimNTf₂ (D).

Table 2 summarizes the thermoelectric transport properties for the four samples synthesized in C₄mimCl, C₄mimBr, C₄mimI and C₄mimNTf₂ at 300 K. The density of the compressed pellets are 5.1 g cm⁻³ (79%, C₄mimCl), 5.2 g cm⁻³ (80%, C₄mimBr), 5.5 g cm⁻³ (85%, C₄mimI) and 5.3 g cm⁻³ (82%, C₄mimNTf₂), respectively.

Table 2 Density, rel. density, Hall carrier concentration n_H and the Hall mobility μ_H overall and in the electrically active volume, electrical conductivity σ, thermal conductivity κ, lattice thermal conductivity κ_L, and Seebeck coefficient α at 300 K

Sample	Cl	Br	I	NTf^2
Ionic liquid	C4mimCl	C4mimBr	C4mimI	C4mimNTf2
Density [g cm^−3]	5.1	5.2	5.5	5.3
Relative density [%]	79	80	85	82
n H [×10^19 cm^−3] – overall	9.7	3.8	5.7	7.1
μ H [cm^2 V^−1 s^−1] – overall	19	43	95	35
– In electrically active volume	24	54	112	42
σ [S cm^−1]	291	264	869	393
κ [W m^−1 K^−1]	0.56	0.72	0.89	1.1
α [μV K^−1]	150	133	135	150

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In Fig. 14 the thermoelectric transport properties are presented. The Seebeck coefficient for all samples ranges from 130 to 170 μV K⁻¹ in the temperature range between room temperature and 573 K, which is comparable to the values of the samples discussed before. The highest electrical conductivity of 870 S cm⁻¹ at room temperature was found for the sample synthesized in C₄mimI, whereas that prepared in C₄mimNTf₂ (397 S cm⁻¹), C₄mimCl (293 S cm⁻¹) and C₄mimBr (264 S cm⁻¹) showed significantly lower values. The thermal conductivity is 1.1 W m⁻¹ K⁻¹ for the sample obtained from C₄mimNTf₂, 0.89 W m⁻¹ K⁻¹ for that from C₄mimI, 0.72 W m⁻¹ K⁻¹ for that from C₄mimBr and 0.56 W m⁻¹ K⁻¹ for that from C₄mimCl. The electrical and thermal conductivities show a dependence on the density of the samples. The highest values for σ and κ are measured for the samples with densities of 85% (C₄mimI) and 82% (C₄mimNTf₂) and are smaller for the Sb₂Te₃ pellets with 80% (C₄mimBr) and 79% (C₄mimCl). The highest zT value of 0.93 at 260 °C is reached for C₄mimI, and for the other samples the zT values are between 0.35 (C₄mimBr) and 0.44 (C₄mimNTf₂, C₄mimCl).

Fig. 14 Thermoelectric transport properties of four Sb₂Te₃ bulk samples synthesized in C₄mimBr, C₄mimCl, C₄mimI, and C₄mimNTf₂.

By correlating the thermal transport data with the particle morphologies it is evident that the concept of controlling the thermal conductivity through phonon phase boundary scattering by a nanotemplating effect of the IL has been successful: in the case where small, individual nano-sandroses could be obtained by using an ionic liquid of high polarity with a strongly coordinating anion, the compacted material exhibited the lowest thermal conductivity.

The electrical Hall mobility shows a clear trend for the samples synthesized in the ionic liquids C₄mimCl, C₄mimBr, and C₄mimI, with increasing μ_H from 24 cm² V⁻¹ s⁻¹ to 112 cm² V⁻¹ s⁻¹. This correlates with the trend found in the morphology of the respective nanoparticles which show an increasing thickness of the nanoparticle platelets with increasing atomic number of the halide anion (compare Fig. 4). It is assumed that the nanoparticle platelets orient – at least partly – during the compaction process perpendicular to the pressing direction. All transport properties are characterized in the pressing direction of the pellets. Therefore, with increasing thickness of the platelets, there are less scattering events for both, electrons and phonons, and consequently the highest values for the electrical Hall mobility and also the thermal conductivity are reached. However, looking at the ionic radii of the IL anions used for the synthesis, it becomes clear that the ionic radius of an I⁻ ion (220 pm) is very similar to that of a Te²⁻ ion (221 pm).³⁰ Thus, it appears possible that small amounts of I⁻ can replace Te²⁻ in the structure of Sb₂Te₃, which could also influence the electronic transport properties. More theoretical and experimental evidence will be needed to further substantiate this hypothesis.

Comparison of zT values. Within the following paragraph we compare our results with the literature state of the art. Table 3 shows the zT of nanostructured Sb₂Te₃ samples for different synthesis routes.

Table 3 Compilation of the Sb₂Te₃ figure of merit data at 400 K, comparing the data with literature references

Synthesis method	zT (300 K)	zT (400 K)	Ref.
a This paper. b EG = ethylene glycol. c SSP = single source precursor. The marked values (*) were interpolated from the literature data for 300 K for comparison.
Thermal decomposition of SSP in the IL C4mimI	0.59	0.65
Commercial alloys, mostly based on Sb2Te3	1	0.8	Snyder (2008)^31
Solvothermal (MW-assisted); SbCl3 + Na2TeO3 + N2H4 hydrate in EG^b	0.15	0.46	Dong (2010)^32
S-doped Sb2Te3 NPs	0.7	0.92	Mehta (2012)^27
Solvothermal, template-free	0.22*	0.35	Sun (2013)^33
Co-precipitation followed by wet Chemical reduction	0.03*	0.12	Saleemi (2014)^34
Thermal decomposition of SSP^c in IL			Heimann (2015)^12a
(a) Thermal (oil bad)	0.4	0.5
(b) MW-assisted	0.75	1.1
Solvothermal; SbCl3 + Te powder + alkaline 2,7-DHN in the presence of CTAB	0.001		Nithiyanantham (2015)^35
Physical vapor deposition (PVD); Sb2Te3−xSx platelets			Thankamma (2015)^36
-Without doping x = 0	0.22
-With the S-content x = 0.3	0.54
Solvothermal; Sb2Te3 wires; Sb + Te in EG	0.32*	0.44	Yang (2016)^37

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Within this comparison, Snyder and Toberer³¹ report the zT data by Marlow Industries that reach zT ≅ 0.8 at 400 K for Sb₂Te₃-based alloys (not further specified). By co-doping Sb₂Te₃ with sulphur, Mehta et al. demonstrate zT ≅ 0.92 at 400 K.²⁷ Phase pure Sb₂Te₃, without any alloying or co-coping, was investigated by Heimann et al. within an earlier work of this group.^12a Hereby, the microwave-assisted decomposition of the SSP (Et₂Sb)₂Te in ionic liquids enhanced the zT value from zT ≅ 0.5 to zT ≅ 1.1 at 400 K. Within this work we systematically studied the influence of different ILs and report zT values that match this range of literature values, see graphical compilation in Fig. 15.

Fig. 15 Compilation of the Sb₂Te₃ figure of merit data at 400 K, comparing the data with literature references. Heimann 2015 describes the thermal decomposition in an oil bad and Heimann 2015* in a laboratory microwave oven.

Conclusions

The morphology of Sb₂Te₃ nanoparticles synthesized in 1-alkyl-3-methylimidazolium- and 1,3-dialkylimidazolium-based ILs strongly depends on the chain length of the alkyl group of the IL cation (Fig. 16) and the Lewis basicity of the IL anion (Fig. 17).

Fig. 16 Morphology-directing effect of the IL cation.

Fig. 17 Morphology-directing effect of the IL anion.

An increasing chain length resulted in better solubility of the single source precursor (Et₂Sb)₂Te, which enhanced the formation of less aggregated nanoparticles. In addition, the role of the anion is mainly attributed to its basicity and its capability to bind to the growing nanoparticle surface. Stronger bases were found to more effectively block the surface, resulting in the formation of thin Sb₂Te₃ nanoplates, while the formation of thicker nanoparticles was observed with decreasing basicity. As a consequence, the thermoelectric properties of the resulting Sb₂Te₃ nanoplates strongly differed. Identification of the distinctive roles of the IL anion and cation may help to further improve the figure of merit for these types of materials in the near future.

Experimental

Synthetic procedures

Synthetic procedures and thermolysis experiments were performed under inert conditions (Ar atmosphere) in a glovebox or using standard Schlenk techniques. Solvents were dried over Na/K alloy and degassed prior to use. 1 was prepared according to a literature method.³⁸ 1-N-Methylimidazole (99%, Sigma Aldrich), 1-halobutanes (99%, Acros) and CH₃CN (99.9+%, Extra Dry, Acros) were commercially available. 1-Alkyl-3-methylimidazolium (C_nmim; n = 3–8) and 1,3-dialkyl-imidazolium-based ILs (C_nC_nim; n = 4, 6 and 8) were prepared by literature methods.^12,39

Materials and methods

Microwave synthesis of Sb₂Te₃ nanoparticles. 1.18 g (2.42 mmol) of 1 was added to 13.7 mmol of the respective ionic liquid. The room temperature solid ILs C₄mimCl, C_xmimBr (x = 3–5) and C_xC_xmimBr (x = 4, 6, 8) were molten by heating to 90 °C before adding 1. The reaction mixture was stirred for 5 min and heated in a laboratory microwave oven (Discover, CEM) for 30 s at 100 °C, subsequently for 5 s at 150 °C and finally for 5 min at 170 °C. The heating was performed with a maximum power of 100 W until the desired temperature was reached and kept at that temperature with a power of 5–12 W. The reaction container was cooled with compressed air with a pressure of 100 kPa. The resulting colloidal solutions were centrifuged (2000 rpm), washed with 10 mL of acetonitrile (7×) and dried at ambient temperature under dynamic vacuum.

Material characterization

Electron microscopy. The particle size and morphology as well as the elemental composition of the nanoparticles and of cross section samples of the Sb₂Te₃ pellets, which were prepared using a Jeol Cross Section Polisher (IB-09010CP), were analysed by scanning electron microscopy (SEM) using a Jeol JSM 6510 microscope equipped with a Bruker Quantax 400 unit (EDX).

Powder X-ray analysis. PXRD patterns were collected on powder filled Lindeman capillaries on a Huber 670 powder diffractometer with Mo Kα radiation (λ: 0.71073 nm, 40 kV and 40 mA) or a Bruker D8 Advance powder diffractometer with Cu Kα radiation (λ: 1.5418 Å, 40 kV and 40 mA) using a Si single crystal as a sample holder to minimize scattering. For better homogenization, the dried powder samples were re-dispersed in ethanol on the Si surface and investigated in the range from 10 to 90° 2θ with a step size of 0.01° 2θ (counting time 0.6 s). Rietveld refinements were performed with the program package TOPAS 4.2 (Bruker) to determine the lattice parameters and average crystallite size by using the Scherrer equation.²⁶ The structure model of Sb₂Te₃ (#192780) from the ICSD database was used. For each Rietveld refinement, the instrumental correction as determined with a standard powder sample LaB₆ from the NIST (National Institute of Standards and Technology) as the standard reference material (SRM 660b; a(LaB₆) = 4.15689 Å) was taken into account.

Photoelectron spectroscopy. The XPS measurements were done at a VersaProbe II by Ulvac-Phi. Monochromatic Al-Kα light with hν = 1486.6 eV was used and the electron emission angle was 45°. All spectra were referenced to the position of the main carbon peak at 284.8 eV binding energy. The Sb 3d signal was fitted by first fitting the 3d_3/2 peaks and constraining the position and intensity of the 3d_5/2 components to these values. This is done in order to estimate the O 1s signal which overlaps with the Sb 3d_5/2 peaks. The samples were transported under an inert gas atmosphere to the XPS machine and were exposed to air for roughly 3 minutes prior to their insertion into a vacuum.

Thermoelectric properties. The nanocrystalline Sb₂Te₃ powder was compressed to a pellet with a diameter of 5 mm applying a pressure of 815 MPa for 30 min. A pressing tool from Atlas Power 25T from SPECAC made from stainless steel was used. The density of the pellets was determined from the mass to volume ratio. Annealing was performed at 573 K in vacuum (10⁻⁵ mbar) with a ramp of 5 K min⁻¹ and a dwell time of one hour. All thermoelectric coefficients were measured in the z-direction corresponding to the pressing direction of the pellet in a temperature range from room temperature to 573 K. The Seebeck coefficient α and the electrical conductivity σ were measured by using a commercial device ZEM-3 provided by Ulvac Technologies, Inc. The thermal conductivity κ is calculated with κ = D_Tρc_p with D_T the thermal diffusivity, ρ the density and c_p the heat capacity. The thermal diffusivity was measured with an LFA 457 Microflash from NETZSCH-Gerätebau GmbH. For the calculation a literature value for the heat capacity was used.⁴⁰ Hall measurements were done at room temperature in the van-der-Pauw geometry with a Physical Property Measurement System (PPMS, Dyna Cool Series) provided by Quantum Design, Inc. From the measured Hall coefficient R_H, the Hall carrier concentration n_H is estimated, which is assumed to be isotropic and temperature independent. The Hall mobility was derived from the dependence σ = eμ_Hn_H.

Acknowledgements

S. Schulz and G. Schierning gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft DFG within the priority program SPP 1708. The authors like to thank M. Sc. Georg Bendt (Faculty of Chemistry, University of Duisburg-Essen) for Rietveld refinements of selected samples.

Notes and references

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