4.3.1.3 Group 15 Chalcogenides

Group 15/16 chalcogenides of the general type M₂E₃ (M = Sb, Bi; E = S, Se, Te) have a long standing history in material sciences. Sb₂S₃ is of particular interest for technical applications in sensitized solar cells (SSCs) due to its attractive bandgap energy (∼1.7 eV) and high extinction coefficient (1.8 × 105 cm-1) in the visible region, allowing the fabrication of heterojunction solar cells with over 5% of power conversion efficiency under 1 sun = 1 kWm-2 illumination,[1] while Sb₂Se₃ is a semiconductor with ~1 eV bandgap,[2] which exhibits a high Seebeck coefficient[3] with a fast amorphous crystalline transition,[4] rendering it very attractive for technical applications in photovoltaic,[5] phase change memory[6] and in thermoelectric (TE) devices.[7] The thermoelectric properties of Sb₂Te₃ are even better compared to Sb₂Se₃, rendering this material a very promising candidate for near room-temperature TE applications.[8] As a consequence, bulk Sb₂Te₃ finds technical applications in minipower-generation systems and microcoolers, charge coupled device (CCD) technology, and infrared detectors.[9]

Sb₂E₃ (E = S, Se, Te) nanoparticles are typically prepared by use of polyol processes, solvothermal routes, and gas-induced reduction routes, respectively. In addition, thermal decomposition of metal organic precursors have been demonstrated to be promising synthetic pathways for the low-tempertaure synthesis of these materials. Recently, the promising potential of so-called single-source precursors for the synthesis of nanostructured M₂E₃ materials was demonstrated.[10] For instance, Kim et al. synthesized Sb₂Te₃ nanoplates by thermal decomposition of Ph₂SbTeR (R = Et, Ph) at 300 °C.[11]

We reported on the wet-chemical synthesis of Sb₂S₃ and Sb₂Se₃ nanowires at temperatures below 180 °C using single source precursors of the type (Et₂Sb)₂E and Et₃SbE (E = S, Se).[12] The influence of the solvent (DIPB, oleylamine) and the capping agent (PVP*) on the morphology, chemical composition and crystallinity of the resulting binary nanomaterials was demonstrated. Figures 1 and 2 show SEM and TEM pictures of Sb₂S₃ nanowires, while figure 3 shows an TEM picture of Sb₂Se₃ nanowires.

Simultaneous decomposition of two different precursors of the type (Et₂Sb)₂E (E = S, Se) yielded the ternary material phase Sb₂(S,Se)₃. The Sb₂(S,Se)₃ nanowires again grew along the <010> direction (d₀₁₀ = 0.39 nm) of the Sb₂(S,Se)₃ stibnite structure (Pnma). Fig. 4 shows a HRTEM image recorded at 250 keV of the Sb₂(S,Se)₃ nanowire, in which the growth direction is indicated by a double arrow-headed line. The lattice spacings perpendicular to the growth direction, as well as the FFT, that can be indexed as [101], prove the growth direction.

Thermal decomposition of the corresponding Te-containing compound [Et₂Sb)₂Te in non-coordinating solvents gave highly agglomerated particles in a controlled stoichiometric reaction (scheme 1), whereas the presence of capping agents such as PVP yielded isolated Sb₂Te₃ hexagons.[13]

In order to demonstrate the potential of the as-prepared nanostructures for thermoelectric application, several cold-pressed pellets were characterized with respect to their thermoelectric properties with and without additional temperature anneal. The Seebeck coefficient a showed high values between 145 μV/K and 170 μV/K, which do not have their origin in the size or shape of the nanoplatelets but rather originate from the specific defect chemistry of the material and are a result of a slightly under-stoichiometric tellurium concentration. The under-stoichiometric tellurium concentration results in the formation of Sb-antisite defects, doping the material. Single crystalline Sb₂Te₃ reference material obtained by travelling heater method has smaller values of the Seebeck coefficient than the ones measured in this study, in the range between around 80 μV/K to 90 μV/K. While the Seebeck coefficients already are in a very favorable range pointing towards ideal composition and defect chemistry of the nanoplates, the relatively low value of the specific electrical conductivity s demonstrates the need for further optimization of the compaction process. For the 'native' sample the electrical conductivity increases from 6 to 9 S/cm, for the annealed powder from 30 to 36 S/cm.

To date the most effective materials for thermoelectric applications near room temperature are bismuth-based materials such as Bi₂Te₃ and the solid ternary solutions (Sb_xBi_1-x)₂Te₃ due to their high effective masses, their high electrical conductivities and high Seebeck coefficients combined with glass-like low thermal conductivities. Nanosized Bi₂Te₃ particles were synthesized in different ways, i.e. by reducing bismuth acetate with oleylamine in dodecanthiole and further treatment with trioctyltellurophosphorane TOPTe, which is a widely used soluble Te-source, at 110 °C. The resulting rhombohedral bismuth-rich Bi₂Te₃-particles showed reduced thermal conductivity after SPS-treatment and a high power factor (5 µW∙K^-2∙cm^-1).[14] In addition, hexagonal und rod-shaped Bi₂Te₃ particles were obtained from the reaction of bismuth nitrate and TOPTe in octadecene and oleic acid and by reaction of BiCl₃ and TOPTe in thioglycolic acid in a microwave assisted synthesis.[15]

Even though TOPTe is a suitable Te-precursor for the solution-based synthesis of bismuth tellurides, alternative Te-precursors have been investigated. A general problem in the wet-chemical synthesis of Bi₂Te₃ nanoparticles is their strong tendency to form so-called antisite defects - the occupation of either Te sites by Bi atoms or Bi sites by Te atoms in the tetradymite lattice, resulting in the formation of either p- or n-doped materials - which strongly influences the carrier concentration and therefore the thermoelectric properties of the resulting material.[16] Moreover, the facile incorporation of excess bismuth into the tetradymite lattice leads to a series of sandwich-like structures of the general form (Bi₂)_n(Bi₂Te₃)_m, in which the quintuple layers are separated by Bi-bilayers as can be observed in tsumoite BiTe, pilsenite Bi₄Te₃ and hedleyite Bi₇Te₃, respectively.

We investigated both single source and dual source approaches for the synthesis of Bi₂Te₃ nanoparticles. (Et₂Bi)₂Te, which is readily accessible by insertion reaction of elemental Te into the weak Bi-Bi bond of tetraethyldibismuthine Et₄Bi₂ in n-pentane at low temperature,[16] was thermolyzed at 80 °C, yielding a black powder which was shown by EDX and XRD to be Bi₄Te₃.[17] All observed peaks in the PXRD pattern were indexed on the basis of rhombohedral Bi₄Te₃ (JCPDS 33-216) and the lattice parameters were refined to (a = 4.43(1) Å, c = 41.84(1) Å and V = 415.5(4) ų). The broad full width half maximum (FWHM) of the peaks indicate a small particle size of roughly 20 nm.

In contrast, thermolysis of in situ generated Et₂BiTeEt in DIPB or OA gave pseudo-cubic Bi₂Te particles, whose size increase with increasing reaction time. We therefore investigated the reaction of Bi(NMe₂)₃ with (Et₃Si)₂Te in different molar ratios.[17] Addition of Bi(NMe₂)₃ to a solution of Te(SiEt₃)₂ in DIPB at ambient temperature resulted in an immediate darkening, indicating a fast reaction. Monitoring the reaction in THF-d₈ by ¹H-NMR spectroscopy proved that the reaction proceeded with elimination of N,N-dimethylaminotriethylsilane Me₂NSiEt₃ according to the following mechanism:

The reaction was found to be strongkle dependant on the molar ratio of the starting reagents. As<a consequence, different bismuth telluride phases were obtained simply by changing the molar ratios. The resulting partickles were clearly identified by powder XRD.

The Bi₂Te₃ phase was of particular interest and therefore were further analysed by TEM, showing small pseudo-cubic particles with edge lengths of roughly 40 nm. The SAED pattern was in good agreement with the literature data and clearly shows the formation of phase pure Bi₂Te₃ as proven by comparison with calculated ED-pattern (ICSD-42546).

Figure 10 shows the HRTEM image of a Bi₂Te₃ nanoparticle in [110] zone axis orientation, thus perpendicular to the stacking of the five membered Bi₂Te₃ building units. Fourier transform of the whole image (Fig. 10b) indicates that the periodicity is perfect and matches perfect with the simulation of the electron diffraction for Bi₂Te₃ (Fig. 10c). A closer inspection of the thin areas of the sample shows periodic 1 nm stacks perpendicular to the c-axis of Bi₂Te₃. Each of the stacks has five rows of well resolved contrast maxima, which match the positions of the atoms of Bi₂Te₃ in this projection. This is illustrated by an overlay of a ball and stick plot of Bi₂Te₃ in the same orientation and proofs the synthesis of pure Bi₂Te₃ (Fig. 10a).

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