Unveiling the synthesis mechanisms of (Bi,Sb,Sn)–Te and observation of the n-type to p-type semiconducting transition in Sb–Bi₂Te₃ nanostructured chalcogenides derived via an aqueous-based reflux method

Abstract

A systematic study of a surfactant-assisted, aqueous-based, low-temperature chemical method for the synthesis of (Bi,Sb,Sn)–Te nanostructures was carried out. A detailed understanding of the chemistry involved in the reaction mechanism and decomposition of ethylenediaminetetraacetic acid was proposed for different precursors of Bi, Sb and Sn, and the possibilities for the aqueous-based, low-temperature synthesis of phase pure Bi₂Te₃, SnTe, and Sb₂Te₃ was demonstrated. The reaction mechanism revealed that the aqueous-based reflux reaction is suitable for the synthesis of Bi₂Te₃, unsuitable for the synthesis of SnTe, and suitable for the partial formation of Sb₂Te₃. Hot-pressed Sb-doped Bi₂Te₃ samples exhibited an n- to p-type semiconducting transition with variations in temperature and Sb dopant concentration. A significant improvement in Seebeck coefficient and power factor values of 215.12 μV K⁻¹ and 7.1 μW m⁻¹ K⁻², respectively, was observed at 465 K for the 5% Sb-doped Bi₂Te₃ nanostructures. This work focuses primarily on the reaction mechanism of Bi-, Sb- and Sn-based tellurides in an aqueous medium, providing further possibilities in the field of chalcogenide nanostructures for thermoelectric applications.

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1. Introduction

Increased energy demand since the last decade has boosted the interest in highly efficient materials for power generation and cooling applications. Thermoelectric (TE) materials are a class of materials that can reversibly convert heat energy into electrical energy. By analyzing the TE principles, the TE performance of materials can be efficiently optimized. A material with a high TE performance should have a good electrical conductivity (σ), high Seebeck coefficient (S) for electronic transport, appropriate carrier concentration and low thermal conductivity (κ) for maintaining the temperature difference. Tuning the carrier concentration and κ through various methods is an efficient way to adjust the TE properties of materials. A dimensionless quantity called the figure of merit (ZT) is a measure of the TE efficiency of any material, which is generally expressed using the following relation:¹

Specifically, the focus is on high-performance TE devices comprising n- and p-type components derived from high ZT materials. A series connection is maintained electrically and a parallel connection is maintained thermally on these TE devices or modules. From the above relation, it is clear that a good TE material should possess a “phonon-glass electron-crystal” (PGEC) behavior, resulting in a large S.² Interestingly, the researchers aim to win the conflict among the interdependent parameters S, σ and κ, and generally, it is an extremely tedious job to satisfy these criteria in any TE material possessing simple crystal structures. With these conflicting properties, any attempt to improve any of the S, σ or κ parameters may significantly deteriorate other properties and thereby the overall TE performance. By considering this key issue, it is interesting to observe that there is a possibility of getting a prominently increased value of power factor (S²σ) when the carrier concentration is nearly 10¹⁹, which is exhibited by the materials in the region where there is a crossover among metals and semiconductors.²

The most studied and interesting TE materials in the chalcogenide category are Bi₂Te₃, Sb₂Te₃ and SnTe, which have a wide temperature range for thermoelectric applications. Reports on the aqueous-based synthesis of chalcogenide nanostructures are limited.^1,2 Due to the inherent transport properties, a hexagonal crystal structure is preferred for the thermoelectric applications of Bi₂Te₃ and Sb₂Te₃.¹

Bi₂Te₃ and Sb₂Te₃ are narrow band gap semiconductors with homologous layered crystal structures having a rhombohedral unit cell with an R3̄m space group, possessing good σ and low κ and are used for power generation and in cooling devices.^3–5Fig. 1(a) shows the layered crystal structure of M₂Te₃ (M = Bi and Sb) and is described by a hexagonal unit cell, as shown in Fig. 1. The unit cell is composed of five covalently bonded monatomic sheets along the c-axis in the sequence –Te(1)–Bi–Te(2)–Bi–Te(1)–Te(1)–Bi–Te(2)–Bi–Te(1)–. Bi₂Te₃ is reported to be an n-type semiconductor, while Sb₂Te₃ is a p-type semiconductor. However, the presence of excess Te may change their type from n-type to p-type semiconductors. Sb₂Te₃ and its doped derivatives have been widely studied due to their excellent ZT in the temperature range of 300–500 K.^6–8 The self-generation of anti-site defects creates a large number of holes in bulk Sb₂Te₃ owing to a low S.⁶ As a consequence, the ZT values are limited to about 1 over the entire temperature range, which corresponds to low efficiencies of the TE device, limiting wider applications. Venkatasubramaniam et al.⁹ proposed a specially constructed Bi₂Te₃/Sb₂Te₃ superlattice with a ZT of 2.4 at room temperature. Recently, Yang et al.¹⁰ synthesized Sb₂Te₃ nanostructures and achieved a ZT of 0.37 at 473 K by employing a surfactant-assisted reflux method and utilizing cold compaction, followed by annealing. Several chemical methods have been employed to prepare nanostructures with different shapes. However, a single phase of Sb₂Te₃ could not be easily achieved due to the spontaneous occupation of partial Sb atoms at the Te lattice sites.¹¹ Wang et al.¹² synthesized hexagonal nanoplates of Sb₂Te₃ using a solvothermal approach. However, they showed a low ZT due to the Te impurity, which reduced the σ of the Sb₂Te₃ matrix. Hence, the majority of research work has been focused on the preparation of nanostructures with various techniques, while studies on the consolidation of Bi₂Te₃, Sb₂Te₃ and Bi_2−xSb_xTe₃ nanocrystals and their thermoelectric properties are relatively rare.

Fig. 1 Crystal structure of (a) Bi₂Te₃/Sb₂Te₃ and (b) SnTe.

SnTe is another narrow band gap semiconductor with a direct band gap of 0.18 eV. SnTe normally forms a p-type semiconductor due to Sn vacancies, and at low temperatures, it becomes a superconductor. SnTe exhibits a very low ZT ∼0.40 at 900 K¹³ due to its intrinsically high carrier concentration, large ΔE_L-Σ of 0.3 eV and high lattice thermal conductivity.^14,15 SnTe exists in 3 crystallographic phases. At low temperatures, where the hole concentration is less than 1.5 × 10²⁰ cm⁻³, SnTe exists in a rhombohedral structure and is known as α-SnTe. At room temperature and atmospheric pressure, SnTe crystallizes in the rock salt structure¹⁶ and SnTe is composed of a face-centered cubic lattice of Te atoms with Sn atoms filling all the octahedral voids and is known as β-SnTe. The β-SnTe transforms to γ-SnTe at 18 kbar pressure, with an orthorhombic structure and Pnma space group. The β-SnTe (referred to as SnTe) with cubic structure and Fm3̄m space group finds application as TE materials. The crystal structure of SnTe is shown in Fig. 1(b).

SnTe has received limited attention as a TE material due to its inability to control its very high carrier concentration, which is of the order of 10²¹ cm⁻³ at 300 K, resulting in a low S and high κ_e.¹⁷ A large concentration of intrinsic Sn vacancies is responsible for the high p-type carrier concentration in SnTe.¹⁸ In SnTe, the energy difference between the light hole valence band (L band) and the heavy hole valence band (Σ band) is 0.3 eV.^17,19–23 The large energy separation between the light and heavy hole valence bands restricts the contribution of heavy hole mass to the Seebeck coefficient. Recently, alloying SnTe with other metal tellurides, such as AgSbTe₂, MgTe, SrSe, and CdSe, was found to improve its TE performance.^24–28 A remarkable enhancement in the ZT was achieved due to the formation of resonance levels in the valence band through In doping in SnTe synthesized by a high energy ball milling and spark plasma sintering.²⁹ Alloying of SnTe with SnSe, which has a high band gap of 0.9 eV, helps reduce the κ value and also results in an improved S due to the convergence of the two valence bands of SnTe. Moreover, the κ value of SnTe can be further decreased by the solid solution alloying with SnSe. Alloying Cd or Hg in SnTe also results in an enhancement in S due to the decreased energy separation between the light and heavy hole valence band.^30,31 In the present investigation, we have synthesized the desired crystal structures of different chalcogenide materials such as Bi₂Te₃, Sb₂Te₃ and SnTe nanostructures using an aqueous-based low-temperature reflux method and investigated the feasibility of phase formation in an aqueous medium. The aqueous-based reflux method using NaBH₄ could successfully lead to the complete reduction of Te without the formation of TeO₂, which is highly desirable for the formation of phase pure compounds. Hence, we attempted to synthesize different chalcogenide materials to achieve a good control over the morphology. Further, the TE properties of Sb-doped Bi₂Te₃ nanostructures are also studied in the present work.

2. Experimental section

The Bi₂Te₃, Sb₂Te₃ and SnTe nanostructures were prepared using BiCl₃ (99.9%, metals basis, supplied by Alfa Aesar), Sb powder (99.999%, metals basis, supplied by Alfa Aesar), SbCl₃ (99.997%, metals basis, supplied by Alfa Aesar), SnCl₂ (99.99%, metals basis, supplied by Sigma Aldrich) and Te powder (99.99%, metals basis, supplied by Alfa Aesar) as metal ion precursors and ethylenediaminetetraacetic acid (EDTA) as the surfactant. Deionised water was used as the reaction medium, sodium borohydride (NaBH₄) as the reducing agent and the reaction was carried out at the boiling point of the medium. The detailed synthesis procedure is reported in our earlier studies.¹ After being stirred for 10 min, the solution was refluxed, which constitutes the primary stage of the reaction, where BiCl₃, SbCl₃ and SnCl₂ could ionize into the corresponding metal ions and chloride ions. EDTA is essential to cap the bismuth ions and to control the morphology over different reaction times. The reaction was carried out using a 4.4 M NaBH₄ and reaction time of 24 h to get stable and phase pure nanostructures based on our previous studies. In order to get the nanostructured Bi₂Te₃, a pH of 10 was maintained during the synthesis (using NaOH). As soon as the reaction was completed, the synthesized products were collected, centrifuged, washed with acetone, ethanol, and deionized water several times, and then dried under a vacuum environment at 373 K for 6 h. The further removal of EDTA in all the samples was ensured by heat treating at temperatures of 473, 533 and 623 K, respectively, for 3 h under inert conditions with intermediate grinding. The samples are labelled as BT, ST-1, ST-2 and TT for Bi₂Te₃, Sb₂Te₃ (using Sb powder), Sb₂Te₃ (using SbCl₃) and SnTe, respectively. The 1%, 3% and 5% Sb-doped Bi₂Te₃ samples are labelled as BS1T, BS3T and BS5T, respectively. The hot pressing of BS1T, BS3T and BS5T samples was carried out at 723 K and 60 Mpa, which are labelled as BS1T-HP, BS3T-HP and BS3T-HP, respectively. Their structures and phase purities were studied by powder X-ray diffraction (XRD) at room temperature using PANalytical X’Pert Pro diffractometer using Cu-Kα radiation and an X-ray wavelength of 1.5406 Å with a step size of 0.0167°. The particle size, morphology and crystallinity are characterized by high-resolution transmission electron microscopy and HR-TEM (FEI Tecnai F20, operated at 200 kV). To investigate the electrical properties, including electrical resistivity (ρ = 1/σ), S and the corresponding thermoelectric power factor S²σ, a rectangular block (dimensions 3 mm × 3 mm × 15 mm) was taken from the hot-pressed pellet and measured in the ZEM-3 (ULVAC-RIKO) system under helium atmosphere from room temperature to 573 K.

3. Results and discussion

3.1. Structural analysis of Bi₂Te₃, Sb₂Te₃ and SnTe samples

Fig. 2 shows the XRD patterns of all the studied samples. A phase pure Bi₂Te₃ is obtained for BT with a hexagonal crystal structure and without any impurity peaks of Bi or Te. The BT sample matches well with the Bi₂Te₃ hexagonal phase with ICSD No: 20289. The observed peaks for ST1 correspond to the diffraction from the hexagonal Sb₂Te₃ material having space group R3̄m with the JCPDS number 15-0874.³² In addition, minor impurity peaks of Sb are noticed in ST1. It is observed that ST2 contains both Sb₂Te₃ peaks and several impurity peaks of intermediate phases such as Te and Sb₂O₃. A similar observation was also made for the TT sample, where impurity peaks such as SnTe₃O₈ and Te are present. Hence, the XRD results suggest that the formation of phase pure compounds of chalcogenides through an aqueous-based chemical method is feasible only for selected cases, especially for BT.

Fig. 2 XRD patterns of BT, ST1, ST2 and TT nanostructures.

3.2. Mechanism of the thermal decomposition of EDTA

Ligands such as EDTA act as a capping agent for tailoring the crystal growth and may also provide the possibility of breaking the crystalline nature, which may lead to different morphologies and wider applications.^33–35 EDTA can coordinate with several inorganic ions to form multinuclear complexes. It is well-reported that such structures can also act as a template in the self-assembly process.^36–38 It is expected that an oriented attachment may occur with the template effect of EDTA by adding appropriate ligands to the reaction mixtures. Also, EDTA added as an anion surfactant in the solution could connect with Bi³⁺ ions to form large molecular groups, which facilitates Bi₂Te₃ nuclei to grow along the surfaces of EDTA agglomerates. When the EDTA selectively binds to one of the facets, the growth rate in either a- or b-axis directions kinetically slows down, with a relative increase in the c-axis direction. The movements of these fine hexagonal crystals together with molecular groups in the solution, make it possible for the particles to connect with each other by suspended bonds according to the definite epitaxy in the c-axis direction and consequently form long nanorods or nanosheets. These rods and sheets are arranged through the template action of the EDTA additive, which is a favourable mechanism as per the recent reports where expensive techniques such as solvothermal/hydrothermal methods and high reaction temperature were used to develop the desired materials.^39–41 Hence, the EDTA-assisted synthesis using EDTA as a surfactant gives good control over the morphology and helps to prevent oxidation of the synthesized nanostructures.

However, EDTA residues in the final product can detrimentally affect its structural integrity, crystallinity and thermoelectric performance. To obtain the nanostructured Bi₂Te₃, a pH of 10 must be maintained during the synthesis (using NaOH) and finally, the precipitated product is to be washed with acetone, ethanol and deionized water several times. The residual EDTA can dissolve well in acetone when the pH is above 8; hence, from the slurry of the aqueous-based Bi₂Te₃ final product, EDTA and other unreacted residues can be removed by treating it initially with acetone till a clear solution is obtained. Again, upon heat treatment, the EDTA primarily decomposes to N-(2-hydroxyethyl) iminodiacetic acid and iminodiacetic acid at 473 K. At higher temperatures, i.e., at 533 K, these primary products can yield ethylene glycol as the final product. Ethylene glycol controls the morphology of our nanostructures even at a sintering temperature of 623 K. Hence, EDTA is removed partially by washing with different solvents and partially by converting it to ethylene glycol at a higher temperature. The temperature-dependent decomposition of EDTA in the aqueous synthesis of chalcogenide nanostructures is explained as follows: At 533 K, EDTA decomposes into N-(2-hydroxyethy1)iminodiacetic acid and iminodiacetic acid, as shown in eqn (1). Further hydrolysis of N-(2-hydroxyethy1)iminodiacetic acid results in the formation of ethylene glycol and iminodiacetic acid (eqn (2)).⁴²

A portion of iminodiacetic acid decomposes to iminodiacetic anhydride with the evolution of water at 512 K (eqn (3)).

At around 593 K, iminodiacetic anhydride boils off. Beyond 593 K, iminodiacetic acid decomposes into water, carbon dioxide, carbon monoxide, acetonitrile and hydrogen gas as by-products, as shown in eqn (4). Acetonitrile boils at 354 K and ethylene glycol vaporises at 470 K.

Deng et al. synthesized Bi₂Te₃ nanorods using EDTA as the surfactant.⁴³ Srashti et al. synthesized Bi₂Te₃ nanostructures by a refluxing method using EDTA as a surfactant, KOH and NaBH₄ and showed that the growth and morphology of Bi₂Te₃ depends upon the surfactant, concentration of KOH and reaction timings.⁴⁴ Yang et al. showed that EDTA-Na₂ could be used as a complexing agent in the synthesis of nanocrystalline Sb₂Te₃ and achieved a power factor (PF) of 2.04 μW cm⁻¹ K² at 140 °C and σ remained in the range of 5–10 × 10³ S m⁻¹.⁴⁵ Dharmaiah et al. synthesized Sb₂Te₃ nanoplates using EDTA and achieved a high S of 181 μV K⁻¹, high σ of 763 Ω⁻¹ cm⁻¹ and low κ of 1.15 W m⁻¹ K⁻¹.⁴⁶

3.3. Mechanism for the formation of secondary phases in Sb₂Te₃ and SnTe samples

To explore further the unstable phases of ST2 and TT, a detailed understanding of the chemistry of these materials is necessary. Let us consider the chemical reactions that could possibly occur in each case. Fig. 3 represents the chemical compounds that can generate Bi³⁺, Sb³⁺ and Sn²⁺/Sn⁴⁺ in an aqueous medium to combine with Te²⁻ to form the corresponding chalcogenide materials. The existence of Te²⁻ ions in the aqueous medium is validated with the chemical equations as represented in eqn (5)–(7).

3Te + 6OH⁻ → 2Te²⁻ + TeO₃²⁻ + 3H₂O (5)

2TeO₃²⁻ + NaBH₄ → Te²⁻ + Te + NaBO₃ + H₂O + 2OH⁻ (6)

5Te + 10OH⁻ + NaBH₄ → 5Te²⁻ + NaBO₃ + 7H₂O (7)

Fig. 3 Possible chemical compounds generating cations of (a) Bi, (b) and (c) Sb and (d) Sn to form the corresponding chalcogenide nanostructures.

While using BiCl₃ in the formation of Bi₂Te₃, it is obvious that Bi(OH)₂Cl, which is stable at room temperature, will generate Bi³⁺ ions in the aqueous medium, whereas Bi₄O₅Cl₂ and BiOCl will not take part in the chemical reaction,⁴⁷ as represented in eqn (8)–(12).⁴⁸

Thus, the formation of stable BT nanostructures is possible using BiCl₃. While using SbCl₃ as the starting material, the following reactions could occur as mentioned below in eqn (13)–(17)

It is well understood from eqn (13) and (15) that the formation of Sb₄O₅Cl₂ and SbOCl is unavoidable, and only the partial synthesis of Sb₂Te₃ is possible. An easy way to obtain phase pure Sb₂Te₃ in an aqueous medium is through Sb metallic powder where Sb(OH)₃ can contribute Sb³⁺, as shown in eqn (18).

A small trace of metallic Sb could be present here; still, this method is valid for eqn (16) and (17) to develop Sb₂Te₃ nanomaterials in an aqueous medium, which corroborates with the XRD results of ST1.

Interestingly, it is observed that the Sn-based reactions are not recommended in aqueous medium as Sn²⁺ can easily undergo oxidation to form Sn⁴⁺, as represented in eqn (19)–(21).

It could be seen that SnCl₂ reacts with water to form an insoluble basic salt Sn(OH)Cl. This basic salt could further combine with Te ions present in the solution to form a secondary phase SbTe₃O₈ in addition to SnTe. Thus, the aqueous-based synthesis using EDTA is not suitable for the synthesis of SnTe. Other methods, such as the one described in the recent report,⁴⁹ must be adopted to synthesize phase pure SnTe nanopowders.

In this regard, it could be presumed that there are limitations to the formation of Sb₂Te₃ and SnTe in aqueous-based reflux synthesis. However, the partial formation of Sb₂Te₃ using EDTA prompted us to synthesize Sb-doped Bi₂Te₃ and to investigate the n-to-p-type behaviour with Sb doping. Hence, we have attempted to synthesize Sb-doped Bi₂Te₃. Small amounts of Sb, such as 1%, 3% and 5%, were doped into the Bi site using the reflux method. The use of Sb powder may not bring satisfactory results, and hence, SbCl₃ was used as the precursor for the synthesis of Sb-doped Bi₂Te₃ as SbCl₃ will be suitable for replacing Bi ions to form the doped compound.

3.4. Structural analysis of Sb-doped Bi₂Te₃

Fig. 4 shows the XRD patterns of the Sb-doped BT samples prepared for 1%, 3% and 5% concentrations by keeping the molarity of NaBH₄ at 4.4 M. It could be seen that BS1T exists in a single phase at lower Sb concentrations and crystallizes into a hexagonal Bi₂Te₃ structure. However, BS3T and BS5T contain small amounts of secondary phases of Sb and Sb₂O₃. Hence, it could be seen that for higher concentrations of Sb, the samples do not become phase pure. Hence, the reflux method alone is not sufficient to produce phase-pure samples of Sb-doped Bi₂Te₃.

Fig. 4 XRD patterns of cold-pressed Sb-doped BT nanostructures.

Hence, as a second step, these samples were hot pressed at a pressure of 50 MPa and a temperature of 723 K and the XRD patterns were obtained for the Sb-doped samples, as shown in Fig. 5(a). Upon hot pressing, all the Sb-doped BT samples crystallized into a combination of hexagonal Bi₂Te₃ and BiTe phases. The Rietveld refinement of all the samples is shown in Fig. 5(b)–(d). No peaks corresponding to Sb or Sb₂O₃ were indexed in the XRD patterns. The structural parameters determined from Rietveld refinement are listed in Table 1. A low value of χ² is observed, which justifies the quality and goodness of the refinement.

Fig. 5 (a) XRD patterns of hot-pressed Sb-doped BT nanostructures. Refined XRD patterns of (b) BS1T-HP, (c) BS3T-HP and (d) BS5T-HP.

Table 1 Refined XRD parameters of BS1T-HP, BS3T-HP and BS5T-HP

BTA structure	BS1T-HP		BS3T-HP		BS5T-HP
Phase	Bi2Te3 + BiTe		Bi2Te3 + BiTe		Bi2Te3 + BiTe
Crystal structure	Hexagonal + rhombohedral		Hexagonal + rhombohedral		Hexagonal + rhombohedral
Space group	R3̄m + P3̄m1		R3̄m + P3̄m1		R3̄m + P3̄m1
Lattice parameters
a (Å)	4.38(3)	4.34(8)	4.37(3)	4.34(8)	4.36(3)	4.35(8)
b (Å)	4.38(3)	4.34(8)	4.37(3)	4.34(8)	4.36(3)	4.35(8)
c (Å)	30.08(5)	24.80(9)	30.08(5)	24.80(9)	30.08(5)	24.80(9)
γ (deg)	120	120	120	120	120	120
Volume (Å)^3	504.99(6)	406.50(1)	504.99(6)	406.50(1)	504.99(6)	406.50(1)
Residual parameters
R p	3.40		3.34		3.27
wRp	2.29		2.13		2.09
χ ^2	1.89		1.68		1.54

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It is seen from the refinement data that there is a decrease in unit cell volume with an increase in Sb concentration. This is attributed to the small size of Sb³⁺ ions substituting Bi³⁺ ions in the lattice. Also, upon hot pressing, there could be a redistribution of ions in the lattice, and the insoluble compounds could have vaporised, leading to the formation of phase-pure Bi₂Te₃ without any impurity peaks.

3.5. Morphological analysis of hot-pressed Sb-doped Bi₂Te₃

Fig. 6(a) and (b) show the TEM images of BS1T-HP. The TEM images of BS1T-HP indicate the formation of nanosheets. The formation of nanosheets in BS1T-HP could be attributed to the template action of EDTA, as a result of which an oriented attachment occurs along a preferred direction, resulting in the formation of nanosheets. The (006) plane and the SAED patterns shown in Fig. 6(c) and (d) correspond to the lattice planes of Bi₂Te₃.

Fig. 6 (a) and (b) TEM images showing the formation of BS1T-HP nanosheets. (c) (006) plane corresponding to Bi₂Te₃ and (d) SAED patterns of BS1T-HP.

3.6. Mechanism for the formation of hot-pressed Sb-doped Bi₂Te₃

The detailed reaction mechanism showing the formation of Bi₂Te₃ nanostructures was reported in our earlier studies.⁴⁸ The mechanism for the formation of Bi_2−xSb_xTe₃ nanostructures using EDTA is demonstrated in Fig. 7. Here, the secondary phases of Sb and Sb₂O₃ present in the as-prepared BS3T and BS5T disappear upon hot pressing. This could be due to the additional energy provided during hot pressing, which results in the phase formation of Bi_2−xSb_xTe₃ nanostructures. The TEM images indicate the formation of sheet-like structures of BS1T-HP. It is well known that EDTA is a multidentate ligand with polyfunctional groups, and it could effectively serve as a bridging ligand to form multinuclear complexes with metal ions above some critical concentration. Then, chains of crystalline seeds would form in the nucleation process, especially with reaction time, which finally yields sheet/rod-like structures of the desired compound. The present approach using the reflux technique could favour the reaction mechanism through nucleation and growth processes. The crystalline Bi_2−xSb_xTe₃ structures could form in the reflux reaction process through a homogeneous nucleation process if the reaction time is a varying parameter along with EDTA concentration. It is well known that the bismuth tellurium-based materials have a highly anisotropic structure, which favours the growth, primarily confined to a particular direction and the crystalline seeds of the material tend to grow into the rod/sheet shape under the influence of EDTA, which would be a soft template, and induce the formation of 1D/2D structures. Hence, EDTA could promote preferential directional growth under the template effect, where the formation of inorganic nanoparticles in liquid media is associated with monomer growth. Previous reports suggest that the nucleus is characterized by various shapes and facets with different surface energies and grows by bonding with other monomers in the solution. Crystal surface energy and facet attachment highly influence nanoparticle growth and shape formation. If any capping agent is present in the solution, it could bind to specific facets of the nucleus to coat it with a monolayer, which precisely occurs between Bi and EDTA in the present technique. These attached surfactants could lower the total surface energy when Bi_2−xSb_xTe₃ nanocrystals are formed, specifically by blocking high-energy facets and exposing low-energy facets.

Fig. 7 Schematic showing the formation of EDTA-assisted (a) ST1, (b) 3% and 5% Bi_2−xSb_xTe₃ and (c) hot-pressed Bi_2−xSb_xTe₃ nanostructures.

3.7. Seebeck coefficient, electrical resistivity and power factor of hot-pressed Sb-doped Bi₂Te₃

Fig. 8 shows the temperature dependence of S in the temperature range of 280–480 K. A negative value of S is obtained for BS1T at temperatures below 425 K, revealing an n-type semiconducting behaviour indicating that electrons are the majority carriers up to 425 K. As the temperature increases to 425 K, a clear transition from a negative to a positive value of S is evident, which marks a crossover from the n-type to the p-type. As the Sb doping concentration increases to 5%, the sample becomes completely p-type, which is evident from the obtained positive values of S from Fig. 8. Hence, an n-type to p-type semiconducting behaviour has been achieved with Sb doping in Bi₂Te₃. However, due to the presence of mixed phases in BS1T-HP, the value of S is quite low, reaching a maximum value of −54.78 μV K⁻¹ at 306 K and 11.2 μV K⁻¹ at 448 K. In the case of BS5T-HP, a maximum value of 215.12 μV K⁻¹ is achieved at 465 K, which is remarkable.

Fig. 8 Temperature dependence of Seebeck coefficient (S) of BS1T-HP and BS5T-HP samples.

To understand the electrical transport behaviour of the synthesized Sb-doped BT nanostructures, a detailed resistivity analysis was performed on the BS1T-HP and BS5T-HP samples in the temperature range of 280–480 K. Fig. 9 presents the temperature-dependent ρ of these samples, and reveals the semiconducting behaviour for BS1T-HP and BS5T-HP. BS1T-HP exhibits a ρ value of 0.00347 Ω m and BS5T-HP exhibits 0.01848 Ω m at 300 K, which is much higher than the values obtained for the BT samples reported recently.⁴⁸ Due to this, a considerable decrease in electrical conductivity can happen, which could ultimately lower the PF.

Fig. 9 Temperature dependence of resistivity (ρ) of BS1T-HP and BS5T-HP.

The temperature variation of PF, which is calculated as S²σ, is plotted and shown in Fig. 10. The PF value of the BS1T-HP sample decreases with temperature, giving a maximum of 0.89 μW m⁻¹ K⁻² at 306 K and this low value of PF is arising due to the decreased S and increased ρ value compared to the Bi₂Te₃ samples discussed in the previous section. With the increase in Sb concentration in BS5T-HP, the PF follows an inverse trend where it increases with an increase in temperature, reaching a maximum value of 7.1 μW m⁻¹ K⁻² at 465 K. The decreased value of PF in BS1T-HP could be attributed to the presence of multiphases of Bi₂Te₃ and BiTe present in the system. It could also be seen that there is a crossover from n-type (negative Seebeck) to p-type (positive Seebeck) in BS1T-HP at 440 K, as shown in Fig. 8. For BS5T-HP, the material becomes completely p-type and Brahmi et al. reported that this transition could be due to the movement of the Fermi level (E_F) away from the conduction band and towards the valence band caused by the increased contribution from acceptors by changing the temperature.⁵⁰ This result could provide a new pathway towards finding the appropriate reaction mechanism and doping scheme for tailoring the TE properties for novel technological applications.

Fig. 10 Temperature dependence of PF of BS1T-HP and BS5T-HP.

4. Conclusion

In summary, the Sb-doped Bi₂Te₃ nanostructures were synthesised successfully by a simple, low-cost and low-temperature reflux method using deionized water as the solvent. Structural and morphological changes were obtained by varying the reaction time in 75 mmol EDTA, which is confirmed by XRD and TEM analyses. The reaction mechanism for Bi₂Te₃, Sb₂Te₃ and SnTe was investigated in detail. Mechanism for the decomposition of EDTA and the concentration of which could facilitate the formation of chalcogenide nanostructures is explained. EDTA behaves not only as a capping agent but also as a soft template for lowering the surface energy, which could be the reason for the formation of sheet-like structures in Sb-doped Bi₂Te₃. An n-type semiconducting behaviour is witnessed for the 1% Sb-doped Bi₂Te₃ at temperatures below 440 K, and above this, a clear transition of semiconducting behaviour from the n-type to p-type is observed. Upon further increase in Sb concentration to 5%, the sample becomes completely p-type. Thus, it is understood that the hot-pressed Sb-doped Bi₂Te₃ shows an n-type to p-type semiconducting behaviour as a function of both temperature and Sb dopant concentration. However, the 5% Sb-doped Bi₂Te₃ exhibits a power factor value of 7.1 μW m⁻¹ K⁻² at 465 K. These results indicate that Bi and Sb-based chalcogenide nanostructures could be synthesized successfully at low temperatures using a simple aqueous-based reflux technique, where appropriate reaction pathways could help in the formation of phase pure compounds, enhancing the overall thermoelectric performance of nanostructured chalcogenide materials.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors would like to acknowledge the financial support received from the Council of Scientific and Industrial Research (CSIR), Govt. of India. V. R. A. is thankful to the Academy of Scientific and Innovative Research and CSIR for granting the Fellowship. The authors would also like to thank the Department of Science and Technology sponsored project number SPF/2023/00018 for partially supporting this work. M Vasundhara acknowledges the support offered by the Department of K&IM (IICT/Pubs./2024/117).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4cp02312a

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