Wet-chemical synthesis and consolidation of stoichiometric bismuth telluride nanoparticles for improving the thermoelectric figure-of-merit

Rapid microwave-assisted synthesis and characterization of a novel CuCoTe nanocomposite material for optoelectronic and dielectric applications

In the realm of nanomaterial research, copper telluride and cobalt telluride have individually attracted considerable attention owing to their unique properties and potential applications. However, there exists a notable gap in the literature when it comes to the exploration of composite materials derived from these elements. From this point of view, a ternary CuCoTe nanocomposite was prepared using the microwave synthesis method. Various characterizations were performed by varying the power and irradiation time. X-Ray diffraction study and transmission electron microscopy analysis confirmed the polycrystalline nature of the material with Cu2Te and CoTe hexagonal phases. Field emission scanning electron microscopy images reveal nanoparticle-like morphology, which remains unchanged even when the time of irradiation increases. In addition, the nanoparticle size of the material lies in the range of 30-39 nm. The differential scanning calorimetry inferred various exothermic and endothermic peaks. Meanwhile, the optical analysis from the UV-visible study shows the red-shifted absorbance, enabling the material for semiconductor and photovoltaic devices. Furthermore, the optical bandgap of the material varies in the range from 2.45 to 3.61 eV, which reveals the tuneable bandgap desiring the material for various optoelectronic applications. The frequency-temperature-dependent dielectric study gives results for dielectric parameters, conductivity, and impedance behaviour. The material's dielectric constant, dielectric loss, and AC conductivity enhance with the increase in temperature. This behaviour of the material broadens the area of applicability in energy storage devices.

Effect of (Sm, In) Doping on the Electrical and Thermal Properties of Sb2Te3 Microstructures

Doped Sb₂Te₃ narrow-band-gap semiconductors have been attracting considerable attention for different electronic and thermoelectric applications. Trivalent samarium (Sm)- and indium (In)-doped Sb₂Te₃ microstructures have been synthesized by the economical solvothermal method. Powder X-ray diffraction (PXRD) was used to verify the synthesis of single-phase doped and undoped Sb₂Te₃ and doping of Sm and In within the crystal lattice of Sb₂Te₃. Further, the morphology, structure elucidation, and stability have been investigated systematically by scanning electron microscopy (SEM), Raman analysis, and thermogravimetric analysis (TGA). These analyses verified the successful synthesis of hexagonal undoped Sb₂Te₃ (AT) and (Sm, In)-doped Sb₂Te₃ (SAT, IAT) microstructures. Moreover, the comparison of dielectric parameters, including dielectric constant, dielectric loss, and tan loss of AT, SAT, and IAT, was done in detail. An increment in the electrical conductivities, both AC and DC, from 1.92 × 10^-4 to 4.9 × 10^-3 Ω^-1 m^-1 and a decrease in thermal conductivity (0.68-0.60 W m^-1 K^-1) were observed due to the doping by trivalent (Sm, In) dopants. According to our best knowledge, the synthesis and dielectric properties of (Sm, In)-doped and undoped Sb₂Te₃ in comparison with their electrical properties and thermal conductivity have not been reported earlier. This implies that appropriate doping with (Sm, In) in Sb₂Te₃ is promising to enhance the electronic and thermoelectric behavior.

Energy-Saving Pathways for Thermoelectric Nanomaterial Synthesis: Hydrothermal/Solvothermal, Microwave-Assisted, Solution-Based, and Powder Processing

The pillars of Green Chemistry necessitate the development of new chemical methodologies and processes that can benefit chemical synthesis in terms of energy efficiency, conservation of resources, product selectivity, operational simplicity and, crucially, health, safety, and environmental impact. Implementation of green principles whenever possible can spur the growth of benign scientific technologies by considering environmental, economical, and societal sustainability in parallel. These principles seem especially important in the context of the manufacture of materials for sustainable energy and environmental applications. In this review, the production of energy conversion materials is taken as an exemplar, by examining the recent growth in the energy-efficient synthesis of thermoelectric nanomaterials for use in devices for thermal energy harvesting. Specifically, "soft chemistry" techniques such as solution-based, solvothermal, microwave-assisted, and mechanochemical (ball-milling) methods as viable and sustainable alternatives to processes performed at high temperature and/or pressure are focused. How some of these new approaches are also considered to thermoelectric materials fabrication can influence the properties and performance of the nanomaterials so-produced and the prospects of developing such techniques further.

Minute-Made, High-Efficiency Nanostructured Bi2Te3 via High-Throughput Green Solution Chemical Synthesis

Scalable synthetic strategies for high-quality and reproducible thermoelectric (TE) materials is an essential step for advancing the TE technology. We present here very rapid and effective methods for the synthesis of nanostructured bismuth telluride materials with promising TE performance. The methodology is based on an effective volume heating using microwaves, leading to highly crystalline nanostructured powders, in a reaction duration of two minutes. As the solvents, we demonstrate that water with a high dielectric constant is as good a solvent as ethylene glycol (EG) for the synthetic process, providing a greener reaction media. Crystal structure, crystallinity, morphology, microstructure and surface chemistry of these materials were evaluated using XRD, SEM/TEM, XPS and zeta potential characterization techniques. Nanostructured particles with hexagonal platelet morphology were observed in both systems. Surfaces show various degrees of oxidation, and signatures of the precursors used. Thermoelectric transport properties were evaluated using electrical conductivity, Seebeck coefficient and thermal conductivity measurements to estimate the TE figure-of-merit, ZT. Low thermal conductivity values were obtained, mainly due to the increased density of boundaries via materials nanostructuring. The estimated ZT values of 0.8-0.9 was reached in the 300-375 K temperature range for the hydrothermally synthesized sample, while 0.9-1 was reached in the 425-525 K temperature range for the polyol (EG) sample. Considering the energy and time efficiency of the synthetic processes developed in this work, these are rather promising ZT values paving the way for a wider impact of these strategic materials with a minimum environmental impact.

Influence of Nanoparticle Processing on the Thermoelectric Properties of (Bix Sb1-X )2 Te3 Ternary Alloys

The synthesis of phase-pure ternary solutions of tetradymite-type materials (Bix Sb1-x )2 Te3 (x=0.25; 0.50; 0.75) in an ionic liquid approach has been carried out. The nanoparticles are characterized by means of energy-dispersive X-ray spectroscopy (EDX), powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), and transmission electron microscopy. In addition, the role of different processing approaches on the thermoelectric properties - Seebeck coefficient as well as electrical and thermal conductivity - is demonstrated.

Scalable colloidal synthesis of Bi2Te2.7Se0.3 plate-like particles give access to a high-performing n-type thermoelectric material for low temperature application

Colloidal synthesis is harnessed for the gram-scale preparation of hexagonal-shaped plate-like Bi2Te2.7Se0.3 particles, yielding nearly 5 g of the product in one experiment. The resultant textured particles are highly crystalline, phase-pure, chemically uniform, and can serve as a starting material for the preparation of bulk thermoelectrics for room temperature applications. The consolidation occurs via spark plasma sintering, which affords nanostructured n-type Bi2Te2.7Se0.3 material exhibiting a high figure of merit ZT ≈ 1 at 373 K with an average ZT ≈ 0.93 (300-473 K). Our experimental and theoretical studies indicate that the high thermoelectric performance is attributed to a favorable combination of the resultant transport properties. Specifically, bottom-up formation of the plate-like particles results in the substantial reduction of thermal conductivity by nanostructuring as observed experimentally and can be ascribed to phonon scattering at grain boundaries and suppressed bipolar conduction. When coupled with high electrical conductivity, which is preserved at the bulk scale as confirmed by ab initio calculations, these factors boost the thermoelectric performance of the as-synthesized n-type Bi2Te2.7Se0.3 bulk nanostructured alloy to the state-of-the-art level. The combination of a newly developed scalable colloidal synthesis with optimized spark plasma sintering constitutes a convenient route to nanostructured bulk thermoelectrics, which is an interesting pathway for the preparation of simple and complex thermoelectric chalcogenides.

Towards Infrared Electronic Eyes: Flexible Colloidal Quantum Dot Photovoltaic Detectors Enhanced by Resonant Cavity

Electronic eye cameras are receiving increasing interest due to their unique advantages such as wide field of view, low aberrations, and simple imaging optics compared to conventional planar focal plane arrays. However, the spectral sensing ranges of most electronic eyes are confined to the visible, which is limited by the energy gaps of the sensing materials and by fabrication obstacles. Here, a potential route leading to infrared electronic eyes is demonstrated by exploring flexible colloidal quantum dot (CQD) photovoltaic detectors. Benefitting from their tunable optical response and the ease of fabrication as solution processable materials, mercury telluride (HgTe) CQD detectors with mechanical flexibility, wide spectral sensing range, fast response, and high detectivity are demonstrated. A strategy is provided to further enhance the light absorption in flexible detectors by integrating a Fabry-Perot resonant cavity. Integrated short-wave IR detectors on flexible substrates have peak D^* of 7.5 × 10¹⁰ Jones at 2.2 µm at room temperature and promise the development of infrared electronic eyes with high-resolution imaging capability. Finally, infrared images are captured with the flexible CQD detectors at varying bending conditions, showing a practical approach to sensitive infrared electronic eyes beyond the visible range.

Room-temperature growth of colloidal Bi2Te3 nanosheets

In this work, we report the colloidal synthesis of Bi₂Te₃ nanosheets with controlled thickness, morphology and crystallinity at temperatures as low as 20 °C. Grown at room temperature, Bi₂Te₃ exhibits two-dimensional morphology with thickness of 4 nm and lateral size of 200 nm. Upon increasing the temperature to 170 °C, the nanosheets demonstrate increased thickness of 16 nm and lateral dimensions of 600 nm where polycrystalline nanosheets (20 °C) are replaced by single crystal platelets (170 °C). Rapid synthesis of the material at moderately low temperatures with controllable morphology, crystallinity and consequently electrical and thermal properties can pave the way toward its large-scale production for practical applications.

Dual-functional aniline-assisted wet-chemical synthesis of bismuth telluride nanoplatelets and their thermoelectric performance

The wet-chemical approach is of great significance for the synthesis of two-dimensional (2D) bismuth telluride nanoplatelets as a potential thermoelectric (TE) material. Herein, we proposed a simple and effective solution method with the assistance of aniline for the fabrication of bismuth telluride nanoplatelets at a low temperature of 100 °C. The choice of aniline with its dual function avoided the simultaneous use of a capping regent and a toxic reductant. The as-synthesized nanoplatelets have a large size of more than 900 × 500 nm² and a small thickness of 15.4 nm. The growth of bismuth telluride nanoplatelets are related to the Bi/Te ratio of precursors indicating that a larger content of the Bi precursor is more conducive to the formation of 2D nanoplatelets. The bismuth telluride nanoplatelets pressed into a pellet show a smaller electrical resistivity (∼6.5 × 10^-3 Ω · m) and a larger Seebeck coefficient (-135 μV K^-1), as well as a lower thermal conductivity (0.27 W m^-1 K^-1) than those of nanoparticles. The next goal is to further reduce the electrical resistivity and optimize the TE performance by disposing of the residual reactant of aniline adsorbed on the surface of the nanoplatelets.

Fabrication of Bi2Te3-x Se x nanowires with tunable chemical compositions and enhanced thermoelectric properties

Uniform Bi₂Te_3-x Se _x nanowires (NWs) with tunable components are synthesized by a modified solution method free of any template, and inter-diffusion mechanism is proposed for the growth and transformation of ternary nanowires. Spark plasma sintering is adopted to fabricate the pellets of Bi₂Te_3-x Se _x NWs and thermoelectric transport properties are measured. As compared to Bi₂Te₃ pellets, Se doping results in lowered electrical conductivity because of the reduced carrier concentration, both the Seebeck coefficient and the power factor are enhanced substantially. The Bi₂Te_2.7Se_0.3 pellet exhibits the highest power factor at room temperature as a result of optimized carrier concentration (4.37 × 10¹⁹ cm^-3) and mobility (60.22 cm² V^-1 s^-1). As compared to Bi₂Te₃, the thermal conductivity of Bi₂Te_3-x Se _x is lowered owing to the enhanced phonon scattering by dopants and grain boundaries. As a result, the ZT value at 300 K is substantially improved from 0.045 of Bi₂Te₃ to 0.42 of Bi₂Te_2.7Se_0.3. It is suggested that Se doping is an effective way to enhance the thermoelectric performance of Bi₂Te₃ based materials.

Graphene Quantum Dots Embedded in Bi2Te3 Nanosheets To Enhance Thermoelectric Performance

Novel Bi₂Te₃/graphene quantum dots (Bi₂Te₃/GQDs) hybrid nanosheets with a unique structure that GQDs are homogeneously embedded in the Bi₂Te₃ nanosheet matrix have been synthesized by a simple solution-based synthesis strategy. A significantly reduced thermal conductivity and enhanced powder factor are observed in the Bi₂Te₃/GQDs hybrid nanosheets, which is ascribed to the optimized thermoelectric transport properties of the Bi₂Te₃/GQDs interface. Furthermore, by varying the size of the GQDs, the thermoelectric performance of Bi₂Te₃/GQDs hybrid nanostructures could be further enhanced, which could be attributed to the optimization of the density and dispersion manner of the GQDs in the Bi₂Te₃ matrix. A maximum ZT of 0.55 is obtained at 425 K for the Bi₂Te₃/GQDs-20 nm, which is higher than that of Bi₂Te₃ without hybrid nanostrucure. This work provides insights for the structural design and synthesis of Bi₂Te₃-based hybrid thermoelectric materials, which will be important for future development of broadly functional material systems.

Phase Segregation and Superior Thermoelectric Properties of Mg2Si(1-x)Sb(x) (0 ≤ x ≤ 0.025) Prepared by Ultrafast Self-Propagating High-Temperature Synthesis

A series of Sb-doped Mg2Si(1-x)Sb(x) compounds with the Sb content x within 0 ≤ x ≤ 0.025 were prepared by self-propagating high-temperature synthesis (SHS) combined with plasma activated sintering (PAS) method in less than 20 min. Thermodynamic parameters of the SHS process, such as adiabatic temperature, ignition temperature, combustion temperature, and propagation speed of the combustion wave, were determined for the first time. Nanoprecipitates were observed for the samples doped with Sb. Thermoelectric properties were characterized in the temperature range of 300-875 K. With the increasing content of Sb, the electrical conductivity σ rises markedly while the Seebeck coefficient α decreases, which is attributed to the increase in carrier concentration. The carrier mobility μ(H) decreases slightly with the increasing carrier concentration but remains larger than the Sb-doped samples prepared by other methods, which is ascribed to the self-purification process associated with the SHS synthesis. In spite of the increasing electrical conductivity with the increasing Sb content x, the overall thermal conductivity κ decreases on account of a significantly falled lattice thermal conductivity κ(L) due to the strong point defect scattering on Sb impurities and possibly enhanced interface scattering on nanoprecipitates. As a result, the sample with x = 0.02 achieves the thermoelectric figure of merit ZT ∼ 0.65 at 873 K, one of the highest values for the Sb-doped binary Mg2Si compounds investigated so far. A subsequent annealing treatment on the sample with x = 0.02 at 773 K for 7 days has resulted in no noticeble changes in the thermoelectric transport properties, indicating an excellent thermal stability of the compounds prepared by the SHS method. Therefore, SHS method can serve as an effective alternative fabrication route to synthesize Mg-Si based themoelectrics and some other functional materials due to the resulting high performance, perfect thermal stability, and feasible production in large scale for commercial application.

Synthesis of Bi2Te3and (BixSb1−x)2Te3nanoparticles using the novel IL [C4mim]3[Bi3I12]

[C₄mim]₃[Bi₃I₁₂] is a promising Bi-source for the ionothermal synthesis of binary (Bi₂Te₃) and ternary tetradymite-type nanoparticles (Bi_xSb_1−x)₂Te₃(x= 0.25, 0.5, 0.75) in ionic liquid.

Thermoelectric Properties of Solution Synthesized Nanostructured Materials

Thermoelectric nanocomposites made by solution synthesis and compression of nanostructured chalcogenides could potentially be low-cost, scalable alternatives to traditional solid-state synthesized materials. We review the progress in this field by comparing the power factor and/or the thermoelectric figure of merit, ZT, of four classes of materials: (Bi,Sb)2(Te,Se)3, PbTe, ternary and quaternary copper chalcogenides, and silver chalcogenides. We also discuss the thermal conductivity reduction associated with multiphased nanocomposites. The ZT of the best solution synthesized materials are, in several cases, shown to be equal to or greater than the corresponding bulk materials despite the generally reduced mobility associated with solution synthesized nanocomposites. For the solution synthesized materials with the highest performance, the synthesis and processing conditions are summarized to provide guidance for future work.

Wet-chemical synthesis of different bismuth telluride nanoparticles using metal organic precursors – single source vs. dual source approach

Thermolysis of metal organicsingle sourceanddual source precursorsyielded phase-pure Bi_xTe_ynanoparticles at low temperatures.