The Scientific Service Units (SSU) of ISTA supported this research through resources provided by the Electron Microscopy Facility (EMF) and the Lab Support Facility (LSF). This work was financially supported by the Institute of Science and Technology Austria and the Werner Siemens Foundation.

Production of CdSe&#45;SnSe Nanocomposites from Consolidated SnSe Particles

The authors have no conflicts of interest to declare.

Critical steps 
Selenium oxidation before mixing with the Sn precursor 
In this work, SnSe is synthesized by co-precipitation of Sn (II) complexes and Se2-. We start by reducing metallic selenium to selenide.
<img alt="Equation 3" src="/files/ftp_upload/66278/66278eq03.jpg" style="margin: auto;" />
Once the selenium (grey) is reduced, it forms a transparent solution. The selenium precursor, once exposed to oxygen, turns red, due to the formation of polyselenides. Thus, it is important to keep all solutions under argon for the duration of the reaction.
<img alt="Equation 4" src="/files/ftp_upload/66278/66278eq04.jpg" style="margin: auto;" />
On heating the tin chloride and sodium hydroxide, the tin precursor dissolves into a colorless solution as well.
<img alt="Equation 5" src="/files/ftp_upload/66278/66278eq05.jpg" style="margin: auto;" />
Upon addition of the selenide, which is in excess (0.9:1; Sn:Se), to the tin precursor, the mixture turns black, indicating the immediate formation of SnSe.
<img alt="Equation 6" src="/files/ftp_upload/66278/66278eq06.jpg" style="margin: auto;" />
As small amounts of the NaBH4 reagent react with the water, it is important to prevent oxidation of the Se by adding an excess of NaBH423,24,25. Even though the formation of SnSe is instantaneous, the reaction is kept at ~100 &#176;C for a further 2 h to allow the particles to grow26,27.
Purification 
The as-synthesized particles are then subjected to a purification procedure since they are in suspension with byproducts such as Na+, Cl-, B(OH)3, B(OH)4-, OH-, and excess BH4- and Se2-/HSe- and potential impurities. This is carried out for six purification steps of alternating water and ethanol as solvents28,29,30,31,32,33,34,35. Deviation in the purification procedure results in pellets with different performances, while the structural characterization looks identical.
Preparing CdSe thiol-amine solution fresh 
CdSe molecular complexes are stable for a limited period in the thiol-amine solution and therefore, should be used within 24 h after the dissolution is completed22.
Vacuum drying 
Vacuum drying creates a lower-pressure environment, which facilitates the rapid removal of solvents from the particles. This is essential to prevent the formation of residual solvent pockets within the particles, which can negatively affect the sintering process and the final pellet properties or stability.
Annealing powders after purification in a reducing atmosphere 
Annealing the particles is important to remove any prevalent volatile impurities, for example, thiol, amine, and excess Se36,37,38. Oxygen exposure of the particles is inevitable and thus, annealing in a reducing atmosphere aids in the reduction of oxides that inherently enhance the thermal conductivity of the material39,40,41.
Evaluate performance in two directions, parallel and perpendicular 
In accordance with the anisotropic nature of SnSe, electrical and thermal transport properties are different in the pressing (parallel) and non-pressing (perpendicular) directions. Therefore, it is important to prepare cylindrical pellets with dimensions that allow for the cutting of a bar and a disk to measure the transport properties in both directions41.
Sample preparation for transport characterization 
A smooth and flat pellet surface is crucial for accurate diffusivity measurements. Imperfections on the pellet surface can lead to heat losses and inaccurate results. Polishing is necessary to achieve a uniform and smooth surface. The orientation of the treated and untreated SnSe when loading is important and crucial for correct transport data analysis. Anisotropic materials such as SnSe must be measured along the same direction and combined (&#963;, S, and &#954;) for an accurate zT. Proper thermal contacts between the pellet and probes are also critical for accurate S and &#961; measurements.
Limitations 
However, due to the use of sodium reagents, the method is limited to producing p-type SnSe as Na+ ions are adsorbed onto the surface of the particles and act as a dopant enhancing the carrier concentration and &#963; of the material42.
Significance of the technique with respect to existing/alternative methods 
Various solution-based techniques have been reported to prepare polycrystalline SnSe such as solvothermal, hydrothermal, and non-pressurized methods in water or ethylene glycol18,19.&#160;In this work, we focused on a surfactant-free aqueous synthesis43, as it is more sustainable than any other reported methods: no organic solvents nor surfactants are used, and it requires a short reaction time (2 h) and low temperatures (~100 &#176;C) compared to those done by melting.
Future applications or directions after mastering this technique 
The method is adaptable in synthesizing other chalcogenides-SnTe, PbSe, and PbTe. In amending the reducing agents and bases to Na-free, pure materials without an intentional dopant can be synthesized. Surface treatments, such as the one done here with CdSe molecular complexes, allow for an added degree of flexibility in the material preparation, where secondary phases can be added in a secondary step to control the microstructure. In the specific case described here, the presence of CdSe nanoparticles not only inhibits the grain growth of the CdSe-SnSe particles compared to that of SnSe, but also lowers the thermal conductivity of the material (Figure 7 and Figure 8, respectively). Explanations that have been reported by Liu et al. 22 support the results postulated from the method we have stipulated in this work.

Thermoelectric (TE) materials, which convert heat into electricity and vice versa, can play an important role in sustainable energy management1. However, the low conversion efficiencies combined with the relatively high production costs of these materials have limited their broad application for industrial and domestic use. To overcome present challenges, cost-effective synthetic methods and the use of abundant and non-toxic materials with significantly improved efficiency must be implemented.
The thermoelectric figure of merit zT= S2&#963;T/&#954;, where S is the Seebeck coefficient, &#963; the electrical conductivity, T the absolute temperature, and &#954; the thermal conductivity, determines the efficiency of these materials. Due to the strong coupling of these properties, maximizing zT is a challenge. It often entails tuning the electronic band structure and microstructural defects to control charge and phonon scattering mechanisms2,3,4,5.
In the last decade, tin selenide (SnSe) has been explored as a non-toxic thermoelectric material due to its outstanding performance in its single crystal form (zT: p-type ~2.6, n-type ~2.8)6,7. However, single crystals are expensive to produce, limiting their applicability to devices. Alternatively, polycrystalline SnSe is cheaper to produce and mechanically more stable. The problem is that attaining high performance presents difficulties due to partial loss of anisotropy, diminishing electrical conductivity, greater ease of oxidation, and imprecise control of the doping level8,9,10.
Polycrystalline inorganic TE materials are usually processed in two steps: preparation of the semiconductor in powder form followed by consolidation of the powder into a dense pellet. Powders can be prepared through high-temperature reactions and grinding or directly by ball-milling11,12,13,14,15,16. Alternatively, powders can be synthesized via solution methods (e.g., hydrothermal, solvothermal, aqueous synthesis), requiring less demanding conditions (i.e., lower reagent purity, lower temperatures, and shorter reaction times)17,18,19,20,21.
This paper describes a method to produce dense SnSe nanocomposites from surface-modified SnSe particles that are synthesized in water. The process commences from the aqueous synthesis of SnSe particles, where reducing agents and bases are used to solubilize the Se and Sn reagents, respectively. When the solutions are combined, SnSe particles immediately start precipitating. After purification, SnSe particles are then functionalized with CdSe molecular complexes. During the annealing process, the molecular complexes decompose; forming CdSe nanoparticles19. The presence of CdSe nanoparticles inhibits grain growth and promotes the formation of many defects at varying length scales. These scattering sources result in low thermal conductivity and a high thermoelectric figure of merit22.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Chemicals</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>1, 2-ethanedithiol</td>
			<td>Thermo Scientific</td>
			<td>75-08-1</td>
			<td>Vaccum distilled</td>
		</tr>
		<tr>
			<td>Absolute Ethanol</td>
			<td>Honeywell</td>
			<td>64-17-5</td>
			<td></td>
		</tr>
		<tr>
			<td>Acetone (extra dry)</td>
			<td>Acros</td>
			<td>67-64-1&#160;</td>
			<td></td>
		</tr>
		<tr>
			<td>Anhydrous ethanol</td>
			<td>Thermofischer</td>
			<td>64-17-5</td>
			<td></td>
		</tr>
		<tr>
			<td>Cadmium oxide</td>
			<td>Alfa Aesar</td>
			<td>1306-23-6</td>
			<td></td>
		</tr>
		<tr>
			<td>Ethylenediamine</td>
			<td>Sigma-Aldrich</td>
			<td>107-15-3</td>
			<td></td>
		</tr>
		<tr>
			<td>N-methylformamide</td>
			<td>Sigma-Aldrich</td>
			<td>123-39-7</td>
			<td>Vacuum distilled, stored over molecular sieves</td>
		</tr>
		<tr>
			<td>Selenium</td>
			<td>Sigma-Aldrich</td>
			<td>7782-49-2</td>
			<td></td>
		</tr>
		<tr>
			<td>Sodium borohydride</td>
			<td>Sigma-Aldrich</td>
			<td>6940-66-2</td>
			<td></td>
		</tr>
		<tr>
			<td>Sodium hydroxide</td>
			<td>Sigma-Aldrich</td>
			<td>1310-73-2</td>
			<td></td>
		</tr>
		<tr>
			<td>Tin chloride dihydrate</td>
			<td>Thermo Scientific</td>
			<td>L0025-69-1</td>
			<td></td>
		</tr>
		<tr>
			<td>Apparatus/Materials</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Reduction adapter</td>
			<td>Bartelt</td>
			<td>9.011 755</td>
			<td></td>
		</tr>
		<tr>
			<td>Adapter with NS stopcock</td>
			<td>Bartelt</td>
			<td>9.012 312</td>
			<td></td>
		</tr>
		<tr>
			<td>Agate mortar and pestle</td>
			<td>Bartelt</td>
			<td>6204102</td>
			<td></td>
		</tr>
		<tr>
			<td>Caliper&#160;</td>
			<td>Sartorius</td>
			<td>5007021150</td>
			<td></td>
		</tr>
		<tr>
			<td>Carbon tape&#160;</td>
			<td>Micro to Nano</td>
			<td>15-000508</td>
			<td></td>
		</tr>
		<tr>
			<td>Centrifuge tubes x 4&#160;</td>
			<td>Sarstedt Ges.m.b.H.</td>
			<td>62.547.254</td>
			<td>50 mL</td>
		</tr>
		<tr>
			<td>Condenser</td>
			<td>Bartelt</td>
			<td>6.203 028</td>
			<td></td>
		</tr>
		<tr>
			<td>Crystallising dishes</td>
			<td>Bartelt</td>
			<td>7.021 089</td>
			<td></td>
		</tr>
		<tr>
			<td>Graphite foil</td>
			<td>Fisher Scientific</td>
			<td>11326967</td>
			<td>0.254 mm</td>
		</tr>
		<tr>
			<td>Measuring cylinder&#160;</td>
			<td>Bartelt</td>
			<td>6.082 194</td>
			<td>250 mL</td>
		</tr>
		<tr>
			<td>Micropipette&#160;</td>
			<td>Eppendorf</td>
			<td>3123000063</td>
			<td>Research plus 100-1000&#181;L (GLP)</td>
		</tr>
		<tr>
			<td>Quartz tube&#160;</td>
			<td>Hansun Electric Technology Co. Ltd</td>
			<td>50ODx 44 ID x 650 L, mm for DIY Tube Furnace</td>
			<td></td>
		</tr>
		<tr>
			<td>Round-bottom flask 2-neck&#160;</td>
			<td>Bartelt</td>
			<td>4.008 387</td>
			<td>500 mL</td>
		</tr>
		<tr>
			<td>Round-bottom flask 3-neck&#160;</td>
			<td>Lactan</td>
			<td>E614.1</td>
			<td>1000 mL</td>
		</tr>
		<tr>
			<td>Rubber septum x 3&#160;</td>
			<td>Bartelt</td>
			<td>9.230 657</td>
			<td></td>
		</tr>
		<tr>
			<td>Sand paper</td>
			<td>RS Components OC</td>
			<td>484-5942</td>
			<td>1 sheet, 1200 grit</td>
		</tr>
		<tr>
			<td>Schlenk line</td>
			<td>Chemglass</td>
			<td>CG-4436-03</td>
			<td></td>
		</tr>
		<tr>
			<td>Separating funnel&#160;</td>
			<td>Bartelt</td>
			<td>9.203 325</td>
			<td>250 mL</td>
		</tr>
		<tr>
			<td>Magnetic stir bars, oval</td>
			<td>Bartelt</td>
			<td>9.197 592</td>
			<td></td>
		</tr>
		<tr>
			<td>Magnetic stir bars, cylindrical</td>
			<td>Bartelt</td>
			<td>9.197 520</td>
			<td></td>
		</tr>
		<tr>
			<td>Magnetic stir bars, octagonal</td>
			<td>VWR</td>
			<td>442-0345</td>
			<td></td>
		</tr>
		<tr>
			<td>Succintillation vials x&#160; 4&#160;</td>
			<td>Sigma-Aldrich</td>
			<td>Z561754-1EA</td>
			<td>20 mL</td>
		</tr>
		<tr>
			<td>Succintillation vials x&#160; 1</td>
			<td>Bartelt</td>
			<td>9.003 482</td>
			<td>4 mL</td>
		</tr>
		<tr>
			<td>Equipment</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>AGUS-Pecs Spark Plasma Sintering (SPS)</td>
			<td>Suga CO., LTD.</td>
			<td>AGUS-PECS</td>
			<td>SPS-210Sx</td>
		</tr>
		<tr>
			<td>Bruker D8 Advance X-ray Diffraction&#160;</td>
			<td>Bruker</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Centrifuge</td>
			<td>Eppendorf</td>
			<td>Centrifuge 5810</td>
			<td></td>
		</tr>
		<tr>
			<td>Cold press</td>
			<td>Specac&#8482;&#160;</td>
			<td>Atlas Manual 15T Hydraulic Press</td>
			<td></td>
		</tr>
		<tr>
			<td>Density Meter</td>
			<td>Bartelt</td>
			<td>6263396</td>
			<td></td>
		</tr>
		<tr>
			<td>Electric saw&#160;</td>
			<td>Amazon</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>FE-SEM Merlin VP Contact</td>
			<td>Carl Zeiss&#160;</td>
			<td>Merlin Compact VP</td>
			<td></td>
		</tr>
		<tr>
			<td>Heating mantle 1000 mL&#160;</td>
			<td>Bartelt</td>
			<td>9.642 406</td>
			<td></td>
		</tr>
		<tr>
			<td>Benchtop Temperature Controller</td>
			<td>Cole-Parmer</td>
			<td>Digi-Sense TC9600</td>
			<td></td>
		</tr>
		<tr>
			<td>Linseis Laser Flash Analyser- LFA-1000</td>
			<td>Linseis</td>
			<td>LFA-1000</td>
			<td></td>
		</tr>
		<tr>
			<td>Linseis LSR-3</td>
			<td>Linseis</td>
			<td>LSR-3/800</td>
			<td></td>
		</tr>
		<tr>
			<td>Magnetic stirrer&#160;</td>
			<td>Heidolph</td>
			<td>MR Hei-Tec</td>
			<td></td>
		</tr>
		<tr>
			<td>Tubular furnace&#160;</td>
			<td>Hansun Electric Technology Co. Ltd</td>
			<td>Compact split tube furnace</td>
			<td></td>
		</tr>
		<tr>
			<td>Software</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>DIFFRAC.COMMANDER</td>
			<td>Bruker</td>
			<td></td>
			<td>Comes with the equipment</td>
		</tr>
		<tr>
			<td>Laser Flash Lenseis-AproSoft v.3.01 c.001</td>
			<td>Lenseis</td>
			<td></td>
			<td>Comes with the equipment</td>
		</tr>
		<tr>
			<td>Laserflash</td>
			<td>Lenseis</td>
			<td></td>
			<td>Comes with the equipment</td>
		</tr>
		<tr>
			<td>Lenseis data evaluation</td>
			<td>Lenseis</td>
			<td></td>
			<td>Comes with the equipment</td>
		</tr>
		<tr>
			<td>LSR Measure</td>
			<td>Lenseis</td>
			<td></td>
			<td>Comes with the equipment</td>
		</tr>
		<tr>
			<td>LSRDistance</td>
			<td>Lenseis</td>
			<td></td>
			<td>Comes with the equipment</td>
		</tr>
		<tr>
			<td>WAVE LOGGER</td>
			<td>Suga CO., LTD.</td>
			<td></td>
			<td>Comes with the equipment</td>
		</tr>
	</tbody>
</table>

solution&#45;processed, surface&#45;engineered, polycrystalline cdse&#45;snse exhibiting low thermal conductivity

In recent years, solution processes have gained considerable traction as a cost-effective and scalable method to produce high-performance thermoelectric materials. The process entails a series of critical steps: synthesis, purification, thermal treatments, and consolidation, each playing a pivotal role in determining performance, stability, and reproducibility. We have noticed a need for more comprehensive details for each of the described steps in most published works. Recognizing the significance of detailed synthetic protocols, we describe here the approach used to synthesize and characterize one of the highest-performing polycrystalline p-type SnSe. In particular, we report the synthesis of SnSe particles in water and the subsequent surface treatment with CdSe molecular complexes that yields CdSe-SnSe nanocomposites upon consolidation. Moreover, the surface treatment inhibits grain growth through Zenner pinning of secondary phase CdSe nanoparticles and enhances defect formation at different length scales. The enhanced complexity in the CdSe-SnSe nanocomposite microstructure with respect to SnSe promotes phonon scattering and thereby significantly reduces the thermal conductivity. Such surface engineering provides opportunities in solution processing for introducing and controlling defects, making it possible to optimize the transport properties and attain a high thermoelectric figure of merit.

Author Spotlight: Advancing Energy Solutions Using Nanocomposites as Processed Thermoelectric Materials

Solution&#45;Processed, Surface&#45;Engineered, Polycrystalline CdSe&#45;SnSe Exhibiting Low Thermal Conductivity

The fabrication of SnSe particles relies on the complete dissolution of the precursors in their stoichiometric ratios. An essential step in the protocol involves the reduction of Se with NaBH4, while under inert conditions. Any slight exposure to air results in the Se precursor changing from colorless to red (formation of polyselenides), as demonstrated in Figure 5.
Following the synthesis of SnSe, the particles are subjected to a purification procedure. The first supernatant of the purification process is yellow but upon exposure to oxygen turns orange. This is the result of unreacted Se, as the precursor was added in excess. In addition, there is a loss of small particles as shown in Figure 6 (steps #3 and onwards). At high ionic strength, the surface charge of the particles is efficiently shielded, allowing particles to be closer together without experiencing repulsion. With every washing step, the ionic strength decreases and the particle surface is not shielded; thus, particles repel and remain colloidally stable and consequently, are lost during the purification procedure.
The synthesis of SnSe yields ~14 g per batch of pure phase SnSe, as confirmed by XRD (Figure 7A). The particles are polydisperse in shape with a size between 50 nm and 200 nm (Figure 7B). After annealing, the average size of the particles increases to 680 nm. The densification using SPS also promotes grain growth, and the resulting pellets have a relative density of &#62;90%. A comparison of grain size is done from the SEM images between the untreated SnSe and SnSe-CdSe nanocomposite (Figure 7B and Figure 7C, respectively). Following the surface treatment results in grains that are considerably smaller compared to the untreated SnSe.
The cut and polished samples are then postannealed to confer stability. The, &#963;, S, and &#945;are measured using the setups in Figure 3 and Figure 4, respectively. From the measurements, the &#954; and zT are calculated with error bars calculated considering the propagation of uncertainties from each measurement (Figure 8).
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/66278/66278fig05.jpg" /> 
Figure 5: Time lapse of Se precursor on exposure to air. (A) Instant exposure to air results in a yellow solution. (B) After 2 min, the solution begins to turn red, and (C) within 3 min, the solution turns reddish as a result of Se oxidation. <a href="https://www.jove.com/files/ftp_upload/66278/66278fig05large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" class="xfigimg" src="/files/ftp_upload/66278/66278fig06.jpg" /> 
Figure 6: Supernatants after each washing step in the purification of SnSe. The colors of the seven supernatants of the different washing steps. <a href="https://www.jove.com/files/ftp_upload/66278/66278fig06large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 7" class="xfigimg" src="/files/ftp_upload/66278/66278fig07.jpg" /> 
Figure 7:&#160;Structural and morphological analysis of the SnSe and CdSe-SnSe particles and pellet. (A)&#160;XRD analysis and SEM images of (B) SnSe and (C) CdSe-SnSe particles obtained after the solution synthesis, annealed powder, and consolidated pellet. Scale bars = 1 &#181;m. This figure has been modified from Liu et al.22. <a href="https://www.jove.com/files/ftp_upload/66278/66278fig07large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 8" class="xfigimg" src="/files/ftp_upload/66278/66278fig08.jpg" /> 
Figure 8: Thermoelectric properties of pure SnSe and CdSe-SnSe. (A) Electrical conductivity, (B) Seebeck coefficient, (C) total thermal conductivity, and (D) thermoelectric figure of merit. <a href="https://www.jove.com/files/ftp_upload/66278/66278fig08large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
Supplemental Figure S1: Die characteristics and dimensions. <a href="https://www.jove.com/files/ftp_upload/66278/Figure S1.pdf" target="_blank">Please click here to download this File.</a>
Supplemental Figure S2: Adapters used to cut the SnSe samples with respect to the pressing directions. <a href="https://www.jove.com/files/ftp_upload/66278/Figure S2.pdf" target="_blank">Please click here to download this File.</a>
Supplemental Figure S3: Density measurement setup for SnSe and CdSe-SnSe samples. The mass of the pellet measured in (A) air and (B) water. <a href="https://www.jove.com/files/ftp_upload/66278/Figure S3.pdf" target="_blank">Please click here to download this File.</a>
Supplemental Table S1: Die characteristics and specifications. <a href="https://www.jove.com/files/ftp_upload/66278/Table S1.xlsx" target="_blank">Please click here to download this File.</a>

CdSe-SnSe nanocomposites are produced by consolidating surface-engineered SnSe particles. A simple aqueous synthesis is employed to produce SnSe particles. Coating SnSe particles with CdSe molecular complexes allows for controlling grain size and increasing the number of defects present in the nanocomposite, thus lowering the thermal conductivity.

In recent years, solution processes have gained considerable traction as a cost-effective and scalable method to produce high-performance thermoelectric ...

solution-processed-surface-engineered-polycrystalline-cdse-snse

Research

JoVE Journal

Chemistry

810 Views.  Institute of Science and Technology Austria (ISTA). CdSe-SnSe nanocomposites are produced by consolidating surface-engineered SnSe particles. A simple aqueous synthesis is employed to produce SnSe particles. Coating SnSe particles with CdSe molecular complexes allows for controlling grain size and increasing the number of defects present in the nanocomposite, thus lowering the thermal conductivity.