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><tbody><tr><td>&lt;strong&gt;Chemicals&lt;/strong&gt;</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&amp;nbsp;</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>&lt;strong&gt;Apparatus/Materials&lt;/strong&gt;</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&amp;nbsp;</td><td>Sartorius</td><td>5007021150</td><td></td></tr><tr><td>Carbon tape&amp;nbsp;</td><td>Micro to Nano</td><td>15-000508</td><td></td></tr><tr><td>Centrifuge tubes x 4&amp;nbsp;</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&amp;nbsp;</td><td>Bartelt</td><td>6.082 194</td><td>250 mL</td></tr><tr><td>Micropipette&amp;nbsp;</td><td>Eppendorf</td><td>3123000063</td><td>Research plus 100-1000&amp;micro;L (GLP)</td></tr><tr><td>Quartz tube&amp;nbsp;</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&amp;nbsp;</td><td>Bartelt</td><td>4.008 387</td><td>500 mL</td></tr><tr><td>Round-bottom flask 3-neck&amp;nbsp;</td><td>Lactan</td><td>E614.1</td><td>1000 mL</td></tr><tr><td>Rubber septum x 3&amp;nbsp;</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&amp;nbsp;</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&amp;nbsp; 4&amp;nbsp;</td><td>Sigma-Aldrich</td><td>Z561754-1EA</td><td>20 mL</td></tr><tr><td>Succintillation vials x&amp;nbsp; 1</td><td>Bartelt</td><td>9.003 482</td><td>4 mL</td></tr><tr><td>&lt;strong&gt;Equipment&lt;/strong&gt;</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&amp;nbsp;</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&amp;trade;&amp;nbsp;</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&amp;nbsp;</td><td>Amazon</td><td></td><td></td></tr><tr><td>FE-SEM Merlin VP Contact</td><td>Carl Zeiss&amp;nbsp;</td><td>Merlin Compact VP</td><td></td></tr><tr><td>Heating mantle 1000 mL&amp;nbsp;</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&amp;nbsp;</td><td>Heidolph</td><td>MR Hei-Tec</td><td></td></tr><tr><td>Tubular furnace&amp;nbsp;</td><td>Hansun Electric Technology Co. Ltd</td><td>Compact split tube furnace</td><td></td></tr><tr><td>&lt;strong&gt;Software&lt;/strong&gt;</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.

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>

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

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.

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 ...

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

Research

JoVE Journal

Chemistry

1.4K 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.

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

Harvesting Solar Energy by Means of Charge-Separating Nanocrystals and Their Solids

Conjoining different semiconductor materials in a single nano-composite provides synthetic means for the development of novel optoelectronic materials offering a superior control over the spatial distribution of charge carriers across material interfaces. As this study demonstrates, a combination of donor-acceptor nanocrystal (NC) domains in a single nanoparticle can lead to the realization of efficient photocatalytic1-5 materials, while a layered assembly of donor- and acceptor-like nanocrystals films gives rise to photovoltaic materials.
Initially the paper focuses on the synthesis of composite inorganic nanocrystals, comprising linearly stacked ZnSe, CdS, and Pt domains, which jointly promote photoinduced charge separation. These structures are used in aqueous solutions for the photocatalysis of water under solar radiation, resulting in the production of H2 gas. To enhance the photoinduced separation of charges, a nanorod morphology with a linear gradient originating from an intrinsic electric field is used5. The inter-domain energetics are then optimized to drive photogenerated electrons toward the Pt catalytic site while expelling the holes to the surface of ZnSe domains for sacrificial regeneration (via methanol). Here we show that the only efficient way to produce hydrogen is to use electron-donating ligands to passivate the surface states by tuning the energy level alignment at the semiconductor-ligand interface. Stable and efficient reduction of water is allowed by these ligands due to the fact that they fill vacancies in the valence band of the semiconductor domain, preventing energetic holes from degrading it. Specifically, we show that the energy of the hole is transferred to the ligand moiety, leaving the semiconductor domain functional. This enables us to return the entire nanocrystal-ligand system to a functional state, when the ligands are degraded, by simply adding fresh ligands to the system4.
To promote a photovoltaic charge separation, we use a composite two-layer solid of PbS and TiO2 films. In this configuration, photoinduced electrons are injected into TiO2 and are subsequently picked up by an FTO electrode, while holes are channeled to a Au electrode via PbS layer6. To develop the latter we introduce a Semiconductor Matrix Encapsulated Nanocrystal Arrays (SMENA) strategy, which allows bonding PbS NCs into the surrounding matrix of CdS semiconductor. As a result, fabricated solids exhibit excellent thermal stability, attributed to the heteroepitaxial structure of nanocrystal-matrix interfaces, and show compelling light-harvesting performance in prototype solar cells7.

A general strategy for the development of charge-separating semiconductor nanocrystal composites deployable for solar energy production is presented. We show that assembly of donor-acceptor nanocrystal domains in a single nanoparticle geometry gives rise to a photocatalytic function, while bulk-heterojunctions of donor-acceptor nanocrystal films can be used for photovoltaic energy conversion.

Conjoining different semiconductor materials in a single nano-composite provides synthetic means for the development of novel optoelectronic materials ...

Engineering

Highly Stereoselective Synthesis of 1,6-Ketoesters Mediated by Ionic Liquids: A Three-component Reaction Enabling Rapid Access to a New Class of Low Molecular Weight Gelators

In organic chemistry ionic liquids (ILs) have emerged as safe and recyclable reaction solvents. In the presence of a base ILs can be deprotonated to form catalytically active N-Heterocyclic Carbenes (NHCs). Here we have used ILs as precatalysts in the addition of &#945;,&#946;-unsaturated aldehydes to chalcones to form 1,6-ketoesters, incorporating an anti-diphenyl moiety in a highly stereoselective fashion. The reaction has a broad substrate scope and several functional groups and heteroaromatics can be integrated into the ketoester backbone in generally good yields with maintained stereoselectivity. The reaction protocol is robust and scalable. The starting materials are inexpensive and the products can be obtained after simple filtration, avoiding solvent-demanding chromatography. Furthermore, the IL can be recycled up to 5 times without any loss of reactivity. Moreover, the 1,6-ketoester end product is a potent gelator in several hydrocarbon based solvents. The method enables rapid access to and evaluation of a new class of low molecular weight gelators (LMWGs) from recyclable and inexpensive starting materials.

Ionic liquids (ILs) mediate fast, simple and cheap access to 1,6-ketoesters in high diastereoselectivities and good yields. The reaction protocol is robust and the 1,6-ketoesters can be obtained in gram scale after a simple filtration protocol. Moreover, the 1,6-ketoesters are potent gelators in hydrocarbon solvents.

In organic chemistry ionic liquids (ILs) have emerged as safe and recyclable reaction solvents. In the presence of a base ILs can be deprotonated to form ...

Making Record-efficiency SnS Solar Cells by Thermal Evaporation and Atomic Layer Deposition

Tin sulfide (SnS) is a candidate absorber material for Earth-abundant, non-toxic solar cells. SnS offers easy phase control and rapid growth by congruent thermal evaporation, and it absorbs visible light strongly. However, for a long time the record power conversion efficiency of SnS solar cells remained below 2%. Recently we demonstrated new certified record efficiencies of 4.36% using SnS deposited by atomic layer deposition, and 3.88% using thermal evaporation. Here the fabrication procedure for these record solar cells is described, and the statistical distribution of the fabrication process is reported. The standard deviation of efficiency measured on a single substrate is typically over 0.5%. All steps including substrate selection and cleaning, Mo sputtering for the rear contact (cathode), SnS deposition, annealing, surface passivation, Zn(O,S) buffer layer selection and deposition, transparent conductor (anode) deposition, and metallization are described. On each substrate we fabricate 11 individual devices, each with active area 0.25 cm2. Further, a system for high throughput measurements of current-voltage curves under simulated solar light, and external quantum efficiency measurement with variable light bias is described. With this system we are able to measure full data sets on all 11 devices in an automated manner and in minimal time. These results illustrate the value of studying large sample sets, rather than focusing narrowly on the highest performing devices. Large data sets help us to distinguish and remedy individual loss mechanisms affecting our devices.

Tin sulfide (SnS) is a candidate material for Earth-abundant, non-toxic solar cells. Here, we demonstrate the fabrication procedure of the SnS solar cells employing atomic layer deposition, which yields 4.36% certified power conversion efficiency, and thermal evaporation which yields 3.88%.

Tin sulfide (SnS) is a candidate absorber material for Earth-abundant, non-toxic solar cells. SnS offers easy phase control and rapid growth by congruent ...

Preparation of Biopolymer Aerogels Using Green Solvents

Although the first reports on aerogels made by Kistler1 in the 1930s dealt with aerogels from both inorganic oxides (silica and others) and biopolymers (gelatin, agar, cellulose), only recently have biomasses been recognized as an abundant source of chemically diverse macromolecules for functional aerogel materials. Biopolymer aerogels (pectin, alginate, chitosan, cellulose, etc.) exhibit both specific inheritable functions of starting biopolymers and distinctive features of aerogels (80-99% porosity and specific surface up to 800 m2/g). This synergy of properties makes biopolymer aerogels promising candidates for a wide gamut of applications such as thermal insulation, tissue engineering and regenerative medicine, drug delivery systems, functional foods, catalysts, adsorbents and sensors. This work demonstrates the use of pressurized carbon dioxide (5 MPa) for the ionic cross linking of amidated pectin into hydrogels. Initially a biopolymer/salt dispersion is prepared in water. Under pressurized CO2 conditions, the pH of the biopolymer solution is lowered to 3 which releases the crosslinking cations from the salt to bind with the biopolymer yielding hydrogels. Solvent exchange to ethanol and further supercritical CO2 drying (10 - 12 MPa) yield aerogels. Obtained aerogels are ultra-porous with low density (as low as 0.02 g/cm3), high specific surface area (350 - 500 m2/g) and pore volume (3 - 7 cm3/g for pore sizes less than 150 nm).

A new way for the production of biopolymer-based aerogels by carbon dioxide (CO2) induced gelation is shown. The technique utilizes pressurized carbon dioxide (5 MPa) for the production of biopolymer hydrogels and supercritical CO2 (12 MPa) to convert gels into aerogels. The only solvents needed besides CO2 are water and ethanol.

Although the first reports on aerogels made by Kistler1 in the 1930s dealt with aerogels from both inorganic oxides (silica and others) and biopolymers ...

Low-energy Cathodoluminescence for (Oxy)Nitride Phosphors

Nitride and oxynitride (Sialon) phosphors are good candidates for the ultraviolet and visible emission applications. High performance, good stability and flexibility of their emission properties can be achieved by controlling their composition and dopants. However, a lot of work is still required to improve their properties and to reduce the production cost. A possible approach is to correlate the luminescence properties of the Sialon particles with their local structural and chemical environment in order to optimize their growth parameters and find novel phosphors. For such a purpose, the low-voltage cathodoluminescence (CL) microscopy is a powerful technique. The use of electron as an excitation source allows detecting most of the luminescence centers, revealing their luminescence distribution spatially and in depth, directly comparing CL results with the other electron-based techniques, and investigating the stability of their luminescence properties under stress. Such advantages for phosphors characterization will be highlighted through examples of investigation on several Sialon phosphors by low-energy CL.

Low-energy Cathodoluminescence for OxyNitride Phosphors

An excellent chemical and luminescence stabilities of (oxy)nitride phosphors present it as an promising alternative to currently used sulfide and oxide phosphors. In this paper, we present the way to investigate its local luminescence properties using low-energy cathodoluminescence (CL).

Nitride and oxynitride (Sialon) phosphors are good candidates for the ultraviolet and visible emission applications. High performance, good stability and ...

A Simple, Low-cost, and Robust System to Measure the Volume of Hydrogen Evolved by Chemical Reactions with Aqueous Solutions

There is a growing research interest in the development of portable systems which can deliver hydrogen on-demand to proton exchange membrane (PEM) hydrogen fuel cells. Researchers seeking to develop such systems require a method of measuring the generated hydrogen. Herein, we describe a simple, low-cost, and robust method to measure the hydrogen generated from the reaction of solids with aqueous solutions. The reactions are conducted in a conventional one-necked round-bottomed flask placed in a temperature controlled water bath. The hydrogen generated from the reaction in the flask is channeled through tubing into a water-filled inverted measuring cylinder. The water displaced from the measuring cylinder by the incoming gas is diverted into a beaker on a balance. The balance is connected to a computer, and the change in the mass reading of the balance over time is recorded using data collection and spreadsheet software programs. The data can then be approximately corrected for water vapor using the method described herein, and parameters such as the total hydrogen yield, the hydrogen generation rate, and the induction period can also be deduced. The size of the measuring cylinder and the resolution of the balance can be changed to adapt the setup to different hydrogen volumes and flow rates.

The study of methods to generate on-demand hydrogen for fuel cells continues to grow in importance. However, systems to measure hydrogen evolution from the reaction of chemicals with water can be complicated and expensive. This article details a simple, low-cost, and robust method to measure the evolution of hydrogen gas.

There is a growing research interest in the development of portable systems which can deliver hydrogen on-demand to proton exchange membrane (PEM) ...

Fabrication of Fully Solution Processed Inorganic Nanocrystal Photovoltaic Devices

We demonstrate a method for the preparation of fully solution processed inorganic solar cells from a spin and spray coating deposition of nanocrystal inks. For the photoactive absorber layer, colloidal CdTe and CdSe nanocrystals (3-5 nm) are synthesized using an inert hot injection technique and cleaned with precipitations to remove excess starting reagents. Similarly, gold nanocrystals (3-5 nm) are synthesized under ambient conditions and dissolved in organic solvents. In addition, precursor solutions for transparent conductive indium tin oxide (ITO) films are prepared from solutions of indium and tin salts paired with a reactive oxidizer. Layer-by-layer, these solutions are deposited onto a glass substrate following annealing (200-400 &#176;C) to build the nanocrystal solar cell (glass/ITO/CdSe/CdTe/Au). Pre-annealing ligand exchange is required for CdSe and CdTe nanocrystals where films are dipped in NH4Cl:methanol to replace long-chain native ligands with small inorganic Cl- anions. NH4Cl(s) was found to act as a catalyst for the sintering reaction (as a non-toxic alternative to the conventional CdCl2(s) treatment) leading to grain growth (136&#177;39 nm) during heating. The thickness and roughness of the prepared films are characterized with SEM and optical profilometry. FTIR is used to determine the degree of ligand exchange prior to sintering, and XRD is used to verify the crystallinity and phase of each material. UV/Vis spectra show high visible light transmission through the ITO layer and a red shift in the absorbance of the cadmium chalcogenide nanocrystals after thermal annealing. Current-voltage curves of completed devices are measured under simulated one sun illumination. Small differences in deposition techniques and reagents employed during ligand exchange have been shown to have a profound influence on the device properties. Here, we examine the effects of chemical (sintering and ligand exchange agents) and physical treatments (solution concentration, spray-pressure, annealing time and annealing temperature) on photovoltaic device performance.

This protocol describes the synthesis and solution deposition of inorganic nanocrystals layer by layer to produce thin film electronics on non-conductive surfaces. Solvent-stabilized inks can produce complete photovoltaic devices on glass substrates via spin and spray coating following post-deposition ligand exchange and sintering.

We demonstrate a method for the preparation of fully solution processed inorganic solar cells from a spin and spray coating deposition of nanocrystal ...

Green and Low-cost Production of Thermally Stable and Carboxylated Cellulose Nanocrystals and Nanofibrils Using Highly Recyclable Dicarboxylic Acids

Here we demonstrate potentially low cost and green productions of high thermally stable and carboxylated cellulose nanocrystals (CNCs) and nanofibrils (CNF) from bleached eucalyptus pulp (BEP) and unbleached mixed hardwood kraft pulp (UMHP) fibers using highly recyclable dicarboxylic solid acids. Typical operating conditions were acid concentrations of 50 - 70 wt% at 100 &#176;C for 60 min and 120 &#176;C (no boiling at atmospheric pressure) for 120 min, for BEP and UMHP, respectively. The resultant CNCs have a higher thermal degradation temperature than their corresponding feed fibers and carboxylic acid group content from 0.2 - 0.4 mmol/g. The low strength (high pKa of 1.0 - 3.0) of organic acids also resulted in CNCs with both longer lengths of approximately 239 - 336 nm and higher crystallinity than CNCs produced using mineral acids. Cellulose loss to sugar was minimal. Fibrous cellulosic solid residue (FCSR) from the dicarboxylic acid hydrolysis was used to produce carboxylated CNFs through subsequent mechanical fibrillation with low energy input.

Here we demonstrate a novel method for green and sustainable productions of highly thermally stable and carboxylated cellulose nanocrystals (CNC) and nanofibrils (CNF) using highly recyclable solid dicarboxylic acids.

Here we demonstrate potentially low cost and green productions of high thermally stable and carboxylated cellulose nanocrystals (CNCs) and nanofibrils ...

Low Pressure Vapor-assisted Solution Process for Tunable Band Gap Pinhole-free Methylammonium Lead Halide Perovskite Films

Organo-lead halide perovskites have recently attracted great interest for potential applications in thin-film photovoltaics and optoelectronics. Herein, we present a protocol for the fabrication of this material via the low-pressure vapor assisted solution process (LP-VASP) method, which yields ~19% power conversion efficiency in planar heterojunction perovskite solar cells. First, we report the synthesis of methylammonium iodide (CH3NH3I) and methylammonium bromide (CH3NH3Br) from methylamine and the corresponding halide acid (HI or HBr). Then, we describe the fabrication of pinhole-free, continuous methylammonium-lead halide perovskite (CH3NH3PbX3 with X = I, Br, Cl and their mixture) films with the LP-VASP. This process is based on two steps: i) spin-coating of a homogenous layer of lead halide precursor onto a substrate, and ii) conversion of this layer to CH3NH3PbI3-xBrx by exposing the substrate to vapors of a mixture of CH3NH3I and CH3NH3Br at reduced pressure and 120 &#176;C. Through slow diffusion of the methylammonium halide vapor into the lead halide precursor, we achieve slow and controlled growth of a continuous, pinhole-free perovskite film. The LP-VASP allows synthetic access to the full halide composition space in CH3NH3PbI3-xBrx with 0&#160;&#8804; x&#160;&#8804; 3. Depending on the composition of the vapor phase, the bandgap can be tuned between 1.6 eV&#160;&#8804; Eg&#160;&#8804; 2.3 eV. In addition, by varying the composition of the halide precursor and of the vapor phase, we can also obtain CH3NH3PbI3-xClx. Films obtained from the LP-VASP are reproducible, phase pure as confirmed by X-ray diffraction measurements, and show high photoluminescence quantum yield. The process does not require the use of a glovebox.

Here, we present a protocol for the synthesis of CH3NH3I and CH3NH3Br precursors and the subsequent formation of pinhole-free, continuous CH3NH3PbI3-xBrx thin films for the application in high efficiency solar cells and other optoelectronic devices.

Organo-lead halide perovskites have recently attracted great interest for potential applications in thin-film photovoltaics and optoelectronics. Herein, ...

Close-Space Sublimation-Deposited Ultra-Thin CdSeTe/CdTe Solar Cells for Enhanced Short-Circuit Current Density and Photoluminescence

Developments in photovoltaic device architectures are necessary to make solar energy a cost-effective and reliable source of renewable energy amidst growing global energy demands and climate change. Thin film CdTe technology has demonstrated cost-competitiveness and increasing efficiencies due partially to rapid fabrication times, minimal material usage, and introduction of a CdSeTe alloy into a ~3 &#956;m absorber layer. This work presents the close-space sublimation fabrication of thin, 1.5 &#181;m CdSeTe/CdTe bilayer devices using an automated in-line vacuum deposition system. The thin bilayer structure and fabrication technique minimize deposition time, increase device efficiency, and facilitate future thin absorber-based device architecture development. Three fabrication parameters appear to be the most impactful for optimizing thin CdSeTe/CdTe absorber devices: substrate preheat temperature, CdSeTe:CdTe thickness ratio, and CdCl2 passivation. For proper sublimation of the CdSeTe, the substrate temperature prior to deposition must be ~540 &#176;C (higher than that for CdTe) as controlled by dwell time in a preheat source. Variation in the CdSeTe:CdTe thickness ratio reveals a strong dependence of device performance on this ratio. The optimal absorber thicknesses are 0.5 &#956;m CdSeTe/1.0 &#956;m CdTe, and non-optimized thickness ratios reduce efficiency through back-barrier effects. Thin absorbers are sensitive to CdCl2 passivation variation; a much less aggressive CdCl2 treatment (compared to thicker absorbers) regarding both temperature and time yields optimal device performance. With optimized fabrication conditions, CdSeTe/CdTe increases device short-circuit current density and photoluminescence intensity compared to single-absorber CdTe. Additionally, an in-line close-space sublimation vacuum deposition system offers material and time reduction, scalability, and attainability of future ultra-thin absorber architectures.

This work describes the complete fabrication process of thin absorber cadmium selenium telluride/cadmium telluride photovoltaic devices for enhanced efficiency. The process utilizes an automated in-line vacuum system for close-space sublimation deposition that is scalable, from fabrication of small area research devices as well as large-scale modules.

Developments in photovoltaic device architectures are necessary to make solar energy a cost-effective and reliable source of renewable energy amidst ...

Solution-Processed, Surface-Engineered, Polycrystalline CdSe-SnSe Exhibiting Low Thermal Conductivity

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