Functional Monolithic Polymeric Organic Framework Aerogel as Reducing and Hosting Media for Ag nanoparticles and Application in Capturing of Iodine Vapors

Monolithic aerogels of polymeric organic framework (Mon-POF) with a high density of OH functional groups were synthesized through solvothermal polymerization of terephthalaldehyde and 1,5-dihydroxynaphthalene. This POF material presents high surface area of 1230 m² g^–1 having micro-, meso-, macropores, and low bulk density of 0.15 g cm^–3. The evolution of the porous properties is controlled with the polymerization rate. Mon-POF is stable under acidic and basic conditions. The presence of high number of OH functional groups provides the monolith with ion-exchange properties as well as reducing properties. The Mon-POF adsorbs Ag⁺ from aqueous solution to deposit Ag nanoparticles into the pores at a high loading content ∼25 wt % of the composite material. The Ag loaded monolith captures significant amount of I₂ vapor and fixes it effectively in the form of β-AgI

Selective Surfaces: Quaternary Co(Ni)MoS-Based Chalcogels with Divalent (Pb²⁺, Cd²⁺, Pd²⁺) and Trivalent (Cr³⁺, Bi³⁺) Metals for Gas Separation

Porous chalcogels with tunable compositions of Co_<i>x</i>M_1<i>–x</i>MoS₄ and Ni_<i>x</i>M_1–<i>x</i>MoS₄, where M = Pd²⁺, Pb²⁺, Cd²⁺, Bi³⁺, or Cr³⁺ and <i>x</i> = 0.3–0.7, were synthesized by metathesis reactions between the metal ions and MoS₄^2–. Solvent exchange, counterion removal and CO₂ supercritical drying led to the formation of aerogels. All chalcogels exhibited high surface areas (170–510 m²/g) and pore volumes in the 0.56–1.50 cm³/g range. Electron microscopy coupled with nitrogen adsorption measurements suggest the presence of both mesoporosity (2 nm < <i>d</i> < 50 nm) and macroporosity (<i>d</i> > 50 nm, where <i>d</i> is the average pore size). Pyridine adsorption corroborated for the acid character of the aerogels. We present X-ray photoelectron spectroscopic and X-ray scattering evidence that the [MoS₄]^2– unit does not stay intact when bound to the metals in the chalcogel structure. The Mo⁶⁺ species undergoes redox reactions during network assembly, giving rise to Mo^4+/5+-containing species where the Mo is bound to sulfide and polysulfide ligands. The chalcogels exhibit high adsorption selectivities for CO₂ and C₂H₆ over H₂, N₂, and CH₄ whereas specific compositions exhibited among the highest CO₂ enthalpy of adsorption reported so far for a porous material (up to 47 kJ/mol). The Co-Pb-MoS₄ and Co-Cr-MoS₄ chalcogels exhibited a 2-fold to 4-fold increase in CO₂/H₂ selectivity compared to ternary CoMoS₄ chalcogels

Seeing Is Believing: Weak Phonon Scattering from Nanostructures in Alkali Metal-Doped Lead Telluride

Alkali metal doped p-type PbTe is a canonical thermoelectric material studied extensively for heat-to-power generation at high temperature. Most reports have indirectly indicated alkali metals to be conventional with PbTe forming homogeneous solid solutions. Using transmission electron microscopy (TEM), we show the presence of platelet-like nanostructures in these systems containing Na and/or K. By combining further TEM and semiclassical theoretical calculations based on a modified Debye model of the lattice thermal conductivity, we explain the lack of efficacy of these nanostructures for strong phonon scattering. These findings are important in the understanding of alkali metals as carriers in p-type lead chalcogenides. These results also underscore that not all nanostructures favorably scatter phonons in a matrix; an insight that may help in further improvements of the power factor and the overall figure of merit

High Thermoelectric Performance Realized in a BiCuSeO System by Improving Carrier Mobility through 3D Modulation Doping

We report a greatly enhanced thermoelectric performance in a BiCuSeO system, realized by improving carrier mobility through modulation doping. The heterostructures of the modulation doped sample make charge carriers transport preferentially in the low carrier concentration area, which increases carrier mobility by a factor of 2 while maintaining the carrier concentration similar to that in the uniformly doped sample. The improved electrical conductivity and retained Seebeck coefficient synergistically lead to a broad, high power factor ranging from 5 to 10 μW cm^–1 K^–2. Coupling the extraordinarily high power factor with the extremely low thermal conductivity of ∼0.25 W m^–1 K^–1 at 923 K, a high <i>ZT</i> ≈ 1.4 is achieved in a BiCuSeO system

Highly Enhanced Thermoelectric Properties of Bi/Bi₂S₃ Nanocomposites

Bismuth sulfide (Bi₂S₃) has been of high interest for thermoelectric applications due to the high abundance of sulfur on Earth. However, the low electrical conductivity of pristine Bi₂S₃ results in a low figure of merit (ZT). In this work, Bi₂S₃@Bi core–shell nanowires with different Bi shell thicknesses were prepared by a hydrothermal method. The core–shell nanowires were densified to Bi/Bi₂S₃ nanocomposite by spark plasma sintering (SPS), and the structure of the nanowire was maintained as the nanocomposite due to rapid SPS processing and low sintering temperature. The thermoelectric properties of bulk samples were investigated. The electrical conductivity of a bulk sample after sintering at 673 K for 5 min using Bi₂S₃@Bi nanowire powders prepared by treating Bi₂S₃ nanowires in a hydrazine solution for 3 h is 3 orders of magnitude greater than that of a pristine Bi₂S₃ sample. The nanocomposite possessed the highest ZT value of 0.36 at 623 K. This represents a new strategy for densifying core–shell powders to enhance their thermoelectric properties

Energetics of Nanoparticle Exsolution from Perovskite Oxides

The presence of active metal nanoparticles on the surface significantly increases the electrochemical performance of ABO₃ perovskite oxide materials. While conventional deposition methods can improve the activity, <i>in situ</i> exsolution produces nanoparticles with far greater stability. The migration of transition metal atoms toward the surface is expected to affect the exsolution process. To study the energetics, we use <i>ab initio</i> computations combined with experiments in a SrTiO₃-based model system. Our calculations show that Ni preferentially segregates toward the (100)-oriented and SrTiO-terminated surfaces, note that this orientation is identical to one reported by the Irvine and Gorte groups. Vacancies in the Sr-site and O-site promote the segregation of Ni, while placing La on the Sr-site has an opposite effect. The corresponding experiments are in agreement with the computational predictions. Fast nanoparticle growth and activity enhancement are found in STO system with Sr vacancies and without La. The approach developed in this Letter could be used to study the mechanism of exsolution in other material systems, and possibly lead to the development of new compositions capable of nanoparticle exsolution with higher activity and stability

Raising the Thermoelectric Performance of p‑Type PbS with Endotaxial Nanostructuring and Valence-Band Offset Engineering Using CdS and ZnS

We have investigated in detail the effect of CdS and ZnS as second phases on the thermoelectric properties of p-type PbS. We report a <i>ZT</i> of ∼1.3 at 923 K for 2.5 at.% Na-doped p-type PbS with endotaxially nanostructured 3.0 at.% CdS. We attribute the high <i>ZT</i> to the combination of broad-based phonon scattering on multiple length scales to reduce (lattice) thermal conductivity and favorable charge transport through coherent interfaces between the PbS matrix and metal sulfide nanophase precipitates, which maintains the requisite high carrier conductivity and the associated power factor. Similar to large ionically bonded metal sulfides (ZnS, CaS, and SrS), the covalently bonded CdS can also effectively reduce the lattice thermal conductivity in p-type PbS. The presence of ubiquitous nanostructuring was confirmed by transmission electron microscopy. Valence and conduction band energy levels of the NaCl-type metal sulfides, MS (M = Pb, Cd, Zn, Ca, and Sr) were calculated from density functional theory to gain insight into the band alignment between PbS and the second phases in these materials. The hole transport is controlled by band offset minimization through the alignment of valence bands between the host PbS and the embedded second phases, MS (M = Cd, Zn, Ca, and Sr). The smallest valence band offset of about 0.13 eV at 0 K was found between PbS and CdS which is diminished further by thermal band broadening at elevated temperature. This allows carrier transport between the endotaxially aligned components (i.e., matrix and nanostructure), thus minimizing significant deterioration of the hole mobility and power factor. We conclude the thermoelectric performance of the PbS system and, by extension, other systems can be enhanced by means of a closely coupled phonon-blocking/electron-transmitting approach through embedding endotaxially nanostructured second phases

Substrateless Welding of Self-Assembled Silver Nanowires at Air/Water Interface

Integrating connected silver nanowire networks with flexible polymers has appeared as a popular way to prepare flexible electronics. To reduce the contact resistance and enhance the connectivity between silver nanowires, various welding techniques have been developed. Herein, rather than welding on solid supporting substrates, which often requires complicated transferring operations and also may pose damage to heat-sensitive substrates, we report an alternative approach to prepare easily transferrable conductive networks through welding of self-assembled silver nanowires at the air/water interface using plasmonic heating. The intriguing welding behavior of partially aligned silver nanowires was analyzed with combined experimental observation and theoretical modeling. The underlying water not only physically supports the assembled silver nanowires but also buffers potential overheating during the welding process, thereby enabling effective welding within a broad range of illumination power density and illumination duration. The welded networks could be directly integrated with PDMS substrates to prepare high-performance stable flexible heaters that are stretchable, bendable, and can be easily patterned to explore selective heating applications

Thermoelectrics with Earth Abundant Elements: High Performance p-type PbS Nanostructured with SrS and CaS

We report high thermoelectric performance in nanostructured p-type PbS, a material consisting of highly earth abundant and inexpensive elements. The high level of Na doping switched intrinsic n-type PbS to p-type and substantially raised the power factor maximum for pure PbS to ∼9.0 μW cm^–1 K^–2 at >723 K using 2.5 at. % Na as the hole dopant. Contrary to that of PbTe, no enhancement in the Hall coefficient occurs at high temperature for heavily doped p-type PbS, indicating a single band model and no heavy hole band. We also report that the lattice thermal conductivity of PbS can be greatly reduced by adding SrS or CaS, which form a combination of a nanostructured/solid solution material as determined by transmission electron microscopy. We find that both nanoscale precipitates and point defects play an important role in reducing the lattice thermal conductivity, but the contribution from nanoscale precipitates of SrS is greater than that of CaS, whereas the contribution from point defects in the case of CaS is greater than that of SrS. Theoretical calculations of the lattice thermal conductivity based on the modified Callaway model reveal that both nanostructures and point defects (solid solution) effectively scatter phonons in this system. The lattice thermal conductivity at 723 K can be reduced by ∼50% by introducing up to 4.0 at. % of either SrS or CaS. As a consequence, <i>ZT</i> values as high as 1.22 and 1.12 at 923 K can be achieved for nominal Pb_0.975Na_0.025S with 3.0 at. % SrS and CaS, respectively. No deterioration was observed after a 15 d annealing treatment of the samples, indicating the excellent thermal stability for these high performance thermoelectrics. The promising thermoelectric properties of nanostructured PbS point to a robust low cost alternative to other high performance thermoelectric materials

Understanding Phonon Scattering by Nanoprecipitates in Potassium-Doped Lead Chalcogenides

We present a comprehensive experimental and theoretical study of phonon scattering by nanoprecipitates in potassium-doped PbTe, PbSe, and PbS. We highlight the role of the precipitate size distribution measured by microscopy, whose tuning allows for thermal conductivities lower than the limit achievable with a single size. The correlation between the size distribution and the contributions to thermal conductivity from phonons in different frequency ranges provides a physical basis to the experimentally measured thermal conductivities, and a criterion to estimate the lowest achievable thermal conductivity. The results have clear implications for efficiency enhancements in nanostructured bulk thermoelectrics