Mapping Li⁺ Concentration and Transport via In Situ Confocal Raman Microscopy

We demonstrate confocal Raman microscopy as a general, nonperturbative tool to measure spatially resolved lithium ion concentrations in liquid electrolytes. By combining this high-spatial-resolution technique with a simple microfluidic device, we are able to measure the diffusion coefficient of lithium ions in dimethyl carbonate in two different concentration regimes. Because lithium ion transport plays a key role in the function of a variety of electrochemical devices, quantifying and visualizing this process is crucial for understanding device performance. This method for detecting lithium ions should be immediately useful in the study of lithium-ion-based devices, ion transport in porous media, and at electrode–electrolyte interfaces, and the analytical framework is useful for any system exhibiting a concentration-dependent Raman spectrum

Evolution of Vibrational Properties in Lanthanum Hexaboride Nanocrystals

Lanthanum hexaboride (LaB₆) is known for its hardness, mechanical strength, thermionic emission, and strong plasmonic properties. However, given the lack of colloidal synthetic methods to access this material, very little is understood about its physical properties on the nanoscale. Recently, a new moderate-temperature synthetic technique was developed to directly synthesize LaB₆ nanoparticles [Mattox et al. Chem. Mater. 2015, 27, 6620]. We report the influence of nanoparticle size on the structural and vibrational properties of LaB₆ using a combination of Raman and Fourier transform infrared spectroscopies. Our studies indicate that the size of the lanthanum salt anion has a larger influence on LaB₆ vibrational energies than particle size. Surprisingly, our work finds that the LaB₆ lattice readily expands to accommodate larger ions and contracts with their removal, while ligand incorporation significantly amplifies and shifts the Raman stretching modes

Effects of Size and Structural Defects on the Vibrational Properties of Lanthanum Hexaboride Nanocrystals

Lanthanum hexaboride (LaB₆) is notable for its thermionic emission and mechanical strength and is being explored for its potential applications in IR-absorbing photovoltaic cells and thermally insulating window coatings. Previous studies have not investigated how the properties of LaB₆ change on the nanoscale. Despite interest in the tunable plasmonic properties of nanocrystalline LaB₆, studies have been limited due to challenges in the synthesis of phase-pure, size-controlled, high-purity nanocrystals without high temperatures or pressures. Here, we report, for the first time, the ability to control particle size and boron content through reaction temperature and heating ramp rate, which allows the effects of size and defects on the vibrational modes of the nanocrystals to be studied independently. Understanding these effects is important to develop methods to fully control the properties of nanocrystalline LaB₆, such as IR absorbance. In contrast to previous studies on stoichiometric LaB₆ nanocrystals, we report here that boron content and lanthanum vacancies have a greater influence on their vibrational properties than their particle size

Size-Dependent Permeability Deviations from Maxwell’s Model in Hybrid Cross-Linked Poly(ethylene glycol)/Silica Nanoparticle Membranes

Currently, separation of gaseous mixtures largely relies on energy intensive and expensive processes, like chemical looping of amines. This has driven research into less energy-intensive, passive methods of performing separations such as the use of polymer membranes. Although pure polymer membranes have demonstrated appealing separation performance, they suffer from an inherent trade-off between permeability and selectivity, which limits overall performance. Recent research efforts have shown that the introduction of a secondary phase, often an inorganic species, is added to selectively boost permeability or selectivity. However, these hybrid organic/inorganic systems have not seen widespread adoption because synthetic control over the size, shape, and dispersion of the inorganic species is poor and understanding of transport in these membranes is largely empirical. Thus, understanding and optimizing hybrid membranes requires development of well-controlled model systems in which size, shape, and surface chemistry of the inorganic species are precisely controlled, leading to homogeneous membranes amenable to careful study. Here, we report on the synthesis, characterization, and gas transport properties of tailored hybrid membranes composed of cross-linked poly­(ethylene glycol) and silica nanoparticles. We show excellent control of nanoparticle size, loading, and dispersibility. We find that permeability deviations from Maxwell’s model increases as the size of silica nanoparticle decreases and loading increases. These size-dependent deviations from Maxwell’s model are attributed to interfacial interactions, which scale with surface area and act to decrease segmental chain mobility

Swelling of Graphene Oxide Membranes in Aqueous Solution: Characterization of Interlayer Spacing and Insight into Water Transport Mechanisms

Graphene oxide (GO) has recently emerged as a promising 2D nanomaterial to make high-performance membranes for important applications. However, the aqueous-phase separation capability of a layer-stacked GO membrane can be significantly limited by its natural tendency to swell, that is, absorb water into the GO channel and form an enlarged interlayer spacing (<i>d</i>-spacing). In this study, the <i>d</i>-spacing of a GO membrane in an aqueous environment was experimentally characterized using an integrated quartz crystal microbalance with dissipation and ellipsometry. This method can accurately quantify a <i>d</i>-spacing in liquid and well beyond the typical measurement limit of ∼2 nm. Molecular simulations were conducted to fundamentally understand the structure and mobility of water in the GO channel, and a theoretical model was developed to predict the <i>d</i>-spacing. It was found that, as a dry GO membrane was soaked in water, it initially maintained a <i>d</i>-spacing of 0.76 nm, and water molecules in the GO channel formed a semiordered network with a density 30% higher than that of bulk water but 20% lower than that of the rhombus-shaped water network formed in a graphene channel. The corresponding mobility of water in the GO channel was much lower than in the graphene channel, where water exhibited almost the same mobility as in the bulk. As the GO membrane remained in water, its <i>d</i>-spacing increased and reached 6 to 7 nm at equilibrium. In comparison, the <i>d</i>-spacing of a GO membrane in NaCl and Na₂SO₄ solutions decreased as the ionic strength increased and was ∼2 nm at 100 mM

Enhancing Separation and Mechanical Performance of Hybrid Membranes through Nanoparticle Surface Modification

Membranes with selective gas transport properties and good mechanical integrity are increasingly desired to replace current energy intensive approaches to gas separation. Here, we report on the dual enhancement of transport and mechanical properties of hybrid cross-linked poly­(ethylene glycol) membranes with aminopropyl-modified silica nanoparticles. CO₂ permeability in hybrid membranes exceeds what can be predicted by Maxwell’s equation and surpasses values of the pure polymer. Furthermore, dynamic mechanical and thermogravimetric analyses reveal increases in both the storage modulus and thermal stability in hybrid membranes, with respect to silica nanoparticle loading

Mechanism and Kinetics of Propane and <i>n</i>‑Butane Dehydrogenation over Isolated and Nested SiOZn–OH Sites Grafted onto Silanol Nests of Dealuminated Beta Zeolite

Zn Lewis acid centers were grafted onto the silanol nest created by dealumination of H-BEA zeolite (DeAlBEA). The resulting material was characterized and investigated for propane dehydrogenation to propene and n-butane dehydrogenation to 1,3-butadiene (1,3-BD). For Zn/Al molar ratios (Al is the molar amount in H-BEA) below 0.12, Zn sites are present as isolated (SiOZn–OH) species, but for Zn/Al ratios between 0.12 and 0.60, the SiOZn–OH species form nests in which enhanced electron transfer between Zn and O atoms of the neighboring SiOZn–OH group and H-bonding interaction between adjacent Zn–OH groups occur. The turnover frequency (TOF) for both propane and n-butane dehydrogenation is virtually identical for Zn-DeAlBEA for Zn/Al < 0.12 and then increases almost linearly with increasing Zn/Al ratio from 0.12 to 0.36, indicating the superior activity of Zn atoms in SiOZn–OH nests. In the case of 1-butene dehydrogenation, identical activity is observed for both isolated and nested SiOZn–OH sites. The kinetics of these three reactions was investigated to clarify the difference in activity. The rate coefficient for the forward reaction (dehydrogenation) was found to be 173 mol propene/(mol Zn sites·bar·h) at 773 K over SiOZn–OH nests, and that for the forward n-butane dehydrogenation was found to be 1193 mol butene/(mol Zn sites·bar·h) at 823 K, a value that is significantly higher than those for most other supported non-noble metal catalysts. Regeneration experiments for propane and n-butane dehydrogenation over 0.60Zn-DeAlBEA suggest a good stability of Zn atom in SiOZn–OH nests

Experimental and characterization data supporting the study from Inkjet-printed SnO_x as an effective electron transport layer for planar perovskite solar cells and the effect of Cu doping

Inkjet printing is a more sustainable and scalable fabrication method than spin coating for producing perovskite solar cells (PSCs). Although spin-coated SnO2 has been intensively studied as an effective electron transport layer (ETL) for PSCs, inkjet-printed SnO2 ETLs have not been widely reported. Here, we fabricated inkjet-printed, solution-processed SnOx ETLs for planar PSCs. A champion efficiency of 17.55% was achieved for the cell using a low-temperature processed SnOx ETL. The low-temperature SnOx exhibited an amorphous structure and outperformed the high-temperature crystalline SnO2. The improved performance was attributed to enhanced charge extraction and transport and suppressed charge recombination at ETL/perovskite interfaces, which originated from enhanced electrical and optical properties of SnOx, improved perovskite film quality, and well-matched energy level alignment between the SnOx ETL and the perovskite layer. Furthermore, SnOx was doped with Cu. Cu doping increased surface oxygen defects and upshifted energy levels of SnOx, leading to reduced device performance. A tunable hysteresis was observed for PSCs with Cu-doped SnOx ETLs, decreasing at first and turning into inverted hysteresis afterwards with increasing Cu doping level. This tunable hysteresis was related to the interplay between charge/ion accumulation and recombination at ETL/perovskite interfaces in the case of electron extraction barriers

Ultralow Thermal Conductivity in Polycrystalline CdSe Thin Films with Controlled Grain Size

Polycrystallinity leads to increased phonon scattering at grain boundaries and is known to be an effective method to reduce thermal conductivity in thermoelectric materials. However, the fundamental limits of this approach are not fully understood, as it is difficult to form uniform sub-20 nm grain structures. We use colloidal nanocrystals treated with functional inorganic ligands to obtain nanograined films of CdSe with controlled characteristic grain size between 3 and 6 nm. Experimental measurements demonstrate that thermal conductivity in these composites can fall beneath the prediction of the so-called minimum thermal conductivity for disordered crystals. The measurements are consistent, however, with diffuse boundary scattering of acoustic phonons. This apparent paradox can be explained by an overattribution of transport to high-energy phonons in the minimum thermal conductivity model where, in compound semiconductors, optical and zone edge phonons have low group velocity and high scattering rates

Robust Interfacial Effect in Multi-interface Environment through Hybrid Reconstruction Chemistry for Enhanced Energy Storage

Electrochemical-oxidation-driven reconstruction has emerged as an efficient approach for developing advanced materials, but the reconstructed microstructure still faces challenges including inferior conductivity, unsatisfying intrinsic activity, and active-species dissolution. Herein, we present hybrid reconstruction chemistry that synergistically couples electrochemical oxidation with electrochemical polymerization (EOEP) to overcome these constraints. During the EOEP process, the metal hydroxides undergo rapid reconstruction and dynamically couple with polypyrrole (PPy), resulting in an interface-enriched microenvironment. We observe that the interaction between PPy and the reconstructed metal center (i.e., Mn > Ni, Co) is strongly correlated. Theoretical calculation results demonstrate that the strong interaction between Mn sites and PPy breaks the intrinsic limitation of MnO2, rendering MnO2 with a metallic property for fast charge transfer and enhancing the ion-adsorption dynamics. Operando Raman measurement confirms the promise of EOEP-treated Mn(OH)2 (E-MO/PPy) to stably work under a 1.2 V potential window. The tailored E-MO/PPy exhibits a high capacitance of 296 F g–1 at a large current density of 100 A g–1. Our strategy presents breakthroughs in upgrading the electrochemical reconstruction technique, which enables both activity and kinetics engineering of electrode materials for better performance in energy-related fields