The commercial feasibility of polymer electrolyte fuel cells (PEFCs) passes by the development of Pt-based, O 2 -reduction catalysts with greater activities and/or lower Pt-contents, as well as an improved stability. In an effort to tackle these requirements, unsupported bimetallic Pt-Ni nanoparticles (NPs) interconnected in the shape of nanochain networks (aerogels) were synthesized using a simple one-step reduction and gel formation process in aqueous solution. The products of this novel synthetic route were characterized by X-ray absorption spectroscopy to elucidate the materials' structure. Using electrochemical experiments, we probed the surface composition of the as-synthesized aerogels and of equivalent materials exposed to acid, and concluded that a Ni-(hydr)oxide side phase is present in the aerogel with a larger Ni-concentration. Regardless of this initial surface composition, the Pt-Ni aerogels feature a ≈3-fold increase of surface-specific ORR activity when compared to a commercial platinum-on-carbon catalyst, reaching the mass-specific requirement for application in automotive PEFCs. 1 to account for the large overpotential of the oxygen reduction reaction (ORR). Thus, Pt contributes significantly to the fuel cell system cost, and progress to reduce its loading is crucial to meet the long-term PEFC cost target of 40 $/kW set by the U. S. Department of Energy (DOE).2 One approach to reduce this excessive Pt-loading relies on increasing the catalysts' ORR activity, e.g. by alloying platinum with other metals like Ni, Cu and Co, to form materials which show up to one order of magnitude higher mass-specific activity than commercial Pt/C catalysts.3 On the other hand, these carbon-supported materials suffer from significant carbonand Pt-corrosion during the standard operation of PEFCs, gradually compromising their efficiency and reliability. 4 To partially overcome stability issues, research focuses on unsupported materials (e.g. Ptcoated Ni, Co or Cu nanowires 5-7 ) besides extended metal surfaces (e.g. 3 M nanostructured thin film catalysts 8 ) or alternative supports (e.g. conductive metal oxides 9-12 ). Naturally, those materials should be processable into catalytic layer architectures that provide reactant and product diffusion pathways similar to those in conventional Pt/C electrodes to guarantee high catalyst utilization and PEFC performance. 13To meet the requirements mentioned above, un-supported bimetallic electrocatalysts with high surface area (up to ≈80 m 2 /g metal ) and nanochain network structure, referred to as aerogels, have been synthesized. [14][15][16] The synthetic routes to prepare such materials vary, but generally involve the use of stabilizing surfactants and/or organic solvents [17][18][19] that can poison the catalyst's surface and decrease its activity. In contrast to those approaches, our groups have developed a facile one-step synthesis for mono-and bimetallic aerogels based on the reduction of metal salts by NaBH 4 in aqueous solution without addition of stabilizing surfac...

2638www.MaterialsViews.com wileyonlinelibrary.com approaches, [ 15,16 ] effi ciency, [ 17,18 ] extending their spectral range [19][20][21] and environmental friendliness using less toxic materials [22][23][24] sparked industrial applications. Those started with the demonstration of a 40 in. display prototype presented by Samsung [ 25 ] and followed by the foundation of QD Vision, Inc., [ 26 ] whose QDs are now used in the latest series of Sony products. All of the mentioned applications require long-term stability of the QDs under various conditions including high temperatures as well as high intensity illumination. Packaging of the QDs within polymer or inorganic matrices is one way to address these issues, while improving the processability of the QDs at the same time. Commonly used polymers, such as polystyrene and poly(methylmethacrylate) [ 27 ] are relatively less stable and tight in comparison to their inorganic counterparts. Inorganic matrices on the other hand can incorporate the QDs directly from their melt, e.g., using the Czochralski approach, [ 28 ] coated as a thin fi lm directly on their surface [ 29,30 ] or, as recently developed by our group, grown as mixed crystals at ambient temperatures from a saturated salt solution. [ 31 ] Using this method, different types of QDs can be incorporated due to the low thermal stress and, by choosing the proper matrix-QD system, the photoluminescence quantum yields (PL-QYs) are enhanced upon incorporation. [ 32,33 ] Here, a new, fast, and versatile method for the incorporation of colloidal quantum dots (QDs) into ionic matrices enabled by liquid-liquid diffusion is demonstrated. QDs bear a huge potential for numerous applications thanks to their unique chemical and physical properties. However, stability and processability are essential for their successful use in these applications. Incorporating QDs into a tight and chemically robust ionic matrix is one possible approach to increase both their stability and processability. With the proposed liquid-liquid diffusion-assisted crystallization (LLDC), substantially accelerated ionic crystallization of the QDs is shown, reducing the crystallization time needed by one order of magnitude. This fast process allows to incorporate even the less stable colloids including initially oil-based ligandexchanged QDs into salt matrices. Furthermore, in a modifi ed two-step approach, the seed-mediated LLDC provides the ability to incorporate oilbased QDs directly into ionic matrices without a prior phase transfer. Finally, making use of their processability, a proof-of-concept white light emitting diode with LLDC-based mixed QD-salt fi lms as an excellent color-conversion layer is demonstrated. These fi ndings suggest that the LLDC offers a robust, adaptable, and rapid technique for obtaining high quality QD-salts.

In this work, a model-experimental feedback approach is developed and applied to fabricate high-quality, warm-white light-emitting diodes based on quantum dots (QDs) as color-conversion materials. Owing to their unique chemical and physical properties, QDs offer huge potential for lighting applications. Nevertheless, both emission stability and processability of the QDs are limited upon usage from solution. Incorporating them into a solid ionic matrix overcomes both of these drawbacks, while preserving the initial optical properties. Here borax (Na2B4O7·10H2O) is used as a host matrix because of its lower solubility and thereby reduced ionic strength in water in comparison with NaCl. This guarantees the stability of high-quality CdSe/ZnS QDs in the aqueous phase during crystallization and results in a 3.4 times higher loading amount of QDs within the borax crystals compared to NaCl. All steps from the synthesis via mixed crystal preparation to the warm-white LED preparation are verified by applying the model-experimental feedback, in which experimental data and numerical results provide feedback to each other recursively. These measures are taken to ensure a high luminous efficacy of optical radiation (LER) and a high color rendering index (CRI) of the final device as well as a correlated color temperature (CCT) comparable to an incandescent bulb. By doing so, a warm-white LED with a LER of 341 lm/Wopt, a CCT of 2720 K and a CRI of 91.1 is produced. Finally, we show that the emission stability of the QDs within the borax crystals on LEDs driven at high currents is significantly improved. These findings indicate that the proposed warm-white light-emitting diodes based on QDs-in-borax hold great promise for quality lighting.

In this work, we propose and develop the inorganic salt encapsulation of semiconductor nanocrystal (NC) dispersion in a nonpolar phase to make a highly stable and highly efficient colour converting powder for colour enrichment in light-emitting diode backlighting. Here the wrapping of the as-synthesized green-emitting CdSe/CdZnSeS/ZnS nanocrystals into a salt matrix without ligand exchange is uniquely enabled by using a LiCl ionic host dissolved in tetrahydrofuran (THF), which simultaneously disperses these nonpolar nanocrystals. We studied the emission stability of the solid films prepared using NCs with and without LiCl encapsulation on blue LEDs driven at high current levels. The encapsulated NC powder in epoxy preserved 95.5% of the initial emission intensity and stabilized at this level while the emission intensity of NCs without salt encapsulation continuously decreased to 34.7% of its initial value after 96 h of operation. In addition, we investigated the effect of ionic salt encapsulation on the quantum efficiency of nonpolar NCs and found the quantum efficiency of the NCs-in-LiCl to be 75.1% while that of the NCs in dispersion was 73.0% and that in a film without LiCl encapsulation was 67.9%. We believe that such ionic salt encapsulated powders of nonpolar NCs presented here will find ubiquitous use for colour enrichment in display backlighting. © The Royal Society of Chemistry 2015

Colloidal semiconductor nanocrystals have gained substantial interest as spectrally tunable and bright fluorophores for color conversion and enrichment solids. However, they suffer from limitations in processing their solutions as well as efficiency degradation in solid films. As a remedy, embedding them into crystalline host matrixes has stepped forward for superior photostability, thermal stability, and chemical durability while simultaneously sustaining high quantum yields. Here, we review three basic methods for loading the macrocrystals with nanocrystals, namely relatively slow direct embedding, as well as accelerated methods of vacuum-assisted and liquid-liquid diffusion-assisted crystallization. We discuss photophysical properties of the resulting composites and present their application in light-emitting diodes as well as their utilization for plasmonics and excitonics. Finally, we present a future outlook for the science and technology of these materials.