Emerging Hierarchical Aerogels: Self‐Assembly of Metal and Semiconductor Nanocrystals

Abstract

Aerogels assembled from colloidal metal or semiconductor nanocrystals (NCs) feature large surface area, ultralow density, and high porosity, thus rendering them attractive in various applications, such as catalysis, sensors, energy storage, and electronic devices. Morphological and structural modification of the aerogel backbones while maintaining the aerogel properties enables a second stage of the aerogel research, which is defined as hierarchical aerogels. Different from the conventional aerogels with nanowire-like backbones, those hierarchical aerogels are generally comprised of at least two levels of architectures, i.e., an interconnected porous structure on the macroscale and a specially designed configuration at local backbones at the nanoscale. This combination "locks in" the inherent properties of the NCs, so that the beneficial genes obtained by nanoengineering are retained in the resulting monolithic hierarchical aerogels. Herein, groundbreaking advances in the design, synthesis, and physicochemical properties of the hierarchical aerogels are reviewed and organized in three sections: i) pure metallic hierarchical aerogels, ii) semiconductor hierarchical aerogels, and iii) metal/semiconductor hybrid hierarchical aerogels. This report aims to define and demonstrate the concept, potential, and challenges of the hierarchical aerogels, thereby providing a perspective on the further development of these materials.

Advanced Materials for High‐Temperature Thermal Transport

“…In particular, the layered structures of graphene or hBN with van der Waals bonds will facilitate the formation of chemical bonding between neighboring layers . Similarly, the prepared hydrogels in the course of typical sol–gel fabrication can be treated to modify the morphology with the interconnected structures . It would reinforce the mechanical stability of the aerogels under the multiple cycles of thermal shock.…”

“…Lastly, in addition to the development in the material designs, scalable and low‐cost fabrication techniques also need to be developed for the more widespread use of aerogels in various applications. A typical process to fabricate aerogels is the sol–gel method . It involves gelation, aging, and drying process.…”

Engineering Self‐Supported Noble Metal Foams Toward Electrocatalysis and Beyond

“…They can provide information of almost all metal elements and certain nonmetal elements with a detection limit down to ppb level. X‐ray photoelectron spectroscopy (XPS) is another prevalently used technique for element analysis for NMFs . With a detection limit of ≈0.1 at%, it is capable of giving surface information about chemical composition and chemical environment of all elements except for hydrogen and helium.…”

“…Combining analytical methods (e.g., energy‐dispersive X‐ray spectroscopy (EDXS) and electron energy‐loss spectroscopy (EELS)) with imaging techniques (e.g., scanning electron microscopy (SEM) and transmission electron microscopy (TEM)) bring the additional advantage to investigate the spatially resolved chemical composition from micrometer down to nanometer scale. Especially, spectrum imaging analysis based on EDXS or EELS in scanning transmission electron microscopy (STEM) delivers 2D element distributions, where it is possible to unveil fine details of certain nanoscale architectures, e.g., core–shell structures, at a very high resolution . This is extremely useful to unravel mechanisms regarding synthesis and structure–property correlations, thus significantly promoting the understanding for both material design and performance optimization.…”

Synergistic Supports Beyond Carbon Black for Polymer Electrolyte Fuel Cell Anodes

“…Soon afterwards, many novel metal aerogels were developed for fuel cell applications, such as PtAg nanotubular aerogel (NTAG) for formic acid oxidation, Au hydrogel networks for glucose oxidation, Ni−Pd 60 Pt 40 aerogel for ethanol oxidation, Pt 3 Ni aerogel and Pd x Au−Pt core‐shell aerogel for oxygen reduction . The significant advances in the design, synthesis, and physicochemical properties of hierarchical aerogels, which contain both macroscale interconnected porous structures and nanoscale configurations, were summarized recently …”