Control over Structure and Properties in Nanocrystal Aerogels at the Nano-, Micro-, and Macroscale

Abstract

Conspectus The assembly of individual colloidal nanocrystals into macroscopic solvogels and aerogels introduced a new exciting type of material into the class of porous architectures. In these so-called nanocrystal gels, the structure and properties can be controlled and fine-tuned to the smallest details. Recently it was shown that by employing nanocrystal building blocks for such gel materials, the interesting nanoscopic properties can be conserved or even expanded to properties that are available neither in the nanocrystals nor in their respective bulk materials. In general, the production of these materials features the wet-chemical synthesis of stable nanocrystal colloids followed by their carefully controlled destabilization to facilitate arrangement of the nanocrystals into highly porous, interconnected networks. By isolation of the synthesis of the discrete building blocks from the assembly process, the electronic structure, optical properties, and structural morphology can be tailored by the myriad of procedures developed in colloidal nanocrystal chemistry. Furthermore, knowledge and control over the structure–property correlation in the resulting gel structures opens up numerous new ways for extended and advanced applications. Consequently, the amount of different materials converted to nanocrystal-based gel structures is rising steadily. Meanwhile the number of methods for assembly initiation is likewise increasing, offering control over the overall network structure and porosity as well as the individual nanocrystal building block connection. The resulting networks can be dried by different methods to obtain highly porous air-filled networks (aerogels) or applied in their wet form (solvogels). By now a number of different applications profiting from the unique advantages of nanocrystal-based gel materials have been realized and exploited in the areas of photocatalysis, electrocatalysis, and sensing. In this Account, we aim to summarize the efforts undertaken in the structuring of nanocrystal-based network materials on different scales, fine-tuning of the individual building blocks on the nanoscale, the network connections on the microscale, and the macroscale structure and shape of the final construct. It is exemplarily demonstrated how cation exchange reactions (at the nanoscale), postgelation modifications on the nanocrystal networks (microscale), and the structuring of the gels via printing techniques (macroscale) endow the resulting nanocrystal gel networks with novel physicochemical, mechanical, and electrocatalytic properties. The methods applied in the more traditional sol–gel chemistry targeting micro- and macroscale structuring are also reviewed, showing their future potential promoting the field of nanocrystal-based aerogels and their applications.

Fabrication and application of macroscopic nanowire aerogels

“…The assembly of crystalline nanoparticles into macroscopic porous 3D networks represents an effective approach to achieve crystalline aerogels with tunable compositions and morphologies, which enriches the properties and applications of aerogels. [5][6][7][8][9][10][11][12] Brock's group pioneered the research on nanoparticle aerogels. 5,13,14 In 2005, her group reported the first nanoparticle aerogels by gelling metal chalcogenide nanocrystals into 3D networks via the partial removal of coating ligands on the nanoparticles through chem/photo-oxidation of capping groups.…”

“…5,7,8 More importantly, the idea of assembling preformed nanoparticles into aerogels has made it possible to control the compositions and structures of aerogels by tuning the compositions, sizes and morphologies of primary nanobuilding blocks and by engineering the gel structures. [10][11][12] As a result, the aerogels prepared from nanoparticles showed significantly enhanced optoelectronic, electro/photo catalytic, and sensing properties in comparison to primary nanobuilding blocks. [10][11][12] Since 2007, many comprehensive reviews regarding nanoparticle aerogels have been published, covering topics on the preparation and application of metal chalcogenide, metal and metal oxide nanoparticles aerogels 5,[7][8][9]11,15,16 as well as structure and property control of nanocrystal aerogels at nano-, micro-and macroscales.…”

“…[10][11][12] As a result, the aerogels prepared from nanoparticles showed significantly enhanced optoelectronic, electro/photo catalytic, and sensing properties in comparison to primary nanobuilding blocks. [10][11][12] Since 2007, many comprehensive reviews regarding nanoparticle aerogels have been published, covering topics on the preparation and application of metal chalcogenide, metal and metal oxide nanoparticles aerogels 5,[7][8][9]11,15,16 as well as structure and property control of nanocrystal aerogels at nano-, micro-and macroscales. 10,12,17,18 However, most of the aerogels were produced by using zerodimensional (0D) spherical nanoparticles as building blocks and the weak interparticle force throughout the nanoparticle networks rendered the resultant aerogels too fragile to be integrated into a practical device without destroying their structural integrity.…”

“…[10][11][12] Since 2007, many comprehensive reviews regarding nanoparticle aerogels have been published, covering topics on the preparation and application of metal chalcogenide, metal and metal oxide nanoparticles aerogels 5,[7][8][9]11,15,16 as well as structure and property control of nanocrystal aerogels at nano-, micro-and macroscales. 10,12,17,18 However, most of the aerogels were produced by using zerodimensional (0D) spherical nanoparticles as building blocks and the weak interparticle force throughout the nanoparticle networks rendered the resultant aerogels too fragile to be integrated into a practical device without destroying their structural integrity. [8][9][10][11][12] In addition, the ligands coating on nanoparticles are essential for making stable nanoparticle dispersion that is critical for gelling the nanoparticle into 3D networks but are detrimental to the application of the nanoparticle aerogels because of the poor electrical conductivity of the organic ligands.…”

Pd‐Doped Cellulose Carbon Aerogels for Energy Storage Applications

“…Therefore, the present paper analyzes the electrochemical and physicochemical properties of Pd‐doped cellulose carbon aerogels as a novel carbon‐metal hybrid material obtained through cryogelation, which is a versatile tool for preparing aerogels from different colloidal nanoparticles. [ 17,36–39 ] In this work, a different strategy compared to other previously reported syntheses of metal‐doped carbon aerogels is used, [ 9 ] via dispersing the previously synthesized PdNPs in the cellulose solution, which is further processed to obtain Pd‐doped carbon aerogels. Thus, Pd nanoparticles are synthesized and then they are dispersed in the cellulose dissolution prior to the cross‐linking and carbonization process.…”