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Multicomponent Aerogels with Tailored Nano-, Micro- Macrostructure

Periodic Reporting for period 4 - MAEROSTRUC (Multicomponent Aerogels with Tailored Nano-, Micro- Macrostructure)

Reporting period: 2021-09-01 to 2022-02-28

There is a strong demand for materials with new and defined physical properties like e.g. conductivity, charge carrier separation ability or multifunctionality, which can have impact for various application fields such as electrodes and batteries, sensors, (photo)catalysis, solar cells, membranes and touch screen devices. Nanotechnology is a promising research field which partially addresses the development of such materials. Aerogels and hydrogels from nanocrystal building blocks are a fascinating novel class of materials with extremely low densities and large specific surfaces, which partially exhibit the advantageous properties of their nanoscopic building blocks (e.g. size quantized fluorescence or catalytic activity).
The aim of the present project was to synthesize multicomponent gels with controlled mechanical properties, plasmon enhanced fluorescence, photocatalytic properties, and with controlled conductivity properties. These new materials did not exhibit "simply" the nanoscopic properties of their building blocks, but also new properties which are neither accessible from nanoparticle nor from bulk material. This was e.g. be achieved due to nanoscopic interactions between the materials or due to synergistic combination effects caused by appropriate material combination.
The project had the following three main objectives:
New synthetic techniques have been developed, which allow a detailed control over the nanoscopic, microscopic and macroscopic composition of such aerogels from multiple components. Accordingly, new multicomponent hydrogel and aerogel materials have been developed with such a high degree of control over the compound distribution in the multicomponent gel materials. A detailed understanding of the influence of the compound distribution on the physical and chemical properties of such materials was achieved.
The laboratories were adapted to the project requirements. The research instruments were acquired and tested. The doctoral students working on the project were hired, so that the starting configuration of the team was ready. In our research, various types of nanoparticles differing in size, shape and composition could be synthesized. We have characterized them by optical spectroscopy, electron microscopy and electrochemically. We were able to synthesize also previously unknown nanoparticles with new physicochemical properties. The nanoparticles are the building blocks for our gels, which can be seen as self-supporting networks from these nanoparticle building blocks extending to the macroscopic size regime. Our research therefore included the investigation of possibilities to form such networks - gel formation routes. Various gel formation routes were tested and new ones developed, and depending on the respective building blocks, the best gelation routes were selected to form such macroscopic gels. One of the gelation routes developed by our group is easily applicable to all types of aqueous solutions of nanoparticle building blocks. In this route, directly aerogels are synthesized without the need of so-called supercrytical drying (the transfer from liquid to gas environment, which is sometimes technically challenging). It was also shown that by simple freezing and subsequent thawing, so-called cryohydrogels can be made directly, so that for aqueous applications no drying and re-hydratization is necessary, which increases the mechanical stability of the gels. We have found out that this new gelation technique is also suitable for making aerogels from two different components of nanoparticles. Our investigations show that we can even control the microscopic distribution of the two components within the aerogels by controlling the nanoparticle surface chemistry. Another important novelty is that all possible mixing ratios of the two components are possible. In recent works we have shown that the physical properties of gels differ significantly even from the same nanoscopic compositions if the structure differs: The nature of the contact between different nanoscopic domains is of utmost importance, as well as the nano- and microscopic distribution of the compounds. Additionally we have conducted spectroelectrochemical and various optical investigations how far generated charge carriers can travel in gels. Of the new gels developed, we were able to investigate the new physical and chemical properties, e.g. by elecron microscopy and spectroscopic as well as spectroelectrochemical techniques. Further works concentrated on developing shaping methods of the gels by means of inkjet printing, additive manufacturing and molding. In several publications, the main results achieved were described and discussed in detail.
As described more in detail in the section above "... main results achieved ...", the progress beyond the state of the art can be summarized as follows: nanoparticles which have not been reported previously have been synthesized and characterized. Lyogels and aerogels which have not been reported previously have been synthesized by employing nanoparticles as building blocks and characterized. We were able to achieve structural control on the nano-, and micro- and macroscale in a way which was formally not achievable. This has lead to materials with new physicochemical properties. We have shown for the first time that the submicroscopic distribution of compounds in such nanoheterostructured nanocrystal aerogels crucially influences their physical properties. By this, new synthetic techniques were developed allowing detailed control over the nanoscopic and microscopic composition of aerogels from multiple components. Various understandings of the influence of the compund distribution on the physicochemical properties were already acquired. We have now already shown that expecially by nanostructuring and microstructuring and by pre- and postgelation modification routes aerogel properties can be tailored with e.g. various optic, spectroelectrochemical and electronic properties. The main results are: New synthetic techniques have been developed, which will allow a detailed control over the nanoscopic, microscopic and macroscopic composition of aerogels from multiple components. Accordingly, a variety of multicomponent hydrogel and aerogel materials have been developed with such a high degree of control over the compound distribution in the multicomponent gel materials. A detailed understanding of the influence of the compound distribution on the physical and chemical properties of such materials has been achieved.
nano-, micro- and macrostructuring of multicomponent aerogels from Acc. Chem. Res. 2020, 53, 10, 241
multicomponent aerogel from cryogelation method