Design, Fabrication And Performances Of 3D Ordered Macroscopic Assemblies From Nanomaterials Via Directional Freezing

With the development of nanotechnology, a variety of high-performance nanomaterials are being produced. Their potential applications have penetrated into all walks of life. However, individual nanostructural unit can’t meet the urgent requirement of practical applications. An alternative approach to realize their practical application is to assemble them into macroscopic three-dimensional (3D) assemblies with hierarchical ordered microstructure. At present, though a series of progresses on the fabrication and application of macroscopic 3D assemblies have been made, precisely regulating their microstructures at macroscopic scale is still an siginificant challenge. Many scientific and technical problems about how to realize their practical applications remain to be solved. Developing convenient, efficient and controllable macroscopic 3D assembly methods for those high-performance nanomaterials should be an important strategy to solve these problems, which attractes much attention in recent years. In this dissertation, we focus our research interest on this topic. We firstly summarized the progress on applications of macroscopic 3D assemblies in all walks of life. Then we particularly highlighted the progress on the assembly methods of macroscopic 3D assemblies, and also highlighted the progress on directional freezing and its advanced technical advantages in controlling the microstructure of materials. Based on this background, we aim to develop new macroscopic assembly methods by using directional freezing technique to regulate the arrangement of nanomaterials in macroscopic 3D space, and build some new types of macroscopic 3D assemblies with hierarchical ordered structure and functionalities to meet the requirements for practical applications. The main achievements can be summarized as follows:1. We developed an simple macroscopic 3D assembling method, which can be applied to a variety of nanomaterials. We first fabricated a macroscopic 3D chitosan foam with highly ordered cellular structure via unidirectional freezing method. This chitosan foam was then used as supported matrix for assembling a series of nanomaterials with different dimensions (including OD nanoparticles, 1D nanowires and 2D nanosheets) into highly ordered macroscopic 3D assemblies, which largely relies on the unique properties (shape memory, strong water absorption, strong absorption of nanomaterials) of the chitosan foam. The results showed that the resulting macroscopic 3D assemblies not only retained the original shape memory property but also be endowed the the unit unique functionalities of the assembled nanomaterials.2. On the basis of the macroscopic 3D chitosan foam, we further investigate its application as a flexible immobilized carrier for nano-catalysts. We found that the highly ordered cellular chitosan foam fixed with nano-catalysts showed excellent catalytic effect in the circulating reaction system. Here, we prepared several cellular chitosan foam with different pore size by regulating the parameters in the freeze process, and systematically studied the influence of the pore size on its recovery rate as well as the catalytic efficiency. The results show that the catalytic efficiency of the macroscopic 3D assemblies of the nano-catalysts show high catalytic efficiency and well stability based on this ordered cellular chitosan foam. This nano-catalyst support material has many advantages compared to traditional nano-catalyst support material, which in attributed to its unique directional porous structure, strong water absorption and fully recovery performance as well as versatility for different kinds of nano-catalysts.3. By using unidirectional freezing process, we successfully assembled 1D Ag nanowires into macroscopic 3D assembly with highly ordered hierarchical structure without using any adhesive polymer. The assembly method is not only easy to be operated and scale-up, but also applicable to other 1D nanowires. The obtained macroscopic Ag nanowires assembly has a good electrical conductivity, as well as an unique ordered microstructure, which can be precisely regulated by adjusting the freezing conditions. Thus, we further successfully made it into a stretchable conductor by infusing polydimethylsiloxane (PDMS) elastomer into its 3D network. The tested results demonstrates that the obtained stretchable conductor has outstanding advantages compared to other stretchable conductors based on 2D and 3D assemblies.4. We developed a new type of unidirectional freezing method. By combining this freezing method with high temperature annealing process, we obtained a macroscopic carbon-graphene assembly with layered multi-arch microstructure. Because the arched mcirostructures act as the basic elastic element of the carbon-graphene assembly and also have consistent orientation. So, though composed of brittle carbon constituent, the resulting macroscopic assembly still has excellent compressibility and elasticity. It was found that the elastic carbon-graphene assembly can rebound a small metal ball like a spring with a speed of about 580 mm/s. Moreover, it can fully recover to its original height even at a large strain of 90%, and only with small energy dissipation in each compression cycle. Secondly, the elastic carbon-graphene assembly also has excellent fatigue resistant ability, it can maintain its structural stability after cyclic compression over one million times at 20% strain, and 250,000 times at 50% strain. This macroscopic 3D carbon assembly composed of unique layered multi-arch microstructure shows a clear advantage compared to the previous reported compressible foams.