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Supramolecular assembly of Janus Carbon Nanotubes into functional 3D microparticles

Periodic Reporting for period 1 - Supra-CNT (Supramolecular assembly of Janus Carbon Nanotubes into functional 3D microparticles)

Reporting period: 2016-04-01 to 2018-03-31

Owing to their remarkable properties (electrical, mechanical, thermal and optical), carbon nanotubes (CNTs) have shown great potential for implementation both in microscopic electronic devices (e.g. field transistors, field emitters, nano-sensors), and macroscopic applications (filtration materials, structural materials, conductors, electrode materials and composites).
To date however, most of the latter applications use disorganized CNTs, which as a result of their random orientation, entanglement and aggregation have poor overall properties. For example, CNTs are extremely attractive filter materials because of their intrinsic properties (e.g. large surface area, sorption of a wide range of pollutants, etc.). The use of powdered CNTs is not pursued as a scalable approach in water remediation because of the difficult recovery of the particles after filtration. A popular alternative is to work with immobilized CNT membranes, referred to as buckypapers. However, because of the random orientation of the CNTs, these do not allow control on geometry, porosity and pore shape, thus suffering from low permeability or low filter capacity.

The production and commercialization of CNT based products is dependent on the possibility of designing and preparing hierarchically-structured CNT assemblies where the CNTs are synergistically collaborating to a real enhancement of the device’s characteristics, with minimal reciprocal perturbation. Tremendous efforts have been deployed for the production of CNT-based macroscopic assemblies, including: 1D yarns or fibers, 2D films, 3D gels and vertically aligned arrays. But their standardized continuous fabrication approach does not allow for the fine-tuning of their structure, and consequently their properties, for integration in composites and devices. A more versatile manufacturing processwould be desirable to achieve the necessary design flexibility required to overcome this limitation.

Supra-CNT project defines a new methodology to precisely and deterministically engineer the order, morphology, and porosity of CNT assemblies at several length scales. At the interface between top-down and bottom-up approaches, Supra-CNT provides exceptional control of CNT manufacturing by addressing: the engineering of the single particles (i.e. surface chemistry); their controlled aggregation into definite microparticles; and the large-scale assembly of the latter, unlocking the preparation of unprecedented macroscopic hierarchized high-tech specialized CNT materials.
During this MSCA Fellowship, I developed an emulsion templated methodology for the preparation of supramolecular aggregates of CNTs (i.e. supraparticles) with defined diameter, shape and porosity. This result was achieved by exploiting a microfluidic platform for the generation of a monodispersed emulsion of an aqueous CNT dispersion in oil. Upon shrinkage of the water droplet during evaporation, the dispersed CNTs experience an inward capillary pressure that compacts the nanoparticles into a stable supraparticle, which can then be employed in large-scale assembly and production of hierarchized CNT materials.

Initially, I worked on the development of a hydrothermal microwave process to produce a stable and concentrated aqueous suspension of oxidized CNTs. This was used to produce a water-in-oil emulsion via a microfluidic droplet generator. The CNT droplets were dried, washed and baked affording the supra particles, characterized by a homogenous spherical morphology with an average diameter of circa 100 μm (microparticle, MP). Their surface area resulted higher than that measured for unstructured ox-MWCNTs, indicating a high porosity.

The MPs were packed into micro column filters, then tested against SDS, a commonly used surfactant in detergents for laundry, shampoos, etc. Packing was performed by pushing a dispersion of the supraparticles into a microfluidic column until optimal assembly was achieved, and tested by pumping DW through the filter under an optical microscope. No structural deformation was observed and the C content in the filtrate was unchanged, indicating the absence of loose CNTs. Interestingly, the MPs were able to adsorb as much as 17 times their weight of SDS. Increase of the flow rate resulted in higher adsorption, while for loose MPs the filtration efficiency decreased by over a tenfold, highlighting the importance of the flow-through approach to foster an interaction between the surfactant and the CNTs. The key advantages of the supraparticle based filtration technology are: (i) porosity of the supraparticles, and possibility to tailor their surface chemistry to target specific contaminants; (ii) effective CNT immobilization; (iii) a reduction in pressure drop over the filter compared to buckypaper.

The microfluidic manufacturing technique was tested on mixtures of CNTs and other nanomaterials, such as graphene oxide (tuning of the adsorption properties for filtration applications) and metaloxides (Fe2O3, for energy storage applications; SiO2 for optical filter applications). In the case of graphene oxide, the two materials mixed homogeneously. Modulation of the concentration of the nanoparticles in suspension led to morphological transition from spherical to discoidal MPs, which could then pack in a different fashion (columnar stacks). With Fe2O3 the blending of the materials was less homogeneous, leading to a patchy spherical microparticle. SiO2 nanoparticles phase segregated from CNTs leading to Janus type MPs.

The scientific results achieved during this Fellowship were disseminated through attendance to international conferences (229th and 231st ECS Meetings), national gatherings (PPG Seminar) and through publication on high impact factor journals (i.e. Adv. Mat. 2018, 30, 1706503).
The research developed in this Fellowship has progressed the field of carbon based advanced materials advancing our understanding of the relationships between material engineering at the nano/micro/macroscale and device properties. The manufacturing approach of hierarchically structuring CNTs into discrete microparticles and packing these to build bottom up bulk materials will enable specific tailoring, addressing all the diffusion-limited applications employing CNTs (e.g. energy storage, catalysis, etc.), not only water filtration.

The results developed in this Fellowship will have numerous impacts affecting a range of beneficiaries:
- Academia will benefit from the scientific advancements in a field of research that interests a wide range of disciplines (materials chemistry, materials science, manufacturing engineering, etc.);
- Industry will gain new knowledge and economic advantage from the technologies developed;
- Besides the targeted application (i.e. filtration), the knock-on effects of this research are wide ranging: for example, replacing metals with cheaper and lighter CNT-based materials in composites for the automotive industry would not only have the obvious manufacturing cost benefits, but also the health and environmental benefits of reduced fuel usage.