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Engineered multi-scale carbon materials

Periodic Reporting for period 1 - Emu Cam (Engineered multi-scale carbon materials)

Reporting period: 2015-08-01 to 2017-07-31

Access to clean water is a human right and is fundamental to maintaining the basic standards of public health. Nevertheless, the United Nations is estimating that over a billion individuals are lacking access to potable water. At the same time, wastewater treatment is becoming increasingly challenging as it needs to address not only classic pollutants but also viral, microbial and other organic contaminants, such as surfactants, disinfection by-products, and pharmaceuticals.

Carbon nanotube (CNT) based filters have the potential to revolutionize water treatment, because of their ability to rapidly remove large amounts of various pollutants from numerous liquids. Despite significant commercial activities, CNTs have not yet found widespread applications in water filtration. Most reports on CNT filters rely on powders, colloidal particles, spherical aggregates, or foams suspended in a beaker to adsorb the pollutants. However, these approaches are not scalable because they have low filtration rates, the recovery of CNTs after filtration is challenging, and their filter regeneration cycles are complex and cumbersome. A popular alternative approach is to work with immobilized CNT membranes, which eliminate the need for dispersing and recovering the nanotubes. However, such membranes offer only very limited retention of pollutants and suffer from extremely low filtration rates. Further, fabrication of CNT membranes with controlled geometries, porosity and pore shape, still remains a challenge.

Water filtration in particular requires engineering of CNT order, morphology, and porosity at several length scales to create highly ordered 3D structures to be used as filters. This project developed a new process that addresses these challenges by assembling CNTs into microstructures using microfluidic emulsification followed by large area into colloidal crystals. This approach provides a novel scalable route to sequentially engineer nano-, micro-, and macroscale material architecture. The CNT microparticles and their macroscale colloidal crystals developed in this project have, for the first time, enabled a high performance CNT filter and will, in the near future, enable high performance catalysts and energy devices.
The goal of this project was to employ a novel engineering approach to hierarchically assemble carbon nanotubes (CNTs), addressing longstanding issues in CNT macroscale assemblies, and to develop a new, low-cost method for scalable liquid processing and structuring of CNTs.

A methodical approach was followed to first create CNT suspensions and use these to produce stable emulations that then could be dried to produce CNT microspheres with a controlled macro- and mesoporosity. These microspheres are then packed into macroscale assemblies and used as filtration columns to demonstrate their record-braking performance in flow-through filtration applications.

1. Formation of stable suspensions and bulk emulsions of CNTs
Initially, standard CNT surfactants (i.e. SDS, SDBS, PVA) and block copolymers were used at various concentrations to obtain water suspensions; however, it proved challenging to obtain suspensions with 0.1 wt% or more CNTs with these systems. Multiwalled carbon nanotubes (MWCNTs) were then oxidized in order to suspend them in water without surfactants. Functional groups were optimized to attain suspensions with up to 0.2 wt% CNTs. Various water-based suspensions with high CNT content were used to produce and investigate bulk water-in-oil emulsions, but these ultimately proved to be unstable without the use of further surfactant in the oil phase. Numerous two-surfactant system compositions were added to the oil phase to stabilize the emulsion. Studies to underpin the factors affecting bulk emulsion stability.

2. Employ microfluidics to precisely assemble carbon nanotube microstructures
Droplet generators were cast from 3D printed master moulds and used to produce homogeneous emulsions. The produced droplets are collected in a petri dish on a leveled surface and were dried in an oven at for multiple days to produce homogeneous, spherical carbon nanotube supraparticles (CNTSPs, 97.2±10.7 μm diameter). These are then extensively washed in EtOAc and baked in Helium to remove any residual solvent or surfactant.

3. Achieve control of CNT microparticle size, shape, and porosity
Particle size could be controlled, on a limited scale, within each droplet generator by adjusting the relative flowrates of the water and oil phases, while droplet generators of various sizes were used to further extend the range of produced droplets up to 1mm. Initially inclusion of sacrificial silica particles and their subsequent etching was demonstrated to control the porosity of the CNTSPs. It was found that the porosity of the final structure could also be controlled by use of excess surfactant in the oil phase. The additional surfactant would become trapped inside the particle and locally reshape the CNT morphology to produce larger pores. This however required additional washing and baking to remove and produce purified CNTSPs.

4. Assemble macroscale colloidal crystal from carbon microparticles.
Blade casting was initially used to assemble multilayer colloidal crystals from the produced CNTSPs. This was accomplished by dispensing the CNTSPs in ethanol and manually blade casting on a ridged support substrate. Vacuum filtration of suspended CNTSPs was also attempted to assemble colloidal CNTSP membranes, but resulted in poor packing.

5. Fabricate and demonstrate a macroscale CNTSP based filter.
CNTSP-based column filters were fabricated by packing of CNT microspheres into 3D microfluidic channels. The channels were fabricated by PDMS replica-moulding of 3D printed master moulds. The CNTSP packing inside the filters was imaged by X-ray tomography. The filter exhibited record-braking capacity and permeability for filtering numerous surfactants from water. 
In this project, a unique approach is demonstrated that bypasses the fundamental CNT-filter permeability and capacity trade-off and, for the first time, achieves CNT filters with buckypaper-like efficiency combined with filter-column-like permeability and capacity. This approach facilitates (1) easy immobilization of CNTs (avoiding need for filter media recover), (2) low pressure drop across the filter (large column permeability), (3) large flow-through filter efficiencies, (4) large capacity, and (5) a simple recharge cycle. This effectively eliminates all drawbacks of CNT based filters. This technology will revolutionize CNT based filters by allowing for improvements in both the filtration efficiency and safety and therefore provide an important step towards implementation of CNT filters outside of the lab.

Filters built using this technology exhibited large flow-through filtration efficiencies for both Congo Red (84.7%) and SDS (61.6%), while maintaining permeabilities more than an order of magnitude larger then CNT buckypaper (232,000 L/m2/h/bar). Moreover, these microstructured CNT column filters are able to remove approximately 20 times their own weight of sodium dodecyl sulphate compared. Future experiments are planned to investigate other pollutants that can be effectively filtered and building of larger devices that could provide adequate point-of-use technologies with form factors that are suitable for disaster relief.
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