Kallista Sears

Recent Developments in Carbon Nanotube Membranes for Water Purification and Gas Separation

Carbon nanotubes (CNTs) are nanoscale cylinders of graphene with exceptional properties such as high mechanical strength, high aspect ratio and large specific surface area. To exploit these properties for membranes, macroscopic structures need to be designed with controlled porosity and pore size. This manuscript reviews recent progress on two such structures: (i) CNT Bucky-papers, a non-woven, paper like structure of randomly entangled CNTs, and (ii) isoporous CNT membranes, where the hollow CNT interior acts as a membrane pore. The construction of these two types of membranes will be discussed, characterization and permeance results compared, and some promising applications presented.

Control of porosity and pore size of metal reinforced carbon nanotube membranes.

Membranes are crucial in modern industry and both new technologies and materials need to be designed to achieve higher selectivity and performance. Exotic materials such as nanoparticles offer promising perspectives, and combining both their very high specific surface area and the possibility to incorporate them into macrostructures have already shown to substantially increase the membrane performance. In this paper we report on the fabrication and engineering of metal-reinforced carbon nanotube (CNT) Bucky-Paper (BP) composites with tuneable porosity and surface pore size. A BP is an entangled mesh non-woven like structure of nanotubes. Pure CNT BPs present both very high porosity (>90%) and specific surface area (>400 m2/g). Furthermore, their pore size is generally between 20–50 nm making them promising candidates for various membrane and separation applications. Both electro-plating and electroless plating techniques were used to plate different series of BPs and offered various degrees of success. Here we will report mainly on electroless plated gold/CNT composites. The benefit of this method resides in the versatility of the plating and the opportunity to tune both average pore size and porosity of the structure with a high degree of reproducibility. The CNT BPs were first oxidized by short UV/O3 treatment, followed by successive immersion in different plating solutions. The morphology and properties of these samples has been investigated and their performance in air permeation and gas adsorption will be reported.

Activation of gold decorated carbon nanotube hybrids for targeted gas adsorption and enhanced catalytic oxidation

Free standing assemblies of carbon nanotubes (CNTs), known as bucky-paper (BP), have been functionalised through the in situ plating of gold nanoparticles within the interstitial spaces in the BP. The nanoparticles are extremely small and well distributed at short plating times, so much so that the specific surface area of the BP is actually increased by the gold incorporation. These well distributed nanoparticles exhibit high enthalpy hydrogen storage and selective carbon dioxide adsorption over other gases, in particular methane. In concert with the conductive BP substrate, it has been demonstrated that these materials can also act as high turnover heterogeneous catalysts.

Influence of the Sonication Temperature on the Debundling Kinetics of Carbon Nanotubes in Propan-2-ol

The effect of sonication temperature on the debundling of carbon nanotube (CNT) macro-bundles is reported and demonstrated by analysis with different particle sizing methods. The change of bundle size over time and after several comparatively gentle sonication cycles of suspensions at various temperatures is reported. A novel technique is presented that produces a more homogeneous nanotube dispersion by lowering the temperature during sonication. We produce evidence that temperature influences the suspension stability, and that low temperatures are preferable to obtain better dispersion without increasing damage to the CNT walls.

Assessing the temporal stability of surface functional groups introduced by plasma treatments on the outer shells of carbon nanotubes.

Plasma treatments are emerging as superior efficiency treatment for high surface to volume ratio materials to tune functional group densities and alter crystallinity due to their ability to interact with matter at the nanoscale. The purpose of this study is to assess for the first time the long term stability of surface functional groups introduced across the surface of carbon nanotube materials for a series of oxidative, reductive and neutral plasma treatment conditions. Both plasma duration dose matrix based exposures and time decay experiments, whereby the surface energy of the materials was evaluated periodically over a one-month period, were carried out. Although only few morphological changes across the graphitic planes of the carbon nanotubes were found under the uniform plasma treatment conditions, the time dependence of pertinent work functions, supported by Raman analysis, suggested that the density of polar groups decreased non-linearly over time prior to reaching saturation from 7 days post treatment. This work provides critical considerations on the understanding of the stability of functional groups introduced across high specific surface area nano-materials used for the design of nano-composites, adsorptive or separation systems, or sensing materials and where interfacial interactions are key to the final materials performance.

A Preliminary Study on the Effect of Macro Cavities Formation on Properties of Carbon Nanotube Bucky-Paper Composites

In this study, we focus on processing and characterizing composite material structures made of carbon nanotubes (CNTs) and reproducibly engineering macro-pores inside their structure. Highly porous bucky-papers were fabricated from pure carbon nanotubes by dispersing and stabilizing large 1 μm polystyrene beads within a carbon nanotube suspension. The polystyrene beads, homogeneously dispersed across the thickness of the bucky-papers, were then either dissolved or carbonized to generate macro cavities of different shape and properties. The impact of adding these macro cavities on the porosity, specific surface area and Young’s modulus was investigated and some benefits of the macro cavities will be demonstrated.

Small angle X-ray scattering study of carbon nanotube forests densified into long range patterns by controlled solvent evaporation

Although emergent properties from self-assembly of carbon nanotubes have been described in various forms there is so far no systematic process for the preparation of dense arrays of aligned nanotubes. Here we present a systematic study on the analysis of the alignment of carbon nanotubes within solvent densified carbon nanotube forests. Highly periodic patterns with length scales of the order of the millimetres were generated and characterized by electron and optical micrographs and compared to results from small angle X-ray scattering performed at various incident beam angles. The impact of the different solvents was also discussed in light of the densification process and in relation to solvent properties.

Dipole-field-assisted charge extraction in metal-perovskite-metal back-contact solar cells

Hybrid organic-inorganic halide perovskites are low-cost solution-processable solar cell materials with photovoltaic properties that rival those of crystalline silicon. The perovskite films are typically sandwiched between thin layers of hole and electron transport materials, which efficiently extract photogenerated charges. This affords high-energy conversion efficiencies but results in significant performance and fabrication challenges. Herein we present a simple charge transport layer-free perovskite solar cell, comprising only a perovskite layer with two interdigitated gold back-contacts. Charge extraction is achieved via self-assembled monolayers and their associated dipole fields at the metal-perovskite interface. Photovoltages of ~600 mV generated by self-assembled molecular monolayer modified perovskite solar cells are equivalent to the built-in potential generated by individual dipole layers. Efficient charge extraction results in photocurrents of up to 12.1 mA cm−2 under simulated sunlight, despite a large electrode spacing.Simplified device concepts may become important for the development of low cost photovoltaics. Lin et al. report solar cells based on interdigitated gold back-contacts and metal halide perovskites where charge extraction is assisted via a dipole field generated by self-assembled molecular monolayers.

Focused Ion Beam Milling of Carbon Nanotube Yarns and Bucky-Papers: Correlating Their Internal Structure with Their Macro-Properties

Focused ion beam (FIB) milling through carbon nanotube (CNT) yarns and bucky-papers followed by scanning electron microscopy has recently emerged as a powerful tool for eliciting details of their internal structure. The internal arrangement of CNTs in bucky-papers and yarns directly affects their performance and characteristics. Consequently this information is critical for further optimisation of these structures and to tailor their properties for specific applications. This chapter describes in detail FIB milling of CNT yarns and bucky-papers and gives a range of examples where FIB milling has enabled a better understanding of how processing conditions and treatments affect the internal structure. Emphasis is placed on how FIB milling elucidates the influence of fabrication conditions on the internal arrangement of CNTs and how this influences the material’s macroscopic properties.