Jason K. Holt

Fast Mass Transport Through Sub-2-Nanometer Carbon Nanotubes

We report gas and water flow measurements through microfabricated membranes in which aligned carbon nanotubes with diameters of less than 2 nanometers serve as pores. The measured gas flow exceeds predictions of the Knudsen diffusion model by more than an order of magnitude. The measured water flow exceeds values calculated from continuum hydrodynamics models by more than three orders of magnitude and is comparable to flow rates extrapolated from molecular dynamics simulations. The gas and water permeabilities of these nanotube-based membranes are several orders of magnitude higher than those of commercial polycarbonate membranes, despite having pore sizes an order of magnitude smaller. These membranes enable fundamental studies of mass transport in confined environments, as well as more energy-efficient nanoscale filtration.

Ion Exclusion by Sub 2-nm Carbon Nanotube Pores

Biological pores regulate the cellular traffic of a large variety of solutes, often with high selectivity and fast flow rates. These pores share several common structural features: the inner surface of the pore is frequently lined with hydrophobic residues, and the selectivity filter regions often contain charged functional groups. Hydrophobic, narrow-diameter carbon nanotubes can provide a simplified model of membrane channels by reproducing these critical features in a simpler and more robust platform. Previous studies demonstrated that carbon nanotube pores can support a water flux comparable to natural aquaporin channels. Here, we investigate ion transport through these pores using a sub-2-nm, aligned carbon nanotube membrane nanofluidic platform. To mimic the charged groups at the selectivity region, we introduce negatively charged groups at the opening of the carbon nanotubes by plasma treatment. Pressure-driven filtration experiments, coupled with capillary electrophoresis analysis of the permeate and feed, are used to quantify ion exclusion in these membranes as a function of solution ionic strength, pH, and ion valence. We show that carbon nanotube membranes exhibit significant ion exclusion that can be as high as 98% under certain conditions. Our results strongly support a Donnan-type rejection mechanism, dominated by electrostatic interactions between fixed membrane charges and mobile ions, whereas steric and hydrodynamic effects appear to be less important.

Nanofluidics in carbon nanotubes

Extremely high aspect ratios, molecularly smooth hydrophobic graphitic walls, and nanoscale inner diameters of carbon nanotubes give rise to the unique phenomenon of ultra-efficient transport of water and gas through these ultra-narrow molecular pipes. Water and gas molecules move through nanotube pores orders of magnitude faster than through other pores of comparable size. The proposed water transport mechanism has a distinct similarity to the transport mechanisms of biological ion channels. Molecular dynamics simulations and experimental measurements of water transport underscore the importance of nanotube structure in enabling ultra-efficient transport through the pore.

Recent advances in nanoelectrode architecture for photochemical hydrogen production

We review recent advances in nanoelectrode architecture for photochemical hydrogen production by water splitting. Today, solar energy is recognized as one of the most important renewable energy sources that humanity must harness in addressing the future energy sustainability issues. Of the different strategies for solar energy conversion, solar fuel or solar hydrogen conversion is attractive in that one can store the harvested energy in chemical bonds. Recent work in this field has focused on the use of nanoarchitecture designs that aim to increase photocatalytic activity, enable visible light harvesting, and ensure chemical stability and cost-effectiveness. In this perspective review, we focus on selected work in the following areas: (1) oxide semiconductor nanoelectrodes; (2) sensitization of semiconductor nanowire/nanotube arrays; (3) bioinorganic conjugate architectures; and (4) hybrid nanoarchitectures.

Identification of endohedral water in single-walled carbon nanotubes by 1H NMR

Water confinement within single-walled carbon nanotubes (SWCNTs) has been a topic of current interest, due in part to their potential nanofiltration applications. Experiments have recently validated molecular dynamics predictions of flow enhancement within these channels, although few studies have probed the detailed structure and dynamics of water in these systems. Proton nuclear magnetic resonance ( (1)H NMR) is a technique capable of providing some of these details, although care must be exercised in separating the confined water of interest from exterior water. By using controlled experiments with both sealed and opened SWCNTs and by providing a quantitative measure of water content through desorption experiments, a signature for confined water in SWCNTs has been positively identified. This endohedral or interior water is characterized by a relatively broad feature located at 0.0 ppm, shifted upfield relative to bulk water. With the identification of a signature for water inside SWCNTs, further studies aimed at probing water dynamics will be enabled.

Nanofluidic Carbon Nanotube Membranes: Applications for Water Purification and Desalination

Publisher Summary The unique geometry and internal structure of carbon nanotubes (CNTs) give rise to newly discovered phenomena of the ultraefficient transport of water through these ultra-narrow molecular pipes. Water transport in nanometer-size nanotube pores is orders of magnitude faster than transport in other pores of comparable size. We discuss the basic physical principles of the ultraefficient transport in CNTs, the fabrication of CNT membranes, and their nanofiltration and ion exclusion properties. A rare combination of transport efficiency and selectivity makes CNT membranes a highly promising technological platform for the next-generation desalination and water purification technologies. This chapter discusses the potential of these applications for improving water quality.

Carbon nanotube-based membranes: a platform for studying nanofluidics

A membrane of multiwalled carbon nanotubes embedded in a silicon nitride matrix was fabricated for use in studying fluid mechanics on the nanometer scale. Characterization by fluorescent tracer diffusion and scanning electron microscopy suggests that the membrane is void-free near the silicon substrate on which it rests, implying that the hollow core of the nanotube is the only conduction path for molecular transport. Nitrogen flow measurements of a nanoporous silicon nitride membrane, fabricated by sacrificial removal of carbon, give a flow rate of 0.086 cc/sec. Calculations of water flow across a nanotube membrane give a rate of 2.1/spl times/10/sup -6/ cc/sec (0.12 /spl mu/L/min).

Chapter 11 – Nanofluidic Carbon Nanotube Membranes: Applications for Water Purification and Desalination

This chapter presents a brief overview of the basic physical processes that govern the structure and transport of water inside CNT pores, basic properties that make nanotube pore technologies attractive for water purification and desalination, the fabrication approaches for producing CNT membranes, and the experimental observations of water transport and ion exclusion properties in CNT membranes.

Mechanism of ion exclusion by sub-2nm carbon nanotube membranes

Carbon nanotubes offer an outstanding platform for studying molecular transport at nanoscale, and have become promising materials for nanofluidics and membrane technology due to their unique combination of physical, chemical, mechanical, and electronic properties. In particular, both simulations and experiments have proved that fluid flow through carbon nanotubes of nanometer size diameter is exceptionally fast compared to what continuum hydrodynamic theories would predict when applied on this length scale, and also, compared to conventional membranes with pores of similar size, such as zeolites. For a variety of applications such as separation technology, molecular sensing, drug delivery, and biomimetics, selectivity is required together with fast flow. In particular, for water desalination, coupling the enhancement of the water flux with selective ion transport could drastically reduce the cost of brackish and seawater desalting. In this work, we study the ion selectivity of membranes made of aligned double-walled carbon nanotubes with sub-2 nm diameter. Negatively charged groups are introduced at the opening of the carbon nanotubes by oxygen plasma treatment. Reverse osmosis experiments coupled with capillary electrophoresis analysis of permeate and feed show significant anion and cation rejection. Ion exclusion declines by increasing ionic strength (concentration) of the feed and by lowering solution pH; also, the highest rejection is observed for the C A z z mn AC salts (A=anion, C=cation, z= valence) with the greatest zA/zC ratio. Our results strongly support a Donnan-type rejection mechanism, dominated by electrostatic interactions between fixed membrane charges and mobile ions, while steric and hydrodynamic effects appear to be less important. Comparison with commercial nanofiltration membranes for water softening reveals that our carbon nanotube membranes provides far superior water fluxes for similar ion rejection capabilities.

Carbon nanotube-based permeable membranes

A membrane of multiwalled carbon nanotubes embedded in a silicon nitride matrix was fabricated for use in studying fluid mechanics on the nanometer scale. Characterization by fluorescent tracer diffusion and scanning electron microscopy suggests that the membrane is void-free near the silicon substrate on which it rests, implying that the hollow core of the nanotube is the only conduction path for molecular transport. Assuming Knudsen diffusion through this nanotube membrane, a maximum helium transport rate (for a pressure drop of 1 atm) of 0.25 cc/sec is predicted. Helium flow measurements of a nanoporous silicon nitride membrane, fabricated by sacrificial removal of carbon, give a flow rate greater than 1x10{sup -6} cc/sec. For viscous, laminar flow conditions, water is estimated to flow across the nanotube membrane (under a 1 atm pressure drop) at up to 2.8x10{sup -5} cc/sec (1.7 {micro}L/min).