Researchers clarify properties of 'confined' water within single-walled carbon nanotube pores

Water and ice may not be among the first things that come to mind when you think about single-walled carbon nanotubes (SWCNTs), but a Japan-based research team hoping to get a clearer understanding of the phase behavior of confined water in the cylindrical pores of carbon nanotubes zeroed in on confined water's properties and made some surprising discoveries.

The team, from Tokyo Metropolitan University, Nagoya University, Japan Science and Technology Agency, and National Institute of Advanced Industrial Science and Technology, describes their findings in the American Institute of Physics' Journal of Chemical Physics.

Although carbon nanotubes consist of hydrophobic (water repelling) graphene sheets, experimental studies on SWCNTs show that water can indeed be confined in open-ended carbon nanotubes.

This discovery gives us a deeper understanding of the properties of nanoconfined water within the pores of SWCNTs, which is a key to the future of nanoscience. It's anticipated that nanoconfined water within carbon nanotubes can open the door to the development of a variety of nifty new nanothings—nanofiltration systems, molecular nanovalves, molecular water pumps, nanoscale power cells, and even nanoscale ferroelectric devices.

"When materials are confined at the atomic scale they exhibit unusual properties not otherwise observed, due to the so-called 'nanoconfinement effect.' In geology, for example, nanoconfined water provides the driving force for frost heaves in soil, and also for the swelling of clay minerals," explains Yutaka Maniwa, a professor in the Department of Physics at Tokyo Metropolitan University. "We experimentally studied this type of effect for water using SWCNTs."

Water within SWCNTs in the range of 1.68 to 2.40 nanometers undergoes a wet-dry type of transition when temperature is decreased. And the team discovered that when SWCNTs are extremely narrow, the water inside forms tubule ices that are quite different from any bulk ices known so far. Strikingly, their melting point rises as the SWCNT diameter decreases—contrary to that of bulk water inside a large-diameter capillary. In fact, tubule ice occurred even at room temperature inside SWCNTs.

"We extended our studies to the larger diameter SWCNTs up to 2.40 nanometers and successfully proposed a global phase behavior of water," says Maniwa. "This phase diagram (See Figure) covers a crossover from microscopic to macroscopic regions. In the macroscopic region, a novel wet-dry transition was newly explored at low temperature."

Results such as these contribute to a greater understanding of fundamental science because nanoconfined water exists and plays a vital role everywhere on Earth—including our bodies. "Understanding the nanoconfined effect on the properties of materials is also crucial to develop new devices, such as proton-conducting membranes and nanofiltration," Maniwa notes.

Next up, the team plans to investigate the physical properties of confined water discovered so far inside SWCNTs (such as dielectricity and proton conduction). They will pursue this to obtain a better understanding of the molecular structure and transport properties in biological systems.

 

Properties of 'confined' water within single-walled carbon nanotube pores clarified

Water and ice may not be among the first things that come to mind when you think about single-walled carbon nanotubes (SWCNTs), but a Japan-based research team hoping to get a clearer understanding of the phase behavior of confined water in the cylindrical pores of carbon nanotubes zeroed in on confined water's properties and made some surprising discoveries.

The team, from Tokyo Metropolitan University, Nagoya University, Japan Science and Technology Agency, and National Institute of Advanced Industrial Science and Technology, describes their findings in the American Institute of Physics' Journal of Chemical Physics.

Although carbon nanotubes consist of hydrophobic (water repelling) graphene sheets, experimental studies on SWCNTs show that water can indeed be confined in open-ended carbon nanotubes.

This discovery gives us a deeper understanding of the properties of nanoconfined water within the pores of SWCNTs, which is a key to the future of nanoscience. It's anticipated that nanoconfined water within carbon nanotubes can open the door to the development of a variety of nifty new nanothings -- nanofiltration systems, molecular nanovalves, molecular water pumps, nanoscale power cells, and even nanoscale ferroelectric devices.

"When materials are confined at the atomic scale they exhibit unusual properties not otherwise observed, due to the so-called 'nanoconfinement effect.' In geology, for example, nanoconfined water provides the driving force for frost heaves in soil, and also for the swelling of clay minerals," explains Yutaka Maniwa, a professor in the Department of Physics at Tokyo Metropolitan University. "We experimentally studied this type of effect for water using SWCNTs."

Water within SWCNTs in the range of 1.68 to 2.40 nanometers undergoes a wet-dry type of transition when temperature is decreased. And the team discovered that when SWCNTs are extremely narrow, the water inside forms tubule ices that are quite different from any bulk ices known so far. Strikingly, their melting point rises as the SWCNT diameter decreases -- contrary to that of bulk water inside a large-diameter capillary. In fact, tubule ice occurred even at room temperature inside SWCNTs.

"We extended our studies to the larger diameter SWCNTs up to 2.40 nanometers and successfully proposed a global phase behavior of water," says Maniwa. "This phase diagram (see image) covers a crossover from microscopic to macroscopic regions. In the macroscopic region, a novel wet-dry transition was newly explored at low temperature."

Results such as these contribute to a greater understanding of fundamental science because nanoconfined water exists and plays a vital role everywhere on Earth -- including our bodies. "Understanding the nanoconfined effect on the properties of materials is also crucial to develop new devices, such as proton-conducting membranes and nanofiltration," Maniwa notes.

Next up, the team plans to investigate the physical properties of confined water discovered so far inside SWCNTs (such as dielectricity and proton conduction). They will pursue this to obtain a better understanding of the molecular structure and transport properties in biological systems.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by American Institute of Physics, via EurekAlert!, a service of AAAS.

Free-standing single-walled carbon nanotube thin films

ScienceDaily (May 13, 2011) — Single-walled CNTs (SWCNTs) are a unique family of materials exhibiting diverse useful chemical and physical properties, researchers in Finland are demonstrating.

Thin films of SWCNTs have many unique properties such as high porosity and specific surface area, low density, high ratio of optical transmittance to sheet resistance, high thermal conductivity and chemical sensitivity, and tunable metallic and semiconducting properties.

Recently researchers from Department of Applied Physics at Aalto University (Finland) in collaboration with Canatu Ltd. (Finland) have discovered a simple and rapid method to prepare thin multifunctional single-walled carbon nanotube films without any substrate (free-standing films). Usually SWCNT films are prepared from suspensions of SWCNTs by a liquid filtration. This method typically involves several time and resource consuming and potentially detrimental liquid dispersion and purification steps ending up with dense SWCNT networks on a filter, which have a transfer issue. Moreover, preparation of free-standing films by the vacuum-filtration method is still a challenging task.

"Our method allows the preparation of SWCNT deposits both on different substrates and in the form of free-standing films during less than 15 s. This becomes possible due to the fact that the SWCNTs produced in the gas phase synthesis process gave very high purity and crystallinity, can be directly deposit from the gas to a substrate and accordingly be directly utilised without additional purification steps," says Dr. David P. Brown, CEO of the company Canatu Ltd, which commercializes the SWCNT films.

The method easily allows Canatu to alter the thickness of multifunctional free-standing SWCNT films from a sub-monolayer (when the amount of SWCNTs is insufficient to create a single continuous layer) to a few micrometers.

Collaboration with other Finnish universities

According to the researchers, the collaboration with other Finnish universities and institutions, Tampere University of Technology and Oulu University, was extremely important to investigate the unique properties and to demonstrate the multifunctionality of this unique material.

"We fabricated the state-of-the-art components for filtration of aerosol nanoparticles, transparent, flexible and highly conductive electrodes, extremely sensitive electrochemical sensors, polymer free saturable laser absorbers, gas heaters, thermo acoustic loudspeakers, and gas flow meters," says professor Esko I. Kauppinen, the leader of the research group.

SWCNT films - wide range of applications

"However, the wide range of applications is not limited to those reported in our paper. The superior mechanical and electrical properties of these films suggest potential uses in a broad range of other devices. As a filter, SWCNT films could be used for filtration of bacteria and viruses. The possibility to heat SWCNT films can be utilized for water or air sterilization. Additionally, since SWCNTs contain iron particles embedded inside them, one could exploit their magnetic properties. Their high strength coupled with high electrical conductivity could be employed in novel energy generators, electromagnetic interference shielding, flexible radio frequency identification tags, touch sensors, flat panel displays and static-charge dissipators. The ultrahigh surface area coupled with high electrical conductivity could be used in advanced solar cells and super capacitors," Dr. Albert G. Nasibulin, the leader of this project and the first author of the article related to the discovery of multifunctional free-standing SWCNTs concludes.

The results has been recently published in the journal ACS Nano.

Even though graphene has recently attracted much attention from the research community, some properties of SWCNTs, such as porosity, mechanical strength, and fine-tunability of optical and electrical properties, provide many applications where the flat, single-layered carbon structure cannot compete with its tubular 'brother'.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Aalto University, via AlphaGalileo.

Journal Reference:

Albert G. Nasibulin, Antti Kaskela, Kimmo Mustonen, Anton S. Anisimov, Virginia Ruiz, Samuli Kivisto¨, Simas Rackauskas, Marina Y. Timmermans, Marko Pudas, Brad Aitchison, Marko Kauppinen, David P. Brown, Oleg G. Okhotnikov, Esko I. Kauppinen. Multifunctional Free-Standing Single-Walled Carbon Nanotube Films. ACS Nano, 2011; : 110310090902026 DOI: 10.1021/nn200338r

Note: If no author is given, the source is cited instead.

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