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Posted: Oct 28, 2009

Carbon nanotube-DNA nanotechnology for improved fuel cell catalysts

(Nanowerk Spotlight) Due to their unique structural and electrical properties, carbon nanotubes (CNTs) have been extensively investigated as promising catalyst supports to improve the efficiency of direct ethanol/methanol fuel cells. CNTs have a significantly higher electronic conductivity and an extremely higher specific surface area in comparison with the most widely-used Vulcan XC-72R carbon support. Several approaches, such as electrochemical reduction, electroless deposition, spontaneous reduction, sonochemical technique, microwave-heated polyol process, and nanoparticle decoration on chemically oxidized nanotube sidewalls, have been reported to form CNT-supported platinum catalysts. Some remarkable progress has been made in synthesis techniques; however, pioneering nanotechnology breakthroughs have not been made yet in terms of cost-effectiveness catalyst activity, durability, and chemical-electrochemical stability.
Nanotechnology researchers in the U.S. have now discovered that platinum nanoparticles selectively grow on carbon nanotubes in accordance with single-stranded DNA (ssDNA) locations. They have demonstrated that not only can ssDNA bind to nanotube surfaces but also disperse bundled single-walled carbon nanotubes (SWCNTs) into individual tubes. This finding suggests a method to synthesize other types of carbon nanotube-supported nanoparticles, such as palladium and gold for applications in fuel cells and nanoscale electronics.
"Major problems hampering the development of CNT-supported platinum catalysts are the lack of reliable approaches for controlling morphology, size, density, and configuration of platinum nanoparticles along carbon nanotubes," Lifeng Dong, Dong, an assistant professor in the Department of Physics, Astronomy, and Materials Science at Missouri State University, explains to Nanowerk. "The paucity of reports on the synthesis of CNT-supported platinum catalysts, demonstrating controlled properties and structural characterization, is due mostly to the complexity of separating nanotubes, especially single-walled carbon nanotubes."
Nanotubes tend to form bundles due to hydrophobic interactions in aqueous solutions and strong inter-tube van der Waals interactions. Consequently, most reported attempts have been limited to multi-walled carbon nanotubes (MWCNTs) and bundles of SWCNTs. SWCNTs are expected to have better characteristics as catalyst supports due to their larger surface area and smaller diameters.
The aim of Dong's research has been to develop an efficient method to synthesize platinum nanoparticles on SWCNTs with controlled properties. He explains that a desirable approach to producing platinum nanoparticles on SWCNTs must include two processes: the separation of bundled SWCNTs into individual tubes and the synthesis of platinum nanoparticles on the nanotubes.
SWCNT/DNA/platinum nanostructures
High angle annular dark-field STEM image of SWCNT/DNA/platinum nanostructures demonstrates the distribution of platinum nanoparticles surrounding the nanotubes. Bright particles are platinum nanoparticles with a uniform size of ≤1-2 nm, and no aggregations exist. (Reprinted with permission from IOP Publishing)
In a recent study published in the October 21, 2009 online edition of Nanotechnology ("DNA-templated synthesis of Pt nanoparticles on single-walled carbon nanotubes"), Dong describes the use of ssDNA molecules to disperse SWCNTs in aqueous solution and as templates for the binding of platinum ions to form platinum nanoparticles along the nanotubes.
With the understanding of the interactions between SWCNTs and ssDNA, researchers can now utilize ssDNA as templates to control the density and location of platinum and other metal nanoparticles formed along nanotube surface for applications in direct methanol/ethanol fuel cells. The understanding also helps us to employ SWCNTs as non-viral transporters for the delivery of DNA and RNA molecules into cells for gene therapeutic applications.
Since, in 2003, Ming Zheng and his colleagues successfully used ssDNA molecules to disperse and sort SWCNTs ("Structure-Based Carbon Nanotube Sorting by Sequence-Dependent DNA Assembly"), a large number of experiments and theoretical simulations have been conducted to study interactions of ssDNA and SWCNTs.
However, according to Dong, there remains a lack of systematic observations regarding the interfacial structure between the DNA and the nanotube surface. "Scanning tunneling microscopy (STM) and atomic force microscopy (AFM) can provide atomic resolution surface and morphology information on DNA/nanotube hybrids, yet it is difficult to use STM and AFM to explore the interfaces," he says.
In October, 2007, Dong was awarded the Visiting Scientist Fellowship from the National Center for Electron Microscopy located at Berkeley Lawrence National Laboratory. Consequently, he has access to state-of-the-art electron microscopy facility located in Berkeley to investigate the morphology and interfacial structures of DNA/SWCNT hybrids and DNA/SWCNT/platinum nanostructures.
"In our study, we used a series of electron microscopy and microanalysis techniques to investigate the morphology and interfacial structures of the hybrids" says Dong. "We observed that ssDNA molecules with a particular nucleotide sequence can wrap around the same nanotube with different morphologies, such as helices and clusters; therefore, the morphology of the SWCNT/DNA hybrids is not controlled solely by the base sequence of the ssDNA molecules or the diameter and chirality of the nanotubes."
It appears that the interactions between SWCNTs and ssDNA can be affected by other parameters, such as perturbations from water molecules in solution, CNT structural defects, electrostatic interactions between DNA charges, van der Waals and hydrophobic interactions between DNA bases and the CNT, and the sugar and phosphate groups in the DNA backbone.
Going forward, besides investigating sequences of ssDNA molecules and the diameter and chirality of SWCNTs, Dong and his team will explore other factors that affect the morphology of DNA/SWCNT hybrids. Meanwhile, they will utilize similar procedures to synthesize other types of metal nanoparticles – such as palladium, iron, and gold – along nanotube surfaces.
"Due to their small sizes and sensitivity to electron beam irradiation, it is very challenging to image the DNA/SWCNT hybrids, especially individual ssDNA molecules" Dong points out. "Even with the use of state-of-the-art double-aberration-corrected TEAM 0.5 (scanning) transmission electron microscope at 80 kV acceleration potential, we exploited the tendency of the focused electron beam to separate nanotube bundles into smaller fascicles as an in situ dissection method, which revealed the configuration of platinum nanoparticles along the nanotubes."
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