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1. Objectives

Fuel cells have been receiving increased attention recently due to the depletion of fossil fuels and the increase in environmental pollution. Among different types of fuel cells, direct methanol and ethanol fuel cells are excellent power sources due to their high energy density, low pollutant emission, low operating temperature (60 - 100 °C) and ease of handling liquid fuel. However, there are still some critical obstacles inhibiting broad applications of direct methanol and ethanol fuel cells that include low electrocatalytic activity of anodes for methanol and ethanol oxidation reactions and the high cost of noble metal platinum (Pt)-based catalysts. In order to enhance the catalytic activity and reduce the usage of Pt- based catalysts, one strategy is to explore novel carbon materials to effectively disperse catalyst particles. In this study, we investigate catalytic properties of single-walled carbon nanotube (SWCNT)-supported Pt nanoparticles that leads to the development of a novel method for synthesizing Pt nanoparticles with controlled properties along nanotube surfaces.

2. Findings

2.1 Development of an Environmental-Friendly Surfactant to Disperse Bundled SWCNTs into Individual Nanotubes

In order to be useful for their promising applications, SWCNTs need to be purified and dispersed into individual nanotubes since synthesized nanotubes occur in the form of bundles with accompanying impurities such as metal catalyst particles and amorphous carbon debris. One method to do this is by surfactant stabilization of the hydrophobic nanotube surfaces, which overcomes the van der Waals forces among the nanotubes and results in suspensions of individual SWCNTs. Several surfactants, such as sodium dodecyl sulfate (SDS), sodium dodecylbenzene sulfonate (SDBS) and sodium cholate (SC), have been demonstrated to efficiently disperse bundled nanotubes into aqueous suspensions of individual nanotubes. It is crucial to understand the toxicity of the nanotube-surfactant conjugates since these reagents are increasingly used in manufacturing industries and research laboratories.

In collaboration with Dr. Michael Craig and Dr. Colette Witkowski of the Department of Biomedical Science, we conducted a series experiment to develop an environmental-friendly surfactant to disperse nanotubes. Our experimental results demonstrate that cytotoxicity of the nanotube-surfactant conjugates is related to the toxicity of surfactant molecules attached on the nanotube surfaces. Both SDS and SDBS are toxic to cells. Exposure to CNT-SDS conjugates (0.5 mg/ml) for less than 5 min caused changes in cell morphology resulting in a distinctly spherical shape compared to untreated cells. In contrast, SC and CNT-SC did not affect cell morphology, proliferation or growth. These data indicate that SC is an environmental-friendly surfactant for the purification and dispersion of SWCNTs.

2.2 DNA-Templated Synthesis of Pt Nanoparticles on SWCNTs

A series of electron microscopy characterizations demonstrate that single-stranded DNA (ssDNA) can bind to nanotube surfaces and disperse bundled SWCNTs into individual tubes. The ssDNA molecules on the nanotube surfaces demonstrate various morphologies, such as aggregated clusters and spiral wrapping around a nanotube with different pitches and spaces, indicating that the morphology of the SWCNT/DNA hybrids is not related solely to the base sequence of the ssDNA nor the chirality and diameter of the nanotubes. In addition to serving as a non-covalent dispersion agent, the ssDNA molecules that bonded to the nanotube surface can provide addresses for localizing Pt(II) complexes along the nanotubes. The Pt nanoparticles obtained by a reduction of the Pt²⁺-DNA adducts are single crystals with a size of ≤ 1 nm to 2 nm. These results expand our understanding of the interactions between ssDNA and SWCNTs and provide an efficient approach for positioning Pt and other metal particles, with uniform sizes and without aggregations, along the nanotube surfaces for applications in direct ethanol/methanol fuel cells and nanoscale electronics.

2.3 SWCNT-Supported Pt and Pt-Ru Nanoparticles with High Electrocatalytic Activity for Methanol and Ethanol Oxidation

Pt and Pt-Ru nanoparticles were synthesized on SWCNTs and their electrocatalytic activity for methanol and ethanol oxidation was investigated. Experimental results demonstrate, in comparison to the widely-used Vulcan XC-72R carbon black catalyst supports, SWCNT-supported Pt and Pt-Ru nanoparticles reveal enhanced efficiency for both methanol and ethanol electro-oxidations with regard to diffusion efficiency, oxidation potential, forward oxidation peak current density and the ratio of the forward peak current density to the reverse peak current density. These findings favor the use of SWCNTs as catalyst supports for both direct methanol and ethanol fuel cells.

3. Student Training and Development

The execution of this proposed research not only has significant impact on the career development of the PI, but also provides invaluable opportunities for both graduate and undergraduate students to participate in cutting-edge research in the areas of nanomaterials and nanoparticle catalysts. During the period from January 1, 2008 to August 31, 2009, eight students (2 high school students, 3 undergraduates, and 3 graduate students) have participated this project.