Timothy J. McDonald

Transparent Conductive Single-Walled Carbon Nanotube Networks with Precisely Tunable Ratios of Semiconducting and Metallic Nanotubes

We present a comprehensive study of the optical and electrical properties of transparent conductive films made from precisely tuned ratios of metallic and semiconducting single-wall carbon nanotubes. The conductivity and transparency of the SWNT films are controlled by an interplay between localized and delocalized carriers, as determined by the SWNT electronic structure, tube-tube junctions, and intentional and unintentional redox dopants. The results suggest that the main resistance in the SWNT thin films is the resistance associated with tube-tube junctions. Redox dopants are found to increase the delocalized carrier density and transmission probability through intertube junctions more effectively for semiconductor-enriched films than for metal-enriched films. As a result, redox-doped semiconductor-enriched films are more conductive than either intrinsic or redox-doped metal-enriched films.

Protonation effects on the branching ratio in photoexcited single-walled carbon nanotube dispersions.

The ensemble PL quantum yield for raw single-walled carbon nanotubes (SWNTs) dispersed in sodium cholate (SC) is approximately 5 times greater than that for the same raw SWNTs dispersed in sodium dodecyl sulfate (SDS) and approximately 10 times greater than the quantum yield of purified SWNTs dispersed in SC. Absorbance and Raman spectra indicate that purified SC-dispersed SWNTs and raw SDS-dispersed SWNTs are hole-doped by protonation. Experiments comparing PL emission efficiency using E2 and E1 excitation show that protonation significantly affects the E2 --> E1 relaxation process, which has typically been assumed to occur with unit efficiency. The E2 --> E 1 relaxation is 5 times more efficient in producing E 1 PL when SWNTs are unprotonated and protected by the SC surfactant. The results provide clear evidence that extrinsic factors, such as residual acids and the specific nature of SWNT-surfactant and SWNT-solvent interactions, can significantly affect measured SWNT luminescence quantum yields.

Near-infrared Fourier transform photoluminescence spectrometer with tunable excitation for the study of single-walled carbon nanotubes

A fast, sensitive, automated Fourier transform (FT) photoluminescence (PL) spectrometer with tunable excitation has been developed for analyzing carbon nanotube suspensions over a wide spectral range. A commercially available spectrometer was modified by the addition of a tunable excitation source, custom collection optics, and computer software to provide control and automated data collection. The apparatus enables excitation from 400to1100nm and detection from 825to1700nm, permitting the analysis of virtually all semiconducting single-walled nanotubes (SWNTs), including those produced by the high pressure carbon monoxide conversion and laser processes. The FT approach provides an excellent combination of high sensitivity and fast measurement. The speed advantage exists because the entire emission spectrum is collected simultaneously, while the sensitivity advantage stems from the high optical throughput. The high sensitivity is demonstrated in the measurement of very dilute SWNT suspensions and the obse...

Raman spectroscopy of charge transfer interactions between single wall carbon nanotubes and [FeFe] hydrogenase

We report a Raman spectroscopy study of charge transfer interactions in complexes formed by single-walled carbon nanotubes (SWNTs) and [FeFe] hydrogenase I (CaHydI) from Clostridium acetobutylicum. The choice of Raman excitation wavelength and sample preparation conditions allows differences to be observed for complexes involving metallic (m) and semiconducting (s) species. Adsorbed CaHydI can reversibly inject electronic charge into the LUMOs of s-SWNTs, while charge can be injected and removed from m-SWNTs at lower potentials just above the Fermi energy. Time-dependent enzymatic assays demonstrated that the reduced and oxidized forms of CaHydI are deactivated by oxygen, but at rates that varied by an order of magnitude. The time evolution of the oxidative decay of the CaHydI activity reveals different time constants when complexed with m-SWNTs and s-SWNTs. The correlation of enzymatic assays with time-dependent Raman spectroscopy provides a novel method by which the charge transfer interactions may be investigated in the various SWNT-CaHydI complexes. Surprisingly, an oxidized form of CaHydI is apparently more resistant to oxygen deactivation when complexed to m-SWNTs rather than s-SWNTs.

Engineered carbohydrate-binding module (CBM) protein-suspended single-walled carbon nanotubes in water

Engineered protein, CtCBM4, the first carbohydrate-binding module (CBM) protein is successfully used to debundle and suspend single-walled carbon nanotubes (SWNTs) effectively in aqueous solution, which opens up a new avenue in further functionalizing and potential selectively fractionating SWNTs for diverse biology- and/or energy-related applications.

Merging [FeFe]-Hydrogenases with Materials and Nanomaterials as Biohybrid Catalysts for Solar H2 Production

The catalysts commonly used for the H2 producing reaction in artificial solar systems are typically platinum or particulate platinum composites. Biological catalysts, the hydrogenases, exist in a wide-variety of microbes and are biosynthesized from abundant, non-precious metals. By virtue of a unique catalytic metallo-cluster that is composed of iron and sulfur, [FeFe]-hydrogenases are capable of catalyzing H2 production at turnover rates of millimoles-per-second. In addition, these biological catalysts possess some of the characteristics that are desired for cost-effective solar H2 production systems, high solubilities in aqueous solutions and low activation energies, but are sensitive to CO and O2. We are investigating ways to merge [FeFe]-hydrogenases with a variety of organic materials and nanomaterials for the fabrication of electrodes and biohybrids as catalysts for use in artificial solar H2 production systems. These efforts include designs that allow for the integration of [FeFe]-hydrogenase in dye-solar cells as models to measure solar conversion and H2 production efficiencies. In support of a more fundamental understanding of [FeFe]-hydrogenase for these and other applications the role of protein structure in catalysis is being investigated. Currently there is little known about the mechanism of how these and other enzymes couple multi-electron transfer to proton reduction. To further the mechanistic understanding of [FeFe]-hydrogenases, structural models for substrate transfer are being used to create enzyme variants for biochemical analysis. Here results are presented on investigations of proton-transfer pathways in [FeFe]-hydrogenase and their interaction with single-walled carbon nanotubes.

Interaction of [FeFe]-hydrogenases with single-walled carbon nanotubes

Single-walled carbon nanotubes (SWNT) are promising candidates for use in energy conversion devices as an active photo-collecting elements, for dissociation of bound excitons and charge-transfer from photo-excited chromophores, or as molecular wires to transport charge. Hydrogenases are enzymes that efficiently catalyze the reduction of protons from a variety of electron donors to produce molecular hydrogen. Hydrogenases together with SWNT suggest a novel biohybrid material for direct conversion of sunlight into H2. Here, we report changes in SWNT optical properties upon addition of recombinant [FeFe] hydrogenases from Clostridium acetobutylicum and Chlamydomonas reinhardtii. We find evidence that novel and stable charge-transfer complexes are formed under conditions of the hydrogenase catalytic turnover, providing spectroscopic handles for further study and application of this hybrid system.