Reports: DNI1050421-DNI10: Interfacial Chemistry of Carbon-Based Photovoltaics

YuHuang Wang, PhD, University of Maryland

The goal of the proposed DNI research is to build an understanding of the effects of surface chemistry on electrical contact using double-walled carbon nanotubes (DWNTs) as a model system. The specific aims include: 1) probing the structural evolution of DWNTs in a chemical reaction by Raman spectroscopy, 2) structural characterization of covalently functionalized DWNTs, and 3) establishing correlations between DWNT chemistry with solubility, thin film conductivity and efficiency of photovoltaic composites. In year 2, we have continued making rapid progress in achieving these objectives, especially objective 3. This work is summarized in a research article in Journal of Materials Chemistry (2011, 21, 18568-15574), which is featured on the back cover of the 46thissue.

This DNI project has provided participating students with interesting research opportunities and the necessary resource to get started. Particularly, the support allows three graduate students (T. Baker, D. Ramsdell and Y. Piao) and one undergraduate student (N. Kim) to participate in summer research on carbon nanotube chemistry. Some of the initial successes also provided aspiration to my National Science Foundation CAREER award project focusing on fundamental studies of semiconducting inner-tubes. Hence, this DNI award has a timely and significant impact on the career development of both my students’ and mine.

Highlight: One-Wall Selective Chemistry Allows Enrichment of Double-Walled Carbon Nanotubes

DWNTs are core-shell structures that are composed of exactly two single-walled carbon nanotubes (SWNTs). This interesting structure gives DWNTs better thermal and chemical stability, as well as higher mechanical strength in comparison to SWNTs and have shown promise in energy applications. Recent studies by our group and others have further demonstrated the capability to covalently functionalize the outer-wall without degrading the electrical properties of the inner- tube. Because of these properties, DWNTs have shown promising potential applications in many important areas such as electrodes, field-effect transitors, biosensors, composites, and energy conversion materials. However, all known synthetic methods for producing DWNTs, examplified by arc-discharge, coalescence of C60 peapods and catalytic chemical vapor deposition (CVD), yield substantial amounts of contaminants, including catalyst particles,  amphorous carbon, SWNTs, few-walled carbon nanotubes (FWNTs, 3-5 walls), and multi-walled carbon nanotubes (MWNTs, >5 walls). The SWNT and FWNT byproducts are particularly difficult to remove because they differ from DWNTs by merely one wall. High purity DWNT materials were unattainable until recently. It has been reported that high temperature air oxidation followed by hydrochloric acid treatment increases the proportion of DWNTs by destroying the chemically more reactive SWNTs, amorphous carbon and catalyst particles. Density differences have been used to separate DWNTs from other carbon nanostructures by density gradient ultracentrifugation (DGU). These methods have provided purified DWNT materials and enabled several important studies on the materials properties of DWNTs. Yet, both methods have limitations. Oxidative removal of MWNTs unavoidably introduces permanent structural defects such as vacancies to DWNTs, since the latter is more prone to oxidation than MWNTs. DGU requires extensive ultrasonication to disperse nanotubes, which cuts them to sub-micron meter length, limiting this approach to shorter nanotubes. From the chemistry side, DWNTs provide an interesting structure for probing surface chemistry of carbon materials because the two walls can be independently monitored by spectroscopy methods such as Raman scattering and fluorescence spectroscopy.

We found that a wall-number selective covalent chemistry, which allows us to devise a chemically reversible and potentially scalable chemical approach to DWNTs separation. The DWNTs were selectively functionalized by Billups-Birch reductive alkylcarboxylation and subsequently enriched in water soluble extracts from other carbon byproducts, especially SWNTs and MWNTs. The separation is made possible due to the high structural selectivity of the chemistry which affords the various carbon structures different degrees of functionalization and thus water solubility. We have previously reported that the Billups-Birch reductive alkylcarboxylation chemistry occurs by reaction propagation from existing defects and exhibits a strong diameter dependence, allowing one to selectively functionalize smaller diameter SWNTs and physically separate them in fractions of enriched diameters by a combination of progressive reaction and water extraction. This structural dependence inspired us to extend the method to the separation of DWNTs from SWNTs since the SWNT contaminants are typically smaller in diameter than the DWNTs. The covalent attachment of alkyl functional groups to nanotube sidewalls converts the carbon atoms from sp2 hybridization to sp3, introducing sp3 defects in the carbon nanotube network. Because this chemistry occurs nearly exclusively by reaction propagation from existing defects, the reaction rate difference between SWNTs and DWNTs is kinetically amplified to afford high selectivity. The water solubility is approximately proportional to the relative number of covalent functional groups. As such, this provides a means to physically separate DWNTs by competitive solvation and partitioning in a hexane/water mixture, in which the more easily functionalized carbonaceous structures, such as amorphous carbon and SWNTs were separated in aqueous extracts while the less functionalized DWNTs and MWNTs were enriched in the insoluble solid. By performing additional alkylcarboxylation on the insoluble solid, DWNTs and FWNTs were then selectively enriched in aqueous extracts leaving larger diameter MWNTs as insoluble solid. The effective separation was confirmed by Raman resonant scattering and quantitative, statistical analysis by transmission electron microscopy (TEM). Raman spectroscopy and fluorescence mapping further suggest that alkylcarboxylation occurred selectively on the outerwall of DWNTs, leaving the inner-tubes intact. The functional groups can be removed as needed by thermal annealing to recover the pristine DWNT structure.

We note that a clean separation of DWNTs from few wall nanotubes remains a challenge, probably due to the overwhelming population of FWNTs in the starting material. However, enrichment of DWNTs is clearly evident by TEM analysis. Additional advantages of the proposed approach include 1) the wet chemistry is straightforward and highly scalable; 2) no structural degradation of DWNTs is evident as compared with thermal or oxidative treatment; and 3) DWNTs can become water soluble while the inner-tube structures remain intact.