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

YuHuang Wang, PhD , University of Maryland

Electrical contact between chemically functionalized carbon nanostructures is a recognized critical issue for the use of these types of nanomaterials as conductive, functional electrodes for high performance electrical energy storages and solar energy harvesting. 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 1 (09/01/2010-08/31/2011), we have achieved most objectives in aim 1 and aim 2, and have made important observation from thin film electrical conductivity measurement. The work is summarized in three research articles and one invited review article, all of which have been published in premier journals including Chemical Communications (2011, 47, 758-760), Journal of Physical Chemistry Letters (2011, 2, 885-888, and 1577-1582), and Nanoscale (2011, 3, 503-518).

This DNI project has significant impact on both my career and on the students who participated in the project. The support has provided participating students with interesting research opportunities and the necessary resource to get started. Particularly, the support allows graduate student Yanmei Piao to collect some high quality Raman spectroscopy data for her PhD candidacy exam, and postdoc Chien-Fu Chen has successfully landed an assistant professorship at the National Chung Hsing University, one of the top universities in Taiwan. 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 impact on the career development of both my students’ and mine.

Highlight of Research Progress

I. Outerwall Selective Diazonium Chemistries Confirmed by Raman Spectroscopy

We demonstrate that 4-bromobenzenediazonium tetrafluoroborate reacts with the outer wall of DWNTs selectively to the exclusion of the inner tube. By following the structural evolution of the reacting DWNTs with Raman spectroscopy, we were able to find the optimal reaction conditions that allowed functionalization of the outer wall to the highest degree possible by the diazonium chemistry. The reaching of a saturation point is evidenced by little further changes in both the optical absorbance and Raman intensity as more diazonium salts were added and the reaction time extended up to seven days. This saturation point is experimentally determined by X-ray photoelectron spectroscopy to be approximately 69 functional groups (covalently attached aryls) per 1000 surface carbons.

A DWNT displays two distinct sets of Raman signatures corresponding to the inner tube and outer wall, respectively. In particular, the inner tube and the outer wall have distinct radial breathing modes (RBMs) positioned ~218*(1/din-1/dout) cm-1 apart because the low frequency Raman shifts are inversely proportional to the nanotube diameter. With an average inner tube diameter of ~0.86 nm and outer wall diameter of ~1.61 nm in our samples, this corresponds to approximately 273 cm-1 and 155 cm-1, respectively. Unambiguous assignment of the spectroscopic features to each wall were made possible by the use of purified DWNT samples, free of SWNTs, multiwall nanotubes (MWNTs) and other by-products, prepared by our collaborators Alex Green and Professor Mark Hersam (Northwestern) using a density gradient ultracentrifugation technique. We followed the evolution of the Raman RBMs of the DWNTs during the course of reaction over a time period of seven days. As the reaction proceeded, the RBM peaks of the outer walls continuously decrease until they vanish completely, while those from the inner tubes remain nearly unchanged. Similar trends were obtained by increasing the concentration from 1:1 to 1:100 carbon to diazonium molar ratio. Although Raman typically is not a quantitative method, the nearly unchanged inner tube intensity suggests that the inner tube can potentially serve as an internal standard to provide relatively quantitative information on the progress of reaction at the outer wall.

These spectroscopic studies unambiguously show that both the diazonium chemistry and the Raman spectroscopic features of the inner tube can be cleanly decoupled from the outer wall. The outer wall selectivity can be attributed to the physical protection of the inner tube by the outer wall and size exclusion from the small inner tubes. The DWNT structures investigated in this study have small opening (< 1 nm) at the ends and high aspect ratio (>1,000), which significantly retard endohedral reactions by minimizing diffusion of the reagents into the inner tubes.

II. Extended Electrical Percolation via Intact Inner Tubes

The optical properties of inner tubes are corroborated with electrical properties that show markedly high conductivity compared to similarly functionalized SWNTs. To probe the electrical properties of the inner tube, we have fabricated DWNT thin film devices and measured their electrical conductivity before and after diazonium functionalization. It is well known that covalently attached functional groups strongly scatter electrons. A single defect can block one of the two transport channels of a SWNT. Because of the high degree of functionalization (approximately 69 functional groups per 1000 carbons), it is not surprising that the SWNT conductivity drops continuously as the reaction proceeds, until it is almost completely lost. In stark contrast to SWNTs, DWNT devices retain a nearly constant conductivity, approximately 50% of the original value of each device after extensive diazonium functionalization. The observed high electrical retention is characteristic of the double-wall structure, not due to thin film inhomogenity or variation in contact resistance. The two walls that constitute a DWNT presumably form two electrically conductive pathways. Diazonium functionalization selectively blocks the outer wall pathway, but the inner tube pathway survives intact.

The high conductivity retention observed in functionalized DWNT networks suggests a promising “outer wall” materials strategy to address the electrical contact problem. This will be a subject of intense research in year 2 and become a main research direction in my group for years to come.

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