Justin Bult

Prolonging Charge Separation in P3HT−SWNT Composites Using Highly Enriched Semiconducting Nanotubes

Single-walled carbon nanotubes (SWNTs) have potential as electron acceptors in organic photovoltaics (OPVs), but the currently low-power conversion efficiencies of devices remain largely unexplained. We demonstrate effective redispersion of isolated, highly enriched semiconducting and metallic SWNTs into poly(3-hexylthiophene) (P3HT). We use these enriched blends to provide the first experimental evidence of the negative impact of metallic nanotubes. Time-resolved microwave conductivity reveals that the long-lived carrier population can be significantly increased by incorporating highly enriched semiconducting SWNTs into semiconducting polymer composites.

Graphene as an Efficient Interfacial Layer for Electrochromic Devices

This study presents an interfacial modification strategy to improve the performance of electrochromic films that were fabricated by a magnetron sputtering technique. High-quality graphene sheets, synthesized by chemical vapor deposition, were used to modify fluorine-doped tin oxide substrates, followed by the deposition of high-performance nanocomposite nickel oxide electrochromic films. Electrochromic cycling results revealed that a near-complete monolayer graphene interfacial layer improves the electrochromic performance in terms of switching kinetics, activation period, coloration efficiency, and bleached-state transparency, while maintaining ∼100% charge reversibility. The present study offers an alternative route for improving the interfacial properties between electrochromic and transparent conducting oxide films without relying on conventional methods such as nanostructuring or thin film composition control.

Atomic Layer Deposition of Platinum onto Functionalized Aligned MWNT Arrays for Fuel Cell Application

High aspect ratio materials, such as carbon nanotubes (CNTs), provide unique opportunities and advantages as catalyst support materials in fuel cells. In particular, CNTs are highly conductive and corrosion resistant; properties which represent limitations for current carbon supports. While most advanced catalysts research focuses on the production of small nanoparticles to increase the percent of surface accessible Pt; here, we specifically attempt to conformally coat Pt in thin layers onto CNT arrays. We present our work on modifying CNT surfaces inside high-density, surface-bound aligned CNT arrays (aspect ratio ~1:750) with non-toxic gas phase chemistries. The number of nucleation sites and the onset of growth of Pt by ALD can be tuned by using Ar plasma, O2 plasma and chemical functionalization. This, in turn, affects the uniformity of the Pt ALD coating down the length of the tubes within the CNT array.

Atomic layer deposition for aligned growth of and conformal deposition onto double and triple walled carbon nanotubes

We present our work on the growth and functionalization of carbon nanotubes (CNTs). A significant challenge in the growth of aligned single, double and triple walled nanotubes is in the deposition of a controlled thickness catalyst layer. Conventional techniques using line of sight deposition such as sputtering and evaporation produce uniform catalyst layers only when extreme care is taken in the placement of flat substrates. Growth of aligned low wall number carbon nanotubes on contoured, complex geometry, or large surface area substrates is simply not technically feasible through these techniques. Using iron atomic layer deposition (ALD) with ferrocene and oxygen precursors for catalyst deposition circumvents the line of sight problems and allows for uniform coverage across almost all substrates. Furthermore the ALD technique allows for extremely accurate and reproducible thickness depositions. Using these ALD catalyst layers reproducible aligned arrays consisting of primarily double and triple wall CNTS can be fabricated. Conformal coatings onto high aspect ratio surfaces are particularly challenging. The walls of single carbon nanotubes in a nanotube array are inaccessible by line of sight techniques. ALD circumvents this problem by relying on a gas-surface reaction to initiate growth. Generally, growth of ALD films on CNTs results in beading of the deposited materials around CNT defects. This is particularly true of high surface energy materials. The number of nucleation sites and the onset of growth of Pt by ALD can be tuned by use of Ar plasma, O2 plasma and chemical functionalization.

A core-level spectroscopic investigation of the preparation and electrochemical cycling of nitrogen-modified carbon as a model catalyst support

The synthesis and electrochemical cycling of platinum–ruthenium nanoparticles sputtered onto nitrogen-implanted highly-oriented-pyrolytic-graphite (HOPG) was studied with soft X-ray spectroscopy. The near edge X-ray absorption fine structure (NEXAFS) of the carbon 1s, nitrogen 1s, and oxygen 1s transitions were measured as a function of sample preparation and electrochemical cycling. The NEXAFS of the C 1s edge indicate defect formation in the graphitic (sp2) network of the carbon support due to implantation. The primary nitrogen species include pyridinic, nitrilic, and graphitic with no evidence of pyrrolic nitrogen. Upon exposure to ambient conditions, the carbon defects react and produce both –CO and –C–OH species. Sputtering Pt : Ru and subsequent air exposure introduces more defects that react with ambient oxygen to increase the number of –CO species. The samples also show signs of oxidization after implantation. Electrochemical cycling of the samples restores the C 1s fine structure associated with graphitic (sp2) carbon and alters the concentration of nitrogen species associated with the nitrile functional groups. The cycling also induces platinum oxidation and ruthenium loss, determined from X-ray photoelectron spectroscopy (XPS) of the Pt 4f, Ru 3d and Ru 3p. The results provide useful evidence of the types of nitrogen species that are present after electrochemical processes which can be used in the rational design of future electrocatalyst systems.