58th Annual Report on Research 2013 Under Sponsorship of the ACS Petroleum Research Fund

The basic research being pursued in this proposal is focused on studying fundamental mass-transport mechanisms involved in heterogeneous catalytic reactions involving bi-metallic nanocrystals with the aim of helping answer significant questions surrounding whether small molecule catalysis occurs on the surface of individual nanocrystals, due to homogenous reactions due to dilute zero-valent metal atoms, or possibly some combination of the two.  The central objective of this proposal is to conduct basic materials science and engineering research to elucidate the fundamental mass transport mechanisms involved in catalytic reactions at the surfaces of individual, optically-isolated metallic nanoscale materials.

Both palladium- as well as solid-solution palladium/gold- nanocrystals recently have received considerable attention in the greater scientific literature as promising heterogeneous catalysts due both to their compositional tunability as well as the extreme reactivity related to the high-Miller-index (hkl) surface planes observed after solution-phase nanocrystal synthesis.  One of the most pressing fundamental questions in this field is whether the observed molecular reactions occur 1) on the surface of these nanoscale metals, 2) away from the nanocrystal surface due to zero-valent metal atoms that leach homogeneously into solvents, or 3) some combination of both heterogeneous and homogeneous catalysis.

The research is designed to employ a suite of advanced optoelectronic methods involving optoelectronic tweezers (OET) as well as single-beam laser tweezers (LT) can be used to confine, transfer, and monitor/observe individual metal nanocrystals over a range of time scales (second-to-days), temperatures (0 – 300°C), and solvents (water, toluene, DMF, acetonitrile), leading to potential breakthroughs in the understanding, design, and ultimate performance of engineered metallic nanomaterials relevant to the petroleum and energy conversion industries.

The first year of research has been focused on the synthesis of multiply-pentagonally-twinned palladium nanowires with well-defined length distributions and high aspect ratios.  Multiple-pentagonal-twinning is important given that twinning planes have been shown to exhibit enhanced catalytic activity relative to flat single-crystal surfaces that are free of extended twinning-plane defects.  The high aspect ratio of the palladium nanowires is important given that the confinement force in optoelectronic tweezer instruments is related to the Clausius-Mossotti factor which is two orders of magnitude larger for high-aspect-ratio (nanowire) morphologies than for particles with small volumes.

Preliminary literature reports from another research groups published in the Journal of the American Chemical Society have claimed that hydrothermal processing may be used to prepare large volumes of palladium nanowires using a mixture of palladium (II) iodide and polyvinylpyrrolidone (PVP) reagent.  During the last year two graduate students in the PI’s research group (Jennifer Hanson and Zach Rousslang) has been working on synthesizing high-aspect-ratio palladium nanowires for use as substrates for Suzuki cross-coupling reactions using either optoelectronic tweezer or single-beam laser trapping instruments.

After numerous attempts to reproduce preliminary literature reports, Jennifer Hanson discovered that the reported hydrothermal processing is not capable of producing large volumes of high-aspect ratio palladium nanowires.  Intensive experiments performed by her using ultra-pure 18.6 mega-Ohm-cm water have conclusively shown that small concentrations of copper (II) ions are required to produce large volumes of palladium nanowires.  Copper is not mentioned as a reagent in prior literature reports of hydrothermal palladium nanowire synthesis, and we speculate that tap water used by the research publication in question was delivered via copper pipes.  Jennifer’s discovery is currently being prepared for submission to a leading international chemistry journal. Her efforts have been very successful leading to a novel copper-mediated hydrothermal method that is capable of producing large volumes of palladium nanowires for use in small molecule catalysis.  Support from the ACS-PRF award has also enabled Jennifer to train to learn how to operate a transmission electron microscope to provide insight into the local composition of palladium nanowires using x-ray fluorescence mapping in tandem with scanning transmission electron microscopy imaging shown in the TOC graphic.  She is currently working towards answering the question whether the copper is incorporated as a metallic solid solution with palladium, or whether it is observable only through surface adsorption to palladium nanowires.

We interpret the central importance of aqueous copper ions in the manuscript in preparation based on recent insights published by Younan Xia’s research lab.  Xia’s recent publications with silver and palladium nanocrystal synthesis have shown convincing evidence that dissolved oxygen molecules act as selective etchants of seed particles that contain twinning planes.  Increasing the concentration of molecular oxygen in a given synthesis leads to a *decrease* in the final concentration of multiply-pentagonally-twinned seeds through oxidative etching in both aqueous and organic solvents.  The reduction of multiply-pentagonally-twinned seeds then leads to a dramatic reduction in the final yield of high-aspect-ratio metallic nanowires.  In particular, experiments focused on the synthesis of silver nanowires have shown that aqueous copper ions act as effective oxygen scavengers during the synthesis of silver nanowires.  Jennifer Hanson’s recent experiments have confirmed the critical importance of scavenging oxygen during the synthesis of high-aspect-ratio palladium nanowires and the manuscript in preparation for submission will help elucidate the critical importance of oxidative etching during the synthesis of multiply-twinned palladium nanocrystals.