Research in our group focuses on several distinct problems including the self-assembly of monolayers at the solid/liquid interface, organometallic surface chemistry, corrosion inhibition chemistry, the role of surface-molecule interactions in efficiency of CO2 reduction, the reaction dynamics of gas-surface interactions of nanotube supported platinum catalysts, nanoplasmonic sensing of nanoparticle nanocatalysts and surface chemistry of topological insulators. These studies vary widely in scope and aim but all share the use of surface science instruments and techniques such as electron spectroscopy, scanning probe microscopy, laser excitation sources, optical spectroscopy, mass spectroscopy and X-ray photoelectron spectroscopy for building a detailed understanding of significant surface related chemical processes. The following paragraphs describe these studies:
- Mechanisms of Self-Assembled Monolayer Formation
- Organometallic Surface Chemistry
- Corrosion Inhibition Chemistry
- Heterogenous Reduction of CO2
- Dynamics of Gas-Surface Interactions
- Nanoplasmonic Sensing on Model Nanocatalysts
- Topological Insulators
Mechanisms of Self-Assembled Monolayer Formation
Self-assembly is a bottom up approach for the construction of highly ordered nanostructure that allows for a high degree of control over surface properties. Surface modification by self assembled monolayers (SAMs) is a broad and expansive field. One way to subdivide this topic is by the nature of how the molecule adsorbs to the surface, namely physisorption versus chemisorption.Our work focuses on mechanisms of self-assembly of both types of monolayers.
Physisorption involves surface adsorption through weak forces such as van der Waal (VDW) interactions. Lateral interactions between adjacent molecules are therefore relatively strong compared to the surface-molecule bond and thus affect morphology. The unique morphologies are imaged with high resolution via Scanning Tunneling Microscopy (STM). The goal is to study how varying the surface-molecule and interlateral interactions affect the mechanism of physisorbed SAM formation. The fascinating aspects of physisorbed monolayers lie in the basic understanding we can gain about crystallization from these two dimensional systems and in their additional applications as scaffolding for guiding three dimensional crystallizations.
Chemisorbed SAMs are strongly attached to the surface through a chemical bond and of interest for use in electronic devices. The morphology of the chemisorbed SAM is determined by the number of molecules and nature of the bond. The goal of our work in this project is to study how the nature of the substituents of a bifunctionalized aromatic molecule affects selectivity and rate of chemisorption to a surface. See, for example:
Esther Frederick, Pearl N. Dickerson, Yu Lin Zhong, and Steven L. Bernasek, ” Substituent Effects on the Kinetics of Bifunctional Styrene SAM formation on H-terminated Si(111)”, Langmuir, 30, 7687 (2014). Abstract Full: HTML/PDF
Pearl N. Dickerson, Amber M. Hibberd, Nuri Oncel and S.L. Bernasek, “Hydrogen-bonding Versus van der Waals Interactions in Self-assembled Monolayers of Substituted Isophthalic Acids.”, Langmuir, 26, (23), 18155 (2010). Abstract Full: HTML / PDF
F. Tao, J. Goswami, and S.L. Bernasek, “Competition and Coadsorption of cis-Unsaturated Carboxylic Acids on Highly Ordered Pyrolytic Graphite”, J. Phys. Chem. B, 110 (9) 4199-4206 (2006). Abstract Full: HTML/PDF
Organometallic Surface Chemistry
The bonding between an organometallic species and an oxide or nitride supporting substrate is important to the chemistry of supported catalysts, electronic device processing, and adhesion and lubrication phenomena. This bonding may be investigated with the aid of ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). When combined with other surface probes such as thermal desorption spectroscopy (TDS), AES, LEED, and vibrational spectroscopic characterization by high resolution electron energy loss spectroscopy (HREELS) or reflection-absorption infrared spectroscopy (RAIRS), a relatively complete picture of the surface properties of the adsorbed species becomes available. We are exploring, in collaboration with Professor Jeffrey Schwartz, the interaction of a number of transition metal and phosphonate complexes with well characterized oxide surfaces. In this work electron spectroscopic measurements are correlated with the chemical behavior of the supported complex, as studied by conventional heterogeneous catalysis methods and mass sensitive kinetic methods. This approach is also extended to the study of important oxide and nitride bound metallic layers, which are of interest in electronic materials chemistry, in bio-compatible materials chemistry, and in adhesion and corrosion inhibition applications. See, for example:
J.E. McDermott, M. McDowell, I.G. Hill, J. Hwang, A. Kahn, S.L. Bernasek, J. Schwartz, “Organophosphonate Self-Assembled Monolayers for Gate Dielectric Surface Modification of Pentacene-Based Organic Thin-Film Transistors: A Comparative Study”, J. Phys. Chem. A, 11, 12333 (2007). Full: HTML/PDF
M. Mc Dowell, I.G. Hill, J.E. McDermott, S.L. Bernasek and J. Schwartz, “Improved Organic Thin-Film Transistor Performance Using Novel Self-assembled Monolayers”, App. Phys. Lett. 88, 1 (2006). Full: PDF
Corrosion Inhibition Chemistry
There are two main methods to inhibit corrosion of structural metals; one focuses on the design of corrosion resistant alloys, while the other involves protecting the surface with corrosion inhibitors.
Many industrial processes involve high-temperature corrosive conditions and require alloys capable of regenerative oxide layer growth to avoid structural failure. The growth of these protective oxide layers is studied with SEM (secondary electron microscopy), EDX (energy dispersive x-ray spectroscopy), and XPS (x-ray photoelectron spectroscopy). With these techniques, the different types of oxides can be distinguished and their kinetics better understood. The goal of this project is to understand in detail the capabilities of different alloys to grow compact, continuous and adherent oxide layers in situ.
Corrosion resistant alloys are not cost effective on the large scale needed for oil and gas wells and refinery applications. For these applications, a more cost effective and commonly used alternative is the injection of corrosion inhibitors into the well. These inhibitors protect the metal by physically blocking the surface corrosion sites. We analyze the interaction between the surface and inhibitor in order to determine the mechanism of corrosion inhibition. Current studies focus specifically on small molecules, mainly organic heterocycles, as corrosion inhibitors using EIS (electrochemical impedance spectroscopy) and XPS (x-ray photoelectron spectroscopy) to assess their effectiveness.
G. Bhargava, I. Gouzman, T.A. Ramanarayanan, C.M. Chun, and S.L. Bernasek,“Characterization of the “Native” Surface Thin Film on Pure Polycrystalline Iron: a High Resolution XPS and TEM Study”, Appl. Surf. Sci., 253, 4322 (2007). Full: HTML/PDF
G. Bhargava, T.A. Ramanarayanan, I. Gouzman, and S.L. Bernasek, “Imidazole as an Inhibitor for Fe Corrosion: a Study Combining Surface Science and Electrochemistry”, ECS Transactions, 1, 195 (2005). Abstract Full: PDF
Dynamics of Gas-Surface Interactions
The collision of a gas molecule with a surface, followed by trapping of the molecule on the surface, must precede any surface reactions which might take place. Some knowledge of this fundamental process, along with the accompanying energy transfer to the surface, is essential to a complete understanding of heterogeneous reaction dynamics.The mechanism by which highly effiecient catalysts achieve their activity is of special interest in the energy sector, where novel catalysts could pave the way for practical fuel cells and other breakthrough technologies. We have investigated the detailed dynamics of a very important prototype surface reaction, the catalytic oxidation of CO on platinum. Our diode laser absorption spectrometer provides much better resolution and is sensitive to the possible production of cold CO2 in the oxidation reaction. We have used this method to map out the detailed ro-vibrational populations of the product CO2 and changes in these populations with changing surface reaction conditions. These studies have been extended to the reaction of CO with NO and methanol with O2. Currently, we are extending the studies to investigate the catalytic oxidation of CO occuring on Pt nanoparticles deposited on bundles of carbon nanotubes. Ab-initio quantum calculations are used to map the electronic structure of the Pt/SWNT system. See, for example:
Nanoplasmonic Sensing on Model Nanocatalysts
Indirect nanoplasmonic sensing, partly based on a novel experimental platform created by Larsson et. al. (2009), allows us to monitor catalyst surface changes under industrially relevant reaction conditions. We are currently studying the direct epoxidation of propene on titania-supported gold nanoparticles as a model catalytic system. A two-dimensional array of gold nanodiscs is prepared on fused silica and coated with a thin layer of dielectric material. Catalyst nanoparticles are deposited upon the dielectric material. Catalyst surface changes can be monitored via position and intensity changes in the gold nanodisc localized surface plasmon peak using in-situ UV-Vis spectroscopy. Surface enhanced Raman spectroscopy is used to characterize surface species formed during reactions. The surface changes are then correlated with activity under various conditions by monitoring the gases produced via mass spectrometry.
Heterogenous Reduction of CO2
We are exploring, in collaboration with Professor Andrew Bocarsly, the mechanisms of heterogenous CO2 reduction at different electrode interfaces. The interactions between CO2 and catalytic electrode under aqueous conditions are probed using cyclic voltammetry and in-situ infrared spectroscopic studies of the electrode. Surface analysis via XPS and HREELS is used to explore the bonding between CO2 and catalytic electrode. Preliminary work indicates that In electrodes are more effective in higher oxidation states. A major goal of this project is to understand what accounts for this behavior. The reductive behaviors of other metals such as Au, Pt and Sn are also being studied. See, for example:
Zachary M. Detweiler, James White, Steven L. Bernasek, Andrew B. Bocarsly, “Anodized Indium Metal Electrodes for Enhanced Carbon Dioxide Reduction in Aqueous Electrolyte”, Langmuir, 30, 7593 (2014). Abstract Full: HTML/PDF
Topological insulators are a new class of materials that conduct in two dimensions along their surfaces, but are electrically insulating in the bulk. Additionally, the momentum of the conducting electrons are linearly correlated to their spin, allowing for spin current along the surfaces. In the Bernasek lab, in collaboration with the Cava and Schwartz labs, our focus is on studying the surface properties of topological insulators to determine the effects of various surface modifications. When modifying these surfaces, two goals must be balanced; protection of the surface from unwanted atmospheric reactions and preservation of the unique surface states. These surfaces are modified through a variety of treatments and investigated primarily through x-ray and ultraviolet photoelectron spectroscopy.