Research Interests

    The objective of this project is to use a novel interdisciplinary approach, which draws diverse strengths from in situ surface electrochemical nuclear magnetic resonance (EC-NMR), surface-enhanced infrared spectroscopy (SEIRAS), surface-enhanced Raman spectroscopy (SERS), and interfacial electrochemistry, to investigate the physical and chemical properties of electrochemically-engineered nanoscale bimetallic surfaces in order to establish relationships between surface electronic/structural/dynamic properties and catalytic reactivity in these real-world bimetallic catalysts with many industrial applications. This project will provide unique insights into electronic structure –– reactivity relationships for real-world bimetallic catalysts and make significant contributions toward establishing a bridge between low-surface-area model and high-surface-area industrial catalysts, thereby furthering our understanding of surface science in general and bimetallic electrocatalysis in particular.
    As pointed out above, our approach draws strengths from different fields. First, the materials-engineering and environment-controlling power of interfacial electrochemistry offers a truly elegant way to prepare nanoscale bimetallic surfaces via electroless or underpotential deposition (UPD), and electrolysis or electro-dissolution. The relatively mild electrochemical approach in preparing bimetallic surfaces can rival the widely-used vapor deposition technique used in UHV and wet impregnation methods used industrially, both of which involve procedures that are rigorous and often technically demanding. Second, EC-NMR technique possesses technical versatility and chemical specificity for in-situ investigations in general and the analytical strength of high-field solid-state NMR in investigating real-world supported nanoscale (gas phase) metal and alloy catalysts in particular. Third, IR spectroscopy and Raman spectroscopy are a well-established major research techniques of the surface/interface science community and can provide quick access to important information regarding the bond strength and geometry of molecular probes such as CO or NO. A joint in situ SEIRAS/SERS/NMR approach will provide new, mutually complementary information on the electronic/molecular structure of the bimetallic surfaces by combining bond strength/geometry (IR/Raman) and quantitative electronic/dynamic (NMR) results. Finally, this electrochemically-based approach offers an extra bonus ability to control the metal surface potential, opening a new route by which processes of practically important, namely alkali promotion/halogen poison and the newly discovered electrochemical pumping promotion in heterogeneous catalysis, may be investigated in detail. In addition, this interdisciplinary project will play an important role in graduate and undergraduate education by providing unique opportunities for training in materials-engineering, interfacial electrochemistry, condensed matter chemistry and physics, heterogeneous and electrocatalysis, solid-state NMR, IR/Raman spectroscopy, and quantum chemistry, all directed towards the improved engineering of novel bimetallic systems.
    Specifically, the following questions will be the main focuses of this project:

• How can changes in surface electronic properties of a substrate caused by interactions with metal adatoms be correlated to physical/chemical properties, such as electronegativity and bulk band structure, of the substrate and of the adatoms?
• What are the changes in chemical reactivity of bimetallic surfaces, i.e., the behavioral changes in chemisorption of simple molecules or ions (like CO, NO, CN-, H2, O2 or other small organic molecules) as compared to monometallic surfaces of individual constituents? How are these changes correlated with changes in physical properties of the binary metal surfaces?
• Can the roles of geometric “ensemble” vs. electronic “ligand” effects on catalytic active sites, ionic vs. covalent heteronuclear metal-metal bond, and charge transfer vs. nd-(n+1)sp orbital rehydridization  be discriminated ?

    Another project that we are interested in is the peculiar catalytic reactivity of oxide-supported Au nanoparticles, in particular CO oxidation. Gold metal is probably the most noble material that us inert to chemical reactions. However, nanoscale Au particles supported on TiO₂ or some other oxides show unusual high catalytic reactivity for CO oxidation, even at low temperatures. The exact mechanism is still subject to animated debate. We are planning to use 13CO as a molecualr probe to carry out 13C NMR and CO IR investigation. We believe such investigation will provide  much  needed mechanistic information for a better understanding of the system.

    A metal QD is an isolated nanoparticle containing hundreds or thousands of metal atoms which forms a small enough geometric 3-dimension confinement of electrons leading to resolvable discrete electronic energy levels, as opposed to the quasi-continuum of band structures in its bulk counterpart. Such metal QDs are the fundamental building blocks for many nanostructured materials expected to show unprecedented physical and chemical properties which are inaccessible using existing materials. This project is directed toward investigation of local electronic/structural properties of ligand-protected metal quantum dots (QDs) as a function of QD size, number of excess electrons that the QD carries (i.e., electric potential), and inter-QD spacing in metal QD superlattices. This project will provide critical data for understanding the physics and chemistry of these systems, in particular the  bonding interactions between the metal surface and the ligand. Such an understanding is crucial for the ultimate rational design of novel nanostructured materials which will be the basis for many future technological applications, for instance, molecular electronics.
    Currently, we are focusing on octanethiol- and octaneselenol-protected metal nanoclusters and using ¹³C₁ of octane carbon chain and ⁷⁷Se as proximal NMR probe to invesigate the physical and chemical properties of underlying metal particles. Simultaneously, we are also using ¹⁹⁵Pt and ¹⁰⁹Ag NMR to investigate directly the metal nanoparticels themselves. The following are some specific targets:

• Optimize further the size distribution by methods, such as fractionation, dialysis, chromatographic separation, centrifugation, or their combination. This is very important for reducing unwanted effects, facilitating NMR experiments and data interpretations, in particular as a function of particle size, and increasing efficiency and precision of electrochemical control because the narrower the size distribution is, the better-resolved quantized DL charging waves are.
• Synthesize a variety of octanethiol-protected metal QDs with various metal core elements (Au, Ag, Cu, Pd, Pt, Rh, etc) for the purpose of investigating elemental specificity of metal cores.
• Carry out the 13C T2 and T1 measurements as a function of metal element, particle size, and temperature (from 300 to 80K by using an Oxford cryostat and homemade probes). These experiments will help to pin down the real mechanism(s) responsible for the relaxations and the chemical/physical reason(s) for the heterogeneous broadening of the spectrum, and may eventually provide insightful information on metal-ligand bonding geometry (atop, bridge, or hollow site) and electronic configuration.
• Carry out NMR measurements as a function of charge controlled via quantized DL charging. Such experiments are essential to answer some critical questions like (i) Do charging affects the metal-ligand interaction or bonding geometry and if so, how (electronically or electrostatically)? (ii) How will surface diffusion respond to the changes in electron density? (iii) How can these electron-density-dependent properties be related to their functionality?
• Use IR spectroscopy to obtain complementary information on the packing density, intermolecular interaction, and geometry of octanethiol SAMs. We are currently designing and testing a transmission mode thin layer configuration for IR spectroelectrochemistry. It uses an ultrathin (<10 nm) gold film deposited on polypropylene film as the working electrode as well as one side of window for transmission thin layer and the layer thickness is precisely controlled by a micrometer.
• Develop electrochemical impedance technique for measuring the PZC (i.e. work function) as a function of particle size and correlate the results with the existing quantum theories. The advantages of the impedance method over the z-plot are (i) it is not particle concentration dependent and (ii) it can be used for particles of any size while z-plot only works for particles less than 3 nm.
• Carry out the 13C T1 measurements as a function of temperature, particle size and alkanethiol dimension for alkanethiol-protected metal QD superlattices. These measurements will provide insightful information on the mechanism of charge transfer, i.e., charge hopping or tunneling or conducting.

    Alkaneselenol-Protected Metal QDs. Selenol offers numerous NMR advantages over thiol in that 77Se is a much better NMR nucleus than 33S and offers potentially a valuable NMR probe that is most proximal to the underlying metal surfaces. Since no commercial alkaneselenols is available, we started synthesizing dioctyl-diselenide via selenolbenzamide. Our current yield is 20%. Although this represents good progress from a yield of 8% from our first tries, it is still far lower than the 67% range reported in the literature. So, our first step is to continue to improve the yield of dioctyl-diselenide. Then we will synthesize octaneselenol-protected metal QDs by following the published method and explore the 77Se NMR by following the footsteps elaborated for the octanethiol-protected metal QDs. This direction will be of high payoff because 77Se has the real potential to become a major local probe, as 13CO for nanoscale heterogeneous catalysts, for investigating chemical/physical properties of ligand-protected metal QDs

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