Research Activities in the Hamers Group
Our research is aimed at understanding and controlling the structure, bonding, reactivity, and electrical properties of surfaces and interfaces. The term "interfacial architecture" is a good description because, just as an architect uses simple materials to build functional complex structures, much of what we do is use the physical and chemical properties of individual molecules with solid-state materials to build more complex interface structures with specific types of functionality. Many of our projects rely on the use of nanoscale materials such as nanoparticles and/or nanorods. Because nanostructured materials have very high surface-to-volume ratios, in many cases it is the surface properties (rather than the bulk properties) that control their behavior. By controlling their surfaces, we control their behavior. Our group has more than 25 years of expertise in the chemical functionalization of semiconductor surfaces. Semiconductors have unique ability to separate charge and are therefore sit at the center of renewable energy, in areas such as solar conversion, solar-initiated photocatalysis. Diamond also plays a prominent role in our group due to its unique role as the hardest known material and the most chemically stable material, thereby giving it unique roles as a substrate for electrocatalysis and photocatalysis.
Interfacial Charge Transfer for Advanced Batteries: The successful implementation of solar and wind power, along with ongoing national efforts to transform to hybrid and all-electric vehicles for transportation are placing increased emphasis on the development of next-generation battery technologies. We are engaged in research activities aimed at understanding the fundamental electrochemical charge-transfer processes that occur at solid-liquid interfaces relevant to next-generation batteries. Examples include lithium intercalation into new cathode materials, the development of novel electrolytes for improved battery safety, and new anode materials such as silicon and silicon-based materials that can provide increased battery capacity. Our research includes use of surface analytical methods to characterize electrochemical processes and the development of new, real-time analytical methods for characterizing the electrochemical and thermal stability of electrochemical interfaces.
Environmental Health and Safety of Nanomaterials: The potentially widespread use of nanomaterials in a range of commercial applications brings questions regarding the potential environmental impact and potentials safety issues associated with release of nanomaterials in the environment. We are engaged in research projects aimed at understanding how the size, shape, and surface functional groups impact the bioavailability and toxicity of nanomaterials. We are particulalarly interested in understanding the molecular basis of nanoparticle toxicity, addressing questions such as "how to nanoparticles impact protein folding processes?" and "do nanoparticles induce genetic mutations or other trans-generational effects?". This work is conducted through collaborations with faculty in multiple departments, including Prof. Joel Pedersen, Prof. Richard Peterson, Prof. Warren Heideman, and Prof. Rebecca Klaper (UW-Milwaukee).
Surface chemistry of TiO2: Titanium dioxide (TiO2) is the most common metal oxide used in renewable energy because of its very high chemical stability. Emerging applications of TiO2 often hinge on the ability to integrate organic molecules (such as organic dyes or molecular catalysts). However, relatively little is known of the molecular-level details of how to form stable TiO2-molecule adducts. We are engaged in research aimed at understanding how to manipulate the surface chemistry of TiO2 and a related structure, SrTiO3, using these as model systems to develop new types of molecule-surface linkages. As one example, oxygen atoms at the surface of TiO2 are in a unique geometry in which they prefer to form three bonds instead of two; this unusual hybridization leads to unique reactivity for surface oxygen atoms. This work fosters new links between the surface chemistry of inorganic solid-state materials and organic chemistry.
Charge transfer at metal oxide semiconductor interfaces: Metal oxides such as WO3, Fe2O3, ZnO, and SnO2 are useful because of their ability to facilitate charge separation and charge transfer. By forming junctions between two oxide materials, a "built-in" electric field is created that greatly facilitates the separation of charge. We are investigating the use of chemical assembly methods to form and characterize the electron-transfer properties of adducts between different oxide materials (such as Fe2O3-WO3 "dyadic" structures) and between light-harvesting molecules and metal oxides. Of particular interest is the use of "click" chemistry to form photoelectrochemically active oxide-oxide junctions and to link light-harvesting molecules to SnO2. SnO2 and ZnO are of particular interest because these materials have electron mobilities several orders of magnitude faster than more commonly used oxides, and both can be grown as highly crystalline nanorods. Time-resolved surface photovoltage spectroscopy and time-resolved photoluminescence are being used to probe the charge transfer on time scales from microseconds to nanoseconds.
Ultrafast dynamics of charge transfer: As part of two larger collaborations, we are using femtosecond laser techniqus to investigate the dynamics of charge transfer processs in several types of different systems, including quantum-confined solid-state heterojunctions, molecule-surface junctions, and the dynammics of carriers within carbon-based materials systems. These projects build on the expertise in surface chemistry but focus on using novel femtosecond laser spectroscopy methods to characterize the electron dynamics of electron injection across heterojunctions. In addition, we are developing new methods for probing charge-transfer dynamics on femtosecond time scales using hybrid optical-electrical measurements. These methods hold promise for eventually being able to probe the carrier dynamics of individual quantum dots on femtosecond time scales, pushing the measurements to the uncertainty-principle limit.
Energy-efficiency catalytic transformations: Diamond is particularly attractive as an ultra-stable substrates for electrocatalysis and photocatalysis. We have developed novel methods for functionalizing surfaces of conductive diamond with a range of functions groups that allow diamond to be used as a versatile platform for making novel types of electrocatalytically active surfaces. These adducts show remarkable chemical and electrochemical stability, in some cases able to withstand millions of repeated cycles of oxidation and reduction even to high potentials. We are engaged in a number of different experiments that leverage the extraordinary chemical stability of diamond. Ultimately our goal is to combine the selectivity of molecular-based catalysts with the stabilty and ease-of-use provided by solid-state heterogeneous catalysts.
Ultra-stable biomolecular interfaces: Diamond is biocompatible and can be readily coated onto a wide range of materials, including sensor chips and prosthetic implants. The strong covalent bonding to diamond allows one to make biologically-modified surfaces with excellent biomolecular recognition properties and unprecedented chemical stability. For example, antibody-modified diamond surfaces can be stored for months in aqueous solutions, whle more commonly used materials such as glass would undergo severe degradation under equivalent conditions. Diamond surfaces coated with small molecules such as ethylene glycol are able to resist nonspecific binding of proteins for long periods of time, making them potentially useful in a wide range of in vivo and in vitro studies.