Since 2003, our group has been developing, modifying, expanding, and diversifying synthetic pathways to complex inorganic materials, with an emphasis on elucidating, understanding, and controlling the reaction pathways by which nanocrystals and other solid-state materials form. For example, we established the “metallurgy in a beaker” approach for synthesizing metallurgical solids and other solid-state materials at low temperatures using nanoparticle reagents and composites. We also demonstrated, for diverse materials systems, that colloidal nanoparticles can serve as reagents that can be chemically transformed in a predictable and controllable manner into derivative nanoparticles with otherwise inaccessible features. We have also contributed to inorganic nanomaterials research involving separations, biotemplating, and shape-controlled metal nanocrystal synthesis. Our current research projects leverage and build on these capabilities. We seek to continue developing new synthetic tools that permit access to increasingly sophisticated and complex nanostructures that map onto emerging nanotechnological applications. We also seek to use our toolbox of nanochemical and solid-state reactions as a platform for synthesizing and discovering new functional materials that have a direct impact on applications involving sustainable and renewable energy technologies. While we frequently launch into new areas as global research needs and collaborative opportunities arise, some of our current projects are highlighted below.
Earth-abundant catalysts for energy applications. The best-performing and most widely used catalytic materials that enable fuel cells, solar cells, water electrolyzers, and other clean-energy devices to operate efficiently often consist of expensive elements that are rare in the Earth’s crust. It is therefore important to identify alternative materials in inexpensive, Earth-abundant systems. For example, platinum is the benchmark catalyst for the hydrogen evolution reaction (HER), which involves the production of molecular hydrogen from aqueous solutions. Recently, through a collaboration with Nate Lewis’s group at Caltech that is funded through the NSF Center for Chemical Innovation on Solar Fuels, we discovered that transition metal phosphides are highly-active HER catalysts that produce operationally relevant current densities at very low over potentials in harsh, highly acidic aqueous solutions. We continue to search for and design new catalytic materials in inexpensive, Earth-abundant systems for the HER, as well as for other energy-relevant reactions that include oxygen evolution, oxygen reduction, hydrogen oxidation, and CO2 reduction.
High-order colloidal hybrid nanoparticles. Hybrid nanoparticle constructs that combine multiple distinct materials with precisely defined configurations, spatial arrangments, and solid-state interfaces underpin a growing number applications that include solar energy conversion, catalysis, photonics, electronics, and theranostics. For example, ternary hybrid nanoparticle systems that couple a hydrogen evolution catalyst with an oxygen evolution catalyst at opposite ends of a light-absorbing semiconductor particle can facilitate overall water splitting, but other configurations of the same nanoparticle materials are functionally inert. We have been developing a “total synthesis” framework for the construction of colloidal hybrid nanoparticles with three, four, and more distinct materials components. To achieve this, we look to synthetic organic chemistry for inspiration. Accordingly, we have developed nanoparticle analogues of orthogonal reactivity, regio- and chemo-selective reactions, protection/deprotection strategies, and an extensive reaction library in order to construct two-component nanoparticle heterodimers, three-component heterotrimers, four-component heterotetramers, and higher-order hetero-oligomers using multi-step reaction sequences. We continue to expand this total synthesis toolkit for hybrid nanoparticle synthesis, with a current emphasis on developing new classes of nanoparticle reactions and hybrid nanoparticle systems, developing tools for selectively targeting precise configurations and spatial arrangements in high yield, and applying methods for separating and purifying complex hybrid nanoparticle mixtures. We are also using our current knowledge of these systems to design highly sophisticated catalytic and enzyme-mimetic hybrid nanoparticle architectures, which includes studies of their surface chemistry and functionalization.
Dimensionally confined colloidal metal chalcogenide nanostructures. Nanostructured metal sulfides, selenides, and tellurides are useful and interesting materials for diverse applications. For example, nanosheets of transition metal dichalcogenides such as MoS2 and MoSe2 exhibit catalytic, optical, and electronic properties that depend sensitively on their layer thicknesses. Colloidal nanostructures of the metal chalcogenides SnS, SnSe, GeS, and GeSe – which are photoactive narrow band-gap semiconductors having large absorption coefficients, band gaps that overlap well with the solar spectrum, and solution processability – are poised as lower-toxicity and more environmentally responsible alternatives to mercury, cadmium, and lead quantum dot systems. We have been studying synthetic pathways to these and other dimensionally confined colloidal metal chalcogenide nanostructures, with an emphasis on targeting desired morphologies and crystal structure modifications that are not typically observed in bulk systems. A key interest area is elucidating and understanding the reaction pathways that lead to the formation of targeted metal chalcogenides and then applying these insights to the design and synthesis of new materials. To achieve this, we exploit chemical transformation reactions of nanoparticles, which allows us to pre-program desired morphological, structural, or compositional features into a precursor nanoparticle and then systematically modify it using a predictable sequence of chemical reactions to form the targeted product nanoparticles. Currently we are focusing our efforts on the synthesis of 3d transition metal chalcogenide phases that are not typically observed in bulk systems, as well as colloidal transition metal dichalcogenides and their heterostructures.
Other projects. We participate in several productive and stimulating collaborations, at Penn State as well as other institutions, with a focus on synthesizing previously elusive or chemically challenging materials for specific targeted properties and applications. For example, we have been studying the synthesis, properties, and assembly of reduced metal oxide nanostructures with switchable optical and electronic properties. We also have been targeting high-pressure polymorphs of carbon-based materials that have desirable properties but that are typically inaccessible as isolatable solids.
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