Our group investigates structural, electronic, magnetic, vibrational, and chemical properties of size- and shape-selected nanostructures. These fundamental physical properties are of great importance to many applications of nanotechnology, including giant magnetoresistance spin valves, magnetic tunnel junctions, single electron transistors, molecular electronic devices, thermoelectric devices, and catalytic systems for energy generation and environmental remediation.
Emphasis is given to: (i) synthesizing metallic nanostructures with well defined geometries, (ii) understanding the mechanisms underlying the enhanced chemical reactivity of metal nanoclusters, (iii) monitoring size- and composition-dependent changes in the electronic structure and vibrational dynamics of nanoscale systems, (iv) investigating the magnetic properties of nearly 1D and 0D systems, (v) exploring chemically- and thermally-induced changes in the mobility and morphology of nanostructures.
Sample undergoing heat treatment in the analysis chamber
Despite more than a decade of intense research on nanoparticle catalysis, fundamental knowledge of key features that influence the catalyst activity and selectivity of a chemical reaction is still lacking. Our group's interdisciplinary research efforts aim at improving the fundamental understanding of the processes taking place in nanoparticle-catalyzed chemical reactions by systematically studying how the chemical reactivity and stability of a nanoparticle (NP) is affected by its size, shape, oxidation state, and particle-support interactions. Our target material systems thus far have been monometallic (Au, Pt, Pd, Ir, Cu, and Fe) as well as bimetallic (Au-M, Cu-M, Pt-M, and Pd-M) NPs. The environmentally and industrially relevant chemical processes we investigate include CO oxidation, NOx reduction and oxidation, alcohol decomposition and oxidation reactions, water-gas-shift reactions, methane oxidation, CH4 reforming, CO and CO2 hydrogenation, and CO2 electro-reduction.
To create ordered arrays of size- and shape-selected nanospheres, nanorods, and triangular nanoprism catalysts we use different ex-situ wet chemistry methods such as nanosphere lithography and the self-assembly of metal loaded block-copolymer micelles. In addition, in-situ nanoparticle shape modifications by pulsed laser irradiation are being conducted.
STM image of Pt NPs on TiO2(110).
Our research combines in situ ultrahigh vacuum (UHV) and high pressure studies in order to gain insight into the parameters affecting the performance of "real-world" nanocatalysts. Temperature-Programmed Desorption (TPD), Scanning Tunneling Microscopy (STM), Atomic Force Microscopy (AFM), X-ray Photoelectron Spectroscopy (XPS) and Ultraviolet Photoelectron Spectroscopy (UPS) are being employed to characterize the changes induced in the metal nanoclusters and their supports upon gas exposure. Additionally, we perform "operando" investigations under reaction conditions via X-ray absorption fine-structure spectroscopy (XAFS), grazing-incidence small-angle scattering (GISAXS), and high pressure X-ray photoelectron spectroscopy (XPS) with synchrotron radiation. The proposed research may lead to the discovery of new catalytic processes while making efficient use of energy and raw materials with minimal influence on the environment.
A deeper insight into the vibrational behavior of nanoscale systems is essential to understand their thermodynamic properties, and could have implications in the design and understanding of new thermoelectric materials. In addition, it can provide new knowledge on temperature-dependent atomic order-disorder transitions, structural phase transitions involving soft phonon modes of NPs, and the physical phenomena underlying electrical and thermal conductivity, heat capacity, vibrational entropy, electron-phonon coupling, Debye temperature, and 1/f noise in electronic devices.
Our group regularly visits the Advanced Photon Source at Argonne National Laboratory, where we use synchrotron-based Nuclear Resonant Inelastic X-Ray Scattering (NRIXS) techniques to investigate the vibrational properties of nanoscale metallic multilayers and size-selected supported mono- and bimetallic 57Fe nanoclusters. Our data have revealed a drastic modification of the measured vibrational density of states (VDOS) of size-selected Fe, FeM (M=Pt, Pd, Au) NPs as compared to the bulk materials. We are currently investigating the influence of the NP size, support, and composition on the vibrational properties of nanoscale systems, and whether low-coordinated atoms (at the NP surface and as defects) induce deviations from the bulk VDOS. Information regarding thermodynamic phase diagrams of binary alloy NPs can also be obtained. The combination of STS (probing local electronic states), XPS, and NRIXS (phonon spectrum) can reveal, in a non-destructive way, the metallurgical state of the alloy NP (phase segregation, homogenous alloying, etc). Such knowledge is a prerequisite for the theoretical description of surface processes such as adsorption, desorption, and in general, catalytic activity.
|NRIXS setup, 3ID beamline, Advanced Photon Source, Argonne National Laboratory (Chicago, IL).|
VDOS [g(E)] of 57Fe nanoclusters coated by Fe2O3 (samples #1, #2), and Fe3C (sample #2b) shells obtained by NRIXS. For reference, the VDOS of bulk bcc-Fe is also shown. Insert: low energy part of g(E).
Many Fe-rich transition metal alloys (e.g. Fe-Pt, Fe-Pd, Fe-Ni) experience a structural phase transformation from fcc-Fe to bcc-Fe with increasing Fe content. These phase transitions have a profound effect on their magnetic properties. For example, instabilities of the magnetic moment with respect to changes in the atomic electron density and volume have been previously established and labeled as the Invar effect. The physics of these phase transitions is directly related to soft phonon modes that can be observed in NRIXS experiments. Phonons play also a role in magneto-caloric materials. An important open question is whether phonon softening and the related lattice instabilities still exist in isolated alloy NPs, or whether they are suppressed by size or surface effects. We plan to continue exploring the effect of NP size and composition on the structural and magnetic properties of nanostructures via Nuclear Resonant Scattering (NRS) with synchrotron radiation, including structural studies via X-ray diffraction.
NRS setup at the BL09XU beamline at the synchrotron facility Spring 8 (Japan).