46444-G5
High Density, Monodisperse Core-Shell Nanoparticle Arrays: A Study of Nanoscale Catalytic Properties
1 Research Activities Overview: Nanocatalysts with particles a few nanometers in size hold great promise because of their large surface area ratios, the availability of enormous of active sites and enhanced resistance against poisoning [1,2]. A fundamental understanding of the physics that increase activity as metal structure scale down to atomic ensembles is not clearly established. Thus a model nanoscale bimetallic system was developed as a platform for measuring correlations between catalytic activity and changes in electronic structure. We first focus on characterizing electronic structure based on a few important observations in the scientific literature. For example, the relationship between electronic structure and activity was demonstrated for Au clusters on TiO2 supports. Cluster with a measured band gap of 0.2 - 0.6 eV, as determined from scanning tunneling spectroscopy, exhibited high activity for CO oxidation while larger Au clusters exhibiting no band gap had lower activity [1]. Another important observation is that catalytic reaction rates, in the context of electrochemical catalysis, have exhibited an exponential dependence on the catalyst work function [3]. Yet the work function of a metal catalyst on a solid support is not as easily tuned as in an electrochemical system, this result has had limited impact outside electrochemical catalysis. 2 Material System Studied: A major challenge of correlating activity with material and structure is the fabrication of metallic nanostructure that have atomically controlled surfaces along with relatively uniform size and shape on substrates. We utilize self-assembly in order to fabricate dense arrays of bimetallic particles composed of an atomic shell of noble metal aggregated on a rare earth disilicide (RESi2) core [4]. We previously published results of the surface after reactive ion etching, core-shell nanostructures with mean particle size and distribution was determined to be 8 nm ± 0.9 nm from analysis of scanning electron microscopy images [5]. 3 Results: During the funding period, we have found via ab initio calculations and experiments that the work function is tunable with material and size. For example, Ag, Au Pd and Pt were predicted to have different charge transfer toward RESi2 nanowires as determined in local density of states calculations [6]. Charge transfer between metal atoms and nanowire surfaces leads to a surface dipole that is known to change the work function [7]. Thus, the choice of noble metal can be used to tune the catalyst's work function. Our scanning Kelvin probe force microscopy (KPFM) data confirms a change in work function due to the aggregation of Pt atoms on DySi2 nanowire surfaces. We have also found that the surface electronic structure of the RESi2 core exhibits large changes on nanometer length scales. In situ scanning tunneling microscopy (STM) and spectroscopy (STS) yield strong evidence that the Fermi wavelength is on the order of 1 nm. For example, STS spectra of DySi2 wire having width of 3.4 nm on n-type Si(001) has nearly Ohmic (linear I-V response) behavior. In comparison, STS of wires having a width of 1 nm shows current rectification. We have also observed a lower work function of DySi2 nanowires versus larger DySi2 island structures that was attributed to quantum size effects. The ACS PRF starter grant was one of the first grants the PI, Regina Ragan, received as an Assistant Professor and has been critical in pushing her research ideas forward. The funding during the grant period provided infrastructure for advanced training of graduate students, Aniketa Shinde, and Sangyeob Lee, as well as undergraduate, Satoru Emori. Aniketa Shinde, had little experience with ab initio calculations at the beginning of the funding period and now she is highly productive. Miss Shinde recently was a finalist in the Nottingham Competition that is sponsored by the American Vacuum Society. Mr. Lee performed high-resolution scanning KPFM measurements and will be finishing his doctoral thesis at the beginning of 2009. Satoru Emori designed and built a new ultrahigh vacuum system deposition system for gas phase molecular depositions for future catalysis experiments. The research training that Mr. Emori has received led to success in obtaining competitive undergraduate fellowships and he is now attending the Massachusetts Institute of Technology as a graduate student in Materials Science. References: (1) Goodman, D. W. Surf. Rev. Lett. 1995, 2, 9. (2) Valden, M. et al. 1998, 281, 1647; Grunes, J. et al. Catal. Lett. 2003, 86, 157. (3) Bebelis, S.; Vayenas, C. G. J. Catal. 1989, 118, 125. (4) Ragan, R. et al. Appl. Phys. A Mater. 2005, 80, 1399. (5) You, J. P. et al. Nano Letters 2006, 6, 1858. (6) Jo, C. et al. Chem. Phys. Lett. 2008, 454, 327. (7) He, T. et al. J. Am. Chem. Soc. 2008, 130, 1699; Glatzel, T. et al. Mat. Sci. Eng. B-Solid 2003, 102, 138.
