Elektrochemische Untersuchungen zur potential-induzierten Oberflächenfacettierung
Theoretical Chemistry: Electronic Structure, Dynamics, Simulation
Final Report Abstract
The formation of well-defined nanostructures or facets on single-crystal surfaces provides a reproducible basis and model systems for studying structural sensitivity in (electro-)catalytic reactions. Surface faceting can be understood as a morphology change from a flat bulk-truncated surface to a hill-and-valley structure. While clean surfaces rarely facet, adsorbate-induced faceting of surfaces, driven by the anisotropy of surface free energy, is a general phenomenon observed in many systems. Usually the facets have more close-packed surface structures than the original surface, resulting in a minimized surface free energy although the total surface area may be increased. Therefore, in order to actively select and control a desired surface morphology, it is necessary to deepen our understanding of adsorbate-induced faceting. Furthermore, this would provide model systems to study structural sensitivity in catalytic reactions and may be used as templates to grow nanostructures. So far experimental studies of adsorbate-induced faceting of metal surfaces have exclusively performed under UHV conditions, focusing mainly on body-centered cubic or face-centered cubic metals. Motivated by these studies, the aim of this project was to better understand the facet formation process and to transfer this phenomenon to the electrochemical regime. The aim of this project was to explore the facating of Ir(210) surfaces under electrochemical conditions. The project consisted of an experimental and a theoretical part. Regarding the theoretical simulations we first investigated the faceting phenomena under UHV conditions, i.e. at the solid/gas interface. Since the facets consist of Ir(311) and Ir(110) surfaces, adsorption of oxygen (which is causing the faceting) was studied intensively. Based on the energetics a corresponding surface phase diagram could be established, indeed supporting the thermodynamic origins of the faceting phenomenon. Furthermore, the existence of surface reconstructions, so far only speculated about based on STM measurements, could be proven. Afterwards a corresponding phase diagram for the electrochemical case was generated, which showed that surface faceting should also be possible under electrochemical conditions by tuning the electrolyte and more importantly the electrode potential. Motivated by these predictions our experimental work concentrated on realizing the electrochemical surface faceting. Here we started by first reproducing the UHV faceting performed by the group of Prof. Madey (Rutgers University). However, instead of simply heating the system under UHV conditions, we could induce faceting of Ir(210) even under ambient conditions using the inductive-heating technique. The faceted surface was then characterizes electrochemically and the measurements compared to equivalent studies on Ir(210), Ir(110), and Ir(311) single crystal surfaces. Here we could show that obtained facets are equivalent to the ones observed under UHV conditions and that the electrochemical behavior is dominated by the Ir(311) face of the facets. After the inductive-heating we moved to the electrochemical faceting and after various trials to overcome the kinetic barriers that are hindering surface restructuring, we could indeed show that faceting of Ir(210) can be achieved electrochemically via application of subsequent (and optimized) potential cycling. The thus-prepared faceted surface was again characterized electrochemically using cyclic voltammetry, while its structure was investigated in-situ using STM. Although the electrochemically faceted surfaces turned out to be not as regularly faceted as the ones obtained by inductive-heating in a gaseous atmosphere, it already showed the same characteristics. Afterwards the electrocatalytic behavior of Ir(210) before and after surface faceting was studied both experimentally and theoretically using the CO oxidation as model reaction. Here we find that the facets are indeed more active. Further, the potentially-induced facets were even more reactive than the facets obtained by inductive-heating. A correlation with the surface morphology showed that this is due to the reduced regularity and roughness of the potentially-induced system, which in addition showed various surface defects. Furthermore, regarding the facets prepared by inductive-heating we found that the surrounding cooling gas has a drastic influence on the facet regularity and finally on the overall catalyst activity. Besides the CO oxidation on Ir surfaces we also studied the decomposition of NO as well as the oxidation of CO by NO. Here our theoretical studies were supported by corresponding UHV- experiments performed by our collaborators at the Rutgers University. Furthermore, we also investigated the O- and N-induced faceting of Re(11−21) surface, where oxygen induced the formation of four-sided nanopyramids and N-adsorption results in the formation of twosided ridges. Performing extensive theoretical studies on the interaction between O/N and various Resurfaces, again corresponding surface phase diagrams could be drawn. Similar to Ir, we also generated electrochemical phase diagrams indicating the possibility to induce surface faceting by applying an electrode potential. Experimental studies in this direction are still performed at the moment. Finally, the calculated interaction energies for O and N at various Re-surfaces were used to study the shape and morphology of Re-nanoparticles under different environmental conditions (this went beyond the scope of the proposed work). Here we found that due to the strong anisotropy in surface energy changing the temperature and/or the partial pressure of the surrounding atmosphere leads to drastic changes in the overall shape of the preferred nanoparticle geometry, indicating that under reaction conditions particles should not be considered as rigid objects.
Publications
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Bridging the Gap between Nanoparticles and Single-Crystal Surfaces, Faraday Discuss., 140, 69-80 (2008)
P. Kaghazchi, K. A. Soliman, F. C. Simeone, L. A. Kibler, T. Jacob
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Nanoscale Surface Chemistry over Faceted Substrates: Structure, Reactivity and Nanotemplates, Chem. Soc. Rev. (special issue), 37(10), 2310-2327 (2008)
T. E. Madey, W. Chen, H. Wang, P. Kaghazchi, T. Jacob
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Electrochemical Behavior of Nano-faceted Ir(210), Electrochem. Comm., 11, 31 (2009)
K. A. Soliman, F. C. Simeone, L. A. Kibler
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First Principles Studies on Adsorbate-Induced Faceting of Re(11-21), Phys. Rev. B (Brief Reports), 79, 132107 (2009)
P. Kaghazchi, H. Wang, W. Chen, T. E. Madey, T. Jacob
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Nanoscale-Faceting of Metal Surfaces Induced by Adsorbates, Phys. Chem. Chem. Phys. (Perspectives), 12, 8669-8684 (2010)
P. Kaghazchi, D. Fantauzzi, J. Anton, T. Jacob
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Nitrogen-Induced Roughening of Re Surfaces on the Atomic Scale, Phys. Rev. B, 82, 165448 (2010)
P. Kaghazchi, T. Jacob
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Oxygen-Induced Reconstruction of Re(10-10) studied by density functional theory, Phys. Rev. B, 81(7), 075431 (2010)
P. Kaghazchi, T. Jacob
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Reduction of NO by CO on unsupported Ir: Bridging the materials gap, Chem. Phys. Chem., 11(12), 2515-2520 (2010)
W. Chen, Q. Shen, R. A. Bartynski, P. Kaghazchi, T. Jacob
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New surfaces stabilized by adsorbate-induced faceting, J. Phys.: Condens. Matter, 24(26), 265003 (2012)
P. Kaghazchi, I. Ermanoski, W. Chen, T. E. Madey, T. Jacob
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Reduction of Nitric Oxide by Acetylene on Ir Surfaces with Different Morphologies: Comparison with Reduction of NO by CO, Langmuir, 29, 1113-1121 (2013)
W. Chen, Q. Shen, R. A. Bartynski, P. Kaghazchi, T. Jacob
