Molecular Studies of Surfaces under Reaction Conditions; Sum Frequency Generation Vibrational Spectroscopy, Scanning Tunneling Microscopy and Ambient Pressure X-Ray Photoelectron Spectroscopy

Molecular Studies of Surfaces under Reaction Conditions; Sum Frequency Generation Vibrational Spectroscopy, Scanning Tunneling Microscopy and Ambient Pressure X-Ray Photoelectron Spectroscopy Gabor A. Somorjai Department of Chemistry and Lawrence Berkeley National Laboratory, University of California, Berkeley Somorjai@berkeley.edu Abstract: Instruments developed in our laboratory permit the atomic and molecular level study of NPs under reaction conditions (SFG, ambient pressure XPS and high pressure STM). These studies indicate continuous restructuring of the metal substrate and the adsorbate molecules, changes of oxidation states with NP size and surface composition variations of bimetallic NPs with changes of reactant molecules. The studies of surfaces during the last forty years became an atomic scale and molecular level science 1 . Using model surfaces, single crystals at first, the evolution of electron, ion and molecular scattering techniques provided information on atomic and molecular structures, composition and oxidation states and energy transfer, adsorption, desorption and scattering of molecules from surfaces. Since the instruments used for these techniques required high vacuum and clean surfaces, surface science at low pressures developed first. However, applications of surfaces in technologies required studies at high pressures and at buried interfaces, solid-gas, solid-liquid, solid-solid and liquid-liquid. These applications included nanomaterial technologies, catalysis, biointerfaces, tribology, electrochemistry, coatings, corrosion, integrated circuitry and sensors; just to mention a few pervasive technologies that we use in our life. Since life on this planet does not operate in high vacuum, the techniques that probed buried interfaces opened up molecular level investigations of many applications of interfaces. Many of the surface scientists trained in our laboratories pursue research in these fields. Our research, at present, focuses on catalysis at surfaces by transition metals and studies of biointerfaces as the human body which may be viewed as a polymer-water interface with a layer of proteins. To this end we developed four techniques in Berkeley that operate under reaction conditions. These are ambient pressure X-ray photoelectron spectroscopy 2 , sum frequency generation vibrational spectroscopy 3 , high pressure scanning tunneling microscopy 4 , and the metal-semiconductor nanodiode 5 . Surface Composition and Oxidation States Under Reaction Condition Revealed by High Pressure XPS The ambient pressure X-ray photroelectron spectroscopy instrument can be operated at total reactant pressure of up to 10 Torr. The key component that makes this technique different from the conventional XPS is the differentially pumped electrostatic lens system that refocuses the photoelectrons from the sample surface into the object plane of a standard electron energy analyzer working under high vacuum. The kinetic energy of the photoelectrons can be tuned by varying the energy of the X-ray source for which we use the Advanced Light Source synchrotron in Berkeley. By tuning the kinetic energy of the photoelectrons to an appropriate value, the electron mean free paths can be minimized for a given sample surface so that the oxidation state and the composition of the surface layer with the thickness of about 1 nm can be determined. HPXPS has been applied to study the oxidation states of rhodium nanoparticles with size in the range of 2-11 nm under CO oxidation reactions 6 . Recent HPXPS