Vacuum interface

A number of methods that provide information about the structure of a solid surface, its composition, and the oxidation states present have come into use. The recent explosion of activity in scanning probe microscopy has resulted in investigation of a wide variety of surface structures under a range of conditions. In addition, spectroscopic interrogation of the solid-high-vacuum interface elucidates structure and other atomic processes. 

The liquid-solid interface, which is the interface that is involved in many chemical and enviromnental applications, is described m section A 1.7.6. This interface is more complex than the solid-vacuum interface, and can only be probed by a limited number of experimental techniques. Thus, obtaining a fiindamental understanding of its properties represents a challenging frontier for surface science. 

Figure A2.4.8. Potential energy profile at the metal-vacuum interface. Bulk and surface contributions to Vare shown separately. From [16].

There are several approaches to gain the required surface sensitivity with diffraction methods. We review several of these here, emphasizing the case of solid/vacuum interfaces some of these also apply to other interfaces. 

HyperChem uses th e ril 31 water m odel for solvation. You can place th e solute in a box of T1P3P water m oleeules an d impose periodic boun dary eon dition s. You may then turn off the boundary conditions for specific geometry optimi/.aiion or molecular dynamics calculations. However, th is produces undesirable edge effects at the solvent-vacuum interface. 

Fig. 1 illustrates possible setups that have been used in various studies. Scheme (a) allows the simulation of two equivalent interfaces between aqueous and non-aqueous phases. Scheme (b) simulates two equivalent aqueous/non-aqueous and two equivalent non-aqueous/vacuum interfaces and can be used to avoid the interactions between the aqueous phase and its images. Scheme (c), usually used when the non-aqueous phase is solid, simulates simultaneously an aqueous/non-aqueous, a solid/vacuum and an aqueous/gas interface. In addition, a confining wall at large distances from the aqueous/gas interface may be employed to prevent the loss of molecules from the simulation cell. 

We have developed a theory that allows to determine the effective cluster interactions for surfaces of disordered alloys. It is based on the selfconsistent electronic structure of surfaces and includes the charge redistribution at the metal/vacuum interface. It can yield effective cluster interactions for any concentration profile and permits to determine the surface concentration profile from first principles in a selfconsistent manner, by... 

For a metal/solution interface, the pcz is as informative as the electron work function is for a metal/vacuum interface.6,15 It is a property of the nature of the metal and of its surface structure (see later discussion) it is sensitive to the presence of impurities. Its value can be used to check the cleanliness and perfection of a metal surface. Its position determines the potential ranges of ionic and nonionic adsorption, and the region where double-layer effects are possible in electrode kinetics.8,10,16... 

Figure 5.7. Schematic representation of the definitions of work function O, chemical potential of electrons i, electrochemical potential of electrons or Fermi level p = EF, surface potential %, Galvani (or inner) potential

The backspillover O species on the Pt surface have an O Is binding energy 1.1 eV lower than on the same surface under open-circuit conditions. The Pt catalyst-electrode is surrounded by isoenergetic oxygen species both at the Pt/YSZ and at the Pt/vacuum interfaces.67... 

Figure 7.13. The definitions of ionization potential, Ie, work function, , Fermi level, EF, conduction level, Ec, valence level Ev, and x-potential Xe without (a) and with (b) band bending at the semiconductor-vacuum interface.

Conductor-insulator and conductor-vacuum interfaces lack a continuous exchange of free charges, and there is no electrochemical equilibrium. For this reason the work that is performed in transferring charged particles from one phase to the other is not zero. The total work, X, which must be performed by the external forces in transferring (extracting) an electron from a metal (M) into vacuum (0) is called the electron work function (or simply the work function). The work function for all metals is always positive, since otherwise the electrons would leave the conductor spontaneously. 

Okamoto Y, Sugino O, Mochizuki Y, Ikeshoji T, Morikawa Y. 2003. Comparative study of dehydrogenation of methanol at Pt(lll)/water and Pt(lll)/vacuum interfaces. Chem Phys Lett 377 236-242. 

Surface Structure and Adsorption Properties at the Soiid/Vacuum Interface Heating Ru(OOOl) surfaces covered by submonolayer amounts of ft... 

Zou S, Gomes R, Weaver MJ. 1999. Infrared spectroscopy of carbon monoxide and nitric oxide on palladium(lll) in aqueous solution unexpected adlayer structural differences between electrochemical and ultrahigh-vacuum interfaces. J Electroanal Chem 474 155-166. 

Electrochemical reactions are driven by the potential difference at the solid liquid interface, which is established by the electrochemical double layer composed, in a simple case, of water and two types of counter ions. Thus, provided the electrochemical interface is preserved upon emersion and transfer, one always has to deal with a complex coadsorption experiment. In contrast to the solid/vacuum interface, where for instance metal adsorption can be studied by evaporating a metal onto the surface, electrochemical metal deposition is always a coadsorption of metal ions, counter ions, and probably water dipols, which together cause the potential difference at the surface. This complex situation has to be taken into account when interpreting XPS data of emersed electrode surfaces in terms of chemical shifts or binding energies. 

These measurements have verified that the work function of an electrode, emersed with the double layer intact, depends only on the electrode potential and not on the electrode material or the state of the electrode (oxidized or covered with submonolayer amounts of a metal) [20]. Work function measurements on emersed electrodes do not serve the same purpose as in surface science investigations of the solid vacuum interface. At the electrochemical interface, any change of the work function by adsorption is compensated by a rearrangement of the electrochemical double layer in order to keep the applied potential i.e. overall work function, constant. Work function measurements, however, could well be used as a probe for the quality of the emersion process. Provided the accuracy of the measurement is good enough, a combination of electrochemical and UPS measurements may lead to a determination of the components of equation (4). 

While the above XPS results give the impression, that the electrochemical interface and the metal vacuum interface behave similarly, fundamental differences become evident when work function changes during metal deposition are considered. During metal deposition at the metal vacuum interface the work function of the sample surface usually shifts from that of the bare substrate to that of the bulk deposit. In the case of Cu deposition onto Pt(l 11) a work function reduction from 5.5 eV to 4.3 eV is observed during deposition of one monolayer of copper [96], Although a reduction of work function with UPD metal coverage is also observed at the electrochemical interface, the absolute values are totally different. For Ag deposition on Pt (see Fig. 31)... 

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