Structure of surfaces

We have considered briefly the important macroscopic description of a solid adsorbent, namely, its speciflc surface area, its possible fractal nature, and if porous, its pore size distribution. In addition, it is important to know as much as possible about the microscopic structure of the surface, and contemporary surface spectroscopic and diffraction techniques, discussed in Chapter VIII, provide a good deal of such information (see also Refs. 55 and 56 for short general reviews, and the monograph by Somoijai [57]). Scanning tunneling microscopy (STM) and atomic force microscopy (AFT) are now widely used to obtain the structure of surfaces and of adsorbed layers on a molecular scale (see Chapter VIII, Section XVIII-2B, and Ref. 58). On a less informative and more statistical basis are site energy distributions (Section XVII-14) there is also the somewhat laige-scale type of structure due to surface imperfections and dislocations (Section VII-4D and Fig. XVIII-14). 

Structure of Surfaces and Interfaces as Studied Using Synchrotron Radiation. Faraday Discussions Chem. Soc. 89, 1990. A lively and recent account of studies in EXAFS, NEXAFS, SEXAFS, etc. 

LEED can be used to determine the atomic structure of surfaces, surface structural disorder, and to some extent, surfiice morphology, as well as changes in structure with time, temperature, and externally controlled conditions like deposition or chemical reaction. Some examples are briefly discussed here ... 

Phase transitions in overlayers or surfaces. The structure of surface layers may undergo a transition with temperature or coverage. Observation of changes in the diffraction pattern gives a qualitative analysis of a phase transition. Measurement of the intensity and the shape of the profile gives a quantitative analysis of phase boundaries and the influence of finite sizes on the transition. ... 

MEIS has proven to be a powerful and intuitive tool for the study of the composition and geometrical structure of surfaces and interfaces several layers below a surface. The fact that the technique is truly quantitative is all but unique in surface science. The use of very high resolution depth profiling, made possible by the high-resolution energy detectors in MEIS, will find increased applicability in many areas of materials science. With continued technical development, resulting in less costly instrumentation, the technique should become of even wider importance in the years to come. 

Although physical studies of the electronic structure of surfaces have to be performed under UHV conditions to guarantee clean uncontaminated samples, the technique does not require vacuum for its operation. Thus, in-situ observation of processes at solid-gas and solid-liquid interfaces is possible as well. This has been utilized, for instance, to directly observe corrosion and electrode processes with atomic resolution [5.2, 5.37]. 

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... 

Electronic Structure of Surfaces and Interfaces in Conjugated Polymers... 

Another way to monitor the expected changes in the metal electronic structure is to look at the adsorbed molecules, which are sensitive in their properties to the changes in the electronic structure of surface metal atoms. Such a molecule is CO and the frequency of the CO stretch vibrations ( v(CO)) is a sensitive detector of the direct- and back-donation upon adsorption of CO. It has been reported, that v(CO) decreases for the VIII group metal by alloying of Pd with Ag (22), Ni with Cu (23), but also when mixing Ni with Co (24). This has been first explained (25) as an indication for an increased backdonation due to an assumed electron shift Cu Pt,... 

High resolution electron microscopy has recently demonstrated the capability to directly resolve the atomic structure of surfaces on small particles and thin films. In this paper we briefly review experimental observations for gold (110) and (111) surfacest and analyse how these results when combined with theoretical and experimental morphological studies, influence the interpretation of geometrical catalytic effects and the transfer of bulk surface experimental data to heterogeneous catalysts. 

Understanding and controlling oxide surfaces are the key issues for the development of industrial oxide catalysts, but oxide surfaces are in general heterogeneous and complicated, and hence have been little studied so as to put them on a scientific basis by traditional approaches. While studies of the structure of surfaces have focused on metals and semiconductors over the past thirty years, the application of surface science techniques to metal oxides has blossomed only within the last decade[l-3]. 

An important future goal of catalytic surface science is to monitor the structure of surfaces and adsorbates at the molecular level in situ under catalytic reaction conditions, to model the more complex technical catalysts, and to undertake the design and tuning of new catalyst surfaces. 

One key aspect of SOMC is the determination of the structure of surface complexes at a molecular level one of the reasons being that our goal is to assess structure-activity relationships in heterogeneous catalysis, which requires a firm characterization of active sites or more exactly active site precursors. While elemental analysis is an essential first step to understand how the organometallic complex reacts with the support, it is necessary to gather spectroscopic data in order to understand what are the ligands and... 

Here, we demonstrate clear and direct evidence for the modified electronic structures of surface Pt atoms in Pt-Co and Pt-Ru by using EC-XPS [Wakisaka et al., 2006]. The sample electrode was transferred between an XPS chamber and an electrochemical (EC) chamber without exposure to air (to minimize contamination of the surface). All photoelectron spectra, including the valence level region) were taken by using a monochromatic Al Ka (hv = 1486.58 eV). The uncertainty of binding energy measurement was less than + 0.03 eV. 

Soon after the invention of the STM as a tool for imaging surfaces in real space, it was discovered that the microscope could also be used (or misused) for surface manipulations, that is, for nano structuring of surfaces [5]. The extremely close vicinity of the STM tip and the sample surface required by the tunnel process... 

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