Work function measurements

Mobility of this second kind is illustrated in Fig. XVIII-14, which shows NO molecules diffusing around on terraces with intervals of being trapped at steps. Surface diffusion can be seen in field emission microscopy (FEM) and can be measured by observing the growth rate of patches or fluctuations in emission from a small area [136,138] (see Section V111-2C), field ion microscopy [138], Auger and work function measurements, and laser-induced desorption... 

Swanson L W and Davis P R 1985 Work function measurements Solid State Physics Surfaces(Methods of Experimental Physics 22) cd R L Park and M G Lagally (New York Academic) chi... 

It seems hard to support the above hypothesis on the basis of work function measurements for Hg in the presence of residual gases. Adsorption of water indeed reduces the work function and this is also the case with inert gases. There remains the possibility of surface oxidation by residual oxygen, but the values of Ayr measured with the Hg stream have been shown42,43 to be stable even in the presence of 02 impurities provided the gas flows rapidly, as was the case during the experiments. The same conclusion has been reached recently by measuring the work function of Hg in ambient gas.46... 

Work function measurements of Sb alloys with Sn, In, and Zn have... 

S. Ladas, S. Bebelis, and C.G. Vayenas, Work Function Measurements on Catalyst Films subject to in-situ Electrochemical Promotion, Surf. Sci. 251/252, 1062-1068 (1991). 

The technique of photoemission electron spectroscopy (PEEM) is a particularly attractive and important one for spatially resolved work function measurements, as both the Kelvin probe technique and UPS are integral methods with very poor ( mm) spatial resolution. The PEEM technique, pioneered in the area of catalysis by Ertl,72-74 Block75 76 and Imbihl,28 has been used successfully to study catalytic oscillatory phenomena on noble metal surfaces.74,75... 

D. Tsiplakides, S. Neophytides, and C.G. Vayenas, Investigation of electrochemical promotion using temperature programmed desorption and work function measurements, Solid State Ionics 136-137, 839-847 (2000). 

R. Imbihl, J. Janek The groups of Professors Imbihl and Janek have made important contributions in the use of PEEM, work function measurement and XPS (Chapter 5) to establish the O2 backspillover mechanism of electrochemical promotion under UHV conditions. 

Metcalfe The group of Professor Metcalfe has combined kinetic and work function measurements to investigate the electrochemical promotion of CO oxidation on Pt/YSZ and, more recently, had made important modeling advances for the fundamental description of electrochemical promotion at the molecular level. 

Kelvin probe technique and work function measurement, 138, 205, 340 experimental details, 340 two-probe arrangement, 340 Kinetics... 

Bertel, E. and Netzer, F.P. (1980) Adsorption of bromine on the reconstructed Au(lOO) surface LEED, thermal desorption and work function measurements. Surface Science, 97, 409-424. 

Figure 2.1 Real-time photoemission study (hv = 6.2 eV) of the interaction of oxygen (Po2 = 10- Torr) with a nickel surface at 300 K. The photocurrent decreases initially (A B), then recovers (B-C), before finally decreasing (CD). Surface reconstruction occurs (B-C) with further support from studies of the work function. The work function measured by the capacitor method15 increases by 1.5 eV with oxygen exposure at 80 K followed by a rapid decrease on warming to 295 K and an increase on further oxygen exposure at 295 K. These observations suggest that three different oxygen states are involved in the formation of the chemisorbed overlayer. (Reproduced from Refs. 15, 42).

In surface science, work function measurements are considered to be rather sensitive towards changes of the sample surface. Work function measurements are used to follow adsorption processes and to determine the dipole established at the surface. During oxygen adsorption and oxide formation the sign of the work function change allows one to distinguish between oxygen atom adsorbed on the surface or sub-surface [30]. 

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). 

It has indeed been found (83a) that the work function of Ag-Pd alloy films equilibrated at 300°C only changes from 4.38 eV at pure silver to 4.50 eV at 86% Pd (increasing to 5.22 at pure Pd). It was proposed (83a) that this work function pattern is a consequence of surface enrichment by silver and so confirmation of the theory awaits further work function measurements on alloy surfaces for which compositions have been determined by, say, Auger electron spectroscopy. 

As pointed out above the comparison between both sets of results relies upon the physical equivalence of the two measureable quantities work function (surface science) and electrode potential (electrochemistry) 111. This equivalence has been realized in electrochemistry some time ago and has been exploited to analyze measured values of the potential of zero charge 111 and of work function changes upon emersion of electrodes at fixed potential 181. In the simulation experiments the approach is quite similar in that one prepares a well-defined composition of the synthetic electrochemical adsorbate layer and then obtains the electrostatic potential drop across it by a work function measurement. 

A water-alone monolayer potential above the pzc is in accordance with an absolute work function measurement for the water monolayer on Pt(lll) of 4.8 eV (29). Comparing this to the hydrogen electrode (4.7 eV below vacuum (30) for the normal hydrogen electrode NHE) corrected by 7x0.059 V for a nominaI pH 7 yields a water-alone mono-layer potential of +0.5 V vs. RHE at pH 7. This lies 0.3 V above our proposed pzc of 0.2 V RHE. This relatively high apparent potential of the water monolayer has been noted previously (Sass, J.K., private communication), and has raised concern about the relevance of the UHV monolayer to real electrochemical conditions, since most electrochemical measurements of the pzc of polycrystalline Pt have been closer to 0.2 V than to 0.5 V (31). By showing that the water monolayer lies above, not at, the pzc, the present H.+H-O data remove part of the apparent discrepancy between the electrochemical and UHV results. If future UHV work function data show a large ( 0.3 V) decrease in the water monolayer work function upon addition of small (<20X saturation) amounts of hydrogen, all of the apparent discrepancy could be quantitatively accounted for. 

The intimate relationship between double layer emersion and parameters fundamental to electrochemical interfaces is shown. The surface dipole layer (xs) of 80SS sat. KC1 electrolyte is measured as the difference in outer potentials of an emersed oxide-coated Au electrode and the electrolyte. The value of +0.050 V compares favorably with previous determinations of g. Emersion of Au is discussed in terms of UHV work function measurements and the relationship between emersed electrodes and absolute half-cell potentials. Results show that either the accepted work function value of Hg in N2 is off by 0.4 eV, or the dipole contribution to the double layer (perhaps the "jellium" surface dipole layer of noble metal electrodes) changes by 0.4 V between solution and UHV. 

In addition to the indirect experimental evidence coming from work function measurements, information about water orientation at metal surfaces is beginning to emerge from recent applications of a number of in situ vibrational spectroscopic techniques. Infrared reflection-absorption spectroscopy, surface-enhanced Raman scattering, and second harmonic generation have been used to investigate the structure of water at different metal surfaces, but the pictures emerging from all these studies are not always consistent, partially because of surface modification and chemical adsorption, which complicate the analysis. 

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