Solids work function

The final technique addressed in this chapter is the measurement of the surface work function, the energy required to remove an electron from a solid. This is one of the oldest surface characterization methods, and certainly the oldest carried out in vacuo since it was first measured by Millikan using the photoelectric effect [4]. The observation of this effect led to the proposal of the Einstein equation ... 

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... 

In hydrodynamic voltammetry current is measured as a function of the potential applied to a solid working electrode. The same potential profiles used for polarography, such as a linear scan or a differential pulse, are used in hydrodynamic voltammetry. The resulting voltammograms are identical to those for polarography, except for the lack of current oscillations resulting from the growth of the mercury drops. Because hydrodynamic voltammetry is not limited to Hg electrodes, it is useful for the analysis of analytes that are reduced or oxidized at more positive potentials. 

Surface ionization. Takes place when an atom or molecule is ionized when it interacts with a solid surface. Ionization occurs only when the work function of the surface, the temperature of the surface, and the ionization energy of the atom or molecule have an appropriate relationship. 

Phofoelectron spectroscopy is a simple extension of the photoelectric effect involving the use of higher-energy incident photons and applied to the study not only of solid surfaces but also of samples in the gas phase. Equations (8.1) and (8.2) still apply buf, for gas-phase measuremenfs in particular, fhe work function is usually replaced by fhe ionization energy l so fhaf Equation (8.2) becomes... 

Measurements [113,368] of interfacial (contact) potentials or calculated values of the relative work functions of reactant and of solid decomposition product under conditions expected to apply during pyrolysis have been correlated with rates of reaction by Zakharov et al. [369]. There are reservations about this approach, however, since the magnitudes of work functions of substances have been shown to vary with structure and particle size especially high values have been reported for amorphous compounds [370,371]. Kabanov [351] estimates that the electrical field in the interfacial zone of contact between reactant and decomposition product may be of the order of 104 106 V cm 1. This is sufficient to bring about decomposition. 

A value close to 4.8 V has been obtained in four different laboratories using quite different approaches (solid metal/solution Ay, 44 emersed electrodes,40,47 work function changes48), and is apparently supported by indirect estimates of electronic energy levels. The consistency of results around 4.8 V suggests that the value of 4.44 V is probably due to the value of 0 not reflecting the actual state of an Hg jet or pool. According to some authors,44 the actual value of 0 for Hg in the stream should be 4.8 V in that the metal surface would be oxidized. 

As discussed in Section I.3(i), AX indicates the variation in the work function of a metal as an interface is created by bringing a solid and a liquid in contact. In principle, it should be possible to compare AX values with A values measured directly in gas phase experiments. This is the aim of UHV synthesis of the electrochemical double layer868 in which the electrode interface is created molecule by molecule, starting with the bare metal surface. It is thus possible to obtain evidence of ion-water interactions that can be envisaged from electrochemical measurements but that are not directly demonstrable. Wagner55 has given a recent comprehensive review of electrochemical UHV experiments. 

By 19884 it became obvious that the NEMCA effect, this large apparent violation of Faraday s law, is a general phenomenon not limited to a few oxidation reactions on Ag. Of key importance in understanding NEMCA came the observation that NEMCA is accompanied by potential-controlled variation in the catalyst work function.6 Its importance was soon recognized by leading electrochemists, surface scientists and catalysis researchers. Today the NEMCA effect has been studied already for more than 60 catalytic systems and does not seem to be limited to any specific type of catalytic reaction, metal catalyst or solid electrolyte, particularly in view of... 

Thus, as will be shown in this book, the effect of electrochemical promotion (EP), or NEMCA, or in situ controlled promotion (ICP), is due to an electrochemically induced and controlled migration (backspillover) of ions from the solid electrolyte onto the gas-exposed, that is, catalytically active, surface of metal electrodes. It is these ions which, accompanied by their compensating (screening) charge in the metal, form an effective electrochemical double layer on the gas-exposed catalyst surface (Fig. 1.5), change its work function and affect the catalytic phenomena taking place there in a very pronounced, reversible, and controlled manner. 

D.A. Emery, P.H. Middleton, and I.S. Metcalfe, The effect of electrochemical current pumping on the work function of solid electrolyte supported catalysts, Surf. Sci. 405, 308-315(1998). 

J. Holzl, and F.K. Schulte, Work Function of Metals, in Solid Surface Physics, Springer-Verlag, Berlin (1979), pp. 1-150. 

D. Tsiplakides, and C.G. Vayenas, Electrode work function and absolute potential scale in solid state electrochemistry, J. Electrochem. Soc. 148(5), E189-E202 (2001). 

C.G. Vayenas, On the work function of the gas exposed electrode surfaces in solid state electrochemistry, J. Electroanal. Chem. 486, 85-90 (2000). 

There is an important point to be made regarding UWr vs t transients such as the ones shown in Fig. 4.15 when using Na+ conductors as the promoter donor. As will be discussed in the next section (4.4) there is in solid state electrochemistry an one-to-one correspondence between potential of the working electrode (UWr) and work function (O) of the gas exposed (catalytically active) surface of the working electrode (eAUwR=AO, eq. 4.30). Consequently the UWr vs t transients are also AO vs t transients. 

CATALYST WORK FUNCTION VARIATION WITH POTENTIAL IN SOLID ELECTROLYTE CELLS... 

As discussed already in Chapter 2 the work function, , of a solid surface is one of the most important parameters dictating its chemisorptive and catalytic properties. The work function, (eV/atom) of a surface is the minimum energy which an electron must have to escape from the surface when the surface is electrically neutral. More precisely is defined as the energy to bring an electron from the Fermi level, EF, of the solid at a distance of a few pm outside of the surface under consideration so that image charge interactions are negligible. 

It is important to notice that the work function, , of a given solid surface changes significantly with chemisorption. Thus oxygen chemisorption on transition metal surfaces causes up to 1 eV increase in while alkali chemisorption on transition metal surfaces causes up to 3 eV decrease in . In general electronegative, i.e. electron acceptor adsorbates cause an increase in 0 while electropositive, i.e. electron donor adsorbates cause a decrease in 0. Note that in the former case the dipole vector P formed by the adsorbate and the surface points to the vacuum while in the latter case P points to the surface (Fig. 4.20). 

The implications of Equation (4.30) for solid state electrochemistry and electrochemical promotion in particular can hardly be overemphasized It shows that solid electrolyte cells are both work function probes and work function controllers for their gas-exposed electrode surfaces. 

The work function, O, of a solid surface (in eV) is the minimum energy required to extract an electron from that (neutral) surface.9 10,16 23 The parameter O/e (in V) is usually called the extraction potential. 

The Work Function of Catalyst Films Deposited on Solid Electrolytes... 

Work function, a quantity of great importance in surface science and catalysis, plays a key role in solid state electrochemistry and in electrochemical promotion. As will be shown in Chapter 7 the work function of the gas-exposed surface of an electrode in a solid electrolyte cell can be used to define an absolute potential scale in solid state electrochemistry. 

Solid electrolyte cells can be used to alter significantly the work function catalytically active, catalyst electrode surface by polarizing the catalyst-solid electrolyte interface. 

Over a wide range of conditions (again i.e., as long as ion backspillover from the solid electrolyte forms a double layer at the metal/gas interface)30"33 the potential difference eUWR is equal to the difference in work functions between the two electrodes... 

Figure 5.10. Transient response of catalyst work function O and potential Uwr upon imposition of constant currents I between the Pt catalyst (labeled26 C2) and the Pt counter electrode p"-A1203 solid electrolyte T = 240°C, p02 = 21 kPa Na ions are pumped to (I<0) or from (I>0) the catalyst surface at a rate I/F.26 Reprinted with permission from Elsevier Science.

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