Kelvin probes

2di = probe oscillation amplitude) because capacitance is inversely related to distance, the oscillation changes the capacitance and alters the charge on the probe tip this generates a measurable current which is used to calculate the potential difference between the tip and sample with a constant work potential seen with the metallic tip, the skin surface potential can be determined. 

Let us make a simple electrical circuit from palladium and copper. First the ends of the two metals are arranged in such a way that they form a parallel plate capacitor (Fig. 6.29). An electric field appears between the two plates, which is the result of the spontaneous separation of electrons, driven by the difference of their electron affinities. 

In the case of a copper/palladium junction, electrons are withdrawn from the copper plate which becomes positively charged. They are then deposited in the palladium plate which becomes negative. A galvanometer and a battery are also placed in series to complete the circuit. It can be shown that the field inside the gap depends only on the materials forming the capacitor and not on material(s) connecting the two plates. 

The importance of surfaces has grown along with the development of chemical sensors in recent years, due to the interaction between a given volatile compound and the surface of a chemically interactive material. 

The Kelvin Probe technique allows measurement of the Work Function of a given surface, not only in stationary conditions but also during absorption - desorption processes. 

In its simple form the Kelvin Probe is shown in figure 13, where the test plate is left fixed while the other plate of the capacitor can 

According to electromagnetic theory, any time a charge capacitor changes its value a displacement current is generated, expressed as I = dQ/dt = CdV/dt + VdC/dt. 

The experiment is conducted measuring the current corresponding to different voltages (positive and negative) applied to the capacitor. Since the overall voltage applied to the capacitance is V—AT the displacement current is given by I = (V — A )dC/dt. 

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Historically, the first and most important capacitance method is the vibrating capacitor approach implemented by Lord Kelvin in 1897. In this technique (now called the Kelvin probe), the reference plate moves relative to the sample surface at some constant frequency and tlie capacitance changes as tlie interelectrode separation changes. An AC current thus flows in the external circuit. Upon reduction of the electric field to zero, the AC current is also reduced to zero. Originally, Kelvin detected the zero point manually using his quadrant electrometer. Nowadays, there are many elegant and sensitive versions of this technique. A piezoceramic foil can be used to vibrate the reference plate. To minimize noise and maximize sensitivity, a phase-locked... 

The work function, , of a metal surface can be measured relatively easily and when using the Kelvin probe technique, in situ, i.e., during catalyst operation.54,55 Three techniques are the most commonly used54-58 ... 

All three techniques are quite straightforward to use. The Kelvin probe technique has the advantage that it does not require vacuum conditions, thus a catalyst can be studied under atmospheric or higher pressure. 

As already shown in Figures 5.10 and 5.11 the equality Aconstant current is applied at t = 0 between the catalyst and the counter electrode and one follows the time evolution of UWr by a voltmeter and of

[Pg.223]

As also already shown in Figures 5.8 to 5.16 the validity of Eqs. (5.18) and (5.19) has been confirmed by several laboratories using the Kelvin probe technique, as well as UPS (via electron cutoff energy) and in a semiquanti-tative manner via the PEEM technique. Experiment has also clearly shown that the validity of these equations, which include only thermodynamic properties, does not depend on which, if any, electrode is grounded.31 The same is clearly tme for electrochemical promotion in general, as should be obvious to every electrochemist reader. 

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... 

Equation (7.12) has been reported since 199031 by several groups31 39,40,42 45 and has been confirmed using both the Kelvin probe technique and UPS, as already discussed in Chapter 5. Only one group46 has reported significant deviations from it, but the SEMs in that work show massive blocking nonporous electrodes which apparently do not allow for ion spillover. 

By comparing Figure 11.9 and the characteristic Po2(Uwr) rate breaks of the inset of Fig. 11.9 one can assign to each support an equivalent potential Uwr value (Fig. 11.10). These values are plotted in Figure 11.11 vs the actual work function G>° measured via the Kelvin probe technique for the supports at po2-l atm and T=400°C. The measuring principle utilizing a Kelvin probe and the pinning of the Fermi levels of the support and of metal electrodes in contact with it has been discussed already in Chapter 7 in conjunction with the absolute potential scale of solid state electrochemistry.37... 

Figure 11.11. Correlation between the equivalent potentials of the supports defined in Figure 11.10 and of the work function or absolute potential of the supports measured via the Kelvin probe technique in po2 =1 atm at 400°C.22...

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

Recently, scanning Kelvin probes and microprobes, as high-resolution surface analysis devices, have been developed. They allow one to investigate the lateral distribution of the work functions of the surfaces of various phases, including the determination of the potential profiles of metals and semiconductors under very thin films of electrolytic solution, and also of the surface potential map of various polymer- and biomembranes [50-56], The lateral resolution and the sensitivity are in the 100 nm and ImV ranges, respectively [54],... 

In the most common LB films with the Y-type structure, the center of inversion exists, and hence they are not suitable for pyroelectric usages. On the other hand, since LB films with X- or Z-type structure have no center of symmetry, it is possible to construct the polar pyroelectric film with permanent dipoles pointing toward one direction. Similar structures can also be formed in hetero LB films with two different amphiphiles stacked altematingly. The first report on the pyroelectric LB film with X-or Z-type structure appeared in 1982 by Blinov et al. [12], It was followed by those of the alternate LB films by Smith et al. [13] and Christie et al. [14]. The polarized structure of the fabricated LB film can be checked by the surface potential measurements using the Kelvin probe [15], the Stark effect measurements [12], or the sign inversion of the induced current between heating and cooling processes. 

Figure 1. The upper curve is a cyclic esnersogram of a rotating oxide-coated Au electrode (outer potential 0 vs electrode potential U), measured with a Kelvin probe. (Here is plotted as 0 minus the ref. electrode Fermi level.) The lower curve is the electrolyte cyclic potentiogram (0S - (-U) vs U), by the same Kelvin probe. Data indicate that 0m 0g 3 50 mV at all times.

Figure 13. Schematic design of Kelvin Probe circuit and its signal output.

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