Scanning vibrating probe techniques

Recent advances in the development of non-invasive, in situ spectroscopic scanned-probe and microscopy techniques have been applied successfully to study mineral particles in aqueous suspension (Hawthorne, 1988 Hochella and White, 1990). In situ spectroscopic methods often utilise molecular probes that have diagnostic properties sensitive to changes in short-range molecular environments. At the particle-solution interface, the molecular environment around a probe species is perturbed, and the diagnostic properties of the probe, which can be either optical or magnetic, then report back on surface molecular structure. Examples of in situ probe approaches that have been used fruitfully include electron spin resonance (ESR) and nuclear magnetic resonance (NMR) spin-probe studies perturbed vibrational probe (Raman and Fourier-transform IR) studies and X-ray absorption (Hawthorne, 1988 Hochella and White, 1990 Charletand Manceau, 1993 Johnston et al., 1993). 

As the nature of the electrified interface dominates the kinetics of corrosive reactions, it is most desirable to measure, e.g., the drop in electrical potential across the interface, even where the interface is buried beneath a polymer layer and is therefore not accessible for conventional electrochemical techniques. The scanning Kelvin probe (SKP), which measures in principle the Volta potential difference (or contact potential difference) between the sample and a sensing probe (which may consist of a sharp wire composed of a conducting, stable phase such as graphite or gold) by the vibrating condenser method, is the only technique which allows the measurement of such data and therefore aU modern models which deal with electrochemical de-adhesion reactions are based on such techniques [1-8]. Recently, it has been apphed mainly for the measurement of electrode potentials at polymer/metal interfaces, especially polymer-coated metals such as iron, zinc, and aluminum alloys [9-15]. The principal features of a scanning Kelvin probe for corrosion studies are shown in Fig. 31.1. 

The physical tools for chemical analysis of surfaces as well as for investigation of their structural, electronic, vibrational, and dynamic properties have been described quite extensively in the literature [5-11], so we refrain here from repetitions. Scanning probe techniques, and in particular the scanning tunneling microscope [12], proved to be most powerful for direct observation of processes on atomic scale. 

The Kelvin probe is a noncontact, nondestructive, vibrating capacitor technique for measuring work functions, or more precisely the difference between the work function of sample and probe. It was first used by Thomson, later Lord Kelvin, in 1862 [93]. This method has been further improved throughout the following decades [94] and is now a well-established method for measuring work functions, or, from a more electrochemical point of view, Volta potentials. Whereas in traditional Kelvin probes, the probe is a small gold plate or mesh of several square millimeters or centimeters, in Scanning Kelvin Probes (SKP), the probe is a small metal tip with a diameter of typically several tens of micrometers, which can be scaimed across the surface of the sample. 

The present version of the book represents a completely revised update of the first edition as it appeared in 1993. Significant new developments in e.g. the scanning probe microscopies, imaging and vibrational techniques called for revision and additions to the respective chapters. But also the other chapters have been updated with recent examples, and references to relevant new literature. Many figures from the first version of the book have been improved to make them more informative. The chapter with case studies has been expanded with an example on polymerization catalysts. 

We first experimented with the Quartz Crystal Microbalance (QCM) in order to measure the ablation rate in 1987 (12). The only technique used before was the stylus profilometer which revealed enough accuracy for etch rate of the order of 0.1 pm, but was unable to probe the region of the ablation threshold where the etch rate is expressed in a few A/pulse. Polymer surfaces are easily damaged by the probe tip and the meaning of these measurements are often questionable. Scanning electron microscopy (21) and more recently interferometry (22) were also used. The principle of the QCM was demonstrated in 1957 by Sauerbrey (22) and the technique was developed in thin film chemistiy. analytical and physical chemistry (24). The equipment used in this work is described in previous publications (25). When connected to an appropriate oscillating circuit, the basic vibration frequency (FQ) of the crystal is 5 MHz. When a film covers one of the electrodes, a negative shift <5F, proportional to its mass, is induced ... 

---