Introduction

Recently, the structure of the solid/liquid interface has been studied with a wide range of in-situ structural techniques. In particular, scanned probe microscopes [1-5] and synchrotron-based methods [6-9] have yielded a wealth of structural information. The ultimate goal of this work is an understanding of the structure and reactivity of the electrode surface at the atomic level. One of the most extensively studied processes is metal underpotential deposition (UPD) [10], which involves the formation of one or more metal monolayers at a potential positive of the reversible Nernst potential for bulk deposition.

One of the most interesting structural aspects of adsorption in an electrochemical environment is the formation of very open ordered structures. For instance, the UPD of copper on gold (111) in sulfuric acid exhibits a honeycomb structure [11-12], and the silver on gold (111) UPD system displays a range of open adlayer structures in different electrolytes [13-15]. These surface structures are markedly different from the UHV metal monolayers found for copper and silver on gold (111) [16-18] in which close-packed surface adlayers are the norm. There are a number of explanations for these structural differences. Firstly, the metal species in several UPD adlayers has been shown to retain a partial positive charge after adsorption [19-21]. These partially charged species will tend to repel one another and consequently prefer a more open adlattice. Alternatively, co-adsorption of negatively charged anions, which is known to occur from coulometric measurements[ll, 12, 20], SPM [13], and extended X-ray absorption fine structure (EXAFS) [9], also tends to favor the formation of a similar adlattice arrangement. In both of these cases the surface is thought to be subject to a compressive stress mediated via Coulombic charge-charge repulsive forces.

Formation of open adlayers via UPD is not a universal phenomena. Other metals such as lead, thallium and bismuth form close-packed incommensurate monolayers and exhibit an effect which has been termed electrocompression. This refers to a gradual compression of the monolayer in the region between the monolayer deposition and bulk deposition regions. There is a slow reduction of the monolayer lattice parameter until it is about 3% compressed with respect to the bulk metal [22 - 23]. This type of behavior is expected for close-packed commensurate monolayers on the basis of effective medium theory [24].

In all of these systems, reduction of surface stress is thought to be one of the most important factors in the structure adopted. In the past some confusion has arisen because of the tendency in ordinary language to mix stress and strain. Only direct measurements of the stress changes on deposition and stripping of metal monolayers and bulk deposits can give information about the lateral forces in the plane of the surface. These stress change measurements are complementary to other structural probes which provide information about the geometry of the surface structure. The stress changes can also be related to the thermodynamics of the interface via the electrocapillary equation [12].

The most commonly used direct method for determination of surface stress is the bending beam method in which a lever bends when subjected to a change in surface stress in one of its faces in order to minimize its stored strain energy. The relationship between the deflection of a cantilever and the different stresses in its surfaces was first determined by Stoney [25] and in surface science it has been used to measure the stress changes associated with the reconstructions of semiconductor surfaces [26 - 27].

Within the electrochemical environment, bending-beam stress measurements have been used to determine the electrocapillary curves for gold and platinum surfaces [28 -29]. This serves as a useful test of the sensor performance and calibration procedure. A number of other methods have also been applied to study interfacial stress changes in an electrochemical environment. For instance, Haiss and Sass [30] have used a STM to measure the stress changes occurring during the UPD of copper on gold (111). Simultaneous mass and surface stress changes have also been made for the lead and thallium UPD processes on gold using a quartz crystal microbalance combined with interferometric measurement of the surface stress change [31].

Semiconductor fabrication processes permit construction of small, sensitive, stress sensors. In fact the levers used in atomic force microscopes are almost ideal for this purpose. The combination of the mechanical properties of silicon nitride and the geometry of the cantilever mean that the lever has a high resonant frequency and a low spring constant [32], The low spring constant is beneficial for sensor applications because it means that a small applied force can be transduced to a measurable deflection, which lies at the heart of any sensor [33], When combined with the highly sensitive optical lever AFM detection system, both of these factors mean that this arrangement is a fast and highly sensitive stress sensor.

The sensor can in principle be used to detect stress changes as small as 10"5 Nm"1 although here the stress changes are several orders of magnitude larger than this. This means that other possible sources of lever deflection such as thermal drift or heating of the electrode can be neglected. Already, several AFM-based sensor devices which measure mass changes [34] humidity changes [35], and surface stress changes [36] working at the lever resonance frequency in air have been designed. A thermal sensor based on bimetallic bending working in a direct current (dc) mode [37], has also been developed for use in air.

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