Theory of electrochemical phase formation

Abstract

In underpotential deposition (UPD), as in other examples of ECPF, many basic questions are yet unresolved. One such question is: Does UPD involve adsorption or phase transition Despite varied experimental observations, especially those obtained using linear potential sweep or optical measurements, a conclusive answer is still to emerge. This is partly due to the fact that proper tests for elucidating nucleation-growth-like phenomena had not been developed before. In this chapter, we have taken up this question in detail and discussed this issue in the light of linear potential sweep for nucleation-growth models. Diagnostic criteria are given in the form of analytic limits and computed curves. The results are compared with the corresponding ones for some simple adsorption models. Though many limits and parametric dependencies seem similar for these two classes of models, we demonstrate the existence of tests that can clearly distinguish adsorption from nucleation-growth mechanisms. It is also shown how the various model parameters like ?, ?, or Bc can be obtained through an analysis of experimental transients. The developments of this chapter, apart from their application to the theory of dynamic Ising models as such, have important bearing on the phenomenology of SGPP and enable one to arrive at kinetic equations starting from a molecular basis. However, no trace of clustering is seen under the MFA that is made use of in Section 5. The exact results of Section 6, on the other hand, refer to one-dimensional transitions where a phase transition is not expected under normal circumstances. The case of two dimensions might have more interesting features to reveal in this regard.