Modification of the Electrochemical Behavior of InP Electrodes Induced By Ionic Bombardment: The Case of the Response of the Ce4+/Ce3+ Couple
Authors: Aureau Damien , Vigneron Jackie , Bouttemy Muriel , Frégnaux Mathieu , Simon Nathalie , Etcheberry Arnaud , Gonçalves Anne-Marie 
Published: February 2020

The concept of modified electrode is very important for a lot of topics related to interfacial electrochemistry. It is particularly true when semiconductor (SC) interfacial electrochemistry is considered. Due to the specificities of the SC/liquid interfaces, modified SC electrodes have to be examined from different angles. The external part of the surface can be strongly modified due to the variations of surface /solution equilibrium inducing drastic transformations of the energy diagram at the interface. A direct consequence is a possible alteration of electronic transfers. Moreover the modification of the SC electrode can also concern deeper region through variation of the depletion layer distribution even of buried diffusion layer. Evolution of these parameters could again modify the energy diagram and the related electronic transfer abilities. Finally surface modifications can induced also evolution of the photoelectrochemical properties through evolution of the optical properties of the SC surface. All this different parameters are more or less related to the global response of the interface. It is in this context that our paper is being taken.

For this study, our tool of surface modification is based on ion bombardment through mono-atomic or cluster Argon beams. Modulation of the perturbation induced by ion / SC surface interaction can be largely modulated by using mono or cluster Argon beam conditions [1]. In UHV the level and the nature of the perturbation can be easily in-situ evaluated from XPS responses evolutions from the core levels and also from the valence band. At this in-situ approach can be also associated optical responses such as ex-situ Ellipsometric Spectroscopy giving a global evaluation of the SC modification.

As supporting SC material we choose InP whose surface, optical and interfacial electrochemical responses are well known in their initial stage before ionic bombardment. Based on well characterized InP (-n or -p) electrodes effect of ionic bombardment can be explored through a lot of electrochemical responses. Among the more attractive modification towards electronic transfer on InP, one can consider the addition in the electrolyte of a redox probe which acts as a perfect tool giving quantitative information for post bombardment. Ce4+/Ce3+ couple is particularly efficient due to its transfer specificities when its oxidant state (Ce4+) is only present [2-6].

Indeed pure holes injection inside the valance band (BV) is observed during the Ce4+ reduction mechanism which is only governed by the Ce4+ diffusion at the surface. As a direct consequence, the evolution of the reduction rate of Ce4+ can be easily measured vs the level of ionic bombardment. Moreover when InP is properly polarized all the holes injected are used in the anodic dissolution process with a very attractive and unusual conduction band (BC) injection phenomenon due to corrosion intermediates [6]. This is again a very sensitive probe to the level of modification of the InP surface after ionic bombardment.

Note that this induced anodic process as a very similar potential distribution as the one obtained under illumination for which the holes creation is nevertheless only associated to the photon absorption. Common and difference points are also a subject of discussion for a characterization of the induced modifications. Finally we will discuss how an anodic and/or a chemical etching are able to regenerate the initial chemical state of the surface. The underlying question is how the anodic process linked to Ce4+/Ce3+ is sufficient to perform the surface regeneration even if it is initially modified by the ionic bombardment or if it requires the assistance of high positive polarization of the modified InP electrode.

References:

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