Surface potential oxide-solution interface equilibrium

Although a family of OgS - Jig8 values are allowed under Equation 7 the actual equilibrium state of the oxide/solution interface will be determined by the dissociation of the surface groups and the properties of the electrolyte or the diffuse double layer near the surface. For surfaces that develop surface charges by different mechanisms such as for semiconductor, there will be an equation of state or charge-potential relationship that is analogous to Equation 7 which characterizes the electrical response of the surface. 

To describe in detail such a specific system as the metal oxide/solution interface, it is necessary to prepare a model describing dependences between potential and surface charge and draw up reactions, the occurrence of which leads to the changes of surface charge o-The reaction equations describing an equilibrium state between the surface and solution as well as values of equilibrium constants of these reactions provide detailed information... 

When a metal, M, is immersed in a solution containing its ions, M, several reactions may occur. The metal atoms may lose electrons (oxidation reaction) to become metaUic ions, or the metal ions in solution may gain electrons (reduction reaction) to become soHd metal atoms. The equihbrium conditions across the metal-solution interface controls which reaction, if any, will take place. When the metal is immersed in the electrolyte, electrons wiU be transferred across the interface until the electrochemical potentials or chemical potentials (Gibbs ffee-energies) on both sides of the interface are balanced, that is, Absolution electrode Until thermodynamic equihbrium is reached. The charge transfer rate at the electrode-electrolyte interface depends on the electric field across the interface and on the chemical potential gradient. At equihbrium, the net current is zero and the rates of the oxidation and reduction reactions become equal. The potential when the electrode is at equilibrium is known as the reversible half-ceU potential or equihbrium potential, Ceq. The net equivalent current that flows across the interface per unit surface area when there is no external current source is known as the exchange current density, f. 

Similar photovoltaic cells can be made of semiconductor/liquid junctions. For example, the system could consist of an n-type semiconductor and an inert metal counterelectrode, in contact with an electrolyte solution containing a suitable reversible redox couple. At equilibrium, the electrochemical potential of the redox system in solution is aligned with the Fermi level of the semiconductor. Upon light excitation, the generated holes move toward the Si surface and are consumed for the oxidation of the red species. The charge transfer at the Si/electrolyte interface should account for the width of occupied states in the semiconductor and the range of the energy states in the redox system as represented in Fig. 1. 

In principle, like all electrochemical reactions initiated by the transfer of an electron across an electrode-electrolyte interface, photoelectrochemical transformations offer the possibility of more precise control than can be attained with reactions that take place in homogeneous solution [62, 63]. This better selectivity derives from three features associated with reactions that take place on surfaces, and hence with the photoelectrochemical event the applied potential (allowing for specific activation of a functional group whose oxidation potential is higher, even in a multifunctional molecule) the chemical nature of the electrode surface (and hence of the adsorption equilibrium constant of a specific molecule present in the double layer) and, finally, control of current flow (and hence a constraint on the number of electrons passed to an adsorbed reactant). 

One obvious interpretation of above results is oxidation at the Si/Cu interface through voids in the Cu layer. As preliminary test to clarify whether these observations may be asssigned to copper and/or silicon oxidation, the porosity of the Cu films was inspected by immersing a Cu/Si junction into an acidic solution of CuSC>4 (pH 2) with increasing HF content. Initially, the open circuit potential (OCP) of the silicon covered by the Cu film was - 0.38V, which is equal to the rest potential of a clean Cu wire. This indicated that the junction n-Si/Cu/CuSC>4 solution was at equilibrium (Fig. 3A). Addition of a small amount of HF, up to 2%, however, induced a rapid shift of the OCP of the n-Si/Cu electrode, the value being intermediate between that of the Cu wire and that of the bare n-Si electrode in contact with the C11SO4 solution (Fig. 3B, the rest potential of bare n-Si is - 0 64 V). Since HF is known to dissolve Si oxide, the negative shift of the OCP means that HF actually reaches the Si surface, i.e. that Cu films are not ideally compact. Porosity of the Cu films is also... 

Although most metals display an active or activation controlled region, when polarised anodically from the equilibrium potential, many metals and perhaps even more so alloys developed for engineering applications, produce a solid corrosion product. In many examples the solid is an oxide that is the stable phase rather than the ion in solution. If this solid product is formed at the metal surface and has good intimate contact with the metal, and features low ion-conductivity, the dissolution rate of the metal is limited to the rate at which metal ions can migrate through the film. The layer of corrosion product acts as a barrier to further ion movement across the interface. The resistance afforded by this corrosion layer is generally referred to as the passivity. Alloys such as the stainless steels, nickel alloys and metals like titanium owe their corrosion resistance to this passive layer. 

It is also possible to measure the potential of semiconductors under various conditions (electrolyte, solution) and applied potentials with and without irradiation when a semiconductor photocatalyst is employed as an electrode [11], The rest potential measurement of a semiconductor should provide a good estimate of the potentials at equilibrium under dark conditions and steady-state photoirradiation. A lack of equilibration is often observed between metal/semiconductor and the redox potential in the solution this can be ascribed to corrosion of the semiconductor, to formation of a surface film (e.g., oxide), or to inherently slow electron transfer across the interface [67], making the situation more complex. It is thus preferable to directly measure the potentials during the photocatalytic process. 

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