Semiconductor/liquid electrolyte interfaces

One additional problem at semiconductor/liquid electrolyte interfaces is the redox decomposition of the semiconductor itself.(24) Upon Illumination to create e- - h+ pairs, for example, all n-type semiconductor photoanodes are thermodynamically unstable with respect to anodic decomposition when immersed in the liquid electrolyte. This means that the oxidizing power of the photogenerated oxidizing equivalents (h+,s) is sufficiently great that the semiconductor can be destroyed. This thermodynamic instability 1s obviously a practical concern for photoanodes, since the kinetics for the anodic decomposition are often quite good. Indeed, no non-oxide n-type semiconductor has been demonstrated to be capable of evolving O2 from H2O (without surface modification), the anodic decomposition always dominates as in equations (6) and (7) for... 

Koval CA, Howard JN (1992) Electron transfer at semiconductor electrode-liquid electrolyte interfaces. Chem Rev 92 411 33... 

The band bending at the semiconductor/liquid (electrolyte solution) interface can be understood by considering the potential distribution at this interface. In a case where the electrolyte solution contains a redox couple (R/Ox), which causes an electrochemical redox reaction,... 

Koval, C. A., Howard, J. N., Electron transfer at Semiconductor Electrode Liquid Electrolyte Interfaces, Chem. Rev. 1992, 92, 411 433. 

Marguerettaz X., O Neill R., Fitzmaurice D.J. Heterodyads—Electron-transfer at a semiconductor electrode liquid electrolyte interface modified by an adsorbed spacer acceptor complex. J. Am. Chem. Soc. 1994 116(6) 2629-2630... 

Equilibrium between the two phases at a semiconductor-electrolyte interface, solid and liquid, can only be achieved if their electrochemical potential is the same, that is ... 

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. 

Solid state materials that can conduct electricity, are electrochemically of interest with a view to (a) the conduction mechanism, (b) the properties of the electrical double layer inside a solid electrolyte or semiconductor, adjacent to an interface with a metallic conductor or a liquid electrolyte, (c) charge-transfer processes at such interfaces, (d) their possible application in systems of practical interest, e.g. batteries, fuel cells, electrolysis cells, and (e) improvement of their operation in these applications by modifications of the electrode surface, etc. 

By contrast, electrolyte states are much more limited in their distribution than metal conduction band states so that in many cases electron transfer through surface states may be the dominant process in semiconductor-electrolyte junctions. On the other hand, in contrast to vacuum and insulators, liquid electrolytes allow substantial interaction at the interface. Ionic currents flow, adsorption and desorption take place, solvent molecules fluctuate around ions and reactants and products diffuse to and from the surface. The reactions and kinetics of these processes must be considered in analyzing the behavior of surface states at the semiconductor-electrolyte junction. Thus, at the semiconductor-electrolyte junction, surface states can interact strongly with the electrolyte but from the point of view of the semiconductor the reaction of surface states with the semiconductor carriers should still be describable by equations 1 and 2. 

Most detailed studies of water photodissociation on SrTi03 and Ti02 have concentrated on photoelectrochemical cells (PEC cells) operating under conditions of optimum efficiency, that is with an external potential applied between the photoanode and counterelectrode. We have become interested in understanding and improving reaction kinetics under conditions of zero applied potential. Operation at zero applied potential permits simpler electrode configurations (11) and is essential to the development of photochemistry at the gas-semiconductor interface. Reactions at the gas-sold, rather than liquid-solid, interface might permit the use of materials which photocorrode in aqueous electrolyte. 

Electrolyte Electroreflectance (EER) is a sensitive optical technique in which an applied electric field at the surface of a semiconductor modulates the reflectivity, and the detected signals are analyzed using a lock-in amplifier. EER is a powerful method for studying the optical properties of semiconductors, and considerable experimental detail is available in the literature. ( H, J 2, H, 14 JL5) The EER spectrum is automatically normalized with respect to field-independent optical properties of surface films (for example, sulfides), electrolytes, and other experimental particulars. Significantly, the EER spectrum may contain features which are sensitive to both the AC and the DC applied electric fields, and can be used to monitor in situ the potential distribution at the liquid junction interface. (14, 15, 16, 17, 18)... 

The recent recognition that the surfaces of n-type semiconductors become effective redox catalysts when irradiated with light has captured the attention of a wide range of scientists interested in solar energy conversion and has spawned innumerable studies by electrochemists, physical chemists, and solid state physicists in the last decade. Most of these investigations have concentrated on detailed depictions of the semiconductor or the interface formed as the semiconductor is brought into contact with a metal, with another semiconductor, or with a liquid phase electrolyte. 

Fig. 5.7 shows schematically the potential across a semiconductor-electrolyte interface. To understand it we have to take two additional effects into account. First, the liquid molecules usually show a preferred orientation at the surface. Their dipole moment causes a jump of the potential. Second, on a solid surface the electrons can occupy surface states. These extra electrons contribute to the potential. 

Electrochemical reactions usually occur at the interface between a solid electrode and a liquid electrolyte. The electrode is an electron conductor, such as metals and semiconductors, and is immersed in an electrolyte. In practice the electrode is partially immersed in an electrolyte, but in theory it is convenient to define that the electrode is a multiphase system in which an electronic conductor is fully immersed in an electrolyte as shown in Fig. 9.4. 

In this chapter we present a few selected results on the nanoscale electrodeposition of some important metals and semiconductors, namely, Al, Ta and Si, in air- and water-stable ionic liquids. Here we focus on the investigation of the electrode/electrolyte interface during electrodeposition with the in situ scanning tunneling microscope and we would like to draw attention to the fascinating... 

In the presence of a redox system dissolved in the electrolyte, as long as there exists an energy difference between the Fermi level of the semiconductor and the redox couple, to reach the equilibrium conditions charge-carrier transfer occurs across the semiconductor-liquid interface via the energy bands, i.e., the conduction or valence band of the semiconductor. At the equilibrium point, the Fermi level of the redox... 

We are investigating the effects of binding non-electroactive molecules to electrode surfaces. The attached layer will be sufficiently thin (ca. 1 monolayer) that electron transfer across the electrode/electrolyte interface will not be inhibited. However, other surface properties may be advantageously modified. For semiconductor electrodes, desirable changes include suppression of the photo-activated surface corrosion and shifts in the flatband potential. We are seeking to improve the performance of semiconductor liquid-junction solar cells by these means. 

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