Photoelectrochemical charge-transfer processes

In the present article, various fundamental photoelectrochemical effects are quantitatively described and discussed, with the main emphasis on the kinetics of charge transfer processes. Although in principle the same reaction mechanisms are valid for extended semiconductor electrodes and particles, different factors govern the reaction rate, as will be discussed in detail. Finally, a brief overview of various applications will be given. 

The equivalent circuit diagram used to model solar cell current-voltage characteristics is shown at the top of Figure 1.1. The schematic energy level diagram of a DSSC at the bottom of Figure 1.1 shows the various charge transfer processes that occur in photoelectrochemical cells and relates these processes to current pathways via components of the model circuit. An illumination current density /l is induced upon photoexcitation of the... 

The photoelectrochemical properties of 283 colloids prepared by chemical solution growth [193] have been demonstrated by carrying out oxidation and reduction processes under visible light irradiation. Charged stabilizers such as Nation were found to provide an effective microenvironment for controlling charge transfer between the semiconductor colloid and the redox relay. 

There are two possible excited state interfacial electron transfer processes that can occur from a molecular excited state, S, created at a metal surface (a) the metal accepts an electron from S to form S+ or (b) the metal donates an electron to S to form S . Neither of these processes has been directly observed. The two processes would be competitive and unless there is some preference, no net charge will cross the interface. In order to obtain a steady-state photoelectrochemical response, back interfacial electron transfer reactions of S+ (or S ) to yield ground-state products must also be eliminated. Energy transfer from an excited sensitizer to the metal is thermodynamically favorable and allowed by both Forster and Dexter mechanisms [20, 21]. There exists a theoretical [20] and experimental [21] literature describing energy transfer quenching of molecular excited states by metals. How-... 

The photoelectrochemical process can be divided into the four reactions (Equations 1-4) involving photon excitation and charge separation in the Pc film (Equation 1) recombination events (Equation 2), charge transfer at the electrode substrate-Pc. interface (Equation 3) and charge transfer at the Pc-solution interface (Equation 4). The net process is the oxidation of hydroquinone with 0 to form quinone and R. If this is normally a thermodynamically uphill process where the dye is superimposed on a semiconductor substrate, then true photosensitized energy conversion has occurred. 

An alternative way is to divide the overall process of water splitting into two stages, each being conducted in a separate photoelectrochemical cell. Both cells are coupled with the aid of a certain intermediate substance, which acts as a charge transfer agent and is not consumed in the course of the overall process, but only provides the connection in series of the chemical potentials developed in both cells. Apparently, such a scheme imitates the combination of two photosystems in photosynthesis occurring in green plants. 

The aqueous corrosion of ceramics may involve a charge-transfer or electrochemical dissolution process. However, in many cases, dissolution or corrosion may take place with no charge transfer yet may be determined by one or more electrochemical factors such as absorbed surface charge or electronic band bending at the surface of narrow-band-gap semiconducting ceramics. The aqueous corrosion of ceramics is important in a number of areas. One of the most important is the stability of passive oxide films on metals. The stability of ceramics is a critical aspect in some aqueous photoelectrochemical applications (12), an example being the photoelectrolytic decomposition of water. Structural, nonoxide ceramics such as SiC or Si3N4 are unstable in both aqueous acid and alkaline environments the latter is virtually unstudied, however. 

The main objective of this chapter is to illustrate how fundamental aspects behind catalytic two-phase processes can be studied at polarizable interfaces between two immiscible electrolyte solutions (ITIES). The impact of electrochemistry at the ITIES is twofold first, electrochemical control over the Galvani potential difference allows fine-tuning of the organization and reactivity of catalysts and substrates at the liquid liquid junction. Second, electrochemical, spectroscopic, and photoelectrochemical techniques provide fundamental insights into the mechanistic aspects of catalytic and photocatalytic processes in liquid liquid systems. We shall describe some fundamental concepts in connection with charge transfer at polarizable ITIES and their relevance to two-phase catalysis. In subsequent sections, we shall review catalytic processes involving phase transfer catalysts, redox mediators, redox-active dyes, and nanoparticles from the optic provided by electrochemical and spectroscopic techniques. This chapter also features a brief overview of the properties of nanoparticles and microheterogeneous systems and their impact in the fields of catalysis and photocatalysis. 

To facilitate a self-contained description, we will start with well-estahlished aspects related to the semiconductor energy hand model and the electrostatics at semiconductor-electrolyte interfaces in the dark . We shall then examine the processes of light absorption, electron-hole generation, and charge separation at these interfaces. The steady state and dynamic aspects of charge transfer are then briefly considered. Nanocrystalline semiconductor films and size quantization are then discussed as are issues related to electron transfer across chemically modified semiconductor-electrolyte interfaces. Finally, we shall introduce the various types of photoelectrochemical devices ranging from regenerative and photoelectrolysis cells to dye-sensitized solar cells. 

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