Photosensitization Electrode processes

An interesting idea has been to prepare the photosensitive electrode on site having the liquid play the dual role of a medium for anodic film growth on a metal electrode and a potential-determining redox electrolyte in the electrochemical solar cell. Such integration of the preparation process with PEC realization was demonstrated initially by Miller and Heller [86], who showed that photosensitive sulfide layers could be grown on bismuth and cadmium electrodes in solutions of sodium polysulfide and then used in situ as photoanodes driving the... 

The MFEs on the photoelectrochemical reactions of photosensitive electrodes modified with nanoclusters containing C oN and MePH were examined as a study of spin chemistry at solid/liquid interfaces. The results can be expected to lead to an epochmaking means of reaction control involving photoelectrochemical processes. The results also provide useful information for designing novel nanodevices whose photofunctions can be controlled by a magnetic field. 

If a solution, being in contact with an electrode, contains photosensitive atoms or molecules, irradiation of such a system may lead to photoelectro-chemical reactions or, to be more exact, electrochemical reactions with excited particles involved. In such reactions the electrons pass either from an excited particle to the electrode (the anodic process) or from the electrode to an excited particle (the cathodic process). In this case, an elementary act of charge transfer has much in common with ordinary (dark) electrochemical redox reactions, which opens a possibility of interpreting certain aspects of photochemical processes under consideration with the use of concepts developed for general quantum mechanical description of electrode processes. 

A substance can be thought to be a catalyst when it accelerates a chemical reaction without being consumed as a reactant that is to say, it appears in the rate expression describing a thermal reaction without appearing in the stoichiometric equation [49], A catalyst is a compound that lowers the free activation enthalpy of the reaction. Then, photocatalysis can be defined as the acceleration of a photoreaction by the presence of a catalyst [38, pp. 362-375], This definition, as pointed out in [29, pp. 1-8], includes photosensitization, a process by which a photochemical alteration occurs in one molecular entity as a result of initial absorption of radiation by another molecular entity called the photosensitizer [13], but it excludes the photoacceleration of a stoichiometric thermal reaction irrespective of whether it occurs in homogeneous solution or at the surface of an illuminated electrode. Otherwise, any photoreaction would be catalytic [29, pp. 1-8]. Depending on the specific photoreaction, the catalyst may accelerate the photoreaction by interaction with the substrate in its ground or excited state and/or with a primary photoproduct. 

In such devices the light-absorbing semiconductor electrode immersed in an electrolyte solution comprises a photosensitive interface where thermodynamically uphill redox processes can be driven with optical energy. Depending on the nature of the photoelectrode, either a reduction or an oxidation half-reaction can be light-driven with the counterelectrode being the site of the accompanying half-reaction. N-type semiconductors are photoanodes, p-type semiconductors are photocathodes, and... 

Recently, the DSSC has been used for an educational demonstration of solar energy-to-electricity conversion system because of its simple fabrication [12,168]. Students could purchase DSSC kits including all components such as TCO-coated glass, 2 electrodes, blackberries (i.e., dye), and electrolyte solution [169,170] and easily demonstrate an artificial photosynthetic process. For detailed studies, it is possible to purchase raw materials, including Ru dye photosensitizers, Ti02 paste, and sealing materials, from Solaronix S. A. [171]. 

This approach can be further extended to photoelectrochemical reactions at modified semiconductor electrodes. In such cases the immobilized substance(s) may serve several functions mediation of the redox process, photosensitization of the semiconductor, and photocorrosion protection. 

This brief review attempts to summarize the salient features of chemically modified electrodes, and, of necessity, does not address many of the theoretical and practical concepts in any real detail. It is clear, however, that this field will continue to grow rapidly in the future to provide electrodes for a variety of purposes including electrocatalysis, electrochromic displays, surface corrosion protection, electrosynthesis, photosensitization, and selective chemical concentration and analysis. But before many of these applications are realized, numerous unanswered questions concerning surface orientation, bonding, electron-transfer processes, mass-transport phenomena and non-ideal redox behavior must be addressed. This is a very challenging area of research, and the potential for important contributions, both fundamental and applied, is extremely high. 

An even simpler duplication process based on the same principle is shown in Figure 5.29(B). If a hole-injecting electrode is used instead of a photosensitive layer, simple electrostatic charging allows the pattern to be read and transferred.148 Other new technologies based on the photopatteming of polysilanes are being developed, and may become commercialized in the future. 

Photoswitchable enzymes could have an important role in controlling biochemical transformations in bioreactors. Various biotechnological processes generate an inhibitor, or alter the environmental conditions (pH, for example) of the reaction medium. Photochemical activation of enzymes that adjust environmental conditions or deplete the inhibitor to a low concentration may maintain the bioreactor at optimal performance. More specifically, integration of the photoswitchable biocataly-tic matrix with a sensory electrode might yield a feedback mechanism in which the sensor element triggers the light-induced activation/deactivation of the photosensitive biocatalyst. 

A significant drawback of metals for photoelectrochemical applications lies in their ability to efficiently quench excited states via energy transfer processes, as discussed below. Direct detection of photosensitized electron transfer to or from a metal electrode surface has been observed [30]. However, unlike dye-sensitized semiconductor systems, little examination of the kinetics of such systems has yet been undertaken. 

Sensitization of electrodes can be defined as the process by which interfacial electron transfers occurs as a result of selective light absorption by an entity called a photosensitizer, or simply a sensitizer [1]. The most common types of sensitizers are organic chromophores and inorganic coordination compounds, generically referred to as dyes. Interfacial electron transfer produces current or voltage response that can be measured in an external circuit. Thus sensitization provides a method for the conversion of a photon into an electrical signal that can be controlled at the molecular level. 

The products of degradation during polarization of HTSC electrodes have not been studied in much detail [44,287,293,476,478], Upon cathodic polarization of cuprates, the reduced forms of copper oxides [44,476], and sometimes metallic copper [293], are formed at considerable rates these processes are photosensitive [453], During anodic polarization, the formation of alkaline-earth compounds on the HTSC surface is accelerated [287,476]. 

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. 

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