Photocurrent photoelectrochemistry

The combination of electrochemistry and photochemistry is a fonn of dual-activation process. Evidence for a photochemical effect in addition to an electrochemical one is nonnally seen m the fonn of photocurrent, which is extra current that flows in the presence of light [, 89 and 90]. In photoelectrochemistry, light is absorbed into the electrode (typically a semiconductor) and this can induce changes in the electrode s conduction properties, thus altering its electrochemical activity. Alternatively, the light is absorbed in solution by electroactive molecules or their reduced/oxidized products inducing photochemical reactions or modifications of the electrode reaction. In the latter case electrochemical cells (RDE or chaimel-flow cells) are constmcted to allow irradiation of the electrode area with UV/VIS light to excite species involved in electrochemical processes and thus promote fiirther reactions. 

Stationary microwave electrochemical measurements can be performed like stationary photoelectrochemical measurements simultaneously with the dynamic plot of photocurrents as a function of the voltage. The reflected photoinduced microwave power is recorded. A simultaneous plot of both photocurrents and microwave conductivity makes sense because the technique allows, as we will see, the determination of interfacial rate constants, flatband potential measurements, and the determination of a variety of interfacial and solid-state parameters. The accuracy increases when the photocurrent and the microwave conductivity are simultaneously determined for the same system. As in ordinary photoelectrochemistry, many parameters (light intensity, concentration of redox systems, temperature, the rotation speed of an electrode, or the pretreatment of an electrode) may be changed to obtain additional information. 

Since photoelectrochemistry is not limited to photocurrent measurements, it may at this point be useful to think about some general new research possibilities to be expected from the combination of electrochemical and microwave measurements. Table 1 shows obvious combination possibilities between electrochemical and microwave measurements. 

The mathematical evolution of the theory is involved, and struggling with its algebra does little to increase our understanding of photoelectrochemistry. The result for a photocurrent provoked by a light with a monochromatic frequency is... 

Thus, it can be seen that a study of the steady state photoelectrochemistry of colloidal semiconductors with the ORDE can provide information relating to the energy distribution of the particle surface states, the photogenerated carrier density and the quantum efficiency of carrier generation. The next section describes how to obtain information pertaining to intraparticle charge carrier dynamics from a study of the behaviour of transient photocurrents at the ORDE. 

Photocurrent multiplication processes are encountered frequently in photoelectrochemistry. Common examples include the photo-oxidation of formic acid and of secondary alcohols at n-type semiconductors [1], and the photoreduction of oxygen at p-type semiconductors [40, 41, 48]. The mechanisms are generally supposed to involve majority carrier injection by a photogenerated intermediate, and IMPS has been used to determined the rate constants for these processes. Earlier work has been reviewed previously in some detail [48]. The first example to be studied by IMPS was the photoreduction of oxygen to H2O2 at p-GaP [40, 41]. Subsequently, the oxidation of formic acid at n-CdS was characterised by the same method [52]. The oxidation of formic acid to CO2 is a two step reaction which involves the following steps... 

Figure 2.21 shows a schematic of the setup for simultaneous measurement of the stationary light-induced excess minority carrier microwave reflectivity and the photocurrent at the semiconductor-electrolyte contact. The sample is illuminated from the front side and photoelectrochemistry is performed using the standard... 

In special cases, inelastic processes contribute too, for example, by the Poole-Frenkel effect [61]. In photoelectrochemistry, finally, the photocurrent is initiated by vertical electron/hole pair generation with following charge separation. 

Schefold, J. (1992). Impedance and intensity modulated photocurrent spectroscopy as complementary differential methods in photoelectrochemistry. J. Electmanal. Chem. 341,111-136. 

Under potentiostatic conditions, photoinduced heterogeneous electron transfer between specifically adsorbed porphyrins and redox couples confined to the organic phase manifests itself by photocurrent responses. As in the case of dynamic photoelectrochemistry, these photoresponses provide information on the dynamics of heterogeneous electron transfer and recombination processes. In addition, we shall demonstrate that photocurrent measurements can be used to characterise the interfacial coverage of the specifically adsorbed porphyrins as well as their molecular orientation. 

In this paper, we have developed new multi-l er film electrodes for solar cell application, composed of PANI and PANI/Ti02 films deposited on Au/p-ATP substrates. Fu er investigations involved photoelectrochemistry and mechanism of photocurrent generation in these electrodes. 

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