Photocurrent, measurement

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 combination of photocurrent measurements with photoinduced microwave conductivity measurements yields, as we have seen [Eqs. (11), (12), and (13)], the interfacial rate constants for minority carrier reactions (kn sr) as well as the surface concentration of photoinduced minority carriers (Aps) (and a series of solid-state parameters of the electrode material). Since light intensity modulation spectroscopy measurements give information on kinetic constants of electrode processes, a combination of this technique with light intensity-modulated microwave measurements should lead to information on kinetic mechanisms, especially very fast ones, which would not be accessible with conventional electrochemical techniques owing to RC restraints. Also, more specific kinetic information may become accessible for example, a distinction between different recombination processes. Potential-modulation MC techniques may, in parallel with potential-modulation electrochemical impedance measurements, provide more detailed information relevant for the interpretation and measurement of interfacial capacitance (see later discus-... 

Lemasson P, Boutry AE, Tiiboulet R (1984) The semiconductor-electrolyte junction Physical parameters determination by photocurrent measurement throughout the Cdi xZnxTe alloy series. J Appl Phys 55 592-594... 

The first report on photocurrent measurements at liquid-liquid interfaees was published by Marecek et al. for the photoreduction of methylviologen in the aqueous phase by Ru(bpy)3 located in the organic phase (DCE or benzonitrile) [48],... 

FIG. 13 Cyclic voltammogram (a) and potential dependence of the photoresponses (b)-(c) to chopped illumination and lock-in detection associated with the photoreaction in Eq. (40). The CV shows that the polarizable window extended to less than 100 mV. The photocurrent measurements carried out were done in the presence (trace 3) and absence (trace 2) of the redox quencher in the organic phase. (Reprinted with permission from Ref 48. Copyright 1989 American Chemical Society.)... 

C. Basic Aspects Concerning Dynamic Photocurrent Measurements... 

The basic experimental arrangements for photocurrent measurements under periodic square and sinusoidal light perturbation are schematically depicted in Fig. 19. In the previous section, we have already discussed experimental results based on chopped light and lock-in detection. This approach is particularly useful for measurement at a single frequency, generally above 5 Hz. At lower frequencies the performance of lock-in amplifier and mechanical choppers diminishes considerably. For rather slow dynamics, DC photocurrent transients employing optical shutters are more advisable. On the other hand, for kinetic studies of the various reaction steps under illumination, intensity modulated photocurrent spectroscopy (IMPS) has proved to be a very powerful approach [132,133,148-156]. For IMPS, the applied potential is kept constant and the light intensity is sinusoid-... 

FIG. 19 Block diagrams for photocurrent measurements with chopped light and lock-in detection (a) as well as for intensity-modulated photocurrent spectroscopy (b). (Adapted from Ref. 85.)... 

The adsorption spectra in the UV-visible range of the extracts exhibit different broad bands depending on the component of the mixtures photocurrent measurements show larger bands in comparison with those derived from adsorbed dyes. In particular, in the case of Mix, a bathochromic shift of about 40 nm was observed the proposed rationale is a band interruption between density states of Ti02 and the HOMO-LUMO in the dye. Moreover, the photocurrent response derives predominantly from the superposition of those of Morus nigra and carminic acid. 

PbO is more stable in alkaline solutions. Anodic oxidation of Pb leads to the selective growth of /I-PbO, which transforms into a-PbO in the course of prolonged polarization, as supported by X-ray diffraction studies and photoelectrochemical experiments [172]. In the presence of sulfate ions, simultaneous formation of basic lead sulfate with PbO has been observed, until the formation of 4Pb0-PbS04. At higher potentials, the oxide film in sulfate ions solution has a strong (110) orientation with microporous structure. X-ray diffractometry and photocurrent measurements have confirmed the presence of... 

It can be seen that the 1 kV transient switching voltages, which would normally occur at the switching on and off times of the bias voltage, were eliminated down to level where they do not interfere with the photocurrent measurement. Figure 4.7... 

Fig. 15. Dependence of photocurrents (measured in the vidicon arrangement) on light inten-sity log /phot = / ( ) (1) Sb2S3 (2) phthalocyanine ( — 5500 A)...

Photocurrent onset, pH = 10.1, and quasi-Fermi level, 633 nFf - 0.85 V vs SCE, were determined from pH-dependent photocurrent measurements in the presence of methylviologen and glucose... 

Size quantization was demonstrated by TEM, absorption spectroscopy, and photocurrent measurements... 

Photocurrent measurements permit the determination of the hole diffusion length Lp. As was already noted, comparison of measured and calculated polarization curves allows Lp to be determined by a fitting procedure. For example, Butler (1977) and Wilson (1977) obtained for W03 and TiOz the values of Lp equal to 0.5 x 10-4 and 4 x 10-4 cm, respectively. 

J.H model is, indeed, the common behavior. Figure 1 reports the degradation rate of phenol [36], a poorly adsorbed compound, and that of CHBA [35], a strongly absorbed compound, as a function of their initial concentration. A peaked reaction rate is observed, in contrast to the saturative LH model, also for CHCI3 and dodecane (see Fig. 2 in Ref. 37). For the photocurrents measured in photoelectro-chemical oxidation experiments of methanol and salicylic acid on anatase film electrodes, a saturation curve for the poorly adsorbed methanol and a peak at an intermediate concentration for strongly adsorbed salicylic acid were also observed as a function of the substrate concentration [38]. 

With estimated ip(X ), the percentage of oxidation of halide ions (X ) can be determined as a ratio of iq to the total disk photocurrent measured ... 

Figure 2 shows that the photocurrent measured at 0.5 V increases with decreasing the oxidation potentials of the olefins examined however, benzylideneacetone, etc. with higher oxidation potentials did not show any photocurrent (32). 

W. Moritz, I. Gerhardt, D. Roden, M. Xu and S. Krause, Photocurrent measurements for laterally resolved interface characterization, Frese-nius J. Anal. Chem., 367(4) (2000) 329-333. 

When amphiphilic dyes are incorporated into such monolayers, their fluorescence properties can be used to characterize the organization of the array [84-87]. An amphiphilic cyanine dye used in this way showed a much lower distance damping factor (P value) between layers containing the donor cyanine and the acceptor viologen (P = 0.3) than is usually observed in fixed donor-acceptor systems [88]. An even smaller p value was obtained from photocurrent measurements in monolayer assemblies containing amphiphilic porphyrins (p = 0.005) [89], The former value was attributed to electron tunnelling occurring in parallel... 

Photocurrent measurements performed for unmodified P25 point to an ra-type semiconductor behavior. Calculated IPCE values for P25... 

Unfortunately, the redox potential of the Pt4 + /3+ couple is not known in literature. Although some stable Ptm compounds have been isolated and characterized (37), the oxidation state III is reached usually only in unstable intermediates of photoaquation reactions (38-40) and on titania surfaces as detected by time resolved diffuse reflectance spectroscopy (41). To estimate the potential of the reductive surface center one has to recall that the injection of an electron into the conduction band of titania (TH) occurs at pH = 7, as confirmed by photocurrent measurements. Therefore, the redox potential of the surface Pt4 + /3+ couple should be equal or more negative than —0.28 V, i.e., the flatband potential of 4.0% H2[PtClal/ TH at pH = 7. From these results a potential energy diagram can be constructed as summarized in Scheme 2 for 4.0% H2[PtCl6]/TH at pH = 7. It includes the experimentally obtained positions of valence and conduction band edges, estimated redox potentials of the excited state of the surface platinum complex and other relevant potentials taken from literature. An important remark which should be made here is concerned with the error of the estimated potentials. Usually they are measured in simplified systems - for instance in the absence of titania - while adsorption at the surface, presence of various redox couples and other parameters can influence their values. Therefore the presented data may be connected with a rather large error. 

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