Illumination, chopped

C60 has been used to produce solvent-cast and LB films with interesting photoelec-trochemical behavior. A study of solvent-cast films of C60 on Pt rotating disc electrodes (RDEs) under various illumination conditions was reported [284]. Iodide was used as the solution-phase rednctant. The open-circuit potential shifted by 74 mV per decade of illumination intensity from a continuous wave (cw) argon-ion laser. The photocurrent versus power was measured at -0.26 V under chopped illumination (14-Hz frequency, vs. SCE) up to 30 mW cm and was close to linear. The photoexcitation spectrum (photocurrent versus wavelength) was measured at 0.02 V (vs. SCE) from 400 to 800 mn and found to be... 

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.)... 

Another point subject to criticism from this work is the apparent absence of DC photocurrents upon constant illumination. Although the responses in Fig. 13(c) appear effectively in phase at the chopping frequency, it is possible that slow photoinduced perturbation of accumulated charge at the interface can be connected with these photoresponses, e.g., photoionization of the dye [72]. In this respect, clearer evidence of photo-induced ET was introduced by Brown et al. for the reaction [49]... 

FIG. 14 On-off photocurrent responses (a) associated with the reaction in Eq. (41) at Ao0 = —0.225 V. In this figure, positive currents correspond to the transfer of a negative charge from water to DCE. The potential dependence of the photocurrent (b) was obtained under chopped illumination and lock-in detection. The maximum in the photocurrent-potential curve contrasts with the small changes in the dark current shown in (c). These responses are developed within the polarizable window described in (d). (From Ref. 49. Reproduced by permission of The Royal Society of Chemistry.)... 

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-... 

Photocurrent analysis under chopped illumination and lock-in detection is largely complementary to IMPS. While the former provides a simple approach for studying the dependence of the photocurrent on applied potential or illumination wavelength (see examples in Figs. 13, 14, and 16), the latter allows reliable kinetic analysis as a function... 

The first estimations of for photoinduced processes were reported by Dvorak et al. for the photoreaction in Eq. (40) [157,158]. In this work, the authors proposed that the impedance under illumination could be estimated from the ratio between the AC photopotential under chopped illumination and the AC photocurrent responses. Subsequently, the faradaic impedance was calculated following a treatment similar to that described in Eqs. (22) to (26), i.e., subtracting the impedance under illumination and in the dark. From this analysis, a pseudo-first-order photoinduced ET rate constant of the order of 10 to 10 ms was estimated, corresponding to a rather unrealistic ket > 10 M cms . Considering the nonactivated limit for adiabatic outer sphere heterogeneous ET at liquid-liquid interfaces given by Eq. (17) [5], the maximum bimolecular rate constant is approximately 1000 smaller than the values reported by these authors. 

Electron injection has been observed during the chemical dissolution of an oxide film in HF [Mai, Ozl, Bi5]. The injected electrons are easily detected if the anodized electrode is n-type and kept in the dark. Independently of oxide thickness and whether the oxide is thermally grown or formed by anodization, injected electrons are only observed during the dissolution of the last few monolayers adjacent to the silicon interface. The electron injection current transient depends on dissolution rate respectively HF concentration, however, the exchanged charge per area is always in the order of 0.6 mC cm-2. This is shown in Fig. 4.14 for an n-type silicon electrode illuminated with chopped light. The transient injection current is clearly visible in the dark phases. 

Fig. 4.14 Anodic potential scan (50mVs ) of an n-type Si electrode in 1% HF under con stant (bold broken line) and chopped illumination (solid line, illumination intensity corresponding toJP). Point A Current transients are minimal belowJPS. Points B, C Current...

Fig. 9.9 Current-voltage characteristics under chopped illumination (= AMI) of a nanocrystalline (4—5 nm) CdSe film, deposited by CD, in a polysulphide electrolyte. The two characteristics are for as-deposited CdSe (top) and after etching with dilute HCl (bottom). (After Ref. 67.)...

Fig. 9.10 Chopped illumination (100 mW-cm ) current-voltage characteristics of a nanocrystalline PbSe fihn deposited from a citrate/selenosulphate bath at 60°C. The electrolyte is the original solution from which the film was deposited. (After Ref. 83.)...

Figure 2. Current-voltage characteristics of the cell cond. glass/Pblg + RbAgJs/ RbAgsls/Ag + RbAgJg/cond. glass under chopped illumination (200 mW/cm2) at 25°C (voltage scan rate 25 mV/s)...

Current-potential measurements, in the dark and under illumination of the semiconductor working electrode, are extremely useful for first defining the charge-transfer behavior across the interface before more sophisticated experiments are undertaken. The irradiation can be either continuous or intermittent (chopped) the latter mode has the distinct advantage that both the dark and light behavior can be examined in the same scan [55, 58]. Even some dynamic information can thus be extracted under the nominally steady-state conditions typical of a cyclic or linear potential sweep experiment. Another useful steady-state experiment is photocurrent spectroscopy (performed at a fixed DC potential) [55], although this can also be dynamically performed via IMPS (see below). Such measurements not only yield the so-called photoaction spectrum of the semiconductor electrode, but also afford information on surface recombination and surface state activity at the interface as discussed below. 

Photovoltage spectra are measured at open circuit using chopped light of low intensity. It might appear that an advantage of photovoltage spectroscopy over photocurrent spectroscopy is that no photocorrosion occurs. However, this is not necessarily correct, because anodic photocorrosion in the illuminated areas may be balanced by cathodic reduction of solution species such as oxygen or protons. 

In practice, the incident illumination is chopped or intensity-modulated to enhance the detection of the photoinduced microwave conductivity (PMC) signal, so that the measurements are not strictly steady state. However, the chopping frequency is normally much lower than the characteristic frequency associated with the relaxation... 

Figure 3. High-throughput photocurrent screening of the WO3 library at zero bias, (a) Photocurrent trends in the library, (b) Cyclic voltammogram of pure WO3 (0.2 cm ) under chopped illumination from a 150 W Xe lamp (2.3 mW/cm ).

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