Light illumination, chopped

The mechanism of photoreduction of CO2 and water at the p-type GaP in contact with aqueous solutions has been studied with PCS under chopped light illumination [748]. Results show a photo-anodic sub-bandgap response and, as a consequence, a photo-cathodic current. 

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-... 

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. 

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. 

As mentioned previously, the dark and illuminated behavior of an electrode can be characterized in a single sweep by chopping the light at a regular frequency. This is presented in Fig. 6.11, which shows the photoresponse of a Fe203-based photoelectrode exposed to periodic illumination. 

In this experiment, the photocurrent (/ph) of the sample is measured versus time (chronoamperometry) with or without an applied bias. In this case, monochromatic light is used over the solar UV-VIS-NIR spectrum (300-1100 nm). Figure 7.1 shows the experimental setup, which consists of an illumination source with a monochromator and a set of filters for order sorting. A mechanical shutter to chop... 

Equipment modifications are needed for a white light bias (see optional equipment in Fig. 7.1). The experiment requires a low noise broadband DC illumination, while simultaneously illuminating the sample with chopped ( AC) monochromatic light. The waveform of the chopped monochromic light must be monitored to verify that the chopping speed is low enough for the sample response to reach steady state. The bias level should be set to 37 % of the expected j c [2-4]. 

Fig. 8.1 Chopped-light 2-electrode j-V curve for a GaInP2/GaAs tandem cell with a Ru02 counter electrode in 3 M H2SO4 under AM 1.5 G illumination...

The fiber-optic spectrometer used for this work is shown in Figure 2. The illumination block consists of a 5-watt quartz-halogen lamp whose output is chopped at 30 Hz and imaged onto the excitation optical fiber. The UV wavelengths are removed from the excitation light with a long-wavelength pass... 

Fig. 3.10 (a) Voltammogram for a spray-deposited BiV04 photoanode rat FTO glass under chopped AM 1.5 illumination, (b) Current vs. time curve for BiV04 under high-intensity illumination with 364 nm light from a continuous-wave Ar laser at a potential of 1.23 Vrhe. In both cases a 0.15 M K2SO4 aqueous electrolyte solution was used... 

In photothermal spectroscopy, a sensitive fast response thermistor is attached to the back of the test electrode with silver epoxy (to achieve a good thermal contact), whilst an identical thermistor is placed in the bulk of the solution [27]. These thermistors are then configured as two branches of a balanced Wheatstone bridge and the electrode is illuminated with a chopped monochromatic light beam. The small periodic changes in the electrode temperature are then detected... 

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