Photocurrents onset potential

The (photo)electrochemical behavior of p-InSe single-crystal vdW surface was studied in 0.5 M H2SO4 and 1.0 M NaOH solutions, in relation to the effect of surface steps on the crystal [183]. The pH-potential diagram was constructed, in order to examine the thermodynamic stability of the InSe crystals (Fig. 5.12). The mechanism of photoelectrochemical hydrogen evolution in 0.5 M H2SO4 and the effect of Pt modification were discussed. A several hundred mV anodic shift of the photocurrent onset potential was observed by depositing Pt on the semiconductor electrode. 

The photocurrent onset potential is often taken as the flatband potential, since the measurement of the flatband potential is typically only good to 100 mV and the onset of photocurrent is often observed with less than 100 mV of band bending. This practice is dangerous, however, since the onset potential is actually the potential at which the dark cathodic current and the photoanodic current are equal. Even though in the case of the p-GaP illustration, the observation of an anodic current and a photocathodic current are separated by several hundred millivolts, in many systems these two currents overlap. In those cases, the relationship between the flatband potential and the onset potential becomes unclear. 

Below, we discuss one more (photoelectrochemical) method for determination of the flat-band potential [40, 172]. The flat-band potential can be determined (i) as the photocurrent onset potential Eomsi (ii) from the dependence of electrode open-circuit photopotential Eocph on light intensity J, as the limiting value of Eoc at a sufficiently high J and (iii) by extrapolating, to zero photocurrent, the potential dependence of the photocurrent jvh squared (see Section 7). These methods are based on the concept... 

To compare these methods, in Fig. 40 we show (a) EIX vs. J and (b) /Ph vs. E plots for a CVD single crystal thin-film electrode. We see that with increase in illumination intensity J, the open-circuit potential E(X approaches a limit of 0.7 V, which is close to the photocurrent onset potential (0.75 V). [The photocurrent density squared vs. potential dependence for this electrode, although far from linear (unlike that of Fig. 38), by the extrapolation to yph -> 0 gives the potential value of approx. 0.65 V.] It is concluded that, on the whole, methods (i) and (ii) are in a good agreement and can be used in the determination of the flat-band potential. Similar results were obtained with HTHP single crystals. 

In I/E curves the onset of photocurrent is expected from classical theories to occur near the Hatband potential as measured in the dark (Efb (d)), i.e. where the majority carrier current starts too. However, a large shift of the onset potential is seen especially if no additional redox couple is present in the aqueous electrolyte, in cathodic direction for p-, in anodic direction for n-type materials (Fig. 1). This shift depends on the light intensity but saturates already at relatively low intensities (Memming, 1987). If minority carrier acceptors (oxidants for p- and reductants for n-type semiconductors) are added to the solution, the onset can be shifted back to Efb (d) if they have the appropiate redox potential. In principal two types of redox couples can be found those which lead to a shift of the photocurrent onset potential and those which don t. The transition between the two classes occurs at a specific redox potential. 

Adding halides to the electrolyte, the shift of bands under illumination and thereby the photocurrent onset potential decreased in accordance with the change in the redoxpotential of the halide couple used. The results can only be understood in terms of Fermi level pinning by the redox couple about 0.5 to 0.8 V below... 

The photocurrent in nonfluoride solutions is affected by the amount of preanodic current passed through the sample as shown in Fig. 5.10. It is also seen that the photocurrent onset potential is shifted to more anodic values with formation of an oxide film and the amount of shift is related to the thickness of the film. This shift is due to the potential drop across a growing oxide layer and is one of the reasons for the difference between the photocurrent onset potential and the flatband potential. ... 

In the absence of surface recombination and with a fast rate of electron transfer, the photocurrent increases with increasing potential when the depletion layer starts to form and a saturation current is quickly reached. On the other hand, with a fast surface recombination or in the case of slow electron transfer reactions, the apparent onset of the photocurrent is shifted to higher bias and the saturation current is only reached at larger band bending. ° " " Among other factors, surface treatment strongly affects the photocurrent onset potential due to its effect on surface states which determine the recombination process. °° " " ... 

M. Handschuh, W. Lorenz, C. Adgerter, and T. Katterle, Determination of the kinetics of photoelec-trochemical processes with minority carriers from photocurrent onset potential on semiconductor electrodes, J. Electroanal. Chem. 144, 99, 1983. 

The combined evidence of an earlier photocurrent onset potential relative to the whole thylakoid membranes and its well defined peak in addition to the voltammograms negative slope at the potential... 

When illuminated with energy equal to or above the band gap hv > Eg) at these operating potentials, minority hole carriers in n-type electrodes drive the OER at the electrode-electrolyte interface while minority electron carriers in p-type electrodes drive the HER at this interface. The potential at which this phenomenon begins to occur is the photocurrent onset potential (Eonset). which is offset relative to the flat-band potential (Efb) by the required kinetic overpotentials for the reaction of interest. The difference between the photocurrent onset potential (Eonset) and the reversible redox potential of interest (E°) is the onset voltage (Eonset)- A band diagram of a n-type photoanode and its hypothetical j-V response is shown in Fig. 6.7. 

To determine Edark-onset. an initial sweep should be performed without illumination. This sweep should be interrupted when dark current becomes significantly large. A second sweep is performed to determine the photocurrent onset potential under illumination between Edark-onset and the illuminated OCP. The potential at which the direction of the photocurrent changes (i.e. from anodic to cathodic for n-type materials) is the photocurrent onset potential. 

Photocurrent onset potential The potential at which there is an onset of photocurrent. 

Several techniques can be used to determine the flatband potential of a semiconductor. The most straightforward method is to measure the photocurrent onset potential, ( onset- At potentials positive of (/>fb a depletion layer forms that enables the separation of photogenerated electrons and holes, so one would expect a photocurrent. However, the actual potential that needs to be applied before a photocurrent is observed is often several tenths of a volt more positive than ( fb- This can be due to recombination in the space charge layer [45], hole trapping at surface defects [46], or hole accumulation at the surface due to poor charge transfer kinetics [43]. A more reliable method for determining ( fb is electrolyte electroreflectance (EER), with which changes in the surface free electron concentration can be accurately detected [47]. The most often used method, however, is Mott- chottky analysis. Here, the 1/ Csc is plotted as a function of the applied potential and the value of the flatband... 

The photocurrent onset potential is at 0.95 V vs. RHE. This is much more positive than the flatband of a-FeaOs, which is at 0.3 Vrhe [34, 35]. The difference is due to slow oxidation kinetics (catalysis) [36] and/or to recombination in the space... 

The first surface challenge can be addressed by strong oxidation conditions [46] and careful hematite preparation, but the slow water oxidation kinetics are probably intrinsic to hematite. Nevertheless, methods have recently been found to increase the oxidation rate and thus reduce the overpotential. For example, the water oxidation by cobalt has been extensively studied and is known to be particularly rapid [114]. Indeed the treatment of Fe203 photoanodes (prepared by APCVD) with a monolayer of Co " resulted in a ca. 0.1 V reduction of the photocurrent onset potential [105]. Since this treatment also increased the plateau photocurrent it is good evidence that the reaction rate was increased, and the Co " did not just fill surface traps. Following the report of a remarkably effective cobalt-phosphate (Co-Pi)- based water oxidation catalyst [115], the overpotential was reduced even further on hematite photoanodes by Gamelin and coworkers [116]. Their results are shown in Fig. 4.11. 

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