Flat-Band Potential From Photocurrent Onset

7 Flat-Band Potential From Photocurrent Onset 

To determine the Eg, the j-V data (considering only the photocurrent, Jptd is plotted in a photocurrent squared versus potential (/ph vs. V) [26]. The linear portion closest to the onset of photocurrent is fitted to a linear line. The x-intercept 

The dependence of Efb as a function of pH can be assessed, and if the pH dependence is —59 mV/pH, then it is a Nemstian dependence, which usually indicates that the surface of the semiconductor may be protonated or hydroxylated depending on the concentration of H and OH [28]. It is useful to compare the pH dependence of (b with the pH dependence of the redox potential of hydrogen and oxygen evolution to determine if the band edges of the semiconductor are aligned to photoelectrochemicaUy split water at any pH. 

Memming, Physical chemistry of semiconductor-liquid interfaces. J. Phys. Chem. 100, 13061-13078 (1996) 

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As shown in Fig. 10-9, the photoexcited reaction current occurs only when an appreciable electric field exists in the space chai ge layer. No photocurrent occurs at the flat band potential because no electric field that is required to separate the photoexcited electron-hole pairs is present. The photocurrent occurs at any potentials different from the flat band potential hence, the flat band potential may be regarded as the potential for the onset of the photocurrent. It follows, then, that photoexcited electrode reactions may occur at potentials at which the same electrode reactions are thermodynamically impossible in the dark. 

As shown in Fig. 10-18, the flat band potential that characterizes the onset potential of photocurrent shifts from the dark flat band potential Em to the photoexcited flat band potential Eukpu as photoexdtation continues. 

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. 

Clearly, analytical solutions for cases (1) and (2) are no longer practicable, but numerical solutions can be obtained and some examples are shown in Fig. 75 for a species that can be oxidised both from the valence band and from the surface state [151]. One interesting feature is that the dependence of the photocurrent onset on (f>0 is much reduced another effect that can be observed is that, if there is a redox couple present in solution that can only be oxidised via surface states, the rise of the photocurrent from flat-band potential becomes very sensitive to the value of ksC2 as shown in Fig. 76. 

Interestingly, iron oxide films prepared by spray pyrolysis of iron nitrate solutions containing 0.05 M titanium tetrachloride were reported to have a band gap of 1.27 eV, which would presumably be too low for solar hydrogen production for the reasons stated above. Indeed, the films prepared with this level of titanium displayed the poorest photocurrents under illumination from 150 mW cm of a Xe arc lamp. Kumari et al. [61] noticed that as the concentration of titanium present in the precursor solution increased, the flat band potential and the photocurrent onset moved to a more cathodic potential. This is in general agreement with Glasscock et al. [68]. 

It is important to determine the conductivity and flat-band potential ( ft) of a photoelectrode before carrying out any photoelectrochemical experiments. These properties help to elucidate the band structure of a semiconductor which ultimately determines its ability to drive efficient water splitting. Photoanodes (n-type conductivity) drive the oxygen evolution reaction (OER) at the electrode-electrolyte interface, while photocathodes (p-type conductivity) drive the hydrogen evolution reaction (HER). The conductivity type is determined from the direction of the shift in the open circuit potential upon illumination. Illuminating the electrode surface will shift the Fermi level of the bulk (measured potential) towards more anodic potentials for a p-type material and towards more cathodic potentials for a n-type material. The conductivity type is also used to determine the potential ranges for three-electrode j-V measurements (see section Three-Electrode J-V and Photocurrent Onset ) and type of suitable electrolyte solutions (see section Cell Setup and Connections for Three- and Two-Electrode Configurations ) used for the electrochemical analyses. 

Next, if process 1) occurred and the surface HO radicals were produced, the radicals should give rise to a considerable amount of photocurrent even in the absence of alcohols. In this context, it is to be noted that the onset potential of the photocurrent (U j ) in acidic solutions (in the absence of alcohols) is largely (about 0.5 V) more positive than the flat-band potential (U (Figure 1), whereas the in neutral and alkaline solutions lies much closer to (12,14,18), The latter fact is explained by taking account of the formation of easily oxidized surface species such as Ti-0 in the neutral and alkaline solutions, followed by their oxidation by the holes. This implies, if considered in the opposite way, that the surface Ti-OH species in acidic solutions is difficult to be oxidized. In other words, if process 1) is assumed to occur, it is quite difficult to explain the large pH-dependence of the deviation of from U. ... 

Fig. 9 Current-voltage curves showing the shift of the onset of photocurrent from the flat-band potential (VfbK...

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