Potential Hatband

McCann JE, Pezy J (1981) The measurement of the Hatband potentials of n-type and p-type semiconductors by rectified alternating photocurrent voltammetry. J Electrochem Soc 128 1735-1740... 

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. 

Another example for the same kind of effect is n-RuS2 studied extensively by Kuhne and Tributsch (1986). For this material (bandgap 1.3 eV) the valence band is located in the dark at about 0 eV (SCE). The bands are shifted downwards by as much as 2 eV upon illumination. Interestingly, the photocurrent resulting in 02 evolution occured around +1.1 V(SCE), the valence band being located, however, at +2.3 eV, i.e. considerably below the E0(H20/02)-value. The bandedge movement is seen best in capacity measurements which give Hatband potentials as shown in Fig. 7. 

Unpinning of band edges at the semiconductor/electrolyte interface is understood as a common phenomenon for n- and p-type materials. Thus, the band edge positions as obtained from Hatband potential measurements in the dark, cannot be taken as a fixed value for the interpretation of charge transfer processes. More investigations in this direction are necessary. 

Under Ti02 electrode polarization slightly anodic from the Hatband potential (Vft), a cathodic current superimposed to the anodic photocunent (transient behaviour) can be observed (Fig. 1). This catohdic effect is attributed to the recombination with 6cb of holes trapped at surface species (mainly OH° radicals and H202 molecules) photogenerated at intermediate steps of oxygen evolution (Salvador, 1985). 

Finally, it should be mentioned that frequently, as in the case of Ti02, a frequency dispersion of the slope of the Mott-Schottky curves has been observed (see e.g. [67,68]), although the Hatband potential was not affected. Modern methods, such as impedance spectroscopy, have shown, however, that this frequency dispersion is an artifact [59]. 

Oskam et al. [82] have used IMPS to investigate the role of surface states at the n-Si(l 1 1)/NH4F interface. In this case, the redox reaction is simpler, and appears not to involve holes trapped at surface states. This is probably due to the presence of a surface oxide layer. However, electron transfer is evidently exceptionally slow in this case, since these authors observed a modulated photocurrent even at potentials far from the Hatband potential where recombination is expected to be negligible. Accumulation of holes modifies the potential drop across the Helmholtz (and presumably also surface oxide region), leading to a capacitive charging current. This effect has also been treated by Peter et al. in more detail [89]. 

Fig. 4.3 Challenges presented by hematite at left is an energy diagram showing a typical value of the Hatband potential, Vp, of n-type hematite, and the PEC water-splitting operation of a hematite photoanode under illumination and with an applied external bias, Vf,. The right graph (from [53], with permission) shows the quantum efficiency vs. wavelength for Nb-doped and Ge-doped hematite single crystals at 0 V vs. SCE in 1 M NaOH (1.06 V vs. RHE). The efficiency is expressed in units of percent...

Chane-Tune, J., Petit, J.-R, Szunerits, S., Bouvier, R, Delabouglise, D., Marcus, B., Mermoux, M. Using scanning electrochemical microscopy to determine the doping level and the Hatband potential of boron-doped diamond electrodes. ChemPhysChem 2006, 7, 89-93. 

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