The n -Type Photoanode

N-type semiconductor consists of electron donor elements and, hence, the Fermi level is higher. N-type photoanode, therefore, has higher Fermi level than that of the solution. The n-type photoanode when brought in contact with the solution, the... 

The ideal device for water-splitting process consists of an n-type semiconductor as photoanode and p-type semiconductor as photocathode (Fig. 2.5). The n-type photoanode consists of excess positive charge due to doping. As a result, the electrolyte solution will have excess negative charge after equilibrium with redox couple (O2/H2O) [42, 43]. 

Various other semiconductor materials, such as CdSe, MoSe, WSe, and InP were also used in electrochemistry, mainly as n-type photoanodes. Stability against photoanodic corrosion is, naturally, much higher with semiconducting oxides (Ti02, ZnO, SrTi03, BaTi03, W03, etc.). For this reason, they are the most important n-type semiconductors for photoanodes. The semiconducting metal oxide electrodes are discussed in more detail below. 

Direct splitting of water can be accomplished by illuminating two interconnected photoelectrodes, a photoanode, and a photocathode as shown in Figure 7.6. Here, Eg(n) and Eg(p) are, respectively, the bandgaps of the n- and p-type semiconductors and AEp(n) and AEF(p) are, respectively, the differences between the Fermi energies and the conduction band-minimum of the n-type semiconductor bulk and valence band-maximum of the p-type semiconductor bulk. lifb(p) and Utb(n) are, respectively, the flat-band potentials of the p- and n-type semiconductors with the electrolyte. In this case, the sum of the potentials of the electron-hole pairs generated in the two photoelectrodes can be approximated by the following expression ... 

The overall reaction of the photoelectrochemical cell (PEC), H2O + hv H2 -I- I/2O2, takes place when the energy of the photon absorbed by the photoanode is equal to or larger than the threshold energy of 1.23 eV. At standard conditions water can be reversibly electrolyzed at a potential of 1.23 V, but sustained electrolysis generally requires -1.5 V to overcome the impedance of the PEC. Ideally, a photoelectrochemical cell should operate with no external bias so as to maximize efficiency and ease of construction. When an n-type photoanode is placed in the electrolyte charge distribution occurs, in both the semiconductor and at the semiconductor-... 

At the n-type interface, the electric field generated causes photogenerated conduction band electrons to move into the bulk of the semiconductor, to the back metal contact, and into the external circuit. The valence band holes access the semiconductor interface due to the influence of the interfacial electric field (Fig. 28.2). Thus, redox species can be oxidized by the excited n-type semiconductor. These materials act as photoanodes. On the other hand, the electric field in a p-type material is reversed in potential gradient therefore, excited electrons move to the semiconductor surface, while holes move through the semiconductor to the external circuit (Fig. 28.2). These materials are photocathodes. The presence of an electric field at the semiconductor-electrolyte interface is usually depicted by a bending of the band edges as shown in Figure 28.2. Elec-... 

The Pt exhibits a reversible wave in the dark whereas the n-type Si exhibits no oxidation current unless illuminated with > E light (632.8 nm, 50 mWtcm2). The photoanodic peak is more negative than the anodic peak on Pt, reflecting the extent to which ferrocene... 

SCE the (EeCp2+/°)surf. ratio is typically >10 and the available oxidant is (FeCp2+)surf. Thus, any material B that is oxidizable with (FeCp2 ) in solution should be oxidizable with the n-type Si photoanode derivatized with I (10-16). N-type Si derivatized with I can be used in l O/electrolyte solution, unlike the naked n-type Si that is rapidly passivated by photoanodic growth of an oxide layer on the surface (10, 11, 12, 16). 

Consider a photo-assisted water electrolysis cell, incorporating a photoanode and dark metal cathode. Illumination of the n-type semiconductor photoanode with a depletion space charge region results in a net flow of positive vacancies, or holes, to the semiconductor/electrolyte interface. Here the hole (h+) may be accepted by the reduced form of the oxygen redox couple. 

As already explained (Section 10.3.1), it is the jt-doped semiconductors that provide cathodic electrons when irradiated with light of sufficient energy and the n-doped type semiconductors that yield the holes to act as photoanodes when semiconductors are used. In cells involving semiconductors, but driven by an outside power source rather than by incident photons, the situation is reversed the n-type emits electrons and thep-type receives them. 

Fig. 4. An interfacial energetic situation in a photoelectrolysis cell where the flat-band potential of the n-type semiconductor photoanode lies positive of the HER potential. 1W is the external bias potential needed in this case to drive the photoelectrolysis process.

The /- F behavior demonstrates that holes are involved in the anodic dissolution reactions the question arises, however, whether charge transfer occurs exclusively over the valence band. A straightforward way to investigate this problem is by comparing the number of photons absorbed in an n-type photoanode to the number of electrons flowing through the external circuit. This procedure has been used by Kohl et al. [33], and its results indicate a non-negligible contribution of the conduction... 

Ey is the photovoltage obtained for the derivatized n-type semiconductor photoanodes. We assume E° to be the values given in brackets and Ey is the extent to which the peak of the photoanodic current is more negative than E° under >Eg illumination. Data are from references given in (a). cWe assume E° to be the same on the n-type semiconductors as on metallic electrodes but these values have not been measured, since the n-type semiconductors generally are not reversible. 

The menace of photocorrosion compels to restrict efficiency of the regenerative PEC cells. In fact, to increase efficiency (through the increase in photopotential) one has to increase the initial (i.e., preexisting in the dark) band bending in the semiconductor. For this reason, the reversible potential (p° of the redox couple in the cell with, say, n-type photoanode should be as positive as possible. Yet, at the same time it should not exceed the decomposition potential for the semiconductor, (p°dec,p Thus, one is forced to deliberately diminish the cell photopotential. As a result, the cell efficiency and stability vary with the solution redox potential in an opposed way. In reality the cell characteristics always are a result of a compromise between the requirements of efficiency and stability. (Therefore, one should take cautiously higher values of efficiency and service life of PEC cells given in published papers, they might be measured not for the same cell.)... 

In fact cathodic protection is a well known electrochemical procedure to prevent slow oxidation. Thus p-type semiconductors do not suffer the magnitude of corrosion problems that n-type photoanodes do. 

Fig. 1.1 Band structure of an n-type photoanode water splitting device, (a) Illustrating the various processes of photon irradiation, electron-hole pair formation, charge transport, and interfadal reactions, (b) Illustrating the energetic requirements associated with the minimum thermodynamic energy to split water, catalytic overpotentials for the HER and OER half-reactions, and photovoltage...

Solutions should be sparged with the gaseous product of the photoelectrode (H2 for p-type photocathodes and O2 for n-type photoanodes) to produce a well-defined redox potential and consistent Schottky barrier height. 

Fig. 6.7 Band diagram of a n-type photoanode at (a) flat-band potential, (b) a potential sufficient to separate charge carriers and drive photocurrent, and (c) large reverse bias potential sufficient to saturate the photocurrent response. The corresponding hypothetical j-V curve is...

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. 

Now we understand the structure of the semiconductor/electrolyte interface, we can draw a detailed band diagram for a complete PEC cell. An example is shown in Fig. 2.16 for a cell composed of a n-type photoanode and a metal cotmter electrode. As usual, the y-axis represent the energy of an electron at a certain point x in the cell. The energy of an electron in vacuum at infinite distance is chosen as a reference. It is important to note that the vacuum level bends in the presence of an... 

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