Photoanodes, n-type

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. 

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-... 

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... 

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... 

Figure 3.9a shows a linear sweep voltammogram of a 0.2% Si-doped a-Fe203 photoanode deposited on FTO glass. At potentials more positive than 1.7 Vrhe> the dark current starts to increase, indicating the electrochemical oxidation of water at the surface. This is due to an increase in the concentration of free holes in the valence band as the Fermi level is pulled down (more positive potential) and starts to approach the valence band in the region near the surface. A positive photocurrent is observed upon illumination, as indeed expected for an n-type photoanode. 

Figure 18.3 Photoassisted electrolysis using an n-type photoanode. A bias voltage is used to raise the Fermi level of the cathode above the reversible hydrogen potential by an amount corresponding to the cathodic overpotential t Q. The biiLS also induced a band bending qA.4) in the space charge region of the photoanode. The broken lines show the quasi-Fermi levels for holes and electrons (see text for details).

If either an n-type photoanode or a p-type photocathode is used in conjimction with a metal counterelectrode and a reversible redox electrolyte (e.g., Fe(CN)6 ), we have the basis for a regenerative photoelectrochemical cell. Alternately both an n-type and a p-type semiconductor may be used in tandem in a twin-photoelectrode geometry for the cell, much like what plants do in photosynthesis (For example [13]). Note that in these case there is no net chemistry occurring in the electrolyte phase in response tophotoexcitation, i.e., what is photooxized (or photoreduced) at one terminal is re-reduced (or re-oxidized) back at the other. The result is conversion or transduction of photon energy to electrical energy. 

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]. 

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