Semiconductor photoanode

Diaz AF, Logan JA(1980)Electroactive polyanihne films. JElectroanalChem 111 111-114 Noufi R, Nozik AJ, White J, Warren LF (1982) Enhanced stability of photoelectrodes with electrogenerated polyanUine films. J Electrochem Soc 129 2261-2265 Noufi R, Tench D, Warren LE (1981) Protection of semiconductor photoanodes with photoelectrochemicaUy generated polypyrrole films. J Electrochem Soc 128 2596-2599 Jaeger CD, Fan FRF, Bard AJ (1980) Semiconductor electrodes. 26. Spectral sensitization of semiconductors with phthalocyanine. J Am Chem Soc 102 2592-2598 Gerischer H (1977) On the stability of semiconductor electrodes against photodecomposition. J Electroanal Chem 82 133-143... 

Fig. 20. Energy level diagram for a semiconductor photoanode (e.g. SrTiOj) doped with Cr(III). KB valence band CB conduction band...

Fig. 21. Energy level diagram for a semiconductor photoanode sensitized by a dye (D) layer. The black arrow indicates the optical transition on the dye. The semiconductor is on the left-hand side...

Fig. 5.62 Scheme of a photovoltaic cell with n-semiconductor photoanode... 

One additional problem at semiconductor/liquid electrolyte interfaces is the redox decomposition of the semiconductor itself.(24) Upon Illumination to create e- - h+ pairs, for example, all n-type semiconductor photoanodes are thermodynamically unstable with respect to anodic decomposition when immersed in the liquid electrolyte. This means that the oxidizing power of the photogenerated oxidizing equivalents (h+,s) is sufficiently great that the semiconductor can be destroyed. This thermodynamic instability 1s obviously a practical concern for photoanodes, since the kinetics for the anodic decomposition are often quite good. Indeed, no non-oxide n-type semiconductor has been demonstrated to be capable of evolving O2 from H2O (without surface modification), the anodic decomposition always dominates as in equations (6) and (7) for... 

A common photoelectrolysis cell structure is that of a semiconductor photoanode and metal cathode, the band diagrams of which are illustrated in Fig. 3.15 together with that of electrolyte redox couples. In Fig. 3.15(a) there is no contact between the semiconductor anode and metal cathode (no equilibrium effects communicated through the electrolyte). As seen in Fig. 3.15(b), contact between the two electrodes (no illumination) results in... 

An approach similar to this avoids the use of a comparative nohle metal electrode and neglects overpotential losses at the electrodes. In this method, the potential applied at the hydrogen (or oxygen) electrode (in a three electrode configuration) is compared with the potential generated at an ideal fuel cell anode (or cathode). In the case of a n-type semiconductor photoanode ... 

Another form of this definition [equation (3.6.15)] has sparked much debate in the scientific community [121-124]. In this approach Vapp (or Vbias) is taken as the absolute value of the difference between the potential at the working electrode measured with respect to a reference electrode (Vmeas) and the open circuit potential (Voc) measured with respect to the same reference electrode under identical conditions (in the same electrolyte solution and under the same illumination). In the case of a semiconductor photoanode where oxygen evolution takes place the efficiency is calculated as ... 

This chapter considers the fabrication of oxide semiconductor photoanode materials possessing tubular-form geometries and their application to water photoelectrolysis due to their demonstrated excellent photo-conversion efficiencies particular emphasis is given in this chapter to highly-ordered Ti02 nanotube arrays made by anodic oxidation of titanium in fluoride based electrolytes. Since photoconversion efficiencies are intricately tied to surface area and architectural features, the ability to fabricate nanotube arrays of different pore size, length, wall thickness, and composition are considered, with fabrication and crystallization variables discussed in relationship to a nanotube-array growth model. 

To compare quantitatively the current-voltage characteristic of an illuminated electrode, given by formula (31), with experimental data, Butler (1977) and Wilson (1977) measured the photocurrent, which arises in a cell with an n-type semiconductor photoanode ( 2, W03) when irradiated with monochromatic light at a frequency satisfying the condition ha>> Eg. In this case a light-stimulated electrochemical reaction of water oxidation with oxygen evolution... 

Inoue, T., Watanabe, T., Fujishima, A., and Honda, K., Competitive Oxidation at Semiconductor Photoanodes, in Semiconductor Liquid--Junction Solar Cells, Heller, A., Ed., The Electrochemical Society, Princeton, N3, 1977, 210. 

Of particular interest in this context has been the finding that the Kolbe reaction, the anodic oxidation of carboxylic acids (Equation 1) (2), can be made to occur at n-type oxide semiconductor photoanodes to the virtual exclusion of oxygen formation (3,4,5). 

Figure 4. Schematic for the photoelectrochemical simulation of the photosynthetic electron-pumping processes ("upper sketch by means of a Chl-semiconductor photoanode and a Chl-metal photocathode...

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. 

The arguments just presented suggest that all n-type semiconductor photoanodes which resist corrosion under conditions of H2O to O2 oxidation must have (a) stable elemental component(s) at low pH. Moreover, since the oxide dissolution reaction is associated with essentially the same H+/0= interactions, the low pH stability limit will be at approximately the same value (P stable 3). 

A photoelectrochemical (photoelectrolysis) system can be constructed using a n-type semiconductor electrode, a p type semiconductor, or even mating n- and p-type semiconductor photoelectrodes as illustrated in Figs. 2a c respectively. In the device in Fig. 2a, OER occurs on the semiconductor photoanode while the HER proceeds at a catalytic counterelectrode (e.g., Pt black). Indeed, the classical n-Ti02 photocell alluded to earlier,53 57 belongs to this category. Alternately, the HER can be photo-driven on a p type semiconductor while the OER occurs on a "dark" anode. 

Next, we define an ideal semiconductor photoanode and photocathode for the solar electrolysis of water. We also briefly examine real world issues related to charge-transfer kinetics at semiconductor/electrolyte interfaces and the need for an external bias to drive the photolysis of water. 

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 p-n photoelectrolysis approach,60 on the other hand, simply combines a n-type semiconductor photoanode and a p-type semiconductor photocathode in an electrolysis cell (Fig. 2c). The pros and cons of this twin-photosystem approach (which mimicks plant photosynthesis) were enumerated earlier in this Chapter (see Section 2). Table 16 provides a compilation of the semiconductor photocathode and photoanode combinations that have been examined. Reference 67 may also be con suited in this regard for combinations involving n WSe2, n MoSe2, n WS2, n TiCH, p InP, p GaP and p Si semiconductor electrodes. 

Table 16. Photoelectrolysis cells using n-type semiconductor photoanodes and p-type semiconductor photocathodes.

R. Noufi, D. Tench, and L. E. Warren, Protection of semiconductor photoanodes with photoelectro-chemically generated polypyrrole films, J. Electrochem. Soc. 128, 2596, 1981. 

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. 

Figure 11. Energy diagram of an illuminated photocell with an n-type semiconductor photoanode for water splitting.

Next, photogeneration of electron-hole pairs leads to the formation of quasi-levels of minority and majority carriers, Fp and F , as shown in Fig. 12. Since, at the surface, Fp < Fs -/sl and F > Fs -/sl, illumination results in the acceleration of both forward and reverse reactions in a sulfide polysulfide couple. If the circuit is closed on an external load R, the anodic and cathodic reactions become separated the holes are transferred from the semiconductor photoanode to the solution, so that ions are oxidized to 82 , and the electrons are transferred through the external circuit to the metal counterelectrode (cathode) where they reduce S2 to The potential difference across a photocell is iphR, where iph is the photocurrent, and the power converted is equal to /phF. 

---