Photoelectrolysis

As already mentioned, the photoelectrolysis of water into hydrogen and oxygen has been an objective of many researchers. The work done since the early 1970s has been reviewed by many authors [4,12, 63-67]. Several approaches to photoelectrolysis are 

A second possible approach, based on semiconductor-liquid junctions, is to adsorb dye molecules onto the semiconductor surface which, upon light absorption, will inject electrons (into n-type semiconductors) or holes (into p-type semiconductors) from the excited state of the dye molecule into the semiconductor. In principle, the photo-oxidized (or photoreduced) dye can then oxidize (reduce) water, and the complementary redox process can occur at the counter electrode in the cell. However, this approach has never been demonstrated experimentally. 

A third approach is to use solid state p-n or Schottky junctions to produce the required internal fields for efficient charge separation and the production of a sufficient photovoltage to decompose water. The solid state photovoltaic structure could be external to the electrolysis device, or it could be configured as a monolithic structure and simply immersed into aqueous solution. 

RUO2 has been deposited. The relevant experiments usually failed, either because the semiconductor was not photostable or the energetic requirements were not fulfilled (for details see, e.g., [14, 27]). 

There are some further aspects which must be considered when photochemical diodes are used. When a metal (catalyst) is deposited on a semiconductor, then frequently a Schottky barrier instead of an ohmic contact is formed at the semiconductor-metal interface. In this case, the latter junction behaves as a photovoltaic system by itself, which may determine or essentially change the properties of the photochemical diode. The consequences have been discussed in detail in [14, 27]. Frequently, colloidal semiconducting particles have been used, their size being much smaller than the thickness of the space charge region expected. 

There are several significant differences between these two systems, as follows  

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The reason for the exponential increase in the electron transfer rate with increasing electrode potential at the ZnO/electrolyte interface must be further explored. A possible explanation is provided in a recent study on water photoelectrolysis which describes the mechanism of water oxidation to molecular oxygen as one of strong molecular interaction with nonisoenergetic electron transfer subject to irreversible thermodynamics.48 Under such conditions, the rate of electron transfer will depend on the thermodynamic force in the semiconductor/electrolyte interface to... 

The prediction of which semiconductor materials are suitable as electrodes for a given photoelectrolysis reaction would be straightforward if the energy position of the bands at the electrode surface remained fixed with respect to the solution redox levels and independent of the redox species in the solution. Gerischer... 

Tomkiewicz M, Woodall J (1979) Photoelectrolysis of water with semiconductor materials. J Electrochem Soc 124 1436-1440... 

Fornaiini L, Nozik AJ, Parkinson BA (1984) The energetics of p/n photoelectrolysis cells. J Phys Chem 88 3238-3243... 

Mavroides JG, Tchemev DI, Kafalas JA, Kolesar DP (1975) Photoelectrolysis of water in cells with Ti02 anodes. Mater Res Bull 10 1023-1030... 

Kiihne HM, Tributsch H (1986) Energetics and dynamics of the interface of RuS2 and implications for photoelectrolysis of water. J Electroanal Chem 201 263-282 Kiihne HM, Jaegermann W, Tributsch H (1984) The electronic band character of Ru dichalcogenides and its significance for the photoelectrolysis of water. Chem Phys Lett 112 160-164... 

FIGURE 29.5 Energy diagram for water photoelectrolysis with a TiOj anode. 

The photoelectrolysis of H2O can be performed in cells being very similar to those applied for the production of electricity. They differ only insofar as no additional redox couple is used in a photoelectrolysis cell. The energy scheme of corresponding systems, semiconductor/liquid/Pt, is illustrated in Fig. 9, the upper scheme for an n-type, the lower for a p-type electrode. In the case of an n-type electrode the hole created by light excitation must react with H2O resulting in 02-formation whereas at the counter electrode H2 is produced. The electrolyte can be described by two redox potentials, E°(H20/H2) and E (H20/02) which differ by 1.23 eV. At equilibrium (left side of Fig. 9) the electrochemical potential (Fermi level) is constant in the whole system and it occurs in the electrolyte somewhere between the two standard energies E°(H20/H2) and E°(H20/02). The exact position depends on the relative concentrations of H2 and O2. Illuminating the n-type electrode the electrons are driven toward the bulk of the semiconductor and reach the counter electrode via the external circuit at which they are consumed for Hj-evolution whereas the holes are dir tly... 

The electrochemical cell can again be of the regenerative or electrosynthetic type, as with the photogalvanic cells described above. In the regenerative photovoltaic cell, the electron donor (D) and acceptor (A) (see Fig. 5.62) are two redox forms of one reversible redox couple, e.g. Fe(CN)6-/4 , I2/I , Br2/Br , S2 /S2, etc. the cell reaction is cyclic (AG = 0, cf. Eq. (5.10.24) since =A and D = A ). On the other hand, in the electrosynthetic cell, the half-cell reactions are irreversible and the products (D+ and A ) accumulate in the electrolyte. The most carefully studied reaction of this type is photoelectrolysis of water (D+ = 02 and A = H2)- Other photoelectrosynthetic studies include the preparation of S2O8-, the reduction of C02 to formic acid, N2 to NH3, etc. 

The direct photoelectrolysis of water requires that the v level be below the 02/H20 level and the ec level be above the H+/H2 level. This condition is satisfied, e.g. for CdS, GaP, and several large-band gap semiconductors, such as SrTi03, KTa03, Nb205 and Zr02 (cf. also Fig. 5.59). From the practical points of view, these materials show, however, other specific problems, e.g. low electrocatalytic activity, sensitivity to photocorrosion (CdS, GaP), and inconvenient absorption spectrum (oxides). 

Fujishima and Honda [16, 158] reported the photodecomposition of water using semiconductor photoelectrolysis cells (Figure 4.10). When the surface of the Ti02 electrode was irradiated with UV light, oxygen evolution was observed at the Ti02 electrode surface and hydrogen at the Pt black electrode. The overall water photodecomposition reaction ... 

Mohapatra, S.K., Raja, K.S., Mahajan, V.K., and Misra, M. (2008) Efficient photoelectrolysis of water using Ti02 nanotube arrays by minimizing recombination losses with organic additives. Journal of Physical Chemistry C, 112 (29), 11007-11012. 

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