Photochemical Diodes and Twin Photosystem Configurations for Water Splitting

As mentioned earlier, photochemical diodes489 can be either of the Schottky type, involving a metal and a semiconductor, or a p-n junction type, involving two semiconductors (which can be the same, i.e., a homojunction or different, a heterojunc-tion). Only the latter type is considered in this Section involving two irradiated semi-conductor/ electrolyte interfaces. Thus n-TiO2 and p-GaP crystal wafers were bonded together (through the rear Ohmic contacts) with conductive Ag epoxy cement.489 The resultant heterotype p-n photochemical diode was suspended in an acidic aqueous medium and irradiated with simulated sunlight. Evolution of H2 and O2 was noted, albeit at a very slow rate.489

This type of device has been contrasted489 with a series connection of a photovoltaic p-n junction solar cell and a water electrolyzer. Unlike the latter which is a majority carrier system (i.e., the n-side of the junction is the cathode and the p-side becomes the anode), in a photochemical diode, minority carriers (holes for the n-type and electrons for the p-type) are injected into the electrolyte. This distinction translates to certain advantages in terms of the overall energetics of the solar energy conversion system (see Ref. 489).

Since this original work in 1977, another study has appeared combining p-GaP and n-Fe2O3.130 Co-catalysts (RuO2 on the n-Fe2O3 surface and Pt on the p-GaP surface) several to enhance H2 and O2 evolution from seawater.130 In a broad sense, arrays of p-n diodes that are embedded in glass and used for the splitting of HBr,131 and the semiconductor/redox electrolyte/ semiconductor photoelectrode configura-tion,586 may be regarded as extensions of the photochemical diode approach. These devices are discussed in the next Section.

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 consulted in this regard for combinations involving n-WSe2, n-MoSe2, n-WS2, n-TiO2, p-InP, p-GaP and p-Si semiconductor electrodes.

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