We have already seen (see Table 4) that p-InP photocathodes are capable of evolving H2 from HCl or HClO4 electrolytes with very high efficiency.66,199,201 Photocathodes made from p-GaInP2 (a solid solution of GaP and InP, see Section 12 of this Chapter) biased with a GaAs p-n junction have also evolved H2 with high efficiency.126 As with their chalcogenide semiconductor counterparts, Group III-V semiconductors, in n-type form, undergo photoanodic corrosion instead of evolving O2 under illumination in aqueous media. On the other hand, these materials are relatively more stable against cathodic photocorrosion and the photogenerated minority carriers (electrons) on the p-type semiconductor surface can be used to reduce water to H2, particularly in the presence of a co-catalyst such as Pt or Ru.6 The reader is referred to chapters in Ref. 6 that provide reviews of work up to ~ 1988 on the HER on irradiated GaS and InP surfaces. Table 15 provides a chronological listing of selected studies up to ~ 1985 on Group III-V compound semiconductors that have been examined from a water photosplitting and H2 generation perspective. The Eg values for GaAs, GaP and InP are 1.43 eV, 2.25 eV and 1.30 eV respectively.
As with the n-TiO2 and n-SrTiO3 counterparts discussed earlier in Section 6.2 of this Chapter (see also Ref. 407), luminescence probes have proven to be very useful for unraveling the mechanistic details of the cathodic processes both at n-type (e.g., n- GaAs)556 and p-type (e.g., p-InP)557,558 Group III-V semiconductor surfaces. Finally, these semiconductors share another trend with those discussed earlier (metal chalcogenides) in that the majority of the studies since ~ 1990 have been directed at solid solutions (alloys of GaP and InP, GaAs and InAs etc.). These newer studies will be addressed in Section 12 of this Chapter.
In summary, Group III-V semiconductors have several positive features that make them attractive for water photosplitting applications. The combination of high carrier mobility and an optimal band gap (particularly for many of the alloys, see below) coupled with reasonable photoelectrochemical stability for the p-type material under HER conditions, should inspire continuing scrutiny of Group III-V semiconductors. The control of surface chemistry is also particularly crucial to avoid problems with surface recombination. For example, the studies on p-InP photocathode surfaces have shown that a (controlled) ultra-thin interfacial oxide layer is critical for minimizing carrier recombination at the surface.66,199,201,554
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