Fig. 7. Energy level scheme for the photoassisted OER from water using AgCl layers after initial self-sensitization to generate Ag clusters.
In the case of alloys, we will consider, in turn, metal oxides, metal chalcogenides and, finally, Group III-V semiconductors. We have seen earlier (Sections 6.3 and 7) how non-metallic elements such as F, N and S can alloy with the metal oxide lattice, these species occupying anion sites within the host framework. The corresponding oxyfluoride, oxynitride and oxysulfide compounds are thus generated (Section 7).
Other solid-solutions involving oxide semiconductors that have been examined include TiO2-MnO2,80 ZnO-CdO,80 TO2-MO2 (M = Nb, Ta),573 TiO2-In2O3,107 TiO2-V2O5,106 and Fe2O3-Nb2O5.80 Tungsten-based mixed-metal oxides, WnOmMx (M = Ni, Co, Cu, Zn, Pt, Ru, Rh, Pd and Ag) have been prepared using electrosynthesis and high-throughput (combinatorial) screening,574 but it is not clear how many of these compounds are true alloys (rather than mixtures). An interesting oxide alloy with lamellar structure, In2O3(ZnO)m, has been reported575 with photocatalytic activity for HER from an aqueous methanol and OER from an aqueous AgNO3 solution. This alloy consists of layers of wurtzite-type ZnO slabs interspersed with InO3 lamella; the band gaps of In2O3(ZnO)3 and ImO3(ZnO)9 are 2.6 eV and 2.7 eV, respectively.575
We have seen several examples of solid solutions or alloys involving metal chal-cogenides (see Table 4, Entries 6-10). Other widely studied systems include CdSxSe1-x and CdxZm-xS, involving, respectively, substitution in the anion and cation sub-lattices. The latter has been especially examined from a water photosplitting perspective (see for example, Refs. 524 and 576).
Amongst the Group III-V semiconductor alloys, AlxGa1-xAs and GaxIm-xP have been most widely studied. In particular, the alloy composites Al0.4Ga0.6As and Ga0.5In0.5P (or equivalently, GaInP2) have band gap values of 1.9 eV—close to the ideal value in terms of photoelectrolysis applications (see above). Further, both these alloys are lattice-matched to GaAs, allowing for epitaxial growth on this substrate. However, the growth of high-quality, oxygen-free AlGaAs and the fabrication of a high conductance cell interconnect have plagued this alloy material.577 It must be noted that tandem, monolithic GaInP/GaAs solar cells have yielded very high efficiencies in the 27.3%-29.5% range.577578
The photoelectrochemical stability of p-GaInP2 has been studied in three different electrolytes of varying pH.579 Tandem cells consisting of a GaInP2 homojunction grown epitaxially on a GaAs homojunction with a GaAs tunnel-diode interconnect, were utilized to photoelectrochemically split water.126128 Subsequent work by the same group modified this hybrid electrode structure with an additional top layer of p-GaInP2.128 To quantify the efficiency gains from a photoelectrolysis system, an integrated photovoltaic cell/electrolysis system was deployed by this group using a GaInP2/GaAs multi-junction cell.125 Finally, a less expensive alternative for fabricating GaInP2/GaAs junctions was explored using a combined close-spaced vapor transport/liquid-phase epitaxy; arrays of mesas of GaInP2/GaAs were selectively grown on Si substrates.580
Much less work has appeared on AlGaAs alloys. A bipolar electrode configuration of Al0.15Ga0.85As (Eg = 1.6 eV) and Si was used in conjunction with OER and HER co-catalysts, RuO2 and Pt, respectively, to drive water photosplitting at 18.3% conversion efficiency.209 Clearly, among all the photoelectrode materials discussed up till now, the Group III-V compounds, namely, InP and the alloyed materials, have yielded the most impressive results.
The incentive for using mixed semiconductors derives from the possibility of securing interparticle electron transfer and thus mitigate carrier recombination. For example, the conduction band of WO3 lies at a lower energy (relative to the vacuum reference level) than TiO2.581,582 Thus, in a TiO2-WO3 composite, the photogenerated electrons in TiO2 are driven to WO3 before they have an opportunity to recombine with the holes in the TiO2 particle. Other examples illustrative of this approach were discussed earlier in this Chapter and include CdS-TiO2583 and CdS-K4Nb6O17.454 Other examples of mixed semiconductors include TiO2-LaCrO3,200 CdS-LaCrO3,436 Fe2O3-TiO2,584 and Cu2O-TiO2.585 However, not all these composites have been examined from a water photosplitting perspective. Note that a bilayer configuration of the two semiconductors is not fundamentally different (at least from an electron transfer perspective) than a suspension containing mixed semiconductor particles (composites) in electronic contact.
Closely related is the so-called photochemical diode,489 consisting of either a metal/ semiconductor Schottky barrier or a p-n junction, which generates the voltage needed on illumination, to split water. Photochemical diodes are discussed along with other twin-photosystem configurations in the next Section.
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