Types of Approaches

Figure 1 illustrates the interfacial energetics involved in the photoelectrochemical evolution of H2. Thus, the electronic energy levels in the semiconductor and in the contacting solution are shown on a common diagram. In a semiconductor, the filled electronic levels (valence band or VB) and the empty levels (conduction band or CB) are separated by a forbidden gap, namely, the band gap energy, Eg.98-100 Photoexcitation of the semiconductor with light of energy equal to or exceeding Eg (i.e., with wavelengths corresponding to or shorter than that corresponding to the energy gap) elicits electron-hole pairs, a fraction of which (as defined by the quantum yield) escape recombination and find their way to the semiconductor/ solution interface. For the photosplitting of water (Figure 1a), the CB and VB edges at the semiconductor surface (Ecb and Evb respectively) must bracket the two redox levels corresponding to the HER and the oxygen evolution reaction (OER) respectively. This is tantamount to stating that the photogenerated electrons have sufficient energy to reduce protons and the photogenerated holes have sufficient energy to oxidize water (Figure 1a).

This is a stringent requirement indeed as further elaborated in the next Section. Instead of actually photosplitting water, sacrificial agents may be added to the solution such that the HER and OER steps may be separately optimized and studied (Figures 1b and 1c). It must be borne in mind that now the overall photoreaction becomes thermodynamically down hill and is more appropriately termed: photocata-lytic (see below). Examples of sacrificial agents include sulfite for the photo-driven HER case (Figure 1b) or Ag+ as the electron acceptor for the photocatalytic oxidation of water (Figure 1c).

Instead of using the semiconductor in the form of electrodes in an electrochemical cell, a wireless water splitting or HER system could be envisioned where particle suspensions are used (instead of electrodes) in a photochemical reactor. Two points regarding such an approach must be noted. First, unlike in the case of a semiconductor electrode, a bias potential cannot be applied in the suspension case. Second, the sites for the HER and OER are not physically separated as in the electrochemical case. Thus, the potential exists in a photochemical system for a highly explosive stoichiometric (2:1) mixture of H2 and O2 to be evolved. Nonetheless, strategies have been devised for immobilizing the semiconductor particles in a membrane so that the HER and OER sites are properly separated (see for example, Refs. 101-108). These include the so-called semiconductor septum photoelectrochemical cells where the n-and p- type semiconductor particles are embedded, for example, in a bilayer lipid membrane.105 The OER and HER sites are thus compartmentalized in this approach. However, claims of enhanced solar conversion efficiency in such devices have been questioned109 on the basis that in many of the cases reported, a galvanic cell (i.e., a sacrificial battery system) was used to drive the photoproduction of H2.

Fig. 1. Interfacial energetics at semiconductor-liquid junctions. D is an electron donor and A is an electron acceptor.

Fig. 1. Interfacial energetics at semiconductor-liquid junctions. D is an electron donor and A is an electron acceptor.

Bifunctional redox catalysts have been investigated in terms of their applicability for the solar-assisted splitting of water.30,110-118 In this approach, Pt (an excellent catalyst for the HER) and RuO2 (an excellent catalyst for the OER) are loaded onto colloidal TiO2 particles. But unlike in the approaches discussed earlier, the oxide semiconductor is not used as a light absorber; instead, an inorganic complex [e.g., amphiphilic Ru(bpy)32+ derivative, bpy = 2,2'-bipyridyl ligand] is used as the sensitizer.30,110 Claims of cyclic and sustained water cleavage by visible light in this system, however, have not been independently verified. Other colloidal systems have also been reported for the OER.119123 Since these microheterogeneous assemblies do not involve photoexcitation of a semiconductor, they are not further discussed in this Chapter.

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-TiO2 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.

Unlike the single photosystem cases in Figs. 2a and 2b, the approach in Fig. 2c combines two photosystems. Both heterotype (different semiconductors) or homotype (same semiconductor) approaches can be envisioned, and it has been shown60 that the efficiency of photoelectrolysis with solar radiation can be enhanced by using simultaneously illuminated n- and p- type semiconductor electrodes (Fig. 2c). It is interesting to note that this twin-photosystems approach mimics the plant photosynthesis system, intricately constructed by nature, albeit operating at rather low efficiency. The approach in Fig. 2c has at least two built-in advantages. First, the sum of two photopotentials can be secured in an additive manner such that the required threshold for the water splitting reaction (Chapter 2) can be more easily attained than in the single photoelectrode cases in Figs. 2a and 2b. Second, different segments of the solar spectrum can be utilized in the heterotype approach, and indeed, many semiconductors (with different Eg's) can be stacked to enhance the overall solar conversion efficiency of the device.44 However, the attendant price to be paid is the concomitant increase in the device complexity. Further, the photocurrents through

Photoelectrolysis Cell Configuration
Fig. 2. Photoelectrolysis cell configurations (refer to text).

the two interfaces will have to be carefully matched since the overall current flowing in the cell must obviously be the same.

Finally, hybrid approaches for water photosplitting can be envisioned. As illustrated in Fig. 3 a, a water electrolyzer can be simply hooked up to a solar panel that delivers the needed photovoltage.40,70,124,125 A conceptually more appealing scenario deploys a p-n junction directly in ohmic (electronic) contact with the electroactive surface where the HER (or less commonly, the OER) occurs (Fig. 3b). A variety of such monolithic configurations have been discussed.126129 A p/n photochemical diode consisting of p-GaP and n-Fe2O3 has been assembled in a monolithic unit and studied for its capability to evolve H2 and O2 from seawater.130 Silicon spheres comprising of p- and n- regions in electronic contact (forming p/n diodes) embedded in glass with a conductive backing have been used for photosplitting HBr into H2 and Br2.131 These and other hybrid approaches are further elaborated in a subsequent Section.

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

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