Non-oxide semiconductors

Fig. 3.11c Band edge positions of several non-oxide semiconductors in contact with a pH 13 aqueous electrolyte.

This chapter considers photo-electrodes consisting of non-oxide semiconductors, alone and in combination with oxide semiconductors for water splitting. 

Fig. 7.1 Position of band edges and photodecomposition Fermi energies levels of various non-oxide semiconductors. E(e,d) represents decomposition energy level by electrons, while E(h,d) represents the decomposition energy level for holes vs normal hydrogen electrode (NHE). E(VB) denotes the valence band edge, E(CB) denotes the conduction band edge. E(H2/H20) denotes the reduction potential of water, and (H2O/O2) the oxidation potential of water, both with reference to NHE.

There are several non-oxide semiconductor electrodes that have been studied for use in photoelectrochemical cells for water splitting. Materials used as photocathodes include p-Si, p-lnP, p-GaAs, and p-CdTe. Materials used as photoanodes include n-Si, n-InP, n-GaAs, n-CdTe and n-CdSe. Similar to CdS, unfortunately most of these non-oxide semiconductors undergo photocatalytic corrosion under the conditions of oxygen evolution. 

Cadmium sulfide (CdS), 2.4 eV bandgap, is one of the most widely studied non-oxide semiconductors. For a semiconductor irradiated by photons with energy equal to or greater than its bandgap, an electron from the valence band jumps to the conduction band leaving a positively charged hole behind in the valance band for CdS this is expressed as... 

Like other non-oxidic semiconductors in aqueous solutions, surface oxidized and photocorrosive InP is a poor photoelectrode for water decomposition [19,27,32,33], To enhance properties several efforts have focused on coupling of the semiconductor with discontinuous noble metal layers of island-like topology. For example, rhodium, ruthenium and platinum thin films, less than 10 nm in thickness, have been electrodeposited onto p-type InP followed by a brief etching treatment to achieve an island-like topology on the surface [27,28]. In combination with a Pt counter electrode, under AM 1.5 illumination of 87 mW/cm the metal (Pt, Rh, Ru) functionalized p-InP photocathodes [27] see a reduction in the threshold voltage for water electrolysis from 1.23 V to 0.64 V, and in aqueous HCl solution a photocurrent density of 24 mA/cm with a photoconversion efficiency of 12% [27]. 

Non-oxide Semiconductors Mixed with Metals or Metal Oxides [34 41,149-156]... 

Other non-oxide semiconductors such as ZnS doped with Cu (Zn0.957Cu0.043S) [42], doped with Ni (Zn0.999Ni0.001S) [155] and codoped with Pb [156] have shown excellent solar hydrogen production in the presence, respectively, of aqueous potassium sulfite... 

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