P-GaAs electrode

The photolytic reduction of N2 at TiO -suspensions was at first reported by Schrauzer et al. Small amounts of NH3 and N2H4 were obtained as products. The highest activity was found with anatase containing 20-30 % rutile. Very low yields were also obtained with p-GaP electrodes under illumination It is much easier to produce NH3 from NO -solutions at CdS- and Ti02-particles using S -ions as hole scavengers . Efficiencies are not reported yet. Recently the formation of NH3 from NO was observed at p-GaAs electrodes under illumination. In this case NH3-formation was only found in the presence of transition metal ions or their complex with EDTA. 

Khader MM, Nasser SA, Hannout MM, El-Dessonki MS (1993) Photoelectrochemical dissociation of water at copper-doped p-GaAs electrodes. Int J Hydrogen Energy 18 921-924... 

Fig. 16. Current-potential curve for a rotating (1000 rpm) p-GaAs electrode in 6 M HCI [93] a) anodic decomposition current b) partial current of Cu -reduction (0.7 mM) c) total current (dotted line) d) total current upon addition of Cu " -ions (50 mM)...

Fig. 22. Current vs potential at a rotating p-GaAs-electrode (6 M HCl 2.5 nM CuCl 1000 ipm) j. anodic dissolution current j n partial current of Cu -oxidation [93]...

The advantage of the model presented here is as follows the quasi-Fermi level of the majority carriers can be determined because it is identical to the electrode potential. Since the cnrrent-potential curve measured with the p-GaAs electrode yields directly the relation between and the hole cnrrent (dark current at p-GaAs), the position of the quasi-Fermi level of holes (minority carriers) at the surface of the n-type electrode is then also known for a given cnrrent (dark or photocurrent). This is illustrated in Fig. 2.22. The saturated anodic photocurrent at the n-electrode has a value of fph = 0.44 mA cm . The same current at the p-electrode occurs at Up = 0.28 V, Le. the quasi-Fermi level of holes occurs for both types of electrodes at F,p — +0.28 eV. 

Fig. 7,10 Current-potential characteristics for n- and p-GaAs electrodes in 0.1 M H2SO4...

Fig. 7.30 Electron transfer at a p-GaAs electrode as illustrated by an energy diagram. (After ref. [50])...

Since a GaAs surface is covered with native oxide, atomic arrangement is not observed in air. When an electrode was immersed in electrolyte solution and a cathodic potential was applied, the oxide layer seemed to be reduced electrochemically as atomic arrangement was observed at the p-GaAs electrode, as shown in Fig. 2 [20]. The top view (Fig. 2(a)) and the Fourier spectrum (Fig. 2(b)) show that the atomic stnicture is of nearly four-fold symmetry with a nearest neighbor distance of about 0.42 0.04 nm. This result shows that the surface has a GaAs(100)-(lxl) structure [21]. This is in contrast with the fact that a GaAs(lOO) surface forms a reconstructed structure such as... 

Fig. 2. (a) An atomically resolved AFM image of the p-GaAs electrode surface in 10 mM HCl obtained at 0 V vs. Ag/AgCl. (b) Two-dimensional Fourier spectrum of the image. 

Fig. 9. The mean depth and the steady current as a function of the electrode potential for a p-GaAs electrode in 10 mM H2SO4 solution.

Semiconductor Electrode, Fig. 6 Current-potential relatirms at a bare (solid line) and Pt-modified (broken line) p-GaAs electrodes measmed in a 1 M NaOH solution (a) in dark and (b) under illiunination [25]... 

Figure 7. Sequentially obtained AFM images of the p-GaAs electrode in the relatively flat region in 9 mM HCl + 1 mM CUCI2 solution (a) at +0.1 V vs. Ag/AgCl, (b) while the potential was pulsed to -0.15 V, (c)-(e) at -0.15 V, and (f) while the potential was pulsed back to the initial potential (+0.1 V). Time after the application of -0.15 V at the beginning of imaging was (c) 4 s, (d) 12 s, (e) 20 s, and (f) 74 s. Thick arrows indicate the onset of deposition (b) and stripping (f) of Cu. A ows beside the figure indicate the scan direction of the tip.

Certainly, there are also cases where surface recombination does play a dominant role. Examples are cathodic processes at p-GaAs electrodes. Here also a considerable overvoltage for the photocurrent onset has been found ( 0.5 V) as shown in Figure 7.29 [49]. Identical photocurrent-potential dependences were obtained with solutions containing redox systems such as Eu = -0.7 V... 

The figure shows how the luminescence decay observed with a p-GaAs electrode varies upon the addition of an electron acceptor such as cobaltocene (CoCpp and ferrocene (FeCpJ). The GaAs surface was passivated in order to keep the surface recombination low. The evaluation of such a decay curve is not easy. In principle, the continuity equation (Eq. (7.62)) must be solved again, this time under conditions of short pulse illumination the resulting minority carrier... 

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