Anodic photocurrents

Figure 16 shows such PMC peaks in the depletion region for electrodes of Si,9 WSez8 and ZnO.12 They all appear near the onset of anodic photocurrents. They have different shapes, which, however, can easily be explained with the assumption of potential-dependent interfacial charge-transfer and charge recombination rates. 

Figured 16. PMC peaks in the depletion region near the onset of anodic photocurrents for Si,9 WSe2,8 and Zn0.iaThe clearly reduced width of the ZnO peak can be seen.

Figure 40. PMC peak measured with a ZnO single-crystal electrode in a microwave resonator near the onset of the anodic photocurrent.5...

Because of the excess holes with an energy lower than the Fermi level that are present at the n-type semiconductor surface in contact with the solution, electron ttansitions from the solution to the semiconductor electrode are facilitated ( egress of holes from the electrode to the reacting species ), and anodic photocurrents arise. Such currents do not arise merely from an acceleration of reactions which, at the particular potential, will also occur in the dark. According to Eq. (29.6), the electrochemical potential, corresponds to a more positive value of electrode potential (E ) than that which actually exists (E). Hence, anodic reactions can occur at the electrode even with redox systems having an equilibrium potential more positive than E (between E and E ) (i.e., reactions that are prohibited in the dark). 

Photoirradiation of the modified electrode with nanoclusters of Cj qN alone or the mixture of C oN and MePH afforded anodic photocurrents. The photocurrent action spectrum was in fair agreement with the absorption spectrum of the THF-H2O (2 1) mixed solution containing nanoclusters of the mixture of CfioN and MePH or C oN alone. These results strongly indicate that the photocurrents can be ascribed to photoexcitation of the nanoclusters of C qN. ... 

If the same experiment is performed with an n-type Si electrode under identical illumination intensity the anodic photocurrent is found to be larger than for the p-type electrode under cathodic conditions. This increase is small (about 10%) for current densities in excess of JPS. Figure 3.2 shows that in this anodic regime injected electrons are also detected at p-type electrodes. This allows us to interpret the 10% increase in photocurrent observed at n-type electrodes as electron injection during anodic oxide formation and dissolution. 

ZnS-CdS (bandgap = 2.3-2.4 eV) composite semiconductor photoelectrodes show a broad spectral response and n-type behavior, with saturation of the anodic photocurrent upon increasing anodic potential making the system suitable for use as a photoelectrochemical cell photoanode [72], Nanostructured ZnS-CdS thin film electrodes show that anodic photocurrent saturation can be attained with the application of a small, 0.1 V, bias [73], while hydrogen evolution is observed at the Pt cathode. The performance of the ZnS-CdS photoanodes appear strongly dependent upon the method of film preparation [72,73], with Zn rich films demonstrating superior photocurrent generation, and stability, in comparison to Cd rich films. 

Methyl-4 -vinyl-2,2 -bipyridyl (7 Vbpy) was prepared and the Ru complex of its trichlorosilylethyl derivative was coated on n-Sn02 by condensation of the surface hydroxyl groups 32). Anodic photocurrent obtained at the Ru(bpy)2+ bound n-Sn02 semiconductor electrode was about twice than that obtained at the bare Sn02 dipped in 0.1 N H2S04 solution of Ru(bpy)f +. ... 

Onset of anodic photocurrent at electrodes, coated by 673 PbS-containing Nafion showed a cathodic shift with increasing band-gap... 

Fig. 8. The anodic photocurrent squared as a function of the W03 electrode potential in 1 M CH3COONa. The wavelength of light (nm) 1—397 2—327 3—280. [From Butler (1977).]...

In principle, the same photoexcited reactant, if it is liable both to oxidation and reduction, can inject both electrons (into an -type semiconductor) and holes (into a p-type semiconductor). Such a material is, for example, the crystal-violet dye. Figure 25 shows the spectra of cathodic photocurrent iph at p-type gallium phosphide and anodic photocurrent at n-type zinc oxide both in a solution, which does not absorb light (dashed lines), and in the presence of crystal violet the absorption spectrum of the latter is also shown for comparison. 

Fig. 25. Absorption spectrum of the crystal-violet dye (a) and the spectra of anodic photocurrent at an electrode of n-type ZnO (b) and of cathodic photocurrent at an electrode of p-type GaP (c) in the presence of crystal violet (in pA/cm2). The dashed line is the photocurrent in the absence of the dye. [From Gerischer (1977b).]...

The importance of the first factor, the concentration, is clear because the anodic photocurrent due to oxidation of halide ions should be proportional to the product of the concentration of positive holes at the electrode surface and that of halide ions in solution. When ip becomes large, the supply of halide ions to the electrode surface by diffusion becomes unable to follow ip, resulting in a decrease of (X ), which depends on the concentration of X". 

The Effect of Illumination. In an alkaline solution, an n-GaP electrode, (111) surface, under illumination shows an anodic photocurrent, accompanied by quantitative dissolution of the electrode. The current-potential curve shows considerable hysterisis as seen in Fig. 2 the anodic current, scanned backward, (toward less positive potential) begins to decrease at a potential much more positive than the onset potential of the anodic current for the forward scanning, the latter being slightly more positive than the Ug value in the dark, Us(dark). 

Figure 3. Mott-Schottky plots for the illuminated (lll)-face of n-GaP (pH 12.9, modulation frequency 1 kHz). The anodic photocurrents flowing during measurements are shown in units of pAcm 2.

Chlorophyll-Coated Semiconductor Electrodes. Chi has first been employed by Tributsch and Calvin (55,56) in dye sensitization studies of semiconductor electrodes. Solvent-evaporated films of Chi a, Chi b, and bacteriochlorophyll on n-type semiconductor ZnO electrodes (single crystal) gave anodic sensitized photocurrents under potentiostatic conditions in aqueous electrolytes. The photocurrent action spectrum obtained for Chi a showed the red band peak at 673 nm corresponding closely to the amorphous and monomeric state of Chi a. The addition of supersensitizers (reducing agents) increased the anodic photocurrents, and a maximum quantum efficiency of 12.5% was obtained for the photocurrent in the presence of phenylhydrazine. 

It was found that the quantum efficiency of the anodic photocurrent in the Chi a-DPL system was increased from 3-4% in the pure Chi a monolayer up to about 25% (65) in highly diluted mixed monolayers. This increase in efficiency was accompanied by red shifts of the photocurrent peak positions in the red and blue bands (e.g., in the red, from 675 nm (pure Chi a) to 665-670 nm (Chi atDPL 1/49)) and probably reflects the change in chromophore-chromophore interaction between Chi a molecules. 

Significant increase in the anodic photocurrent was observed at pHs higher than 7. Since the photooxidized Chi a (cation radical) is considered to oxidize water better at higher pH above pH 7 (see Section 3), water probably behaves as a reducing agent (supersensitizer) for Chi a in alkaline solutions. 

Takahashi and co-workers (69,70,71) reported both cathodic and anodic photocurrents in addition to corresponding positive and negative photovoltages at solvent-evaporated films of a Chl-oxidant mixture and a Chl-reductant mixture, respectively, on platinum electrodes. Various redox species were examined, respectively, as a donor or acceptor added in an aqueous electrolyte (69). In a typical experiment (71), NAD and Fe(CN)g, each dissolved in a neutral electrolyte solution, were employed as an acceptor for a photocathode and a donor for a photoanode, respectively, and the photoreduction of NAD at a Chl-naphthoquinone-coated cathode and the photooxidation of Fe(CN)J at a Chl-anthrahydroquinone-coated anode were performed under either short circuit conditions or potentiostatic conditions. The reduction of NAD at the photocathode was demonstrated as a model for the photosynthetic system I. In their studies, the photoactive species was attributed to the composite of Chl-oxidant or -reductant (70). A p-type semiconductor model was proposed as the mechanism for photocurrent generation at the Chi photocathode (71). 

Photoelectrochemical behavior of metal phthalocyanine solid films (p-type photoconductors) have been studied at both metal (93,94,95,96) and semiconductor (97,98) electrodes. Copper phthalocyanine vacuum-deposited on a Sn02 OTE (97) displayed photocurrents with signs depending on the thickness of film as well as the electrode potential. Besides anodic photocurrents due to normal dye sensitization phenomenon on an n-type semiconductor, enhanced cathodic photocurrents were observed with thicker films due to a bulk effect (p-type photoconductivity) of the dye layer. Meier et al. (9j>) studied the cathodic photocurrent behavior of various metal phthalocyanines on platinum electrodes where the dye layer acted as a typical p-type organic semiconductor. 

Another model compound, the tris(2,2 -bipyridine)ruthenium(II) complex, has prompted considerable interest because its water-splitting photoreactivity has been demonstrated in various types of photochemical systems (77,99,100,101). Memming and Schroppel (102) have attempted to deposit a monolayer of a surfactant Ru(II) complex on a Sn02 OTE. In aqueous solution, an anodic photocurrent attributable to water oxidation by the excited triplet Ru complex was observed. A maximum quantum efficiency of 15% was obtained in alkaline solution. 

The experiments were performed with single crystal (111) p-Si electrodes with a resistivity of about 5.5 ohm cm non-aqueous electrolytes were used consisting of absolute methanol containing tetramethylammonium chloride (TMAC) or acetonitrile containing tetraethyl ammonium perchlorate (TEAP). The flat-band potentials or p-Si in the two electrolytes were determined from Mott-Schottky plots (in the dark) in the depletion range of the p-Si electrode, from open-circuit photopotential measurements, and from the values of electrode potential at which anodic photocurrent is first observed in n-type Si electrodes. These three methods all yielded consistent flat-band potential values for p-Si of + 0.05V (vs SCE)... 

---