Illumination hydrogen evolution

Under anaerobic conditions with a low partial pressure of hydrogen and under low intensity illumination, hydrogen evolution takes place and the overall reaction can be represented by (2.4.1) [172] the electron transfer route is as follows [141] ... 

More recently, Ikeda et a/.108 have examined C02 reduction in aqueous and nonaqueous solvents using metal-deposited p-GaP and p-InP electrodes under illumination. Metal coatings on these semiconductor electrodes gave much improved faradaic efficiencies for C02 reduction. In an aqueous solution, the products obtained were formic acid and CO with hydrogen evolution at Pb-, Zn-, and In-coated electrodes, while in a nonaqueous PC solution, CO was obtained with faradaic efficiencies of ca. 90% at In-, Zn-, and Au-coated p-GaP and p-InP, and a Pb coating on a p-GaP electrode gave oxalate as the main product with a faradaic efficiency of ca. 50% at -1.2 V versus Ag/AgCl. 

In the cathodic regime the silicon atoms of the electrode do not participate in the chemical reaction. Therefore, an n-type or a strongly illuminated p-type silicon electrode behave like a noble metal electrode and hydrogen evolution or metal plating reactions are observed. For the case of an aqueous electrolyte free of metal ions the main reaction is electrochemical hydrogen evolution according to ... 

Li, H. Zhang, X. Cui, X. Lin, Y., Ti02 nanotubes/MWCNTs nanocomposite photocatalysts synthesis, characterization and photocatalytic hydrogen evolution under UV-vis light illumination./. Nanoscience andNanotechn. 2012,12 1806-1811. 

Figure 16. Photocurrent/cell potential difference for n-type GaP anode and p-type GaP cathode in O.IM HfSOi, illuminated as in Figure 15. Hydrogen evolution occurred at the GaP cathode without visible degradation, but in this cell the anodic reaction is oxidation of P to HsPOs and dissolution of the anode material (16).

Consider now the processes caused by the formation of quasilevels. As was noted above, the shift of Fn relative to F is very small for majority carriers (electrons) and can usually be neglected precisely, this was done in constructing Fig. 16b. But for minority carriers (holes) the shift of Fp can be very large. The shifts of both Fnx F and Fp increase with the growing intensity of semiconductor illumination, so that for a certain illumination intensity Fp may reach the level of the electrochemical potential of anodic decomposition Fdec, p, and Fn—the level of a certain cathodic reaction (for example, reduction of water with hydrogen evolution FHljH20). These reactions start to proceed simultaneously, and their joint action constitutes the process of photocorrosion. 

The rate of flow of electrons from such a charged particle depends on the availability of an accessible site for this transfer. Although it is known that lattice defects provide such sites and that conduction band electrons can trickle down through solid dislocation levels reduction sites for electron accumulation are usually provided by metallization of the semiconductor particle. This can be achieved through photo-platinization or by a number of vapor transfer techniques and the principles relevant to hydrogen evolution on such platinized surfaces have been delineated by Heller The existence of such sites will thus control whether single or multiple electron transfer events can actually take place under steady state illumination. 

The effect of platinum has been studied mainly in relation to hydrogen generation from water because it is essential for hydrogen evolution. Similarly, Pt enhances the degradation of the polluting substance (Fig. 9.11).28) The function of Pt is assumed to promote the transfer of the conduction band electron to 02 (Eq. (9.2)). Other metals such as rhodium, palladium and silver are also effective. Copper was studied to replace these expensive metals. However, copper is less effective, and dissolves in water by illumination. The loading method is also... 

An example of the analysis of IMPS data based on the competition between electron transfer and recombination is provided by a detailed study of hydrogen evolution on illuminated p-InP in acid solution [29]. Since the photogenerated minority carriers are electrons in the case of p-InP, they are driven to the surface under depletion conditions, where they can reduce protons to hydrogen. Figure 8.9 is a set of experimental IMPS responses measured for p-InP in 1.0 mol dm-3 H2S04 at different potentials. The measurements were performed using a small ac modulation of the illumination intensity superimposed on a larger steady component. 

Fig. 8.10. Variation of k,r and krcc for hydrogen evolution on illuminated p-InP. The ratio k,J(k,r + km) represents the fraction of photogenerated electrons that are transferred across the interface. Note that k appears to depend weakly on potential. The non-ideal variation of km with potential is interpreted as evidence for partial Fermi level pinning.

An example of recent achievement in this area is a flexible, thin film Cu(In,Ga)Se2 solar cell deposited on a titanium foil, which was combined with a TiC>2 photocatalyst layer and modified by a niobium-doped titanium oxide front electrode to function as a photoelectrochemical tandem cell/membrane for a direct light-driven hydrogen evolution from an aqueous solution [48], Under illumination with UV/vis light, the system produced up to 0.052 pLH2/scm2 (e.g. the hydrogen formation rate was approximately 7,250 pmol/h g relative to the amount of TiC>2 used). Several aspects of the operating principles of the photoelectrochemical devices, the materials requirements, main bottlenecks, and the various device concepts (in relation to H2... 

The n-type character of these zinc sulfides is indicated by the observation of an anodic photocurrent upon illumination of a powder-coated electrode. The action spectrum of the photocurrent parallels the wavelength dependence of the apparent quantum yield of the photoreduction of water by sodium formate, except that hydrogen evolution starts already in the sub-bandgap region at 370 nm, a wavelength where no photocurrent was observable [98]. 

The current-potential curve for the p-InP photocathode under illumination in CO2 (40 atm)-methanol exhibited a relatively large photocurrent (solid line), while the dark current was negligibly small (dotted line, < 1 mA cm 2) at potentials down to -2.0 V vs. Ag-QRE (Fig. 1). The onset photopotential was approximately -0.6 V. When CO2 was replaced with Ar, the onset of the cathodic photocurrent shifted toward the negative direction by 0.4 V (dashed line). This indicates that, in the highly concentrated CO2 solution, CO2 reduction on the p-InP surface occurs in preference to the reaction occurring under Ar atmosphere, which is predominantly hydrogen evolution. The cathodic photocurrent reached 20 mA (approximately 100 mA cm 2) at a potential of -2.4 V vs. Ag-QRE. 

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