Semiconductor photocathode

No naked semiconductor photocathode has been demonstrated to have good kinetics for the evolution of H2, despite the fact that the position of Eqb in many cases has been demonstrated to be more negative than E° (H2O/H2). This means that electrons excited to the conduction band have the reducing power to effect H2 evolution, but the kinetics are too poor to compete with e - h+ recombination. The demonstration that N,N -dimethyl-4,4 -bipyridinium, MV2+, could be efficiently photoreduced at illuminated p-type Si to form MV+ in aqueous solution under conditions where E° (MV2+/+) = E° (H20/H2) when no H2 evolution occurs establishes directly that the thermodynamics are good, but the kinetics are poor, for H2 evolution.(23,47)... 

Heller A, Aharon-Shalom E, Bonner WA, Miller B. (1982) Hydrogen-evolving semiconductor photocathodes. Nature of the junction and function of the platinum group metal catalyst. J Am Chem Soc 104 6942-6948... 

Dominey RN, Lewis NS, Bruce JA, Bookbinder DC, Wrighton MS (1982) Improvement of photoelectrochemical hydrogen generation by surface modification of p-type silicon semiconductor photocathodes. J Am Chem Soc 104 467-482... 

Bokris and Uosaki (1) have studied transient photo-assisted electrolysis current for systems including a p-type semiconductor photocathode and dark Pt anode. A set of current vs. time scans taken with a ZnTe photocathode system is shown in Figure 6. 

A review of photo-assisted electrolysis studies performed with p-type semiconductor photocathode/dark Pt anode systems suggests that a complementary phenomena arising from the presence of OH ions produced during the reduction half-cell reaction,... 

Conversion of Sunlight into Electrical Power and Photoassisted Electrolysis of Water in Photoelectrochemical Cells Advances in the development of efficient regenerative photoelectrochemical cells reviewed with a brief discussion of p-type semiconductor photocathodes for the HER. 29... 

The p-n photoelectrolysis approach,60 on the other hand, simply combines a n-type semiconductor photoanode and a p-type semiconductor photocathode in an electrolysis cell (Fig. 2c). The pros and cons of this twin-photosystem approach (which mimicks plant photosynthesis) were enumerated earlier in this Chapter (see Section 2). Table 16 provides a compilation of the semiconductor photocathode and photoanode combinations that have been examined. Reference 67 may also be con suited in this regard for combinations involving n WSe2, n MoSe2, n WS2, n TiCH, p InP, p GaP and p Si semiconductor electrodes. 

Table 16. Photoelectrolysis cells using n-type semiconductor photoanodes and p-type semiconductor photocathodes.

Catalysis of H Generation from P-Type Semiconductor Photocathodes... 

The FWHM of the system response is between 200 ps and 350 ps. There is a weak secondary peak 1 ns to 2.5 ns after the main peak. A peak prior to the main peak can appear at low discriminator thresholds. The width of the response of the 0 and -50 versions does not noticeably depend on the CFD threshold and the CFD zero cross. This is an indication that the response is limited by the intrinsic speed of the semiconductor photocathode. 

Fig. 2.8a and b. Photoemissive processes (a) Metal photocathode, (b) Semiconductor photocathode with positive electron affinity... 

Fig. 2.9. Photoemissive process semiconductor photocathode having negative electron affinity...

CO2 reduction. Four different schemes of photoassisted reduction of CO2 using a semiconducting photocathode have been approached (i) direct heterogeneous CO2 reduction by a biased semiconductor photocathode [27-35], (ii) heterogeneous CO2 reduction by metal particles on a biased semiconductor photocathode [36-40], (iii) homogeneous CO2 reduction by a molecular catalyst through a semiconductor/molecular catalyst junction [41-45], and (iv) heterogeneous CO2 reduction by a molecular catalyst attached to the semiconductor photocathode surface [46-48],... 

There are some limitations to the effectiveness of metal emitters that were improved by the introduction of semiconductor photocathodes. The disadvantages of metal emitters include the following ... 

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