Semiconductor powder metallized, electron-hole

Platinum-loaded Ti02 systems can be considered as a short-circuited photo-electrochemical cell where the Ti02 semiconductor electrode and metal Pt counterelectrode are brought into contact [159]. Light irradiation can induce electron-hole (e -h +) pair formation and surface oxidation and also reduction reactions on each Pt/Ti02 particle (Figure 4.11). These powder-based systems lack the advantage of... 

Similar to the molecular photosensitizers described above, solid semiconductor materials can absorb photons and convert light into electrical energy capable of reducing C02. In solution, a semiconductor will absorb light, and the electric field created at the solid-liquid interface effects the separation of photo-excited electron-hole pairs. The electrons can then carry out an interfacial reduction reaction at one site, while the holes can perform an interfacial oxidation at a separate site. In the following sections, details will be provided of the reduction of C02 at both bulk semiconductor electrodes that resemble their metal electrode counterparts, and semiconductor powders and colloids that approach the molecular length scale. Further information on semiconductor systems for C02 reduction is available in several excellent reviews [8, 44, 104, 105],... 

Figure 2. Electron-hole separation on a metallized (M) semiconductor (SC) powder.

Y. Nosaka Y. Ishizuka H. Miyama, Separation mechanism of a photo-induced electron-hole pair in metal-loaded semiconductor powders. Ber. Bunsenges. Phys. Chem. 1986, 90, 1199-1204. 

Illumination to powdery semiconductor suspended in an aqueous solution creates holes and electrons in the valence band (VB) and conduction band (CB), respectively. Imbalance in the rates of their reaction with solution species leads to electron accumulation in the powder, and the potentials (Eq 0 and Ey0) are shifted negatively (Fig. 3). By measuring photocurrents at a potential-controlled metal electrode, the position of E00 can be determined [8]. 

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