Porous semiconductor surface

Cf are the resistance and capacitance due to the particulate semiconductor film R m and are the resistance and capacitance of the parts of the BLM which remained unaltered by the incorporation of the semiconductor particles Rsc and Csc are the space charge resistance and capacitance at the semiconductor particle-BLM interface and Rss and Css are the resistance and capacitance due to surface-state on the semiconductor particles in the BLM. Electrolytes short circuit the porous semiconductor particles (Rf = Rsol = 1.4 kO) such that their contribution, along with that due to the Helmholtz layer, can be neglected. This allows the simplification of the equivalent circuit to that shown in Fig. 108c. As seen, the working electrode is connected (via ions) to the semiconductor particulate film. 

Investigations of metals (Ag [28-30], Au [31, 33], Pt [18, 22, 30], Pd [11], Cu, Fig [41] etc.) photoreduction at surfaces of porous samples and colloidal particles of Ti02 shows, that in such systems metal is deposited on the semiconductor surface as separate particles of subnanometric - nanometric size. Such metal particles have ohmic contact with semiconductor surface [11, 24, 32] and developed electronic structure [24, 28-32], So, we concluded that photocatalytic nickel(II) reduction takes place at irradiation of suspensions, containing mesoporous Ti02, Ni2+ and ethanol, this process resulting in the... 

It is reasonable to assume that in a nanoporous electrode (Fig. 7), the time required for a photogenerated minority carrier to reach the semiconductor surface by drift or diffusion is not very different from that estimated for a flat semiconductor/ electrolyte interface. However, the photogenerated majority carrier must travel through the porous structure to reach the back contact. Much work has been devoted recently to the study of electronic transport through nanostructured electrodes (cf. sections 3 and 5). It has been found that the characteristic time required for a majority carrier to travel through the porous network can be extremely long (in the ms to s range). 

IMPS measurements of electronic transport in a GaP networks have been studied by using a modulated UV laser [78]. The light was incident from the electrolyte side, and in all cases the penetration depth of the light (1/a a 100 nm) was much smaller than the thickness of the porous film d. Photogenerated holes are driven to the semiconductor surface and consumed in photoanodic oxidation of the sample. Photogenerated electrons are driven to the interior of the GaP filaments by an electric field perpendicular to the surface. Transport of photogenerated electrons through a fully depleted GaP network can be measured with IMPS in the frequency domain well below (l/tf(,//) [78]. A typical IMPS plot is shown in Fig. 43. 

Photocurrent generation is one of the most interesting direct applications of photosynthetic studies. The adsorption of sensitizers onto semiconductor surfaces has been found to be an efficient way to generate photocurrents and has been studied extensively. Ruthenium bipyridyl complexes, in particular, have been the focus of recent research [137-139]. In these cases, only the first layer of molecules, which is in direct contact with the surface, is active. A highly porous semiconductor material was therefore employed to compensate for the low level of absorption of the single molecular layer. Other varieties of chromophores, semiconductor materials, and electron carriers for totally solid systems have been the subjects of extensive studies. The present... 

Surface Modification of Porous Semiconductors to Improve Gas-Sensing Characteristics... 

As shown in the previous section, to achieve the essential parameters of gas sensors, it is necessary to use porous layers with optimal thickness and porosity. However, it should not be forgotten that the surface chemistry of the inner walls of the pores controls the adsorption of gases as well as the capillarity condensation. Therefore, in designing sensors based on porous materials, the opportunity to control these processes, using various treatments for surface functionalizing and stabilization, should not be ignored. As demonstrated earlier for metal oxide-based sensors, such an approach makes it possible to optimize better the parameters of gas sensors. The results of numerous research projects, which can be found in Table 26.2, have shown that such an approach for the design of gas sensors based on porous semiconductors is effective as well. 

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