Pore electrochemical mechanisms

Up to now there is still not much research on the mechanism of how the MPS enhance the catalysis of proteins and how the direct electron transfer occur while the proteins encapsulated in the MPS pore. The mechanism is not clear. Besides, as this chapter referred above, many scientists found that the protein/enzymes encapsulated on normal MPS surface and in the pore were not stable and lots of endeavor was devoted to solve this problem [34-38], Abundant of work is currently carrying out on the study of the electrochemical... 

Corrosion in soil is aqueous, and the mechanism is electrochemical (see Section 1.4), but the conditions in the soil can range from atmospheric to completely immersed (Sections 2.2 and 2.3). Which conditions prevail depends on the compactness of the soil and the water or moisture content. Moisture retained within a soil under field dry conditions is largely held within the capillaries and pores of the soil. Soil moisture is extremely significant in this connection, and a dry sandy soil will, in general, be less corrosive than a wet clay. 

The fundamental reason for the uneven distribution of reactions is that the rate of electrochemical reactions on a semiconductor is sensitive to the radius of curvature of the surface. This sensitivity can either be associated with the thickness of the space charge layer or the resistance of the substrate. Thus, when the rate of the dissolution reactions depends on the thickness of the space charge layer, formation of pores can in principle occur on a semiconductor electrode. The specific porous structures are determined by the spatial and temporal distributions of reactions and their rates which are affected by the geometric elements in the system. Because of the intricate relations among the kinetic factors and geometric elements, the detail features of PS morphology and the mechanisms for their formation are complex and greatly vary with experimental conditions. 

A basic requirement for electrochemical pore formation is passivation of the pore walls and passivity breakdown at the pore tips. Any model of the pore formation process in silicon electrodes has to explain this difference between pore tip and pore wall conditions. Three different mechanisms have been proposed to explain the remarkable stability of the silicon pore walls against dissolution in HF, as shown in Fig. 6.1. 

According to the macropore formation mechanisms, as discussed in Section 9.1, the pore wall thickness of PS films formed on p-type substrates is always less than twice the SCR width. The conductivity of such a macroporous silicon film is therefore sensitive to the width of the surface depletion layer, which itself depends on the type and density of the surface charges present. For n-type substrates the pore spacing may become much more than twice the SCR width. In the latter case and for macro PS films that have been heavily doped after electrochemical formation, the effect of the surface depletion layer becomes negligible and the conductivity is determined by the geometry of the sample only. The conductivity parallel to the pores is then the bulk conductivity of the substrate times 1 -p, where p is the porosity. 

An interesting question is whether such well-ordered pore arrays can also be produced in other semiconductors than Si by the same electrochemical etching process. Conversion of the macropore formation process active for n-type silicon electrodes on other semiconductors is unlikely, because their minority carrier diffusion length is usually not large enough to enable holes to diffuse from the illuminated backside to the front. The macropore formation process active in p-type silicon or the mesopore formation mechanisms, however, involve no minority carrier diffusion and it therefore seems likely that these mechanisms also apply to other semiconductor electrodes. 

The reasons for formation of bubbles in packed column CEC has not been explained satisfactorily. Bubbles may be formed due to local differences in EOF velocity (e.g. between unpacked and packed sections of the capillary [6,7]), by local differences in field strength (leading to "hot spots"), by release of gas trapped in the pores or electrochemically formed [8]. Whichever mechanism applies, it was suggested by early workers in CEC, to pressurize the inlet and outlet vials, in order to keep the gas dissolved [3,9,10], Once the bubbles form, the detector base line becomes very noisy and the current unstable. This may lead to break down of the current and the flow stops. Robson et al. illustrated that using pressurization of the solvent vials CEC can be carried out routinely at high fields with high speed and high efficiency [11],... 

The Mechanism. A closer look reveals that the processes in such an operation involve electro-osmosis (Section 6.11.3). The contaminants in the soil are adsorbed on the surface of the soil particles, and there is an equilibrium between what is adsorbed and what is dissolved in the aqueous solution in the pores of the soil. One can present the electrochemical processes in a schematic form as shown in Fig. 15.30. 

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