Semiconductor particle charge transfer

Fig. 9.1. (a) Semiconductor particle charge transfer processes induced by direct band gap excitation. 

The change in the electronic properties of Ru particles upon modification with Se was investigated recently by electrochemical nuclear magnetic resonance (EC-NMR) and XPS [28]. In this work, it was established for the first time that Se, which is a p-type semiconductor in elemental form, becomes metallic when interacting with Ru, due to charge transfer from Ru to Se. On the basis of this and previous results, the authors emphasized that the combination of two or more elements to induce electronic alterations on a major catalytic component, as exemplified by Se addition on Ru, is quite a promising method to design stable and potent fuel cell electrocatalysts. 

Thus, the major conclusions of tiie early studies by Volkenshtein and his colleagues applicable to the theory of the method of semiconductor gas sensors are the following a) chemisorption of particles on a semiconductor surface can be accompanied by a charge transfer between adsorption-induced surface levels and volume bands of adsorbent and b) only a certain fraction of absorbed particles is charged, the fraction being dependent on adsorbate and adsorbent. 

It should be mentioned that one can detect two types of equilibrium in the model of charge transfer in the absorbate - adsorbent system (i) complete transition of chemisorbed particles into the charged form and (ii) flattening of Fermi level of adsorbent and energy level of chemisorbed particles. The former type takes place in the case of substantially low concentration of adsorbed particles characterized by high affinity to electron compared to the work function of semiconductor (for acceptor adsorbates) or small value of ionization potential (for donor adsorbates). The latter type can take place for sufficiently large concentration of chemisorbed particles. 

Memming, R. Photoinduced Charge Transfer Processes at Semiconductor Electrodes and Particles. 169, 105-182 (1994). 

It should also be briefly recalled that semiconductors can be added to nanocarbons in different ways, such as using sol-gel, hydrothermal, solvothermal and other methods (see Chapter 5). These procedures lead to different sizes and shapes in semiconductor particles resulting in different types of nanocarbon-semiconductor interactions which may significantly influence the electron-transfer charge carrier mobility, and interface states. The latter play a relevant role in introducing radiative paths (carrier-trapped-centers and electron-hole recombination centers), but also in strain-induced band gap modification [72]. These are aspects scarcely studied, particularly in relation to nanocarbon-semiconductor (Ti02) hybrids, but which are a critical element for their rational design. 

Memming R (1994) Photoinduced charge transfer processes at semiconductor electrodes and particles. Top Curr Chem 169 105-181... 

Methodologies developed for the in situ formation of semiconductor particulate films on BLMs have been applied to monolayer systems [638-641]. There are several advantages to the use of monolayer matrices for semiconductor particle generation. Monolayers are considerably more stable than BLMs and they also possess surface areas and charges which are two-dimensionally controllable. They may also, in association with the semiconductor paniculate films grown in their matrices, by conveniently transferred to solid supports. 

Upon illumination, semiconductor particles become charged, allowing even for electrophoretic mobility under an applied electrical field When appropriately prepared, colloidal TiO2 can apparently accumulate charge to effect directly multiple quanta redox reactions The efficiency of such charge accumulation is surely related to doping level for the doping level can alter band positions and may improve the efficiency of photoinduced electron transfer. For example, the dispersal of FcjOa... 

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. 

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