Electrons surface-trapped

In addition, the results of adsorption experiment in Fig. 4 revealed that H2O2 promotes the adsorption of 4-NP on the Cr-Ti-MCM-41 surface. From considering above results, it can be said that H2O2 increases the reaction rate by the promotion of adsorption of reactant and the removing of surface-trapped electrons. 

The photoexcited electrons are trapped at the surface by TiIV sites, subsequent to this process resulting Tim sites, which may further transfer charge carriers to molecular oxygen resulting in the formation of a superoxide radical ... 

An unusual type of cationic chemisorption occurs in the C(9 and Cimpurity levels. In this case an electron is lost from an impurity level for each foreign atom adsorbed, and two electrons are trapped in the C(P level. Now the wave functions for these electrons are small on the foreign atom, which exists therefore on the surface as the cation. This chemisorption is depletive. If the C<9 level lies between the bottom of the conduction band and the impurity levels, the chemisorption is still cationic, but the electrons in the impurity levels play no part in the process, and only one electron is trapped in the C(P level in the neighborhood of the first crystal atom. This chemisorption is cumulative. 

An atom or molecule that approaches the surface of a solid always experiences a net attractive potential ). As a result there is a finite probability that it is trapped on the surface and the phenomenon that we call adsorption occurs. Under the usual environmental conditions (about one atmosphere and 300 K and in the presence of oxygen, nitrogen, water vapor and assorted hydrocarbons) all solid surfaces are covered with a monolayer of adsorbate and the build-up of multiple adsorbate layers is often detectable. The constant presence of the adsorbate layer influences all the chemical, mechanical and electronic surface properties. Adhesion, lubrication, the onset of chemical corrosion or photoconductivity are just a few of the many macroscopic surface processes that are controlled by the various properties of a monolayer of adsorbates. 

Surface O- species have also been generated via X-ray irradiation of polycrystalline MgO14 as well as via the reaction of surface trapped electrons with N2015,16 according to ... 

As discussed earlier, surface trapped electrons are not only important as paramagnetic surface probes but, being localized at a solid-gas interface, they are extremely reactive and can act as powerful reducing agents leading to surface stabilized radicals. An account of these radicals has been given in the past volume of this series1 and we limit ourselves here to report on the 1702 radical stabilized on the surface of MgO for which the hyperfine tensor was fully... 

Energy levels corresponding to electrons localized near the surface may also be present. These are termed surface states. For example, adsorbed ions are one type of surface state they may be of the form of donors, such as hydrogen, which yield electrons to the material, or in the form of acceptors, such as oxygen, which accept, or trap electrons from the material. In Fig. 1, surface traps of the acceptor type are shown, and it is indicated that there are two possible levels present. Other possibilities are impurity atoms at the surface, which are introduced in the preparation of the sample or diffuse from the interior during heat treatment, or nonstoichiometry of the surface layers of the compound. Surface... 

A modification of this simple adsorption theory must be made if there are other surface levels present. If the surface concentration of these levels is very large compared to the concentration of ions to be adsorbed, one would expect the adsorption to more closely resemble that on a clean metal, as electron transfer between the various surface traps may predominate over transfer between the adsorbed ions and the bulk semiconductor. If the number of these traps is small compared to the amount of adsorption, one would expect the adsorption characteristics to resemble those for the theory discussed above. Intermediate cases are also possible. 

The physical properties necessary for the control of the electrical conductivity by surface traps may be indicated by a brief calculation. If we assume that the surface levels are deeper than the bulk donor levels, a large fraction of the electrons from the donors may be trapped on the surface, and the conductivity will be strongly dependent on the properties of the surface traps. 

The existence of other deep surface levels, for example Tamm levels (discussed in the preceding section), on the surface of zinc oxide is placed in doubt by an experiment of Bevan and Anderson (32) on sintered zinc oxide. They observed that the activation energy of the conduction electrons (of the order of an electron volt when the sample is subjected to high oxygen pressure) decreases to a few hundredths of an electron volt if the measurements are taken at low pressure (less than 10 mm.) and high temperature (the order of 600°C). Surface traps other than those asso-... 

In many semiconductors the majority of the photoelectrons are produced by excitation from the valence band, the process thus simultaneously producing holes. Surface traps may act as recombination centers for electron-hole recombination and a change in the number or energy of these surface traps, or a change in the height of the surface barrier, may change the rate of recombination. For example Bube (9,10) has concluded that it is through this effect that the adorption of water vapor influences the photoconductivity of cadmium sulfide. 

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