Surface electron traps

Paganini, M.C., Chiesa, M., Giamello, E., Coluccia, S., Martra, G., Murphy, D.M. and Pacchioni, G. (1999) Colour Centres at the Surface of Alkali-Earth Oxides. A new Hypothesis on the Location of Surface Electron Traps, Surf. Sci. 421, 246-262. 

The concentration of surface electrons trapped in anion vacancies can be considerably increased by doping the MgO surface with metals with low ionization energies, such as alkali metals (230,237) or Mg (230,238-240). 

Photoadsorbed species can act (i) as surface-hole trapping and photoelectrons can be trapped in the bulk of the solid or (ii) as surface-electron trapping and holes can react with OH surface groups and/or H2O. Both the alternatives depend on the chemical nature of the molecule to be adsorbed and on the type of the solid adsorbent. It is worth noting that in the gas-solid regime only gaseous species or lattice ions can be involved, whereas in the liquid phase also the interaction with the solvent (often H2O) should also be considered. 

In Section 1.2.9, a case study was presented on how EPR was used to identify and characterize the NO2 radical supported on an oxide surface. To further illustrate the generic nature of this analytical approach in EPR to the investigation of the properties of surface radicals, the case of CO2 adsorbed on an MgO surface will be presented. This radical can be easily formed by exposure of CO2 to MgO containing excess surface electron trapped species (that is the (H )(e ) centers discussed in the previous section). Although it has been studied on different oxides over the years [38, 39], a study by Chiesa and Giamello [40] demonstrates the wealth of information that can be obtained from the powder EPR spectrum. The EPR spectrum for the surface (MgO) supported C02 species is shown in Figure 1.19. 

Rapid e / h recombination, the reverse of equation 3, necessitates that D andM be pre-adsorbed prior to light excitation of the Ti02 photocatalyst. In the case of a hydrated and hydroxylated Ti02 anatase surface, hole trapping by interfacial electron transfer occurs via equation 6 to give surface-bound OH radicals (43,44). The necessity for pre-adsorbed D andM for efficient charge carrier trapping calls attention to the importance of adsorption—desorption equihbria in... 

Gratzel and Serpone and co-workers recently reported on a picosecond laser flash photolysis study of TiO. They observed the absorption spectrum immediately after the 30 ps flash and attributed it to electrons trapped on Ti" " ions at the surface of the colloidal particles. The absorption decayed within nanoseconds, the rate being faster as the number of photons absorbed per colloidal particle increased. This decay was attributed to the recombination of the trapped electrons with holes. 

Electron-trap on germanium surface produced by heating in oxygen. Physic. Rev. [2] 94, 1420 (1954). 

A detailed study of the C02- species on MgO has been carried out by Lunsford and Jayne 26). Electrons trapped at surface defects during UV irradiation of the sample are transferred to the CO2 molecule upon adsorption. By using 13C02 the hyperfine structure was obtained. The coupling constants are axx - 184, am = 184, and a = 230 G. An analysis of the data, similar to that carried out in Section II.B.2 for N02, indicates that the unpaired electron has 18% 2s character and 47% 2p character on the carbon atom. An OCO bond angle of 125° may be compared with an angle of 128° for CO2- in sodium formate. 

The concept of color centers has been extended to surfaces to explain a number of puzzling aspects of surface reactivity. For example, in oxides such as MgO an anion vacancy carries two effective charges, V(2. These vacancies can trap two electrons to form an F center or one electron to form an F+ center. When the vacancy is located at a surface, the centers are given a subscript s, that is, Fs+ represents a single electron trapped at an anion vacancy on an MgO surface. As the trapping energy for the electrons in such centers is weak, they are available to enhance surface reactions. 

Metal to insulator Semiconductor to insulator, or Insulator to insulator Light contact (touching) Ion migration (due to inherent or unavoidable ion contamination electron traps (a) by random adhesion of ions for contact of dissimilar materials (b) by diffusion due to differences in ion concentration or mobilities (c) by image attraction Anomalous (due to avoidable surface contamination)... 

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