Particles electron injection

Spanhel L, Weller H, Henglein A (1987) Photochemistry of semiconductor colloids. 22. Electron injection from illuminated CdS into attached TiOa and ZnO particles. J Am Chem Soc109 6632-6635... 

The reaction was studied for all coinage metal nanoparticles. In the case of GMEs the rate follows zero-order kinetics with IT for all the coinage metal cases. The observed IT for the Cu catalyzed reaction was maximum but its rate of reduction was found to be minimum. Just the reverse was the case for Au and an intermediate value was obtained for the Ag catalyzed reaction (Figure 7). The adsorption of substrates is driven by chemical interaction between the particle surface and the substrates. Here phe-nolate ions get adsorbed onto the particle surface when present in the aqueous medium. This caused a blue shift of the plasmon band. A strong nucleophile such as NaBH4, because of its diffusive nature and high electron injection capability, transfers electrons to the substrate via metal particles. This helps to overcome the kinetic barrier of the reaction. 

I.2. The Tandem Accelerator. As indicated in the diagram of Figure 4.13, a tandem accelerator uses a positive terminal located in the centre of the device. Negatively charged He- particles are injected into the accelerator and attracted to the terminal, where a stripper element removes two or more electrons from each... 

The LEC structure that involves the addition of ionic dopants and surfactants to the printable inks enables the ability to print a top electrode without restriction by the work function of the metal. Silver, nickel, or carbon particle-based pastes are generally the preferred printable electron injecting electrodes however, the shape and size of the particles combined with the softening properties of the solvent can create electrical shorts throughout the device when printed over a thin polymer layer that is only several hundred nanometers thick. For optimal performance, the commercially available pastes must be optimized for printing onto soluble thin films to make a fully screen-printed polymer EL display. 

In region III, the discharge is maintained only by ionization in the gas phase without electron injection from the cathode. Because of the inertial effect of ions and electrons, only small part of charged particles in the gas phase can arrive on the electrode. Therefore, polymerization may be induced principally by diffused free radicals and/or ion-electron pairs ... 

Dye sensitization plays an important role in photography. The sensitization mechanism for ZnO-materials as used in electro-photography is obviously in complete correspondence with these electrochemical experiments as shown for single crystals under high vacuum conditions by Heiland 56> and for imbedded ZnO-particles by Hauffe 57). Even for silver halides where electron injection as sensitization mechanism has been questioned by the energy transfer mechanism 58> electrochemical experiments have shown that the electron injection mechanism is at least energetically possible in contact with electrolytes 59>. Silver halides behave as mixed conductors with predominance of ionic conductivity at room temperature. These results will therefore not be discussed here in any detail since such electrodes are quite inconvenient for the study of excited dye molecules. 

Charge alteration on the surfaces of nanosized metallic silver particles has been investigated by simultaneously monitoring absorption and conductivity changes during pulse-radiolytic experiments [506]. Pulse radiolysis of a nitrous-oxide-(N20) saturated aqueous solution of 3.0 nm diameter metallic silver particles containing 0.2 M 2-propanol resulted in electron injection to the colloid. NzO functions to double the yield of hydroxyl radicals ( OH) generated in water... 

Separation of bulk and surface properties in macroscopic semiconductors is less than straight forward and requires highly sensitive experimental techniques. In contrast, the large surface-to-volume ratios in nanosized semiconductor particles render the examination of surface processes in and/or on these colloids to be experimentally feasible. Advantage has been taken of pulse radiolysis to inject electrons (in aqueous, N20-saturated solutions which contained 2-propanol see Eqs. 22,23, and 25) or holes (in aqueous, N20-saturated solutions which did not contain 2-propanol see Eqs. 22 and 23) into nanosized semiconductor particles [601, 602], Electron injection into CdS particles, for example, decreased the extinction coefficient at 470 nm (the absorption onset) by — 5 x 104 M-1cm-1 (Fig. 98) [576]. Hole injection resulted in the appearance of a transient absorption band in the long-wavelength region and in much less... 

Focusing on the shorter time-scale component, the characteristic recovery time shows a strong dependence on the pump-laser power or, equivalently, the number of electrons injected The higher the power, the shorter the recovery time. Similar behavior has been noted by Ford et al. [40]. If 1>app is plotted versus the number of electrons injected per particle (Fig. 4), a linear correlation is obtained. In other words, the reaction appears to be first order in electrons (and first order in the oxidized dye). What does this mean mechanistically The simplest interpretation—sketched in Scheme 1—is that the injected electrons are free to return to any available dye molecule, not just the molecule from which they originated. This would be the case if injected electrons avoided surface states (at least at these shorter times) and remained in the conduction band. (Notably, the power-dependent kinetic behavior persists in a rigid glass matrix. Consequently, possible... 

Figure 4 Plot of k1>app for back ET to Os(phen)3 versus Tiep, the number of electrons injected per colloidal Sn02 particle. Note that Eq. (4) contains r e, the number of electrons injected per unit volume. The estimated average volume for one particle is 1800 nm3. (Adapted from Ref. 36.)...

In this study we use the dye Coumarin 343 (C343) adsorbed on the surfaces of ZrC>2 nano particles in aqueous solution to study the solvation dynamics close to these surfaces. Zr02 is, in many respects, very similar to Ti02 and serves as a suitable model substance since, due to its higher band gap energy, electron injection from adsorbed dyes does not occur. To measure the time resolved Stokes shift, we used femtosecond frequency-resolved upconversion. 

Bilayered polysilane LEDs have been obtained by inserting a SiOx thin layer between the cathode and a Wurtz synthesized PMPS emitter film.94 The SiOx layers were prepared by 02 plasma treatment of the PMPS film surfaces. It was found that the external quantum efficiency was significantly enhanced by this treatment. This enhancement has been attributed to an increased electron injection via tunneling, resulting in a reduced hole current caused by the blocking effect of the thin SiOx layer. The weak visible emission observed from single-layer polysilane LEDs is almost completely eliminated. It was concluded that the visible emission is caused by the erosion of the PMPS surfaces due to the collision with hot metal particles during the vacuum deposition of the cathode, and this erosion process is avoided by the SiOx layer. 

Ion activation takes place by admitting particles (photons, ions, electrons) into the cell containing the ion of interest. These activating particles are injected along the z-axis so that their trajectory is interrupted by the disk, and the ions in the first cell are preserved. 

Fig. 7.2. Schematic representation of the forward reactions (steps 1-4, indicated by plain arrows) and recombination routes (steps 5-7, indicated by dotted arrows) taking place in the nc-DSC. (1) Optical excitation of the sensitizer. (2) Electron injection from the excited sensitizer (S ) to the conduction band of Ti02. (3) Electron percolation through the network of Ti02 particles. (4) regeneration of the oxidized sensitizer (S+) by iodide (I ). (5) Deactivation of the excited state of the sensitizer (S ). (6) Recombination of injected electrons with oxidised sensitizer (S+). (7) Recombination of conduction band electrons with triiodide (Ig ) in the electrolyte. Al/max is the maximum voltage that can be generated under illumination and corresponds to the difference between the Fermi level of the conduction band of TiC>2 under illumination and the electrochemical potential of the electrolyte...

Cyclic water cleavage by visible light was also achieved in electron relay free systems (48). In this case the fraction of sensitizer that Is absorbed onto the particle surface is photoactive and electron injection occurs directly from its excited state into the Ti02 conduction band. Using the surfactant ruthenium complex depicted in Figure 10, a quantum yield of 7% was obtained for the water splitting process. 

It should be emphasized that the experiments have been performed with ZnS-particles synthesized with an excess of S in order to avoid complications by surface states (see previous section). As shown above, ZnS was a very suitable semiconductor for these investigations because of its high energy position. The particle size-effect did not occur for instance with CdS, the energy bands of which occur at relatively low energies. In this case, current doubling occurs, i.e. the radical formed by a photoexcited hole can be further oxidized at the same particle by electron injection into the conduction band of CdS (189]. 

Proton adsorption/desorption was utilized by Clark and Sutin to control electron injection yields and probe Gerisher s distributions at planar rutile electrodes. More recently, Gratzel and co-workers found that excited-state electron injection from tetrakis(4-carboxyphenyl)porphyrinato]Zn to colloidal Ti02 particles was negligible at pH > 8 but increased substantially when the pH was lowered [211]. However, in this case it was difficult to rule out a trivial mechanism where the sensitizer simply desorbed from the surface at higher pH. 

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