Interfacial electron transfer recombination

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... 

The addition of a second species can cause a decrease in charge recombination and an increase in the TiOz photocatalytic efficiency. Such behavior was examined by loading a series of species on the surface or into the crystal lattice of photocatalysts inorganic ions [148-152], noble metals [153,154], and other semiconductor metal oxides [155], It was thus proven that modifications produced by these species can change semiconductor surface properties by altering interfacial electron-transfer events and thus the photocatalytic efficiency. 

This case is shown in Fig. 10.6c and d where through absorption of light a photohole in the vb and a photoelectron in the cb are formed. The probability that interfacial electron transfer takes place, i.e. that a thermodynamically suitable electron donor is oxidized by the photohole of the vb depends (i) on the rate constant of the interfacial electron transfer, kET, (ii) on the concentration of the adsorbed electron donor, [Rads]. and (iii) on the rate constants of recombination of the electron-hole pair via radiative and radiationless transitions,Ykj. At steady-state of the electronically excited state, the quantum yield, Ox, ofinterfacial electron-transfer can be expressed in terms of rate constants ... 

Acceptor and donor related processes involve interfacial electron transfer from free/trapped charge carries, including back reactions 3b and 3d [22, 27]. These latter processes are essentially donor/acceptor mediated recombination reactions. 

The overall process performance, as measured by photon efficiency (number of incident photon per molecule reacted, like the incident photon to current conversion efficiency, or IPCE, for PV cells), depends on the chain from the light absorption to acceptor/donor reduction/oxidation, and results from the relative kinetic of the recombination processes and interfacial electron transfer [23, 28]. Essentially, control over the rate of carrier crossing the interface, relative to the rates at which carriers recombine, is fundamental in obtaining the control over the efficiency of a photocatalyst. To suppress bulk- and surface-mediated recombination processes an efficient separation mechanism of the photogenerated carrier should be active. 

Fig. 16.3 Quantum yield (QY) for electron and hole transfer to solution redox acceptors/donors as a function of the reduced variables y (related to the surface properties of the catalyst, i.e., ratio between interfacial electron transfer rate and surface recombination rate) and w (related to the ratio between surface migration currents of hole and electrons to the rate of bulk recombination), according to the proposed kinetic model [23],...

Studies on luminescence of CdS colloids provide useful knowledge on the energy and nature of recombination sites of charge carriers in the colloidal particles. The regularities of the colloid photoluminescence quenching provide the information on the dynamics of electrons and holes in semiconductor particles as well as on the kinetics of interfacial electron transfer. Of a particular interest are studies on the luminescence of colloidal solutions of the so-called Q-semiconductors, their properties depending on the size of semiconductor particles due to the quantum size effects. 

Fig. 18. The parameter A as a function of the deepness (in energetic scale) of the level of recombination sites for CdS particles. Parameter A is calculated from Eq. (2.19). A is proportional to the ratio between the rate of interfacial electron transfer to the rate of trapped carriers recombination. The location of energy level of recombination site is measured from the bottom of conductivity band.

Gosh HN, Ashbury JB, Weng Y, LianT. Interfacial electron transfer between [Fe(CN )6]4-and Ti02 nanoparticles direct electron injection and nonexponential recombination. J Phys Chem B 1998 102 10208-15. 

This section shows that in order to separate klr and kTec it is necessary to carry out non-steady-state measurements. A simple example of a non steady-state measurement is switching the illumination on and off. The photogenerated flux of holes, g, towards the surface feeds into three processes surface charge storage, interfacial electron transfer and recombination. The magnitudes of the corresponding components of the total current density can be written in terms of the surface hole charge Qs ... 

Fig. 8.6. Components of the current response of a n-type semiconductor electrode to an illumination step. ch is the charging current, is the current due to interfacial electron transfer and rc,. is the current due to electrons recombining with holes via surface states. The total current, given by the sum of and j , is equal to qg - /rec, where g is the flux of minority carriers given by the Gartner equation. The dimensionless normalised time axis is ( lr + k,c<.)t. The dimensionless normalised current axis is jlqg.

Charge Carrier Recombination and Interfacial Electron Transfer... 

Ghosh H. N., Asbury J. B., Weng Y. X. and Lian T. Q. (1998), Interfacial electron transfer between Fe CNe " and TiOi nanoparticles direct electron injection and nanoexponential recombination , J. Phys. Chem. B 102, 10208-10215. 

Figure 12.14 Band diagram of the semiconductor-electrolyte interface, illustrating photogeneration and loss of minority carriers (holes) via surface recombination and interfacial electron transfer. The balance between these processes determines the steady-state surface concentration of minority carriers...

Equation (7) is valid if the field in the space-charge region is sufficient to prevent recombination but, in disordered materials, we might expect some recombination effects even when migration is rapid. Space-charge recombination will also be important if slow interfacial electron transfer leads to a build-up of minority carriers. 

---