Interfacial Hole Transfer

A significant body of literature proposes that the photocatalytic oxidation of organic or inorganic solutes may occur by either indirect oxidation via a surface-bound hydroxyl radical (i.e., a trapped hole at the particle surface) or directly via the valence-band hole before it is trapped either within the particle or at the particle surface.Interfacial hole transfer from titanium dioxide to organic and inorganic solutes has been studied recently in [4f, 6c, 7]. An example of the latter paper is shown in Fig. 7.5. 

Grabner et al. have shown that in titanium dioxide sols containing chloride (which is either introduced into the solution as HC1 to adjust the pH or is present on the particle surface when TiCU is used as starting compound to prepare Ti02) Cl2 radical anions are formed. Their formation was postulated to occur by direct valence-band hole oxidation of surface adsorbed Cl (reactions (7.18), (7.19)) [4f]. 

It has been observed that these Cl2 - radical anions oxidize phenol yielding phenoxyl radicals (reaction (7.20)) [4f]. 

Interfacial hole transfer dynamics from titanium dioxide (Degussa P 25) to SCN has been investigated by Colombo and Bowman using femtosecond time-resolved diffuse reflectance spectroscopy [6c]. A dramatic increase in the population of trapped electrons was observed within the first few picoseconds, demonstrating that interfacial charge transfer of an electron from the SCN to a hole on the photoexcited titanium dioxide effectively competes with electron-hole recombination (reactions (7.12) - (7.15)) on an ultrafast time scale [6c]. 

Since Bahnemann and co-workers have observed that a comparatively high amount of trapped holes are formed when partially platinized titanium dioxide particles are subjected to ultra band gap irradiation (cf. Fig. 7.6), they have chosen this system to study the dynamics of the photocatalytic oxidation of the model compounds dichloroacetate, DCA , and SCN- [7]. To explain their experimental observations these authors have used a model assuming two energetically different types of hole traps (see our detailed discussion above). 

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By employing a 337 nm pulsed laser as the excitation source for exciting Ti02 colloids and initiating the redox reactions at the interface one can monitor the interfacial hole transfer process (reactions (1) and (2)). 

Furube A, Asahi T, Masuhara H, Yamashita H, Anpo M (2001) Direct observation of interfacial hole transfer from a photoexcited Ti02 particle to an adsorbed molecule SCN-by femtosecond diffuse reflectance spectroscopy. Res Chem Intermed 27(1) 177-187... 

Yang XJ, Tamai N (2001) How fast is interfacial hole transfer In situ monitoring of carrier dynamics in anatase Ti02 nanoparticles by femtosecond laser spectroscopy. Phys Chem Chem Phys 3 3393-3398... 

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... 

Figure 13. Numerically calculated PMC potential curves from transport equations (14)—(17) without simplifications for different interfacial reaction rate constants for minority carriers (holes in n-type semiconductor) (a) PMC peak in depletion region. Bulk lifetime 10" s, combined interfacial rate constants (sr = sr + kr) inserted in drawing. Dark points, calculation from analytical formula (18). (b) PMC peak in accumulation region. Bulk lifetime 10 5s. The combined interfacial charge-transfer and recombination rate ranges from 10 (1), 100 (2), 103 (3), 3 x 103 (4), 104 (5), 3 x 104 (6) to 106 (7) cm s"1. The flatband potential is indicated.

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 ... 

It is also assumed (Hoffmann 1990) that the adsorbed sulfite is oxidized by the valence band holes, h+b, that are formed through absorption of light with photon energies exceeding the band-gap energy (ca. 2.2 eV) of an iron(III)(hydr)oxide, e.g., hematite (a-Fe203). This interfacial electron transfer reaction results in formation of the SO radical anion which reacts with another radical to form S20 , one of the end product, if the reaction is carried out under nitrogen. 

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],...

A further increase in anodic polarization lowers still further the Fermi level ersc)(ti) which gradually approaches the valence band edge Cy at the electrode interface as shown in Fig. 8-21. As the anodic polarization increases, the concentration of interfacial holes in the valence band increases, thus causing the anodic electron transfer to change from the conduction band mechanism to the valence band mechanism. 

When the transport current of electrons or holes in semiconductor electrodes more or less influences the interfacial electron transfer current, the overvoltage T) consists of an overvoltage of space charge layer iisc, an overvoltage of compact layer t]h, and a transport overvoltage tit in semiconductors as expressed in Eqn. 8-68 ... 

The cathodic current of electron transfer is proportional to the concentration of interfadal electrons, n and the anodic current of hole transfer is proportional to the concentration of interfacial holes, p., in semiconductor electrodes as described in Sec. 8.3. Since the concentration of interfacial electrons or holes depends on the quasi-Fermi level of interfacial electrons or holes in the electrode as shown in Eqn. 10-3 or 10—4 (n, = n + dra and p, =p + 4P ), the transfer current of cathodic electrons or anodic holes under the condition of photoexdtation depends on the quasi-Fermi level of interfadal electrons, nCp, or the quasi-Fermi level of interfadal holes, pEp It also follows from Sec. 8.3 that the anodic current of electron transfer (the ipjection of electrons into the conduction hand) or the cathodic current of hole transfer (the ipjection of holes into the valence band) does not depend on the... 

With n-type semiconductor electrodes, the anodic oiQ en reaction (Euiodic hole transfer) will not occur in the dark because the concentration of interfacial holes in the valence band is extremely small whereas, the same reaction will occur in the photon irradiation simply because the concentration of interfadal holes in the valence band is increased by photoexcitation and the quasi-Fermi level pEp of interfadal holes becomes lower than the Fermi level the o en redox... 

Fig. 10-21. Quasi-Fermi levels of holes in transfer reaction of anodic holes, from the valence band (a) of a photoexcited n-type electrode and (b) of a dark p-type electrode of the same semiconductor, to redox particles pCp, = quasi-Fermi level of interfacial holes in a photoexcited n-type electrode where pCp, is lower than the Fermi level cp and in a dark n-type electrode where pCp, equals the Fermi level sp.

For p-type electrodes in the dark and in the photoexdted state, the concentration of majority charge carriers (holes) is sufficiently great that the Fermi level eptso of the electrode interior nearly equals the quasi-Fermi level of interfacial holes hence, the overvoltage Up sc for the generation and transport of holes in the space charge layer is zero even as the transfer of anodic holes progresses as expressed in Eqn. 10-30 ... 

For n-type electrodes in the dark, the transfer of anodic holes reduces the concentration of interfacial holes (minority carriers) so that the quasi-Fermi level pEfj of interfacial holes is raised beyond the Fermi level efcso (pej > ertso) of the electrode interior, hence, a positive overvoltage Up sc emerges due to the diffusion of holes in the electrode as shown in Eqn. 10-31 ... 

Figure 10-23 shows the electron levels and the polarization curves for the transfer of anodic redox holes both at a photoexcited n-fype electrode and at a dark p-type electrode of the same semiconductor. The range of potential where the anodic hole current occurs at the photoexcited n-type electrode is more cathodic (more negative) than the range of potential for the anodic hole current at the dark p-type electrode. The difference between the polarizatitm potential aE(i) (point N in the figure) of the photoexcited n-type electrode and the polarization potential pE(i) (point P in the figme) of the dark p-type electrode at a constant anodic current i is equivalent to the difference between the quasi-Fermi level pej of interfacial holes and the Fermi level bEf of interior holes (electrons) in the photoexcited n-type electrode this difference of polarization potential, in turn, equals the inverse overvoltage rip.sc(i) defined in Eqn. 10-46 ... 

From this discussion, it can be seen that while control over semiconductor potential does not provide control over the interfacial energetics, it does allow one to control the efficiency with which electron-hole pairs are allowed to recombine or alternatively undergo interfacial charge transfer. This process is usually discussed in terms of a quantum yield for electron flow, d>e, as defined in... 

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