induces a splitting of the Fermi level into quasi-Fermi levels of
electrons and holes indicated as nEF andpEF, respectively.
Since redox reactions are connected with reorganization energies
(w), the density of states has a maximum not at the standard
potential (Dred,oxand Ared,ox) but at an energy higher (Doxand
Aox) and lower (Dredand Ared) by the amount ofw. As a conse-
quence, both reductive and oxidative IFET reactions are thermo-
dynamically feasible only when the energies of Aoxand Dredare
equal or below and above the quasi-Fermi level of electrons
and holes, respectively. Whereas standard potentials are usually
easy obtainable, this is not true for reorganization energies which
may reach values of up to 0.5 eV. Thus, a large uncertainty is
connected with a precise calculation of the free energy change
of such type of interfacial electron exchange reactions. These
basic aspects are often overlooked when discussing the energetic
requirements for water splitting. Instead of taking the ideal
value of 1.23 eV, a value of about 2 eV seems more realistic.
To estimate the thermodynamic feasibility of interfacial redox
reactions at a hitherto unknown semiconductor surface, it is
therefore of basic importance to know the position of the quasi-
Fermi level. The quasi-Fermi level of electrons of powders or thin
films of modified titania photocatalysts can be easily obtained by
the “suspension method” developed by Bard et al. (9,10) and
modified by Royet al.( 11 ) for titania and cadmium sulfide. It is
based on the pH-dependence of the flat-band potential of TiO 2.
SCHEME3. Thermodynamics of interfacial electron transfer (IFET)
between a photoexcited n-type semiconductor solid and dissolved donor
and acceptor molecules. The depicted density of states maxima apply
for equal concentrations of reduced and oxidized forms.
376 HORST KISCH