radical cation generated in the reaction step according to Eq. (4)
undergoes a fast and irreversible decomposition (Eq. 6) rendering
the BET (Eq. 7) too slow to successfully compete with formation
of Ared(Eq. 5), which corresponds to the reduction of water by
the reduced methylviologen ( 1 ). In summary, the function of the
transition metal complex is to photosensitize two consecutive
homogeneous electron exchange reactions with a donor and
acceptor.
½M L!hn
½Mþ!L ð 1 Þ½MþL!½ M Lþhn=heat ð 2 Þ½M
þ
LþA!½M
þ
LþA ð 3 Þ½MþLþD!½ M LþDþ ð 4 ÞA!Ared ð 5 ÞDþ!Dox ð 6 ÞAþDþ!AþD ð 7 ÞThe reaction sequence discussed above differs significantly from
photosensitization by a semiconductor, in general just named
photocatalysis, a reaction system wherein a solid photocatalyst
simultaneouslysensitizes twoheterogeneousredox reactions( 2 ).
By analogy with Eqs. (1)–(7), the basic reaction steps may be
summarized in a simplified way according to Eqs. (8)–(10). Light
absorption generates, inter alia, reactive electron–hole pairs
trapped at the surface. It is expected that the distance between
these redox centers should be larger than in a molecular sensi-
tizer and therefore charge recombination may become slow
enough to allow the desired interfacial electron transfer (IFET)
between the solid and adsorbed or dissolved substrates. The
subsequent reaction steps are described by Eqs. (5)–(7).
SC!hn
SC er;hþr
ð 8 ÞSC er;hþr
!SCþhn=heat ð 9 ÞSC er;hþr
þAþD!SCþAþDþ ð 10 ÞScheme 1 summarizes the primary processes occurring after
light absorption by a titania crystal. Due to the band gap of about
3.2 eV, ultraviolet light of wavelength shorter than 391 nm is
VISIBLE LIGHT PHOTOCATALYSIS 373