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Electronic interactions occur at the point of contact
of the different phases (heterojunction), leading to the
transfer of charge carriers across this junction, when
two or more semiconductors are in contact. The basic
concept is that the photogenerated charge carriers in
the excited semiconductor can be transferred to the
second semiconductor if this is thermodynamically
feasible.
When examining the relative positions of the valence
band and the conduction bands in Table 2, it is
evident that it is in fact thermodynamically feasible for
the generated electrons in titanium dioxide to be transferred
to the lower lying conduction band of any the
iron oxides phases present, while the photogenerated
holes may also be transferred to the upper lying valence
bands of the iron oxides. The narrower bandgap
of the iron oxides is thought to lead to an increase in
the incidence of electron-hole recombination, lowering
the photoactivity, while the continuous hopping
between Fe2+ and Fe3+ in the magnetite lattice has
been reported to enhance electron–hole recombination
[8]. The lower oxidising power and reducing
power of the of the available electrons and holes once
transferred to the iron oxide phase, as suggested by
Litter and Navio [9], can also be translated into lower
photoactivity.
The photodissolution of the iron oxide phase puts
forward the possibility of the occurrence of competing
reactions with the oxidation reaction of sucrose.
The main competing reaction of concern is the oxidation
of the dissolved Fe2+ back to Fe3+. The possible
occurrence of this back reaction has been suggested
by a number of people and is thought to involve the
Fe2+ competing with the sucrose molecule for the
photogenerated holes (or OH•) [6,10–13]. These back
reactions may hinder the oxidation of the organic (even though the photodissolution mechanism itself
might in fact lead to an enhanced charge separation).
The photodissolution results enabled us to make
a very important conclusion regarding the driving
force behind the photodissolution observed. It is clear
that the stability of the coated particles is highly
dependent on the nature of the titanium dioxide.
This conclusion was made upon comparison between
the levels of photodissolution of uncoated magnetite
samples (sample 1), coated-uncalcined samples
(sample 2), and coated-calcined samples (sample
3). While both the bare magnetite particles and the
uncalcined-coated samples exhibited minimal photodissolution,
the coated-calcined samples, made up of
iron oxide in direct contact with a crystalline titanium
dioxide phase, showed very high photodissolution
levels.